Nonconventional Thermodynamic Cycles for the Nineties: Comparisons and Trends

Abstract

Ways to improve the performance of power cycles by using different working fluids will be a major objective in coming years. One of these is to modify the Brayton cycle by using water and steam injection. Such applications are comparable to gas-steam combined cycles (GSCC) in which the air and steam flows are separated. The introduction of water-steam in the typical “dry” process of the Brayton cycle affords notable improvement to system performance in terms of power, efficiency, and/or NOx emissions. Some of these solutions, among them STIG and Cheng, have been patented, with plants already onstream worldwide.

A new cycle, the humid air turbine cycle (HAT), has recently been proposed and patented. Its main innovation is that steam is produced along the airflow, thus eliminating the heat recovery boiler. A special component of this cycle is a multistage saturator, where the air and hot water mix to produce vaporization at a variable temperature, with some beneficial thermodynamic effects.

This paper presents the state-of-art of the possible modifications to the gas turbine cycle by means of watersteam adduction. The trends and developments of the Eighties will be discussed and the advantages and disadvantages of the various solutions will be pointed out. Special attention is devoted to a) increase in power output, b) variations in efficiency, and c) effects on NOx release. Both energy and exergy approaches are employed. In addition, the problem of water consumption, or recovery, is examined, since water shortages and rising costs are aspects that will become increasingly important in the near future.

In addition to Brayton cycles, a second group, Kalina cycles, will be discussed. In these, heat recovery is enhanced by means of a suitable mixture of water and ammonia.

The main difference between these cycles and the conventional Rankine cycle as a bottomer is related to different composition in the water-ammonia mixture in the various plant components, which optimize heat transfer and reduce exergy losses. An analysis of the practical possibilities of these cycles is presented and discussed.