Potential of emerging glazing technologies for highly glazed buildings in hot arid climates

Energy and Buildings 40 (2008) 720–731 www.elsevier.com/locate/enbuild

Potential of emerging glazing technologies for highly glazed buildings in hot arid climates AbuBakr S. Bahaj, Patrick A.B. James *, Mark F. Jentsch Sustainable Energy Research Group, School of Civil Engineering and the Environment, University of Southampton, Highfield, Southampton SO19 1BJ, UK Received 22 March 2007; received in revised form 14 May 2007; accepted 14 May 2007

Abstract In order to improve the sustainability of buildings one of the challenges is to address the role of the building envelope as the key climate moderator between the internal and external environments. The envelope is exposed to the elements and needs to control air exchange as well as sunlight and sound passing through to the occupants. Therefore, it has a major impact not only on the energy utilisation within the space it controls but also on the quality of comfort. However, inside highly glazed modern buildings, achieving good comfort is often at the cost of high-energy consumption. Therefore, in the light of ever increasing energy costs, improved fac¸ade design can contribute to a reduction of operational costs. The aim of this paper is to explore technical, economic, environmental and indoor comfort implications of emerging glazing technologies for energy control of highly glazed buildings in arid Middle Eastern climates, which is one of the harshest climates for this building type. The work includes predictions through thermal simulation of the impact of electrochromic glazing, holographic optical elements (HOE), aerogel glazing and thin film photovoltaics on two example buildings. Potential reductions in cooling demand are assessed. # 2007 Elsevier B.V. All rights reserved. Keywords: High rise buildings; Glazing; Fac¸ade; Air conditioning; Thermal simulation; Arid climate; BiPV

1. Background 1.1. Daylight architecture and sunlight architecture The primary function of a building is to provide a secure shelter from the elements, regardless of the climatic zone within which it is located [1,2]. In this respect, the fac¸ade, which acts as the primary climate moderator, is a key component in ensuring comfortable indoor conditions [3]. Over centuries the fac¸ade, in vernacular architecture, has been optimised towards specific regions and climates [4]. The temperature level and the availability of sunlight play key roles in determining the architectural appearance of traditional building forms. It is possible to distinguish between daylight architecture as it is found in central Europe (Fig. 1) and the sunlight architecture of hot arid climates such as the Arabian Peninsula (Fig. 2). Daylight architecture is characterised by window openings of a large height to maximise daylight penetration into the building,

* Corresponding author. Tel.: +44 2380 593941; fax: +44 2380 677519. E-mail address: [email protected] (P.A.B. James). 0378-7788/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2007.05.006

whilst sunlight architecture tries to avoid solar gain by using small windows and appropriate shading devices such as the traditional Arabic window opening with transmission reducing ornamental structures (arabesques). For centuries, window glass was an expensive product and therefore large glazed areas were seen as a visible indicator of the wealth of a building owner. For example, in the UK between 1696 and 1851 building taxation was imposed based on the number of windows above 6 on the main fac¸ade. This wealth indicator tradition continues today with glass still being considered to be a prestigious material. It is furthermore associated with positive values of openness, transparency, inside–outside connection, freshness, modernity and brightness. During the industrial revolution the availability of glass as a mass product at a considerably lowered price alongside new construction possibilities in steel and concrete allowed larger glazed areas to be realised. However, highly glazed buildings constructed in Europe and America within the modern architecture movement at the beginning of the 20th century soon revealed negative effects in terms of indoor comfort. Buildings were difficult to heat in winter and tended to overheat

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Fig. 1. Daylight architecture—Old market place in Stralsund, Germany.

in summer. This was compensated by advances in central heating systems, glazing technology (double glazing) and the invention of compressor-driven air-conditioning systems in the beginning of the 20th century [5]. Furthermore, these technologies enabled the construction of fully glazed skyscrapers, as first conceived by Mies van der Rohe in the 1920s [6]. However, extensive utilisation of heating and cooling facilities in order to compensate for the negative effects caused by large glazed areas implicitly leads to high energy consumption and, increasingly, high costs. Nevertheless, highly glazed buildings, which have their roots in daylight architecture, have become a world-wide standard for non-domestic constructions such as offices and hotels.

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emergence of the Emirates Airline as a major world player, since its establishment in 1985 [7]. The future market for glazing technologies in this region is therefore likely to be dominated by hotels, holiday homes and retail with a smaller office component. The fac¸ade performance requirements of hotels (room accommodation) and office spaces (computer terminals) are potentially very different. The majority of office users work within a specific space for a prolonged period and so it is, therefore, important that the conditions are appropriate to their needs. Daylight issues such as glare impact on office users, but are of far less of a concern to the hotel industry. A comparison between the modern high-rise glass structures being constructed in emerging Middle Eastern cities and areas such as Dubai today and the traditional buildings of countries such as Yemen (Fig. 2.) reveal stark differences. The latter represent the vernacular architecture of the region, which is based on principles of high thermal mass and natural ventilation with small openings in the fac¸ade. This is in complete contrast to steel framed, curtain walled towers often being constructed in the region today, which are essentially a development of the daylight architecture of moderate climates. In the Middle East, such structures can only function through extensive mechanical support, essentially in the form of air-conditioning which is in turn reliant on low cost, fossil fuel derived electricity [5]. 1.3. High rise glass buildings

Many regions in the Middle East; most notably Dubai; are attempting to ‘reinvent’ themselves as major international holiday and shopping destinations to reduce their future dependence on income from oil. This can be seen not only from the continual and rapid construction in the region but the

All around the world, be it in for example, New York, London, Frankfurt, Hong Kong or Houston, the skyline is dominated by glass towers. It is surprising that there is apparently one building form appropriate for such different climates and cultures. This inconsistency becomes evident when comparing the global horizontal irradiance levels and temperature profiles of for example, Dubai and London (Figs. 3 and 4). Whilst the majority of high-rise tower buildings in Europe, America and the Far East are constructed as office buildings in order to give a company statement, in the Middle East the high rise building market is dominated by hotels and residential towers. For example, the tallest hotel in the world is

Fig. 2. Sunlight architecture—Buraa District, Tahamah Province, Yemen.

Fig. 3. Monthly horizontal irradiance distribution (kWh/m2 month) of Dubai (top) and London (bottom) (data source Meteonorm weather database [8]).

1.2. Contemporary architecture in the Middle East

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create a major potential market for emerging glazing technologies. A radical change of built form in the near future appears unrealistic even though it is perhaps desirable from a sustainability perspective. Therefore, advanced glazing and solar control technologies are key to improving current performance levels and represent a first step towards higher sustainability of highly glazed tower buildings if these continue to be constructed. Section 2 gives an overview of the development in glazing related fac¸ade technologies in terms of solar control, daylighting and power generation (photovoltaics) and their potential benefit to the Middle Eastern market. Emerging technologies are compared with solar control, low-e glazing which is the current industry standard in this market. Fig. 4. Annual temperature profiles of Dubai (top) and London (bottom) (data source Meteonorm weather database [8]).

currently the Burj-al-Arab in Dubai at 321 m (see Fig. 9 schematic), which will be surpassed by the Rose Rotana Suites, also in Dubai, which are currently under construction and will be 333 m high. Whilst there are advantages to high-rise glass towers such as small land take and constructability (reduced risk) their adoption in certain climates appears at best inappropriate and at worst cavalier in attitude. In essence, tower blocks can be considered to be the most vulnerable building type to climate variations and in particular solar impact (Fig. 5), since they have large surfaces exposed to the elements and often an unfavourable volume to surface ratio [2]. Studies in Germany of highly glazed buildings show that glass towers in particular consume large amounts of energy and/or are at risk of having an uncomfortable room climate if no full mechanical cooling system is provided [9]. It is perhaps the case, that in areas such as Dubai building form is often driven by status and prestige rather than more fundamental environmental control, which vernacular architecture was forced to address as its focus. Whilst this can be seen as a failing – predominantly driven by people’s expectations that building form should be the same regardless of climate, with glass the building material of choice – it does

Fig. 5. Schematic representation of a tower block and a low rise building with identical volume showing why tower blocks are considered to be the most vulnerable building type to extreme climate conditions.

1.4. Comfortable indoor environment Assessment of comfort requirements for hotels and residential buildings is potentially more difficult than for offices as user perception may vary more significantly depending on circumstances, experience, cultural and intellectual background. However, basic needs for a well-received interior space can be derived from studies on office environments. Extensive UK office post occupancy evaluation (POE) studies have identified key parameters, which are crucial in determining how successful a workspace is in realising the target of ‘happy and productive’ office users. Studies by Leaman and Bordass [10,11] concluded that an ideal UK office space should be naturally ventilated and shallow plan with manual components for the users to operate (windows, heating, ventilation ducts, etc.). This helps to provide good comfort, which appears to be closely related to ability for personal control. Users need to be able to easily comprehend these component functions (window openings, lighting scheme, etc.) in order to prevent failure in performance and to achieve high satisfaction levels inside the building. Furthermore, it is important that the design intent of a building is understood by the facility manager and the users. In the event of building related problems being noted by users, a quick reaction by the facility management is vital to prevent dissatisfaction. Studies at the University of Southampton [12,13] have shown that a failure to respond quickly to users’ needs can result in a reinforcement of negative perceptions over time even if performance improvements in the environmental room conditions are ultimately achieved. Furthermore, Leaman and Bordass [11] have identified ventilation type as one of the key components related to productivity and comfort. In particular, inside air-conditioned spaces, office users can experience health problems, the so called ‘sick building syndrome’ or ‘building related ill-health’. Generally it can be stated that a controlled environment such as in an air-conditioned space requires higher levels of facilities management and committed staff to compensate for the lack of personal control. In conclusion, the Leaman and Bordass work [10,11] suggests that air conditioned spaces are less likely to be favoured by office users if a naturally ventilated/mixed mode operation of a building is possible.

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Highly glazed tall buildings in the Middle East, have little choice but to commit themselves to a fully mechanically controlled ventilation and cooling strategy. Buildings often become fully automated spaces controlled by complex building management systems (BMS). The potential for personalisation of space through manual intervention and in particular user interaction with the fac¸ade is therefore at best reduced if not impossible. However, the majority of these Middle Eastern buildings can be considered as thoroughly managed, high-end constructions with a fast responding facility management regime which means that building related ill-health is less likely to happen. However, this strategy implies a cost premium in operation as well as a potentially high energy consumption to satisfy user needs. Such a high level of automation in many aspects enhances the potential of certain emerging glazing technologies, in particular, those which enable the regulation of daylight and solar gain, such as switchable electrochromic glazing (see Section 2.1). This may help to save both costs and energy. 2. Overview of emerging glazing technologies The fac¸ade of a building should address thermal, acoustic and visual comfort [14,15]. The glazing in particular needs to contribute to providing appropriate thermal and visual comfort. In terms of energy performance, avoiding solar gain whilst providing the required level of air exchange, are the key parameters for an air-conditioned space in Middle Eastern locations such as Dubai. Consequently, emerging glazing technologies with potential to enhance energy performance of highly glazed buildings have been reviewed across the areas of (Section 2.1) high performance insulation (HPI), solar control (SC) and daylighting (DL). In addition, the potential of (Section 2.2) photovoltaic fac¸ades (PV) has been assessed. Certain technologies such as aerogel glazing (see Section 2.1) can be viewed as potentially providing performance benefits across more than one area. In general, solar control is achieved using two or more fac¸ade components such as an openable window in conjunction with an internal blind. A ‘simple’ smart fac¸ade progression would be, for example, the ‘interpane’ blind where the blind elements are incorporated within a double glazed unit. In this case, combining the two technologies has clear advantages producing an aesthetically pleasing solution, with low maintenance cost (no dust cleaning issues for the blind lamellae) and reduced solar heat gain to an office space. However, the integration does create problems of high window pane temperature resulting in the potential for radiant discomfort for office users close to the fac¸ade. At present, for many of the ‘smart’ glazing technologies which are discussed, there appear to be these ‘incidental’ problems which can serve to reduce their true market potential. Seven technologies have been assessed across a range of criteria essentially addressing performance (user comfort and impact on building carbon footprint), operation and maintenance (O&M), availability, lifetime and risk. The desire is for new glazing and fac¸ade technologies to

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create energy efficient spaces in which people wish to live and work. If they cannot deliver this, these technologies will have failed. A simple overview of the seven investigated glazing technologies in terms of performance and risk criteria is shown in Fig. 6 comparing them to state of the art low-e glazing and tinted glass. No one glazing technology can be viewed as a ‘magic bullet’ which can be considered to ‘tick all the right boxes’. Indeed high performance, low-e glazing with an appropriate shading solution to reduce solar gain can still in many respects be viewed as superior. 2.1. High performance insulation, solar control and daylighting solutions 2.1.1. Aerogel glazing (HPI and DL) Aerogels are a class of open celled mesoporous solids with a minimum porosity of 50% by volume. They have a density ranging from 1 to 150 kg/m3 and are typically 90–99.8% air. Aerogels can be formed from a variety of materials including silica, alumina, transition and lanthanide metal oxides, metal chalcogenides, organic and inorganic polymers and carbon. Aerogel, as an insulation material was originally developed by NASA [16,17] and is the lowest density solid known—often termed solid air. Significant capacity of translucent curtain walling using aerogel started to appear on the market in 2006 [18]. Polycarbonate construction panels which enclose the granular aerogel weigh less than 20% of the equivalent glass unit and have 200 times the impact strength. Light transmission and Uvalue of aerogel panels are a function of panel thickness. For example, a 25 mm thick panel (equivalent to a standard 6/12/ 6 mm double glazed unit, typical U-value 1.4 W/m2 K) would have a U-value of 0.57 W/m2 K, a light transmission of 45% and a g-value 0.43 (solar heat gain coefficient of 43%) [19]. Such high performance, coupled with low density and excellent light diffusing properties make them particularly

Fig. 6. Simple evaluation of glazing technologies in terms of performance and risk criteria. For example, electrochromic glazing has good solar control performance (+), but has an unproven lifetime ( ).

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appropriate for rooflight applications (e.g. shopping malls and swimming pools). Clear aerogel glazing offers the promise of exceptionally low U-value windows (0.1 W/m2 K), better insulating than a modern wall construction. However, at present this technology is far from market and remains stubbornly at the R&D phase. Originally developed by NASA [18], aerogel glazing is often portrayed as the ‘holy grail’ of future glazing. However, the largest aerogel windows produced to date are approximately 1 m2 in size [20]. In general, they have poor optical clarity and are brittle. Despite these problems, the long-term potential of the technology is very high. 2.1.2. Vacuum glazing (HPI and DL) At present high performance, low-e double glazing can produce U-values of the order of 1.0 W/m2 K. Vacuum glazing available today offers a step change reduction down to 0.2 W/ m2 K [21]. There are however, vacuum lifetime issues with the technology at present, primarily as a result of market caution associated with the limited number of long-term case study examples. The technology is effectively competing with aerogel to produce mass market ‘super-insulation’ as it can also be used to produce high performance vacuum insulation panels (VIP). 2.1.3. Switchable reflective glazing (SC and DL) Active thin films on glazing have long been seen as a very attractive approach to solar control and daylight regulation in highly glazed buildings. Whilst external shading devices are almost always the ‘best’ approach to solar control there are many reasons why their application is considered undesirable such as capital cost, aesthetics, maintenance, structural restrictions and wind loading, especially on high rise buildings. Reflection of solar gain from a fac¸ade would be the best alternative but this can create glare issues for the exterior surrounding which is critical for architectural applications, in particular for high-rise buildings. Therefore, a possible application may be light guiding elements such as switchable reflective light shelves [22]. At present, switchable reflective glazing is at the early stage of laboratory development [23,24]. 2.1.4. Electrochromic glazing (SC and DL) Electrochromic glazing and associated technologies such as user controllable photochromic [25] and gasochromic [24] windows are essentially variable tint glazing. The active layer is a film which either requires a low dc voltage (electrochromics, user controllable photochromics) or hydrogen (gasochromic) to change from a bleached to a coloured state. The most advanced of these technologies are electrochromic devices which are commercially available at small scale. To reduce the solar gain a dc voltage is applied to the glass and the tint of the glazing becomes progressively darker. However, as the glass becomes darker, the radiant temperature of the window pane will rise due to the greater absorption of light. Nevertheless, such a technology has strong potential for air-conditioned buildings in particular where the problems associated with the high

window pane temperature can, to a certain extent, be moderated. The visual light transmittance of electrochromic glazing is typically between about 50 and 60% in the bleached and 5 and 15% in the coloured states respectively [26,27]. The g-value is typically around 0.5 for the bleached state and 0.1 for the coloured state [27]. A commercial assessment of electrochromic glazing by Lawrence Berkley National Laboratory suggests that for air-conditioned offices, reductions in cooling loads of 19–26% would be expected [28]. However, at present there are a range of both technical and non-technical issues that need to be addressed to enable significant market penetration. Technical issues include: (i) Switching time: 5–10 min from fully coloured to bleached states. (ii) Glare: many switchable glazings require secondary glare protection, in particular for office use. (iii) Colour rendering: the luminous colour provided is very different to the normal spectrum and so could be considered to be inappropriate for many building applications. Non-technical issues include: (i) Cost: premium over low-e double glazing is too high at present. (ii) Lifetime: early adopter risk is high with typical warranties of only five years being offered at present on the switching performance of the glazing. This is very short compared with the 30 years lifetime of a more traditional fac¸ade technology. Research is also in progress to address some of the ‘hot pane’ drawbacks of electrochromics by combining them with for example vacuum panel glazing [29]. Furthermore, user acceptance of electrochromic windows is also being investigated [26]. 2.1.5. Suspended particle devices (SC and DL) Suspended particle devices (SPD) essentially perform the same function as electrochromics. An SPD film is laminated between two panes of glass in which light absorbing molecules are randomly orientated in their normal (unenergised) state, creating a light absorbing film. If a current is applied, the molecules align perpendicular to the plane of the glazing, creating a transparent glass [22,30]. Unlike electrochromics, the switching time is very fast (1 s) but the issues of radiant temperature, glare, colour rendering, clearness and lifetime are still potential problems. 2.1.6. Holographic optical elements (DL or SC) Holographic optical elements (HOE) are light guiding elements consisting of a holographic film laminated between two sheets of glass. Direct sunlight incident on a fac¸ade is redirected at a predefined angle through diffraction at the holographic interlayer (Fig. 7). The primary application is seen

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Fig. 7. Function of a holographic optical element (HOE) for redirecting incident direct sunlight.

as enhanced daylighting where natural light can be redirected onto the ceiling of an internal space. This natural light should enhance comfort for users and reduce artificial lighting requirements. This in turn reduces the air-conditioning loads associated with the heat gains that artificial lighting produces. However, the diffraction efficiency of the transmitted visible light is only in the range of 25–55% which means that glare effects cannot be prevented [31]. Further potential problems include edge effects with light dispersion into rainbow colours and the transparency of the holographic film which appears slightly milky. Reflection holograms, which reflect incident solar radiation whilst transmitting diffuse daylight, can in principle be used to reduce the solar gain of a fac¸ade whilst still allowing users a view to the exterior. Their operation is restricted by the hologram exposure to certain solar azimuth and zenith angles and so a fixed solution may be used only for ‘peak clipping’ of solar gain [32,33]. To ensure high reflection or light guiding efficiency for a prolonged period of the day, light guiding holograms must be applied as external, tracked systems [31,32]. The high maintenance burden associated with tracked systems makes their use unattractive and it is difficult to see how this technology will progress beyond building demonstration unless this can be overcome. At present reflection holograms for building integration are not yet available as a commercial product.

Fig. 8. Fac¸ade with building integrated photovoltaics (BiPV) at the Umwelt Campus Birkenfeld, Germany using thin film solar cells. The thin film material has been etched following deposition to increase visual and light transmission.

3. Case study—Jumeriah beach complex, Dubai Of the technologies introduced in Section 2, the potential of electrochromic glazing, aerogel glazing, holographic optical elements (reflection holograms) and thin film photovoltaics (present and future technologies) has been analysed and compared to low-e glazing when applied to a hotel development in Dubai. The Jumeirah beach complex comprising the Burj-al-Arab Tower (Fig. 9) and the Jumeirah beach hotel (Fig. 10) is a prestigious development utilising the latest building management systems (BMS) to control temperature, lighting and humidity. The set point conditions of the rooms of both hotels

2.2. Photovoltaic fac¸ades (PV) At present, the majority of building integrated photovoltaics (BiPV) consists of mono-crystalline (MC) silicon wafer solar cells connected in series to produce glass panels (modules) of the appropriate power rating. In the future it is hoped that thin film (TF) solar cells (a series of active layers deposited onto glass or other appropriate substrates) will become a common fac¸ade element (Fig. 8). At present, cost, lifetime (especially for thin film solar cells) and cell efficiency (conversion of sunlight into electricity 10% for TF, 18% for MC) are all drawbacks [34]. Theoretically, third generation thin films with efficiencies of 60% or greater are possible [35]. This offers excellent potential for BiPV fac¸ades in regions such as the Middle East which are solar resource rich and air-conditioning hungry.

Fig. 9. Schematic of the Burj-al-Arab Tower, Dubai. Glazed areas on the East (A), South (B and C) and West (D) elevations are highlighted. The variation in slope of the glazing in area C is shown as dotted lines on the West elevation at 45, 60 and 908.

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Fig. 10. Schematic of the North elevation of the Jumeirah Beach Hotel, Dubai. Glazed areas are highlighted. Fac¸ade ‘E’ curves convexly towards the West, ‘F’ curves concavely towards the East.

are the same at 23  1 8C and 55% RH. The building designers, WS Atkins, estimated that the thermal heat gains of the central atrium of the Burj-al-Arab tower were of the order of 650 kW [36]. 3.1. General modelling approach 3.1.1. Modelling of the Jumeirah beach hotel To estimate the air conditioning loads of the Jumeirah beach hotel a computer model consisting of a 3  3 array of thermally linked hotel rooms was produced for transient thermal simulation analysis as shown in Fig. 11. Floor to ceiling glazing was applied to both the North and South vertical fac¸ades. The same wall construction was applied throughout the model and there were no doors or walkways defined between the nine zones. This approach enabled the central room of the array of nine to be considered as representative of the average solar gain within the hotel complex. A series of simulations were undertaken to predict the electrical air conditioning savings that could be achieved by the application of various fac¸ade technologies. No occupancy or lighting loads were included in the simulations. These can be neglected because the ambient temperature (Fig. 4) almost always exceeds the setpoint cooling temperature (+23 8C) of the hotel rooms. Therefore, the heating effect of these loads will simply be an additional air conditioning load, which will be the same in each simulation.

The baseline reference was taken to be that of a standard low-e coating, double glazed window construction. The installed glazing was simulated with a U-value of 1.76 W/m2 K and a visible light transmittance, VT, of 0.77. The g-value or solar hear gain coefficient (SHGC) for the window is 0.60. Glazing calculations were undertaken using Window, a fenestration package produced by the Lawrence Berkley National Laboratory [37]. A frame with an absorption coefficient of 0.6 and a Uvalue of 1.7 was defined. Table 1 below details the Jumeirah beach hotel room baseline simulation parameters. A fraction of the solar gain, which enters a building, will inevitably exit through the glazing due to reflection and incomplete absorption by the building interior. The simulation package (TRNSYS) [38] estimates the percentage of light, which enters a room and subsequently becomes incident on the inner pane of a room’s windows by treating all light within the room as diffuse. The distribution of the diffuse light to the walls, ceilings and windows is determined by the relative solar absorption coefficients of the surfaces. As a second reference, the same South fac¸ade with a thermally isolated external blind was simulated, which allowed no diffuse or direct solar radiation to reach the fac¸ade, essentially representing a daylight ‘blocker’. Therefore, for the South elevation, wind pressure effects and the temperature difference between ambient and the hotel room were the only drivers for the air conditioning load. 3.1.2. Modelling of the Burj-al-Arab Tower The Burj-al-Arab Tower is a far more complex structure than the Jumeirah Beach hotel but it is interesting to consider the glazed areas either side of the semi-transparent ‘sail’ on the South elevation of the building. Four different inclinations have been considered (30, 45, 60 and 908 (vertical)) for the Eastern side of the South fac¸ade (308 east of South, B in Fig. 9). The simulation within TRNSYS is achieved by taking the hotel model of the Jumeirah beach hotel and modifying the glazing orientation and slope of the ‘South fac¸ade’ within the simulation. 3.2. Modelling of emerging glazing technologies 3.2.1. Electrochromic glazing The benefit of applying electrochromic glazing was predicted by undertaking TRNSYS simulations with a range of glazings of the same U-value but different g-values. A relationship between air-conditioning load and glazing g-value was determined. This enabled the performance of glazing at specific g-values representative of electrochromic glazing [27] Table 1 Simulation parameters for a hotel room at the Jumeirah Beach Hotel, Dubai

Fig. 11. A 3  3 array of thermally linked rooms to simulate the Jumeirah Beach Hotel, Dubai with North and South glazing. The air-conditioning requirement of the highlighted central hotel room is used in the analysis.

Room volume Room surface area S glazing 6.5 m 2 N glazing 6.5 m 2 Wall and floor construction Cooling set point temperature Humidity set point

92.4 m 3 30.8 m 2 U-value 1.76 W/m2/K U-value 1.76 W/m2/K U-value 0.36 W/m2/K, solar absorbance 0.1 23 8C 55%

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in both tinted (g = 0.12) and bleached states (g = 0.59) to be predicted. To model the performance of electrochromic glazing as it switches between bleached and coloured states a switching threshold of 200 W/m2 external horizontal irradiance was assumed. 3.2.2. Holographic optical elements: reflection holograms HOE were simulated as both fixed glazing and tracked reflection holograms. As stated previously, HOE have a performance function which is dependant on the incident angle of the solar radiation relative to the designed working angle of the HOE, i.e. the angle of best light redirecting function relative to the hologram plane (Fig. 7). The performance function of reflection HOE follows a Gaussian distribution with a 258 difference between the designed working angle of the HOE and the solar beam radiation reducing the HOE performance (ability to redirect direct radiation) by approximately 50% [31,32]. To estimate the performance of a fixed glazing HOE, the solar incident angle difference was calculated every 15 min to produce an hourly average of HOE function for an entire year. This function was used to ‘attenuate’ the level of beam radiation incident on the simulated elevation’s glazing. The selection of the most appropriate hologram, i.e. the HOE with the most suitable designed working angle, for each orientation and elevation was determined by predicting hourly HOE function for a range of solar zenith angles (108 increments from 08 to 908). Therefore, rather than using the designed working angle relative to the HOE plane, the working angle relative to the zenith was chosen as a comparative measure. The hourly HOE function was then used to scale the value of incident beam radiation, which was then integrated over an entire year. Table 2 shows the predicted effect of a range of zenith working angles on the annual incident beam radiation at different orientations and elevations for Dubai. In each case, the working azimuth angle of the reflection hologram will be normal to the glazing orientation. The optimum zenith angle HOE for each surface is underlined in Table 2.

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In general, the optimum zenith working angle of an HOE increases with a rise in the inclination of the glazed elevation. Fig. 12 highlights this effect, showing the direct radiation (kWh/m2 annum) transmitted through a fixed reflection hologram applied at four different inclinations to area B of the Burj-al-Arab Tower (data in Table 2, locations highlighted in Fig. 9). For example, at a slope angle of 608 (30E60), the optimum working zenith angle is shown to be between 30 and 408 (arrow in Fig. 12). In this case the simulated annual direct radiation reaching the fac¸ade would be reduced from 1147 to 707 kWh/m2 annum. For a vertical fac¸ade (30E90), the optimum working zenith angle is shown to be approximately 458. For the shallower sloping glazed areas of the South fac¸ade of the Burj-al-Arab Tower (upper sections of B and C in Fig. 9) a shallower zenith angle (308, defined as 30E30 in Table 2) is predicted to be more efficient. 3.2.3. Aerogel glazing The benefits of applying clear, low U-value aerogel glazing in future were estimated by undertaking TRNSYS simulations with a range of glazings with different U-values (range 0.52– 1.40 W/m2 K) but with the same g-value (0.59). A good fit between air-conditioning load and glazing U-value was determined. This enabled the performance of a glazing at a specific low U-value (0.2 W/m2 K), which is representative of potential aerogel glazing, to be predicted. 3.2.4. Thin film photovoltaics (microgeneration fac¸ades) The energy yield from 40% coverage of the South fac¸ade glazing with thin film technology of 10% and 60% efficiency has been estimated. The g-value of the 60% efficiency thin film glazing is assumed lower, because the majority of the solar gain absorbed by the solar cell is converted to electricity rather than to heat in the case of present day technology. The better cell efficiency not only reduces the heat gain transmitted to the building but also lowers the cell working temperature. This is important because the electrical output of a solar cell drops with temperature rise. The performance ratio (yield factor which

Table 2 The influence of the designed working angle of the HOE, here represented through the zenith working angle, on the level of direct radiation transmitted through a fixed reflection hologram at different orientations and elevations for the latitude of Dubai HOE zenith working angle (8)

0 10 20 30 40 50 60 70 80 90

Direct solar energy transmitted through a fixed reflection hologram (a range of glazing orientations and inclinations is shown); (30E30 = HOE 308 offset from South to the East with 308 slope) [direct radiation per annum] (kWh/m2/annum) S30 [1175]

S45 [1111]

S60 [972]

S90 [531]

30E30 [1147]

30E45 [1077]

30E60 [941]

30E90 [541]

881 800 745 728 758 831 930 1026 1100 1143

849 769 708 681 699 766 861 958 1033 1078

762 687 623 588 595 650 736 827 896 939

459 411 358 313 296 318 369 428 477 507

858 782 729 707 725 782 867 958 1036 1091

822 745 684 651 657 705 785 876 957 1015

738 665 601 559 553 588 659 743 821 878

468 421 367 318 291 295 332 389 445 489

Note: The simulations assume a perfectly recorded HOE (if the angle between the incident beam radiation and the designed working angle of the HOE is zero, all the direct radiation is reflected). Transmission through the HOE of direct radiation, which is not reflected, is 100% in the above example. A zenith working angle of 08 requires the sun to be directly overhead for the HOE to function perfectly. A zenith working angle of 908 requires the sun to be just rising or setting in the sky.

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Fig. 12. Influence of zenith working angle on the direct radiation (kWh/ m2 annum) transmitted through a fixed reflection hologram applied at four different inclinations to area B of the Burj-Al-Arab Tower, Dubai. 30E90: azimuth angle 3308, slope 908 (vertical), 30E30: azimuth angle 3308, slope 308.

accounts for system power losses associated with inverters, cabling, mismatch and temperature effects) of the PV system with the 60% efficiency cell is therefore assumed to be better at 0.85, compared with the typical 0.72 assumed for the present day technology. 3.3. Results and discussion 3.3.1. Jumeirah beach hotel The predicted air conditioning load in kWh per annum for a Jumeirah beach hotel room with North and South glazing is

shown in Table 3. As a simulation control, a baseline room has been defined as having double glazed, low-e windows. This is an informative guide to quantify potential air conditioning savings resulting from alternative fac¸ade options in relation to this baseline control fac¸ade. The effect of external solar radiation blocking blinds, fixed and tracked reflection HOEs, aerogel glazing and its combined use with HOEs as well as electrochromic glazing is shown. A 408 zenith working angle HOE was chosen for the fixed HOE glazing simulation (optimum for S90 as shown in Table 2.). The potential benefit of thin film PV applied to 40% of the glazed area is estimated from the combined effects of electricity generation and solar control. This is shown for present day (2006) and future cell technology (2020). The baseline low-e glazed room is predicted to have an air conditioning load of 4768 kWh per annum (excluding occupancy and lighting loads). The greatest air-conditioning load reduction is achieved when external blinds, which block all incident solar radiation to the South elevation, are applied. Although such an approach is clearly not realistic, as these blinds essentially create a closed wall, it does provide an indication of a target reduction level. It is estimated that external blinds placed in front of the control glazing would reduce the air conditioning load of each room of the Jumeirah beach hotel by 2156 kWh per annum in comparison to the control room. For the entire 600 rooms of the Jumeirah beach hotel, the predicted reduction in air conditioning load achievable with the use of external radiation blocking blinds is 600  2156 = 1,294,000 kWh per annum. If a well maintained air conditioning system is used, a typical coefficient of

Table 3 The annual air conditioning load of various glazing configurations for the vertical fac¸ades of a Jumeirah beach hotel room, Dubai Glazing system

N + S glazed room

N + S glazed room

6.5 m glazing per elevation, room surface area 30 m . No shading system applied unless specified

TRNSYS calculation of room air conditioning load per annum (kWh)

Reduction in cooling demand per annum compared to control windows (kWh) [%] reduction compared to control

(a) Control fac¸ades Control low-e glazing Control 85% transmissiona External blinds 0% trans.

4768 4411 2612

n/a 357 [7] 2156 [45]

4280 3991 3572 4089 3670 4800 2450 2609 4174 (3303b)

488 [10] 777 [16] 1196 [25] 321 [7] 1098 [26] 32 [ 1] 2318 [49] 2159 [45] 594 [12] (1465 [31]b)

3980 ( 2189c)

788 [17] (Net Generation)

2

2

(b) Emerging glazing technologies Fixed reflection HOE 100% trans. a Fixed reflection HOE 85% trans.a Tracked reflection HOE 85% trans.a Aerogel glazing (U-value, 0.2) Aerogel + fixed reflection HOE 85% trans. a Electrochromic glazing bleached mode Electrochromic glazing tinted mode Electrochromic glazing switching at 200 W/m2 horizontal irradiance 60% area glazed with control glazing and 40% area glazed with TF PV 2006 (considering air conditioning load offset by PV generation of 218 kWhe/annumb) 60% area glazed with control glazing and 40% area glazed with TF PV 2020 (considering air conditioning load offset by PV generation of 1542 kWhe/annumc)

A 408 zenith working angle hologram has been used in HOE simulations. Hotel room temperature is maintained at +23 8C. a Percent transmission is defined as percentage of light transmission of the standard control low-e glazing b Reduction in thermal cooling load = PV generatione  COP of air-conditioning = 218  4 = 871 kWht. c Reduction in thermal cooling load = PV generatione  COP of air-conditioning = 1542  4 = 6169 kWht.

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performance (COP) of 4 would be expected. (The COP is defined as waste heat rejected/electrical air conditioning load.) The total reduction in air conditioning load would therefore be approximately 323,000 kWh per annum. The Dubai electricity unit price is 0.055 US$/kWh [39], so perfect external radiation blocking blinds would reduce the air conditioning cost to the hotel by approximately US$ 18,000 per annum. However, light blocking external blinds, even if they could be operated by the user, are clearly not an option for a hotel where unobstructed views are amongst the key criteria. For a fixed HOE solution on the South facing vertical fac¸ade of the Jumeirah beach hotel, a 408 zenith angle working hologram is predicted to provide the best efficiency. In this case a reflection HOE would reduce the incident beam radiation from 531 to 296 kWh/m2/annum (column S90 in Table 2). A fixed HOE glazing solution with a reduced transmission of 85% compared to the low-e baseline glazing for all diffuse and direct light not redirected by the HOE function is estimated to reduce the annual air conditioning load by 777 kWh (16%), approximately a 1/3rd of that which can theoretically be achieved with external radiation blocking blinds. However, the true benefit of the hologram is much lower for this South facing vertical application. An HOE with 100% light transmission of the baseline glazing (i.e. the same) would deliver a 10% reduction compared to the low-e glazing solution. An 85% transmission, tracked HOE system which continuously follows the sun and so reflects all incident beam radiation away from the fac¸ade reduces the air conditioning load by 50% more than that of the non-tracked system (1196 kWh compared to 777 kWh). Such an incremental benefit would not however, justify the additional capital and maintenance cost of the tracked solution even if the building structure could accommodate it. Aerogel glazing with the same g-value as the control glazing is predicted to reduce air conditioning demands by approximately 17%. The simulations of advanced glazing solutions detailed here do not have any internal or external shading applied. Therefore, in reality the relative contribution that aerogel would make to air conditioning reductions would be greater since the importance of the thermal gradient between ambient and the room space will have been underestimated in the simulation. An aerogel glazing incorporating a fixed reflection HOE in the glass laminate is predicted to produce air conditioning savings of approximately 26%. When operating continuously in maximum light transmission mode (bleached mode) electrochromic glazing is predicted to produce an air conditioning load similar, but slightly more than conventional glazing (1% more). In permanent maximum solar control mode (g value = 0.12) air conditioning load reductions resulting from the application of electrochromic glazing (49%) would be double those of tracked HOE glazing (25%). Solar control mode electrochromic glazing absorbs the majority of incident solar gain (both diffuse and direct), whereas a tracked HOE system actively interacts only with the direct component. However, a permanently tinted electrochromic glazing is not a realistic scenario. In addition, projected air conditioning/electricity savings are overestimated as a permanently tinted state will necessitate the use of artificial

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lighting at certain times of the day/year. For a window switching at 200 W/m2 external horizontal irradiance, a 45% reduction in the air-conditioning load of the room is predicted. When in its bleached (high light transmission) state it is assumed that the internal lighting is not required. This electrical saving can therefore be offset against the air-conditioning load. Thin film PV (2006 technology) can provide an annual electricity generation of 218 kWh per room from the South fac¸ade. If a COP of 4.0 is assumed for the air-conditioning, this PV generation effectively reduces the room cooling load to 3303 kWh/annum, a reduction of 31%. If third generation thin film PV can achieve 60% efficiency, this would produce a fac¸ade which can address the externally driven air-conditioning load in its entirety with a surplus electricity generation of 1542 kWhe/annum. This surplus would provide an additional thermal cooling capacity per room of 6168 kWht/annum equivalent to 704 Wt continuous. However, a glazing integrated PV solution covering 40% of the fac¸ade area would always compromise the vision to the exterior. 3.3.2. Burj-al-Arab Tower Inside the Burj-al-Arab Tower the air conditioning load increases as the slope pitch approaches the horizontal, due to the increased solar gain. At each slope studied a simulation of external blinds, which do not transmit any solar radiation, is, as would be expected, predicted to have an identical air conditioning load. Therefore, the annual air conditioning load of a room is considered relative to a room with external radiation blocking blinds, as shown in Fig. 13 for each of the fac¸ade elevations. At this orientation (308E), the annual solar gain (diffuse and direct) is highest for the shallowest pitch glazing studied (308 slope surface). This effect is highlighted by comparing the air conditioning load of the optimum fixed HOE glazing with that of the reduced light transmission normal glazing (normal glazing 85% trans.), as shown in Fig. 13. For example, at 308 slope, the fixed HOE (100% trans.) has a lower air conditioning load than reduced light transmission normal glazing (85% trans.). However, the

Fig. 13. Effect of glazing slope on the predicted air conditioning loads (kWh/ annum) for a room in the Burj-al-Arab Tower, Dubai with different fac¸ade options. Glazed areas on the South elevation (azimuth 3308, 308E) have been studied.

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Fig. 14. Effect of slope angle on solar gain and HOE function of a glazed surface for a low latitude such as Dubai. The schematic shows a South East facing fac¸ade with sunlight meeting the fac¸ade at an altitude angle of 608 and an azimuth angle of 308 East of South (azimuth 3308). For the majority of the day the projected surface (A) of the slope window is larger than the projected surface (B) of the vertical window and, therefore the solar gain is higher.

difference in air conditioning load steadily decreases with glazing slope. This is due to the greater percentage of direct radiation reaching the 308 slope (55% of 2083 kWh/m2 annum total irradiance) than the vertical one (44% of 1219 kWh/m2 annum total irradiance) and the interaction of the HOE function with this incident radiation (Fig. 14). Over the year, the vertical slope HOE, reflects a higher percentage of the direct radiation it receives than the 308 slope (46% as opposed to 38%), but due to the relatively small direct radiation component at this inclination, the HOE fac¸ade is better suited to the 308 slope. From a solar control perspective, the vertical elevation would appear to be more suitable for electrochromic glazing, which regulates both diffuse and direct radiation in tandem. 4. Conclusions Fixed glazing, reflection HOE are predicted to reduce the air conditioning loads of comparable buildings to the Jumeirah beach hotel by approximately a 1/3rd of the level that can be achieved using radiation blocking external blinds. This is accomplished by reflecting the incident beam radiation away from the window whilst allowing diffuse light to pass through. The view from the window remains relatively unobstructed and the HOE glazing can be incorporated into the normal fac¸ade construction. The light transmission of such glazing would be

approximately 85% of the equivalent standard low-e glazing solution applied. However, the larger part of the air conditioning energy saving would be attributable to this lower light transmission. In a fixed vertical application, only a quarter of the total expected saving would be due to the hologram. Tracked HOE, which follow the solar zenith angle, can provide higher levels of solar control (56% of external blinds) but this can only be achieved using external louvers making this solution unattractive from a maintenance perspective. Aerogel glazing, once market ready, promises to deliver small air conditioning savings (7%) in vertical fac¸ade applications approximately half that of fixed HOE solutions. However, in many ways aerogel can be considered to be the more promising technology as HOEs require extensive care in planning, production and installation in order to obtain the designed working angles to match the location. Electrochromic glazing can be continually varied in light transmission, to as low as 5%, enabling such glass to reduce air conditioning loads far lower than any of the HOE or aerogel systems described. However, such glazing has negative impacts on the luminous colour of the light provided through a window, since it creates a blue light when in solar control mode. This can be unattractive to building occupants. The simulations suggest that, if a balanced mix between tinted and bleached mode is assumed, air conditioning savings of around 25% appear realistic. This in broad agreement with data published by the Lawrence Berkley National Laboratory which suggests airconditioning savings are between 19 and 26% [28]. Glazing integrated thin film PV solutions are potentially the most promising solution for fully glazed buildings in the Middle East, especially if the goal of a 60% efficient, third generation cell can be achieved. The simulations predict that such a PV solution covering about 40% of the area of a fully glazed high rise building in the Middle East would yield a net energy gain over the air conditioning loads. This could help to create truly sustainable glass buildings in hot, arid climates by saving fossil fuel for their operation. Acknowledgements Aspects of this work were funded by the European Commission as part of a Framework 5 programme ‘‘Holographic Optical Elements (HOE) for High Efficiency Illumination, Solar Control and Photovoltaic Power in Buildings’’ (ENK6-CT-2000-00327) and the UK, Engineering & Physical Sciences Research Council (EPSRC) under the Sustainable Urban Environment programme ‘‘IDCOP: Innovation in the Design, Construction and Operation of buildings for People’’. References [1] G. Battle, C. McCarthy, Visionen—Die Fassade der Zukunft, in: D. Danner, F.H. Dassler, J.R. Krause (Eds.), Die klima-aktive Fassade, Verlagsanstalt Alexander Koch GmbH, Leinfelden-Echterdingen, 1999 , pp. 174–183. [2] S. Roaf, D. Crichton, F. Nicol, Adapting Buildings and Cities for Climate Change—A 21st century survival guide, Architectural Press—An Imprint of Elsevier, Oxford, 2005.

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