Summary

1. Introduction

Solar energy coming from the sun’s radiation can be used to produce heat, cold, or electricity and has been used since ancient times as an essentially inexhaustible renewable source. The irradiance at the outer atmosphere is ~1361 W/m², which can be reflected, absorbed, or dispersed on the way to the ground. On its side, incident radiation can be captured with different technologies and is composed of direct radiation, which does not suffer any change on the way to the ground; diffuse radiation, scattered by the atmosphere clouds and dust; and reflected radiation, from objects or by the ground itself.

According to authors such as Ozturk et al. [1], Mahmud et al. [2], and Herrando et al. [3], this energy source presents minimal environmental impact, as demonstrated throughout the life cycle analyses of the three most widely used solar technologies: solar photovoltaic (PV), solar thermal (ST), and combined PV/thermal (PVT) technologies. These authors conclude that solar energy has an environmental impact up to 51% lower than that of grid electricity, and the energy and CO₂ payback time of these solar collectors varies between 2 and 12 years and 1.6 and 3.6 years, respectively.

Nowadays, solar energy is one of the renewable energies with the most installed capacity with a total value of 1700 GW [4], divided mainly into ground- and roof-mounted installations. Highlighting roof-mounted installations, in 2020, the estimated situation by researchers was that the total global rooftop area was 0.25 million square kilometers, with Asia having the largest share, 48%, followed by Europe, with almost 19%. By 2050, this area is expected to increase by 20%–52% with respect to 2020, with Africa projected to see the highest growth, potentially doubling its rooftop area [5]. Focusing on the European buildings sector, in March 2024, European legislators adopted the EU Solar Standard in the European Parliament, which dictated that member states would have to deploy solar installations progressively in public and nonresidential buildings and in all new residential buildings by 2029 [6]. This, in addition to helping member states meet the proposed 2030 climate targets [7], will also empower citizens, who will power themselves with their own generated energy, resulting in economic savings on their energy bills. The latter has been growing over the last few years, installing 38 GW of PV capacity by 2023 [8], for example, when policies and financial support were introduced across Europe that boosted this growth. In addition to these policies and subsidies, the “Clean energy for all Europeans package” [9] promoted in 2019 the concept of energy communities, whose main objective was to empower citizens in the fight for the energy transition to clean energy, which would increase public acceptance of renewable energy projects [10]. The motivation of the European Union in this action was so strong that from 2020 to 2024, 107 new energy community projects, with around 12 GW of installed capacity, were registered throughout Europe, most of them based on solar energy production [11].

According to the International Energy Agency (IEA), solar energy technologies need to get on track with the Net Zero Emissions by 2050 Scenario [12], which expects that for 2025, 240 million household would have installed PV panels on their rooftops and that ST technology, together with geothermal energy, would reach the 12% of energy share in buildings so that it is clear the objective worldwide of increasing the use of this energy source.

Given the prospects for the deployment of these technologies, it is important to address key issues such as the resilience of solar technologies to climate change. This can be done by exploring how extreme weather events affect the durability and efficiency of PV and ST systems, highlighting advances in more resilient materials and adaptive designs. On the other hand, the performance affection can also be analyzed by looking at the impact of high temperatures and extreme cold on the efficiency of solar panels, as well as storm resilience. Another key option to ensure the adaptability of solar technologies to future energy demands and climate scenarios is to consider the development of hybrid systems, improvements in energy storage, predictive maintenance, and long-term planning with adaptive policies. Accordingly, some researchers, such as Bosnjakovic et al. [13], warn about the effects that extreme weather conditions generated by climate change could have on solar technologies, especially on PV and PVT panels, as they are electricity-generating technologies. In this case, Bosnjakovic warns that, at high air temperatures, the temperature of the panel frame can reach about 70°C, which would cause a drop in electricity production of up to 30% without posing a fire risk to the cables. Therefore, this factor is something to consider for plans or strategies involving this type of technology.

However, even with the trends in favor, there are still social and cultural barriers that limit the adoption of solar technologies in buildings. These include lack of knowledge and awareness, perceived high upfront costs, and esthetic resistance, especially in traditional buildings. In addition, mistrust of government policies and lack of appropriate incentives can hold back investment. Resistance to change, technical problems of integration into existing buildings, and misinformation about the environmental benefits of these technologies also play a role.

This review describes in detail the beginnings of each of these technologies, some current options for their application in buildings for future improvements, and the current measures proposed by the research community to overcome the different barriers, whether social, technological, or managerial, currently presented by the aforementioned solar technologies, with a special focus on the situation in Europe. In addition, all published literature on PV, solar collectors, PVT, and buildings is analyzed in a general way, listing the countries with the most research and the review works carried out.

2. Solar PV Panels

Alexandre Edmond Becquerel first observed the PV effect in 1839 by means of an electrochemical experiment with silver and platinum electrodes [14]. After, the same PV effect was observed also in selenium, which led to the design of the first solar cell. Albert Einstein proposed in 1905 the photoelectric theory [15], winning a Nobel Prize in 1921. From there on, other materials (especially silicon) replaced the selenium and, in 1955, solar cells were first commercialized [16]. Although until 1973, only monosilicon cells were used, the oil crisis led to the emergence of new materials, so polysilicon and amorphous silicon began to be also commercialized. As a result, the efficiency of solar cells increased and other cells (CdTe, CdS, Multi-Junction, Polymer, etc.) classified into three generations were proposed [14].

Currently, the solar PV capacity in 2022 accounts for almost 1055 GW and 1300 TWh, and it is the 4th energy source with more power capacity, after coal, natural gas, and hydropower [17], with clear future projections. By 2050, solar PV is expected to be capable of meeting 25% of the world’s electricity needs, with an installed capacity of more than 8500 GW worldwide and ranking as the second most important energy source behind wind power [18].

In Europe, the PV capacity increased by 27% from 2022 to 2023, arriving to 263 GW, and being Germany the leading country in this context, with a total installed capacity of 82.1 GW in 2023, adding 14 GW with respect to the previous year [19]. However, 90% of all building roofs in Europe still do not contain PVs, although these buildings could generate 25% of Europe’s electricity demand [19]. Therefore, Europe has proposed the European Solar Rooftop Initiative [20] to harness the potential of building rooftops for installing solar technology. The situation in Spain, the second country in the race for the expansion of this technology, is similar to that in Europe, as the PV capacity increased by 23% from 2022 to 2023, adding 8.2 GW and reaching a total installed capacity of 35.6 GW [19]. Although Spanish policies had revived the installation of PV panels on the roofs of residential buildings in 2022, achieving a peak annual installed capacity of close to 3 GW, this momentum declined in 2023 due to prolonged delays in the disbursement of subsidies to install such panels. This negatively impacted public perception of the nascent PV rooftop market segment in Spain, as highlighted in the SolarPowerEurope report [19]. However, the National Integrated Energy and Climate Plan [21] is projected to rise PV power generation to 39 GW by 2030 and to promote self-consumption.

Regarding the (1) cell efficiency, Maleki et al. [22] review the performance of solar cells as a function of active and passive cooling methods, drawing two main conclusions. On the one hand, passive cooling methods are advantageous due to their self-reliance and lack of requirement for additional energy input to operate, while, on the other hand, active cooling methods are preferred when efficient heat transfer and improved cooling are required. According to the same topic, Salem et al. [23] and Zhou, Zheng, and Zhang [24] analyzed the cooling effects of phase change materials (PCMs) hybridized with active cooling, water, and water + air injection. Salem et al. [23] found that the efficiency of PV increases if combined techniques were used, such as Al₂O₃ (φ = 1%)/PCM mixture (λPCM = 25%) + 75% water (5.31 kg/s·m² mass flow). Meanwhile, Zhou, Zheng, and Zhang [24] determined that combining techniques reduces the temperature by almost 40°C and increases the PV cell power above 1 MW, with an efficiency increase of 11.5%–15%. Similarly, Kabeel and Abdelgaied [25] explored the impact of cooling methods, focusing on the reflectors, water and air injection. They concluded that this cooling system could achieve an overall efficiency of 27.15% with an economic cost of $0.082 per kWh for energy production and $0.012 per liter for freshwater production. In this way, Marudaipillai et al. [26] studied the cooling effect of the polyethylene glycol/expanded graphite form stable PCM on the PV cells, increasing the efficiency by 3.67 points. Another technique for increasing efficiency deals with changing the structure or the material of the PV cells. For years, State Key Laboratory of Polymer Physics and Chemistry Research Group from Beijing, Cui et al. [27–30] studied the efficiency of the organic PV cells, and they increased it from 16% to 19% using different methods, such as using chlorinated acceptors or combining material design with a ternary blending strategy. Finally, in 2023, they obtained an efficiency of 31% under a specific situation of an organic PV cell [31] (PB4:FTCC-Br-based cell with an effective area of 1 cm² with 2700 K illumination of 1000 Lx). This shows that the “ternary copolymerization” technique can adjust the properties of high-performance polymer materials, which work well in solar uses.

On the other hand, many researchers discussed methods to (2) reduce the costs of PV technology. Some linked it to research and development (R&D) efforts, such as Ding et al. [32], which emphasized the impact of government-sponsored R&D initiatives and demonstrated that cost reduction is associated with the integration of solar electricity into the energy market. They claim of this penetration results from well-structured policy plans, with special emphasis on the government’s role and policies. Similarly, Zhou and Gu [33] and Benda and Černá [34] reached the same conclusion by stating that, although increased R&D&I funding may require more capital investment, it can effectively reduce the economic cost, the total social cost of deploying renewable energy, and the price of carbon. Shukla, Sudhakar, and Baredar [35] noted that some countries have a limited stock of PV panels and expected technological advancements to reduce PV cell prices. They noted that prices have decreased since the last decade for rooftop solar PV systems in India, even if there is still much work to do, emphasizing the importance of government subsidies. In addition, Green [36] turns his attention to the latest PV panels, particularly those equipped with bifacially responsive cells, and predicts that these will represent the future PV technology and will reduce costs for several more decades.

According to the (a) PV potential at the urban level, Romero Rodriguez et al. [37] present a novel methodology employing 3D city models and assessing accurately the rooftops and their solar radiation, which includes different factors such as the separation of the PV panels or the shadow, slope, and orientation. Tercan et al. [38] introduce a sustainable planning framework for PV solar farms in Central Anatolia, Turkey, incorporating GIS and multicriteria assessment methods. The authors assure that their approach significantly reduces the soft costs of PV farm investments and creates opportunities for a novel investment model that allows regular individuals to become shareholders in PV farms. Bódis et al. [39] offer a detailed geospatial analysis of rooftop solar PV potential across the European Union, showing that at least five countries (Cyprus, Portugal, Malta, Greece, and Italy) should immediately establish realistic targets and roadmaps for implementing PV roof systems and that Spain could cover more than the 20% of the electricity demand.

Regarding the (b) solar Energy Potential Assessment, Qiu et al. [40] assessed the PV potential for power generation in China, with a focus on its expansive energy landscape, and proposing a highly precise methodology that considers the meteorological and solar radiation data from ERA5. Zhang et al. [41] introduced a comprehensive framework that integrates not only technical but also geographic and economic indices for conducting solar energy potential in China.

In terms of (c) spatial optimization and analysis, Zhong et al. [42] enhanced the precision of rooftop solar energy potential assessments with a method that offers a promising avenue for improving reliability. Furthermore, Ren et al. [43] presented an innovative approach that combines 3D geographic information systems and deep learning techniques to achieve a high level of accuracy in characterizing rooftop solar energy potential, especially in densely populated urban areas. Schunder et al. [44] performed a comprehensive spatial analysis to evaluate the rooftop and community solar energy potential, measuring the influence of spatial factors at both the individual rooftop and community levels. As a result, understanding these spatial dynamics is crucial for optimizing the deployment of solar energy solutions and achieving sustainable energy goals.

Finally, different authors analyze the (d) metropolitan areas and rooftop PV potential, such as Cuesta-Fernández et al. [45], who directed the attention to the metropolitan areas, particularly to their capacity to reduce carbon emissions in residential structures. They employed a combined geographic information system approach (PVGIS + QGIS) to calculate the rooftop PV potential of Valencia, Spain. Gómez-Navarro et al. [46] assessed comprehensively and multidimensionally the potential for rooftop PV producer–consumer generation also in Valencia, Spain. This multifaceted analysis spans the technical, economic, and environmental aspects, understanding holistically their viability and advantages within an urban setting. The cumulative findings provide vital insights for policymakers and urban planners to drive sustainable energy practices in metropolitan areas. Besides, Gomez-Exposito, Arcos-Vargas, and Gutierrez-Garcia [47] analyzed in-depth the potential contribution of rooftop PV to achieve a sustainable electricity mix, with a specific focus on the Spanish context. The research examined the favorable feasibility and implications of incorporating rooftop PV systems within Spain, providing critical insights for formulating sustainable energy strategies and policies.

As a result, it is concluded that, although PV technology is advanced and widely adopted, the PV panels efficiency can still be enhanced and the cost reduced as well as to implemented in current unused spaces, majorly roofs. This technology should be pushed by governments and policies to be implemented by individuals.

3. ST Panels

Horace-Bénédict de Saussure invented the first solar heater in 1767, employing a well-insulated box protected by three glass layers [48]. In 1891, Clarence Kemp introduced for the first time the solar water heater in the market, as a pioneering technology known as the Climax Solar Water Heater [49]. In 1909, William J. Bailey patented a solar water heater that transformed the industry [50] by redesigning its concept and splitting it into two parts: in an open-air heating element that absorbs sunlight and in an insulated storage unit inside the home. This arrangement allowed to access solar-heated water during the whole day. The heating element was composed of narrow pipes attached to a black-painted metal sheet inside a glass-covered enclosure that, by passing water through these, instead of storing it in a large tank, the water heating-process was accelerated. At that time, the United States began to use these solar water heaters for domestic hot water (DHW), and later on, after World War II, nations such as Israel and Australia, initiated an extensive process of research, development, manufacturing, and implementation of solar water heaters [51]. Hottel and Whillier proposed a thermal model for flat plate ST collectors and published several papers emphasizing their performance and their use for house heating [52–55]. Similarly, in 1973, during the oil crisis, solar energy emerged as a viable technology, leading researchers to explore various methods for enhancing solar heater efficiencies [56]. As a result, Duffie and Beckman [57] introduced a design-oriented model for flat plate ST collectors in 1980. This model assumed steady-state heat transfer conditions and inspired the analogy between one-dimensional heat transfer and electrical networks. As Cadafalch and Cònsul [58] pointed out, various researchers proposed new models, contributing to technological advancements and the attainment of higher efficiencies through an extensive research effort. Currently, this technology has a great role for the energy transition, as demonstrated in The Solar Heat Worldwide report [4].

In 2021, the total installed capacity worldwide was 524 GWth, with ~748 million m², considering unglazed water collectors, flat-plate collectors (FPCs), evacuated tube collectors, and unglazed and glazed air collectors, of which FPC account for 25.2% (131.9 GWth and 188 million m²), being China the leading country, see Figure 1a. As a whole, the trend in FPC technology incorporation has been decreasing from 2014; even so, the installed capacity is still increasing, albeit at a slower rate, see Figure 1b.

The Solar Heat Worldwide report [4] states that due to the increasing impact of global warming and heightened concerns about energy security triggered by the war in Ukraine, the heating sector is gaining political attention. The IEA Renewables 2022 report [59] predicts that global heat consumption, excluding ambient heat utilized by heat pumps, will grow by almost 3.9 million GWh from 2022 to 2027. While the electrification of the heating sector will contribute to a certain extent, most of the demand will have to be met by geothermal energy, modern use of biomass, and ST energy. In addition, the data shows that the demand for large-scale ST systems in 2023 has increased, especially in the district heating sector, as it is a very cost-effective way to decarbonize, with costs ranging between 20 and 50 €/MWh under favorable conditions (considerably lower than current end-user prices for district heating).

According to the EurObserv’ER 2023 report [60], in Europe in 2023, the installed capacity increased by 11.9% compared to 2022 and reached 41.2 GWth (58.8 million m²) being Germany the leading country, with more than 38% of the total installed capacity, followed by Greece (9.2%), Italy (8.5%), Austria (7.8%), and Spain (7.7%). Similar to what happened with PV technology, the European Solar Rooftops Initiative [61] promotes installing thermal solar panels on rooftops to achieve the goals of the RePowerEU Plan [62] of (1) save energy, (2) diversify energy sources, and (3) accelerate the clean energy transition. Therefore, Spain is the fifth country in Europe in terms of the largest deployment and installed capacity of thermal solar energy. As seen in Figure 2, since 2005 the installed capacity area has grown [60], as has the building sector consumption supplied by ST technology [63].

Nowadays, the installed capacity in Spain exceeds 3153.7 MWth, and it seems that it will continue to grow, because as the European Commission states [64]: “Solar thermal technologies can be deployed in most European regions and are a particularly good option in Europe’s Eastern and South-Eastern countries, where solar thermal heat is often the cheapest option to replace fossil-fuel heating. The integration of solar collectors in energy-efficient renovations of housing and buildings can contribute to the expansion of these technologies.” Moreover, the initiatives and the guidelines promoted by Spain, such as the Solar Energy Technical Guide [65], the Regulation of Thermal Installations in Buildings [66] or the Technical Building Code-Basic Document [67] indicate that this technology is booming, and they give a hint that the social perception toward this technology is more than acceptable.

The first technique, (1) combining FPCs with TES, can be done using sensible heat TES (SHTES) or latent heat TES (LHTES). Alptekin and Ezan [68] combined SHTES with FPCs and analyzed the system from an energy and exergy point of view, varying the tank height, the inner diameter of the sphere, and the mass flow rate. Results show that the optimum working situation for the charging process, under real meteorological data, occurs with a high mass flow rate (0.4 kg/s), small tank height (0.8 m), and small sphere diameter (20 mm). In addition, Yang et al. [69] assessed exergy flow and destruction rates within the ST system, composed of three solar FPCs connected in series and a SHTES tank, by experimentally analyzing different temperature and irradiation values. The research revealed that exergy destruction rates in the SHTES were relatively small comparing to those of the FPCs, mostly due to the mixing of the fluid inside the tank coming from the FPCs and the tank reservoir. Anacreonte et al. [70] evaluated the dimensionless exergy ξ ^∗ parameter to characterize the ability of the storage to generate and preserve optimal temperature stratification. They used an experimental stratified TES of water in a real industrial application, and they developed and validated a 2D axisymmetric CFD model, analyzing the tank under various realistic loading conditions. Based on the results, the inlet temperature impacts more than the inlet velocity regarding the optimal performance in thermal storage. Lastly, Hassan et al. [71] studied the behavior of the thermal storage tank in two types of FPCs, a novel partitioned ducts solar collector (PDSC) and a conventional flat plate solar water collector (FPSC), with the same dimensions and materials. To this aim, storage water temperature, energy storage capacity, energy efficiency, energy losses, and exergy efficiency are analyzed, varying the tilt angle (17°, 27°, and 37°). The findings suggest that the PDSC has greater storage water temperature, energy storage, energy efficiency, exergy efficiency, and lower energy losses than the FPSC for all studied cases.

Regarding the case of LHTES coupled with FPCs, it is shown that including PCMs to store thermal energy improves the efficiencies of different types and combinations of FPC. Among others, Madhavan et al. [72], Fan et al. [73], and Vengadesan et al. [74] improved the efficiencies by including PCMs in different parts of the solar collector system. Elbahjaoui and Hanchi [75] researched the impact of incorporating three PCMs storage tanks in a cascade, showing that the average efficiency increases by 3.47% compared to a single PCM storage. Lamrani et al. [76] compared a PCM tank with a sensible water storage tank, both coupled to an FPC system, under the same operational conditions. According to the results, PCM increases by 35% the stored ST energy and the storage duration in charging periods, at constant thermal power and using the same volume; while in discharging mode, it provides a longer constant thermal power production, and the storage duration increases 65% using the same volume.

In terms of (2) the use of reflectors, Abubakkar et al. [77] conducted a numerical analysis using the Duffie-Beckman [57] formulas justifying that reflectors can enhance the efficiency by 32% with a wind speed of 3.5 m/s. Additionally, the outlet temperature of the thermal solar panel can reach 97°C, saving 0.66 kWh of energy. Ramesh et al. [78] analyzed the impact of the reflectors on different types of FPCs, varying the mass flow rate, collector angle, and reflector angle in terms of thermal efficiency. The findings suggest that the maximum thermal efficiency can be obtained at 1.28 l/min, 43.89° of collector angle, and 44.92° reflector angle. El-Assal, Irshad, and Ali [79] studied the influence of introducing side reflectors on FPCs, using TRNSYS software, in a region of Saudi Arabia. They concluded that the thermal efficiency increases from 46% (without using reflectors) to 58% (using reflectors) and that the output water temperature increases by 12°C compared to the input water temperature.

According to (3), the design of solar collectors, Balamurali and Natajaran [80] analyzed how the thermal efficiency of the FPC varies with different duct shapes by considering three internal grooved profiles (trapezoidal, plain, and rectangular). They concluded, as Kundu [81] did previously, that the best profile is the trapezoidal one, with a maximum thermal efficiency of 77.3%, although Kundu obtained 60.85%. Shridharan [82] experimentally configured a new water collector system for an FPC by increasing the contact surface length, which results in 12.41% higher performance than in a conventional one, by adding 5.35% of contact surface length. The research of Biswas and Tripathy [83] parametrized three alternative baffle/fin configurations (cross-flow, channel, and twist designs). The cross-flow parametrization involved variations in the angle of the solid baffle (30°, 35°, and 40°), while for channel and twist designs, the distance between the fins was varied 50, 100, and 150 mm. The average overall thermal efficiency for cross-flow, channel, and twist design was 76.16%, 74.26%, and 70.65%, respectively.

Lastly, (4) adding nanofluids into the heat transfer fluids (HTFs) enhance the performance of FPCs, as demonstrated by Ajeena, Farkas, and Vig [84], Ajeena, Farkas, and Vig [85], Yijie et al. [86], and Choudhary, Kumar, and Singhal [87] by analyzing different nanofluids and concentrations. Ajeena, Farkas, and Vig [84] studied firstly the behavior of ZrO₂-SiC nanoparticles dispersed in distilled water (DW) and concluded that the maximum thermal efficiency could reach to 75.21%. 1 year later, Ajeena, Farkas, and Vig [85] studied the behavior of SiC nanoparticles dispersed in DW and concluded that this new nanofluid had a better performance with a maximum thermal efficiency of 77.53%. Yijie et al. [86] compared Al₂O₃ and CuO nanoparticles showing that Al₂O₃ gives the maximum thermal efficiency of 21.9%, higher than just using water. Besides, Choudhary, Kumar, and Singhal [87] also analyzed and compared ZnO and MgO nanofluids, concluding that both enhance the thermal efficiency of the FPC, being ZnO better (67.98%) than MgO (65.22%).

On the whole, the current research studied how to improve the performance of ST technology, and in particular, the FPC, as it is a very versatile technology with a very large applicability.

4. Solar PVT Panels

Professor Karl Wolfgang Böer first combined solar PVT technology in 1973 with an experimental prototype that simultaneously generated heat and electricity, called Solar One [93, 94]. The technology has been advancing and, in 2017, uncovered PVT water collectors, air PVT collectors, and covered PVT water collectors were commercialized. According to the latest IEA Report [4], the panel with the best performance in 2022 was the uncovered PVT water collector, with 60% of installed thermal capacity and 40% of electric capacity, followed by the air PVT collector (37% thermal capacity). In addition, the total installed PVT collector area in the world was 1,524,945 m², with 789 MW_th thermal installed capacity and 276 MW_peak electrical power; where Europe accounts for 62% of the total installed, followed by Asia (excluding China) with 21% and China with 10%.

Therefore, there are 950,155 m² of PVT in Europe, with 474,575 MW_th thermal and 165,363 MW_peak electric capacity, where 64% was in France (the country with the most installed PVT in the world), followed by Germany (24%), Netherlands (18%), Spain (4.5%), Switzerland (3.2%), and Italy (3%). As mentioned, the commercialization of PVT technology was growing rapidly between 2017 and 2021, but in 2022, it declined by 52% compared to the previous year due to the beginning of the War in Ukraine and the restrictive and discontinued PVT subsidies in some countries, such as the change in France’s financing scheme. Despite that, other European countries increased their installed capacity by structuring regulations and funding programs, such as Italy, Germany, Switzerland, or Spain (Figure 3), and also by developing their market and deepening technological research. Accordingly, the following paragraphs describe the innovative contributions of researchers mostly aimed at improving the efficiency of PVT panels by different methods, which is the main barrier of this technology at the moment.

Alikhan, Kazemi, and Soroush [95] integrated PCMs as storage systems to enhance the overall efficiency of the PVT panels by proposing a series-connected PVT and ST panels with integrated PCMs and analyzed it from an energy and exergy point of view. They also compared the system with the one with no PCM, resulting that PCMs effectively regulate PV cell temperature and enhance electrical power and exergy rate.

Nanofluids are also included to enhance the efficiency of this technology. Al Shamani et al. [96] studied the optimal design of the tube and sheet of a thermal absorber (rectangular, 24 mm with and 15 mm depth) and compared three nanofluids (CuO, SiO₂, and ZnO) to be integrated in the HTF (water) and pure water as HTF. According to the results, the SiO₂ nanofluid has the best thermal, electric, and overall efficiency (64.40%, 12.70%, and 77.10%). Alktranee et al. [97] integrated the WO₃ nanofluid into the HTF at different volume concentrations (0.5 vol%, 0.75 vol%, and 1 vol%), showing that the nanofluid reduces the PV temperature by 21.4%, improves the PVT overall efficiency by 29.6%, and increases the electrical power output by 11.15 W compared to deionized water.

Other researchers analyzed the efficiency of the panel depending on its design, as it is the case of Aghakhani and Afrand [98], who suggested two methods for cooling the PVT panel and enhancing the efficiency, one for water-cooling, based on a cooper pipe system, and the other one for air-cooling, using some fans under the panel. In addition, some porous were also integrated under the panel of both systems, and their performance was compared by changing the volumetric flow-rate in a range of 0.5–2.5 L/min and the number of fans (1, 2, and 3). According to the results, the maximum electric, thermal, and overall efficiencies were 11.99%, 70.59%, and 81.61%. Chan et al. [99] proposed a method based on a thermal regulation strategy for PVT collectors, which used a switchable backplate that adjusts the seasonal heat-demands by opening or closing. In addition, they compared this panel with the conventional one, concluding that the maximum temperature of the PVT panel could be reduced by 18.1°C and the electrical efficiency improved by 8.6%.

Al-Aasam et al. [100] tested a novel PVT panel with a twisted absorber tube and a nano-PCM in five configurations, including or discarding elements to improve the efficiency of the PVT panel, varying the mass flow rates (0.008–0.04 kg/s) and using a constant solar irradiance of 800 W/m². Among the tested configurations, the Twisted-PVT-nano-PCM configuration demonstrated the highest performance, at a mass flow rate of 0.04 kg/s with an electrical, thermal, and overall efficiency of 9.46%, 79.40%, and 88.86%.

According to research advances, this technology continues to grow due to its versatility, ease of installation, and high-energy efficiency compared to other solar technologies.

5. Results

This work addresses the technological alternatives of PV, thermal panels, and PVT to capture and transform solar energy into different types of energy in buildings, that is, electricity and heat/cold, and analyses the past, current, and future lines of research dealing with the enhancement of the efficiency and reduction of costs. To do that, we highlight three current research lines in the literature regarding the PV panels, which are the improvement of cell efficiency, the reduction of costs, and the assessment of space energy-potential, see Table 2.

In addition, the efficiency of FPCs has been enhanced in the literature by combining FPCs with TES, using reflectors, modifying the design of the collectors, and using nanofluids, see Table 3.

This work also describes some ways to combine PV and ST with storage technologies, see Table 4.

The last review section analyses how to enhance the efficiency of the combined PVT technology, see Table 5.

6. Discussion and Global Bibliographic Review

Current bibliographic databases allow obtaining a global perspective on the research related to the topic of this review; thus, if we analyze the works on Scopus associated with the four keywords “Photovoltaic,” “Collector,” “PVT,” and “Building,” we found 785 research papers since 1976, see Figure 4. It is important to note that each keyword has been found in different terms: 8 for PV, 6 for solar collectors, 7 for PVT, and 3 for buildings.

Besides, regarding this database, 96% of the papers (754) are related to PV technology, 65% to solar collectors, 53% to PVT hybridization, and 31% to building applications. In addition, research has increased since 2000, decreasing slightly, as mentioned above, from 2022 onward, with China being the country with the most research (12%), followed by Italy (8%) and the USA (7%).

As a noteworthy fact, there are 35 review articles from 2011 to 2023 containing at least one of the previous keywords (77% consider PVT technology, 40% PV; 37% mention its application in buildings, and 26% review solar collectors), see Table 6; a fact that highlights the importance of the topic of this work.

In addition, in the last 5 months of 2024, in terms of scientific contribution, 21 papers have been found associated with the four keywords (Table 7), meaning that this technology still generates a lot of expectation and has a lot of progress ahead. These works show the predominant research trend for the near future, dealing with technological advances and performance improvement (7 papers) and theoretical analysis of the different improvement proposals (another 7 papers). Even so, the application and optimization of these systems in buildings is still something to be investigated, as six of the papers said.

As a whole, solar radiation is an inexhaustible renewable resource capable of supplying energy demands through multiple alternatives and combinations. Furthermore, its application in buildings is advantageous because it does not require more space than that currently built and does not affect the biodiversity of the environment, allowing energy users to be energy producers, thus promoting energy self-sufficiency in society.

Recommendations for solar technologies in buildings can include improving the efficiency and architectural integration of PV, ST, and PVT systems, as well as optimizing energy storage. Besides, policies should encourage economic incentives, mandatory integration regulations, and the development of smart grid infrastructures. While during its implementation, it is key to promote its adoption in the residential, commercial and industrial sectors, with an emphasis on self-consumption and the use of hybrid solutions. In addition, progress must be made in accessible financing and research into complementary technologies. Besides, the satisfaction level of the end users and producers and the global viewpoint of policymakers, between others, must be considered for advancing all together in energy usage patterns, policies, subsidies, and regulations that support and promote this type of technology, which are the main social barriers.

Conflicts of Interest

The authors declare no conflicts of interest.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.