The Fuel Cell - Ready?

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

This table evaluates the cost to generate one kilowatt (kW) of power 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. As can be seen, power obtained through the electrical utility grid is most cost effective, while the cost comparison between a mobile petrol engine and mobile fuel cells is daunting!

Energy Source	Investment of equipment to generate 1kW (US$)	Lifespan of equipment before major overhaul or replacement	Cost of fuel per kWh (US$)	Total Cost per kWh, incl. fuel, maintenance and equipment replacement. (US$)
NiCd For portable use	$7,000 based on 7.2V, 1000mAh at $50/pack	1500h based on 1C discharge	$0.15 electricity for charging	$7.50
Petrol Engine For mobile use	$30 based on $3,000/100kW (134hp)	4000 h	$0.10	$0.14
Diesel Engine For stationary use	$40 based on $4,000/100kW (134hp)	5000 h	$0.07	$0.10
Fuel Cell For portable use For mobile use For stationary use	$3,000-7,500	2000 h 4000 h 40,000 h	$0.35 $0.35 $0.35 $0.35	$1.85-4.10 $1.10-2.25 $0.45-0.55
Electricity From electric grid	All inclusive	All inclusive	0.10	0.10

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 that converts the fuel to hydrogen. Reformers for PEM-type cells 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 that which comes from the tailpipe of a car.

Fuel Cell History

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 20^th century when scientists learned how to better utilize 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. 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 favoured 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 boot was utilized to store the fuel cell and back-up batteries. According to Dr. Kordesch, there was enough room in the car for four people and a dog.

Type of Fuel Cells

Several variations of fuel cell systems have emerged. The most common are 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. The following table 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; relatively long operating life; adapted by major automakers; offers quick start -up, low temperature operation, operates at 50% efficiency.	High manufacturing costs, needs heavy auxiliary equipment and pure hydrogen, no tolerance for contaminates; complex heat and water management.	Most widely developed; limited production; offers promising technology.
Alkaline (AFC)	Space (NASA), terrestrial transport (German submarines).	Low manufacturing and operation costs; does not need heavy compressor, fast cathode kinetics.	Large size; needs pure hydrogen and oxygen; use of corrosive liquid electrolyte.	First generation technology; has renewed interest due to low operating cost.
Molten Carbonate (MCFC)	Large-scale power generation.	Highly efficient; utilizes 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; utilizes 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 natural gas directly, no reformer needed. Operates at 60% efficiency; utilizes 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 compressor or humidification needed; feeds directly off methanol in liquid form.	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. The efficiency is approximately 50 percent. (In comparison, the internal combustion 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 consumes 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 contributes 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 sulphur 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, 50kW stack contains about 250 cells.

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 centres 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 40,000kW 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.

Advantages and Limitations

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 NiCd battery or a diesel engine, which perform well at a 100 percent load factor. So 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 super-capacitor 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 are found. While an internal combustion engine requires an investment of $35 to $50 to produce one kilowatt (kW) of power, the equivalent cost in a fuel cells is still a whopping US$3,000 to US$7,500. The goal is a fuel cell that would cost equal or less than diesel engines...

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