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

orc2019 Tracking Number 45

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 [1, 2]. However, this advantage is often at the expense of mission capability, such as available electrical power. A possible solution is to change from the conventional solar photovoltaic and battery configuration to a micro-Organic Rankine Cycle (ORC) and thermal energy storage system. This unique approach has the potential to offer higher system efficiency and power density. However, limited literature is available on micro-ORC systems, which are capable of producing a few hundred Watts of electrical power, especially for small satellite applications. A feasibility study of these systems and a fluid selection study were conducted. This was done by using a multi-objective genetic algorithm to optimise an on-board micro-ORC system for various working fluids such as Toluene (C7H8), Hexamethyldisiloxane (MM), and Octamethylcyclotetra-siloxane (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, including case study validations to increase model fidelity. 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 the design of the micro-ORC system is dependent on the mission designer, and various design configurations are provided from the Pareto frontier. It was also found that when using high energy density materials, such as silicon, for the thermal energy storage system, the evaporator operates in the inverted annular film boiling regime which reduces the heat transfer coefficient. Additional challenges include high micro-turbine 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 on-board small satellites. Future work will focus on improving the fidelity of the model, especially the heat transfer coupling of the evaporator by more in-depth modelling or experimental investigations.