Optimization of the process design parameters in cascaded heat pump systems for steam generation using natural refrigerants
The reliance on fossil fuels in industrial steam generation necessitates innovative decarbonization strategies. Highly efficient heat pump systems present a promising alternative, where the selection of the refrigerants and the process design are crucial factors for economic and environmental sustainability. This paper investigates the systematic optimization of the internal process parameters (superheating, intermediate pressure level, mass flows) of various alternative double stage heat pump circuits and the intermediate temperature level between the high-temperature heat pump and the vapor compression system for maximizing the system performance like the COP or the VHC with respect to the selected refrigerant and the process restrictions. Based on the heat demand of industrial processes, such as drying and rectification, specific process conditions on the source and sink sides are defined. The analysis is conducted using steady-state thermodynamic simulation models, relying on accurate thermophysical property data and (semi-)physical models of relevant plant components like compressors, heat exchangers and flash tanks. A particular focus is placed on the adaptation of advanced circuit concepts using hydrocarbons as refrigerant. The resulting highly non-linear optimization problems are solved using the Nelder-Mead method (Downhill-Simplex). Objective of this study is to quantify the overall performance gains and sustainability advantages of electrified industrial steam generation through the utilization of natural refrigerants and adaptive system integration, thereby enabling an accelerated market uptake of these novel technologies. Compared to conventional heat pumps, significantly higher temperature lifts of up to 90 K and COP improvements of up to 50% at specific operating points can be achieved. When combined with mechanical vapor recompression systems (MVR) to achieve higher temperature lifts, these configurations enable a substantial reduction in both the size and number of stages of the MVR units, while maintaining the overall COP of the cascade.