Solar thermoelectric generators: Pushing the efficiency up

Concentrated solar thermoelectric generators offer an intriguing alternative to wind turbines and photovoltaic modules for the production of electricity from renewable sources^1,2. Such thermoelectric generators are solid-state heat engines³, directly converting the heat from sunlight to electrical power by exploiting the large temperature difference that develops across a thermoelectric generator under concentrated solar irradiation. While the solar thermoelectric generator literature dates back to 1888, it has been challenging to realize efficient devices, and a modest efficiency value of 3.4% under 50-fold concentration achieved in the mid-1950s remained a high water mark until this century⁴. However, a flurry of recent activity in solar thermoelectric generators has been inspired by improvements in thermoelectric materials, including advanced models of system performance, optical design optimization, and occasional bench-top demonstrations with efficiencies of 3–5% (refs 1,2,5,​6,​7). Writing in Nature Energy, Gang Chen, Zhifeng Ren and colleagues at MIT and the University of Houston now report on the design, construction, and testing of a 7.4%-efficient concentrated solar thermoelectric generator utilizing the segmented leg geometry of Bi₂Te₃ and CoSb₃ that operates with a 600 °C hot side temperature⁸.

The researchers have implemented innovations at both the thermoelectric unicouple and system level. The transition from measurements of individual thermoelectric properties to fabrication of a device is highly nontrivial. Critical challenges in this transition include mitigating thermomechanical stresses arising from temperature gradients and developing non-reactive interconnects and bonds that retain their desired electrical and mechanical properties upon cycling. Chen, Ren and colleagues not only address these challenges using state-of-the-art materials, but do so in an unusually transparent way. Optimized ingots of Bi₂Te₃ and CoSb₃ were fabricated in-house, metallized and bonded into a unicouple (a single pair of n- and p-type legs). A spectrally selective absorber was then bonded onto the unicouple and integrated into a custom optical test-bed equipped with a ∼200 sun source. A schematic illustration of the device is shown in Fig. 1a. Test measurements under such illumination require care, as the optical flux at high concentration must be well-calibrated and thermal stresses during cycling can destroy the generator.