(Tianjin Geothermal Exploration and Development Design Institute)
Abstract: Shallow geothermal energy refers to the heat energy widely existing in shallow strata (from the constant temperature layer to hundreds of meters), and it is a low-grade energy source (< 25℃). However, due to its advantages of wide distribution, large reserves, remarkable energy saving and recycling, it has attracted more and more attention. With the development of heat pump technology, shallow geothermal energy is considered as the most realistic and promising new energy for building heating (cooling). This paper mainly introduces the resource potential, development and utilization prospect and optimized mining scheme of this kind of energy.
1 Introduction
With the improvement of living standards, people have higher and higher requirements for indoor comfort. The proportion of building energy in China's energy demand has also increased from 1/6 to 1/4, in which the energy consumption of heating, refrigeration and domestic hot water accounts for a considerable proportion. The growing contradiction between energy supply and demand and the enormous pressure brought by the use of traditional energy sources make people have great enthusiasm for the development of green energy such as wind energy, solar energy and geothermal energy. Shallow geothermal resources are highly regarded in the world because of their huge reserves, wide distribution and recycling. Heat pump technology, which mainly uses shallow geothermal energy, has been quite mature in developed countries such as Europe and America after decades of development. Therefore, some energy experts at home and abroad believe that collecting low-temperature renewable energy from nature, especially shallow geothermal energy (heat energy), is the most realistic and promising technology to replace traditional heating (refrigeration) in 2 1 century.
2 characteristics of shallow geothermal energy
Shallow geothermal energy refers to low-grade thermal energy stored in shallow strata hundreds of meters underground, and the temperature is between the reference temperature (the temperature of the local constant temperature zone) and 25℃. It is the result of the upward conduction of deep heat energy and solar radiation. The depth of constant temperature zone in each region and its corresponding reference temperature are mainly related to latitude, and the reference temperature is equivalent to the local annual average temperature. China has a vast territory, and there is a big gap between north and south: the measured depth of stratosphere in China is between 15 ~ 30m, and the reference temperature is between 10 ~ 23℃.
Shallow geothermal energy is characterized by huge reserves, wide distribution and stable parameters, which are hardly affected by environment and climate. It is a renewable and clean energy with relatively constant energy, and it is also the geothermal resource layer with the most application of heat pump technology [1].
3 Tianjin shallow geothermal energy reserves estimation
Tianjin covers an area of11919.7km2. Except for the northern mountainous area of 775km2, the vast areas are covered by loose Cenozoic strata. However, the thickness of the loose layer north of Baodi fault is only 100 ~ 300m, the temperature at the bottom of the loose layer is generally less than 20℃, and the shallow low-temperature energy reserve is limited, so it is not calculated here. Within the range of 8700km2 south of the fault, the thickness of loose layer is greater than km, the buried depth of constant temperature zone is about 30m, and the reference temperature is 13.5℃. Affected by the fluctuation of bedrock surface and fault structure, the average geothermal gradient of caprock is between 2 ~ 8.3℃/ 100 m; At 25℃, the maximum buried depth is 600m (except the area north of Baodi fault), and the minimum buried depth is170 m. According to the previous geothermal geological data, it is estimated that the shallow low-temperature energy reserves within the range of 8700km2 reach 3.6×10/9j, which is equivalent to the power generation1.
4 shallow geothermal energy development and application examples
Under the situation that our government proposes to speed up the development of circular economy and build a conservation-oriented city, some public buildings such as government agencies, hospitals and schools should give priority to the use of renewable energy such as shallow geothermal energy, solar energy and biomass energy to partially replace or supplement electric energy, so as to change the single situation of urban energy structure, promote the diversification of energy structure and effectively alleviate the power shortage problem. For example, Beijing began to use shallow geothermal energy a few years ago. At present, the total construction area of replacing coal-fired boilers with shallow geothermal energy has reached 2 million square meters, and it is expected to reach 3 million square meters in 2006.
Tianjin is one of the earliest cities in China to develop and utilize geothermal resources, but the deep geothermal resources are facing increasing exploitation pressure due to slow replenishment, and the exploitation in some areas has been strictly restricted; With the strong support of relevant departments, the development of heat pump technology that can recover shallow geothermal energy is ushered in a rapid development stage.
The fuel conversion project of Tianjin Geology, Minerals and Jewellery Company successfully adopted the new technology of deep energy storage and anti-seasonal recycling, which met the requirements of about 6200m2 buildings in warm winter and cool in summer. Two wells in the same layer were dug (well spacing is 34m) with a depth of about 200m, and shallow geothermal energy was utilized (20 ~ 100 m in Q3 formation of Upper Pleistocene, water temperature14). Middle Pleistocene Q2 formation 1 10 ~ 140m, water temperature15.5℃; 190 ~ 230 m, water temperature 18℃), and the underground water is used as a carrier to contact the deep energy storage stratum and the above-ground energy conversion equipment system for energy conversion, and the irrigation yield is stable at 40 ~ 50m3/h, basically reaching a balance. After the completion of the system, the operation shows that although the outdoor temperature changes greatly in winter and summer, the indoor temperature is controlled at about 20℃ in winter and 26℃ in summer, which can not only fully meet the temperature requirements of the project in different seasons, but also save about 270 thousand yuan in operating expenses and increase the income of office buildings and hotels by about 500 thousand yuan every year. At the same time, the annual emissions of pollutants such as sulfur dioxide, nitrogen oxides, carbon dioxide and coal dust are reduced by about 4800kg, 1400kg, 120m3, 1 100kg respectively, and the economic and environmental benefits are quite remarkable.
Ancient Culture Street, Haihe Business District, downtown Tianjin. In this project, three deep wells of 900 ~ 1000 m are used as heat source wells, and three shallow wells of 400 ~ 450 m are used as cold source wells, forming three production and irrigation well groups with different layers, and the well spacing is 5 m. The three well groups are arranged in a triangle, and each group is 270 meters apart. In summer, the cold water temperature of the fourth water-bearing group in shallow wells is 19 ~ 20℃, and the tail water at 38℃ is pumped into deep wells for heat storage through heat pump refrigeration. In winter, the underground hot water and water temperature of Minghuazhen Formation in deep wells are 36 ~ 4 1℃, which are supplied by heat pump, and the tail water at 9℃ is reinjected into shallow wells for cold storage. The project has been running safely for nearly two years, and the effect is good. Although this kind of stratified mining and irrigation is still a new thing, it can realize the balance of mining and compensation through one mining and one irrigation, realize the anti-season energy storage, not only make the valuable geothermal resources sustainable use, but also control the land subsidence by adjusting the recharge pressure, and also make full use of the shallow geothermal energy with low temperature and high temperature for circulating heating and refrigeration, which is the real embodiment of circular economy.
5 problems and countermeasures in the utilization of shallow geothermal energy
The main purpose of utilizing geothermal resources is to make use of the characteristics of the earth's surface constant temperature layer to make meaningful application and development for human survival and activities. The development and utilization of geothermal resources are mainly based on water (or fluid medium), which can be utilized by direct exploitation of recharge groundwater or artificial injection circulation and formation heat exchange.
The traditional energy collection method of shallow groundwater at home and abroad is pumping well. Its advantage is that the water temperature of pumping well is basically stable during the operation cycle of heat pump, which can make the heat pump unit run close to the rated load. However, pumping wells or multiple wells not only increases the number of wells, but also has the main disadvantage of low recharge rate. However, recharging in different places changes the distribution of groundwater, which is easy to cause desanding and siltation (especially in the construction with poor drilling and well completion technology, when the sand content of groundwater exceeds the standard, hundreds of thousands of tons of water will be circulated in a heating period, which can take away more than several tons of sediment). In order to overcome the difficulty of groundwater recharge, relevant scientific research departments independently developed the technology of "single well pumping irrigation" in combination with practice. This shallow geothermal energy (heat) collection technology has been widely used in all kinds of building heating (cooling) combined supply projects, reaching 2 million m2 nationwide, and achieved good results. At present, many related experiments and studies have been done on low-temperature heat collection of shallow soil at home and abroad. There are three main acquisition methods: horizontal buried pipe (straight pipe and spiral pipe), vertical buried pipe (U-shaped pipe and spiral pipe) and buried pipe of foundation pile (integrated with building foundation pile) [3].
6 matters needing attention in the development of shallow geothermal energy
Although it is convenient to collect the low-temperature energy of surface water (rivers, lakes, oceans, etc.). Due to the influence of climate, the temperature changes greatly, especially in the cold areas in the north, where the water temperature and temperature are very low. Therefore, the problems of freezing and antifreeze must be considered in energy collection. At the same time, due to the low temperature, the energy efficiency ratio (COP) of the heat pump system decreases, and its rated heat output power is affected. Although the energy collection of shallow geothermal energy (groundwater and soil) is not as convenient as surface water, its low-temperature energy is relatively stable, the temperature level is slightly higher than the annual average temperature of the local climate, and it is basically unchanged in spring, summer, autumn and winter. As long as the technical means of energy collection are appropriate and reasonable, the energy balance of heat pump system is relatively stable.
Although shallow geothermal energy has huge reserves and can be regenerated in four seasons, its development and utilization should be selective according to different hydrogeological conditions in specific areas, such as where it is suitable to collect low-temperature energy from shallow groundwater and soil, how large the collection scale is, and how to use it. Otherwise, it will not only affect the economy and safety, but also cause the destruction of underground hydrogeological conditions. Therefore, in the supporting design and construction of the project, we should not only consider the stable and reasonable collection of energy, but also pay special attention to environmental benefits, not to pollute groundwater, not to destroy underground geological structure, to protect groundwater resources [1], and pay attention to the following aspects:
(1) When collecting geothermal energy, we must only use its heat without consuming its moisture, and all the heat must be reinjected after use, and the implementation of reinjection should be monitored.
(2) The depth of the groundwater collection well should be limited within 400m of the shallow surface, away from the national second-class water quality area below 400m, and protect the drinking water area. The drilling depth should be determined according to the specific hydrogeological conditions, and the shallow groundwater within underground 100 meters should be used as much as possible. When using underground water source heat pump, the choice of water source should be: sufficient water, suitable water temperature, good water quality, constant water supply and convenient recharge. Specifically, it should be considered that the depth of groundwater intake is mostly within 100m, the thickness of each aquifer should generally be more than 5m, the temperature of groundwater should not be lower than 10℃ in winter, and the sediment concentration of groundwater is small.
(3) Recharge as close as possible. Under the condition of not affecting thermal power, actively promote the "single well pumping and irrigation" technology. If the condition of "single well pumping and irrigation" is not available, pumping and irrigation can be adopted for lateral wells. The position of the opposite well should be far away from the urban regional water supply station (the distance should be greater than 100m) and located in its downstream area, and the recharge well should be located in the upstream area of the pumping well with a well spacing of 65438. It is generally equivalent to the depth of pumping well (reinjection in the same layer), and at the same time, the circulation system is strictly closed, and the quality of reinjection water is closely monitored to prevent pollution, sediment discharge, evacuation and siltation.
(4) Strictly examine and approve the multi-wells that are combined with pumping and irrigation, limit the number of wells, prohibit over-exploitation of groundwater, eliminate waste and pollution of water resources, and effectively and rationally utilize groundwater resources.
(5) Reasonably design the spacing between wells and the relative position between wells and buildings. The system adopts large temperature difference and small flow rate to reduce power consumption.
refer to
[1] Chengren. Development and utilization of shallow geothermal energy (heat) 38880.88888888686
[2] Paragraph. Beijing will promote the use of shallow ground temperature. Geological Exploration Guide of China Geological Environment Information Network .2005