Analysis of a Thermal Energy Storage (TES) System Using Phase Change Materials
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축열조(TES, Thermal Energy Storage) 해석
Thermal Energy Storage (TES) Analysis
◇ Research Background
As the use of renewable energy becomes increasingly important in modern society, the advancement of energy storage technologies has become essential. Efficient storage and utilization of energy play a critical role in minimizing energy loss and addressing the imbalance between energy supply and demand. Among emerging technologies, Power-to-Heat (P2H) systems have received significant attention. P2H converts surplus renewable energy into thermal energy for storage, and the key component enabling this process is the Thermal Energy Storage (TES) tank.
To enhance TES performance, Phase Change Materials (PCMs) are applied. PCMs provide high energy density by storing latent heat during phase transitions, making them especially valuable in applications where space efficiency is important. Previous studies have shown that TES systems incorporating PCMs outperform conventional water-based storage tanks but tend to exhibit longer charging times. Other research also suggests that an appropriate ratio of cooling PCM to heating PCM is important to maximize storage performance. However, limited research has been conducted on how the size of the PCM capsules affects TES performance.
Therefore, this study aims to compare the stored energy capacity of TES tanks containing PCM capsules of different sizes, with the PCM ratio for cooling and heating applications fixed at 5:5. The ultimate goal is to propose the optimal PCM capsule size for TES systems.
◇ Research Methods and System Description
In this study, ANSYS Fluent simulations were conducted to analyze TES charging and discharging performance according to PCM capsule size. The TES tank used for the simulation was a scaled-down experimental prototype, smaller than those typically installed in real facilities. The tank has a diameter of 1,150 mm and a height of 1,220 mm, and contains an internal fluid distributor.
Three PCM sizes were compared. The reference (medium-size) PCM capsule has a diameter of 130 mm, while the small and large capsules have diameters of 65 mm and 195 mm, respectively. All PCM capsules are spherical and encapsulated in 3 mm-thick polyethylene (PE). Each PCM size was filled into the TES tank in equal volume ratios. Considering temperature gradients and heat transfer behavior, heating PCMs were placed at the upper section of the tank, and cooling PCMs were placed at the lower section.
The phase change temperatures were set to 47.3°C for heating PCMs and 4.16°C for cooling PCMs. During charging, the inlet fluid temperature was set to 55°C, whereas during cooling it was set to 3.5°C. The simulations evaluated TES performance by analyzing fluid flow and PCM phase-change behavior.
◇ Research Results
Simulation results revealed significant performance differences depending on PCM capsule size. During the charging process, the total stored energy was similar across all PCM sizes. However, the energy stored per individual PCM increased with capsule size. Smaller PCMs stored energy more quickly but reached lower overall capacity, whereas larger PCMs stored more energy per unit but required longer charging times.
The cooling (discharging) process exhibited similar trends: small PCMs cooled rapidly but had lower energy density, while large PCMs had higher energy storage capacity but required longer cooling times. These results indicate that larger PCM capsules have the potential to enhance total TES capacity; however, excessively large PCMs may increase both charging and discharging times, which could reduce system response speed.
This study demonstrates the significant impact of PCM capsule size on TES charging and discharging performance. Small PCMs offer fast thermal response but limited energy capacity, while large PCMs provide higher energy density at the expense of slower responsiveness. Thus, selecting an optimal PCM size requires balancing total energy storage capacity with operational response speed.
Future work will include experimental validation under real operating conditions and exploration of various PCM combinations and TES configurations to develop an optimized TES design. This study is expected to contribute to improving the performance of PCM-based thermal energy storage systems.