Organic Rankine cycle waste heat recovery systems for aircraft auxiliary power units

Abstract

The prime mover of current-day auxiliary power units (APUs) onboard passenger aircraft is a small single-shaft gas turbine. Given the low overall pressure ratio and turbine inlet temperature of these engines, their thermal efficiency is low and typically in the range of 15–20%. For a short-range flight, the fuel consumption of the APU amounts to approximately 1.0–1.5% of the mission fuel mass. Therefore, an improvement in APU efficiency is desirable. This paper documents an investigation of the feasibility of adding an organic Rankine cycle (ORC) waste heat recovery (WHR) system to the APU. Furthermore, the simulations and resulting analysis of this simple configuration provide a test case to verify a newly developed multidisciplinary design method based on reduced-order models of the aircraft, the gas turbine, and the ORC system in preparation for more complex studies on so-called combined-cycle power units and engines. The simulation infrastructure is implemented in Python and allows for the analysis of the thermodynamic performance and the estimation of the system size and mass. This method is coupled with an optimizer to identify the combined-cycle APU (CC-APU) design leading to the lowest mission fuel mass, under the assumption that the CC-APU is only used to provide power during ground operation. The case study considers the replacement of the 250 kW APU of an Airbus A320neo with the envisaged CC-APU and investigates its impact on mission fuel burn for a 600 NM mission. Results indicate that the fuel consumption of the CC-APU associated with the provision of ground power can be 50% lower than the fuel consumption of the currently installed APU. This corresponds to a reduction of overall mission fuel mass of 0.7%. The thermal efficiency of the optimal CC-APU design is 34% and the dry mass is 148 kg. The ORC WHR unit features an estimated mass-specific power of 1:7 kW=kg. The design of the ORC WHR unit is driven by system size and mass constraints rather than achieving optimal thermodynamic performance. This is in contrast to conventional applications of such systems for stationary applications. In the future, the developed simulation infrastructure will be extended to assess the feasibility of ORC WHR systems for larger power-capacity aircraft engines.