Retrofitting Computer Room Air Conditioner Fans with Variable-Speed Drives: Three Case Studies

Steve Greenberg, Lawrence Berkeley National Laboratory

This presentation documents three retrofits to existing constant-speed fans in computer-room air conditioners (CRACs), all located in California: the Lawrence Berkeley National Laboratory (LBNL), the Electric Power Research Institute (EPRI), and NetApp.

Computer room air conditioners (CRACs) are often used to provide cooling to the Information Technology (IT) loads. CRACs use a vapor-compression refrigeration cycle to remove heat from the data center air, and pump it either into water (in turn cooled by a cooling tower or a dry cooler located outdoors) or directly into the outdoor air with a remote condenser. While the compressors in the CRAC units are the largest energy consumer of the cooling system, the fans located in the CRACs (used to circulate room air through the units' cooling coils) are also large energy users. CRACs were historically designed to operate at one fan speed even though many units have multiple (up to 4) stages of cooling. Running the fans at full speed even at reduced cooling loads wastes energy.

In general, the power required by the CRAC fan is proportional to the cube of the fan speed. For example, if a fan requires 6.5 kW at 100% speed, it will require only 3.3 kW at 80% speed. Not only will the fan power be reduced, but since the fan power all becomes heat in the space and must be removed by the cooling system, there are significant secondary savings in the compressor(s) and condenser(s) as well. Manufacturers do not provide retrofit systems for CRACs, so third-party retrofit schemes must be used to realize the savings from variable-speed fans. The retrofits of variable-speed drives and control systems are described in terms of the steps involved and the variations between the three locations.

In terms of results, at LBNL, the variable-speed fan controls caused a drop of 20 kW in the cooling power, even though the IT power increased by about 40 kW over the same period. This represents an improvement in the partial Power Usage Effectiveness (pPUE) for cooling from 0.46 to 0.35, a 24% reduction in cooling power. At EPRI, the fan controls saved up to 19% including the fans and compressor power (located in the CRACs themselves), and 31% when the outdoor units were included. The outdoor units are refrigerant-to-air condenser coils with fans; the reduced heat rejection meant less condenser fan energy, adding to the savings. The pPUE of the cooling system was improved from 0.64 to 0.44. At NetApp, results were nearly identical to those at EPRI.

Biography:

Steve Greenberg holds a bachelor's degree in Mechanical Engineering and a master's degree in Energy and Resources, both from the University of California at Berkeley, and is a registered Mechanical Engineer in California. He is also a LEED Accredited Professional by the U.S. Green Building Council and a Certified Energy Manager. Now a Senior Energy Management Engineer at the Lawrence Berkeley National Laboratory, he has researched and applied energy-efficient building and industrial systems, for a variety of clients on three continents over the past 33 years. He is currently working with the Lab's Environmental Energy Technologies Division, performing research on energy efficiency in laboratories, data centers, and other high-tech buildings, as well as working with the Lab's Facilities Division on several new building projects. He has been involved in design, design review, commissioning, and retrofit.

Note: I²SL did not edit or revise abstract or biography text. Abstracts and biographies are displayed as submitted by the author(s).