Tech Talk: How Pool Covers Save Energy

Where do pool-blanket energy savings really come from? This question is the most significant yet least understood consideration when pool owners look into the use of pool-blanket systems. In the simplest terms, swimming pool blankets stop evaporation. It's only incidental that they insulate to a small degree. Insulation, the property usually attributed to blankets, contributes from one to five percent of the energy savings for pools (a little more when used on outdoor spas). Evaporation retardation is overwhelmingly the larger contributor.

Without delving into the physics too deeply, a brief "scientific" explanation helps our pool operator to understand the significance of evaporation: Water, that master-standard chemical, requires just one calorie per milliliter (cubic centimeter) to raise its temperature exactly one degree Centigrade. Conversely, it loses one calorie as the temperature drops one degree. However, vastly increased amounts of energy are either given off or required to be "taken in" for changes of state – that's the transition between the vapor, liquid and solid states of water. For the shift from liquid to vapor, the energy required is called the "heat of vaporization" – 540 calories. It takes 540 times as much energy to vaporize any quantity of water than it does simply to raise that same quantity of water one degree Centigrade! That's a killer of a statement. Read it again...

Here is an example: We have an outdoor spa at a ski resort with snow on the ground. Water temperature is 42° C (103° F) and the air temperature is freezing – zero degrees C (32° F).

The zero-Centigrade air that is in contact with the warm water, (42° C,) naturally takes energy form the water. Ultimately, 42 calories are lost for each milliliter of water if it cools to the temperature of the air (or this much energy must be replaced by the heater to avoid that temperature drop). At the same time, each tiny milliliter of water that evaporates absorbs fully 540 calories from surrounding water... or 12 times the energy-replacement requirement as the conduction energy exchange. A spa cover was essential in this true-life example in order that the skiers could shush up and take a warm, relaxing soak.

Wind and humidity as well as radiant energy and ground conduction affect this ratio, but not enough to invalidate the example. The point is this: an impervious membrane stretched across the surface can halt evaporation entirely, thus saving a huge portion of heat energy which otherwise would be lost! And that membrane can be anything; it could even be Visqueen or Saran Wrap! Something more substantial, removable and reusable might be a better idea, of course.

Now let's take an ordinary pool on an average evening, when the air and water are within, say, 10° C of each other with low humidity and some air movement is present, significant energy loss still must be arrested. And stopping evaporation is now hundreds of times more important than insulating.

Enter the commercial pool cover.

It was near' hilarious, seeing the earliest pool covers hustled to California outdoor pool owners in 1976 or so. They were lightweight white foam slabs, often two feet by six feet in dimension, some two inches thick, strapped together in sequence somewhat like the old trick dominoes that would go click-clacking down a ribbon. Removed they stood folded in head-high stacks in the corner of the pool; deployed they covered the pool in neat, stiff little soldier rows of white with parallel streaks of black two-inch web strap, keeping in that heat by "insulating" the nasty cold air from the nice, warm pool... Did they work? Sure', but not by virtue of any perceived R-factor. Were they easy to install on the pool's surface? No way, especially when a breeze was kickin' up. Throw in an honest wind and you'd end up flying away with your magic polyfoam carpet, ending up with it in a pile of criss-crossed slab strips somewhere under the diving board. Every multi-thousand-dollar pool cover of this type that was ever sold found quick demise in pieces, parts and tangled ribbon against the chain-link fence, on the dressing-room roof, or down the block in the elementary school's front yard. It was a massive failure, all because some well-meaning, uneducated soul thought insulation was required for heat retention.

In defense of a bit of foam, a thin layer of closed-cell foam – maybe one-sixteenth of an inch thick or a bit more – laminated between green or black poly scrim surface material performs a few roles far more important than insulation: those of flotation, ease of handling, enhancement of strength and, mostly, keeping the blanket high and dry on the water's surface.

Speaking of "dry", does all that talk about hanging onto – avoiding the loss of – that energy called "heat of vaporization" give you a clue as to the worth of a wet pool cover after a rain or one puddled because it was improperly pulled across the pool? No worth at all. That wet blanket is no blanket, for a while. A few puddles are insignificant but larger wet areas result in proportionately more energy loss, most of it right through the blanket, as these wet patches evaporate and the blanket dries. "A puddle is as bad as a hole the same size" is a statement close to correct.

Evaporation cannot take place without that energy conversion we've talked about. Cooling, therefore, of that which contributes the energy always takes place. That contributor is, mostly, the densest body around. It's your pool! (The thin bit of foam insulation in a modern blanket actually does help – it slows the loss through the wet cover and "forces" the evaporation process to take more energy from the surrounding air.)

In any case it is clear that procedures to keep a blanket dry, mostly during careful deployment, are in the pool owner's best interest.

To reduce humidity at indoor pools, blowers driven by huge electric motors circulate and dry air through the natatorium. This air carries heat away from pool water. Owners then have to buy the fuel to replace that heat, as they maintain the pool at a comfortable swimming temperature. Even with the modern super-efficient air-handling systems, they burn energy dollars keeping the air just right. (See air-quality articles, PrP #11 and PrP #12.) This process of heat dissipation and replacement goes on at varying levels, 24 hours a day, though every season of the year. Put on that cover! While vaporization-loss savings may be a little less, you can match the savings of a lot of those outdoor-pool owners just by cycling down those huge blower motors.

Because these blowers are not required to be running nearly so much as they would without a pool blanket, there is a substantial reduction in electrical cost with cover use. Depending on the electrical rate, electric motors cost between 10 cents and 25 cents per hour per horsepower to operate. As an example, if air handler motors were rated at 50 horsepower total, the savings would be between $50 and $150 per day for a pool blanket used 8 hours each night. At $75, that's 27,000!

(We've even heard designers of such air-handling systems say "don't use pool covers; our system is designed to extract heat and needs the humidity." That's like saying, "We need you to be inefficient, please. If you try to save money, our system won't work!")

Energy conservation with a pool blanket is a complex subject, and accurate calculations are difficult. But the proof is in the puddin' (or the savings) and pool owners are finding that even eight hours of "cover time" a day is often saving up to one-half of their heating costs – usually paying for the blanket in six months of use or less. It's all possible if the blanket is carefully maintained, thoroughly understood, and properly (and religiously) used.

Can't get that maintenance staff or swim team to put the cover back on? The maintenance director for a well-known western university got his entire staff out on the pool deck on start-up day for his brand-new, expensive pool cover. He took his time, scanned the group, then pointed at the big blue rolls on shiny stainless reels. He said "See these pool covers? They are more valuable to me and to this university than any one of you... If I don't see them layin' on that pool, neat and dry, every night, then off and safely stowed each day, It's you that will go, not the cover! At a high school not far away, the aquatics director told the parents' group a similar story; there would be no evening Tadpole swim time if that cover wasn't in place at closing time. Wouldn't you guess those covers are in place, saving money, to this day!

~kw