Characterization of Thermophotovoltaics and Materials for High-Temperature Thermal Energy Storage

Abstract

To achieve a decarbonized electricity system, energy storage capacity must grow significantly to mitigate the intermittency of wind and solar energy. Thermal energy grid storage (TEGS) can meet the low cost per energy needed for wind and solar paired with storage to be cost-competitive with fossil fuels. The TEGS concept uses thermophotovoltaics (TPVs) to convert stored heat back to electricity. TPVs have long had the potential to reach high efficiencies, but experimental demonstrations of devices have not met these predictions. Prior work has largely focused on ~0.5 – 0.75 eV devices paired with emitter temperatures <1300 °C. Operating at higher emitter temperature is advantageous because it has the potential to boost TPV efficiency as well as power density, which lowers costs. However, the operation and characterization of devices under >2000 °C light sources and at view factors required to achieve high power densities presents many challenges.

In this thesis, we characterize high-bandgap 1.4/1.2 eV and 1.2/1.0 eV two-junction devices for TEGS. First, we integrate and test devices in a graphite cavity system and discuss challenges for high-temperature systems. Next, we demonstrate the devices with a tungsten emitter across a temperature range of 1900 – 2400 °C. By reflecting ~93% of unusable sub-bandgap light, a 1.4/1.2 eV device reached an efficiency of 41.1%±1% under a 2400 °C tungsten emitter at an electric power density of 2.39 W/cm². A 1.2/1.0 eV device reached an efficiency of 39.3%±1% under a 2127 °C emitter at an electric power density of 1.8 W/cm². Through the combination of high bandgaps paired with high emitter temperatures, multiple junctions, and high back surface reflectivity, devices reached efficiencies up to 9.1 percentage points higher than prior work.

In the next part of this thesis, we develop a testing platform for the characterization of TPVs at high power density. By reaching a view factor of 0.36 from the cell to the emitter and using a carbon/carbon emitter, this setup tested 1.2/1.0 eV devices under a maximum irradiance of 92.7 W/cm² with a 2350 °C emitter producing electric power densities of 8.26 W/cm². Using this calorimetry platform, we measured a peak efficiency of 38.7%±1.4% under a 2250 °C emitter and at an electric power density of 6.73 W/cm².

This work addresses a number of high-temperature/high-irradiance TPV testing challenges and demonstrates the viability of TPVs to be both an efficient and power-dense heat engine. This can enable grid-scale thermal energy storage and can be used in a variety of applications including portable or stationary power generation and heat recovery. In the last part of this thesis, we address the challenge of creep in graphite components in the TEGS system. We characterize compressive creep rates in graphite insulation and propose mitigation strategies.