Fuel Cells and the Use of Fuel Cells to Power Cars and Other Vehicles

Background
How Would a Fuel Cell Powered Car Compare to One Powered by a Battery?
How Efficient Will a Fuel Cell Car Be and How Many Miles per Gallon Will it Get?
What's Holding Back Use of Fuel Cells?
What About Hydrogen Safety?
Can a Fuel Cell Vehicle Use Other Fuels Besides Hydrogen?
If All Those Fuel Cell Cars are Emitting Water, Won't That Create Other Problems?

Background

The first fuel cell was built in 1839 by Sir William Grove, a Welsh judge and gentleman scientist. Serious interest in the fuel cell as a practical generator did not begin until the 1960's, when the U.S. space program chose fuel cells over riskier nuclear power and more expensive solar energy. Fuel cells furnished power for the Gemini and Apollo spacecraft, and still provide electricity and water for the space shuttle. This article looks at the use of fuel cells in vehicles.

How Would a Fuel Cell Powered Car Compare to One Powered by a Battery?

Fuel cell automobiles are an attractive advance from battery-powered cars. They offer the advantages of battery-powered vehicles but can also be refueled quickly and could go longer between refuelings.

Fuel cells utilizing hydrogen as a fuel would be zero emission vehicles, and those using other fuels would produce near-zero emissions. They are also more efficient than "grid"-powered battery vehicles. In addition, fuel cell cars could produce fewer "system-wide" releases of greenhouse gases -- taking into account all emissions associated with resource recovery, fuel processing and use.

Studies by General Motors and Ford noted that fuel cell car engines could be built for about the same price as an internal combustion engine.

How Efficient Will a Fuel Cell Car Be and How Many Miles per Gallon Will it Get?

Fuel cell vehicles (FCVs) are achieving energy efficiencies of 40 to 50 percent in current testing and demonstrations; through extensive research and development, these numbers are improving every day. Increased energy efficiency, which holds the promise of reducing dependence on foreign oil and increasing energy security, makes FCVs a very attractive replacement for internal combustion engines (ICEs), which are between 10 to 16 percent efficient.

Exact calculations vary from study to study, but many automotive manufacturers have released data showing that FCVs are much more efficient than comparable ICE vehicles. Toyota has published research showing its conventional gasoline vehicle with a vehicle efficiency of only 16 percent, while its FCVH-4, running on hydrogen, is projected to achieve 48 percent vehicle efficiency - three times more efficient. General Motors (GM) claims that its fuel cell prototypes running on hydrogen have more than twice the efficiency of their conventional gasoline vehicles.

With vehicle emissions and fuel efficiency, it is important to look at the complete picture - from the time the fuel is first taken from the ground, produced, refined, manufactured, transported, and stored, until it actually powers a vehicle, as well as the overall safety risks of handling the fuel along the way. This approach is known as the complete fuel cycle or "well-to-wheels" analysis. A well-to-wheels analysis factors in the fuel production efficiency (well-to-tank) and the vehicle efficiency (tank-to-wheel). Looking at this complete picture offers a more thorough comparison.

Thermodynamic laws limit ICEs and all other combustion engines. Having no flame, fuel cells avoid the efficiency losses associated with the ignition, burning, heat transfer to the gases, and exhaust. Fuel cells convert the chemical energy in the fuel directly into electrical energy, which is fed into an electric motor to power the wheels of a FCV.

As gasoline enters an ICE, about 85 percent of the energy released by burning it in the engine is lost, mainly as waste heat. The remaining energy is converted to mechanical energy to rotate the engine's shafts and gears; some of this mechanical energy is lost through friction, as it passes through the transmission to the wheels. Even worse, when a car idles, the efficiency is zero. A practical way to think of your vehicle's efficiency is through your own pocketbook. Sport Utility Vehicles (SUVs) have been tested with efficiencies of around 10 percent. When you drive your SUV to the gas station and fill the tank with $20.00 of gasoline, or chemical fuel, only $2.00 actually goes towards moving your vehicle. The rest, $18.00 of your money, is wasted as heat or pollution.

Battery powered electric vehicles demonstrate the importance of looking at the entire well-to-wheels picture, since no energy conversion takes place on board. Toyota has shown its pure electric vehicle having a vehicle efficiency of 80 percent, twice that of FCVs. If you take into account the well-to-tank efficiency of 26 percent and the efficiencies associated with charging the battery; the overall (well-to-wheels) efficiency becomes 21 percent - better than today's vehicles, but not as efficient as a FCV.

Even today, with alternative fuel generation and distribution in its infancy, FCVs have higher well-to-wheels efficiencies than any other type of vehicle, including ICE and battery hybrids. Three independent analyses have reached similar, but not identical conclusions. Toyota's in-house testing has published 13 percent overall, well-to-wheels, fuel cycle efficiency for its gasoline ICE vehicles. The Methanol Institute (MI) has released very similar overall numbers. MI's research shows gasoline ICE vehicles have a 15 percent overall, well-to-wheels, efficiency. Compare that to Toyota's FCHV-4 running on compressed hydrogen overall efficiency of 30+ percent (58 percent for well-to-tank and 48 percent tank-to-wheel respectively), and MI's 31 percent overall efficiency for the average hydrocarbon fuel cell vehicle (85 percent for well-to-tank and 36 percent tank-to-wheel respectively.)

GM conducted a well-to-wheels study with Argonne National Laboratory, BP, ExxonMobil and Shell. The study found that hydrogen-powered fuel cell vehicles are the cleanest and most efficient combination of fuel and propulsion system for the long term, offering zero vehicle tailpipe emissions, greater efficiency and lower CO2, well-to-wheels, than other vehicles. FCV prototypes also have promising long-term potential for weight, size and cost reductions to make them competitive with current ICE cars.

What's Holding Back Use of Fuel Cells?

Many technical and engineering challenges remain; scientists and developers are hard at work on them. The biggest problem is that fuel cells are still too expensive. One key reason is that not enough are being made to allow economies of scale. When the Model T Ford was introduced, it, too, was very expensive. Eventually, mass production made the Model T affordable.

What About Hydrogen Safety?

Many questions have been raised regarding hydrogen's safety as an energy carrier. Hydrogen is highly flammable and requires a low hydrogen to air concentration for combustion. However, if handled properly hydrogen is as safe or safer than most fuels, and hydrogen producers and users have generated an impeccable safety record over the last half-century.

There are many myths about hydrogen, which have recently been dispelled. A study of the Hindenburg incident found that it was not the hydrogen that was the cause of the accident.

Comprehensive studies have shown that hydrogen presents less of a safety hazard than other fuels including gasoline, propane, and natural gas. In 1997, Ford Motor Company in conjunction with the Department of Energy published a "Hydrogen Vehicle Safety Report" in which it concluded, "the safety of a hydrogen [Fuel Cell Vehicle] system to be potentially better than the demonstrated safety record of gasoline or propane, and equal to or better than that of natural gas." The study cited hydrogen's higher buoyancy, higher lower flammability limit, and much higher lower detonation limit as major contributors to hydrogen's greater safety potential.

Specifically, the study compared the safety of the various fuel systems during collisions in open spaces, collisions in tunnels, and over the fuels' entire lifecycle. The study found that in an open space collision, hydrogen powered fuel cell vehicles were safer than gasoline, propane, or natural gas powered internal combustion engine (ICE) vehicles because of four factors.

  • Hydrogen's carbon fiber composite tanks are very resilient to rupture even upon high impact. In general, hydrogen tanks and operating systems are designed to withstand without rupture or puncture pressures 2.25 to 3.5 times their operating pressure, high-speed collisions, and direct shots from high-powered rifles and handguns.
  • Hydrogen possesses a density only 7% that of air, and has a high buoyancy so that it will rise and dissipate without wind or ventilation. Natural gas' density is 55% that of air while both gasoline (3.4 to 4 times heavier) and propane (1.52 times heavier) vapors are heavier than air. Hydrogen also has a diffusion coefficient 3.8 times greater than natural gas, 6.1 times greater than propane vapor, and 12 times greater than gasoline vapor. Consequently, hydrogen gas rises and diffuses laterally much faster than natural gas, propane, or gasoline. In open spaces, hydrogen's greater dispersion rate should translate into fewer fires. Also, for hydrogen to burn downward, i.e. when the point of ignition is above the gas, the hydrogen/air mixture must be at least 9% hydrogen or higher. ("if the ignition source is above a 10% or less flammable mixture of hydrogen, then the hydrogen below the source will not be ignited."). In comparison, methane has a downward propagating lower flammability limit of 5.6% making methane more likely than hydrogen to be ignited by a source point located above the gas/air mixture.
  • A fuel cell vehicle could carry approximately 60% less energy than an internal combustion vehicle because a fuel cell vehicle is more efficient. If combusted, a fuel cell vehicle's hydrogen would generate less thermal energy than the comparable amount of natural gas, propane, or gasoline for an internal combustion engine vehicle. The hydrogen gas would also burn quicker in the event of a fire because it has a burning velocity 7 times greater than natural gas or gasoline. The result could be a quick plume of fire that does not cause as much damage as a gasoline fire.
  • A hydrogen powered fuel cell vehicle will possess many safety sensors and devices that will stop the flow of hydrogen through the system if a leak is detected or in the event of an impact. By sealing the tank, the safety measures will decrease the chance that a rupture in a line will cause a continuous leak that would lead to a hydrogen concentration sufficient for ignition. The vehicle design will also cut electrical power from the battery eliminating an ignition source.

In a tunnel collision, the same properties that made hydrogen safer for open-air collisions should also make hydrogen safer. Hydrogen gas will disperse quicker than other fuels, although it could create a larger initial plume of gas potentially coming into contact with more ignition sources than a natural gas plume.

If handled properly, the entire lifecycle of the hydrogen should prove to be safer than those of natural gas, propane, and gasoline. The production and transportation of hydrogen would pose fewer direct public hazards because hydrogen gas pipelines or hydrogen tanker trucks present less of a public risk than oil tank trucks (see above). Moreover, hydrogen is not toxic and will not contaminate the environment like a propane, gasoline, or even a natural gas spill could.

Hydrogen's safety record provides no evidence of an unusual safety risk. Liquid hydrogen trucks have carried on the nation's roadways an average 70 million gallons of liquid hydrogen per year without major incident. A high hydrogen gas mixture called "town gas" used to light streetlights and houses has been determined to have an equal safety rating as similarly used natural gas. Hydrogen has been handled and sent through hundreds of miles of pipelines with relative safety for the oil, chemical, and iron industries. Moreover, NASA has used liquid hydrogen as its major fuel source for the last half-century without major incident.

Can a Fuel Cell Vehicle Use Other Fuels Besides Hydrogen?

Fuel cells run on hydrogen, the most abundant element on Earth. The simplest and most efficient vehicle designs store hydrogen on board, either as compressed gas, liquid, or in metal hydride. Many automotive manufacturers have used a transition fuel in earlier models of their fuel cell vehicles, with the long-term vision of strictly hydrogen-powered vehicles. Some have demonstrated vehicles running on methanol and sodium borohydride. The very first FCVs in demonstration are powered by hydrogen. They are fleet vehicles that refuel at a centrally located fuel station.

If All Those Fuel Cell Cars are Emitting Water, Won't That Create Other Problems?

According to calculations by Jason Mark of the Union of Concerned Scientists: Assuming all hydrogen input turns into water, and that all water is released (either as liquid or vapor), "If the entire U.S. passenger vehicle fleet were powered by hydrogen FCVs, the amount of water emitted annually (assuming no losses) would be 0.005% the rate of natural evapotranspiration (water that evaporates or is transpired by plants) in the continental U.S."

Many people are concerned about the amount of water produced by a fuel cell vehicle. They worry "where will the water go?" "Will it cause fog or ice?" and what we can do with it to make it useful. Some discussion of what we have now (the internal combustion engine) and what we will have in a few years (the fuel cell vehicle) can help to put this into perspective.

It is important to remember that gasoline engines also produce water. The hydrogen in gasoline (and the hydrogen in diesel fuel and the hydrogen in natural gas) all combine with oxygen in the flame to produce water. The production of water is one of the big reasons combustion happens since forming water releases heat that makes the reaction possible. It is not a new thing to produce water while making power and energy. Burning or chemically oxidizing any hydrogen bearing fuel produces water. The only fuel that may be an exception to this rule is pure-carbon (coal). For the sake of comparison sake we will use a C6H18 (octane) baseline for gasoline. We will base our calculations of the current situation on an internal combustion engine burning octane.

The classical hydrogen fuel cell uses hydrogen as its fuel. Where does the hydrogen come from? Natural gas! Yes, the vast majority of hydrogen sold in the world today is made from natural gas, (natural gas is mostly methane, CH4). The conversion is done by combining the CH4 with H2O (water!) to make H2 and CO2, so the manufacturing of hydrogen actually USES water! But we will account for this by using the energy units for comparison, just to make it simpler.

So we are comparing the energy from a fuel cell using Hydrogen derived from natural gas to the energy from a gasoline engine using gasoline (octane). What is the difference? The heat of formation of water is - 69 kcal/mole and that of carbon dioxide is - 94 kcal/mole. The heat of combustion of octane in air at perfect stoichiometry with no unburned hydrocarbon is 1806 kcal/mole and the potential chemical energy contained in the same amount of methane is 370 kcal/mole. We must reduce the methane energy by 15% to account for an 85% efficient (energy basis) reformer. The reduction leaves us with 315 kcal/mole in the methane. Comparing the energy content to the hydrogen content allows us to get at the difference in water production between the two fuels.

The ratio of heat produced by chemically oxidizing each one is 1806/315 = 5.7. That means one mole of octane will produce almost six times the energy of one mole of methane (converted to hydrogen and) used in a fuel cell, and it weighs more too.

The ratio of water formed is the same as the ratio of hydrogen atoms or 18/4 = 4.5. That means the octane makes 4.5 times the amount of water as the methane does to make 5.7 times the energy. Computing a relative ratio of water production for a common unit of energy (cal or btu) gives 4.5/5.7 = 0.78. So the octane makes less water (22% less) than the methane does, on a per unit of energy basis. But energy doesn't take into account the energy conversion device (the fuel cell versus the internal combustion engine). We have to take the energy conversion efficiency into account. Fuel cells are typically 30%-40% efficient in automotive sizes.

They are even higher in efficiency in some instances running on pure hydrogen. Some automotive applications running on pure hydrogen have achieved 50% efficiency using fuel cells. Gasoline internal combustion engines are lucky to get 15%-20%. This means that for the same energy in the fuel, the fuel cell car will do twice the work, and the car will travel twice as far, or conversely that the fuel cell car will need only half the energy to do the same work (move the same miles). So divide the 5.7 in half to get 5.7/2 = 2.85 (you only need half the energy to do the same work!) and now you have the FINAL ANSWER. 4.5/2.85 = 1.6. So the internal combustion engine actually makes 1.6 times MORE water than the fuel cell for the same miles traveled in the same car with the same passenger and luggage load. On a "miles traveled" basis, the fuel cell produces LESS water than an internal combustion engine running on gasoline. This is mostly due to the much higher efficiency of the fuel cell compared to the internal combustion engine.

While it is true that the internal combustion engine will make more water, it does so at a higher temperature and this might tend to keep the water in the vapor phase longer than the low temperature fuel cell exhaust. It remains to be seen how the now fuel cell cars will fare in use, but the California Fuel Cell Partnership will certainly find out. But keep in mind that on cold days, the relative humidity is usually VERY low, even if it is snowing, so the chances of condensation on the road are reduced. In Chicago and Vancouver, when they tested the Ballard buses, they put the exhaust up at the top of the bus to help make sure the water vapor didn't cause a problem, and it didn't! It made a "plume" of water vapor on cold days, but no condensation problems at all.

Source: Fuel Cells 2000

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