Heat as an Energy Source
This series provides a look at different phenomena that can produce electrical energy. In this article, the sixth in our series, we’ll discuss how heat produces thermoelectricity through the Seebeck Effect and the Peltier Effect.
Heat can be used to generate electric potential energy. Thermoelectricity is electrical energy produced by the action of heat. The two thermoelectric effects are the Seebeck effect and the Peltier effect.
The Seebeck Effect
The Seebeck effect occurs when a temperature difference between two joined electrical conductors consisting of different metals produces a voltage difference. The Seebeck effect occurs when the junction of two dissimilar metals with mobile charges is heated, and an electrical voltage occurs between the two open ends. This voltage is directly proportional to the temperature difference between the ambient temperature and the heat source. See Figure 1. The Seebeck effect is also referred to as the thermoelectric effect. The Seebeck effect occurs in liquid and solid materials. The materials include metals and semiconductors.
Figure 1. The Seebeck effect occurs when the junction of two dissimilar metals is heated. This produces a voltage difference between the two open ends.
The Peltier Effect
The Peltier effect causes heat to be absorbed or emitted at the junctions of two different metals as electrons pass through the junctions. Peltier heat is the heat either emitted or absorbed at the junction of these two dissimilar metals. Whether heat is emitted or absorbed at the junctions depends on the direction of electron flow. See Figure 2. Peltier heat is different from Joule heat, which is the heat caused by current flow through a material.
Figure 2. The Peltier effect occurs when the heat is either emitted or absorbed at the junctions of two dissimilar metals, depending on the direction of electron flow.
The Peltier effect is named after the French physicist Jean C. Peltier for his work on cooling and heating effects at the junction of unlike materials.
With the Peltier effect, a net motion of charged particles exists. These charges transport energy. The energy levels differ in the two types of materials. As the charges move from one material to the other, they either give energy to or absorb energy from the junction. Whether a material gives energy or absorbs energy depends on the direction of the electron flow.
Understanding Thermionic Emissions
The emission of electrons from a liquid or solid material depending on the material's heat energy is known as thermionic emission. In 1883, Thomas Edison, while working on the development of the incandescent light bulb, inserted a metal plate close to the filament of the bulb. He then connected a meter to the plate and to the positive terminal of a battery. The negative terminal of the battery was connected to the bulb filament. Edison detected a current flow when the filament was heated. This surprised Edison because the circuit appeared open since the metal plate was not in contact with the filament. This phenomenon is referred to as the thermionic (Edison) effect. See Figure 3.
Figure 3. The thermionic (Edison) effect occurs when metal is heated, and electrons are emitted into space.
Thermionic emission of electrons can be compared to boiling water. When water is placed in a container and heated, steam is given off. As the temperature rises to the boiling point, the amount of steam increases. If a metal is heated to the point of being red or white hot, electrons are emitted (boiled off) into space.
Most metals have many free electrons. Although these electrons are designated as free, they are only free to move from atom to atom within the lattice structure of the metal. When the surface of a solid is in contact with a liquid or gas, a thin film develops on the surface of the solid. Under normal conditions, this film prevents the electrons from escaping and is referred to as the surface barrier. Theoretically, in a perfect vacuum, electrons would not be bound to the metal. Because a perfect vacuum cannot be achieved, the surface barrier continues to exist.
Energy must be supplied to an electron for it to escape into a vacuum. This escape energy is called the work function of the particular metal. The unit of measure of the work function is the electron volt (eV). For example, the amount of kinetic energy required for an electron to escape from tungsten, the element used in an incandescent light bulb, is 4.53 eV. Tungsten produces approximately 7 mA per watt when heated to 2227°C. Other materials, such as the rare earth oxides, are more efficient at producing free electrons in a vacuum.
Thermoelectric Device Applications
The interaction of heat and electricity has many practical applications in modern technology. Thermal energy is used to generate electrical energy for many diverse purposes. Simultaneously, electron flow can generate heat or cause a cooling effect. Many thermoelectric applications use a thermocouple.
A thermocouple is a temperature sensor that consists of two dissimilar metals joined at the end where the heat is measured and produces a voltage output at the other end proportional to the measured temperature. Thermocouples are Seebeck devices. See Figure 4. If heat is applied to the junction, the voltage increases across the open ends. If the junction is cooled, the voltage decreases across the open ends. Thermocouples are useful in taking temperature measurements.
Figure 4. A thermocouple uses two dissimilar metals connected at a welded junction.
Porcelain insulation beads are used with some thermocouples so that they can be used for high-temperature applications. Thermocouples constructed for use as temperature probes consist of an inner conductor of metal welded at its end to an outer tube made of another kind of metal. Usually, a protective coating covers the probe to protect it from the environments in which it is used.
Thermocouples are designed for specific applications. Some thermocouples are designed to be immersed in liquids, while others are designed for use in air. Others may be designed to operate in particular types of gases. The thermocouple head may be threaded for use in a tapped hole (well). A variety of other attachment methods are also available.
Different combinations of dissimilar metals are used in thermo-couples. These metals include tungsten, platinum, rhodium, constantan, iron, copper, and alloys such as Alumel® (nickel-aluminum). Thermocouples are listed with an alphabetical code to designate the combination of metal used. For example, a J-type thermocouple is made of iron/constantan, and a T-type thermocouple is made of copper/constantan. A different potential per temperature degree is generated by each unique combination.
The most common application of thermoelectricity is the measurement of temperature. Thermocouples and thermopiles can be calibrated to measure a wide range of temperatures in very small increments, exceeding the performance capabilities of traditional mercury and alcohol thermometers. A thermopile is an array of thermocouples connected in series, parallel, or series/parallel combination to provide a higher voltage and/or current output than an individual thermocouple. The small voltage and current outputs of each thermocouple in a thermopile are added together, permitting a greater response to a given temperature change. For example, an antimony/bismuth thermopile with its unheated ends kept at a constant temperature can detect temperature changes of a hundred-millionth of a degree. Single temperature probes are commonly used to measure temperatures from –150°C to 1000°C. Specifications are available for thermocouples that measure temperatures as low as –200°C and as high as 1800°C.
A thermopile can be used as an electrical source to switch on a gas furnace's main gas valve. A thermocouple is an example of a Seebeck device. See Figure 5. The thermopile is located in the heat of the pilot light, where it can generate the necessary voltage difference to activate the main gas valve. If the pilot light goes out, the power is removed from the control circuit so that the main gas valve cannot be activated. This provides a safety feature in that the pilot light must be burning when the thermostat calls for heat, guaranteeing that gas supplied to the burner will be ignited.
Figure 5. A thermopile can be used for activating the main gas valve on a furnace
Another device used in electrical and electronic circuits is the thermistor. A thermistor is a device that changes resistance in response to a change in temperature. Some thermistors are small enough to fit through the eye of a needle. Most thermistors are made of semiconductor materials. Manganese and nickel oxides are often used for thermistors. Thermistors are manufactured with positive and negative temperature coefficients and may be directly or indirectly heated. See Figure 6.