Heat Gain from Lights

Heat contribution from lights may have major impacts on air condition systems

The heat gained from lights in a modern office or production area may be of an significant amount. Heat emitted to the room depends on

• preferred light level in the room
• type of lights and their construction
• location of the light equipment

Light Level - Illuminance

• Illuminance is the total luminous flux incident on a surface - per unit area.

The preferred light level in a room depends primarily on the type of activity. For common office work the level may be in the range of 500 - 1000 lux.

• Recommended Light Levels in Rooms

Required Installed Electric Power for Lights

Unless special arrangements as local cooling or air outlets through the lighting equipment are used, the electric power to the lights are converted to heat emitted to the room. Required electric power to achieve a recommended light level can be estimated as

P = b / (η_e η_r l_s)         (1)

where

P = installed electric power (W/m² floor area)

b = recommended light level (lux, lumen/m²)

η_e = light equipment efficiency

η_r = room lighting efficiency

l_s = emitted light from the source (lumen/W)

Emitted Light From Source - l_s

The purpose of a lamp is to convert electrical power (Watts) into light (lumens). Lamps do this with varying efficiencies and the light emitted from a source depends on the type of source.

Typical efficiency of different lamp types are indicated in the table below:

• A typical incandescent GLS light bulb emit approximately 10 lumen/Watt
• A typical fluorescent tube emits up to approximately 60 lumen/Watt

Light Equipment Efficiency - η_e

Light equipment efficiency express how much of the light is really emitted from the light to the room.

A bare fluorescent tube emits 100% to the room. A shielded tube emit less - 50% to 80% is common.

Room Lighting Efficiency - η_r

The room lightning efficiency express how much of the light is absorbed by the room before entering the activity area.

Light Equipment Efficiency and Room Lightning Efficiency influences each other. Common values of the product η_e η_r are in the range 0.3 - 0.6.

Example - Heat Load from Lights

1,000 lux is recommended light level in a office where detailed drawing work is performed. The room and lightning equipment efficiency an be set to 0.5.

Using standard GLS bulbs - the electric power for lightning can be calculated

P = (1,000 lumen/m²) / (0.5 (10 lumen/W))

    = 200 W/m²

Using standard fluorescent tubes - the electric power for lightning can be calculated

P = (1,000 lumen/m²) / (0.5 (60 lumen/W))

    = 33.3 W/m²

Due to the high energy consumption and the major impact on air condition heat loads, standard GLS bulbs is not an alternative in high illuminance applications.

The table below indicates typically installed electrical effect with different light levels

Note! Datasheets from manufacturers should always be consulted before detailed engineering. The numbers above do for rough preliminary calculations.

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