Geothermal energy refers to the renewable energy stored in the soil, rock and water source of the shallow surface layer when the temperature is below 25 ℃.The technology of using this energy in the field of building air conditioning is called geothermal technology.[1]
Geothermal energy is an energy concept relative to geothermal resources. Both belong to the category of geothermal energy. The difference is that the former is the definition of shallow low temperature (<25 ℃) geothermal energy, and the latter is the definition of deep high temperature (≥ 25 ℃) geothermal energy.[2]
Chinese name
Geothermal energy
Foreign name
geotemperature energy
Alias
Shallow geothermal energy
Introduction
Heat energy extracted and utilized in groundwater or soil
Features
Fast recovery, pollution-free, high energy utilization
Geothermal energy is an energy concept relative to geothermal resources. Both belong to the category of geothermal energy. The difference is that the former is the definition of shallow low temperature (<25 ℃) geothermal energy, and the latter is the definition of deep high temperature (≥ 25 ℃) geothermal energy.Geothermal energy is one of the renewable energies encouraged to be developed and utilized according to the Renewable Energy Law, and plays an important role in China's energy development strategy.The extensive utilization of geothermal energy will inevitably reduce the dependence on conventional energy and alleviate the serious shortage of conventional energy in China.[2]
From the perspective of energy carrier, geothermal energy (shallow geothermal energy) is contained in the geological environment such as groundwater, surface water, soil and rock. Therefore, the development and utilization of geothermal energy is a production activity directly related to the protection of geological environment, which is related to the protection of the earth environment and must be given high attention.Naturally, harmless utilization, recycling and sustainable utilization have become the three basic principles that must be adhered to in geothermal energy utilization. Any technology, process and equipment related to geothermal energy utilization must first follow these three basic principles.
Geothermal energy has four outstanding characteristics: shallow, low temperature, constant temperature and renewable.In view of the current level of scientific and technological development, especially the mature application of heat pump equipment technology, these characteristics of geothermal energy are mainly used to regulate the indoor environment of human settlements, that is, to create a comfortable temperature environment.On the basis of integrating other technologies, the indoor environment of human settlements can achieve a comprehensive and comfortable effect.[2]
The main technical form of the geothermal circulating cooling and heating system is to use a completely closed high-strength pipeline system and combine necessary equipment to connect the geological environment with the indoor environment of human settlements, so that geothermal energy can be recycled, heating in winter and cooling in summer, creating a comfortable living and living environment for human beings, completely getting rid of the dependence on groundwater, scientific, environmental protectionThe three basic principles of geothermal energy utilization are perfectly practiced by comprehensively utilizing geothermal energy in various carriers.
The application of geothermal cycle cooling and heating system is of great strategic significance to China, where the building energy consumption is much higher than that of developed countries, especially in areas where the energy consumption of heating and air conditioning is too high. It can largely replace conventional heating methods such as coal, gas, oil, etc., save energy consumption, reduce pollutant emissions, and improve the quality of the atmospheric environment.[2]
The concept of shallow geothermal energy
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As an invisible natural resource accumulated underground (0-200m), shallow geothermal energy is a deepheat conductionandThermal convectionAndsolar radiationA product of mutual action.It refers to the energy formed by the perennial temperature difference between the temperature in the depth of 0~200m below the surface and the temperature in the place 1.5m away from the surface without direct sunlight (louver box). It is the potential energy between two temperature fields in different states, also called shallow layergeothermal energy, a part of geothermal resources.[3]
Shallow geothermal energy is recyclable, clean and environmentally friendly, widely distributed, with huge reserves, shallow burial, and can be developed and utilized nearbyFossil energyCan reduce pollutant emissions.
At present, there are still many disputes on the understanding of shallow geothermal energy resources. At present, there are mainly two academic schools. One is the view of energy storage, which is non renewable energy. In the process of project design and application, cold and heat balance must be considered.Another view is that the renewable low-temperature energy is the result of the combined action of solar energy and deep geothermal energy.Although the understanding of shallow geothermal energy resources has not yet been unified, the use of such resources for heating and cooling buildings has become popular. Since 2004Ground source heat pumpThe annual growth rate of market size is more than 30%, far higher than the average development rate of 20% - 22% in the world over the same period, and it has now jumped to the second place in the world.
From the perspective of energy conservation, environmental protection and sustainable development, shallow geothermal energy has become one of the hot issues concerned by the HVAC sector and the geological system.[3]
Characteristics of shallow geothermal energy
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Shallow geothermal energy belongs to renewable green energy, which has the characteristics of rapid recovery of geological environment and groundwater, and no pollution to the atmosphere;High energy utilization rate, saving 50% - 75% energy compared with traditional methods;It really realizes zero emission and zero pollution in the heating (cooling) building use area;One set of equipment, heating in winter, cooling in summer, and providing daily domestic hot water, can realize three functions of one machine, save overall investment, and occupy less land.[4]
Overview of development and utilization of shallow geothermal energy abroad
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Current situation of development and utilization abroad
The concept of shallow geothermal energy has not been reported in foreign literature, butHeat pump technologyThe development and utilization of shallow geothermal energy resources have been going on for nearly a hundred years.
In 1948, the first underground water source heat pump system was put into operation in the Federal Building in Portland, Oregon, the United States. The system has received extensive attention since its design.The failure of the system due to corrosion and rust within 10 years after its completion and the low energy price at that time led to the failure of the heat pump system to be further promoted.[3]
At the end of the 1970s, due to the world oil crisis, European and American countries once again paid attention toGround source heat pumpSystem.For example, with the support of the US Department of Energy,oklahoma state university Such research institutions have conducted in-depth research on the thermophysical properties of rock and soil and the performance of different types of buried heat exchangers, solved corrosion and other problems, improved the utilization rate of energy, and gradually made groundwater heat pump widely used.
Since the 1990s,Ground source heat pumpIt is rapidly popularized in North America and Europe.Due to the cold climate in central and northern Europe, ground source heat pumps are mainly used for heating and providing domestic water.In 1994, 1995, 1996, 1997, 2006, and 2007, the production of ground source heat pumps in the United States was 5924, 8615, 7603, 9724, 64000, and 50000 units, basically showing a straight upward trend. As of 2009, the number of ground source heat pump systems in operation in the United States was about 1 million, thanks to many corresponding measures issued by the local government of the United States to encourage the development of ground source heat pumps.[3]
Canada from 1990 to 1996Ground source heat pump20% annually.It is estimated that the installed units of ground source heat pumps in Canada were 35000 in 2004 and 37000 in 2005. Since 2005, the market of ground source heat pumps in Canada has increased sharply, mainly due to the rise in energy prices, federal government support and targeted subsidies from local governments.
Sweden's ground source heat pump has developed at the fastest speed in the world in the past five years. In 2000, the direct utilization of geothermal energy ranked 10th in the world, and in 2005, it quickly jumped to the second place in the world. In addition, the market of ground source heat pump in Germany, Austria, Finland and other countries has also increased rapidly.[3]
Some municipal construction projects and public welfare buildings in Japan (such as hospitals, nursing homes, roads, etc.) have used ground heat pump systems to provide comprehensive services such as heating, cooling, hot water supply, and snow melting on roads. The effect is very obvious. Due to the problems of groundwater recharge, land subsidence, and high initial investment costs,Ground source heat pumpThe development of the system is constrained by certain conditions and has not been fully popularized.After the 1980s, Japan used the water source heat pump system of surface water, urban domestic waste water and industrial waste water to provide central heating or cooling for buildingsSeawater source heat pumpSystem, 2001Heat pump water heaterWhen entering Japanese families, the government gives certain subsidies to consumers, which is very popular with users.
Ground source heat pumpIt has just entered the Russian market and has not yet been accepted. It is considered as a foreign thing and not a reasonable substitute for traditional heat sources. The main reason is that domestic organic fuels are abundant and cheap.[3]
Development status of heat pump technology abroad
After 50 years of development, the heat pump technology in North America and Europe has been relatively mature, and a complete set of calculation methods, standards and specifications and construction processes have gradually formed. At present, various theories and methods play an important role in production practice.In 1912, zoelli of switzerland first proposed the concept of using the soil source as the low-temperature heat source of the heat pump system, and applied for a patent.In 1948, higersoll and Plass put forward the line heat source theory of buried pipe heat transfer according to the Kdvin line source concept. In 1950, they measured the thermal conductivity of soil with this method. At present, most of the design of ground source heat pump pipes are based on this theory.
The American Society for Testing and Materials (1963, 1992) standardized the thermal conductivity test method in 1963 and 1992.In 1983, Mogensen P. (Mogensen P., 1983) put forward the idea of on-site geothermal response test. In 1996, Eklof C (Eklof C, 1996) and others developed on-site geothermal response test equipment based on Mogensen's idea, and began to test the formation thermal conductivity coefficient in various places in Sweden. Later, the United States (Austin et al., 1998) Germany, Canada, Norway, SwedenFrance, Britain and Japan also have test equipment.[3]
Since the 21st century, heat pump technology has been constantly enriched and improved. New technologies and methods represented by information technology have been widely used and are currently moving towards quantitative development.
The underground heat exchanger design and analysis software, which is the most symbolic for the development and utilization of shallow geothermal energy resources, is widely used in the world, and is developing in the direction of simplification, rapidity and interaction.[3]
For example, there are on-site rock and soil thermal physical parameter measurement and geothermal monitoring systems in Sweden, and there are geothermal field models established with the help of mathematiciansGround source heat pumpThe system has carried out accurate calculation and prediction simulation before construction.The vertical buried pipe performance of ground source heat pump and the simulation and efficiency evaluation of heat pump system were studied by experimental methods (ArifHe Pbash, 2004). The numerical solution of ground source well and hole heat exchanger and the improved finite linear source model (Louis et al, 2007) were proposed. The efficiency prediction and evaluation of ground source heat pump system were studied by using neural network and fuzzy theory (Hikmct et al, 2007).
Tokyo, Japan uses geological, hydrogeological and thermal conductivity of rock and soil massGeographic Information SystemCarry out numerical simulation and prepare potential maps of surface and ground water heat pump systems.[3]
Overview of development and utilization of shallow geothermal energy in China
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Domestic development and utilization status
The development and utilization of shallow geothermal energy resources in China started late, but in recent ten yearsGround source heat pumpThe number of engineering projects using technology to heat (cool) buildings has increased rapidly, and it has ranked second in the world up to now.In addition to Liaoning and Beijing, in Hebei, Tianjin, Shandong, Henan, Inner Mongolia and other regions, a total of dozens of cities will begin to use ground source heat pump technology for building heating (cooling) pilot work. Ground source heat pump projects using shallow geothermal energy resources for heating and cooling have exceeded 7000 in China, with a total area of 2.1 × 108m by the end of 2010twoThe projects are mostly concentrated in North China and Northeast China, and the building types are mainly concentrated in office buildings, hotels, hospitals, shopping malls, schools and residential buildings.[3]
Development status of domestic heat pump technology
The research on the development and utilization technology of shallow geothermal energy resources in China really started from the late 1980s to the 1990s.1989Qingdao Institute of Architecture and EngineeringFirst, we set up a test platform in China to start the research work of ground source heat pump system.Since the introduction of ground source heat pump technology from the United States in 1997 as China's "11th Five Year Plan" scientific and technological breakthrough plan, the Ministry of Science and Technology, the Ministry of Construction and other ministries and commissions have supported and guided ground source heat pumps from the national level. The heat pump technology has attracted the attention of professionals and management departments. Researchers have published a large number of documents detailing the composition, working principle, classification and research status of ground source heat pumps,The key problems in the research and development of ground source heat pump are pointed out, and the development trend of heat pump is predicted.[3]
In the 21st century, heat pump technology has been attached great importance and has developed rapidly in application research.With the continuous strengthening of renewable energy application and energy conservation and emission reduction in China《Renewable Energy Law of the People's Republic of China》As well as other laws and supporting policies, heat pump technology has been highly valued by the national and local governments.In 2005, the Ministry of Construction launched the national standard《Technical code for ground source heat pump system engineering》At the same time, many scientific researchers have carried out in-depth research on key technologies such as different backfill media, different water flow rates, different connection methods and heat transfer models, providing multi-directional scientific support for optimizing the buried pipe heat exchanger to improve the efficiency of shallow geothermal energy development and utilization.In 2004, the Ground Source Heat Pump Research Institute of Shandong Jianzhu University was the first to develop a portable test instrument for thermal properties of rock and soil, which is the size of a suitcase. In 2008, Beijing Municipal Bureau of Geological Exploration developed a vehicle mounted shallow geothermal energy thermal response tester, which has been widely used in engineering practice.In 2006, the "First China"Ground source heat pumpThe "High level Forum on City level Application of Technology" was held in Beijing. The forum focused on the relationship between the promotion and application of ground source heat pump technology and future urban economic development, as well as the experience of regional promotion and application of ground source heat pump technology at home and abroad.In 2007, the Ministry of Land and Resources held the first field experience exchange meeting of shallow geothermal energy - national geothermal (shallow geothermal energy) development and utilization. After the meeting, a collection of papers was published. In 2008, Wei Wanshun presided over the publication of Beijing Shallow Geothermal Energy《Evaluation of shallow geothermal energy resources》This is the first monograph with shallow geothermal energy resources as its title and theme in China.In 2009, the revised version of Engineering Technical Specifications for Ground Source Heat Pump System issued by the Ministry of Construction was officially implemented, incorporating underground conditions into the implementation rules of ground source heat pump.2010China Geological SurveyThe Evaluation Specification for Shallow Geothermal Energy Resources has been formulated to make specific requirements for regional exploration and evaluation of shallow geothermal energy resources and project site exploration and evaluation. The introduction of these two specifications provides a solid guarantee for efficient use of shallow geothermal energy resources and scientific management of heat pump systems.[3]
Key problems in the development and utilization of shallow geothermal energy resources
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As a new technology, ground source heat pump system has entered the practical stage in Europe, North America and other countries. On the basis of experiments, various heat transfer model theories have been proposed, which are mainly applied to independent villas in rural areas where there is no other energy supply;Although China started relatively late and the number of projects has jumped to the forefront of the world in just a few decades, there is still a certain gap between China and foreign countries in technical research. In order to promote the efficient and rational development and utilization of shallow geothermal energy resources, the following research work should be strengthened as soon as possible.[3]
Investigation and evaluation of shallow geothermal energy resources
Although the shallow geothermal energy resources have the characteristics of wide distribution, huge reserves, shallow burial, and can be developed and utilized nearby, the development mode, system design, and utilization effect are restricted by the local geological, hydrogeological, meteorological, climate, and other conditions, which are manifested in different geographical locations, and the thermophysical properties of rock and soil mass are different, resulting in great differences in heat transfer efficiency,As a result, the energy-saving effect of some projects is not significant.
Therefore, the investigation and evaluation of shallow geothermal energy resources should be carried out as soon as possible in accordance with the Code for Geological Exploration of Shallow Geothermal Energy, and the suitable areas should be delineated to provide a basis for the sustainable, efficient and reasonable use of shallow geothermal energy resources and the preparation of special plans in the future.[3]
Supervision, management and environmental monitoring
Because the ground source heat pump system needs continuous cycle heat exchange, the original temperature field, chemical field and underground flow field will inevitably be changed. In addition, the redox environment of the water source heat pump system for groundwaterGroundwater microorganismThe balance and water quality will have a certain impact, so the relevant departments should mandatorily require the design of monitoring holes during the construction of heat pump projects to carry out long-term monitoring of ground temperature, groundwater level and water quality, ground elevation and other projects at different depths, so as to timely grasp the dynamic change of ground temperature, water and soil quality and ground deformation, and prevent geological environment problems.[3]
Experimental study on efficiency of shallow geothermal energy development and utilization
The thermophysical properties of rock and soil mass are the key factors of shallow geothermal energy resources and heat pump operation efficiency, while rock and soil mass have the characteristics of porosity and random structure, which is reflected in the difference of the thermophysical parameters of the stratum between different geological units and even between the upstream and downstream of the same geological unit, which can lead to the calculation results being difficult to meet the actual engineering use,In addition, the development and utilization of shallow geothermal energy resources in China has a trend of regionalization and large-scale development.
Therefore, it is necessary to strengthen the experimental research and simulation analysis of various rock and soil mass structures, thermal physical parameters of strata, formation mechanism of shallow geothermal energy and heat transfer process, and establish corresponding heat transfer models toGround source heat pumpThe engineering design provides correct and reliable technical data.[3]
Establish a technology R&D and training base equivalent to the international level
With the support of national energy conservation and emission reduction policies, the development and utilization of shallow geothermal energy resources has developed rapidly, and has jumped to the forefront of the world. Relevant scientific research is also very active. However, compared with European and American countries, there is no national comprehensive utilization laboratory of shallow geothermal energy in China to carry out special research on thermal properties, heat transfer mechanism and monitoring of rock and soil mass.[3]
Environmental assessment of shallow geothermal energy utilization
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Shallow geothermal energy is a kind of clean energy.In the exploration of shallow geothermal energy, based on the identification of shallow geothermal energy resources, the environmental impact of the design, construction and operation of shallow geothermal energy utilization projects should be evaluated according to the selected mining and utilization plan. The possible positive and negative environmental effects caused by the construction and operation of the system should be evaluated and predicted to provide a scientific basis for the management of the geological environment.The scope of environmental impact assessment for shallow geothermal energy utilization shall be based on the principle of meeting the needs of geological environment protection, and shall be determined according to the nature, project scale, layout, production process of shallow geothermal energy utilization system, combined with local environment, geological conditions and other factors through comprehensive analysis.Based on the investigation of the current situation of the geological environment within the assessment scope, the impact of shallow geothermal energy utilization on the atmospheric environment is assessed;Influence of groundwater heat exchange system on shallow groundwater;The impact of chemical components in the discharged fluid on the groundwater environment, whether land subsidence, karst collapse and ground fissures can be generated;The impact of circulating water leakage of soil source heat exchange system on groundwater quality, especially the impact of shallow geothermal energy development on shallow geothermal field should be evaluated.
On the basis of environmental assessment, measures to prevent the negative impact of shallow geothermal energy development are proposed.The work program for environmental impact assessment of shallow geothermal energy development includes prying to collect and sort out historical data and establish environmental impactEvaluation index system。Set up observation points to collect system data such as water quality, water level and temperature.Obtain the environmental impact assessment parameters, establish the environmental prediction model, and correct the model to assess the environmental impact of shallow geothermal energy utilization.
The assessment of atmospheric environmental impact should evaluate the environmental benefits of developing shallow geothermal energy in reducing air pollution and cleaning the environment.Include estimates of reduced emissions of combustion products, such assulfur dioxideEmissions, nitrogen oxide emissions, carbon dioxide emissions, coal dust emissions, etc.According to the current internationally accepted calculation method, burning 1 ton of coal will produce 22.7kgcarbon monoxide、3.67kgcarbon dioxide。In addition, about 60kg of sulfur dioxide, about 10kg of dust (total suspended load) and 11kg of other substances are also generated.The reduction of greenhouse gas emissions can be calculated and evaluated according to the amount of coal replaced by the development and utilization of shallow geothermal energy.
The groundwater environmental impact assessment shall be based on the change of circulating water quality during the operation of shallow geothermal energy system to assess the groundwater quality.The evaluation should be made on the geological environment problems such as land subsidence, karst collapse and ground fissures during the utilization of shallow geothermal energy.In accordance with the principle of maintaining geothermal balance in the thermal stratum within a hydrological year, evaluate whether there will be thermal pollution or the ground temperature will continue to decline.Ground source heat pumpThe project shall not affect the original local ecological environment and shallow geothermal development in adjacent areas.Radioactive pollution assessment can be carried out according to the regulations on radioactive protection.
The development and utilization of geothermal energy cannot only consider the current demand for energy. The ecosystem on which human beings depend for survival has a basic demand for geothermal energy, which can be called "ecological heat".Judging from the limited research results at present, the ecosystem is relatively fragile in areas where the earth heat flow is lower than 57mw/mZ. If human beings develop geothermal from these areas by heating, they may risk aggravating the local ecological degradation.However, it is possible to improve the local ecological environment by using this area as a cold source to develop a refrigeration system and transferring excess heat from the surface to the ground.Therefore, the area where geothermal energy is developed only for heating should be the area where the terrestrial heat flow is higher than 57mw/mZ, and the area where the terrestrial heat flow is lower than 57mw/mZ should maintain the balance between heating and cooling.[5]
Economic evaluation of shallow geothermal energy utilization
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Economic evaluation should be carried out in shallow geothermal energy evaluation of ground source heat pump project.The content includes: prediction and evaluation of investment, operation, income and benefit.
Ground source heat pumpThe project includes three parts: equipment system of machine room, outdoor heat exchange system and indoor air conditioning terminal system.Including: equipment system in machine room includesGround source heat pump unitElectric automatic control device, circulating pump unit, full-automatic constant pressure instrument, softened water and make-up water system device and auxiliary equipment, pipelines, valves, etc. in the machine room.The outdoor heat exchange system includes drilling, heat exchange pipe installation, and ground connection pipeline from the heat exchange site to the machine room.Indoor air conditioning terminal system includes indoor pipes, air ductsFan coilEtc.The investment estimate mainly includes civil works, pump room equipment, drilling and preliminary demonstration fees (design fees, supervision fees, etc.).
The operation cost includes various costs required to maintain the normal operation of the system.It mainly includes the power consumption cost for facility operation, and others include maintenance cost, overhaul cost, labor cost, management cost, etc.Income calculation: calculate the heating and cooling area according to the building area.The heating fee and cooling fee are calculated according to the local charging standard, and the total heating fee and total cooling fee that should be charged each year are calculated.According to the initial investment and operation cost of the project, the static andDynamic payback period。At the same time, we should also calculate theFinancial internal rate of return, financial net present value, net present value rate and other economic parameters.Fully evaluate the investment income of the project.Benefit comparison: estimated separately according to local oil, gas, coal and other pricesGas fired boilerHeatingCoal fired boilerHeating andOil fired boilerThe initial investment and annual operation cost of heating are compared with the total cost of heating and cooling of ground source heat pump project.[5]
expectation
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The key to the utilization of ground temperature lies in the conversion technology of resources. Its technological breakthrough can be compared with the social and economic benefits brought by the invention of the steam engine.The development of this technology in Changchun, China has yielded fruitful results. It is a major leap in China's utilization of new energy resources and heating and cooling projects, and a new economic growth point. The ground temperature heat exchange technology can be widely used for heating, cooling and heat supply of urban living quarters, office buildings, office buildings and various public facilities.The geothermal heat exchange technology makes full use of renewable and sustainable geothermal resources on the surface of the earth's crust, which is characterized by less energy consumption, "small footprint" and pollution-free, and can replace various boiler heating, gas heating and power consuming central air conditioning.First, heating does not produce air and environmental pollution caused by coal, oil and gas.
Second, refrigeration can reduce the high hanging air conditioners in high-rise residential buildings in cities, and save a lot of electricity costs for each household due to refrigeration. People don't have to worry about buying one, two, or three air conditioners, and it also solves the problem of temperature increase in downtown areas caused by indoor air conditioning cooling on outdoor heating.Extensive application of geothermal heat exchange technology can bring huge economic, social and environmental benefits.[6]
