THERMO-ELECTRICAL APPROACH

In the focus of parabolic concentrator of SPS multi modules thermoelectric generators (TEG) will be installed. TEG the work of which is based on Seebeck effect present semiconductor thermo pairs and are aimed for direct transformation of heat energy into electric energy [2-3]. Solar energy, focused with the help of solar concentrators, may be heat sources for hot junctions of TEG.

The advantages of TEG are the following: a long term of work, high reliability, stability of parameters, noiselessness, stability to vibration.

The drawbacks of TEG are the following: low relative energetic indices: specific mass from 10 to 15 kg/kW, surface density of power — 10 kW/m2, volume density of power from 200 to 400 kW/m3 and low efficiency of energy transformation up to 8%.

TEG may be made as batteries of silicon-germanium thermoelectric elements (TEE), connected by matrix principle in succession in branches and branches may have parallel connections between themselves. TEE batteries are put into hermetic containers of flat or cylindrical forms and filled with inert gas to avoid oxidation and semiconductor aging. Construction of power electro outlet of TEG must provide simultaneously thermo density and electric isolation from container body, which are complicated technical problems. Cascade connection of TEG makes it possible to raise transformation efficiency up to 13 %. The quantity of electric energy generated by module is directly proportional to the quadrant of difference of temperatures on module junctions. That is why it is important to have maximum high temperature difference.

To reach these results it is necessary, firstly, to provide maximum heat quantity on the hot side of the module and, secondly, to make use of effective radiator on the cold side of the module, for example, with water cooling. Alongside with this, the temperature of hot junction should not be higher than the maximum temperature value allowed for given module. Load resistance should be chosen according to the module resistance. TEG will work in the regime of maximum power at equal resistances.

And TEG will work in the regime of maximum efficiency when the load resistance is approximately equal to 130 % of module resistance.

For practical purposes the following thermoelectric materials are used: alloys based on Bi2Te3 and Bi-Sb for intervals of low temperatures up to 300°C, alloys based on PbSb, PbTe, GeTe, AgSbTe2 for medium temperatures from 300°C to 600°C and alloys based on SnTe and GeSi for high temperatures over 600°C.

The TEG will be made in a special form to be installed in the focus of a parabolic concentrator. The calculated concentrator surface area is 10 m2 for the output electrical power of about 1 kW. One of the technical solution may be the performance of TEG in the form of a cylindrical construction (a quarter of its surface), placed into transparent vacuum glass packet and with internal injected cold water. At the system output direct current is generated.