
Drones will fly for days with new photovoltaic engine - dnetesn
https://techxplore.com/news/2019-07-drones-days-photovoltaic.html
======
dwighttk
"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."

^interesting part, the drone stuff is just click-bait

~~~
flanbiscuit
left out the actual science part of this interesting part:

> According to Yablonovitch, this finding builds on work that he and students
> published in 2011, which found that the key to boosting solar cell
> efficiency 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.

~~~
mac01021
Perhaps a stupid question:

Is this simply a matter of getting a second chance to capture the photon (by
the same mechanism) as it makes a second pass through the cell after
reflection?

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duckqlz
I was unclear how this differed from what I would traditionally consider as
solar power (photovoltaic power) as I had never heard of thermophotovoltaic
power. After a quick search I found the following helpful:

“The basic principle [of thermophotovoltaics] is similar to that of
traditional photovoltaics (PV) where a p-n junction is used to absorb optical
energy, generate and separate electron/hole pairs, and in doing so convert
that energy into electricity. The difference is that the optical energy is not
directly generated by the Sun, but instead by a material at high temperature
(termed the emitter), that causes it to emit light. In this way thermal energy
is converted to electrical energy.” [1]

[1]
[https://en.m.wikipedia.org/wiki/Thermophotovoltaic](https://en.m.wikipedia.org/wiki/Thermophotovoltaic)

~~~
lightedman
Basically a highly-inefficient method of converting energy. I've looked at the
article and the pictures, and what I see is basically slapping a solar cell
with a gold mirror backing right near a hot ribbon of graphite - eg we're
trying to use highly inefficient incandescence to generate light.

Want to know what works better? Just the simple mix of gold into the p-n
junction matrix and finger lines. Takes a silicon solar cell up to about 35%
efficiency when properly doped into the junction material.

This technology? Well, given incandescence isn't going to break ~30%
efficiency (excepting in the IR-glass-sandwiched tungsten filament) you're
literally just wasting energy. Best we've seen in general incan is ~12%
efficiency, so enjoy your 30-ish percent of that (4%).

You're better off with other technologies. This is neat, but you're looking at
pretty much over-simulating the conditions of a solar cell welding line, which
means that solar cell is going to likely bust over time under the heat stress.
I foresee tons of problems with this tech, from the incan ribbon breaking to
the cell harvesting energy breaking from the heat/cool cycling.

------
IanCal
People seem to be misinterpreting this a lot.

This is not about solar cells. This is about thermophotovoltalics, and the
application here is using a burning fuel source to generate electricity.

The "drone" the paper talks about seems to be a reference to another paper on
unmanned sea vehicles though. Edit - to be fair the flying drone part is from
the corresponding author.

~~~
pjc50
Exactly. The paper is here:
[https://www.pnas.org/content/early/2019/07/15/1903001116](https://www.pnas.org/content/early/2019/07/15/1903001116)

The context of thermophotovoltalics is very interesting - conventional heat-
to-electricity engines are limited by Carnot efficiency, which relies on a
"cold side" and generally has low theoretical maximum efficiencies. This
approach takes _any_ hot emitter heated by _any_ process - fuel burning, solar
concentration, nuclear - and extracts electricity directly from the mostly IR
photons it emits.

> Here, we present experimental results on a thermophotovoltaic cell with 29.1
> ± 0.4% power conversion efficiency at an emitter temperature of 1,207 °C.
> This is a record for thermophotovoltaic efficiency.

~~~
jcfrei
So (one of) the advantages compared to a Peltier element is that you no longer
need a cold side?

~~~
Diggsey
You still need to keep the thermophotovoltaic cells cool, it doesn't violate
thermodynamics (energy can only be extracted from a temperature differential).

------
agumonkey
Surprisingly never heard of Thermophotovoltaics.

Here's a simple diagram
[https://proxy.duckduckgo.com/iu/?u=http%3A%2F%2Fanilyuksel.f...](https://proxy.duckduckgo.com/iu/?u=http%3A%2F%2Fanilyuksel.files.wordpress.com%2F2013%2F03%2Ffig1.jpg&f=1)

It seems like 'impedance matched' photovoltaics.. very interesting.

~~~
PakG1
Hah, now I have a vision of one of these things hooked up to every air
conditioner in the world to claw back some of the electricity that the air
conditioner uses. I imagine the heat would need to be a bit more than what an
air conditioner generates though, but I know nothing about the topic, so who
knows.

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zaarn
It reads a bit overhyped, considering they claim to power a house with a
generator the size of an envelope; that's a hard no since there is a limit on
how much energy hits every square meter and it's not _THAT_ much.

~~~
asteli
To elaborate, solar energy hitting the Earth’s surface is about a kilowatt/m^2
if conditions are perfect. If you were able to convert solar power at the
maximum theoretical PV efficiency (the Shockley-Queisser limit, 33.7%), you’d
need about 3 square meters to just to make toast.

~~~
numlock86
Is this supposed to sound bad or good? Well, 3m² to make toast intuitively
sounds bad I guess. Well, consider this: It's enough to run 2 small office PCs
with all peripherals. Considering that it sounds much more positive. Really
just anything producing heat would sound bad because of power dissipation. A
toaster is probably one of the most extreme ... (Imagine putting 1kW into a
slice of bread without power dissipation!)

~~~
asteli
Someone else wrote the comment I meant to, essentially doing the math to show
that one would need an awkwardly large solar panel to power something like a
quadrotor drone.

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IshKebab
Terrible article. "Scientists have invented a new thing which could (i.e.
doesn't yet) power drones, and houses and space ships and cars, and computers
and ..... (10 paragraphs later). Oh btw it works by adding a mirror or
something. The end."

------
clouddrover
Researchers from Rice University have an approach they think can get to 80%
efficiency using a carbon nanotube film:

[https://news.rice.edu/2019/07/12/rice-device-channels-
heat-i...](https://news.rice.edu/2019/07/12/rice-device-channels-heat-into-
light/)

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crusso
I really dislike headlines like this on HN that imply that lab research mixed
with theory are somehow practical. It would be great if there were a
"Theoretically" in there along with removing the hype-included "drone"
reference.

~~~
baking
Eli Yablonovitch's company, Alta Devices, manufactures a thin film high
efficiency PV cell but they can't compete right now against cheap Chinese
silicon wafer cells in the solar panel market, so they are focusing on niche
markets, primarily drones at first.

His 2017 talk to the MIT Energy Initiative
([https://www.youtube.com/watch?v=u_K1URyarE0](https://www.youtube.com/watch?v=u_K1URyarE0))
goes into the economics of solar and why his company is using this strategy.
Basically, they need to find niche markets until they can work their way down
the learning curve and achieve economies of scale.

He also discusses thermophotovoltaics. Very efficient light-weight PV allows
for longer drone flights but night is an obstacle to multi-day flights. Given
the choice between batteries and fuel to keep aloft until dawn this does seem
like a possible breakthrough, deserving of the article's title, although the
text could be a lot more clear.

~~~
baking
[https://www.youtube.com/watch?v=lDxJsa8miNQ](https://www.youtube.com/watch?v=lDxJsa8miNQ)
has a lot more information on thermophotovoltaics, including drone
applications, and some more info on solar panel economics.

------
kmacdough
For the record, to sustain a modern small drone at 50% conversion, you still
need at least a 50cm x 50cm panel with optimistic estimates. This is
completely ignoring the fact that such panels would need to be mounted without
destroying the aerodynamics of the propellers

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EGreg
So what does this mean, we will be able to have drones flying continuously and
getting energy from the sun?

~~~
dvh
DJI Phantom 3 is 0.3m wide and needs 120W to fly. Solar radiation at surface
is 1000W/m2. 0.3m _0.3m_ 1000 = 90W at 100% efficiency. They claim 29%
eficiency* (compared to regular 23%) which is only 26W out of 120W in ideal
circumstances, realistically it would be maybe 1/3 of it. It will not fly.

You can safely ignore ALL articles about breakthroughs in solar cell
efficiency and look only at what is sold in shops because that's the only
thing that matters. For years this has been 21-23% despite 5 or more articles
about solar panel efficiency breakthrough every month for the last 20 years.

~~~
walrus01
I essentially second everything that the person above is saying.

There are an endless number of press releases and public relations bullshit
articles put out by people who have some new "breakthrough" in photovoltaics.
Whether it's special weird cells, or flower shaped ground mount things, or
whatever.

What I believe in, is what I can pull out my visa card and buy right now. And
at the moment, here's what that looks like:

One pallet load (22 panels), of high quality 156mm monocrystalline silicon
cells, assembled into a 1.99 x 0.99 meter sized panel. Rated at 370W STC
(standard test conditions) per panel. Under $0.60/watt.

STC: [https://www.altestore.com/blog/2016/04/how-do-i-read-
specifi...](https://www.altestore.com/blog/2016/04/how-do-i-read-
specifications-of-my-solar-panel/)

Everything else is either so high priced that you have to contact a sales
person to buy it (weird fresnel lens concentrator and triple junction GaAs
cells intended for use on spacecraft), or is not manufactured in sufficient
quantity to gain even 1% of market share, and therefore is not stocked by
major photovoltaic equipment dealers.

~~~
IanCal
The article is not about solar cells though. It's about thermophotovoltalics,
where you'd use a high temperature heat source.

------
rodolphoarruda
I've got an immediate idea reference from the movie Interstellar, which shows
a drone that supposed to be flying non stop around the globe for X number of
years until it had its navigation system hacked by people and then landed.

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shyam-geek
"Fly for Days", Wondering what are the trade-offs here? Can those drones carry
a load or cover certain distances to be of any functional use.

~~~
emiliobumachar
Surveillance, spying, communications. Such drones could encroach a lot into
roles currently assigned to satellites.

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acje
I wonder if it is feasible to make PV panels that concentrate the IR part of
the light spectrum and guide it to a TVP unit through fiber optics.

~~~
pjc50
I'm having trouble envisaging how this would work?

What might work though is what I'm going to call Pink Floyd Concentrated
Solar: collimate light from the sun over a large area into a beam, then _run
that beam through a prism_. That would allow the use of solar cells with
different efficiency bands.

(I doubt this would make overall sense given the cost and size of optics, but
it's fun to think about)

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bwang008
Just increasing the efficiency to 30pct is a pretty big deal. If these could
be mass produced cheaply to market, that would be incredible.

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mattsears18
In 6th grade I connected a 9V battery to my mechanical pencil lead and lit my
science folder on fire. Basically the same thing...

~~~
Intermernet
Not really, as it didn't use any energy not present in the 9v battery. This is
about using solar energy in a more efficient arrangement, through reflection
of waste solar energy to heat a medium and create a higher efficiency thermo-
voltaic cell based on a photo-voltaic cell (I'm probably very wrong with my
terminology in the last sentence, but, either way, it ain't no burning
graphite connected to a battery.)

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amelius
So did they show an actual drone flying for days?

~~~
kgantchev
No, but they could... someday... in the future!

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lowdose
I heard the raw cocaine price of a kg dropped by 20k last 10 years. I bet they
are already flying.

