Design and Fabrication of a Phase Change Material Heat Storage Device for the Thermal Control of Electronics Components of Space Applications
Abstract
This work was supported by the Spanish Ministry of Economy and Competitiveness through the financial support given to the project “Soluciones térmicas para componentes espaciales basadas en materiales con cambio de fase” (ref: AYA2010-18663).
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Citation: Garmendia, I.; Vallejo, H.; Seco, M.; Anglada, E. Design and Fabrication of a Phase Change Material Heat Storage Device for the Thermal Control of Electronics Components of Space Applications. Aerospace 2022,9, 126. https:// doi.org/10.3390/aerospace9030126 Academic Editor: Ephraim Suhir Received: 20 January 2022 Accepted: 25 February 2022 Published: 28 February 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). aerospace Article Design and Fabrication of a Phase Change Material Heat Storage Device for the Thermal Control of Electronics Components of Space Applications Iñaki Garmendia 1,* , Haritz Vallejo 2, Miguel Seco 2and Eva Anglada 3 1Mechanical Engineering Department, Engineering School of Gipuzkoa, University of the Basque Country UPV/EHU, Plaza de Europa, 1, E-20018 Donostia-San Sebastian, Spain 2TECNALIA, Basque Research and Technology Alliance (BRTA), Mikeletegi Pasealekua, 7, E-20009 Donostia-San Sebastian, Spain; [email protected] (H.V.); [email protected] (M.S.) 3TECNALIA, Basque Research and Technology Alliance (BRTA), Mikeletegi Pasealekua, 2, E-20009 Donostia-San Sebastian, Spain; [email protected] *Correspondence: [email protected]; Tel.: +34-43-018630 Abstract: In this paper, the design and validation of a heat storage device based on phase change materials are presented, with the focus on improving the thermal control of micro-satellites. The main objective of the development is to provide a system that is able to keep electronics within safe temperature ranges during the operation of manoeuvres, while reducing mass and volume in comparison to other thermal control techniques. Due to the low thermal conductivity of phase change materials, the conductivity of the device as a whole is one of the major challenges of the development. This issue has been solved by means of the use of a lattice of aluminium fins. The thermal behaviour of the proposed solution is assessed with numerical simulation tools, and the results prove that the developed phase change material-based thermal control technique is able to provide the suitable integrated thermal management of micro-satellites. Fabrication challenges found in the project are also explained. Numerical results are validated through a testing stage. The predicted temperature profiles are in good agreement with experimental data and inside the range foreseen for the heat storage device. Keywords: phase change materials; space thermal control; heat storage devices 1. Introduction Space is a very harsh and difficult environment for spacecraft and their payloads. From a thermal point of view, spacecraft and payloads must undergo extreme temperatures, heat fluxes and variations in these flows depending on the orbit type [1,2]. Thermal control refers to the set of techniques employed to maintain the temperatures of the spacecraft and its components inside the range of allowed temperatures [ 3 , 4 ]. This must be achieved in all possible scenarios (hot cases, cold cases, steady state or transient cases, etc.) because, otherwise, individual components can fail due to the very high or very low temperatures reached. A failure in one component, e.g., the electronics, can jeopardize a mission or even make it a failure [5]. Phase change materials (PCM) have been used for thermal control techniques since the early years of the 1960s. Several Apollo missions carried components that used PCMs to stabilize their temperatures [ 6 ]. Other components were also used that relied on PCM behaviors [ 7 ]. The general appreciation in the space industry for PCMs continued also during the 21st century, as was shown by the call from the European Space Agency (ESA) to the European space industries to design, calculate and fabricate a phase change material heat storage device (PCM-HSD) [8]. The physics and behavior of PCM materials have been widely studied in the literature [ 9 – 20 ]. One interesting study showed the possibility of using PCMs to stabilize the Aerospace 2022,9, 126. https://doi.org/10.3390/aerospace9030126 https://www.mdpi.com/journal/aerospace
Aerospace 2022,9, 126 2 of 16 temperature of some components, for instance, the electronics [ 1 ]. When the different electronic circuits are switched on, heat is produced, and temperatures start to rise. The heat produced must be conducted to the radiator and, from there, must be evacuated into outer space. If the power to be disposed of is big, the surface of the radiator must also be big and, consequently, its weight will be high. However, if the power produced by the electronics is somehow ‘stored’ in the PCM, and the temperature is maintained at a constant level, it will be possible to dispose of the heat when the circuits are switched off. This means that the area of the radiator will not be that large, the weight will be lower and the design will be better. In fact, what the PCM achieves is an increase in the thermal inertia of the spacecraft or payload and, in this way, it will have a more stable temperature and a reduced weight of the payload or spacecraft. 2. Phase Change Material Heat Storage Device (PCM-HSD) 2.1. Description This study aimed to assess the possibility of using PCMs in space to stabilize the temperatures of components and to reduce the size of the radiator in order to reduce weight. The system considered, shown in Figure 1, was based on a PCM contained in a house, which was attached to the electronics whose temperature is to be controlled and the radiator. Every surface but the radiator was assumed to be covered by multi-layer insulation (MLI) blankets to avoid heat leaks. Moreover, the radiator was assumed to operate just when the heat needed to be released into the space. The device was considered, therefore, to be insulated from the environment to make the study conditions more unfavorable. Aerospace2022,9,1262of16 ThephysicsandbehaviorofPCMmaterialshavebeenwidelystudiedinthelitera‐ ture[9–20].OneinterestingstudyshowedthepossibilityofusingPCMstostabilizethe temperatureofsomecomponents,forinstance,theelectronics[1].Whenthedifferentelec‐ troniccircuitsareswitchedon,heatisproduced,andtemperaturesstarttorise.Theheat producedmustbeconductedtotheradiatorand,fromthere,mustbeevacuatedintoouter space.Ifthepowertobedisposedofisbig,thesurfaceoftheradiatormustalsobebig and,consequently,itsweightwillbehigh.However,ifthepowerproducedbytheelec‐ tronicsissomehow‘stored’inthePCM,andthetemperatureismaintainedataconstant level,itwillbepossibletodisposeoftheheatwhenthecircuitsareswitchedoff.This meansthattheareaoftheradiatorwillnotbethatlarge,theweightwillbelowerandthe designwillbebetter.Infact,whatthePCMachievesisanincreaseinthethermalinertia ofthespacecraftorpayloadand,inthisway,itwillhaveamorestabletemperatureand areducedweightofthepayloadorspacecraft. 2.PhaseChangeMaterialHeatStorageDevice(PCM‐HSD) 2.1.Description ThisstudyaimedtoassessthepossibilityofusingPCMsinspacetostabilizethe temperaturesofcomponentsandtoreducethesizeoftheradiatorinordertoreduce weight.Thesystemconsidered,showninFigure1,wasbasedonaPCMcontainedina house,whichwasattachedtotheelectronicswhosetemperatureistobecontrolledand theradiator.Everysurfacebuttheradiatorwasassumedtobecoveredbymulti‐layer insulation(MLI)blanketstoavoidheatleaks.Moreover,theradiatorwasassumedtoop‐ eratejustwhentheheatneededtobereleasedintothespace.Thedevicewasconsidered, therefore,tobeinsulatedfromtheenvironmenttomakethestudyconditionsmoreunfa‐ vorable. Figure1.Phasechangematerialheatstoragedevice(PCM‐HSD)considered. ThekeyideabehindthisPCMthermalcontrolconceptistoconvertthethermalen‐ ergyintoaphasechangereaction,storingheatwhenitisproducedandreleasingthis energywhentheelectronicsisswitchedoff.Sincethephasechangeprocessoccursatal‐ mostconstanttemperature,suchthermalcontrolmeansthatthesystemtemperaturedoes notchangesignificantlyduringthemelting/solidification,sothatifthemeltingpointis appropriate,theelectronicscanbeefficientlyprotected. Ingeneralterms,micro‐satellites(between10and100kginweight)operateonlow Earthcircularorbits(LEO,450–1200km)withawiderangeofNASAβangles,andon highellipticalorbits,andareexposedtotheSun,albedoandinfraredEarthradiation. Typicalmaximalincidentfluxesfor550kmorbitforflatsurfaceswithnormaltonadir trajectoriesareQ IR (infrared)~200W/m 2 andQ AL (albedo)max~450W/m 2 (averagedover orbit<150W/m 2 ).Theeclipsetimecanvarybetween0.5h(circular)anduptoseveral Figure 1. Phase change material heat storage device (PCM-HSD) considered. The key idea behind this PCM thermal control concept is to convert the thermal energy into a phase change reaction, storing heat when it is produced and releasing this energy when the electronics is switched off. Since the phase change process occurs at almost constant temperature, such thermal control means that the system temperature does not change significantly during the melting/solidification, so that if the melting point is appropriate, the electronics can be efficiently protected. In general terms, micro-satellites (between 10 and 100 kg in weight) operate on low Earth circular orbits (LEO, 450–1200 km) with a wide range of NASA β angles, and on high elliptical orbits, and are exposed to the Sun, albedo and infrared Earth radiation. Typical maximal incident fluxes for 550 km orbit for flat surfaces with normal to nadir trajectories are Q IR (infrared) ~200 W/m 2 and Q AL (albedo) max ~450 W/m 2 (averaged over orbit <150 W/m 2 ). The eclipse time can vary between 0.5 h (circular) and up to several hours for elliptic orbits. According to Baturkin [ 21 ], typical requirements of average heat generation inside the satellite are in the range of 15–40 W. This power is produced mainly by housekeeping equipment (on board computer, transmitter, the attitude and control
Aerospace 2022,9, 126 3 of 16 system, batteries). Peak heat generation coincides with payload operation and can reach up to 200 W. In our case, the specifications that were to be followed are those stated by the ESA [ 8 ]. There were requirements for functional and performance (FPR), interface (IR), environmental (ER), operational (OR), design (DR) and verification and testing (VTR) operations. However, the main requirements are those related with thermal capacity and mechanical behavior, listed below: •The device must be capable of absorbing 30 W for 45 min; •Operational temperature range must be (−20/+40 ◦C); •The device’s mass shall be less than 0.50 kg; •The device’s first resonance frequency must be higher than 140 Hz; •The device shall sustain a mechanical environment characterized by dynamic loads. 2.2. Governing Equations and Numerical Methods It is very common for thermal control in spacecraft to describe the heat transfer process through the thermal lumped method (TLP). Details about the method can be found elsewhere [22–24]. The set of nonlinear equations that describe the temperatures of a transient thermal mathematical model is (for a node i) j=n ∑ j=1 GL(i,j)Ti−Tj+ j=n ∑ j=1 σGR(i,j)T4 i−T4 j+MiCi dTi dt =qi(1) where n is the number of nodes of the thermal mathematical model (TMM), GL(i,j) is the conductive conductance (W/m) between nodes i and j , σ is the Stefan–Boltzmann constant (5.67 × 10 − 8 W/(m 2· K 4 )), GR(i,j) is the radiative conductance (m 2 ) between nodes i and j , Ti and Tj are the temperatures (K) of nodes i and j , MiCi is the product of the i node mass (kg) times the heat capacity (J/(kg · K) and qi is the power (W) that enters into node i . The subscripts i and j go from 1 to n . It is usual to ascribe thermal inertia to the product MiCi as it describes the “opposition” to changing the temperature of i node when subjected to a power input. The time derivative of the temperature of node ican be approximated by dTi dt =Tit+∆t−Tit ∆t(2) For a node iand for a general time step t+∆t, it is possible to write j=n ∑ j=1 GL(i,j)Tit+∆t−Tjt+∆t+ j=n ∑ j=1 σGR(i,j)Ti4t+∆t−Tj4t+∆t +MiCi(Tit+∆t−Tit) ∆t−qi=0 (3) In order to simplify the notation, the following will be used Tit+∆t=Ti;Tit=Tit(4) Additionally, the following set of equations is obtained j=n ∑ j=1 GL(i,j)Ti−Tj+ j=n ∑ j=1 σGR(i,j)Ti4−Tj4+MiCiTi−Tit ∆t−qi=0 (5) Each equation represents the thermal instant equilibrium of a node. An in-housedeveloped computer program called TK was used to solve the set of n non-linear equations. These equations are solved for each time step where temperatures of the different nodes
Aerospace 2022,9, 126 4 of 16 are calculated. The heat power (W) that goes from one node to another is also calculated, as well as the heat power (W) that goes into each node, which is employed in increasing its temperature. The TK computer program was modified to be able to deal with phase change materials. In the following, we will assume that the initial state of each node is solid and that the phase change will be melting. For those nodes made of PCMs, additional information must be supplied to the computer program. The PCM has a latent heat L , measured in J/kg. The mass of each node (kg), as well as the specific heat (J/(kg · K)) of the material are also known. By multiplying the mass of the node by the latent heat L , the program can obtain the total energy needed by the node when passing from a solid to liquid phase. The program uses a predictor–corrector method to take into account the latent heat of nodes made of PCM. For each time step, a set of temperatures is calculated (predicted) with the previously explained equations. Then, the program checks each node made of PCM. If the temperature predicted for this node is higher than the starting temperature of the change of state from solid to liquid, then the energy used (J) is calculated, the temperature of the node is corrected and the accumulated energy used in that node is calculated, as is the liquid fraction of PCM in that node. These calculations are performed taking into account the product of the mass times the specific heat of the node. If the calculated liquid fraction is lower than 1, the program continues with the next node. When the total energy accumulated in one node is higher than the total energy that the node needs to change state, the state of the node is considered liquid and, in the following time steps, it will not have influence over the predicted temperatures. It is clear that time steps short enough must be considered when phase change takes place, otherwise significant errors can appear. It is not unusual to need to make several trials before fixing an appropriate time step length. Finally, the procedure for considering the change from liquid to solid is similar, but with decreasing temperatures. 3. Design of the Phase Change Material Heat Storage Device (PCM-HSD) 3.1. Conceptual Design The design efforts are aimed at creating an appropriate heat management technique to keep the electronics within the safe temperature range and a suitable housing design able to withstand the structural loads specified. Together with the specifications mentioned in the previous section, the main issues considered for the conceptual design were the PCM selection and the PCM thermal conductivity enhancement. It has not been previously mentioned but, in general, thermal conductivity of PCMs is quite low, and it is necessary to improve it to obtain an appropriate thermal behavior for the device. 3.1.1. PCM Selection Regarding the PCM selection, a first calculation has been performed to find the minimum value of the latent heat needed for the PCM. Following the specifications, Equation (6) states: λmin =Emin mmax =30 W·45 min·60 s 1 min 0.5 kg =81000 J 0.5 kg =162 kJ kg . (6) In fact, the minimum value for the latent heat of the PCM must be bigger, because the PCM will be positioned inside a container that will add mass to the device but almost no latent heat (not taking into account the specific heat). A box-shaped aluminum container with a,b,cdimensions and tthickness has been considered. Aluminum has very good thermal conductivity and a relatively low density, so as a first approximation, it has been selected as the container material. Values for dimensions aand bhave also been selected as 80 mm and 20 mm, respectively. The reason is that these dimensions are adequate for one of the mechanical interfaces considered. A thickness of tequal to 2 mm has also been selected because of the fabrication constraints, so the only unknown dimension left is the height c. It is possible to demonstrate that the c
Aerospace 2022,9, 126 5 of 16 maximum value (in meters) that can be accepted for the box if the maximum weight of the PCM-HSD has to be 0.5 kg must be c=0.5 −2t·ρcontainer ab +2t(a+b)+4t2 ab·ρPCM +2t·ρcontainer(a+b+2t), (7) where ρcontainer and ρPCM are the densities of the container and the PCM. Once the general dimensions of the PCM-HSD are known, it is possible to calculate the energy that the PCM-HSD can handle due to the latent heat for a particular PCM. If this energy is higher than 81 kJ (162 kJ/kg 0.5 kg), it is possible to conclude that the selected PCM is appropriate for the application. Several organic and inorganic PCMs have been studied, and the first selection criterion has been that the energy calculated with Equation (2) is bigger than 81 kJ. In addition, the good chemical compatibility between the PCM and the structural material has been considered. EPCM =mPCMλPCM =a·b·c·ρPCM·λPCM (8) 3.1.2. Enhancement of the Thermal Conductivity As it has already been mentioned, the thermal conductivity of the PCMs is, in general, low. This can pose a problem for the PCM-HSD because higher than desired temperatures could be reached while the phase change occurs, endangering the functioning of the electronics. In this context, an introductory calculation has been performed, comparing the behavior of the PCM alone with its behavior if aluminum fins are present, if aluminum foam is used or if even both of them, fins and foam, are used simultaneously (Figure 2). The calculation is based on the mixture rule and gave an initial idea of what to expect from each solution. Aerospace2022,9,1265of16 soasafirstapproximation,ithasbeenselectedasthecontainermaterial.Valuesfordi‐ mensionsaandbhavealsobeenselectedas80mmand20mm,respectively.Thereason isthatthesedimensionsareadequateforoneofthemechanicalinterfacesconsidered.A thicknessoftequalto2mmhasalsobeenselectedbecauseofthefabricationconstraints, sotheonlyunknowndimensionleftistheheightc.Itispossibletodemonstratethatthec maximumvalue(inmeters)thatcanbeacceptedfortheboxifthemaximumweightof thePCM‐HSDhastobe0.5kgmustbe 𝑐. ,(7) where𝜌and𝜌arethedensitiesofthecontainerandthePCM. OncethegeneraldimensionsofthePCM‐HSDareknown,itispossibletocalculate theenergythatthePCM‐HSDcanhandleduetothelatentheatforaparticularPCM.If thisenergyishigherthan81kJ(162kJ/kg0.5kg),itispossibletoconcludethattheselected PCMisappropriatefortheapplication. SeveralorganicandinorganicPCMshavebeenstudied,andthefirstselectioncrite‐ rionhasbeenthattheenergycalculatedwithEquation(2)isbiggerthan81kJ.Inaddition, thegoodchemicalcompatibilitybetweenthePCMandthestructuralmaterialhasbeen considered. 𝐸 𝑚𝜆 𝑎𝑏𝑐𝜌 𝜆(8) 3.1.2.EnhancementoftheThermalConductivity Asithasalreadybeenmentioned,thethermalconductivityofthePCMsis,ingen‐ eral,low.ThiscanposeaproblemforthePCM‐HSDbecausehigherthandesiredtemper‐ aturescouldbereachedwhilethephasechangeoccurs,endangeringthefunctioningof theelectronics.Inthiscontext,anintroductorycalculationhasbeenperformed,compar‐ ingthebehaviorofthePCMalonewithitsbehaviorifaluminumfinsarepresent,ifalu‐ minumfoamisusedorifevenbothofthem,finsandfoam,areusedsimultaneously(Fig‐ ure2).Thecalculationisbasedonthemixtureruleandgaveaninitialideaofwhatto expectfromeachsolution. Figure2.Fromlefttoright,PCM,PCMwithfins,PCMwithfoam,PCMwithfinsandfoam. Tocalculatetheequivalentthermalconductivity,wehavetakenintoaccountthecon‐ ductivityofthePCM,theconductivityofthefinsandalsothetransversalareaofthefins. Inthefoamcase,theapparentconductivityhasbeenconsidered,whichisafunctionof thematerialconductivityandtheporedensity: 𝑘 _ _ _ ,(9) performingsomealgebrainEquation(3),itispossibletowrite: 𝑘 𝑘 _ 𝑘 1 _ _ 1𝑥𝑘 𝑥,(10) where: 𝑘:PCM‐HSDequivalentthermalconductivity(W/(m∙K)); 𝑘:Thephasechangematerial’sthermalconductivity(W/(m∙K)); 𝑘:Thefins’thermalconductivity(W/(m∙K)); Figure 2. From left to right, PCM, PCM with fins, PCM with foam, PCM with fins and foam. To calculate the equivalent thermal conductivity, we have taken into account the conductivity of the PCM, the conductivity of the fins and also the transversal area of the fins. In the foam case, the apparent conductivity has been considered, which is a function of the material conductivity and the pore density: ke f =kPCM·APCM +kfin·Afin +kreal_foam·Areal_foam APCM +Afin +Areal_foam , (9) performing some algebra in Equation (3), it is possible to write: ke f =kapparent_foam +kPCM·1− ρapparent_foam ρreal_foam ·(1−x)+kfin·x, (10) where: ke f : PCM-HSD equivalent thermal conductivity (W/(m·K)); kPCM: The phase change material’s thermal conductivity (W/(m·K)); kfin: The fins’ thermal conductivity (W/(m·K)); kreal_foam: The real foam’s thermal conductivity (W/(m·K)); kapparent_foam: The apparent foam’s thermal conductivity (W/(m·K)); APCM: The transversal area of the PCM (m2);
Aerospace 2022,9, 126 6 of 16 Afin: The transversal area of the fins (m2); Areal_foam: The transversal area of the foam (m2); ρapparent_foam: The apparent density of the foam (kg/m3); ρreal_foam: The real density of the foam (kg/m3). For this preliminary analysis, a paraffin called RT5HC was selected as the PCM (properties in Table 1), with a metallic foam Duocel ® (Table 2) and aluminum alloy 6101 for the fins (Table 3). Table 1. PCM RT5HC material properties (supplier: Rubitherm). PCM Type Tfusion (◦C) λ(kJ/kg) ρ(kg/m3)K (W/(m·K)) Cp (J/(kg·K)) EPCM (J) RT5HC Organic 6 214 880/763 0.2 1800/2400 60,553 Table 2. Duocel®foam material properties. Material ρapparent (kg/m3) ρreal (kg/m3) Cp (J/(kg·K)) K (W/(m·K)) Aluminum 6101 Duocel®foam 207 2700 895 218 Table 3. Aluminum material properties. Material ρ(kg/m3)Cp (J/(kg·K)) K (W/(m·K)) Aluminum 2700 895 218 It is worth to noting that the PCM (RT5HC) selected for this first approximation does not fulfil the condition imposed regarding the minimum energy (i.e., be higher than 81 kJ). However, the purpose of this initial investigation is to estimate the variation of the equivalent conductivity in relation to the percentage of filler volume content. This variation, as well as the equivalent latent heat, can be seen in Figure 3. The dot lines express the values when foam is present, as well as the fins. The continuous lines show values with only fins. Aerospace2022,9,1266of16 𝑘_:Therealfoam’sthermalconductivity(W/(m∙K)); 𝑘_:Theapparentfoam’sthermalconductivity(W/(m∙K)); 𝐴:ThetransversalareaofthePCM(m 2 ); 𝐴:Thetransversalareaofthefins(m 2 ); 𝐴_:Thetransversalareaofthefoam(m 2 ); 𝜌_:Theapparentdensityofthefoam(kg/m 3 ); 𝜌_:Therealdensityofthefoam(kg/m 3 ). Forthispreliminaryanalysis,aparaffincalledRT5HCwasselectedasthePCM (propertiesinTable1),withametallicfoamDuocel ® (Table2)andaluminumalloy6101 forthefins(Table3). ItisworthtonotingthatthePCM(RT5HC)selectedforthisfirstapproximationdoes notfulfiltheconditionimposedregardingtheminimumenergy(i.e.,behigherthan81 kJ).However,thepurposeofthisinitialinvestigationistoestimatethevariationofthe equivalentconductivityinrelationtothepercentageoffillervolumecontent.Thisvaria‐ tion,aswellastheequivalentlatentheat,canbeseeninFigure3.Thedotlinesexpress thevalueswhenfoamispresent,aswellasthefins.Thecontinuouslinesshowvalues withonlyfins. Table1.PCMRT5HCmaterialproperties(supplier:Rubitherm). PCMTypeT fusion (°C)𝝀(kJ/kg)𝝆(kg/m 3 ) 𝐊 (W/(m∙K))Cp(J/(kg∙K))E PCM (J) RT5HCOrganic6214880/7630.21800/240060,553 Table2.Duocel ® foammaterialproperties. Material𝝆𝒂𝒑𝒑𝒂𝒓𝒆𝒏𝒕 (kg/m 3 )𝝆𝒓𝒆𝒂𝒍 (kg/m 3 ) Cp (J/(kg∙K)) K (W/(m∙K)) Aluminum6101Duocel ® foam2072700895218 Table3.Aluminummaterialproperties. Material𝝆(kg/m 3 )Cp(J/(kg∙K))K(W/(m∙K)) Aluminum2700895218 Figure3.Variationofequivalentconductivityandequivalentlatentheatasafunctionofthe%of fillervolumecontent. Figure 3. Variation of equivalent conductivity and equivalent latent heat as a function of the % of filler volume content. Figure 3shows that the equivalent conductivity of the device improves a lot when foam and fins are present but, at the same time, the equivalent latent heat of the device
Aerospace 2022,9, 126 7 of 16 decreases significantly. The final design should be a trade-off between these two aspects of the heat transfer mechanism. 3.1.3. Initial Conceptual Design Reference [ 1 ] provides a good method for the geometrical initial design of the PCMHSD. Assuming a geometry similar to the one shown in Figure 4, a spreadsheet has been prepared to evaluate different initial designs in a quick and efficient way. Aerospace2022,9,1267of16 Figure3showsthattheequivalentconductivityofthedeviceimprovesalotwhen foamandfinsarepresentbut,atthesametime,theequivalentlatentheatofthedevice decreasessignificantly.Thefinaldesignshouldbeatrade‐offbetweenthesetwoaspects oftheheattransfermechanism. 3.1.3.InitialConceptualDesign Reference[1]providesagoodmethodforthegeometricalinitialdesignofthePCM‐ HSD.AssumingageometrysimilartotheoneshowninFigure4,aspreadsheethasbeen preparedtoevaluatedifferentinitialdesignsinaquickandefficientway. Figure4.InitialbasicdesignofthePCM‐HSD. Thebasicequationstakenintoaccountaretheenergyconservation,Fourier’slaw, themixtureruleandmassconservation.TheinputsforthespreadsheetarethePCMprop‐ erties,generaldimensionsofthehousing,numberandthicknessofthefinsandtheirprop‐ ertiesandthepropertiesofthefoam.Theobtainedresultsarethetotalmassofthedevice (maximum0.5kg),thethermalenergythatcanbedealtwith(minimum81kJ)andthe equivalentthermalconductivity. Itcanbeseenthatthedevice’sweightis0.5kgandthatthetotalenergyis87.9kJ: bothconstraintsarethenfulfilled. 3.2.PreliminaryDesignofTwoPossiblePhaseChangeMaterialHeatStorageDevices Theprocessofelaboratinganewdesignis,withoutadoubt,aniterativeprocessthat onlyconvergeswhenthefinaldesignisbuilt.Inourcase,twopreliminarydesigns,based ontwophasechangematerials(RT5HCandKF.4H 2 O),havebeendeveloped.Thegeom‐ etriesarequitedifferent,andbothdesignsneededtobecalculatedthermallyandmechan‐ ically.Forthesakeofcompleteness,thematerialpropertiesofbothmaterialsarecollected inTable4. Table4.Materialpropertiesforthepreselectedphasechangematerials. PropertyRT5HCKF.4H 2 O Meltingpoint(°C)618.5 Latentheat(kJ/kg)245231 Solidphasedensity(kg/m 3 )8801455 Liquidphasedensity(kg/m 3 )7631447 Specificheatsolidphase(J/kg/°C)18001840 Specificheatliquidphase(J/kg/°C)24001850 Thermalconductivity(W/m/°C)0.20.2 BothmaterialscomplywiththeESAspecificationspreviouslymentioned.Perhaps themostrelevantpointofinterestoftheKF.4H 2 Osaltisthatitundergoesalmostno changeinvolumewhenmelting. Figure 4. Initial basic design of the PCM-HSD. The basic equations taken into account are the energy conservation, Fourier’s law, the mixture rule and mass conservation. The inputs for the spreadsheet are the PCM properties, general dimensions of the housing, number and thickness of the fins and their properties and the properties of the foam. The obtained results are the total mass of the device (maximum 0.5 kg), the thermal energy that can be dealt with (minimum 81 kJ) and the equivalent thermal conductivity. It can be seen that the device’s weight is 0.5 kg and that the total energy is 87.9 kJ: both constraints are then fulfilled. 3.2. Preliminary Design of Two Possible Phase Change Material Heat Storage Devices The process of elaborating a new design is, without a doubt, an iterative process that only converges when the final design is built. In our case, two preliminary designs, based on two phase change materials (RT5HC and KF · 4H 2 O), have been developed. The geometries are quite different, and both designs needed to be calculated thermally and mechanically. For the sake of completeness, the material properties of both materials are collected in Table 4. Table 4. Material properties for the preselected phase change materials. Property RT5HC KF·4H2O Melting point (◦C) 6 18.5 Latent heat (kJ/kg) 245 231 Solid phase density (kg/m3)880 1455 Liquid phase density (kg/m3)763 1447 Specific heat solid phase (J/kg/◦C) 1800 1840 Specific heat liquid phase (J/kg/◦C) 2400 1850 Thermal conductivity (W/m/◦C) 0.2 0.2 Both materials comply with the ESA specifications previously mentioned. Perhaps the most relevant point of interest of the KF · 4H 2 O salt is that it undergoes almost no change in volume when melting.
Aerospace 2022,9, 126 8 of 16 3.2.1. RT5HC Material The proposed design for this material can be seen in Figure 5. The previously mentioned spreadsheet was used, with some minor adaptations. Aerospace2022,9,1268of16 3.2.1.RT5HCMaterial TheproposeddesignforthismaterialcanbeseeninFigure5.Thepreviouslymen‐ tionedspreadsheetwasused,withsomeminoradaptations. Figure5.PreliminarydesignfortheRT5HCmaterial. Thetrapezoidalgeometrytriestomaximizethesurfacecontactbetweenthehotelec‐ tronicdeviceandthePCMmaterial.Thedesignhastwoinnerfinstoimprovethethermal conductivityofthedevice.Thecontainermaterialisthe6101aluminummaterial. 3.2.2.KF.4H 2 OMaterial Afairlydifferentpreliminarydesignwasdepictedforthehydratedsaltmaterial,see Figure6.Thedesignwasconceivedtotakeadvantageofthealmost‐constantdensityof thePCMmaterialforsolidandliquidphases.Thenumberofinnerfinsisbiggerinthis case,againtoimprovethethermalconductivity.Thematerialcontainerisagain6101alu‐ minummaterial,andtheestimatedweightis0.5kg. Figure6.PreliminarydesignfortheKF.4H 2 Omaterial. 3.3.DetailedDesign:ThermalandMechanicalModellingofthePhaseChangeMaterialHeat StorageDevices Thepreliminarydesignsproposedintheprevioussectionofthepapermustundergo adetailedthermalandmechanicalanalysisbeforebeingfabricated.Thisisperformedin ordertocheckwhetherthedesignsfulfilltherequirementsexpressedinSection2. Figure 5. Preliminary design for the RT5HC material. The trapezoidal geometry tries to maximize the surface contact between the hot electronic device and the PCM material. The design has two inner fins to improve the thermal conductivity of the device. The container material is the 6101 aluminum material. 3.2.2. KF·4H2O Material A fairly different preliminary design was depicted for the hydrated salt material, see Figure 6. The design was conceived to take advantage of the almost-constant density of the PCM material for solid and liquid phases. The number of inner fins is bigger in this case, again to improve the thermal conductivity. The material container is again 6101 aluminum material, and the estimated weight is 0.5 kg. Aerospace2022,9,1268of16 3.2.1.RT5HCMaterial TheproposeddesignforthismaterialcanbeseeninFigure5.Thepreviouslymen‐ tionedspreadsheetwasused,withsomeminoradaptations. Figure5.PreliminarydesignfortheRT5HCmaterial. Thetrapezoidalgeometrytriestomaximizethesurfacecontactbetweenthehotelec‐ tronicdeviceandthePCMmaterial.Thedesignhastwoinnerfinstoimprovethethermal conductivityofthedevice.Thecontainermaterialisthe6101aluminummaterial. 3.2.2.KF.4H 2 OMaterial Afairlydifferentpreliminarydesignwasdepictedforthehydratedsaltmaterial,see Figure6.Thedesignwasconceivedtotakeadvantageofthealmost‐constantdensityof thePCMmaterialforsolidandliquidphases.Thenumberofinnerfinsisbiggerinthis case,againtoimprovethethermalconductivity.Thematerialcontainerisagain6101alu‐ minummaterial,andtheestimatedweightis0.5kg. Figure6.PreliminarydesignfortheKF.4H 2 Omaterial. 3.3.DetailedDesign:ThermalandMechanicalModellingofthePhaseChangeMaterialHeat StorageDevices Thepreliminarydesignsproposedintheprevioussectionofthepapermustundergo adetailedthermalandmechanicalanalysisbeforebeingfabricated.Thisisperformedin ordertocheckwhetherthedesignsfulfilltherequirementsexpressedinSection2. Figure 6. Preliminary design for the KF·4H2O material. 3.3. Detailed Design: Thermal and Mechanical Modelling of the Phase Change Material Heat Storage Devices The preliminary designs proposed in the previous section of the paper must undergo a detailed thermal and mechanical analysis before being fabricated. This is performed in order to check whether the designs fulfill the requirements expressed in Section 2.
Aerospace 2022,9, 126 9 of 16 3.3.1. Thermal Behavior Modelling The commercial thermal software NX-TMG from Siemens has been used for the transient calculation of the system temperatures. The employed mesh can be seen in Figure 7 . Aerospace2022,9,1269of16 3.3.1.ThermalBehaviorModelling ThecommercialthermalsoftwareNX‐TMGfromSiemenshasbeenusedforthetran‐ sientcalculationofthesystemtemperatures.TheemployedmeshcanbeseeninFigure7. Figure7.Detailedmeshemployedinthethermalcalculation. TheNXTMGsoftwareautomaticallyconvertsthefiniteelementmeshshowninFig‐ ure7intoanetworkofthermalconductancevaluesemployedinthethermallumpedpa‐ rameter(TLP)method,whichisverywellknowninthespaceindustry[24].Thermalre‐ sistanceshavebeenaddedmanuallytothemodelstotakeintoaccountthecontactbe‐ tweentheinterfaceandthePCMcontainer.Theemployedvalueis0.070∙10 −4 m 2 K/W, takenfrom[25].Themeshrepresentsaquarteroftherealgeometryduetothesymmetry presence.Boththermalinterfaces(smallandbig)havebeenmodelledforcompletenessof thecalculations.Theobjectiveistomaintainthetemperaturebetween−20°Cand+40°C. Thethermalloadcaseappliedineachofthefourgeometriesisaheatflowof30W producedbytheelectronics,appliedfor45min.Theinitialtemperatureconsideredis−20 °Candatimestepof0.1swasused,whichmakesatotalof27,000steps.Theresultsfor themostcriticalpointofthePCM‐HSD(apointintheinterface,neartheelectronics)are showninFigure8. Figure8.HighesttemperatureofthePCM‐HSDinatransientcalculation. Ascanbeseen,evenforthemostcriticalpointofthedesign,themaximumallowed temperatureof40°Cisnotreached.ItcanbealsoconcludedfromFigure8thatthesmall interfacegenerateshighertemperaturesthanthelargeone,asitwouldbeexpected. Figure 7. Detailed mesh employed in the thermal calculation. The NX TMG software automatically converts the finite element mesh shown in Figure 7into a network of thermal conductance values employed in the thermal lumped parameter (TLP) method, which is very well known in the space industry [ 24 ]. Thermal resistances have been added manually to the models to take into account the contact between the interface and the PCM container. The employed value is 0.070 × 10 −4 m 2 K/W, taken from [ 25 ]. The mesh represents a quarter of the real geometry due to the symmetry presence. Both thermal interfaces (small and big) have been modelled for completeness of the calculations. The objective is to maintain the temperature between − 20 ◦ C and +40 ◦ C. The thermal load case applied in each of the four geometries is a heat flow of 30 W produced by the electronics, applied for 45 min. The initial temperature considered is −20 ◦C and a time step of 0.1 s was used, which makes a total of 27,000 steps. The results for the most critical point of the PCM-HSD (a point in the interface, near the electronics) are shown in Figure 8. Aerospace2022,9,1269of16 3.3.1.ThermalBehaviorModelling ThecommercialthermalsoftwareNX‐TMGfromSiemenshasbeenusedforthetran‐ sientcalculationofthesystemtemperatures.TheemployedmeshcanbeseeninFigure7. Figure7.Detailedmeshemployedinthethermalcalculation. TheNXTMGsoftwareautomaticallyconvertsthefiniteelementmeshshowninFig‐ ure7intoanetworkofthermalconductancevaluesemployedinthethermallumpedpa‐ rameter(TLP)method,whichisverywellknowninthespaceindustry[24].Thermalre‐ sistanceshavebeenaddedmanuallytothemodelstotakeintoaccountthecontactbe‐ tweentheinterfaceandthePCMcontainer.Theemployedvalueis0.070∙10 −4 m 2 K/W, takenfrom[25].Themeshrepresentsaquarteroftherealgeometryduetothesymmetry presence.Boththermalinterfaces(smallandbig)havebeenmodelledforcompletenessof thecalculations.Theobjectiveistomaintainthetemperaturebetween−20°Cand+40°C. Thethermalloadcaseappliedineachofthefourgeometriesisaheatflowof30W producedbytheelectronics,appliedfor45min.Theinitialtemperatureconsideredis−20 °Candatimestepof0.1swasused,whichmakesatotalof27,000steps.Theresultsfor themostcriticalpointofthePCM‐HSD(apointintheinterface,neartheelectronics)are showninFigure8. Figure8.HighesttemperatureofthePCM‐HSDinatransientcalculation. Ascanbeseen,evenforthemostcriticalpointofthedesign,themaximumallowed temperatureof40°Cisnotreached.ItcanbealsoconcludedfromFigure8thatthesmall interfacegenerateshighertemperaturesthanthelargeone,asitwouldbeexpected. Figure 8. Highest temperature of the PCM-HSD in a transient calculation. As can be seen, even for the most critical point of the design, the maximum allowed temperature of 40 ◦ C is not reached. It can be also concluded from Figure 8that the small interface generates higher temperatures than the large one, as it would be expected. 3.3.2. Mechanical Behavior Modelling Both static and dynamic loads are foreseen for the different mission phases where the PCM-HSD can be used. As a consequence, different load cases are analyzed.
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