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Strategies for reducing energyconsumption in existing office buildingsT. Itani a , N. Ghaddar a & K. Ghali aa Department of Mechanical Engineering , American University ofBeirut , Beirut , LebanonPublished online: 17 Nov 2011.

To cite this article: T. Itani , N. Ghaddar & K. Ghali (2013) Strategies for reducing energyconsumption in existing office buildings, International Journal of Sustainable Energy, 32:4, 259-275,DOI: 10.1080/14786451.2011.622765

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International Journal of Sustainable Energy, 2013Vol. 32, No. 4, 259–275, http://dx.doi.org/10.1080/14786451.2011.622765

Strategies for reducing energy consumption in existingoffice buildings

T. Itani, N. Ghaddar* and K. Ghali

Department of Mechanical Engineering, American University of Beirut, Beirut, Lebanon

(Received 5 July 2011; final version received 8 September 2011)

This work establishes viable low-cost building system energy conservation measures (ECMs) while main-taining thermal comfort and good indoor air quality for existing office buildings in Mediterranean climate,taking an eight-storey office prototype building for analysis. The low-cost ECMs explored include raisingindoor temperature cooling set point, lighting control, air economizer, night purging, and utilization ofcondensate drain to cool the direct expansion unit condenser to improve the overall system coefficientof performance. A standard system audit methodology and advanced energy modelling techniques wereused to replicate the building base case. It is found that the building electrical energy consumption canbe reduced by more than 16% energy without compromising thermal comfort when implementing thelow-cost ECMs with minimal disruption and installation cost on existing building systems.

Keywords: energy conservation strategies; night purging; condenser evaporative cooling

Nomenclature

AHU air-handling unitCAV constant air volumeCOP coefficient of performanceCp specific heat coefficient at constant pressure (J/kg K)DX direct expansionh enthalpy (J/kg)hFG latent heat of evaporation (kJ/kg)mC condensate drain water flow rate (kg/s)RH relative humidity (%)T temperature (◦C)Tcond−air adjusted ambient dry bulb temperature (◦C)Vair−condenser condenser intake air flow rate (m3/s)W humidity ratio (kg H2O/kg of air)�W humidity ratio difference (kg H2O/kg of air)

*Corresponding author. Email: farah@aub.edu.lb

© 2013 Taylor & Francis

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Greek

ρ density (kg/m3)

μ efficiency (%)

Subscripts

a airadj adjustedc condensatecc cooling coilcon condenser inlet airdb dry bulbFR fresh airpv water vapour

1. Introduction

Building retrofits and implementation of energy conservation measures (ECMs) can be cost-effective in reducing energy consumption in buildings. For example, Carriere et al. (1999) showedthat changing building heating ventilation and air conditioning (HVAC)-operating strategies canresult in savings through reduced equipment sizes as a consequence of peak reductions. In addi-tion, Fuller and Luther (2002) have shown that ECMs often provide better indoor air quality andPlympton and Conway (2000) have shown that ECMs enhance productivity. Buildings are respon-sible for approximately 42% of the world’s total annual energy consumption (US Department ofEnergy 2005). A study by Clarke (1993) estimated that nearly 30% of building energy consump-tion for heating and cooling could be saved by energy conservation and/or sustainable buildingdesign and operation. Mathews et al. (2001) have shown that air conditioning is responsible fora substantial share of energy use (50%). Thus, energy efficiency of air-conditioning systems isclearly of global importance.

The energy sector in Lebanon is totally dependent on imported oil products and energy demandis forecasted to increase with around 70% of total electric energy supplied consumed by the resi-dential building sector (Houri and Ibrahim-Korfalli 2005, Dagher and Ruble 2010, El-Fadel et al.2010, Ruble and Dagher 2011). The average total demand for electricity in 2009 in Lebanon wasestimated to range between 2000 and 2450 MW. According to Houri and Ibrahim-Korfalli (2005),the average total energy demand was 15,000 GWh of which 3478 GWh is from self or backupgeneration using diesel generators which emit a significant amount of CO2. Any improvementin the building energy consumption would result in a significant impact on the electrical powerconsumption. Existing buildings present a great potential to reduce energy demand. A survey con-ducted by the Central Administration of Statistics (1996) has shown that the number of buildingsuntil the year 1996 was 518,858 buildings, and the building stock indicates that 75% of buildingswere for residential use, and 25% for commercial and institutional uses (Chedid et al. 2001, Che-did and Ghajar 2004). Hence, it is reasonable to concentrate in saving energy in existing buildingsby applying low-cost energy measures without changing installed electromechanical systems.

Many studies have been done on the effectiveness of ECMs that help in reducing energy con-sumption in office buildings. These measures target enhancing building envelope using moreinsulation on roofs and walls as well as enhanced glazing performance (Miyazaki et al. 2005).Successful adoption of ECMs targets the following key waste areas: building envelope, lighting,HVAC systems, and energy storage (Surapong and Bundit 1997, Saidur 2009). Preliminary audits

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carried out by Markis and Paravantis (2006) on a sample of 12 representative small businessenterprises in Greece prioritized energy conservation in the area of air conditioning followed byspace/water heating and the building envelope. Moreover, a review on energy-saving strategiesby Abdelaziz et al. (2010) in industrial sector based on energy audit shows that most cost-effectiveenergy-saving technologies include the use of high-efficiency motors, variable speed drives, econ-omizers, leak prevention, and reducing pressure drop. Kofoworola and Gheewala (2009) reportedthat electricity used for lighting and HVAC systems in the operation phase and the manufactureof concrete and steel were the most significant elements in the buildings’ life cycle in Thailand.These findings were also confirmed by Chirarattananon et al. (2010) and Saidur (2010). Mung-wititkul and Mohanty (1997) showed that idle losses from office equipment can reach 2–5% ofbuilding energy consumption. Other building ECMs exist such as pre-cooling the inlet air to thecondenser of the direct expansion (DX) unit. Yu and Chan (2010), Hajidavalloo (2007), and Walyet al. (2005) considered pre-cooling the inlet air to the condenser of the AC unit as an effectivemeans to improve system energy performance. WhileYu and Chan (2010) applied this strategy ona large-scale air-cooled chiller, Waly et al. (2005) applied it to a 2.8 t split AC unit, and Hajidaval-loo (2007) did that for a window-air-conditioning unit. In the work of Yu and Chan (2010), theresearch is based on a calibrated simulation model that covered two kinds of air-cooled chillers,the condensing temperature control (CTC) and head pressure control (HPC) and showed thatmist pre-cooling improves the performance of both HPC (by 9.8%) and CTC (by 60.9%) units.The experimental and modelling results of Waly et al. (2005) and Hajidavalloo (2007) show adecrease in the power consumption by 16% and an increase in the coefficient of performance(COP) by 55%.

This study focuses on various ECMs that have low investment value and minimal disruptionand installation aspects on existing building systems without making changes on the buildingenvelope. The objective of this study is to investigate, using commercial energy analysis software,the opportunities for ECMs in office building systems for Lebanon’s hot and humid climate. Low-cost investment measures which will be examined in this work include: raising indoor coolingset point, lighting control, air economizer, and night pre-cooling, in addition to a non-standardECM for pre-cooling condenser air using exhaust air and spraying condensate drain to improvethe overall system COP. Building data collection through the review of design drawings, buildingaudit, and utility bills are used to calibrate the base case of the existing building using IntegratedEnvironmental Solutions – Virtual Environment (IES – VE) v-6.0.5 simulation software and toguide the analysis and recommendations of the low-cost ECMs.

2. Research methodology

An energy audit is performed on an existing eight-storey office building located in Beirut, Lebanon.A virtual building model is created using simulation software IES v-6.0.5. Analysis of buildingmain characteristics is performed, simulated, and calibrated. Based on the energy audit and basecase energy model, relevant low-investment ECMs are selected for study by simulating theirpotential effect on energy savings without compromising thermal comfort and indoor air qualityin the space.

2.1. Building and system description

An eight-storey office building is considered for this case study. The building is located in Beirut,Lebanon, at latitude 33.9◦N and longitude 35.5◦E. The total floor area of the building is 5180 m2

and its floor-to-floor height is 3.3 m. The building floor plan is identical for all the floors except

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for the ground floor. The total number of occupants is about 500. External walls, roof, and glazingcharacteristics are summarized in Table 1. The HVAC system used in the building is a decentralizedconstant air volume (CAV) system with 16 air-handling units (AHUs). The cooling of the buildingis provided by DX air-cooled condensers with a total capacity of 200 t and a COP of 2.2. Each floorhas two separate thermostat controls with the temperature set point of 23◦C throughout the year.Heating is not required because of high internal gains from office equipment including computers,printers, scanners, photocopy machines, water dispenser, and other electric equipment. Most ofthe building is used as offices with one conference room in each floor. Finally, the lighting servingthe building consists mainly of fluorescent lamps T5.

2.2. Building energy audit

Each floor of the building was physically investigated with the co-operation of the operation andmaintenance personnel in order to obtain information about existing lighting, system equipmentrating, and actual occupancy. For the purpose of getting details of building envelope, the buildingarchitectural and engineering drawings were reviewed (Table 1) and were found in compliancewith the Ministry of Public Works, Republic of Lebanon (2005) which directed us to explore othermeasures for enhancing overall building energy consumption.

The actual power consumed by computers, photocopiers, printers, and water dispensers wasobtained on each floor using ohmmeter and equipment tags. The schedules of operation foroffice equipment, occupancy, lighting, and elevators were estimated to reflect the actual operationwhere the building is occupied from 7.30 a.m. till 7.30 p.m. on Monday–Friday and from 7.30a.m. till 2.30 p.m. on Saturday. Finally, the total number of occupants and lighting fixtures werecounted. The details of building lighting, equipment, and people are shown in Table 2. Data onHVAC systems, mainly AHUs and condenser DX units, were collected as per design data, aswell as operation and maintenance manuals information provided by the building maintenance

Table 1. Physical characteristics of the building.

Component Description

External wall 150 mm block wall50 mm insulation boardU-value = 0.35 W/m2 K

Roof 200 mm concrete50 mm insulation boardU-value = 0.43 W/m2 K

Glazing Double glazed 6/12/6 mmU-value = 2.98 W/m2 KShading coefficient = 0.59

Window to wall ratio 17%People 484Lighting 10.23 W/m2

Type of lighting Fluorescent T5 and CFLHVAC system CAV

Total AHUs = 16Each AHU cooling capacity = 10 tFan airflow capacity per unit = 1800 l/sNumber of zones = 16

Condenser DX unit Reciprocating air-cooledNumber of DX units = 16

Ventilation rate 8.5 l/s·personPressurization rate 0.3 l/s m2

Set point temperature 23–24◦CHVAC system operation 7.30–19.30

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Table 2. Details of building lighting, equipment, and people.

Lighting power Equipment power Number ofFloor density (W/m2) density (W/m2) people

Ground 7 32 20First 11 42 38Second 11 37 58Third 11 35 55Fourth 11 34 75Fifth 10 39 62Sixth 12 20 68Seventh 12 34 54Eighth 10 32 54

personnel. Indoor air temperature and humidity were monitored from the return air stream of thetwo vertically mounted AHUs of the fourth floor (typical floor) over 1 week in April (5–12 April2010) and the data were recorded over 15 min using the HHF11 Omega data logger having anaccuracy of ±0.6◦C and ±3% error in relative humidity. The utility bills data provided valuableinformation on building energy use which was used for the calibration of energy model such thatthe corresponding energy costs match those in the utility bills. For the investigated office building,utility bills for the year 2009 were collected on request from building officials.

Monthly energy consumption billing data in kWh for the year 2009 is shown in Table 3. It isclear that the electric energy use peaks during summer months when the outdoor air temperature ishigh. During winter months, electric energy use is lower due to a lower cooling demand. From theanalysis, it is observed that there is a variation in monthly electric energy due to seasonal weathereffects. Interestingly, the annual energy use index for the building is 255 kWh/m2·year and isrelatively high compared with Commercial Building Energy Survey (CBECS) (2003) electricintensity of 207 kWh/m2 for office buildings.

2.3. Building base case energy simulation model and performance

In order to simulate the existing building performance and establish a base case scenario forwhich sensitivity analysis can be used to assess ECMs’ impact on energy savings, an energymodel of the building has been created and validated for the base case using IES (Corson 1992,Crawley et al. 2008). Specifically, the commercial package IES – VE was chosen mainly due to

Table 3. Building average monthly energy consumption.

Month Electricity (kWh)

January 79,749February 63,811March 75,174April 95,297May 89,177June 92,343July 128,175August 106,477September 120,190October 125,559November 106,832December 86,977

Total 1,170,000

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Figure 1. Office building geometry and façade.

its accuracy, versatility, and user-friendliness (Hammad and Abu-Hijleh 2010). At the core of ourmodel is the 3D geometric representation of the building to which application-specific data areattached as shown in Figure 1. The software IES qualifies as a dynamic model in the CharteredInstitution of Building Services Engineers (CIBSE) system of model classification (DOE/EIA2007, Azhar et al. 2011). It incorporates ApacheSim, a dynamic thermal simulation tool based onfirst-principles mathematical modelling of building heat transfer processes. The program providesan environment for the detailed evaluation of building and system designs, allowing them to beoptimized with regard to comfort criteria and energy use.

The IES model was structured in order of building site, block, zone, and surface area to assemblea 3D model similar to the actual building. In addition, thermal zones were modelled accordingto the HVAC drawings. For each zone, the people, equipment, and lighting load were includedas per energy audit. The building energy simulation was performed using Beirut weather file thathas been acquired from Global Meteorological Database – Meteonorm version 6.1 (2011).

Upon surveying input parameters and schedules of people, lightening, and equipment, the basecase monthly energy consumption matched closely the utility bill data of 2009 for the building.Figure 2 shows a comparison between (a) the measured and simulated indoor air temperaturefor a typical floor on 8 April 2010, (b) the measured and simulated indoor relative humidity on8 April 2010, (c) the measured and simulated hourly energy consumption of the whole buildingon 8 April 2010, and (d) the building 2009 monthly utility billing electrical use versus energyconsumption predicted by the IES v-6.0.5 software. The results show that the energy simulationprogram predicts the indoor air temperature and the energy use pattern of the building fairly well.The measured and predicted relative humidity in the space remained between 43% and 54%.The difference between the simulation program and the utility bills data ranges between 3% and13%. These results are considered reasonably accepted in light of the uncertainty in the buildingoperational parameters.

To assess opportunities for energy savings, the base case model energy performance is analysed.The energy consumption breakdown for the modelled building is shown in Figure 3 where 46%of the total building energy consumption is due to space cooling while office appliances consume36% and lighting system consumes 16%. The high energy consumption for space cooling indicatesthat targeting reduction of cooling loads and improvements on the operation of the cooling systemmay lead to energy savings. Potential ECMs that we will address in this study include raisingindoor design conditions, use of air economizer and night purging, and use of condensate drainfor mist cooling of condenser air to improve system efficiency.

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Figure 2. A plot of (a) the measured and simulated indoor air temperature for a typical floor on 8 April 2010,(b) the measured and simulated indoor relative humidity for 8 April 2010, (c) the measured and simulated hourly energyconsumption for 8April 2010, and (d) the building 2009 monthly utility billing and predicted electrical energy consumption.

Lighting is a major contributor to energy consumption with respect to total energy. Lightingaccounts for 16% of total energy consumption. Efficient lighting fixtures already installed inthe building include: fluorescent lamps ‘T5’ and compact fluorescent lighting (CFL). As there isno need to change existing fixtures, lighting loads can be reduced via other control strategies.Lighting controls including programmable timing control occupancy sensors in meeting roomswill be modelled to check their contribution in saving energy, knowing that any saving in lightingload reflects directly on energy savings in space cooling.

In our study, we have concentrated on low-cost investment ECMs that can be implemented atno cost or at minor alteration of building structures and systems at low cost without compromis-ing occupants’ thermal comfort. In addition, the utilization of condensate drain for pre-cooling

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Figure 3. Building end-use energy consumption breakdown.

condenser air using exhaust air will be modelled using basic energy and mass balance equationsto predict and improve system COP. The above retrofit options are investigated for the buildingHVAC and lighting systems. The ECMs are analysed and evaluated in the following section basedon the seasonal energy use pattern and schedules in Beirut climate.

3. Classification of proposed zero and low-cost energy-saving measures

3.1. ECM-1: raising temperature comfort settings

Set point temperature change is a zero investment measure. The temperature set point is raisedto 24◦C taking into consideration that the indoor conditions do not result in exceeding comfortlimits for moderate activity. At 23◦C, the predicted mean vote (PMV) was in the range of ±0.2.When the set point is raised to 24◦C from the base case value, the PMV remained below 0.5 andthe space cooling decreased to 532 MWh compared with the base case value of 574 MWh, whichcorresponds to 7.3% savings in building cooling load. The overall energy consumption decreasesto 1214 MWh corresponding to a 3.4% savings in total annual energy consumption.

3.2. ECM-2: lighting controls

Lighting accounts for a significant portion of the energy use in commercial buildings. In typicaloffice buildings, Crawley et al. (2008) reported that 30–50% of the electricity consumption isused to provide lighting. Typically, energy retrofits of lighting equipment are very cost-effectivewith payback periods of less than 2 years in most applications (Krarti 2000). The lighting energyconsumption in the building studied in this work consumes less percentage of total energy dueto the use of efficient lights and the large day lighting available from the building glazed façade.However, substantial energy savings are realized by improving the present lighting control systemby focusing on scheduled lighting controls and on placing occupancy sensors in meeting roomsand private offices (ASHRAE 2007). For the investigated case study building, lighting was usedall the time during unoccupied and low occupancy hours. Table 4 summarizes the occupancy

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Table 4. Occupancy and lighting schedule for the base case and modified lighting schedule in% of maximum load forweek days and weekends.

Schedule for lighting receptacle(percent of maximum load)

Schedule for occupancy(percent of maximum load) Existing Modified as per ECM-2

Hours of day (time) Week Sat Sun Week Sat Sun Week Sat Sun

12–1 a.m. 0 0 0 5 5 5 5 5 51–2 a.m. 0 0 0 5 5 5 5 5 52–3 a.m. 0 0 0 5 5 5 5 5 53–4 a.m. 0 0 0 5 5 5 5 5 54–5 a.m. 0 0 0 5 5 5 5 5 55–6 a.m. 0 0 0 5 5 5 5 5 56–7 a.m. 5 5 0 30 30 5 30 30 57–8 a.m. 30 30 0 70 70 5 70 70 58–9 a.m. 90 90 5 90 90 20 90 90 209–10 a.m. 90 90 5 90 90 20 90 90 2010–11 a.m. 90 90 5 90 90 20 90 90 2011–12 p.m. 90 90 5 90 90 20 90 90 2012–1 p.m. 80 90 5 90 90 20 90 90 201–2 p.m. 30 30 5 90 90 20 50 90 202–3 p.m. 90 30 5 90 90 20 90 90 203–4 p.m. 90 20 0 90 90 5 90 70 54–5 p.m. 90 20 0 90 90 5 90 40 55–6 p.m. 60 20 0 90 70 5 90 40 56–7 p.m. 30 10 0 90 40 5 70 20 57–8 p.m. 20 0 0 90 10 5 50 10 58–9 p.m. 10 0 0 90 5 5 30 5 59–10 p.m. 10 0 0 40 5 5 30 5 510–11 p.m. 0 0 0 10 5 5 5 5 511–12 p.m. 0 0 0 5 5 5 5 5 5

and the lighting schedule of the base case and the modified lighting load with additional lightingcontrols in meeting rooms. In the energy model, the schedule of lighting was adjusted by turningoff some lighting during unoccupied and low occupancy hours as per the schedule shown inTable 4. These adjustments resulted in the reduction in the lighting energy from 204 to 180 MWh.This corresponds to 11.8% savings in lighting energy or a 2.6% saving in overall building energyconsumption due to a simultaneous decrease in lighting and cooling loads.

3.3. ECM-3: air economizer system

Due to the mild climate of Beirut, providing an air economizer will reduce the cooling demandfor extended periods of the year (Jones and Stoecker 1982). The air economizer system can beintegrated with the mechanical cooling system to provide partial cooling even when additionalmechanical cooling is required to meet the remainder of the cooling load. When the outdoor airconditions are favourable, excess ventilation air can actually be used to condition the buildingand, thus, reduce the cooling energy use for the HVAC system.

The temperature economizer cycle is modelled such that the fresh air intake mixing ratio isincreased whenever the outside air temperature is colder than the return air temperature and themixing air temperature does not exceed the air temperature economizer set point. The operationof the economizer follows a differential dry-bulb control strategy of three modes. The first modeof operation is to supply the minimum required fresh air flow rate for mixing with return air whenthe outdoor temperature is higher than the return air temperature. The second mode of operationis to use 100% fresh air (recirculation air damper is fully closed) when the outdoor temperature

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Table 5. Space cooling energy savings due to air economizer.

Space cooling energy (MWh)

Date Base case ECM-3: air economizer % savings

January 30.6 12.7 58February 28.6 11.1 61March 36.1 17.9 50April 39.4 25.6 35May 47.8 45.9 4June 58.3 58.3 0July 69.1 69.1 0August 69.2 69.2 0September 62.9 62.9 0October 53.5 53.1 1November 42.4 37.1 13December 36.4 21.2 42

Total 574.8 484.6 15.50

is less than the return air temperature but is higher than 14◦C. The third mode of operationalstrategy is applied during winter when the outdoor air temperature is less than 14◦C where adirect digital controller modulates the mixing ratio of fresh and recalculated air flow to maintain anearly constant mixed air temperature of 14 ± 0.5◦C (Wang 2001). With temperature economizercycle, space cooling savings amount to 15.5%, as presented in Table 5. Note that the economizeris not operated for the four summer months of June–September, when humidity is generally above60%. Since the building is air-conditioned all year-round, the economizer is used in fall, winter,and spring, and the latent load associated with fresh air introduction in the building is minimal.The overall energy savings for the building energy consumption is reduced by 7.2% comparedwith the base case.

3.4. ECM-4: night purging

In many climates, night temperatures are cool even when daytime temperatures exceed economizerlimits. Thus, the air handler can flush the building with night air to cool down the building mass thatacts as a heat sink the following day. Setting controls for night pre-cooling can save a significantamount of energy, depending on location (Frankle et al. 2008). DX unit control is modelled,allowing fresh air to circulate in the space at night, thus flushing the building with night air. Thiscorresponds to 20% energy consumption of the total space cooling load. This strategy is appliedby adjusting the fresh air schedule entering the space, thus allowing fresh air to circulate at nightas long the outdoor air temperature is lower than 21◦C. The annual power consumption of thefans predicted at169 kWh/y is included in the analysis since the building HVAC system is shutoff at night.

Night purging does not function during July, August, and September since the fresh air temper-ature exceeds 21◦C at night and the relative humidity is high and would increase the latent load.Figure 4 shows the hourly indoor (a) temperature and (b) humidity variation with and withoutnight purging operation on March 15. The relative humidity does not exceed 61.7% with the useof night purging and hence has no adverse effect on comfort or on the system latent load duringthe off-summer months of operation. Table 6 shows the monthly space cooling summary with andwithout night purging. Energy savings of 1.9% are realized in space cooling load reflecting 0.9%energy savings of overall building base case energy consumption.

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Figure 4. The hourly indoor (a) temperature and (b) humidity variation with and without night purging operation on 15March.

Table 6. Night purging space cooling energy summary.

Space cooling energy w/o fan energy consumption (MWh)

Date ECM-4: night purging Base case % savings

January 18.30 21.11 13February 16.97 19.80 14March 23.12 26.17 12April 27.29 29.79 8May 36.83 38.33 4June 48.46 48.69 0July 59.27 59.29 0August 59.64 59.64 0September 53.34 53.34 0October 43.77 44.00 1November 31.39 32.83 4December 24.01 26.47 9

Total 442.40 459.45 –

3.5. ECM-5: pre-cooling condenser air

This ECM utilizes air-conditioning units’ condensate drain and exhaust air to cool the condenserair intake, thus improving unit COP. Guinn and Novell (1993) showed that by spraying waterdirectly on the condenser surface of a 3 t air-conditioning split unit, the energy efficiency canbe improved by 12–19%. Similarly, Huan et al. (2000) studied the effect of evaporative coolingon an air-cooled chiller, where the inlet condenser air temperature is decreased to its wet-bulbtemperature by means of an evaporative cooler pad. Their theoretical and experimental studiesshowed that the use of a cooler pad caused a decrease in the chiller’s condensation temperature,

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an increase in the cooling capacity, and a reduction in the electric demand. However, since wateris not a cheap commodity, we propose to use the building’s own system condensate with mistcooling system to ensure the effective use of the condensate in cooling the air. The water mistsystem is coupled to air-cooled condensers where the air temperature entering the condenser fromthe outdoor will decrease.

The condensate quantity is estimated in IES software, by computing the hourly humidity ratiodifference �Wcc across the cooling coil between the mixed air on the cooling coil and the airleaving the cooling coil for each DX unit. The condensate drain flow rate is calculated as per theequation below:

mC = ρAVcc�W , (1)

where Vcc is the supply flow rate over the cooling coil. The condensate volume is calculated bysumming the hourly condensate rate for all working days as per IES simulation. The decrease inthe outdoor air temperature depends on the condenser air flow rate, outdoor humidity, and mistwater flow rate evaporated in the air stream (Yu and Chan 2010). The increase in humidity ratioof the ambient air, as long as saturation is not attained, is given by

�W = mC

ρAVair-condenser, (2)

where mC is the condensate drain flow rate, ρA the density of the air, and Vair-condenser the condenserair volume flow rate. Note that we assumed that all mist is vapourized by the condenser air stream.This assumption is checked in the simulations against the degree of saturation of exit air and itwas always below 70% in all the months due to the limited rate of condensate water drain. Thenew condenser air humidity is given by

Wcon = WFR + �W , (3)

where WFR is the humidity ratio of ambient air and Wcon is the new humidity of the condensercooling air. When air is sprayed with water, the air drives towards the saturation line at the wettedsurface temperature as it passes through saturator (ASHRAE 2007). The energy balance to getthe final state is given by

mah1 + mChw = mah2, (4)

where ma is the condenser air mass flow rate, h1 the enthalpy of entering ambient air, and h2

the air leaving the evaporative cooling system. The new air temperature for the condenser intaketemperature Tcond−air using Yu and Chan (2010) model is given by

Tcond−air = 1.006 × TFR + (CpvTFR + hFG) × WFR − hFGWcon

1.006 + 1.805Wcon, (5)

where hFG is the heat of evaporation/fusion in kJ/kg K. To include the effect of reduced condensersink temperature in the simulation software, the outdoor ambient temperature is adjusted to thenew air temperature for a known mist flow rate and exhaust air flow rate implemented for thesummer months of June till September. The energy savings will be reflected in the improvementof the COP of the individual DX units which is a function of the ambient temperature and isalready embedded in the software functions (Steven-Brown et al. 2002).

The key parameter for determining the new condenser air temperature is the available mistwater flow rate. As the source of water is system condensate, a system is proposed which is shownin Figure 5. The condensate water from the floor AHUs is proposed to be collected through drainpans into one tank that is placed at ground level with overflow to drain. Condensate water can bepumped from the ground level to spray nozzles in each of the two condenser units at each floor. The

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Figure 5. Schematic of condenser air mist-spray system.

condensate quantity has been estimated on hourly basis for the summer months in Lebanon andthe collected water over 1 day is used the next day. The mist flow rate is distributed equally to the16 condenser air intakes and is kept constant for the 12 h of operation. The condensate quantity isestimated by IES software, by computing the hourly humidity ratio difference across the coolingcoil for each DX unit (Equation (1)). The condensate volume is calculated by summing the hourlycondensate flow rate for all working days as per IES simulation. Figure 6 shows the monthlyvariation of condensate volume for the whole building with the highest collected condensateobserved in July at 20,240 l. Figure 6 implies that water spraying is most feasible for months withthe most condensate generation; specifically, June, July, August, and September. Figure 7 showsthe sample variation of condensate drain on 17 July for the fourth floor during operational hoursfrom 7.30 to 19.30 h. The pump work for the mist system is extremely small at 0.25 hp.

At the condenser air intake at 4000 l/s/floor, mist water is injected. The mist water flow rate tocondenser intakes is estimated for the building at 0.0103 l/s in the months of June and Septemberand at 0.015 l/s in the months of July and August. The equivalent outdoor dry-bulb temperatureTcon-air exiting the mixer to the condenser was then estimated. This temperature is dependent on

Figure 6. Monthly variations of condensate drain volume.

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Figure 7. Sample variation of condensate drain on 17 July for a typical floor during operational hours from 7.30 to 19.30.

the air temperature, relative humidity, and velocity of air (Kutscher and Costenaro 2002). The dropin the condenser intake air temperature ranged from 2.5◦C to 4◦C. Saturation effectiveness is akey factor in determining evaporative cooler performance. For climate conditions during summerin Beirut, no saturation of air occurs since relative humidity ranges between 72% and 88% forthe critical months of July and August, where the most condensate is collected. It is noticed thatsaturation effectiveness ranged from 24% to 55%.After quantifying the impact of blowing exhaustair on the inlet condenser temperature, the effect of spraying water at the condenser air intakeand the consequent air temperature drop is studied for our selected building by adjusting theambient temperature for the condenser intake in the IES energy model enabling the determinationof energy savings during June, July, August, and September.

Energy savings for this ECM shows 10.4% improvement in the air-conditioning unit efficiencyand 5.5% energy saving with respect to base case for the above-mentioned 4 months. This ECMreduces the electrical energy consumption by 2.1% due to an enhanced COP. Note that more watercan be sprayed since saturation effectiveness is relatively low. No additional water was used sinceour objective is to make use of condensate generated from AHUs.

4. Energy savings and payback period with combined ECM

The annual energy use for the combined ECMs is shown in Figure 8 where annual electricalenergy savings can be achieved at 7.2% using the air economizer, 3.4% by raising temperature setpoint, 2.6% using occupancy sensors and scheduling of lighting, 2.1% by cooling condenser air,and 0.9% for night purging. The combined yield of using a low-investment ECMs reaches 16.5%of annual energy savings.

An economic assessment based on simple payback analysis was performed to evaluate the cost-effectiveness of the proposed ECMs. The capital cost of each ECM was calculated by referringto prices from local suppliers and RS Means Mechanical Cost Data book (Mossman et al. 2011)taking into account material, labour cost operation, and maintenance cost. Due to uncertainties incapital cost, a sensitivity analysis is done considering a cost variation of±20%.Table 7 summarizesthe ECMs capital investment, operating and maintenance cost, and payback period. The electrical

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Figure 8. Annual energy consumption for individual ECMs considered in this study and their combined effect onenergy savings.

Table 7. Capital investment, operating cost, and payback period per ECM.

ECM-1: ECM-2: ECM-3: air ECM-5:increase temp. lighting economizer and condenser Combined

Cost category set point control night purging pre-cooling ECMs

Annual operating costa $ 157,924 159,146 150,293 159,952 136,513Total capital investment cost $ 0 550 ± 100 9200 ± 1800 1670 ± 335 11,420 ± 2285Operation and maintenance costb $ 0 0 184 33.4 217.4Annual energy savings MWh 42 33 101 27 207

$ 549 426 1312 346 2690Payback period years – 1.3 ± 0.2 7.2 ± 1.4 4.9 ± 1 4.3 ± 0.9

Notes: aElectrical price is $0.13/kWh.bO&M cost is estimated to be 2% of capital cost.

energy cost is rated at $0.13/kWh. The economizer and night purging are combined into one ECMsince the installation of the economizer and its control can also be used to implement the nightpurging strategy. Lighting control and increasing temperature set point should be implementedbecause of short payback period of 1.3 ± 0.2 and 0 years, respectively. The economizer and nightpurging represent a major upgrade of the existing HVAC system and their payback period is7.2 ± 1.4 years. It is still recommended since the remaining DX unit life time is about 10 years.The pre-cooling ECM payback period is 4.9 ± 1 years and this is acceptable.

The reductions in CO2 emissions associated with the proposed low-cost ECMs are significantwhen noting that commercial air-conditioned buildings represented 27% of the overall buildingbuilt-up area in Lebanon in 1995 which corresponds to 42,703 commercial units (AdministrationCentral de la Statistique of Lebanon 1996). In order to estimate an equivalent office area in 2011,we assume 3% annual growth rate in commercial buildings’ permits; thus, the total commercialbuildings’ area would be estimated to be 1,637,415 m2 (Ministry of Public Works and Transport,Republic of Lebanon 2005). Our case study reflects the savings of 207 MWh for a 5180 m2 officebuilding. Implementing such ECMs may result in annual savings of 65,433 MWh in Lebanonwhich would reduce the CO2 emission by 77,080 t assuming 1.178 kg of CO2 per kWh emissions(El-Fadel et al. 2010).

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5. Conclusion and recommendations

An extensive energy audit was performed to collect building energy data and identify potentialenergy-saving measures. This was then recalibrated and a base case model was established usingIES – VE v-6.0.5 simulation software. Several ECMs were selected to reduce energy consumptionin existing office buildings at a low investment cost.

Annual electrical energy savings can be achieved at 7.2% using the air economizer, 3.4% byraising temperature set point, 2.6% using occupancy sensors and scheduling of lighting, and0.9% by night purging. Moreover, using ECM-5 by which condenser air is cooled by condensatedrain results in an additional annual saving of 2.1%. More energy can be saved using ECM-5 inapplications where water is available and fresh air is at higher percentage of total supplied air flowwhen high occupant density exists. The combined yield of using low-investment ECMs reaches16.5% of annual energy savings. The payback period for individual ECMs ranged from 0 to 7.2years while for the combined ECMs the payback period was 4.3 years.

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