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Economizers

The term “economizer” refers to methods of cooling that do not use mechanical or chiller cooling. The two most common are air-side and water-side economizers.

Air-side Economizer

An air-side economizer assures that the large air handling units have adequate openings to allow them to run at 100% using outside air. That way, whenever the outside air is cool enough (generally, at least 5 degrees below the inside set point), outside air can be used directly to cool the space. Thus the only energy cost is for the fan power. In Beaverton’s climate, this practice reduces the cooling energy by about 50% while ensuring that building tenants benefit from an abundance of fresh air.

Water-side Economizer

A water-side economizer provides cooling by evaporation. When the weather is cold enough, water is circulated through the cooling towers, which are, in effect, giant evaporative coolers. As the tower water is sprayed in the air, some of it evaporates, cooling the rest down. A large heat exchanger transfers the “cool” to the regular chilled water loop. The heat exchanger prevents cross-contamination of the chilled water loop with water that has been exposed to the air and air-borne dust, etc. in the towers. Here, the only energy cost is for the tower pumps. The advantage of the water-side economizer is that it can cool the water to lower temperatures, since it can go down to the wet bulb, or dew point temperature, not just the regular dry bulb, or air temperature. This makes it very valuable for meeting the cooling loads in the data rooms, where cooling loads are high and air flow is limited, reducing effectiveness of air-side economizers. In Beaverton’s climate, this might save 25% to 35% of the cooling energy. (Note: the energy savings for the two economizer modes are not strictly additive.)