At the opposite end of the spectrum, NASA has looked to the development of component technologies and conceptual design of large (hundreds of passengers) hybrid-electric aircraft . These concepts use turboelectric generators for electrical power, and, depending on the particular technology target, may use either cryogenic or traditional liquid hydrocarbon fuels. The use of conventional fuels ensures compatibility with existing infrastructure, and generating electrical power from Brayton cycle gas turbines for use in high-efficiency electric motors improves the overall efficiency of the concept. Such concept studies are often meant to drive component technology development, such as NASA’s recent solicitation for a megawatt-class non-cryogenic electric motor .
A promising approach for faster adoption of electrically-propelled aircraft is to consider something between the size of flight trainers and large transports, perhaps biased initially towards the light aircraft market. For success in this market, the vehicles will need to cruise at higher speeds, at longer ranges, and with larger payloads than required for flight training. Targeting light aviation as an “early adopter” market makes sense from a certification perspective. In the United States, light aircraft under 6,000 pounds need to demonstrate less than one catastrophic failure per million flight hours , which is 1,000 times less stringent than the requirement for larger commercial aircraft. Experience gained from the light aircraft market can provide data for future certification of larger commercial aircraft. This has occurred in the past for different technologies, ranging from GPS, flat-panel displays, and most recently, the use of consumer-grade tablet computers as electronic flight bags.
1. Electric Power from Fuel
The use of an infrastructure-friendly fuel, as opposed to a battery or gaseous/cryogenic source, ensures that the electric product will “drop in” to the existing infrastructure with little or no change in utility. At the power levels required for light aircraft (<1MW), turboelectric generators are less efficient and potentially costly from both an acquisition and operational standpoint. Instead, a promising energy storage mechanism for electric aircraft is the fuel cell. Fuel cells convert energy from a chemical fuel into electricity by combining the hydrogen in the fuel with oxygen, and produce water as a byproduct. If the fuel source is other than pure hydrogen, the fuel must be “reformed” by stripping the hydrogen from the fuel, which leads to additional byproducts in the exhaust. With ongoing development, fuel cells have promise to replace ICEs as they offer potential for higher efficiencies and lower emissions.
There are several different types of fuel cells, with the principal discriminator being the type of electrolyte. The selection of the electrolyte also determines whether hydrogen or oxygen ions will transport across the electrolyte. The typical fuel cells of interest in aerospace applications are the Proton Exchange Membrane Fuel Cell (PEMFC) and Solid Oxide Fuel Cell (SOFC), as they tend to have higher efficiency and lower mass than other options. PEMFCs operate at lower temperatures (<100°C) than SOFCs (600-800°C), enabling a faster start. Unfortunately, PEMFCs are best suited for operating on pure hydrogen, which can be an issue when the hydrogen source is a reformed hydrocarbon fuel. In this case, other products of the reformation process (especially sulfur and carbon monoxide) can “poison” the PEMFC. Hence, the SOFC is the preferred aerospace fuel cell when hydrocarbon fuels are used .
The use of a SOFC-enabled electric propulsion architecture has significant promise to enable transition to electric propulsion. The SOFC can utilize hydrogen reformed from traditional fuels to generate electricity, thus adding no infrastructure cost or utility penalty – the aircraft can use the same airports and fuel depots as before. The high efficiency of the SOFC vs. the ICE results in less fuel usage (and therefore lower operating costs), and is directly correlated with lower carbon emissions. A fuel cell architecture that uses infrastructure-friendly fuels enables a hybrid power generation approach for larger aircraft that still may use combustion for primary propulsion, but instead generate electricity for onboard system use at a greatly increased efficiency over power extraction from the propulsion cycle.
An added benefit of the hybrid SOFC-electric power system is the potential for a major reduction in emissions. The SOFC operates at 600-800°C, which is certainly a high temperature, but is much lower than the temperatures
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