The selection of suitable working fluids for use in Organic Rankine Cycles (ORC) is strongly addicted to the intended application of the ORC system. The design of the ORC, the kind of heat source and the ambient condition has an influence on the performance of the Organic Rankine Cycle and on the selection of the working fluid. It can come to a discrepancy between the best candidate from the thermodynamic point of view and the transformation into a real machine design. If an axial turbine design is considered for expansion and energy conversion within the ORC, the vapor volume flow ratios within the expansion path, the pressure ratio and of course the number of stages have to be considered within the fluid selection process and for the design parameters. Furthermore, environmental aspects have to be taken into account, e.g. the global warming potential (GWP) and the flammability of the selected fluid.
This paper shows the results of the design and fluid selection process for an Organic Rankine Cycle for application in a combined operation with a 2MW class industrial gas turbine.
The gas turbine contains two radial compressor stages with an integrated intercooler. To further increase the thermal cycle efficiency, a recuperator has been implemented to the gas turbine cycle, which uses the exhaust gas waste heat to preheat the compressed air after the second compressor, before it enters the combustion chamber. The shaft power is generated by a three stage axial turbine, whereby the first stage is a convection cooled stage, due to a turbine inlet temperature of 1100°C.
To further increase the electrical efficiency and the power output of the energy conversion cycle, a combined operation with an organic Rankine cycle is intended. Therefore the waste heat from the GT compressor intercooler is used as first heat source and the waste heat of the exhaust gas after the recuperator as second heat source for the Organic Rankine Cycle. It is intended that the ORC fluid acts as heat absorption fluid within the compressor intercooler. Due to these specifications for the ORC, a detailed thermodynamic analysis has been performed to determine the optimal design parameter and the best working fluid for the ORC, in order to obtain a maximum power output of the combined cycle.
Due to the twice coupling of the ORC to the GT cycle, the heat exchange between the two cycles is bounded by each other and a detailed analysis of the coupled cycles is necessary. E.g. the ambient temperature has an enormous influence on the transferred heat from the intercooler to the ORC cycle, which itself affects the heat transfer and temperatures of the transferable heat from the second heat source. Thus, a detailed analysis by considering the ambient operation conditions has been performed, in order to provide a most efficient energy conversion system over a wide operation range.
The performance analysis has shown that by application of an ORC for a combined operation with the intercooled and recuperated gas turbine, the combined cycle efficiency can be increased, for a wide ambient conditions range, by more than 3 %pts. and the electrical power output by more than 10 %, in comparison to the stand alone intercooled and recuperated gas turbine.
