Analysis Of Intermediate Pressure And Exergy For Low-temperature Air Source Heat Pump System

Since recent years, Sino-housing constructions are growing tremendously, and energy consumption becomes a real challenge for building engineers, especially in terms of the heating and air conditioning energy consumption. An Air-Source Heat Pump (ASHP) may be taken as alternative choice for diminishing the building energy consumption relying on his advantages. However, under low-temperature environment, the general air-source heat pump encounters some problems such as low efficiency, high compression ratio and unstable operation. To face the latter situations, two-stage compression method is applied. Therein, intermediate pressure is a critical parameter since it is linked with system performance and stability. With regard to intermediate pressure, investigations should be made to get high efficiency and stable conditions.In order to expand the low-temperature adaptation, a new low-temperature ASHP test bed with two rolling piston compressors is formerly developed in our previous work and can mimic outdoor environment. Based on the principle of the error analysis, the appropriate test instruments are selected. The system error is highly depending on temperature difference while slightly correlated with the flow rate. The results prevailed that the system error is less than5%. In fact, the test bed can be well reproduced via experimental process with discrepancy error3.65%for heating capacity and3.79%for heating Coefficient Of Performance (COP).The variations of intermediate pressures with evaporative pressures, condensate pressures and injected mass flow rate are determined from experimentation. For the two-stage compression system with no injection, the experimental results show that the intermediate pressure increases in high rate with evaporative pressures when the condensing pressure is2410kPa and the evaporation pressures change from270to480kPa, and in low rate with regard to condensate pressures when the evaporation pressure is400kPa and the condensing pressures change from2130to3053kPa. The heating COP for the unit is going uphill until its peak and then downhill with intermediate pressure, and. For the two-stage compression system with injection, similar trends are seen in terms of intermediate pressures with evaporative pressures and condensate pressures in the same condition. When the evaporative and condensate pressures are constant, the intermediate pressure rises exponentially first and then slowly and finally flatten as the injection mass flow rate goes up. The heating COP increases until its summit, the maximum value can reach2.5, then decreases with respect to refrigerant injection.In this paper, the Engineering Equation Solver (EES) software and the experimental data are used to setup the exergy analysis of ASHP in low-temperature environment. The results demonstrate that the compressor has the largest exergy loss (more than60%of the total exergy loss). The high-stage throttle is ranked in second position. And the plate heat exchangers are in third place where heat transfer exists, and the evaporative exergy loss is higher than the condensate one. In addition, the input exergy, output exergy and exergy loss of the unit simultaneously tone up with evaporative and condensate pressures. The exergy efficiency goes up with evaporative pressure, whereas the condensation pressure is slowly inversely proportional to exergy efficiency.