Generally speaking, solar thermal power generation technology refers to solar steam thermal power generation technology. It uses a large-scale array of parabolic or dish-shaped mirrors to collect solar thermal energy, provides steam through a heat exchange device, and combines the technology of traditional steam turbine generators to achieve the purpose of generating electricity. It avoids the expensive silicon crystal photoelectric conversion process in the photovoltaic power generation system, and greatly reduces the cost of solar power generation. At the same time, the working fluid heated by solar energy can be stored in a huge container, and it can still drive the steam turbine to generate electricity a few hours after the sun sets, which has the incomparable advantage of photovoltaic power generation.
The principle of solar thermal power generation is similar to that of traditional thermal power generation, and the generator sets and power cycles used are basically the same. The difference is that the heat source for steam generation is solar energy, not fossil energy such as coal. The light radiation of solar energy is collected and converted into heat energy through the light-concentrating and heat-collecting device, and a certain working fluid is heated to a high temperature of several white degrees Celsius, and then high-temperature and high-pressure superheated steam is generated through the heat exchanger, which then drives the steam turbine to rotate and drive the generator to generate electricity.
A typical solar thermal power generation system is mainly composed of a concentrating and collecting subsystem, a heat transfer subsystem, a heat storage and heat exchange subsystem, and a steam turbine power generation subsystem, as shown in Figure 1. Among them, the heliostat and heat collector realize the heat collection function. The heat accumulator is the main equipment for heat storage and heat exchange. The steam turbine and generator are the core equipment for power generation, and the condenser and water pump are the thermal power.
(1) Concentrating and collecting subsystem. Because solar energy is relatively scattered, the role of heliostats (or concentrating systems) is to focus solar radiation to increase the power density of solar energy. The concentrating system of large-scale solar thermal power generation usually consists of a combination of multiple heliostats to form a huge solar collection field. In order to gather and track the sun’s rays, a solar tracking system is generally required to ensure the efficient use of the sun’s rays.
The function of the heat collector is to absorb the focused solar radiation and convert it into heat energy and transfer it to the working fluid. The collector is a key component in the solar thermal power generation system, and it is usually necessary to connect multiple scattered collectors in series or in parallel to form a square array of collectors. At present, the commonly used heat absorption devices include vacuum tube type and cavity type.
For the solar thermal power generation system, the reflector (condensing device), solar tracking device, and heat collector together constitute the concentrating and heat collecting subsystem.
(2) Heat transfer subsystem. The heat transfer subsystem transfers the heat energy collected by the heat collectors of each unit to the heat storage part. The heat transfer working medium is usually pressurized water, carbon dioxide or organic working medium. In order to reduce the heat loss of the heat transfer pipes, heat-transferring materials are generally added to the heat transfer pipes or special heat pipes are used to transfer heat.
(3) Heat storage and heat exchange subsystem. Because solar energy is affected by weather conditions, day and night, and seasons, it is volatile and intermittent. In order to ensure the stability of the heat source of the solar thermal power generation system to achieve stable operation, a heat accumulator is required. There are four types of heat storage: low temperature (<100℃), medium temperature (100~500℃), high temperature (500~1000℃) and extremely high temperature (above 1000℃). Hydration salt, heat transfer oil, molten salt, oxide diamond refractory ball, etc. are used as heat storage materials. In order to meet the needs of steam turbines for power generation, the transmitted and stored heat energy needs to be converted into high-temperature and high-pressure steam through heat exchange devices to drive the steam turbines to do work.
(4) Steam turbine power generation subsystem. The steam turbine power generation subsystem is mainly composed of a steam turbine and a generator, and is the core component to realize thermal power generation. After heat collection, storage and heat exchange, the high temperature and high pressure steam drives the turbine to rotate, thereby driving the generator to generate electricity. The output power can directly supply power to the load or can be connected to the grid. In addition to the steam turbine generator sets commonly used in solar thermal power generation systems, there are also gas turbine generator sets that use solar energy to heat the air, low boiling point working fluid steam turbines, and Stirling heat engines.