User:Pelham88/Daylight Harvesting Draft

Daylight Harvesting is the term used in the building controls industry for a control system that reduces artificial (electric) light in building interiors when natural (day)light is available, in order to reduce energy consumption.

System Design and Components
Daylight harvesting systems are typically designed to maintain a minimum recommended light level. This light level will vary according to the needs and use of the space; for example, the commonly recommended light level for offices in North America is 500 Lux (or around 50 footcandles) on the desktop. All daylight harvesting systems use a light level sensor (or photosensor) to detect the prevailing light level. In an open-loop system, the photosensor detects the amount of available daylight only, and might be positioned on the building exterior, or inside the building facing the window or skylight. In a closed-loop system, the photosensor detects the total amount of light (natural + electric) in the space. For example, in an office a closed-loop photosensor might be positioned on the ceiling facing the desktop in order to detect the amount of light on the desktop, a key location for this application (placing the sensor on the desktop itself would be impractical). In both the open- and closed-loop configurations, the signal from the photosensor must be carefully calibrated to accurately indicate the effect of exterior daylight variations on the light level on important areas in the space.

The signal from the photosensor is interpreted by a lighting control module in the electric lighting system, and the electric lighting can be reduced, if appropriate. If the electric lighting is dimmable, then the artificial lighting may be continually reduced in proportion to the amount of daylight available. If the electric lighting is on-off only, then the electric lighting must remain on at full output until daylight can meet the entire recommended light level for the space. A variant of on-off switching is step (sometimes referred to as "bi-level") switching, in which multiple lamps in an individual light fixture can be switched on and off independent of eachother. This allows for typically one or two steps between full output and zero.

Dimming systems are generally more expensive that on-off systems. They have the potential to save more energy, because they can reduce electric light output when daylight can only partially meet the needs of the space. However, dimming systems may also require a little more energy for their basic operation. If a dimming system is well-calibrated, the occupants of the space will not notice changes in electric lighting due to daylight harvesting, whereas they are very likely to notice the changes due to a on-off or step switching.

Energy Savings
Several studies have recorded the energy savings due to daylight harvesting. Energy savings for electric lighting in the range of 20-60% are common. Savings are very dependent on the type of space the control system is deployed in, and its usage. Clearly, savings can only accrue in spaces with substantial daylight, and only if electric lighting would have been used otherwise. Therefore daylight harvesting works best in spaces with access to windows or skylights (or other sources of daylight) and where electric lighting would otherwise be left on for long periods. Such spaces include offices, atria, and schools.

It is too simplistic to try to increase energy savings by increasing the size of windows. Excessive daylight may be glary for occupants, causing them to deploy blinds or other shading devices, and compromising the daylight harvesting system. Even partially-deployed Venetian blinds can cut energy savings in half.

Impressive energy savings estimates may not be realized in practice due to poor system design, calibration, or commissioning. Systems that dim or switch electric lighting in a distracting manner, or that produce overall light levels that are perceived as too low, can be sabotaged by occupants. (For example, simply taping over a sensor will create constant electric lighting at maximum output.)

Payback, and Drivers for Adoption
Obviously, there is an incremental cost to daylight harvesting systems. Dividing this cost by the annual energy savings provides a "simple payback", the number of years for the system to pay for itself. The shorter the calculated payback period, the more likely it is that a building owner witll invest in the system. Costs vary for a whole host of local factors, but generally if energy costs rise, or the cost of the control hardware and installation falls, the payback period will be reduced.

The "green building" movement seeks to encourage sustainable building practices. Various green building certification schemes exist around the world, such as LEED, BOMA Best, BREEAM, HKBeam, and Green Star. All of these schemes offer points for various building design features that promote sustainability, and certification at various levels is offered for the accumulation of a given number of points. One of the principal ways to gain points is through energy saving measures. Therefore, daylight harvesting is a common feature of green buildings. Thus green building practices are increasing the production of daylight harvesting components, leading to lower prices.

Many electric utilities provide financial incentives for their customers to save energy. One such incentive is rebates on daylight harvesting systems , which also reduces payback periods.