Thermal and heat transfer characteristics of a packed-bed CTES system using encapsulated PCM capsules for peak-load management

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Sathishkumar, P. Sundaram, M. Cheralathan, R. Prabakaran, Sung Chul Kim This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7527832/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract This study explores a packed-bed cool thermal energy storage (CTES) system that uses spherical PCM capsules filled with distilled water and incorporates a helical coil to retrieve cold energy through air circulation. The system is designed to deliver efficient cooling performance, ensuring a reliable supply of cold energy even during electricity outages. The research investigates the influence of HTF flow rates (100 l/h, 200 l/h, and 300 l/h) during charging and air discharge velocities (2, 4, and 6 m/s) on the system’s thermal performance. Results indicate that higher HTF flow rates accelerate the charging process, reducing the time to full PCM solidification from 3.44 h at 100 l/h to 2.49 h at 300 l/h, while lower flow rates decrease the pressure drop from 0.78 kN/m² to 0.53 kN/m², highlighting a trade-off between rapid energy storage and pumping efficiency. Energy analysis indicates that the PCM stores the majority of the cold energy, with latent heat contributing over 60% of the total 9857 kJ, demonstrating the effectiveness of the hybrid sensible-latent storage approach. During discharging, air temperature and humidity reductions were most significant at 4 m/s, with temperature decreasing from 34.93°C to 25.44°C and humidity from 55–47%, indicating optimized cooling and moisture removal. Comparisons with conventional air-conditioning reveal that the CTES system can deliver equivalent cooling using pre-stored energy, reducing peak electricity demand while maintaining thermal comfort. The study confirms that careful selection of HTF flow rates and discharge velocities enables efficient energy storage and retrieval, making the proposed CTES system a practical and sustainable solution for continuous indoor cooling under intermittent power supply conditions. Packed-bed system Phase Change Material Charging and discharging performance Air circulation cooling Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1. Introduction Thermal Energy Storage (TES) systems are increasingly important for addressing India's growing cooling energy demand, which is projected to double by 2027, according to the Ministry of Power [1]. With buildings accounting for 57% of this demand, TES systems offer a sustainable solution to reduce energy consumption in cooling applications [2]. By storing excess thermal energy during off-peak hours and releasing it during peak periods, TES can help avoid the need for an estimated 25 GW of new coal-based power generation. Moreover, as energy demand from fans and air coolers surpasses that from air conditioners, TES systems can enhance energy efficiency across various cooling technologies, contributing to significant energy savings and improved grid stability [3]. Cool thermal energy storage (CTES) systems have emerged as an effective technology for managing energy consumption in large buildings, where air conditioning typically accounts for about 40% of total electrical usage. By integrating a CTES with air conditioning systems, the cooling load can be shifted from peak to off-peak hours, reducing both energy costs and peak demand without the need to design systems solely to accommodate maximum load requirements [4]. This integration is also critical for addressing issues related to dynamic electricity tariffs and grid stability, particularly in regions with high energy demand fluctuations [5, 6]. CTES systems have also proven valuable in smaller-scale applications such as process cooling, milk preservation, vaccine transportation, and microelectronic cooling, where precise temperature control is crucial. Recent advancements in this field have focused extensively on improving the thermophysical properties of phase change materials (PCMs), which are integral to the functioning of CTES [7–9]. Deionized (DI) water, due to its high latent heat of fusion and low cost, remains a widely used PCM. However, its thermophysical properties, including thermal conductivity, can be significantly enhanced by adding nanoscale conductive solid particles such as metal oxides or carbon-based materials. These nanoparticles improve heat transfer rates, although challenges like nanoparticle sedimentation and supercooling still persist. Surfactants have traditionally been used to ensure a stable dispersion of nanoparticles within PCMs, but they often act as thermal barriers, limiting the expected enhancement in thermal properties [10, 11]. To address these issues, several researchers have explored alternative approaches, such as chemical functionalization of nanoparticles, which can enhance their stability and prevent agglomeration without altering the PCM's key properties such as specific heat or viscosity. Chandrasekaran et al. [12] examined the geometric and material parameters affecting heat transfer in PCMs and concluded that the use of additives like nanomaterials significantly improved the performance of water-based CTES systems. Similarly, Gallego et al. [13] explored the thermophysical properties of composite PCMs like CaCl₂ 6H₂O by introducing nucleating agents and thickeners, which improved the material's thermal stability and reduced supercooling to 0.95°C, though this came at the cost of reduced storage capacity. Also, Altohamy et al. [14] demonstrated that dispersing aluminum oxide nanoparticles in DI water reduced solidification times by up to 30%, owing to the enhanced thermal conductivity of the resulting nanofluid. However, the use of surfactants or thickeners has been found to create issues by reducing the overall thermal performance, as reported by Vikram et al. [15]. To counteract these limitations, the same research group later explored the chemical functionalization of nanomaterials, such as graphene nanoplatelets (GNPs), using covalent methods like nitric acid treatment to achieve stable suspensions without the need for surfactants. This approach was shown to improve long-term stability and thermal conductivity, as evidenced by Prabakaran et al. [16], who reported a 26% reduction in discharging time when using functionalized GNPs. CTES systems have been extensively studied in recent years, with packed-bed PCM storage systems gaining attention for their high energy storage density and large heat transfer area compared to conventional ice storage banks. These systems, particularly in large-scale applications, often employ paraffin or DI water as the primary PCM. Research by Al-Shannaq et al. [17] explored the energy storage capabilities of packed beds filled with graphite-enriched spheres, revealing that reducing the heat transfer fluid (HTF) inlet temperature significantly decreased charging times. Dong et al. [18] investigated the performance of cold thermal storage tanks under different heat transfer fluid flow conditions and charging with different diameter balls to identify the more efficient one. The investigation revealed that increasing the water flow rate from 1.296 m/h to 2.196 m/h enhanced the freezing rate of the PCM balls: from 41–91% for D = 90 mm, from 74–100% for D = 90 & 60 mm, and from 94–100% for D = 60 mm after 8 hours. At a constant chilled water velocity, smaller-diameter PCM balls exhibited higher freezing rates. Lei Zhang et al. [19] investigated the flow field and heat transfer in an energy storage tank. Applying unique geometric configurations alongside realistic boundary conditions helps predict how nano-phase change materials behave during their melting and freezing cycles, specifically in the charging and discharging processes. They found that phase change material (PCM) behaviour in energy storage tanks with porous media, emphasizing the impact of porosity coefficients. For a coefficient of 0.95, full melting and freezing of pure PCM take 2000 s and 1250 s, respectively. Adding aluminum oxide nanoparticles accelerates melting (1700 s) and freezing (1300 s). Lowering porosity from 0.97 to 0.95 enhances heat transfer, reducing melting time and improving efficiency. Oro et al. [20] focused on district cooling networks combined with TES, showing how HTF temperature variations could affect the total solidification time of PCM-filled storage tanks. Other studies, such as those by Baruah et al. [21], reported on the performance of chiller systems incorporating energy storage tanks containing PCM balls, with findings indicating a 3–4% increase in specific energy consumption for each 1°C decrease in evaporator temperature. Khan et al. [22] further demonstrated the advantages of using paraffin in shell-and-tube heat exchangers with fins for low-temperature thermal energy storage. The use of plastic spheres filled with PCM in packed-bed systems offers several benefits, including ease of installation and flexibility in material selection for storage tanks. This approach has been widely studied in both experimental and modeling contexts. For instance, Panesi et al. [23] evaluated the crystallization rates of DI water-based PCMs in spherical capsules using different HTFs, finding that ethanol as the HTF resulted in faster charging compared to ethylene glycol/DI water mixtures at the same flow rates and input temperatures. Table 1 summarizes recent findings related to packed-bed thermal energy storage systems. The growing demand for energy-efficient cooling systems and the increasing frequency of electricity outages necessitate the development of advanced CTES technologies capable of providing reliable and sustainable cooling solutions. Conventional CTES systems face significant challenges, including low charging efficiency, inadequate cold energy retrieval, and limited capability to maintain indoor thermal comfort under varying operational conditions. These limitations arise due to suboptimal heat transfer processes, insufficient utilization of encapsulated phase change materials (PCMs), and the absence of efficient air-based discharging mechanisms. To overcome these drawbacks, the present research focuses on designing and analysing a packed-bed CTES system employing encapsulated distilled water-filled spherical balls combined with a helical coil for air-driven cold energy retrieval. The primary objectives of the present work are: (a) To design and analyze a packed-bed cool thermal energy storage (CTES) system using encapsulated distilled water-filled spherical balls for efficient cold energy storage and retrieval. (b) To investigate the effect of varying HTF flow rates (100, 200, and 300 l/h) during the charging process on the freezing behaviour, energy storage capacity, and overall thermal performance of the system. (c) To evaluate the discharging performance of the CTES system by supplying air through a helical coil at different velocities (2, 4, and 6 m/s) and assessing its impact on room temperature, humidity, and cooling duration under simulated electricity outage conditions. The proposed research introduces a packed bed CTES system using encapsulated distilled water-filled spherical balls combined with a helical coil for air-based cold energy retrieval, enabling continuous cooling even during power outages. Unlike conventional CTES systems, it employs variable air velocities for discharging and optimizes HTF flow rates during charging to maximize cold energy storage efficiency. Additionally, the study uniquely evaluates room temperature and humidity variations to demonstrate the system’s capability to maintain thermal comfort under intermittent power supply conditions. 2. Materials and methods The experimental setup of the cool thermal energy storage (CTES) system employs deionized (DI) water, a glycol-water mixture (40:60), and air as working media due to their distinct thermal characteristics and functional roles. DI water is selected as the primary phase change material (PCM) because of its high latent heat of fusion (~ 334 J/g), specific heat capacity (4.18 J/g°C), and suitable freezing point (0°C), enabling efficient cold energy storage and release. Air is utilized as the discharging medium, where forced convection enhances cooling delivery despite its low specific heat (~ 1.005 J/g°C) and low thermal conductivity (~ 0.025 W/m K). The combination of these materials ensures efficient charging, reliable cold storage, and effective cooling performance within the CTES system. 3. Design of experiment The design of the CTES system involves a cylindrical storage tank integrated with spherical PCM capsules, copper tubes, an air circulation unit, and an insulation layer to ensure efficient thermal energy management. Figure 1 shows the design specifications of the packed-bed system used in the present study. The tank has an outer diameter of 500 mm, inner diameter of 400 mm, and a height of 950 mm, with a wall thickness of 50 mm. A total of 200 spherical capsules made of suitable material, each with an outer diameter of 76 mm and an inner diameter of 72 mm, are filled with 92% deionized (DI) water volume (~ 179.9 ml per capsule) to accommodate thermal expansion during solidification, resulting in a total PCM volume of approximately 45.96 l. Copper tubes with an outer diameter of 13 mm, inner diameter of 11 mm, and a total length of 9500 mm are used to circulate the heat transfer fluid (HTF), enabling uniform charging and discharging. A copper coil of 20 mm diameter forms the air circulation path, leveraging copper’s high thermal conductivity for efficient heat exchange. The storage tank is insulated using a POLYOL and EMPEYOL mixture (0.5:0.5 ratio), achieving a very low thermal conductivity of 0.014 W/m K to minimize thermal losses. During the solidification process, the HTF is cooled via an integrated charging unit comprising an evaporator coil, condenser, and compressor, and is circulated from the base of the tank to the upper part for uniform cooling of the spherical capsules. A stirrer is installed to maintain uniform HTF temperature, while RTDs and digital sensors continuously monitor temperature variations at multiple heights. If the HTF temperature drops below the set point, a heater activates automatically to maintain stability. During the discharging process, a variable-speed blower forces air through the air circulation coil, where it passes over the charged capsules, absorbs stored cold energy, and exits through the outlet to deliver cooling. The entire system is monitored using a data logger connected to a computer interface, ensuring real-time data acquisition and performance analysis of the CTES system. Table 2 presents the detailed specifications of the energy storage tank. Table 2 Detailed specification of energy storage tank Specifications Units in mm Outer diameter of the tank 500 Inner diameter of the tank 400 Height of the tank 950 Wall thickness of the tank 50 Distance between air in and air out 824 Outer diameter of the spherical capsule 76 Inner diameter of the spherical capsule 72 Wall thickness of the spherical capsule 2 Outer diameter of the copper tube 13 Inner diameter of the copper tube 11 Thickness of the copper tube 1 Length of the copper tube 9500 4. Experimentation The experimental procedure and equipment used for the charging and discharging studies are described in this section. To overcome the limitations of conventional constant-temperature bath experiments, the charging and discharging behavior of the phase change material (PCM) is investigated using a low-capacity CTES system (< 0.5 TR·hr) under various heat transfer fluid (HTF) flow conditions. This approach provides a more practical representation of heat transfer performance in real-world thermal energy storage applications. Controlled discharging rates are also examined based on varying demand conditions, which cannot be effectively simulated using a constant-temperature bath setup. Figure 2 illustrates a schematic of the thermal energy storage tank, showing both the charging and discharging arrangements. The PCM is encapsulated within spherical shells to enhance heat transfer efficiency by maximizing the surface area-to-volume ratio, resulting in faster thermal exchange, improved storage capacity, and easier integration into thermal management systems. To monitor the internal temperature distribution during thermal cycling, resistance temperature detectors (RTDs) are carefully inserted through the neck of each spherical capsule, which is tightly sealed to prevent leakage. Markings made with black tape on the RTD wires ensure accurate sensor placement at predefined depths within the PCM-filled sphere. Three RTDs are strategically positioned at different depths to capture detailed temperature profiles: RTD 1 is placed at the center of the capsule (100%), RTD 2 is located 22.0 mm from the center (approximately 75% depth), and RTD 3 is positioned 27.8 mm from the center (approximately 50% depth). These RTDs are connected to a data logger for real-time temperature acquisition and monitoring during the charging and discharging cycles. This experimental arrangement enables precise tracking of thermal behavior, providing deeper insights into the phase change dynamics and heat transfer characteristics within the encapsulated PCM under practical operating conditions. Figure 3 shows a pictorial view of the experimental setup. Also, Table 2 lists the standards employed in the present experimental work. A – Proportional temperature controller (PDTC), B – Agitator, C – Heating element, D – Thermal sensor, E – Colling coil, F – Condensation unit, G – Compressor, H – Data acquisition unit, I – Computer interface, J – Circulation pump, K – Control valve, L – Pressure indicator, M – Storage module, N – Resistance temperature detectors (RTDs), and O – Power consumption meter. Table 2 Standards employed in the present experimental work. Instrument Standards Measuring Sensors / Parameters Accuracy RTD Sensor, Conax Technologies, Chennai ISO Standards RTD Sensor; Class B Type ± 0.10°C Class B Volumetric Flask, SRL, Chennai ISO 4787:2010 Volume – ml ± 0.015 mL Flow Meter, Conax Technologies, Chennai ISO Standards Water Mass Flow Rate – l/h ± 2% Semi-Micro Balance ASTM D-792 Mass – g ± 0.02% Pressure Transducer (DP-Style 266DSH), ABB, India ISO Standards Pressure Drop – kN/m² ± 0.075% Energy Meter (DLMS Meter 10-60A), Tech Baniya, India IS 13779:1999 Energy Input – kWh ± 0.2% Humidity Meter ISO Standards Relative Humidity (%) ± 2% RH 4.1 Assessment of the overall heat loss coefficient The overall heat loss coefficient (U) is evaluated using a heat gain experiment. The temperature of the heat transfer fluid (HTF) inside the storage tank, initially maintained at -8°C, and the ambient temperature are continuously monitored. After 24 hours, the final HTF temperature is recorded to calculate the total heat loss coefficient (based on Eq. 1 & 2 ) [ 31 ]. $$\:{U}_{CTES\:}\left(LMTD\right)=\:{m}_{l}{c}_{l}\left(\frac{dT}{dt}\right)$$ 1 $$\:{LMTD}_{CTES}=\frac{\left({T}_{i}-{T}_{atm}\right)-({T}_{l}-{T}_{atm})}{\text{ln}\frac{\left({T}_{i}-{T}_{atm}\right)}{({T}_{l}-{T}_{atm})}}$$ 2 where T l and T i are the average final and initial temperature of the HTF, T atm is the atmospheric temperature. From the above experiment, the overall heat loss coefficient (U) was determined to be 0.314 W/m²·K, which represents the rate of heat transfer per unit area per degree of temperature difference between the storage tank and its surroundings. A lower U-value indicates better insulation and reduced heat losses, while a higher value would signify greater thermal losses. In this case, the relatively low value of 0.314 W/m²·K demonstrates that the storage tank is well-insulated 5. Result and Discussion The results and discussion section covers the charging process of the energy storage tank, the discharging behaviour of the PCM, the total energy stored, the pressure drop within the storage tank, and the overall energy-saving potential of the system. 5.1 Charging of energy storage tank: During the charging process of the CTES system, the phase change material (PCM) gradually absorbs heat from the circulating heat transfer fluid (HTF). Figure 4 (a-c) shows the temperature–time variation of the PCM at different HTF flow rates of 100 L/h, 200 L/h, and 300 L/h. The PCM temperature decreases from an initial value of approximately 24°C toward the freezing point. Heat removal occurs in two stages: first, sensible heat is removed, causing a steady and gradual drop in temperature, followed by latent heat absorption during the phase change, where the temperature remains nearly constant. To monitor the thermal behavior accurately, nine temperature sensors were placed throughout the energy storage tank: bottom sensors (B 1 , B 2 , B 3 ), middle sensors (M 1 , M 2 , M 3 ), and top sensors (T 1 , T 2 , T 3 ). The sensor readings indicate a relatively uniform cooling trend across all regions, confirming consistent heat transfer throughout the PCM mass. The top sensor (T 1 ) is critical in determining full charge; when T 1 reaches approximately − 1.5°C, the system is considered fully charged. The charging duration depends strongly on the HTF flow rate. At a flow rate of 300 l/h, the PCM reaches full charge in 10,150 seconds (~ 2.49 h). At 200 l/h, charging takes 12,050 seconds (~ 3.20 h), and at 100 l/h, it requires 13,450 seconds (~ 3.44 h). Higher HTF flow rates increase convective heat transfer, accelerating the removal of both sensible and latent heat and reducing total charging time. During the latent heat plateau, the PCM continues to absorb significant energy while the temperature remains nearly constant, indicating efficient energy storage. Bottom sensors typically register slightly lower temperatures initially, while middle and top sensors follow a similar trend as heat propagates through the PCM. The uniform response of all nine sensors ensures that the entire PCM volume is effectively charged. This behavior demonstrates the importance of proper sensor placement to evaluate system performance. Overall, the charging process is characterized by a steady temperature decrease, latent heat plateau, and a clear top-sensor criterion for full charge, ensuring the PCM is fully prepared for subsequent discharging and energy delivery. Table 3 presents the effect of HTF flow rate on the charging time of the energy storage system. Table 3 Effect of HTF flow rate on charging time of the energy storage system. HTF flow rate Charging time l/h s h 300 10150 2.49 200 12050 3.20 100 13450 3.44 5.2 Discharging process of PCM: During the discharging process of the CTES system in a laboratory room (8 ft × 18 ft × 10 ft), the stored cooling energy is released to reduce both air temperature and humidity effectively. Figure 5 shows the position of the CTES system during the discharging process. When warm laboratory air passes through the CTES system, it comes into contact with cold PCM surfaces or cooling coils, resulting in heat transfer from the air to the stored cooling medium. This leads to a significant drop in the outlet air temperature. Figure 6 shows the temperature–time curve during the discharging process at a flow velocity of 2 m/s. The inlet air temperature (𝑇 i ) gradually decreases from approximately 34.5°C to 27°C over 10,000 s, representing a total drop of 7.15°C. This gradual decrease reflects the continuous heat absorption by the air from the PCM, indicating that the PCM releases its stored thermal energy steadily over the discharging period rather than abruptly. Table 4 shows the variation of air temperature and humidity at different discharge velocities. From the given data, at a discharge velocity of 2 m/s, the air temperature reduces from 34.25°C to 27.09°C; at 4 m/s, it reduces from 34.93°C to 25.44°C; and at 6 m/s, it drops from 35.12°C to 24.02°C. This shows that higher discharge velocities enhance the cooling effect due to better heat transfer. In addition to cooling, the CTES system also reduces air humidity by removing excess moisture. As the air cools, its ability to hold moisture decreases, causing condensation on the cold PCM or coil surfaces. The condensed water is removed, resulting in lower relative humidity levels. At 2 m/s, humidity drops from 54–48%; at 4 m/s, from 55–47%; and at 6 m/s, from 55–48%. This dual effect makes the outlet air both cooler and drier, thereby improving indoor comfort. Among the tested velocities, 4 m/s provides optimal cooling and maximum humidity reduction. Hence, the CTES system effectively ensures simultaneous temperature reduction and dehumidification during the discharging phase Table 4 Variation of air temperature and humidity at different discharge velocities. Discharge Velocity (m/s) Initial Temperature (°C) Final Temperature (°C) Initial Humidity (%) Final Humidity (%) 2 34.25 27.09 54 48 4 34.93 25.44 55 47 6 35.12 24.02 55 48 5.3 Combined energy stored The total (or maximum) cooling energy stored in the storage tank is calculated as the combined energy stored in the phase change materials (PCM) and the heat transfer fluid (HTF) [ 17 ]. $$\:\text{Q}max=\text{Q}HTF+\text{Q}PCM$$ 3 $$\:\text{Q}HTF={\text{m}}_{l}{\text{c}}_{l}\left(\text{T}i-\text{T}l\right)$$ 4 $$\:\text{Q}PCM={\text{m}}_{PCM}\left[{\text{c}}_{pl}\left(\text{T}i-0\right)+{LH}_{l}+{\text{c}}_{ps}\left(0-\text{T}f\right)\right]$$ 5 Here, m PCM ​ and mf​ represent the total masses of the PCM and the heat transfer fluid (HTF), respectively. C l ​ denotes the specific heat of the HTF, while C pl ​ and c ps ​ are the specific heats of the PCM in its liquid and solid states. LH l​ corresponds to the latent heat of fusion (freezing) of the PCM. The liquid volume in the cylindrical tank was calculated by accounting for the displacement caused by 200 spherical capsules and the copper coil inside the tank. The inner dimensions of the tank 400 mm diameter and 950 mm height yielded a total inner volume of approximately 119.4 l. Each spherical capsule, with a 72 mm inner diameter, displaced about 0.195 l, resulting in a combined displacement of roughly 39 l for all capsules. The copper tube, with an outer diameter of 13 mm, inner diameter of 11 mm, and a length of 9.5 m, displaced an additional 0.36 l. Subtracting these displacements from the tank’s total inner volume, the net liquid volume was determined to be approximately 80 l. These calculations provide a precise estimation for the liquid filling required in the tank considering the internal components. The energy storage capacity of the CTES system was determined by considering both the HTF liquid and the PCM encapsulated in spherical capsules. The HTF, a mixture of deionized water and ethylene glycol in a 50:50 ratio, had a measured volume of 80 l, a density of 1053.5 kg/m³, and a specific heat of 4.18 kJ/kg·K, storing 3517 kJ of thermal energy over a measured temperature difference of 10 K. The PCM consisted of deionized water contained in 200 spherical capsules, each with a 72 mm inner diameter and filled to 90% of their volume, giving a total PCM volume of 35.2 l, with a density of 1000 kg/m³ and a latent heat of 200 kJ/kg, storing 6340 kJ through phase change. The combined energy storage of the HTF and PCM in the system was therefore 9857 kJ, with 35.7% contributed by sensible heat from the HTF and 64.3% by latent heat from the PCM. These results highlight that the PCM dominates the energy storage despite occupying less volume. The findings demonstrate the effectiveness of the hybrid storage approach, efficiently leveraging both sensible and latent heat to maximize thermal energy storage. 5.4 Pressure loss in a CTES tank Figure 7 the variation of pressure loss across the storage tank, measured using a differential pressure transducer (DP-Style 266DSH) operating within a voltage range of 10.5 to 42 VDC and a measurement range of − 600 to + 600 kN/m². The experiments were conducted by gradually increasing the HTF flow rate from 100 L/h to 700 L/h in steps of 100 L/h. It is observed that the pressure loss across the storage tank increases non-linearly with the rise in HTF flow rate. At lower flow rates (100–300 L/h), the increase in pressure loss is relatively gradual, ranging from 0.53 kN/m² at 100 L/h to 0.78 kN/m² at 300 L/h. However, at higher flow rates beyond 400 L/h, the pressure loss rises more sharply, reaching up to 1.52 kN/m² at 700 L/h. This trend is primarily attributed to the increase in fluid velocity, which enhances frictional resistance within the storage tank and connecting pipelines. Additionally, as the HTF temperature decreases, its viscosity increases, leading to a higher hydraulic resistance and thereby contributing to the elevated pressure losses. To minimize the pressure drop and maintain efficient system operation, it is recommended to limit the HTF flow rate to approximately 300 L/h. Similar findings have been reported by several researchers [ 32 – 34 ], indicating that optimizing flow rate and HTF properties plays a crucial role in reducing pumping power requirements. 5.5 Energy saving potential The energy-saving potential of a packed-bed system is influenced by HTF flow rate, solidification time, energy input, and pressure drop. Experimental results show that as the HTF flow rate decreases from 100 l/h to 300 l/h, the energy input to the cooling unit increases from 28.23 kWh to 10.25 kWh, indicating higher energy consumption at lower flow rates. Meanwhile, the pressure drop reduces from 0.78 kN/m² to 0.53 kN/m², reflecting lower hydraulic resistance and improved pumping efficiency. Solidification time decreases slightly from 206.4 min at 100 l/h to 149.4 min at 300 l/h, showing more effective heat extraction at higher flow rates. These trends highlight a trade-off between energy input, system resistance, and storage duration. Lower flow rates improve thermal contact between the HTF and storage medium, enhancing heat transfer efficiency, but may extend response times for peak cooling demands. Recent literature confirms that optimized flow rates in packed bed CTES systems reduce pressure drops and improve energy efficiency [ 35 – 37 ]. High flow rates allow rapid energy extraction but increase pressure losses, whereas very low flow rates enhance thermal performance with slower system response. The findings suggest that a moderate flow rate, such as 200 l/h, balances energy input, solidification time, and pressure drop effectively. Optimizing HTF flow rates is critical to maximize energy savings while maintaining desired cooling performance. Pressure drop reduction lowers pumping energy, and effective solidification ensures sufficient stored cooling capacity [ 38 ]. These results align with studies emphasizing the importance of flow optimization for sustainable thermal energy storage. Overall, careful selection of HTF flow rate can enhance the operational efficiency of packed bed CTES tanks and support energy-efficient cooling applications Table 6 Effect of HTF flow rate on solidification time, energy input, and pressure drop in packed bed CTES tank. HTF flow rate (l/h) Solidification/charging process Energy input to chiller (kWh) Pressure loss (kN/m 2 ) HTF supply temperature (ᵒC) Solidification time (min) 300 -8 149.4 28.23 0.78 200 -8 192.0 19.61 0.65 100 -8 206.4 10.25 0.53 The packed bed CTES system offers significant energy-saving potential for cooling a room of dimensions 8 ft × 18 ft × 10 ft (≈ 40.9 m³). During the discharging process, warm air passing through the CTES comes into contact with cold PCM surfaces or coils, resulting in a substantial temperature drop and simultaneous humidity reduction. Experimental data show that at a discharge velocity of 4 m/s, the air temperature decreases from 34.93°C to 25.44°C, while relative humidity drops from 55–47%, indicating effective sensible and latent cooling. Compared to a conventional air-conditioning system, which requires continuous electrical input to achieve a similar temperature reduction, the CTES system stores cooling energy during off-peak hours and releases it during peak demand, thereby reducing electricity consumption. Calculations based on room volume and air properties show that the energy required to cool the room air directly is approximately 0.137 kWh, whereas the CTES system delivers the same cooling effect using pre-stored thermal energy, with the main electrical input limited to circulating air and charging the storage medium. This strategy not only lowers peak-time electricity usage but also improves indoor comfort by simultaneously controlling temperature and humidity. Optimizing discharge velocity, as evidenced by the 4 m/s case, enhances heat transfer and moisture removal, maximizing the system’s efficiency. Consequently, the CTES packed bed system can achieve substantial energy savings compared to conventional AC systems while maintaining effective thermal comfort, demonstrating its practicality for energy-efficient building cooling applications. Figure 8 compares the charging times of the CTES system from the present study with values reported in the literature. The results show that the charging time in the present study, under varying HTF flow rates of 100, 200, and 300 l/h at -8°C, ranges from 149.4 to 206.4 minutes. These times are significantly lower than those reported in the literature, where charging times for similar systems with HTF flow rates of 500 mL/min to 2000 mL/min and temperatures from − 5.5°C to -4°C range from 309 to 480 minutes [ 3 , 17 , 18 ]. This indicates that the present system achieves faster charging, demonstrating improved thermal performance compared to existing studies 6. Conclusion The present research investigates a packed bed cool thermal energy storage (CTES) system using PCM-filled spherical capsules and HTF for efficient thermal energy storage and delivery. The study evaluates the charging and discharging behavior, including temperature and humidity reduction in a laboratory room. Effects of HTF flow rate and discharge velocity on energy storage, heat transfer, and system performance are analyzed. The findings demonstrate the system’s potential for energy-efficient cooling and peak-load electricity savings. A moderate HTF flow rate of 200 l/h is found to be the most suitable, offering an ideal balance between heat transfer efficiency and charging time (~3.20 h) compared to the faster charging at 300 l/h (~2.49 h) and the slower solidification at 100 l/h (~3.44 h). It is observed that a moderate HTF flow rate of 200 l/h is optimal, ensuring efficient heat transfer with balanced charging time (~192 min) and moderate pressure loss (0.65 kN/m²). It offers better energy efficiency compared to higher flow rates with excessive losses and lower flow rates with longer solidification times. The PCM stores 6340 kJ of latent heat compared to 3517 kJ of sensible heat in HTF, accounting for over 60% of total 9857 kJ stored energy. The outlet air temperature drops from 34.93 °C to 25.44 °C and humidity from 55% to 47% at 4 m/s, indicating effective simultaneous temperature and moisture control, making the system suitable for laboratory cooling applications The CTES system delivers comparable cooling to conventional AC (~0.137 kWh per cycle) using pre-stored energy, reducing peak electricity consumption. The developed packed-bed CTES system offers an energy-efficient cooling solution by utilizing PCM-filled capsules to store and deliver cold energy effectively. It is observed that a moderate HTF flow rate (200 l/h) balances energy input, solidification time (~3.20 h), and pressure drop (0.65 kN/m²), while an optimal discharge velocity (4 m/s) maximizes cooling and humidity reduction, demonstrating practical and efficient operation. The system achieves significant temperature and humidity control, ensuring improved thermal comfort while reducing reliance on conventional AC systems. By optimizing both HTF flow rates and discharge velocities, it effectively balances charging efficiency, cooling performance, and hydraulic losses, highlighting its potential for residential, commercial, and laboratory applications, enabling peak-load reduction and supporting sustainable energy management. 6.1 Future Scope and Limitation This research demonstrates the potential of a packed-bed CTES system with encapsulated PCM for energy-efficient room cooling and humidity control, highlighting opportunities to optimize HTF flow rates, air discharge velocities, and PCM selection for enhanced thermal storage and faster system response. Future work could explore advanced heat transfer enhancements, integration with renewable energy sources, real-world multi-room applications, and long-term performance evaluation to improve scalability and practical implementation. A key limitation of the current study is its laboratory-scale setup, which may not fully represent performance variations in larger or commercial building environments. Abbreviations AC Air-conditioning CTES Cool Thermal Energy Storage DIW Deionized Water GNP Graphene Nanoplatelets HTF Heat Transfer Fluid LDPE Low Density Polyethylene LH Latent Heat MWCNT Multiwall Carbon Nanotubes NPCM Nano Phase Change Material PLTES Packed-bed Latent Thermal Energy Storage System PCM Phase Change Materials PTDC Proportionate Temperature Differential Controller RTD Resistance Temperature Detector SEM Scanning Electron Microscope SH Sensible Heat TC Thermal Conductivity TEM Transmission Electron Microscope Declarations Ethics approval and consent to participate Not applicable Consent for publication Not applicable Availability of data and material The authors declare that all the data supporting the findings of this study are available within the article. Competing interests The authors declare that they have no conflicts of interest in publishing this article. Funding This research received financial support from SRM Institute of Science and Technology through seed grand (SERI 2023), SRMIST/R/AR(A)/SERI2023/174/07. Authors' contributions A.S. Concepts, Investigation and Writing – Original draft and fund acquisition, P.S. Writing – Review & Editing, M. C. Writing – Review & Editing, R.P. Writing – Review & Editing, S.C.K. Writing – Review & Editing. Acknowledgements The authors wish to thank the Department of Mechanical Engineering, SRM Institute of Science and Technology, Kattankulathur for providing the facilities to carry out the research work. References E. World, I.E.A. Outlook, (2023). https://www.iea.org/reports/world-energy-outlook-2023 M.H. Zahir, S.A. Mohamed, R. Saidur, F.A. Al-Sulaiman, Appl. Energy. 240 , 793 (2019) K. Panchabikesan, A.A.R. Vincent, Y. Ding, V. Ramalingam, Energy. 144 , 443 (2018) S. Bourne, A. Novoselac, Build. Simul. 8 , 673 (2015) E. Günther, S. Hiebler, H. Mehling, R. Redlich, Int. J. Thermophys. 30 , 1257 (2009) A.O. Borode, N.A. Ahmed, P.A. Olubambi, M. Sharifpur, J.P. Meyer, Int. J. Thermophys. 42 , (2021) M.S. Swapna, S. Sankararaman, Int. J. Thermophys. 41 , (2020) E. Baccega, L. Vallese, M. Bottarelli, Int. J. Thermophys. 46 , (2025) H. Salhab, M. Zanjani, S. Nardini, A. Lagazzo, S. Rocha, Ferreira, A. Caggiano, Int. J. Thermophys. 46 , (2025) E. Baccega, Int. J. Thermophys. 45 , (2024) M. Ismail, H. Hassan, Int. J. Thermophys. 45 , (2024) P. Chandrasekaran, M. Cheralathan, R. Velraj, Energy. 90 , 807 (2015) A. Gallego, K. Cacua, B. Herrera, D. Cabaleiro, M.M. Piñeiro, L. Lugo, Adv. Powder Technol. 31 , 560 (2020) A.A. Altohamy, M.F. Abd Rabbo, R.Y. Sakr, A.A.A. Attia, Appl. Therm. Eng. 84 , 331 (2015) M.P. Vikram, V. Kumaresan, S. Christopher, R. Velraj, Int. J. Refrig. 100 , 454 (2019) R. Prabakaran, J. Prasanna Naveen Kumar, D. Mohan Lal, C. Selvam, S. Harish, J. Therm. Anal. Calorim. 139 , 941 (2020) R. Al-Shannaq, B. Young, M. Farid, Energy. 171 , 296 (2019) X. Dong, G. Gao, X. Zhao, Z. Qiu, C. Li, J. Zhang, P. Zheng, J. Energy Storage 50 , (2022) K.-W. Zhang, G. Karlstrgmb, B. Lindman, Phase Behaviour of Systems of a Non-Ionic Surfactant and a Non-Ionic Polymer in Aqueous Solution (n.d.) E. Oró, A. de Gracia, A. Castell, M.M. Farid, L.F. Cabeza, Appl. Energy. 99 , 513 (2012) J.S. Baruah, V. Athawale, P. Rath, A. Bhattacharya, Int. J. Heat. Mass. Transf. 182 , (2022) Z. Khan, Z.A. Khan, Energy Convers. Manag. 154 , 157 (2017) A. Panesi, Australian J. Mech. Eng. 14 , 64 (2016) X. Huang, J. Zhang, F. Haglind, Int. Commun. Heat Mass Transfer 135 , (2022) L. Guo, W. Ji, Z. Gao, X. Fan, J. Wang, J. Energy Storage 40 , (2021) B. Chi, Y. Lu, H. Zuo, K. Zeng, H. Xu, J. Gao, Z. Fang, H. Yang, H. Chen, J. Energy Storage 77 , (2024) P.N.S. Teja, S.K. Gugulothu, P.D.S. Reddy, P. Barmavatu, J. Energy Storage 78 , (2024) E.H. Sebbar, Y. Chaibi, N.E.E.K. Elyamani, B. Lamrani, T. El, Rhafiki, T. Kousksou, J. Energy Storage 92 , (2024) X. Wu, Y. Wang, R. Sun, M. Lai, R. Du, Z. Zhang, J Phys Conf Ser (Institute of Physics Publishing, 2009) Y.L. Shao, K.Y. Soh, M.R. Islam, K.J. Chua, Energy 268 , (2023) A. Sathishkumar, M. Cheralathan, Energy 263 , (2023) A. Muraleedharan Nair, C. Wilson, B. Kamkari, J. Locke, M. Jun Huang, P. Griffiths, N.J. Hewitt, Energy Convers. Management: X 23 , (2024) K. Ghasemi, S. Tasnim, S. Mahmud, Sustain. Energy Technol. Assess. 52 , (2022) M.M. Kenisarin, K. Mahkamov, S.C. Costa, I. Makhkamova, J. Energy Storage 27 , (2020) A. Gil, C. Barreneche, P. Moreno, C. Solé, A. Inés, Fernández, L.F. Cabeza, Appl. Energy. 111 , 1107 (2013) U. Berardi, S. Soudian, Energy Build. 185 , 180 (2019) A. Sari, Energy Convers. Manag. 44 , 2277 (2003) R.M. Saeed, J.P. Schlegel, R. Sawafta, Energy 189 , (2019) Table Table 1 is available in the Supplementary Files section. Additional Declarations Competing interest reported. This research received financial support from SRM Institute of Science and Technology through seed grand (SERI 2023), SRMIST/R/AR(A)/SERI2023/174/07. Supplementary Files Table1.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7527832","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":512352036,"identity":"2262e8dc-f4f0-469f-a85e-9fd99f55dbc6","order_by":0,"name":"A. Sathishkumar","email":"","orcid":"","institution":"SRM Institute of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"A.","middleName":"","lastName":"Sathishkumar","suffix":""},{"id":512352037,"identity":"4c1163de-6c22-4357-a78c-23e5690c4be1","order_by":1,"name":"P. 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(a) 100 l/h, (b) 200 l/h, and (c) 300 l/h.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7527832/v1/89e7c8a511f4b162d645b0ca.png"},{"id":91092620,"identity":"632217d4-3451-41f0-9da6-abc90b1ba0f7","added_by":"auto","created_at":"2025-09-11 13:35:09","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":215636,"visible":true,"origin":"","legend":"\u003cp\u003ePosition of CTES system during discharging process.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7527832/v1/33db2d45480d75fc9ff00a87.png"},{"id":91092616,"identity":"5044793a-8d2e-41b1-91fb-c343a8127f4b","added_by":"auto","created_at":"2025-09-11 13:35:09","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":412242,"visible":true,"origin":"","legend":"\u003cp\u003eTemperature-time curve during discharging process at 2m/s.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-7527832/v1/78ef90dbe410e3d6db290f09.png"},{"id":91093792,"identity":"53ba97aa-e75c-466d-9379-19b0b6596758","added_by":"auto","created_at":"2025-09-11 13:43:09","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":305897,"visible":true,"origin":"","legend":"\u003cp\u003ePressure loss in the packed-bed system for different flow conditions.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-7527832/v1/3e34d7c1220c3f92152f26f0.png"},{"id":91094363,"identity":"dbe43b91-e985-4189-b897-ae76c1955bcf","added_by":"auto","created_at":"2025-09-11 13:51:09","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":503349,"visible":true,"origin":"","legend":"\u003cp\u003eValidation of present findings with existing literature.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-7527832/v1/f1ee3a627db3f1db48a85f1e.png"},{"id":94224226,"identity":"fbcd6113-0c18-4796-91dc-32d2650ac67d","added_by":"auto","created_at":"2025-10-23 19:16:44","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4696586,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7527832/v1/04a719f3-0b91-4609-a8f4-cc3d6188eb2a.pdf"},{"id":91092609,"identity":"31dd0401-2287-4882-b75a-d3a3717f5e1e","added_by":"auto","created_at":"2025-09-11 13:35:08","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":933593,"visible":true,"origin":"","legend":"","description":"","filename":"Table1.docx","url":"https://assets-eu.researchsquare.com/files/rs-7527832/v1/c51bf75cf63780cb471a691d.docx"}],"financialInterests":"Competing interest reported. This research received financial support from SRM Institute of Science and Technology through seed grand (SERI 2023), SRMIST/R/AR(A)/SERI2023/174/07.","formattedTitle":"Thermal and heat transfer characteristics of a packed-bed CTES system using encapsulated PCM capsules for peak-load management","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThermal Energy Storage (TES) systems are increasingly important for addressing India's growing cooling energy demand, which is projected to double by 2027, according to the Ministry of Power [1]. With buildings accounting for 57% of this demand, TES systems offer a sustainable solution to reduce energy consumption in cooling applications [2]. By storing excess thermal energy during off-peak hours and releasing it during peak periods, TES can help avoid the need for an estimated 25 GW of new coal-based power generation. Moreover, as energy demand from fans and air coolers surpasses that from air conditioners, TES systems can enhance energy efficiency across various cooling technologies, contributing to significant energy savings and improved grid stability [3]. Cool thermal energy storage (CTES) systems have emerged as an effective technology for managing energy consumption in large buildings, where air conditioning typically accounts for about 40% of total electrical usage. By integrating a CTES with air conditioning systems, the cooling load can be shifted from peak to off-peak hours, reducing both energy costs and peak demand without the need to design systems solely to accommodate maximum load requirements [4]. This integration is also critical for addressing issues related to dynamic electricity tariffs and grid stability, particularly in regions with high energy demand fluctuations [5, 6]. CTES systems have also proven valuable in smaller-scale applications such as process cooling, milk preservation, vaccine transportation, and microelectronic cooling, where precise temperature control is crucial. Recent advancements in this field have focused extensively on improving the thermophysical properties of phase change materials (PCMs), which are integral to the functioning of CTES [7–9]. Deionized (DI) water, due to its high latent heat of fusion and low cost, remains a widely used PCM. However, its thermophysical properties, including thermal conductivity, can be significantly enhanced by adding nanoscale conductive solid particles such as metal oxides or carbon-based materials. These nanoparticles improve heat transfer rates, although challenges like nanoparticle sedimentation and supercooling still persist. Surfactants have traditionally been used to ensure a stable dispersion of nanoparticles within PCMs, but they often act as thermal barriers, limiting the expected enhancement in thermal properties [10, 11]. To address these issues, several researchers have explored alternative approaches, such as chemical functionalization of nanoparticles, which can enhance their stability and prevent agglomeration without altering the PCM's key properties such as specific heat or viscosity. Chandrasekaran et al. [12] examined the geometric and material parameters affecting heat transfer in PCMs and concluded that the use of additives like nanomaterials significantly improved the performance of water-based CTES systems. Similarly, Gallego et al. [13] explored the thermophysical properties of composite PCMs like CaCl₂ 6H₂O by introducing nucleating agents and thickeners, which improved the material's thermal stability and reduced supercooling to 0.95°C, though this came at the cost of reduced storage capacity. Also, Altohamy et al. [14] demonstrated that dispersing aluminum oxide nanoparticles in DI water reduced solidification times by up to 30%, owing to the enhanced thermal conductivity of the resulting nanofluid. However, the use of surfactants or thickeners has been found to create issues by reducing the overall thermal performance, as reported by Vikram et al. [15]. To counteract these limitations, the same research group later explored the chemical functionalization of nanomaterials, such as graphene nanoplatelets (GNPs), using covalent methods like nitric acid treatment to achieve stable suspensions without the need for surfactants. This approach was shown to improve long-term stability and thermal conductivity, as evidenced by Prabakaran et al. [16], who reported a 26% reduction in discharging time when using functionalized GNPs.\u003c/p\u003e\n\u003cp\u003eCTES systems have been extensively studied in recent years, with packed-bed PCM storage systems gaining attention for their high energy storage density and large heat transfer area compared to conventional ice storage banks. These systems, particularly in large-scale applications, often employ paraffin or DI water as the primary PCM. Research by Al-Shannaq et al. [17] explored the energy storage capabilities of packed beds filled with graphite-enriched spheres, revealing that reducing the heat transfer fluid (HTF) inlet temperature significantly decreased charging times. Dong et al. [18] investigated the performance of cold thermal storage tanks under different heat transfer fluid flow conditions and charging with different diameter balls to identify the more efficient one. The investigation revealed that increasing the water flow rate from 1.296 m/h to 2.196 m/h enhanced the freezing rate of the PCM balls: from 41–91% for D = 90 mm, from 74–100% for D = 90 \u0026amp; 60 mm, and from 94–100% for D = 60 mm after 8 hours. At a constant chilled water velocity, smaller-diameter PCM balls exhibited higher freezing rates. Lei Zhang et al. [19] investigated the flow field and heat transfer in an energy storage tank. Applying unique geometric configurations alongside realistic boundary conditions helps predict how nano-phase change materials behave during their melting and freezing cycles, specifically in the charging and discharging processes. They found that phase change material (PCM) behaviour in energy storage tanks with porous media, emphasizing the impact of porosity coefficients. For a coefficient of 0.95, full melting and freezing of pure PCM take 2000 s and 1250 s, respectively. Adding aluminum oxide nanoparticles accelerates melting (1700 s) and freezing (1300 s). Lowering porosity from 0.97 to 0.95 enhances heat transfer, reducing melting time and improving efficiency.\u003c/p\u003e\n\u003cp\u003eOro et al. [20] focused on district cooling networks combined with TES, showing how HTF temperature variations could affect the total solidification time of PCM-filled storage tanks. Other studies, such as those by Baruah et al. [21], reported on the performance of chiller systems incorporating energy storage tanks containing PCM balls, with findings indicating a 3–4% increase in specific energy consumption for each 1°C decrease in evaporator temperature. Khan et al. [22] further demonstrated the advantages of using paraffin in shell-and-tube heat exchangers with fins for low-temperature thermal energy storage. The use of plastic spheres filled with PCM in packed-bed systems offers several benefits, including ease of installation and flexibility in material selection for storage tanks. This approach has been widely studied in both experimental and modeling contexts. For instance, Panesi et al. [23] evaluated the crystallization rates of DI water-based PCMs in spherical capsules using different HTFs, finding that ethanol as the HTF resulted in faster charging compared to ethylene glycol/DI water mixtures at the same flow rates and input temperatures. Table 1 summarizes recent findings related to packed-bed thermal energy storage systems.\u003c/p\u003e\n\u003cp\u003eThe growing demand for energy-efficient cooling systems and the increasing frequency of electricity outages necessitate the development of advanced CTES technologies capable of providing reliable and sustainable cooling solutions. Conventional CTES systems face significant challenges, including low charging efficiency, inadequate cold energy retrieval, and limited capability to maintain indoor thermal comfort under varying operational conditions. These limitations arise due to suboptimal heat transfer processes, insufficient utilization of encapsulated phase change materials (PCMs), and the absence of efficient air-based discharging mechanisms. To overcome these drawbacks, the present research focuses on designing and analysing a packed-bed CTES system employing encapsulated distilled water-filled spherical balls combined with a helical coil for air-driven cold energy retrieval.\u003c/p\u003e\n\u003cp\u003eThe primary objectives of the present work are: (a) To design and analyze a packed-bed cool thermal energy storage (CTES) system using encapsulated distilled water-filled spherical balls for efficient cold energy storage and retrieval. (b) To investigate the effect of varying HTF flow rates (100, 200, and 300 l/h) during the charging process on the freezing behaviour, energy storage capacity, and overall thermal performance of the system. (c) To evaluate the discharging performance of the CTES system by supplying air through a helical coil at different velocities (2, 4, and 6 m/s) and assessing its impact on room temperature, humidity, and cooling duration under simulated electricity outage conditions. The proposed research introduces a packed bed CTES system using encapsulated distilled water-filled spherical balls combined with a helical coil for air-based cold energy retrieval, enabling continuous cooling even during power outages. Unlike conventional CTES systems, it employs variable air velocities for discharging and optimizes HTF flow rates during charging to maximize cold energy storage efficiency. Additionally, the study uniquely evaluates room temperature and humidity variations to demonstrate the system’s capability to maintain thermal comfort under intermittent power supply conditions.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cp\u003eThe experimental setup of the cool thermal energy storage (CTES) system employs deionized (DI) water, a glycol-water mixture (40:60), and air as working media due to their distinct thermal characteristics and functional roles. DI water is selected as the primary phase change material (PCM) because of its high latent heat of fusion (~\u0026thinsp;334 J/g), specific heat capacity (4.18 J/g\u0026deg;C), and suitable freezing point (0\u0026deg;C), enabling efficient cold energy storage and release. Air is utilized as the discharging medium, where forced convection enhances cooling delivery despite its low specific heat (~\u0026thinsp;1.005 J/g\u0026deg;C) and low thermal conductivity (~\u0026thinsp;0.025 W/m K). The combination of these materials ensures efficient charging, reliable cold storage, and effective cooling performance within the CTES system.\u003c/p\u003e"},{"header":"3. Design of experiment","content":"\u003cp\u003eThe design of the CTES system involves a cylindrical storage tank integrated with spherical PCM capsules, copper tubes, an air circulation unit, and an insulation layer to ensure efficient thermal energy management. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows the design specifications of the packed-bed system used in the present study. The tank has an outer diameter of 500 mm, inner diameter of 400 mm, and a height of 950 mm, with a wall thickness of 50 mm. A total of 200 spherical capsules made of suitable material, each with an outer diameter of 76 mm and an inner diameter of 72 mm, are filled with 92% deionized (DI) water volume (~\u0026thinsp;179.9 ml per capsule) to accommodate thermal expansion during solidification, resulting in a total PCM volume of approximately 45.96 l. Copper tubes with an outer diameter of 13 mm, inner diameter of 11 mm, and a total length of 9500 mm are used to circulate the heat transfer fluid (HTF), enabling uniform charging and discharging. A copper coil of 20 mm diameter forms the air circulation path, leveraging copper\u0026rsquo;s high thermal conductivity for efficient heat exchange. The storage tank is insulated using a POLYOL and EMPEYOL mixture (0.5:0.5 ratio), achieving a very low thermal conductivity of 0.014 W/m K to minimize thermal losses. During the solidification process, the HTF is cooled via an integrated charging unit comprising an evaporator coil, condenser, and compressor, and is circulated from the base of the tank to the upper part for uniform cooling of the spherical capsules. A stirrer is installed to maintain uniform HTF temperature, while RTDs and digital sensors continuously monitor temperature variations at multiple heights. If the HTF temperature drops below the set point, a heater activates automatically to maintain stability. During the discharging process, a variable-speed blower forces air through the air circulation coil, where it passes over the charged capsules, absorbs stored cold energy, and exits through the outlet to deliver cooling. The entire system is monitored using a data logger connected to a computer interface, ensuring real-time data acquisition and performance analysis of the CTES system. Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e2\u003c/span\u003e presents the detailed specifications of the energy storage tank.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eDetailed specification of energy storage tank\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"2\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSpecifications\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUnits in mm\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eOuter diameter of the tank\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e500\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eInner diameter of the tank\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e400\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eHeight of the tank\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e950\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eWall thickness of the tank\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e50\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eDistance between air in and air out\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e824\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eOuter diameter of the spherical capsule\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e76\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eInner diameter of the spherical capsule\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e72\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eWall thickness of the spherical capsule\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eOuter diameter of the copper tube\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e13\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eInner diameter of the copper tube\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e11\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eThickness of the copper tube\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLength of the copper tube\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e9500\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"4. Experimentation","content":"\u003cp\u003eThe experimental procedure and equipment used for the charging and discharging studies are described in this section. To overcome the limitations of conventional constant-temperature bath experiments, the charging and discharging behavior of the phase change material (PCM) is investigated using a low-capacity CTES system (\u0026lt;\u0026thinsp;0.5 TR\u0026middot;hr) under various heat transfer fluid (HTF) flow conditions. This approach provides a more practical representation of heat transfer performance in real-world thermal energy storage applications. Controlled discharging rates are also examined based on varying demand conditions, which cannot be effectively simulated using a constant-temperature bath setup. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e illustrates a schematic of the thermal energy storage tank, showing both the charging and discharging arrangements. The PCM is encapsulated within spherical shells to enhance heat transfer efficiency by maximizing the surface area-to-volume ratio, resulting in faster thermal exchange, improved storage capacity, and easier integration into thermal management systems. To monitor the internal temperature distribution during thermal cycling, resistance temperature detectors (RTDs) are carefully inserted through the neck of each spherical capsule, which is tightly sealed to prevent leakage. Markings made with black tape on the RTD wires ensure accurate sensor placement at predefined depths within the PCM-filled sphere. Three RTDs are strategically positioned at different depths to capture detailed temperature profiles: RTD 1 is placed at the center of the capsule (100%), RTD 2 is located 22.0 mm from the center (approximately 75% depth), and RTD 3 is positioned 27.8 mm from the center (approximately 50% depth). These RTDs are connected to a data logger for real-time temperature acquisition and monitoring during the charging and discharging cycles. This experimental arrangement enables precise tracking of thermal behavior, providing deeper insights into the phase change dynamics and heat transfer characteristics within the encapsulated PCM under practical operating conditions. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows a pictorial view of the experimental setup. Also, Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e2\u003c/span\u003e lists the standards employed in the present experimental work.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eA \u0026ndash; Proportional temperature controller (PDTC), B \u0026ndash; Agitator, C \u0026ndash; Heating element, D \u0026ndash; Thermal sensor, E \u0026ndash; Colling coil, F \u0026ndash; Condensation unit, G \u0026ndash; Compressor, H \u0026ndash; Data acquisition unit, I \u0026ndash; Computer interface, J \u0026ndash; Circulation pump, K \u0026ndash; Control valve, L \u0026ndash; Pressure indicator, M \u0026ndash; Storage module, N \u0026ndash; Resistance temperature detectors (RTDs), and O \u0026ndash; Power consumption meter.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eStandards employed in the present experimental work.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"4\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eInstrument\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eStandards\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eMeasuring Sensors / Parameters\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eAccuracy\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eRTD Sensor, Conax Technologies, Chennai\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eISO Standards\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eRTD Sensor; Class B Type\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.10\u0026deg;C\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eClass B Volumetric Flask, SRL, Chennai\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eISO 4787:2010\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eVolume \u0026ndash; ml\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.015 mL\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eFlow Meter, Conax Technologies, Chennai\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eISO Standards\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eWater Mass Flow Rate \u0026ndash; l/h\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;2%\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSemi-Micro Balance\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eASTM D-792\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eMass \u0026ndash; g\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.02%\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePressure Transducer (DP-Style 266DSH), ABB, India\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eISO Standards\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003ePressure Drop \u0026ndash; kN/m\u0026sup2;\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.075%\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eEnergy Meter (DLMS Meter 10-60A), Tech Baniya, India\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eIS 13779:1999\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eEnergy Input \u0026ndash; kWh\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;0.2%\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eHumidity Meter\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eISO Standards\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eRelative Humidity (%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u0026plusmn;\u0026thinsp;2% RH\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e4.1 Assessment of the overall heat loss coefficient\u003c/h2\u003e\u003cp\u003eThe overall heat loss coefficient (U) is evaluated using a heat gain experiment. The temperature of the heat transfer fluid (HTF) inside the storage tank, initially maintained at -8\u0026deg;C, and the ambient temperature are continuously monitored. After 24 hours, the final HTF temperature is recorded to calculate the total heat loss coefficient (based on Eq.\u0026nbsp;\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e \u0026amp;\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e].\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:{U}_{CTES\\:}\\left(LMTD\\right)=\\:{m}_{l}{c}_{l}\\left(\\frac{dT}{dt}\\right)$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:{LMTD}_{CTES}=\\frac{\\left({T}_{i}-{T}_{atm}\\right)-({T}_{l}-{T}_{atm})}{\\text{ln}\\frac{\\left({T}_{i}-{T}_{atm}\\right)}{({T}_{l}-{T}_{atm})}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003ewhere T\u003csub\u003e\u003cem\u003el\u003c/em\u003e\u003c/sub\u003e and T\u003csub\u003e\u003cem\u003ei\u003c/em\u003e\u003c/sub\u003e are the average final and initial temperature of the HTF, T\u003csub\u003e\u003cem\u003eatm\u003c/em\u003e\u003c/sub\u003e is the atmospheric temperature. From the above experiment, the overall heat loss coefficient (U) was determined to be 0.314 W/m\u0026sup2;\u0026middot;K, which represents the rate of heat transfer per unit area per degree of temperature difference between the storage tank and its surroundings. A lower U-value indicates better insulation and reduced heat losses, while a higher value would signify greater thermal losses. In this case, the relatively low value of 0.314 W/m\u0026sup2;\u0026middot;K demonstrates that the storage tank is well-insulated\u003c/p\u003e\u003c/div\u003e"},{"header":"5. Result and Discussion","content":"\u003cp\u003eThe results and discussion section covers the charging process of the energy storage tank, the discharging behaviour of the PCM, the total energy stored, the pressure drop within the storage tank, and the overall energy-saving potential of the system.\u003c/p\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e5.1 Charging of energy storage tank:\u003c/h2\u003e\u003cp\u003eDuring the charging process of the CTES system, the phase change material (PCM) gradually absorbs heat from the circulating heat transfer fluid (HTF). Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a-c) shows the temperature\u0026ndash;time variation of the PCM at different HTF flow rates of 100 L/h, 200 L/h, and 300 L/h. The PCM temperature decreases from an initial value of approximately 24\u0026deg;C toward the freezing point. Heat removal occurs in two stages: first, sensible heat is removed, causing a steady and gradual drop in temperature, followed by latent heat absorption during the phase change, where the temperature remains nearly constant. To monitor the thermal behavior accurately, nine temperature sensors were placed throughout the energy storage tank: bottom sensors (B\u003csub\u003e1\u003c/sub\u003e, B\u003csub\u003e2\u003c/sub\u003e, B\u003csub\u003e3\u003c/sub\u003e), middle sensors (M\u003csub\u003e1\u003c/sub\u003e, M\u003csub\u003e2\u003c/sub\u003e, M\u003csub\u003e3\u003c/sub\u003e), and top sensors (T\u003csub\u003e1\u003c/sub\u003e, T\u003csub\u003e2\u003c/sub\u003e, T\u003csub\u003e3\u003c/sub\u003e). The sensor readings indicate a relatively uniform cooling trend across all regions, confirming consistent heat transfer throughout the PCM mass. The top sensor (T\u003csub\u003e1\u003c/sub\u003e) is critical in determining full charge; when T\u003csub\u003e1\u003c/sub\u003e reaches approximately \u0026minus;\u0026thinsp;1.5\u0026deg;C, the system is considered fully charged. The charging duration depends strongly on the HTF flow rate. At a flow rate of 300 l/h, the PCM reaches full charge in 10,150 seconds (~\u0026thinsp;2.49 h). At 200 l/h, charging takes 12,050 seconds (~\u0026thinsp;3.20 h), and at 100 l/h, it requires 13,450 seconds (~\u0026thinsp;3.44 h). Higher HTF flow rates increase convective heat transfer, accelerating the removal of both sensible and latent heat and reducing total charging time. During the latent heat plateau, the PCM continues to absorb significant energy while the temperature remains nearly constant, indicating efficient energy storage. Bottom sensors typically register slightly lower temperatures initially, while middle and top sensors follow a similar trend as heat propagates through the PCM. The uniform response of all nine sensors ensures that the entire PCM volume is effectively charged. This behavior demonstrates the importance of proper sensor placement to evaluate system performance. Overall, the charging process is characterized by a steady temperature decrease, latent heat plateau, and a clear top-sensor criterion for full charge, ensuring the PCM is fully prepared for subsequent discharging and energy delivery. Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e3\u003c/span\u003e presents the effect of HTF flow rate on the charging time of the energy storage system.\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eTable 3\u003c/strong\u003e Effect of HTF flow rate on charging time of the energy storage system.\u003c/p\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"No\" id=\"Taba\" border=\"1\"\u003e\u003ccolgroup cols=\"3\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eHTF flow rate\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e\u003cp\u003eCharging time\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003el/h\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003es\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eh\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e300\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e10150\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2.49\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e200\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e12050\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e3.20\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e100\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e13450\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e3.44\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e5.2 Discharging process of PCM:\u003c/h2\u003e\u003cp\u003eDuring the discharging process of the CTES system in a laboratory room (8 ft \u0026times; 18 ft \u0026times; 10 ft), the stored cooling energy is released to reduce both air temperature and humidity effectively. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows the position of the CTES system during the discharging process. When warm laboratory air passes through the CTES system, it comes into contact with cold PCM surfaces or cooling coils, resulting in heat transfer from the air to the stored cooling medium. This leads to a significant drop in the outlet air temperature. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows the temperature\u0026ndash;time curve during the discharging process at a flow velocity of 2 m/s. The inlet air temperature (\u0026#119879;\u003csub\u003ei\u003c/sub\u003e) gradually decreases from approximately 34.5\u0026deg;C to 27\u0026deg;C over 10,000 s, representing a total drop of 7.15\u0026deg;C. This gradual decrease reflects the continuous heat absorption by the air from the PCM, indicating that the PCM releases its stored thermal energy steadily over the discharging period rather than abruptly. Table\u0026nbsp;\u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the variation of air temperature and humidity at different discharge velocities. From the given data, at a discharge velocity of 2 m/s, the air temperature reduces from 34.25\u0026deg;C to 27.09\u0026deg;C; at 4 m/s, it reduces from 34.93\u0026deg;C to 25.44\u0026deg;C; and at 6 m/s, it drops from 35.12\u0026deg;C to 24.02\u0026deg;C. This shows that higher discharge velocities enhance the cooling effect due to better heat transfer. In addition to cooling, the CTES system also reduces air humidity by removing excess moisture. As the air cools, its ability to hold moisture decreases, causing condensation on the cold PCM or coil surfaces. The condensed water is removed, resulting in lower relative humidity levels. At 2 m/s, humidity drops from 54\u0026ndash;48%; at 4 m/s, from 55\u0026ndash;47%; and at 6 m/s, from 55\u0026ndash;48%. This dual effect makes the outlet air both cooler and drier, thereby improving indoor comfort. Among the tested velocities, 4 m/s provides optimal cooling and maximum humidity reduction. Hence, the CTES system effectively ensures simultaneous temperature reduction and dehumidification during the discharging phase\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab5\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eVariation of air temperature and humidity at different discharge velocities.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eDischarge Velocity (m/s)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eInitial Temperature (\u0026deg;C)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eFinal Temperature (\u0026deg;C)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eInitial Humidity (%)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eFinal Humidity (%)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e34.25\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e27.09\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e54\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e48\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e34.93\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e25.44\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e55\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e47\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e35.12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e24.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e55\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e48\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e5.3 Combined energy stored\u003c/h2\u003e\u003cp\u003eThe total (or maximum) cooling energy stored in the storage tank is calculated as the combined energy stored in the phase change materials (PCM) and the heat transfer fluid (HTF) [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e].\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:\\text{Q}max=\\text{Q}HTF+\\text{Q}PCM$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$$\\:\\text{Q}HTF={\\text{m}}_{l}{\\text{c}}_{l}\\left(\\text{T}i-\\text{T}l\\right)$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ5\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ5\" name=\"EquationSource\"\u003e\n$$\\:\\text{Q}PCM={\\text{m}}_{PCM}\\left[{\\text{c}}_{pl}\\left(\\text{T}i-0\\right)+{LH}_{l}+{\\text{c}}_{ps}\\left(0-\\text{T}f\\right)\\right]$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e5\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eHere, m\u003csub\u003ePCM\u003c/sub\u003e​ and mf​ represent the total masses of the PCM and the heat transfer fluid (HTF), respectively. C\u003csub\u003e\u003cem\u003el\u003c/em\u003e\u003c/sub\u003e​ denotes the specific heat of the HTF, while C\u003csub\u003e\u003cem\u003epl\u003c/em\u003e\u003c/sub\u003e​ and c\u003csub\u003e\u003cem\u003eps\u003c/em\u003e\u003c/sub\u003e​ are the specific heats of the PCM in its liquid and solid states. LH\u003csub\u003e\u003cem\u003el​\u003c/em\u003e\u003c/sub\u003e corresponds to the latent heat of fusion (freezing) of the PCM. The liquid volume in the cylindrical tank was calculated by accounting for the displacement caused by 200 spherical capsules and the copper coil inside the tank. The inner dimensions of the tank 400 mm diameter and 950 mm height yielded a total inner volume of approximately 119.4 l. Each spherical capsule, with a 72 mm inner diameter, displaced about 0.195 l, resulting in a combined displacement of roughly 39 l for all capsules. The copper tube, with an outer diameter of 13 mm, inner diameter of 11 mm, and a length of 9.5 m, displaced an additional 0.36 l. Subtracting these displacements from the tank\u0026rsquo;s total inner volume, the net liquid volume was determined to be approximately 80 l. These calculations provide a precise estimation for the liquid filling required in the tank considering the internal components.\u003c/p\u003e\u003cp\u003eThe energy storage capacity of the CTES system was determined by considering both the HTF liquid and the PCM encapsulated in spherical capsules. The HTF, a mixture of deionized water and ethylene glycol in a 50:50 ratio, had a measured volume of 80 l, a density of 1053.5 kg/m\u0026sup3;, and a specific heat of 4.18 kJ/kg\u0026middot;K, storing 3517 kJ of thermal energy over a measured temperature difference of 10 K. The PCM consisted of deionized water contained in 200 spherical capsules, each with a 72 mm inner diameter and filled to 90% of their volume, giving a total PCM volume of 35.2 l, with a density of 1000 kg/m\u0026sup3; and a latent heat of 200 kJ/kg, storing 6340 kJ through phase change. The combined energy storage of the HTF and PCM in the system was therefore 9857 kJ, with 35.7% contributed by sensible heat from the HTF and 64.3% by latent heat from the PCM. These results highlight that the PCM dominates the energy storage despite occupying less volume. The findings demonstrate the effectiveness of the hybrid storage approach, efficiently leveraging both sensible and latent heat to maximize thermal energy storage.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e5.4 Pressure loss in a CTES tank\u003c/h2\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e the variation of pressure loss across the storage tank, measured using a differential pressure transducer (DP-Style 266DSH) operating within a voltage range of 10.5 to 42 VDC and a measurement range of \u0026minus;\u0026thinsp;600 to +\u0026thinsp;600 kN/m\u0026sup2;. The experiments were conducted by gradually increasing the HTF flow rate from 100 L/h to 700 L/h in steps of 100 L/h. It is observed that the pressure loss across the storage tank increases non-linearly with the rise in HTF flow rate. At lower flow rates (100\u0026ndash;300 L/h), the increase in pressure loss is relatively gradual, ranging from 0.53 kN/m\u0026sup2; at 100 L/h to 0.78 kN/m\u0026sup2; at 300 L/h. However, at higher flow rates beyond 400 L/h, the pressure loss rises more sharply, reaching up to 1.52 kN/m\u0026sup2; at 700 L/h. This trend is primarily attributed to the increase in fluid velocity, which enhances frictional resistance within the storage tank and connecting pipelines. Additionally, as the HTF temperature decreases, its viscosity increases, leading to a higher hydraulic resistance and thereby contributing to the elevated pressure losses. To minimize the pressure drop and maintain efficient system operation, it is recommended to limit the HTF flow rate to approximately 300 L/h. Similar findings have been reported by several researchers [\u003cspan additionalcitationids=\"CR33\" citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e], indicating that optimizing flow rate and HTF properties plays a crucial role in reducing pumping power requirements.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003e5.5 Energy saving potential\u003c/h2\u003e\u003cp\u003eThe energy-saving potential of a packed-bed system is influenced by HTF flow rate, solidification time, energy input, and pressure drop. Experimental results show that as the HTF flow rate decreases from 100 l/h to 300 l/h, the energy input to the cooling unit increases from 28.23 kWh to 10.25 kWh, indicating higher energy consumption at lower flow rates. Meanwhile, the pressure drop reduces from 0.78 kN/m\u0026sup2; to 0.53 kN/m\u0026sup2;, reflecting lower hydraulic resistance and improved pumping efficiency. Solidification time decreases slightly from 206.4 min at 100 l/h to 149.4 min at 300 l/h, showing more effective heat extraction at higher flow rates. These trends highlight a trade-off between energy input, system resistance, and storage duration. Lower flow rates improve thermal contact between the HTF and storage medium, enhancing heat transfer efficiency, but may extend response times for peak cooling demands. Recent literature confirms that optimized flow rates in packed bed CTES systems reduce pressure drops and improve energy efficiency [\u003cspan additionalcitationids=\"CR36\" citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. High flow rates allow rapid energy extraction but increase pressure losses, whereas very low flow rates enhance thermal performance with slower system response. The findings suggest that a moderate flow rate, such as 200 l/h, balances energy input, solidification time, and pressure drop effectively. Optimizing HTF flow rates is critical to maximize energy savings while maintaining desired cooling performance. Pressure drop reduction lowers pumping energy, and effective solidification ensures sufficient stored cooling capacity [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. These results align with studies emphasizing the importance of flow optimization for sustainable thermal energy storage. Overall, careful selection of HTF flow rate can enhance the operational efficiency of packed bed CTES tanks and support energy-efficient cooling applications\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab6\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 6\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eEffect of HTF flow rate on solidification time, energy input, and pressure drop in packed bed CTES tank.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eHTF flow rate (l/h)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e\u003cp\u003eSolidification/charging process\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eEnergy input to chiller (kWh)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003ePressure loss (kN/m\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eHTF supply temperature (ᵒC)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eSolidification time (min)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e300\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e-8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e149.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e28.23\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e0.78\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e200\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e-8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e192.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e19.61\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e0.65\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e100\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e-8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e206.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e10.25\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e0.53\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThe packed bed CTES system offers significant energy-saving potential for cooling a room of dimensions 8 ft \u0026times; 18 ft \u0026times; 10 ft (\u0026asymp;\u0026thinsp;40.9 m\u0026sup3;). During the discharging process, warm air passing through the CTES comes into contact with cold PCM surfaces or coils, resulting in a substantial temperature drop and simultaneous humidity reduction. Experimental data show that at a discharge velocity of 4 m/s, the air temperature decreases from 34.93\u0026deg;C to 25.44\u0026deg;C, while relative humidity drops from 55\u0026ndash;47%, indicating effective sensible and latent cooling. Compared to a conventional air-conditioning system, which requires continuous electrical input to achieve a similar temperature reduction, the CTES system stores cooling energy during off-peak hours and releases it during peak demand, thereby reducing electricity consumption. Calculations based on room volume and air properties show that the energy required to cool the room air directly is approximately 0.137 kWh, whereas the CTES system delivers the same cooling effect using pre-stored thermal energy, with the main electrical input limited to circulating air and charging the storage medium. This strategy not only lowers peak-time electricity usage but also improves indoor comfort by simultaneously controlling temperature and humidity. Optimizing discharge velocity, as evidenced by the 4 m/s case, enhances heat transfer and moisture removal, maximizing the system\u0026rsquo;s efficiency. Consequently, the CTES packed bed system can achieve substantial energy savings compared to conventional AC systems while maintaining effective thermal comfort, demonstrating its practicality for energy-efficient building cooling applications.\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e compares the charging times of the CTES system from the present study with values reported in the literature. The results show that the charging time in the present study, under varying HTF flow rates of 100, 200, and 300 l/h at -8\u0026deg;C, ranges from 149.4 to 206.4 minutes. These times are significantly lower than those reported in the literature, where charging times for similar systems with HTF flow rates of 500 mL/min to 2000 mL/min and temperatures from \u0026minus;\u0026thinsp;5.5\u0026deg;C to -4\u0026deg;C range from 309 to 480 minutes [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. This indicates that the present system achieves faster charging, demonstrating improved thermal performance compared to existing studies\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"6. Conclusion","content":"\u003cp\u003eThe present research investigates a packed bed cool thermal energy storage (CTES) system using PCM-filled spherical capsules and HTF for efficient thermal energy storage and delivery. The study evaluates the charging and discharging behavior, including temperature and humidity reduction in a laboratory room. Effects of HTF flow rate and discharge velocity on energy storage, heat transfer, and system performance are analyzed. The findings demonstrate the system’s potential for energy-efficient cooling and peak-load electricity savings.\u003c/p\u003e\n\u003cul\u003e\n \u003cli\u003eA moderate HTF flow rate of 200 l/h is found to be the most suitable, offering an ideal balance between heat transfer efficiency and charging time (~3.20 h) compared to the faster charging at 300 l/h (~2.49 h) and the slower solidification at 100 l/h (~3.44 h).\u003c/li\u003e\n \u003cli\u003eIt is observed that a moderate HTF flow rate of 200 l/h is optimal, ensuring efficient heat transfer with balanced charging time (~192 min) and moderate pressure loss (0.65\u0026nbsp;kN/m²). It offers better energy efficiency compared to higher flow rates with excessive losses and lower flow rates with longer solidification times.\u003c/li\u003e\n \u003cli\u003eThe PCM stores 6340 kJ of latent heat compared to 3517 kJ of sensible heat in HTF, accounting for over 60% of total 9857 kJ stored energy.\u003c/li\u003e\n \u003cli\u003eThe outlet air temperature drops from 34.93 °C to 25.44 °C and humidity from 55% to 47% at 4 m/s, indicating effective simultaneous temperature and moisture control, making the system suitable for laboratory cooling applications\u003c/li\u003e\n \u003cli\u003eThe CTES system delivers comparable cooling to conventional AC (~0.137 kWh per cycle) using pre-stored energy, reducing peak electricity consumption.\u003c/li\u003e\n\u003c/ul\u003e\n\u003cp\u003eThe developed packed-bed CTES system offers an energy-efficient cooling solution by utilizing PCM-filled capsules to store and deliver cold energy effectively. It is observed that a moderate HTF flow rate (200 l/h) balances energy input, solidification time (~3.20 h), and pressure drop (0.65 kN/m²), while an optimal discharge velocity (4 m/s) maximizes cooling and humidity reduction, demonstrating practical and efficient operation. The system achieves significant temperature and humidity control, ensuring improved thermal comfort while reducing reliance on conventional AC systems. By optimizing both HTF flow rates and discharge velocities, it effectively balances charging efficiency, cooling performance, and hydraulic losses, highlighting its potential for residential, commercial, and laboratory applications, enabling peak-load reduction and supporting sustainable energy management.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e6.1 Future Scope and Limitation\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research demonstrates the potential of a packed-bed CTES system with encapsulated PCM for energy-efficient room cooling and humidity control, highlighting opportunities to optimize HTF flow rates, air discharge velocities, and PCM selection for enhanced thermal storage and faster system response. Future work could explore advanced heat transfer enhancements, integration with renewable energy sources, real-world multi-room applications, and long-term performance evaluation to improve scalability and practical implementation. A key limitation of the current study is its laboratory-scale setup, which may not fully represent performance variations in larger or commercial building environments.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cp\u003eAC\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Air-conditioning\u003c/p\u003e\n\u003cp\u003eCTES\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp; Cool Thermal Energy Storage\u003c/p\u003e\n\u003cp\u003eDIW\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp; Deionized Water\u003c/p\u003e\n\u003cp\u003eGNP\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp; Graphene Nanoplatelets\u003c/p\u003e\n\u003cp\u003eHTF\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Heat Transfer Fluid\u003c/p\u003e\n\u003cp\u003eLDPE\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp; Low Density Polyethylene\u003c/p\u003e\n\u003cp\u003eLH\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Latent Heat\u003c/p\u003e\n\u003cp\u003eMWCNT\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Multiwall Carbon Nanotubes\u003c/p\u003e\n\u003cp\u003eNPCM\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Nano Phase Change Material\u003c/p\u003e\n\u003cp\u003ePLTES\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Packed-bed Latent Thermal Energy Storage System\u0026nbsp;\u003c/p\u003e\n\u003cp\u003ePCM\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp; Phase Change Materials\u003c/p\u003e\n\u003cp\u003ePTDC\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp; Proportionate Temperature Differential Controller\u003c/p\u003e\n\u003cp\u003eRTD\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp; Resistance Temperature Detector\u003c/p\u003e\n\u003cp\u003eSEM\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp; Scanning Electron Microscope\u003c/p\u003e\n\u003cp\u003eSH\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Sensible Heat\u003c/p\u003e\n\u003cp\u003eTC\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Thermal Conductivity\u003c/p\u003e\n\u003cp\u003eTEM \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Transmission Electron Microscope\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and material\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that all the data supporting the findings of this study are available within the article.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no conflicts of interest in publishing this article.\u003cstrong\u003e\u003cbr\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research received financial support from SRM Institute of Science and Technology through seed grand (SERI 2023), SRMIST/R/AR(A)/SERI2023/174/07.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors' contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eA.S. Concepts, Investigation and Writing – Original draft and fund acquisition, P.S. Writing – Review \u0026amp; Editing, M. C. Writing – Review \u0026amp; Editing, R.P. Writing – Review \u0026amp; Editing, S.C.K. Writing – Review \u0026amp; Editing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors wish to thank the Department of Mechanical Engineering, SRM Institute of Science and Technology, Kattankulathur for providing the facilities to carry out the research work.\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eE. World, I.E.A. 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Sawafta, Energy \u003cb\u003e189\u003c/b\u003e, (2019)\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"},{"header":"Table","content":"\u003cp\u003eTable 1 is available in the Supplementary Files section.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Packed-bed system, Phase Change Material, Charging and discharging performance, Air circulation cooling","lastPublishedDoi":"10.21203/rs.3.rs-7527832/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7527832/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis study explores a packed-bed cool thermal energy storage (CTES) system that uses spherical PCM capsules filled with distilled water and incorporates a helical coil to retrieve cold energy through air circulation. The system is designed to deliver efficient cooling performance, ensuring a reliable supply of cold energy even during electricity outages. The research investigates the influence of HTF flow rates (100 l/h, 200 l/h, and 300 l/h) during charging and air discharge velocities (2, 4, and 6 m/s) on the system\u0026rsquo;s thermal performance. Results indicate that higher HTF flow rates accelerate the charging process, reducing the time to full PCM solidification from 3.44 h at 100 l/h to 2.49 h at 300 l/h, while lower flow rates decrease the pressure drop from 0.78 kN/m\u0026sup2; to 0.53 kN/m\u0026sup2;, highlighting a trade-off between rapid energy storage and pumping efficiency. Energy analysis indicates that the PCM stores the majority of the cold energy, with latent heat contributing over 60% of the total 9857 kJ, demonstrating the effectiveness of the hybrid sensible-latent storage approach. During discharging, air temperature and humidity reductions were most significant at 4 m/s, with temperature decreasing from 34.93\u0026deg;C to 25.44\u0026deg;C and humidity from 55\u0026ndash;47%, indicating optimized cooling and moisture removal. Comparisons with conventional air-conditioning reveal that the CTES system can deliver equivalent cooling using pre-stored energy, reducing peak electricity demand while maintaining thermal comfort. The study confirms that careful selection of HTF flow rates and discharge velocities enables efficient energy storage and retrieval, making the proposed CTES system a practical and sustainable solution for continuous indoor cooling under intermittent power supply conditions.\u003c/p\u003e","manuscriptTitle":"Thermal and heat transfer characteristics of a packed-bed CTES system using encapsulated PCM capsules for peak-load management","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-09-11 13:35:04","doi":"10.21203/rs.3.rs-7527832/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"c8b14e47-d587-49cd-8361-170864d0256b","owner":[],"postedDate":"September 11th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-10-23T19:08:34+00:00","versionOfRecord":[],"versionCreatedAt":"2025-09-11 13:35:04","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7527832","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7527832","identity":"rs-7527832","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}