New energy generation method: geothermal power generation - geothermal energy

Geothermal energy comes from a large amount of heat generated by the decay of natural radioactive isotopes such as uranium, thorium and potassium in the Earth's interior, which is transferred to the surface through carriers such as volcanic eruptions, hot springs and groundwater. The heat energy transported to the surface from inside the earth is equivalent to 28 billion tons of standard coal every year, because of its large reserves, wide distribution, clean environmental protection, stability and reliability and many other advantages and has been widely used, at present, more than 88 countries and regions in the world use geothermal energy for power generation and heating. In 2020, China proposed for the first time to achieve "carbon peak" in 2030 and "carbon neutrality" in 2060, and clean energy is an indispensable part of achieving this goal.

Hydrothermal geothermal resources in China are equivalent to 1.25 trillion tons of standard coal, and the annual recoverable resources are equivalent to 1.865 billion tons of standard coal, which can be divided into convective hydrothermal system, conductive hydrothermal system and conduction-convective composite hydrothermal system according to heat transfer mode, and can be divided into high-temperature geothermal (150 ° C) and medium-low temperature geothermal (<150 ° C) according to temperature. Hydrothermal medium and low temperature geothermal resources are mainly distributed in the North China Plain, Songliao Basin, Sichuan Basin, Jiaodong Peninsula, Liaodong Peninsula and other regions, with an annual mining capacity equivalent to 1.85 billion tons of standard coal, mainly used for heating, tourism, industrial drying and so on. High-temperature hydrothermal geothermal energy is mainly distributed in the southwest of China, such as South Tibet, west Yunnan and west Sichuan, with an annual mining capacity equivalent to 18 million tons of standard coal, which is mainly used for power generation and industrial utilization.

Dry hot rock refers to the high-temperature rock mass with no or little water content at a depth of 3 ~ 10km from the surface, which is mainly metamorphic or crystalline rock with low porosity and poor fracture permeability. Therefore, geothermal reservoirs need to be formed by artificial fracturing to be exploited and utilized. The temperature range is 150℃ ~ 650℃. The basic resources of dry hot rock buried less than 10,000 m in China can be equivalent to 856 trillion tons of standard coal, mainly distributed in Xizang, Yunnan, Guangdong, Fujian and other regions.

New energy generation method: geothermal power generation - geothermal energy

Geothermal power system

Flash evaporation electric system

The flash evaporation electric system, also known as the vacuum expansion power generation system, transforms the ground hot water under low pressure from liquid to gas by using the principle of different boiling points of water under different pressures. The working process is shown in Figure 2(a). The steam and water mixture with a certain pressure extracted from the geothermal well is transported to the flash evaporator through the pipeline for pressure reduction and expansion; the expanded water is pumped underground through the pipeline; the expanded steam is dehumidified by the dehumidifier and then sent to the steam turbine through the pipeline to do work; the exhaust gas discharged from the steam turbine is condensed by the condenser and then transported to the recharge well and pumped underground. According to the different times of geothermal water passing through the flash evaporator, it can be divided into single-stage flash system and two-stage flash system. The two-stage flash system is an improvement based on the single-stage flash system, and its working principle is shown in Figure 2(b). The expanded water is sent to the flash evaporator again for secondary flash expansion, and the steam generated by expansion is sent to the low pressure end of the turbine to continue generating power.

New energy generation method: geothermal power generation - geothermal energy

Flash evaporation power system is the most commonly used power generation system for geothermal power generation. When the power generation system is in normal operation, the separated brine contains some high concentration of dissolved minerals, which will cause serious water pollution if it is mixed with the surface or groundwater. The concentration of waste brine in the two-stage flash evaporation power station is generally higher than that in the single-stage flash power station. In order to prevent water pollution, the wastewater needs to be recharged, which can effectively restore the fluid in the reservoir and maintain the pressure of the reservoir. The single-stage flash evaporation electric system is simple in structure and easy to manufacture, but the conversion efficiency is low. The equipment of the two-stage flash evaporation system is complex, but the conversion efficiency can be increased by 20% to 30% compared with the single-stage flash evaporation system under the same heat source condition. The use of single-stage or two-stage flash evaporation system depends on the characteristics of geothermal resources, the economy of geothermal power station and the loss of equipment.

Dry steam power generation system

Dry steam power generation system is a power generation system in which the geothermal fluid extracted from the ground is dominated by dry steam. Its working principle is shown in Figure 3. First, dry steam extracted from geothermal Wells is filtered out of solid particles with large diameter through a purification separator, and then sent to the turbine for power generation. Finally, the exhaust steam from the turbine is pumped back underground through the condenser and cooling tower, and the equipment used is the same as that of conventional thermal power plants. The power generation system is mainly aimed at dry steam geothermal fields with high parameters, and has the advantages of safety and reliability, little impact on the environment, and is generally suitable for high temperature geothermal energy. Comparing Figure 2 and Figure 3, it can be seen that the dry steam power generation system is very similar to the flash evaporation power system. The difference is that the dry steam power generation system uses a purification separator instead of a flash evaporator. The power generation process only uses steam and does not produce any mineral-containing brine, so the impact on the environment is lower than that of the flash evaporation power system. Currently, there are 63 dry-steam geothermal power plants in the world, mainly in countries such as the United States, Italy and Japan, accounting for about 22% of the world's total installed geothermal capacity.

New energy generation method: geothermal power generation - geothermal energy

Dual fluid power generation system

The dual-working medium cycle power generation system uses low-boiling organic working medium as the circulating working medium, and geothermal water does not directly participate in the thermal cycle. According to the different circulating working medium, it can be divided into organic Rankine cycle (ORC) and Kalina power generation system. ORC power generation system is the use of low boiling point organic working substances, such as halogenated hydrocarbons (CFCs), hydrochlorofluorocarbons (HCFCS), hydrofluorocarbons (HFCs), alkane (HCs), organic oxides and cyclic organic compounds. The working principle is shown in Figure 4. The low-boiling organic working medium exchanges heat with geothermal fluid through the heat exchanger to complete preheating and evaporation, and then generates power through the steam turbine, and finally returns to the heat exchanger through the working medium pump after condensation by the condenser to complete the cycle. The low boiling point organic working medium is mostly flammable and explosive, which requires higher sealing of equipment.

Kalina cycle uses ammonia mixture as the circulating working medium. At a lower temperature, ammonia gas will evaporate out and the components of the ammonia mixture in the circulating solution will change, resulting ina change in the boiling point temperature. The mixture of ammonia and water is exchanged with geothermal water in the evaporator, the gas-liquid mixture is generated and then enters the separator for gas-liquid separation. The separated saturated ammonia vapor is sent to the steam turbine to expand and do work to drive the generator to generate electricity. The separated ammonia water is sent to the regenerator to recover the heat. The exhaust gas discharged from the steam turbine is sent to the condenser to condense into ammonia, which is then sent to the evaporator through the working medium pump for re-circulation.

The dual-medium power generation system is widely used in geothermal power generation, and has the advantages of compact equipment, small turbine size and low operating cost. When the temperature of the local thermal reservoir is low, the use of flash evaporation electric system has large investment and low efficiency, and the dual-medium power generation system can not only use the geothermal fluid of 85 ° C ~ 170 ° C, but also in the cycle process, because there is no direct contact between the geothermal fluid and the power production equipment, it can effectively prevent the corrosion and scale of the power generation equipment. The power generation system can make use of low-grade energy of low-temperature geothermal resources, promote steam turbine to do power generation, and make reasonable use of low-temperature geothermal resources.

New energy generation method: geothermal power generation - geothermal energy

Enhanced geothermal system

Enhanced Geothermal Systems (EGS) refer to the construction of artificial geothermal reservoirs through hydraulic fracturing and other technical means to build cracks in the rock to form a heat exchange space between the rock and the fluid, which is generally used in hot and dry rock geothermal resources. The process of EGS power generation is to build artificial heat storage by pressurizing cold water through water injection Wells. Cold water penetrates cracks in rock strata and contacts with high-temperature rock mass to absorb heat. Then hot water or water vapor is extracted to the ground by production Wells and heat is exchanged through heat exchangers. Hot and dry rock resources are abundant in China, but only stay in the exploration and development stage at present. In May 2015, China Geological Survey organized the implementation of China's first dry hot rock scientific drilling in Zhangzhou, Fujian, which marked the official start of China's national dry hot rock practice. In 2017, the hydrogeological team of the Hebei Coal Geology Bureau implemented a dry hot rock pre-survey project, drilling a well 4000 m deep at 110℃. Subsequently, China successfully drilled 236℃ hot dry rock at a depth of 3,705 meters in Gonghe Basin, Qinghai Province, and is expected to successfully build one to two hot dry rock demonstration projects in 2035 to achieve power generation from hot dry rock. In June 2021, the key technology research and demonstration project of dry hot rock development in Matouying Uplift Area of Tangshan City, Hebei Province, realized the experimental power generation of dry hot rock, which is the first time to realize the experimental power generation of dry hot rock in China.

In recent years, the utilization of geothermal energy in China is mainly direct utilization. With the continuous breakthrough of key technologies of geothermal power generation, geothermal development and utilization gradually extend to the direction of geothermal power generation. Geothermal power generation is the extraction of underground heat energy into usable electrical energy, almost zero emissions in the power generation process, compared with thermal power generation, hydropower generation is more competitive. Considering the distribution of geothermal resources in China, Sichuan, Yunnan and Tibet, which are distributed in the Mediterranean-Himalayan mountain tropics, are the main distribution areas of high-temperature geothermal resources, which have a very large power generation potential. With the breakthrough in the research and development of technical means and equipment for medium and low temperature geothermal power generation, the use of medium and low temperature geothermal resources for power generation continues to grow; China is rich in dry hot rock resources, the use of dry hot rock power generation is still in the research and development stage, compared with western countries slow development. Key technological breakthroughs in medium and low temperature geothermal power generation and enhanced geothermal power generation systems will accelerate the development and utilization of geothermal resources and contribute to the construction of a clean, low-carbon, safe and efficient modern energy system in China.