Will energy regulations in the Gulf States make buildings more comfortable – A scoping study of residential buildings

Applied Energy 86 (2009) 2531–2539

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Will energy regulations in the Gulf States make buildings more comfortable – A scoping study of residential buildings Hassan Radhi a,*, Ali Eltrapolsi b, Stephen Sharples b a b

College of Engineering, UAE University, P.O. Box 17555, Al-Ain, United Arab Emirates School of Architecture, University of Sheffield, The Arts Tower, Western Bank, Sheffield S10 2TN, United Kingdom

a r t i c l e

i n f o

Article history: Received 11 September 2008 Received in revised form 16 March 2009 Accepted 3 April 2009 Available online 9 May 2009 Keywords: Envelope energy regulations Summer comfort Residential buildings

a b s t r a c t Building envelope impacts upon energy consumption and indoor environment. The relationship between envelope components and indoor environment has become increasingly important, especially with the new emphasis on visual comfort, thermal comfort and indoor air quality. This paper examines the interaction between occupant thermal comfort and envelope component regulations in the Gulf States. The country chosen for this study is the Kingdom of Bahrain, the smallest country in the Gulf region. Simulation results and comparative studies were employed to investigate the impact of the current envelope component regulations on the internal environment. The paper focuses on residential buildings and concludes that the envelope component regulations contribute positively to the internal thermal performance. Although these envelope components are not generally the primary elements that impact upon internal thermal comfort there are circumstances when the components become very warm and occupants positioned close to them will experience discomfort. This paper shows that the thermal insulation regulation makes a small impact on thermal comfort, whereas the window regulation, particularly glazing, is more influential and that for most window areas, solar impacts are generally large. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Buildings respond to their local environment and, therefore, different building designs are found in different climatic regions. In any region, however, the ultimate objective of buildings is to avoid the extreme outside conditions and to provide a comfortable internal environment in an economic way. The building envelope represents the connection between the internal environment and the outside conditions, and hence, a key function of this envelope is to reduce the need to modify the indoor environment to be more suitable for habitation than the outdoor. Sometimes, the envelope fails to meet its objective due to one or more reasons, such as insufficient design or extreme outdoor conditions that probably make it impossible for any certain level of comfortable indoor environment to be achieved through passive means. Then, it is necessary to rely upon mechanical means to achieve the comfort level. To avoid this situation or, at least, to reduce the reliance upon mechanical means, the design of the building envelope should be in a harmony with environment. In practice, an effective strategy to enforce optimum design is through appropriate legislation for energy efficiency. This is notable in Bahrain where cooling is a problem and expensive energy * Corresponding author. Tel.: +971 3 7133105; fax: +971 3 7636925. E-mail address: [email protected] (H. Radhi). 0306-2619/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2009.04.003

consuming air-conditioning systems are used to cool poorly designed buildings. Bahrain has recently been responding with suitable energy regulations. It began planning building standards for energy efficiency in 1998 [1]. As a first step of the task, new envelope standards were regulated. In terms of energy savings, the standards have long-reaching effects on the next generation of buildings. However, there is a serious lack of experience of their impact on the internal environment. This may result in an energy efficient building that does not provide thermal comfort. 2. Factors impacting thermal comfort in buildings It is often said that the most effective assets for achieving a comfortable environment are to design with climate and with a sense of place [2,3]. Two types of factors come in to play to achieve such environments: firstly, the climatic elements, which building designer has no control on and secondly, the design parameters which are affected by the climate and can be controlled by the building designer. 2.1. Climatic elements impacting internal conditions The outdoor conditions have a significant impact on the indoor environment. Building designers have no control over such conditions. However, they can reduce their impact on buildings. The

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H. Radhi et al. / Applied Energy 86 (2009) 2531–2539

outdoor conditions are represented by a number of climatic elements including the outdoor air temperature, relative humidity, wind speed, wind direction and solar radiation. Fig. 1 shows an analysis of climatic elements provided by the Directorate of Meteorology of Bahrain. The analysis shows an annual average temperature of 26.5 °C with a monthly average maximum temperature of 41 °C (August) and a monthly average minimum temperature of 14.4 °C (January). The cold season in Bahrain spans from late December through January and February. March until early May is considered the transitional period between the mild winter and hot summer. This season is characterised by sudden and daily variations in temperature. The summer season spans from late May to September, during which the monthly mean temperature is around 30–35 °C. In October and November sunny skies are dominant, with dry conditions and monthly mean temperatures between 29 °C and 24 °C, respectively. The monthly average relative humidity is 62%, with a maximum monthly average of 72% (January) and a minimum monthly average of 50% (June). Winds from a north-east direction throughout the year are characteristic of Bahrain. The wind speed average shows slight variation, being generally low from April to December with a monthly average of 4.2 m/s, while from January to March it is well above 5.1 m/s, reaching a monthly average of 5.2 m/s in February. Bahrain experiences high solar radiation levels. The highest monthly averages of total and direct radiation are 585 W/m2 and 383 W/m2 in June and September, respectively. The lowest monthly averages of the same parameters are in December and January with 373 W/m2 and 267 W/m2, respectively. Bahrain has an average of 3468 sunshine hours per year as has been measured during the last 10 years [4]. 2.2. Envelope parameters impacting internal conditions There are many building component parameters that impact thermal comfort in buildings. For example, designing the form of the building to respond to the sun and wind is with the most influence on the thermal performance of building envelope. Building regulation in Bahrain has recognised the important role that envelope components play in preventing or reducing the impact of the climate on building energy consumption. Therefore, new Bahraini envelope standards were developed. Article-32 is the Bahraini building energy regulation. It is an envelope components design standards that considers the heat flow through individual components of the building shell (e.g. external wall, roof and window). It considers the maximum U-value for two elements of the building envelope. For roofs and walls these are 0.6 and 0.75 W/m2/°C, respectively. Article-32 allows the use of single glass in buildings with an area of glazing less than 20% of the facade. When the area exceeds this percentage, the regulation requires the use of double glazing. Similar to Bahrain, many countries set some requirement about thermal transmittance values of walls, windows and in some

700

90 80

600

70 60

400

50

300

40 30

200

20 100

Total radiation Mean air-temperature

Direct radiation Mean relative humidity

0

10

( % ) - ( oC )

(W/m2)

500

cases, as in Part L of the UK regulation [5], for floors and doors as well. Although this approach may be easy to implement and comply with, it is not adequately comprehensive. Furthermore, the actual heat flow through the envelope is subject to many more variables of the building design. It is, therefore, not surprising that existing traditional buildings often perform much better than the standards for new buildings. Nevertheless, the current trend in Bahrain imposes a new style that is quite different from the traditional Bahraini architecture. For a country with little experience in building energy regulations, like Bahrain, the envelope component standards (walls and windows) may represent the first step towards a more comprehensive approach. It is worth mentioning that the envelope components are not the primary elements influencing the comfort of a building’s occupants. However, when they are hot or cold and in close proximity to the occupants then they are important. 2.3. Research into thermal comfort in hot climates Over the last few decades, considerable research has been undertaken on the impact of building elements on the indoor environment and how can envelope design be improved in order to provide comfortable internal conditions [6]. A recent study was carried out in India to examine the impact of glazing on human comfort. It was found that in warm and hot climates solar impacts can be controlled by glazing parameters [7]. In Australia, a study was conducted using DesignBuilder software to examine cooling technologies in order to provide comfortable internal thermal conditions [8]. With respect to the use of simulation software to assess the thermal comfort of residential buildings, a parametric study [9] indicated that the congenital algorithms are inadequate in such buildings due to the wide range of possibilities to adapt the thermal environment. It linked the comfort temperatures to the form of outdoor temperature providing that there is a strong dependency between thermal conditions in residences and weather data, more specific recent outdoor temperatures. In countries that are characterised as areas with hot and humid climates, much study has investigated the envelope elements and how they impact thermal comfort. Some of those studies examined the typical strategies to naturally improve comfort and showed that a well-insulated envelope can provide thermal comfort without using air-conditioning system [10]. Other studies explored how the use of envelope natural ventilation strategies helps in providing thermal comfort in hot humid climates [11]. In another study into the effect of passive techniques on interior temperature in small houses found that using passive techniques, such as window shading, orientation and thermal inertia, can reduce the interior air temperature and consequently increase the level of thermal comfort [12]. To compare the impact of the traditional and modern designs of building envelope, two studies were conducted in the hot arid climate of Ghadames – Libya [13,14]. It was found that the optimum thermal design of the traditional houses provided a thermal comfort for building occupants due to the use of high thermal mass. The results of those studies suggested that people have an overall impression of a higher standard of thermal comfort in old buildings than in new buildings. In Iraq, a study examined the effect of size, orientation, glass quality and shading of windows on the indoor thermal environment of buildings [15]. This study showed that the indoor conditions do not improve significantly due to changing the glazing type and that a south facing window has the largest potential to improve winter and summer indoor comfort. 2.4. Research into thermal comfort in the Gulf States

0 Jan Feb Mar Apr

May Jun

Jul Aug Sep Oct Nov Dec

Fig. 1. Analysis of Bahrain’s climate.

The economic and architectural boom coupled with the hot arid climate of the Gulf States demand studies on the thermal and

H. Radhi et al. / Applied Energy 86 (2009) 2531–2539

environmental performance of buildings. A brief analysis of the conducted studies shows that the majority focused on building design and how to reduce electricity consumption in buildings, while a few researches concentrated on the thermal conditions of interior spaces. In Kuwait, for example, a recent study investigated the thermal insulation and clothing area factors of typical Arabian Gulf clothing ensembles and determined the thermal insulation values of a number of Arabian Gulf garments [16], while In Bahrain, a study [17] into the requirements of thermal comfort found that there was a reasonable agreement between recorded values obtained from a field study and the predicted PMV of Fanger. A study was conducted in Saudi Arabia to inveterate the effect of height on thermal performance of a multi-storey building [18] and indicated that there was a considerable difference in the thermal performance due to changing in height. The indoor air temperature and internal surface temperatures were relatively higher in the top floor than the values recorded for the lower levels, mainly due to the exposure of the roofs to the intense heating effect of the direct solar radiation. Another recent study in Saudi Arabia [19] utilized the basic chart of thermal-comfort in order to provide the highest comfort-level while maximising energy savings. This study examined the amount of heating and cooling energy savings due to altering the set-point temperature. Based on the above, on the one hand, the majority of studies in the Gulf region focused on reducing the energy consumption of buildings through improving building design and operation. This approach may result in energy efficient buildings that do not provide comfortable internal environment. On the other hand, with exception to the study in Ref. [18] most researches in thermal comfort in the Gulf region focused on the internal conditions without any consideration to the role of building envelope as a key function to provide a comfortable internal environment or at least to reduce the need to modify the internal conditions to be more suitable for habitation than the outdoor. This current study investigates the influence of envelope component regulations including thermal insulation and window parameters on the indoor thermal comfort of residential buildings in the hot arid climate of the Gulf States. The country chosen for this study is the Kingdom of Bahrain, the smallest country in the Gulf region. The principle advantage of this study is that the results might lead to interesting recommenda-

2533

tions to building practice concerning the improvement of internal thermal conditions with minimum energy consumption. 3. Methodology The impact of Article-32 is unknown, as there are no methods to assess the impact on the internal conditions. Recently, a systematic methodology has been introduced [20] to assess the impact of envelope component standards on the overall performance of buildings. This current study uses the same methodology to examine the thermal performance of residential buildings in Bahrain. For this purpose, a simulation model of a real building was first constructed and then the model was validated using measurement data. Finally, a PMV indicator was used to assess the thermal comfort. These processes are described in more details below. 3.1. Characteristics of the case study For appropriate evaluation, a typical residential building was used. Fig. 2 illustrates the architectural characteristics of the studied building. It is a single storey building with a floor to floor height of 3.5 m. The walls consist of mainly 150 mm concrete block, 24 mm of plaster inside and outside and 30 mm of polystyrene insulation positioned to the outside. The roof consists of a 50 mm screed, 35 mm polyurethane and 150 mm concrete slab. The windows are double-glazing with a 6 mm air gap and an approximate 0.20 window–wall ratio. The building has multi-thermal zones with a spilt unit system. Complete details of the building’s physical and operational characteristics are given in Table 1. 3.2. Assessment criteria According to ASHRAE standard 55P [21] the thermal comfort is ‘the condition of mind which expresses satisfaction with the thermal environment’. This environment is typically described by two types of variables. The first type is represented by four physical parameters – air temperature, mean radiant temperature, relative humidity and air velocity. To quantify the impact of windows on the comfort of building occupants, one more variable should be considered, namely the direct solar radiation. This is because solar

Fig. 2. Architectural characteristics of the studied buildings.

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H. Radhi et al. / Applied Energy 86 (2009) 2531–2539

Table 1 Description of the case building.

3500 Electricity bills

3000

Parameters

Value

No. of floor Total area Floor height Orientation Wall

1 215 m2 3.5 m North–south 150 mm concrete block-24 mm of plaster inside and outside 30 mm of polystyrene insulation. 0.75 W/m2/°C 50 mm screed, 35 mm polyurethane and 150 mm concrete slab 0.60 W/m2/°C WWR 0.2 Double glazing 1.6 W/m2/°C SC 0.57 5.0 m3/h/m2 6 W/m2 12 W/m2 Spilt unit system 27 m2/person

Simulation results

Roof Window

Infiltration rate Equipment Lighting HVAC Occupancy

(kWh/m2)

2500 2000 1500 1000 500 0 Jan Feb Mar Apr

May Jun

Jul

Aug Sep Oct

Nov Dec

Fig. 3. Calibration of the simulation program base case building.

radiation falling directly on occupants significantly impacts upon their perception of thermal comfort. It is a potent determinant of comfort or discomfort, especially in the summer months. The second type of thermal comfort variable is represented by two personspecific variables – the thermal insulation value of clothing and metabolic rate. All of these variables can be combined into values that represent the thermal sensations of building occupants such as Predicted Mean Vote (PMV) or Predicted Percentage of Dissatisfied (PPD). These indices are able to illustrate the quantitative values of the degree of discomfort and the influence of not only environmental factors but also human factors. In this study, the assessment of thermal sensation was based on simple measures of indoor temperature, mean radiant temperature, relative humidity, air velocity and solar gains and are represented by the Predicted Mean Vote (PMV) and Predicted Percentage of Dissatisfied (PPD). These measures were determined using the DesignBuilder software [22]. It is a user interface for the Energy Plus detailed modelling program. This program uses the heat balance method to calculate response factors for transient heat flow in walls and finite difference techniques for the thermal response of building spaces. An approach based on the Fanger comfort model has been utilised by Energy Plus for predicting the thermal comfort. This model applies an energy balance to a person and uses the energy exchange mechanisms along with experimentally

derived physiological parameters to predict the thermal sensation and the physiological response of a person as a function of the environment. The comfort boundary conditions in this study are an activity level of 1.2 met and an indoor clothing level of 0.75 clo. To determine the PMV and PPD two points of assessment for thermal comfort were considered, first, the centre point of the space and second, a point near to the walls and windows (200 mm away). 3.3. Procedure Based on monthly utility bills and building design and operation, the base case of the simulation program was first calibrated, as shown in Fig. 3. The calibration was based on real weather data of Bahrain. Although increasing the number of time steps slows the simulation, for improving the accuracy, the output was requested at hourly, daily and monthly intervals. As the Bahraini regulation has been set as a strategy to minimise the cooling load, this study focuses on the summer months. Parametric analysis was undertaken to examine the variation of PMV and PPD for the summer period from June to September, four thermal insulation values, up to three glazing systems and four window sizes. As mentioned earlier, this study deals with the two Bahraini envelope component standards (thermal insulation and window parameters) and briefly discusses some issues concern with shading devices and thermal mass. Table 2 illustrates the examined parameters. The thermal insulation was examined by varying the U-values of the walls of the test building. The U-value of the roof was kept fixed in all cases.

Table 2 Tested parameters. Abbreviation

U-factor (W/m2/K)

Roughness

Absorption

Specification from outside to inside

Insulation Wall-1 Wall-2 Wall-3 Wall-4 Roof (fixed)

2.32 0.7 0.5 0.3 0.6

3 3 3 3 0.9

0.7 0.7 0.7 0.7 0.5

Plaster 12 mm, block, 150 mm, plaster 12 mm Plaster 12 mm, polystyrene 5 mm, block, 150 mm, plaster 12 mm Plaster 12 mm, polystyrene 10 mm, block, 150 mm, plaster 12 mm Plaster 12 mm, polystyrene 35 mm, block, 150 mm, plaster 12 mm Screed 50 mm, polyurethane 35 mm, Concrete 150 mm

Abbreviation

U-value (W/m2/K)

SC

SHGC

Visible transmittance

Summer inside glazing surface (°C)

Window Single Double Low-e Window area Shading devices

6.3 2.78 1.48 10%

1.00 0.89 0.65 20%

0.86 0.77 0.56 40%

0.9 0.81 0.75 60%

23.9 31.8 29.7

500 mm overhang 300 mm louvers Removable internal shading devices Thermal mass Mass-150 Mass-200 Mass-250

Plaster 12 mm, polystyrene 35 mm, light weight concrete block 150 mm, plaster 12 mm Plaster 12 mm, polystyrene 35 mm, medium weight concrete block 200 mm, plaster 12 mm Plaster 12 mm, polystyrene 35 mm, heavy weight concrete block 250 mm, plaster 12 mm

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H. Radhi et al. / Applied Energy 86 (2009) 2531–2539 Case-1:window gain Case-2:wall gain

Case-2:window gain Case-2:wall gain 2500

1400 1200

2000

1500

800

48.1%

600

48.2%

47%

47.8%

1000

400

53.6%

200

52.7%

(kWh)

1000

(kWh)

The impact of window parameters was examined in two steps: firstly, altering the window area without changing the construction of the wall and properties of the glazing and secondly, by varying the glazing type and keeping the window area and the construction of the wall properties fixed. Further examination was carried out on 10 types of glass to determine their influence on the internal thermal comfort. The surface-temperature data of glazing were derived from the WINDOW 6.2 program [23]. The direct solar heat gain data were obtained using the DesignBuilder software. This software was then used to calculate the PMV for each combination. Results were transferred into a spreadsheet. The PPD determined by the simulation program was derived in each case and the results were plotted graphically in order to illustrate trends for the dependence of thermal discomfort on thermal insulation, window parameters, thermal mass and shading devices.

54.5% 52.2%

0

500

0 June

July

August

September

Fig. 4. Drop in heat gains through walls and solar gain through windows.

4. Results and discussion 2

with standards

1.6

1.2

0.8

4.1. Thermal comfort in Bahrain

0.4

0 Jun

Jul

Aug

Sep

Fig. 5. PMV of the two cases in the centre of the spaces.

no standards

2

with standards

1.6

1.2

PMV

In general, most people feel comfortable at indoors temperature ranging from 22 °C to 27 °C along with a range of 40–60% relative humidity. The climatic analysis carried out earlier showed that during the four months from June, where the cooling season starts, to September, where the cooling season ends, the ambient temperature exceeds 34 °C. Therefore, the internal summer comfort conditions are often achieved using air-conditioning systems. It is important to note that the monthly average ambient air temperatures of the summer months range from 34.2 °C in June to 37.8 °C in August. Simulating the test building with respect to two cases, given in Table 3 (Case-1: no standards and Case-2: with standards), under the four summer months shows that Case-1 has average indoor temperatures of 27.9 °C in June and 28.7 °C in August, while Case-2 has average indoor temperatures ranges from 27.0 °C to 27.5 °C for the same period. In the latter case, the consequence of ambient air swing is reduced by 10.3 °C and 6.6 °C in August to September, respectively. The inside air temperature for this case is approximately 1 °C lower than Case-1, while the operative temperature is approximately 2.4 °C less. This reduction can be related to the drop in heat gains through walls and solar gain through windows as can be seen in Fig. 4. It is therefore, clear that the relatively low temperatures of the wall and glazing tend to lower the indoor temperature of the second case due to the use of thermal insulation and double glazing. PMV and PPD were calculated to find the thermal conditions for the two cases (Case-1: no standards and Case-2: with standards). Figs. 5 and 6 show a comparison of the PMV of the two cases with

no standards

PMV

Envelope component standards impact upon energy consumption and indoor environment. The impact on energy consumption was discussed in details in Ref. [24]. This paper first studies the thermal comfort in Bahrain and then discusses the impact of envelope components, namely thermal insulation, thermal mass, window parameters and shading devices, on the summer thermal comfort of residential buildings in Bahrain.

0.8

0.4

0 Jun

Jul

Aug

Sep

Fig. 6. PMV of the two cases close to buildings surfaces.

respect to the two assessment points. Each bar in the illustrations represents an average vote of one summer month. As illustrated, the PMV is reduced as the distance from the wall is increased. It is clear that the occupants feel warm in the building with no

Table 3 Characteristics of the studied buildings. Abbreviation

Case-1: no standards

Case-2: with standards

Buildings Wall Roof Glazing area Glazing Type

Plaster 12 mm, block, 150 mm, plaster 12 mm 2.32 W/m2/K Screed 50 mm, polyurethane 35 mm, Concrete 150 mm 0.6 W/m2/K 30% of the wall area Single 6.3 W/m2/K SC: 1.00 SHGC: 0.86

Plaster 12 mm, polystyrene 5 mm, block, 150 mm, plaster 12 mm 0.7 W/m2/K Screed 50 mm, polyurethane 35 mm, Concrete 150 mm 0.6 W/m2/K 20% of the wall area Double 2.78 W/m2/K SC: 0.89 SHGC: 0.77

H. Radhi et al. / Applied Energy 86 (2009) 2531–2539

standards, while they feel neutral to slightly warm in the building that complies with the regulation, even though the operative temperature inside the building reaches 28 °C. It can be noted that using the envelope component standards to optimising the thermal performance of buildings can positively impact the occupant comfort. However, are all of these components working in the same way and how do the building internal thermal conditions respond to them?

3.0 2.5 2.0

PMV

2536

1.5 1.0

4.2. Thermal comfort as a function of envelope components 0.5

There is a fundamental difference between the thermal comfort impacts of walls and windows in summer and winter. On the one hand, the impact in winter depends largely on inside surface temperature, which is subject to the outside temperature and the thermal transmittance value. This is because the solar impact is either small or unavailable. On the other hand, the impact in the summer depends largely on a combination of the inside surface temperature and transmitted sol–air temperature. These two parameters are tightly correlated with the characteristics of the wall, windows and shading devices. 4.2.1. Summer comfort and wall characteristics – insulation and thermal mass There is significant evidence about the benefits of reducing the thermal transmittance value of building envelopes in typical cold climates, when well-insulated envelopes directly reduce heating energy. However, in hot climates, insulation may cause a rise in the internal temperature and consequently lead to the use of large capacity air-conditioning systems that would not necessarily be needed with less insulation. This problem can be found more frequently in internal load dominated buildings, such as offices, where the internal loads are often large coupled with long summer periods with high outdoor temperatures and intense solar radiation. It is important to emphasis that the amount of internal heat gain in such buildings depends largely on the type of equipment and lighting system. Using low U-values of thermal insulation may trap the heat inside the buildings making a sense of that the inside temperature is higher than the outside one. In skin load dominated buildings, such as those buildings with a residential profile, the internal gains are small and the use of low thermal transmittance elements reduces the external heat gain and may provide comfortable conditions, especially in spaces close to the external walls. In general, residential buildings experience a difference in temperature between internal surface and internal air due to the cold outdoor conditions. The opposite is true in the case of hot outdoor conditions. The difference in temperature can be large when building envelope is not appropriately insulated. Thermally, as the difference between outdoor and indoor temperature increases, the impact of the internal air temperature close to the surface becomes greater. This is because the internal air is connected to inside surfaces such as those of external walls. When these surfaces are hot the internal air temperature rises and consequently creates hot flows close to those walls. These flows may cause discomfort for people occupying the space. In Fig. 7 it is illustrated that the thermal insulation comfort impact, in summer, is sensitive to the U-value of the walls. The air temperature, mean radiant temperature, relative humidity and air velocity, represented by an average PMV of the four summer months, become more comfortable as the fraction of the thermal insulation is increased. The lower thermal insulation is effective in reducing indoor maximum temperatures below the high outdoor maxima. This is because of the drop in conductive and solar heat gain that leads to a drop in the internal air temperature. For example, the PPD due to the use of 0.75 W/m2/°C is 40.3% and

0.0 0.7 5

0.5

0.3

0.1

U-Value Fig. 7. PMV as a function of envelope thermal insulation.

35.2% in August and September, respectively; while it drops to 35.3% and 30.5% when 0.1 W/m2/°C is used for the same months. As can be seen the thermal insulation standard influences the internal comfort of buildings. Nevertheless, its impact is only moderate considering the high level of insulation. As a great amount of heat can be reduced by thermal insulation an appropriate careful and treatment of building thermal mass offer a considerable opportunity to control heat flow. In climates where cooling is of primary concern, thermal mass can reduce energy consumption. Because thermal mass stores and releases heat, it interacts with the building operation more than the simple addition of insulation. In general, insulation can be more effective in climates with extreme seasonal variations and small daily variations while thermal mass of the building plays a more critical role in balancing the indoor temperatures in hot-dry climates with large diurnal ranges such as those of the Gulf area [25]. In Bahrain, as in most Gulf States, there is no specification or reference to any types and constructions of walls and roofs. The only criterion is that the walls and roofs of buildings should be insulated to a particular level. In this study, two types of thermal mass of walls are examined. These are heavy (250 mm), heavy (200 mm) and heavy (150 mm) weight concrete blocks. Table 4 illustrates the impact of using different thermal mass with respect to the heat gain, cooling load and internal parameters. It is shown that the cooling load decreases as the thermal mass becomes higher. This reduction can be related to the drop in heat gains through walls. It is therefore, clear that the relatively low temperatures of the wall tend to lower the indoor temperature due to the use of thermal mass. As illustrated in Fig. 8 that the thermal mass discomfort impact, in summer, is sensitive to the mass of the walls. The air temperature, mean radiant temperature, relative humidity and air velocity, represented by an average PMV of the

Table 4 Increase due to thermal mass. Mass-150 MRT (°C) Reduction (%) Air temperature (°C) Reduction (%) RH (%) Reduction (%) Operative temperature (°C) Reduction (%) Wall gain (k W h) Reduction (%) cooling (k W h) Reduction (%) ( ) Increase

29.4 27.3 48.5 28.4 704.7 2390

Mass-200

Mass-250

29.3 0.2 27.2 0.4 48.6 0.1 28.3 0.1 602.4 14.5 2340 2.1

29.2 0.7 27.1 0.7 48.8 0.6 28.2 0.5 392.4 44.3 2240 6.3

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H. Radhi et al. / Applied Energy 86 (2009) 2531–2539 Table 5 Tested 10 glass types.

2.5

Abbreviation 2.0

PMV

1.5

1.0

0.5

0.0 Mass-150

Mass-200

Mass-250

Thermal mass

Window types Clear Blue Grey Bronze Iron Green LoE-(e2-1) LoE-(e2-2) Ref-a-Tint Ref-d-Tint

Total solar transmission (SHGC)

Direct solar transmission

Light transmission

U-value (W/m2/K)

0.69 0.48 0.46 0.48 0.81 0.49 0.56 0.62 0.14 0.34

0.60 0.37 0.35 0.37 0.79 0.37 0.47 0.53 0.03 0.23

0.78 0.50 0.38 0.47 0.83 0.66 0.74 0.73 0.04 0.22

3.16 3.15 3.15 3.15 3.18 3.15 2.40 2.53 2.76 3.10

Fig. 8. PMV as a function of envelope thermal mass. 1.0

four summer months, become more uncomfortable as the thermal mass becomes lower.

0.9

Blue

Green

Bronze

LoE-e2-1

LoE-e2-2

Clear

Iron

0.5

Grey

0.6

Ref-d-tint

PMV

0.7

0.14

0.35

0.47

0.49

0.49

0.49

0.56

0.63

0.70

0.82

0.4 0.3 0.2 0.1 0.0

SHGC Fig. 10. PMV as a function of glazing system with different SHGCs.

1.0 0.9

Clear

Bronze

Blue

Green

2.53

2.77

3.11

3.16

3.16

3.16

3.16

Iron

Ref-d-tint

2.40

Grey

Ref-a-tint

0.6

LoE-e2-2

0.7

LoE-e2-1

0.8

PMV

4.2.2. Summer comfort and window parameters The window impacts building internal conditions by three mechanisms – long-wave radiation from the internal glazing surface, solar heat gain and air velocity (air movements resulting from the temperature difference between the glass internal surface and the internal air temperature). Two types of properties must be considered where these mechanisms are concerned – the thermal and optical properties, which can be represented by the U-value and solar heat gain coefficient (SHGC). Fig. 9 shows the simulation results of the studied building using glazing systems with different U-values and SHGCs. It is clear that the LoE-system provides the best internal conditions as can be seen in the PMV scale, which has a value of 0.8. However, these glazing systems have different thermal and optical properties. To determine which properties make more impact on the thermal conditions, 10 double glazing systems, given in Table 5, were used for further examination. They examined glazing system were simulated under the same weather data as were used with the case base building. Fig. 10 shows the PMV under the summer period as a function of glazing system with different SHGC values, while the results in Fig. 11 illustrate the sensitivity of comfort conditions to the glazing U-value. The solar gain seems to be the dominant factor with respect to the perception of comfort. Discomfort increases with higher SHGC. Coupled with this is the secondary impact resulting from the absorption of vertical solar radiation by the glazing system. For instance, the solar gain by clear double-glazing is 55.6% and 41%

Ref-a-tint

0.8

3.16

3.18

0.5 0.4 0.3 0.2 0.1 0.0

U-Value Fig. 11. PMV as a function of glazing system with different U-values. 3.0 2.5

PMV

2.0 1.5 1.0 0.5 0.0 Single

Double

LoE

Glazing system Fig. 9. PMV as a function of glazing system.

higher than that of grey and tinted glazing, respectively. This is in addition to the rise in the inside surface temperature of the glazing which leads to additional long-wave radiation, and consequently causes the PPD to increase from 48% to 49% and 52% when tinted glass, grey glass and clear glass are used, respectively. Different scenarios in PMV are seen for different U-values. Technically, there is on going trade-off between two sides. The first is the glazing U-value and its impact on long-wave heat loss from the occupant. The second is the solar body heating that is resulted from the absorption of solar radiation. Calculated value of 28.6 °C is the radiant temperature of the lowest PMV, while 29.2 °C is the calculated one of the highest PMV. The difference is only 0.52 °C. However, the PPD increases by 10.5% due to replacing the Ref-a-tint system with the Iron system. Although the U-value of

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Ref-a-tint system is higher than that of LoE-e2-2 system it still gives better internal conditions. Therefore, a lower U-value of glazing is not necessary more effective than a higher one. Based on the above, there is no strong correlation between the PPD and the U-value, but there is a tightly connection with the SHGC. A large window area implies a large hot surface in the summer months where the average maximum outside temperature reaches above 41 °C in August along with intense solar radiation. As shown in Table 6 that altering the window area from 20% to 10% without changing the thermal and optical properties of the glazing leads to a reduction in the inside air temperature of 0.3 °C, while enlarging the area to 40% and 60% increase the inside air temperature by 0.5 °C and 1.0 °C, respectively. Consequently, the window solar gain is reduced by 31.7% in the first case, but increases by 52.3% and 84.7% when the area enlarges to 40% and 60%. The difference in the radiant temperature between 10% and 60% reaches 2.9 °C. Fig. 12 shows that the discomfort increases with larger window areas. The analysis shows that the radiant asymmetry and associated temperature has a significant impact on thermal comfort close to the window. As the window area becomes larger this impact becomes greater and that the difference of comfort vote is often attributed to the radiant temperature. This may imply that for improving the internal thermal conditions the solar gains need to be carefully controlled, especially when low thermal insulation is used. A sufficient method to control the solar gains is the use of shading devices. 4.2.3. Summer comfort and shading devices Although Bahrain has only two mandatory standards, the regulation encourages the use of architectural elements such as shading

devices. The main purpose of the shading device is to protect building envelope and occupants from sun glare and radiation. In principle, the shading device should prevent the solar radiation from entering the facing window during the summer, but lets it through during winter when heat is needed. Three types of shading devices were examined with respect to the studied building including overhang shading devices, louvers and removable internal shading devices Fig. 13 shows a comparison of the PMV with respect to the examined shading devices relative to the studied building. It can be noted that the three types of shading devices have a significant impact on the internal thermal conditions. However, the overhang device seems to have more impact where the PPD is reduced from 40% to 35% in June and from 35% to 30% in September. It is clear that the use of shading devices can improve the internal thermal conditions especially when they are used in the south, east and west facades. It is important, however, to mention that the western shading device, sometimes, is not effective. This is simply because the window in this orientation is difficult to be shaded with overhang shading devices due to the low position of the sun in the sky. Therefore, the louver type may be more effective in this orientation. An argument can be made that the interior shading is more useful than the outside shading device. Changing the shading device to an internal one such as curtains and removable blinds may have a positive impact on thermal comfort, but leads to deteriorate other aspects of comfort such as the visual comfort and consequently consume more energy to illuminate and air-condition the space. Table 7 shows the impact of different shading devices on the cooling load, lighting load and internal conditions. As shown, the lighting load is increased when an internal shading devise is used and

1.40

Table 6 Increase due to window area.

1.20

Window area

1.00

10%

20%

40%

60%

28.2 4.73 27.2 0.73 49.1 1.66 28.2 1.05 312.0 31.73 767.0 11.16

29.6

30.4 2.7 27.8 1.5 47.0 2.7 29.1 2.1 696.0 52.3 558.0 19.1

31.1 5.1 28.1 2.6 46.0 4.8 29.6 3.9 844.0 84.7 455.0 34.1

27.4 48.3 28.5 457.0 690.0

PMV

MRT (°C) Increase (%) Air temperature Increase (%) RH Increase (%) Operative temperature Increase (%) Glazing solar gain (k W h) Increase (%) Wall solar gain (k W h) Increase (%)

no shading internal shading louvers overhang

0.80 0.60 0.40 0.20 0.00 June

July

August

September

Fig. 13. PMV as a function of deferent types of shading devices.

( ) Reduction Table 7 Reduction due to shading devises. No shading 3.0 2.5

PMV

2.0 1.5 1.0 0.5 0.0 10%

20%

40%

Glazing area Fig. 12. PMV as a function of window area.

60%

Types of shading devices RTM (°C) Reduction (%) Air temperature (°C) Reduction (%) RH (%) Reduction (%) Operative temperature (°C) Reduction (%) window solar gain (k W h) Reduction (%) Lighting energy (k W h) Reduction (%) Cooling energy (k W h) Reduction (%) ( ) Increase

29.6 27.4 48.3 28.5 456.5 312.4 2464.6

Internal shading

Louvers

Overhang

29.4 0.8 27.3 0.2 48.5 0.5 28.3 0.5 569.6 24.8 327.7 4.9 2387.0 3.1

29.3 1.2 27.2 0.5 48.9 1.3 28.2 0.9 447.4 2.0 316.9 1.4 2278.1 7.6

29.2 1.4 27.2 0.7 49.1 1.6 28.2 1.1 434.8 4.8 327.8 4.9 2251.8 8.6

H. Radhi et al. / Applied Energy 86 (2009) 2531–2539

that the amount of energy used for cooling the building in this case is larger than in the overhang and louvers. This is simply because the solar radiation is stopped after penetrating the glazing. In other words, the heat will be transferred inside the building and a proportion of this heat will be trapped behind the glass and, therefore, extra energy will be used to remove it. 5. Conclusions and recommendations Summer comfort conditions are adversely affected by the presence of large, hot internal building surfaces. Improving the thermal performance of the building envelope through the use of energy standards can provide a comfortable indoor environment. The analysis carried out in this paper has shown that an improvement in the internal conditions can be achieved through the use of envelope component regulations. The thermal transmittance value and thermal mass of building envelope, window areas, glazing type and shading devices effectively contribute to the internal thermal comfort. However, not all of these components work in the same way, and some of them have more impact over the building response than others. Although a lower value of thermal insulation influences the internal thermal conditions of residential buildings, its impact is only moderate, while a higher thermal mass can provide more comfortable internal environment. In contrast, the window parameters, particularly glazing, is more significant. This is simply because the temperatures of the glazing surface often fluctuate much more than those of other surfaces in buildings. Even when the set point temperature is adjusted to a comfortable level, occupants may experience significant discomfort because of radiant heat exchange with glazing surfaces. The window area can make a significant impact on the internal thermal conditions. The use of overhang and louvers devices can also improve the internal environment of buildings. However, the louvers may be more effective in the western facades. The following recommendations can be suggested: 1. In residential buildings, when the external heat gains are not effectively controlled, there is a tendency towards more comfort as the envelope insulation increases and the thermal mass becomes higher. To reduce the heat gain and consequently improve the internal conditions in such buildings, a lower U-value should be used and a higher thermal mass applied. 2. In the case of the window parameters, the dominant factor is not the outdoor air temperature but the solar radiation. To improve the solar performance of glazing the optical properties of glass, including the shading coefficient, light reflection and absorption, should be considered where glazing system is concerned. Large window areas increase the amount of solar heat gains. To improve the internal conditions, the window area should be reduced and shading devices should be used as much as is reasonably possible. A trade-off between optical properties

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and window areas and shading devices should be taken into account when large glazing areas are desirable. References [1] National Committee of Buildings in Bahrain. Thermal insulation for buildings. Ministry of Electricity and Water, Kingdom of Bahrain; 2002. [2] Oktay D. Design with the climate in housing environment – an analysis in Northern Cyprus. Build Environ 2002;37(10):1003–12. [3] Olgyay F. Design with climate – bioclimatic approach to architectural regionalism. Princeton, NJ: Princeton University Press; 1963. [4] Radhi H. A comparison of the accuracy of building energy analysis in Bahrain using data from different weather periods. Renew Energy 2009;34:869–75. [5] Triker R, Algar R. Building regulations in brief. 5th ed. London: Butterworth– Heinemann; 2006. [6] Oral GK, Yener AK, Nurgun TB. Building envelope design with the objective to ensure thermal, visual and acoustic comfort conditions. Build Environ 2004;39:281–7. [7] Singh MC, Garga SN, Jha R. Different glazing systems and their impact on human thermal comfort – Indian scenario. Build Environ 2008;43:1596–602. [8] Chowdhury AA, Rasul MG, Khan MMK. Thermal-comfort analysis and simulation for various low-energy cooling-technologies applied to an office building in a subtropical climate. Appl Energy 2008;85:449–62. [9] Peeters L, Dear RD, Hensen J, D’haeseleer W. Thermal comfort in residential buildings: comfort values and scales for building energy simulation. Appl Energy 2009(86):772–80. [10] Zain MZ, Taib MN, Baki SMS. Hot and humid climate: prospect for thermal comfort in residential buildings. Desalination 2007;209:261–8. [11] Feriadi H, Wong NH. Thermal comfort for naturally ventilated houses in Indonesia. Energy Build 2004;36:614–26. [12] Porta-Gandara MA, Rubio E, Fernandez JL, Munoz VJ. Effect of passive techniques on interior temperature in small houses in the dry, hot climate of north-western Mexico. Technical note. Energy Build 2002;26:121–35. [13] Ealiwa MA, Taki AH, Howarth AT, Seden MR. An investigation into thermal comfort in the summer season of Ghadames, Libya. Build Environ 2001;36:231–7. [14] Ahmad I, Khetrish E, Abughres SM. Thermal analysis of the architecture of old and new houses at Ghadames. Build Environ 1985;20(l):39–42. [15] Al-Hamdani NI, Ahmad AI. Effect of window parameters on indoor thermal environment of buildings. Short communication. Solar Wind Technol 1987;4(1):71–5. [16] Al-ajmi FF, Loveday DL, Bedwell KH, Havenit G. Thermal insulation and clothing area factors of typical Arabian Gulf clothing ensembles for males and females: measurements using thermal manikins. Technical note. Appl Ergon 2008;39:407–14. [17] Saeed SAR. Thermal comfort requirements in a hot humid region with special reference to Bahrain. In: World renewable energy congress VI; 2000. p. 1792– 95. [18] Saeed SAR. The effect of vertical location on thermal performance of multistorey apartments in Riyadh, Saudi Arabia. Energy Build 1989;14:51–9. [19] Al-Sanea SA, Zedan MF. Optimized monthly-fixed thermostat-setting scheme for maximum energy-savings and thermal comfort in air-conditioned spaces. Appl Energy 2008;5(85):326–46. [20] Radhi H. A systematic methodology for optimising the energy performance of office buildings in Bahrain. Energy Build 2008;40(7):1297–303. [21] ASHRAE. BSR/ASHRAE standard 55P. Thermal environmental conditions for human occupancy. Atlanta: American Society of Heating, Refrigerating, and Air-Conditioning Engineers Inc.; 2004. [22] DesignBuilder Software Ltd. DesignBuilder 1.2 – user manual [online]. . [23] LBNL. WINDOWS computer software-V6.2 [online]. . [24] Radhi H. Can envelope codes reduce electricity and CO2 emissions in different types of buildings in the hot climate of Bahrain. Energy 2009;34:205–15. [25] Al-Homoud MS. Performance characteristics and practical applications of common building thermal insulation materials. Build Environ 2005;40(3):353–66.