Redefinition of smartness

The definition of “smartness” has been somewhat ambiguous. For a thermochromic window, the word “smart” has usually been used to describe the window's capacity to regulate its properties, especially its solar transmittance, in response to changing temperature1. The goal of regulating the solar transmittance of a thermochromic window is actually to regulate the energy loads for both space cooling and heating and, ultimately, to achieve energy efficiency. Consequently, we would like to define “smartness” directly on the basis of the corresponding energy performance. Our new definitions of the different behavior types for thermochromic windows are listed in Table 2: The word “smart” is now used to describe the energy performance over an entire year. “Inefficient” thermochromic windows should not be used in building applications. “Efficient” windows behave like common energy-efficient windows without a smart regulation capacity, unlike smart windows, which can reduce the energy load intelligently. It should be noted that the word “advantageous” or “disadvantageous” in Table 2 implies a better or worse energy performance than a given standard. Here, the standard window is chosen to be an ordinary window of plain glass whose solar transmittance, solar absorptivity and long-wave thermal emissivity are 77.1, 15.9 and 83.7%, respectively. This type of glass is commonly used in residential buildings. If the energy consumption of a room with a particular thermochromic window is less during a period of time than that with the standard window, i.e., the use of this thermochromic window is advantageous for energy efficiency, then the thermochromic window is labeled as advantageous; otherwise, it is disadvantageous. Using the new definitions, the performance of a particular thermochromic window can be labeled as a particular type from a simple perspective rather than from the complicated relationship between temperature and solar radiation properties.

Table 2 Definitions of three window behavior types according to their energy performance Full size table

Assumptions for the discussion on potential thermochromic windows

Our goal is to perform an analysis of all potential thermochromic windows to discover the general relationship between smartness and energy performance. Several parameters are needed to evaluate each thermochromic window, such as the transition temperature, the solar radiation properties in both states and the long-wave thermal radiation properties. Therefore, the number of potential thermochromic windows is theoretically infinite due to the infinite possible combinations of these parameters. To make the analysis feasible, the transition temperature is set as 10, 20, 30, 40 or 50°C. The radiation properties of each of the potential thermochromic windows at each transition temperature should then obey the following assumptions:

1 The radiation properties vary from zero to unity at an interval of 0.1. 2 Although the long-wave thermal emissivity of thermochromic windows can be regulated with a multilayered structure19,20, the basic feature of thermochromic materials is their ability to modulate solar radiation. In this study, the long-wave thermal emissivity is set at a value of 83.7% under the assumption that the thermochromic material is plated with an ordinary glazing substrate and the thermochromic window's emissivity is equal to that of a standard window. 3 In most current thermochromic windows, undesirable solar radiation is regulated through absorption by the thermochromic material21,22,23. Although it has been revealed that the high solar absorptivity in the opaque state was disadvantageous for energy efficiency5, for these thermochromic materials, the variation in the absorptivity with temperature is actually the fundamental and dominant cause of the regulation of the transmittance. Consequently, the solar absorptivity in the opaque state is greater than that in the transparent state and the solar transmittance in the opaque state is less than that in the transparent state. 4 According to the law of conservation of energy, the sum of the solar transmittance and absorptivity is not greater than unity in either state.

Consequently, at each transition temperature, there are 1716 potential types of thermochromic windows, each with a particular combination of solar transmittance and absorptivity in the transparent state and the opaque state, respectively.

The transition temperature with the greatest proportion of smart thermochromic windows

The energy consumption performance of a typical residential room containing one of the potential thermochromic windows was simulated using an energy modeling program called BuildingEnergy (see Supplementary Figure S1 online). This room has internal dimensions of 4 m × 3.3 m × 2.8 m (length × width × height). It contains a single exterior wall facing south, in the center of which there is a 1.5 m × 1.5 m single-glazed window (see Supplementary Figure S2 online). The thermochromic coating is on the outdoor surface of the glazing. The room is assumed to be in Shanghai, which is located at latitude 31N and longitude 121E and has a cooling season from June 24th to September 15th and a heating season from December 6th to March 22nd of the following year. The indoor temperature of the room is maintained with heating, ventilation and air conditioning (HVAC) facilities at 18 and 26°C in the heating and cooling seasons, respectively.

At each transition temperature, the energy consumption of each of the potential thermochromic windows during cooling and heating was calculated first. The energy consumption behavior type of each window was then labeled according to the information in Table 2. The type statistics associated with each transition temperature are shown in Figure 2. The number of smart thermochromic windows first increases and then decreases as the transition temperature decreases. At a transition temperature of 50 or 40°C, there are no smart windows and the inefficient windows represent approximately 88% of the 1716 potential thermochromic windows. The percentage of smart windows at a transition temperature of 10°C is only 0.3%. According to the definition of smart windows, the energy consumption of a smart window should be less than that of an ordinary window in both the cooling and heating seasons. Because the appropriate radiation properties for energy efficiency during cooling and heating are distinct, a state transition is necessary. Transition temperatures of 10, 40 or 50°C, however, are either too low or too high for a state transition to occur, maintaining the thermochromic window predominantly in a single state; consequently, it is non-smart. The negligible number of smart windows indicates the inappropriate nature of these transition temperatures.

Figure 2 Type statistics associated with the transition temperature. The percentages of each type are shown for different transition temperature values. Full size image

The percentage of smart thermochromic windows increases to ~5% at a transition temperature of 30°C and reaches a peak of ~11% at 20°C. The set points for the indoor temperature are 18 and 26°C for heating and cooling, respectively and the transition temperature of 20°C is within this temperature range, making it convenient for state transition between different seasons. The number of efficient windows is also the largest at a transition temperature of 20°C. As Figure 2 shows, the optimal transition temperature, at which the largest number of both smart and efficient windows exist, is 20°C. It should also be noted that the discussed transition temperatures have a relatively large interval of 10°C, which means that another transition temperature between 10 ~ 20 or 20 ~ 30°C might be even better than 20°C. Our previous study5 investigated the optimal transition temperature for an ideal thermochromic window and we found that the optimal value was 21°C, which is very close to 20°C. Consequently, a transition temperature of 20°C may be considered appropriate for this study. The following discussion will focus on the energy performance associated with a transition temperature of 20°C.

Energy Consumption Index frequency distribution

The Energy Consumption Index (ECI) was calculated to obtain the frequency distribution shown in Figure 3 for a transition temperature of 20°C. The ECI is a dimensionless parameter used to evaluate energy performance5. The ECI of a particular window is defined as the ratio of the energy consumption of the room containing the window to the corresponding value of the same room with a perfect window. The ECI of a perfect window is unity and the ECI of the standard window in this study is calculated to be 1.115 and 1.630 for the cooling and heating seasons, respectively. A higher ECI results in greater energy consumption and an ECI closer to unity produces energy performance closer to that of the perfect window.

Figure 3 Energy Consumption Index (ECI) frequency counts for the cooling season, the heating season and an entire year at a transition temperature of 20°C. Each thermochromic window has an ECI value. The ECI frequency counts represent the energy performance distribution for all of the thermochromic windows for (a) the cooling season, (b) the heating season and (c) an entire year. The ECI values of the perfect windows (PW) and the standard window (SW) are also shown. Full size image

As Figure 3 (a) shows, at a transition temperature of 20°C, many thermochromic windows have a lower ECI than that of the standard window during the cooling season. There are also many windows whose ECI is less than that of the perfect window during the summer. This phenomenon seems puzzling because the perfect window should, in theory, have the lowest ECI. According to Table 1, in summer, the perfect window can meet the demand of providing illumination with a visible transmittance of 100%. Because the visible part of the spectrum carries approximately 42.8% of the total solar energy24, the solar transmittance of the perfect window for summer can be regarded as 42.8%. However, thermochromic windows are discussed from the perspective of energy consumption for HVAC in this study and the visible transmittance is integrated into the solar transmittance. As a result, if a thermochromic window has a solar transmittance that is substantially less than 42.8% (at the expense of illumination), it is possible for its ECI to be less than that of the perfect window. The thermochromic windows whose ECIs are much less than unity are generally poor choices for natural light illumination.

The distribution of ECI frequency counts in the heating season, as shown in Figure 3 (b), is quite different from that in the cooling season. Most of the thermochromic windows have higher ECIs than the standard window and there is no ECI lower than unity. The solar transmittance of the standard window has a relatively high value of 77.1%, which means that many thermochromic windows have a lower solar transmittance. Consequently, there are only a few energy-efficient windows in the heating season. With a perfect window for winter, long-wave thermal radiation can only enter but cannot leave the room; with thermochromic windows, long-wave thermal radiation can be absorbed by the window and then emitted to the outdoor environment. Because of this difference in long-wave thermal radiation characteristics, the ECIs of the thermochromic windows are always greater than that of the perfect window during the heating season.

The total energy consumption over an entire year includes both cooling and heating requirements. Considering the difference in the coefficient of performance (COP), it is inappropriate to directly sum the energy consumption for cooling and heating. In engineering design, cooling and heating consumption are generally transformed into electric power by dividing by the COP and they can then be summed. In Shanghai, the recommended COP values for cooling and heating are 2.3 and 1.9, respectively25. The ECI of a thermochromic window over an entire year is obtained by calculating the ratio of the total energy consumption of the thermochromic window to that of the perfect window. Figure 3 (c) shows that the distribution of the annual ECI frequency is approximately normal. The ECI of the standard window is 1.384, which is greater than approximately one-third of the potential thermochromic windows, meaning that approximately one-third of the windows are efficient or smart, consistent with the results shown in Figure 2. No thermochromic windows have a lower ECI than the perfect window. However, there are a few ECIs that are very close to unity, which indicates the possible existence of thermochromic windows with excellent energy performance. These ideal windows will be discovered step by step.

Figure 3 shows that there are many more thermochromic windows that are efficient in the cooling season (~98% of all thermochromic windows) than in the heating season (~12%). This poor performance during winter leads to only ~36% of all thermochromic windows being efficient over the entire year, meaning that performance in the heating season is the critical factor for annual performance. According to the definition, smart windows should be efficient in both the cooling and heating seasons, which indicates that the number of smart windows should not exceed 12% of the total number of windows. As Figure 2 shows, the percentage is ~11%.

Effect of solar modulation ability on energy consumption

The overall energy consumption performance is shown in Figure 3 from a qualitative perspective. A more important issue now is the relationship between the radiation properties of the windows and their smartness. The solar modulation ability (Δτ solar ) is a key parameter for thermochromic material estimation and it is defined as the difference in solar transmittance between the transparent state and the opaque state. Figure 4 shows the effect of the solar modulation ability on the thermochromic windows' performance at a transition temperature of 20°C.

Figure 4 Effect of the solar modulation ability on performance at a transition temperature of 20°C. (a) The percentages of each behavior type at different solar modulation abilities. The percentages at each solar modulation ability are relative values. The absolute number of thermochromic windows decreases as the solar modulation ability increases. (b) The ECI ranges of smart and efficient windows at each solar modulation ability. Full size image

Figure 4 (a) shows the percentages of each behavior type at different solar modulation abilities. As Δτ solar increases, the percentage of inefficient windows decreases and the percentages of efficient and smart windows increase. At an appropriate transition temperature, as Δτ solar increases, the solar transmittance will decrease during the cooling season and increase during the heating season, leading to an increased possibility for a window to be smart. This tendency confirms the advantage from a smartness perspective of enhancing a window's solar modulation ability. Such enhancement is the focus of research of many materials scientists. Smart windows appear achievable when Δτ solar is greater than or equal to 10%, indicating that Δτ solar needs to be enhanced to at least approximately 10% to have the possibility of being a smart window. All of the windows are smart when Δτ solar is 90 or 100%; however, such high solar modulation ability values are obtained by having a very high solar transmittance in the transparent state and very low solar transmittance in the opaque state. In the extreme case, a Δτ solar of 100% means a solar transmittance of 100% in the transparent state and one of zero in the opaque state. Note that the ordinate in Figure 4 (a) is a percentage rather than a number. The number of thermochromic windows decreases as Δτ solar increases.

Of the three behavior types, we are most interested in the smart and efficient types and we are especially interested in the energy consumption of these types. Figure 4 (b) shows the ECI ranges at different solar modulation abilities for the smart and efficient window types. In the figure, the height represents the ECI level on the vertical axis and the length represents the range: the longer the bar, the larger the ECI range. The lowest ECI appears when Δτ solar is 100%. The bars for the smart and efficient types overlap one another except at Δτ solar levels of 0, 90 and 100%. For each Δτ solar value where overlap of the ECI bars occurs, the highest and lowest ECIs are presented by the efficient and smart windows, respectively, indicating that the smart windows provide better performance than the efficient ones under optimized cases. The overlap indicates that the ECIs of the smart windows are not necessarily lower than those of the efficient ones, which raises the questions of how to identify the behavior type of a particular thermochromic window and how to determine its specific ECI.

Evaluation of the behavior type and performance of a particular thermochromic window

An overview of the smartness and energy performance of all of the thermochromic windows at a transition temperature of 20°C was provided above. Additionally, we want to connect a particular thermochromic window with its behavior type and energy performance. It is quite a challenging task to determine a concise relationship because four parameters (the solar transmittance and absorptivity in the transparent and opaque states) are needed to characterize a particular thermochromic window in this study. It is observed from Figure 4 that the ECI is greatly affected by Δτ solar . In fact, Δτ solar contains two of the four parameters, suggesting that the remaining two parameters can similarly be integrated into one parameter. Here, we define the solar absorption variation (Δα solar ) as the difference in the solar absorptivity between a thermochromic window's two states. As shown in Figure 5 (a), with Δτ solar as the abscissa and Δα solar as the ordinate, a strong regularity is observed in the color contour between the annual ECI and the thermochromic windows' radiation properties. Each thermochromic window can be located on the contour according to its radiation properties and its ECI can then be estimated. Note that a specific combination of Δτ solar and Δα solar may represent more than one thermochromic window, especially when Δτ solar and Δα solar are small. The overlapping ECI values for identical coordinates are averaged to obtain a statistical value. The coefficient of variation, defined as the ratio of the standard deviation to the mean, can be used to measure the dispersion of ECIs with identical coordinates (see Supplementary Figure S3 online). The largest coefficient of variation for the averaged ECIs is 5.64% (with a corresponding standard deviation and mean of 0.084 and 1.489, respectively), which demonstrates the acceptability of using the averaged values.

Figure 5 ECI contour for a particular thermochromic window. (a) The annual ECI contour. (b) ECI contours for the cooling season and the heating season. Using (a), inefficient windows can be identified, whereas smart or efficient windows can be identified using (b). Full size image

As Figure 5 (a) shows, the ECI decreases as Δτ solar increases and Δα solar decreases. The lowest ECI for the entire year is obtained when Δτ solar and Δα solar are 100% and 0%, respectively. For this ideal case, the solar absorptivity remains unchanged between the window's different states, meaning that the dramatic change in the solar transmittance is achieved by varying the solar reflectance. This mechanism of changing the solar transmittance is similar to that of the perfect window, so that the ECI for this case is close to unity and can be lowered further by the long-wave thermal radiation properties. There are two special lines in Figure 5 (a): the 1.384 contour (dash-dot line) and the line on which Δτ solar is equal to Δα solar (dashed line). The former represents the annual ECI of the standard window. The ECIs to the left of the dash-dot line are greater than those of the standard window, making the thermochromic windows located in the left domain inefficient. The ECIs on the other side are lower, making the windows in the domain on the right efficient or smart. It is observed that the inefficient windows have relatively small Δτ solar values and as Δα solar increases, Δτ solar must also be increased to make the window efficient or smart. Regarding the other (dashed) line, the thermochromic windows located in the domain above the line have a greater Δα solar than Δτ solar . Because the solar transmittance in the transparent state (τ solar,t ) is higher than that in the opaque state (τ solar,o ) and the solar absorptivity in the transparent state (α solar,t ) is less than that in the opaque state (α solar,o ), the solar reflectance relationship between these two states, ρ solar,t and ρ solar,o , is determined by the relationship between Δτ solar ( = τ solar,t − τ solar,o ) and Δα solar ( = α solar,o − α solar,t ). The difference between ρ solar,t and ρ solar,o should be equal to the difference between Δα solar and Δτ solar . If Δα solar is greater than Δτ solar , then ρ solar,t will be greater than ρ solar,o . Otherwise, a higher Δτ solar means a higher ρ solar,o . A higher solar reflectance in the opaque state indicates that the reduced solar radiation transmitted into the building is partially reflected, rather than entirely absorbed, after the window transforms into its opaque state from the transparent state. The reflection of the solar radiation has a greater effect on the energy performance than the absorption due to the reduction in the transmitted solar radiation of the perfect window. Consequently, in the domain above the dashed line, the inefficient area is much larger than elsewhere and Δτ solar must be larger to make the window efficient or smart. In the domain below the dashed line, the inefficient area is much smaller. This difference between the domains separated by the dashed line emphasizes the importance of reflection in improving thermochromic materials.

The energy performance of a particular thermochromic window can be found using Figure 5 (a). However, the smartness cannot be identified completely because only inefficient windows are distinct, whereas efficient and smart windows are mixed together. As Table 2 shows, efficient and smart windows cannot be distinguished using the annual ECI because both types have better energy performance than the standard window over the entire year. According to the definitions, they must be distinguished further by their performance during the cooling and heating seasons. Similarly to Figure 5 (a), the seasonal performance is shown in Figure 5 (b) using a color contour for the ECI. The abscissa and ordinate are the solar transmittance and solar reflectance, respectively. At a transition temperature of 20°C, a window is primarily in its transparent state and its opaque state during the heating and cooling seasons, respectively, with the window radiation properties corresponding to the state in each season. The ECIs for the cooling and heating seasons are located in the lower left and upper right domains, respectively. In the cooling season, the ECI decreases as the solar transmittance decreases and the solar reflectance increases; in the heating season, the ECI decreases as the solar transmittance increases and the solar reflectance decreases. The ECIs during the cooling season are lower than those in the heating season, indicating that thermochromic windows provide better performance in the cooling season. The ECI contour lines for the standard window are shown with dash-dot lines and the contour for the perfect window for summer is shown with a dotted line. The thermochromic windows located to the left of the dash-dot lines have lower ECIs than the standard window. It is apparent that the area in which the ECIs for the cooling season are less than 1.114 is much larger than the area in which the ECIs for the heating season are lower than 1.630, meaning that thermochromic windows are more likely to be advantageous during the cooling season. A particular thermochromic window's energy performance and smartness can be estimated using Figure 5: if the window is located in the advantageous regions for both seasons in Figure 5 (b), it is a smart window; if it is located in the advantageous region for only one season in Figure 5 (b), it will be an efficient window if it is to the right of the dash-dot line in Figure 5 (a) and an inefficient window otherwise; if the window is not located in the advantageous region for either season in Figure 5 (b), it is an inefficient window. The window's ECI over an entire year can be estimated from Figure 5 (a).