Evaporative Cooling

Two-stage evaporative coolers operate at a fraction of the energy costs of conventional residential air conditioning and can keep a home comfortably cool when outside temperatures soar.
 

The two-stage evaporative coolers pictured above are hidden in the roof well of this award-winning house in Phoenix--a classic "two-stage-cooled/passive-solar-heated" home.

Depending on the design, an evaporative cooler can use as little as 10% of the energy consumed by refrigeration air conditioning systems.  While the savings are substantial, only certain geographic areas are suitable for evaporative cooling. In general, evaporative coolers work best where annual humidity levels are low and summer temperatures are mild (see "Installing and Maintaining Evaporative Coolers," HE May/June '96, p. 23).

Figure 1. Schematic of one type of two-stage evaporative cooler.

But what about very hot climates? Is evaporative cooling a viable option for desert residences? If the evaporative cooler is of the direct, single-stage variety, then the answer is no. Single-stage evaporative coolers are approximately 60% to 85% effective in reaching wet-bulb temperature (the coldest the incoming air can become by evaporating water into it). In regions where temperatures frequently exceed 100°F, the capacity of a two-stage evaporative cooler is necessary to meet the greater cooling demands.

Two-stage evaporative coolers are commonly 100%-115% effective in reaching wet-bulb temperatures. They can reduce the temperature of outside air by as much as 50°F (for example, from 115°F to 65°F), while delivering air that is less humid than that from a single-stage evaporative cooler.

Two-stage coolers (also called indirect/direct coolers) usually use a rigid pad and have an indirect evaporative "pre-cooler." As illustrated in Figure 1, the indirect evaporative cooler cools the tubes in a heat exchanger. At this stage, the air moving down the inside of the tubes is precooled without contacting water. This precooled air then flows into a direct evaporative cooler, where it is cooled again, and finally enters the house.

The process of two-stage cooling similar to that of refrigeration-based air conditioning, in that heat is transferred (or pumped) from the primary air to a secondary airstream. The primary air goes into the house, while the secondary air is discarded to the outside. Not surprisingly, indirect coolers resemble refrigeration air conditioners, having external fans that draw a secondary airstream through the unit and discard it to the atmosphere.

The advantage of this two-stage process is illustrated on the psychro-metric chart (see Figure 2). The graph shows that the two-stage cooler produces air that is not only colder than that of the single-stage cooler but drier, as the precooled air cannot hold as much moisture.

Figure 2. Psychrometric chart showing an example of two-stage cooling under hot-dry conditions. Note that outside air is cooled from I I 0°F (I) to 91 °F (2) through indirect cooling, then cooled further to 65°F (3) through direct cooling.

Evaporative coolers are popular in Phoenix, Arizona. Frequent high temperatures and low relative humidity levels make the city prime "two-stage country." Also important is the area's relatively high energy costs, which affect the cooler's payback period. For example, to install a two-stage cooler as an add-on in a home with an existing conventional air conditioner takes an average initial investment of $2,000. Annual energy savings are approximately $200-$300 per year (based on electricity costs of 12¢/kWh). Payback is thus possible within 10 years.

Vegas' climate is also very suitable for two-stage cooling. Summer temperatures frequently exceed 100°F and the air is exceptionally dry; Las Vegas rarely experiences seasonal influxes of moist air from the coast that would bring thundershowers and hinder evaporative cooler performance. Nevertheless, few Las Vegas residences use two-stage coolers. The reason is that energy is relatively cheap in Nevada. This can extend the payback period beyond the expected life (15-20 years) of an average two-stage cooler.

Energy savings associated with a two-stage evaporative cooler will vary depending on whether the cooler is used alone or in combination with conventional cooling systems. Table 1 compares energy use at a sample house in select cities using evaporative coolers and/or conventional air conditioning systems.

While there are several two-stage coolers available for commercial applications, the typical two-stage cooler available for the residential market relies on a wet-surface air-to-air heat exchanger. The only current manufacturer of residential evaporative coolers is Adobe Air. However, Davis Energy Group in California has designed an advanced two-stage cooler that should be on the market this summer (see "New Two-Stage Design Completes Field Test").

Flat plate cross flow and tube bundles cross flow are two common configurations available in the wet-surface indirect cooler design. The former is slightly more compact and is the more popular of the two; it uses stacked plates of plastic or aluminum, set about iA inch apart and lined with fabric or flocking. Primary air moves horizontally through alternate passages between the plates and secondary air is drawn vertically through the other passages.

There are currently no residential two-stage coolers that can perform in humid climates. However, in the commercial/industrial market, desiccant-and refrigeration-enhanced two-stage evaporative coolers are becoming more common. A desiccant-enhanced two-stage cooler is one that simultaneously dries and heats air with a dessicant before cooling. Dessicant-enhanced coolers have no climatic limitations, rely on natural gas, and can reduce electrical peak loads. A natural gas-based desiccant cooler could use simple solar collectors to reduce the use of natural gas in the drying process and would not require heat storage, since peak solar collection occurs at almost the same time as peak cooling requirements.

Similarities in appearance between an air condi-tioner (right) and a two-stage evaporative cooler (left) are evident here. Both exhaust waste heat to the atmosphere out the top, and supply cooled air through the roof to the house below.