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A Systematic Literature Review of the Solar Photovoltaic Value Chain for a Circular Economy

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1. Introduction

Solar photovoltaic (PV) energy, or the capture of solar radiation through photovoltaic panels to produce electricity, is considered one of the most promising markets in the portfolio of renewable energies, due to its potential to mitigate global warming and meet the CO 2 reduction targets imposed by national governments and international agreements. The PV industry has grown nearly exponentially in recent years, as showcased by the increasing production volumes and the growing networks of solar installers and financing schemes worldwide. In 2018, global cumulative installed PV capacity reached almost 480 GW, representing about 2% of the world’s electricity output [1]. By 2030, it is estimated that global capacity could reach 2840 GW, while by 2050, it may reach 8500 GW [2]. Other things being equal, and assuming the average lifetime of a PV panel is 25 years, one can expect the accelerated growth in PV deployment to be translated into an exponential increase in end-of-life (EOL) PV waste in the years to come. PV panel waste projections, in fact, show that between 1.7–8 and 60–78 million tons of waste will be lying in landfills by 2030 and 2050, respectively [3]. Up until now, PV systems have operated under a linear “take-make-use-dispose” model, whereby natural resources are extracted, panels are manufactured, commercialized, used, and then mostly disposed of in landfills, where soil and groundwater contamination can occur [4]. For the PV industry to reduce and eliminate waste altogether, a circular, lifecycle perspective needs to be incorporated. Such a perspective will demand changes throughout the PV value chain, from product design to product end-of-life and the collaboration of a wide range of stakeholders, namely businesses, governments, customers, and academia.

Aimed at supporting an informed transition of the PV industry towards a circular economy (CE), this article proposes a systematic literature review (SLR) to understand the current configuration and functioning of the PV value chain, including the issue of reusing electric vehicle (EV) batteries for small-scale solar energy storage, in order to identify potential areas where circular strategies could be implemented. We deem the study of the PV value chain necessary for various reasons. First, current literature has seldom looked at the PV value chain as one holistic entity and at the stakeholders that play a role in its functioning [5,6]. Research focusing on photovoltaic systems has been studied mostly from a forward flow supply chain perspective (i.e., polysilicon production, cell and module manufacturing, PV system installation and recycling), while paying little, although increasingly more, attention to other equally important value chain stages such as R&D for circular product design and circular business models, as well as PV refurbishing, reuse, and recycle. Second, if one is to understand the dynamics guiding the evolution of PV systems, and to expect large-scale PV deployment in the future, a value chain view of the industry is necessary. Failing to account for the different networks that are part of the PV ecosystem can prevent policymakers and new market entrants from recognizing, for instance, that changes in public policy, technology, market conditions or consumer behavior will affect the relative attractiveness and diffusion trajectories of the different PV technologies [7] and the extent to which different circular paths for PV can be realized.

Third, to deal with PV waste effectively, innovation from product design to product recovery is essential. It has been argued that innovation in the renewable energy sector is increasingly distributed and interdependent, as it requires cooperation from incumbent and start-up firms, governments, research institutions, service providers, and so on [8]. Because these actors often display their own value chain structures and interactions, knowledge about these structures can provide opportunities for joint value discovery and creation [9,10]. Fourth, the projected scarcity of critical materials, such as tellurium, gallium, indium, and selenium in thin-film solar cell technologies, or lithium, cobalt, nickel, and natural graphite in EV batteries, also calls for a value chain view. By identifying risks and volatilities along the PV and EV battery value chains, stakeholders can prepare for imminent supply disruptions and ensure the sustainability and resilience of their supply chains [11,12].

Finally, if the goal is to aim for high-value PV reuse and recycling, circular product design and business model strategies must be incorporated in industry practices. For instance, current PV panel designs do not facilitate the effective separation of materials upon disposal, which provides incentives for low-value recycling and landfilling. Similarly, current product-focused business models rarely allow for product maintenance, refurbishing, take-back, or recycling. The development of a circular mindset among the actors of the PV value chain, not only among the actors within the boundaries of the firm, is therefore critical to secure the availability of secondary raw materials and to prevent, delay, or mitigate environmental damage.

Considering the ever-increasing attention given to PV energy and PV waste, this study’s main goals are: (i) to understand how the PV value chain operates, including its main stages and processes, stakeholders, and the interactions among them; and (ii) to investigate which factors inhibit the incorporation of circular economy principles in the existing PV value chain. The remainder of this article is organized as follows: Section 2 briefly introduces the value chain framework, Section 3 provides a detailed view of the methodological framework used to plan and execute this systematic literature review, Section 4 presents a descriptive analysis of the reviewed article database, while Section 5 describes the main stages of the value chain for PV systems. Finally, Section 6 presents the conclusions of this study, and Section 7 poses some questions for future analysis.

2. Analytical Framework and Related Literature

Understanding how PV systems are developed, manufactured, sold, and managed throughout their lifetimes demands a value chain view. The concept of a value chain was coined by Michael Porter as a means of breaking down the activities of the firm into strategically relevant stages, processes, and relationships related to a product or service during the process of delivering “value” for a customer. Such activities involve product manufacturing, product delivery to consumers, and product disposal and/or reprocessing after use [13]. Although initially developed to help understand the value creation process at the firm level, the value chain concept is now also used as a tool for understanding value creation in industries and countries [14,15]. At the industry level, a value chain analysis provides a comprehensive view of an industry, thereby supporting strategic and technology planning for incumbents and new entrants, as well as policy making at a higher level [16].

The accelerated transition to a CE requires research on all relevant aspects of the value chain. In the literature, terms such as “circular value chain,” “circular supply chain,” “supply chain management in a circular economy,” or “closed-loop supply chain” are sometimes used interchangeably. Different from the traditional linear value chain explained above, we define the value chain concept in this article as the myriad of activities involved both in the supply and the take-back chain of the PV industry. Our base definition therefore covers “all stages of the life cycle from idea/concept, raw material sourcing, production, distribution, and end customer use to the point where the product returns to a biological or technical cycle, thus closing the loop” [17]. A value chain perspective has therefore been chosen as means to identify hotspots for value creation at different stages of the PV lifecycle.

Despite the holistic view proposed in this article, most of the published systematic literature reviews linked to solar PV have showed a technical focus, covering topics such as: advances in solar cell research and testing [18,19,20,21], energy losses and degradation of PV modules [22,23,24], forecasting of solar photovoltaic radiation and electricity generation [25,26], digital technologies for PV monitoring [27], and leaching of metals from EOL PV waste [28,29]. Other review articles have been more market-oriented, highlighting the need for government interventions in supporting PV diffusion [30]; the factors influencing residential households’ adoption of PV systems [31,32]; and descriptions of the current PV market, its associated costs, and available technologies [33]. Finally, a growing stream of literature focusing on the management of EOL PV modules has also emerged. For instance, [34] suggests that monitoring and reporting systems at the national and regional level can support the identification and management of current and future streams of PV waste. The authors also stress the need for reverse logistics between geographically close nodes and recycling centers. Furthermore, while analyzing the drivers, barriers, and enablers for the EOL management of PV and battery energy storage systems, [35] suggests that besides technology-related research, socio-economic research is also necessary to boost successful EOL implementation. Different from other review publications, the contribution of this article lies not only in showcasing the current barriers that impede PV and LIB reuse and recycling, and the overall achievement of industry circularity, but also in unveiling untapped opportunities for different stakeholders along the PV value chain.

3. Methodology

To investigate the research questions introduced in Section 1, we conducted a systematic literature review (SLR) by following the methodological framework proposed by Denyer and Tranfield [36] and Tranfield, Denyer and Smart [37]. A systematic literature review is a self-contained research project that uses existing studies to provide answers to research questions, which are usually derived from policy or practice. A systematic review differs from a traditional, more general, literature review in that it proposes a replicable, scientific, and transparent process, thereby creating a foundation for advancing knowledge in a particular field and facilitating theory development [38,39]. A summary of the employed methodology is displayed in Table 1.

3.1. Phase 1: Planning the Review

As a first step, and to ensure the validity, reliability, and replicability of the results, we developed a draft protocol for carrying out the literature review process. After agreeing on a structured process flow for the review, we decided to embark on an informal preliminary literature scan to better outline the scope of our research. The preliminary scan was instrumental in: (i) confirming that there was indeed a gap in the literature, (ii) delineating the thematic focus for the review as well as the exclusion criteria for the selection of articles, and (iii) defining the time frame and the set of keywords to be employed in the systematic search.

3.2. Phases 2 and 3: Location, Selection, and Evaluation of Studies

3.2.1. Location

The literature search was undertaken using two of the largest abstract and citation databases of peer-reviewed literature, namely Web of Science and Scopus. A search in both databases ensured that the review results considered all the available evidence and were based on quality contributions [36]. To identify the most reliable types of publications, we adhered to the “fit-for-purpose” rationale, which suggests that rather than a hierarchy of evidence (i.e., ranking of the publication outlet), the criteria for the selection of articles must rely on the purpose and context of the research. Hence, we delimited our search to peer-reviewed articles in English, published in academic journals; proceedings of international conferences; and book chapters. The inclusion of conference papers ensured that the results covered the most recently available knowledge, especially concerning the use of EV batteries for the stationary storage of PV energy. Industry reports and other grey literature were intentionally excluded due to the challenges of collecting them systematically.

We set the keyword search’s timeframe from 2000 to 2020 because solar energy markets only regained momentum from the early 2000s [40]. Similarly, sales of EVs also started to take off since the beginning of the 21st century. Finally, we extended our search to the year 2020 after realizing that a significant number of scholarly articles had been published just recently. The choice of time frame was confirmed during the preliminary literature scan, when the search results returned almost no publications before the year 2000 and a rising number of published articles within the past three years.

3.2.2. Selection and Evaluation

After conducting the actual search using the set of keywords displayed in Table 2, all information from the resulting articles (e.g., title, abstract, keywords, publication year, and publication outlet) was exported to two Excel spreadsheets (i.e., one for Web of Science results and another for Scopus results). The two databases were then merged into one ( n = 371) and the combined results analyzed to identify duplicate entries ( n = 27). Once identified, duplicates were tagged and removed from the merged database.

To determine whether an article met the inclusion criteria, we read the article’s title and abstract, and, when necessary, scanned the article’s complete content. This filtering process resulted in the exclusion of 215 articles, with a final number of 129 articles being considered for further analysis. Articles were removed from the merged database for various reasons, including: (i) their content did not match the topic of this study; (ii) access was restricted; (iii) they were deemed to be too technical or not fitting the scope of this review (i.e., a great number of publications addressed PV panel manufacturing and deployment exclusively from an engineering, materials science, chemical or electrical perspective or discussed only one stage of the PV value chain); (iv) they referred to alternative types of solar energy, such as solar thermal or concentrating solar power, or discussed solar PV only marginally (i.e., photovoltaic energy was mentioned along with other renewable energy sources such as hydropower, wind power, biomass, biogas, and biofuel); and (v) they referred to off-grid, small-scale PV applications, such as water heating, lightening or mobile charging, primarily in remote areas in Africa, or to related, yet different, value chains, such as refrigeration chains for food fueled by PV energy. Finally, the articles’ list of cited references served as a secondary and additional source of analysis. Cross-referencing resulted in 19 articles being added to the primary database, resulting in a total of 148 articles being analyzed for the present review (see Figure 1).

3.3. Phase 4: Data Analysis and Coding Scheme

In this final stage, we imported all the articles that met the inclusion criteria ( n = 148) to the coding software Atlas.ti and read them all in detail to perform an open coding content analysis [41]. Using this technique, we coded the article’s content inductively (i.e., open coding) and then structured the incoming data according to its relationship to the PV and EV battery value chains (i.e., axial coding). The established coding system included such labels as: UPSTREAM PV cell raw material, UPSTREAM PV cell manufacturing, DOWNSTREAM PV module recycling, and STAKEHOLDER equipment manufacturers. Throughout the coding process, we also kept a diary to track our thought process and to structure our analysis and findings. At the end of the coding stage, we reviewed each code and merged or deleted some of them for clarity. This process resulted in 214 codes that served as the basis for the content in this article.

4. Analysis and Results: Descriptive Analysis

4.1. Number and Sources of Publications

The trend indicates a growing interest within the academic community in solar photovoltaic-related research, especially since 2013. Around 88% of the articles ( n = 135) written between 2000 and 2020 have been published since 2013 (see Figure 2). The greatest number of publications occurred in 2018 ( n = 34), exhibiting approximately a 140% increase since 2017. Overall, between 2013 and 2017, publications increased by an average of 14% per year. Before 2013, it seems academic research was undergoing an incubation period, especially in relation to the analysis of value chain actors and dynamics.

Figure 3 indicates that journal publications constituted the major avenue for disseminating research results. The academic journals with the highest number of publications were the Journal of Cleaner Production ( n = 15), Energy Policy ( n = 9), Renewable and Sustainable Energy Reviews ( n = 8), Renewable Energy ( n = 8), Sustainability ( n = 6), and Applied Energy with 5 publications (see Table 3 and Table 4). The six most prolific journals accounted for 35% of the analyzed records. Finally, results show that the topic of the PV value chain is suitable for publishing in a range of specialized journals ( n = 68) that focus mainly on sustainability, the environment, and energy issues.

4.2. Methodological Trends

Upon examination of the article database, we observed four approaches to research, namely: (i) literature review (i.e., a study that collects, reviews, and analyzes previously published research); (ii) modelling and simulation (i.e., a study that uses mathematical functions for decision-making); (iii) case study (i.e., a study that uses qualitative data to build a case exploring a problem); and (iv) theoretical and conceptual (i.e., a study that proposes a theory or a conceptual framework) [42] (see Figure 4).

Modelling and simulation was by far the preferred methodology among researchers in the database (48%). Some of the most popular modelling techniques included optimization, life cycle assessment, financial modelling, and techno-economic modelling. These methods were used to study a broad range of problems, such as environmental sustainability, competing supply chains, R&D cooperation among actors in the value chain, and growth evolution of the PV industry in a specific location or for specific firms. Theoretical and conceptual papers ranked second in the list of the most preferred methods (32%), followed by case studies (16%) and literature reviews (4%). As expected, the share of case study and literature review articles has been increasing only lately, following the recent boom in PV installations and scholarly publications on the topic.

4.3. Geographical Trends

European countries have been at the forefront of academic publishing in relation to solar PV systems (see Figure 5). Out of all the reviewed articles, 39% had first authors who work at a European institution, with Germany, the United Kingdom, and Italy being the most active players. Asia and North America are also strong centers of publication, with 35% and 18% of the publications originating in these regions, respectively. China, which captures 55% of the articles in Asia, has had a much-publicized increase in its share of scientific output after becoming the largest producer of solar cells in 2007 and the largest producer of solar panels in 2008 [43]. Furthermore, developing countries present a marked contrast to the European and Chinese cases. While governmental sponsorship has been pivotal in fostering the establishment of a solar PV industry in Europe and China, policies for PV deployment in developing countries have been much less robust or even nonexistent, with data and academic research output being scant. For a summary of research subjects, methodologies employed, and author geography see Table 5.

5. Analysis and Results: Discussion

The solar PV value chain can be regarded as complex, not only technologically, but also because of the various supply chains, stakeholders, installation sizes, business models, and customer segments that it encompasses. Despite its inherent complexity, there is no comprehensive study that describes the interdependencies between the different value networks that play a role in the PV value chain [6]. Before presenting a detailed description of each stage of the PV value chain, however, Table 6 presents a classification of each reviewed publication according to the main themes that were identified in this SLR.

5.1. Upstream PV Value Chain

5.1.1. Research and Development (R&D)

Review results show R&D efforts in the PV industry are mostly concentrated on new material development and cell efficiency improvements (i.e., chemical process industries), as well as on specialized machinery and robotics for manufacturing [95], rather than on new recycling technologies or design for EOL. Currently, one can distinguish between three categories of PV technologies: (i) 1st generation technologies of mono- and multicrystalline silicon solar cells (c-Si); (ii) 2nd generation technologies of thin-film technologies; and (iii) newer, 3rd generation technologies of multi-junction, organic PV cells, and concentrating photovoltaics (CPV) [58,72]. Each PV technology features its own cell type, based on different semiconductor materials, module efficiency and area requirements for installation. For all technologies, large jumps in conversion efficiencies are expected in the long term. As an example, it is reported that the layer thickness in a CdTe (cadmium telluride) module could be reduced to around 1.0 µm, resulting in an efficiency gain of around 18% [50]. The same can be forecast for polycrystalline single-junction modules that could see their material intensity and weight lowered and their efficiency increased through basic research. Nowadays, almost all the leading countries in PV production (i.e., China, Japan, Germany, and the United States) devote government funds to enhancing and strengthening their basic PV research capabilities and infrastructure.

Besides the emphasis on efficiency improvements, research and implementation efforts also need to target eco-design strategies for PV to ensure, for instance, that modules are built with delamination in mind, allowing the recovery of high-quality silicon wafers at EOL. Digital technologies also play a role, not only by supporting the storage of product information, i.e., material composition and technical parameters), but also the monitoring and maintenance of PV modules for further reuse and recycling.

5.1.2. Solar Grade Silicon Production

Pure silicon is the dominant semiconductor material used in the production of solar cells because of its abundance, non-toxicity, high and stable cell efficiency, and the maturity of its production infrastructure [83]. Silicon is also reported as the only element that can help the PV industry achieve the number of terawatts needed for renewables to make a substantial contribution to global energy use [44]. Although this element is the second most abundant in the crust of the earth, it is not pure in its natural state and must be refined before it is used in the production of solar cells, which require high-purity silicon of at least 99.999999% (6N).

The first reported step in the overall silicon PV production process thus involves the conversion of high-purity silica sand into silicon. The resulting metallurgical grade silicon (MG-Si), of about 98.5% purity, is obtained by the carbothermic reduction of silicates in electrode arc furnaces at temperatures above 1900 °C. Most of the MG silicon at this point is used for aluminum casting or in the chemical industry. The remainder MG-Si is further refined and converted into semiconductor or solar grade silicon (SOG-Si), by using, among others, the modified Siemens process or the fluid bed reactor process [52,92].

5.1.3. Crystallization, Ingot Molding, and Wafering

Before solar cells are manufactured, a silicon ingot is grown by different crystallization methods. Crystallization is one of the first steps in the silicon solar-cell value chain and can be differentiated by monocrystalline and poli- or multicrystalline processes [6,51,52]. Although multicrystalline silicon cells exhibit lower conversion efficiencies than monocrystalline ones (13–16% vs. 15–20%), around 56% of the world’s solar cells today are produced by multicrystalline processes. This is because they are cheaper to manufacture and, thus, more preferred in the market. Once either type of silicon ingot has been manufactured, it is sliced into thin disks or wafers, and then chemically treated, doped, coated, and provided with electrical contacts in order to produce solar cells [181]. Most of the published research at this early stage of the value chain deals with examining the effects of defects and impurities on material property and ultimately on solar cell performance [44,51].

5.1.4. Solar Cell Manufacturing

Prior research suggests that solar cell and module manufacturing have been studied from a technological point of view only [52,169]. In this spirit, the following lines provide a quick review of the technical characteristics of silicon and thin-film solar cells.

Silicon-Based

Crystalline silicon is the most prevalent in the global market, accounting for around 90% of PV production [119]. To turn wafers into c-Si solar cells that can convert solar power into electric power, wafers are first cleaned and placed in a phosphorous diffusion furnace, resulting in a P-N junction for the photovoltaic effect. Next, the top surface of the wafer is covered with an anti-reflective coating to reduce the reflectivity of light and raise efficiency. Afterwards, electrical contacts are imprinted on the entire front surface of the wafer, while aluminum-based conductive material is deposited on the back surface. To finish, each cell is electrically connected to other cells to form cell circuits for assembly in PV modules [6,52,62]. Sets of cells or “strings” usually connect 10–12 cells in a silicon-based module and 60–100 cells in a thin-film module.

Originally introduced in the 1970s, thin-film cells are an alternative PV technology aimed at reducing the cost and price of solar cells by using little or no silicon in the manufacturing process. Although they exhibit an easier manufacturing process and lower costs than c-Si cells, they also present lower light-to-voltage conversion rates (10–11%), and therefore require more physical space to generate the same amount of power. Among the several types of thin-film cells that exist nowadays (e.g., cadmium telluride (CdTe), copper indium gallium selenide (CIGS), gallium arsenide (GaAs), and amorphous silicon (a-Si)), CdTe cells are the most prevalent [33]. Otherwise identical in structure and function, the difference between c-Si and thin-film solar cells resides in their thin and flexible layers and in the semiconductor material they use: CdTe, CIGS or GaAs instead of silicon.

The reviewed literature in connection to thin-film cells reported concerns regarding the scarcity of the base metals that make up CdTe thin-film cells (i.e., tellurium, indium, and gallium) and, therefore, on the suitability of this technology for large-scale PV deployment [11,50]. Some authors even suggest that the current base of critical elements in thin-film cells is not large enough to support large-scale PV deployment, even if the industry were to somehow monopolize the reserves of each element. Technological improvements involving material reduction and cheaper byproduct recovery processes, as well as circular strategies to recover critical elements from decommissioned PV panels, could offset the potential supply limitations and imminent price increases associated with the critical materials in thin-film cells.

Emerging PV Cell Technologies

Both crystalline silicon and thin-film technologies are single-junction. Multi-junction solar cells, or cells with multiple P-N junctions, promise to drastically increase solar cell efficiency because of their ability to absorb different multiple light wavelengths. Grau, Huo and Neuhoff [75] report, for instance, that for two- (tandem), three- and four-junction devices, maximum efficiencies of 55.9%, 63.8%, and 68.8% are predicted, respectively. Besides multi-junction cells, organic materials also offer the potential for low cost and high energy absorption. These cells can be of various natures, namely: petrochemical cells, dye sensitized solar cells, organic and polymer solar cells, and other emerging technologies such as quantum dot solar cells. These technologies are still under investigation and have not yet been widely commercialized.

5.1.5. Module Manufacturing and Balance of Systems (BOS)

After silicon has been casted into ingots and wafers have been sliced from the ingot blocks and turned into solar cells through etching and polishing, cells are put together into modules [72]. Solar modules are the core components of PV systems and account for about 40% of the PV system price [116]. Modules are assemblies of typically 6 × 10 or 6 × 12 series-connected solar cells, which are packaged into a protective multilayered structure of five main components: (i) the front cover (tempered glass), (ii, iii and iv) the interconnected solar cells matrix in an envelope of two encapsulant layers (front/back), and (v) a back cover (back sheet or tempered glass). Such a structure provides electrical insulation and long-term protection against external environmental stresses.

Solar modules together with BOS components (e.g., inverters, batteries, controllers, and trackers) are assembled into solar PV systems by installers [62,72,116]. Of all the BOS elements, the inverter is the most important as well as the most expensive and the most technically complicated. An inverter transforms direct current (DC) from the PV array into a form of alternating current (AC) electricity that can be connected to the electric utility grid. Most reviewed articles concerning module manufacturing focus on the PV module manufacturing process (see Figure 6) as well as on the key technological improvements resulting in the ever-decreasing costs of silicon PV modules.

5.1.6. PV Installations

This stage is concerned with the installation of the PV system and the delivery of electricity at the customer’s premises. Small rooftop installations are typically planned by the installer, whereas larger rooftop and open space installations are handled by a planner who takes care of various aspects such as system design and installation, permit and license acquisition, construction, operation, maintenance services, insurance, and so on [72]. The few publications focusing on this particular topic highlight that the installation of a PV system is a labor-intensive process, because qualified personnel are needed to connect the solar panels and to provide after-sale services to customers [85,90]. The personnel needed for a PV installation will depend on the project size, namely whether the PV project is residential (i.e., 1–10 kW), commercial (i.e., 11–500 kW), or industrial (i.e., >500 kW). Figure 7 presents an overview of all stages in the upstream PV value chain and their relevant stakeholders.

5.1.7. Geography and Composition of the Global PV Supply Chain

Research showed that knowledge and technology-intensive R&D and capital equipment segments have been traditionally located in Europe, the United States, and Japan. This scenario, however, has recently changed as many segments of the PV value chain, from polysilicon production to module manufacturing, have become part of a global value network now featuring new players such as China and Taiwan [72]. China’s success in the PV industry as a rapid innovation follower has been the result of government-sponsored import-substitution policies where the infant Chinese PV industry first produced for the local market before then exporting worldwide when firms reached an international level of competitiveness [65,72,83,89]. Despite the new competitive landscape, low-labor, high-value added activities, such as polysilicon production and capital equipment manufacturing, are still led by European and American players [91,130].

In terms of industry structure, the supply chain for c-Si modules is described as being fragmented, because it is comprised of a plethora of firms specializing in either polysilicon feedstock, wafer, cell, or module manufacturing [52]. The articles in our sample suggest that the number of companies at the lower-end of the upstream supply chain (i.e., firms in charge of sales and installation of PV systems) exceeded those located in the upper-end (i.e., suppliers of raw materials and manufacturers of wafers, ingots, and cells) [124]. However, upstream manufacturers, particularly the suppliers of capital equipment, silicon materials, and silicon wafers, provided the most value added and achieved the highest profits, because upstream activities required more firm and labor expertise, rather than standardized, routine tasks [72,89]. In contrast, manufacturers and installers of modules and panels achieved the lowest profits because barriers to entry were low and competition high [6].

5.2. Midstream PV Value Chain: Business Models

Barriers to PV system adoption, among them high up-front costs, long payback periods, and the difficulty of planning and installing a PV system, spurred the need to look for new business models (BM), or for new ways of creating, capturing, and delivering value in the industry [71,182]. The literature on business models for the PV industry was not vast. In the few identified publications, authors focused on exploring the characteristics surrounding three main PV BMs: (i) home-owned systems, (ii) third-party ownership (TPO) models, and (iii) community solar systems.

5.2.1. Home-Owned Systems and Feed-in-Tariffs (FITs)

In a home-owned system, customers own and finance (directly or indirectly) the upfront costs of their PV system [6] (see Figure 8). This type of business model is targeted at households and SMEs who own a sufficiently large roof (with a good solar orientation and no shadows) and have incentives to reduce the financial burden caused by high electricity costs [69,70]. The Netherlands, Denmark, China, and Germany are part of the fifty plus countries that have implemented FIT schemes [6]. Currently, most of the electricity generated by these home-owned systems has been connected to the grid and is reimbursed by utilities according to a regulated feed-in-tariff rate (FIT) [71]. A FIT is an energy-supply policy aimed at attracting investments in renewable energies by means of a payment ($/kWh) to PV owners for any electricity that is fed back to the grid. This payment is always embedded in a long-term guaranteed purchase agreement that can last up to 25 years.

The German solar industry is reportedly a prime example of how feed-in-tariffs have helped the PV solar industry flourish. Germany, which held the world’s number one place in PV installations from 2004 to 2012, was a pioneer in passing the “Renewable Energy Sources Act” (EEG) that guaranteed a minimum 20-year FIT for customers. The German EEG states that the PV electricity fed into the grid by PV installation owners has to be purchased by utility companies at an enhanced price [77]. Variations in the payment rates for FITs depend on multiple factors including energy prices, the state of the domestic electric infrastructure, and the capacity and nature of the PV installation [75].

5.2.2. Third-Party Ownership Models (TPOs)

Third-party ownership (TPO) models were born as a response to the “high up-front costs, low operating costs” profile of PV energy provision. In a TPO model, solar service firms plan, build, own, operate, and maintain solar PV installations at the customers’ premises, selling electricity to them for a predetermined period [9] (see Figure 9). Solar service companies provide a full-service solution that includes the inspection of the potential installation site, the evaluation of providers and installers (they might be the same entity), the arrangement of financing, insurance, and permits, the negotiation with utilities to sell surplus electricity to the grid, the maintenance of the solar system, and eventually the responsibility for scrapping [9]. While customers or hosts benefit by not having to deal with the high upfront costs associated with PV installations, and by passing the long-term operation and maintenance of the solar installation onto the TPO provider, the service provider benefits from the tax credits and revenues resulting from the sale of electricity [9].

Under a TPO model, the literature differentiates between two types of financing methods: (i) leasing and (ii) Power Purchase Agreements (PPA). In the lease model, customers (i.e., property owners or lease holders) consume the electricity generated by the PV system and pay the installer/developer a fixed monthly installment, regardless of the system’s energy production. Conversely, in the PPA case, customers buy electricity from installers at a predetermined price each month, usually at a rate lower than the one offered by the local utility. This is a way for residential and business customers to incorporate predictability in volatile electricity markets. Contracts under the PPA model usually range from 15 to 25 years, after which customers can buy or return the PV system to the service provider or renew their contracts [71].

The TPO model was found to be reliant on a set of contextual conditions (e.g., tax credits, and tariffs, as well as market and consumer characteristics) that determined its financial viability and deployment trajectory in different locations [71,116]. After its particular success in the United States, other countries, such as the UK, the Netherlands, and Singapore, have implemented similar systems over the past few years [9,56].

5.2.3. Community Solar Model

In a community model, multiple users purchase electricity from an off-site PV park or garden without having to host their own PV systems on-site [70]. Users that subscribe to this model either lack a suitable roof for installing a PV system (e.g., shaded, aged or damaged rooftops) or property ownership rights (e.g., people who rent or lease instead of owning, people who are planning to move). Under a community-shared business model, participants can purchase rights of the total output of the solar system without the need to pay any upfront costs or deal with the technical complexity of the PV installation (see Figure 10). In return, subscribers receive credit on their energy bills. Alternatively, customers can pay an upfront fee to finance the costs of the project, thereby purchasing an equity stake in the revenues from a portion of the plant [69].

5.3. Downstream: End-of-Life Management of PV Systems

The appropriate end-of-life management of PV waste is of utmost importance, not only for the collection and recycling of important raw materials such as aluminum and glass, but also for the effective disposal of hazardous elements such as lead (from silicon modules) and cadmium (from thin-film modules). Growing volumes of PV waste also represent an opportunity to incorporate new value-added activities across the PV value chain and an avenue for achieving combined environmental and socio-economic benefits for multiple stakeholders. Downstream, most publications focused on PV panel recycling, with no mention of other circular strategies such as PV refurbishing or reuse.

5.3.1. PV Panel Reuse

Although the second life use of PV panels represents a way to slow material loops in a CE, there is no indication of significant academic research output in this area. Overall, a lack of reliable data on module degradation and yearly aging, as well as on failure rates and types, prevents relevant parties from analyzing which types of module failures warrant repair, and at what cost and performance levels. Another important unknown relates to the business case for reuse: (i) given the dramatic fall in PV module prices and increasing module efficiency over time, would consumers opt for second life modules instead of new ones? (ii) Depending on the cost structure and the technical performance of second life PV panels, which customer segments would be suitable marketing targets (e.g., residential vs. industrial customers, B2C vs. B2B, system owners pursuing the replacement of a broken module at existing installations vs. investors pursuing optimized energy output at a new, large-scale PV installations)? (iii) What would the value proposition and the value capture formula for the target customer segments be? These are questions that are yet to be investigated.

Finally, it is not yet clear whether the definition for second life PV involves panels that are a product of warranty returns, early defects, natural disaster damage, or production scrap, just to mention a few possibilities. There is a crucial need to develop a terminology to define the state and prospective use of second life panels so policy makers and industry partners can encourage reuse and refurbishment through legislation, certifications, and industry practices.

5.3.2. PV Panel Recycling

As pointed out before, most R&D funds in the PV industry have targeted improving the efficiency of crystalline silicon panels, with less effort being devoted to devising cost-effective, innovative processes for dismantling and recycling PV panel waste (see Figure 11). Not much has been discussed about PV recycling because most of the PV systems that are currently in operation have only been installed since 2010. Therefore, PV waste today consists primarily either of pre-consumer waste (i.e., processing scrap from manufacturing) or decommissioned failed panels, and not of end-of-life PV modules [10,54,123]. With no substantial volume of panels to recycle, little research has been encouraged on this topic.

Current academic output in connection to recycling has mainly focused on: (i) the probability that PV panels containing recycled materials are likely to generate reduced levels of electricity, (ii) the presence of various manual activities that undermine the effectiveness of the recycling process, (iii) the probability of cross-contamination when PV waste is mixed with other types of waste [5], and finally (iv) the high dismantling, transportation, and recycling costs resulting from the presence of hazardous elements in PV panel waste [119].

In terms of regulations, the European Union introduced the EU Waste from Electrical and Electronic Equipment (WEEE) directive to regulate the collection and recovery of end-of-life PV modules in 2012. The so-called “WEEE Recast” demanded that all EU member states encode the directive in national law by February 2014 and required that all PV panel manufacturers, regardless of their geographical location, finance the costs of collecting, recovering, and recycling all the PV panels sold in Europe [5,119].

Environmental Issues Related to the Disposal of PV Panels

Some of the critical environmental issues associated with the disposal of EOL PV panels include losses of scarce metals (e.g., silver, gallium, indium, and germanium) and conventional materials (e.g., aluminum and glass), plus the release of hazardous metals (e.g., cadmium, lead, tellurium, and selenium) and toxic gases (e.g., hydrofluoric acid) into the environment [119,120,121]. The leaching of hazardous materials such as Pb and Cd, which takes place when the glass that encapsulates the PV cells is broken down or damaged, has been of particular concern [119]. Cadmium, for instance, is believed to cause itai-itai disease and to be toxic to fish and wildlife, as well as to the human body. Once absorbed, cadmium can cause lung, kidney, and bone damage [124].

Furthermore, many elements critical to emerging PV technologies, such as indium, tellurium, and gallium, today exhibit near-zero recycling rates [12]. Indium, for instance, is present in amorphous silicon and copper indium gallium selenide panels, while gallium is present in copper indium gallium selenide panels, concentrated photovoltaic panels, and other emerging panel technologies [119]. Although these metals account for about 1% of the panel volume, their value is significant, and their non-recirculation would signify a loss for manufacturers and the industry in general. The main challenge is to find methods that allow for recovery at the highest possible purity level.

Various authors report on the loss of profitability that will result from recycling PV panels. Current low volumes of decommissioned panels not only make recycling expensive, but also decrease the incentives that manufacturers have to proactively engage in recovery and recycling schemes [5]. High collection, dismantling, transportation, and capital costs (including machinery, chemicals, and other materials) associated with establishing recycling infrastructure mean that it is not economical to recycle at low waste volumes today. High recycling costs also increase competition for landfilling (i.e., it is cheaper to landfill than to recycle) and the incentives for low value recycling (i.e., no material separation before recycling). The question is also whether the recovery of certain precious materials might profitably offset overall recovery costs, thereby supporting the competitive position of PV manufacturers and PV technologies.

5.4. Electric Vehicle (EV) Batteries for PV Energy Storage

Giving electric vehicle batteries a second life as a stationary unit for renewable energy storage not only helps the PV industry become more circular, but it also prolongs the lifespan of batteries (through reuse) and delays costly recycling by 3–15 years. Lithium-ion batteries (LIBs) are removed from the EV when their maximum capacity has degraded to 70–80% of the original capacity, which occurs about 8–10 years after the vehicle has entered into operation [139]. At this point, when the vehicle is no longer suitable for automotive purposes, EV batteries can be re-purposed and given a second life use in less stressful applications such as stationary storage units for PV energy [149]. A business model that couples PV technology with storage devices could help fit intermittent renewable technologies into the existing power generation system and increase solar energy dispatchability (see Figure 12).

According to the few articles that simultaneously discuss LIBs and PV systems, the second life use of LIBs can only be accomplished once some issues are resolved. This would specifically involve: (i) finding out what the costs of refurbishing an EV battery are; (ii) dealing with the uncertainty surrounding the reduction in capacity or efficiency of the LIB after its first life; and (iii) dealing with warranties, reliability and safety concerns, as well as regulatory barriers that hinder customer trust and the adoption of second life LIBs [140,146]. A separate study that details the EV battery value chain could shed light on more concrete answers to such questions.

6. Discussion and Conclusions

Photovoltaic installations have experienced explosive growth globally following the increasing attention of industry and policy on climate change mitigation, the decarbonization and diversification of the energy sector, and energy security. The expected expansion of global solar PV generation capacity will inevitably translate into a large volume of solar panel waste in the future. A similar growth/waste scenario is expected for lithium-ion batteries, which end their automotive life when their maximum capacity has degraded to 70–80% of their original capacity.

In this article, we posit that a closer look at the current functioning and structure of the PV value chain is necessary to highlight critical improvement areas to achieve circularity in the PV industry. To paint a more refined picture of the PV value chain, we conducted a systematic literature review based on 148 articles published between 2000 and 2020. Results showed that most of the academic research output related to the studied topic has: (i) increased since 2013; (ii) been primarily published by European research institutions, with Germany, the United Kingdom, and Italy at the forefront; and (iii) been technology-focused, concentrating on ways to achieve more efficient and competitive, brand-new PV systems. Furthermore, almost all the papers that matched the search criteria for the SLR provided a narrow view, describing the PV value chain as starting with raw material procurement and ending with the installation of PV systems at the customers’ premises.

Table 7 provides a summary of some of the main issues, in connection with circularity and throughout each stage of the PV value chain, that emerged during the SLR. These issues can be understood as barriers to circularity from an industry perspective and have been classified according to different criteria, such as technical, financial, customer-related, and infrastructure-related. Upstream, both the PV and the EV industry allocate R&D funds for efficiency improvements in the asset’s first life, disregarding investments in design of easier-to-recover panels or more cost-effective recycling technologies. Ensuing module circularity and smartness to enable module repairability (e.g., replacement of bypass diodes in the junction box or the complete junction box), dismantlability (e.g., separation and recovery of the semiconductor from the frame, glass, encapsulants and back sheet) and material disclosure (e.g., metals and polymers) should be a priority for industry players and policymakers if circularity in PV is to be achieved.

Midstream in the PV value chain, business models catered to the needs of brand-new PV system owners only, e.g., home-owned, with no mention of innovative business models supporting the deployment of second life modules decommissioned due to technical failures, insurance claims, repowering or early replacement. BMs that enable the diffusion of second-hand PV modules in low-income economies, where the low-cost feature could compensate for the lower remaining lifetime and lower performance of used modules, could be an alternative. Because of their geographical location, developing countries tend to be greatly endowed with renewable resources, including solar irradiation. Tighter budgets, lower requirements for panel aesthetics, increased tolerance towards modules with no warranties, and the need for still efficient, yet affordable modules, make second life PV a suitable option for low-income areas that lack access to grid electricity. Examples of use cases include not only home energy applications, battery charging and solar Wi-Fi, but also solar irrigation and refrigeration for agriculture. The latter are particularly critical for developing countries, where households rely on small-scale agriculture for sustenance, income, or both. All in all, off-grid solar solutions represent a clean energy alternative to replace environmentally harmful energy sources (e.g., charcoal from fuelwood) and reduce carbon-related emissions, increase rural electrification levels, and help provide income-enhancing opportunities and raise living standards for disadvantaged communities. Recent environmental and health-related shocks are also a reminder of the need for establishing off-grid energy preparedness to increase self-sufficiency and systemic resilience for energy provision. This is especially true in the mentioned areas where the impact of such shocks tends to be of a higher magnitude.

We also highlight the fact that scarce statistical data on PV module failures and the costs of the corresponding repairs hinder the emergence of BMs for PV and battery reuse. We therefore believe that additional research is necessary to more accurately estimate not only the possible volume trajectories of second life PV and EOL waste but also the levelized cost of electricity (LCOE) (i.e., the net present value of the total cost of a system divided by the total amount of energy it produces) for both new and second life PV and batteries. Only when a second life PV system has a LCOE that is lower or at least the same as the LCOE of a system with new panels, is it financially attractive for customers in all market segments. With decreasing costs and increased efficiencies for newer PV panel technologies, one could argue that new PV systems will be the preferred choice in developed economies, where consumers rely heavily on high efficiency, aesthetics, and warranties. If this is the case, and early replacement takes place, volumes of decommissioned PV panels will be higher than expected. Finally, business models dealing with new PV systems were found to be contextual and adapted to the market needs and regulatory landscape of the country where they were the most prevalent (e.g., home-owned systems in Germany or third-party ownership models in the United States).

At the downstream end of the PV value chain, lack of design for refurbishing, disassembly, and recycling, current low volumes EOL panel waste, differences in PV panel architectures, and infant recycling technologies and infrastructure, currently turn PV recycling into an unattractive and unprofitable activity for manufacturers and recyclers. Similarly, low volumes of waste combined with different battery chemistries and configurations, as well as uncertainties surrounding the economic, technical, and environmental viability of repurposed EV batteries for energy storage, represent some of the main hurdles to the cost-efficient deployment of EV batteries for second life. When these uncertainties are eliminated, public policy could support the development of certification schemes that can boost customer trust and accelerate market adoption for both second life PV and LIBs. Finally, the barriers presented in Table 7 evidence the extent to which, from the perspective of the customer, both intrinsic (e.g., knowledge and perception of circular products) and extrinsic attributes (e.g., product infrastructure, pricing, warranties) must be addressed if the diffusion of circular business models in the PV industry is to be secured. All in all, we posit that value chain challenges and barriers can be taken as opportunities for the creation of future innovative value formulas and policies that address current technical, socio-economic and regulatory hurdles.

7. Future Research

What will be the impacts of raw material scarcity, price fluctuations, or other external shocks such as pandemics or extreme weather events on the resilience of the PV and battery supply chain? Additionally, what are the implications of scarcity and fluctuating prices for R&D activities and high-value material recovery activities at EOL (i.e., at the raw material stage)?

Which PV technologies and battery chemistries will triumph over others in the quest to dominate market share over the medium and the long term? Additionally, what are the effects of these trajectories on the adoption, uptake, second life use, and decommissioning of PV systems and LIBs (i.e., PV and EV battery cell/module manufacturing stage)?

How will the mix of dominant PV and battery technologies affect different policy options and industry arrangements for the deployment of innovative business models (that facilitate monitoring, collection, reuse, and recycling)? How do new BMs create simultaneous value for manufacturers, service providers, end-customers, and utilities (i.e., at the deployment and business model stage)?

For both reuse and recycling scenarios, what are the estimated recovery rates, costs, and performance indicators for each PV technology? Additionally, at what rate will the recovered materials be used in new manufacturing cycles? Moreover, with new “circular tasks” to be performed (i.e., refurbishment for reuse, recycling, and so on) new ecosystem actors are likely to emerge. If so, what will be the nature of the work performed by these actors and what is their connection with the traditional actor network of the PV and LIB value chain (i.e., circular economy strategies)?

Given the complexity of a circular PV industry, answering some of these questions with the aid of quantitative complex system methods such as system dynamics or agent-based modelling might be appropriate. These tools have the power to capture the many dynamic relationships (e.g., feedbacks, non-linearities, individual actor behavior) and the various impact types (e.g., social, environmental, and economic) inherent in PV and EV value chains. For instance, a time-dependent analysis of how price changes in conventional and alternative energy sources, coupled with how different business models and government policies, as well as customer-related behavioral factors, affect the competitiveness of the PV industry, the uptake of different PV technologies, and the subsequently available resource types and qualities for reuse, refurbishment, and recycling, is a concrete example of a modelling application. Additionally, simulation models could not only enhance understanding about different industry development scenarios but could also help identify key circularity metrics at the firm and industry level.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/su13179615/s1 , an Excel file with a record of all the reviewed publications can be found as supplementary data to this article.

Author Contributions

Conceptualization, methodology, and software, M.A.F. and S.N.G.; formal analysis and original draft preparation, as well as writing, reviewing and editing, M.A.F.; funding acquisition, project administration, and article proof-reading, S.N.G. All authors have read and agreed to the published version of the manuscript.

This research has received funding from the European Union’s Horizon 2020 research and innovation programme CIRCUSOL under grant agreement No 776680.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Data Availability Statement

Acknowledgments

We are thankful to all CIRCUSOL partners for their initial feedback on the content of this article, as well as to the anonymous reviewers, whose comments and suggestions helped improve and clarify this manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

Figure 1. Flow diagram of the systematic review.

Figure 2. Distribution of reviewed publications over time (n = 148).

Figure 3. Proportion of journal, conference and book chapter publications (n = 148).

Figure 4. Temporal distribution of research methodologies.

Figure 5. Temporal distribution of the geographical location of the first author.

Figure 6. Manufacturing process for silicon PV modules.

Figure 7. View of the upstream or supply side of the PV value chain.

Figure 8. Business model 1: Home-owned PV systems.

Figure 9. Business model 2: TPO model.

Figure 10. Business model 3: Community-solar model.

Figure 11. Recycling process for c-Si modules. Taken from [19].

Figure 12. Repurposing flow for second life LIBs.

Summary of the methodology.

Database search summary.

Number of publications per journal.

Number of publications per conference.

Distribution of published articles.

Publications per thematic group.

* Cross-reference.

Summary of CE-related challenges derived from the SLR.

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As the solar photovoltaic market booms, so will the volume of photovoltaic (PV) systems entering the waste stream. The same is forecast for lithium-ion batteries from electric vehicles, which at the end of their automotive life can be given a second life by serving as stationary energy storage units for renewable energy sources, including solar PV. The main objective of this paper is to systematically review the “state-of-the-art” research on the solar PV value chain (i.e., from product design to product end-of-life), including its main stages, processes, and stakeholder relationships, in order to identify areas along the value chain where circular strategies could be implemented, thereby advancing the transition of the PV industry towards circularity. To achieve this goal, we conducted a systematic literature review of 148 peer-reviewed articles, published in English between 2000 and 2020. Results show the PV value chain has been studied from a forward flow supply chain perspective and mostly from a technological point of view, with little regard for circular design, circular business models, and PV reuse. This article thus takes an integrated value chain perspective, introduces some of the barriers to circularity that industry players face, and argues that these barriers represent future opportunities for incumbent and new entrants to innovate within a circular PV industry.

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1 Introduction

2 review methodology, 3 results and discussion, 4 conclusions, acknowledgements.

• List of tables
• List of figures

Review Article

A literature review on Building Integrated Solar Energy Systems (BI-SES) for façades − photovoltaic, thermal and hybrid systems

Karol Bot 1 * , Laura Aelenei 1 , Maria da Glória Gomes 2 and Carlos Santos Silva 3

1 Laboratório Nacional de Energia e Geologia (LNEG), 1649–038 Lisbon, Portugal 2 CERIS, Department of Civil Engineering, Architecture and Georesources (DECivil), Instituto Superior Técnico, Universidade de Lisboa, 1049–001 Lisbon, Portugal 3 IN+, Center for Innovation, Technology and Policy Research /LARSyS, Department of Mechanical Engineering (DEM), Instituto Superior Técnico, Universidade de Lisboa, 1049–001 Lisbon, Portugal

* e-mail: [email protected]

Received: 9 August 2021 Received in final form: 11 November 2021 Accepted: 11 November 2021

The building façade has a crucial role in acting as the interface between the environment and the indoor ambient, and from an engineering and architecture perspective, in the last years, there has been a growing focus on the strategic development of building façades. In this sense, this work aims to present a literature review for the Building Integrated Solar Energy Systems (BI-SES) for façades, subdivided into three categories: thermal, photovoltaic and hybrid (both thermal and photovoltaic). The methodology used corresponds to a systematic review method. A sample of 75 works was reviewed (16 works on thermal BI-SES, 37 works on photovoltaic BI-SES, 22 works on hybrid BI-SES). This article summarises the works and later classifies them according to the type of study (numerical or experimental), simulation tool, parametric analysis and performance when applied.

© K. Bot et al., Published by EDP Sciences, 2022

In order to overcome the substantial challenges faced by building sector in European Commission, being responsible for approximately 40% of the energy consumption and 36% of the greenhouse gas emissions, the scientific community together with policy makers are continuously working on delivering and adopting innovative solutions, advanced practices and regulations, respectively. In recent years, building regulations have gradually introduced new requirements to ensure a phased decarbonization of building sector and an increasing of its energy performance. In 2010, the Energy Performance of Buildings Directive (EPBD) recast [ 1 ] introduced requirements with the objective of attaining environmental and energy efficiency goals adopting the nearly Zero Energy Buildings (nZEB) Performance for new and existing buildings. A special attention was given to the public building sectors in terms of energy efficiency measures, drivers and barriers [ 2 ] and their optimal calculation [ 3 ].

For a building to be considered nZEB, it must reduce its energy consumption and generate energy from renewable sources, which can compensate for the majority of the building's consumption assuring at the same time thermal comfort. Taking into account the specific requirements and specifications for nZEB performance, a special attention has been paid to the integration of renewable systems in the buildings footprint or nearby. At building level these renewable technologies are mostly integrated in the building envelope (walls and roofs).

Usually, the building façade has a crucial role in performing as the interface between the environment and the indoor ambient. With the integration of renewable energy (especially solar), the buildinǵs facade has a significant impact on the occupant's comfort, building energy demands, and the aesthetics of the building. Commonly, designing a building façade takes into consideration several factors, as the climatic conditions and surrounding structures, indoor and spatial characteristics, needs of the building occupants regarding comfort and costs, among others. From an engineering and architecture perspective, in the last years, there has been a growing focus on the strategic development of building façades, it is, to contribute to meet the requirements of the high-performance regulations while being sustainable and aesthetically pleasant. This strategic development brings new experiments, innovative systems, and technology to be integrated into the formal functions of the envelope [ 4 ].

The façade elements may improve the energy flexibility of the building by the adequation of the constructive elements thermal performance to climate and building usage profile, also by being adaptive or automated to adapt to the different boundary conditions. Given this context and the flexibility that façade elements can offer in the design process, innovative façade elements based on solar energy systems can significantly reduce the building energy demand [ 5 ].

Entire buildings are broad, multi-scale, multi-material, with exceptionally unique analysis approach frameworks with vast influences. When addressing the design, applications and control of Building Integrated Photovoltaic System (BIPV) and its relationship with the building itself, it becomes very complex to create functional systems that are adaptable and generally relevant to the improvement of energy performance; once there must be a trade-off between factors as life-cycle assessment and real improvement it brings to the energy demand reduction [ 6 ].

The present article provides a concise review of a sample of studies concerning Building Integrated Solar Energy Systems integrated into façades published in the last five years. This article presents the main scope of the works, a comparison of the outcomes through a table classification, and a discussion about trends in the field.

The present study presents a systematic review concerning innovative systems on façade BI-SES. The source of information used to acquire the data is the Clarivate Analytics Web-of-Science. Figure 1 presents in detail the survey method and rationale used for the systematic review. In summary, the eligibility criteria and study selection are based on the published material within the search terms, period, the relevance, keywords and abstract pertinent to the objectives, and consideration through the screening of appropriate content throughout the text. Thus, Figure 1 also presents the results regarding the number of publications filtered through the adopted survey methodology. The data items, summary measures and report characteristics are based on the study details (reference), study characteristics (study type and technology type), extension of analysis, among others.

In the survey step previous to the detailed consideration of the title and abstract pertinent to the objective of the study (resulting in 115 articles), the results obtained by the source of information were segmented concerning the year of publication ( Fig. 2 ), journal of publication ( Fig. 3 ) and country of submission ( Fig. 4 ).

The remaining 75 articles were later segmented in the three mentioned categories: thermal, photovoltaic and hybrid BI-SES. The results are then presented in terms of summary of the manuscripts, and classification concerning the detailed system type, study type (experimental and/or numerical), simulation tool or technique, parameters under study in case of existence of a parametric analysis, and performance of the system considering its thermal, electrical and total efficieny.

The obtained results were segmented into three categories: integrated solar thermal systems, integrated photovoltaic systems and integrated hybrid systems (both thermal and photovoltaic). The thermal system converts the solar radiation into thermal energy, the photovoltaic converts it into electricity and the hybrid converts both in electricity and thermal energy. The results presented here are described concerning their core information and are further classified in a table to compare the different studies.

3.1 Integrated solar thermal systems

A sample of 16 scientific articles was considered representative innovative solar thermal systems pertinent, among the 75 articles reviewed. A summary of the most pertinent is presented here, followed by a table summarising the studies.

In Prieto et al. [ 7 ], there is a detailed review concerning the possibilities of using solar cooling integrated façades by exploring their feasibility concerning orientation, efficiency, and climate in which it is used. It is concluded that warm-dry climates and east/west orientations are the best situations for solar cooling façade applications, reaching a theoretical solar fraction of 100% [ 7 ]. In Maurer et al. [ 8 ], a review is done on the most important contributions of recent years of building-integrated solar thermal systems, in terms of systems being designed, results being achieved in terms of thermal characterization, and simple models to evaluate the systems − being this publication an interesting compilation of studies to have an overall view of the current technology status for building integration. Valladares-Rendon et al. [ 9 ] developed a review of shading thermal solutions to decrease direct solar gains and improve energy savings, balanced with visual comfort. This publication emphasizes the importance of employing the solar thermal elements with more than one purpose in a single element, reinforcing that solar façade elements shall not have a static goal.

Velasco et al. [ 10 ], a Venetian blind double-skin façade with the integrated solar thermal collector is analysed through CFD software. The authors emphasize that the system would promote energy efficiency through avoiding direct solar gains while being aesthetically pleasant. In Sun et al. [ 11 ], the authors present a façade system with parallel transparent plastic slats sandwiched between glass panes to form a parallel slat transparent insulation material to reduce coupled convective and radiative heat transfer inside the air cavity of the panes of a double glazed window. It contributes to increasing the thermal resistance without constraining the daylight access to the point of visual comfort reduction. In Li et al. [ 12 ], the work focuses on the innovation of building-integrated solar thermal shading systems to reduce the energy demand and improve the daylight levels through modelling and simulations.

In O’Hegarty et al. [ 13 ], the authors review and analyse solar thermal façades in terms of the type, technology used, and the materials that constitute it. Daily efficiency models are presented based on a combination of analysis methods, comprising a good data resource for comparison among technologies. Lamnatou et al. [ 14 ] give a critical review of building-integrated solar thermal systems' simulation methods and usage. Not only thermal but other types of BI solar configurations such as photovoltaic and hybrid systems are covered.

In Buonomano et al. [ 15 ], the design and the thermodynamic analysis of a new prototype of a flat-plate water-based solar thermal collector are developed, to integrate the system in building façades. The innovation is based on inexpensive materials and simplified design, aiming to reduce production and installation costs to improve market penetration. The applications are the production of hot water for domestic uses and space thermal comfort. This study contrasts with academia's tendency to develop expensive prototypes, as it aims to reach buildings in a faster manner by implications and technology transfer. As in the previously mentioned work, in Agathokleous et al. [ 16 ], it is also possible to find a flat-plate based thermal collector integrated into building façade envelopes but based on using air as the fluid. The authors also focused on the use of cost-effective materials and simple design solutions. They developed an energy dynamic simulation model and economic performance analyses and concluded that the system payback would be close to six years.

In Garnier et al. [ 17 ], a novel incorporated solar collector with storage for water heaters was created, followed by a praiseworthy CFD investigation. The proposed project is composed of a heating component to give household independence through the high-temperature water system and considers the coordination of the system and the rooftop configuration, enabling the system unit to be inserted inside an auxiliary protected material board framework. In Resch-Fauster et al. [ 18 ], the proposition focuses on an integrated solar thermal collector and latent heat storage modules. The overheating protection supplied by this system has high efficiency of the optimized configuration, calculated in function of other thermophysical characteristics. This study also reinforces the modularity that the BI-SES systems have been adopting in recent years. In Ibanez-Puy et al. [ 19 ], a ventilated active thermoelectric envelope component is studied. It focuses on a modular active ventilated façade prototype with a thermoelectric system to be installed in the building envelope and provide a high comfort level. The system integrates a passive design strategy through the ventilation and an active strategy through an active thermoelectric solution. This study is an example of coupling passive and active techniques to improve the overall system performance.

In Guarino et al. [ 20 ], the authors study the performance of a building-integrated thermal storage system, intending to improve the energy performances of the system in a cold climate. Navarro et al. [ 21 ] presented a novel phase change material (PCM) system inside the structural horizontal building component. The structural element was a composed concrete with micro-encapsulated PCM located into 14 channels, coupled to a solar air collector to melt and induce the phase change. The technique presented by these authors may be considered more intrusive once coupled with hard materials of civil construction (concrete), which per si deliver a reliable structural performance throughout the whole building lifetime. In Hengstberger et al. [ 22 ], a solution is presented by using PCM embedded into the absorber insulation which buffers the heat during the day and releases it at night. A parametric analysis is developed using a dynamic simulation tool to find the best melting temperature of a thin layer of PCM at different positions.

In Shen et al. [ 23 ], the authors introduce an innovative compact solar thin film with an interiorly extruded pin-fin flow channel convenient for building integration. A simulation model was used, and a prototype of the solar thin film was fabricated to test the system under different controlled conditions. The methodology presented by this work is pertinent once it discusses the process of designing and testing. In He et al. [ 24 ], an innovative tile-shaped dual-function solar collector is analysed for water heating. The study is developed using CFD software and aims not only to provide optimal designs but also to meet pleasing aesthetics. In Giovanardi et al. [ 25 ], a modular unglazed solar thermal façade system was developed to aid the installation of active solar façades, with a particular focus on the renovation of existing buildings. In He et al. [ 26 ], the authors investigate the loop-heat-pipe water heating performance of an innovative heat pump assisted solar, using theoretical and experimental methods.

Table 1 presents the complete list and classification of the solar thermal systems reviewed in this work, considering the system type, existence/non-existence of experimental and numerical analysis, existence/non-existence of parametrical analysis and details, reached efficiency of the system under study. The terminology (N.S.) stands for “not stated”, meaning that the article did not mention the feature.

The results obtained show that there is no specific trend concerning the systems under study in the most recent publications concerning the solar thermal systems. However, the focus can be given to the integration between passive and active techniques and the modularity and multiple purposes of the same element. The technologies vary from innovative system design to innovative methods of operation or material combination. Also, the thermal efficiency ( η t ) of the systems is, in most cases, not directly assessed. Most of the studies using dynamic simulation evaluated the impact of the systems in thermal behaviour by calculating nominal energy needs for heating and cooling of the thermal zone, based on determined setpoints. Others use computational fluid dynamics analysis to have a detailed profile of the thermal behaviour of the systems given the specified boundary conditions and evaluate the systems in terms of temperatures (primarily based on the outlet-inlet differences). Parametric analysis is not always done in the reviewed studies. Still, in the studies that develop this component, the geometry, inlet velocity and inlet temperature ( T inl ) are the most used variables of variation.

Summary of the studies − solar thermal systems.

3.2 Integrated photovoltaic systems

A sample of 37 scientific articles presented innovative solar photovoltaic systems (working only with the photovoltaic effect), among the 75 articles reviewed. A summary of the most pertinent is presented here, followed by a table summarising the studies. In Shukla et al. [ 28 ], an extensive review is given concerning the design of Building Integrated Photovoltaic (BIPV) systems. It focuses on developing the technology, classification of cells and products, and industry/research opportunities. Another study, developed by Tripathy et al. [ 29 ], presents a review of the state-of-the-art PV products for building different components of envelopes, their properties and their accordance with international standards.

In Aguacil et al. [ 30 ], the work aims to provide a methodology to contribute to the decision-making process concerning the use of BIPV in the urban renewal process. It considers the surface types and trade-offs between self-consumption and self-sufficiency. It is a straightforward approach that aims to facilitate the analysis of suitability concerning different factors. Chen et al. [ 31 ] explored the impact of archetypes and confounding factors in optimising the design. They focus specifically on high-rise buildings with BIPV façades, using data-driven models incorporating qualitative and quantitative analysis. It intends to facilitate the analysis by defining typical types of façades in which the buildings In Biyik et al. [ 32 ], the authors reviewed the BIPV and BIPVT possible uses in terms of types, supply, generation power, performance characterization, and approaches of analysis. They identify two crucial research areas concerning this subject: (i) increase in system efficiency utilizing ventilation while reducing the modules temperature; (ii) use of thin-film applicable for integration in buildings. This study is an excellent source to assess the comparison between BIPVT and BIPVT. In Shukla et al. [ 33 ], the study also presents a comprehensive review of the BIPV commercial solutions and their characteristics and a comparison of international testing and operation standards and instructions. The authors focus on BIPV solutions for different façade elements.

In Agathokleous and Kalogirou [ 34 ], the authors study a naturally ventilated BIPV system, and the assessment is based on experimental thermal analysis. This study is particularly attractive, and further results obtained by the authors are presented in Table 2 . In Agathokleous et al. [ 16 ], the authors continued the previous work by introducing a simulation-based thermal analysis of the same system. In Wang et al. [ 45 ], a ventilated PV double-skin façade and a PV insulating glass unit are studied through comparative experiments to evaluate the systems' solar heat gain and U-value. In Cipriano et al. [ 54 ], the focus was on a PV ventilated component and a data-driven approach to iteratively identify the unknown parameters, determine their impact in the simulation outputs and ultimately, assess the deviations of the computational outcomes against the measured data. In Peng et al. [ 56 ], the authors used EnergyPlus and developed a whole-year energy performance evaluation and saving potential of a ventilated photovoltaic double-skin façade in a cool-summer Mediterranean climate zone. The work developed a sensitivity analysis over the numerical model, considering different air gap width and operation models of the ventilation. In Pantic et al. [ 57 ], they present a theory-based and experimental investigation of electricity generation potential concerning different orientations of the modules in the façade elements.

In Asfour [ 37 ], the study focuses on the association of the PV modules in shading devices, and the investigation is oriented to hot climates. They also develop a parametric simulation to evaluate the potential of different designs. In Luo et al. [ 60 ], PV-blind embedded double skin façade is studied by coupling thermal-electrical-optical models. The aim was to evaluate and optimize the system by using ray-tracing, radiosity and net radiation methods, and other usual thermal models for buildings. In Tablada et al. [ 40 ], the authors also study the use of PV coupled to shading devices for farming plants growing application − focusing on windows and balconies. In [ 35 ], the study derived a new metric for assessing the daylight quality by comparing different coverage ratios of the PV cell and window-wall ratios. They also compared different orientations and estimated the net electricity use of the building. Karthick et al. [ 42 ], they investigated semi-transparent building integrated photovoltaic modules on façades, focusing on different coverage ratios. In Zhang et al. [ 43 ], the authors also explore the potential savings generated by the use of PV associated with shading elements, developing a parametric analysis concerning tilt angles and orientation of the system.

In Connelly et al. [ 48 ], the idea of semi-transparent BIPV with concentrator is additionally investigated. They propose a “smart window” framework comprising a thermotropic layer with integrated PV modules. The authors propose a system that naturally reacts to climatic conditions and analyse the power generation, natural light availability and heat transfer from the system to the building structure through parametric analysis of different solar energy ratios incident on the PV. In Wang et al. [ 49 ], they evaluated the energy performance of an a-Si semi-transparent PV insulating glass unit via numerical simulation and experimental tests. Considering the measured optical and electrical features of the PV, an integrated model was made to simulate the system's energy performance under analysis. In Favoino et al. [ 50 ], they propose a novel simulation framework for the performance evaluation of a responsive structure based on envelope advances in the switchable photochromatic coating. The analysis is done by incorporating building energy simulation and lighting simulation and varying parameters as the climate in which it is inserted.

In Wu et al. [ 52 ], a novel static concentrating PV system, reasonable for use in windows or coating exteriors, has been proposed. The proposed concentrating PV system is lightweight, with minimal economic effort and ready to produce power. Moreover, this system consequently reacts to atmospheric conditions by changing the parity of power created by the PV with the measure of sunlight-based light and heat allowed through it into the structure. It also offers the possibility to control the energy utilization in the building. Liu et al. [ 58 ], improved the structure of a commonplace semitransparent PV module and investigated the utilization of three sorts of high-reflectivity heat protection movies to frame the BIPV. Hence, the creators broke down the impact of the system structures on the optical, heat, and control time execution of the semitransparent PV module and how much the execution improved.

Qiu et al. [ 36 ] investigate mergers of vacuum glazing and BIPV integration and analyse its capacity to reduce the energy needs of the buildings. Huang et al. [ 38 ] also present a detailed investigation of a similar novel system's thermal and power efficiencies, a combined design improvement of photovoltaic envelope solutions. In Sun et al. [ 48 ], they combine optical, electrical and energy models to assess the integration of semi-transparent photovoltaic in commercial buildings. The publication assesses the effect of window design on the energy needs of the building. In Tak et al. [ 44 ], the authors structured a semi-transparent sun-powered cell window, in which the transparency can be changed by modifying its temperature and dissolvable vapour pressure. Further details may be seen in the reference. A modelling test with the proposed system was led to look at the impacts on energy utilization, power generation, and inhabitant comfort. The outcomes demonstrate that the proposed window has a significant potential to generate electrical energy.

In Sornek et al. [ 41 ], a Fresnel lens is used to increase the efficiency of BIPV systems. The analysis of the system is made both employing dynamic simulations and experimental campaigns. They improved the general productivity of the building integrated photovoltaic systems by the use of a Fresnel lens. During the tests, the efficiency of the photovoltaic module increased by about 7% (reaching an η e of 22%). In Bunthof et al. [ 47 ], they build up the examination dependent on three Concentrator Photovoltaic (CPV) systems arrangements that consider the development of semi-straightforward structure veneer components. The systems likewise are a Fresnel focal point based concentrator and a novel level planar optic concentrator. In Correia et al. [ 51 ], Luminescent Solar Concentrators are displayed as financially savvy parts effectively incorporated in PV that can improve and advance the integration between PV components and building structures, with considerable potential outcomes for energy generation in façades, while improving urban aesthetics. In Sabry [ 53 ], a range of prismatic total interior reflection low concentration PV façades with different head angles has been evaluated, dependent on the location and characteristics of the surrounding areas of the building. Every veneer design is mimicked by ray-tracing procedure. Its presentation is examined against sensible direct sun-based radiation information in two clear sky days representing the summer and winter of the area under study. Ray-tracing recreations uncovered that most of the chosen arrangements could gather the vast majority of the direct solar radiation in summer.

Kang et al. [ 59 ] developed a light-catching system connected to BI-SES based on the PV use, which naturally promotes light exposure during the entire year. The structure is streamlined for the precise scope of the occurrence light by breaking the underlying symmetry. The authors show the viability of the designed light-catching structure for different occurrence point ranges employing exhaustive reproduction studies and trial results utilizing organic photovoltaic elements. In Hofer et al. [ 55 ], they present a modelling framework, coupling parametric 3D with high-resolution electrical modelling of the shading devices composed by thin-film PV modules, to reenact electric energy of geometrically complex PV applications. The proposed modelling framework can foresee with high spatial-transient resolution the shading positioning and adapt it over each PV module, being critical to improving the electricity generation through the adequate positioning of the modules and contributing to the control of direct solar gains in the building.

In Palacios-Jaimes et al. [ 46 ], a plan to transform a university building into NZEB is presented. It demonstrates that the BIPV system may provide the power needs and lessen the structure's energy use in a financially savvy way. The investigation emphatically centres around the life cycle assessment, surveying the net emissions of CO 2 and the harms caused in a near setting with traditional power sources. In Yang [ 61 ], they identify the technical barriers and risks related to the utilization of BIPV in different building life-cycle stages, together with the proposal of potential arrangements. When a straightforward answer could not be proposed, suggestions for future innovative work are made. The proposed approach incorporates assessment of past productions and gathering of criticism from the business experts.

Table 2 presents the complete list and classification of the solar photovoltaic systems reviewed in this work, considering the system type, existence/non-existence of experimental and numerical analysis, existence/non-existence of parametrical analysis and details, reached efficiency of the system under study.

The results of this sub-section show a considerable amount of studies being made concerning BI-SES based on photovoltaic technology. Based on this review, three main design trends were identified: (i) improvement of standard BIPV configurations through smart ventilation; (ii) use of photovoltaic technology integrated into building façades as shading devices; and (iii) use of concentrators in the PV systems integrated into building façades and rooftop. As in the previous category, many studies do not approach the systems in direct terms of efficiency (in this case, η e ). They are approached in terms of nominal energy needs, energy balances (demand and on-site supply), and system temperatures. Also, a parametric analysis is done mainly by varying parameters as orientation, cell coverage ratio, air gap width, ventilation rates, and geometries.

Summary of the studies − solar photovoltaic systems.

3.3 (Building) integrated hybrid systems

Compared with solar thermal collectors and photovoltaic systems, the integrated hybrid systems employ both technologies in the same system, generating both thermal energy and electricity. A sample of 22 scientific articles was considered as presenting coupled innovative solar photovoltaic and thermal systems, among the 75 are reviewed. A summary of the most pertinent is presented here, followed by a table summarising the studies.

In Lee et al. [ 62 ], an extensive review is presented on PV/T systems, being of particular interest to works concerning the design of innovative energy façade elements due to the novelty of the strategies presented. The study reviews the structure guidelines and working instruments of the PV/T façade systems, execution, control procedures and building applications. They highlight the use of electrochromic coating as the most used smart coating for thermal applications in PV systems and also stress that concerning PV shading, the external shading is the most utilized due to its low initial costs. The authors also state that algae growth façades and folding façades (complex geometry) shading systems are rising solutions, with high initial investment costs and requiring professional installers. They are, indeed, a promising arrangement because of their multi-purpose capabilities. Dynamic shading systems were found to spare 12% to 50% of the structure cooling power utilization. In Lai and Hokoi [ 63 ], a survey of a significant number of shading systems on the main façades facing south or north (depending on the hemisphere, referred to as sun-oriented façades) is presented, considering studies that have been published after 2010, segmenting the study in opaque and translucid elements.

In a most recent study by Lai and Hokoi [ 64 ], the state of the art sun-oriented control systems for façades are introduced, with a comparative assessment of sun-powered control systems and guidelines for improving new ones. It incorporates multifunctional frameworks and modelling with BIPV and thermal energy generation. In complement, in Debbarma et al. [ 65 ], the authors survey the BIPV and BIPVT advancements and energy, and the exergy examination of BIPV and BIPVT systems are likewise discussed. This work reviews the ongoing betterment of innovation around the world. In Agathokleous and Kalogirou [ 66 ], the work presents state of the art on thermal analysis of double skin façades with BIPV in terms of the published studies on these systems. In Zhang et al. [ 67 ], an in-depth review of the recently emerging active building-integrated solar thermal/PV technologies is also provided. The authors elaborate on the concept, parameters of classification and assessment, among other topics.

In Nagy et al. [ 68 ], they propose a modular adaptive solar façade to couple the element with the very dynamic environment surrounding the building boundaries. The energy behaviour and aesthetic expression of the façade can be managed to employ high Spatio-temporal resolution responses. The design process and operational plan are described, along with simulation results of the thermal behaviour and power production/consumption. In Peng et al. [ 69 ], the authors elaborate on the energy performance of a ventilated photovoltaic façade under varied ventilation modes and controlling modes for different climacteric conditions, aiming to improve the energy conversion efficiency.

In Chialastri and Isaacson [ 70 ], a prototype of a BIPVT was constructed based on thermal and electrical energy, aiming to achieve visual comfort and shading control through the system application. In this article, the prototype was evaluated under various conditions to characterize its performance. Dehra [ 71 ] presents a study on energy evaluation of a photovoltaic wall using either natural convection incited or fan-helped ventilation system. The vertical photovoltaic sun-oriented wall was introduced on the façade of a pre-assembled outside test room. The prototype was developed with two economically accessible photovoltaic modules, an air cavity and an insulated back layer.

In Smyth et al. [ 72 ], the authors propose a modular hybrid photovoltaic/solar thermal façade technology that uses an Integrated collector storage solar technology. In light of a patented solar thermal diode concept and shaped into a flat modular profile incorporating PV cells/module, the proposed system aims to heat the indoor environment, provide hot water, and generate electricity. In Luo et al. [ 60 ], the authors proposed a building-integrated photovoltaic, thermoelectric wall solution. It is examined by a numerical model comprising a PV framework and thermoelectric brilliant wall element. The thermal and electrical components of the system under cooling prevailing atmospheres was numerically researched utilizing an iterative system model. The presentation of the system is optimized by a comparative investigation with a traditional solid wall.

In Barman et al. [ 73 ], the study investigates the outcomes of a solar transparent photovoltaic window, focusing on angles of incidence, thermal gains using direct solar gains and energy generation. In Ahmed-Dahmane et al. [ 74 ], the proposed BIPVT system prototype comprises air collectors connected to an air handling unit to manage the airflow. The solution works based on two applications, namely for heating and cooling needs.

In Gaur and Tiwari [ 75 ], a BIPVT system is analysed. There is a focus on improving the articulation between electrical and thermal efficiencies and heat transfer through the structure. These thermal and electrical efficiencies articulations are crucial for various climatic conditions and diverse façade BI-SES designs. The system modules have been intensely studied for their energy, exergy and operational attributes with and without associated air pipe. Buonomano et al. [ 76 ], a BIPVT system has been analysed for residential applications, assessing active and passive operational applications. In Oh et al. [ 77 ], they built up an incorporated model for evaluating the techno-financial execution of the BIPVT on façades, emphasising energy demand and supply. In [ 78 , 79 ], the authors develop an experimental study of a Building-Integrated Photovoltaic system combined with a water storage tank prototype. The authors achieve a thermal efficiency of nearly 8% during the winter and 40% during the summer. In [ 80 ], a CFD study is presented for the prototype with an interior module of insulation instead of the water tank. This new modular prototype constituted a next step study of previous prototypes proposed by the research group, as may be consulted in [ 81 , 82 ]. Also to note is the work presented in [ 83 ], in which they assess a BIPVT-PCM prototype via genetic algorithm optimization. Having as case study the same living lab in which these prototypes were tested, in [ 84 ] it is possible to find a numerical study of a full scale BIPVT system. In [ 85 ], the experimental results for this BIPVT system are presented.

Table 3 presents the complete list and classification of the hybrid solar systems reviewed in this work, considering the system type, existence/non-existence of experimental and numerical analysis, existence/non-existence of parametrical analysis and details, reached efficiency of the system under study.

The hybrid systems presented by the sample of publications reviewed in the scope of this work are, mainly, façade elements of BIPVT walls, in which the principal analysis is made through numerical simulation via a finite element of CFD analysis. Also, as in the previous sub-sections, many of the studies do not present the results in terms of system efficiency, and parametric analysis is developed in nearly half of them. The parameters under examination in the parametric analysis are ventilation nodes and velocity, geometry (duct width, for example) and glazing type.

Summary of the studies − hybrid systems.

This article intended to present a literature review to contribute to increasing knowledge and systematization of different building-integrated solar energy systems. The façades of the buildings offer huge potential to increase the sustainability of the built sector. Its association with building-integrated solar energy systems demonstrates that they can not only increase the comfort of the building and reduce the energy consumption but also respond to the necessities of the grid, especially concerning adaptive systems. A sample of 71 studies was reviewed in this study, and the results were segmented into three categories: thermal systems, photovoltaic systems, and hybrid systems integrated into the façades. When applicable, the studies were further classified regarding the type of study, the tool used, parametric analysis parameters, and performance.

Concerning the solar thermal systems, the results show that there is not a specific trend concerning the systems under study in the most recent publications. However, the focus can be given to the integration between passive and active techniques and the modularity and multiple purposes of the same element. The technologies vary from innovative system design to innovative methods of operation or material combination. The results concerning the photovoltaic systems presented three main design trends were identified based on this review: i) improvement of standard BIPV configurations through smart ventilation; ii) use of photovoltaic technology integrated into building façades as shading devices, and iii) use of concentrators in the PV systems integrated into building façades and rooftop. The hybrid systems presented by the sample of publications reviewed in the scope of this work are, mainly, façade elements of BIPVT walls, in which the principal analysis is made through numerical simulation via a finite element of CFD analysis.

NZEB_LAB—Research Infrastructure on Integration of Solar Energy Systems in Buildings” (Refª. LISBOA-01-0145-FEDER-022075)” is financed by national funds FCT/MCTES (PIDDAC) and European FEDER from Regional Operation Program of Lisbon.

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Cite this article as : Karol Bot, Laura Aelenei, Maria da Glória Gomes, Carlos Santos Silva, A literature review on Building Integrated Solar Energy Systems (BI-SES) for façades − photovoltaic, thermal and hybrid systems, Renew. Energy Environ. Sustain. 7 , 7 (2022)

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Introduction and Literature Review

• First Online: 14 November 2020

Cite this chapter

• Ibrahim Moukhtar 5 ,
• Adel Z. El Dein 5 ,
• Adel A. Elbaset 6 , 7 &
• Yasunori Mitani 8  

Part of the book series: Power Systems ((POWSYS))

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1 Citations

As the world’s supply of fossil fuels shrinks, there is a great need for clean and affordable renewable energy sources (RES) in order to meet growing energy demands. Furthermore, the conventional plants based on fossil fuel have serious environmental and financial problems, and therefore, the dependency of the distribution networks on the RES such as solar power systems for generating electrical power is significantly promoted. In the past few decades, solar energy systems have been received great attention as an important type of RES. Nowadays, solar energy sources constitute appropriate commercial options for small and large power plants. The two mainstream categories of solar energy systems utilized for this purpose are concentrated solar power (CSP) and photovoltaic (PV). This chapter presents a brief introduction about the role, important need, and advantages of renewable energies for today and the future, especially solar energy such as PV and CSP systems. In addition, it introduces a survey for all types of CSP technologies. As well as, it presents a literature review for the LCOE and cost reduction of CSP and PV systems, CSP modeling, and the application of ANN technologies in various SF systems. Further, it presents the problem definition, objectives, and outlines of this thesis.

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Ibrahim Moukhtar & Adel Z. El Dein

Faculty of Engineering, Electrical Engineering Department, Minia University, El-Minia, Egypt

Adel A. Elbaset

El-Arish High Institute for Engineering and Technology, El-Arish, North Sinai, Egypt

Electrical and Electronic Engineering, Kyushu Art Institute of Technology, Tobata-ku, Fukuoka, Japan

Yasunori Mitani

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Moukhtar, I., El Dein, A.Z., Elbaset, A.A., Mitani, Y. (2021). Introduction and Literature Review. In: Solar Energy. Power Systems. Springer, Cham. https://doi.org/10.1007/978-3-030-61307-5_1

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This study examined the optimal size of an autonomous hybrid renewable energy system (HRES) for a residential application in Buea, located in the southwest region of Cameroon. Two hybrid systems, PV-Battery and PV-Battery-Diesel, have been evaluated in order to determine which was the better option. The goal of this research was to propose a dependable, low-cost power source as an alternative to the unreliable and highly unstable electricity grid in Buea. The decision criterion for the proposed HRES was the cost of energy (COE), while the system’s dependability constraint was the loss of power supply probability (LPSP). The crayfish optimization algorithm (COA) was used to optimize the component sizes of the proposed HRES, and the results were contrasted to those obtained from the whale optimization algorithm (WOA), sine cosine algorithm (SCA), and grasshopper optimization algorithm (GOA). The MATLAB software was used to model the components, criteria, and constraints of this single-objective optimization problem. The results obtained after simulation for LPSP of less than 1% showed that the COA algorithm outperformed the other three techniques, regardless of the configuration. Indeed, the COE obtained using the COA algorithm was 0.06%, 0.12%, and 1% lower than the COE provided by the WOA, SCA, and GOA algorithms, respectively, for the PV-Battery configuration. Likewise, for the PV-Battery-Diesel configuration, the COE obtained using the COA algorithm was 0.065%, 0.13%, and 0.39% lower than the COE provided by the WOA, SCA, and GOA algorithms, respectively. A comparative analysis of the outcomes obtained for the two configurations indicated that the PV-Battery-Diesel configuration exhibited a COE that was 4.32% lower in comparison to the PV-Battery configuration. Finally, the impact of the LPSP reduction on the COE was assessed in the PV-Battery-Diesel configuration. The decrease in LPSP resulted in an increase in COE owing to the nominal capacity of the diesel generator.

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Introduction, motivation and incitement.

Self-sufficiency in electrical energy is a pillar of development for a country aspiring to be an emerging power. Indeed, the availability of electricity promotes the delivery of essential services such as education, health care, access to communication technologies, and increased agricultural and economic productivity. Nevertheless, according to the International Energy Agency (IEA), the proportion of Cameroon’s population with electricity access in 2021 was merely 65% 1 . The Cameroonian government’s electrification projects have mostly resulted in the electrification of urban centers. Similarly, the towns served by the power grid face issues with reliability and stability. Power outages, load shedding, and voltage drops are common on the electrical grid, causing significant social and economic consequences for the population. In 2021, Cameroon’s power network experienced an average system interruption duration index (SAIDI) of 162.6 h and an average system interruption frequency index (SAIFI) of 41.8 2 . These two indices assess the reliability of Cameroon’s power grid, highlighting its inferior performance compared to more advanced countries such as the United States, where the SAIDI was 4.7 h in 2019 3 . Renewable energies, particularly solar photovoltaic energy, are critical for expanding the population’s access to electricity in a sustainable basis. PV systems produce decarbonized and environmentally friendly electricity, which helps fight global warming. Cameroon has significant solar photovoltaic (PV) potential across its territory. The annual mean solar radiation varies across the country, with the north receiving 5.8 kWh/m 2 and the south receiving 4.9 kWh/m 2 4 , 5 . Utilizing this significant potential could allow for both large-scale energy production for grid-connected systems and smaller, stand-alone systems. However, solar energy is not a panacea for Cameroon’s lack of access to high-quality energy. Solar panel output is highly dependent on the erratic nature of both solar radiation and ambient temperature, which frequently leads to an imbalance between supply and demand. In order to mitigate this issue, storage systems are interconnected with photovoltaic systems to ensure the uninterrupted provision of energy to diverse loads in the event of solar panel failure. Furthermore, adding supplementary power sources, such as diesel generators, can improve the HRES’s reliability. The incorporation of diverse energy sources and storage systems into renewable energy systems significantly impacts the expenses associated with their installation, operation, and maintenance. So, to be appealing, these renewable energy systems must be optimally dimensioned in order to reduce the investment required for installation. This work developed an optimal sizing approach for a HRES composed of batteries, solar panels, and a diesel generator for a residential application in Buea, Cameroon’s southwest region. The system is intended to operate independently of the electrical grid.

Related works

The literature extensively discusses the optimal design of Hybrid Renewable Energy Systems (HRESs), which integrate various energy sources such as solar panels, wind turbines, diesel generators, biogas generators, and storage systems. These systems are designed to provide a reliable and sustainable energy supply by leveraging the complementary nature of different energy sources. For instance, solar panels generate electricity during the day, while wind turbines can produce power both day and night, depending on wind conditions. Diesel and biogas generators serve as backup power sources, ensuring a continuous energy supply even when renewable sources are insufficient.

Determining the appropriate size and configuration of these HRESs is a complex task that involves balancing cost, efficiency, and reliability. Effective methods for sizing these systems often utilize advanced software tools like HOMER (Hybrid Optimization of Multiple Energy Resources) 6 , 7 . HOMER allows users to simulate and optimize the performance of various HRES configurations under different scenarios, helping to identify the most cost-effective and reliable setup 8 .

Additionally, metaheuristic algorithms, such as genetic algorithms, particle swarm optimization, and simulated annealing, are frequently employed to solve the optimization problems associated with HRES design 9 , 10 . These algorithms can efficiently search through large solution spaces to find near-optimal configurations that might be impractical to identify through traditional methods 11 . By combining these tools and techniques, researchers and engineers can design HRESs that meet specific energy demands, minimize costs, and enhance sustainability.

Al-Shamma’a et al. 12 conducted an investigation to look into the possibility of transitioning from diesel-powered to hybrid energy systems in remote areas of Saudi Arabia. This study proposes using PV and wind turbines to reduce costs and increase the use of energy from renewable sources. The researchers used an optimization framework with a genetic algorithm to determine the most efficient hybrid system, taking into account factors such as solar panel rated power, wind turbine nominal power, battery count, and diesel generator rated capacity. The study found that a PV/battery/diesel system is the most cost-effective option for remote locations. Finally, an HRES with PV/battery/diesel saved approximately 60% of fuel compared to a diesel-only system while lowering carbon emissions by approximately 43%.

Rashid et al. 13 sought to develop and assess the feasibility of hybrid energy systems that incorporate biomass energy sources in a remote part of Bangladesh. The study looked at different hybrid system configurations, optimized their sizing and components, and evaluated their techno-economic aspects using a genetic algorithm. The findings show that a solar-based photovoltaic (PV) system with wind, diesel, and biomass backup sources has the lowest levelized cost of energy (LCOE). Furthermore, the study demonstrates that the genetic algorithm (GA) method delivers long-term and cost-effective results when contrasted with HOMER Pro software.

As reported by Ferrari et al. 14 , HRESs have the potential to reduce supply costs and incorporate renewable energy sources for remote off-grid users. By implementing a new sizing strategy for hybrid PV-wind-diesel systems that considers real-world constraints and machine data, the authors were able to optimize the energy mix configurations. This study contributes to the development of dependable techniques for designing hybrid systems, including renewables, energy-storage batteries, and fossil fuel generators. Olatomiwa et al. 15 found that the PV/diesel/battery combination was the most economically feasible choice, exhibiting the smallest fuel consumption and CO 2 emissions.

Makhdoomi and Askarzadeh 16 performed an investigation to assess the techno-environmental feasibility of various HRESs, such as PV/diesel, PV/diesel/battery, and PV/diesel/pumped hydro storage (PHS) configurations. The optimization problem involved two objective functions: LCOE and total primary energy (TPE). The optimal results were determined using multi-objective crow search algorithm (MOCSA). The study indicated that the most economical choice was a PV/diesel/PHS system utilizing fixed panels. Olatomiwa et al. 17 assessed the most suitable dimensions of HRES in various off-grid locations to power healthcare facilities in rural areas of Nigeria. The renewable energy systems considered in this study include solar PV, wind turbines, diesel generators, and storage batteries. The HOMER software was utilized for High Renewable Energy Systems (HRES) sizing. The optimal results revealed that for some locations, the PV-wind-diesel-battery configuration was optimal, while for others, the PV-diesel-battery configuration was most appropriate. This research 18 aimed to conduct an extensive technical and economic evaluation to determine the best approach for hybrid photovoltaic/wind systems integrating various types of energy storage to provide electricity to three particular areas in Cameroon: Fotokol, Figuil, and Idabato. The study utilized the cuckoo search algorithm to identify the optimal hybrid system combination with the cheapest cost of energy and net present cost, while ensuring it meets the load deficit probability. The findings indicated that a battery-based photovoltaic/wind HRES is the most efficient, economical, and dependable configuration for all levels of activity and locations. The researchers in this study 19 sized and analyzed stand-alone HRES that included PV modules and/or wind turbines in Cameroon’s Far North region, where many rural areas lack access to electricity. An analysis of sensitivity was conducted to assess the performance of HRES by examining different parameters such as LPSP, surplus energy ratio, and level of self-sufficiency. Optimization algorithms constitute popular and effective methods to figure out the optimal size of HRES. The Cuckoo Optimization Algorithm (COA) metaheuristic, despite being relatively new, has been successfully applied to solve numerous intricate engineering problems because of its simplicity and resilience 20 , 21 . This algorithm is inspired by the brood parasitism of some cuckoo species and is known for its ability to find optimal solutions efficiently in complex problem spaces 22 .

In addition to COA, other metaheuristic methods such as the Whale Optimization Algorithm (WOA), Sine Cosine Algorithm (SCA), and Grasshopper Optimization Algorithm (GOA) have demonstrated their effectiveness in addressing HRES sizing issues 23 , 24 . These algorithms mimic natural processes and behaviors to explore and exploit search spaces, making them highly suitable for optimizing the design and operation of Hybrid Renewable Energy Systems 25 , 26 . Each of these methods brings unique strengths; for instance, WOA mimics the bubble-net hunting strategy of humpback whales, SCA leverages mathematical sine and cosine functions to navigate the search space, and GOA is inspired by the swarming behavior of grasshoppers 27 . Their application to HRES design helps in achieving balanced, efficient, and sustainable energy systems.

Based on insights gained from an extensive review of the current literature, this study reveals a key gap in solving the energy challenges faced by residential areas in Buea, Cameroon. Despite the abundance of renewable energy resources, the region continues to grapple with an unreliable and unstable electricity grid, leading to frequent power outages and hindered socio-economic development. Existing studies have highlighted the potential of Hybrid Renewable Energy Systems (HRES) to mitigate these issues by harnessing solar energy alongside other renewable sources.

However, the literature reveals a lack of comprehensive research focused on optimizing the design and configuration of HRES specifically tailored for the residential context in Buea. Existing studies often overlook the unique socio-economic and environmental dynamics of the region, thus failing to provide practical solutions that address the specific needs and constraints of local residents. Furthermore, the existing energy management strategies in Buea primarily rely on traditional fossil fuel-based generators as backup power sources, contributing to environmental degradation and exacerbating the region’s vulnerability to climate change impacts. There is a pressing need for innovative, sustainable alternatives that not only ensure reliable energy access but also minimize greenhouse gas emissions and promote resilience in the face of evolving environmental challenges.

Therefore, the research problem addressed in this study revolves around the formulation of an optimal HRES configuration that balances cost-effectiveness, reliability, and environmental sustainability. By integrating insights from the literature review with empirical data and advanced optimization techniques, this research aims to contribute a tailored solution to the energy challenges faced by residential communities in Buea. The study seeks to bridge the gap between theoretical knowledge and practical implementation, thereby offering valuable insights for policymakers, energy planners, and stakeholders involved in sustainable development initiatives in the region.

Objective of the study

This study sought to figure out the optimal dimension of an autonomous PV/Battery/Diesel hybrid system for residential use in Buea, Cameroon, with the goal of enhancing the community’s access to dependable and quality energy. This study utilized the cost of energy as a decision criterion and the loss of power supply probability as a system dependability criterion.

Research contributions of the work

This work makes the following contributions to the literature:

A PV/Battery/Diesel hybrid system was suggested for residential use in Buea, southwest Cameroon. An energy management approach has been proposed to boost the proportion of renewable energy in order to meet demand and restrict greenhouse gas emissions.

The suggested HRES’s size was optimized using the COA meta heuristic in accordance with LPSP and energy cost criteria. Following that, the results were examined in relation to those given by the WOA, SCA, and GOA algorithms.

A comparison was conducted between PV/Battery/Diesel and PV/Battery configurations to identify the most efficient configuration according to cost of energy and LPSP.

A thorough examination was conducted to assess the impact of gradually decreasing the LPSP on energy costs and the capacity of diesel generators. The purpose was to analyze how enhancing the reliability of the proposed hybrid system would affect the cost of energy.

The next sections of this study follow a systematic framework: Section “ Materials and method ” elaborates on the materials and methodology used, outlining the methods and techniques applied. Section “ Result and discussion ” carefully examines the obtained results, providing a thorough analysis of the findings. Finally, Section “ Conclusion and future research directions ” summarizes the study’s findings, combining the insights gained from previous parts.

Materials and method

Load assessment.

Buea is the capital of Cameroon’s Southwest region, located at latitude 4°14″ north and longitude 9°20″ east. The University of Buea, situated in the city, is one of the nation’s top institutions. The city has a significant student population, most of whom reside in private dormitories located near the university. The energy consumption profile of this private university residence, located next to the University of Buea, varies depending on the time period considered. When the university was open, students occupied the majority of accommodations, and energy consumption was the highest. When the university is closed for vacations, energy consumption is low. Figure  1 shows the two measured daily energy consumption profiles. The long vacations take place from mid-July to mid-September, while the inter-semester vacations take place in February. The highest demand was 8.066 kW, while the lowest was 1.7 kW.

Hourly load profile during vacation period and during normal period.

Solar resource assessment

Cameroon receives abundant sunshine across its territory. Despite being situated in the southern region, Buea receives ample sunlight, which makes it suitable for the implementation of photovoltaic systems. Figure  2 displays the monthly solar radiation data and monthly average ambient temperature, acquired from 28 .

Buea monthly solar radiation data and monthly average ambient temperature.

Proposed hybrid renewable energy system

The autonomous HRES consisted of solar photovoltaic panels, lithium batteries, and a diesel generator is shown in Fig.  3 .

Layout of the proposed HRES.

Mathematical modeling of renewable system

Pv solar system.

Equation ( 1 ) provides the power generated by the photovoltaic system 29 , 30 .

where, \({P}_{STC}\) represents the nominal power of the solar panel, \({N}_{PV}\) indicates the number of solar panels, Ir is the solar radiation at the chosen location, \({T}_{C}\) stands for the cell temperature, \({\alpha }_{p}\) indicates the temperature coefficient and \({F}_{losses}\) quantifies the reduction in efficiency of solar panels caused by factors such as dirt and shading. In this study, this variable was assigned a value of 0.95 31 .

The temperature \({T}_{C}\) of the cells is determined by solar radiation, the ambient temperature, and the normal operating cell temperature ( \(NOCT)\) .

• Diesel generator

The quantity of fuel used by a diesel generator significantly impacts its performance. This study assumes a linear fuel consumption curve, as referenced in 32 . Equation ( 3 ) calculates the fuel consumption per hour ( \({D}_{f}(t)\) ).

where the diesel generator’s nominal capacity is denoted by \({P}_{Dgr}\) , while the power generated at time t is represented by \({P}_{Dg}\) . The fuel curve’s slope \({\alpha }_{D}\) is expressed in L/kW, and its intercept coefficient is denoted by \({\beta }_{D}\) . The values of \({\alpha }_{D}\) and \({\beta }_{D}\) considered in this study were 0.246 and 0.08145 respectively 32 . The diesel generator is assumed to run at least 30% of its rated capacity for the purposes of this study. The lifespan of the diesel generator is determined by the number of hours it runs per year. Equations ( 4 ) and ( 5 ) represent the running time ( \({RT}_{Diesel}\) ) and the lifespan of the diesel generator, respectively.

Storage system

The battery system has two modes.

Charging mode. Surplus energy is utilized for charging the batteries. The battery-charging power was calculated using two equations.

If production (PV system and diesel generator) surpasses demand and the diesel generator’s output alone is insufficient to meet demand, Eq. ( 6 ) is used to calculate how much energy needed for charging the batteries 63 , 64 .

When the power produced by the diesel generator is higher than the load demand, Eq. ( 7 ) is utilized to charge the batteries.

The inverter under consideration is bidirectional, with the assumption that its efficiency is the same in both DC/AC and AC/DC conversion modes.

Discharging mode. Batteries are discharged when the PV panels and/or diesel generator are unable to meet the entire load requirement 61 .

The energy available in the batteries is given by Eq. ( 9 ) 33 .

In Eqs. ( 6 )–( 9 ), the symbols \({\eta }_{ch}\) and \({\eta }_{dis}\) are used to represent the battery charging efficiency and battery discharging efficiency, respectively. The inverter conversion efficiency is represented by \({\eta }_{inv}\) , while the nominal battery capacity is denoted as \({E}_{bat}^{max}\) . The minimum energy quantity is referred to as \({E}_{bat}^{min}\) , and the battery self-discharge is represented by \(\sigma\) . The minimum battery energy depends on the depth of discharge (DOD). By utilizing Eq. ( 10 ), the value can be computed.

The amount of energy available over time in the batteries is constrained by the following factors:

The DC/AC converter has to be capable of supplying the power required by the load.

Here, \({P}_{invn}\) denotes the inverter’s rated power, and \({k}_{SF}\) is a safety factor that must be greater than one.

Reliability model

The metric employed to evaluate the reliability of the HRES is the loss of power supply probability (LPSP), which measures the probability of power supply failure.

Loss of power supply probability

The Loss of Power Supply Probability (LPSP) criterion has been used in numerous studies 34 , 35 . This criterion is essential for evaluating the reliability of Hybrid Renewable Energy Systems (HRESs), as it measures the probability that the system will fail to meet the energy demand 36 , 37 .

Equation ( 13 ) was employed to compute this criterion in this study 38 , 39 .

Economics models

The economic feasibility of the new hybrid system was evaluated based on two criteria: total annualized cost (TAC) along with the cost of energy (COE).

The total annualized cost (TAC)

The total annualized cost of the HRES encompass the initial capital expenditure required for its installation ( \({C}_{CAP}\) ), the cost associated with replacing the batteries and diesel generator ( \({C}_{REP}\) ), and the ongoing operational and maintenance costs ( \({C}_{O\&M}\) ) for all components of the HRES 40 , 41 .

The TAC is calculated for each component of the HRES.

PV system. The total yearly cost of the photovoltaic system encompasses solely the initial cost ( \({C}_{cap}^{PV}\) ) and the yearly expenses for maintenance and operation ( \({C}_{O\&M}^{PV}\) .), as the solar panels’ lifespan aligns with the project’s lifespan.

The \(CRF\) , which stands for capital recovery factor, is determined by utilizing Eq. ( 16 ).

where r represents the real interest rate while N denote the duration of the project.

The real interest rate is calculated based on the nominal interest rate \({i}_{n}\) and the inflation rate \({i}_{f}\) 42 .

Batterie system. The total annualized cost of the batteries includes the cost of the capital \({C}_{cap}^{Bat}\) , required for installing the batteries, the cost of replacing the batteries \({C}_{rep}^{Bat}\) , and the costs of operating and maintaining the batteries \({C}_{O\&M}^{Bat}\) 58 , 59 .

\({k}_{r}\) represents the discount factor for component replacement cost. It is determined by the project lifespan, real interest rate, and component lifespan. Equation ( 19 ) describes this expression.

where \({N}_{rep}\) is the number of component replacements and \(n\) is the lifetime of the component.

Diesel generator. The overall cost of the diesel generator takes into account the initial investment, the expenses for maintenance and operation, as well as the fuel costs ( \({Cost}_{fuel}\) ). The calculation was made using Eq. ( 20 ).

Inverter. The TAC of the inverter encompasses the initial capital, subsequent replacement costs, and ongoing maintenance and operational costs.

Cost of energy (COE)

The cost of energy is calculated by dividing the total annualized cost by the amount of energy delivered to the load ( \({Energy}_{served}\) ). Equation ( 22 ) is utilized to calculate it 43 , 44 .

Optimization problem

The single-objective optimization problem is to minimize the energy cost of the HRES. The problem of optimization is mathematically represented by Eq. ( 23 ).

This single objective optimization problem is subject to the constraints presented in Eq. ( 24 ).

\({N}_{PV}^{L}\) , \({E}_{Bat}^{L}\) , \({P}_{diesel}^{L}\) represent the lower bounds of the optimization variables. \({N}_{PV}^{U}\) , \({E}_{Bat}^{U}\) , \({P}_{diesel}^{U}\) , represent the upper bounds of the optimization variables.

Optimization algorithms

Crayfish optimization algorithm (coa).

The Crayfish Optimization Algorithm (COA) is a computational model that emulates the behavioral patterns of crayfish, specifically focusing on their resorting, competitive, and foraging abilities. To ensure equilibrium between the algorithm’s exploration and operation, the three behaviors were segregated into three separate stages 62 . The sequential procedures involved in the COA for addressing optimization problems are outlined in reference 45 :

Step 1 Setting parameters and initializing the population.

The values for the number of iterations (T), population (N), dimension (dim), and upper and lower bounds (ub, lb) are established. Establish the initial population X by considering the upper and lower bounds. The population initialization is determined using Eqs. ( 25 ) and ( 26 ).

Here, the variable \(X\) denotes the initial population, \({X}_{i,j}\) denotes the position of individual \(i\) within dimension \(j\) , \({lb}_{j}\) and \({ub}_{j}\) represent the lower and upper limits of dimension j, and the variable rand is a randomly generated number 65 .

Step 2 Setting the temperature.

Equations ( 27 ) and ( 28 ) specify the ambient temperature ( \(temp\) ) of the crayfish, which influences the progression of COA through different stages.

Here, the variable \(\mu\) denotes the ideal temperature for crayfish, while \(\sigma\) and \({C}_{1}\) govern the consumption of crayfish at different temperatures.

Step 3 Summer resort and competition stages.

If \(temp>30\) and rand drops below 0.5, the COA transitions to the resort stage. The COA computes a new position ( \({X}_{i,j}^{t+1}\) ) by utilizing the cave position ( \({X}_{shade}\) ) and crayfish position ( \({X}_{i,j}^{t}\) ). Equations ( 29 ), ( 30 ), and ( 31 ) allow computation of the new position. Next, we proceed to Step 5.

Here, \({X}_{G}\) indicates the current optimal position attained after a specific number of iterations, \({X}_{L}\) indicates the current population’s optimal position, \(t\) denotes the present iteration’s number and \(t+1\) denotes the iteration number for the following generation. \({C}_{2}\) exhibits a declining trend.

The COA entered the competitive phase when \(temp>30\) and rand was 0.5 or higher. Currently, the two crayfish engage in a competition for the cave by utilizing Eqs. ( 32 ) and ( 33 0. This competition leads to the establishment of a new position, denoted as \({X}_{i,j}^{t+1}\) , which is determined by the cave position ( \({X}_{shade}\) ) and the positions of the two crayfish ( \({X}_{i,j}^{t}\) , \({X}_{z,j}^{t}\) ). Then proceed to Step 5.

Here, \(z\) represents a randomly selected crayfish individual.

Step 4 foraging phase.

If \(temp\le 30\) , the COA transitions into the foraging phase, where the consumption of food p and the size of the food \(Q\) are determined by Eqs. ( 34 ) and ( 35 ) 60 .

If \(Q>{(C}_{3}+1)/2\) , the food is shared using Eq. ( 36 ). Then use Eq. ( 37 ) to calculate the new position and proceed to Step 5.

If \(Q<{(C}_{3}+1)/2\) , use Eq. ( 38 ) to find a new position and move on to Step 5.

Step 5 Evaluation function.

Assess the population and decide if to break the cycle. Otherwise, go back to Step 2.

Step 6 Displaying the optimal fitness value.

Other algorithms

The WOA 46 , SCA 47 , and GOA 48 are three additional meta-heuristics employed to address this optimization problem.

The initial settings of the algorithms are displayed in Table 1 .

Energy management of the hybrid system

The most suitable design of the hybrid renewable energy system is heavily dependent on the strategic management of power flow between demand and the two sources of energy production, which include batteries. Solar panels are the primary source of power for meeting load demand, with lithium batteries serving as a secondary option. The diesel generator serves as a backup energy source, providing power to the load only when the PV system and batteries are unable to do so. This reduces the emission of greenhouse gases into the environment. The various stages of the energy management strategy are as follows:

If \({P}_{PV}\left(t\right)=\frac{{P}_{Load}\left(t\right)}{{\eta }_{inv}}\) , the solar panels’ generation is adequate to meet the load demand.

If \({P}_{PV}\left(t\right)>\frac{{P}_{Load}\left(t\right)}{{\eta }_{inv}}\) , excess energy is utilized for the purpose of charging the batteries.

If \({P}_{PV}\left(t\right)<\frac{{P}_{Load}\left(t\right)}{{\eta }_{inv}}\) , the state of charge of the battery is assessed.

If the amount of energy stored in the battery at the preceding moment is adequate to satisfy the demand \(\left({P}_{PV}\left(t\right)-\frac{{P}_{Load}(t)}{{\eta }_{inv}}\right)\) , then the batteries are discharged.

Insufficient energy stored in the batteries triggers the activation of the diesel generator to supply the required power for the load, and any surplus energy will be utilized to recharge the batteries.

In the event that the diesel generator is incapable of meeting the entirety of the load demand, and the battery state-of-charge does not meet the minimum threshold established by the DOD, the combined efforts of the diesel generator and the diesel generator and batteries work together to fulfill the entire load demand.

The energy management applied to the proposed HRES is depicted in Fig.  4 .

Proposed energy management strategy for the HRES.

Economic and technical specifications of the different components

Table 2 lists the economic, technical, and financial features of the diesel generator, battery, and photovoltaic solar panel.

Result and discussion

Figure  5 displays the meteorological data for Buea, including hourly solar radiation and hourly ambient temperature over a 1-year timeframe 53 .

Hourly solar radiation and ambient temperature for the city of Buea.

Two different hybrid renewable energy configurations were tested to determine the best configuration. These are the PV-Battery-Diesel and PV-Battery configurations. Four metaheuristics are used to find the best size for each HRES configuration. Next, the best results for each configuration are compared. Figure  6 depicts the results of using the four metaheuristics to solve optimization problems.

Results obtained for COA, WOA, SCA, and GOA algorithms.

In accordance with the results depicted in Fig.  6 , the COA algorithm yielded the best outcomes irrespective of the HRES type being evaluated. The costs of energy determined with the four algorithms for the PV-Battery hybrid option converge to their optimal values as the number of iterations increases. Using the COA, WOA, SCA, and GOA algorithms, respectively, COEs of 0.1599 $/kWh, 0.16 $/kWh, 0.1601 $/kWh, and 0.1615 $/kWh were determined. Furthermore, the energy costs of the COA algorithm were reduced by 0.06%, 0.12%, and 1% compared to the energy costs achieved using the WOA, SCA, and GOA algorithms, respectively.

In the context of the PV-Battery-Diesel hybrid option, the COA algorithm demonstrated superior performance, yielding COEs of 0.1530$/kWh, 0.1531$/kWh, 0.1532$/kWh, and 0.1536$/kWh when the COA, WOA, SCA, and GOA algorithms were employed, respectively. It can be observed that the energy costs of the COA algorithm are 0.065%, 0.13%, and 0.39% less compared to the WOA, SCA, and GOA algorithms, correspondingly. In both the PV-Lithium Battery and PV-Lithium Battery-Diesel hybrid system configurations, the superior performance of the COA technique was observed compared to the WOA, SCA, and GOA algorithms. Figure  6 depicts the comparison of the results produced by the COA algorithm for the two configurations.

After conducting a comparative analysis of the optimal outcomes for each configuration, it came out that the PV-Battery-Diesel system exhibited superior performance with regard to both cost of energy and total annualized cost. The COE for the PV-Battery System was 0.1599 $/kWh, while the PV-Battery-Diesel System had a COE of 0.153 $/kWh. The COE achieved for the PV-Battery-Diesel system was 4.5% less compared to the PV-Battery system. Additionally, the total annualized cost (TAC) achieved using the PV-Battery configuration was $4444.90, while the PV-Battery-Diesel configuration yielded a TAC of $4251.98. Nevertheless, the PV-Battery-Diesel system releases a total of 1409.6 kg of carbon dioxide annually, based on a CO2 emission rate of 1.94 kg/kWh 54 , as a result of employing a diesel generator.

The PV-Battery-Diesel configuration exhibited superior economic performance compared to the PV-battery configuration. Prior research has yielded similar findings 55 , 56 . Regarding the COE obtained in this study compared to those reported in the literature, there is a wide range of results depending on the geographical location of the case study, the type of load profile, and the economic parameters taken into account for the analysis. For example, research conducted in China 57 resulted in a COE of $0.26/kWh. However, in reference 52 , the cost of energy for a specific study conducted in Iraq was $0.152/kWh. The COE achieved in this study is $0.153/kWh, which is within the same range of values.

Table 3 presents the detailed results generated by each algorithm. Due to the main focus of minimizing energy costs, the PV-Lithium Battery-Diesel setup was selected, and the subsequent analysis exclusively concentrated on this configuration.

The effect of the gradual reduction in LPSP on COE was investigated. The goal is to investigate the potential cost implications of increasing the reliability of the PV-Battery-Diesel hybrid system. The COA algorithm is used to generate the results shown in Fig.  7 .

Results obtained when decreasing LPSP toward zero.

As shown in Fig.  7 , the gradual decrease in LPSP led to a gradual increase in COE from 0.153$/kWh for an LPSP of 0.998% to 0.1670 $/kWh for an LPSP of 0.009%. Table 4 presents the detailed results of the LPSP reduction in the COE.

The data in Table 4 show that as the LPSP decreases, there is a corresponding increase in the COE and the total annualized cost of the system. The rise in energy costs as LPSP approaches zero is due to the growth in the nominal capacity of the diesel generator. The diesel generator’s nominal capacity increases from 1.76 at a 0.998% LPSP to 5.74 kW at a 0.009% LPSP. The findings indicated that the gradual decrease in LPSP primarily affected the rated capacity of the diesel generator. The solar PV system capacity ranged from 36.56 kW for 0.998% LPSP (LPSP < 1%) to 38.02 kW for 0.009% LPSP (LPSP < 0.01%). The storage system capacity showed minimal variation despite the decrease in LPSP. The renewable energy fraction decreased as LPSP decreased. The CO 2 emissions were approximately 1,953.6 kg for the 0.009% LPSP, in contrast to 1,409.6 kg for the 0.99% LPSP. By maintaining the LPSP results at 0.009% and aligning the nominal capacity of the diesel generator with the maximum load demand (8.066 kW), a system with complete reliability (LPSP = 0%) was achieved. Figure  8 shows the relationship among diesel generator rated capacity, energy cost, and LPSP. This energy management strategy utilized a diesel generator as an energy backup system to minimize GHG emissions.

Impact of the size of diesel generator on COE and LPSP.

Increasing the rated capacity of the diesel generator in Fig.  8 results in a higher COE and a decrease in the LPSP towards zero. At a diesel generator rated power output of 8.066 kW, which represents the peak load demand, the LPSP dropped to zero and the energy cost was 0.1718 kWh. The findings of the study indicate that the diesel generator plays a significant role in enhancing the dependability of the hybrid system under investigation. A comprehensive examination of the outcomes acquired for LPSP < 1% and LPSP = 0% was conducted. Figure  9 illustrates the yearly amounts of energy generated/consumed by the PV, diesel, and battery systems.

Annual energy generation/consumed for the LPSP = 0% and LPSP = 0.99%.

When LPSP was 0%, the PV system produced 49.04 MWh annually. Of this, 16.92 MWh was surplus energy, 20.55 MWh was used for battery charging, and 11.57 MWh was directly supplied to the load by the solar panels through the inverter. The battery supplied 16.66 MWh of energy to the load via the inverter annually, while the diesel generator produced 1.042 MWh of energy per year. The annual energy demand was 27.80 MWh. The diesel generator provides approximately 3.7% of the total annual energy demand.

When LPSP was 0.99%, the PV system produced 47.26 MWh annually. Of this, 15.2 MWh was surplus energy, 20.32 MWh was used to charge the batteries, and 11.70 MWh was directly supplied to the load via the inverter. The diesel generator produced 0.727 MWh of energy annually, while the battery supplied 16.52 MWh of energy to the load through the inverter. The diesel generator provided approximately 2.6% of the overall annual energy demand.

The annual generation of the diesel generator was minimal compared to the energy produced by the solar panels and batteries in the two LPSPs analyzed. The energy management strategy implemented in this study aimed to optimize the utilization of renewable energy sources instead of diesel. The diesel generators ran for 441 h and consumed 241.96 L of diesel with LPSP = 0.99%, whereas they ran for 348 h and consumed 485.91 L of diesel with LPSP = 0%.

Figures  10 and 11 show the energy production of the PV system, battery charging/discharging energy, and diesel generator for LPSP = 0% and LPSP = 0.99% over a 1-day period with minimal sunlight.

24-h solar PV, battery, and diesel generator energy production for LPSP = 0%.

24-h solar PV, battery, and diesel generator energy production for LPSP = 0.99%.

Figure  10 illustrates that the diesel generator is utilized only when the storage system can no longer fulfill the demand in the LPSP = 0% scenario. Given that the LPSP is 0%, the nominal capacity of the diesel generator is adequate to fulfill all demands, resulting in no energy deficit. Conversely, when the LPSP is 0.99% (LPSP < 1%), the nominal capacity of the diesel generator is significantly lower than demand. When the batteries can no longer meet the demand, an energy deficit occurs, as shown in Fig.  11 .

Figure  11 demonstrates that the diesel generator is utilized more when LPSP is 0.998% compared to when it is zero. In the 0.998% LPSP scenario, the diesel generator operated for a greater duration compared to the zero LPSP scenario.

Conclusion and future research directions

In conclusion, this study explored the optimization of an autonomous hybrid renewable energy system for residential use in Buea, Cameroon. The optimal dimensions of a HRES intended for residential use were investigated in this study. The evaluation involved two hybrid system configurations: PV-Battery and PV-Battery-Diesel. The techno-economic decision criterion selected for the single-objective optimization problem was the cost of energy (COE), while the dependability constraint used was the loss of supplied power probability (LPSP). To optimize and compare the two hybrid system configurations, the COA algorithm was employed. A comparison was made between the outcomes of the COA algorithm and those of established algorithms such as the WOA, SCA, and GOA. Based on the findings, the algorithms yielded energy costs of 0.1599$/kWh, 0.160$/kWh, 0.1601$/kWh, and 0.1615$/kWh for the COA, WOA, SCA, and GOA configurations, respectively. Compared to the COE produced by the WOA, SCA, and GOA algorithms, the COE derived from the COA algorithm is 0.06%, 0.12%, and 1% lower. In a similar vein, the algorithms yield COE values of 0.1530 $/kWh, 0.1531 $/kWh, 0.1532 $/kWh, and 0.1555 $/kWh for the COA, WOA, SCA, and GOA configurations, respectively. The COE achieved through the COA algorithm was 0.065%, 0.13%, and 0.39% less than the COEs achieved from the WOA, SCA, and GOA algorithms, separately. Upon comparing the optimal outcomes yielded by the COA algorithm for the two hybrid system configurations, it was Witnessed that the PV-Battery-Diesel configuration exhibited the most favorable coefficient of efficiency (COE). Furthermore, an analysis of the effect of gradually decreasing LPSP on COE in the PV-Battery-Diesel configuration revealed that COE tends to rise as LPSP decreases. The rise in the COE can be attributed to an augmentation in the nominal capacity of the diesel generator. Therefore, once the diesel generator’s nominal capacity reached the highest energy demand, the hybrid system achieved full reliability, with LPSP = 0.

In considering future research directions, several promising paths emerge to advance the field of hybrid renewable energy systems. Firstly, exploring advanced optimization techniques such as genetic algorithms, particle swarm optimization, or machine learning-based approaches could refine HRES design and operation. Secondly, integrating emerging technologies like advanced energy storage systems and smart grid technologies into HRES could enhance system performance and reliability. Thirdly, investigating the socio-economic impacts and policy implications of HRES deployment, along with field studies and pilot projects, can provide valuable insights into the practical feasibility and benefits of renewable energy adoption. By addressing these areas, future research can contribute to the development of innovative solutions for sustainable energy provision and accelerate the transition towards a low-carbon and resilient energy future.

Data availability

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.

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Amoussou, I., Paddy, E.Y., Agajie, T.F. et al. Enhancing residential energy access with optimized stand-alone hybrid solar-diesel-battery systems in Buea, Cameroon. Sci Rep 14 , 15543 (2024). https://doi.org/10.1038/s41598-024-66582-0

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Bioinformatics in neonatal/pediatric medicine—a literature review.

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Rallis, D.; Baltogianni, M.; Kapetaniou, K.; Kosmeri, C.; Giapros, V. Bioinformatics in Neonatal/Pediatric Medicine—A Literature Review. J. Pers. Med. 2024 , 14 , 767. https://doi.org/10.3390/jpm14070767

Rallis D, Baltogianni M, Kapetaniou K, Kosmeri C, Giapros V. Bioinformatics in Neonatal/Pediatric Medicine—A Literature Review. Journal of Personalized Medicine . 2024; 14(7):767. https://doi.org/10.3390/jpm14070767

Rallis, Dimitrios, Maria Baltogianni, Konstantina Kapetaniou, Chrysoula Kosmeri, and Vasileios Giapros. 2024. "Bioinformatics in Neonatal/Pediatric Medicine—A Literature Review" Journal of Personalized Medicine 14, no. 7: 767. https://doi.org/10.3390/jpm14070767

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