Thermoelectrics refers to the phenomena of a temperature difference in a conductive material creating an electric potential. Thermoelectrics encompasses three separate effects, the Seebeck effect, the Peltier effect and the Thomson effect. About 60% of the energy produced in the U.S. is lost in the form of heat and the field of thermoelectrics looks to utilize some of this lost energy.
The Seebeck Effect
The thermoelectric effect was actually discovered by Thomas Seebeck in 1821. He found that a compass needle deflected when placed near a closed loop formed by two dissimilar conductors when the junctions were at different temperatures. The magnitude of this deflection was proportional to the temperature difference and also depended on the conductive material. The Seebeck coefficient was then defined as the open circuit voltage between two points on a conductor where the difference in temperature between the two points is 1 K. Good thermoelectric materials should possess large Seebeck coefficients, high electrical conductivity and low thermal conductivity. High electrical conductivity is necessary to minimize Joule heating, and low thermal conductivity helps to retain heat at the junctions and maintain a large temperature gradient. These three properties were later combined in the so-called figure-of-merit, Z, defined as:
S = Seebeck coefficient of the material (microvolts/K)
σ = the electrical conductivity of the material
λ = the total thermal conductivity of the material.
This equation is more commonly expressed as a dimensionless factor ZT, which is simply Z multiplied by the average temperature Tavg = (T1+T2)/2. Materials with ZT≈1 are considered effective, but values between 3 and 4 are essential to compete with traditional mechanical power generation.
The Peltier Effect
In 1834, Jean-Charles Peltier discovered the calorific effect of an electrical current at the junction of two different metals. He found that heat adsorption or generation at the junction of two different materials depends on the polarity of the current and that reversing this polarity will change the direction of transfer.
The Thompson Effect
In 1851, William Thomson observed the cooling and heating of a conductor resulting from the flow of an electrical current caused by a temperature gradient. The Thomson effect is defined as the rate of heat generated or absorbed in a single current carrying conductor in the presence of a temperature gradient.
There are two types of materials used in thermoelectric power generation, p-type and n-type. P-type materials cause a positive potential to form at the cold side, while n-type materials cause a negative potential to form at the cold side. When the hot side of the n-type and p-type materials are electrically connected, with a load connected across the cold ends, the voltage produced by the Seebeck effect will cause current to flow through the load, generating electrical power.
The main challenge in the search for efficient materials is to optimize the electrical transport while minimizing the thermal conductivity. Current research is focused on the development of advanced thermal electric materials with improved figure-of-merit, Z. One area of research focuses on the introduction of various phonon scattering mechanisms in an attempt to reduce the thermal conductivity without adverse reduction in electrical conductivity. Phonons play a major role in many of the physical properties of solids, including a material's thermal and electrical conductivities. Recent advances in thermoelectric devices show the opportunities offered by the development of complex materials for use in high-efficiency devices.
A major contributor to waste heat is in the transportation sector where only 25% of the fuel's energy ends up as useful energy. Roughly 75% of the energy produced during combustion is lost through the exhaust or engine coolant in the form of heat. Thermoelectrics are ideal for vehicle applications as they are small, have no moving parts, and are relatively efficient at the engine exhaust temperatures that are typically 300-500°C. By utilizing a portion of the lost thermal energy to charge the battery instead of using an alternator, the overall fuel economy of the vehicle can be increased by about 10%.
Commercial & Residential Power Generation
A typical Carnot efficiency limited steam power plant is only 40% efficient, losing most of the available energy in the form of waste heat. A thermoelectrically-assisted steam power plant utilizing this waste heat to produce energy can increase its energy efficiency to about 90%. Thermoelectric systems are ideal for small cogeneration such as in a single family home because they could be small and silent. A small cogeneration plant in the home would produce electricity whenever the heat is needed. The added fuel consumed to produce the electricity has essentially the same energy content as the electricity produced. Also the application of thermoelectric devices to furnaces and other heat dissipating devices in a home can greatly increase the energy efficiency of a home.
An interesting application of thermoelectric power generation is in space exploration. In this application, a radioisotope thermoelectric generator is used to convert radioisotope heat into electricity . In such a device, the heat released by the decay of a radioactive material is converted into electricity by the Seebeck effect using an array of thermocouples. Radioisotope thermoelectric generators have been used to provide power in satellites, space probes and unmanned remote facilities. Radioisotope thermoelectric generators are usually the most desirable power source for unmanned or unmaintained situations needing a few hundred watts or less of power for durations too long for fuel cells, batteries or places where solar cells are not viable.
Electrochemical Deposition of Nanowires
New research is focused on electro-deposition of thermoelectric nano-wires using porous aluminum oxide as a template. In this process, the electrically conducting thermoelectric material is electrodeposited from solution within the empty channels of an electrically insulating alumina template. This yields dense arrays of parallel, cylindrical, high aspect ratio nano-wires allowing for increased electron flow, thus increasing the electrical output of the thermoelectric device.
Super Lattice Improves Thermoelectric Response Times
The Office of Naval Research and the Defense Advanced Research Projects Agency are funding the development of new thermoelectric materials using alternating layers of bismuth and telluride antimony that allow the material to respond 23,000 times faster than existing thermoelectric materials. A super lattice was created that allows electrons to flow freely allowing for faster response times but limits thermal processes.
Advantages vs. Disadvantages
One big advantages of thermoelectrics is that does not have any materials that need to be replenished. It also operates with almost no noise and does not have any moving parts so it can have up to 100,000 hours of steady state operation. Another advantage is that it can conveniently be reversed from heating to cooling. The main disadvantage is its efficiency. Current research is in developing materials that can operate at high temperature gradients and conduct electricity without conducting heat. This research will hopefully lead to thermoelectrics with higher efficiencies.