Dear Solar Guy,

         I have heard that solar energy can be collected using lakes of
water.  How do they work?  

Sally Ponders, Shallowater, TX.

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Dear Sally,

        It is interesting that one living in a town of Shallowater should be
inquiring about a solar energy concept which uses a shallow pond of water.
In any case, you are correct, these so called "Solar Ponds" are able to
collect and store solar energy for later use either as "heat" or to power an
engine to produce electricity.

        There are different solar pond concepts, but the most widely used
was developed in Israel and it works this way.  An excavation is constructed
that is about 8 to 10 feet deep and is lined with a material to prevent
water leakage.  The pond area can be of almost any size, but the area is
dependent on the power expected to be realized from it.  It is then filled
with a brine (salt solution, water and common salt) that is almost saturated
with salt at the bottom and nearly pure water near the surface.  In any
case, with this salt-gradient pond, the solar energy falling on it largely
passes through the water to the bottom where it is absorbed.  If this were
done with pure water, the warmed water at the bottom, being less dense,
would mix with the water above it thus losing energy to the surroundings.
However, with the denser fluid near the bottom, even if heated it is denser
than the water above it and little or no mixing takes place.  Therefore the
water at the bottom gets quite hot and the only heat losses are to the
ground (small) and by conduction up through the pond to the surface.  But
since the pond is rather deep (6 to 10 feet) this loss is also not great.
Hot water is extracted from  the bottom of the pond and circulated to a heat
exchanger where the heat is extracted by another fluid and used as needed.
This could be for producing hot water for washing purposes, or it could be
used as the heat source for an engine to produce mechanical/electrical
power.

        The annual average daily solar radiation falling on a horizontal
surface in (west) Texas is about 18,000 kJ/day per square meter.  This is
about 0.20 kW/square meter, and if the pond is about 20% efficient in
collecting energy, this means that about 25 square meters of area is needed
to produce an average annual kilowatt of power.

        The several aspects of solar pond operation which are not trivial,
such as producing the "salt gradient" initially and maintaining the gradient
during operation, but these are left to the experts to discuss.  There are
also some naturally occurring salt gradient ponds located in west Texas
which may have potential for exploitation.  The University of Texas at El
Paso has been doing solar pond research for several years and Andy Swift or
Steve Cook at UTEP can be contacted for further information.    The Solar
Guy.