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FUEL CELLS

By Jay Grant,2014-07-16 17:37
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FUEL CELLSFuel,Cells,fuel,cells,cell,Cell

Group: Rajvinder Malhi Personal tutor: Dr. E. Sorensen

     Nadira Jayasuriya

     Jaina Shah

    FUEL CELL VEHICLE ENGINES

     [11, 12] Introduction

    Fuel cells are energy conversion devices which produce electricity from a fuel and oxidant. The reaction takes place in the presence of an electrolyte, with a fuel such as hydrogen or hydrocarbons react with an oxidant such as oxygen or chlorine.

    The concept of Fuel cells were first developed in 1838 by a German Scientist Christian Friedrich Schönbein but was not further developed till the mid 1900’s, with its first commercial use in 1958, for NASA project Gemini.

    Fuel cells have recently been used to power car engines as opposed to traditional internal combustion engines. They are desirable as they are more environmentally sustainable then other methods of energy production as they create less or no by products and pollutants. They are more energy efficient and Fuel cells are also relatively light and compact with no major moving parts.

    The use of fuel cell engines will allow vehicles to be twice as efficient as their traditionally powered counter parts. They are a continuous, open system as they consume reactants from an external source where as battery cells are thermodynamically closed systems which store electrical energy chemically.

    FreedomCAR and California Fuel Cell Partnership (CaFCP) are two of the largest collaborations currently underway between car manufactures, fuel providers and government agencies to manufacture Fuel cell vehicles (FCVs) with fuel cell vehicle engines. Many of the larger car manufacturers are now getting involved in the manufacture and research of FCV engines including BMW, Daimlerchrysler and Ford.

    Although not expected to hit the mass market till 2010, FCVs are believed to have the potential to revolutionise on road transportation.

Needs

    The need for cars which consume less fossil fuels and emit less pollutants have clearly been identified over the past decade as the population of the world and hence the number of vehicle users increase. As wealth grows and the GDP of developing countries increases, the average number of cars owned by a family has also increased. Traditional Internal combustion engines which run on fuels such as petrol and diesel must be replaced as fuel sources run out and pollution as a result of these internal combustion engines have a dire effect on the environment. Hence the need for more sustainable sources of transport is clearly identified.

    Needs can be further analysed, addressing the needs of the product and what it must achieve. Ergonomic needs-the ergonomic needs of this product include its ease of use and maintenance, safety issues and user interactions with the product.

    On board hydrogen storage- there will have to be adequate room on board for the storage of hydrogen (which is to react with an oxidiser to form electricity) to give the equivalent power of a full tank of gas. This requires a large volume of space however the hydrogen can be stored in high pressure tanks specially developed for safety.

    Safety-Hydrogen, like any other fuel, is hazardous and needs to be handles with caution. While consumers are now comfortable with handling gasoline the handling of compressed hydrogen is new. Therefore, fuel storage and delivery systems must be optimized for safe everyday use and consumers must be informed and educated about hydrogen’s properties and risks.

    Cold weather operation- since the fuel cell systems contain water (both by-products and to humidify the fuel cells) cold weather operation can be problematic and cause freezing. The cells must be maintained at an optimal temperature for efficient performance.

    Aesthetic needs- this involves the overall image of the product including brand promotion, differentiation and loyalty. Brand promotion- FCV’s and FCV engines may be promoted as the smarter more environmentally conscious

    alternative. This will appeal to niche market of environmentally conscious consumers. However as responsibility and

    sustainable use of resources has been gaining more media coverage and becoming a heatedly discussed current issue, FCVs can appeal to an image conscious, chicer market as well.

    The car manufactures who will be using FCV engines can tap into their already existing brand loyalty (BMW, Ford and Daimlerchrysler) as well as make the vehicles more aesthetically pleasing with innovative design and accessories.

    [13] Alternatives- Hybrid vehicles engines

    Hybrid car engines are those which combine the effects of both gasoline engines and electric motors. They are configured to attain different objectives such as improved fuel efficiency or increased power. Most commonly used is an internal combustion engine, combined with electric batteries, to power an electric motor.

    There are many types of hybrid engines from the ‘full hybrid’ to ‘mild hybrids’ HEVs became freely available to the mass

    markets in the late 1990s with big car manufactures like Honda Insight and Toyota Prius realising many vehicles. HEV’s have many advantages when compared with traditional internal combustion engine vehicles such as a lower

    consumption of fossil fuels than an internal combustion engine and less polluting emissions and by-products. However we can further compare Hybrid Vehicle engines (HEVs) with fuel cell vehicle engines (FCVs).

    ADVANTAGES OF FCVs OVER HYBRIDS DISADVANTAGES OF FCVs OVER HYBRIDS

    o They are almost 100% emission free (when o Current costs to permanently run an electric

    hydrogen and oxygen are used) and have motor using just a fuel cell are exorbitant.

    no polluting by products (where as hybrids o The mileage between re-charging is still

    have usage of fuels) relatively low and hence not practical

    o Fuel cell engines are more lightweight and o The storage space required of enough hydrogen

    compact. to give a feasible mileage is very high.

    o Cars powered by fuel cell engines are three

    times more efficient than gas engines.

    Hybrid cars are the best median between conventional cars and fuel cell vehicle cars displaying a balance of advantages of both systems.they achieve lower emissions and consume less fossil fuels while maintaining economic feasibility, however they are not as environmentally beneficial as fuel cell Vehicle engines.

    Ideas

Fuel cells are classified primarily by the kind of electrolyte they employ. This determines the kind of chemical reactions that

    take place in the cell, the kind of catalysts required, the temperature range in which the cell operates, the fuel required, and

    other factors. These characteristics, in turn, affect the applications for which these cells are most suitable. There are several

    types of fuel cells currently under development, each with its own advantages, limitations, and potential applications. A few of the most promising types include

    ; Polymer Electrolyte Membrane (PEM)

    ; Phosphoric Acid

    ; Direct Methanol

    ; Alkaline

    ; Molten Carbonate

    ; Solid Oxide

    Polymer electrolyte membrane (PEM)

    Polymer electrolyte membrane (PEM) fuel cells (also called proton exchange membrane fuel cells) deliver high power density [1]and offer the following advantages and disadvantages:

    ADVANTAGES DISADVANTAGES

    o Low weight o High system cost due to the use of the platinum

    o Low volume catalyst

    o Do not require corrosive fluids to operate o Additional reactor may be needed to reduce

    o Operate at relatively low temperatures- carbon monoxide in the fuel cell- this adds to

    80?C (176?F) costs

    o Warm-up quickly

    o Good durability

    o Low sensitivity to orientation

    o Favourable power-to- weight ratio

    Due to their fast start-up time, low sensitivity to orientation, and favourable power-to-weight ratio, PEM fuel cells are

    particularly suitable for use in passenger vehicles, such as cars and buses. Phosphoric acid

    Phosphoric acid fuel cells use liquid phosphoric acid as an electrolyte. The acid is contained in a Teflon-bonded silicon carbide

    matrix and porous carbon electrodes containing a platinum catalyst. The phosphoric acid fuel cell (PAFC) is considered the "first generation" of modern fuel cells. This type of fuel cell is typically

    used for stationary power generation, but some PAFCs have been used to power large vehicles such as city buses. The table [1]below lists the advantages and disadvantages of the PAFC’s .

    ADVANTAGES DISADVANTAGES

    o Tolerant to impurities in the reformate o Less efficient at generating electricity alone

    compared to PEM cells o Less powerful than other fuel cells of the same

    o Very efficient for co-generation of electricity and weight and volume

    heat o Large and heavy

    o Very expensive due to expensive platinum

    catalyst- average cost of fuel cell between

    $4000-$4500 per kW

    Direct methanol

    Most fuel cells are powered by hydrogen, which can be fed to the fuel cell system directly or can be generated within the fuel

    cell system by reforming hydrogen-rich fuels such as methanol, ethanol, and hydrocarbon fuels. Direct methanol fuel cells [1](DMFCs), however, are powered by pure methanol, which is mixed with steam and fed directly to the fuel cell anode. The

    table below lists the advantages and disadvantages of the DMFC’s.

    ADVANTAGES DISADVANTAGES

    o Do not have the same fuel storage problems as o This technology is fairly new compared to fuel

    most fuel cells- methanol has higher energy cell technology powered by hydrogen, hence

    density than hydrogen research and development are roughly 3-4 years

    o Methanol easier to transport behind

    o Large quantity of expensive platinum catalyst

    required than in PEM

    o Methanol is toxic

    These cells have been tested in a temperature range from about 50ºC-120ºC. This low operating temperature and no

    requirement for a fuel reformer make the DMFC an excellent candidate for very small to mid-sized applications, such as [2]mobile phones and other consumer products, up to automobile power plants .

    Alkaline

    Alkaline fuel cells (AFCs) were one of the first fuel cell technologies developed, and they were the first type widely used in the

    U.S. space program to produce electrical energy and water onboard spacecraft. These fuel cells use a solution of potassium

    hydroxide in water as the electrolyte and can use a variety of non-precious metals as a catalyst at the anode and cathode.

    High-temperature AFCs operate at temperatures between 100ºC and 250ºC (212ºF and 482ºF). However, more-recent AFC

     [1]designs operate at lower temperatures of roughly 23ºC to 70ºC (74ºF to 158ºF) . The table below summarises the advantages and the disadvantages of the AFC’s.

    ADVANTAGES DISADVANTAGES

    o Have a high performance o Easily poisoned by carbon dioxide- a small

    o Very efficient amount of CO in the air can affect the cells 2

    o Sufficient stable operation operation

    o Cheapest of all fuel cells o Purification process very costly

    o Any catalyst can be used, which are inexpensive

     Figure 1. Alkaline fuel cell

    Molten carbonate

    Molten carbonate fuel cells (MCFCs) are currently being developed for natural gas and coal-based power plants for electrical

    utility, industrial, and military applications. MCFCs are high-temperature fuel cells that use an electrolyte composed of a

    molten carbonate salt mixture suspended in a porous, chemically inert ceramic lithium aluminum oxide (LiAlO2) matrix. Since

    they operate at extremely high temperatures of 650ºC (roughly 1,200ºF) and above, non-precious metals can be used as [1]catalysts at the anode and cathode, reducing costs. The table below lists the advantages and disadvantages of the MCFC’s.

    ADVANTAGES DISADVANTAGES

    o Improved efficiencies compared to PAFC’s o High corrosion rates due to the high

    o Do not require an external reformer-internal temperatures at which the cells operate

    reforming occurs reducing costs o Low durability

    o Not prone to CO or CO poisoning 2

    o Resistant to impurities

    Solid Oxide

    Solid oxide fuel cells (SOFCs) use a hard, non-porous ceramic compound as the electrolyte. Since the electrolyte is a solid, the

    cells do not have to be constructed in the plate-like configuration typical of other fuel cell types. SOFCs are expected to be [3]around 50-60 percent efficient at converting fuel to electricity.

    ADVANTAGES DISADVANTAGES

    o High temperatures reduce cost as precious-o Slow start-up due to the high temperature

    metal catalysts do not need to be used operation

    o Internal reforming occurs, reducing cost o Low durability

    associated with adding a reformer

    o Resistant to CO poisoning

    A significant problem in using these fuel cells in vehicles is hydrogen storage. Most fuel cell vehicles (FCVs) powered by pure

    hydrogen must store the hydrogen onboard as a compressed gas in pressurised tanks. Due to the low energy density of

    hydrogen, it is difficult to store enough hydrogen onboard to allow vehicles to travel the same distance as gasoline-powered vehicles before refuelling. Higher-density liquid fuels such as methanol, ethanol, natural gas, and gasoline can be used for fuel, but the vehicles must have an onboard fuel processor to reform the methanol to hydrogen. This increases costs and maintenance requirements. The reformer also releases carbon dioxide, though less than that emitted from current gasoline-powered engines.

    Selection

    Due to the nature of the product and its end use, the PEM fuel cell showed to be the most applicable. After careful consideration of all candidate technologies, the only real contenders were the PEM fuel cell and the DM fuel cell. The other technologies all have great advantages in many applications which have been listed above, however for the desired operating [4]characteristics of a vehicle engine (operating temperature range of 40?C - 80?C and a durability rating of 150,000 miles) ,

    the only feasible options were chosen.

    The DMFC, also known as the solid polymer fuel cell (SPFC), is different from most other fuel cell types as its electrolyte consists of a layer of solid polymer across which protons are transmitted. DMFCs use methanol in place of other SPFCs where since in liquid form, it can be relatively easy to transport. However, because of its low energy density and low efficiency, the [1]DMFCs are regarded as less of a contender for the winner of our product.

The PEM fuel cells are also a type of SPFCs. They are exceptionally responsive to varying loads (such as driving) and are cheap

    to manufacture becoming cheaper as technology advances. Using an advanced plastic electrolyte to shuttle protons from the anode to the cathode, the solid electrolyte is much easier to handle and use than a liquid counterpart whilst its low operating [5]temperature allows for a quick start-up time.

As a result of its desirable operating conditions and high efficiency rate, the PEM fuel cell makes it ideal for use in vehicle

    engines. Also with the potential of toxicity problems in DM fuel cells, the PEM is preferred.

    Replacement of Internal Combustion Engines

    Fuel cells vehicle engines could replace internal combustion engines due to the many advantages of them listed above, however it is unlikely that fuel cells will replace existing internal combustion engines in the next 10 years as the UK government had once planned. One reason for this being that fuelling stations will have to be modified across the country in order for public acceptance since convenience is key to securing widespread exposure e.g. Since the introduction of liquefied petroleum gas (LPG) in the 1940s, LPG has been unable to reach the awesome potential that it once was thought to have in reducing the amount of petrol and diesel used on the roads today simply because it is not as easily obtainable as petrol or diesel. This was probably due to the fact that legislation was not passed to incorporate LPG into fuelling stations and make it

    a mandatory fuel to be held across the country.

    Another reason for this is due to the integration of fuel cell engines into existing vehicles. Unlike that of changing the battery

    of a mobile phone or a laptop, the integration of fuel cells into a vehicle engine would be a much more involved process and could not be done by the average joe thus would become costly and with a higher cost comes a decreased desire to integrate the fuel cells into ones existing engine.

    These factors and others show that is it acceptable to say that fuel cells will not be dominating the vehicle engine market for

    at least a decade unless the UK government passes legislation such as the one above or others which would promote this technology or world oil reserves become scarce.

    Manufacture

    The voltage of a fuel cell is quite small, about 0.7V when drawing a useful current. This means that to produce a useful voltage many cells have to be connected in series. Such a collection of fuel cells in series is known as a ‘stack’. The most obvious way to do this is by simply connecting the edge of each anode to the cathode of the next cell, all along the line. The problem with this method is that the electrons have to flow across the face of the electrode to the current collection point at

    the edge. A much better method of cell interconnection is to use a ‘bipolar plate’. This makes connections all over the surface [6]of one cathode and the anode of the next cell .

    Most PEM fuel cells are constructed as multiple cells connected in series with bipolar plates. Internal manifolding is almost universally used. However, there are many variations in the way the bipolar plate is constructed and the materials they are

made from. An important issue is that there are different ways of making stacks, which avoid the need for a bipolar plate. [6]These are sometimes used, though only for very small cells .

    The bipolar plate has to collect and conduct the current from the anode of one cell to the cathode of the next, while evenly distributing the fuel gas over the surface of the anode, and the oxygen/air over the surface of the cathode. In addition to this,

    it often has to carry a cooling fluid through the stack and keep all these reactant gases and cooling fluids apart. If this was not

    enough, it also has to contain the reactant gases within the cell, so the edges of the cell must be of sufficient size to allow

    space for sealing.

The material must also be able to be manufactured within the following requirements:

    o The bipolar plate must be slim, for minimum stack volume.

    o It must be light, for minimum stack weight.

    o The production cycle time should be reasonably short.

    One feature of bipolar plate manufacture that is common to many is that the plate is often made in two halves. This makes the manufacture of the channels for the cooling air or water that pass right through the middle of the cell much easier. We will now consider a range of the different materials and processes used. One of the most established is the machining of the graphite sheet. Graphite is electrically conductive and reasonably easy to machine. Fuel cell stacks made in this way have achieved a competitive power density. However, they have three major disadvantages:

    o The machining of the graphite may be done automatically, but the cutting still takes a long time on an expensive

    machine.

    o Graphite is brittle, and so the resulting plate needs careful handling, and assembly is difficult.

    o Finally, graphite is quite porous, and so the plates need to be a few millimeters thick to keep the reactant gases

    apart.

    o This means that although the material is of low density, the final bipolar plate is not particularly light.

    Using injection moulding of graphite-filled polymer offers a cheap solution with the benefits of a corrosion resistant end product. The main alternative to graphite is metal plate. This is susceptible to corrosion within the PEMFC since the surroundings contain water, air and warmth, resulting in a corrosive location. The only real disadvantage to this method of manufacture is the poor conductivity. This can be counteracted, however, by forming a composite of graphite and [6]thermoplastics to make its properties more suitable.

    Another key manufacturing process is the addition of the platinum catalyst to the anode and cathode. This is done by using carbon supporting platinum within the electrolyte. This reduces the quantity of platinum required for the fuel cell to operate [6]correctly .

    Displacement

    Hydrogen fuel-cell vehicles are seen as one possible way to dramatically cut the quantity of greenhouse gas emissions at [7]some point in the future. The cells produce no pollution and leave only two by-products: heat and water . The use of

    hydrogen is of increasing interest to countries, companies and individual due to the range of benefits it provides. At this present time, you can not buy a fuel cell vehicle as the primary obstacles to using fuel cells in vehicles are hydrogen storage

    issues and component cost. There is a current market for the hybrid vehicles, which at present are found to be greener than a fuel cell vehicle, as there are concerns about the CO emissions. 2

    Widespread use of hydrogen as an energy source could help address concerns about energy security, global climate change, and air quality. Fuel cells are an important enabling technology for the hydrogen future and have the potential to revolutionise the way we power our nations, offering cleaner, more-efficient alternatives to the combustion of gasoline and [8]other fossil fuels.

    As the research and development of fuel cell technology continues, fuel cells have the potential to strengthen energy security by reducing our dependence on foreign oil because hydrogen can be derived from a variety of domestically available sources. The primary sources include fossil fuels, renewable, and nuclear power. This flexibility would make us less dependent upon [8]oil from foreign countries .

    In the future of fuel cells, there will be fuel cell refuelling stations, as refuelling takes up much less time than recharging, with

    the current vehicles running on battery power. The fuel cell vehicles can typically go further before refuelling due to the [8]storage limitations of current battery designs .

Conclusion

    As discussed in the section above, peculiarities with the manufacturing process of the fuel cell are apparent. The whole process seems to be very lengthy and intensive, not to mention the extensive investment required. The integration of manufacturing fuel cell vehicle engines into existing plants would initiate high start up costs for the product and trying to gain

    a competitive edge with market prices whilst still trying to stay in business (with these harsh times right now) requires patience to see a significant return of investment. On the contrary, the UK government and many others across the globe have proposed legislation for tax benefits and subsidies for both the users of the product and manufacturer of the product which have been increasing the size of the market year on year. Statistics have also shown how consumers have grown to the idea of fuel cell vehicle engines where studies in America have shown that people are willing to pay more for an increase [9]efficiency fuel cell vehicle rather than an existing internal combustion vehicle.

    The extensive investment requirement mentioned earlier is down to a number of factors coupled together such as the integration of the fuel cell engine scenario previously mentioned and increased research costs to find solutions to problems such as that of integration. However the costs spent on research will hopefully pay off and reduce costs elsewhere, where advancements are made ultimately to reduce costs. A prime example of this would be where solid catalysts were being used but would decrease in efficiency very quickly due to poisoning, thus research showed that by using some material such steel and coating it with the same material as that of the catalyst would not only decrease the products cost but increase the products efficiency where a larger surface area is present due to the coating rather than a solid lump of catalyst. Some of the fundamental issues which need to be rectified before fuel cells become main stream would be that of the distribution and storage of hydration, the size for the cells and the weaker volume to power ratio when compared with [10]internal combustion engines.

    With many multi-national companies and firms pushing their research and development departments hard so as they can tap into the alternate fuel market as soon possible, it will be difficult for any product to gain dominance on the market. Therefore

    to determine whether or not this product will dominate the market cannot be determined as this moment in time however with the current state of climate change and the global awareness of it, the environmentally friendly tag held on fuel cell vehicles creates an excellent, unique selling point which gives the product a great advantage in a very dynamic market.

References

    1) http://www.fuelcellsworks.com/Typesoffuelcells.html

    2) http://www.fctec.com/fctec_types_dmfc.asp

    3) http://www.risoe.dk/rispubl/reports/ris-r-1608_327-338.pdf

    4) http://www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_challenges.html

    5) http://www.e-sources.com/fuelcell-PEMFC.htm

    nd6) Larminie. J and Dicks. A, 2003, Fuel Cell Systems Explained, John Wiley and Sons, 2 edition

    7) http://news.bbc.co.uk/1/hi/sci/tech/2840191.stm

    8) http://www.shell.com/home/content/hydrogen-en/faq/fuel_cell_1204.html

    9) http://www.the-infoshop.com/study/wg19804_fuel_cell_trans_market.html

    10) www.fuelcelltoday.com

    11) http://www.fueleconomy.gov/feg/fuelcell.shtml

    12) http://www.efcf.com/reports/E04.pdf

    13) http://www.allabouthybridcars.com/suv-hybrid-autos.htm

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