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Evaluation of colour properties due to switching behaviour of a PDLC glazing for adaptive building integration
Renewable Energy 120 (2018) 126e133
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Evaluation of colour properties due to switching behaviour of a PDLC glazing for adaptive building integration A. Ghosh*, T.K. Mallick Environmental and Sustainability Institute, University of Exeter, Penryn, Cornwall, UK
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 28 December 2017
In this study, intermediate transmissions, colour-rendering index (CRI) and correlated colour temperature (CCT) of an electrically actuated, switchable polymer dispersed liquid crystal (PDLC) glazing have been investigated. This 0.03 m2 PDLC glazing changes its state from translucent to transparent in the presence of a 20 V AC power supply. Modulation of visible and NIR transmissions were observed for different applied voltages and no modulation was found in the UV range. For this particular type PDLC glazing, the CCT and CRI varied between 5430 K and 6100 K and 93 to 98, while luminous transmittance varied from 0.27 to 0.71 respectively. Low contrast ratio between the translucent and transparent states of this PDLC glazing offered a strong linear correlation between CCT and CRI. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Adaptive PDLC glazing Daylight CCT CRI Colour
1. Introduction Buildings consume 40% of the global energy for heating, cooling and lighting energy demand. This high demand is also a root cause of high CO2 emissions. In the UK, if residential houses and buildings reduce one third of their existing energy demand, then a 60% CO2 reduction is possible by 2050 [1]. To achieve sustainable environment and society, energy efficient buildings are essential. Compared to the rest of the building envelope, fenestrations get higher attention as it is multi-functional allowing incoming daylight into the building's indoor space as well as offering visual amenity, privacy, solar heat gain, heat loss, and control of light and air [2,3]. Single and double glazing are the most widely available fenestration technology for building window applications [4,5]. To mitigate glare issues due to excessive daylight, shading devices are often used [6]. However, these have limitations due to cleaning and maintenance. Advanced smart glazing is a preferable replacement to single or double glazing as it possesses higher energy efficiency and is also aesthetically suitable [7,8]. These smart glazing systems include non-switchable types (static or constant transparency) such as aerogel [9], vacuum [10e12], and photovoltaic [13] and switchable types, which include thermally actuated phase-change materials [14], thermochromic [15], thermotropic [16], hydrogen actuated gasochromic [17], electrically
* Corresponding author. E-mail address: [email protected] (A. Ghosh). https://doi.org/10.1016/j.renene.2017.12.094 0960-1481/© 2017 Elsevier Ltd. All rights reserved.
actuated electrochromic (EC) [18,19], liquid crystal (LC) [20] and suspended particle devices (SPD) [21,22]. Except constant transparency vacuum (evacuated) glazing [12], other aforementioned non-switchable and switchable glazing systems have the potential to allow comfortable daylight and glare control for indoor spaces. In general, buildings experience dynamic weather throughout both the year and the course of a day. Diurnal variation of ambient temperature and solar radiation causes changes to the indoor temperature and light levels [23]. Alleviation of these changes by using constant transparency smart glazing is not feasible. For buildings, adaptive glazing is required so that transparency can be altered in response to the internal environment and transient external conditions [23]. Thus, smart switchable glazing systems are gaining priority over non-switchable glazing. Switchable glazing offers more than one transparency level and intermediate transmission states are possible. However, controlling of the intermediate transmission states of non-electrically switchable glazing requires complex processes. Thus, electrically actuated glazing can be more attractive than non-electrically actuated switchable glazing as they offer easily accessible user control. EC glazing systems are activated by low (0e5 V) DC power supply [24,25]. It consumes power to become coloured and becomes opaque both in the absence of and in reversing the power supply. Direct coupling with photovoltaic (PV) devices is possible as EC glazing systems are actuated by direct current (DC) supplies and PV generates DC power [26e29]. In addition with vacuum glazing, it offers switchable-low heat-loss glazing [30e34]. Control of the NIR solar spectrum [35,36] and intermediate transmission [37,38]
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also makes it a viable glazing material for low-energy building applications. Low-cost manufacturing [18,39,40], large durability [41], large-scale (higher than 1 m2) application [42], low-voltage requirement at higher-surface temperature [43,44] are all added advantages of EC glazing. However, EC glazing needs power converter to connect with mains supply. SPD glazing is made using dihydrocinchonidine bisulfite polyiodide or herapathite-type particles suspended in a plastic film [45]. In the presence of a high, 100e110 V AC power supply, this SPD glazing becomes transparent. Absence of a power supply renders this glazing opaque [46]. SPD glazing integration needs an inverter to connect it with a PV [47]. The high contrast ratio between the opaque and transparent states [48], high durability [45], simpler connection with household mains [46] and low heat-loss switchable potential makes this glazing a potential candidate [23,49] for low-energy building applications. SPD glazing has a high NIR transmission [22], high-cost and high-voltage requirement to operate. In liquid crystal (LC) glazing, LC films are sandwiched between two glass panes. The LC materials can be twisted nematic, ferroelectric, guest host, and polymer dispersed liquid crystal (PDLC) types [50e53]. Since they do not require polarizers to operate, PDLCs are the most suited for glazing applications [54]. For PDLC type glazing, micron sized liquid crystal droplets are contained in a polymer matrix. In the presence of an AC power supply, particle orientation becomes aligned, parallel to the applied electric field and they admit light [20]. In the absence of a power supply, the LC particles scatter the light as shown in Fig. 1. The power supply required to make PDLC glazing transparent depends on LC particle size, shape, dielectric and conductive anisotropy, molecular weight and chemical nature of the polymer and anchoring effects at the polymer boundary [50,51,55]. PDLC glazing having reverse mode operation transparent OFF state and opaque ON state, was also investigated [51]. Daylight characterisation using PDLC glazing was investigated using outdoor test cell for Dublin weather condition was investigated [20]. This particular type of PDLC glazing had 71% transmission in the transparent state 27% transmission in the translucent state. Presence of high 82% haze of this PDLC glazing allowed higher daylight transmission in the translucent state [56]. However, for clear sunny day, this PDLC glazing was not able to control glare. For intermittent cloudy and overcast cloudy day PDLC glazing performance was acceptable to control glare. To generate haze free and low driving voltage PDLC glazing, polymer was replaced by glass material. This type of LC glazing was introduced as glass dispersed liquid crystal (GDLC). One 5 5 cm2 GDLC device was fabricated which offered 10 times lower switching time, 85% low voltage (15 V) was required
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to achieve 80% transmission compared to similar type PDLC [57]. Daylight is classified as solar irradiance with a spectral power distribution in the visible range (380e780 nm), which depends on time of day, season, latitude, weather, dust and pollutants. Filtered daylight into an indoor space due to a PDLC window can affect the color rendering of interior objects. As PDLC glazing is potentially considered for adaptive, low-energy building envelopes, it should not distort the daylight spectrum significantly. Thus, investigation is required before marketing this glazing for building integration. Correlated color temperature (CCT) and colour rendering index (CRI) are the colour properties which quantify the quality of daylight [58]. In this work, intermediate transparency levels for 0.03 m2 of PDLC glazing in the solar, visible and UV range were investigated. Solar factors for different solar transmission levels were calculated. Colour properties (CCT and CRI) of this PDLC glazing for its different visible (luminous) transmission states were also calculated to find out its suitability for adaptive building window integration. CCT and CRI results were also compared with those of an SPD glazing, vacuum glazing and air-filled double-glazing. Building engineers and architects can use these results to design an aesthetic, adaptive-low-energy building with PDLC glazing or façade. 2. Methodology Building occupants spend a considerable amount of time at indoor workplaces and at home. However, they prefer daylight than artificial light in an indoor environment. Daylight has been shown to increase productivity at work, improve well-being and offer protection from health issues [59]. Thus, it is desirable, particularly for temperate and higher latitude countries that windows should have good-quality view and allow natural daylight to enter providing the primary source of light for more healthy indoor space environment. The quality and quantity of daylight should be controlled to ensure the physical and mental well-being of building occupants [59]. The transmitted daylight properties of a glazing are characterized by a correlated colour temperature (CCT) and colour-rendering index (CRI). The CCT of the sky varies mostly between 6000 K (overcast sky) and 10,000 K (light blue sky) [60]. The color temperature of dark blue sky can be higher than 20,000 K. A CCT needs to be equivalent to that of a blackbody source at temperatures between 3000 and 7500 K. CRI values of 90 or higher are considered acceptable whereas close to 100 indicates an excellent visual quality [61,62]. Good color rendering indicates no significant perceived color difference between objects illuminated by daylight and by the same illuminant transmitted through a glazing [63e65].
Fig. 1. Schematic presentation of PDLC glazing for switched-on transparent state and switched-off translucent state.
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X, Y and Z are the tristimulus values which represent the threecolor perception values of the human eye response, Luminous transmittance values tv, D65(l) is the spectral power distribution of CIE standard illuminant D65, V(l) is the photopic luminous efficiency function of the human eye and Dl ¼ 10 nm. CRI is given by Ref. [68].
CRI ¼ Fig. 2. Photographs of PDLC glazing for switched-off, “translucent” and switched-on, “transparent” states.
" 8 1X 100 8 i¼1 (rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi )# 2 2 2 U *t;i U *r;i þ V *t;i V *r;i þ W *t;i W *r;i 4:6 (2)
Good quality of light in an indoor space is an essential feature for health, aesthetic taste and interpersonal relationships of its occupants. Thus indoor comfort of a building depends on light colour [66]. CCT was calculated using McCamy's equation (1) [67].
CCT ¼ 449n3 þ 3525n2 þ 6823:3n þ 5520:33 where ðx0:3320Þ n ¼ ð0:1858yÞ X Y x ¼ XþYþZ y ¼ XþYþZ P780nm X ¼ 380nm D65 ðlÞ tðlÞ xðlÞ Dl P Y ¼ 780nm 380nm D65 ðlÞ tðlÞ yðlÞ Dl P780nm Z ¼ 380nm D65 ðlÞ tðlÞ zðlÞ Dl
(1)
Conversion into the CIE 1964 uniform colour space system for each test colours the conversion is performed using colour space * , U * , V * whereas W * , U * , V * represents for each test system Wt;i t;i t;i r;i r;i r;i colours, lighted by the standard illuminant D65 without the glazing
100Yt;i 1=3 * Wt;i ¼ 25 17 Yt
(3)
* * u0t;i 0:1978 Ut;i ¼ 13Wt;i
(4)
* * 0 Vt;i ¼ 13Wt;i 0:3122 Vt;i
(5)
Fig. 3. Electrical connection to obtain variable voltage across PDLC glazing and an internal schematic of the double beam type Perkin Elmer® Lambda 1050 UV/vis/NIR spectrophotometer.
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Fig. 4. Variable solar transmission of PDLC glazing for a range of applied voltages.
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Fig. 5. Variable luminous transmission of PDLC glazing for a range of applied voltages.
3. Experiment PDLC glazing of 0.2 m 0.15 m and thickness of 0.01 m (glass thickness 0.004 m each and PDLC film 0.002 m thick) from Polytron technology was employed for investigation. It requires 20 V AC to become transparent and becomes translucent in the absence of a power supply. Fig. 2 shows the switched-on, transparent and switched-off, translucent states of the PDLC glazing. Intermediate UV, visible and solar transparencies were measured for this PDLC glazing using a spectrophotometer. To obtain variable transmission, a variable voltage using a VARIAC was applied to the PDLC glazing and the experimental set up is shown in Fig. 3. The VARIAC is a variable transformer that provides variable AC voltage. Spectral transmittance measurements have been conducted with a double beam type Perkin Elmer® Lambda 1050 UV/ vis/NIR spectrophotometer. Measurements were taken at five different positions on the glazing sample and average transmittance was calculated to obviate measurement error and to obtain accurate values [69,70].
Fig. 6. Correlation between PDLC glazing solar and luminous transmission and variable applied AC voltage.
4. Results & discussions 4.1. Voltage dependent variable transmissions Fig. 4 illustrates the variable spectral solar transmission and Fig. 5 illustrates the variable spectral luminous transmission of the PDLC glazing for different applied voltages. In the 300 nme2500 nm range, variation of the voltage source from 0 V to 20 V caused changes in the PDLC glazing solar transparency from 23% to 50%. From 0 to 10 V applied voltage, modulation of optical transmission in the range between 300 nm and 2500 nm, was much more significant than that of the 15 Ve20 V range. Luminous transmission (380 nm-780 nm) changed from 27% to 71% due to applied 0 Ve20 V supply. PDLC glazing offered similar modulation at visible and NIR wavelengths. Maximum transparency of this PDLC glazing was 34.9% at 0 V, which gradually increased to 82.55% at 20 V for a wavelength of 550 nm. Fig. 6 shows the wavelength-integrated solar and luminous transmittance as a function of voltage and indicates that the transmittance ratio between 0 V and 20 V for luminous and solar transmittance were 1: 2.6 and 1: 2.1 respectively. Optical modulation was insignificant in the UV range for a variable applied voltage.
The voltage dependence of the solar factor (SF) has been shown in Fig. 7. The SF was calculated using equation (6). The solar factor
Fig. 7. Different solar factors of PDLC glazing for variable applied voltages.
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(SF) or solar heat gain coefficient indicates the transmitted solar energy through glazing [56].
SFðSHGCÞ ¼ ts þ qi ¼ ts þ a ¼ ts þ ð1 ts rs Þ
hi hi þ he
hi hi þ he
(6)
Where he and hi are the external and internal heat transfer coefficient,ts is solar transmittance and rs is solar reflectance [56]. The SF at 20 V was 0.53 and at 0 V was 0.39. Variable solar transmission offered a variable SF, which has the added advantage of being electrically activated switchable glazing. Intermediate SF is achievable by using an electrical switch, which makes it more feasible for low-energy new or retrofit building integration. 4.2. 4.2CCT and CRI for switchable PDLC glazing Fig. 8 shows the relationship between CCT, CRI with luminous transmittance and the correlation between CCT and CRI. Daylight
filtered by switchable PDLC glazing is characterized by its CCT and CRI. The CCT and CRI values were calculated by applying the CIE procedure. Increase luminous transmission offered higher CCT and CRI. For this PDLC glazing with a transmission between 27% and 71%, the CRI was always higher than 90. Thus, this PDLC glazing, even in translucent states, offered good color rendering. A strong linear correlation was found between CCT and CRI for this PDLC glazing. Low contrast ratio of PDLC glazing between translucent and transparent states offered strong correlation between the CCT and CRI. Previous investigation using high contrast ratio (1:11) switchable SPD [58], EC [37] and gasochromic [64], showed no correlation between CCT and CRI. 4.3. CCT and CRI for PDLC and SPD The CCT and CRI of PDLC glazing was compared with those of an AC-powered, switchable, suspended particle device (SPD) glazing as shown in Fig. 9. Transmission data for the SPD glazing was collected from our previous work [47,58]. As both glazing types are AC-powered, electrically activated, switchable glazing the
Fig. 8. (a) Relations between luminous transmittance (tvis), CCT, and general CRI of the light filtered through PDLC switchable units. (b) Correlation between CCT and CRI values of PDLC glazing.
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Fig. 9. (a) Luminous transmission (b) CRI and CCT of 52% transparent PDLC and 55% transparent SPD glazing.
differences between the CCTs and CRIs for similar transmission states were investigated. To achieve 52% transmission, the PDLC glazing requires 5 V whereas for SPD glazing the voltage requirement was 110 V. For 52% transparent PDLC, the CCT and CRI were 6989 K and 97.5 whereas for 55% transparent SPD the values were 5762 K and 95.5 respectively. For a 3% change in transmission of light, CCT changed 17% and CRI changed 2%. PDLC glazing has an advantage in that it requires a low-voltage supply, to achieve higher CCT and CRI compared to the SPD glazing, which requires a mains supply. It can also be concluded that CCT and CRI are both spectrally dependent characteristics and less dependent on the single luminous transmittance value. 4.4. CCT and CRI for PDLC and double glazing filled with vacuum and air
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Fig. 10. (a) Luminous transmission (b) CRI and CCT of 71% transparent PDLC, 72% transparent vacuum glazing and 77% transparent air-filled double-glazing.
ranging between 0 and 20 V, solar transmission of PDLC glazing varied from 23% to 50% and luminous transmission changed from 27% to 71%. No modulation was found due to voltage variation in the UV range. Variation of solar transmission from 23% to 50% causes a variation in the solar factor from 0.39 to 0.53. The CRI of this PDLC glazing for its translucent and transparent states was always higher than 90, which indicates a comfortable light level for an indoor space. A strong linear correlation was found between CCT and CRI for this PDLC glazing. A comparison between 52% transparent PDLC and 55% transparent SPD showed that PDLC glazing offers similar CRI and CCT with a lower applied voltage. A comparison was also conducted between 72% transparent vacuum glazing, 77% transparent double-glazing and 71% transparent PDLC glazing. The similar CCT and CRI for these three glazing types indicates that PDLC glazing shows promise since its transparent state offers daylighting similar to that of vacuum and double glazing. In addition, variable transmission enhances the suitability for integration of this glazing type into adaptive, low-energy buildings.
The CCT and CRI of this PDLC glazing were also compared to that of vacuum and air-filled, double-glazing. Details of these two glazing types can be found in Ref. [12]. Fig. 10 shows the luminous spectrum for a 72% transparent vacuum glazing, a 77% transparent double -glazing and 71% transparent PDLC glazing. The CRIs for the PDLC, vacuum and air-filled double glazings were 97.6, 97.7 and 98.7 and the CCTs for PDLC, vacuum, double-glazing were 6089 K, 6532 K and 6545 K respectively as shown in Fig. 10. The transparent state colour render and CCT of the PDLC glazing matches closely to that of both the vacuum and air-filled double-glazings.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. The author is thankful to the respected reviewers of this paper for their valuable comments and suggestions. The author would like to thank Alice Perrett for very useful discussions.
5. Conclusions
References
Voltage dependent transmission, correlated color temperature (CCT) and colour rendering index (CRI) of 0.03 m2 PDLC glazing from Polytron technology were investigated for its transparent, translucent and intermediate states. For an AC power supply
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Evaluation of colour properties due to switching behaviour of a PDLC glazing for adaptive building integration A. Ghosh*, T.K. Mallick Environmental and Sustainability Institute, University of Exeter, Penryn, Cornwall, UK
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 28 December 2017
In this study, intermediate transmissions, colour-rendering index (CRI) and correlated colour temperature (CCT) of an electrically actuated, switchable polymer dispersed liquid crystal (PDLC) glazing have been investigated. This 0.03 m2 PDLC glazing changes its state from translucent to transparent in the presence of a 20 V AC power supply. Modulation of visible and NIR transmissions were observed for different applied voltages and no modulation was found in the UV range. For this particular type PDLC glazing, the CCT and CRI varied between 5430 K and 6100 K and 93 to 98, while luminous transmittance varied from 0.27 to 0.71 respectively. Low contrast ratio between the translucent and transparent states of this PDLC glazing offered a strong linear correlation between CCT and CRI. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Adaptive PDLC glazing Daylight CCT CRI Colour
1. Introduction Buildings consume 40% of the global energy for heating, cooling and lighting energy demand. This high demand is also a root cause of high CO2 emissions. In the UK, if residential houses and buildings reduce one third of their existing energy demand, then a 60% CO2 reduction is possible by 2050 [1]. To achieve sustainable environment and society, energy efficient buildings are essential. Compared to the rest of the building envelope, fenestrations get higher attention as it is multi-functional allowing incoming daylight into the building's indoor space as well as offering visual amenity, privacy, solar heat gain, heat loss, and control of light and air [2,3]. Single and double glazing are the most widely available fenestration technology for building window applications [4,5]. To mitigate glare issues due to excessive daylight, shading devices are often used [6]. However, these have limitations due to cleaning and maintenance. Advanced smart glazing is a preferable replacement to single or double glazing as it possesses higher energy efficiency and is also aesthetically suitable [7,8]. These smart glazing systems include non-switchable types (static or constant transparency) such as aerogel [9], vacuum [10e12], and photovoltaic [13] and switchable types, which include thermally actuated phase-change materials [14], thermochromic [15], thermotropic [16], hydrogen actuated gasochromic [17], electrically
* Corresponding author. E-mail address: [email protected] (A. Ghosh). https://doi.org/10.1016/j.renene.2017.12.094 0960-1481/© 2017 Elsevier Ltd. All rights reserved.
actuated electrochromic (EC) [18,19], liquid crystal (LC) [20] and suspended particle devices (SPD) [21,22]. Except constant transparency vacuum (evacuated) glazing [12], other aforementioned non-switchable and switchable glazing systems have the potential to allow comfortable daylight and glare control for indoor spaces. In general, buildings experience dynamic weather throughout both the year and the course of a day. Diurnal variation of ambient temperature and solar radiation causes changes to the indoor temperature and light levels [23]. Alleviation of these changes by using constant transparency smart glazing is not feasible. For buildings, adaptive glazing is required so that transparency can be altered in response to the internal environment and transient external conditions [23]. Thus, smart switchable glazing systems are gaining priority over non-switchable glazing. Switchable glazing offers more than one transparency level and intermediate transmission states are possible. However, controlling of the intermediate transmission states of non-electrically switchable glazing requires complex processes. Thus, electrically actuated glazing can be more attractive than non-electrically actuated switchable glazing as they offer easily accessible user control. EC glazing systems are activated by low (0e5 V) DC power supply [24,25]. It consumes power to become coloured and becomes opaque both in the absence of and in reversing the power supply. Direct coupling with photovoltaic (PV) devices is possible as EC glazing systems are actuated by direct current (DC) supplies and PV generates DC power [26e29]. In addition with vacuum glazing, it offers switchable-low heat-loss glazing [30e34]. Control of the NIR solar spectrum [35,36] and intermediate transmission [37,38]
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also makes it a viable glazing material for low-energy building applications. Low-cost manufacturing [18,39,40], large durability [41], large-scale (higher than 1 m2) application [42], low-voltage requirement at higher-surface temperature [43,44] are all added advantages of EC glazing. However, EC glazing needs power converter to connect with mains supply. SPD glazing is made using dihydrocinchonidine bisulfite polyiodide or herapathite-type particles suspended in a plastic film [45]. In the presence of a high, 100e110 V AC power supply, this SPD glazing becomes transparent. Absence of a power supply renders this glazing opaque [46]. SPD glazing integration needs an inverter to connect it with a PV [47]. The high contrast ratio between the opaque and transparent states [48], high durability [45], simpler connection with household mains [46] and low heat-loss switchable potential makes this glazing a potential candidate [23,49] for low-energy building applications. SPD glazing has a high NIR transmission [22], high-cost and high-voltage requirement to operate. In liquid crystal (LC) glazing, LC films are sandwiched between two glass panes. The LC materials can be twisted nematic, ferroelectric, guest host, and polymer dispersed liquid crystal (PDLC) types [50e53]. Since they do not require polarizers to operate, PDLCs are the most suited for glazing applications [54]. For PDLC type glazing, micron sized liquid crystal droplets are contained in a polymer matrix. In the presence of an AC power supply, particle orientation becomes aligned, parallel to the applied electric field and they admit light [20]. In the absence of a power supply, the LC particles scatter the light as shown in Fig. 1. The power supply required to make PDLC glazing transparent depends on LC particle size, shape, dielectric and conductive anisotropy, molecular weight and chemical nature of the polymer and anchoring effects at the polymer boundary [50,51,55]. PDLC glazing having reverse mode operation transparent OFF state and opaque ON state, was also investigated [51]. Daylight characterisation using PDLC glazing was investigated using outdoor test cell for Dublin weather condition was investigated [20]. This particular type of PDLC glazing had 71% transmission in the transparent state 27% transmission in the translucent state. Presence of high 82% haze of this PDLC glazing allowed higher daylight transmission in the translucent state [56]. However, for clear sunny day, this PDLC glazing was not able to control glare. For intermittent cloudy and overcast cloudy day PDLC glazing performance was acceptable to control glare. To generate haze free and low driving voltage PDLC glazing, polymer was replaced by glass material. This type of LC glazing was introduced as glass dispersed liquid crystal (GDLC). One 5 5 cm2 GDLC device was fabricated which offered 10 times lower switching time, 85% low voltage (15 V) was required
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to achieve 80% transmission compared to similar type PDLC [57]. Daylight is classified as solar irradiance with a spectral power distribution in the visible range (380e780 nm), which depends on time of day, season, latitude, weather, dust and pollutants. Filtered daylight into an indoor space due to a PDLC window can affect the color rendering of interior objects. As PDLC glazing is potentially considered for adaptive, low-energy building envelopes, it should not distort the daylight spectrum significantly. Thus, investigation is required before marketing this glazing for building integration. Correlated color temperature (CCT) and colour rendering index (CRI) are the colour properties which quantify the quality of daylight [58]. In this work, intermediate transparency levels for 0.03 m2 of PDLC glazing in the solar, visible and UV range were investigated. Solar factors for different solar transmission levels were calculated. Colour properties (CCT and CRI) of this PDLC glazing for its different visible (luminous) transmission states were also calculated to find out its suitability for adaptive building window integration. CCT and CRI results were also compared with those of an SPD glazing, vacuum glazing and air-filled double-glazing. Building engineers and architects can use these results to design an aesthetic, adaptive-low-energy building with PDLC glazing or façade. 2. Methodology Building occupants spend a considerable amount of time at indoor workplaces and at home. However, they prefer daylight than artificial light in an indoor environment. Daylight has been shown to increase productivity at work, improve well-being and offer protection from health issues [59]. Thus, it is desirable, particularly for temperate and higher latitude countries that windows should have good-quality view and allow natural daylight to enter providing the primary source of light for more healthy indoor space environment. The quality and quantity of daylight should be controlled to ensure the physical and mental well-being of building occupants [59]. The transmitted daylight properties of a glazing are characterized by a correlated colour temperature (CCT) and colour-rendering index (CRI). The CCT of the sky varies mostly between 6000 K (overcast sky) and 10,000 K (light blue sky) [60]. The color temperature of dark blue sky can be higher than 20,000 K. A CCT needs to be equivalent to that of a blackbody source at temperatures between 3000 and 7500 K. CRI values of 90 or higher are considered acceptable whereas close to 100 indicates an excellent visual quality [61,62]. Good color rendering indicates no significant perceived color difference between objects illuminated by daylight and by the same illuminant transmitted through a glazing [63e65].
Fig. 1. Schematic presentation of PDLC glazing for switched-on transparent state and switched-off translucent state.
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X, Y and Z are the tristimulus values which represent the threecolor perception values of the human eye response, Luminous transmittance values tv, D65(l) is the spectral power distribution of CIE standard illuminant D65, V(l) is the photopic luminous efficiency function of the human eye and Dl ¼ 10 nm. CRI is given by Ref. [68].
CRI ¼ Fig. 2. Photographs of PDLC glazing for switched-off, “translucent” and switched-on, “transparent” states.
" 8 1X 100 8 i¼1 (rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi )# 2 2 2 U *t;i U *r;i þ V *t;i V *r;i þ W *t;i W *r;i 4:6 (2)
Good quality of light in an indoor space is an essential feature for health, aesthetic taste and interpersonal relationships of its occupants. Thus indoor comfort of a building depends on light colour [66]. CCT was calculated using McCamy's equation (1) [67].
CCT ¼ 449n3 þ 3525n2 þ 6823:3n þ 5520:33 where ðx0:3320Þ n ¼ ð0:1858yÞ X Y x ¼ XþYþZ y ¼ XþYþZ P780nm X ¼ 380nm D65 ðlÞ tðlÞ xðlÞ Dl P Y ¼ 780nm 380nm D65 ðlÞ tðlÞ yðlÞ Dl P780nm Z ¼ 380nm D65 ðlÞ tðlÞ zðlÞ Dl
(1)
Conversion into the CIE 1964 uniform colour space system for each test colours the conversion is performed using colour space * , U * , V * whereas W * , U * , V * represents for each test system Wt;i t;i t;i r;i r;i r;i colours, lighted by the standard illuminant D65 without the glazing
100Yt;i 1=3 * Wt;i ¼ 25 17 Yt
(3)
* * u0t;i 0:1978 Ut;i ¼ 13Wt;i
(4)
* * 0 Vt;i ¼ 13Wt;i 0:3122 Vt;i
(5)
Fig. 3. Electrical connection to obtain variable voltage across PDLC glazing and an internal schematic of the double beam type Perkin Elmer® Lambda 1050 UV/vis/NIR spectrophotometer.
A. Ghosh, T.K. Mallick / Renewable Energy 120 (2018) 126e133
Fig. 4. Variable solar transmission of PDLC glazing for a range of applied voltages.
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Fig. 5. Variable luminous transmission of PDLC glazing for a range of applied voltages.
3. Experiment PDLC glazing of 0.2 m 0.15 m and thickness of 0.01 m (glass thickness 0.004 m each and PDLC film 0.002 m thick) from Polytron technology was employed for investigation. It requires 20 V AC to become transparent and becomes translucent in the absence of a power supply. Fig. 2 shows the switched-on, transparent and switched-off, translucent states of the PDLC glazing. Intermediate UV, visible and solar transparencies were measured for this PDLC glazing using a spectrophotometer. To obtain variable transmission, a variable voltage using a VARIAC was applied to the PDLC glazing and the experimental set up is shown in Fig. 3. The VARIAC is a variable transformer that provides variable AC voltage. Spectral transmittance measurements have been conducted with a double beam type Perkin Elmer® Lambda 1050 UV/ vis/NIR spectrophotometer. Measurements were taken at five different positions on the glazing sample and average transmittance was calculated to obviate measurement error and to obtain accurate values [69,70].
Fig. 6. Correlation between PDLC glazing solar and luminous transmission and variable applied AC voltage.
4. Results & discussions 4.1. Voltage dependent variable transmissions Fig. 4 illustrates the variable spectral solar transmission and Fig. 5 illustrates the variable spectral luminous transmission of the PDLC glazing for different applied voltages. In the 300 nme2500 nm range, variation of the voltage source from 0 V to 20 V caused changes in the PDLC glazing solar transparency from 23% to 50%. From 0 to 10 V applied voltage, modulation of optical transmission in the range between 300 nm and 2500 nm, was much more significant than that of the 15 Ve20 V range. Luminous transmission (380 nm-780 nm) changed from 27% to 71% due to applied 0 Ve20 V supply. PDLC glazing offered similar modulation at visible and NIR wavelengths. Maximum transparency of this PDLC glazing was 34.9% at 0 V, which gradually increased to 82.55% at 20 V for a wavelength of 550 nm. Fig. 6 shows the wavelength-integrated solar and luminous transmittance as a function of voltage and indicates that the transmittance ratio between 0 V and 20 V for luminous and solar transmittance were 1: 2.6 and 1: 2.1 respectively. Optical modulation was insignificant in the UV range for a variable applied voltage.
The voltage dependence of the solar factor (SF) has been shown in Fig. 7. The SF was calculated using equation (6). The solar factor
Fig. 7. Different solar factors of PDLC glazing for variable applied voltages.
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(SF) or solar heat gain coefficient indicates the transmitted solar energy through glazing [56].
SFðSHGCÞ ¼ ts þ qi ¼ ts þ a ¼ ts þ ð1 ts rs Þ
hi hi þ he
hi hi þ he
(6)
Where he and hi are the external and internal heat transfer coefficient,ts is solar transmittance and rs is solar reflectance [56]. The SF at 20 V was 0.53 and at 0 V was 0.39. Variable solar transmission offered a variable SF, which has the added advantage of being electrically activated switchable glazing. Intermediate SF is achievable by using an electrical switch, which makes it more feasible for low-energy new or retrofit building integration. 4.2. 4.2CCT and CRI for switchable PDLC glazing Fig. 8 shows the relationship between CCT, CRI with luminous transmittance and the correlation between CCT and CRI. Daylight
filtered by switchable PDLC glazing is characterized by its CCT and CRI. The CCT and CRI values were calculated by applying the CIE procedure. Increase luminous transmission offered higher CCT and CRI. For this PDLC glazing with a transmission between 27% and 71%, the CRI was always higher than 90. Thus, this PDLC glazing, even in translucent states, offered good color rendering. A strong linear correlation was found between CCT and CRI for this PDLC glazing. Low contrast ratio of PDLC glazing between translucent and transparent states offered strong correlation between the CCT and CRI. Previous investigation using high contrast ratio (1:11) switchable SPD [58], EC [37] and gasochromic [64], showed no correlation between CCT and CRI. 4.3. CCT and CRI for PDLC and SPD The CCT and CRI of PDLC glazing was compared with those of an AC-powered, switchable, suspended particle device (SPD) glazing as shown in Fig. 9. Transmission data for the SPD glazing was collected from our previous work [47,58]. As both glazing types are AC-powered, electrically activated, switchable glazing the
Fig. 8. (a) Relations between luminous transmittance (tvis), CCT, and general CRI of the light filtered through PDLC switchable units. (b) Correlation between CCT and CRI values of PDLC glazing.
A. Ghosh, T.K. Mallick / Renewable Energy 120 (2018) 126e133
Fig. 9. (a) Luminous transmission (b) CRI and CCT of 52% transparent PDLC and 55% transparent SPD glazing.
differences between the CCTs and CRIs for similar transmission states were investigated. To achieve 52% transmission, the PDLC glazing requires 5 V whereas for SPD glazing the voltage requirement was 110 V. For 52% transparent PDLC, the CCT and CRI were 6989 K and 97.5 whereas for 55% transparent SPD the values were 5762 K and 95.5 respectively. For a 3% change in transmission of light, CCT changed 17% and CRI changed 2%. PDLC glazing has an advantage in that it requires a low-voltage supply, to achieve higher CCT and CRI compared to the SPD glazing, which requires a mains supply. It can also be concluded that CCT and CRI are both spectrally dependent characteristics and less dependent on the single luminous transmittance value. 4.4. CCT and CRI for PDLC and double glazing filled with vacuum and air
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Fig. 10. (a) Luminous transmission (b) CRI and CCT of 71% transparent PDLC, 72% transparent vacuum glazing and 77% transparent air-filled double-glazing.
ranging between 0 and 20 V, solar transmission of PDLC glazing varied from 23% to 50% and luminous transmission changed from 27% to 71%. No modulation was found due to voltage variation in the UV range. Variation of solar transmission from 23% to 50% causes a variation in the solar factor from 0.39 to 0.53. The CRI of this PDLC glazing for its translucent and transparent states was always higher than 90, which indicates a comfortable light level for an indoor space. A strong linear correlation was found between CCT and CRI for this PDLC glazing. A comparison between 52% transparent PDLC and 55% transparent SPD showed that PDLC glazing offers similar CRI and CCT with a lower applied voltage. A comparison was also conducted between 72% transparent vacuum glazing, 77% transparent double-glazing and 71% transparent PDLC glazing. The similar CCT and CRI for these three glazing types indicates that PDLC glazing shows promise since its transparent state offers daylighting similar to that of vacuum and double glazing. In addition, variable transmission enhances the suitability for integration of this glazing type into adaptive, low-energy buildings.
The CCT and CRI of this PDLC glazing were also compared to that of vacuum and air-filled, double-glazing. Details of these two glazing types can be found in Ref. [12]. Fig. 10 shows the luminous spectrum for a 72% transparent vacuum glazing, a 77% transparent double -glazing and 71% transparent PDLC glazing. The CRIs for the PDLC, vacuum and air-filled double glazings were 97.6, 97.7 and 98.7 and the CCTs for PDLC, vacuum, double-glazing were 6089 K, 6532 K and 6545 K respectively as shown in Fig. 10. The transparent state colour render and CCT of the PDLC glazing matches closely to that of both the vacuum and air-filled double-glazings.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. The author is thankful to the respected reviewers of this paper for their valuable comments and suggestions. The author would like to thank Alice Perrett for very useful discussions.
5. Conclusions
References
Voltage dependent transmission, correlated color temperature (CCT) and colour rendering index (CRI) of 0.03 m2 PDLC glazing from Polytron technology were investigated for its transparent, translucent and intermediate states. For an AC power supply
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