1. Introduction
The radical increase in construction of highly glazed facades for commercial buildings in the UAE over the last years has caused an extraordinary increase in energy consumption and emissions. Highly glazed facades are increasingly constructed to meet the requirements of an international city like Dubai. Since the aesthetic quality of the buildings enhances the context of a city and adds to its culture, this causes an increase in the use of glass due to its pleasing appearance; however, this has resulted in buildings being responsible for a large amount of devastation. High amounts glazing will conduct more solar heat in comparison to an insulated wall which can transmit 30% to 70% of solar radiation to the interior [
1]. These massive highly glazed building in the UAE increase the amount of solar gains, and the penetration of heat indoors dramatically adds to the cooling loads during summer. The annual average temperatures are predicated to rise between 1.6 °C and 2.9 °C by the year 2050 compared to the years 1961–1990, while further increases in temperature are expected between 2.3 °C and 5.9 °C by 2100 [
2]. Moreover, the electricity consumption for lighting and cooling has increased drastically over the past 24 years, from 5 to 50 billion kWh according to Aboulnaga [
3].
In the hot climate of the UAE, the façade materials utilised are the buildings’ key protection layer for occupants from the harsh external climate. Therefore, its essential to control the solar gain transmittance indoors, as the enormous use of glass affects the visual and thermal comfort of occupants, since excessive sunlight penetration has a negative effect on the individual’s health, causing fatigue, seasonal affective disorder, and insomnia [
3]. In addition, glazing was misused in 70% of the buildings, causing the daylight factor in commercial buildings to be tremendously beyond the recommended levels [
3]. These buildings increase the cooling loads significantly, resulting in glare and visual discomfort, thus the combination of daylighting with high performance artificial lighting can achieve 30%–50% energy savings [
3].
Commercial buildings require more energy to maintain thermal comfort, and to this day, highly glazed buildings are still being constructed in the UAE regardless of their environmental impacts. Therefore, this can be offset by implementing shading techniques to provide a sustainable urban design, reduce the heat island effect, and minimise energy consumption [
4]. As shadings has a crucial role in reducing the energy consumed for cooling and artificial lighting by declining the transmittance of solar gains, shading can also enhance the daylighting to improve visual comfort, cutting down the requirement for artificial lighting. Since commercial buildings have higher consumption due to the high requirements for the cooling to compensate the continuous external and internal heat gains, shading can be integrated in the buildings design to balance the aesthetic appearance of the building with the context of the city, while controlling the solar gains and radiation to reduce the energy required to maintain the thermal and visual comfort and enhance natural ventilation.
The aim of this paper is to investigate and compare the effects of different external and internal shading techniques, to shed light on the role of shading for the enhanced sustainable design of high-rise glazed buildings. This study first reviews the literature concerning the effect of shading devices on energy efficiency, daylighting, and natural ventilation. Then, it presents the methodology of the building and shading design for external overhangs, horizontal and vertical louvers, and egg crate shading, in addition to internal venetian blinds, roller blinds, vertical internal louvers, and light shelves. This study uses Integrated Environmental Simulation Virtual Environment (IES-VE) for the energy modelling and daylighting, and Computational Fluid Dynamics (CFD) for wind flow simulation. The results were presented for all the shadings examined and further analysis of the results are included to discover the ideal shading. In addition to discussing these results against the existing literature and benchmarks, conclusions are presented with respect to the most suitable shading technique in the UAE.
2. Literature Review
Many studies have highlighted the significance of adopting shading techniques to reduce energy consumption for a more sustainable design. The impacts of various shading devices have been studied individually to find the optimum solution to improve the energy efficiency, thermal comfort, lighting control, and wind flow.
Li, et al. [
4] discussed how buildings are one of the major sources of greenhouse gas (GHG) emissions due to the energy required for building operation. Energy consumption can also be influenced by the urban form, which includes solar access and the heat island effect. Therefore, shading is important in controlling energy consumption and (GHG) emissions, which improves building sustainability.
The energy savings were examined by applying three different types of shading at different orientations for an office building in Malaysia, studied by Lau, et al. [
5]. The study investigated different cooling loads with different thermal performance and configuration. (IES-VE) was used for simulating shadings, such as horizontal, vertical and egg crate shading, for both studies [
5,
6]. The glazing was modelled as single 6 mm with a U-value of 6.38 W/m
2k, different from the actual building of double glazing and low-e coating, to compare the difference in performance. Results showed energy savings of 1%–3.4% and savings increased to 5%–9.9% when shading is applied to all orientations of low-e glazing [
5]. Similarly, a study in Malaysia conducted by Fadzil and Al-Tamimi [
6] examined the reduction of cooling loads and solar gain. A fixed shading width of 600 mm was applied to ventilated and unventilated conditions for comparison. The results showed that a reduction in air temperature leads to increasing number of comfort hours by 66.8% and 67% for horizontal and egg crate shading, respectively. While, the number of comfort hours in unventilated and ventilated conditions was 26% and 4.7% for egg crate shading [
6].
By implementing active strategies, such as increasing the temperature to 24 °C and upgrading the coefficient of performance from 2.88 to 5.3, the total energy savings reach 65% as discussed by Alkhateeba and Abu Hijlehb [
7]. They used (IES-VE) software for evaluating the impact of active and passive measures in a federal building in the UAE, the Ministry of Infrastructure Development (MOID), in Ras Al Khaimah to decrease the electricity demand. Different shading devices were implemented on the southeast and southwest windows, such as the egg crate, vertical fins, and cantilever, concentrating on elevations exposed to solar gain. The results illustrated that egg crate shading had the highest energy savings compared to other shadings. Both passive and active optimal retrofits resulted in reducing the energy from 415.22 MWh to 69.23 MWh.
Freewan [
8] investigated the outcome of using shadings on southwest oriented offices at Jordan university for temperature reductions, users’ interface, and visual comfort. (IES-VE) and Radiance software were used for simulation and real experiments performed to study the effect of vertical, diagonal fins and egg crate shadings. The vertical shading was 10 cm width with 7.5 cm gap, diagonal fins of 30 cm, 45° and 17 cm gap, while an egg crate of 10 cm in width with 15 cm gap between vertical fins and 7.5 cm gap between horizontal fins. It was found that the temperature reduction was similar till 13:00; however, diagonal shading showed better performance after that time. Egg crate and vertical fins shading allowed some penetration of light at 15:30 to 16:30 and 13:00 to 18:00, respectively. Diagonal fins performed better when it comes to air temperature and daylight quality, and the egg crate improved the illuminance level and interaction of occupants, while vertical fins allowed more contact with the outdoor environment.
Kim, et al. [
9] studied the configuration of an external shading device that was applied to an apartment in South Korea. (IES-VE) was used for a sequence of simulations to compare the energy savings in terms of external shadings and daylighting. Four cases were studied, namely overhang, blind system, light shelves, and experimental shading. For cooling loads, a short overhang of 0.63 m reduced the loads by 1.1 MWh, while longer overhangs of 1.53 m reduced it by 18% (1.9 MWh). While, blinds with between −10° and −80° slat angle had a reduction that ranged between 5.12 and 5.47 MWh, whereas long light shelves of 1.53 m had cooling reduction of 34% with slat angle of 0°, while 0.5 m shelves had reduction of 26%. This shows that the length of the shadings effects the energy consumption.
Peng and Ying [
10] discussed the effectiveness of internal shadings devices using Grey relational analysis (GRA) and EnergyPlus. To compare the difference in performance with internal shading, external shading and without shading. The experiment took place in Shanghai, China, in two similar rooms, with and without internal shading, where cooling loads were inspected. Energy saving by external shading was 64.1% compared to internal shading of 56.2%. However, using EnergyPlus for further simulation, it showed that solar transmittance, distance between the window and shading, and solar reflectivity are main aspects that can change the performance of internal shadings. While utilising GRA it was found that visible reflectivity, solar reflectivity, thickness, and infrared hemispherical emissivity are the most important factors to improve internal shading performance.
A comparison between the impacts of different internal shadings on the lighting intensity in office rooms was investigated by Khalid and Othman [
11]. Two internal shadings, roller and venetian blinds were experimented by observation and measurement with lux meters in Malaysia. The lux meters were placed at different distances, 1 m, 2 m, 3 m, and 4 m from both closed blinds. Results exemplified that roller blinds had the highest illuminance at noon and the average illuminance level for all distances were 261 lux. The illuminance levels reduced by 37%, 81%, 90% and 96% at each distance measured, respectively. While, the average was 295 lux for venetian blind, the reductions of illuminance level were 30%, 79%, 88% and 94% at distance 1 m, 2 m, 3 m, and 4 m, respectively. This showed that venetian blinds performed better in keeping the illuminance at an ideal level.
Two studies were conducted in Qatar by Ouahrani and Touma using mock up and EnergyPlus [
12,
13] to inspect the effects of shading and daylighting control on energy savings for external venetian and brise-soleil [
12]. The temperatures were set to 24 °C with no internal load and Styrofoam insulation of 0.3 W/m
2k. The results illustrated that brise-soleil with inclination 45° and vertical shading had 7.7% and 18.6% of energy savings to south and north orientations, respectively. Meanwhile, the inclination of 90° increased energy savings to 9.1% and 20.6% when blinds were applied, eliminating glare to the south orientation. Energy saving reached 26.1% with shading and lighting control [
12]. On the other hand, the energy savings and reduction in carbon emissions were examined by the application of shadings and light control on all orientations [
13]. The results showed energy savings of 23.8% and 23.4 kg of CO
2 in the south orientation, which further reduced with lighting control to 28.2% and 27.7 kg of CO
2. Similar savings in the north orientation, where shading saved 11.6% of energy and 8.7 kg of CO
2, however, after combining it with light control, savings increased to 14.1% and 10.5 kg of energy and CO
2, respectively. The east and west orientations had savings of 28.3% and 27.9 kg of energy and CO
2 when shading is combined with lighting control [
13].
Lee and Alshayeb [
14] examined the optimal control of shading in a naturally ventilated double skin façade building while maintaining the natural ventilation efficiency with energy saving in University of Kansas, USA. The CFD numerical method was utilised to simulate the air flow. Horizontal and vertical shadings were tested at 0°, 30°, 60°, and 90°. The results illustrated that at 90° degrees the heat transfer into the building is minimised. While, the air velocity at 0° shades was higher than that at 90°. For energy consumption, the horizontal shading at 30°, 60° and 90° had energy savings of 0.4%, 2.6% and 6.4%, respectively. The cooling consumption increased, and heating consumptions dropped at higher angle for both shadings. This showed that horizontal shadings at 90° resulted in the lowest energy consumptions.
Testing the practicality of implementing passive cooling strategies to improve the thermal performance and cut down the consumptions of a residential building in Dubai, UAE, was the purpose of this study conducted by Taleb [
15]. Using (IES-VE) and Design Builder software to measure the performance where eight different passive cooling approaches were implemented with shading. Both the software results were compared with the actual measurements. Horizontal louver shadings were placed in the south east orientation with 45° to block solar gains. The results displayed that good shading techniques with double glazing, natural ventilation, in addition to the green roof, resulted in reduction of energy by 23.6% and cooling loads by 9%.
Hammad and Abu-Hijleh [
16] investigated the influence of dynamic louvers on energy for an office located in Abu Dhabi. Utilising (IES-VE) to estimate the energy performance by implementing louvers to south, east and west facades. The results illustrated that the savings was 34.02%, 28.57% and 30.31%, in south, east, and west orientations, respectively. Comprehensive analysis demonstrated that the optimal façade static angle was −20°. While the light dimming strategy only saved 24.4%, 24.45%, and 25.19% for south, east, and west orientations, applying fixed louvers at 20° resulted in savings of 31.28%, 26.08% and 25.97% to each orientation, and when glazing with a high shading coefficient was applied, it resulted in savings close to dynamic louvers. This showed that for the east and west façades, the light dimming strategy would be adequate.
Horizontal shading showed a reduction in velocity by 0.3 m/s when implemented on side hung sash window (SHW) with 84% opening and 0.28 m/s when applied to horizontal sliding window (HSW) with 54% opening as investigated by Kannan et al. [
17]. Using the CFD numerical method to determine the wind velocities, 30% was the opening of the inlet and outlet windows from the total floor area and 7 m/s was assigned as the outdoor air velocity. This study showed that horizontal shading does influence the air flow when implemented on different window types.
Aerodynamic implication related with shading and windows should be considered when weighing different design decisions as windows and shading techniques can either minimize or enhance the flow of air indoors according to Hildebrand and Wankaeo [
18]. A classroom model of dimensions 4.25 × 4.5 × 2.9 m was created to examine the air flow of two types of shade (perforated panel and exterior louvers) in a warm humid climate. The shades had 53% porosity of 100 × 13 mm tilted downwards at 22.5° and spaced at 95 and 190 mm on centre. The results showed that perforated panel had higher velocity ratio at 90° while the louvers had higher velocity ratio at 0° and 45° when tested individually. When combined with double hung and awning windows, it resulted in reduction of 15.8% and 33.2% in velocity ratio for plan view. These results demonstrated that each shading type will result in some change to the velocity of air flow indoors.
There is different literature that has successfully performed energy modelling for some external and internal shadings but failed to extensively provide a data comparison between internal and external shades in terms of energy savings and daylighting. Moreover, there was lack of comprehensive studies that adapted these shading techniques when allocated to high rise commercial buildings in a hot climate. On the other hand, very few studies have investigated the effect of external shading techniques on natural ventilation, as most of the literature only emphasised different window types and positions, but not shading devices. The point of differentiation for this study is the examination of different shading techniques, to compare the energy savings, reduction in cooling demand, and daylighting improvements for external and internal shades, in addition to investigating the influence of different external shades on natural ventilation.
5. Analysis of Results
The results demonstrate that all the external and internal shading does influence the energy and cooling loads when compared to base model with no shadings. The horizontal louvers and overhang shades showed a reduction in energy by 6.3% and 6%, and in cooling by 13.4% and 12.6%, respectively. Vertical louvers show the least savings among external shades, reducing the consumptions by 5.5% and 11.7%. Thus, it can be known that the vertical shade can be effective during the lower sun angle time but does not affect the solar gains and radiation falling directly on the façade in an extreme climate like in the UAE. However, it was noted that if the vertical shades were titled to an angle it may have resulted in higher savings as a larger area of the glazing will be covered.
While the egg crate showed the highest reduction in energy as it dropped down to 405.8 kWh/m
2 from 445.5 kWh/m
2, (
Figure 14) which lies under the good practice value mentioned in ECON guide 19 [
24]. This demonstrated that the building is designed to follow the guidelines requirements and the implementation of egg crate shading have further improved the savings in energy consumed. Additionally, this resulted in savings of 9% and 16% of energy consumption and cooling loads, moving towards a more sustainable building design. As it decreases the solar gains penetrating indoors from the south orientation, where the horizontal shading will have more influence on the solar gains and radiation. Furthermore, for the east and west orientations when the sun is at a lower angle, vertical shades would be more effective. This would have higher energy savings in cooler climate countries. Hence, the combination of vertical and horizontal louvers will be the most effective shading type in the UAE for cutting off additional energy cooling demand. Subsequently, this will result in lower cooling loads and energy consumed to maintain the thermal comfort to occupants within the building and increase sustainability by avoiding high carbon emissions.
As shading devices applied to buildings influences other factor apart from energy consumption and cooling loads, most importantly the building occupants. Since the windows are the major source of daylighting to building users. Hence, it is critical to take daylighting into consideration when designing shading techniques; therefore, daylighting was considered in this study. All external and internal shades mention above where examined in terms of daylight factor and illuminance levels. It showed that the internal shades had more influence on daylighting as it can totally block the sunlight if the shadings are fully closed. The internal shades such as roller blind, vertical internal louvers, and venetian blind dropped down to 2%, 2.95%, and 3.7%. On the other hand, the external shades allowed higher daylighting, where the egg crate has the most influence on the daylighting levels between the external shading investigated as the daylight factor and lighting levels reduced to average of 4.08% and 309.87 lux,
Figure 14. Egg crate shades will only allow diffused lighting indoors that avoids glare. While horizontal louver shading showed higher value of daylight factor and illuminance level of 6.46% and 483 lux. However, according to CIBSE guide A, if the daylight factor is closer to 5% on the horizontal plane, the interior will be daylit even during cloudy days. In addition, CISBE guide A, Table 1.5 recommends a lighting level of 300 lux in offices to avoid glare within the indoor space. Hence, the egg crate would be the most appropriate shading technique to provide the most adequate daylight levels to building users.
Furthermore, as the shading will also influence the air flow through the space in the case of natural ventilation during the winter time, so it was added to this study to ensure appropriate shading type for the UAE’s climate. Since the air velocity within a space effects the convective heat exchange, which effect the thermal comfort of occupancy, therefore the higher velocity can be utilised to offset the warmth sensation that will be caused by high temperatures in the summer. In addition, since the internal shades will be functionable only with the windows are closed, so the benchmark model and the four external shades were only simulated on CFD to determine the effect of the shadings on the air velocity.
The results revealed that the egg crate shade has the highest influence on the wind velocity since it blocks most of the windows vertically and horizontally reducing the average velocity from 3.5 m/s to 1.56 m/s (
Figure 14). However, as noticed in the UAE many commercial buildings do not have openable windows, so no natural ventilation and buildings relay fully on mechanical ventilation. In addition, in the extreme climate of the UAE, building occupants prefer to keep the air conditioning on and windows closed for most of the year. Furthermore, during the winter time, the external air velocity may be higher than 3.5 m/s, which will cause higher average velocities within the building space. Therefore, to avoid any discomfort due to wind velocity during the winter, egg crate shading can break down the high air velocities, hence it would be the optimum shading technique in terms of air speed within the indoor space of high-rise commercial buildings.
Based on all those factors mentioned, the ideal shading for high rise commercial buildings is egg crate since it showed the optimum results in almost all the factors that was considered in this study. Where it demonstrated the lowest energy consumption and cooling loads and enhanced the daylighting by allowing diffused light and avoiding glare within the indoor space. In addition to not fully blocking the wind flow through the windows in case it was kept open in the winter time. Therefore, egg crate shading is the optimum shade to be utilised in a hot climate like the UAE.
6. Model Comparison with Literature
The office building was modelled using IES-VE and CFD numerical method to replicate the baseline design of typical commercial buildings in the UAE, as an example of a region with hot climate conditions. The validation of the model was performed by comparing it against pervious literature and industry benchmarks.
Dynamic simulation results of the benchmark model showed that the lowest annual energy consumption to be 405.8 khW/m
2 for egg crate model followed by horizontal louvers. This could be validated by both the studies conducted by Fadzil and Al-Tamimi [
6] and Alkhateeba and Abu-Hijlehb [
7] which showed that the egg crate shading presented the ideal energy savings followed by horizontal shades, similar to this study. Moreover, the energy saving of the shadings simulated in this study ranged from 5.5% to 9% for the external shades, it was confirmed by the study conducted by Lau et al. [
5] were the savings ranged between 5% to 9.9%. On the other hand, among internal shades, the light shelves illustrated the highest saving in energy which could be validated with the study undertaken by Kim et al. [
9], showing that overall energy saving for longer depth light shelves were the highest against overhang and venetian blinds. Overall, the external shades demonstrated higher savings in energy compared to internal shades as validated by Peng and Ying [
10].
In term of daylight factor and illuminance level, again egg crate illustrated the optimum illuminance level of 309 lux, which was validated against the CIBSE guide A unit of 300 lux for office buildings. In addition, it could be confirmed from an experiment conducted by Freewan [
8], showing that egg crate had better improvement for illuminance level and interaction of occupants. On the other hand, when comparing different internal shades such as roller blind and venetian blind, venetian blind showed better daylight factor of 3.7% closer to 5% as validated against CIBSE guide A. Moreover, it was confirmed from the experiment conducted by Othman and Khalid [
11], showing that the venetian blind shading performs better in maintaining the ideal illuminance level compared to roller blinds.
On the other hand, the egg crate helped in cutting down the wind flow of the office by 55.4% to become 1.56 m/s from inlet velocity of 3.5 m/s. This is the lowest wind flow velocity between all shades examine which is closest to the acceptable value in accordance with ASHRAE (American Society of Heating, Refrigerating and Air Conditioning Engineers) for air velocity to be between 0 to 0.8 m/s [
25]. While, ISO (International Organization for Standardization) recommended air velocity to range between 0 to 1 m/s [
26]. In addition, this can be validated against both study [
17,
18] that also showed reduced in the air flow when different shading types were applied with different windows. Since egg crate shading demonstrated the closest value to 1 m/s it was the ideal shading technique among the other shadings simulated.