Fuel Cells

 Fuel Cells have emerged as one of the most promising technologies for the power source of the future. Fuel cell is an electrochemical device that converts energy into electricity and heat without combustion

Fuel cells are modular in construction, their efficiency is independent of size and have flexibility of using a wide range of fuels to produce hydrogen/reformate suitable for fuel cell applications. Fuel cells require relatively pure hydrogen free of contaminants such as sulphur and carbon compounds etc. When hydrogen rich reformate gas mixture is used, it results in some emissions similar to lean hydrogen – air mixture burning in IC engines. The fuel cell systems are more efficient than IC engines and turbines. Fuel cells are considered suitable for a broad spectrum of applications ranging from a few milli watts to several kilowatts. 6.1.3 Fuel cells are the long term option for hydrogen applications both for transportation and power generation. Fuel cells especially for vehicular applications are in the development stage and the country needs to identify the path for fuel cell development taking into account technology development efforts going on worldwide and the country’s specific priorities and the achievements in this area so far.

Fuel Cell Technologies :There are six major fuel cell technologies of interest viz., Proton Exchange Membrane Fuel Cells (PEMFC), Phosphoric Acid Fuel Cells (PAFC), Alkaline Fuel Cells (AFC), Direct Methanol Fuel Cells (DMFC), Molten Carbonate Fuel Cells (MCFC); and Solid Oxide Fuel Cells (SOFC). These fuel cells operate with different fuels in a wide range of temperatures (600 C–10000 C) depending on the technology. These technologies are at different stages of development and demonstration. 

Phosphoric Acid Fuel Cell (PAFC)

The Phosphoric Acid Fuel cell (PAFC) technology is the most commercially developed fuel cell technology. PAFC stacks use phosphoric acid as an electrolyte. PAFC is being used in applications such as hospitals, hotels, offices, schools, etc. It can also be used in larger vehicles such as buses, etc. The efficiency of PAFC is about 40%. Phosphoric acid fuel cells (PAFC) were the first to be developed in the country more than a decade ago. PAFC power packs of various capacities have been developed, demonstrated and evaluated in actual field conditions. 

Proton Exchange Membrane Fuel Cell (PEMFC)

Polymer electrolyte membrane fuel cells (PEMFC) have been developed for use in stationary and transport applications. The mechanism of PEMFC is same as PAFC. They differ in that PEMFCs operate at relatively low temperatures (about 90 0 C). In view of low operating temperatures PEM fuel cells are considered appropriate for use in automobiles. They have high power density and can vary their output quickly to meet quick shifts in power demand

Molten Carbonate Fuel Cell (MCFC)

 Molten Carbonate Fuel Cells (MCFC) uses an alkali metal carbonate (Li, Na, K) as the electrolyte. However an alkali metal carbonate must be in the liquid phase to function as an electrolyte. This cell operates at higher temperature of about 600 0 C. The high operating temperature is required to achieve sufficient conductivity of the electrolyte. The higher operating temperature of MCFCs provides the opportunity for achieving higher overall system efficiencies and greater flexibility in the use of available fuels. The high operating temperature, however, impose limitations and constraints on choosing materials suitable for long life time operations. This technology also require significant improvements 

Solid Oxide Fuel cells 

Solid Oxide Fuel cells (SOFC) use solid, nonporous metal oxide electrolytes. The metal electrolyte normally used in manufacturing SOFCs is stabilized Zirconia. This cell operates at a higher temperature of about 1000 0 C. This high operating temperature allows internal reforming, promotes rapid kinetics with non-precious materials and produces high quality heat. The solid state character of SOFC components implies that there is no restriction on the cell configuration. It is possible to shape the cell according to the criteria such as overcoming design or application issues. The combined heat and power efficiencies of SOFC could be as high as 80%.

Direct Methanol Fuel Cell (DMFC  

 Direct Methanol Fuel Cells use methanol as the fuel. These fuel cells work at lower temperatures and have found application in mobile power generation applications such as laptops etc

Alkaline Fuel Cell (AFC) 

AFC uses alkaline potassium hydroxide as the electrolyte. These cells can achieve power generating efficiencies of upto 70%. The low operating temperatures of Alkaline fuel cells is one of the main advantages for pursuing further improvements in the technology for terrestrial applications.  

Issues and Barriers  

Develop fuel cell systems :

•  Fuel cell technologies are still in the early stages of development. Several technological issues concerning choice of materials, improvements in design and performance of fuel cell stacks and systems are yet to be fully resolved. A better understanding of the issues concerning purity of materials, gas diffusion/distribution system, contamination, temperature tolerance and durability is also required. There are several engineering issues that need to be resolved to make fuel cell systems compact and cost effective. In addition, issues concerning reliability, reproducibility and life expectancy of fuel cell system need to be resolved. Their performance under different climatic conditions remains to be evaluated. Air, thermal and water management are other important issues from the system integration point of view. The performance requirements for stationary use of fuel cells are different from the transport application.

Key Issues 

i Durability ii Cost reduction iii New catalysts substituting platinum iv New membranes performing at higher temperatures v Stack design vi Power density vii Cooling arrangement viii Reduction in auxiliary power consumption ix Compact size x Load tracking system xi Fuel cell system development

Fuel cells are still in early stages of development and are not yet cost competitive. Considerable research efforts are necessary to develop various materials used for electrodes, catalysts, membranes and separators. Low cost replacement for expensive noble metals, used as catalysts, have to be found. The life expectancy and reliability of fuel cells also needs to be significantly improved