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Development of novel roof structures for thermal comfort and

energy savings in buildings with and without PCM’s

Development of novel methods for energy generation, utilization, storage and conservation has been a matter of concern among researchers for many years.

The technologies in the area of energy storage and conservation in buildings are being gaining increased attention over the years.

A technology that can be used to store large amounts of heat or cold in a definite volume is a matter of concern among the researchers around the world.

Sensible heat storage systems have been practiced for so many years.

Overview

Energy conservation through energy storage in buildings has become an exciting and attracting method for domestic, industrial and commercial sector applications.

Reducing the dependency on fossil fuels in view of the threat of their depletion in another five to ten decades has really made the researchers to find the ways and means to develop the sustainable energy dwellings with the use of PCM’s.

In this work it is aimed to attempt on the development of novel roof structures with and with out phase change materials(PCM’s) for building applications.

Overview

In view of the above, the present work is focused on feasibility of developing novel roof structures for thermal comfort and energy savings in buildings with and with out phase change materials(PCM’s).

In India cooling of buildings consume considerable amounts of energy due to the climatic conditions. Sensible heat storage(SHS) has been used since prehistoric times.

To overcome some of the inherent problems with sensible heat storage systems such as excessive mass and undesirable temperature excursions during and prolonged periods of high and low ambient temperatures.

Overview

To overcome the above mentioned problems the use of phase change materials(PCMs) as latent heat storage(LHS) medium in buildings began to receive serious consideration in the last two decades.

These materials absorb heat in changing from the solid to liquid state and release it as they change in the opposite direction.

Latent heat storage in a phase change material(PCM) is very attractive because of its high-energy storage density and its isothermal behaviour during the phase change process.

Overview

Thermal storage plays a major role in building energy conservation which is greatly assisted by the incorporation of latent heat storage in building products.

Increasing the thermal storage capacity of a building can enhance human comfort by decreasing the frequency of internal air temperature swings so that the indoor air temperature is closer to the desired temperature for a longer period of time.

Overview

Based on the above background in this paper

1. Attempt is made to study the thermal performance of the two roof structures i.e., a simple RCC roof and a PCM integrated roof and the feasibility of other two proposed roof structure models.

2. The theoretical simulation results obtained for RCC and PCM roof using Ansys 10 are validated by comparing the experimental results.

Aim of the paper

3. To study the influence of solar flux on the indoor temperatures, thermal flux, thermal gradient and the heat flow across the RCC and PCM roofs.

4. Validating the theoretical results with the experimental data and to draw the conclusions and making suggestions and recommendations based on the findings.

Aim of the paper

Literature review

The earlier works done by the researchers L.E.Bourdeau[1], P.Braousseu et al[2], R.Velraj et al[3] and A.Pasupathy et al[4] were focussed on theoretical simulations on the use of PCMs(small blocks of 50mmx50mm size) for passive thermal storage for heating and cooling of buildings.

Most of the works were carried out on heating applications. In India heating is never a problem. Some of the works were limited to provide only the temperature distribution analysis for the small sample models of PCM blocks.

Literature review

In view of the above literature review, in the present work a computer simulation and modeling using Ansys 10 software which provides the complete analysis for the two modeled roof structures (2mx2m in area).

The experimental validations made with the simulation results for the two models is seem to be promising.

In this work, an attempt is made to study the effect of phase change materials(PCMs) on the indoor room temperatures of a residential building.

A comparative study on the thermal performance of an inorganic PCM(CaCl26H2o) as phase change material has been carried out both experimentally and through a simulation study using Ansys Software version 10.

Theoretical simulation and modeling analysis using Ansys10

Concrete Slab (RCC)

Roof Top slab

PCM Panel

10 cm

2.5 cm

12 cm

RCC 12 cm

Fig.1 simple RCC roof

Fig.2 PCM integrated roof

ConvectionWind

SUN

Radiation

PCM Panel

Concrete Slab

Roof top (brick mixture + lime)

Fig.3 Modeled PCM integrated roof

MATERIAL PROPERTY DATA

Material Density (kg/m3)

Thermal conductivity(W/mK)

Specific Heat (J/kg K)

Concrete slab(RCC)

2300 1.279 1130

Roof top slab(mixture of brick + lime)

1300 0.25 800

Phase change material (PCM) CaCl26H20

1500 1.01

1440

Latent heat of PCM(KJ/kg)

188

TECHNICAL SPECIFICATIONS OF USED PCM

PCM material :CaCl26H20

Appearance (color) :GreyPhase change temperature (0C) : 290CDensity (kg/m3) : 1500 Latent heat of fusion (kJ/kg) : 188Thermal conductivity (W/mK)Solid : 1.09 [0-29 0C]Liquid : 0.54 [ 29 – 600C ]Specific heat (J/kg K)( 0 – 290C ) :1440( 290C – 300C ) :125,000(300C – 600C ) :1440

ASSUMPTIONS MADE The following assumptions are made in the analysis.The heat conduction in the composite wall is one

dimensional and the end effects are neglected.The thermal conductivity of the concrete slab and

the roof top slab are considered constant and not varying with respect to temperature.

The PCM is homogenous and isotropic.The convection effect in the molten PCM is neglected.The interfacial resistances are negligible.The material properties are constantRadiation heat exchange with in the room is neglected The thermo physical properties of the PCM are different for the solid and liquid phases but are independent of temperature.

PROBLEM FORMULATION The physical system considered is a galvanized

iron panel filled with PCM placed between the roof top slab and the bottom concrete slab, which form the roof of the PCM room.

In each cycle, during the charging process(sunshine hours), the PCM in the roof changes its phase from solid to liquid.

As melting requires a large quantity of heat at its phase change temperature, the temperature of the concrete slab normally will not exceed the PCM phase change temperature.

During the discharging process(night hours), the PCM changes its phase from liquid to solid(solidification) by rejecting heat to the ambient and to the air inside the room. This cycle continues every day.

The composite wall described in the above Fig. is initially maintained at a uniform temperature.

The boundary condition on the outer surface of roof is considered due to the combined effect of radiation and convection.

In order to consider the radiation effect, the average monthly solar radiation heat flux data(measured values) for every 1-h in Pulivendula town, A.P, India is used.

For convection, the heat transfer coefficient(h0) on the outer surface is calculated based on the prevailing velocity of wind using Nusselt correlation[NuL=0.664(ReL)0.5(Pr)0.33] and the inner surface is considered having natural convection inside the roof [NuL=0.54(Gr.Pr)0.25]

Using these two correlations the local heat transfer coefficients are calculated. However, in the theoretical analysis the heat transfer coefficients are assumed as 10W/m2 and 5W/m2 at outside and inside the roofs respectively.

The boundary condition on the inner surface of the concrete slab is considered to be natural convection.

As the temperature difference between the room and the wall is very less, most of the earlier researchers have approximated the bottom wall as insulated. However, when the temperature difference becomes appreciable, the effect of heat flow is considerable and hence this convection effect is also taken into account in the present work with a suitable Nusselt correlation [NuL=0.54(Gr.Pr)0.25]

ANSYS 10 is a general purpose finite element analysis (FEA) software developed to solve the problems of both structural and thermal streams.

It is an user-friendly software that can be used for modeling the building roof structures and besides providing the complete thermal analysis such as variation of temperature distribution, thermal gradient, thermal flux, heat flow across the roof etc.,

For comparing the theoretical simulations obtained using Ansys software, two experimental identical test rooms have been constructed and the performance of both have been analysed.

Ansys 10 software

MATHEMATICAL EQUATIONS AND METHODS FOR LHTES

The latent heat thermal energy storage systems[LHTES] have been developed for the applications of cooling and heating of buildings and for many other applications.

To carryout the theoretical and thermal performance analysis of such type of systems invariably require a mathematical model or a computer simulation software.

The following governing equations and boundary conditions for one-dimensional heat transfer through the two roof models were used.

In the present research work, a computer simulation software FEA Ansys version 10.0 is used to solve the two modeled roof structures.

Governing Equation used

kmð2 Tm = ρm cpm ð Tm [ 0< x<L] ; m = 1, 2, 3

ð x2 ðt

where m = 1 for roof top slab m = 2 for PCM panel m = 3 for bottom concrete slab.

The same equation holds good for all the three material regions by incorporating suitable k, ρ, cp. In the exterior boundary where the floor is exposed to solar radiation, the boundary condition is,

k1 ðT1 / ðx|x=0 = q rad + h0 ( Ta- Tx=0 )

The radiation effect is considered during sunshine hours. In the bottom layer of the concrete slab x = L the boundary condition is,

k3 ðT3 / ðx|x=L = hi (Tx=L – T room )

The governing equations may be either solved by i) Finite volume method

ii) Finite difference method (Crank-Nicholson method)

iii) Finite element method

or by using a computer simulation softwares such as FEA ANSYS, MATLAB

TYPES OF ROOFS MODELED AND PROPOSED

Roof -1(a) RCC Simple RCC roof

(concrete slab) 12 cm thick Roof -1(b) PCM integrated Roof : PCM

Panel of 2.5 cm thick placed between RCC (12cm thick) and Roof top

slab(mixture of broken bricks + lime mortar) 10 cm thick.

Roof – 2 A corrugated roof structure with air gap in the middle and insulated at the bottom.

Roof – 3 a) A simple RCC b) RCC with WC c) RCC with Hollow Clay Tile – no air flow

d) RCC with Hollow clay tile with air gap and free flow of air.

Fig.4&5 Mesh generation for RCC(right) and PCM roofs(left)

0

100

200

300

400

500

600

700

0 2 4 6 8 10 12 14 16 18 20 22 24

Time (Hrs)

Sol

ar f

lux(

W/m

2)

solarf lux

Fig.6 Solar flux for the month of January 2009

Fig.7 Two identical Experimental Test rooms(8ftx4ftx4ft) one with out PCM panel and another with PCM panel

Constructed at JNTU College of Engineering, Pulivendula

Fig.8 Digital indicator with thermocouples for the measurement of temperature across the two roofs

Fig.9 Temperature distribution across the RCC roof at 13hr January 2009

Results and discussion

Fig.10 Temperature distribution across the PCM roof at 13hr January 2009


0

10

20

30

40

50

0 4 8 12 16 20 24

Time(Hrs)

Tem

pera

ture

(0C

)

roof top roof bottom roof middle ambient

J anuary 2009

0

10

20

30

40

50

60

0 4 8 12 16 20 24

Time(Hrs)

Tem

pera

ture

(0C

)


February 2009

0

10

20

30

40

50

60

0 4 8 12 16 20 24

Time(Hrs)

Tem

pera

ture

(0C

)

roof top roof bottom roof bottom ambient

March 2009

0

10

20

30

40

50

60

0 4 8 12 16 20 24

Time(Hrs)

Tem

pera

ture

(0C

)


April 2009

A

Fig.11 Temperature Distribution across the RCC roof January 2009

Fig.12 Temperature Distribution across the RCC roof February 2009

Fig.13 Temperature Distribution across the RCC roof March 2009

Fig.14 Temperature Distribution across the RCC roof April 2009

0

10

20

30

40

50

60

0 4 8 12 16 20 24

Time(Hrs)

Tem

pera

ture

( 0 C

)


May 2009

0

10

20

30

40

50

60

0 4 8 12 16 20 24

Time(Hrs)

Tem

pera

ture

( 0C

)

roof top roof bottom PCM panel ambient

J anuary 2009

0

10

20

30

40

50

60

0 4 8 12 16 20 24

Time (Hrs)

Tem

pera

ture

(0C

)


February 2009

0

10

20

30

40

50

60

70

0 4 8 12 16 20 24

Time(Hrs)

Te

mp

era

ture

(0 C)


March 2009

Fig.15 Temperature Distribution across the RCC roof May 2009

Fig.17 Temperature Distribution across the PCM roof February 2009

Fig.18 Temperature Distribution across the PCM roof March 2009

Fig.16 Temperature Distribution across the PCM roof January 2009

0

10

20

30

40

50

60

0 4 8 12 16 20 24

Time(Hrs)

Tem

pera

ture

(0C

)


April 2009

0

10

20

30

40

50

60

0 4 8 12 16 20 24

Time(Hrs)

Tem

pera

ture

(0C

)


May 2009

Fig.19 Temperature Distribution across the PCM roof April 2009

Fig.20 Temperature Distribution across the PCM roof May 2009


0

5

10

15

20

25

30

35

40

45

0 4 8 12 16 20 24

Time(Hrs)

Tem

pera

ture

(0C

)

RCC room PCM room ambient

0

5

10

15

20

25

30

35

40

45

50

0 4 8 12 16 20 24

Time(Hrs)

Tem

pera

ture

(0C

)

RCC room PCM room ambient

Fig.21 Experimental Temperature variation in the ceiling (roof bottom) January 2009

Fig.22 Experimental Temperature variation in the roof top slab January 2009


12

18

24

30

36

42

0 4 8 12 16 20 24

Time(Hrs)

Tem

pera

ture

(0C

)

Sim PCM Exp.Ceilg Sim Ceilg

Ambient Exp PCM

J anuary 2009

Fig.23 Comparison of Experimental and Simulated Temperature variations in the ceiling of RCC and PCM rooms January 2009


15

20

25

30

35

40

45

50

0 1 2 3

Tem

pera

ture

( 0C

)0hr

4hr

6hr

8hr

10hr

12hr

14hr

16hr

18hr

20hr

Roof top slab PCM panel RCC(Ceiling)

Fig.24 Temperature variation across the roof of PCM room January 2009


20

22

24

26

28

30

32

34

36

0 0.2 0.4 0.6 0.8 1

RCC slab thickness (Y*)

Tem

pera

ture

(0 C)

0hr

4hr

6hr

10hr

12hr

14hr

18hr

20hr

Fig.25 Temperature variation across the roof of RCC room January 2009


0

10

20

30

40

50

60

70

80

0 0.2 0.4 0.6 0.8 1

Y*

Hea

t Tra

nsfe

r(W

)

0hr

4hr

6hr

8hr

10hr

12hr

14hr

16hr

18hr

20hr

24hr

J anuary 2009

Fig.26 Heat transfer variation across the roof of RCC room January 2009


0

10

20

30

40

50

60

0 0.2 0.4 0.6 0.8 1

Y*

The

rmal

gra

dien

t(dT

/dx)

0hr

4hr

6hr

8hr

10hr

12hr

14hr

16hr

18hr

20hr

24hr

J anuary 2009

Fig.27 Thermal gradient variation across the roof of RCC room January 2009


0

50

100

150

200

250

300

0 0.2 0.4 0.6 0.8 1

Y*

The

rmal

gra

dien

t(dT

/dx)

0hr

4hr

6hr

8hr

10hr

12hr

14hr

16hr

18hr

20hr

24hr

J anuary 2009(PCM)

Fig.28 Thermal gradient variation across the roof of PCM room January 2009


0

10

20

30

40

50

60

70

0 0.2 0.4 0.6 0.8 1

Y*

Hea

t tra

nsfe

r(W

)

0hr

4hr

6hr

8hr

10hr

12hr

14hr

16hr

18hr

20hr

24hr

J anuary 2009(PCM)

Fig.29 Heat transfer variation across across the roof of PCM room January 2009


0

15

30

45

60

75

90

105

0 0.2 0.4 0.6 0.8 1

Y*

Hea

t T

ran

sfer

( W

)

0hr

4hr

6hr

8hr

10hr

12hr

14hr

16hr

18hr

20hr

March-2009

Fig.30 Heat transfer variation across the roof of PCM room March 2009


0

50

100

150

200

250

300

350

400

0 0.2 0.4 0.6 0.8 1

Roof top thickness(Y*)

The

rmal

Gra

dien

t (

dT/d

x)

0hr

4hr

6hr

8hr

10hr

12hr

14hr

16hr

18hr

20hr

March-2009

Fig.31 Thermal gradient variation across the roof of PCM room March 2009


0

20

40

60

80

100

120

140

RCC PCM

Hea

t flu

x, W

/m2 d

ay

Fig.32 Comparison of Heat flux entering the RCC and PCM rooms January 2009


Heat Flux entering the room

0

50

100

150

200

250

300

350

RCC room PCM room

Hea

t fl

ux

W/m

2- d

ay

Fig.33 Comparison of Heat flux entering the RCC and PCM rooms March 2009


Effect of various parameters on the performance of the PCM roof

0

1

2

3

4

5

6

7

Jan

Feb

Mar

Apr

May Ju

n

Jul

Aug

Sep Oct

Nov

Dec

Wind Speed m/s h value W/m2 K

0

10

20

30

40

50

0 4 8 12 16 20 24

Time (Hrs)

Te

mp

era

ture

(0C

)

Ambient

PCM panel

roof top

ceiling

Fig.34 Variation of heat transfer coefficient with wind speed

Fig.35 Effect of PCM Panel thickness for 3cm and 3.5cm


0102030405060708090

60 180 300 420 540

Melting time(min)

% M

elt f

ract

ion

r=20mm

r=25mm

r=30mm

r=40mm

r=50mm

r=60mm

0

10

20

30

40

50

60

70

80

90

60 180 300 420 540

Time(min)

% S

olid

frac

tion

r=20mm

r=25mm

r=30mm

r=40mm

r=50mm

r=60mm

Fig.36 Melt fraction of the capsule for various capsule radii

Fig.37 Solid fraction of PCM for various radii


0

100

200

300

400

500

600

700

1 2 3 4 5 6 7 8 9 10 11

Time (Hrs)

Sol

ar f

lux

W/m

2

solar f lux w ith reflective coatings solar f lux w ith out reflective coatings

Fig.38 Effect of reflective coatings on incident solar flux

Effect of various parameters on the performance of the PCM roof Proposed Roof Structure-II

Insulation

Solar Reflective surface coatings

PCM

Air gap

Figure.Corrugative PCM integrated roof with air gap at the middle and insulation at the bottom

Fig.39 A Corrugative PCM integrated roof with air gap at the middle and insulation at the bottom

Proposed Roof Structure-III

Fig. 40. Roof structures for investigation (uniform width of 75 mm) (material: 1-RCC, 2-WC, 3-HCT, 4-air).

ConclusionsConclusions

Several promising developments are taking place in the field of thermal storage for thermal comfort and energy savings using PCMs in buildings.

In the present work investigations have been carried out experimentally to study and analyze the thermal performance of the roof of a building incorporating PCM for thermal comfort and energy savings in a residential building. The other two models were presented as proposed roof structures.

Two models were used and the theoretical performance of both is compared by considering one as the reference case. Several simulation runs were made using this model for the average ambient conditions that prevail at Pulivendula town, A.P during January to May 2009.

The various parameters that affect the performance of PCM integrated roof are wind speed, PCM panel thickness, capsule size, reflective roof coatings.

A PCM integrated roof has the potential to maintain a fairly constant temperature inside the room due to its large heat absorbing and storing capacity in a passive manner.

Where as the ceiling temperatures always fluctuate in a Non-PCM room(RCC room) throughout the day.

It is observed from the analysis that the ceiling temperatures in the Non-PCM room fluctuate between 210C and 360C(simulated), 210C and 350C (experimental).

The heat flux entering the Non-PCM room is observed to be 312W/m2 . On the other hand, in the PCM room the ceiling temperatures are maintained at a constant value of 280C(simulated) throughout the day and 28 (+/_) 30C(experimental).

The heat flux entering the PCM room is estimated as 84W/m2 . The roof integrated with PCM is noticed to be better than the RCC roof in terms of less transfer of heat into the room due to the incident solar heat flux during the day time.

The roof installed with PCM can reduce the heat entering the room about more than two-thirds as compared to that of RCC laid roof.

A reduction of 73.1% of heat transmission is observed with the PCM roof as compared to the RCC roof.

It is quite evident from the preceding studies that the thermal improvements in a building due to the inclusion of PCMs depend on the ceiling temperature of the PCM, large latent heat storage capacity and thermo-physical properties of the PCM.

The reduction in heat transmission in to the room is directly proportional to the corresponding reduction in the cooling load in case of an air-conditioned building or reduction in the fluctuation of inside room temperatures in case of a non air-conditioned building.

Therefore it is observed that a reduction in power consumption required to maintain the room at any desired temperature with in the human comfort temperature limits.

For the latent heat thermal storage(LHTS) systems are to be commercialized, it is necessary to go for experimentation.

Careful design and development is needed for use in residential buildings in the near future to replace conventional A/C systems completely with an exception of maintaining required levels of R.H(Relative Humidity).

The thermal storage systems with PCM will be useful for those regions of India where the temperatures exceed 400C in summer.

It is concluded that for the purpose of narrowing indoor air temperature swing a PCM incorporated in the roof of a building is suggested and recommended.

The other two proposed roof structure models may be developed in near future for thermal comfort and energy savings in buildings with simulations followed by experimentations.

Technology

4 p.vijaya