Introduction

Fuel cells are electrochemical devices that convert chemical energy directly into electricity. The process continues as long as fuel is fed to the device. The output from fuel cell is dc electric current and heat. Fuel cells consist of two electrodes, an anode and a cathode, in contact with an electrolyte. They essentially operate by reverse electrolysis. Usually the anode is supplied with hydrogen fuel and the cathode with air (oxygen), the cells sets up a voltage between the electrodes. Once an external circuit is connected fuel cells generate electric power. It is also to be noted that the unlike heat engines, fuel cells are not limited by Carnot cycle efficiency. Fuel cells have very high efficiencies particularly when used as CHP (combined heat and power) units. In this article we focus on the major fuel cell technologies, their operational characteristics, applications, status of development process and installed capacity.

Application/Types of fuel cells

Fuel cells are used for on site power generation, Combined heat and power for industries, portable power units (military use), electric Vehicles and in the long term also for residential use. The most widely proposed fuel cell technologies are,

PAFC - Phosphoric Acid Fuel Cell

MCFC - Molten Carbonate Fuel Cell

SOFC - solid oxide Fuel Cell and

PEM or SPFC - Proton Exchange Membrane or Solid Polymer Fuel Cell.

AFC - Alkaline Fuel Cell

Electrode reaction of H₂/O₂ fuel cells can be broadly classified as acid electrolyte reaction and alkaline electrolyte reaction.

Acid electrolyte reaction

In acid electrolyte reaction, hydrogen is supplied to anode, where hydrogen disassociates into hydrogen ions and electrons. The hydrogen ions migrate through the electrolyte to cathode and electrons flow through an external circuit connecting cathode. At the cathode the oxygen in air supplied to it reacts with the hydrogen ions and electrons from the external circuit to form water.

Alkaline electrolyte reaction

In alkaline electrolyte reaction, oxygen in air supplied to the cathode reacts with water and electrons from the external circuit to form hydroxyl ions and migrate to the anode through the electrolyte. At the anode, the hydrogen fuel reacts with the hydroxyl ions to form water the other products formed are recycled.

PAFC and SPFC undergo acid electrolyte reaction and MCFC, SOFC, AFC undergo alkaline electrolyte reaction.

Fuel cell power generation system

The fuel cell power generation system essentially consists of four major components. They are,

Fuel processing section.

Power generation section.

Power conditioning section and

Heat recovery section.

Fig 1Block Diagram of Fuel Cell Power Generation System

Fuel processing section

For power generation using fuel cell it requires reformed hydrogen rich fuel to be supplied at the anode. So in the fuel processing section the fuel gas say LNG(liquefied Natural Gas), methanol, coal bed methane, coal gob gas, land fill gas etc which contain hydrogen are enriched and the impurities like sulfur etc are removed. The reformed gas is then passed on to the anode of the fuel cell power section. The fuel processing section may either be a separate component of the Fuel cell power generating system or even may be incorporated directly in the fuel cell power section.

The power generation section

This is the heart of the whole assembly it houses the Fuel cell. The fuel cell constitutes normally of separator, electrodes (anode, cathode) and an electrolyte matrix. The design of the constituents may vary according to the manufactures but usually the basic elements are the same. Unlike acid batteries which involve chemical reactions fuel cells undergo electrochemical reactions. At the anode the hydrogen disassociates into hydrogen ions and electrons. The hydrogen ions migrate through the electrolyte matrix towards cathode where, it reacts with air to produce water and electrons. Thus power is generated.

Power conditioning section

The output of the fuel cell is dc electricity and Heat. The heat is utilized through heat exchangers. The dc electricity is fed to the power conditioning section where it is converted to normal ac power as most of the electrical appliances operate on ac supply. The functioning is as that of the UPS (uninterruptible power supply). The power conditioning unit also takes into account the required power factor and the frequency of operation so as to protect the connected load from damages due to power mismatches. It essentially consists of inverters, transformers, capacitors and control buses etc.

Heat recovery section

The heat recovery section captures the used heat involved in the process of generating power and transports it for useful purpose. Most of the advanced cells operate in the range of temperature about 180°C – 210°C degree for phosphoric acid, 630°C-650°C for molten carbonate and 900°C –1000°Cfor solid oxide cells. Hence the heat can be utilized for domestic and industrial purpose. The efficiency of fuel cell exceeds 80% if used as a co-generation plant.

For the major fuel cell technologies being discussed, the following table describes the principle of operation; operational characteristics; status of research/commercialization and their applications.

Major Fuel Cell Technologies-Principle of operation; operational characteristics; status of research,commercialization and their applications

	PAFC - Phosphoric Acid Fuel Cell	MCFC - Molten Carbonate Fuel Cell	SOFC - Solid Oxide Fuel Cell

PRINCIPLE	A reformed hydrogen rich gas is supplied to the anode where hydrogen molecules disassociate into hydrogen ions and electrons. The hydrogen ions migrate through the electrolyte to the cathode. At the cathode oxygen in air supplied to it reacts with the migrated hydrogen ions and the electrons supplied by an external circuit to produce water.	Oxygen in air and carbon-dioxide are supplied to the cathode, where the gases react with electrons supplied by an external circuit to produce CO₃²', which migrates to the anode through the electrolyte. A reformed gas that contains hydrogen and carbon-mono-oxide is supplied to the anode. At the anode the gases react with CO₃²' to produce water, carbon-dioxide and electrons. The carbon produced at anode is recycled to the cathode.	Oxygen in air is supplied to the cathode, where the oxygen reacts with electrons supplied through an external circuit to form O₂", which migrates to the anode through the electrolyte. A reformed gas that contains hydrogen and carbon-mono-oxide is supplied to the anode. At the anode these gases react with O₂" to form water, carbon-dioxide and electrons.
OPERATIONAL FEATURES
Electrolyte	concentrated phosphoric acid	carbonates Li₂CO₃/Na₂CO₃	Stabilized zirconia ZrO₂
Charge carrier	H⁺	co₃^2-	Mobile O₂⁻ ions migrating through crystal lattice.
Fuel	H₂	H₂, CO	H₂, CO
Anode reaction	H₂ → 2H⁺ +2e"	H₂ + CO₃²⁻ → CO₂ + H₂O + 2e-	H₂ + O₂⁻ → H₂O + 2e-
Cathode reaction	2H⁺ + 2e"+ ½O₂→ H₂O	2e- + CO₂ + ½O₂ → CO₃²−	2e- + ½O₂ → O₂−
Overall reaction	H₂ + ½O₂ → H₂O	H₂ +CO₂ + ½O₂ → CO₂ + H₂O	H₂ + ½O₂ → H₂O
Power generation Efficiency	40–45%	45–60%	50–60%
Temperature in °C	150–210	600–650	800–1100
Power level (kW)	10–110 kW	5 kW-2MW	10–200 kW
Start up time (minutes)	300 minutes	500 minutes	100 minutes
Materials used	Carbon, Platinum. Et	Nickel, stainless steel, etc.	ceramics, etc.
STATUS	These types of fuel cells operate at low temperature around 200?C, "PAFC cells are commercially available today. More than 200 fuel cell systems have been installed all over the world - in hospitals, nursing homes, hotels, office buildings, schools, utility power plants, an airport terminal, even a municipal waste dump. Phosphoric acid fuel cells generate electricity at more than 40% efficiency — and nearly 85% of steam this fuel cell produces is used for co-generation." ['Fuel Cells 2000′s' -www.fuelcells.org] PAFC cells face strong competition from other fuel cell technologies. The major disadvantages realized are limited lifetime, expensive catalyst requirement and low overall efficiency.	MCFC are more suitable for large scale power generation and co-generation. They require fuel reforming unit that can be either external or integral part of the cell structure. This type of fuel cells operation is at high temperatures between 600–650?C. Heat recovered from the system can be utilized in bottoming cycles for fuel reforming. To date, molten carbonate fuel cells have been operated on hydrogen, carbon monoxide, natural gas, propane, landfill gas, marine diesel, and simulated coal gasification products. 10 kW to 2 MW molten carbonate fuel cells have been tested on a variety of fuels. MCFC cells for stationary applications have been successfully demonstrated in Japan and Italy.	The SOFC type fuel cells operate at as high as 1000?C, they have the same features as a MCFC cell. There is no problem related to corrosion, cathode dissolution and loss of electrolyte as it is an 'all-solid-type'. SOFC is expected to have stable operation with long life. SOFC cells could be used in medium to big, high-power applications including industrial and large-scale central electricity generating stations. Some developers also see solid oxide use in motor vehicles and are developing fuel cell auxiliary power units (APU) One type of SOFC uses an array of meter-long tubes, and other variations include a compressed disc that resembles the top of a soup can. Tubular SOFC designs are closer to commercialization and are being produced by several companies around the world. Demonstrations of tubular SOFC technology have produced as much as 220 kW
INSTALLED CAPACITY/TEST UNITS	Fuji Electric and Mitsubishi Electric from Japan have developed cost-effective PAFC-systems up to 500 kWp. 200 kWp PC25 system manufactured by ONSI, USA -more than 200 systems installed around the world, [from: http://www.fuelcell-eur.nl]	Germany: 250 kWp field test in co-generation mode. USA: Fuel Cell Energy Corp. 263 kWp in lab. MC-Power tests 75 kWp Italy: Ansaldo in co-operation with Spanish Comp. 100 kWp tested in Spain and in Italy. Japan: 1 MWp (500 kWp each of IHI & Hitachi) tested in 1999/2000; 200 kWp Mitsubishi tested in 1 999/2000; Low-cost MCFC 10 kWp Toshiba. Korea: 25 kWp External & 5 kWp Internal Reforming.	Germany: 100 kWp tubular systems USA: 25 kWp Westinghouse, 250 kWp NFCRCSwitzerland: Four 1 kWp planar stapled cylindrical format Micro CHP systemsJapan: several units of 10 kWp systems using tubular SOFC by Mitsubishi Electric Comp
APPLICATION	Military: CHP-units at military sites. Civil: Fork lift truck & urban transit bus; CHP & UPS systems fuelled with hydrogen, natural gas, LPG and methane from waste water purification plants; dispersed electricity production.	Natural gas and coal fuelled dispersed electricity and heat/steam production.	Power pack for outdoor recreation (small tubular system); Micro CHP-systems in residences (flat plates stapled in cylindrical format); Dispersed CHP production (tubular cells).

Major Fuel Cell Technologies-Principle of operation; operational characteristics; status of research, commercialization and their applications

	PEM/SPFC - SOLID POLYMER ELECTROLYTE FUEL CELL	AFC-ALKALINE FUEL CELL

PRINCIPLE	At the anode, hydrogen is split into electrons and protons. The protons migrate through the ion-conducting membrane and the electrons travel through an external circuit. Oxygen from air combines at the cathode with the protons and electrons, producing water and closing the electronic circuit. Platinum is used as catalyst. The proton exchange membrane is a thin plastic sheet that allows hydrogen ions to pass through it. The membrane is coated on both sides with highly dispersed metal alloy particles (platinum) that are active catalysts. Hydrogen is fed to the anode side of the fuel cell where the catalyst encourages the hydrogen atoms to release electrons and become hydrogen ions (protons).	Oxygen in air supplied to the cathode reacts with water and electrons from the external circuit to form hydroxyl ions and migrate to the anode through the electrolyte. At the anode, the hydrogen fuel reacts with the hydroxyl ions to form water and electrons
OPERATIONAL FEATURES
Electrolyte	Per-fluorinated solid polymer membrane	Concentrated KOH solution
Charge carrier	H ⁺	OH⁻
Fuel	H₂	H₂
Anode reaction	H₂ → 2H⁺ +2e⁻	H₂ + 2OH -→2H₂O +2e⁻
Cathode reaction	2H⁺ + 2e⁻+ ½O₂→ H₂O	2H₂O + 2e- + ½O₂ → 2OH⁻
Overall reaction	H₂ + ½O₂ → H₂O	H₂ + ½O₂ → H₂O
Power generation Efficiency	32–40%	55–60%
Temperature in °C	70–90	70–90
Power level (kW)	0.1–250kW	50-100kW
Start up time (minutes)	5 minutes	5 minutes
Materials used	Platinum. Etc	&
STATUS	Pre-commercial, receiving the most attention for mo- bile and portable applications. Stationary applications to benefit from progress in other areas.	Mature technology struggling to penetrate markets and facing strong competition form PEM.
STATUS		Long used by NASA on space missions, these cells are capable of achieving power generating efficiencies of up to 70 percent. Though they are too costly for commercial applications, but several companies are examining ways to reduce costs and improve operating flexibility.
INSTALLED CAPACITY/TEST UNITS	Mobile application: Germany, Italy, Japan, Canada, USA: NECAR, NEBUS operating on Hydrogen and Methanol fuel.Ranging from 50 kWp to 250 kWp systems.	Germany: 100 kWp submarineUK: 5 kWp London city cab
INSTALLED CAPACITY/TEST UNITS	Stationary application:Germany: Four 250 kWp system for assembly and manufacturing facility. ALSTOMUSA: several small portable power packs and micro CHP systems.Canada: 250 kWp Ballard power system Italy: 240 kWp for H2 recovery from chemical plant.	Germany: 100 kWp submarineUK: 5 kWp London city cab
APPLICATION	Military: submarines, portable units, power packs, etc.Civil: Portable phones & computers, power packs for outdoor recreation; Micro Combined Heat & Power (CHP) production in residences (few kWp), small-scale UPS-systems; Dispersed CHP-generation (first 250 kWp being tested as field experiments); Forklift trucks, passenger cars, four wheel drives, multi-purpose vans, mini vans, urban transit buses, Boats of all sizes.	Military: Submarines. Civil: Gemini & Apollo space-crafts; London taxi & park maintenance van.

Conclusion

PAFC are suitable for commercial or light industry applications. Systems of 200 kW are presently commercially available. Pilot plants of 1.3 MW in Italy and of 11 MW in Japan have been built, but their cost is still high.

MCFC type face a number of technical problems such as corrosion and reliability still have to be solved. It is expected that commercialization will take 5-10years. MCFC systems up to 2 MW have been constructed and demonstrated but the reliability of such systems has not been encouraging.

SOFC have a comparable efficiency and the same utilization sectors as MCFC. Operating temperature is between 700°Cand 1,000°C. Further materials research and cost reduction will be needed. Commercialization is expected to take place in around 10 years.

PEM/SPFC offer potential for significant cost reduction and are expected to be commercialized in 5–10 years. They are highly suited for both mobile and stationary applications.

AFC face competition from PEM type cells. Being mature technology and offering high efficiency still research is going on to reduce the cost. They are suited for mobile applications.

World's largest solar project unveiled

If all goes according to plan, a partnership between SES and Southern California Edison (SCE), would see the construction of an expansive 4,500-acre solar generating station in Southern California. When completed, the proposed power station would be the world's largest solar facility, capable of producing more electricity than all other U.S. solar projects combined.

The 20-year power purchase agreement signed today, which is subject to California Public Utilities Commission approval, calls for development of a 500-MW solar project 70 miles northeast of Los Angeles using innovative Stirling dish technology. The agreement includes an option to expand the project to 850 MW. Initially, Stirling would build a one-MW test facility using 40 of the company's 37-foot-diameter dish assemblies. Subsequently, a 20,000-dish array would be constructed near Victorville, Calif., during a four-year period.

Although Stirling dish technology has been successfully tested for 20 years, the SCE-Stirling project represents its first major application in the commercial electricity generation field. Experimental models of the Stirling dish technology have undergone more than 26,000 hours of successful solar operation. A six-dish model Stirling power project is currently operating at the Sandia National Laboratories in Albuquerque, New Mexico.

The Stirling dish technology converts thermal energy to electricity by using a mirror array to focus the sun's rays on the receiver end of a Stirling engine. The internal side of the receiver then heats hydrogen gas which expands. The pressure created by the expanding gas drives a piston, crank shaft, and drive shaft assembly much like those found in internal combustion engines but without igniting the gas. The drive shaft turns a small electricity generator. The entire energy conversion process takes place within a canister the size of an oil barrel. The process requires no water and the engine is emission-free.

Fuel Cell Technology