Numerical study of a latent heat thermal energy storage system enhanced by varying fin configurations

Latent heat thermal energy storage (LHTES) has been used to deal with the cyclical nature of energy production through solar means. One problem with LHTES systems is the PCM having a low thermal conductivity. This results in low heat transfer rate prolonging the charging and discharging cycles. In the current study, the thermal characteristics of a latent heat thermal energy storage system enhanced with annular and radial fins are investigated experimentally. Rubitherm RT-55 is used as the phase change material (PCM) and is enclosed within a vertical cylindrical container. Water is used as the heat transfer fluid (HTF) which is circulated in a copper pipe that passes through the center of the container. Different numbers of annular and radial fins attached to the central pipe, all containing the same volume of copper are studied. The effects of these configurations on the thermal performance of the latent heat thermal energy storage system during the charging and discharging processes is monitored. The no-fin benchmark case took 47.87 h to charge the LHTES unit and 42.5 h to discharge. The ten-annular fins charged the system 84.1% faster and discharged the system 68.21% faster. The twenty-annular fin case reduced the charging time by 85.8% and the discharging by 68.58%. The four-fin radial case was found to decrease the charging time by 81.9% and 70.0%; whereas the eight-fin radial case was found to have the greatest decrease the charging and discharging times, being 86.6% and 80.1%, respectively.

An experimental study on the effect of annular and radial fins on thermal performance of a latent heat thermal energy storage unit


 Latent heat thermal energy storage (LHTES) systems are used to store thermal energy and release it for later use by melting or solidifying a phase change material (PCM). One problem associated with latent heat thermal energy storage systems is the low thermal conductivity of most commercially aviable phase change materials. This can have a significant negative effect on the thermal performance of the system by leading to a longer charging or discharging process. Several passive heat transfer enhancement techniques are used to resolve this issue. Common passive heat transfer enhancement techniques include inserting fins and extended surfaces into the PCM, embedding heat pipes or other two-phase heat transfer devices within the PCM, dispersion of highly conductive nanoparticles in the PCM, and impregnation of highly conductive porous media with the PCM. The current study analyzes the effect of a fin-based enhancement technique on the thermal performance of a latent heat thermal energy storage unit. Copper fins are attached annually around the central pipe inside the PCM. A transient two-dimensional numerical model technique is developed using ANSYS FLUENT 19.0 to simulate the operation of the system. Baseline tests have been conducted experimentally for a system without fins to validate the numerical model. The results obtained from the numerical modeling are in good agreement with those of the experimental testing. Based on the experimental testing, the total charging time of the system using hot water at 70°C and flow rate of 7.57 L/min is around 47.9 hours which is very close to the prediction by the numerical model which is 48 hours. Numerical modeling of the system with 10 fins and 20 fins found that the charging time was decreased by 68.9% and 73.7%, respectively. The discharging time was also decreased by 73.2% and 79.1%, respectively.

Experimental and Numerical Study of a Latent Heat Thermal Energy Storage Unit Enhanced by Fins

Extended Abstract The lack of a dispatchable energy supply makes transitioning from fossil fuels to green energy alternatives such as solar and wind energy extremely difficult. One solution to remedy this problem is the introduction of a latent heat thermal energy storage (LHTES) unit into an energy system to continue to provide energy long after the original source is gone. LHTES units utilize phase change materials (PCM) as storage media that melt and solidify in the presence of thermal energy. LHTES units using only pure PCMs take a long time to charge and discharge due to PCM low thermal conductivity, making their application in energy systems more difficult. The introduction of thermal conductivity enhancement techniques such as fins, nanoparticles and metallic foam decreases charging and discharging time and increases the thermal efficiency of LHTES units [1-3]. The current study numerically and experimentally analyses a LHTES unit containing Rubitherm RT55 as the PCM and enhanced with fins. Different configurations of metallic fins were examined to maximize the heat transfer to the PCM. These configurations were compared to the operation of the LHTES unit without any enhancement. Both the charging and the discharging cycles of the LHTES unit were studied. The experimental results were compared to numerical simulations completed by ANSYS Fluent 17.0 commercial CFD package. Finding an optimal configuration of a LHTES unit containing PCM and fins allows such units to be integrated with an alternative energy system making the alternative energy more reliable and comparable to their fossil fuel counterpart. The PCM in this study is stored in a cylindrical container which was of 30.48 cm tall with an inner diameter of 19.05 cm, with a copper pipe that transports the heat transfer fluid at the center of the container. The system is initialized from room temperature (22 °C) and is charged with water at 70 °C. Firstly, the numerical data was compared to the experimental to provide verification for the model. The comparison indicated that the numerical predictions were in good agreement with the experimental data. Two configurations of 10 annular fins and 20 annular fins attached to the central pipe were analysed numerically. These two cases were compared to a benchmark case using no fins. The no-fin case using 70°C HTF took approximately 48 hours to charge. This benchmark was then compared to the finned cases. The 10-fin case took roughly 15 hours to charge, while the 20-fin case took 12 hours and 40 minutes to fully charge. The resulting percent decreases in charging times were 68.9% and 73.7%, respectively. When evaluating the discharging cycle of the heat exchanger, it was found that the case utilizing no fins took about 42.5 hours to completely discharge. The 10-fin case took 11 hours and 20 minutes, a 73.2% decrease in total charging time. The 20-fin case had the fastest discharge cycle, which was 8 hours and 50 minutes, a 79.1% decrease when compared to the no-fin case. A case can be made that gains in efficiency when using the 20-fin case are offset by the manufacturing cost and difficulty when creating and assembling the fin geometry.

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Experimental and Numerical Study of a Latent Heat Thermal Energy Storage System Enhanced with Fins

Extended Abstract A major drawback of latent heat thermal energy storage (LHTES) systems is the low thermal conductivity of phase change materials (PCMs) used as storage media in them. Different passive heat transfer techniques such as using embedded heat pipes [1], impregnation of highly conductive porous media with PCMs [2], dispersion of highly conductive nanoparticles into PCMs [3], and the inclusion of fins into PCMs have been employed to improve the thermal performance of these systems. In the current study, effects of annular and radial fins on the thermal characteristics of a LHTES system during the charging and discharging processes are studied experimentally. Rubitherm RT-55 is used as the phase change material (PCM) and is enclosed within a vertical insulated cylindrical container. The container is manufactured from a 1.27 cm thick clear acrylic tube with a height of 30.48 cm and an inner diameter of 17.78 cm. Water is used as the heat transfer fluid (HTF) which is circulated in a copper pipe that passes through the center of the container. For the charging process, hot HTF at 70 °C is circulated through the system until the entire mass of solid PCM inside the container is fully molten. For the discharging process, cold 20 °C HTF is pumped through the system until all the molten PCM had solidified. Twelve k-type thermocouples are inserted into the PCM container at different heights and depths to monitor the PCM temperature. The inlet and outlet temperature are monitored by two 100 Ohm RTDs. The effects of four different fin configurations attached to the central pipe are studied with the same amount of material used for each configuration. This allows for the understanding of system efficiency while maintaining the same material cost and loss of volume of PCM for energy storage. The two annular fin configurations have fins of 4.0 cm in length with the first configuration having ten fins with a thickness of 1.58 cm. While the second configuration has twenty fins of thickness 0.79 cm. The two radial configurations are rectangular in shape being 7.77 cm in length and 22.94 cm in height. Configuration three has four fins of thickness 1.58 cm while configuration four has eight fins of thickness 0.79 cm. The effects these fin configurations on the total charging and discharging times as well as the PCM temperature distribution have been studied during charging and discharging cycles. The no-fin benchmark case took 48 hours to fully charge and 42.5 hours to discharge. The 10 annular fins charged the system 84.1% faster than the nofin case. The 20 annular fin case reduced the charging time by 85.8%. The 10 annular fins decreased discharging time by 68.21% and the 20 annular fins decreased charging time by 68.58%. The radial fin configurations of four and eight fins were found to decrease the charging time by 81.9% and 86.6%, respectively. The discharging time was improved by 4 radial fins by 70.0% and by the 8 radial fins by 80.1%. Overall, it was found for the same amount of fin material, the 8-fin radial design produces the best results in accelerating the charging and discharging processes.

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Samantha Moretti

Papers

Experimental and Numerical Study of a Latent Heat Thermal Energy Storage Unit Enhanced by Fins

Experimental and Numerical Study of a Latent Heat Thermal Energy Storage System Enhanced with Fins

Thermal Analysis of Latent Heat Thermal Energy Storage SystemsEnhanced with Annular and Radial Fins