Fuel Cell Principles

Fuel cells are simply devices that convert fuel (such as hydrogen, methane, propane, etc.) directly into D.C. electricity. The process is an electrochemical reaction that is similar to a battery. Unlike the battery though, fuel cells do not store the energy with chemicals internally. Instead, they use a continuous supply of fuel (chemical) from an external storage tank.

The original fuel cell was invented in 1839 by Sir William Robert Grove. It remained a scientific curiosity only until the United Stated National Aeronautics and Space Administration (NASA) developed it for use in the United States space program in the mid-1960's. Fuel cells convert stored energy to electricity with about 60-70% efficiency, with higher efficiency thought to be theoretically possible.

HOW IT WORKS

A fuel cell works on the basic principle that when an element is oxidized chemically combined with oxygen), it releases energy. Most fuel cells involve a simple chemical reaction between two materials, (a fuel and an oxidizer). Various designs operate using many different fuel/oxidizer combinations with the most common being hydrogen and oxygen.

Generally, fuel cells require a catalyst, which is a material that helps the chemical reaction, but is not part of it. There are many types of catalysts, but for this discussion, we will concentrate on something called a "Proton Exchange Membrane" (PEM). This PEM is the catalyst, or magic that does the work.

In a PEM based fuel cell, the hydrogen gas is fed into a sealed chamber, where one wall of the chamber is a PEM membrane coated with a catalyst. The other side of the PEM membrane also, coated with a catalyst typically platinum, is exposed to Oxygen (possibly from air). The membrane allows the small hydrogen proton to pass through, but rejects the larger H2 molecule. When the single hydrogen proton passes through the membrane, it leaves behind an electron. These electrons that are left behind can be collected in a conductor. The concentrated electrons in a conductor caused a potential negative voltage on that conductor due to the excess of electrons (electrons are negatively charged). The backside of the membrane (PEM) is a second chamber that allows oxygen (or air) to reach PEM. When the radical oxygen molecule (O) meets the hydrogen proton (H+), they easily combine to produce OH. (H2O). This combination however requires electrons to make it happen, so a shortage of electrons is created. The needed electrons can be supplied by a second conductor that can supply electrons. (Of course it can only supply electrons if it has some to spare). With the 2 conductors now on the cell, (the hydrogen conductor has an excess of electrons and the oxygen needs some) an electrical potential exists between the 2. The net result is positive for oxygen, and negative for hydrogen.

Note: This discussion assumes an electron current flow from negative to positive to explain the behavior of the cell. It is recognized that the majority of the world follows the 'conventional current flow' that states electric current flows from positive to negative.)

The reaction process is: 4H2 + 4e + 2O2 - 4e = 2H2O = heat (losses) + electrical energy. Since the electrical energy is used as it goes around the loop, the only byproducts of this arrangement are water vapor and heat lost through inefficiency of the cell itself, or about 30% of the power. With a 70% efficiency, this process is significantly more attractive for extracting stored energy than an internal combustion engine that typically extracts only about 20-30% of the stored fuel energy.

This drawing shows a typical Solar Hydrogen based generator and point of use system. PV > electrolyser > H2 storage > point of use. The system shown above has several energy conversions to enable storing energy. The solar panel shown could be any other renewable source, such as wind, or water. The energy source for all the power on Earth originates from the Sun. To make the energy useful, we need to be able to store, concentrate, or move it to a different location. The following section is a description of each element in the energy conversion system and what it does:

  1. Photovoltaic cells convert the Sunlight directly into electricity (usually direct current or DC power). Energy in the form of electricity can be easily moved and concentrated, but not easily stored, so it is moved to the electrolysis system when storage is needed.

  2. Water is needed in the electrolyzer for chemical conversion.

  3. Electrolysis converts the electrical energy into chemical energy in the form of Hydrogen via the electrolyzer. This stage is important, as the Hydrogen can be stored as well as moved, or concentrated. For simpler systems, the Oxygen (O2) that is also broken down from the water (H2O) is simply released back to the atmosphere. To maintain pressure in the system, a pressure regulator is used to limit flow and maintain a higher Oxygen pressure in the electrolyzer.

  4. Hydrogen is a gas, and so storage is accomplished by compressing it and forcing it into a tank to lower the required volume. The electrolysis process can pressurize the tank over a limited range as shown. If higher pressures are needed, it must be compressed by a compressor pump. A one-way valve prevents the pressurized Hydrogen from going back to to the electrolysis system. A pressure regulator on the output side of the Hydrogen tank drops the higher storage pressure to a lower pressure (typ. <10 psi) for usage.

  5. The Hydrogen gas may be used in a variety of ways:

This Hydrogen usage completes the energy cycle, by combining the Hydrogen with Oxygen, thus returning it to water.