Similar to a battery-electric car, a fuel cell car dispenses with the internal combustion engine altogether. Fuel cells are electro-chemical devices that convert the energy stored in chemical form directly into electrical energy, water and heat.
The fundamental principle of a fuel cell is that an electro-chemical reaction is used to produce electricity. As is the case for an electric cell, fuel cells are not limited by the laws of thermodynamics. This means that they are able to achieve higher conversion efficiencies than conventional engines that only make use of 20%-25% of the fuel's energy – fuel cells can achieve up to 60%. However, unlike a battery the reactants (fuel and oxygen) have to be continually supplied for an electric current to be produced.
The fuel cell with the greatest potential for automotive applications is the Proton Exchange Membrane Fuel Cell (PEMFC). The principal advantage of the PEMFC is its ability to operate at relatively low temperatures (which reduces start-up times). The cell's conducting electrodes are made of graphite, which are grooved to allow easy passage of the reactants while maintaining electrical contact with the electrolyte. At the anode, hydrogen is catalytically disassociated to leave hydrogen ions. An external circuit conducts electrons while the positive ions migrate through the electrolytic membrane to the cathode. There they combine with oxygen and electrons from the external circuit to form water.
If a fuel cell could state a preference for its favourite fuel it would be hydrogen due to the ease with which the element can form ions. The gas is highly combustible and has a high energy-content. However, hydrogen's low density has presented a technological challenge to the design of on-board hydrogen storage systems. At room temperature and pressure, to store an equivalent amount of energy as contained in a typical petrol tank would require a hydrogen tank with around 800 times the volume. However, three main solutions to hydrogen storage have been devised: Compression – the gas being stored in cylinders at up to 7000 times atmospheric pressure; Cryogenic systems – these retain the low temperature required for hydrogen liquefaction (-253degC); and Metal-hydrides – special metal alloys absorb hydrogen when under pressure.
One approach that avoids the problems of on-board hydrogen storage is to reform a hydrogen-rich fuel on-board the car, so generating gas on-demand. As reformers need to have fast response times, fuels that can be processed at relatively low temperatures are preferred. Of the liquid fuels, methanol is unique in that it can be reformed at 260degC, as compared to 600-900degC for petrol, ethanol, natural gas, and propane. Therefore methanol is considered to be the prime candidate for on-board fuel reforming.
The method of refuelling a fuel cell car depends on the type of fuel used. As discussed above, a number of on-board fuels are possible including hydrogen, methanol and petrol. As the latter two are liquids, using these would be like filling a conventional car.
However, if hydrogen is widely adopted, refuelling becomes a very different process. Although hydrogen gas refuelling is still being developed, they all involve the use of a flexible connection between the dispenser and the car that creates a sealed system.
Given the novelty of hydrogen cars, there are still relatively few hydrogen-refuelling stations worldwide. However, the number of stations is increasing with the advent of fuel cell vehicle demonstration programmes in the USA, Europe and the Far East. As a result, more and more hydrogen stations (some of which are publicly accessible) are being built in cities throughout Europe, already including Amsterdam, Barcelona, Hamburg, London, Luxembourg, Madrid, Porto, Reykjavik, Stockholm and Stuttgart.
Visit H2 Stations for a list of hydrogen filling stations worldwide.
If non-renewable energy is used, the impact on emissions is difficult to quantify, depending on the method of on-board fuel storage and fuel production. However, considering the main options, and accounting for carbon dioxide and methane emissions, fuel cell cars are predicted to show a significant reduction in life cycle greenhouse gas emissions of up to 55% as compared to petrol.
Fuel cell cars are significantly more energy efficient than conventional vehicles – electric power-trains are well suited to stop-start operation and use almost no energy when stationary. Also regenerative braking improves fuel efficiency by up to 20%. If renewable energy is used to generate hydrogen, then lifecycle greenhouse gas emissions are virtually zero. With the exception of water vapour, this is a true zero-emission car.
Estimates based on modelling suggest very low lifecycle regulated emissions associated with fuel cell car use. Regulated emissions from UK hydrogen fuel cell vehicles are predicted to be significantly lower than petrol cars with NOx emissions being cut by over 70%. As is the case with greenhouse gas emissions, if renewable energy is used to manufacture hydrogen fuel, then regulated emissions are again virtually zero.
As fuel cell vehicles have yet to go into commercial production, no one can predict with certainty how much a fuel cell car will cost to own, but it is likely that they will cost significantly more than petrol or diesel equivalents (by up to 100%). However, the price will fall if sufficient numbers of fuel cell cars are produced.
Predicting running costs is difficult due to the uncertainties about the production method and demand for hydrogen fuel. At least in principle, the higher purchase costs could be offset by lower fuel costs (due to the high fuel economy of fuel cell cars). Servicing, maintenance and repair costs for fuel cell cars also remain unknown, although these are predicted to be less than for conventional cars due to the low number of moving parts in a fuel cell engine.
One way fuel cell cars are likely to reduce running costs is if used within urban areas in which a Congestion Charge applies. As is the case with battery-electric cars, fuel cell cars are likely to receive a 100% discount on the London Congestion Charge (although owners will need to register with Transport for London and pay an annual £10 fee). With a £11.50 payable daily charge, this could provide a potential annual saving of up to £2000.
At present, there are only a handful of vehicles powered by fuel cells on the UK market. However, the situation is likely to change dramatically over the coming years. Already demonstration fuel cell vehicles are in use on UK roads and include fuel cell black cabs and Citaro fuel cell buses in London. The buses were used as part of the Cleaner Urban Transport for Europe (CUTE) programme, which ran from 2003 until 2007 and successfully demonstrated the potential of hydrogen with 30 fuel cell buses across Europe.
Worldwide, several hundred fuel cell car prototypes have been developed, and small fleets of fuel cell cars are being used by companies and government agencies – these include the Mercedes-Benz 'F-Cell' cars based on the A-Class car (in Berlin) and the Honda FCX Clarity (in California). Many of these use a fuel cell engine developed by Ballard Power Systems of Canada, one of the first companies to see the potential fuel cell engine technology. Indeed, almost every car manufacturer has a fuel cell car development programme. The only question that remains is how long it will be before the first fuel cell car appears in a car showroom.
In the UK, the London Hydrogen Partnership is coordinating activities to explore some of the barriers associated with hydrogen vehicle adoption. The Partnership is researching conditions required for a hydrogen refuelling infrastructure in London, as well as investigating costs and timescales for implementation of fuel cell vehicles in the capital.