Fuel Cells: fuelling cars to the future

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1. NEED FOR FUEL CELLS


After a century of constant improvements, the internal combustion engine still only converts on average about 16 percent of the energy in gasoline to turn the car’s wheels. All heat engines have efficiencies limited by the Carnot Cycle. The theoretical thermodynamic derivation of the Carnot Cycle shows that even under ideal conditions, a heat engine, used to power a vehicle or generator, cannot convert all the heat energy supplied to it into mechanical energy. Some of the heat energy is rejected. In an internal combustion engine, the engine accepts heat from a source at a high temperature (T1), converts part of the energy into mechanical work and rejects the remainder to a heat sink at a low temperature (T). The greater the temperatures difference between source and sink, the greater the efficiency


Maximum Efficiency = (T1 � T) / T1


Where the temperatures T1 and T are given in degrees Kelvin


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Fuel cell vehicles, not limited by the Carnot Cycle, are expected to achieve energy efficiencies of 40 to 45 percent and very possibly higher. Thus Fuel cell vehicles have proven to be much more efficient than similar internal combustion vehicles


Secondly, the conventional ICE vehicle, however refined they may be, emit harmful exhaust, whose effects is not evident today will be bourn by our children in the future. Given the significant improvement in energy efficiency, fuel cell vehicles offer substantial reductions in greenhouse gas emissions, and higher mileage too.


Thirdly, the conventional fuel resources are limited on this planet. Today or tomorrow they will be exhausted bringing the entire world to a standstill. So it is imperative that we look for other avenues to power our modern transportation systems. Fuel cells provide an excellent alternative to today’s gasoline. Since it uses simple elements to produce energy which are abundantly available, there is no question of depletion these resources.


All the above reasons emphasis the importance of fuel cells in today’s world. In the further sections of the paper, we will trying explain the basic working of fuel cells, their important types along with advantages and drawbacks and further scope of development.





.INTRODUCTION


In principle, a fuel cell operates like a battery. Unlike a battery, a fuel cell does not run down or require recharging. It will produce energy in the form of electricity and heat as long as fuel is supplied.


A fuel cell consists of two electrodes sandwiched around an electrolyte. Oxygen passes over one electrode and hydrogen over the other, generating electricity, water and heat. Refer Figure 1.


This type of a fuel cell is called Proton Exchange Membrane (PEM) fuel cell. This is the most basic fuel cell based on its principle other fuel cell are made.


A basic overview of the component and the working of a PEM fuel cell is given below


.1 Parts of a fuel cell (Refer Fig.)


• The anode, the negative post of the fuel cell, has several jobs. It conducts the electrons that are freed from the hydrogen molecules so that they can be used in an external circuit. It has channels etched into it that disperse the hydrogen gas equally over the surface of the catalyst.


• The cathode, the positive post of the fuel cell, has channels etched into it that distribute the oxygen to the surface of the catalyst. It also conducts the electrons back from the external circuit to the catalyst, where they can recombine with the hydrogen ions and oxygen to form water.


• The electrolyte is the proton exchange membrane. This specially treated material, which looks something like ordinary kitchen plastic wrap, only conducts positively charged ions. The membrane blocks electrons.


• The catalyst is a special material that facilitates the reaction of oxygen and hydrogen. It is usually made of platinum powder very thinly coated onto carbon paper or cloth. The catalyst is rough and porous so that the maximum surface area of the platinum can be exposed to the hydrogen or oxygen. The platinum-coated side of the catalyst faces the PEM.


. Working of a PEM fuel cell (Refer Fig no.)


Pressurized hydrogen gas (H) enters the fuel cell on the anode side. This gas is forced through the catalyst by the pressure. When an H molecule comes in contact with the platinum on the catalyst, it splits into two H+ ions and two electrons (e-). The electrons are conducted through the anode, where they make their way through the external circuit (doing useful work such as turning a motor) and return to the cathode side of the fuel cell.


Meanwhile, on the cathode side of the fuel cell, oxygen gas (O) is being forced through the catalyst, where it forms two oxygen atoms. Each of these atoms has a strong negative charge. This negative charge attracts the two H+ ions through the membrane, where they combine with an oxygen atom and two of the electrons from the external circuit to form a water molecule (HO).


This reaction in a single fuel cell produces only about 0.7 volts. To get this voltage up to a reasonable level, many separate fuel cells must be combined to form a fuel-cell stack.


. Chemistry inside the fuel cells


Anode side H ͖ 4H+ + 4e-


Cathode side O +4H+ +4e- ͖ HO


Net reaction H + O ͖ HO


.TYPES OF FUEL CELLS


There are several different types of fuel cells but they are all based around a central design which consists of two electrodes, a negative anode and a positive cathode. These are separated by a solid or liquid electrolyte that carries electrically charged particles between the two electrodes. A catalyst, such as platinum, is often used to speed up the reactions at the electrodes.


Fuel cells are classified according to the nature of the electrolyte. Each type requires particular materials and fuels and is suitable for different applications


The different types of fuel cells are as given in the next page


.1 Proton Exchange Membrane Fuel Cells (PEMFC)


In the PEM fuel cell the electrolyte is a thin polymer membrane (such as poly [perfluorosulphonic] acid, NafionTM which is permeable to protons, but does not conduct electrons, and the electrodes are typically made from carbon. Hydrogen flows into the fuel cell on to the anode and is split into hydrogen ions (protons) and electrons. The hydrogen ions permeate across the electrolyte to the cathode, while the electrons flow through an external circuit and provide power. Oxygen, in the form of air, is supplied to the cathode and this combines with the electrons and the hydrogen ions to produce water.


The chemical reactions taking place in the PEMFC is same as basic fuel cell reactions given in the previous section.


. Alkaline Fuel Cells (AFC)


.


The design of an alkali fuel cell is similar to that of a PEM cell but with an aqueous solution or stabilized matrix of potassium hydroxide as the electrolyte. The electrochemistry is somewhat different in that hydroxyl ions (OH-) migrate from the cathode to the anode where they react with hydrogen to produce water and electrons. These electrons are used to power an external circuit then return to the cathode where they react with oxygen and water to produce more hydroxyl ions.


Anode Reaction H + 4OH- ͖ 4HO + 4e-


Cathode Reaction O + HO + 4e- ͖ 4OH-


. Phosphoric Acid Fuel Cells (PAFC)


The phosphoric acid fuel cell is currently the most commercially advanced fuel cell technology. As the name suggests, these cells use liquid phosphoric acid as the electrolyte, usually contained in a silicone carbide matrix. Phosphoric acid cells work at slightly higher temperatures than PEM or alkaline fuel cells - around 150 to 00°C - but still require platinum catalysts on the electrodes to promote reactivity. The anode and cathode reactions are the same as those in the PEM fuel cell with the cathode reaction occurring at a faster rate due to the higher operating temperature


.4 Molten Carbonate Fuel Cells (MCFC)


Molten carbonate fuel cells work quite differently from those discussed so far. These cells use either molten lithium potassium or lithium sodium carbonate salts as the electrolyte. When heated to a temperature of around 650°C these salts melt and generate carbonate ions which flow from the cathode to the anode where they combine with hydrogen to give water, carbon dioxide and electrons. These electrons are routed through an external circuit back to the cathode, generating power on the way.


Anode Reaction CO- + H+HO ͖ CO + e-


Cathode Reaction CO+ ½ O + e- ͖ CO-


.5 Solid Oxide Fuel Cells (SOFC)


Solid oxide fuel cells work at even higher temperatures than molten carbonate cells. They use a solid ceramic electrolyte, such as zirconium oxide stabilised with yttrium oxide, instead of a liquid and operate at 800 - 1,000°C.


In these fuel cells, energy is generated by the migration of oxygen anions from the cathode to the anode to oxidize the fuel gas, which is typically a mixture of hydrogen and carbon monoxide. The electrons generated at the anode move via an external circuit back to the cathode where they reduce the incoming oxygen, thereby completing the cycle.


Anode Reactions H + O ͖ HO + e-


CO + O ͖ CO + e-


Cathode Reaction O + 4e- ͖ O-


.


.6 Direct Methanol Fuel Cells (DMFC)


The direct methanol fuel cell is a variant of the PEM fuel cell which uses methanol directly without prior reforming. The methanol is converted to carbon dioxide and hydrogen at the anode. The hydrogen then goes on to react with oxygen as in a standard PEM fuel cell.


Anode Reaction CHOH+ HO ͖ CO + 6H+ + 6e-


Cathode Reaction /O + 6H+ + 6e- ͖HO


Cell Reaction CHOH+ /O ͖CO + HO.


4. BENEFITS OF FUELS CELLS IN THE AUTOMOBILE SECTOR


4.1 Efficiency


The efficiency of an conventional ICE engine is limited by the carnot cycle. Practically, we can get an efficiency of about 16% from an ICE. The fuel cells are on limited by the carnot cycle and tests by different companies have shown that the efficiency of car powered by fuel cell can be as high as 48%. This makes fuel cells as an interesting and attractive idea to pursue for the future.


4. Energy security


If the current rate of increase in number of ICE automobiles is maintained, the oil imports in India will reach alarming value. This would then make automobiles out of the reach of common man as petrol and diesel would become very costly. More over the country will lose valuable foreign exchange. We have to look into alternative sources like Fuel cells to fuel our future automobiles. Studies shows that if we replaced 0% of all our ICE automobiles in the world with Fuel cell powered cars, we can reduce petrol consumption by 40% (source Automotive Fuel Cells Markets, Allied Business Intelligence - May 000). Fuel cells will increase national energy security by reducing and eventually eliminating the reliance on foreign fossil fuels. Fuel price instability and international tensions due to competition for limited fossil fuel resources will be reduced.


4. Clean and helps to save our Planet


About 5% of all human-generated greenhouse gases come from transportation - more than half of that from light-duty vehicles. Because fuel cells significantly reduce greenhouse gas and other pollutant emissions, it is an important strategy/technology in the fight against global warming, smog and any other pollution problems.


Unlike air pollutants (carbon monoxide, nitrogen oxides, hydrocarbons, and particulates - soot, smoke, etc.), greenhouse gas emissions (primarily carbon dioxide, methane, nitrous oxide, water vapor, etc.) from vehicles cannot be easily or inexpensively reduced by using add-on control devices such as a catalytic converter.


Fuel cells will reduce local air and noise pollution, groundwater contamination, and improve public health and safety from reduced exposure to fuel and emissions dangers.


Fuel cells will also be responsible for reduced motor oil spills and disposal into groundwater and streams; reduced gasoline tank leakage and resulting groundwater contamination (except for FCVs using future pump grade gasoline); and creation of a long-term pathway toward an environmentally sustainable transportation energy future based on renewable natural resources.


Fuel cells could dramatically reduce urban air pollution,


4.4 Ease of Manufacturing and Maintenance


A fuel cell powered car is easier to manufacture than much more complex conventional ICE cars. Because fuel cell vehicles operate with electric motors which have very few moving parts (only those pumps and blowers needed to provide fuel and coolant), vehicle vibrations and noise will be vastly reduced and routine maintenance (oil changes, spark plug replacement) will be eliminated..


5. CURRENT SCENARIO


Keeping in mind the benefits of fuel cell powered vehicles, huge amounts of money has been put into its research by various automobile giants


• Ballard is the leading supplier of PEM fuel cells for transportation. The


company has received orders from auto manufacturers around the


world.


• DaimlerChrysler has unveiled a fuel cell powered Town & Country minivan, the Natrium, which uses Millennium Cells Hydrogen on Demand system. The unique feature of the Natrium is that the hydrogen for the fuel cell is generated from sodium borohydride, which is derived from borax


• Ford unveiled the TH!NK FC5, a family size sedan powered by a Ballard fuel cell electric powertrain using methanol fuel. Based on the 000 Ford Focus, the TH!NK FC5s fuel cell powertrain is located beneath the vehicle floor, so it doesnt compromise passenger or cargo space.


• General Motors unveiled the fuel cell AUTOnomy, a platform that looks like a giant skateboard in which the entire propulsion and electrical systems are built into a 6-inch-thick chassis. The chassis, long and flat, could be built in varying lengths and widths to accept a wide array of body types, from family sedan to SUV or from station wagon to hot little sports car


• BMW announced plans to unveil a hydrogen-powered Mini Cooper, featuring an internal combustion engine (ICE) similar to its Clean Energy cars. The Mini Cooper features an advanced hydrogen fuel storage tank that utilizes the same space as a conventional fuel storage tank


6. KEY ISSUES


6.1 Generation of hydrogen fuel


While the operation of these vehicles is environmentally beneficial, it’s also the method of producing the fuel to power them that will determine the extent of the environmental benefits. Fuel cell vehicles can be designed to run on a variety of fuels, including hydrogen, and methanol. But not all fuels, or all fuel


Technologies, are created equal.


One way of producing hydrogen is a process called electrolysis � passing an electrical current through water, breaking down water into its components of hydrogen and oxygen. It is found that hydrogen produced by electrolysis using electricity from renewable energy sources, such as wind power and


Hydroelectricity, had the fewest negative environmental and social impacts compared with any conventional or alternative vehicle technology in the market, however, the economic costs remain relatively high.


Steam methane reforming technology � using steam to release hydrogen from a methane molecule at a large plant or local fuel station � is the next best environmental choice for powering fuel cells. However, there remain challenges to easy distribution and small-scale production of hydrogen produced in this way for powering vehicles.


Currently, fuel cell cars running on methanol have near zero tailpipe emissions, but, compared to a standard gasoline car, they do not significantly reduce life-cycle emissions of nitrogen oxides, acid rain precursors or greenhouse gases. This is expected to change as the technology matures.


Using fossil fuel-generated electricity, such as burning coal or natural gas, to produce hydrogen via electrolysis for use in fuel cell vehicles is not clearly environmentally beneficial.


Using electricity from nuclear power to produce hydrogen via electrolysis has positive and negative environmental and social attributes. Nuclear power-based systems have near zero life-cycle air emissions, but create radioactive waste with long-term negative safety, security and environmental impacts.


6. Storage of hydrogen


A major issue for automotive applications is the way in which the hydrogen will be supplied to the cells. It is possible to supply hydrogen gas directly and store in tanks on the vehicle. it has the lowest storage density of all fuels. A tank of hydrogen gas at atmospheric pressure would need to be around 000 times larger than a tank of gasoline for a comparable driving distance. This is clearly unworkable and three more space efficient ways of storing hydrogen are being considered the use of compressed gaseous hydrogen, liquid hydrogen or hydrogen fixed in the form of species such as metal hydrides.


Compressed gaseous hydrogen dramatically reduces the amount of fuel that can be stored and hence the driving range. As with compressed gas, liquefying the gas presents containment problems which add to the weight, volume and cost of storage. The boiling point of hydrogen is - 5°C so it needs to be contained in well insulated tanks to halt evaporation. Even then, around 1 - per cent of the hydrogen will boil off each day. The other downside is that significant energy is required to liquefy the gas - around 0 to 40 per cent of the energy content of the gas. To overcome these drawbacks, One idea is to trap hydrogen by forming metal hydride complexes. These metal structures tend to be very heavy, Other storage methods like Carbon nano tubes and Glass microspheres


are being looked into bout they are still in there experimental stages.


6. Market drivers for Fuel cells.


To many peoples eyes, fuel cells suffer the same problems as numerous other technological advances that of being a technology push rather than a market pull. It has certainly been the case throughout much of the history of the fuel cell (FC) that development has been carried out by people and companies with significant scientific curiosity but less commercial skill. As a result, there has been a belief that the fuel cell would be a great product if only we could find a market for it. Even if the numerous technological and commercial barriers to this concept are overcome, a whole new infrastructure will need to be created in order to deliver the hydrogen to the fuel cell.


7. SCOPE


1) Though the fuel cell powered vehicles will not completely replace the ICE vehicles at this point of time, at least till a new technique to produce hydrogen is developed, the fuel cell vehicles will run on hydrogen generated from hydrocarbon sources, either by external or internal reforming.


) This will still impart a number of environmental benefits compared to the internal combustion engine and may well be an intermediate stage on the way to a fully fledged renewable hydrogen economy.


) This will also reduce the dependence on non renewable sources of energy and conserve them for a longer length of time.


4) Fuel cell powered cars will reduce the import of crude oil and generate employment opportunities in many fields like in the R&D areas.


5) Since hydrogen and oxygen are available in all countries in almost the same amount, it will reduce the dependence of the country on foreign countries for fuel.


6) FC cars can be efficiently used for city driving. It can be used as on effective mode of transport in small cities.


7) Once FC cars are used in daily life many more benefits will come to light. These results can be used to improve the cars and finally perfect them. One day, a fuel cell will be an indispensable part of our lives.


All the above reasons provide a great scope in the field of Fuel cell research and development.


8. CONCLUSION


The ultimate goal of fuel cell vehicle is to serve mankind as a totally non-polluting vehicle with improved energy efficiency. To achieve this, the fuel cell must run on hydrogen generated by renewable means. Fuel Cell Vehicle, running on Methanol and Hydrocarbons to produce hydrogen, are still more environmental friendly than their ICE counterparts. Fuels are boon to mankind which has to judiciously used to harness maximum potential out of it. In fuel cell technology, every body is a winner from the consumers to the manufacturer. The economic advantages will make the country more self sufficient without relying much on others. Like once William C. Ford, Jr. said


“I believe fuel cell vehicles will finally end the hundred-year reign of the internal combustion engine as the dominant source of power for personal transportation. Its going to be a winning situation all the way around - consumers will get an efficient power source, communities will get zero emissions, and automakers will get another major business opportunity - a growth opportunity”


Fuel cells have the power to change our future. The fuel cell harnesses the chemical energy of hydrogen and oxygen to generate electricity without combustion or pollution. Fuel cells will power the car of tomorrow -- quieter, cleaner and more energy efficient, with equivalent range and performance. The benefits will be extraordinary, in national energy security, cleaner air, and economic opportunity


1. www.fuelcelltoday.com


. www.fuelcells.org


. Report on issues facing fuel cell research, Pembina institute, USA in collaboration with Ballard power systems.


4. www.howstuffworks.com.


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