Off-Design Heat Exchanger Modelling for Transcritical CO2 Heat Pump Cycles

In previous publications, MAN Energy Solutions discussed the steady-state [1], dynamic modelling and validation of large-scale transcritical CO2 heat pump cycles [2]. The current study focuses on the off-design modelling of heat exchangers and presents new models with the aim of capturing the performance of a sea water heat pump subject to large variation in operating and boundary conditions following seasonal variations. This study shows the impact of novel heat exchanger models on overall heat pump performance throughout its operating envelope as well as on the boundary of the latter restricted by components limitation such as the motor envelope or compressor surge. This allows the identification of the most impactful heat exchanger performance parameters on the overall cycle performance, namely the evaporator and gas cooler HTC. Both steady and unsteady model predictions are validated against manufacturer data and show a relative difference below 1% and 3% for respectively the duty and pressure drop prediction, illustrating the capabilities of off-design models. Moreover, the dynamic modelling of a shell- and tube evaporator is presented and depicts the impact of heat source temperature variation on the evolution of the refrigerant level and its performance. All the above findings contribute to providing reliable and efficient heating and/or cooling services in a wide range of process conditions.