Net Energy Costs of Skylights

Skylights in industrial facilities have been documented to improve worker productivity and reduce absenteeism. Though productivity improvements alone may justify skylights, it is also useful to consider the net energy costs associated with skylights. This paper describes a methodology for calculating net energy savings from skylights as a function of skylight area, the required lighting level, and type of lighting. The methodology can be applied to plants located anywhere in the world, by using typical meteorological data from TMY2 or EPW files, which are available free-of-charge over the internet. The method uses the LightSim hour-by-hour daylighting simulation program to calculate the number of hours that daylighting can meet a target lighting level. Energy balances are employed to calculate net heating and cooling loads through a ceiling with and without skylights. The net energy cost savings are calculated as the difference between lighting and space conditioning costs with and without skylights. Results show that the optimum skylight to floor area ratio and net energy cost savings increase as the target lighting level increases. For Dayton, Ohio, results indicate optimum skylight to floor area ratios range from about 1% to 6%, and net energy savings ranging from about 0.5 to 25 cents per square foot of floor area per year using average 2005 industrial energy costs. Net cooling energy savings are higher than net heating energy savings. This analysis demonstrates that an economically optimal area of skylights exists that results in net energy cost savings, but installing either too many or too few skylights can result in an increase in net energy costs. Thus, it is important to perform an analysis such as this to guarantee that skylights actually reduce net energy costs.