Upon realization that no fixed solution would solve the daylighting for the rare books library, Lam presented the option of employing electrochromic glass (EC) to control the illuminance. After visiting other projects with the technology, the university decided to move forward with the concept.
STEP 1: PROVE THAT IT WORKS
The first test was to assure stake-holders that in the worst case times, the EC glass would be able to maintain the required 500lux threshold. In the simulation results shown below, all angled glazing (1-4) are set to 1% VLT, and the vertical glass (5) is set to 60% VLT. End glazing is set at 34% VLT:
This temporal map shows that in the worst case, direct sun is cut down to below the 500lux threshold, except for times earlier than 7 AM from light entering through the east wall, when the library is closed.
One July 11th at 2pm, a typical ‘worst case’ time based on the orientation of the building, the 1% electrochromic tinting successfully mitigates the daylight to required thresholds.
STEP 2: DEVELOP A CONTROL STRATEGY
Sage’s EC glass has 4 tint levels: 60% (base VLT of the IGU), 18%, 6%, and 1% visible light transmission. While 1% transmission can accommodate the worst case as seen above, in a fully glazed skylight, a control scenario must be created to determine which zones are tinted at what time. Typically, EC glazing is used in smaller window openings, vertical glazing, or small skylights, where the options are limited. However the Rare Books Reading Room is a fully glazed skylight over 140′ long, with over 350 individual glass panels. A new solution was required.
Given that 1% glass can handle direct sun penetration, we propose programming the electrochromic skylights to respond to the sun path and geometry, such that the patches of darkest glass are no larger than required, and the contrast at their edges is softened by bands of glass of the middle tones . Pre-programming the system using the algorithm developed for the representations below, the skylight would become a dynamic, slowly shifting work of art throughout the day and year.
To accomplish this, we proposed subdividing all larger glass cells into sub-units such that all cells are approximately 24″ square, creating 923 individual control nodes:
This slider was developed to sell this control concept, showing how the glazing tint responds to solar angle as well as building geometry and context. Try moving the sliders – during the winter months, little adjustment is required, but during summer months, much of the skylight must be dropped to 1% VLT. By maintaining dynamic control, the space never feels dark and gloomy.
Three views of May 21 / July 21st:
Upper left shows the rays inside the space, where red lines indicate that a cell is intercepting direct sun hitting the task plane, orange, is light hitting the lower walls, yellow, the upper walls. In the rendered set of images created using iRay in 3ds Max, transmissions and color are approximate and representational. This slider view also does not represent the fade transitions correctly, which may take 2-15min between states depending on temperature. Note also that in the final version it was determined that we’d transition the states on an hourly basis.
STEP 3: OPTIMIZATION
After gaining initial approval from the project team, Lam Partners, in collaboration with Sage Glass, now had the task of assuring that the solar-oriented scene would actually perform throughout the year, maintaining no more than 500lux on the task plane at any given time. Implicit in this proposal was to also attempt to maximize the illuminance at any given time up to 500lux. To tackle this problem, the team first assessed each cell of the system to see how much each cell contributed to the overall illuminance. The images below show % contribution at 3 arbitrary sensor points.
In this analysis, a generic CIE uniform sky was used, and a grasshopper definition using Honeybee iteratively opened up one cell at a time to assess the contribution of each cell on a particular point. From this analysis we learned that, while none of the cells outside the actual reading room space contributed significantly, within the reading room all cells will ultimately have an impact on the illuminance at the task plane.
Ultimately, a comprehensive scene has been developed that controls each cell’s tint based on time of day, date and sky condition. The primary goal of this scene is to maximize un-tinted, or minimally tinted glass at any given time, therefore maximizing the interior illuminance while limiting it to the 500 lux threshold, preserving views, and in the mid-summer months, maintaining at least 10% untinted glass for color rendition. The scene will be fully automated, and able to operate throughout the year.
At any given time, one of four scene states is activated dependent upon sky condition (sunny, partially sunny, partially cloudy, and cloudy). Light sensors detect the sky condition, and inform the system which state to apply. If the sky condition changes within the hour for a long enough period to warrant adjustment, the scene may adjust. As the sun moves across the sky, the scene adjusts the tint pattern on an hourly basis to maintain required shading and illuminance levels in the Reading Room. Each day therefore has one scene state for each hour, from 7am-6pm (which may be truncated based on actual operating hours).
It should also be noted that the patterning at any given scene state has been greatly simplified since earlier studies. Changes of state of the glass tint will never be abrupt, and will in fact be so slow as to be barely perceptible. The dynamic states could be even further simplified, however, such modifications will necessarily require over-shading to maintain required light levels, particularly in the summer months, meaning unnecessarily low light levels.
The optimization strategy works by iteratively testing one shade level at a time. The below sliders show the complex movements of the shading algorithm for a particular time, where Step 0 would be akin to a fully retracted blind, and Step 44 = a fully pulled blind. However instead of simple up-down movements, this algorithm accounts for the location of the sun, and adjusts the tint in each cell accordingly.
The money shot:
Rendered video of May 21/July 21 using iRay+ in 3ds Max.
A single control point of glazing. All angled glass panels in the skylight are subdivided into 3 cells, while the vertical “clerestory” glazing panels are only one cell per panel. Therefore all cells are approximately 24” square. The new skylight system will have 923 discrete cells that may all be programmed to dynamically shade the reading room.
A programmed method to set and change the tints of each cell. A scene could be static (akin to a still image), or dynamic, changing the pattern over time (also called a ‘show’). While scenes may be saved indefinitely, they can also be edited if operating experience dictates changes. The development and modification of scenes after construction and into the foreseeable future is accomplished through programming. Custom scenes may be deployed for particular events or activities, or scenes may be modified due to changing needs over time.
One ‘moment’ in a scene – a static pattern across all cells that exists for a particular time period (one hour in the current programming).
Amount of tinting required at any given time based on sky conditions and solar angle. Given the size of the reading room, homogeneous tinting of all cells rarely results in appropriate shading level. 4 shading levels have been pre-determined: Sunny, partially sunny, partially cloudy, and cloudy, for each time throughout the year, and light sensors tell the system which level to choose at a given time.
The color and transmission levels available to each cell (60%, 18%, 6%, and 1% VLT). The glass tints to blue as it darkens to minimize solar penetration.
Dependent on both temperature and size of panel – given the size of the panels in this installation the transition between any two states on a warm day would be as little as 2-5 minutes, and 5-10 minutes on a cold day. Even with the fastest of these transition times, the transitions will be so slow as to be virtually imperceptible.
A group of cells controlled together. Zoning is only necessary if implementing manually controlled scenes, where occupants may desire to adjust areas of glazing independently. Computer programming allows more dynamic scene development not bound to zones.