At the simplest level, thermocells convert heat into electricity. There are many ways of converting heat into electricity that are currently being used, such as that seen at nuclear power plants. Thermocells can be distinguished from the devices currently in use by how the conversion from one form of energy to another is completed. A thermocell is made up of an anode and a cathode, both of which are electrodes. Electrodes are materials that are capable of conducting electricity.
The conversion from heat into electricity that occurs as electrons flow within a thermocell depends on a phenomenon first noticed in the 19th century by a Prussian scientist called Thomas Johann Seebeck. He observed that circuits comprised of two dissimilar metallic conductors will conduct electricity when the corresponding areas where the conductors touch are maintained at different temperatures. As heat is applied to one of the two conductors, the heated electrons flow toward the cooler one. This produces a small, yet measurable, amount of voltage. Although Seebeck mistakenly assumed that the effect he observed was a magnetic one, scientists later determined it to be an electrical one.
The kind of reaction that takes place inside a thermocell is also called a redox, or oxidation and reduction reaction. This type of chemical reaction is characterized by the loss or gain, respectively, of one or more electrons by an atom or molecule. In the case of thermocells, it is the flow of electrons from the heated conductor to the cooler conductor that produces the electrical current, or the heated conductor is losing electrons to the cooler one.
Ordinary cell batteries also produce electrical energy using redox reactions. Sulfates from two dissimilar metals, such as copper and zinc, are brought into contact so that the chemical reaction created causes electrons to flow from one point of contact to another. The electrical current that is generated flows in the opposite direction to the electrons. Currently, this reaction produces electrical energy much more efficiently than a thermocell does.
In spite of present day inefficiencies, a great deal of research is being carried out into the potential uses of thermocells. This is due to the fact that many other forms of energy production still produce large amounts of waste heat. A thermocell is one of the few devices that produce electrical energy from heat, so one of the goals is that thermocells may be used to recover waste heat and convert it into to usable amounts of electrical energy.