Simulation and preliminary experimental study on the sea water source tandem heat pumps

The sea water is one of promising heat sink or source for a heat pump throughout the year. In order to utilize the sea water as a heat sink or source, tandem heat pumps, which are composed of two or more heat pumps in series, were investigated by simulation and preliminary experiments. For a simulation, the heating supply and return temperatures, and heat source inlet and outlet temperatures were fixed at 60°C, 40°C, 7°C and 4°C, respectively. The performance of 2-, 3- and 4- tandem heat pumps were simulated and compared with that of a single-unit screw heat pump. The simulations were conducted at the minimum approach temperature of 4°C and 5°C in heat exchangers. A heating capacity was assumed to be about 3.5 MW. The results show that a heating COP of 4-tandem screw heat pump is about 12% higher than that of the single-unit screw heat pump under the simulation conditions considered in the present study. For a preliminary experiment, a 2-tandem heat pump which has a rated capacity of 16 kW was designed and installed. Each single-unit heat pump has a HFC-134a reciprocating compressor which has a displacement of 16.2m³/h at 1750 RPM, brazed plate type evaporator and condenser and an electronic expansion valve. An experimental was designed and built. A heating performance test was carried out at the heat source inlet temperature range from 5 to 20°C and at the heating supply temperature range from 50 to 60°C. The experimental results show that the 2-tandem heat pump is cable of providing hot water of 60°C with a heating COP of higher than 3.1, which is approximately 15% greater than the heating COP of conventional heat pumps.