Data center designers and operators have a choice of strategies that can result in significant reduction in chiller plant connected power and consequently energy consumption. Two options involving relaxing some of the tight temperature ranges under which plants were traditionally designed and operated in particular, have proven effective:
Raising the leaving chilled water temperature (LCHWT) to make use of reduced lift and hence lower the connected power consumption
- Increasing the temperature Delta T (ΔT) between entering chilled water temperatures (ECHWT) and LCHWT to make use of pump laws and reduce pump horsepower
These strategies, combined with proper aisle containment practices, will result in a 10 to 14 percent reduction in data center energy consumption at the low end and 15 to 18 percent at the high end.
WHY DATA CENTERS?
Servers in computer rooms require a stream of cool air to avoid overheating and shutdown of the microprocessor chip. Supplying 55°F air to the servers for comfort cooling should not be an objective; this will result in oversized chiller plants and unnecessary data center energy consumption. Both ASHRAE TC9.9 guidelines and Lawrence Berkeley National Lab (LBNL) publications allow up to 80°F at the server inlet. Studies by server manufacturers have shown that their equipment can accept higher inlet air temperatures without failure. Since most cooling in data centers is sensible heat with very low latent loads, raising chilled water temperature in the data center is a guaranteed means of reducing chiller energy consumption, which is accomplished by lowering chiller lift and avoiding unnecessary dehumidification during the sensible cooling process.
Designers paid attention to ASHRAE guidelines and thus were born hot-aisle (HAC)/cold-aisle (CAC) containment layouts. The hot-aisle/cold-aisle room layout allowed a suitable amount of cool air to reach the front of the servers, and it minimizes the wasteful mixing of hot and cold airstreams in the aisles. Unfortunately, designers and operators continue to be leery of implementing raising LCHWT and room air supply temperatures, and they continue to be “stuck” to narrow temperature Delta practices, typically 10°F to 12°F.
RAISING SUPPLY TEMPERATURES
Raising temperatures (water and air) in a data center environment has many benefits. Raising the LCHWT reduces chiller lift. Commercial comfort-cooling practices tend to emphasize lowering lift by varying condenser water temperature with ambient temperatures while maintaining a constant evaporator temperature, which reduces head pressure on the condenser side. Chilled water can be distributed in data centers at higher LCHWT, which reduces the head pressure on the evaporator side and achieves the same lower lift effect.
Higher air temperatures reduce energy for cooling, which also lowers fan energy
Higher air or water temperatures increases the number of hours per year in which economizers will be effective. The compressor is either off or at partial loading during full or partial free cooling and energy consumption is decreased
- Lower chiller power may allow smaller generators can satisfy the electrical load of the building
In present-day energy “lingo,” these benefits can lead to lower power usage effectiveness (PUEs) rations reduced carbon footprints.
The effectiveness of raising LCHWT can be illustrated using a test case by raising temperatures in increments of 1°F up to 60°F starting at 42°F, using a 10,000-ton centrifugal chiller with the evaporator temperature differential kept constant at 14°F Delta across the selections. For purposes of the example, condenser water conditions are kept at a 12°F ΔT with 85°F leaving and 97°F entering condenser water conditions. Trane provides the actual selections for each temperature.
Figure 1 plots the primary chiller connected power in kilowatts (kW) for each LCHWT. The results are as anticipated; the reduction in power between the two extreme leaving water supply temperatures is approximately 115 kW or 29 percent. Considering that most chiller plants operate in the 42°F to 46°F at a constant Delta, elevating the LCHWT to the mid-50°F’s reduces power consumption by 14 to 18 percent. Taking a more aggressive approach and raising the LCHWT to the low 60°F’s results in power reductions in the range of 25 to 32 percent. With the proper CAC/HAC strategies and chiller controls, raising the supply air temperature in conjunction with the LCHWT is not only achievable but proves to have significant benefits for data center power reduction.
WIDER TEMPERATURE DELTA
Increasing the temperature Delta (ΔT) between LCHWT and ECHWT is another energy-saving strategy. Flow and ΔT are inversely related according to the formula:
Mass flow rate = Heat load
Specific heat X (ΔT)
For a constant-cooling load, as ΔT increases, flow decreases linearly (1:1). However pump horsepower is related to the cube of flow as shown in the pump equation:
HP1 = (GPM1)3
As flow decreases, pump horsepower decreases to the third power. This translates to a considerable reduction in pump horsepower and energy consumption for the overall plant. Subsequently, reduced flow and smaller pumps translate to smaller variable frequency drives (VFDs) and pipes throughout the facility.
A simulated 4,000-ton chilled-water plant serving a 100,000-square-foot (sq ft) data center at 150 watts/sq ft in Washington, DC, is served by four 1,000-ton centrifugal chillers, four 1,000-ton induced draft cooling towers, and a variable primary pumping system. The simulation varied LCHWTs in increments of 2°F starting at 42˚F up to 56°F.
Two extreme temperature Deltas provide a good picture to interpolate temperatures in between these Deltas. The base plant (table 1) runs at 10°F chilled water ∆T. The alternate plant (table 2) runs at 16°F ∆T chilled-water. Both plants operate at 12°F ∆T for the condenser-water side with 85°F leaving condenser water temperature. Energy-cost estimates use a utility rate of $0.08/kWh.
Figure 2 depicts the results for total energy consumption for both temperature Deltas, but an important observation needs to be highlighted. Figure 2 plots the LCHWTs and the corresponding return air (RA) temperature at the computer room air-handling (CRAH) units. RA temperature is a snapshot of the overall room temperature, and what is important here is the direct correlation with LCHWT. Room air temperature increases proportionately with LCHWT and that will have a negative impact on a data center thermal environment. Simply put, there has to be proper HAC/CAC containment to route the hot air back to the CRAH units. If containment is not implemented, more CRAH units will be required to offset the higher return air temperatures and the CRAH unit fan motors will consume more energy than the chillers and pumps. Containment is factored into the computer models and hence RA temperatures floated up with the LCHWT without additional fan horsepower at the CRAH units.
Figure 3 plots chilled-water pump brake horsepower (BHP) starting at 10°F ∆T and up to 16°F ∆T in increments of 1°F. BHP is reduced by 57 percent at the extreme temperature range points. This power reduction will in turn have a profound effect to reduce overall plant energy consumption.
Total plant energy consumptions for the Base (10°F ∆T) and the Alternate (16°F ∆T) are plotted in figure 4. In concurrence with the first strategy, energy is reduced along a constant ∆T selection (follow either the green or the red lines for any LCHWT). The reduction in energy between 44°F and 56°F is ±13% for either the Base or the Alternate scenarios. The reduction in energy is an additional 6 to 8 percent as ∆T is increased-a jump from the green to the red line along any of the LCHWT points.
Since most chiller plants are designed around a 44°F to 45°F LCHWT with 10°F ∆T, a user may want to know what percentage energy is reduced between any LCHWT at the 16°F ∆T and the Base temperature (44°F, 10°F ∆T). The results plotted in figure 5 show a reduction in energy between 6 to 18 percent. A “sweet spot” is shown where LCHWTs are in the low 50°F range; the energy reduction is 10 to 14 percent.
The sweet spot exists because these temperatures are reasonably easy to achieve without dramatically affecting either the chiller controls or the interior thermal conditions within the data center. Increasing LCHWT temperatures to the upper 50°’s or low 60°’s results in further energy reduction between 15 to 18 percent. This practice, while achievable, should be closely coordinated with the chiller manufacturers in order to mitigate the risks of low lift conditions. Lower ∆T lines, while not simulated, are sketched on the graph to give the user an idea of where the percentage reductions will in turn fall on the chart and hence interpolated.
Design engineers and operators can reap significant energy and cost savings when selecting proper chilled water set points for LCHWT and temperature deltas. Data centers are ideal candidates for LCHWT in the mid 50°F’s with high temperature Delta. These combined strategies can reduce data center power and energy consumption from a moderate low of 10 to 14 percent up to an aggressive high between 15 to 18 percent.