Introduced three main double-sided photovoltaic modules in the photovoltaic market: single crystal n-type double-sided photovoltaic modules, single crystal PERC double-sided photovoltaic modules, heterojunction (HIT or HJT) double-sided photovoltaic modules, analysis of double-sided photovoltaic modules The application characteristics, combined with the current PV industry policy and development direction, analyzed the application prospects of double-sided PV modules.
1, double-sided photovoltaic module structure and characteristics
1.1 structure
Conventional PV modules can only receive solar light on the front side to generate electricity. Due to the special battery structure and transparent backplane material, the double-sided PV module can effectively generate electricity by using the received light in addition to the front side. It includes reflected light from the ground, scattered light in the atmosphere, reflected light from dust in the air, reflected light from surrounding buildings, etc. The double-sided power generation characteristics of double-sided photovoltaic modules make it possible to emit more electric energy than conventional photovoltaic modules.
In the 1970s, the structure of a double-sided light-receiving crystalline silicon solar cell based on p+pn+ or p+nn+ structure was formally proposed.
In 1994, Moehlecke et al. introduced a double-sided solar cell based on p+nn+ structure at the first World Photovoltaic Conference. The front conversion efficiency of the battery reached 19.1%, and the back conversion efficiency was 18.1%. Researchers from all over the world were blunt Progress in technology, screen printing, doping diffusion, etc., to achieve industrial production of double-sided photovoltaic modules.
At present, the double-sided photovoltaic modules on the market mainly include single crystal n-type double-sided photovoltaic modules, single-crystal PERC double-sided photovoltaic modules, and heterojunction (HIT or HJT) double-sided photovoltaic modules.
1) Single crystal n-type double-sided photovoltaic module. Figure 1 is a double-sided solar cell based on phosphorus-doped n-type silicon prepared into p+nn+ structure, which is prepared by boron diffusion doping, and phosphorus diffusion doping is used to prepare n+ Back field. Since the n+ phosphor back field replaces the aluminum back field formed by the aluminum paste printing technology of the conventional p-type silicon solar cell, the back electrode also adopts the same grid line structure as the front electrode, so that the front and back surfaces of the battery can absorb light '2'. At the same time, the component back plate uses 2.5mm thick transparent glass to enable the back light to enter the cell. The single-crystal n-type double-sided PV module has a front conversion efficiency of 18.34% and a back conversion efficiency of 15.59%. The comprehensive conversion efficiency reached 19.90%. The manufacturers of this type of components mainly include Yingli Green Energy Holdings Co., Ltd., Suzhou Zhonglai Photovoltaic New Materials Co., Ltd., etc.
2) Single crystal PERC double-sided photovoltaic module. Figure 2 shows the structure of single-crystal PERC double-sided solar cell. PERC battery is passivated emission and back partial contact battery. Al2O3 film is used to passivate the back surface of the battery to improve the conversion efficiency of the battery. The ordinary PERC battery can only generate electricity on the front side. The PERC double-sided battery replaces the back aluminum with the opaque light of the ordinary PERC battery into a partial aluminum grid line, which realizes the light transmission on the back of the battery. At the same time, the transparent glass back plate of 2.5mm is used to make the PERC. Double-sided components, so that reflected light from the ground and the like can be absorbed by the module. The single-sided PERC double-sided photovoltaic module has a front conversion efficiency of 18.31%, a back conversion efficiency of 11.90%, and a component conversion efficiency of 19.50%. The main manufacturers are Leye Photovoltaic Technology Co., Ltd., Jingao Solar Co., Ltd., Trina Solar Energy Co., Ltd., etc.
3) Heterojunction (HIT or HJT) double-sided photovoltaic module. Figure 3 is a heterojunction (HIT or HJT) solar cell structure in which the intersubstrate is n-type crystalline silicon, n-type c-Si after cleaning and texturing An intrinsic amorphous silicon film (ia-Si:H) and a p-type amorphous silicon film (pa-Si:H) having a thickness of 5 to 10 nm are sequentially deposited on the front side to form a pn heterojunction. The ia-Si:H film with a thickness of 5-10 nm is deposited, and the n-type amorphous silicon film (na-Si:H) forms a back surface field. On both sides of the doped a-Si:H film, a transparent conductive oxide is deposited. The film (TCO) finally forms a metal collector '3' on the layers on both sides by screen printing technology. The structure of the battery is symmetrical, and the front and back sides can generate electricity after receiving light, and the component back plate adopts 2.5 mm thick transparent glass. The double-sided photovoltaic module is fabricated. The entire preparation process is carried out at a temperature of 200 ° C to avoid damage to the silicon wafer by the high temperature process. The heterojunction double-sided photovoltaic module is a process in three double-sided photovoltaic modules. The highest cost, mass production cost is above 4.2 yuan / W. The manufacturers of this type of components are mainly Xinao Energy Holdings Co., Ltd., Jinneng Clean Energy Co., Ltd., etc.
1.2 Features
1) The back side can generate electricity. The back side of the double-sided PV module can generate electricity by using reflected light from the ground. The higher the ground reflectivity, the stronger the light received on the back of the battery, and the better the power generation effect. Common ground reflectances are: Grassland is 15 %~ 25%, concrete is 25%~35%, wet snow is 55%~ 75%. Double-sided PV modules can increase power generation by 8%~10% in grassland, and the highest power generation in snow. Increase by 30%.
2) Accelerate the melting of winter components. After the conventional PV modules are covered with snow in winter, if the snow cannot be cleaned up in time, the components will easily freeze in a continuous low temperature environment, which not only seriously affects the power generation efficiency, but also is likely to The components cause unpredictable damage. The double-sided PV modules are heated by the back of the module to receive heat from the snow, which accelerates the melting and slipping of the snow, which can increase the power generation.
3) Double glass components. The back plate of double-sided PV modules is generally made of transparent glass, which can be called double glass components. Double glass components can reduce the amount of combiner boxes, cables, etc. in 1500V photovoltaic system, and reduce the initial system investment cost. Because the glass permeability is almost zero, there is no need to consider the problem of the output power drop caused by the water vapor entering the component induced PID; and the components are more adaptable to the environment, and are suitable for construction in areas with more acid rain or salt spray. Photovoltaic power station.
4) Flexible installation direction and location. Since both the front and the back of the module can be powered by light, the power generation benefit is more than 1.5 times that of the general components under vertical placement conditions, and is less affected by the installation direction. , such as guardrails, sound insulation walls, BIPV systems, etc.
5) Special bracket form requirements. The conventional bracket form will block the back side of the double-sided PV module, which not only reduces the back light, but also causes the series mismatch between the cells in the module, which affects the power generation effect. The bracket of the double-sided PV module should be designed as ' Frame ' form, to avoid blocking the back of the component. Figure 4 is a complete installation of the double-sided PV module.
2, the certification progress of double-sided photovoltaic components
At present, the international PV industry test standards are written for traditional single-sided power generation products. They are not suitable for evaluating the performance of PV modules with double-sided power generation, nor for accurately testing the electrical performance parameters of double-sided PV modules. This affects The indication of component power makes the buyers and sellers have great differences in understanding of power; such cognitive differences in power identification will directly affect the terminal system, making design and planning difficult. Domestic and foreign inspection agencies have already addressed this issue. Research work has been carried out and certain results have been achieved. TÜV, an international third-party testing and certification organization, is working with domestic component companies such as Yingli, Zhonglai on research and development of double-sided component PV test procedures, power calibration methods, test reports and component cards. It is expected that the standard specification for double-sided photovoltaic modules will be introduced in the near future. Domestic certification bodies such as Beijing Certified Certification Center (CGC) have released the “Test Method for Electrical Performance Parameters of Double-sided Photovoltaic Modules” in November 2016. The method defines a double-sided photovoltaic module, double-sided power generation gain rate, and summarizes the test method for double-sided photovoltaic components. In December, Yingli's 'Panda' n-type double-sided photovoltaic module was awarded the industry's first double-sided power generation product certification by CGC.
3, double-sided photovoltaic module policy
The “13th Five-Year Plan for Solar Energy Development” issued by the National Energy Administration once again clearly defines the realization of photovoltaic power generation cost reduction and efficiency as the primary goal of industrial development. By 2020, the photovoltaic power generation price level will reach 0.4~0.5 yuan/kWh. To achieve the goal of using electricity on the electricity side. In order to achieve this goal, it is necessary to continuously innovate in equipment and application solutions to reduce the cost of electricity. According to the National Energy Administration, Ministry of Industry and Information Technology, National Certification Committee, "Improve the technical indicators of major photovoltaic products." And to strengthen the supervision work notice, the advanced photovoltaic power generation technology application base used polycrystalline silicon photovoltaic modules and monocrystalline silicon photovoltaic modules, the photoelectric conversion efficiency 'leaders' technical indicators increased to 17% and 7.8% respectively. And conventional photovoltaic components can not meet This technical indicator must use high-efficiency photovoltaic modules such as double-sided photovoltaic modules.
4, the application prospects of double-sided photovoltaic modules
The back side of the double-sided PV module can bring additional power generation, realize the cost reduction and efficiency increase of the photovoltaic power station, not only meet the technical characteristics of the 'leading runners', but also meet the goal of the photovoltaic level price, which is of great significance for accelerating the realization of the photovoltaic level price. The domestic production capacity of pre-single-crystal n-type double-sided PV modules and single-crystal PERC double-sided PV modules has exceeded GW, and the price is gradually approaching the conventional PV modules. The testing and certification methods are improving, and the application solutions are becoming more mature. Domestically, there are large-scale applications. Cases, such as Datong Panda Power Station using 30MW single crystal n-type double-sided photovoltaic modules, according to PERC photovoltaic modules per W power generation is about 10% higher. It is foreseeable that double-sided PV modules will have broad application prospects in the future. The power generation data shows that it is about 10% higher than the single-sided PERC PV module per W. It is foreseeable that the double-sided PV module will have broad application prospects in the future.
