by Dick Aidt
Build a Solar Thermal Water
The weather in my part of
Florida on the Space Coast
is usually very nice in the winter.
That being said, it is not unusual
for the temperature to get into
the mid 30’s on an occasional
winter night. It was one of those
rare instances that prompted me
to create this project.
My solar collector is a good heat
absorber, and unfortunately, equally
good at radiating that same heat
back out to the sky. This means the
collector can drop below ambient
temperature during cool clear nights.
On one such night, the water in
the collector froze and the pipes
burst. My old solar controller failed to
perform its job, which is to pump hot
water from the tank in the utility room
into the collector on the roof for
freeze protection — the circuit had
apparently become unreliable. Thus
my plan to build another controller!
The collector is made up of
copper tubing and copper flashing
sheets, assembled in an aluminum box
FIGURE 1. Assembled and working.
with a glass cover. Its dimensions are
10 feet by four feet and about three
inches thick. To start this project, I
installed LM35DT sensors at the inlet
and outlet of the collector. I also
opened an access hole at the exact
center of the collector and installed
another sensor there.
A neat way to mount the
LM35DT (which is available in a
TO-220 package) is to solder a 6-32
flat head brass screw directly onto
copper pipe to be sensed. I used
brass nuts and thermal compound to
complete that installation.
The pump for the system draws
about 100 watts while running, which
is a lot better than the 1,500 watts
used for the heating element. I found
that with my system, this heating
element operates after about two days
of no sun. Its thermostat is set at
120°F on the element, however, it
only takes about two hours of full sun
to get up to this temperature, and
even in mid winter, water temperature
is usually far above that.
The typical temperature is somewhere above 160°F in winter and
higher for summer. With the original
installation, the pressure/temp relief
valve (195°F) at the collector top
would occasionally pop off, therefore
during collector repairs, I installed a
high temperature version (210°F).
This lets you know what kind of
temperatures can be expected in
the summer around here (if you’re a
copper heat absorber).
The hot water tank is the
“standard solar water heater” which
has four pipes of different lengths fed
into the top of the tank housing. The
house feed line takes hot water from
the top of the tank, and the water feed
into the tank goes approximately half
way down into the tank. The water to
the collector is pumped from about
six inches above the bottom and the
collector output is inserted about
18 inches from the bottom. The hot
water migrates to the top of the tank
through the convection process.
These tanks are available from
most solar utility companies here in
Florida. They can also be obtained
on-line or through special order from
a plumbing supply company. My
tank is about five feet high, with a 24
inch diameter and holds 80 gallons.
A thermal sensor is mounted on
the exterior surface at the bottom
of the tank. The output of this
sensor is included in my code as the
FIGURE 2. Ready
The old controller is
one of those old-fashioned
transistorized versions that
has a slide switch that
was constantly getting dust
contamination. It uses a
transistor flip flop clock and
several other transistors for
relay control. It also has an
advanced 741 op-amp used
as a comparator and some
4066 DIP solid-state switches.
I decided to use a programmable device rather than go