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Hydrogen as a fuel has received widespread attention in the media of late, particularly ever since the Bush administration proclaimed that developing a hydrogen economy would clean our air, and free us of oil dependence. There are many problems with using hydrogen as a fuel. The first, and most obvious, is that hydrogen gas is extremely explosive. To store hydrogen at high pressures for as a transportation fuel, it is essential to have tanks that are constructed of rust-proof materials, so that as they age they won't rust and spring leaks. Hydrogen has to be stored at very high pressures to try to make up for its low energy density. Diesel fuel has an energy density of 1,058 Btu/cu.ft. Biodiesel has an energy density of 950 Btu/cu.ft, and hydrogen stored at 3,626 psi (250 times atmospheric pressure) only has an energy density of 68 Btu/cu.ft.4 Hydrogen's energy density is only 7.2% of that of biodiesel. Even if the hydrogen fuel cell is twice as efficient as a diesel engine, running on hydrogen stored at 250 atmospheres would yield an equivalent vehicle only 14% of the range of a vehicle running on biodiesel, with equivalent space set aside for fuel storage. To get a 1,000 mile range, a tractor trailer running on diesel needs to store 168 gallons of diesel fuel. When the greater efficiency of the engine running on biodiesel is taken into account, it would need roughly 175 gallons of biodiesel for the same range. But, to run on hydrogen stored at 250 atmospheres, to get the same range would require 2,360 gallons of hydrogen. Dedicating that much space to fuel storage would drastically reduce how much cargo trucks could carry. Additionally, the cost of the high pressure, corrosion resistant storage tanks to carry that much fuel is astronomical.
There are two options for using hydrogen in a fuel cell - using compressed hydrogen produced by electrolyzing water, and extracting hydrogen from other fuels. I will look at each individually, and then analyze the use of hydrogen as a fuel in general. Currently, most hydrogen used industrially is extracted from natural gas. At current usage rates, the United States will deplete its natural gas reserves in 46 years. If the use of natural gas for transportation (whether directly, or as hydrogen extracted from natural gas) increases dramatically, the time it will take before we use up all of our reserves will decrease correspondingly. One of the primary reasons for looking for alternatives to petroleum is to decrease our dependence on foreign fuels. If we spend trillions of dollars converting to using natural gas, only to use up our own reserves in a decade or two, we would find ourselves back in the exact same position of being dependent on foreign sources.
Thus, the focus needs to be on renewable fuels. For hydrogen, it is only renewable when it is extracted from biofuels, or when the hydrogen is produced by electrolyzing water using renewable energies (wind, solar, etc.). The most logical biofuel to use in fuel cells would be biodiesel, due to its high energy balance and energy density. But, let's consider the option of producing hydrogen through electrolysis.
VI. Hydrogen electrolyzed from water
The first way to look at a potential transportation fuel is to examine the overall energy balance. The energy balance tells you how much energy you get back for each unit of energy you put into developing the fuel. The higher the energy balance, the better the fuel source. The lower the energy balance, the more energy that has to be put into producing the fuel per amount of energy yielded when using the fuel.
When discussing hydrogen as a fuel, people usually take a very simplified approach. When used in a fuel cell, the only by-product of using hydrogen as a fuel is water. However, that completely ignores the issue of where the hydrogen came from in the first place. It is tempting to think that this hydrogen would be produced by electrolyzing water using renewable energy sources, such as wind. To see how realistic this approach is, it is important to analyze the overall energy balance, and henceforth the amount of energy that would need to be produced for the fuel to be used on a wide scale.
The place to start with hydrogen is electrolysis (directly separating the hydrogen and oxygen atoms in water molecules, using electricity). While biodiesel is produced by growing crops and transesterifying the oils, hydrogen as a fuel could be produced by electrolyzing water. Electrolysis systems are around 60% efficient. That means that for each unit of energy you put in, the amount of recoverable energy in the hydrogen produced is equal to 0.6 units. The hydrogen then needs to be compressed to high pressures for storage in fuel tanks (due to the low energy density, hydrogen has to be stored at high pressures so that vehicles can have a reasonable range). Compressing the hydrogen takes energy, but for the moment we will ignore this energy cost, as well as the cost of transporting hydrogen (likewise, we will ignore the cost of transporting biodiesel. Transporting biodiesel should be more efficient, since hydrogen needs to be stored and shipped in high pressure stable metal containers (which are very heavy), whereas biodiesel can be shipped in the same fuel trucks used today. The mass of the fuel tanks for transporting hydrogen is a greater percentage of the energy yield in the fuel than for biodiesel)).
So, the hydrogen fuel can be produced with an energy balance of 0.6:1 (0.6 units produced per unit of energy input, a 60% efficiency). Current generation fuel cells are 50-60% efficient. Assuming a 60% efficiency, that reduces the overall energy yield from 0.6:1 down to 0.36:1. That means that for each unit of energy that goes into producing the fuel (hydrogen), 0.36 units of energy gets used for moving a vehicle.
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