MAXIMUM POWER FROM A HOT STREAM

This paper reports the solution to the fundamental problem of how to m aximize the mechanical power extracted from a hot single-phase stream when the total heat transfer area bathed by the stream is constrained. It is shown that the optimization has two degrees of freedom: the sha pe of the stream temperature distribution as a function of the length (x) traveled along the heat transfer surface, and the position of this distribution on the absolute temperature scale. The optimal stream te mperature distribution is exponential in x, and so is the temperature distribution along the hot end of the system that converts the heat tr ansfer into mechanical power. At any x, the temperature difference acr oss the heat exchanger is proportional to the local absolute temperatu re. Similar conclusions are reached for the cold end heat exchanger, w hen the power system rejects heat to a cold single-phase stream. It is shown that the optimal solution can be implemented in practice by usi ng two counterflow heat exchangers. Each counterflow is imbalanced to a degree recommended by thermodynamic optimization. The effect of the sizes and capacity rates of the two heat exchangers is documented. (C) 1998 Elsevier Science Ltd. All rights reserved.