Simple Tool for Energy Analysis of Day Lighting and Artificial Lighting for a Typical Office Building Lighting System Design

Today, energy shortage and carbon emission are the major challenges faced by all countries. There are many ways to tackle this issue by saving energy in many sectors. Lighting is used in many applications and it is one of the areas to be addressed for improving the energy efficiency thereby reducing the energy consumption. In this paper, a typical office building in Dubai is considered for analysis. The first section of the paper reports an efficient use of natural lighting into an office building to improve the energy efficiency. This study is carried out using Simulation software VELUX Daylight Visualizer 2 and simulation results are discussed in detail. The study showed that an ideal office space in Dubai has ample potential to harvest natural light to reduce the need for art ificial lighting. In the second section, artificial lighting is used to complement natural lighting to meet the required illumination criteria. DIALux 4.1 Lighting Wizard software is used to design and evaluate artificial lighting scheme with similar illumination abilities for the same office space through the use of different types of lamps and the simulation results are highlighted. Its cost effectiveness is also studied. The impact of use of natural lighting and artificial lighting on HVAC system is examined and highlighted.


Introduction
Energy consumption is a major concern of global discussions among engineers, scientists and planners. Lighting is one of the sectors of power consumption in residential and co mmercial buildings. Demand for energy consumption in every country is increasing everyday and there is a scope to improve energy efficiency.
In Dubai, as with the rest of the world, artificial lighting fo r b o th fu n ctio n al an d d eco rativ e p u rp os es is a considerable part of total electrical energy consumed in office buildings although scope of effective utilization of natural lighting is existent. An important issue with few b u ild in gs is o ver-illu min at io n and s ubs eq uent cost effect iveness. These costs can be great ly reduced using light ing strat eg ies such as light ing contro l systems to efficiently harvest day light fo r indoor lighting purposes. As a result, lighting represents a serious component of energy use today and has attracted a lot o f attention, especially in office build ings. The main objectives of this paper are to explore the possibility of optimizing the use of day lighting in the office building and to reduce the energy consumption through the use of artificial lighting while limiting its effects on the cooling load. The ultimate goal is to achieve energy efficiency in lighting system design.
Reducing lighting energy consumption is a major concern for building owners, governments and utilities. A study presented by Nicol, Wilson and Chiancarella (2006, pp.802-813) says that lighting is one of the largest energy users in domestic and commercial bu ild ings [1] and another research presented by Ryckaert et. al. (2010, pp.341-347) concluded that indoor lighting fixtures accounts for 5-15% of the total electric energy consumption [2]. Govern ments, utilit ies and all other building stakeholders have already started implementing strategies to replace existing lighting with more energy efficient lighting sources. In this paper, a typical office space in Dubai in the M iddle East is considered for the analysis to find a balance between different strategies that could be used to reduce the electrical lighting energy consumption. The building orientation and local weather conditions were taken into consideration for the effective utilizat ion of day lighting followed by a comparison study of conventional lighting lu minaires such as Fluorescent and LED (Light Emitting Diode) la mps. Cost analysis for each technology is discussed. This study can then be used as a baseline to other buildings within this region for further validation.
The structural layout of the office building used for the analysis is shown in Figure 1 and Figure 2 respectively. The building has six floors. Each floor has twelve offices of the dimension 7.5 m length by 5 m width.

Methodology
Dubai in the Middle East is situated in the Persian Gu lf and is the north-eastern state of the United Arab Emirates (UA E). The weather in Dubai is usually warm and sunny because of its closeness to the Tropic of Cancer [3]. Since the Tropic of Cancer line crosses through the United Arab Emirates, the weather in Dubai is hot and humid for a majority of the year. Rainfall is occasional and does not last for a long period. If it does rain, it's mostly in the winter period in the form of short gushes and an occasional thunderstorm. Hu midity is over 90% in coastal areas and an average rain fall of only five days a year in Dubai [3]. Summer season is extremely hot, windy and dry. According to Dubai Meteorological Office website, Dubai has recorded an average high of about 40 (104 F) and the night lows are about 30 (86 °F) , the highest recorded temperature in Dubai is 49 (119 °F) on 27 July 2012. Winters are generally cool but of short duration with an average high of 23 (73 °F) and overnight lo ws of 14 (57 F) [4]. The orientation of the building is a major factor in optimizing the building's energy perfo rmance. For Dubai, it is reco mmended that build ing be designed to be oriented on the east-west axis, with any glazing on the north and south sides complemented with appropriate shading and glare control [5]. Shading is another important aspect in the UAE considering the high heat gains experienced through solar radiation. Shading is done through a range of strategies such as planting large trees, build ing clustering, overhangs or blinds on windows among other shading features. Operable shading devices offer the flexibility through the control of altering shading blades to permit ventilation and day lighting into interior spaces without allowing any direct heat gain [5]. The selection of the facade, the orientation of the building will play a vital part in p redicting the thermal gain calculations of the structure.

Simulation of Day Lighting Using VELUX Daylight Visualizer 2 Software
The simu lation software VELUX Day light Visualizer 2 is used to evaluate the building's orientation to assess the illu minance levels (lu x). The assumption used in this section is that there is no external shading due to neighbouring trees or high rise buildings. Th is software is used to simulate day lighting conditions and to later provide valuable inputs into the development of an efficient, integrated day lighting strategy.   Figure. 3 show the office space and its 3D view is shown in the Figure.   The CIBSE Code for Lighting recommends a maintained illu minance of 500 lu x for general offices [6].  The results of the simulat ion were used to carry out analysis using Green Building criteria (LEED Day light) for optimu m daylight within the chosen sample office space [7].

Side Lighting Vision Gl azing
Windows located between 0.762m and 2.286m off the floor (sill height) are considered to be vision glazing. The location of windows in the simulat ion is 0.9m above the floor [7].

Visible Transmittance
Visib le light transmittance (VT, Tvis, or VLT) is an optical property that indicates the amount of visible light transmitted. Most Tvis values are between 0.3 and 0.8. Higher the value, more light is transmitted. A high Tv is is sought to maximize daylight but possible glare problems should be taken into consideration for optimu m balance [7] . In this design, the Tvis is 0.78 which is within the acceptable range.

Glazing Factor
Glazing factor is the ratio of interior illu minance at a given point on a given plane (usually the work p lane) to the exterior illu minance under known overcast sky conditions. The glazing factor requirement is a minimu m o f 2 % over 75% o f the regularly occupied areas [7]. Tab le 1 summarizes the calculations.  Fro m the results it is clear that a glazing factor of 3.59% is achieved in 78.8% of all regularly occupied areas which is well above the minimu m requirement of 2% for 75% of regularly occupied area. A minimu m daylight illu mination level of 203.3 lu x has been achieved.

Side Lighting Zone
The value achieved in the proposed design for side lighting zone comp lies with the LEED requirement (0.150 < Window to floor area rat io x T vis < 0.180) [8].
Window to Floor area rat io = 0.2304, T vis = 0.78 0.150 < 0.179 < 0.180 Hence it is clear fro m the above result that the proposed design meets the standard.

Effecti ve Sky Angle
Sky Angle is amount of unobstructed daylight received by the office space. We assume no obstructions at this stage. So, the Effective Sky Angle is 90 0 [9].

Daylight Factor
The daylight factor (DF) is a measurement used to quantify the quantity of daylight available in a space. It is measured at the height of the work plane; under a standardized CIE overcast sky. Daylight factor is given by equation (3)[9]. DF = (E in / E ext ) × 100 (3) E in : Interior illu minance at a fixed point on the work plane. E ext : Exterior illu minance under an overcast sky. The 'mean daylight factor' of a roo m is the average daylight factor value of a grid of sensors at work p lane height that extends across the room. According to the Brit ish Standards Institution, BS 8206 part 2, a space with a mean daylight factor between 2% and 5% is considered well lit and requires little or no addit ional lighting during daytime. A space with a daylight factor of less than 2% appears dimly lit [10]. VELUX simu lation o f Daylight factors under overcast conditions are shown in Figure 9 and Figure 10 respectively. Table 2 summarizes the results.
The DF mean is much higher than the DF mean0.75 because of values taken near the windows. It would be safe to assume the DF mean0.75 since it takes the spaces away from the window into consideration to be more accurate.

Window to Wall Rati o (WWR)
The min imu m WWR required for a side lit space is given by the equation (4)[9]. WWR > 0.088 x DF x 90°/(sky angle) (4) The recommended DF minimu m is 2%, sky angle is assumed to be 90° and the WWR in the office space dimensions is 0.288. Therefore the above calculation shows that the minimu m WWR of 0.225 for a side lit space has been satisfied.

Di mensional Constraints -Daylight Uniformity/ Li miting Depth
It is the distance at which the uniformity of daylighting levels drops throughout the space. Limit ing depth can be calculated using equation (5)  Spaces with depths lower than this limit ing depth show relatively uniform levels of daylight throughout. As per calculation, the max limit ing depth is 14.4m while the proposed office space depth is 5m.
Thus, from the simu lation results it is evident that the office space designed is ideal for day lighting as it meets the required standards. The intent is not only to harvest day lighting but also to provide the building occupants a connection between indoor spaces and the outdoors through the introduction of daylight and views into the regularly occupied areas of the buildings.

Simulation of Indoor Lighting
This section deals with the artificial lighting to supplement the day lighting for the selected office space. The DIA Lu x Light Wizard software is used for analysis of the indoor lighting schemes using both Fluorescent and LED lu minaries. It is assumed that the offices are lit by artificial light sources throughout normal office hours (9A.M to 5P.M) without harvesting the day light. The illu minance level of 500 lu x as per CIBSE Code for lighting offices is used for the analysis [6], [12].
The following lu minaries were chosen for analysis because of their similar illu mination ability : The office room is illu minated through the use of GE make fluorescent lamps TL 5000 5506/236/ 6/ EB. Figure 12 shows the polar candela of the lu minaire. Figure 13 shows DIA Lu x 4.1 simulat ion results with fluorescent luminary -TL 50005506/ 236/ 6/EB.  It is clear fro m this result that twelve fittings were required to illu minate the office space to achieve the reco mmended lu x level of 500.

LED Lumi nary -ENDO GXLX7009W of (Make, ENDO)
The office room is illu minated through the use of LED Lamps ENDO GXLX7009W. Figure 15 shows the polar candela of the luminaire. Figure 16 shows DIALu x 4.1 simu lation results with LED lu minary. Wattage is 125 W, Lu minous flu x is 6279 lu men.  It is clear fro m th is result that six fittings were required to illu minate the office space to achieve the recommended lux level of 500.
Co mputer simulat ions of both the lighting levels in the office space are obtained using the DIAlu x Lighting Wizard software and the results are summarized in the Table 3. The maintenance factor was assumed to be 0.67. The power tariff and fuel surcharge in Dubai as of June 2012 are 23 fils/kWh and 6 fils/kWh respectively. These values are used for the calculation purposes and the results are tabulated in Table 3.

Results and Conclusions
A typical office space in Dubai in the Middle East is considered for the analysis to find a balance between different strategies that could be used to reduce the electrical Lighting for a Typical Office Building Lighting System Design lighting energy consumption. The building orientation and local weather conditions were taken into consideration for the effective utilizat ion of day lighting using VELUX Daylight Visualizer 2 software followed by a co mparison study of conventional lighting lu minaires such as Fluorescent and LED (Light Emitting Diode) la mps.
The office space was designed to meet day lighting harvesting criteria for Dubai. The intention being not only to harvest day lighting but also to give the building occupants a connection between indoor spaces and the outdoors through the introduction of daylight and views into the regularly occupied areas of the buildings. It is evident fro m the VELUX simu lation results that the office space designed is ideal for day lighting as it meets the required standards.
The DIA lu x Light Wizard software is used for analysis of the indoor lighting schemes using both Fluorescent and LED lu minaries. If Fluorescent Lu minaires are used for interior illu mination throughout working hours, the annual operating costs would be around 745.65 A ED (Arab emirates Dirhams) for a single office space. There are twelve offices in a single floor, wh ich brings the cost to be around 8950 AED (appro ximately). If the LED lu minaires are used, the operating costs would be about 646.36 A ED and the annual cost for a single floor co mes around 7756 AED (appro ximately). The difference between both the costs is 1194 A ED for a single floor and the building has five floors typical to the one in consideration, this brings the figure to approximately 6000 A ED, quite a substantial amount in terms o f savings just through operating costs and this being a liberal estimate. The savings through the LED light fixtures can increase even more through the option of controls.
With effective use of thermal insulation and thermal automation controls the use of day lighting can have no or the least impact on the space cooling system within the building. Hence a control strategy with control logic and coupling algorith m to integrate the lighting and space cooling system was reco mmended for practical imple mentation and further validation of this study. The idea is to create a baseline study for an ideal office space which can then be used as a reference while being applied to other buildings in Dubai or M iddle East in general. This study can be discussed further by establishing mathematical equations to couple daylight harvesting and space cooling. This will also contribute towards Green building practices and help in cutting down the energy consumption and lo wer the CO 2 emissions.