Geothermal power generation first appeared in 1904. At that time, Ladriro in Italy used geothermal heat to drive a small generator of 0.75hp (1hp=745.7W) to provide lighting power for five 100W electric lamps, and then built the world’s first 500kW small geothermal power station . In the early 1970s, with the support of the National Science and Technology Commission, geothermal power stations began to appear in various parts of China.
Geothermal power generation is a new type of power generation technology that uses underground hot water and steam as power sources, and its basic principles are similar to thermal power generation. According to the principle of energy conversion, first convert underground thermal energy into mechanical energy, and then convert mechanical energy into electrical energy.
To make use of underground heat energy, a “heat carrier” is first needed to bring the underground heat energy to the ground. At present, the heating medium that can be used by geothermal power stations is mainly underground natural steam and hot water. According to the different types of heating medium, temperature, pressure and other characteristics, the methods of geothermal power generation can be divided into steam-type geothermal power generation and hot water-type geothermal power generation.
1. Steam-type geothermal power generation
Steam-type geothermal power generation is the direct introduction of dry steam from the steam field into a steam turbine generator set for power generation, but the rock debris and water droplets contained in the steam should be separated before being introduced into the generator set. This kind of power generation method is the simplest, but the dry steam geothermal resources are very limited and mostly exist in the deeper stratum. The mining technology is difficult, so the development is restricted. There are mainly two types of power generation systems, back pressure type and condensing type.
2. Hot water-type geothermal power generation
Hot water-type geothermal power generation is the main method of geothermal power generation. At present, there are two types of circulation systems in hot water-type geothermal power stations:
(1) Flash evaporation system. When high-pressure hot water is pumped from the hot water well to the surface, the part of the hot water with reduced pressure will boil and “flash” into steam, which is sent to the steam turbine to perform work; the separated hot water can be reused and then discharged. Of course, it is best to be injected back into the formation.
(2) Double circulation system. The geothermal water first flows through the heat exchanger to transfer the geothermal energy to another working fluid with a low boiling point to make it boil to produce steam. The steam enters the steam turbine to perform work, enters the condenser, and then passes through the heat exchanger to complete the power generation cycle. Geothermal water is injected back into the formation from the heat exchanger. This system is particularly suitable for geothermal resources with large salt content, strong corrosiveness and high non-condensable gas content. The key technology for the development of dual-cycle systems is the development of high-efficiency heat exchangers.
Geothermal power generation has the following characteristics:
(1) The geothermal energy reserves are large, not affected by the weather, and the cost of power generation is low. Geothermal energy is from the center of the earth and can be used to replace part of nuclear energy. The geothermal energy stored in the earth’s crust within 10km below the surface is 5000 times that of all oil and natural gas reserves in the world. Compared with wind energy and solar energy, geothermal energy is stable and reliable, and is not affected by the weather. Depending on the geographical structure, the long-term cost of geothermal energy may be lower than that of coal. Once the location of the geothermal energy is determined and the power generation turbines are built, the cost of power generation is very low.
(2) The initial investment of geothermal energy mining is large, and the investment return period is long. Geothermal projects are different from petroleum or mining projects. The size of the geothermal source is unknown before development, and the initial investment of the project is huge. 1MW of geothermal energy requires an investment of 23 million yuan, while the same capacity of thermal power generation only costs 7.85 million yuan. In addition, it usually takes 5 to 7 years for geothermal resources to go from exploration to commercial operation. The long return period is one of the reasons why many investors are unwilling to intervene.