ANALYSIS OF THE DYNAMIC THERMAL PERFORMANCE OF FIBEROUS INSULATIONS CONTAINING PHASE CHANGE MATERIALS Jan Kośny, David W. Yarbrough, William A. Miller, and Kenneth E. Wilkes Oak Ridge National Laboratory, Oak Ridge, TN 37831-6070, USA Tel: 1-865-574-9353, e-mail: kosnyj@ornl.gov Edwin S. Lee Oklahoma State University, School of Mechanical and Aerospace Engineering 218 Engineering North, Stillwater, OK 74078, USA ABSTRACT PCM can be distributed in thermal insulation, incorporated into structural and sheathing materials, or packaged for localized application. Results from a series of small-scale laboratory measurements and field experiments, indicate that a new generation of PCM-enhanced fiber insulations could have an excellent potential for successful application in U.S. buildings because of their ability to reduce energy consumption for space conditioning and reduction of peak loads. New PCM applications require careful selection of materials, identification of PCM location, bounding thermal resistances, and specification of the amount of PCM to be used. This paper describes the use of a small-scale heat-flow meter apparatus for dynamic testing, and calibration of a computer model for evaluating the performance of PCMenhanced insulations. Whole-house energy simulations have been used to optimize the use of PCMs in the building envelope and to quantify energy savings resulting from the use of dispersed PCMs in Southern California US climate. 1. TECHNICAL BACKGROUND Since 2002, an ORNL research team has been working to evaluate fiber insulations blended with microencapsulated PCMs [Kosny et al. 2006, 2007 a, b also Miller and Kosny 2007]. These PCM-insulation mixtures function as lightweight thermal-mass components. This type of insulation system will contribute to the objective of reducing energy use in buildings and development of zero-net energy buildings because of their ability to reduce energy consumption for space conditioning and reshape peak-hour loads. Other anticipated advantages of PCMs are improvement of occupant comfort, compatibility with traditional wood and steel framing technologies, and potential for application in retrofit projects. Previous studies [Feustel 1995, Kissock 1998, Kosny 2006, Slayer 1989, Tomlinson 1992, Zhang 2005] have demonstrated that the use of PCM in well-insulated buildings can reduce heating and cooling energy in U.S. residential buildings by as much as 25%. If PCM-enhanced building envelope components are installed in about 10% of U.S. homes, then the anticipated energy savings are between 211000 and 528000 terajoules/year. PCM-enhanced interior sheathing based on n-octadecane has been considered in the past [Kissock 1998, Slayer 1989, Tomlinson 1992] but flammability has been a concern. Therefore, the project team decided to pursue the development of ignition
resistant microencapsulated PCMs. In addition, this project dealt with PCM displaced from the interior sheathing. In a series of experiments carried out in a manufacturing environment, PCM was blended with cellulose insulation intended for use in walls and attics. The first PCM-enhanced cellulose insulation is close to introduction in the US. The PCM-enhanced cellulose being discussed has successfully passed the smoldering combustion test required by the standard specification ASTM C 739 and a series of critical radiant flux tests done in accordance with ASTM C1485 [Kosny 2007b]. The goal of this work was experimental and numerical analysis of the energy performance of PCM-enhanced fibrous insulations with relatively complex and nonuniform blends of two or more different materials, including cellulose fibers, very fine PCM powders, adhesives, and fire retardants. In numerous publications, the thermal characteristics of PCM materials are obtained using a differential scanning calorimeter (DSC). The investigated PCM-enhanced cellulose doesn t contain pure PCM. The 5 to 50 µm PCM pellets contain about 75 to 85 wt% of the organic PCM with the balance of the weight being an acrylic or composite skin. When mixed with the cellulose insulation, these micro pellets usually create 100 to 200 µm clusters. The amount of PCM in the insulation blend has to be accurately determined in order to perform any further thermal analysis. This is not a trivial task. During the manufacturing process, a specific amount of PCM is added to the fibrous insulation. However, at the end of the production process, due to many different reasons, the amount of PCM material in the cellulose blend can be only roughly estimated. That is why a special testing method to determine PCM content was developed. It is anticipated that a similar method can be used in the future for quality control purposes during the production of the PCM-enhanced fiber insulations. 2. THERMAL CONDUCTIVITY OF CELLULOSE - PCM BLENDS During production of the PCM-enhanced cellulose, cellulose fibers are mixed with specific amounts of PCM powder making composite blends of two significantly different materials. In previous ORNL studies paraffinic-pcm products were used. These PCMs were produced by a process that held microscopic droplets of n- octadecane inside hard acrylic polymer shells. In the current project, in addition to n- octadecane, the fatty acid esters were used due to their lower flammability. Like paraffinic products, they undergo a solid-liquid transition with most of the phase change process between 25 C to 29 C. In case of the popular paraffinic PCMs the phase change enthalpy h fus is close to 120 J/g. Since apparent thermal conductivity (k a ) of the PCM-cellulose composite is critical for its proper thermal performance, a series of k a measurements were performed. These measurements were conducted on 5.1-cm. thick specimens of cellulose insulation and PCM-enhanced cellulose insulation using a heat-flow meter apparatus operated in accordance with ASTM C 518. The k a data were determined as a function of the mean specimen temperature as shown in Figure 1. At 24 C, the k a of a 30wt% PCM-cellulose blend is 0.0388 W/m K. This result is the same as the k a for cellulose without PCM. The addition of microencapsulated PCM does not result in a significant change in the k a of the insulation. These results were obtained for a test specimen with density 33.6 kg/m 3 tested with a temperature difference of 22.2 C and are represented by Equation 1. k a = 0.000130 T + 0.0357 (1)
0.044 Apparent Thermal Conductivity, W/mK 0.043 0.042 0.041 0.040 0.039 0.038 0.037 Cellulose with 30% PCM at 33.6 kg/m 3 Delta T = 22.2 C except first point at 10.0 C 0.036 0 10 20 30 40 50 60 Temperature, C Figure 1. Apparent Thermal Conductivity of PCM-Enhanced Cellulose as a Function of Temperature. 3. DYNAMIC HEAT FLUX TESTING OF PCM-ENHANCED CELLULOSE The dynamic thermal characteristics of fiber insulations are relatively well known, but the dynamic thermal characteristics of the PCM-enhanced insulations are difficult to predict. The unknown factors affecting these characteristics are the concentration of PCM, size and amount of clusters constituted of PCM micro-pellets, amounts of fire retardant, and adhesives. During the manufacturing process, a specific amount of PCM is added to cellulose insulation. In the final product, however, the amount of PCM in the cellulose blend can be different from the intended amount. For this reason a procedure that uses a heat-flow meter apparatus (HFM) to determine the actual PCM content in a blend was developed [Kosny et al. 2007a, ASTM C518]. The method utilizes heat flux measurements for cellulose insulation with known amounts of PCM material to determine the amount of PCM in an unknown blend. The following procedure was used for this testing. The 7.6-cm thick test specimen was brought to a uniform temperature of 15 C. This was accomplished by setting both plates in the HFM to the same temperature. At time zero, the bottom plate of the HMF was changed to 37.7 C with a resulting inward heat flux to the test specimen. This is the charge of the PCM which has a phase change temperature of about 25.8 C. The bottom plate is changed back to 15 C when steady-state conditions are observed. Heat flux data have been collected for cellulose specimens containing 0, 10, 20, 30, and 40 wt.% phase change material to cellulose having fiber density of 25.6 kg/m 3. A complete set of heat flux-time data for first 80 minutes of these experiments (first dynamic thermal ramp) is presented on Figure 2 for the hot plate of the HFM apparatus. This set of data can be utilized to estimate the amount of PCM in an unknown specimen by generating a curve to compare with the data in Figure 2. The data collected during these experiments helped in development of a numerical algorithm for temperature-dependent enthalpy which was utilized later in wholebuilding energy simulations.
50 PCM Charging Process (flux in) heat flux [W/m2] 45 40 35 30 25 10% PCM 20% PCM 30% PCM 40% PCM 20 15 10 0 10 20 30 40 50 60 70 80 time [minutes] Figure 2. Measured heat fluxes for PCM-enhanced cellulose. 4. WHOLE BUILDING ENERGY SIMULATIONS Energy Plus is a whole building energy simulation program for estimating building energy consumption [Crowley 2000]. The program includes a finite difference routine for the effect of PCMs exhibiting variable thermal storage capacities in building materials [Pedersen 2007]. This feature was exploited for simulating PCM-enhanced attic insulation added to an approximately 129 m 2 single-story house. Weather data for Bakersfield, CA was used. The key input data for the structure that was simulated is shown in Table 1. Analyses of PCM-enhanced building envelopes have demonstrated a potential for reductions of cooling loads in southern US locations [Kosny et al. 2008]. In order to explore further these findings, a whole building energy model of a house containing PCM-enhanced attic insulation was developed. A series of Energy Plus simulations were completed using the enthalpy data shown in Figure 3 for a 20 wt% blend of cellulose insulation. Table 1. Building Characteristics Used in Whole Building Energy Modeling. Building Feature Basic Characteristics Dimensions One story, 129 m 2 of floor area, 8.0 m x 16.1 m Floor 10 cm. concrete slab, R-1.0 foam perimeter insulation 60 cm. wide Insulation - wood-frame walls with R 2.3 cavity insulation, - attics with R-values ranging from R 2.1 to R 8.8, - attic with PCM-enhanced insulation R 6.7 plus 20 wt% of PCM Exterior Walls 5x10 cm. wood framing with 25% framing factor Roof Attic - 30 degree pitch, 5x15 cm.. wood rafters and joists - Roof rafters and joists installed 60 cm. OC Windows Glazing - 20% of floor area (total window area about 25 m 2 - including 3 south facing windows, 2 north facing, 1 west facing, and 1 east facing window) Doors 91 x 200 cm. south facing, 91 x 200 cm. north facing Interior Walls 5x10 cm. wood framing with studs representing 10% of wall area HVAC System Multizone (to allow temp variations between rooms). Cooling Temp 25.6 o C, Heating Temp 23.9 o C, Single Speed AC Unit - Autosized, Gas Furnace -Autosized, 77% Eff.
Enthalpy [J/g] 140 120 100 80 60 40 20 0-20.00 0.00 20.00 40.00 60.00 Temperature [C] Figure 3. Enthalpy data for 20 wt% blend of cellulose insulation and PCM. In Energy Plus simulations presented in Figures 4 and 5 roof-generated loads represented about 25% of the total whole building HVAC loads. Figure 5 shows heat fluxes computed for four summer days for attic floor area. 1600 1400 1200 1000 800 600 kj 400 200 0-200 -400-600 -800 3-Jul 4-Jul 5-Jul 6-Jul 7-Jul R-2.1 (R-12 US) R-3.3 (R-19 US) R-5.3 (R-30 US) R-6.7 (R-38 US) R-8.8 (R-50 US) PCM R-6.7 (R-38 US) Figure 4. Total Ceiling Heat Conduction Attic Simulations Bakersfield, CA (Summer). Simulation results showed that during the peak time hours, attic floor heat conduction for the case of the R SI -6.7 PCM-enhanced attic insulation is significantly lower, when compare to the heat fluxes generated for R SI -8.8 conventional insulation. The all-day attic heat flow amplitudes in house containing PCM are also notable lower from the conventional R SI -8.8 insulation cases. A 1.5 hour shift in peak heat flow can be also observed for PCM attic compared to a R SI -8.8 conventionally insulated attic. For the same days as shown in Figure 5, a comparison of the whole house HVAC loads was made between conventional R SI -6.7 and R SI -8.8 attic insulations and PCMenhanced R SI -6.7 insulation. It can be observed that a superior energy performance of
the PCM-enhanced insulation was transformed into the notable lower whole house cooling loads. In the house containing R SI -8.8 conventional attic insulation, overall cooling loads are notably higher than the case where R SI -6.7 PCM-enhanced insulation was used. Maximum peak-hour cooling loads in the house containing PCM are 35% to 40% lower than the loads in the conventional house using R SI -8.8 attic insulation. 13000 R-8.8 (R-50 US) PCM R-6.7 (R-38 US) 11000 9000 7000 kj 5000 3000 1000-1000 3-Jul 4-Jul 5-Jul 6-Jul 7-Jul Figure 5. Whole Building Sensible Loads Attic Simulations Bakersfield, CA (Summer). The results of the Energy Plus simulations are plotted in Figure 6 to show a relationship between the attic insulation R-value and annual whole building energy consumption. For Bakersfield, CA a R SI -6.7 attic containing 20 wt% PCM-enhanced insulation had the same annual energy use as an attic with R SI -13.2. An annual savings of 556 kwh were saved by adding the 20% by weight PCM to the R SI -6.7 attic insulation. The value of this saved energy is estimated at $70 using a cost of electricity of 0.1282 $/kwh [Miller & Kosny 2007]. Therefore, in terms of available United States construction cost data [BNI 2008, Means 2005], about $14 per square meter of attic footprint [i.e., $1830 for the 129 m 2 ranch style home] would be spent to increase the thermal resistance of conventional blown-in fiberglass insulation from R SI -6.7 to R SI -13.2 (considering 18% floor area coverage by structural attic components). For comparison, for the same attic footprint, adding 20% by weight of the microencapsulated PCM to the R-6.7 attic insulation would cost about $1360 (assuming cost of PCM as $8.83 per kg.).this is about 26% saving comparing to the cost of adding extra R-6.5 insulation on top of the R SI -6.7 insulation (to reach overall R SI -13.2). In addition, as showed on Figure 6, PCMenhanced insulation generates significant shavings and shifting of cooling peak loads, which is not the case for conventional insulations. Thus, the above cost comparison does not show the full benefit of using blends of the fiber insulations and microencapsulated PCM.
Whole Building HVAC Energy [GJ] 110 108 Dynamic R-value Equivalent for PCM attic tells what attic R-value a house with conventional attic 106 should have, to obtain the same space heating 104 and cooling energy consumption as a similar house using PCM-enhanced attic insulation. 102 100 98 96 0 5 10 13.2 15 20 Attic R-value m 2 K/W Typical attic insulation R-6.7 attic insulation with 20% of PCM Figure 6. Relationship between attic R-value and whole building energy consumption for a 129 m 2 single-story house located in Bakersfield, CA. 5. CONCLUSIONS This paper presents results from a series of small-scale laboratory experiments and numerical analysis that deal with PCM-enhanced fiberous insulation. These results indicate that PCM insulation blends have a high potential for successful adoption in U.S. buildings because of their ability to reduce energy consumption for space conditioning and reduce peak loads. Steady-state heat-flow meter testing was performed on specimens of PCM-cellulose blends to analyze its thermal conductivity. At 24 C, the k a of a 30wt% PCM-cellulose blend was the same as the k a for cellulose without PCM. The addition of microencapsulated PCM didn t result in a significant change in the k a of the insulation. A new testing method was developed for estimation of the PCM content in the PCMcellulose composite. This dynamic test method was using modified heat flow apparatus which is traditionally utilized for thermal conductivity testing (ASTM C518). Dynamic heat flux measurements have been made for cellulose insulation with and without microencapsulated phase change material (PCM). Experimental heat fluxes recorded during the test on the top and bottom plates of the HFM apparatus were compared for different PCM-cellulose blends. These data helped in development of numerical algorithms which were utilized in whole house energy simulations. Test results demonstrated that it is possible to use this method to determine the amount of PCM in fiber insulations. A series of whole building Energy Plus simulations was performed for Bakersfield CA - southern California US location. In house containing R SI -8.8 conventional attics insulated, overall cooling loads were notably higher from the cases where R SI -6.7 PCM-enhanced insulation was used. For the Bakersfield CA climate, maximum peaktime cooling loads in the house containing PCM were 35% to 40% lower comparing to the loads in the conventional house using R SI -8.8 attic insulation. In addition, these simulation results for demonstrated that conventional attic insulation must be about R SI -13.2 in order to yield the same annual whole house energy consumption compared to the home with R SI -6.7 attic insulation with PCM-enhancement. Cost-
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