Day Light Harvesting

What is Daylight Harvesting?

Electrical energy usage for lighting is a most important factor in the power consumption of buildings. Commercial buildings consume 23% of their electricity for electric lighting. For school and hospital, the percentage is 42%. Electrical energy saving and control is more easy for existing buildings, by switching lights off and dimming as per the demand of light and by using high efficient equipment for system.

Daylight harvesting is also known as automatic daylight dimming or daylighting control. Daylight harvesting is most advanced lighting control system used to minimize energy costs. Daylight harvesting is the scheme that new buildings are configured to take advantage of natural sunlight in the interior of buildings, when it is available, to scale down the need for energy usage of artificial electrical lighting. All buildings have windows which can provide some daylight through. Old buildings have artificial electrical lighting systems that are switched on. But the buildings which uses daylight harvesting have automated lighting control systems that monitor the amount of natural sunlight in all area and adjust the artificial electric lights, to provide just that which is needed to provide the correct level of lumens. The need for artificial electric lighting will changes depending on the outside condition. It occurs a ceiling mounted or wall mounted light sensor to monitor the amount of illumination at the task surface in the space or at the daylight aperture and signals a control to light output from the electric lighting system to maintain the desired level of illumination. Different daylight levels are automatically harvested as energy savings through electric lighting diming.

Why to Use it?

In most general buildings, lighting energy usage is around 43% of all consumed electrical energy. We can do to cut down this percentage can have a positive hit on our electricity costs. But we can’t directly turn of all the lights to save the energy and suppose people to work in dim lighting; we need some natural sunlight to use in area or in combination with our electric setup. At that time daylight harvesting enters in show. In fact, depending on the depth of daylight harvesting system, we can save our lighting energy bill anywhere from 8% to 47%. Also, because daylight harvesting has a lower cooling load than electric lighting, we can also see a reduction in your air conditioning costs.

Daylighting Control Types

Daylight harvesting controls has two type “open loop” or “closed loop” systems. Each type of system works differently, and gives advantages for certain applications.

Open-loop systems monitor only the natural light, and do not count the electric lighting contributions. The light sensor is not exposed to any electric light and is typically mounted in exterior of the building or near a window or skylight facing opposite from the controlled light fixtures. Because there is no feedback from the electric lighting, this is considered an open loop. With open loop systems, the sensor is affected by passing light level changes, but it does not monitor electric light levels. Open loop systems are often preferred for applications that are not dependent on accuracy or in outdoor environments. Some examples include hallway or atrium, and parking lots or garages. Open loop systems are typically programmed by setting lighting set points that activate specific lighting presets or looks. As the light level changes to a specific range, the electric light output is adjusted to a predetermined level. It is typical to set a threshold, or set point, that triggers a system action.

Closed-loop systems measure the combined input from both natural and the electric lighting. Because the light sensor measures the electric lighting system’s light output, it is exposed to a combination of electric light and sunlight. The system is giving itself feedback, making a closed loop. With closed loop systems, the light sensor monitors real-time light levels, and is more accurate than open loop. Control is limited to a single zone per sensor and the system must be properly designed so that transient light level changes to do not cause frequent dimming or switching. Closed loop is commonly used for applications where a specific target light level will be maintained. Some area includes small offices, classrooms, museums and studios. Closed loop systems are designed to be dynamic based on the light levels seen by the sensor. The design is different for switched and dimmed systems because of differences in capabilities.