Solar Cell Installation and Lake Reflection
Location: Outside U.S.
Date: Spring 2013
We are thinking of installing solar cells and are wondering if we would gain any efficiency by placing the panels at the edge of a lake where the sun would reflect off of the water or snow instead of putting the panels on our roof. My husband and I are discussing this but need and expert opinion.
There would appear to be no advantage to placing your solar cells
near a horizontal reflective surface such as a lake. The light from the
lake would be reflected upwards, and at best, only illuminate the non-
functioning rear surface of the solar cells. For maximum efficiency,
your solar cells require light that traveling downward, perpendicular to
the surface of the solar cells. Light reflected from a nearby lake is
traveling upward, in the opposite direction needed, and likely would
never be "caught" by upward-facing solar cells.
I believe you will get the best result by pointing the solar cells so that they get the greatest exposure to the sun. Ideally, a line from the sun to the solar array will be perpendicular to the surface at all times. If the array cannot change direction during the day, then I would recommend getting it to point as directly as possible at the sun when it is directly overhead. If your array is fixed, you can get somewhat better results by adjusting the tilt twice a year to account for seasonal variation of the sun’s relative location. As for the lake/snow idea, consider that placement as secondary to the optimal angle relative to the sun. My reason comes from a simple thought experiment. On a bright day, how long can you stare at the sun? How long at the snow or the lake? The brightness from the sun far outweighs that of the latter two, and thus, intuitively, we would expect the greatest gain from optimizing the location to get the most direct light from the sun. That being said, if you can locate the array to get some indirect light as well, it will not hurt, but I would focus on placing the array such that you are pleased with its aesthetics. Do not disrupt your view of the lake in order to get a minor boost in energy. If, however, you really want to find the ideal placement, you can buy a simple photometer (an instrument to measure light intensity) to use to find the most available light during the day and during the season. Buy a cheap one with a flat surface, and treat the surface as your solar cell. Point it where the meter shows the greatest intensity, and place your array accordingly.
Kyle Bunch, PhD, PE
Direct sunlight is better than reflected sunlight because the reflecting surfaces like snow and lake water will absorb some of the energy. Some Solar furnace sites have an array of mirrors over a wide area that concentrate the sunlight up to a boiler that produces steam that runs turbines, but that is not what you are talking about here.
The concept you are describing is called solar concentration --
basically collecting light from a larger area than just that of the
panels. There are substantial efforts under way to make CSP
(concentrated solar power) a viable improvement to traditional PV
(photovoltaics, another word for 'solar' power).
In your case, the lake may provide a trivial additional amount of
power, but given the angle that the panels need to be at to collect
sun, they are going to be at a very oblique angle to the lake. If there
are any trees nearby that would obstruct the sun, they would
counteract any benefit. And, most people don't like to interrupt their
view of a beautiful lake!
All in all, I suggest you mount them up high, where there are no
obstructions (blocking the sun, or blocking your views).
Hope this helps,
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