INTRODUCTION
A fuel cell is an electrochemical energy conversion device that continuously converts the chemical energy of a fuel directly into electrical energy. Continuous operation requires a supply of fuel and oxidant and removal of water vapour, spent fuel, spent oxidant, inert residue and heat, etc. It is known as a cell because of some similarities with a primary cell. Like a conventional primary cell, it also has two electrodes and an electrolyte between them and produces dc power.
Emission from Fuel Cell
The only exhaust of a fuel cell is water vapour which is not a pollutant if pure hydrogen is used as fuel (and pure oxygen as an oxidant). In the case of hydrocarbon fuel, carbon dioxide is also produced. If air is used as an oxidant, nitrogen (spent air) is also produced in the exhaust.
The efficiency of Fuel Cell
No cooling water is required, unlike conventional thermal power conversion devices where a substantial quantity of cooling water is required.
Fuel Cell Construction
The average cell voltage is typically about 0.7 V (on rated load) und several cells may be connected in series to increase the voltage. The current depends on the electrode area and can be increased by connecting several cells in parallel.
Advantages
- It is quiet in operation as it is a static device.
- It is less pollutant.
- Its conversion efficiency is more due to direct single stage energy conversion.
- Fuel cell plant can be installed near point of use; thus, transmission and distribution losses are avoided.
- No cooling water is needed as required in condenser of a conventional steam plant. The heat generated can be easily removed and discharged to atmosphere or used locally.
- Because of modular nature, any voltage/current level can be realized and the capacity can be added later on as the demand grows
- Fuel cell plants are compact and require less space
- Availability of choice from large number of possible fuels
- Can be used efficiently at part load from 50 per cent to 100 per cent
- No charging is required.
CLASSIFICATION OF FUELS CELLS
Fuel cells can be classified in several ways.
1) Based on the Type of Electrolyte
(a) Phosphoric Acid Fuel Cell (PAFC)
(b) Alkaline Fuel Cell (AFC)
(c) Polymer Electrolytic Membrane Fuel Cell (PEMFC)
or Solid Polymer Fuel Cell (SPFC)
or Proton Exchange Membrane Fuel Cell (PEMFC)
(d) Molten Carbonate Fuel Cell (MCFC)
(e) Solid Oxide Fuel Cell (SOFC)
2) Based on the Types of the Fuel and Oxidant
(a) Hydrogen (pure) - Oxygen (pure) fuel cell
(b) Hydrogen rich gas - air fuel cell
(c) Hydrazine - Oxygen/hydrogen peroxide fuel cell
(d) Ammonia - air fuel cell
(e) Synthesis gas - air fuel cell
(f) Hydrocarbon (gas) - air fuel cell
(g) Hydrocarbon (liquid) - air fuel cell
3) Based on Operating Temperature
(a) Low temperature fuel cell (below 150°C)
(b) Medium temperature fuel cell (150°C–250°C)
(c) High temperature fuel cell (250°C–800°C)
(d) Very high temperature fuel cell (800°C–1100°C)
4) Based on Application
(a) Fuel cell for space applications
(b) Fuel cell for vehicle propulsion
(c) Fuel cell for submarines
(d) Fuel cell for defense applications
(e) Fuel cell for commercial applications
5) Based on the Chemical Nature of Electrolyte
(a) Acidic electrolyte type
(b) Alkaline electrolyte type
(c) Neutral electrolyte type
FUEL CELL POWER PLANT
The block diagram showing main components of a fuel cell power plant is given in Fig. 7. Electrical energy is generated from primary fossil fuels through fuel cell. Fuel is managed and supplied by fuel processing unit. In this unit, fuel is received, stored, reformed, purified and supplied to fuel cell module. Fuel cell module convert fuel energy electrochemically into dc power using ambient air as oxidant. Basic configurations of cell, module and plant are shown in Fig. 8. A number of cells are stacked to form a module. Several modules are interconnected to form a power-generating unit. Fuel gas and air are supplied to modules from common supply pipes. The exhaust is collected in a common pipe and discharged to atmosphere either directly or after recovery of heat in a cogeneration unit. Power generating unit generates electrical power as dc. Industrial/commercial loads are normally rated for standard ac supply such as 3 ph., 400 V, 50/60 Hz or 1 ph., 230/110 V, 50/60 Hz. The electrical power-conditioning unit, converts dc output of fuel cell to ac using inverter and also controls and regulates it.
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