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Fuel Cells: Potential Renewable Energy Sources

By Gordon Shaw

Fuel Cells promise so much in the way of carbon-neutral energy in many applications but their full potential has yet to be realised and that will take time.

What are Fuel Cells?

All energy generators are basically engines which transform one type of energy into another. Fuel Cells serve the primary purpose of generating electricity and resemble the cells found in electrical batteries (conventional batteries and cells are described in a subsection of Hazardous Waste; see the main menu on the left). The basic similarity is that they have two electrodes separated by an electrolyte. As with batteries the energy is converted to electrical form by chemical reactions but unlike battery cells the energy is derived from fuel which is dissipated in the discharge process and the fuel cell is "charged" by providing fresh fuel. In operation fuel cells run hot or very hot. By comparison "battery" type cells have a static electrolyte which is recharged by the application of a charging current (except primary cells which are discarded once they are discharged). [Note that the potentially important Innogy Regenesys ® fuel cell system is exceptional because it is charged electrically.]

Fuel Cell Powered Bus (German ). Courtesy of Neoplan
German Fuel Cell Powered Bus. Click for full image

What fuels are suitable for use in fuel cells?

The basic fuel is hydrogen and in the cell this combines with oxygen from the atmosphere. From the emissions point of view this paints a Utopian picture, because the only exhaust emissions are water vapour and heat, with none of the other gases associated with accelerated global warming, notably CO2 and CO. Unfortunately hydrogen is a light gas and needs to be compressed to about 250 bar or cryogenically cooled to be of practical use and this is not normally convenient.
Other fuels can be used, indirectly, providing they are rich in hydrogen, such as methane, methanol, ethanol, natural gas, petrol and diesel. However, these substitutes normally require a reformer which releases the hydrogen.
Mostly these latter compounds are derived from fossil fuels although ethanol and methanol may be bio derived. Even where the fossil fuels are used it is claimed that the process is significantly cleaner than when they are burned (combusted). In part this is due to the higher efficiency of fuel cells especially where Combined-Heat-and-Power (CHP), or Cogeneration as it is called in the US, is incorporated.
[see the CHP reference page under Renewable Energy in the main Menu on the left].
 

What are the advantages and disadvantages of Fuel Cells?

The main advantages of Fuel Cells are that they can be environmentally friendly and can operate with high efficiency (for example compared to the internal combustion engine which operates at about 30%). They also operate silently.
The downside is that they are expensive and technologically they pose quite complex problems. So far they have not proven commercially viable in common usage compared to the alternatives but there are a number of successful prototypes which give cause for optimism. Now with the emphasis on environmental considerations they are being actively researched and developed. Countries which are investing heavily include Canada, Japan, the US and Germany.

What types of Fuel Cells are worth considering?

There are several types which can be identified. Normally they are categorised by the electrolyte and each type has different properties which makes them suitable for different applications. Most types are identified below:
  • Solid Oxide 
    There are several versions of this type. Different fuels are specified and the construction of the cells differ. Operating temperatures approach 1,000° C. Applications are in high-power areas such as industry or generating stations and it has potential for motor vehicles.
  • Molten Carbonate 
    The cathode is supplied with Carbon Dioxide and the fuel is normally fossil fuel, and can achieve high efficiencies. The operating temperature is about 650° C. Regarded as having high potential in large scale power generation where the high temperatures can be put to use.
  • Phosphoric Acid
    Relatively well developed and used for small scale power generation in hospitals, offices, hotels etc. High efficiencies with CHP. The temperatures are moderate (about 200° C).
  • Solid Polymer (aka Proton Exchange Membrane or PEM) 
    Has a high power density, high efficiency and can adapt to varying demands. Operating temperatures are low (around 90° C). This is currently the favourite for motor vehicles. Other possible applications include air conditioning and even domestic sized electronic equipment.
  • Direct Methanol 
    Not a particularly efficient system, based on the solid polymer system. If the research is successful could have application in automotive areas because the fuel is convenient.
  • Alkaline  
    Costly with fairly high efficiencies. Have been used in space missions and there have been experiments in zero emission taxis but the preference for pure hydrogen as the fuel presents problems
Summary

Fuel cells have enormous potential for cleaner energy, including the motor vehicle sector, but they are still in the research and development stage. The need to provide more environmentally friendly energy sources has made them more attractive than they were and there is considerable international investment in developing the technologies. The chemistry of the various types leads to specific applications which are best suited to each type. Development and unit costs are high, so acceptance of successful prototypes is necessary to encourage a mass market to deliver cost reductions and their eventual commercial success. We are somewhat optimistic if slightly impatient.
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Originated: January 2001, Last Amended: 29 October, 2013