| Air-source heat pump and ground-source heat pump have been widely developed in the market. However, the essences of their problems, such as frosting in winter of air-source heat pump and high initial investment upon ground-source heat pump, need to be further investigated and resolved. Recently, it shows good prospects about solar-assisted heat pump because it has higher evaporation temperature than air-source heat pump. But it would not be employed all day unless heat storage device was installed. Heat-Source Tower heat pump(HSTHP) employed cooling tower to absorb heat from air in winter. In practice, however, the problem of low evaporation temperature in HSTHP existed. Based upon the analysis above, a novel solar thermal heat-source tower heat pump(STHSTHP) is proposed.The choice of heat medium fluid is a key problem in STHSTHP. In this paper, glycol/water solution, ethanol/water solution, glycerol/water solution, methyl-carbonate/water solution were proposed as heat medium fluids. Their mixture ratios were presented and six aspects of properties including freezing point, density, specific heat capacity and viscosity were obtained by experiments. The calculation model of head loss was presented and the analysis of the effects of viscosity and density upon head loss was carried out. It concludes that the factors viscosity and density could be ignored as a standard of choosing the adequate heat medium fluid when the mass flow rate was low enough. Freezing point of methyl-carbonate/water solution is -6.7℃, so it can be utilized in the southern area. The others can be applied in Tianjin.In order to identify the performance of the different heat medium fluids in solar collector, solar collector heat collection performance testing system was established according to GBT17581-2007 and ISO9801. The results said that the absorbed heat and the instantaneous collector efficiency were affected by heat medium fluids, solar irradiance and solar collector inlet temperature. Heat medium fluids contained glycol/ water solution, ethanol/water solution and glycerol/water solution. Solar irradiance varied from 450W/m~2 to 800W/m~2. Solar collector inlet temperature was respectively set to be -12℃, -9℃and -5℃. Heat collection performance of solar collector varied according to different heat medium fluids. Given that glycol/water solution and glycerol/water solution were heat medium fluid and solar irradiance was constant, the instantaneous collector efficiency and the absorbed heat would be lower as solar collector inlet temperature decreased. However, when ethanol/water solution was heat medium fluid and solar irradiance was constant, the instantaneous collector efficiency and the absorbed heat would be higher as solar collector inlet temperature decreased. Solar irradiance in the range of 550-700W/m~2 had possessed of highest absorbed heat change rate, followed by solar irradiance in the range of 700-800 W/m~2. The range of 450-550 W/m~2 was at least of three ranges.Given that there was 4kW as heat output in the existed heat pump of the lab, the results of theoretical analysis said that solar collector would absorb heat 1.11kW, 1.13kW or 1.17kW when collector inlet temperature was -12℃,-9℃and -5℃respectively. Correspondingly, the adequate solar collector area was obtained. Ethanol/water solution as heat medium fluid needed the minimum area 5.88m~2, glycerol/water solution 6.78 m~2 and glycol/water solution 6.97 m~2.According to comprehensive information about heat medium fluid and heat collection performance, it concludes that glycerol/water solution as heat medium fluid applied by STHSTHP was suggested in Tianjin area.
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