Devices and Design

Date: 2015-10-07; view: 465.

Fig.3. An illustration of the manner in which semiconductors are intrinsic at critical temperatures

Despite these developments, the increasing knowledge of semi­conductors or mixed-valence materials does not solve all problems of thermoelectricity, for materials are not an end in themselves; they must be fabricated as thermocouples and then be assembled in fin­ished devices. For example, assemblies of thermoelectric materials must be joined so that contact resistance is not excessive, for this would have the same effect as high internal resistivity of the material and would reduce the efficiency.

Also, above 300 degrees C, thermoelectric materials must be shielded from the air to prevent corrosion of materials and joints. Another aspect of design is the need to mount thermoelectric devices so that they will withstand shock and vibration. One method used for accomplishing this is to apply compressive forces through spring-load­ing.

Other design problems with high priority grow out of a desire to narrow the gap between the efficiency that is theoretically available from known materials and the efficiency that is actually available when these materials are applied in equipment. Materials available today are capable of an efficiency of about 17 percent, but when as­sembled as elements of complete generators, the overall efficiency then becomes about six percent. Much of this loss is due to such factors as the stack losses, represented by the discharge of heat-bear­ing gases from the generator's "chimney", and the fact that some of the energy transferred through the walls of the chimney passes around but not through the thermoelectric elements.

Although continued progress in generator design will reduce losses and increase total efficiency, nuclear reactors seem certain to be much more efficient in thermoelectric applications than conventional heat sources. With nuclear reactors, the heat source can be completely sur­rounded by thermoelectric elements to eliminate stack losses.

An interesting aspect of the efficiency of thermoelectric generators is that it is independent of power rating, which is in contrast to the power-efficiency relation for conventional machines. As Fig.4 shows, small conventional power supplies have an efficiency of roughly five percent, the automobile engine is about 15 percent efficient, and large diesel engines and marine steam turbines have efficiencies of about 20 percent. As the most efficient units, large central station power plants have efficiencies of about 42 percent. At present, the efficiency of today's thermoelectric generators is constant at about six percent re­gardless of rating. Viewed from the standpoint of efficiency only, ther­moelectric devices are thus comparable to conventional power sources in applications up to about 10 horsepower.

By S.J. Angello