Reducing the environmental impact of aircraft in terms of carbon dioxide, noise, and pollutant emissions is an important objective. Fuel-cell systems deliver electrical energy with high efficiency—almost noiseless and without pollutant emissions. Due to these properties, fuel cells are a promising energy source, especially for “more-electric aircraft.”
But the integration of fuel-cell systems as an independent energy source makes an additional cooling system necessary. The same applies for new electrical environmental control systems architectures and power electronics.
A joint system architecture development aims to reduce the cooling system impact on weight, power consumption, and aerodynamic drag. The sharing of cooling loops and heat sinks may reduce overall system impact. Further, the development of alternative heat sinks improves heat dissipation to the environment.
A combined system development starts with analyzing heat loads and temperature levels of the main thermal aircraft systems. In PEM fuel cells, chemical energy from hydrogen and atmospheric oxygen is transformed to electrical energy. Because of irreversible processes inside the fuel cell, waste heat has to be rejected. Exceeding temperature limits leads to water management problems and eventually destruction of the fuel-cell membrane. Therefore, an additional cooling loop is necessary. Liquid cooling is state of the art for high-density fuel cells.
An environmental control system is necessary to maintain a safe and comfortable atmosphere in an aircraft cabin. Recent research projects and new aircraft programs have shown potential for bleed-less environmental control systems. Architecture concepts for electrical environmental control systems (EECS) use a combined approach with an air cycle machine (ACM) and a vapor cycle system.
Higher heat loads in different aircraft zones increase the need for alternative heat sinks. Ram air channels are state of the art for providing cooling air for an aircraft. In flight, air flows over a flush inlet into the ram air channel. On ground, fans can be used to create a cooling air stream. A disadvantage of the ram air channel is an increase in aerodynamic drag.
One approach to thermal management is integration of a heat exchanger into the aircraft skin (Fig. 1). Possible integration areas are the aircraft tail cone and the belly fairing.
Another promising approach is a liquid skin heat exchanger in which cooling pipes are installed on the inner aircraft skin. The installation of this heat exchanger is a possible solution to reduce the aerodynamic drag caused by cooling air demand. The heat from the fluid passes through the pipe wall and is then released to the aircraft skin. The aircraft skin is used as a heat exchanger surface and transfers the heat energy to ambient air.
A cooling architecture should be designed in a way that cooling loops and heat sinks are used by different heat sources, researchers at the Hamburg University of Technology say. Furthermore, it should be taken into account whether components or subsystems have to be heated and can be used as a heat sink.
The cooling architecture design starts with identifying and analyzing fluid streams that need to be cooled or heated. As this may vary within the flight mission, all relevant flight phases have to be considered. Essential data for designing cooling architectures are the temperature levels, heat loads, and used fluids. Furthermore, limiting factors such as different installation zones have to be considered.
A possible combined system architecture (Fig. 2) consists of liquid-cooled fuel cell stacks, dc-dc converters, and a moist air condensation unit for the fuel-cell exhaust gas. The heat sinks are a liquid hydrogen tank, the tail cone cooling system, and a liquid skin heat exchanger The EECS consist of an electrical driven ACM and a vapor cycle. The ACM heat exchanger and the vapor cycle condenser are cooled by ram air. The condensed air from the fuel-cell inert gas system is injected in the ram air. The water evaporates, cools the ram air, and reduces the necessary mass flow rate. Pressurized air from the ACM compressor is used to feed the fuel-cell cathode.
The air leaving the ACM is mixed with recirculated cabin air and the vapor cycle system is used to cool the mixed air. Moist air water condensation might occur and increases the evaporator heat load. The vapor cycle condensation and evaporation temperatures directly influence vapor cycle compressor power and performance. Varying these temperatures also influences the temperature differences to the cabin and ambient air, and therefore the heat exchanger size.
Analyzing large simulation models and identifying the most influential parameters is time-consuming work. Sensitivity analyses are used to quantify the parameters’ influence on key performance indicators. The applied sensitivity analyses are variance-based methods. By using sensitivity indices, the influence of the input parameter variance on the output variance can be measured; it can be determined whether parameters are essential or unessential.
Figure 3 shows the system mass sensitivity for four flight phases. The control of the air cycle machine has the highest sensitivity in all flight phases. As the ACM is an essential consumer of electrical current, ACM design influences the EECS mass as well as the fuel-cell system mass. So, the combined system development is necessary to find the optimal system architecture, the ACM being the driving subsystem.
This article is based on SAE International technical paper 2013-01-2274 by Enno Vredenborg and Frank Thielecke of Hamburg University of Technology.
Continue reading »