Kenneth McEnaney

High-performance flat-panel solar thermoelectric generators with high thermal concentration

The conversion of sunlight into electricity has been dominated by photovoltaic and solar thermal power generation. Photovoltaic cells are deployed widely, mostly as flat panels, whereas solar thermal electricity generation relying on optical concentrators and mechanical heat engines is only seen in large-scale power plants. Here we demonstrate a promising flat-panel solar thermal to electric power conversion technology based on the Seebeck effect and high thermal concentration, thus enabling wider applications. The developed solar thermoelectric generators (STEGs) achieved a peak efficiency of 4.6% under AM1.5G (1 kW m(-2)) conditions. The efficiency is 7-8 times higher than the previously reported best value for a flat-panel STEG, and is enabled by the use of high-performance nanostructured thermoelectric materials and spectrally-selective solar absorbers in an innovative design that exploits high thermal concentration in an evacuated environment. Our work opens up a promising new approach which has the potential to achieve cost-effective conversion of solar energy into electricity.

Studies on the Bi2Te3–Bi2Se3–Bi2S3 system for mid-temperature thermoelectric energy conversion

Bismuth telluride (Bi2Te3) and its alloys have been widely investigated as thermoelectric materials for cooling applications at around room temperature. We report a systematic study on many compounds in the Bi2Te3–Bi2Se3–Bi2S3 system. All the samples were fabricated by high energy ball milling followed by hot pressing. Among the investigated compounds, Bi2Te2S1 shows a peak ZT ∼0.8 at 300 °C and Bi2Se1S2 ∼0.8 at 500 °C. The results show that these compounds can be used for mid-temperature power generation applications. The leg efficiency of thermoelectric conversion for segmented elements based on these n-type materials could potentially reach 12.5% with a cold side at 25 °C and a hot side at 500 °C if appropriate p-type legs are paired, which could compete well with the state-of-the-art n-type materials within the same temperature range, including lead tellurides, lead selenides, lead sulfides, filled-skutterudites, and half Heuslers.

Modeling of concentrating solar thermoelectric generators

The conversion of solar power into electricity is dominated by non-concentrating photovoltaics and concentrating solar thermal systems. Recently, it has been shown that solar thermoelectric generators (STEGs) are a viable alternative in the non-concentrating regime. This paper addresses the possibility of STEGs being used as the power block in concentrating solar power systems. STEG power blocks have no moving parts, they are scalable, and they eliminate the need for an external traditional thermomechanical generator, such as a steam turbine or Stirling engine. Using existing skutterudite and bismuth telluride materials, concentrating STEGs can have efficiencies exceeding 10% based on a geometric optical concentration ratio of 45.

Modeling of thin-film solar thermoelectric generators

Recent advances in solar thermoelectric generator (STEG) performance have raised their prospect as a potential technology to convert solar energy into electricity. This paper presents an analysis of thin-film STEGs. Properties and geometries of the devices are lumped into two parameters which are optimized to guide device design. The predicted efficiencies of thin-film STEGs are comparable to those of existing STEG configurations built on bulk materials.

Thermoelectric energy conversion using nanostructured materials

High performance thermoelectric materials in a wide range of temperatures are essential to broaden the application spectrum of thermoelectric devices. This paper presents experiments on the power and efficiency characteristics of lowand mid-temperature thermoelectric materials. We show that as long as an appreciable temperature difference can be created over a short thermoelectric leg, good power output can be achieved. For a mid-temperature n-type doped skutterudite material an efficiency of over 11% at a temperature difference of 600 °C could be achieved. Besides the improvement of thermoelectric materials, device optimization is a crucial factor for efficient heat-to-electric power conversion and one of the key challenges is how to create a large temperature across a thermoelectric generator especially in the case of a dilute incident heat flux. For the solar application of thermoelectrics we investigated the concept of large thermal heat flux concentration to optimize the operating temperature for highest solar thermoelectric generator efficiency. A solar-to-electric power conversion efficiency of ~5% could be demonstrated. Solar thermoelectric generators with a large thermal concentration which minimizes the amount of thermoelectric nanostrucutured bulk material shows great potential to enable cost-effective electrical power generation from the sun.

The Promise of Nanocomposite Thermoelectric Materials

The concept of using nanocomposite thermoelectric materials in bulk form for practical applications is presented. Laboratory studies have shown the possibilities of nanostructures to yield large reductions in the thermal conductivity while at the same time increasing the power factor. Theoretical studies have suggested that structural ordering in nano-systems is not necessary for the enhancement of ZT, leading to the idea of using nanocomposites as a practical scale-up technology for making bulk thermoelectric materials with enhanced ZT values. Specific examples are presented of nanocomposite thermoelectric materials developed by our group based on the familiar silicon germanium system, showing enhanced thermoelectric performance through nano-structuring.

Exceeding Solar Cell Efficiency Limit by Thermal Upconversion of Low-Energy Photons

We present a conceptual design of a new thermo-photovoltaic solar power conversion system with directionally- and spectrally-selective properties that enables the Shockley-Queisser efficiency limit to be exceeded via thermal upconversion of below-bandgap photons.