DIY Solar Pool Heater

Wow I can't believe it! I made one nearly exactly like this last year except mine is 4'x8' and my coil is actually a snaking back and forth instead of concentric coils. That way it drained pretty easily at the end of the summer. When I first hooked it up I got about a 18 degree F raise in water temperature inlet to output. However, I don't think it actually raised the pool temp more than a degree or two. Just too little flow volume. Mine is also not covered because I'm trying to keep it as dirt cheap as possible and the plexi is so expensive. However, this summer I'm going to try covering the entire thing with Saran Wrap and see if it makes a difference. Should be able to cover the entire 32 sft for a buck or two.

I tried using natural convection on mine and while it works, the flow is just too slow. It doesn't move enough water volume and so the net temp raise in the pool is pretty small. Of course, pumping it through the heater coil will increase the volume flow rate but will also decrease the heat absorbed by the water flowing through it. But the tradeoff is well worth it.

I've seen a couple of different mentions on this thread about creating a termal mass, absorbing heat from pavers, filling with mud/sand, covering with plastic to block the wind, etc. I don't want to insult anyone's intelligence (sincerely I don't) but I wanted to clarify a couple of things on how these work to help everybody improve the efficiency of your system.

The water is heated by only one source of heat... the hot pipe through which it flows. However, there are two sources of heat that causes that black pipe to get hot. You need to be careful that in the course of changing one heat source you don't affect the other. The first heat source is solar radiation. This is the direct "sunlight" radiating energy into the pipe. I say "sunlight" but it's really not mostly coming from visible light and will still have an effect on a cloudy day. Think of the pipe floating in the cold vacuum of outer space. It will still heat up if exposed to the sun, even though there is no termal mass around it. This is why astronaut suits are white (in part) to reflect as much of the solar radiation as possible as compared to darker colors. The second heat source, which is entirely separate and independent of the solar radiation, is direct conduction of heat from whatever material surrounds the tube. Think of putting the pipe in the oven where this is no solar radiation. It will, obviously, still absorb heat and get hot.

If your pipe coil is on a board, exposed to the air, with no clear plastic cover, the conduction heat transfer to the pipe will be minimal because the ambient air is only mildly warmer than the pipe and any wind will tend to cool it off. Painting the backer board black will help because it will absorb the solar radiation heat, and then conduct that heat to the pipe. So that's a little conduction heat. Covering the whole panel with a sheet of glass (or similar clear material) will help becuase it will help trap the hot air, causing the air to become somewhat "superheated" and increasing conducation to the pipe. However, any cover you put on the panel, even clear, will reduce the solar radiation to the pipe a little. A good, clear sheet of glass or plexi will improve conduction more than it will negatively impact radiation, but just beware that as it gets dirty, yellows with age, etc., the solar radiation impact becomes more severe.

If you were to paint the glass black, you would block all the radiation for example. This is exactly the same effect as burying the pipe in say 2" of black mud. If the pipe is completely buried you will eliminate direct solar radiation heating of the pipe. However, now the mud itself is heated from solar radiation and has the further benefit of having a greater "thermal mass" than what used to be air around the pipes meaning that it just doesn't cool off as easily. This is why the brick pavers stay hot into the evening, because they have a substantial thermal mass and don't cool easily and quickly like the ambient air does. So, surrounding the pipes with mud, or sand, or whatever, will increase the thermal mass around the pipe and improve conduction, but will eliminate radiation heat transfer. At northern lattitudes where solar radiation is reduced anyway, this is probably a good tradeoff. In Colorado where I live, and at high altitudes where the air is relatively thin and the solar radiation is particularly intense, this is a bad tradeoff.

There are a number of solutions to this. One, you could only bury the pipe half way, or maybe 2/3 deep. This would increase the thermal mass around them improving conduction but would leave a substantial surface area exposed directly to the sun for continued radiation benefits. Two, if you want to get really creative, you could bury them entirely but in a transparent material (I'm thinking gallons of clear epoxy) which would increase the termal mass around them but at the same time would not block much radiation. There are lots of other solutions I'm sure and again, depending on your geography, your normal air temperatures, or what you might use to bury the pipes in, the tradeoff may, or may not be worth it. Just beware as you make changes as to the effect you may have on both conduction heating as well as radiation heating since they are entirely independent of each other.

Danny

My design is very similar to yours. Black plastic tubing attached to a plywood board. My tubing is spaced more loosely than yours and so I did paint the board black. 80% of my heating, if not more, comes from the solar radiation I'm sure. I'm hoping to make some tweaks this summer though. Pool goes up this weekend!

How much heat do you add? I set up my pool this weekend along with my ipswh and took some measurements to determine how much heat I'm able to capture and add to the pool. I'm curious as to your calcuations of the same to see if I may want to make changes to the flow volume rate, or perhaps other adjustments like to a larger diameter tubing. Here are my calculations
Pool temp - 54 degrees (Farenheit)
Water temp out of the ipswh - 66 degrees
Flow rate - .833 gpm (6 minutes for 5 gallons)
Density of water 8.33 pounds per gallon
1 BTU equals the heat required to raise 1 pound of water 1 degree Farenheit.
It was a sunny day and pretty warm, about 70F. I took the measurements about 2 hours before my "solar noon".
If you run all that math...
12 degree F rise X (8.33 ppg * .833 gal per min) X 60 minutes
you come up with almost exactly 5,000 BTUs per hour added to the pool. Of course it would be more when I get closer to solar noon and even more when it's closer to the summer solstice, but how does that compare to what others are seeeing? Am I undershooting what's possible with these ipswh's?
Thanks,
Danny

Intuitively I wouldn't really expect to see much difference between the high and low flow rates down to a point, all other things being equal. In either case, high or low flow rate, the inlet water temperature on an intensive sunshine day will be low relative to the energy absorbed by the IPSW. Therefore, in either flow rate case the IPSW should absorb all the solar energy available. Actually, not all available, but rather all the energy that it's capable of absorbing. Getting down into really slow flow rates we'll run into different effects like Ian suggests, but short of that my assumption is it ought to be relatively close. Slow flow rates do have the advantage of more accurate measurements both in terms of temperature change and duration of test. The other advantage of low flow rate is lower inlet pressure, and thus lower back pressure on the pump and longer pump life.

The big test that I wonder about, though, is 1/2" vs. 5/8" vs. 3/4" tubing. But I don't have the money to buy three different sets of tubing to test.

Rob,
Can you post an Excel file spreadsheet on this blog? If so, I can send you one (though you probably already have one) that can accept either metric or English measurements and spit out BTUs per hour or other conversion like Joules per minute or whatever.

Your math is all correct Dale. There is another factor though, that of the pump and the backpressure created by your choice of tubing. The more narrow the tube, and the longer the tube, the more pressure required from the pump to produce the desired flow rate. I used 1/2" tubing but my Intex pump is limited in that the max flow rate I can create through the heater is about 5 gal per 3 minutes. Too much back pressure. I would have been better off with 3/4" or 1" for sure, but then it's quite a bit more costly. Four 1/2" tubes in parallel would flow the same volume (given the same flow rate) as a single 1" but would have double the visible surface area as the 1". That's why I went with the 1/2". More surface area yields more heating. But more flow yields more heating too.