The direct utilization of geothermal energy is developing very rapidly, and has been widely used in industrial processing, civil heating and air conditioning, bathing, medical care, agricultural greenhouses, agricultural irrigation, soil heating, aquaculture, livestock and poultry rearing, and other aspects of the good economic and technical benefits received, saving energy. The direct utilization of geothermal energy, technical requirements are lower, and the required equipment is simpler. In the direct use of geothermal system, although sometimes due to the geothermal heat flow in the salt and sediment content is very low and can be directly utilized for geothermal, but usually with a pump will be the geothermal heat flow pumped up through the heat exchanger into the hot gas and hot liquid and then use. These systems are the simplest, using conventional off-the-shelf components.
The temperature of the heat source used in the direct utilization of geothermal energy is mostly above 40 degrees Celsius. If heat pump technology is utilized, a hydrothermal source with a temperature of 20°C or less can also be used as a heat source (as is done in the United States, Canada, France, Sweden and other countries, for example). A heat pump works on the same principle as a domestic refrigerator, except that while a refrigerator is actually a one-way heat pump, a geothermal heat pump can deliver heat in both directions. In the winter, it extracts heat from the earth and provides it to the home or building (heating mode); in the summer, it extracts heat from the home or building and provides it back to the earth for storage (air conditioning mode). In either cycle, the water is heated and stored, performing all or part of the function of a stand-alone hot water heater. Since electric current can only be used to transfer heat, not to produce it, a geothermal heat pump will be able to provide three to four times more energy than it consumes on its own. It can be used over a wide range of earth temperatures. In the U.S., geothermal heat pump systems are growing at a rate of 20% per year and will continue to grow at a good double-digit rate in the future. The U.S. Energy Information Administration predicts that geothermal heat pumps will provide up to 68 Mt of oil equivalent for heating, cooling and water heating by 2030.
For geothermal power, if the geothermal resource is hot enough, a good way to utilize it is to generate electricity. The electricity generated can either feed the public **** grid or power local industrial processes. Normally, it is used for base-load power generation, and only under special circumstances is it used for peak-load power generation. The reasons for this are, firstly, the difficulty of controlling peak loads, and then the problems of scaling and corrosion of the vessel, once the liquid inside the vessel and turbine is not full and air is allowed to enter.
Summarizing the above, 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. Geothermal power generation and thermal power generation is the same principle, are the use of steam heat energy in the turbine into mechanical energy, and then drive the generator to generate electricity. The difference is that geothermal power generation does not need a huge boiler like thermal power generation, nor does it need to consume fuel, the energy it uses is geothermal energy. The process of geothermal power generation is to transform the underground thermal energy into mechanical energy first, and then transform the mechanical energy into electrical energy. In order to utilize the underground thermal energy, first of all, there needs to be a "heat carrier" to bring the underground thermal energy to the ground. The heat carriers that can be utilized by geothermal power plants are mainly natural steam and hot water in the ground. According to the type of heat carrier, temperature, pressure and other characteristics of the different ways of geothermal power generation can be divided into steam-type geothermal power generation and hot water-type geothermal power generation two categories.
Schematic diagram of geothermal power generation
(1) Steam-type geothermal power generation
Steam-type geothermal power generation is the dry steam in the steam field directly lead to the turbine generator to generate electricity, but before the lead generator should be separated from the steam contained in the rock debris and water droplets. This type of power generation is the simplest, but the dry steam geothermal resources are very limited, and most of them are stored in deeper strata, and the extraction technology is difficult, so the development is restricted (refer to the relevant articles in the "Resources" column). There are mainly back-pressure and condensing two power generation systems.
(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 circulation systems in hot water type geothermal power station: a. Flash vaporization system. Flash vaporization system is shown in Figure 1. When the high-pressure hot water from the hot water wells pumped to the ground, in the pressure reduction part of the hot water will boil and "flash" into steam, steam sent to the turbine to do work; and after the separation of the hot water can continue to use the discharge, of course, it is best to inject back into the stratum. b. Double-cycle system. The flow of double-cycle system is shown in Figure 2. Geothermal water first flows through the heat exchanger, the geothermal energy to another low boiling point of the working fluid, so that it boils and produces steam. The steam enters the turbine to do work, then enters the condenser and passes through the heat exchanger to complete the power generation cycle. The geothermal water is injected back into the ground from the heat exchanger. This system is particularly suitable for geothermal resources with high salt content, high corrosivity and high non-condensable gas content. The key technology for the development of dual-cycle systems is the development of highly efficient heat exchangers.
Figure 1 Flashing system for hot-water geothermal power generation
Figure 2 Dual-cycle system for hot-water geothermal power generation
The future of geothermal power generation depends on the development of dry-heat-rock resources, which have a large geothermal reserve. Figure 3 is a schematic diagram of power generation using dry heat rock. The key technology is the ability to drill deep wells into hot rock formations. The Los Alamos Science Laboratory in New Mexico, USA, is conducting a visionary test of such a system.
Figure 3 Schematic diagram of power generation from hot rocks
2. Geothermal heating
Direct use of geothermal energy for heating, heating and hot water supply is the second most popular way of geothermal utilization after geothermal power generation. Because of this way of utilization is simple, good economy, doubly valued by the countries, especially located in the alpine region of the western countries, of which Iceland development and utilization of the best. The country as early as 1928 in the capital city of Reykjavik built the world's first geothermal heating system, now this heating system has been developed very well, every hour from the ground to extract 7740t80 ℃ of hot water for the city's 110,000 residents to use. With no towering chimneys, Iceland's capital has been described as "the world's cleanest and smokeless city". In addition to the use of geothermal heating to the factory, such as used as a heat source for drying grains and food, used as a heat source for diatomaceous earth production, timber, paper, tannery, textiles, brewing, sugar and other production processes is also promising. At present, the world's two largest geothermal application plant is Iceland's diatomaceous earth factory and New Zealand's paper paddle processing plant. China's use of geothermal heating and hot water development is also very rapid, in Beijing and Tianjin has become the most common way of geothermal utilization.
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3. Geothermal agriculture
Geothermal in agriculture has a wide range of applications. Such as the use of geothermal water at a suitable temperature to irrigate farmland, can make crops mature early and increase yields; the use of geothermal water for fish farming, in 28 ℃ water temperature can accelerate the fertilization of fish, improve fish yield; the use of geothermal construction of greenhouses, rice seedlings, planting and flower; the use of geothermal heating to the biogas digester to raise the biogas production, and so on. The direct use of geothermal energy for agriculture is becoming more and more widespread in China, with geothermal greenhouses of varying sizes being built in Beijing, Tianjin, Tibet and Yunnan. Various places also use geothermal energy to develop aquaculture, such as cultivation of fungus, breeding of African carp, eel, tilapia, tilapia and shrimp.
4. Geothermal medicine
Geothermal application in the medical field has attractive prospects, hot mineral water is regarded as a valuable resource, the world are very valuable. As the geothermal water from the deep underground extraction to the ground, in addition to the higher temperature, often contains some special chemical elements, so that it has a certain medical effect. Such as carbonated mineral water for drinking, can regulate stomach acid, balance the body's pH; iron-containing mineral water for drinking, iron deficiency anemia can be treated; hydrogen springs, sulfur water hydrogen spring baths can treat neurasthenia and arthritis, skin diseases, and so on. Due to the medical effects of hot springs and the special geological and geomorphological conditions that accompany them, hot springs often become tourist destinations, attracting large numbers of healers and travelers. In Japan, there are more than 1,500 hot spring sanatoriums, attracting 100 million people every year. China has a long history of using geothermal treatment of disease, containing a variety of mineral elements of the hot springs, so give full play to the role of geothermal medicine, the development of hot springs sanatorium industry is promising.
In the future, with the development of high and new technology related to geothermal utilization, will enable people to more accurately identify more geothermal resources; drilling deeper drilling wells will be taken out from the depths of the earth, so the geothermal utilization is also bound to enter a phase of rapid development.