The rate-dependent performance and optimization of stratified thermal energy storage devices

Applications of stratified thermal storage include heat storage in solar energy and water-heating appliance, and cool storage applications. Since buoyancy is the only mechanism separating the incoming flow from the fluid initially in the storage vessel, mixing is unavoidable between the hot and cold fluid volumes present in the storage vessel. The efficiency for both the energy charging and discharging processes, inherently time-dependent as well as rate-dependent, depends on the extent of the thermal mixing. This thesis first examines the performances of several representative existing simplified models which provide basis for the sizing and rating of the storage devices. The limitation of the existing models is the relatively low flow rates which could not satisfy the desired higher energy transfer rates and therefore also flow rates. Useful volume fraction and internal entropy generation are then used to characterize the extent of the thermodynamics losses and their variations with flow rate ranging from well-stratified to highly-mixed conditions. The thermally-stratified bottom filling process is simulated by a two-dimensional K - epsilon turbulent model. Transition from the gravity-dominated to the inertia-dominated regime is observed for an inlet Froude number less than approximately one. The final study utilizes mass transfer process of a scale-model salinity-stratified system to simulate full-scale thermally-stratified storage systems. Inlet diffusers with a side inlet and multiple inlets are investigated for the bottom filling process. Performances for multiple inlets and a side inlet are compared and a correlation for predicting performance is proposed for the range encompassing from well-stratified to fully-mixed conditions. The inlet diffuser with multiple inlets produces relatively less mixing and therefore enables higher storage efficiency. In addition, the turbulent kinetic energy obtained from the measured dimensional velocity data is strongly attenuated for the well-stratified condition and is less damped for higher level of mixing.