Solar-driven TEG advances via fabrication, not materials

Solar thermoelectric generators (STEGs) are used for direct conversion of impinging solar and thermal energy into electricity. It can be an alternative to photovoltaic cells in some cases, which can only make use of sunlight. STEGs consist of a hot side and a cold side separated by semiconductor materials, and the temperature difference between them generates electricity through the well-known Seebeck effect, Figure 1.

Figure 1 New, high-efficiency STEGs were engineered with three strategies: black metal technology on the hot side, covering the black metal with a piece of plastic to make a mini-greenhouse, and laser-etched heat sinks on the cold side. Source: University of Rochester / J. Adam Fenster

However, widespread use of STEGs has been limited by their extremely low efficiency, typically under 1 percent; in contrast, standard consumer-grade solar panels have an energy-conversion rate of roughly 20 percent.

A team at the University of Rochester has focused on this low-efficiency challenge, but not by seeking to develop more advanced or esoteric materials. Instead, they used enhanced spectral engineering and thermal management methods in three ways to create a STEG device that generates 15 times more power than previous devices, Figure 2.

Figure 2 Theoretical design of spectral engineering and thermal management strategies for the STEG hot and cold sides: a) Schematic of enhancing STEG output power through hot- and cold-side thermal management. The hot-side thermal management system consists of a W-SSA and a greenhouse chamber to reduce heat loss. The cold-side thermal management system consists of a μ-dissipator, which enhances the cold-side heat dissipation. b) Four cases of STEG with (I) no thermal management, (II) hot-side thermal management, (III) cold-side thermal management, and (IV) both sides thermal management. c) Simulated STEGs’ peak output power with different thermal management strategies. d) Simulated energy flows in the four STEGs. The blue bars represent the energy flow through the STEG. Source: University of Rochester / J. Adam Fenster

By focusing on the hot and cold sides of the device, and by combining better solar energy absorption and heat trapping at the hot side with better heat dissipation at the cold side, they improved efficiency to about 15%.

First, they applied a specialized black metal technology developed in their lab to the hot side of the device, by modifying ordinary tungsten to selectively absorb light at solar wavelengths. They did this by using intense femtosecond laser pulses to etch nanoscale structures into the metal’s surface, which increased its ability to capture energy from sunlight while limiting heat loss at other wavelengths.

Second, the researchers covered the black metal with a piece of plastic to make a mini greenhouse. This minimized the convection and conduction to trap more heat, increasing the temperature on the hot side.

Third, on the cold side of the STEG, they once again used femtosecond laser pulses, but this time on regular aluminum. This created a heat sink with tiny structures that improved the heat dissipation through both radiation and convection, Figure 3. Doing so doubles the cooling performance of a typical aluminum heat dissipator.

Figure 3 A close-up of laser-etched nanostructures on the surface of a solar thermoelectric generator. Source: University of Rochester / J. Adam Fenster

Their tests and analysis separated the three improvement changes they implemented, so they could confirm the impact of each individual enhancement and compare it to their simulations, Figure 4.

Figure 4 Synergistic effect of STEG hot- and cold-side spectral and thermal management: a) Schematics of four cases of STEG with different thermal management strategies. b) STEG weight increases when adding the μ-dissipator, W-SSA, and greenhouse chamber to the TEG. c) STEG power-current curves under 3 suns. d) STEG peak output power under 1–5× solar concentrations. e) STEG power enhancement and TEG average temperatures under 1–5× solar concentrations by applying spectral and thermal management on both sides. f) Photos of LED illumination when powered by the four STEGs in (a). Source: University of Rochester / J. Adam Fenster

It’s obviously not possible to say how successful or practical this STEG approach will be. Nonetheless, it’s interesting to see their focused approach to the weaknesses of STEGs and how they avoided working on the materials-science aspects, but instead concentrated on design improvements. The work is detailed in their paper “15-Fold increase in solar thermoelectric generator performance through femtosecond-laser spectral engineering and thermal management” published in Light: Science & Applications.

Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.

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