1. ABSTRACT
    Fuel cells are an old technology. Problems have plagued their introduction. Present material science may make them a reality soon in specialized applications. The Solid Oxide Fuel Cell appears to be the most promising technology for small electric powerplants over 1 kw. The Direct Alcohol Fuel Cell appears to be the most promising as a battery replacement for portable applications such cellular phones and laptop computers. It is difficult to tell at this moment whether fuel cells will be practical for transportation applications such as automobiles and buses or whether hybrid or electric vehicles will be more popular in the future. It is unclear whether hydrogen fuel will be widely used or whether a mostly electric economy will develop. Fuel cells are analyzed theoretically using the carnot ratio which, it is explained, applies to both heat engines as well as fuel cells. A simple second law analysis shows where the loss of efficiency in different fuel cells occurs. Energy concepts are based on the web-book "Energy Science Made Simple".

2. INTRODUCTION
    I received some really good experience with hydrogen fuel cells during the several years I worked at Ballard Power in BC Canada where I was involved in stack development, small powerplants, the hydrogen fuel cell bus, submarine studies, and design of portable manpacks. Later I worked on development of micro formic acid fuel cells for the portable electronic market at Tekion Solutions in BC Canada. I was also involved with Solid Oxide Fuel Cells with AAA Power of Alberta Canada.

3. BACKGROUND
    Fuel cells convert the chemical energy of fuels directly into electricity. The principle of the fuel cell was developed by William Grove in 1839. Already around 1900 scientists and engineers were predicting that fuel cells would be common for producing electricity and motive power within a few years. That was over 100 years ago. Contrast this with the roughly 2 years that it took Nikolaus Otto to bring his Otto cycle 4-stroke internal combustion engine from the invention stage to a commercial success. Still development length has little to do with whether technology will be eventually successful. The gas turbine was invented by John Barber in 1791. It took roughly 150 years for it to reach the point of being reasonably efficient. Today however gas turbine combined cycle powerplants are about the most efficient type of engine available.
    Fuel cells are in about the fifth cycle of attempts to turn them into commercial reality. In the past the attempts often failed to the point where few companies continued development. Will these companies be successful this time around?
    During the period from 1990-2000 fuel cells were constantly in the news. At the time many experts predicted that around 2005 a large percentage of vehicles would be powered by hydrogen fuel cells, many buildings would be partially or totally powered by fuel cells, and a considerable percentage of electronic applications would be powered by micro fuel cells. Yet in the year 2008 we really only have a few trial vehicles on the road, fuel cells powering buildings are not common, and micro fuel cells are struggling to get into the market. What is going on? There really are many issues to blame but here are some very rough suggestions.
    With vehicles the major issue is that advanced piston engine hybrid vehicles are much closer to the efficiency of fuel cell vehicles than was ever thought possible and by some miracle are almost as clean as fuel cells vehicles. There is more focus now on plug in hybrids which can use green electricity more efficiently than converting electricity to hydrogen. I would like to see fuel cells used in vehicles, but my idea is to use them in a different way than they are used presently.
    The main problem with stationary fuel cells is that they are either not reliable enough or too expensive at the present time. The Solid Oxide Fuel Cell appears ideal for this application because it can easily use fossil fuel, however the high temperatures can cause severe material problems. None of these issues however should be stopping the development of stationary fuel cells, I think the problem here is one of perception by the public and government.
    A few years ago it looked like micro fuel cells would soon be powering many portable electronic products. But this has not come to pass. One issue is that batteries have become much more powerful, and electronic devices smaller. Also, it has been hard to fit the fuel cell into the same thin profile of the battery. Another issue is that here is a problem with certain fuels being transported by passengers on aircraft. There are still some technical issues to be solved. The present price of fuel cells is higher than batteries. In my opinion the reason why micro fuel cells haven't penetrated the market however has nothing to do with the above factors.

4. HOW A FUEL CELL WORKS
    A fuel cell works similar to a battery. In a battery there are two electrodes which are separated by an electrolyte. At least one of the electrodes is generally made of a solid metal. This metal is converted to another chemical compound during the production of electricity in the battery. The energy that the battery can produce in one cycle is limited by the amount of this solid metal that can be converted. In the fuel cell the solid metal is replaced by an electrode that is not consumed and a fuel that continuously replenishes the fuel cell. This fuel reacts with an oxidant such as oxygen from the other electrode. A fuel cell can produce electricity as long as more fuel and oxidant is pumped through it.

Fig 1 Alkaline fuel cell often uses hydrogen and oxygen as fuel

    The alkaline fuel cell as shown in Fig 1 is one of the oldest and most simple type of fuel cell. This is the type of fuel cell that has been used in space missions for some time. Hydrogen and oxygen are commonly used as the fuel and oxidant. The electrodes are made of porous carbon plates which are laced with a catalyst...which is a substance that accelerates chemical reactions. The electrolyte is potassium hydroxide. At the anode, the hydrogen gas combines with hydroxide ions to produce water vapor. This reaction results in electrons that are left over. These electrons are forced out of the anode and produce the electric current. At the cathode, oxygen and water plus returning electrons from the circuit form hydroxide ions which are again recycled back to the anode. The basic core of the fuel cell consisting of the manifolds, anode, cathode and electrolyte is generally called the stack.

5. TYPES OF FUEL CELLS
    There are numerous types of fuel cells that have been made. The most common are shown below. Each type uses different materials and operates at a different temperature.

    Scientists keep changing their minds every few years about which of the above fuel cells will be the most popular in the future. As of 2002May there are three types of fuel cells that appear to be the most promising. The Solid Oxide Fuel Cell or SOFC is the most likely contender for both large and small electric powerplants in the 1 kw and above size. The Direct Alcohol Fuel Cell or DAFC appears to be the most promising as a battery replacement for portable applications such as cellular phones and laptop computers. It is difficult to tell at this moment which fuel cell will be most practical for transportation applications such as automobiles and buses. The Polymer Electrolyte Fuel Cell PEFC is the most practical if we have a developed hydrogen economy. Many automobile manufacturers however believe that the DAFC will be much simpler than the PEFC so it will be the winner for vehicular applications. Others say that the much higher efficiency of the SOFC and its ability to use most any fuel will make it a logical choice for vehicular applications as well. Proponents claim the startup time problem of the SOFC can be overcome by using supercapacitor batteries for the first few minutes of operation.
    At the moment there are several fuel cells that are in limited production. The Polymer Electrolyte Fuel Cell PEFC is at the point of limited commercial production. The Phosphoric Acid Fuel Cell PAFC has been produced for several years already for medium sized electric powerplants. The Alkaline Fuel Cell AFC has been produced in limited volumes for decades already. Both the Solid Oxide Fuel Cell and the Direct Alcohol Fuel Cell are being produced in limited quantities.
    The SOFC is considered to be superior to the PAFC and so would likely replace it in time. The Molten Carbonate Fuel Cell MCFC was thought by many scientists to be the logical choice for electric powerplants due to the perceived problems with the SOFC. Now that it appears these problems may be solved, development of the MCFC will likely be shelved. The Alkaline Fuel Cell AFC has been used in space applications where hydrogen and oxygen are available. By using carbon dioxide scrubbers, several of these fuel cells are being operated successfully on hydrogen and air.

6. FUEL CELLS FOR ELECTRIC POWER PRODUCTION

Fig 3 Chart showing projected efficiencies of different future electricity generating powerplants

    There is a rapid trend in North America to deregulate the production of electric power. One of the benefits of deregulation is that it will promote CHP...combined heat and power, also known as cogeneration. North America will likely generate much of its electricity by burning fossil fuel for the next 10-40 years. CHP will conserve fuel by utilizing the thermal energy that is produced as a result of generating electricity. Because thermal energy cannot be piped efficiently for long distances, CHP powerplants will generally need to be much smaller than the present ones which are often around 200,000 kw.
    Fuel cells will likely be the favored technology of the future for small electric powerplants. Not only do they produce reasonable efficiencies in 30 kw sizes, they will likely be able to run quietly, need infrequent maintenance, emit little pollution and have high efficiency even at part load conditions.
    Electricity is used by many of our modern high technology devices. Presently batteries are used in these devices. Batteries do not have a long enough life for these applications. Fuel cells could provide continuous power for these devices. Every week or month a new supply of liquid fuel would be injected into the fuel cell.
    Fuel cells are most ideal for electric power production because electricity is both the initial and final form of energy that is produced.

7. FUEL CELLS FOR TRANSPORTATION

Fig 4 Estimated efficiencies [1] of different automobiles using liquid hydrocarbon fuel

    Fuel cells are being proposed to replace Otto or Diesel engines because they could be reliable, simple, quieter, less polluting, and have even greater economy.
    The internal combustion Otto or Diesel cycle engine has been used in automobiles for 100 years. It is a reasonably simple and reliable mechanical device which nowadays has a lifespan of up to 400,000 km or roughly 10,000 hrs of operation in automobiles and over 1,000,000 km or 25,000 hrs or more in larger applications such as buses, trucks, ships and locomotives. Therefore life span is not a problem.
    Automobile manufactures are finding new ways of improving the Otto and Diesel engines. Toyota for example has unveiled an Otto cycle automobile that has tailpipe emissions that are 5 times cleaner than typical Los Angeles air. In other words the gasoline engine cleans up the air, at least the present dirty air.
    Volkswagen has a prototype compact 4 seater Diesel cycle automobile that gets 100 mpg or 35 km/liter. This would be roughly 570 liters/year for average drivers. In Canada this would result in $285/yr in fuel costs. At present fuel costs, buying a car with incredible efficiency is not an issue yet in North America.
    Fuel cells have the potential to be considerably quieter than Otto or Diesel cycle powerplants. This would especially reduce the noise on quiet neighborhood streets. At speeds higher than 50 km/hr however there is still the problem of road noise.
    Fuel cells produce electricity. This is not the desired form of energy for transportation. The electricity must be converted into mechanical power using an electric motor. The Otto or Diesel cycle produces the required mechanical power directly. This gives them an advantage compared to fuel cell powered automobiles.
    Presently Otto and Diesel cycle engines seem to be able to comply with extremely stringent pollution regulations, are inexpensive to produce, produce reasonable fuel economy, and use readily available liquid fuels. Fuel cell vehicles have a much greater chance of being accepted however in the future when fuel prices are higher and liquid fossil fuels are in short supply. However fuel cell vehicles will then be competing with electric vehicles which will be cheaper to operate but have problems with recharging.

8. SOLID OXIDE FUEL CELL (SOFC)

Fig 5 Simple type [2] SOFC suitable for 1-30 kw powerplants

    The Solid Oxide Fuel Cell is considered to be the most desirable fuel cell for generating electricity from hydrocarbon fuels. This is because it is simple, highly efficient, tolerant to impurities, and can at least partially internally reform hydrocarbon fuels.
    The SOFC runs at a red-hot temperature of 700-1000°C. Westinghouse has worked at developing a tubular style of SOFC for many years which operates at 1000°C. These long tubes have high electrical resistance but are simple to seal. Many companies such as Global Thermoelectric are now working on a planar SOFC composed of thin ceramic sheets which operate at 800°C or even less. Thin sheets have low electrical resistance and possible high efficiencies. Cheaper materials can be used at these lower temperatures. Experts previously predicted that the SOFC was a long way to becoming commercial reality. Many now believe that these lower temperatures may lead to a quicker solution to these problems.
    One of the big advantages of the SOFC over the MCFC is that the electrolyte is a solid. This means that no pumps are required to circulate hot electrolyte. Small planar SOFC of 1 kw could be constructed with very thin sheets and result in a very compact package.
    A big advantage of the SOFC is that both hydrogen and carbon monoxide are used in the cell [3]. In the PEFC the carbon monoxide is a poison, while in the SOFC it is a fuel. This means that the SOFC can readily and safely use many common hydrocarbons fuels such as natural gas, diesel, gasoline, alcohol and coal gas. In the PEFC an external reformer is required to produce hydrogen gas while the SOFC can reform these fuels into hydrogen and carbon monoxide inside the cell. This results in some of the high temperature waste thermal energy being recycled back into the fuel.
    Because the chemical reactions in the SOFC are good at the high operating temperatures, air compression is not required. Especially on smaller systems this results in a simpler system, quiet operation, and high efficiencies. Exotic catalysts are not required either.
    Many fuel cells such as the PEFC require an expensive liquid cooling system but the SOFC requires none. In fact insulation must be used to maintain the cell temperature on small systems. The cell is cooled internally by the reforming action of the fuel and by the cool outside air that is drawn into the fuel cell.
    Because the SOFC does not produce any power below 650°C, a few minutes of fuel burning is required to reach operating temperature. While the SOFC is also being proposed as an automotive powerplant, this time delay is considered to be a disadvantage. Because electric powerplants run continuously, this time delay is not a problem. The SOFC may well be suited to at least certain vehicles which run more continuously.
    Because of the high temperatures of the SOFC, they may not be practical for sizes much below 1,000 watts or when small to midsize portable applications are involved.
    Small SOFC will be about 50% efficient [4] from about 15%-100% power. To achieve even greater efficiency, medium sized and larger SOFC are generally combined with gas turbines. The fuel cells are pressurized and the gas turbine produces electricity from the extra waste thermal energy produced by the fuel cell. The resulting efficiency of the medium SOFC could be 60% and large one's up to 70%.
    A SOFC suitable for producing 1-30 kW and using natural gas as its fuel is shown in Fig 5. On the anode side, natural gas is first ejected into a reforming chamber where it draws waste thermal energy from the stack and is converted into hydrogen and carbon monoxide. It then flows into the anode manifold where most of the hydrogen and carbon monoxide is oxidized into water and carbon dioxide. This gas stream is then partly recycled to the reforming chamber where the water is used in the reforming chamber. On the cathode side, air is first blown into a heat exchanger where it reaches nearly operating temperature. The air is brought up to the operating temperature of 800°C by combustion of the remaining hydrogen and carbon monoxide gas from the anode. The oxygen in the cathode manifold is converted into an oxygen ion which travels back to the anode.

9. DIRECT ALCOHOL FUEL CELL (DAFC)
    Several companies around the world are presently working on DAFC. In 1999 there was a marked shift away from developing the PEFC in favor of the DAFC [5]. ]. In this type of fuel cell, either methyl DMFC or ethyl DEFC alcohol is not reformed into hydrogen gas but is used directly in a very simple type of fuel cell. Its operating temperature of 50-100°C is low and so is ideal for tiny to midsize applications. Its electrolyte is a polymer or a liquid alkaline. This type of fuel cell was largely overlooked in the early 1990s because its efficiency was below 25%. Most companies rather pursued the PEFC because of its higher efficiency and power density. There has been tremendous progress made in the last 7 years. Efficiencies of the DMFC are much higher and predicted efficiencies in the future may be as high as 40% [6] for a DC automobile powerplant. Power densities are over 20 times as high now as in the early 1990s. It is expected that the DMFC will be more efficient than the PEFC for automobiles that use methanol as fuel. Presently the power density of the DEFC is only 50% of the DMFC but hopefully this can be improved in the future.
    Fuel crossing over from the anode to the cathode without producing electricity is one problem that has restricted this technology from its inception. One company, Energy Ventures Inc claimed in Dec1999 that it has completely solved this cross-over problem. Another problem however is that there are often chemical compounds formed during operation that poison the catalyst.
    There are already working DMFC prototypes used by the military for powering electronic equipment in the field.

Fig 6 A small simple 30 kw Direct Methanol Fuel Cell

    Figure 6 illustrates a type of DMFC that could be used in a 30 kw system. Even smaller ones for use as battery replacements do away with the air blower and the separate methanol water tank and pump. Such fuel cells are not much different than batteries in construction.
    Recently there has been much concern about the poisonous aspects of methanol--methyl alcohol. As of 2001 methanol is "out" and ethanol is "in". Already several companies are now working on DEFC. Presently the power density is only 50% of the DMFC but hopefully this can be improved.

11. PHOSPHORIC ACID FUEL CELL (PAFC)
    The Phosphoric Acid Fuel Cell has been under development for 15 years as an electric powerplant. While it has a lower real efficiency than the MCFC or SOFC, its lower operating temperature of 160-220°C was considered more ideal for small and midsize powerplants. Midsize 200 kW AC powerplants are 40% efficient and large 11MW units are 45% efficient when running on natural gas. These efficiencies are comparable to the PEFC.

12. MOLTEN CARBONATE FUEL CELL (MCFC)
    The Molten Carbonate Fuel Cell has also been under development for 15 years as an electric powerplant. The operating temperature of 600-650°C is lower than the SOFC. It is considerably more efficient that the PAFC. It already has the advantage of reforming inside the stack. Its disadvantage is the corrosiveness of the molten carbonate electrolyte. Large AC powerplants using gas turbine bottoming cycles to extract the waste heat from the stack could be up to 60% efficient when operating on natural gas. When problems with the SOFC are solved, work on the MCFC may be disbanded.

13. ALKALINE FUEL CELL (AFC)
    The Alkaline Fuel Cell cannot operate with carbon dioxide in either the fuel or oxidant. Even the small amount of carbon dioxide in the air is harmful. Carbon dioxide scrubbers have been successfully used to allow these fuel cells to operate on air. The cost of the scrubber is considered to be reasonable. This fuel cell operates at various temperatures, 250°C was chosen for space vehicles. The DC efficiency is as high as 60% (lhv) at rated power and because there are low system losses, the part load efficiency can be even higher.

14. FUELS
    The hydrogen economy which was popularized in the 1970s was based on producing hydrogen using nuclear powerplants. Now that nuclear power is unpopular, we have eliminated any present method of making large amounts of hydrogen for a reasonable price. Society has however held on to the wonders of having a hydrogen economy, where hydrogen would be used for everything from generating electric power to heating homes and powering industry.
    Hydrogen is admittedly a wonderful fuel because only water is produced in operating the fuel cell. Hydrogen is however a difficult fuel to store. It is difficult and costly to liquefy. It has lower energy content than natural gas when pressurized in tanks. There has been increasing success in storing hydrogen gas in metal hydrides and carbon compounds but many of these techniques require either pressure or temperature swings during storage and extraction. Many require cryogenic refrigeration.
    There is presently no way of making cheap hydrogen. Laws of energy demand an equal or larger amount of another form of energy to produce it. Presently hydrogen is mostly made from natural gas. Because this process is only 65% efficient when storage losses are considered, this results in a loss of efficiency compared to using the natural gas in a SOFC. Producing hydrogen by electrolysis is generally even less efficient because the electricity is generated by a gas turbine which is no more than 57% efficient.
    Of course if hydrogen would be made from the electricity produced by solar panels or fusion powerplants, the situation would be somewhat different. Presently however the cost of making hydrogen from the electricity of solar panels is much higher than making it from natural gas. As well electricity is presently sold for about 3x the cost of fuel--which makes selling electricity more viable than producing hydrogen. Fusion power has not been perfected presently.
    It is true that carbon dioxide is considered a greenhouse gas. It is not a local type of pollution however. There is no advantage in producing hydrogen from natural gas far away from city areas. The carbon dioxide quickly mixes throughout the globe. There are benefits in using renewable hydrocarbon fuels rather than hydrogen.
    Ethanol or butanol are presently viewed by many scientists as the perfect fuels for portable fuel cells. Ethanol, butanol, and methanol presently can be made from either natural gas or biomass. This process is also about 65% efficient. Therefore hydrogen and alcohol cost about the same to produce and store. The DMFC however is slightly less efficient than a PEFC operating on stored hydrogen gas. Many consider that the benefits of storing a liquid fuel more than offset this loss of efficiency. In the future it may also be possible to produce alcohol directly in solar panels or in fusion powerplants. It is possible to extract carbon dioxide from the atmosphere in the process just as plants do. Artificial photosynthesis in a type of solar panel is being worked on extensively by the Japanese. If this could be accomplished, carbon dioxide would not be "produced" during use. It would simply be emitted temporarily before being recycled.

15. FORMS OF ENERGY

Fig 7 Different forms of energy shown in a chart

    To properly evaluate different types of fuel cells it is desirable to understand basic theoretical energy concepts. To understand energy concepts, it is beneficial to have a proper naming system that covers all the basic types of different energy in the universe. This is because it is often difficult or impossible to convert certain types of energy into different forms. The system of energy used in this web-booklet is based on a plural energy system where all the different types of energy are two word forms such as chemical energy. The basis of this two word naming system is borrowed from chemistry, however typically it is not labeled as the plural energy system. Engineers do not like to use this chemistry naming system, however it is the simplest and easiest to understand. The plural energy system is shown as a bar chart in Fig 7. At the head of the chart of "simple forms" is einstein energy which is...a term for the concept of the total energy in the universe or a particular system. When referring to the fact that energy is conserved in the universe it should be mentioned that it is einstein energy that is conserved, because other forms may not be. All einstein energy can be logically divided into either external energy or internal energy. The major difference between external energy and internal energy is the fact that internal energy can never be completely converted into external energy.
    External energy is logically divided up between the major forms of kinetic energy and potential energy. Internal energy is logically divided up between different types that are the most used. Thermal energy...is the motion or translational energy of the molecules. Chemical energy ... is the energy stored due to the bonding of the atoms in the molecules. Radiant energy...is the energy contained in the moving photon wavicle.
Caloric energy...is a term which represents the amount of internal energy that will flow between two reservoirs. Caloric energy can never be completely converted into external energy. It can be split up into two parts for analysis. The helmholtz energy...is the part of caloric energy that could be converted into external energy in a future process. The bound energy is the caloric energy that could have been converted into external energy if the conversion had started at an infinite temperature or temperament and progressed till the present point
    Substances usually contain mixtures of external energy and internal energy. Scientists have developed many terms for combinations and of these two types as shown in the chart of "complex forms" in Fig 7. Gibbs energy...is composed of helmholtz energy plus expansion energy. Exergic energy is composed of external energy plus helmholtz energy.

16. TEMPERAMENT vs TEMPERATURE

Fig 8 Virtual photons are closely coupled and real photons travel alone through space

    A fuel cell creates electricity, which is a form of external energy, directly from the energy in chemical fuels without an intermediate conversion into thermal energy. When a hydrogen atom bonds to an oxygen molecule, not as much total energy is required in the newly formed water molecule as in the separate hydrogen and oxygen molecules. A certain amount of energy can be released. When the hydrogen-oxygen bonding occurs, the excess energy under ideal conditions can be released in a single package for each newly created bond. In other words the excess energy is not dribbled out in multiple randomly sized amounts of energy. This single package is called a virtual photon and is illustrated in Fig 8. Photons are not marble like objects but rather tiny localized vibrations of energy that travel through the substratum or continuum of space. They cannot be detected and so are called virtual. They are referred to as a package because this energy does not split up traveling to its destination and neither do two packages join together. This virtual photon can under ideal conditions be transferred directly to other chemical system through, for example close contact, without being spilled to the surroundings. Such a transfer of energy can be equated to the transfer of grains of sugar from one tank to another through a pipe. No sugar would be spilled to the outside environment. Real photons on the other hand are packages of energy that have broken away as separate entities. It is as if the pipe between the tanks of sugar is missing and the sugar spills on the floor. Light is composed of such real photons.
    We could use Joules or BTU as a measure of the amount of energy that each real or virtual photon contains but it would be a very small fraction of a Joule indeed. It is simpler to use a scale that merely represents the amount. We already use the scale called temperature to measure thermal energy. This represents the average collision energy between molecules. Real photons are created during these collisions which are equal in energy to each collision as shown in Fig 8. Therefore radiant energy can already be thought of as having a certain temperature. This thinking can be extended and a different term temperament can be used to represent the amount of energy in all types of photons.

17. FUEL CELLS vs HEAT ENGINES

Fig 9 Heat engines are theoretically at a disadvantage compared to fuel cells

    The virtual photons that are transferred during the chemical reactions in a fuel cell have a very high temperament somewhere between 3,500° and 20,000° Kelvin. It is this extremely high temperament that allows the fuel cell to be theoretically so efficient. Generally textbooks relate Carnot's Law only to the amount of external energy that can be extracted from thermal energy systems. The same law however does apply to all internal energy systems whether nuclear, chemical or thermal etc. The amount of external energy that can be extracted from all types of internal energy is called the carnot ratio. The carnot ratio for virtual photons of 3,500°K is however about 92% under normal conditions. This is much higher than for real photons in a gas turbine with a mean temperament of 1000°K and a carnot ratio of 72%. The carnot ratio is based on a particular ambient temperature of the surroundings. The carnot ratio only relates to the absolute temperature scale where 0°C=273.15°K degrees.
    Heat engines such as gas turbines are considered to be inferior to fuel cells because they must convert the high temperament chemical energy into low temperature thermal energy first. A gas turbine cannot operate at the temperament of the chemical energy without melting. As can be seen from the graph in Fig 9, when the temperament is reduced, the carnot ratio is reduced. A large percentage of the helmholtz energy that was available at the higher temperament is lost, it is converted into useless bound energy. The fuel cell does get hot but only because of the resistance and inefficiencies during the ion and electron flow during the production of electricity. So, many types of fuel cells can run efficiently at low temperatures while at the same time converting very high temperament energy.
    Present highly advanced gas turbines do not achieve a mean temperature of more than 1150°K or 877°C. In spite of this gas turbines (with addition of heat exchanging or steam turbines) can be highly efficient in the large sizes and produce little pollution. The latest are 57% efficient in converting fuel to electricity. In the future, ceramic gas turbines could reach 70% efficiency. This would result in a higher efficiency than what the fuel cell can achieve by itself. There is even some possibility of using energy transformers in the combustion process to increase the efficiency to 80%.
    Unfortunately very small gas turbines are not nearly as efficient. Present microturbines in the 30 kw range are only about 25% efficient even when heat exchanging is employed, though future ceramic microturbines in this size may achieve 35% efficiency.
    In the future, medium and large powerplants using SOFC will be fuel cell gas turbine combined cycles. In this way the benefits of each type of conversion technology is utilized.

18. SECOND LAW ANALYSIS OF FUEL CELLS

Fig 10 Exergic energy loss diagram for proposed 30 kw AC powerplants operating on hydrocarbon fuel

    In Fig 10 the exergic energy efficiency of three proposed fuel cells are compared when operating on hydrocarbon fuel. The fuel cell process is divided into six subsystems. In each subsystem there are inefficiencies involved that reduce the exergic energy that is left in the system. In all cases, the electricity that is extracted is still considered to be part of the exergic energy of the system. It appears that the SOFC 30 kw system will have an efficiency of 1.4 times that of the PEFC and 1.3 times that of the DMFC.
    If Fig 10 is examined in more detail, it is apparent that the SOFC is the most efficient largely because of the low reformer and air pressurization losses. This is because the SOFC can reform fuel inside the stack and utilize some of the stack waste thermal energy. Because the PEFC operates at a lower temperature this is not possible. The SOFC does not need to operate at higher than ambient air pressure. It only uses a low pressure blower to drive air through the cell. The PEFC runs at a high air pressure. In a small 30 kw powerplant this pressure energy cannot be readily recovered. The DMFC stack efficiency is very low, but because there are no reformer losses and less air pressurization and system losses, the final efficiency is still higher than the PEFC.
    A more detailed breakdown of the three types of fuel cells is shown in Fig 11. For each subsystem there are three columns. The system efficiency shows how efficient each subsystem is in retaining the exergic energy YE. The bound energy BE produced is equal to the loss of exergic energy YE in each subsystem. The YE leaving is the amount of exergic energy that is passed on to the next subsystem.
    It can be readily seen why the SOFC is the most desirable fuel cell of the three for ultimate efficiency in a fuel cell gas turbine powerplant. Notice that after the electricity extraction process in the stack, there are still 82 units of exergic energy YE retained in the SOFC. The PEFC has only 51.5 units and the DMFC has only 46.8 units.
    Not shown however is that the PEFC operating at ambient air pressure and using hydrogen as its fuel would be the most efficient fuel cell without using a bottoming cycle such as a gas turbine. It would achieve 57% efficiency, while the SOFC would be 53% and the DMFC would be 43%.

19. CONCLUSIONS
    Fuel cells are still a few years away from commercialization on a large scale. It is very difficult to tell which fuel and which technology will be predominant in the future. There are some problems to be solved in the SOFC and the DAFC. If these can be solved then these will become the predominant fuel cells being developed in the future.

20. NOTES
[1] "Today's internal combustion engine converts only 19% of the useful energy in gasoline to turning a car's wheels", from American Methanol Institute report pIIII (Nowell)
[2] Many SOFC use a separate pre-reformer as opposed to the integral reformer as shown (Stimming)
[3] SOFC generally use carbon monoxide as a fuel because most transport oxygen ions. (Minh p29)
[4] Based on extensive laboratory tests. Some of the fuel cells achieved even higher efficiencies than this (Stimming)
[5] Based on recent reports for example Nissan and Suzuki (Comline)
[6] From JPL Website "Description of Direct Oxidation, Liquid Feed Methanol Fuel Cell", updated 14Jun1996 "efficiency is projected to increase to >40% with the use of advanced materials". A more recent report in the Hydrogen and Fuel Cell letter predicts 45% efficiency.

21. REFERENCES
Buswell, Clause, Cohen, Louie, Watkins 1994 Ballard US Patent 5,360,679 ...Hydrocarbon Fueled Solid Polymer Fuel Cell Electric Power Generation System
Kordesch, K., Simader, G. 1996 Fuel Cells and their Applications VCH Press NY USA
Stimming, U. et all 1997 Proceedings of the fifth International Symposium on Solid Oxide Fuel Cells Vol 97-40 pg 69 The Electrochemical Society NJ USA.
Minh, Nguyen Quang, Takahashi, Takehiko 1995 Science and Technology of Ceramic Fuel Cells Elesevier Science B.V. Amsterdam, Netherlands
Comline News Service 09Feb1999 Nissan, Suzuki Join Effort for Direct Methanol Fuel Cell
Nowell, G.P. 1998 The Promise of Methanol Fuel Cell Vehicles American Methanol Institute Washington DC USA

22. SYMBOLS
B-energy=bound energy, is the energy that can never be converted to external energy
X-energy=external energy, is total of potential and kinetic forms
Y-energy=exergic energy, or exergy, forms that could be converted to external energy
AC=alternating current
DC=direct current
°K=degrees Kelvin, most textbooks write this only as K
°C=degrees Celsius, 0°C=273.15°K

23. REVISION HISTORY
1999May17 First printing, 8 pages, 9 illustrations
1999Dec12 Largely rewritten, new fuel cell companies added, expanded on theoretical conversion section, added two illustrations.
2000Jun27 Some changes in first chapters regarding popularity of different types of fuel cells.
2001Mar19 Made numerous updates throughout document. Mentioned that methanol is considered poisonous and so ethanol is now favored and some companies are now working on DEFC.
2002May03 Changed name from hyphenated energy system to plural energy system and made associated naming changes.
2008Apr08 Updated information.

COPYRIGHT © 1999-2008 by Ben Wiens Energy Science Inc. All rights reserved.