Energy Performance of Switchable Glazing IEA Solar Heating and Cooling Programme, Task 27

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1 10DBMC International Conférence On Durability of Building Materials and Components Energy Performance of Switchable Glazing IEA Solar Heating and Cooling Programme, Task 27 A. Roos 1, D. Covalet 2, X. Fanton 3, M.-L. Persson 1, W. Platzer 4, T. R. Nielsen 5, H. R. Wilson 6, M. Zinzi 7, M. Köhl 4, M. Heck 4 and B Chevalier 8 arne.roos@angstrom.uu.se 1) Uppsala University, The Ångström Laboratory, Box 534, Uppsala, Sweden 2) EDF R&D, Moret sur Loing Cedex, France, 3) Saint Gobain, Paris, France, 4) Fraunhofer ISE, Freiburg, Germany, 5) Technical University of Denmark, Lyngby, Denmark, 6) Interpane, Lauenförde, Germany, 7) ENEA, Rome, Italy, 8) CSTB, Grenoble, France TT6-156 ABSTRACT Approximately 40% of the energy consumption in Europe is connected to heating, cooling and artificial lighting of buildings. Most of this consumption generates CO 2 emissions into the atmosphere, which we want to reduce. Traditionally most of the energy that goes into a building is consumed during its service lifetime and only a smaller part during its construction and destruction. As energy efficiency is improved, this may change and by investing more energy during the construction phase, the total energy load can be reduced. Unless they are carefully designed and appropriate glazing is installed, buildings with large glazed areas may need more energy for heating during the winter and also need electricity to run air-conditioning systems. Windows using modern coated glazing products with either high or low values of the total solar energy transmittance in combination with a low thermal emissivity can reduce these problems considerably and save large amounts of energy both for heating and cooling in our built environment A new emerging window technology for energy efficient windows makes use of switchable glazing products. These are coated glass or polymer sheets, for which the light and solar transmittance can be varied by exposing the active coating to a modified gas fill or to a low electrical voltage. Windows with a light transmittance that can be varied between around 60 and 10 percent have been produced and evaluated within the International Energy Agency, Solar Heating and Cooling Programme, Task 27. These windows have been tested for durability and the energy performance has been simulated using building simulation software. By having them in the dark state whenever cooling is needed and in the light state whenever heating is needed passive solar energy is utilised for heating and the cooling load is reduced by blocking solar irradiation. Control strategies are in general more complex and daylighting, glare and thermal comfort are aspects that need to be taken into account, as well as personal preferences. In this presentation some of these aspects are discussed, and some results of building simulation of the energy performance are presented. For a standard reference office block, the energy saving potential in three different climates has been simulated. The results indicate large savings in cooling load and, perhaps more important, a reduction of the cooling power needed. If installed airconditioning power can be reduced, or even eliminated, the investment cost of switchable glazing can be justified. Sensitivity studies using building simulation also play an important role in assessing the effect of product durability on building energy performance over the entire product lifetime. During Task 27 tests have been performed to attempt to relate outdoor durability tests with accelerated ageing tests under controlled switching conditions. It is important to note that the emphasis was on developing the testing procedure, and that the results obtained from prototypes are not representative of technically mature products. KEYWORDS Switchable glazing, Energy performance, Durability testing

2 1 INTRODUCTION Modern advanced glazing materials include products with optimised optical properties for different applications. In particular, recent developments in glass coating technology have led to a large number of coated glazing products with high light transmittance and low emittance of thermal radiation for energy efficient window applications. A special group of advanced windows are the so-called switchable windows. These are windows that can switch between high and low transmittance of solar and/or visible radiation. Thus, they can adapt to varying solar irradiation conditions and maintain a comfortable level of light and thermal transmittance. Owing to this ability to switch, these window have a further energy saving capacity compared to static windows. The switching can be between high and low transmittance or between transparent and translucent states. This short presentation is limited to the high/low transparent case. Presently there are two competing technologies to achieve the switching. Either the switching is controlled by a low electrical voltage between transparent electrodes (electrochromic) or by the addition of hydrogen in the gas phase to a coated surface (gasochromic). In both cases the coatings become absorbing upon reduction. An oxidation process leads to bleaching. In the electrochromic case hydrogen can be replaced by lithium. For a detailed description of the function of these surfaces the readers are referred to the literature. [Granqvist, 1995, Wilson et al, 2002]. This presentation is a summary of the activities carried out within the International Energy Agency, Solar Heating and Cooling Programme - Task 27. Switchable glazing was studied with respect to its energy performance and long term durability. The performance and life time of these products depend on a number of external parameters and many of the results are still only preliminary. A very important goal of the task is thus the identification of recommended procedures to evaluate switchable glazing. 2 TESTED WINDOWS Prototypes of switchable glazing from three European manufacturers have been tested and evaluated within the task. The object has not been to compare products, but instead to evaluate energy performance and durability test procedures. The switchable prototypes are therefore not identified in this report, and the results are simply referred to windows A, B or C. Neither of the switchable coatings provides a surface with low thermal emittance. In order to get a window with a low U-value, the switchable panes have to be combined with a separate low-e coated pane in a double or triple glazed unit. A list of the relevant optical and thermal parameters of the tested windows is shown in table 1. The task has lasted for 5 years and product specification has changed over these years. Building simulations performed by the different participants presented in this report may therefore use slightly different input data. Window A B C Ref TSET (g) T vis U value (W/m 2 K) Table 1. Solar and thermal parameters for window simulations. TSET=g=Total solar energy transmittance (including absorbed and re-radiated energy), Tvis = light transmittance, U- value=thermal conductivity of window. A, B and C correspond to switchable windows and ref is a standard double glazed low-e window.

3 3 ENERGY PERFORMANCE The performance of a window depends on a large number of parameters and is in most cases difficult to evaluate. In the winter when it is much colder outside than inside, the net energy flow through the window is outward and the room needs to be heated. In the summer it is often warmer outside than inside and then the net energy flow is inward. In addition to the thermal flow driven by a temperature difference, there is also the solar irradiation transmitted through the window and this is where switchable glazing can help to control the energy flow. Inside the building, heat is generated by people, lighting and various electrical appliances as well as by the heating radiators. It is desirable to maintain an indoor temperature between 20 and 26 degrees. To achieve this we generally need both a heating and a cooling system. Since more than one third of the total energy consumption in Europe today is associated with temperature control of our buildings, it is easy to understand the importance of keeping the consumption down. In later years cooling has become more common and the summer of 2003 was very hot in Europe with temperatures reaching 40 degrees in many places. The trend with an increasing use of air conditioning is alarming since it inevitably increases the consumption of electricity as well as the peak demand. In order to calculate the total energy balance of buildings, simulation tools have been developed. They make it possible to simulate the temperature inside the rooms of a building hour by hour throughout the year. The physical properties of all building elements need to be known, together with the meteorological data for the location of the building and occupant behaviour. All simulation tools are based on models and simplifications of the building structure. The simulation tools so far used in Task 27 are based on the software tools TRNSYS [Platzer, 2003], CLIM2000 [Covalet and Greiffier, 2003], BSIM [Nielsen, 2002] and DEROB [Kvist, 1999] The simulations were performed for three different locations in Europe, representing Mediterranean, central European and northern European climate zones, Rome, Brussels and Stockholm, respectively. Meteorological data were generated by the program Meteonorm. T out north T 1 room office T 1 T 1 corridor T 2 room T 3 office T 3 T 3 T out south Fig 1 Standard reference office for building energy simulation within IEA SHC Task 27. The simulations were applied to a standard reference office, defined in Fig. 1. This office consists of identical office modules facing north and south separated by a corridor. The offices can be seen as independent units unaffected by adjacent offices, but interacting with the corridor and the opposite office. Lighting, heating, cooling, office equipment, occupancy and several other parameters are specified, so that different simulation methods can be compared using identical input parameters.

4 Switchable glazing needs a control strategy for when it should be in the dark state and when it should be in the bleached state. This strategy can be based on a number of parameters, one of which is the unpredictable preference of the occupants. Minimising power consumption is not necessarily what the occupants want. Glare reduction may be the prime factor for the staff who sit in front of computer screens, while temperature is more important to others. Physical parameters that can be used for the control are solar irradiation, outside temperature, indoor temperature, light level inside the office, occupancy, time of day and time of year. For each one of these parameters, different set points and different control algorithms can be used. The switching time is of the order of several minutes so a time constant must be used to avoid too frequent switching. All this means that the simulation of the energy and power consumption of the office block is complex. 4 SIMULATION RESULTS Only a small selection of the obtained results is presented in this summary report. These can be seen as examples of how data can be presented and give an indication of the results. In table 2 the results from the different simulation tools can be seen for some of the glazing options for heating and cooling and for the south and the north office in Fig. 1. All results can be separated into, for instance, monthly values and for different control. For simplicity the presented results are only calculated for the switchable glazing either in a permanent bleached state or in a permanent dark state. Heating BRUSSELS < Heating - south office >< Heating - north office > Window clim bsim derob trnsys clim bsim derob trnsys Low-e A(bleached) A(coloured) B(bleached) B(coloured) Cooling BRUSSELS < Cooling - south office >< Cooling - north office > Window clim bsim derob trnsys clim bsim derob trnsys Low-e A(bleached) A(coloured) B(bleached) B(coloured) Table 2. Annual heating and cooling energy demand (kwh/m 2 year) for different glazing options. Simulation tool from Section 2 indicated in table head. For optimum energy performance, seasonal switching between these two states is probably close to what is the best choice, since cooling is only required in the summer and heating in the winter. Thus the windows can be kept in a permanently dark state during the summer months and vice versa for the winter. As can be seen from the simulated values in the table there are considerable differences in the energy demand as obtained from the different simulation tools. The exact reasons for these differences have not yet been fully evaluated. What is interesting to see is the low values for cooling for the case when the switchable glazing is left in the dark state and that the energy needed for heating is of the same order as for the low-e reference glazing. In the simplest possible control strategy the window is set in the dark state when cooling is needed and in the bleached state otherwise. 4 DURABILITY OF CHROMOGENIC GLAZING As chromogenic glazing is a very new product that is just entering the market, the data base on its durability is very limited. In an attempt to gain information that can be used in simulation

5 sensitivity studies, two approaches have been followed in parallel: newly defined accelerated ageing test procedures for chromogenic glazing, and outdoor exposure in test stands. It must be emphasised that because the research field is so new, the results presented here were obtained with prototypes that were still in the process of development. 4.1 Accelerated Ageing Tests A number of accelerated ageing tests for chromogenic glazing have been defined during the duration of IEA Task 27, in the U.S.A. as ASTM standards or drafts, and in Europe as exploratory tests within the EU-funded SWIFT project [Wilson, 2003]. A series of tests has been defined, in which the samples cycle continuously between the bleached and coloured states (representing one form of acceleration), and are subjected to different temperature and/or radiation conditions. The ASTM tests and the SWIFT tests differ significantly in the specified number of cycles. If the criterion is the number of cycles expected during a 20-year lifetime, 3600 is probably too low (less than one cycle per two days - SWIFT). However, cycles (ASTM), corresponding to more than 6 cycles per day, is much too high. The experience of one industrial partner with chromogenic windows installed in offices was that they were switched for one or two cycles per day, or even less frequently. This corresponds with surveys of office workers, which indicated that in most cases, lighting conditions were changed (by switching on lights or operating mechanical blinds) only when the occupant entered the room in the morning and afternoon, or if there were disturbing glare. On this basis, it is suggested that 7000 cycles would be a more suitable limit. However, it may be more appropriate to choose a still lower number of cycles, so that the total testing time remains with acceptable limits, but the duration of the cycling period is long enough to ensure cycling over at least e.g. 80 % of the maximum transmittance switching range. Depending on the size of the sample, up to 60 minutes may be appropriate for a complete switching cycle. It is probable that 3500 cycles between transmittance values corresponding to a given change in optical density represents a greater stress to the active material in the chromogenic unit than 7000 cycles covering only half the change in optical density. Investigations by one industrial partner have indicated the importance of using samples of different areas for the cycling tests. A minimum area of 400 mm x 400 mm was recommended (preferably 600 mm x 600 mm), although a longer time is needed for one complete cycle than with smaller samples. This is because the number of cycles successfully completed by larger samples was found to be larger than that for smaller samples which were otherwise identical in composition, i.e. the smaller samples did not adequately represent the performance of the larger ones. Two thermal cycling tests were also defined analogously to existing standards such as pren , which was specified to test the durability of the seal in conventional insulating glazing units. In accordance with the aim of testing the complete system, the scientists in the SWIFT project propose that the chromogenic glazing unit remain connected to the supply and control units during the test, although the latter need not be subjected to the test conditions if they are to be installed indoors. 4.2 Outdoor testing Site conditions Measurements were made at two different sites, with two different types of samples, C and B. One site was the Fraunhofer Institute for Solar Energy Systems in Freiburg, Germany (48 00' N, 7 51' E, altitude: 269 m). The measurement period is from 1st December, 2001 to 31st August, The other site was the Centre Scientifique et Technique du Bâtiment (CSTB) in Grenoble, France (45 11' N, 5 43' E, altitude: 212 m). Results are presented from to for this site. As the statistics for the average temperature and total radiation values indicated, the second year included an unusually hot and dry summer, resulting in a high frequency of high radiation values at both sites.

6 1.0 T vis [-] Sample C, colouring original value 10 minutes after colouring start 20 minutes after colouring start 30 minutes after colouring start 40 minutes after colouring start 50 minutes after colouring start Transmittance Initial T T+10' T+20' T+30' date [dd.mm.yyyy] Time after (days) i) ii) T vis [-] original value, Sample B 10 minutes after colouring start 20 minutes after colouring start 30 minutes after colouring start 40 minutes after colouring start 50 minutes after colouring start Transmittance Initial T T+10' T+20' T+30' date [dd.mm.yyyy] Time after (days) iii) iv) Fig. 2: Visible transmittance values at 10-minute intervals after the beginning of colouring or bleaching for the prototype chromogenic glazing samples C and B, exposed outdoors in Freiburg and Grenoble. i) Window C, Freiburg, ii) Window C, Grenoble, iii) Window B, Freiburg, iv) Window B, Grenoble Test Stand At both test sites, the chromogenic glazing system prototypes were exposed outdoors in a test stand which is tilted 45 and orientated due south. The total glazing area for each type of sample, each represented by two IGUs, was 1.08 m 2. The glazing is cycled twice a day during the daylight hours. In addition to standard meteorological data, the glazing surface temperature, the UV radiation, solar radiation and daylight incident on the plane parallel to the glazing are monitored continuously. The visible transmittance is used as the performance parameter, and is monitored with luxmeters located outside and behind the glazing units. As the solar incidence angle varies throughout the day and year, the transmittance values originally calculated from the measured luxmeter ratios reflect the angular dependence of the glazing transmittance. The transmittance values presented here from Freiburg were subsequently corrected to the values for near-normal incidence on the basis of laboratory measurements. To reduce extrapolation errors, the transmittance values used for analysis were restricted to those measured with angles of incidence less than 65 and reference illuminance values exceeding 4500 lux. The transmittance values from Grenoble still include the variation due to the angle of incidence. Figure 2 gives a representation of each switching process by showing the transmittance value at different times after the start of bleaching or colouring processes. Despite filtering the data to remove points corresponding to the lowest light levels and highest incidence angles, the prevalence of low light levels and high incidence angles clearly reduces the accuracy of the transmittance values determined in winter, causing wider scatter of the points then. However, qualitative trends can be

7 recognised, including the similarity in behaviour shown at the two different sites for samples of type C or B. 4.3 Comparison of results from accelerated ageing and outdoor exposure It is important to note that the results presented here are from prototypes in the development process, not from technically mature products. The aim of the comparison is to see whether the information from the accelerated tests correlates adequately with that from outdoor exposure. On the basis of the graphs for sample C at both sites in figure 2, it is clear that the transmittance values for the coloured and bleached states have converged and the switching rate has decreased so much that the chromogenic glazing system prototype has failed. This agrees qualitatively with the results from the accelerated SWIFT tests 1 and 3 at 5 C and 23 C. However, the high-temperature accelerated test, SWIFT 2, on this sample did not reveal the same type of marked degradation as had been seen in the outdoor test after high temperatures in spring and summer. By contrast, the severe SWIFT 2 test caused catastrophic failure of sample B, although the outdoor performance is good. This raises the question of an appropriate temperature and radiation level for the accelerated ageing test. Obviously, the temperature should not be so high that the activation energy threshold for new degradation mechanisms is exceeded. However, even if a temperature is chosen which is lower than the peak which can be experienced in outdoor exposure, it can be questioned whether the continuous cumulative load on the switching system only accelerates degradation or whether it enables a sequence of mechanism steps that would be terminated and may even reverse during the nocturnal conditions of natural exposure. In choosing testing conditions for architectural chromogenic glazing systems, there may be a case for distinguishing the requirements according to the application. For example, safety standards differentiate between vertical and overhead glazing - for absorptive glazing such as coloured chromogenic glazing, the difference in maximum possible incident solar radiation intensity could be significant. Similarly, a classification according to climatic zones, as is the case for energy rating systems, could also be considered. Finally, the flexibility associated with the control unit, which is an integral part of a chromogenic glazing system, means that precautionary measures can be taken to prevent the glazing from reaching a temperature known to be harmful. A test specifying a higher temperature then may become irrelevant. 5 CONCLUSIONS The results of the simulation work performed within Task 27 clearly indicate a large saving potential for buildings, especially in warm climates. An important result not specifically shown by the results here is that the cooling power is considerably reduced. This can help to reduce peak loads for the local energy system, and also in many cases make expensive and energy consuming air conditioning units obsolete. In central and northern European climates, it is likely that buildings designed with proper solar shading can maintain a comfortable indoor climate without air conditioning. The results of these investigations are not fully evaluated. It is, however, of interest to compare different tools having different degrees of built-in approximations and simplifications, using the same input parameters. The correct answers will have to wait until switchable glazing is more readily available and can be installed in more than just a few test offices. Based on observations of architecturally dimensioned, chromogenic glazing systems exposed to natural weathering, some questions are raised on the appropriate choice of loads, switching cycle duration and number of cycles in current and proposed accelerated ageing tests. Definitive answers to these questions will require further time-consuming accelerated ageing and natural exposure tests, and careful analysis and comparison of their results. Although it is clear that the results presented here are still of a provisional nature, it is suggested that they provide a useful basis for sensitivity studies on the effect of changing technical specifications on building energy performance. Given the lack of clear correlation between accelerated ageing and outdoor exposure results, at present it seems more appropriate to draw on the results gained in outdoor exposure.

8 6 REFERENCES Covalet, D. and Greffier, V., Modelling a reference office building proposed in T27 of IEA - Application to chromogenics, Task 27 report, EDF HE-14/03/006 (2003) Granqvist, C.G., Handbook of inorganic electrochromic Materials, Elsevier, Amsterdam, 1995 Nielsen, T.R., Simulation results for reference office with electrochromic and gasochromic glazings, IEA Task 27 report (Sept 2002) Kvist, H., User Manual for DEROB-LTH, Lund University, Lund, Sweden, 1999 Platzer, W.J., Switchable Façade Technology - Energy Efficient Office Buildings with Smart Facades, Proceedings ISES Solar World Congress, Göteborg, Sweden, 2003 Wilson, H.R., Blessing, R., Hagenström, H., Hutchins, M.G., Dvorjetski, D., Platzer, W.J., 2002, 'The optical properties of gasochromic glazing', Conf. Proc. 4th ICCG, Braunschweig, Germany, 3rd -7th November, 2002, pp Wilson, H.R., 2003, 'Steps toward Appropriate Accelerated Ageing Tests for Architectural Chromogenic Glazing', Proc. IEA SHC Task 27 Dissemination Workshop, Freiburg, Germany, 6th October, 2003