Thermogenerator test facility for efficiency determination of thermoelectric energy converters
The most important performance parameter of thermoelectric generators (TEG) is the efficiency, which is the ratio of the heat flowing through and the electric output power. The efficiency, in combination with other technical parameters of a TEG (device figure of merit, effective thermo-power, internal electrical resistance and thermal resistance), can be measured under variable thermal and electric boundary conditions on generator elements of various geometry as well as on complete TEG modules in a test facility developed at DLR. The selectable conditions include a wide field of real operation conditions – large temperature difference, selectable hot and cold side temperature and selectable electric load.
The generator test facility employs a comparative technique for heat flow determination. The heater plate reaches up to 1100 °C, the temperature of the cooled lower plate can be chosen between 5 °C and 100 °C. The TEG is sandwiched within the heat conductive track, thermally connected in series to reference blocks of the same cross-section possessing known heat conductivity. The temperature profile is measured in the reference blocks by thermocouples. Thus, the overall heat flow is determined and the interface temperatures of the generator are extrapolated.
An exchangeable probe head with a specially prepared test specimen allows a similar measurement on functionally graded or segmented single TEG elements. Several thermocouples arranged in line along the temperature gradient over the heat flow track and also inside the specimen allow for recording the temperature and potential profiles not only within the reference blocks but also in the specimen. In this way the electric and thermal interface resistance between reference block and TEG can be separated, thus eliminating and accordingly correcting substantial sources of error in measurement.
The layout of the system equipped with a ceramic heater and optional radiation shielding allows for measuring operation under vacuum, inert gas or on air. Additional mechanical components including linear positioning unit and force sensor allow for TEG testing under variable mechanic pressure. So it is possible to determine the optimal load force for TEG modules in a removable assembly.