A new physical phenomenon was discovered by the scientists of our laboratory while studying high-temperature electrical properties of rare-earth semiconductors. They noticed that a sample generated spontaneous voltage while being uniformly heated. A collective process of rare-earth ions (Sm) valence changing lies in the basis of the effect which is accompanied by an abrupt increase of free electrons number. We produced and tested a model of a thermoelectric generator (TEG) which performed the conversion in the temperature range of 150 - 450°C. Experimentally determined efficiency of that energy converter was ~ 47% at T = 150°C and ~ 30% at T = 450°C. The generated voltage was ~ 0.5 V, weight of the sample was only 10 g. The closest analogue to SmS converter is a classical thermoelectric converter based on the Seebeck effect which is widely used in practice. Efficiency of thermoelectric converters is considered to be the main parameter which characterizes their quality. According to this parameter even the first model sample of our converter exceeds the best among existing TEGs in 3 - 4 times.
It should be noted that theory and practice of classical thermoelectric energy conversion are already obsolete and haven’t show any significant progress in the last. The limits for a new principle to be improved are not clear yet though the results are sufficient enough to initiate the development of a commercial generator. Such type of generators will be designed for all areas which use TEGs. They will have such unique qualities as complete autonomy, high reliability, simplicity of operation, durability and small size. There are no moving parts in these converters and it is not needed to create large temperature gradients. It simplifies the production technology and makes it cheaper than thermoelectric or other energy sources.
It is economically proved that TEG can be highly competitive when it reaches 15% efficiency (actual numbers are less than 10%). The proposed converter allows to give a 2 or 3-fold increase, now its efficiency is nearly 25%. It affords ground for affirming that TEG will be effectively applied in many fields of technology. There can be made a solar TEG, TEG on fossil fuel, reactor and isotope TEG (working material of a converter has record radiation resistance among the semiconductors). Due to relatively low operating temperature of the converter it can be used for utilization of heat losses (internal combustion engines, garbage burning, nuclear waste, etc.), in aerospace objects, automotive, shipbuilding, petroleum industry and other areas where it is necessary to have independent sources of electricity.
Operating principle of a thermoelement is based on generation effect of electromotive force while heating semiconductor material based on samarium sulphide (SmS) in the absence of a temperature gradient. Structurally SmS TEG can be designed and manufactured in two versions. Schematically they are shown below: radial (Fig. a), and flat (Fig. b). Radial version is much more functional whereas flat is more technological.
Construction of thermoelements. Thermoelectric converter consists of:
- Solid metal body (heat storage). It is used to transfer heat from a heat-source to converting elements, to maintain a converting element in operating thermal conditions when temperature fluctuates, to protect a transforming element from external factors (1);
- Converting elements of single crystalline or polycrystalline SmS (it is also possible to use polycrystalline film) doped with donor impurities in the direction of the electrodes (2);
- Metal electrodes (3).
A figure below shows the curve of voltage generation based on the thermovoltaic effect in SmS while heating a model of thermoelectric element in a steady state (in this experiment heating was carried out for more than 5 hours).
Thermoelectric generator (TEG) based on SmS works on a completely new principle. Presence of the tensoresistive and thermovoltaic effects in SmS is caused by unique physical and electrical properties of SmS.
Features of semiconductor SmS:
- Unique semiconductor rare earth material with pronounced phase transition “semiconductor-metal” (6,5 Kbar)
- Possible high level of electrons concentration in conduction band is 1019-1021cm-3
- Isotropy of physical properties allows to use polycrystalline phase
- Temperature coefficient of linear expansion is suitable for deposition on surface of different materials/substrates
- High radiation and magnetic hardness (long life-time) allows to use devices based on SmS in extreme conditions.
- High melting (2300°C) and operating temperature (-100÷400°C)
Existing analogues need a temperature gradient to operate. TEG based on SmS does not require it since it operates at uniform heating.
The table visually demonstrates the advantages of thermoelectric concerter (TEC) based on the thermovoltaic effect in comparison with TEC based on the Seebeck effect.
|Ordinary TEC (Seebeck)||Parameter||SmS TEC|
|<0,2||Power density of a generator, W/g||до 1,8|
|<0,1||Voltage of a single element V||до 5|
|0,2-2||Internal resistance, Ohm||0,2-2|
|до 18||Efficiency, %||30-40|
Nowadays there is a growing interest in the use of thermoelectric generator modules in domestic devices. First of all, it concerns the possibility of low-power supply to consumers of electricity from existing sources of heat. TEG allows to receive electricity from any source of heat and, consequently, generation occurs due to transformation of collateral types of energy into electric energy (charging of mobile devices from domestic heat sources). Energy Harvesting solutions often allow to refuse power battery.
Even highly effective internal combustion engines can convert only a third of fuel energy into mechanical, which provides car movement. Nearly 60% of generated energy is spent for heat losses that are uniformly distributed among exhaust gases and a cooling liquid. When in a passenger compartment required current is generated not by a generator but directly from energy of heat output there occur additional opportunities to reduce fuel consumption.
- Autonomous ground and underground remote systems
Soilthermoelectric generators were designed to provide power to small autonomous ground and underground remote systems that include a variety of sensors and communication devices.
- Petroleum industry, meteorology, navigation, agriculture, army, home consumption
Thermoelectric generator on fossil fuel can be used to supply heat and electricity to autonomous objects of the listed sectors. As a heat source they use combustion products of solid, liquid and gaseous fuel.
- Nuclear reactors
Currently nuclear-power engineering is looking for ways to improve the effectiveness and efficiency of power plants. Reactor thermoelectric generators (RTEG) convert thermal fission energy into electrical and allow to obtain high efficiency and power density at high efficiency, reliability and compactness.
At present the existing model of thermoelectric generator operates by the Seebeck effect, discovered in the early 19th century and named after its discoverer. Gist of the effect lies in occurrence of e.m.f. in a closed circuit that consists of series-connected dissimilar conductors, contacts of which are of different temperatures. Thus, when temperature difference occurs across the ends of the sample, we can see the process of electricity generation whereat the magnitude of this difference influences the efficiency of these devices.
The key difference between the analogues and the proposed solution is that voltage generation of the latter occurs due to a qualitatively new physical phenomenon, called thermovoltaic effect. The principal difference between the observed effect in samarium sulfide from the classical Seebeck effect is that the conversion of thermal energy into electrical one occurs at a uniform heating of the sample, i.e. in the absence of the temperature difference.