Waterside economizers typically pump cold condenser water from a cooling tower through a heat exchanger to generate chilled water in lieu of operating chillers. They are utilized in process cooling applications or when airside economizers are not available. They are also used in lieu of airside economizers when there are significant humidification requirements, like in a data center (this is a subject for another day). In addition to saving on utility costs, waterside economizer systems offer the added benefit of extended chiller downtime, which is a great opportunity for tube cleaning and other scheduled maintenance. Waterside economizers work best in cool, dry climates where the wet bulb temperature is consistently below 40 degrees.
For example, consider a large data center in Atlanta with a fairly constant 24-hr load of 1,000 tons. An airside economizer system cannot be used because humidity control is required in the data center. Does a waterside economizer system make sense? Assume the existing chillers operate at 0.60 kW/ton and the electric rate is $.10/kWh. The total available waterside hrs/yr in Atlanta is 1,686, so the annual savings would be 1,000 x .60 x 1,686 x $0.10 = $101,160; that's nothing to shake a stick at. What if sufficient space existed for the new equipment, and it could be built for less than $250,000? A 2.47-yr payback is worth considering, even if the local utility is not offering rebate incentives.