Status of BIPV and BAPV System for Less Energy-Hungry Building in India—A Review

The photovoltaic (PV) system is one of the most promising technologies that generate benevolent electricity. Therefore, fossil fuel-generated electric power plants, that emit an enormous amount of greenhouse gases, can be replaced by the PV power plant. However, due to its lower efficiency than a traditional power plant, and to generate equal amount of power, a large land area is required for the PV power plant. Also, transmission and distribution losses are intricate issues for PV power plants. Therefore, the inclusion of PV into a building is one of the holistic approaches which reduce the necessity for such large land areas. Building-integrated and building attached/applied are the two types where PV can be included in the building. Building applied/attached PV(BAPV) indicates that the PV system is added/attached or applied to a building, whereas, building integrated PV (BIPV) illustrates the concept of replacing the traditional building envelop, such as window, wall, roof by PV. In India, applying PV on a building is growing due to India’s solar mission target for 2022. In 2015, through Jawaharlal Nehru National Solar Mission, India targeted to achieve 100 GW PV power of which 40 GW will be acquired from roof-integrated PV by 2022. By the end of December 2019, India achieved 33.7 GW total installed PV power. Also, green/zero energy/and sustainable buildings are gaining significance in India due to rapid urbanization. However, BIPV system is rarely used in India which is likely due to a lack of government support and public awareness. This work reviewed the status of BIPV/BAPV system in India. The BIPV window system can probably be the suitable BIPV product for Indian context to reduce the building’s HVAC load.


Introduction
India's energy consumption has increased to 931 billion kWh which is double than of the level marked in the year 1990 making it one of the largest energy consumers in the world along with China, the USA and, Russia. In 2014, India's per capita electricity consumption was 900 kWh which was 1/3 of the average worldwide consumption [1,2]. Also, compared to 1971, Indian per capita energy use has increased from 3116.84 to 7408.31 kWh in 2014. India's energy sources primarily depend on non-renewable coal-based sources which contribute to a massive amount of greenhouse gases [3][4][5].
• Due to low level of silicon purity, performance is only around 13-16%.

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The cell can be produced in various shapes. • At high temperature, performance degradation is lower than crystalline silicon.

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They have shorter lifetime compared to other solar cells.

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Required twice, the space to get same PV power than that of crystalline silicon.

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The manufacturing process is simple.

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It can absorb light of shorter wavelength.
• The efficiency operates in the range 9-11%. • Tellurium is not abundant.

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Cadmium telluride is toxic and not environmentally benign.

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The efficiency ranges in between 12-14%. • High fabrication and production costs.
Organic solar cells [36][37][38] • The PV modules are low weight and flexible. • Lower production costs than traditional inorganic technologies, such as silicon solar cells.
DSSC [68][69][70] • It consists of low-cost materials easy fabrication. • It works even in low light conditions such as the cloudy weather. • Tunable transparency is possible by tuning the thickness and dye. • Use of flexible substrate makes it flexible and suitable for BIPV.

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The liquid electrolyte has temperature stability problems.

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The liquid will quickly dry up. • Long term stability is questionable.

Solar cell Advantages Disadvantages
Perovskite solar cells [71][72][73] • Perovskite uses smaller quantity of material to absorb the equivalent amount of light in comparison to c-Si.

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Perovskite materials, like methylammonium lead halides are cheap and easy to produce. • Semi-transparent/transparent cells are possible, which makes it suitable for aesthetic building application.
• Not stable at ambient condition. • Not fully matured technology. • Thermal performance of this technology is not well.

Building Integrated and Applied Photovoltaic (BIPV/BAPV) Technologies
BAPVs are an addition to the traditional or new PV system, on an existing or new building whereas BIPVs replace the existing traditional building envelope, such as window, roof, and wall [74]. Hence, BIPV has a greater impact on the building's indoor environment. BIPVs are often transparent or semi-transparent by nature, which allows incident daylight and solar heat to pass through, thereby directly modifying the indoor ambiances. Additionally, it also has a variety of capabilities like control solar heat gain or loss, daylight glare and offset the window, roof or wall material cost [75]. On the other hand, BAPVs have no such contribution to the building environment other than the production of green power. Currently, available BIPV products include BIPV tile, foil and glazing [76,77]. BIPV foils and tiles are primarily applied on the roof while BIPV glazings are mostly employed for vertical semi-transparent and transparent windows, façade and wall applications. Presently, 80% of the BIPV market contributes to rooftop-mounted and only 20% of it is in accord for façade-mounted [2,[78][79][80][81][82]. Generally, rooftops, standing without any hindrance from nearby tall buildings or trees, are the ideal solutions to harvest the best energy when pitched at certain elevation angles. BIPV foil products are best suited for building applications due to their flexibility and light-weight properties. PV cells for BIPV products are mostly thin films, which possess low power generation due to the high electrical resistance of the thin film. However, due to a low temperature coefficient of thin-film BIPV foil, power degradation is comparatively less at high temperatures to silicon types. Alwitra GmbH and Co, which uses amorphous silicon cells and Uni-Solar cells are the present manufacturers of BIPV foil. Next, BIPV tiles are most prominently used as roof integration, which includes covering the entire roof or selected part of the roof with BIPV tiles. Some of these tiles also appear to be similar to that of a ceramic curved tile, which might be aesthetically pleasing but are not effective in terms of power generation, due to its curved surface area [76,77]. SRS Energy, Solar Century, Luma solar, Suntegra and, Sunflare Tesla are few of the present BIPV tile manufacturers. Figure 1 shows the presently available BIPV and BAPV products. BIPV windows are one of the most fascinating applications, responsible for maintaining visual comfort between the external world and building interior, modulating available daylight and heat. Crystalline silicon [83,84], amorphous silicon [85][86][87][88], CdTe [89,90], DSSC [91,92] and perovskite [44] are the few materials, which when intensely investigated for BIPV window applications (shown in Figure 2), have found impressive possibilities towards building integration.  Concentrator-based BIPV windows are also very attractive for less energy-hungry building integration. Low concentrating compound parabolic concentrator (CPC) [94] and luminescent solar concentrators (LSC) [95] are now dominating the major building-integrated concentrating photovoltaic research activity. Low concentrators are static which reduces the cost of the expensive solar tracker. The thermal effect is lower than a high concentrator, due to a low concentrator on PV cells, which reduces the necessity of a cooling system and makes a low concentrator a suitable candidate for building's window and façade application [96][97][98]. For northern latitude location, diffuse solar radiations are higher, CPC and LSC both work efficiently. Concentrating PV came into a scenario to reduce the usage of the costly silicon material by replacing low-cost material, which concentrates a higher amount of incident solar light on a lesser PV material [99]. For crystalline  Concentrator-based BIPV windows are also very attractive for less energy-hungry building integration. Low concentrating compound parabolic concentrator (CPC) [94] and luminescent solar concentrators (LSC) [95] are now dominating the major building-integrated concentrating photovoltaic research activity. Low concentrators are static which reduces the cost of the expensive solar tracker. The thermal effect is lower than a high concentrator, due to a low concentrator on PV cells, which reduces the necessity of a cooling system and makes a low concentrator a suitable candidate for building's window and façade application [96][97][98]. For northern latitude location, diffuse solar radiations are higher, CPC and LSC both work efficiently. Concentrating PV came into a scenario to reduce the usage of the costly silicon material by replacing low-cost material, which concentrates a higher amount of incident solar light on a lesser PV material [99]. For crystalline Concentrator-based BIPV windows are also very attractive for less energy-hungry building integration. Low concentrating compound parabolic concentrator (CPC) [94] and luminescent solar concentrators (LSC) [95] are now dominating the major building-integrated concentrating photovoltaic research activity. Low concentrators are static which reduces the cost of the expensive solar tracker. The thermal effect is lower than a high concentrator, due to a low concentrator on PV cells, which reduces the necessity of a cooling system and makes a low concentrator a suitable candidate for building's window and façade application [96][97][98]. For northern latitude location, diffuse solar radiations are higher, CPC and LSC both work efficiently. Concentrating PV came into a scenario to reduce the usage of the costly silicon material by replacing low-cost material, which concentrates a higher amount of incident solar light on a lesser PV material [99]. For crystalline silicon-CPC-based BIPV window, regular distribution of spacing between PV cells offer semi-transparency (silicon solar cells are opaque), as shown in Figure 3. Different geometries of CPCs were investigated for BIPV window and façade application [100][101][102][103]. Recently, the performance of DSSC and perovskite solar cells was also investigated, using low concentrating CPC, which enhanced the PV performance than non-concentrating counterpart [104,105].
Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 24 silicon-CPC-based BIPV window, regular distribution of spacing between PV cells offer semitransparency (silicon solar cells are opaque), as shown in Figure 3. Different geometries of CPCs were investigated for BIPV window and façade application [100][101][102][103]. Recently, the performance of DSSC and perovskite solar cells was also investigated, using low concentrating CPC, which enhanced the PV performance than non-concentrating counterpart [104,105]. A typical LSC consists of a glass or plastic-based square/rectangular-shaped waveguide luminophores, which absorbs a short-wavelength photon and convert them into long-wavelength. Further, due to total internal reflection, these photons finally reach the PV cells attached at the edge of the waveguide [106][107][108][109][110], as shown in Figure 4. The advantage of LSC-BIPV system is that the PV cells are placed at the edge, which does not create an obstacle for viewing. Also, this waveguide plate can be made semi-transparent to fully-transparent or different colors, which is aesthetic for building application and suitable for building window integration [111][112][113][114]. Promising results, using LSCthin film integration, was also reported by [115]. LSC does not possess any thermal effect on PV cells, which is an added advantage over low concentrating CPC. The performance degradation of both BIPV and BAPV is possible at higher ambient temperature and exposure to higher incident radiation, which increases the PV cell's temperature. Crystalline silicon and thin-film both work, with poor efficiency, at higher PV cell temperature. Thermal regulation BAPV system is possible by employing forced water flow, forced airflow or phase change material (PCM) at the back of the system [116,117]. At the back of the BPAV system, copper pipes are integrated to flow the air [118][119][120] or water [121,122]. This typical BAPV is also known as BAPVthermal (BAPV/T) water or air collector, where water or air will extract the additional heat energy A typical LSC consists of a glass or plastic-based square/rectangular-shaped waveguide luminophores, which absorbs a short-wavelength photon and convert them into long-wavelength. Further, due to total internal reflection, these photons finally reach the PV cells attached at the edge of the waveguide [106][107][108][109][110], as shown in Figure 4. The advantage of LSC-BIPV system is that the PV cells are placed at the edge, which does not create an obstacle for viewing. Also, this waveguide plate can be made semi-transparent to fully-transparent or different colors, which is aesthetic for building application and suitable for building window integration [111][112][113][114]. Promising results, using LSC-thin film integration, was also reported by [115]. LSC does not possess any thermal effect on PV cells, which is an added advantage over low concentrating CPC.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 24 silicon-CPC-based BIPV window, regular distribution of spacing between PV cells offer semitransparency (silicon solar cells are opaque), as shown in Figure 3. Different geometries of CPCs were investigated for BIPV window and façade application [100][101][102][103]. Recently, the performance of DSSC and perovskite solar cells was also investigated, using low concentrating CPC, which enhanced the PV performance than non-concentrating counterpart [104,105]. A typical LSC consists of a glass or plastic-based square/rectangular-shaped waveguide luminophores, which absorbs a short-wavelength photon and convert them into long-wavelength. Further, due to total internal reflection, these photons finally reach the PV cells attached at the edge of the waveguide [106][107][108][109][110], as shown in Figure 4. The advantage of LSC-BIPV system is that the PV cells are placed at the edge, which does not create an obstacle for viewing. Also, this waveguide plate can be made semi-transparent to fully-transparent or different colors, which is aesthetic for building application and suitable for building window integration [111][112][113][114]. Promising results, using LSCthin film integration, was also reported by [115]. LSC does not possess any thermal effect on PV cells, which is an added advantage over low concentrating CPC. The performance degradation of both BIPV and BAPV is possible at higher ambient temperature and exposure to higher incident radiation, which increases the PV cell's temperature. Crystalline silicon and thin-film both work, with poor efficiency, at higher PV cell temperature. Thermal regulation BAPV system is possible by employing forced water flow, forced airflow or phase change material (PCM) at the back of the system [116,117]. At the back of the BPAV system, copper pipes are integrated to flow the air [118][119][120] or water [121,122]. This typical BAPV is also known as BAPVthermal (BAPV/T) water or air collector, where water or air will extract the additional heat energy The performance degradation of both BIPV and BAPV is possible at higher ambient temperature and exposure to higher incident radiation, which increases the PV cell's temperature. Crystalline silicon and thin-film both work, with poor efficiency, at higher PV cell temperature. Thermal regulation BAPV system is possible by employing forced water flow, forced airflow or phase change material (PCM) at the back of the system [116,117]. At the back of the BPAV system, copper pipes are integrated to flow the air [118][119][120] or water [121,122]. This typical BAPV is also known as BAPV-thermal (BAPV/T) water or air collector, where water or air will extract the additional heat energy from the PV system and allow PV system to operate efficiently. Hot air or water can be used by building purposes [123]. Notably, researchers often misuse the BIPV term [121,122,[124][125][126][127][128][129][130]. As BIPV is attached as a building envelope, natural airflow and PCM are the only two available and investigated options to diminish the elevated PV cell temperature [131][132][133]. The inclusion of collector to BIPV was forced to compromise with buildings aesthetic. For a system which is BAPV/T system is most often referred to as BIPV/T in the articles. Hence, care should be taken when referring to a PV/T system as BAPV/T or BIPV/T. A typical BAPV/T with a water flow system is shown in Figure 5. Details of BIPV and BAPV are listed in Table 2.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 24 from the PV system and allow PV system to operate efficiently. Hot air or water can be used by building purposes [123]. Notably, researchers often misuse the BIPV term [121,122,[124][125][126][127][128][129][130]. As BIPV is attached as a building envelope, natural airflow and PCM are the only two available and investigated options to diminish the elevated PV cell temperature [131][132][133]. The inclusion of collector to BIPV were forced to compromise with buildings aesthetic. For a system which is BAPV/T system is most often referred to as BIPV/T in the articles. Hence, care should be taken when referring to a PV/T system as BAPV/T or BIPV/T. A typical BAPV/T with a water flow system is shown in Figure  5. Details of BIPV and BAPV are listed in Table 2.

Potential of BIPV/BAPV in India
India lies between 68°7′ to 97°25' east longitude and 8°4′ to 37°6′ north latitude, has 2.9 million Km 2 of landmass, and is the seventh largest country in the world. It is in the tropical region and receives maximum solar radiation in summer, and experiences about 300 sunny clear days in a year. Ambient conditions vary from 45 0 C in summer while 4 0 C in winter and has a hot-dry, warm-humid, composite, temperate, and cold climatic zones [135]. India's rich solar radiation profile shows 4.5-5.0 KWh/m 2 /Day of annual average direct normal irradiance in most of the Indian states and around 5.0-5.5 KWh/m 2 /Day average global horizontal irradiance [136,137]. This makes India one of the most

Potential of BIPV/BAPV in India
India lies between 68 • 7 to 97 • 25 east longitude and 8 • 4 to 37 • 6 north latitude, has 2.9 million Km 2 of landmass, and is the seventh largest country in the world. It is in the tropical region and receives maximum solar radiation in summer, and experiences about 300 sunny clear days in a year. Ambient conditions vary from 45 • C in summer while 4 • C in winter and has a hot-dry, warm-humid, composite, temperate, and cold climatic zones [135]. India's rich solar radiation profile shows 4.5-5.0 KWh/m 2 /Day of annual average direct normal irradiance in most of the Indian states and around 5.0-5.5 KWh/m 2 /Day average global horizontal irradiance [136,137]. This makes India one of the most potential candidates to contribute to PV power generation. Figure 6 shows the solar radiation intensity throughout India. India's projected electricity demand in 2047 is expected to be 5518 TWh. India's present energy demand is supplied by 70% of imported crude oil and coal. Indian thermal power plants, that are run by coal, are the most inefficient ones. Hence, in order to become an energy-secured country and dependent from oil-import, India should use its solar PV power potential. Although, India has a higher solar radiation, being a developing country, PV power generation faces issues, including high initial installation cost of PV power generation, the lack of suitable storage devices or unavailable during an instant power supply demand. Backup power supply from fossil fuel-generated kerosene oil lamps, diesel generators, etc. are still low cost in India [138]. However, this fossil fuel generated power emits considerable amount of green house gas ( GHG). India is committed to lowering its GHG to 30 India's present energy demand is supplied by 70% of imported crude oil and coal. Indian thermal power plants, that are run by coal, are the most inefficient ones. Hence, in order to become an energysecured country and dependent from oil-import, India should use its solar PV power potential.
Although, India has a higher solar radiation, being a developing country, PV power generation faces issues, including high initial installation cost of PV power generation, the lack of suitable storage devices or unavailable during an instant power supply demand. Backup power supply from fossil fuel-generated kerosene oil lamps, diesel generators, etc. are still low cost in India [138]. However, this fossil fuel generated power emits considerable amount of green house gas ( GHG). India is committed to lowering its GHG to 30-33% by 2030, compared to 2005 level. Interestingly, energy generation from PV devices was in discussion in India since 1960. However, progress was limited until 2010 [140,141]. The first significant move was taken in 2010 to priorities the PV power generation through the National Action Plan on Climate Change, by launching the JNSSM scheme [142]. In 2010, the total installed capacity from PV was only 39.6 MW. After Prime Minister Narendra Modi came into power, India's 2022 target changed from 20 GW to 100 GW, which included grid-connected projects, off-grid projects, and solar parks. It was also fixed that out of 100 GW, rooftop PV should produce 40 GW by 2022 [143]. Under the JNSSM mission, gridconnected rooftops and small solar power plant programs have been launched to obtain the 40 GW rooftop power generation. The minimum and maximum limit of installing PV power capacity are to 1 kWp, and 500 kWp, respectively [139,144]. India has 29 states and 7 union territory out of which Madhya Pradesh, Gujarat Ladakh, Andhra Pradesh, Maharashtra, and Rajasthan receive the maximum amount of average annual solar radiation, as compared to other states of India.  Interestingly, energy generation from PV devices was in discussion in India since 1960. However, progress was limited until 2010 [140,141]. The first significant move was taken in 2010 to priorities the PV power generation through the National Action Plan on Climate Change, by launching the JNSSM scheme [142]. In 2010, the total installed capacity from PV was only 39.6 MW. After Prime Minister Narendra Modi came into power, India's 2022 target changed from 20 GW to 100 GW, which included grid-connected projects, off-grid projects, and solar parks. It was also fixed that out of 100 GW, rooftop PV should produce 40 GW by 2022 [143]. Under the JNSSM mission, grid-connected rooftops and small solar power plant programs have been launched to obtain the 40 GW rooftop power generation. The minimum and maximum limit of installing PV power capacity are to 1 kWp, and 500 kWp, respectively [139,144]. India has 29 states and 7 union territory out of which Madhya Pradesh, Gujarat Ladakh, Andhra Pradesh, Maharashtra, and Rajasthan receive the maximum amount of average annual solar radiation, as compared to other states of India. Gujarat is the first Indian state which implemented a solar policy in the year 2009, well before the initiation of JNNSM. Rajasthan started its solar mission in 2011 to meet the national target. Karnataka started its solar mission for the period of 2014 to 2021. Madhya-Pradesh started its solar policy in 2012 and provided the incentives and benefits to the Private Sector to encourage the PV installation [145]. Although, in India, 83.3 crore reside in a rural area out of 121 crores, urbanization is occuring at a rapid pace. Every minute, 30 Indians move from a rural area to a city, seeking better-paying jobs. Population and economic growth have fostered urbanization in the country and the number of urban towns and cities has drastically increased [146,147]. By the end of 2030, 590 million Indians will be in city for which new buildings are required. It is expected that five-fold built space will be in 2030 than 2005 level in India, of which 60% will be air-conditioned space. Maintaining similar conditions, Indian's building can consume energy and emits GHG with a 700% increment by 2050, compared to 2005 levels [135]. Presently building consumes 30% of electricity in India [148]. The reduction of building energy enhances the demand for sustainable building, which will perform as low energy or less energy-hungry building by trim down the HVAC load demand. To attain such a building, envelopes need to be energy efficient and responsive to an outdoor conundrum. To assess the performance of the buildings, The Energy and Resource Institute of India (TERI) and MNRE has created Green Rating for Integrated Habitat Assessment (GRIHA), and the Leadership in Energy and Environmental Design (LEED) rating tools to help curtail the substantial resources consumed by the building industry, and to reduce the overall environmental impact within tolerable limits. GRIHA evaluates the ecological performance of the building comprehensively by controlling energy consumption, reducing carbon dioxide gas emissions and reinforce the use of inexhaustible and processable sources to the best possible extent [149,150]. MNRE also encourages now passive Although, in India, 83.3 crore reside in a rural area out of 121 crores, urbanization is occuring at a rapid pace. Every minute, 30 Indians move from a rural area to a city, seeking better-paying jobs. Population and economic growth have fostered urbanization in the country and the number of urban towns and cities has drastically increased [146,147]. By the end of 2030, 590 million Indians will be in city for which new buildings are required. It is expected that five-fold built space will be in 2030 than 2005 level in India, of which 60% will be air-conditioned space. Maintaining similar conditions, Indian's building can consume energy and emits GHG with a 700% increment by 2050, compared to 2005 levels [135]. Presently building consumes 30% of electricity in India [148]. The reduction of building energy enhances the demand for sustainable building, which will perform as low energy or less energy-hungry building by trim down the HVAC load demand. To attain such a building, envelopes need to be energy efficient and responsive to an outdoor conundrum. To assess the performance of the buildings, The Energy and Resource Institute of India (TERI) and MNRE has created Green Rating for Integrated Habitat Assessment (GRIHA), and the Leadership in Energy and Environmental Design (LEED) rating tools to help curtail the substantial resources consumed by the building industry, and to reduce the overall environmental impact within tolerable limits. GRIHA evaluates the ecological performance of the building comprehensively by controlling energy consumption, reducing carbon dioxide gas emissions and reinforce the use of inexhaustible and processable sources to the best possible extent [149,150]. MNRE also encourages now passive building which will use solar energy by the suitable orientation of building for daylighting, heating and cooling load demand [151].
BIPV and BAPV both can contribute a considerable amount of energy and improve the building's indoor environment in India [152,153]. However, the dearth of BIPV experts and BIPV marketing professionals, limited in-house consumption data, dearth of ability in planning, commissioning, operation, and maintenance of solar PV/BIPV projects, inadequate training and capacity building, not enough available information about BIPV for policy-making and mobilizing civil society are the barrier for Indian BIPV/BAPV market [81,82]. Another major barrier for widespread PV in India is the lack of resources of raw materials for PV manufacturing. BIPV technology, which is mainly thin-film-based, did not have much uptake in India due to the same reason. For crystalline silicon, India depends on import of the silicon wafer. Currently, in India, the thin-film PV industries are run by US-based First Solar (22% share), Canadian Solar (6% share) and 6% share of Trina Solar Chinese manufacture (6%). India also cannot support CdTe production as India's copper refining industry size is not big enough and upgradation is required to enhance tellurium recovery rates from the copper refining process [154]. The Indian developer, Vikram Solar, has a 3.5% share followed by Moser Baer, Tata Power Solar and Lanco [155]. China controls over 97% of rare earth material which makes them capable to control the price of thin-film [155]. Poor performance of thin-film PV system compared to silicon PV system creates a negative impact on thin-film BIPV system. Thin film degradation occurs in higher rate than crystalline PV over 25 years. Also, thin-film PV cells possess micro-cracks after few years of operation, due to the temperature gradient differences between bottom and top, which cause additional cost of replacement. In India, utilization of BIPV and BAPV is still not fully well established and primarily most of the major integration types are BAPV. Solar rooftop PV application which is BAPV technologies are predominant in India. According to census 2011, in India, there are 331 million households, with urban settlement area of 77,370 km 2 , which can be a huge potential of 124 GW for rooftop BAPV to satisfy 40 GW rooftop PV power generation target by 2022. Rooftop PV installations grew at robust pace adding 1,836 MW in the financial year 2018-2019, with a total becoming 10 GW. Figure 8a shows the spaced type semitransparent crystalline solar BIPV module integrated on the rooftop having installed capacity of 1.68 kWp. Each module had dimensions of 1963 mm × 0.987 mm × 40 mm covered with 36 c-Si panels with a transparent area of 49% and rated power of 150 W. Figure 8b shows the India's first zero energy building, which was constructed in 2014. PV panels occupy 4600 m 2 area and annual energy generation cost is 14 lakh ($19k) Unit kWh, while the cost of installation was Rs 18 crore ($2533k). Coal India Limited's Corporate Headquarters at Rajarhat in Kolkata (22. The government buildings in India are also encouraged to use solar energy in an aesthetic approach by using BIPV/BAPV technology. The government is also focused on increasing the roof top systems and streamlining policy implementation processes. In 2015, Novus green installed a 1MW BAPV system (4000 PV system each had 250 Wp) at the rooftop of IIT Delhi. Energy Efficiency Services (EESL) had planned to invest INR 800 crore for rooftop solar in Maharashtra across 5000 state-owned buildings to install 200 MW grid-connected systems. EESL estimates that about 100 million units would be saved per year by replacing energy-inefficient ceiling fans (6 lakh), LED bulbs (11 lakh) and air-conditioners (7000) along with streetlights (14,000) and retrofitting 3000 buildings. approach by using BIPV/BAPV technology. The government is also focused on increasing the roof top systems and streamlining policy implementation processes. In 2015, Novus green installed a 1MW BAPV system (4000 PV system each had 250Wp) at the rooftop of IIT Delhi. Energy Efficiency Services (EESL) had planned to invest INR 800 crore for rooftop solar in Maharashtra across 5,000 state-owned buildings to install 200 MW grid-connected systems. EESL estimates that about 100 million units would be saved per year by replacing energy-inefficient ceiling fans (6 lakh), LED bulbs Another barrier in India for poor BIPV/BAPV -standalone system is the unorganized Indian electricity market. The PV power generation sector includes three different customers. The first customer state distribution companies (DISCOMs) who have renewable purchase obligations (RPO) to buy PV power and meet 10.5% PV electricity generated, second is the rooftop PV consumers (RPVCs) and third is large buyers of power who are also known as open access consumers (OACs) [156]. Under open access (OA), consumers are capable of buying electricity from producers who generate electricity independently. Indian rail started exploring PV power from OA. Developers feel that RPO is not same for all state as State Electricity Regulatory Commissions (SERCs) has different benchmark for each state. PV power electricity price varies from INR 7.5 to INR18.5/kWh. In 2010, Central Electricity Regularity Commission (CERC) has implemented the PV/BIPV feed-in tariff of INR 17.9/kWh [157]. In India, the coal power electricity price is about INR 5.5/kW h, while the PV power price is about INR6.5/kW h. Hence, project developers are compelled to offer discounts [158]. Thus, DISCOMs are failing to comply with RPO requirements, due to their poor financial health and lower solar tariff support. Hence, they prefer to wait for buying low price PV electricity. DISCOMs also argued that BIPV/BAPV based standalone systems increase their financial burden as RPV customers prefer to buy grid electricity over BIPV/BAPV power due to intermittency. Intermittent PV power generation is also an issue for the promotion of OA [159]. To rectify this issue, energy banking can be created where DISCOM will facilitate OA transactions through electricity banking, between an independent power producer and OAC. DISCOM can generate less power whenever an independent power producer generates a higher amount of power than the OAC's demand, in order to use surplus PV power and generate when PV generation is reduced and shortfalls arise [159,160]. The presence of multiple electricity regulatory boards like MNRE (Ministry of New and Renewable Energy), CERC (Central Electricity Regulatory Commission), SERC (Ministry of Power and State Electricity Regulatory Commission) also make the legal process a cumbersome task [161].
By the end of Sep 2019, India's cumulative installed solar capacity stood at 33.8 GW, of which 88% are PV plants and 12% rooftop installations. Ambiguity in incentive implementation, non-availability of storage systems incentives, lack of consumer awareness and research studies are the reason behind this sluggish movement for rooftop PV application in India [162]. Dust accumulation, which reduces PV power generation, should also be taken care of as air quality in India can pose a negative impact [163][164][165].

Perspective and Discussion
It is evident that in India, solar PV power generation got heads up after the year of 2015. Rooftop installation got priority however it included rooftop of any large construction whether it is a building (residential/commercial) or other area, such as a stadium or car park. Hence, it is difficult to differentiate or estimate the installed percentage for only rooftop building. Until now, power generation from PV has only just become a higher priority than aesthetic application. In India, BAPV system is prevalent and they are wrongly termed as BIPV systems. Actual, BIPV technology is not particularly popular in India, which could be the reason for lack of government plan and policy, and awareness to the public. BIPV window tiles or foil technology is not popular in India. Thermal regulation of BAPVT in the name of BIPVT is available in India [129]. The primary goal of this work to enhance PV power generation, rather than reducing the building cooling load demand. Improvements in the building environment, using BIPV, should be in government policy. Concentrating PV in India is mainly higher in concentrators [166]. To the best of our knowledge, no work has been reported on a low concentrator using LSC-or CPC-based BIPV/BAPV in India. To meet the GRIHA rating, large commercial buildings are now growing, however, they do not use BIPV to improve the built environment. Most green buildings, which meet the GRIHA rating, use rooftop BAPV system, while external shading devices control the admitted daylight and heat for those large glazed façades [167]. Table 3 listed the availability of BIPV and BAPV products in India.

Future Pathway of BIPV/BAPV in India
Weak BIPV implementation and national planning, lack of energy policy and details of BIPV products, fewer BIPV experts and market professionals are the key responsible factors for slow or no progress of BIPV, for less-energy hungry building in India. The Indian Central government should motivate and support deployment of BIPV research and development by removing non-economic issues to BIPV uses, creating building codes for BIPV integration in building assembly. New training programs related to BIPV can educate the builders, developers, and engineers. Support from central or state-level government organizations, such as NISE, SECI, NIWE, MNRE, IIT, NITs and the state nodal agencies should work together.
Windows are one of the weakest components in a building. It allows external heat to come inside (enhance the air condition load), internal heat to outside (enhance heating load) and offer visual connection to building interior to exterior [172][173][174][175][176][177][178][179][180][181][182][183][184][185]. Building window systems are affected by an overall heat transfer coefficient (U-value) and solar energy transmittance (g-value) [186][187][188][189]. For warmer place, high U-value and low g-value are required, while for colder area, low U-value and high g-value are suitable. Generally, single pane glass possesses higher U-value (U-value 3-5 W/m 2 K) and higher g-value followed by double (U-value-2-3 W/m 2 K g-value lower than single glass) and triple (U-value < 2W/m 2 K; g-value lower than single and double) glass window [190][191][192][193]. This clear and highly transparent window is not able to limit the heat entering from exterior ambient of buildings. Thus, building interior temperature often crosses over occupants' comfort limit (thermal comfort temperature 18-20 • C). Hence, in order to maintain thermal comfort level, an excessive amount of grid power is consumed to run air-condition (AC). Integration of PV system into the single or double glass can create a single glass BIPV window or double glass BIPV window, which will, not only generate benevolent electricity, but also contro heat and restrict its flow into the exterior, where required, as shown in Figure 9. Semitransparent c-Si PV based BIPV windows can reduce 5.3% heating and cooling load compared to standard BIPV [194] and has ability to limit up to 65% in total heat gains compared to traditional clear glass [195]. Previous investigation of BIPV window in cooling load dominated climate such as Singapore, Hongkong showed a positive impact on load reduction [196][197][198].
Appl. Sci. 2020, 10, x FOR PEER REVIEW 14 of 24 by employing BIPV window to obtain less energy-hungry building. Also, replacing the traditional window system by BIPV window is easier than replacing other building components, such as roof or wall [201,202]. Figure 9. Schematic of a semitransparent BIPV window (left) and Sankey diagram while BIPV window is integrated into a building (right).

Conclusions:
The following conclusion can be drawn from this review article: • Building integrated photovoltaic (BIPV) replaces traditional building envelop, such as window, wall, roof and most often they are thin film, or third-generation based transparent or semi-transparent in nature. • Building attached/applied photovoltaic (BAPV) indicates when PV systems are attached to a building without replacing its traditional envelops. • India's solar mission which geared up from 2015, accelerated the rooftop PV integration.
For building, the rooftop application's majority are BAPV types and capacity higher than kW level. India's total installed solar capacity reached only 33.8 GW by the end of September 2019, which is way behind to achieve India's 40 GW rooftop PV power generation by 2022. • New technologies, such as PV tiles, foil and windows, as part of BIPV or BPAV, are not popular in India. • Indian electricity market needs a complete reform to allow smooth penetration of BIPV /BAPV in building. Also, due to rapid urbanization and development of zero and sustainable building industry, BIPV will keep pace in India soon. • Single and double glass BIPV window systems have been identified as one of the most potential candidates for India. Funding: This work didn't receive any specific grants.

Acknowledgments:
The authors would like to thank the anonymous reviewers for their valuable comments which were useful to enhance the quality of paper.

Conflicts of Interest:
The authors declare no conflict of interest. In India, buildings' AC load is excessively high, due to thehigh g-value of window. In summer, buildings' windows are closed, in order to abate hot air and sunlight [199,200]. This also creates a dearth of light in an indoor setting, which encourages occupants to employ artificial light. Small-to-medium office buildings use air conditioners during the day and peak summer, and are not in use at night or off-peak season. Hence, advanced single or double glass-based semitransparent BIPV window systems, which possess lower g-value compared to clear glass, are favorable in India as they can be particularly be influential in limiting excess usage of AC and lighting load for a less-energy hungry building.The inclusion of this semi-transparent BIPV window, not only lowers the g-value, but allows sufficient daylight and generates benign electricity concomitantly. India has primarily cooling load (AC load) demand climate, but can trim down this excessive grid power consumption by employing BIPV window to obtain less energy-hungry building. Also, replacing the traditional window system by BIPV window is easier than replacing other building components, such as roof or wall [201,202].

Conclusions
The following conclusion can be drawn from this review article:

•
Building integrated photovoltaic (BIPV) replaces traditional building envelop, such as window, wall, roof and most often they are thin film, or third-generation based transparent or semi-transparent in nature.

•
Building attached/applied photovoltaic (BAPV) indicates when PV systems are attached to a building without replacing its traditional envelops. • India's solar mission which geared up from 2015, accelerated the rooftop PV integration. For building, the rooftop application's majority are BAPV types and capacity higher than kW level. India's total installed solar capacity reached only 33.8 GW by the end of September 2019, which is way behind to achieve India's 40 GW rooftop PV power generation by 2022.

•
New technologies, such as PV tiles, foil and windows, as part of BIPV or BPAV, are not popular in India.
• Indian electricity market needs a complete reform to allow smooth penetration of BIPV/BAPV in building. Also, due to rapid urbanization and development of zero and sustainable building industry, BIPV will keep pace in India soon.

•
Single and double glass BIPV window systems have been identified as one of the most potential candidates for India. Funding: This work didn't receive any specific grants.