7 ways to use geothermal energy resources

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Humans have been using geothermal energy for a long time, such as using hot springs for bathing, medical use of underground hot water for heating, building greenhouses for crops, aquaculture, and drying grains. However, the real understanding of geothermal resources, and the development and utilization of large-scale development and utilization began in the middle of the 20th century. In the utilization of geothermal energy, high temperature geothermal resources are mainly used for power generation, and medium and low temperature geothermal resources are mainly used for non-electric direct utilization. The range of possible utilization of geothermal fluids of different temperatures is generally as follows:
①200~400℃ direct power generation and comprehensive utilization;
②150~200℃ double cycle power generation, refrigeration, industrial drying, industrial thermal processing;
③100~150℃ double cycle power generation, heating, cooling, industrial drying, dehydration processing, salt recovery, canned food;
④50~100℃ heating, greenhouse, domestic hot water, industrial drying;
⑤20~50℃ bathing, aquaculture, livestock raising, soil heating, dehydration processing.

In recent years, foreign countries have attached great importance to the non-electrical utilization of geothermal energy, that is, direct utilization. Because of geothermal power generation, thermal efficiency is low and temperature requirements are high. The so-called low thermal efficiency means that due to different geothermal types and different types of steam turbines, the thermal efficiency is generally only 6.4~18.6%, and most of the heat is consumed in vain. The so-called high temperature requirement means that, when using geothermal energy to generate electricity, the temperature requirement for underground hot water or steam is generally above 150℃; otherwise, its economy will be seriously affected. The direct use of geothermal energy not only consumes much less energy, but also has a much lower temperature requirement for underground hot water, which can be used in a wide temperature range from 15-180 °C. Among all the geothermal resources, such medium and low temperature geothermal resources are very abundant, much larger than the high temperature geothermal resources. In order to improve the utilization rate of geothermal heat, many countries have adopted the method of cascade development and comprehensive utilization, such as cogeneration of heat and power, cogeneration of heat, electricity and cooling, heating first and then breeding.

When geothermal energy is used directly, most of the heat sources used are above 40°C. If heat pump technology is used, a hydrothermal source with a temperature of 20°C or less can also be used as a heat source (as is the case in the United States, Canada, France, Sweden and other countries). The working principle of a heat pump is the same as that of a household refrigerator, except that the refrigerator is actually a one-way heat pump, while the geothermal heat pump can transfer heat in both directions. In winter, it extracts heat from the earth and supplies it to the house or building (heating mode); in summer, it extracts heat from the house or building and supplies it to the earth for storage (air conditioning mode). Regardless of the cycle, the water is heated and stored, performing all or part of the function of a stand-alone hot water heater. Since electricity can only be used to transfer heat, not to generate heat, a geothermal pump will be able to provide 3-4 times more energy than it consumes, and it can be used in a wide range of earth temperatures. In the United States, geothermal pump systems are growing at a rate of 20% annually, and will continue to grow at a double-digit rate in the future. According to the U.S. Energy Information Administration, geothermal heat pumps will provide up to 68Mt of oil-equivalent energy for heating, cooling and water heating by 2030.
The utilization of geothermal energy mainly includes the following aspects.

Geothermal power generation is the most important way of geothermal utilization. High-temperature geothermal fluids should first be used for power generation. The geothermal fluid used for power generation requires a high temperature, which is generally more economical when it is above 180°C or even 200°C. The principle of geothermal power generation and thermal power generation is the same, both use the thermal energy of steam to convert into mechanical energy in the steam turbine, and then drive the generator to generate electricity. The difference is that geothermal power generation does not require huge boilers like thermal power generation and does not need to consume fuel. The energy it uses is geothermal energy. The process of geothermal power generation is the process of first converting underground thermal energy into mechanical energy, and then converting mechanical energy into electrical energy. To use underground thermal energy, first of all, a "heat carrier" is required to bring the underground thermal energy to the ground. At present, the heat carriers that can be used by geothermal power plants are mainly underground natural steam and hot water. Geothermal power generation can be divided into two categories: steam type geothermal power generation and hot water type geothermal power generation according to the type of heat carrier, temperature, pressure and other characteristics.

①Steam-type geothermal power generation
Steam-type geothermal power generation is to directly introduce the dry steam in the steam field into the steam turbine generator set to generate electricity, but before introducing into the generator set, the cuttings and water droplets contained in the steam should be separated. This power generation method is the simplest, but the dry steam geothermal resources are very limited, and most of them exist in deeper formations, so the mining technology is difficult, so the development is limited. There are mainly two power generation systems of back pressure type and condensing type.

② Hot water type geothermal power generation
Hot water type geothermal power generation is the main way of geothermal power generation. There are two kinds of circulation systems in the hot water type geothermal power station:

(1) Flash evaporation system. When the high-pressure hot water is pumped from the hot water well to the ground, some of the hot water will boil and "flash" into steam when the pressure is reduced, and the steam will be sent to the steam turbine to do work; and the separated hot water can be continuously used and then discharged. back into the formation.

(2) Double circulation system. The geothermal water first flows through the heat exchanger, which transfers the geothermal energy to another low-boiling working fluid, which boils and produces steam. The steam enters the steam turbine to do work and then 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 high salt content, strong corrosiveness and high content of non-condensable gases. The key technology for the development of dual-cycle systems is the development of high-efficiency heat exchangers.
China's high-temperature geothermal resources are mainly distributed in Yunnan, Tibet, and western Sichuan, with a total power generation potential of 5,800MW/a. Since the first experimental geothermal power station in China was completed and put into operation in Fengshun, Guangdong in October 1970, geothermal power stations such as Hunan Huitang, Tibet Yangbajing, Tibet Nagqu and Tibet Langjiu have been built successively. Yangbajing Geothermal Power Station had a total installed capacity of 25.18MW in 14 years from 1977 to 1991. The last 3MW unit was put into operation in early 1991. Since 1993, the annual power generation has been maintained at about 100 million kWh. The total amount of geothermal power generation in Yangbajing reaches 1.6 billion kWh, and the power station operates for an average of 4,300 hours per year. The Yangbajing Geothermal Power Station supplies 41% of Lhasa's electricity throughout the year and more than 60% in winter. Two other smaller geothermal power plants have also been built in Langjiu and Nagqu, with installed capacity of 2MW and 1MW respectively, which also play a considerable role in local economic development.

Geothermal power generation has low operating cost, easy transmission of electricity, is not limited by the location of hot fields, and belongs to high-grade energy without environmental pollution. Therefore, the utilization value of geothermal power generation is significantly higher than other utilization forms. The prospect of geothermal power generation depends on how to develop and utilize hot dry rock resources with large geothermal reserves. The key technology is whether deep wells can be drilled into hot rock formations. The system is being tested in the future at the Los Alamo Science Laboratory in New Mexico.

The direct use of geothermal energy for heating, heating and hot water supply is second only to geothermal power generation. Because this method of utilization is simple and economical, it is highly valued by various countries, especially in western countries located in alpine regions, among which Iceland is the best developed and utilized. As early as 1928, the country built the world's first geothermal heating system in the capital Reykjavik. Today, this heating system has been developed very well. It can extract 7740t80 ℃ of hot water from the ground every hour. For the city's 110,000 residents. The Icelandic capital has been dubbed "the cleanest smoke-free city in the world" due to its absence of towering chimneys. In addition, the use of geothermal energy to heat factories, such as heat source for drying grains and food, as a heat source for diatomite production, wood, paper, leather, textile, wine, sugar and other production processes is also promising. At present, the two largest geothermal application plants in the world are the diatomite plant in Iceland and the pulp processing plant in New Zealand.

In the 1990s, with the enhancement of the global awareness of environmental protection, the upsurge of direct utilization of geothermal energy in China, especially in the high-latitude and cold Three North (Northeast, North, Northwest) regions, increased the use of geothermal heating (heating and domestic water). )-based development efforts. The development of this work not only reduces the discharge of a large number of harmful substances, but also achieves obvious economic benefits. By the end of 1999, the flow of hot water for non-electric direct utilization was 64416L/s, which is equivalent to providing 162009MJ of thermal energy per year. This figure shows that the level of direct utilization of geothermal energy in China ranks first in the world. The national geothermal heating was 190× 104m² in 1990, and increased to 1100× 104m² in 2000. Mainly in Tianjin Tanggu, Hangu, Dagang and other places in North China. There are more than 50 geothermal wells, some of which have a maximum flow rate of 300m³/h, a water temperature of up to 97°C, and a heating area of ​​80.5× 104m². The geothermal heating area in the entire North China area reaches 131.38× 104m².

The use of geothermal heating and hot water supply in China has developed very rapidly. The application of geothermal heating is mainly in northern China, and it has become the most common way of geothermal utilization in the Beijing-Tianjin area. This utilization method is not only an effective method to save coal, reduce coal consumption, reduce smoke and dust pollution, and improve the environment, but also is well received by people due to the stable temperature of geothermal water and high heating quality. At present, there are Hebei, Liaoning, Shandong, Henan, Shanxi and other places that use medium and low temperature geothermal heating, and the national geothermal heating area exceeds 5 million square meters. The extensive use of coal-fired boilers is an important cause of serious air pollution. At present, the Beijing Municipal Government has clearly stipulated that coal-fired boilers should be eliminated in major urban areas and replaced by oil or gas to reduce air pollution. However, the initial investment and operating costs of gas and fuel oil are very expensive. The development of geothermal resources provides a feasible way to solve this problem. Vigorously advocating and promoting geothermal heating will make an important contribution to the cause of environmental protection.