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Hydrogen fuel cells offer a clean and efficient technology to replace the current internal combustion engines used in the majority of today's vehicles. The internal combustion engine has come a long way in becoming as efficient and reliable in its current state; however, the new hydrogen fuel cell technology is more than three times as efficient and has completely clean emissions. The fuel for the hydrogen cell is hydrogen and oxygen. When they are combusted or reacted the emission is water and heat. Hydrogen and oxygen are also the most abundant elements in the world. The advantages of the hydrogen fuel cell include clean emissions, higher efficiency, and better performance.
The hydrogen fuel cell
The most common type of hydrogen fuel cells being developed for vehicles is the PEM (proton exchange membrane) fuel cells. They are constructed from a solid polymer which acts as an electrolyte and porous carbon acting as electrodes which contain platinum as a catalyst (“US Department of Energy,” 2009). PEM fuel cells work using hydrogen and oxygen as a fuel and produce electrical energy from the electrochemical reaction taking place inside the fuel cell. The unused hydrogen fuel is then recycled back through the system to be reused on the next cycle through the PEM cell, as can be seen in Figure A: PEM hydrogen fuel cell. PEM fuel cells only produce water and heat by-products which can be further used to increase the efficiency of the fuel cell system. The average heat output form a PEM fuel cell is eighty degrees Celsius. One PEM fuel cell on average generates 0.7 volts of electricity so multiple PEM cells must be used together to produce a more usable voltage that could drive an electric motor. Typical PEM hydrogen fuel cells produce an efficiency of 40% to 60%; however they have the potential to produce up to 80% efficiency (“US Department of Energy,” 2006). This means that the fuel cell converts 60% of the energy content of hydrogen into electricity. PEM fuel cells are the preferred fuel cell type for automobiles because they offer high efficiency energy conversions and don't use corrosive chemicals like other cells (“US Department of Energy,” 2009). PEM cells also are lightweight and take up a smaller volume which means more can be stacked into a car to produce more power.
Source: Transportation for London, 2009, < http://www.tfl.gov.uk/assets/images/landscape/bus-fuel-cell-diagram-large.GIF>
Internal Combustion Engine
The internal combustion engine uses fossil fuels to produce usable mechanical energy. This is a major drawback because fossil fuels are a non renewable resource. They are also one of the major contributors to greenhouse gas pollution. The internal combustion engine works by creating a fuel air mixture and then combusting the mixture to produce mechanical energy. Converting thermal energy into mechanical energy produces a large waste of energy in the form of heat and sound. Typical combustion engines have an efficiency of 18% to 20 % (“US Department of Energy,” 2006). This means that only 20% of the total energy is being used to produce usable mechanical energy from the gasoline and the rest is wasted on sounds and heat.
When comparing the hydrogen fuel cell to the internal combustion engine, there is a clear difference in efficiencies. The PCM fuel cell can produce a 60% energy efficiency rating and the combustion engine can only at obtain a maximum of 20% efficiency (“US Department of Energy,” 2006). The difference in efficiency means that it would take less fuel in a fuel cell car to go the same distance in a gasoline vehicle. A different perspective is to look at the energy densities of different fuels seen in Figure B. Energy density of different fuel types. Since gasoline is generally used in volumes it can be seen as a more efficient fuel; however, hydrogen is better measured in terms of mass because as a gas it consumes a larger volume then a liquid. The trade off is that hydrogen requires nearly four times the space of gasoline but has almost a third of the mass (“Stanford University,” N.D.). This would require larger fuel tanks or packing the hydrogen into gas tanks at a higher pressure. The hydrogen fuel has the highest energy density in terms of mass and produces the zero CO2 emissions.