A fuel cell is defined as an electrochemical device that continuously converts the chemical energy of fuel into electricity and heat without combustion.
Difference between Fuel Cell and Battery
A fuel cell is similar to other electrical cells (battery) in respect that both have the positive and negative electrodes with an electrolyte between them and both produce dc power.
The essential difference between the two is that a fuel cell provides continuous energy input and output while the battery stores the electric charge.
A fuel cell supply system requires continuous supply of fuel and oxidant and generates dc power and heat. For example in hydrogen — oxygen fuel cell, H2, is supplied as fuel and O2, as oxidant continuously. It generates dc power with water and heat as by-product. Generated heat can be transferred to the atmosphere.
While a battery stores the electrical charge and after supply of power it discharges and needs recharging.
Advantages of Fuel Cells
In case of conventional power generation methods, the energy of the fuel is first converted into heat energy and then it is converted into electrical power. During power generation, lot of cooling water is needed for rejection of heat which is not utilized.
While in case of fuel cells no cooling water is needed since the heat can be either rejected directly to atmosphere or it can be used as process heat.
The efficiency of conventional power generation methods depends on the temperature range of the cycle. Whereas, the conversion of chemical energy into electrical power in case of fuel cells is an isothermal process.
Therefore, the efficiency of chemical conversion into electric power of fuel cells is higher than the conventional power plants.
This advantage of fuel cells makes it an attractive proposition to use in applications like space and ocean ships, automobiles, portable power plants, military use and for power production.
Advantages of fuel cells over conventional power generation methods can be listed as under:
- It is quite in operation and less polluting.
- Energy conversion efficiency into electrical power is higher.
- No cooling water needed.
- Heat generated can be rejected directly to atmosphere, the process being isothermal.
- Fuel cells can be installed at the point of use thus reducing the power losses due to transmission and distribution.
- By use of series-parallel combination of fuel cells, the output power rating at any voltage and current can be obtained.
- The capacity can be increased as the demand grows.
- Space requirement is much less.
- It has long life.
Regenerative Fuel Cell (RFC)
The Regenerative Fuel Cell (RFC) is a system that can operate in a closed loop and could serve as the basis of a hydrogen economy operating on renewable energy.
Fuel cells generating electricity, heat, and water from hydrogen and oxygen would be used throughout the economy, powering factories, vehicles, and houses.
The hydrogen would be generated from the electrolysis of water, splitting it into its constituent components of hydrogen and oxygen, using renewable energy sources such as wind, solar, or geothermal.
Such a system would not require any specific type of fuel cell, but would need on infrastructure to deliver hydrogen to the many fuel cells in use. Little to no new technology is required to implement a renewable-based system.
However, at present, the infrastructure for hydrogen delivery does not exist. Also, the cost of electricity would be high, but will decrease as costs decrease for each of the components: Fuel cells, hydrogen storage, and delivery, renewable energy collection, etc.
Currently, there is a project led by NASA to develop an efficient and lightweight regenerative fuel cell system for use on board an airplane called the Helios that can fly at altitudes near 30000 m.
The predecessor aircraft during the day and generate a supply of hydrogen that would be stored for use by the fuel cell overnight. Such a system would then be capable of flights lasting many days.
If the fuel cell is designed to operate also in reverse as an electrolyzer, then electricity can be used to convert the water back into hydrogen and oxygen. This dual function system is known as a Reversible Fuel Cell or Unitised Reversible Fuel Cell (URFC).
Lighter than a separate electrolyzer and generator, a URFC is an excellent energy source in situations where weight is a concern.
The regenerative fuel cell, coupled with lightweight hydrogen storage, had by far the highest energy density about 450 watt-hours which is more than ten times of lead acid battery and more than twice that forecast for any chemical for chemical batteries. The Prototype Fuel cells have been used since 1960s when they supplied on-board power for the Gemini and Apollo spacecraft.
Present Status and Limitations of Fuel Cells
Fuel cell technology has been ignored in the recent past due to success of I.C. engines and batteries since the fuel cells were costly and uneconomical. However, the interest in fuel cells have been revived due to their direct energy conversion, high efficient with negligible or no environmental pollution.
Fuel plants are modular and available in wide range of size 5 kW to 10 MW. These are ideally suited as standalone plants and use of power in remote areas. Fuel cell technology was revived in 1980s.
Presently Phosphoric acid fuel cells (PAFC) are operating in Japan, MCFC and SOFC fuels cells are being used in USA and European countries are favoring PAFC, MCFC and SOFC fuel cells.
A number of companies are engaged in manufacturing of fuel cells like UTC fuel cell, A vista Labs. Other companies like Casio, Motorola, Sony, Panasonic are also developing fuel cells for their various applications.
Application of fuel cells is limited by their high initial cost and low service life. There are problems in productions of pure hydrogen gas and the inert gases having in supply air.