Reprinted by permission
Isidor Buchmann, President
© Cadex Electronics Inc.
part 1 of 3
Battery experts agree that the battery, as we know it today, will remain a ‘weak link’ for the foreseeable future. Given its relatively short life span, the battery is also the most expensive and least reliable component of a portable device.
An innovative new approach will be needed to satisfy the ever-increasing thirst for mobile power. The ideal battery, which would provide an inexhaustible pool of energy carried in a small package, is still far from reality. Will this miracle battery be based on the classic electro-chemical concept, the evolving fuel cell or some groundbreaking new technology? This answer is anyone’s guess.
In this article we focus on the emerging fuel cell and examine its suitability in stationary, mobile and portable applications. But first, we make some general cost comparisons on available power sources.
Cost of Mobile Power
Among the common power sources, energy from non-rechargeable batteries is the most expensive. This cost increases with smaller battery sizes. Figure 1 reflects the cost per kWh using non-rechargeable batteries, also referred to as primary batteries.
Figure 2 evaluates the cost to generate one kilowatt (kW) of energy by means of a rechargeable battery, combustion engine, fuel cell and electricity from the utility grid. We take into account the initial investment, add the fuel consumption and include the eventual replacement of each system.
Power obtained through the electrical utility grid is most cost effective. Consumers in industrialized countries pay between $0.05 and 0.10US per kWh. The typical daily energy consumption of a household is 25 kilowatt-hour (kWh).
The Fuel Cell
A fuel cell is an electrochemical device, which combines hydrogen fuel with oxygen to produce electric power, heat and water. In many ways, the fuel cell resembles a battery. Rather than applying a periodic recharge, a continuous supply of oxygen and hydrogen is supplied from the outside. Oxygen is drawn from the air and hydrogen is carried as fuel in a pressurized container. As alternative fuel, methanol, propane, butane and natural gas can be used.
The fuel cell does not generate energy through burning; rather, it is based on an electrochemical process. There are little or no harmful emissions. The only release is clean water. In fact, the water is so pure that visitors to Vancouver’s Ballard Power Systems drank clear water emitted from the tailpipes of buses powered by a Ballard fuel cell.
The fuel cell is twice as efficient in energy conversion through a chemical process than through combustion. Hydrogen, the simplest element consisting of one proton and one electron, is plentiful and is exceptionally clean as a fuel. Hydrogen makes up 90 percent of the composition of the universe and is the third most abundant element on the earth’s surface. Such a wealth of fuel would provide an almost unlimited pool of energy at relatively low 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.
Hydrogen must be carried in a pressurized bottle. If propane, natural gas or diesel is used, a reformer is needed to extract the hydrogen. Reformers for PEMs are bulky and expensive. They start slowly and purification is required. Often the hydrogen is delivered at low pressure and additional compression is required. Some fuel efficiency is lost and a certain amount of pollution is produced. However, these pollutants are typically 90 percent less than what comes from the tailpipe of a car.
The fuel cell concept was developed in 1839 by Sir William Grove, a Welsh judge and gentleman scientist. The invention never took off, partly because of the success of the internal combustion engine. It was not until the second half of the 20th century when scientists learned how to better utilize materials such as platinum and TeflonÔ that the fuel cell could be put to practical use.
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 PEM system, the oxygen is ionized and migrates across the electrolyte to the anodic compartment where it combines with hydrogen. The byproduct is electricity, some heat and water. A single fuel cell produces 0.6 to 0.8V under load. Several cells are connected in series to obtain higher Voltages.
The first practical application of the fuel cell system was made in the 1960s during the Gemini space program when this power source was favored over nuclear or solar power. The fuel cell, based on the alkaline system, generated electricity and produced the astronauts’ drinking water. Commercial application of this power source was prohibitive at that time because of the high cost of materials. In the early 1990s, improvements were made in stack design, which led to increased power densities and reduced platinum loadings at the electrodes.
High cost did not hinder Dr. Karl Kordesch, the co-inventor of the alkaline battery, to convert his car to an Alkaline fuel cell in the early 1970s. Dr. Kordesch drove the car for many years in Ohio, USA. The hydrogen tank was placed on the roof and the trunk was utilized to store the fuel cell and back-up batteries. According to Dr. Kordesch, there was enough room for four people and a dog.
Several variations of fuel cell systems have emerged. The most common is the previously mentioned and most widely developed PEM System using a polymer electrolyte. This system is aimed at vehicles and portable electronics. Several developers are also targeting stationary applications. 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 applications such as power generating plants for electric utilities. Among these stationary systems, the Solid Oxide is the least developed but has received renewed attention due to breakthroughs in cell material and stack designs. Figure 3 compares the most common fuel cell systems in development.
The 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 about 80°C (176°F). The efficiency is approximately 50 percent. (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.
The PEM fuel cell has a limited temperature range. Freezing water can damage the stack. Heating elements are needed to keep the stack within an acceptable temperature range. The warm-up is slow and the performance is poor when cold. Heat is also a concern if the temperature rises too high.
The PEM fuel cell requires heavy accessories. Operating compressors, pumps and other apparatus consume 30 percent 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 contribute to membrane stress. If run continuously, the stationary stack is good for about 40,000 hours. The replacement of the stack 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. Testing and repairing a stack are difficult. The complexity to service a fuel cell 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 high operating temperatures (about 1000°C). Newer systems are being developed which can run at about 700°C.
A significant advantage of the SOFC is its leniency to fuel. Due to the high operating temperature, hydrogen is produced through a catalytic reforming process. This eliminates the need for an external reformer to generate 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 percent, one of the highest among fuel cells.
Higher stack temperatures demand specialized and exotic materials, which adds to manufacturing costs. Heat also presents a challenge for longevity and reliability because of increased material oxidation and stress. However, high temperatures offer a benefit by enabling co-generation by running steam generators. This improves the overall efficiency of this fuel cell system.
The Alkaline Fuel Cell (AFC) has received renewed interest because of low operating cost. Although larger in physical size than the PEM system, the alkaline Fuel Cell has the potential of lower manufacturing and operating costs. The water management is simpler; no compressor is usually needed, and the hardware is cheaper. Whereas the separator for the PEM stack costs between $800 and $1,100US per square meter, the equivalent of the alkaline system is almost negligible. (In comparison, the separator of a Lead-acid battery is $5 per square meter.) Operating costs of $100 to 200 per kW are feasible. Start and stop (wetting and drying) is more forgiving than with most 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 Alkaline Fuel Cell. It should be noted, however, that carbon dioxide is easier to scrub than carbon monoxide, a deterrent of the PEM system.
Applications — The fuel cell is being considered
as an eventual replacement for the internal combustion engine for cars, trucks and buses. Major car manufacturers have teamed up with fuel cell research centers or are doing their own development. There are plans for mass-producing cars running on fuel cells. Because of the low operating cost of the combustion engine, and some unresolved technical challenges of the fuel cell, however, experts predict that a large scale implementation of the fuel cell to power cars will not occur before 2015, or even 2020.
Large power plants running in the 40,000 kW range 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 absence of shock and vibration.
Residential power supplies are also being tested. Such a unit would sit in the basement or outside the house, similar to an air-conditioning unit of a typical middle class North American home. The fuel would be natural gas or propane, a commodity that is available in many urban settings.
Fuel cells may soon compete with batteries for portable applications such as laptop computers and mobile phones. However, today’s technologies have limitations in meeting the cost and size criteria for small portable devices. In addition, the cost per Watt-hour is less favorable for small systems than large installations.
Let’s examine once more the cost to produce one kiloWatt (kW) of power. In Figure 3 we learned that the investment to provide 1kW of power using rechargeable batteries is around $7,000US. This calculation is based on 7.2V; 1000mAh Nickel-cadmium packs costing $50 each. High energy-dense batteries that deliver fewer cycles and are more expensive than the Nickel-cadmium will double the cost.
The high cost of portable power opens vast opportunities for the portable fuel cell. At an investment of $3,000 to $7,500 to produce one kiloWatt of power, however, the energy generated by the fuel cell is only marginally less expensive than that produced by conventional batteries.
Direct Methanol (DMFC), the fuel cell designed for portable applications, would not necessarily replace the battery in the equipment but serve as a charger that is carried separately. The feasibility to build a mass-produced fuel cell that fits into the form factor of a battery is still a few years away.
The advantages of the portable DMFC are: its relatively high energy density (up to five times that of a Li-ion battery); its use of liquefied fuel as an energy supply; its environmental cleanliness; its fast recharge and its long runtimes. In fact, continuous operation is feasible. Miniature fuel cells have been demonstrated that operate at an efficiency of 20 percent and run for 3,000 hours before a stack replacement is necessary. There is some degradation during the service life of the fuel cell. Portable fuels cells are still in experimental stages.
Advantages and limitations of the fuel cell — A less known limitation of the fuel cell is the marginal loading characteristic. On a high current load, mass transport limitations come into effect. Supplying air instead of pure oxygen aggregates this phenomenon.
The issue of mass transport limitation is why the fuel cell operates best at a 30 percent load factor. Higher loads reduce the efficiency considerably. In terms of loading characteristics, the fuel cell does not match the performance of a Nickel-cadmium battery or a diesel engine, which perfrom well at a 100 percent load factor.
Ironically, the fuel cell will not eliminate the chemical battery — but promote it. Similar to the argument that the computer would make paper redundant, the fuel cell needs batteries as a buffer. For many applications, a battery bank will provide momentary high current loads, and the fuel cell will serve to keep the battery fully charged. For portable applications, a supercapacitor will improve the loading characteristics and enable high current pulses.
Most fuel cells are still handmade and are used for experimental purposes. Fuel cell promoters remind the public that the cost will come down once the cells are mass-produced and lower cost material is found. While an internal combustion engine requires an investment of $35 to $50 to produce one kiloWatt (kW) of power, the equivalent cost in fuel cells is still a whopping $3,000 to $7,500. The goal is a fuel cell that would cost equal or less than diesel engines.
The fuel cell will find applications that lie beyond the reach of the internal combustion engine. Once low cost manufacturing is feasible, this power source will transform the world and bring great wealth potential to those who invest in this technology.
It is said that the fuel cell is as revolutionary in transforming our technology as the microprocessor has been. Once fuel cell technology has matured and is in common use, our quality of life will improve and the environmental degradation caused by burning fossil fuel will be reversed. It is generally known that the maturing process of the fuel cell will not be as rapid as that of microelectronics.
This completes the series of three sections on Fuel Cells by Isidor Buchmann, The series began in the January issue, continued in February and is completed here. Mr. Buchmann will answer your questions on fuel cells or battery subjects in his column, ‘Ask Isidor’. E Mail him at [email protected]