16-02-2010, 09:55 AM
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FUEL CELLS-Evolving Concepts and Developments
Introduction
Output of conventional batteries are limited.
They are polluting.
Continuous operation results in fall in efficiency.
Output not stable under long durations of operations.
Fuel Cells
An Electrochemical energy conversion device
Produces electricity from external fuel and oxidant.
Can operate virtually continuously as long as the necessary flows are maintained.
General Structure
Two electrodes sandwiched around an electrolyte
Hydrogen fuel is fed into the anode
Oxygen enters through the cathode.
Solid Oxide Fuel Cell
Permeable electrodes are used.
Hydrogen from fuel and oxygen permeate the corresponding electrodes.
Electrons flow through the external circuit.
Features
Advantages
High efficiency
Fuel flexibility
Can use a variety of catalysts
Solid electrolyte reduces electrolyte management problems
Suitable for CHP
Disadvantage
Slow start-up
Brittleness of ceramic electrolyte with thermal cycling
Proton Exchange Membrane Fuel Cell
A Hydrogen- Oxygen fuel cell.
Uses a polymer membrane along with electrolyte.
Anode splits hydrogen into proton and electron.
The membrane allows only the proton to pass.
Electrons take the external circuit, forming current.
Membrane prevents leakage of gas from either side into electrolyte.
Microbial Fuel Cells
Bio-electrochemical system that uses microbes for oxidation of fuel, normally organic matter.
In anode compartment, fuel is oxidised by bacteria, to release protons and electrons.
Protons pass through a cation- specific membrane and reach the cathode through the electrolyte.
Electrons flow through the external circuit.
The result of the reaction is clean water, so can be used to treat waste water, etc.
Types
Mediator Microbial Fuel Cell
Most of the microbial cells are electrochemically inactive. The electron transfer from microbial cells to the electrode is facilitated by mediators such as thionine, methyl blue, neutral red, etc.
Mediator-less Microbial Fuel Cell
Do not need mediators.
Use electrochemically active bacteria like Shewanella putrefaciens or Aeromonas hydrophila to transfer electrons to the electrode (electrons are carried directly from the bacterial respiratory enzyme to the electrode).
Protonic Ceramic Fuel Cell
Uses ceramic electrolyte that shows high protonic conductivity at high temperatures.
Suitable for use at elevated temperature.
Reforming of fuel to hydrogen not necessary
Since electrolyte is solid, no danger of drying up or leakage.
Applications
Stationary
Either connected to power grid as back up or supplementary power or independent in inaccessible areas.
Waste heat generated can be used for other purposes.
Since silent, no noise pollution.
Telecommunications.
To provide highly reliable power to telecommunication systems, either as back-up or primary.
Independent of weather and durable.
Used in telecom switching nodes, mobile phone towers, etc.
Landfills/Wastewater Treatment Plants/Breweries
Methane gas evolving can be used as fuel for fuel cells.
Can use microbial fuel cells also.
Reduces effluents and poisonous gases.
Transportation
Passenger Cars: Initiatives like Freedom Car and California Fuel Cell Partnership have brought close the dream of a fuel cell powered car, expected to release commercially in 2010.
Buses: Over 50 buses already tested in various country.
Use of fuel cells for trains, aeroplanes and boats are under research.
Auxilliary Power in vehicles.
To keep amenities like air conditioning and heater running when vehicle is stopped.
Also used in most submarines.
Portable Power
May be used where there is no power network.
May be used by defense personnel in remote places of combat.
Consumer Electronics
For use as laptop and cellular phone batteries, to last much longer.
Can be used to work portable power tools such as drills.
Space
Used in Apollo mission, as it provides power, heat and potable water.
Since then used in most manned space missions.
