Solar Cover and Reel Recommendations?

[duraleigh]

Hi, Kay,

If I read your post correctly, "that solar input would ever be greater than losses from evaporation." wouldn't that mean that a pools' temperature would never rise during the daytime? Mine sure does.

As to whether the cover adds or subtracts from heat gain is, I think, dependent on all the other variables associated with the cover off (namely wind, RH, color of the pool, etc.)

[kaybinster]

No, did not say that. Just stating that there is a huge loss of heat due to evaporation. I strongly believe that you are better off with the cover on as it reduces evaporative losses. I guess you could try it for a week with the cover on and a week with the cover off and (if conditions are the same) see which works better for you. I have done it both ways and find when I leave the cover on during the summer my heatpump almost never runs, but when I leave it off during the day the heatpump has to run more.

[matt4x4]

HEAT energy is needed for evaporation to occur, this means that you're putting heat into your pool, only, some of that heat is used to evaporate some of the water. The remaining heat will warm the remaining water, meaning it is inputting BTUs , not removing. This is why your pool will warm up during the day and your water level will drop at the same time.
You cannot input BTU and take away BTU at the same time, there are many factors at play here, wind itself plays a big role, so does relative humidity, since the humidity is high where we are, we lose way less water in a day than some person in Arizona on an identical day, yet, both our pools managed to warm up over the course of the day - mine probably warmed up more because i did not lose as much water to evaporation, but both will have warmed to some degree.
You could never get actual numbers using a pool since the environment is not controlled and outside factors are ever changing, however, in general terms, it has been shown numerous times in this forum that a pool with the cover off will warm up more than a pool with the cover on.
Mind you, these tests were likely performed on two days back to back where the weather was similar, yet never identical.
If you use a non scientific logical approach, you can deduct that this is true. Placing a cover over the water does stop evaporation, BUT, the cover reflects much of the heat so you have a big heat loss due to reflection. This is heat that NEVER even gets to your water, so you've just lost efficiency, it also does not allow much heat to penetrate through the cover, essentially the cover does warm up and transfer some of that heat into the water directly below it and also loses some of that heat back into the air above it. The water below the cover does NOT heat the water beneath it very deeply since heat rises, so it just reheats the cover which reheats the water BUT also the air above it.....so you get a 1-2" warm layer under your cover and the remaining 4 feet are unaffected.
If you take the cover off, the heat travels through the water without much loss since water is relatively opaque for the depth of a pool and not much is lost to reflection or absorption, these energy rays (heat) are now heating the floor and some of the walls of the pool - adding the floor area and partial wall area together also shows that there is more surface area now absorbing heat (yes, there will also be some reflection), this heat will transfer to two adjacent surfaces as well, the ground beneath the floor and the water above it. BUT, the heat that transfers to the ground will help warm the water since it will RISE back into the floor of the pool rewarming the ground and WATER above it etc etc...
The water warming at the BOTTOM of the pool will help heat the water above it as the heat rises, the water at the surface of the pool is affected the least since most of your heat transfer happens at the bottom and dissipates as it rises leaving the top surface at a relatively stable temperature while the lower water warms at a much higher rate because you have a lot less wasted energy to the outside environment.

In the end, all I care about is that my pool warms up in the summer, not cools down.

[kaybinster]

Well this is contrary to my experience. Further, I disagree with your statement that heat input is needed for evaporation. If you believe that then explain to me how if you put a glass of water on your kitchen counter the level will drop over time. The water and the kitchen are at the same temperature. Water will evaporate, cooling the water in the glass, which then absorbs heat from the air in the kitchen to maintain an equal temperature with the room. Thus, although trivial in amount, the evaporation process is removing heat from the air in the kitchen. The same happens with the pool. If the evaporation causes the temperature in the pool to drop below the air/ground then it too will absorbe heat, if it is higher it will not.

HEAT energy is needed for evaporation to occur, this means that you're putting heat into your pool, only, some of that heat is used to evaporate some of the water. The remaining heat will warm the remaining water, meaning it is inputting BTUs , not removing. This is why your pool will warm up during the day and your water level will drop at the same time.
You cannot input BTU and take away BTU at the same time, there are many factors at play here, wind itself plays a big role, so does relative humidity, since the humidity is high where we are, we lose way less water in a day than some person in Arizona on an identical day, yet, both our pools managed to warm up over the course of the day - mine probably warmed up more because i did not lose as much water to evaporation, but both will have warmed to some degree.
You could never get actual numbers using a pool since the environment is not controlled and outside factors are ever changing, however, in general terms, it has been shown numerous times in this forum that a pool with the cover off will warm up more than a pool with the cover on.
Mind you, these tests were likely performed on two days back to back where the weather was similar, yet never identical.
If you use a non scientific logical approach, you can deduct that this is true. Placing a cover over the water does stop evaporation, BUT, the cover reflects much of the heat so you have a big heat loss due to reflection. This is heat that NEVER even gets to your water, so you've just lost efficiency, it also does not allow much heat to penetrate through the cover, essentially the cover does warm up and transfer some of that heat into the water directly below it and also loses some of that heat back into the air above it. The water below the cover does NOT heat the water beneath it very deeply since heat rises, so it just reheats the cover which reheats the water BUT also the air above it.....so you get a 1-2" warm layer under your cover and the remaining 4 feet are unaffected.
If you take the cover off, the heat travels through the water without much loss since water is relatively opaque for the depth of a pool and not much is lost to reflection or absorption, these energy rays (heat) are now heating the floor and some of the walls of the pool - adding the floor area and partial wall area together also shows that there is more surface area now absorbing heat (yes, there will also be some reflection), this heat will transfer to two adjacent surfaces as well, the ground beneath the floor and the water above it. BUT, the heat that transfers to the ground will help warm the water since it will RISE back into the floor of the pool rewarming the ground and WATER above it etc etc...
The water warming at the BOTTOM of the pool will help heat the water above it as the heat rises, the water at the surface of the pool is affected the least since most of your heat transfer happens at the bottom and dissipates as it rises leaving the top surface at a relatively stable temperature while the lower water warms at a much higher rate because you have a lot less wasted energy to the outside environment.

In the end, all I care about is that my pool warms up in the summer, not cools down.

[matt4x4]

Sorry, I should not have said heat input, I should have said energy input, energy is needed, this energy is converted to heat which in turn evaporates your water.
Leaving a glass sitting on your counter doesn't mean the world stops turning, there is air circulating (energy), there is light (energy), your house still warms and cools during the day (energy).....
Just like a microwave, you don't just put in something, press a button and voila - magically it's now warm - it must be magic, because the inside of the microwave is still cool, so we didn't add heat, and really it just sat there for 2 minutes and now it's hot, it must be magic.
The microwaves that are generated transfer energy to the particles that make up the food, the particles get excited by this energy and convert it to heat.

Here, this is from the US govenment....

Heat (energy) is necessary for evaporation to occur. Energy is used to break the bonds that hold water molecules together, which is why water easily evaporates at the boiling point (212° F, 100° C) but evaporates much more slowly at the freezing point. Net evaporation occurs when the rate of evaporation exceeds the rate of condensation. A state of saturation exists when these two process rates are equal, at which point the relative humidity of the air is 100 percent. Condensation, the opposite of evaporation, occurs when saturated air is cooled below the dew point (the temperature to which air must be cooled at a constant pressure for it to become fully saturated with water), such as on the outside of a glass of ice water. In fact, the process of evaporation removes heat from the environment, which is why water evaporating from your skin cools you.

[CarlD]

Matt:
You have forgotten a major component in your analysis of solar covers: Forced circulation--the colder lower water is pulled in low down and returned just under the cover where it mixes with the warmer water and heat exchange occurs.

A solar cover performs several functions:
1) Insulation--heat rises, whether it is warm air or warm water. That's because the heat forces a given mass of the fluid to expand to a larger volume, therefore making it lighter--convection. That heat will pass into the air warming it, and losing it from the water. So you insulate the water for the same reason you insulate your roof.
2) Conduction. Opaque covers transfer heat by absorbing light, hopefully at a greater rate than water, and the water cools the underside, transferring the heat energy to the water--then the circulation Matt missed has to kick in, or he will be right--it is fairly ineffective.
3) Transmission. The more light a cover can pass the more heat energy the water directly absorbs--greenhouses have used this principle for at least 200 years (George Washington's Mount Vernon has a wonderful greenhouse the First President used for his botanical experiments). True, some light is blocked, but the trapping of the fluid under the greenhouse (in this case our clear solar cover) more than makes up for that loss.

Evaporation is partly dependent on heat energy, but also on the absorbtion capability of the air--cold air doesn't hold much water, that's why when it's really cold--low teens, single digits or sub-zero, your joints don't ache nearly as much as in that nasty wet high 20's low 30's cold. The air is dry. But evaporation is still at work (check your freeze-dried instant coffee or cup'o'noodles). Evaporation due to non-heating--wind-- pulls heat out of the water. It's basic to the laws of thermodynamics and your fridge and A/C wouldn't work if that wasn't true.(there's more to it, but this is still critical).

Notice that even if your hands are warm, rub alcohol on them and they get cool. Shake them and they get a lot cooler. Put them in front of a fan and they get downright uncomfortable. Wind-based evaporation is endothermic so it drops the water temp. The solar cover prevents that.

OK. the bottom line: I'm now of the personal opinion that solar covers are necessary to keep your pool warm and I personally prefer the clear ones to the transparent blue ones or the opaque.

[huskerfan]

What size is your pool? AG or IG? I have a brand new (tested a few times only) Odyssey reel I'd sell pretty reasonable. I have 2 reels; long story .

[matt4x4]

Hi Carl,
Well said, I too agree that an opaque solar cover is probably the best of all worlds since it allows the rays THROUGH as well as stop evaporationa and evaporative cooling - my entire argument though was just based on the fact that leaving a regular cover off does NOT decrease the warming effect on a pooll, rather it allows more energy to get into the water causing more heat buildup from the bottom up which is probably your best case scenario, yes, some is lost again due to evaporation etc, but in the grand scheme of things the WHOLE pool gets warmer with the cover off.
Now all this said, I'm mainly oing off my pool experiences and those i read about over the last few years (all AG), I don't read much of the IG stuff - it has no bearing on my pool, unfortunately, having an AG pool, I don't have the luxury of taking my cold water from below and mixing it in above - it stays below, (probably a whole different scenario with a whole different set of parameters giving a whole different set of results) the only mixing that occurs is on the top, so leaving the blue cover on causes a 2" warm spot on top with a 4 foot darn cold area below.
Oh, and pointing my return down to help mix things up doesn't work well with my pool, I end up with bad circulation over most of the pool...

[huskerfan]

The reels are so expensive... any recommendations on reels and where to get a good one cheap?

What type of pool do you have- AG or IG, how big?? I have an extra reel that is only tested- brand new I'd sell reasonable.

[MaxxFusion]

What type of reel is it? I may be looking for one for a 18' Round AG.

You can shoot me an email at frank@maxxfusion.com

Thanks,