Radial Thermoelectrics

Thermoelectric generators are solid-state devices that generate power when subjected to a temperature gradient. Conventional TEGs consist of p- and n-type semiconductor legs sandwiched between two ceramic plates. Upon applying a temperature gradient across the device, majority carriers diffuse through the semiconductor legs, thereby generating a voltage. TEGs have numerous applications ranging from powering wearable electronics to waste heat recovery in automobiles and power plants.

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 Figure 1: Schematic of a Flat-Plate TEG

A major limitation in the widespread use of this technology for power generation is the high $/W cost comprising material, manufacturing, and heat exchanger costs. We tackle this by focusing on electrically conducting polymers that possess desirable properties such as low thermal conductivity, low cost, and scalable processability compared to inorganic materials [1]. We have developed a novel radial device geometry that has advantages for polymer TEs. Each TE module consists of discs of p- and n-type polymers on a thermally insulating circular substrate. At the center of the disc a channel of warm fluid flows through a heat pipe as the source of heat, which creates a radial temperature gradient across the TE. Many discs can be stacked and connected electrically in series, thus generating an appreciable output voltage. External flow across the device allows for natural convection, thereby eliminating the cold-side HX cost. Figure 2 below shows the proposed radial design.

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Figure 2: Schematic of a Radial TEG

We have developed a numerical thermal and electrical model to present an optimized device geometry for maximum power, maximum efficiency, and low $/W scenarios. The non-linear resistance of the radial device offers a higher power density and greater thermal insulation than traditional rectangular TEs. Another advantage for our thin-film device is the negligible electrical contact resistance in the radial geometry, owing to a larger area of contact. The manufacturing and assembling costs associated with this design offer significant savings as polymers can be screen printed and spin coated via solution processing.  Preliminary results anticipated using dedoped PEDOT:PSS [2] are displayed in Figure 3 below.

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Figure 3: Anticipated Performance of Radial TEG

For a given set of input parameters, the optimum leg length is about 5 cm, resulting in power densities of around 2 mW/cm2. While the equations for maximum power and efficiency are identical to that of a flat-plate, the geometry matching condition for the radial device becomes:

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where tn and tp are the thickness of the n and p-type legs, rout and rin are the outer and inner radii, k is the thermal conductivity and s is the electrical conductivity of the respective leg.

  References

[1] O. Bubnova and X. Crispin, “Towards polymer-based organic thermoelectric generators,” Energy & Environmental Science, vol. 5, pp. 9345-9362, 2012.

[2] G. H. Kim, L. Shao, K. Zhang, and K. P. Pipe, “Engineered doping of organic semiconductors for enhanced thermoelectric efficiency,” Nat Mater, vol. 12, pp. 719-723, 2013.