The fuel cell
A fuel cell is an electrochemical device that combines
hydrogen fuel with oxygen to produce electric power, heat and water. In
many ways, the fuel cell resembles an electro-chemical battery. Rather
than applying a periodic recharge, a continuous supply of oxygen and hydrogen
is provided from the outside. Oxygen is commonly drawn from the air and
hydrogen is carried as fuel in a pressurized container. As alternatives,
methanol, propane, butane, natural gas and diesel can be used.
Alternative fuels require a reformer to extract the hydrogen. This allows
tapping into existing distribution systems. However, reformers are bulky,
expensive and sluggish. Some fuel efficiency is lost and a small amount
of pollution is produced, but this is 90% less than from a regular car.
The fuel cell does not generate energy through burning; rather, it is
based on an electrochemical process. The energy conversion is twice as
efficient than through combustion. There are little or no harmful emissions.
The only release is clean water. The water is so pure that visitors to
Vancouver's Ballard Power Systems drank the water emitted from the tailpipes
of buses powered by a Ballard fuel cell.
Hydrogen, the simplest element consisting of one proton and one electron,
is plentiful and is exceptionally clean as a fuel. Hydrogen makes up 90%
of the universe and is the third most abundant element on the earth's
surface. Such wealth of energy would provide an almost unlimited amount
of energy at relatively low fuel cost. But there is a price to pay. The
fuel cell core (or stack), which converts oxygen and hydrogen to electricity,
is expensive to build and maintain.
A fuel cell is electrolysis in reverse, using two electrodes separated
by an electrolyte. Hydrogen is presented to the negative electrode (anode)
and oxygen to the positive electrode (cathode). A catalyst at the anode
separates the hydrogen into positively charged hydrogen ions and electrons.
On the Proton Exchange Membrane (PEM) system, the oxygen is ionized and
migrates across the electrolyte to the anodic compartment where it combines
with hydrogen. A single fuel cell produces 0.6-0.8 volts under load. Several
cells are connected in series to obtain higher voltages.
The fuel cell concept was developed in 1839 by Sir William Grove, a Welsh
judge and gentleman scientist. The invention did not take off, partly
due to the success of the internal combustion motor. The revival occurred
when the first fuel cell was used in the Gemini space program during the
1960s. Based on the alkaline system, the fuel cell generated electricity
and produced the astronauts' drinking water. Commercial application of
this power source was prohibitive at that time because of high material
costs. Improvements in the stack design during the 1990s led to reduced
costs and increased power densities.
High cost did not deter Dr. Karl Kordesch from converting his car to an
alkaline fuel cell in the early 1970s. Dr. Kordesch, the inventor of the
reusable alkaline, drove the car for many years in Ohio, USA. The hydrogen
tank was mounted on the roof and the trunk contained the fuel cell and
back-up batteries. According to Dr. Kordesch, there was enough room for
four people and a dog. Long up-hills were a struggle.
Types of fuel cells
Several variations of fuel cell systems have emerged. The PEM is the most
developed system and is aimed for vehicles and portable power units. The
Alkaline System, which uses a liquid electrolyte, is the preferred fuel
cell for aerospace applications, including the Space Shuttle. Molten Carbonate,
Phosphoric Acid and Solid Oxide Fuel Cells are reserved for stationary
power generation. The Solid Oxide is the least developed but has received
renewed attention due to new cell materials and improvements in stack
designs. Figure 1 compares the most common fuel cell systems.
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Figure 1: Advantages and disadvantages of the various fuel cell systems. The PEM is the most widely developed system today. |
· The Proton Exchange Membrane (PEM) system allows compact
designs and achieves a high energy to weight ratio. Another advantage
is a quick start-up when hydrogen is applied. The stack runs at a relatively
low temperature of 80°C (176°F). The efficiency is approximately
50%. (In comparison, the internal compaction motor has an efficiency of
about 15%).
The limitations of the PEM system are high manufacturing costs and complex
water management issues. The stack contains hydrogen, oxygen and water.
If dry, the input resistance is high and water must be added to get the
system going. Too much water causes flooding. Freezing can damage the
stack. The warm-up is slow and the performance is poor when cold. The
cooling systems are extensive.
The PEM fuel cell requires heavy accessories. Operating compressors, pumps
and other apparatus consumes 30% of the energy generated. The PEM stack
has an estimated service life of 4000 hours if operated in a vehicle.
The relatively short life span is caused by intermittent operation. Start
and stop conditions induce drying and wetting, which contributes to membrane
stress. If run continuously, the stationary stack is estimated at 40,000
hours. Stack replacement is a major expense.
The PEM fuel cell requires pure hydrogen. There is little tolerance for
contaminates such as sulfur compounds or carbon monoxide. Carbon monoxide
can poison the system. A decomposition of the membrane takes place if
different grade fuels are used. The complexity of repairing a fuel cell
stack becomes apparent when considering that a typical 150V, 50 kW stack
contains about 250 cells.
· The Solid Oxide Fuel Cell (SOFC) is best suited for stationary
applications. The system requires a high operating temperature of 1000°C.
Newer systems are being developed that run at about 700°C.
A significant advantage of the SOFC is leniency to fuel. Due to the high
operating temperature, hydrogen is produced through a catalytic reforming
process. This eliminates the external reformer to provide hydrogen. Carbon
monoxide, a contaminant in the PEM systems, is a fuel for the SOFC. In
addition, the SOFC system offers a fuel efficiency of 60%, one of the
highest among fuel cells.
Higher stack temperatures demand exotic materials, which add to manufacturing
costs. Heat also presents a challenge for longevity and reliability because
of increased material oxidation and stress. High temperatures enable co-generation
by running steam generators to improve overall efficiency.
· The Alkaline Fuel Cell (AFC) has received renewed interest
because of low operating costs. Although larger in physical size than
the PEM system, the AFC has the potential of lower manufacturing and operating
costs. The water management is simpler, the compressor can be eliminated,
and the hardware is cheaper. Whereas the separator for the PEM stack costs
between $800-1,100US per square meter, the equivalent of the alkaline
system is almost negligible. (The separator of a lead-acid battery is
$5 per square meter.) Start and stop (wetting and drying) is more forgiving
than with other systems.
As a negative, the AFC needs pure oxygen and hydrogen to operate. The
amount of carbon dioxide in the air can poison the system. It should be
noted that carbon dioxide is easier to scrub than carbon monoxide, a deterrent
of the PEM system.
· The Direct Methanol (DMFC) is aimed for portable applications.
The system provides a relatively high energy density (up to five times
that of lithium-ion); uses liquefied fuel as energy source, is environmentally
clean and offers continuous operation through replacements of fuel cartages.
Miniature fuel cells are operating at 20% efficiency and running for 3000
hours before a stack replacement is necessary. There are some performance
degradations during the service life.
Applications
The fuel cell is intended to replace the internal combustion engine of
cars, trucks and buses. Major car manufacturers have teamed up with fuel
cell research centers or are doing their own development. Because of pending
technical issues of the fuel cell, and the low cost of the combustion
engine, experts predict mass-produced fuel cell powered cars to arrive
by 2015, or even 2020. Some experts go as far as to say that the commercial
viability of the fuel cell is not proven.
Large fuel cell plants running at 40,000 kW will likely out-pace the automotive
industry. Such systems could provide electricity to remote locations within
10 years. Many of these regions have an abundance of fossil fuel that
could be utilized. The stack on these large power plants would last longer
than in mobile applications because of steady use, even operating temperatures
and the absence of shocks and vibrations.
Fuel cells may soon compete with batteries for portable applications,
such as laptops. The energy will be cheaper than that of a conventional
battery and lengthy recharging will become redundant. However, the size
and price of today's portable fuel cells do not yet meet customer's expectations.
Limitations
The efficiency of a new power source is often compared with a diesel engine
or a nickel-cadmium battery, both of which perfrom well at 100% load factor.
This is not the case with the fuel cell, which operates best at 30%. Higher
loads reduce the efficiency considerably. Supplying pure oxygen instead
of air improves the load factor.
The fuel cell is intended to replace the chemical battery. Ironically,
it will promote the battery. Most fuel cell applications need batteries
as a buffer to provide momentary high load currents. The fuel will keep
the battery charged. For portable applications, a supercapacitor will
improve the loading characteristics and enable high current pulses.
One of the major limitations of the fuel cell is the high-energy cost.
While an internal combustion engine requires an investment of $30 to produce
one kilowatt (kW) of power, the equivalent cost in a fuel cell is a whopping
$3,000 (Refer to The cost of portable power). Part of that cost is due
to experimental production since the fuel cell is not yet mass-produced.
The goal is developing a fuel cell that is par with a diesel engine in
terms of cost.
Once the current difficulties have been solved, the fuel cell is bound
to find applications that lie beyond the reach of the internal combustion
engine. It is said that the fuel cell is as revolutionary as the microprocessor
but the maturing process will take longer.
About the Author
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc., in Vancouver
BC. Mr. Buchmann has a background in radio communications and has studied
the behavior of rechargeable batteries in practical, everyday applications
for two decades. Award winning author of many articles and books on batteries,
Mr. Buchmann has delivered technical papers around the world.
Cadex Electronics is a manufacturer of advanced battery chargers, battery
analyzers and PC software. For product information please visit www.cadex.com.