Energy savings has become one of the most important issues these days. About 20% of total energy consumption in the world accounts for lighting system alone. Most of the office space electric lights are turned on during the day, even in cases where enough sunlight is available. In such cases daylight integration with electric light offers greater opportunity to reduce lighting consumption. For the integration of daylight with artificial light, lighting control systems are usually employed. Various studies show that the energy savings obtained by daylight integration is between 20% and 60%. More over the openings for daylight improves the lighting conditions in the interior spaces considerably. Natural light not only saves energy but also it releases greater levels of serotonin in the human body which is called as good mood hormone. Natural light works best with indirect or electric lighting systems because with this system, occupants cannot easily notice the changes in electric light output.
Light sensors accurately measure the amount of light in its detecting range. Connecting a light sensor to a microcontroller the amount of light in a room can constantly be measured. The light sensors thus determine whether the light fixtures should increase or decrease their light output. In the microcontroller the maximum and minimum light levels can be set. As soon as the light level is lower or higher than the predetermined limits which given in the software, the microcontroller will adjust the lighting fixtures light output by means of PWM signals accordingly. Thus energy savings can be achieved. Various types of photo sensors or light sensors are available in the market. Electronic photo sensors usually use a light sensitive silicon diode to detect daylight and provide more precise control.
Placement of the light sensor is an important factor in the case of daylight integration. The light sensor manufactures will have specific recommendations regarding the location of photosensor.Placing the sensor can directly measure the task illuminance, but wiring of the sensor to the microcontroller will be difficult and there is a chance of damaging the sensor due to the task materials. The most common method of placing the sensor to ceiling, so that it is oriented towards the task. In case of a room with only one task area, the sensor which is mounted on the ceiling should be placed directly above the task. If the room is having more than one task area, the sensor is usually placed above the task that best represents the available daylight.
In case of direct lighting systems the light sensor is placed in a recessed position in the ceiling. The light sensor chosen should be sensitive to small changes in light or its field of view should not be too narrow. Usually ceiling mounted closed loop sensors are having large field of view and are shielded from direct light through the window. Some sensors are attached with sun shields and are used for cases where the sensor cannot be placed far enough from the window.
Providing daylight alone in a building may not by itself lead to energy efficiency. If the lighting controls are inappropriate, even a well daylit building may have a high level of lighting energy use. Various studies have shown that the choice of control alone can make difference of about 30-40% to the resulting lighting use in a conventionally daylit commercial building.
When daylight illuminance measured using photosensor reaches its station point, the electric lights are switched off and are switched them on again when the daylight illuminance falls down the set value. The user’s reaction to this on/off operation is due to the photoelectric switch. When daylight levels are fluctuating, it will result in the rapid switching of photoelectric switch to turn lights on and off around the switching illuminance which can annoy occupants and also reduces the lamp life. To solve this problem, various techniques have been developed which will reduce the number of switch offs. They are:
Differential switching control:This mode of control has two levels of switching illuminance; Eoff level at which the lights are switched off, and lower illuminance Eon level at which the lights are switched on. As there are 2 levels, the daylight illuminance available at the task area has to traverse the whole of the illuminance level between Eon and Eoff before the lights are switched on and off in succession. By this method rapid switching problem can be reduced. In this case switching off is less prominent, as it is occurred when a higher proportion of daylight illuminance to which the human eye is adapted is available.
Photoelectric switching with time delay: Another method is the introduction of a time delay with which the frequency of switching operations is reduced. Two types of time delay can be applied:
Switching linked time delay, in which the switching off cannot occur until atleast `n' minutes after the last switch on, where `n' is a preset delay.
Daylight linked time delay, in which switching off cannot occur until the daylight illuminance has exceeded the target value Es for `n' minutes. That is even if the the daylight illuminance has reached above Es, the lamp will be turned on for ‘n’ minutes to avoid sudden on-off.
In case of switch ‘on’ delay is not considered because that may lead to the falling of illuminance value below desired levels. Among the two time delay strategy, switching is significantly less in case of daylight linked time delay. Also energy usage is less for switching linked time delay than daylight linked time delay.
Differential switching control
Switching Linked time delay photoelectric control
Daylight linked time delay photoelectric control
Dimming hardware is required to control the light output according to the available daylight. Even though dimming control of lights is twice as expensive as switching control, it is the best for implementing dimming as the energy saving is more and is good for lamp life than the other one. Dimming is preferred by most of the customers as the changes in the light output are least disturbing. Since the daylight and light output or lumen maintenance strategies use the same hardware, they can be integrated well. Dimming is cost effective only in case of areas where daylight is well available. Switching control is suitable for spaces with adequate daylight available during whole day long, as the lights may need to adjust only once or twice during daylight hours.
In case of daylight integration, the rooms under consideration have to be divided into different zones. Zones are areas in the building under consideration that uses electric lighting and daylight jointly to do task. The size of a zone depends upon the arrangement of openings or windows, condition of sky and solar location. In order to set up the lighting zones, illuminance measurements of different months each from four different seasons such as summer, spring, winter and rainy are needed. For establishing the usual minimum and maximum range of performance, only winter and summer season measurements need to be analyzed.
The readings are taken usually in two time periods, one in noon and the other is taken at least 3 hours before or after noon. If the measurements taken in the two periods of the same day in same location vary considerably (ie., when there is variation in daylight) both the set of readings should be reviewed. From that the data sets of clear and overcast sky conditions can be recognized.
After analyzing the data set, the lighting zones with similar daylighting distribution characteristics can be linked. Within a particular zone the light at the point of maximum illuminance should not be more than about 3 times than that at the point of minimum illuminance. This ensures a sensible contrast ratio within the zone. If the ratio of maximum to minimum illuminance is greater than 9:1 then the area should be divided into more zones.
The figure below shows the division of a room into different zones.
Thus in a room if greater the number of zones, then greater will be the opportunity for energy savings. But the initial costs will be high as the number of zones increases. If the number of zones in a room is less, then the initial cost will be less, but the performance characteristics of the integrated lighting system will be poor. Daylighting is said to be works better with indirect lighting systems because with indirect lighting, occupants in the room are less likely to notice the changes in electric light output.
One of the major factors to be considered along with the daylight integration is the rise of HVAC load inside the room. Since sun is powerful source it can produce tremendous amount of heat. So if the orientations of windows or openings are not planned properly, the incoming daylight into the rooms will results in undesirable heat gains. This in turn increases the need air-condition and cooling systems. But this problem can be solved to an extent with the use of window films and glazing’s which will avoid the entering of sunlight directly.
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