Thermal performance of a multiple PCM thermal storage unit for free

Thermal performance of a multiple PCM thermal storage unit for free coolingA.H.Mosaffa a ,b ,⇑,C.A.Infante Ferreira b ,F.Talati a ,M.A.Rosen caFaculty of Mechanical Engineering,University of Tabriz,IranbDelft University of Technology,Department Process &Energy,Delft,2628CA,Netherlands cFaculty of Engineering and Applied Science,University of Ontario Institute of Technology,Oshawa,ON,Canada L1H 7K4a r t i c l e i n f o Article history:Received 16July 2012Received in revised form 2October 2012Accepted 30October 2012Keywords:Thermal energy storage (TES)Phase change material (PCM)Latent heat thermal storage (LHTS)Free coolingEffective heat capacitya b s t r a c tAs demand for refrigeration and air conditioning increased during the last decade,the opportunities have expanded for using thermal energy storage (TES)systems in an economically advantageous manner in place of conventional cooling plants.Many cool storage systems use phase change materials (PCMs)and achieve peak load shifting in buildings.This work presents numerical investigations of the perfor-mance enhancement of a free cooling system using a TES unit employing multiple PCMs.The TES unit is composed of a number of rectangular channels for the flowing heat transfer fluid,separated by PCM ing the effective heat capacity method,the melting and solidification of the PCM is solved.The forced convective heat transfer inside the channels is analyzed by solving the energy equation,which is coupled with the heat conduction equation in the container wall.The effect of design parameters such as PCM slab length,thickness and fluid passage gap on the storage performance is also investigated using an energy based optimization.The results show that a system which can guarantee comfort conditions for the climate of Tabriz,Iran has an optimum COP of 7.0.This could be achieved by a combination of CaCl 2Á6H 2O with RT25with the optimum air channel thickness of 3.2mm,length of 1.3m and PCM slab thickness of 10mm.Ó2012Elsevier Ltd.All rights reserved.1.IntroductionElectrical energy consumption varies significantly during the day and night according to the demand by the industrial,commer-cial,residential sector and other activities.In hot and cold climate countries,a major part of the load variation is due to loads for air conditioning and space heating respectively.Cooling demand has been increasing due to the developing comfort expectations and technological developments around the world.Climate change has brought additional challenges for cooling systems designers.Significant economic benefits can be achieved by using thermal en-ergy storage (TES)for heating and cooling in residential and com-mercial buildings.These demands can be satisfied by smoothing the temporal variations with the help of latent heat thermal stor-age (LHTS)systems.LHTS in general,and phase change material (PCM)storage in particular,have been investigated for over 20years and are described in various references.Important LHTS applications and advances in LHTS materials and heat transfer have recently been reviewed [1–8].Due to the advantages offered by LHTS such as low temperature variation during melting and solidification cycles and high TES capacity,PCMs have been utilized in numerous applications including solar heating and cooling [9–11],conventional air condi-tioning [12–14],below-floor heating [15]and building envelopes [16–20].Saman et al.[21]employed a two-dimensional numerical mod-el based on an enthalpy formulation and analyzed the thermal per-formance of a flat thermal storage unit.The outlet air temperatures and heat transfer rates predicted by this model were compared with experimental data,and showed close agreement.Halawa et al.[22]developed a one-dimensional model to study the heat transfer in the thermal storage unit with the same arrangement as Saman et al.[21].They proposed a phase change processor algo-rithm to solve the phase change of the PCM in melting and solidi-fication processes and also to determine the liquid fraction of the PCM node.The model has been verified with experimental results with air as the heat transfer fluid (HTF).Due to the small thickness and the high length to thickness ratio of the slabs under investiga-tion,they used a one dimensional model for quick computation zaro et al.[23,24]presented an experimental set up for testing PCM–air real-scale heat exchangers.The results showed that a heat exchanger using a PCM with lower thermal conductiv-ity and lower total stored energy,but adequately designed,has higher cooling power and can be applied for free cooling.The free cooling potential of using PCMs in buildings in various climate con-ditions was studied by Takeda et al.[25]and Medved and Arkar0196-8904/$-see front matter Ó2012Elsevier Ltd.All rights reserved./10.1016/j.enconman.2012.10.018⇑Corresponding author at:Faculty of Mechanical Engineering,University ofTabriz,Iran.Tel.:+984113392498.E-mail address:mosaffa@tabrizu.ac.ir (A.H.Mosaffa).[26].Both studies indicated that the potential of free cooling mainly depends on the amplitude of the ambient air temperature variations.In spite of various advantages of PCMs,LHTS systems possess a major disadvantage:low melting and solidification rates owing to the low thermal conductivity of the PCM.To overcome this disad-vantage a number of studies have been carried out to enhance the performance of LHTS systems.The various techniques adopted for enhancing the thermal performance of the LHTS units include the dispersion of high conductivity particles in the PCM[27],using a high thermal conductivity porous matrix[28],the addition offins to the external surface of the HTF passage[29–31],and employing multiple families of PCMs in the unit[32,33].Multiple PCM units are those LHTS systems employing more than one PCM to store or release thermal energy.The heat transfer rate in a LHTS system and thus the performance of the system dur-ing melting and solidification mainly depend on the difference be-tween the HTF temperature and the PCM melting point.If a single PCM is used in the system,then this temperature difference would decrease in theflow direction of the HTF.This results in a decrease in heat transfer rate and thus poor performance of the system.If multiple PCMs with different melting temperatures are packed in the system in decreasing order of their melting points,then nearly a constant temperature difference can bemelting process,even though the HTFtheflow direction.This leads to an almostthe PCM.During solidification,if the HTFflowthe PCMs remain in increasing order of theiragain a nearly constant heatflux from the PCM tosible.Farid and Kanzawa[34]employed threemelting points in a LHTS.Air was used as theshowed about10%increment in heat transfercharging and discharging relative to a singleand Mujumdar[35]developed afinite elementconduction model for cyclic melting/solidificationphase change material slabs.Numericalstrated that the melting/solidification rates couldhanced by using composite PCMs with differentcompared with using a single PCM in a slab.The purpose of the present study is to performoptimization of free cooling systems usingunits.Recently an indirect evaporative cooler,point cooler,has been introduced by Esfandiari Nia[36].This heat exchanger is manufactured from plastic material plates.Air(the HTF)flows through a large number of small parallel channels. The idea is tofill the space between the air channels with the PCM material.This paper presents a two-dimensional numerical analysis for the system illustrated in Fig.1.The system consists of several layers of PCM slabs placed parallel to each other.Each slab contains multiple PCMs.By selecting the PCMs with appropri-ate melting points,the system can be used for free cooling applica-tions.The effective heat capacity method is used as the numerical model[37]and is applied to determine the effects of design param-eters on system performance.The theoretical analyses are based on thefinite element method.2.Mathematical formulationThe solidification and melting processes of the PCM and heat transfer in the HTF are unsteady two-dimensional problems for the system studied.Due to the negligible variation of the container wall temperature,temperature variations normal to theflow direc-tion are ignored[13,38].To develop a mathematical model,it is as-sumed that the effect of natural convection is negligible.AsNomenclatureA cross sectional area,m2a thickness of PCM slab,mb thickness of air channel,mc specific heat,J kgÀ1KÀ1D h hydraulic diameter,mh convective heat transfer coefficient,W mÀ2KÀ1 k thermal conductivity,W mÀ1KÀ1L latent heat of fusion,J kgÀ1l length of storage,mNu Nusselt number=hD h/k fp pressure,PaP total energy consumption,kW hq00heatflux,W mÀ2_Q heat rate,WRe Reynolds numberS location of solid–liquid interface,mT temperature,°Ct time,su velocity,m sÀ1_V volumetricflow rate,m3hÀ1_W power,Wx,y spatial coordinates,mGreek symbolsg efficiencyq density,kg mÀ3Subscripts1begin melting2end meltingb bulki initialf HTFl liquidm meltings solidw wallFig.1.Schematic diagram of the TES unit.2 A.H.Mosaffa et al./Energy Conversion and Management67(2013)1–7The governing energy equation for the PCM withboundary conditions can be written as@T ¼k q @2T 2þ@2T2 !;t >0T ðx ;y ;0Þ¼T i k@T ðx ;0;t Þ@y ¼Àk @T ðx ;a ;t Þ@y¼h ðT w ÀT f Þ@T ð0;y ;t Þ@x ¼ÀT ðl ;y ;t Þ@x¼0where T w and T f are wall and HTF temperatures energy balance for the solid–liquid interface with the tions is (see [40]):1þ@S @x 2"#k s @T s @y Àk l@T l @y¼q L @S@t at y ¼S ðx ;t Þð5ÞS ðx ;y ;0Þ¼0ð6Þwhere S is the location of the solid–liquid interface as a function oftime.The energy conservation equations and boundary conditions governing heat transfer for incompressible and laminar flow with no viscous dissipation are [41]:@T f þu @T f ¼k fq f f @2T f2!;t >0ð7ÞT f ðx ;y ;0Þ¼T ið8Þk f @T f ðx ;0;t Þ¼Àk f @T f ðx ;Àb ;t Þ¼h ðT w ÀT f Þð9ÞT f ð0;y ;t Þ¼T inletð10ÞThe local Nusselt number is defined asNu ðx Þ¼q 00D hk f ðT f ;b ÀT w Þð11Þwhere T f,b is the fluid bulk temperature defined asT f ;b ¼Ru ðy ÞT f ðx ;y Þdy Rð12ÞIn reality phase change usually takes place over a non-isother-mal temperature range.In such cases,the most commonly usednumerical method is the effective heat capacity method.The effec-tive heat capacity of the material is directly proportional to the en-ergy stored and released during the phase change and also to the specific heat capacity.Therefore,the heat capacity in heat Eq.(1)is defined as [37]:c ¼c s ;T <T 1L T 2ÀT 1þc s þc l2;T 16T 6T 2c l ;T >T 28><>:ð13Þwhere T 1is the temperature at which the melting begins and T 2is the temperature at which the PCM is totally molten.Fig.2shows a phase change process over a temperature interval and isothermal phase change.Curve (a)represents solidification or melting process over a temperature interval,where the slope of the curve is in-creased during the phase change.However,in theoretical studies the solidification or melting process appears at a constant temper-ature and is shown by the vertical curve (b).In the present method,it is assumed that the melting range is 1K.The power related to all friction losses can be calculated as fol-lows [42]:_W fan ¼ðD p total _V Þ=g fanð14ÞPressure losses are calculated using classical relations for flows in ducts and expansion and contraction cross section changes of ducts [42]:Frictional pressure loss :D p fric¼64D h l h q u 2ð15ÞExpansion cross section loss :D p ecs¼21ÀA s Lq u 2ð16ÞContraction cross section loss :D p ccs¼1:11ÀA s Lq u 2ð17Þwhere A s /A L is the ratio of the cross-sectional area of the smaller pipe to that of the larger pipe.Also,air filter and distribution equip-ment create an air-side pressure drop in the system.Therefore they affect the fan power.The total energy consumption of the system P fan ,is the sum of the energy consumption during daytime (melting)and energy consumption during night-time (solidification):P fan ¼ð_W fan Ât Þdaytime þð_W fan Ât Þnight-timeð18ÞThe cooling load of the system,_Qc ,can be calculated as follows:_Qc ¼Q PCM t 0¼R t 00_Q PCM ðt Þdt t 0ð19Þwhere Q PCM is the total heat which is absorbed by the PCM and t 0is the operating time of the system during daytime.The coefficient of performance (COP)of the system can be de-fined as:COP ¼Q PCM P fanð20Þ3.Results and discussion 3.1.PCM selectionFor free-cooling systems,PCMs have to be selected such that the cooled air temperature is within the range of human comfort.Appropriate selection of a PCM for a building application requires knowledge of the melting temperature relevant to the application.In building applications,PCMs with a phase change temperature of 18–30°C are preferred to meet the need for thermal comfort.Fur-Phase change process:(a)over a temperature interval and (b)isothermal.A.H.Mosaffa et al.thermore,the PCMs have to solidify at the outside temperature during the night.The PCMs considered in this investigation are cal-cium chloride hexahydrate,CaCl2Á6H2O,paraffin C18and RT25. Thermophysical properties of the PCMs are listed in Table1.Theo-retical analyses for the evaluation of the thermal performance system were carried out using COMSOL Multiphysics.TheThe validity of the present method is verified by comparing the temperature–time history of the PCM during the melting process at the center of a rectangular container with the experimental data presented by Zivkovic and Fujii[46](see Fig.3).The properties listed in Fig.3have been reported by the authors of[46].The PCM used in the experimental study is CaCl2Á6H2O.From the re-sults shown in Fig.3,it can be observed that the agreement between numerical and experimental data is well within experi-mental uncertainties(positioning of the thermocouple’s tip exactly in the center of the container is quite difficult and experimental data were read from charts,which reduces accuracy)[46].Further-more,the model validation was performed by comparison of theTable1Thermal properties of selected PCMs[6,43,44].Property CaCl2Á6H2O(PCM-1)Paraffin C18(PCM-2)RT25(PCM-3) Density,q(kg mÀ3)1530(liquid)774(liquid)749(liquid)1710(solid)814(solid)785(solid)À1À1parison of experimental temperature–time history of the PCM atcenter of a rectangular container with those obtained by present model.4.Inlet and outlet air temperature profiles of thermal storage unit for present model and experimental data(u=3.42m sÀ1).5.Effect of different composition of PCMs in the slabs on outlet temperature.PCMs1–3have been specified in Table1.6.Effect of the length and thickness of the PCM slabs on the outlet temperature from the storage.For the geometry consult Fig.1.4 A.H.Mosaffa et al./Energy Conversion and Management67(2013)1–7predicted exit air temperature values with the experimental mea-surements by Vakilaltojjar [47].Fig.4shows the inlet temperatureof the air flowing into the PCM (CaCl 2Á6H 2O)storage unit,the ac-tual outlet temperature from the experiment and the outlet tem-peratures calculated by the present model.The figure shows a good agreement between the present model and experimental data.However,the present model predicts slightly higher values in the initial period of melting and lower values in the later stage of melting.One possible reason for this is the impurity of the test PCM which degrades its heat transfer performance,and another may be the presence of air in the PCM solid before it melts [48].3.3.Multiple PCM thermal storage unitFor the first set of calculations,the storage system specifications shown in Table 2were used.These specifications correspond to the geometry considered by Esfandiari Nia [36].The inlet air tempera-ture and initial temperature of the PCM are 36°C and 25°C respec-tively (according to summer design condition of Tabriz).Fig.5shows the effect of composition of the PCMs in the slabs on outlet air temperature.In these cases,the volume of the PCMs in the slabs is equal.The results show that the system with composi-tion of PCMs (1+3),which are CaCl 2Á6H 2O and RT25,has the best performance.By using RT25the outlet air temperature can remain lower than 27°C for about 8h.In addition,when this composition is used,because of the larger amount of CaCl 2Á6H 2O,the heat ab-sorbed by the PCMs is high.Therefore,for further optimizations,the system with composition of PCMs (1+3)has been used.Fig.6shows the effect of the length and thickness of the PCM slabs on the outlet air temperature.Initially,due to the large tem-perature difference between the air and the PCMs,the outlet air temperature approaches the initial temperature of the PCM and rises sharply shortly afterward.When the surface temperatureofFig.7.Effect of the length and thickness of the PCM slabs on the heat transfer rateduring the solidification process (T i =36°C,T inlet =25°C and _V¼2250m 3h À1).Table 3COP for the cases in Figs.6and 7(operating time during day is 8h).Case l (m)a (mm)PCM mass (kg)Solidification time (h)Absorbed heat by PCMs (MJ)P fan (kW h)_Q c (kW)COP 1 1.28484.69.44111.46 3.84 3.878.062 1.38525.09.72115.49 4.02 4.017.973 1.48557.310.11116.64 4.22 4.057.684 1.29545.210.53117.22 4.17 4.077.805 1.39590.611.03120.10 4.42 4.177.546 1.49627.011.94121.54 4.80 4.227.027 1.210605.711.42119.52 4.45 4.157.468 1.310656.212.51121.25 4.89 4.21 6.8891.410696.613.17122.405.194.256.55(a)(b)(c)(d)) t =2) t =4) t =6) t =82 hr4 hr6 hr 8 hr rr8.Variation of wall,air and PCM temperature along the centerline of the through the thermal storage system (l =1.3m and a =10mm).and Management 67(2013)1–75the PCMs reaches the melting point,the outlet air temperature rises gradually because a large amount of the heat is still used to melt the PCMs.The outlet air temperature suitable for cooling is limited by the necessity to achieve thermal comfort.It is seen that larger slab lengths and thicknesses result in lower outlet air tem-peratures.These systems contain a large quantity of PCMs to ab-sorb the heat.Fig.7shows the effect of the length and thickness of the PCM slabs on heat transfer rate during solidification.Because of the low-er thermal conductivity of the liquid PCMs,the solidification pro-cess takes place more slowly than the melting process.So,to solidify PCMs completely within a reasonable period,the airflow rate must be increased.The air velocity in the channels during fan,P fan,the operating time during daytime is taken to be8h and during night-time is equal to total solidification time.In all cal-culations,the efficiency of the fan and the total pressure drop for airfilters and distribution equipment have been assumed60% and180Pa respectively[42].It is clear that the cooling capacity of the larger systems is greater than of the smaller ones.However, increasing the length and PCM slab thickness causes an additional pressure drop and larger operation time during the solidification period.The results show that the effect of increasing the fan energy requirement,P fan,is larger than that of the increasing of absorbed heat,Q PCM.Therefore,larger systems have lower COPs.The variation of air,wall and PCM temperature along the cen-terline of the slab with time and distance from the entrance for9.Variation of local Nusselt number with time along theflow direction 1.3m,a=10mm,b=3.6mm and number of plates=80).Fig.10.Effect of thickness of air channels on:(a)cooling load and(b)energy consumption(l=1.3m,a=10mm and number of plates=80).11.Effect of thickness of air channels on COP(l=1.3m,a=10mm and number plates=80).6 A.H.Mosaffa et al./Energy Conversion and Management67(2013)1–74.ConclusionsA computer model has been developed for the evaluation of a multiple PCM LHTS unit which is composed of a number of rectan-gular channels for the air,separated by PCM slabs.The perfor-mance of a LHTS unit employing multiple PCMs is studied numerically using the effective heat capacity method.The model employed in this study has been validated using existing experi-mental data and the comparison is satisfactory.The results show 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