I have been investigating recapturing energy from house lighting using solar panels.
Are there solar panels specifically designed to capture indoor lighting with a higher efficiency?
If so, what are the differences between the outdoor and indoor solar panels designed for this purpose?
References:
I don't think they existed, and it has its reason.
First: a solar panel can be characterized mainly by its efficiency spectrum: on which wavelength, which ratio of light energy can it convert to electric power.
This needs to have its maximum around the visible light, because the Sun gives most of its energy in this wavelength interval.
This is because our eyes can see best in this spectrum. We simply evolved to the sunlight.
And this is why home light is also in this wavelength: this is what we, human, like the most.
There was no need for different solar panels.
But the power of the sunlight is around some hundred $\frac{W}{m^2}$, although it varies very heavily:
The light power of a house bulb is around sometimes 10W - not for a $m^2$, but for a whole room! Maybe, we see in a well lighted room just as good as in sunlight, but it is only because our eye is very adaptive. The actual light power density is a tenth, or even hundredth smaller, compared to sunlight.
And the efficiency of most solar panels is around 10-20%. There are experimental, very costly versions reaching 40%. A solar panel in a room couldn't produce valuable energy, at most some watts - at the cost of the price of a solar panel on the roof. And the cost is their main problem even with the many times bigger solar power.
My gut feel is that you would be able to save more energy than you could re-capture by eliminating light wastage and lowering the energy usage of the light source accordingly.
Let me use a thought experiment to explain:
If you were in a white cube-shaped room with a light in the roof and a large window in one wall with black curtains. At night when there's no light from the window if your curtains are open, you lose all that light, if you draw the curtains it's no better as they are black so they absorb it and the room is no lighter. However, if you make the curtains white the room will be a bit brighter and you can lower the power consumption of your light to get the same brightness in the room.
So, if you put a solar panel all over one of the white walls of your room, you now have something "re-capturing" the energy, but the room is darker because that light is no longer adding to the brightness of the room by being reflected, so you need to increase the energy of your light to get the same brightness.
Due to inefficiencies of energy conversion from electricity to light and in re-capturing that and converting it back to electricity, you will always lose more than the extra you have to put in in order to maintain the same room-brightness. Therefore it's more efficient to lower the light usage and optimise the room for reflecting more of the light back in (white surfaces).
Some materials are somewhat better than others in lower light levels BUT in a typical indoor situation, even an extremely optimised material simply lacks the availability of input energy to convert. eg Mono-crystalline silicon is somewhat better at very low light levels than Poly-crystalline but neither can overcome the fundamental lack of energy.
Full sunlight ~= 100,000 lux (lumens/metre^2).
Room-light is say 250 lux (quite bright).
A modern LCD monitor displaying all white and set to "nice and bright" will have a brightness at the screen surface of about 250 lux.
The difference between full sunlight & this is 400:1.
A good PV (photovoltaic) panel in bright sunlight will provide 150W per square metre delivered. (Over 200 W at the cell in the best cases). At 250 lux the BEST you can expect is around 500 milliWatts per square metre.
To get the 250 lux = 250 Watts/m^2 I mentioned above you'd need 250 Watts of actual light energy shining on exactly one square metre. The very best LEDs available convert about 50% of their DC input to light. A good commercial product is very approximately 25% efficient. So you'd need to illuminate a one square metre area with 1,000 Watts DC input** of good commercial LED lighting to get 500 milliWatts output. An about 2000:1 ratio of power in to power out.
** eg a top "150 Watt replacement" bulb (Philips, Cree, ...) may have about 25 Watts of input. You'd need 1000 / 25 = 40 of these "150 Watt replacement" bulbs shining on a good 1 square metre PV panel to produce about 500 milliWatts output.
I have this 150 Watt panel, 12 volt(said to be). In testing it with two 100 Watt (equal too regular 100 watt old kind) bulbs used for lighting the room. This one panel can light up two other LED bulbs from the two on the ceiling. WOW, I thought that was impossible. That you only get 10% from the panel. Not so...
Two of these to power solar panel. The LEDs look a little dimmer then normal. But the panel is only getting 11.61 Volts (about).
Plugged LEDs in a 12 volt power supply and there's a big difference in lighting strength.
