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

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.

• The lux is the SI unit of illuminance and luminous emittance - measuring luminous flux per unit area.

• 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, lm/m²)

η_e = light equipment efficiency

η_r = room lighting efficiency

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

• The lumen is the SI derived unit of luminous flux - a measure of the total visible light emitted by a source

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:

Lamp Type	Emitted Light from The Source (lumen/Watt)	Lifetime (hours)
GLS Bulbs	10 - 15	1,000
Low Voltage Halogen	20	2,000 - 5,000
Mercury Vapor	40 - 60	22,000
Fluorescent	50 - 90	more than 7,000
Metal Halide	70 - 90	more than 12,000
White LED	80+
High Pressure Sodium	90 - 125	25,000
Low Pressure Sodium	120 - 200	20,000

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

Light Equipment Efficiency and Room Lighting Efficiency influences each other. Common values of the product η_e η_rare 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 lighting equipment efficiency an be set to 0.5.

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

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

    = 200 W/m²

Using standard fluorescent tubes - the electric power for lighting 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

Installed effect (W)	Illumination - Light Level (lux)
	200	400	600	800	1000
Incandescent GLS bulb lamp	40	80	120	160	200
Fluorescent tubes	6.7	13.3	20	26.7	33.3

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