Lights

Lighting constitutes about 7 percent of the total energy use in residential buildings and 25 percent in commercial buildings. Therefore, lighting can have a significant impact on the heating and cooling loads of a building. Not counting the candle light used for emergencies and romantic settings, and the kerosene lamps used during camping, all modern lighting equipment is powered by electricity. The basic types of electric lighting devices are incandescent, fluorescent, and gaseous discharge lamps.

The amount of heat given off per lux of lighting varies greatly with the type of lighting, and thus we need to know the type of lighting installed in order to predict the lighting internal heat load accurately. The lighting efficacy of common types of lighting is given in Table 9. Note that incandescent lights are the least efficient lighting sources, and thus they will impose the greatest load on cooling systems (Fig. 28). So it is no surprise that practically all office buildings use high-efficiency fluorescent lights despite their higher initial cost. Note that incandescent lights waste energy by (1) consuming more electricity for the same amount of lighting and (2) making the cooling system work harder and longer to remove the heat given off. Office spaces are usually well lit, and the lighting energy consumption in office buildings is about 20 to 30 W/m² (2 to 3 W/ft²) of floor space.

The energy consumed by the lights is dissipated by convection and radiation. The convection component of the heat constitutes about 40 percent for fluorescent lamps, and it represents the instantaneous part of the cooling load due to lighting. The remaining part is in the form of radiation that is absorbed and reradiated by the walls, floors, ceiling, and the furniture, and thus they affect the cooling load with time delay. Therefore, lighting may continue contributing to the cooling load by reradiation even after the lights have been turned off. Sometimes it may be necessary to consider time lag effects when determining the design cooling load.

The ratio of the lighting wattage in use to the total wattage installed is called the usage factor, and it must be considered when determining the heat gain due to lighting at a given time since installed lighting does not give off heat unless it is on. For commercial applications such as supermarkets and shopping centers, the usage factor is taken to be unity.