Concentrated solar energy systems are mainly composed of concentrator components, high-efficiency solar cells, active or passive cooling devices, and single-axis or dual-axis tracking devices. The functions of each component are described as follows:
The function of the concentrator assembly is to increase the radiation intensity of the incident light on the solar surface, thereby increasing the output power of the solar cell per unit area. Taking silicon solar cells as an example, the short-circuit current increases with the increase of light intensity, and increases in a linear proportional relationship with the light intensity; the open-circuit voltage increases exponentially with the increase of light intensity, and increases rapidly under weak light, and under strong light. tends to be saturated; so the output power of the solar cell increases with the increase of the light intensity.
Solar concentrators can be divided into two categories according to the different forms of concentrating:
The first type is the refractive concentrator system. The advantage is that there are many degrees of freedom in optical design, and it can solve the problem of greater sensitivity to the incident angle of light, the manufacturing difficulty is not high, and the cost is low; the disadvantage is that it is easy to produce dispersion. Represented by Fresnel lens.
The second type is the reflective concentrating system. The advantages are: mature manufacturing technology, low cost, no dispersion, easy optical design, and can be designed into a folded structure, which is conducive to lightening and thinning. For example, “Compound Parabolic Mirror Concentrator” (CPC), which consists of two trough parabolic mirrors, does not need to track the sun, at most only needs to be adjusted slightly according to the season, it can condense light and obtain a higher temperature . In addition, there is a type of condenser used in tower solar power plants – heliostats. Heliostats are composed of many flat or curved mirrors. Under computer control, these mirrors reflect sunlight to the same absorber. The absorber can reach a very high temperature and obtain a large amount of energy. Hybrid concentrators also have applications in photovoltaic systems, one of which utilizes refraction, reflection, and internal reflection. In a thermal photovoltaic concentrator, the sun heats a radiator to a high temperature, and then the radiator emits radiation to the solar cell. The long-wave radiation that cannot be used by the battery is returned to the radiator, which can theoretically achieve high efficiency. Fluorescent concentrators and holographic concentrators are two immature technologies. The disadvantage is that it is sensitive to the incident angle of light, represented by parabolic reflective concentrators.
At present, the commonly used condensers in CPV systems mainly include: Fresnel lens, compound parabolic condenser (CPC), dish condenser, etc.
(1) Fresnel lens
Fresnel lens is a new type of optical component invented and named after Fresnel, a famous French physicist. One side of the condenser is a plane, and the other side is a sawtooth edge designed according to a certain width and angle. Sunlight is transmitted from one side of the plane to the other side, and is refracted by sawtooth edges and concentrated into a small area to form a concentrating effect. It can also be designed into point focus (disk mirror) and line focus (stripe mirror) according to actual needs. Compared with the parabolic reflector, the Fresnel lens device condenses light more uniformly and does not produce shadows, so it is widely used in early applications and demonstration projects. At present, the Fresnel lens concentrating system is developing towards the application of multi-junction cells and miniaturization. The Fresnel concentrator is basically made by the process of extruding transparent plastic, the manufacturing method is simple, and the cost is very low, but the cost increases sharply after adding the tracking device. Therefore, reducing the cost of the tracking device and improving its tracking accuracy are the keys to determining the large-scale use of the Fresnel lens condensing system. Compared with the traditional optical glass lens, the Fresnel lens has the advantages of excellent optical performance, anti-aging, rich material sources, low cost, convenient production, light weight, thin thickness, etc. The root cause of the application.
(2) Disc condenser
The dish condenser is the most used and studied concentrator in the current solar engineering, and its technology is relatively mature.
Disc condensers can be divided into three categories: glass plate type, integral parabolic type and stretch film type. The reflective materials used are aluminum film, silver film and thin silver glass, etc. During operation, a dual-axis automatic tracking system is generally used to control its orientation, so that it always obtains the most solar radiation. No matter what kind of parabolic mirror, it can be divided into two types: long focal length and short focal length. The mirror with the focus falling inside the opening surface is called the short focal length mirror, and the focus falling outside the opening surface is called the long focal length mirror.
The butterfly-type concentrating photovoltaic module model adopts multiple plane mirrors and ingenious structural design, so that the sunlight is evenly irradiated on the photovoltaic cell array on the corresponding side after being reflected by the plane mirror, realizing several times of concentrating function, thereby improving the unit Electricity generation efficiency of area solar cells. The shape of the concentrator is like a butterfly, the condensing multiple can reach 10~1000 times, and the utilization rate of the flat mirror is as high as 85%, which is especially suitable for photovoltaic power generation systems of various scales.
Compared with the Fresnel lens refractor, the dish condenser is cheaper, and allows the use of single-axis tracking or tracking-free, and the component cost is greatly reduced, so it has developed rapidly in recent years (Figure 1). The parabolic mirror concentrator has complex processing technology, high processing error, high manufacturing cost, and also has the problems that the reflective layer is easy to fall off during use, the light concentration is uneven, and the performance decreases significantly over time. Therefore, future research should focus on improving its stability and durability.
(3) Compound Parabolic Condenser (CPC)
The compound parabolic concentrator (CPC) is a non-imaging concentrator designed according to the principle of edge optics. It consists of two trough parabolic mirrors, and the absorber is installed at the bottom. It can collect the sun’s rays within a given incident angle range to the receiver with the ideal concentration ratio. This concentrator can receive not only direct radiation, but also scattered radiation (approximately 29% of the total scattered radiation can be utilized). This concentrator concentrates light without imaging, so it doesn’t need to track the sun, and at best requires only a small tilt adjustment with the seasons. Its possible condensing ratio is generally below 10. When the condensing ratio is below 3, it can be used as a fixed concentrator. Its performance is similar to that of the single-axis tracking trough parabolic concentrator, but the complicated tracking system is omitted.
Edge ray principle: For a condenser, all rays incident at the maximum incident half angle θ must exit from the edge of the exit aperture. That is, the outermost ray in the incident beam should also be the outermost ray at the exit aperture. The above design completely determines the shape of the compound paraboloid by the diameter 2b of the exit aperture and the maximum incident half angle θ. According to the above principles, CPC can be designed. The focal length of this paraboloid is:
The diameter of the entrance aperture is:
The main function of the heat sink is to cool down the solar cell. The optimal working temperature of the solar cell is around 25°C. While the concentrator increases the light intensity, the temperature of the surface of the solar cell also increases due to the increase of the energy flow density. much higher. Taking silicon solar cells as an example, as the temperature increases, the short-circuit current of the silicon solar cell increases, while the open-circuit voltage decreases, and the magnitude of the voltage drop is larger than that of the current increase, so the output power of the silicon solar cell increases with the temperature. and decline. Specifically, taking silicon solar cells as an example, the output power decreases by 0.35%~0.45% for every 1°C increase in temperature.
The solar cells used in concentrating photovoltaics include monocrystalline silicon solar cells, polycrystalline silicon solar cells, and III-V solar cells. Among them, the most promising one is the arsenide rubbed tandem battery. The efficiency of triple-layer GaAs cells can reach more than 35%. Multi-junction arsenide wiper cells are also the most commonly used cells for concentrating solar energy. Their temperature characteristics are stable, and their photoelectric conversion efficiency is also maintained at a high level at high temperatures.
The main function of the tracking device is to ensure that the surface of the photovoltaic module is always perpendicular to the incident sunlight, prolong the direct irradiation time of the photovoltaic module, and increase its power generation per unit time. Tracking devices are generally used in concentrating photovoltaic power generation systems. Since the concentrator is sensitive to the incident angle of incident light, and the incident angle determines the focus position of the concentrator, the tracking device can ensure that the concentrator cell works in the best state.
The concentrator has the advantages of large power generation per unit area and small investment per unit power, but in actual operation, there are many technologies such as hot spot effect, difficult heat dissipation of closed concentrators, and performance of open concentrators after many years of operation. question.
Comparing the power generation of the low-power concentrating tracking type and the fixed photovoltaic power generation system, it can be seen from Figure 2 and Figure 3 that under the same power conditions, the annual power generation of the low-power concentrating tracking power generation system is higher than that of the flat-panel fixed photovoltaic power generation system. Increase by 20%~30%. The main reason is that the cosine effect can be eliminated by the low-magnification light-concentrating tracking cell module, thus increasing the power generation of the system.