We’re long past the End of Owgust. The End of September is upon us, and my pool is still at 93° F. Why? It has nothing to do with global warming, and everything to do with a 20′ by 40′ sheet of blue bubble wrap. Back toward the end of spring, Carol and I bought a swimming pool cover. It came in a box, and it was just what I described: a 20′ X 40′ sheet of bubble wrap. I had to modify the corners a little to make it fit my idiosyncratically shaped in-ground pool, but that took less than an hour with a pair of ordinary scissors.
Now, in a riproaring Phoenix summer, you don’t need no steenking pool cover to keep your pool upwards of 90°. Just sitting there in the sun all day, my 44,000 gallon diving pool hit 95 degrees in early July all by its lonesome. But as we got into September and the days got shorter, the water temp soon fell to 85°. This feels great when the air temp is 110, but the air temp fell with the length of the day, and especially in our late-evening dips before bed, the air temp was in the high 80s and the water actually felt better warm. So two weeks ago, with the water temp at 85°, we dragged out the pool cover and wrestled it into place on the water.
Then, day by day, we boggled as the water temperature rose. By this afternoon, with a daily high a mere 100° (lukewarm by Phoenix standards) the water was at 93°. Carol’s sister Kathy is coming down for a visit in ten days, and she’s expecting warm days and warm pool water. The days will be in the 90s, which, if your’re accustomed to Chicago weather, is plenty warm. The challenge is to keep the water above 90° into the middle of October. I was a little worried about that.
Not anymore.
It’s been an interesting science experiment. At the end of a sunny day, the water immediately under the pool cover comes very close to 100°. That’s just for the top 2″ or so. When the pool pump kicks in at 8PM, it mixes that hot top layer with the cooler water beneath it. Come morning, the water is all mixed, and has gained half a degree or more throughout. The pool cover prevents most of the radiation by which pools lose their warmth at the end of summer. With the Sun to add new heat every day, and the cover to prevent it from radiating into the night sky, the pool accumulates heat. I think that 93° is the equilibrium temperature when the daily highs hover around 100°. We’re heading into a cooler week, so I don’t know precisley how that’s going to go, but as long as I can maintain 90° I’ll be more than happy.
The cover cost about $200. This may seem high for a sheet of bubble wrap, but in truth, it’s not the same kind of bubble wrap you get at the UPS Store to stuff in around the knicknacks you’re shipping to your godmother. The plastic is heavier and more rugged, and with some luck and careful handling could last 3-4 years.
Kathy’s visit will be a good test, but the primary experiment is to find out how long the cover can extend pool season, which by our definition is when the pool is at 82°or higher. We’re expecting to make it to Halloween, and–given reasonably warm weather and all sunny days–hoping to make it to Thanksgiving. There will be another experiment next March or April, to see when the cover brings the pool temp up to 82° for the first time in the season.
Sometime this winter, we’re going to have the pool “depth-modified,” which means that they’re going to jackhammer out the plaster, fill in the 9′ deep end, and replaster it to become a “play pool,” which will be 5′ in the center, and 3-4′ deep at both ends. I was never much of a diver, and I think we may score a discount on our homowner’s policy for getting rid of that 9′ depth. With only 30,000 gallons or so in the modified pool, who knows? We could be in the water 9 months out of the year. Maybe more.
Solar power rocks. It isn’t all photovoltaics.
Wow. In the unlikely event you needed to heat your house, you could use that to collect the heat. IMHO the circulation pump mixing the warm water with the cold water also lowers the overall water temperature, which (if my weak grasp of physics holds) should also reduce radiant losses through the cover and walls of the pool.
Re: reducing the depth of your pool: what would happen if you just put a concrete grate at the desired depth and used the rest of the depth as a thermal reservoir? Thinking on that it seems like it’d trap the cold water, but in the depth of summer when the pool’s getting too hot, being able to circulate a shaded, cold mass of water into it at will might be good.
Expect a call soon.
-JRS
When the pool pump kicks in at 8PM, it mixes that hot top layer with the cooler water beneath it.
I wonder if the water would be heated more efficiently if there was some circulation while the sun was on it. If the heat transfer is sun to cover to water it should. If, however, the sun passes through the cover and heats the water directly I doubt it would matter.
Actually, yes. I found by experimentation that warm water collects in a layer 2-3″ thick immediately under the plastic cover. About 2 PM I run the pump for an hour to mix the warmer water with the cooler water below it. (Ordinarily the pump runs at night.) If I left the warm later right under the cover, the cooler night air would take some of the heat by simple conduction. As I write this (10/25/2018) the pool is at 85 degrees, and we’re looking at a warmer weekend, so it may get up to 87. That’s actually pretty warm by any measure, and remarkably warm for this time of year, even by Arizona standards. We were in it yesterday, and will likely be in it tomorrow.
The pool cover has another profound effect. In your low-humidity climate, it substantially reduces water loss due to evaporation.
Have you talked to Guy Wicker lately? He was working on a project to float PV panels on top of water reservoirs in arid regions to stop water evaporation *and* generate power. One of the uses for the power was for pumps to push the water uphill.
As a certified, or should I say certifiable :-), germphobe the thought of this pond of water sitting there for months on end near body temperature gives me the willies. You must have to do some fairly heavy duty treatments of the water to keep down the germs.
Well, there is all that nice, UV-laden sunlight. Nice clear water for the UV to penetrate, too.
Chlorine is pretty good at killing the stragglers.
I don’t think the UV will penetrate the bubble-wrap. UV is blocked by most plastics.
Pools have been around for a long time, and there’s a lot of compiled experience in books and on the Web. Carol and I had pools from 1987-2003. The current one we’ve had from 2015 to the present. I have test strips, and I test the water every day. Chlorine tablets work well. We’ve never had any trouble with bacteria or algae, even when the water gets obnoxiously warm in early July.
Sam, water strongly absorbs UV radiation therefore the radiation only penetrates a few millimeters below the surface leaving all the germs below that unharmed. See the bottom graph on the first page of the wiki article. The graph plots the attenuation coefficient as a function of wavelength. The half value layer, i.e. the thickness at which half the radiation is absorbed is 0.7/attenuation-coeff. Notice that at 100 nm, it is about the attemuation coefficient is about 10^8 1/mm resulting in L_0.5 ~ 10^-8 mm . It does drop steeply so at 200 nm it is 10 1/mm so L_0.5 = 0.1 mm, still very small penetration. So the UV in the sunlight does not have much germicidal effects. Jeff, what do your test strips measure? Is it the Cl concentration?
There is total chlorine and free chlorine, and the strips I use test for both.
Now, I’m pretty sure that most UV goes through a LOT more water than a few millimeters, especially if the water is clean and clear, and I keep the pool damned clean. Also, unless I misrecall, in the developing world they purify water by putting it in plastic soda bottles and setting the bottles out in bright sunlight.
I’m not saying you’re wrong, but this goes against what I’ve read for decades. It doesn’t matter in the current discussion, because I don’t count on UV to assist in sanitization. I do know that some recent hot tub designs use a UV sanitizer, but I don’t know precisely how they do it. In a narrow piece of tubing, a centimeter or so may be all the penetration needed. I’ll look into this more when time allows.