Numerical Investigations On Dynamic Performance Of Packed-Bed Thermocline Tank For Thermal Energy Storage

Thermal energy storage is the most promising energy storage technology,which can play a key role in the fields of solar thermal power generation,cogeneration unit peakshaving operation,and industrial waste heat utilization.Single tank thermal energy storage(TES)with thermocline layer has the advantages of little land occupation and significant reduction of cost.However,further investigation is necessary for its dynamic performance.The present work is directed towards the numerical investigations of the dynamic characteristics of packed-bed thermocline energy storage systems applicable for low and high-temperature applications.Three different numerical models are established to simulate the heat transfer and fluid flow characteristics inside the packed-bed thermocline tank,such as a simplified model for pure water and molten-salt tanks without a packedbed;a transient two-dimensional model based on the local non-equilibrium thermal approach;and a transient one-dimensional concentric-dispersion numerical model.The latter two models are formulated for a dual-medium heat storage system.The numerical results obtained from different models are validated with the experimental results reported in the literature.Water is used as heat transfer fluid(HTF)for a thermocline tank that is employed for combined heat and power unit(CHP),while molten-salt is used as HTF for a storage system integrated with concentrated solar power(CSP)plant.For a solid packed-bed tank system,three different solid filler materials are adopted such as quartzite rock,steelmaking slag pebbles,and alumina ceramics.While for a latent heat storage system,encapsulated phase change materials(PCM)having different melting points are adopted based on the operating temperatures of the charging and discharging processes.Different operation strategies of thermocline tanks are discussed,such as single charging/discharging process,cyclic,and simultaneous operations throughout investigating the effects of various operating parameters on the dynamic characteristics.For a water tank(without packed-bed),results indicated that the thermocline thickness increases as the inlet flow rate increases,and consequently,the heat storage/release period decreases.The effect of the mixing ratio on the performance of thermocline thickness is a good performance parameter to investigate the thermocline thickness and the operation of periodic charging with steady discharging resulted in improved discharge performance.For a solid-PCM multi-layered packed-bed water tank,the results show that the higher the volume fraction of both PCM layers than the solid filler layer,the higher heat storage and release periods and,therefore,the higher the energy storage and release.As the PCMs volume fraction increases from 10%to 40%,the amount of energy storage,energy release,and latent utilization ratio increased by 82.65%,73.94%,and 55%,respectively.In contrast,the overall exergy efficiency falls by 6.3%.Also,the optimum selection of both charge and discharge cut-off temperatures associated with the melting-point of two PCMs layers positioned at both extremes has a significant effect on the dynamic performance in terms of capacity ratio,total utilization ratio,recovered energy,and overall efficiency.For a molten-salt solid packed-bed tank(single-layer),results revealed that the thermocline thickness of the molten-salt packed-bed tank is higher than that of a pure molten-salt tank,while the latter is better in thermal stratification.As the inlet flow rate increases,both the discharging power and thermocline thickness increase.Moreover,decreasing particle size diameter results in a decrease in thermocline thickness,and the discharging performance becomes more stable.Slag pebble as a filler material is more effective than quartzite rock in thermal energy storage(TES).Compared with quartzite rock and slag pebbles,alumina ceramics achieves a higher thermal gradient.For a molten-salt packed-bed tank with multiple-PCMs during cyclic operation with certain cut-off temperatures,the results show that as the volume fraction of the bottom PCM increases compared to the top PCM,the system’s performance enhances both in terms of energy storage and recovered.Besides,the arrangement wherein the bottom PCM occupies half of the total bed height has the highest capacity ratio and the total utilization ratio of 83%and 40.5%,respectively.The scenario wherein the top and the bottom PCMs have a high inverse Stefan number,exhibits the greatest benefits compared to other scenarios.The results also indicated that the smaller capsule size exhibits the highest performance and enables the system to recover for a longer time.The results can be beneficial for the design and optimization of the packed-bed thermocline tank.