Annual Lighting Cost of a Classroom
The lighting needs of a classroom are met by 30 fluorescent lamps, each consuming 80 W of electricity (Fig. 2–52). The lights in the classroom are kept on for 12 hours a day and 250 days a year. For a unit electricity cost of 11 cents per kWh, determine the annual energy cost of lighting for this classroom. Also, discuss the effect of lighting on the heating and air-conditioning requirements of the room.
The lighting of a classroom by fluorescent lamps is considered. The annual electricity cost of lighting for this classroom is to be determined, and the lighting’s effect on the heating and air-conditioning requirements is to be discussed.
Assumptions The effect of voltage fluctuations is negligible, so each fluorescent lamp consumes its rated power.
Analysis The electric power consumed by the lamps when all are on and the number of hours they are kept on per year are
\quad\quadLighting power =( Power consumed per lamp ) \times( No. of lamps )
\begin{aligned}&=(80 \mathrm{~W} / \text { lamp })(30 \text { lamps }) \\&=2400 \mathrm{~W}=2.4 \mathrm{~kW} \\\text { Operating hours } &=(12 \mathrm{~h} / \text { day })(250 \text { days } / \text { year })=3000 \mathrm{~h} / \text { year }\end{aligned}
Then the amount and cost of electricity used per year become
Lighting energy =( Lighting power )( Operating hours )
\quad\quad\quad\quad\quad\quad\quad\quad\quad =(2.4 \mathrm{~kW})(3000 \mathrm{~h} / \text { year })=7200 \mathrm{kWh} / \mathrm{year}
Lighting cost = (Lighting energy)(Unit cost)
\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad=(7200 \mathrm{kWh} / \text { year })(\$ 0.11 / \mathrm{kWh})=\$ 792 / \text { year }
Light is absorbed by the surfaces it strikes and is converted to thermal energy. Disregarding the light that escapes through the windows, the entire 2.4 \mathrm{~kW} of electric power consumed by the lamps eventually becomes part of thermal energy of the classroom. Therefore, the lighting system in this room reduces the heating requirements by 2.4 \mathrm{~kW} but increases the air-conditioning load by 2.4 \mathrm{~kW}.
Discussion Note that the annual lighting cost of this classroom alone is close to \$ 800 . This shows the importance of energy conservation measures. If incandescent lightbulbs were used instead of fluorescent tubes, the lighting costs would be four times as much since incandescent lamps use four times as much power for the same amount of light produced.