According to data from the Ministry of Housing and Construction, building energy consumption in China accounts for about 30% of the total energy consumption of the whole society. Building energy consumption includes building materials manufacturing, building construction, heating, air conditioning and lighting in buildings. The transformation from "energy-consuming city" to "energy-saving city" and even "capacity city" is also a major issue for the low-carbon development of smart cities.

On August 10, the research results of CIGS-BIPV technology and application were displayed at the summit forum and exhibition of photovoltaic industry development in Xi'an, Shaanxi Province. The project, led by the National Energy Group, integrates four innovations: CIGS-BIPV integrated assembly technology, comprehensive utilization of photovoltaic, photothermal and solar energy, DC power supply system and intelligent building control system.

Photovoltaic power generation using copper, indium, gallium and selenium thin films (CIGS) solar cells converts light energy into electricity. CIGS solar cell is a crystalline thin film solar cell composed of copper (Cu), indium (In), gallium (Ga) and selenium (Se) in the absorption layer.

Established in August 2017, State Energy Investment Group Co., Ltd. is the largest coal production, thermal power generation and renewable energy power generation company in the world, which is formed by the merger and reorganization of China Electricity Corporation and Shenhua Group Co., Ltd.

The CIGS-BIPV exhibition unit in Xi'an, Shaanxi Province, is 6.6 meters long, 4 meters wide and 3.7 meters high. Its building area is 26.15 square meters, which is equivalent to a small residential unit with 90 photovoltaic modules. Relevant head of National Energy Group said that the actual CIGS conversion efficiency of the unit was 14.2%. Under the sunshine conditions in Xi'an, the display unit 1 can generate about 15 kilowatt-hour electricity per day.

"Current (CIGS) solar cells have a maximum conversion efficiency of 22.9% in the laboratory." Dr. Tang Yang, Center for Green Energy and Architecture Research, National Energy Group, said.

Thin film solar cells have a hierarchical structure, the substrate is usually glass, and then deposited on the glass, copper, indium, gallium and selenium absorption layer, cadmium sulfide buffer layer, zinc oxide window layer and other multi-layer films, the top cover glass, so it has inherent advantages in the integration with the building. Traditional solar photovoltaic panels are separated from buildings, with glass curtain walls and solar panels. CIGS-BIPV means that the solar panel itself is a glass curtain wall that can generate electricity, while meeting the seismic, corrosion and wind resistance targets of building walls.

Tang Yang introduced that amorphous silicon thin film solar cells have conversion efficiency of less than 10% among the three industrialized thin film solar cells, and their attenuation is large, and they are in the market withdrawal state.

"Cadmium telluride and copper, indium, gallium and selenium are potential thin film solar cells in the future." Tang Yang said.

Te, In, Ga and Se are rare dispersed elements in the raw materials used for these two thin film solar cells. They are low in abundance in the crust and do not form independent minerals in nature, but disperse in other minerals as impurities.

The photoelectric conversion efficiency of CIGS solar cells ranks first among all kinds of thin film solar cells, exceeding 14%, close to that of crystalline silicon solar cells, and the cost is one third of that of crystalline silicon solar cells. Tang Yang introduced that the current market mainstream of crystalline silicon solar cells, the use of cutting and assembling technology. That is, irregular crystal silicon blocks are heated and melted to form orderly atom-arranged silicon ingots, which are cut into a battery plate and connected by tin-plated solder strip. The second generation of thin film batteries can use the coating technology to make the materials directly "long" on the glass, which greatly reduces the production cost.

In addition, thin film batteries do not produce photoinduced degradation effect which crystal silicon batteries are difficult to overcome, and the service life of thin film batteries can be more than 30 years; at the same time, the weak light performance of thin film batteries is better than that of crystal silicon batteries.

"With the same calibrated conversion efficiency, the power generation of CIGS solar cells is much higher than that of monocrystalline silicon and polycrystalline silicon solar cells." Ling Wen, General Manager of National Energy Group, introduced.

Ling Wen said that CIGS solar cells can be widely used in remote mountainous areas and islands of independent power stations, agricultural and photovoltaic complementarity, fishing and photovoltaic complementarity and other precision poverty alleviation projects, photovoltaic building integration (BIPV) and large-scale terrestrial photovoltaic power plants and other fields.

Because of the harsher natural conditions in remote mountainous areas and islands, Ling Wen proposed that the next goal of CIGS solar cells is not to marketize immediately, but to "enhance the strength of wind resistance and corrosion resistance".

"In five years'time, the cost of photovoltaic panels will be greatly reduced and the efficiency will be greatly improved compared with the present one." Ling Wen said that the task now is to continue to invest in R&D and research how to improve efficiency and reduce costs.

In January 2018, Shenhua Group, Shanghai Electric and German photovoltaic equipment manufacturer Manz AG jointly funded the construction of Chongqing Shenhua thin film solar energy project, with an annual design capacity of 306 MW. Tang Yang said the cost of CIGS solar cells would be more competitive when the plant was built and localized.