A Flow Rate Dependent 1D Model for Thermally Stratified Hot-Water Energy Storage

Abstract

Stratified tank models are used to simulate thermal storage in applications such as residential or commercial hot-water storage tanks, chilled-water storage tanks, and solar thermal systems. The energy efficiency of these applications relates to the system components and the level of stratification maintained during various flow events in the tank. One-dimensional (1D) models are used in building energy simulations because of the short computation time but often do not include flow-rate dependent mixing. The accuracy of 1D models for plug flow, plug flow with axial conduction, and two convection eddy-diffusivity models were compared with experimental data sets for discharging a 50-gal residential tank and recharging the tank with hot water from an external hot-water source. A minimum and maximum relationship for the eddy diffusivity factor were found at Re <2100 and >10,000 for recirculation of hot water to the top of the tank and vertical tubes inletting cold water at the bottom. The root mean square error decreased from >4 °C to near 2 °C when considering flow-based mixing models during heating, while the exponential decay of the eddy diffusion results in a root mean square error reduction of 1 °C for cone-shaped diffusers that begin to relaminarize flow at the inlet.

Heat Transfer, Refrigeration and Heat Pumps

“…In the next section, we provide a brief review of the seven published papers [1][2][3][4][5][6][7] and their potential scope of application.…”

“…However, their energy efficiency depends on system components as well as internal water stratification in the storage tank during water draws. The paper by Rendall et al [6] examined the impact of temperature stratification based on a 1D model because such simulations are not as computationally intense as 2D or 3D models. To capture the essential physics of the problem and perform a comparative study, the authors included pure plug flow, plug flow with axial conduction, and two convection eddy diffusivity models.…”