An alternative architecture of the Humphrey cycle and the effect of fuel type on its efficiency

Abstract

Conventional gas turbines are a very mature technology, and performance improvements are becoming increasingly difficult and costly to achieve. Pressure-gain combustion (PGC) has emerged as a promising technology in this respect, due to the higher thermal efficiency of the respective ideal gas turbine cycles. The current work analyzes two layouts of the Humphrey cycle for gas turbines with pressure-gain combustion. One layout replicates the classical layout of gas turbine cycles, whereas an alternative one optimizes the use of pressure-gain combustion by ensuring the operation of the combustor at stoichiometric conditions. In parallel, both cycle layouts are studied with two different fuels (hydrogen and dimethyl ether)to account for differences in combustion specific heat addition and its effect on cycle efficiency. The current work concludes with an attempt to benchmark the maximum losses of a plenum to achieve efficiency parity with the Joule cycle, for a given pressure gain over a PGC combustor. It is found that the cycle layout with stoichiometric combustion results in an increase in thermal efficiency of up to 7 percentage points, compared to the classic cycle architecture. Moreover, the thermal efficiency of the new layout is less sensitive to the turbine inlet temperature, especially at low compressor pressure ratios. The study of the two fuels has shown that the larger mass specific heat addition leads to higher cycle thermal efficiency and should be considered during the fuel choice. Finally, the maximum allowable plenum pressure loss that results to efficiency parity with the Joule cycle has been computed for a given combustor pressure gain. For turbine inlet temperatures above 1500°C, pressure gain above 1.6 would allow for at least 20% relative pressure drop in the plenum. The respective pressure gain becomes considerably higher for lower turbine inlet temperatures.