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

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Numerical Analysis of Building Envelope with Movable Phase Change Materials for Heating Applications applied sciences Article Numerical Analysis of Building 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: Latent heat storage materials have been tested by several researchers for decades to be used as passive heating and cooling systems in buildings 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 thermal comfort [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 walls 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 passive cooling, 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
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