Dear Solar Guy,

        I live along the Colorado River near Bastrop and am thinking next
year of watering my garden (about 40 feet by 80 feet) with a PV pumping
system.  A friend of mine says solar energy is unreliable and expensive.
Does anybody do this kind of thing ?   What equipment do I need if I'm not
out to lunch on this thing ?   

 A Green Thumb.

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Dear Green,

        First of all, although there are many uses of solar energy that
are not practical, there are several that are.  Secondly, the idea you have is
a great application for solar energy.  I'd like to say that you are the
first to think of it, but truthfully this is a rather well know
application.

        To begin with, your garden serves as the "storage vessel", so you
don't need a tank to store the water.  Also, you need most water on sunny
days when the pump works best.  On the negative side it is not used in the
off-season, so that tends to reduce its economic attractiveness.  Maybe
you have some chickens that needs a drink also.  Hope they survive the river
water coming down from Austin !  On the whole though its a good
application.

        Assume you want to use 2 inches of water per week on you garden
(about 0.3 inches per day), which corresponds to about 630 gallons per
day. You didn't say how high you are above the river, but I'll assume 50 feet.
The daily energy required to pump the water is 0.10 kW-hrs.  Most pumping
systems will be fairly inefficient, so I'll assume 25%, meaning about 0.4
kW-hr/day is needed.  Typical PV panels are about 10 - 15 % efficient (say
12%) meaning the required solar energy per day would need to be about 3.3
kW-hr.  Since the solar energy on a tilted surface in central Texas is
typically about 6 kW-hr/m2, then one would need a PV panel of about 0.55
square meter.  So you see that it doesn't take a big collector to do what
you need.

        If you want to follow up on this, I suggest you contact either:
Photocomm, (713) 933-1578, or Southwest PV Systems (713) 351-0031 (both in
Houston), as they can give you better details on what you need, including
the proper panel size, type of pump and controls.  You may also have a
local supplier with that information.

        Good luck, and look forward to a great "solar crop" next year.

The Solar Guy.