To truly enable the soil source heat pump to replace the traditional air conditioning system, there are still many core technical issues to be further resolved. Figure 1 shows the dynamic thermal characteristics of a complete soil source heat pump. From a system perspective, the following issues need to be resolved in terms of micro to macro order:
1 Study on heat transfer and mass transfer in soil
At the bottom of the overall system is the secondary subsystem of the buried heat exchanger. In this system, the main concern is the heat exchange process between the buried heat exchanger and the surrounding soil. Since the soil is a multiphase dispersion consisting of a solid soil skeleton, liquid and gaseous water, and air, the simple composite unsteady heat conduction used in most of the current researches attribute the transport of water and air to The added value of a thermal conductivity coefficient to describe the thermal mass coupling in the soil obviously brings about a large error. Corresponding results, the size of the buried heat exchanger of the soil source heat pump is too large, and the initial investment of the heat pump device is increased ( Buried heat exchangers typically cost 20% to 30% of the total cost of a heat pump system and cannot compete with conventional air conditioning systems.
Therefore, the urgent solution is to describe the heat exchange process between the buried heat exchanger and the soil by using a more complete mathematical model, and comprehensively consider the heat transfer and mass transfer process. The porous media hydrodynamics approach may be a powerful tool. Some researchers have proposed to use irreversible thermodynamics to study, and it is also a new and feasible method. Other researchers have proposed using fractal methods to study the thermal conductivity in soil, which may simplify the mathematical model and make the research less difficult.
Another purpose of studying the heat and mass transfer process in soil is to find a new type of filling material for heat transfer enhancement. Researchers in foreign countries have conducted research in this area and have reported it. Of course, the ultimate goal is the same, that is, to minimize the initial investment cost of the soil source heat pump buried heat exchanger, but no similar research reports have been seen in China.
2 Study on the coupling process with heat pump device
The purpose of studying the coupling process with the heat pump device is to optimize the performance of the heat pump device subsystem. Due to the use of soil as a heat source, whether in winter or summer, the operating conditions of the heat pump system (the operating point of the outdoor side heat exchanger) are different from the working points of the conventional air heat pump or the general water source heat pump, thus causing the whole The operating characteristics of the heat pump system vary. Specifically, in the temperature of the new outdoor side heat exchange fluid, how to configure the corresponding evaporator, condenser, compressor and even the entire system components, so that the heat cycle performance of the heat pump is optimal, and the soil source heat pump is maximized. Energy saving potential. Therefore, it is necessary to adopt the thermodynamic method of the refrigeration system.
3 Economic analysis of the annual energy consumption of the system of soil source heat pump
From the thermal system diagram of the soil source heat pump, it can be seen that the third level of the problem to be studied is the annual energy consumption analysis of the entire heat pump system. For the case where air conditioning is required throughout the year, the conclusion that the soil source heat pump is superior should be based on the results of the annual energy consumption analysis.
This is especially important when determining the solution for a soil source heat pump. For different meteorological conditions and different building functional requirements, the heating and cooling load of the building in winter and summer may be different. Winter heating should be given priority. The shortage of summer cooling load is supplemented by traditional air conditioning systems (such as the installation of cooling towers). Is it based on the summer cooling load and the auxiliary heat source mode in winter? Based on the annual energy consumption analysis, the initial investment and operating costs of the system should be fully considered, and the investment recovery period should be used as the basis for judgment.
Therefore, based on the understanding and understanding of the heat transfer process of the underground heat exchanger and the dynamic thermal characteristics of the heat pump system, combined with the annual dynamic load of the building, the superiority of the soil source heat pump can be achieved. It can be fully utilized to achieve energy conservation and environmental protection.
1 Study on heat transfer and mass transfer in soil
At the bottom of the overall system is the secondary subsystem of the buried heat exchanger. In this system, the main concern is the heat exchange process between the buried heat exchanger and the surrounding soil. Since the soil is a multiphase dispersion consisting of a solid soil skeleton, liquid and gaseous water, and air, the simple composite unsteady heat conduction used in most of the current researches attribute the transport of water and air to The added value of a thermal conductivity coefficient to describe the thermal mass coupling in the soil obviously brings about a large error. Corresponding results, the size of the buried heat exchanger of the soil source heat pump is too large, and the initial investment of the heat pump device is increased ( Buried heat exchangers typically cost 20% to 30% of the total cost of a heat pump system and cannot compete with conventional air conditioning systems.
Therefore, the urgent solution is to describe the heat exchange process between the buried heat exchanger and the soil by using a more complete mathematical model, and comprehensively consider the heat transfer and mass transfer process. The porous media hydrodynamics approach may be a powerful tool. Some researchers have proposed to use irreversible thermodynamics to study, and it is also a new and feasible method. Other researchers have proposed using fractal methods to study the thermal conductivity in soil, which may simplify the mathematical model and make the research less difficult.
Another purpose of studying the heat and mass transfer process in soil is to find a new type of filling material for heat transfer enhancement. Researchers in foreign countries have conducted research in this area and have reported it. Of course, the ultimate goal is the same, that is, to minimize the initial investment cost of the soil source heat pump buried heat exchanger, but no similar research reports have been seen in China.
2 Study on the coupling process with heat pump device
The purpose of studying the coupling process with the heat pump device is to optimize the performance of the heat pump device subsystem. Due to the use of soil as a heat source, whether in winter or summer, the operating conditions of the heat pump system (the operating point of the outdoor side heat exchanger) are different from the working points of the conventional air heat pump or the general water source heat pump, thus causing the whole The operating characteristics of the heat pump system vary. Specifically, in the temperature of the new outdoor side heat exchange fluid, how to configure the corresponding evaporator, condenser, compressor and even the entire system components, so that the heat cycle performance of the heat pump is optimal, and the soil source heat pump is maximized. Energy saving potential. Therefore, it is necessary to adopt the thermodynamic method of the refrigeration system.
3 Economic analysis of the annual energy consumption of the system of soil source heat pump
From the thermal system diagram of the soil source heat pump, it can be seen that the third level of the problem to be studied is the annual energy consumption analysis of the entire heat pump system. For the case where air conditioning is required throughout the year, the conclusion that the soil source heat pump is superior should be based on the results of the annual energy consumption analysis.
This is especially important when determining the solution for a soil source heat pump. For different meteorological conditions and different building functional requirements, the heating and cooling load of the building in winter and summer may be different. Winter heating should be given priority. The shortage of summer cooling load is supplemented by traditional air conditioning systems (such as the installation of cooling towers). Is it based on the summer cooling load and the auxiliary heat source mode in winter? Based on the annual energy consumption analysis, the initial investment and operating costs of the system should be fully considered, and the investment recovery period should be used as the basis for judgment.
Therefore, based on the understanding and understanding of the heat transfer process of the underground heat exchanger and the dynamic thermal characteristics of the heat pump system, combined with the annual dynamic load of the building, the superiority of the soil source heat pump can be achieved. It can be fully utilized to achieve energy conservation and environmental protection.
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