A Rankine-cycle engine that contains a binary working fluid (ammonia + water) and a recuperative heat exchanger has been built and tested. This engine is a prototype of "bottoming"-cycle engines that would be used to extract additional useful power from the exhaust heat of gas turbine engines. Its advantages are well suited to vehicles, where volume and weight are important constraints.

The described binary-fluid Rankine cycle has higher efficiency and smaller volume than Rankine cycles with single-component working fluids. It has nearly the same efficiency but much smaller volume than the binary-fluid Kalina Rankine cycle, which entails the use of separators and additional heat exchangers to vary the proportions of ammonia and water in the working fluid at various points in the cycle. The present cycle does not vary the composition of the working fluid. Because the separators and additional heat exchangers are not needed, an engine based on the present recuperated, binary-fluid Rankine cycle has only a fraction of the volume of the corresponding Kalina-cycle system.

This Recuperated, Binary-Fluid Rankine-Cycle Engine exploits the unique characteristics of a binary working fluid to recycle some of the heat that would otherwise be dumped out through the condenser. This improves the system efficiency and reduces its size. The thermodynamic parameters indicated in the figure were calculated for a working fluid of 80 weight percent ammonia and 20 weight percent water.

The present recuperated, binary-fluid, Rankine-cycle engine (see figure) includes a boiler/superheater, a turbine that drives an electric-power generator, a condenser, a pump, and a recuperative heat exchanger ("recuperator" for short). The boiler/superheater transfers heat from the gas-turbine exhaust or other source to the working fluid. The turbine extracts useful power from the heat in the working fluid. The condenser removes the final waste heat that is of too low a temperature to be worth recovering. The recuperative heat exchanger transfers heat from the turbine-outlet/condenser-inlet junction (a higher-temperature, lower-pressure location) to the pump-outlet/boiler-inlet junction (a lower-temperature, higher-pressure location). In so doing, the recuperator recycles some of the heat that would otherwise be dumped out through the condenser as waste heat; thus, the main effect of the recuperator is to increase the energy-conversion efficiency.

A large variation (≈100 K) in the saturation temperature of the ammonia/water mixture with the vapor fraction during boiling makes recuperation possible in this cycle. In designing the recuperator, one must consider the details of heat transfer and the behavior of the binary working fluid. The flow on the low-temperature side of the recuperator proceeds through the sequence of single-phase liquid, two-phase boiling, and superheated vapor. On the high-temperature side of the recuperator, the flows at the corresponding positions in the recuperator may be two-phase condensing or superheated vapor. Thus, the operation of the recuperator entails several different two-phase heat-transfer phenomena, all of which must be taken into account.

Both theoretical calculations and experiments have shown that this recuperated, binary-fluid Rankine-cycle engine operates with energy-conversion efficiency 1.5 or 2.0 times that of an otherwise identical engine that contains either ammonia (only) or water (only) as the working fluid. It has also been found that the total volume of the three heat exchangers (the boiler/superheater, the condenser, and the recuperator) in this engine is 10 percent less than the total volume of the two heat exchangers (the boiler/superheater and the condenser) for a nonrecuperated engine using the binary fluid at the same source and sink temperatures.

This work was done by Christopher J. Crowley and Martin A. Shimko of Creare, Inc., for Glenn Research Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Glenn Research Center
Commercial Technology Office
Attn: Steve Fedor
Mail Stop 4 —8
21000 Brookpark Road
Cleveland
Ohio 44135.

Refer to LEW-16834.

NASA Tech Briefs Magazine

This article first appeared in the September, 2000 issue of NASA Tech Briefs Magazine.

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