1, 2O0 ~ 400℃ direct power generation and comprehensive utilization;
2. 150 ~ 200℃ dual-cycle power generation, refrigeration, industrial drying and industrial thermal processing;
3. 10o ~ 15o℃ dual-cycle power generation, heating, refrigeration, industrial drying, dehydration processing, salt recovery and canned food;
4, 50 ~ 100℃ heating, greenhouse, domestic hot water, industrial drying;
Bathing at 5.20 ~ 50℃, aquaculture, raising livestock, and heating and dewatering the soil;
Nowadays, in order to improve the utilization rate of geothermal energy, many countries adopt the methods of cascade development and comprehensive utilization, such as cogeneration, cogeneration, heating first and then breeding.
In recent years, foreign countries attach great importance to the non-electric utilization of geothermal energy, that is, direct utilization. Because geothermal power generation has low thermal efficiency and high temperature requirements. The so-called low thermal efficiency. That is to say, due to different types of geothermal energy and different types of steam turbines used, 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 geothermal energy is used to generate electricity, the temperature requirement for underground hot water or steam is generally above 150℃. Otherwise, it will seriously affect its economy. The direct utilization of geothermal energy not only has much less energy loss, but also has much lower temperature requirements for underground hot water, so it can be used in a wide temperature range of 15 ~ 180℃. Among all geothermal resources, this kind of medium-low temperature geothermal resources is very rich, far greater than high temperature geothermal resources. However, the direct utilization of geothermal energy also has its limitations. Generally speaking, due to the limitation of hot water transportation distance, the heat source should not be too far away from the town or residential area where heat is used. Otherwise, too much investment, too much loss and poor economy are not worth it.
At present, the direct utilization of geothermal energy has developed very rapidly, and has been widely used in industrial processing, civil heating and air conditioning, bathing, medical treatment, agricultural greenhouse, farmland irrigation, soil heating, aquaculture, livestock and poultry breeding, etc., and has received good economic and technological benefits and saved energy. The direct utilization of geothermal energy has low technical requirements and simple equipment. In the system of directly utilizing geothermal energy, although geothermal energy can sometimes be directly utilized because of the low content of salt and sediment in geothermal energy, it is usually pumped up by a pump and turned into hot gas and hydrothermal solution through a heat exchanger before being used. These systems are the simplest and use traditional off-the-shelf components.
Most of the heat sources directly utilized by geothermal energy are above 40℃. If the heat pump technology is adopted, the water heat source below 20℃ can also be used as the heat source (for example, the practices of the United States, Canada, France, Sweden and other countries). The working principle of heat pump is the same as that of household refrigerator, except that refrigerator is actually a one-way heat pump, while geothermal heat pump can transfer heat in two directions. In winter, it extracts heat from the earth and then provides it to houses or buildings (heating mode); In summer, it extracts heat from houses or buildings and then provides it to the earth for storage (air conditioning mode). No matter what kind of circulation, water is heated and stored, which plays all or part of the functions of independent water heaters. Because current can only be used for heat transfer, but not for heat generation, ground source heat pump will be able to provide 3-4 times more energy than its own consumption. It can be used in a wide temperature range of the earth. In the United States, geothermal heat pump system is developing at an annual rate of 20%, and will continue to develop with good double-digit growth momentum in the future. According to us energy information administration's forecast, by 2030, geothermal heat pump will provide 68 million tons of oil equivalent energy for heating, cooling and water heating.
For geothermal power generation, if the temperature of geothermal resources is high enough, a good way to use it is to generate electricity. The generated electricity can not only supply the public power grid, but also provide electricity for local industrial processing. Generally, it is used for basic load power generation, and only under special circumstances is it used for peak load power generation. One of the reasons is that it is difficult to control the peak load, and the other is the scaling and corrosion of the container. Once the liquid in the container and turbine is not satisfied and air is allowed to enter, scaling and corrosion will occur.
To sum up, geothermal energy utilization plays an important role in the following four aspects.
1. Geothermal power generation
Geothermal power generation is the most important way of geothermal utilization. High temperature geothermal fluid should be first applied to power generation. The principle of geothermal power generation and thermal power generation is the same, which uses the thermal energy of steam to convert into mechanical energy in the steam turbine, and then drives the generator to generate electricity. The difference is that geothermal power generation is not equipped with 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 make use of underground heat energy, we need a kind of "heat carrier" to bring underground heat energy to the ground. At present, the main heat carriers available for geothermal power stations are underground natural steam and hot water. According to the characteristics of heat carrier type, temperature and pressure, geothermal power generation can be divided into steam geothermal power generation and hot water geothermal power generation.
Schematic diagram of geothermal power generation
(1) Steam geothermal power generation
Steam geothermal power generation is to directly introduce the dry steam from steam field into the steam turbine generator set for power generation, but before introducing the steam 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 deep strata, so the development is limited (refer to the related articles in the column of resources). There are mainly two kinds of power generation systems: back pressure type and condensed steam type.
(2) Hot water geothermal power generation
Hot water geothermal power generation is the main way of geothermal power generation. At present, there are two kinds of circulating systems in hot water geothermal power station: A. Flash system. The flash system is shown in figure 1. When high-pressure hot water is pumped from the hot water well to the ground, the hot water will boil and "flash" into steam in the decompression part, and the steam will be sent to the steam turbine to do work; The separated hot water can be discharged after continuous utilization, and it is of course best to re-inject it into the formation. Double circulation system. The flow of the dual-cycle system is shown in Figure 2. Geothermal water first flows through the heat exchanger, transferring geothermal energy to another low-boiling working medium, boiling it and generating steam. Steam enters the steam turbine to do work, then enters the condenser, and then passes through the heat exchanger to complete the power generation cycle. Geothermal water is re-injected into the formation from the heat exchanger. The system is especially suitable for geothermal resources with high salt content, strong corrosiveness and high non-condensable gas content. The key technology of developing dual-cycle system is to develop high-efficiency heat exchanger.
Figure 1 Hot water geothermal power generation flash system
Fig. 2 Dual-cycle system of hot water geothermal power generation
The prospect of geothermal power generation depends on how to develop and utilize dry-hot rock resources with large geothermal reserves. Fig. 3 is a schematic diagram of power generation using dry-hot rocks. The key technology is whether the deep well can be driven into the hot rock. The Los Alamo Science Laboratory in New Mexico, USA is conducting long-term experiments on this system.
Fig. 3 is a schematic diagram of thermal rock power generation.
2. Geothermal heating
Using geothermal energy to directly heat, warm and make hot water is the second only to geothermal power generation. Because of its simple utilization and good economy, it has attracted the attention of all countries, especially the western countries located in the alpine region, among which Iceland is the best to develop and utilize. As early as 1928, the country built the world's first geothermal heating system in Reykjavik, the capital. Now, this heating system has developed very well, and it can extract 7740t80℃ hot water from underground every hour for the use of 1 10000 residents in the city. Because there are no towering chimneys, Iceland's capital is known as "the cleanest and smokeless city in the world". In addition, it is also promising to use geothermal energy to provide heat for factories, such as as the heat source for drying grains and food, diatomite production, wood, paper making, leather making, textile, wine making, sugar making and other production processes. 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. Geothermal heating and utilization of hot water have also developed very rapidly in China, and have become the most common geothermal utilization methods in Beijing and Tianjin.
3. Geothermal agriculture
Geothermal energy is widely used in agriculture. If the farmland is irrigated with geothermal water with suitable temperature, the crops can be matured and the yield can be increased. Using geothermal water to raise fish can accelerate the fattening of fish at 28℃ and increase the output of fish. Using geothermal energy to build greenhouses, raise seedlings, grow vegetables and flowers; Using geothermal energy to heat biogas digesters to increase biogas production. The direct use of geothermal energy in agriculture is increasingly widespread in China, and geothermal greenhouses of different sizes have been built in Beijing, Tianjin, Tibet and Yunnan. Various places also use geothermal energy to develop aquaculture, such as cultured strains, African crucian carp, eel, tilapia and Macrobrachium rosenbergii.
4. Geothermal practice
The application prospect of geothermal energy in medical field is attractive. At present, hot mineral water is regarded as a valuable resource and cherished by all countries in the world. Because geothermal water is extracted from the deep underground to the ground, it often contains some special chemical elements besides high temperature, which makes it have certain medical functions. If you drink carbonated mineral water, you can adjust your stomach acid and balance your pH; Drinking iron-containing mineral water can treat iron deficiency anemia; Bathing with hydrogen spring and sulfur water hydrogen spring can treat neurasthenia, arthritis and dermatosis. Due to the medical function of hot springs and the accompanying special geological and geomorphological characteristics, hot springs often become tourist attractions, and dare not approve convalescents and tourists. There are more than 65,438+0,500 hot spring sanatoriums in Japan, attracting 6,543.8+0 billion people to these sanatoriums every year. China has a long history of using geothermal energy to treat diseases, and there are many hot springs containing various mineral elements. Therefore, it is promising to give full play to the role of geothermal energy in medical practice and develop hot spring industry.
In the future, with the development of high technology related to geothermal utilization, people will be able to identify more geothermal resources more accurately; Drilling deeper wells will bring geothermal energy out of the deep layer, so the utilization of geothermal energy will surely enter a stage of rapid development.