Two stage evaporative coolers are a relatively new addition to the family of alternative cooling appliances. The advantage two stage coolers have over conventional, single stage models is the production of cool air with 10%-30% less humidity (depending on your climate).
The stages of a two stage cooler are described as "indirect" for the first stage, and "direct" for the second stage.
A conventional swamp cooler, by its nature, adds moisture to the air it cools. To offset this, a pre-cooling, or indirect stage, was added.
In the indirect stage, warm, dry air is pre-cooled by passing through a heat exchanger cooled by evaporation on the outside. Because the air supply to the second stage evaporator is pre-cooled, the end result is cool air with 50%-70% relative humidity compared to 80% humidity from a conventional single stage cooler.
An additional benefit of first stage pre-cooling and reduced humidity is extending the product's usefulness into hotter and more humid conditions.
Two stage evaporative coolers are most often installed directly into an outside wall or at ground level supported by a small concrete pad. The units require a 115 VAC electrical connection, a supply water connection and, if they have a water bleed-off mechanism to reduce mineral deposits, a method for water disposal.
The largest units have a cooling capacity of 3 tons, a supply air flow of 1,400+ cubic feet per minute and consume around 3 gallons of water per hour. This is sufficient cooling capacity for up to 1,700 square feet of living space.
Since a two stage evaporative cooler brings outdoor air into the home at a relatively high rate and doesn't recycle indoor air, positive air pressure builds up inside the home. In order for the system to function effectively, the pressure must be relieved by opening a few windows a couple of inches, or installing an exhaust duct or pressure operated damper in the attic or an outside wall.
According to the American Society of Heating and Engineers (ASHRAE), two-stage evaporative coolers can reduce energy consumption by 60 to 75 percent over conventional air conditioning systems.
Coincidently, I found a study by the Department of Energy simulating the annual power consumption of different cooling systems in a prototype house in Borrego Springs, California.
The two systems I chose for comparison from the study are a 13 SEER, 3 ton, DX (Direct Expansion) air conditioner and a variable speed, 3 ton Oasys brand two stage evaporative cooler.
According to the study, the air conditioner consumed 3248 kWh (kilowatt hours) of electricity during the cooling season. Using the average California retail electric rate (2007) of Body.1435 per kWh, the resulting cost is 6.
By comparison, the two stage evaporative cooler consumed 667 kWh of electricity for a seasonal cooling bill of , or 79% less than the air conditioner.
The new two stage evaporative coolers have successfully taken the "swamp" out of swamp cooler but may face problems overcoming the image of its less comfortable predecessor; especially in light of a ,000 price tag.
However, based on the energy cost comparison above, without taking into account future rate increases, a two stage cooler would pay for itself in less than 8 years while consuming 79% less fossil fuel.
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