Session: 12-16-01: Decarbonization and Renewable Energy

Paper Number: 173497

Flameless Ammonia Combustion Coupled With Thermophotovoltaics for Decarbonized Electricity Generation 

Renewable generation through solar photovoltaics is carbon-free and cheap, but intermittent. Instead of using the sun as the light source for photovoltaics, we could use combustion of a fuel, therefore gaining control over the light source and making photovoltaics dispatchable. This approach is called thermo-photovoltaics (TPV) which uses the same physics as solar panels (taking advantage of their low cost and scaled production) but uses a glowing hot object as the light source instead of the sun. 

In this work, we use combustion of hydrogen (H2) and ammonia (NH3), fuels that can be renewably-derived through electrochemical methods, have no carbon in their chemistry, and can be stored easily, to power TPV cells. We have designed and built a combustor made of silicon carbide (SiC) that takes in the fuel and air, preheats them above the auto-ignition temperature of the fuel, combusts the mixture in a flameless reactor, and uses the heat released to make its walls glow and emit light towards TPV cells. The TPV cells themselves are designed to limit absorption of below-bandgap photons, and are also actively cooled to prevent overheating.

Our experimental demonstrations indicate a uniform wall temperature of 1500C and heat output of 2kW. Next, we maximized heat-to-electricity conversion efficiency. This efficiency can be broken down into thermal efficiency and TPV efficiency. Thermal efficiency was improved by ensuring all heat leaving the system was absorbed by the TPV and converted to electricity. That meant limiting other heat loss by using insulation, and reducing the outlet gas temperature by including a heat exchanger that uses the hot exhaust (at ~1500C) to preheat the fuel and air. Combining these two enabled experimental demonstration of a thermal efficiency of 60%. We next utilized III-V TPV cells with an air-bridge reflector to limit parasitic heat absorption on the cell, enabling experimental demonstration of 44% cell efficiency. Combining these two in a system demonstration showed system efficiency of 25% (electricity output of 500W), with improvements slated to increase this to 35%.

One main challenge with using these fuels for combustion is their production of NOx (a toxic gas) and N2O (a potent greenhouse gas). To mitigate the production of these gases, we designed the combustion zone to prevent hot spots where these gases are rapidly produced, limiting NOx production to <10 ppm.

We next evaluated the techno-economics of our system. The high efficiencies result in low fuel consumption, and the cheap materials result in low capital cost. After including additional expenses such as fuel production and storage costs, and balance of plant costs, we end up with a levelized cost of electricity around 8 cents / kWh, competitive with natural gas power plants but without carbon emissions.

Overall, in this work we show high-efficiency combustion-powered thermophotovoltaics for carbon-free, low-NOx, cheap and reliable electricity generation. Our current experimental setup enables 25% overall efficiency, with our modeling suggesting improvements up to 35% are possible. We believe this approach can solve the reliability problem of photovoltaics and make clean energy more widely adopted.

Presenting Author: Shomik Verma Massachusetts Institute of Technology

Presenting Author Biography: Shomik Verma is a Mechanical Engineering PhD student at the Massachusetts Institute of Technology, working with Prof. Asegun Henry in the Atomistic Simulation & Energy Research Group, as a PD Soros and NSF GRFP Fellow. At MIT, he works on thermophotovoltaic systems, including testing high-performing emitters, techno-economic analysis, and designing low-cost heating infrastructure such as thermal energy storage or clean fuel combustion.
Previously, Shomik completed 2 MPhils in Materials Science at Imperial College London and the University of Cambridge as a Marshall Scholar. He obtained his Bachelors in Mechanical Engineering from Duke University in 2019.

Authors:

Shomik Verma Massachusetts Institute of Technology
Asegun Henry Massachusetts Institute of Technology