1、 2016 ASHRAE 215ABSTRACTEnvironmental concerns and limited energy supply todaymakeenergystorageveryimportant,especiallyinsolarenergyutilization. The latent heat storage method has the advantageof storing a large amount of energy in a relatively smallvolume. Achieving thermal energy storage with late
2、nt heatapplication using phase-change materials (PCMs) involvestheheatoffusionatthesolid-liquidphasetransition.Theprob-lem with todays PCMs is that their very low thermal conduc-tivityvaluesseverelylimittheirenergystoragecapability.Thisalso makes the melting and solidification times too long formeet
3、ingthedesiredresults.Investigationstosolvethisprobleminclude improved design configurations and addition ofnanoparticlestothePCMtoenhancethethermalconductivity.This study is on the effects of nanoparticle dispersion inthe melting of a PCM in a triplex-tube heat exchanger heatedunder constant surface
4、-temperature conditions. The govern-ing equations for the configuration and process were discret-ized via the finite volume method and solved numerically. Thedevelopedmodel,whichwasvalidated,showsgoodagreementwhen compared to a previous related experimental study. Thecomputations were performed for
5、nanoparticle volume frac-tions ranging from 1% to 3%. The results, shown in the formof isotherms and contours of the solid-liquid interface overdifferent periods of charging time, are presented anddiscussed.Theresultsshowanenhancementinthemeltingratewith doping nanoparticles of different volumetric
6、concentra-tions. The results also show melting time savings of 17% as aresult of adding nanoparticles to the PCM for a nanoparticlevolumefractionof1%.Higher-volumefractionswerefoundtonot result in significant melting time savings for the process inthe triplex-tube heat exchanger.INTRODUCTIONHigh ove
7、rall cost is one of the major obstacles in thepromotion of renewable energy technologies in energymarkets. Energy storage technology is one path to increasethe efficiency and reduce the cost of all renewable energytechnologies. In solar energy utilization, thermal energystorage (TES) is important du
8、e to its high potential tocorrect the intermittent nature of solar energy. TES is basi-cally the temporary holding of energy for later use (Dincerand Rosen 2011). Storing thermal energy in TES devicescan be done by one of three methods: a) sensible storage bycausing a material to increase or decreas
9、e in temperature,b) latent storage by phase change from solid to liquid orfrom liquid to vapor, and c) storage by reversible chemicalreactions.Latentstoragematerialsarecommonlyknownasphase-change materials (PCMs) and are commercially availablewitha widerange of meltingand freezingpoints. Stability o
10、fthermophysical properties such as melting point and latentheat of fusion during repeated operation cycles is an import-ant factor in selection of suitable PCMs for latent TES appli-cations. Generally, PCMs with a melting point within 15Cto 90C (59F to 194F) are considered suitable for solarheating/
11、cooling applications (Farid et al. 2004). Storageusing PCMs has two main advantages over other storagemethods: higher energy storage density and the isothermalnature of the storage process. However, most PCMs sufferfrom the undesirable property of relatively low thermalconductivity, which strongly s
12、uppresses the energycharging/dischargingrates,thusmakingthesystemresponsetime too long to meet design requirements.Melting of PCM with Nanoparticles in aTriplex-Tube Thermal Energy StorageSystemJasim M. Mahdi Emmanuel C. Nsofor, PhDStudent Member ASHRAE Member ASHRAEJasim M. Mahdi is a doctoral stud
13、ent and Emmanuel C. Nsofor is a professor in the Department of Mechanical Engineering and EnergyProcesses, Southern Illinois University Carbondale, Carbondale, IL.ST-16-022Published in ASHRAE Transactions, Volume 122, Part 2 216 ASHRAE TransactionsThe system response time for melting and solidificat
14、ion isan important factor in designing an effective TES system. If thesystem response time does not reach the required value, serioussafety issues may emerge. Therefore, many investigations arebeing conducted to improve the thermal conductivity of PCMsor to increase the heat transfer performance. Se
15、veral conceptshave been proposed, including a) packing the PCM within ahigh-thermal-conductivity solid matrix (Wu and Zhao 2011),b) using fins and/or heat pipes (Gharebaghi and Sezai 2008;Shabgardetal.2014),andc) addinghigh-thermal-conductivityparticles to the PCM (Khodadadi et al. 2013; Wu et al. 2
16、012).Wu and Zhao (2011) studied experimentally the use ofporous materials such as metal foams and expanded graphiteto enhance heat transfer performance of PCMs in a high-temperature TES system. Results showed that the heat transferrate can be enhanced 2.5 times (from 250C to 300C 482F to572F) throug
17、h the addition of porous materials in the heatingprocess. The investigators also considered the use of extendedsurfacesand/orheatpipestoempowerheattransferperformance.Gharebaghi and Sezai (2008) investigated numerically theenhancement of the energy storage rate of a TES unit filled withaPCMbyinserti
18、ngafinarraysystemintothestoragedevice.Theresults showed that the decrease in fin spacing leads to a signifi-cant decrease in the time required to complete melting of thePCM. Also, it was found that the heat transfer rate can beincreasedasmuchas80timesbyaddingafinarrayintothePCMmodule. Shabgard et al
19、. (2014) investigated numerically a heat-pipe-assisted latent heat thermal energy storage (LHTES) unitintegrated with a dish-Stirling system. The unit was mainly acontainer that houses a PCM with a set of heat pipes embeddedin the PCM. It was found that minimum heat pipe spacing yieldsless fluctuati
20、on in the thermal power delivered to the engine andleads to a maximum amount of PCM melting and solidification.With the development of nanotechnology, many papershave been published on research that studied the effects ofaddingnanoparticlestoPCMstomodifytheirthermophysicalproperties such as specific
21、 heat and thermal conductivity.Researchers have started to study the effects of thermalconductivity improvement by adding nanoparticles to PCMson the heat transfer efficiency of the LHTES. Wu et al. (2012)numerically investigated the melting processes of copper/paraffin nanofluids. The results revea
22、led that with 1 weight %copper/paraffin, the melting time can be shortened by 13.1%.It was concluded that adding nanoparticles is an efficient wayto enhance the heat transfer in a LHTES. Sciacovelli et al.(2013) numerically studied the thermal behavior of theLHTESunitchargedwithnanoparticle-enhanced
23、PCM(nano-PCM). A melting time reduction of 15% was reported bydoping nano-PCM with a particle volume fraction of 4%.Chieruzzi et al. (2013) used the direct-synthesis method todevelop nanofluids with phase-change behavior by mixing amolten salt base fluid (selected as a PCM) with nanoparticles.Theres
24、ultsshowthattheadditionof1%ofnanoparticlestothebasesaltincreasesthespecificheatby15%to57%inthesolidphase and by 1% to 22% in the liquid phase.Al-Abidi et al. (2013) experimentally investigated thePCM melting process in the triplex-tube heat exchanger withparaffin RT82 as a PCM. The study used three
25、heatingapproaches totally dependent on solar energy: inside heatingmethod, outside heating method, and heating on both sides.Compared to the double-pipe heat exchanger, the triplex-tubeheat exchanger has a larger heat-transfer area, which conse-quently improves heat transfer and saves melting time.
26、Thestudy reported that heating on both sides is the preferablemethod because it requires lower heat transfer fluid (HTF)inlet temperature and less melting time for the PCM.The purpose of the present study is to numerically inves-tigate the effect of nano-alumina (Al2O3) particle addition onthe therm
27、al behavior and melting performance of paraffinRT82 in a triplex-tube heat exchanger, especially with regardto savings in time for the melting process. The authors believethat the enhancement of the melting (charging) rate and thesolidification(discharging)rateisveryimportantinachievingan improved l
28、atent thermal storage process in the triplex-tubesystem. This particular study is only on nano-PCM melting inthe triplex-tube system; the nano-PCM solidification study inthe system is the subject of an ongoing investigation by theauthors.PROBLEM STATEMENT AND FORMULATIONA schematic diagram of the ex
29、perimental apparatus usedby Al-Abidi et al. (2013) is shown in Figure 1. It includes atriplex-tube heat exchanger comprising three horizontallymounted concentric tubes with lengths of 500 mm (19.7 in.).Theinner,middle,andoutertubeshavediametersof50.8,150,and 200 mm (2, 5.91, and 7.87 in.), respectiv
30、ely. Both themiddle and outer tubes have a thickness of 2 mm (0.079 in.)while the inner one has a thickness of 1.2 mm (0.047 in.) only.Theannularspacebetweentheinnerandmiddletubesisfilledwith the PCM and the HTF is circulated inside both the innerand outer tubes. The charging process for PCM melting
31、wholly depends on solar energy. The physical domain isselected to be an annulus representing the annular space thathouses the nano-PCM, with roand ribeing the radii of theouter and inner tubes, respectively. Due to symmetry in the -direction, the computation has been restricted to the right-halfof t
32、he domain only, as shown in Figure 2.Initially, at t = 0, the PCM is exposed to an ambienttemperatureof300K(540R)asasubcooledtemperaturethatis 50 K (90R) below solidus temperature Tsto ensure that thePCM is at the solid phase. So, the initial condition can bedefined asFortimet0,boththeinnerandouters
33、urfacesoftheannu-lus are exposed to a constant temperature (Tw= 363 K653.4R) throughout the melting process. Selection of thistemperature was based on the minimum operation temperature(T 65C 149F) to power the solar-powered liquid-desic-at t 0 T Tint300 K (540R)=Published in ASHRAE Transactions, Vol
34、ume 122, Part 2 ASHRAE Transactions 217cant air-conditioning systems (Al-Abidi et al. 2013). So, theboundary conditions can be defined asBecause Twis higher than the liquidus temperature Tl,melting initiates around the inner cylinder and then the meltlayer grows outward into the solid region, causin
35、g what iscalled the mushy region. During the melting of the PCM anddue to temperature gradients, buoyancy-driven convectiveflow develops in the melt by density variations across theannulus. The flow in the melting process was assumed tran-sient, laminar, and incompressible. The following assump-tion
36、s are made in developing the current mathematical model:The temperature variation in the HTF is negligible (i.e.,Twis constant)The melt is an incompressible Newtonian fluidBuoyancy-driven flow in the melt is laminarViscous dissipations are negligibleNo-slip conditions exist for velocities at the bou
37、ndariesAll thermophysical properties of the PCM are constantsexcept density in the momentum equationThe Boussinesq approximation is used to account fordensity variation as = m/(T Tm) + 1), where Tm=(Tl+ Ts)/2Volume variation associated with the phase change isneglectedThere is no heat loss or gain f
38、rom the surroundingsUnder the above assumptions, the equations governingthe fluid motion and the temperature distribution inside theannulus are governed by the standard Navier-Stokes andenergy equations:(1)(2)(3)(4)Figure 1 Schematic diagrams of the PCM thermal storage with thermocouple locations (A
39、l-Abidi et al. 2013).Figure 2 Schematic diagram of the computational modelwith coordinate system.at rriT Tw363 K (653.4R)=at rroT Tw363 K (653.4R)=.V 0=ut- V .u+1npcm- P npcm2u+Cu1 23+-+=vt- V .v+1npcm- P npcm2v npcmgT Tref+Cv1 23+-+=ht-Ht- . Vh+ .knpcmcpnpcm-h=Published in ASHRAE Transactions, Volu
40、me 122, Part 2 218 ASHRAE TransactionsIn these equations, u is the velocity component in the r-direction, v is the velocity component in the -direction, isthedynamicviscosity,Pisaneffectivepressure,Cisamushy-zone constant of 106(it is usually set between 104and 107), istheliquidfraction,andisasmalln
41、umber(0.001)topreventdivision by zero. Enthalpy in the energy equation is computedby decomposing it into sensible enthalpy, h, and latent heat,H:H = h + HThe sensible enthalpy can be represented bywhere hrefis the reference enthalpy at the reference tempera-ture (Tref= 273 K 491.4R) and cpis the spe
42、cific heat atconstant pressure.Thelatentheatcanbewrittenintermsofthemeltingheat,L,asH = Lwhere is the liquid fraction during the phase change in thetemperature interval TsTl).Ascanbeobservedfrom Figure 4, the present results are generally in acceptableagreement with those of the experimental study.
43、The fewdiscrepancies in the early stage of the melting may be due tothe sudden settlement of semimolten PCM molecules in thefully solid PCM area. It is noteworthy to mention that thetime period t 40 min refers to when T is still less than orequal to Ts= 350 K (77C), meaning that the melting is notwe
44、ll established yet (semisolid zone). Thus, the relativelyhigh disagreement in this period may be of less importancewhen compared to the next period, where the PCM meltoccupies the most annular space (mushy and liquid zones),considered as the stages for melting evolution and domi-nance, the focus of
45、this present study.Figure 3 Effects on the variation of liquid fraction versus time of (a) grid size and (b) time step.Published in ASHRAE Transactions, Volume 122, Part 2 220 ASHRAE TransactionsThe accuracy of the numerical simulation was quantita-tivelycheckedviarelativeerror=(TExpTNum)/TExp,where
46、the indices Exp and Num represent the experimental and thenumerical data, respectively. The maximum relative error(%) obtained for the melting process was not more than 8%for the time period t 40 min and less than 1% (a very goodagreement) for the time period t 40 min (for the mushy andliquid zones)
47、. For the same reasoning, a number of studies,suchasthosebyHossainetal.(2015)andTasnimetal.(2015),have also ignored the sensible energy storage stage andassumed that the start-up process is from the melting point.RESULTS AND DISCUSSIONFollowing the procedures described above, severalnumerical runs w
48、ere conducted to study the behavior of thenano-PCM system during its charging by a hot HTF underisothermal conditions of 363 K (653.4R). The numericalsimulation is restricted to 363 K (653.4R) as an averagetemperature of the HTF that meets the requirements of theliquid-desiccant air-conditioning system. The results in theformofisotherms,posi
