Energy Advisor
Your guide to energy products for commercial buildings
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Energy Advisor |
Evaporative cooling occurs when moisture is added to air that has a relative humidity of less than 100 percent. The lower the relative humidity, the greater the temperature drop when moisture is added. The technology is a versatile and energy-efficient alternative or adjunct to compressor-based cooling. In favorable climates (most of the western United States and other dry-climate areas worldwide), evaporative cooling can fully satisfy building cooling loads using one-fourth the energy of conventional equipment. It can also be applied cost-effectively when integrated with conventional chiller systems. Using evaporative technology can also improve a facility load profile, which can enhance a company's negotiating position in a retail electricity market.
Direct. Direct evaporative cooling is the term applied to comfort cooling applications that simply add moisture to a moving airstream to cool the air while increasing its humidity (see Figure 1). The process only works for a moving airstream, so this approach requires a source of air drier than the air in the space to be cooled. In most applications, moisture is added to a moving stream of outdoor air that is delivered to the indoors while an equal quantity of indoor air leaves the building.
Wet-surface direct evaporative coolers typically use pumped recirculating water systems to keep the media wet. A fan blows air through the media, thereby cooling the air and increasing its humidity.
Source: Platts
In operation, pumped recirculating water systems typically keep a pad of woven fibers or corrugated paper wet while air flows through the pad. The pads absorb water by capillary action and maximize contact between the airstream and the wet medium. To ensure that all surfaces are wet, more water is usually pumped than can be evaporated, and excess water drains from the bottom of the media into a sump. An automatic refill system replaces evaporated water.
Indirect. Indirect systems cool air without adding moisture (see Figure 2). They are more expensive and use more energy than direct systems, but they have many applications. In operation, a direct evaporative process cools air or water on one side of an impermeable heat-exchange surface such as a thin plastic plate or tube. The wet side cools the dry side without adding moisture. Water sprayed on a hot roof, for example, evaporates at the roof surface and cools the roof deck, which cools indoor space below without increasing indoor humidity.
In a typical indirect evaporative air cooler, the essential element is a heat exchanger in which dry air contacts heat-exchange surfaces whose other sides are cooled evaporatively.
Source: Platts
Heat exchangers for indirect evaporative coolers were usually made of aluminum or stainless steel until plastic models were introduced in the 1970s and 1980s to reduce costs. Plastic heat exchangers aren't as effective at heat transfer, but they weigh less, are easy to manufacture, and resist corrosion.
Some indirect systems use evaporatively cooled water rather than air to cool a building or to supplement compressor-based cooling. In these cases, the evaporative cooler can be distant from the airstream being cooled. Cooling towers, for instance, evaporatively cool the water used to efficiently discharge heat that has been removed by compressor-based air-conditioning systems, and the cooling tower is sometimes far from the chiller. Water from cooling tower outlets is increasingly used in dry climates to directly cool air in heat-exchange coils or in radiant cooling applications using hydronic circuits in the floor or ceiling. These indirect evaporative cooling strategies, although they are often used to cool ventilation air, do not require that an outdoor airstream pass through the building.
Two-stage. Two-stage evaporative cooling supplies air that is cooler than either a direct or indirect single-stage system can provide. In many cases, these two-stage systems can provide better comfort than a compressor-based system because they maintain a more favorable indoor humidity range. Two-stage systems place an indirect cooling section on the upstream side of a direct cooling stage. The first stage cools without adding moisture. Primary air leaving the first stage can be directly evaporatively cooled to a lower temperature than is possible with direct cooling alone, with less moisture addition. The indirect stage can use either an evaporative heat exchange approach or a tower/coil approach, which connects a cooling tower to a finned cooling coil in the supply air ahead of the direct stage.
Assess the potential for evaporative cooling in your climate. Figure 3 shows average July daily wetbulb and drybulb temperature ranges for 13 U.S. cities. The drybulb temperatures indicate the severity of cooling loads and—for loads with wide daily ranges—the opportunities for night ventilation cooling. In five of the cities shown (Albuquerque, Boston, New York, Salt Lake City, and Tucson), the average wetbulb range is fully below the drybulb range, and in all five the wetbulb range is below 70°F. These five cities are thus excellent candidates for full or hybrid (integrated with compressor-based systems) evaporative cooling of commercial buildings, although occasional humid summer weather in these cities may require auxiliary compressor-based cooling or desiccant dehumidification. The integrated choice is necessary where occasional spells of hot, humid weather substantially alter conditions from the averages shown in Figure 3. These occur in both Boston and New York.
Cities where the wetbulb range is fully below the drybulb range are excellent candidates for evaporative cooling of commercial buildings.
Source: Davis Energy Group
The five cities with wetbulb ranges extending to or below 55°F are all in the West in regions that are ideal for evaporative cooling. (However, Seattle's low drybulb temperature range means cooling loads can usually be satisfied with outdoor air.) Locations with average July wetbulb temperature ranges extending above 70°F (Atlanta, Houston, and Miami) are not good candidates for evaporative cooling in July.
Consider eliminating compressor-based cooling. Prospects for completely eliminating compressor-based cooling are best in high-altitude climates that have dry air and lower summer daytime temperatures, as represented by Denver in Figure 3. In very hot summer climates like Phoenix (not shown), where afternoon July wetbulb temperatures often exceed 75°F, there are times when even a good two-stage evaporative cooler cannot cool air to desired indoor temperatures without exceeding the ASHRAE relative humidity limits. However, in these climates direct or indirect evaporative cooling (or a combination) can usually satisfy full cooling loads for 10 months of the year and can be applied to ventilation air all year. Cost-effectiveness in these locales depends on local utility rates, the duration of the cooling season, cooling load patterns, and ventilation air quantities.
In moderate climates with high daytime temperatures (like the California valleys, where the highest July daytime wetbulb temperatures seldom exceed 70°F), two-stage evaporative cooling can fully satisfy peak cooling loads. However, large components may be needed. In these climates, a particularly beneficial cooling system is evaporative or roof-spray radiative cooling at night, when wetbulb temperatures are lowest, to store cooling in the floor mass, ground, or water for use the next day. Systems using this approach are in their infancy, but they should make substantial progress in the next decade, with development spurred by environmental concerns and by market-transformation efforts related to restructuring.
Copyright © 2004 - Platts, a Division of The McGraw-Hill Companies, Inc.
