Extend the Swim Season & Save Money
By Terry Arko
Water is a precious resource that we all must work to preserve. The reduction of evaporation helps to conserve water in swimming pool environments and reduces the amount of topping off needed during the season. Recreational aquatic facilities can save water, chemicals and energy by using a liquid solar cover product as a part of their pool maintenance program. Classified as evaporation suppressants, despite the name, liquid solar covers do not heat the pool themselves. They do, however help prevent the loss of heat from the vaporization of water. Simply put, when water evaporates it requires energy in the form of heat for the process to occur. This process is the heat of vaporization. Energy is needed as water molecules begin to dance around and then disassociate into a vapor gas. Evaporation suppression is the reduction of evaporation by controlling the rate at which vapor escapes from the water surface. In other words, the dance of the molecules is slowed down considerably. The use of a liquid solar cover serves to save both heat and water reduction from evaporation. In addition, liquid solar covers help to hold the heat of radiation from the sun and thus keep the pools warmer longer.
Have you ever walked out to a pool to see water just steaming away? This process is evaporation, and it occurs when a liquid becomes a gas or vapor. Water closest to the surface is constantly changing into water vapor. Four main factors contribute to evaporation:
- The larger the surface area, the more water will evaporate.
- Water temperature in relation to the air.
- Air movement above the surface such as windy conditions.
- Relative humidity. The drier the air is the more evaporation occurs.
The strength of the radiation from the sun plays a vital role in evaporation. In the Northern Hemisphere, evaporation rates are highest during the months of June, July and August. During dry summer months, evaporation rates increase. This is because water needs heat to reach the point of evaporation. It is possible for a 15,000-gallon pool to lose as much as 1,500 gallons a month during dry hot months. Liquid solar covers are ideal for these times in reducing water loss.
The Department of Energy (DOE) states that 70 percent of total energy loss in a pool is due to evaporation (http://energy.gov/energysaver/swimming-pool-covers). According to the National Swimming Pool Foundation (NSPF) Pool and Spa Operator Handbook, 50 percent of relative heat loss is due to evaporation. When pools lose water to evaporation, continual top-offs are needed. When water evaporates from a pool, only pure water leaves. Because of this, additional solids are left behind and calcium hardness increases along with the total dissolved solids (TDS). In some cases, the contents of source water can cost a pretty penny with the necessary adjustments needed due to metal or phosphate content from tap water. The balancing process has to be started again. This leads to additional labor time and the added expense of additional chemicals. Protecting pool water from evaporation is a cost-effective way to reduce unnecessary top-offs and water chemistry adjustments. Another benefit to the use of liquid solar covers is not only the savings in heat and water, but also chemicals and precious time.
The traditional solution to heat loss and evaporation has been to use a solid cover, which does significantly decrease heat loss and evaporation. According to the U.S. Department of Energy, solid covers can prevent from 30 percent to 50 percent of water loss from the pool. Solar bubble covers are especially great at the beginning of the season to help the water heat up more quickly. There are, however, some downfalls to using a solar cover that you often don't realize until your staff is wrestling to put it on the pool each evening and removing each morning. They are bulky and tough to move, especially on large pools—not to mention the additional dirt and debris that might be introduced into the pool when dragging the cover across the deck. If a bubble solar cover is left on too long on hot days, it may even overheat the pool. This can consume chlorine more quickly and could lead to green pools.
For these reasons, recreational aquatic facilities should consider using a liquid solar product to help maintain heat and reduce evaporation. Liquid solar covers can also increase the length of the swimming season without having to deal with any of the inconveniences of a solid cover. Liquid solar covers can help in savings of heat when pools are using a heater.
According to the DOE report, it takes one BTU (British thermal unit) to raise 1 pound of water by 1 degree. However, each pound of 80-degree water that evaporates takes 1,048 BTUs of heat from the pool.
Types of liquid solar evaporation suppressants used in the pool industry include cetyl alcohol, stearyl alcohol and ethyl alcohol (ethanol). The technology of evaporation suppression uses monomolecular films. The films are a single molecule in thickness and create an additional surface tension. All of these are fatty alcohols so that they readily float on the surface. Dispersants are also used in the formulas to spread the material across the surface of the water. This technology comes from the drinking water reservoir industry. Studies on evaporation suppression go back to the early 1950s. One paper from the U.S. Geological Survey published in 1960 reports on numerous studies done. Many ingredients were tested, but the studies all concluded that the use of cetyl alcohol or stearyl alcohol was found to be effective. In some cases, combining both cetyl alcohol with stearyl was shown to be the most effective at reduction of evaporation. All three types of liquid solar products are tested and proven non-toxic. Cetyl alcohol and stearyl are both found in numerous cosmetics and detergents. They are also used for food preservation. The levels of the ethanol-based liquids have been tested and shown to be relatively benign in pool use. Ethanol can be flammable, however. Both cetyl alcohol and stearyl are not flammable and are food-grade ingredients. A study in 1956 of cetyl alcohol in reservoirs pointed out that there was no adverse effect to the quality of the water or taste and odor.
Liquid evaporation suppressants create an invisible liquid barrier at the surface of the swimming pool to help trap heat and reduce evaporation. This barrier is called a film. The film is very thin, only one molecule thick. Thus, the film is called a monomolecular film. The products are added to the skimmer with the circulation system running to disperse the monomolecular layer on the surface. Think of these products like putting a lid on a pot of boiling water. Some concerns regarding the formation of chloramines and the inability of oxidation gases to escape the pool due to the surface tension have been expressed by some professionals. It should be noted that the evaporation films are broken up when swimming or high winds or other means of breaking surface tension occur. At these times of surface tension, interruption such as heavy swimming there is plenty of opportunity for gases to escape to the atmosphere. In addition, there is some natural permeability of the films.
The purpose of liquid solar technology is to reduce heat loss and evaporation, and it works even when bathers are swimming in the pool. The monomolecular film creates extra surface tension on the water that prevents release of water vapor to the atmosphere. While the surface tension can be broken when swimmers are present, the tight molecular arrangement of the film causes it to quickly reform. The typical evaporation reduction rate of liquid covers runs from 15 percent to 40 percent. So, consider the 15,000-gallon pool that during the summer loses 1,500 gallons a month. A liquid solar cover can save between 225 to 600 gallons of water a month. Over the 3 months of summer, that is a savings of between 675 to 1,800 gallons of water. The number one difference between a physical solar cover and a liquid product is that a physical cover does nothing if it is sitting bunched up on the side of the pool. If someone forgets to put the solid cover on when the temperature drops on a cool night there will be considerable evaporation loss.
Pool professionals are now realizing the importance of offering these relatively new products to their clients. Many pool service companies are now including a liquid solar product with startup packages and explain to their customers that they should include the product in the weekly maintenance of their pool in order to remove evaporation and heat loss. A liquid solar cover product is a terrific alternative to a cumbersome and oftentimes unsightly solar blanket that can be difficult to remove and put back on, not to mention tough to store.
Evaporation of pool water, especially in the dry summer months, is also a very real problem. Evaporation will cause both heat and water loss. High evaporation rates could lead to low water levels and possible equipment damage. Pure water evaporates leaving behind calcium, salts and TDS, which will need to be addressed with additional chemicals or pool draining. Liquid solar covers work 24/7 to prevent evaporation up to 40 percent. These products can create convenience by extending the pool season. The liquid cover not only helps maintain the pool water heat and reduces evaporation, but also reduces chemical loss and helps filters run better. Liquid cover products are a great addition to save money, energy and water so recreational aquatic facilities operate more efficiently and everyone enjoys the pool more, with less work.
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