General lighting

Lighting tab in model data

General lighting accounts for the main lighting in the room. It is assumed to be able to be replaced by daylight through lighting control.

Units

When the Gain definition level model option is set to 2-Zone by zone in model data you can select the units to be used for lighting here on the Lighting tab. Choose from the options:

• 1-Power density - where the General lighting gains are defined per floor area. This method has the advantage that the level of gains can clearly be understood by looking at W/m2 or W/ft2 values and floor area. However it can cause problems when loading data from lighting templates in that if the activity is changed subsequently with a different illuminance requirement, the lighting power density is not updated. This issue has led to the introduction of the second option.
• 2-Normalised power density - Although the units for Normalised power density (W/m2-100 lux or W/ft2-fc) may seem quite obscure, this method of defining lighting gains has advantages over using simple Power density. Namely that the actual lighting level is associated with the lighting system type without the need to refer to the zone activity and hence illuminance requirement. So if a particular lighting system type is installed throughout a building, but the building has a range of activities each with different illuminance requirements, it will usually be possible to enter the lighting Normalised power density value once only at building level, set the activity for each zone and obtain realistic lighting gains.
• 3-Absolute zone power - where the lighting gains are defined as W without reference to the zone floor area. This option is used when the actual absolute lighting gains levels are known in W. Unless you need to enter specific known lighting equipment gains zone by zone, this option is best avoided as it increases the amount of work involved due to the inheritance mechanism not working effectively for gains that are not normalised by floor area.

Note: When using the 3-Absolute zone power option, the lighting gains data is normally entered individually for each zone because values inherited from building and block levels will typically not be correct.

Gain

Set the design level of energy consumption due to general lighting. Depending on the Lighting gain units model option setting, the gains data is entered as Power density (in W/m2 or W/ft2) or as Normalised power density (W/m2-100lux or W/ft2-fc) or as Absolute zone power (in W).

The fuel is assumed to be electricity.

Power density

When the 1-Power density Lighting gain units model option is selected gains are entered in W/m2 or W/ft2.

Note: When loading lighting data from template and using the 1-Power density option, the actual lighting gain per area is calculated by dividing the Normalised power density data in the template by the required illuminance level as currently set on the Activity tab. If you select a new Activity with different illuminance level requirements, the Lighting tab does not automatically update the Power density.

Normalised power density

When the 2-Normalised power density Lighting gain units model option is selected the maximum lighting gains are defined as W/m2-100lux or W/ft2-fc and the actual lighting energy used for the zone in the simulation is based on this value plus floor area and illuminance requirements as follows:

Max Lighting power (W) = Lighting energy (W/m2-100lux) x Zone floor area (m2) x Zone Illuminance requirement / 100

Absolute zone power

When the 3-Absolute zone power Lighting gain units model option is selected the lighting gains are defined as W without reference to the zone floor area. This option used when the actual absolute lighting gains levels are known in W. Unless you need to enter specific known lighting equipment gains zone by zone, this option is best avoided as it increases the amount of work involved due to the inheritance mechanism not working effectively for gains that are not normalised by floor area.

Calculations

For all of the above 3 options, the lighting energy should be the maximum electrical power input to lighting in a zone, including ballasts, if present. This value is multiplied by a schedule fraction (below) during the simulation to get the lighting power in a particular time step.

The electrical input to lighting ultimately appears as heat that contributes to zone loads or to return air heat gains. In EnergyPlus this heat is divided into four different fractions. Three of these are given by the input fields Return Air Fraction, Fraction Radiant and Fraction Visible. A fourth, defined as the fraction of the heat from lights convected to the zone air, is calculated by EnergyPlus as:

Convected Fraction = 1.0 – (Return Air Fraction + Radiant Fraction + Visible Fraction)

Note: An error message is displayed if Return Air Fraction + Radiant Fraction + Visible Fraction exceeds 1.0. These fractions depend on the type of lamp and luminaire, whether the luminaire is vented to the return air, etc.

Schedule

If the Internal gains operate with occupancy model option is not set (the default state) then General lighting operation is governed by the operation schedule selected here.

The level of lighting gains are multiplied by the time-varying factor in the schedule to give the gain for each timestep in the simulation. Otherwise, if the Internal gains operate with occupancy model option is set then these gains are synchronised with the Occupancy schedule.

Luminaire type

Select from:

When you select using the drop-down list an appropriate set of defaults for the luminaire type is loaded according to the table below, assuming fluorescent lighting.

Approximate values of Return Air Fraction, Radiant Fraction and Visible Fraction for overhead fluorescent lighting for different luminaire configurations. These values assume that no light heat goes into an adjacent zone. Source: Lighting Handbook: Reference & Application, 8th Edition, Illuminating Engineering Society of North America, New York, 1993, p. 355.

See below for more Luminaire Performance Data.

Return air fraction

For Return Air Ducted luminaires this is the fraction of the heat from lights that is convected out of the room and into the zone return air (normally into a return plenum). If the return air flow is zero or the zone has no return air system, the program will put this fraction into the zone air. Return Air Fraction should normally be greater than zero for 5-Return-air ducted luminaires. This value is not used for non-return air ducted luminaires, i.e. for Luminaire types 1-4.

Radiant fraction

The fraction of heat from lights that goes into the zone as long-wave (thermal) radiation. The program calculates how much of this radiation is absorbed by the inside surfaces of the zone according the area times thermal absorptance product of these surfaces.

Fraction visible

The fraction of heat from lights that goes into the zone as visible (short-wave) radiation. The program calculates how much of this radiation is absorbed by the inside surfaces of the zone according the area times solar absorptance product of these surfaces.

Note: selecting an option from the Luminaire type drop-down list loads default Return Air Fraction, Radiant Fraction and Visible Fraction data from the table above.

Fraction convected

The fraction of the heat from lights convected to the zone air is calculated from:

Fraction convected = 1.0 – (Return Air Fraction + Fraction Radiant + Fraction Visible)

You will get an error message if Return Air Fraction + Fraction Radiant + Fraction Visible exceeds 1.0.

This data is provided on screen for information purposes.

Luminaire Performance Data

 

Data on luminaire performance can also be obtained from ASHRAE Fundamentals 2013, 18.6 Table 3:

 

 

The Space fraction in the above table from ASHRAE is the fraction of the gain entering the room and corresponds to 1 - Return air fraction.

 

Also, approximate values of Return Air Fraction, Fraction Radiant and Fraction Visible for overhead fluorescent lighting for a variety of luminaire configurations are provided below. The data is based on ASHRAE 1282-RP “Lighting Heat Gain Distribution in Buildings” by Daniel E. Fisher and Chanvit Chantrasrisalai.

 

Fixture No.	Luminaire Feature	Return Air Fraction	Fraction Radiant	Fraction Visible	fconvected
1	Recessed, Parabolic Louver, Non-Vented, T8	0.31	0.22	0.20	0.27
2	Recessed, Acrylic Lens, Non-Vented, T8	0.56	0.12	0.20	0.12
3	Recessed, Parabolic Louver, Vented, T8	0.28	0.19	0.20	0.33
4	Recessed, Acrylic Lens, Vented, T8	0.54	0.10	0.18	0.18
5	Recessed, Direct/Indirect, T8	0.34	0.17	0.16	0.33
6	Recessed, Volumetric, T5	0.54	0.13	0.20	0.13
7	Downlights, Compact Fluorescent, DTT	0.86	0.04	0.10	0.00
8	Downlights, Compact Fluorescent, TRT	0.78	0.09	0.13	0.00
9a	Downlights, Incandescent, A21	0.29	0.10	0.6	0.01
9b	Downlights, Incandescent, BR40	0.21	0.08	0.71	0.00
10	Surface Mounted, T5HO	0.00	0.27	0.23	0.50
11	Pendant, Direct/Indirect, T8	0.00	0.32	0.23	0.45
12	Pendant, Indirect, T5HO	0.00	0.32	0.25	0.43