Research On Water Heat Pollution Caused By Closed-loop Lake Water Heat Pump

With the promotion of energy saving and emission reduction in our country,surface water heat pump(SWHP) as a kind of energy conservation and environmental protection technology, which has been developed and applied in heating and cooling field. However, there are a series of problems is highlighted in the study and application of SWHP. One of the most serious is water heat pollution caused by discharge water from SWHP, it has attracted a lot of attention.From the overseas and domestic research status of water heat pollution, it pointed out the insufficiency of them and the research direction and emphasis of this paper. a lake water heat pump(LWHP)in Nanjing Institute of Technology is taken as the research object, we has carried out the theoretical research, simulation research and experimental research on it. Theoretical research is based on exergy analysis, the advantage of exergy used in thermal process analysis is put forward. Besides, we studied the influence of variable load operation of LWHP on lake temperature and effects lake temperature makes on the performance of LWHP. Theoretical studies shows that with the increase of load operation of LWHP, the average temperature lake rises. Besides, higher lake temperature results in lower performance of LWHP. Moreover, simulation study is based on the theoretical research. For the lake water heat pump in Nanjing Institute of Technology, both two dimensional model and three dimensional model are established for different simulation parameters. Two dimensional model is used to simulate the influence of discharge water flow and buried pipes space on lake temperature. While three dimensional model is used to simulate the influence of load rate and buried pipes depth on lake temperature. Meanwhile, COP in corresponding conditions is calculated.The simulation results show the higher cooling water flow rate results in the bigger temperature rise area and higher COP. With the increase of the spacing of buried pipes,heat accumulation in lake is reduced, and COP is increased. In order to avoid water heat pollution, and take the performance of closed-loop lake water heat pump into consideration, the appropriate cooling water flow rate is 0.36m3/s, and the corresponding buried pipes spacing should be more than 0.35m; Besides, the greater depth of buried tube leads to the less influence discharge water from closed-loop lake water heat pump makes to lake temperature. In addition, with the increase of buried tubedepth, the performance of closed-loop lake water heat pump is rose; With the increase of load rate of LWHP, the temperature of lake water around buried pipe is rose. When comparing the temperature variation in vertical and horizontal direction, we find the temperature rise of lake water in horizontal direction is obvious. Later, we conduct an experiment based on simulation results. Comparing the result of the experiment and simulation, it shows two results within the margin of error, so it verifies the correctness of the simulation. Subsequently, the COP of LWHP under different load operation is calculated, the results show that load operation affects the performance of LWHP. And the higher load rate results in the higher COP of LWHP. Finally, for the shortcomings of the existing water heat pollution evaluation method, we put forward a new water heat pollution evaluation method based on exergy analysis, it presents a unified thermodynamic measure of objective evaluation of water heat pollution. The influence of discharge water from LWHP is analyzed, it shows no water heat pollution in lake.According to the theory, simulation and experimental results, some suggestions about the way and operation of buried tube in closed-loop lake water heat pump system are put forward.