Fuel Cells: Heat and Electricity


• A fuel cell is a device that converts the chemical energy of a fuel (hydrogen, natural gas, methanol, etc.) and an oxidant (air or oxygen) into electricity. In principle, a fuel cell operates like a battery. Unlike a battery however, a fuel cell does not run down or require recharging. It will produce electricity and heat as long as fuel and an oxidizer are supplied.

• Though only just becoming commercially available, fuel cells appear to be ideally suited for use in micro Combined Heat and Power (CHP) applications in buildings. In these applications 3 to 10kW electricity can be generated by a fuel cell, and the heat produced can be used to heat the building. The ability to utilise the heat greatly increases the efficiency of the fuel cell system and provides greater environmental benefits than electricity generated by centralised fossil fuelled power stations.

• Fuel cell systems have an advantage over conventional CHP systems in that their heat to power ratios are lower. Engine-based systems (typically 1.5:1 heat to power) produce more heat than power, which can often lead to ‘heat dumping’ – a particular problem in buildings with low heat demand. Fuel cells have an even 1:1 heat to power ratio.

 

How a fuel cell works


A fuel cell contains an anode and a cathode with an electrolyte sandwiched between them.

How a fuel cell works

1 When a hydrogen atom (from the fuel source) is in contact with the negative anode catalyst layer, it splits into a proton and an electron.
2 The proton passes straight through the electrolyte, whilst
3 The electron produces electricity as it passes through the external circuit.
4 The circuit returns the electrons to the positive side of the electrolyte layer, where they bond and join with an oxygen molecule.
5 Creating water and heat in the positive cathode catalyst layer.

 

 

Types of fuel cells being developed for use in buildings


• Fuel cells are classified by the type of electrolyte they use.

 

Phosphoric Acid Fuel Cell (PAFC)


• Phosphoric Acid Fuel Cells (PAFC) use a phosphoric acid electrolyte. The ionic conductivity of phosphoric acid is low at low temperatures, so PAFCs are operated at around 150–220ºC.

• Phosphoric Acid Fuel Cells (PAFC) were the first fuel cells to be commercialized. Developed in the mid-1960s and field-tested since the 1970s, they have improved significantly in stability, performance, and cost. Such characteristics have made the PAFC a good candidate for early stationary applications.

• Approximately 75 MW of PAFC generating capacity has been installed and is operating. Typical installations include buildings, hotels, hospitals, and electric utilities mainly in Japan, Europe and the United States.

Pro

More widely tested in building applications than other types of fuel cell

Pro

Fuel flexibility (including natural gas)

Pro

Operated at around 85% efficiency in cogeneration applications

Con

Relatively low electrical efficiency

 

Proton Exchange Membrane Fuel Cell (PEMFC)


PEM fuel cells use a solid polymer membrane (a thin plastic film) as the electrolyte. This polymer is permeable to protons when it is saturated with water, but it does not conduct electrons.

• The PEMFC is perhaps the most common form of fuel cell. First used in the NASA Gemini programme in the 1960s, PEMFCs have continued to develop in a size range of 1 to 5kWe for use in transport such as cars and buses.

• Compared to other types of fuel cells, PEMFCs generate more power for a given volume or weight of fuel cell. This high-power density characteristic makes them compact and lightweight.

• Of all fuel cell technologies, the PEMFC has the lowest operating temperature, around 90 degrees – necessitating a highly purified fuel source. If using natural gas as a source, the gas needs to be ‘reformed’ to render it suitable.

• PEMFCs are only now being developed for use in buildings

Pro

Lightweight and portable

Pro

Fast start-up time

Pro

Manufactured using relatively cheap components

Con

At around 40% they have a relatively low electrical efficiency

Con

Fuel types restricted to ‘highly reformed fuels’ such as hydrogen

 

Solid Oxide Fuel Cell (SOFC)


• ‘Solid Oxide Fuel Cells’ are only just becoming available commercially.

• The use of a hard ceramic electrolyte allows operating temperatures to run as high as 980ºC. High temperatures allow for a wider range of fuel types.

• SOFC will suit a wide range of applications from household units up to 1MWe capacity.+ fuel flexibility (including natural gas)

Pro

High gas output temperatures are more suitable for absorption heating systems

Pro

Relatively high electrical efficiency (around 50%)

Pro

Inexpensive catalyst compared with low temperature fuel cells

Pro

Can reach an overall efficiency of 85% within a CHP system

Con

Produces emissions

Con

Relatively un-tested

Con

Higher operating temperatures require longer start-up times (typically 8 hours)

 

Fuel Cells in the UK


There are very few examples of installed CHP fuel cells. Woking Park Leisure Centre was the UK’s first hydrogen fuel cell plant launched in 2003. This 200kwe system was designed to support the public swimming pool’s heating and power systems and Woking Park’s lighting. Excess heat produced is used to power the centre’s air conditioning, cooling and dehumidification requirements via heat fired absorption cooling. The CHP station is also designed to provide energy services for the leisure centre with surplus electricity exported to the Council’s sheltered housing schemes. The project has a total CHP capacity of 1.2 MWe and 1.6 MWth interconnected by heat and chilled water mains and private wire.

 

Expanding the use of fuel cells


• A gradual shift from the current centralised power generation distributed across a national grid to more distributive networks may well provide fuel cells with a niche market, particularly in the long term when a greater proportion of the energy supply originates from renewable sources.

• The widespread take-up of fuel cell systems will require an effective framework of legislation, standards and codes of practice governing commercial manufacture, planning, and safe operation and maintenance.

 

Movies

 

 


Birmingham University leads fuel cell research. Test your stamina to understand the science here:

 


Fuel cell CHP at Woking Leisure Centre

 

 

 

Further information


• Fuel Cells: The Online Fuel Cell information Resource (www.fuelcells.org)
• Fuel Cell Knowledge (www.fuelcellknowledge.org)
 


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