HOW Fuel CellS WORK

A Fuel Cell is an electrochemical device that converts chemical energy directly into electricity, in contrast to more conventional electric generation technologies such as natural gas turbines or fossil fueled boilers. Direct electrochemical conversion is environmentally attractive because of inherently low emissions, lower carbon intensity, as well as less noise and vibration because it is a “solid state” process.

While Fuel Cells have been known since the 1800’s, practical development began in the 1960’s under the impetus of the Gemini and Apollo space programs. Fuel Cells occupied a key role providing on board power and environmental support due to their high energy density to provide both heat and power, and because their by-product water was needed for drinking.

Fuel Cells have been in continuous development since the mid-1960’s incorporating substantial governmental and private industry funding. The major developers today have focused on four distinct technologies. The following chart outlines these technologies and the developers active in each category.

Proton Exchange Membrane (PEMFC)

Ballard Hydrogenics IdaTech Intelligent Energy Nuvera Plug Power ReliOn

Molten Carbonate (MCFC)

FuelCell Energy Corp. GenCell Corp.

Phosphoric Acid (PAFC)

UTC Fuel Cells Hydrogen, LLC

Solid Oxide (SOFC)

Rolls Royce Fuel Cells Siemens Westinghouse Acumentrics Worldwide Energy Bloom Energy Ceres Power

Fuel Cell CONFIGURATION

From a practical point of view, a Fuel Cell is much like a battery with one important difference. A battery essentially has a fixed amount of fuel and runs down when the chemical energy stored in it is consumed. In contrast, a Fuel Cell has its energy continually replenished and, thus, will continue to generate power as long as fuel is supplied.

Unlike a battery, as shown in Figure 1, hydrogen fuel is continuously supplied to the cell’s negative electrode called the anode. Concurrently, oxygen from ambient air is continuously supplied to the cathode or positive electrode. Like a battery, a Fuel Cell consists of two conductive plates separated by an insulating barrier containing an electrolyte. The reaction of hydrogen at the fuel anode is:

H2 —› 2H+ + 2e–
and of oxygen at the cathode, air side:
1/2 O2 + 2H+ + 2e– —› H2O

FIGURE 1
Basic Fuel Cell Configuration of a
FC with External Fuel Processor

Since the electrolyte is actually an electrical insulator, the electrons are forced to travel in an external circuit and thereby produce electric power. In contrast, the hydrogen ions travel through the electrolyte to recombine with the electrons and oxygen to produce water vapor.

To encourage reasonable rates and efficiency, a highly dispersed platinum catalyst “ink” is printed on the anode and cathode layers. The electrolyte, considered a relatively mild material from a chemical viewpoint, is contained in “blotter like” separators inside the cells.

The POWER SECTION of a practical Fuel Cell, shown in the insert to Figure 1, needs to generate electric power at reasonable voltage and current outputs. The production of 1400 amps at 155 volts DC requires an assembly of 256 cells. This assembly is called a Fuel Cell stack and, in case of the UTC Power PureCell 200PC25 Model C, is a package approximately 36 inches (0.9 m) square and 9.5 feet (2.9 m) tall.

Since various cell stacks operates from 65°C to over 800°C thermal management systems within the power plant provides a means: to heat the unit to operating temperature during startup, to extract heat during operation, and to provide a thermal source for Combines Heat and Power HP (CHP) applications.

High temperature Fuel Cell technologies typically use an internal reforming process where the source fuel is introduced directly to the anode plate. Lower temperature Fuel Cells rely upon an integrated fuel processor that converts the source fuel into a hydrogen rich reformate needed by the cell stack.

The cell stack produces variable voltage direct current due to its battery-like characteristic of lower voltage at higher current draws. The POWER CONDITIONER section, shown in the inset of Figure 1, converts this variable DC from the cell stack into a very high quality AC electricity for customer and/or grid use. Also integrated in the Power Conditioner are computerized controls and protective interlocks to assure that the power plant operates smoothly during any condition and properly interfaces with the site’s electrical network during any abnormalities caused by customer or grid events.

Depending upon the Fuel Cell technology incorporated into a power plant, the process converts from 30 to 49 percent of the fuel’s energy to electricity for customer or grid use. Also, between 30 and 40 percent of the fuel input can be recovered as useful thermal energy for site use, such as for water heating, space heating, and even air conditioning.

The Fuel Cell power plant is self contained, capable of fully automatic startup and operation, and of maintaining an appropriate grid/customer interface under all conditions. Required Quarterly Maintenance can actually be done while the unit is operating. While the cell stack is an important part, it becomes just one of a number of other system components that operate together in a fully integrated, unattended manner.

Indeed, a fully operational and automatic Fuel Cell power plant is an extremely impressive mix of chemical, mechanical, electrical, control, computer, and manufacturing engineering. In fact, if an example were to be selected to epitomize a leading edge state of multi-disciplined engineering, a Fuel Cell power plant would be one of the best examples today!