Chapter 2. Wearable solar thermoelectric based on Bi-Te based ink printing technique
2.5 Application of wearable solar thermoelectric generator
2.5.2 Flexibility of the W-STEG
Figure 2.25 demonstrates the internal resistance change for the bending test to see the effect of parylene C. The bending test was repeated for 150 cycles, and the resistance of a single TE leg was measured at a bending radius of 12 mm to 6 mm. The data showed a notable difference in electrical resistance up to 150 cycles depending on the presence of the parylene C. The resistance of the BiTe legs without parylene C showed a significant increase of electrical resistance by 28% at 12 mm and 141% at 10 mm, and eventually, the BiTe leg failed to withstand the repeated deformation at 8 mm. However, for the sample with parylene C, there is a low variation of 2.9% at 6 mm and 1.5% at 12 mm. From these results, it can be deduced that although parylene C is as thin as 500 nm, it strongly reduces strain and therefore contributing to the minimization of tensile and compressive strain of BiTe leg.
Figure 2.26 is a schematic illustration showing a mechanism which shows the improvement of flexibility by parylene coating. As seen in the cross-sectional SEM image of the BiTe leg (Figure 2.27), microcracks are present on the surface of BiTe. With tensile stress applied to the surface, the microcrack ultimately destroys the linkage between the BiTe surface. However, with Parylene C coating of the BiTe surface, the microcracks are filled by Parylene C. The parylene C coating enables the stress to be spread evenly, therefore improving the flexibility of the substrate.
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Fig. 2.25. Internal resistance stability for repeated bending cycle along the different bending radius.
Fig. 2.26. The schematic illustration of the stress during bending cycle with / without parylene C.
Each stress line shows the stress concentration and stress relaxation.
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Fig. 2.27. The cross-sectional SEM image of the parylene C coated TE legs, which shows the micro crack filling.
2.5.3 Application of the W-STEG
The W-STEG of this study can be easily attached to various exposed surfaces such as clothes for human bodies and windows for buildings. W-STEG was tested in a real environment to see the real- world output voltage produced by the sunlight. The test was executed at ambient temperature of 20 °C and a wind speed of 4 m / s. The TE legs were covered to maximize the ΔT between the high and low temperature sides. Under outdoor conditions, the output voltage was measured at 52.3 mV with a ΔT of 18.54 °C, which is very close to the measured output voltage and ΔT using the 1-sun Solar Simulator.
Fig. 2.28. Photograph of the W-STEG attached to clothes and windows.
Figure 2.29 shows the output voltage monitored over time after the W-STEG is exposed to sunlight.
Previous wearable TEGs that are driven by body temperature tend to produce a high output voltage momentarily upon contact with the skin, and the output value decreases over time due to thermal equilibrium.
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Fig. 2.29. Comparison of wearable TEGs application results, voltage and ∆T.
However, this W-STEG maintains the initial output voltage after reaching a steady state within ~ 20 seconds. These results are expected to be used as self-running technology of wearable electronic devices by promoting the application of WTEG in the early stage of development.
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Fig. 2.30. Voltage-time curve showing immediate response to light and retention of the voltage over time.
2.5.4 The optimum temperature difference considering the heat flux
To optimize ΔT, we covered the hot side with PDMS to block convection of air to hot side. For our heat sink material, we used porous-copper foam to actively generate heat flow and promoting convection (Fig. 2.31). It can be expected to increase of the device efficiency by securing the maximized structure of temperature difference considering heat flux.
Fig. 2.31. Schematic illuttration of the strategies to enhance temperature difference.
We need to consider the heat flux and thermal resistance to understand the temperature gradient in the structure. When the heat flows and ΔT is formed through a plane wall, heat transfer occurs by convection from the hot side to cold side through the planar substrate. Heat transfer is indirectly proportional to the length of the thermal circuit. And also, thermal resistance can be expressed as 𝑅𝑡=𝑘𝐴𝑙 . Lower thermal conductivity would result in the increase of resistance which essentially increases the ΔT. The following,
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in terms of structural conditions corresponding to the cross-sectional area and length in the eq. of thermal resistance. Depending on the direction of thermal flow, the cross-sectional area and length can be expressed as follows. When the heat flows in the horizontal direction, the cross-sectional area is relatively small and the length is relatively large as compared with the vertical direction, it caused larger thermal R, and larger ΔT. Therefore, higher heat flux per unit area and thermal resistance contributes to the increase of ΔT which implies that it is more efficient to form a temperature gradient on a planar structure. Based on this principle, the air trapped in the PDMS cover has the effect of increasing the hot-side temperature up to 5 °C as a medium with low thermal conductivity by application of insulation with PDMS cover. In addition, by applying porous copper foam to more actively generate convection, the temperature of the cold side, which is edge side of the device, could be lowered by 5 °C.
Fig. 2.31. Comparison graph of the temperature difference according to wtepwise strategy for increasing temperature difference in a planar substrate.
2.6 Conclusion
In summary, I proposed a new way to surmount the low ΔT problem encountered by general WTEGs.
This is one of the major disadvantages of WTEG technology in which heat-induced from surface of the human body. The proposed wearable solar TEG can generate a high value of ΔT to ~ 21 °C by a locally deposited solar absorber on the substrate. The solar absorber is a five-period consisting of Ti / MgF2
multilayer structure, where each layer designed by optimum dimension so that the structure effectively reduce the reflectance and induce the absorption by interference effects. The thicknesses of each layers
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were 7.3 nm for Ti and 96.5 nm for MgF2 in the multilayer structure of solar absorber. And the absorbance of the structure was 97.53% at the wavelength of 520 nm which has the highest peak of solar irradiance. TE inks for printing were fabricated by mixing ball milled BiTe-based TE powder, sintering aid Sb2Te4 and a glycerol solvent for high viscosity. The TE legs were printed on a flexible substrate, which have similar electrical conductivities of ~ 25,000 S / m, and the Seebeck coefficients of -116.4 and 166.4 μV / K, respectively. The output characteristics of 10 pairs of TE legs with Voc that is exposed to the irradiation condition at AM 1.5G was 55.15 mV and of 4.44 μW. The ΔT value between the hot solar absorber and the cold PI edge is as high as 20.9 °C, the highest ΔT of all wearable TEGs reported to date. In addition, wearable STEGs can easily be mounted on various surfaces exposed to sunlight, such as clothing, windows and exterior walls, and are expected to be useful in a variety of applications, such as self-propelled, wearable electronics.
**Parts of this chapter were published in the article “Wearable solar thermoelectric generator driven by unprecedentedly high temperature difference” Nano Energy 40 (2017) 663–672.
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