Solar Pool Heater - Cheap and Efficient

Before jumping into the build, I should get your attention by giving you real numbers as tested. Too many solar projects are way too complicated, ugly, expensive or inefficient. This build is ~$50 per panel. Here is the data from real world testing:

• Full sun at 900 W/m3 as measured by my Abient weather station
• Panel area (test units) 84" x 24" = 2,016 sqin or 14 sqft. This build is for 16 sqft panels.
• Flow rate was measured as 33 seconds per quart which equates to .45 gal/min
• Temp of pool was 84* F, temp of water (steady state) exiting panel was 99* F. Delta of 15*.

Crunching the numbers yields the panel was exposed to 3,994 BTU per hour from the sun. The heat energy transferred to the pool was 3,375 BTU per hour. This is an 85% efficiency.

I got interested in trying this by watching a YouTube video of a fellow in Europe somewhere doing something very similar but with materials available over there. I tried to find the video again but couldn't or I would give him credit! I also tried the ever popular coiled tube method but the efficiency was nowhere near as good. They ended up bulky and unwieldly too.

Polycarbonate panel (Home Depot) See Here

Plastic "U channel" (Home Depot, along with panels) See Here

Flat black spray paint

GOOD quality silicone adhesive

3/8" vinyl hose

3/8" barbed fitting (right angle, T, straight....your choice)

Jig saw

These corrugated polycarbonate panels have 58 channels running their entire length. We are going to make some cuts and seal up the ends to make a very long channel that our water will travel through to pick up the sun's energy and get it into the pool. Whether you follow these instructions precisely or loosely is not important so long as you are diligent at each step. I tried various ways to make this work and this ended up being the least prone to leaking and generally the easiest.

Start by marking your dividers that will NOT be removed. The concept here is to introduce water into a channel, let it get to the other end, flow sideways into another channel through some cuts we are going to make and come back this way. Do this over and over. I found that going through just a single channel created a lot of friction and reduced the amount of water I could flow using a small pump. It also built up pressure making for greater risk of leak. I settled on using 2 adjacent channels except for each of the two outsides, which used 3. It looks like this:

up,up,up | down,down | up,up | down,down |.....| down,down,down

The two sides use 3 channels for mechanical reasons; as you'll see in the latter steps we will be inserting a round barbed fitting into a square hole. The 3/8" fitting is a very tight fit and by putting it into the center channel of 3, we minimize the risk of cracking the neighboring dividers, but if we do, they are all common anyway.

Remember, the two ends of your panel have different cuts! The 'water end' that has the connection for water in and open channels for the water to come and the 'land end' which will be totally sealed. If you are having difficulty visualizing this, look at the schematic.

I used a jigsaw. I tried one of those oscillating tools but I found this to be easier. Secure your panel so that it doesn't jump around. Insert the blade and put it flush against the bottom side of the hole. I found that high speed works best than trying to go slowly. Cut the divider to the full depth of the blade. Flip the panel over and repeat. Be sure to keep your saw perpendicular to the bottom so that you don't break through the face of the panel. This is the trickiest part of the build. If you mess up, don't worry. Saw off a couple of inches of the panel and start again. Now you know why the prototype panels were only 84" long and not 96"......

DO NOT pull the blade out before it stops moving, it will catch on one of the faces and ruin things....don't ask how I know.

You should now have about 2" of floppy plastic dividers that need to come out. I used a pair of needle nosed pliers. I found if you slide them in and grab the loose divider right near where is still connected and push the pliers in a bit further, the divider breaks right off. This is easier than the quick tug method. Pull/pick any straggling pieces and don't worry if there are minor burrs.

You should now have wide channels separated by the remaining dividers. Make sure your flow configuration is correct.

Don't forget this important step. Silicone will stick MUCH better to a roughed up surface than to a slick one. Get the sandpaper into the first inch of the channels and rough them up. Also rough up the first inch of the exterior of the panel faces.

I used a silicone named Novagard that my swimming pool contractor got me. It is not commercially available (who knows why, maybe it's too good for us mortals?) but I'm sure someone can get it for you. Whatever you use, make sure it is SILICONE and that it is listed as being an adhesive and not just a caulk. 'Below the water line' is a good indicator as to whether it will stand up to the task or not. A panel like this will leak if you don't do this step correctly. That's not the end of the world for the end that will hang over the pool, but you don't want the dry land end leaking.

I didn't use the caulking tip for this part, I wanted a fat bead. Squeeze the silicone into the channels about 1/2 to 3/4 inch. Pinch the panels to ensure good contact. Let it sit a bit and come back and fill in any areas you may have missed. Work it. Make sure you've filled the gaps. The 'land end' of the panel ends up with all the gaps filled. For the 'pool end' of the panel leave the 3 exit holes open and the center hole of the other triple channel so you can push the fitting in later. If you get some silicone in there, just fish it out. Even though we have one more layer of protection coming, try to make this sealing job as perfect as possible.

The plastic U channel should be cut to length; across the whole 'land side' and leaving space for the water-in fitting and the exit channels on the 'water side'. Lay a good bead of silicone in the bottom of the channel and a schmear of silicone on the top and bottom faces of the panel. Push the U channel into place and seal up the ends with more silicone.

The 90* fitting that I used could stand to be longer so that it is pushed into the channel deeper. If I find it leaks with time I may replace it. Depending on how many panels you make you may want to use T fittings so that you have several panels in parallel from one hose and pump. EDIT*** It did drip a bit so I replaced it with a T fitting that I was able to push further into the channel and use a clip over it to keep it secure.***

I then slipped on the vinyl hose and siliconed it to the bottom of the U channel to minimize any force that might make the fitting come loose. I even rigged up a clip to serve as a strain relief.

Once everything has cured, peel off the remainder of the protective plastic on the panel and paint with flat black spray paint. The paint will NOT stick well to any stray silicone. I had some luck with using several light coats but it comes out looking like water droplets. No big deal. I don't know if a quick wipe with acetone or roughing it up with sandpaper would help, but I may try in the future.

When these panels are laying in the sun without any water running through them, they get very hot! The first few minutes of water flowing through them will be extremely hot, so be careful. My data presented in the intro was collected long after startup and therefore in steady state. How you choose to hook your panels up is up to you as is what you use for a pump. I just use a simple submersible pond pump on a GFCI protected circuit. You could also integrate the panels into your pools return lines, but I didn't want to do that.

Remember, the goal is to get as much of the available energy out of the panel and into the pool. The best way to do that is to flow enough water to keep the panels cool to the touch. More water at a lower output temperature is more efficient than smaller volumes at higher temperature. Likewise, don't put the panels in series with one another, keep them in parallel.

When full these are quite heavy and will stay put even in a strong wind. When you shut them off they partially drain and get lighter, potentially needing to be fastened down. When empty they are feather light, stack easily and can be stored for the winter against the wall in a closet or somewhere out of the way.

I calculated that my 15,000 gallon pool would require about 6 panels of this size to raise the temperature one degree per day (6 hours of full sun). Not bad. Combine these with a pool cover to keep the heat in at night and prevent evaporation and you should be able to extend your swimming season materially for ~$50 per panel.