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Solar water heating

solar heater using black plastic irrigation hoseI recently created a solar water heater for a pool in France. Since I had a limited set of tools and could not make anything permanent I had to use materials that were cheap and reusable or recyclable. What I ended up with was a 50 meter long by 13 millimeter in diameter irrigation hose.

Plastic is an absolutely horrible heat conductor. Most people who use irrigation hose for solar water heating live in areas where it is hot and sunny and/or does not get very cool. Or they do not care about the absolute best efficiency of their solar collector.

Even though I knew that plastic is a terrible conductor of heat, I still pressed on with the heater.  Last summer the pool was too cold to swim for very long so any, _any_, additional heat would be better than nothing.

I laid out the pipe in an elongated spiral as I did not have the room for a closely packed spiral like The Sietch’s irrigation pipe solar water heater in Hawaii.

Once the pool was actually opened, the water temperature was 14 degrees Celsius. Damn, that is cold. I set the one of the pool’s pumps to push about one liter (litre [sic]) of water per minute through the hose. However, flow rate is really not important as the amount of energy the pipe collects would remain the same.

Now that we have flow through the pipe on a nice sunny day I was quite pleased to feel hot water come out of the end of the pipe!
To calculate the effectiveness of the heater I need to do some calculations. We will need to know the amount of water in the irrigation pipe, the input temperature to the pipe, the output temperature of the pipe or collection of water, the flow rate, and the time it took to raise the temperature.

Temperature change = 13 degrees Celsius
Pipe length = 50 meters
Pipe diameter = 13 mm
Volume of water in the pipe = 5000cm*pi*0.65cm^2 = 6,636.62 cubic centimeters = 6.64 liters
Flow rate = 0.7 liters per min
It takes 9.48 minutes to exchange the water in the pipe
The specific heat of water at 14 degrees Celsius is about 4.186 kJ/kgK
The density of water at 14 degrees Celsius is about 999.2 kg/m3
There are 3.6 kilojoules per Watt hour

Q = cmT Where:
Q = heat added in kilojoules
c = specific heat in kilojoules/kilogram degrees Celsius
m = mass in kilograms
T = temperature change

For a 1 degree change we see:
Q = 4.186*(6.64*.9992)*1
Q = 27.75 kilojoules
Q = 7.71 Watt hours

For a 13 degree change we see:
Q = 4.186*(6.64*.9992)*13
Q = 360.68 kilojoules
Q = 100.19 Watt hours

My father says “Efficiency doesn’t matter. It is what it is”
My step mom says “You have TPS – Tiny Pipe Syndrome”

Interestingly, with the pool open (no solar bubble wrap cover) and this heater running, the temperature in the pool rose 3 degrees Celsius the first day even though it has been about 8 degrees Celsius at night.
The pool has about 50000 liters of water in it. It’s 5 meters by 10 meters by ~1 meter.
Q = 4.186*(50000*.9992)*3 = 627,397.68 kilojoules = 174,277.1335 Watt hours

I know that this small heater didn’t do that! I’ll bet that Earth’s solar flux of 1380 Watts/meter^2 has a little something to do with it though.

Overall, for 30 Euro’s this might mean that the pool is more comfortable and can stay open a few days longer this summer.

I’d like to make a collector out of copper painted flat black to see what it can do in the Pacific Northwest.

*UPDATE 10 May 2008* looks like this heater is working quite well. I had to change the pump flow so it is much faster but I am seeing a three to four degree rise in temperature from the input to the output. That is 23 to 30 Watt hours of heat being added to the pool eight to 10 hours per day. The pool is now up to 18 degrees C – four degrees warmer in three days.

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