2nd International Engineering Mechanics and Materials Specialty Conference le 2

è Congrès international de mécanique et des matériaux

Ottawa, Ontario

June 14-17, 2011 / 14 au 17 juin 2011

EM-024-1

Improving the Energy Efficiency of Buildings with Hollow Core Slabs: A Numerical Investigation H.B. Gunay

1, O.B. Isgor

1, A.G. Razaqur

2, and Simon Foo

3

(1) Carleton University, Dept. of Civil and Env. Eng. Ottawa, ON, Canada (2) McMaster University, Dept. of Civil Engineering, Hamilton, ON, Canada (3) Public Works and Government Services Canada, Gatineau, QC, Canada

Abstract: Thermal mass is the capacity of a material to store heat. Concrete or masonry has a higher heat storage capacity than air; therefore, there is significant potential in using the natural thermal mass of buildings to reduce and to shift peak load energy demands. Most residential and commercial buildings have adequate thermal mass that can be utilized to reduce and shift peak energy load. In particular, hollow core slabs that utilize air passing through the slabs to transfer heat in and out of concrete, have the potential to reduce and to shift peak load requirements. This paper presents a numerical investigation that aims to investigate design parameters of hollow core slabs for the maximum energy efficiency, particularly with respect to peak energy demand reduction and shifting. Results reveal that hollow core slab system can be actively used to improve the energy efficiency of buildings. The use of phase change materials (PCM) along with the thermal mass of hollow core slabs enhances both peak load reduction and phase shift; therefore, composite systems that combine the thermal mass of concrete with PCMs emerge as feasible design alternatives to commonly used flat slab systems. 1. Introduction Today’s economic and environmental challenges have compelled building owners, developers, engineers, architects and policy makers to reflect on these figures more carefully than before and to come up with less energy consumption alternatives. One such alternative that has emerged is the concept of the net-zero energy building – a commercially viable building that uses zero net energy and is carbon neutral. In a typical commercial building, over 80% of total energy consumption can be attributed to heating, cooling and lighting (Buildings Energy Data Book 2009). Therefore, th “net-zero energy building” concept implies that the energy demand for heating, cooling and lighting is reduced by active and passive methods, and this reduced demand is met on an annual basis from a renewable energy supply that is typically integrated into the building design. An area that has been receiving renewed attention in recent years is the use of thermal mass of buildings to reduce and shift peak energy loads/demand of buildings. Thermal mass is the capacity of a material to store heat. Concrete or masonry has a higher heat storage capacity than air; therefore, there is significant potential in using the natural thermal mass of buildings to reduce and to shift peak load energy demands. For example, in winter, due to their mass buildings can absorb heat from sunlight either directly or by means of heat pumps; at night the process is reversed as heated mass gives up its stored heat, warming the building by radiation, convection and conduction. During summer months, the part of the mass that is properly shaded can absorb the heat from air in the building and reduce the active HVAC requirements. Most residential and commercial buildings have adequate thermal mass (e.g. as concrete slabs or masonry walls) that can be utilized to reduce and shift peak energy load. In particular, hollow core slabs that utilize air passing through the slabs to transfer heat in and out of concrete, have significant potential to reduce and shift peak load requirements (Barton et al. 2002). In the winter months, for example, the air that is heated using natural sun light through solar panels can be circulated through the ducts of the hollow concrete slab to transfers energy to the thermal mass of concrete for storage and its subsequent release to reduce the heating requirements during evenings.

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Previous studies showed that buildings with hollow core slabs are better in reducing and shifting peak energy requirements than buildings with conventional slab systems (Sodha et al. 1980; Winwood et al. 1996; Barton et al. 2002). Banu et al. (1998) also demonstrated that the use of phase change materials (PCMs) in combination with the thermal mass of concrete has the potential to produce further energy efficiency. PCM is a material with a high heat of fusion which is capable of storing and releasing large amounts of energy through melting and solidifying at a specific and narrow temperature range. Heat is absorbed (or released) when PCM experiences phase changes (e.g. from solid to liquid); therefore, they can be considdered as latent heat storage units. Hawes et al. (1990) showed that PCMs can be introduced in concrete in different ways, either during dry mixing of the concrete mix or after pouring. To prevent interference with the hydration process and the aggregate-cement bond reactions, the PCMs can be added relatively homogeneously into the concrete mix in capsules. The final product can be considered as a homogeneous composite material, which has unique properties with respect to thermal efficiency. This paper presents a numerical investigation that aims to investigate design parameters of hollow core slabs, in particular the effects of hollow core geometry and the use of PCMs, for the maximum energy efficiency with respect to peak energy demand reduction and shifting.

2. Numerical Model

The numerical investigation is carried out within a domain representing a room in a typical building as illustrated in Figure 1. The heat transfer in the domain of analysis is assumed to be governed by the conduction equation:

[1] ∇ (k∇T)+Q=ρCp

T

t

in Ω

where k (W/K-m) is thermal conductivity, T (K) is the temperature, Q (W/m3) is a sink or source, (kg/m

3)

and Cp (J/K-kg) are the density and the specific heat of the material of domain , and t (s) is time. The effect of convection inside the room air domain is reflected by magnifying the thermal conductivity coefficient by Nusselt Number (Cengel 2007). The heat transfer on the core surface is governed by forced convection. On the façade of the room, the heat transfer is due to natural convection and radiation. The boundary conditions in the domain of the analysis are given by equations [2] – [6]: [2] n

.(kcomp∇T)=hforced(T-Tair) on Г1

[3] n

.(kbrick∇T)=εσ(T

4-Tamb

4)+hnat(T-Tamb) on Г2

[4] n.(kcomp∇T-kair∇T)=0 on Г3

[5] n.( kair∇T- kbrick∇T)=0 on Г4

[6] n.(k∇T)=0 otherwise

where n is the unit vector perpendicular to the boundary surface, kbrick (W/K-m) is the thermal conductivity of the brick wall, kair (W/K-m) is the thermal conductivity of air, kcomp (W/K-m) is the thermal conductivity of PCM and concrete composite structure inside the slab, hforced (W/m

2°K) is coefficient of forced convection,

hnat (W/m2°

K) is coefficient of natural convection, Tamb (K) is the ambient temperature, Tair (K) is the function of temperature along the duct of the hollow core slab, σ is the Stefan-Boltzmann Constant for radiation heat transfer and, ε is the emissivity of concrete.

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Figure 1: Domain of analysis In this study, the PCM application is assumed to be carried out by mixing the encapsulated PCM micro-packages into the concrete pore solution during the casting process in the plant at 10% of the total concrete dry weight. Since PCMs are assumed to be homogeneously distributed in the concrete, their thermal properties are incorporated in the analysis through equations [2] and [4]. The thermal properties of PCMs as a function of temperature are defined in detail by Alawadhi and Amon (2003). Equations [7] – [9] provide the composite thermal properties of concrete and PCMs as used in the current numerical model: [7] Ccomp=(1-PCMratio)Cconc+PCMratioCpcm

[8] Cpcm=

p melt

melt melt

p melt

C T<T

L T <T<T +ΔT

ΔT

C T <T

[9] kcomp=(1-PCMratio)kconc+PCMratiokpcm

4

where Ccomp (J/(K-kg) is the specific heat of the PCM and concrete composite material inside the slab, Cconc (J/(K-kg) is the specific heat of concrete, Cpcm (J/(K-kg) is the specific heat of PCM, L (J/kg) is the latent heat of fusion of PCM, ΔT (K) is the range of temperature at which the PCM experiences phase change, PCMratio is the mass participation ratio of the PCMs into the hollow core slab domain and, Tmelt is the onset temperature of PCM melting. The properties of materials that are used in the numerical model, i.e., of concrete, brick and air, are presented in Table 1. The properties of the PCM used in this numerical investigation are presented in Table 2.

Table 1: Properties of the materials used in the numerical investigation

Material Density,

ρ kg/m

3

Thermal Conductivity, k

W/(K-m)

Specific Heat, Cp

J/(K-kg)

Air 1.205 0.0257 Nu 1005

Concrete 2300 1.8 840

Brick 375 0.1 900

Table 2: Material properties of the PCM used in the numerical investigation

PCM Type Specific Heat,

Cp

J/(K-kg)

Latent Heat of Fusion, L

kJ/kg

Melting Temperature

Tmelt,K

Thermal Conductivity, k

W/K-m

PCM1 1500 134 294.15 0.2

Temperature differentials in the room air domain create different densities and enhance air circultion. In this study, convection of air in the room is defined by magnifying the thermal conductivity of the air by Nusselt Number, Nu, which is a function of the gradient of the field variable, T: [10] Nu=C1Ra

C2

where Ra is the Rayleigh Number, and C1 and C2 are constants which are given as 0.479 and 0.171, respectively. The relationship between the Nusselt Number and the Rayleigh Number is defined by Warrington and Powe (1984) via:

[11] Ra=GrPr=gβ υ -1∇(T)LiLc3Pr

where Gr is the Grashof Number, Pr is the Prandtl Number, g is the gravitational acceleration constant, β is the coefficient of thermal expansion, υ is the kinematic viscosity, Li is the dimensions of the room air domain in the corresponding direction, and Lc is the characteristic length. 3. Numerical Analysis The numerical investigation presented in this paper is focused on three design parameters: (1) geometry of the hollow core slab, represented as duct diameter, (2) thermal mass of concrete, and (3) the use of PCM. To investigate the effect of slab geometry, the core diameter of the slab is changed while keeping thermal mass of concrete constant, as shown in Table 3. The effect of thermal mass of concrete is studied by changing the cross sectional area of the slab while the core diameter is kept constant, as shown in Table 4. Finally, the thermal mass of the hollow core slab system is modified by applying PCM and the corresponding effects are numerically investigated.

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The ambient temperature is taken from National Climate Data and Information Archive for Ottawa. June extreme daily data and August average data are selected for effect of diameter analysis and for the effect of thermal mass analysis, respectively. The summer season is analyzed to figure out the effect of a PCM having a PCM melting/freezing range at 294.15 K. In June, summer temperature has the highest temperature values and in August, daily temperature fluctuations reach the largest of the summer season. The June extreme temperature data is fluctuating between 293.15 K and 305.15 K, August average temperature is fluctuating between 287.15 K and 301.15 K. The data is then fitted into a sinusoidal function of time. The function brings the ambient temperature to the lowest at 5 AM in the morning. The velocity of the air in the hollow core slab duct is constant at 1 m/s. Air intake section of hollow core slabs are closed when the ambient air temperature is above 294.15 K. Hence, the air is only running in the hollow core slab duct when the ambient is lower than 294.15K. A Heaviside step function is utilized for this purpose. This step function brings the forced convection heat transfer coefficient to zero when there is no air flow inside the duct.

Table 3: Analysis cases for varying slab core diameter

Diameter Width Thickness Core Area

Thermal Mass

Hforced

mm mm mm mm2

kJ/°K W/(m2°

K)

Flat Slab 133 90 0 46.5 0

25 133 94 245 46.5 5.07

50 133 106 981 46.5 6.49

75 133 124 2208 46.5 6.34

100 133 150 3925 46.5 6.07

Table 4: Analysis cases for effect of thermal mass analysis

Thickness Width Core

Diameter

Core

Area

Thermal

Mass

mm mm mm mm2

kJ/K

150 133 100 3925 46

200 133 100 3925 72

250 133 100 3925 98

300 133 100 3925 123

1000 133 100 3925 482

4. Results and Discussion

4.1. Effect of core diameter size

Results shown in Table 5 and Figure 2 reveal that increase in the diameter enhances the damping effect of hollow core slabs on peak temperature cycles. However, effect of diameter change on phase shift is negligible. The peak load reduction increases 77% when a hollow core having a diameter of 100 mm is utilized instead of a flat slab. The increase is 30% for a 25 mm diameter duct, 60% for a 50 mm diameter duct, 70% for a 75 mm diameter duct. An increase in the surface of active heating and cooling at the times of air flow are in favour of energy efficiency. Hence, designing the slab core diameter as large as the local structural requirements permit improves the peak load reduction capacity of the room.

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0 25 50 75 100Core Diameter Size/mm

5

6

7

8

9

10

11

Peak L

oa

d R

edu

ction/K

Table 5: The summary of the core diameter analysis

Diameter Width Slab

Thickness Core Area

Thermal Mass

Peak Load Reduction

Max.Temp.in the Room

Air

mm mm mm mm2

kJ/K K K

Flat Slab 133 90 0 46.453 5.99 26.01

25 133 94 245 46.453 7.79 24.21

50 133 106 981 46.453 9.59 22.41

75 133 124 2208 46.453 10.26 21.74

100 133 150 3925 46.453 10.62 21.38

Figure 1: Core diameter size versus peak load reduction

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0 50 100 150 200 250 300 350 400 450 500Thermal Mass/ (kJ/K)

2

2.5

3

3.5

4

4.5

5

5.5

6

6.5P

eak L

oad

Red

uction

/K

14

13

12

11

10

9

8

7

6

5

4

3

Pha

se S

hift/

ho

urs

Peak Load Reduction

Phase Shift

0 3 6 9 12 15 18 21 24Time/hours

16

17

18

19

20

21

22

23

24

25

26

Te

mpera

ture

/K

Thermal Mass 46kJ/K

Thermal Mass 72kJ/K

Thermal Mass 98kJ/K

Thermal Mass 123kJ/K

Thermal Mass 482kJ/K

4.2. Effect of thermal mass and PCM utilisation

The results for the effect of thermal mass analysis are illustrated in Figures 3 and 4 indicate that an increase in thermal mass improves both peak load reduction and phase shift. As the ambient temperature follows a sinusoidal function, temperature behaviour in the domain is that way also. Though, the peak temperatures reached in higher thermal mass is damped to a lower temperature and shifts to further time of the day. The peak load reduction for 46 kJ/K thermal mass with respect to ambient temperature is 2.1 K and the phase shift is 3.4 hours. The peak load reduction and phase shift values become 3.3 K and 4.5 hours when it is 72 kJ/K, they will become 4.1 K and 5.2 hours when it is 98 kJ/K, 4.7 K and 5.9 hours when it is 123 kJ/K, and, 6.4 K and 13.3 hours for 482 kJ/K. However, it is economically and aesthetically unfeasible to use massive concrete slabs just for thermal efficiency; therefore, PCM utilisation is tested as a possible solution to decrease the slab thickness. This way thermal mass of the slabs are increased without changing the dimensions of the slab. In Figure 4, a relationship between thermal mass and peak load reduction and phase shift is illustrated.

Figure 3: Temperature behaviour at various Figure 4: Thermal mass versus peak load reduction thermal masses & phase-shift properties

The results in Table 6 indicate that peak load reduction follows a curve converging to the amplitude of the daily temperature fluctuations. However, phase shift increases linearly with the increasing thermal mass. An unrealistically thick slab (1000 mm thick) is analysed to show an extreme case of the effect of thermal mass of slab. The results in Table 7 show that PCM utilisation improves the thermal mass of the hollow core slab substantially. For example, 300 mm thick slab without PCM utilization shows the similar thermal responses with 150 mm thick slab with PCM utilisation.

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Table 6: Summary of thermal mass analysis without PCM use

Diameter Width Thickness Thermal

Mass

Min.Temp

in Room

Peak Load

Reduction

Phase

Shift

mm mm mm kJ/K K K Hours

100 133 150 46 16.07 2.1 3.4

100 133 200 72 17.25 3.3 4.5

100 133 250 98 18.09 4.1 5.2

100 133 300 123 18.67 4.7 5.9

100 133 1000 482 20.36 6.4 13.3

Table 7: Summary of thermal mass analysis with PCM use

Diameter Width Thickness Min.Temp. in Room

Peak Load Reduction

Phase Shift

Effective Thermal

Mass

mm mm mm K K Hours kJ/K

100 133 150 18.96 4.98 5.2 121

100 133 200 19.63 5.65 5.9 181

100 133 250 19.86 5.88 6.3 217

100 133 300 19.98 6.00 6.6 240

5. Conclusions In this study, the design-stage parameters that affect the thermal performance of the hollow core slabs are investigated. The thermal responses of the hollow core slabs at various design-stage parameters are compared with respect to peak load reduction and phase shift capacity. Increase in the slab core diameter at a given thermal mass is found to increase peak load reduction. Hence, the core diameter of the hollow core slab ducts should be maximized as much as the structural requirements permit. Analyses also reveal that increase in thermal mass yields phase shift and peak load reduction. Since it is unfeasible to provide massive slabs to improve the thermal performance of buildings, PCMs can be utilized effectively to increase the thermal mass of the slab without significantly changing the structure and architecture of the strcutral units. 6. References Alawadhi, and, E.M., Amon, C.H. 2003. PCM thermal control unit for portable electronic devices:

experimental and numerical studies. Components and Packaging Technologies, IEEE, 26: 116-125. Banu, D., Feldman, D., and, Hawes, D. 1998. Evaluation of thermal storage as latent heat in phase

change material wallboard by differential scanning calorimetry and large scale thermal testing. Thermochimica Acta, 317: 39-45.

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Barton, P., Beggs, C.B., and, Sleigh, P.A. 2002. A theoretical study of thermal performance of the

thermodeck hollow core slab systems. Applied Thermal Engineering, 22: 1485-1499. Bejan, A. 1984. Convection heat transfer. John Wiley and Sons, Ltd., N.Y. Braham, G.D. 2000. Mechanical ventilation and fabric thermal storage. Indoor Built Environment, 9: 102-

110. Buildings Energy Data Book. 2009. U.S. Department of Energy, Office of Energy Efficiency and

Renewable Energy. Cengel, Y.A. 2002. Heat transfer: a practical approach. McGraw-Hill, N.Y. Corgnati, S.P., and Kindinis, A. 2007. Thermal mass activation by hollow core slab coupled with night

ventilation to reduce summer cooling loads. Building and Environment, 42: 3285-3297. Halford, C.K., and Boehm, R.F. 2007. Modeling of phase change material peak load shifting, Energy and

Buildings, 39: 298-305. Hawes, D.W., Banu. D., and Feldman D. 1990. Latent heat storage in concrete II, Solar energy Materials,

21: 61-80. Kakac, S., Shah, R.K., and, Aung, W. 1987. Handbook of single-phase convective heat transfer. John

Wiley and Sons, Ltd., N.Y. Lewis, R.W., Morgan, K., and, Zienkiewicz, O.C. 1981. Numerical methods in heat transfer. John Wiley

and Sons, Ltd., N.Y. New Practice Final Report 106, The Elizabeth Fry Building, University of East Anglia Ð feedback for

designers and clients, Best Practice Program, BRESCU,1998. PROBE 14: Elizabeth Fry Building, Building Services Journal (1998) 3-42. Sinha, S.L., Arora, R.C., and, Roy, S. 2000. Numerical simulation of two-dimensional room air flow with

and without buoyancy. Energy and Buildings, 32: 121-129. Spiegel, E.A., and Veronis, G. 1959. On the Boussinesq approximation for a compressible fluid, American

Astronomical Society, 131: 442-447. Sodha, M.S., Seth A.K., and Kaushik S.C. 1980. Periodic heat transfer through a hollow concrete slab:

optimum placement of the air gap. Applied Energy, 6: 113-123. Tyagi, V.V., and, Buddhi, D. 2007. PCM thermal storage in buildings: A state of art. Renewable and

Sustainable Energy Review, 11: 1146-1166. Vallejos, P.A., and, Duston, C. 2005. Carbon foam filled with phase change materials for passive

temperature management. In Proceedings of the COMSOL Multiphysic User’s Conference, Boston, MA.

Warrington, R.O., and, Powe, R.E. 1984. The transfer of heat by natural convection between bodies and their enclosures. International Journal of Heat and Mass Transfer, 28: 319-330.

Winwood, R., Benstead, R., and, Edwards, R. 1997. Advanced fabric energy storage II- computational fluid dynamics modeling. Chartered Institutions of Building Services Engineers, 18: 7-16.

Zmerenue, and, R., Fazio, P. 1988. Thermal performance of a hollow core concrete floor system for passive cooling. Building and Environment, 23: 243-252