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Status, barriers and perspectives of building integrated photovoltaic systems
Energy xxx (xxxx) xxx
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Status, barriers and perspectives of building integrated photovoltaic systems Rafaela A. Agathokleous*, Soteris A. Kalogirou Department of Mechanical Engineering and Materials Science, Cyprus University of Technology, Kitiou Kyprianou 36, 3041, Limassol, Cyprus
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 August 2019 Received in revised form 29 October 2019 Accepted 1 November 2019 Available online xxx
Many countries apply measures for the limitation of the conventional energy technologies and try to expand the use of renewables. As the building sector accounts for almost 40% of the energy consumption in Europe, building integrated photovoltaic (BIPV) systems gain peoples’ interest lately concerning the replacement of the conventional construction materials of the buildings envelope with photovoltaic (PV) panels which can serve at the same time as construction material and energy producer. The aim of this study is to present an overview of the available published research on the BIPV systems and identify the barriers and risks associated with the application of BIPV and discuss the future perspectives and solutions through recommendations for future research and development. The most important barriers of the BIPV systems are the feed in tariff implementation, the public acceptance, the governmental economic support in terms of subsidies and technical aspects like the power losses and the architectural considerations. The future perspectives of the BIPV systems proposed are based on the barriers discussed. It is stated that new solutions in the PV industry are many and various and there is room for improvement regarding design, configuration, ventilation, positioning, guidelines, monitoring and performance prediction. In total, more than 100 articles have been identified and analysed since 2000. Many of these articles have a predominant focus on the investigation of the performance of the system, and the ventilation of the PV panels in BIPV applications for electricity production, due to the negative role of the temperature on PVs electrical efficiency, and the heat transfer behavior of the system. This paper shows that although research in the adoption of BIPV systems in terms of their performance and optimization is fairly new, it has gained attention in the last decades. However, their practical applications have been slow in comparison with the conventional rack-mounted PV panels. © 2019 Elsevier Ltd. All rights reserved.
Keywords: BIPV BIPV/T PV Barriers Building integration Perspectives
1. Introduction The trend for transforming buildings from energy users to energy producers is not something that has only just appeared. Architectural, structural, and aesthetic solutions involving integrating PV into the building envelope have been sought since photovoltaics (PV) first entered the market. There are two ways of incorporating PV into the building envelope, BAPV (building applied PV) and BIPV (building integrated PV). In a BAPV system the PV modules are fixed onto the existing building façade but in a BIPV system, PV modules are part of the building envelope. BIPV systems can be used mainly to produce electricity, but in some applications, they can also provide hot air
* Corresponding author. E-mail address: [email protected] (R.A. Agathokleous).
for space heating. When the heated air is used to heat the building, then the system is called Building Integrated Photovoltaic/Thermal (BIPV/T). The first installation of building integrated photovoltaic (BIPV) was realized in Aachen, Germany in 1991 where the PV elements were integrated into a curtain wall façade with isolating glasses [1]. BIPV systems ensure many functions. They are known to provide weather and noise protection, thermal insulation and even structural strength [2]. Additionally, they may allow daylight entry, providing outside view and most important, they provide electrical power and thermal energy in some configurations. Additionally, BIPV systems can also function as an attractive feature of the architectural appearance of a building. The advantages of BIPV systems intensified the interest of engineers and architects to investigate them both in terms of performance and appearance. Today PV panels are produced as standard building products and a whole new market is created
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around BIPV systems [3] and it is one of the fastest growing market segments in photovoltaics. BIPV installations are expected to grow in 2020, with a rate of 30% each year. The installed capacity is expected to be more than 8000 MW by the end of the year 2020 [4]. This study aims to present an overview of the available published research on the BIPV systems and identify the technical barriers and risks associated with the application of BIPV from building design through to operation stages and give future perspectives and solutions through recommendations for future research and development. This paper shows that although research in the adoption of BIPV systems in terms of their performance and optimization is fairly new, it has gained increasing attention during the last years, but their practical applications have been slow in comparison with the conventional rack-mounted PV panels. The main reason is the technical barriers which are discussed in the last two sections of this paper. The paper concludes with the identification of the future perspectives of the BIPV systems. Given the above research aims, in this study, an overview of the available published research on the BIPV systems aspects since 2000 is presented. More than 100 articles have been identified and analysed since 2000, which were clearly focusing on BIPV systems. Regarding the current status of research, until now this has been published in a variety of academic journals, which proves the mainstreaming of the topic and the separation into various research disciplines. The most common journal for BIPV systems is Energy Procedia, followed by Energy in Buildings and Solar Energy, then Renewable Energy, Renewable and Sustainable Energy Reviews, Applied Energy and Energy. Additional articles on the topic have been published in more than 10 other journals.
Fig. 1. BIPV system with PV panels in contact with building’s skin.
2. BIPV systems and applications Many countries have established promotive mechanisms for the application of BIPV systems in order to increase their development [5]. The present size of the global BIPV market is about 2.3 GW (or ~ 1%of the global PV market) [6], with Europe constituting the largest market (42% of global market) in particular due to attractive incentives in France, Italy and Germany. The annual installed capacity of BIPV systems worldwide is expected to reach 32.3 GWh by 2024 [7]. The efforts undertaken on BIPVs have revealed that the systems can meet the partial or complete energy demand of buildings [8]. The four main options for building integration of PV cells are on inclined roofs, flat roofs, facades, and shading systems. As time passes and more systems are being installed, the people need to have better design and higher performance, and thus various design configurations were developed. There are references for double skin systems with passing air between the two skins for overheating prevention, transparent and semi-transparent PV panels, roof integrated PV panels replacing the conventional roof tiles called PV shingles, façade applications for curtain walls, glazing windows applications, and shading elements applications. According to Shukla et al. [9], roof top mounted systems hold 80% of the BIPV market while the façade mounted hold the rest. The roof mounted systems include the roof integrated shingles, skylights, atria and roof panels, and the façade mounted systems include the shading systems, cladding and curtain walls. Schematics of the mostly used configurations of BIPV systems are shown in Figs. 1e9. Fig. 1 shows the PV panels in full contact with the building’s wall. Then Figs. 2 and 3 show a BIPV system and BIPV/T system respectively. The first rejects the heated air and the second uses it for space heating through an air duct in the ceiling. Figs. 4e6 show roof BIPV and BIPV/T systems; the first one rejects the heated air behind the PV panels and the other two, use the
Fig. 2. BIPV system with PV panels separated from the building’s skin with an air gap.
heated air for space heating with ducts or roof air circulation. The latter is similar to the system shown in Fig. 7 which is a BIPV/T system which circulates the indoor space air to the PV air gap. Figs. 8 and 9 show BIPV applications with semi-transparent PV modules which provide indoor natural light as well. The various applications can be configured in wall elements, or windows. Other BIPV applications which are not shown here, are the use of BIPV as shading elements and BIPV systems which are also used to provide hot water, connected with pumps and water storage tanks. Another interesting aspect gaining attention recently, is the use of phase change materials (PCM) for thermal regulation enhancement of BIPV/T systems. An approach with two PCMs on the thermal regulation performance of BIPV systems is given by Huang [10]. It is shown that the PV/PCM system with two types of PCMs can maintain the PV at operating temperature closer to its characteristic value of 25 C and thus lead to an improvement in solar-toelectrical conversion efficiency under variable diurnal insolation. Another configuration of the system is studied by Hu et al. [11] named building integrated photovoltaic trombe wall (BIPVTW).
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Fig. 3. BIPV/T system with air gap between panels and wall, and air duct at ceiling to provide the hot air to the interior space.
Fig. 5. Roof BIPV/T system with air gap between the panels and the roof substructure, and air ducts to provide hot air to the interior space.
Fig. 4. Roof BIPV system with air gap between the panels and the roof substructure.
These systems can provide heating/cooling and generate electricity simultaneously. The mostly used PCM BIPV system in the market looks like the schematic representation of Fig. 10, which provides heat transfer enhancement in the interior, with radiation. In many BIPV applications like the system shown in Fig. 1, the PV modules are installed in close contact to building materials like wall insulation, roof tiles or roof substructure, hence, lack of circulating air which increases the module temperature and reduces its performance because of the drop of their conversion efficiency. Accordingly, double skin BIPV systems with air gap for PV cooling are preferable. With growing tendency in recent years, an increasing number of researchers underline the importance of
Fig. 6. Roof BIPV/T system with air gap between the panels and the roof substructure, which circulates the hot air and provide it to the interior space.
ventilation of the BIPV systems and this is seen by the published papers which are mainly focused on the modeling and simulation of the thermal behavior and performance of the systems with natural or forced ventilation. The ventilation of the air gap can be
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Fig. 9. Vertical semi-transparent BIPV system as wall component or windows.
Fig. 7. Roof BIPV/T system with air gap between the panels and the roof substructure, which circulates the hot air within the interior space.
Fig. 10. BIPV/T system with PCM layers for heat transfer enhancement.
gap. Concerning ventilated systems, or double skin ventilated PV facades, various researchers studied the air flow and heat transfer characteristics of the BIPV systems. Although the natural convection analysis of the BIPV systems seems very complex and inconvenient, it is believed that it has many potentials and worth better examination due to their advantages [12]. Fig. 8. Roof semi-transparent BIPV system with natural light in the interior space.
3. Studies and investigations on BIPV systems natural or mechanically driven (fan) depending on the needs of the building or the use of the system. Natural ventilation has a number of advantages, such as the avoidance of energy to power the fans and the operation with no noise, but on the other hand, mechanical ventilation can be more effective to remove excess heat from the
There are various researchers who tried to collect the knowledge around BIPV systems in review papers [13e17]. Various researchers are referring to case studies, investigating real size BIPV and BIPV/T applications or prototypes [18e25] and life cycle
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assessment of BIPV [20,26]. It is known that BIPV systems often reach higher temperatures than stand-alone ones and this leads to performance drop. The operating cell temperatures, however, can usually be controlled with ventilation in the double skin BIPV systems where air passes behind the PV panels to cool them down. Thus, the air gap configuration and sizing are other important parameters which have been investigated related to the performance of the BIPV systems. It can be clearly seen that there is a big number of researchers who attempted to improve the knowledge on the behavior of the BIPV systems by investigating the flow, the heat transfer and optical characteristics of the systems [27e32]. Apart from the studies focusing on the heat transfer characteristics of the system, there are also various studies which examine the performance investigation of BIPV systems [33e40]. These can be categorized on those with simulation modeling studies, experimental studies and case studies. 4. Barriers of BIPV Although BIPV systems installations show an increasing trend every year, there are many problems that have to be solved. As can be seen from the literature review presented earlier, many researchers investigated the performance of the systems and thermal behavior of the systems, always trying to increase their efficiency, to isolate the parameters that affect more their efficiency and optimize the design configuration or identify the problems that affect their efficiency. However, most of the problems are common with the whole PV industry not only for the BIPV systems. The most important barriers of the BIPV systems are the feed in tariff implementation, the public acceptance, the governmental economic support in terms of subsidies and technical aspects like the power losses and the architectural considerations. The barriers and risks of the BIPV systems are very well discussed in Ref. [41] mentioning that if we want the upward trend of the PV systems to continue, we need to pay attention to the current barriers. Goh et al. [42] studied the awareness and initiatives of BIPV implementation in Malaysian Housing industry. Special attention is given to the basic stakeholders. The results show that many established developers have employed green technologies, but none has considered BIPV since 2015. The reason of not being an option for them is the high cost as they claimed. A cost benefit analysis of integrating BIPV/T air systems into energy efficient homes is given by Delisle and Kimmert [43]. After an extensive research based on published articles, Yang [44] identified the barriers and categorized them in terms of stages in the BIPV application lifecycle, from design, construction and installation to commissioning and maintenance. To validate those barriers, the BIPV industry professionals were invited on a workshop to interview them. It was concluded that the professionals knew most of the barriers but also concluded that clients and building professionals lack knowledge on BIPV design, installation and maintenance. Curtius [45] investigated the barriers and facilitators of BIPV adoption based on 43 qualitative interviews with stakeholders across the BIPV value chain. Even in 2018, it is surprising that many architects do not want to adopt BIPV. Based on the findings of the literature review presented earlier, the most important barriers of the BIPV systems are identified and sorted according to their importance as shown below: 1. Heat transfer issues due to the inherent design of BIPV. This is the main problem which is investigated by the most researchers. The overheating of the PV panels and the transfer
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of this heat to the building. There are many cases which required additional cooling due to the PV system. A very important issue is the fact that there is no system performance monitoring system was used after the installation. Even for commissioning purposes to make sure that the system functions as intended, and for long term monitoring, this is very important. Probably a monthly comprehensive monitoring procedure is necessary in order to show any fault alerts and allow for changes to the system set-up to ensure maximum performance is achieved for a long time. Another important issue which is related to the monitoring is the system modeling before the installation. Most of the times, the system is just designed and installed based on the space availability for installation or the building needs. Even if the engineers know that the system is complex and require proper designing decisions, usually, there is no study prior to the installation to show the expected behavior of the system. This would allow better design, and better performances as well. Designers and architects do not think the maintenance and the possibility to replace a single module when designing BIPV systems. Most of the times there is no easy access to external fixing and wiring and there are serious difficulties when there is a need to replace BIPV modules. Replacements are complex because of the amount of wiring that connect each module to the others. There are no standards, codes or guidelines for buildings or generally integrated materials to the buildings. Engineers and installers make most of the times custom made structures to install the systems depending on the position of installation. There are also barriers related to the installation, such as the failure of fixings, rain effects, cabling and connections, silicone waterproofing, waterproof sealants condition and islanding. Issues like water penetration, islanding and heat transfer due to poor design integration considerations and difficult maintenance procedures have all contributed to the low uptake rates of BIPV systems [44]. Regarding standards and regulations, there are also concerns in relation with health and safety. There are no building codes or standards regarding BIPV application in relation with health and safety, to cover the cases of fire, electricity shortcut, wires failures. On the standardization issue, it should be also noted that the BIPV system will be part of the building envelope since PVs will replace conventional materials. Thus, the noise control should be under examination as well. Standards for noise protection by integrating PV in buildings are not clear in the building codes [44]. The inappropriate positioning is another barrier. As mentioned in Ref. [45], when architects choose to use BIPV system they do it mainly for aesthetic reasons. However, it is important to see what kind of system does the building needs or can support. Even the type of PV panels has an important role with respect to the inclination, orientation, shadows etc. Apart from the BIPV system weight and loads, there are other loads that occur from time to time and are very important and should be considered as well. These are the snow, ice and wind. Lack of allowance of extra loads can cause photovoltaic modules bending and this will lead to various failures requiring repairs or replacement. It is important to follow a standard installation procedure to ensure the safety of the system under extreme weather conditions. In cases where BIPV system separates the indoor from the outdoor spaces, attention needs to be paid in safety and protection.
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10. Another issue that concerns the potential users of BIPV systems is the pricing of the system. It is found that economical evaluations for BIPV systems are usually made for retrofits and renovations in order to identify the payback time of the new suggested system for installation. In new buildings, the economic evaluation is easier because it is not affected by the cost of materials that will be replaced by the BIPV system. Nevertheless, in both systems, the total cost, the payback time, and the feed in tariff need to be considered for the economic optimization of the application. Concerning the purpose served by the renewable energy systems for sustainability, a life cycle analysis is also very important.
5. Future perspectives It is obvious that BIPV products development has been ongoing for the past 30 years, but their practical applications have been slow in comparison to conventional rack-mounted solar PV. One of the main reasons is that the technical barriers, mentioned in the previous section, which span from design phase through to commissioning and maintenance phases, have not been understood by stakeholders. Jelle et al. [46] stated that the new solutions in the PV industry are many and of various types. There is usually room for improvement in each specific system, e.g. regarding ventilation rate, positioning, removing of snow, etc. Based on the barriers occurring during BIPV installation mentioned previously, there are also some collective strategies from stakeholders and BIPV industry people that need to pay attention. A good solution would be the joint development of technical training programs by the building and PV industries. A good example of this point is the research project BFIRST [47] where five new BIPV products were designed and installed in demo buildings with the cooperation of architects, engineers, researchers and manufacturers. The collaboration between manufacturers, building professionals and clients is of a major importance for future development. 5.1. System appearance and design configuration development Modern architectural style is a trend and the BIPV systems must be modern and flexible in order to be selected by the architects and make them part of their designs. Some practices to improve the use of the BIPV systems are: Create lightweight panels which will increase the installation in existing buildings. Eliminate the standard PV panel view and cables view for safety issues but also from aesthetic point of view. Incorporate the panels to the envelope of the building. They should be part of the building’s envelope e.g. BFIRST PV roofing shingles [47]. This will increase the acceptance of the building’s users as well as the acceptance of the architects to use them as a building construction material. Eliminate glare with the use of proper coatings so that glare will no longer be an excuse for not choosing the systems. Develop more color options, thickness options, shape options e.g. curved panels. Consider the idea to cover other building elements e.g. balconies and canopies. A design that will allow tubes of water to pass behind the PV panels to heat water would be a good option and the system could serve a triple purpose of electricity production, space heating and water heating.
Integrate PV cells in materials at an early stage e.g. in prefabricated concrete plates [48]. Concrete is the most used material in construction, and it could be a good idea for BIPV market to grow. Use of thin laminate or paint layer solar cell materials to create PV smart windows in a way that the PV elements will provide shading when it is needed. This is already being explored and used. Focus need to be given to the existing buildings and retrofitting market because these are more than the volume of buildings to be constructed in a foreseen future. Easy application of PV cells in existing materials is essential, and this may in the future be performed by various painting techniques as mentioned by Jelle et al. [48]. Enhance the naturally ventilated BIPV systems so to increase their efficiency and benefit from their positive specifications e.g. easier installation, no maintenance, no need for extra energy etc. An optimum air gap for each slope and each location is required as a guideline for engineers and installers to avoid PV panels overheating which is not efficient neither for the system itself nor the building which absorbs the heat created and increases its cooling loads. Another important consideration is the height of the system. The optimum air gap may vary for different system heights or the idea of open joints [49,50] has to be implemented in this kind of applications if conditions allow.
5.2. Realistic performance prediction with modeling tools Advanced graphical analysis for building level installations is of the greatest importance. There are numerous models developed by researchers for the prediction of the BIPV systems performance before the installation, but they remain only in scientific publications since they are not directed to the market to be used by the relevant people involved in the design and installation of BIPV systems. The study carried out by Yang [44] highlighted the importance to apply advanced simulation tools and energy performance monitoring platforms in practice, and encourage stakeholder collaborations in the whole supply chain. This action will: Prevent any negative impact on the building and the system itself e.g. shading, damage etc. Prevent overheating of the building (simulation with weather data of the specific location that the installation will take place). Prevent overheating of the system (allow system design configuration to find the optimum air gap for air circulation of the system for each particular application with respect to slope, height etc.) Predict the amount of produced energy and avoid lower efficiency than expected because of orientation, slope and position.
5.3. Guidelines through regulatory regimes and BIPV policies Since BIPV is a rapidly changing market, it must be acknowledged that this situation is subject to changes as well. Shifting regulatory regimes highly influence the composition of the market and its active stakeholders. Governments and other stakeholders in the building supply chain need to understand not only the benefits of BIPV systems, but the technical issues, and move towards developing unified building regulations and accredited training programs, to encourage an increase in BIPV application rates. It is important to develop guidelines and standards for BIPV
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implementation regarding all the issues from their lifetime, from their installation to the operation and maintenance. Single panel replacement is difficult in the current stage as well as other parts replacements such as structure parts, fixing connections etc. A methodology for a smart installation should be developed for each type of system (roof, façade etc.). The most important aspect that needs to be covered by regulations is the safety issue. There are regulations on various building components regarding fire safety, but they are missing for the PV panels which are being used as a construction element in the BIPV systems. 6. Conclusions The potential of solar energy to make significant contribution to the electricity demand and green energy production is widely recognized. BIPV systems allowed buildings to be transformed from energy consumers to energy producers. The objectives of this study were to identify the current status and research on BIPV systems, identify the barriers that slow down their expansion and give the future perspectives to exploit their usage at maximum level. The literature review identified the most studied topics related to the BIPV systems which are mainly on the performance of the system and the heat transfer mechanisms taking place in the systems. From the literature review, 10 barriers were identified that prevent the usage of the system and the people acceptance. Then future suggestions are given based on the discussed barriers, with the ultimate purpose to increase the penetration of the systems. It was concluded that there is a need to develop standards and building codes regarding the BIPV systems and also a need for collaboration between manufacturers, engineers and architects to overcome the barriers and increase the market share of the BIPV systems. References [1] Heinstein P, Ballif C, Perret-Aebi L-E. Building integrated photovoltaics (BIPV): review, potentials, barriers and myths. Green 2013;3(2). [2] Zhang X, Lau S-K, Lau SSY, Zhao Y. Photovoltaic integrated shading devices (PVSDs): a review. Sol Energy Aug. 2018;170:947e68. [3] Benemann J, Chehab O, Schaar-Gabriel E. Building-integrated PV modules. Sol Energy Mater Sol Cells 2001;67(1e4):345e54. [4] Richhariya G, Kumar A. Chapter 4 review on performance affected parameters for dye sensitized solar cell. May. 2016. [5] Assoa YB, et al. Thermal analysis of a BIPV system by various modelling approaches. Sol Energy Oct. 2017;155:1289e99. [6] I. Global Industry Analysts, “BIPV Market analysis, trends and forecasts.” [Online]. Available: https://www.strategyr.com/market-report-buildingintegrated-photovoltaics-bipv-forecasts-global-industry-analysts-inc.asp. [Accessed: 03-Jul-2019]. [7] I. Global Industry Analysts, “BIPV Market analysis, trends and forecasts.” . [8] Debbarma M, Sudhakar K, Baredar P. Thermal modeling, exergy analysis, performance of BIPV and BIPVT: a review. Renew Sustain Energy Rev Jun. 2017;73:1276e88. [9] Shukla AK, Sudhakar K, Baredar P. Recent advancement in BIPV product technologies: a review. Energy Build Apr. 2017;140:188e95. [10] Jun Huang M. The effect of using two PCMs on the thermal regulation performance of BIPV systems. Sol Energy Mater Sol Cells Mar. 2011;95(3): 957e63. [11] Hu Z, et al. Comparative study on the annual performance of three types of building integrated photovoltaic (BIPV) Trombe wall system. Appl Energy May 2017;194:81e93. [12] Agathokleous RA, Kalogirou SA. Double skin facades (DSF) and building integrated photovoltaics (BIPV): a review of configurations and heat transfer characteristics. Renew Energy Apr. 2016;89:743e56. [13] Biyik E, et al. A key review of building integrated photovoltaic (BIPV) systems. Eng Sci Technol Int J Jun. 2017;20(3):833e58. [14] Shukla AK, Sudhakar K, Baredar P. A comprehensive review on design of building integrated photovoltaic system. Energy Build Sep. 2016;128:99e110. [15] Debbarma M, Sudhakar K, Baredar P. Comparison of BIPV and BIPVT: a review. Resour Technol Sep. 2017;3(3):263e71. [16] Saretta E, Caputo P, Frontini F. A review study about energy renovation of building facades with BIPV in urban environment. Sustain Cities Soc Jan.
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Energy Procedia Nov. 2015;78: 1974e9. [38] Yang T, Athienitis AK. Performance evaluation of air-based building integrated photovolta-ic/thermal (BIPV/T) system with multiple inlets in a cold climate. Procedia Eng Jan. 2015;121:2060e7. [39] Yadav S, Panda SK, Tripathy M. Performance of building integrated photovoltaic thermal system with PV module installed at optimum tilt angle and influenced by shadow. Renew Energy Nov. 2018;127:11e23. [40] Martellotta F, Cannavale A, Ayr U. “Comparing energy performance of different semi-transparent, building-integrated photovoltaic cells applied to ‘reference’ buildings. Energy Procedia Sep. 2017;126:219e26. [41] Azadian F, Radzi MAM. A general approach toward building integrated photovoltaic systems and its implementation barriers: a review. Renew Sustain Energy Rev Jun. 2013;22:527e38. [42] Goh KC, Yap ABK, Goh HH, Seow TW, Toh TC. Awareness and initiatives of building integrated photovoltaic (BIPV) implementation in Malaysian housing industry. 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Energy journal homepage: www.elsevier.com/locate/energy
Status, barriers and perspectives of building integrated photovoltaic systems Rafaela A. Agathokleous*, Soteris A. Kalogirou Department of Mechanical Engineering and Materials Science, Cyprus University of Technology, Kitiou Kyprianou 36, 3041, Limassol, Cyprus
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 August 2019 Received in revised form 29 October 2019 Accepted 1 November 2019 Available online xxx
Many countries apply measures for the limitation of the conventional energy technologies and try to expand the use of renewables. As the building sector accounts for almost 40% of the energy consumption in Europe, building integrated photovoltaic (BIPV) systems gain peoples’ interest lately concerning the replacement of the conventional construction materials of the buildings envelope with photovoltaic (PV) panels which can serve at the same time as construction material and energy producer. The aim of this study is to present an overview of the available published research on the BIPV systems and identify the barriers and risks associated with the application of BIPV and discuss the future perspectives and solutions through recommendations for future research and development. The most important barriers of the BIPV systems are the feed in tariff implementation, the public acceptance, the governmental economic support in terms of subsidies and technical aspects like the power losses and the architectural considerations. The future perspectives of the BIPV systems proposed are based on the barriers discussed. It is stated that new solutions in the PV industry are many and various and there is room for improvement regarding design, configuration, ventilation, positioning, guidelines, monitoring and performance prediction. In total, more than 100 articles have been identified and analysed since 2000. Many of these articles have a predominant focus on the investigation of the performance of the system, and the ventilation of the PV panels in BIPV applications for electricity production, due to the negative role of the temperature on PVs electrical efficiency, and the heat transfer behavior of the system. This paper shows that although research in the adoption of BIPV systems in terms of their performance and optimization is fairly new, it has gained attention in the last decades. However, their practical applications have been slow in comparison with the conventional rack-mounted PV panels. © 2019 Elsevier Ltd. All rights reserved.
Keywords: BIPV BIPV/T PV Barriers Building integration Perspectives
1. Introduction The trend for transforming buildings from energy users to energy producers is not something that has only just appeared. Architectural, structural, and aesthetic solutions involving integrating PV into the building envelope have been sought since photovoltaics (PV) first entered the market. There are two ways of incorporating PV into the building envelope, BAPV (building applied PV) and BIPV (building integrated PV). In a BAPV system the PV modules are fixed onto the existing building façade but in a BIPV system, PV modules are part of the building envelope. BIPV systems can be used mainly to produce electricity, but in some applications, they can also provide hot air
* Corresponding author. E-mail address: [email protected] (R.A. Agathokleous).
for space heating. When the heated air is used to heat the building, then the system is called Building Integrated Photovoltaic/Thermal (BIPV/T). The first installation of building integrated photovoltaic (BIPV) was realized in Aachen, Germany in 1991 where the PV elements were integrated into a curtain wall façade with isolating glasses [1]. BIPV systems ensure many functions. They are known to provide weather and noise protection, thermal insulation and even structural strength [2]. Additionally, they may allow daylight entry, providing outside view and most important, they provide electrical power and thermal energy in some configurations. Additionally, BIPV systems can also function as an attractive feature of the architectural appearance of a building. The advantages of BIPV systems intensified the interest of engineers and architects to investigate them both in terms of performance and appearance. Today PV panels are produced as standard building products and a whole new market is created
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around BIPV systems [3] and it is one of the fastest growing market segments in photovoltaics. BIPV installations are expected to grow in 2020, with a rate of 30% each year. The installed capacity is expected to be more than 8000 MW by the end of the year 2020 [4]. This study aims to present an overview of the available published research on the BIPV systems and identify the technical barriers and risks associated with the application of BIPV from building design through to operation stages and give future perspectives and solutions through recommendations for future research and development. This paper shows that although research in the adoption of BIPV systems in terms of their performance and optimization is fairly new, it has gained increasing attention during the last years, but their practical applications have been slow in comparison with the conventional rack-mounted PV panels. The main reason is the technical barriers which are discussed in the last two sections of this paper. The paper concludes with the identification of the future perspectives of the BIPV systems. Given the above research aims, in this study, an overview of the available published research on the BIPV systems aspects since 2000 is presented. More than 100 articles have been identified and analysed since 2000, which were clearly focusing on BIPV systems. Regarding the current status of research, until now this has been published in a variety of academic journals, which proves the mainstreaming of the topic and the separation into various research disciplines. The most common journal for BIPV systems is Energy Procedia, followed by Energy in Buildings and Solar Energy, then Renewable Energy, Renewable and Sustainable Energy Reviews, Applied Energy and Energy. Additional articles on the topic have been published in more than 10 other journals.
Fig. 1. BIPV system with PV panels in contact with building’s skin.
2. BIPV systems and applications Many countries have established promotive mechanisms for the application of BIPV systems in order to increase their development [5]. The present size of the global BIPV market is about 2.3 GW (or ~ 1%of the global PV market) [6], with Europe constituting the largest market (42% of global market) in particular due to attractive incentives in France, Italy and Germany. The annual installed capacity of BIPV systems worldwide is expected to reach 32.3 GWh by 2024 [7]. The efforts undertaken on BIPVs have revealed that the systems can meet the partial or complete energy demand of buildings [8]. The four main options for building integration of PV cells are on inclined roofs, flat roofs, facades, and shading systems. As time passes and more systems are being installed, the people need to have better design and higher performance, and thus various design configurations were developed. There are references for double skin systems with passing air between the two skins for overheating prevention, transparent and semi-transparent PV panels, roof integrated PV panels replacing the conventional roof tiles called PV shingles, façade applications for curtain walls, glazing windows applications, and shading elements applications. According to Shukla et al. [9], roof top mounted systems hold 80% of the BIPV market while the façade mounted hold the rest. The roof mounted systems include the roof integrated shingles, skylights, atria and roof panels, and the façade mounted systems include the shading systems, cladding and curtain walls. Schematics of the mostly used configurations of BIPV systems are shown in Figs. 1e9. Fig. 1 shows the PV panels in full contact with the building’s wall. Then Figs. 2 and 3 show a BIPV system and BIPV/T system respectively. The first rejects the heated air and the second uses it for space heating through an air duct in the ceiling. Figs. 4e6 show roof BIPV and BIPV/T systems; the first one rejects the heated air behind the PV panels and the other two, use the
Fig. 2. BIPV system with PV panels separated from the building’s skin with an air gap.
heated air for space heating with ducts or roof air circulation. The latter is similar to the system shown in Fig. 7 which is a BIPV/T system which circulates the indoor space air to the PV air gap. Figs. 8 and 9 show BIPV applications with semi-transparent PV modules which provide indoor natural light as well. The various applications can be configured in wall elements, or windows. Other BIPV applications which are not shown here, are the use of BIPV as shading elements and BIPV systems which are also used to provide hot water, connected with pumps and water storage tanks. Another interesting aspect gaining attention recently, is the use of phase change materials (PCM) for thermal regulation enhancement of BIPV/T systems. An approach with two PCMs on the thermal regulation performance of BIPV systems is given by Huang [10]. It is shown that the PV/PCM system with two types of PCMs can maintain the PV at operating temperature closer to its characteristic value of 25 C and thus lead to an improvement in solar-toelectrical conversion efficiency under variable diurnal insolation. Another configuration of the system is studied by Hu et al. [11] named building integrated photovoltaic trombe wall (BIPVTW).
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Fig. 3. BIPV/T system with air gap between panels and wall, and air duct at ceiling to provide the hot air to the interior space.
Fig. 5. Roof BIPV/T system with air gap between the panels and the roof substructure, and air ducts to provide hot air to the interior space.
Fig. 4. Roof BIPV system with air gap between the panels and the roof substructure.
These systems can provide heating/cooling and generate electricity simultaneously. The mostly used PCM BIPV system in the market looks like the schematic representation of Fig. 10, which provides heat transfer enhancement in the interior, with radiation. In many BIPV applications like the system shown in Fig. 1, the PV modules are installed in close contact to building materials like wall insulation, roof tiles or roof substructure, hence, lack of circulating air which increases the module temperature and reduces its performance because of the drop of their conversion efficiency. Accordingly, double skin BIPV systems with air gap for PV cooling are preferable. With growing tendency in recent years, an increasing number of researchers underline the importance of
Fig. 6. Roof BIPV/T system with air gap between the panels and the roof substructure, which circulates the hot air and provide it to the interior space.
ventilation of the BIPV systems and this is seen by the published papers which are mainly focused on the modeling and simulation of the thermal behavior and performance of the systems with natural or forced ventilation. The ventilation of the air gap can be
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Fig. 9. Vertical semi-transparent BIPV system as wall component or windows.
Fig. 7. Roof BIPV/T system with air gap between the panels and the roof substructure, which circulates the hot air within the interior space.
Fig. 10. BIPV/T system with PCM layers for heat transfer enhancement.
gap. Concerning ventilated systems, or double skin ventilated PV facades, various researchers studied the air flow and heat transfer characteristics of the BIPV systems. Although the natural convection analysis of the BIPV systems seems very complex and inconvenient, it is believed that it has many potentials and worth better examination due to their advantages [12]. Fig. 8. Roof semi-transparent BIPV system with natural light in the interior space.
3. Studies and investigations on BIPV systems natural or mechanically driven (fan) depending on the needs of the building or the use of the system. Natural ventilation has a number of advantages, such as the avoidance of energy to power the fans and the operation with no noise, but on the other hand, mechanical ventilation can be more effective to remove excess heat from the
There are various researchers who tried to collect the knowledge around BIPV systems in review papers [13e17]. Various researchers are referring to case studies, investigating real size BIPV and BIPV/T applications or prototypes [18e25] and life cycle
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assessment of BIPV [20,26]. It is known that BIPV systems often reach higher temperatures than stand-alone ones and this leads to performance drop. The operating cell temperatures, however, can usually be controlled with ventilation in the double skin BIPV systems where air passes behind the PV panels to cool them down. Thus, the air gap configuration and sizing are other important parameters which have been investigated related to the performance of the BIPV systems. It can be clearly seen that there is a big number of researchers who attempted to improve the knowledge on the behavior of the BIPV systems by investigating the flow, the heat transfer and optical characteristics of the systems [27e32]. Apart from the studies focusing on the heat transfer characteristics of the system, there are also various studies which examine the performance investigation of BIPV systems [33e40]. These can be categorized on those with simulation modeling studies, experimental studies and case studies. 4. Barriers of BIPV Although BIPV systems installations show an increasing trend every year, there are many problems that have to be solved. As can be seen from the literature review presented earlier, many researchers investigated the performance of the systems and thermal behavior of the systems, always trying to increase their efficiency, to isolate the parameters that affect more their efficiency and optimize the design configuration or identify the problems that affect their efficiency. However, most of the problems are common with the whole PV industry not only for the BIPV systems. The most important barriers of the BIPV systems are the feed in tariff implementation, the public acceptance, the governmental economic support in terms of subsidies and technical aspects like the power losses and the architectural considerations. The barriers and risks of the BIPV systems are very well discussed in Ref. [41] mentioning that if we want the upward trend of the PV systems to continue, we need to pay attention to the current barriers. Goh et al. [42] studied the awareness and initiatives of BIPV implementation in Malaysian Housing industry. Special attention is given to the basic stakeholders. The results show that many established developers have employed green technologies, but none has considered BIPV since 2015. The reason of not being an option for them is the high cost as they claimed. A cost benefit analysis of integrating BIPV/T air systems into energy efficient homes is given by Delisle and Kimmert [43]. After an extensive research based on published articles, Yang [44] identified the barriers and categorized them in terms of stages in the BIPV application lifecycle, from design, construction and installation to commissioning and maintenance. To validate those barriers, the BIPV industry professionals were invited on a workshop to interview them. It was concluded that the professionals knew most of the barriers but also concluded that clients and building professionals lack knowledge on BIPV design, installation and maintenance. Curtius [45] investigated the barriers and facilitators of BIPV adoption based on 43 qualitative interviews with stakeholders across the BIPV value chain. Even in 2018, it is surprising that many architects do not want to adopt BIPV. Based on the findings of the literature review presented earlier, the most important barriers of the BIPV systems are identified and sorted according to their importance as shown below: 1. Heat transfer issues due to the inherent design of BIPV. This is the main problem which is investigated by the most researchers. The overheating of the PV panels and the transfer
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of this heat to the building. There are many cases which required additional cooling due to the PV system. A very important issue is the fact that there is no system performance monitoring system was used after the installation. Even for commissioning purposes to make sure that the system functions as intended, and for long term monitoring, this is very important. Probably a monthly comprehensive monitoring procedure is necessary in order to show any fault alerts and allow for changes to the system set-up to ensure maximum performance is achieved for a long time. Another important issue which is related to the monitoring is the system modeling before the installation. Most of the times, the system is just designed and installed based on the space availability for installation or the building needs. Even if the engineers know that the system is complex and require proper designing decisions, usually, there is no study prior to the installation to show the expected behavior of the system. This would allow better design, and better performances as well. Designers and architects do not think the maintenance and the possibility to replace a single module when designing BIPV systems. Most of the times there is no easy access to external fixing and wiring and there are serious difficulties when there is a need to replace BIPV modules. Replacements are complex because of the amount of wiring that connect each module to the others. There are no standards, codes or guidelines for buildings or generally integrated materials to the buildings. Engineers and installers make most of the times custom made structures to install the systems depending on the position of installation. There are also barriers related to the installation, such as the failure of fixings, rain effects, cabling and connections, silicone waterproofing, waterproof sealants condition and islanding. Issues like water penetration, islanding and heat transfer due to poor design integration considerations and difficult maintenance procedures have all contributed to the low uptake rates of BIPV systems [44]. Regarding standards and regulations, there are also concerns in relation with health and safety. There are no building codes or standards regarding BIPV application in relation with health and safety, to cover the cases of fire, electricity shortcut, wires failures. On the standardization issue, it should be also noted that the BIPV system will be part of the building envelope since PVs will replace conventional materials. Thus, the noise control should be under examination as well. Standards for noise protection by integrating PV in buildings are not clear in the building codes [44]. The inappropriate positioning is another barrier. As mentioned in Ref. [45], when architects choose to use BIPV system they do it mainly for aesthetic reasons. However, it is important to see what kind of system does the building needs or can support. Even the type of PV panels has an important role with respect to the inclination, orientation, shadows etc. Apart from the BIPV system weight and loads, there are other loads that occur from time to time and are very important and should be considered as well. These are the snow, ice and wind. Lack of allowance of extra loads can cause photovoltaic modules bending and this will lead to various failures requiring repairs or replacement. It is important to follow a standard installation procedure to ensure the safety of the system under extreme weather conditions. In cases where BIPV system separates the indoor from the outdoor spaces, attention needs to be paid in safety and protection.
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10. Another issue that concerns the potential users of BIPV systems is the pricing of the system. It is found that economical evaluations for BIPV systems are usually made for retrofits and renovations in order to identify the payback time of the new suggested system for installation. In new buildings, the economic evaluation is easier because it is not affected by the cost of materials that will be replaced by the BIPV system. Nevertheless, in both systems, the total cost, the payback time, and the feed in tariff need to be considered for the economic optimization of the application. Concerning the purpose served by the renewable energy systems for sustainability, a life cycle analysis is also very important.
5. Future perspectives It is obvious that BIPV products development has been ongoing for the past 30 years, but their practical applications have been slow in comparison to conventional rack-mounted solar PV. One of the main reasons is that the technical barriers, mentioned in the previous section, which span from design phase through to commissioning and maintenance phases, have not been understood by stakeholders. Jelle et al. [46] stated that the new solutions in the PV industry are many and of various types. There is usually room for improvement in each specific system, e.g. regarding ventilation rate, positioning, removing of snow, etc. Based on the barriers occurring during BIPV installation mentioned previously, there are also some collective strategies from stakeholders and BIPV industry people that need to pay attention. A good solution would be the joint development of technical training programs by the building and PV industries. A good example of this point is the research project BFIRST [47] where five new BIPV products were designed and installed in demo buildings with the cooperation of architects, engineers, researchers and manufacturers. The collaboration between manufacturers, building professionals and clients is of a major importance for future development. 5.1. System appearance and design configuration development Modern architectural style is a trend and the BIPV systems must be modern and flexible in order to be selected by the architects and make them part of their designs. Some practices to improve the use of the BIPV systems are: Create lightweight panels which will increase the installation in existing buildings. Eliminate the standard PV panel view and cables view for safety issues but also from aesthetic point of view. Incorporate the panels to the envelope of the building. They should be part of the building’s envelope e.g. BFIRST PV roofing shingles [47]. This will increase the acceptance of the building’s users as well as the acceptance of the architects to use them as a building construction material. Eliminate glare with the use of proper coatings so that glare will no longer be an excuse for not choosing the systems. Develop more color options, thickness options, shape options e.g. curved panels. Consider the idea to cover other building elements e.g. balconies and canopies. A design that will allow tubes of water to pass behind the PV panels to heat water would be a good option and the system could serve a triple purpose of electricity production, space heating and water heating.
Integrate PV cells in materials at an early stage e.g. in prefabricated concrete plates [48]. Concrete is the most used material in construction, and it could be a good idea for BIPV market to grow. Use of thin laminate or paint layer solar cell materials to create PV smart windows in a way that the PV elements will provide shading when it is needed. This is already being explored and used. Focus need to be given to the existing buildings and retrofitting market because these are more than the volume of buildings to be constructed in a foreseen future. Easy application of PV cells in existing materials is essential, and this may in the future be performed by various painting techniques as mentioned by Jelle et al. [48]. Enhance the naturally ventilated BIPV systems so to increase their efficiency and benefit from their positive specifications e.g. easier installation, no maintenance, no need for extra energy etc. An optimum air gap for each slope and each location is required as a guideline for engineers and installers to avoid PV panels overheating which is not efficient neither for the system itself nor the building which absorbs the heat created and increases its cooling loads. Another important consideration is the height of the system. The optimum air gap may vary for different system heights or the idea of open joints [49,50] has to be implemented in this kind of applications if conditions allow.
5.2. Realistic performance prediction with modeling tools Advanced graphical analysis for building level installations is of the greatest importance. There are numerous models developed by researchers for the prediction of the BIPV systems performance before the installation, but they remain only in scientific publications since they are not directed to the market to be used by the relevant people involved in the design and installation of BIPV systems. The study carried out by Yang [44] highlighted the importance to apply advanced simulation tools and energy performance monitoring platforms in practice, and encourage stakeholder collaborations in the whole supply chain. This action will: Prevent any negative impact on the building and the system itself e.g. shading, damage etc. Prevent overheating of the building (simulation with weather data of the specific location that the installation will take place). Prevent overheating of the system (allow system design configuration to find the optimum air gap for air circulation of the system for each particular application with respect to slope, height etc.) Predict the amount of produced energy and avoid lower efficiency than expected because of orientation, slope and position.
5.3. Guidelines through regulatory regimes and BIPV policies Since BIPV is a rapidly changing market, it must be acknowledged that this situation is subject to changes as well. Shifting regulatory regimes highly influence the composition of the market and its active stakeholders. Governments and other stakeholders in the building supply chain need to understand not only the benefits of BIPV systems, but the technical issues, and move towards developing unified building regulations and accredited training programs, to encourage an increase in BIPV application rates. It is important to develop guidelines and standards for BIPV
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implementation regarding all the issues from their lifetime, from their installation to the operation and maintenance. Single panel replacement is difficult in the current stage as well as other parts replacements such as structure parts, fixing connections etc. A methodology for a smart installation should be developed for each type of system (roof, façade etc.). The most important aspect that needs to be covered by regulations is the safety issue. There are regulations on various building components regarding fire safety, but they are missing for the PV panels which are being used as a construction element in the BIPV systems. 6. Conclusions The potential of solar energy to make significant contribution to the electricity demand and green energy production is widely recognized. BIPV systems allowed buildings to be transformed from energy consumers to energy producers. The objectives of this study were to identify the current status and research on BIPV systems, identify the barriers that slow down their expansion and give the future perspectives to exploit their usage at maximum level. The literature review identified the most studied topics related to the BIPV systems which are mainly on the performance of the system and the heat transfer mechanisms taking place in the systems. From the literature review, 10 barriers were identified that prevent the usage of the system and the people acceptance. Then future suggestions are given based on the discussed barriers, with the ultimate purpose to increase the penetration of the systems. It was concluded that there is a need to develop standards and building codes regarding the BIPV systems and also a need for collaboration between manufacturers, engineers and architects to overcome the barriers and increase the market share of the BIPV systems. References [1] Heinstein P, Ballif C, Perret-Aebi L-E. Building integrated photovoltaics (BIPV): review, potentials, barriers and myths. Green 2013;3(2). [2] Zhang X, Lau S-K, Lau SSY, Zhao Y. Photovoltaic integrated shading devices (PVSDs): a review. Sol Energy Aug. 2018;170:947e68. [3] Benemann J, Chehab O, Schaar-Gabriel E. Building-integrated PV modules. Sol Energy Mater Sol Cells 2001;67(1e4):345e54. [4] Richhariya G, Kumar A. Chapter 4 review on performance affected parameters for dye sensitized solar cell. May. 2016. [5] Assoa YB, et al. Thermal analysis of a BIPV system by various modelling approaches. Sol Energy Oct. 2017;155:1289e99. [6] I. Global Industry Analysts, “BIPV Market analysis, trends and forecasts.” [Online]. Available: https://www.strategyr.com/market-report-buildingintegrated-photovoltaics-bipv-forecasts-global-industry-analysts-inc.asp. [Accessed: 03-Jul-2019]. [7] I. Global Industry Analysts, “BIPV Market analysis, trends and forecasts.” . [8] Debbarma M, Sudhakar K, Baredar P. Thermal modeling, exergy analysis, performance of BIPV and BIPVT: a review. Renew Sustain Energy Rev Jun. 2017;73:1276e88. [9] Shukla AK, Sudhakar K, Baredar P. Recent advancement in BIPV product technologies: a review. Energy Build Apr. 2017;140:188e95. [10] Jun Huang M. The effect of using two PCMs on the thermal regulation performance of BIPV systems. Sol Energy Mater Sol Cells Mar. 2011;95(3): 957e63. [11] Hu Z, et al. Comparative study on the annual performance of three types of building integrated photovoltaic (BIPV) Trombe wall system. Appl Energy May 2017;194:81e93. [12] Agathokleous RA, Kalogirou SA. Double skin facades (DSF) and building integrated photovoltaics (BIPV): a review of configurations and heat transfer characteristics. Renew Energy Apr. 2016;89:743e56. [13] Biyik E, et al. A key review of building integrated photovoltaic (BIPV) systems. Eng Sci Technol Int J Jun. 2017;20(3):833e58. [14] Shukla AK, Sudhakar K, Baredar P. A comprehensive review on design of building integrated photovoltaic system. Energy Build Sep. 2016;128:99e110. [15] Debbarma M, Sudhakar K, Baredar P. Comparison of BIPV and BIPVT: a review. Resour Technol Sep. 2017;3(3):263e71. [16] Saretta E, Caputo P, Frontini F. A review study about energy renovation of building facades with BIPV in urban environment. Sustain Cities Soc Jan.
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Please cite this article as: Agathokleous RA, Kalogirou SA, Status, barriers and perspectives of building integrated photovoltaic systems, Energy, https://doi.org/10.1016/j.energy.2019.116471
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Please cite this article as: Agathokleous RA, Kalogirou SA, Status, barriers and perspectives of building integrated photovoltaic systems, Energy, https://doi.org/10.1016/j.energy.2019.116471