Thermal Analysis of Aircraft Auxiliary Power Unit: Potential of Super-Critical CO ₂ Brayton Cycle

An “APU” (Auxiliary Power Unit) is a small gas turbine engine to provide supplementary power to an aircraft and is located at the tails of larger jets. APU generators provide auxiliary electrical power for running aircraft systems on the ground. Applications include powering environmental systems for pre-cooling or preheating the cabin, and providing power for crew functions such as preflight, cabin cleanup, and galley (kitchen) operation and long-haul airliners must be started using pneumatic power of APU compressor. The Honeywell 131-9A gas turbine APU has 440 kW shaft power and 90 kW electric generator consuming 120 kg fuel/hour. Hybrid power systems based on fuel cells are promising technology for the forthcoming power generation market. A solid oxide fuel cell (SOFC) is the perfect candidate for utilizing waste heat recovery. This case deals with waste heat recovery from fuel cell exhaust using Brayton cycle as bottoming cycle for additional power production. Here in this paper the traditional combustor of the APU is proposed to be replaced by a hybrid system which integrates a solid oxide fuel cell and a super-critical carbon di-oxide Brayton cycle. In this work, carbon dioxide is proposed as the working fluid for a closed supercritical bottoming cycle, which is expected to perform better for intermediate temperature heat recovery applications than the air cycle and also thermal analysis of hybrid system consisting of SOFC and super-critical carbon di-oxide is proposed to be reported. The super-critical CO₂ power cycle has numerous advantages such as high power cycle efficiency, small turbo-machinery size (due to high density of carbon -dioxide near the critical point), lower critical pressure (7.38MPa) and temperature (304.25K), stability, non-toxic and abundant. The main advantage of this kind of hybrid system, in addition to efficiency improvement, is its ability to reduce harmful emissions and negative impacts on the environment.