Turbo-compressors for R-718: Experimental evaluation of a two-stage steam compression cycle

Water (R-718) is a safe and energy-efficient refrigerant. Mechanical vapour recompression (MVR), an open-loop heat pump using R718, can significantly reduce the energy consumption for steam-heated processes like drying, pasteurization, evaporation or distillation. However, the existing compression technology is not cost-efficient, especially in the capacity range from 500 kW to 4 MW. Therefore, a novel two-stage turbo-compressor system, developed for application in industrial superheated steam drying and based on mass-produced automotive turbocharger technology, was developed. Its performance was evaluated in a test facility, showing that it is possible to compress superheated steam from atmospheric pressure up to 3 bar, delivering 300 kW at 133°C, with a COP of 5.9, an isentropic efficiency of 74% and a Carnot efficiency of 48%. With an estimated investment cost of 150 €/kW installed heating capacity, the system clearly has the potential of being a cost-effective solution for heat recovery in steam-heated industrial processes.