Maximum possible efficiency for energy recovery from low-grade heat sources

Gas-to-gas heat exchangers are increasingly being used to recover low-grade heat (or cooling) to reduce energy requirements for space heating, drying and other applications. The efficiency of heat recovery is often limited by practical considerations which could – in principle – be reduced or eliminated by development of new materials and technology. However, this paper posits a fundamental limitation on the efficiency of heat recovery that may be achieved, determined by the temperature difference between the inlet streams, the rate of heat transfer per unit volume and properties of the particular gases (or other fluids) between which heat is transferred.

Keywords: heat recovery efficiency, heat exchangers, heat transfer, thermodynamics, exergy, laminar flow, counter-current, low-grade heat, temperature difference, inlet streams