Green Manufacturer - July/August 2010

Fuel Cell Types

There are several types of stationary fuel cells, four of which have received significant interest for large stationary power applications. They vary in power output, electrical efficiency, and operating temperatures.

Proton exchange membrane (PEM) fuel cells have a high energy density for their weight and volume. Operating between 50 and 100 degrees C (122 and 212 degrees F), this fuel cell type starts up quickly and is suitable for load following or for adjusting the power output to meet the power demand in the plant. Terry Howe, manager of solutions engineering for Ballard Power Systems, Burnaby, B.C., Canada, a manufacturer of PEM fuel cells, said the company has targeted chemical companies as a near-term potential stationary application.

customer or the grid, and the fuel cell module itself.

The fuel cell has an anode (a negative electrode that provides electrons), an electrolyte in the center, and a cathode (a positive electrode that accepts electrons). As hydrogen flows into the anode, a catalyst on the anode helps to separate the gas into protons ( hydrogen ions) and electrons (see Figure 1). Individual fuel cells can be combined into a fuel cell stack to increase the total electrical output.

Figure 1

A fuel cell comprises an anode (a negative electrode that provides electrons), an electrolyte in the center, and a cathode (a positive electrode that accepts electrons). As hydrogen flows into the anode, a catalyst on the anode helps to separate the gas into protons (hydrogen ions) and electrons. The electrolyte in the center allows only the protons to pass through the electrolyte to the cathode side of the fuel cell. The electrons cannot pass through this electrolyte and therefore must flow through an external circuit in the form of electrical current that can power an electric load. As oxygen flows into the cathode, another catalyst layer helps the oxygen, protons, and electrons combine to produce water and heat. Image courtesy of UTC Power, South Windsor, Conn.

Phosphoric acid fuel cells are one of the most mature fuel cells and are among the first to be used commercially. They have a relatively low operating temperature of between 150 and 200 degrees C (302 and 392 degrees F) and are well-suited for load-following applications, according to Bob Tierney, manager of business development and strategic planning for UTC Power, South Windsor, Conn. He said phosphoric acid fuel cells have long stack life—85,000 hours or 10 years of service, up from 40,000 hours five years ago.

Molten carbonate fuel cells operate at 600 to 700 degrees C (1,112 to 1,292 degrees F). According to Tony Leo,

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vice president of applications and OEM engineering at fuel cell manufacturer FuelCell Energy, Danbury, Conn., the high operating temperature of this fuel cell type produces plenty of heat that can be put to other uses in the plant to produce steam, hot water, or chilled water. The high operating temperature also allows for the re-forming process, which extracts hydrogen from natural gas, to take place right in the fuel stack, saving the cost of an external re-former.

Solid oxide fuel cells are the most recent to be commercialized for stationary applications. Historically, solid oxide fuel cells, which operate at up to 1,000 degrees C (1,832 degrees F), have posed

Figure 2

In this cutaway of a phosphoric acid fuel cell, the fuel processor re-forms natural gas into hydrogen gas, which is fed into the fuel cell stack. In the stack, hydrogen gas and air are combined in an electrochemical process that produces direct current (DC) power, water, and heat. The byproduct water is used in the operation of a power plant. The byproduct heat is available for meeting other requirements in a facility, such as creating hot water, space heating, or cooling. The DC power provided by the fuel cell stack is conditioned to provide alternating current (AC) power output. Image courtesy of UTC Power, South Windsor, Conn.