Kroger Data Center
Existing building retrofit: Kroger Data Center; PEDCO E+A Services Inc.
Location: Cincinnati, Ohio
Firm name: PEDCO E+A Services Inc.
Project type, building type: Existing building retrofit, data center
Project duration: 1.3 years
Project completion date: May 29, 2007
Project budget for mechanical, electrical, plumbing, fire protection engineering only: $288,000
The Kroger Company (Kroger), one of the nation’s largest retail grocery chains, operates data centers across the United States. One such data center, located in Cincinnati, Ohio, where Kroger is headquartered, was in need of an infrastructure upgrade. The existing facility houses two distinct data centers known as the North Data Room and South Data Room.
The original building, constructed in 1978, comprised the 7,500-sq-ft North Data Room and support areas such as offices, UPS and battery rooms, utility spaces, and a generator room. An expansion project in 2006 added the 6,400-sq-ft South Data Room and associated support spaces. That project also included a design for upgrading the North Data Room HVAC system, which was to be executed as a second phase. It mimicked the new South Data Room with a multiple glycol loop system with roof-mounted dry coolers, but the North Data Room system was never installed. As Kroger developed a new IT strategy, the strategy for the North Data Room’s utilization also changed.
Because the North Data Room was never renovated from the previous project, the design team was challenged to provide a robust design that was more energy efficient. Cooling capacity criteria for the North Data Room was 100W/sq ft, which converted to 215 tons. An allowance of 25 tons for future in-rack cooling in either Data Room brought desired capacity to 240 tons. Against the backdrop of reduced energy consumption, the redundancy requirements of a Tier III data center needed to be considered. Located in the Midwest, renown for summer humidity, this data center’s options did not include an airside economizer alternative. Space temperature and humidity criteria (75 F, 50% RH), however, did allow for warmer conditions than had previously been considered normal.
When comparing options for the cooling plant, the previously designed multiple glycol loop system was utilized as the baseline. Ultimately, a concept using modular chillers was developed for two main reasons. First, chilled water provides greater flexibility in terms of adding in-rack cooling, which Kroger anticipates in the future. Second, the renovated North Data Room was anticipated to be loaded over an extended period of time, so there would seem to be an inherent advantage to providing capacity on an incremental basis just ahead of its need.
As PEDCO looked further into the modular chiller approach, the prospect of placing free-cooling modules upstream of compressorized modules was immediately appealing. Depending on ambient conditions, this would enable the use of at least partial cooling of the return chilled water stream without compressors, therefore using less energy to cool the chilled water to the desired supply temperature. This contrasted to the more conventional approach of using either free cooling or compressorized cooling, but not a combination, at any given time. Based on available standard sizes, 30-ton modules were selected to be the design basis. An added advantage of this concept was the ability to achieve N+1 redundancy of chiller capacity with only one module. Thus, the desired redundancy was achieved with only an additional 30 tons of capacity. Had a more conventional approach of two 120-ton air-cooled chillers been used, N+1 redundancy would have required the addition of 120 tons of capacity.
The final design of the chiller assembly included nine 30-ton compressorized modules and eight 30-ton free cooling modules. The ninth compressorized module provided the desired N+1 redundancy. That redundancy was not required for the free coolers, since the compressorized modules served that function. Computer models were created for this concept and for one utilizing glycol computer room cooling equipment. The models predicted the chilled water/free cooling system would consume 92% as much energy as the glycol approach. A cost comparison of the two alternatives indicated first cost for the chilled water approach would be approximately 30% lower. Maintenance costs and system life expectancy were deemed to be roughly equivalent between the two alternatives. It thus became apparent that a modular chilled water system was the approach to pursue for this project. The potential for significant energy savings prompted Kroger and PEDCO to contact the energy rebate representative from Duke Energy. The project was submitted through Duke’s Smart$aver Custom Incentive program, which resulted in Kroger receiving a $130,000 rebate check. Further, Kroger measured a savings of over $100,000 in utility bills during the first year of operation.