Effect of the insulation level on the thermal response of a PCM-modified envelope of a dwelling in Chile. Tom´ as Venegas, Diego A. Vasco∗ Departamento de Ingenier´ıa Mec´ anica, Universidad de Santiago de Chile, Av. Lib. Bernardo O’Higgins 3363, Santiago, Chile.
Fabi´ an E. Garc´ıa, Carlos Salinas Departamento de Ingenier´ıa Mec´ anica, Universidad del B´ıo B´ıo, Av. Collao 1202, Concepci´ on, Chile.
Abstract Chile exhibits a continuous growth of energy demand, and for this reason energy saving approaches in the commercial, public, and residential sectors, which share 21% of the final energy consumption in Chile, have been encouraged. One of the solutions to increase energy savings in buildings and dwellings is to increase the thermal performance of their envelope. The use of phase change materials (PCMs) has gained attention during recent years, especially in northern climates, since they can be used to enhance the thermal inertia of light building materials. Usually, the thermal envelope of a dwelling in Chile is made of brick or wood together with light building materials such like fiber-cement, plasterboard, and thermal insulating materials as polystyrene foam. The experimental part of this work deals with the thermal characterization of an organic PCM (hexadecane), which has a relatively low phase transition temperature. The characterization involves the measurement of density, thermal conductivity, and heat capacity as a function of temperature, and the determination of phase change temperature and latent heat. The thermophysical properties were implemented in a set of thermal simulations of an actual project dwelling located in Santiago and Puerto Montt. The numerical results in terms of heat storage and temperature profiles in the envelope, and heating and cooling demands are shown for different insulation levels. Keywords: Thermal envelope, Phase change material, Energy demands, EnergyPlus.
∗ Corresponding
author Email address:
[email protected] (Diego A. Vasco)
Preprint submitted to Applied Thermal Engineering
May 22, 2018
1. Introduction
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In Chile, the commercial, public and residential sectors share 21% of the total of energy consumption, with electric energy as the most important energy source (34%), followed by biomass (32%), LPG (18%), and NG (11%) [1]. Specifically, the residential sector use 16% of the total electric energy use in Chile [2]. The average Chilean home has an annual total energy consumption of 10,232 kWh/year, which is considered high mainly because of the use of wood as an energy source in the south of the country, mainly due to its low price and availability compared to other sources. In southern Chilean cities like Puerto Montt, wood is used mainly to maintain thermal comfort conditions in houses [3]. Due to its geographical position with respect to high pressure zones, the presence of a polar front, and the influence of the Pacific Ocean and the Andes mountains, Chile has a broad variety of climates. In 2006 the Ministry of Housing and Urban Development of Chile set down act 192, which establishes that all housing must meet the requirements of thermal conditioning. The details of this act were shown and discussed in a previous paper [4]. Phase change materials (PCMs) have high latent heat values, therefore they store large quantities of thermal energy per unit volume. The heat storage of PCMs as latent heat is three to four times higher than heat storage as sensible heat [5]. One of the main characteristics of these materials is that during phase change processes, the temperature varies within a narrow range while the material absorbs or releases thermal energy. PCMs are classified as inorganic, organic and eutectic mixtures [6]. Among organic PCMs (O-PCMs), three different groups of substances may be found: paraffins, fatty acids, and organic mixtures [7]. These materials have melting temperature close to indoor temperature, thus could be potentially used in residential buildings. O-PCMs are chemically stable, melt and solidify conveniently, without adding nucleating agents, and therefore they are less prone to subcooling [8]. One of the possible applications of the thermal storage capacity of O-PCMs is the increase of the thermal inertia of the envelope materials, the shifting of thermal loads, and the regulation of temperature, helping to decrease temperature variations in a building during a period of time. Some authors have studied the inclusion of PCMs in the envelope of buildings or in additional building components. Omari et al. [20] made numerical simulations of an one-dimensional heat transfer problem of a modified insulation material with paraffin PCM particles (100 µm), and optimized the thickness of the insulation material and the PCM load. The effect of melting and solidification of the PCM particles and of the environmental conditions on the temperature and heat transfer across the modified material were studied. The authors found that the thermal conductivity of the insulation material is increased by the addition of PCM and that the melting temperature is a relevant parameter in the optimization of the performance of the modified material according to the season. Moreover, the authors found that the optimal conditions for summer are not suitable for winter. Hichem et al. [18] performed an experimental validation of 2D numerical simulations of heat flux through bricks modified with 2
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R four different organic PCMs and an inorganic PCM using ANSYS/FLUENT . The authors analyzed the effect of the position of the PCM inclusion on the modified bricks and the type of PCM on heat flux. Using the inorganic PCM, an 82.1% reduction was found in the heat flux through the brick. However, only 28% of the liquid fraction of the PCM was reached. Kuznik et al. [17] developed a numerical code to simulate a wall containing a commercially available PCMenhanced material. The authors analyzed different levels of thermal insulation in the wall and the thickness of the PCM-enhanced layer. The results showed that the thickness of the insulation layer does not affect the optimal PCM layer thickness. Bastani et al. [19] performed an one-dimensional numerical study about the effect of a PCM wallboard on shifting cooling energy demands. The boundary conditions of the PCM wallboard corresponds to an adiabatic surface and a time dependent interior set-point temperature. The authors analyzed ten cases in which the thickness and thermal properties of the PCM were varied. The study focused on the analysis of the loading cycle of the PCM wallboard in terms of the thermophysical properties, which allows the optimization of both PCM and building materials properties to generate the desired displacement of the peak cooling load and to reduce the costs of energy consumption. Other authors have focused on the effects of including PCMs in building materials on thermal comfort. Kusama et al. [11] prepared and thermally characterized modified plaster boards with PCM, which were evaluated in boxshaped test samples and two dwellings with different heating systems during winter. The authors observed that by including PCMs, room temperature fluctuation rate is reduced by up to 40%. Rodriguez-Urbinas et al. [10] studied the effect of the use of microencapsulated PCM (30 wt% and melting temperature of 26◦ C) in drywall panels. The authors performed numerical simulations in five cities with different weather conditions in Spain, analyzing the effect of window to wall ratio and different shading factors. The results show an increase of up to 31% of hours within the comfort range by using PCM materials, compared to reference cases without PCM. The effect of the inclusion of PCM on the thermal envelope of buildings on thermal comfort and energy consumption has also been studied using commercially available building performance simulation software, which includes modules specifically designed for the simulation of PCM applications. Saffari et al. [12] performed a review of the studies that evaluate the possibilities of using PCM to reduce cooling energy and to increase the number of hours inside the thermal comfort range in passively cooled buildings. The authors observed that warm temperature weathers were studied more often due to the potential of PCM to reduce cooling energy. Soares et al. [23] reviewed studies regarding PCM applications in buildings, showing that EnergyPlus, TRNSYS, and ESP-r are the most widely used software for numerical simulations of PCM in buildings. EnergyPlus is a commonly used Building Performance Simulation. By having an incorporated validated PCM model, EnergyPlus includes the analysis of PCM alongside different energy saving approaches, during the building design stage. Tabares-Velasco et al. [24] performed a validation of the PCM model implemented in EnergyPlus following the ASHRAE Standard 140, which
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requires an analytic verification, comparative testing, and empirical validation of the numerical results. Panayiotou et al. [25] analyzed the effect of using PCM enhanced bricks in a dwelling under the weather conditions of Cyprus on energy consumption and free-floating temperature during a whole year. The authors performed numerical simulations using TRNSYS, in which they analyzed different cases with thermal insulation and PCM element implementation and position. The results showed that the lower energy consumption was observed in the case with PCM elements on the exterior surface of the bricks. Moreover, the case with thermal insulation and PCM elements presented a slightly higher energy consumption than in the case with only thermal insulation, but a lower free floating temperature during the summer was observed in the former case. Ramakrishnan [26] performed numerical simulations of dwellings to evaluate the effect of inner linings of PCM materials in walls and ceilings for reducing heat stress during heat waves. The simulations were performed using the CondFD object of EnergyPlus. The simulations consider a night forced ventilation strategy. The authors studied a commercially available PCM (Bio-PCM) and a fictitious PCM with melting temperatures between 25 ◦ C and 31 ◦ C. The authors found that during a heat wave in Melbourne (Australia), the use of Bio-PCM along with night ventilation reduced the hours during which the interior temperature is out of the comfort range by 32% and up to 62% for the best performing fictitious PCM. Marin et al. [27] performed numerical studies with EnergyPlus implementing plaster boards with PCM with a melting temperature of 25 ◦ C in air-conditioned (packaged terminal heat pumps) containers placed in several locations. The authors found that the energy savings in air conditioning depends on the weather conditions. Of the assessed locations, the highest reductions in cooling and heating loads were observed in Calama (Chile). Furthermore, the authors evaluated the free floating temperatures in the containers at the same locations, and found that the use of PCM increases the number of hours within the thermal comfort range, except in tropical locations. Chernousov et al. [28] studied numerically with EnergyPlus the effect of implementing PCM elements in an office building envelope located in a subtropical climate to reduce cooling energy during the summer morning hours. The study considered two stages. The first one consisted in simulations to determine the effects of PCM position within the envelope, the amount of solar radiation received and PCM layer thickness. After these analyses, the authors studied the PCM enhanced PCM building including the chiller of the cooling plant, to evaluate the effect of the PCM on cooling energy consumption. The results showed that using the PCM on the interior surfaces of the building reduces the energy consumption by 1% to 4%, and that increasing the thickness of the PCM layer increases cooling energy consumption. The present work is focused first on the thermal characterization of hexadecane as a phase change material of a relative low phase transition point (18 ◦ C), near to the transition point of some fatty acids [31], which may be of interest in near to room temperature applications mainly thanks to their condition of no oil-derived product. Thermal conductivity, heat capacity, and density were measured as a function of temperature, in both the liquid and the solid phase. 4
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These properties are required in the second part of the paper that deals with the thermal simulation with EnergyPlus of an actual dwelling project in Chile to be built. Different levels of thermal insulation along with a layer of the PCM on the exterior walls were analyzed. The thermal simulations were performed in two different thermal zones of Chile, Santiago (Zone 3), and Puerto Montt (Zone 6). Both cities are important urban centers in Chile, and they were selected because of their significant latitude difference and therefore climate conditions. The numerical results in terms of cooling and heating power, stored heat in the envelope, and heating and cooling demands of the dwelling are analyzed. 2. Thermophysical properties of hexadecane
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The experiments were performed with reagent grade hexadecane of 99% purity (Sigma-Aldrich). Both specific heat and latent heat were measured by the DSC technique (TA 2920 Modulated) in the temperature range of 283 K to 313 K. The thermal conductivity was determined by the transient line source method (KD2-Pro), which consists in the immersion of a sensor (SH-1 needle) that generates heat from its surface. The fluid absorbs a fraction of this heat and the remainder increases the surface temperature. The temperature of the sensor in comparison to the added heat allowed the measurement of the thermal conductivity of the fluid of interest. The measurement of the density of hexadecane was determined by using a pycnometer (Marinfield) with a volume of 9.869 mL. The pycnometer was equipped with a mercury thermometer (± 0.1 K), which allowed the measurement of the temperature of the sample. The temperature was controlled by immersion of the hexadecane sample in a thermoregulated water bath (Hilab BL-20) for the measurement of thermal conductivity (283-313 K) and density (283-307 K). The measurements of thermal conductivity and density were performed in triplicate. The results in the temperature range of 10 ◦ C to 40 ◦ C for the thermal conductivity of hexadecane are shown in Figure 1. In the solid zone (I), the thermal conductivity may be considered nearly constant at 0.285 W/mK, and in the liquid zone (III) the thermal conductivity is lower and almost constant at 0.143 W/mK. These values are in agreement with those reported by V´elez et al. [32]. Figure 2 shows the density variation as a function of temperature. In the solid (I) and liquid (II) zones, density decreases almost linearly with temperature according to the equation ρ = α+βT , where α is equal to -0.89 and -0.67 for the solid and liquid zone, respectively. Meanwhile, β is equal to 1051.6 and 968.6 for the solid and liquid zone, respectively. The implemented techniques for the determination of thermal conductivity and density did not allow a more precise determination of these properties in the narrow mushy zone. The values obtained for the heat capacity as a function of temperature are shown in Figure 3, it can be seen the heat capacity of hexadecane presents one peak at 17◦ C (II). In the solid region (I), the heat capacity of hexadecane increases with temperature, and such increase turns steeper as the temperature gets closer to the mushy zone (II). Meanwhile, in the liquid region (III), the 5
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Figure 1: Thermal conductivity of pure hexadecane.
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heat capacity decreases with temperature to 25.5 ◦ C and then it increases to 29 ◦ C, and from this temperature the property becomes almost constant (IV). Finally, a latent heat of 216 kJ/kg is obtained from the DSC curve tracing a base line and calculating the area under the curve. This value is 8% lower than that reported by V´elez et al. [32]. Then specific enthalpy as a function of temperature was obtained from the heat capacity data using equation 1, and the obtained results of enthalpy are shown in Figure 4. Z h=
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Despite of their relative low melting temperature, in comparison for instance with octadecane, hexadecane is more chemically stable and therefore less prone to ageing. Another advantage is its lower cost next to octadecane. When performing transient thermal simulation the thermal properties as a function of
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temperature are required. The performed simulations are based on the thermal balance method that allows the instantaneous sensible load to be calculated for the air in a zone. 3. Simulation tool and numerical formulation
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The computational analysis proposed in this paper is based on building computational simulations using EnergyPlus, which is a well-recognized and accepted building energy analysis software tool [33, 34]. EnergyPlus has been used by engineers, architects and researchers to model energy consumption (heating, cooling, ventilation) in buildings. To obtain the transient temperature values within a zone, EnergyPlus implements an energy balance equation for each zone (z). The transient term of the equation is the thermal energy stored in zone air, and the other terms of the balance corresponds to: i) sum of the convective internal loads, ii) convective heat transfer from the zone surfaces, iii) heat transfer due to infiltration of outside air, iv) heat transfer due to inter-zone air mixing, and v) air systems output. The main goal of the present work is to evaluate the thermal performance of a PCM-enhanced projected dwelling under two different weather conditions in Chile and different thermal insulation levels. No actual HVAC equipment was considered, and instead an ideal load system has been included to determine the instantaneous heating and cooling loads. For the calculation of the transient variation of temperature in the thermal envelope of a building, EnergyPlus implements a finite difference discretized version of the conduction transfer equation. For the formulation of the transient term, the Crank-Nicholson scheme, which it is second-order in time, was implemented. To simulate PCM on EnergyPlus, objects detailing the enthalpy and thermal conductivity as a function of temperature have to be included. These objects are called MaterialPropertyPhaseChange and MaterialPropertyVariableThermalConductivity, respectively. In the MaterialPropertyPhaseChange, a set of data points of enthalpy of the PCM as a function of temperature have to be incorporated, and EnergyPlus interpolates between the given points. The same procedure is used by the MaterialPropertyVariableThermalConductivity object to describe thermal conductivity. The values of the temperature-dependent enthalpy and thermal conductivity used in the simulation were obtained from the experimental results shown in Section 2. 4. Description of the dwelling
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Due to its geographical position, the presence of a polar front and the influence of the Pacific Ocean and the Andes mountains, Chile has a wide variety of climates. The elevation of the western Coast mountain chain does not allow the flow of the sea winds and the Andes mountains inhibit the influence of the continental climate, therefore cities like Santiago (Csb) has a dry Mediterranean climate. The long coast and the Humboldt current turn the climate of the
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country in predominantly marine, which moderates temperature through the development of clouds and fresh winds. Puerto Montt (Cfb) has a rainy marine climate with lower average temperatures and higher rainfall than in Santiago. The TMY3 weather data were obtained from the EnergyPlus webpage for Santiago [35]. For Puerto Montt, it was used a Chilean weather database generated by the software Meteonorm. The location of both cities is shown in Figure 5.
Figure 5: Location of both Chilean cities. 240
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The project dwelling under simulation is made up of ten zones, and it has a built area of 183 m2 (Figure 6). These kind of dwellings are becoming very common in the Chilean housing project market, even in cities of different climates like Puerto Montt and Santiago. The dwelling has one floor below ground and two floors above ground, with 8% of the walls below ground. The window to wall ratio is 19%, windows are double glazed with a thermal transmittance of 1.7 W/m2 K and a solar heat gain coefficient (SHGC) of 0.59. The structural elements of the dwelling (walls and slabs) are made of concrete. Thermal insulation is implemented in all the dwelling surfaces. The configuration and the thermal transmittance values of the walls, floors and roof, according to the Chilean thermal regulation, are shown in Tables 1 and 2, respectively. Each zone of the dwelling has internal loads, which are specified in Table 3. The appliance power density and the occupancy correspond to default values provided by the DesignBuilder software, and the lighting power density value corresponds to the living quarters value of ASHRAE standard 90.1-2007 (Table 9.6.1) [36]. The SurfaceConvectionAlgorithm:Inside object was used to obtain the indoor convection coefficient. This model defines the convection heat transfer coefficient for all inside surfaces. In this case, the TARP algorithm has been selected, which is the default model in EnergyPlus and combines natural and wind-driven convection correlations from laboratory measurements on flat plates. The convective coefficients used for the calculation of the thermal transmittance for the outer environment were obtained using the DOE-2 model, based on the ex9
Figure 6: Blueprint of the dwelling under simulation with EnergyPlus with an orientation rotated 110◦ respect to the North.
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Table 1: Configuration of the thermal envelopes of the dwelling. Thermal envelope
Below grade wall
Wall
Ground floor
External floor
Roof
Material Brickwork XPS Concrete Gypsum plaster Cement plaster XPS Concrete EPS Concrete Tiles Plaster Stone wool Timber Clay tile Concrete EPS Concrete
Thickness [cm] 10 7.95 10
k [W/mK] 0.890 0.04 1.4
ρ [kg/m3 ] 1920 15 2100
Cp [J/kgK] 790 1400 840
1.3
0.25
900
1000
0.034 1.95 0.890 0.25 0.04 0.119
35 2240 1920 900 30 1636
1400 900 790 1000 840 447
1 5 20 5 10 1.5 0.025 14.8 0.5 1 5 12 20
Table 2: Thermal transmittance [W/m2 K] of the thermal envelope of the dwelling in the different analyzed cases (TR: Chilean Thermal Regulation). Thermal Envelope Roof Walls Below ground walls Floor Windows
Project 0.303 0.501 0.356 0.571 1.786
TR Santiago 0.47 1.9 0.356 0.571 1.786
TR Puerto Montt 0.28 1.1 0.356 0.571 1.786
Table 3: Internal loads of the dwelling. Zone Bedroom Hall Basement Kitchen Sitting room Living room
Appliances power density [W/m2 ] 5.00 1.57 3.58 30.28 12.00 12.00
Lighting power density [W/m2 ] 12.00 12.00 12.00 12.00 12.00 12.00
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Occupancy [W/m2 ] 7.14 64.5 43.5 42.19 0 0
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perimental measurements performed by Kleims and Yazdanian for roughened surfaces [37]. DOE-2 is based on the MoWiTT and BLAST models [38, 39]. The details of the calculation of the outer convective coefficient was described in a previous work [4]. The outdoor radiative transfer is calculated as two different terms, short wavelength, which models direct and diffuse radiation heat flux, and long wavelength that models radiation flux exchange with the sky and surroundings. Short wavelength is influenced by location, the surface facing angle and tilt, surface face material properties, and weather conditions. Short wavelength comprises beam radiation from the sun and diffuse radiation from the sky. Radiation from the sky includes beam radiation, diffuse anisotropic from the Skydome, and concentrated horizon radiation. The external longwave radiation between surface, sky and the ground is influenced by surface absorptivity, surface temperature, sky and ground temperatures and sky and ground view factors. The inside radiation is comprised of two different elements: shortwave radiation, which is the radiation that reaches the internal surfaces of the building through the windows, and longwave radiation that accounts for absorption and emittance of low-temperature radiation surfaces such as other surfaces or equipment. EnergyPlus uses a grey interchange model for the longwave radiation among zone surfaces. This model is based on the concept of ScriptF that relies on a matrix of exchange coefficients between pairs of surfaces that include all exchange paths between the surfaces, considering that all radiation surfaces are grey and all radiation is diffuse. For more details about the modeling of radiation refers to the documentation of EnergyPlus [38]. 5. Results and discussion
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The effect of the thickness of the thermal insulation on the performance of a layer of pure hexadecane (10 mm) inserted in the walls (Figure 7) was analyzed from a minimum thermal insulation thickness of 0.85 mm and 12 mm (case 2), according to the Chilean Thermal Regulation (TR) for Santiago and Puerto Montt, respectively, to a maximum value of 50 mm implemented in the project dwelling (case 7). Each of the additional cases with PCM (cases 2 to 7) corresponds to the dwelling with five different levels of thermal insulation (Table 4). Finally, cases 1 and 8 correspond to the baseline case according to the thermal regulation and the project house, both without PCM. Solar energy gains through windows are a relevant element of the annual energy balance of most buildings. They are especially important in housing because houses tend not to have high internal loads, therefore besides mechanical heating, solar gains through windows could be the main energy gain of houses. By this reason, it is important to appropriately design the windows and its external sun control elements to provide adequate solar gains during the colder season and to reduce the solar gains during the warmer period. The area of the windows of the project dwelling and their orientation are shown in Table 5. The project dwelling has already a significant amount of thermal mass in its envelope. The sensible heat storage capacity of the main parts of the envelope 12
Table 4: Studied cases of the dwelling with different thickness of thermal insulation located in Santiago (SCL) and Puerto Montt (PMC). Case U [W/m2 K], SCL U [W/m2 K], PMC Insulation [mm], SCL Insulation [mm], PMC PCM layer
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1 1.908 1.14 0.85 12.0 No
2 1.908 1.14 0.85 12.0 Yes
3 0.833 0.676 23.0 32.5 Yes
4 1.103 0.785 13.0 25.5 Yes
5 1.37 0.887 7.0 20.5 Yes
6 1.633 0.991 3.0 16.5 Yes
7 0.501 0.501 50 50 Yes
8 0.501 0.501 50 50 No
(per built area of the dwelling), considering a temperature gradient of 5 ◦ C, have been calculated and compared to the latent heat store capacity of the PCM (Table 6). It may be seen how the energy stored as sensible heat is 20% higher than the energy stored as latent heat in the PCM, mainly due to the contribution of the concrete layer (0.809 kWh/m2 of built area), but it is important to clarify that the mass of the concrete layer is almost six times higher than the mass of PCM.
Figure 7: Wall configuration of the outer walls of the dwelling.
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Finally, to the better understanding of the results presented in this section, it is performed an energy analysis of the project dwelling (case 8) in its original location (Santiago), to evaluate the importance of the different energy gains (Figure 8). The main sources of energy gains of the dwelling are the internal ones (equipment, lighting, occupancy) and the solar gains (R) through the windows, which is indeed the most important, sharing a 58% of the total energy gains. 5.1. Cooling and Heating power
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The cooling (Zone Ideal Loads Zone Total Cooling Rate) and heating power (Zone Ideal Loads Zone Total Heating Rate) are obtained directly by EnergyPlus when the user explicitly asks for these variables to be written in the output file. The cooling power is the total (sensible plus latent) cooling power delivered to the zone, and the heating power is the total (sensible plus latent) heating power delivered to the zone. The following results show the thermal power demand for cooling and heating of an ideal air conditioning system. The cooling and heating power demands should not be confused with the cooling and heating power consumption of an actual air conditioning system. 13
Table 5: Area of the windows (SHGC=0.598) of the dwelling and their orientation. Window
Area [m2 ]
Azimuth [◦ ]
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
2.58 4.83 5.64 0.28 3.32 3.62 5.97 6.73 2.25 1.76 0.28 0.28 2.95 0.56 1.68 1.68 0.2 0.28 0.61 5.88
20 20 200 290 290.15 200 20 20 200 110 290 290.12 110 290 200 110 110 110 20 20
Cardinal orientation N N S W W S N N S E W W E W S E E E N N
Table 6: Stored thermal energy in some elements of the thermal envelope of the project dwelling. Thermal Envelope Walls above ground Walls below ground Roof Ground floor Exterior floor PCM
kWh/m2 of built area 0.973 0.0450 0.290 0.0728 0.00852 0.728
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Energy gain [MWh/year]
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Figure 8: Annual energy gains of the project dwelling located in Santiago. C: Convections, and R: solar gains.
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Figures 9 and 10 represent the variation of the cooling power demand during the week with the warmer day for each location. For Santiago and Puerto Montt, this day corresponds to December 20 and February 22, respectively. Broadly speaking, the cooling power demand values exhibit the same behavior for both locations, showing minimum cooling demand values during the early hours of the day and during the mid-day hours. On the contrary, the higher values of cooling demands are observed during the late afternoon and the evening. It is seen that the cooling demands are higher in Santiago than in Puerto Montt, especially from 16:00 hours to 22:00 hours. Moreover, the effect of the different thermal insulation levels on the cooling power demand in Santiago is more noticeable. In this analysis, case 1, corresponding to the case with thermal insulation according to the Chilean thermal regulation, is considered the baseline case. It is seen than the cooling power demands in the dwelling located in Santiago are higher in case 2 than in case 1, which is explained by having the minimum thermal insulation level and a PCM that stores the maximum amount of energy during the sunlight hours. The results show that case 2, corresponding to PCM-modified walls and the same level of thermal insulation as the baseline case 1, presents an increase of maximum cooling power demand of 15.5% and 2.4% in the cities of Santiago and Puerto Montt, respectively. Comparing the case for the maximum level of thermal insulation (Case 7: Project) in Santiago de Chile with both baseline
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Figure 9: Cooling power demands during the week of the warmer day (December 20) in Santiago. The detail shows the higher cooling power required in cases 7 and 8.
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mal insulation level is more noticeable in both locations than in the former analysis. In the results corresponding to Santiago, it is seen that the heating power demands are higher for the dwelling with the minimum level of insulation and PCM-modified walls (case 2), and this effect is reduced by increasing the level of thermal insulation up to that of the project level (case 7). This trend may be explained by the reduction of heat loss to the exterior by the increased thermal insulation. In Puerto Montt a different behavior is observed. The maximum heating power demands correspond to the baseline case, though the heating power demand decreases as the thermal insulation level is increased. In comparison with the baseline case, it was found that case 2 presents a max17
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imum increase and reduction of the maximum heating power demand of 32% and 18.7% in the cities of Santiago and Puerto Montt, respectively. Comparing the project case 7 in Santiago with both baseline cases 1 and 2, it seen that the maximum heating power demand is reduced by 58.4% and 68.5% with respect to case 1 and case 2, respectively. The same comparison for the city of Puerto Montt shows an increase of the maximum cooling power demand of 55.9% and 45.8% compared to cases 1 and 2, respectively. The outdoor variation of the temperature in both location is shown in Figures 9 to 12. The inner temperature is maintained almost constant at 25 ◦ C and 15 ◦ C during summer and winter, respectively.
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Figure 11: Heating power demands during the week of the colder day (June 2) in Santiago. The detail shows the higher heating power required in cases 2 and 3.
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The heat storage (Surface Heat Storage Energy) is the difference between the inside and outside face conduction, with the sign convention arranged so that positive values indicate heat being added to the core of the surface. In Figures 13 and 14, it is seen that the heat stored in the walls of the dwelling during the summer day reaches its peak during the morning, after a fast increase coincident with the sunlit hours. In Santiago, after the peak of heat storage, the amount of stored energy is reduced hourly, and depending on the level of thermal insulation, it ceases between 16:00 and 18:00 hours. After this period of accumulation, the walls start to discharge, losing part of the stored heat. The peak of discharge occurs at 20:00 hours for the different levels of insulation. 19
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After this point, the hourly rate of energy loss by the walls is reduced at a slow rate. It can be seen that the case with the higher level of insulation and PCM (case 7) has the lower values in both accumulation and discharge. The case with the lowest insulation level and PCM (case 2) presents the highest levels of both accumulation and discharge rates, even higher than the values of the case with the same insulation level but without PCM (case 1). This observation maybe be explained by the fact that the inclusion of PCM increases the heat capacity and therefore the amount of heat that can be stored in the walls, which is reduced by increasing the amount of thermal insulation, as expected. A similar behavior was observed in Puerto Montt, but in this case the absolute amount of energy stored in the walls is lower than in Santiago, which is attributable to the lower temperatures of Puerto Montt. It can be seen that the differences between the cases are lower than those founded in Santiago. Another difference between Puerto Montt and Santiago is that after the discharge peak (approximately at 20:00 hours), the level of discharge is reduced for all cases after 21:00 hours. The same behavior between cases 1 and 2 may be observed in Puerto Montt, where the case with the lower insulation increases its level of both accumulation and discharge by including PCM in the walls (case 2), but this effect is reduced by increasing the level of insulation, and by reaching the highest level of insulation analyzed (case 7), the lower peaks of accumulation and discharge can be seen. During winter in both locations (Figure 15 and 16), the highest amount of stored energy during the colder day is lower than during the warmer summer days. Another common observation for both locations is that the time of highest level of accumulation is shifted from the morning during summer (10:00 hours) to the afternoon (14:00 hours in Santiago and 13:00 hours in Puerto Montt). In Santiago, the walls stop losing heat and start accumulating energy at approximately 9:00 hours, increasing their level of energy accumulation in a slower way than during summer, reaching the peak at 14:00 hours, after which the walls start accumulating less energy hourly until 17:00 hour, when they start releasing the energy accumulated during the day. The same as during the summer, the case with the lowest variations in accumulation and discharge is that with PCM and the highest level of insulation (case 7), and the case with the highest amplitude between accumulation and discharge is the case with the lowest level of insulation and PCM (case 2). For Puerto Montt, the same period of accumulation as in Santiago may be observed (between 9:00 hours and 17:00 hours), but the same as in summer, the different cases present a more similar behavior between them than in Santiago. In Puerto Montt, it can also be seen that the peak energy accumulation has absolute values similar to those of the discharge, which does not happens in the other analyzed cases, even though the peak during discharge is reduced faster. Except for the peak in discharge at 18:00 hours, the case with the highest insulation and PCM (case 7) has the most stable behavior between accumulation and discharge, in contrast with the case with the lowest level of insulation and PCM (case 2), which presents a greater accumulation and discharge than the other cases.
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The Energy demand is obtained by calling for two variables, cooling energy (Zone Ideal Loads Zone Total Cooling Energy) and heating energy (Zone Ideal Loads Zone Total Heating Energy) to the output file of EnergyPlus. The annual heating and cooling demands for both locations are shown in Figure 17. In general, it is seen that the heating and cooling demands decrease and increase, respectively, when the level of thermal insulation is augmented, respectively. Moreover, when a layer of PCM is added to the envelope according to the thermal regulation (case 2) the heating and cooling demands are increased and decreased compared to the baseline case 1. With respect to the cooling demands 21
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in Santiago and Puerto Montt, case 2, i.e., thermal envelope according to the thermal regulation and a PCM layer, show a reduction of the energy demand of 8.3% and 28.7% compared to the baseline case 1 (thermal envelope according to the thermal regulation without a PCM layer), respectively. Meanwhile, the project case 7 presents an increase of 47.8% (Santiago) and 107.3% (Puerto Montt) of the cooling demand, compared to the same baseline case 1. When a layer of PCM is incorporated in the thermal envelope in compliance with the thermal regulation (case 2), the heating demands are increased by 64.9% and 43.7% compared to the baseline case 1 for the locations of Santiago and Puerto Montt, respectively. But if the level of insulation is increased to the project value
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(case 7) the heating demand is reduced by 83.5% and 59% in comparison to the baseline case for Santiago and Puerto Montt, respectively. Regarding the total demands (heating and cooling), the study in Santiago shows that case 7 and case 2 present lower (5.7%) and higher (21.4%) values compared to the baseline case 1, respectively. The same analysis in Puerto Montt shows a decrease of 9% (case 7) and an increase of 21.9% (case 2) of the total demands compared to case 1. The obtained results of the energy demand are of the same order of the average annual total energy consumption of 10,232 kWh/year, but these results do not consider the electrical consumption of household appliances. In comparison to the average house in Chile, the energy demands for air conditioning of the project dwelling are higher given its relative large built area. 23
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The effect of the insulation level of a PCM-modified thermal envelope of a project dwelling located in Santiago and Puerto Montt on the cooling and heating power demands, stored energy in the envelope, and energy demands was analyzed. With respect to the cooling power demands, the effect of the insulation level is only noticeable in the case of the dwelling located in Santiago during the night hours, showing that the cooling demands decrease with the insulation level. It is important to remark that the weather in Santiago during summer is warmer than in Puerto Montt. When the heating demands are an-
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alyzed, a more noticeable effect of the insulation level is seen. In Santiago, it may be noticed that the heating power demand decreases as the insulation level is increased, while in Puerto Montt the same trend is observed but to a lower extent. The energy stored in the walls during the warmer and colder days was also analyzed in both locations. The effect of the insulation level is more noticeable in the dwelling located in Santiago, especially during the warmer summer day. The incorporation of PCM to the thermal envelope has an increasing effect in the amount of both stored and released heat during the warmer and colder hours 25
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of the day, respectively. The effect of the insulation level in Puerto Montt is less noticeable since the level of insulation in this city is already high, according to the Chilean Thermal Regulation. The addition of PCM to the thermal envelope according to the Chilean Thermal Regulation, increases and decreases the heating and cooling energy demand, respectively. Specifically, the cooling energy demand is decreased in both Santiago (8%) and Puerto Montt (29%), while the heating demand is increased by 64% and 43% in Santiago and Puerto Montt, respectively. As the level of insulation is increased, the heating demand is reduced and the cooling demand is increased, which is more evident in the dwelling located in Santiago. According to these results, a PCM with a relatively low phase change temperature, like hexadecane works, better for cooling purposes, especially in Puerto Montt. In a future work an interesting study may be the incorporation of two different PCMs, with low and high phase change temperatures, in the thermal envelope, analyzing the effect over the cooling and heating demands of each PCM and different proportions of both in a combined element. 7. Acknowledgements This work was funded by CONICYT under Fondecyt Project 11130168.
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