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Energy and Buildings 86 (2015) 104–117

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Optimal design of residential building envelope systems in the Kingdom of Saudi Arabia Alaa Alaidroos, Moncef Krarti ∗ Civil, Environmental, and Architectural Engineering Department, University of Colorado at Boulder, Boulder, CO 80309, United States

a r t i c l e

i n f o

Article history: Received 7 April 2014 Received in revised form 31 August 2014 Accepted 7 September 2014 Available online 16 October 2014 Keywords: Net-zero energy Optimization Residential buildings KSA

a b s t r a c t In this paper, a comprehensive analysis study is presented in order to improve the energy performance of residential buildings in the Kingdom of Saudi Arabia (KSA) through optimizing the building envelope elements. The building envelope energy conservation measures included in the study are wall insulation, roof insulation, window area, window glazing, window shading, and thermal mass. The optimization process was based on life cycle cost and energy savings. Optimum packages of energy efficiency measures for a residential building located in five climate zones in KSA have been determined for subsidized and non-subsidized energy costs. The results showed optimal energy savings of 39.5%, 33.7%, 35%, 32.7% and 22.7% for Riyadh, Jeddah, Dhahran, Tabuk and Abha, respectively, can be obtained for the subsidized energy cost case. For the non-subsidized energy cost case, the optimal energy savings were 47.3%, 41.5%, 43.19%, 41.1% and 26% for Riyadh, Jeddah, Dhahran, Tabuk and Abha, respectively. Moreover, a sensitivity cost analysis indicated that the cost of energy has more influence on optimal energy savings and life cycle costs than the initial costs of the energy efficiency measures. Finally, this study concluded that substantial savings in annual energy costs subsidies can be achieved by the KSA government if it aggressively promotes, through investments and incentives, energy efficiency programs for both existing and new buildings. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Buildings are increasingly requiring high energy demands in the Kingdom of Saudi Arabia (KSA), especially during summer season, due to high air conditioning demands associated to very high outdoor temperatures during the summer even at night throughout most of the Kingdom. In 2010, buildings in KSA consumed about 65% of the total electricity, 47% higher than the world average during 2010 [1]. The annual rate of growth of energy consumption continues to increase mainly due to the increase in population with an annual growth rate of 1.54% [2]. Fig. 1 illustrates the electricity consumption distribution by sector in 2010. As noted in Fig. 1, 52% of the total KSA electricity consumption is attributed to the residential buildings. Moreover, it has been estimated that 2.32 million new residential buildings will be built by 2020, indicating an even more significant increase in electricity demand associated to residential buildings for KSA in the coming years [3]. In addition, more than a quarter of the produced oil is consumed locally in KSA, and large portion of this oil consumption is needed for the electricity

∗ Corresponding author. Tel.: +1 303 492 3389; fax: +1 303 492731. E-mail address: [email protected] (M. Krarti). http://dx.doi.org/10.1016/j.enbuild.2014.09.083 0378-7788/© 2014 Elsevier B.V. All rights reserved.

production [4]. It is well known that Saudi Arabia’s economy depends solely on oil export revenues. Therefore, this continuous growth of local energy consumption would jeopardize the ability of KSA to maintain its reliance on oil export revenues [5]. Some studies have indicated that if energy efficiency measures are considered for new buildings, KSA annual electricity demand from residential buildings can be reduced by 10%, while reducing air conditioning requirements alone can have an equivalent return in investments up to the cost of building 500 MW power plant [6]. Building envelope components such as walls, roof, floor and windows have other functions than just structural or architectural elements. Indeed, building envelope components can be designed to maintain safe and comfortable indoor environment. In particular, building envelope components affect the energy required for thermal comfort within buildings. For instance, heat storage capability of some building envelope components, such as walls, can help in controlling the indoor temperatures without the need of mechanical systems. So there are sustainable approaches to achieve thermal comfort in buildings without utilizing significant amounts of energy especially for cooling or heating. In addition, the efficiency of cooling systems has a significant impact on the total energy consumption of any building as well. The less is the efficiency of the cooling system, the more energy it will

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Fig. 1. Total energy consumption per sector in Saudi Arabia in 2010 [3].

use, thus, resulting in a large contribution of environmental pollution. Al Anzi and Al-Shammeri discussed the potential of energy consumption reduction of Mosques in Kuwait due to increasing the efficiency of the air-conditioning system [7]. The results indicated that energy savings of 17% can be achieved when increasing the energy efficiency ratio (EER) from 8 to 12. Commonly, heat avoidance is the first approach to design buildings that can minimize heat gains especially those associated with direct solar radiation and high outdoor temperatures. The impact of building envelope energy efficiency measures that are investigated in the analysis described in this paper include exterior wall and roof insulation, window shading, window size, glazing type, and thermal mass. Detailed energy analysis is carried to define an optimal design for high performance thermal residential building envelope system in Riyadh and some other locations representatives of different KSA climate zones. First, the existing literature is reviewed to assess previous studies and their findings related to improving energy performance of residential building envelope systems in KSA. Then, the analysis method used in the study presented in this paper is outlined. Finally, selected results are presented and discussed. 2. Literature review A large number of research studies have been reported to assess the benefits of energy conservation measures for residential buildings in Saudi Arabia. In this paper, the review covers in particular the research efforts in investigating optimum insulation, thermal mass, window glazing and window shading for Saudi Arabia’s hot climates. Several studies have specifically investigated the appropriate thickness and location of thermal insulation for KSA buildings. Saleh [8] studied the impact of three different thickness of wall and roof insulation (5, 7.5 and 10 cm) in Riyadh’s climate based on cooling and heating loads. The National Bureau of Standards Load Determination (NBSLD) was used to conduct the energy simulation. This study concluded that thermal insulations with thickness of 5–10 cm located within the outside layer of the exterior wall gave the best energy performance in reducing cooling loads. However, for an air-conditioned space, insulation located in the inside layer of the wall performed better than the one located in the outside layer. Abdelrahman and Ahmad [9] developed a design procedure

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to select the type and thickness of the insulation material. The study involved the investigation of optimum position of polyurethane board and expanded polystyrene board for either clay bricks wall or hollow concrete block wall. The assessment also included a life cycle cost analysis to find the optimum insulation thickness. The findings of this study indicate that walls with clay bricks require less insulation thickness. For example, in Dhahran’s hot and humid climate a wall with hollow concrete block needs an insulation thickness of 5.5 cm, while a wall built with clay bricks requires 5.0 cm of insulation thickness. Moreover, it was recommended that the insulation layer placed on the outside of the exterior wall for locations with high diurnal range of wall surface temperature to prevent the development of thermal stresses. Al-Sanea and Zedan [10] performed a study to optimize the thickness and location of wall insulation under steady periodic conditions in Riyadh. Three wall configurations were examined: walls with one, two and three layers of insulation. The optimization is based on energy consumption and the present worth method. The results showed that the optimum thickness of the insulation depends on its location within the wall. The wall with three layers of insulation achieved the best energy performance; each had a thickness of 2.6 cm placed at the inside, outside and at the middle of the wall. The wall with two insulation layers came next, where each insulation layer had a thickness of 3.9 cm placed at the middle and outside of the wall. The performance of the wall with the three insulation layers was compared to the wall that consists of just one insulation layer with thickness of 7.8 cm placed on the inside. The comparison showed an increase in time lag from 6 to 12 h and a reduction of 20% in peak cooling load. Thermal performance of building roof elements was investigated by Al-Sanea [11]. The study compared the thermal characteristics of six typical roof structures used in building constructions in Saudi Arabia. Energy simulations performed using a validated numerical model based on an implicit finite-volume method. It was found that the contribution of solar radiation is more than twice than that of heat convection and conduction through the building envelope components. The simulation results were compared with an un-insulated roof that consists of heavy weight concrete foam as a leveling layer. The comparison showed a 32% reduction of the daily average heat transfer load when using a 5.0 cm of molded polystyrene insulation, and a 27% reduction for extruded polystyrene. Polyurethane insulation with a thickness of 5.0 cm gave a 22% reduction in the daily average heat transfer load. In addition, the impact of thermal mass on the energy performance of buildings in Saudi Arabia’s hot climate has been investigated extensively as well. Abdelrahman et al. [12] have examined the cost effectiveness and energy consumption of four masonry materials used in building construction in Saudi Arabia. The energy simulation carried out using DOE-2 software. The masonry materials that were investigated in the study include clay bricks, concrete blocks, sandlime bricks, and prefabricated walls. The clay bricks provided the best energy performance in terms of capital investments and running cost of a typical residential building in KSA. Furthermore, it is found that the clay brick consumed less energy by 16% compared to the concrete block, 23% compared to the sandlime bricks, and 25% compared to the prefabricated walls. Al-Sanea et al. [13] have investigated different types of masonry materials and its effect on optimal thermal mass thickness in insulated buildings for a fixed wall nominal thermal resistance. A validated computer model has been used to calculate the transmission loads assuming steady periodic conditions for Riyadh’s climate. The location of the thermal mass was tested (inside and outside relative to the insulation layer), and the thickness of the thermal mass was varied between 0 and 50 cm while fixing the value of the wall resistance. The results of this study showed that walls with solid concrete blocks achieved higher energy savings, and the walls with

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inside thermal mass layers performed better than the walls that have thermal mass located in the outside. In another study, AlSanea et al. [14] have developed the concept of critical thermal mass thickness and thermal mass energy savings potential to be utilized in estimating the thermal mass thickness that can achieve a desired level of energy savings. Simulations are performed using a validated numerical model under steady periodic conditions in Riyadh. The thickness of heavyweight concrete has been varied between 6 and 30 cm. Results showed annual cooling load savings between 17% and 35% based on the thermal mass thickness. The authors recommended placing the insulation on the outside for applications requiring continuous air-conditioning operation throughout the year. Evaluating the performance of window glazing and shading to enhance energy efficiency of buildings had received some attention among the researchers in KSA. One of these studies carried by Aldawoud [15] explored the effectiveness and performance of conventional overhang shading and compared it with the shading performance of the electrochromic glazing for hot and dry climates. Energy model of a typical office building in Dammam has been created using Design-Builder software. The analysis was performed for the south, east and west orientations. Four alternative models are investigated: 1—double glazing windows with no shading, 2—double glazing windows with 1.5 m overhang projection, 3—double glazing windows with overhang and side fines having 1 m projection, 4—electrochromic glazing windows with no shading. The simulation results showed that window shading and glazing properties have a great impact on reducing solar heat gains to the buildings in hot climates. Among the shading alternatives, the electrochromic glazing offered the best energy performance. Nonetheless, optimum size of exterior shading, such as overhangs and side fins, is a key shading strategy to reduce cooling energy especially during summer season. Aldossary et al. [16] analyzed energy consumption patterns in the hot and humid climate of Jeddah. The study consists of six selected family homes with three are typical detached houses and the other three are typical apartment units. Detailed energy modeling is performed using IES-VE software. Annual energy simulation is carried out to investigate some energy conservation measures including shading devices, efficient window glazing and domestic renewable energy sources. The study results indicate that the average energy consumption of the typical residential building is around 185.4 kW h/m2 . The high energy consumption is due to the lack of thermal insulation for walls and roof, in addition to the poor performance of the single glazing windows. According to the simulation results, reduction in energy consumption ranged between 21% and 37% when installing shading devices, upgrading window glazing to double-glazing and utilizing onsite PV system. Al-Homoud [17] investigated an optimal thermal design of air-conditioned residential buildings in different climatic regions with the objective of minimizing annual energy consumption of those buildings. ENEROPT building thermal design optimization tool have been used in this study. Fourteen design variables were optimized including wall and roof insulation, and window glazing. Optimization results suggested a U-value of 0.06 and 0.04 BTU/h F ft2 for the wall and roof, respectively, regardless of climate. However, the optimization tool is based on energy consumption rather than economical cost analysis. For several decades, the Saudi Building Code (SBC) was known as the major source of minimum building standard and construction technical specifications and regulations in KSA to ensure public safety [18]. However, energy efficiency performance of buildings was initially neglected by the SBC since energy consumption was low and no serious threat from peak loads was expected. Nonetheless, due to the very fast economic development and the high population growth, there was a sharp increase in electricity consumption in KSA from both building and industrial sectors. The lack

of energy efficiency standards and regulations made the increase in KSA energy consumption even more significant in the last decade. Therefore, there is an urgent need to redevelop the building energy code to establish minimum requirements for buildings and promote the construction of high-energy performance buildings. The energy conservation requirements section (SBC 601) in the Saudi Building Code has been developed to provide the required standard of energy efficient building components such as the building envelope, mechanical systems, electrical and lighting systems, and domestic water heating systems [18]. The KSA energy conservation code was based on the International Energy Conservation Code (IECC). The design and selection of building components are defined for various climates using the cooling degree-days (base 18 ◦ C) of each location estimated from 1993 to 2003 data obtained from the Meteorology and Environmental Protection Administration in Saudi Arabia [19]. The energy conservation requirement section (SBC 601) covers both residential and commercial buildings with chapters 3–5 covers specifically residential buildings [18]. Detached family dwellings with a window to wall ratio (WWR) of 15% and less as well as townhouses and other residential buildings with WWR less than 25% have to meet the requirements outlined in chapter 5. In the other hand, chapters 3 and 4 are used for detached family dwellings that have WWR more than 15% and townhouses and any other residential buildings that have WWR of 25% or more. Specifically, the U-factor requirements of the exterior wall assemblies are provided in chapter 3 with selection criteria based on degree-days. The lowest U-factor is 0.483 W/m2 K for buildings located in sites with cooling degree-days less than 1400 ◦ C-days, while the highest U-factor is 0.216 W/m2 K and is required for locations with cooling degree-days higher or equal to 7230 ◦ C-days. The fenestration U-factor is also selected according to the location cooling degree-days. The U-factor ranges from 4.2 W/m2 K to 1.42 depending on the cooling degree-days. It should be noted that exterior shading is not required in the standard design specifications. However, it is recommended to add exterior window shading in the proposed design. The solar heat gain coefficient (SHGC) for the fenestration system in the standard design is required to be equal to 0.4 for cooling degree days (CDD) less than 1950 ◦ C-days and 0.68 for CDD more or equal to 1950 ◦ Cdays. It should be noted that the fenestration system consists of both glazing and frame of the windows. On the other hand, chapter 4 of the SBC-601 defines the energy performance compliance approach for residential buildings. For this approach, the U-factor of the exterior walls, roofs and floor slabs are selected based on degree-days. The fenestration solar heat gain coefficient (SHGC) should not exceed 0.4 for locations with CDD less than 1950 ◦ C-days. For a high thermal mass wall with a heat capacity of 1.0 kJ/m2 K or greater, U-factor is selected based on cooling degree-days and the position of the thermal insulation layer. There is also a possibility to select the R-values of the exterior walls, roofs, floors and also the U-factor of the windows based on the window to wall ratio (WWR) for the building. R-values and U-factors are provided in tables listed within the SBC-601 for buildings with window area less or equal to 8%, 12%, 15%, 18%, 20% and 25% for the detached family dwellings.

3. Analysis approach In order to assess the optimal building envelope design for residences through KSA climates, a prototypical residence is defined. The residential building that has been considered in this study is a detached family house (villa). According to a survey study conducted by Opoku and Abdul-Muhmin [20] to determine housing preferences and attributes in major KSA cities, about 40% of the residential building stock is classified as villa type (i.e., detached

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Table 1 Building construction specifications for the base case KSA villa [22]. Number of stories Total height Floor dimensions Gross floor area Gross wall area Window area Type of glass External walls Roof Floor Number of occupants Lighting Appliances

2 7.0 m 15.0 m × 17.5 m 525 455 13.29% of gross wall area Single pane window 20 mm plaster outside + 200 mm concrete hollow block + 20 mm plaster inside 10 mm built-up roofing + 150 mm concrete roof slab + 12.7 mm plaster inside 150 mm slab on grade 6 3.0 kW (lower level), 2.0 kW (upper level) 2.0 kW (lower level), 1.0 kW (upper level)

Fig. 2. 3-D Rendering for the base case building energy model.

single family houses), while apartment buildings represent 35% and duplexes 12%. Therefore, two fifth of the available residential buildings are considered detached family houses in Riyadh. Consequently, improving the energy performance of this type of residential buildings can have a major impact on reducing overall energy consumption of the building sector in KSA. In this study, EnergyPlus was selected to perform wholebuilding energy simulation analysis. The advantage of using EnergyPlus over other energy simulation tools, such as DOE 2.2, is that it uses a heat balance method for heat transfer calculations. This method is known for its accuracy comparing to other methods such as weighting factor approach especially when modeling advanced systems such solar passive systems with high thermal mass, radiant panels, and chilled beams. In addition, EnergyPlus accounts simultaneously and iteratively for all building loads and HVAC systems at each time step rather than sequentially as is the case for DOE2.2 [21]. A baseline energy model for a detached single family home has been developed for assessing the various energy efficiency options to improve the design of villas in KSA. Ahmad [22] presented an energy model for a typical house built with masonry materials in Dhahran, KSA. The characteristics of the energy model for typical villa were based on the findings of a study performed by KACST [23]. The KACST study data were obtained from three sources: (i) review of building plans filed with municipal authorities, (ii) site visits to buildings under construction, (iii) interviews with owners and contractors. The same energy model defined by Ahmad is used in this study as the base-case energy model for a villa in KSA [22]. Moreover, schedules for occupancy, lighting

Table 2 HVAC system specifications for the base case model for a KSA villa [22]. Characteristics

Description

System type

Cons. volume DX air-cooled A/C system with electric heating Two-position with dual (heating and cooling) set point 72 F for heating and 76 F for cooling 55 F (average) Available throughout the year 2.17 None

Thermostat type Thermostat setting Min. supply air temp. Heating and cooling COP Ventilation

and equipments are specified based on previous energy analysis studies of residential buildings in KSA [22]. The building construction characteristics for the base-case energy model for the villa are summarized in Table 1. In addition, the air-conditioning system specifications for the base-case model are shown in Table 2. A rendering for the base-case building energy model is shown in Fig. 2. Each floor was defined as a separate thermal zone. One thermostat was located in the second floor to control the cooling and heating systems for both thermal zones. The monthly energy simulation results obtained for the basecase model for the prototypical villa located in Riyadh are summarized in Fig. 3. As expected, most of the electrical energy consumption is attributed to space cooling. Indeed, as clearly shown in Fig. 4, space cooling represents 66% from the total annual electrical energy used by the villa. There is some space heating during the winter season (mostly in December and January) but it only

Fig. 3. Monthly electrical energy consumption for a villa in Riyadh.

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A. Alaidroos, M. Krarti / Energy and Buildings 86 (2015) 104–117 Table 4 Properties of different types of window glazing [25].

Fig. 4. Energy end-use distribution for base-case model for a villa in Riyadh.

No.

Glazing type

SHGC

U-value (W/m2 K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Single clear Double clear air Double clear argon Double LoE clear air Double LoE clear argon Double LoE TINT air Double LoE TINT argon Double LoE sel clear air Double LoE sel clear argon Double LoE sel TINT air Double LoE sel TINT argon Triple clear air Triple clear argon Triple LoE clear air Triple LoE clear argon

0.86 0.76 0.76 0.60 0.59 0.39 0.37 0.42 0.42 0.30 0.28 0.68 0.68 0.47 0.47

6.31 3.23 2.61 2.47 1.48 2.43 1.46 2.32 1.30 2.32 1.30 2.19 1.64 1.55 0.77

Detailed simulation analysis to take into account the interactive effects between various building energy systems is warranted to determine the optimal design specifications for residential buildings in KSA. 4.1. Building envelope sensitivity analysis

Table 3 Cooling and heating degree-days for the five cities in KSA [24]. City

CDD (◦ C-days)

HDD (◦ C-days)

Jeddah Dhahran Riyadh Tabuk Abha

6587 5953 5688 4359 3132

0 142 291 571 486

accounts for 4% of the total annual energy consumption. The total annual energy consumption of the base-case energy model is 119,700 kW h, which corresponds to an energy use intensity (EUI) of 228 kW h/m2 per year when the villa is located in Riyadh. The same base-case energy model for the villa was simulated in Jeddah, Dhahran, Tabuk and Abha. The simulation results for the five KSA cities are compared in Fig. 5. In particular, the results show that even higher space cooling energy is needed when the villa is located in Jeddah rather than Riyadh with almost no space heating load during the winter, while when located in Abha, the villa had the lowest space cooling energy use and the lowest space heating energy use comparing to Tabuk. The simulation results for space cooling and heating energy end-uses are consistent with the cooling and heating degree-days provided in Table 3. Fig. 6 shows the energy end-use distribution when the villa is located in for KSA locations: Jeddah, Tabuk, Abha, and Dhahran. For all the locations including Riyadh, it is clear that space cooling is the dominant contributor to the total annual villa energy consumption. Indeed, space cooling accounts from 40% (Abha) to 71% (Jeddah) of the total annual electrical energy use for a typical villa in KSA. 4. Discussion of selected results Since all five KSA sites, considered in this analysis, are space cooling dominated climates, it is important to investigate measures to enhance the building envelope performance to reduce cooling thermal loads through thermal insulation, window glazing, solar shading, and other measures. However, identifying the optimum selection of building envelope components requires detailed and comprehensive thermal and economical analyses to insure reaching optimal design specifications. For instance, adding significant thermal insulation to the exterior walls and roof may not be costeffective due to the diminishing return of the added insulation.

The results of the impact of wall or roof insulation R-value on the annual energy use of the baseline villa model show similar pattern for all KSA climates with most of the energy savings are achieved by adding the first R-5 insulation as shown in Fig. 7. After R-5 insulation level, a diminishing return pattern can be observed for all five KSA sites. However, it is clear that more savings are observed for sites with hot climates (i.e., high cooling degree-days) including Riadh and Jeddah. For instance, adding R-5 wall insulation for a villa located in Abha results in 5.5% energy savings, while the same insulation in Riyadh gives almost 12% savings in the total annual energy consumption. The simulation results aslo shows that the villa in Jeddah achieves the lowest energy savings due to the addition roof insulation compared to other KSA cities. This finding is attributed to the fact that Jeddah has the lowest direct solar radiation levels, especially during the summer months, among the other locations resulting in lower outdoor surface roof temperatures and consequently less transmission gains through the roof. As a result, the addition of roof insulation is less effective in Jeddah than in the other KSA locations. Fig. 8 shows selected results of the combined and interactive effects when both wall insulation and roof insulation are added for a villa located in Riyadh and Jeddah. The percent energy savings shown in Fig. 8 are estimated based on the un-insulated baseline model for the villa (i.e., no roof and wall insulation). The results outlined in Fig. 8 indicate that higher energy savings can be obtained when insulation is applied to both walls and roof compared to insulation applied only to either building envelope components. In Jeddah, a maximum energy savings of 25% can be achieved by adding R-15 insulation for both wall and roof, while the same amount of insulation in Riyadh will achieve 31% energy savings. A maximum energy savings of 35% can be achieved in Riyadh from adding R-30 insulation for both wall and roof. Fig. 9 illustrates selected results of the impact of two main window glazing specifications: solar heat gain coefficient (SHGC) and U-value on the energy use savings for the prototypical villa. Fig. 9 also indicates the performance of 14 different glazing types listed in Table 4 on total annual energy use savings compared to the baseline glazing (i.e., single pane clear glazing). For all KSA climates except for Abha, the best glazing to reduce energy use of the villa is Aragon filled double pane tinted selective low-e glazing characterized by low SHGC and U values. Indeed, low SHGC glazing will

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Fig. 5. Total annual energy consumption, space cooling, and space heating for a villa located in five KSA cities.

reduce solar heat gains, which is desirable for KSA hot climates. In addition, since U-value is the measure of transmission heat transfer through the windows (mainly by conduction and convection), low U-value glazing results in reduced cooling thermal loads for the villa. The energy use savings obtained by adding window-shading devices are estimated for the villa located in all five KSA sites as

a function of the overhang projection as shown in Fig. 10. The lowest energy use savings from overhang shading are obtained for the villa located in Dhahran. For instance, a 0.5 m overhang projection results in 3.6% energy savings in Dhahran, while the same overhang leads to 5% energy savings in Riyadh. This result is due to the fact that Dhahran has less solar radiation and thus less solar heat gains through windows than Riyadh. The weather data used in the

Fig. 6. Annual energy end-use distribution of the base-case villa located in five KSA cities.

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Fig. 7. Energy Savings from wall and roof insulation for a villa located in five KSA cities.

Fig. 8. Combined effects of both wall and roof insulation on energy use savings for a villa located in Riyadh and Jeddah.

Fig. 9. Energy use savings as a function of SHGC and U-value of window glazing for a villa located in Riyadh and Jeddah.

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Fig. 10. Effect of window overhang projection on total annual energy use savings for a villa located in five KSA sites.

analysis for Riyadh and Dhahran showed an average global solar radiation of 6109 W/m2 and 4862 W/m2 , respectively. It should be noted that the results of Fig. 10 are obtained for the base-case villa model with a relatively low window to wall ratio of 13%. Thus, the impact of the shading devices is generally not significant in reducing the annual villa energy consumption with a maximum reduction of 6.3% achieved for an overhang projection of 1.0 m in Jeddah. Villas with larger window areas would benefit greatly from shading devices as illustrated in Fig. 11. In particular, Fig. 11 illustrates the impact of both window size (WWR) and overhang shading on annual energy use savings for a villa located in Riyadh and Jeddah. The energy savings of Fig. 11 are estimated relative to a villa configuration with no windows. For all KSA locations, the addition of windows increases the villa energy use independently of the overhang specifications. In general, the results of Fig. 11 show that larger windows are more sensitive to the overhang design. The impact of thermal mass on energy use is investigated as well. Specifically, the thickness of the exterior concrete walls of the villa is varied from 5 cm to 40 cm to assess if the thermal mass has

any significant effect on the total energy consumption of the villa. The results of the analysis are presented in Fig. 12 for the case of a villa with no wall insulation. As the results of Fig. 12 indicate, the site that benefits the most from the addition of wall thermal mass is Abha characterized by its mild climate. Meanwhile, Jeddah with very hot climate has the lowest energy savings using thermal mass. These results are expected since the highest benefit from thermal mass is achieved for climates with a high diurnal temperature range, which means a large temperature difference between daytime and nighttime so heat can be stored and then released during a day cycle. 4.2. Building envelope optimization analysis Optimization analysis is performed to evaluate the costeffectiveness of adding energy efficiency measures to design a villa in various KSA climates using the life cycle cost analysis method. This method accounts for both the initial costs of implementing energy efficiency measures and energy costs associated to the villa over its lifetime [26]. The optimization approach applied in this

Fig. 11. Impact of WWR and the length of overhang on energy savings for a villa located in Riyadh and Jeddah.

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Fig. 12. Impact of exterior wall thermal mass on energy use savings for a villa located in five KSA sites.

analysis uses a sequential search methodology as described by Ihm and Krarti [27]. Genetic algorithm optimization was also used and compared to the sequential search optimization results. In addition, Photovoltaic (PV) solar energy are considered to meet the electrical energy consumption requirements for the optimally designed residential building; thus, the life cycle cost and energy savings attributed to the PV system is included in the optimization analysis. To calculate the life cycle cost (LCC), the initial cost (IC) for each energy measure is needed and the annual energy cost (EC) is obtained based on the simulation results obtained for the villa energy model described previously. In addition, the uniform series present worth factor (USPW) is used to convert all the annual energy costs to the present as shown in the following equation [26]: LCC = IC + USPW × EC

(1)

USPW is a function of both the lifetime (N) of the villa and the discount rate (rd ) of the economy as stated by the following equation [26]: USPW =

1 − (1 + rd )−N rd

(2)

The lifetime span of residential buildings in KSA is assumed to be 30 years and the discount rate is set to 5%. The cost of electricity production in Saudi Arabia is $0.10/kW h, while the sales price of electricity for residential buildings is $0.02/kW h [4]. This difference in energy cost shows a large amount of subsidies ($0.08/kW h) that the KSA government provides to lower the domestic energy prices. In the analysis presented in this chapter, the impact of subsidies on the cost-effectiveness of energy efficiency measures for residential building in KSA is investigated. The cost of materials used in this analysis are estimated from various sources [28] and [29]. A sensitivity analysis for the capital costs related to material costs has been implemented to investigate their influence on the optimization results. First, a brute force optimization analysis has been applied for the subsidized energy cost case to verify the results obtained from other optimization techniques. A brute force optimization analysis consists of a full parametric simulation analysis where all possible combinations of energy efficiency measures are considered. A discretization scheme is used for continuous parameters such as insulation R-value and WWR. The simulation of a brute force optimization is generally time consuming and is usually not practical

Table 5 Optimal energy efficiency measures for building envelope of a villa in Riyadh. Energy measure

Cumulative energy savings (%)

Shading overhang (70 cm) Roof insulation (R-25) Wall insulation (R-15) Glazing (double air)

5.4 23.1 36.5 39.5

LCC ($) 34,390 30,922 26,595 24,173

especially when several efficiency measures have to be evaluated. In this study, the brute force optimization analysis is applied only for the case of the villa located in Riyadh with subsidized energy costs. The results of the brute force optimization analysis as well as the sequential search technique optimization for designing building envelope systems for the prototypical villa located in Riyadh using the subsidized energy rate of $0.02/kW h are presented in Fig. 13. As we can see the optimization results obtained from the sequential search technique for both the optimal path and the absolute optimal solution match exactly the brute force optimization analysis results. Specifically, the optimum cost effective energy efficiency package for the villa in Riyadh can achieve a 39.5% energy savings relative to the base-case villa design configuration with a life cycle cost (LCC) of $24,173, a reduction of 34.2% relative to base-case LCC. The optimal energy efficiency measures selected by the sequential search optimization and the brute force analysis consists of window shading with a 70 cm overhang projection, R-25 roof insulation, R15 wall insulation, and double glazing window as summarized in Table 5. Fig. 14 shows the optimal path found by the sequential search technique as well as the energy savings and LCC values associated with installing PV systems either based the available roof space (defined by the circle in Fig. 14) or to reach net-zero energy design for the villa located in Riyadh (defined by the cross and 100% energy savings in Fig. 14). In the analysis summarized in Fig. 14, the PV solar system cost is assumed to be $4.5/W based on the National Renewable Energy Laboratory (NREL) open PV project [30]. The area of one solar panel is assumed to be 1.9 m2 , and the energy output per month per panel based on 5 sun hours/day is assumed to be 35.3 kW h. The PV system performance has been evaluated using Energy Plus [31]. The optimization results of the other four KSA locations for subsidized electricity cost are summarized in Table 6

A. Alaidroos, M. Krarti / Energy and Buildings 86 (2015) 104–117

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Fig. 13. Brute force and sequential search optimization results for subsidized energy rates and for a villa located in Riyadh.

Fig. 14. Building envelope EEM optimization results for a villa in Riyadh using subsidized KSA energy rates.

. Moreover, the optimal solution obtained by the genetic algorithm optimization technique is shown in Fig. 14 (defined by the red solid symbol) and corresponds well to the optimal solution by both the brute force optimization analysis and sequential search technique. It should be noted that when additional energy efficiency measures are considered such as airtight building envelope, efficient lighting fixtures, and high performance HVAC systems, the optimal package could lead to even higher energy savings. From the optimization results, we can conclude that the energy savings and life cycle costs of the optimum building envelope

design can vary based on the climate zone within KSA. As provided in the results summary of Table 6, higher the energy savings can be achieved for hotter climates. Mild climates such as those in Abha and Tabuk can achieve higher energy savings and lower life cycle cost when adding solar energy to the optimum building envelope package. With the existing optimum solutions that include only envelope energy efficiency measures, achieving netzero-energy (NZE) residential buildings with solar energy is very expensive and in some cases it will be impossible to achieve due to lack of enough area for PV installation. Therefore, additional energy efficiency measures should be evaluated besides building

Table 6 Summary of optimization results for a villa located in five KSA cities using subsidized electricity rates. Location

EE-optimum Energy savings (%)

Riyadh Jeddah Dhahran Tabuk Abha

39.5 33.7 35.0 32.7 22.7

EE and PV—roof LCC ($) 24,173 28,268 26,459 21,283 16,724

Energy savings (%) 79.2 74.6 75.9 83.5 91

EE and PV—NZE LCC ($) 172,700 174,360 174,200 168,400 165,020

Energy savings (%) 100 100 100 100 100

LCC ($) 259,610 289,110 278,000 223,560 185,980

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Fig. 15. Building envelope EEM optimization results for a villa in Riyadh using non-subsidized energy rates.

envelope enhancement. The energy efficiency measures selected for the optimum solutions for each climate zone in KSA are provided in Table 7. In order to assess the impact of electricity rates on the optimal building envelope design for the prototypical villa, non-subsidized energy rates are used to perform the optimization analysis. In particular, the non-subsidized electricity rate of $0.10/kW h is considered for all KSA locations. The results of the sequential search optimization for a villa in Riyadh using electricity rate of $0.10/kW h are presented in Fig. 15. The optimum package of building envelope energy efficiency measures can achieve energy savings of 47.3%, which are 7.8% more than the energy savings for the optimal design determined using subsidized energy rate of $0.02/kW h as shown in Fig. 14. Table 8 provides a summary of the optimization analysis results when the villa is located in any of the five KSA cities using

non-subsidized energy rate. The specific energy efficiency measures selected for optimal villa envelope design are listed in Table 9 for all five KSA cities considered in the analysis when non-subsidized electricity rate is utilized. The optimal design configurations are similar in all five locations except for some variations in the glazing type selected for the windows. 5. Discussion of energy efficiency policies implications As discussed previously, there is a significant energy savings potential that can be obtained from solely upgrading building envelope components of residential buildings in KSA. Both homeowners and the KSA government can benefit when energy efficiency improvements are made to the residential buildings. In this section, the financial benefits to the KSA government are specifically discussed to highlight possible policies that can be adopted to improve

Table 7 Summary of the selected energy efficiency measures for optimum solutions for villas located in five KSA sites using subsidized electricity rates. Location

Wall insulation

Roof insulation

Thermal mass (cm)

Shading (cm)

Glazing

Riyadh Jeddah Dhahran Tabuk Abha

R-15 R-20 R-20 R-15 R-5

R-25 R-20 R-20 R-15 R-10

20 20 20 20 20

70 80 70 50 50

Double glazing Double glazing Double glazing Double glazing Double glazing

Table 8 Summary of optimization results for a villa located in five KSA cities using non-subsidized electricity rate. Location

Energy savings (%) Riyadh Jeddah Dhahran Tabuk Abha

EE and PV—roof

EE-optimum

47.3 41.5 43.2 41.1 26.4

LCC ($) 104,130 122,640 114,170 94,067 75,787

Energy savings (%) 81.6 74.8 77.2 84.8 91.0

EE and PV—NZE LCC ($)

Energy savings (%)

198,960 213,250 206,710 186,690 173,650

100 100 100 100 100

LCC ($) 249,650 288,340 271,760 220,470 187,480

Table 9 List of energy efficiency measures for optimal envelope design of a villa located in five KSA cities using non-subsidized electricity rate. Location

Wall insulation

Roof insulation

Thermal mass (cm)

Shading (cm)

Glazing

Riyadh Jeddah Dhahran Tabuk Abha

R-30 R-30 R-30 R-30 R-30

R-30 R-30 R-30 R-30 R-30

20 20 20 20 20

80 80 80 80 80

Double LoE sel clear air Double LoE TINT air Double LoE sel clear air Double LoE TINT air Double glaze

A. Alaidroos, M. Krarti / Energy and Buildings 86 (2015) 104–117 Table 10 Estimates of the economic benefits for KSA government associated to the improvement in the energy efficiency performance of one house. Description

Value

Government subsidy ($/kW h) Price of electricity production (unsubsidized price of electricity) ($/kW h) Subsidized electricity rate ($/kW h) Energy consumption of a typical un-insulated family house (kW h/year) Annual energy cost for consumer with electricity rate $0.02/kW h ($) = 119,700 × 0.02 Energy cost subsidized by the government per house ($/year) = 119,700 × 0.08 Percent energy savings of the optimum design for the non-subsidized energy cost Energy consumption for the optimum design (kW h/year) = (1 − 0.473) × 119,700 Annual energy cost for consumer with electricity rate $0.1/kW h ($) New subsidized electricity rate ($/kW h) = 2394/63,082 Government subsidies per kW h ($/kW h) = 0.1 − 0.038 Government subsidies per house ($/year) = 63,082 × 0.062 Initial cost of energy measures ($) = LCC − (6308 × USPW) Life cycle cost for 30 years of un-insulated house ($) = 0 + (9576 × USPW) Life cycle cost for 30 years of optimum design ($) = initial cost + (3914 × USPW) Percent life cycle cost savings for government = (147,470 − 67,266)/147,470

Reference 0.08 0.1

0.02 119,700

[4] [4]

[4] Fig. 3

2394



9576



47.3%

Fig. 15

63,082



6308



0.038



0.062



115

that the government would be committed to provide for each family house. 2 The government would lower subsidies for the electricity rate to be $0.062/kW h but would provide incentives to cover the initial costs associated with implementing energy efficiency enhancements of any house. This option, as indicated in Table 10, would result in a total life cycle cost of $67,266 for each house. The difference between the two policy options is 54% savings for the KSA government. The amount of savings in the long term can be significant when considering all housing units in KSA. It should be noted that the above analysis is carried out using only one segment of KSA building sector without considering other types of buildings such as apartments and office buildings or multi-use buildings. Therefore, KSA government can save significantly in three counts: (i) on subsidies for energy costs every year, and (ii) on national energy consumption and thus reduction in local oil consumption, and (iii) on capacity and investment needed to build new electrical power plants to meet increasing electricity demand. These benefits can be obtained if KSA can aggressively promote investments and incentives programs to improve energy efficiency performance of buildings for both existing and new buildings. 6. Material cost sensitivity analysis

3914



6987



147,470



67,266



54%



energy efficiency performance of the residential buildings. Using the results of the analyses presented in this paper, Table 10 provides basic estimates of the energy cost subsidies that the KSA government could save per one house. As shown in Table 10, the KSA government can save 47.3% in annual energy consumption and 54% in subsidized life-cycle energy cost per house, when the government invests $6987 per house to improve its energy efficiency and the electricity rate that the homeowners would pay can be increased to $0.038/kW h. Under this scenario, the annual energy cost for a typical homeowner remains unchanged. From the estimates shown in Table 10, two policy options can be considered and compared: 1 The KSA government would continue to provide the same subsidy level for the electricity rate ($0.08/kW h). As indicated in Table 10, this option would result in a life cycle cost of $147,470

The cost of the energy efficiency measures is a major factor when performing optimal design of building envelope systems using the life cycle cost analysis. However, materials costs (associated for instance to thermal insulation and window glazing) are not well defined and can change within KSA regions depending on the contractors and with time depending on various economical sectors. In order to assess the impact of initial costs for various energy efficiency measures on the optimal design for the building envelope systems for the prototypical KSA villa, the costs for all energy efficiency measure have been varied by 90%: both by increasing these costs by 90% or by decreasing them by 90% to investigate the impact of the material cost on life cycle cost and energy savings. Fig. 16 shows the sequential search optimization results when the villa is located in Riyadh for the non-subsidized electricity rate after applying the material cost variations described above. The results for the impact of the material cost variation associated with the energy efficiency measures on the optimization of the villa envelope system designs for the five climate zones in KSA are provided in Table 11. The optimization results indicate that even after increasing or decreasing the initial cost of the energy efficiency measures by 90%, the variations in life cycle cost and energy savings associated with the optimal solutions are rather minimal. Table 12 summarizes the percentage difference of life cycle costs and energy savings between the case of baseline material costs and 90% variations of material costs. When increasing the material costs by 90%, energy savings will be slightly reduced (reduction of 1.1%) for Jeddah and Dhahran, and 0.6% for Abha. However, the energy savings reduction is 5.7% and 5.4% for Riyadh and Tabuk, respectively. Meanwhile, the life cycle cost increased by around 7% for

Table 11 Impact of material cost variation on EEMs optimization results for non-subsidized energy rate. Location

Normal material cost Energy savings (%)

LCC ($)

Energy savings (%)

LCC ($)

Energy savings (%)

LCC ($)

Riyadh Jeddah Dhahran Tabuk Abha

47.3 41.5 43.2 41.1 26.4

104,130 122,640 114,170 94,067 75,787

41.6 40.4 42.1 35.6 25.8

111,121 131,630 122,799 98,214 78,001

48.3 42.8 44.3 41.9 31.9

93,423 111,354 103,175 84,044 68,958

90% Uncertainty—high

90% Uncertainty—low

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Fig. 16. Optimization results with different costs of energy efficiency measures using non-subsidized energy rate for a villa located in Riyadh.

Table 12 Difference in energy savings and life cycle cost when increasing and decreasing material costs by 90%. Location

Riyadh Jeddah Dhahran Tabuk Abha

Material cost ($)

10,261 10,066 10,261 10,066 8329

After increase ($)

19,496 19,125 19,496 19,125 15,825

Difference (%)

After decrease ($)

Energy savings (%)

LCC (%)

−5.7 −1.1 −1.1 −5.4 −0.6

6.7 7.3 7.6 4.4 2.9

Riyadh, Jeddah and Dhahran, while Abha had the lowest increase in LCC by 2.9%. On the other hand, lowering the material costs by 90% had a minimal effect on energy savings for Riyadh, Jeddah, Dhahran and Tabuk, where the energy savings increased by an average of 1% only. Energy savings in Abha are increased by 5.5% due to change of optimal glazing from double glazing to a higher performance glazing. Decreasing the material cost had almost the same impact on the life cycle cost for all climate zones, where LCC lowered by about 10%. 7. Summary and conclusions Improving the energy performance of residential buildings in Saudi Arabia has been investigated through various sensitivity and optimization analyses of the building envelope systems. Five energy efficiency measures of the building envelope have been considered in this study: wall insulation, roof insulation, thermal mass associated with exterior walls, window shading, and glazing type. The analysis is applied for five different climate zones in KSA. The optimization approach was based on life cycle cost and energy savings to identify optimal and cost effective package of energy efficiency measures. The sequential search optimization and the genetic algorithm optimization are both used to determine optimal villa envelope system design solutions for both subsidized and non-subsidized electricity rates and for all five KSA locations. The analysis results showed that KSA government could save up to 36% on subsidies during the life time of each residential building without affecting the current electricity costs for homeowners if through incentives and grants, the government covers the initial costs associated with the implementation of the energy efficiency measures and provide $0.042/kW h subsidies for electricity instead of the current subsidy level at $0.08/kW h. Thus, significant annual savings can be achieved by KSA on: (i) energy cost subsidies, (ii) national oil consumption, and (iii) investments for new power plants. The sensitivity analysis has indicating that the impact of

1026 1007 1026 1007 833

Difference (%) Energy savings (%)

LCC (%)

1.0 1.3 1.1 0.9 5.5

−10.3 −9.2 −9.6 −10.7 −9.0

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