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Controlling the transmission of radiant energy through windows: a novel ventilated reversible glazing system
Building and Environment 35 (2000) 433±444
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Controlling the transmission of radiant energy through windows: a novel ventilated reversible glazing system Yair Etzion, Evyatar Erell* The Center for Desert Architecture and Urban Planning, The Jacob Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, Sede-Boqer Campus, Israel, 84990 Received 8 June 1998; received in revised form 29 April 1999; accepted 17 May 1999
Abstract State-of-the-art glazing systems can provide very good solutions for cold climate conditions, and fairly eective ones for warm climates. However, there is still no window system on the market that can oer the ¯exibility required to provide a comfortable visual environment and an ecient energy response in climates where heating is required in winter, and cooling is required in summer. This paper describes an experimental investigation of a novel glazing system, designed to overcome glare and radiation damage to interior furnishings, yet which causes no reduction in the energy eciency of the glazed opening compared with a conventional window used in direct gain systems. The proposed glazing system (patent pending) incorporates a rotatable frame holding two glazing components: transparent glazing providing a weatherproof seal, and absorptive glazing with a low shading coecient1. The absorptive glazing is ®xed at a small distance from the clear glazing, forming an airspace between them which is sealed at the sides but open at the bottom and top, so that air ¯ows freely through it. In summer, the absorptive glass faces the exterior of the building, absorbing excessive solar radiation and dissipating the heat to the ambient air. In winter, the glazing assembly is rotated so that the absorbing glass faces the interior, reducing glare but allowing eective convective and radiative heating of the adjacent space. # 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Glazed openings are a unique building element since they alone are designed to allow the passage of energy through the building envelope Ð whether in the form of visible light or of infra-red energy (`heat'). Properly designed windows may provide cost-free natural lighting and a signi®cant proportion of winter heating requirements through the utilization of solar energy. Yet data suggest that more than 30% of all energy use in buildings may nevertheless be attributed to undesirable heat transfer through windows and to arti®cial lighting [1]. Windows thus present a great challenge to * Corresponding author. Tel.: +972-7-6596875; fax: +972-76596881. E-mail address: [email protected] (E. Erell). 1 A patent application describing the window was ®led with the Israel Patent Oce in November 1997.
designers. How to provide the best possible visual environment in the building interior, while maintaining an optimum energy balance under varying environmental conditions throughout the year. The primary functions of glazed openings are to let natural light into the building and to allow a view of the outdoors. However, the uncontrolled in¯ux of solar energy through glazed openings has a number of drawbacks (not least where passive solar heating is used). . Large glazed areas, characteristic, for example, of modern oce construction but also typical of some residential and commercial buildings, may result in extreme overheating in summer. . In spaces ¯ooded with intense solar radiation, interior furnishings often suer from accelerated deterioration, such as fading of fabrics or degradation of plastics. . In locations and periods characterized by intense
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solar radiation, daylight levels in spaces with large openings are usually much higher than needed. Visual discomfort is also caused by glare resulting from the contrast between extremely bright window areas and more subdued interior surfaces. The development of advanced glazing materials provides solutions to some of these drawbacks in some but not all locations and environmental conditions. The new glazings were designed to modulate the passage of radiation by intercepting part of it, allowing only a certain component through the glass. This may be done either by altering the properties of the glass itself, or by applying a coating to the surface of the material. The problems associated with the energy performance of windows in cold climates, where solar radiation is nearly always welcome and illumination levels are rarely excessive, has been solved by the development of insulated glass windows with low-emissivity (low-e ) coatings. Transparent low-e coatings on glass Ð the UK Building Regulations de®ne a low-e coating as a surface having an emissivity of less than 0.2 Ð became widely available in the 1980s, and by 1990 accounted for over a quarter of the double-glazing market in the US [2]. The importance of low-e coatings lies in their capacity to reduce signi®cantly radiative heat loss through glazings Ð the best products claim a surface emissivity (center of glass) of less than 0.05. For example, by suspending two thin polyester ®lms with a low-e coating in an insulated space ®lled with krypton gas between two panes of glass, an American manufacturer, Southwall Technologies, was able to create a window with a center-of-glass conductivity (U-value) of less than 0.7 W/K m2. Since the shading coecient of such a window was 0.52, it was possible to generate a net seasonal energy gain from just northern light, even in fairly severe winter climates [2]. However, warm climates pose a dierent problem Ð providing natural lighting without imposing a severe penalty in terms of unwanted heat. The solution to this problem lies in the development of spectrally selective tints and coatings. Since the visible part of the solar spectrum accounts for only 47% of the energy received from the sun, an ideal selective surface for warm climates would transmit all (or most) of the visible spectrum, from about 0.39 to 0.74 m, while re¯ecting all of the ultra-violet and infra-red radiation. The appropriateness of a glazing for warm climates may be measured by an index called the glazing luminous ecacy, Ke, sometimes called the `coolness index', which is de®ned as the visible transmittance divided by the shading coecient [3]: Ke Tvis =SC where Tvis is the proportion of visible light at normal
incidence transmitted by a glazing, and the shading coecient (SC) is the ratio between the solar heat gain factor of a particular glazing and that of a standard, double strength clear glass sheet 3 mm thick. Tinted glasses vary in their capacity to transmit light of dierent wavelengths. Blue and green glasses tend to transmit a high fraction of the visible light, absorbing some of the infra-red radiation, while gray and bronze glasses do the opposite [4]. Both types of glazing have been used in warm climates as the outer pane in a double glazed unit, to provide what has been called `cool daylight'. An even better solution has been developed with the improvements in the manufacturing technology of lowe coatings. Low-e coatings were originally produced to transmit most of the solar radiation, up to a wavelength of about 3 m, re¯ecting long wave radiation from colder objects. However, it is now possible to produce low-e coatings which transmit only the visible part of the solar spectrum, re¯ecting not only the far infra-red, but also approximately half of the solar spectrum, above 0.74 m. The application of such coatings to the inner surface of the outboard pane of a double glazed window, particularly if this glass is a high-iron heat absorbing glass, results in a window well-suited to warm climates: The overall solar heat gain coecient of such a unit may be only 0.31, while the visual transmissivity may be over 60% [5]. The preceding glazing types are all characterized by ®xed optical properties. In either very cold or very hot climates, this presents no problem. However, in most temperate zone climates, summer conditions are such that solar radiation imposes an unwanted heat load on buildings, while winters are cold, so that passive heating by solar radiation is welcome. Research into socalled `electro-chromic windows' shows promise of eventually providing a glazing system with dynamic optical properties, capable of adapting to changing environmental conditions. This technology is based on the tendency of a number of solids, such as the oxides of tungsten, vanadium, molybdenum or titanium, to perform a reversible color change when an electric current or ®eld is applied to them. These materials may be suitable for use in doping sol-gel glass to create a glazing system which can alter its transmissivity in response to an electric signal [6±8]. However, research into electro-chromic windows is still at a fairly early stage. While the technology holds great promise for the future, there is no commercial application of this concept at the present. The problems yet to be solved include theoretical issues such as the means of controlling the spectral properties of the sol-gel in order to achieve the desired transmissivities, as well as the diculties associated with the increase in scale from very small laboratory samples to large scale commercial glazing of suciently high uniformity.
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Fig. 1. Schematic drawing of glazing system in: (a) winter con®guration and (b) summer con®guration.
2. The proposed glazing system Research conducted at the Center for Desert Architecture and Urban Planning2 culminated in the development and experimental evaluation of a novel design for an improved glazing system (patent pending) which is appropriate in locations where solar radiation is welcome in winter but may be undesirable in summer. The application consists of a reversible, ventilated glazing system. It incorporates a rotatable frame holding two glazing components: transparent glazing providing a weatherproof seal, and absorptive glazing with a low shading coecient. The absorptive glazing is ®xed at a small distance from the clear glazing, forming an airspace which is sealed at the sides but open at the bottom and top. The airspace may be ventilated either by thermosyphoning or by means of a small, solar powered electric fan. In winter, the window frame is rotated so that the absorptive glazing faces the interior. Solar radiation is transmitted through the clear exterior glazing, and is absorbed by the interior glazing, which is heated by this energy (Fig. 1a). Space heating is provided both by long wave radiation and by convection. Long wave radiation is emitted from the absorptive glass, and air is heated by contact with both its surfaces. Convective 2 The Jacob Blaustein Institute for Desert Reseach, Ben-Gurion University of the Negev, Sede-Boqer Campus, Israel.
heating is enhanced by the air ¯ow between the two panes of glass. The air ¯ow in the space between the two glass panes may be either purely thermodynamic, or fan assisted. When fans are employed, the heated air can be exhausted through the lower part of the system, reducing thermal strati®cation of the air in the heated space. Space heating is achieved but visual discomfort and damage to furnishings by short-wave solar radiation is reduced signi®cantly. In summer, the window frame is rotated so that the absorptive glass faces the exterior, intercepting incoming short wave solar radiation. The energy absorbed by the exterior glazing is dissipated by long wave radiation, and is prevented from being transmitted to the building interior by the clear interior glazing, since it is nearly opaque at wavelengths above 4 m (Fig. 1b). The energy released by the warm glass also sets up a thermosyphonic air ¯ow in the space between the two glazing components, preventing overheating of the air and removing unwanted energy. Overheating of the adjacent interior space is prevented and visual comfort is improved.
3. Experimental setup The experimental evaluation of the concept for the new glazing system was carried out in the test facilities of the Center for Desert Architecture and Urban Planning at Sede-Boqer, Israel. A glazing
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system constructed according to the principles previously described was installed in an existing test building, and its performance monitored in summer and in winter. In the building used for the experiment, the walls were constructed of 20 cm thick hollow silicate blocks, with 5 cm thick polystyrene insulation on the exterior and an acrylic plaster ®nish, painted white. The roof was a 12 cm thick concrete slab, with foamed concrete sloped to the drains. The building was insulated from the earth by 10 cm polystyrene. Interior ®nishes included plastered, whitewashed walls and a terrazzo tiled ¯oor. Each of the test rooms measured 2.7 3.5 m, with the long wall facing due south. The experimental glazing was installed on a large window measuring 1.4 2.1 m, at the middle of the south-facing wall of one of the rooms. Both rooms were exposed on three sides to the ambient air, with one of the short walls common to a service space in the building interior. The building was unoccupied during the experiment, and there were no internal sources of heat. The reference window had a standard aluminum frame, with ®xed panes at the bottom and top, and a central section with horizontal sliding panes. The ®xed panes consisted of hollow (double-skinned) polycarbonate sheet glazing 12 mm thick. The sliding panes were glazed with 4 mm clear glass. These were replaced with double glazed units with a 6 mm air space for part of the winter experiment. In the test window, a sheet of absorptive glass was ®xed parallel to the clear glazing; 10 cm high openings at the top and at the bottom of the window assembly allowed free movement of air through the gap between the two sheets of glass. The absorptive glass Ð a dark brown safety glass manufactured by the Phoenicia Co., Israel, model no. 510 (international code 3609) Ð was 8 mm thick, had a visible transmissivity of 9% and a shading coecient of 42%. The net glazed area was 2.25 m2. . In the summer mode, the clear glazing was ®xed on the interior, 35 mm away from the absorptive glass. . In the winter mode, four wooden frames with clear 4 mm glass were attached to the exterior of the frame holding the absorptive glass, forming an air gap. The width of this gap was initially 22 mm, but was increased during the experiment to 75 mm. As in the reference room, the clear glass was later replaced by double glazed units during the second part of the winter experiment. (Minor dierences in the geometry of the window between the summer and winter modes, especially in the width of the air gap, were due to technical reasons associated with the installation of the experimental setup in an existing building.)
The data collection system recorded the following data: . environmental conditions Ð dry bulb temperature, relative humidity, global solar radiation incident on the plane of the glazing and wind velocity and direction; . internal conditions Ð dry bulb temperature, light levels on a working surface near the window and black bulb radiation at a ®xed point in the test room; . conditions characterizing the glazing system itself Ð temperature pro®le of each of the glazed surfaces, vertical temperature pro®le inside the ventilated air space, air ¯ow rate through air space and global radiation on a plane parallel to the glazing.
4. Results and discussion The glazing system was monitored for a extended periods in winter and summer. The performance data for each mode were analyzed for sequences of at least a week, each following several days of climatization to the season's conditions. 4.1. Penetration of solar energy Solar radiation was measured on the exterior of a vertical, south-facing wall and inside the building, 20 cm away from the center of the two windows being tested, parallel to the plane of the glass. In summer, the primary function of the glazing system is to prevent the penetration of excess solar energy while allowing good daylighting performance, and thus to reduce the electricity requirements for space cooling and arti®cial light. The eect of the absorbing glass, when ®xed on the exterior of the window, was to ®lter solar radiation almost entirely (Table 1, Fig. 2). Noontime interior radiation levels were reduced by the absorptive glazing to approximately 5% of exterior levels, compared with about 37% of exterior levels, for standard 3 mm transparent glazing. In winter, the absorptive glazing faces the building interior, and does not interfere with the penetration of solar radiation through the clear exterior glazing. The solar energy transmitted through the experimental window is therefore identical to that of the reference window, and depends on the optical properties of the external glazing. On a typical, sunny winter day (13 January 1997) the solar radiation measured near the window on the interior of the reference room amounted to nearly 77% of the peak solar radiation measured at noon on the wall surface outside the window, and to nearly 72% of the daily total (Table 1).
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Table 1 Solar radiation on a vertical south-facing plane: interior and exterior data for typical sunny days at Sede-Boqer in winter (13 January 1997), and summer (2 August 1997) Summer
Interior Ð reference glazing Interior Ð experimental glazing Exterior
Winter
Daily total [Wh/m2]
Daily maximum [W/m2]
Daily total [Wh/m2]
Daily maximum [W/m2]
1280 140 2850
142 17 406
3734 3734a 5190
641 641a 837
a
In winter, solar radiation is considered as having penetrated to the interior of the room with the experimental glazing once it has been transmitted by the clear, exterior glass. The values in the table are therefore identical to those in the reference room.
The eect of the clear, reference glazing on solar radiation levels on the interior warrants a short discussion. Clear glazing has a transmissivity of about 85% at normal incidence, but at lower incidence angles, particularly below 458, transmittance drops o, having a value of close to zero at highly oblique angles [9]. During summer, the eect of solar elevation Ð over 708 at noon on 1 August Ð is such that a high proportion of direct (beam) radiation incident on a south facing wall is re¯ected even by clear glazing. In winter, when noontime solar elevation is lower Ð at SedeBoqer about 358 on 1 January Ð most of the direct solar radiation incident on a south facing wall may be transmitted through clear glass. Solar radiation surveys carried out at Sede-Boqer [10] indicate that beam radiation accounts for about 45% of the insolation on a vertical, south facing surface during August, but over 70% of the (much higher) insolation on such a surface in January. Thus the solar radiation measured near the window in the reference room in summer and winter is in accordance with other experimental data. 4.2. Black bulb temperature Human thermal comfort is in¯uenced by several fac-
tors, only some of which are aected by the properties and performance of the glazing system installed in the building in question. While the commonly available data refer to air temperature Ð dry bulb temperature, to be precise Ð the eect of radiative heat exchange is often overlooked. The eect of radiation can be assessed by calculating the mean radiant temperature (MRT), which is the solid-angle-weighted average temperature of the surrounding surfaces. While mean radiant temperature cannot be measured directly, the reading of a globe thermometer, also known as the black bulb temperature, gives a very close approximation of the MRT, provided there is no air movement. In this experiment, temperature was measured in a black bulb suspended at the center of each room, about 1.5 m above the ¯oor and 1 m from the window. The eect of the experimental glazing on MRT was evident both in summer and winter Ð the absorptive glazing eliminated the asymmetric overheating which results from exposure to intense solar radiation. In summer, the in¯ux of solar radiation is re¯ected in the black bulb temperature in the center of the reference room, which shows a marked increase from about 07:30 h, remaining above the air temperature till
Fig. 2. Eect of glazing on global radiation measured on a vertical surface parallel to the glass, on a typical sunny summer day (2 August 1997).
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about 18:00 h. At noon on a typical day, the black bulb temperature in the reference room was about 348C, or nearly 68C higher than the air temperature (Fig. 3a). In the test room, the eect of solar radiation was negligible, since nearly all of it was absorbed by the exterior glass. The black bulb temperature was within 18C of the air temperature throughout the whole day. In winter, the eect of the absorptive glazing was even more pronounced. The black bulb temperature measured in the test room was a little above the air temperature, and even on a sunny day (11 January 1997) reached a maximum of only 288C. In the reference room, where the clear glazing transmitted a much higher proportion of the incident solar radiation, the black bulb temperature on the same day reached a maximum of 478C. (Fig. 3b). The dip in the graph, noticeable especially in the curve for the reference room, was caused by partial shading of the black
globe by a vertical divider in the aluminum window frame. 4.3. Illumination levels The quality of daylight depends on many factors, such as the size and position of the window, room geometry, re¯ectance of room surfaces and type of glazing. Levels of illumination were measured in a grid of nine points evenly spaced throughout the room, at a height of 1 m above the ¯oor, and at several more points near the window, where variations in the intensity of the light were more pronounced. At noon on a clear summer day, nearly all of the test room had an illumination level of between 270 and 350 lux. The highest light level was only 390 lux, immediately adjacent to the center of the window. The very even illumination is attributed to the dimensions of the room, which is only 2.75 m deep; to the size of
Fig. 3. Eect of glazing on black bulb temperature measured near the center of the room, about 1 m away from the window: (a) on a typical summer day (2 August 1997) and (b) on a sunny winter day (11 January 1997).
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the window, which occupies 27% of the south wall; to the absorptivity of the external glass, which reduces the glare and intense light near the window which is characteristic of clear-glazed rooms under similar conditions; and to the high angle of incidence of the beam radiation with the exterior glazing, which resulted in much of the solar radiation being re¯ected. The level of illumination in the test room during sunny winter days was less uniform; the portion of the room near the window received direct solar radiation, albeit attenuated by the absorptive glazing, and was brighter than the rest of the room. However, the highest level of illumination recorded on a bright sunny day in this room was only 3600 lux, compared with over 60,000 lux in the reference room, and over 80,000
439
lux outdoors (Figs. 4a and b). The levels of illumination in the test room, outside the patch of direct sunlight, ranged from about 200 to 600 lux. The comparable levels in the reference room were about 4000±10,000 lux. Recommended levels of illumination vary according to the type of activity. Most spaces, such as schools and oces, require illuminance of 300±500 lux on desks or work areas, with slightly higher levels, 500± 750 lux, being required for some specialized tasks [11]. Thus the level of illumination provided in the test room on a sunny day is adequate. However, it is important to note that the actual level of illumination in a room equipped with this type of glazing system may be adapted to local levels of insolation through the selection of an appropriate absorptive glass. The one used in this experiment is probably one of the most highly absorptive glazings available today, with a visible transmissivity of only 9%. 4.4. Temperature of the absorptive glass On clear, sunny winter days, when peak global solar radiation on a vertical surface parallel to the test window was close to 900 W/m2 at noon, the maximum temperature of the absorptive glazing was generally 45±508C, with temperatures exceeding 508C recorded on several occasions. The width of the air gap, 22 or 75 mm, did not have a signi®cant eect on the temperature of the absorptive glass, a 15% dierence in the mean maximum air speed inside the channel not withstanding. Thermal images of the window con®rm the results of point measurements taken at four locations on the glass surface, indicating a very uniform temperature distribution Ð the warmest part of the glass was only 2±38C warmer than the coolest part. In summer, the absorptive glass, which faced the exterior in this mode, was cooler than in winter. The maximum daily temperature was generally between 40 and 458C, or about 108C above ambient air temperature. The dierence in glass temperature compared with winter is explained by two factors. In Sede Boqer, mean daily levels of solar radiation incident on a vertical south-facing wall are much lower in summer, between 2.5 and 2.7 kWh/m2/day, than those in winter, 4.0±4.6 kWh/m2/day [10]; and since in summer the outer face of the absorptive glass is exposed to the wind, rather than to still interior air, as in winter, radiant energy absorbed in the glass is dissipated more readily by convection. 4.5. Convective heat output (winter)
Fig. 4. Illumination level (in lux) on a typical sunny winter day: (a) in the test room, showing the eect of the experimental glazing system and (b) in the reference room.
Energy absorbed in the absorptive glazing is released to the building interior by two mechanisms. It may be emitted in the form of long wave radiation, and it may
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warm the air in contact with the glass. As air in the ventilated space between the clear and absorptive components of the glazing system was heated in this manner, a thermodynamic ¯ow was set up. The temperature gradient created in this airspace was measured using radiation shielded thermistors placed at three points Ð near the inlet at the bottom, near the outlet at the top, and midway between them. The velocity of this air¯ow was measured by means of a hot-wire anemometer. The width of the air gap was initially ®xed at 22 mm. This width, about twice the maximum width of the gap used in conventional air-®lled double glazed windows [2,5], was considered sucient to allow free movement of air. At a later stage, the distance between the clear glazing and the absorptive glass was increased to 75 mm, in the expectation that air¯ow would increase. The width of the air channel had a pronounced eect on the operation of the glazing system. Table 2, based on average data for a one-week period of operation in each con®guration, summarizes the eects of increasing the distance between the clear glazing and the absorptive glass. The environmental conditions during the two weeks in question, in particular solar radiation, ambient air temperature and interior dry bulb temperature, were nearly identical. . The narrow (22 mm) gap allowed what appeared to be an unimpeded air ¯ow, with airspeeds of up to 0.7 m/s measured at the center of the channel. When the width of the air gap was increased to 75 mm, the mean airspeed in the channel, measured at several points, was reduced by 15%. The combined eect, however, was an increase in the mass ¯ow rate through the channel by a factor of 3. . When the air gap was narrow, the temperature dierence between the channel inlet and outlet on sunny days was over 218C (Fig. 5). The mean daily maximum outlet temperature under these conditions was 42.38C, reaching temperatures of over 458C on several days. When the air gap was increased to
75 mm, the increase in the air temperature was limited to an average of 11.68C, and did not exceed 138C. The temperature of the air at the outlet of the channel under these conditions was about 358C on sunny days (Table 2). . The temperature dierence between the air at the inlet of the air gap and at its outlet, multiplied by the mass ¯ow rate and the heat capacity of air gave the net convective heat output of the system. This quantity varied in response to the environmental conditions, particularly the availability of solar radiation. When the air channel was narrow, the maximum daily convective heat output was a little over 200 W/m2; increasing the gap width resulted in an increase in peak convective heat output to an average of nearly 300 W/m2, reaching 350 W/m2 or more on some days (Fig. 6). The convective heat output is the result of the formation of a natural thermosiphon between the two glazing components. The vertical temperature pro®le and the resulting air¯ow are aected by the vertical dimension of the air gap. Thus, the convective heat output reported here, measured in a window with an unobstructed height of nearly 2 m, should be regarded only as an indication of the extent of this phenomenon. Figs. 5 and 6 also demonstrate a reversal of the ¯ow pattern in the air gap at night, between about 18:00 and 08:00 h. In the absence of solar radiation, the exterior glazing cooled down rapidly. As interior air in contact with this glazing was cooled through contact with it, it became denser and a thermosiphon was established. Cool air, as much as 58C below room air, was exhausted from the lower opening of the air channel, and warmer air was drawn in through the upper opening near the ceiling. The air speed during the night hours was up to 0.2 m/s, and the resulting convective cooling rate was between 10 and 15 W/m2. The eect of increasing the distance between the clear glazing and the absorptive glass is summarized in Table 2.
Table 2 The eect of the distance between the clear glazing and the absorptive glass on various operating parameters of the experimental glazing system in the winter mode (mean values are based on experimental data for a period of 1 week in each of the two con®gurations) Width of air gap
Maximum air speed in channel [m/s] Maximum temperature dierence between outlet and inlet [8C] Maximum temperature of absorptive glass [8C] Maximum air temperature at channel outlet [8C] Peak convective heat output [W/m2] Daily convective heat output [Wh/m2 day] Daily convective heat output as proportion of the daily total solar radiation transmitted to the building interior [%]
22 mm
75 mm
0.64 21.1 48.9 42.3 186 682 19.6
0.54 11.6 49.5 34.1 298 1242 37.3
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Fig. 5. Vertical temperature pro®le inside the air channel in the experimental glazing on a sunny winter day (11 January 1997); width of the air gap 22 mm.
It should be noted that the overall energy performance of the glazing system is aected by the width of the air gap only marginally; its main eect is to alter the proportion of the two main components of the energy given o Ð long wave radiation and convection. When the air gap was narrow, the convective component was equivalent to nearly 20% of the total solar radiation measured inside the reference room window (which had clear, single glazing). Increasing the distance between the two main components of the window produced a convective output equivalent to about 37% of the incoming solar radiation.
4.6. Internal air temperature The dry bulb temperature of a building interior depends on many factors, including the amount of insulation installed and the thermal capacity of the structure. The building available for this experiment was very massive, being constructed of concrete and 20 cm silicate building blocks, and very well insulated. It was therefore expected that the temperature variations between the room ®tted with a reference window and the one ®tted with the test window would be relatively small.
Fig. 6. Convective heat output of the experimental window in W/m2 of glazed surface on a sunny winter day (21 January 1998), compared with the solar radiation incident on the glass; width of the air gap 75 mm.
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Dry bulb temperature was measured in each room at three elevations above the ¯oor Ð 0.5, 1.5 and 2.5 m, using thermistors shielded from the eects of radiation by highly re¯ective aluminum foil housings. Mean values were calculated from these sensors. In winter, the function of the absorptive glazing, which faced the building interior, was to convert incoming short-wave solar radiation transmitted by the exterior glazing into two main components: long-wave radiation and warm air. The overall energy balance of the room was not expected to show signi®cant dierences compared with the reference room. The dierence in mean air temperature between the rooms was in fact quite small, and is attributed to minor dierences in the thermal conductivity of the ceiling of the two rooms (Table 3). While the mean air temperature of the test room was similar to that of the reference room, the thermosiphonic movement of air created by the experimental window coupled with the position of the air outlet at the top of the window near the ceiling, resulted in pronounced thermal strati®cation. The extent of this phenomenon was related to the overall convective heat output of the window, and was therefore more noticeable on sunny days. The maximum dierence between the temperature recorded near the ceiling and that measured near the ¯oor was generally about 68C when the gap between the two glazing components was wide (Fig. 7), and about 48C when it was narrow. In the reference room, there was no noticeable thermal strati®cation, and the three temperature sensors were within 0.58C of each other. While the vertical temperature pro®le in the test room appears excessively uneven, it should be noted that for the duration of the experiment, the room was closed and unoccupied. It is expected that normal occupancy would result in better mixing of the air. The thermal imbalance may also be reduced by de¯ecting the air at the outlet of the window down by means of a bae, by lowering the elevation of the outlet, which in this case was about 2.5 m above ¯oor level, or through the use of an electric ceiling fan. Another possibility is the installation of a small fan in the glazing component, powered by a photo-voltaic cell integrated in the frame, which would
force the air down the channel between the two glass sheets. In summer, the air temperature in the two rooms was nearly identical. The large thermal inertia and highly insulated exterior walls of the test building resulted in a daily amplitude of only 1±28C. The eect of the experimental glazing was barely discernible, with daytime maximum temperatures about 0.58C lower in the test room relative to the reference room. 4.7. Discussion The main advantage of the glazing system described is its ¯exibility in response to the con¯icting demands of winter and summer conditions in many locations. In most temperate climates, solar energy is welcomed in winter as a means of reducing heating loads. Summer overheating, though, is a problem wherever large glazed areas are incorporated into the exterior envelope of buildings. The proposed glazing system addresses each of these con¯icting requirements. It provides the full bene®ts of passive solar heating by direct gain, widely acknowledged as the most practical method for solar heating; it practically eliminates most of the undesired side-eects which characterize buildings with extensive glazed areas, such as glare and visual discomfort; and it provides a simple means of reducing the penetration of solar radiation in summer, including the non-directional diuse component, without obstructing the view through the glazed area. The successful application of the experimental design requires the manufacture of an appropriate window frame. Such a frame must have the following properties. . The two glazing assemblies, incorporating the clear glass and the absorptive glass, must be able to rotate together through 1808, so that the absorbing glass will face either inwards or outwards, depending on the season of the year. Rotation of the window would normally be expected to take place only twice a year, i.e. on a seasonal basis. . In each of the two opposing con®gurations, the glazing assembly incorporating the clear glass must provide a weatherproof seal, capable of ful®lling current performance codes in the window industry.
Table 3 Air temperature in test room and reference room during winter experiment
Test room Reference room Ambient DBT
Temperature [8C] Single glazing experiment
Temperature [8C] Double glazing experiment
Daily mean
Daily maximum
Daily minimum
Daily mean
Daily maximum
Daily minimum
13.9 13.4 10.6
18.0 17.9 15.4
12.0 11.6 5.9
15.1 14.3 11.6
21.3 20.6 16.7
13.0 12.4 6.4
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Fig. 7. Thermal strati®cation in the test room, produced by the experimental window on a sunny winter day (21 January 1998). Curves indicate air temperature at speci®ed height (in cm) above the ¯oor.
. The air gap between the two glazing assemblies should be accessible for cleaning. This may be done by mounting the absorptive glazing on a hinged frame. . The rotation of the glazing system, from winter to summer mode or vice versa, should be convenient and must not require the use of special equipment. The bene®ts of the proposed glazing system may be realized through careful selection of the absorptive glass. The optical properties of this glass, which may be one of a large number of glazing types currently being manufactured, should be speci®ed in response to local climatic conditions. For example, in high-latitude countries, where winter levels of solar radiation are low, selective coated glass may be used to absorb radiation mainly in the near infra-red part of the spectrum, but transmitting suciently high levels of visible light to permit eective daylighting of the interior space. In such a glazing assembly, the summer con®guration will be less ecient than one where the absorbing glass has a lower shading coecient, but it may still prevent overheating. In areas where winter insolation levels are often intense, such as in most of the Mediterranean areas, an appropriate glazing would be required to absorb some of the visible light, as well as all of the near infra-red spectrum. 5. Conclusion State-of-the-art glazing systems can provide very good solutions for cold climate conditions, and fairly eective ones for warm climates. However, there is still
no window system on the market that can oer the ¯exibility required to provide a comfortable visual environment and an ecient energy response in climates where heating is required in winter, and air-conditioning in summer. The proposed ventilated reversible glazing system is a technically viable solution to these requirements: its energy eciency is equal to that of solar heating by direct gain, yet visual comfort and protection from excessive short wave radiation may be provided if the appropriate absorptive glazing is selected in each locality. In summer, it may improve visual comfort even in spaces with large glazed areas facing east or west, while reducing undesired energy gains to a minimum. Acknowledgements The authors wish to thank Prof. Amos Zemel for his helpful comments on the manuscript. The Phoenicia Co. of Israel donated the absorptive glazing used in the experiment. References [1] Selkowitz SE. Windows and daylighting group 1990 annual report. Lawrence Berkeley Laboratory, California, 1990. [2] Johnson TE. Low-e glazing design guide. Boston: Butterworth Architecture, 1991. [3] Arasteh D, Johnson R, Selkowitz S. De®nition and use of a daylight `coolness' index. In: Proceedings of International Daylighting Conference, ASHRAE, Atlanta, 1986. [4] Schuman J. Cool daylight, Progressive Architecture, 4(92), 136± 41.
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[5] PPG Solutions 1996. Product information sheet, 1996. [6] Reisfeld R, Jorgensen CK. Optical properties of colorants or luminescent species in sol-gel glasses. In: Reisfeld R, Jorgensen CK, editors. Chemistry, spectroscopy and applications of sol-gel glasses, 1991. [7] Reisfeld R. Theory and applications of spectroscopically active glasses prepared by the sol-gel method. In: Proceedings of the SPIE International Symposium on Optical and Optoelectronic Applied Science and Engineering, Sol-Gel Optics, San Diego, California, 8±13 July, 1990.
[8] Donnadieu A. Electro-chromic materials, Mat. Sci. and Eng. 1985;B3. [9] ASHRAE. 1989 Fundamentals handbook. Atlanta: ASHRAE, 1989. [10] Faiman D, Feuermann D, Ibbetson P, Zemel A. Data processing for the Negev radiation survey: fourth year. Jerusalem: Israel Ministry of Energy and Infrastructure, 1996. [11] Baker N, Fanchiotti A, Steemers K. Daylighting in architecture, a European reference book. London: James & James, 1993.
www.elsevier.com/locate/buildenv
Controlling the transmission of radiant energy through windows: a novel ventilated reversible glazing system Yair Etzion, Evyatar Erell* The Center for Desert Architecture and Urban Planning, The Jacob Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, Sede-Boqer Campus, Israel, 84990 Received 8 June 1998; received in revised form 29 April 1999; accepted 17 May 1999
Abstract State-of-the-art glazing systems can provide very good solutions for cold climate conditions, and fairly eective ones for warm climates. However, there is still no window system on the market that can oer the ¯exibility required to provide a comfortable visual environment and an ecient energy response in climates where heating is required in winter, and cooling is required in summer. This paper describes an experimental investigation of a novel glazing system, designed to overcome glare and radiation damage to interior furnishings, yet which causes no reduction in the energy eciency of the glazed opening compared with a conventional window used in direct gain systems. The proposed glazing system (patent pending) incorporates a rotatable frame holding two glazing components: transparent glazing providing a weatherproof seal, and absorptive glazing with a low shading coecient1. The absorptive glazing is ®xed at a small distance from the clear glazing, forming an airspace between them which is sealed at the sides but open at the bottom and top, so that air ¯ows freely through it. In summer, the absorptive glass faces the exterior of the building, absorbing excessive solar radiation and dissipating the heat to the ambient air. In winter, the glazing assembly is rotated so that the absorbing glass faces the interior, reducing glare but allowing eective convective and radiative heating of the adjacent space. # 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Glazed openings are a unique building element since they alone are designed to allow the passage of energy through the building envelope Ð whether in the form of visible light or of infra-red energy (`heat'). Properly designed windows may provide cost-free natural lighting and a signi®cant proportion of winter heating requirements through the utilization of solar energy. Yet data suggest that more than 30% of all energy use in buildings may nevertheless be attributed to undesirable heat transfer through windows and to arti®cial lighting [1]. Windows thus present a great challenge to * Corresponding author. Tel.: +972-7-6596875; fax: +972-76596881. E-mail address: [email protected] (E. Erell). 1 A patent application describing the window was ®led with the Israel Patent Oce in November 1997.
designers. How to provide the best possible visual environment in the building interior, while maintaining an optimum energy balance under varying environmental conditions throughout the year. The primary functions of glazed openings are to let natural light into the building and to allow a view of the outdoors. However, the uncontrolled in¯ux of solar energy through glazed openings has a number of drawbacks (not least where passive solar heating is used). . Large glazed areas, characteristic, for example, of modern oce construction but also typical of some residential and commercial buildings, may result in extreme overheating in summer. . In spaces ¯ooded with intense solar radiation, interior furnishings often suer from accelerated deterioration, such as fading of fabrics or degradation of plastics. . In locations and periods characterized by intense
0360-1323/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 1 3 2 3 ( 9 9 ) 0 0 0 3 9 - 6
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solar radiation, daylight levels in spaces with large openings are usually much higher than needed. Visual discomfort is also caused by glare resulting from the contrast between extremely bright window areas and more subdued interior surfaces. The development of advanced glazing materials provides solutions to some of these drawbacks in some but not all locations and environmental conditions. The new glazings were designed to modulate the passage of radiation by intercepting part of it, allowing only a certain component through the glass. This may be done either by altering the properties of the glass itself, or by applying a coating to the surface of the material. The problems associated with the energy performance of windows in cold climates, where solar radiation is nearly always welcome and illumination levels are rarely excessive, has been solved by the development of insulated glass windows with low-emissivity (low-e ) coatings. Transparent low-e coatings on glass Ð the UK Building Regulations de®ne a low-e coating as a surface having an emissivity of less than 0.2 Ð became widely available in the 1980s, and by 1990 accounted for over a quarter of the double-glazing market in the US [2]. The importance of low-e coatings lies in their capacity to reduce signi®cantly radiative heat loss through glazings Ð the best products claim a surface emissivity (center of glass) of less than 0.05. For example, by suspending two thin polyester ®lms with a low-e coating in an insulated space ®lled with krypton gas between two panes of glass, an American manufacturer, Southwall Technologies, was able to create a window with a center-of-glass conductivity (U-value) of less than 0.7 W/K m2. Since the shading coecient of such a window was 0.52, it was possible to generate a net seasonal energy gain from just northern light, even in fairly severe winter climates [2]. However, warm climates pose a dierent problem Ð providing natural lighting without imposing a severe penalty in terms of unwanted heat. The solution to this problem lies in the development of spectrally selective tints and coatings. Since the visible part of the solar spectrum accounts for only 47% of the energy received from the sun, an ideal selective surface for warm climates would transmit all (or most) of the visible spectrum, from about 0.39 to 0.74 m, while re¯ecting all of the ultra-violet and infra-red radiation. The appropriateness of a glazing for warm climates may be measured by an index called the glazing luminous ecacy, Ke, sometimes called the `coolness index', which is de®ned as the visible transmittance divided by the shading coecient [3]: Ke Tvis =SC where Tvis is the proportion of visible light at normal
incidence transmitted by a glazing, and the shading coecient (SC) is the ratio between the solar heat gain factor of a particular glazing and that of a standard, double strength clear glass sheet 3 mm thick. Tinted glasses vary in their capacity to transmit light of dierent wavelengths. Blue and green glasses tend to transmit a high fraction of the visible light, absorbing some of the infra-red radiation, while gray and bronze glasses do the opposite [4]. Both types of glazing have been used in warm climates as the outer pane in a double glazed unit, to provide what has been called `cool daylight'. An even better solution has been developed with the improvements in the manufacturing technology of lowe coatings. Low-e coatings were originally produced to transmit most of the solar radiation, up to a wavelength of about 3 m, re¯ecting long wave radiation from colder objects. However, it is now possible to produce low-e coatings which transmit only the visible part of the solar spectrum, re¯ecting not only the far infra-red, but also approximately half of the solar spectrum, above 0.74 m. The application of such coatings to the inner surface of the outboard pane of a double glazed window, particularly if this glass is a high-iron heat absorbing glass, results in a window well-suited to warm climates: The overall solar heat gain coecient of such a unit may be only 0.31, while the visual transmissivity may be over 60% [5]. The preceding glazing types are all characterized by ®xed optical properties. In either very cold or very hot climates, this presents no problem. However, in most temperate zone climates, summer conditions are such that solar radiation imposes an unwanted heat load on buildings, while winters are cold, so that passive heating by solar radiation is welcome. Research into socalled `electro-chromic windows' shows promise of eventually providing a glazing system with dynamic optical properties, capable of adapting to changing environmental conditions. This technology is based on the tendency of a number of solids, such as the oxides of tungsten, vanadium, molybdenum or titanium, to perform a reversible color change when an electric current or ®eld is applied to them. These materials may be suitable for use in doping sol-gel glass to create a glazing system which can alter its transmissivity in response to an electric signal [6±8]. However, research into electro-chromic windows is still at a fairly early stage. While the technology holds great promise for the future, there is no commercial application of this concept at the present. The problems yet to be solved include theoretical issues such as the means of controlling the spectral properties of the sol-gel in order to achieve the desired transmissivities, as well as the diculties associated with the increase in scale from very small laboratory samples to large scale commercial glazing of suciently high uniformity.
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435
Fig. 1. Schematic drawing of glazing system in: (a) winter con®guration and (b) summer con®guration.
2. The proposed glazing system Research conducted at the Center for Desert Architecture and Urban Planning2 culminated in the development and experimental evaluation of a novel design for an improved glazing system (patent pending) which is appropriate in locations where solar radiation is welcome in winter but may be undesirable in summer. The application consists of a reversible, ventilated glazing system. It incorporates a rotatable frame holding two glazing components: transparent glazing providing a weatherproof seal, and absorptive glazing with a low shading coecient. The absorptive glazing is ®xed at a small distance from the clear glazing, forming an airspace which is sealed at the sides but open at the bottom and top. The airspace may be ventilated either by thermosyphoning or by means of a small, solar powered electric fan. In winter, the window frame is rotated so that the absorptive glazing faces the interior. Solar radiation is transmitted through the clear exterior glazing, and is absorbed by the interior glazing, which is heated by this energy (Fig. 1a). Space heating is provided both by long wave radiation and by convection. Long wave radiation is emitted from the absorptive glass, and air is heated by contact with both its surfaces. Convective 2 The Jacob Blaustein Institute for Desert Reseach, Ben-Gurion University of the Negev, Sede-Boqer Campus, Israel.
heating is enhanced by the air ¯ow between the two panes of glass. The air ¯ow in the space between the two glass panes may be either purely thermodynamic, or fan assisted. When fans are employed, the heated air can be exhausted through the lower part of the system, reducing thermal strati®cation of the air in the heated space. Space heating is achieved but visual discomfort and damage to furnishings by short-wave solar radiation is reduced signi®cantly. In summer, the window frame is rotated so that the absorptive glass faces the exterior, intercepting incoming short wave solar radiation. The energy absorbed by the exterior glazing is dissipated by long wave radiation, and is prevented from being transmitted to the building interior by the clear interior glazing, since it is nearly opaque at wavelengths above 4 m (Fig. 1b). The energy released by the warm glass also sets up a thermosyphonic air ¯ow in the space between the two glazing components, preventing overheating of the air and removing unwanted energy. Overheating of the adjacent interior space is prevented and visual comfort is improved.
3. Experimental setup The experimental evaluation of the concept for the new glazing system was carried out in the test facilities of the Center for Desert Architecture and Urban Planning at Sede-Boqer, Israel. A glazing
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system constructed according to the principles previously described was installed in an existing test building, and its performance monitored in summer and in winter. In the building used for the experiment, the walls were constructed of 20 cm thick hollow silicate blocks, with 5 cm thick polystyrene insulation on the exterior and an acrylic plaster ®nish, painted white. The roof was a 12 cm thick concrete slab, with foamed concrete sloped to the drains. The building was insulated from the earth by 10 cm polystyrene. Interior ®nishes included plastered, whitewashed walls and a terrazzo tiled ¯oor. Each of the test rooms measured 2.7 3.5 m, with the long wall facing due south. The experimental glazing was installed on a large window measuring 1.4 2.1 m, at the middle of the south-facing wall of one of the rooms. Both rooms were exposed on three sides to the ambient air, with one of the short walls common to a service space in the building interior. The building was unoccupied during the experiment, and there were no internal sources of heat. The reference window had a standard aluminum frame, with ®xed panes at the bottom and top, and a central section with horizontal sliding panes. The ®xed panes consisted of hollow (double-skinned) polycarbonate sheet glazing 12 mm thick. The sliding panes were glazed with 4 mm clear glass. These were replaced with double glazed units with a 6 mm air space for part of the winter experiment. In the test window, a sheet of absorptive glass was ®xed parallel to the clear glazing; 10 cm high openings at the top and at the bottom of the window assembly allowed free movement of air through the gap between the two sheets of glass. The absorptive glass Ð a dark brown safety glass manufactured by the Phoenicia Co., Israel, model no. 510 (international code 3609) Ð was 8 mm thick, had a visible transmissivity of 9% and a shading coecient of 42%. The net glazed area was 2.25 m2. . In the summer mode, the clear glazing was ®xed on the interior, 35 mm away from the absorptive glass. . In the winter mode, four wooden frames with clear 4 mm glass were attached to the exterior of the frame holding the absorptive glass, forming an air gap. The width of this gap was initially 22 mm, but was increased during the experiment to 75 mm. As in the reference room, the clear glass was later replaced by double glazed units during the second part of the winter experiment. (Minor dierences in the geometry of the window between the summer and winter modes, especially in the width of the air gap, were due to technical reasons associated with the installation of the experimental setup in an existing building.)
The data collection system recorded the following data: . environmental conditions Ð dry bulb temperature, relative humidity, global solar radiation incident on the plane of the glazing and wind velocity and direction; . internal conditions Ð dry bulb temperature, light levels on a working surface near the window and black bulb radiation at a ®xed point in the test room; . conditions characterizing the glazing system itself Ð temperature pro®le of each of the glazed surfaces, vertical temperature pro®le inside the ventilated air space, air ¯ow rate through air space and global radiation on a plane parallel to the glazing.
4. Results and discussion The glazing system was monitored for a extended periods in winter and summer. The performance data for each mode were analyzed for sequences of at least a week, each following several days of climatization to the season's conditions. 4.1. Penetration of solar energy Solar radiation was measured on the exterior of a vertical, south-facing wall and inside the building, 20 cm away from the center of the two windows being tested, parallel to the plane of the glass. In summer, the primary function of the glazing system is to prevent the penetration of excess solar energy while allowing good daylighting performance, and thus to reduce the electricity requirements for space cooling and arti®cial light. The eect of the absorbing glass, when ®xed on the exterior of the window, was to ®lter solar radiation almost entirely (Table 1, Fig. 2). Noontime interior radiation levels were reduced by the absorptive glazing to approximately 5% of exterior levels, compared with about 37% of exterior levels, for standard 3 mm transparent glazing. In winter, the absorptive glazing faces the building interior, and does not interfere with the penetration of solar radiation through the clear exterior glazing. The solar energy transmitted through the experimental window is therefore identical to that of the reference window, and depends on the optical properties of the external glazing. On a typical, sunny winter day (13 January 1997) the solar radiation measured near the window on the interior of the reference room amounted to nearly 77% of the peak solar radiation measured at noon on the wall surface outside the window, and to nearly 72% of the daily total (Table 1).
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Table 1 Solar radiation on a vertical south-facing plane: interior and exterior data for typical sunny days at Sede-Boqer in winter (13 January 1997), and summer (2 August 1997) Summer
Interior Ð reference glazing Interior Ð experimental glazing Exterior
Winter
Daily total [Wh/m2]
Daily maximum [W/m2]
Daily total [Wh/m2]
Daily maximum [W/m2]
1280 140 2850
142 17 406
3734 3734a 5190
641 641a 837
a
In winter, solar radiation is considered as having penetrated to the interior of the room with the experimental glazing once it has been transmitted by the clear, exterior glass. The values in the table are therefore identical to those in the reference room.
The eect of the clear, reference glazing on solar radiation levels on the interior warrants a short discussion. Clear glazing has a transmissivity of about 85% at normal incidence, but at lower incidence angles, particularly below 458, transmittance drops o, having a value of close to zero at highly oblique angles [9]. During summer, the eect of solar elevation Ð over 708 at noon on 1 August Ð is such that a high proportion of direct (beam) radiation incident on a south facing wall is re¯ected even by clear glazing. In winter, when noontime solar elevation is lower Ð at SedeBoqer about 358 on 1 January Ð most of the direct solar radiation incident on a south facing wall may be transmitted through clear glass. Solar radiation surveys carried out at Sede-Boqer [10] indicate that beam radiation accounts for about 45% of the insolation on a vertical, south facing surface during August, but over 70% of the (much higher) insolation on such a surface in January. Thus the solar radiation measured near the window in the reference room in summer and winter is in accordance with other experimental data. 4.2. Black bulb temperature Human thermal comfort is in¯uenced by several fac-
tors, only some of which are aected by the properties and performance of the glazing system installed in the building in question. While the commonly available data refer to air temperature Ð dry bulb temperature, to be precise Ð the eect of radiative heat exchange is often overlooked. The eect of radiation can be assessed by calculating the mean radiant temperature (MRT), which is the solid-angle-weighted average temperature of the surrounding surfaces. While mean radiant temperature cannot be measured directly, the reading of a globe thermometer, also known as the black bulb temperature, gives a very close approximation of the MRT, provided there is no air movement. In this experiment, temperature was measured in a black bulb suspended at the center of each room, about 1.5 m above the ¯oor and 1 m from the window. The eect of the experimental glazing on MRT was evident both in summer and winter Ð the absorptive glazing eliminated the asymmetric overheating which results from exposure to intense solar radiation. In summer, the in¯ux of solar radiation is re¯ected in the black bulb temperature in the center of the reference room, which shows a marked increase from about 07:30 h, remaining above the air temperature till
Fig. 2. Eect of glazing on global radiation measured on a vertical surface parallel to the glass, on a typical sunny summer day (2 August 1997).
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about 18:00 h. At noon on a typical day, the black bulb temperature in the reference room was about 348C, or nearly 68C higher than the air temperature (Fig. 3a). In the test room, the eect of solar radiation was negligible, since nearly all of it was absorbed by the exterior glass. The black bulb temperature was within 18C of the air temperature throughout the whole day. In winter, the eect of the absorptive glazing was even more pronounced. The black bulb temperature measured in the test room was a little above the air temperature, and even on a sunny day (11 January 1997) reached a maximum of only 288C. In the reference room, where the clear glazing transmitted a much higher proportion of the incident solar radiation, the black bulb temperature on the same day reached a maximum of 478C. (Fig. 3b). The dip in the graph, noticeable especially in the curve for the reference room, was caused by partial shading of the black
globe by a vertical divider in the aluminum window frame. 4.3. Illumination levels The quality of daylight depends on many factors, such as the size and position of the window, room geometry, re¯ectance of room surfaces and type of glazing. Levels of illumination were measured in a grid of nine points evenly spaced throughout the room, at a height of 1 m above the ¯oor, and at several more points near the window, where variations in the intensity of the light were more pronounced. At noon on a clear summer day, nearly all of the test room had an illumination level of between 270 and 350 lux. The highest light level was only 390 lux, immediately adjacent to the center of the window. The very even illumination is attributed to the dimensions of the room, which is only 2.75 m deep; to the size of
Fig. 3. Eect of glazing on black bulb temperature measured near the center of the room, about 1 m away from the window: (a) on a typical summer day (2 August 1997) and (b) on a sunny winter day (11 January 1997).
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the window, which occupies 27% of the south wall; to the absorptivity of the external glass, which reduces the glare and intense light near the window which is characteristic of clear-glazed rooms under similar conditions; and to the high angle of incidence of the beam radiation with the exterior glazing, which resulted in much of the solar radiation being re¯ected. The level of illumination in the test room during sunny winter days was less uniform; the portion of the room near the window received direct solar radiation, albeit attenuated by the absorptive glazing, and was brighter than the rest of the room. However, the highest level of illumination recorded on a bright sunny day in this room was only 3600 lux, compared with over 60,000 lux in the reference room, and over 80,000
439
lux outdoors (Figs. 4a and b). The levels of illumination in the test room, outside the patch of direct sunlight, ranged from about 200 to 600 lux. The comparable levels in the reference room were about 4000±10,000 lux. Recommended levels of illumination vary according to the type of activity. Most spaces, such as schools and oces, require illuminance of 300±500 lux on desks or work areas, with slightly higher levels, 500± 750 lux, being required for some specialized tasks [11]. Thus the level of illumination provided in the test room on a sunny day is adequate. However, it is important to note that the actual level of illumination in a room equipped with this type of glazing system may be adapted to local levels of insolation through the selection of an appropriate absorptive glass. The one used in this experiment is probably one of the most highly absorptive glazings available today, with a visible transmissivity of only 9%. 4.4. Temperature of the absorptive glass On clear, sunny winter days, when peak global solar radiation on a vertical surface parallel to the test window was close to 900 W/m2 at noon, the maximum temperature of the absorptive glazing was generally 45±508C, with temperatures exceeding 508C recorded on several occasions. The width of the air gap, 22 or 75 mm, did not have a signi®cant eect on the temperature of the absorptive glass, a 15% dierence in the mean maximum air speed inside the channel not withstanding. Thermal images of the window con®rm the results of point measurements taken at four locations on the glass surface, indicating a very uniform temperature distribution Ð the warmest part of the glass was only 2±38C warmer than the coolest part. In summer, the absorptive glass, which faced the exterior in this mode, was cooler than in winter. The maximum daily temperature was generally between 40 and 458C, or about 108C above ambient air temperature. The dierence in glass temperature compared with winter is explained by two factors. In Sede Boqer, mean daily levels of solar radiation incident on a vertical south-facing wall are much lower in summer, between 2.5 and 2.7 kWh/m2/day, than those in winter, 4.0±4.6 kWh/m2/day [10]; and since in summer the outer face of the absorptive glass is exposed to the wind, rather than to still interior air, as in winter, radiant energy absorbed in the glass is dissipated more readily by convection. 4.5. Convective heat output (winter)
Fig. 4. Illumination level (in lux) on a typical sunny winter day: (a) in the test room, showing the eect of the experimental glazing system and (b) in the reference room.
Energy absorbed in the absorptive glazing is released to the building interior by two mechanisms. It may be emitted in the form of long wave radiation, and it may
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warm the air in contact with the glass. As air in the ventilated space between the clear and absorptive components of the glazing system was heated in this manner, a thermodynamic ¯ow was set up. The temperature gradient created in this airspace was measured using radiation shielded thermistors placed at three points Ð near the inlet at the bottom, near the outlet at the top, and midway between them. The velocity of this air¯ow was measured by means of a hot-wire anemometer. The width of the air gap was initially ®xed at 22 mm. This width, about twice the maximum width of the gap used in conventional air-®lled double glazed windows [2,5], was considered sucient to allow free movement of air. At a later stage, the distance between the clear glazing and the absorptive glass was increased to 75 mm, in the expectation that air¯ow would increase. The width of the air channel had a pronounced eect on the operation of the glazing system. Table 2, based on average data for a one-week period of operation in each con®guration, summarizes the eects of increasing the distance between the clear glazing and the absorptive glass. The environmental conditions during the two weeks in question, in particular solar radiation, ambient air temperature and interior dry bulb temperature, were nearly identical. . The narrow (22 mm) gap allowed what appeared to be an unimpeded air ¯ow, with airspeeds of up to 0.7 m/s measured at the center of the channel. When the width of the air gap was increased to 75 mm, the mean airspeed in the channel, measured at several points, was reduced by 15%. The combined eect, however, was an increase in the mass ¯ow rate through the channel by a factor of 3. . When the air gap was narrow, the temperature dierence between the channel inlet and outlet on sunny days was over 218C (Fig. 5). The mean daily maximum outlet temperature under these conditions was 42.38C, reaching temperatures of over 458C on several days. When the air gap was increased to
75 mm, the increase in the air temperature was limited to an average of 11.68C, and did not exceed 138C. The temperature of the air at the outlet of the channel under these conditions was about 358C on sunny days (Table 2). . The temperature dierence between the air at the inlet of the air gap and at its outlet, multiplied by the mass ¯ow rate and the heat capacity of air gave the net convective heat output of the system. This quantity varied in response to the environmental conditions, particularly the availability of solar radiation. When the air channel was narrow, the maximum daily convective heat output was a little over 200 W/m2; increasing the gap width resulted in an increase in peak convective heat output to an average of nearly 300 W/m2, reaching 350 W/m2 or more on some days (Fig. 6). The convective heat output is the result of the formation of a natural thermosiphon between the two glazing components. The vertical temperature pro®le and the resulting air¯ow are aected by the vertical dimension of the air gap. Thus, the convective heat output reported here, measured in a window with an unobstructed height of nearly 2 m, should be regarded only as an indication of the extent of this phenomenon. Figs. 5 and 6 also demonstrate a reversal of the ¯ow pattern in the air gap at night, between about 18:00 and 08:00 h. In the absence of solar radiation, the exterior glazing cooled down rapidly. As interior air in contact with this glazing was cooled through contact with it, it became denser and a thermosiphon was established. Cool air, as much as 58C below room air, was exhausted from the lower opening of the air channel, and warmer air was drawn in through the upper opening near the ceiling. The air speed during the night hours was up to 0.2 m/s, and the resulting convective cooling rate was between 10 and 15 W/m2. The eect of increasing the distance between the clear glazing and the absorptive glass is summarized in Table 2.
Table 2 The eect of the distance between the clear glazing and the absorptive glass on various operating parameters of the experimental glazing system in the winter mode (mean values are based on experimental data for a period of 1 week in each of the two con®gurations) Width of air gap
Maximum air speed in channel [m/s] Maximum temperature dierence between outlet and inlet [8C] Maximum temperature of absorptive glass [8C] Maximum air temperature at channel outlet [8C] Peak convective heat output [W/m2] Daily convective heat output [Wh/m2 day] Daily convective heat output as proportion of the daily total solar radiation transmitted to the building interior [%]
22 mm
75 mm
0.64 21.1 48.9 42.3 186 682 19.6
0.54 11.6 49.5 34.1 298 1242 37.3
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Fig. 5. Vertical temperature pro®le inside the air channel in the experimental glazing on a sunny winter day (11 January 1997); width of the air gap 22 mm.
It should be noted that the overall energy performance of the glazing system is aected by the width of the air gap only marginally; its main eect is to alter the proportion of the two main components of the energy given o Ð long wave radiation and convection. When the air gap was narrow, the convective component was equivalent to nearly 20% of the total solar radiation measured inside the reference room window (which had clear, single glazing). Increasing the distance between the two main components of the window produced a convective output equivalent to about 37% of the incoming solar radiation.
4.6. Internal air temperature The dry bulb temperature of a building interior depends on many factors, including the amount of insulation installed and the thermal capacity of the structure. The building available for this experiment was very massive, being constructed of concrete and 20 cm silicate building blocks, and very well insulated. It was therefore expected that the temperature variations between the room ®tted with a reference window and the one ®tted with the test window would be relatively small.
Fig. 6. Convective heat output of the experimental window in W/m2 of glazed surface on a sunny winter day (21 January 1998), compared with the solar radiation incident on the glass; width of the air gap 75 mm.
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Dry bulb temperature was measured in each room at three elevations above the ¯oor Ð 0.5, 1.5 and 2.5 m, using thermistors shielded from the eects of radiation by highly re¯ective aluminum foil housings. Mean values were calculated from these sensors. In winter, the function of the absorptive glazing, which faced the building interior, was to convert incoming short-wave solar radiation transmitted by the exterior glazing into two main components: long-wave radiation and warm air. The overall energy balance of the room was not expected to show signi®cant dierences compared with the reference room. The dierence in mean air temperature between the rooms was in fact quite small, and is attributed to minor dierences in the thermal conductivity of the ceiling of the two rooms (Table 3). While the mean air temperature of the test room was similar to that of the reference room, the thermosiphonic movement of air created by the experimental window coupled with the position of the air outlet at the top of the window near the ceiling, resulted in pronounced thermal strati®cation. The extent of this phenomenon was related to the overall convective heat output of the window, and was therefore more noticeable on sunny days. The maximum dierence between the temperature recorded near the ceiling and that measured near the ¯oor was generally about 68C when the gap between the two glazing components was wide (Fig. 7), and about 48C when it was narrow. In the reference room, there was no noticeable thermal strati®cation, and the three temperature sensors were within 0.58C of each other. While the vertical temperature pro®le in the test room appears excessively uneven, it should be noted that for the duration of the experiment, the room was closed and unoccupied. It is expected that normal occupancy would result in better mixing of the air. The thermal imbalance may also be reduced by de¯ecting the air at the outlet of the window down by means of a bae, by lowering the elevation of the outlet, which in this case was about 2.5 m above ¯oor level, or through the use of an electric ceiling fan. Another possibility is the installation of a small fan in the glazing component, powered by a photo-voltaic cell integrated in the frame, which would
force the air down the channel between the two glass sheets. In summer, the air temperature in the two rooms was nearly identical. The large thermal inertia and highly insulated exterior walls of the test building resulted in a daily amplitude of only 1±28C. The eect of the experimental glazing was barely discernible, with daytime maximum temperatures about 0.58C lower in the test room relative to the reference room. 4.7. Discussion The main advantage of the glazing system described is its ¯exibility in response to the con¯icting demands of winter and summer conditions in many locations. In most temperate climates, solar energy is welcomed in winter as a means of reducing heating loads. Summer overheating, though, is a problem wherever large glazed areas are incorporated into the exterior envelope of buildings. The proposed glazing system addresses each of these con¯icting requirements. It provides the full bene®ts of passive solar heating by direct gain, widely acknowledged as the most practical method for solar heating; it practically eliminates most of the undesired side-eects which characterize buildings with extensive glazed areas, such as glare and visual discomfort; and it provides a simple means of reducing the penetration of solar radiation in summer, including the non-directional diuse component, without obstructing the view through the glazed area. The successful application of the experimental design requires the manufacture of an appropriate window frame. Such a frame must have the following properties. . The two glazing assemblies, incorporating the clear glass and the absorptive glass, must be able to rotate together through 1808, so that the absorbing glass will face either inwards or outwards, depending on the season of the year. Rotation of the window would normally be expected to take place only twice a year, i.e. on a seasonal basis. . In each of the two opposing con®gurations, the glazing assembly incorporating the clear glass must provide a weatherproof seal, capable of ful®lling current performance codes in the window industry.
Table 3 Air temperature in test room and reference room during winter experiment
Test room Reference room Ambient DBT
Temperature [8C] Single glazing experiment
Temperature [8C] Double glazing experiment
Daily mean
Daily maximum
Daily minimum
Daily mean
Daily maximum
Daily minimum
13.9 13.4 10.6
18.0 17.9 15.4
12.0 11.6 5.9
15.1 14.3 11.6
21.3 20.6 16.7
13.0 12.4 6.4
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Fig. 7. Thermal strati®cation in the test room, produced by the experimental window on a sunny winter day (21 January 1998). Curves indicate air temperature at speci®ed height (in cm) above the ¯oor.
. The air gap between the two glazing assemblies should be accessible for cleaning. This may be done by mounting the absorptive glazing on a hinged frame. . The rotation of the glazing system, from winter to summer mode or vice versa, should be convenient and must not require the use of special equipment. The bene®ts of the proposed glazing system may be realized through careful selection of the absorptive glass. The optical properties of this glass, which may be one of a large number of glazing types currently being manufactured, should be speci®ed in response to local climatic conditions. For example, in high-latitude countries, where winter levels of solar radiation are low, selective coated glass may be used to absorb radiation mainly in the near infra-red part of the spectrum, but transmitting suciently high levels of visible light to permit eective daylighting of the interior space. In such a glazing assembly, the summer con®guration will be less ecient than one where the absorbing glass has a lower shading coecient, but it may still prevent overheating. In areas where winter insolation levels are often intense, such as in most of the Mediterranean areas, an appropriate glazing would be required to absorb some of the visible light, as well as all of the near infra-red spectrum. 5. Conclusion State-of-the-art glazing systems can provide very good solutions for cold climate conditions, and fairly eective ones for warm climates. However, there is still
no window system on the market that can oer the ¯exibility required to provide a comfortable visual environment and an ecient energy response in climates where heating is required in winter, and air-conditioning in summer. The proposed ventilated reversible glazing system is a technically viable solution to these requirements: its energy eciency is equal to that of solar heating by direct gain, yet visual comfort and protection from excessive short wave radiation may be provided if the appropriate absorptive glazing is selected in each locality. In summer, it may improve visual comfort even in spaces with large glazed areas facing east or west, while reducing undesired energy gains to a minimum. Acknowledgements The authors wish to thank Prof. Amos Zemel for his helpful comments on the manuscript. The Phoenicia Co. of Israel donated the absorptive glazing used in the experiment. References [1] Selkowitz SE. Windows and daylighting group 1990 annual report. Lawrence Berkeley Laboratory, California, 1990. [2] Johnson TE. Low-e glazing design guide. Boston: Butterworth Architecture, 1991. [3] Arasteh D, Johnson R, Selkowitz S. De®nition and use of a daylight `coolness' index. In: Proceedings of International Daylighting Conference, ASHRAE, Atlanta, 1986. [4] Schuman J. Cool daylight, Progressive Architecture, 4(92), 136± 41.
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[5] PPG Solutions 1996. Product information sheet, 1996. [6] Reisfeld R, Jorgensen CK. Optical properties of colorants or luminescent species in sol-gel glasses. In: Reisfeld R, Jorgensen CK, editors. Chemistry, spectroscopy and applications of sol-gel glasses, 1991. [7] Reisfeld R. Theory and applications of spectroscopically active glasses prepared by the sol-gel method. In: Proceedings of the SPIE International Symposium on Optical and Optoelectronic Applied Science and Engineering, Sol-Gel Optics, San Diego, California, 8±13 July, 1990.
[8] Donnadieu A. Electro-chromic materials, Mat. Sci. and Eng. 1985;B3. [9] ASHRAE. 1989 Fundamentals handbook. Atlanta: ASHRAE, 1989. [10] Faiman D, Feuermann D, Ibbetson P, Zemel A. Data processing for the Negev radiation survey: fourth year. Jerusalem: Israel Ministry of Energy and Infrastructure, 1996. [11] Baker N, Fanchiotti A, Steemers K. Daylighting in architecture, a European reference book. London: James & James, 1993.