Influence of PCMs in thermal insulation on thermal behaviour of building envelopes

Transcription

1 Journal of Physics: Conference Series PAPER OPEN ACCESS Influence of PCMs in thermal insulation on thermal behaviour of building enveloes To cite this article: K Dydek et al 016 J. Phys.: Conf. Ser View the article online for udates and enhancements. Related content - Structure, mechanical and thermal behaviour of mixtures of olyester resin and dental ceramic waste G Peña Rodríguez, L Martínez Maldonado and H J Dulce Moreno - Modelling of thermal behaviour of iron oxide layers on boiler tubes J D Angelo, A Bennecer, S Kaczmarczyk et al. - Advanced study of thermal behaviour of CSZ comaring with the classic YSZ coating A Dragomirescu, N Constantin, A tefan et al. This content was downloaded from IP address on 4/0/018 at 3:4

2 Influence of PCMs in thermal insulation on thermal behaviour of building enveloes K Dydek, P Furmański 1 and P Łaka Institute of Heat Engineering, Warsaw University of Technology, 1/5 Nowowiejska St, Warsaw, Poland furm@itc.w.edu.l Abstract. A model of heat transfer through a wall consisting of a layer of concrete and PCM enhanced thermal insulation is considered. The model accounts for heat conduction in both layers, thermal radiation and heat absortion/release due to hase change in the insulation as well as time variation in the ambient temerature and insolation. Local thermal equilibrium between encasulated PCM and light-weight thermal insulation was assumed. Radiation emission, absortion and scattering were also accounted for in the model. Comarison of different cases of heat flow through the building enveloe was carried out. These cases included resence or absence of PCM and thermal radiation in the insulation, effect of emissivity of the PCM microcasules as well as an effect of solar radiation or its lack on the ambient side of the enveloe. Two ways of the PCM distribution in thermal insulation were also considered. The results of simulations were resented for conditions corresonding to the mean summer and winter seasons in Warsaw. It was found that thermal radiation lays an imortant role in heat transfer through thermal insulation layer of the wall while the resence of the PCM in it significantly contributes to daming of temerature fluctuations and a decrease in heat fluxes flowing into or lost by the interior of the building. The similar effect was observed for a decrease in emissivity of the microcasules containing PCM. 1. Introduction Heat losses or gains by building enveloes contribute to the increased energy consumtion. In order to reduce energy demand materials having low values of thermal conductivity are used. Due to time variation in the ambient conditions enveloes should also have high heat caacity. Nowadays the buildings are characterized by thin walls searating the interior of the building from the ambient air and even when they are covered with thermal insulations of high quality and low density they lack sufficient heat caacity [1]. One of the ways to increase heat caacity of the building enveloes is to use hase change materials (PCMs). The hase change rocess involves a large amount of energy accumulated or released at a constant temerature or a narrow range of temeratures []. Both of these features are attractive for heating, cooling, and temerature stabilization uroses. PCMs have found alications in a wide range of areas including thermal energy storage [3], building energy efficiency, food roduct cooling, sacecraft thermal systems, solar ower lants, microelectronics [4], thermal rotection and 1 To whom any corresondence should be addressed. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by Ltd 1

3 waste heat recovery. In the case of buildings these materials have been tested as a thermal mass comonent for more than 40 years. Many otential PCMs were used including inorganic salt hydrates, organic fatty acids and eutectic mixtures, fatty alcohols, neoentyl glycol, and araffinic hydrocarbons [5]. It was found that PCMs imrove building energy erformance by significantly reducing eak loads and shifting eak-demand time [6]. Therefore PCMs are exected to assist in develoment of zero-net energy buildings. PCMs in buildings can assume different form and be located in different locations in the enveloe. The studies focused on imregnating concrete, gysum, or ceramic masonry or blending microencasulated PCMs in building materials [7, 8]. The micro-encasulation technology holds microscoic wax drolets inside hard acrylic olymer shells. Some roblems such as high initial cost, toxic behaviour, loss of hase change caability, corrosiveness (some inorganic PCMs) and PCM leaking have hamered widesread adotion of PCMs in buildings. Some of the PCMs (araffinic hydrocarbons) also relatively chea and having high heat caacity were at the same time found highly flammable. In the former alications, the chosen locations for flammable araffinic PCMs were the interior surfaces of the wall, ceiling, or floor. Subsequent research demonstrated that the micro-encasulated PCMs can be mixed with fibre insulations, incororated into structural and sheathing materials, or ackaged for localized alication. These locations are exected to significantly reduce flammability roblems [9]. One of the first successful develoments was PCM-enriched cellulose. Studies concentrated an effect of the PCM content in the insulation and its caability of heat accumulation were also carried out [10]. These studies were followed by develoment of PCMs blended with blown fiberglass and lastic foams. Theoretical redictions of PCMs alications in buildings enveloes were often resented in literature [11, 1, 13] but studies devoted to their use in thermal insulations are sarse and only account for heat conduction and hase change rocesses [14]. It is however known that thermal radiation can be an imortant mode in the light-weight fibrous insulations. The study on an imact of external solar radiation as well as thermal radiation within thermal insulation on heat losses or gains from building enveloes containing PCM-enriched insulation is the main toic of the resent aer.. Heat transfer through a building enveloe.1. Statement of the roblem and ambient conditions Two-layer building enveloe directed southward was considered. The inner layer was made from a 4 cm thick aerated concrete, while the second layer was a 1 cm thermal insulation made from mineral wool, which can be enhanced with the encasulated PCM uniformly sread across art or whole of its thickness. The internal sace of the building was ket at constant temerature of T i(t) = 4 C, corresonding to the thermal comfort. The ambient conditions were assumed to vary not only with time but also with the season of a year. Both ambient temerature and insolation flux were determined on the basis of data obtained from the Polish Institute of Meteorology and Water Management table 1 and. The ambient temerature was aroximated by the formula: e ( ) sin T t = T + A ωt (1) where: T av mean temerature during the season, A temerature amlitude during the season, ω = π/τ, with the eriod τ corresonding to 4 hrs. The amlitude of temerature variation was averaged over five years eriod. It was determined using extreme temeratures, which differed by 5% from the lowest and highest temerature in the resective season. av

4 Table 1. Mean temerature in Warsaw neighbourhood in the years [15]. Season\Year Mean Sring Summer Fall Winter Table. Temerature amlitude in Warsaw neighbourhood in the years [15]. Season/Year Sring Summer Fall Winter Mean Amlitude Figure 1. Variation of total irradiation in Warsaw neighbourhood during a day. Total solar irradiation, see Figure 1, was aroximated using the Gaussian function, i.e. assuming the normal distribution. Then the OriginPro 8 software was alied to generate the following exressions for summer and winter season, resectively: t ex - for the summer season 6.17 qs ( t) = t ex - for the winter season.58 () 3

5 .. Governing equations Heat in the building enveloe was assumed to be transferred by two modes: heat conduction in the concrete layer and heat conduction and thermal radiation in the insulation. All thermal and radiative roerties were assumed constant in each layer. The solar radiation was absorbed on the external surface of the building enveloe treated as the blackbody. Grey body model was assumed for thermal radiation in the insulation layer, in which small sherical articles with the encasulated PCM were distributed. The effect the microcasules olymer shells is neglected. The PCM resent in the articles undergoes the solid-liquid hase transformation in the range between the solidus and liquidus temeratures. The external and internal surfaces of the building enveloed exchanged heat with the ambient air by convection. The one-dimensional heat transfer was assumed due to large transverse dimensions of the building enveloe in comarison to its thickness. Moreover, the thermal equilibrium, i.e., equality of temeratures of the thermal insulation and encasulated PCM articles was declared. Then the energy equation took the following form, when the enthaly formulation was used: ( ρef hef ) T 4 ( id 4 T ) = λef + σ a, ef 4 Ω σ b (3) t x π where: (ρ efh ef ) is the effective volumetric enthaly, λ ef the effective thermal conductivity and σ a,ef stands for the effective linear absortion coefficient of the PCM enriched insulation. The second term on the right hand side of the equation above is associated with a heat source following from difference between radiation energy absorbed and emitted by the thermal insulation. The symbol i denotes total radiation intensity, which is integrated over the sherical angle Ω while σ b is the blackbody constant. The effective volumetric enthaly is related to the volume fractions of the insulation υ i and PCM containing articles υ, their densities ρ i and ρ as well as their secific enthalies h i and h : ρ h = υ ρ h + υ ρ h (4) ef ef i i i The secific enthalies can be determined from the formulae where the second one accounts not only for temerature but also the hase change resent in the PCM: h i = c T (5) i cst for T < Tsol h = ( 1 υ ) c T + υ ( c T + L) for T T < T cstsol + L + cl ( T Tliq ) for T > Tliq l s l s sol sol liq (6) where: c s and c l are the secific heats of the solid and liquid PCM, L is the latent heat of PCM melting while T sol and T liq denote the solidus and liquidus temerature, resectively. The symbol υ l is the volume fraction of liquid PCM in the microcasules. The liquid fraction varies with temerature and can be calculated from the exressions: 0 for T < Tsol T Tsol υ = for T T < T Tliq Tsol 1 for T > Tliq l sol liq (7) 4

6 In the case of the aerated concrete the energy equation reduces to: T ρ c t c c c T = λ x where the symbols ρ c, c c and λ c stand for the density, secific heat and thermal conductivity of the aerated concrete. The radiation intensity was determined from the Radiative Transfer Equation [16, 17]: di di σ 4 s, ef = ( eω e x ) = ( σ a, ef + σ s, ef ) i + σ a, ef σ bt + idω ds dx 4π (9) 4π which resents variation of the intensity along the ath s in sace (direction). The first term on the right hand side of the equation above is resonsible for intensity attenuation due to absortion and outscattering, the second term contributes to intensity enhancement due to radiation emission while the last term due to radiation in-scattering from other directions. The scattering was assumed to be isotroic. The symbols e Ω and e x denote the unit vectors associated with direction Ω and the coordinate axis x. In the PCM enriched thermal insulation radiation absortion and scattering occurs on fibres of a diameter of a few µm and PCM articles with the diameters usually lying between 50 to 100 µm. The effective linear absortion and scattering coefficients were determined from the following exressions: a, ef a, i a, (8) σ = σ + σ (10) σ = σ + σ (11) s, ef s, i s, where the second subscrit denotes the radiative roerty of the thermal insulation i or PCMcontaining microcasules, resectively. The radiative coefficients were taken from [18] while these for the PCM microcasules were evaluated from the formulae valid for the otical limit [19]: σ a, 3υ = ε (1) d σ s, 3υ ε = 1 d where: d denotes the article diameter and ε emissivity of its surface. The boundary condition at the internal A i and external A e surfaces of the building enveloe assumed the following form: T λ ( T T ) = α c i i x Ai (13) (14) T λi + qrad = α A e Te T + qs t e x ( ) ( ) where the radiative heat flux was determined from the exression: rad ( ) (15) e 4π Ω e x (16) q = i dω 5

7 It was also assumed that temerature distribution is continuous across the aerated concrete thermal insulation interface and that this interface corresonds to the black surface comletely absorbing thermal radiation. Table 3. Material roerties of building materials [0]. Proerty/Material Aerated concrete Mineral wool Density [kgm 3 ] Secific heat [Jkg 1 K 1 ] Thermal conductivity [Wm 1 K 1 ] Numerical solution and material roerties Numerical simulations were erformed using ANSYS Fluent code. The roblem was assumed to be quasi-d with surfaces erendicular to the y-axis treated as adiabatic. The regular mesh was generated using the quadratic elements (QUAD), the mesh quality was checked and the accuracy of calculations was verified by increasing the mesh density. The time ste was assumed to be 60 s while the convective heat transfer coefficients at the internal and external surface of the enveloe were secified to be α i = 7.69 Wm K 1 and α e = 9.09 Wm K 1, resectively [0]. The Radiative Transfer Equation was solved using Finite Volume Method [16, 17] with the sherical angle discretization corresonding to N θ N ϕ = 4 8. The absortion and scattering coefficients of the thermal insulation had the values σ a,i = 00 m 1 and σ s,i = 800 m 1 [18, 19]. The regular emissivity of the microcasules was assumed to be 0.7. Table 4. Thermo-hysical roerties of selected PCMs [1]. Proerty/Material Paraffin C18 Paraffin RT18_HC Tye Organic Organic Particiation mass in insulation [%] Diameter of the microcasules [µm] 5 5 Emissivity of the microcasules Solid hase temerature [K] Liquid hase temerature [K] Latent heat [kjkg 1 K 1 ] Secific heat [kjkg 1 K 1 ] Thermal conductivity [Wm 1 K 1 ] Initial condition for the simulations was obtain from the steady state solution of the considered roblem by fixing the constant external temerature to its mean value for each season. When the model has reached the steady state, the external constant temerature was relaced by the temerature variable in time using the sol-air temerature [14]: ( ) T T t q ( t) s e = e + (17) αe The system attained the fully eriodic state after three days of the real time. Temerature and heat flux at each oint of the mesh were recorded every 15 minutes of the real time. Proerties of the aerated concrete and mineral wool used are resented in table 3. 6

8 Phase change material was selected according to the following methodology. Distributions of the maximum and minimum temerature in the thermal insulation were found for no PCM resent in the thermal insulation. The observed difference in temerature distributions between the summer and winter seasons excluded using a single PCM. Therefore a mixture of two different hase change materials was alied assuming that only art of the PCM will work in each season (summer/winter) in accordance to the material characteristic. Proerties of the selected hase change materials are shown in table 4. For simlicity the secific heat and thermal conductivity of the solid and liquid hases were assumed equal. Figure. Variation of heat loss from the room during a winter day. Figure 3. Distribution of the maximum temerature in the thermal insulation for absence and resence of solar irradiation. 3. Results and discussion In order to investigate the effect of different modes of heat transfer and resence of the PCM in the thermal insulation on the distribution of the temerature, fraction of PCM that undergoes hase change and heat flow, a series of different simulations were carried out: Only heat conduction in the insulation layer and the variable ambient temerature, Heat conduction and thermal radiation in the insulation layer and the variable ambient temerature, Heat conduction and thermal radiation in the insulation layer and the variable ambient temerature and solar heat flux, Heat conduction and thermal radiation in the insulation layer and the variable ambient temerature and solar heat flux, resence of PCM microcasules across the whole insulation layer, Heat conduction and thermal radiation in the insulation layer and the variable ambient temerature and solar heat flux, resence of PCM microcasules only in the 3 cm outer layer of thermal insulation, Heat conduction and thermal radiation in the insulation layer and the variable ambient temerature and solar heat flux, resence of PCM microcasules across the whole insulation layer with variable emissivity of the microcasule surface. At first the influence of radiation on the building enveloe behaviour is considered. Figure shows heat lost from the room in the winter season for the cases when thermal radiation is absent or resent in the insulation. The significant increase in the heat loss is observed when the thermal radiation is accounted for. Distribution of the maximum temerature across the thermal insulation layer in the summer season, Figure 3, demonstrates the role of solar irradiation incident on the external surface of 7

9 the building enveloe. The noteworthy difference in temeratures observed was imortant for selection of the aroriate PCM. In addition the greater difference between the minimum and maximum temerature (about 6 C) were found when the solar irradiation was included in the simulation. Figure 4. Variation of temerature with time at the concrete-thermal insulation interface for a day in the summer season. Figure 5. Variation of heat flux with time at the concrete-thermal insulation interface for a day in the summer season. Figure 6. Variation of temerature with time at the concrete-thermal insulation interface for a day in the winter season. Figure 7. Variation of heat flux with time at the concrete-thermal insulation interface for a day in the winter season. The next four figures illustrate how resence of the PCM microcasules and it distribution in the thermal insulation affects temerature and heat flux at the interface between the aeriated concrete and insulation layers during a day in the summer season (Figures 4 and 5) and in the winter season (Figures 6 and 7). The figures resent two cases of the PCM distribution: PCM uniformly distributed across the whole layer, PCM uniformly distributed only in the outer 3 cm layer of insulation. They are comared with the case where no PCM occurs in the thermal insulation. Presence of the PCM in the thermal insulation leads to a decrease in on of temerature fluctuations and heat flux. The extremum temeratures are shifted more to longer times the more art of the insulation layer been enriched in the PCM. The ositive values of the heat flux corresond to heat flow from the concrete layer while the negative values into the concrete layer. The decrease in temerature and heat flux values is more visible for the summer than for the winter season. 8

10 Figure 8. Effect of the microcasule emissivity on temerature variation with time at the concrete-thermal insulation interface for a day in the summer season, PCM distributed in the whole insulation layer. Figure 9. Effect of the microcasule emissivity on temerature variation with time at the concrete-thermal insulation interface for a day in the winter season, PCM distributed in the whole insulation layer. Finally, influence of the emissivity of micro-casules containing PCM was examined. The emissivity of the microcasules surface was decreased from ε = 0.7 to ε = 0.5 and ε = 0.3 resectively. Figures 8 and 9 show, that the decrease in emissivity causes small decrease in temerature fluctuations when the PCM is sread over the whole insulation layer. The decrease is greater for the summer season. The heat flux fluctuations on the room side of the building enveloe are decreased by about 8% if the emissivity of the microcasule surface is reduced to ε = 0.3. Furthermore, if the PCM only aears in the outer layer of the insulation, changes in temerature distribution and heat flux are almost unnoticeable. 4. Conclusions Temerature distribution in the two-layer building enveloe and heat flux leaving it deend on thickness and thermal and radiative roerties of the layers, ambient atmosheric condition, volume fraction, diameter and emissivity of the microcasules containing PCM. Comarison of different cases of heat flow through the enveloe was carried out. These cases included resence or absence of PCM and thermal radiation in the insulation, effect of emissivity of the PCM microcasules as well as an effect of solar radiation or its lack on the ambient side of the enveloe. Two ways of the PCM distribution in thermal insulation in the insulation were also considered. The choice of the aroriate PCM or its mixture as in the studied case should be erformed using the redicted minimum and maximum temerature in the insulation accounting for different variation of the ambient temerature and solar irradiation in the summer and winter seasons of a year. The results of calculation show that radiative heat transfer in the light-weight thermal insulation lays an imortant role in heat transfer and that resence of the PCM in the thermal insulation contributes to the significant reduction in temerature and heat flux fluctuations in the building enveloe. Therefore influence of radiative roerties of the PCM-filled microcasules should be further studied including different microcasule diameter and the wavelength deendence of their radiative roerties. Due to relatively long duration of the PCM melting/freezing rocess, oor heat transfer between the microcasules and the surrounding gas resent in the insulation as well as the long-distance interaction of thermal radiation the microcasules may not be in thermal equilibrium with the surrounding insulation therefore two temerature model of heat transfer through the PCM enriched insulation should be considered in the future. 9

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