# Why not use our own light production to produce new energy instead of wasting it?

Why don't we use our own light production at night (I mean home, buildings, streets,..., lighting) to charge photovoltaic panels instead of wasting it?

• What do you mean by "light production at night?". – giulio bullsaver Jan 3 '16 at 12:12
• @giulio, I just mean we are lighting homes, buildings, streets, etc. This is light, so photo voltaic energy, no? Does it have any sense to get back part of this energy, or it just not worth because the energy is too low or something like that? – Eddy Khemiri Jan 3 '16 at 12:18
• I am not an expert, probably is not worth the cost...but interesting idea – giulio bullsaver Jan 3 '16 at 12:27
• Sounds like yet another perpetual motion idea. – kasperd Jan 3 '16 at 16:40
• @kasperd OP means recycling the light from streetlights etc. currently being uselessly radiated into the sky. That's not perpetual motion, although it's not an especially efficient way to recycle. – AGML Jan 3 '16 at 19:11

Solar panels work with sunlight. The energy per square meter of light from the sun , depending on the geographic area etc is of order of

8 hour summer day, 40 degree latitude 600 Watts per sq. meter

In one hour a photovoltaic cell of one square meter will provide energy of 600 watthours

Take a light bulb of 100 Watthour . To gather all that irradiance one would need to cover all the walls of the room with photovoltaics, which typically are 30% efficient. So one would gather 30 watthour of that "wasted energy" for an enormous cost in photovoltaics. ( disregarding that photovoltaics should be specially developed for low intensity conditions and as pointed out in the comments that a lot of that energy is in the infrared spectrum ).

In analogy will be the economics for other situations, Take a stadium with its large light sources, the power falls as $1/r^2$, where r is the distance between the light source and the panels.

• I think this answer misses the two most important points: 1) Generating light and then turning it back to electricity via photovoltaics is guaranteed to be less than 100% efficient, so there's no reason to doing it unless the point is to reclaim "wasted" photons. 2) Consider a street lamp at midnight. If nobody is around the photons are wasted, but if you can detect that nobody is around you should just turn the lamp off. – DanielSank Jan 3 '16 at 16:42
• Especially considering that 100w light bulbs are actually incredibly inefficient themselves and generate mostly heat. If you're dealing with 20w (energy efficient) light bulbs then your 30% efficiency when collecting makes it an even lower number. – Matthew Steeples Jan 3 '16 at 16:45
• What is a "bulb of 100 Watts/hour"? Strange unit. Also, as Matthew wrote, a 100W incandescent bulb has a light output of about 5W only. 30% efficient solar panels are rare, 20% is a more accurate estimate for what one can buy in the store nowadays. Added together, the full room covered in photovoltaic cells produces about 1W of electricity only. Additionally, usual photovoltaics are not built to efficiently use the spectrum of a normal light bulb. – asdfex Jan 3 '16 at 17:48
• @annav "Watthour" is wrong as well. It's just "100 Watt", no hours involved. – asdfex Jan 3 '16 at 18:19
• Still has the incorrect assumption that a 100W lightsource and a 30% efficient panel produces 30W of electricity. This ignores the light source efficiency, which is far lower than 100%. Also, the r^-2 effect mentioned in the last paragraph is a fall in flux, not power. At a greater distance, you just need r^2 more panels to harness the same power. – MSalters Jan 5 '16 at 8:23

We don't harvest waste artificial light, because it would be ridiculously expensive to do so. The energy in sunlight is, at full sun, $1000W/m^2$. That's way higher than any artificial light in normal circumstances. So it's far more economic to position solar panels to optimise collection of daylight, rather than to capture artificial light at night.

Any energy collection is going to have significantly less than 100% energy efficiency, unless it's low-grade thermal energy you're collecting. And photovoltaics are typically about 20% efficient. Whereas energy efficiency is pretty much 100% energy efficient: when you cut your energy demand by a gigajoule, the amount of energy consumed is going to drop by a gigajoule (even more if your electricity comes from thermal plant). So if there's an economic incentive to harvest waste light, the economic incentive is going to be even higher to reduce the amount of light created in the first place.

As MSalters noted in the comments - if light isn't going in the direction you want it, the cheapest way to harness it is to use a reflector to redirect it to a useful direction. This generally requires very little investment, and will be 60-90% efficient.

Only in the rarest, freakiest cases will it make sense to use PV to turn artificial light back into electricity: if there's a barrier which will let light pass through, but through which you can't run electric cable, and you've got electricity only on one side of the barrier, but need it on the other. Once every few years, in some freaky lab circumstances or extreme hazardous area, this happens.

well it dosen't make any sense to first use energy to light up a lamp that would radiate energy more the threshold energy and use it to produce energy which is less than the consumed energy, rather it an other way to waste energy.

Maybe when all the stars start to fade this is necessary, unless our eyes have evolved into something else.

Edit: we probably do not do it because the sun is still active and photocells require rare materials (the ones I have heard of) and if you would have photocells everywhere where would you nail that nice painting you made?

Our light collection systems are horrible. A reliable source says that the best commercial solar panel out there work at a whopping 22% efficiency.

The power generated even in a major city like London is very low and also highly angle dependent.

An interesting idea, however, could be to use a mirrored lens to focus the low-light from various angles into a centralised point rather than focusing on maximising surface area and collecting a significant flux of light.

Perhaps this could be a "leech" system for low-power LEDs from nearby sources.

For sure, even having the best photo voltaic panel produced so far, you would not be able to convert back all the energy. You will have always loss. Anyhow, this idea is very similar to what is currently done with hybrid cars, here part of the kinetic energy is converted back to battery energy when the car is slowing down.

It might be an eyeopener (pun intended) to learn how little light there is at night. Street lights might be as bright as the full moon, 1 lux whereas sunlight is about 50.000 lux.

With 50.000 times less light to work with, that photo-volatic panel isn't going to do much.