Study On Thermal Performance Of Lake And Open-Loop Lake-Source Heat Pump System

Due to lack of the fundamental research, the utilization of lake-source heat pump system is still on the demonstration stage. And, it is difficult to use widely the energy-efficeint technology of lake-source heat pump system. In this paper, under the fund of the Tenth Five-year and the Eleven Five-year national scientific and technological project, the study of the thermal performance of lake used with open-loop water source heat pump is systematically analyzed and performance of system is proposed. Meanwhile, the factors influencing the performance of lake and heat pump system are investigated. The results are presented as follows:(1) A 2-D numercial model based on the Navier–Stokes equations is used to simulate the temperature of long narrow stratified lake without inflow and outflow. According to the test data of water pond in Chendu, the accuracy of k-εstandard model and prantl modified k-εmodel are compared. The results show that prantl modified k-εmodel can accurately simulate the thermal stratification of the water.(2) Because thermal characteristic of the lake shows that it has ability to store cool water easily than heat water over seasons, the emphasis is put on the ability of cool storage of the lake. The temperature of the lake under three different conditions from spring to summer is simulated with 2-D temperature model. The analysis shows that the shielding method of solar radiation and wind over water surface and bad water quality make significant influence on the promotion of cool storage of the lake.(3) In order to maximize use of low temperature water under the stratified lake, it is assumed that vertical location of outtake diffuser changes in time in such a way to interact with the ambient water temperature most closely matches the inlet water temperature. The lake temperature change is predicted when specific loads are applied, then ideal cooling capacity of lake when used as heat sink is determined. The factors influencing the ideal cooling capacity of lake are investigated. It is showed that: (ⅰ) Increase of temperature between inflow and outflow is beneficial to increase of cooling capacity of lake. (ⅱ) Climate exerts some effect on the cooling capacity of lake. (ⅲ) water depth has significant influence on unstratified lake. (ⅳ) Inflow is discharged according to same temperature of ambient water can promote considerably the cooling capacity. (ⅴ) Water quality influences significantly on cooling capacity.(4) A 2-D temperature model with eclipse form intake-outlet method is established to analyze the factors influencing the intake water temperature. The results show that: (ⅰ) Increase of outlet height can decrease the change of intake water temperature, only under the condition that the ratio of lake depth to width of outlet must be more than critical value. (ⅱ) the greater distance of intake and outlet, the smaller change of the intake temperature. However, maximum ratio doesn't mean the better effect. So, it is suitable to adopt the critical value. (ⅲ) the greater Fr, the smaller change of intake water temperature. (ⅳ) By comparing the inlet temperature of march-past method and eclipse form, the superiority of the latter only exist with suitable distance of intake and outlet.(5) Open-loop lake source heat pump system model is established to predict different factors affecting the performance of system. The analysis is proposed that: (ⅰ) Winter weather has more significant influence on system performance than summer weather. (ⅱ) Water depth has unimportant affect on it. (ⅲ) Water quality exert some influence on it. (ⅳ) the deeper outlet place, the greater performance efficiency. (ⅴ) Operating model has some affect on performance.(6) An annual energy consumption calculation model of open-loop lake-source heat pump is established. And the comparison of annual energy use of lake-source heat pump system with air source heat pump system is presented. SEER of lake-source heat pump system in winter and summer are higher than air-source heat pump system. It prove that open loop lake source heat pump is more energy efficient than air-source heat pump. However, SEER of open-loop water source heat pump systems used with lake decrease in winter and summer included by water pump. So, it is necessary to decrease the energy consumption of water pump for higher energy efficiency of open-loop lake source heat pump system.