Abstract: In view of the current situation of energy utilization in our country and the demand of building green building, this paper introduces the advantages and disadvantages of soil source heat pump and its application scope. In view of the current situation and development background in Tianjin, The advantages and problems that should be noted.
Key words: Ground source heat pump Scope of application Tianjin area
Abstract: Aiming at the actuality of energy using in our country and the request of green building, this paper introduce the advantage and disadvantage of Geothermal Heat Pump and its using areas. With the background of Tianjin area, we point out the advantages of Tianjin area in the development of Geothermal Heat Pump and some present issues
Keywords: Geothermal Heat Pump using area Tianjin area
1 Introduction
In today's world, environmental pollution and the energy crisis have become the top priorities that threaten human survival. With the improvement of human living standards, central air conditioning energy consumption accounts for the proportion of the world's energy consumption continues to increase, and most of the central air conditioning energy consumption is consumed in the cold source. Therefore, the majority of professionals at home and abroad have been working hard to seek efficient and environmentally friendly air-conditioning cold and heat sources, building green buildings.
For green building should be reflected in the following aspects [1]:
1, the symbiosis of architecture and nature: that is, the requirements of maintaining the environment, using the environment and preventing natural disasters. Protect the ecological system and reduce emissions of CO and other atmospheric pollutants, and maintain the balance of the ecological environment around the building; make full use of solar energy, geothermal energy for heating, heating, lighting and ventilation, give full consideration to green configuration, soften the artificial building environment; Consider the direction of the building and so on.
2, the application of building energy-saving technologies to reduce the environmental load: that is, to reduce energy consumption, extend the service life, the use of environmentally friendly materials, focus on energy reuse, the use of durable building materials and recyclable materials.
3, recycling-based construction career. Recycling runs through the entire building life.
4, to create a healthy and comfortable indoor environment. Including a healthy and lasting living environment, excellent air quality and so on.
5, so that the building into the history and culture of the human environment. Inherit and protect the landscape features of cities and regions, and maintain the permanent charm and vitality of the city.
Ground-source heat pump system is a use of shallow underground heat resources (also known as geology, including groundwater, soil or surface water, etc.), by entering a small amount of high energy (such as electricity) , In order to achieve energy-efficient green air-conditioning system that can both heat and cool.
In building green buildings, ground-source heat pumps are a form of system that can be used in a wide range of applications.
2 ground source heat pump advantages and disadvantages and classification
2.1 The advantages of ground-source heat pump
Soil source heat pump using the characteristics of soil temperature stability throughout the year, the winter soil heat pump as a heat source, that is, higher than the ambient temperature in the energy out of the heat to supply indoor heating, summer soil can be used as a cold source of air conditioning , That is, to remove the heat inside the room to release the soil at the end of the ambient temperature [2]. Usually, the ground-source heat pump consumes 1 kilowatt (kW) of heat, and the user can get about 4 kilowatts of heat or cold so as to achieve the purpose of saving energy. In addition, no harmful substances are generated during the operation of the system, and the environmental protection effect is achieved. Ground source heat pump working principle shown in Figure 1:
　　
Figure 1 ground source heat pump working principle
A-underground circulation loop; B-heat pump working loop; C-indoor heat exchange loop
The system has the following characteristics [3]:
1, renewable resources, soil-source heat pump technology using the Earth's surface shallow geothermal resources (ground energy) as a cold and heat source for energy conversion, while the shallow surface is a huge solar collector, collecting 47% of the solar energy, Equivalent to more than 500 times the amount of energy used by mankind each year, and is not limited by geographical areas, resources, etc., and it is truly vast in quantity and ubiquitous. This is an almost unlimited renewable energy stored in the shallow surface of the earth and a clean energy source. Compared with the ambient air on the ground, the soil temperature below 5m above the ground is basically stable throughout the year and slightly below the annual average temperature, which can provide relatively lower condensation temperature and higher evaporation temperature respectively in summer and winter. Therefore, thermodynamically speaking, soil is a hot and cold source of heat pump system better than ambient air. And ground source heat pump system will not heat, water vapor and bacteria in the atmosphere, in line with the current strategic requirements for sustainable development. Usually ground source heat pump consumes lKW of energy, users can get more than 4KW of heat or cold, the extra energy is the energy from the soil. In addition, the relatively constant temperature to the nature of the characteristics of the heat pump makes the operation more reliable and stable, but also to ensure system efficiency and economy. According to the EPA of the United States Environmental Protection Agency, the design and installation of a good ground source heat pump, on average, can save users 30% to 40% of the heating and cooling air conditioning operating costs. Efficient ground-source heat pump units produce an average of only 1 cold ton of cold energy consumption of only 0.88kW power consumption, the power consumption is only ordinary chillers plus boiler system 3O% ~ 60%;
2, less investment, low operating costs, compared with the traditional air-conditioning system, the one-time investment can save 15% to 25%, annual operating costs can save about 40%. With the ground-source heat pump system, the ground-source heat pump can have higher efficiency and better reliability than the air-cooled heat pump because of the temperature of the soil. The temperature of the heat source is relatively stable throughout the year, generally at 10-25 ° C. And the ground source heat pump system can be used for heating, air conditioning, but also provide hot water, a system can replace the original boiler, air conditioning and refrigeration systems or systems, a multi-purpose machine; not only for hotels, shopping malls, office buildings, schools and other buildings , More suitable for heating and air conditioning in the villa house. In addition, the long life of the unit, are in 20 years or so; compact unit, saving space: low maintenance costs; a high degree of automation and control, unattended. The heat source in the ground source heat pump does not refer to the hot gas or hot (warm) water in the geothermal field, but refers to the normal ambient temperature soil. Therefore, there is no special requirement for underground heat sources and it can be applied in most areas of China.
The COP value of the soil source heat pump system is generally about 3 to 6, which is about 40% higher than that of the conventional air source heat pump, and the operating cost thereof is 50% -60% of the normal central air space.
3, covers an area of ​​?? less, the engine room covers an area of ​​small, space-saving, can be located in the ground;
4, green, ground-source heat pump system uses shallow geothermal resources on the surface of the Earth, there is no combustion, no smoke and waste, environmental plot, without any pollution, soil-source heat pump pollutant emissions, compared with air-source heat pump, To reduce by more than 40%, compared with electric heating, equivalent to a decrease of more than 70%, if combined with other energy-saving measures will be more obvious energy-saving. Although the refrigerant is also used, it is 25% less than the conventional air conditioning unit. The ground source heat pump system is a self-contained system, which can be packaged and sealed well in the factory workshop. Therefore, the probability of refrigerant leakage is high To reduce. The unit is operated without any pollution and can be built in residential areas. It is installed in green spaces and parking lots. There is no combustion, no smoke emission and no waste. There is no need to store fuel waste and do not need to transport heat over long distances. Ground source heat pump system without cooling towers and other outdoor equipment, there is no centralized land-based central air-conditioning problem, saving space and land, generating extra profits for developers, resulting in additional economic benefits and improve the external image of the building;
5, high degree of automation, the unit and the unit and the system can be automated, according to the outdoor temperature and indoor temperature control unit start and stop, to achieve the best energy-saving effect, while saving manpower and resources;
6, self-regulating unit, can arbitrarily adjust, investors can adjust the supply time and temperature as required, completely autonomous;
7, a multi-purpose machine, you can heating, but also cooling, waste heat generated in the cooling can also provide live production of hot water or heating for the swimming pool, maximize the use of energy.
2.2 The disadvantage of ground source heat pump
1, buried heat exchangers are greatly affected by soil properties, soil thermal performance, energy balance, soil heat transfer and humidity have a greater impact on heat transfer [4];
2, continuous operation of the heat pump condensation temperature and evaporation temperature fluctuations in soil temperature fluctuations;
3, smaller soil thermal conductivity, heat exchange capacity smaller. Experience has shown that its continuous heat absorption rate is generally 25W / m2, so when a certain amount of heat, heat exchanger coil covers an area larger, buried pipe laying either horizontal excavation arrangement or vertical drilling installation , Will increase the cost of civil engineering.
2.3 Ground source heat pump classification
2.3.1 according to the underground pipe system classification
Underground buried pipe heat pump system according to the form of buried pipe can be divided into horizontal buried heat pump and vertical buried heat pump. Horizontal pipe is often used in engineering examples, and more for heating. The vertical buried pipe generally considered its performance better than the horizontal buried pipe system, but the construction of a relatively high degree of difficulty.
1, buried pipe heat pump system
Research on horizontal borehole heat pumps began in 1930-1940. Such systems, commonly used in Europe today, are mostly used for heating purposes only. Horizontal coil system has a single layer and double the two forms, can be U-shaped, snake-shaped, single-slot single-slot single tube and other forms. Monolayer is the earliest and most commonly used form, the general design depth of buried pipe between 0.5m to 2.5m. Due to different soil saturation, the depth of the trenches is also different. If the entire winter soil are in saturation, the depth of the trench must be greater than l.5m at the same time for heating. Tube burial depth of more than 1.5m heat storage slow, and less than 0.8m, the coil will be affected by ground cooling and freezing, the other tube spacing of less than 1.5m, coil may produce solid ice crystals and reduce the heat storage in the spring . The double-layer coil system is about 1.2m deep in one layer and about 1.9m deep in the other layer, that is, laying a layer of pipe at a height of 1.9m and then backfilling to a depth of 1.2m to lay another layer of double-layer laying drastically reduced Excavation depth and fill the amount of sand required.
2, vertical buried hot spring system
Vertical buried hot spring system has two kinds of shallow buried and buried. Shallow depth of 8m to 10m. Installed in a ring, hexagonal or rectangular shape, and the use of coaxial flexible sleeve. This design was proposed by Ambrose in 1946 to increase the shell diameter to improve heat exchange performance. The depth of the deep borehole is determined by the drilling conditions and economic conditions of the site, generally ranging from 33 m to 180 m. The solution circulates in a vertical U-bend. In order to reduce the pump capacity, it is necessary to use parallel buried in the vertical buried pipe system. Pipeline into the ground, the soil thermal characteristics will not be affected by the surface temperature. Therefore, it can ensure the heat balance between winter heat and summer heat. A balanced approach would be to use a heat collector to concentrate the heat in the summer and into the ground to heat the soil or to reverse the heat pump to heat the soil in the summer to prepare it for winter. Collector equipment generally uses solar energy and wind energy collector, such equipment has high thermal storage capacity, temperature rise capability.
Vertical buried heat pump system than the horizontal system has many advantages. First of all, it does not require as much floor space as a horizontal pipe system. Second, in many areas, the soil is saturated with moisture at a distance below the surface, and this distance is exactly the location of the heat exchanger and is therefore beneficial to heat exchange. During the cooling season, the solution from the horizontal system flowing into the coil heats the saturated soil layer to reduce the moisture content, thereby reducing the soil thermal conductivity. So that the heat exchange efficiency is also reduced. In vertical tubes, this moisture transfer is only a small fraction. And the stability of the vertical heat pump tube and part of the load operating efficiency than the full load condition, and the general air conditioning system design conditions at full load, but the actual but rarely operate in this case, the efficiency Difficult to guarantee is in the high-performance area.
2.3.2 by whether the intermediate fluid classification
According to the presence or absence of intermediate fluid classification, the ground-source heat pump is divided into a primary fluid, an even heat pump, that is, there is an intermediate fluid between the refrigerant and the earth, mostly water, brine or ethylene glycol solution; System is a direct-expansion, even-heat pump system that utilizes a large amount of refrigerant to directly exchange heat with the environment in an underground coil.
2.3.3 Press and soil surrounding pipe material classification
Pipe materials used in soil systems are now mostly made of hot-melt plastics, including polyethylene pipes, polybutylene pipes and polyvinyl chloride pipes (PVC). Visible, PVC pipe thermal conductivity is relatively low, it is not suitable for such systems under the thermal conductivity of materials. Experiments have shown that if the soil thermal conductivity is doubled, the heat exchange of the polyethylene pipe is increased by 25% in continuous operation, while the PVC pipe is only increased by 12%. Therefore, we should try to use high-density polyethylene material, although the metal has a good thermal conductivity, their performance is not much higher than the high-density polyethylene, but expensive. In addition, due to the high strength and corrosion resistance of HDPE, the lifetime of a ground-source heat pump system using this type of flexible material as a heat exchanger for underground boreholes can last up to 50 years. As the soil heat exchange and the diameter is not very obvious, so the choice of diameter is out of the pipeline pressure loss arising from the operating costs and pipeline cost compromise, generally take 20mm to 50mm.
3 soil source heat pump scope
Geothermal energy technology to the collection of geothermal resources, mainly concentrated in the shallow layer within 100 meters. This range of geological structure is diverse, both clay and sand, both coarse and fine sand sand, pebbles and sand, and some even bedrock. These different structures, the permeability and thermal conductivity are different, high seepage water only applies to water source heat pump technology, high thermal conductivity for ground-source heat pump technology.
4 Tianjin area application
4.1 Tianjin use of ground-source heat pump background
The existing system of heating (including bathing, drinking water, etc.) in Tianjin is basically undertaken by the coal-fired boilers of various administrative units. The heating pipe network is also arranged within the scope of this unit. Soot pollution is serious, energy waste, security risks more prominent phenomenon. In recent years, Tianjin has been carried out boiler room network transformation, so that the above-mentioned phenomenon is being gradually improved
4.2 Tianjin Port area using ground source heat pump status quo
Geothermal resources in Tianjin exploration and development started in the 1970s, the large-scale geothermal field, high reserves, equivalent to more than 50 megawatts of power. Tianjin geothermal direct use of the walk in the forefront of the country, to 2001, geothermal wells for heating more than 200 ports, geothermal heating area has reached 8 million square meters. The city has a good research foundation and engineering development experience in effectively reducing the geothermal tail water temperature, geothermal heat pump technology in medium and high temperature, geothermal energy utilization in the shallow surface of the ground, energy-saving technologies of both ground-source heat pumps that are both heating and cooling.
Representative projects include: Tianjin Meijiang residential area a comprehensive office building, construction area of ​​2991m2, building thermal load 147KW, building cooling load 320KW. After one month in winter and nearly three months in summer, the ground source heat pump system runs stably and reliably, and the overall effect has reached the expected practical goal. The indoor temperature in winter heating room is stable above 18 ℃. The air conditioning room in summer The indoor temperature is basically stable at about 25 ℃, have reached the design temperature requirements. [5]
4.3 The advantages and disadvantages of using ground-source heat pumps in Tianjin Port area
In addition to the common advantages of a practical ground-source heat pump when using ground-source heat pumps in Tianjin Port area, Tianjin Port has the advantages that are not available in many areas: Tianjin Port is located in a temperate zone with little difference between winter heating load and summer cooling load and is easy to balance , Will not lead to excessive accumulation of underground heat and cold.
Disadvantages and issues that should be noticed: 1. The stratum conditions in Tianjin Port are complex. About 80% of the area is covered by salt water and the depth of salt water is shallow. The buried pipe-type ground-source heat pump project in the construction site can easily lead to the formation of salt-freshwater series, resulting in groundwater pollution and high requirements for the sinking process. Moreover, the domestic equipment material, the level of production technology and foreign countries still have some gaps, once the circulating fluid temperature control leaks, will also cause serious pollution of groundwater. In the design and construction should pay attention to this issue.
2, in the northern cold areas, the liquid temperature of underground heat exchangers entering the ground in winter is generally below 0 ° C, and the soil with moisture content around the heat exchanger may freeze. According to the qualitative analysis, when the water is frozen, a large amount of latent heat is released. Therefore, when the same quantity of heat is absorbed, the temperature range of the soil reduction is small, and the more the water is, the more the latent heat is released and the smaller the temperature decrease is , The soil temperature near the heat exchanger buried tube is higher. Therefore, in the design to take into account the above factors.
5 Conclusion
Although the ground source heat pump still exists the initial investment and construction difficulties, the buried heat exchanger is greatly influenced by the soil physical properties. When the continuous operation, the condensation temperature or evaporation temperature of the heat pump fluctuates due to the soil temperature and the soil thermal conductivity is small Leading to large area of ​​buried heat exchangers and other shortcomings, but some well-known international organizations and researchers engaged in heat pumps are generally considered: Due to the wide range of soil resources, in the present and future, ground-source heat pump is the most promising energy-saving device And air-conditioning system, is one of the forefront topics in the international air-conditioning and refrigeration industry and an important form of energy utilization. In 1998, ASHRAE Technology Award from the American HVAC Institute awarded the application of ground-source heat pump.
At present, all countries in the world pay more attention to issues of energy, environment and sustainable development than ever before, and the traditional coal-based energy structure can not meet the needs of the current situation. Therefore, to achieve sustainable economic development, it is necessary to make full use of clean and renewable energy sources, and the ground source heat pump is a good choice because of its energy saving, environmental protection and operational stability. At the same time, after many years of research and development, the ground-source heat pump is also technically mature, and proven by a large number of practices: the ground-source heat pump does have the advantages of energy conservation, environmental protection, renewable resources, comfort and safety, and stable performance. At present, a large amount of energy resources available to us in underground soils are present in vast quantities and are almost unlimited in quantity and are not limited by geographical resources. If they can be utilized by a properly designed ground-source heat pump, they will be alleviated Because of its energy conservation and environmental protection, the ground source heat pump will become one of the most promising HVAC systems in the 21st century.
references
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[2] H. L. von cooper waiting. Prince translation. Heat pump theory and practice [M]. Beijing: China Building Industry Press, 1986.
[2] Jiang Nengzhao. Air conditioning heat pump technology. Beijing: Mechanical Industry Press, 1997.
[3] Shih-heng and others. DYNAMIC SIMULATION OF HEAT MIGRATION OF MULTIPLE COMPONENTS IN UNSATURATED SOILS. Journal of Engineering Thermophysics, 1998
[4] Zhao Jun, Zhang Chunlei, Wang Jian and so on. Ground source heat pump in the actual engineering application and research [J]. Tianjin Construction Science and Technology, 2003, (5): 14-16.
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