DESIGN CONDISERATIONS FOR FULL-SCALE STRATIFIED CH

Chilled water thermal storage is used extensively in the United States for load management and peak electric demand reduction in large chilled water systems. The great majority of storage vessels are naturally stratified tanks, most of which are vertical cylinders. Design guidelines for stratified tanks are based upon an incomplete parameterization that does not account explicitly for tank geomery and that relies upon limited experimental measurements in tanks that are nearly two orders of magnitude smaller than typical full-scale tanks. Consequently, it is not surprising that observed performance of full-scale systems does not accord well with this design theory. This paper considers thermal performance prediction for the purpose of designing cylindrical naturally stratified tanks with radial parallel plate diffusers. The existing design methodology for stratified chilled water storage tanks is reviewed and critiqued in light of field data gathered from full-scale tanks. An improved method is developed through three steps. First, a complete set of parameters is generated by rigorous dimensional analysis. Next, a 2k factorial experiment is conducted on thermal performance results derived from charging simulations performed with a computational fluid dynamic (CFD) model validated by field data. Statistical analysis of these results identifies parameters of first order significance: the inlet Richardson number, the ratio of diffuser radius to tank radius ,and the ratio of diffuser radius to diffuser inlet height. The inlet Reynolds number is found not to have significant influence within the parameter ranges investigated. Finally, linear regression is used to develop simple thermal performance predictors as functions of significant parameters. The use of the resulting relations in design is discussed and illustrated. These new relationships provide a quantitative basis for optimizing diffuser design and take into account tank as well as diffuser and flow parameters.