Applied Thermal Engineering

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1 Applied Thermal Engineering 50 (2013) 896e907 Contents lists available at SciVerse ScienceDirect Applied Thermal Engineering journal homepage: Improved design for heat transfer performance of a novel phase change material (PCM) thermal energy storage (TES) Jundika C. Kurnia a,b, Agus P. Sasmito b,c, *, Sachin V. Jangam a,b, Arun S. Mujumdar a,b a Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore , Singapore b Mineral, Metal and Material Technology Centre, National University of Singapore, 9 Engineering Drive 1, Singapore , Singapore c Mechanical Engineering, Masdar Institute of Science and Technology, Masdar City, Abu Dhabi, P.O. Box 54224, United Arab Emirates highlights < We develop conjugate mathematical model of phase change material (PCM) and heat transfer fluid HTF). < Novel festoon thermal energy storage (TES) design yields improved heat transfer rate during charging/discharging. < Multiple PCMs arrangement enhances the heat transfer performance further. < TES has potential to be used as thermal capacitor in industrial application. article info abstract Article history: Received 30 September 2011 Accepted 11 August 2012 Available online 27 August 2012 Keywords: Festoon Fins Heat transfer performance Mathematical model Thermal energy storage U-tube This study evaluates numerically various configurations of PCM thermal energy storage devices, e.g., U-tube, U-tube with in-line fins, U-tube with staggered fins and a novel festoon design. The conjugate heat transfer between the heat transfer fluid (HTF) and PCM, which undergoes a cyclic melting and freezing process, is solved numerically using the computational fluid dynamic approach utilizing enthalpy-porosity formulation. The results indicate that our novel festoon channel design yields improved heat transfer rate for both charging and discharging stages. To further enhance heat transfer performance, we introduce use of multiple PCMs with various configurations of different PCM arrangement. Advantages and limitations of each design are evaluated with respect to their heat transfer performance vis-à-vis heat storage capacity. Further R&D and experimentation is needed to arrive at commercially viable designs of PCM heat storage units. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Recent years, phase change material (PCM) has received significant attention due to their potential for thermal energy storage. This is attributed to their high latent heat of fusion during a phase change. PCM undergo melting (also known as charging) and solidification (also known as discharging) as it is exposed to hot and cold environment respectively [1]. Despite its huge potential, application of PCM as thermal energy storage has certain drawbacks: the requirement of high heat transfer rates during solidification, lower thermal conductivity of PCMs, need of external nucleating agents, limited cycles of melting and solidification. * Corresponding author. Mechanical Engineering, Masdar Institute of Science and Technology, Masdar City, Abu Dhabi, P.O. Box 54224, United Arab Emirates. Tel.: þ address: ap.sasmito@gmail.com (A.P. Sasmito). In attempt to enhance the performance of PCM assisted thermal storage, numerous study and investigation have been conducted and various methods have been proposed. One route to enhance the performance of PCM assisted thermal storage is by changing the thermal properties of PCM and/or operating conditions, arrangement of thermal energy storage e such as use of extended surfaces [1e5]. Various ways of increasing the thermal properties have been reported, including the use of multiple PCMs, micro-encapsulated or nanoencapsulated PCM (MEPCM or NEPCM) slurries in which the encapsulated particles are suspended in continuous phase such as water. The use of multiple PCM allows sufficient driving force for heat transfer along the flow direction (the difference between the heat transfer fluid and the melting temperature of PCM) which otherwise is not possible if single PCM is used [5]. Considerable effort has been devoted to study the use of multiple PCM to enhance the performance of thermal heat storage systems [6e9]. The later system of encapsulated PCM was experimentally and numerically shown to perform better in terms of heat transfer characteristics [10e13]. This was attributed to a larger /$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.

2 J.C. Kurnia et al. / Applied Thermal Engineering 50 (2013) 896e heat transfer area provided by smaller PCM particles resulting in increased heat transfer rates. Our recent numerical study [14] showed that the use of water suspension of microencapsulated phase change material (n-octadecane) increases the heat transfer at the cost of higher pumping cost in a coiled heat exchanger of square cross-section. In addition, a vast number of studies have shown that extended surfaces, metal matrix, metal rings, steel balls, carbon fibers and/or other geometric modifications also results in enhancement of the charging and discharging rates in PCM assisted heat storage [2,4,5,15,16]. These extended surfaces (fins) are located on either heat transfer fluid side or PCM side based on the relative heat transfer coefficient value. Most of the times, it is located at the PCM side where the heat transfer coefficient is low. The configuration and orientation of fins is very important and has significant effect on the heat transfer enhancement. There are number of experimental and numerical research work carried out to study the effect of longitudinal, circular extended surfaces and other arrangements [17,18]. To date, several experimental [19e22] and numerical [23e27] studies have been conducted to investigate conjugate heat transfer of PCM; several review papers [1,2,4,5,28,29] are also available in literature. Despite numerous studies that have been conducted and reported, none has arrived at definitive conclusion yet. There is thus still room to further enhance the performance of PCMbased thermal energy storage by utilizing extended surfaces, new design and multiple-pcm. Therefore, the aim of the study presented here is threefold: (i) to evaluate several geometrical designs via conjugate model of PCM and HTF including conventional U-tube design, U-tube design with different fins placement, i.e., inline and staggered and a novel festoon design; (ii) to investigate the effect of multiple PCMs and its placements scenarios, e.g., vertical and horizontal, for further heat transfer enhancement; (iii) to study the cyclic charging and discharging of PCM and its potential to be used as thermal capacitor. The most significant aspect of this study is to determine the potential advantages and limitations of heat transfer enhancement in PCM thermal energy storage and provide design guidelines for their applications through mathematical modeling. The comparison was carried out not only based on the liquid fraction and solidification rates but also on the pressure drop required. 2. Model formulation Here, we consider heat transfer fluid flows through a U-pipe of circular cross-section immersed in a pool of PCM, as illustrated in Fig. 1. The PCM used for the current study is paraffin wax. Various configurations considered in this study are: U-tube, U-tube with inline fins, U-tube with staggered fins and a novel festoon design (please refer to Fig. 1 for detail schematics). Implementation of multiple layers of PCM, as illustrated in Fig. 2, will also be investigated Governing equations The mathematical model is based on the validated model that was developed in our group [30e34] comprises two component, viz., water flows and PCM storage, which allows for conjugate heat transfer between carrier fluid (water) and PCM. During charging, the hot water flows through the channel, the heat is then transferred to the PCM to store the energy as latent heat (melt). While at discharging, cool water flows at the channel to take heat from PCM. Initially, the PCM at the molten state and then solidified once the heat is taken away. Note that the three-dimensional physical model is reduced to two-dimensional model to save computational cost. Note that the two-dimensional model is easier to scale-up in practical application by extruding the geometry in span-wise direction. Fig. 1. Schematic representation of a) U-tube, b) U-tube with in-line fins c) U-tube with staggered fins and d) festoon design phase change material thermal energy storage (PCM-TES) Heat transfer fluid In the HTF, fluid flow and convective heat transfer are taken into consideration. A forced convection Newtonian laminar flow with the conservation equations of mass, momentum and energy are given by [35,36] vr w vt þ V$ðr wuþ ¼0; (1) vðr w uþ h þ V$ðr vt w u5uþ ¼ VpþV$ mw Vu þðvuþ Τi ; (2) v rw c p;w T þ V$ r vt w c p;w ut ¼ V$ðk w VTÞ (3) where r w is the fluid density, u is the fluid velocity, p is the pressure, m w is the dynamic viscosity of the fluid, c p,w is the specific heat of the fluid and T is the temperature. The inlet water velocity is set uniform and constant for all cases

3 898 J.C. Kurnia et al. / Applied Thermal Engineering 50 (2013) 896e907 Table 1 Base case and operating parameters. Parameters Value Unit c p,pcm 2890 J kg 1 K d pipe m k ðsþ pcm; k ðlþ pcm 0.21, 0.12 W m 1 K 1 L 173,400 J kg 1 H e h TES 0.3 m p out 1,011,325 Pa Re HTF, in 1000 e T ref K T min, T max 300, 350 K T solidius, T liquidius , K T solidius, lo, T liquidius, lo , K T solidius, hi, T liquidius, hi , K w TES 0.1 m where r pcm is the density of PCM and m pcm is the dynamic viscosity of the PCM. In the above equations, we implement enthalpyporosity formulation which is represented by additional source terms in momentum equation, S mom and accounted for latent heat of PCM in the enthalpy term of energy equation, H pcm Constitutive relations Thermophysical properties of water were obtained as polynomial functions of temperature [35,36]. The water density is defined by r w ¼ 3: T 2 þ 1:88T þ 753:2; (8) while the water viscosity is given by m w ¼ 2: :3 T 143:2 ; (9) and the thermal conductivity of water is calculated from k w ¼ 8: T 2 þ 6: T 0:5981; (10) The specific heat of water is considered constant at c p;w ¼ 4200: (11) The PCM considered in this study is paraffin wax; the density, r pcm, is given by [31] Fig. 2. Schematic representation of various configurations of phase change material thermal energy storage (PCM-TES) with multiple PCMs PCM storage In the storage, fluid flow, heat transfer and phase change processes of PCM is taken into consideration. The conservation of mass, momentum and energy are given by vr pcm þ V$ rpcm u ¼ 0; (4) vt vr pcm þ V$ rpcm u5u ¼ V$s þ r vt pcm g þ S mom ; (5) h s ¼ pιþv$ mpcm Vu þðvuþ Τi ; (6) v rpcm H pcm þ V$ rpcm uh pcm ¼ V$ k pcm VT ; (7) vt Fig. 3. Liquid fraction average of charging (melting) phase change material for U-tube design [/]; U-tube with inline fins [e e]; U-tube with staggered fins [e $e]; and novel festoon design [d].

4 J.C. Kurnia et al. / Applied Thermal Engineering 50 (2013) 896e Fig. 4. Liquid fraction distribution for (a) U-tube design; (b) U-tube with inline fins; (c) U-tube with staggered fins; and (d) novel festoon design [d] during charging at t ¼ 300 s. 750 r pcm ¼ 0:001ðT 319:15Þþ1 ; (12) The PCM thermal conductivity is estimated by k pcm ¼ ( k ðsþ pcm if T < T solidius k ðlþ ; (13) pcm if T>T liquidius where superscript (s) and (l) represent solid and liquid phase of the PCM, respectively. The PCM viscosity is defined as m pcm ¼ 0:001expð 4:25 þ 1790=TÞ: (14) In equations (5) and (7), we have introduced an additional source term in momentum equation, S mom and accounted for latent heat of PCM in the enthalpy term of energy equation, H pcm which defined as H pcm ¼ h pcm þ DH pcm ; (15) where h pcm is the sensible heat, given by Fig. 5. Velocity vector for (a) U-tube design; (b) U-tube with inline fins; (c) U-tube with staggered fins; and (d) novel festoon design [d] during charging at t ¼ 300 s.

5 900 J.C. Kurnia et al. / Applied Thermal Engineering 50 (2013) 896e907 Fig. 6. Temperature contour for (a) U-tube design; (b) U-tube with inline fins; (c) U-tube with staggered fins; and (d) novel festoon design [d] during charging at t ¼ 300 s. h pcm ¼ h ref pcm þ Z T Tref c p;pcm dt; (16) here, c p,pcm is the specific heat of the PCM. The latent heat of PCM is calculated by DH pcm ¼ bl; (17) where L is the latent heat of PCM and b is the melted mass fraction of PCM given by 8 0 if T < T solidius >< T T solidius b ¼ if T T liquidius T solidius < T < T liquidius (18) solidius >: 1 if T>T liquidius Furthermore, the enthalpy-porosity formulation treats the mushy region (partially solidified) as a porous medium. The porosity in each cell is set to equal to the liquid fraction in the cell. In fully solidified region, the porosity is equal to zero, which extinguishes the velocities in this region. The source term due to the reduced porosity in the mushy zone is approximated by S mom ¼ ð1 bþ 2 b 3 þ 0:001 Hu; (19) where H is mushy zone constant Boundary and initial conditions The boundary conditions are defined as follows: a) Inlet: At the inlet, we specify inlet velocity and inlet temperature: u ¼ U in ; T ¼ T max u ¼ U in ; T ¼ T min for charging; for discharging: (20) b) Outlet: At the outlet, we set pressure and stream-wise gradient of temperature to zero; the outlet velocity is not known a priori but needs to be iterated from the neighboring computational cells: p ¼ p out ; n$ðk w VTÞ ¼0: (21) c) Walls: At walls, we set no slip and no heat flux condition: u ¼ 0; n$ðkvtþ ¼0: (22) Fig. 7. Liquid fraction average of discharging (solidification) phase change material for U-tube design [/]; U-tube with inline fins [e e]; U-tube with staggered fins [e $e]; and novel festoon design [d]. d) Interface: At the interface between HTF and PCM, we specify no-slip condition for velocities and coupled temperature to allow for conjugate heat transfer:

6 J.C. Kurnia et al. / Applied Thermal Engineering 50 (2013) 896e Fig. 8. Liquid fraction distribution for (a) U-tube design; (b) U-tube with inline fins; (c) U-tube with staggered fins; and (d) novel festoon design [d] during discharging at t ¼ 1000 s. u ¼ 0; T þ itf ¼ T itf : (23) The initial conditions are defined as follows: a) Charging: During charging, we set initial temperature to be the same as ambient temperature: at t ¼ 0; T ¼ T min ; u ¼ 0: (24) b) Discharging: During discharging, the initial temperature is set to be equal to maximum temperature of hot water: at t ¼ 0; T ¼ T max ; u ¼ 0: (25) In this paper, a constant velocity of water at Re 1000 is prescribed at the inlet for comparison purposes. The values for the above parameters are summarized in Table 1 Fig. 9. Velocity vector for (a) U-tube design; (b) U-tube with inline fins; (c) U-tube with staggered fins; and (d) novel festoon design [d] during discharging at t ¼ 1000 s.

7 902 J.C. Kurnia et al. / Applied Thermal Engineering 50 (2013) 896e907 Fig. 10. Temperature contour for (a) U-tube design; (b) U-tube with inline fins; (c) U-tube with staggered fins; and (d) novel festoon design [d] during discharging at t ¼ 1000 s. 3. Numerics The computational domains were created in the commercial pre-processor software GAMBIT It was also used for meshing, labeling boundary conditions and determines the computational domain. To ensure grid independent results, we have carried out grid independence test where we compare three different mesh sizes: , and in terms of melting and solidification rate. We found that the mesh amount of results in 2% deviation in terms of melting and solidification time compared to the mesh amount of ; meanwhile, results from the mesh size of deviates up to 12% as compared to those from the finest one. Therefore, a mesh of around elements was sufficient for the numerical investigation purposes: a fine structured mesh near the wall to resolve the boundary layer and an increasingly coarser mesh in the rest of the domain in order to reduce the computational time. The pressure-based method within version of the commercial code FLUENT was utilized for solving the governing equations. User-defined functions (UDF) were written in C language to account for temperature-dependence of the thermo-physical properties of paraffin wax and water. The time step for integrating the temporal derivatives was set to 0.1 s. The first-order upwind differencing scheme was used for solving the momentum and energy equations, whereas the PRESTO scheme was adopted for the pressure correction equation. The under-relaxation factors for the velocity components, pressure correction and thermal energy were 0.5, 0.3 and 1 respectively. Convergence criteria were set at 10 3 for continuity and momentum, and 10 6 for thermal energy. An enthalpy-porosity technique is used in FLUENT for modeling the solidification/melting process. In this technique, the liquid melt fraction in each cell is computed every iteration, based on enthalpy balance. The mushy zone is the region where the porosity increases from 0 to 1 as the PCM melts. When the region is complete solid, the porosity is zero and also the flow velocity in that zone also drops to zero. 4. Results and discussion The numerical simulations were carried out for typical conditions found in PCM thermal energy storage. The base-case conditions together with the physical parameters are listed in Table 1. In the following, we will examine the performance of various design of thermal energy storage Effect of geometrical design Fig. 11. Liquid fraction average of charging (melting) of novel festoon design with multiple phase change materials for type 1 [/]; type 2 [e e]; type 3 [e $e]; type 4 [e ]; and single PCM design [d]. The time evolution of melting of PCM for various designs of TES is presented in Fig. 3. As can be seen, the novel festoon design offers better heat transfer performance, indicated by largest melting fraction at early stage of melting process. It also reaches steadystate significantly faster as compared to other design. This is attributed to the fact that the festoon design has larger surface contact area than other designs. Furthermore, it is found that U- tube with fins perform better as compared to the U-tube without

8 J.C. Kurnia et al. / Applied Thermal Engineering 50 (2013) 896e Fig. 12. Liquid fraction distribution for novel festoon design with multiple PCMs configuration for (a) type 1; (b) type 2; (c) type 3; and (d) type 4 during charging at t ¼ 300 s. fin. This is expected since addition of fin will enhance heat transfer surface and in turn improve heat transfer rate. Looking further, we find that U-tube with staggered fins has slightly higher heat transfer rate than U-tube with inline fins. This can be attributed to the natural convection occur during charging which strongly influenced by the fin arrangement. As can be inferred from Figs. 4e 6, inline fin arrangement blocks the flow of the melting PCM and hence resists natural convection that occurs during charging. Meanwhile, staggered arrangement allows natural convection stream to spread to the upperside of the TES. Novel festoon design performs superior as compared to other designs: higher melted fraction (Fig. 4), better natural convection heat transfer represented by higher velocity magnitude inside PCM container (Fig. 5) and higher temperature rise (Fig. 6). With regards to the heat transfer performance of HTF, we note that the outlet water temperature difference for novel festoon design is around 30%, 22% and 18% lower than that of U-tube, inline and staggered fins design, respectively, during charging, although the pressure drop required for festoon design is around 80% higher than others. This clearly indicates that the novel festoon design is superior to other designs at an expense of higher pumping power. Similar to those for melting process, the novel festoon design provides the highest heat transfer during solidification process. This can be inferred from Fig. 7, where the festoon design has the Fig. 13. Velocity vector for novel festoon design with multiple PCMs configuration for (a) type 1; (b) type 2; (c) type 3; and (d) type 4 during charging at t ¼ 300.

9 904 J.C. Kurnia et al. / Applied Thermal Engineering 50 (2013) 896e907 Fig. 14. Temperature contour for novel festoon design with multiple PCMs configuration for (a) type 1; (b) type 2; (c) type 3; and (d) type 4 during charging at t ¼ 300 s. lowest liquid fraction during early stage of discharging, indicating faster solidification. The PCM inside this design also reaches steadystate significantly faster compared to other design. This is due to longer HTF flow passage which allow for heat to be transferred from PCM to the HTF, as seen Fig. 8. It should be noted, however, that the time frame required for complete discharging is about three to four times longer compared to charging: that is around 4000 s for complete discharging compared to around 1000 s for complete charging of festoon design e which is the major drawback for PCM TES. Closer inspection reveals that that the novel festoon design enhances the discharge rate by 50% and 40% compared to U- tube and U-tube with fins design, respectively. It is also found that U-tube with fins design performs better as compared to the U-tube without fin; whereas the fins arrangement has insignificant effect to the discharge rate. This is attributed to the fact that, unlike charging for which the heat transfer is dominated by natural convection, the discharge heat transfer is dominated by heat conduction mechanism; thus the fins arrangement does not significantly contribute to the heat transfer performance. Fig. 9 shows the PCM velocity vector during solidification. We note that Fig. 15. Liquid fraction average of discharging (solidification) of novel festoon design with multiple phase change materials for type 1 [/]; type 2 [e e]; type 3 [e $e]; type 4[e]; and single PCM design [d]. the PCM velocity is much smaller (around two to three order-ofmagnitude) than that of the natural convection velocity during charging (Fig. 5). Aside from the fact that addition of fin will enhance heat transfer surface, addition of fin allows for heat transfer from the PCM to the U-tube when a layer of frozen PCM blocks heat transfer from the melting PCM to U-tube. The fins could reach melting section of PCM which may be far from the U-tube as the solidification is controlled by heat conduction mode. In addition, longer flow passage in festoon design is beneficial for heat transfer performance: It results in significantly smaller temperature along the passage than that at the PCM container, as can be seen in Fig. 10. These results suggest that the novel festoon design is more desirable for heat transfer enhancement during charging and discharging. However, further optimization may be needed for specific applications Effect of multiple PCM arrangement A further point of interest in this study is the implementation of multiple PCM with different melting point in TES. Here we utilize the novel festoon design as it is found to provide highest heat transfer. Properties of these PCM are summarized in Table 1 for which the melting and solidification point is 10 C higher and lower than that of base-case PCM. Four arrangements are investigated in this study (please refer to Fig. 2): vertical arrangements with either low or high melting point PCM on top and horizontal arrangements with either low or high melting point on the inlet side. Fig. 11 presents the liquid fraction average of the various multiple PCM arrangements and single PCM design during melting process. Here, several features are apparent; foremost among them is that the multiple PCM arrangement yields higher heat transfer rate as compared to single PCM design. It is also noted that the horizontal arrangement performs slightly higher heat transfer rate as compared to the vertical counterpart. Charging rate for type 2 arrangement improves by up to around w30% followed by type 1 (w25%), type 3 (w15%) and type 4 (w12%) compared to the single PCM design. Looking further, we find that multiple PCMs arrangement with lower melting point PCM located at the inlet side offers higher heat transfer rate compared to that with higher melting point PCM at the inlet (Fig. 12). This is due to the fact that lower

10 J.C. Kurnia et al. / Applied Thermal Engineering 50 (2013) 896e Fig. 16. Liquid fraction distribution for novel festoon design with multiple PCMs configuration for (a) type 1; (b) type 2; (c) type 3; and (d) type 4 during discharging at t ¼ 1000 s. melting point PCM is easier to melt even at low temperature difference. Furthermore, placing lower melting point at the top of the TES has advantages as it allows for natural convection to develop; whilst placing the PCM at the bottom hinders the natural convection to occur especially at the area bellow the HTF pipe, see Fig. 13 for details of the development of velocity vector due to buoyancy. We note that PCM with lower melting temperature has slightly higher natural convection due to higher buoyancy which is mirrored by faster melting. Furthermore, the velocity vector for horizontal arrangement is seen to be somewhat higher than vertical counterpart as there are no separators which block the flow as in vertical arrangement. The melted fraction and natural convection are mirrored by higher temperature distribution at the PCM container as illustrated in Fig. 14, for which Type 2 design yields the highest average temperature. In contrast to those during charging, multiple PCMs arrangement with high melting point at the inlet (type 1) side offers the highest heat transfer rate among others, indicated by smaller liquid fraction in Fig. 15. This is attributed to the fact that high melting point also has high solidification temperature which, in turn, speeds up the solidification process. Fig. 16 shows the local liquid fraction distribution throughout each design. It is seen that type 1 has the larger solidified region at the top; whereas type 2 has the larger solidified area at the bottom of TES. Similarly, for horizontal designs type 3 has larger solidified area at the left region whilst type 4 yields the larger solidified area at the right region. On closer Fig. 17. Velocity vector for novel festoon design with multiple PCMs configuration for (a) type 1; (b) type 2; (c) type 3; and (d) type 4 during discharging at t ¼ 1000 s.

11 906 J.C. Kurnia et al. / Applied Thermal Engineering 50 (2013) 896e907 Fig. 18. Temperature contour for novel festoon design with multiple PCMs configuration for (a) type 1; (b) type 2; (c) type 3; and (d) type 4 during discharging at t ¼ 1000 s. inspection, however, the deviation of heat transfer between each design is not as significant as those during charging: the difference is up to 20% and around 5% compared to single PCM and other multiple PCMs design, respectively, at the first 1000 s and become smaller e even under-perform for some cases e thereafter. We also note that the development of natural convection velocity during discharging (Fig. 17) is much smaller, around two to three order-ofmagnitude, than those during charging which is consistent with previous finding. Fig. 18 shows temperature distribution for each case. It is observed that Type 1 yields the lowest temperature distribution which indicates faster discharging rate. Therefore, it can be deduced that the vertical placement of multiple PCMs with high melting point placed at the top has potential to be used to enhance heat transfer performance during discharging Effect of thermal cycling In practical application, the thermal energy storage may undergo thermal cycling of melting and freezing several times. Here, we simulate thermal cycling of chargingedischarging of novel festoon design with single and multiple PCMs. Fig. 19 shows the average liquid fraction of PCM during thermal cycling for every 400 s. It is seen that that the behavior of melting-solidification became cyclic steady-state after few cycles. Closer inspection reveals that the festoon design with multiple PCM performs slightly better especially during first cycle. As the required time for melting is faster than solidification, TES also has potential to be used as thermal capacitor in industrial application. 5. Concluding remarks In the present work, a numerical investigation is carried out to investigate and enhance the heat transfer performance in thermal energy storage (TES) system, viz. geometric modification and use of multiple layers of PCM. The results indicate that the novel festoon design provide highest heat transfer performance followed by U- tube with fins. It is found that U-tube with staggered fins perform better as compared to U-tube with in-line fins. Another finding is that the implementation of multiple PCMs could enhance heat transfer performance within TES. Different arrangement of PCMs significantly affects the heat transfer performance: placement of high melting point PCM at the inlet side during discharging improves heat transfer but it slightly underperform during charging. It has also been shown that TES has potential for thermal capacitor in industrial application to store excess/sudden surge of thermal energy as it has fast charging rate. Nomenclature Fig. 19. Liquid fraction average of cyclic melting and freezing of novel festoon design with single PCM [d] and multiple PCMs type 1 design. c p specific heat [J kg 1 K 1 ] d pipe diameter of pipe [m] g gravity [m s 2 ] h sensible heat [J kg 1 ] H total enthalpy [J kg 1 ] DH enthalpy of phase change [J kg 1 ] k thermal conductivity [W m 1 K 1 ] H mushy zone constant L length [m]

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