Investigating the performance of heat exchangers in absorption heat pump systems using both numerical and experimental methods

Abstract

To achieve better heating efficiency and lower CO2 emission, this study has proposed an air source absorption heat pump system with a tube-finned evaporator, a vertical falling film absorber, and a generator. To analyzeboth heat and mass transfer performances and optimize the sizes of both the absorber and the generator, a distributed parameter model and a two-dimensional numerical model have been adopted, both validated. To develop an environmentally efficient working fluid pair for absorption heating for cold climate, a calculation method adopting fugacity and activity models was developed. The defrosting control strategy of this system was developed based on a spatial and temporal frost development model, which determines the characteristics of frost distribution, frost growth, and frost inhomogeneity. To evaluate the functionality of this system, a test rig was constructed, with a heating capacity of 36.88 kW, a coefficient of performance of 1.54 under evaporation temperature and supply water temperature of −9.2 °C and 38.4 °C, respectively. Validation results showed a 1.5 % higher prediction accuracy for the two-dimensional model with correction, comparing to the distributed parameter model. R134a-DMF and R161-DMF were recommended at an ambient temperature of −2 °C. This is because with ambient temperature of −7 °C and supply water temperature of 41 °C, the predicted coefficient of performance was 1.04 and 1.06 for R134a-DMF and R161-DMF, respectively. The frost prediction indicated that at the time of 3,600 s, the thickness of the frost layer in the heavy frost area was 0.94 mm, with a total frost mass of 3,995 g. Compared with the initial stage, the sensible and latent heat transfer rates decreased by 22.1 % and 24.2 %, respectively.