Energy assessment of CRAH bypass for enclosed aisle data centers

Temperature non-uniformities in traditional data centers can be eliminated or at least reduced by utilizing containment systems. As all servers receive the same inlet air temperature in a contained configuration, the cooling system can be operated more efficiently at a higher temperature, which also increases the potential for free cooling through various economizer modes. However, enclosed aisle configurations require computer room air handler (CRAH) fans to operate at a higher speed and provide entire rack air flow through the perforated tiles, unlike open aisle data centers that can make up a fraction of server air from the data center air space. Hence, the traditional enclosed aisle configuration is likely to consume more fan power. This study confirms that enclosing the aisle does not guarantee optimum cooling infrastructure power in air cooled data centers. Proposed CRAH bypass configuration for enclosed aisle data centers provides a fraction of the tile airflow rate through a set of bypass fans while CRAH fans operate at lower speeds. These low-lift fans operate across a pressure difference between the room and plenum, which is significantly less than the flow resistance of CRAH units. Meanwhile, CRAH fans operate at lower speeds and consume less energy. Accordingly, a certain bypass air fraction with respect to total rack air flow rate leads to a minimum cooling infrastructure power for a particular configuration. This study investigates energy savings potential of the enclosed aisle data centers with CRAH bypass configuration utilizing a calibrated flow network model for estimating the energy consumption of air movers as well as a thermodynamic modeling tool to evaluate the off-design performance of major components of data center cooling infrastructure. Hour-by-hour annual energy simulations complement the energy assessment for 7 U.S. cities considering indirect air side economizer operation.