Energy Conversion and Management 73 (2013) 37–50

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Energy Conversion and Management

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HVAC system operational strategies for reduced energy consumptionin buildings with intermittent occupancy: The case of mosques

0196-8904/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.enconman.2013.04.008

⇑ Corresponding author.E-mail address: adel@kfupm.edu.sa (A. Abdou).

I. Budaiwi, A. Abdou ⇑Architectural Engineering Department, College of Environmental Design, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia

a r t i c l e i n f o

Article history:Received 5 June 2012Accepted 2 April 2013Available online 13 May 2013

Keywords:MosquesIntermittent occupancyHVAC operationEnergy conservationThermal zoningEnergy efficiency

a b s t r a c t

Mosques are places of worship for Muslims with unique functional requirements and operational char-acteristics. They are partially or fully occupied for about an hour for five intermittent periods duringthe day. In hot climates, maintaining indoor thermal comfort requires a considerable amount of energythat can be reduced by proper operational zoning and effective HVAC operation strategies. The objectiveof this paper is to investigate the impact of operational zoning and HVAC system intermittent operationstrategies on the energy performance of mosques while thermal comfort is maintained. Energy simula-tion modeling is used for evaluating alternative zoning and HVAC operation strategies. Results indicatethat up to 23% reduction in annual cooling energy is achieved by employing suitable HVAC operationstrategy and system over-sizing, and 30% reduction is achieved by appropriate operational zoning. Com-paring the cooling energy consumption of HVAC summer continuous operation of an un-insulated mos-que with the consumption of the insulated mosque with properly oversized HVAC system operated for1 h during each prayer, indicated that as much as 46% of cooling energy reduction can be achieved. Fur-thermore, utilizing proper operational zoning and HVAC operation strategies is expected to bring aboutan additional significant energy reduction. Guidelines for mosque HVAC systems operation and thermalzoning are developed. These design/operation guidelines would provide a source of information for pro-fessionals to improve the thermal and energy performance of mosques. This knowledge can also contrib-ute towards the development of future energy-related design codes for mosques.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Mosques, places of worship for Muslims, are unique buildings interms of their function and operation. They are used intermittentlyfor performing prayers five times a day. Mosques are normallybuilt in a simple, rectangular-shaped, walled enclosure with aroofed prayer-hall. The long side of the enclosure is normally direc-ted towards Qibla (i.e., the direction of Makkah). Mosque design hasbeen influenced by regional, cultural and climatic differences.However, the development in materials and environmental controlsystems (e.g. air-conditioning) has greatly influenced contempo-rary mosque architecture. The interior surfaces of contemporarymosques are mostly finished with reflecting materials such as plas-ter or marble, and the floor is usually carpeted. Hard painted con-crete ceilings with simple to elaborate decorations are commonlyused. Electro-acoustic sound reinforcement systems have also beenimplemented in mosques of all sizes to improve the audio condi-tions in the space particularly after the introduction of air-condi-tioning systems.

In hot climates, like Saudi Arabia’s, maintaining thermal com-fort in mosques is an important functional requirement and is acritical determinant of their performance. Most mosques in suchclimates are equipped with air-conditioning systems in conjunc-tion with ceiling fans for achieving thermal comfort resulting inthe use of substantial amounts of energy. When improperly de-signed or operated, additional, unnecessary amounts of energycan be used by the air-conditioning system without achievingthe desired thermal comfort.

Assessment of thermal comfort in mosques has been the subjectof a number of studies. Saeed [1] studied the thermal comfortrequirements for Friday prayer during the hot season of Riyadh.The study reported the results of a survey conducted on peopleperforming Friday prayer which indicated that most people arecomfortable and few prefer cooler conditions. Al-Homoud et al.[2] conducted an investigation on the thermal comfort conditionsof several mosques located in the Eastern Province of Saudi Arabiawhich is characterized by a hot-humid climate. Results indicatethat thermal comfort conditions are generally not maintained dur-ing the periods of occupancy due to design and operational rea-sons. Recently, Al-Ajmi [3] presented statistical data about theindoor environmental conditions in mosques in Kuwait. An analy-sis of worshippers’ thermal comfort sensation for a total of 140

38 I. Budaiwi, A. Abdou / Energy Conversion and Management 73 (2013) 37–50

subjects was conducted and related sets of physical measurementsand subjective questionnaires were used to collect data. Resultsshowed that the neutral temperature perceived by the worshippersin the mosque was found to be 26.1 �C, i.e. 2.8 �C higher than thosepredicted by the Predicted Mean Vote (PMV) model.

Given the unique functional and operational characteristics ofmosques, it has been a challenging task to achieve thermal comfortat reduced energy consumption. Many related studies have beencarried out to investigate this relationship and explore ways toconserve energy without compromising comfort [4,5]. The impactof various energy conservation measures and HVAC system andcomponent characteristics on building thermal performanceincluding thermal comfort has been investigated. Results haveindicated that adaptation of a higher temperature set point in sum-mer can lead to a significant reduction in cooling energy withoutloss of thermal comfort [4]. Al-Mofeez and Numan [6] investigatedthe impact of a wall-mounted fan on savings of the energy con-sumption and sizing of HVAC for a bedroom. The study found thatfan application achieves significant savings in annual electricityuse, by reducing HVAC sizes and operating hours. Tanabe and Kim-ura [7] reviewed and summarized the effects of air temperature,humidity and air movement on thermal comfort under hot and hu-mid conditions with a view to energy conservation. Their studiesshowed that under hot and humid conditions, air movement mightbe one of the least expensive methods of providing thermalcomfort.

In Saudi Arabia, buildings, including mosques, consume a sub-stantial amount of energy, which reached about 76.5% of the totalelectric energy produced in the country for the year 2010 [8]. Thebulk of this energy is consumed by air-conditioning due to the pre-vailing harsh climatic conditions. Limited research has been carriedout to investigate energy performance in mosques. The thermalbehavior of a typical mosque in the state of Kuwait using VisualDOE 4.1 (building energy simulation software) was analyzed [9].The simulation model was used to assess potential energy conser-vation opportunities for future mosque design. An annual energyuse saving up to 72% was possible through improvements of themosque envelope design and operating strategies. A saving ofaround 40% by the use of life-cycle cost assessment was foundachievable with a simple payback period of less than 4 yr.

On the other hand, many studies were conducted on other typesof buildings [10–12]. Al-Saadi and Budaiwi [13] have introduced anenergy performance-based envelope design approach and investi-gated the impact of envelope thermal design of residential build-ings on energy performance. Results revealed that substantialenergy savings could be achieved under hot-humid climatic condi-tions when the proper envelope design is employed. Energy perfor-mance of different office building envelope designs under fiveclimatic zones in China was investigated [14] utilizing the overallthermal transfer value method and the heating degree-day tech-nique. Results indicated that major variations in energy require-ments were influenced by climatic conditions and thermalenvelope design. Recently, Abdou et al. [15] and Al-Homoudet al. [16] have investigated the energy consumption trends of sev-eral mosques located in the Eastern Province of Saudi Arabia byanalyzing utility bills and monitoring energy end uses. Results re-vealed that due to the prevailing harsh climatic conditionsthroughout most of the year, the bulk of energy is used by air-con-ditioning for maintaining acceptable indoor thermal quality. Al-Homoud [17] described the physical and operating characteristicstypical for intermittently occupied mosques and reported the re-sults of the thermal optimization of a medium-size mosque inhot-dry and hot-humid climates represented by the cities ofRiyadh and Jeddah in Saudi Arabia. Budaiwi [18] investigated theenergy performance and potential energy savings associated withthe influential envelope design parameters of mosques under

hot-humid climatic conditions. The exterior envelope design wasfound to be a major determinant of a mosque’s thermal and energyperformance and, consequently, considerable savings can beachieved when properly designed.

The required thermal comfort is not necessarily achieved due toimproper operation of air-conditioning systems where under- orovercooling frequently occur in many mosques and this may begreatly reflected on the comfort status of worshippers. The chal-lenge remains to achieve the required thermal comfort while con-suming the least energy. Many measures related to air-conditioning system operation and design can be considered formeeting this challenge. However, in no circumstances should com-fort be compromised for the sake of reducing energy consumption.

Air-conditioning system size and operation strategies, particu-larly for buildings with intermittent occupancy, are expected tohave a major influence on mosque thermal and energy perfor-mance. Furthermore, operational thermal zoning which is nor-mally dictated by the variable occupancy of mosques cansignificantly influence their energy performance.

The operation of an HVAC system with five functions of energymanagement control (EMC) and optimal set points was modeledand simulated [19]. The simulation results manifested energy sav-ings of 17% compared with the system without such functions. Xuet al. [20] investigated the impacts of switching off HVAC duringunoccupied hours in an office on the thermal comfort of occupants.Measurement and assessment results indicated that up to 11% ofannual energy consumption of HVAC for the office could be savedwithout thermal comfort being compromised during occupiedhours. Kima et al. [21] dynamically simulated an intermittent cen-tral heating system of a university building. The simulationshowed that by implementing on–off control, the indoor spacecould be maintained within a comfortable range with less energy.Recently, Badran et al. [22] comparatively studied continuous vs.intermittent heating in homes, and recommended that high-insu-lated apartment buildings can be operated intermittently (on/off)with the boiler at high water temperature for a maximum of 14-h a day in order to provide the same comfort conditions offeredby 24-h continuous heating with less energy. It was also indicatedthat continuous operation of the boiler at relatively low water tem-perature becomes more economical than when it is operated inter-mittently for more than 14-h per day. However, the resulting dailyprofile of indoor temperatures associated with each strategy alongwith thermal comfort assessment was not given. It should be notedthat the heating system in the modeled home was automaticallyset to respond (on/off scheme) to the space heating requirementsin contrast with a predefined scheduled intermittent operation ofthe HVAC system in the case of mosques. It is also worth mention-ing that in homes maintaining thermal comfort is continuously re-quired throughout the day, whereas in the case of mosques,comfort is required to be maintained within scheduled five periodscoinciding with the prayer times during the day. One should alsotake into account the differences in occupancy, and lighting useprofile in homes compared to mosques.

Optimization methods of HVAC system and control strategies tominimize energy use while maintaining comfort, with a specialemphasis on the control of humidity in commercial buildings weredescribed. Results indicated that minimum energy use typically oc-curred at low airflow rates, with indoor humidity levels below theupper comfort limit [23]. A multi-step systematic approach to for-mulate and verify the energy performance model for a representa-tive commercial building was described [24]. The impact of energyconservation measures (ECMs) on energy savings and thermalcomfort status was investigated. Major energy savings of about25% can be achieved with a combination of various HVAC opera-tion strategies in commercial buildings provided the HVAC systemis properly selected and operated, ensuring the occupants’ comfort

I. Budaiwi, A. Abdou / Energy Conversion and Management 73 (2013) 37–50 39

and preserving environmental quality. In order to have a broaderappreciation and better understanding of the relative impact ofthese parameters, further investigation of HVAC system designand operational strategies as well as thermal zoning is required.The objective of this study is to investigate the impact of HVAC sys-tems operation on energy consumption in mosques in order to ex-plore potential energy savings while maintaining acceptableindoor thermal quality for occupants’ comfort.

2. Methodology

Energy simulation modeling is a powerful and economical toolfor evaluating the energy performance of alternative systems de-signs and operational strategies. In this study, energy simulationmodeling is used to investigate the impact of operational zoningand HVAC systems operation strategy on energy consumption inmosques. In order to develop a realistic model for the mosque, ac-tual physical, thermal and operational characteristics of typicallybuilt mosques in the Eastern Province of Saudi Arabia were col-lected. A comprehensive survey of 132 mosques was conducted.The mosques were then classified into six categories according tothe specific criteria such as the mosque’s type of use (i.e. five dailyprayers vs. Friday’s prayer and its preceding speech); floor area;full capacity (persons); aspect ratio of the floor plan and type of

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HVAC system used (e.g. central, packaged, wall- or floor-mountedunits, fan-coil units, window units). The most commonly usedHVAC systems were a simple fan-coil system or packaged system.A detailed energy audit including monitoring of energy consump-tions was conducted for three types of mosques. Base case modelsrepresenting commonly used mosques with potential energy sav-ings were developed based on surveyed results. They were attunedto make their predictions comparable to the actual energy con-sumption behavior.

The HVAC system is not necessarily operated intermittently inall mosques; usually for central HVAC, it is continuously operating.When a unitary system is used, it is normally operated in an inter-mittent manner. In such cases, in reality, the start and end of theHVAC operation usually occurs haphazardly, which may notachieve thermal comfort while reducing energy consumption. En-ergy savings while maintaining thermal comfort can be realizedthrough many design and system operation/control strategiesranging from simple operational ones (e.g., operation scheduling)to more complicated system control strategies (e.g., fuzzy logic).Considering the HVAC systems characteristics of the surveyedmosques and the available maintenance and operational support,only practical and implementable strategies are considered forassessment. In this study, the impacts of operational zoning andoperation strategies of HVAC system on energy consumption whilemaintaining acceptable thermal comfort are investigated.

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2.1. Preparation of simulation tool

The selection of a suitable building energy simulation tool canbe a challenging task considering the wide range of options andcapabilities characterizing available models. However, when theobjectives are clear and the level of accuracy and detailing is iden-tified, options for selecting a suitable energy tool will be limited.

Most available detailed energy simulation models are based ona 1-h time step. The recently developed EnergyPlus software is anexception with a simulation time step of less than an hour. Giventhe unique operational characteristics of the mosque in whichsome thermal events (e.g. occupancy) are repetitive (five times aday) and may last for less than an hour with considerable varia-tions, a modeling tool with a simulation time step of less than1 h seems to be more appropriate. However, considering the rela-tively small contribution of these thermal events to the overallthermal load because of their short duration and the high thermalinertia of the space, an accurate account of their variations withinthe hour is not expected to have a significant impact on the overallthermal load and subsequently on the accuracy of energy perfor-mance prediction provided that the thermal load of the event isappropriately described. Therefore, an energy simulation toolbased on a 1-h time step is considered appropriate for modelingthe thermal behavior of the mosque.

In light of the above, and taking into account the current re-search objectives, the unique operational characteristics of thebuildings under investigation (i.e. mosques) as well as other factorspertaining to reliability of results and previous experience with thesoftware, the VisualDOE building energy simulation program wasselected. It has been widely validated for accuracy and consistency.VisualDOE is an easy-to-use ‘‘front end’’ interface to DOE2.1E (fromLawrence Berkeley National Laboratory) offering a great capabilityto accurately estimate the performance of building design alterna-tives, and to simulate a wide range of design features and energyconservation measures. It also covers all major building systemsand the building envelope with hourly simulation results for de-tailed analysis [25].

To verify the reliability of the simulation tool and modelingassumptions, simulated and actual energy consumptions of differ-ent existing mosques are compared. Fig. 1a and b illustrates a com-parison between measured monthly energy consumption and

Fig. 2. Geometrical configuration of comm

model prediction for two existing mosques of different sizes withdifferent HVAC systems under hot-humid climatic conditions. Itcan be seen that modeling and simulation results are quite reason-able and fairly accurate. Variations of measured and predicted tem-perature for the two mosques during a summer day (15th July)relative to a specified comfort temperature range is shown inFig. 1c and d. In determining the comfort range i.e. from 23.0 to26.5 �C, occupants are considered to be performing light activitiesand dressed in light summer clothing. Additionally, occupants areassumed to stay in the mosque for sufficient periods of time so thata steady state model can be applicable. Examination of Fig. 1c andd reveal that in spite of the noticeable deviations during the unoc-cupied periods the predicted and measured temperature values arefollowing the same trend and closely match during the operationof the air-conditioning system. The temperature measurementsand the fine-tuning process of the base model and its detailed in-put were described and elaborated upon in a recent study [26].The study explained the match of the produced results close tomeasurements considering the sensitivity of results to climaticconditions.

The ability of the model of building energy simulation to accu-rately predict energy performance depends largely on the accuracyof the weather data. All DOE-2 based building energy simulationprograms require detailed hourly weather data for a full year. Aformatted weather data file for Dhahran, Saudi Arabia for the year2002, representing the hot-humid climatic conditions was devel-oped utilizing available weather data obtained from local weatherstations. The weather data of Dhahran (Lat. 26�27’N, Long. 50�17’E,17.0 m above sea level) were used as representative of typicalweather variations in the Eastern Province of Saudi Arabia. Dhah-ran’s climate is characterized by hot and humid summers (i.e.43.3 �C dry-bulb temperature with 22.9 �C mean coincident wet-bulb design temperature), and mild cool winters (9.0 �C dry-bulbdesign temperature). The annual heating and cooling degree-daysat 18.3 �C base temperature are 205 and 3284 �C-days respectively(ASHRAE Fundamentals 2009). The weather data for the year 2002representing the hot-humid climatic conditions and matching themonitoring year of sample existing mosques were used. The usedweather data was intentionally prepared for the year 2002 sincethe measurement data of energy consumptions and indoor tem-peratures of the year 2002 were available to the authors. It was

only built mosques in Saudi Arabia.

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Fig. 3. Typical daily example of occupancy periods, lighting, and air infiltrationschedules during weekdays for the summer (Jun, July, August) season.

I. Budaiwi, A. Abdou / Energy Conversion and Management 73 (2013) 37–50 41

advantageous to utilize such data in fine-tuning the base model[26] and this accordingly contributed to explaining the match inpattern and magnitude between predicted and measured databased on the data for the same year.

2.2. Model development

Mosques, in general, have simple geometrical configurationsbut vary in size. Fig. 2 shows the geometrical configuration andzoning schemes of the different types of mosques that are com-monly built in Saudi Arabia. Type-I represents a small-size dailyprayer mosque with a floor area of 218 m2. Type-II is a larger size(506 m2 floor area) daily prayer mosque which is assumed to bepartially occupied during some prayers. Type-III represents a Fridayprayer mosque with a floor area of 1052 m2 and is partially occu-pied during all prayers except Friday prayer when it is fully occu-pied. In order to accommodate the partial occupancy of Type-IIand Type-III mosques and investigate the impact of operationalzoning on energy consumption, two zoning schemes of each typewere modeled. For all mosque types, thermal and operational char-acteristics are similar.

2.3. Base case model development

A base case model for each of the above types of zoningschemes was developed. Because of similarity in the thermal andoperational characteristics of the modeled mosque types, Type-Iwill be taken as a base model for conducting and investigatingthe impact of HVAC operations strategies on energy consumptionand indoor thermal conditions. Results from Type-I mosque wereutilized to establish the base cases for Type-II and Type-III whichare devoted to investigating the impact of operational zoningschemes on energy consumption.

For the purpose of this study, the input data include informa-tion about the physical and thermal properties of the building,the performance and operational characteristics of the air-condi-tioning systems and the pattern of occupancy. Information relatedto loads includes: mosque orientation and geometrical configura-tion; wall, roof and window areas and thermal properties; lighting,equipment and other energy systems loads and operating profiles;infiltration/ventilation rates and profiles; and occupancy numberand operating profiles. Information regarding the air-conditioningsystem includes: type, performance and operation characteristicsof air-conditioning systems. Input data were based on information

Table 1Thermal and operational characteristics of the base case model of the mosque.

Component Type-I mosque Type-II mosque

Air-conditioned floor area (m2) 218 506Window area (m2) 25 50Floor-to-ceiling height (m) 3.8 5.5Wall U-value (W/m2 �C) 2.41 0.66Thermal insulation placement Un-insulated Thermal insulatRoof U-value (W/m2 �C) 1.93 0.5Window type Clear single glazingWindow shading Movable interiorLighting power density (W/m2) 6.5Equipment power density (W/m2) 5.5Air infiltration rate (ACH) 1.0 (during occupied periods)Maximum occupancy (person) 218 506Type of HVAC system ResidentialHVAC operation Continuous during summer IntermittentTotal cooling capacity (kW) 35.5 96Type of cooling coil DX (Direct Expansion)Type of fan Constant, high efficiencyHeating capacity NoneCooling temperature (�C) 24 (Summer), 21 (Winter)

obtained from detailed auditing of existing mosques and relevantliterature. Thermal and physical characteristics of the modeledmosque are given in Table 1. For the base model of Type-I, the exte-rior envelope is kept un-insulated and HVAC is assumed to operatecontinuously.

In relevant studies, the air leakage rate in air-conditionedhomes in hot-humid climate was found to range from 0.5 to 1.0ACH (change per hour) [27]. The model mosque is assumed at aver-age air tightness with an air leakage rate of 1.0 ACH during occu-pancy as door operation continuously introduces outdoor air tothe air-conditioned space. During unoccupied periods, the air infil-tration rate is assumed at 50% of the value during occupancy sinceall doors are closed and air infiltration can take place only throughcracks and openings around windows and doors. An air infiltrationrate of 0.5 ACH is used to represent a relatively tight enclosure.However, when considering the relatively large volume of the mos-que compared to the area of its exterior envelope, this air infiltra-tion level can be justified.

Prayer times vary throughout the year as they are dependent onsolar time. In order to simulate variations in loads and schedulingtime for the different thermal events, the year is segmented intofour periods, each 3 months long, which roughly represent sea-sonal variations. The first period includes the months of December,January and February, the second period includes the months fromMarch till the end of May, the third period represents the summerperiod starting in June and ending in August and finally, the fourth

Type-III mosque

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ion is placed on the inside surface of the walls behind interior gypsum boards0.5

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42 I. Budaiwi, A. Abdou / Energy Conversion and Management 73 (2013) 37–50

period is from September until the end of November. For the sim-ulated year (i.e. 2002) the month of ‘‘Ramadan’’ mainly coincidedwith November. Consequently, special prayers were included inthe scheduling. The prayer times during each period are scheduledin accordance with the total occupancy period which correspondsto the 15th day of the middle month of each period.

Occupancy is an important input parameter for a building’s en-ergy simulation. Mosques have a unique occupancy schedule as itrandomly varies within the hour and differs from one prayer to an-other. It is, therefore, very difficult to accurately define an occu-pancy profile which characterizes the exact variations inoccupancy load and patterns and considers the incremental in-crease in the number of occupants from the call to prayer (Azan)until the occupants leave the mosque. Since the utilized simulationtool can consider only constant values during a full hour period, areasonable approximation is to devise a representative adjustedoccupancy profile that considers variations in the maximum occu-pancy load and the occupancy periods, and takes into account theless than 1-h duration of the different daily prayers. A weighteduniform occupancy percentage was used for each prayer accordingto the following formula.

OP ¼ 0:5PD=60 Nmax

where OP is the occupancy percentage (%), PD the prayer duration(min); calculated from Azan time until occupants leave the mosque,Nmax is the maximum number of occupants for a particular prayercalculated as a percentage of mosque capacity.

The formula assumes that the average variation of occupancyover the prayer duration is 50% of the maximum occupancy.

The lighting schedule is assumed to coincide with the occu-pancy schedule. The percentage of the lighting power for eachprayer is determined by the duration of the prayer and the per-centage of lamps turned on compared to the total number of lampsavailable in the mosque as identified by the audit process. Fig. 3illustrates typical daily scheduling schemes for occupancy, infiltra-tion and lighting. A single HVAC system (i.e. residential system)with a Coefficient of Performance (COP) of 2.7 was assumed to con-trol each zone instead of the actually used multi-system as the pro-gram modeling approach does not support the use of more thanone system for a single zone. The residential system is the simplestand is selected as it represents the closest system to the most com-monly used ones in mosques including window air conditioners,

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split systems and fan-coil systems where no outside air isprovided.

Other potentially energy-efficient systems that are based ondisplacement ventilation where air is introduced at low velocitythrough horizontal low-velocity supply or floor-mounted diffusersand radiant cooling are not considered in this study. A displace-ment ventilation system is preferable for situations where the spe-cific airflow rate per unit of floor area is high (as in offices, lobbies,theatres, conference rooms, and industrial applications). These sys-tems are rarely used in buildings in Saudi Arabia, and are not suit-able to be used in mosques. In mosques worshippers are eitherstanding very closely or seated in continuous rows (i.e. 1.2 mapart) directly on the floor carpet. Additionally, since cool air issupplied directly to the occupied zone, there is a high potentialfor draught risk close to the units (the near zone) which in turnlimits suitable locations of the wall and floor units with regard tothe floor-seated worshippers.

Similarly, the use of radiant cooling in hot-humid climates isnot recommended since there is a need to continuously removethe high moisture load generated by worshippers in addition tothat moisture added to the indoor space through infiltration. Nat-ural ventilation without combined control is problematic for radi-ant cooling in humid climates due to condensation [28]. Themodeled HVAC system and suggested measures are based on ac-tual or potential HVAC systems to be used in mosques taking intoconsideration the applicability, typical operation, maintenance andenvironmental constraint that may limit utilizing other systems.

For determining the level of indoor thermal comfort, air tem-perature and humidity can be considered the two most importantand indicative parameters. The impact of other important parame-ters such as air velocity and mean radiant temperature (MRT) canalso contribute to thermal comfort conditions. It should be noted,however, that a comprehensive thermal comfort analysis of mos-ques is not intended as part of this study, but rather to obtain a fairidea about thermal comfort conditions as related to the proposedHVAC operation strategies. Considering the relative importanceof the air temperature in determining comfort, while recognizingthe importance of other parameters, particularly air velocity, thequality and condition of thermal comfort can be fairly assessedbased on air temperature assuming that normal air movementand air humidity are maintained within the space. Air movementpatterns within an existing mosque are influenced by many factors

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Fig. 5. Predicted indoor air temperatures for the new mosque design Type I (a) when HVAC operation precedes occupancy and (b) when HVAC is operated for 1 h for eachprayer and equipment is oversized.

I. Budaiwi, A. Abdou / Energy Conversion and Management 73 (2013) 37–50 43

determined by the type and location of the HVAC outlets, ceilingfans type, installation and operation scheme. This would result ina complicated pattern that cannot be quantified using the utilizedsimulation tool which provides only a daily profile of predicted airtemperature for an insight assessment. In the simulation tool, thezone temperature is predicted from the air heat balance withinthe air mass and interior components. The model assumes the tem-perature is perfectly mixed uniformly within the space volume. Itpredicts the indoor hourly temperature at a given time based onthe history of the space indoor temperature and the different com-ponents of the space thermal load.

The comfort temperature range taken into account in this studyis from 20 to 24 �C during the winter season while it is determinedfrom 23 to 26.5 �C during the summer, autumn and spring seasons.This thermal comfort range is determined according to ASHRAEStandards 55 [29] considering occupants engaged in light activitiesand dressed in light summer clothing. Additionally, occupants areassumed to stay in the mosque for sufficient periods so that a stea-dy state model can be applicable. The comfort range is bounded bya relative humidity limit of 30–60% for the four seasons of winter,spring, summer, and autumn. Al-Homoud et al. [2] presented theresults of a study designed to monitor energy use and thermalcomfort conditions of a number of mosques in a hot-humid climate

so that both energy efficiency and the quality of thermal comfortconditions especially during occupancy periods in such intermit-tently operated buildings can be assessed accurately. The relativelyhigh energy use for some mosques was not necessarily translatedinto better thermal comfort conditions. Thermal comfort was ob-served not to be achieved in most of the investigated mosquesespecially the un-insulated ones during times of peak thermalloads. Acceptable thermal comfort conditions can be greatly en-hanced by using mosque envelope thermal insulation due to theirskin-load dominated load, especially during the long unoccupiedperiods, as well as their intermittent operation.

3. Results and discussion

Energy performance of the modeled mosque types and associ-ated variations in indoor air temperature at various air condition-ing system designs and operational strategies are investigated.

3.1. Impact of HVAC system operation and size

The HVAC operation and size are crucial in determining energyrequirements and thermal comfort conditions. The HVAC operation

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65% 45% *

(-36% )

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%*

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+

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(-46%)

A/C operation

Fig. 6. Annual cooling energy requirements using various HVAC strategies for un-insulated and insulated mosque.

44 I. Budaiwi, A. Abdou / Energy Conversion and Management 73 (2013) 37–50

is assumed to operate continuously during the summer seasonwhich is deemed necessary if thermal comfort is to be achievedwith the HVAC equipment size determined by the building peakthermal load. With a total cooling capacity of 35.5 kW for an un-insulated mosque of Type-I, the HVAC equipment was able toachieve the set temperature all the time when operated continu-ously over the summer period as indicated in Fig. 4. However,operating the HVAC continuously may not be justifiable consider-ing the mosque operation and the high amount of energy needed. Adramatic decrease in cooling energy can be achieved when theHVAC system is intermittently operated for 1 h for each prayer.Nevertheless, with the equipment size based on peak space coolingload, thermal comfort was not achieved during any prayer as indi-cated in Fig. 4.

By allowing the HVAC operation to precede occupancy for a cer-tain number of hours, thermal comfort is achieved as shown inFig. 5a but with much less reduced energy. Limiting the HVACoperation to 1 h for each prayer is the most effective strategy in

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Fig. 7. Predicted indoor air temperatures during a normal day (15th July, 2002) for theoversized equipment.

reducing energy. On the other hand, in order to achieve comfortduring the 1-h operation, HVAC equipment needs to be suitablyoversized to increase its cooling capacity and heat extraction rate.When the HVAC equipment is oversized by 45% (total capacity of51.5 kW), thermal comfort is achieved during all prayers as seenin Fig. 5b at slightly higher cooling energy requirements than the1-h operation and at much reduced energy consumption comparedto continuous operation. Although an extra initial cost is associatedwith over-sizing of the equipment, intermittent operation is em-ployed in order to achieve thermal comfort during short operationperiods during each prayer time (i.e. short occupancy events sepa-rated by long unoccupied periods). The purpose of equipmentover-sizing is to provide sufficient extra cooling capacity to over-come the thermal load in a short period. During summer, theequipment needs to deal with accumulated load, so it is expectedto work at full capacity most of the time except for at certainprayer times e.g. Fajr prayer. During moderate climatic conditions,the equipment operates under partial load for a short time com-

19 20 21 22 23 24

single zone of mosque design Type II with 1-h HVAC operation for each prayer and

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Fig. 8. Predicted indoor air temperatures during a normal day (15th July, 2002) for (a) front zone and (b) back zone of mosque design Type II.

I. Budaiwi, A. Abdou / Energy Conversion and Management 73 (2013) 37–50 45

pared to continuous operation. This can be avoided by installingmultiple units with the total oversized cooling capacity. All willoperate only at peak load to achieve thermal comfort.

Fig. 6 compares cooling energy requirements for the variousHVAC operation strategies. It can be seen that for the un-insulatedmosque when HVAC operation precedes occupancy to achievethermal comfort, only about 17% reduction in cooling energy isachieved. On the other hand, when the HVAC equipment is over-sized by 45% but operated for 1 h during each prayer, a reductionof about 36% is achieved. The same relative reductions of the var-ious HVAC strategies are obtained when the mosque is insulated,as also shown in Fig. 6.

For the insulated mosque, the maximum reduction of about 23%in cooling energy consumption is achieved while maintaining ther-mal comfort when the HAVC equipment is oversized by 65% (totalcapacity of 43 kW) and the system operates for 1 h during eachprayer while only a 13% reduction is achieved when the system’soperation precedes occupancy. The indicated percentage of 65%over-sizing in the case of a small-size single zone insulated mosque

is relative to the peak cooling load design based on equipment siz-ing which was found to achieve thermal comfort during all prayerperiods. The same applies for the 45% over-sizing of the un-insu-lated mosques. Since in the case of the insulated mosque the basesize of the HVAC equipment (based on peak cooling load) is smallerthan that corresponding to the un-insulated mosque, the actual in-crease of the equipment size is expected to be smaller because ofthe presence of thermal insulation.

It is obvious that the impact of HVAC strategy on energy reduc-tion has diminished when the mosque is insulated. Comparing theenergy consumption of HVAC summer continuous operation of anun-insulated mosque with the energy consumption of the insu-lated mosque when the HVAC system is oversized and operatedfor 1 h during each prayer, it can be concluded that as much as46% of cooling energy reduction is achieved. The resulting energyperformance index of the insulated mosque with oversized HVACequipment operating 1 h during each prayer is about 71 kW h/m2/yr compared to 131 kW h/m2/yr for the un-insulated mosquewith continuous HVAC operation.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 241 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Fig. 9. Predicted indoor air temperatures during Friday for (a) the single zone, (b) front zone (1), and (c) back zone (2) of mosque design Type II.

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Fig. 10. Annual cooling energy requirements for different operational zoningschemes for mosque design Type II.

46 I. Budaiwi, A. Abdou / Energy Conversion and Management 73 (2013) 37–50

3.2. Impact of operational thermal zoning

Large-size mosques, particularly those designed to accommo-date Friday prayers, are likely to be partially occupied during mostprayers. This offers a major opportunity to reduce energy con-sumption by zoning the mosque and appropriately designing theHVAC system so that part of the mosque is operated during periodsof reduced occupancy. The zones were separated by a light parti-tioning wall system (i.e. drywall with gypsum boards on bothsides) with glazed areas and light doors of adequate sizes. Thismeans that heat transfer from adjacent unused (non-conditioned)zones with higher indoor temperature is accounted for during thesimulation period. To investigate the impact of thermal zoning onenergy consumption, the two mosque types, Type-II and Type-III,with their thermal, physical and operational characteristics are de-scribed in Table 1 and Fig. 2. In order to appropriately compare theimpact of zoning, thermal and operational characteristics were ad-justed to ensure minimum variations in thermal loads of the differ-ent zoning schemes of the same mosque type. Window area andlighting, infiltration and occupancy schedules are accordingly ad-justed. Both types of mosques are operated for 1 h during eachprayer and served by appropriately oversized HVAC equipment.The HVAC equipment size is selected for each zoning scheme toprovide comfortable conditions during summer for all prayers inthe operated zones. Because of variations in thermal zones, differ-

ent HVAC equipment sizes need to be identified to ensure thatthermal comfort conditions are achieved during the 1-h operation.Since mosque Type-II does not operate on Fridays, the Asr prayeron Friday presents a critical time that may determine the equip-ment size needed to achieve thermal comfort. Figs. 7 and 8a andb illustrate thermal comfort status during a normal day (i.e. any

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Fig. 11. Predicted indoor air temperatures during Friday for (a) the single zone, (b) front zone (1), (c) middle zone (2) and (d) back zone (3) of mosque design of Type III.

I. Budaiwi, A. Abdou / Energy Conversion and Management 73 (2013) 37–50 47

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Fig. 12. Requirements of annual cooling energy for different operational zoningschemes for mosque design Type III.

48 I. Budaiwi, A. Abdou / Energy Conversion and Management 73 (2013) 37–50

day of the week except Friday) for the single zone scheme and ineach zone for the two-zone scheme of Type-II mosque whenappropriate equipment sizes are selected. Fig. 9a–c illustrate ther-mal comfort status for the same zones on Friday during the sum-mer period. In all cases, thermal comfort is achieved during allprayers using appropriately sized HVAC equipment. For the sin-gle-zone mosque, the HVAC equipment is oversized by 35% to havea capacity of 96 kW. However, for the two-zone scheme of thesame mosque, the HVAC system is oversized by 50% for the frontzone and 60% for the back zone resulting in a total cooling capacityof 108.5 kW, which is only 13% more than the single-zone capacity.By implementing the two-zone scheme for a Type-II mosque, areduction of about 13% in cooling energy requirement is achievedas illustrated in Fig. 10.

The energy performance index of mosque Type-II is about65 kW h/m2 yr for the single-zone operation scheme compared toaround 56 kW h/m2 yr for the two-zone operation scheme. This le-vel of reduction can be justified when considering the fact that thedifference in operation between the single zone scheme and thetwo-zone scheme is limited to three prayer periods on normal daysand to two prayer periods on Friday while having identical opera-tion when fully occupied during Maghrib and Isha prayers as as-sumed in the mosque operation scheme.

A more significant reduction in the cooling energy requirementcan be achieved by operational zoning when the mosque is oper-ated on a reduced partial occupancy for longer periods. Large-sizemosques, designed for Friday prayer, are partially occupied most ofthe time with a very low percentage of occupancy relative to theirmaximum capacity, consequently representing greater potentialfor energy reduction by operational zoning. Type-III mosque repre-

Table 2Suggested lead and operation hours of the HVAC in the investigated types of mosques for

Pray

Mosque type Fajr

Type-I: Small size: ‘‘Daily Prayers’’ single zone [1]0

Type-II: Medium size: ‘‘Daily Prayers’’ single zone [1]0

Type-II a: Medium size: ‘‘Daily Prayers’’ two zones Front [1]0

Back NAType-III: Large size: ‘‘Friday Prayer’’ Single zone [1]0

Type-IIIb: Large size: ‘‘Friday and Daily Prayers ’’ three zones Front NAMiddle NABack [1]0

Note: [1]0 [Number] = TotalSuperscript = Num

a Front zone is operated for all prayers. Back zone is operated during Maghrib and Ishb Type-III: Back zone is operated for all prayers. Middle zone is operated during Magh

sents a Friday prayer mosque with a maximum of 66% of its capac-ity used during daily prayers while the remaining 33% of itscapacity is used once a week during Friday prayer. The thermaland energy requirements of the mosque were investigated basedon a single zone scheme and a three-zone scheme according tothe configuration in Fig. 2. For all zoning schemes, the mosque isoperated based on 1-h intermittent operation under oversizedHVAC equipment for most prayers throughout the year. However,in order to achieve thermal comfort during Friday prayer, HVACoperation hours were additionally adjusted. For the single zonemodel, the HVAC equipment is oversized by 25% to have a totalcapacity of 213 kW and an additional 1-h HVAC operation is re-quired for Friday prayer. An oversized equipment of 35%(61.5 kW) and an additional three HVAC operation hours arerequired for the front zone which is used only during Fridayprayer.

Both the middle and back zones require 40% additional coolingcapacity (57.5 kW for the middle zone and 114 kW for the backzone) as well as 1-h additional HVAC operation during Fridayprayer to achieve thermal comfort. The total capacity of thethree-zone model consequently amounts to about 233 kW, whichis only 9% more than the total capacity of the single zone model.

With the above design and operational measures, thermal com-fort is achieved during Friday prayer as well as other prayers on thesame day for the single zone mosque as shown in Fig. 11a and in allzones in the three-zone mosque as indicated in Fig. 11b and c. TheFriday corresponding to July 14 is selected as it represents a daywith maximum thermal load. For the three-zone operationalscheme, thermal comfort is achieved as illustrated in Fig. 11d ata substantially reduced cooling energy requirement. The requiredannual cooling energy is reduced by about 19,400 kW h represent-ing 30% savings when compared with the single-zone energyrequirements as depicted by Fig. 12. The resulting energyperformance index of the three-zone model is 42.5 kW h/m2 yrcompared to 56 kW h/m2 yr for the two-zone scheme of Type-IImosque.

3.3. Guidelines for energy-efficient HVAC operation

Although various HVAC control strategies are potential mea-sures for improving energy efficiency and comfort for intermittentoperation of HVAC systems, the scope of the study is to examinesimple HVAC system controls e.g. start time, ON/OFF which canbe practically implemented (automated) in view of the availableoperation and maintenance support. It has been shown that con-siderable energy reduction can be achieved in mosques whilemaintaining thermal comfort when proper HVAC operation strate-gies and appropriate zoning are implemented. Based on simulationresults, the following guidelines are recommended for HVAC oper-

achieving thermal comfort.

er’s name

Dhuhr Asr Maghrib Isha Friday prayer and speech

[3]2 [3]2 [3]2 [1]0 NA[3]2 [3]2 [2]1 [1]0 NA[3]2 [3]2 [3]2 [1]0 NANA NA [3]2 [2]1 NA[2]1 [2]1 [1]0 [1]0 [3]2

NA NA NA NA [5]4NA [4]3 [1]0 [1]0

[3]2 [2]1 [2]1 [1]0

number of hours of HVAC operationber of hours of HVAC operation preceding the start of prayer

a prayers.rib and Isha prayers. Front zone is operated for Friday prayer.

I. Budaiwi, A. Abdou / Energy Conversion and Management 73 (2013) 37–50 49

ation and thermal zoning strategies for mosques designed for hot-humid climatic conditions:

� Operate the HVAC system intermittently but its operationshould sufficiently precede occupancy when the HVAC systemis sized according to peak cooling load in order to achieve ther-mal comfort during prayer times. The lead time is dependent onthe prayer time, mosque size and zoning. Table 2 presents sug-gested lead time and total operation hours during the differentprayers in the three types of mosque designs.� Operate the HVAC intermittently for 1 h during the prayer time

when it is properly oversized to achieve thermal comfort duringoccupancy. The recommended mosque HVAC equipment over-sizing depends on mosque size and zoning. Oversized designsof 65%, 50% and 25% are recommended for small-, medium-and large-size mosques, respectively.� Implement proper operational thermal zoning especially in

‘‘Friday’’ and large ‘‘Daily’’ only mosques, where a small portionof the mosque is frequently used, and design the HVAC systemaccordingly.� Provide efficient HVAC system control that will allow easy oper-

ation of certain parts of the mosque as needed without the needto turn on all units at all times or the need to operate them oneby one.� Use timers to control the ‘‘On’’ and ‘‘Off’’ operation of the HVAC

system.� Utilize low-height air supply system (i.e. supply air outlets are

low in height) in order to avoid providing conditioned air tothe volume high above the worshippers. This in turn directlyinduces conditioned air to the occupied zone, hence requiringless energy to achieve thermal comfort.

4. Conclusions

Significant reduction in a mosque’s annual energy consumptioncan be achieved while maintaining thermal comfort when the air-conditioning system is properly operated and designed. Further-more, the operational zoning of mosques, when carefully consid-ered in the design stage and appropriately implemented, can leadto a significant reduction in energy consumption and to consider-able enhancement of a mosque’s thermal comfort conditions. Areduction of about 36% in cooling energy consumption is achievedfor the un-insulated mosque and 23% for the insulated mosque,while maintaining thermal comfort, when the HVAC system isappropriately oversized and operated intermittently for 1 h duringeach prayer compared to continuous operation during the summerseason. This results in an energy performance index of about71.0 kW h/m2 yr for an insulated mosque compared to131.0 kW h/m2 yr for the same mosque un-insulated with continu-ous HVAC operation. In order to achieve the desired thermal com-fort conditions with intermittent operation, the HVAC equipmenthas to be appropriately oversized or its operation should precedeoccupancy. A properly oversized HVAC system with 1-h intermit-tent operation during each prayer can achieve thermal comfort atreduced energy requirements compared to the continuous opera-tion in summer. Moreover, mosque operational zoning can bringabout additional and significant reduction in the annual coolingenergy requirement (up to 30%, Type III) while maintaining accept-able thermal comfort conditions when carefully considered in thedesign stage and well implemented. The level of energy reductionsas a result of mosque operational zoning is more pronounced inFriday and large mosques with partial daily occupancy comparedto medium- and small-size mosques. When HVAC intermittentoperation is combined with the appropriate operational zoningstrategy, it is expected that greater savings (i.e. >46%) in coolingenergy can be achieved. Although the study was conducted for

mosques designed for operation in the hot-humid climates of theeastern region of Saudi Arabia, the above conclusions can be appli-cable to mosques in general. However, the relative effectiveness ofthe findings may vary according to the mosque type, size, opera-tion, and location.

Acknowledgements

This paper is part of Project Grant No. AT-13-18 funded by KingAbdul-Aziz City for Science and Technology (KACST), Saudi Arabia.The financial support of KACST as well as the support and facilitiesprovided by King Fahd University of Petroleum and Minerals(KFUPM) are highly appreciated. Thanks are also due to ProfessorM. S. Al-Homoud at KFUPM for his valuable input during the mod-eling and simulation phase of the study.

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