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Economizer for Data Center

©2013 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 55, no. 10, October 2013.

By Mehrdad Alipour, P.E., Member ASHRAE

About the Authors
Mehrdad Alipour, P.E., is a mechanical engineer at Therma Corp., San Jose, Calif. He is a member of the San Jose ASHRAE chapter.

Adding an airside economizer to a data center significantly reduced the energy consumed and utility costs at Building 4 of the Marvell Semiconductor U.S. headquarters campus in Santa Clara, Calif.

Building 4 is a two-story, 129,000 ft2 (12 000 m2) facility with a second floor consisting of open office spaces, conference rooms and a central R&D lab area. The first floor has a test area, data center, electrical room (UPS room), and a shipping area. Most of the building is occupied during normal business hours Monday through Friday and the data center operates 24/7.

An ASHRAE Level I and II energy audit was performed to identify potential high-value savings opportunities. One recommendation was the addition of an airside economizer for the data center.

 

Design an Energy-Efficient System

The 5,000 ft2 (465 m2) data center is served by three uninterruptable power supply (UPS) units located in a separate electrical room. The data center is cooled by eight computer room air handlers (CRAH) using common supply and return air plenums that provide a total of 104,000 cfm (49 000 L/s) air to data center. Two independent CRAH units cool the electrical room by providing 18,800 cfm (8800 L/s) of air.

The UPS power meters showed a steady total of 720 kW feeding the servers at data center. The data center’s UPS units are easily the largest energy consumer in the building, accounting for 44.4% of total electricity use (Table 1).

Chilled water is supplied to the CRAHs by three air-cooled chillers with reciprocating compressors rated at 340, 340 and 110 tons (1200, 1200, 390 kW). The chilled water distribution is a primary-secondary system with variable frequency drives (VFDs) on secondary pumps and a dedicated primary pump for each chiller.

The chilled water CRAH units (Figure 1 ) operate as 100% air recirculation units with no economizers. Other rooftop air handlers in the building use chilled water, but they are equipped with airside economizers. 

Table 2 shows historical electricity consumption of Building 4 over the 12 months prior to the installation of the airside economizers for the data center. The steady consumption of electricity through the year is notable.

During the energy audit, engineers collected nameplate and operational data for mechanical equipment and lighting systems to identify potential measures. Engineers also reviewed the control sequences for mechanical systems as well as drawings, facility personnel observations, photographs, and data trends. Energy models were generated to estimate energy savings from various energy conservation measures.

The data center energy model was calibrated based on the 12 months of utility bills and the predicted energy savings from adding the airside economizer.  As a result of the simulations, the following capital intensive measures were recommended:

  • Install a data center outdoor air economizing system to provide free cooling; and
  • Install a 100% outdoor air rooftop unit and economizer exhaust for the UPS room to provide free cooling.

The data center CRAH units draw return air from the data center hot aisles at about 90 °F (32 °C), cool it using chilled water, and supply 65 °F (18 °C) air to the cold aisles. The chilled water temperature is 45°F (7°C) The CRAH units are located in a mechanical chase between the data center and an exterior wall. This is an ideal location to introduce the airside economizer to the data center cooling is system.

The airside economizer requires dampers to regulate the intake of outside air and return air by each CRAH unit, as well as exhaust openings and fans to exhaust hot return air from data center to control the space pressure. The energy savings from the economizer will be particularly high because the data center has robust cold aisle enclosures and the warmer the supply air setpoint, the more hours that the economizer will run.

 

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Figure 1