Angle-resolved optical characterisation of an electrochromic device

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Solar Energy Vol. 68, No. 6, pp. 493–497, 2000  2000 Elsevier Science Ltd S 0 0 3 8 – 0 9 2 X ( 0 0 ) 0 0 0 2 1 – 9 All rights reserved. Printed in Great Britain 0038-092X / 00 / $ - see front matter

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ANGLE-RESOLVED OPTICAL CHARACTERISATION OF AN ELECTROCHROMIC DEVICE JOAKIM KARLSSON† and ARNE ROOS

˚ ¨ Laboratory, Uppsala University, P.O. Box 534, S-751 21 Department of Materials Science, The Angstrom Uppsala, Sweden Communicated by VOLKER WITTWER

Abstract—An electrochromic prototype with WO 3 and NiO as electrochromic layers was analysed in an absolute spectrophotometer. The electrochromic glazing was measured in combination with a clear float glass and a low-e glass in order to simulate a ‘real’ window. Similar measurements were performed on a commercial electrochromic product, i.e., a Gentex Night Vision SafetyE (NVS  ) mirror from Gentex Corporation, and the results were compared. The spectral transmittance was measured, in bleached and coloured state, over the solar wavelength range at the angles of incidence, f 5 0, 40, 60 and 708. The direct solar transmittance, T sol , the visual transmittance, T vis , and the angular dependence for these parameters were calculated.  2000 Elsevier Science Ltd. All rights reserved.

group showed that both when assessing energy savings and visual quality, there is room for improvement of the electrochromic products available and that sophisticated control systems are required to maximise the benefit. Furthermore, they showed that an electrochromic window with a high reflectance in the coloured state performs the best. It may also be a question if occupants will accept low transmission states because of reduced exterior view quality. Considering the complex nature of the energy and daylight simulation, it is necessary to have reliable input data for the optical properties of the windows. In particular, it is necessary to have optical data for variable angles of incidence. The most typical angle of incidence for the solar irradiation is often in the range 45–708. Moreover, the optical data should be correct for the actual window, usually a double-glazed configuration. This report presents the energy- and visual performance of an existing electrochromic prototype, produced at Uppsala University, in terms of solar- and visual transmittance at different angles of incidence. A report on the directional properties of an electrochromic prototype (WO 3 / V2 O 5 ) has previously been reported by van Nijnatten (1997). The direct solar transmittance, T sol , gives an indication of the energy performance, but since the ec-device absorbs in the coloured state, the total solar energy transmittance, or the g-value, should be calculated in order to get the true energy performance. Basically, no straightforward method to measure the g-value

1. INTRODUCTION

Several papers on electrochromics have been reported during the last decade, mostly on the materials development that lies behind this phenomenon. Granqvist (1995) has published a very thorough book on inorganic electrochromic materials. The current status of the development of electrochromic prototypes and commercial devices can be found in a survey by Lampert (1998). A number of reports on the system aspects of the electrochromic (ec-) devices have been made by Selkowitz et al. (1994) and Sullivan et al. (1994, 1995, 1996a,b) showing, in principal, that, in a cooling dominated climate, a large amount of energy can be saved with the use of electrochromics. However, the same group shows that, in a heating dominated climate, an electrochromic window should be left in its bleached state during the heating season and thus does not improve the energy performance. These results were obtained from simulations on a 100 3 100 ft. building module. Moeck et al. (1998) showed that it is possible to use electrochromic windows to control the visual quality, e.g. maintain a stable daylight workplane illumination level, under most circumstances. When optimising the electrochromic window for visual quality, the energy efficiency decreases and vice versa. Simulations performed by the Selkowitz †

Author to whom correspondence should be addressed. Tel.: 1 46-18-4713-134; fax: 1 46-18-500-131; e-mail: [email protected] 493

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exists. It is necessary to have spectral information of the transmittance and reflectance in both s and p polarisation for the panes in the configuration. Then, calculations like in ISO 9050 (1990), or similar, need to be performed in order to see how much of the radiation is absorbed and re-emitted inwardly. There are calorimetric methods to determine the g-value, however, they are often timeconsuming and give no spectral information. 2. EXPERIMENTAL

A 5 3 5 cm electrochromic prototype with WO 3 and NiO as electrochromic layers and a proton electrolyte was analysed [for detailed description, see Azens et al. (1998)]. There was a problem with gas formation when a high voltage (approximately 2 V) was applied for a long time. In order not to stress the ec-device, the voltage was kept at a relatively low level leading to a relatively high T sol and T vis in the dark state. The effect of gas formation has been reported after cycling (Azens et al., 1998) and it has also been reported that, with a lithium electrolyte, no gas evaluation occurred after cycling up to 10 4 times. For the NiO / WO 3 prototype, the voltage was regulated in the coloured state so that the transmittance was held at an approximately constant level at a reference wavelength (590 nm). Fig. 1a illustrates the situation when 1.6 V is first applied for a certain time and then 1.2 V. Since the angular-dependent measurements are time-consuming, these types of regulations were made iteratively a couple of times in order to reduce the time dependence. Fig. 1b shows that when the voltage was disconnected, the transmittance increases at a rate of 2.5 3 10 24 min 21 , which gives an increase in transmittance of about 1.5% per hour. On the NVS  -mirror, the mirror layer was removed with sulphuric acid in order to obtain a ‘window’. During the coloured state, 1 V was applied constantly, as it has no optical memory, and it showed constant time dependence in each state. All measurements were performed using an absolute spectrophotometer with a small integrating sphere as detector (Roos, 1997). With this instrument, it is possible to measure both reflectance and transmittance for two and three pane configurations up to a 708 angle of incidence. The ability to measure on ‘‘real’ window units can sometimes be preferable since calculations would require spectral transmittance and reflectance, s and p polarised, for the panes in the configuration. It was assumed that the ec-devices are non-scattering.

Fig. 1. (a) Arranging voltage level (at 590 nm) so that an approximate reference transmittance level is stable in the coloured state; (b) Transmittance increases as the voltage is removed with a speed of about 1.5% per hour. The curves refer to the NiO / WO 3 prototype.

3. RESULTS AND DISCUSSION

Fig. 2 shows the results for the NVS  (Fig. 2a), the results for NVS  in combination with a clear float glass (Fig. 2b) and results for NVS  in combination with a low-e glass (Fig. 2c). The curves show transmittance versus wavelength in the coloured and the bleached state, respectively. The uppermost curve, in the two sets of four, is at normal incidence and the following curves are for 40, 60 and 708 angles of incidence, respectively. It should be noted that the NVS  is not produced for use in energy-saving applications but for vision safety in cars, and it shows, consequently, a big difference in transmittance in the visible region (T vis , 78–6.7 at normal incidence), which is the region of interest for this product. The results from the calculations of T sol and T vis are presented in Table 1. Fig. 3 illustrates the similar curves, for similar configurations, for the NiO / WO 3 prototype as for the NVS  in Fig. 2. Since these angle-dependent measurements are time-consuming, the difficulties in finding an equilibrium voltage leads to a change in transmittance with time, which induces

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has a larger span between the bleached and coloured state, both in T sol and T vis . As no gas formation occurs upon cycling, however, the results from the NiO / WO 3 prototype in the coloured state should be lowered if a Li-electrolyte is used and, thus, a higher span in T sol and T vis should be possible (see optical transmittance curves in Azens et al., 1998). From Table 1, it is seen that not only the absolute values, but also the span in T sol and T vis decreases as the clear float glass is combined with the ec-device. It decreases even more when the low-e is combined together with the ec-device. It is also seen that the angular dependence of the span is not significant until the angle approaches a 708 angle of incidence. In Fig. 4, the linearly interpolated normalised T sol - and T vis -values are plotted versus the angle of incidence. It is illustrated that the two investigated ec-devices show similar angular dependence, except for T vis in the coloured state, Fig. 4b. For comparison, the angle dependence for some, theoretically assembled, common glazings were calculated using Fresnel equations (ISO 9050 standard). For T sol , it was found that, in the bleached state, the angular dependence of the NiO / WO 3 prototype followed the same angular dependence as a clear float glass (4 mm) with a thick silver coating (30:20:60 SnO 2 /Ag / SnO 2 ). In the dark state, the angular dependence was similar to that for an absorbing ‘grey’ glass (6 mm) with the same silver coating. The deviation for NVS  in the coloured state could be due to a high error caused by low signals. In Fig. 5, the normalised span, i.e. normalised T(bleached) 2 T(coloured), is plotted versus the angle for all of the configurations investigated. The figures illustrate an almost constant behaviour until quite high angles, 708, are reached. 4. CONCLUSIONS Fig. 2. Transmittance versus wavelength for Gentex NVS  (EC) in bleached and coloured states at 0, 40, 60 and 708 angles of incidence. (a) single NVS  ; (b) NVS  in combination with clear float glass (CF); (c) NVS  in combination with low-e glass (LE).

some errors. An estimate of the relative error is about 5%. The figures from the coloured state can be lower if a slightly higher voltage is applied. It should be noted that measurements were only performed on one prototype and, thus, the demonstrated data do not have the statistical foundation that would be preferred. Comparing this prototype with the NVS  in Table 1, it is seen that NVS 

The measured solar transmittance values for the ec-devices in this investigation are comparable with the first electrochromic window on the market, considering that a darker state than obtained here is achievable with the NiO / WO 3 prototypes. This commercial EC window (made by Pilkington FLABEG GmbH) has T sol values of 35 and 9% in the bleached and coloured states, respectively, when used in combination with a K-glass low-e glazing. The solar transmittance values are also comparable with the ones in Sullivan et al. (1996a,b), where simulations show that, compared to conventional glazings, these prototypes can save up to 90 kWh / m 2 year, in a

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Table 1. T sol and T vis for the different systems studied. The thicknesses of the types of glass used were NVS  , 4.5 mm; clear float, 2.8 mm (low iron); hard coating on float glass, 3.9 mm and NiO / WO 3 prototype, 2.4 mm T sol , bleached / coloured NVS  NiO / WO 3 NVS  1 clear float NiO / WO 3 1 clear float NVS  1 low-e NiO / WO 3 1 low-e

T vis , bleached / coloured

08

408

608

708

08

408

608

708

56 / 23 53 / 25 49 / 20 45 / 23 42 / 17 39 / 22

54 / 21 49 / 23 46 / 18 43 / 21 39 / 15 37 / 20

48 / 17 44 / 19 39 / 14 37 / 17 34 / 11 32 / 16

40 / 14 36 / 15 29 / 10 27 / 12 26 / 8.3 24 / 12

78 / 6.7 74 / 38 70 / 6.8 68 / 35 63 / 6.2 61 / 36

76 / 5.1 73 / 35 68 / 5.2 66 / 33 61 / 4.7 60 / 34

70 / 3.6 66 / 30 59 / 3.5 59 / 29 54 / 3.2 54 / 30

60 / 2.7 56 / 25 45 / 2.4 45 / 21 43 / 2.1 42 / 22

Fig. 4. (a) Interpolation of the angular dependence of T sol for NVS  and the NiO / WO 3 prototype in bleached and coloured states and (b) the same for T vis . Note that the y-scale gives the normalised transmittance and not the actual transmittance.

Fig. 3. Transmittance versus wavelength for the NiO / WO 3 (EC) electrochromic prototype in bleached and coloured states at 0, 40, 60 and 708 angles of incidence. (a) single NiO / WO 3 ; (b) NiO / WO 3 in combination with clear float glass (CF); (c) NiO / WO 3 in combination with low-e glass (LE).

cooling dominated climate. Considering T sol and T vis , it is largely dependent on the application and / or location whether one would prefer a large span in one or in both of those parameters. For example, for the NVS  mirrors, a large span in T vis is preferable (as in Moeck et al., 1998), or one could focus on a large span in T sol in order to improve the energy performance of the window. The decrease in T sol , T vis and in the span when the ec-device is combined with complementing glazing is important to bear in mind as the ecdevice is not likely to be used as a single glazing. The angular dependence of T sol seems to be quite similar for the two ec-devices. Compared with static solar control windows, the angular dependence of the electrochromics is not extreme. An exception to this was in the T vis value of the NVS  in the dark state which, if correct, indicates

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that the regulating performance of the ec-devices is consistent, until quite high angles, 708, are reached.

Acknowledgements—A. Azens kindly supported the authors by providing the NiO / WO 3 prototypes. This work was performed under the auspices of the graduate school Energy Systems, supported by the Swedish Foundation for Strategic Research, SSF.

REFERENCES

Fig. 5. (a) Interpolation of the angular dependence of the span in T sol for NVS  and the NiO / WO 3 prototype; (b) the same for T vis . Note that the y-scale gives the normalised span and not the actual one.

that an EC that can go to a very dark state may have an extremely strong angle dependence, even at quite low angles of incidence. This will probably only benefit such a window since its purpose in the dark state is to block the sun, which normally impinges at high angles of incidence. In the bleached state, the angular dependence is still not extreme, compared to static solar control windows, which is also positive since the solar energy is probably needed when the EC is in the bleached state. The angular dependence of the span (Fig. 5) is fairly constant, which indicates

Azens A., Kullman L., Vaivars G., Nordborg H. and Granqvist C. G. (1998) Sputter-deposited nickel oxide for electrochromic applications. Solid State Ionics 113–115, 449–456. Granqvist C. G. (1995) Handbook of Inorganic Electrochromic Materials. In Elsevier, Amsterdam, International standard: ISO 9050 (1990). Lampert C. M. (1998) Smart switchable glazing for solar energy and daylight control. Solar Energy Materials and Solar Cells 52, 207–221. Moeck M., Lee E. S., Rubin M., Sullivan R. and Selkowitz S. (1998) Visual quality assessment of electrochromic and conventional glazings. Solar Energy Materials and Solar Cells 54, 157–164. van Nijnatten P. A. and Spee C. I. M. A. (1997) Determination of directional optical properties of an electrochromic device. Journal of Non-Crystalline Solids 218, 302–306. Roos A. (1997). Optical characterisation of coated glazings at oblique angles of incidence: measurements versus model calculations. Journal of Non-Crystalline Solids, 218. Selkowitz S., Rubin M., Lee E. S. and Sullivan R. (1994) A review of electrochromic window performance factors. SPIE 2255, 226–248. Sullivan R., Lee E. S., Papamichael K., Rubin M. and Selkowitz S. (1994) Effect of switching control strategies on the energy performance of electrochromic windows. SPIE 2255, 443–455. Sullivan R., Rubin M. and Selkowitz S. (1995) Reducing residential cooling requirements through the use of electrochromic windows. In Thermal Performance of the Exterior Envelopes of Buildings VI Conference, Clearwater Beach, USA, LBNL-report NR: LBNL-37211. Sullivan R., Lee E. S., Rubin M. and Selkowitz S. (1996a) The energy performance of electrochromic windows in heatingdominated geographic locations. In LBNL-report NR: LBNL-38252. Sullivan R., Rubin M. and Selkowitz S. (1996b) Energy performance analysis of prototype electrochromic windows ASHRAE, Boston, USA. In LBNL-report NR: LBNL-39905.