28 November 2014

Techno-economic evaluation of combining heat pump and mechanical steam compression for the production of low pressure steam from waste heat

Most of the industrial use of energy is for thermal applications such as drying and heating. A large percentage of the energy used is lost or dissipated into the environment as heat through warm liquid effluents and gaseous effluents. We evaluated an approach relying on combined use of a mechanical closed-cycle heat pump and mechanical steam
compression. The heat pump produces steam at moderate vacuum conditions which is afterward compressed above atmospheric pressure. This sequence is to be used for upgrading of waste heat at temperatures of 60 °C, with the subsequent production of low pressure steam (110 °C or above) as a hot utility. A techno-economic evaluation of such a sequence is presented in the context of heat recovery from wet air of a paper drying process. A COP higher than three is achievable. Although such a sequence is currently hardly economically justifiable in a context of low natural gas prices, it could eventually be beneficial, especially where renewable energy is available and where the price of electricity compares to fuel prices.