Probe into the Design of Geothermal Heat Pump System

As a heating and cooling system utilizing renewable energy, ground energy heat pump system is rapidly being promoted domestically because of its advantages of energy conservation, environmental protection and stability. However, in the promotion, due to differences in understanding of the system, the groundwater heat pump system design and research, the basic exploration period, due to some manufacturers of non-optimal design, also led to the ground to heat pump system operating efficiency decline. In this paper, the development of geothermal heat pump system is introduced, the design steps of geothermal heat pump are introduced, and some common problems in the design are analyzed. It is pointed out that the design of geothermal heat pump system needs an overall optimization plan, rather than a simple optimization of indoor heat pump system, Simple combination of unit and tube. 1. Background Geothermal heat pump systems (Geothermal heat pump systems) are closed-circuit circulation systems that utilize underground geotechnical rock as a heat accumulator and use heat pump principles to achieve quarter-heat storage and summer heat storage. Ground to heat pump system generally consists of three parts: earth heat exchanger, heat pump host and air conditioning terminal. The key of the system is the design and construction of earth heat exchanger. Geothermal heat pump systems are popular in North America and Europe. According to the statistics of The geothermal heat pump consortium, by the end of 2003, the ground floor area using heat pump technology for cooling and heating in the United States was 37.2 million square meters 20 million square meters in Sweden, 5.6 million square meters in Germany and 4.35 million square meters in Canada. However, North American applications and the application of Europe there is a clear difference. North American applications, ground heat pump more emphasis on building air-conditioning and cooling to solve the problem. In the United States, the government has invested a lot of power to support the promotion of geothermal heat pump systems. After years of efforts, the government and schools have established a database of geological parameters throughout the country and established approved geothermal heat pump project providers in all states. ASHERE also set standards for unit design in response to system specific requirements. At the same time, the government supports research in the design of earth heat exchangers and construction, and in different states, each has its own policy to encourage the promotion of heat pump systems. If the special subsidies, government promotion sites. From a system design point of view, although there are also small water-heat pump units in North America, North American ground heat pump systems are more likely to be water ring heat pump systems, especially for large commercial and industrial buildings that are being designed with water ring heat pumps Mainstream trend. Well-known ground energy heat pump manufacturers in the United States are CLIAMTMASTER, WATER FURNACE, etc. They provide ARI's standard series of products specifically for geosystems. For the earth heat exchanger, North America is mostly used single U-vertical buried pipe and horizontal buried pipe way. Drilling depth of 50-160 meters. In Europe, due to environmental protection and energy saving requirements, at present, in Europe, ground energy heat pump systems have accumulated rich experience in heating. From a system design point of view, more water systems are used in Europe and more water-heat heat pump units in Europe Emphasis on heating, but no special ground heat pump units standard and specialized ground energy heat pump equipment manufacturers. For earth heat exchangers, European technology is more dual-U vertical pipe buried. In China, ground heat pump is still a new cooling heating method just started. Due to the concern of Beijing and other cities on air pollution, the removal of coal-fired boiler heating, in 1998-2003, prompting the rapid popularity of water source heat pump, water source heat pump in practice abroad shows that only in specific conditions, In order to achieve better results. More often than not, LCA analysis of buildings using water source heat pumps is not optimal due to geological constraints such as potential groundwater mining, water level subsidence, water pollution, well aging, and recharge difficulties. So in 2000, some domestic universities and manufacturers started to establish some research prototype projects on the basis of foreign technologies. Since 2004, more and more projects have been devoted to heat pump projects. However, the status quo in China is that various technical schools are trying their best to adopt various solutions. The technical methods promoted by different manufacturers vary widely. Although the overall performance is good compared with air and water source heat pumps, the differences among different systems are considerable Large, in the system design, also exposed a lot of problems. 2, to heat pump system design status to heat pump system due to the complexity of the underground soil heat exchange and ground heat exchanger underground buried pipe construction cost is high, so ground to heat pump system design requirements and conventional air-cooled heat pump and water There are many differences between heat pumps. The most important difference is the selection of parameters of underground heat exchangers, and the cumulative hourly load of buildings. These are for general design institutes and engineering companies are not based on experience, and there is no theoretical methods and tools to solve the problem. In general, the conventional air conditioning system, according to design conditions, determine the maximum load of the building, choose to match the appropriate unit can be. As for the ground heat pump, not only the maximum load needs to be determined according to the working conditions, but also the hourly load and the cumulative sum, and the balance of the system can be determined in consideration of the geothermal heat capacity of the geotechnical earth. So to heat pump system design for different regions, different construction purposes, the design of the program should be different. Instead of simply copying past design parameters. At present, the publicity and research on heat pump in China mainly focus on its advantages and foreign applications. Among them, the research on design has just started, and the academic circles focus more on the analysis and comparison of different heat exchange models. However, the research results have not been formed yet for the practical application. Many projects are exploratory attempts. Although there are foreign models to be used as design guides, due to the different applications and the lack of understanding of the essential differences between the heat pump systems in the ground and their industry resources, Applied design standards and specifications. As the theory can not guide the practice, there are some problems such as the design of the first construction and then the construction of the heat pump system based on the experience of others. In theory, geothermal heat pump system design needs to go through the following steps: First, the type of building and the basic geographical analysis of the building type analysis includes the following: the shape of the building, structure, maintenance and insulation, the number of layers High, Uses, Customers' Current and Future Possible Needs, etc. The basic geographical analysis includes the following: geographical location, surrounding landscape, meteorological parameters, future regional development plans, underground structures and so on. These two aspects of the analysis, can be macroscopically decides whether the heat pump system can be used. Only after the above conditions are comprehensively analyzed to meet the requirements of geothermal heat pump system, the application of geothermal heat pump system can achieve the expected economic results. Calculation and assessment of building load After determining the application of ground energy heat pump system, the first step of the design is to calculate the hourly load of the building, and it is necessary to simulate the cooling capacity and heat demand of different periods of one year. And distinguish between base load and peak load. As well as sustained time and intensity. In determining the construction load, we must take into account the longer term climate change. Through the calculation and assessment of building load, the absolute value of endothermic and exothermic heat of the earth heat exchanger and the bearing capacity and mode of spike impact can be determined. III. ANALYSIS OF UNDERGROUND ROCKS Estimating the underground geological layers and depths that may exist, the types and characteristics of underground rock can be estimated based on the geological exploration reports and geological data of other sources, so as to initially determine the composition of underground rock layers Physical parameters, and possible influences and assessments of geotechnical temperature and humidity, geothermal parameters, and uncertainties. Underground rock analysis is an important foundation for the design of earth heat exchangers and the key to determining system options and costs. Fourth, the initial design of the program First, select a reasonable model, based on the experience of preliminary design, and then use the professional calculation software for system optimization and verification, and ultimately, choose from one to three options that can be applied. And to do a preliminary system design. In the initial program stage, basically determine the type of heat exchanger, pipe, layout, velocity distribution, back to the Tim method. V. Drilling Experiments In the absence of sufficient geological data or large-scale project design, the completion of the initial program design, drilling experiments must be conducted to verify the design input parameters correct or not. For drilling experiments, the physical composition and thermal parameters of the geological structure should be obtained. When necessary, you can also enter the thermal changes affect the experiment. To determine the carrying capacity of the earth. Sixth, the formal program design On the basis of the above five steps, review all the input parameters, and communicate with the confirmation of Party A, began formal program to determine and design the entire system. Determine the earth heat exchanger and ground heat pump system, and determine the key process parameters. Air Conditioning Xu Wei has compiled the American Association of Heating, Refrigeration and Air Conditioning Engineers, "Ground Source Heat Pump Engineering Guide", Sweden, Professor Eskilson developed Earth Energy Designer software, can be used as a reference design and tools. 3. Some key issues of geothermal heat pump system design For geothermal heat pump system design, according to the application example of pre-buried pipe system, it is found that the operating effects of different systems are quite different. In order to summarize and compare the advantages and disadvantages of different systems, it is better To optimize the system, to play the energy-efficient heat pump features, of which some of the key design issues, are described as follows: 3.1 The type of heat exchanger selection. In theory, the earth heat exchanger can be horizontal buried pipe, vertical pipe buried and immersed in the rivers and lakes and sea forms, and for each form, there are different forms, such as vertical buried single U, Double U, three U, casing and other forms. Each form has its own characteristics and advantages. However, from the engineering application point of view, the application of single U and double U in the application of the northern region of our country is a more suitable form. Which is the mainstream of the choice of PE80 or PE100 grade HDPE pipe, pipe diameter is generally selected DN32, mainly due to the one hand, the diameter of the series of special fittings for the rich, on the other hand, the tube can meet the design of heat exchange Volume requirements and strength requirements. DN25 pipe in the application of strength has been found in the phenomenon of inadequate. For the design of the drilling depth, the actual application, from 50 meters to 160 meters have, but need to point out that the drilling depth is not as deep as possible. To determine the depth of reasonable optimization, it is especially necessary to consider the interaction between pipelines and the temperature difference design of each single road besides stratigraphic considerations. Generally for the double U system, using 5 degrees temperature difference, the ideal depth of about 100 meters. As for the spacing between different holes, according to determine the heat effect, generally not less than 4 meters. 3.2 heat exchanger horizontal pipe connection. However, in fact, due to the unreasonable design of horizontal joints, the uneven distribution of flow velocity in the pipe has caused some failures of system operation. Especially for large systems, horizontal linking is even more critical. Some projects in Beijing, the initial design of the use of out-of-the-range design