Fuel cell vehicles
A fuel cell vehicle (FCV) is an electric car, but instead of using a battery, energy is stored as hydrogen and a fuel cell converts that hydrogen into electricity to run the electric motor. A small battery is also used as an energy store for regenerative braking. The general design of an FCV is shown in Figure 17.
Hydrogen fuel cells are widely viewed as the eventual future technology that will power cars and other vehicles, and are the technology to which the NAIGT roadmap leads. As a fuel, hydrogen has the highest energy-to-weight ratio of all fuels, with 1 kg of hydrogen containing the same amount of energy as 2.5 kg of natural gas or 2.7 kg of petrol. This is in stark contrast to the low energy density storage of even the best batteries.
Like electricity, hydrogen can be manufactured in a wide variety of ways. Although today it is largely produced using natural gas as feedstock, hydrogen can be manufactured using renewable energy through the electrolysis of water. (For details of how fuel cells work see Everett et al 2012, pp. 587–91).
Hydrogen can be used in an ICE vehicle (an option explored by car manufacturer BMW), but it is largely being developed for use with fuel cells to power an electric motor. Fuel cells convert chemical energy directly into electrical energy, with the only by-products being water and heat. There are no pollutants emitted in use.
Most fuel cells consist of two electrodes, an 'anode' and a 'cathode', which are separated by an electrolyte that allows the transfer of ions. When the reactants are fed into the cell, chemical reactions occur between the fuel/oxidant and the electrolyte. The main charge carriers (usually H+) cross the electrolyte and the electrons are transferred via an external circuit. The electric current produced can be used to drive a motor. This is demonstrated in the fuel cell animation [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] provided by the U.S. Department of Energy.
There are a number of types of fuel cell, but the polymer electrolyte membrane (PEM) fuel cell is the type best suited to transport traction purposes. Individual fuel cells designed for use in vehicles each produce a power output of under 150 watts. Larger outputs are achieved by assembling cells in series or parallel to form a 'stack'. Fuel cells are able to achieve higher conversion efficiencies than heat engines, and efficiencies of up to 80% have been demonstrated in the laboratory – though in practical vehicle use, 45% is the sort of efficiency achieved. For further details, see the Howstuffworks basics on fuel cells.
As with biofuels and BEVs, fuel cell cars are an example of an approach towards shifting to a fuel that has the potential to be produced sustainably – but rather than the fuel being stored as electricity, as in a BEV's battery, it is stored in a hydrogen tank, with the hydrogen converted to electricity using the fuel cell.