applied sciences

Article Numerical Analysis of Envelope with Movable Phase Change Materials for Heating Applications

Alvaro de Gracia *

GREiA Research Group, INSPIRES Research Centre, University of Lleida, Pere de Cabrera s/n, 25001 Lleida, Spain

 Received: 26 July 2019; Accepted: 3 September 2019; Published: 5 September 2019 

Abstract: storage materials have been tested by several researchers for decades to be used as passive heating and cooling systems in but their implementation into building components is still stacked as is facing specific technical limitations related to difficulties to be charged both in heating and cooling periods. This paper presents a numerical analysis to evaluate the potential of a disruptive system, which is designed to solve the main drawbacks and to convert phase change materials (PCM) passive heating technology into a competitive solution for the building sector. The novel technology moves PCM layer with respect to the insulation layer inside the building component to maximize solar benefits in winter and be able to actively provide space heating. Design variables such as PCM melting point and control schemes were optimized. The results demonstrated that this technology is not only able to limit heat losses towards outdoors but it can provide space heating from stored solar energy when required. The promising numerical results endorse the possibility to build a future experimental prototype to quantify more in detail the benefits of this system.

Keywords: phase change material; low energy buildings; climate adaptive building shell; solar energy

1. Introduction Building envelopes have a crucial impact on building energy use for space heating and cooling systems and their designs are crucial to reduce these loads while maintaining the occupants [1]. Currently, both heating and cooling demands represent 50% of the total global energy consumed in European buildings and this demand is increasing with the comfort requirements in developing countries [2]. Within this context, the use of phase change materials (PCM) for passive heating and cooling systems in buildings has attracted the attention of the scientific community as an appropriate technology which can promote net-zero energy buildings [3]. These materials, when integrated into building or roofs, can provide thermal benefits due to its high thermal energy storage capacity, which can prevent or delay the cooling peak load during summer and can absorb part of the solar radiation in winter. In building envelope applications, PCM stores latent heat as the ambient temperature rises to the melting point (most PCM changes from a solid to a liquid state) and as the temperature cools down, the PCM returns to a solid phase, releasing the stored latent heat [4]. Several researchers have investigated and developed different ways to incorporate PCM into building envelopes [5], such as microencapsulated [6–8], impregnated [9], or installed in macro-encapsulated panels [10]. Numerical and experimental research have emphasized the benefits of using passive PCM technology in buildings: (1) reduction of maximum room air temperature during the cooling period [11], (2) reduction of thermal oscillation during the cooling period, (3) cooling load reduction [12], and (4) possible solar absorption and heating load reduction [13].

Appl. Sci. 2019, 9, 3688; doi:10.3390/app9183688 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, x FOR PEER REVIEW 2 of 12

Appl.the cooling Sci. 2019 ,period9, 3688 [11], (2) reduction of thermal oscillation during the cooling period, (3) cooling2 load of 12 reduction [12], and (4) possible solar absorption and heating load reduction [13]. However, there existexist specificspecific technicaltechnical barriers that limit the benefitsbenefits of both PCM heating and cooling passive technologies and do not allow them to become an attractive technology able to justify their initialinitial economiceconomic investment.investment. These These technical ba barriersrriers are blocking the implementation of this technology into the market.market. In In case case of of PCM , the solidification solidification process of PCM is limited as the PCMPCM layerlayer isis notnot directlydirectly exposedexposed outdoorsoutdoors butbut thermallythermally protected by insulationinsulation (Figure(Figure1 1),), which which makes makes thethe technology technology completelycompletely uselessuseless inin periodsperiods whenwhen solidificationsolidification cannotcannot bebe ensured. Within this context, Mandilaras et al. [14] [14] experimentally tested passive PCM in two-storey buildings and showed that PCM did not provide benefitsbenefits from July to August (when cooling is highly demanded) as solidificationsolidification was not achieved during night time. Moreover, in PCM passive cooling applications, thethe building building peak peak cooling cooling load load is accumulated is accumulated in the in PCM the andPCM therefore and therefore its effect its indoors effect isindoors delayed, is delayed, however, however, that latent that heat latent is mainly heat is discharged mainly discharged indoors becauseindoors thebecause insulation the insulation strongly limitsstrongly its limits dissipation its dissipation to outdoors. to outdoors. Finally, passive Finally, cooling passive PCM cooling technology PCM technology only interferes only interferes in outdoor in coolingoutdoor loadscooling transmitted loads transmitted by conduction by conduction but it cannot but it cannot deal with deal internal with internal heat loads heat orloads solar or gainssolar fromgains openings.from openings. In fact, In this fact, technology this technology provides prov higherides higher energy energy consumption consumption for space for coolingspace cooling in the in the presence of significant internal heating loads [15]. presence of significant internal heating loads [15]. On the other hand, the use of PCM for heating purposes is based on its capability to absorb solar On the other hand, the use of PCM for heating purposes is based on its capability to absorb solar gains coming through the glazing and to release stored solar energy during the night [16]. This gains coming through the glazing and to release stored solar energy during the night [16]. This limited limited activation conditions limits: (1) its energy benefits (quantified around 4–5% [17]), (2) its activation conditions limits: (1) its energy benefits (quantified around 4–5% [17]), (2) its implementation implementation mainly to internal solar exposed surfaces (internal walls or floor), and (3) its use in mainly to internal solar exposed surfaces (internal walls or floor), and (3) its use in climatic regions climatic regions with non-severe winter conditions (otherwise, only operative during early and late with non-severe winter conditions (otherwise, only operative during early and late winter as well as winter as well as spring and autumn seasons) [13]. spring and autumn seasons) [13].

Figure 1. Figure 1. Implementation of passive phase change materialsmaterials (PCM) in static building envelope. The idea behind the system presented in this paper relies on changing the age-old concept that The idea behind the system presented in this paper relies on changing the age-old concept that building envelopes are static and fixed during its use. Research has been developed in the frame of building envelopes are static and fixed during its use. Research has been developed in the frame of developing climatic adaptive building shells (CABS) [18–20]. One can imagine that it would provide developing climatic adaptive building shells (CABS) [18–20]. One can imagine that it would provide extraordinary energy benefits if building envelopes (roofs or walls) can modify its thermal properties, extraordinary energy benefits if building envelopes (roofs or walls) can modify its thermal properties, such as thermal resistance or heat storage capacity, according to the heating and cooling demand such as thermal resistance or heat storage capacity, according to the heating and cooling demand daily profiles. daily profiles. For example, it will be interesting to remove the insulating layers of building envelopes (roofs For example, it will be interesting to remove the insulating layers of building envelopes (roofs and walls) in an office building during summer nights so the building can dissipate the heat stored and walls) in an office building during summer nights so the building can dissipate the heat stored from external and internal loads to outdoors. However, insulation is required to prevent peak cooling from external and internal loads to outdoors. However, insulation is required to prevent peak cooling load during the day. Similarly, in winter it will be interesting to remove the insulation during the load during the day. Similarly, in winter it will be interesting to remove the insulation during the sunny hours so thermal massive elements (brick or concrete layers facing indoors) would be able to sunny hours so thermal massive elements (brick or concrete layers facing indoors) would be able to store solar heat and later provide space heating. Insulation layers have to be present after these sunny store solar heat and later provide space heating. Insulation layers have to be present after these sunny hours to avoid or limit thermal losses to outdoors. Within this context, some work has been done in hours to avoid or limit thermal losses to outdoors. Within this context, some work has been done in the development of active insulating layers. Simko et al. [21] tested numerically and experimentally Appl. Sci. 2019, 9, 3688 3 of 12 thermally active walls with water pipes arranged in milled channels in the , which is able to alternate between a thermal barrier system to a space heating supplier. The ability to modify the thermal properties depending on the weather conditions and building use becomes even more interesting when phase change material (PCM) is integrated into the building envelope. This paper presents an innovative system, which is designed to solve the previously cited drawbacks and to convert PCM passive heating and cooling technology into a competitive and attractive solution for the building sector. The proposed system relies on the ability of roofs and walls to adapt the position of their layers and hence their morphology during its operation. This new ability of building components will revolutionize building designs as multiple technical, functional, and aesthetic options arise. This new system varies the position of the PCM with respect to the insulating layer to maximize solar benefits in winter and night in summer. de Gracia [22] numerically analyzed the benefits and performance of such technology when used for passive cooling purposes, showing that the system cannot only reduce cooling loads from outdoors but can provide space cooling when required by the demand. The previous paper [22] evaluated the performance of the system during summer when it moves the PCM to face outdoors at night time to facilitate its solidification and once the PCM is charged, it moves the PCM back to face indoors to prevent peak cooling load during the following day. This paper will use the previously developed numerical tool to evaluate the potential of this new building system to reduce energy consumption for space heating taking advantage of thermal storage of solar energy and to optimize key design and control variables.

2. Methodology

2.1. Dynamic PCM Concept The proposed dynamic PCM system relies on the ability of the PCM layer to change its position inside the building envelope (vertical or horizontal). The Boolean variable POS describes the position of the PCM layer with respect to the insulation layer at each time. Therefore, the system is able to place PCM facing outdoors during sunny hours to store incident solar energy and moves back PCM to face indoor space and release stored solar energy to the indoor environment when space heating is required (usually at night time). The addition of PCM in the movable layer can be achieved by means of different techniques [23], such as impregnation, tubes filled with PCM attached to continuous layers, or honeycomb systems. A rotatable electrical servomotor can be implemented on top and bottom of the insulation layer to guide the movement of the PCM layer from one face to the other. The use of timing belts and/or gears are expected in this sort of system to ensure appropriate control of movement without slip. Moreover, the use of endless screw might be appropriate to link electrical servomotor with the rotatable gears. More details about the possible design and integration of PCM can be found in de Gracia [22]. Figure2 shows in detail a possible mechanical design of the system, including the position of the servomotor and the endless screw connected to the rotatable gear. Appl. Sci. 2019, 9, 3688 4 of 12 Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 12

FigureFigure 2.2. PossiblePossible mechanicalmechanical designdesign ofof thethe rotatablerotatable system.system.

2.2.2.2. NumericalNumerical ModelModel DescriptionDescription TheThe numericalnumerical modelmodel hashas beenbeen previouslypreviously usedused toto analyzeanalyze thethe performanceperformance ofof thethe presentedpresented systemsystem forfor cooling cooling purposes purposes [22 [22],], which which is anis an implicit implicit one one dimensional dimensional finite finite control control volume volume model model [24]. The convective between the surfaces and both environments (hin and hout) are assumed [24]. The convective heat transfer between the surfaces and both environments ( hin and hout ) are constant, being 8 W/m2 K for the indoor environment and 25 W/m2 K for outdoor surfaces [25]. The assumed constant, being· 8 W/m2·K for the indoor environment and· 25 W/m2·K for outdoor surfaces melting and solidification processes are implemented through an equivalent heat capacity method [26] [25]. The melting and solidification processes are implemented through an equivalent heat capacity with a triangle shaped distribution over temperature [27,28]. The phase change range is 4 K, and method [26] with a triangle shaped distribution over temperature [27,28]. The phase change range is hysteresis and subcooling are not considered. Moreover, the temperature of the indoor environment 4 K, and hysteresis and subcooling are not considered. Moreover, the temperature of the indoor (Tindoor) is set constant to 20 ◦C[29]. environmentThe finite ( volumeTindoor ) modelis set constant uses uniform to 20 discretized°C [29]. spatial step of 1 mm [24] and a time step of 60 s, ensuringThe meshfinite independencevolume model anduses consistency uniform discretized of the numerical spatial step scheme. of 1 mm A detailed [24] and description a time step of of the 60 numericals, ensuring model mesh canindependence be found in and de Graciaconsistency et al. [of22 ].the numerical scheme. A detailed description of the numerical model can be found in de Gracia et al. [22]. 2.3. Optimization of Design and Control Parameters 2.3. OptimizationA suitable selection of Design of and PCM Control melting Parameters temperature depending on the climatic conditions, building constructiveA suitable system, selection and of heating PCM melting demand temperature is mandatory depending for the correcton the climatic performance conditions, of the building system. Moreover,constructive the system, activation and of heating the system demand and an is appropriate mandatory schedule for the correct of positioning performance PCM facing of the outdoors system. orMoreover, indoors playsthe activation a crucial role.of the system and an appropriate schedule of positioning PCM facing outdoorsDesign or (phaseindoors change plays a PCM crucial temperature) role. and control (POS[t]) variables have been optimized usingDesign a generic (phase optimization change PCM program temperature) (GenOpt v3.11)and co [30ntrol]. A (POS[t]) particle variables swarm optimization have been algorithmoptimized withusing constriction a generic optimization coefficient (PSOCC) program [31 (GenOpt] was used v3.11) in this [30]. study. A particle The algorithm swarm optimization was linked to algorithm the finite volumewith constriction numerical coefficient model and (PSOCC) was set with[31] thewas following used in this settings: study. (i) The 20 generations,algorithm was (ii) linked 20 particles to the perfinite generation, volume numerical and (iii) 2 seeds.model and was set with the following settings: (i) 20 generations, (ii) 20 2 particlesThe PSOCCper generation, varies these and three(iii) 2 variablesseeds. to minimize the heating load (Qload in J/m day): · 2 The PSOCC varies these three variables to minimize the heating load ( Qload in J/m ·day): (i) PCM peak melting temperature (TPCM) exploring integer values between 8 to 29 ◦C. (ii) Hour to start exposure of PCM to solar energy, POS [t] is set to 1 (t1). Continuous exploration (i) PCM peak melting temperature (TPCM ) exploring integer values between 8 to 29 °C. between 0 to 24 h. (ii) Hour to start exposure of PCM to solar energy, POS [t] is set to 1 (t1 ). Continuous exploration (iii) Hour to finish exposure of PCM to solar energy, POS [t] is set to 0 (t0). Continuous exploration between 0 to 24 h. (iii) Hour to finish exposure of PCM to solar energy, POS [t] is set to 0 (t0 ). Continuous exploration Itbetween has to be 0 to noticed 24 h. that control was optimized based on the average values of a specific period, meaning that t1 and t0 were set to the same value for all days during the analyzed period. Finally, the It has to be noticed that control was optimized based on the average. values of a specific period, daily heating load is computed as shown in Equations (1) and (2). Where Qheating is the rate of heating loadsmeaning per that unit t area1 and (W t0 /werem2) at set each to the time same step, valueh is for the all heat days transfer during coe theffi analyzedcient of the period. internal Finally, surface, the in  daily heating load is computed as shown in Equations (1) and (2). Where Qheating is the rate of Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 12 Appl. Sci. 2019, 9, 3688 5 of 12 2 heating loads per unit area (W/m ) at each time step, hin is the heat transfer coefficient of the internal surface, Tindoor is the temperature of the indoor environment, Tsi is the temperature of the interior Tindoor is the temperature of the indoor environment, Tsi is the temperature of the interior surface, and surface,Qload is theand heating Qload load is the for heating a specific load period for a specific between period t0 and between tend, t0 and tend,  . = − QQheating hhin((Tindoor Tsi)) (1)(1) heating in indoor − si t Ztendend = . (2) Q load = QQheating ·dtdt (2) load  heating· t0 t0

2.4. Case Case Studies Studies InIn this this study, the novel system has been tested numerically considering the registered weather conditions during during November November 1st 1st to to 10th 10th 2017 2017 (Figur (Figuree 33)) inin thethe experimentalexperimental set-upset-up ofof PuigverdPuigverd dede Lleida [32] [32] which which is is classified classified as as Csa Csa according according to to Köppen-Geiger Köppen-Geiger [33]. [33]. Furt Furthermore,hermore, the the performance performance of the dynamic PCM technology has has been been analyzed analyzed in in the the south south wall wall of the following different different constructive systems: • Wall 1: Typical Mediterranean constructive system based on two layers of bricks and 5 cm of • Wall 1: Typical Mediterranean constructive system based on two layers of bricks and 5 cm of mineral wool in between. • Wall 2: Case-600 building prototype from ASHRAE Standard 140-2011 [29]. • Wall 2: Case-600 building prototype from ASHRAE Standard 140-2011 [29]. • Wall 3: Ideal envelope, which is based on a single layer of insulation surrounded by a polymeric • Wall 3: Ideal envelope, which is based on a single layer of insulation surrounded by a polymeric sheet with and without PCM.

1600 25 ] 2 Solar Radiation Outdoor temperature 1400 20 1200

1000 15 800

600 10

400 5 Outdoortemperature [ºC] 200

Vertical Global Vertical SolarRadiation [W/m 0 0 0 24 48 72 96 120 144 168 192 216 240 Time [h]

Figure 3. Weather conditions during the analyzed period. Figure 3. Weather conditions during the analyzed period. The three selected morphologies of the building envelope represent different levels of thermal storageThe capacity three selected and thermal morphologies resistance. of Withoutthe building considering envelope the represent PCM layer, different Wall 2 presentslevels of athermal storage storagecapacity capacity of 156.1 kJand/m 2thermalK, while resistance. Wall 1 presents Without more considering the andPCM can layer, store Wall 200.7 2 kJ presents/m2 K. On a · 2 · storagethe other capacity hand, Wall of 156.1 3 is analyzed kJ/m ·K, as while a case Wall study 1 withpresents minimum more storagethermal capacity mass and (10,4 can kJ/ mstore2 K) 200.7 other 2 · kJ/mthan the·K. On one the provided other hand, by the Wall use of3 is PCM. analyzed Regarding as a case the thermalstudy with resistance, minimum Wall storage 2 presents capacity a thermal (10,4 2 kJ/mtransmittance·K) other inthan steady the one state provided (U-value) by of the 0.22 use W of/m PCM.2 K, while Regarding U-value the of thermal both Wall resistance, 1 and Wall Wall 3 is2 · 2 presents0.66 W/m a2 thermalK. The layertransmittance distribution, in steady thickness state of(U-value) each layer, of 0.22 and W/m thermo-physical·K, while U-value properties of both of Wall used · 2 1materials and Wall are 3 givenis 0.66 in W/m Table·K.1 for The each layer tested distribution, constructive thickness system. of each layer, and thermo-physical propertiesFurthermore, of used thematerials specific are heat given capacity in Table of the1 for PCM each layer tested presents constructive a heat storagesystem. capacity of 2,000 Furthermore, the specific heat capacity of the PCM layer presents a heat storage capacity of 2,000 J/kg K outside the phase change range (solid and liquid sensible region), and an of fusion J/kg·K· outside the phase change range (solid and liquid sensible region), and an enthalpy of fusion of 200 J/g, similar to those available in the market. It might occur that the amount of PCM able to be of 200 J/g, similar to those available in the market. It might occur that the amount of PCM able to be added in the polymeric sheet is limited, therefore this study will evaluate the influence that the amount Appl. Sci. 2019, 9, 3688 6 of 12 of PCM (0%, 25%, 50% 75%, and 100% w/w) has in the performance of the static (conventional system in which PCM is always placed in the layer facing indoors) and the proposed dynamic system.

Table1. Thermo-physical properties and constructive systems description (listed from outside to inside).

Construct Thermal Conductivity Heat Capacity Density Layer Thickness (m) System (W/m K) (J/kg K) (kg/m3) · · Hollow Brick 0.07 0.375 1000 930 PCM 0.007 0.2 f(T) 800 Wall 1 Insulation 0.05 0.05 1000 35 Plastic sheet 0.007 0.3 1250 900 Perforated 0.14 0.543 1000 900 brick Wood siding 0.01 0.14 900 530 PCM 0.007 0.2 f(T) 800 Wall 2 Insulation 0.0615 0.04 1400 40 Plastic sheet 0.007 0.3 1250 900 Concrete 0.1 0.51 1000 1400 block PCM 0.007 0.2 f(T) 800 Wall 3 Insulation 0.072 0.05 1000 35 Plastic sheet 0.007 0.3 1250 900

For the proposed amount of PCM (5.6 kg of PCM/m2), the current design suggests the use of an electrical motor (100 W) for 2 s for changing the position of the PCM layer, which means a daily electrical consumption of 400 J.

3. Results and Discussion

3.1. Thermal Behaviour of Dynamic PCM System Figure4 shows the solar radiation and outdoor temperature evolution, and how the variation of the position of PCM layer (0 or 1) affects to the PCM temperature when implemented in ASHRAE envelope (Wall 2). As it can be seen, once the position of the system is set to 1 and PCM is located facing outdoors with respect to the insulation layer, its temperature starts to increase as solar radiation is stored through the melting process of PCM. After the sunny hours, the PCM is set back to its original position (facing indoors with respect to insulation, POS = 0) and discharges the stored heat to the indoor environment. In this period, the PCM layer remains inside the phase change range (21 to 25 ◦C) being warmer than indoor set point temperature (20 ◦C), that allows the system not only to be used as a thermal barrier to outdoor conditions, but to provide space heating due to stored solar energy. Appl. Sci. 2019, 9, 3688 7 of 12 Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 12

30

27

24

1 21 18

15 ] and POS[-] 2 12

0.5 [ºC] Temperature 9 Iso[W/m 6

3

0 0 168 174 180 186 192 198 204 210 216 Time [hour] POS Iso T_PCM Toutdoor

FigureFigure 4. 4. EvolutionEvolution of ofPCM PCM temperature temperature in dynamic in dynamic system system when whenimplemented implemented in the ASHRAE in the ASHRAE (Wall 2) envelope.(Wall 2) envelope.

Moreover, as it was previously discussed in the introduction, the current state of the art of passive Moreover, as it was previously discussed in the introduction, the current state of the art of PCM systems for heating purposes is limited as PCM cannot be melted unless it is directly exposed passive PCM systems for heating purposes is limited as PCM cannot be melted unless it is directly to solar energy coming from openings or . This issue can be clearly seen in Figure5, which exposed to solar energy coming from openings or windows. This issue can be clearly seen in Figure shows the thermal evolution of the polymeric layer (with or without PCM) in case it is implemented in 5, which shows the thermal evolution of the polymeric layer (with or without PCM) in case it is a static or dynamic system. No significant differences were observed between the static system with or implemented in a static or dynamic system. No significant differences were observed between the without PCM with temperatures below set point (20 C) almost all the time. On the other hand, the static system with or without PCM with temperatures◦ below set point (20 °C) almost all the time. On dynamic system is able to charge the PCM partially six days (phase change range between 21 and 25 C) the other hand, the dynamic system is able to charge the PCM partially six days (phase change range◦ out of ten, and to fully charge it three days. It is important to highlight, that the control of the system between 21 and 25 °C) out of ten, and to fully charge it three days. It is important to highlight, that was optimized based on the average results of the ten analyzed days, meaning the optimal decision the control of the system was optimized based on the average results of the ten analyzed days, of positioning PCM facing outdoors and indoors is the same for all the analyzed days, therefore, a meaning the optimal decision of positioning PCM facing outdoors and indoors is the same for all the daily-based control would even increase the performance. These activations of the PCM result in a analyzed days, therefore, a daily-based control would even increase the performance. These dramatic reduction of the heating load coming from the outer environment. Figure6 presents the activations of the PCM result in a dramatic reduction of the heating load coming from the outer evolution of the heating load for static and dynamic systems, showing that the use of PCM does not environment. Figure 6 presents the evolution of the heating load for static and dynamic systems, reduce heating load unless it is implemented in the dynamic technology. When the PCM layer is able to showing that the use of PCM does not reduce heating load unless it is implemented in the dynamic move with respect to the insulation, it provides space heating to indoor environment (negative heating technology. When the PCM layer is able to move with respect to the insulation, it provides space loads), specially during afternoons and night periods. It has to be noticed that since the schedule of heating to indoor environment (negative heating loads), specially during afternoons and night operation of the system is optimized to minimize the overall heating load and activations are set at the periods. It has to be noticed that since the schedule of operation of the system is optimized to same time for each day, in some particular days with low solar radiation (first two tested days), the minimize the overall heating load and activations are set at the same time for each day, in some dynamic system can provide extra heating loads in some specific periods. particular days with low solar radiation (first two tested days), the dynamic system can provide extra heating loads in some specific periods. Appl.Appl. Sci. Sci.2019 2019, ,9 9,, 3688 x FOR PEER REVIEW 88 ofof 1212 Appl. Sci. 2019, 9, x FOR PEER REVIEW 8 of 12 32 32 Static_NoPCM 30 Static_NoPCM 30 Static_PCM 28 Static_PCM 28 26 Dynamic_PCM 26 Dynamic_PCM 24 24 22 22 20

Temperature20[ºC] 18 Temperature18[ºC] 16 16 14 14 12 12 0 24 48 72 96 120 144 168 192 216 240 0 24 48 72 96Time 120 [h] 144 168 192 216 240 Time [h]

Figure 5. Evolution of polymeric layer temperature when implemented in the ASHRAE (Wall 2) Figure 5. Evolution of polymeric layer temperature when implemented in the ASHRAE Figureenvelope. 5. Evolution of polymeric layer temperature when implemented in the ASHRAE (Wall 2) (Wallenvelope. 2) envelope. 6 6 4 4 2 2 0 ] 2 0

] 0 24487296120144168192216240 2 -2 0 24487296120144168192216240 [W/m -2 [W/m -4

-4heating Q heating -6 Q -6 -8 -8 -10 -10 -12 -12 Time [h] Time [h] Static_NoPCM Static_PCM Dynamic_PCM Static_NoPCM Static_PCM Dynamic_PCM Figure 6. Evolution of heating load for the different analysed systems when implemented in the

ASHRAEFigure 6. (Wall2) Evolution envelope. of heating load for the different analysed systems when implemented in the FigureASHRAE 6. Evolution (Wall2) envelope. of heating load for the different analysed systems when implemented in the 3.2. HeatingASHRAE Load (Wall2) Reduction envelope. by Using Dynamic PCM Wall 3.2. TableHeating2 presents Load Reduction the heating by Using load Dynamic reduction PCM provided Wall by the dynamic system when implemented 3.2. Heating Load Reduction by Using Dynamic PCM Wall into diTablefferent 2 typespresents of envelopes. the heating The load results reduction show provided reductions by higher the dynamic than 100% system in all when studied implemented envelopes forinto the Tabledifferent analyzed 2 presents types period, theof heating whichenvelopes. means load The reduction that results when provided sh usingow reductions the by dynamic the dynamic higher system, system than the 100%when envelope implementedin all is ablestudied to provideintoenvelopes different more for heattypesthe analyzed from of envelopes. solar period, energy Thewhich than results means what sh isthatow transferred whenreductions using to higher the dynamic outdoor than environment100%system, in theall envelopestudied due to thermalenvelopesis able to losses. providefor the Results analyzed more alsoheat period, demonstratefrom solarwhich energy means that the than that system whatwhen performsis using transferred the better dynamic to whenthe system,outdoor implemented the environment envelope into buildingisdue able to to thermal walls provide with losses. more low thermalResultsheat from also mass, solar demonstrate as energy PCM is than more that what directlythe system is transferred exposed performs to solarto thebetter radiation outdoor when and environmentimplemented it can also dischargedueinto to building thermal stored walls losses. solar with heatResults low easily thermal also to demonstrate the mass, indoor as PCM environment. that isthe more system directly Table performs2 alsoexposed shows better to thatsolarwhen optimum radiation implemented PCM and it meltingintocan buildingalso temperature discharge walls with dependsstored low solarthermal on the heat constructive mass, easily as PCMto systemth eis indoormore in which directly environment. the exposed dynamic Table to system solar 2 isalsoradiation implemented, shows and that it withcanoptimum also its melting discharge PCM peakmelting stored at 21 temperature ◦solarC in theheat case easilydepends when to itthon ise theindoor implemented constructive environment. in wallssystem Table with in which high2 also thermal theshows dynamic mass that (Walloptimumsystem 1) andis PCMimplemented, 23 ◦ Cmelting in cases temperaturewith with its lower melting thermaldepends peak mass aton 21 the (Wall °C constructive in 2 the and case 3). when system it isin implemented which the dynamic in walls systemwith high is implemented, thermal mass with(Wall its 1) meltingand 23 °Cpeak in casesat 21 with°C in lower the case thermal when mass it is (Wallimplemented 2 and 3). in walls with high thermal mass (Wall 1) and 23 °C in cases with lower thermal mass (Wall 2 and 3). Appl. Sci. 2019, 9, 3688 9 of 12

Table 2. Heating load reduction by using static and dynamic PCM for each constructive system.

2 Construction System System Optimized TPCM (◦C) Qload [kJ/m ] Heating Load Reduction (%) Static - 2085.2 - NoPCM Wall 1 Static PCM 23 2085.2 0,00 Dynamic 21 311.6 114.94 − Static - 2033.7 - NoPCM Wall 2 Static PCM 22 2035.1 0,07 − Dynamic 23 2452.4 220.59 − Static - 2869.6 - NoPCM Wall 3 Static PCM 22 2871.0 0,05 − Dynamic 23 5994.4 308.89 −

3.3. Parametric Analysis of Amount of PCM As it was previously stated, the implementation of PCM in the movable layer can be achieved by means of using a polymeric matrix, or other system such as tubes or honeycomb layers. In any case, the addition of PCM will not achieve 100% w/w, as some matrix will be used to hold the PCM and prevent its leakage. Within this context, Table3 presents how the amount of PCM in the rotatable layer affects the performance of the whole system in terms of energy savings and optimal control strategy (t0 and t1). Results show that the dynamic system is able to provide considerable heating benefits even without using any PCM in the movable layer and just storing solar energy in the polymeric matrix in a sensible form. Thus, this can reduce the heating load by 36% when the dynamic system is implemented in Wall1 and Wall3 construction systems and by more than 50% when implemented into the Wall 2 envelope. Furthermore, the use of any amount of PCM enhances dramatically this effect, as the system achieves heating load reductions higher than 100% when using 50% w/w of PCM in Wall 1 and when using 25% w/w of PCM in Wall 2 and Wall 3 construction systems. That means that with these proportions of PCM, the system provides a positive energy balance, as the amount of solar energy dissipated indoors is higher than heat losses to the outdoor environment.

Table 3. Sensitivity analysis of PCM amount in heating load and control parameters by using dynamic system for each constructive system.

2 Constructive System PCM % w/w t1 t0 Qload [kJ/m ] Heating Load Reduction (%) 0 14:14 16:01 1319.8 36.71 25 13:29 16:36 278.9 86.63 Wall 1 50 12:37 16:44 189.8 109.10 − 75 12:17 16:53 309.9 114.86 − 100 12:26 16:58 311.6 114.94 − 0 13:12 15:07 949.1 53.33 25 11:25 15:01 425.5 120.92 − Wall 2 50 10:43 15:31 1359.8 166.86 − 75 10:42 15:24 2041.6 200.39 − 100 10:32 15:47 2452.3 220.59 − 0 12:02 13:11 1833.6 36.10 25 13:08 15:15 95.0 103.31 − Wall 3 50 11:49 14:59 2281.3 179.50 − 75 11:52 16:41 4260.8 248.48 − 100 11:49 15:49 5994.4 308.89 −

As expected, the more amount of PCM added in the movable layer, the higher heating benefits that the system can provide. However, this improvement is not always linear, as shown in Figure7. Appl. Sci. 2019, 9, 3688 10 of 12

The system cannot improve benefits when adding more PCM than 50% in the case when the system is Appl.implemented Sci. 2019, 9, x in FOR Wall PEER 1, asREVIEW the high thermal mass of the constructive system also plays an important10 of 12 role. On the other hand, lightweight systems (Wall 3) present a linear relation between thermal benefits benefitsand the amountand the ofamount PCM; of numerical PCM; numerical results suggest results that suggest thicker that rotatable thicker layersrotatable with layers PCM with could PCM be a couldgood optionbe a good to evenoption maximize to even maximize the heating the benefits. heating Thus,benefits. this Thus, remarks this theremarks importance the importance of a proper of adesign proper of design the system of the depending system depending on the morphology on the morphology of the building of the element building ( element or wall) (roof in or which wall) it inwill which be implemented it will be implemented and the weather and the conditions weather underconditions which under it will which be exposed. it will be exposed.

350 Wall 1 Wall 2 Wall 3 300

250

200

150

100

50 Heating load reduction [%] reduction load Heating

0 0 255075100 PCM amount [%]

Figure 7. Heating load reduction in case of different amount of PCM. Figure 7. Heating load reduction in case of different amount of PCM. 4. Conclusions 4. Conclusions A numerical proof of concept of a novel PCM heating technology implemented into building wallsA and numerical/or roofs proof is given. of concept The system of a is novel able toPCM change heating the positiontechnology of the implemented PCM layer with into respectbuilding to wallsthe insulation and/or roofs inside is thegiven. building The system component. is able Therefore,to change the systemposition can of the locate PCM PCM layer facing with outdoors respect toduring the insulation sunny hours inside to maximizethe building solar component. absorption Therefore, and the position the system of the can PCM locate facing PCM indoors facing to outdoorsprovide space during heating sunny when hours required to maximize by the demand. solar absorption The novel and dynamic the position system hasof beenthe PCM numerically facing indoorstested when to provide implemented space heating into three when diff erentrequired construction by the demand. morphologies The novel under dynamic Csa weather system conditions, has been numericallyproviding extraordinary tested when benefits implemented in all of them.into three Numerical different results construction demonstrate morphologies that the system under can Csa not weatheronly be used conditions, as a thermal providing barrier extraordinary from the outer benefits environment in all of but them. can provide Numerical active results space demonstrate heating from thatstored the solar system energy. can not Moreover, only be inused case as of a thermal using movable barrier layersfrom the with outer limited environment quantity ofbut PCM can provide (25–50% activew/w), space the system heating is ablefrom to stored provide solar positive energy. net Moreov energyer, balance, in case meaningof using movable that it releases layers indoors with limited more quantitystored solar of PCM energy (25–50% than all w/w), the heat the lossessystem to is the able outdoor to provide environment. positive net energy balance, meaning that itThe releases study indoors also highlighted more stored that solar the energy performance than all of the the heat system losses is highlyto the outdoor influenced environment. by certain designThe key study parameters, also highlighted such as melting that the temperature performance and of amount the system of PCM, is highly as well influenced as on control by schedule certain designto determine key parameters, activations such during as the melting day. These temperature variables and should amount be examined of PCM, andas well optimized as on forcontrol each schedulecase, considering to determine constructive activations system, during building the day. use, These and variables weather should conditions. be examined and optimized for eachThe case, high considering potential to constructive reduce energy system, consumption building for use, space and heatingweather applications conditions. showed by the numericalThe high results potential supports to reduce the future energy development consumptio of ann experimentalfor space heating prototype applications to quantify showed the benefits by the numericalfrom this technology results supports under relevant the future environment development and evaluateof an experimental its economic andprototype environmental to quantify benefits. the benefits from this technology under relevant environment and evaluate its economic and environmentalFunding: This work benefits. was partially funded by the Ministerio de Ciencia, Innovación y Universidades de España (RTI2018-093849-B-C31). The authors at the University of Lleida would like to thank the Catalan Government for the quality accreditation given to their research group (2017 SGR 1537). GREA is certified agent TECNIO in Funding:the category This of work technology was partially developers funded from by the Ministerio Government de ofCiencia, Catalonia. Innovación A.D.G. y has Universidades received funding de España from (RTI2018-093849-B-C31). The authors at the University of Lleida would like to thank the Catalan Government for the quality accreditation given to their research group (2017 SGR 1537). GREA is certified agent TECNIO in the category of technology developers from the Government of Catalonia. A.D.G. has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant Appl. Sci. 2019, 9, 3688 11 of 12 the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 712949 (TECNIOspring PLUS) and from the Agency for Business Competitiveness of the Government of Catalonia. Conflicts of Interest: The author declares no conflict of interest.

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