Fuel Cells.

Fuel Cells,Principle of Operation, 
Construction and Types.

A

A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanolare sometimes used. Fuel cells are different from batteries in that they require a constant source of fuel and oxygen to run, but they can produce electricity continually for as long as these inputs are supplied.

CONSTRUCTION

There are many types of fuel cells, but they all consist of an anode (negative side), a cathode (positive side) and an electrolyte that allows charges to move between the two sides of the fuel cell. Electrons are drawn from the anode to the cathode through an external circuit, producing direct current electricity. As the main difference among fuel cell types is the electrolyte, fuel cells are classified by the type of electrolyte they use. Fuel cells come in a variety of sizes. Individual fuel cells produce relatively small electrical potentials, about 0.7 volts, so cells are "stacked", or placed in series, to increase the voltage and meet an application’s requirements. In addition to electricity, fuel cells produce water, heat and, depending on the fuel source, very small amounts ofnitrogen dioxide and other emissions. The energy efficiency of a fuel cell is generally between 40-60%, or up to 85% efficient if waste heat is captured for use.

The most important design features in a fuel cell are:

 The electrolyte substance. The electrolyte substance usually defines the type of fuel cell.

 The fuel that is used. The most common fuel is hydrogen.
 The anode catalyst, which breaks down the fuel into electrons and ions. The anode catalyst is usually made up of very fine platinum powder.

 The cathode catalyst, which turns the ions into the waste chemicals like water or carbon dioxide. The cathode catalyst is often made up of nickel but it can also be a nanomaterial-based catalyst.

Basic principles of operation:-

The fuel cell is an electrochemical system which converts the chemical energy of a conventional fuel, directly into d.c. electrical energy. The basic principle of operation is illustrated in Fig.1. A fuel cell comprises two porous electrodes, with a conducting electrolyte betwen them. At the anode,the hydrogen gives up electrons to the electrode, and enters the electrolyte as a positive ion (H+), while at the cathode, the oxygen takes electrons and enters the electrolyte as a negative ion (O2- or OH-). The respective ions combine in the electrolyte and form water, while the electrons move through the external circuit, to produce electric current. Since these systems do not rely on thermal energy conversion, they are not bounded by Carnot efficiency limitations.

When fuel other than hydrogen is used, fuel processing or reforming is required. The role of the reformer is to convert any fuel into a hydrogen rich stream of gas. This is attained by mixing the fuel with steam (typical steam-to-carbon-ratio: 2.5). An additional role of fuel processing, is to ensure that CO is converted to CO2 (gas shift conversion). Thus, steam temperature has to be high enough to favour the above chemical reaction.

TYPES OF FUEL CELLS:_

 

Proton exchange membrane fuel cells

The archetypical hydrogen–oxygen proton exchange membrane fuel cell (PEMFC) efficient frontier design, a proton-conducting polymer membrane, (the electrolyte), separates the anode and cathode sides. This was called a "solid polymer electrolyte fuel cell" (SPEFC) in the early 1970s, before the proton exchange mechanism was well-understood. (Notice that the synonyms "polymer electrolyte membrane" and "proton exchange mechanism" result in the same acronym.)

On the anode side, hydrogen diffuses to the anode catalyst where it later dissociates into protons and electrons. These protons often react with oxidants causing them to become what is commonly referred to as multi-facilitated proton membranes. 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 electrically insulating. On the cathode catalyst, oxygen molecules react with the electrons (which have traveled through the external circuit) and protons to form water — in this example, the only waste product, either liquid or vapor.

High temperature fuel cells

SOFC

Solid oxide fuel cell

Solid oxide fuel cells use a solid material, most commonly a ceramic material called yttria-stabilized zirconia (YSZ), as the electrolyte. Because SOFCs are made entirely of solid materials, they are not limited to the flat plane configuration of other types of fuel cells and are often designed as rolled tubes. They require high operating temperatures (800°C to 1000°C) and can be run on a variety of fuels including natural gas.

SOFCs are unique in that negatively charged oxygen ions travel from the cathode (negative side of the fuel cell) to the anode (positive side of the fuel cell) instead of positively charged hydrogen ions travelling from the anode to the cathode, as is the case in all other types of fuel cells. Oxygen gas is fed through the cathode, where it reacts with electrons to create oxygen ions. The oxygen ions then travel through the electrolyte to react with hydrogen gas at the anode. The reaction at the anode produces electricity and water as by-products. Carbon dioxide may also be a by-product depending on the fuel, but the carbon emissions from an SOFC system are less than those from a fossil fuel combustion plant.[ The chemical reactions for the SOFC system can be expressed as follows:

Anode Reaction: 2H2 + 2O–2 → 2H2O + 4e–

Cathode Reaction: O2 + 4e– → 2O–2

Overall Cell Reaction: 2H2 + O2 → 2H2O

MCFC

Molten carbonate fuel cell

Molten carbonate fuel cells (MCFCs) require a high operating temperature, 650 °C (1,200 °F), similar to SOFCs. MCFCs use lithium potassium carbonate salt as an electrolyte, and at high temperatures, this salt melts into a molten state that allows for the movement of charge (in this case, negative carbonate ions) within the cell.

Like SOFCs, MCFCs are capable of converting fossil fuel to a hydrogen-rich gas in the anode, eliminating the need to produce hydrogen externally. The reforming process creates CO2 emissions. MCFC-compatible fuels include natural gas, biogas and gas produced from coal. The hydrogen in the gas reacts with carbonate ions from the electrolyte to produce water, carbon dioxide, electrons and small amounts of other chemicals. The electrons travel through an external circuit creating electricity and return to the cathode. There, oxygen from the air and carbon dioxide recycled from the anode react with the electrons to form carbonate ions that replenish the electrolyte, completing the circuit.The chemical reactions for an MCFC system can be expressed as follows:

Anode Reaction: CO3-2 + H2 → H2O + CO2 + 2e-

Cathode Reaction: CO2 + ½O2 + 2e- → CO3-2

Overall Cell Reaction: H2 + ½O2 → H2O

APPLICATION:-

1:-Power

Stationary fuel cells are used for commercial, industrial and residential primary and backup power generation. Fuel cells are very useful as power sources in remote locations, such as spacecraft, remote weather stations, large parks, communications centers, rural locations including research stations, and in certain military applications. A fuel cell system running on hydrogen can be compact and lightweight, and have no major moving parts. Because fuel cells have no moving parts and do not involve combustion, in ideal conditions they can achieve up to 99.9999% reliability.This equates to less than one minute of downtime in a six year period.

2:-Cogeneration

Combined heat and power (CHP) fuel cell systems, including Micro combined heat and power (MicroCHP) systems are used to generate both electricity and heat for homes (seehome fuel cell), office building and factories. The system generates constant electric power (selling excess power back to the grid when it is not consumed), and at the same time produces hot air and water from the waste heat. MicroCHP is usually less than 5 kWe for a home fuel cell or small business.

3:-Automobiles

Although there are currently no Fuel cell vehicles available for commercial sale, over 20 FCEVs prototypes and demonstration cars have been released since 2009. Demonstration models include the Honda FCX Clarity, Toyota FCHV-adv, and Mercedes-Benz F-Cell  As of June 2011 demonstration FCEVs had driven more than 4,800,000 km (3,000,000 mi), with more than 27,000 refuelings. Demonstration fuel cell vehicles have been produced with "a driving range of more than 400 km (250 mi) between refueling". They can be refueled in less than 5 minutes.

4:-Forklifts

Fuel cell powered forklifts are one of the largest sectors of fuel cell applications in the industry. Most fuel cells used for material handling purposes are powered by PEM fuel cells, although some direct methanol fuel forklifts are coming onto the market. Fuel cell fleets are currently being operated by a large number of companies, including Sysco Foods, FedEx Freight, GENCO (at Wegmans, Coca-Cola, Kimberly Clark, and Whole Foods), and H-E-B Grocers.

5:-Motorcycles and bicycles

In 2005 the British firm Intelligent Energy  produced the first ever working hydrogen run motorcycle called the ENV (Emission Neutral Vehicle). The motorcycle holds enough fuel to run for four hours, and to travel 160 km (100 mi) in an urban area, at a top speed of 80 km/h (50 mph)

6:-Airplanes

Boeing researchers and industry partners throughout Europe conducted experimental flight tests in February 2008 of a mannedairplane powered only by a fuel cell and lightweight batteries. The Fuel Cell Demonstrator Airplane, as it was called, used a Proton Exchange Membrane (PEM) fuel cell/lithium-ion battery hybrid system to power an electric motor, which was coupled to a conventional propeller.^[106] In 2003, the world's first propeller driven airplane to be powered entirely by a fuel cell was flown. The fuel cell was a unique FlatStack^TM stack design which allowed the fuel cell to be integrated with the aerodynamic surfaces of the plane.

7:-Submarines

The Type 212 submarines of the German and Italian navies use fuel cells to remain submerged for weeks without the need to surface.

The latest in fuel cell submarines is the U212A—an ultra-advanced non-nuclear sub developed by German naval shipyard Howaldtswerke Deutsche Werft, who claim it to be "the peak of German submarine technology."^[112] The system consists of nine PEM (Proton Exchange Membrane) fuel cells, providing between 30 kW and 50 kW each. The ship is totally silent giving it a distinct advantage in the detection of other submarines.

Other applications

 Providing power for base stations or cell sites

Distributed generation

Emergency power systems are a type of fuel cell system, which may include lighting, generators and other apparatus, to provide backup resources in a crisis or when regular systems fail. They find uses in a wide variety of settings from residential homes to hospitals, scientific laboratories, data centers,

 telecommunication equipment and modern naval ships.

• Notebook computers for applications where AC charging may not be readily available.
• Portable charging docks for small electronics (e.g. a belt clip that charges your cell phone or PDA).
• Smartphones, laptops and tablets.
• Small heating appliances