Effects of Manifold Deformation and Permeability on the Performance of the Flexible and Rigid Porous Stratification Manifolds for Solar Storage Tanks

Abstract

Promoting and maintaining a high degree of thermal stratification in solar storage tanks has well documented benefits for increasing the solar energy gain from solar heating systems [1–3]. This dissertation investigated the flexible fabric porous manifold and rigid porous-tube manifold proposed and tested in prior studies [4–10] for stratification enhancement. A mathematical model for the flexible fabric manifold is developed that accounts the interaction between the surrounding fluid in the tank and the flexible manifold wall. The relationship between the tube deformation and the pressure difference across the wall is described by the semi-empirical “tube law”. This manifold model provides a physical understanding of the working mechanism of the flexible fabric manifold and disapproves the widely held hypothesis that the deformation of the manifold is beneficial. Modeling results indicate that the dimensionless permeability can be optimized for improved performance. Following above findings, the effect of dimensionless permeability on the performance of the rigid porous-tube manifold are investigated in conditions representative of residential solar hot water systems. 2-D CFD simulation of the charging process reveals that optimizing the selection of the dimensionless permeability can improve the effectiveness of the manifold by eliminating suction (during intermediate charging) and releasing the fluid in the upper portion of the tank (during top charging). Furthermore, I show that the 1-D manifold model provides adequate prediction of the radial velocity distribution on the manifold wall compared with the results from the 2-D simulation. Therefore, the 1-D model can be used as an efficient design tool. With simulation results from the 1-D model over a wide range of the Richardson number and the dimensionless permeability, a design guideline for selection of the dimensionless permeability according to the Richardson number and the charging mode are developed. A prototype of the rigid porous-tube manifold is constructed and tested in comparison to conventional inlets. Temperature distribution in the tank is measured by thermocouple trees and the velocity field near the manifold is measured by a PIV system. For intermediate charging, the manifold achieves 0.5 dimensionless exergy efficiency after 90 minutes of charging, while the exergy efficiency for inlet diffuser and inlet pipe are 0.15 and 0, respectively. For top charging, the performance of the manifold is comparable to the best performing inlet diffuser. The measured velocity field is consistent to the model predication, indicating a 90% reduction of the suction rate during intermediate charging compared with conventional inlet pipe, and showing that the fluid is released approximately in the upper 25% of the tank during top charging.