Reposted article from Western's Energy Services Bulletin

The efficiency of standard, direct-expansion (DX) compressor-based air conditioners can decline sharply as outdoor temperature increases.  In contrast, the efficiency of air conditioners using evaporative cooling increases as outdoor temperature increases.  Hybrid systems that make use of both types of cooling, switching between them based on outdoor temperature, can increase overall efficiency significantly.  Such hybrids are often used in larger commercial-scale applications, but until recently have not been available in a size with features suitable for residential and small commercial buildings.  

Where hybrids work best

Evaporative cooling technology takes advantage of the fact that when water evaporates it cools the surrounding air.  Evaporative cooling can be accomplished directly or indirectly.  In direct evaporative coolers ("swamp coolers"), moisture is evaporated directly into the air supplied to the conditioned space. This cools the air but also increases the humidity.  In indirect systems, moisture is evaporated into a second air stream, which is then used to cool supply air to the space via a heat exchanger. 

Indirect evaporative cooling has lower efficiency than direct cooling, but avoids the high, uncomfortable humidity that can occur with direct evaporative cooling.  Commercially available hybrid evaporative/DX compressor systems generally use indirect evaporative cooling, with some units providing a second stage of direct evaporative cooling.  

While direct evaporative cooling is only practical in dry climates, hybrid indirect evaporative/DX systems can be used over a wider range of climate zones.  During hot, humid weather, evaporative cooling must be supplemented by compressor-based cooling.  Even with the addition of compressor-based cooling, the technology is still most effective in dry climates.  One of the first steps in deciding if this type of system is a good fit for a specific project is to determine if the local climate is suitable for indirect evaporative cooling. A color-coded performance table from Coolerado shows the ability of the manufacturer's five-ton hybrid unit to meet comfort conditions in locations throughout the world.   

Increased efficiency

The increased efficiency of hybrid indirect evaporative/DX systems compared to conventional systems can be significant, but varies widely depending on the application and location. Generally, in hot dry climates, the systems can reduce cooling power demand by up to 80 percent and cooling energy use by as much as 70 percent.

A study by the National Renewable Energy Laboratory found that the Coolerado's five-ton rooftop hybrid air conditioner has an energy efficiency ratio (EER) of 21.7 at 105°F and 25 percent relative humidity. At 90°F and 25 percent relative humidity the unit has an EER of 15.8.  As a point of reference, similarly sized light commercial air conditioners must have a minimum rated EER of 11 to qualify for an Energy Star label.   

Water use

Water use by evaporative coolers is a point of concern, especially since this technology is most energy-efficient in dry climates. However, from the conservation standpoint, keep in mind that water is also used in making electricity.

Nevertheless, it is important to consider the cost of water use when choosing the equipment and calculating the payback.  Water consumption varies, but typically ranges from 2 to 6 gallons per hour per ton of cooling.   A modern evaporative cooler in a typical home in the Southwest United States would likely increase water use about 3 percent averaged over the year—small compared to typical water use for other needs. In many cases, electricity cost savings can offset the increase in water costs.   

Operation, maintenance

Evaporative cooling systems require special routine maintenance in addition to what is normally required for standard air conditioners. Those tasks may include periodic water filter cleaning or replacement, an annual drain-down and dry-out (winterization) and a spring tune-up prior to the cooling season.

Historically, evaporative coolers were prone to scaling due to mineral deposits and mold growth, which can block and damage the unit.  Running purge cycles at controlled intervals and designing units to keep solids in solution until the water exits the core are successfully controlling scaling in newer models without increasing water consumption. Mold growth has been reduced by using mold resistant materials and by applying biocide treatment in the manufacturing process. 

Cost, payback

Hybrid evaporative/DX systems have greater first cost than standard air-conditioners.  The increased first cost and savings are site and equipment specific, but payback on investment can be in the range of one to five years.   

Ventilation benefits

Most evaporative cooling systems can deliver 100-percent outside air, improving indoor air quality without an energy penalty. Energy savings can be particularly large in facilities with high ventilation requirements, such as commercial kitchens, laboratories and assembly and conference spaces.    

Other considerations

With evaporative cooling taking much of the load, the DX compressor of a hybrid unit can often be smaller.  This allows electrical service to the unit to be sized for lower amps, possibly reducing installed costs.   

More information

• Assessment of Market-Ready Evaporative Technologies for HVAC Applic...
  New Buildings Institute, 2006 

• "Going 'Back to the Future' of Evaporative Cooling"
  John Dieckmann, Kurtis McKenney and James Brodrick, ASHRAE Journal, May 2009 

• SWEEP/WCEC Workshop on Modern Evaporative Cooling Technologies
  Southwest Energy Efficiency Project and U.C. Davis Western Cooling Efficiency Center, Sept. 14, 2007