Miscellaneous/Ask Isidor/What are Fuel Cells page 051013
Q: What are Fuel Cells?
(Oct: 2005) Answer: 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 Vancouverís 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 earthís 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 astronautsí 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.
Type 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. 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 consume 30% of the energy generated. The PEM stack has an estimated service life of 4,000 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 cartridges. 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.
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 todayís portable fuel cells do not yet meet customersí expectations.
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.