4 p.vijaya

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4 p.vijaya

  1. 1. Development of novel roof structures for thermal comfort and energy savings in buildings with and without PCM’s
  2. 2. Overview  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.
  3. 3. 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.
  4. 4. 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.
  5. 5. 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.
  6. 6. 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.
  7. 7. Aim of the paper 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.
  8. 8. 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.
  9. 9. Literature review 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.
  10. 10.  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.
  11. 11.  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.
  12. 12. Theoretical simulation and modeling analysis using Ansys10 RCC 12 cm Fig.1 simple RCC roof Roof Top slab 10 cm PCM Panel Concrete Slab (RCC) 2.5 cm 12 cm Fig.2 PCM integrated roof
  13. 13. SUN Wind Radiation Convection Roof top (brick mixture + lime) PCM Panel Concrete Slab Fig.3 Modeled PCM integrated roof
  14. 14. 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
  15. 15. TECHNICAL SPECIFICATIONS OF USED PCM PCM material Appearance (color) Phase change temperature (0C) Density (kg/m3) Latent heat of fusion (kJ/kg) Thermal conductivity (W/mK) Solid [0-29 0C] Liquid [ 29 – 600C ] Specific heat (J/kg K) ( 0 – 290C ) ( 290C – 300C ) (300C – 600C ) :CaCl26H20 :Grey : 290C : 1500 : 188 : 1.09 : 0.54 :1440 :125,000 :1440
  16. 16. 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 constant Radiation 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.
  17. 17. 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.
  18. 18.  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]
  19. 19.  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]
  20. 20. Ansys 10 software  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.
  21. 21. 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.
  22. 22.  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 ð x2 [ 0< x<L] ; m = 1, 2, 3 ðt where m = 1 for roof top slab m = 2 for PCM panel m = 3 for bottom concrete slab.
  23. 23.  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 )
  24. 24.  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
  25. 25. 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.
  26. 26. Fig.4&5 Mesh generation for RCC(right) and PCM roofs(left)
  27. 27. 700 solarflux Solar flux( W/m2) 600 500 400 300 200 100 0 0 2 4 6 8 10 12 14 16 18 20 22 24 Time (Hrs) Fig.6 Solar flux for the month of January 2009
  28. 28. 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
  29. 29. Fig.8 Digital indicator with thermocouples for the measurement of temperature across the two roofs
  30. 30. Results and discussion Fig.9 Temperature distribution across the RCC roof at 13hr January 2009
  31. 31. Results and discussion Fig.10 Temperature distribution across the PCM roof at 13hr January 2009
  32. 32. roof top roof bottom roof middle ambient roof top 50 ambient 16 24 50 Temperature(0C) Temperature(0C) roof middle 60 40 30 20 10 40 30 20 10 0 0 0 4 8 12 16 20 24 Time(Hrs) Fig.11 Temperature Distribution across the RCC roof January 2009 roof top roof bottom 0 4 8 12 roof bottom 20 Time(Hrs) February 2009 January 2009 Fig.12 Temperature Distribution across the RCC roof February 2009 roof top ambient roof bottom roof middle ambient 60 60 50 Temperature(0C) 50 Temperature(0C) roof bottom 40 30 20 40 30 A 20 10 10 0 0 0 4 8 12 16 20 24 Time(Hrs) March 2009 Fig.13 Temperature Distribution across the RCC roof March 2009 0 April 2009 4 8 12 16 20 24 Time(Hrs) Fig.14 Temperature Distribution across the RCC roof April 2009
  33. 33. roof top roof bottom roof middle ambient roof top Temperature( 0C ) 50 40 0 PCM panel ambient 60 60 Temperature ( C) roof bottom 30 20 10 50 40 30 20 10 0 0 0 4 8 12 16 20 0 24 4 8 12 16 20 24 Time(Hrs) Time(Hrs) May 2009 January 2009 Fig.15 Temperature Distribution across the RCC roof May 2009 Fig.16 Temperature Distribution across the PCM roof January 2009 roof top roof top roof bottom PCM panel roof bottom PCM panel ambient ambient 70 60 60 Temperature( 0C) Temperature (0C) 50 40 30 20 10 50 40 30 20 10 0 0 0 4 8 12 16 20 24 0 4 Time (Hrs) February 2009 Fig.17 Temperature Distribution across the PCM roof February 2009 March 2009 8 12 16 20 Time(Hrs) Fig.18 Temperature Distribution across the PCM roof March 2009 24
  34. 34. Results and discussion roof top roof bottom PCM panel ambient roof top PCM panel ambient 50 Temperature(0C) 60 50 Temperature(0C) 60 roof bottom 40 30 20 40 30 20 10 10 0 0 0 April 2009 4 8 12 16 20 Time(Hrs) Fig.19 Temperature Distribution across the PCM roof April 2009 24 0 May 2009 4 8 12 16 20 Time(Hrs) Fig.20 Temperature Distribution across the PCM roof May 2009 24
  35. 35. Results and discussion RCC room PCM room ambient RCC room 40 30 35 Temperature(0C) 45 35 ambient 50 40 Temperature(0C) 45 PCM room 25 20 15 10 30 25 20 15 10 5 5 0 0 0 4 8 12 16 20 Time(Hrs) Fig.21 Experimental Temperature variation in the ceiling (roof bottom) January 2009 24 0 4 8 12 16 20 Time(Hrs) Fig.22 Experimental Temperature variation in the roof top slab January 2009 24
  36. 36. Results and discussion Sim PCM Exp.Ceilg Ambient Sim Ceilg Exp PCM 42 Temperature(0C) 36 30 24 18 12 0 4 8 12 16 20 Time(Hrs) January 2009 Fig.23 Comparison of Experimental and Simulated Temperature variations in the ceiling of RCC and PCM rooms January 2009 24
  37. 37. Results and discussion 50 45 0hr 4hr Temperature ( 0C) 40 6hr 8hr 35 10hr 12hr 30 14hr 16hr 25 18hr 20hr 20 Roof top slab PCM panel RCC(Ceiling) 15 0 1 2 3 Fig.24 Temperature variation across the roof of PCM room January 2009
  38. 38. Results and discussion 36 34 0hr Temperature(0C) 32 4hr 30 6hr 10hr 28 12hr 14hr 26 18hr 24 20hr 22 20 0 0.2 0.4 0.6 0.8 1 RCC slab thickness (Y*) Fig.25 Temperature variation across the roof of RCC room January 2009
  39. 39. Results and discussion 80 0hr 70 4hr Heat Transfer(W) 60 6hr 8hr 50 10hr 40 12hr 14hr 30 16hr 18hr 20 20hr 10 24hr 0 0 January 2009 0.2 0.4 0.6 0.8 1 Y* Fig.26 Heat transfer variation across the roof of RCC room January 2009
  40. 40. Results and discussion 60 0hr Thermal gradient(dT/dx) 50 4hr 6hr 40 8hr 10hr 30 12hr 14hr 16hr 20 18hr 20hr 10 24hr 0 0 January 2009 0.2 0.4 0.6 0.8 1 Y* Fig.27 Thermal gradient variation across the roof of RCC room January 2009
  41. 41. Results and discussion 300 0hr Thermal gradient(dT/dx) 250 4hr 6hr 200 8hr 10hr 150 12hr 14hr 16hr 100 18hr 20hr 50 24hr 0 0 0.2 January 2009(PCM) 0.4 0.6 0.8 1 Y* Fig.28 Thermal gradient variation across the roof of PCM room January 2009
  42. 42. Results and discussion 70 0hr 60 4hr 6hr Heat transfer(W) 50 8hr 10hr 40 12hr 30 14hr 16hr 20 18hr 20hr 10 24hr 0 0 0.2 January 2009(PCM) 0.4 0.6 0.8 1 Y* Fig.29 Heat transfer variation across across the roof of PCM room January 2009
  43. 43. Results and discussion 105 90 0hr 4hr Heat Transfer( W ) 75 6hr 8hr 60 10hr 12hr 45 14hr 16hr 30 18hr 20hr 15 0 0 March-2009 0.2 0.4 0.6 0.8 1 Y* Fig.30 Heat transfer variation across the roof of PCM room March 2009
  44. 44. Results and discussion 400 Thermal Gradient ( dT/dx) 350 0hr 300 4hr 6hr 250 8hr 200 10hr 12hr 150 14hr 16hr 100 18hr 20hr 50 0 0 March-2009 0.2 0.4 0.6 0.8 1 Roof top thickness(Y*) Fig.31 Thermal gradient variation across the roof of PCM room March 2009
  45. 45. Results and discussion 140 Heat flux, W/m2 day 120 100 80 60 40 20 0 RCC PCM Fig.32 Comparison of Heat flux entering the RCC and PCM rooms January 2009
  46. 46. Results and discussion Heat Flux entering the room 350 Heat flux W/m2- day 300 250 200 150 100 50 0 RCC room PCM room Fig.33 Comparison of Heat flux entering the RCC and PCM rooms March 2009
  47. 47. Effect of various parameters on the performance of the PCM roof Wind Speed m/s h value W/m2 K 50 Temperature( 0C) 7 6 5 4 3 2 40 Ambient PCM panel 30 roof top 20 ceiling 10 1 0 Dec Nov Oct Sep Aug Jul Jun May Apr Mar Feb Jan 0 Fig.34 Variation of heat transfer coefficient with wind speed 0 4 8 12 16 20 24 Time (Hrs) Fig.35 Effect of PCM Panel thickness for 3cm and 3.5cm
  48. 48. 90 80 70 60 50 40 30 20 10 0 r=20mm r=25mm r=30mm r=40mm r=50mm r=60mm 60 180 300 420 540 Melting time(min) Fig.36 Melt fraction of the capsule for various capsule radii % Solid fraction % Melt fraction Effect of various parameters on the performance of the PCM roof 90 80 70 60 50 40 30 20 10 0 r=20mm r=25mm r=30mm r=40mm r=50mm r=60mm 60 180 300 420 540 Time(min) Fig.37 Solid fraction of PCM for various radii
  49. 49. Effect of various parameters on the performance of the PCM roof solar flux w ith reflective coatings solar flux w ith out reflective coatings 700 Solar flux W/m 2 600 500 400 300 200 100 0 1 2 3 4 5 6 7 8 9 10 Time (Hrs) Fig.38 Effect of reflective coatings on incident solar flux 11
  50. 50. Effect of various parameters on the Proposed Roof Structure-II performance of the PCM roof Solar Reflective surface coatings PCM Air gap Insulation 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
  51. 51. Proposed Roof Structure-III Fig. 40. Roof structures for investigation (uniform width of 75 mm) (material: 1-RCC, 2-WC, 3-HCT, 4-air).
  52. 52. Conclusions  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
  53. 53.  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).
  54. 54.  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 twothirds as compared to that of RCC laid roof.
  55. 55.  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.
  56. 56.  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.
  57. 57.  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.

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