How ready is the fuel cell?
The battery as we know it 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.
Innovation 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?
Let’s focus on the emerging fuel cell and examine its suitability in stationary, mobile and portable applications.
The fuel cell
An electrochemical device that combines hydrogen fuel with oxygen, the fuel cell produces 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 a fuel in a pressurized container. As alternative fuel, methanol, propane, butane and natural gas can be used.
Energy is not generated through burning with the fuel cell; rather, it is based on an electrochemical process. There are little or no harmful emissions — the only release is clean water.
The fuel cell is twice as efficient as combustion. Hydrogen, the simplest element, is an exceptionally clean fuel. It makes up 90% 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 that will convert the fuel to hydrogen. Reformers for proton exchange membranes (the heart of the fuel cell, also called polymer electrolyte membranes) 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% 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 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 use materials such as platinum and Teflon, 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. In 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 0.6V 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 plant 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 stalled because of the prohibitive 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.
Type of fuel cells
Several variations of fuel cell systems have emerged. There 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. The molten carbonate, the phosphoric acid and the solid oxide fuel cell 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. Table 1 below compares the most common fuel cell systems in development.
|Type of Fuel Cell||Applications||Advantages||Limitations||Status|
|Proton Exchange Membrane (PEMFC)||Mobile (buses, cars,) portable power, medium to large-scale stationary power generation (homes, industry).||Compact design; long operating life; adapted by major automakers; offers quick start-up, low temperature operation, operates at 50% efficiency (25%-35% in system).||High manufacturing costs, needs heavy auxiliary equipment and pure hydrogen, no tolerance for contaminates; complex heat & water management.||Most widely developed, production; offers promising technology.|
|Alkaline (AFC)||Space (NASA), terrestrial transport (German submarines).||Low manufacturing & operation costs; does not need heavy compressor, fast cathode kinetics.||Large size; needs pure hydrogen & oxygen, use of corrosive liquid electrolyte.||First-generation technology.|
|Molten Carbonate (MCFC)||Large-scale power generation.||Highly efficient; uses heat to run turbines for co-generation.||Electrolyte instability; limited service life.||Well-developed; semi-commercial.|
|Phosphoric Acid (PAFC)||Medium to large-scale power generation.||Commercially available; lenient to fuels; uses heat for co-generation.||Low efficiency, limited service life, expensive catalyst.||Mature but faces competition from PEMFC.|
|Solid Oxide (SOFC)||Medium to large-scale power generation.||High efficiency, lenient to fuels, takes a natural gas directly, no reformer needed. Operates at 60% efficiency; uses heat for co-generation.||High operating temperature; requires exotic metals, high manufacturing costs, oxidation issues; low specific power.||Least developed. Breakthroughs in cell material and stack design sets off new research.|
|Direct Methanol (DMFC)||Suitable for portable, mobile and stationary applications.||Compact design, no compress or humidification needed; feeds directly off methanol.||Complex stack structure, slow load response times; operates at 20% efficiency.||Laboratory prototypes.|
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 about 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.
The PEM fuel cell also has a limited temperature range. Freezing water can damage the stack. Heating elements are needed to keep the stack within an acceptable temperature range because freezing water can cause damage. 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 consumes 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 contribute to corrosion and leakage of the delicate seals. 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 with little tolerance for contaminates, such as sulfur compounds or carbon monoxide. Carbon monoxide can poison the system. Decomposition of the membrane takes place if different grade fuels are used.
To reduce the internal resistance of the stack, the membrane is made thin. Over time, pinholes can develop in the membrane, which cause a crossover of hydrogen. Once present, the pinholes grow in size and eventually lead to a cell failure due to electrical shorting. Testing and repairing a stack are difficult. The complexity to service a fuel cell becomes apparent when considering that a typical 150V, 50kW stack has about 250 cells.
The solid oxide fuel cell
The solid oxide fuel cell is best suited for stationary applications. The system requires high operating temperatures (about 1,000°C). Newer systems are being developed that 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%, one of the highest among fuel cells.
Higher stack temperatures demand specialized and exotic materials, which add 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.
An effort exists to further explore the potential of the alkaline fuel cell. Although larger in physical size than the PEM system, the alkaline fuel cell has the potential to lower manufacturing and operating costs. Water management is simpler, no compressor is usually needed and the hardware is cheaper.
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. The cost per watt-hour is less favorable for small systems than large installations.
Note the cost to produce 1kW of power. The investment to provide 1kW of power using rechargeable batteries is around $7,000. This calculation is based on 7.2V; 1,000mAh NiCd packs costing $50 each. High energy-dense batteries that deliver fewer cycles and are more expensive than the NiCd.
The high cost of portable power opens vast opportunities for the portable fuel cell. At today’s cost of $3,000 to $7,500 per kilowatt, however, energy generated by the fuel cell is only marginally less expensive than that generated by conventional batteries.
A fuel cell for portable applications would not necessarily replace the battery in the equipment but could 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 fuel cell are: high energy density (as much as five times that of a Liion battery); environmental cleanliness, fast recharge and long runtimes. In fact, continuous operation is feasible. Miniature fuel cells have been demonstrated that operate at an efficiency of 20% and run for 3,000 hours before a stack replacement is necessary. There is, however, some degradation during the service life of the fuel cell. Portable fuels cells are still in experimental stages.
Ironically, the fuel cell will not eliminate the chemical battery but will promote it. 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 keep the battery fully charged. For portable applications, a supercapacitor will improve the loading characteristics and enable high current pulses.
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 to those who invest in this technology.
|Energy source||Investment of equipment to generate 1kW||Lifespan of equipment before major overhaul or replacement||Cost of fuel per kWh||Total cost per kWh, incl. fuel, maintenance & equipment replacement|
|NiCd For portable use||$7,000 based on 7.2V, 1,000mAh at $50/pack||500h 1 based on IC discharge||$0.15 electricity for charging||$7.50|
|Gasoline engine For mobile use (134hp)||$30 based on $3,000/100kW||4,000h||$0.10||$0.14|
|Diesel engine Stationary use (134hp)||$40 based on $4,000/100kW||5,000h||$0.07||$0.10|
|Electricity From utility grid||All inclusive||All inclusive||$0.10||$0.10|
The fuel cell may be as revolutionary in transforming 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. However, the maturing process of the fuel cell may not be as rapid as that of microelectronics.
This article is an excerpt from Batteries in a Portable World — A Handbook on Rechargeable Batteries for Non-Engineers (second edition). In the book, Buchmann evaluates the battery in everyday use and explains their strengths and weaknesses in laymen’s terms. The 300-page book is available from Cadex Electronics through [email protected], tel. 604- 231-7777 or most bookstores. For more details on the book visit www.buchmann.ca.
Buchmann is the founder and chief executive of Cadex Electronics, Richmond, British Columbia, Canada.