“…This trend is due to the fixed turbine exit temperature, i.e., between 873.15 K and 923.15 K for the machines under consideration in Figure 5, and variable speed control to find the optimal operation line to achieve highest possible efficiency at full and part load. The optimal operation line is obtained by matching the compressor and turbine characteristic curves for a set of variables such as rotational speed, pressure ratio, inlet and outlet temperature, mass flow of air and fuel [24,35,36]. As most of the CHP machines operate either at full load or above 50% part load, the variation in the η em,gross , i.e., from the gross mechanical power output to the electrical power output, within this range is little owing to low frictional losses.…”