Thermal efficiency of direct, inverse and sCO2 gas turbine cycles intended for small power plants

Abstract

The aim of this paper is to study whether the efficiency loss that takes place at low Reynolds numbers (Re) can be a problem for small power gas turbines and, if this is the case, to assess whether the use of alternative power cycles such as supercritical sCO2 or inverse Joule–Brayton cycles might solve this problem.First, two different methods for the estimation of the Reynolds efficiency loss are explained and compared. Then, the design parameters of a number of existing commercial gas turbines are used in order to show that this efficiency loss begins to be significant when the shaft power is lower than about 1.5 MW. Efficiency and Re calculations are carried out subsequently for inverse and sCO2 1 MW cycles intended for medium range temperatures. For such small powers, the supercritical sCO2 closed loop Brayton cycle seems to be a promising alternative, as the Reynolds number experiences an increase due to the high fluid density. Also, sCO2 cycles achieve good efficiencies due to the compressor work reduction obtained near the critical point. The inverse cycle is a less interesting option because its efficiency is low and there is no significant improvement in the Reynolds number.