Design of a heat upgrade system for heat recovery in a paper factory
High-temperature heat pumps play a key role in industrial decarbonization by enabling the recovery and upgrading of waste heat to process-relevant temperatures. This paper presents the design of a heat upgrade system for a paper factory developed within the PUSH2HEAT project, demonstrating the industrial application of high-temperature heat pumps. The system upgrades waste heat at 90 °C from a cogeneration plant to produce saturated process steam at 6.5 bar and 162 °C. Due to the high temperature lift, the process is implemented in two stages: a high-temperature heat pump with a screw compressor using R1233zd(E) generates steam at 1.7 bar (105 °C), followed by a mechanical vapor recompression unit that increases the steam pressure to 6.5 bar. The non-flammable refrigerant enables direct steam generation in the condenser, eliminating the need for an intermediate heat exchanger. The condenser and evaporator are shell-and-tube type heat exchangers, with the refrigerant flowing in the tubes. The compressor for further vapor compression is lubricated with saturated water, which prevents oil contamination in the steam circuit and keeps the outlet temperature low. At design conditions, the system recovers 600 kW, with electrical power of 143 kW and 149 kW required for the heat pump and vapor recompression stages, respectively, resulting in a heat pump COP of 5.2 and an overall system COP of 2.85.