Modern Evaporative Coolers
Not all evaporative coolers are created equal. Modern machines operate more efficiently and with much less fuss than the old swamp coolers did.
Dave Emmitt owns Direct Drive Service, a company in Colorado that specializes in efficient lowmass boilers—and high-efficiency evaporative coolers. We’re not talking swamp coolers on the roof of a mobile home; Emmitt’s staff install coolers in the attics of large, site-built homes. The coolers pull in air from large gable vents, cool it by 30°F or so, and distribute it via several large ducts, typically to a hallway below. Cool air is directed by the patterns of window openings or by backdraft dampers, also known as up ducts, in the ceilings of rooms on the top story. The process is controlled by a multifunction thermostat that has the smarts to throttle back the fan speed when the setpoint temperature is about to be met, rinses out the reservoir to keep water and air clean, and partially automates much of the end-of-season maintenance.
“We install modern evaporative cooler systems in new homes that cost $2 million,” reports Emmitt. “These people can afford conventional compressor-based cooling and the higher bills that go with it, but they prefer the better comfort and fresh, clean air our systems give them. The $300 we save them on their utility bill each summer just pays for a barbeque with 50 of their closest friends.”
Emmitt’s retrofit business is also doing quite well. Colorado homeowners who have never had cooling systems generally tend to install conventional A/C systems—as do those who are disenchanted with their old swamp coolers. But if Emmitt gets to them first, they are more likely to upgrade to one of the high-quality evaporative coolers he installs, which deliver two or three ACH and wash the pollen out of that air before it gets inside. Emmitt is enjoying a growing customer base, fueled largely by word of mouth from happy customers.
Evaporative Cooling Innovations
Emmitt wouldn’t be getting the same good word of mouth if he were selling old-fashioned swamp coolers, but modern evaporative coolers are several technological leaps ahead of those old machines.All evaporative coolers— old and new—rely on the same design principle:Water can be used to cool air.Air blowing through a wet medium—a tee shirt, aspen fibers (excelsior) or treated cellulose,fiberglass, or plastic—evaporates some of the water and its dry bulb temperature is lowered (see “Wet’s the Difference:Dry Bulb and Wet Bulb Temperatures”). The cooling effect of an evaporative cooler depends on two factors. The first is the local difference between the air’s dry bulb and wet bulb temperatures; the second is the cooler’s efficiency (see Table 1, p. 27). Exactly how an evaporative cooler takes advantage of water’s chilling effect—the pathway and the velocity of the air as it passes over the media combined with the condition of the media—determines how efficient it is as a cooling device.
Today’s evaporative coolers come in one of three general designs: direct, indirect, and indirect/direct. Modern direct evaporative coolers couple high-performance media—generally plastic-coated cellulose—with low-velocity air flow. This combination maximizes moisture transfer from the wet media to the hot, dry air.The effectiveness with which an appliance makes this transfer—known in the trade as its direct saturation effectiveness— is a key measurement of the appliance’s cooling efficiency.
Direct evaporative coolers humidify the incoming air, making them inappropriate for use in humid climates. Indirect evaporative coolers take advantage of evaporative cooling effects, but use an air-to-air heat exchanger to cool without raising the humidity of the cooled airstream. Indirect/direct coolers cool in two stages. In the first stage, the air passes through an indirect cooler, which lowers the temperature without adding humidity.The air that enters the second, direct cooling stage is already substantially cooler than the outside air, and it can be cooled a good deal more because it’s still largely dry. Combining these two techniques enables efficient indirect/direct units to deliver air that is cooler than the outside wet bulb temperature.
After losing a good deal of market share to conventional cooling, there’s optimism in the evaporative cooling industry that higher-quality units will lead the way to recovery (see “Copious Refinements,” p. 28). One particularly promising new design is being manufactured by Speakman CRS—short for Clean,Renewable, Sustainable—a branch of the Speakman Company, a firm that has been producing showerheads and other water-related products for more than 130 years. The company manufactures an indirect/direct evaporative cooler, called the OASys, that was developed by the Davis Energy Group in Davis, California (see Figure 1).
The OASys system uses a single blower that pulls in outside air and directs most of it—about 70%—through the dry side of a heat exchanger that uses 14- inch-thick media to efficiently and indirectly cool the airstream without adding moisture. This partially cooled air then passes through a direct-cooling module before being directed into the home. About 30% of the outside airstream is used in the other, wet side of the counterflow heat exchanger, where it is cooled, gathers moisture, and is then discharged to the outdoors. Water from both the indirect- and the direct-cooling processes gathers in a single reservoir, which is purged with a frequency that is tied to the amount of scale-causing minerals in local tap water and the rate of water use by the system. The rate of water use in turn depends on the blower speed, which is controlled by a multifunction thermostat.
This machine incorporates a number of improvements over earlier indirect/direct evaporative coolers designed for residential use. It employs a single polyethylene cabinet that houses all parts of the system. This substantially simplifies the overall design, helps maintain tolerances, shortens assembly time, and ensures a long lifetime. The OASys also uses an electronically commutated motor (ECM) controlled by a smart thermostat, so blower speed can be changed while maintaining high motor efficiency. This is important, because overall system efficiency of the OASys is best at low fan speeds and low air flow rates (see Figure 2).
Engineers at the Davis Energy Group took these plots of efficiency at different flow rates and other test results—such as temperatures delivered under different conditions of wet and dry bulb temperatures— and performed simulations of a very efficient 1,600 ft2 home in eight of California’s climate zones.The results for Fresno are most pertinent for the Southwest, as Fresno has a hot, arid climate not unlike many locations in the Southwest (1% dry bulb 101ºF, mean coincident wet bulb 70ºF).The base case home with a conventional DX air conditioning system rated at 12-SEER uses 1,890 kWh per year with a peak of 3 kW, while the OASys uses 135 kWh per year with a peak of 0.52 kW. This amounts to an annual energy savings of 93% and a peak demand savings of 83%.
In another evaporative cooling development, AdobeAir has recently introduced its own series of efficient evaporative coolers that fit in a sidewall (see Figure 3).These units are easy to install either in new dwellings or as retrofits and have clear aesthetic and mechanical advantages over rooftop or concrete-pad-mounted coolers.
Water and Energy Use
There’s no getting away from the fact that an evaporative cooler relies on, and so consumes, a significant amount of water, but the newer units use up much less than the old-fashioned coolers do. Evaporating 1 lb of water yields about 1,060 Btu of cooling. Accordingly, if an evaporative cooler were 100% effective, 1 gallon of water would yield roughly 8,800 Btu of evaporative cooling. If the flow of water and the flow of air are well matched in a carefully designed evaporative cooler, the air is cooled efficiently and most of the water in the media is evaporated. However, some extra water is needed to flush out the residue of air pollutants and scale in the water. In inefficient units,water that is not evaporated by the cooler is continuously diluted by makeup water in the
reservoir or sump, with the residue going down an overflow drain. This bleed system continuously dilutes the water and reduces the concentration of scale and impurities, but this method of cleaning wastes water.
Higher-quality units use a more effective and less wasteful batch process to deal with impurities. The sump is typically sloped so that heavier pollutants and scale tend to collect at the bottom. There is no continuous dilution; instead, after the cooler has run for several hours, the reservoir is drained and flushed automatically. Well-designed machines key the need for dumping to the elapsed run time of the pump so as to keep the sump full, but not overflowing.The residue of several gallons from this sump dump may be piped to a nearby garden. With this system of periodic purging, almost all of the water—over 95% in most places—is used to provide cooling.And the amount of water that is discharged is well matched to the needs of a garden; more water is delivered on hot days when the evaporative cooler works the most and plants are especially thirsty.
The trade-off for this water use at a home is that evaporative coolers reduce a power plant’s use of water to generate electricity, because they use substantially less energy for cooling than conventional direct expansion (DX) air conditioning systems would. Generating 1 kWh of electricity with a thermoelectric plant in the Southwest uses about 0.5 gallons of water.
I ran simulations to estimate the energy and water used for cooling in six cities in the Southwest (see Table 2). The homes modeled are efficient 1,800 ft2 structures whose overall energy use is 48% lower than that of homes that just meet the requirements of the year 2000 International Energy Conservation Code for the weather conditions associated with each city. I assumed that the DX systems have an energy efficiency rating (EER) of 11.1—roughly corresponding to a seasonal energy efficiency rating (SEER) of 12.9—and a thermostat setpoint of 76°F.
According to this analysis, modern residential evaporative coolers in the Southwest use an average of 5,800 gallons of water per year at the site, ranging from 2,400 gallons in Cheyenne to 8,600 gallons in Phoenix. For singlefamily households, this figure represents an average of only 3.3% of annual water use. However, from the overall environmental point of view, which takes into account water used at the power station, net water use for evaporative cooling averages 3,900 gallons of water per year, ranging from 1,700 gallons in Cheyenne to 5,900 gallons in Phoenix.
Most important by far is the savings in electricity use—and cost to the consumer— achieved by using evaporative instead of DX-based cooling (see Table 3). I compared the annual cost to the end user of using either a DX-based or an evaporative system to cool an 1,800 ft2 new home that slightly exceeds Energy Star standards in five Southwestern cities.When local water rates are higher with increased consumption, the computations shown assume the higher marginal cost per gallon of water used. Water and electricity rates applicable to singlefamily residences in each city in 2003 were used to estimate costs.
First costs of cooling equipment tend to be a function of their efficiency, whether the systems are conventional or evaporative coolers. In the case of conventional A/C units, split systems have more than 3 times the market share of packaged systems. Average costs weighted for market share are $1,771 for A/C equipment and $3,265 for installed costs.
Single-stage evaporative cooling systems that have a saturation effectiveness of greater than 80% under all operating conditions, variable-speed (or at least two-speed) motors, and a sump dump feature for effective cleaning with minimal water use, cost from $600 to $1,120, depending on saturation effectiveness and blower horsepower. Blower horsepower is the principal factor that determines air flow rates. Equipment for indirect/direct evaporative coolers whose saturation effectiveness is in the 105%–110% range cost from $1,700 to slightly less than $3,000. Installation costs are lower than they are for central A/C systems, largely because ductwork is substantially simplified. Installations on a concrete pad next to a home cost from $600 to $1,000, while attic installations run from $800 to $1,400, depending on the number of upducts that must be installed, and on such factors as access to plumbing and electricity.
Considering these cost ranges, the total installed cost for an efficient single-stage evaporative cooling system is typically between $1,600 and $2,200. The total installed cost for an efficient indirect/direct evaporative cooler is on the order of $2,500 to $3,500. In general, installed costs for efficient evaporative equipment are lower than installed costs for comparable compressor-based central cooling systems. Lifetime (20- year) costs—including first costs, maintenance costs, and energy costs over 20 years—are on the order of $5,500 in the Southwest. For a comparable compressor- based cooling system, lifetime costs would be roughly double, depending on the local climate.
Utility Incentive Programs
Meeting demand for electric power during peak periods in the summer is a major—and burgeoning—problem for most utilities in the fast-growing Southwest. Indeed, peak demand is rising faster than total electricity sales throughout the region. Most new homes in the Southwest include air conditioners whose demands are at least 3 kW—sometimes much more—and existing housing is increasingly being retrofitted with conventional air conditioning. Given these considerations, a number of utility companies have initiated demand-side management (DSM) programs that provide incentives to owners of both existing and new homes to install energy-efficient evaporative cooling equipment.
In California, both the Pacific Gas and Electric Company (PG&E) and Southern California Edison (SCE) have programs that provide incentive payments of $300 to $500 for the purchase of energy-efficient evaporative coolers. To qualify, units must have a saturation efficiency of 85% or better; must have sump water removal systems (no water-wasting continuous bleeding systems); and must be configured to automatically exhaust air through pressure relief dampers (up ducts) into the attic, and then through attic vents to the outdoors. SCE requires a variable-speed fan and a dedicated thermostat remote from the cooler. Both utilities offer an additional rebate of $100 for the installation of up ducts in the attic.
In Colorado, Utah Power and Xcel Energy are expanding their programs that support evaporative cooling installations. Both utilities are experimenting with ways to steer consumers toward buying highly efficient units while still providing some incentives for lower-end evaporative coolers. Other utilities in the Southwest either have small-scale programs in operation or are in the planning stages.
I am hopeful that these types of program, as well as partnerships between programs like DOE’s Building America and those conducted by local utility companies, will persuade production builders to construct model homes that illustrate the advantages of excellent evaporative cooling.These examples could help to establish the credibility of modern evaporative cooler systems that are appropriately integrated into welldesigned homes.
To bring the technology to full fruition, designers and builders need to think of evaporative cooler systems as systems thoroughly integrated into energy-efficient structures. Techniques for sealing them carefully and simply during shoulder and winter seasons, and to eliminate the risk of freezing, need to be developed. Up ducts need to be redesigned to be thoroughly insulated and positively sealed during times when cooling is not needed, and optimized to ensure good distribution of cooling air. Further, controls need to be developed that not only vary fan speeds and control water-cleaning cycles, but also monitor efficiency performance to signal the need for maintenance. Finally, there is room for improvement in the heat exchanger technology used in indirect cooling systems. Several companies are working to develop more efficient systems that require less pressure drop across indirect media while achieving more effective cooling.
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