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Smart glazing solutions to glare and solar gain: a ‘sick building’ case study
Energy and Buildings 37 (2005) 1058–1067 www.elsevier.com/locate/enbuild
Smart glazing solutions to glare and solar gain: a ‘sick building’ case study P.A.B. James *, A.S. Bahaj Sustainable Energy Research Group, School of Civil Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, UK Received 29 October 2004; received in revised form 9 November 2004; accepted 27 December 2004
Abstract Holographic optical elements (HOE) can provide solar control by reflecting/redirecting the beam (direct) radiation incident on a window. This paper considers HOE applied for solar control in an office development at Southampton University, UK. In 2000, a new University campus was constructed through the renovation of existing Victorian school buildings coupled to modern, highly glazed office extensions. However, the combination of the low thermal mass of the extensions and the high level of glazing led to excessive office temperatures and occupant discomfort. Office users are requesting the installation of individual air conditioning units which would represent an unacceptable indicator of the building’s design failings. Simulation of the office structure has been undertaken using transient thermal analysis to model possible solutions. Forced air convection, louver systems and fac¸ade changes such as electrochromic glazing are considered in addition to solar control holograms. The simulations highlight the need for the elimination of solar glare within offices and for some form of control of artificial lighting within the building. It is predicted that HOE can produce a comfortable working environment whilst maintaining daylighting and external views from the office: a combination of benefits which competing technologies such as blinds cannot provide. # 2005 Elsevier B.V. All rights reserved. Keywords: Simulation; Fenestration; Holographic optical elements
1. Introduction: a ‘sick building’ It is somewhat surprising that a new UK office development should apparently suffer from excessive solar gain, particularly one on the east elevation of a building (fac¸ade orientated 268 from east to south). Although, in terms of legislation, no upper temperature limit exists for office workers in the UK, 27 8C is usually considered as the upper level for comfort [1]. A combination of factors has brought about a situation where during the summer months office temperatures in the East elevation of the James-Parkes building can regularly exceed 30 8C, with the ambient temperature being usually no more than 23 8C. The experienced high level of discomfort is a result of not only the absolute office temperature but also the high temperature differential with ambient. * Corresponding author. Tel.: +44 2380 593941; fax: +44 2380 677519. E-mail address: [email protected] (P.A.B. James). 0378-7788/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2004.12.010
The low thermal mass of the building structure coupled with a high degree of air tightness and the extensive glazing on the fac¸ade are the root causes of the building overheating during the summer months. However, the problem is exacerbated by the actions of the occupants. The high level of solar glare which can occur early in the morning forces office users to close their blinds to enable use of a computer monitor. However, such an action blocks the majority of the incoming irradiance, necessitating the use of artificial lighting within the office. Although the solar gain has been reduced, a new lighting load and associated heat source has been introduced within the office. The period of the day when solar glare would be a problem is short at such an orientation in comparison to the length of the working day. However, once closed, office users prefer not to adjust the blinds and they are likely to remain in the closed position throughout the year. This behaviour is a common problem and can be seen to a certain level in most UK offices with traditional blinds. Such an effect is highlighted in Fig. 1,
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Fig. 1. James-Parkes building, east elevation, Southampton University, UK. Exterior view on an overcast day. Note 8 out of 15 office blinds are fully closed (arrowed highlight).
which shows one of the four blocks of the east elevation on a rainy, overcast day during late autumn (October). Of the 15 offices shown, 8 have all their blinds closed, with a further 3 showing some level of blind use, on a day when glare or excessive solar gain is clearly not a problem for the occupants.
2. Modelling the office environment Transient thermal computer simulations using TRNSYS [2] were undertaken to understand the problems of the east elevation of the building and to model possible solutions. A 3D model of one of the office blocks was created and a detailed analysis of the first floor undertaken. The ground and second floors were simulated as single thermal zones with each office and corridor on the first floor modelled as one of 10 thermally linked zones. Fig. 2 shows a 3D depiction of the first floor of the building, with the office on the east elevation highlighted for which analysis is shown. Custom wall and glazing constructions were created within the TRNSYS supporting programmes (PreBid and Window 5.1 [3]) to model the present building construction as closely as possible. The Estates and Facilities Department supplied occupancy, electrical and lighting load data to enable their incorporation within the simulations. As an initial step a TRNSYS simulation of the office in its present state was undertaken. The load profile within the offices was scheduled between 09:00 and 18:00 h during the working week (Monday–Friday) with no office use at weekends. All simulation times are GMT. The air exchange rate with ambient was set at 0.5 air exchanges/h throughout all the simulations unless otherwise stated. The loads within the offices for a working day were as follows: Lighting 10 W/m2, over 12 m2 of floorspace of east and west offices = 120 W (24%).
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Fig. 2. James-Parkes building, Southampton University. TRNSYS simulation modelling of the first floor of the office block is shown. East facing office used in simulations is highlighted (arrow).
One computer and monitor = 230 W (46%). One person occupancy, defined as seated, light work, typing, ISO 7730 [1] = 150 W (30%). Simulations were undertaken using a 1 h timestep over the entire year [4]. Irradiance and temperature data was inputted into TRNSYS from Meteonorm [5] a solar irradiance software package. The behaviour of the smart glazing technologies studied was modelled by allowing TRNSYS to dynamically change the level of beam and diffuse radiation incident on the office glazing at each timestep. Window 5.1 and Optic, fenestration packages produced by LBNL [3] enabled custom glazing constructions to be made to simulate two representative office glazings (‘at-present’ and ‘at-present low-e’) and retrofit thermal solutions such as window film. Table 1 highlights the relative effects of the irradiance and the combined occupancy + computers + artificial lighting load through a simple month by month analysis. Approximately 5 times as much sunlight is received in June (equivalent continuous irradiance, ICONTINUOUS, striking outer surface of glazing 116 W/m2) compared with December (24 W/m2) for the east facing office. The equivalent continuous irradiance, ICONTINUOUS being defined as I CONTINUOUS ðW=m2 Þ kWh in plane irradiance per month 1000 ¼ number of hours in month
(1)
Clearly, the solar gain received by the office does not occur as a time averaged value, but as a narrow peak during the morning (see Fig. 10). The low infiltration rate (0.5 air exchanges/h) combined with the low thermal mass of the building means that in reality a rapid increase in office temperature occurs during the morning. Table 1 serves to highlight the importance of the introduced, additional thermal loads, through office use.
Beam irradiance as a percentage of the room loads (%)
13 23 19 28 25 26 24 24 24 16 19 13 25 40 45 56 59 60 59 58 52 39 33 23
The solar heat gain resulting from the equivalent continuous irradiance incident on the exterior of the office window during June is estimated to be 277 W, this is of a similar order to the average introduced continuous thermal load due to office working of 188 W. Elimination of the incident beam irradiance without modifying the other loads can; at most; reduce the office thermal loads by 26%. Additional changes such as control of the artificial lighting may therefore be required to achieve the desired temperature reductions. In essence, if the effects of glare could be eliminated there would be no need for the current internal blinds to be closed throughout the day and so the artificial lighting loads could be reduced.
3. Solar gain control options A variety of remedial options are available to reduce the solar gain problems of the James-Parkes building. These include window film, external louvers or blinds and smart glazing solutions such as electrochromics and holographic optical elements [6]. Essentially the solutions can be considered as being three different distinct types: mechanical, shading and glazing as shown in Fig. 3.
a
b
Solar heat gain coefficient of glazing = 0.63 (63% of solar heat gain transmitted). Equivalent solar heat gain (W) = (equivalent continuous irradiance)(solar heat gain coefficient)(window area).
252 312 343 432 463 465 463 446 389 307 279 245 188 188 188 188 188 188 188 188 188 188 188 188 64 124 155 244 275 277b 275 258 201 119 91 57 27 52 65 102 115 116 115 108 84 50 38 24 January February March April May June July August September October November December
14 30 27 51 48 51 46 45 39 20 22 14
Equivalent continuous irradiance striking outside of office window (W/m2) Month
Equivalent continuous beam irradiance striking outside of office window (W/m2)
Equivalent continuous solar heat gain into office; window area 3.79 m2 a (W)
Occupancy, lighting and computer loads (W)
Total equivalent continuous thermal load (W)
Irradiance as a percentage of the room loads (%)
P.A.B. James, A.S. Bahaj / Energy and Buildings 37 (2005) 1058–1067 Table 1 A comparison of the relative monthly contribution of irradiance and occupancy loads to the thermal gain of an office on the east elevation of the James-Parkes building (assuming every day is a working day)
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3.1. Mechanical Air conditioning is energy and maintenance intensive but does result in far lower office temperatures than any of the other options. Users are able to set the office temperature to their specific requirements and additional thermal loads (e.g. change of office use resulting in more heat generating equipment) can easily be accommodated. However, the use of individual office air conditioning units (only practical option for retrofit) would be an expensive and highly visible indicator of the building’s environmental failings and so is not an option for the university. Moreover, such an approach creates a dangerous precedent leading to similar requests from occupants of older university buildings.
Fig. 3. Solar gain control options to improve the thermal comfort of the ‘atpresent’ office. Three distinct types: mechanical, shading and glazing are available.
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Forced air convection relies on increasing the air exchange rate with ambient to lower the office temperature. Ideally, forced air convection buildings have a large thermal mass. This enables the use of high air exchange rates at night with the low ambient temperature to create a cooled building structure. During the day the building acts as a heat sink to the solar gain on the building envelope. However, the JamesParkes building is a low thermal mass structure and so forced convection would be required throughout the day to maintain the cooling effect. 3.2. Shading External blinds enable the control of either direct or diffuse and direct radiation. Fixed louvers mounted horizontally above the window reflect the incident beam radiation eliminating glare and the majority of solar gain whilst allowing diffuse light to enter providing lighting. Horizontal fixed louver systems also maintain an unobstructed view from the office, an important feature of a ‘healthy’ working environment. However, the function of a horizontal lamellae is dependant on the zenith angle of the sun (angle from the vertical, b in Fig. 5). If we consider a horizontal louver protruding out 1 m above a 1 m high window for example, the zenith angle must be 458 or less to ensure a shadow is cast over the entire window. Fig. 4 shows the azimuth and zenith angle paths of the sun for the summer (21 June) and winter (21 December) equinox from 05:00 to 21:00 h. The sun is normal to the east elevation of the James-Parkes building (648 azimuth) at approximately 09:30 GMT (‘A’ in Fig. 4). To perform a useful function the horizontal louver must provide full solar protection by this time throughout the year. However, the corresponding zenith angles for the summer and winter equinox are 42 (‘B’ in Fig. 4) and 828 (‘C’ in Fig. 4), respectively. At this orientation it is clear that only limited
Fig. 4. Zenith and azimuth angle paths of the sun for Southampton, UK (longitude 1.348W, latitude 50.48N) for the longest (21 June) and shortest (21 December) days of the year. The azimuth angle (648) when the sun is normal to the glazing of the east elevation is reached at approximately 9.30 GMT. The corresponding zenith angle for the longest and shortest days is shown.
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shading will occur in the summer months and no glare protection at all during the winter. To overcome this problem angled mechanical roller blinds (Fig. 3.) could be applied with their level of use controlled by the office user or sensors although such a scenario has not been modelled in this paper. 3.3. Glazing 3.3.1. Electrochromic glazing Electrochromic windows [7,8] enable the visible light transmission and solar heat gain of a window to be varied by the application of a dc voltage to an electrochromic coating. The electrochromic manufacturer, SageGlass [7] for example, produces an electrochromic double glazing unit with a visible light transmission, VT, which is varied between 0.70 and 0.04. The corresponding solar heat gain coefficient (SHGC) varies between 0.52 and 0.10. The SHGC representing the thermal fraction of the incident sunlight, which is transmitted through the glazing. Therefore, electrochromics offer occupants or building services managers the ability to continuously vary the solar thermal and illuminance gain in an office. 3.3.2. Holographic optical elements The simulated HOE system consists of a series of horizontal louvers mounted externally to the window. A single axis tracking system is employed which enables the louvers to follow zenith angle of the sun throughout the day (Fig. 5). Each louver would, for example, consist of a glass laminate of upper solar absorber glass, HOE film and underside solar absorber glass. The louvers would utilise a
Fig. 5. Schematic illustration of the function of lightguiding HOE in solar control louvers. Two possible schemes are shown: (1) top louvers—HOE redirects light within laminate at angle greater than the critical angle— energy dissipated by solar absorber glass, diffuse radiation is transmitted (2). (3) Lower louver—light is redirected to solar absorber attached to back surface of HOE laminate.
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transmitting, lightguiding HOE [9,10], which enables incident beam radiation to become trapped and eventually absorbed within the solar absorber glass. The incident radiation is redirected within the glass louver such that the light strikes the glass–air interface of the louver at an incident angle greater than the critical angle, causing total internal reflection (1). The refractive index of the hologram is similar to that of the glass allowing the reflected beam to undertake multiple reflections across the two solar absorber panes of the laminate before being absorbed (2). An alternative arrangement, would be, for example, a solar absorber mounted on the back surface of a light guiding HOE louver. The light would be redirected onto the absorber, as shown for the lower louver in Fig. 5 (3). Diffuse radiation does not actively interact with the HOE and so passes through the louver elements. The level of diffuse solar energy radiation transmission is determined only by SHGC of the HOE louver and not the diffraction properties of the HOE. The diffraction efficiency of both (a) transmitting and (b) reflecting holograms is dependant on the angle of incidence of the beam radiation. (a) Transmission HOE: Transmission (lightguiding) holograms in particular are critically dependant on the incident radiation striking the hologram at the designed working angle (Bragg angle). Typically, if the incident beam radiation is 58 away from the working angle of the HOE, the diffraction efficiency is reduced by 50% [11]. A tracked system is therefore, essential for transmitting hologram applications if the HOE is to function efficiently for more than a few minutes per day [11]. (b) Reflecting HOE: Reflecting holograms have a greater angle tolerance (258), which enables their use in certain applications without the need for tracking [11]. However, reflection HOE can produce glare effects on surrounding buildings which may make their use unacceptable for certain applications such as the offices considered here. 3.3.3. Window film Window film is often added to glazing as a retrofit application to try and compensate for glazing that is already causing problems for the occupants. The film is generally added to the interior side of the glazing to ensure a long lifetime. A variety of films are available from simple tints, to low-e coatings and spectrally selective films. This paper considers the use of a low SHGC window film, SunGuard Silver 20, produced by Guardian [12]. The film when applied to a 3 mm thick clear glass pane, for example, has a visible light transmittance, VT of 0.20 and SHGC of 0.15. 3.3.4. Solar control glazing A latest generation, titanium low-e coating glazing (TiAC-low-e, International Glazing Database Number, IGDB 965) manufactured by AFG glass [13] was simulated.
The coating has a very low emissivity of 0.034 compared to 0.84 for normal glazing.
4. Simulation model The east elevation of the James-Parkes building at Southampton University is orientated 268 from east towards south (azimuth = 648). Within TRNSYS the exterior wall of the east elevation is modelled as a composite construction (5 parts) of thickness 0.195 m and u-value 0.353. The interior partition walls of the building have a u-value of 4.73. Two representative office glazings (‘at-present’ and ‘atpresent low-e’) were created using the software package Window [3] to simulate the double glazing units. The atpresent glazing consisted of two Pilkington Optifloat, clear glass panes with a 12 mm air gap. A PVC frame was used to create a window of u-value 2.59, solar heat gain coefficient (SHGC) of 0.63 and visual light transmittance, VT, 0.80. To enable a fairer comparison between traditional glazing and smart fac¸ade technologies an alternative low-e coating glazing was defined to produce an ‘at-present low-e’ glazing with a u-value of 1.69. The SunGuard window film when used on the inner pane of the ‘at-present low-e’ window lowered the u-value from 1.69 to 1.65, with a SHGC of 0.124 and VT of 0.19. The holographic optical element louvres were assumed to have a SHGC and VT of 0.85 for diffuse light and an efficiency of 100% in the simulation, i.e. all incident beam radiation is diffracted and subsequently absorbed within the HOE louvre. An overview of the glazing types used in this study is shown in Table 2. An additional benefit of the HOE system is the elimination of glare, which will enable office users to avoid the need for blinds. The artificial lighting load can therefore, potentially be decreased, since incident diffuse light can now provide natural office daylighting. A reduction in artificial lighting from a continuous level of 10 W/m2 to an on–off switched mode for the 12 m2 illuminated floor area of the east facing offices was defined to assess the impact of utilising diffuse natural lighting. To determine the threshold incident diffuse light level below which artificial lighting is required in an office of the James-Parkes building a daylighting simulation package, Radiance [3] was used. Fig. 6 shows the predicted horizontal illuminance at desk height along the centre line of the room (2.5 m wide) as a function of distance from the window. The illuminance distribution within the office for a range of external diffuse illuminance levels is shown (5000– 13500 lx). The glazing construction, HOETRACKED-85, was simulated as at-present low-e glazing with external HOE louvers (85% transmission for diffuse light) producing an overall light transmission, VT, of 0.64. At an external horizontal diffuse illuminance of 7000 lx, it is predicted that in the centre of the room (2 m from window), the light level will be 500 lx, the minimum required level for office
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Table 2 Centre of a glass pane optical and thermal properties of the glazing types used in the James-Parkes building analysis Glazing type At-present 5.9 mm Pilkington Optifloat, IGDB 4009 12 mm air gap 5.9 mm Pilkington Optifloat, IGDB 4009 At-present low-e 3.9 mm Comfort Ti-PS low-e on clear, IGDB 933 12 mm air gap 3.0 mm, generic clear glass, IGDB 102 Electrochromic At-present low-e with modified SHGC and VT Light mode Dark mode HOEDIFFUSE-85 At-present low-e glazing, HOE interaction with diffuse radiation HOETRACKED-85 At-present low-e glazing, tracked HOE interaction with direct radiation Window film At-present low-e glazing with Guardian Sunguard Silver 20 on inside of inner pane IGDB 5.8 mm Pilkington K glass, IGDB 4013 12.0 mm air gap 5.6 mm SGSR20 on clear, IGDB 3138 Solar glazing TiAC-low-e coating glazing on inner side of outer pane 3.0 mm TiAC-low-e, IGDB 965 12.0 mm air gap 3.0 mm, generic clear glass, IGDB 102
Solar heat gain coefficient (SHGC)
Visual transmission, VT
u-Value (W/m K)
Emissivity
0.71
0.80
2.70
0.84 0.84
0.50
0.75
1.69
0.048 0.84
1.69
0.048 0.84
1.69
0.048 0.84
0.52 0.10
0.70 0.04
0.85 0.50 = 0.43
0.85 0.75 = 0.64
0
0
1.69
0.40
0.17
1.69
0.048 0.84
0.17 0.42 0.37
working [14]. Therefore, TRNSYS simulations which utilised internal lighting control or glazing changes (electrochromics) used an external horizontal illuminance level of 7000 lx as the threshold level for switching. The
Fig. 6. Predicted horizontal illuminance (lx) at desk height along the centre line of the room (2.5 m wide) as a function of distance from the window. Dependency on external horizontal diffuse illuminance level is shown. Glazing construction: at-present low-e with external HOE louvers, VT = 0.64; radiance simulation.
0.68
1.69
0.034 0.84
7000 lx corresponding to the typical illuminance level of a dull, overcast day. Fig. 7 shows the effect of glazing type on the predicted horizontal illuminance (lx) in the centre of an office, 2 m from the window for a clear, summer’s day (radiance
Fig. 7. Effect of glazing type on predicted horizontal illuminance (lx) in the centre of an office, 2 m from the window. Radiance simulation, clear sky, 14 June. (A) 07:00 GMT illuminance exceeds 500 lx with HOE system, (B) 17:00 GMT illuminance falls below 500 lx with HOE system.
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5. Simulation results
Fig. 8. Predicted ambient and average office temperature (‘at-present’) within an office on the east elevation of the James-Parkes building. A 7-day period of high irradiance is shown starting on the 21 June (Day 172 of the year). Lower office temperatures are predicted for the weekend due to the non-occupancy of the building. The upper temperature limit for comfort (27 8C) is highlighted.
simulation for 14 June) when glare and solar gain will be a problem. The HOE system (HOETRACKED-85 absorbs all direct radiation and has a visual light transmission, VT of 0.64 for diffuse light. The threshold light level of 500 lx is achieved between 07:00 and 17:00 GMT (‘A’ and ‘B’ in Fig. 7). The high level of incident beam radiation results in a peak illuminance at 08:00 GMT which, without attenuation, is too high for normal office working (at-present 35,000 lx, solar glazing 28,000 lx, window film 7000 lx), resulting in glare which would probably result in the internal blinds being closed by the office user. The low light transmission of the window film means that once the solar beam radiation has crossed the fac¸ade there is insufficient diffuse light to provide the required illumination level.
Figs. 8 and 9 represent the environmental conditions the office is subjected to for a period of a week in June. Fig. 8 shows the predicted ambient and office temperature distributions for the highlighted office (arrowed in Fig. 2) for a 7-day period starting on 21st June (Day 172 of the year). The 27 8C maximum thermal comfort level for an office is indicated as a dashed line. The corresponding predicted irradiance distribution (diffuse and direct) is shown in Fig. 9. Throughout the working week shown, the temperature in the office modelled under either its ‘atpresent’ or ‘at-present low-e’ state is shown to consistently exceed the 27 8C comfort limit (Fig. 8). Day 177, the Wednesday of the week shown will now be considered in detail. The predicted ambient and office temperatures and the corresponding irradiance distribution for Day 177 are shown in Fig. 10. The peak office temperature is reached at 18:00 h when the internal working day loads (occupancy, computers and artificial lighting) are switched off in the simulation. 5.1. Glazing/lighting scheme definitions at-present low-e: The James-Park office glazing as constructed, standard double glazing unit, low-e coating. HOETRACKED-85: A tracked HOE solar absorber external to the window, light transmission for diffuse light 85%, all beam radiation is absorbed. HOETRACKED-100: Tracked HOE unit with 100% transmission for diffuse radiation, all beam radiation is absorbed. Highlights the HOE ‘effect’ due solely to light guiding and not to the reduced light transmission of the HOE unit. HOETRACKED-85-LIGHTS-SWITCH: The high light transmission of the HOE for diffuse light enables the artificial
Fig. 9. Irradiance distributions (in plane beam and diffuse) for the east elevation of the James-Parkes building. A 7-day period is shown starting on the 21 June (Day 172 of the year).
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Fig. 10. Predicted ambient and office temperature and irradiance distributions (beam and diffuse) for the 21 June (Day 172 of the year).
interior lighting to be switched off when the external horizontal diffuse illuminance is greater than 7000 lx. ELECTROCHROMICCLEAR-MODE: Electrochromic glazing in high light transmission mode. ELECTROCHROMICDARK-MODE: Electrochromic glazing in low light transmission mode. ELECTROCHROMICVARIABLE-MODE: Electrochromic glazing light transmission mode changes to maintain required interior lighting level. ELECTROCHROMICVARIABLE-MODE-LIGHTS-SWITCH: Electrochromic glazing light transmission mode changes and interior lights switched on and off to maintain required interior lighting level.
5.2. Holographic optical elements The effect of adding a tracked HOE system mounted external to the fac¸ade is now considered. The predicted effect of a tracked HOE system for Day 172 is shown in
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Fig. 11. Of the selected glazings shown, the highest office temperature is reached with the at-present low-e condition, which represents the current double glazed office using a low-e coating. The addition of glazing mounted external to the fac¸ade will lead to a lower office temperature due to a reduction in incident solar radiation. To simulate the thermal effect of the additional glazing created by the HOE unit, the at-present low-e condition was modelled with reduced incident diffuse and direct radiation (85% of the at-present level). A perfectly functioning tracked HOE system which absorbs all the incident beam radiation and reduces the diffuse radiation level to 85% of the at-present low-e level (HOETRACKED-85) can be compared with an HOE system which allows all diffuse light to pass through (HOETRACKED100). The relative performance of these two systems in comparison to the at-present low-e glazing enables an assessment of the effect of HOE function and reduced diffuse light transmission on solar gain. The majority of the solar gain (direct radiation) occurs during the morning, which results in a rapid rise in temperature for a normally glazed office. An office with the HOE system for example, is approximately 58 cooler than an at-present low-e office at 10:00 h, this difference gradually reduces during the day. If internal lighting control (HOETRACKED-85-LIGHTS-SWITCH) is used in conjunction with the HOE an incremental reduction in temperature of approximately 1 8C is predicted. The internal lights are switched on when, during a working day, the external horizontal diffuse illuminance falls below 7000 lx. Over the entire year the internal lighting would be required for 586 office hours, compared to 2346 h for lighting throughout each working day. For an illuminated area of 12 m2, with a lighting load of 10 W/m2, such a reduction corresponds to 210 kWh of electricity per annum. 5.3. Ventilation Two typical forced ventilation schemes were considered for the east elevation with the second applying a lower air exchange rate during the day to reduce noise disruption. In each case the proposed solution was to cut ventilation holes below the windows in all the offices of the east elevation and to force additional ambient air through to the west side of the building. Continual ventilation is required due to the very low thermal mass of the building. The two schemes would operate as follows:
Fig. 11. Comparison of the predicted office temperature for 21 June (Day 172) with normal glazing and three possible tracked HOE solutions. The ambient temperature reaches a maximum at 15.30, with the office temperature peaking at 18:00 h, regardless of the modelled glazing type or level of office load (lighting, computers and occupancy).
Ventilation scheme 1 (vent 1): Time of day 02:00–18:00 h, 8 air changes per hour offices on east elevation. Ventilation scheme 2 (vent 2): Time of day 02:00–09:00 h, 8 air changes per hour offices on east elevation. Time of day 09:00–18:00 h, 4 air changes per hour offices on east elevation.
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Fig. 12. Comparison of the predicted office temperature for Day 177 of the year following solar gain control with either forced ventilation (vents 1 and 2) or a tracked HOE system.
Fig. 14. Comparison of the predicted office temperature for Day 177 of the year following solar gain control with a range of fac¸ade options. Electrochromic and tracked HOE which combine solar and interior lighting control show superior performance to traditional glazing options.
The predicted performance of the two schemes for Day 177 of the year is shown in Fig. 12. The high night air exchange rate of vent 2 lowers the temperature of the office to approximately 22 8C. At 9 a.m. the air exchange rate of vent 2 is halved to reduce noise disturbance, leading to a temperature rise in comparison to vent 1. The peak daytime working office temperature predicted by the scheme vent 1 is approximately 28 lower than that of the best HOE system, HOETRACKED-85-LIGHTS-SWITCH.
chromic working range. The ‘light’ mode of a Sage electrochromic is shown as ELECTROCHROMICCLEARMODE in Fig. 13 and is similar in profile to the at-present low-e glazing. In ‘dark’ mode, the SHGC is reduced from 0.52 to 0.10. The predicted temperature profile in the office (ELECTROCHROMICDARK-MODE) is very similar to that of a tracked HOE system (HOETRACKED-85). A key feature of electrochromics is the ability to continually vary the light transmission. Fig. 13 shows that switched electrochromic glazing, which varies in light transmission to provide natural lighting (ELECTROCHROMICVARIABLE-MODE) can be as effective at controlling office temperature as dark mode electrochromics if combined with lighting control (ELECTROCHROMICVARIABLE-MODE-LIGHTS-SWITCH). The predicted temperature profile would be slightly lower if the glazing was switched to dark mode outside office hours, reducing undesirable solar gain still further.
5.4. Electrochromic glazing Electrochromics enable continual variation in the SHGC and light transmission of the glazing. If the interior lighting of an office is maintained throughout the working day the maximum and minimum of solar control performance can be estimated by considering the extremes of the electro-
5.5. Alternative fac¸ade options
Fig. 13. Comparison of the predicted office temperature for Day 177 of the year following solar gain control with a range of electrochromic options (clear, dark and variable transmission mode).
A comparison of HOE with electrochromics, solar glazing, window film and internal blinds options for the working hours of the 26 June (Day 177) is shown in Fig. 14. The window film attenuates both the incident diffuse and direct radiation at the inner pane of the existing glazing, which, in comparison to HOE leads to a higher percentage of the attenuated radiation leading to heat gain within the office. In addition, the low visible light transmission of the proposed window film (VT of 0.19) means that the current level of artificial lighting within the office will have to be maintained. This is highlighted in Fig. 7, where after 10:00 h on a clear day in June, interior lighting is required to achieve a 500 lx illuminance level within the office. The predicted temperature profile of the window film (window film) is similar to that of an internal blinds (int-blinds). The tracked HOE systems, which incorporate a lower level of interior
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lighting (HOETRACKED-85-LIGHTS-SWITCH) predict significantly lower office temperatures. The TiAC-low e solar control glazing is predicted to have far superior solar control performance than either normal low-e glazing or the selected window film options. The temperature profiles of both HOE and electrochromics utilising switched internal lighting is lower than that of solar control glazing, especially during the morning when the majority of the solar gain is incident on the fac¸ade.
6. Discussion The TRNSYS simulations highlight the need to control both external and internal heat gains within an office to create a comfortable working environment. Lightguiding HOE which enable solar control of the incident beam radiation whilst controlling glare and retaining vision through the window or switchable electrochromics have been shown to be potential alternative to traditional solar control approaches. None of the fac¸ade solutions can maintain the office below the upper working temperature for comfort of 27 8C throughout the year. However, for non-airconditioned buildings, there should perhaps be a degree of tolerance in this upper limit, accommodating higher temperatures for a limited number of days. The glare problems of an East facing offices of the JamesParkes building studied mean that it is not possible to utilise blinds or louvers to block the incident beam radiation without obstructing the office view. The forced ventilation schemes were shown to outperform the glazing solutions but this is at the expense of higher energy consumption, noise and maintenance costs. Moreover, the ventilation schemes do not address the issue of glare and so office users would still have to work behind blinds. Window film can lead to lower office temperatures in a cost effective manner but may however, result in higher artificial lighting loads for the remainder of the year. Moreover, removing 80% of the incident illuminance level is a somewhat crude tool and is not ideally suited to the highly variable UK weather. The simulations shown here assume that the HOE systems functions at 100% diffraction efficiency. In reality however, HOE have a spectrum dependant efficiency [15] and are not perfectly recorded films which leads to a reduction in efficiency. In addition, the HOE requires true alignment between the incident beam radiation and the working angle of the hologram. If the tracking system does
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not perform to within 28, this loss in HOE function increases rapidly, rising to 50% for a 58 error [11]. The resulting glare and spectral dispersion effects (rainbow colour spectrum) that may result from a poorly functioning HOE are significant from a comfort rather than absolute office temperature perspective.
Acknowledgements Aspects of this work were undertaken as part of a European Commission Framework 5 programme ‘Holographic Optical Elements (HOE) for High Efficiency Illumination, Solar Control and Photovoltaic Power in Buildings’, project no. ENK6-CT-2000-00327.
References [1] ISO7730, Moderate thermal environments—Determination of the PMV and PPD indices and specification of the conditions for thermal comfort, International Organisation for Standardisation, 1994. [2] TRNSYS, Thermal Energy System Specialists, 2916 Marketplace Drive, Suite 104, Madison, WI, USA. [3] LBNL, Lawrence Berkley National Laboratory, Fenestration simulation programme, Window 5.1 and Daylighting simulation programme, Radiance. [4] J. Kos´ny, E. Kossecka, Multi-dimensional heat transfer through complex building envelope assemblies in hourly energy simulation programs, Energy and Buildings 34 (5) (2002) 445–454. [5] Meteonorm, Solar simulation software, http://www.meteonorm.ch. [6] LBNL, IEA Report Task 21, A Source Book on Daylighting Systems and Components, 2000. [7] Sage Electrochromics Inc., 2150 Airport Drive, Faribault, MN, USA, http://www.sage-ec.com. [8] LBNL, Electrochromic Window Tests in U.S. Office Show Promise, EETD Newsletter, vol. 5, 2000. [9] H.F.O. Muller, Application of holographic optical elements in buildings for various purposes like daylighting, solar shading and photovoltaic power generation, Renewable Energy (5) (1994) 935–941. [10] H.F.O. Muller, Innovative use of light directing holograms with solar cells, Solar Architecture (1994). [11] P.A.B. James, A.S. Bahaj, Holographic optical elements: various principles for solar control of highly glazed buildings, Solar Energy 78 (3) (2005) 441–454. [12] Guardian, http://www.sun-guardglass.com. [13] AFG glass, http://www.afg.com. [14] DIN EN 12464 Light and lighting—lighting of work places, Part 1, Indoor work places. [15] J. Breitenbach, J.L.J. Rosenfeld, Goniospectrometer measurements of the optical performance of a holographic optical element, Solar Energy 68 (5) (2000) 427–437.
Smart glazing solutions to glare and solar gain: a ‘sick building’ case study P.A.B. James *, A.S. Bahaj Sustainable Energy Research Group, School of Civil Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, UK Received 29 October 2004; received in revised form 9 November 2004; accepted 27 December 2004
Abstract Holographic optical elements (HOE) can provide solar control by reflecting/redirecting the beam (direct) radiation incident on a window. This paper considers HOE applied for solar control in an office development at Southampton University, UK. In 2000, a new University campus was constructed through the renovation of existing Victorian school buildings coupled to modern, highly glazed office extensions. However, the combination of the low thermal mass of the extensions and the high level of glazing led to excessive office temperatures and occupant discomfort. Office users are requesting the installation of individual air conditioning units which would represent an unacceptable indicator of the building’s design failings. Simulation of the office structure has been undertaken using transient thermal analysis to model possible solutions. Forced air convection, louver systems and fac¸ade changes such as electrochromic glazing are considered in addition to solar control holograms. The simulations highlight the need for the elimination of solar glare within offices and for some form of control of artificial lighting within the building. It is predicted that HOE can produce a comfortable working environment whilst maintaining daylighting and external views from the office: a combination of benefits which competing technologies such as blinds cannot provide. # 2005 Elsevier B.V. All rights reserved. Keywords: Simulation; Fenestration; Holographic optical elements
1. Introduction: a ‘sick building’ It is somewhat surprising that a new UK office development should apparently suffer from excessive solar gain, particularly one on the east elevation of a building (fac¸ade orientated 268 from east to south). Although, in terms of legislation, no upper temperature limit exists for office workers in the UK, 27 8C is usually considered as the upper level for comfort [1]. A combination of factors has brought about a situation where during the summer months office temperatures in the East elevation of the James-Parkes building can regularly exceed 30 8C, with the ambient temperature being usually no more than 23 8C. The experienced high level of discomfort is a result of not only the absolute office temperature but also the high temperature differential with ambient. * Corresponding author. Tel.: +44 2380 593941; fax: +44 2380 677519. E-mail address: [email protected] (P.A.B. James). 0378-7788/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2004.12.010
The low thermal mass of the building structure coupled with a high degree of air tightness and the extensive glazing on the fac¸ade are the root causes of the building overheating during the summer months. However, the problem is exacerbated by the actions of the occupants. The high level of solar glare which can occur early in the morning forces office users to close their blinds to enable use of a computer monitor. However, such an action blocks the majority of the incoming irradiance, necessitating the use of artificial lighting within the office. Although the solar gain has been reduced, a new lighting load and associated heat source has been introduced within the office. The period of the day when solar glare would be a problem is short at such an orientation in comparison to the length of the working day. However, once closed, office users prefer not to adjust the blinds and they are likely to remain in the closed position throughout the year. This behaviour is a common problem and can be seen to a certain level in most UK offices with traditional blinds. Such an effect is highlighted in Fig. 1,
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Fig. 1. James-Parkes building, east elevation, Southampton University, UK. Exterior view on an overcast day. Note 8 out of 15 office blinds are fully closed (arrowed highlight).
which shows one of the four blocks of the east elevation on a rainy, overcast day during late autumn (October). Of the 15 offices shown, 8 have all their blinds closed, with a further 3 showing some level of blind use, on a day when glare or excessive solar gain is clearly not a problem for the occupants.
2. Modelling the office environment Transient thermal computer simulations using TRNSYS [2] were undertaken to understand the problems of the east elevation of the building and to model possible solutions. A 3D model of one of the office blocks was created and a detailed analysis of the first floor undertaken. The ground and second floors were simulated as single thermal zones with each office and corridor on the first floor modelled as one of 10 thermally linked zones. Fig. 2 shows a 3D depiction of the first floor of the building, with the office on the east elevation highlighted for which analysis is shown. Custom wall and glazing constructions were created within the TRNSYS supporting programmes (PreBid and Window 5.1 [3]) to model the present building construction as closely as possible. The Estates and Facilities Department supplied occupancy, electrical and lighting load data to enable their incorporation within the simulations. As an initial step a TRNSYS simulation of the office in its present state was undertaken. The load profile within the offices was scheduled between 09:00 and 18:00 h during the working week (Monday–Friday) with no office use at weekends. All simulation times are GMT. The air exchange rate with ambient was set at 0.5 air exchanges/h throughout all the simulations unless otherwise stated. The loads within the offices for a working day were as follows: Lighting 10 W/m2, over 12 m2 of floorspace of east and west offices = 120 W (24%).
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Fig. 2. James-Parkes building, Southampton University. TRNSYS simulation modelling of the first floor of the office block is shown. East facing office used in simulations is highlighted (arrow).
One computer and monitor = 230 W (46%). One person occupancy, defined as seated, light work, typing, ISO 7730 [1] = 150 W (30%). Simulations were undertaken using a 1 h timestep over the entire year [4]. Irradiance and temperature data was inputted into TRNSYS from Meteonorm [5] a solar irradiance software package. The behaviour of the smart glazing technologies studied was modelled by allowing TRNSYS to dynamically change the level of beam and diffuse radiation incident on the office glazing at each timestep. Window 5.1 and Optic, fenestration packages produced by LBNL [3] enabled custom glazing constructions to be made to simulate two representative office glazings (‘at-present’ and ‘at-present low-e’) and retrofit thermal solutions such as window film. Table 1 highlights the relative effects of the irradiance and the combined occupancy + computers + artificial lighting load through a simple month by month analysis. Approximately 5 times as much sunlight is received in June (equivalent continuous irradiance, ICONTINUOUS, striking outer surface of glazing 116 W/m2) compared with December (24 W/m2) for the east facing office. The equivalent continuous irradiance, ICONTINUOUS being defined as I CONTINUOUS ðW=m2 Þ kWh in plane irradiance per month 1000 ¼ number of hours in month
(1)
Clearly, the solar gain received by the office does not occur as a time averaged value, but as a narrow peak during the morning (see Fig. 10). The low infiltration rate (0.5 air exchanges/h) combined with the low thermal mass of the building means that in reality a rapid increase in office temperature occurs during the morning. Table 1 serves to highlight the importance of the introduced, additional thermal loads, through office use.
Beam irradiance as a percentage of the room loads (%)
13 23 19 28 25 26 24 24 24 16 19 13 25 40 45 56 59 60 59 58 52 39 33 23
The solar heat gain resulting from the equivalent continuous irradiance incident on the exterior of the office window during June is estimated to be 277 W, this is of a similar order to the average introduced continuous thermal load due to office working of 188 W. Elimination of the incident beam irradiance without modifying the other loads can; at most; reduce the office thermal loads by 26%. Additional changes such as control of the artificial lighting may therefore be required to achieve the desired temperature reductions. In essence, if the effects of glare could be eliminated there would be no need for the current internal blinds to be closed throughout the day and so the artificial lighting loads could be reduced.
3. Solar gain control options A variety of remedial options are available to reduce the solar gain problems of the James-Parkes building. These include window film, external louvers or blinds and smart glazing solutions such as electrochromics and holographic optical elements [6]. Essentially the solutions can be considered as being three different distinct types: mechanical, shading and glazing as shown in Fig. 3.
a
b
Solar heat gain coefficient of glazing = 0.63 (63% of solar heat gain transmitted). Equivalent solar heat gain (W) = (equivalent continuous irradiance)(solar heat gain coefficient)(window area).
252 312 343 432 463 465 463 446 389 307 279 245 188 188 188 188 188 188 188 188 188 188 188 188 64 124 155 244 275 277b 275 258 201 119 91 57 27 52 65 102 115 116 115 108 84 50 38 24 January February March April May June July August September October November December
14 30 27 51 48 51 46 45 39 20 22 14
Equivalent continuous irradiance striking outside of office window (W/m2) Month
Equivalent continuous beam irradiance striking outside of office window (W/m2)
Equivalent continuous solar heat gain into office; window area 3.79 m2 a (W)
Occupancy, lighting and computer loads (W)
Total equivalent continuous thermal load (W)
Irradiance as a percentage of the room loads (%)
P.A.B. James, A.S. Bahaj / Energy and Buildings 37 (2005) 1058–1067 Table 1 A comparison of the relative monthly contribution of irradiance and occupancy loads to the thermal gain of an office on the east elevation of the James-Parkes building (assuming every day is a working day)
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3.1. Mechanical Air conditioning is energy and maintenance intensive but does result in far lower office temperatures than any of the other options. Users are able to set the office temperature to their specific requirements and additional thermal loads (e.g. change of office use resulting in more heat generating equipment) can easily be accommodated. However, the use of individual office air conditioning units (only practical option for retrofit) would be an expensive and highly visible indicator of the building’s environmental failings and so is not an option for the university. Moreover, such an approach creates a dangerous precedent leading to similar requests from occupants of older university buildings.
Fig. 3. Solar gain control options to improve the thermal comfort of the ‘atpresent’ office. Three distinct types: mechanical, shading and glazing are available.
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Forced air convection relies on increasing the air exchange rate with ambient to lower the office temperature. Ideally, forced air convection buildings have a large thermal mass. This enables the use of high air exchange rates at night with the low ambient temperature to create a cooled building structure. During the day the building acts as a heat sink to the solar gain on the building envelope. However, the JamesParkes building is a low thermal mass structure and so forced convection would be required throughout the day to maintain the cooling effect. 3.2. Shading External blinds enable the control of either direct or diffuse and direct radiation. Fixed louvers mounted horizontally above the window reflect the incident beam radiation eliminating glare and the majority of solar gain whilst allowing diffuse light to enter providing lighting. Horizontal fixed louver systems also maintain an unobstructed view from the office, an important feature of a ‘healthy’ working environment. However, the function of a horizontal lamellae is dependant on the zenith angle of the sun (angle from the vertical, b in Fig. 5). If we consider a horizontal louver protruding out 1 m above a 1 m high window for example, the zenith angle must be 458 or less to ensure a shadow is cast over the entire window. Fig. 4 shows the azimuth and zenith angle paths of the sun for the summer (21 June) and winter (21 December) equinox from 05:00 to 21:00 h. The sun is normal to the east elevation of the James-Parkes building (648 azimuth) at approximately 09:30 GMT (‘A’ in Fig. 4). To perform a useful function the horizontal louver must provide full solar protection by this time throughout the year. However, the corresponding zenith angles for the summer and winter equinox are 42 (‘B’ in Fig. 4) and 828 (‘C’ in Fig. 4), respectively. At this orientation it is clear that only limited
Fig. 4. Zenith and azimuth angle paths of the sun for Southampton, UK (longitude 1.348W, latitude 50.48N) for the longest (21 June) and shortest (21 December) days of the year. The azimuth angle (648) when the sun is normal to the glazing of the east elevation is reached at approximately 9.30 GMT. The corresponding zenith angle for the longest and shortest days is shown.
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shading will occur in the summer months and no glare protection at all during the winter. To overcome this problem angled mechanical roller blinds (Fig. 3.) could be applied with their level of use controlled by the office user or sensors although such a scenario has not been modelled in this paper. 3.3. Glazing 3.3.1. Electrochromic glazing Electrochromic windows [7,8] enable the visible light transmission and solar heat gain of a window to be varied by the application of a dc voltage to an electrochromic coating. The electrochromic manufacturer, SageGlass [7] for example, produces an electrochromic double glazing unit with a visible light transmission, VT, which is varied between 0.70 and 0.04. The corresponding solar heat gain coefficient (SHGC) varies between 0.52 and 0.10. The SHGC representing the thermal fraction of the incident sunlight, which is transmitted through the glazing. Therefore, electrochromics offer occupants or building services managers the ability to continuously vary the solar thermal and illuminance gain in an office. 3.3.2. Holographic optical elements The simulated HOE system consists of a series of horizontal louvers mounted externally to the window. A single axis tracking system is employed which enables the louvers to follow zenith angle of the sun throughout the day (Fig. 5). Each louver would, for example, consist of a glass laminate of upper solar absorber glass, HOE film and underside solar absorber glass. The louvers would utilise a
Fig. 5. Schematic illustration of the function of lightguiding HOE in solar control louvers. Two possible schemes are shown: (1) top louvers—HOE redirects light within laminate at angle greater than the critical angle— energy dissipated by solar absorber glass, diffuse radiation is transmitted (2). (3) Lower louver—light is redirected to solar absorber attached to back surface of HOE laminate.
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transmitting, lightguiding HOE [9,10], which enables incident beam radiation to become trapped and eventually absorbed within the solar absorber glass. The incident radiation is redirected within the glass louver such that the light strikes the glass–air interface of the louver at an incident angle greater than the critical angle, causing total internal reflection (1). The refractive index of the hologram is similar to that of the glass allowing the reflected beam to undertake multiple reflections across the two solar absorber panes of the laminate before being absorbed (2). An alternative arrangement, would be, for example, a solar absorber mounted on the back surface of a light guiding HOE louver. The light would be redirected onto the absorber, as shown for the lower louver in Fig. 5 (3). Diffuse radiation does not actively interact with the HOE and so passes through the louver elements. The level of diffuse solar energy radiation transmission is determined only by SHGC of the HOE louver and not the diffraction properties of the HOE. The diffraction efficiency of both (a) transmitting and (b) reflecting holograms is dependant on the angle of incidence of the beam radiation. (a) Transmission HOE: Transmission (lightguiding) holograms in particular are critically dependant on the incident radiation striking the hologram at the designed working angle (Bragg angle). Typically, if the incident beam radiation is 58 away from the working angle of the HOE, the diffraction efficiency is reduced by 50% [11]. A tracked system is therefore, essential for transmitting hologram applications if the HOE is to function efficiently for more than a few minutes per day [11]. (b) Reflecting HOE: Reflecting holograms have a greater angle tolerance (258), which enables their use in certain applications without the need for tracking [11]. However, reflection HOE can produce glare effects on surrounding buildings which may make their use unacceptable for certain applications such as the offices considered here. 3.3.3. Window film Window film is often added to glazing as a retrofit application to try and compensate for glazing that is already causing problems for the occupants. The film is generally added to the interior side of the glazing to ensure a long lifetime. A variety of films are available from simple tints, to low-e coatings and spectrally selective films. This paper considers the use of a low SHGC window film, SunGuard Silver 20, produced by Guardian [12]. The film when applied to a 3 mm thick clear glass pane, for example, has a visible light transmittance, VT of 0.20 and SHGC of 0.15. 3.3.4. Solar control glazing A latest generation, titanium low-e coating glazing (TiAC-low-e, International Glazing Database Number, IGDB 965) manufactured by AFG glass [13] was simulated.
The coating has a very low emissivity of 0.034 compared to 0.84 for normal glazing.
4. Simulation model The east elevation of the James-Parkes building at Southampton University is orientated 268 from east towards south (azimuth = 648). Within TRNSYS the exterior wall of the east elevation is modelled as a composite construction (5 parts) of thickness 0.195 m and u-value 0.353. The interior partition walls of the building have a u-value of 4.73. Two representative office glazings (‘at-present’ and ‘atpresent low-e’) were created using the software package Window [3] to simulate the double glazing units. The atpresent glazing consisted of two Pilkington Optifloat, clear glass panes with a 12 mm air gap. A PVC frame was used to create a window of u-value 2.59, solar heat gain coefficient (SHGC) of 0.63 and visual light transmittance, VT, 0.80. To enable a fairer comparison between traditional glazing and smart fac¸ade technologies an alternative low-e coating glazing was defined to produce an ‘at-present low-e’ glazing with a u-value of 1.69. The SunGuard window film when used on the inner pane of the ‘at-present low-e’ window lowered the u-value from 1.69 to 1.65, with a SHGC of 0.124 and VT of 0.19. The holographic optical element louvres were assumed to have a SHGC and VT of 0.85 for diffuse light and an efficiency of 100% in the simulation, i.e. all incident beam radiation is diffracted and subsequently absorbed within the HOE louvre. An overview of the glazing types used in this study is shown in Table 2. An additional benefit of the HOE system is the elimination of glare, which will enable office users to avoid the need for blinds. The artificial lighting load can therefore, potentially be decreased, since incident diffuse light can now provide natural office daylighting. A reduction in artificial lighting from a continuous level of 10 W/m2 to an on–off switched mode for the 12 m2 illuminated floor area of the east facing offices was defined to assess the impact of utilising diffuse natural lighting. To determine the threshold incident diffuse light level below which artificial lighting is required in an office of the James-Parkes building a daylighting simulation package, Radiance [3] was used. Fig. 6 shows the predicted horizontal illuminance at desk height along the centre line of the room (2.5 m wide) as a function of distance from the window. The illuminance distribution within the office for a range of external diffuse illuminance levels is shown (5000– 13500 lx). The glazing construction, HOETRACKED-85, was simulated as at-present low-e glazing with external HOE louvers (85% transmission for diffuse light) producing an overall light transmission, VT, of 0.64. At an external horizontal diffuse illuminance of 7000 lx, it is predicted that in the centre of the room (2 m from window), the light level will be 500 lx, the minimum required level for office
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Table 2 Centre of a glass pane optical and thermal properties of the glazing types used in the James-Parkes building analysis Glazing type At-present 5.9 mm Pilkington Optifloat, IGDB 4009 12 mm air gap 5.9 mm Pilkington Optifloat, IGDB 4009 At-present low-e 3.9 mm Comfort Ti-PS low-e on clear, IGDB 933 12 mm air gap 3.0 mm, generic clear glass, IGDB 102 Electrochromic At-present low-e with modified SHGC and VT Light mode Dark mode HOEDIFFUSE-85 At-present low-e glazing, HOE interaction with diffuse radiation HOETRACKED-85 At-present low-e glazing, tracked HOE interaction with direct radiation Window film At-present low-e glazing with Guardian Sunguard Silver 20 on inside of inner pane IGDB 5.8 mm Pilkington K glass, IGDB 4013 12.0 mm air gap 5.6 mm SGSR20 on clear, IGDB 3138 Solar glazing TiAC-low-e coating glazing on inner side of outer pane 3.0 mm TiAC-low-e, IGDB 965 12.0 mm air gap 3.0 mm, generic clear glass, IGDB 102
Solar heat gain coefficient (SHGC)
Visual transmission, VT
u-Value (W/m K)
Emissivity
0.71
0.80
2.70
0.84 0.84
0.50
0.75
1.69
0.048 0.84
1.69
0.048 0.84
1.69
0.048 0.84
0.52 0.10
0.70 0.04
0.85 0.50 = 0.43
0.85 0.75 = 0.64
0
0
1.69
0.40
0.17
1.69
0.048 0.84
0.17 0.42 0.37
working [14]. Therefore, TRNSYS simulations which utilised internal lighting control or glazing changes (electrochromics) used an external horizontal illuminance level of 7000 lx as the threshold level for switching. The
Fig. 6. Predicted horizontal illuminance (lx) at desk height along the centre line of the room (2.5 m wide) as a function of distance from the window. Dependency on external horizontal diffuse illuminance level is shown. Glazing construction: at-present low-e with external HOE louvers, VT = 0.64; radiance simulation.
0.68
1.69
0.034 0.84
7000 lx corresponding to the typical illuminance level of a dull, overcast day. Fig. 7 shows the effect of glazing type on the predicted horizontal illuminance (lx) in the centre of an office, 2 m from the window for a clear, summer’s day (radiance
Fig. 7. Effect of glazing type on predicted horizontal illuminance (lx) in the centre of an office, 2 m from the window. Radiance simulation, clear sky, 14 June. (A) 07:00 GMT illuminance exceeds 500 lx with HOE system, (B) 17:00 GMT illuminance falls below 500 lx with HOE system.
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5. Simulation results
Fig. 8. Predicted ambient and average office temperature (‘at-present’) within an office on the east elevation of the James-Parkes building. A 7-day period of high irradiance is shown starting on the 21 June (Day 172 of the year). Lower office temperatures are predicted for the weekend due to the non-occupancy of the building. The upper temperature limit for comfort (27 8C) is highlighted.
simulation for 14 June) when glare and solar gain will be a problem. The HOE system (HOETRACKED-85 absorbs all direct radiation and has a visual light transmission, VT of 0.64 for diffuse light. The threshold light level of 500 lx is achieved between 07:00 and 17:00 GMT (‘A’ and ‘B’ in Fig. 7). The high level of incident beam radiation results in a peak illuminance at 08:00 GMT which, without attenuation, is too high for normal office working (at-present 35,000 lx, solar glazing 28,000 lx, window film 7000 lx), resulting in glare which would probably result in the internal blinds being closed by the office user. The low light transmission of the window film means that once the solar beam radiation has crossed the fac¸ade there is insufficient diffuse light to provide the required illumination level.
Figs. 8 and 9 represent the environmental conditions the office is subjected to for a period of a week in June. Fig. 8 shows the predicted ambient and office temperature distributions for the highlighted office (arrowed in Fig. 2) for a 7-day period starting on 21st June (Day 172 of the year). The 27 8C maximum thermal comfort level for an office is indicated as a dashed line. The corresponding predicted irradiance distribution (diffuse and direct) is shown in Fig. 9. Throughout the working week shown, the temperature in the office modelled under either its ‘atpresent’ or ‘at-present low-e’ state is shown to consistently exceed the 27 8C comfort limit (Fig. 8). Day 177, the Wednesday of the week shown will now be considered in detail. The predicted ambient and office temperatures and the corresponding irradiance distribution for Day 177 are shown in Fig. 10. The peak office temperature is reached at 18:00 h when the internal working day loads (occupancy, computers and artificial lighting) are switched off in the simulation. 5.1. Glazing/lighting scheme definitions at-present low-e: The James-Park office glazing as constructed, standard double glazing unit, low-e coating. HOETRACKED-85: A tracked HOE solar absorber external to the window, light transmission for diffuse light 85%, all beam radiation is absorbed. HOETRACKED-100: Tracked HOE unit with 100% transmission for diffuse radiation, all beam radiation is absorbed. Highlights the HOE ‘effect’ due solely to light guiding and not to the reduced light transmission of the HOE unit. HOETRACKED-85-LIGHTS-SWITCH: The high light transmission of the HOE for diffuse light enables the artificial
Fig. 9. Irradiance distributions (in plane beam and diffuse) for the east elevation of the James-Parkes building. A 7-day period is shown starting on the 21 June (Day 172 of the year).
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Fig. 10. Predicted ambient and office temperature and irradiance distributions (beam and diffuse) for the 21 June (Day 172 of the year).
interior lighting to be switched off when the external horizontal diffuse illuminance is greater than 7000 lx. ELECTROCHROMICCLEAR-MODE: Electrochromic glazing in high light transmission mode. ELECTROCHROMICDARK-MODE: Electrochromic glazing in low light transmission mode. ELECTROCHROMICVARIABLE-MODE: Electrochromic glazing light transmission mode changes to maintain required interior lighting level. ELECTROCHROMICVARIABLE-MODE-LIGHTS-SWITCH: Electrochromic glazing light transmission mode changes and interior lights switched on and off to maintain required interior lighting level.
5.2. Holographic optical elements The effect of adding a tracked HOE system mounted external to the fac¸ade is now considered. The predicted effect of a tracked HOE system for Day 172 is shown in
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Fig. 11. Of the selected glazings shown, the highest office temperature is reached with the at-present low-e condition, which represents the current double glazed office using a low-e coating. The addition of glazing mounted external to the fac¸ade will lead to a lower office temperature due to a reduction in incident solar radiation. To simulate the thermal effect of the additional glazing created by the HOE unit, the at-present low-e condition was modelled with reduced incident diffuse and direct radiation (85% of the at-present level). A perfectly functioning tracked HOE system which absorbs all the incident beam radiation and reduces the diffuse radiation level to 85% of the at-present low-e level (HOETRACKED-85) can be compared with an HOE system which allows all diffuse light to pass through (HOETRACKED100). The relative performance of these two systems in comparison to the at-present low-e glazing enables an assessment of the effect of HOE function and reduced diffuse light transmission on solar gain. The majority of the solar gain (direct radiation) occurs during the morning, which results in a rapid rise in temperature for a normally glazed office. An office with the HOE system for example, is approximately 58 cooler than an at-present low-e office at 10:00 h, this difference gradually reduces during the day. If internal lighting control (HOETRACKED-85-LIGHTS-SWITCH) is used in conjunction with the HOE an incremental reduction in temperature of approximately 1 8C is predicted. The internal lights are switched on when, during a working day, the external horizontal diffuse illuminance falls below 7000 lx. Over the entire year the internal lighting would be required for 586 office hours, compared to 2346 h for lighting throughout each working day. For an illuminated area of 12 m2, with a lighting load of 10 W/m2, such a reduction corresponds to 210 kWh of electricity per annum. 5.3. Ventilation Two typical forced ventilation schemes were considered for the east elevation with the second applying a lower air exchange rate during the day to reduce noise disruption. In each case the proposed solution was to cut ventilation holes below the windows in all the offices of the east elevation and to force additional ambient air through to the west side of the building. Continual ventilation is required due to the very low thermal mass of the building. The two schemes would operate as follows:
Fig. 11. Comparison of the predicted office temperature for 21 June (Day 172) with normal glazing and three possible tracked HOE solutions. The ambient temperature reaches a maximum at 15.30, with the office temperature peaking at 18:00 h, regardless of the modelled glazing type or level of office load (lighting, computers and occupancy).
Ventilation scheme 1 (vent 1): Time of day 02:00–18:00 h, 8 air changes per hour offices on east elevation. Ventilation scheme 2 (vent 2): Time of day 02:00–09:00 h, 8 air changes per hour offices on east elevation. Time of day 09:00–18:00 h, 4 air changes per hour offices on east elevation.
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Fig. 12. Comparison of the predicted office temperature for Day 177 of the year following solar gain control with either forced ventilation (vents 1 and 2) or a tracked HOE system.
Fig. 14. Comparison of the predicted office temperature for Day 177 of the year following solar gain control with a range of fac¸ade options. Electrochromic and tracked HOE which combine solar and interior lighting control show superior performance to traditional glazing options.
The predicted performance of the two schemes for Day 177 of the year is shown in Fig. 12. The high night air exchange rate of vent 2 lowers the temperature of the office to approximately 22 8C. At 9 a.m. the air exchange rate of vent 2 is halved to reduce noise disturbance, leading to a temperature rise in comparison to vent 1. The peak daytime working office temperature predicted by the scheme vent 1 is approximately 28 lower than that of the best HOE system, HOETRACKED-85-LIGHTS-SWITCH.
chromic working range. The ‘light’ mode of a Sage electrochromic is shown as ELECTROCHROMICCLEARMODE in Fig. 13 and is similar in profile to the at-present low-e glazing. In ‘dark’ mode, the SHGC is reduced from 0.52 to 0.10. The predicted temperature profile in the office (ELECTROCHROMICDARK-MODE) is very similar to that of a tracked HOE system (HOETRACKED-85). A key feature of electrochromics is the ability to continually vary the light transmission. Fig. 13 shows that switched electrochromic glazing, which varies in light transmission to provide natural lighting (ELECTROCHROMICVARIABLE-MODE) can be as effective at controlling office temperature as dark mode electrochromics if combined with lighting control (ELECTROCHROMICVARIABLE-MODE-LIGHTS-SWITCH). The predicted temperature profile would be slightly lower if the glazing was switched to dark mode outside office hours, reducing undesirable solar gain still further.
5.4. Electrochromic glazing Electrochromics enable continual variation in the SHGC and light transmission of the glazing. If the interior lighting of an office is maintained throughout the working day the maximum and minimum of solar control performance can be estimated by considering the extremes of the electro-
5.5. Alternative fac¸ade options
Fig. 13. Comparison of the predicted office temperature for Day 177 of the year following solar gain control with a range of electrochromic options (clear, dark and variable transmission mode).
A comparison of HOE with electrochromics, solar glazing, window film and internal blinds options for the working hours of the 26 June (Day 177) is shown in Fig. 14. The window film attenuates both the incident diffuse and direct radiation at the inner pane of the existing glazing, which, in comparison to HOE leads to a higher percentage of the attenuated radiation leading to heat gain within the office. In addition, the low visible light transmission of the proposed window film (VT of 0.19) means that the current level of artificial lighting within the office will have to be maintained. This is highlighted in Fig. 7, where after 10:00 h on a clear day in June, interior lighting is required to achieve a 500 lx illuminance level within the office. The predicted temperature profile of the window film (window film) is similar to that of an internal blinds (int-blinds). The tracked HOE systems, which incorporate a lower level of interior
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lighting (HOETRACKED-85-LIGHTS-SWITCH) predict significantly lower office temperatures. The TiAC-low e solar control glazing is predicted to have far superior solar control performance than either normal low-e glazing or the selected window film options. The temperature profiles of both HOE and electrochromics utilising switched internal lighting is lower than that of solar control glazing, especially during the morning when the majority of the solar gain is incident on the fac¸ade.
6. Discussion The TRNSYS simulations highlight the need to control both external and internal heat gains within an office to create a comfortable working environment. Lightguiding HOE which enable solar control of the incident beam radiation whilst controlling glare and retaining vision through the window or switchable electrochromics have been shown to be potential alternative to traditional solar control approaches. None of the fac¸ade solutions can maintain the office below the upper working temperature for comfort of 27 8C throughout the year. However, for non-airconditioned buildings, there should perhaps be a degree of tolerance in this upper limit, accommodating higher temperatures for a limited number of days. The glare problems of an East facing offices of the JamesParkes building studied mean that it is not possible to utilise blinds or louvers to block the incident beam radiation without obstructing the office view. The forced ventilation schemes were shown to outperform the glazing solutions but this is at the expense of higher energy consumption, noise and maintenance costs. Moreover, the ventilation schemes do not address the issue of glare and so office users would still have to work behind blinds. Window film can lead to lower office temperatures in a cost effective manner but may however, result in higher artificial lighting loads for the remainder of the year. Moreover, removing 80% of the incident illuminance level is a somewhat crude tool and is not ideally suited to the highly variable UK weather. The simulations shown here assume that the HOE systems functions at 100% diffraction efficiency. In reality however, HOE have a spectrum dependant efficiency [15] and are not perfectly recorded films which leads to a reduction in efficiency. In addition, the HOE requires true alignment between the incident beam radiation and the working angle of the hologram. If the tracking system does
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not perform to within 28, this loss in HOE function increases rapidly, rising to 50% for a 58 error [11]. The resulting glare and spectral dispersion effects (rainbow colour spectrum) that may result from a poorly functioning HOE are significant from a comfort rather than absolute office temperature perspective.
Acknowledgements Aspects of this work were undertaken as part of a European Commission Framework 5 programme ‘Holographic Optical Elements (HOE) for High Efficiency Illumination, Solar Control and Photovoltaic Power in Buildings’, project no. ENK6-CT-2000-00327.
References [1] ISO7730, Moderate thermal environments—Determination of the PMV and PPD indices and specification of the conditions for thermal comfort, International Organisation for Standardisation, 1994. [2] TRNSYS, Thermal Energy System Specialists, 2916 Marketplace Drive, Suite 104, Madison, WI, USA. [3] LBNL, Lawrence Berkley National Laboratory, Fenestration simulation programme, Window 5.1 and Daylighting simulation programme, Radiance. [4] J. Kos´ny, E. Kossecka, Multi-dimensional heat transfer through complex building envelope assemblies in hourly energy simulation programs, Energy and Buildings 34 (5) (2002) 445–454. [5] Meteonorm, Solar simulation software, http://www.meteonorm.ch. [6] LBNL, IEA Report Task 21, A Source Book on Daylighting Systems and Components, 2000. [7] Sage Electrochromics Inc., 2150 Airport Drive, Faribault, MN, USA, http://www.sage-ec.com. [8] LBNL, Electrochromic Window Tests in U.S. Office Show Promise, EETD Newsletter, vol. 5, 2000. [9] H.F.O. Muller, Application of holographic optical elements in buildings for various purposes like daylighting, solar shading and photovoltaic power generation, Renewable Energy (5) (1994) 935–941. [10] H.F.O. Muller, Innovative use of light directing holograms with solar cells, Solar Architecture (1994). [11] P.A.B. James, A.S. Bahaj, Holographic optical elements: various principles for solar control of highly glazed buildings, Solar Energy 78 (3) (2005) 441–454. [12] Guardian, http://www.sun-guardglass.com. [13] AFG glass, http://www.afg.com. [14] DIN EN 12464 Light and lighting—lighting of work places, Part 1, Indoor work places. [15] J. Breitenbach, J.L.J. Rosenfeld, Goniospectrometer measurements of the optical performance of a holographic optical element, Solar Energy 68 (5) (2000) 427–437.