Available online at www.sciencedirect.com
ScienceDirect Solar Energy 120 (2015) 55–64 www.elsevier.com/locate/solener
Performance demonstration and simulation of thermochromic double glazing in building applications Linshuang Long a, Hong Ye a,⇑, Haitao Zhang a, Yanfeng Gao b a
Department of Thermal Science and Energy Engineering, University of Science and Technology of China, Hefei, Anhui 230027, PR China b School of Materials Science and Engineering, Shanghai University, Shanghai 200072, PR China Received 11 March 2015; received in revised form 19 June 2015; accepted 10 July 2015
Communicated by: Associate Editor Mario A. Medina
Abstract The application performance of a type of thermochromic double glazing is first demonstrated in a full-scale room. The experimental results show that the low-mass room with a VO2 double window consumes approximately 11.1% less cooling energy than that with an ordinary double window. Three types of windows, a VO2 double window, an ordinary double window and a VO2 single window, are then analyzed as improvements for an ordinary single window. The analysis indicates that in hot climate conditions, the application of the VO2 single window uses less energy than that of the ordinary double one but uses more energy than that of the VO2 double window. Two widely investigated methods to enhance the energy performance of a window, optimizing the capacity for the regulation of solar radiation and improving the thermal insulation performance, are embodied in applications of two hypothetical windows and then further discussed. The results indicate that, in a hot climate, whether a window with an extremely low U-value is favorable for energy efficiency will depend on the set point of the indoor temperature: an indoor temperature lower than 23 °C makes the window more efficient than an ordinary single window. However, the low U-value window still has worse energy performance than a window with an appropriate solar heat gain coefficient, which highlights the effect of regulating the solar radiation. Ó 2015 Elsevier Ltd. All rights reserved.
Keywords: Thermochromic double glazing; Vanadium dioxide; g-value; U-value
1. Introduction The building sector is responsible for 30–40% of the primary energy used in developed countries (Mlyuka et al., 2009) and is also one of the main sources of carbon dioxide emissions. The corresponding proportion in China was approximately 25% in 2010 (International-Energy-Agency). Up to 60% of the total energy losses are from windows (Gustavsen, 2008), giving windows great potential for energy efficient development. ⇑ Corresponding author. Tel./fax: +86 551 63607281.
E-mail address:
[email protected] (H. Ye). http://dx.doi.org/10.1016/j.solener.2015.07.025 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.
A window impacts the energy balance in a building mainly from two aspects: radiation (generally including solar radiation and long-wave thermal radiation) and thermal transmittance. A window transmits solar radiation and emits long-wave thermal radiation and conducts heat between the indoor and outdoor environments. These properties imply two approaches to improve the energy performance of a window: optimizing the capacity for the regulation of radiation (radiation includes both solar and long-wave thermal radiation, but the latter is beyond the scope of this study) and improving the thermal insulation performance. In recent years, windows with improved insulation performance have been widely used to reduce the energy
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consumption of buildings. A multilayer window is one such window that generally contains glazing panes, a gas fill, a frame, a spacer and an edge seal (Van Den Bergh et al., 2012). With gas (or a vacuum) sealed in the cavity between the panes, a multilayer window can achieve a low thermal transmittance (U-value) of 0.28 W/m2 K (Jelle et al., 2012). Another approach to reduce the building energy from the window perspective lies in the application of smart windows that are able to regulate the radiation gain based on the inhabitants’ demand (Jonsson and Roos, 2010). Smart windows may be further divided into two types: active windows with mechanical devices (Sabry et al., 2014; Ulavi et al., 2014; Yi et al., 2014) and passive windows with chromogenic materials. As the typical passive type, the thermochromic window has been widely researched (Baetens et al., 2010; Nitz and Hartwig, 2005; Zheng et al., 2015). First reported by Morin (1959), vanadium dioxide (VO2) is the most promising thermochromic material. VO2 is able to undergo a reversible transition at a phase transition temperature (Ts); when the temperature of the material is lower than Ts, it is monoclinic, semiconducting and rather infrared transparent, and when the temperature is higher than Ts, it is tetragonal, metallic and near infrared reflecting. However, three inherent shortcomings block its application to building energy efficiency (Li et al., 2012): (1) the solar modulation ability, which is defined as the difference in solar transmittance between its two states, is modest; (2) its luminous transmittance is relatively low compared with that of a float-glass window; and (3) the transition temperature of bulk VO2 (approximately 68 °C) is too high for building applications. For the first two issues, the regulation of the microstructure of the VO2 film (Madida et al., 2014; Yao et al., 2013) may be a solution. For the third issue, doping with transition metal ions, e.g., tungsten (Tan et al., 2012), can significantly decrease Ts. A VO2 film with a Ts as low as 30 °C has been reported (Huang et al., 2011). In addition to transmittance and Ts, the transition hysteresis width and gradient of VO2 also play crucial roles in the energy performance (Saeli et al., 2013; Warwick et al., 2013). The performance of a VO2 single-glazed window has been demonstrated (Long and Ye, 2014; Ye et al., 2013a), showing that the application of the VO2 film could reduce cooling energy consumption in a low- or heavy-mass room by approximately 15.1% or 9.4%, respectively. Attaching a VO2 film onto a singleglazed window only optimizes the solar radiation of the window. Enhancing the thermal insulation performance needs to be simultaneously considered for a comprehensive improvement. In this study, we combined the VO2 film and double-glazed window to form a VO2 double window. Theoretically, the VO2 double window will share the advantages of both VO2 and double glazing: the ability to dynamically control the solar radiation into the building and blocking the heat from entering or leaving the building in summer or winter, respectively. We demonstrated its application performance in a full-scale room whose opaque
envelope is made of materials with a low volumetric heat capacity, i.e., a low-mass room. Then, we simulated the performance in a residential building constructed with heavy materials such as bricks and reinforced concrete. The effects of the regulation of the radiation properties and improvement in thermal insulation of a window on the energy performance were also discussed. 2. Materials and methodology 2.1. Experimental setup The application performance was demonstrated with the Testing and Demonstration Platform for Building Energy Research. The platform is located at the campus of the University of Science and Technology of China, Hefei, China, and contains two identical testing rooms, Room A and B. One of the rooms can be setup as an experiment unit by adopting a building component or material of interest, while the other one can be setup as a control. The testing rooms are 2.9 m in length, 1.8 m in width and 1.8 m in height. The non-transparent envelopes of the rooms are made of polyurethane wrapped with metal boards; the polyurethane has a density of 37 kg/m3, a specific heat of 1385 J/(kg K) and a thermal conductivity of 0.00228 W/(m K). The thicknesses of the walls and the roofs are 10 cm. The indoor temperature of the rooms can be maintained at a particular temperature by delivering cool or hot air via fan coil units. With the measurements of the parameters of the fan coil units, the instantaneous cooling load q is obtained from _ out T in Þ q ¼ cwater mðT
ð1Þ
where cwater denotes the specific heat of water in the coil with a value of 4200 J/(kg K); m_ represents the mass flow rate of the water measured by an electromagnetic flow meter; and Tout and Tin are the water temperatures at the outlet and inlet of the coil, respectively, which were measured by PT100 resistance thermometers. Due to the inherent errors from instrument precision, the value of q shows an uncertainty of Dq, which can be determined from the function of the propagation of uncertainty. sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 Dq Dm_ DT out DT in ð2Þ ¼ þ þ q m_ T out T in _ DTout and DTin denote the relative uncerIn Eq. (2), Dm, tainties of the corresponding measurements. According to the nameplates of the instruments, the accuracy of the mea_ m_ is 5%, and those surement of the mass flow rate, i.e., Dm= of the water temperatures, i.e., DTout/Tout and DTin/Tin, are 0.2%. With the consideration of uncertainty, the value of the load is expressed as q ± Dq. 2.2. VO2 double window A type of thermal-stable PET film covered with VO2 was prepared through an all-solution process (Gao et al., 2012).
L. Long et al. / Solar Energy 120 (2015) 55–64
The Ts of the VO2 particles on the film is 41.3 °C. At a temperature above or below Ts, which is in the metallic or semiconductor state, the spectral transmittance of the film was monitored by a Hitachi U-4100 UV–visible–NIR spectrophotometer equipped with a film heating unit in the wavelength range of 240–2600 nm (Ye et al., 2013a). The normal-incidence hemispherical reflectance was measured using a Bruker Equinox 55 Fourier transform infrared (FTIR) spectrometer equipped with a thermoregulated environmental cell, where an Au film was used as a reference. By sticking the film on an ordinary glass substrate, a VO2 glazing can be formed, which is convenient for refurbishing an existing window system. The window system of Room A and B contained two 4.6 mm thick glass panes of ordinary float glass. The foam spacer provided an air cavity with a thickness of 7.0 mm between the interior and exterior panes. The frames were made of wood with stainless steel cladding. The double glazing was on the south wall of the testing rooms with a size of 1.65 m 1.65 m, making the south wall a glass curtain wall. The VO2 film was pasted, for convenience, onto the outside surface of the exterior pane of Room A to form a VO2 double-glazed window, and Room B was set as the control with an ordinary double window. The photos of the rooms are shown in Fig. 1. The solar spectral properties of the film were measured and employed to acquire the integral properties within the solar or the visible spectrum. These properties along with the long-wave emissivity of the widow systems are listed in Table 1, in which the total radiation properties of the double glazing were calculated via the ray tracing method with the properties of the glazing panes.
2.3. Simulation tool In addition to an experimental demonstration, the application performance of the VO2 double-glazed window was
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also simulated through a building modeling program, BuildingEnergy (Ye et al., 2012, 2013a,b). The program is based on a non-steady-state heat transfer model. For the room that contained a single-glazed window, the program was validated via ANSI/ASHRAE Standard 140-2004 (Standard Method of Test for the Evaluation of Building Energy Analysis Computer Programs) (Ye et al., 2012) as well as a set of experiments (Ye et al., 2013a). However, the heat transfer process within double glazing is rather distinct from that within single glazing. As a result, before simulation, the program was verified through the data from the demonstration, details of which are also provided in Appendix.
3. Results and discussion 3.1. Performance demonstrations in a low-mass room The demonstration started on May 20th, 2013 and _ Tout and Tin, lasted for five days. With the measured, m, the cooling load that was needed to maintain the indoor temperature at 20 °C was computed from Eq. (1) and is plotted in Fig. 2(a). The figure shows that during the daytime, the cooling load in the room that contained the VO2 double glazing, Room A, is lower than that in the room that contained the ordinary glazing, Room B; the daily peak load of the former is approximately 100 W less than that of the latter. The lower cooling load in Room A is because the solar transmittance of the VO2 glazing is lower. A decrease in the solar transmittance in turn leads to a decrease in the heat gain from the solar radiation transmitted through the glazing, meaning that less cooling is needed in Room A compared with Room B. Despite a decrease in the cooling load, a decrease in the visible light illuminance for Room A may simultaneously occur because, from Table 1, the visible transmittance of the VO2 double
Fig. 1. Photos of Room A with VO2 double glazing (left) and Room B with ordinary double glazing (right).
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Table 1 Radiation properties of the windows. Properties
With VO2 film Semiconductor state
Metallic state
Glazing pane
Solar absorptivity Solar reflectance Solar transmittance Visible transmittance Long-wave emissivity
0.460 0.102 0.438 0.434 0.880
0.576 0.071 0.353 0.420 0.880
0.159 0.070 0.771 0.837 0.840
Double glazing
Total Total Total Total
0.544 0.115 0.341 0.382
0.646 0.080 0.274 0.369
0.291 0.112 0.597 0.761
solar absorptivity solar reflectance solar transmittance visible transmittance
glazing is also lower. The measured data indicate that the visible light illuminance in Room A is approximately 60% lower than that in Room B, implying that additional lighting load may be needed in the room with the VO2 double glazing. At night, the presence of the VO2 film on the glazing makes little difference due to the absence of solar radiation and the low mass envelope, which makes the cooling load in the rooms close to each other. In addition to the instantaneous cooling loads, the cumulative energy consumption obtained through a time integration of the cooling loads is also significant for evaluating the application performance. Along with the errors of the loads from Eq. (2), the consumptions are within a range of values rather than at a single value. The cumulative energy consumptions for cooling in Room A and B are 168.6 ± 6.0 and 190.3 ± 7.0 MJ, respectively. The results indicate that the application of the VO2 double glazing in the testing room reduced the cooling energy consumption by 11.1 ± 6.4%. In addition to the system parameters (e.g., water temperature, mass flow rate of water, etc.), meteorological data (e.g., dry-bulb temperature, solar irradiance, wind speed, etc.) were also recorded during the demonstration. With
Ordinary
an input of the meteorological data, the BuildingEnergy program can simulate the instantaneous cooling loads in Room A and B. Fig. 2(b) and (c) shows the comparison between the measured and simulated loads, which show an acceptable agreement between the measured and simulated results. The simulated cumulative consumptions were 173.3 and 187.6 MJ in Room A and B, respectively, within the range of the measured values. Instead of a time-consuming and costly demonstration, the performance of the double glazing can now be conveniently predicted through the verified program. 3.2. Performance comparison with a single-glazed window So far, the performances of both the VO2 single- and double-glazed windows have been demonstrated in the platform, which naturally raises an interesting question: what is the difference between these two types of windows in energy saving performance? To answer this question, one must compare their performances on a common basis. However, as they were not demonstrated simultaneously, the meteorological conditions were different, making a direct comparison inappropriate. An available approach
Fig. 2. Cooling load comparisons (a) between the rooms with and without VO2 and (b, c) between the simulated and measured results.
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is to simulate their performance under the same meteorological conditions. In BuildingEnergy, we set the window as a single-glazed window, whose properties are listed in Table 1. With an input of the meteorological data during May 20th to 24th, 2013, the cooling loads in the testing room that contained the single window were then output. The loads were compared with those of the double window in Fig. 3. The figure indicates that the cooling load of the double window is lower than that of the single window during both day and night. Theoretically, the double window, whose thermal insulation performance is better than that of the single window, reduces the heat gain from the hot outdoor air to the cool indoor air. In addition, the lower total solar transmittance of the double glazing (see Table 1) decreases the heat gain from the solar radiation in the daytime. A remarkable decrease in the heat gain of the room with the double window leads to a decrease in cooling loads of up to 300 W compared with the room with the single window. The cumulative consumption of the room with the VO2 double glazing window is 23.3% lower than that of the single one. The simulation shows that the application of the VO2 double window to the testing room can significantly reduce the cooling energy consumption compared with that of the single glazing.
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are made of reinforced concrete, with a density of 2500 kg/m3, a specific heat capacity of 920 J/(kg K) and a thermal conductivity of 1.74 W/(m K). According to the design standard (JGJ75, 2003), the inner heat gain from the occupants and equipment is set at 4.3 W per unit of floor area and the heat gain from lighting at 3.5 W per unit of floor area when the lights are on from 18:00 to 22:00 every day. The standard also recommends the ventilation rate to be
3.3. Application performances to residential buildings The above results are associated with low-mass rooms. The performance in a room with heavy mass will be discussed in this subsection. The standard room, which has internal dimensions of 4 3.3 2.8 m3 (length width height), is located in a middle story of a multi-story apartment building and contains a 1.5 1.5 m2 window in the center of the south-facing wall, which is the only external wall of the room. The walls are made of bricks with a density of 1400 kg/m3, a specific heat capacity of 1050 J/(kg K) and a thermal conductivity of 0.58 W/(m K). The roof and floor
Fig. 3. Variations in the simulated cooling loads with the VO2 single glazing and the VO2 double glazing.
Fig. 4. Performance simulations in a standard room. (a) Variations in cooling load (Aug. 1st to Aug. 7th). (b) Temperature of the exterior pane from Aug. 26th to Sep. 1st and Sep. 8th to Sep. 14th. (c) Energy consumptions of rooms with different types of windows.
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1.0 air change per hour (ACH) when the space cooling is under operation and 10.0 ACH at the other times. Saeli et al. (2010a,b) have stated that the VO2 glazing is inappropriate for cold climates, so the room is assumed to be in a city with a hot climate. The city of Guangzhou, China located at 23 N latitude and 113 E longitude has a cooling period from May 13th to October 17th. During the cooling period, the indoor temperature of the room is maintained at the recommended 26 °C through space cooling (JGJ75, 2003). The climate data used in BuildingEnergy are the typical meteorological year data offered by the Chinese Architecture-specific Meteorological Data Sets for Thermal Environment Analysis. The cooling loads during the entire cooling period were simulated in BuildingEnergy, and those within one week are displayed in Fig. 4(a). The figure shows that the cooling load in the room with the VO2 double glazing is lower than that with the ordinary glazing. The peak cooling load of the VO2 glazing is approximately 8.9% lower than that of the ordinary one, so a refrigeration facility with a lower load capacity can be used for a room with VO2 glazing. The cooling loads then can be integrated into the cumulative cooling energy consumption. The results indicate that during the entire cooling period of a year, the application of the VO2 double glazing to the standard room can reduce the energy consumption by 11.6% or 26.4 kWh without considering the lighting energy. In addition to the cooling loads, the temperature of the film is another key parameter for estimating the window’s performance because it reveals whether the transition of the VO2 occurs. Although the transition temperature (41.3 °C) seems high compared with the indoor temperature (26 °C), the film temperature shown in Fig. 4(b) can be much higher than the indoor temperature due to intense solar irradiation and high absorptivity. The film’s temperature varies with the weather conditions each day, leading to different states of the VO2: it may be completely in its metallic state as its temperature reaches 50 °C or it may be mostly in its semiconductor state when the highest temperature is only 36 °C. Considering that the phase transition process does not occur at a particular temperature but within a range of temperatures, the seemingly high transition temperature is actually suitable for residential applications.
The performances of the single windows were also simulated to be compared with the double ones. As Fig. 4(c) shows, either adding a VO2 film to the ordinary single window or changing it into a double window decreases the corresponding cooling energy consumption. The former effort gains an energy efficient benefit of approximately 9.8%, while the later reduces the energy by approximately 1.2%. From an energy perspective, the application of the VO2 film has priority over the double glazing. Furthermore, a combination of the efforts, if possible, is the best choice because the application of the VO2 double window reduces the consumption by approximately 12.7%. This value is greater than the sum of the addition of the VO2 film and the double window because the solar transmittance of the VO2 double window is lower than that of the single one. 3.4. Performance comparison of ideal windows As mentioned in the Introduction, the applications of VO2 film and double glazing can be separately labeled as one of the two methods for improving the performance of an ordinary single window. A VO2 film optimizes the regulation of radiation, and a double-glazed window improves the thermal insulation. The solar radiation properties and the thermal insulation performance can be evaluated by the solar heat gain coefficient (SHGC, g-value) and the thermal transmittance (U-value), respectively. The g- and U-values of varieties of windows obtained through the WINDOW 6.3 software are listed in Table 2. To display the improvements in the g- and U-values, two hypothetical windows were simulated based on the ordinary single glazing. With the same g-value as the single glazing, the first window was a low U-value window of 0.28 W/(m2 K), which was achieved by a double glazing window filled with xenon (Jelle et al., 2012). The other theoretical concept was an ideal smart window (Ye et al., 2013b) that is able to dynamically decrease or increase its g-value in summer or winter, respectively, with the same U-value as the ordinary single glazing. The window parameters are provided in Table 2, where the ideal smart window in the near infrared (NIR) reflection state transmits exclusively the visible part of the solar radiation (400–700 nm, approximately 42% of the solar radiation reaching the
Table 2 U- and g-values of windows. Window
U-value [W/(m2 K)]
g-value (%)
Ordinary single glazing VO2 single glazing
5.8 5.8 5.8 3.0 3.0 3.0 0.28 5.8 5.8
82 58 53 70 47 41 82 42 100
Ordinary double glazing VO2 double glazing Low U-value window (double panes) Ideal smart window (single pane)
Semiconductor state Metallic state Semiconductor state Metallic state NIR reflection state NIR transmission state
L. Long et al. / Solar Energy 120 (2015) 55–64
earth’s surface) and in the NIR transmission state permits the whole solar radiation spectrum into the room. Fig. 5(a) depicts the energy consumptions in the rooms with these hypothetical windows and the original single glazing. In this figure, the room with the low U-value window requires more energy than that with the ordinary window, so a higher U-valve improves the energy performance. In summer, a low U-value decreases the cooling load by
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obstructing the heat gain from the hot outdoor air to the cool indoor air. However, a low U-value may also increase the load by impeding the heat loss from the indoor air to the outdoor air when the outdoor air is cooler, typically at night. Fig. 5(b) shows that temperature of the outdoor air is generally higher than the indoor air temperature during the day and lower at night; therefore, compared with an ordinary single window, the low U-value window increases the cooling load at night and decreases the load during the day. If the increase is greater than the decrease, then the low U-value window has an overall poorer performance than the ordinary window. In other words, the set point of the indoor temperature may determine whether the heat will transfer from the outdoor air into the room, and in turn decide the performance of the low U-value window. Fig. 5(c) shows the relationships between the energy consumptions of the windows and the set points that drop from 28 to 20 °C. In the figure, the consumption increases with decreasing set point, and the ideal smart window always displays the best performance. The room with the ordinary single window consumes less cooling energy than that of the low U-value window when the indoor temperature is higher than a critical point, which is between 23 and 24 °C, and is nearly always lower than the outdoor air in Fig. 5(b). When the set point is equal to or lower than 23 °C, the performance of the low U-value window becomes better than that of the ordinary window, Table A.1 Nomenclature of the appendix. A cp hj-air k rvent Ri–j S0 T Dt V Dx
area of the surface [m2] specific heat capacity at constant pressure [J/kg] convection heat transfer coefficient between surface j and air [W/ (m2 K)] thermal conductivity [W/m K] ventilation rate [ACH] thermal resistance for radiation between surface i and j [K/W] solar irradiation absorbed by the wall or window per unit area [W/m2] temperature [K or °C] time interval [s] volume [m3] spatial interval [m]
Greek letters q mass density [kg/m3] e thermal emissivity r Stefan–Boltzmann constant
Fig. 5. Discussions of hypothetical windows. (a) Energy consumptions in rooms with ordinary single window, low U-value window and ideal smart window. (b) Temperature variations in Guangzhou during the cooling period. (c) Energy consumption variations along with the indoor temperature set point.
Subscripts air,i indoor air air,o outdoor air conv convection i ith node of a wall in indoor side of a wall interior interior of a wall j surface in a room m surface of outdoor surroundings (e.g., the sky, the ground) out outdoor side of a wall rad radiation wa wall wi window
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signifying that a low set point provides the advantage to the low U-value window in summer. In summary, a better U-value will decrease the cooling energy consumption if the indoor temperature is set at a relatively low value, which is fairly common in practical applications because in a hot summer, the residents are sometimes willing to experience better thermal comfort by setting a low indoor temperature. However, the energy consumption of the room with the ideal smart window is always the lowest in both Fig. 5(a) and (c), which reveals that improving the regulation capacity of solar radiation is more efficient than improving the thermal insulation for a hot climate. Although these rules were summarized from conceptual windows, they may provide some practical references to the design of VO2 double window. With the information from Table 2, the U-value of the VO2 double window discussed in this study is much higher than that of the low U-value window, implying that the VO2 double window may be improved by decreasing its U-value. If the U-value is lowered to 1.2 W/(m2 K), which is the new window U-value required by the Norwegian government (Jelle et al., 2012), the energy performance can be improved by 6.14% from the current VO2 double window.
Furthermore, if the gas fill of the double glazing, which is air for the current window, is replaced by argon, producing a U-value of 0.64 W/(m2 K) (Jelle et al., 2012), the improvement will be 6.7% compared with the current window. Despite the large gap of the U-values between the current VO2 double window and the low U-value window, the g-value of the current window, especially in its metallic state, is already close to that of the ideal smart window. Nevertheless, this does not mean that the VO2 window is as effective as the ideal smart window because their mechanisms to decrease the g-value are totally different: the undesirable solar radiation is reflected by the ideal window, while it is absorbed by the current VO2 Window. An absorption effect may increase the glazing temperature and result in more heat gain from the glazing, leading to an increased cooling load. This inherent disadvantage of VO2 may be overcome by applying low-emissivity (low-e) coatings (Jelle et al., 2015), the performance of which is outside the scope of this study. From these discussions, the following improvements would decrease the energy consumptions of the VO2 double window: improving the thermal transmittance of the double window system and employing low-e coatings.
Fig. A.1. (a, b) Meteorological data and (c, d) system parameters during the demonstration.
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4. Conclusions The application performance of a thermochromic double-glazed window has been demonstrated in a full-scale room for the first time. In a low-mass room, the adoption of the VO2 double-glazed window reduced the cumulative cooling energy consumption by approximately 11.1% compared with an ordinary double-glazed window. The percentage was simulated to be 11.6% in a conventional residential room in a hot climate. For the residential room with an ordinary single window, attaching a VO2 film onto the single glazing reduced more cooling energy than simply changing the single window into a double-glazed window. Further discussion on conceptual windows showed that the low U-value window outperforms the ordinary single window when the set point was lower than 23 °C, but both windows were surpassed by the ideal smart window. The results mean that, among the two widely investigated methods to improve the window’s performance, regulation of solar radiation and improvement in thermal insulation, the former had much better performance in a hot climate. Acknowledgment This work was funded by the National Basic Research Program of China (Grant No. 2009CB939904).
the other surfaces in the room by long-wave thermal radiation and that from the indoor air by convection. For the nodes at the outdoor surface of the walls, the energy balance equation is expressed as qcp A
pþ1 p T pwa;interior T pwa;out Dx T wa;out T wa;out ¼ kA 2 Dt Dx X T p T pwa;out m þ S0A þ Rwam m
þ hwaair;o AðT pair;o T pwa;out Þ ðA:2Þ where the four terms on the right-hand side correspond to the heat flows entering the node from the adjoining node by conduction, the heat flows from the sun by absorption, that from the outdoor surroundings by long-wave thermal radiation and that from the outdoor air by convection. For the internal nodes, the energy balance equations can be written in a single discrete form as p p p T ipþ1 T pi k T i1 þ T iþ1 2T i ¼ qcp Dt Dx2
Here, we will elaborate on the validation methodology of the BuildingEnergy software and the heat transfer model used in the software. This modeling program was compiled based on a non-steady-state heat transfer model, in which the building envelope and the indoor and outdoor air were divided into hundreds of nodes. The energy balance equations of the nodes are introduced below. For the nodes at the indoor surface of the walls, the energy balance equation can be expressed as pþ1 p T pwa;interior T pwa;in Dx T wa;in T wa;in qcp A ¼ kA 2 Dt Dx X T pj T pwa;in þ Rwaj j
þ hwaair;i AðT pair;i T pwa;in Þ
where the superscript p is used to denote the time dependence of T, and the time derivative is embodied in terms of the difference in temperatures associated with the new (p + 1) and previous (p) times. Other symbols in the equation are explained in Table A.1. From the energy balance perspective, the left-hand side of Eq. (A.1) represents the rate of increase of the thermal energy stored in the node-controlled volume. The terms on the right-hand side are, in sequence, the heat flows entering the node from the adjoining node by conduction, the heat flows from
pþ1 T wi T pwi X T j T wi ¼ þ S0A Dt R wij j
ðA:4Þ
For a double window system, the thermal resistance between the panes, Rgap, is also a key parameter, which is obtained through 1 1 1 ¼ þ Rgap Rrad Rconv
ðA:5Þ
where Rrad and Rconv denote the thermal radiation and the thermal convection resistance between the panes, respectively. They can be written in a unified form as R¼
ðA:1Þ
ðA:3Þ
which is also known as the discrete heat diffusion equation. The subscript i denotes the ith node of the wall’s interior, and i + 1 and i 1 designate the adjacent nodes of the ith node. The energy balance equation for the window is qcp ADx
Appendix A
63
1 hA
ðA:6Þ
where h is the coefficient of heat transfer. The heat transfer via thermal radiation between the panes can be simplified as that between two infinitely large plates, whose coefficient of heat transfer, hrad, follows hrad ¼
rðT 1 þ T 2 Þ ðT 21 þ T 22 Þ 1 þ e12 1 e1
ðA:7Þ
where T1 and T2 represent the surface temperature of the panes, and e1 and e2 are the thermal emissivity of the panes. The heat transfer via thermal convection, hconv, can be acquired through hconv ¼ Nu
k d
ðA:8Þ
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where d designates the thickness of the air gap; and Nu stands for the Nusselt number, which is calculated by the method described in the literature (Wright, 1996). These equations of all of the nodes in the temperature field formed a matrix. By solving this matrix, the temperature field can be determined. When validating the program from experiments, we also input the meteorological data measured by the platform to serve as the variables, e.g., Tair,o and S0 , which is required in Eqs. (A.1), (A.2) and (A.4), some of which are provided in Fig. A.1(a and b). Based on the verified temperature field, the instantaneous load for space cooling or heating, q, is simulated by X q¼ hjair;i Aj ðT air;i T j Þ j
þ rvent qair;i cp;air;i V air;i ðT air;o T air;i Þ
ðA:9Þ
The measured load can be obtained from Eq. (1), the parameters in which were recorded by the platform and are shown in Fig. A.1(c and d). With the measured and simulated results, the program can then be validated, and the comparisons are shown in Fig. 2(b and c). References Baetens, R., Jelle, B.P., Gustavsen, A., 2010. Properties, requirements and possibilities of smart windows for dynamic daylight and solar energy control in buildings: a state-of-the-art review. Sol. Energ. Mat. Sol. C 94, 87–105. Gao, Y., Wang, S., Luo, H., Dai, L., Cao, C., Liu, Y., Chen, Z., Kanehira, M., 2012. Enhanced chemical stability of VO2 nanoparticles by the formation of SiO2/VO2 core/shell structures and the application to transparent and flexible VO2-based composite foils with excellent thermochromic properties for solar heat control. Energ. Environ. Sci. 5, 6104–6110. Gustavsen, A., 2008. State-of-the-art highly insulating window frames – research and market review. Lawrence Berkeley National Laboratory.
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