Geothermal energy has become one of the important alternative energy sources after coal and petroleum, and is also an important member of the new energy family such as solar energy, wind energy, and biomass energy. It is a clean and green energy source with no or very little pollution. Geothermal energy is renewable heat energy from the depths of the earth, which originates from the decay of molten magma and radioactive materials on the earth. The deep circulation of groundwater and the intrusion of magma from extremely deep into the earth's crust bring heat from deep underground to the near surface. Its reserves are much more than the total amount of energy used by people at present, most of which are concentrated in the edge of tectonic plates, which is also an area prone to volcanoes and earthquakes. It is not only a pollution-free clean energy, but also if the heat extraction rate does not exceed the replenishment rate, then the heat energy is renewable.
Geothermal resources integrate heat, minerals, and water. In addition to geothermal power generation, they can also be directly used for heating, bathing, medical care, recreation, breeding, agricultural planting and breeding, textile printing and dyeing, and food processing. In addition, the development and utilization of geothermal resources can drive the development of geothermal resource survey, geothermal well construction, surface development and utilization engineering design and construction, geothermal equipment production, water treatment, environmental engineering, catering, tourism and vacation, etc. It is a new industry that can increase in a large amount. Social employment, promotion of economic development, and improvement of people’s quality of life. Therefore, countries with geothermal resources in the world regard it as a priority for new energy development, and cultivate their own distinctive geothermal industries, which has played a significant role in alleviating the shortage of conventional energy supply and improving the ecological environment.
Geothermal energy is natural thermal energy extracted from the earth's crust. This energy comes from the lava inside the earth and exists in the form of heat. It is the energy that causes volcanic eruptions and earthquakes. The temperature inside the earth is as high as 7000°C, and at a depth of 80 to 100 kilometers, the temperature drops to 650 to 1200°C. Through the flow of groundwater and lava flow to the earth's crust 1 to 5 kilometers above the ground, the heat can be transferred to a place closer to the ground. The hot lava heats nearby groundwater, and this heated water will eventually seep out of the ground. The simplest and most cost-effective way to use geothermal energy is to directly use these heat sources and extract their energy.
Geothermal energy at different depths
The temperature of lava erupted by volcanoes on the earth is as high as 1200℃~1300℃, and the temperature of natural hot springs is mostly above 60℃, and some even as high as 100℃-140℃. This shows that the earth is a huge thermal reservoir with huge heat energy. The earth can be regarded as a solid sphere with a radius of approximately 6370km. It is structured like a half-boiled egg, mainly divided into three layers. The outer surface of the earth is equivalent to an eggshell. This part is called the "crust". Its thickness varies from several kilometers to 70km. Below the crust is "the middle layer is equivalent to egg white, also called the "mantle". It is mainly composed of molten magma. The thickness is about 2900km. The inner part of the crust that is equivalent to the egg yolk is called the "core" and the core is divided into outer regions. Nuclear and Mainland nuclear.
The temperature of each layer of the earth is very different. The temperature rises by 3°C for every 100 meters from below the surface of the ground. In geothermal anomalous areas, the temperature increases faster with depth. When a drilling well in the North China Plain reached 1,000 meters, the temperature was 46.8°C; when it reached 2100 meters, the temperature rose to 84.5°C. The other well was 5,000 meters deep, and the bottom hole temperature was 180°C. According to various data, the temperature at the bottom of the crust and the upper part of the mantle is about 1100℃-1300℃, and the temperature of the core is about 2000℃~5000℃.
It is estimated that in the history of the earth, the heat generated by the decay of radioactive elements inside the earth has averaged 5 trillion calories per year. In August 1981, the United Nations New Energy Conference was held in Nairobi, the capital of Kenya. According to the technical report of the conference, the potential resources of global geothermal energy are about 450,000 times the current global energy consumption. The total amount of underground thermal energy is about 170 million times the heat released by all coal combustion.
Due to structural reasons, the uneven distribution of heat flow on the earth's surface has formed a geothermal anomaly. If geological conditions such as cap rocks, reservoirs, heat conduction, and water conduction are available, the development and utilization of geothermal resources can be carried out.
The total heat content of rocks and liquids at a depth of 5,000 meters from the earth's surface and above 15°C is estimated to be 14.5×1025 joules, which is equivalent to the heat of 4948 trillion tons of standard coal. Geothermal resources can be divided into four types according to their occurrence forms: one is hot water or steam, that is, the hot water or steam seen in the shallow earth (100~4500m underground); the other is geothermal energy, which is There are high-temperature and high-pressure fluids in the depths of some large sedimentary basins (3~6km), which contain a lot of methane gas; the third is hot dry rock geothermal energy, due to the special geological structure conditions, the hot dry rock mass with high temperature but little or no water is caused ; Fourth, magma heat energy is the huge heat energy stored in high temperature (7001-200℃) molten magma.
According to the temperature of geothermal water, geothermal energy can be divided into three types: high temperature (>150℃), medium temperature (90-150℃) and low temperature (<90℃). High temperature geothermal resources are mainly used for geothermal power generation; Low-temperature geothermal resources are mainly used for direct use of geothermal energy, usually directly used for heating, industrial and agricultural heating, aquaculture, medical treatment, and bathing.