Design and Analysis of a Closed Chilled Water Thermal Storage Tank

ASHRAE Conference Paper (under review)

Author:

Christopher Kirchner and Kishor Khankari

Abstract:

The increasing dependence on all things cyber has resulted in robust systems being designed and installed in mission critical facilities to provide uninterruptable power and cooling to critical equipment in case of primary power and/or cooling supply loss. These systems typically utilize uninterruptible power systems (UPS) and often chilled water thermal storage to provide “ride” through power and cooling capacity until back-up power is online and primary cooling systems can be restored. Chilled water thermal storage tanks are often utilized for “ride” through capacity because cooling can be delivered from the tank(s) using just a distribution pump which can be powered by a UPS. However, estimating the performance of the thermal storage tank is critical to determining the required tank size, quantity, space requirements and associated capital costs necessary to meet the anticipated cooling load and duration. The principal factors in determining tank performance are temperature differential, flow rate, tank geometry (width and height) and how water is introduced and withdrawn from the tank which is related to the design of diffuser.
This paper with the help of Computational Fluid Dynamics (CFD) analysis will present the results of an evaluation of two different diffuser designs for a fixed geometry closed chilled water thermal storage tank that is part of a large multi-tank ride through cooling system. The CFD analysis compared the performance of a traditional octagonal diffuser and with the radial style diffuser. The impact of the diffuser geometry was evaluated to understand whether the diffuser design can make significant improvement in the tank performance with respect to maintaining the discharge temperature for a longer period of time. This information is an essential part of understanding the cost benefit of each of diffuser design and for their application in smaller scale closed thermal storage tanks. This paper will provide valuable insights to design engineers in understanding the importance of the diffuser design for the thermal storage tanks.