The thermodynamic potential of high-temperature transcritical heat pump cycles for industrial processes with large temperature glides

Abstract

Industrial heat pumps up to 200 °C are an emerging technology with the potential to reshape the industrial heating supply. For large heat sink temperature glides, transcritical cycles are able to increase the power-to-heat efficiency. Its potential is however yet to be unlocked. To examine this potential, a thermodynamic optimization model is proposed. The model includes robust cycle optimization, is able to screen a large set of working fluids, and includes proper post-processing. This model is applied to three highly relevant industrial cases, namely thermal oil heating, superheated steam drying and spray drying. The heat sink temperature glides for the respective case studies are 60 K, 81 K and 105 K. The results show that a temperature glide larger than 60 K is desired to achieve a better coefficient of performance (COP) with transcritical cycles compared to the classical subcritical cycles. Moreover, potential working fluids were identified for these high operational temperatures. For the case study with a heat sink temperature glide of 81 K, transcritical cycles allowed for a COP increase of 4.6%, whereas this increased to 7.3% for a heat sink temperature glide of 105 K. Furthermore, transcritical cycles introduce a much larger volumetric heating capacity, a lower compressor discharge temperature and a substantially lower pressure ratio. In addition, the best performing working fluids for subcritical cycles are highly flammable, which is only the case for some transcritical working fluids. Therefore, these cycles can be beneficial for temperature glides below 60 K. The compressor for transcritical cycles should however be able to cope with pressures up to 60 bar. If these compressors are available, transcritical cycles are shown to be superior compared to classical subcritical cycles.