06-06-2012, 04:51 PM
RENEWABLE ENERGY RESOURCES
INTRODUCTION:
Basically the energy sources are two types; they are conventional energy sources like coal, petroleum, natural gas etc. & non-conventional energy sources like solar cells, fuel cells, thermo-electric generator, thermionic converter, solar power generation, wind power generation, geo-thermal energy generation, tidal power generation etc.
Fast depletion of conventional energy sources made us to look after alternate energy sources such as magneto-hydro-dynamic (MHD) power generation and other non-conventional methods of power generation.
FUEL CELLS
INTRODUCTION TO FUEL CELLS:
The basic concept involved in any modern fuel cell for electric power generation is the electrochemical reaction between hydrogen and oxygen in the presence of catalysts, which produces electrical energy in the form of a DC current. The by-products are heat and water. Since hydrogen has one of the highest chemical reactivity’s, it is commonly used as either pure hydrogen or hydrogen rich fuel in most modern fuel cells. The fuel is supplied on the anode side. The anode reaction in fuel cells is either direct oxidation of hydrogen or methanol or indirect oxidation via a reforming process for hydrocarbon fuels. The cathode reaction is oxygen reduction from air in most fuel cells. Since the electricity in a fuel cell is not produced through the use of thermal energy, fuel cell efficiency is not limited by the Carnot efficiency.
BRIEF THEORY AND OPERATION:
For the hydrogen/oxygen (air) fuel cell, the overall reaction is: H2 + ½ O2 = H2O. The product of this reaction is water released at the cathode or anode depending on the type of fuel cell. For PEM fuel cell, hydrogen is oxidized at the anode as given by the reaction, H2 = 2 H+ + 2 e-. Electrons generated at the anode are flown through external load to the cathode. Protons (H+) are migrated through the proton exchange membrane to the cathode. The protons and electrons reached at the cathode react with oxygen from air as given by the reaction, 2 H+ + 2 e- + ½ O2 = H2O.
THE CELL:
The proton exchange membrane fuel cell (PEMFC) is a low temperature fuel cell operating at around 80°C. It has a polymer (plastic) membrane (Nafion/Dow) which when hydrated with water becomes the electrolyte for the proton transfer from the anode to cathode. The gas diffusion layer of anode and cathode are porous thin carbon papers (graphite sheets) or cloth. Thin layer (about 10-100 µm) of Pt or Pt/Ru catalyst layer is deposited on each side of the membrane.
The Stack:
The flow distribution and associated pressure drop of hydrogen and air in the cell stack is a real issue for high voltage and high power density performance. Novel designs of the gas flow fields (channel designs) on the anode and cathode sides are needed to make cost effective high performance gas flow field plate designs that also reduces parasitic losses of flowing the fuel and oxidant.
