ANALOG REGULATOR FOR ELECTROCHROMIC WINDOWS Janez Krč^ Marko Topič^ Franc Smolei, Urša Opara Krašovec2, Urška Lavrenčič Štangar2, Boris Orel2 1 Faculty of Electrical Engineering, University of Ljubljana, Slovenia 2National Institute of Chemistry, Ljubljana, Slovenia Keywords: chemistry, EC glasses, ElectroChromic glasses, EC windows, ElectroChromic windows, all sol-gel structures, colouring, analog three state regulators, optical transmittanoe, variable optical transmittance, bleaching Abstract: Optical transmittance of electrochromic (EC) windows can be varied by means of electrical signals, which enables a simple way to control the intensity of light transmitted through the window. For "smart" windows in buildings it is desired that the transmitted light is automatically kept on a certain level, regardless the changes in outdoors daylight illumination. Therefore we designed and built a prototype of analog three-state regulator, which automatically controls the optical transmittance of EC window - decreases, increases or keeps unchanged - maintaining the transmitted light intensity on a preselected value. In the paper, an electrical circuit and performances of the regulator are presented. Finally, demonstrationai EC glasses controlled with handy battery-powered regulator are shown. Analogni regulator za elektrokromna stekla Ključne besede: kemija, EC stekla elektrokromna, EC okna elektrokromna, aH sol-gel strukture, obarvanje, regulatorji analogni tri-stanjski. prepustnost svetlobe, prepustnost svetlobe spremenljiva, razbarvanje Povzetek: Elektrokromna (EC) stekla omogočajo, da s pomočjo električnih signalov spreminjamo njihovo svetlobno prepustnost in s tem uravnavamo jakost prepuščene svetlobe. Ko se EC stekla uporabljajo kot "inteligentna" okna v zgradbah, je zaželeno, da se jakost prepuščene svetlobe avtomatsko ohranja na izbrani vrednosti, ne glede na zunanje spremembe v dnevni osvetlitvi. V ta namen smo načrtali in izdelali prototip tri-stanjskega regulatorja, ki avtomatsko krmili prepustnost EC okna - jo zmanjšuje, povečuje ali pa ohranja nespremenjeno - tako, da se jakost prepuščene svetlobe ohranja na izbrani prednastavljeni vrednosti. V članku sta prikazani električna shema in delovanje regulatorja. Na koncu so predstavljena demonstracijska EC očala, ki jih krmili prenosni baterijsko napajani regulator. 1. Introduction Electrically controllable optical transmittance of electro-chromic (EC) windows present an advantageous feature which can be beneficially used in different light applications. One of the most promising application are "smart" buildings' windows, which in contrastto simple glass windows enable a control over the intensity of daylight, transmitted through the window, by varying their optical transmittance. Thus, in the case of obscure and cloudy days high transmittance can assure that more daylight enters the building interior, which may lead to lower lightning and heating consumption, while in case of cloudless and sunny days low transmittance can prevent excessive solar illumination and decreases the heating of the inside space, resulting in lower cooling energy consumption. In other words, a constant level of daylight inside the building, regardless the outdoors illumination level is desirable in terms of energy saving as well as building occupants' comfort. Different processing methods and types of EC devices have been investigated /1-4/ in order to produce low cost EC windows "with good performances. Publications indicated that among several processing methods a sol-gel technique exhibits many advantages over other traditional techniques /3/, The performances of the EC devices produced with the sol-gel method are also very good, therefore we based our applications on "all sol-gel" EC devices /3,4/. Their structure and some performances are briefly presented in the paper. For automatic control of the EC device a prototype of analog three-state ECW (electrochromic window) regu- lator was developed. It provides three different voltage states: negative for window colouring (towards lower transmittance), positive for window bleaching (towards higher transmittance) and open-circuit state for keeping the transmittance of EC window unchanged. The desired level of transmitted light is adjustable with a potentiometer. In this work a detailed electrical schematic of the prototype and the results of regulation are given. Finally, a demonstrationai application of EC devices -EC glasses - and corresponding handy battery-pow-ered regulator are presented. 2. "All Sol-gel" Electrochromic Device The crossection of a typical EC device is shown in Fig. 1. The basic structure - WO3 as an active layer, organically modified electrolyte (ormolyte) with Li+ ions as an ionic conductor and Lio,3CeV04 as a counter electrode - is to be found between two glass substrates covered with Sn02:F transparent conductive oxide (TCO). All three layers of the basic structure were processed by the sol-gel technique therefore the device is called "all sol-gel" EC device /3,4/. Applied voltage, U, is connected to the top and bottom TCO layers. Fig. 1 shows a situation when the value of U is negative (positive potential at top and negative potential at bottom TCO layer). In this case Li+ ions are inserted from ionic conductor to WO3 layer. Higher concentration of Li+ ions in WO3 layer results in higher colouration of the WO3 film (it becomes blue) decreasing its optical transmittance. The structure remains in coloured state, even if the voltage supply is discon- nected (memory effect) because the Li+ ionic conductor has low electronic conductivity (-10"^ S/cm). When the positive polarity of U is applied, Li+ ions are extracted from WO3 layer and transported through the ionic conductor and become inserted into the counter electrode. Since WO3 layer does not contain Li+ ions anymore, blue colour typical for tungsten bronze (MetaixWOa) is lost (bleaching effect) and becomes transparent layer. Fig. 1: Structure of the EC window The variation of the transmittance of EC device between coloured and bleached state when applied voltage is changed from -2 V to -t-2 V is shown in Fig. 2. Higher applied voltage potentials would lead to larger difference between coloured and bleached state, but should be avoided because too high intercalation levels destroy the basic structure and consequently worsen the device performance. Another limitation which affects the device durability /5/ is electrical current, I. Higher electrical current, supplied from the regulator, results in higher colouring/bleaching speed rate, but it also detrimentally influences the device stability. Therefore, for a particular EC window the optimal voltage level and current limitation should be pre-determined. For our "all sol-gel" EC devices of size 3x3 cm^ the corresponding values were found to be U = ± 2 V and Imax —±15 mA, respectively. 1.0 Fig. 2: 300 400 500 600 700 Wavelength, Ä (nm) Transmittances of the EC window in coioLired and bieached state 800 3. ECW Regulator The primary task of our prototype ECW regulator is to automatically control the transmittance of EC window to keep the transmitted light intensity on a constant preselected level during the variation of the intensity of incident light (Jph incident). Fig. Sshematicallyshowsthe complete EC system: EC device and regulator with a photodiode for detection of transmitted light intensity. J, ph If :uien( EC window // i ,/....... ^^ photodiode j)h irammined i desired value of 7. ph ircinsmiltad ECW regulator Fig. 3: Schematic view of EC system To assure simple but efficient control over the EC window colouring/bleaching changes, we designed a prototype of three-state regulator, which consists of customary analog electronics components (resistors, potentiometers, capacitors, transistors, operational amplifiers). The output signal can assume three different voltage states: negative (U = -2 V, Imax — -15 mA) for colouring, positive (U — -1-2 V, Imax — -f 15 mA) for bleaching and open-circuit state (1 = 0 mA) for stand-by position. Negative state is activated when the intensity of transmitted light exceeds the preselected value (colouring requirement), while positive state is applied in the case of too low intensity of light (bleaching requirement). When the intensity matches the preselected value, the open-circuit state appears. To avoid the continuous switching from bleaching to colouring or vice versa, which may occur if the transmitted light slightly deviates around the preselected value, we implemented a hysteresis circuits in the regulator. Therefore after the particular preselected value Jph transmitted has been achieved the minor fluctuations of the light intensity do not affect the regulator's output if relative deviations of transmitted light (AJph transmitted /Jph transmitted) are smaller than the negative (-5 %) or positive (-t-5 %) hysteresis gap (Fig. 4). Fig. 5 shows electrical circuit of the regulator. The basic sub-circuits are: photodiode, the circuit for adjusting the desired value of Jph transmitted, error circuit, low-pass RC filter, an amplifier with adjustable gain, hysteresis circuit, time limitation circuit and an output actuator. measun ■edJ, vh ira/i.wii/iul ph /IXlll.\lll!fK'(l negative U \ open circuit (colouring) I positive hysteresis gap negative : L hysteresis gap positive U (bleaching) Fig. 4: Hysteresis behaviour of the regulator Photodiode provides electrical ctjrrent, Imi which is linearly dependent on measured Jph transmitted- 'm IS connected to the same node as iadj, which can be varied by potentiometer Pia in the circuit for adjusting the desired value of Jph transmitted, thus the difference between measured (Im) and desired (Iadj) Jph transmitted can be established. The error circuits transforms Im -Iadj to voltage error signal, Uerror. If the current difference Im - Iadj is positive, negative or zero the error voltage Uerror is higher, lower or equal to Vcc/2, respectively. To smoothen the undesirable sudden short-term fluctuations of Jph transmitted, voltage, Uerror is smoothened by low-pass RC filter. Thus smoothening effect is necessary in EC devices with faster colouring/bleaching transition times (e.g. gasochromic EC devices /6,7/). The smoothened signal is then amplified by IC2A. The gain of this amplifier is controlled by potentiometer Rib, which is rotated simultaneously witli potentiometer Ria (for adjusting the desired Jph transmitted), therefore the gain is light intensity dependent. In case of low Jph transmitted the gain is set to high value, while at higher Jph transmitted amplification is attenuated. This results in relative error si gnal, Uerror rei, which means that e.g. 1 % deviation between measured and prescaled Jph transmitted cause the same Uerror rei regardless the absolute value. Such relative form of the error signal is more appropriate for regulation, because it shows significance of the error with regard to the value of primary signal. Uerror rei is connected to the positive (IC1B, Re, Ry, Di) and negative (ICic, Rs, R9, D2) part of the hysteresis circuit. In case that Uerror rei exceeds the margin of positive (negative) hysteresis - too high (low) value of measured Jph transmitted -, the output of ICib (ICtc) becomes low (high) indicating colouring (bleaching) request. If zero error is achieved (Uerror rei = 0) and afterwards Uerror rei is found to be inside the hysteresis margins, the open-circuit state is activated keeping the transmittance of EC window unchanged. Time limitation circuits restricts the duration of the active output signal (negative and positive voltage state) to the time period which is required for transition of EC device from one saturated state (full coloured state) to the other one (full bleached state). The application of the active output signal longer than the transition time period would increase power consumption but does not affect the transmittance of EC device, therefore the limitation is justified. In the circuit, maximum time duration of positive and negative output voltage is determined by (Rio + Ri2)C3 and (Rn -1- Ri3)C4, respectively. In the case of low, 0 V, (high, Vcc) output level of ICib (ICic), the capacitor C3 (C4) starts charging through resistors R10 + Ri2 (R11 + R13), respectively. As long as the adjusting the de,sired value of./,,;,,,,„„■„„„,.