Drones will fly for days with new photovoltaic engine

Drones will fly for days with new photovoltaic engine
A drone flying by UC Berkeley's Campanile. Credit: Noah Berger

UC Berkeley researchers just broke another record in photovoltaic efficiency, an achievement that could lead to an ultralight engine that can power drones for days.

For the past 15 years, the efficiency of converting heat into electricity with thermovoltaics has been stalled at 23 percent. But a groundbreaking physical insight has allowed researchers to raise this efficiency to 29 percent. Using a novel design, the researchers are now aiming to reach 50 percent efficiency in the near future by applying well-established scientific concepts.

This breakthrough has big implications for technologies that currently rely on heavy batteries for power. Thermophotovoltaics are an ultralight alternative power source that could allow and other to operate continuously for days. It could also be used to power deep space probes for centuries and eventually an entire house with a generator the size of an envelope.

Their work was described in a paper published this week in Proceedings of the National Academy of Sciences.

"Thermophotovoltaics are compact and extremely efficient for a wide range of applications, from those that require as little as 100 watts, [such as] a lightweight unmanned aerial vehicle, to 100 megawatts, [providing] electricity for 36,000 homes. In comparison, a 100-megawatt combined cycle power plant is massive," said Eli Yablonovitch, professor of electrical engineering and computer science (EECS) and corresponding author on the paper.

Drones will fly for days with new photovoltaic engine
Graphite ribbon (glowing bar) heating the thermophotovoltaic cell sitting under it. Credit: Luis M. Pazos Outόn, UC Berkeley

According to Yablonovitch, this finding builds on work that he and students published in 2011, which found that the key to boosting was not by absorbing more photons (light) but emitting them. By adding a highly reflective mirror on the back of a photovoltaic cell, they broke efficiency records at the time and have continued to do so with subsequent research.

Graphite ribbon (glowing bar) heating the thermophotovoltaic cell sitting under it. (Photo by Luis M. Pazos Outόn, UC Berkeley)

"What the mirror does is create a dense infrared luminescent photon gas within the solar cell, a phenomenon that adds voltage," said Yablonovitch.

Recently, his team recognized that this mirror could serve double duty. In fact, it solves one of the biggest challenges in thermophotovoltaics: how to exploit the thermal (heat) photons that have too little energy to produce electricity. It turns out that the mirror can reflect those small photons to reheat the thermal source, providing a second chance for a high energy photon to be created and generate electricity. This phenomenon leads to unprecedented efficiency.

"We have achieved this record-breaking result even though we are just using a simple gold mirror. Now, we'll add a dielectric layer above the gold, and that will improve our efficiency to 36 percent," said Luis M. Pazos Outόn, a postdoctoral researcher in EECS and one of the lead authors on the paper.

"Just by increasing the reflectivity, we will get 36 percent efficiency. But by making other tweaks to the cell, using proven techniques in the scientific literature, we know we can get to 50 percent efficiency," said Zunaid Omair, a graduate student researcher in EECS and first author on the paper. "Before our result, thermophotovoltaic efficiency had stalled at 23 percent for a long time, so to get from 23 to 29 percent is a really big deal."


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More information: Zunaid Omair et al. Ultraefficient thermophotovoltaic power conversion by band-edge spectral filtering, Proceedings of the National Academy of Sciences (2019). DOI: 10.1073/pnas.1903001116
Citation: Drones will fly for days with new photovoltaic engine (2019, July 26) retrieved 24 August 2019 from https://techxplore.com/news/2019-07-drones-days-photovoltaic.html
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Jul 26, 2019
Reflections are important.

Jul 26, 2019
I'm surprised this isn't published in Nature or Science if it is such a breakthrough.

Jul 26, 2019
You need a source of heat for this. It's still not clear how this compares with other sources of energy for practical use.

Jul 28, 2019
"Thermophotovoltaics are an ultralight alternative power source that could allow drones and other unmanned aerial vehicles to operate continuously for days."

Considering one has to have enough power to operate during the night, it's hard to see how this can be true, the alternative being flying a drone at a latitude where the light shines all day, or flying 12,500 miles every 12 hours (I don't see drones breaking the sound barrier any time soon) circumnavigating the globe and remaining in the sunlight for days.

Jul 28, 2019
"Thermophotovoltaics are an ultralight alternative power source that could allow drones and other unmanned aerial vehicles to operate continuously for days."

Considering one has to have enough power to operate during the night, it's hard to see how this can be true, the alternative being flying a drone at a latitude where the light shines all day, or flying 12,500 miles every 12 hours (I don't see drones breaking the sound barrier any time soon) circumnavigating the globe and remaining in the sunlight for days.


This is true the the predominant 4 blade design, but by applying a more loose definition of a drone, that which hovers within 1km (as oppose to that which hovers within cm or a few meters) in other words an airplane drone could drift with as little as 1/3 power, if AI and wind could also be used effectively.

Jul 29, 2019
The article hints to a few things; a partial reflection doesn't cut it because the reflected electrons are insufficiently energized to be captured. With a great reflective surface the infrared as well as the visible light are reflected such that the infrared re-energizes the visible photons to offer another chance at recapture. So what if we made glass to convert 5-15% of visible photons to infrared so that we could have 3 chances to capture the photons due to the additional infrared energy thereby easily achieving 40% efficiency; in other words who says we can't cheat and manipulate visible light to serve our efficiency purposes, none of this must be captured as is. This makes a lot of sense; why stick with 30 out of 100 when you get 40 out of 95.

Jul 29, 2019
Considering one has to have enough power to operate during the night


The term "photovoltaics" is misleading here.

What they're really talking about is converting heat directly to electricity using the photon radiation of a hot object. In the case of drones, that could be the sun, but the power density is still too low - what they're really talking about is a reasonably efficient (50%) way to convert fuel into electricity by burning it.

Or in the case of space probes, radionuclear generators where a piece of radioactive material glows red hot next to a thermovoltaic panel.

in other words an airplane drone could drift


It's already been done with conventional photovoltaics and batteries, and a glider drone with the wingspan of a jumbo jet.

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