- Some special requirements of BIPV for solar cell modules
(1) Color requirements
When the solar cell module is used as the curtain wall or skylight of the south facade, the color of the solar cell module will be required. For monocrystalline silicon solar cells, the surface can be turned into black by etching suede. When installed on the roof or south facade, it looks solemn, basically does not reflect light, and there is no problem of light pollution. For polycrystalline silicon solar cell modules, the method of etching suede cannot be adopted, but some trace elements can be added during evaporation of antireflection film to change the color of solar cell surface, which can turn into yellow, pink, light green and other colors. For amorphous silicon solar cells, their natural color is the same as that of brown glass, which is very suitable for making glass curtain wall and skylight glass.
(2) Light transmission requirements
When solar cell modules are used in skylights, sunshades and curtain walls, there are certain requirements for their light transmittance. Generally speaking, crystalline silicon solar cells are opaque. When light transmission is required, the components can only be encapsulated with double-layer glass, and the light transmission can be adjusted by adjusting the gap between single cells. Because the single cell itself is opaque, when it is used as a glass curtain wall or skylight, its projection presents uneven spots. Crystalline silicon solar cell modules can also be made into transparent type, that is, many small holes are punched in crystalline silicon solar cells, but the manufacturing process is complex and expensive, and has not reached the degree of commercialization at present.
Amorphous silicon solar cell module can be made into the same effect as brown glass, with good light transmission effect and uniform and soft projection. If the solar cell module is used as a glass curtain wall and skylight, amorphous silicon solar cell module is more suitable.
(3) Size and shape requirements
Because the solar cell module should be combined with the building, the shape of the solar cell module will be required in some special applications, which is no longer just a conventional rectangle: for example, the circular roof requires the solar cell module to be a circular strip; Buildings with beveled edges require that solar cell modules also have beveled edges. Arched roof requires solar cell modules to have a certain degree of curvature.
- Electrical connection mode of BIPV
Most of the photovoltaic roof plans in Germany and the Netherlands are distributed systems installed on residential buildings, and the power is generally 1 ~ 50kwp. Since the compensation price of photovoltaic power generation is different from the electricity price of users, the dual meter system is adopted. One meter is used to record the electricity that can be fed into the power grid by the solar power generation system and the other to record the electricity consumption of users, as shown in Figure .
Roof grid connected PV also has some systems with high power, such as the BIPV system of 2mwp on the roof of Munich Exhibition Center in Germany and the 200kWp system of Berlin railway station. For small systems, generally only one grid connected inverter is used; For large systems, multiple inverters can be used. The BIPV system of 216kwp in Berlin railway station is divided into 12 solar cell sub arrays, and each sub array is composed of 60 300wp solar cell modules. Each square array is connected with a 15kV · a inverter, which is connected to the grid for power generation respectively. The BIPV project of Munich 2mwp is different. 2mwp is built by two 1mwp systems in phase I and phase II. Each 1mwp system adopts common DC bus. Three 300kV · a inverters operate in the master-slave mode. When the solar irradiance is weak, only one inverter works. When the sunlight is strongest, all three inverters work, which will have higher conversion efficiency.
Grid connected photovoltaic power generation can adopt the wiring mode of separate pricing for power generation and consumption, or the wiring mode of “net electricity meter” pricing. Germany and most European countries adopt the double price system. Power companies purchase the power of solar photovoltaic power generation at a high price [average 0.55 euro / (kW · h)], and the user side only pays the conventional low price [0.06 ~ 0.1 euro / (kW · h)]. This policy is called “on grid price” policy. In this case, the photovoltaic power generation system shall be connected to the grid before the user’s meter. The United States and Japan adopt initial investment subsidies. During operation, the electricity price of photovoltaic power generation is no longer supplemented, but the electricity generated by photovoltaic power generation is allowed to offset the user’s electricity consumption from the power grid. The power company charges according to the net value of the user’s electricity meter, which is called the “net electricity meter” measurement system. At this time, the photovoltaic power generation system shall be connected to the power grid after the user’s meter. Of course, with the continuous reduction of the cost of photovoltaic power generation, the subsidies of various countries are also decreasing.
Since China has not yet implemented high electricity prices for photovoltaic power generation systems installed on the urban distribution side, most projects adopt the “net meter” distribution mode.