Simulation Study On The Refrigerating Performance Of The CO₂ Direct Expansion Ground Source Heat Pump

The pollution problem caused by refrigerant leakage of direct expansion ground source heat pump has been affecting its promotion. CO₂, the natural working substance,is considered to be used as the refrigerant of direct expansion ground source heat pump system. This paper analyzes the model of CO₂ direct expansion ground source heat pump when it is in refrigeration condition. And it also analyzes the optimization measures to improve its performance.Firstly, through making thermodynamic analysis on heat transfer process of the CO₂ direct expansion ground source heat pump's buried tube when it is in refrigeration condition, this paper shows us how to calculate the length of the buried tube. Based on the analysis of buried tube's mathematical model which is established by energy balance method, it is found that there is thermal short-circuiting in the buried tube which is working as gas cooler. Increasing the distance of the U-tube's two branches, the thermal conductivity of the soil and backfill material, the inlet temperature and mass flow rate of the refrigerant respectively in a certain degree can improve the heat transfer effect of the buried tube.Secondly, the mathematical models of buried tube, compressor, throttle and evaporator are set up respectively. Then the model of the system is built up. Through the theoretical analysis of the system model, it is found that the optimal high-side pressure of the system is 14.6MPa and the maximum value of COP is 0.966. On the other hand, the increase of the inlet temperature of the chilled water, the flow of the chilled water and the length of the evaporator could increase the refrigerating capacity, the power consumption of compressor and COP in a certain degree. The increase of the initial temperature of soil has a negative effect on the system when it is in refrigeration condition. The refrigerating capacity and COP would decline, but the power consumption of compressor would rise.Lastly, four measures are showed in this paper to improve the performance of the original system. And study on them respectively. The results show that using two-stage compression can enhance COP more obviously when the high-side pressure is high. The lifting range of refrigeration capacity is not high, and it leads to the increase of the optimal high pressure. But it can effectively reduce the power consumption and exhausttemperature of the compressor. The system using regenerator has a certain improvement on COP and refrigeration capacity when the high-side pressure is low. But it increases the power consumption and exhaust temperature of the compressor. Adding expander in original system can significantly improve COP and refrigerating capacity. It can also reduce power consumption of the compressor. And its optimal high-side pressure is the lowest of the four improvement measures which are presented in this paper. But it doesn't have much effect on the exhaust temperature of the compressor. Adding two-stage compression and expansion in original system at the same time can comprehensively improve the performance of the system. Comprehensive comparison and analysis in these four kinds of optimization measures can show us that the performance improvement of using two-stage compression and expansion in original system at the same time is the most significant. But in the premise of considering the cost factor, using regenerator or expander is better.