For the introduction to Daylighting, ways to introduce light into the space, and some of the benefits, please see Daylighting 101 (should be about 2 posts ago). Any comments or questions, please feel free to post below. For now, we are going to pick up where we left off with and take a look at how the systems work, energy codes and the effect on daylighting, and what makes a particular system successful.
So we look at Daylighting, bringing daylighting into the space, we have to start talking about Daylight Harvesting. Harvesting is defined by the use of sustainable architecture that reduces the use of artificial lighting when natural daylight is available. When using Harvesting techniques, the daylighting sensors are set to measure in one of two ways: an open loop or a closed loop.
Let’s start with an open looped sensor. An open looped system is designed to measure only daylight contribution to the space. An open loop sensor is typically placed in a position where the sensor can see only direct sunlight: on the exterior wall (facing out), on the roof (measuring daylight), or inside a skylight (placed to measure the light in the skylight).
Alright, so going back to the closed loop system, we have a second type of system. A closed loop system measures daylight in a space as well as the artificial light. Closed loop systems differ from open loop, as they are typically used to measure light within a room. Placed in a location where they will not get over exposed to sunlight or indirect light, the photocell measures all the light received on a surface.
So now we have the options for the types of photocell that can be used to determine the daylighting in the space. Closed loops should be utilized in spaces like offices and conference rooms where lighting levels need to be carefully monitored for important tasks. Open loops will be utilized for spaces such as lobbies and exterior spaces. But now that we have the proper type of sensor for our space, we need to look at the type of daylighting control these sensors will utilize.
When controlling lights in spaces with daylight, there are three options to control the space: Switching, Bi-level switching, and dimming.
Switching is the simplest solution, turning the lights off when there is enough daylight contributed to the space. It is the simplest design, carries the lowest design cost, and easiest to commission. Draw backs to the design have the most effect on the occupant of the space. The occupant is limited to two light levels (on and off), high amounts of cost savings are hard to come by, there is little flexibility in the design, and lights switching on/off can be quite irritating. These systems are typically used in Corridors, Atriums, and Bathrooms, spaces with non-stationary tasks and typically void of workers.
The next level past switching is bi-level switching. Bi-level switching still turns the lights on/off, but also offers levels of control between on/off. These systems have more than 3 levels of lighting, the systems are cheaper than a full dimming system, and they will achieve higher levels of cost savings than standard on/off. Disadvantages to the system are similar to that of the switching system. The lights can be as distracting, and the ballasts need additional wiring with more commissioning. Bi-level systems are typically installed in factories, gyms, warehouses, and other spaces were non-detailed tasks take place and where switching is not distracted.
The final option is a dimming system. Dimming systems gives an unlimited range of control in the space. The benefits are that the system can be programmed to have an exact FC level at all times, it is the most comfortable for clients, and has the highest energy savings of any system. The disadvantage to the system is the cost of the ballasts and the wiring of the ballasts as well as the commissioning level required to perfect the operation. These systems are best utilized in classrooms, laboratories, office spaces, and libraries, spaces that have highly detailed tasks.
With daylighting becoming more prominent in buildings, Codes have begun to put standards and requirements in place for monitoring the installation of daylight systems. With California leading the US in Green Standards and design, requirements for ASHRAE 90.1 have been developed in conjuncture with California Title 24. Daylit areas must have 2 levels of ouput, 0%-35% and 50%-70% or continuous dimming. Additional lighting power is allowed per code if daylighting is incorporated into various spaces when mandatory and advanced lighting controls are used in the space.
So the last piece to discuss is what makes a good daylighting system. When you strip away the parts and pieces, the open and closed loops, the switching and the dimming, the system is really only as good as the commissioning that was put in place to tell the system to operate. All the dimming ballasts in the world, all the sensors, and all the dimming curves, none of it means anything unless the system is set up right.
Commissioning an open loop system, a single adjustment setting on a proportional control can be set at any time that daylight is present. The adjustment should be made when the daylight distribution is representative of what is typical. You cannot commission this system during a period of time when direct sunlight is streaming in, if that is not a norm.
Commissioning of a closed loop system becomes a two step process. Maintained illuminance level is set by making an adjustment with only the electric lights on (no daylight). Once the level is set, the ratio between the photo sensor optical signal and the desired light level is set by dimming or switching the electric lights until the desired light level is achieved.
Bottom line, make sure you have a licensed commissioning agent commission your system for proper operation.
So, Daylighting points to think about. ASHRAE 90.1 2010 will be the first year of mandatory Daylight control requirements. Options for daylighting system include open/closed loop and switched/bi-level/dimmed solutions. Benefits of daylight systems vary from energy saving to improved occupant productivity.
Daylighting is a reality that is fast approaching, and the more you can learn now…the more enlighting you will be later…