Technology

What is a fuel cell?

Originally invented by Sir William Grove in 1839, fuel cells are now becoming a viable source of power.

Fuel cells can in their simplest from be regarded as generators. But whereas conventional generators use internal combustion engines to rotate an alternator, fuel cells generate power by producing electrons directly, with no moving parts. As a result, they are very efficient and reliable. Moreover, they are almost silent and, other than electricity and heat, they produce only water vapour. This makes them ideal for indoor use.

Fuel cell systems are clean, very quiet and produce no exhaust gases other than water vapour. As there are very few moving parts, maintenance is minimal.

How does it work?

There are many different types of fuel cell technologies adopted today, each having their own characteristics and each being suited to differing applications. The basic principles of any fuel cell are essentially the same, with all fuel cells using an electrochemical reaction to split hydrogen gas molecules (H2) to form hydrogen ions and electrons. 

The Cell

A fuel ‘cell’ essentially consists of two plates (the anode and cathode) separated by an ‘ion-conducting’ electrolyte which can come in many different forms.

A fuel (the simplest form being hydrogen) is passed across the anode where it is split by a catalytic reaction into hydrogen ions (H+) and electrons. 

Simultaneously an oxygen source (usually either air or pure oxygen) is passed across the cathode side of the cell.

As the fuel cell’s electrolyte layer is designed only to allow a flow of hydrogen ions through it from anode to cathode, these flow across the membrane forcing the electrons to flow through an external electrical circuit to the cathode side where they then combine with the hydrogen ions and oxygen to form water.

This flow of electrons creates the electrical current or power from the fuel cell.

The Stack

Each individual fuel ‘cell’ produces only a small amount of power, each cell’s output differing dependent on their design and size.

To provide the higher levels of power output required from a fuel cell system for practical use, the cells are then combined to form a fuel cell “stack” the size of which is dependent on its manufacture and requirements. 

The systems available on the  market today are more correctly termed as fuel cell systems as they comprise all the cells in a ‘stack’ plus the electronic controls required to meter the fuel and oxygen, and control the process.

Technologies

Fuel Cells offer a wide choice of power outputs dependent on the system chosen.

These vary from smaller portable fuel cells which produce low power outputs from 1W to 150W running on gaseous hydrogen or methanol, to standby power units from 10kW to 100kW running on hydrogen through to prime power 250kW+ units mainly fuelled by natural gas, with alternative sources of methane such as biogas now becoming available.

Today’s fuel cell development continues apace with new technologies appearing on a regular basis. In terms of those systems which could be deemed commercially available, there are several variants all of which can be chosen, dependent on the requirement of the user.

High Temperature Fuel Cells

These fuel cells have long start-up times and are generally higher power units from 200kW upwards. As such they are best suited to continuous use. 

Even though fuel cells in general are considerably more fuel efficient than other forms of power production, use of the high-grade heat produced by these types of fuel cell for combined heat and power (CHP) or cooling via absorption chillers, can increase efficiency further by up to 85% in total.

Solid Oxide Fuel cell (SOFC)
– operating at 500-1000°C

Molten carbonate fuel cell (MCFC)
– operating at 600-650°C

Phosphoric Acid fuel cell (PAFC)
– operating at 150-200°C

Low Temperature Fuel Cells

These fuel cells have rapid start-up but produce little in the way of usable heat. They are generally of lower power output (up to 20kW) and are ideal for standby and low-power, long runtime, prime power applications.

Fuel cell technologies are continuing to develop, but whilst they share the fundamentals and the technology adopted within them, their differing power outputs make each one suitable for differing applications.

Alkaline fuel cell (AFC)
– operating around 80°C

Direct methanol fuel cell (DMFC)
– operating around 70°C

Proton exchange membrane fuel cell (PEM)
– operating around 75°C