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 rccognized 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 generaI method, which requests the knowledge of a limited information about the fluid properties: speci/ic heat in the ideai gas state, a portion of the saturation curve, and the criticaI pararn- eters. On the ground of such a theory the adoption of fluids with a relatively complcx rnolecular structure and condensation at the lowest practically admissible reduccd temperature allow a better efficiency than achievable with the lise of toluene, which is taken as a reference fluido The influence of turbine cfficicncy actually achievable in real machines on cycle performance is then addrcsscd; 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 dcfinition of an optimum working Iluid and power cycle. A c1ass of fluids is exarnincd, that of the rnethyl-substituted benzenes, offering a wide variation of thcrrnnl prope ties. 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 maxirnum pressure, rcasonable turbine dimensions, and rotating speed. A thermodynamic conver- sion efficiency in excess of 30 percent seems achievable at a rnaxirnurn tempera- ture of 360Q C for a condensation temperature of 60°C. Such energy perfor- mance suggests that ORC systems could represent a viable multifuel prime mover option also for terrestrial power generation. Thermal stability of the proposcd fluid is experimentall; investigated and found to be similar to that of tolucne, but its definite evaluation is shown to require further testing