Closed-cycle compression heat pumps are described in the section Heat pump technology . Some have been realised to recover heat from refuse incineration plants, notably in Sweden and Denmark. The COP typically ranges from 1.2 to 1.4. With a heat transformer waste heat can be upgraded, virtually without the use of external drive energy. Useful heat of a higher temperature is given off in the absorber.
All current systems use water and lithium bromide as working pair.
COPs under these conditions range from 0.45 to 0.48. Reverse Brayton-cycle heat pumps recover solvents from gases in many processes. Solvent loaden air is compressed, and then expanded. The air cools through the expansion, and the solvents condense and are recovered. When heat pumps are used in drying, evaporation and distillation processes, heat is recycled within the process.