(40g) Shaping the Spectrum of Thermal Radiation Using Nanostructured Materials for Efficient Thermophotovoltaic Power Generation

Authors: 
Lenert, A., University of Michigan
By manipulating the structure of materials at the nanoscale, we can tune the spectrum of thermal radiation. In doing so, we can selectively enhance or suppress certain modes linked to either desirable or undesirable properties. Such spectrally-selective transport holds promise for more efficient solar and thermal power generation.

Here, we enhance spectrally-selective radiative transport using nanostructured materials to improve the efficiency of thermophotovoltaics (TPVs). TPVs are solid-state heat engines that convert energy from a high temperature radiant sourceâ??the emitterâ??into excited charge carriers in a semiconductor diodeâ??the cell. TPVs are a promising technology for applications in concentrated solar power and distributed combined heat and power because they are solid-state, scalable and compatible with a variety of high-temperature heat sources. To achieve high efficiencies, it is well known that spectral control over thermal radiation is critical. Nevertheless, several questions remain: 1) What spectral features are most important in TPVs when accounting for parasitic losses and non-idealities?, and 2) How can nanostructured materials be designed to meet the stringent spectral and thermal requirements in TPVs?

To address these questions, we developed radiative transport and energy conversion models to identify the most important spectral features in realistic TPVs. Based on these models, we show that suppressing radiative exchange between the emitter and the cell at energies below the bandgap of the cell (i.e., at long wavelengths) is critical for high efficiency. This is explained by the fact that, at the optimal operating temperature, the emissive power below bandgap is much higher than above the bandgap. Operation at higher temperatures is undesirable because of the increased high-temperature radiative losses and excited charge carrier thermalization in the cell.

Based on these insights, we developed thin-film and nanostructured materials for TPV applications where spectrally-selective transport is based on the interaction of thermal radiation with wavelength-scale structures. The structures are chosen to minimize radiative transport at energies below the bandgap and then optimized to achieve high efficiencies in the presence of realistic losses. By suppressing radiative exchange at low energies, we demonstrate a solar TPV device that is an order of magnitude more efficient than previous solar TPVs and has the potential to exceed the limit of single-junction solar cells. Further, we show how a scalable and thin-film cell design promotes spectral selectivity using a back-side surface reflector to return sub-bandgap radiation back to the hot emitter. Spectroscopy measurements confirm that the reflectance below the bandgap for these thin-film cells is significantly higher than for cells on bulk substrates. Based on these spectral properties we show that the heat engine efficiency can be significantly higher than the state-of-the-art, exceeding 30%.

This work provides a detailed understanding of the relative importance of spectral features in TPVs and offers strategies for how to best engineer thin-film and nanostructured surfaces to shape the spectrum of thermal radiation for TPVs. With integration of thermal storage, TPVs can become an important part of distributed energy systems and may lead to increased adoption of intermittent renewables.