New device can harvest indoor light to power electronics | Hacker News Reader

New device can harvest indoor light to power electronics

arstechnica.com

58 pointstdrnd35 comments

How much efficiency improvement over calculator solar-cells are we talking about? It's not like we couldn't power basic electronics (like solar powered calculators) indoor before.

Apparently solar cells larger than a calculator one can give about 30mA, 5V at peak power output [1]. We really need to know surface area and lumen to know how good this thing performs.

[1] https://www.amazon.com/53X30mm-Micro-Power-Solar-Panels/dp/B...

Units point of interest, the luminous flux per area is called "illuminance" and measured in lux (lumens per square meter) or footcandles (lumens per square foot)

`footcandle` is a ridiculous name for a unit.

it's not even the right dimension.

Catchy though. Same style: Toebright?[0] Fingerbling? Nosefire?

This sort of fundamentally physical compound noun has a sort of medieval ring to it.

[0] In homage to Madeleine

Originally comes from 1 fc being the illuminance on a surface 1 foot away from a 1 candela light source.

A candela being roughly the intensity of light from a candle.

Intensity (candelas) breaking down to lumens per steradian. They're a measure of how bright a light is in a particular direction, rather than its total light output (which is lumens).

And steradians, for the unfamiliar, being a measure of "solid angle". Steradians are to 3D angles as radians are to 2D angles. A 1 steradian solid angle projects out to an area of 1 unit^2 on the surface of a unit sphere.

Fun stuff!

Other fun fact, 1 fc = 10.7639 lux, because that's how many square feet make up a square meter.

But in the lighting industry we just pretend 1 fc = 1 lux for most conversions, because eh, close enough.

Interesting side note: A lot of those calculators have a fake solar panel and just run for years and years on a single battery.

I've been using Logitech's solar powered keyboard (uses traditional cells) for years and I love the product. The biggest problem are:

1) doesn't charge well in low light

2) doesn't charge well from LED light

At first glance, the new approach looks to address one of those problems and possibly both.

Claimed 30 mW may not seem a lot, but most of electronic sensors (even displays) are only used for short period of time. You continuously collect charge over time, often in a supercapacitor, while most of device circuitry is dormant. At some time the device wakes to take measurements, transmit data over some wireless technology, schedule next wake-up and then goes back to sleep.

Even if you need measurements every minute, 30 mW is plenty for these kinds of devices to do their work.

TL;DR it's a special kind of solar panel that works better in low light conditions. Super cheap solar panels are around 11% efficient, average cells are around 18%, and these are around 29% efficient. So about a 50%-ish improvement.

It is a fascinating breakthrough, but unfortunately even if it was 100% efficient indoor solar cells would still be pretty useless. As a rule of thumb indoor illuminance is 200-1000 lux, vs outdoor illuminance up to 120,000 lux. A lux is slightly less than 1.5e-7 watts per centimeter. So for a 500 lux room (which I'd call pretty well lit) with one of these devices the size of a smartphone you'd be harvesting 1.5e-7w/cm2/lux500 lux7cm14cm0.29 efficiency = 0.00213 watts.

As cool as this breakthrough is, it's not going to get anywhere. Current solar-powered household devices (pretty much just calculators) don't really need 50% more power, and other household devices need many orders of magnitude more power.

Even if it was 100% efficient, it'd still be gathering less than a hundredth of a watt. That's not coming anywhere close to useful.

I love the idea of reclaiming some of the energy from our lighting. Perhaps one day solar cell wall paints will be a thing.

If those solar cells are efficient, then they'll look you've painted the walls black. Not only will the quality of the light become worse (harsher shadows from less diffuse light), you'll probably install more light bulbs to compensate.

Exactly. Now that we're putting out primarily visible light from our bulbs, there isn't a bunch of free energy floating around there. Far better to save the energy by making the bulb as efficient as possible in the first place than to try to capture it back.

Technically speaking the solar panel walls would get most of their output from the daylight, assuming you have windows, considering that it's thousand times more intense than light bulbs.

But yeah, I wouldn't want to live in black glass coated rooms.

Ha, yeah, but that's kind of like putting your jacket on under your clothes.

The EU incidentally created a similar problem when banning most incandescent bulbs.

Well, at least in some countries. Here in Norway, most buildings are heated by electricity - so replacing incandescents with LED or fluorescent bulbs effectively means that you spend less on lighting, more on heating and the sum stays the same - except the new bulbs are costlier and (mostly) provide less comfy light.

Sigh. Free lunches remain elusive.

Using electricity for heating is a terrible idea. Unless you are a rich country like Norway which can afford it.

You would be much better using LED for lighting and gas burning for heating.

So the sum does not remain the same if you do things properly.

Considering practically all of electricity production in Norway is renewable (99% hydro) the less efficiency in electrical heating can be traded off the more efficient but CO2 emitting gas burning.

It's true though that in countries where there's a significant fossil dependent electrical sector it's better to burn the fuel directly.

Except it is not as cold everyday, and not every room has the light and heating demands perfectly proportional. And incandescent bulbs also generates more trash.

Tying lighting to heating is inherently inefficient.

Wasting electricity like that... At least put it through some computer before throwing it away as heat.

Heat pumps are way more efficient than just resistive heating.

How is that possible?

They steal heat from outside. It takes less energy to move heat from a cold place to a warm place, than to create warmth "from scratch".

A heat pump extracts thermal energy from, say, air. Electricity is not converted into heat, but used to create a temperature difference (one part of the system gets colder, the other gets hotter). This is vastly more efficient than using resistive heating elements. Such a system can appear to be more than 100% (up to around 400% [1]) efficient, if you look at the electricity going in, and the heat coming out.

[1] https://en.wikipedia.org/wiki/Heat_pump

I don't think they are more efficient in Norway harsh winters..

That's actually a reasonable point, and it may be true. There's got to some temperature delta makes a heat pump a losing proposition, but I'm not sure how big that threshold is.

I accidentally replaced one of my heat pumps with an A/C only unit. So now I have to run the emergency heat strip all winter. I haven't seen any real increase in my power usage. The heat strip was running most of the time anyway because of low temps.

Recent winter pumps work pretty well down to -20C, and don't get any worse than resistive heating after that.

In the north of Norway, Tromso, the lowest monthly average temperature is -6C for January. Coastal Norway is surprisingly warm due to the ocean and Gulf stream.

Good point. Maybe things that are frequently dark - like baseboard trim?

A bit like a perpetuum mobile, right, light is captured by the walls which generate electricity for the lights. Free lighting for everyone...

Well the point of lighting a room is that you want the room `light` and making walls that absorb light makes them not reflect light and then your room is dark again.

So even if this was possible (which it is not), the idea would still be rather useless (until you managed to achieve efficiency over 100 % ;) ).

If we paint the outside it might make sense. As long as you like black houses.

It's more efficient to wear night vision attire everywhere. Why illuminate a whole room when you only care about the light that makes it onto your retinas?

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