The extraction of power from a hot stream

The solution to the problem of maximizing the extraction of exergy from a stream of hot gas showed that the hot stream must be cooled in a counterflow heat exchanger with optimal imbalance of capacity rates, i.e. with an optimal capacity rate on the cold side. This paper outlines the first few steps toward making this solution practical, by combining the optimized counterflow with conventional components for compressing and expanding the cold stream. In the first part of the paper, the cold stream is compressed in an isothermal compressor, expanded in an adiabatic turbine, and discharged into the ambient. In the second part, the cold stream is compressed in an adiabatic compressor. Both designs are optimized with respect to two degrees of freedom, the capacity-rate imbalance of the counterflow, and the pressure ratio maintained by the compressor. The effect of other constraints is documented, e.g. heat transfer area size, hot gas initial temperature and compressor and turbine efficiencies. This study shows the tradeoff between simplicity and increased performance, and outlines the path for further conceptual work on the extraction of exergy from a hot stream that is being cooled gradually. Copyright © 2001 John Wiley & Sons, Ltd.