Daylight harvesting
Encyclopedia
Daylight Harvesting is the term used in sustainable architecture
and the building controls and active daylighting
industries for a control system that reduces the use of artificial lighting
with electric lamps
in building interiors when natural daylight
is available, in order to reduce energy consumption.
is 500 Lux
(or around 50 footcandles) on the desktop.
or brightness
, in open-loop or closed-loop systems. Photosensors are used to integrate an electric lighting system with a daylighting system so lights operate only when daylighting is insufficient. 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
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. 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 functions' areas in the space.
module, an automated light switching
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.
Savings are very dependent on the type of space the light harvesting control system
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
window
s, skylight
s, light tube
groups, glass block walls, and other passive daylighting
sources from sunlight
; and where electric lighting would otherwise be left on for long periods. Such spaces have included office
s, atria
, interior public multistory plaza
s and shopping mall
courts, and school
s.
It is too simplistic to try to increase energy savings by increasing the size of windows. Daylight over-illumination
may cause glare
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 it to use commercially-available software programs, such as the (freeware) DOE-2, which considers thermal loads.
, 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.
-sustainable building movement encourages sustainable architecture
design and building practices. Various green building ecolabel
certification marks 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
, 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.
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Sustainable architecture
Sustainable architecture is a general term that describes environmentally conscious design techniques in the field of architecture. Sustainable architecture is framed by the larger discussion of sustainability and the pressing economic and political issues of our world...
and the building controls and active daylighting
Active Daylighting
Active daylighting is a system of collecting sunlight using a mechanical device to increase the efficiency of light collection for a given lighting purpose...
industries for a control system that reduces the use of artificial lighting
Lighting
Lighting or illumination is the deliberate application of light to achieve some practical or aesthetic effect. Lighting includes the use of both artificial light sources such as lamps and light fixtures, as well as natural illumination by capturing daylight...
with electric lamps
Lamp (electrical component)
A lamp is a replaceable component such as an incandescent light bulb, which is designed to produce light from electricity. These components usually have a base of ceramic, metal, glass or plastic, which makes an electrical connection in the socket of a light fixture. This connection may be made...
in building interiors when natural daylight
Daylight
Daylight or the light of day is the combination of all direct and indirect sunlight outdoors during the daytime. This includes direct sunlight, diffuse sky radiation, and both of these reflected from the Earth and terrestrial objects. Sunlight scattered or reflected from objects in outer space is...
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 AmericaNorth America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
is 500 Lux
Lux
The lux is the SI unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is used in photometry as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface...
(or around 50 footcandles) on the desktop.
Photosensors
All daylight harvesting systems use a light level sensor, a photosensor, to detect the prevailing light level, luminanceLuminance
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 or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...
or brightness
Brightness
Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target...
, in open-loop or closed-loop systems. Photosensors are used to integrate an electric lighting system with a daylighting system so lights operate only when daylighting is insufficient. In an open-loop system
Open-loop controller
An open-loop controller, also called a non-feedback controller, is a type of controller that computes its input into a system using only the current state and its model of the 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
Photometry (optics)
Photometry is the science of the measurement of light, in terms of its perceived brightness to the human eye. It is distinct from radiometry, which is the science of measurement of radiant energy in terms of absolute power; rather, in photometry, the radiant power at each wavelength is weighted by...
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
Ceiling
A ceiling is an overhead interior surface that covers the upper limit of a room. It is generally not a structural element, but a finished surface concealing the underside of the floor or roof structure above....
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. 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 functions' areas in the space.
Control modules and dimming
The signal from the photosensor is interpreted by a lighting control systemLighting control system
A lighting control system consists of a device that controls electric lighting and devices, alone or as part of a daylight harvesting system, for a public, commercial, or residential building or property, or the theater...
module, an automated light switching
Light switch
A light switch is a switch, most commonly used to operate electric lights, permanently connected equipment, or electrical outlets. In torches the switch is often near the bulb, but may be in the tail, or even the entire head itself may constitute the switch .-Wall-mounted switches:Switches for...
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
X10 (industry standard)
X10 is an international and open industry standard for communication among electronic devices used for home automation, also known as domotics. It primarily uses power line wiring for signaling and control, where the signals involve brief radio frequency bursts representing digital information...
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
Lighting control system
A lighting control system consists of a device that controls electric lighting and devices, alone or as part of a daylight harvesting system, for a public, commercial, or residential building or property, or the theater...
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
Clerestory
Clerestory is an architectural term that historically denoted an upper level of a Roman basilica or of the nave of a Romanesque or Gothic church, the walls of which rise above the rooflines of the lower aisles and are pierced with windows. In modern usage, clerestory refers to any high windows...
window
Window
A window is a transparent or translucent opening in a wall or door that allows the passage of light and, if not closed or sealed, air and sound. Windows are usually glazed or covered in some other transparent or translucent material like float glass. Windows are held in place by frames, which...
s, skylight
Skylight
Skylight may refer to:* Skylight * Skylight , by David Hare* Skylight of a lava tube, a hole in the ceiling of the tube* Skylight, Arkansas* Skylight, a short film by David Clayton Rogers* Skylight Pictures, a film company...
s, light tube
Light tube
Light tubes or light pipes are used for transporting or distributing natural or artificial light. In their application to daylighting, they are also often called sun pipes, sun scopes, solar light pipes, sky lights or daylight pipes.Generally speaking, a light pipe or light tube may refer to:* a...
groups, glass block walls, and other passive daylighting
Passive Daylighting
Passive daylighting is a system of both: collecting sunlight using static, non-moving, and non-tracking systems such as Windows, Sliding glass doors, most skylights, light tubes, and reflecting the collected daylight deeper inside with elements such as light shelves...
sources from sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...
; and where electric lighting would otherwise be left on for long periods. Such spaces have included office
Office
An office is generally a room or other area in which people work, but may also denote a position within an organization with specific duties attached to it ; the latter is in fact an earlier usage, office as place originally referring to the location of one's duty. When used as an adjective, the...
s, atria
Atrium (architecture)
In modern architecture, an atrium is a large open space, often several stories high and having a glazed roof and/or large windows, often situated within a larger multistory building and often located immediately beyond the main entrance doors...
, interior public multistory plaza
Plaza
Plaza is a Spanish word related to "field" which describes an open urban public space, such as a city square. All through Spanish America, the plaza mayor of each center of administration held three closely related institutions: the cathedral, the cabildo or administrative center, which might be...
s and shopping mall
Shopping mall
A shopping mall, shopping centre, shopping arcade, shopping precinct or simply mall is one or more buildings forming a complex of shops representing merchandisers, with interconnecting walkways enabling visitors to easily walk from unit to unit, along with a parking area — a modern, indoor version...
courts, and school
School
A school is an institution designed for the teaching of students under the direction of teachers. Most countries have systems of formal education, which is commonly compulsory. In these systems, students progress through a series of schools...
s.
It is too simplistic to try to increase energy savings by increasing the size of windows. Daylight over-illumination
Over-illumination
Over-illumination is the presence of lighting intensity beyond that required for a specified activity. Over-illumination was commonly ignored between 1950 and 1995, especially in office and retail environments; only since then has the interior design community begun to reconsider this practice.The...
may cause glare
Glare (vision)
Glare is difficulty seeing in the presence of bright light such as direct or reflected sunlight or artificial light such as car headlamps at night. Because of this, some cars include mirrors with automatic anti-glare functions....
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
Building Commissioning
Building commissioning is the process of verifying, in new construction, that all the subsystems for HVAC, plumbing, electrical, fire/life safety, building envelopes, interior systems , cogeneration, utility plants, sustainable systems, lighting, wastewater, controls, and building security achieve...
. 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 it to use commercially-available software programs, such as the (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 periodPayback period
Payback period in capital budgeting refers to the period of time required for the return on an investment to "repay" the sum of the original investment. For example, a $1000 investment which returned $500 per year would have a two year payback period. The time value of money is not taken into account...
, 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 buildingGreen building
Green building refers to a structure and using process that is environmentally responsible and resource-efficient throughout a building's life-cycle: from siting to design, construction, operation, maintenance, renovation, and demolition...
-sustainable building movement encourages sustainable architecture
Sustainable architecture
Sustainable architecture is a general term that describes environmentally conscious design techniques in the field of architecture. Sustainable architecture is framed by the larger discussion of sustainability and the pressing economic and political issues of our world...
design and building practices. Various green building ecolabel
Ecolabel
Ecolabels and Green Stickers are labelling systems for food and consumer products. Ecolabels are often voluntary, but Green Stickers are mandated by law in North America for major appliances and automobiles. They are a form of sustainability measurement directed at consumers, intended to make it...
certification marks exist around the world, such as LEED
Leadership in Energy and Environmental Design
Leadership in Energy and Environmental Design consists of a suite of rating systems for the design, construction and operation of high performance green buildings, homes and neighborhoods....
, BOMA
Building Owners and Managers Association
The Building Owners and Managers Association , founded in 1907, is a professional organization for commercial real estate professionals and is the oldest and largest in its field. Its membership includes building owners, managers, developers, leasing professionals, medical office building managers,...
Best, BREEAM
BREEAM
BRE Environmental Assessment Method is a voluntary measurement rating for green buildings that was established in the UK by the Building Research Establishment . Since its inception it has since grown in scope and geographically, being exported in various guises across the globe...
, HKBeam, and Green Star
Green Star (Australia)
Green Star is a voluntary environmental rating system for buildings in Australia. It was launched in 2003 by the Green Building Council of Australia....
. All of these programs offer points for various building design features that promote sustainability
Sustainability
Sustainability is the capacity to endure. For humans, sustainability is the long-term maintenance of well being, which has environmental, economic, and social dimensions, and encompasses the concept of union, an interdependent relationship and mutual responsible position with all living and non...
, 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
California Energy Code
The California Energy Code, or Title 24, Part 6 of the California Code of Regulations, also titled The Energy Efficiency Standards for Residential and Nonresidential Buildings, were established in 1978 in response to a legislative mandate to reduce California's energy consumption...
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
ASHRAE 90.1
ASHRAE 90.1 is a standard that provides minimum requirements for energy efficient designs for buildings except for low-rise buildings. The original standard ASHRAE 90 was published in 1975 and had multiple editions to it in years after...
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.
See also
- DaylightingDaylightingDaylighting is the practice of placing windows or other openings and reflective surfaces so that during the day natural light provides effective internal lighting. Particular attention is given to daylighting while designing a building when the aim is to maximize visual comfort or to reduce energy...
- Passive daylightingPassive DaylightingPassive daylighting is a system of both: collecting sunlight using static, non-moving, and non-tracking systems such as Windows, Sliding glass doors, most skylights, light tubes, and reflecting the collected daylight deeper inside with elements such as light shelves...
- Active daylightingActive DaylightingActive daylighting is a system of collecting sunlight using a mechanical device to increase the efficiency of light collection for a given lighting purpose...
- Light intensityLight intensitySeveral measures of light are commonly known as intensity. These are obtained by dividing either a power or a luminous flux by a solid angle, a planar area, or a combination of the two...
- BrightnessBrightnessBrightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target...
- SustainabilitySustainabilitySustainability is the capacity to endure. For humans, sustainability is the long-term maintenance of well being, which has environmental, economic, and social dimensions, and encompasses the concept of union, an interdependent relationship and mutual responsible position with all living and non...
- Sustainable architectureSustainable architectureSustainable architecture is a general term that describes environmentally conscious design techniques in the field of architecture. Sustainable architecture is framed by the larger discussion of sustainability and the pressing economic and political issues of our world...
- Sustainable designSustainable designSustainable design is the philosophy of designing physical objects, the built environment, and services to comply with the principles of economic, social, and ecological sustainability.-Intentions:The intention of sustainable design is to "eliminate negative environmental...
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
- Daylight Harvesting Implementation by Rainbow Grocery Co-op
- National Research Council Institute for Research in Construction (NRC-IRC) Lighting Research
- Harvest Daylight and Reap Rewards by Daintree NetworksDaintree NetworksDaintree Networks, Inc. is a clean technology company that provides wireless control solutions for commercial buildings.Commercial building control and lighting control solutions can provide benefits including reduced energy consumption and carbon footprint, compliance with new "green" building...
- Dayswitch technology
- Daylight Dividends, a research organization
- Welch Allyn Headquarters Renovations a renovation project utilizing daylight harvesting technology
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