Two-stage evaporator for R744 heat pumps using greywater as heat source

Heat pumps are pinpointed as a key technology in decarbonizing buildings' heat supply, which is currently dominated by fossil-fuel based systems. To minimize the environmental impact of heat pumps over their lifetime, selecting natural working fluids (refrigerants) is the only viable long-term choice. Independent of the refrigerant, the efficiency of a heat pump is closely linked to temperature of the applied heat source, and to this end, wastewater is a promising candidate that is abundant in all urban environments. This experimental study investigates a novel two-stage, ejector-supported, brazed plate evaporator for its use in R744 (CO2) heat pumps with greywater as a heat source and compares it to more conventional evaporators. The work shows that splitting the evaporation process in two stages with an ejector in between allows reducing the evaporation pressure while the compressor suction pressure can be increased, in some cases significantly. Even if several possibilities for further optimization of the novel two-stage evaporator were identified, the results show heat pump efficiency improvements above 10 % compared to an equivalent heat pump with a well-performing one-stage evaporator. Heat pump systems aiming at utilizing maximum temperature difference for the greywater should use a high pressure lift ejector.