Energy supply problems and the realization that a carbon-based economy cannot be sustained indefinitely have prompted us to look for alternatives. One such alternative is hydrogen, a noble gas abundant in water (H2O). Combustion of hydrogen releases only pure water and hydrogen fuel cells have a theoretical efficiency of 83%. Why is there not a fuel cell in every car and every basement? As this emerging technology matures, prices will decrease and fuel cells will become increasingly prevalent. Is hydrogen fuel cell technology suitable for use in vehicles? Or will the internal combustion engine remain the vehicle propulsion of choice? Bill Ford, Chairman of the Ford Motor Company says, “I believe fuel cells will finally end the 100-year reign of the internal combustion engine.”
W. Ostwald first theorized the fuel cell, an electrochemical conversion device, in 1894. A fuel cell is similar to a battery because it converts chemical energy to electrical energy. Fuel cells are classified by the type of electrolyte they use. These several types include:
- The molten carbonate fuel cell (MCFC) operates at around 600C, making it unsuitable for use in vehicles but excellent as a cogeneration plant because it can also be used to power a steam turbine or heat water.
- The phosphoric-acid fuel cell (PAFC) has a relatively high start-up time and is therefore not practical for use in cars.
- The solid oxide fuel cell (SOFC) operates at 1000C and is only viable for large-scale stationary power plants and can also make steam for a turbine.
- The Alkaline fuel cell (AFC) has been used since the 1960’s in the space program. However, it requires pure hydrogen and oxygen, making it very expensive.
The most promising fuel cell type is the Proton Exchange Membrane (PEMFC). The PEMFC is the technology that is most likely to power cars, busses and distributed generation units in the future. These devices have four main components.
- Anode – is the negative terminal of the “battery”. It conducts the electron released by the hydrogen to the load.
- Cathode – is the positive terminal of the “battery”. It returns electrons from the circuit to be recombined with the hydrogen and oxygen to form water.
- Catalyst – a thin, platinum powder coated material that facilitates the reaction of oxygen and hydrogen.
- Electrolyte – conducts only ions with a positive charge, blocks electrons.
Pressurized hydrogen enters the cell on the anode side and is split by the platinum powder in the catalyst. Two electrons are released in this process and are conducted to the external load through the anode. On the cathode side oxygen is split by the catalyst, attracting the H+ ions through the proton exchange membrane. The hydrogen and oxygen ions join with the electrons from the external load and form water.
Anode: 2H2 => 4H+ + 4e-
Cathode: O2 + 4H+ + 4e- => 2H2O
Net Reaction: 2H2 + O2 => 2H2O
Each fuel cell creates a voltage difference of less than one volt, so several cells must be combined in series to produce a sufficiently high voltage. This is called a fuel cell stack. These stacks produce DC voltage that can be used to power a vehicle. When converter to AC with an inverter they can also be used to supplement or replace domestic (in home) power load.
Hydrogen powered PEM fuel cells have the potential of being around 80% efficient at converting chemical energy into electrical energy. The efficiency of a typical electric vehicle motor, which turns electrical energy into mechanical energy, is also 80%. Generally around 5% is also lost in mechanical systems within the drive train. Combined, this makes fuel cell-powered vehicles around 61% efficient which is considerably better than internal combustion and electric vehicles. Regular gasoline engines are about 20% efficient at converting thermal energy into mechanical energy. A battery-powered electric vehicle is about 76% efficient at converting electrical energy into mechanical energy but the net efficiency is far lower. A typical power plant, such as the Morro Bay Power Plant, is 33% efficient and charging the batteries is only about 90% efficient. Therefore an electric vehicle can be expected to have a net efficiency of only 23%.
Of course hydrogen, being a “secondary” fuel and “not an energy resource” , is not naturally occurring in large concentrations and must be produced. The efficiency of the process of making hydrogen must also be taken into account. “The principle of electrolysis was first formulated by Michael Faraday in 1820.” By applying electrical energy water can be dissociated into hydrogen and oxygen. Making hydrogen through the electrolysis of water is at best 55% efficient. The efficiency of the energy source must also be considered in the net efficiency of a fuel cell vehicle. If the energy for the electrolysis comes from a typical power plant the vehicle’s net efficiency would be a dismal 11%. Using a renewable energy source the efficiency can be increased to around 34%.
Another option for producing hydrogen is using a reformer to extract the fuel from methane, ethanol or even gasoline. “The use of methanol is the most efficient source of ‘wells-to-wheels’ energy known for transportation. It has the added bonus of being able to manage emissions, such as CO2 centrally, rather than at each individual tail pipe.” The efficiency of reformers is around 50% at best making hydrogen reformer powered fuel cell vehicles 40% efficient, or roughly twice as good as a conventional automobile. “Even when taking into account the CO2 which is formed during reformation of the methanol, emissions of CO2 are less than 50% of those of a conventional vehicle of similar size. This low level is due to the inherently better energy conversion efficiency of the fuel cell, and also in part because methanol contains relatively less carbon per unit of chemical energy than petrol.”
Other, experimental methods for producing hydrogen exist but are currently largely unproven. These techniques include anaerobe digesters that use small organisms to break down biomass and release hydrogen as a byproduct. Several chemical techniques are being developed. These include a NaH (Sodium Hydride) pellet that reacts with water to form hydrogen and a fuel cell that creates hydrogen as salt water corrodes its anode.
After the Hindenburg accident in 1937 fear of hydrogen prevented further development of the hydrogen-filled Zeppelin air ship. The stigma of hydrogen remains today and is a hurdle that the fuel cell industry must overcome. A recent study determined that it was not the hydrogen that caught fire on the Hindenburg but the material that covered the air ship. This was proven by careful review of eyewitness accounts that indicated that the flames were orange-red in color, while a hydrogen flame is white in color. Also, the air ship remained level as it burst into flames, further discounting a large hydrogen release.
Although the truth behind the Hindenburg disaster has been discovered, fears continue for various reasons. “In some respects hydrogen is a more dangerous fuel than natural gas, because it forms an explosive mixture with air over a very wide range of concentrations, from 4 to 75% hydrogen; natural gas is flammable only in a range of 5 to 15% concentrations in air.” However, proponents maintain that hydrogen is safer than natural gas and methane because it dissipates at a much higher rate due to its light weight.
One difficulty of hydrogen storage is that it is a gas at standard temperature and pressure (STP). Hydrogen has almost three times the energy density than gasoline, 38kWh/kg compared to 13kWh/kg, but gasoline is a liquid at STP making it much easier to store the equivalent amount of energy. In order to store sufficient amounts of fuel to power a vehicle for a reasonable amount of time a substantial pressure vessel is required. Several systems are being developed that use a modified metal hydride powder to store the hydrogen. Generally these systems can only store 1-2 grams of fuel for every hundred grams of hydride. By adding high levels of magnesium, Energy Conversion Devices, Inc. says that it can store up to 7% hydrogen by weight. This, they claim, is actually more efficient than liquid or compressed hydrogen storage systems.
The United States and the rest of the world have an extensive infrastructure for supplying the carbon-based economy. “The hydrogen carrier methanol – a liquid that can be sold in a manner similar to gasoline through the existing filling station network – is the first fuel in the 115-year history of the automobile in Europe that isn’t derived from a fossil source and can even be produced from renewable sources.” Switching the entire planet to a hydrogen-based economy signifies a major paradigm shift in worldwide energy policy. This shift will occur slowly at first until the necessary hydrogen production and supply facilities have been built.
Trends in passenger vehicles have been towards increasing luxury and comfort. Car companies added radios, air conditioning and power seats for an additional cost. Now those luxuries are generally standard equipment and CD players, GPS navigation and seat heaters have taken their place. The increase in gadgets also means an increase in electrical load. This power is produced by the engine and is considered almost negligible. However, in vehicles powered by electric motors, this power draw is not insignificant. Can fuel cell vehicles produce enough power to compete with current cars as well as run on-board equipment without compromising performance?
Sophisticated power management systems as well as hybrid systems will enhance efficiency and performance. Current hybrid vehicles use “regenerative braking,” slowing the vehicle by converting kinetic energy into electrical energy, to charge batteries that provide a power assist when the car starts from a stop or at low speed. Concepts being developed include a flywheel design, which stores the vehicles kinetic energy by spinning a disk. The rotational kinetic energy is then turned into kinetic energy through the drive train. A hybrid fuel cell electric vehicle powered by renewably produced hydrogen could reach an amazing pinnacle in efficiency.
Car manufacturers are working to make fuel cell vehicles available to the public. “Eight major automakers plan to commercialize fuel cell vehicles in the 2004-2005 time frame.” They must overcome several technological hurdles and embrace many ideological changes within their industry. Government subsidies or tax credits may be necessary to promote this new breed of vehicles until demand increases and costs normalize.
In order to make fuel cell vehicles more feasible several goals must be realized. Public fear over hydrogen and legitimate safety concerns must be addressed and remedied. A reliable and efficient production system must be developed and a supply infrastructure needs to be developed. On-board storage of hydrogen needs to be made completely safe and competitive in size with current gasoline tanks. Finally, it is imperative that fuel cell vehicles do not sacrifice the performance and amenities that customers demand from their cars. Acceleration and speed as well as air conditioning, on-board computers and navigation systems will need to be considered if consumers are to embrace this new technology. Jason Mark from the Union of Concerned Scientists says, “I think there’s broad agreement among environmentalists, policy makers and even the auto industry that fuel cells are the next-generation technology. It’s the car we’ll be driving into the next millennium.”
- Nave, R., Hydrogen Fuel Cell. http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/electrol.html
- Ford, Bill, Chairman Ford Motor Company, October 5, 2000. http://www.methanol.org/fuelcell/quotes/mfcvc.html
- Ristinen, Robert A. and Kraushaar, Jack J., Energy and the Environment, John Wiley and Sons, Inc., 1999, p264.
- Flury, Richard, Chief Executive BP Amoco Gas & Power, World Energy Council Conference, May 19, 2000. http://www.methanol.org/fuelcell/quotes/mfcvc.html
- Toyota Motor Company, January 1999. http://www.methanol.org/fuelcell/quotes/mfcvc.html
- Ristinen, Robert A. and Kraushaar, Jack J., Energy and the Environment, John Wiley and Sons, Inc., 1999, p267.
- DaimlerChrysler at the unveiling of NECAR 5, November 7, 2000. http://www.methanol.org/fuelcell/quotes/mfcvc.html
- Fuel Cells Investors Portal. http://www.loonycat.com/fc_transportation.htm
- Mark, Jason, Union of Concerned Scientists, 1999. http://www.methanol.org/fuelcell/quotes/mfcvc.html