Complete Thermoelectric Generator Performance Measurement
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Complete Thermoelectric Generator Performance Measurement Jan D. Koenig, Ulrike Nussel, Markus Bartel, Uwe Vetter Fraunhofer-Institute Physical Measurement Techniques IPM, Thermoelectric Systems, Heidenhofstraße 8, 79110 Freiburg, Germany
ABSTRACT The standardization of thermoelectric generator (TEG) performance measurement is of a major interest for a usage of them in industrial energy harvesting application. The effects influencing the TEG measurement are regarded and are addressed in this article. The dramatic influences of different measurement parameters are pointed out. The output power dependence on the contact pressure is investigated as a major attempt. The influence of contact pressure on the temperature of the TEG-substrate at the hot side is observed with a high resolution IR camera. Different approaches are discussed in order to obtain the accurate efficiency of a TEG. The heat flux method is influenced by many effects that can falsify the results. The Harman method is used as another approach to determine the figure of merit to calculate the efficiency.
INTRODUCTION Thermoelectricity is classified as one of the most promising technologies to exploit waste heat for the production of electrical energy [1-3]. Thermoelectric generators are being used in increasing numbers to provide electrical power in deep space applications and military, where combinations of their desirable properties outweigh their relatively high cost and low generating efficiency. Nowadays a new demand exists in automotive application to save fuel and reduce CO2 emission. Thermoelectric materials are used in thermoelectric generators for a direct conversion of heat energy into electricity. Compatible materials of n- and p-type semiconductors are used to build up a thermoelectric generator. The material performance e.g. the conversion efficiency of the material depends on the material figure of merit ZM defined by ZM =
α2 λ ⋅ ρM
,
(1)
(α - Seebeck-coefficient or thermopower, ρΜ - specific resistivity of the material, λ - thermal conductivity). New thermoelectric materials with a higher figure of merit are developed at the present and new TEGs are built up with these materials to show the enhanced performance in experiment. Beside the electrical power output the conversion efficiency is of a major interest. The TEG conversion efficiency ηTEG could be determined by the formula
η TEG ≡
Pel (T − TC ) = H ⋅ QH TH
TH + TC −1 2 TH + TC TC + 2 TH
1 + Z TEG 1 + Z TEG
(2)
(Pel – generated electrical output power; QH – input heat flow into the TEG; T - absolute temperature of the hot and cold side respectively). ZTEG describes the TEG figure of merit, which is defined as (α n + α p )2 Z TEG = , (3) K ⋅ Ri where K is the thermal conductance and Ri the internal resistance of the TEG. The internal resistance is the sum of the resistances of the thermoelectric legs, the metal bridges, and the contact resistances. Therefore the TEG figure of merit is smaller any time than the material figure of merit which results directly i
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