Solar pool heater an experiment

[C.R.J.]

So I got bored. here is the result.
solar_pool_1[1].jpg
A bit of background.
I had a solar water heater on the roof of my house a few years ago. (pic 6)
solar_pool_6[1].jpg A massive hailstorm put paid to the glass tubes. i did replace them, but after numerous leaks I finally gave up and removed it.
being a member of the 'secret squirrel club' i stored all the glass tubes. I finally decided to use them

I re-framed the tubes, added a small pump, some garden hose and a couple of 10w solar panels to make a solar water heater for my pool.
It is a circulation system. The water from the pool comes out from the drain to a vertical stand pipe. (pic 3).
A small 12v pump is fixed to the stand pipe. pic 2
. solar_pool_2[1].jpg
The water is pumped through the tubes back into the pool. (pic 4)
solar_pool_4[1].jpg
The amount of sun decides how fast the pump runs. (No sun, and its off.)
The temperature difference between the water leaving the pool and then returning to the pool is quite variable. As high as 15 degrees C, as low as 5 degrees C.

The PVC pipes and T pieces do not leak due to high temperatures, as the maximum temperature I have measured at the output is 42 degrees C in August.
solar_pool_5[1].jpg
It does not mean I can use the pool in winter! It does however extend the use of the pool by a couple of weeks.
I am getting a pool cover next year, so expect that to increase the time the pool is usable.

I don't think for a moment that this is the answer to all pool heating solutions. It was just an experiment, that surprisingly seems to work better than i expected.
If i was to repeat this experiment, I would make a few alterations. I would insulate and bury the return pipes, use better connections to the input and output of the glass tubes. Use more tubes and a different pump (this one was a bit flimsy).
However, overall i am quite pleased with the result.
Is it commercially viable, I don't think so!

ok back to the pool for me!

CRJ

PS, First time i have posted pics so hope its ok.

[Logan5]

Do you have your PV panels mounted covering part of your thermal collector?

[J.P.M.]

That seems a small collector to be adding what sounds like a rather noticeable amount of heat to such a relatively large pool. Looks like about 5ft^2 of net collector surface which might get you about 7,500 BTU/day (~ 2.2 kWh/day.), and raise the temp. of a 10,000 gal. pool by ~ 0.1 F. or so. Or is there more to the collector system than shown ?

[C.R.J.]

LOGAN5. yes i put the panels on the tubes. It was a quick way of making sure that when the panels get the sun, and start heating up, the panels also run the pump at the same time.
later i will mount the panels as close as possible to the tubes. I did not want the tubes heating up too much before the PV panels produce power for the small pump.

[Logan5]

Ok, as long as you are aware that this arraignment is counter-productive, especially with already inadequate thermal surface area.

[C.R.J.]

J.P.M. I do not disagree too much with your figures. As you say it is not making a great difference to the overall pool temperature. The figures I gave were to show the difference between the temperature of the water going into the collector, and the temperature of the water returning to the pool. It would require a lot more tubes, and a corresponding increase in flow, to make a real difference to the overall temperature of the pool. It is also necessary to fit a pool cover to help maintain the temperature. ( My next project!). We do get an awful lot of sunshine throughout the whole year here in my part of Spain, which helps of course. The average temperature this year from June to middle of September was 37 degrees C. With a high of 42 degrees C on August 20th. It goes without saying, that if I had to buy the tubes and most of the other items, I would not have built this. The tubes and panels were taking up space in my crowded workshop and I cannot throw anything away. ( my wife will testify to this fact!). If i come across some more 'free' tubes, i may add to the collector.

[J.P.M.]

J.P.M. I do not disagree too much with your figures. As you say it is not making a great difference to the overall pool temperature. The figures I gave were to show the difference between the temperature of the water going into the collector, and the temperature of the water returning to the pool. It would require a lot more tubes, and a corresponding increase in flow, to make a real difference to the overall temperature of the pool. It is also necessary to fit a pool cover to help maintain the temperature. ( My next project!). We do get an awful lot of sunshine throughout the whole year here in my part of Spain, which helps of course. The average temperature this year from June to middle of September was 37 degrees C. With a high of 42 degrees C on August 20th. It goes without saying, that if I had to buy the tubes and most of the other items, I would not have built this. The tubes and panels were taking up space in my crowded workshop and I cannot throw anything away. ( my wife will testify to this fact!). If i come across some more 'free' tubes, i may add to the collector.

Fun to play with but know that plastic/vinyl collectors made for the purpose of pool water heating will outperform vac. tubes for pool heating any day of the week. After the cover, expect to need/start with a solar pool heater that's ~ 1/2 the surface area of the pool and bump the size until adequate. That vac. tube assembly looks like about 0.5 m^2 effective area.

Your small PV is similar to a design I came up with as an undergrad engineering student. It worked OK for a student project, but had limitations. One thing I did figure out was that the flowrate could be optimized a bit for time of day (irradiance level and to some extent exterior temp. influences) by not necessarily having the PV in the same plane as the collector. The bigger problem however, was getting some vague linearity of flowrate with irradiance. Turns out that the PV was not a good match with the collector flow parameters when trying to maximize thermal output. It was, and still is, much easier and more efficient to slightly oversize an A.C. powerd pump and throttle the output a bit if needed/desired.

Vac. tubes are mostly best used for temps. closer to process heat applications - say > 100 C. For domestic H2O heating and pool heating they are gross technical overkill for the application, very especially in warm climates. For overall heat collection, efficiency and cost as well as maint., for pool heating, cheap(er) Fafco type heaters will outperform vac. tubes every time.

Do not make the common mistake of equating quantity of heat (lots of water heated a little bit), with quality of heat (temperature). The best and most cost effective designs always go for as much of the first (quantity) as needed/possible and no more of the second (temperature) than necessary for the application. Put another way, if you need 30 C. water, don't use equipment that will produce 100 C. water. That's a waste of equipment, resources and money.

As for performance of vac. tubes as f(flow rate), the net heat (not temp.) output is much less sensitive to flow rate for vac. tubes than for flat plate or pool heaters, mostly because of the relatively low overall heat loss coefficient that vac. tube or heat pipe tech. produces. The benefit of that improved thermal efficiency (and the cost of achieving it) that the low loss coeff. achieves is mostly wasted at DHW application and most certainly very low pool heating application temps. The cutting butter with a chain saw analogy applies.

Have fun learning.I sure did and still do, but, as you imply, a pool cover is necessary and almost always a more cost effective and usually more practical first step to pool water heating.

Good luck.