Performance analysis of thermal storage unit with possible nano enhanced phase change material in building cooling applications

Solomon G Ravikumar，Ravikumar T S，Raj V Antony Aroul，Velraj R

（1 Department of Mechanical Engineering，Hindustan Institute of Technology and Science，Padur，Kelambakkam，Chennai 603103，India；2 Department of Mechanical Engineering，Easwari Engineering College，Ramapuram，Chennai 600089，India；3 Institute for Energy Studies，Anna University，Chennai 600025，India）

1 Introduction

The thermal energy storage system employed in building applications, and refrigeration and air conditioning units, acts as a thermal flywheel to storethe surplus energy when the demand is less, and to deliver the same at other times.This reduces the peak demand, with the advantages of downsizing the capacity of the units, and also operating the same at optimum efficiency.In recent years thermal energy storage systems have received greater research attention,for several applications.Among the various storage systems, the latent heat thermal energy storage systems (LHTES) with phase change materials have dominated the sensible heat storage systems, due to their large storage capacity and nearisothermal charging/discharging behaviour.During phase change, in the LHTES, the solid liquid interface moves away from the heat transfer surface.In this process, the surface heat flux decreases with respect to time, due to the increasing thermal resistance of the growing layer of the molten/solidified medium, as the thermal conductivity of the solidified phase change material (PCM) is abnormally low.In the case of solidification, conduction is the sole transport mechanism, and in the case of melting, natural convection occurs in the melt layer, and this generally increases the heat transfer rate as compared to the solidification process.

The study carried out by Shamsundar and Sparrow [1] inferred that the super cooling of the PCM during solidification fritters away the advantages of isothermal operation.For most of the available PCMs,the Biot number (Bi= hR/k) becomes larger as the thermal conductivity is very low, and the surface temperature of the PCM drops within a short period after solidification is started, and this results in a very low heat flux thereafter.Therefore, a major portion ofthe heat is extracted at a very low temperature difference.Lowering the Biot number value can be achieved, either by decreasing the radius of the PCM storage tube, or by increasing the effective thermal conductivity of the PCM.As reducing the storage tube radius makes it uneconomical, the use of proper heat transfer enhancement techniques in LHTES systems becomes necessary.

There are various techniques available to improve the thermal performance of the LHTES, such as the use of fin configurations, introduction of a metal matrix, graphite compounded material, the addition of high conductivity particles and nano particles, etc.Velraj and Seeniraj [2] reported that the internal fin configuration gives the maximum benefit of the fin to the PCM, farther away from the convectively cooled surface.Padmanabhan and Krishna Murthy [3] studied the phase change process occurring in a cylindrical annulus, in which rectangular, uniformly spaced axial fins spanning the annulus are attached to the inner isothermal tube,while the outer tube is made adiabatic.Velraj et al.[4]investigated three different heat transfer enhancement techniques, and compared the total solidification time and the total quantity of heat stored for the three configurations analysed.They reported that comparing the volume occupied by the fins and the lessing rings,the latter occupy more volume, without a proportionate reduction in time for complete solidification.Rajesh Baby and Balaji [5] studied the results of an experimental investigation of the performance of finned heat sinks filled with PCM, for thermal management of portable electronic devices.

Mehling et al.[6] and Py et al.[7] proposed a graphite compound material, where the PCM is embedded inside a graphite matrix.The main advantages of such a material is the increased heat conductivity in the PCM without much of reduction in the energy storage, also the other advantages include a decrease in the sub cooling of the salt hydrates and a decrease in the volume change of the paraffins.This technique is being employed in building material applications.The other methods were to embed the PCM in a metal matrix structure suggested by Tong et al.[8] and the use of thin aluminium plates filled with a PCM, as developed by Bauer et al.[9].Experiments were performed by Cabeza et al.[10] in a small thermal energy storage device to studythe heat transfer improvement inthe PCM (water/ice) with three different heat transfer enhancement methods.These were the addition of stainless steel pieces, copper pieces, and a graphite matrix impregnated withthe PCM.The use of graphite composite allows an even larger increase inthe heat transfer than with copper.The heat flux is about four times larger on heating and three times larger on cooling as compared to using pure ice.

Kumaresan et al.[11] observed that the nano fluid phase change material (NFPCM) dispersed with 0.6%(by volume) multi-walled carbon nano tube enhances the heat transfer, with 33.64% reduction in solidification time.An enhancement in the thermal conductivity of 30%～45% was achieved both in the liquid and solid states of the NFPCM compared to that of pure PCM.Increasing the concentration of the MWCNT had steadily increased the thermal conductivity of the NFPCM, upto a volume fraction of 0.6%.Chandrasekar et al.[12] reported that there is an enhancement inthe heat transfer by reduction inthe solidification time of 45%, when iron-water nano fluid is used asthe PCM, compared tothewater PCM.Further, it is inferred that the addition of nanoparticles,has much influence in enhancing the heat transfer in the passive way than increasing the temperature difference between the HTF andthe freezing temperature of the PCM.In the study carried out by Zhang et al.[13], multiwall carbon nano-tube(MWCNT) particles were dispersed in an organic liquid (n-hexadecane), and reported that with the addition of 0.1% （by mass）MWCNT, the super cooling of hexadecane can decrease by 43%, which produced the most significant effect among the test samples.It is also interesting to note that there was an effective concentration range of nanoparticles for super cooling reduction, and better results cannot be obtained by continuously increasing the nanoparticle concentration.Kalaiselvam et al.[14] comparedthe reduction the solidification time of various PCMs embedded with alumina and aluminium nanoparticles to the pure PCM and showed that the solidification time for the 60% n-tetradecane: 40% n-hexadecane PCM dispersed with the aluminium and alumina nanoparticles were expected to reduce by 12.97% and 4.97% than at its pure form respectively.Besides,increasing the mass fraction of the nanoparticles beyond the limiting value of 0.07, the rate of solidification was not significant further.

In recent years the phenomenal enhancement in thermal conductivity and other properties has been achieved with the addition of nano particles in the PCM, referred to asthe nano enhanced PCM(NEPCM). Considering the possibilities of enhancement in the thermal conductivity of the NEPCM, the increase in the heat transfer achievable under various conditions of the heat transfer fluid(HTF), will be very useful for researchers and engineers to design energy efficient storage systems for various applications.Hence, in the present work,initially, the solidification time obtained, by using the scheme(1), one dimensional outward cylindrical solidification equation [15], which was fundamentally derived from the electrical analogy principle is validated with the experimental results.

The validated equation was further extended to study the solidificationtime for various conventional PCMs and NEPCMs, with practical ranges of the convective heat transfer coefficient of flowing fluids.

2 Experimental investigation

2.1 Experimental setup

The experimental setup consisted of a test section,and the HTF flow control section, that regulates the flow through the inner tube of the test section.The sectional view of the experimental setup is shown in fig.1 (a).The test section was a double pipe annular heat exchanger, comprising an inner copper tube with an OD of 75 mm with 1 mm thickness, and 300 mm height, and an outer acrylic tube of OD of 150 mm with 5 mm thickness, and 280 mm height.The PCM(RT21), which was in the liquid state at room temperature, was filled to a height of 245 mm in annular space.The phase change material in which the heat energy stored was paraffin, commercially known as RT21, obtained from Rubitherm, Germany.It is a chemically inert and stable, environmentally harmless,non-toxic, and organic compound.The thermo-physical properties of the PCM are given in table 1.The liquid PCM does not require any clearance volume in annular space, as the volume contracts about 14%during solidification.

Table 1 Thermo-physical properties of RT21(Manufacturer’ data)

The bottom side of the inner tube was externally threaded, and assembled with an acrylic bottom face plate of 12.7 mm thickness, which is internally threaded.The outer acrylic tube was also fixed on the bottom face plate, in a ring groove of 5 mm width.A synthetic rubber solution was applied on the groove to ensure proper sealing.The PCM filled outer acrylic container was covered with an acrylic lid of 75 mm ID,140 mm OD and 12.7 mm thickness.Three layers of 3 mm thickness thermorex insulation were tightly wrapped on the outer acrylic tube to provide perfect insulation.The concentric cylinders were fastened by 4 bolts and nuts.

The HTF flow control section consisted of an entry length tube, a conical diffuser and a flow straightening section.The entry length tube made of copper, identical in dimensions to the inner tube, was attached at the bottom of the face plate.The conical GI diffuser that had a major diameter of 300 mm and a minor diameter of 75 mm was fixed at the bottom of the entry length tube.At the bottom of the conical GI diffuser, the flow straightener that had 400 holes, each of 3 mm diameter, and 150 mm height, was fixed.The above arrangement was well supported on a 50 mm thick wooden platform.

The cool air supply was maintained by a variable speed axial flow fan to suit the cooling requirements.The inlet velocity of air through the inner copper tube was measured by the vane-type anemometer.The accuracy of the anemometer was ± 2% of the reading,and it had a resolution of 0.1 m/sec.At the start of each experiment, the required inlet velocity of the HTF was obtained by applying the set voltage, which is measured using the anemometer and voltmeter, and this velocity was maintained throughout the experimentation.The entire experimental setup was placed in a 7 kW climatic stimulator that controls the space temperature up to a range of ±1℃ of the desired temperature, through a temperature controller unit.

Fig.1 Experimental setup (a) sectional view (b) cross sectional view of test section (c) thermocouple location at one axial position

The temperatures of the PCM at 3 radial, one angular and 4 axial position, and the temperatures of the HTF at the inlet and the outlet, totalling 16 locations, were measured with J type thermocouples.The crosssectional view of the test section and the thermocouple locations at one of the axial positions,are shown in fig.1 (b) and fig.1(c) respectively.The thermocouples at one axial location were fixed using a 6 mm thick acrylic plate of 45° sector, and the other three similar acrylic plates were located at three different axial locations at alternate angular positions,considering symmetry.The HP-Agilent 34970A, data acquisition system (DAS) was used to record the temperatures, at specified time intervals throughout the experiment.The DAS had an accuracy of ± 0.05%in the reading, and a resolution of 0.01℃.

2.2 Experimental procedure

During the start of each experimental trial, the ambient air is circulated through the test section until all the thermocouples located in the PCM regions attain the same temperature, whereby the thermal equilibrium of the PCM is ensured.The climatic stimulator is switched on, to reduce the space temperature to the required level.Then the blower is switched on, and the flow rate is obtained by applying the pre-determined set voltage, which is maintained at a constant level throughout the experiment.The temperature measurements at all the thermocouple locations are continuously monitored using the DAS.The experiments are conducted at two HTF inlet temperatures of 12 ℃, 14 ℃ and two HTF inlet velocities of 3 m/s, 6 m/s.The surface heat flux is varied with the combination of the HTF inlet temperature and its velocity.A higher inlet temperature and lower inlet velocity is considered as low heat flux (LHF), while lower inlet temperature and higher inlet velocity is considered as high heat flux (HHF), among the tested parameters.

3 Results and discussions

3.1 Validation of the theoretical equation

The results of the experimental investigation were initially compared with the results obtained from the one dimensional outward cylindricalsolidification equation, neglecting the effects of free convection and other solidification dynamics.In the experiment the temperature measurements were made at four axial heights.Considering the minimum free convection and other end effects at axial location C, the solidification time obtained at three different radial positions, at this height, were used to validate the results obtained from the equation.

Fig.2 shows the time for solidification obtained experimentally and theoretically at three radial positions of 45 mm, 52.5 mm and 60 mm radius,which were at 7.5 mm, 15 mm, and 22.5 mm respectively from the heat transfer surface in the annular gap of 32.5 mm.These positions were normalized with respect to the annular gap that corresponds to 0.23, 0.46 and 0.69 respectively.Fig.2(a) and 2(b) are drawn for the cases of HHF and LHF respectively.It is seen from both the figures that the time for solidification obtained from the theoretical equation were in close agreement, with the experimental results for the first two radial positions,and the theoretical results over predicts the solidification time, at the farthest radial position.The free convection which prevailed during the initial sensible cooling of the liquid PCM decreased the time for solidification atthe farthest radial position, as it was in the liquid state for longer duration and this free convection effect was not considered in the theoretical equation and hence the actual time for solidification was lesser than the theoretically evaluated time, at the furthest radial position from the heat transfer surface.

Fig.2 Comparison of experimental and theoretical time for solidification at different radial positions, under (a)HHF conditions (b) LHF conditions

Since the equation considered shows good agreement with the experimental results in the absence of free convection, this equation was further used to analyze the solidification performance for various practical ranges of thermal conductivity of the PCM and the flow conditions of the HTF, which are primarily influencing the solidification behaviour of the PCM.

3.2 Parametric studies

In general the thermal conductivity of the PCMs is very low and the recent advancement in nano technology encourages researchers to explore the possibilities of using the NEPCM, for various applications. Hence the study on the heat transfer enhancement potential using the possible NEPCM will be of very useful, for engineers to select the PCM for various applications.Further the heat transfer coefficient possible with certain gaseous HTF, may not help in increasing the heat transfer even with the higher thermal conductivity achieved with the NEPCM. Hence, a parametric study is carried out for the combination ofthe practical ranges of the thermal conductivity, possible with paraffin and waterthe PCM with and withoutthe addition of nano particles, andthe heat transfer coefficient possible under different fluid.Kumaresan et al.[11] already reported that there is a possibility of increase of thermal conductivity of the paraffinic PCM in the range of 45% and Chandrasekaret al.[12] reported that there is an enhancement inthe heat transfer by reduction inthe solidification time of 45% by the addition of ferrous nano particles in water PCM.

The results of the parametric studies given in the present investigation are valid, when there is no free convection during sensible cooling of the PCM.The effect of free convection is normally insignificant,when the TES systems are effectively designed,considering only the latent energy for storage and retrieval.In addition, the sub cooling may also have an effect, on the heat transfer during solidification.Further, there is a misunderstanding among the heat transfer scientists about the enhancement in heat transfer of NEPCM, as the nano particles are not having any physical contact with the heat transfer surfaces.However it is evident from the literature that the improvement in effective thermal conductivity of the NEPCM augments the heat transfer during solidification process considerably.Since, the mechanism of heat transfer with NEPCM is on the evolving stage, there may be difference in enhancement with the combination of base material and nano particles, irrespective of effective thermal conductivity evaluated from the rule of mixing ratio.Under such circumstances, the enhancement in heat transfer could only be predicted with experimental investigation.

3.2.1 Effect of thermal conductivity with gaseous HTF

Fig.3 shows the time for solidification at various radial locations, by varying the thermal conductivity of the PCMs, and NEPCMs, for the range of heat transfer coefficient possible with air as HTF.Fig.3(a),3(b), 3(c) and 3(d) are shown for the heat transfer coefficients of 10, 20, 30 and 100 W/ (m2·℃ )respectively, which are normally possible with air as HTF, under different air velocities and for various configuration with turbulators.It is seen from fig.3(a)that the increase in thermal conductivity from 0.2 to 0.3 W/(m·℃ ), decreased the time required for solidification, from 4.5% at the normalized radial position of 0.2, to 12.2% at the normalized radial position of 0.8, when the heat transfer coefficient is maintained at 10 W/(m2·℃).Further, the increase in the thermal conductivity did not show the similar enhancement in reducing the time for solidification at all radial positions, and beyond the thermal conductivity of 2.25 W/(m·℃), there was absolutely no effect in reducing the time for solidification.It is inferred from the above results that the internal conductive resistance offered by the PCM is much lesser, compared to the surface convective resistance,with heat transfer coefficient of 10 W/(m2·℃), when the thermal conductivity exceeds 2.25 W/(m·℃).It is seen from fig.3 (b), 3(c) and 3(d) that the increase in heat transfer coefficient proportionately increased the percentage reduction in solidification time, as the thermal conductivity of PCM increased.Table 1 shows the percentage reduction in solidification time at various radial positions, for all possible heat transfer coefficients considered, with air as HTF, when the thermal conductivity increased from 0.2 to 0.3 W/ (m·℃).

Fig.3 Solidification time at various radial locations, for different thermal conductivity of PCMs and NEPCMs under heat transfer coefficient of (a) 10 W/(m2·℃), (b) 20 W/(m2·℃), (c) 30 W/(m2·℃), (d) 100 W/(m2·℃)

Table 1 Percentage reduction in solidification time for the increase in thermal conductivity from 0.2 to 0.3 with air HTF

It is seen from the table 1 that the percentage reduction in solidification time, increased as the radial distance from the heat transfer surface increased.This was due to the percentage variation in the internal conductive resistance was appreciable with respect to the radius, where magnitude of the constant, surface convective resistance is of equal order.Though, the maximum percentage enhancement in thermal conductivity was 50%, as it increased from 0.2 to 0.3 W/(m·℃), which was normally possible with NEPCM,the maximum percentage reduction in solidification time was only 28%.

3.2.2 Effect of thermal conductivity with liquid HTF

Fig.4 shows the time for solidification at various radial locations, by varying the thermal conductivity of the PCMs, and NEPCMs, for the range of heat transfer coefficient possible with liquid HTF.Fig.4(a),4(b), 4(c) and 4(d) are shown for the heat transfer coefficients of 200, 350, 500 and 5000 W/(m2·℃)respectively, which are normally possible with liquid fluid as HTF.It is seen from the figures that the increase in thermal conductivity value from 0.2 to 0.3 W/(m·℃), had significant effect at all heat transfer coefficients and reduces the solidification time from 20% to 33%, with higher percentage reduction at the farthest distance from the heat transfer surface.

Fig.4 Solidification time at various radial locations, for different thermal conductivity of PCMs and NEPCMs under heat transfer coefficient of (a) 200 W/ (m2·℃), (b) 350 W/ (m2·℃), (c) 500 W/ (m2·℃), (d) 5000 W/ (m2·℃)

Table 2 shows the percentage reduction in solidification time at various radial positions, for all possible heat transfer coefficients considered, with liquid fluid as HTF, when the thermal conductivity increased from 0.2 to 0.3 W/(m·℃).It is observed from tables 2 that, at higher heat transfer coefficients the percentage reduction in solidification time was uniform at all radial positions.This is due to the negligible surface convective resistance, and very high internal conductive resistance experienced by the PCM nearer to the heat transfer surface. Hence the internal conductive resistance due to the increase in radius was not showing any further observable variation in solidification time, along the radius.

Table 2 Percentage reduction in solidification time for the increase in thermal conductivity from 0.2 to 0.3 with water as HTF

3.2.3 Effect of surface heat transfer coefficient for PCMs of different thermal conductivity

Fig.5 shows the time for solidification at various radial positions by varying the surface heat transfer coefficient, possible with air as HTF, for the range of thermal conductivity of the PCMs and NEPCMs.Fig.5(a), 5(b), 5(c), 5(d), and 5(e) are drawn for the thermal conductivity of the PCMs of 0.2, 0.3, 0.5, 2.25 and 4 W/(m·℃) respectively.

Fig.5 Solidification time at various radial locations, under different heat transfer coefficients possible with air as HTF, for thermal conductivity of (a) 0.2 W/ (m·℃), (b) 0.3 W/ (m·℃),(c) 0.5 W/ (m·℃), (d) 2.25 W/ (m·℃), (e) 4 W/ (m·℃)

Table 3 Percentage reduction in solidification time for the increase in heat transfer coefficient from 10 to 100 for PCMs of different thermal conductivity with air HTF

It is seen from the fig.5 (a) that the increase in heat transfer coefficient reduced the time for solidification appreciably at all heat transfer coefficients, when the thermal conductivity of the PCM was of 0.2 W/(m·℃).Similar trends were seen for all thermal conductivity, which is evident from the fig.5(b), 5(c), 5(d), and 5(e).Table 3 shows the percentage reduction in solidification time at various radial positions, for all thermal conductivity of the PCM considered, when the heat transfer coefficient increased from 10 to 100 W/(m2·℃).It was observed from the table 3 that the percentage reduction in the solidification time has a negative trend as the radial distance from the heat transfer surface increases.This is because, the increase in heat transfer coefficient enhances the heat transfer of the PCM near to the heat transfer surface, while the PCM at the farthest distance was more influenced by the lower thermal conductivity of the PCM.However, at higher thermal conductivity of the PCM, this variation was not appreciable, as it is seen from table 3 for the thermal conductivity of 4 W/(m·℃).

Fig.6 shows the time for solidification at various radial positions, by varying the surface heat transfer coefficient, possible with liquid fluid as HTF, for the range of thermal conductivity of the PCMs and NEPCMs.Fig.6(a), 6(b), 6(c), 6(d), and 6(e) are drawn for the thermal conductivity of the PCMs of 0.2,0.3, 0.5, 2.25 and 4 W/(m·℃) respectively.It is seen from fig.6(a) that the increase in heat transfer coefficient had not much influence in reducing the time for solidification, when the thermal conductivity of the PCM is 0.2 W/(m·℃). However, the reduction in time for solidification, for the increase in heat transfer coefficient was observable as the thermal conductivity of the PCM increased from 0.3 W/(m·℃) to 4 W/(m·℃), as seen in fig.6(b) to fig.6(e) respectively.This increasing effect is highest, when the thermal conductivity of the PCM was 4 W/(m·℃). It is seen from fig.6(e) that the increase in heat transfer coefficient from 200 W/(m2·℃) to 350 W/(m2·℃),decreases the time required for solidification from 37% at the normalized radial position of 0.2, to 27% at the normalized radial position of 0.8, when the thermal conductivity is 4 W/(m·℃).This decreasing trend, in the percentage reduction of solidification time, with respect to the radial position was in similar pattern, observed with air as HTF as already explained,using the table 3.Further, it is observed from the figure that the increase in heat transfer coefficient from 200 W/(m2·℃) to 5000 W/(m2·℃), decreased the time required for solidification from 83% at the normalized radial position of 0.2, to 60.8% at the normalized radial position of 0.8.

Fig.6 Solidification time at various radial locations, under different heat transfer coefficients possible with liquid fluid as HTF, for thermal conductivity of (a) 0.2 W/(m·℃),(b) 0.3 W/(m·℃), (c) 0.5 W/(m·℃), (d) 2.25 W/(m·℃),(e) 4 W/(m·℃)

3.2.4 Efficient heat transfer conditions for paraffinic PCM

Fig.7 shows the time for solidification for the paraffinic PCMs, which are known for their abnormally low thermal conductivity, and with the addition of nano particles / nano tubes that enhances the effective thermal conductivity to an appreciable extend, for the practical ranges of heat transfer coefficients.Normally, the paraffinic PCMs have thermal conductivity in the range of 0.2W/(m·℃) and the recent development in the nano technology makes it possible, to enhance the thermal conductivity in the range of 0.3W/(m·℃).The values of 10 W/(m2·℃)and 30 W/(m2·℃) are practically possible values of heat transfer coefficients, with lower and higher velocities of air respectively.Further, it is possible to increase the heat transfer coefficient of air to 100 W/(m2·℃), when turbulators are used, to break the thermal boundary layer in the flow passage.The higher value of heat transfer coefficient above 200 W/(m2·℃) was possible, with liquid as HTF.

Fig.7 Solidification time for paraffinic PCM and NEPCMs, under possible heat transfer coefficients with gaseous fluid as HTF

It is seen from fig.7 that the percentage reduction in solidification time, by comparing the enhancement in thermal conductivity from 0.2W/(m·℃) to 0.3W/(m·℃) and the enhancement in heat transfer coefficient from 10 W/(m2·℃) to 30 W/(m2·℃), the percentage reduction in solidification time is appreciable, for the enhancement in theheat transfer coefficient than the enhancement in thermal conductivity.However, the enhancement in thermal conductivity has its influence, at the higher heat transfer coefficient of 100 W/(m2·℃).Similarly, when the liquid is used as the HTF, the heat transfer coefficient with the flowing fluid will be in the range of 5000 W/(m2·℃).Under such circumstances, every increase in thermal conductivity had its proportionate influence in enhancing the heat transfer.

3.2.5 Efficient heat transfer conditions for water PCM

Fig.8 Solidification time for water based PCM and NEPCMs, under possible heat transfer coefficients with liquid fluid as HTF

Fig.8 shows the time for solidification for the water PCM, and nano enhanced water PCM, for the practical range of heat transfer coefficients, possible with liquid fluid.A range of values of heat transfer coefficient of 200 W/(m2·℃) to 500 W/(m2·℃), is possible with different combinations of water and ethylene glycol as HTF, under stationary conditions and range of 5000 W/(m2·℃) is possible, with above said HTF, under flowing conditions.The water PCM and nano enhanced water PCM have thermal conductivity in the range of 2.25 W/(m·℃) to 4 W/(m·℃), at the solid state.It is seen from fig.8 that the increase in both thermal conductivity and heat transfer coefficient has appreciable effect, in reducing the time for solidification.This is due to the equal influence of both internal conductive resistance and surface convective resistance.

4 Conclusion

The theoretical equation, used to determine the time for outward cylindrical solidification of the PCM,shows good agreement with the experimental results in the absence of free convection and hence this equation is used to analyse the solidification performance under various parametric conditions.

The following conclusions are arrived, based on the parametric studies carried out, on the thermal conductivity of paraffinic and water based materials dispersedwith nano particles / nano tubes and the heat transfer coefficient possible, under various flow conditions.

（1）During solidification of paraffinic PCM under air as HTF, the heat transfer enhancement achieved by increasing the heat transfer coefficient,was much greater thanthe heattransfer enhancement achieved by, increasing the thermal conductivity with addition of nano particles / nano tubes.However,when the heat transfer coefficient of the flowing HTF was high, the increase of thermal conductivity with the addition of nano particles / nano tubes had a visible effect in reducing the time for solidification and the enhancement in heat transfer.

（2）During solidification of water based PCMs,the increase in thermal conductivity with the addition of nano particles / nano tubes had considerable effect in enhancing the heat transfer.Also it was observed that, for a given thermal conductivity of the PCM,when the heat transfer coefficient of the flowing HTF is increased, an appreciable enhancement in heat transfer was achieved.The enhancement in heat transfer achieved by, both the increase in thermal conductivity and heat transfer coefficient, infers that the internal conductive resistance and the surface convective resistance are of same magnitude.

（3）It is concluded from the parametric studies that for paraffinic PCM with air as HTF, the first attempt should be made to increase the heat transfer coefficient to the maximum extent, before making any attempt to increase the thermal conductivity of the PCM, with the addition of nano particles.When water is used as the PCM, with the addition of nano particles is recommended to achieve better heat transfer, when liquid fluid is used as HTF.

[1] Shamsundar N，Sparrow E M.Storage of thermal energy by solid-liquid phase change temperature drop and heat flux.ASME Trans.J.Heat Transfer，1974，96：541-543.

[2] Velraj R，Seeniraj R V.Heat transfer studies during solidification of PCM inside an internally finned tube.ASME Journal of Heat Transfer，1999，121：493-497.

[3] Padmanabhan P V，Murthy M V Krishna.Outward phase change in a cylindrical annulus with axial fins on the inner tube.International Journal of Heat and Mass Transfer，1986，29：1855-1868.

[4] Velraj R，Seeniraj R V，Hafner B，F(xiàn)aber C，Schwarzer K.Heat transfer enhancement in a latent heat storage system.Solar Energy，1999，65(3)：171-180.

[5] Rajesh Baby，Balaji C.Experimental investigations on phase change material based finned heat sinks for electronic equipment cooling.International Journal of Heat and Mass Transfer，2012，55：1642-1649.

[6] Mehling H，Hiebler S，Ziegler F.Latent heat storage using a PCM-graphite composite material.in: Proceedings of Terrastock 2000-8th International Conference on Thermal Energy Storage，Stuttgart，Germany，2000：375-380.

[7] Py X，Olives R，Mauran S.Paraffin/porous-graphite-matrix composite as a high and constant power thermal storage material.Int.J.Heat Mass Transfer，2001，44：2727-2737.

[8] Tong X ，Khan J A ，Amin M R.Enhancement of heat transfer by inserting a metal matrix into a phase change material. Numer Heat Transfer：Part A，1996，30：125-141.

[9] Bauer C A ，Wirtz R A.Thermal characteristics of a compact，passive thermal energy storage device.in: Proceedings of the 2000 ASME IMECE，Orlando，F(xiàn)lorida，USA，2000.

[10] Cabeza L F，Mehling H ，Hiebler S，Ziegler F.Heat transfer enhancement in water when used as PCM in thermal energy storage.Applied Thermal Engineering，2002，22：1141-1151.

[11] Kumaresan Vellaisamy，Velraj Ramalingam，Das Sarit K.The effect of carbon nanotubes in enhancing the thermal transport properties of PCM during solidification.Heat Mass Transfer，2012，48 (8)：1345-1355.

[12] Chandrasekaran P，Kumaresan V，Cheralathan M，Velraj R.Effective method of using PCM encapsulated spherical container for short duration charging and discharging applications.The 12th International Conference on Energy Storage，Innostock，2012

[13] Zhang Shuo，Wu Jian Yong，Tse Chi Tat ，Niu Jianlei.Effective addition of multi-wall carbon nano-tubes in hexadecane through physiochemical modification and decrease of super cooling.Solar Energy Materials & Solar Cells，2012，96：124-130.

[14] Kalaiselvam S，ParameshwaranR，Harikrishnan S.Analytical and experimental investigations of nanoparticles embedded phase change materials for cooling application in modern buildings.Renewable Energy，2012，39：375-387.

[15] Lunardini V J.Heat transfer in cold climates.Van Nostrand Reinhold Co.，1981.