Infrastructure relates to the equipment and systems needed to produce, distribute, store, monitor and dispense fuel, specifically hydrogen, for fuel cells.
Hydrogen as a Fuel
Most types of fuel cells require hydrogen as a fuel source. The long-term goal of fuel cell research is to produce a totally non-polluting power source. In order to achieve this, the fuel cell must run on hydrogen generated by renewable means. The technology to do this does exist (see below), but the infrastructure to achieve this efficiently and cheaply is still under development.
In the meantime, fuel cells will be powered by hydrogen extracted from fossil fuels (oil and natural gas) by reforming. Reforming can either take place on a very large scale at the source or locally at the point of use by small reformers integrated with the fuel cell. More detail on this is given below.
Producing Hydrogen by Electrolysis
Hydrogen can be produced by the electrolysis of water: splitting of water into its component elements. This process takes place in an electrolyzer, which can be described as a ‘reverse’ fuel cell: instead of combining hydrogen and oxygen electrochemically to produce electricity and water as a fuel cell does, an electrolyzer uses an electrical current and water to produce hydrogen and oxygen.
The key issue here is the source of the electrical current. If grid electricity is used, the hydrogen has a carbon footprint associated with it due to the coal or gas that must be burnt to produce the necessary electricity. However, if the electricity is obtained from renewable energy such as wind or solar power, the hydrogen can be produced in a completely carbon-free way.
Electrolysers exist and many commercial versions of various capacities are available on the market. A number of companies have called for these to be used in combination with wind or solar power to produce hydrogen for fuel cells.
Producing Hydrogen by Fuel Reforming
Hydrogen can be generated by reforming of hydrocarbon fuels such as natural gas, methanol, gasoline or ethanol. These are not necessarily fossil fuels; reforming of bio-ethanol, for instance, is equally possible and this would then also be a source of renewable hydrogen.
Generally, there are two different kinds of reforming: external reforming, which is carried out before the fuel reaches the fuel cell itself, and internal reforming, which takes place within the fuel cell stack.
External reforming could be carried out at a refinery or chemical plant and the hydrogen delivered by pipeline to filling stations. External reforming can also take place in a reformer integrated with the fuel cell so that the fuel cell system can be fed a hydrocarbon fuel (town gas for instance). The reformer then extracts the hydrogen from the fuel and feeds it to the fuel cell. In this instance, there will still be emissions from the system at the point of use, but due to the higher efficiency of fuel cells, these will be less than if the gas were simply combusted.
Fuel is mixed with steam in the presence of a base metal catalyst to produce hydrogen and carbon monoxide. This method is the most well-developed and cost-effective for generating hydrogen and is also the most efficient, giving conversion rates of 70% to 80% on a large scale.
Partial Oxidation Reforming
Partial oxidation can be used for converting methane and higher hydrocarbons but is rarely used for alcohols. This method involves the reaction of the hydrocarbon with oxygen to liberate hydrogen and produces less hydrogen for the same amount of fuel than steam reforming. The reaction is, however, exothermic and therefore generates heat. This means that the reaction can be initiated by a simple combustion process leading to a quick start-up. Once the system is running it then requires little external heating to keep going. The technology is preferred where there is little access to natural gas or an abundance of oil.
Autothermal reforming combines the endothermic steam reforming process with the exothermic partial oxidation reaction, therefore balancing heat flow into and out of the reactor. These systems can be very productive, fast-starting and compact, and have been demonstrated with methanol, gasoline and natural gas. A number of auto and oil companies are also working on proprietary versions of this technology.
Hydrogen is the lightest chemical element and offers the best energy to weight ratio of any fuel. The major drawback to using hydrogen is that it has the lowest storage density of all fuels. However, it is possible to store large quantities of hydrogen in its pure form by compressing it to very high pressure and storing it in containers which are designed and certified to withstand the pressures involved. In this way, it can either be stored as a gas or cooled to below its critical point and stored as a liquid.
Hydrogen can also be stored in solid form, in chemical combination with other elements (there are a number of metals which can ‘absorb’ many times their own weight in hydrogen). The hydrogen is released from these compounds by heating or the addition of water.
Other storage mediums are being investigated, for example, carbon nanotubes and glass microspheres.
For high-temperature systems such as molten carbonate and solid oxide cells, it is possible to supply a hydrocarbon (e.g. natural gas or methanol) directly to the fuel cell without prior reforming. The high temperature allows the reforming stage to take place within the fuel cell structure. In practice, some preliminary reforming or purifying of the fuel is often carried out.
The exception to this is direct methanol fuel cells, in which a catalyst on the anode draws the hydrogen from liquid methanol, eliminating the need for a fuel reformer. Therefore, as the name suggests, pure methanol can be used as fuel.
Source: Fuel Cell Today (www.fuelcelltoday.com)