**1. 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 [3–5].

Currently, India is facing urbanization due to the migration of people into larger cities from smaller towns and villages. This transition enhances the necessity of developing new buildings. During 2014-2015, India consumed almost 840 million m<sup>2</sup> floor space for commercial use. Buildings in India, consume 29% of the total energy, of which residential contributes to 20% and commercial to 9% [6]. In residential buildings, lighting and space cooling accounts to one-third of the energy consumption (1–3 kWh/m<sup>2</sup>/month), whereas commercial buildings consume two-third of the total energy (5–25 kWh/m<sup>2</sup>/month) [7]. The phenomenal growth in the building sector will be witnessed by the year 2030 with an annual building rate of 700–900 million sq. m. These buildings consume a considerable amount of energy for heating, ventilation and air conditioning (HVAC) load demand. Further, the indoor air condition rate is growing at a rate of 30% every year. Projected energy usage for 2050, with the current scenario, shows an 85% increment compared to the energy level in 2005 [8].

The present energy consumption scenario, along with future projections, has forced India to take some necessary actions. In 2015, the international energy association (IEA) has set a target to limit the ambient temperature increment to below 2 ◦C than the pre-industrial levels. Hence, India should focus on finding out the energy-e fficient ways to generate power and also reduce building energy consumption rates. Fortunately, India is blessed with high solar radiation, which receives 6 billion GWh equivalent energy potential per year. The average incident solar radiation in India is 5.1 kWh/m<sup>2</sup>/day (with large regional di fferences). This makes India deploy solar photovoltaic (PV) technology to meet the IEA target. The PV device is one of the most promising renewable energy technologies, which converts solar energy into environment-friendly electrical energy by using abundant incident solar radiation. Replacing fossil fuel-generated power by secure, clean and suitable PV generated power can mitigate issues like climatic changes [9,10].

The Ministry of Power, which controls the power sector in India, created an impressive mission through Jawaharlal Nehru National Solar Mission (JNNSM). Previously, JNNSM has set a target to install a PV capacity of 22 GW by the year 2022 which later increased to a more ambitious target of 100 GW [11]. Subsequently, to reduce building energy consumption and generate power from renewable sources in the buildings; zero energy buildings (ZEB) or net-zero energy buildings are also getting a promotion. Hence, addition of the PV system into the building is one of the most holistic approaches, where, PV will generate a benevolent amount of energy, su fficient for the building-energy requirements. The inclusion of PV technologies into buildings include building-integrated photovoltaics (BIPV) and building-applied photovoltaics (BAPV). For the BIPV system, the PV system replaces the traditional building envelopes, such as windows, roofs, walls and itself acts as a building envelope, whereas for the BAPV system, PVs are applied or attached to the building walls or roofs. Both BAPV and BIPV works as an onsite green power generation, reducing the transmission losses, and improving the building's overall performance.

In this paper, various technologies involving BIPV and BAPV approaches have been discussed and their potential application for Indian context has been critically analyzed in detail. Moreover, solar potential and PV power electricity market in India are also discussed.

#### **2. PV Technologies for BIPV**/**BAPV**

Presently PV technologies include first-generation opaque silicon type, second-generation transparent or semitransparent thin film and third or emerging types [12]. Until now, the first generations are employed for BIPV and BAPV applications, whereas second and third generations are primarily considered for BIPV application.

Crystalline silicon (c-Si) PV cells are the most widely used and predominant technology in the market due to their mature and long-term durability. Monocrystalline PV cells are made from a single crystal, developed using the Czochralski process with the best-reported e fficiency of nearly 22%. Polycrystalline solar cells are developed by melting several fragments of silicon together to form a wafer. Typical e fficiency is in the range of 14–18% for polycrystalline PV cells, which is less efficient than the monocrystalline counterparts, since electrons have less freedom of movement due

to grain boundaries of many crystals in each cell. However various anti-reflective coatings can be applied onto the surface to change the color of the PV cells. Presently colored silicon PV is also under investigation [13,14]. The major constraints of crystalline silicon PV cells are power losses due to the shading and at elevated temperature [15–22].

Thin films include (i) amorphous silicon (a-Si) (ii) Copper – Indium Selenide (CIS) or Copper-Indium-Gallium- Selenide (CIGS), (iii) Cadmium-Telluride (CdTe). The thickness of the film could be a few nanometers to micrometers. These technologies have meager e fficiencies in comparison to c-Si, typically 11–12%. However, they have several advantages such as (a) less loss in performance under overcast cloudy climatic conditions and partial shading from obstacles [23,24] (b) employ lower semiconductor material and hence lower production cost (c) manufacture of transparent or translucent modules using laser scribing [25–27]. Amorphous silicon is the non-crystalline form of silicon, with atoms disoriented in a random network structure. The major advantage of it is being able to be deposited as thin films on to a moldable substrate like plastic at less than 300 ◦C of manufacturing temperature. Moreover, its absorptivity is higher (~40 times) and needs only 1% (about 1 μm) of material of crystalline silicon, which results in lower making cost/unit-area. Due to its flexible nature, it can be molded into any suitable complex shape for building integration. Although it has high e fficiency in comparison to other thin-film technologies, it su ffers from degradation due to hydrogenation (Staebler-Wronski e ffect) [28–33]. Cadmium telluride (CdTe) is a single-junction solar cell having 1.45 eV bandgap energy. It is a direct bandgap semiconductor nearly ideal for optimal conversion of solar radiation into electricity. An e fficiency exceeding 20% has been reported CdTe PV. The major limitations of CdTe cells are its instability and toxicity of cadmium which makes it less suitable for PV application. Copper Indium Gallium Diselenide (CIS) is a polycrystalline compound consisting of copper, indium, gallium, sulphur and selenide elements, with the highest reported conversion efficiency of about 25% in combination with perovskites [34] CIS has high light absorptivity and 0.5 μm of CIS can absorb 90% of the solar spectrum [35]. Similar to other thin-film technology, CIGSs are semi-transparent and flexible.

Emerging third generations are gaining importance due to their low fabrication cost, and transparent and semitransparent makes them a potential candidate for aesthetic building integration. Organic photovoltaics uses organic polymer as the light-absorbing layer. Organic PVs are lightweight, and flexible which allows them to be applied in building as a BIPV system [36–38]. O'Regan and Gratzel carried out seminal work on dye-sensitized solar cells (DSSC) [39]. Since its inception, extensive research was carried out to improve the e fficiency and stability of DSSC. DSSCs are considered for BIPV application due to its simpler and low-cost fabrication process, flexible, have potential to operate at di ffuse solar radiation [40,41]. Colored and semi-transparent windows are popular for BIPV application [42,43]. However, factors inhibiting to its commercialization are long term- stability and durability. Table 1 listed the advantage and disadvantages of various PV technologies. Recently, Perovskite PV gained attention due to its e fficiency improvement in 10 years. However, they are mostly operated and fabricated at inert atmospheric condition. Tunable transparency [44], and low temperature fabrication [45] makes it fascinating to researcher for BIPV application [46,47].



