Window Thermal Performance Optimization in Governmental Emirati Housing Prototype in Abu Dhabi, UAE

Window Thermal Performance Optimization in Governmental Emirati Housing Prototype in Abu Dhabi, UAE Abuimara, Tareq A 1 ; Tabet Aoul, Kheira A. 1 1 Department of Architectural Engineering, United Arab Emirates University-Al Ain, UAE. Abstract: This paper focuses on the thermal optimization of windows in houses of the Emirati Housing program, launched by the government of Abu Dhabi, UAE. Typical units are implemented to suit planning objectives despite the impact of orientation on the building energy performance. Thus, the objective of this study is to explore window glazing specifications that provide optimum thermal performance for any building orientation. The dominant housing type in a representative Government Housing Program was identified along its construction components and windows characteristics. The simulated energy performance of the existing design indicated variable annual energy consumption per orientation with the west carrying 9.7% more load than the east. Thereafter, thermally efficient glazing types were tested including; double reflective glass, double tinted Low-E and double squared Low-E. The double tinted squared Low-E glass yielded an overall measurable annual energy consumption reduction ranging from about 4 to 8% respectively for the east and west orientations. Keywords: Window Design, Thermal Performance, Optimization, Housing, Hot Climate, UAE Introduction Presently, the United Arab Emirates (UAE) has one of the world s largest energy consumption per capita, with the building sector accounting for 70% of the consumed energy, primarily used for cooling. The building sector in the UAE has experienced a tremendous expansion due to population growth and economic development. Recently, the UAE government launched several housing programs intended for Emirati beneficiaries, while decisively targeting energy efficiency. The Emirati Housing, a representative housing program, aims at providing 13,000 detached houses by 2017. It targets the accommodation of cultural values and environmental adaptation while providing modern, sustainable homes that meet the local sustainability framework (Estidama) requirements. Often enough, in these housing programs for architectural unity the housing units are implemented based on planning objectives with no specific accommodation of the different orientations, despite the impact of orientation on the building energy performance. Therefore, the objective of this study is to explore and identify window glazing specifications that provide optimum thermal performance for all building orientation. Housing and Energy Efficiency Governmental housing was launched in the era that followed the discovery of oil. These housing projects were in the form of large numbers of detached houses with all related services and infrastructures. Houses were typical units with a limited number of typologies, sharing the same construction characteristics. Detached houses as skin loaded building type 1

[1], feature a large envelop that is exposed to solar radiation [2]. Under the extreme hot climate of the UAE, these detached houses are the most energy demanding type of buildings for cooling [2]. The nature of housing projects imposed typical designs with variable orientations in a way that meets planning objectives. However, each unit orientation may result in a variable thermal load and consequently excessive energy consumption for some. Windows are a critical building component and play an important role in determining the building energy efficiency. They can be a weak thermal link between the indoor and outdoor if they are not adequately specified [3]. Hence, designing a window that can mitigate the admission of heat under extreme hot regions such as the UAE is of great relevance. The optimization of the window thermal performance can be achieved through the improvement of several windows aspects such as size, component material and shading of the window. In governmental housing projects, where planning and allocation of houses are set according to planning objectives, the range of window thermal optimization lays mainly in the area of window s components; namely glass and frame. There has been a rapid improvement in construction material since the environmental issues; advanced types of framing and glass in terms of thermal efficiency, light and sound control have been introduced. For glass; tinted and coated glass, insulated glass, and smart glass types were introduced as thermally efficient glazing materials. Vinyl frames, wooden frames, cladded frames and thermal break frames were types of thermally efficient ones. Window material selection should provide the optimum elimination of all forms of heat transfer through the window [4]. This calls for a combination of glass treatments that satisfies the heat elimination requirement. Case Study: Al Falah Community in Abu Dhabi Al Falah community is a project of the Emirati Hosing Program which was launched in Abu Dhabi as part of the plan Abu Dhabi 2030. It is designed to provide community facilities along with alternative housing options for the Emirati citizens. It consists of five residential villages with approximately five thousand residential detached houses coming in nine different designs in terms of size and architectural style [5]. The diversity in design aims at providing multi choices for local families to meet different family sizes and personal needs. Villas are located in large plots of over 1000 m 2 (30m x 35m), and surrounded by 2.5m high boundary walls [6]. Houses at Al Falah have nine different designs varying in terms of size and architectural style. Villas come in 3-bedrooms, 4-bedrooms, and 5-bedrooms typology ranging respectively from 300m 2, to 350 and slightly over 450m 2. Figure 1 presents the various architectural styles that otherwise share the same layout. The units are implemented based on planning objectives with no consideration of orientation implication (Figure 2). Surveying available design and construction materials used revealed that only general sustainability measures were implemented in Al Falah houses. It includes materials with low heat transmittance (low U values), the selection of efficient HVAC equipment and the selection of water efficient fixtures. Windows frames are made of powder 2

coated aluminum profiles while the selected glass is tinted low reflective double glass with 6mm thick glass panes and 12mm air gap [8]. Figure 1: Architectural styles of the 5 bedroom houses in Al Falah community. Source [7] Figure 2: Aerial view of the first phase of Al Falah community. Source: [9] Experiemental Investigation The five bedroom villa was selected for the window optimization study as it accounts for about 50% of the total number of villas (Figures 3 & 4). An evaluation of ratios of glazing for each style per façade was carried out. The modern style emerged as the one that has the highest window wall ratio (WWR). Table 1 presents the detailed characteristics of the house. Figure 3: Typical ground gloor plan for the five bedroom Villa Figure 4: Base case model; Al Falah typical five bedroom villa (modern style) Thermal Performance of the Existing Design The impact of orientation on the thermal performance of the existing design has been tested using Home Energy Efficient Design (HEED 4.0 build 34) software developed at the 3

University of California in Los Angles. HEED is purposely dedicated energy evaluation tool for houses thermal performance and its output includes among other data the annual electrical energy consumption, cooling loads and lighting loads. Table 1: The Base Case; Specifications of Existing Building Category Item Description Site Location Abu Dhabi 24.42 0 N, 54.65 0 E Total Area (GF+FF) 402.36 m 2 (4331 ft 2) Area Envelop Construction Material Floor Area (Footprint) 212.56 m 2 (2288 ft 2 ) Overall Dimensions of the floor plan Windows Walls Roof Reflectances 17* 15.85 m (56 * 52 ft). Double tinted low reflective glass with air gap Insulated concrete panels. (20cm thick concrete panel with 6cm polystyrene insulation) R=11; Calculated using Opaque (Version 2) software based on the existing construction detail. Wall/ ceiling: Reflectance 70%. Colors range between white and cream (construction documents: ALDAR,2009) Hollow core concrete slab with water proofing and heat insulation layers R=18; Calculated using Opaque (Version 2) software based on the existing construction detail. Front Façade 26.55% Ratio of Glazing per Façade Air Conditioning System Indoor Temperature Right Side Façade 13.4% Left Side Façade 9.03% Rear Façade 16.2% Package Unit Seasonal Energy Efficiency Rate (SEER) = 13 Lowest indoor comfort degree= 21.1 C (70F) Highest indoor comfort degree= 23.88C (75F) According to California Residential Code The annual electrical energy consumption, cooling loads and lighting loads for each cardinal orientation of the existing design are presented in table 2 and figure 5. The base case testing has revealed a similar energy consumption rates when the model is oriented either to the north or to the south. The west facing model had the highest energy consumption rate. While, the east oriented model had the lowest consumption rate. The recorded difference between the highest and the lowest consumption rates is 8.85%. This reduction in annual electrical energy consumption refers to the reduction in cooling loads as it is reduced by 10.65% when rotating building form west to face east. 4

This reduction in cooling loads with the main façade orientated east can be easily explained because the east elevation receives the least amount of solar radiation. This radiation strikes the east during the morning hours, when temperature is still low. During the day, the east façade heat gain happens through heat transmission from the hot ambient air. The transmitted heat was mitigated because of insulated wall panels and insulated glass panels used for windows (double glazing). On the other hand, when the building is facing west, it starts to receive direct solar radiation during the afternoon until evening when the temperature is at its highest in addition to the transmitted heat from ambient hot air. Table 2: The base case annual energy consumption, cooling loads and lighting loads per orientation Power Usage (Kwh) Orientation North South East West Total Annual Electrical Energy 46,087.18 46,048.50 43,200.68 47,393.95 Cooling Loads 29,455.98 29,434.24 27,163.91 30,400.48 Lighting Loads 1,677.42 1,671.27 1,868.10 1,714.18 The first observation confirms the impact of orientation on heat gains and consequently on energy load, highlighting the impact of window design on the total energy used. The impact of orientation carried an additional 10% annual energy consumption from the west orientation compared to the east in this case. The higher ratio of glazing for the main façade is therefore directly affected by the change of orientation leading to higher heat gains. 50.000,00 48.000,00 46.000,00 44.000,00 42.000,00 Base case; Total Electrical Consumption Kwh 40.000,00 North South East West Figure 5: The base case annual energy consumption, cooling loads and lighting loads per orientation Glass Thermal Optimization The glass optimisation investigation considered a number of energy efficient alternate options including: double reflective glass, double Low-E glass and double squared Low-E of which the thermal characteristics are listed in Table 3. The overall annual energy consumption, cooling and lighting load were evaluated for each type of glass in the four cardinal orientations. The total annual electrical consumption indicates a differential result for the thermally efficient glass considered per orientation. The results reveals that the highest rate of savings 5

where obtained when using double tinted squared glass with all orientations (Table 4 & Figure 6). Table 3: Thermally Improved Glass Alternatives Glass Type U Value SHGC Tvis Tinted double pane glass (Existing Design) 0.81 0.45 0.57 Tinted double pane low E glass 0.69 0.39 0.53 Tinted double pane low E squared glass 0.67 0.25 0.38 Tinted double pane reflective glass 0.81 0.16 0.09 The savings ranged between 4% when facing east and about 8% for the west facing model. On the other hand the least savings were obtained when using double reflective glass with even an adverse impact in the east orientation where it generated about 1% increase when facing east in the total energy consumption due to the low visible transmittance of reflective glass that created significant increase in lighting loads. Table 4: Annual Energy Consumption for the Thermally Improved Glass Alternatives. Variable North South East West Base case Total (kwh) 46,087.18 46,048.50 43,200.68 47,393.95 Tinted, Double, Low E Glass Tinted, Double, Squared Low E Glass Double Reflective Glass. Total (kwh) 44,626.92 44,580.91 42,225.15 45,709.24 Reduction % 3.17% 3.19% 2.26% 3.55% Total (kwh) 43,072.78 43,045.74 41,846.74 43,663.94 Reduction % 6.54% 6.52% 3.13% 7.87% Total (kwh) 44,665.09 44,616.05 43,573.18 44,869.22 Reduction % 3.10% 3.11% -0.86% 5.33% 48.000,00 46.000,00 Base case (kwh) 44.000,00 42.000,00 40.000,00 38.000,00 North South East West Tinted, Double, Low E Glass (kwh) Tinted, Double, Squared Low E Glass (kwh) Double Reflective glass. (kwh) Figure 6: Annual Energy Consumption for Alternatives with Thermally Optimized Glass 6

The results in table 4 and figure 6 indicate an optimum thermal performance of windows with the highest savings. These savings were achieved because of the properties of double tinted squared Low-E glass combination. The multiple low emittances coating that minimize the radiative heat and the double panes that resist the transmitted heat. Of more relevance to the objective of this study i.e. identify a glass that mitigates the impact of heat gain associated with orientation; it is interesting to note that the double tinted squared Low-E glass has achieved a similar thermal behaviour for all orientation (Table 4), which in turn enables planning flexibility along with measurable energy savings. Conclusion This research has investigated the thermal optimization of windows glass in relation to orientation in a representative governmental housing project. This target enables flexibility in housing planning and at the same time offers energy savings. The thermal performance of the dominant house type (five bedrooms with total built up area of 402 m 2 ) has been initially tested in relation to the cardinal orientation. The west orientation consumed 9.7% more total annual energy than the east orientation. Subsequently, more efficient glass alternatives were identified, and tested including: double reflective glass, double Low-E glass and double squared Low-E. The Double Tinted Low E glass provided the optimum performance with saving ranging from 4 to 8% of the total annual energy consumption was achievable along with similar consumption rates for all orientations. Hence, some advanced types of glass can provide designers and planners with the required planning flexibility and the house owner with the additional energy savings. References [1] Lyons, P, (2001), The Energy Impact of Windows in Building Design, Environment Design Guide, Note PRO3, PEDP. [2] Clair, P., (2009), Low-Energy Design in the United Arab Emirates Building Design Principles, PEDP. [3] Smith, P., (2005). Architecture in a Climate of Change. 2nd Ed. Architectural Press. [4] Cardomy, J, Haglund, K., (2012) Measure Guideline: Energy-Efficient Window Performance and Selection University of Minnesota, U.S. Department of Energy. [5] G.H.M, April. (2009), Detailed Planning Review Application, Villages_2, 3, 4, 5, Part_1. [6] G.H.M, Nov.(2008), Detailed Planning Review Application, Village one, Part_1. [7] UPC (2011). Emirati Family Housing Program. Retrieved from: http://www.upc.gov.ae/media/100294/upc%20emirati%20housing%20brochure%20final1. pdf. [Accessed 20 May 2014]. [8] ALDAR, (2009), Architectural Drawings Issued for Construction, Abu Dhabi: UAE. [9] RIAS. (2013). Al Falah New Town Masterplan, Abu Dhabi: RIAS. Retrieved from: http://www.rias.org.uk/directory/practices/hypostyle-architects/al-falah-new-townmasterplan-abu-dhabi/. [Accessed 20 May 2014] 7