Home project - DIY Large Solar Pool Heater

[cwinhall]

Hi all,

I live in Perth, Australia and have just moved houses to a place with a relatively small pool, unfortunately it is shaded for the majority of the day by a big tree and it is quite cold and unwelcoming (unless you have built of a sweat).

I want to make a solar pool heating system to bring the temperature of the pool up by about 4C using submersible pumps and poly tube piping.

Now I've already done a basic system for this already with this pump (http://www.bunnings.com.au/products_product_pump-submers-clean-commer-elec-550w-15500lhr-pcw550_P4813554.aspx?search=pump&searchType=any&searchSubType=products&page=2) and 100 metres of 13mm poly tubing coiled and held in place on a 2x4 plank. With 10m hose pipe feeding from the pool to the piping and then a 2nd 10m hose pipe feeding from the piping back to the pool. (these hoses are not getting direct sunlight and are not black)

At mid day this system is getting a 7C gain in temperature from the intake to the output. and is pumping at a rate of 240L an hour.

(The pump is rated at pumping 15,500L/H - is mine being so severely reduced due to using 13mm poly pipe? --- And what would be best? Higher volume with lesser temperature gain or lower volume and higher temperature gain?)

The pool is a free form shaped pool where I have estimated the volume to be around 52,000 litres.

Now obviously my current setup is no where near enough to heat the pool at all, as the 7C gain from the system is dissipated almost immediately by the share volume of water in the pool of cold water.

How could I optimize my system to get the most gain efficiently. Presumably just increasing the amount of piping won't help that much because there won't be enough water circulating to make a difference.

How about 3 pumps all in separate locations of the pool each working on 200m of poly piping?

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Secondly, instead of just poly piping, as an alternative I have been tying with the idea of using a fresnel lens where it heats some copper pipes after passing through the initial heating from the poly tubing. This would make the heat gain back into the pool incredibly higher, but would it still dissipate too quickly due to the large volume of the pool?

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Thirdly, I have also been toying with the idea of having some copper piping running side by side, layered, sitting on top of the Gas BBQ grill, with a similar submersible pump and piping leading to it and back to the pool. Presumably the heat gain would be tremendous but obviously there is a large cost in running this compared to free solar energy :)

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I look forward to hearing everyone's suggestions for this project.

I have seen a lot of these projects online in my research but most of the systems are just for small pools not large ones and I feel there isn't a good solution to this yet for large pools.

Thanks for reading! :)

 

[Engineer_Phil]

If your pipe is small in diameter, the system resistance will be high. This will decrease flow and increase the pressure gain of the pump. However, you are only getting 1.5% of rated flow of the pump. I would not be suprised to hear that you burn up the pump or motor. Not to mention the pump efficiency at that level is probably terrible. Do you have the means to measure the pressure of the water leaving the pump?

Even if you do increase the flow of the water, the amount of energy you gain from the system is dependent upon the surface area that is exposed to direct sunlight.

First, I would reccomend determining why your flow is so low, and correcting the problem.
Secondly, you will probably have to increase the area of your solar collector or use mirrors to focus more light on the collector.
Thirdly, make sure the absorptivity of the collector is high (flat black paint on the pipes will help), and that pipe that is not exposed to the sunlight should be insulated.

I don't have time to run any calculations right now, but maybe later if you still need help. Let me know.

 

[johnbbahm]

First, there is something up with the pump, Your flow through 13 mm hose should be
about 24 l/ min. (about what a garden hose puts out).
Second your hose should be mounted on a reflector board like this,
http://www.homedepot.com/Building-Materials-Insulation-Rigid-Insulation/h_d1/N-5yc1vZbaxx/R-100572981/h_d2/ProductDisplay?catalogId=10053&langId=-1&storeId=10051#.UTe2B6wXU40
With a glass top, and pointed north with a 32 degree tilt.
The idea is to capture the heat, and get it to the water.
Also, look at what others have done.
http://www.thecasualtraveler.com/solar.htm
good luck!

 

[SteamKing]

You also don't state how much you want to heat the water. What is the maximum temperature rise you will need to make the pool comfortable?

 

[cwinhall]

I want to gain a temperature change of about 4-5C.as for the pump, the incline it is pumping to (roughly 2m higher than the pump lies inside the pool), would that cause such a loss in pressure?

 

[etudiant]

The setup you have starves the pump at the intake.
The intake pressure for the 100 m of 13mm poly tubing will be less than 1 atmosphere, from the pump suction.
So your poor pump is trying to pull 15,000l/hr through a half inch coiled straw 300 feet long. That is a non starter.
You have to connect the pump intake directly to the pool and use the pump pressure to push the water through the poly tubing. Hopefully the poly will not pop because of the pressure.
The other aspect is that you have coiled the poly, which limits the heat it can absorb to that provided by the sun to the area it is coiled on. The sun gives about 1kw/m2 at max, so you can estimate how much heating power your coil can provide. Spreading the poly over a larger area or concentrating the sunlight with reflecting surfaces, as has been suggested, would help.

 

[cwinhall]

The setup you have starves the pump at the intake.
The intake pressure for the 100 m of 13mm poly tubing will be less than 1 atmosphere, from the pump suction.
So your poor pump is trying to pull 15,000l/hr through a half inch coiled straw 300 feet long. That is a non starter.
You have to connect the pump intake directly to the pool and use the pump pressure to push the water through the poly tubing. Hopefully the poly will not pop because of the pressure.
The other aspect is that you have coiled the poly, which limits the heat it can absorb to that provided by the sun to the area it is coiled on. The sun gives about 1kw/m2 at max, so you can estimate how much heating power your coil can provide. Spreading the poly over a larger area or concentrating the sunlight with reflecting surfaces, as has been suggested, would help.

Sorry, i think you misunderstood maybe? The pump is a submersible pump, it is about 20cm below the water level. The 13mm tubing is set on top of the pool shed, about 2m above ground.

Could you explain how coiling the piping gets less exposure to the sun? Most of the examples I found online show coiled set ups? What would be a better shape to make?

 

[etudiant]

Thank you for the clarification.
As the pump is submerged and therefore has ample intake, the problem is most likely due to back pressure in the pipe setup.
Have you checked for kinks or similar constrictions? It is easy for poly to get distorted or pinched when coiling it. But apart from that, 300 ft of half inch hose is a lot of friction.
My guess is that you would get much better flow if you could set up 3x100 foot independent coils, fed from your 10 m input pipe. If you have them coming out of a common plenum, they should have equal pressures and flow.
Separately, the concern with coiling the tubing is that it may shade itself if packed tight.
There is a fixed amount of sunlight per square meter, so adding more tubing beyond some point will only reduce performance. You may need more area to boost your system performance.
Rough estimates to figure that out below.
You get about 1000 watts/m**2 from the sun, so about 250 calories/second/m**2. Your pool is about 50,000 liters, so 50x10**6 grams of water, so you need 250x10**6 calories to raise your pool temperature 5 degrees. At 100% efficiency, a 1m**2 collector will take 1 million seconds to heat your pool water 5 degrees. Clearly you need a much bigger collector, but even a big collector, say 100 m**2, will still take a long time to heat up your pool, especially as evaporation really chills the remaining water.
Your best bet is a solar cover, that cuts evaporation and heats the pool simultaneously.
As your pool is free form, permanent covers would be a bother to deal with, but you can get most of the benefit by solar pads or bubbles, small floating elements that absorb solar heat and also inhibit evaporation by reducing the free surface area of the pool.