Before I get into how I built the heater here is some basic information about
it:
The solar collector is approximately 27ft2 (2.5 m2)
Building the Heater
The solar water heater consists of two basic parts, the hot water tank and the
solar collector. The tank is 310 liters (80 gallons), it sits in the attic and
is insulated with spray foam insulation.
The solar collector is made up of 1/2 inch and 3/4 inch PVC pipe and florescent
light bulbs. The collector is encased in an insulated box with glass on one
side.
The solar collector is approximately 2.5 m2 (27 ft2), the pipes that are
exposed to the sun have a total volume of about 18.5 liters (5 gallons), which
is equivalent to 6% of the volume of the hot water tank. (Meaning that if the
water circulates 16-17 times all the water in the tank will have been heated
once, in theory).
To make the plumbing part of the heater I bought 64 T fittings and with them
made 2 pipes, 32 T's each with the T's butt up against each other. Then
between the two I put a pipe that was just a bit longer than a florescent bulb.
To prepare the bulbs I tore off the metal parts on each end, then poked a large
hole in both ends so that a 1/2 inch pipe could fit through. (Click Here to read more about this step) Then I pushed a
piece of a sponge through a couple of times to clean out the powder. When I
was done I had a long glass tube. While I was taking apart the bulbs I used a
mask and was sure to wash my hands and cloths afterwards, since they contain
mercury.
Close up of the bulbs and pipes, The entire array, The solar collector in
the insulated box.
Click for larger images.
The bulbs were painted black on the back side, and the pipes were completely
painted black. Then I closed off the ends of the bulbs with tin foil and a bit
of spray foam, since they are just used as a basic insulator its not that
important that they be air tight. The solar collector was lined with black
plastic, underneath were some Styrofoam sheets to help insulate the heater.
The Heater installed (before glass I tried transparent plastic sheeting but
it wilted in the heat), Installing the Water tank
Click for larger images.
The panel was installed at a 35 degree angle and about 1 foot below the bottom
of the water tank. It is below the tank so that it does not act as a water
cooler at night. Basically, the cold water is already at the bottom, so it
should not circulate with the water in the hot water tank. Click here for more on how to connect it to the water tank.
On sunny days when the outside temperature is in the mid 80's the water heats
up to about 135 or more. Even on cloudy days the water heats up to the 90's.
How to prepare the florescent bulbs
There are two ways to break open the ends.
Method #1:
The simple, but tedious way is to puncture them with a screw driver. Here's
how I did it:
First I tore off the metal caps, then I put a screw driver in the end and
wrapped a rag around. The rag is dual purpose. One, it helped protect me from
the mercury inside (I also wore a mask and did it in a ventilated area), the
rag also helped slow down the air as it flowed into the bulb. The bulbs seem
to have a near vacuum inside them, so when they are punctured the air enters
pretty quickly, and can propel some of the debris from the puncturing into the
other end and break it. The rag helps slow the air down. After the bulb was
broken on both ends I tapped out just enough glass so that a ½ inch pipe could
fit in I found G.E. bulbs to be the easiest to work with. Afterwards be sure
to wash your hands.
In the end the bulbs are just for insulation so they don’t have to be perfect.
They just keep the hot air around the pipe, and the ends are sealed off with
foil, which covers up a lot of the imperfections. Of the bulbs in my collector
about ½ of them have breaks or small imperfections. But it doesn’t seem to
matter, it still increases efficiency.
Method #2:
Here is a link to the other way to break open the ends. It isn’t nearly as
simple. I haven’t tried it, but it was published in a magazine so I am sure it
works. I found it with an article about a florescent bulb solar air heater.
It uses common items to build a glass cutter. From what the site says, it
should make nice looking cuts. Link to cutter instructions
Connecting it to the water tank - I put 5 holes into the
tank. They are listed here from highest to lowest.
Supply to the Solar Heater. This pipe connects to the bottom of the solar
heater, it also connects to the bottom of the tank. It supplies the solar
heater with the cooler water that settles to the bottom of the water tank.
Cheap $30 Backup System for Cloudy Days:
On mostly cloudy days the heater reaches about 90 degrees, on very cloudy and
windy days it wont
get above 80 and may only reach 70. So I came up with a backup. (Note: while
I have this
tested and installed, I actually have it unplugged to save electricity). I
bought a 'Bucket
Heater' that automatically turns on at 80 degrees and turns off at 110.
I suspended it inside the water tank so that it only heats the upper portion of
the water. I
assume that if it were resting on the bottom that the hot water would rise. By
the time the hot
thermostat measured 110, the water at the top of the tank would be well over
that.
Why I built the solar water heater this way
After doing a lot of research on the internet, and a couple experiments I came
to a few conclusions.
A solar heater would heat the water using two forms of heat transfer,
radiation and convection. Radiation would reach at best 180 degrees of the
pipe, convection would reach 360 degrees of the pipe, doubling the surface
area. In other words, as well as trying to expose the pipes to as much sun as
possible, I needed to also trap hot air produced by the radiation around the
pipe to help heat it even more.
With this in mind I set out to design a solar collector. I priced copper but
found it to be very expensive. PVC was much cheaper, but it does not withstand
more that 135 degrees. This I found out not to be entirely true.
After some tests I found that the PVC pipe itself will withstand higher
temperatures without losing rigidity, however I assume that the fittings would
begin to fail at lower, but still hot, temperatures. I should also note that
the water pressure in the collector is very low, just as much pressure as is
created by about 2-4 feet of fall, others who had failures in their PVC
fittings had put their heater inline before their water heater, as a primer.
Unlike houses in the USA where the water enters the house already pressurized,
houses in rural Brazil have water tanks in their attics to create pressure. So
I figured that the max temp for my PVC would be higher than the standard since
I have sub standard pressure and will subject the pipes to much less stress.
So decided to go with PVC and to insulate the fittings from the temperature
inside the collector and protect them from sunlight. That way only the pipe
itself would be exposed to the sun light and the highest temperatures.
Since I wanted to try to keep the heat close to the pipes I decided to use old
florescent bulbs to act as a second glazing. The wind will cool the glass
panels that cover the solar collector, which in turn cools the air inside the
collector. The florescent bulbs act as another barrier. The trap in the
hotter air around the pipes and keep it from circulating with the cooler air
outside of the bulbs. In tests the pipes with bulbs heated up faster and to a
higher temperature than those without.
