Cyclic Methylsiloxanes as Working Fluids for Space Power Cycles

Abstract

The potential merits of cyclic polymethylsiloxanes, particularly those conventionally denominated D4 and D5, as working fluids for space power cycles are discussed. The attractive technical characteristics of these substances which are fully nontoxic, moderately flammable, and stable at high temperature are presented. Some experimental results on vapor pressure and on thermal stability are reported. A maximum operating temperature of about 400°C appears achievable. A comprehensive thermodynamic analysis comparing siloxanes with other classes of high temperature fluids is performed. The peculiar characters of siloxane cycles are found to be: a good overall efficiency achieved through a massive regeneration, a moderate expansion work, and an abundant volume flow at turbine exhaust. A number of two-stage turbines for two power levels (i.e., 30 and 5 kW) were designed using an appropriate optimization program. The resulting main features of such expanders were a satisfactory efficiency, a low rotating and peripheral speed, and a comparatively large wheel diameter. These characteristics seem of particular interest for low capacity systems where, with other fluids, turbines tend to be impractically small and fast rotating and where a high level of regeneration becomes more acceptable. In considering for the sake of comparison the thermodynamic performance of many classes of organic fluids, it becomes apparent that the full potential of organic power cycles in view of the variety of future needs has not yet been thoroughly investigated.

Thermal Stability of Hexamethyldisiloxane (MM) for High-Temperature Organic Rankine Cycle (ORC)

“…Colonna et al [16] report a limit of 400˝C for siloxanes, however, the duration of the experiments, the heating rate and the pressure remain unclear. For cyclic siloxanes, Angelino [17] gives a similar value of 400˝C. Dvornic [18,19] investigates extensively thermal properties of polysiloxanes, however, he does not give quantitative results for the degradation temperature of MM.…”

Heat recovery from Diesel engines: A thermodynamic comparison between Kalina and ORC cycles

“…Methylsiloxanes were proposed some years ago as working fluids in Rankine cycles [18] and are today 9 actually used in several engines [19].…”

Active subspaces for the optimal meanline design of unconventional turbomachinery

“…1 shows the corresponding process flow diagram and an exemplary thermodynamic cycle in the temperature-entropy plane. Table 1 lists the fluids selected because they feature a critical temperature > 240 C, and because of their high thermal stability [19].…”

“…In order to better illustrate how x is defined, an exemplary application of this design method is first introduced, namely the preliminary design of a mORC unit (power capacity < 20 kW) for the conversion of medium-temperature thermal sources (maximum cycle temperature < 240 C), and adopting a single-stage radial inflow turbine. This design problem, representative of applications like waste heat recovery from long-haul truck diesel engines [2], or solar power conversion in space [18,19], is particularly challenging because the choice of system operating conditions, working fluid, and components design, are mutually constrained [16]. Fig.…”