Solar thermal power generation mainly involves taking the gathered solar radiation energy and generating steam through a heat exchanger to drive a steam turbine to generate electricity. Solar thermal power generation and conventional fossil energy in the thermal power generation method is the same principle, both through the Rankine cycle, Brayton cycle or Stirling cycle to convert thermal energy into electricity, the difference is the heat source is different, solar power generation heat source from the solar radiation, and therefore how to collect solar energy with concentrating devices is one of the key technologies of most solar thermal power generation .
In addition, considering the intermittent nature of solar energy, thermal storage systems are needed to store the collected solar energy for power generation at night or when the radiation is insufficient, so the mature thermal storage technology becomes another key technology in solar thermal power generation.
Direct photovoltaic and indirect photovoltaic are the most commonly used classifications in solar thermal power generation.
Direct photovoltaic power generation can be divided into solar thermal ion power generation, solar temperature difference power generation and solar thermal magnet power generation; indirect photovoltaic power generation can be divided into concentrating and non-concentrating types, among which concentrating types can be divided into solar tower power generation, solar trough power generation and solar dish power generation according to the solar harvesting method; non-concentrating types mainly include solar vacuum tube power generation, solar thermal airflow power generation and solar thermal pool power generation, etc.
The commonly referred to solar thermal power generation mainly refers to indirect light power generation, and direct light power generation is still in the experimental stage. At present, the mainstream solar thermal power generation technologies are concentrated in tower, trough and dish types, which have received great attention because of their great development prospects.
The main classification of concentrated solar thermal power generation technology.
1、Tower type solar thermal power generation
Tower solar power generation is mainly composed of a large number of sun-tracking directional mirrors (sunset mirrors) and thermal receivers mounted on the central tower. Thousands of sunset mirrors focus sunlight on the central receiver, which converts the gathered solar radiation energy into thermal energy, and then transfers the thermal energy to the thermal cycle tool to drive the heat engine to do work to generate electricity. As the number of heliostat mirrors in the mirror field increases, the concentrating ratio of the tower solar power system rises up to 1500 and the operating temperature is 1000°C to 1500°C. It is extremely suitable for grid-connected solar power generation because of its high concentrating ratio, simple energy concentration process, and high thermal conversion efficiency. Figure 1 shows the system diagram of tower solar power generation. As can be seen from Figure 1, the tower solar power system consists of the following parts: sunlight tracking fixed mirrors, receivers, mass heaters, energy storage systems and turbine units. The collection device consists of multi-surface heliostat mirrors, tracking devices, support structures, etc. The system achieves the best tracking of the sun through the control of the collection device, so that the reflected light from the sun is accurately focused into the heat absorber in the central receiver, which heats up the heat transfer medium and enters the steam generator to produce steam, which eventually drives the turbine unit to generate electricity. In addition, in order to ensure continuous power supply, a heat storage device is needed to store the heat during peak hours for use in the morning and evening and between rainy days.
2、Trough type solar thermal power generation
Trough solar power generation uses multiple trough-shaped paraboloidal concentrators to gather sunlight onto the collector tubes of the receiving device, which heats the working mass and generates high-temperature steam to drive the turbine for power generation. The geometric characteristics of the collecting device determine that the concentration ratio of trough solar power is lower than that of the tower type, usually between 10 and 100, and the operating temperature reaches 400°C. As shown in Figure 3, trough solar power generation includes a concentrating collector section, a heat exchange section, and a
Power generation and storage part. Among them, the power generation and energy storage part is basically similar to the tower type, but the difference lies in the concentrating collector and heat exchange part. Concentrating heat collection is the core of the whole tank type power generation system, which consists of concentrating array, collector and tracking device. In this part, the collectors are mostly arranged in series and parallel, and can track the sunlight in 3 directions: north-south, east-west and polar axis in one dimension. In the heat exchange part, the 4 components of preheater, steam generator, superheater and reheater realize the process of heating the work mass, exchanging heat, generating steam and performing power generation. Due to the relatively compact structure of the trough type power generation system, the footprint of its collection device is relatively smaller than that of the tower and dish type, thus laying the foundation for the development of trough type solar power generation towards industrialization
3、Dish type solar thermal power generation
As the most efficient way of thermal power generation, dish solar power generation integrates multiple mirrors to form a parabolic butterfly concentrator, through its rotation, the sunlight is gathered into the receiver, and the receiver absorbs heat and heats the mass to further drive the generator set to generate electricity. The application of rotating parabolic butterfly concentrator makes the concentrating ratio of dish solar power generation reach more than 3000, which on the one hand effectively improves the efficiency of heat and light conversion, but on the other hand also puts higher requirements on the material and process of the receiver due to its higher receiving temperature. As seen in Figure 4, the dish type solar power system includes parabolic butterfly concentrator mirror, high temperature receiver, tracking drive, and power generation and storage device.
Unlike the tower and trough type, dish solar power generation mainly uses Stirling (Stirling) thermal cycle to complete the conversion of thermal energy to mechanical energy, but because the Stirling (Stirling) thermal machine technology development is not yet mature, so the dish solar power generation is still in the test demonstration stage.
4、Other
Way recently, a new design of solar thermal power generation system has attracted widespread attention. The design uses a row of coaxially arranged mirrors instead of the traditional sense of parabolic mirrors, the sunlight will first focus on the upper central mirror, and then reflected downward by the central mirror, the sunlight will be focused into the ground receiver, this new way of concentrating light called downward reflection or Fresnel reflection (such as Figure 5). Due to the secondary focus, a higher concentration ratio is guaranteed; at the same time, the downward-reflecting method not only avoids the risk of installing receivers on high towers, but also solves the problems of high heat loss from the top of the tower and high installation and maintenance costs, which is bound to become an important research direction for future solar thermal power generation.
5、Comparison of the three solar thermal power generation technologies
Each of the above three solar thermal power generation methods has its own advantages. In terms of theory, tower solar power generation is more suitable for large-scale production because of its high concentrating ratio, high operating temperature, large system capacity and high thermal conversion efficiency; trough solar power generation has become the first thermal power generation method to enter commercial production because of its relatively simple system structure and more mature technology; and dish solar power generation has the highest thermal efficiency, compact structure and easy installation. Compact structure, easy installation, etc., is very suitable for distributed small-scale energy systems. On the other hand, the high initial investment and the difficulty in reducing costs make tower solar power generation has never been widely put into commercial production; the small concentrating ratio, low system operating temperature, the core component vacuum tube technology has not yet matured, the selective coating performance of the absorber tube surface is unstable and other problems have hindered the promotion of trough solar power generation; disc power generation system, Stirling heat engine key technology difficulties, short development time and other reasons In the dish type power generation system, the key technology of Stirling heaters is difficult and the development time is short, resulting in it is still in the test demonstration stage.
