5.1 exercise: daylighting analysis of a single office · tutorial on the use of daysim/radiance...

Tutorial on the Use of Daysim/Radiance Simulations for Building Design – version: Aug-06 Page 53

5.1 Exercise: Daylighting Analysis of a Single Office

This first exercise introduces you to the DAYSIM JAVA interface and guides you through the steps necessary to setup and run a daylighting analysis of a single office located in Ottawa, Canada. Daylight autonomy, daylight factor, and annual electric lighting use are the daylighting performance measures used in this exercise.

Your Task

You are involved in the design of an office building located in Ottawa, Canada. The building is mainly oriented along the West-East axis with sixty identical private offices bordering either the North or South facades (Figure 5-1-1). The two facades are not shaded by surrounding buildings or landscape. The offices are connected through a central aisle that runs along the center of the building on all three storeys.

Figure 5.1-1: Sketchup Visualization of the investigated office building.

Your Task is to use Daysim to

! predict the daylight availability (daylight autonomy and daylight factor) in the offices and on the central aisle, and

! estimate the lighting energy savings from an occupancy sensor versus a regular on/off wall switch.

Step 1: prepare the DAYSIM simulation

Before you start with the Daysim simulation, you need to prepare (a) a CAD model of the building that can be imported it into Daysim, and (b) a sensor point file. Looking at Figure 5-1-1, you will realize that the office building is highly repetitive, consisting of 30 identical blocks with each block consisting of a Northern and a Southern offices linked by a piece of aisle (Figure 5-1-2).


Figure 5.1-2: Sketchup Visualization one of the thirty identical blocks out of which the building is made up.

Since the daylight availabilities are identical within each of the individual blocks and since these blocks are –as far a daylighting is concerned – largely independent of each other, you may use the model shown in Figure 5-1-2 for your analysis.

Note: Working with a smaller model reduces the memory requirements for your simulation and allows you to use less stringent Radiance parameters, as the resolution at which the raytracing algorithm “scans” surfaces within your scene depends on the size of bounding box of your scene

3. Remember, the

time required to generate a 3 dimensional building model may be substantial. Include only those details into your building model that are relevant for the daylight simulation.

The model shown in Figure 5-1-2 happens to coincide with the Sketchup model used in chapter 4.1. Please refer to the relevant sections in chapter 4 to learn what to consider when preparing a Radiance/Daysim model in Sketchup and how to export the Sketchup files into 3d Studio (3ds) format. A 3ds file of the geometry shown in Figure 5-1-2 is also provided with this design exercise. It is stored under C:\Daysim\projects\Ex5.1DaylightingAnalysisOfASingleOffice/ ExternalFiles/.

As mentioned earlier, you also need a sensor point file for your project, i.e. a file with the coordinates and orientations of the points of interest in the building. A description of how to generate the sensor file is given in section 4.1. For this

3 The bounding box of a Radiance/Daysim scene is the smallest cube which holds the

scene’s complete geometry.


exercise you will use the sensor point file from chapter 4.1. A copy is already stored under C:\Daysim\projects\Ex5.1DaylightingAnalysisOfASingleOffice

/pts/center_line.pts. As explained in 4.1, the file contains a line of sensors facing upwards, that are located on the center axis of the offices and the aisle at desk height (85cm). The sensor are one meter apart from each other. The file is shown in Figure 5.1-3.

Figure 5.1-3: Radiance sensor point file

You will use Daysim to calculate daylight autonomies and daylight factors at these sensor points. You are now prepared to start Daysim.

Step 2: start DAYSIM

Under Windows: go to START > PROGRAMS > DAYSIM2.1 > DAYSIM or use the DAYSIM shortcut on your desktop

Under Linux: at the command line type: daysim

The DAYSIM graphical user interface (GUI) should appear on your screen (Figure 5-1-4). The interface functions as:

• an editor to read/write a DAYSIM project header file that contains all information relevant for your Daysim project.

• a platform to execute the different DAYSIM subprograms.

• an editor to create shell scripts (Linux) or batch files (Windows) that execute the different DAYSIM subprograms. An overview of the relationship between the different RADIANCE subprograms is provided in Appendix A.


Figure 5.1-4: DAYSIM startup screen.

WARNING: In some rare case, you will get an error message that your PC does not recognize the JAR file extension. In that case, please refer to the trouble shooting section in chapter 3.

Step 3: start a new project

Under the FILE > NEW PROJECT dialogue choose NEW and pick a directory under which you want to store the files for your new DAYSIM project. As you will be using the scene and sensor files that were discussed in step 1, please go to /projects/Ex5.1DaylightingAnalysisOfASingleOffice/ and name the project header file header1.hea (Figure 1-2). The name of the project header file will be used as a prefix for the results file created by DAYSIM (see below). The project header file contains all the information for your DAYSIM projects. It is an ASCII file with a number of keywords that are explained in the DAYSIM documentation accessible via the HELP menu. More information will be added to this file as you enter more information in the different GUI menus. You can always view a current version of the file by left-clicking on the FILE menu.

Note: For DAYSIM to run properly, project directories and Daysim header file names must not have any blanks in them, e.g. call you Daysim project “version_1” instead of “version_1”.

Tutorial on the Use of Daysim/Radiance Simulations for Building Design – version: Aug-06 7

Figure 5.1-5: Create a New Project Directory dialogue box.

In the directory under which you store your project header file DAYSIM automatically creates the following subdirectories:

/rad - imported RADIANCE scene files

/tmp - temporary files

/wea - project climate files

/pts - sensor point file

/res - simulation results

Step 4: load climate data

You now need to import the climate data for Ottawa, Canada. A climate file contains annual time series of direct and diffuse irradiances. In Daysim, this data is combined with the Perez sky model to predict the luminous distribution of the sky at different times of the year (see also sections 1.6 and 2.1.1). The luminous distribution is a luminance mapping that describes the amount of daylight incident onto a building from the different parts of the sky. Climate data is stored in test reference years which also include a variety of other climate data.

Under the SITE > NEW SITE dialogue you can specify the climate data for your building site. DAYSIM supports two climate file formats:

• DAYSIM weather file (*.wea)

• EnergyPlus weather data file (*.epw)

You can pick these files either directly from your local hard drive or you can first open your browser (Figure 1-3) and download weather data for over 680 locations world wide from the EnergyPlus weather data site (Figure 5.1-7).


Figure 5.1-6: Pick a Site dialogue box.

You should save the downloaded epw files under C:\Daysim\wea\ or any other directory under which you want to stores the raw weather data files for your Daysim projects

4.

Figure 5.1-7: EnergyPlus weather data site.

Browse to a climate file of your choice and press next.

Figure 5.1-8: Load a climate file dialogue box.

4 You can change the name of your default climate directory under FILE->

PREFERENCES.


You can pick a simulation time step for your annual daylight simulation between 1 minute and 1 hour. For calculations of the electric lighting use you should pick 5 minutes (default). Press FINISH and wait until the subprogram ds_shortterm has created your project weather data file and stored it under the project subdirectory /wea. Your final SITE screen should look like Figure 5-1-10.

Figure 5.1-9: Choose Simulation Time Step.

Figure 5.1-10: Final Site dialogue box.

Note:

! Within the GUI you can left-click on the blue underlined labels for additional help.

! When you chose a time step smaller than one hour, a stochastic auto-correlation model is used to generate down to one minute time series of direct and diffuse irradiance from hourly means (see chapter 6.2).

! For this exercise the simulation should only take a couple of seconds as the Ottawa 5 minute file comes with the Daysim distribution. Depending on the speed of your computer, this calculation can take up to 20 minutes. The resulting short time step weather data file is centrally stored on your computer so that you only need to carry out the calculation once for each climate file.


Step 5: import building model and sensor point file

You now need to import the 3d studio file (*.3ds) that was previously exported from SketchUp (chapter 4.1). Go to BUILDING > IMPORT 3D BUILDING MODEL. As you can see in Figure 5.1.11, you have the choice of either importing a 3d Studio file, importing a Radiance rif-file or manually importing Radiance material and geometry files. An example of how to import a rif file is given in chapter 5.3. To import a Radiance file, please refer to design exercise 5.2.

Figure 5.1-11: Import 3D Building Model dialog box.

Choose “import a 3D Studio file (*.3ds)” and click on “continue>>”. Select PrivateOffices.3ds under subdirectory External Files (Figure 5.1.12).


Figure 5.1-12: Import 3D Building Model dialog box (continued).

When importing a 3d Studio file, Daysim first converts the file using via the mgf format into Radiance file format. mgf stands for “Materials and Geometry Format”. Once your 3ds file has been successfully converted into the Radiance format, a filter (rad2daysim.exe) runs over the Radiance scene. The filter erase all light sources from the building model converts all materials to grayscale. In case a material layer name corresponds to a material in the Daysim database, the material description used in the 3ds Studio file is replaced with the material from the Daysim database (see section 4.2 for details).

After a few seconds, the following message screen should appear on your screen.

Figure 5.1-13: Report from the conversion from Radiance to Daysim.

The message indicates that the material layers GenIntFloor, GenIntWall, GenIntCeiling, DblGlazSpecSel72, and SingGlazClear90 have been replaced with the material files of the same name stored in the Daysim material database (default: C:\Daysim\materials).


By clicking “OK” you finalize the import of the building model into Daysim. The building menu should now look similar to Figure 5.1.14.

Figure 5.1-14: Building menu after a successful import of a 3ds file.

On the left hand side you see a visualization of the building model you just imported. At this point you should

! verify whether the import into Daysim was complete (was the complete scene geometry imported into Daysim?) and

! review the Daysim material file as described in chapter 4.2.

As discussed in chapter 4.2, the Daysim material file for this building model already consists of realistic material properties that have been taken from the Daysim material database.

Next you need to import the sensor point file. As explained above, the sensor point file is an ASCII file that contains the location and orientations of particular points of interest in the building. Click on PICK A SENSOR FILE to choose .../ExternalFiles/center_line.pts.


Figure 5.1-15: Pick center_line.pts.

Note: Tips on how to generate a sensor point file are given in section 4.1.1.

Afterwards you need to specify the unit measured by each sensor in your sensor point file using the SPECIFY SENSOR UNITS button that appeared in the building menu after you imported the sensor point file. The corresponding dialog is shown below.

Figure 5.1-16: Specify sensor units dialog.

The dialog file shows the coordinates and orientations of all sensors in the sensor point file. Under sensor unit you can characterize the type of the individual sensors within the simulation by using a pull down menu. You can choose between luminance, illuminance, radiance, and irradiance sensors. By default, all sensor are illuminance sensors. In this exercise all sensors are illuminance sensors. Therefore, you can leave dialog 5.1.16 unchanged.

Finally, you need to pick your shading device model using the SHADING DEVICE MODE pull-down menu. Depending on the amount of detail you want to provide, DAYSIM allows three modes to model shading devices:


• static shading devices (e.g. light shelves): in this mode DAYSIM either assumes that the shading device is already part of your basic RADIANCE scene or that there is no shading device.

• dynamic shading device model (simple): in this mode DAYSIM uses a simplified model to consider the effect of a generic venetian blinds system on the annual daylight availability: DAYSIM uses the basic RADIANCE scene to calculate indoor illuminances when the blinds are retracted. During times of the year when the blinds are lowered due to direct glare, DAYSIM simply assumes that a generic blind system blocks all direct sunlight and transmits 25% of all diffuse daylight. The use of this simulation mode is recommended at an early design stage as explicitly creating and simulating a geometric blind model is very time consuming.

• dynamic shading device model (advanced): in this mode DAYSIM uses an explicit RADIANCE model of the shading device both in retracted and lowered positions. Please note that choosing this mode can more than double the required simulation time since two sets of daylight coefficients need to be simulated (shading device open and closed) and additional raytracing is necessary to simulate a lowered blind system. An example of how to use this mode is given in design exercise 5-3.

For this exercise please choose the second option (simple blinds). You BUILDING menu should now look like Figure 5.1.17. You can now run an actual daylight simulation.

Figure 5.1-17: BUILDING menu after the building model has been successfully entered.


Step 6: run a simulation

Under the SIMULATION menu (Figure5-1-18) annual indoor illuminance profiles for all sensors in the sensor point file are calculated. As shown in Figure A-1 in Appendix A, this calculation involves the use of two subprograms:

(1) Subprogram gen_dc calculates one or two sets of daylight coefficients for all sensor points depending on the underlying blinds model.

(2) Subprogram ds_illum combines the daylight coefficients with the project climate file to yield annual indoor illuminance profiles for all sensor points.

The second step usually only take a couple of minutes (depending on the size of your sensor point file) whereas the first can take hours up to days.

Before starting a simulation you need to pick an adequate set of RADIANCE simulation parameters. For this exercise, please choose the simulation parameters shown in Figure 5.1-18. The simulation parameters correspond to those for scene 1 in chapter 2.1.3. The simulation will take about 1 hour on a 1GHz processor. In case you first want to get a feeling of how the program works, you can set the ambient bounces to 2 to bring the simulation time down to a couple of minutes.

Figure 5-1-18: SIMULATION main dialogue box.

Via SIMULATION > RUN A SIMULATION you can start a simulation. The first dialogue box (Fig 5.1-19) allows you to pick which files you want to generate/re-generate. Usually all two boxes should be activated. Please left-click on NEXT.


Figure 1-19: First RUN SIMULATION dialogue box.

The second dialogue box (Figure 5.1-20) allows you to start the simulation either from within the DAYSIM GUI or independently as a batch file under Windows or as a shell script under a Linux/Unix environment.

Figure 5.1-21: Second RUN SIMULATION dialogue box.

Pick the first option and click FINISH. The simulation will take about 1 hour on a PC with a 1GHz processor.

Note: During the simulation under Windows a number of DOS windows will pop up on your screen. These DOS windows mark the different simulation steps namely:

- calculation of diffuse daylight coefficients: This simulation step is accompanied with a WARNING: “no light sources found”. This is perfectly normal as the Radiance scene does not contain any direct light sources during the calculation of the diffuse daylight contribution.

- calculation of direct daylight coefficients: This simulation step will take the longest since involves calculations with some 60 direct light sources which correspond to the typical sun position for your building site that appear over the course of the year.

- calculation of annual illuminance/luminance profiles (*.ill)

WARNING: Most Daysim users find out at this point if Daysim has not been properly installed on their computer. In that case the Daysim simulation will usually finish within a couple of seconds and the message below is displayed.


This failure to run properly execute Radiance is usually the result of either your path and/or directory names containing blanks “ “, or that the Windows installation program did not properly set all required environmental variables. To remedy the problem either rename your files or go to the Troubleshooting section in chapter 3.

Once the simulation is finished, the following result files should be stored in the directory C:\Daysim\projects\Ex5.1DaylightingAnalysisOfASingleOffice/res:

header1.dc – daylight coefficient file

header1.ill – annual illuminance profile (blinds up)

header1_down.ill – annual illuminance profile (blinds down)

The format of these files is explained in Appendix A. Note that the file prefix corresponds to the project header file name.

Step 7: carry out a daylighting analysis

After the raytracing run from the previous step is finished and after you verified that the two annual illuminance profiles (*.ill) and the daylight coefficient file (*.dc) are in the “res” subdirectory of your Daysim project, you can go to the ANALYSIS menu (Figure 5.1-22). This menu allows you to carry out an in-depth analysis of the annual daylight availability and electric lighting energy use in the investigated building. Entry fields are divided into three groups:


Figure 5.1-22: ANALYSIS dialogue box.

• Occupancy Profile: information on typical hours of occupancy

• User Requirements and Behavior: here you need to specify both, the amount of lighting typically required by the users of the space as well as general behavioral tendencies of the users: Daysim allows you to choose an “active user ” a “passive user” or an occupant population that is a mix of both basic user types. An active user considers interior daylight levels when setting the lighting and blinds as opposed to a passive user who keeps blinds lowered lighting switched on during occupied hours. Both behavioral patterns have been observed in field studies. Obviously, the two behavior patterns results in considerably different energy use. As a designer usually cannot anticipate the ratio of active to passive users in a future building, a hands-on approach is to assume an evenly mixed population (default setting: ‘mix of both’). If this user behavior is chosen, the electric lighting use is calculated for both types of users individually an the predicted energy use corresponds to a mean of both values. This user behavior option is recommended, when the investigated building zones can repetitively be found throughout the building. This requirement is met in this exercise, as the two office and the aisle can be found 30 times in the building (see Figure 5.1-1).

• lighting and shading control system: These entries allow you to describe the type of lighting and shading controls investigated. You can enter the installed lighting power density either in Wm

-2 or in Wft

-2 or in whatever floor unit you

choose. The simulation results will accordingly be in the corresponding unit, i.e. W/m

2 yr or W/ft

2 yr. You also need to specify where the work plane is

located within the space using the button: “Specify Work Plane”.


Please take some time to familiarize yourself with the input options by left-clicking on the blue field labels and set the lighting power density and zone size to 12Wm

-2 and 15m

2 respectively.

What is the “work plane”?

You need to specify which of the illuminance sensors in the sensor point file correspond to sensors on the work plane of the occupant who is switching the electric lighting and manipulating the shading device. A work plane illuminance sensor is usually facing upwards and located at about desk height (0.85cm). At each time step, Daysim will calculate the minimum illuminance of all work plane sensors. This minimum work plane illuminance will be used to determine whether the occupant manually activates the electric lighting at a particular time step.

The work plane sensors are also used to predict the appearance of direct glare. Direct glare is detected when direct sunlight above 50Wm

-2 (exterior direct irradiance) is

incident on the work plane. The Daysim subprogram gen_directsunlight predicts for each time step of the year whether direct glare conditions appear at the work place. This information will be stored under der “res” subdirectory in a direct glare profile called (header1.dir) .

Before you start a daylighting analysis, you need to specify the work plane sensors. A Daysim simulation report concentrates on one building section at a time. As the daylighting situation and requirements vary in both offices and the central aisle, a simulation report has to be generated for all three sections independently.

We will first concentrate on the South office. As shown in Figures 5.1-2 and 5.1-3, the first four sensors in the sensor point file are located in the South office. Assuming that the occupant will usually be seated between 2 and 3 meter away from the facade, we will choose the second and third sensor to be work plane sensors in the South office (see Figure 5.1-23).

Figure 5.1-23: ”Specify work plane” dialogue.


Note: If you do not specify any work plane sensors, Daysim will assume that all illuminance sensor in your sensor point file are on the work plane. In this exercise, this would lead to misleading predictions of the electric lighting use and the shading device setting as illuminance sensors are located in both offices and on the aisle.

Once you specified the work plane sensors, please click on “Start Daylighting Analysis” using the default options from Figure 5.1-22. This will prompt Daysim to generate a simulation report similar to the one shown below.

Table 5.1-1: Daysim Simulation Report for the South office.

Daysim Simulation Report Notes...

The predicted annual electric lighting energy use in the investigated zone is: 20 kWh/unit area Assuming a lighting zone size of 15 unit area, this corresponds to a total annual lighting energy use of 300 kWh/a

Site Description The investigated building is located in Ottawa (45.32 N/ 75.67 E). Daylight savings time lasts from April 1st to October 31st. The picture below shows a visualization of the building model.

User Description The zone is occupied Monday through Friday from 8:00 to 17:00. The occupant leaves the office three times during the day (30 minutes in the morning, 1 hour at midday, and 30 minutes in the afternoon). The total annual hours of occupancy at the work place are 1805.6.The electric lighting is activated 2356.3 hours per year. The occupant performs a task that requires a minimum illuminance level of 500 lux. The predicted annual electric lighting energy use of 2.5 kWh/unit area corresponds to the mean energy use in an ensemble of identical offices that are occupied by four user types:

! a user who operates the electric lighting in relation to ambient daylight conditions, opens the blinds in the morning (upon arrival), and lowers them when direct sunlight above 50 Wm-2 hits the seating position (to avoid direct glare),

! a user who operates the electric lighting in relation to ambient daylight conditions, and keeps the blinds lowered throughout the year to avoid direct sunlight,

! a user who keeps the electric lighting on throughout the working day, opens the blinds in the morning (upon arrival), and lowers them when direct sunlight above 50 Wm-2 hits the seating position (to avoid direct glare), and

! a user who keeps the electric lighting on throughout the working day, and keeps the blinds lowered throughout the year to avoid direct sunlight.

The coordinates of work place sensors are marked in blue in the table below.


x y z daylight factor [%]

daylight autonomy [%] (active user)

daylight autonomy [%] (passive user)

annual light exposure [luxh]

1.500 1.000 0.850 12.1 89.5 71.5 20769910

1.500 2.000 0.850 5.4 78.1 41.8 6436636

1.500 3.000 0.850 3.0 63.4 8.0 3696022

1.500 4.000 0.850 1.9 51.3 0.0 2508053

1.500 6.000 0.850 0.2 0.0 0.0 278091

1.500 7.000 0.850 0.2 0.0 0.0 261861

1.500 9.000 0.850 1.9 42.0 0.0 1605234

1.500 10.000 0.850 3.0 52.5 0.0 2348671

1.500 11.000 0.850 5.4 62.1 4.1 3758828

1.500 12.000 0.850 12.2 76.0 48.2 7142613

Each report lists some key simulation assumptions followed by a table with simulation results. Within the results table, the first three columns correspond to the x, y and, z coordinates of the sensors from the sensor point file. Column 4 shows the daylight factors for the individual sensor points. The last column shows the annual light exposure of the sensor points in luxh for active blind usage.

An analysis of the simulation report is provided in the following.

daylight factor distribution

The daylight factor only depends on the building model and is therefore independent of all entry fields in the ANALYSIS menu. Figure 5.1-24 shows an EXCEL graph of the daylight factor distribution from Table 5.1-1

#.

Figure 5.1-24: Daylight factor distribution in the office. (Figure generated with Microsoft Excel.)

The figure reveals that the daylight factor near the work plane lies between 3.0 and 5.4% for both offices. Note that the daylight factor distribution is identical in

# Please note that Daysim does not have the capability to display graphs. You have to

import the data generates in the *.el.htm file and import it into a spreadsheet of your choice.


the North and South office, the reason for this symmetry is that the reference CIE overcast sky is rotationally invariant. The daylight factor near the work planes (2-3m from the facade) lies above the 2% mark required by LEED. It rises above 5% closer to the window, which is relatively high for an office daylight factor (see Table 1-1 in chapter 1). This finding suggests that there is a need for a glare protection device in the offices for a VDT work place lose to the facade. The daylight factor analysis further suggests that there is only a negligible amount of daylight on the central aisle.

daylight autonomy distribution

As discussed in Table 1-1, daylight factor predictions are of limited use for design purposes, as they are based on a single sky condition. The daylight autonomy has been developed to provide a more holistic daylighting analysis in a building. It depends on the minimum illuminances threshold, the specified user occupancy, and the type of blind control used. The daylight autonomy is defined as the percentage of occupied hours during the year when the minimum illuminance level is provided at a sensor by daylight alone.

In the default setting, Daysim assumes that the offices are occupied weekdays from 8AM to 5PM with a one hour lunch break and two 30 minute breaks throughout the day. The minimum illuminance threshold is 500 lux which coincides with recommended minimum illuminance levels for type b desk work stipulated by the Canada Labour Code, Part II - Canada Occupational Health and Safety Regulations. Two daylight autonomies are given in the results table: one for an active and a second for a passive blind user. The results in table 5.1-1 refer to the daylight autonomy in the South office, as the work plane chosen is located in the South office. To calculate the daylight autonomy for the North office as well you need to do the following:

! save the Daylight Simulation report for the South office under a different name

! change the work plane sensor to a work place in the North office (Figure 5.1-25)

! rerun “Start Daylighting Analysis”.


Figure 5.1-25: Reset the work plane sensor for an analysis of the North office.

Finally, to calculate daylight autonomy on the aisle, you need to set the minimum illuminance level to 100 lux which corresponds to the recommended level for “a service area with frequent usage” according to the Canada Labour Code. To get a conservative estimate of the daylight autonomy on the aisle, you should configure the work plan sensors in the South and North office synchronously.

Recommended illuminance levels and maximum lighting power densities

In Daysim the electric lighting system is characterized through the choice of lighting control and the installed lighting power density. Recommended values for according to the IESNA Lighting Handbook, the Canadian Labor Code and German DIN 3035 can be accessed by clicking on the minimum illuminance level label. Similarly, recommended maximum lighting power densities can be accessed under installed lighting power density.

The resulting daylight autonomy distribution in the three spaces are shown in the EXCEL graph below.


Figure 5.1-26: Daylight autonomy distribution in the offices(minimum illuminance level of 500 lux and manually control blinds) and on the aisle (minimum illuminance level of 100lux). (Figure generated with Microsoft Excel.)

The figure reveals that in both office the occupants can in principle work between 40% and 80% of the year by daylight alone depending on how they use their blinds. It is also worth mentioning, that the daylight autonomy in the North office is marginally larger than in the South office. The reason for this is that glare is less of an issue for the North office. In the South office, reduced window size and/or a more advanced shading device such a split blind system might provide a more effective way to reduce glare than the default venetian blind system investigated in this example.

The figure also predicts a daylight autonomy over 30% on the aisle. This reveals that sufficient lighting levels are routinely reached on the aisle by daylight alone. A convenient way to reduce the electric lighting use on the aisle - if allowed by local safety regulations - could be through manual switches combined with a timer.

electric lighting use

The second part of your task is to estimate the energy saving potential of an occupancy sensor in the two offices. The predicted annual electric lighting use is provided at the beginning of each simulation report. As shown in Table 5.1-1, the predicted annual electric lighting use for the South office is 20 kWh/ unit area which corresponds to 300kWh/a per office assuming an installed lighting power density of 12Wm

-2 in the 15m

2 offices (width x depth =3m x 5m). If you rerun the

simulation for a switch-off and a switch on/off occupancy sensor, you will get the following lighting energy uses for the north and South office.


Figure 5.1-27: Annual electric lighting use in the north and South offices for three different lighting control strategies. (Figure generated with Microsoft Excel.)

Figure 5.1-27 reveals that the lighting use for both office orientations will be very similar. Introducing an occupancy sensor that switches the electric lighting off when absence has been detected for more than 5 minutes saves about 30% of lighting energy in both offices. If on the other hand an occupancy sensor is installed that switches the electric lighting on and off, the lighting energy use rises as such a lighting control systems hinders occupants from ever working by daylight alone.

Step 8: summing up

The daylight factor analysis in the offices yielded a level between 3 and 5% near the work plane. Assuming occupancy during regular office hours (Mo-Fr. 8.00-17.00) and a work that requires a minimum desktop illuminance of 500 lux on the desk, the occupants could work 40-80% of the year by daylight alone depending on the type of shading device used. A further going analysis should concentrate on either reducing window sizes or using a more advance shading device. For an installed electric lighting power density of 12 Wm

-2, the mean annual electric

lighting use in all the offices would be around 300kWh/yr in both offices. An occupancy sensor that switches the lighting automatically off after a delay time of 5 minutes would reduce the mean annual electric lighting use in the offices by roughly 30%. Assuming an additional investment cost of $25 for such an occupancy sensor and electricity costs of 10cent/kWh, the payback time for the occupancy sensor would be around 2.8 years.


Note: If you want to present Daysim simulation results in your report, you can open res/SingleOffice.el.htm directly in MS-Word and quickly integrate the simulation report in your standard report format.

5.1 exercise: daylighting analysis of a single office · tutorial on the use of daysim/radiance...

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