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.
While glass does block/reflect some of the light, it also creates an
insulating buffer. The benefits of the insulating buffer out ways the lost
light. This is essentially the same insulating concept used in double glazed
windows.
Since the temperature loss increases when the difference between the
outside temperature and the temperate inside the water heater increases, it is
better to heat a lot of water to 110-115 degrees, than to heat a little water
to 150 degrees. For example, if the hot water tank is at 150 and the outside
temperature is 80, there is a 70 degree difference and more heat will be lost
through the insulation than if the temperature difference was 30 degrees, and
the hot water was at 110.
Passive solar heating requires (as the name implies) no moving parts. As
water warms up it becomes less dense and moves upward, pulling cold water into
the collector. This requires no pump, making it very simple and energy
efficient, as well as there being less to go wrong.
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.
