PLEA 2016 Los Angeles - 36th International Conference on Passive and Low Energy Architecture. Cities, Buildings, People: Towards Regenerative Environments

Light Zones vs. HVAC Loads: evaluating energy efficiency for shading devices in commercial buildings

SYMA HAQUE TRISHA1, DR. ZEBUN NASREEN AHMED2

1 Lecturer, Dept. of Architecture, Bangladesh University of Engineering and Technology (BUET), Dhaka, Bangladesh

2 Professor, Dept. of Architecture, Bangladesh University of Engineering and Technology (BUET), Dhaka, Bangladesh ABSTRACT: This paper addresses the compromises that can be struck, using the passive architectural feature of sunshades, when balancing daylight and the accompanying thermal gains. Daylight inclusion subdivides the interior luminous environment into varying illumination level of light zones. Under and over-lit zone may affect visual and thermal comfort, and consequent energy consumption. However, if daylight can reduce the building's use of electric light, while keeping HVAC load under control, it can reduce peak energy use, as well as total energy consumption. This paper presents a simulation study of evaluating luminous and thermal performance of most commonly used shading devices, for recent tall office buildings of Dhaka, in the context of the most preferred, south orientation, for this location. Six selected fixed external shading devices, from a field survey, have been evaluated, during the overheated period of summer, based on the light zone distribution vs HVAC load. The simulation results revealed that, both the geometrical, as well as the material characteristics of shading devices, can have significant influence on the desired performance. The results clearly indicate that, modification of the dimensional relations of light zone, by proper selection of shading devices, can enhance energy efficiency in offices of similar tropical areas. Keywords: HVAC load, light zone, energy efficiency, shading device, commercial buildings.

INTRODUCTION In the present World, artificial lighting already forms 30% of overall energy consumption (Brotas and Rusovan 2013) in office buildings. Merely by reducing the lighting load through daylighting, 10–30% of reduction in electricity consumption for building cooling can be achieved (Mayhoub and Carter 2011). Moreover, buildings incorporating daylighting can reach an overall balanced energy savings of 15% to 40% (Dubois and Blomsterberg 2011). Therefore, daylight is being encouraged in office/commercial buildings all around the world, and Dhaka, where this study is based, being no exception. The recent trend is seen to be high rise, deep plan, open offices, with extensive use of curtain glass envelopes, using large apertures (Rahman and Ahmed 2008).

Daylight inclusion into large interiors creates two types of luminous areas or light zones within a space: a daylight zone of abundant daylight and a dark or under-lit zone, requiring artificial light (Merete 2007).In the Tropics, unwanted heat may enter in the daylight zone, especially with direct sun light in over-lit areas, causing thermal discomfort. Too much or unguided daylight may cause glare with visual discomfort, creating over-lit zone (Mayhoub 2012). Sometimes, daylight itself causes extreme HVAC load to the building, resulting in energy consumption in excess of the savings from artificial light. Therefore, improper and unconsidered addition of daylight may cause harm, rather than any targeted benefit (Joarder, Price and Mourshed 2010).

Shading provision should be considered, as an

integral part of fenestration system design for office buildings, in order to balance daylighting requirements of light zone, versus the need to reduce HVAC loads (Tzempelikos and Athienitis 2005). A recent study in Delhi, India shows that, heat load from envelope can be reduced from 67% to 71% by the use of shading devices (Gulati 2012). In another study the National Institute of Building Science(USA) has reported that, reductions in annual cooling energy consumption of 5% to 15% can be achieved, through the use of well-designed sun control and shading devices. However, the extent of shading devices hasn’t yet been investigated (RAJUK 2008) for the Dhaka context, which has the highest consumption of electricity in Bangladesh (Joarder 2009).

This research paper is based on luminous vs. thermal

conflicts of tall air conditioned office buildings of Dhaka, with fenestrations, using the most common, external fixed shading devices, for south orientation. Daylight or luminous effects have been evaluated by computer simulation program, ‘Ecotect’ (version 5.50), with ray trace based software ‘Radiance’, and thermal simulation was performed only using ‘Ecotect’ (version 5.50).Along with these simulation tools, ‘Microsoft Excel 2007’ was also used for data processing and comparative analysis of simulation results. The aim of the study was to identify the efficiency of shading devices in respect to lighting and to evaluate their thermal energy efficiency ,on the basis of HVAC loads.

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METHODOLOGY Quantitative determination of average luminous and thermal variables was used, to evaluate energy efficiency of shading devices in this research, in three distinct phases; considering extent of Light Zone, HVAC loads and an evaluation of Light Zone vs. HVAC Load. The phases are elaborated below.

Light Zone: According to the International Energy Agency (IEA 2000), ideally within the visual field the luminance ratio should: Central field (5): background (2): environment (1). However, whenever this ratio exceeds 10:3:1, the visual problem of glare occurs, due to over-lit zone. The minimum standard illumination level for general office is considered to be 300lux (CIBSE 2002)(BNBC 1993).Therefore, on the basis of distribution of the illumination level (Joarder et al. 2009) in deep plan open office spaces, lighting efficiency of shading devices can be evaluated, under the fulfillment of the four luminous/lighting criteria of Table 1. Table 1: luminous variables for lighting efficiency

Area of daylight zone: acceptable illumination level

Area of over-lit zone

Area of under-lit zone

Maximum depth-accepted illumination level/daylight zone

≤900-≥300lux ≥900lux ≤300lux ≤300lux Maximizing area of accepted daylight zone is the

prime criteria for good quality for daylighting (BNBC 1993). Both glare prone over-lit and under-lit or artificial light zone, are considered under efficient, in daylit buildings, for office tasks, and should be minimized, compared to acceptable area of daylight zone. However, the over-lit zone is given preference, compared to the under-lit zone, as it requires no energy consumption for artificial lighting. Moreover, it may be converted into usable area, merely by changing the direction of seating in the interior layout (Jakubiec and Reinhart 2012).

. HVAC Load: Thermal efficiency of shading devices

was measured, based on minimum values of HVAC load(Burdic 2011),ensuring thermal comfort. The following space heat gains (Ameen 2006) that constitute the HVAC load, were taken into account, for the analysis:

Solar radiation through transparent surfaces or

Solar Heat Gain Fabric Heat Gain of opaque surfaces

Ventilation and Infiltration Heat Gain Inter-zonal loads from heat flow between

adjacent zones Heat from occupancy, lights and appliances

Light zone vs. HVAC load: Finally, for each of the

tested shading devices, the balance of energy efficiency in luminous vs. thermal aspect was calculated, by the ratio of area of accepted daylight zone(where light level is ≥300≤900lux)to the HVAC load (energy consumption in sqm/kWh). The greater the magnitude of the ratio, for a particular shading device, the greater is the usable daylit area, while consuming lower values of electricity for HVAC or cooling load (all other parameters considered constant). The shading device, ensuring maximum lighting efficiency(under criteria of Table 1), along with the maximum ratio of accepted daylight area(≤900-≥300lux) to HVAC load, has been determined as the most energy efficient.

Figure 1: Flow diagram of simulation process Figure 1 summarizes the methodology adopted for

this study. The office period (9.00-17.00hrs.) of 15 April and 'sunny with sun' sky condition was chosen for simulation, charactering extreme climatic features (U.S. dept. of Energy) of the hottest month of the year, for the

selection of shading devices (field

survey)

identifying simulation parameters (literature review)

division of model's working plane into grid(test) points

introducing the 3D model and climatic

data into the simulation program

Daylight simulation-Ecotect +Radiance

simulation data processing (M.S. Excel)

identifying values of lighting efficiency variables of light zone

setting thermal parameters

Thermal simulation-Ecotect

simulation data processing

(M.S. Excel)

identifying values of thermal efficiency

variables of HVAC load and space heat gain

comparative result analysis: lighting efficiency, thermal efficiency, balancing lighting vs. thermal efficiency

conclusion and recommendation

general parameters of tall office buildings of Dhaka

thermal parameters

formation of 3D model with surveyed shading device

formation of model without shading device

division of working plane into grid(test) points

introducing the 3D model and climatic data into the simulation program for parametric study

Daylight simulation-Ecotect +Radiance

simulation data processing (M.S. Excel)

identifying maximum depth of accepted daylight zone 

climate based

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study region of Dhaka. For the simulation, 3D models were generated, using parameters of tall office buildings of Dhaka (Table 2), and parameters of shading devices (Fig. 3), all derived from a field survey. As there is no defined aspect of tallness (CTBUH 2014), buildings above six stories were regarded as tall buildings, considering walk up limit and fire escape provision(RAJUK 2008, BNBC 2003).After a pilot survey, involving 106 buildings, six were finally selected, based on the most commonly found parameters of shading devices (Fig. 3).

Table 2: General parameter of tall office building of Dhaka Model dimension Model Width 6000mm typical column grid Floor height 3000mm typical clear height Aperture Width 5750mm the whole span

between two columns up to full floor height

Aperture head height 2875mm

Working plane height 760mm ergonomics standard Material specification Ceiling white painted on 12.5mm plaster, 150mm

RCC (reflectance:0.7, U value:2.05w/m2k)

Wall 150mm brick work with 18.5mm plaster (reflectance:0.5, U value:2.602w/m2k)

Floor 200mm thick concrete slab plus tiles finishes (reflectance:0.3, U value:2.9w/m2k)

Glazing Single glazed clear with aluminum frame for maximum visual transmittance with thermal gain. (reflectance:0.89, U value:6w/m2k)

Shading device

Concrete Aluminum

reflectance:0.55, U value:1.8w/m2k

reflectance:0.796, U value:1.7w/m2k

The depth of the simulation model, was derived from the maximum depth reached, of accepted illumination level of 300 lux, for a ‘model without shading’. Using daylight thumb rule, i.e. daylight penetrates about 2.5 times the head height of the aperture into the room from a window (Robbins 1986), parametric daylight simulation studies for South, East and West orientations, were carried out (considering severity of direct sun light). The results revealed that 300lux reaches its maximum depth after 15.00 hours in the West orientation, and at 9.00hoursin the East (Fig. 2).With aperture/window heights of 2875mm, the maximum depth 2.5x2xaperture head height, 13750mm, was considered the depth of the simulation model. In the thermal simulations, all variables of thermal comfort and operating measures of air conditioning, were considered constant (Table 3), to get the HVAC load and corresponding space heat gain.

Daylight simulation steps study 1: depth=2.5x aperture head height simulation model

Division of grid points

study 2: depth=2x2.5x aperture head

height

Radiance simulation results in Ecotect

Figure 2: Identification of maximum depth of 300lux through parametric daylight simulation Table3:Thermal parameters for hourly heat gain/loss simulation, considered comparing standards (ASHRAE 2008, BNBC 1993, ecotect v 5.50) Thermal parameters considered

Temperature(°C) 24.5-26

RH (%) 53

Air speed(m/s) .2

Clothing (clo) .6

No. of people 7(Typical-12 m²ea.)

Activity-avg. biological heat output(W)

70(Clerical)

Sensible gain(W/m² ) 40(small power load without artificial lighting)

Latent gain(W/m² ) 0(no evaporation)

Air change rate (ach) 0.37

Wind sensitivity (ach) 0.1(well protected)

Schedule(Occupancy, internal gain, infiltration)

100% occupied during the whole operation hour.

Efficiency of A.C. plant 95%

Hours of operation Full office hour

17.00

10.00 11.00 12.00

13.00 14.00 15.00 16.00

9.00 hrs.

South East WestGrey denotes illumination level ≥300lux MS Excel data processing

17.00

10.00

12.00

13.00 14.00 15.00 16.00

9.00 hrs.

11.00

South East WestGrey denotes illumination level ≥300lux MS Excel data processing

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Selected building Section of the case shading device Model &daylight simulation

Nuruzzaman Biswas Tower, Gulshan

Horizontal concrete cornice(ID:H01)

I-Center, Dhanmondi

Horizontal aluminum louvers with concrete boundary(ID:H02)

The Alliance Building, Baridhara

Horizontal aluminum louvers in overhang(ID:H03)

Uday Tower, Gulshan

Horizontal aluminum louvers in vertical plane(ID:H04)

Mika-Cornerstone, Uttara

Horizontal angled aluminum louvers in vertical plane (ID:H05)

NCC-Bank, Head office, Motijheel

Horizontal concrete louvers (ID:H06) *Grey at left denotes over-lit zone, at middle black denotes accepted daylight zone and at right white denotes under-lit zone for daylight simulation results of case shading devices

Figure 3: simulation model and case shading devices from field survey

RESULT ANALYSIS AND DISCUSSION The comparative evaluation of the simulation results of the shading devices, followed three steps:

Step 1: Luminous efficiency of light zone: The

average of daylight simulation results (Fig.4) show that, in general, over and under-lit zones, constitute greater

area, than accepted daylight zone, for all the tested shading devices.

Figure 4: Comparison of different light zones from average of daylight simulation results for lighting efficiency

Among the shading devices tested, only H02 yields

smaller under-lit or artificial light zone, compared to its over-lit glare area. The over-lit area for H02, as well as for H04 and H05, is smaller than that of their accepted daylight zone. However, area of under-lit zone, for H04 and H05 is much greater, compared to the rest of the tested shading devices. H05 also presents the worst case scenario, characterizing the least values under other considered criteria. On the other hand, H02 yields a much smaller under-lit zone. Moreover, the area of accepted daylight zone, over-lit zone and maximum depth of accepted daylight zone, for H02 has the highest value. H01 shows the second highest value, for these three criteria, though the values are markedly smaller than that of H02. H01 also occupies the second highest area of under-lit zone, though it is much greater compared to H02. With lower values, H03 and H06 yield similar comparative results among their variables, as do H04 and H05. H04 and H06 yields very close results. H04 fulfils the prime criteria of greater accepted daylight zone. However, its performance is not as good as H06, considering the other three criteria. Considering the above discussion on light zones distribution, the identified selection order of the shading devices for lighting efficiency, is specified in Table 4. Table4:Shading device selection order for lighting efficiency Selection order 1 2 3 4 5 6 Shading device H02 H01 H03 H06 H04 H05

Step 2: Checking results of step 1 through

thermal energy efficiency of HVAC load :The average thermal simulation results (Fig.5) show, for space heat

9 10 11 12 13 14 15 16 17

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gain, only solar and fabric heat gain have significant differences.

Figure 5: Comparison of average HVAC load and space heat gain from thermal simulation results

H02 yields the highest solar heat gain, but with much lower fabric heat gain and HVAC load, compared to the others. The rest of the tested shading devices present negligible differences, in their values of fabric heat gain. Similarities in their solar heat gain and HVAC load can also be identified from Fig. 5.

According the HVAC load, the identified selection order of the shading devices for thermal energy efficiency, is given in Table 5.

Table 5: Shading device selection for thermal energy efficiency Selection order 1 2 3 4 5 6 Shading device H02 H05 H04 H06 H03 H01

In Table 5, only H02 secures its position 1, similar to Table 4. The selection order of the rest of the shading devices for thermal energy efficiency, in Table 5 shows a reverse order from the lighting efficiency order (Table 4). The average HVAC loads, as well as solar heat gain order(Fig. 5), is also reverse to the order of the average area of over-lit zone(Fig. 4). This clearly signifies the over-lit zone is associated with the inclusion of heat.

Step 3: Rechecking the results of Steps 1 and 2,

balancing lighting vs. thermal energy efficiency: Variation in selection order of shading devices for

lighting and thermal energy efficiency (steps 1 and 2), requires checking lighting vs. thermal energy balance through the comparison of average ratio of accepted daylight zone per HVAC load (Fig. 6).

Figure 6: Comparison of average ratio of the area of accepted daylight zone per HVAC load Table 6: Shading device selection order for the ratio of average area of accepted daylight zone per HVAC load Selection order 1 2 2 3 4 5 Shading device H02 H03 H04 H01 H06 H05

According to the selection order, considering the average ratio of the area of accepted daylight zone, per unit HVAC load, in Table 6, H02 characterizes the most balanced shading device for lighting vs. thermal energy efficiency. H05 secures the worst position under this consideration. The results verify those in Table 4, regarding these two shading devices. The performance of H03 and H04 when considering average ratio of the area of accepted daylight zone per HVAC load, is the same. However, on the basis of lighting efficiency selection order (Table 4),H03 is more suitable than H04 for daylighting. In both Tables 4 and 6, H01 is more efficient, compared to H06. However, in Table 6, H04 and H01 acquire opposite positions to Table 4.On the basis of the ratio, balancing lighting and thermal energy, H04 should be preferable to H01. On the basis of the above evaluation, Table 7 finally identifies, the selection order of the tested shading devices for energy efficiency in total, ensuring lighting efficiency with balanced lighting vs. thermal energy. Table 7: selection order of energy efficient case shading devices for commercial buildings in south orientation Selection order 1 2 2 3 4 5 Shading device H02 H03 H04 H01 H06 H05

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CONCLUSIONS AND RECOMMENDATIONS In this paper, evaluation of shading devices (Fig. 3), reveals that their proper selection, in controlling distribution of light zone and space heat gain of HVAC load, can advance energy efficiency, in commercial office buildings of tropical cities significantly. Moreover, proper physical characteristics of shading devices can be significant in minimizing solar, fabric, as well as, HVAC load, and maximizing glare-free acceptable daylight zone. The summary of the findings, from results, discussion and analysis of Table 7, are given below: Under-lit zone of unusable area needs to be reduced,

through the reduction of depth in space. It should be limited from 1.5 to 3.5 times of the aperture head height(calculated from Fig. 4) or 2.5 times of the aperture head height on average.

Geometric and material combination of horizontal

aluminum louvers in horizontal plane with concrete boundary, display highest luminous, thermal and overall energy efficiency. Controlling solar heat gain and over-lit zone, will further increase its efficiency.

Arrangement of horizontal aluminum louvers in

horizontal plane, should be preferred, to that in vertical plane, considering both lighting and thermal energy efficiency.

Aluminum louvered arrangements in concrete

cornice are more suitable than solid ones. Horizontal concrete louvers containing multiple

number of cornices, are not as energy efficient as a single concrete cornice.

Aluminum louvered screens <1800angle is less

beneficial than those of 1800,though, both are preferable for daylighting in shallow plan open spaces.

1800 horizontal aluminum louvers can be added, with

bare concrete cornice, in vertical planes, to increase energy efficiency

Increasing number of horizontal aluminum louvers

covering vertical plane, helps to decrease solar heat gain, along with over-lit zone control. However, its adverse effect on daylight zone of accepted illumination level and under-lit zone, needs to be taken into account, to increase energy efficiency.

This research assesses the energy efficiency of the fixed external commonly used shading devices of tall office buildings of Dhaka, facing South, giving

guidelines to increase their efficiency. Clearly, shading devices in other orientations also needs to be explored, with parametric design, to ensure optimized energy consumption and comfort, all the year round. Acknowledgements The authors deeply acknowledge the role of BUET Authorities, and the Department of Architecture, in particular, for the support extended in the pursuance of the research, which formed the basis of this paper. REFERENCES Ameen, A.(2006) Refrigeration and Air Conditioning, Prentice Hall of India Private Ltd. ,New Delhi. p. 242 Brotas, L. and Rusovan, D., (2013). Parametric Daylight Envelope. In the proceedings of PLEA 2013-29th Conference. Munich, Germany, September 10-12. American Society Of Heating, Refrigerating And Air Conditioning Engineers, Inc.(2008) BSR/ASHRAE Addendum d to ANSI/ASHRAE Standard 55-2004, Thermal Environmental Conditions for Human Occupancy, First Public Review Draft , September 12, 2008 Burdic, A., (2011). Strategy guideline: accurate heating and cooling load calculations, Energy efficiency and renewable energy, Building technologies program, U.S. Department of Energy.p.8 Chartered Institution of Building Services Engineers, (2002). Code for interior lighting, (CIBSE), UK, http://www.arca53.dsl.pipex.com/index_files/lightlevel.htm Council on Tall Buildings and Urban Habitat (2014), Criteria for the Defining and Measuring of Tall Buildings, (CTBUH), Illinois Institute of Technology, Chicago, USA,p.1-5 Dubois, M.C. and Blomsterberg, A., (2011). Energy saving potential and strategies for electric lighting in future North European, low energy office buildings: A literature review. Energy and Buildings, 43(10): p. 2572-2582. Gulati, N., (2012). Cost Effectiveness In HVAC By Building Envelope Optimization. Revista AUS, (11):p.14-17. Housing and Building Research Institute and Bangladesh Standards and Testing Institute, (1993). Bangladesh National Building Code(BNBC),(HBRI and BSTI), p.8.3-8.13 International Energy Egency, Energy Conservation in buildings and community systems(ECBCS)(2000); Daylight in buildings: a source book on Daylighting systems and components; IEA SHC Task 21/ECBCS Annex 29 report, July 2000; Lawrence Berkeley National Laboratory; CA,USA; p3-5 Jakubiec, J. A., Reinhart, C. F., (2012). The ‘adaptive zone’–A concept for assessing discomfort glare throughout daylit spaces. Lighting Research and Technology, 44(2): p. 149-170. Joarder, M.A.R., (2009). A Survey on Daylighting Potentiality in the Offices of Dhaka, Bangladesh. Global Built Environment Review(GBER), 7(1):p.6. Joarder, M.A.R., Ahmed, Z.N., Price, A.D.F. and Mourshed, M., (2009). A simulation assessment of the height of light shelves to enhance daylighting quality in tropical office buildings under overcast sky conditions in Dhaka, Bangladesh. IN: Proceedings of the Eleventh International IBPSA Conference, (Building Simulation 2009). Glasgow, Scotland, July 27-30, p.920-927. Joarder, M.A.R., Price, A.D.F. and Mourshed, M., (2010). The changing perspective of daylight design to face the challenge

PLEA2016 Los Angeles - Cities, Buildings, People: Towards Regenerative Environments, 11-13 July, 2016

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