MODULE DESIGN

Thermoelectric phenomena arises out of the intercoupled electrical and thermal currents in a material as illustrated in this animation:

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TXL uses thermoelements from the bismuth telluride family of thermoelectric materials. These materials provide the highest known conversion efficiencies in the temperature range of 0 degrees C to +200 degrees C. The n-type and p-type thermoelements are doped so as to provide well matched thermoelectric properties of thermopower, electrical conductivity and thermal conductivity. A two-element generator is constructed using an n-type thermoelectric element (or thermoelement)in electrical series and thermal parallel with a p-type thermoelement.


The electrical connections at the top and bottom of the thermoelements are metallic conductors, generally copper. In order to give


mechanical support, a white ceramic top made out of alumina is often used on the top and bottom sides of a module. Each thermoelement generates a relatively low voltage per degree of temperature difference. For the thermoelements in TXL's TEG modules, the generated voltage (or thermopower, S) per thermoelement is approximately 215 microvolts per degree C. In order to obtain higher voltages, it is common to connect many pairs of thermoelements in electrical series as illustrated in this 254 element "topless" module, where a single n-p thermocouple is highlighted in red.


The formula for the no load, or open-circuit, generated voltage is VOC=nS(dT), where n is the total number of series connected thermoelectric elements, S is the thermopower and dT is temperature difference in C across the module. So, for a 254 element module as depicted above, there is 6.5 mV of voltage generated for each degree of delta T across the module. To generate higher voltages,


either use a module with more elements or place a higher dT across the module.

TXL Group sells TEGs that are constructed from melt drawn ingots of bismuth telluride alloys. The "quality index" for the n-type and p-type thermoelements is given by

where T is the absolute temperature in Kelvin and sigma and kappa are respectively, electrical and thermal conductivity. There are two loss components for a thermoelectric module. Joule heating losses are the I2R losses that occur as electrical current passes through the series connected thermoelements. Diffusive heating losses reflect the heat energy flux that passes through the module without being converted to electricity.

There are many trade-offs in module design. For a given total area of module, having many series connected elements allows the generation of higher voltages, but at the expense of higher internal electrical resistance, which limits the power delivery capacity of the module and represents a loss component. Having long thermoelements allows isolation of the heat source from the heat sink but at the expense of higher electrical resistance.

TXL Group offers TEGs that have been tailor made for electrical generation from environmental heat sources.

© 2015 TXL-Group, Inc.