Organic Rankine Cycle Waste Heat Recovery Systems for Aircraft Engines

Abstract

Since the advent of commercial aviation, advancements in propulsion system technology
have been the main cause of the reduction of fuel consumption. Modern turbofan
engines typically achieve a thermal efficiency of approximately 50%, implying that
roughly half of the chemical energy released by fossil fuel combustion is lost to the environment as hot exhaust gas.

For gas turbine engines of stationary power plants, it is common practice to use bottoming
units based on the Rankine cycle to recover part of this energy and increase thermal efficiency by up to 20%. The concept of the combined-cycle engines is also suitable for applications with low power capacity, however, organic compounds instead of water must be used as the working fluid of the bottoming unit. The combined-cycle concept is in principle also suitable for aircraft engines, however, adding an organic Rankine cycle (ORC) waste heat recovery system to an aircraft gas turbine engine is challenging because the thermodynamic benefit is counterbalanced by the increased aircraft mass and drag. The few studies conducted so far on combined-cycle aircraft engines indicate a possible net benefit on fuel consumption, however, these results are based on low-fidelity models, neglecting or only partially considering the effect of the new engine configuration on aircraft design and performance.

The work documented in this dissertation aims to provide reliable information on the feasibility of the combined-cycle engine concept based on complex system models, enabling the optimization of preliminary designs and formulating design guidelines. For this purpose, a simulation framework that considers the interaction of the gas turbine, the bottoming unit, and the aircraft was developed. This software package is named ARENA framework, and it can provide the preliminary design of combined-cycle engines optimized for minimized fuel consumption while considering their effect on aircraft design and performance.

ARENA was used to model the effect of this novel technology on the fuel consumption of three exemplary aircraft adopting different combined-cycle configurations and mission scenarios. These cases are 1) a medium-range aircraft employing a combined-cycle auxiliary power unit (CC-APU) instead of a conventional APU to provide power on the ground, 2) a medium-range turboelectric aircraft employing combined-cycle turboshaft engines (CC-TS) in place of conventional turboshaft engines, and 3) a medium-range partial-turboelectric aircraft replacing conventional turbofan engines with combined-cycle turbofan engines (CC-TF). All combined-cycle engine configurations are based on an ORC waste heat recovery unit implementing a non-recuperated cycle, using cyclopentane as the working fluid, whereby the ORC turbogenerator converts the recovered heat into electrical power. The simulated CC-APU engine consumes approximately 50% less fuel to provide ground power compared to a conventional APU, which corresponds to mission fuel savings of approximately 0.6%. The power output of the CC-APU engine is 250 kW, of which 60kW are provided by the ORC turbogenerator. The optimized ORC unit features a mass-specific power of 1.5kW/kg and an efficiency of 15%, while the overall combined-cycle efficiency is 34%. The fuel savings calculated in the case of the CC-TS engine are 1.5%, if compared to a single-cycle turboshaft engine. The combined power output is 5.4MW of which 340kW are contributed by the ORC turbogenerator. The optimized ORC unit mass-specific power is 1.3kW/kg and its efficiency
at cruise is 17%, while the CC-TS engine efficiency is 53%. The CC-TF engine burns 4% less fuel if compared to a single-cycle engine. It contributes 60% of the cruise thrust and 2.6MW of shaft power of which 570 kW are provided by the ORC turbogenerator. The shaft power is converted to thrust by the electrical distributed propulsion system. The optimized ORC unit has a mass-specific power of 1kW/kg and an efficiency of 18%. The performance difference between the CC-TS and CC-TF engines is mainly due to different condenser integration architectures. The condensers of the CC-TF engine are integrated into the engine bypass duct downstream of the fan, whereas the condensers of the CC-TS engine are placed into ram-air ducts. The combination of pressure rise and thermal energy input into the bypass air stream increases the propulsive efficiency and the specific thrust of the CC-TF. According to these preliminary studies, the optimized CC-TS and CC-TF engines have no appreciable impact on the lift-to-drag ratio of the aircraft and the maximum take-off mass only increases by a few percent.
It can be concluded that according to the results of this work, the thermodynamic benefit of adopting an ORC system to recover the thermal energy of the exhaust of gas turbines onboard aircraft can outweigh the penalties of the increased aircraft mass and drag. However, the uncertainty due to modeling limitations and simplifying assumptions suggests that further research and development are needed before decisions regarding the development of this engine concept can be taken. Such a drastic change in engine configuration would only be justifiable if the fuel consumption reduction is larger than what was estimated. Further performance improvements may be possible if advanced heat exchanger technology is considered. Furthermore, as well known from theory and practice regarding ORC power plant technology, the identification of an optimal organic working fluid (pure or mixture) may result in considerable performance and operational improvements. Another research direction worth investigating is the optimization of the design of the combined-cycle engine to minimize environmental impact and not fuel consumption. Preliminary considerations show that the benefit of waste heat recovery in this case may be even larger.