Comparative effects of two injection paths on performance of a heat pump with a sub-cooler injection configuration

Abstract

As one of the most promising and convenient heating service to replace coal-fired boilers and electric heaters due to its high energy efficiency, a heat pump has received more and more attention for the economical application and extensive research. The injection technology can be used to improve the heat pump performance. Currently, the injection ratio and pressure ratio are usually used to characterize the effects of injection styles on its performance. This is not conducive to its practical operation control because of additional cost increase. In this work, the effects of two injection cycles due to two injection paths were characterized with openings of a sub-cooling electronic expansion valve, comparatively tested and analyzed. Test results demonstrate that the heating capacities (Qh), electricity power (EP) and coefficient of performance with the refrigerant injection into compressor (CRI) are more than those with the injection into accumulator (ARI), and the overall effect with the CRI is positive because the Qh improvement outperforms EP increase. For example, the maximum Qh with the CRI at 55, 60 and 85 rps were 8.1%, 7.7% and 18.3% higher than those without the CRI, and 7.3%, 6.7% and 19.0% higher than those with the ARI. The thermodynamic model was used to analyze the experimental results and expound the mechanism of the influence of injection path on the performance of heat pump. This work provides guidance for improving energy efficiency in common areas and enhancing Qh in cold areas.

Effects of injection styles on heating performance of a heat pump with sub-cooler refrigerant injection: Experimental and thermodynamic analysis

“…In Figure 3, K indicates a cycle state at a certain SEEV opening, m 0 is the refrigerant mass flow rate discharged from the compressor without injection cycle and m K and m i, K are, respectively, the refrigerant mass flow rate circulating in the system with the SCRI through the evaporator and injection refrigerant mass flow rate passing through the SEEV into the intermediate pressure chamber of the compressor. For liquid injection, two-phase injection and 49 T d : discharge temperature; T s : suction temperature; T ao : outdoor ambient temperature; T def : evaporator coil temperature for defrosting; T liq;in : liquid pipe temperature in inlet of sub-cooler; T liq;out : liquid pipe temperature in outlet of sub-cooler; T w;in : inlet water temperature; T w;out : outlet water temperature; P d : discharge pressure; P inj : injection pressure; T inj;in : temperature in inlet of injection pipe; T inj;out : temperature in outlet of injection pipe; P s : suction pressure; MEEV: main electronic expansion valve; SEEV: subcooling electronic expansion valve; SV: solenoid valve. vapour injection, Point K 7 , respectively, falls in the subcooled zone, two-phase zone and super-heated zone of the pressure-enthalpy diagram.…”

“…The system principle diagram of the heat pump with the SCRI is shown in Figure 2.

Figure 2.Schematic diagram of a heat pump with SCRI. 49 T d : discharge temperature; T s : suction temperature; T ao : outdoor ambient temperature; T def : evaporator coil temperature for defrosting; T liq;in : liquid pipe temperature in inlet of sub-cooler; T liq;out : liquid pipe temperature in outlet of sub-cooler; T w;in : inlet water temperature; T w;out : outlet water temperature; P d : discharge pressure; P inj : injection pressure; T inj;in : temperature in inlet of injection pipe; T inj;out : temperature in outlet of injection pipe; P s : suction pressure; MEEV: main electronic expansion valve; SEEV: sub-cooling electronic expansion valve; SV: solenoid valve.

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