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Simulation of phase change drywalls in a passive solar building
Applied Thermal Engineering 26 (2006) 853–858 www.elsevier.com/locate/apthermeng
Simulation of phase change drywalls in a passive solar building K. Darkwa *, P.W. OÕCallaghan School of the Built Environment, The Applied Energy and Environmental Engineering Group, Nottingham Trent University, Burton Street, Nottingham NG1 4BU, UK Received 22 April 2005; accepted 7 October 2005 Available online 18 November 2005
Abstract Integration of phase change materials (PCMs) into building fabrics is considered to be one of the potential and effective ways of minimizing energy consumption and CO2 emissions in the building sector. In order to assess the thermal effectiveness of this concept, composite PCM drywall samples (i.e. randomly-mixed and laminated PCM drywalls) have been evaluated in a model passive solar building. For a broader assessment, effects of three phase change zones (narrow, intermediate and wide) of the PCM sample were considered. The results showed that the laminated PCM sample with a narrow phase change zone was capable of increasing the minimum room temperature by about 17% more than the randomly-mixed type. Even though there was some display of non-isothermal phase change process, the laminated system proved to be thermally more effective in terms of evolution and utilization of latent heat. Further heat transfer enhancement process is however required towards the development of the laminated system. 2005 Elsevier Ltd. All rights reserved. Keywords: Phase change material; Phase change zone; Heat transfer
1. Introduction Passive architecture can be interpreted as architecture which tempers the external environment in order to create a relatively stable environment internally. Therefore, a passively designed building incorporating such features as exposed walls, ceiling and floor slabs with energy storage capabilities could help stabilize the internal environment and thus minimise energy consumption. Integration of phase change material (PCM) into building fabrics have been discussed and reported as potential method of reducing energy consumptions in passively-designed buildings [1–6]. The characteristics of PCMs make them inherently suitable for use for energy conservation purposes without the complications
*
Corresponding author. Tel.: +44 115 848 2557; fax: +44 115 848 6438. E-mail address: [email protected] (K. Darkwa). 1359-4311/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2005.10.007
brought about by other thermal storage devices requiring separate plant and space. Although the principles of latent heat storage can be applied to any porous building material, most PCM research work have been concentrated on integration with gypsum wallboard and concrete blocks. For instance experiments have been carried out by Feldman et al. [7] and found gypsum wallboard to be compatible with a broad range of PCMs, including fatty acids and esters. Studies conducted by Athientis et al. [8] showed that an integrated PCM wall board could reduce the maximum room resultant temperature in a passive solar building by up to 4 C during the daytime and also significantly reduce the heating load at night. With respect to thermal comfort criteria, Lamberg et al. [9] have investigated the effects of integrated PCM concrete structures. Even though there was the recognition for a mechanical ventilation system to improve upon poor heat transfer rates, considerable improvement in indoor temperature was noticed.
854
K. Darkwa, P.W. O’Callaghan / Applied Thermal Engineering 26 (2006) 853–858
Nomenclature A C I N Q U V T t
area (m2) specific heat capacity (J/kg K) intensity of solar radiation (W/m2) number of air changes (Ac/h) heat (W) U-value (W/m2 K) volume (m3) temperature (C) time (s)
Greek symbols a heat transfer coefficient (W/m2 K) q density (kg/m3) s transmission coefficient through glass
Darkwa et al. [10–13] have analytically and experimentally evaluated two integrated gypsum-based PCM systems (i.e. randomly-mixed and laminated PCM systems) and found the thermal performance of the laminated-PCM system to be about 18% better than the randomly-mixed type. However these studies were limited to narrow boundary conditions and assumptions. The present study is therefore intended to simulate the practical performance of both the randomly-mixed and laminated PCM systems in a passively-designed model room.
2. Model room description Fig. 1(a) shows a diagram of the model room measuring 3 m · 4 m · 2.5 m high. It also has one window measuring 1.5 m · 1 m and facing south. The walls are of lightweight construction with the interior surfaces lined with 12 mm thick PCM wallboard. Both drywall samples contain the same amount (16.7 mass%) of PCM but the laminated sample (Fig. 1(b)) consists of 2 mm and 10 mm separate layers of PCM and gypsum respectively whilst the randomly-mixed type (Fig. 1(c)) is made up of one layer of PCM and gypsum mixed together.
Subscripts and superscripts eff effective g glazing i initial j nodal point L latent l liquid phase m melting phase r a room air s solid phase w wall su sundry
o oT o oT o oT oH k k k þ þ ¼q ox ox oy oy oz oz ot Z o ¼q C eff dT . ot
ð1Þ
The laminated type was reduced to one-dimensional energy equation as Z o oT oH o k ¼q C eff dT ; ¼q ox ox ot ot
ð2Þ
where H is the enthalpy and expressed for isothermal phase change process as Z H ðT Þ ¼ qC eff ðT ÞdT . ð3Þ For phase change process over an interval of Ts to Tl, the enthalpy is expressed as Z Ts Z T dQL H ðT Þ ¼ qC s ðT ÞdT þ q þ qC m ðT Þ d dT Tj Ts T ðT s < T 6 T l Þ; Z Ts Z H ðT Þ ¼ qC s ðT ÞdT þ qQL þ Tj
þ
Z
ð4Þ Tl
qC m ðT ÞdT Ts
T
qC l ðT ÞdT ðT P T l Þ.
ð5Þ
Tl
3. Mathematical modelling 3.1. Energy equations for PCM wall boards The equations presented here are based on previous studies by Darkwa et al. [9,10] as follows: For the randomly-mixed type, a three-dimensional energy equation was considered as
Now by considering identical properties for both liquid and solid phases the effective heat capacity can be, written in the Gaussian format as C eff ¼ C s þ ae0:5ð
T T m 2 b
Þ;
ð6Þ
where a is the total amount of latent heat and b is the width of phase change zone.
K. Darkwa, P.W. O’Callaghan / Applied Thermal Engineering 26 (2006) 853–858
855
Fig. 1. (a) Skeleton view of model room. (b) Laminated PCM system. (c) Randomly-mixed PCM system.
3.2. Room air temperature distribution
ð7Þ
4. Thermal simulation 3.3. Outdoor temperature distribution
ð8Þ
3.4. Solar radiation intensity (I) t12 2
I ¼ 500e0:8ð8000Þ ;
ð9Þ
where the maximum solar intensity at noon is 500 W/m2.
Thermal simulations for the room were done numerically by applying an implicit finite difference method (FDM) based on the fixed mesh method. This method involves the solution of a continuous system with an implicit representation of the phase change. In order to reduce errors in the analysis, each conduction term was therefore introduced to cover series connections for different materials. If one of the surfaces of the finite volumes was exposed to the convection zone, corresponding conduction term was subsequently replaced by the convection term. In this study the step size of uniform square grid was fixed to 2 mm and the time step at
856
K. Darkwa, P.W. O’Callaghan / Applied Thermal Engineering 26 (2006) 853–858
60 s. The main assumptions considered for the simulation process are as follows: • All thermophysical properties (except the heat capacity of PCM) were kept constant. • There was no convective heat transfer in the liquid PCM phase. • Heat transfer to and from the ceiling and floor were neglected. • Thermal capacity of window was neglected. • Radiant heat onto the outer wall envelope was not considered. Tables 1 and 2 represent the simulation data and thermophysical properties of the materials.
4.1. Effective of heat capacity Fig. 2 shows the variations of effective heat capacity with temperature based on Eq. (6) and data from Table 1. It is clear that in order to avoid an infinite heat capacity and also promote maximum evolution of latent heat an interval of temperature has to be maintained. Even though the effective method cannot accurately model an isothermal phase change process, the Gaussian distribution enables the numerical oscillations to be filtered and smoothened thus reducing errors. For the benefit of this investigation the effects of all the three defined phase change zones (narrow, intermediate and wide) would be considered in the simulation process. 4.2. Environmental conditions
Coefficient of solar radiation (s) Sundry heat load, Qsu (W) Number of air changes per hour, N (Ac/h) Variable zone bands Wide-width of phase change zone Intermediate-width of phase change zone Narrow-width of phase change zone Melting temperature, Tm C Initial temperature, Ti C (for room, concrete walls and PCM wallboards)
0.8 200 (from 8 a.m. to 6 p.m.) 1
a = 50 000, b = 1.6 a = 100 000, b = 0.8 a = 200 000, b = 0.4 18 17
Table 2 Thermophysical properties Density, kg/m3
Wall Gypsum PCM (solid and liquid) Indoor air
2400 900 900a 1.2
Heat capacity, J/kg K
Wall Gypsum PCM (solid and liquid) Indoor air
1000 900 2000b 1000
Heat of fusion, J/kg U–value, W/m2 K
PCM Window
205 000 2.25
Thermal conductivity, (k) W/m K
Wall Gypsum board PCM (solid and liquid)
1.7 0.15 0.15c
Heat transfer coefficient, (a) W/m2 K
Internal wall surface External wall surface
5d 25e
a,c Density and conductivity for liquid and solid phases were assumed to be identical to each other. b The heat capacity value above represents the sensible heat component only. d Room air velocity <0.5 m/s; smooth wall surface. e Wind velocity <15 m/s; normal exposed rough surface.
The outdoor temperature was assumed to vary with a uniform sinusoidal pattern as expressed in Eq. (8). The time (t) was set over 24 h (i.e. from 1.40 p.m. to 1.40 a.m.) with corresponding maximum and minimum temperatures of 22 C and 12 C, respectively. The intensity of solar radiation was assumed to have a uniform daily Gaussian distribution pattern as represented by Eq. (9) over a period of 12 h (i.e. from 6 a.m. to 6 p.m.) with a peak intensity of 500 W/m2 at noon. The total sundry heat gain was estimated as 200 W for a period of 10 h (i.e. from 8 a.m. to 6 p.m.). This is equivalent to heat gains from one person, one source light, and one small office equipment. It must be noted that the radiant heat effect on the outer wall envelope was neglected and that only radiation through the window was considered. The variations of these environmental parameters are presented in Fig. 3.
Effective heat capacity (J/kg) ˚C
Table 1 Simulation data
200000 Narrow phase change zone
150000
Intermediate phase change
100000
Wide phase change zone
50000
0 -5
-3
-1
1
3
5
Differential temperature (T-Tm) ˚C Fig. 2. Variation of effective heat capacity with differential temperature.
K. Darkwa, P.W. O’Callaghan / Applied Thermal Engineering 26 (2006) 853–858 Narrow zone
857
Inter. zone
Wide zone
48
72
Board without PCM
Wall surface temperature (˚C)
21
19
17
15 0
24
Inter. zone
Wide zone
Board without PCM
Room temperature (˚C)
25
Wall surface temperature (˚C)
The simulation results for 120 h are presented in Figs. 4–7. Since the study relates to both storage and release capabilities of PCM-wallboards, the discussions would be focussed on the thermal response factor and the extent to which the lower and higher peaks of room tem-
Narrow zone
21
Inter. zone
Wide zone
48
72
Board without PCM
19
17
15
23
0
24
96
120
Time (Hours) 21
Fig. 7. Phase change zones versus surface temperature of a randomlymixed PCM drywall.
19 17 15 0
24
48
72
120
96
Time (Hours)
Fig. 4. Phase change zones versus room temperature—laminated PCM drywall room.
Narrow zone
Inter. zone
Wide zone
Board without PCM
25
Room temperature (˚C)
120
Fig. 6. Phase change zones effects versus surface temperature of a laminated PCM drywall.
5. Results and discussion
Narrow zone
96
Time (Hours)
Fig. 3. Variations of environmental parameters.
23 21 19
peratures are affected by factors such as phase change zones and sample configuration. By analysing the lower peak temperatures for each cycle it can be seen in Figs. 4 and 5 that the overall thermal performance of the laminated PCM type was much better than the randomlymixed type during night times. It also did response faster in stabilising the room temperature during peak periods. Similar thermal performance can also be observed for the wall surface temperatures in Figs. 6 and 7. Regarding the effect of phase change zones the narrow type displayed the best thermal performance for the laminated PCM type thus supporting the analysis in Fig. 2. On the contrary there were no significant changes among the phase change zones for the randomly-mixed PCM wallboard. This is attributed to the multi-dimensional heat transfer phenomenon that exists in randomly-mixed PCM system.
17
6. Conclusions
15 0
24
48
72
96
120
Time (Hours)
Fig. 5. Phase change zones versus room temperature—randomlymixed PCM drywall.
From Section 5 it can be concluded that the laminated wallboard with a narrow phase change zone would be more effective in moderating night time
858
K. Darkwa, P.W. O’Callaghan / Applied Thermal Engineering 26 (2006) 853–858
temperature in a passively designed room. Other specific conclusions reached were: • Laminated PCM-wallboard performed thermally better than the randomly-mixed type in terms of efficient way of utilizing latent heat. • The laminated PCM-wallboard did increase the minimum room temperature at night by about 17% more than the randomly-mixed type. References [1] K. Darkwa, Mathematical evaluation of phase change wall systems: for reducing energy consumption in buildings and CO2 emissions in the united kingdom, Architectural Science Review 43 (2000) 221–228. [2] M. Hadjiwva, R. Stouyov, Tz. Filipova, Composite salt-hydrate concrete system for building energy storage, Renewable Energy 19 (1–2) (2000) 111–115. [3] T. Haussmann, H.-M. Henning, P. Schossig, Phase change materials in wall integrated systems, in: Proc. IEA, ECES IA Annex 17, Advanced Thermal Energy Storage and TechniquesFeasibility Studies and Demonstration Projects 2nd Workshop, Ljubljana, 2002. [4] H.E. Feustel, C. Stetiu, Thermal performance of phase change wallboard for residential cooling application, Laurence Berkeley National Laboratory, LBL-38320, 1997.
[5] K. Darkwa, Evaluation of regenerative phase change drywalls: low-energy buildings application, International Journal of Energy Research 23 (1999) 1205–1212. [6] D.A. Neeper, Thermal dynamics of wallboard with latent heat storage, Solar Energy 68 (5) (2000) 393–403. [7] D. Feldman, M.A. Khan, D. Banu, Energy storage composite with an organic PCM, Solar Energy Materials 18 (1989) 333– 341. [8] P. Lamberg, A. Jokisalo, K. Siren, The effects on indoor comfort when using phase materials with building concrete products, in: Proc. IEA, ECES IA Annex 10, 6th Workshop, Stockholm, Sweden, November 2000, pp. 22–24. [9] A.K. Athienitis, C. Liu, D. Hawes, D. Hanu, D. Feldman, Investigation of thermal performance of a passive solar test-room with wall latent heat storage, Building and Environment 32 (5) (1997) 405–410. [10] J.-S. Kim, K. Darkwa, Simulation of an integrated PCMwallboard system, International Journal of Energy Research 27 (3) (2003) 215–223. [11] K. Darkwa, J.S. Kim, Heat transfer in neuron-composite laminated phase change drywall, Journal of Power and Energy, Proc. Instn. Mech. Eng. (Part A) 218 (2004) 83–88. [12] K. Darkwa, J.-S. Kim, Enhanced performance of laminated PCM wallboard for thermal energy storage in buildings, in: Proc. 37th Intersociety Energy Conversion Engineering Conference, 2002, Washington, USA. [13] K. Darkwa, J.S. Kim, Dynamics of energy storage in phase change drywall systems, International Journal of Energy Research 29 (2005) 335–343.
Simulation of phase change drywalls in a passive solar building K. Darkwa *, P.W. OÕCallaghan School of the Built Environment, The Applied Energy and Environmental Engineering Group, Nottingham Trent University, Burton Street, Nottingham NG1 4BU, UK Received 22 April 2005; accepted 7 October 2005 Available online 18 November 2005
Abstract Integration of phase change materials (PCMs) into building fabrics is considered to be one of the potential and effective ways of minimizing energy consumption and CO2 emissions in the building sector. In order to assess the thermal effectiveness of this concept, composite PCM drywall samples (i.e. randomly-mixed and laminated PCM drywalls) have been evaluated in a model passive solar building. For a broader assessment, effects of three phase change zones (narrow, intermediate and wide) of the PCM sample were considered. The results showed that the laminated PCM sample with a narrow phase change zone was capable of increasing the minimum room temperature by about 17% more than the randomly-mixed type. Even though there was some display of non-isothermal phase change process, the laminated system proved to be thermally more effective in terms of evolution and utilization of latent heat. Further heat transfer enhancement process is however required towards the development of the laminated system. 2005 Elsevier Ltd. All rights reserved. Keywords: Phase change material; Phase change zone; Heat transfer
1. Introduction Passive architecture can be interpreted as architecture which tempers the external environment in order to create a relatively stable environment internally. Therefore, a passively designed building incorporating such features as exposed walls, ceiling and floor slabs with energy storage capabilities could help stabilize the internal environment and thus minimise energy consumption. Integration of phase change material (PCM) into building fabrics have been discussed and reported as potential method of reducing energy consumptions in passively-designed buildings [1–6]. The characteristics of PCMs make them inherently suitable for use for energy conservation purposes without the complications
*
Corresponding author. Tel.: +44 115 848 2557; fax: +44 115 848 6438. E-mail address: [email protected] (K. Darkwa). 1359-4311/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2005.10.007
brought about by other thermal storage devices requiring separate plant and space. Although the principles of latent heat storage can be applied to any porous building material, most PCM research work have been concentrated on integration with gypsum wallboard and concrete blocks. For instance experiments have been carried out by Feldman et al. [7] and found gypsum wallboard to be compatible with a broad range of PCMs, including fatty acids and esters. Studies conducted by Athientis et al. [8] showed that an integrated PCM wall board could reduce the maximum room resultant temperature in a passive solar building by up to 4 C during the daytime and also significantly reduce the heating load at night. With respect to thermal comfort criteria, Lamberg et al. [9] have investigated the effects of integrated PCM concrete structures. Even though there was the recognition for a mechanical ventilation system to improve upon poor heat transfer rates, considerable improvement in indoor temperature was noticed.
854
K. Darkwa, P.W. O’Callaghan / Applied Thermal Engineering 26 (2006) 853–858
Nomenclature A C I N Q U V T t
area (m2) specific heat capacity (J/kg K) intensity of solar radiation (W/m2) number of air changes (Ac/h) heat (W) U-value (W/m2 K) volume (m3) temperature (C) time (s)
Greek symbols a heat transfer coefficient (W/m2 K) q density (kg/m3) s transmission coefficient through glass
Darkwa et al. [10–13] have analytically and experimentally evaluated two integrated gypsum-based PCM systems (i.e. randomly-mixed and laminated PCM systems) and found the thermal performance of the laminated-PCM system to be about 18% better than the randomly-mixed type. However these studies were limited to narrow boundary conditions and assumptions. The present study is therefore intended to simulate the practical performance of both the randomly-mixed and laminated PCM systems in a passively-designed model room.
2. Model room description Fig. 1(a) shows a diagram of the model room measuring 3 m · 4 m · 2.5 m high. It also has one window measuring 1.5 m · 1 m and facing south. The walls are of lightweight construction with the interior surfaces lined with 12 mm thick PCM wallboard. Both drywall samples contain the same amount (16.7 mass%) of PCM but the laminated sample (Fig. 1(b)) consists of 2 mm and 10 mm separate layers of PCM and gypsum respectively whilst the randomly-mixed type (Fig. 1(c)) is made up of one layer of PCM and gypsum mixed together.
Subscripts and superscripts eff effective g glazing i initial j nodal point L latent l liquid phase m melting phase r a room air s solid phase w wall su sundry
o oT o oT o oT oH k k k þ þ ¼q ox ox oy oy oz oz ot Z o ¼q C eff dT . ot
ð1Þ
The laminated type was reduced to one-dimensional energy equation as Z o oT oH o k ¼q C eff dT ; ¼q ox ox ot ot
ð2Þ
where H is the enthalpy and expressed for isothermal phase change process as Z H ðT Þ ¼ qC eff ðT ÞdT . ð3Þ For phase change process over an interval of Ts to Tl, the enthalpy is expressed as Z Ts Z T dQL H ðT Þ ¼ qC s ðT ÞdT þ q þ qC m ðT Þ d dT Tj Ts T ðT s < T 6 T l Þ; Z Ts Z H ðT Þ ¼ qC s ðT ÞdT þ qQL þ Tj
þ
Z
ð4Þ Tl
qC m ðT ÞdT Ts
T
qC l ðT ÞdT ðT P T l Þ.
ð5Þ
Tl
3. Mathematical modelling 3.1. Energy equations for PCM wall boards The equations presented here are based on previous studies by Darkwa et al. [9,10] as follows: For the randomly-mixed type, a three-dimensional energy equation was considered as
Now by considering identical properties for both liquid and solid phases the effective heat capacity can be, written in the Gaussian format as C eff ¼ C s þ ae0:5ð
T T m 2 b
Þ;
ð6Þ
where a is the total amount of latent heat and b is the width of phase change zone.
K. Darkwa, P.W. O’Callaghan / Applied Thermal Engineering 26 (2006) 853–858
855
Fig. 1. (a) Skeleton view of model room. (b) Laminated PCM system. (c) Randomly-mixed PCM system.
3.2. Room air temperature distribution
ð7Þ
4. Thermal simulation 3.3. Outdoor temperature distribution
ð8Þ
3.4. Solar radiation intensity (I) t12 2
I ¼ 500e0:8ð8000Þ ;
ð9Þ
where the maximum solar intensity at noon is 500 W/m2.
Thermal simulations for the room were done numerically by applying an implicit finite difference method (FDM) based on the fixed mesh method. This method involves the solution of a continuous system with an implicit representation of the phase change. In order to reduce errors in the analysis, each conduction term was therefore introduced to cover series connections for different materials. If one of the surfaces of the finite volumes was exposed to the convection zone, corresponding conduction term was subsequently replaced by the convection term. In this study the step size of uniform square grid was fixed to 2 mm and the time step at
856
K. Darkwa, P.W. O’Callaghan / Applied Thermal Engineering 26 (2006) 853–858
60 s. The main assumptions considered for the simulation process are as follows: • All thermophysical properties (except the heat capacity of PCM) were kept constant. • There was no convective heat transfer in the liquid PCM phase. • Heat transfer to and from the ceiling and floor were neglected. • Thermal capacity of window was neglected. • Radiant heat onto the outer wall envelope was not considered. Tables 1 and 2 represent the simulation data and thermophysical properties of the materials.
4.1. Effective of heat capacity Fig. 2 shows the variations of effective heat capacity with temperature based on Eq. (6) and data from Table 1. It is clear that in order to avoid an infinite heat capacity and also promote maximum evolution of latent heat an interval of temperature has to be maintained. Even though the effective method cannot accurately model an isothermal phase change process, the Gaussian distribution enables the numerical oscillations to be filtered and smoothened thus reducing errors. For the benefit of this investigation the effects of all the three defined phase change zones (narrow, intermediate and wide) would be considered in the simulation process. 4.2. Environmental conditions
Coefficient of solar radiation (s) Sundry heat load, Qsu (W) Number of air changes per hour, N (Ac/h) Variable zone bands Wide-width of phase change zone Intermediate-width of phase change zone Narrow-width of phase change zone Melting temperature, Tm C Initial temperature, Ti C (for room, concrete walls and PCM wallboards)
0.8 200 (from 8 a.m. to 6 p.m.) 1
a = 50 000, b = 1.6 a = 100 000, b = 0.8 a = 200 000, b = 0.4 18 17
Table 2 Thermophysical properties Density, kg/m3
Wall Gypsum PCM (solid and liquid) Indoor air
2400 900 900a 1.2
Heat capacity, J/kg K
Wall Gypsum PCM (solid and liquid) Indoor air
1000 900 2000b 1000
Heat of fusion, J/kg U–value, W/m2 K
PCM Window
205 000 2.25
Thermal conductivity, (k) W/m K
Wall Gypsum board PCM (solid and liquid)
1.7 0.15 0.15c
Heat transfer coefficient, (a) W/m2 K
Internal wall surface External wall surface
5d 25e
a,c Density and conductivity for liquid and solid phases were assumed to be identical to each other. b The heat capacity value above represents the sensible heat component only. d Room air velocity <0.5 m/s; smooth wall surface. e Wind velocity <15 m/s; normal exposed rough surface.
The outdoor temperature was assumed to vary with a uniform sinusoidal pattern as expressed in Eq. (8). The time (t) was set over 24 h (i.e. from 1.40 p.m. to 1.40 a.m.) with corresponding maximum and minimum temperatures of 22 C and 12 C, respectively. The intensity of solar radiation was assumed to have a uniform daily Gaussian distribution pattern as represented by Eq. (9) over a period of 12 h (i.e. from 6 a.m. to 6 p.m.) with a peak intensity of 500 W/m2 at noon. The total sundry heat gain was estimated as 200 W for a period of 10 h (i.e. from 8 a.m. to 6 p.m.). This is equivalent to heat gains from one person, one source light, and one small office equipment. It must be noted that the radiant heat effect on the outer wall envelope was neglected and that only radiation through the window was considered. The variations of these environmental parameters are presented in Fig. 3.
Effective heat capacity (J/kg) ˚C
Table 1 Simulation data
200000 Narrow phase change zone
150000
Intermediate phase change
100000
Wide phase change zone
50000
0 -5
-3
-1
1
3
5
Differential temperature (T-Tm) ˚C Fig. 2. Variation of effective heat capacity with differential temperature.
K. Darkwa, P.W. O’Callaghan / Applied Thermal Engineering 26 (2006) 853–858 Narrow zone
857
Inter. zone
Wide zone
48
72
Board without PCM
Wall surface temperature (˚C)
21
19
17
15 0
24
Inter. zone
Wide zone
Board without PCM
Room temperature (˚C)
25
Wall surface temperature (˚C)
The simulation results for 120 h are presented in Figs. 4–7. Since the study relates to both storage and release capabilities of PCM-wallboards, the discussions would be focussed on the thermal response factor and the extent to which the lower and higher peaks of room tem-
Narrow zone
21
Inter. zone
Wide zone
48
72
Board without PCM
19
17
15
23
0
24
96
120
Time (Hours) 21
Fig. 7. Phase change zones versus surface temperature of a randomlymixed PCM drywall.
19 17 15 0
24
48
72
120
96
Time (Hours)
Fig. 4. Phase change zones versus room temperature—laminated PCM drywall room.
Narrow zone
Inter. zone
Wide zone
Board without PCM
25
Room temperature (˚C)
120
Fig. 6. Phase change zones effects versus surface temperature of a laminated PCM drywall.
5. Results and discussion
Narrow zone
96
Time (Hours)
Fig. 3. Variations of environmental parameters.
23 21 19
peratures are affected by factors such as phase change zones and sample configuration. By analysing the lower peak temperatures for each cycle it can be seen in Figs. 4 and 5 that the overall thermal performance of the laminated PCM type was much better than the randomlymixed type during night times. It also did response faster in stabilising the room temperature during peak periods. Similar thermal performance can also be observed for the wall surface temperatures in Figs. 6 and 7. Regarding the effect of phase change zones the narrow type displayed the best thermal performance for the laminated PCM type thus supporting the analysis in Fig. 2. On the contrary there were no significant changes among the phase change zones for the randomly-mixed PCM wallboard. This is attributed to the multi-dimensional heat transfer phenomenon that exists in randomly-mixed PCM system.
17
6. Conclusions
15 0
24
48
72
96
120
Time (Hours)
Fig. 5. Phase change zones versus room temperature—randomlymixed PCM drywall.
From Section 5 it can be concluded that the laminated wallboard with a narrow phase change zone would be more effective in moderating night time
858
K. Darkwa, P.W. O’Callaghan / Applied Thermal Engineering 26 (2006) 853–858
temperature in a passively designed room. Other specific conclusions reached were: • Laminated PCM-wallboard performed thermally better than the randomly-mixed type in terms of efficient way of utilizing latent heat. • The laminated PCM-wallboard did increase the minimum room temperature at night by about 17% more than the randomly-mixed type. References [1] K. Darkwa, Mathematical evaluation of phase change wall systems: for reducing energy consumption in buildings and CO2 emissions in the united kingdom, Architectural Science Review 43 (2000) 221–228. [2] M. Hadjiwva, R. Stouyov, Tz. Filipova, Composite salt-hydrate concrete system for building energy storage, Renewable Energy 19 (1–2) (2000) 111–115. [3] T. Haussmann, H.-M. Henning, P. Schossig, Phase change materials in wall integrated systems, in: Proc. IEA, ECES IA Annex 17, Advanced Thermal Energy Storage and TechniquesFeasibility Studies and Demonstration Projects 2nd Workshop, Ljubljana, 2002. [4] H.E. Feustel, C. Stetiu, Thermal performance of phase change wallboard for residential cooling application, Laurence Berkeley National Laboratory, LBL-38320, 1997.
[5] K. Darkwa, Evaluation of regenerative phase change drywalls: low-energy buildings application, International Journal of Energy Research 23 (1999) 1205–1212. [6] D.A. Neeper, Thermal dynamics of wallboard with latent heat storage, Solar Energy 68 (5) (2000) 393–403. [7] D. Feldman, M.A. Khan, D. Banu, Energy storage composite with an organic PCM, Solar Energy Materials 18 (1989) 333– 341. [8] P. Lamberg, A. Jokisalo, K. Siren, The effects on indoor comfort when using phase materials with building concrete products, in: Proc. IEA, ECES IA Annex 10, 6th Workshop, Stockholm, Sweden, November 2000, pp. 22–24. [9] A.K. Athienitis, C. Liu, D. Hawes, D. Hanu, D. Feldman, Investigation of thermal performance of a passive solar test-room with wall latent heat storage, Building and Environment 32 (5) (1997) 405–410. [10] J.-S. Kim, K. Darkwa, Simulation of an integrated PCMwallboard system, International Journal of Energy Research 27 (3) (2003) 215–223. [11] K. Darkwa, J.S. Kim, Heat transfer in neuron-composite laminated phase change drywall, Journal of Power and Energy, Proc. Instn. Mech. Eng. (Part A) 218 (2004) 83–88. [12] K. Darkwa, J.-S. Kim, Enhanced performance of laminated PCM wallboard for thermal energy storage in buildings, in: Proc. 37th Intersociety Energy Conversion Engineering Conference, 2002, Washington, USA. [13] K. Darkwa, J.S. Kim, Dynamics of energy storage in phase change drywall systems, International Journal of Energy Research 29 (2005) 335–343.