Solar cooling has good season matching, that is, the hotter the weather, the better the solar radiation and the greater the cooling capacity of the system. This feature makes solar refrigeration technology pay attention and development. There are "light-heat-cold", "light-electric-cold", "light-heat-electric-cold" and other ways to realize solar cooling. Solar semiconductor refrigeration is a special cooling method that uses the electrical energy generated by solar cells to drive semiconductor refrigeration devices to achieve thermal energy transfer. Its working principle is mainly the photovoltaic effect and the Peltier effect.
The solar-powered semiconductor refrigeration system has a compact structure and is easy to carry. It can be made into a miniaturized special refrigeration device according to user needs. It has the characteristics of simple use and maintenance, good safety performance, decentralized power supply, convenient energy storage, and no environmental pollution. In addition, compared with the general mechanical refrigeration, the semiconductor refrigeration system using the Peltier effect does not require moving parts such as pumps and compressors, so there is no wear and noise. It does not require refrigerant, so it will not cause environmental pollution, and it also omits the complicated transmission pipeline. It only needs to switch the current direction to change the system from cooling to heating. These unparalleled advantages have made people interested in solar semiconductor refrigeration technology.
At present, the efficiency of solar semiconductor refrigeration systems is still relatively low, and some important technical issues of the system need to be further studied.
1 Working principle and basic structure of solar semiconductor refrigeration
Semiconductor refrigeration is a cooling method that utilizes the thermoelectric cooling effect, so it is also called thermoelectric cooling or thermoelectric cooling. The basic element of a semiconductor refrigerator is a thermocouple pair, that is, a thermocouple formed by connecting a p-type semiconductor element and an n-type semiconductor element.
When the DC power supply is connected, the current direction of the upper connector is np, the temperature decreases, and the heat is absorbed, forming a cold end; the current direction of the lower connector is pn, the temperature rises, and the heat is released, forming the hot end. Several thermocouples are connected to form a commonly used thermopile. With the help of various heat transfer devices, the hot end of the thermopile is continuously radiated and maintains a certain temperature. The cold end of the thermopile is placed in the working environment to absorb Heat produces low temperatures, which is how semiconductor refrigeration works. The solar semiconductor refrigeration system utilizes the thermoelectric cooling effect of semiconductors to directly supply the required DC power from the solar cells to achieve the effect of cooling and heating.
The solar semiconductor refrigeration system is composed of 4 parts: solar photovoltaic converter, numerical control matching device, energy storage equipment and semiconductor refrigeration device. The solar photovoltaic converter outputs direct current, part of which is directly supplied to the semiconductor refrigeration device, and the other part is stored in the energy storage equipment for storage on cloudy days or at night, so that the system can operate normally around the clock.
The solar photovoltaic converter can choose crystalline silicon solar cells or nanocrystalline solar cells, and choose the type of solar cell according to the capacity of the cooling device. On a sunny day, the solar photovoltaic converter converts the solar radiation irradiated on its surface into electrical energy for use by the entire system.
The numerical control matching device makes the energy transmission of the whole system always in the best matching state. At the same time, it controls the overcharge and overdischarge of energy storage equipment.
Energy storage equipment generally uses batteries, which store part or all of the energy output by the photoelectric converter for use when the solar photovoltaic converter has no output, so that the solar semiconductor refrigeration system can reach all-weather operation.
2 Key issues of solar semiconductor refrigeration
The biggest shortcoming of solar cooling system is the low cooling efficiency and the high cost. This has seriously affected the promotion and application of solar refrigeration systems. If you want to improve and improve the performance of solar cooling system, you should start with the following key issues
(1) Improve the performance of semiconductor refrigeration materials
The core of the solar semiconductor refrigeration system is the semiconductor refrigeration material. The main reason for the low efficiency of the semiconductor refrigeration system is that the thermoelectric conversion efficiency of the semiconductor refrigeration material is not high.
It is the figure of merit Z that ultimately determines the performance of thermoelectric materials
Among them: α- Seebeck coefficient of semiconductor refrigeration element;
R—the resistance of the refrigeration element;
Kt—The thermal conductivity of the refrigeration element.
The product ZT of the figure of merit coefficient Z and temperature T is a commonly used parameter for evaluating the performance of materials. As far as semiconductor refrigeration is concerned, if its refrigeration performance is to be comparable to that of mechanical refrigeration, the dimensionless parameter ZT must be above 3 At present, the semiconductor materials commonly used in various countries are far from this level. The ZT value of the most commonly used thermoelectric materials (Bi-Sb-Te-Se series solid solutions) at room temperature is about 1. Therefore, how to improve the performance of materials and find more ideal materials have become an important issue for solar semiconductor refrigeration.
(2) Energy optimization of the system
There are energy losses in the solar semiconductor refrigeration system itself. How to reduce these losses and ensure the stable and reliable operation of the system is a very important issue. Photoelectric conversion efficiency and cooling efficiency are the main indicators to measure energy loss. The higher the photoelectric efficiency, the smaller the area of ​​the solar cell required under the same power output, which is conducive to the miniaturization of the solar semiconductor refrigeration system. Currently, the photovoltaic efficiency of solar cells commonly used is up to 17%. For any refrigeration system, the cooling efficiency COP is the most important operating parameter. At present, the COP of a semiconductor refrigeration device is generally about 0.2 to 0.3, which is much lower than that of compression refrigeration. After experimental research, it was found that the temperature difference between the cold and hot ends has a great influence on the efficiency of semiconductor refrigeration. The performance of the semiconductor refrigeration system can be greatly improved by enhancing the heat end heat dissipation method.
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