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Fuel cells are electrochemical energy conversion devices that process oxygen and hydrogen to produce electricity, heat and water. They operate much like a battery, but rather than running down and requiring re-charging or replacement, they can be refuelled.
Fuel cells generate electrical power quietly and efficiently and are virtually pollution-free at the point of use. The by-products from a fuel cell system are water and heat. Whilst more traditional combustion technologies typically have an efficiency of around 35%, fuel cells can achieve double this, extracting more energy from the same amount of fuel.
However, fuel cells are not necessarily ‘clean’ in relation to the fuel source they use. The hydrogen fuel itself has to be produced, usually from hydrocarbons such as natural gas or alternatively by being electrolysed from water. This use of natural gas, is not sustainable and does generate emissions. In addition, the hydrogen fuel must be transported, and stored.
Fuel cells are composed of two electrodes. The anode is a negative electrode that provides electrons and the cathode is a positive electrode that accepts electrons. An electrolyte is found in the middle of a fuel cell. The reaction in the fuel cell is typically as follows:
- Hydrogen atoms enter the fuel cell at the anode where a chemical reaction strips them of their electrons.
- A catalyst layer on the anode helps to separate the hydrogen atoms into electrons and protons.
- The hydrogen atoms then become ionised and carry positive electrical charge.
- Negatively charged electrons provide the current.
- Protons pass through the electrolyte to the cathode side.
- Oxygen enters the fuel cell at the cathode.
- Oxygen combines with electrons from the circuit and hydrogen ions that have travelled through the anode and electrolyte.
- The electrolyte only allows the required ions to pass between the anode and cathode.
- Another catalyst layer in the cathode helps to combine hydrogen, oxygen, protons and electrons to form water and heat.
- The water drains from the cell.
Individual fuel cells can be combined into a stacked fuel cell to increase electrical output. A fuel cell system is made up of a number of components that may include:
- Fuel delivery module (from a hydrogen storage tank or a fuel processor).
- Fuel cell stack.
- Balance of plant.
- Power electronics module.
- Control system.
Fuel cells offer a number of advantages over other energy sources:
- A clean, reliable and high-yield energy source.
- Energy security and resiliency – the use of fuel cells can increase reliability of the grid by reducing demand at peak times.
- Energy responsibility and efficiency – they can reduce the level of on-site energy consumption to counter the effects of climate change and achieve energy efficiency targets.
- Combined heat and power – fuel cells can provide power and heat simultaneously which can be useful for some types of demand, and can significantly lower greenhouse gas emissions.
- Fuel cells can reach up to 90% CHP efficiency with low life-cycle costs. The power is distributed constantly. This system allows energy to be independent from the grid operation.
- Renewable energy such as solar photo voltaic systems can operate in tandem with fuel cell technology, giving the ability to manage energy usage and reduce emissions further.
The question of the ‘sustainability’ of fuel cells is a complex one, depending on an number of factors, such as:
- The alternative technology that would be adopted if fuel cells were not used.
- The end use for the fuel cell.
- The source of the hydrogen fuel.
- The method used for storing and transporting hydrogen.
- The efficiency of the cell.
- The ability to connect to the grid.
For more information on fuel cells see types of fuel cells.
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