Daylight HarvestingElectrical power consumption for lighting is a major factor in the energy use of buildings. Office buildings typically use 17% of their energy consumption for electric lighting. For schools, the figure is an even higher 30%. Savings in lighting costs are among the most easily achievable for existing buildings, by dimming and switching lights off (demand reduction) and by installing more efficient light bulbs (system efficiency).
Daylight harvesting is the way that new green buildings are designed to collect and utilize natural daylight in the interior of buildings, when available, to reduce the need for energy-consuming artificial lighting. All buildings have windows, of course, that provide some daylight nearby, but green buildings are designed to maximize that light and distribute it effectively deeper into the building. Older buildings have electric light systems that are turned on whenever the building is occupied – day or night; but buildings that utilize daylight harvesting have automatic lighting control systems that measure the amount of natural light in each space and adjust the electric lights accordingly, to provide just that which is needed to provide the correct level of illumination. The need for electric lighting will change depending on the time of day – day or night, the position of the sun – and with the weather.
A common design of daylight harvesting uses passive devices known as “light shelves” to direct daylight back up and further into a space by bouncing it off the ceiling. The light shelf may be polished or reflective, and the ceiling must be of a light color with good reflective qualities. This method of providing “indirect” natural light provides a good quality of light – diffuse and glare-free – that is superior to direct daylight and especially direct sunlight. Another, more efficient type of light shelf is shown below. In both of these diagrams, it is shown that the lower window could have a blind to shield the room from the glare of direct sun (but allowing outdoor views) yet each of the desks would benefit from receiving natural light.
Natural light is more pleasing, healthful and psychologically beneficial than artificial light. Daylight harvesting can reduce energy consumption and costs of building lighting by over 25%. As usual, the “green” choice is the best choice for several reasons, reminding us of the “Triple Bottom Line” of green building: environmental benefit, economic benefit, and health & welfare benefit!
Images from www.schorsch.com