A fuel cell is an electrochemical device that converts hydrogen fuel directly into electricity and heat without combustion.
By the nature of its electrochemical reaction, a fuel cell can be more than twice as efficient as an internal combustion engine (approximately 60% compared to about 30%).
A conventional engine burns fuel to create heat and in turn converts heat into mechanical energy and finally electricity. A fuel cell produces electricity, water and heat directly from hydrogen and oxygen.
Conventional engine
Fuel → (combustion) → Heat → Mechanical energy → Electricity
Fuel cell
Fuel → (chemical reaction) → Electricity + Water + Heat
A number of fuel cells can be combined to form a fuel cell stack. The power output will depend on the size of the stack.
A stack may be used on its own with a direct hydrogen source, or integrated with a number of other components to provide an operational power system tailored for a specific application or market.
Construction of a fuel cell stack
To view an animation of the workings of a fuel cell, click here
Sources of hydrogen
Fuel cells are fuelled by hydrogen, one of the most abundant elements on the planet. The hydrogen fuel can come from a wide range of sources:
- Hydrogen can be produced through reformation of a range of hydrocarbon-based feedstocks such as natural gas, propane, methanol butane or propane (liquid petroleum gas or LPG).
- Hydrogen can be derived from sustainable, carbon-neutral sources of methane such as biomass or land-fill gas.
- Hydrogen can be produced through a process of electrolysis using renewable power sources such as wind or solar power.
Fuel cell types
Fuel cell technology embraces a number of different fuel cell types with different characteristics and applications.
PEM fuel cells:
- Efficiency ~55%
- Operating temperature 80°C
PEM is considered to be the most promising of fuel cell types for mass market application and is being developed for stationary, portable and vehicle markets.
Phosphoric acid fuel cells (PAFC):
- Efficiency ~55%
- Operating temperature 180°-210°C
This was the earliest fuel cell technology to achieve commercialisation it remains a costly technology.
PAFCs are large and heavy compared to PEM cells.
Used in medium to high power stationary applications including CHP.
Alkaline fuel cells (AFC):
- Efficiency ~60%
- Operating temperature 60°-90°C
The main disadvantage of AFC technology is its sensitivity to contaminants in the gas streams – carbon dioxide must be removed, adding to the scale and complexity of the system.
AFC technology is used in space applications.
Molten carbonate fuel cells (MCFC):
- Efficiency ~55%
- Operating temperature 650°C
MCFC technology employs a liquid electrolyte which has a negative impact on the stability and life of the fuel cell components.
MCFC is in early development but promises high fuel-to-electricity efficiencies and the ability to consume coal-based fuels.
MCFC is being developed for natural gas and coal-based power plants for use in heavy industrial complexes and for CHP applications.
Solid oxide fuel cells (SOFC):
- Efficiency ~50%
- Operating temperature 1000°C
SOFCs are constructed using high temperature ceramics and metals which can be costly.
SOFC technology is being designed and demonstrated for stationary applications, particularly for industrial and large-scale central electricity generating stations and CHP applications.
Fuel cell benefits
Fuel cells of all types provide a range of benefits when compared to both conventional and other renewable technologies:
Efficiency Conventional technologies reach maximum efficiency at a single operating point. Fuel cells however achieve high efficiency both under partial loads and at full capacity, and have rapid load following capability.
This gives fuel cell technology a distinct advantage over other technologies in both stationary and automotive power applications, as they are capable of rapid start-up and immediate response to changes in demand.
Non-polluting Unlike conventional technologies, when fuelled by hydrogen, fuel cells emit no pollutant by-products. Even when operating on hydrocarbon fuels, pollutant levels can be significantly lower.
With few moving parts, fuel cells are also a quiet source of power.
Modularity Fuel cells are modular in construction and can be economic across a wide range of scales, with multiple units easily constructed to match demand, whereas conventional technologies are often least expensive and more efficient only at larger scales.
This modular feature also has advantages in the manufacturing process where operating efficiencies can be achieved by replicating a small number of core fuel cell stack designs to meet a range of power demands.
Power quality The modular design also mitigates against operations failure. The failure of a single cell or stack (in a group of more stacks) would only result in some loss of power rather than a complete shutdown, as is the case with other conventional technologies.
Power independence Fuel cells can be operated without the need for connections to an electricity grid infrastructure. This makes them a relatively cheap source of power for remote locations and applicable for portable uses.
Fuel flexibility Fuel cell systems can be tailored to accept a wide range of hydrogen-based fuels, from pure hydrogen through to heavy hydrocarbon feedstocks such as diesel. Systems can also be adapted for multi-fuel capability.
Capability as battery replacement Unlike batteries in which the reactants are contained within the case, fuel cells draw their energy from an external supply (hydrogen) which can be easily and quickly replaced.
Weight for weight, fuel cells contain around 1000 times the energy of a lead-acid battery and 200 times that of the latest lithium ion technology.