First, the heat pump and building heating and air conditioning With the economic development and improvement of living standards, public buildings and residential heating and air conditioning has become a universal demand. In developed countries, heating and air conditioning energy consumption can account for 25-30% of the total social energy consumption. The energy mix in our country depends mainly on fossil fuels, especially coal. A large number of pollutants generated by the combustion of fossil fuels, including a large number of harmful gases such as SO2, NOX and greenhouse gases such as CO2. The environmental problems arising from burning large quantities of fossil fuels have increasingly become the focus of attention of all governments and the public. China's heating has experienced a change from a small coal-fired boiler to a coal-fired boiler. Now further prohibit the construction of small and medium-sized coal-fired boiler rooms in urban areas, reflecting the government attaches great importance to the protection of the atmospheric environment. Therefore, in addition to the type of district heating, there is an urgent need to develop alternative alternative heating methods. Heat pump is a new technology for heating and air-conditioning that can effectively save energy, reduce air pollution and CO2 emissions.
Heat pump (refrigerator) is through the work of the heat from the low temperature medium to a medium of high temperature medium. Building air-conditioning system should generally meet the winter heating and summer cooling two opposite requirements. Traditional air-conditioning systems usually require separate sets of cold sources (chillers) and heat sources (boilers). Because of the need to have a cold source (chiller) in a building air conditioning system, boiler and boiler rooms can be eliminated if it is run in heat pump mode in winter, saving not only initial investment, but also the use of clean energy such as electricity throughout the year, Greatly reduce the air pollution caused by heating problems.
Using heat pumps to heat buildings can greatly reduce primary energy consumption. Normally we generate heat by burning direct fossil fuels (coal, oil, natural gas) and eventually heat the building through several heat transfer links. In the ideal case where there is no heat loss from the boiler and heating pipelines, primary energy utilization (ie, the ratio of the heat of the building heating to the heating of the fuel) can be up to 100%. However, burning fossil fuels typically produces temperatures in the 1500-1800 ° C range, which are high-grade thermal energy, whereas building heating ultimately requires low-grade thermal energy at 20-25 ° C; direct burning of fossil fuels for building heating means a significant amount Available energy loss. If the first use of high-temperature thermal energy combustion power generation, and then use the electric energy-driven heat pump to absorb low-grade thermal energy from the surrounding environment, the appropriate increase in temperature and heating the building, you can take full advantage of high-grade fuel energy, greatly reducing the Primary energy consumption for heating. The coefficient of performance of the heat-supply heat pump, which is the ratio of the heat supplied to the energy consumed, can now reach 3-4; the efficiency of thermal power plants can reach 35-58% (high value gas-fired combined cycle power station). The use of fuel power generation and then heat pump heating, under the conditions of the existing advanced technology, an energy efficiency can reach more than 200%. Therefore, the use of heat pump technology for building heating can greatly reduce the heating fuel consumption, not only saving energy, but also greatly reduce the emissions of CO2 and other pollutants caused by burning fossil fuels.
Low-temperature heat sources utilized by heat pumps can generally be the environment (air, surface water and ground) or a variety of waste heat. It should be noted that the heat absorbed by these heat sources by heat pumps is a renewable energy source.
Second, the classification of air-conditioning heat pump and its advantages and disadvantages of building air-conditioning (including heating and cooling) for the purpose of the heat pump system there are many, such as the use of building ventilation system heat (heat) heat recovery heat pump and application Water ring heat pump systems in different sections of large buildings. Here mainly discusses the use of the surrounding air conditioning as a heat source heat pump system. By their nature, foreign literature usually divides them into two categories: air source heat pump (ASHP) and ground source heat pump (GSHP). Ground-source heat pumps can be further divided into surface-water heat pumps (SWHPs), groundwater heat pumps (GWHPs) and ground-coupled heat pumps (GCHPs). Our country on the heat pump system terminology has not yet formed a normative usage. For example, commercial names for "ground temperature air conditioning" are used for groundwater heat pump systems; underground heat pumps are referred to in some literature as "ground source heat pumps," or directly as "ground source heat pumps."
Air source heat pump to outdoor air as a heat source. In the heat condition outdoor air as a low temperature heat source, absorb heat from the outdoor air, the heat pump to raise the temperature into the indoor heating. Air source heat pump system is simple, the initial investment is low. The main disadvantage of air-source heat pumps is that the efficiency of the heat pump is greatly reduced during high temperatures in summer and cold weather in winter. Moreover, the heating capacity decreases with decreasing outdoor air temperature, which is opposite to the trend of building thermal load demand. Therefore, when outdoor air temperature is lower than the balance of the heat pump temperature, you need to use electricity or other auxiliary heat source to heat the air. In addition, air-source heat pump evaporators frost on heating conditions and require regular defrost, which also consumes a lot of energy. Frosting of heat pump evaporators in cold and high humidity areas can be a major technical obstacle. In summer hot weather, as its cooling capacity decreases with outdoor air temperature, the same may cause the system can not work properly. Air source heat pump is not suitable for cold areas, in the more temperate winter climate regions, such as China's midstream and downstream areas of the Yangtze River, has been widely used.
Another heat pump using the earth (soil, groundwater, groundwater) as a heat source, can be called "ground source heat pump." Ground-source heat pumps overcome the technical barriers to air-source heat pumps because they remain constant throughout the year in undisturbed conditions, well above the outdoor temperature in winter and outside the summer, and the efficiency Greatly improve. In addition, in winter, the heat in the ground is raised by the heat pump to heat the building and the temperature in the ground is reduced, that is, the cold energy is stored for use in summer. In summer, the heat in the building is transmitted to The earth, cooling the building, while storing heat in the earth for winter use. This ground source heat pump system in the earth played the role of accumulator to further improve the air conditioning system annual energy efficiency.
Groundwater source heat pump system is the heat source from the wells or abandoned mine groundwater extraction. The heat-exchanged groundwater can be discharged into the surface water system, but for larger applications it is often required to recharge the groundwater to the groundwater level through the recharge well. Groundwater source heat pump system has been rapidly developed in our country in recent years. However, the application of such groundwater heat pump systems also suffer from many limitations. First of all, such systems require a rich and stable groundwater resource as a prerequisite. Therefore, before deciding to adopt the groundwater heat pump system, a detailed hydrogeological survey must be conducted and drilling wells must be drilled to obtain the data of underground temperature, groundwater depth, water quality and water discharge. The economics of a groundwater heat pump system have a lot to do with the depth of the groundwater layer. If the groundwater level is low, not only the cost of going to a well increases, but also the power consumption of the pump in operation will greatly reduce the efficiency of the system. In addition, although theoretically extracted groundwater will be recharged into the groundwater layer, at present, groundwater recharge technology in China is not yet mature and the rate of recharge under many geological conditions is much lower than that of pumping water. The water extracted from underground passes After the heater is difficult to be fully recharged into the aquifer, causing the loss of groundwater resources. In addition, it is also a thorny issue that how to ensure that the groundwater layer is free of pollution even if all the extracted groundwater can be recharged. Water resources are the most scarce and most precious resources at present. Any waste or pollution of water resources is absolutely unacceptable. Abroad due to environmental protection and the use of groundwater regulations and legislation more stringent, the application of groundwater heat pump has been gradually reduced.
A source of heat for surface water heat pump systems is surface water in ponds, lakes or streams. The use of these natural waters as low-temperature heat sources in heat pumps near large rivers, lakes and seas, etc., is a type of air conditioning heat pump to be considered. Of course, this surface water heat pump system is also subject to natural conditions. In addition, as the surface water temperature is greatly affected by the climate, similar to the air source heat pump, the heat pump heat quantity is smaller when the ambient temperature is lower, and the heat pump performance coefficient is also reduced. Certain surface water body can bear the heat load and its area, depth and temperature and other factors, need to be calculated according to the specific circumstances. The heat exchanger on the impact of water on the ecological environment sometimes need to be considered in advance.
The underground coupled heat pump system is a ground-source heat pump system that utilizes the closed circuit of heat in the underground rock. The name of "underground coupled heat pump" is literally translated from English, not popular. Often referred to as a "closed-loop ground source heat pump," to distinguish it from a groundwater heat pump system or directly as a "ground source heat pump." It is through the circulation of liquid (water or water as the main component of the antifreeze) in the closed underground pipe flow, the system and the ground to achieve heat transfer. During the heating process in winter, the fluid collects heat from the ground and brings it to the room through the system. The system operates in reverse during summer cooling, which takes the heat away from the room and sends the heat to underground soil through the system. Therefore, the underground coupled heat pump system maintains the advantages of groundwater heat pumps using the earth as a source of heat and heat, without the need to extract groundwater as a heat transfer medium. It is a sustainable new technology of building energy saving. In 1998, the United States Department of Energy promulgated regulations requiring the promotion and application of underground coupled heat pump heating and air conditioning systems in the construction of the federal government agencies throughout the country. In an effort to showcase this new energy-saving and environmental-friendly technology, the U.S. groundbreaking heat pump air-conditioning system was also installed by President Bush in his Texas home (see for example May 18, 2001).
Third, the ground source heat pump heating air conditioning system of economic analysis Ground source heat pump system can be achieved on the building heating and cooling, but also for domestic hot water, a multi-purpose machine. A system can replace the original boiler plus cooler two sets of devices or systems. Compact system, eliminating the boiler room and cooling tower, saving construction space, but also conducive to the appearance of the building. Another notable feature of the ground-source heat pump system is that it greatly increases the utilization of primary energy and therefore has the advantage of being energy efficient. Ground source heat pump than the traditional air conditioning system operating efficiency of about 40-60% higher. In addition, the relatively constant temperature of the ground source makes the operation of the heat pump unit more reliable and stable. The maintenance cost of the entire system is also greatly reduced compared with the boiler-refrigerator system, which ensures the high efficiency and economy of the system.
The obstacles that have hindered the application of underground coupled heat pump systems in our country are mainly high initial investment in buried underground pipes and lack of understanding of this technology by the government, architects and the general public. The economics of GSHP air conditioning systems depend on many factors. Different regions, different geological conditions, different energy structures and prices will have a direct impact on their economy. According to foreign experience, due to the low operation cost of ground-source heat pumps, the increased initial investment can be recovered within three to seven years. The average cost of the ground-source heat pump system during the entire service cycle will be lower than that of a traditional air-conditioning system.
IV Conclusion In the building heating and air conditioning using heat pump technology can effectively improve the primary energy efficiency and reduce emissions of greenhouse gases CO2 and other pollutants generated by combustion is a sustainable development of new building energy-saving technologies. Among the main heat pump systems discussed in this paper, the air-source heat pump has the lowest initial investment, but its efficiency is low and its application conditions are limited. It will still be used more often in some areas with mild winter temperatures. Groundwater heat pump and surface water heat pump system is subject to water resources conditions, the scope of application is limited. Ground source heat pump (underground heat pump system) a wide range of applications, low operating costs, energy saving and environmental benefits significantly. Ground-source heat pump has been widely used in many countries in North America and Europe and is a mature technology. However, our country has just started the application of ground-source heat pump. The promotion of ground source heat pump technology requires government policy guidance, training of design and construction personnel, equipment and materials needed, and public awareness of ground source heat pump technology. The main constraints in the application of heat pump technology in heating and air conditioning used to be the lack of electricity supply and the low level of people's consumption. The market demand for heat pump air-conditioning systems has not yet been established. With the economic development and improvement of people's living standards in our country since the reform and opening up, the above two constraints have ceased to exist. Air conditioning and heat supply have become the needs of ordinary people and have gradually expanded to the rural and southern areas with good market prospects. Through the joint efforts of government departments, research institutions and engineering and technical personnel, learn from the successful experience of other countries, China's application of ground source heat pump will be faster to promote and develop.