Abstract: Ground-source heat pump is a new energy-saving, environment-friendly and sustainable technology that neither pollutes the groundwater nor affects land subsidence. This paper discusses several key issues in the design of ground source heat pump household air conditioning system, and analyzes the system design method and operation control technology.
I. Introduction
Ground-source heat pump is the underground soil layer for cold (heat) source of building heating, hot water supply and air-conditioning technology. It is well-known that the stratigraphy maintains a relatively stable temperature throughout the year. In summer, the temperature in the ground is lower than the temperature in the ground and higher in winter than in the ground. Ground source heat pump is the use of this feature of the earth, through the heat exchanger buried in the ground, and soil or rock exchange heat. Ground source heat pump operating conditions throughout the year stable, without the need for other auxiliary heat and cooling equipment to achieve winter heating, summer cooling. Therefore, the ground-source heat pump is a new type of energy-saving and environment-friendly technology that can achieve sustainable development. It will neither pollute the groundwater nor affect the land subsidence. In winter, the liquid in the pipeline draws underground heat and is then directed into the building through the system while storing cold for future use. In the summer, heat is drawn from the building and discharged into the ground through the system Save heat for winter use. Ground-source heat pumps can reliably provide high-quality warm air all year round, creating a very comfortable indoor environment for us. Therefore, ground source heat pump air conditioning has been widely developed, especially suitable as a household central air conditioning cold (heat) source.
Ground-source heat pump has a long history. As early as the beginning of the last century, the United States started the study of the performance of ground-source heat pumps. At present, ground-source heat pumps in the United States have become a mature and completely industrialized technology. As of 2001, 400,000 ground-source heat pumps have been installed and are still steadily growing at a rate of 10% per annum. In Nordic countries like Sweden, 90% of houses have ground-source heat pumps. Relatively speaking, China's research on ground-source heat pumps was much later, until some units in the 1980s did not start systematic research on it. The research on ground source heat pump in China began to be explored. However, no breakthrough has been made on how to effectively reduce the initial investment of the system and ensure the reliable operation of the system. The main reason for this is that the vast majority of the research conducted has been limited to the performance test of the established experimental system and the technical and economic comparison with the traditional air-to-air heat pump performance to draw a general conclusion on the energy saving of the ground-source heat pump. Due to the lack of complicated heat transfer and the mass transfer process of buried heat exchangers in soil, these conclusions are only applicable to a specific experimental system, which provides less basic data and can not be used as design basis. Therefore, there are still many problems to be solved in the development and design of ground-source heat pump units and systems. This article will briefly discuss the development and design of household water-water ground source heat pump units, soil heat exchangers and the entire air conditioning system.
Second, ground source heat pump air conditioning system design
1. Ground source heat pump system cycle introduction and selection
Ground source heat pump system can be divided according to their cycle form: closed cycle system, open cycle system and mixed cycle system.
1.1 closed cycle system
Closed-loop system refers to the cold (heat) source side of the circulating water in the unit outdoor heat exchanger and ground source heat exchanger to form a closed loop. Pipes can be buried 150-200 feet underground through vertical wells or 4-6 feet underground, or at the bottom of the pond. In winter, the fluid in the pipe extracts heat from the ground and into the building, while in the summer it heats the building's heat through underground pipelines; the pipes used are high-density polyethylene pipes or other corrosion-resistant pipes And ground source heat exchanger material. Most ground source heat pump cold (heat) source side heat exchanger system is closed loop.
1.2 open circulation system
The open circulation system is where the water in its pipeline comes from a source of water in a lake, river or shaft and after it has exchanged heat with the building in the same way as a closed cycle, the water is returned to its original location or discharged to another suitable location .
1.3 mixed cycle system
For mixed-cycle systems, underground heat exchangers are typically calculated as heat load, with the additional cooling load required by summer being provided by a conventional cooling tower.
1.4 cycle system selection
Closed-loop system is a more stable and reliable conventional circulation system, no pollution to groundwater and underground environment, the general design should give priority to the circulatory system. For underground design heat exchange space is not sufficient, or vertical buried difficulties and other underground special circumstances, consider the design of mixed cycle system.
2. System Design Parameter Discussion
Regarding (cold) heat source side water flow, it is determined by the maximum heat gain and the maximum heat release amount. The selection of water flow rate in the buried pipe depends on the length of the buried pipe cycle, the material of the buried pipe, the diameter of the pipe, the local ground source conditions and the characteristics of the unit. In general, such as increasing the water flow rate can be properly increased heat transfer coefficient, heat transfer enhancement, reduce the heat transfer area and heat pipe supplies, but the flow rate is too fast will increase the circulating pump energy consumption. Generally desirable flow rate of 0.65-1.5m / s. Specific conditions can be optimized for local analysis and design, optimization design considerations of the parameters are as follows.
Composite energy consumption N = f (length LLT, buried pipe material Ma, pipe diameter D, ground temperature Te, ground source heat index Ke, unit characteristics Type)
In the selection of units, the inlet water temperature of buried pipe is set, and the outlet water temperature of buried pipe is calculated according to the temperature difference between inlet and outlet measured by logging, so as to determine the evaporating temperature and condensing temperature of working fluid in heat pump unit in winter. In short, China has a vast territory and is located in a temperate zone. The climatic conditions in different regions vary greatly and the loads are also quite different. Therefore, we can not copy the technological achievements of other countries and develop technologies suitable for the climate characteristics of our country.
Third, the design of the unit
Ground source heat pump in the form of more, of which the most widely commercialized vapor compression heat pump. For household ground source heat pump system, water-water system, for example, by an outdoor unit and a number of indoor units. The system can be individually adjusted for each air-conditioned room to meet the requirements of each air-conditioned room, with better energy-saving effect. The variable frequency household ground source heat pump air conditioning system plus an independent fresh air system is a promising future energy-saving comfortable type of residential central air conditioning system. Therefore, its optimal design has extremely important value.
The traditional method of refrigeration system design is based on experience and experiment. Usually experience design method is simple and easy, the theoretical knowledge and experimental conditions, the dependence is relatively small. However, the empirical design method inevitably has the shortcomings of direct and low reliability, poor stability, only suitable for the initial development of the product. Optimized design techniques based on theoretical prediction can be effective. The optimization method is to choose the best solution among all feasible solutions. In the optimization design, all independent variables that characterize the scheme are design variables. The optimization approach is to study how to reasonably determine these variables. The evaluation of the merits of the program depends on the design of the program selected variables, that is, the indicator of the design variables function - the objective function. In system optimization design, the value of design variables is often limited by various conditions, namely constraints.
Inverter household ground source heat pump air conditioning system consists of inverter compressor, condenser, evaporator, electronic expansion valve, indoor unit, refrigerant pipeline and Water Pump system. According to the thermodynamic theory of refrigeration system, the dynamic equations of the system components and the dynamic parameters of the operating parameters are established by means of the dynamic distribution of parameters and the interrelated methods. The equations of the system operating parameters are established and the system is simulated dynamically. Simulation of the dynamic characteristics of the system, to provide the basis for optimizing the design. To meet the energy-saving, thermal comfort and cooling and heating effects of the air-conditioning system, the energy efficiency ratio, temperature drop rate and temperature drop rate of the air-conditioning system should reach the target requirements. Therefore, in the process of optimal design, multi-objective optimization methods with energy efficiency ratio, temperature drop rate and temperature drop rate as objective function are respectively selected.
At the same time, the optimization method is adopted to meet the requirements of the constraints of condenser and evaporator structure, area range, oncoming wind speed range, system temperature and pressure range, water and refrigerant flow range, supercooling superheat range and indoor unit number. Multi-objective optimization of the above objectives, so as to achieve the purpose of optimizing the design of geothermal heat pump system for different regions.
Fourth, ground source heat pump underground heat exchanger form and layout
Soil heat exchanger is the key to ground source pump design. Geothermal heat exchangers have many forms, such as horizontal buried pipe, vertical buried pipe and so on. Both types of buried pipe have their own characteristics and application environment. Vertical pipe buried in China also shows its superiority: saving land area, heat transfer performance, can be installed in the building foundation, roads, green spaces, playgrounds, etc. without affecting the use of the upper part of the function, and even in Buried pipes are placed in the pile foundations in the building, making use of the available land area. The following vertical borehole heat exchanger design for a simple discussion.
1. Vertical pipe material and depth
Pipe material is best to use plastic pipe, compared with the metal pipe, plastic pipe with corrosion-resistant, easy processing, heat transfer performance to meet the heat requirements, the advantages of low prices. Available for selection of high-density polyethylene pipe (PE pipe), such as aluminum-plastic pipe. Vertical buried pipe diameter may have different options, such as DN20, DN25, DN32, DN50 and so on. Vertical buried pipe according to local geological conditions may be, from 20m-200m. Determine the depth should take into account the area, drilling equipment, drilling costs and project size. If the surface soil layer is very thick, drilling costs are relatively cheap, should adopt a deeper vertical buried pipe, on the contrary, the use of shallow buried. Buried pipe spacing generally 5-6 meters and above, to consider the local geology and soil heat transfer situation.
2. Vertical buried heat exchanger backfill, sensitivity
The vertical borehole heat exchanger was formed by drilling down from the ground to the expected depth, placing the fabricated U-tube into the hole, and backfilling different materials in the hole. Close to the surface at the level of water pipes, sub-pipes will be all the U-shaped tubes constitute the underground heat exchanger. Depending on the geological structure, the backfill material can be cast concrete, backfill sand and sand or backfill soil. Material selection should take into account engineering costs, heat transfer properties, construction convenience and other factors. Casting concrete from the actual test to compare the best heat transfer performance, but the high cost, construction difficult, but with the construction of the pile together.
3 vertical borehole heat exchanger heat attenuation
The temperature of circulating water circulating in vertical borehole heat exchangers is constantly changing. When the cooling conditions in summer are underway, the temperature of the water in the unit is continuously rising due to the increase of the heat storage ground temperature, and the water temperature drops again when the heat is stopped. When the heat gain reaches the maximum, the water temperature reaches the highest point. On the other hand, when the heating conditions in winter are running, on the contrary, the water temperature in the heat exchanger reaches the lowest point when the heat loss in the building reaches the maximum due to the drop in the ground temperature for taking heat. The sign pipe is particularly serious. Design, the first should be set in the heat exchanger tube circulating water maximum temperature and minimum temperature. At the same time, due to the surface of the heat exchanger surface scaling and other effects, the design must consider the attenuation. Set value should be selected through the economic comparison of the best state point.
Fifth, the conclusion
Several key issues, system selection, heat pump and soil heat exchanger design of ground source heat pump residential air conditioning system are discussed, and the design method and operation control technology of the system are analyzed.
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