Case study: Modular data center units improve energy efficiency

The National Bank of Abu Dhabi (NBAD)’s project to create a modular data center (MDC) solution, also provides energy-saving benefits.

By Debra Vieira, PE, LEED AP, ATD, CH2M, Portland, Ore. December 30, 2016

The key building blocks for the The National Bank of Abu Dhabi (NBAD) concept are the MDC units (Figure 4). These premanufactured modular units are factory-constructed, tested, and then shipped to the site for installation. Each MDC accommodates either five 23.6-in. cabinets or four 31.5-in.-wide by 86.6-in.-tall cabinets. This feature provides the NBAD with the flexibility to accommodate either form factor to support different IT equipment hardware sets. IT equipment with specialized requirements, such as bottom-to-top or side-to-side airflow, can also be accommodated with special provisions. Service aisles for equipment move-in and service have been provided in the cold and hot aisles and 47.2 in. has been allowed for cabinet depth.

The MDCs use hot-aisle segregation for energy performance. When coupled with a centralized humidity-control system, there are major advantages in energy usage when compared to standard, open architectures. These MDC systems lower energy usage by reducing or eliminating bypass air from the facility. Thereby, all air supplied to the MDCs will provide cooling to the IT equipment. Energy-saving opportunities include:

  • Isolation allows for warmer supply-air temperature than would be found in an open architecture.
  • Warm supply air temperature means even warmer chilled water may be used.
  • Warmer chilled water reduces the energy required by the chiller.
  • The MDC unit fan output can be controlled to actively track the mass airflow rate demand by the IT equipment, thereby reducing the MDC fan energy.
  • Airflow from the supply fans to the cold aisle (Figure 5).
  • Air is drawn from the cold aisle through the IT equipment by the server fans.
  • Discharge air enters the enclosed hot aisle from the server fans, where it flows up into the return plenum.
  • From the return plenum, the air flows back down through duct risers to the air-mixing box.
  • From the air-mixing box, the air flows through filters and a cooling coil back to the supply fans.

MDC units are designed to be linked together in groups of four, with the doors removed to create a POD. The POD configuration allows continuous rows of cabinets, cold aisles, and hot aisles for IT equipment access in both the front and back of each cabinet. Between two PODs, there is a single MDC core unit that acts as a vestibule and airlock between MDCs and the ambient office climate. The MDC core unit will provide central connections for IT cabling and switch panels.

Each MDC is designed with its own N+1 redundant power and cooling systems, thereby enabling installation of dual-corded equipment in any cabinet. Two different critical-load profiles have been selected by NBAD for the MDC units. Twenty-four MDCs will be provisioned to support 40 kW (five cabinets at 8 kW/cabinet) while 8 MDCs will be provisioned to support 75 kW (five cabinets at 15kW/cabinet). When fully loaded, the critical power capacity of these two MDC profiles will total 1.596 MW. With two different MDC profiles, NBAD will have the flexibility to locate higher-density IT equipment to operate most efficiently. Also, it is important to note that since each MDC unit has its own mechanical and electrical systems, any power density can be accommodated in an individual cabinet as long as the total critical power for the MDC unit is not exceeded.

The power supply to the MDC consists of two external sources to support either 40 kW or 75 kW of IT load. Both sources are supported by the central UPS system. The power connection supports all onboard power requirements including server cabinets, cooling fans, controls, and lights.

The power connection to the MDC is through the two safety switches at the front exterior of the MDC. Two-branch circuit panels are provided to enable the dual-cord power scheme to each cabinet and redundant power to the cooling fans. A wireway with integral twist-lock receptacles is installed above the cabinets to provide convenient and flexible power connections.

A key feature of the MDC design is that service access to all electrical and mechanical equipment components has been planned into the design so that normal procedures for filter access and other routine maintenance can be performed while the MDC unit is in operation. The detailed planning of the MDC design is expected to minimize annual downtime and disruptions for the bank’s IT systems.

The MDC units have a self-contained cooling system onboard. The units are equipped with chilled-water cooling coils, return-air filters, and fans with electronically commutated motors.

The MDC includes fully redundant chilled-water coils and pipes to allow for concurrent maintainability. The coil and piping systems are completely independent of each other, which includes control valves. There are four supply-air fans (three plus one redundant). All four fans operate simultaneously to meet airflow requirements. If one fan fails, the others will accommodate and continue to meet the airflow requirements of the MDC.

Access to the mechanical equipment components has been planned into the design such that routine maintenance including filter replacements can be performed while the MDC is in operation.

Debra Vieira is a senior electrical engineer at CH2M. She specializes in data center and mission critical environments.