Continuing last month's subject of economizer "free cooling": November is the first month for initiating waterside economizer systems in many cities. In fact, the wet bulb temperature in Denver was below 40 degrees F for 708 of the 720 hours in November, essentially allowing for a permanent chiller shutdown in that city.

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.

In Atlanta? Anything is Possible!

When considering the feasibility of designing a new economizer system or retrofitting an existing plant to save energy, the two primary variables to consider are the hours of cooling load operation and number of available wet bulb hours. In colder climates, with a substantial off-season cooling load, it's usually a no-brainer, especially with the help of utility rebate programs. But every system is unique and must be evaluated on a case-by-case basis.

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.

How Did November 2001 Measure Up?

The waterside economizer hours in this report are the total number of hours when the outside air wet bulb temperature is less than 40 degrees during the month of November. Based on this data, Figure 2 illustrates the potential cost savings per ton of economizer cooling load for November.

Figure 3 ranks the most extreme from normal. Salt Lake City experienced a 31% increase from normal. That's almost one additional week of economizer cooling. On the downside, the economizer hours in Boston were off 38% from normal or about one extra week of mechanical cooling. In large-load applications, these differences could have a noticeable cost impact on the utility bills when compared to previous years. Assume a 500-ton data center chilled water system, 0.6 kW/ton, and $0.10/kWh. One less or one more week of mechanical cooling could save or cost 500 x 168 x 0.6 x $0.10 = $5,040. A better understanding of how the weather impacts the operation of building heating and cooling systems can definitely help explain fluctuations in operating costs. ES

Editor's Note: Some charts/figures associated with this article can not be posted on this website. To view these items, please refer to the print version of this issue.