According to the different ways of concentrating solar energy, solar thermal power generation systems can be divided into three types, namely trough solar thermal power generation systems, tower solar thermal power generation systems and dish solar thermal power generation systems.
(1) Trough solar thermal power generation system. Trough solar thermal power generation system uses trough-shaped parabolic or cylindrical mirrors to focus sunlight on a tubular receiver, heats the heat transfer working medium in the tube, and then generates steam in the heat exchanger, which pushes the turbo-generator unit to perform work and generate electrical energy. Suitable for large-scale solar thermal power generation applications.
Figure 1 shows the light-concentrating and heat-collecting system of a trough solar thermal power generation system, which is composed of a plurality of trough-shaped mirrors curved in a parabolic shape. Sometimes a parabolic cylindrical structure can also be used for the convenience of production. Each mirror concentrates the sunlight it receives to a tubular receiver located on the line of the focal point of its cross-section, thereby collecting heat energy.
Because the trough thermal power generation system has the worst wind resistance, the current trough power plants are mostly located in areas with little or no wind. Northwest China is rich in sunshine, but it is often windy and windy, even with frequent sandstorms. Therefore, if this application or demonstration is carried out in the northwest region, the wind resistance of the trough system must be enhanced, and its cost will also increase greatly.
(2) Tower solar thermal power generation system. The tower-type solar thermal power generation system usually builds a high tower on an open flat ground, installs a receiver on the top of the tower, and places a large number of solar mirror groups (also called heliostat groups) on the surrounding ground with the high tower as the center. Each reflector is equipped with an automatic tracking system, which can accurately concentrate solar energy reflection on the receiver on the top of the tower to collect heat, and then heat the working fluid to generate high temperature and high pressure steam to drive the turbine generator set to generate electricity, as shown in Figure 2.
For solar thermal power generation systems, there is an important performance evaluation parameter: the concentration ratio, which refers to the ratio of the average energy flow density of the absorber to the incident energy flow density. The tower solar thermal power generation system has a high concentration ratio, is easy to achieve a higher operating temperature, has high system efficiency and large capacity, so it is suitable for large-scale solar thermal power generation systems. However, the cost of the tower system has remained high, and the road to industrialization is still full of difficulties.
(3) Dish-type solar thermal power generation system. The dish-type solar thermal power generation system, also known as the Stirling system of parabolic mirrors, is a large parabola composed of many mirrors, similar to a parabolic radar antenna. Install a thermal energy receiver on the focal point of the parabola, and collect the incident sunlight on the thermal energy receiver through a reflector, heat the working fluid, and then drive the Stirling engine unit to generate electricity, as shown in Figure 3.
Due to the dish-type concentrating form, the dish-type solar thermal power generation system’s concentrating ratio can reach hundreds to thousands of times. Therefore, it can produce a very high temperature at the focal point, which is higher than the concentrating temperature of the other two thermal power generation methods. The operating temperature can reach about 750~1500℃, and the efficiency is the highest. However, due to the size limitation of the concentrating and heat collecting device, the power of the dish-type solar thermal power generation system is smaller, and it is more suitable for the distributed power generation system. At present, the dish-type solar thermal power generation method is still in the early stage, but because of its high efficiency, many countries pay more attention to it, and the corresponding research activities are being actively carried out.