Directionally selective shading control in maritime sub-tropical and temperate climates: Life cycle energy implications for office buildings

Accepted Manuscript Directionally selective shading control in maritime sub-tropical and temperate climates: Life cycle energy implications for office buildings Myles E. Bunning, Robert H. Crawford PII:

S0360-1323(16)30161-5

DOI:

10.1016/j.buildenv.2016.05.009

Reference:

BAE 4483

To appear in:

Building and Environment

Received Date: 15 February 2016 Revised Date:

4 May 2016

Accepted Date: 5 May 2016

Please cite this article as: Bunning ME, Crawford RH, Directionally selective shading control in maritime sub-tropical and temperate climates: Life cycle energy implications for office buildings, Building and Environment (2016), doi: 10.1016/j.buildenv.2016.05.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Myles E. Bunning a, Robert H. Crawford a* a

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Directionally selective shading control in maritime sub-tropical and temperate climates: life cycle energy implications for office buildings Faculty of Architecture, Building and Planning, The University of Melbourne, Victoria 3010, Australia

Abstract

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Scheduling directionally selective shading devices to increase or decrease their level of occlusion relative to the total incoming solar radiation has the benefit of controlling solar heat gain during a variety of sky conditions and allowing more constant illuminance levels to be achieved within a building. In this study, hourly sky condition and annual solar angles were used to describe the tilt of the slats of an external directionally selective shading control for an external venetian blind on an office building in Melbourne and Brisbane, Australia. The life cycle energy demand associated with this shading control was compared to a static base case with an external overhang and internal venetians. The analysis was extended to the HVAC system which was sized to account for the effect of the shading on solar gain and the artificial lighting requirement. It was found that the embodied energy of the HVAC and shading components accounted for between 21.7% and 25.5% of the total life cycle energy of these systems over 25 years. There was a reduction in embodied and operational energy requirements over a 25 year life cycle for the external venetian blind control of 24.9% for Melbourne and 24.0% for Brisbane relative to the static base case. Based on the simulation results, office buildings with equator facing facades located in similar climates and latitudes may have the potential for equivalent life cycle energy reductions when external directionally selective shading controls are employed to moderate overheating and daylighting.

1. Introduction

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Keywords: Venetian shading, Embodied energy, Directionally selective shading, Life cycle energy, office buildings

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Occupants of office buildings depend on the careful consideration of solar penetration, provision of views to the outside and access to daylight in design of fenestration systems. For several decades the design trend in Europe and America has been towards highly-glazed facades [1]. Equator facing and western facades are particularly vulnerable to overheating causing occupant thermal discomfort as well as non-uniform daylight distribution. A sensible shading strategy can assist in reducing the energy costs associated with solar gains; however, additional shading devices have their own embodied energy demands that contribute to a building’s life cycle energy demand. Because shading also has a direct relationship to heating and cooling requirements for a building, if the shading can be controlled in a way that reduces the requirement for heating and cooling, energy reductions can also be translated to other components of the building which are sized to distribute heating and cooling. Examining the interaction between shading and the equipment for heating and cooling an office building can assist in selecting systems which use less energy over the building life cycle. The aim of this study was to calculate and analyse the life cycle energy demands of an external directionally selective shading control which responds to real-time weather conditions in multiple locations. The contribution that external directionally selective shading control can make to reducing the HVAC systems’ sizing and hence embodied energy has also been considered.

2. Background 2.1. Directionally selective shading of office buildings The energy consumed by a newly installed HVAC system differs according to how directionally selective shading is operated and positioned relative to the glazing. Venetian blinds are one type of shading that have the

* Corresponding Author [email protected], +61 (0)3 8344 8745 (Robert H. Crawford)

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potential to redirect sunlight to both improve the uniformity of daylight distribution within a space while at the same time reducing potential sources of glare from the sky or directly from the sun. Venetians perform a number of functions that affect the operation of office artificial lighting and HVAC systems. During periods of high incident solar radiation when overheating is likely to occur, venetian slats can be angled to absorb and radiate direct solar radiation to the exterior. A venetian blind is also capable of reflecting daylight to either the interior or exterior. The proportion of available light that is redirected to the interior or the exterior can be controlled according to the sun’s position throughout the day. Shading of the equator facing façades is necessary for reducing the cooling load throughout much of the year in Australia. The requirement of a portion of summer solar radiation to be restricted can also conflict with the occupant’s desire for views to the outside or direct sunlight. Overemphasis on the restriction of radiation at certain periods at the expense of exposure to views, natural light or to make adjustments to uniformity of lighting within a room, may lead to increased energy demand as dissatisfied occupants seek to over-ride automated fenestration controls. In a previous study of blind use in office buildings, blinds were re-opened by the occupants in 45% of cases when the blind was closed in response to a façade illuminance of 28 kLux [2]. While the subjects of the study were not formally interviewed about why they chose to retract blinds at such high illuminance levels, a greater likelihood for acceptance of blinds being automatically retracted than lowered was demonstrated. For venetian blinds, providing the facility for tuning the view angle to either slightly upward towards the sky or downward towards the ground during periods without direct incident radiation appears to reduce dissatisfaction with automated shading [3]. Without a system that integrates façade shading, artificial lighting and the HVAC design, office workers are subject to thermal and visual discomfort if these systems interact unfavourably. Venetian blinds offer a range of possibilities of control of daylight that can improve internal thermal and visual comfort in comparison to glazing with low visual light transmittance or fixed shading. The unique aspect of a venetian is its ability to rotate each slat to allow for the simultaneous provision of daylight and glare protection [4]. While some level of automation of slat tilt or raising and lowering of the blind can offer a level of interactivity and intelligence to the operation of an office building’s thermal and visual control system, without a shading system affording a degree of human control, occupants are likely to find certain aspects either visually uncomfortable or off-putting [3]. The need for shading of existing Australian office buildings is compounded by the typically high percentage of glazing within the overall area of the facade. Based on a survey of 127 Australian office buildings, approximately three quarters were found to have glazing to between 41-70% of their equator or west facing façade area. The majority of this existing glazing was found to be tinted with either a medium or dark tint [5] with much of this glazing initially installed before the 1990s when low-e coatings became more commonplace. The properties of low-emissivity (low-e) glass with their higher visual light transmittance relative to solar heat gain provide for greater transparency than solar heat absorbing glass, such as non-low-e coated tinted glass. As 6 mm thick low-e glass with a visual light transmittance value of 0.8 or above can often be substituted for what might have previously been specified as a non-low-e coated tinted glass to enhance a building’s thermal performance, the need to control daylight levels and residual solar gains with shading devices still remains a priority to maintain the expected level of comfort for occupants. From a review of twelve behavioural studies of manual shading between the latitudes of 37°N and 55°N, those studies that had blinds on both orientations established that occupants operated venetian blinds differently for equator facing and non-equator facing façades [6]. While there are varying reasons for the use of blinds, user surveys consistently report that orientation of the window and sky conditions are the two most important factors for their use [7]. More studies have concluded that independent of orientation, sunnier conditions resulted in greater levels of occlusion [8-10] than studies which find no significant correlation [11]. Discomfort caused by sky conditions has not always been found to coincide with clear sunny days. A bright cloudy sky can often produce a vertical illuminance incident on a window which is higher than that from the sun when there is a clear sky [12]. In locations where partly cloudy days are common, such as in Sydney, Australia [13], manual shading preferences are likely to be more influenced by higher vertical illuminance at the window plane during partly cloudy days. Discomfort from overheating is another factor that is understood from thermal models and occupant surveys to be a factor in manual control of blinds. Based on an earlier study of manual control of blinds in Japan, it was proposed that some of the observed variation in roller blind use could be modelled according to the occupant’s distance from the window [14]. The study used simplified heat balance equations predicting the percentage of dissatisfied occupants according to the Fanger model of thermal comfort [15]. By analysing this heat exchange between the room and occupants as part of the building thermal model, it was possible to demonstrate how proximity of occupants to the window might influence both blind control and building energy usage. In a

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separate survey in which participants filled out a daily questionnaire on reasons for their venetian blind control behavior, it was found that their primary motivation was not excessive solar heat gain but the need to protect against glare [10]. Since two-thirds of the survey sample was located in air-conditioned offices, the sample exhibited a reduced need for protecting against solar gains whilst within acceptable thermal comfort conditions. Such narrow thermal comfort bands, which tend to be the primary conditions in which studies of manual venetian blind control are conducted, are unlikely to provide conditions which will allow conclusions to be drawn about the effect of thermal conditions on manual blind control [6]. Nevertheless, during high levels of daylight, occupant assessments of daylight conducted in the UK suggest that there is a strong psychological link between glare and overheating [16]. The colour temperature variation, or the shifts within the daylight spectrum between noon and late afternoon daylight, may also contribute to an occupant’s ability to stay alert. Certain shorter wavelengths within the visible spectrum, have been found to suppress melatonin release which is a key modulator of the circadian system [17]. Based on this relationship, the controlled transmission of daylight within tolerable ranges through venetian blinds rather than relying on constant longer wavelength artificial light might help to reduce fatigue. 2.2. Quantifying the energy demand associated with directionally selective shading

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Due to the need to adjust to momentary changes in weather and daylighting, in addition to occupant comfort requirements, to determine operational energy it is necessary to use computers for the large number of interrelated calculations involved [18]. Radiosity models which describe the interplay of light from purely diffuse surfaces are also suited to computational analysis. Blinds with specular reflectivity are best modelled with raytracing methods which are outside the scope of this study; however surfaces with low specularity still approximate the transmission calculated using radiosity models [19]. EnergyPlus uses radiosity models to describe light transmission through flat slat venetian blinds. The software also provides about 20 possible shading control schemes based on weather variables such as glare and solar radiation [6] and has a large variety of templates for modelling active heating and cooling systems. The determination of peak load requirements for the HVAC system, as well as the level of shading of the established base case, the characteristics of the building and the type of HVAC system is fundamental to the embodied energy calculation. The peak load energy requirements of a 17 m2 room in Oakland, California has been reported to be reduced by between 6-15% with dynamically adjusting internal venetian slats at 30 second intervals [20]. Such a short interval between blind adjustments was not generally accepted by occupants with four out of 14 respondents preferring less blind movement. This was in comparison to slats for which the outside edges were angled at 45 degrees downward from horizontal without any dynamic adjustment to maintain internal illuminance levels. This is a substantial reduction in peak load which could be expected to be greater with external venetians and hence, potentially lead to reductions in the size of the HVAC system and associated infrastructure for generating its supply energy. However, there is also the potential that using multiple embodied energy-intensive engineering solutions to reduce operational energy, such as automated venetian blinds or metal louvres, may sacrifice much of their operational energy benefits as a consequence and even bring about an increase in overall life cycle energy [21, 22]. The energy demand associated with non-operational stages of the life cycle of an office building is often omitted from studies of the energy performance of venetian blinds. However, the inclusion of life cycle energy requirements is critical to determine the full extent of the energy demand of venetian shading controls. Methods of quantifying embodied energy can vary according to how energy demands associated with production, delivery and maintenance processes are accounted for. The energy demands of complex components within a building are not generally confined to one region, but are accrued within a globally distributed network of resource suppliers, manufacturers and service providers. In this interconnected supply network, compilation of a process tree for even a simple component, where each process is responsible for a quantifiable amount of energy, is likely to exhibit inherent ‘system incompleteness’ [23]. Quantification of embodied energy using process data from manufacturing processes alone, whilst accurate for the limited system boundary of individual industrial processes, can result in an underestimation of the embodied energy of a product. However, inputoutput (I-O) data which describes monetary transactions throughout an economy can provide a more comprehensive framework for analysing energy usage of products or building components than using an approach based on process data alone. Because an I-O analysis can provide a disaggregated snapshot of a nation’s economy, all energy flows can be traced. Combining process analysis with I-O analysis, in the form of a hybrid analysis, is considered the most preferred approach for quantifying embodied energy, and is used in this study.

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ACCEPTED MANUSCRIPT 3. Case study building

North Perimeter Zone

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The case study building was based upon an existing office building located in Melbourne, Australia, for which detailed information about the building envelope and HVAC system was available. The total floor area of the building, spread over 12 floors and 2 basement levels, was 8,343 sqm. Seven floors (from five through to eleven) were used as offices. The remainder of the building served non-office functions and no directionally selective shading control was to be used for this section of the building. As is common at intervals of 20-25 years, the HVAC system recently required upgrading and the scope of the upgrade was extended to the facade. The design for the new building had 43% of the equator facing façade area glazed and was orientated 19 degrees west of north. Other façades had much lower percentages of the façade area glazed with the easterly façade being 21% glazed and the westerly and southerly facades being only 3% glazed with vertical slot windows. The low emissivity coating to face 2 of the double glazing unit had an infrared hemispherical emissivity of 0.2. For the largest 2.4m high double glazing units in the north and east perimeter zones, the U-value at the centre of the glazing was 1.89 W/m2K and the SHGC without blinds was 0.58. Fig. 1 shows a typical floor plan of the case study building.

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Fig. 1 Typical floor plan of the case study building showing perimeter and interior zones (unconditioned zones shown hatched)

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The HVAC system for the building relies on a combination of active chilled beams and skirting heaters. The chilled water loop from the water-cooled chiller and cooling tower supplies a secondary chilled water loop providing chilled water to the active chilled beams. These beams act as an air-water system transferring heat away from the interior through the circulation of chilled water as well as providing supplementary cooling using re-circulated and outside air. A flat plate heat recovery system runs throughout the year to reclaim heat from return air unless outdoor temperatures are above 8.5°C during the winter months. The base case shading was formulated to be a static shading configuration which combined a fixed louvred overhang projecting 700 mm from the facade and a manually operable internal venetian blind (Fig. 2). The outside edges of the venetian blind slats were maintained at a constant angle of 45 degrees downward from horizontal throughout the year with artificial lighting operating continuously during operational periods with an energy requirement of 12 W/m2. This is equivalent to the upper limit of what is permissible for achieving an illuminance of up to 400 lux in accordance with Australian standards. For situations in which cooling loads dominate the annual energy budget, positioning venetian shading on the external side of the glazing is recommended. The external location of shading allows for potential solar gains to be absorbed externally or reflected away from the glazing without either contributing further to the absorption and re-radiation of heat at the facade or to the perimeter zone cooling load. An external responsive motorised venetian for which the slat tilt was programmable in response to predefined conditions was investigated as an alternative to the base case. The external venetian was spaced 100 mm from the outside surface of the glazing. Splitting the venetian into two independently controlled sections at a point above average head height allowed the opportunity, in cases where daylighting could be promoted over reduction in heat gain, to tilt the outside face of the slats upward towards the interior from the horizontal position. Tilting the slats towards the interior in this way reflects sunlight onto the ceiling and increases both the level of illumination at the work surface

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Fig. 3 Directionally selective shading scenario: external motorised venetian with control modes for upper slat tilt and lower slat tilt based on relative solar angle and sky condition

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Fig. 2 Base case shading scenario: internal static venetian slat tilted to 45 degrees downward from the horizontal position

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(Fig. 3). These reflecting slats within the upper section of the venetian above eye level also serves to increase the illuminance contrast within the occupant’s surroundings to reduce the potential for the window to become a glare source.

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Both scenarios were analysed in two distinctively different climatic regions of Australia, Melbourne and Brisbane, in order to demonstrate the effect of climate on the life cycle energy performance of the shading scenarios. Melbourne has been described as having a temperate, mid-latitude climate with maximum temperatures occasionally reaching above 40 degrees during mid to late summer. Maximum temperatures in Brisbane rarely exceed 40 degrees, but average monthly minimum temperatures are between 6-7 degrees higher than Melbourne. Climate conditions in Brisbane are characterised by narrower diurnal temperature ranges and higher relative humidity in comparison to Melbourne, being considered to be a maritime sub-tropical climate. The climates of Brisbane and Melbourne were considered suitably divergent in terms of incident solar radiation, whilst still being located within coastal urban areas which include a significant amount of existing office building stock. Mean daily maximum temperatures during May to October in Brisbane’s climate are within the range of 20 to 26 degrees Celsius [24], which exceeds the temperatures that are typically comfortable for office work. These warm external temperatures combined with a bright sky, with a design sky illuminance level of approximately 8,900 lux, can result in highly glazed buildings having an imbalance in daylight illumination contributing to both glare and solar heat gain. However, despite higher diurnal temperature swings, Melbourne’s temperature has a weaker correlation with global solar irradiation than Brisbane [25].

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4. Approach for determining blind slat tilts This section describes the approach used to determine the slat tilt for the shading scenarios. The base case was to be consistent with a scenario in which occupants who are compelled to draw a blind down to control solar radiation will then leave them down for periods in which conditions are appropriate for daylighting, bringing about an unnecessary dependence on artificial lighting. Such a scenario follows reports of this being a common behavior in offices. One behavioural study by the BRE in the UK found that blinds were left down and lights left on unless blinds were opened routinely by cleaners [26]. This pattern of blind usage is consistent with a study of six office buildings in Maryland, USA showing 80% of the glazed façade having been found to be covered by venetian blinds on a typical day [27] and a study of a 16 storey building in Ottawa showing 65.5% of equator facing glazing was obscured by venetian blinds [28]. In the Maryland study, venetian blinds were placed in the fully open and fully occluded positions during the weekend, since few venetian blinds were being opened or closed before the study was initiated. Interventions in blind usage patterns to provoke responses have also been deemed necessary in another survey of venetian blind usage in which participants were asked to retract their venetians when they first arrived for the day and only then adjust them according to their preference [10]. Studies have indicated the need for some form of provocation to bring about a blind response other than keeping the blinds closed. In the absence of a more deliberate manual shading model, a base case in which both upper

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and lower sections of the venetian blinds are left at 45 degrees downward from horizontal for much of a given day, could be considered to be a common state for manually operated blinds to be left in for extended periods. The position of the outside edge tilted at 45 degrees downward from horizontal, which was adopted as the internal blind position for the base case of this study, is a relatively closed state whilst still maintaining some access to modest views, which are both nearby and below the occupant’s horizontal line of sight. To isolate the effects of the controlled external venetian relative to the static base case, the width of the slats (80 mm) and the optical properties of the downward facing or back face of the slat were kept constant for both. The solar reflectance of 0.486 and visible reflectance of 0.488 were the same for the upward facing or front face of the slat to match the properties of ‘silver cloud’ aluminium slats [29]. The upward facing or front face of the upper section of the two-part external blind differed in its reflectivity. The reflectivity was increased to maximise reflection of daylight to the ceiling, the white surface of the slat having a solar reflectance of 0.762 and visible reflectance of 0.863. The distance from the pivot point of the venetian slat to the glass was 110 mm for the base case and 140 mm for the external venetian scenario. The control of the external venetian was defined by hourly schedules of slat tilt for the upper and lower section of the blinds. The slat tilt schedule varied according to an hourly choice based on sun position and calculated clearness index from the relevant diffuse and global radiation ratios. The hourly global and diffuse radiation parameters make up part of the weather file used to calculate energy transfers occurring throughout a simulated year. Together with the sun position, the hourly global and diffuse radiation provided enough information to calculate an hourly value for the clearness index for each hour in the year. Days were categorised into both cloudy and non-cloudy days, with cloudy days defined as days where the threshold number of daylight hours where the Perez clearness index exceeded 1.9 [30] using data from the Bureau of Meteorology. The approach for determining the blind slat tilts took sun position and clearness index into consideration for the external venetian shading case and a static 45 degree tilt was assumed for the internal venetian base case. 4.1. Blind slat tilt calculations for building energy simulation

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The energy simulation program, Design Builder was used to develop an EnergyPlus model to compare the response of external venetian shading control and the static base case to changes in weather and sun position. To limit the number of EnergyPlus schedule inputs, hourly slat tilts were averaged over a month and collated into the 24 hour slat tilt schedule for each month of the year. This allowed a pattern of blind usage to be responsive to prevailing conditions within the weather file. The lower venetian blind section performed the function of blocking beam radiation and moderating window luminance. During months that were predominantly overcast, the slat tilt for the lower window would be tilted more towards the horizontal (0 degrees). During periods where there was potential for greater vertical illuminance such as during partly cloudy or clear periods when direct solar radiation was incident on the glazing, slat tilt would be a minimum of 45 degrees downward from horizontal to obscure transmission of direct sunlight and associated glare from the bright diffuse sky. The upper venetian blind section was to perform several functions depending on the position of the sun. One function was to block beam radiation on the vertically orientated window pane when: • the solar angles of incidence normal to the glazing were between horizontal and 15 degrees above horizontal by tilting the slat to an angle of 85 degrees downward from horizontal during the winter half year, or • the solar angles of incidence normal to the glazing were between 15 and 45 degrees above horizontal by calculating the slat tilt to keep three-quarters of the lower slat shaded. Another function was to reflect light to increase illumination. During sunny periods, when the angle of incidence normal to the glazing was greater than 45 degrees from horizontal, the slat angle was calculated to reflect the light back to the ceiling at an angle of 10 degrees above horizontal except during summer when the angle of incidence normal to the glazing was less than 60 degrees above horizontal. In the summer case the slat angle was 85 degrees downward from horizontal while in the winter case, the slats were kept in the horizontal position. During partly cloudy weather, slat tilt was always angled to be 45 degrees downward from horizontal to reduce glare. Otherwise the upper venetian slats were kept in the horizontal position to promote daylighting. A series of rules were established so that each hour of the day could be assigned a slat tilt according to its occurrence on a cloudy day or a non-cloudy day and whether the conditions during that hour are cloudy, partly cloudy or clear. Similar to the upper blind, the lower venetian slats are tilted to 45 degrees downward from horizontal until 1pm during partly cloudy weather on cloudy days or partly cloudy weather during all noncloudy days. Table 1 shows the relationship between the solar angle of incidence normal to the vertical glazing

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ACCEPTED MANUSCRIPT (φ), the Perez sky clearness index (ε) and slat tilt established for Brisbane’s daylight climate. A variation to this was deemed necessary for the Melbourne daylight climate to allow more light during partly cloudy afternoon periods on cloudy days during the winter half year when φ was greater than 15 degrees.

Table 1 Relationship between lower section slat tilts, sun positions and Perez sky clearness index for east and north orientations for Brisbane Cloudy Day Sunny

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Sun position range

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To reduce glare from reflected light off unshaded slats, unless it was a cloudy period during a cloudy day, if the relationships in Table 1 caused more than three quarters of the top of the slat to be unshaded, the result was adjusted so that at least one quarter of the slat cross section normal to the glazing remained in shade. The conditions for venetian shading to have its maximum range of utility is likely to be when the solar angle of incidence is relatively low throughout the day and external temperatures exceed the preset thermal comfort band. For the case study building’s equator facing façade, this period occurred during March. The calculated hourly slat tilt for selected days in March for Brisbane is illustrated in Fig. 4 and the average for the month is illustrated in Fig. 5. Fig. 4 shows that during the selected week for Brisbane, partly cloudy and cloudy skies are dominant. During the afternoon, the control mode ensures that during conditions that are partly cloudy, especially when the sun is out on an otherwise cloudy day, slat tilt is set to 45 degrees downward from horizontal. North 0

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Partly cloudy hour (cloudy day after 1pm) Count=2 Partly cloudy hour (non-cloudy day) Count=21 Cloudy hour (non-cloudy day) Count=20

Fig. 4 Lower slat tilt for hourly azimuths during varied

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Fig. 5 Lower slat tilt for hourly azimuths averaged

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over the month of March for Brisbane

5. Comparing the life cycle energy performance of shading scenarios

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The slat tilts were aggregated as an hourly slat tilt for each month as shown for the month of March for Brisbane in Fig. 5. Whilst aggregating slat tilts by month is no longer fully representative of tilt responses to daily and hourly variations in sky condition, longer term changes in average slat tilt of 15 degrees or more between morning and afternoon were able to be captured for the energy calculation when these aggregated slat tilt choices were used as a daily control schedule in the building energy simulation. During the month of March in Brisbane, the limited response to high sun altitudes is evident in the high frequency of slat tilts between horizontal and 15 degrees downward from horizontal. With the orientation of the northern façade being 19 degrees towards the west, the altitude of the sun relative to the glass was lower during the sun’s afternoon trajectory. This also translates to average slat tilts being more occluded and closer to being tilted 45 degrees downward from horizontal.

5.1. Energy for artificial lighting

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The energy demands associated with maintaining thermal and visual comfort within an office building occur at various stages of a building’s life. A full replacement of major HVAC components, such as the chiller, only occurs on average once every 25 years [31]. This duration of 25 years was selected as the timeframe over which to perform the life cycle energy analysis. This study focuses on the effect of external venetian shading on the operational energy requirements (for artificial lighting, cooling, heating and ventilating) and the embodied energy for the case study office building.

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In keeping with the reported manual blind usage pattern of leaving blinds down and artificial lights on during the day, the base case was defined to maintain a constant artificial lighting load throughout the office space. Because the external venetian scenario has the potential to provide more uniform and greater levels of daylight, the use of daylight controls for artificial lighting in combination with the external venetian control mode allowed for reduced power consumption. The venetian blind control was configured to provide more light during overcast days to daylit areas so that the use of artificial lighting could be limited accordingly to maintain a work plane illuminance of approximately 400 lux. Illuminance levels were assessed using the DELight module which is integrated with EnergyPlus. The illuminance was calculated at points between 0.5 and 4.4 metres from the façade. This allowed confirmation of the proportion of the perimeter zones below the required threshold for illuminance at selected times during the day.

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The peak load design determined the quantity of active chilled beams and the combined length of beam for each zone of the seven office floors where chilled beams were installed. Additional load for the ground floor and four non-office floors were kept the same. The designated date for calculating HVAC sizing for summer was February 20th for Melbourne and January 20th for Brisbane, influenced by the recent occurrences of higher than average monthly maximum temperatures in these locations [32, 33]. Although records show maximum outdoor temperatures typically occurring in mid to late January in Melbourne [34], because the case study building has large expanses of equator facing glass, the solar angles that are more normal to the glazing at the latter stages of a hot dry spell during February were considered to provide the conditions for the greatest heat load. Rather than considering the external venetian blinds as user operable and therefore not able to be relied upon during a peak load event, a credit for venetian blinds as suggested by Klems [35] was allowed. This allowance was satisfied through applying the aggregate monthly slat tilt described in the preceding section as the slat tilt for the summer design day. The active chilled beams installed in the case study building transfers the cooling load via a system of pipes and pumps rather than through the air if an all air system had been used. This brings about a major advantage of greater energy efficiency due to lower chiller and fan energy [36, 37]. Flow rates of water and air through the active chilled beams were controlled within manufacturer’s guidelines to insure that the set points of ± 2 degrees around a fixed value of 22 degrees Celsius were maintained during the occupancy period.

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Natural gas fired boilers were sized for the ground floor entry and 11 storeys of conditioned space based on the summation of conductive heat losses to the outside and the heat required to heat outside air during the winter design conditions. Outside air heating and return air reheating was based on 10 sqm/person occupancy and a supply air rate of 10 L/s/person. Simulation of the annual heating energy was based on a system which included a flat plate heat recovery loop reclaiming heat from return air which would otherwise be returned to the outside, and a convective model of a baseboard hydronic heater that was sized for each conditioned zone. To determine the total life cycle energy, operational energy demands of either electricity or natural gas were multiplied by corresponding primary energy factors. The primary energy factors used for the electricity component were 3.4 for Melbourne and 3.1 for Brisbane which is representative of their respective brown coal and black coal-fired electricity networks. A primary energy factor of 1.4 was used to convert the delivered natural gas value into primary energy terms [38]. 5.4. Embodied energy

6. Results and Discussion

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The embodied energy associated with the production and ongoing maintenance of the HVAC and shading systems was determined with the use of an I-O-based hybrid approach and hybrid material coefficients as per Crawford [39]. These hybrid material coefficients were developed by Treloar and Crawford [40] using the latest available SimaPro database for the most common basic materials. These were multiplied by the quantities of materials constituting the two different HVAC and shading options. Data gaps were then filled in accordance with the I-O-based hybrid approach using I-O data taken from the Australian National Accounts [41] in combination with energy intensity factors by fuel type from the Australian Energy Accounts [38]. ‘Other machinery’, ‘Fabricated metal products’ and ‘Structural steel’ sectors of the Australian economy were used to represent the various HVAC and shading components. To make allowance for material wastage during construction or assembly, wastage factors of between 1.02 and 1.10 were applied consistently to each component material for the HVAC and shading systems for both Melbourne and Brisbane. Further details of the method and validation of the I-O-based hybrid model for determining embodied energy have been documented elsewhere [39, 42, 43] and shall not be repeated here. To determine the recurrent embodied energy associated with maintenance over 25 years, life spans for components of the HVAC and shading system were established [31, 44]. The manufacturer of the active chilled beams estimated that their chilled beams would have a life expectancy of 25 years [45]. As shown in Fig. 7, cooling towers and chillers were considered to need replacement after 25 years.

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This section presents and discusses the results of the life cycle energy analysis of the two shading scenarios. Table 2 shows the difference in sizing of the chilled beams between Melbourne and Brisbane and isolates beam sizes for the northern and eastern perimeter zones of the office space which were the most highly glazed zones bringing about the need for a greater surface area of chilled beams. Flow rates were restricted to no greater than an air supply rate of 14.3 l/s/m at a supply air nozzle pressure of 80 Pa for each building location. As shown in Table 2, standard beam size was 3 metres, but a reduction in increments of 300 mm down to no smaller than 1.8 metres was necessary if the beams could not fit within certain room sizes.

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ACCEPTED MANUSCRIPT Table 2 Active chilled beam sizing and supply air rates for Melbourne and Brisbane Total length per floor (m)

Base Case

External Venetian

Base Case

2.1 3 -

21 112 161

14 49 161

6.3 48 62.4

4.2 21 62.4

25.2 27.0 -

2.1 3 -

28 119 182

14 63 182

8.4 51 70.8

6.3 27 70.8

30.0 27.0 -

External Venetian

22.0 25.0 -

Hrs cooling set point not met Base Case

External Venetian

147 123 -

85 55 -

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Melbourne East Perimeter North Perimeter Other conditioned zones Brisbane East Perimeter North Perimeter Other conditioned zones

Supply Air Flow Rate (l/s/beam) Base External Case Venetian

No. of beams for 7 floors

Beam length (m)

25.2 27.0 -

303 257 -

124 202 -

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When multiplied over the seven office floors conditioned with chilled beams, the difference in the number of beams required for the base case in comparison to the external venetian scenario was significant. The size of the two chillers to supply chilled water to the beams was reduced from 650 kW for the Melbourne base case to 550 kW with external venetians. In Brisbane, the size of the two chillers was reduced from 650 kW to 600 kW. Boiler sizes for each scenario in Melbourne and Brisbane were found to be 450 kW and 400 kW, respectively. Heat gains transferred to the interior remained much lower for the external venetian scenario than those of the base case throughout the year. Throughout the months of January, March, July and September, there was found to be only 6% and 4% variation in average daily heat gain through the northern glazing for the external venetian scenario in Melbourne and Brisbane, respectively. The reduction in average daily heat gain through the northern glazing for the external venetian scenario relative to the base case is shown in Table 3. Table 3 Reduction in average daily heat gain through glazing for external venetian scenario compared to the base case, by month for non-cloudy and cloudy days

Brisbane

Weather condition Non-cloudy Cloudy Non-cloudy Cloudy

Jan 17.0% 22.0% 20.1% 20.2%

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Mar 57.6% 58.9% 35.3% 28.1%

July 60.2% 60.9% 62.5% 62.4%

Sept 49.7% 49.8% 43.5% 44.8%

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Comparison of heat loads during the year long simulation shows that the period during which the reduction in loads was the greatest for the external venetians for the main north facing glazing was during the shoulder seasons. Table 4 shows the difference between monthly heat gain through northern windows calculated for noncloudy days and monthly heat gain through northern windows on cloudy days. Where there is greatest disparity between the increase of heat gains on non-cloudy relative to those on cloudy days, for the external venetians and the base case, the effect of the monthly slat tilt control mode is most evident. For example, in March in Melbourne the control mode designates that the average tilt of the upper section of the blind is upward from horizontal during the morning and horizontal during the afternoon while the bottom section of the blind does not exceed an average tilt of 25 degrees downward from horizontal throughout. The larger heat gain through the glazing on non-cloudy days relative to cloudy days for the external venetian during March in Melbourne, shown in Table 4, demonstrates that in comparison to the base case, the control mode is calculated to preference daylight available during non-cloudy periods while still maintaining significant heat gain reductions through the glazing for both conditions, as shown in Table 3. During September in Melbourne and March in Brisbane, the greatest relative differences occur between heat gain during cloudy and non-cloudy days for the base case. Due to the orientation of the case study building towards the northwest, the greatest potential for energy savings for the external venetian control mode is when the venetian is able to absorb the lowest angle solar radiation which occurs in the afternoon. In March in Brisbane, these potential gains coincide with the afternoon cloud build up which is described as characteristic of the Brisbane sky climate.

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ACCEPTED MANUSCRIPT Table 4 Increases in average daily heat gain through glazing by month for non-cloudy days relative to cloudy days Base Case

External Venetian Cloudy days Increase in heat gain on

increase in heat

non-cloudy days

increase in heat

non-cloudy days

gain on non-cloudy

relative to heat gain on

gain on non-cloudy

relative to heat gain on

days relative to heat

cloudy days as a % of

days relative to

cloudy days as a % of

gain on cloudy days

heat gain on cloudy

heat gain on cloudy

heat gain on cloudy

(W)

days

days (W)

Jan

149

7.9%

219

Mar

401

16.0%

268

July

178

7.8%

86

Sept

715

34.9%

363

Jan

293

16.6%

235

Mar

587

26.3%

221

during the month as derived from weather file data

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Average daily

days

14.9%

42%

20.9%

39%

9.7%

48%

35.3%

37%

16.6%

35%

13.8%

32%

July

284

7.4%

102

7.1%

13%

Sept

-267

-7.4%

-104

-5.2%

23%

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Brisbane

Increase in heat gain on

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Melbourne

Average daily

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Based on the solar radiation data within the Brisbane weather file, during the pre-monsoon season between November and March a large number of daylight hours were recorded as being within the partly cloudy Perez clearness index range of 1.9 to 2.8. During March, reductions in heat gains through the glazing were observed in the average monthly simulation results during the afternoon, particularly for the base case on cloudy days. This heat gain reduction, likely to be as a result of early morning or late afternoon convective weather patterns over land which is not uncommon in tropical regions, may have translated to a reduced difference in heat gain for the external venetian control mode during cloudy days. This would explain the heat gain on cloudy days being only 12.1% lower than non-cloudy days for Brisbane in comparison to 17.3% for Melbourne during the same month of March. Only a week’s worth of cloudy days or less were identified in Brisbane’s weather data during July and September thereby limiting the significance of a comparison of non-cloudy days with cloudy days for these months. Artificial lighting dominated the annual operational energy demand in both climates, even when daylight controls were introduced in perimeter zones for the external venetian scenario. The small increase in energy demand for heating was outweighed by the more significant energy savings in pump, fan and cooling energy. Lower heat loads from lighting and reduced solar gain both contributed to the minimal increase in heating energy. Natural gas consumption increased by 17% and 20% for the external venetian scenario in Melbourne and Brisbane, respectively. Fig. 6 shows the operational primary energy by end use over 25 years. External venetian shading with the scheduled blind controls was found to bring about a reduction in cooling services energy of 42% for Melbourne relative to the base case.

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Artificial lighting Primary heating

54

100

Pumps and fans

59

60

33

46 1.9

40

41

Cooling services

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0.19

TJ

0.23 25

27

2.3 20

20

42

28

23

13

0 External Venetian Melbourne

Base Case Brisbane

External Venetian Brisbane

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Base Case Melbourne

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Fig. 6 Primary operational energy consumption for base case and external venetian shading scenarios over a 25 year period

6.2. Daylight illumination

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In Brisbane, reductions in cooling services energy were limited to 33%. There were limited differences in artificial lighting energy associated with the change in daylight climate between Brisbane and Melbourne although the combination of daylight controls and adjustable blinds contributed to more than 20% of the total operational energy savings for external venetian controlled shading in both locations. In terms of the overall amount of operational energy delivered, relative savings for the external venetian control mode for both Melbourne and Brisbane were comparable.

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Illumination levels from daylight were consistently higher for the external venetian blind control mode compared to the illumination levels at the same point for the static base case. For the external venetians, the percentage increase in total hours during a typical year that was above the 400 lux threshold was 11.9% for Melbourne and 9.5% for Brisbane when calculating the lux level at a point 3.7 metres from the facade. The workplane illuminance levels modeled were considered to allow the possibility of higher vertical illumination at the window plane caused by less occlusion of the blind without causing excessively high luminance contrast within the occupant’s visual field. 6.3. Embodied energy

The embodied energy analysis showed that within the established system boundary of this case study, the greatest variation in embodied energy between the external shading control mode and the static base case for a given location was attributable to the HVAC system, especially the active chilled beams. Fig. 7 shows the breakdown of initial embodied energy by component and illustrates the small difference in embodied energy between the shading components of the two options. When the HVAC and shading were combined for each scenario, the larger size of the base case HVAC system led to a higher total embodied energy than for the external venetian scenario. The copper heat exchanging elements in the chiller and active chilled beams as well as the aluminium in the chilled beams represented a large proportion of the embodied energy. For the base case, the initial embodied energy of the HVAC component ranged from 6 to 9 times greater than the embodied energy of the shading components for Melbourne and Brisbane, respectively.

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10 B_Chilled beams

8

M_Chilled beams

7

B_Chilled beams

6

M_Chilled beams

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The external venetians, including motors, weighed only 51% of the combined weight of the fixed aluminium louvre and internal manually operated venetians of the base case. Whilst this represented a modest net saving in material resources for the external venetian scenario, the heavier louvre and internal venetian base case scenario was found to be only 18% more energy intensive than the exterior venetian scenario. The reason for this is due to the more energy intensive materials and components that are required for the additional items including the tubular motor and cover panel used within the motorized venetians. The significance of the shading device as a proportion of the total embodied energy might have been greater if the curtain wall glazing of the case study building had not been restricted to only two facades, the window to wall ratio had been higher or the area of the equator facing glazing in the case study building had been larger.

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TJ 5 4

B_Chillers M_Chillers

3

B_Ext venetian

M&B_ Chillers

M_Ext venetian B_Int venetian M_Int venetian AHUs

2

Water pumps

1 0 0

5

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Cooling towers

10

15

20

25

30

35

40

B_Louvre M_Louvre M_Boilers B_Boilers

45

50

Replacement cycle (years)

External venetians

Both categories

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Fig. 7 Initial embodied energy and replacement cycle for shading and HVAC components for base case and external venetian shading scenarios in Brisbane (B) and Melbourne (M) Since the average life cycle for HVAC equipment such as a chiller may extend to approximately 23 years, the impact of replacing differently sized equipment is only evident over several decades. For example, the requirement for these high embodied energy HVAC components to be replaced during retrocommissioning means that more than half of the embodied energy requirement is accrued after the 25 year mark for a 50 year life cycle. Based on average replacement cycles for HVAC equipment and shading, the Melbourne and Brisbane base cases were only found to have used up approximately 41-42% of the total embodied energy after the first 25 years of a 50 year life cycle, whereas approximately 43-44% of the total embodied energy was realised during the latter half of the life cycle for the external venetian option. Fig. 8 combines the recurrent and initial embodied energy for both HVAC and shading systems over a 25 year building life cycle. For the case of shading, embodied energy does not recur because, as shown in Fig. 7, venetian and louvred overhang shading are expected to last longer than 25 years.

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ACCEPTED MANUSCRIPT 40 35 30 25

4.7

4.1

3.9 3.4

13.3

12.6

TJ 20

11.3

10.0

10

17.1

17.7

14.4

5 0 External Venetian Melbourne

HVAC initial EE

Base Case Brisbane

HVAC recurrent EE

15.6

External Venetian Brisbane

SC

Base Case Melbourne

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15

Shading initial EE

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Fig. 8 Initial and recurrent embodied energy for the base case and external venetian shading scenarios for Melbourne and Brisbane

6.4. Life cycle energy

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The external venetian control mode for Melbourne appears to be slightly more effective in reducing the total amount of embodied energy than for Brisbane. Allowing for the uncertainties associated with the embodied energy assessment method used, the combined initial and recurrent embodied energy reduction for the exterior venetian control mode in Melbourne was between 16.7 and 19.3% compared to the base case. This is in contrast with Brisbane where there was only a 13 to 15% reduction in the combined initial and recurrent embodied energy demands for the external venetian scenario.

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Over a 25 year period, the case study buildings in both Melbourne and Brisbane had a lower life cycle energy demand when an external venetian with control modes was used in lieu of a static shading base case. The life cycle energy was 14.1% and 15.4% higher in Brisbane than in Melbourne for the base case and external venetian scenario, respectively. This is despite the efficiency of electrical generation being higher for Brisbane and based on the same levels of efficiency being achieved as the building is operated and components are replaced. Fig. 9 shows the total life cycle energy demand for both shading scenarios over a 25 year period, including the energy required for artificial lighting. The greatest uncertainty for the life cycle energy results was 20% for the external venetian scenario in Melbourne. This uncertainty includes an estimate based on Macdonald and Strachan [46] of 11% standard deviation for operational energy calculations, and uncertainty associated with the embodied energy of 50% for the I-O data used and 20% for the process data component.

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150 TJ 100

129.6 111.3

50

21.2

10.0 17.8

Base Case Melbourne

External Venetian Melbourne

0

13.3 22.4 Base Case Brisbane

11.3 19.5

External Venetian Brisbane

SC

12.6

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94.8

81.2

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Heating, pumps, fans, cooling services & artificial lighting over 25 years Recurrent embodied energy over 25 years Initial embodied energy of HVAC and shading

Fig. 9 Life cycle energy for base case and external venetian shading scenarios

7. Conclusions

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This study demonstrates that for the case study building analysed, the combination of external venetians and the adjustment of slat tilt according to sun position and sky condition delivered substantial energy savings when compared to a static overhang and static internal venetian blind base case. In comparison to static shading systems, which are designed for maximum heat gain reductions for a specific day of the year, the ability to make adjustments to slat tilts with the use of external directionally selective shading that compensates for heat transfers occurring as a result of monthly and diurnal fluctuations in solar radiation, was shown to result in energy savings of 24.9% and 24.0% for Melbourne and Brisbane, respectively. The greatest difference in solar heat gain between the external venetian control mode and the static shading base case were observed during September in Melbourne and during March in Brisbane. This difference is likely to be attributable to the geometric limits of directionally selective external venetian blinds and louvred overhangs. For situations in which solar angles of incidence normal to the glazing are less than 45 degrees from the horizontal, louvred overhangs are limited in the area of shade they can provide without the depth of the overhang reducing illumination by diffuse lighting. The situation of an equator facing façade in Brisbane during January is an instance in which external venetians are limited in the potential reductions in solar heat gain in comparison to a fixed overhang. For simplicity, the modelling conducted in this study considers the directionally selective function of the venetian blind, but could be incorporated with control modes for venetian blind opening and closing to allow greater solar radiation exposure to varying depths in perimeter zones. Further research is necessary to determine the effect of a variation in service life of components on life cycle energy demand. For instance, the high frequency with which internal fitouts occur in office buildings as a result of churn could result in internal venetians being replaced at a faster rate than external venetians. Poor durability or irregular maintenance of components may also be found to be significant factors affecting the demand for recurrent embodied energy. The case study adapted for the climates of Melbourne and Brisbane shows the importance of considering the broader implications of shading strategies. The choices available when specifying directionally selective shading have been shown to have a significant effect upon the thermal and visual comfort of occupants. The size and embodied energy of the HVAC system has also been shown to be effected by the type and control of façade shading. The life cycle energy savings found for the external venetian scenario suggests that building practitioners should give greater consideration to the use of automated shading controls in their attempt to

15

ACCEPTED MANUSCRIPT reduce energy demands associated with buildings. Such consideration can result in reduced sizing of HVAC systems within office buildings and overall life cycle energy reductions without compromising occupant comfort.

Acknowledgements

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The authors wish to acknowledge the contribution made to reviewing and improving this work by Dr Eckhart Hertzsch, Dr Eddy Rusly, Adrian Rowe and the anonymous reviewers.

References

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ACCEPTED MANUSCRIPT Directionally selective shading control in maritime sub-tropical and temperate climates: life cycle energy implications for office buildings. Highlights

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We analyse the performance of internal and external blinds for an office building Life cycle energy demand of external directionally selective blinds is determined We include the effect of shading on HVAC sizing and life cycle energy demand Embodied energy accounts for up to 25% of energy demand over 25 years External venetian blinds reduce life cycle energy demand by up to 25% over 25 years

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