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 overview
Geothermal heat pump systems (Geothermal heat pump systems) is the use of underground rock and soil as a stable regenerator, the use of heat pump principle to achieve the quarter heat storage, the summer take cold storage heat closed loop system. 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 floor area of ​​the United States with 37.4 million U.S. dollars of cooling and heating using geothermal heat pump technology 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 to air and water source heat pumps, the differences among different systems are quite considerable Large, in the system design, also exposed a lot of problems.
2, to heat pump system design status quo
Ground heat pump system due to the complexity of underground soil heat exchange and ground heat exchanger underground buried pipe higher construction costs, so the ground heat pump system design requirements and conventional air-cooled heat pump and water source heat pump compared to many different. 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 guidelines, due to different applications and the lack of understanding of the essential differences between 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 basic geographical analysis
Analysis of building types includes the following: shape, structure, maintenance and insulation of buildings, levels, storey, usage, current and future customer needs, etc. Analysis of basic geo-physical conditions includes the following: Geographical location, surrounding landscape , Meteorological parameters, future regional development planning, underground structure 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.
Second, the calculation of the load on the building and assessment
The first step in designing a heat pump system that can be applied to earth is to calculate the hourly load on the building by simulating the amount of cooling and heat required to determine the demand for the different periods of the year and to distinguish the base load from the 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.
Third, the analysis of underground rock
Based on the geological survey reports and geological data of other sources, the types and characteristics of the underground rock can be estimated to determine the formation and physical parameters of the underground rock, as well as the geothermal temperature and Humidity, geothermal thermodynamic parameters and the possible impact and assessment of 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 the initial design, and then use the professional calculation software, system optimization and verification, and ultimately, choose 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.
Fifth, drilling experiments
In the absence of sufficient geological data or design of large projects, after completing the preliminary design of the project, drilling experiments must be conducted to verify that the design input parameters are correct. 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 official program design
Based on the above five steps, review all the input parameters, and confirm with the communication of Party A, formal plan determination is started and the whole system is designed. 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, to heat pump system design of some of the key issues
For the ground heat pump system design, based on the application of pre-buried pipe system research and found that different systems have very different operating results, in order to summarize the advantages and disadvantages of different systems to better optimize the system, play a ground energy heat pump energy efficient Features, some of the key design issues, are described separately as follows:
3.1 heat exchanger type of choice.
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-way design, leading to different loops, serious uneven water flow, and finally must rely on the flow control valve to balance the various channels of water flow. But for a single vertical buried circuit, the basic can not be adjusted. Based on computational simulations, changes in water flow rate can attenuate heat exchanger capacity by up to 20-25%.
The basic way of horizontal connection with the same program, different programs and type of manifold type.
The advantage of this method is that it saves the pipe and reduces the construction cost. However, because of its inability to avoid the uneven flow of water in different circuits, it may even cause 20-25% loss of heat exchange capacity for some circuits in large-scale system design. Therefore, The connection is not recommended to use the same way.
It solves the water flow balance of each circuit by ensuring the same flow for each circuit, but for buried systems it increases the flow for each circuit and also leads to an increase in the overall pressure drop. At present, it is the mainstream of buried systems horizontal connection.
The basic principle is to change the horizontal tube into a manifold. On the one hand, this mode can ensure the uniform distribution of the system, reduce the pressure drop of the loop system, and can reduce the loop design water temperature difference.
According to the design experience of Beijing Chongchuang Power Co., Ltd., the horizontal connection of the type manifolds can well achieve the system hydraulic balance and reduce the pump power consumption. Such as a project in Beijing using the first way to connect the 70-hole system, the pump design power consumption 55KW, the actual operation of water flow uneven, reach the design capacity, increase the flow balance valve, the system to improve, but the water flow in and out Pressure difference up to 0.3MPa. The similar project in Tianjin using the third way to link the 100 hole system, the pump design power consumption 30KW, the system is running well, the actual operation of the water pressure difference of 0.1MPa.
3.3 heat exchanger design uses a large flow of small temperature difference
For the earth heat exchanger design, the basic parameters of the pipe and the distribution of the determination of the form, the most critical is the flow rate of flow design. Earth heat exchanger design should be designed with a large flow of small temperature difference, according to the basic requirements of heat transfer, the minimum flow rate needs to ensure that the Reynolds number greater than 2300. The design temperature difference, the best guarantee at 3-5 degrees. Buried system is different from the well drilling system, because it does not take groundwater, it should not be small flow large temperature difference water source heat pump system design.
The use of large flow rate of small temperature difference, has the following advantages: First, can improve the efficiency of the unit, whether refrigeration or heating, the use of large flow small temperature difference, can improve the working conditions of units in the summer, you can reduce the condensation temperature , While in winter, the evaporation temperature can be increased. Second, you can improve the heat transfer efficiency, the use of large flow design, can improve the overall heat exchange capacity of a single heat exchanger. Third, you can choose a deeper hole depth, the temperature difference is small, then reduce the influence of the tube, you can use deeper holes. Fourth, you can better play each single circuit heat exchange capacity. Due to the large amount of water, to ensure uniform water temperature of each circuit, will not reduce the capacity of the circuit because of mixing water.
However, the application of large flow system design, in order to prevent increased pump consumption, we must tie in with a good level.
3.4 unit selection
According to the above introduction, for the ground to the heat pump host must meet the requirements of the entire system. It is recommended to use a specially designed geothermal heat pump unit. For the water ring heat pump mode unit, you must be able to work at low water temperature, you can choose according to the US ARI standards. Or reference to foreign product samples for selection.
For water and water systems, you should choose a small temperature difference between the custom unit. At present, the water source heat pump on the market according to the national standard, both with large temperature difference of small flow design, can not simply do to heat pump host.
Domestic heat pump system has just started, but mainly by engineering companies to promote, a dynamic commitment to providing the overall system solutions. Can provide design and supporting the dedicated unit.
4 Conclusion
Ground energy heat pump system due to its energy saving and environmental protection features, rapid development in the world. The application of ground energy heat pump system in China is just beginning, but in design, there is no unified standard yet.
Geothermal heat pump system design should be based on the type of construction and geology and geology, based on the preliminary simulation with software design, and on the basis of the input parameters to verify the formal program design.
Ground heat pump system is the most important thing is the need for an overall optimization program, rather than simply a combination of indoor, unit and buried pipe. Ground heat pump design should be designed to use large flow rate of small temperature difference. The corresponding type should be used to set the level of water pipe type junction design and dedicated ground heat pump units.
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