Development of copper metal wool incorporated in a latent thermal energy storage tank for improved charging and discharging

Applied Thermal Engineering 264 (2025) 125549 Available online 12 January 2025 1359-4311/© 2025 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/bync/4.0/). Research Paper Development of copper metal wool incorporated in a latent thermal energy storage tank for improved charging and discharging Saranprabhu Mani Kala a , Alessandro Ribezzo b , Gabriel Zsembinszki a , Emiliano Borri a , Cristina Prieto c , Luisa F. Cabeza a,* a GREiA Research Group, University of Lleida, Pere de Cabrera 3, 25001 Lleida, Spain b Department of Energy, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy c University of Seville, Department of Energy Engineering, Camino de los Descubrimientos s/n, 41092 Seville, Spain ARTICLE INFO Keywords: Charging and discharging time Copper metal wool Experimental study Latent heat thermal energy storage system N-octadecane Phase change material ABSTRACT The thermal energy storage tank is an essential component of a conventional thermal energy storage system. Nevertheless, low charging and discharging rates is one of the different problems associated with them. Many research studies were conducted to improve the charging and discharging characteristics of thermal energy storage tanks. Those include the addition of nanoparticles, heat pipes, metal foams, and encapsulation of the phase change material. Even though those approaches improved the thermal performance of the storage tank, they also promoted some problems, like nanoparticle deposition, suppression of natural convection, and reduced energy density. This study, used a different approach to improve the thermal performance, integrating copper metal wool of two discrete thicknesses perpendicular to the heat transfer fluid carrying tubes. Accordingly, with two different packaging factor four different configurations of metal wool-packed tanks were developed and compared against the reference, i.e., a conventional shell and tube tank. The thermal performance of the developed cases was tested using a custom-built test rig. The integration of copper metal wool reduced the charging and discharging time significantly. However, the fully stacked fine fibre wool packed tank showed a remarkable 85% reduction in solidification time and a 37.4% reduction in melting time compared to the conventional shell and tube tank. Moreover, it showed a higher mean power than all the developed cases, with 414% enhancement during the discharging cycle and 42% during the charging cycle. Thus, the incorporation of metal wool into a tank is an effective way to enhance its thermal performance. 1. Introduction Renewable energy sources are recognized as a vital source for the clean energy transition, in order to meet the growing energy demand and carbon monoxide emissions [1]. Utilization of renewable energy sources for electricity production, transportation, and heat generation for industry/buildings helps to maintain the rise in the global average temperature to less than 1.5 ◦C. Its effect on electricity production reached an all-time high in 2022, producing 340 GW. Utilizing effective techniques to harness renewable energy sources lowers the depletion rate of fossil fuels and consequent emissions [2,3]. Large-scale industries and building networks require thermal energy in the form of heat for their process heating needs [4]. Thermal energy is needed in the pulp and paper sector to produce steam [5]. Moreover, thermal energy is required for space heating applications in building structures. In each of these scenarios, if thermal energy is generated for end-use applications by using fossil fuels and electricity as energy sources, it further accelerates the demand to produce electricity and the depletion of fossil fuel resources. To address the aforementioned issues, solar energy – a sustainable clean energy source, needs to be made useful for the building and industrial sector thermal energy needs [6]. To solve these problems, the use of thermal energy storage (TES) system is required, which stores solar energy in the form of heat and releases it when needed [7]. TES systems were mainly classified in literature into three categories: (i) sensible, (ii) latent, and (iii) thermo-chemical TES systems [8,9]. Sensible heat TES systems store and release energy by utilizing the sensible heat component of the material. On the other hand, latent heat TES systems store and release energy by utilizing the latent heat component of a phase change material (PCM) [10]. Latent heat TES system uses PCM as an energy storage medium [11,12]. Such systems * Corresponding author. E-mail address: [email protected] (L.F. Cabeza). Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng https://doi.org/10.1016/j.applthermaleng.2025.125549 Received 19 July 2024; Received in revised form 10 January 2025; Accepted 11 January 2025 Applied Thermal Engineering 264 (2025) 125549 2 can store a significant quantity of thermal energy during the phase transitions from solid to liquid and from liquid to vapour and release it during its phase transitions from vapour to liquid and liquid to solid [8,10]. The field of sensible and latent heat TES systems is vibrant, with numerous research studies to maximise the use of clean energy sources in industry and building applications. Agarwal et al. [13] recently conducted an experimental study on a phase change material incorporated atmospheric water extraction, intending to generate water even during off-sunshine hours. This is an example of the dynamic nature of the field and the continuous quest for innovative solutions. While thermal energy storage systems have many advantages, they also need to be improved because of their low charging and discharging rates [14]. TES systems charging and discharging rates are typically determined by three TES system components: (i) PCM, (ii) heat transfer fluid, and (iii) TES tank design. Several research studies were extensively done on these components to enhance the charging and discharging characteristics [15–19]. This involves improving the thermophysical properties of the PCM and heat transfer fluid (HTF) as well as modifying the TES tank design. Several methods were also proposed to enhance the effective thermal conductivity of the PCM housed in the TES tank; those include the addition of nanoparticles [20,21], heat pipes [22], fins [17,19], metal foams [22–24] and encapsulation [15,25,26] of PCMs. Though they tend to increase thermal performance, they also have problems associated with them. Long-term stability, large-scale synthesis and costeffectiveness are the major problems with the use of nanoparticles. Whereas the fins, metal foams, and encapsulation of PCM highly suppress the natural convection effects [27]. A TES tank might benefit from enhancers that boost the system efficiency without compromising its energy density. Metal wool would be an ideal candidate in this phenomenon [28]. Without compromising significantly the energy density, the coarsely bound metal wool tends to contain the same volume of PCM as a conventional TES tank. The major benefit of employing metal wool is that it facilitates the heat transfer between the HTF tubes and the contained PCM [29]. It also maintains a uniform PCM temperature throughout the TES tank. Favache et al. [28] experimentally studied the effect of adding aluminum metal wool to the RT28HC PCM. By varying the mass of PCM infiltrated and the loaded aluminum metal wool, the author developed four distinct TES tanks. According to the author, the addition of aluminum metal wool causes the melting to occur more uniformly and reduces the radial temperature difference within the PCM. It was also reported by the author that the solidification time was reduced to half on inclusion of metal wool. Prieto et al. [29] examined both numerically and experimentally the thermal performance of a modified shell and tube TES tank using stainless steel metal wool. It was observed that the addition of stainless-steel metal wool increased the effective thermal conductivity of PCM (NaNO 3 ) by 300 %. Gasia et al. [30] examined the differences between a conventional TES tank and the impact of adding fins vs metal wool. The energy storage material for the TES tank was noctadecane. The authors reported that the TES tank containing fins performed better than the metal wool. Cu foam and Al/Cu wire spun metal structures were added by Prasanth et al. [23] to increase a paraffin wax’s effective thermal conductivity. The author found that adding foam and wire-woven structures shortened the melting time of the paraffin wax by 18.9 %. The impact of adding finned heat pipe and metal foam to the KNO 3 PCM was numerically investigated by Tiari et al. [22]. The addition of metal foam lowered the discharging time to a maximum of 75 %. It was also stated that the charging and discharging processes were accelerated by decreasing the metal wool porosity size. The charging and discharging characteristics of two distinct copper foams—gradient porous and homogenous—placed within a shell and tube TES tank were examined by Wang et al. [24]. The author reported a 37.6 % reduction in melting time for the gradient porous copper foam employed TES unit. According to the authors, PCM reached the desired temperature considerably more quickly when gradient copper metal foam was used. Gupta et al. [31–33] performed a series of numerical investigations on using copper metal foam to increase the thermal performance of a TES system. The author developed various PCM metal wool triplex-tube TES configurations in his recent studies and reported a maximum 91 % reduction in melting time. This literature review reveals that the integration of metal wool into a TES tank would be a potential to increase its power density. A vast majority of numerical studies were reported on enhancing the thermal performance of TES tanks; however, they lack experimental investigations. This study stands alone by exploring the feasibility of employing aligned copper metal wools as heat transfer enhancers to the TES tank. The advantage of such a technique is that the individual copper metal fibre strands composing the metal wool are oriented in a specific direction and bound to each other. This bounding helps to enhance the uniform thermal energy transport by transferring heat homogeneously within the TES tank. Copper is a highly thermally conductive material; hence, a higher heat transfer rate can be realized in addition to phase change material. Considering these significant benefits, five TES tank configurations, including the reference for performance evaluation were developed. Copper metal wool of two different thicknesses were used in this study. The charging and discharging performance of the individual TES tanks were tested using a custom-built experimental facility. This study also highlights the possible mechanism that happens with the incorporation of copper metal wool into the TES tanks. A very recent study from Cabeza et al. [34] reported a maximum 16 % reduction in melting time with the introduction of copper metal wool. The novelty of this study lies on the method of packing the metal wool, which in the present study was placed perpendicular to the HTF tubes. 2. Material and methods 2.1. Material The organic PCM n-octadecane, which has eighteen carbon atoms, Nomenclature Pmean power (W) mmass (g) Cp specific heat (J/g⋅K) TTemperature (◦C) tTime (hours) ˙ mmass flow rate (l/min) Uuncertainty Subscripts ch charging dis discharging HTF heat transfer fluid HTF, in heat transfer fluid related to the tank inlet HTF, out heat transfer fluid related to the tank outlet Auncertainty related to Type-A Buncertainty related to Type-B Ttotal uncertainty Pmean power Abbreviation TES Thermal energy storage PCM Phase change material HTF Heat transfer fluid SEM Standard error of mean S. Mani Kala et al. Applied Thermal Engineering 264 (2025) 125549 3 was chosen as the energy storage medium for the intended study due to its well-known properties and good phase change behaviour without subcooling. Therefore, this PCM allowed to study the heat transfer problem independently without the issues associated with more complex PCM. This PCM melts at 28 ◦C with a phase transition enthalpy of about 243.5 J/g, which is advantageous for a low-temperature TES system [35]. Aligned copper metal wool with two different thicknesses (fine: 60 µm and coarse: 120 µm) were obtained from STAX [36], as heat transfer enhancers. The thermal conductivity of the copper metal wool reported by the manufacturer is 383 W/m⋅K. 2.2. Methods A custom-built TES facility was used to test the charging and discharging characteristics of the developed TES tank configurations. A refrigeration unit, a heating element, a buffer tank, a TES tank, and a data acquisition system are the primary elements of the experimental setup. A conventional shell and tubes TES tank is used in this experimental study, which can hold up to 2,400 g of PCM. The buffer tank and the TES tank are connected through 12.7 mm diameter insulated copper pipes. The buffer tank is insulated to minimize heat losses to the surroundings. Since the operating temperature of the present study ranges between 15 ±1 ◦C and 45 ±1 ◦C, water has been selected as the heat transfer (HTF) fluid. The buffer tank has a capacity of 200 L, and it supplies hot/cold water to the TES tank during charging/discharging cycle. To obtain the TES tank discharge procedure, the refrigeration unit (GCU2030ED01B, Zanotti uniblock) cools the water stored in the buffer tank. On the other hand, for the charging procedure, two electric heaters (3 kW) that are mounted to the buffer tank help to raise the temperature of the water that is being stored. The water pump (PM45, 0.5 HP, Pentax) connected in series with the buffer tank, pumps the hot/cold water to the TES tank during the charging/discharging cycle. The volumetric flow rate of water is measured by the flow meter (M1000, ModMAG) that is linked in series between the pump and the TES tank. The entire operation of the experimental setup is managed by the application that was created using the Indusoft software (Version 8.1). Fig. 1 shows the schematic representation of the experimental setup. Several temperature sensors measure the HTF temperature within the buffer tank as well as at the HTF inlet and outlet points of the TES tank. In addition, the temperature of the PCM contained inside the TES tank was monitored using ten temperature sensors, as shown in Fig. 2 The temperature sensors were placed between the HTF tubes at various positions along the TES tank height. Four temperature sensors were placed in the TES tank anterior area and five in the tank posterior area. The temperature sensor T10 was placed deep within the TES tank, where the temperature difference between the PCM and the HTF will be maximum. The temperature data was measured with an accuracy of ± 0.3 ◦C using Pt-100 class B temperature sensors. The data acquisition system (DL01-CPU) records the temperature vs time data at a rate of 10 s. As shown in Fig. 3, the TES tank was properly insulated to minimize heat loss to the surroundings. 2.3. TES unit design Four distinct TES tank design configurations were developed by employing copper metal wool of two different thicknesses (fine and coarse fibres). The shell and tube TES tank with no metal fibres has been used as a reference for performance evaluation, in addition to the four metal wool configurations. The shape of the copper metal wool was modified from the purchased size (30 cm ×30 cm) to a tailored size matching the shell and tube TES tank lateral dimensions (20 cm ×8 cm). Moreover, they were also trimmed at the intersection with the heat transfer tubes. As Fig. 1. Schematic representation of the experimental setup. S. Mani Kala et al. Applied Thermal Engineering 264 (2025) 125549 4 illustrated in the Fig. 4, the individual trimmed metal wool sheets were stacked inside the shell and tube TES tank. In order to maximize heat transfer in the radial direction, the direction of the copper metal wool was purposefully made perpendicular to the HTF tubes in each case. The developed TES tank configurations are shown in Fig. 5. The amount of metal wool added to the tank was based on a specific criterion called packing factor, defined as the amount of metal wool contained in the TES tank to the volumetric capacity of the TES tank. The packing factor was calculated using the expression shown in Eq. (1): Packingfactor =Massofmetalwool TEStankvolume (1) TES tanks with different metal wool packing factors were developed to assess the suitable packing factor required to improve the heat transfer process. In this study metal wool was incorporated to the TES tank at different packing factor: (i) packing factor 1, the maximum number of metal wool sheets that the TES tank can hold, and (ii) packing factor 2, the minimum number of sheets required to fill the tank. In the case 1 configuration, twenty trimmed fine fibre metal wool sheets were stacked within the TES tank, covering the whole volume of the tank. In the case 2 configuration, the trimmed fine fibre metal wool sheets were manually extrapolated before stacking. So, only ten sheets of fine fibres were required to fill the entire TES storage volume. In case 3 and case 4 configurations, an analogous procedure was adopted to fill the TES tank with coarse fibre metal wool sheets. The case 3 configuration held a maximum of sixteen trimmed coarse fibre metal wool sheets, covering the whole volume of the tank. For the case 4 configuration, only eight extrapolated trimmed coarse fibre metal wool sheets were required to fill the TES tank volume. The Table 1 details the specifications of the developed TES tanks. 2.4. Charging and discharging cycle The refrigeration unit, heating element, buffer tank, water pump, and TES tank are the main components of the custom-built thermal cycling setup. The following methodology was followed to charge and discharge the TES tanks. To start the charging cycle, some prerequisite steps need to be Fig. 2. Position of temperature sensors in the TES tank. Fig. 3. Pictures of the insulated TES tank. S. Mani Kala et al. Applied Thermal Engineering 264 (2025) 125549 5 performed. The temperature of the PCM must be decreased to less than 15 ±1◦C since the temperature of interest for the charging cycle ranges from 15 ◦C to 45 ◦C. This preliminary step was done by cooling the water in the buffer tank with the refrigeration unit and subsequently pumping it to the TES tank lowered the PCM temperature. Water circulation to the TES tank was stopped as soon as the PCM temperature reached the desired temperature of 15 ±1◦C. To initiate the charging cycle, the water in the buffer tank was then heated to 45 ±1 ◦C. Once the required water temperature was attained, the TES tank was charged by continuously pumping hot water at a rate of 3 L/min till the PCM temperature reached 45 ◦C, thus obtaining the charging process of the TES tank. This procedure allowed the PCM in the TES tank to undergo a phase transition from solid phase to liquid phase with a simultaneous increase in temperature. The discharging cycles were immediately conducted after charging to utilize the melted PCM in the tank at 45 ±1 ◦C, which is the required condition for the discharging process. For the discharging cycle, some prerequisite steps need to be followed, such as that of the charging cycle but vice versa. The stored water temperature was cooled down to 15 ± 1 ◦C using the refrigeration unit to facilitate the discharging cycle. The contained PCM temperature was 45 ±1 ◦C and once the buffer tank water temperature was brought down to 15 ±1 ◦C the water was then circulated at 3 L/min through the tube side of the TES tank until the PCM temperature dropped to 15 ±1 ◦C. By doing so, the heat stored in the TES tank was discharged by the pumped cold water with a simultaneous phase transition of the PCM from liquid phase to solid phase. The procedure was repeated thrice for uncertainty estimates. The following equations, Eq. (2) and Eq. (3), were used to determine the mean power that each TES tank acquired during the charging and discharging cycle: (I) Charging Pch =∑TPCM=29◦C TPCM=15◦C ˙ mHTFCp(THTF,in −THTF,out)Δt tch (2) (II) Discharging Pdis =∑TPCM=23.5◦C TPCM=45◦C ˙ mHTFCp(THTF,out −THTF,in)Δt tdis (3) where Pch and Pdis refers to the mean power developed by the TES tanks during the charging and discharging cycle, respectively, ˙ mHTF refers to the HTF mass flow rate, Cp refers to the HTF specific heat capacity and THTF,in & THTF,out refers to the HTF temperatures at the inlet and outlet of the TES tank, respectively, TPCM refers the temperature of the contained PCM, tch and tdis refers to the charging and discharging time, respectively, and Δt refers the time interval between two readings. The two extreme temperatures were chosen based on the latent heat measurements done in a previous work [34]. 2.5. Uncertainty analysis The uncertainty in the measurements largely depends on the precision and accuracy of the instrument used for measuring [37]. The uncertainties were generally populated under Type-A uncertainty and Type-B uncertainty. Type A uncertainty (U A ) is the error that evolved from repeated measurements, and Type B uncertainty (U B ) is the systemic error coupled with the instrument. These two types of error were used to estimate the total uncertainty (U) associated with the measurements, as shown in Eq. (4). U= U2 A+U2 B √(4) The uncertainty associated with the parameters of mass flow rate (manufacturer-reported value) and temperature sensors is listed in Table 2. Hence, the total uncertainty associated with mean power (U P ) was estimated from the randomness error that evolved during the mean power measurement and the systemic error related to the mass flow rate, temperature (U T ) and time (U t ). The following equation, Eq. (5), was Fig. 4. Stacking process of metal wool fibres inside the TES tank. S. Mani Kala et al. Applied Thermal Engineering 264 (2025) 125549 6 Fig. 5. Pictures of the custom-built TES tank configurations: (a) Baseline (b) Case 1–20 fine fibre metal wool sheets; (c) Case 2–10 fine fibre metal wool sheets; (d) Case 3–16 coarse fibre metal wool sheets; and (e) Case 4–8 coarse fibre metal wool sheets. S. Mani Kala et al. Applied Thermal Engineering 264 (2025) 125549 7 used to estimate the total uncertainty of mean power [37]: UP= SEM2 P+( ∂ P ∂ ˙ m⋅U˙ m)2 +2⋅( ∂ P ∂ T⋅UT)2 +2⋅( ∂ P ∂ t⋅UT)2 √(5) 3. Results and discussion 3.1. Charging and discharging behavior of n-octadecane Fig. 6, shows the temperature vs time profile of the reference TES tank configuration temperature sensors (T1 to T10). From the Fig. 6, the T10 appears more reliable than all the temperature sensors, and the phase change process was distinguishable. Therefore, this particular temperature sensor was used for assessing the thermal performance of the developed TES tank configurations. Fig. 7 depicts the temperature vs. time profile obtained by the temperature sensor T10 for characterizing the PCM charging and discharging behaviour in the baseline case (bulk n-octadecane). The temperature vs. time profile of pure n-octadecane shows three charging stages. The temperature range of 15 ◦C to 24 ◦C makes up the first stage. The temperature range of 24 ◦C to 29 ◦C makes up the second stage. And the temperature range between 29 ◦C and 45 ◦C makes up the third stage. In the first stage, the charging rate was higher because of the significant temperature difference between the HTF and the contained PCM. The PCM is in a solid state at this point, and conduction is the only means of heat transfer. In the second stage, PCM undergoes a phase change from solid to liquid, accompanied by a slow rate of temperature increase over time. Two phases of PCM, solid and liquid exist at this point. Close to the HTF tube walls are the liquid phases of PCM, and farther away from the HTF tubes are the solid phases. Natural convection effects play a major role to transport heat from the PCM liquid phase to its solid phase [38]. Nevertheless, the PCM will be in the liquid phase in the third stage, and the rate of charging will decrease from the initial to the final phases of the third stage due to the temperature difference decrease between the PCM and HTF. The pure n-octadecane temperature vs. time profile clearly shows three stages of discharge. The temperature range of 45 ◦C to 27 ◦C makes up the first stage. The temperature range of 27 ◦C to 23.5 ◦C makes up the second stage. And the temperature range between 23.5 ◦C and 15 ◦C makes up the third stage. Similar to the charging cycle, there was a greater temperature difference between the HTF and the contained PCM during the first stage, which led to a higher discharging rate. At this point, convection is the phenomenon that transfer heat within the PCM, which is in liquid phase. The phase change of PCM from a liquid to a solid state occurs in the second stage, associated with a slower rate of temperature decrease over time. Two phases of PCM, solid and liquid exist at this point. The major problem associated with PCM is their lower solid phase thermal conductivity, which reduces the discharging performance of the TES tank. Conduction is the primary mechanism that governs the solidification process of the PCM. During the discharging cycle, the PCM located along the walls of the HTF tubes solidifies first. This solid layer with lower thermal conductivity is now a barrier to transfer heat from liquid PCM to the HTF, lengthening the discharge time. Finally, the rate of discharging is higher in the initial phase of the third stage, and it gets decreases as the temperature difference between the HTF and the solid PCM decreases. 3.2. Effect of metal wool on charging and discharging behavior The charging and discharging behaviour of the TES tanks were plotted Fig. 8 for the baseline configuration and for the TES tank loaded with fine fibre (case 1) and coarse fibre (case 3) with maximum packing factor. From Fig. 8, it is evident that the inclusion of copper metal wool to the TES tank has a positive impact on the charging and discharging behaviour of n-octadecane. The performance of the addition of fine fibre copper metal wool was greater compared to the one obtained with the coarse fibre copper metal wool. During the charging cycle, the temperature vs time profiles for the fully stacked fine and coarse metal wool TES tank configurations were nearly similar in the first stage. Though the addition of copper metal Table 1 Specification of the developed TES tanks. Description Filler Number of metal wool sheets Mass of metal wool (g) Packing factor (g/ cm 3 ) Baseline None − − − Case 1 Fine fibre metal wool 20 sheets of size 20 cm £8 cm 764.90 0.24 Case 2 Fine fibre metal wool 10 sheets of size 20 cm £8 cm 305.34 0.09 Case 3 Coarse fibre metal wool 16 sheets of size 20 cm £8 cm 687.76 0.22 Case 4 Coarse fibre metal wool 8 sheets of size 20 cm £8 cm 348.16 0.11 Table 2 Parameter and their accuracy. Parameter Unit Accuracy Temperature ◦C±0.3 Mass flow rate L/min ±0.3 % Fig. 6. Temperature vs time profile of the reference conventional shell and tube TES tank obtained from T1 to T10 sensors. S. Mani Kala et al. Applied Thermal Engineering 264 (2025) 125549 8 wool has a positive impact on the solid state effective thermal conductivity of the PCM, the effect of metal wool thickness is negligible when the PCM exists as the solid phase. However, the effect of metal wool thickness is clearly distinguishable during the second and third stages of the TES tank charging process. As the TES tank progresses through the second stage of the charging process, the PCM undergoes phase transition from solid phase to liquid phase. Once the PCM turns to liquid phase, natural convection comes into play by increasing heat transfer within the contained PCM. The metal wool containing TES tank performed way better during these stages. Despite the natural convection effects, the addition of copper metal wool has also increased the heat transfer within the PCM. Comparing among the configurations, the TES tank containing fine copper metal wool fibre was found to have better charging characteristics compared to the coarse fibre copper metal wool contained TES tank configuration. Favache et al. [28] observed a similar behaviour, i.e., the inclusion of metal wool reduces heat transfer by preventing natural convection from progressing on a wider scale. In the present study, though in both the configurations (fine and coarse) the copper metal wool were tightly stacked (fully packed) inside the TES tank, the trade-off between the level of natural convection suppression and the enhanced effective thermal conductivity caused by the addition of metal wool determines the percentage reduction in charging time. Yang et al. [39] studied the natural convection effect associated with micrometer-sized wire using the hot wire method. The authors reported that the convection heat transfer coefficient increased with the decreased wire diameter. The authors summarised the following key occurrence as the reasons behind the variations in heat transfer coefficient: i. A boundary layer was generated around the wire when the liquid phase PCM rose to the top section of the tank. ii. The thickness of the boundary varied with the wire diameter; the more significant the wire diameter, the larger the thickness of the boundary layer. iii. Larger boundary layer thickness, low-temperature gradient across the boundary layer, and low heat transfer coefficient. iv. The smaller-diameter wire had a higher convective heat transfer coefficient than the higher-diameter wire. A similar mechanism might evolve due to the presence of metal wool in this present study, which is the reason behind the variation of thermal performance on the inclusion of different thicknesses of metal wool fibres. Hence, the fine fibres promote the effective thermal conductivity of the PCM in the liquid phase without supressing the natural convection effects to a large extent. The temperature vs time profile for metal wool loaded TES tank during the initial stage of discharging cycle were similar as like the charging cycle. However, in the second stage of the discharging cycle, the effect of metal wool is clearly distinguishable. Once the PCM undergoes a phase transition from liquid phase to solid phase, the heat transfer largely relies on the solid-state properties of the contained PCM. The inclusion of metal wool increases the solid state effective thermal conductivity of the PCM thereby increasing the discharging characteristics of the PCM. The TES tank containing fine copper metal wool was Fig. 7. Temperature vs. time profile of the PCM contained in conventional shell and tube configuration (a) charging and (b) discharging cycle. Fig. 8. Temperature vs time profile during the (a) charging and (b) discharging cycle for the TES tank configurations: Baseline, Case 1 and Case 3. S. Mani Kala et al. Applied Thermal Engineering 264 (2025) 125549 9 found to have better discharging characteristics compared to the coarse metal wool fibre contained TES tank configuration. Though in both the configurations the metal wool has been tightly stacked (fully sacked) inside the TES tank, the fine fibre metal wool contained TES tank configuration has the highest fibres contained per cm 3 . The higher the number of metal wool fibres, the higher the pathway created for the heat to get transported from one part of the TES tank to another part. 3.3. Effect of metal wool packaging factor on charging and discharging behavior The charging and discharging behaviour of the TES tanks for the TES tank configurations loaded with fine fibre (case 1, case 2) and coarse fibre (case 3 and case 4) are shown in Fig. 9 and Fig. 10. On comparing the charging and discharging temperature vs time profile between the fine fibre metal wool TES configurations, it is evident that the packaging factor has a significant effect on the profile (Fig. 9). Table 3 provides the comparison of charging and discharging rates during the successive stages among the TES tank configurations that use fine fibre metal wool as an enhancer. The average rate of charging and discharging was higher for the configuration 1 compared to configuration 2. The case 1 TES tank has higher rates of charging during each stage compared to that of the case 2 TES tank configuration. In each case, the volume of stacked metal wool determined the quantity of heat transfer within the TES tank, in addition to the contribution by natural convection effects. The case 1 configuration was stacked with 20 fine fibre metal wool sheets; however, the case 2 configuration was stacked with 10 fine fibre metal wool sheets (almost half). And this is the underlying factor responsible behind the increase in charging and discharging rates of the case 1 configuration. On comparing the charging and discharging temperature vs. time profile between the coarse fibre metal wool TES configurations it came to light that the packaging factor has a negligible effect on the temperature vs time profile (Fig. 10). Table 4 provides the comparison of charging and discharging rates during successive stages among the TES tank configurations (case 3 and case 4) that use coarse fibre metal wool as an enhancer. The average rate Fig. 9. Effect of metal wool packing factor on the (a) charging and (b) discharging behaviour of the TES tank configurations: Case 1 and Case 2. Fig. 10. Effect of metal wool packing factor on the (a) charging and (b) discharging behaviour of the TES tank configurations: Case 3 and Case 4. Table 3 Charging and discharging rates of the case 1 and case 2 configurations during three stages. Charging cycle Charging rate (K/min) Discharging cycle Discharging rate (K/min) Case 1 Case 2 Case 1 Case 2 Stage 1 (ΔT: 15 to 24 ◦C) 1.39 0.84 Stage 1 (ΔT: 45 to 27 ◦C) 3.53 1.67 Stage 2 (ΔT: 24 to 29 ◦C) 0.43 0.41 Stage 2 (ΔT: 27 to 23.5 ◦C) 0.17 0.12 Stage 3 (ΔT: 29 to 40 ◦C) 0.96 0.65 Stage 3 (ΔT: 23.5 to 17 ◦C) 0.23 0.15 Average of three stages 0.93 0.63 Average of three stages 1.31 0.64 S. Mani Kala et al.