Most of the energy use of commercial buildings in the United States is electricity and natural gas. Electricity accounts for 56 percent of the energy use, and natural gas accounts for 32 percent (Figure 22.1). The natural gas is primarily used for heating, which suggests that increasing the thermal resistance of the building envelope can save significant energy. The electricity use in commercial buildings is 38 percent lighting, 14 percent cooling, and 12 percent ventilation (Figure 22.2). Lighting, being the largest usage of electricity in commercial buildings deserves considerable attention in any energy saving schemes.
There are multiple dimensions involved in good energy efficient lighting. The efficacy of the light source is important. Efficacy is the lumens of light produced per watt of input energy (see Table 22.1). The quantity of light distributed onto working surfaces needs to be enough for the work that needs to be performed, but this quantity should be concentrated if possible only on the work area. The amount of light needed to walk around in lobbies and circulation spaces is lower than needed on work surfaces. When daylight is available to illuminate spaces, the electric lighting needs to be dimmed to save energy. When rooms, such as conference rooms, are not always occupied, there should be occupancy sensors to automatically turn off the lights.
Commercial buildings are primarily illuminated with linear fluorescent lamps. Newer fluorescent lamps are thinner, five eighths of an inch in diameter, than the traditional inch and a half diameter fluorescent lamps. Fluorescent lamps produce light by running an electrical current through a gas with mercury vapor in it. When electrons hit the mercury atoms the outer valence electrons of the mercury atoms are pushed to a higher level. When they fall back to their standard level they give off a limited multiline spectrum of light. This light is barely visible. The mercury spectrum is transformed to a useful spectrum of visible light with phosphors on the inside surface of the fluorescent lamp. Current phosphors provide a spectral mix of red, green, and blue light, which mixes together to produce white light with good color rendering, and high efficacy.
FIGURE 22.1 Commercial building energy use by source for the United States.
Source: United States Energy Administration, Annual Energy Review 2011. www.eia.gov/are.
FIGURE 22.2 Commercial building electricity use by function in the United States.
Source: United States Energy Administration, Annual Energy Review 2011. www.eia.gov/are.
TABLE 22.1 Lamp wattage, efficacy, life, and color rendering index (CRI).