Novel concept of composite phase change material wall system for year-round thermal energy savings

Energy and Buildings 42 (2010) 1759–1772

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Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild

Novel concept of composite phase change material wall system for year-round thermal energy savings Bogdan M. Diaconu a,b,∗ , Mihai Cruceru b a b

IST-ICIST, Universidade Técnica de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal University “Constantin Brancusi” of Tg-Jiu, Calea Eroilor 30, Tg-Jiu, Romania

a r t i c l e

i n f o

Article history: Received 19 February 2010 Received in revised form 21 April 2010 Accepted 13 May 2010 Keywords: Latent heat thermal energy storage PCM building materials Energy saving Passive building Building simulation

a b s t r a c t A new type of composite wall system incorporating phase change materials (PCMs) was proposed and its potential for air conditioning/heating energy savings in continental temperate climate was evaluated. The novelty of the wall system consists of the fact that two PCM wallboards, impregnated with different PCMs are used. The structure of the new wall system is that of a three-layer sandwich-type insulating panel with outer layers consisting of PCM wallboards and middle layer conventional thermal insulation. The PCM wallboard layers have different functions: the external layer has a higher value of the PCM melting point and it is active during hot season and the internal layer with a PCM melting point near set point temperature for heating is active during cold season. A year-round simulation of a room built using the new wall system was carried out and the effect of PCM presence into the structure of the wall system was assessed. It was found that the new wall system contributes to annual energy savings and reduces the peak value of the cooling/heating loads. The melting point values for the two PCMs resulting in the highest value of the energy savings were identified. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Heat transfer of the built environment through the building envelope has a significant weight in the overall energy balance contributing to a high extent to the cooling and heating load. Passive methods used for the thermal energy management of the built environment include the thermal mass, which can contribute to downsizing of the AC/heating equipment and reducing the AC/heating demand. In addition, increasing the thermal mass of the built environment can contribute to increasing the indoor thermal comfort. The use of PCMs in building elements with the purpose of increasing the thermal mass was extensively studied [1–4]. A review of PCMs application in the built environment can be found in [5]. PCMs can be integrated into building materials or prefabricated building elements such as concrete, gypsum wallboards, plaster, etc. Gypsum wallboards incorporating PCMs such as fatty acids [6], mixture of lauric and myristic acids [7], eutectic mixtures of capric

Abbreviations: AC, air conditioning; DSC, differential scanning calorimetry; PCM, phase change material. ∗ Corresponding author at: IST-ICIST, Universidade Técnica de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal. Tel.: +351 218412216; fax: +351 218470762. E-mail address: [email protected] (B.M. Diaconu). 0378-7788/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2010.05.012

acid and lauric acid [8], mixture of capric acid and stearic acid [9], butyl stearate [1], etc. A key issue in developing building materials and building elements incorporating PCMs is the integration of the PCM into the structure of the matrix material. The method of incorporation depends mainly on the nature of the building material. Khudhair and Farid [10] described the most commonly used methods for integration of PCMs into the building materials and building elements. Feldman et al. [11] used immersion of gypsum wallboards into a mixture of 93–95 wt% commercial Methyl Palmitate with 7–5 wt% commercial Methyl Stearate at 45 ◦ C for approximately 25 s. A PCM mass absorption degree of approximately 24% was found. The thermo-physical properties of the resulting impregnated wallboard were assessed by means of DSC analysis. A phase transition temperature range of approximately 2 ◦ C and a latent heat capacity varying between 35 and 55 kJ/kg were found. Rozanna et al. [6] used immersion of gypsum boards with the thickness values of 6 and 12.5 mm into an eutectic mixture of lauric and stearic acids for 1 h. A mass absorption degree of 27–28.5% and a maximum value of the latent heat of 56 kJ/kg were identified. The phase transition process was affected by a hysteresis-type behaviour in the sense that the melting temperature range was different from solidification temperature range. Sari et al. [9] developed a phase change wallboard by immersing gypsum wallboards into an eutectic mixture of commercial 83 wt% capric acid and 17 wt% lauric acid for approximately 1 h. A mass absorption degree of 25 wt% was found and no separa-

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Nomenclature A ACH c Fo H h k L M n q Q t U V x

surface area (m2 ) air changes per hour (h−1 ) specific heat capacity (J kg−1 K−1 ) Fourier number Fo = ax−2 specific enthalpy (J kg−1 ) convection coefficient (W m−2 K−1 ) thermal conductivity coefficient (W m−1 K−1 ) latent heat of fusion (J kg−1 ) melting peak factor (J kg−1 ) number of nodes heat flux (W m−2 ) heat flow (W) temperature (◦ C or K) overall heat transfer coefficient (W m−2 K−1 ) volume (m3 ) spatial coordinate (m)

Greek symbols ˛ thermal diffusivity (m2 s−1 ) ı layer thickness (m)  density (kg m−3 )  time (s or h) Subscripts 0 initial a air (indoor environment) amb ambient (temperature) H/C heating/cooling (load) HVAC heating or air conditioning equipment I/V infiltration/ventilation int internal (room) l layer L infiltration and ventilation lq liquid (phase) m melting r room rad radiative ref reference s solid (phase) SET set point t (phase) transition W window w wall

tion of the PCM from the matrix material was observed after 5000 melting/solidification cycles. Shilei et al. [12] used the gypsum wallboard immersion into a mixture of 82% capric acid and 18% lauric acid as a method of PCM incorporation. The immersion of the gypsum wallboards into the liquid PCM was maintained for 6–10 min achieving a PCM absorption degree of approximately 26 wt%. Feldman and Banu [13] developed PCM wallboards by incorporating various PCMs directly into the gypsum paste prepared in a way similar to production of commercial wallboards. A PCM mass percentage of the resulting PCM wallboard was between 20 and 25 wt%. It was found that the PCM incorporation at the time of mixing resulted in a more even PCM distribution in the wallboard. Zhang et al. [14] described a method of PCM incorporation into concrete. It was found that the thermo-physical properties of the concrete containing PCM (butyl stearate), including the latent heat and phase transition temperature range, were influenced to an important extent by the physical properties of the aggregates (especially porosity).

Schossig et al. [15] investigated the incorporation of PCM microcapsules into gypsum wallboard. PCM microencapsulation prevents the interaction between the matrix material and the PCM that could change the properties of the first. Another advantage of PCM microencapsulation is leakage prevention that could occur during the lifetime of the building element. An attenuation of the indoor temperature fluctuations of approximately 2 ◦ C was found by means of simulations compared to the case of the walls without PCM microcapsules. The potential of PCM building materials of energy saving and improving the indoor thermal comfort was confirmed by both experimental and numerical studies. Darkwa and Callaghan [16] carried out a simulation of a passive room with PCM drywalls modelling the phase transition process by an increase of the effective heat capacity around the PCM melting point. A Gaussian-type variation of the effective heat capacity with various values of the phase transition temperature range was considered. Two methods of PCM integration into the building materials were considered: laminated PCM boards and randomly mixed PCM boards. An attenuation of the indoor temperature profile of approximately 2 ◦ C was found compared to an identical room built of drywalls without PCM. Ismail and Castro [4] conducted a simulation of a three-layer wall with the exterior layers consisting of conventional building materials and the middle layer consisting of PCM. It was found that the presence of the PCM in the structure of the exterior wall resulted in downsizing of the heating equipment and decreasing the energy demand for AC. Neeper [17] conducted a simulation of a wallboard with latent storage modelling the effective heat capacity of the PCM using a Gaussian function. It was found that the maximum diurnal energy storage occurs when the PCM melting point equals the average room temperature (for a sinusoidal variation of the room temperature) and for a narrow phase transition temperature range. Xu et al. [18] investigated numerically and experimentally a test room with shape-stabilized PCM floor. It was found that the suitable PCM melting point should be approximately equal to the average indoor temperature during winter sunny days. Athienitis et al. [1] developed a mathematical model for the transient heat conduction through a PCM-gypsum board. The simulation results were compared with experimental measurements obtaining satisfactory agreement, which indicated that the explicit one-dimensional nonlinear finite difference model can be used successfully in simulation of the PCM-gypsum wallboard room. Composite wall systems incorporating PCMs were investigated by Pasupathy and Velraj [19]. Chen et al. [20], Zhou et al. [21], Kuznik and Virgone [22], Ahmad et al. [23], Halford and Boehm [24], Darkwa [25], Carbonari et al. [26]. Modelling thermo-physical properties of PCMs poses some difficulties. Diaconu et al. [27] determined experimentally the apparent heat capacity of a microencapsulated PCM slurry by means of DSC. It was found that the apparent heat capacity was DSC scanning rate dependent and it followed a hysteresis-type pattern. A key conclusion was that the hysteresis of the microencapsulated PCM slurry H–t curve made impossible to predict the variation of enthalpy with temperature. Moreover, it was found that the magnitude of the hysteresis in the H–t curve was DSC scanning rate dependent. Complex and unpredictable behaviour of the H–t curves was found in cases in which temperature did not sweep completely the phase transition temperature range. Arkar and Medved [28] used DSC to determine the thermophysical properties of a paraffin type (RT20). It was found that the scanning rate influences to a significant extent both thermophysical properties and the phase transition temperature range. To the authors’ knowledge, an accurate analytical model of the transient heat conduction in a PCM impregnated building material was not developed so far. Various simplifying assumptions were made

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Table 1 Energy savings and peak load reduction–literature review. Reference

Object configuration and PCMs

Location

Results

Analysis type

Zhang et al. [35]

Two 1.83 m × 1.83 m × 1.22 m test rooms. Frame wall with macroencapsulated PCM – cylinders (Rubitherm)

Kansas, USA

Experimental

Halford and Boehm [24]

Three-layer sandwich panel with outer layers consisting of insulation and middle layer PCM

USA

Stetiu and Feustel [36]

One room in a building office, PCM wallboard interior walls. PCM type/mass concentration: paraffin/20% Outdoor test room with gypsum wallboards attached on the vertical walls of the room. PCM type/mass concentration: butyl stearate/25% Test room with interior walls, ceiling and floor consisting of PCM layers

CA, USA

Average reduction of the wall heat flux from 11% to 21% for 10% PCM concentration and from 1% to 15% for 20% PCM concentration. Heat flux reduction rate depended on the wall orientation and on weather and climate conditions. Daily cooling load was reduced with 8.6% up to 10.8%, depending on the PCM concentration 11–25% maximum reduction in peak load compared to thermal mass and no phase change; 19–57% reduction in peak load compared to the case with insulation only Peak cooling load reduction by 28%

Montreal, CA

Total heating load reduction of approximately 15%

Experiment and simulation

Beijing, CH

Experimental

Existing building with brick walls and PCM layer, prefabricated ceiling of 100 mm thickness and a PCM layer of 20 mm Test room 3.9 m × 3.3 m × 2.7 m. PCM composite plates attached on the inner walls and ceiling

Campinas, BR

Energy saving rate of the heating season reached 10% during the whole winter Capacity of the central air conditioning unit reduced with 31%

Experimental

Lightweight passive solar house with PCM (fatty acids) impregnated plasterboard Multilayer attic insulation system with microencapsulated PCM-enhanced insulation foam. Two types of PCM with melting point 26 ◦ C and 32 ◦ C

WI, USA

Energy savings (heating) counted throughout the whole heating season of 10% compared to the case with no PCM corresponding to a PCM with phase change enthalpy 60 kJ/kg and optimal thickness 30 mm Annual energy savings of 15%

20% reduction of the peak hour heat flow Total summertime peak heat flow crossing the roof deck was reduced by about 90% compared with the heat flow penetrating a conventional roof Electricity consumption for space heating during four consecutive winter months reduced by 32%

Experimental

Athienitis et al. [1]

Chen et al. [20]

Ismail and Castro [4]

Zhou et al. [21]

Peippo et al. [38]

Kosny et al. [37]

Hammou and Lacroix [39]

Walls containing PCM spherical capsules with the diameter 0.064 m. Room with the dimensions 5 m × 5 m × 3 m, PCM storage wall thickness 0.192 m

in the existing models [17,29]. The enthalpy method [16,29,20] is widely used for modelling the transient heat conduction in PCM wallboards. While it is generally agreed that passive use of PCMs in buildings improves thermal comfort and reduces global energy consump-

Beijing, CH

USA

Montreal, CA

Simulation

Simulation, RADCOOL

Experimental and simulation

Simulation

Simulation

tion [5], results concerning the actual values of energy savings for heating and air conditioning are scarcely available in the literature. A synthesis of the results found in the literature concerning energy savings and peak load reduction resulting form application of various PCM systems in the built environment is given in Table 1. A new type of PCM composite wall system for year-round thermal energy management in the built environment is proposed in this paper. The novelty of the concept consists of the fact that two different PCMs with different values of the thermo-physical properties are integrated into the structure of the wall system. The key thermo-physical properties that influence the efficiency of the new wall system were the PCM melting point and latent heat.

2. System description

Fig. 1. The structure of the composite PCM wallboard wall system.

The new type of composite wall system consists of three functional layers denoted 1 to 3 and is presented in cross-section in Fig. 1. The outer layers consist of a building material incorporating a PCM and the middle layer consists of conventional thermal insulation. The exact nature of layers 1 and 2 is not discussed here since

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the present study attempts to assess the potential of the new wall system for energy savings. A fundamental assumption, explained in the next section, is tm1 > tm2 . Layer 1 is active during the cooling season and its function is to attenuate the amplitude of temperature fluctuations on the wall side exposed to the ambient. The typical temperature cycle for layer 1 starts at the beginning of the day and consists of an initial rise (due to the ambient temperature solar radiation intensity rise) until the beginning of the PCM phase transition process. During the PCM phase transition the layer 1 temperature rise rate is significantly slowed down (due to the PCM latent heat of fusion). By the end of the phase transition process the ambient temperature and solar radiation intensity are expected to drop. During night layer 1 releases the heat accumulated during the day (the night time temperature must drop enough for efficient cooling of layer 1). Layer 2 is active during the heating season and contributes to increasing the indoor thermal comfort attenuating the indoor temperature fluctuations, shifts the peak load, reduces the cycling of the heating equipment and the amplitude of the building envelope indoor side temperature. Layer 2 temperature is affected by the ambient heat flow to a relatively little extent during the heating season since it is in contact with the indoor environment. Layer 3 consists of thermal insulation (expanded polystyrene board). The actual implementation of the wall system (developing of PCM impregnated wallboards, assembly of the wall system) is out of the scope of this study. A system consisting of a test room built of the composite PCM wall system equipped with HVAC systems was considered for a realistic assessment of the PCM composite wall system energy saving potential. A mathematical model of the test room – wall system and indoor environment – was developed and a year-round simulation of the indoor thermal environment under ambient conditions for a specific location was carried out. 3. Mathematical description of the composite PCM wall–indoor environment system The mathematical model of the heat transfer through the composite PCM wall system and indoor air environment took into account the following assumptions: • The heat conduction process in the composite panel is onedimensional. • All layers of the wall system are homogenous and isotropic. • Constant thermo-physical properties except specific heat capacity of the PCM wallboards. • Only sensible heat of the air (and not latent) in the test room was considered. • Lumped-capacity model for air in the test-room. • The contact thermal resistances between wall system layers are negligible. • The PCM phase transition occurs over a temperature interval rather than at a point. • The PCM wallboard enthalpy is an invertible function of temperature (this assumption excludes the presence of phenomena such as PCM supercooling or hysteresis). • The hysteresis-type pattern thermo-physical properties variation that PCMs usually exhibit is not taken into account; the hysteresis occurs during phase transition cannot be taken into account unless the PCM temperature would sweep the whole phase transition temperature range. In such case the H–t dependence can be experimentally determined (e.g. by DSC) resulting in two curves, one corresponding to heating and one to cooling [27]. Due to the operating conditions of the system considered (the composite

PCM wall system temperature is governed mainly by ambient temperature evolution) it is not possible to guarantee that the temperature cycling sweeps completely the PCM phase transition temperature range. Moreover, Diaconu et al. [27] found experimentally that the hysteresis magnitude (defined as the difference between the enthalpy values corresponding to the same temperature value for cooling and for heating) decreased in the case of temperature cycling inside the PCM phase transition temperature range and the temperature history influenced the H values making their prediction impossible in the case of arbitrary temperature evolution patterns. To the authors’ knowledge, there is no temperature history dependent model to describe uniquely the temperature dependence of the PCM thermo-physical properties and especially H–t curve. Bony and Citherlet [33] developed a mathematical model for a storage tank with water and PCM modules considering the hysteresis-type pattern of the thermophysical properties variation and sub-cooling effects. However, in contrast to the situation presented here, the PCM water storage considered underwent a temperature cycling that swept completely the phase transition temperature range. Other similar studies exist in which the hysteresis and sub-cooling effects were not considered (Koschenz and Lehmann [34]). The enthalpy method was used for modelling the transient heat transfer process instead of the transient heat conduction equation in order to account for the variable specific heat capacity of the PCM wallboards (layers 1 and 2). The transient enthalpy equations for layers 1 and 2 and the boundary conditions are the following: Layer 1:

1

dH1 d2 t1 = k1 2 d dx

(1)

Boundary conditions: x = 0 (outer side of the wall system, convection and radiation): −k1

 dt  1

dx

x=0

= hext (tamb − t1 |x=0 ) + qrad,ext Aw

(2)

x = ı1 (layer 1 – thermal insulation interface, conservative interface heat flux): −k1

 dt  1

dx

x=ı1

= −k3

 dt  2

dx

(3) x=ı1

Ignoring the contact thermal resistance between layers 1 and 3: t1 |x=ı1 = t3 |x=ı1

(4)

Layer 2: 2

dH2 d2 t2 = k2 2 d dx

(5)

Boundary conditions: x = ı1 + ı3 (interface layer 2 – thermal insulation, conservative interface heat flux): −k2

 dt  2

dx

x=ı1 +ı3

= −k3

 dt  3

dx

x=ı1 +ı3

(6)

Ignoring the contact thermal resistance between layers 2 and 3: t2 |x=ı1 +ı3 = t3 |x=ı1 +ı3

(7)

x = ı1 + ı2 + ı3 (inner side of the wall system, convection): −k2

 dt  2

dx

x=ı1 +ı2 +ı3

= hint ( t2 |x=ı1 +ı2 +ı3 − ta ) + qrad,int Aw

(8)

B.M. Diaconu, M. Cruceru / Energy and Buildings 42 (2010) 1759–1772 j

j

x = ı1 , −k1

t1,n − t1,n 1

1763

1 −1

x1

j

= −k3

j

t3,2 − t3,1

(17)

x3

Ignoring the contact thermal resistance between layers 1 and 3: j

j

t1,n = t3,1

(18)

1

Layer 2: j

2

j−1

H2,i − H2,i 

j−1

= k2

j−1

j−1

t2,i+1 − 2t2,i + t2,i−1

(19)

x22

Boundary conditions: j

x = ı1 + ı3 , −k2

j

t2,n − t2,n

1 −1

1

x2

j

= −k3

j

t3,n − t3,n

3 −1

3

x3

j

j

, t2,1 = t3,n

3

(20)

Fig. 2. Discretization mesh.

j

x = ı1 + ı3 + ı2 , −k2

j

t2,n − t2,n 2

For layer 3 (thermal insulation) the transient heat conduction equation can be used since no considerable variation of the thermophysical properties occurs for this layer: (9)

Boundary conditions: x = ı1 (thermal insulation – layer 1 interface, conservative interface heat flux):

 dt  3

dx

x=ı1

= −k1

 dt  1

dx

x=ı1

and t3 |x=ı1 = t1 |x=ı1

(10)

x = ı1 + ı3 (thermal insulation – layer 2 interface, conservative interface heat flux): −k3

 dt  3

dx

x=ı1 +ı3

= −k2

 dt  2

dx

x=ı1 +ı3

and t3 |x=ı1 +ı3 = t2 |x=ı1 +ı3 (11)

Indoor environment:

j

2

j

+qrad,int Aw

(21)

j

j−1

j−1

t3,i − t3,i

= ˛3



j−1

j−1

t3,i+1 − 2t3,i + t3,i−1

(22)

x32

Boundary conditions: x = ı1

Eq.(17)

x = ı1 + ı3

Eq.(20)

For nodes 2, . . ., n3 − 1 the nodal temperature at the time step j is given by j

j−1

j−1

j−1

j−1

t3,i = t3,i + Fo(t3,i+1 − 2t3,i + t3,i−1 )

(23)

The following solution algorithm for solving Eqs. (15) and (19) was used:

dta = hint ( t2 |x=ı1 +ı2 +ı3 − ta )Aw a Vr ca d + qrad,int Aw + QI/V − QHVAC + QW

(12)

˜ j = H j−1 +  f j−1 H l,i l,i l,i

(24)

with

where QI/V = a Vr ca × ACH ×

j−1

(tamb − ta ) 3600

(13)

QW = UW AW (tamb − ta )

(14)

A one-dimensional, computational domain with an equally spaced mesh shown in Fig. 2 was used. An explicit Euler time stepping algorithm was used for solving Eqs. (1), (5) and (9)—with boundary conditions (2)–(4), (6)–(8) and (10) and (11). The discrete forms of equations describing the transient heat conduction through the wall system components are the following (superscript denotes the time step and the subscript denotes the space nodes):

j

j−1

j−1

H1,i − H1,i

= k1



j−1

j−1

t1,i+1 − 2t1,i + t1,i−1

= kl

j t1,2

j − t1,1

x1

xl2

, l = 1 or 2

(25)

j j−1 j Hl,i = Hl.i +  f˜l,i

(26)

with j j j t˜l,i+1 − 2t˜l,i + t˜l,i−1

(27)

xl2 j

Finally, tl,i with i = 2,. . .,nl are determined based on Hl,i . Initial conditions for Eqs. (1), (5) and (9) are



j

j−1

˜ j the nodal temperatures t˜j with i = 2,. . .,nl are deterBased on H l.i l.i mined taking the inverse of the function H = f(t), t = f−1 (H). Then, a j more accurate estimation of Hl,i is obtained from:

t1 ( = 0) j

j−1

tl,i+1 − 2tl,i + tl,i−1

j

(15)

x12

Boundary conditions: x = 0, −k1

j−1

fl,i

j f˜l,i = kl

Layer 1:

1

j

= hint (t2,n − ta )Aw

Layer 3

d2 t3 dt3 = ˛3 2 d dx

−k3

2 −1

x2

j

= hext (tamb − t1,1 )Aw + qrad,ext Aw

(16)

j

a Vr ca



0≤x<ı1 j−1

ta − ta 

= t2 ( = 0)

j



ı1 +ı3 ≤x≤ı1 +ı3 +ı2

j

j−1

= t3 ( = 0)

j−1

ı1 ≤x<ı1 +ı3

j−1

= t0

j−1

= hint (t1,N2 − ta )Aw + qrad,int Aw + QI/V − QHVAC + QW

(28) (29)

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Fig. 3. Apparent specific heat capacity profiles used in the simulation.

The cooling/heating load was calculated from the energy balance equation: QC/H = hint ( t2 |x=ı1 +ı2 +ı3 − tSET )Aw + a Vr ca × ACH × + UW AW (tamb − tSET ) + qrad,int Aw

(tamb − tSET ) 3600 (30)

The enthalpy of the PCM wallboard is ideally modelled by the following equation:

t

H(t) =

cs ( − tref ) d

(31)

tref

tm

H(tm , x) =

cs ( − tref ) d + xL

(32)

tref



tm

t

cs ( − tref )d + L +

H(t) = tref

clq (t − ) d

(33)

tm

• Constant values of the PCM wallboard material with PCM in liquid and solid phase. • The phase transition process occurs within a temperature range centred around tm , [tm − tt ,tm + tt ], and not at a single point. • The phase transition process is modelled by an increase of the apparent specific heat capacity over the phase transition temperature range. The function c = f(t) used for modelling the phase transition process is assumed continuous and integrable. The following profile types of the specific heat capacity modelling function were considered (see Fig. 3):

    t − tm 2

M exp − tt

tt

(34)

The melting peak factor M was determined from the condition: +∞

−∞

−0.7 ◦ C 42.6 ◦ C 35% 48% 20.9 ◦ C

 

M t − tm exp − tt tt

2 

=L

M tt

(36)

tm −tt +∞



2

exp[−((t − tm )/tt ) ] 2

exp[−((t − tm )/tt ) ]

= 0.8423

(35)

√ The integral is M1/2 and the melting peak factor is M = L/ 

(37)

Alternatively, tt can be defined such as at tm + tt the phase transition process is completed to 92.1%:

 tm +tt −∞ +∞ −∞

2

exp[−((t − tm )/tt ) ] 2

exp[−((t − tm )/tt ) ]

= 0.921

(38)

b. Linear function [32] For t < tm − tt and t ≥ tm + tt c = cs,lq

(39)

For tm − tt ≤ t < tm , c = cs,lq + M +

M (t − tm ) tt

(40)

For tm ≤ t < tm + tt , c = cs,lq + M −

M (t − tm ) tt

(41)

The melting peak factor M was determined from the condition: M=

L tt

(42)

c. Constant value of the specific heat capacity (no PCM in the structure of layers 1 and 2) – for reference c = const.

a. Gauss function [16,17,29]



 tm +tt



The model for the PCM wallboard used in this paper considers the following:

c = cs,l +

cmax = cs,lq +

−∞

Liquid phase x = 1:



Value

Minimum temperature Maximum temperature Percentage of time with tamb > 25 ◦ C Percentage of time with tamb < 20 ◦ C Average temperature

In the case of Gaussian function tt is defined such as in the temperature interval [tm − tt , tm + tt ] 84.23% of the latent heat occurs, since:

with  dummy variable for integration Phase transition 0 < x < 1:



Parameter

cmax is given by:

Solid phase x = 0:



Table 2 Main characteristics of the climate considered in simulation.

(43)

4. Simulation approach and parameters The weather data (ambient temperature and solar radiation intensity) was generated using TRNSYS Type 109-TMY2 [30] for Béchar (Algeria). The main characteristics of the temperature profile are presented in Table 2. Such climate was considered adequate for the purpose of this simulation since it requires both AC during hot season and heating during cold season.

B.M. Diaconu, M. Cruceru / Energy and Buildings 42 (2010) 1759–1772

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Table 3 Thermo-physical properties of the building materials and PCMs. Thermo-physical property

k  c ı

Layer 1

Layer 2

Insulation

Matrix material

PCM

Matrix material

PCM

0.33 1050 1050 0.050

0.15 800 2000

0.33 1050 1050 0.050

0.15 800 2000

0.035 200 1500 0.100

A test room was considered with the dimensions 28 m × 10 m × 3 m. The heat gain/loss through the ceiling and floor were not included into the analysis. All lateral walls of the room were built of PCM composite wallboard panels. A reference wall system was considered in which layers 1 and 2 did not contain PCM in order in order to asses the influence of PCM presence in the matrix material on the indoor temperature and cooling/heating load profile. First, simulations of the test room were carried out in the absence of heating/cooling (passive room, QHVAC = 0) and neglecting air infiltrations/ventilation (QI/V = 0), heat gain/loss through the glazed surface (QW = 0), without and with PCM in the structure of building material layers 1 and 2. The cooling/heating load was calculated at each time step based on the wall temperature, ambient corresponding to the previous time step: Fig. 5. Effect of c profile on the indoor temperature evolution for the passive room.

j−1

j

j−1

QH/C = hint (t2,N − tSET )Aw + a Vr ca × ACH × 2

j−1 + UW AW (tamb

j−1 − tSET ) + qrad,int Aw

(tamb − tSET ) 3600 (44)

5. Results and discussion 5.1. Analysis of layer temperatures

The set point temperature was selected 20 ◦ C for heating and 25 ◦ C for cooling. No assumption was made concerning the occupancy pattern and the occupancy gains were neglected. Thermo-physical properties of the matrix materials and PCMs are presented in Table 3. The convective heat transfer coefficients hint and hext were chosen according to ASHRAE recommendations [31] hint = 8.3 W/m2 K and hext = 17 W/m2 K. Hourly values of the ambient temperature and solar radiation intensity resulting from TRNSYS simulation were interpolated in order to refine the time stepping. The time step used in simulations was 144 s and the mesh size was 0.005 m for layers 1 and 2 and 0.01 m for layer 3. The values of time step and mesh size were found to fulfil Fo < 0.5.

The effect of PCM presence in the matrix in layer 1 matrix material was assessed first considering the passive room (no AC/heating systems). Then a comparison was carried out with the temperature evolution patterns considering the presence of PCMs with various values of the specific parameters in layers 1 and 2. The points where temperature values were monitored were the outer and inner sides of the wall system (exposed to the ambient and to the indoor environment, respectively) and the layer interface temperatures. The following parameters of the specific heat capacity profile were used: tm2 = 20 ◦ C, L1 = L2 = 38.4 kJ/kg, tt1 = tt2 = 0.5 ◦ C. The temperature at the insulation interface is presented in Fig. 4 for three situations in which the external PCM wallboard limits the temperature at the insulation interface. However, it was found that the maximum value of the indoor temperature is practically uninfluenced by the presence of the PCM in layer 1 matrix material. Only

Fig. 4. Effect of tm1 on layer 1–insulation interface temperature.

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Fig. 8. Temperature profile in layer 1 during a discharge cycle – passive room. Fig. 6. Charging phase of the daily temperature cycle – passive room.

the PCM with tm1 = 32 ◦ C influences to little extent the indoor temperature compared to the specific heat capacity profile c (Fig. 3c, wall system with no PCM in layer 1), as shown in Fig. 5. The apparent specific heat capacity profile does not influence noticeably the indoor temperature evolution (Fig. 5). The potential of PCM presence and PCM melting point on reducing the temperature at the insulation interface was assessed by comparing the temperature profile in layer 1 in three cases: no PCM in the matrix material, PCM with Gaussian variation of the specific heat capacity and PCM with linear variation of the specific heat capacity. A typical charging phase of the daily temperature cycle is shown in Fig. 6 in the case of tm1 = 20 ◦ C. In Fig. 6 a typical time interval was selected for plotting ambient and layer 1 temperature values (see also the electronic version of Fig. 6 for the whole simulation interval). Outside the time interval presented in Fig. 6 the temperature evolution pattern was similar. Therefore, the temperature evolution presented in Fig. 6 was considered representative. The temperature evolution on the external side and insulation interface for layer 1 is presented in Fig. 7 for a typical discharging phase of the daily temperature cycle (see also the electronic version of Fig. 7 for the whole simulation interval). The corresponding spatial temperature profile for the beginning of the discharging phase ( = 3234 h) and end of the discharging phase

Fig. 7. Temperature evolution during a discharge cycle – passive room.

( = 3246 h) is shown in Fig. 8. The PCM maintains at the insulation interface a temperature closed to the melting point for the ambient temperature considered. A typical situation in which layer 1 fails to maintain the temperature at the insulation interface due to inadequate choice of tm1 (20 ◦ C) is presented in Fig. 9. In cases of failure such as the one presented in Fig. 9 the maximum value of the temperature at the insulation interface was found to rise as high as 37 ◦ C (e.g. in the interval 4842–4848 h, as presented in the electronic version of Fig. 9). In the case of tm1 = 32 ◦ C a charging phase during which tamb reaches approximately 41 ◦ C is presented in Fig. 10 and the corresponding spatial temperature profile in layer 1 is presented in Fig. 11 (see also the electronic version of Fig. 10 for the whole simulation interval). The function of layer 2 is to attenuate the descending tendency of the insulation – layer 2 interface temperature occurring at ambient temperature drop, thus limiting the heat loss through insulation. The typical evolution of insulation – layer 2 interface temperature if shown in Fig. 12 for three values of tm2 , including the case of layers 1 and 2 without PCM (see also the electronic version of Fig. 12 for the whole simulation interval). For tm2 = 19.5 ◦ C layer 2 maintains a higher value of the insulation–layer 2 interface temperature than for the other tm2 values. A temperature profile in

Fig. 9. Typical situation in which layer 1 fails to prevent an increase of layer 1 – insulation interface temperature.

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Fig. 10. Temperature evolution during a charging cycle in case of tm1 = 32 ◦ C.

Fig. 13. Typical temperature profile in all layers of the wall system –  = 7883 h.

all layers of the wall system corresponding to  = 7883 h is presented in Fig. 13. However, situations such as the one presented in Fig. 14 can occur when the daily average ambient temperature has an ascending tendency (see also the electronic version of Fig. 14 for the whole simulation interval). Other values of tm2 can have the effect of tm2 = 19.5 ◦ C presented in Figs. 13 and 14 for other values of tamb (other time intervals). Therefore, the criteria for selecting the optimum value of the melting point for both layers will be minimization of cooling/heating loads. 5.2. Thermal energy savings

Fig. 11. Temperature profile in layer 1 at the beginning (t = 4854 h) and end (t = 4866 h) of a charging cycle for tm1 = 32 ◦ C.

Fig. 12. Typical evolution of insulation – layer 2 interface temperature.

Two main effects resulting from using PCMs in the structure of wall system layers 1 and 2 were investigated: (1) annual energy savings for heating and AC and (2) reduction of the peak value of the heating/cooling loads, resulting eventually in downsizing of the heating and AC equipment. The annual energy savings were calculated comparing the annual energy demand for heating/AC of the

Fig. 14. Situation in which layer 2 prevents an increase of insulation – layer 2 interface temperature.

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Fig. 15. Cooling load profile: (a) wall system without PCMs; (b) tm1 = 24.5 ◦ C; (c) tm1 = 33.2 ◦ C.

Fig. 16. Effect of tm1 on QC,annual (a) and peak value of the cooling load (b).

room without PCMs in the layers 1 and 2 structure with the annual energy demand for heating/AC of the room with PCMs in the layers 1 and 2 structure. The relative value of the energy savings value was calculated with: Annual energy savings for heating/AC =

The annual energy savings and the peak load reduction are presented in Table 7. The influence of the PCMs characteristics (especially tm,l ) was assessed, varying the values of tm,l , Ll and tt,l (l = 1, 2).

Wallboards without PCM − Q PCM wallboards QH/C,annual H/C,annual Wallboards without PCM QH/C,annual

× 100

The relative value of the peak load reduction was calculated with: Peak load reduction =

Wallboards without PCM ) − max(Q PCM wallboards ) max(QH/C H/C Wallboards without PCM ) max(QH/C

× 100

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Fig. 17. Typical evolution of temperature on wall sides and layer interfaces: (a) tm1 = 24.5 ◦ C and the corresponding cooling load (b).

5.2.1. Cooling load The annual energy demand for air conditioning was calculated by integrating the cooling load values over the cooling season:

 QC,annual =

QC d

Table 4 Influence of c profile and tt on the values of QC,annual . Layer 2

Layer 1

c profile

tt

c profile

tt ( C)

tm1 = 24.5 ◦ C

tm1 = 33.5 ◦ C

Gaussian

2 2 2

Gaussian

0.50 1.75 3.00

917.7 919.8 921.1

928.1 928.2 928.1

Linear

2 2 2

Linear

0.50 1.75 3.00

921.7 922.8 924.2

932.3 932.3 932.3

(45)

cooling season

The cooling load corresponding to the wall system without PCM and neglecting radiative exchange and air infiltration is shown in Fig. 15a (see also the electronic version of Fig. 15 for the whole simulation interval) The annual energy demand for AC was found to be dependent on tm1 as shown in Fig. 16a, where QC,annual was plotted against tm1 for L1 = L2 = 57.3 kJ/kg ([32], paraffin) and tt1 = tt2 = 0.5 ◦ C. The tm1 value for which QC,annual reached its minimum value was approximately 24.5 ◦ C (Fig. 15a). However, layer 1 incorporating PCM with tm1 = 24.5 ◦ C does not flatten significantly the cooling load profile (Fig. 15b) compared to the wall system without PCMs (Fig. 15a). tm1 value for which the peak value of QC reached a minimum (approximately 540 W) was approximately 33.2 ◦ C as shown in Fig. 16b. The cooling load profile is shown in Fig. 15c for tm1 = 33.2 ◦ C. The influence of c profile and tt on the values of QC,annual is resumed in Table 4. The typical evolution of temperature in the wall system for tm1 = 24.5 ◦ C is presented in Fig. 17a and the corresponding cooling load values in Fig. 17b.

Qc,annual (kWh) ◦

5.2.2. Heating load analysis The annual energy demand for heating was calculated with:



QH,annual =

QH d

(46)

heating season

QH,annual corresponding to the wall system without PCM and neglecting radiative exchange and air infiltration was 1567 kWh and the peak value of the heating load was 925 W. The annual energy demand for heating and the minimum peak value of the cooling load were plotted against tm2 in Fig. 18a and b, respectively. The input parameters for which the plots in Fig. 18 were drawn are presented in Table 5. tm2 values for which minimum value of QH,annual and lowest peak value of the heating load were reached were 19.8 and 19.0 ◦ C, respectively. QH,annual reduced with approximately 12% for tm2 = 19.8 ◦ C and the peak value of the heating load reduced with approximately 35% for tm2 = 19.0 ◦ C.

Fig. 18. Effect of tm2 on QH,annual (a) and peak value of the heating load (b).

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Table 5 Layers 1 and 2 parameters for plots in Fig. 18. Parameter

Layer 1

Layer 2

L (kJ/kg) tt (◦ C) c profile tm

57.3 0.5 Gaussian 24.5

57.3 0.5 Gaussian Variable

Table 7 Maximum values of energy savings and peak load reduction and the corresponding melting point values.

Maximum energy savings Maximum reduction of the peak load

AC [%]

Heating [%]

1 24.3

12.8 35.4

6. Conclusions

Fig. 19. Effect of PCM presence in layer 2 on the heating load profile – tm2 = 19.8 ◦ C.

The heating load profile is shown in Fig. 19 for two cases, (1) no PCM in layers 1 and 2 and (2) layer 2 containing PCM with melting point 19.8 ◦ C (see also the electronic version of Fig. 19 for the whole simulation interval). The asymmetric shape of QH,annual − tm2 curve with the right ascending branch less steep than the left descending branch can be explained by the delay of the heating season onset that the PCM layer 2 with tm2 higher than heating set point induces, as shown in Fig. 20a (see also the electronic version of Fig. 20 for the whole simulation interval). Another shift of the heating load occurs at the end of the cooling season (Fig. 20b).

A new type of composite PCM wall system was designed and its potential for AC/heating energy savings was assessed by means of numerical simulation of indoor environment thermal conditions for a test room built using the new wall system. The new wall system energy savings potential was estimated by comparing the annual energy demand for AC and for heating corresponding to an identical test room built using a wall system with identical geometry and layer structure but without PCMs. A finite difference method was used to solve the transient heat conduction equation through the composite PCM wall system. The enthalpy method was used in order to account for variable thermo-physical properties of the PCM wallboards (specific heat capacity). It was found that the outer PCM wallboard layer prevents an excessive increase of the temperature at the insulation interface, thus limiting the heat gain through the wall system. This effect occurs for a range of melting point values of the PCM incorporated into layer 1. It was thought that the most relevant way to assess quantitatively the layer 1 effect of limiting the heat gain was the annual energy demand for AC. Other effect induced by the presence of the PCM in layer 1 structure is a decrease of the cooling load peak value. The lowest peak value of the cooling load was found for a tm1 value higher than the one for which annual energy demand for cooling reached its minimum. The energy savings for AC more than doubled when the thickness of PCM layers 1 and 2 doubled. Although the annual energy savings for AC is relatively small it is expected that in combination with other energy saving passive methods (such as night ventilation) could result in significant performance. More significant energy savings were found in the case of heating load. The most important results of the present study are synthesised in Figs. 16 and 18. The presence of PCMs in layers 1 and 2 (1) reduces

Fig. 20. Effect of heating load shifting induced by the presence of PCM in layer 2 and influence of tm2 on the magnitude of the shifting. Table 6 The specific values of melting point temperature tm1 and tm2 . Heating Condition Melting point

QH,annual = min tm2 = 24.4 ◦ C

Cooling max(QH ) = min tm2 = 33.4 ◦ C

QC,annual = min tm1 = 20.0 ◦ C

max(QC ) = min tm1 = 19.3 ◦ C

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the annual energy demand for heating/cooling (Figs. 16a and 18a) and (2) reduces the peak heating/cooling loads (Figs. 16b and 18b). It is interesting to notice that an optimum value of the PCM melting point exists for which (1) the annual energy demand for heating/cooling reaches a minimum value and (2) the peak value of the heating/cooling loads reaches a minimum. The two values of the melting point for each of the layers 1 and 2 are different, as presented in Table 6. The maximum values of energy savings and peak load reduction (with reference to the no PCM wall system) are presented in Table 7. Compared to the results from other authors (see Table 1), the following conclusions come out: • Peak cooling load reduction found in this study (35.4%) is consistent with the findings of other authors (Stetiu and Feustel 28% [36], Ismail and Castro 31% [4]). • The reduction of the total cooling load (energy savings for AC) found in this study was 1%. Other authors also found relatively small values (Zhang et al. [35], 8.6% up to 10.8%, calculated on a daily basis and not annual, therefore a comparison is not relevant). However, no attempt to optimize the configuration of the wall system was attempted in this study. It is expected that once the parameters of the wall system are adjusted, higher value of annual energy savings for cooling can be achieved. • The value of the annual energy savings for heating found in this study (12.8%) is consistent with the findings of other authors (Athienitis et al. 15% [1], Chen et al. 10% [20]). • No information was reported in the literature concerning the peak load reduction for heating. The value found in this study (35.4%) could not be compared. Both curves QH/C = f(tml ) with l = 1, 2 have a similar profile with the left branch (in the case of cooling load) and right branch (in the case of heating load) having a less steep descend and ascend, respectively than the other branch. The asymmetric profile of the two curves is explained by the transition from the heating season to the cooling season and from the cooling season to the heating season, respectively, which contributes to shifting the onset of the cooling and heating season, respectively and eventually reducing the annual energy demand for AC and heating. The specific heat capacity variation profile and the phase transition temperature range were found to influence to little extent the temperature evolution in the wall system layers. The influence of the PCM latent heat and layer thickness values was not considered in this study, however it is expected that higher values of latent heat and layer thickness values would increase the efficiency of the wall system. The new wall system is designed for use in buildings located in areas with climates requiring both AC and heating (such as continental temperate climate). Proper selection of the optimum wall system characteristics depend on the climate for which the simulation was carried out. Different values of optimum melting point values can be obtained for other climate conditions than those considered in this paper. The heat loss/gain through ceiling/roof, floor and windows, the air infiltration and ventilation and the gain due to occupancy were not included in the analysis in order to reach conclusions not affected by other heat loss/gain than the heat flow through walls. The enthalpy method employed in this qualitative study and the simplifying assumptions provide an approximation of the heat transfer mechanism through the PCM impregnated building material. PCM impregnated building materials represent a complex heat transfer medium not fully investigated and understood. However, it is thought that the results of this study demonstrate that the new wall system has the potential of reducing the annual energy demand for AC and heating.

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It was not considered essential for the results of this study to use PCM and PCM wallboard actual thermo-physical properties data since the objective was to identify optimum values for these properties and to provide guidelines for further investigation and development of the PCM wallboards. Further investigation and actual implementation of the new wall system requires on the on hand selection of adequate PCMs, incorporation of the PCMs into an appropriate building material and on the other hand development of an advanced model for the PCM wallboard thermo-physical properties including PCM hysteresis and heat transfer process. The results of the present study could be considered a start point for an experimental investigation that would provide more realistic results and confirm the technical feasibility of the new wall system. The objective of the study was to demonstrate by using the proposed wall system and carefully selecting the PCM layers characteristics year-round energy savings and downsizing of the heating and AC equipment are possible. No attempt to optimize the characteristics of the wall system was made, therefore the actual value of the energy savings is of little relevance. The factors that can induce model errors, possible ways of correction, optimization and fine tuning are: • Hysteresis pattern of the PCM thermo-physical properties. The effect of PCM thermo-physical properties hysteresis pattern could be assessed considering a regular pattern of the ambient temperature evolution that sweeps completely the phase transition temperature range (i.e. reaches values lower than the end of solidification in the case of cooling and higher than end of melting in the case of heating); comparing the results obtained first considering and then ignoring the hysteresis could provide some insight into its influence. However, due to the ambient temperature evolution, it is thought that this approach would not add to the relevance of the results presented; on the other hand, the influence of hysteresis on the final results (energy savings and peak load shaving) is negligible compared to other factors. • Wall orientation and exposure to solar radiation; boundary condition expressed by Eq. (2) could be modified to consider the actual position of the Sun and sky temperature. • Effect of occupancy pattern and ventilation. • Estimation of layer thickness effect, especially in the case of layer 1. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.enbuild.2010.05.012. References [1] A.K. Athienitis, C. Liu, D. Hawes, D. Banu, D. Feldman, Investigation of the thermal performance of a passive solar test-room with wall latent heat storage, Building and Environment 32 (5) (1997) 405–410. [2] M. Ahmad, A. Bontemps, H. Salee, D. Quenard, Experimental investigation and computer simulation of thermal behaviour of wallboards containing a phase change material, Energy and Buildings 38 (2006) 357–366. [3] F. Kuznik, J. Virgone, J.-J. Roux, Energetic efficiency of room wall containing PCM wallboard: a full scale experimental investigation, Energy and Buildings 40 (2008) 148–156. [4] K.A.R. Ismail, J.N.C. Castro, PCM thermal insulation in buildings, International Journal of Energy Research 21 (1997) 1281–1296. [5] N. Zhu, Z. Ma, S. Wang, Dynamic characteristics and energy performance of buildings using phase change materials: a review, Energy Conversion and Management 50 (2009) 3169–3181. [6] D. Rozanna, A. Salmiah, T.G. Chuah, R. Medyan, S.Y. Thomas Chong, M. Sa’ari, A study on thermal characteristics of phase change material (PCM) in gypsum board for building application, Journal of Oil Palm Research 17 (2005) 41–46. [7] S. Keles¸, K. Kaygusuz, A. Sari, Lauric and myristic acids eutectic mixture as phase change material for low-temperature heating application, International Journal of Energy Research 29 (2005) 857–870.

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