meeting report

PEM Fuel Cell Progress Reported at International Energy Conversion Engineering Conference

• by Henry Oman
• Consulting Engineer, Seattle, WA

In the past the efficiency with which fuel energy can be converted to electric power has been limited by two important factors. The first is the Carnot cycle which limits the efficiency of fuel-burning heat engines to the ratio of the temperature difference between the heat source and sink temperatures, to the absolute temperature of the heat source. The second has been the high cost of platinum, currently around $700 per troy ounce. The cost of fuel-cell power generation must include the costly rare-earth platinum catalyst in its fuel-to-hydrogen converter. Now two new discoveries suggest that fuel cells can become the major converter of fuel-energy to electric energy in the future as the world’s petroleum production diminishes.

On June 27, 2003 chemical and biological engineers at the University of Wisconsin reported that they have found a cost-effective nickel-tin catalyst that can replace the expensive platinum metal in a new process for making hydrogen from corn syrup, sugar beets, and biomass waste such as paper-mill sludge, cheese whey, and wood waste (1). In the 15 August issue of Science, a team in the Department of Chemical and Biological Engineering at Tufts University reported results from work on ceria-based water-gas shift catalysts. (2)

News reports about producing hydrogen fuel with catalysts of nickel-tin, instead of high-cost platinum, produced great excitement in the fuel-cell sessions of the International Energy Conversion Engineering Conference (IECEC) that was held in Portsmouth, Virginia, August 18 to 21, 2003. This conference, sponsored by the American Institute of Aeronautics and Astronautics, dealt with energy conversion for generating electric power in Earth-orbit satellites, space stations, Mars-surface exploration, propulsion of deep-space probes, earth-surface vehicles and other applications. The latest developments in energy and power fields were described in the 206 technical papers presented. In seven panel sessions government and industry leaders described future energy- and power-related developments.

Fuel Cells That Power Spacecraft

Proton-membrane (PEM) fuel cells, rated 1.0kW, powered the Gemini spacecraft and produced drinking water for its crew on eight flights from 1965 to 1966. Alkaline fuel cells, rated 1.5kW and weighing 250 pounds, powered 18 flights of the Apollo space vehicles. These fuel cells operated for nearly 11,000 hours total between 1966 and 1978. The Shuttles, that have flown 113 flights since 1981, were powered by 12kW alkaline fuel cells that have now accumulated more than 90,000 hours of operation. The reliability of fuel cell power made possible the success of these missions. With new developments the cost and weight of fuel cells for coming missions can be reduced, but reliability and safety must be assured.

M. A. Hoberecht in his IECEC presentation reviewed the difficulties encountered during operation of the Shuttle’s three 12kW fuel cells, each of which powered a 28V DC bus (3). Each fuel cell consisted of a power section, where the chemical reaction occurs, and an accessory section that monitors the power section’s performance. The power section, where hydrogen and oxygen are transformed into electric power, water, and heat, consisted of 96 cells that were contained in three substacks. Manifolds run the length of these substacks to distribute hydrogen, oxygen, and coolant to each cell. Each cell contains an electrolyte consisting of potassium hydroxide and water, an oxygen electrode (cathode) and a hydrogen electrode (anode). Each fuel cell power plant is 14 inches high, 15 inches wide, and 40 inches long, and weighs 260 pounds.

Reactant consumption in a fuel cell is directly proportional to the current produced. If no load is connected to the cell, no reactants are consumed. Leaks can be detected from the fuel being consumed by an unloaded cell. The water produced by the power-generating reaction must be removed or the cells will become saturated with water and their efficiency will drop. With a 7kW load it takes only a few minutes to flood the fuel cell with product water, halting power generation. Therefore, hydrogen is pumped through the stack. It picks up the water vapor, which is next condensed and separated from the hydrogen. This water is available for life support and environmental control. A relief valve set at 45psia discharges water overboard if it is not used. Each fuel cell is serviced between flights, and is usually replaced after 1400 to 1500 hours of operation due to accessory component problems.

The hydrogen and oxygen are stored as cryogenic liquids in double-walled, thermally insulated spherical tanks with a vacuum annulus between the inner pressure vessel and outer shell of the tank. Having cryogenic liquid hydrogen and oxygen on board complicates the orbiter processing after it lands. Only essential personnel are allowed onboard until the fuel cells are shut down, a detank is completed, and systems are inerted. This takes seven eight-hour shifts.

Fuel Cells for Next Generation Spacecraft

Fuel cells were not being used for commercial power generation at the time when non-engine-generated power was needed in space vehicles that carried personnel. Electrochemical research data showed that combining hydrogen and oxygen in a fuel cell could produce electric power. The short development time that was available did not permit thorough testing of alternatives to the basic electrochemical process that had worked in laboratories. Consequently, the designers of fuel cells had to incorporate complicated features, such as sophisticated operating controls and complex procedures that made fuel cells the power source for decades of successful manned spacecraft programs. However, fuel cell operation required costly procedures. For example, general work on a landed Apollo spacecraft had to be delayed for up to four days while the fuel cells and their hydrogen and oxygen tanks were de-activated. Today, with fuel cell power being used to propel buses and other vehicles, the previous costly servicing processes can be simplified and the total cost of the fuel cell systems for generating spacecraft can be reduced.

NASA Glenn is leading a five-year program in which PEM fuel cell power plants for next-generation space vehicles are being developed. Two contractors are developing hardware on NASA Glenn contracts. The current power plant designs are based on modular 30V substacks that have a 4-to-1 peak power capability within voltage regulation. These power plants will have at least a 6-to-1 total peak power capability when surge response times are less than 1 millisecond.

In Phase 1 of the program the contractors will develop breadboard units that incorporate advanced technology that will raise the total readiness level from 4 to 5. Vendor testing will be completed during 2003, and the program will down-select to one vendor who will develop an engineering model and start testing it in late 2004. The final power plant will consist of multiple stacks with maximum design flexibility for a wide variety of missions. The PEM fuel cell can be: optimized to minimize power-plant weight, reactant consumption, and configured for multiple voltages.

One of the primary objectives of the PEM fuel cell technology development is to demonstrate improved capability over existing Shuttle alkaline fuel cell technology. Lab tests indicate that the PEM fuel cell has the potential for three times the power and four times the life of an alkaline fuel cell that has the same weight and volume. The PEM fuel cell also uses fewer reduced hazardous materials and has fewer critical failure modes, enabling a significant reduction in ground processing turnaround. For example, since the PEM fuel cell does not use potassium hydroxide as an electrolyte in the cell membranes, there is no need to store it in an inert environment or shipping container. Orientation of the PEM fuel cell is not critical when unpressurized, nor is it sensitive to minor pressure unbalances with respect to hydrogen and oxygen pressure. However, it does require freeze protection.

The PEM fuel cell currently under development uses water as a coolant instead of FC-40, which is used on the Shuttle’s alkaline fuel cells. This eliminates the cost of processing the FC-40 coolant, which requires an expensive infrastructure in the Shuttle program. Also eliminated with PEM fuel cells is the corrosive liquid electrolyte that is presently used on Shuttle fuel cells.

A highly competitive commercial market for PEM fuel cells is developing in producing power for automotive and residential use. Production costs of fuel cell components will fall as their production grows. The PEM fuel cell is based on separating the positive and negative electrodes with a thin sheet of polymer, which is chemically bound with acid radicals. Thus it is not expected to require special ground processing for transportation and storage, nor precise orientation during vehicle installations and removals.

Simplification of Operational Requirements

By achieving the above operational requirements, many of the work tasks previously required on ground for supporting an Orbital Space Plane’s fuel system can be eliminated or simplified. Hoberecht cited the following possible reductions in operational requirements:

• Eliminate the single fuel cell diagnostic test on the ground because the fuel cell instrumentation in the vehicle’s health-monitoring system can record the voltage of every cell on ground as well as during flight.

• Design the fuel cell purge system to purge all cells in parallel when making reactant purges prior to start and fuel cell purges after shutdown.

• Streamline the fuel cell “start” processes so that they are started in parallel. This would cut the 30-to-45-minute start time by one-third.

• New fuel cells could be started on cold gas from on-board storage tanks.

• Design the fuel cells to be less sensitive to flight and ground operations handling. For example, the cells should be less susceptible to damage from cell flooding. Water in the cell should be removable without removing the fuel cell from the vehicle.

• PEM technology makes the fuel cell tolerate differences in hydrogen and oxygen pressure. This avoids the need for sophisticated pressure control and expensive ground support equipment.

• Design the fuel cell assembly so that components in the accessory section can be replaced without removing the fuel cell from the vehicle.

• Specify all fluid-line connections to the fuel cell to be flexible hoses and quick disconnects wherever feasible.

• Provide electrical test points on the fuel cell to ease troubleshooting for component failures

Challenges in PEM Fuel Cell Development

PEM fuel cells offer potential advantages of increased power and life, reduced content of hazardous materials such as KOH and asbestos, and reduced ground turnaround operations. However, many challenges need to be overcome. Consequently, PEM fuel cell flight qualification will take three years and cost between $25 million and $30 million. For example, present PEM fuel cells depend on gravity for removal of water from the oxygen gas stream on the cathode side of the fuel cell. The water must be separated from the oxygen stream for two reasons: (1) the oxygen stream needs to be recirculated back to the fuel cells to avoid wasting the gas, and (2) the spacecraft crew needs the water. The water-separator must be able to operate in accelerations that range from three-times earth-surface gravity during launch to microgravity during flight in space.

The energy conversion efficiency of PEM fuel cells is about 5% less than the efficiency of alkaline fuel cells. However, the PEM fuel cells are capable of generating power with much greater current densities, and, consequently, have greater peak-power capacity than the alkaline cells have. This offsets the slightly lower efficiency. The next-generation fuel cells for space use need to have robust components, and they need to be repairable in place. Problems must be easily identified with correction achievable locally. These cells will also need a robust proton-exchange membrane that will withstand the rigors of launch while retaining its voltage-producing efficiency.

The power consumption in spacecraft started with 1.5kW in the Apollo, and grew to 12kW in the Shuttle. The next generation vehicles will require even more power for their flight elements. The targets for next-generation fuel-cell development are illustrated in Figure 1.

New Catalysts for Hydrogen Production

Efficient electric propulsion, which will be needed as the world’s oil resources continue to diminish, is best achieved with motors powered by hydrogen-consuming fuel cells. The hydrogen can be carried on-board in renewable biomass-derived oxygenated hydrocarbons, such as ethylene glycol, glycerol, and sorbitol. Pure hydrogen is being released from these compounds by steam reforming in the presence of a catalyst that contains platinum that costs around $700 a troy ounce, and is a very scarce mineral. The need for lower-cost hydrogen production has motivated many research projects.

The catalyst evaluating team at the University of Wisconsin, headed by G. W. Huber, tested more than 300 materials to find a nickel-tin-aluminum combination that reacts with oxygenated hydrocarbons derived from biomass, to produce hydrogen and carbon dioxide without emitting large amounts of methane (1). The single-step process uses temperature, pressure, and a catalyst to convert hydrocarbons such as glucose, the energy-source used by most plants and animals, into hydrogen, carbon dioxide, and gaseous alkanes, with hydrogen constituting 50 percent of the product. Glucose is manufactured in the form of corn syrup, but it can be made from sugar beets or cheese whey. Other low-cost sources are biomass waste streams like paper-mill sludge and wood waste.

References

The following are among over 200 papers presented at the International Energy Conversion Engineering Conference (IECEC) August 18 to 21, 2003. Copies of papers can be obtained from the American Institute of Aeronautics and Astronautics, 1801 Alexander Bell Drive, Suite 500, Reston, VA 20191-4344. The next IECEC will be held in Provincetown, Rhode Island, August 16 to 19, 2004.

1. Huber, G.W., Shabaker, J.W., and Dumesic, J.A., “Raney Ni-Sn Catalyst for H2 Production from Biomass-Derived Hydrocarbons,” Science, June 27, 2003, pages 2075 to 2078.

2. Qi Fu, Howard Saltsburg, and Maria Flytzani-Stephanopoulos, “Active Nonmetallic Au and Pt Species on Ceria-Based Water-Gas Shift Catalysts,” Science, June 27, 2003, pages 935 to 938.

3. Reaves, W.F., and Hoberecht, M.A., “Proton Exchange Membrane (PEM) Fuel Cell Status and Remaining Challenges for Manned Space Flight Applications,” AIAA-2003-5963.

Figure 1. Since the mid-1960s the electric load in spacecraft has grown from 1.5kW (Apollo) to 12kW (Shuttle). The weights of the spacecraft fuel cell power plants have grown only slightly.