1. Classification of building integrated photovoltaics
Building integrated photovoltaics is a new concept of using solar energy to generate electricity. Simply put, solar photovoltaic arrays are installed on the outer surface of the building envelope to provide electricity. According to the combination of photovoltaic arrays and buildings, it is usually divided into two categories. The first category is the combination of photovoltaic arrays and buildings, called BAPV. In this way, the photovoltaic array is attached to the building, and the building acts as the carrier of the photovoltaic array and plays a supporting role. The second category is the integration of photovoltaic arrays and buildings, called BIPV. In this way, the photovoltaic modules appear in the form of a building material, and the photovoltaic array becomes an integral part of the building, such as photovoltaic tile roofs, photovoltaic curtain walls and photovoltaic lighting roofs, as shown in Figure 1.
In these two ways, the combination of photovoltaic array and building is a common form, especially the combination with building roof. Since the combination of photovoltaic arrays and buildings does not occupy additional ground space, it is the best installation method for photovoltaic power generation systems widely used in cities, so it has attracted much attention. The integration of photovoltaic arrays and buildings has higher requirements for photovoltaic modules. Photovoltaic modules must not only meet the functional requirements of photovoltaic power generation, but also take into account the basic functional requirements of buildings.
2. Features of BIPV
It can meet the requirements of architectural aesthetics; it can meet the lighting requirements of buildings; it can meet the safety performance requirements of buildings; it can meet the requirements of convenient installation; it can have the advantage of long life; it has the effect of green environmental protection; it does not need to occupy valuable land resources; It can effectively reduce building energy consumption and achieve building energy saving; it can reduce the temperature rise of walls and roofs.
3. Problems existing in building integrated photovoltaic
Although solar building integrated photovoltaic has many advantages such as high efficiency, economy and environmental protection, and has been used in Shanghai World Expo venues and demonstration projects, photovoltaic buildings have not yet entered the homes of ordinary people, and the residential communities using this technology have not yet appear. This is because there are several major problems with solar building integrated photovoltaic:
1) High cost: The cost of building integrated solar photovoltaic buildings is high.
2) High cost: The cost of solar power generation is more than double the cost of conventional power generation.
3) Unstable: Solar photovoltaic power generation is unstable, greatly affected by weather, and fluctuates. This is because the sun is not available 24 hours a day, so how to solve the intermittency of solar photovoltaic power generation and how to store electricity are also urgent problems to be solved.
4 Forms of building integrated photovoltaic
Building integrated photovoltaics is suitable for most buildings, such as flat roofs, sloping roofs, curtain walls and ceilings. Below, the following briefly introduces several application forms.
(1) Flat roof
From a power generation perspective, flat roof economics are the best:
1) It can be installed according to the best angle to obtain the maximum power generation.
2) Standard PV modules can be used with the best performance.
3) There is no conflict with the function of the building.
4) The cost of photovoltaic power generation is relatively low, and it is the best choice from the perspective of power generation economy.
(2) Pitched roof
South-facing sloping roofs are more economical:
1) It can be installed at the optimum angle or close to the optimum angle, so the maximum or greater power generation can be obtained.
2) Standard photovoltaic modules can be used, with good performance and low cost.
3) There is no conflict with the function of the building.
4) The cost of photovoltaic power generation is relatively low or low, and it is one of the preferred installation solutions for photovoltaic power generation systems.
(3) Photovoltaic curtain wall
The photovoltaic curtain wall must meet the BIPV requirements. In addition to the power generation function, its mechanical, aesthetic and safety characteristics must meet all functional requirements of the curtain wall; the photovoltaic curtain wall should be designed, constructed and installed at the same time as the building, and its project progress is restricted by the overall progress of the building; the installation of photovoltaic arrays is generally based on the building’s The structure and orientation often deviate from the optimal installation angle, so the output power is low and the power generation cost is high; the photovoltaic curtain wall reduces the energy consumption of the building, enhances the social value of the building, and brings the effect of the green concept. Figure 2 shows the effect of the building integrated photovoltaics and the environment in the German solar cell and solar cell module factory.
(4) Photovoltaic ceiling
Photovoltaic ceilings require components to be transparent and have low efficiency; in addition to the power generation function and light transmission performance, the ceiling must also meet certain architectural requirements such as mechanics, aesthetics, and structural connections. The cost of components is high; the cost of power generation is high; Enhance social value and bring the effect of green concept.
5. Key Points for BIPV Design
(1) PV module performance
As an ordinary photovoltaic module, the photovoltaic module used as a curtain wall panel and a lighting top panel must meet the performance requirements of the photovoltaic module, and at the same time, it must meet the three-property experimental requirements of the curtain wall (the three properties of the curtain wall are wind pressure deformation performance, rainwater penetration performance, air permeability) and building safety performance requirements, so higher mechanical properties and different structural methods are required. For example, ordinary photovoltaic modules with a size of 1200mm × 530mm generally use 3.2mm thick ultra-white tempered glass and aluminum alloy frame to meet the requirements. However, components of the same size are used in BIPV buildings, different locations, different floor heights and different installation methods may have completely different requirements for its glass mechanical properties. The components used in the external circulation double-layer curtain wall of CSG Building are photovoltaic modules made of two 6mm thick ultra-white tempered glass sandwiched by glue, which is the result of strict mechanical calculation.
(2) The aesthetic requirements of the building
BIPV is first and foremost a building, which has high requirements for light and shadow. However, the glass used in ordinary photovoltaic modules is mostly cloth-patterned ultra-white tempered glass, and its cloth pattern has the effect of frosted glass to block sight. If the BIPV module is installed in the sightseeing part of the building, this position needs to be transparent, then the smooth ultra-white tempered glass should be used to make the double-sided junction glass module to meet the function of the building. At the same time, in order to save costs, the glass on the back of the battery panel can be made of ordinary smooth tempered glass.
Whether a building is successful or not, the key point is the appearance of the building. Sometimes subtle inconsistencies cannot be tolerated. However, the junction box of ordinary photovoltaic modules is generally glued to the back of the panel. The junction box is large, which can easily destroy the overall sense of coordination of the building. It is usually not acceptable to architects. Therefore, BIPV requires the junction box to be omitted or hidden. At this time, the bypass diode does not have the protection of the junction box. It is necessary to consider other methods to protect it. The bypass diode and the connecting wire need to be hidden in the curtain wall structure. For example, bypass diodes are placed in the curtain wall skeleton structure to prevent direct sunlight and rain erosion.
(3) Structural performance coordination
When designing BIPV, it is necessary to consider whether the voltage and current of the panel itself is convenient for the selection of photovoltaic system equipment, but the facade of the building may be composed of some geometric figures of different sizes and forms, which will cause problems between components. The voltage and current are different. At this time, you can consider partitioning and adjusting the grid of the building facade, so that the BIPV modules are close to the electrical performance of the standard components. You can also use cells of different sizes to meet the requirements of the grid, so as to meet the requirements of the building to the greatest extent. Facade effect. In addition, the cells on a few corners may not be connected into the circuit to meet the electrical requirements.
6. The development direction of building integrated photovoltaic
As the junction of the huge construction market and the photovoltaic market with huge potential, BIPV has infinitely broad development prospects. It can be predicted that the combination of photovoltaics and buildings is one of the most important fields of photovoltaic applications in the future, with very broad development prospects and huge market potential.
7 Future research priorities
Building air temperature regulation consumes a lot of energy, it accounts for about 70% of the total energy consumption of the building.
Using air conditioners and burning coal to control the room temperature not only consumes energy and brings environmental pollution, but also does not bring a healthy and healthy environment to indoor personnel (although it is temporarily comfortable). The use of solar energy for heating has not yet been applied on a large scale, except for some passive solar houses with high cost and demonstration buildings. Active solar energy is more expensive due to its higher cost. Therefore, building energy supply should be a combination of active and passive, solar and conventional energy. According to the function of the room, the cooperation and intersection of different schemes can be adopted, which can greatly reduce the one-time investment and operation cost of solar energy for building energy supply, and make the whole scheme feasible in the sense of commercialization. The significance of passive heating and cooling is to greatly reduce the energy load of the building itself (the energy saving rate is about 70%), so that the energy provided by the active energy supply device is required to be large and greatly reduced. That is, it will require less expensive equipment. In addition, passive energy supply is a clever use of changes in natural conditions to adjust the indoor temperature. We believe that the development direction of air temperature regulation technology in buildings should not be to change the natural environment to meet people’s requirements, but to make clever use of and adapt to the natural world to meet people’s requirements for health and comfort. The purpose of studying air conditioning should be to minimize artificial environment, not the other way around. The significance of active energy supply is to ensure the building and increase the indoor comfort. In the case of active energy supply and passive energy supply to form an energy supply system, the entire building is supplied with energy, the equipment performance of the system will be improved, and the size and cost will be reduced.
With the continuous development of new energy sources and the increasing demand for energy conservation, emission reduction, and green environmental protection in cities, building integration of solar photovoltaics has increasingly become a new trend in solar power generation applications.