The Future of Fuel Cells
CONTENTS (16 WEB PAGES to PRINT, 68 Kbytes of TEXT FILE and 43 KBbytes for 11 IMAGES)
4. How does a fuel cell work?
5. Types of fuel cells
6. Fuel cells for electric power production
7. Fuel cells for transportation
8. Solid oxide fuel cell (SOFC)
9. Direct alcohol fuel cell (DAFC)
10. Polymer electrolyte fuel cell (PEFC)
11. Phosphoric acid fuel cell (PAFC)
12. Molten carbonate fuel cell (MCFC)
13. Alkaline fuel cell (AFC)
15. Forms of energy
16. Temperament vs temperature
17. Fuel cells vs heat engines
18. Second law analysis of fuel cells
19. Companies involved in research
24. Revision History
©2002May03 by Ben Wiens...applied energy scientist
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 which fuel cell will be most practical for transportation applications such as automobiles and buses. Fuel cells used as electric powerplants may be successful before vehicular ones are. This is because a fuel cell produces electric power which is what is required in this case. In transportation applications the electricity produced must then be converted to mechanical power. It is unclear whether hydrogen fuel will be widely used. This is because solid oxide fuel cells will be become extremely popular and these can cleanly convert renewable hydrocarbon fuels. 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".
I have been interested in different aspects of energy since I was young. I got some really good experience with fuel cells during the several years I worked at Ballard Power in BC Canada. Soon after I started work at Ballard I was assigned to work on stack development. My boss David Watkins felt the basic fuel cell stack design needed to be changed right away but could not get the project approved. He pegged me as a maverick and so assigned the project to me as a black project...under the table. To do my job right I needed to ask a lot of questions and of course I faced some angry managers who felt they shouldn't be wasting their time on a project that many felt was not necessary. It wasn't long before the existing fuel cell stack design failed in trial runs with some of the licensees just as Watkins had predicted. The best design from the black project was picked and a team of us developed a new workable stack in 30 days. I remember making most of the cast in place seals on the 40 or so fuel cell electrodes under a microscope using a foot controlled syringe. See my article Understanding Patents for diagrams of the new electrode design.
Quite some time later I was assigned to work on the bus project. There my bosses were more used to the military style of working. I was used to working on black projects. I was used to thinking that fighting the status quo was how to get things done. It very often is, but my new managers didn't like my style. I have learned a lot about developing technology since then. I was so ideal then, thinking that good technology is all that counts. The real world is made of people and they have a lot of different ideas. Sometimes people with similar education and background can have quite opposite ideas. More time is often spent on trying figure out how to please everyone. But that is the real world.
Today fuel cells are much in the news. Some say hydrogen will be widely used. Others say methanol or ethanol is more appropriate. Even others say we will be using gasoline for a long time. Fusion power could be a reality tomorrow. Some of the hype is even a conspiracy by people wanting to force their views on the public or make money on their stock options. Make up your own mind. Just remember back. The Wankel engine and 20 hr work-week were talked about as almost sure things, while the photocopy machine and computer were initially viewed as something few people would need. In the end quite the opposite of what was predicted became the norm.
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 roughly 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 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? Do we have more advanced materials than were available in the 1960s when the last wave of development occurred? It appears not that much has changed. But human society seems to advance itself in waves; the next time around it is possible to do things just a bit better. And this time around this little bit better may result in success. Scientists and engineers also have had their fling with many of the other known energy conversion technologies. The current trend in research is to do things directly. Vacuum tubes are not used in computers today, simpler solid-state transistors are used in their place. Liquid crystal screens are replacing tube type monitors for computers. Solid-state film type solar panels are replacing solar concentrator dishes. Scientists are quickly developing an understanding of how to cut out the intermediate steps.
There have been several world events that have shaped how we think about energy technology. The Chernobyl disaster made the world think differently about nuclear fission power. After many years of developing fusion power many scientists are not sure it will happen. The cheap nuclear solution therefore is not a reality right now. The new focus instead is for efficient and clean technology that uses fuel. This focus may only last as long as abundant fuel is available, but it is the focus now. Just around the corner fusion power may be developed. Costs of solar panels may be brought to the point where all electricity is generated with solar energy and super batteries are the thing to power cars. Batteries are roughly twice as efficient in storing "electricity" than many fuel cells are. Will the fuel cell survive with these changes? Who knows?
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.
Type Abbreviation Operating temp Uses Solid Oxide SOFC 500-1000°C All sizes of CHP Direct Alcohol DAFC 50-100°C Buses, cars, appliances, small CHP Polymer Electrolyte PEFC 50-100°C Buses, cars Phosphoric Acid PAFC 200°C Medium CHP Molten Carbonate MCFC 600°C Large CHP Alkaline AFC 50-250°C Used in space vehicles
Fig 2 Different types of fuel cells
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  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.
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. It is not likely that the PEFC operating on methanol or gasoline will be able to compete with them easily. Such fuel cells using reformers do not produce much less pollution than the future advanced Otto and Diesel cycle engines with complex catalytic converters. If vehicles use hydrogen as fuel, a hydrogen supply system would need to be installed. This would be extremely expensive. The DAFC however would likely be simpler than the internal combustion engine, produce superior efficiency and be less polluting. The liquid fuel could be handled by slightly modifying the present distribution equipment. When the DAFC is perfected there may be a major swing away from the Otto and Diesel cycle automobiles. There are some vehicle manufacturers who are betting on this. Even Ballard who has been the major proponent of the hydrogen economy and hydrogen fuel cells has bought major DAFC technology in Aug1999, but it appears that recently they have practically shelved work on DAFC fuel cells.
8. SOLID OXIDE FUEL CELL (SOFC)
Fig 5 Simple type  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 . 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. Still you may be interested to know that the manager for bus development at Ballard, who develop PEFC, is now working for Global, who develop SOFC. 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  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 . ]. 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%  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.
10. POLYMER ELECTROLYTE FUEL CELL (PEFC)
The PEFC is considered the darling fuel cell by proponents of the hydrogen economy. Automobiles emitting pure water from their tailpipes are envisioned. It is not likely that there will be hydrogen pipelines supplying homes, businesses and service stations in the near future however. Many companies are proposing that PEFC systems would extract hydrogen from hydrocarbon fuels such as methanol or natural gas. While the efficiency of the PEFC when running on hydrogen and no air pressurization is high, practical systems that use fuel reforming and air compression suffer in efficiency. Small 30 kW AC powerplants will likely be 35% fuel to electricity efficient, 200 kW units 40% and large units 45%. Figure 4 shows that an automobile powerplant including an electric motor would have an efficiency of about 35%. There has been some progress made in storing hydrogen in different materials such as hydrides or carbon. If such materials can be perfected this would dramatically increase the chances for the PEFC success for automotive applications. The complex reformer would not be necessary, however unless hydrogen is universally available through pipelines across the country, the hydrogen would have to be manufactured locally by service stations. This is possible for larger city service stations but not really practical for small out of the way ones.
The PEFC generally operates at 80°C which makes it ideal for small applications and allows reasonably inexpensive materials to be used. Unfortunately, this low a temperature is quite near the ambient temperature which hampers disposing of excess heat--present automobile engines dispose of heat at up to 100°C. A catalyst is also required to promote the chemical reaction at the low temperatures involved. Previously the platinum catalysts used in the stack made this type of fuel cell expensive. New techniques for coating very thin layers of catalyst on the polymer electrolyte have reduced the cost of the catalyst to around $150 per automobile.
The PEFC is particular in that only hydrogen fuel can be used in the cell. Hydrocarbon fuels must be reformed carefully. Even small amounts of carbon monoxide in the cell can poison the catalyst--often permanently. If a reformer is used, this also requires a few minutes warm up time. Stored hydrogen must be used in the startup phase. Such problems make the PEFC running on stored hydrogen sound more appealing. A larger manufacturing plant running continuously has a much better chance of supplying very pure hydrogen.
A liquid cooling system is required. This means that there is pure water inside the cells. Ballard has tested the fuel cell at below freezing temperatures and there was no damage to the stack. It appears that the stack coolant must be drained after shutdown. I do not know what repeated freeze-thaw cycling would do to the hydrated stack even if drained.
Larger than 1 kw PEFC are generally pressurized to increase the chemical reaction at the low temperatures involved. Air compression to about 3 atmospheres or higher must be used for the fuel cell to have a reasonable power density. On small systems this results in a substantial loss of efficiency. The air compressors also add considerable complexity to the fuel cell. On automobiles and buses two air compressors are often used. One is a turbocharger and the second is a supercharger.
Many experts feel that the DAFC will replace the PEFC once problems are solved. There is however a chance that a gasoline reformer will be perfected. If such a fuel cell system can be made to be reliable and inexpensive, then the PEFC will have a much better chance of being successful. Many experts however are not sure this is possible.
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.
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 is presently viewed by many scientists as the perfect fuel for portable fuel cells. Methanol and ethanol 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%.
SOFC PEFC DMFC Subsystem Y-eff BE YE Y-eff BE YE Y-eff BE YE 0. Hydrocarbon fuel - - 100.0 - - 100.0 - - 100.0 1. Reformer/Burner 95% 5.0 95.0 80% 20.0 80.0 100% 0 100.0 2. Stack electrical 86% 14.0 82.0 64% 28.5 51.5 47% 53.2 46.8 3. Stack thermal 0% 27.0 55.0 0% 1.5 50.0 0% 1.4 45.4 4. Pressurization 98% 1.0 54 78% 10.8 39.2 90% 4.6 40.8 5. System 98% 1.0 53.0 95% 2.0 37.2 98% 0.8 40.0 6. Inverter 94% 3.0 50.0 94% 2.2 35.0 94% 2.5 37.5
I>Fig 11 Exergic energy efficiency of subsystems in 30 kw AC powerplants operating on hydrocarbon fuel
19. SOME COMPANIES INVOLVED IN RESEARCH
Ballard Power, Canada...predominantly working on PEFC for transportation and electric powerplants. Most of the PEFC technology was developed in house and they own over 200 patents. They are working closely with DaimlerChrysler and Ford. According to Merill Lynch, the PEFC fuel cell cars powered by Ballard will not commence mass production until 2004. In Aug1999 however they announced purchase of a worldwide, non-exclusive license to DMFC intellectual property from the California Institute of Technology (Caltech) and the University of Southern California (USC) through DTI Energy, Inc. which holds exclusive licensing rights to the intellectual property. The license is based on technology developed at the Jet Propulsion Laboratory of Caltech and the Loker Hydrocarbon Research Institute at USC. Ballard also acquired the right to sublicense the intellectual property to its alliance members under certain conditions.
CSIRO, Australia...large scientific research agency, working on planar SOFC, claim to be making good progress.
Energy Ventures Inc, Canada...is developing DMFC, AFC and lithium ion batteries for portable power. Dec1999 the company claimed its new DMFC technology has resolved the historic problem of methanol cross-over and would result in an initial 30-40% improvement in output. Wayne Hartford, President of EVI stated that "We feel that the marketplace has hugely underestimated the difficulty in developing a broadly based consumer friendly fuelling infrastructure for hydrogen. Without that, there is a need to reform any other fuel into hydrogen on board a vehicle and this equipment is costly, takes up space, and requires a significant percentage of the power output to operate."
Fuel Cell Energy, USA...working on MCFC of 300 kW, 1.5 MW and 3 MW for electric power generation. This technology cannot be scaled down below 300 kW because of their need for significant amounts of auxiliary equipment such as pumps. They target Jan2001 as their date of market entry.
Global Thermoelectric, Canada...working on planar SOFC operating at 800°C. In July 1997, Global signed a fuel cell agreement with Forschungszentrum Julich, one of the world's leading authorities on solid oxide fuel cells. In early 1999 Global reported that they had achieved high levels of power output. A new type of seal was reportedly working well. As well they have been pursuing mass production of an inexpensive variety of ceramic plates for the stack. Global is already a manufacturing company which has produced small thermoelectric powerplants for many years. In Apr1999 Global stock value increased by 15x in one week. In Nov1999 Global announced that Brian Gorbell, former Chief Engineer for Ballard's passenger bus program, will now head Global's automotive related fuel cell program.
JPL, USA...a division of NASA. They have been working on DMFC extensively since 1992. Many of the increases in efficiency and power density are as a result of their efforts.
Nissan & Suzuki, Japan...Nissan announced on Feb1998 that they would develop a PEFC automobile based on the Ballard technology, one year later however they have announced they will be joining Suzuki to develop their own DMFC automobile.
Plug Power, US...is a joint venture between DTE Energy Co., the parent of Detroit Edison and Michigan's largest electric utility, and Mechanical Technology Inc. or MTI, an early developer of fuel cell technologies. Their goal is to develop and manufacture affordable fuel cell systems for residential, small commercial and automotive applications. They target Jan2001 as their date of market entry.
Siemens Westinghouse, Germany...working on tubular SOFC operating at 1000°C. In Oct1998, Siemens halted work on its own planar solid oxide fuel cells and bought out Westinghouse's gas turbine and tubular solid oxide fuel cell division. Siemens planar design suffered from leaky seals of its window frame type design which reportedly had 16 small SOFC cells in each layer. Siemens is reportedly also working on DMFC for automobiles and PEFC for specialty applications.
Sulzer, Germany...working on a 3 kW SOFC for CHP.
Toyota, Japan...have their own PEFC technology they are working on.
United Technologies, USA...working on AFC, PEFC, PAFC and DMFC.
Zevco, UK...Zevco recently delivered a fuel cell van to the city of Westminster, UK.
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.
 "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)
 Many SOFC use a separate pre-reformer as opposed to the integral reformer as shown (Stimming)
 SOFC generally use carbon monoxide as a fuel because most transport oxygen ions. (Minh p29)
 Based on extensive laboratory tests. Some of the fuel cells achieved even higher efficiencies than this (Stimming)
 Based on recent reports for example Nissan and Suzuki (Comline)
 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.
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
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
°K=degrees Kelvin, most textbooks write this only as K
°C=degrees Celsius, 0°C=273.15°K
24. 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.
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