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, if this option is selected.

Normalised power density

There are two ways to define the lighting energy use in a zone depending on the Lighting gain units setting made in model options:

1. Watts per m2 - where the maximum lighting gains are defined as W/m2 independent of the required illuminance level.
2. Watts per m2 per 100lux - where the maximum lighting gains are defined as W/m2-100lux 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
   
   Example to calculate W/m2/100lux: For a lighting system emitting 40 W/m2 and providing 500 lux, the value to enter is 40/5 = 8W/m2-100lux

For both the above 2 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 during the simulation to get the lighting power in a particular time step.

Note: When loading lighting data from template and using the W/m2 option, the actual lighting W/m2 gain to be loaded is calculated by dividing the W/m2/100lux data in the template by the required illuminance level as currently set on the Activity tab. Once the lighting template has been loaded, the W/m2 value can be overridden at zone level. Also if you select a new Activity with different illuminance level requirements, the Lighting tab does not automatically update the W/m2 unless you reapply the lighting template.

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)

You will get an error message 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.

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.

More up to date data on luminaire performance can also be obtained from ASHREA Fundamentals 2013, 18.6 Table 3:

Note: 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.

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 but it can also be greater than zero when not return air ducted through natural ventilation etc.

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.

Operation

If 'Internal gains operate with occupancy' under model options is not set General lighting has a specific operation schedule set under General Lighting, otherwise these gains are synchronised with occupancy.