Question

The "lumen method" is to be applied to design an overhead lighting system for a room, with basic illuminance equation: total = Ewp · Awp/(Cu · LLF) The room has floor area = 60 m². The target illuminance at the work-plane is 400 lux. It is intended the lighting power density for the room should be 7.5 W/m2 or less. The design will be fluorescent lamps with electronic ballasts that together achieve "system-efficacy" = 85 lm/W. A light-loss-factor Ly = 0.80 is to be used in the calculation. a) Estimate the power consumption rate of the overhead lighting system if the design achieves LPD = 7.5 W/m? b) Determine the acceptable range of the coefficient of utilization that will be required for the design to achieve LPD = 7.5 W/m? or less. c) Calculate the annual cost of the electricity used by these lights (LPD = 7.5 W/m²) if they are powered on 10 hours/day (365 days/year) and the average cost of electricity is $0.16/kWh.

Answer 1

2. Given

N= 500 200 2700*0.64*0.8

Floor area

Awp = 60m2

Target illuminance Ewp = 400 lux

Light loss factor = 0.8

System efficacy = 85 lm/W

power density =7.5 W/m2

To find

a) Power consumption rate at power density 7.5 W/m2

=NF = System efficacy * Power density * Area

Tot

=85*7.5*60

= 38250 lumins

Power consumption =

= Lumin s Efficacy

38250 88

= 450 W

b) Range of coefficient of utilization

CL, 400*60 38250= C *0.8

Cu= 0.784 to 1

C) Annual cost of electricity

= 356*10*0.45*0.15

= $262.8/year