Thermoelectric energy conversion
In thermoelectric devices, direct and reverse conversion of thermal energy to electricity is carried out based on the effects of Seebeck, Pelt and Thomson. These effects are manifested in the presence of a potential difference due to the diffusion of current carriers (electrons and holes) when creating a temperature gradient in it, which is used in thermoelectric power generators, and, conversely, in the generation or absorption of heat when moving in such a structure of current carriers. The latter has found application in refrigeration appliances.
In thermoelectric generators (TEGs), organic or nuclear fuel, radioactive isotopes, the scattered body of the exhaust gases of internal combustion engines, industrial plants, etc. can be used as heat sources.
Despite the relatively low efficiency of thermoelectric energy conversion, which currently amounts to 5-8%, due to the absence of moving parts, noiselessness and reliability, allowing such systems to operate in maintenance-free mode for a long service life, which can reach decades, TEGs have found their application when creating backup or emergency sources of electricity in areas of decentralized electricity supply, in particular in the Far North, fossil fuel generators for protection of pipelines against corrosion (cathodic protection stations) and power supply to gas distribution points. To date, there is virtually no alternative to such generators in deep space exploration. Launched in 1977, 2 Voyager spacecraft with radioisotope thermoelectric generators (RTGs) on board, having successfully studied the distant planets of the solar system, are currently continuing to transmit data for the study of transition regions between the solar and interstellar plasma, being the most distant, long and productive space objects created by human hands.
Currently, new more efficient radioisotope thermoelectric generators with a greater specific electric power and service life are being developed for future space programs.
The successful application of thermoelectric energy conversion in space systems, the high reliability of the RTG, the huge amount of heat dissipated in the atmosphere, as well as the emerging global trend to improve energy efficiency and environmental friendliness of technology, prompt researchers to expand the scope of thermoelectric generators, to search for and develop new, more efficient thermoelectric materials, optimization of design and technological solutions, reducing the cost of such systems. In 2006, for example, to study Pluto and its satellite Charon, the New Frontiers automatic interplanetary station with RTGs on board was launched.
One of the areas of work of FMN Laboratory in this area has been the development of thermoelectric generators that convert the dissipated heat of transport power plants and industrial enterprises into electricity.
In 2016, together with the Conventional Engines Department, BMSTU, the team conducted a project to develop an experimental model of an electric power source with direct heat conversion for transport systems for various purposes. The project aimed to increase engine efficiency by utilizing a portion of the thermal energy generated by the exhaust gas, which accounts for up to 37% of the energy of combustible fuel. Part of this energy can be converted into useful work by installing a thermoelectric generator in the exhaust system, which can increase its energy efficiency, reduce fuel consumption by up to 7%, and in some cases abandon the standard generator.
Within the framework of the project, a mathematical model of TEG for internal combustion engines was developed, taking into account the combination of hydraulic, thermal, electrical and mechanical processes in a power plant. The model allowed the calculation of generators with both air and water cooling, taking into account the cost of electric power for the operation of the control electronics. This allowed considering the inverse effect of TEG on the internal combustion engine due to the creation of hydraulic resistance in the exhaust gas channel, select the most efficient heat exchanger designs for various types of internal combustion engines including stationary installations.
An experimental bench has been developed and manufactured that allowing determining the parameters of the mathematical model and verify it, as well as investigating the features of work and refining the thermoelectric generator when installing it on various engines. The hub power stand and load device included in the stand allow testing TEG as a part of the vehicle, simulating in laboratory conditions various modes of vehicle movement. Designed and manufactured TEG mockups for cars and trucks with power: up to 500 W and up to 1 kW.
It is worth noting that the development of automotive thermoelectric generators is conducted by almost all the world's largest automakers, including Ford, GM, Toyota, BMW, Mercedes. At the same time, there are currently no serial samples of such generators, which is due to the need to resolve many technical inconsistencies in the design of effective systems, such as providing, in limited dimensions, a simultaneously intense heat flux through thermoelements and low hydraulic resistance. The resolution of these contradictions requires a complex whole set of energy conversion processes in such a generator.
Within the framework of the completed project, several ways were suggested to overcome the conflict between the positive and negative effects of TEG on ICE; a methodology was developed for the rational design of the heat exchanger design, and individual design and technological solutions were developed that increase the efficiency of installing automobile TEGs, including the use of a heat exchanger with variable geometry of the ribs to reduce resistance at high exhaust speeds and increased heat flux at low speeds.
In addition to the development of complete thermoelectric energy conversion systems, FMN Laboratory also develops design and technological solutions to improve the efficiency and reliability of thermoelectric modules for both refrigeration and generator purposes. The team develops methods for measuring the physicomechanical properties of semiconductor thermoelectric materials, thermoelements and thermoelectric batteries, and works on the methods for monitoring the manufacturing process, including the assessment of reliability indicators.