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)
