Multi-criterion comparison of compression and absorption heat pumps for ultra-low grade waste heat recovery

Abstract Heat pump is effective to recover ultra-low grade waste heat, and includes compression and absorption heat pumps. However, the driving source of these heat pumps are different. This makes the efficiency comparison unfair, and multi-criterion comparison is necessary. In this paper, the compression and absorption heat pumps are compared under the same condition, where 30 °C waste heat is recovered to provide 60 °C domestic heating supply. Analyses with coefficient of performance (COP), second law efficiency, exergy efficiency and exergy rate are carried out. Exergy-to-energy ratio of driving source, instead of temperature, is used to unify the evaluation of different driving sources. Results show that compression heat pump has higher COP but lower exergy efficiency, indicating more irreversible loss. This is followed by double effect, single effect and double lift absorption heat pumps. The high COP lead to effective recovery of exergy from waste heat, with higher exergy rate. However, the strong sensitivity of COP versus temperature lift in compression heat pump makes it more effective under small temperature lift, while absorption heat pumps are more effective under higher temperature lift. The multi-criterion comparisons provide both deeper understanding about heat pumps and useful framework for waste heat recovery analysis.