Mike Flaherty introduces the concept of optimizing chilled-water production for data center cooling. He outlines the optimal configuration of the water chiller system, control strategies, and equipment selection in this first part of a three-part series.
While no one cools a data center with gasoline, the $4 per gallon price tag should serve as a wakeup call to data center operators. The price of all nonrenewable energy sources is linked together through market forces, so the price of the coal, oil or gas required to cool most data centers has increased as well.
Every ton of cooling consumed by servers must be generated by the chilled-water system. Efforts to improve cost should look at both reducing the amount of cooling needed as well as efficiently producing the chilled water.
If this is obvious to you, then ask, "Why are technical developments and trade press coverage almost exclusively directed toward reducing cooling consumption, but little attention has been paid to improving production efficiency?" This three-part series sheds light on this overlooked area and offers solutions capable of reducing electricity usage by .1 to.4 kW per ton of cooling.
Because many data center chilled-water plants produce upward of 5,000,000 ton-hours of cooling annually, the need to invest in additional capacity can be significantly delayed or even totally eliminated through effective implementation.
Optimizing chilled water production efficiency
Effectively optimizing a chilled-water plant requires attention in three fundamental areas: piping or hydronic configuration, control system and strategy, and the mechanical equipment selection. These areas collectively contribute to the .1 to .4 kW per-ton savings, with the savings impact of each depending on the plant's original design, control strategy, current operating methods and equipment condition.
Hydronic configuration. the plant must have an energy efficient piping design to connect the chillers, pumps, cooling towers and CRACs, and the design must maximize the use of water-side economizers for free cooling. Unfortunately, most chilled-water systems use the outdated primary/secondary (P/S) model, a scheme developed during the 1970s when chillers needed constant evaporator flow and when electricity cost a penny a kilowatt, so economizers were an afterthought.
During the mid-1990s, when chillers evolved to handle variable flow, the variable primary (VP) model was created to take advantage of the VP design's lower initial cost. Since then, smart owners have discovered that VP flow systems are much more energy efficient than P/S systems because they maximize chilled water differential temperature (ΔT) which in turn minimizes pumping energy and allows optimum chiller loading. They have also found that it pays to maximize the economizer operation by using outdoor air to produce "free" cooling in colder months. So, if you really want an efficient chilled-water system, make sure you start with a VP hydronic configuration and maximize water-side economizing capability for free cooling.
Control strategy. Unless you take specific steps to avoid it, your plant follows a traditional make-it-work sequence of operation. A make-it-work sequence is designed to meet cooling demand by simply producing ample chilled water without regard for efficient production. Consequently, the control program consists of simple "If, then" logic and fixed set points, and the only subsequent program modification is limited to tinkering by the operating staff.
For efficient cooling production, insist on a sequence of optimization. This is a sequence designed to meet cooling demand at the lowest kW per ton. It is equally important for the control program to use adaptive algorithms to automatically and continuously adjust equipment operation based on actual load and conditions. A word to the wise: The more sophisticated sequences and control techniques recommended here are not the norm, so scrutinize the consulting engineer and control contractor to avoid getting the standard fare.
Equipment selection. If you have a piping configuration and a control system that allow for efficient operation, three basic equipment decisions account for the rest of the difference:
- Electric centrifugal chillers represent the best overall balance of first cost, energy usage and reliability.
- Variable speed cooling towers maximize heat rejection and minimize chiller lift while expending the least amount of energy.
- Two-way, high performance rotary valves on the CRAC/AHU coils provide the range and shut-off pressure necessary for proper flow control at all system loads and pressures.
Next: Optimizing your existing chilled-water plant
The recommendations and resulting savings are hardly limited to new plants. Many existing chiller plants have been successfully retrofitted to accommodate the hydronic, control and equipment improvements. The next installment of this series will focus on how to retrofit your plant and start saving energy right away.
ABOUT THE AUTHOR: Mike Flaherty is general manager of tekWorx, a Cincinnati-based engineering and control firm that specializes in optimizing energy usage in chiller and boiler plants. Prior to founding tekWorx 2000, Mike had over 20 years experience in computer control and software technology for production machinery and industrial processes. He has served in executive positions at leading industrial automation companies including Allen-Bradley and Parker-Hannifin Corp.