Daylighting. Note: Daylight is typically defined as diffuse light and is very different from sunlight (direct solar radiation).

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1 systems can significantly reduce both lighting consumption and cooling-energy consumption by reducing the electric-lighting heat gain in the building. However, daylighting can also cause increased heating-energy consumption, something that must be considered when evaluating adding daylighting to a building. Different control strategies, window configurations, surface reflectivities, and so forth, can greatly influence the energy savings provided by these systems. Note: Daylight is typically defined as diffuse light and is very different from sunlight (direct solar radiation). The TRACE 700 algorithm is identical to that used by the DOE-2 computer program except for the user interface. It is comprised of three main stages, as follows. 1 A preprocessor calculates a set of daylight factors (interior illuminance divided by exterior horizontal illuminance) for later use in the hourly-loads calculation. The program automatically calculates the three-dimensional coordinates of one or two reference points in a space, using either the room and glass dimensions or optional aspect ratios defined by the user. The algorithm then integrates over the area of each window to obtain the contribution of direct light from the window to the illuminance at the reference points, and the contribution of light from sky and ground, which enters the window and reflects from the walls, floor, and ceiling before reaching the reference points. Taken into account are such factors as window size and orientation, glass visible transmittance, inside surface reflectance of the space, sun-controlled devices such as blinds and overhangs, and the illuminance distribution of the sky. Because this distribution depends on the position of the sun and the of 20 different solar altitude and azimuth values, covering the annual range of sun positions. 2 An hourly daylight-illuminance glare calculation is performed. The illuminance contribution from each window is found by interpolating the stored daylight factors using the current-hour sun position and cloud cover, and then multiplying by the current- CDS-PRM001-EN TRACE 700 User s Manual System Control Strategies 5 33

2 hour exterior horizontal illuminance obtained from the Step 1 calculation. If applicable, the solar-gain threshold is tested to determine whether the window blinds or drapes should be closed to maintain a user-defined comfort level. Adding the illuminance contributions from all the windows (including skylights) then gives the total illumination (daylight effect) at each reference point. 3 Stepped and continuously dimming control systems are simulated to determine the electrical lighting energy needed to make up the difference, if any, between the daylighting level and the required illuminance. The effect of the diminished lighting level (and possibly reduced solar heat gain if the internal shading device is employed) is calculated and passed to the load phase. Application considerations does not necessarily mean an increase in the amount of fenestration on a building. Proper placement and configuration for the fenestration is as important to a properly designed daylighting system as the amount of fenestration. When designing a building with daylighting, be aware of the possible increase in solar heat gain and the possibility of glare from sunlight. Related reading For additional information related to daylighting, refer to Daylight in Buildings: A Source Book on Systems and Components at: Sample scenario controls will be added to all of the perimeter rooms in a building. All windows were input using dimensions (rather than percent glass), and a standard stepped controller will be used. The user would like to find the savings that daylighting will provide for the building. Note: Assuming that the other default inputs are acceptable, this scenario can be modeled in TRACE 700 with as few as three inputs System Control Strategies TRACE 700 User s Manual CDS-PRM001-EN

3 To model daylighting: 1 Start by modeling the first alternative completely. It is required that all glass be input using the same method (percent or dimensions) for daylighting. For more-accurate modeling of daylighting, the dimensions method is recommended. After Alternative 1 has been completed, a second alternative can be created with daylighting. Two alternatives are not required to model daylighting. However, to determine the effects of daylighting on a model, two alternatives are recommended and will be used in this example. 2 To create the second alternative, click Copy Alternative on the Alternatives Setup menu to create a copy of Alternative 1. 3 With the second alternative created, the templates from Alternative 1 must be copied so that the design and system section of Alternative 2 will be recalculated with daylighting. Right-click Alternative 2 and click Templates on the list. To make the copy of the templates editable, select Create Templates for this Alternative based on Alternative 1 and click OK. CDS-PRM001-EN TRACE 700 User s Manual System Control Strategies 5 35

4 4 To activate, click Create Definition on the Actions menu. The Parameters screen should open, displaying where the daylighting definitions will be created and added to the spaces. The Parameters screen should open to the General tab. Note: Inside Visible Reflectivity for Floor, Wall, and Partition are defined via the Advanced tab in the Construction library and can be overridden on the Construction Parameters tab System Control Strategies TRACE 700 User s Manual CDS-PRM001-EN

5 5 Geometry Type, Atmospheric Moisture, Atmospheric Turbidity, and Inside Visible Reflectivity are input from the General tab. Note: Select the Geometry Type that reflects how the glass was input, in this example, as GLAS-DIM. Note: Recommended values and typical ranges for these values can be obtained by accessing Help (press F1). Note: Inside Visible Reflectivity for Floor, Wall, and Partition are defined via the Advanced tab in the Construction library and can be overridden on the Construction Parameters tab. a The Geometry Type tells the program how the glass was input and is used to determine the fraction of the available daylight, passing through the glazing area, that can be used to offset the power consumption of the electric lights. b The Atmospheric Moisture represents the amount of perceptible water vapor in the atmosphere and affects the calculation of the luminance of clear skies (this is not the amount of rainfall). c The Atmospheric Turbidity is a measure of aerosols (particulate pollutants and water droplets) in the atmosphere and affects the calculation of the luminance of clear skies. d The Inside Visible Reflectivity is the fraction of visible (not solar) light that is reflected, rather than absorbed, by the opaque surface. This value is used to help calculate the luminance of both natural and electric lighting within a space. CDS-PRM001-EN TRACE 700 User s Manual System Control Strategies 5 37

6 6 Click the Controls tab, which contains standard and userdefined Control Strategies. No inputs are required on this tab for the example scenario because it is using a default control type. Note: To create a custom controls template, click Copy and enter a description. This activates the fields under the Controls and Geometry sections. Note: Values under the Controls section should be input starting at the top and working down: a The Percent of Space Affected represents the fraction of the space floor area for which the lights are controlled by the specified daylighting controls. b The Lighting Setpoint represents the design lighting level in the space, in footcandles. c The Lighting Control Type is the type of daylighting control being used in the template (continuous or stepped). Depending on the Lighting Control Type selected, either input the Minimum Power and Lighting Percent or the Light Control Steps to simulate how the selected controls will operate. d The Light Control Probability is the probability that manually controlled, stepped controls are utilized. Note: Under the Geometry section, input the Reference Height (A) of the daylight sensor. All other values under the Geometry section will be autocalculated if not input. Multiple templates can be created and used in the same file and even in the same space System Control Strategies TRACE 700 User s Manual CDS-PRM001-EN

7 6 Click the Room Parameters tab to select the Alternative, the Schedule, the Reference Points, and the Perimeter Depth. 7 For this schedule, choose Available (100%) and click Yes to apply this schedule to all rooms. a The Schedule acts as an on/off switch. Utilization percent hours > 1 percent means the daylighting controls are available, while utilization percent hours = 0 means the daylighting controls are deactivated and no daylight reduction of lighting energy is allowed that hour. Typically, the schedule will be available 100 percent, off 0 percent, or a custom schedule that neglects daylighting during design but applies it during energy analysis (for details on creating schedules, refer to Create schedules on page 2 19). b The Reference Points are used to select the templates created on the Controls tab. Up to two Reference Points are allowed per space. c The Perimeter Depth is used to calculate the distance of the Reference Point from the perimeter wall. In addition, if the glass dimension method is used, Perimeter Depth is used to estimate the field of view seen by the daylighting sensor. Perimeter Depth can be selected as the room length or width, or manually input for each space. 8 For Reference Point 1, choose Std Stepped Controller and click Yes to apply this to all rooms. Note: If daylighting is added to a space that has no fenestration, then the daylighting controls will be ignored for that space. CDS-PRM001-EN TRACE 700 User s Manual System Control Strategies 5 39

8 10The Glass Parameters tab allows the user to input the Visible Transmissivity and the Inside Visible Reflectivity. No inputs are required on this tab for the example scenario. Only glass types used in the file will appear under the Glass Library Member menu. Refer to online Help for a table with typical Inside Visible Reflectivity values. 11The Internal Shade Parameters tab allows the user to input the Visible Transmissivity and the Inside Visible Reflectance of the glass and internal shade combination at normal incidence. No inputs are required on this tab for the example scenario. 12 When completed, click OK and the Parameters will close. The second alternative can now be calculated and the effects of daylighting can be seen by comparing the output reports. The Visible Transmissivity represents visible transmittance of the glass at normal incidence. Do not confuse visible (daylight) transmittance with total solar transmittance, which determines how much solar radiation is transmitted. The Inside Visible Reflectivity is the fraction of visible (not solar) light that is reflected on the inside surface of the glazing. This value is used to help calculate the luminance due to both natural and electric lighting within a space. The Overall Visible Transmittance can be obtained by multiplying the Visible Transmittance of the shading device by the Visible Transmittance of the unshaded glass System Control Strategies TRACE 700 User s Manual CDS-PRM001-EN

9 Additional Items 1 User-defined templates are available only for the file in which they were created. 2 Typical values and explanations for all fields can be found by pressing F1 to access online Help. 3 can affect all sections of the building simulation (depending on the daylighting schedule selected on the Room Parameters tab). For cooling-load design, the cooling load and airflow will both change based on the lower lighting load, possible increased solar loads, changes in plenum temperature, and changes in time of peak. The heating-design simulation will also be affected, but the effects may be smaller than the effects on the cooling-design simulation, due to the typical neglecting of the solar and internal loads in heating-design calculations. In TRACE 700, two alternatives (one with daylighting and one without daylighting) can be used compare the overall effects of daylighting on the rooms, systems, plants, and life-cycle costs. The checksums reports can be used to view the effects on the design calculations for the rooms and systems usually a reduction in cooling capacity and airflows occurs. The energy consumption reports can be used to view the effects on the plants usually lower lighting energy, lower cooling energy, and possibly higher heating energy (because the heat given off by the lights in the spaces is less). The economic reports can be used to see the reduction in utility costs and the effects on the life-cycle cost for the building. Note: For conservativeness, it may be desirable to disregard daylighting during the sizing calculations. This can be done by creating a custom utilization schedule, which can be selected on the Room Parameters tab (refer to step 8 of the daylighting example for the location of the daylighting utilization schedule). The custom schedule would have the cooling-design and heatingdesign daytypes input as midnight-to-midnight 0 percent (to turn off the daylighting controls during the design simulation). The Weekday, Saturday, and Sunday daytypes in the custom schedule would define the hours that daylighting controls should be available to operate as Available 100%. For example, Weekday to Sunday could be set to 100 percent from midnight to midnight, to have the daylighting controls available to reduce the lighting power during any hour of the energy-analysis simulation. It is recommended that a full 8760 simulation be performed if the CDS-PRM001-EN TRACE 700 User s Manual System Control Strategies 5 41

10 daylighting controls are going to be neglected during the design simulation. 4 For additional information related to creating custom schedules, refer to Creating schedules on page For additional information related to performing a full 8760 simulation, refer to Adding weather locations and activating the 8760 calculation methodology on page For additional information related to daylighting, visit: or System Control Strategies TRACE 700 User s Manual CDS-PRM001-EN