There's a secondary effect, which is that the more sunlight is blocked from the water, the less FC you lose each day. People with dark/opaque covers seem to have distinctly less FC loss than people with more transparent covers (or no covers).
--paulr

Even a transparent cover should block almost all of the UV, which is what breaks down chlorine. {EDIT}Except that some of them don't. See below.{/EDIT}

Plus there is the fundamental trade-off of getting enough sunlight into the pool to heat it while not getting in so much UV to breakdown chlorine.

The ideal solar cover would do the following:

1. insulate to retain heat by reducing conduction, convection and evaporation
2. let virtually all of the infrared and visible light (380 - 2500 nm) in from sunlight
3. block virtually all of the relevant UV from sunlight (290-380 nm), and would block the longer infrared (2500+ nm) to prevent radiation of heat from the pool.

Unfortunately, no such cover exists, though it would truly be something worthy to pursue and I'm a bit surprised no one has tried to figure out a chemical combination that satisfies at least the most important of these requirements (i.e. add an organic compound to absorb UV while letting visible and infrared light through. Most bubble-type solar covers insulate reasonably well, but the ones that let a lot of light in also tend to let a lot of UV as well while the ones that are darker or more opaque tend to block the light that heats the pool as well as the UV that breaks down chlorine.

Richard

I'm not so sure about this. I can't find it right now, but I do remember reading reports of those with clear covers that had higher chlorine loss and then they switched to the more opaque bluer covers and had less loss. It's true that glass will tend to block a lot of UV, but that isn't necessarily the case with pool cover plastics. I've never seen a spectral absorption (or transmittance) curve of pool covers -- that would, of course, be very interesting to see.

Looks like I spoke too quickly. A quick search turns up manufacturers advertising several clear plastics that absorb UV and several that don't. It isn't obvious which kind of plastic (and with which additives) any given solar cover is made out of, so results will probably vary.

Can you explain a little more please? I was planning on covering at night only, but I am looking for the most efficient heating (until the water temperature hits 90 degrees then I'm looking for the efficient cooling...). I am interested in how the humidity, wind, and temperature change the game. What is that general turning point where the pool water starts evaporating? Thanks!

The primary form of heat loss is through evaporation. A solar cover stops nearly all of the evaporation, but it also blocks some of the sunlight. With a solar cover during the day, you don't lose as much heat due to evaporation, and you don't gain as much heat from sunlight. How that balances out depends on how much evaporation the cover is preventing. The amount of evaporation depends on the water temperature, air temperature, humidity level, and wind speed.

On windy very low humidity days, a solar cover is worth leaving on during the day. On calm very humid days a solar cover should be removed during the day. Exactly where that boundary is on in between days depends on a number of factors and is difficult to predict.

I have a blue/silver (diamond bubble) cover. I've learned to take it off on still, hot, humid days for more solar gain (and so I can get in the pool and cool off, of course). On the dry, windy days I leave it on to reduce evaporative cooling. Religiously cover the pool at night.

When this blanket needs replacing I'm thinking about trying a clear one, just to experiment with solar gain.

kelly

Thanks to great replies!
One last question, is it better to run the filter (to mix the warm water on the top with the colder on the bottom for a lukewarm mix) while the cover is on or off?

On our pool it really doesn't matter. I can run the pump with the cover on or off and it makes no difference in the mixing effect.

If you're trying to warm up the water, circulating during the day should help a tiny bit. Whether it's enough to notice is another question.

My reasoning goes like this. If you have solar panels, you should run water through them fast enough that the water coming out is only a degree or two higher than what's going in. That is, the heat transfer is most efficient when the temperature difference is greatest. If you have a solar cover, a lot of the heat transference is by conduction, same as in solar panels; therefore if the cover is on during the day, you also want to be circulating during the day, to improve the efficiency of that heat transfer. In my own covered pool, just the other day I measured 75F an inch or two down, and 65F elbow deep; this is with the pump off. So if I ran the pump during the day, I'd get a bit more efficient warming.

Without a cover, more of the heating is by radiation, so the benefit of mixing is less clear; although I hear the radiation effect is mostly in the top foot or two of the water, so circulating during the day should still help some.

Again, hard to know whether this has any practical benefit.
--paulr