Fuel cell

by

P.M.Prasanna Shankar,Final Year EEE,Saranathan College of Engg.

INTRODUCTION:

A fuel cell is an electrochemical device similar to a battery, but differing from the latter in that it is designed for continuous replenishment of the reactants consumed; i.e. it produces electricity from an external fuel supply as opposed to the limited internal energy storage capacity of a battery.

Typical reactants used in a fuel cell are hydrogen on the anode side and oxygen on the cathode side (a hydrogen cell). In contrast, conventional batteries consume solid reactants and, once these reactants are depleted, must be discarded, recharged with electricity by running the chemical reaction backwards, or, at least in theory, by having their electrodes replaced. Typically in fuel cells, reactants flow in and reaction products flow out, and continuous long-term operation is feasible virtually as long as these flows are maintained.

Fuel cells are very attractive in modern applications for their high efficiency and ideally emission-free advantage, where the only by-product of a hydrogen fuel cell is water vapor. However, using more-available fuels such as methane or natural gas will still generate some carbon dioxide. The other concern is the energy-consuming process of manufacturing hydrogen, which may generate pollution. However, this argument can be countered that for example, drilling for oil also causes environmental headaches.

Some applications that have been suggested include

• baseload utility power plants,
• emergency backup power,
• off-grid power storage,
• portable electronics,
• electrically-powered vehicles, and
• cellular phone power.

Types of fuel cells

There are several types of fuel cells:

• Solid-oxide fuel cells;
• Proton-exchange fuel cell;
• Reversible Fuel Cell;
• Direct-methanol fuel cell;
• Direct Borohydride Fuel Cells
• Molten-carbonate fuel cells;
• Phosphoric-acid fuel cells;
• Alkaline fuel cells.

Science

Fuel cells are electrochemical devices, so they are not constrained by the maximum thermal (Carnot) efficiency as combustion engines are. Consequently, they can have very high efficiencies in converting chemical energy to electrical energy.

In the archetypal example of a hydrogen/oxygen proton-exchange membrane (or "polymer electrolyte") fuel cell (PEMFC), a proton-conducting polymer membrane separates the anode and cathode sides. Each side has an electrode, typically carbon paper coated with platinum catalyst.

On the anode side, hydrogen diffuses to the anode catalyst where it dissociates into protons and electrons. The protons are conducted through the membrane to the cathode, but the electrons are forced to travel in an external circuit (supplying power) because the membrane is electronically insulating.

On the cathode catalyst, oxygen molecules react with the electrons (which have travelled through the external circuit) and protons to form water.

In this example, the only waste product is water vapor and/or liquid water.

Fuel cells cannot store energy like a battery, but in some applications, like stand-alone power plants based on discontinuous sources (solar, wind power), they are combined with electrolyzers and storage systems to form an energy storage system. The round-trip efficiency (electricity to hydrogen and back to electricity) of such plants is between 30 and 40%.

History

The principle of the fuel cell was discovered by Swiss scientist Christian Friedrich Schönbein in 1838 and published in the January 1839 edition of the "Philosophical Magazine" [1]. Based on this work, the first fuel cell was developed by Welsh scientist Sir William Grove. A sketch was published in 1843, but it wasn't until 1932 that British engineer Francis Thomas Bacon developed successful fuel cell devices. Twenty-seven years later in 1959, Bacon and his colleagues demonstrated a practical five-kilowatt unit capable of powering a welding machine. In the 1960s Bacon's patents were licensed by Pratt and Whitney from the U.S. where the concepts were used in the U.S. space program to supply electricity and drinking water (hydrogen and oxygen being readily available from the spacecraft tanks). Extremely expensive materials were used and the fuel cells required very pure hydrogen and oxygen. Early fuel cells tended to require inconveniently high operating temperatures that were a problem in many applications. However, fuel cells were seen to be desirable due to the large amounts of fuel available (hydrogen & oxygen).

Further technological advances in the 1980s and 1990s, like the use of Nafion as the electrolyte, and reductions in the quantity of expensive platinum catalyst required, have made the prospect of fuel cells in consumer applications such as automobiles more realistic.

The fuel cell industry

Ballard Power Systems is a major manufacturer of fuel cells and claims to lead the world in automotive fuel cell technology. Ford Motor Company and DaimlerChrysler are major investors in Ballard. In 2003, most automobile companies were customers of Ballard, with only General Motors and Toyota pursuing internal development of fuel cells for automotive use; in 2004 Nissan and Honda started similar research programs.

Following the two-year 3-bus demonstration projects in Chicago, Illinois and Vancouver (1998-2000), DaimlerChrysler fuel cell buses went into public use in nine cities across the European Union in 2004. The fuel cells in the buses were manufactured by Ballard Power Systems. The EU's CUTE (Clean Urban Transport for Europe) project is the largest of its type anywhere in the world. These buses reduce pollution and noise, and give a smooth vibration-free ride. London's trial, for example, co-financed by the European Commission Directorate-General for Energy and Transport, runs on Route 25 from Oxford Circus (in the centre of town) out to Ilford in the East End. The oil company BP is providing the hydrogen refuelling facilities in 5 of the 9 trial cities, including London. See the UK Government's Transport for London Fuel Cell Buses PDF for a description of the London trial.

Perth in Western Australia is also participating in the trial with three fuel cell powered buses now operating between Perth and the port city of Fremantle. The trial is to be extended to other Australian cities over the next three years.

United Technologies (UTX) was the first company to manufacturer and commercialize a large, stationary fuel cell system for use in co-generation power plants in hospitals and large office buildings, but has since discontinued [2]this product due to the high costs of the phosphoric acid technology used. UTX's UTC Fuel Cells subsidiary [3] continues to be the sole supplier of fuel cells to NASA for use in space vehicles and is also developing fuel cells for cars and buses.

In late 2004, Mechanical Technology Inc.'s subsidiary, MTI MicroFuel Cells debuted its first Direct Methanol Fuel Cell (DMFC) for commercial use. MTI's Mobion™ cord-free rechargeable power pack technology consists of a fuel cell which runs on 100% (neat) Methanol. MTI's Mobion line is being released in industrial, consumer, and military markets as a low-cost replacement for lithium-ion batteries.

Advantages and disadvantages of fuel cells in various applications

Environmental effects of hydrogen fuel cells

A common misconception among the public is that hydrogen is a source of energy, and that there are "mines" or "reservoirs" of hydrogen to find. However, all hydrogen is not a primary source of energy: it is only an energy carrier, and must be manufactured using energy from other sources.

Some critics of the current stages of this technology argue that the energy needed to create the fuel in the first place may reduce the ultimate energy efficiency of the system to below that of the most efficient gasoline internal-combustion engines; this is especially true if the hydrogen has to be compressed to high pressures or liquified, as it does in automobile applications (the electrolysis of water is itself a fairly efficient process). It has to be remarked, though, that even the most efficient internal-combustion engines are not very efficient in absolute terms; furthermore, gasoline is neither a primary energy source, because crude oil has to be treated in a refinery to obtain gasoline.

As an alternative to electrolysis, hydrogen can be generated from methane (the primary component of natural gas) with approximately 80% efficiency, or with other hydrocarbon to a varying degree of efficiency. The hydrocarbon-conversion method releases greenhouse gases, but, since the production is concentrated in one facility, and not distributed on every single vehicle or utility, it is possible to separate the gases and dispose of them properly, for example by injecting them in an oil or gas reservoir. A CO₂ injection project has been started by Norwegian company Statoil in the North Sea, at the Sleipner field. [4]

Other types of fuel cells do not face these problems, however. For example, biological fuel cells take glucose and methanol from food scraps and convert it into hydrogen and food for the bacteria.

However, another environmental problem faced by all types of hydrogen fuel cells has been pointed out in a paper published in Science magazine by a group of Caltech scientists. They note that if hydrogen fuel cell usage becomes widespread enough to replace gasoline internal-combustion engines, small amounts of hydrogen leaking from storage containers and pipelines will have a detrimental impact on the Earth's ozone layer. However, their findings remain controversial, and their assumptions regarding the amount of hydrogen leaked have been disputed by industry officials.

Fuel cell design issues

To make fuel cells economically competitive, there are many practical problems to be overcome as well. Water management remains a key problem in Proton Exchange Membrane Fuel Cells (PEMFCs), where generated water will need to be disposed of. Not enough water and the polymer loses its ability to conduct protons across the cell; too much water in the fuel cell and the electrodes will flood, stopping the reaction. Methods to dispose of the excess water are being developed by fuel cell companies.

At the same time many other variables must be juggled, including temperature throughout the cell (which changes and can sometimes destroy a cell through thermal loading), reactant and product levels at various cells. Materials must be chosen to do various tasks which none fill completely. Durability and lifetime of the cells can be serious issues for some cells, low power densities for others. Putting all of these factors together hasn't been accomplished decisively yet, and remains the challenge.

In vehicle usage, many problems are amplified. For instance, cars must be required to start in any weather conditions a person can reasonably expect to encounter: about 80% of the world's car park is legally subject to the requirement of being able to start from sub-zero temperatures. Fuel cells have no difficulty operating in the hottest locations, but the coldest do present a problem. Honda's FCX is the first fuel cell powered vehicle to reliably start in freezing temperatures, but temperatures below -10 degrees Celsius still prohibit the fuel cell stack from starting.

Fuel cell applications

Fuel cells are very useful as power sources in remote locations, such as spacecraft, remote weather stations, and in certain military applications. A fuel cell system running on hydrogen can be compact, lightweight and has no major moving parts.

A future application is combined heat and power for homes and office buildings. This is economically possible in areas where the cost of natural gas is much lower than that of electricity. This type of system would give nearly constant electric power (selling it to the grid when it is not consumed within the building), and at the same time produce hot water from the waste heat. The most likely technique to be used for this is SOFC, but prototypes also exist for PEM-based systems.

Because fuel cells have a high cost per kilowatt, and because their efficiency decreases with increasing power density, they are usually not considered for applications with high load variations. In particular, they are not suited for energy storage systems in small and medium scale. An electrolyzer and fuel cell would return less than 50 percent of the input energy (this is known as round-trip efficiency), while a much cheaper lead-acid battery might return about 90 percent.

However, since a fuel cell/electrolyser system does not store fuel, but relies on an external storage unit, they can be successfully applied in large-scale energy storage, for example for remote villages: in this case, batteries would have to be largely oversized to meet the storage demand, but fuel cells only need a larger storage unit (typically cheaper that an electrochemical device).

The first hydrogen refuelling station was opened in Reykjavík, Iceland on April 2003. This station serves three buses built by DaimlerChrysler that are in service in the public transport net of Reykjavík. The station produces the hydrogen it needs by itself, with an electrolysing unit (produced by Norsk Hydro), and does not need refilling: all that enters is electricity and water. Shell is also a partner in the project. The station has no roof, in order to allow any leaked hydrogen to escape to the atmosphere.

Currently, a team of college students is planning to take a hydrogen fuel cell powered boat around the world. Their venture is called The Hydrogen Expedition.

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