Benefits and Costs of Radiant Barrier and Roof Insulation Used under Tropical Climate

Transcription

1 The 2 nd Joint International Conference on Sustainable Energy and Environment (SEE 6) F-38 (O) November 6, Bangkok, Thailand Benefits and Costs of Radiant Barrier and Roof Insulation Used under Tropical Climate Surapong Chirarattananon * and Duc H. Vu Energy Field of Study, Asian Institute of Technology, Pathum Thani, Thailand Abstract: In tropical region, heat gain through roof during daytime contributes significantly to cooling load and cooling cost for an air-conditioned space, and can cause thermal discomfort in case where air-conditioning is not used. Radiant barrier, roof tiles of low solar absorptance, and various types of insulation are increasing introduced to ameliorate the problems. Studies on heat gain through roof for specific configurations and under specific weather conditions as have been reported do not seem to offer conclusive results on comparative costs and benefits in each case. This paper reports an approach used to compute heat gain through roof through separating the mechanism of heat gain to the interior surfaces in the roof from thermal radiation heat transfer between surfaces in the roof. A method is used to compute form factors between surfaces that are then used in the radiosity computation. The results from a series of experiments conducted based on applications of radiance barrier and different types of insulation were then used to compare with calculation results in order to validate the approach of this paper. The method was then applied to calculate comparative energy consumption and space temperature under roof for a number of configurations of application of radiant barrier and roof insulation when the space under roof is used to serve as office, bedroom, and other functions. This same method of life-cycle economic evaluation has been used in the development of building energy code. Keywords: Heat Gain Through Roof, Radiant Barrier, Radiosity, Thermal Network, Roof Insulation 1. INTRODUCTION Heat gain through roof contributes substantially to cooling load of an air-conditioned building and to raising the interior temperature of a naturally ventilated building. This problem is addressed specifically in the DSM Plan of the electric utilities, [1], and dealt with in the energy performance requirements for buildings in the Ministerial Regulations, issued as part of the Energy Conservation Promotion Act, [2], in Thailand. Since 1982, Fairey reported that thermal radiation from roofs that absorbed solar radiation was the dominant mode of heat transfer in the attic and was responsible mainly for heat gain across ceiling to the space below, [3]. Experiments involving the use of radiant barrier (RB), a thin sheet of thermally reflective surface(s) (single or double sided) at Florida Solar Energy Center found that the temperatures on the surface and in the middle of fiberglass insulation mounted on the ceiling was higher than the temperature of air in the attic space, [3], indicating that the insulation transferred heat to air and not vice versa. In USA, it was believed earlier that air under roof deck transferred heat to ceiling and that movement of air through the attic would help remove heat from it. Subsequent studies found that use of RB helps reduce cooling energy in summer and heating energy in winter, []. It was also reported that accumulated dust on RB reduced its effectiveness, but it still performed its function, [5]. Moser etal., [6], developed a model for calculation of heat transfer in the attic zone that included a computational fluid dynamic model, a thermal radiation exchange model, and conduction heat gain through wall model. The authors applied their model to an atrium model and obtained results for a particular time. Moujaes and Brickman, [1], used backward difference for approximating the partial derivative of the heat conduction equation to find solution of energy balance equations they developed for the attic, the zone below the attic, and the whole house. Comparison of numerical results obtained from their model with experimental results show good correspondence. The authors then applied their model to show the results of application of radiant barrier under a number of situations. More recently, Soubdhan etal., [8], conducted experiments using four test cells to determine comparative effectiveness of radiant barrier against typical insulation materials used under roofs of different colors and for ventilated and unventilated attic space under roof deck. The authors also developed a model of the mechanisms of heat flow. The authors concluded that thermal radiation heat transfer was dominant during daytime, that the performance of the RB was comparable to that of fiber glass or polystyrene insulation when there was no ventilation and was comparatively superior when there was ventilation. This paper presents development of a model of mechanisms of heat transfer for a room being subject to external environment, with specific attention to radiation and air movement in the attic under the roof deck. The paper illustrates a computational procedure used to solve equations of energy balance of the model. The calculated results are compared to experimental results and shown that the roof mean square values of the differences between experimental and calculated results are mostly in the same order as instrument errors. Furthermore, values coefficient of determination fall mostly in the range.8 to.9. The model is then used to calculate heat transfer in three otherwise identical room models, except that one comprises a plain ceiling, one comprises ceiling with RB under roof deck, and one comprises ceiling with reflective insulation under roof deck. Economic analysis shows that net present values of the RB and insulation cases are positive under certain condition of use of the room. When there is no air-conditioning, use of insulation and RB helps reduce temperature in the room. 2. CONFIGURATION OF THE TEST ROOM AND THE ROOFS A room was constructed in the Energy Park in the campus of the institute for the experiments. It was first constructed with a 3 o pitched roof with the roof ridge aligned along east-west direction. Flat cement boards were used to cover the gables. The attic under the roof deck was divided into two equal compartments with glass wool insulation and gypsum boards used for the separator. Two electrical fans each with a capacity of 3 /min were installed on the parts of the roof above each compartment. Three experiments were conducted for the room with pitched roof. After completing the three experiments, the roof was reconstructed into a flat concrete roof. The roof attic was similarly divided into two equal compartments. Two experiments were performed with the room Corresponding author: surapong@ait.ac.th 1

2 The 2 nd Joint International Conference on Sustainable Energy and Environment (SEE 6) F-38 (O) November 6, Bangkok, Thailand under this configuration. Figure 1 shows the configurations of the room. The two compartments in each roof configuration are aligned into east-west direction to minimize the difference in daily reception of solar radiation. In all cases the room was cooled by an air-conditioner with the room temperature kept to 26 O C. Surface heat flux sensors and type K thermocouples were placed at various points in the attic and on surfaces with data logged at every 3 minutes, during 8: and 16: hours. Measurements of direct and diffuse solar radiation, wind speed, ambient air conditions and sky temperature are routinely taken and recorded at 5-minute interval in a station in the Energy Park. Data corresponding to days the experiments were conducted were processed and merged with the heat flux and temperature data from the experiments for the analysis to be described. Gypsum board and glass wool insulaton Concrete roof tile Insulation Flat cement board 15o Fan 3.m 3. m 3 o 3 o m w s m Fig. 1 The configurations of the experimental room 3. MODELING THE MECHANISMS OF HEAT TRANSFER THROUGH ROOF Figure 2 illustrates an electrical network used to represent the mechanism of thermal heat transfer on the roof and in the roof attic. The exterior surfaces of roof at temperature T ro absorbed an α s fraction of solar radiation E etθ on its surface. The roof also convects heat to the ambient air that has a temperature T o. The thermal resistance of convection heat transfer accounts for the velocity and temperature of air and the temperature of the surface involved in the heat transfer. The roof exchanges thermal radiation with sky and ground, each at temperature T s and T g respectively. Thermal resistances of radiation heat transfer account for the temperatures involved, surface emittances, and view factors between the surfaces. Energy balance consideration on the exterior surface of the roof, or around temperature node T ro, gives ( T ( ) ( ) T ) Tsky T ro ro Tg Tro ( Tro Tr ) αreet θ =. (1) R R R R ro rs rg r1 The net heat flux is conducted into the roof material. Here we use finite difference to approximate partial derivative of the heat conduction equation. Such approximation leads to the use of an analogous resistor-capacitor circuit to represent, in terms of heat conduction behavior, a section each of roof material. In the figure, the pitched roof is represented by one section, while for the thicker flat concrete roof more than one section would be used. The equation around temperature node T r is given as T ro Tr Tri Tr Tr + = C r + qr, (2) Rr1 Rr2 t where Cr = ρcp x and ρ, c p, x are density, specific heat, and thickness of the roof section respectively, and q r is the thermal radiation flux leaving the lower roof surface. Heat transfer from the lower surface of roof through the air to the upper surface of the RB or insulation comprises conduction, convection, and thermal radiation transfer through air. Conduction heat transfer through RB or a section of insulation material to its lower surface is represented by an equation similar to Equation (2). Convection heat transfer from all interior surfaces to air, and heat removed by ventilation are represented by Equation (3), given as Ai( Tsi Ta) dta + mc a pa( To Ta) = Macpa, (3) i Rai dt where A i, T si, and R ai are surface area, surface temperature and thermal resistance of convection heat transfer from each relevant surface, T a is the temperature of air in the attic, m a is the rate of ventilation air flow through the attic, M a is the mass of air in the attic, and c pa is the specific heat of air. Equation (1) to (3) and all other related heat conduction and convection equations form a set of network equations. Total thermal radiation flux, or radiosity J i, from a surface i with surface temperature T si is related to the radiosity of other surfaces in the attic as n i σε i si ρi ij j j= 1 J = T + F J, () where σ is the Stefan-Boltzmann constant, ε i and ρ i are emittance and reflectance of surface i, and F ij is the view factor from surface i to surface j. The net thermal radiation q ri from surface i is then given as qri = Ji Fij J j (5) j Equation () and (5) form a set of radiation exchange equations. When values of environmental variables (such as T o, T sky, and E etθ ) are known, the two set of equations above are solved by first assuming initial values of temperature variables for evaluation of 2

3 The 2 nd Joint International Conference on Sustainable Energy and Environment (SEE 6) F-38 (O) November 6, Bangkok, Thailand thermal resistances. With assumed values of surface temperatures, the set of radiation exchange equations is then solved to give values of net surface radiation fluxes. These are then used in the solution of network equations. With the resultant values of temperature variables, the steps of computation are repeated until successive iterations give negligible changes. Network part E etθ T o α R ro r R rs R rg T g R r1 T ro C r T r R r2 T ri T sky Roof q r Air gap R rag T ag R iag Insulation T io E etθ T sky R ss Side wall Radiation exchange part R i1 T i R i2 C i T ii R ia Radiation exchange part α s R so R s1 T s T so R s2 T si R sa T a C a R ca Attic T o T g R sg C s T co C c R c1 T c R c2 Ceiling T ci R room T room q c Fig. 2 A network representing mechanisms of heat transfer through roof. VALIDATION OF THE MODEL USING EXPERIMENTAL RESULTS Modeling the mechanisms of heat transfer through roof and assessment of roof performance had received interest for a long time, especially in association with the implementation of the mandatory requirements of the Building Energy Code, adopted in Thailand since Although RB had been introduced and used in buildings for some time, its effectiveness had not been evaluated systematically before. Experiments were then conducted during May to July of 1999 essentially to evaluate its performance under a number of configurations. During that time, the simple model of heat transfer mechanisms that the authors employed produced results that could not quite reconcile with the results from experiments. When a methodology and a computer program to compute view factors between arbitrary polygonal surfaces was developed, as reported in Hien and Chirarattananon, [9], the authors were then able to develop and employ the model described in Section 3. Five experiments were conducted with the room under pitched roof and with the room under flat roof, each for one or more days. This paper report results of one selected day from each experiment. Although temperature and heat flux were measured at several points, only heat flux from the lower surface of ceiling (that entered the room below), temperature on the lower (exterior of attic) surface of ceiling, and temperature on the upper (exterior) surface of roof are reported here. Table 1 gives details on physical and thermal properties of materials of each roof component. Thermal properties of flat cement board, concrete roof tile, and concrete roof are all similar to those of cement, while cellocrete and acoustic board are based on organic fibers and have low thermal conductivity. Table 1 Thermal properties of materials of the roofs of the experimental room Thickness Conductivity Density Specific heat Emissivity solar Component Material W d, (m) K d,, (Wm -1 K -1 ) ρ, (kg/m 3 ) c p,(j/kgk) ε absorptance, α gable Flat cement board roof Concrete roof tile ceiling Cellocrete separator Gypsum board separator Glass wool RB Aluminized flat roof Concrete ceiling Acoustic board Table 2 gives summary results from the experiments. The table shows values of coefficients of determination or R 2, and values of root meansquare difference, or RMSD, between measured and calculated results from the model. Both are used as measures of the performance of the model described in Section 3. The values of R 2 fall in the range.69 to.99 with most values larger than.8. The values of RMSD for heat flux fall in the range of 1. to 3.7, which are near the figure of accuracy of the heat flux sensor. 3

4 The 2 nd Joint International Conference on Sustainable Energy and Environment (SEE 6) F-38 (O) November 6, Bangkok, Thailand The values of RMSD of temperatures fall in the range of. to 3.1, the upper value of which are just above the figure of accuracy of the thermocouple sensor. These illustrate that the model produce sufficiently accurate results. Table 2 Summary results of experiments Experiment 1 Experiment 2 Experiment 3 Experiment Experiment 5 Date conducted 31 May, June 3, -26 June 8 June 16 and 17 July 19, July Date of data selected 31 May June 8 June 17 July July R 2 : heat flux R 2 : ceiling temperature R 2 : roof temperature RMSD: heat flux RMSD: ceiling temperature RMSD: roof temperature Experiments with pitched roof Experiment 1: RB under roof truss, free VS forced ventilation. Radiant barriers were placed under roof truss in each compartment. The electric fan in compartment 1 was turned off, while that in compartment 2 was turned on. Figure 3 shows graphs of heat fluxes through ceiling, temperatures at lower surfaces of ceiling, and temperatures on upper surfaces of roofs of both compartments. Calculated results agree well with measured results. The graphs also show that forced ventilation (FV) case shows in marginally lower heat flux than that of the naturally ventilated (NV) case, with a difference of 2.8 for computed values and 6.9 for measured values TCeil-NaturV-M sr TCeil-ForcV-M sr TCeil-NaturV-Cal TCeil-ForcV-Cal TRoof-NaturV-M sr TRoof-ForcV-Msr TRoof-NaturV-Cal TRoof-ForcV-Cal HFlux-NaturV-Msr HFlux-ForcV-Msr HFlux-NaturV-Cal HFlux-ForcV-Cal TCeil-FlatC-M sr TCeil-UnderT-M sr TCeil-FlatC-Cal TCeil-UnderT-Cal TRoof-FlatC-Msr TRoof-UnderT-Msr TRoof-FlatC-Cal TRoof-UnderT-Cal HFlux-FlatC-Msr HFlux-UnderT-Msr HFlux-FlatC-Cal HFlux-UnderT-Cal TCeil-RB-M sr TCeil-Cello-Msr TCeil-RB-Cal TCeil-Cello-Cal TRoof-RB-Msr TRoof-Cello-M sr TRoof-RB-Cal TRoof-Cello-Cal HFlux-RB-Msr HFlux-Cello-M sr HFlux-RB-Cal HFlux-Cello-Cal Fig. 3 RBs under roof truss, natural ventilation versus forced ventilation Fig. RBs as flat ceiling versus RBs under roof truss, both with forced ventilation Fig. 5 RBs versus cellocrete as flat ceiling, both with forced ventilation Experiment 2: RB under roof truss VS RB flatly laid, forced ventilation for both cases. Radiant barrier was laid out as flat ceiling in compartment 1, while it was placed under roof truss in compartment 2. Both electric fans were turned on. Figure shows that calculated results agree well with measured results. The compartment with flatly laid RB shows lower

5 The 2 nd Joint International Conference on Sustainable Energy and Environment (SEE 6) F-38 (O) November 6, Bangkok, Thailand values of heat flux through ceiling, with a difference of for computed values and for measured values. Experiment 3: RB flatly laid VS cellocrete ceiling, forced ventilation for both cases. Radiant barrier was laid out as flat ceiling in compartment 1, while cellocrete was used as ceiling in compartment 2. Figure 5 shows that calculated results agree well with measured results. The thermal emissivity of cellocrete and that of RB are given in Table 1 as.9 and. respectively. The RB case shows lower values of heat flux through ceiling, with a difference of 76 for computed values and 7 for measured values..2 Experiments with Flat Roof Experiment : RB flatly laid VS cellocrete ceiling. Radiant barrier was laid out as flat ceiling in compartment 1 and cellocrete was used as ceiling in compartment 2, both without ventilation. Figure 6 shows the results that are similar to those in Experiment 3. The RB case shows lower values of heat flux through ceiling, with a difference of 7 for computed values and 7 for measured values. Experiment 5: RB flatly laid VS acoustic board ceiling. Radiant barrier was laid out as flat ceiling in compartment 1 and acoustic board was used as ceiling in compartment 2, both without ventilation. Figure 7 shows the results that are similar to those in Experiment. The emissivity of the surface of acoustic board is given in Table 1 as.91. The RB case shows lower values of heat flux through ceiling, with a difference of 68 for computed values and 68 for measured values TCeil-RB-M sr TCeil-Cello-M sr TCeil-RB-Cal TCeil-Cello-Cal TRoof-RB-Msr TRoof-Cello-Msr TRoof-RB-Cal TRoof-Cello-Cal HFlux-RB-Msr HFlux-Cello-M sr HFlux-RB-Cal HFlux-Cello-Cal TCeil-RB-M sr TCeil-Acous-M sr TCeil-RB-Cal TCeil-Acous-Cal TRoof-RB-Msr TRoof-Acous-Msr TRoof-RB-Cal TRoof-Acous-Cal HFlux-RB-Msr HFlux-Acous-Msr HFlux-RB-Cal HFlux-Acous-Cal Fig. 6 RBs versus cellocrate as flat ceiling Fig. 7 RBs versus acoustic as flat ceiling 5. ECONOMIC BENEFITS AND COSTS OF RB AND REFLECTIVE INSULATION We apply the model described in Section 3 and validated in Section to evaluate the benefits and costs of RB and insulation used in a room under a pitched roof when the room is used for each of three functions. The room is constructed with light weight concrete with two glazed windows and a wooden door. Three alternative configurations of roof are used. In all cases, the pitched roof is covered with concrete roof tile, for which material properties in Table 1 applies. In the base case, gypsum board is used as flat ceiling. In the alternative options, either RB or a reflective insulation (RI) is placed under roof truss in addition to the gypsum board ceiling. Table 3 summarizes the three configurations. This comparative benefit and cost analysis is made to examine under what condition RI, a newly introduced material, performs better than the economical RB, an already well-known material. Table 3 Three configurations of roofs Case Composition Cost Base Gypsum board ceiling N/A RB Gypsum board ceiling with RB under roof truss RB 85 Bm -2 Insulation Gypsum board ceiling with glass wool covered with reflective sheet (RI) RI 185 Bm -2 Thermal emissivities of both surfaces of RB and of reflective insulation are.5. Other properties are identical to those corresponding items in Table 1. The schedules of use of the room under each function are shown in Table. Table Schedules of use of room in each function Function Schedule of use of room Weekday Weekend Office None Bed room Living room

6 The 2 nd Joint International Conference on Sustainable Energy and Environment (SEE 6) F-38 (O) November 6, Bangkok, Thailand Net Present Values of Alternative Options Air-conditioning is used to cool the air in the room to when the room is occupied. We calculate the load on the cooling coil of the fan coil unit that is used in the room. We then sum the load of the cooling coil for all of the usage hours in a year. Weather data of year is used. We use a value of 2.5 of the coefficient of performance of the cooling plant to obtain total electrical energy (kwh) required by the room when it is used under each function and use a marginal electricity cost of B(kWh) -1 to calculate total cost of electricity required for the year for each case. The resulting difference between electricity cost of base case and that of the RB or RI cases is the saving due to the use of RB or RI in the roof, the cost of which is the cost of RB or RI. Table 6 summarizes the savings. The costs of RB and RI appear in Table 3. Table 6 Savings of electricity costs due to RB and RI, Bm -2 Y -1 Savings Office Bed room Living room from RB RI The life of RB and RI each is given as 1 years, and the electricity escalation rate is 3 per year, net present values of each case are given in the followings. Office The graphs of heat fluxes through ceiling and temperatures of the interior of the room for the base case, RB case, and RI cases are as shown in Figure 8a) for the warmest day in the year, that is 2 May. The exterior air temperature reaches.. Heat flux for the base case reaches 1. The use of RB reduces heat flux to 7, and that from RI to. The graphs of NPVs of both cases for various discount rates are given in Figure 8b). The NPVs are highly positive, especially for the case of RI. Temperature (oc) Office Hours 16 Tin-Ins Ta Tin-RB Tin-Base C-flux-Ins C-flux-RB C-flux-Base Fig. 8a) Heat fluxes and temperatures, office Fig. 8b) Graphs of NPVs, office Bed Room Since the interior room temperature is allowed to float, heat fluxes during daytime are lower for this situation as seen in Figure 9a). Night time heat fluxes are low. The use of RI and RB reduces air temperature in the room during day time. Figure 9b) shows reduced NPVs for both cases. The case of RB offers higher NPV when discount rate exceeds about 7. At discount rate of 15 the NPV for RI becomes zero. NPV (B/m2) Net Present Values Office Ins-Base Discount () RB-Base Temperature (oc) 3 Bed Room 8 16 Hours Tin-Ins Ta Tin-RB Fig. 9a) Heat fluxes and temperatures, bed room Tin-Base C-flux-Ins C-flux-RB C-flux-Base NPV (B/m2) Net Present Values Bed Room 8 16 Discount () Ins-Base RB-Base Fig. 9b) Graphs of NPVs, bed room Living Room The duration of use of the room is short. The NPV for RI becomes negative when discount rate exceeds 1 while that for RB remains positive. 6. CONCLUSION This paper has demonstrated that the model for heat transfer through roof is accurate and appropriate for the tropical climate. It also exhibits the usefulness of the model by applying it to evaluate the comparative benefits and costs of different construction materials used on buildings when it is used to serve different functions. 6

7 The 2 nd Joint International Conference on Sustainable Energy and Environment (SEE 6) F-38 (O) November 6, Bangkok, Thailand 3 Base Case Hours Tin-Ins Ta Tin-RB Tin-Base C-flux-Ins C-flux-RB C-flux-Base Fig. 1a) Heat fluxes and temperatures, living room REFERENCES NPV (B/m2) Net Present Values Living Room Ins-Base Discount () RB-Base Fig. 1b) Graphs of NPVs, living room [1] International Institute for Energy Conservation (Asia Office). (1993) Demand-Side-Management Plan: Pre-investment Proposal, a report prepared for the World Bank/GEF, UNDP, Bangkok. [2] Surapong C. and Bundit L., (199) A New Building Energy-Efficiency Law in Thailand: Impact on New Buildings Energy-The International Journal, 19, pp [3] Fairey, F.W. (1982) Efffects of Infrared Radiation Barriers on the Effective Thermal Resistance of Building Envelopes, ASHRAE/DOE Conference on Thermal Performance of the Exterior Envelopes of Buildings II, Las Vegas. [] Levins, W.P. and Herron, D.L. (199) Radiant Barrier Field Tests in Army Family Housing Units at Fort Benning, Georgia, ASHRAE Transactions Part II, pp [5] Levins, W.P. and Hall J.A., (199) Measured Effects of Dust on the Performance of Radiant Barriers Installed on Top of Attic Insulation, ASHRAE Transactions, Part II, pp [6] Moser, A, Frank, O., Schalin, A, and Yuan, X. (1995) Numerical Modeling of Heat Transfer by Radiation and Convection in a Atrium with Thermal Inertia, ASHRAE Transactions: Symposia, pp [7] Moujaes, S.F. and Brickman, R.A. (1998) Effect of Radiant Barrier on the Cooling Load of a Residential Application in a Hot and Arid Region: Attic Duct Effect, Heating Ventilating Air-conditioning and Refrigeration Research,, pp [8] Soundhan, T., Feuillard, T., and Bade, F. (5) Experimental Evaluation of Insulation Material in Roofing Sysytem under Trropical Climate, Solar Energy, 79, pp [9] Hien, V.D. and Chirarattananon, S., (5). Triangular Subdivision for the Computation of Form Factor. LEUKOS, The Journal of the Illuminating Engineering Society of North America, 2 (1),