Model Based Experimental Study on Next Generation Hybrid Environmental Control Systems (HECS)

The environmental temperature range of the avionics is specified at between -31degreesC and +70degreesC. Operation outside the range leads to exponentially increases in failure rate. Controlling to the lower limit of the range requires the Environmental Control System (ECS) to supply surplus conditioned air. The generation of the surplus requires increased use of bleed and ram air, demands extra engine power and reduces aircraft aerodynamic performance. Controlling to the upper limit of the range significantly reduces the amount of the conditioned cold air, increases engine power output, improves fuel efficiencies, and improves aircraft performance.

Conventional - bleed air ECS uses engine bleed air and external ram air. It is a proven and reliable technology but largely inefficient. It demands extra engine power, reduces aircraft aerodynamic performance, and results in an enormous of thermal power penalty over the engine in addition to other maintenance problems due to overheating on ground and during ascent/descent (when ram air flow must be forced with a fan, the engine fan or a dedicated one, for example).

Electric ECS uses dedicated electric compressor(s) that feed on ram air to produce the conditioned air. It eliminates engine bleed, offers potentials such as: (1) improve airframe systems utilization and implement more efficient power units, (2) reduce engine core size, overall pressure ratio and the turbine inlet temperature, thus improving engine performance which lead to more efficient aircraft, and (3) eliminate bleed air systems lead to overall aircraft mass reduction. However, the increases in high avionics and radar heat loads demand excessive cooling power. The current electric system is relatively immature to satisfy the high demands.

A potential solution of optimisation can be achieved through a hybrid environmental control system (HECS) concept by adding a small-scale electric ECS to the conventional bleed air system. In doing so, the bleed air ECS is reduced to only provide the conditioned air to maintain the avionics at the upper operating temperature range. The add-on electric ECS will be a start-stop design, which only supply the excessive conditioned air when it is needed. The HECS therefore provides an energy efficient yet low risk solution. It keeps the demand of engine bleed air to minimum, therefore significantly increases engine power output, improves fuel efficiencies, and improves aircraft performance. It is an intermediate step towards realisation of electric ECS for future aircrafts.

The aim of this project is to develop the HECS concept into a proof-of-concept demonstration unit based on our current research outputs on avionic thermal demands, ECS modelling studies and experimental ECS evaluations. The HECS concept developed from this project will offer the potential of improving the ECS efficiency while maintaining a low failure rate. In addition, the optimised HECS can improve the aircraft performance and increase the engine power.