The Effect of Inlet Conditions and Tank Geometry on the Thermal Performance of Stratified Chilled Water Storage
Growth in peak demand and declining load factors coupled with the high cost of new generating capacity has prompted electric utilities to search for new ways to meet electric demand with existing generating capacity and to impose demand charges on consumption during peak hours. Chilled water storage has emerged as one of the simplest and most effective methods for shifting on-peak cooling loads to off-peak hours. With estimates of the electricity consumed by space cooling at nearly 50% of the summer peak, chilled water storage technology has potential to greatly benefit the environment both through reduced fossil fuel consumption and reduced pollutant emissions by shifting load away from low efficiency peaking units. Development of this potential has been limited by uncertainties in the relationship between system design and performance due, in part, to difficulties associated with the large scale of these systems. The present research has shown that the ideal, diffusion-limited efficiency of stratified chilled water storage is upwards of 90%. A method has been developed for comparing the performance of installed systems with this ideal limit using quantities which can be easily measured in the field. Based on published data, the method reveals potential for significant improvements in storage system efficiency. Using numerical solutions of the two-dimensional Navier-Stokes equations for parameters typical of full-scale systems, the impact of tank geometry has been examined for both the charge and discharge processes of stratified storage. The long time limit of the flowfield for the buoyant charge process, isothermal flow of a wall jet into a large width tank, is dominated by the jet vortex which scales with Reynolds number in both strength and size. Agreement with the classic similarity description of a slender wall jet is limited by the effect of overall geometry on the size and orientation of the jet recirculation cell. During the buoyant charge process, an order one Froude number leads to the establishment of a thermocline having sufficient strength to restrain the growth of the jet recirculation cell and effectively isolate the inlet and outlet flowfields unless very near either the inlet or outlet. Thermal mixing is greatest during the formation of the thermocline and during the time in which the thermocline remains near the jet inlet. In the discharge process, the initial formation of the thermocline leads to gravity-induced oscillations which are of an amplitude greater than those of the charge process due to lower shear stress at the free surface. The increased strength of oscillation leads to slight increases in thermocline thickness.
K. Homan, "The Effect of Inlet Conditions and Tank Geometry on the Thermal Performance of Stratified Chilled Water Storage," University of Illinois at Urbana-Champaign, Jan 1996.
Mechanical and Aerospace Engineering
Keywords and Phrases
Civil Engineering; Mechanical Engineering; Energy
© 1996 University of Illinois at Urbana-Champaign, All rights reserved.