Organic Working Fluid Optimization for Space Power Cycles

The merits of organic fluid space power cycles are surveyed and compared with those of alternate options. Selection of an optimum working fluid is recognized as an important tool to improve system performance. The main characteristics of organic power cycles are shown to be predictable with a good level of accuracy through a general method, which requests the knowledge of a limited information about the fluid properties: specific heat in the ideal gas state, a portion of the saturation curve, and the critical parameters. On the ground of such a theory the adoption of fluids with a relatively complex molecular structure and condensation at the lowest practically admissible reduced temperature allow a better efficiency than achievable with the use of toluene, which is taken as a reference fluid. The influence of turbine efficiency actually achievable in real machines on cycle performance is then addressed; performance diagrams of optimized turbines in the power range of interest for space cycles are calculated and presented. It is shown that only the combined optimization of thermal and fluid dynamic variables leads to the definition of an optimum working fluid and power cycle. A class of fluids is examined, that of the methyl-substituted benzenes, offering a wide variation of thermal properties. A thorough optimization that considers a wide range of power outputs, one-and two-stage turbines, saturated and superheated cycles is performed. For a power output of about 30 kW trimethylbenzene is found to offer the best overall efficiency, a moderate maximum pressure, reasonable turbine dimensions, and rotating speed. A thermodynamic conversion efficiency in excess of 30 percent seems achievable at a maximum temperature of 360°C for a condensation temperature of 60°C. Such energy performance suggests that ORC systems could represent a viable multifuel prime mover option also for terrestrial power generation. Thermal stability of the proposed fluid is experimentally investigated and found to be similar to that of toluene, but its definite evaluation is shown to require further testing.