Thermophotovoltaics

Project Title: Multi-scale modeling and design of an efficient thermophotovoltaic system

Project Dates: September 2021 - May 2026

Collaborators: Minok Park, Kyle Buznitsky, Alina LaPotin, Ashwin Sandeep, Santosh Shanbhogue, Colin Kelsall, Vasillia Zorba, Ahmed Ghoniem

Location: Atomistic Simulation & Energy Research Group, Massachusetts Institute of Technology

Project Summary:

Decarbonizing electricity production is critical for mitigating climate change. While solar and wind are cheap and clean, their intermittency requires supplemental on-demand generation to ensure grid reliability. Current on-demand generators including turbines and engines are designed for specific hydrocarbon fuels like natural gas and diesel. This limits fuel flexibility, a barrier to not only the energy transition, but also energy security and affordability.

Thermophotovoltaics (TPV) offer a promising alternative i.e., a fuel-flexible, solid-state heat engine with high efficiency and power density. TPV cells convert light emitted from a glowing hot object (>1200°C) into electricity using the same physical principles as photovoltaic cells. Importantly, they decouple heat generation from power production – allowing any heat source to supply the emitter. However, TPV systems have never been deployed at the industrial scale because of low performance. There is a large drop in performance going from cells to systems, including low efficiencies, poor power densities, and high costs. This thesis addresses this issue through contributions along the TPV power generation chain.

A techno-economic framework is developed using the levelized cost of electricity (LCOE) to evaluate TPV systems against conventional generators. This framework shows that achieving cost-competitive TPV electricity requires > 5 W/cm2 power density and >5¢/kWh for the cost-of-light input, as heat – but these target metrics are underexplored in the literature and require key technological advances to meet.

To improve TPV power density, an emitter engineering approach leveraging surface texturing to maximize emissivity is presented. Near-black surfaces are created on a variety of emitter materials including refractory metals, nickel superalloys, and carbides. This enables a doubling of TPV power density when using pristine vs. textured emitters, from 2.5 to 5 W/cm2. Further, these emitters retain emissivity of >0.9 after 100 hours at 2000°C, the highest-temperature and longest-duration stability test conducted to date.

Finally, two system designs for TPV power generation are presented, each addressing different grid needs. The first is a thermal energy storage system using cheap day-time electricity to heat graphite to high temperatures, providing baseload power by discharging at night to balance daily generation-demand variation. Such a system produces electricity at 9.44 ¢/kWh, cost-competitive with other baseload power sources. However, using such a system to meet the ~300 hours of peak demand would result in massive overbuilding of storage capacity that sits idle for 95% of the year, making it prohibitively expensive. To efficiently meet peak demand, an ammonia-fired combustor coupled to TPV cells is designed, built, and tested. Ammonia can be stockpiled over months in cheap tanks to meet these short bursts of peak demand. The proof-of-concept prototype achieved the first experimental demonstration of power generation (~1 W/cm2) from low-NOx ammonia combustion (20 ppm @ 15% O2). This is used as an anchor for estimating performance at scale, predicting 40% efficiency, $0.50/W capital cost, and 20¢/kWh levelized cost, competitive with existing natural gas peaker plants.

Overall, this thesis outlines contributions on moving from TPV cells to systems – advances in power density and system-level design enable TPV systems to compete with conventional on-demand heat engines while enabling cleaner electricity generation.