Application design of ground source heat pump air conditioning?

The application design of ground source heat pump air conditioning is very important. The quality of the design directly affects the effect of subsequent use, and every detail treatment will bring different effects. Zhong Da Consulting introduces the application design of ground source heat pump air conditioning.

I. Introduction

With the development of economy and the improvement of people's living standards, heating and air conditioning in public buildings and houses have become a universal demand. On the premise of meeting people's health and comfort requirements, rational utilization of natural resources, environmental protection and reduction of conventional energy consumption have become important issues facing the HVAC industry. Ground source heat pump air conditioning system can save energy by absorbing the cold heat of the earth (including soil, well water, lakes, etc.). ), take heat from the earth in winter and cold from the earth in summer, and then the heat pump unit can provide cooling and heating for the building. It is an efficient, energy-saving and pollution-free new air conditioning system, which can both heat and cool.

Second, the ground source heat pump air conditioning system

Ground source heat pump (GSHP) is an efficient and energy-saving air conditioning system, which can use shallow geothermal resources for heating and cooling. The system extracts heat energy from the environment by ground source heat pump to heat the building or release the heat energy from the building to the environment to realize the cooling of the building, and the excess heat energy can be stored in the stratum in summer for use in winter; In winter, excess cold energy can be stored in the stratum for use in summer. In this way, the principle of energy exchange between buildings and the environment is realized by using the characteristics of the stratum itself (as shown in figure 1).

Advantages and application status of ground source heat pump

Because of its technical advantages, ground source heat pump has obvious energy-saving and environmental protection benefits, which mainly have the following advantages: (1) The operating efficiency of ground source heat pump system is about 40% higher than that of traditional air conditioning system, with energy saving and low operating cost. (2) The ground source heat pump system can provide heating, air conditioning and domestic hot water. One system can be used for multiple purposes, and one system can replace the original two sets of boilers and air conditioners. (3) The development and popularization of ground source heat pump air conditioning technology can completely abolish small and medium-sized coal-fired boiler rooms, which are free from combustion, waste and pollution, and will not affect the environmental quality. (4) The shallow geothermal resources on the surface are abundant and ubiquitous, which is a clean and renewable energy.

With the progress of ground-source heat pump technology, by the end of 2000, there were more than 400,000 platform-source heat pump systems in the United States used in homes, schools and commercial buildings, providing about 8,000 ~11000 gwh of terminal energy every year. The design of ground source heat pump air conditioning system in China mainly includes two parts: one is the design of water ring air conditioning system in buildings; The second is the design of underground part of ground source heat pump air conditioning system, that is, the design of underground heat exchanger of underground coupled heat pump system, surface water heat exchanger of surface water heat pump system and well system of underground heat pump system. The underground coupled heat pump system was first applied to the office building of Minhang Development Zone in Shanghai (4305m2, cooling load 4532kW, heating load 23 1kW), and it was put into operation in June 1989. Its technology and equipment are provided by the United States and are well used. At present, in China, the groundwater heat pump system is mature in technology, feasible in utilization and has many engineering projects. At present, there are twenty or thirty domestic manufacturers of water source heat pump units, because the technical standards for domestic production of water source heat pump units have not yet been promulgated, and the quality of products produced by domestic manufacturers is quite different. Judging from the product samples of some manufacturers, the technical parameters are incomplete and inaccurate.

Fourthly, the design of ground source heat pump air conditioning system.

1. Classification of ground source heat pump system. Ground source heat pump system can be divided into open cycle system, closed cycle system and mixed cycle system according to its cycle form. (1) open circulation system. Open circulation system means that the water in its pipeline comes from the water source in lakes, rivers or shafts. After heat exchange with the building in a closed cycle, the water flows back to its original place or is discharged to other suitable places. (2) Closed circulation system. Closed circulation system means that the circulating water on the cold (hot) source side forms a closed circulation between the outdoor heat exchanger of the unit and the ground source heat exchanger. The pipeline can be buried underground through the shaft 150 ~ 200 feet or horizontally 4 ~ 6 feet, or at the bottom of the pond. In winter, the fluid in the pipeline extracts heat from the ground and brings it into the building. In summer, the heat energy in the building is transported to the ground for storage through pipelines. The pipes used are high-density polyethylene pipes or other anticorrosion pipes, which are used as materials for transportation and ground source heat exchangers. Closed circulation system is a relatively stable and reliable conventional circulation system, which has no pollution to groundwater and underground environment and should be given priority in overall design. (3) Mixed circulation system. The underground heat exchanger of mixed circulation system is generally calculated according to the heat load, and the extra cooling load required in summer is provided by the conventional cooling tower. For special underground conditions, such as insufficient heat exchange space in underground design, or difficulty in vertically burying pipes, we can consider designing a mixed circulation system.

2. Discussion of system design parameters. The water flow at the (cold) heat source side should be determined by the maximum heat gain and maximum heat release. The choice of water velocity in buried pipe depends on the length of buried pipe circulation process, buried pipe material, pipe diameter, local ground source conditions and the characteristics of the unit. Generally speaking, if the water velocity is increased, the heat exchange coefficient can be appropriately increased, the heat exchange capacity can be strengthened, and the heat exchange area and consumables of the heat exchange tubes can be reduced. However, if the water flow speed is too fast, it will increase the energy consumption of the circulating water pump. Generally, the acceptable flow speed is 0.65 ~1.5m/s. Specifically, the optimization analysis and design can be carried out according to local conditions, and the parameter relationship considered in the optimization design is as follows. Compound energy consumption N=f (length LLT, buried pipe material Ma, pipe diameter D, ground source temperature Te, ground source thermal index Ke, unit characteristic type) In the unit selection, the water inlet temperature of the buried pipe is set, and the water outlet temperature of the buried pipe is calculated according to the temperature difference between the inlet and outlet water measured by logging, so as to determine the evaporation temperature and condensation temperature of the working medium of the heat pump unit in winter. In a word, China has a vast territory and is located in the temperate zone. The climatic conditions in different regions are very different, and its load is also very different. Therefore, we should not copy foreign technological achievements, but develop technologies suitable for China's climate characteristics.

3. Design of the device. There are many forms of ground source heat pump, among which the most widely commercialized is steam compression heat pump. Take the water-water system as an example, it consists of an outdoor unit and several indoor units. The system can adjust each air-conditioned room independently, meet the requirements of each air-conditioned room, and has good energy-saving effect. Frequency conversion household ground source heat pump air conditioning system plus independent fresh air system is an ideal energy-saving and comfortable household central air conditioning system with great development prospects, so its optimal design has extremely important value. The traditional design method of refrigeration system is based on experience and experiment. Generally, the empirical design method is simple and easy, and it is relatively less dependent on theoretical knowledge and experimental conditions. However, the empirical design method inevitably has the shortcomings of directness, low reliability and poor stability, and is only suitable for the early development of products. The optimal design technology based on theoretical prediction is effective.

Optimization method is a method to select the best scheme from all feasible schemes. In the optimization design, all the independent variables representing the scheme are design variables, and the optimization method is to study how to determine these variables reasonably. The index of evaluating the quality of the scheme depends on the design variables selected, that is, the index is the functional objective function of the design variables. In system optimization design, the value of design variables is often restricted by various conditions, that is, constraints. The variable frequency household ground source heat pump air conditioning system consists of variable frequency compressor, condenser, evaporator, electronic expansion valve, indoor unit, refrigerant pipeline and water pump pipeline system. According to the thermodynamic theory of refrigeration system, the mathematical model of each part of the system and the dynamic equation of operating parameters are established by using the method of dynamic distribution and correlation of parameters, and the operating parameter equation of the system is formed to simulate the system dynamically. The dynamic characteristics of the simulation system provide a basis for optimal design. In order to meet the energy saving, thermal comfort and good refrigeration and heating effect of air conditioning system, the energy efficiency ratio, temperature drop (temperature rise) rate and temperature drop (temperature rise) range of air conditioning system should meet the index requirements. Therefore, in the optimization design, the energy efficiency ratio, temperature drop (temperature rise) rate and temperature drop (temperature rise) amplitude are selected as the objective functions of the multi-objective optimization method. At the same time, considering the requirements of satisfying the constraints such as condenser and evaporator structure, area range, head-on wind speed range, system temperature and pressure variation range, water and refrigerant flow range, supercooling and superheat range, number of indoor units, etc., multi-objective optimization calculation is carried out by using the optimization method to achieve the purpose of optimizing the design of ground source heat pump systems in different regions.

4. Form and arrangement of underground heat exchanger of ground source heat pump. Soil heat exchanger is the key to the design of ground source heat pump unit. There are many forms of ground source heat pump-soil heat exchanger, such as horizontal buried pipe and vertical buried pipe. These two types of buried pipes have their own characteristics and application environment. The application of vertical buried pipe in China shows its advantages: saving land area and good heat exchange performance. Can be installed under building foundations, roads, green spaces, squares, playgrounds, etc. Under the condition of not affecting the use function of the upper part, buried pipes can even be installed in the pile foundation of the building to make full use of the available land area. The design of vertical buried tube heat exchanger is briefly discussed below.

(1) Material and depth of vertical buried pipe. The best buried pipe material is plastic pipe, because compared with metal pipe, plastic pipe has the advantages of corrosion resistance, easy processing, heat transfer performance meeting heat transfer requirements, low price and so on. Available pipes are high-density polyethylene pipe (pe pipe) and aluminum-plastic pipe. The diameter of the vertical buried pipe can also be different, such as DN20, DN25, DN32, DN50, etc. Vertical buried pipes can be determined according to local geological conditions, ranging from 20m to 200m. The floor space, drilling equipment, drilling cost and project scale should be considered comprehensively when determining the depth. If the surface soil layer is thick and the drilling cost is relatively cheap, deep vertical buried pipes should be adopted, otherwise shallow buried pipes should be adopted. The spacing between buried pipes is generally 5 ~ 6m or more, and the local geology and soil heat transfer should be considered comprehensively.

(2) Backfilling and sensitivity of vertical buried heat exchanger. The vertical buried tube heat exchanger is formed by drilling down from the ground to the expected depth, lowering the manufactured U-tube into the hole, and then backfilling the hole with different materials. Near the surface, all U-shaped pipes are connected in parallel with horizontal water collecting pipes and water distribution pipes to form underground heat exchangers. According to different geological structures, backfill materials can be cast concrete, backfill sand or backfill soil. The material selection should consider the factors such as project cost, heat transfer performance and convenient construction. Compared with the actual test, cast-in-place concrete has the best heat transfer performance, but it is expensive and difficult to construct, but it can be constructed together with building pile foundation.

(3) Heat transfer attenuation of vertical buried tube heat exchanger. The temperature of circulating water flowing in vertical buried heat exchanger is constantly changing. During the cooling operation in summer, due to the increase of the ground temperature of the heat accumulator, the water temperature rises continuously during the operation of the unit, and drops again during the shutdown, and rises to the highest point when the heat gain of the building reaches the maximum. On the contrary, when heating works in winter, when the building loses the most heat due to the decrease of ground temperature, the water temperature in the heat exchanger reaches the lowest point. It is especially serious for signing buried pipes. When designing, the maximum and minimum temperatures of circulating water in the buried pipe of heat exchanger should be set first. Due to the influence of scaling on the surface of buried heat exchanger, attenuation should be considered in design, and the best state point should be selected through economic comparison.

Verb (abbreviation of verb) conclusion

As an environmental protection and energy-saving air conditioning method, ground source heat pump is a multi-disciplinary and interdisciplinary energy comprehensive utilization technology, which requires the joint efforts of relevant professional and technical personnel to do a good job in the design, installation, operation and maintenance of ground source heat pump units. In recent ten years, especially in the last five years, the ground source heat pump air conditioning system has made rapid development in the United States, Canada, France, Switzerland, Sweden and other North American countries, and the ground source heat pump market in China has become increasingly active. It can be predicted that this technology will become the most effective heating and cooling air conditioning technology in 2 1 century.

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