Thermal Behavior of a Storage Tank of Solar Water Heater in Cases of Charge and Discharge

Abstract

Abstract-Stratification in a storage tank is the key to proper functioning of any installation designed to meet the thermal requirements. This work deals with the modeling of a storage tank to better understand the thermal behavior of the latter. The impact of the injection rate in the case of charge and discharge of the tank, the presence of auxiliary camp systems and / or an internal heat exchanger, on the temporal evolution of the temperature in the tank is examined. Modeling the stratification is undertaken by subdividing the tank N (20) layers. An energy and mass balance is drawn in each node. The overall system of equations is solved by Crank-Nicolson' method. The simulation results show that in the case of tank load, the last layer heats all the more rapidly as the injection flow rate is high. Furthermore the load by a heat exchanger placed in the bottom of the tank does not lead to a stratification in the latter. This charge mode thus tends uniformed the temperature in the tank. In the opposite, the use of electric boost during tank charging leads to establishment stratification.

Development and Analysis of a Multi-Node Dynamic Model for the Simulation of Stratified Thermal Energy Storage

“…The latter is represented in Equation (4) by the terms k • (T i 1 − T i ) and k • (T i − T i+1 ), which can be appointed as "pseudo-conduction terms" since they are used to represent the thermal exchange that would take place between bordering nodes by convection. This is a simplified way to consider these contributions (as suggested in [17,28]), but it has been noted by a preliminary analysis, which is not reported herein for the sake of brevity, that their influence on the global energy balance could be neglected. For this reason, this is an approach which is commonly used when dealing with multi-node models [15,16].…”

“…Finally, the multi-node models are developed following a 1D finite-volume method that considers a uniform horizontal temperature in each layer (called a node). Therefore, a temperature gradient in the vertical direction only (i.e., stratification) and a 1D flow inside the tank [27,28] can be modeled. Using this 1D approach, González-Altozano et al [17] have proposed a new methodology for the estimation of numerous temperature-dependent indices employed for the characterization of thermal stratification in water storage tanks (as temperature profile and thermocline thickness) during the charging phase, while Li et al [18] have investigated the influence of several inlet structures on the stratification effectiveness and discharging performance of a tank.…”

“…It is widely known that several physical phenomena occur in a water storage system that affect its performance. Among them, stratification has particularly raised the interest of researchers [27,28,33] for its strong influence on the thermal performance of plants [34,35]. For instance, Van Koppen et al [36] and Furbo and Mikkelsen [37] found that thermal stratification in solar heating systems allows a reduction in temperature at the collector inlet (which increases its efficiency) and leads to a limited operation of the auxiliary energy supply.…”

“…• Incoming mass flow rates and their related temperatures for each node; • Outgoing mass flow rates for each node but one (orange arrow in Figure 3) that is calculated by the model through the mass conservation equation. This is a simplified way to consider these contributions (as suggested in [17,28]), but it has been noted by a preliminary analysis, which is not reported herein for the sake of brevity, that their influence on the global energy balance could be neglected. For this reason, this is an approach which is commonly used when dealing with multi-node models [15,16].…”