STEG output power enhancement strategies
A STEG generates electrical power when there’s a ΔT across its hot and cold sides. The generated power is expressed as38
$${P}_{{STEG}}={\left({S}_{{TEG}}\varDelta T\right)}^{2}\frac{{R}_{L}}{{\left({R}_{L}+R\right)}^{2}}$$ (1)
where \({S}_{{TEG}}\) is the effective Seebeck coefficient of the TEG, R is the total electrical resistance between the TEG terminals, and \({R}_{L}\) is the load resistance. For a given TE material, an effective way to increase \({P}_{{STEG}}\) is to enlarge the ΔT across it. To achieve this goal, proper spectral and thermal managements at the hot and cold sides are needed.
To predict the effect of the hot- and cold-side thermal management, we conducted a numerical simulation of the heat transfer and thermoelectric effects for a STEG device (see Supplementary Note 1). Starting with a bare STEG attached with an ideal broadband solar absorber (BBA) and a regular metal heat dissipator, we investigated the enhancement of the STEG output power by reducing the inevitable thermal loss at the hot side and increasing the heat dissipation at the cold side (Fig. 1b). Assuming the hot-side thermal management minimizes the radiative loss and reduces the convective heat transfer coefficient by half. Similarly, we assume that the cold-side thermal management maximizes the radiative cooling and doubles the convective heat transfer coefficient. When the absorber converts solar energy to heat, part of the energy is lost due to hot-side radiation and convection. The rest is conducted through the STEG to the cold side, and this portion is utilized for power generation. Figure 1c shows the STEG peak output power for each case, and Fig. 1d shows the corresponding energy flow. The enhanced \({P}_{{STEG}}\) with thermal management is due to more thermal energy utilized by the STEG (represented by the blue bars). To have more thermal energy utilized by the STEG, the hot-side thermal loss needs to be minimized, while the cold-side heat dissipation needs to be maximized.
Hot-side thermal management
As discussed before, the STEG output power is proportional to the ΔT across the TE material. The ΔT can be enhanced by maximizing the solar-thermal energy generation efficiency and minimizing the energy losses at the STEG hot side through effective spectral engineering and thermal management.
Spectral engineering
The manipulation of absorption/emission spectra of a solar absorber is essential for solar-thermal devices to minimize the radiative heat dissipation and maximize the solar-thermal energy conversion efficiency. An ideal solar absorber would exhibit near-complete absorbance within the solar spectrum while maintaining minimal thermal emittance in the IR region, characterizing it as an SSA39. In contrast, nonselective BBAs have inferior solar energy harvesting ability because of their significant radiative heat loss40. The creation of SSA that exhibits selective absorption over the solar spectrum (300–2500 nm) requires careful control of the hybridized surface plasmon resonances in the surface nanostructures. To determine the SSA material and the optimal fs-laser processing parameters, we create SSA on a variety of metals—nickel (Ni), copper (Cu), aluminum (Al), and W—using different laser power, scanning speed, and interline spacing. We measure their spectral absorption/emission (see Supplementary Note 2) using a UV-vis spectrometer and a Fourier transform infrared (FTIR) spectrometer (see details in the “Materials and methods” section), and we evaluate the solar absorption efficiency (\({\eta }_{{abs}}\)) as40
$${\eta }_{{abs}}=\bar{\alpha }-\frac{\bar{\varepsilon }\sigma \left({T}_{{abs}}^{4}-{T}_{{amb}}^{4}\right)}{C\bullet {I}_{{solar}}}$$ (2)
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