6 Daylighting Example



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Next: 7 Exercises Up: PT Chapter Previous: 5.4 Program Execution

6 Daylighting Example

 

Daylighting is an emerging area of Monte Carlo surface to surface transport that is gaining increased acceptance from the building design community. The essence of daylighting is to use the structure of the building to capture and distribute internally within the building the natural sunlight, in an effort to live in harmony with the environment as opposed to butting heads against it. Daylighting design involves the careful design and placement of windows to capture the sunlight and the implementation of an internal distribution system within the building. Issues which obtain are: (1) effecting the design so as to prevent excessive glare and (2) the degree to which electrical lighting energy is displaced, resulting in reduced energy consumption and cost savings. As we shall indicate, Monte Carlo offers some pronounced advantages over traditional design techniques, due to its ability to simulate accurately the interaction of sunlight with sophisticated building materials

To illustrate the daylighting process, we provide Figure 22, depicting a cross sectional view of a south-facing room of a typical commercial building. A lower view window 3' in height is located 4' above the floor. This view window provides light to about the front one-third of the room. Indeed, the problem with previous daylighting designs is that strategies to get the light into the rear of the building have been woefully ineffective. To address this limitation, our strategy is to use a specular light duct to light the rear two-thirds of the room. The light shelf begins 8' above the floor and is about 2'' thick (a suspended ceiling). The height above the light shelf is 3', making the total height of the room 11'-suitable for running ducts for a heating/cooling system in the space (11' heights are common in commercial construction). Sunlight entering the upper light duct window encounters the aluminized mylar coating the inside of the light duct, is specularly reflected (analogous to the reflection from a mirror) and traverses the light duct until it strikes the tilted diffuse reflector at the rear of the light duct. Normally, this is a white surface, painted with a flat paint to reflect the light diffusely down and forward toward the front of the room. Monte Carlo offers the ability to simulate specular reflections, which provide just such a means to get the light into the rear of the room.


Figure 22 Cross Sectional View of Daylighting Geometry. View Figure

The Monte Carlo simulation process proceeds as follows. Individual photons of daylight are emitted from a plane outside the room. Each is traced from birth to death, through possibly many intermediate interactions (reflections or transmissions) with surfaces. An event proceeds as follows. First, all photons are traced to their surface of interaction. Next, the interaction occurs, resulting in the photons being absorbed (death) whence tracing ceases, reflected or transmitted. If reflected or transmitted, the outgoing direction is determined. The tracing then recommences with the next event.

The objective of each complete simulation is to compute the lluminance of the work surface. A completely transmitting ``work'' surface at a height of 3 ft. (1 meter) above the floor is subdivided into a grid. The number of photons which pass through each subdivision of the work surface are tallied. A series of simulations are performed, varying the geometry and the material properties, to optimize versus cost the parameters of the design.

In the accompanying video, we illustrate this process by displaying photons as they enter the geometry of Figure 22 and interact with interior surfaces. There are multiple views of the process, from the rear of the room, and various side perspectives. Photons which are emitted from the exterior ``solar'' surface are initially colored yellow. If they succeed in passing through the windows into the room, their color is changed to red for those which are transmitted through the upper light duct window or green for those which are transmitted through the lower view window.

In this particular example, there are mini-blinds inside the view window, which serve to reflect the strong sunlight up onto the bottom of the light shelf, which is a diffuse white surface. This diffuses the strong daylight and distributes it to the front one-third of the room. A close-up view of this process is also shown on the video.

The video has been very effective in illustrating the process to non-technical audiences. It has received particular attention from the Architectural community.

Photon animation - mpeg.

(See exercise 5.)



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Next: 7 Exercises Up: PT Chapter Previous: 5.4 Program Execution



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