A novel hybrid-energy heat pump with refrigerant injection: Performance characterization and injection optimization

Heat pumps have been widely used for space heating towards sustainable buildings. Thermally-driven absorption heat pumps can greatly reduce electricity consumption but suffer from low reliability, low applicability or low efficiency under some circumstances. To address these problems, this study proposes a novel hybrid-energy heat pump (HEHP) with refrigerant injection, offering a high flexibility to gradually transform from an absorption cycle to a compression cycle. The refrigerant injection can reduce the required driving temperatures of the absorption sub-cycle, aiming to utilize energy with lower temperatures. Thermodynamic models have been established with verified accuracy to characterize the heating capacity, COP (coefficient of performance) and PEE (primary energy efficiency) under various refrigerant injection ratios (R_AHP) and injection pressure ratios (CR). Results show that the constant-frequency HEHP yields more stable heating capacities while the variable-frequency HEHP yields more stable COPs. The COP increases significantly with R_AHP, reaching 17.19~42.34 with a R_AHP of 0.8 for the two HEHPs. As the CR increases, the heating capacity increases and the COP decreases, while the PEE decreases under better conditions (high T_gin and T_ein). After injection optimization, the optimal CR is found to increase significantly as T_gin or T_ein decreases, with maximum CR of 1.4~2.5. The maximum PEEs are 0.99~1.62, 1.02~1.52, 1.04~1.48, and 1.045~1.46 for R_AHP of 0.2, 0.4, 0.6, and 0.8, respectively. The turning T_ein from which a higher R_AHP yields higher maximum PEEs rises from −10 °C to 8 °C as T_gin increases from 100 °C to 130 °C.