Thermoelectricity

Thermoelectric phenomena allow the direct conversion of heat into electricity. Because these phenomena take place in solids and the "working fluid" is the carrier of the electric charge, it is typical for thermoelectric devices:

• simplicity
• operation without chemicals and moving parts
• reliability
• long life

The essential structural element of thermoelectric devices is the so-called thermoelectric pairs (Fig. 1) formed by a semiconductor p-n junction. In principle, their function can be described as follows: if heat is applied to the p-n junction, most of the charge carriers in the semiconductors are redistributed. Electrons move in n-type semiconductors, and positive holes move in p-type semiconductors. These charges are then concentrated at the cooler ends of the semiconductors, between which there is a potential difference. When the circuit thus formed is closed, electrons move through the transition and flow of electric current. Thermoelectric pairs are grouped into larger units, thermoelectric modules and subsequently thermoelectric generators, which can operate as electricity sources. In cases where the heat source will be otherwise unused waste heat, the use of thermoelectric devices can be a benefit and a way to obtain quality electricity. The production of which by conventional conversion methods could be challenging to implement. The use of the obtained thermoelectricity is wide. At present, thermoelectric generators are often used as small sources of autonomous devices operating in hard-to-reach or dangerous areas.

The efficiency of thermoelectric conversion is determined by the materials used. A dimensionless coefficient ZT is used to express its size, which affects the characteristic properties of the semiconductor material used and the effect of operating temperatures (Fig. 2). Commonly available thermoelectric materials have approximately the parameter ZT ≤ 1 and therefore, low efficiency. In low-temperature applications, typically around 5%. Efficiency does not disqualify thermoelectricity in low-power applications. It is a disadvantage in the mass production of electricity. Thermoelectric applications using higher potential heat are acceptable when there are no more suitable ways of converting waste heat. On the contrary, the use of waste heat with temperatures below 140 °C significantly increases the competitiveness of the thermoelectric conversion.

Possible applications

The amount of waste heat produced released uselessly into the environment, especially in the transport and energy industries, is considerable. In connection with the oil crisis in 1973 and 1979, with rising fuel prices and interest in environmentally friendly technologies, interest in thermoelectricity also increased. Efforts have been made to use thermoelectricity from waste heat as a source of electricity. Obtaining electricity is ideal in many cases. Concerning thermoelectric efficiency, the application should be preceded by careful consideration. The thermoelectric system is often used in a so-called parasitic configuration, such as a heat exchanger/generator (Fig. 3). Part of the absorbed heat is converted into electrical energy and the remaining part, except for the heat released to the surroundings, is used to preheat the substance. The relatively low efficiency of thermoelectric conversion is then not so significant. It is possible to produce heat and electricity at the same time, with little or no effect on the heating system’s resulting efficiency. The use of thermoelectricity is an advantage, especially in low-power applications in which there is no source of electricity or the supply of electricity is unstable. One of the promising areas is the use of thermoelectricity in the automotive industry. The goal of automotive thermoelectric generators is to use the fuel’s energy, which is uselessly released through the exhaust pipe and cooling system of the car into the atmosphere. In previous years, several experimental thermoelectric generators were created, the aim of which was to verify the possibilities of autonomous operation of small combustion plants or covering their electricity consumption by thermoelectricity produced from waste heat. Such a system can also be used as a source of electricity.

The idea of using thermoelectric conversion and thermoelectric phenomena as a source of electricity is not new. There are a number of commercially available thermoelectric generators using fossil fuels, capable of reliable long-term operation. The current development, the construction of generators using waste heat, is a long-term trend. The aim is to build autonomous systems, independent of the supply of electricity from the grid. In low-power applications, thermoelectric microgenerators can significantly affect or eliminate the use of chemical sources of electricity. Research and development of thermoelectric materials and thermoelectric generators are concentrated mainly in large industrial countries in Europe, North America, Russia, China and Japan. Research and development focus intensively on automotive thermoelectric generators and energy-harvesting applications. In Japan, efforts are being made to use thermoelectric generators in municipal waste incinerators and industrial incinerators. The aim of the efforts, in general, is to find suitable thermoelectric materials and to market thermoelectric generators in conjunction with conventional equipment commercially. In the long term, thermoelectric technologies are expected to increase the efficiency of existing conversion systems and contribute to fuel savings and reduce harmful emissions.

Examples of assembled thermoelectric generators

• Automatic hot water boiler with a thermoelectric generator
• Fireplace insert with a thermoelectric generator
• Microgenerator (energy-harvesting application)

Automatic hot water boiler with a thermoelectric generator. A simple thermoelectric generator (Fig. 4) using the waste heat of the flue gas of an automatic hot water boiler for pellets with an output of 25 kW was designed and constructed to verify the possibility of using thermoelectricity from waste heat and electricity production. It is an experimental device connected to an existing combustion plant without interfering with its construction. The generator is composed of segments. One segment consists of 4 low-temperature thermoelectric modules. The power characteristics of the generator can be found in Fig. 5 and Fig. 6. Depending on the flue gas temperature, it would be possible to increase the generator’s resulting power by connecting other segments.

Fireplace insert with a thermoelectric generator. In large thermoelectric systems, depending on the thermal conditions, losses can occur that degrade the generator’s resulting power. Therefore, a low power combustion unit (fireplace insert) and a thermoelectric generator were constructed (Fig. 5). It is an experimental device used to detect the occurrence of losses of thermoelectric modules, their measurement and verification of the possibility of their elimination for more efficient thermoelectric power generation. The thermoelectric generator contains 10 low-temperature thermoelectric modules. During the initial measurements, the power of the generator reached 21 W. Thermoelectric modules can be connected in series, in parallel, or a series-parallel combination. The generator itself includes a direct converter with maximum power point tracking (MPPT) technology. The combustion device is equipped with a software-controlled valve, limiting the thermoelectric generator’s temperature dynamics, which prevents damage to the thermoelectric modules due to high temperature.

Test equipment

Thermoelectric generators could be used in applications where it is not possible to use waste heat by more efficient methods: in cases where the mass flow and temperature of the working substance fluctuate, where it is not possible to ensure a stable energy supply with the required parameters or heat sources are small and scattered. Thermoelectric modules in such applications can be cyclically stressed. To test the service life and monitor individual thermoelectric modules’ performance, a measuring stand (Fig. 7) was built for long-term tests in a vacuum.

Implemented research projects related to thermoelectricity

• FSI-S-10-33 Low power polygeneration
• FSI-S-11-7 Selected components of trigeneration processes
• FSI-J-12-1780 Use of thermoelectric generators in microcogeneration
• FSI-S-14-2403 Comprehensive research of combustion equipment and heat utilisation
• FSI-S-17-4531 Increasing the efficiency of modular components of power plants

Download

Applications using thermoelectricity (PDF to download here)

Further information

Possibilities of using thermoelectric phenomena for the production of electricity from waste heat (link to TZB website - info)

Contact

prof. Ing. Jiří Pospíšil, Ph.D.
Energy Institute, FME BUT
e-mail: pospisil.j@fme.vutbr.cz
tel.: +420 541 142 581

Ing. Marian Brázdil, Ph.D.
Energy Institute, FME BUT
e-mail: brazdil@fme.vutbr.cz
tel.: +420 541 142 591