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Q & A: solar-powered outdoor lights

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Most recent answer: 10/22/2007
Q:
A friend of mine bought solar-powered outdoor lights - the type you stick in the ground to light a walk way. Obviously, they store the solar energy in a cell during the day, then come on at night - usually they are only lit for a few hours before the charge runs out. My question is this: Given 2 of these lights, fully charged, and in the dark, - would they each stay lit longer if they were positioned right beside each other, as opposed to say 20 feet from one another? Put another way: could they "feed" off of one another? Why or why not? thanks, Keith
- Keith McGill (age 31)
Lancaster, PA, USA
A:
Hi Keith,

Yes, you probably can collect some energy from the light given off by one of these lamps. There may be a feature of them that they cannot charge up their batteries and discharge them at the same time (I don't know, but their designers may not have foreseen that the solar panels may be in a light enough place to collect enough energy and still have a need for the light to be on). So as long as charging and discharging are not exclusive in some way, some fraction of the energy can be recovered.

But that's not the end of the story! The energy recovery procedure here is rife with inefficiency. Typical solar cells are only 10% efficient or so. 90% of the light energy striking a solar panel is reflected back, or converted to heat, or lost in some other way. The light source is also inefficient -- much of the energy stored in the battery goes to heating up the light bulb, and another fraction goes to making light. Light can also be absorbed and scattered by the diffusers that are often on outdoor lamps.

There is also a simple geometrical effect -- most of the light emitted from one of these lamps will miss the solar panel of its neighbor unless extraordinary measures are taken to collect the light going off in all directions and focus it down on the solar panel of the other lamp.

My guess is that you can get a small fraction of a percent of extra lighting time out of these things if they "feed off of each other" provided that there is no switch in these things keeping it from happening. And if you collect the light from one to focus on the other, you miss out on having that light for its original purpose.

Tom

(published on 10/22/2007)

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