An Integrated Assessment of an Organic Rankine Cycle Concept for Use in Onboard Aircraft Power Generation

Abstract

It is well established that there are advantages in moving towards non-pneumatic engine secondary systems. Such systems are used primarily to provide pressurization, cabin climate control, and de-icing; however, as bypass ratios continue to grow and engine cores become more efficient, the engine fan diameter is increased and core size is diminished. As a consequence, pneumatic off-takes require a larger percentage of the core flow leading to larger performance penalties. One current solution is to drive the aircraft environmental control systems (ECS) with large engine driven electric compressors rather than to use high pressure air from the core. Since cores are generally less sensitive to electrical power off-takes than to pneumatic off-takes this results in a smaller performance penalty. [F1] Using electrical air compressors also ensures fresh, clean air is delivered to the ECS thereby eliminating the risk of engine bleed contaminated cabin air. This research uses the Environmental Design Space (EDS) to examine the feasibility of recovering engine core exhaust heat to perform useful work within the aircraft. EDS serves to capture interdependencies at the conceptual design level of fuel burn, emissions, and noise for conventional and advanced engine and airframe architectures [F2]. Recovering exhaust heat is accomplished through a novel concept that makes use of an organic Rankine cycle (ORC).The concept is similar in principle to heat recovery steam generators used in power plant applications to improve combined cycle efficiency [3]. The main difference is the ORC system is relatively lightweight and appropriate for use onboard an aircraft. The waste heat in this application is used to generate electricity to drive external air compressors to supply flow to the ECS. As a result pneumatic bleeds within the engine can be eliminated, thereby eliminating growing performance penalties associated with shrinking core size and increased fan diameters. An ORC is considered because ORC cycles are ideal for extracting low grade heat. As an additional benefit the ORC vapor cycle can use the fan inlet and wing leading edge anti-ice devices as a condensation heat transfer mechanism that could also allow the system to provide anti-icing capabilities, further reducing engine pneumatic off-takes. The current research focuses on the system as applied the ORC concept to a CFM56 sized engine and has analytically demonstrated from a 0.9% to a 2.5% benefit in vehicle fuel burn relative to a conventional, pneumatically driven ECS. Actual fuel burn savings are dependent on the net installation weight of the ORC cycle.