Abstract
Thermal energy storage (TES) technologies are becoming vitally important due to intermittency of renewable energy sources in solar applications. Since high energy density is an important parameter in TES systems, latent heat thermal energy storage (LHTES) system is a common way to store thermal energy. Though there are a great number of experimental studies in the field of LHTES systems, utilizing computational codes can yield relatively quick analyses with relatively small expense. In this study, a numerical investigation of a LHTES system has been studied using ANSYS FLUENT. Results are validated with experiments, using hydroquinone as a phase-change material (PCM), which is external to Therminol VP-1 as a heat transfer fluid (HTF) contained in pipes. Energy efficiency and entropy generation are investigated for different tube/pipe geometries with a constant PCM volume. HTF inlet temperature and flow rate impacts on the thermodynamic efficiencies are examined including viscous dissipation effects. Highest efficiency and lowest entropy generation cases exist when when flow rates are lowest due to low viscous heating effects. A positive relation is found between energy efficiency and volume ratio while it differs for entropy generation for higher and lower velocities. Both efficiency and entropy generation decreased with decreasing HTF inlet temperature. The novelty of this study is the analysis of the effect of volume ratio on system performance and PCM melting time which ultimately proved to be the most dominant factor among those considered herein. However, as PCM solidification and melting time is of primary importance to system designers, simply minimizing entropy generation by decreasing volume ratio in this case does not lead to a practically optimal system, merely to decrease heat transfer entropy generation. Therefore, caution should be taken when applying entropy analyses to any LHTES storage system as entropy minimization methods may not be appropriate for practicality purposes.
Highlights
Thermal energy storage (TES) is a popular tool to help match short and long-term energy demands.There are a variety of applications for TES, such as space heating and cooling and hot water storage [1].The underlying premise behind TES is that thermal energy, or lack thereof, can be stored by heating or cooling a storage substance at high or low temperatures
A three-dimensional numerical latent heat thermal energy storage (LHTES) model has been created to investigate the impacts of heat transfer fluid (HTF) inlet temperature, volumetric flow rate (VFR), and volume ratio (VR) of phase-change material (PCM) to HTF
The authors note that several simulations were conducted to ascertain the effects of buoyancy on simulated parameters
Summary
Thermal energy storage (TES) is a popular tool to help match short and long-term energy demands. Peiro et al [10] performed an experimental study to examine the efficiency of heat exchangers of a LHTES systems in parallel and counter flow conditions comparing HTF inlet temperatures. Performed a numerical study to examine the storage characteristics of a LHTES system during charging and discharging periods changing the HTF inlet temperature, and flow rate. The authors claimed that entropy generation decreases with decreasing flow rate and increasing the inlet temperature for charging process In this present work, a three-dimensional numerical LHTES model has been created to investigate the impacts of HTF inlet temperature, volumetric flow rate (VFR), and volume ratio (VR) of PCM to HTF domain on energy efficiency and entropy generation. There are no known studies considering this volume ratio in this geometrical setting, and weighing losses comparing volume ratio to other factors such as HTF flow rate and temperature, on overall system performance in terms of energy efficiency and entropy generation
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