I have measured the 200 lux with this instrument, cheap but very useful for such measurements: http://cgi.ebay.de/182356676136
The light source are 3 halogen bulbs, 50 watt, 1.5 m distance and no light from outside (was evening). But the lux value looks like it is standard for normal home lighting:
http://worldofleds.co.uk/lux-levels.htm
Agreed, but in many homes living areas may be more like 100 lux or less - depends on personal preferences.
Problem is, I measured it lying on my workbench, will be less when sticking on the fridge, as one of my application idea is. But even with 1 mW it should be possible to do an update once per hour, which is my goal.
I think there are big differences for the solar cells, not only the efficiency, but also what part of the spectrum can be used. My halogen bulbs are very yellowish, such warm light is typical for many home lights.
Ah-ha! You didn't provide that info in your blog. So....
Some solar cells are more optimized for the colder sun light spectrum.
I swapped the LED bulb for a 150W incandescent. Perhaps 2200 lumens so approx 45% more than the LED. The output of the first panel, on the desk, was now 541uW - approx 30x as much. Moving it to 1m below the bulb increased that to 1410uW or approx 17x LED bulb.
Scaled up to your panel that would be 1.05mW and 2.73mW. In the second case at 1m from the bulb lux = lumens/area = 2200 lumens / (4 x pi x r^2) = 175lux. 2.73mW/175*200 = 3.12mW. If your panel is 22% efficient that makes mine 13.7% which is about what I'd expect given their age and being
cheap polycrystalline cells from eBay So from this I think the conclusion is that you should expect
*much* lower outputs with LED lighting at similar Lux levels - which you'd expect as they put out much less power in total. The colour temperature is almost irrelevant - a 100W incandescent bulb puts out 100W of energy, of which the panel is responsive to perhaps 35% of the spectrum - ie. visible light plus some of the near infra-red. That means a solar panel completely surrounding the bulb could receive up to 35W of light plus near infra-red.
An equivalent 13W LED bulb may lose 3W in the driver; of the remaining 10W the LEDs may be around 14% efficient so perhaps 1.4W would be available for the solar panel with rest being dissipated as low temperature heat which the panel can't utilise. 1.4W is 1/25 of the power aailable from the incandescent - which isn't far of what I observed notwithstanding the very crude experimental conditions and broad assumptions.
This is made worse by the severe drop off in efficiency of a solar cell, including monocrystalline, at low light levels. At 200 lux when provided by the sun or an incandescent bulb, the drop off may be minimal but 200 lux from an LED is a very much lower power level and the efficiency may well be much worse. I say *may* as the spectra are very different so more research is needed to be sure. Its easy to measure though using your panel of choice and a suitably LED lit room.
Some/many people may still use halogen bulbs in downlights, but lots have switched to LEDs and it's probably only a matter of time before they are banned altogether. Unfortunately I think you are going to have to rethink this project.