FUEL TECHNOLOGIES

Hydrogen fuel cell technology has potential for aviation use

System’s sole byproduct is clean, harmless water vapor.

By Steven Barceló and Samuel Mao
Univ of California at Berkeley


Schematics of an airplane that uses a proton exchange membrane fuel cell/lithium-ion battery hybrid system to power the propeller.

High prices, scarce resources, pollution, climate change and national security are all driving the push for newer, cleaner sources of energy to soften US dependence on fossil fuels. The transportation industry as a whole accounts for about 50% of total liquid fuel usage and almost 30% of overall energy consumption in the US.

This has inspired a long list of innovative technologies, from solar-powered cars to jet fuel derived from corn and specially designed bacteria, that could inspire any technophile. However, in the search for newer and better power sources, a promising technology of the future isn’t so new after all—hydrogen fuel cells.

Fuel cells generate electricity through an electrochemical process first reported in 1838, but the technology is still being refined today while obstacles exist. As a fuel, hydrogen can be used for either conventional combustion engines or fuel cells—but, while hydrogen combustion typically has an efficiency of about 40%, fuel cells can have an efficiency of greater than 70%.

Another advantage of hydrogen is that the reaction is pollutant-free, as the only byproduct is water. Finally, fuel cells can be relatively lightweight and compact, since hydrogen gas has the highest energy density per unit mass of any fuel.

These advantages make fuel cells attractive for both mobile transportation and stationary applications. Capable of power output of up to a few hundred kilowatts, they are clear alternatives for distributed power for either industry or residential buildings.

They are also well suited for applications where quick startup and response to load changes are required, such as transportation or auxiliary power systems. The high power density and low operating temperature permit intermittent operation without significant energy loss.

This is different from batteries, where rapid cycling and load changes can degrade performance and lead to shorter system life times. There are a number of fuel cell technologies, each with a different range of operating conditions best suited to a particular application.

For example, polymer electrolyte membrane (PEM) fuel cells (which are also known as proton exchange membrane fuel cells) typically operate just below 100° C, making them convenient for mobile applications.

Solid oxide fuel cells (SOFCs) operate at much higher temperatures—typically over 500° C—but their high efficiency, low cost and long life make them suitable for stationary large-scale power generation.

Molten carbonate fuel cells (MCFCs) are another high-temperature variety, typically operating above 650° C. MCFCs have characteristics similar to those of SOFCs—like high efficiency and resistance to poisoning—but have reduced durability due to their use of corrosive high-temperature liquids.

At present, PEM fuel cells are the most commonly investigated fuel cell technology for mobile transportation applications, so it’s worth describing how they work in more detail. In PEM fuel cells, hydrogen and oxygen are combined electrochemically to produce electricity, heat and water, as shown in the figures on this page and p 72.

The heart of a PEM fuel cell is the membrane-electrode assembly (MEA), which consists of an electrolyte membrane sandwiched between 2 porous and highly conductive carbon electrodes. A thin catalyst layer—usually platinum particles—is distributed across the interface between the membrane and the electrodes.

Basic electrochemical reactions at anode and cathode for PEM fuel cells.

In operation, hydrogen gas is ionized on the anode to produce electrons and protons. The protons move through the membrane, while the electrons move through an external circuit. The protons and electrons recombine on the cathode, reducing oxygen to water in the process.

Therefore, the MEA must be made of matrices with high ionic and electronic conductivity. Furthermore, the MEA must be durable under fuel cell operating environments. This calls for composite matrices that can resist oxidation, reduction and degradation at a broad range of temperatures in the presence of H2 and O2.

The electrodes must have high gas permeability, electron and proton conductivity. And they must be able to repel water to prevent flooding. Ultimately, the performance of the MEA is determined at the interfaces where the assembly’s components intersect and electrochemical reactions take place.

In the ground transportation industry, many leading automobile manufacturers have already developed vehicles powered by PEM fuel cells. The emission-free nature of these vehicles is an attractive way to reduce smog problems in large cities.

As an added bonus, their quiet operation could significantly reduce noise pollution near major roadways. Fuel cell vehicles first hit the streets in 2002 in the form of buses developed by Daimler-Chrysler and Mercedes-Benz.

Honda currently leases hydrogen fuel cell cars to government operators in Japan and California, and many other companies have passenger car designs or prototypes in development. An even more recent development is the application of fuel cells to air transportation.

While emissions from airplanes represent a small proportion of transportation industry total emissions, they have a more pernicious impact, being deposited directly into the troposphere. Researchers R Miake-Lye et al (“Aviation and the Changing Climate,” Aerospace America, Sep 2000) have reported that airplane contrails—a mix of water vapor, unburned hydro­carbons, particulates, sulfates, nitrogen oxides and CO2—can have 2–3 times the warming effect of CO2 emitted on the planet’s surface.

The use of fuel cells would eliminate all but the water vapor from this list of emissions. At first glance, the application of fuel cells to aircraft appears to be a difficult task. While the substitution of electric motors for internal combustion engines represents an efficiency improvement in cars, there is no electrical analogy for jet engines, which generate thrust by discharging a stream of high-pressure gas.

An airplane powered by a fuel cell system must rely on a propeller driven by an electric motor, limiting its maximum speed and altitude. An even more important challenge is the weight of fuel cell systems and the corresponding limit on the total power available from a system that can achieve flight.

It requires extreme attention to weight reduction of other components in the design of a fuel cell airplane, although this factor can be somewhat offset by the elimination of the generator required to power an airplane’s electric auxiliary systems.

 

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