Unit 6 Fuel Cells http://www.youtube.com/watch?v=esuAlB4NVi0

The fuel cell is a unique power converter That is efficient, nonpolluting, and flexible. For many, it is the technology that will wean Us from our strong dependence upon Imported oil, especially the transportation sector. It combines a fuel (usually natural gas or hydrogen) With oxygen from the air by an electrochemical Process to produce electricity.

Fuel cells are being developed to power passenger Vehicles, commercial buildings, homes, and Even small devices such as laptop computers. These systems are effective over a large range Of sizes, from 1kW to hundreds of MW, with Over all efficiencies of 80% or more! (when heat production is combined with The power generation) They are clean, highly reliable (no moving parts), And offer a unique opportunity for energy Independence.

The fuel cell was invented more than a hundred Years ago, but didn’t come of age in The 1970’s when it was first used on space missions. They offer a large power-to-weight ratio, Compactness, and reliability. This makes it a popular power source for such Uses as vehicle propulsion, and Small scale electric facilities.

The high cost of fuel cells and future Unknowns have hindered their commercialization. The present price of about $4000 per kilowatt Can be offset by fading government grants. In its most basic form, it is a battery That has a constant supply of fuel to Always “recharge” it. It provides direct current through an Electro-chemical process.

H2 + ½ 02  H22+ 02- + energy The 2 chemical reactants in a fuel cell Are generally hydrogen and oxygen, which Are fed into the cell through Porous electrodes. The fuel cell reaction can be thought of as A very slow hydrogen gas combustion process. H2 + ½ 02  H22+ 02- + energy

In a fuel cell the electrons are transferred Slowly to the oxygen via an external circuit. At the positive electrode, hydrogen atoms are Stripped of their electrons and enter into the Electrolyte. At the negative electrode, the hydrogen ions Combine with the oxygen atoms and electrons to Form water and heat.

The electrons flow through an external circuit Connecting the electrodes. The net result is the reaction of Oxygen and hydrogen to form Water.

There are 5 different types of fuel cells that Are in research, testing, or development stages. The proton exchange membrane (PEM), the Molten carbonate cell, the solid-oxide ceramic Cells are all in the demonstration stage. The phosphoric acid cell has been operated In a commercial size of 200 kW and tested With an 11MW unit.

The alkaline process has been used by NASA in its space program, in the Gemini space capsules in the 1960s.

The phosphoric acid cell (PAFC) is considered to Be the “1st” generation of fuel cells. It was also the first to be used commercially With over 200 units currently in use. They are usually used for stationary power generation Since they are heavy. They are 85% efficient when used for Electricity and heat.

Polymer electrolyte membrane (PEM) fuel cells Deliver high power density and are lower in Weight compared to other types. PEM uses a solid polymer as an electrolyte and Porous carbon electrodes containing a platinum catalyst. They require only hydrogen, oxygen from the air And water to operate. Can also use methane and methanol but a Fuel processor is necessary. These are the most popular choice for small Electronics and vehicle propulsion.

Alkaline fuel cells were the first to be used in the Space program to produce electricity and Onboard spacecraft. They use a solution of potassium hydroxide in Water as the electrolyte and a variety of precious Metals as the anode and cathode. They have demonstrated efficiencies as High as 60%.

Molten metal carbonate fuel cells use an Electrolyte of molten carbonate salt mixture in A porous, chemically inert ceramic lithium Aluminum oxide (LiAlO2). They operate at a high temperature, so they Can use lower quality fuels. Efficiencies reach 60%.

Solid Oxide fuel cells (SOFC) use a hard, nonporous Ceramic compound as the electrolyte, allowing Solid construction. These also operate at a very high temperature, So they require a long start up time, making Them more suitable for stationary functions. Efficiencies of around 50%.

Potassium hydroxide solution Hydrogen Oxygen 60-90 Fuel Cell Type Electrolyte Anode Fuel Cathode Fuel Operating Temp. (°C) Applications Alkaline (AFC) Potassium hydroxide solution Hydrogen Oxygen 60-90 Spacecraft Submarines Proton exchange membrane (PEMFC) 50% Proton-conductive polymer electrolyte (in air) Transportation Stationary power plants Direct methanol (DMFC) Proton-conductive polymer Methanol 90-120 Phosphoric-acid (PAFC) 50% Phosphoric acid 200 Molten carbonate (MCFC)60+% Molten alkaline carbonate Hydrogen, methane 650 Solid oxide (SOFC) 60+% Ceramic solid electrolyte Hydrogen, natural gas 800-1000

Direct methanol fuel cell (DMFC) And regenerative fuel cells. There are more types: Direct methanol fuel cell (DMFC) And regenerative fuel cells. Methanol uses methanol, which has a Higher energy density than H2. Regenerative fuel cells use solar panels to Split water through electrolysis and use Those gases to run the fuel cell.

The fuel stack is the heart of the system. A single cell produces only about 1 volt DC and A small current. Therefore, fuel cells use layers of cells to Produce the desired voltage and current needed.

The costs of fuel cells are not yet competitive with Conventional power technologies. In general, they are $30/kW, so they Cannot replace the ICE. At the moment, there are only a handful of Fuel cell systems in the U.S. The largest one being in Anchorage, Alaska.

Questions?

THE END