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What is Fuel cell car?

By admin in , | November 25, 2009 | Comments Off on What is Fuel cell car?

      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.

How do fuel cell cars work?

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 from the air) have to be continually supplied for an electric current to be produced. The fuel cell is more versatile than the electric cell in two ways: it can continue to produce electricity as long as the reactants are supplied, and its design maximises its energy conversion characteristics without also having to be optimised for energy storage.

The fuel cell thought by most analysts to have 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 uses solid polymer materials such as Nafion (related to Teflon) that eliminates the safety considerations associated with other cells that use liquid acid or alkali electrolytes. Its conducting electrodes are made of graphite, which form the terminal of each cell. The electrodes are grooved to allow easy passage of the reactants while maintaining electrical contact with the electrolyte-catalyst-gas interface. 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. In principle the gas offers good potential as a green car fuel. The gas is highly combustible and has a high energy- content (1 kg of hydrogen contains the same energy as 2.8 kg of gasoline). 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 gasoline 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 (-253oC) – although liquefaction provides a range comparable with conventional cars, it requires large amounts of energy to liquefy the hydrogen gas; and Metal-hydrides – special metal alloys absorb hydrogen when under pressure. To release the gas heat is applied and the pressure reduced. Although this method is probably the safest, hydrides significantly increase a vehicle’s weight.

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 260oC, as compared to 600-900oC for gasoline, ethanol, natural gas, and propane. Therefore methanol is considered to be the prime candidate for on-board fuel reforming (see below).

How do I refuel a fuel cell electric car?

The method of refueling 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 gasoline. As the latter two are liquids, if these were used, refueling a fuel cell car would be similar to filling-up a conventional car.

However, if hydrogen is widely adopted, refueling becomes a very different process. Although hydrogen gas refueling systems are still being developed, they all involve the use of a flexible connection between the dispenser and the car that is locked into place creating a sealed system. The amount of hydrogen required is then pre-selected before the gas is automatically dispensed.

Given the novelty of hydrogen cars, it will come as little surprise that there are only a few hundred hydrogen-refueling stations worldwide. However, the number of stations is rapidly increasing with the advent of fuel cell vehicle demonstration programmes in the US (mainly in California), Europe and the Far East.

Are fuel cell electric cars better for the environment?

If non-renewable primary energy is used, the impact on emissions is difficult to quantify, depending as it does 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 (per mile) of up to 55% as compared to gasoline.

The benefits are a result of the that fact that 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.

For life cycle regulated emissions, estimates based on modelling suggest very low regulated emissions associated with fuel cell car use. Regulated emissions from UK hydrogen fuel cell vehicles (using hydrogen produced from reformed natural gas) are predicted to be significantly lower than gasoline 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.

How much do fuel cell electric cars cost to own and run?

As FCVs have yet to go into commercial production, no one can predict with certainty how much a fuel cell car will cost to own. It is likely, however, that some ownership costs will be significantly higher than for their gasoline or diesel equivalents. This will be principally due to a higher purchase price, predicted by many to be more than a conventional car by 80%-100% (capital subsidies are likely to become available as and when fuel cell cars come on to the market). However, the price will fall if sufficient numbers of fuel cell cars are produced. Rates of depreciation and resale values for fuel cell cars are also impossible to predict until more experience is gained.

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. However, it remains to be seen from the initial fleets what the running costs turn out to be.

Where can I buy a fuel cell electric car?

The short answer is that, in the US as of 2009, you probably can’t! However, the situation is likely to change dramatically over the next two to three years as manufacturers want to gain experience by operating demonstration programs with real drivers. In fact Honda are already doing this – after leasing the Honda FCX (in San Francisco) to a small number of selected customers, they are now rolling a trial of the Honda Clarity FCV.

Indeed, almost every car manufacturer has a fuel cell car development program – over 170 fuel cell car prototypes have been developed, the most significant of which include: the Mercedes-Benz ‘F-Cell’ A-Class, the Honda FCX, the Honda Clarity, the GM/Vauxhall HydroGen3 (based on a Zafira platform) and the Ford Focus FCV. Most of these have used a fuel cell engine developed by Ballard Power Systems of Canada, one of the first companies to see the potential fuel cell engine technology. The only question that remains is how long it will be before the first fuel cell car appears in a real showroom with a sale sticker.



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