The thermoelectric generator is electrically separated from the hot and cold sides by a suitable insulator so that there is no short circuit or leakage of electricity. Due to temperature difference, electron-hole pairs are created at the hot end, absorbing heat in the process. The pairs recombine and reject heat at the cold ends. A voltage potential, the Seebeck voltage which drives the hole/electron flow, is created between the hot and cold ends of the thermoelectric element legs.
The Seekbeck EMF depends only upon the material and temperature. Figure of merit ‘Z’ indicates the efficiency of the thermoelectric couple. It can be expressed as Z=S2.σ/λ, where ‘S’ is Seebeck coefficient, ‘σ’ electrical conductivity and ‘λ’ thermal conductivity.
As a solid-state device without any moving part, a thermoelectric generator can be completely silent and extremely reliable. It can be used for years to provide electrical power. The existing thermoelectric modules are expensive and their conversion efficiency is low in non-concentrated solar energy technologies. In order to improve the performance, the efficiency is to be maximised by selecting the best semiconducting materials with controlled doping. Materials made of heavy atoms like lead and bismuth have low lattice thermal conductivity. Hence compound semiconductors like bismuth telluride (Bi2Te3) and lead telluride are useful thermoelectric materials.
Another factor that reduces thermal conductivity is induction of disorderliness of the atomic arrangement in the crystals. In bismuth telluride, if some atom of bismuth is replaced with antimony or some of tellurium is replaced with selenium, the thermal conductivity reduces by a factor of ten, but the electrical properties are retained. As efficiency is inversely proportional to thermal conductivity (λ), it would increase efficiency by a factor of more than two.
Nanotechnology and its potential quantum-scale synthesis lead to new super thermoelectric materials, which give a higher efficiency due to confinement of electrons to two-dimensional quantum wells while suppressing the thermal conductivity. In today’s commercial thermoelectric modules, efficiency is about 1.0. But emerging nanotechnology in thermoelectric cells has changed the situation. Recently, researchers working on this concept have developed super thermoelectric materials which can achieve efficiency of about 2.4 for a nanoscale structure (made by alternating layers of thermoelectric materials that both enhance Seebeck coefficient and suppress thermal conductivity at room temperature) with conversion efficiency of about 20 per cent.
Concept of hybrid system
The hybrid system integrates PV, thermoelectric and hot-water modules in a composite structure, which can be installed on the rooftop or in an open ground that receives sunlight. The PV module generates electricity directly from sunlight, whereas the thermoelectric module harvests the solar energy which is otherwise wasted in the PV module through heat dissipation and converts it into electricity. The hot-water module cools the PV module, thereby increasing its efficiency. It also provides a temperature gradient for the thermoelectric module to operate. In addition, it supplies hot water for domestic and other uses by recovering the waste heat.
Typically, the energy payback time (EPBT) for a solar thermal system is less than for PV systems. The EPBT of a PV system can be reduced by using it in a hybrid system integrating PV with solar thermal components, such as hot-water tubes and thermoelectric generators. This approach provides a viable solution to significantly increase the overall energy utilisation efficiency, while alleviating the disadvantages of a single approach. A PV thermal collector enables heat harvesting while improving the PV utilisation efficiency by controlling the temperature of PV modules.
Design. Fig. 2 shows the design of a hybrid solar panel utilising PV, thermoelectric and hot-water modules in a multi-layered configuration. The PV surface layer is followed by the thermoelectric layer. The PV cells convert the sun’s electromagnetic radiation into electricity, while the thermoelectric layer converts the sun’s heat into electricity. The thermoelectric layer is bonded to a plastic lumber plate through a functionally-graded material interlayer where hot-water tubes are cast. The functionally graded material layer contains aluminium powder dispersed in a high-density polythelene (HDPE) matrix with a graded microstructure. Water pipelines are cast within the functionally graded material layer to control the panel’s temperature.
The plastic lumber made of recycled polymeric materials provides mechanical support and heat insulation of the building skin. The design has the following advantages:
1. The temperature difference between the PV module and the water tubes provides a considerable temperature gradient within the thermoelectric layer for a higher efficiency of thermoelectric utilisation.
2. The hot-water tubes whose temperature is partially controlled by the flow rate can be directly utilised by water-heating systems for domestic usage.
3. Due to the temperature control on the roof, the room temperature can be significantly reduced and the thermal comfort in the building improved.