Free Cooling Water-Side Economizer

Free cooling can be provided via a water-side economizer, which uses the evaporative cooling capacity of a cooling tower to indirectly produce chilled

Data centers present an almost constant, 24-hour, internal cooling load. Free cooling can be provided via a water-side economizer, which uses the evaporative cooling capacity of a cooling tower to indirectly produce chilled water to cool the data center during mild outdoor conditions (particularly at night in hot climates).

While a bin analysis using local weather data is required to properly assess the potential, free cooling is usually best suited for climates that have wet bulb temperatures lower than 55°F for 3,000 or more hours per year. It most effectively serves chilled water loops designed for 50°F and above chilled water or lower temperature chilled water loops with significant surplus air handler capacity in normal operation. Often, existing data centers can capitalize on redundant air handler capacity with chilled water temperature reset controls to retrofit free cooling.

Principles of Free Cooling Water-Side Economizer

While free cooling is operating, chilled water plant energy consumption costs are cut by up to 70%1.
Data centers require cooling 24 hours a day every day of the year – even when it is cold outside. This makes data centers very well suited to waterside economization. For example, in San Jose, free cooling would be expected to operate for over a third of the year, significantly reducing cooling bills and noticeably reducing chiller run hours and maintenance costs.
Free cooling utilizing a water-side economizer can usually be economically retrofitted to existing chilled water cooled facilities.
Isolation between the space air and outside air is not impacted by water-side free cooling, making it an alternative to air-side economization when this is a concern.
A flat plate heat exchanger is used to isolate the chilled water loop from the open tower condenser water to prevent fouling of coils.
A low approach temperature cooling tower plant design is critical for best results. Use of redundant tower capacity can provide low approach temperature operation in a high-reliability ‘spinning reserve’ operation configuration.
A traditional chiller is used to provide cooling during hot periods and as an always-available emergency backup. For a portion of the year, free cooling increases reliability by offering a non-compressor based backup to the traditional chiller, particularly at night when plant monitoring and staffing are liable to be lower.

Data centers often have redundancy in their cooling tower plants. Through the use of VFDs and common condenser water header systems and/or sumps, the redundant tower capacity can be used to achieve a lower approach temperature. With variable speed fans, it is efficient to operate as many towers as the tower minimum flow requirements allow, maximizing the natural convective cooling while achieving a lower approach capability.

Free cooling requires that the cooling tower produce low temperature water, often lower than a chiller will accept for condenser water. There are two common design approaches to address this concern. One approach is to hydraulically isolate a tower and dedicate it to free cooling only. This is the best approach, but requires careful piping configuration and control valve placement and operation. A redundant backup tower can be provided with automatic isolation valves and used for free cooling. Since free cooling operates during low temperature weather conditions, the chilled water plant load is often low enough that even non-backup towers are available for free cooling use provided the proper automatic valves and a control sequence that gives the chillers priority for tower use (in case of free cooling failure) is implemented.