Integrating phase change materials (PCMs) in thermal energy storage systems for buildings

Integrating phase change materials (PCMs) in thermal energy storage systems for buildings

13

F. Kuznik, K. Johannes, D. David Université de Lyon, France

13.1 Introduction As demand for thermal comfort in buildings is steadily rising, the energy consumption is correspondingly increasing. For example, in France, the energy consumption of buildings has increased by 30% during the last 30 years. Housing and tertiary buildings are responsible for the consumption of approximatively 46% of all energy and approximatively 19% of the total CO2 emissions (Climate Plan, 2004). Nowadays, thermal energy storage systems are essential for reducing dependency on fossil fuels and then contributing to a more efficient, environmentally benign energy use (Dincer and Rosen, 2002). Thermal energy storage can be accomplished either by using sensible heat storage or latent heat storage. Sensible heat storage has been used for centuries by builders to store/release passively thermal energy, but a much larger volume of material is required to store the same amount of energy in comparison to latent heat storage. The principle of phase change material (PCM) use is simple. As the temperature increases, the material changes phase from solid to liquid. The reaction being endothermic, the PCM absorbs heat. Similarly, when the temperature decreases, the material changes phase from liquid to solid. The reaction being exothermic, the PCM desorbs heat. The use of PCM integrated in building walls (PCMIBW) is a way to enhance the storage capacity of the building envelope and then to rationalize the use of renewable and non-renewable energies.

13.2 Integration of phase change materials (PCMs) into the building envelope: physical considerations and heuristic arguments 13.2.1 Physical considerations The building is a quite complex object submitted to internal and external solicitations (see Figure 13.1). External solicitations are due to the local external weather. Internal Advances in Thermal Energy Storage Systems. http://dx.doi.org/10.1533/9781782420965.2.325 Copyright © 2015 Elsevier Ltd. All rights reserved.

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Convection Radiation

Convection

Convection

Radiation

Convection Radiation

Radiation Convection

Conduction

Figure 13.1 Schematic of heat transfers in a building (from Kuznik et al., 2011a, with permission from Elsevier).

solicitations come from solar radiative flux entering the building and internal loads. A highly energy-efficient building must have an energy-efficient envelope that can ensure comfort of occupants with a minimum system energy requirement. From this point of view, thermal energy storage in the envelope is a key factor. Inside a room, the heat transfer processes between the surface of the wall and the solicitations are: ∑ convective heat transfer between the air and the surface, ∑ shortwave radiative heat transfer, ∑ longwave radiative heat transfer.

The heat transfer in the wall is conduction. Outside the envelope, the heat transfer processes are the same as inside the room. The effect of thermal energy storage in the building envelope is to reduce the indoor temperature fluctuations and to delay the air temperature extremes. Thermal energy is usually stored in the building envelope by sensible heat of the materials. The storage capacity is related to the mass-specific heat capacity and the mass of the materials used in the building envelope. Of course, the storage capacity of the envelope is also related to the composition of the walls and the technological solutions. For example, a wall composed of concrete with external insulation has a higher storage capacity than the same wall with internal insulation. For example, lightweight buildings have low thermal energy storage capacity because of the materials used for the envelope. In that case, integration of PCM enhances the storage capacity (see Figure 13.2): as the temperature increases, the material changes phase from solid to liquid and the PCM absorbs heat. Similarly, when the temperature decreases, the material changes phase from liquid to solid and the PCM desorbs heat. PCM can also be used to control the air temperature

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Insulation

Glazing window Solar flux

PCMIBW

Figure 13.2 A schematic of PCMIBW in a house with solar gain (from Kuznik et al., 2011a, with permission from Elsevier).

(operative, temperature or radiative temperature or air temperature): contrary to sensible heat storage, latent heat storage occurs at the phase change temperature without a significant rise in temperature. On the whole, a highly energy-efficient building envelope must have the following characteristics: ∑ few thermal energy losses through the exterior by thermal insulation, ∑ use of renewable energy using, for example, sun radiation through glazing windows, ∑ limitation of overheating or energy demand peaks using PCMIBW.

13.2.2 Heuristic arguments Using very simplified assumptions, Peippo et al. (1991) presented approximate formulae for optimum phase change temperature and thickness of the PCMIBW: Q ht stor st

(13.1)

td h (Tm, opt – Tn ) rD DH H

(13.2)

Tm,opt = Te +

Dopt = Tr =

t d T + t nTn td + tn

(13.3)

where: Tm,opt is the optimal phase change point of the PCM [°C], Tr is the average room temperature [°C], Q is the heat absorbed by unit area of the room surface [J/m2], h is the average heat transfer coefficient between wall surface and surroundings [W/m–2/°C–1]

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Td is the daytime room temperature [°C], Tn is the nighttime room temperature [°C], td is the charging time, day [s], tn is the discharging time, night [s], tstor is the diurnal storage, cycle = td + tn [s] (24h), Dopt is the optimal thickness of the PCM slab [m], DH is the latent heat of fusion of PCM [J/kg]. The previous formulae are very simplified and cannot be used to optimize a PCM wall used in a building. For example, the optimal thickness of the wall depends greatly on the thermal diffusivity of the PCM composite. The reason is that the penetration time for transient heat conduction tp can be evaluated by (Kuznik et al., 2008a): tp ª

(e/2)2 a

(13.4)

where a is the thermal diffusivity of the medium [m2/s] and e the thickness of the wall [m]. Of course, the penetration time must have a value lower than 12h if diurnal heat storage is required.

13.3 Organic and inorganic PCMs used in building walls The phase change materials used in building wall applications can be either organic materials or inorganic materials.

13.3.1 Organic PCMs Organic PCMs include paraffins, fatty acids and polyethylene glycol (PEG). They present a congruent phase change, they are not dangerous, and they have a good nucleation rate. Table 13.1 presents the thermal properties of organic materials found in the literature, which may meet the specifications listed above. Tf is the temperature of fusion, Hf is the latent heat of fusion, Cps and Cpl are the heat capacities of the solid and liquid phases, ks and kl are the thermal conductivity of the solid and liquid phase. The advantages of organic PCMs are: ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑

availability in a large temperature range, freeze without much supercooling, ability to melt congruently, self-nucleating properties, compatibility with conventional construction materials, no segregation, chemically stable, high heat of fusion,

Tf [°C]

Hf [kJ/kg] Cps [kJ/ kg/K]

Cpl [kJ/ kg/K]

ks [W/ m2/K]

kl [W/ m2/K]

Reference

GR25

23.2-24.1

45.3

1.2

1.2

n.a.

n.a.

Ahmad et al. (2006a, 2006b)

PEG600

22

127.2

n.a.

2.49

n.a.

0.189

Ahmad et al. (2006a)

n-octadecane

27

243.5

1.934

2.196

0.358

0.148

Alawadhi (2008)

n-eicosane

37

241

2.01

2.04

0.15

0.15

Alawadhi (2008)

P116

47

225

2.4

1.9

0.24

0.24

Alawadhi (2008)

Butyl-stearate

19

140

n.a.

n.a.

n.a.

n.a.

Athienitis et al. (1997), Lee et al. (2000)

ERMEST2325

17-20

138

n.a.

n.a.

n.a.

n.a.

Banu et al. (1998), Scalat et al. (1996)

RT27

26-28

179

1.8

2.4

0.2

0.2

Borreguero et al. (2010), Castell et al. (2010), Castellon et al. (2009), Evers et al. (2010)

MICRONAL26

26

110

n.a.

n.a.

n.a.

n.a.

Castellón et al. (2009), Cabeza et al. (2007)

RT20

22

172

n.a.

n.a.

n.a.

n.a.

Fang and Zhang (2006)

MP65%-MS35%

21.8-24.5

175

n.a.

n.a.

n.a.

n.a.

Feldman et al. (1995)

MP77%-MS23%

22.4-23.8

177

n.a.

n.a.

n.a.

n.a.

Feldman et al. (1995)

MP93%-MS7%

22.2-22.8

182

n.a.

n.a.

n.a.

n.a.

Feldman et al. (1995)

GR41

43

63

n.a.

n.a.

0.15

0.15

Huang et al. (2006)

GR27

28

72

n.a.

n.a.

0.15

0.15

Huang et al. (2006)

155

n.a.

n.a.

n.a.

n.a.

Karaipekli and Sari (2008)

Eutectic capric-myristic 21.7

(Continued)

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Materials

Integrating phase change materials (PCMs) in thermal energy storage systems for buildings

Organic PCMs in the literature (MP: methyl palmitate; MS: methyl stearate; U: unknown; n.a.: not available)

Table 13.1

(Continued)

330

Table 13.1

Tf [°C]

Hf [kJ/kg] Cps [kJ/ kg/K]

Cpl [kJ/ kg/K]

ks [W/ m2/K]

kl [W/ m2/K]

Reference

MICRONAL 5001

26

110

n.a.

n.a.

n.a.

n.a.

Konuklu and Paksoy (2009)

MICRONAL 5008

22

110

n.a.

n.a.

n.a.

n.a.

Konuklu and Paksoy (2009)

heptadecane

22

214

n.a.

n.a.

n.a.

n.a.

Koschenz and Lehmann (2004)

MPCM28-D

28

180-195

n.a.

n.a.

n.a.

n.a.

Lai et al. (2010)

UNICERE55

45-60

185

n.a.

n.a.

n.a.

n.a.

Lee et al. (2000)

n-nonadecane

31.8

160

n.a.

n.a.

n.a.

n.a.

Li et al. (2010)

Eutectic capric-stearic

24.7

179

n.a.

n.a.

n.a.

n.a.

Sari et al. (2008)

Non-eutectic capriclauric

19.2-20.3

144-150

n.a.

n.a.

n.a.

n.a.

Shilei et al. (2007)

U3

28

244

n.a.

n.a.

0.28

0.22

Voelker et al. (2008)

U4

13.6-23.5

104.5107.5

4

4.1

0.18

0.22

Kuznik et al. (2008a, 2008b), Kuznik and Virgone (2009a,b), Liu and Awbi (2009)

RT25

25

147

2.9

2.1

1.02

0.56

Weinlder et al. (2005)

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Materials

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∑ safe and non-reactive, ∑ recyclable.

The disadvantages of organic PCMs are: ∑ low thermal conductivity, ∑ low volumetric latent heat storage capacity, ∑ flammable (depending on containment).

13.3.2 Inorganic PCMs The inorganic PCMs are salt hydrates. Table 13.2 lists some inorganic PCMs. The advantages of inorganic PCMs are: ∑ ∑ ∑ ∑ ∑

high volumetric latent heat storage capacity, low cost and easy availability, sharp phase change, high thermal conductivity, non-flammable.

The disadvantages of inorganic PCMs are: ∑ ∑ ∑ ∑

high volume change, supercooling, segregation, corrosion.

13.4 PCM containment 13.4.1 Impregnation of building materials with PCM The simplest method consists in the direct impregnation of the PCM into gypsum, concrete or other porous materials to form mixed-type PCMIBW. Khudhair and Farid (2004) explained the different impregnation techniques. The volume occupied by the PCM in the pores is small enough to prevent the isolation of the solid PCM crust. The structure of the porous material transports the heat to the pores. Unfortunately, important leakage has been observed, in particular by Xiao et al. (2002). Cabeza et al. (2007) also reported an interaction between the PCM and its porous container. This interaction can deteriorate the mechanical properties of the container. The materials used for impregnation are: ∑ plaster: Athienitis et  al. (1997), Banu et  al. (1998), Scalat et  al. (1996), Feldman et  al. (1991, 1995), Koschenz and Lehmann (2004), Sari et al. (2008), Shilei et al. (2007), Hawes et al. (1993), Feldman and Banu (1996) ∑ concrete: Lee et al. (2000), Hadjieva et al. (2000), Hawes et al. (1993) ∑ vermiculite: Karaipekli and Sari (2008)

332

Table 13.2

Inorganic PCMs in the literature (U: unknown)

Materials

Tf [°C]

Hf [kJ/kg] Cps[kJ/kg/K] Cpl [kJ/kg/K] ks [W/m2/K] kl [W/m2/K]

Reference

Eutectic salt

32

216

Carbonari et al. (2006)

25–26

180

2.5

26–29

175

2.3

1.4

Sodium thiosulfate pentahydrate

40–48

210

1.46

2.4

U1

30–32.5

131

0.6

0.6

Castell et al. (2010)

1

1

Evers et al. (2010) Hadjieva et al. (2000) Medina et al. (2008), Zhang et al. (2005a)

U2

26–28

188

CaCl2.6H2O

29.8

191

1.44

1.44

1.09

0.54

Pasupathy et al. (2008)

S27

27

190

1.5

2.22

0.79

0.48

Weinlder et al. (2005)

L30

30

270

1.23

1.79

1.02

0.56

Weinlder et al. (2005)

Voelker et al. (2008)

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SP-25-A8 Calcium chloride hexahydrate

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∑ wood: Li et al. (2009) ∑ cement: Li et al. (2010) ∑ compound: Lin et al. (2005), Xu et al. (2005), Zhang et al. (2006a)

13.4.2 Micro-encapsulation of PCM The micro-encapsulation consists in enclosing the PCM in a microscopic polymer capsule. The microcapsules form a powder, which is then included in the recipe of a building construction material. Special attention has to be given to the choice of the capsule material to avoid chemical reactions between the capsules and the building material. The PCM is trapped should no longer leak, and the size of the capsules should be small enough to prevent a disproportionate isolation of the solid crust of the PCM. The quality of the process of micro-encapsulation is evaluated by the ratio between the mass of the satisfying capsules (hermetic capsules containing PCM) and the total mass of the powder. Hawlader et al. (2003) investigated the influence of this ratio on several parameters such as the duration of the process, the quantity of PCM and reticulation agent introduced in the solution, for a coacervation microencapsulation. Three characteristics of the capsules are relevant to appreciate the quality of the powder: their mean diameter, the thickness of their shell, and the mass percentage of PCM compared to the total mass of the capsule. For an in-situ polymerization, Zhang et  al. (2005b) varied the strength of the beater, which caused a variation in the mean size of the capsules. Sarier and Onder (2007) performed a statistical study on the size of the capsules to evaluate the degree of inhomogeneities in a powder. The PCM powder has to be included into the mixture of a building material, such as concrete, a polymer or gypsum, to form the improved building phase change material. Thus, the thermal behaviour of the PCMIBW depends on the thermal specifications of the building material. However, differential scanning calorimetry (DSC) measurements have been performed directly on the PCM powder to determine its specific latent heat and its temperature of fusion. Yamagishi et al. (1996) observed a supercooling effect during the solidification of micro-encapsulated material. When the size of the microcapsules decreases below a few microns, the nucleation agents, which are necessary to the start of the solidification, rarefy. The solidification is delayed. Zhang et al. (2005b) attenuated the supercooling effect by adding nucleation agents into the PCM. The nucleation agent they used was 1-tetradecanol for C14 PCM, and 1-pentadecanol for C15 PCM. In the literature, many papers deal with microencapsulated PCM in building material, most of them being plaster material. For example, Schossig et al. (2005) built gypsum boards containing microencapsulated PCM, of which the temperature of fusion was around 25°C. Figure 13.3 is an SEM photograph of the PCM in the concrete.

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Figure 13.3 SEM photograph of a concrete wall containing micro-encapsulated PCM (from Schossig et al., 2005, with permission from Elsevier).

10 mm

(a)

(b)

Figure 13.4 Pictures of shape-stabilized PCM: (a) the PCM plate; (b) SEM picture (from Zhou et al., 2007, with permission from Elsevier).

13.4.3 Shape-stabilized PCM Shape-stabilized PCM are prepared from a liquid mixture of the PCM and a supporting material. The mixture is then cooled below the glass transition temperature of the supporting material, until it becomes solid. An appropriate choice of the supporting material allows PCM mass proportions up to 80%. Figure 13.4 shows two pictures of a plate made of shape-stabilized PCM. On the first one, one can notice that the shape-stabilized PCM looks like a homogeneous material. The second picture shows the microstructure of the material. The most common supporting materials found in the literature are high-density polyethylene (HDPE) and styrene-butadiene-styrene (SBS). Sari (2004) did not observe

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any leakage of the phase change material by using HDPE as supporting material. Xiao et al. (2002) made the same remark for SBS. Zhang et al. (2006a) report that the PCM mixes better into SBS than HDPE, but the shape-stabilized PCM is more rigid when using HDPE. The thermal conductivity of a shape-stabilized PCM is not very high, which is a problem in latent heat storage systems. Thus, researchers added some materials into their shape-stabilized PCM composition to improve the conductivity. The most complete study of those additives has been carried out by Zhang et  al. (2006b). They found the that most efficient conducting material was ex-foliated graphite. The conductivity of their shape-stabilized PCM evolved from 0.150 W/m.K to 0.229 W/m.K by adding 10 wt.% graphite into the mixture. Zhang et  al. (2006b) developed a model to predict the thermal conductivity of the material from its composition.

13.4.4 Other containers Other containers can also be used for the integration of PCM into building walls. Ahmad et  al. (2006a, 2006b) used PVC panels filled with PCM. Carbonari et  al. (2006) used sandwich panels with plastic rigid containers of PCM. Castell et  al. (2010) used CSM panels. Konuklu and Paksoy (2009) tested aluminium foils to incorporate PCM in a multi-layer panel. Medina et al. (2008), Voelker et al. (2008), Zhang et al. (2005a) and Guceri and Faunce (1979) filled tubes with PCM that was integrated in the wall. Pasupathy et  al. (2008) filled a steel container with PCM for being included in the roof slab. Jin et al. (2013) filled polyethylene sheets with macroscopic flat bubbles covered with aluminium foil.

13.5 Measurement of the thermal properties of PCM and PCM integrated in building walls Arkar and Medved (2005) and Cho and Choi (2000) showed that a perfect knowledge of the thermal properties of the PCM and the way those properties are measured is necessary to correctly analyse a latent heat storage system. Tyagi and Buddhi (2007) warn the reader about data provided by the manufacturer, which could be erroneous (usually overoptimistic). Lazaro et  al. (2013) carried out a round robin test to determine the enthalpy curve of octadecane. Their results show that it is still difficult to have the same curves as they depend highly on the protocol of measurement and interpretation. In Dumas et al. (2014), numerical results show that the temperature and the heat flux cannot be correctly predicted if special attention is not paid to the interpretation of the calorimetry experiments. Thus, measurement methods have been developed in order to obtain the thermodynamic characteristics of PCMs. Even though several measurement methods exist, differential scanning calorimetry (DSC) is the most common.

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13.5.1 Differential scanning calorimetry (DSC) This measurement method was initially developed to characterize the heat exchanges between some materials and their environment during transformations such as polymerization or phase change of polymers. The name differential scanning calorimetry is very explicit: ∑ Calorimetry: the calorimetry is the measurement of the quantity of heat that can be absorbed or released by a body subjected to a change in temperature. In our case, the heat transfer is due to conduction. ∑ Differential: the measurement setup is designed to have two different samples in identical conditions. The thermal reaction of the sample being characterized is obtained by comparison with the thermal reaction of the reference sample (whose properties are known). ∑ Scanning: the thermal excitation is a ramp of temperature. The temperature rate has to be determined by the researcher.

Two kinds of DSC setups exist. The power compensation DSC consists in two independent calorimeters. The heat flux DSC has a Siamese structure: the two samples are connected to the same metal disc; the behaviour difference between the samples submitted to the same temperature excitation leads to a voltage difference between the samples; the absorbed heat in the PCM sample is deduced from the voltage. When the weight of the DSC measurement sample is only a few grams, DSC provides information about the local properties of the material. It does not characterize the thermal behaviour of the bulk BIPCM. Bigger sample sizes are required to test composite materials. Also, the reference capsules may be filled with the container material in order to amplify the phase change effect (Bareneche et al., 2013a). DSC measurements are highly sensitive to the sample preparation procedure and measurement calibration. Several temperature cycles are needed to check the repeatability of the measurement (Barreneche et al., 2012, 2013b). Castellón et al. (2008) and Lazaro et al. (2013) organized international intercomparative measurement campaigns to define a rigorous DSC measurement procedure. Annex 17 of ECES-IEA (International Energy Agency) (2005) observed the response of several samples with different masses to a temperature scanning with different rates. The material of the sample did not suffer from supercooling. Results are shown in Figure 13.5. The equivalent heat capacity calculated using the DSC curves is clearly influenced by the sample mass and heating rate. But from a thermodynamic point of view, the enthalpy, which is an intensive variable, cannot depend on the mass sample or the heating rate. The DSC is a complex system and the direct use of the measured curves is not physically correct because some heat transfer phenomena are omitted: the convection in the sample (i.e., capsule), the non-uniformity of the temperature in the sample (conduction), the time needed to heat or cool the sample (inertia), etc. Therefore an inverse method based on these physical phenomena is necessary to qualitatively enhance the results of DSC (Dumas et al., 2014; Gibout et al., 2013; Franquet et al., 2012). Figure 13.6 (from Kuznik and Virgone, 2009a) shows the DSC curves obtained for a paraffin mixture. The heating and cooling curves are of course different and

Integrating phase change materials (PCMs) in thermal energy storage systems for buildings 100

Result of cp depends on: – sample mass – heating rate

0.5K/min Serie A 2K/min Serie B

90 80 70

1K/min Serie B

60

0.5/min Serie B

50 40

Serie A 13 0 mg Serie B 22 6 mg

cp (J/gK)

2K/min Serie A 1K/min Serie A

337

30 20 10

20

21

22

23

24 25 26 Temperature (°C)

27

28

29

30

0

Figure 13.5 Temperature scanning responses depending on sample mass and heating rate (from Annex 17-ECES-IEA, 2005). 17.8°C Freezing curve

3 2 Heat flow (W/g)

33.5 J/g

23.5°C 71 J/g (–104.5 J/g)

1 0

13.6°C 72.4 J/g (107.5 J/g)

–1 –2

35.5 J/g Melting curve

–3 –20

22.2°C –10

0 10 Temperature (°C)

20

30

Figure 13.6 DSC melting and freezing curves for the composite PCM (from Kuznik and Virgone, 2009a, with permission from Elsevier).

the melting and freezing temperatures are 13.6°C and 23.5°C, respectively. The mixture phase change depends on the phase diagram but the DSC curves are not sufficient to determine it. Further investigations are needed to calculate the physical characteristics needed to model the phase change of such PCM mixtures.

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13.5.2 The T-history method The T-history method has been designed to test large samples. It also provides information about the thermal conductivity of the PCMIBW, and enables several samples to be tested at the same time. The method is explained by Zhang et  al. (1999): samples of PCMIBW are put in different vertical tubes; a reference material is also put in a vertical tube. The temperature of the sample is measured with a thermocouple located at the centre of the tube. At the beginning of the test, all the materials are in the liquid phase. The tubes are suddenly immersed into a controlled atmosphere (usually cold water), in which the temperature is regulated and is below the fusion temperature of the PCMIBW. The temperature inside each tube and in the controlled atmosphere is monitored; an example of the curve obtained is shown in Figure 13.7(b). A convective coefficient is deduced from the temperature curve of the reference material. Three steps appear in the temperature curve of the PCMIBWs: the cooling

Tube

Thermocouple PCM sample

Data logger system Water bath I

Software

Water bath II (a)

T (°C) T0

Tm,1 Tm,2

A1

Tr

A2 A3

Ta,* or Tw,* 0

t 1

(b)

t 2

t (s)

Figure 13.7 (a) T-history experimental setup; (b) T-history temperature curves (from Kuznik et al., 2011a, with permission from Elsevier).

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of the liquid phase, the solidification of the PCM, and the cooling of the solid phase. The latent heat of the material and the heat capacity of its different phases are obtained after the calculation of the areas between the sample temperature curve and the atmosphere temperature curve (A1, A2 and A3 in Figure 13.7(b)) for each step. The thermal conductivity of the sample is obtained by using an inverse method on the total solidification time. The method was improved by Marin et al. (2003) to obtain the heat capacity as a function of the temperature. Peck et al. (2006) proposed laying the tubes horizontally in order to minimize disparities of the heat flux on their surface.

13.5.3 The guarded hot-plate setup Darkwa and Kim (2005a) used the guarded hot-plate to compare the storage performances of two different PCM wallboards. The setup contained a stack composed of a cold source, a hot source, a heat flux meter and the wallboard sample. The stack is isolated on every side. Figure 13.8 shows the setup. The specificities of the wallboard sample are obtained by integration of the measured heat flux during the phase change. Schossig et al. (2005) also designed a guarded hot-plate type measurement setup. The stack was composed of the wall sample and one copper plate on each side.

13.6 Experimental studies Table 13.3 summarizes the experimental studies concerning measurements held in a room with walls containing PCM. Most of these experiments were carried out in outdoor conditions with no internal gains due to occupation (i.e., real use of the building). The phase change temperature of the materials tested varies between 20°C and 30°C, which is the usual thermal comfort zone of buildings. The PCM are mainly contained in plasterboards.

Insulated

Insulated Sample HFM Hot plate Cold plate

TC TC

Data logger

Heater

Chiller

Figure 13.8 The guarded hot-plate setup (from Darkwa and Kim, 2005a, with permission from Wiley InterScience).

Experimental studies involving PCM wallboards; the material refers to Tables 13.1 and 13.2 Material

Container

Cell size

Number of cells

Conditions

Ahmad et al. (2006b)

PEG600

PVC panel

0.9m ¥ 0.9m ¥ 0.9m

2

Outdoor

Athienitis et al. (1997)

Butyl stearate

Gypsum

2.88m ¥ 2.22m ¥ 2.24m

1

Outdoor

Banu et al. (1998)

Butyl stearate palmitate; Gypsum ERMEST2325

2.27m ¥ 2.29m ¥ 2.45m

2

Laboratory

Cabeza et al. (2007)

MICRONAL26

Gypsum

2.4m ¥ 2.4m ¥ 2.4m

2

Outdoor

Carbonari et al. (2006)

Eutectic salt

Sandwich panel

4.37m ¥ 3.39m ¥ 2.7m

1

Laboratory

Castell et al. (2010)

RT25; SP25A8

CSM panel

2.4m ¥ 2.4m ¥ 2.4m

2

Outdoor

Castellón et al. (2009)

MICRONAL26; RT27

2.4m ¥ 2.4m ¥ 2.4m

2

Outdoor

Fang and Zhang (2006)

RT20

Gypsum

0.7m ¥ 0.7m ¥ 0.7m

3

Laboratory

Fang et al. (2008)

RT20

Gypsum

0.7m ¥ 0.7m ¥ 0.7m

3

Laboratory

Jin et al. (2013)

Salt hydrate

Polyethylene flat bubbles

1.2m ¥ 1.2m ¥ 1.2m

1

Laboratory

Konuklu and Paksoy (2009)

PC5001; PCM5008

Aluminium foils

2.7m ¥ 2m ¥ 1.5m

3

Outdoor

Kuznik et al. (2008b), Kuznik and Virgono (2009a)

U4

ENERGAIN

3.1m ¥ 3.1m ¥ 2.5m

1

Laboratory

Kuznik et al. (2009b)

U4

ENERGAIN

0.5m ¥ 0.5m ¥ 0.5m

2

Laboratory

Li et al. (2009), Zhang et al. (2006a) Paraffin

Shape-stabilized

0.575m ¥ 0.453m ¥ 0.463m

1

Outdoor

Lin et al. (2005)

Paraffin

Shape-stabilized

3m ¥ 2m ¥ 2m

1

Outdoor

Liu and Awbi (2009)

U4

ENERGAIN

4m ¥ 3m ¥ 2.5m

1

Laboratory

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Table 13.3

MICRONAL23

Gypsum

–

–

Real building

Medina et al. (2008), Zhang et al. (2005a)

U1

Tube

1.83m ¥ 1.83m ¥ 1.22m

2

Outdoor

Meng et al. (2013)

RT18; SP29

CSM panel

1m ¥ 1m ¥ 1m

2

Outdoor

Pasupathy et al. (2008)

U2

Steel

1.22m ¥ 1.22m ¥ 1.44m

2

Outdoor

Scalat et al. (1996)

ERMEST2325

Gypsum

2.27m ¥ 2.29m ¥ 2.45m

2

Laboratory

Gypsum

Room of a building

2

Outdoor

Shilei et al. (2007)

Capric-lauric non eutectic

Gypsum

5m ¥ 3.3m ¥ 2.8m

1

Outdoor

Tyagi et al. (2012)

CaCl2.6H2O

HDPE container

9.26m ¥ 2.41m ¥ 2.6m

1

Laboratory

Voelker et al. (2008)

U3; CaCl2.6H2O

Gypsum; tube

2.95m ¥ 4.43m ¥ 2m

2

Outdoor

Kuznik et al. (2011a)

U4

ENERGAIN

5.2m ¥ 3.5m ¥ 2.5m

2

Real building

Schossig et al. (2005)

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0.25

In the majority of the experimental studies, the measurements concern the air temperature in the test cell (usually one point) and, sometimes, the wall temperature. In all cases, the major effect of PCMIBW is to reduce the temperature fluctuations with a more or less important time lag concerning the temperature maximum. There are few studies with heat flux measurements, which is an interesting way to calculate the thermal energy stored/release. On the whole, there is a lack of indicators allowing evaluation of the real effectiveness of the solutions tested. The thermal comfort of occupants is driven in particular by the air temperature (convective heat transfer) and the surface temperature using the mean radiant temperature (radiative heat transfer). Special attention must be paid to these two parameters to really assess the effect of PCM on thermal comfort. In Kuznik and Virgone (2009a), the laboratory experimental test cell MINIBAT (presented in Figure 13.9) has been monitored with and without PCM in the same temperature and radiative solar flux conditions. The temperatures and external 12

7

5

4

Test cell 1

2.2

5

Test cell 2

5

3

3.1

2

2.5

12

3.43

8

3.1

9

10.5 1

Section view

Z

Test cell 1 5 2

3

10

Test cell 2 3.1

11

2 0.12

3.1

3.1

Top view

Figure 13.9 Scheme of the experimental setup MINIBAT: (1 cooling unit: 2 climatic chamber; 3 simple glazing; 4 protection glass of the solar simulator; 5 test ceIL; 6 concrete; 7 air blowing plenum; 8 solar simulator’s heat removal ventilators; 9 air extraction plenum; 10 I-IVAC unit of the thermal guard: 11 solar simulator; 12 thermal guard (metric units)) (from Kuznik and Virgone, 2009a, with permission from Elsevier).

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conditions are presented in Figure 13.10. In summer conditions, the indoor air temperature for the case without PCM fluctuated from 36.6°C to 18.9°C, whereas for the PCM case varied from 32.8°C to 19.8°C. It proves that the PCM walls decreased the temperature fluctuations by 4.7°C in our tests. An interesting observation for the same case concerns the differences between temperatures T1 and T2. A thermal stratification exists in the case without PCM (a difference of 1°C for the temperature maxima between the two probes) which did not happen for the PCM case. This was mainly due to higher natural convection effects because of the lower vertical PCM wall temperatures. 35

Tcl summer Tcl mid-season Tcl winter

Temperature (°C)

30 25 20 15 10 5

0

10

20 30 Time (h)

40

20 30 Time (h) (a)

40

Radiative flux density (W/m2)

E 150

100

50

0

0

10

Figure 13.10 (a) Experimental conditions in the climatic chamber and (b) temperatures in the room for summer case: T1 at height 0.85 m and T2 at height 1.70 m (from Kuznik and Virgone, 2009a, with permission from Elsevier). (Continued)

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T1 without PCM T2 without PCM

38 36 34 Temperature (°C)

32 30 28 26 24 22 20 18

0

10

20 30 Time (h)

40

(b)

Figure 13.10 Continued

Concerning the energy, Castell et al. (2010) measured a reduction of about 15% of electricity consumption during summer 2008. A more systematic study of energy reduction coupled with life cycle analysis is necessary to really assess the performance of PCMIBW. To go further in the analysis of the environmental potential of PCMIBW, de Gracia et al. (2010) and Menoufi et al. (2013) performed life cycle assessement on tests cells with PCM wallboards. They showed that the embodied energy (and other environmental impacts) contained by the PCM was fully compensated by the energy (and other environmental impacts) saved during the lifetime of the building. In order to really assess the potential of PCM wallboards (ENERGAIN®), a renovated office building was monitored for approximately one year by Kuznik et al. (2011b). A room was equipped with PCM wallboards in the lateral walls and in the ceiling. Another room, identical to the first one, was not equipped but also monitored. This study is the first to deal with the results obtained in real use conditions. Figure 13.11 shows the evolution of the air temperature Ta and the equivalent globe temperature Tg for the two rooms. The globe temperature is effectively affected by the cooling effect of the surface temperatures. For the PCM office, the effect of the surface cooling is a little more important than without PCM. The globe temperature is about 3°C lower than the air one. The maximum globe temperature is reached after about 1 h after the maximum air temperature.

Integrating phase change materials (PCMs) in thermal energy storage systems for buildings Ta without PCM

Tg without PCM

Ta with PCM

Tg with PCM

345

Temperature (°C)

30

25

20

0 07

-1 1-

19

00

.0

0 -1 1-

18

12

.0

0 07

07

-1 1-

18

00

.0

0 .0 12 17 -1 107

07

-1 1-

17

00

.0

0

15

Figure 13.11 Ta and Tg evolution on 17–19 November 2007 (from Kuznik et al., 2011a, with permission from Elsevier).

13.7 Numerical studies The phase change can be taken into account in the heat equation using either the effective heat capacity method or the enthalpy method. These two methods have been extensively studied in the literature, for example: Goodrich (1978), Comini et  al. (1974) and Minwu and Chait (1993) for the effective heat capacity method and Kakac and Yener (2008), Voller and Cross (1981) and Date (1991) for the enthalpy formulation method. The two methods have the advantages of allowing one formulation of the heat equation to be used for the entire domain and of avoiding solving the melting front position. The numerical studies involving PCM integrated in building walls can be roughly categorized as follows: ∑ unidirectional heat equation in a single wall: Kuznik et al. (2008a, 2008b), Ahmad et al. (2006a, 2006b), Koschenz and Lehmann (2004), Weinlder et al. (2005), Pasupathy et al. (2008), Darkwa and Kim (2005a, 2005b), Darkwa (2007), Halford and Boehm (2007), Kim and Darkwa (2003), Mathieu-Potvin and Gosselin (2009), Zhang et al. (2008), Kaushik et al. (1981), Zhou et al. (2010) ∑ two-dimensional heat equation in a single wall: Alawadhi (2008), Carbonari et al. (2006), Darkwa (1999)

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∑ unidirectional heat equation in the wall, energy balance in a room: Kuznik and Virgone (2009b), Li et  al. (2009), Chen et  al. (2008), Darkwa and O’Callaghan (2006), Heim (2010), Ibanez et al. (2005), Kuznik et al. (2010), Neeper (2000), Zhou et al. (2007, 2009), Chandra et al. (1985), Evola et al. (2013) ∑ two- or three-dimensional heat equation in the wall, energy balance in a room: Ahmad et al. (2006b), Mozhevelov et al. (2006)

Most of the studies concerning unidirectional heat equation in a building wall with PCM deal with the problem of PCM optimization: phase change temperature, position of the PCM, and thickness. One of the most important features is the thickness of the PCM wall: the thicker the wall, the higher the price. Of course, when the thickness is large, the time needed for the heat to penetrate the PCM becomes larger than 12 h and the storage process cannot be complete during a day (Kuznik et al., 2008a). This optimal thickness depends on the diffusivity of the medium and then must be calculated for each PCMIBW. The unidirectional conductive heat transfer in walls is a common assumption in building simulation. In low energy buildings, this assumption is not realistic, so attention must then be paid in future studies concerning this assumption, especially for thermal bridge reduction. The heat transfer between the PCM wall and the air is due to convection. For external wall surfaces, the convective heat transfer is driven by forced convection, but as the walls are insulated, this transfer process is not prevalent. The convective heat transfer between the internal face of the wall and the indoor air is important to evaluate the store/release process in PCM. Liu and Awbi (2009) found that the correlation used to evaluate the convective heat transfer for ordinary walls underestimates this coefficient for PCM walls (by a factor of two in their experiment). In David et al. (2011), different correlations are used in a model of walls with PCMs. The study deals with natural convection and mixed convection. The results show a factor up to three for the energy storage in the wall depending on the convective heat transfer coefficient used! This is a very important problem because there is a lack of knowledge concerning the convective heat transfer with PCM walls, whereas numerical simulations need the convective heat transfer value! Most of the studies deal with unoccupied rooms. Of course, the evaluation of air temperature in a building is clearly affected by internal heat loads. One way to evaluate the optimum phase change temperature is to calculate the thermal evolution of a building without PCM and calculate the mean surface temperature of the walls for the storage period. This optimization can only be done if internal loads due to occupation are taken into account with realistic scenarios.

13.8 Conclusions This chapter presents the state-of-the-art of phase change material integrated in building walls. On the whole, PCM have a good potential for reducing heating and cooling loads by enhancing the storage capacity of the building envelope. However,

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this storage capacity can be enhanced by an increase in the PCMIBW thermal conductivity. From a practical point of view, a more systematic evaluation of the various PCM integrated in the building structure is needed, in particular in real use conditions. Such analysis can be numerical but attention must be paid to numerical modelling assumptions: e.g., convective heat transfer coefficient, use of the phase diagram, etc., Moreover, there is a lack of clear indicators to effectively assess the PCMIBW.

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