Feasibility of an On-Board Micro-ORC System for Small Satellites

Small satellites are receiving increased recognition in the space domain due to their reduced associated launch costs and their shorter lead time when compared to larger satellites. However, this advantage is often at the expense of mission capabilities, such as available electrical power and propulsion. A possible solution is to change from the conventional solar photovoltaic and battery configuration to a microOrganic Rankine Cycle (ORC) and thermal energy storage system that uses the waste energy from a solar thermal propulsion system. This unique approach has the potential to offer higher system efficiency and power density. However, limited literature is available on microORC systems, which are capable of producing a few hundred Watts of electrical power, especially for small satellites. A feasibility study of these systems and a fluid selection study were conducted. This was done by using a multiobjective genetic algorithm to optimise an onboard microORC system for various working fluids such as Toluene (C7H8), Hexamethyldisiloxane (MM), and Octamethylcyclotetrasiloxane (D4). The two objective functions were to minimise the total volume and maximise the thermal energy storage capacity. This paper describes the proposed system layout and model of the integrated microORC system. The specific objectives of this study are: i) the working fluid selection, and ii) the optimisation of the proposed system incorporating the design of the thermodynamic cycle and the sizing of the turbine and heat exchangers. Results show that the design of the microORC system is dependent on the mission designer requirements, and various design configurations are provided from the Pareto frontier. It was also found that when the surface wall temperature of the evaporator is near the thermal stability limit of the working fluid, the evaporator operates in the dispersed film boiling regime which reduces the heat transfer coefficient. Additional challenges include high microturbine rotational speeds, large thermal cycling, small blade heights, and large condensers. Finally, the storage configuration of the concentrator was identified as crucial for the feasibility of the system onboard small satellites.