Daylight harvesting systems use

daylight Daylight is the combination of all direct and indirect sunlight during the daytime. This includes direct sunlight, diffuse sky radiation, and (often) both of these reflected by Earth and terrestrial objects, like landforms and buildings. Sunlig ...

to offset the amount of electric

lighting Lighting or illumination is the deliberate use of light to achieve practical or aesthetic effects. Lighting includes the use of both artificial light sources like lamps and light fixtures, as well as natural illumination by capturing daylight. ...

needed to properly light a space, in order to reduce

energy consumption Energy consumption is the amount of energy used. Biology In the body, energy consumption is part of energy homeostasis. It derived from food energy. Energy consumption in the body is a product of the basal metabolic rate and the physical acti ...

. This is accomplished using lighting control systems that are able to dim or switch electric lighting in response to changing daylight availability. The term Daylight Harvesting has become the standard in the fields of

sustainable architecture Sustainable architecture is architecture that seeks to minimize the negative environmental impact of buildings through improved efficiency and moderation in the use of materials, energy, development space and the ecosystem at large. Sometimes, su ...

, and active daylighting industries.

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 An office is a space where the employees of an organization perform administrative work in order to support and realize the various goals of the organization. The word "office" may also denote a position within an organization with specific dut ...

is 500 Lux (or around 50 foot-candles) on the desktop.

Photosensors

All daylight harvesting systems use a light level sensor, a

photosensor Photodetectors, also called photosensors, are devices that detect light or other forms of electromagnetic radiation and convert it into an electrical signal. They are essential in a wide range of applications, from digital imaging and optical c ...

, to detect the prevailing light level,

luminance Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls wit ...

brightness Brightness is an attribute of visual perception in which a source appears to be radiating/reflecting light. In other words, brightness is the perception dictated by the luminance of a visual target. The perception is not linear to luminance, and ...

, in open-loop or closed-loop systems. Photosensors are used to adjust electric lighting based on the available daylight in the space.http://www.lrc.rpi.edu/programs/NLPIP/PDF/VIEW/SR_Photosensors.pdf In an open-loop system, the photosensor detects the amount of available daylight only, and can be positioned on the building's exterior wall or roof, or inside the building facing the window or skylight. In a closed-loop system, the photosensor detects the total photometric amount of light, from both daylight and electric sources in the space. For example, in an office a closed-loop photosensor can be positioned on the

ceiling A ceiling is an overhead interior roof that covers the upper limits of a room. It is not generally considered a structural element, but a finished surface concealing the underside of the roof structure or the floor of a story above. Ceilings can ...

facing the desktops in order to detect the amount of light on the work surface, as placing the sensor on the desktop itself would be impractical. When sensors were placed on desktops in an office building, an penalty of 24% on energy saving occurred. 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 function' areas in the space.

Control modules and dimming

The signal from the photosensor is interpreted by a lighting control system module, an automated

light switch In electrical wiring, a light switch is a switch most commonly used to operate electric lights, permanently connected equipment, or AC power plugs and sockets, electrical outlets. Portable lamps such as table lamps may have a light switch mounte ...

ing device, in the electric lighting system which can reduce the electric lighting, by shutting off or dimming fixtures as appropriate. If the electric lighting is dimmable, then the artificial lighting may be continuously adjusted in proportion to the amount of daylight available. If the electric lighting is on-off only, then an electric lighting fixture or lamp must remain on at full output until daylight can meet the entire recommended light level for the space. Non-dimming variants include having multiple non-adjacent light fixtures such as alternate units in the ceiling 'grid layout,' or daylight source adjacent fixtures near windows or skylights, linked for module on-off switching. Another variant of on-off switching is step switching (sometimes referred to as "bi-level switching"), in which multiple lamps in a single light fixture can be switched on and off independent of each other. This allows for typically one or two steps between full output and zero. Dimming systems are generally more expensive than 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 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 light harvesting

control system A control system manages, commands, directs, or regulates the behavior of other devices or systems using control loops. It can range from a single home heating controller using a thermostat controlling a domestic boiler to large industrial ...

is deployed in, and its usage. Clearly, savings can only accrue in spaces with substantial daylight where electric lighting would have been otherwise used. Therefore, daylight harvesting works best in spaces with access to conventional or

clerestory A clerestory ( ; , also clearstory, clearstorey, or overstorey; from Old French ''cler estor'') is a high section of wall that contains windows above eye-level. Its purpose is to admit light, fresh air, or both. Historically, a ''clerestory' ...

window A window is an opening in a wall, door, roof, or vehicle that allows the exchange of light and may also allow the passage of sound and sometimes air. Modern windows are usually glazed or covered in some other transparent or translucent ma ...

skylight A skylight (sometimes called a rooflight) is a light-permitting structure or window, usually made of transparent or translucent glass, that forms all or part of the roof space of a building for daylighting and ventilation purposes. History O ...

s, light tube groups, glass block walls, and other passive daylighting sources from

sunlight Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible spectrum, visible light perceptible to the human eye as well as invisible infrare ...

; and where electric lighting would otherwise be left on for long periods. Such spaces have included

office An office is a space where the employees of an organization perform Business administration, administrative Work (human activity), work in order to support and realize the various goals of the organization. The word "office" may also denote a po ...

s, atria, interior public multistory

plaza A town square (or public square, urban square, city square or simply square), also called a plaza or piazza, is an open public space commonly found in the heart of a traditional town or city, and which is used for community gatherings. Rela ...

s and

shopping mall A shopping mall (or simply mall) is a large indoor shopping center, usually Anchor tenant, anchored by department stores. The term ''mall'' originally meant pedestrian zone, a pedestrian promenade with shops along it, but in the late 1960s, i ...

courts, and

school A school is the educational institution (and, in the case of in-person learning, the Educational architecture, building) designed to provide learning environments for the teaching of students, usually under the direction of teachers. Most co ...

s. It is too simplistic to try to increase energy savings by increasing the size of windows. Daylight over-illumination may cause

glare Glare may refer to: * Glare (vision), difficulty seeing in the presence of very bright light * Glaring, a facial expression of squinted eyes and look of contempt * A call collision in telecommunications * GLARE, Glass reinforced aluminium, an ...

for occupants, causing them to deploy blinds or other window 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.) The adoption of daylight harvesting technologies has been hampered by high costs and imperfect performance of the technologies. However, studies have shown that by using daylight harvesting technologies, owners can see an average annual energy savings of 24%. One method of predicting energy savings is to use commercially available software programs, such as TRACE 700 or (freeware) DOE-2, which considers thermal loads.

Payback, and drivers for adoption

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 will 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.

Sustainability

The

green building Green building (also known as green construction, sustainable building, or eco-friendly building) refers to both a structure and the application of processes that are environmentally responsible and resource-efficient throughout a building's li ...

-sustainable building movement encourages

design and building practices. Various green building

ecolabel Ecolabels (also "eco-Labels") and Green Stickers are labeling systems for food and consumer products. The use of ecolabels is voluntary, whereas green stickers are mandated by law; for example, in North America major appliances and automobiles us ...

certification marks Certification is part of testing, inspection and certification and the provision by an independent body of written assurance (a certificate) that the product, service or system in question meets specific requirements. It is the formal attestatio ...

exist around the world, such as LEED, BOMA Best, BREEAM, HKBeam, and Green Star. All of these programs offer points for various building design features that promote

sustainability Sustainability is a social goal for people to co-exist on Earth over a long period of time. Definitions of this term are disputed and have varied with literature, context, and time. Sustainability usually has three dimensions (or pillars): env ...

, and certification at various levels is awarded for reaching 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. In addition, energy codes and standards are beginning to address daylight harvesting. For example, the California Energy Code Title 24-2008 recognizes primary and secondary daylight zones. At least 50% of the general lighting in primary zones must be controlled separately from other lighting, with automatic control required for larger zones. The code encourages automatic daylight harvesting in secondary zones by awarding power adjustment factor credits that can be applied to the lighting design. The 2009 International Energy Conservation Code (IECC) recognizes daylight zones around vertical fenestration and skylights, and requires that the lighting in these zones be controlled separately from the general lighting in the space. The 2010 ASHRAE 90.1 energy standard, expected to be published in the fall 2010, is also expected to address daylight harvesting. Meanwhile, ASHRAE 189.1, the first of a generation of sustainable construction codes, defines daylight zones and requires daylight harvesting control.

References

External links

Cost Effective Simplified Controls for Daylight Harvesting
California Lighting Technology Center, University of California, Davis
Daylight Harvesting Made Simple
California Energy Commission
Daylighting
Whole Building Design Guide

Implementation b

Co-op

* ttp://www.daintree.net/downloads/whitepapers/daylighting.pdf Harvest Daylight and Reap Rewardsby Daintree Networks
Dayswitch technology

Daylight Dividends, a research organization

Welch Allyn Headquarters Renovations
a renovation project utilizing daylight harvesting technology {{Clear Lighting Architectural lighting design Energy-saving lighting Building engineering Sustainable building Solar-powered devices