Hydrogen is the smallest of all atoms. It has the atomic number 1 and is the lightest, most abundant chemical in the universe, making up roughly 75% of all matter. In its molecular form, hydrogen is a gas, however most hydrogen on earth is combined with oxygen to form water (H2O).
Hydrogen is highly flammable and will burn at concentrations ranging from 4 to 75% in air. It is used in the Space Shuttle main engine. The best way to compare fuels in terms of the energy they deliver is to use energy density. This term standardizes the amount of energy in a fuel for a given volume it contains. Compressed hydrogen has an energy density of 143 megajoules per kilogram, but only 5.6 megajoules per liter. Gasoline, on the other hand has 47.2 megajoules of energy per kilogram, but 34 meagjoules per liter. Diesel is even more dramatic at 45.4 megajoules per kilogram and 38.6 per liter. Energy density is one of the reasons hydrogen has not yet replaced hydrocarbon fuels.
Benefits and Use
One of the biggest advantages of hydrogen as a fuel is that burning it produces only water. Burning hydrogen is a less efficient means of extracting energy than are hydrogen fuel cells. Current fuel cells are roughly 60% efficient, though when heat trapping features are included, they can reach 83% efficiency. Typical combustion engines, whether burning hydrogen or petroleum are 25% efficient at a maximum.
Fuel cells are similar to batteries in many ways. The major difference between a fuel cell and a battery is that the chemicals in a battery are finite while in a fuel cell, constant supplies of hydrogen and oxygen are fed in. Other than that, the principle is generally the same. Here is how a fuel cell works.
The cell is divided in half by an electrolyte. On one half of the cell, hydrogen gas is injected. On the other side of the electrolyte, oxygen is injected. Only hydrogen can cross the electrolyte, but when it does, its electron is removed. The remaining proton travels across the electrolyte to where the oxygen is waiting. The electron, on the other hand, is conducted through wiring where it does useful work as electricity. Once it reaches the end of the wire, it combines with the proton and oxygen to form water.
There are several major drawbacks to the use of hydrogen as a fuel. The first is economic, both fuel cells and standard internal combustion engines that burn hydrogen are not predicted to be competitive in terms of price with standard fossil fuels until well beyond 2040. Fuel cells are expensive to produce. Overall, electricity generated from a fuel cell costs approximately $100 per kilowatt, compared to $0.15 to $0.30 per kilowatt for standard fuels. Current goals are to have costs down to $35 per kilowatt by 2020.
In addition to expense, there is no infrastructure in place for delivering hydrogen to consumers, particularly for transportation. Hydrogen requires unique storage conditions and is considerably more explosive than hydrocarbon fuels are. Most nations would have to completely reinvent their fuel delivery infrastructures to accommodate hydrogen.
Storage is also a major drawback of hydrogen. In its gas form, enough cannot be stored to make it of practical use and it is also exceptionally flammable. In liquid form, hydrogen is much less explosive. However, creating liquid hydrogen requires cooling it in cryogenic tanks or compressing it. While the energy per ass is three times that of gasoline, current technology only allows liquid hydrogen to carry one sixth of the energy per liter than standard gasoline. Some research has been performed on special crystalline storage materials, but they are expensive.
The biggest drawback to hydrogen, however, is production. There is little molecular hydrogen on the planet, so most of it has to be produced by alternative means.
Ways Hydrogen Can be Generated
On an industrial scale, hydrogen can be produced in several different ways. The most efficient of these is through removal from hydrocarbon. Often this is accomplished through stream reforming of natural gas in which steam at 700 – 1100 C is reacted with methane to produce carbon monoxide and hydrogen gas. Currently, energy input is greater than energy gained for this process. In addition, hydrocarbon fuel is needed to heat the water to create steam. Producing hydrogen from fossil fuels does not solve the problem of greenhouse gas emissions and does not end society’s reliance on fossil fuels.
Thermochemical techniques use heat cycles and various metal combinations to produce hydrogen from water without using electricity. Currently, solar is being investigated in order to separate completely this production means from hydrocarbons. Geothermal means have also been employed in a similar manner. Iceland is the global leader in hydrogen production via geothermal processing.Electrolysis is the splitting of water into hydrogen and oxygen using electricity. Electricity must be produced through either conventional burning of fossil fuels or through renewable means such as hydro, wind, or solar. A kilogram of hydrogen can be produced by wind-powered electrolysis for approximately $5.55 per gallon, making this a viable method if enough wind can be harnessed and the problems listed above can be overcome.