Compressor-assisted heat transformer for low-grade heat recovery

• Two alternative compressor heat transformer variants are described and compared. • The application considered was heat recovery from 35 to 90 °C. • The high pressure compressor variant was found to be the most efficient. • Electric COPs between 3.8 and 14.5 were found in the selected application. • Electric COPs up to twice that of a reference heat pump were found. The recovery of low-temperature waste heat can be promoted by upgrading its temperature. Heat transformers can increase the temperature of a heat stream while using little electrical energy, at the price of rejecting part of the heat input to a lower temperature sink. However, the lifting capability of heat transformers are limited by the operating conditions and often insufficient to meet the temperature requirements of many applications. An option to overcome this limitation is to use a compressor-assisted heat transformer (CHT), which benefits from the mechanical work of a compressor to expand the lifting capability. This work analyzes two alternative variation of CHT, characterized by a different positioning of the compressor in the thermodynamic cycle: CHTa, with the compressor at low pressure and CHTb with the compressor at high pressure. The CHTb variation showed higher electrical efficiencies (in the range 5–15 vs 1.5–10) and lower maximum compressor temperature (180 vs. 230 °C) than the CHTa, when recovering heat at 60 °C to deliver upgraded heat up to 120 °C. The use of the CHTb is therefore further investigated in four practical applications, assuming heat recovery from data centers, combined heat and power plants, district heating networks and industrial steam as a high temperature sink. In all the applications the CHTb has a higher electrical COP than the one of a reference heat pump, with values ranging from 4.5 to >25.