Photovoltaic concentrating structure is the use of concentrating optical elements and solar cells together to form concentrating solar cells, which can greatly improve the photoelectric conversion efficiency and use small-area solar cells. A “focal spot” or “focal zone” is formed within the range, and the solar cell is placed on the “focal spot” or “focal zone” to increase the light intensity, overcome the defect of low solar radiation energy density, and obtain more power output . Therefore, some people foresee that the future power generation mode should be “high-conversion-efficiency solar cells with low-cost concentrating optical elements”.
The integrated high-efficiency concentrating silicon solar cell power generation system (IHCPV) developed by a company in the United States has been applied to many places. The core technology of the system is: the photoelectric conversion efficiency of 10mm² point-contact suede silicon solar cells is as high as 25%~27%, and the condensing Fresnel lens used is made of ordinary acrylic plastic, which is simple to manufacture and cheap. Therefore, the integrated high-concentration photovoltaic technology is the one with the lowest power generation cost among the existing practical photovoltaic technologies. A German company conducted a model test of a concentrating photovoltaic system in 2008. The concentrating system used a FLATCON (fresnel lens all-glass tandem cell concentrator) concentrator module, which is essentially a Fresnel lens injection-molded from glass. The light ratio is 500, using circuit board technology and insulating glass technology, making it relatively cost-effective and the system can maintain stable and reliable performance for many years, and the efficiency of the demonstration model is higher than 27%.
The radiation power density received by the earth’s surface from the sun is about 1kW/m². If we use an ideal black body to absorb the radiation power for the purpose of collection, the equilibrium temperature T of the ideal black body is expressed as:
In the formula: σ——Boltzmann constant, about K=1.3806505(24)×10-23, J/K;
T is the equilibrium temperature of an ideal black body, K;
S——The radiation power density of the sun, W/m².
It can be concluded from the above formula that by increasing the radiation power density of sunlight, the equilibrium temperature of the heat sink can be increased, which is the purpose of the concentrator. Concentrating conventional solar cells can increase the amount of solar radiation per unit area, thereby increasing the output power of solar cells per unit area, which overcomes the dispersion of solar energy to a certain extent and has a good application prospect. By making concentrators made of inexpensive materials, the sunlight can be concentrated on a small area of solar cells, which can reduce the use of expensive solar cell materials and the cost of photovoltaic power generation systems.
Concentrated photovoltaic systems include many disciplines and technologies, such as optics, control, electronics, mechanics, and so on. According to the classification of concentrating magnification, 1~10 times is low condensing, 10~100 times is medium concentrating, and more than 100 times is high condensing. Because the main purpose of concentrating photovoltaic technology is to reduce costs by reducing the use of solar cell materials, the main development direction is focused on high-power concentration technology.
For concentrated photovoltaic systems, a large number of theoretical and experimental studies have been started in the 1990s and the technology has been used in practical engineering applications. Researcher Chen Yingtian of the Institute of Theoretical Physics, Chinese Academy of Sciences independently developed the “4 times concentrating + tracking + n-type photovoltaic cell” turntable power generation technology, which is a new type of solar photovoltaic technology. Reduce the amount of silicon and non-silicon materials used in photovoltaic cells. The outstanding advantages of its designed heat sink include: it can greatly reduce the working temperature of the silicon wafer, increase the power generation, prolong the service life of the battery, and reduce the usage of polysilicon. Researcher Chen Yingtian developed a “photovoltaic power generation system with several times of concentration” on the basis of 4 times the concentration of light, which aroused strong repercussions in the world. The theoretical breakthrough will bring breakthroughs in many technical fields applicable to optics, such as large telescopes, new radars, laser beams, ultra-precision aiming and zoom cameras, solar concentrating, etc. In solar energy, photovoltaic power generation and photothermal power generation can be used. , solar furnace, solar cooker and many other technological inventions or major improvements. According to the new concentrating and tracking theory, Chen Yingtian, He Zuoxiu and others invented a light concentrator composed of octahedral mirrors, also known as “light funnel”. Designed with light theory, the highest concentration ratio can reach 7 times.
At present, the research on concentrated photovoltaic power generation in various countries mainly focuses on the following aspects:
(1) Analysis and research on the principle, optical path and concentrating effect of various concentrators used for concentrating photovoltaic power generation;
(2) Design and study the solar tracking system required by the concentrating system;
(3) Conduct theoretical and experimental research on the output characteristics of solar cells such as open-circuit voltage, short-circuit current, power, and efficiency after concentrating light;
(4) Research on the working temperature and reasonable cooling method of solar cells after concentrating light.
The HiPerfPV program being implemented by the US Department of Energy is to explore the performance limits of photovoltaic technology in order to better provide cost-effective clean energy. The program has two focuses, one of which is to study the performance of multijunction concentrator photovoltaics. The concentrating system cannot adopt the same standard as the non-concentrating system, which is related to the characteristics of the concentrating system itself – usually only direct light can be used, the illumination on the surface of the photovoltaic module is uneven, and an effective radiator is required. The EUCLIDES concentrating photovoltaic power generation system project completed by the European Union in 1998, including 14 cell arrays, adopts single-axis tracking, the geometric concentration rate is 38.2, the power is 480kWp, and the heat dissipation method is passive; the IDEOCONTE project carried out from 2002 to 2006, For the CPV systems used in different regions, the relationship between energy cost, different system types, sun tracking methods, concentration rates, and climate types was compared, and the actual cost and cost of CPV systems in the range of 100~2000kWp were clarified. optimal system structure. The United States has built a 1.5MWp concentrating solar cell system in the desert of Prescott, using a 250-fold concentrating solar cell produced by Amonix Company in the United States, combined with a horizontal coordinate dual-axis sun tracking system. The APS Star Center peak shaving power station in the United States has installed a 300kWp high-power concentrating photovoltaic system. The Australian National University and Solahart jointly established the Rockingham concentrating photovoltaic system in 1999. The total concentrating area of this concentrating system is 154m², the geometric concentration ratio is 30, and the actual concentration ratio is 23. Together with the dual-axis sun tracking system The battery is a high-efficiency monocrystalline silicon battery with a power of about 20kW and an operating temperature of about 70°C. In addition, there are many demonstration projects of concentrating systems, such as the 350kW Soleras project established in Saudi Arabia (1981), and the 300kW concentrating system established in Austin, USA (1990).