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Sun Machines PUBLIC ACCESS

By Duplicating the Sun’s Trace through the Sky or its Dispersion Among Clouds, Sky Simulators Help Architecture Harmonize with the Four Seasons.

[+] Author Notes

Associate Editor

Mechanical Engineering 123(09), 46-50 (Sep 01, 2001) (5 pages) doi:10.1115/1.2001-SEP-1

Pacific Energy Center in San Francisco offers building designers access to its heliodon. A machine that imitates the rotation and orbit of the Earth, a heliodon helps architects wanting to analyze passive solar heating options, site solar panels, or control solar heat gain. Sun machines are likely to always have a part in architecture. As instrumentation improves, sky simulators will see their roles increasingly directed toward comparative analyses. A silvered mesh, for example, while difficult to describe optically, could be tested easily in a sky simulator. Then, designers could feed the material’s optical characteristics into a program like Radiance, and proceed to simulate its effect on a room’s light. The heliodon provides an almost intuitive method of studying the sun as it makes its way across the seasonal skies. Into the sun machine are built the mathematics of solar position and seasonal variation, of night and day.

It may be that architects take a certain gritty delight in handling physical models. Perhaps making cardboard houses somehow pays homage to the craft’s tradition. To mechanical engineers, whose models today exist in 2-D and 3-D images or as rapid prototypes, the labor of modeling a building in cardboard must seem terribly overbearing.

It’s conceivable, though, that any reluctance by architects to make virtual models benefits mechanical engineers. Because cardboard-and-glue structures are not digitally rendered in computers, they aren’t manipulated there either. So architects come to rely on mechanical systems to study their designs.

The Pacific Energy Center in San Francisco offers building designers access to its heliodon for just such purposes. A machine that imitates the rotation and orbit of the Earth, a heliodon helps architects wanting to analyze passive solar heating options, site solar panels, or control solar heat gain.

The Energy Center, which is sponsored by Pacific Gas and Electric Co., uses a fixed-sun, moving-Earth arrangement for its heliodon. A table holds the building model at various angles to a 1,000-watt theater spotlight, which mounts in the ceiling about 32 feet from the table. The spotlight uses a Fresnel lens to approximate the sun’s parallel rays, helping designers to distinguish between illuminated areas in and around a building and those regions falling in the shadows. The light, with a one-degree angular width, diverges a mere 2 feet by the time it reaches the table.

A sun simulator tests off-axis performance of a full-size solar panel. Replacing the usual sun bulb, a 4-kW arc lamp casts nearly 800 watts/m2 of shortwave radiation onto the prototype.

Grahic Jump LocationA sun simulator tests off-axis performance of a full-size solar panel. Replacing the usual sun bulb, a 4-kW arc lamp casts nearly 800 watts/m2 of shortwave radiation onto the prototype.

The heliodon swivels in three directions for setting latitude, season, and time of day. Using the device, an architect first clamps a model to the tabletop, then turns the table to the coordinates of interest.. Usually, the winter and summer solstices receive strong attention, for they represent extreme cases.

“We try to make the heliodon experience as realistic as possible,” said Ryan Stroupe, who coordinates the Whole Building Performance program for the center. The lamp that serves as the sun, for example, appears to be about the size of a quarter held at arm’s length, just like the real thing.

Often what’s needed is a view from the building interior. A CCD point-of-view camera fulfills this need by taking up position inside the model. There, it records on video the interaction of walls and light as the model tilts through its range of seasons and hours of the day.

Any number of computer programs can depict the sunlight falling on a building over the course of a year, Stroupe said. The programs work quite accurately, he added. But the existence of such programs doesn’t diminish the heliodon’s usefulness for architects wanting to show clients how the sun will shine on a particular structure in true, rather than virtual, three dimensions.

Over the nearly 10 years that the heliodon has operated at the Energy Center, hundreds of clients have used it, Stroupe said. This being California, applications frequently center around sun and shade analysis rather than passive heating. The benefit of a real model for such an analysis is that it can be quickly modified right on the table, then reevaluated instantly.

A more recent installation at the Welsh School of Architecture at Cardiff University adds to its heliodon an array of lamps to recreate the illumination level of a cloudy sky. Commissioned in 1999, the Sky dome suspends 640 lamps from a geodesic hemisphere. The dome wears no covering, minimizing reflection. However, every triangle in the structure frames a single fluorescent lamp.

According to Phil Jones, director of architectural research at the school and an engineer himself, the most popular application for the Skydome is daylighting studies.

The Skydome can duplicate sunny or cloudy skies— even fast-changing patterns—for any day, place, or time, he said.

Architects evaluate building models in the Skydome for two main reasons. One is to see how a building will shadow itself and its neighbors at various times. Another is to measure the amount of light entering a building from the outside.

The economic benefit of daylighting—using the sun to illuminate a building’s interior—comes from turning out the lights. At the same time, a good building design blocks any direct rays from entering. Otherwise, the savings realized through natural light fall prey to increased solar radiation and higher cooling bills. Sun glare needs eliminating, too.

The school of architecture recently put the Skydome to work evaluating a low-energy factory design, Jones said. The plan called for the building to rely on solar energy for both electricity and lighting. By providing glazing for daylighting and solar panels for electricity and shade, the building’s designers hoped to balance passive illumination with power production and heat gain.

A church model studied under blue sky. Architects, wanting to minimize glare, also wanted to emphasize sunbeams. The black box—a window—is dark because the sky lighting is off.

Grahic Jump LocationA church model studied under blue sky. Architects, wanting to minimize glare, also wanted to emphasize sunbeams. The black box—a window—is dark because the sky lighting is off.

The designers actually built two models. According to Don Alexander, a researcher in the department of architecture, a small-scale model served the daylighting evaluation. Then, a larger model, representing a single facade of the building, sat in during the solar panel studies.

Unlike real buildings, the models studied on this heliodon must be able to withstand gravity's pull from several directions. A camera ties the viewpoint to the site. Two monitors project what the camera sees.

Grahic Jump LocationUnlike real buildings, the models studied on this heliodon must be able to withstand gravity's pull from several directions. A camera ties the viewpoint to the site. Two monitors project what the camera sees.

The smaller, full-building model went onto the Skydome platform with many of its walls left intentionally transparent. As the daylighting study progressed, instruments inside measured the varying interior light, while designers added and removed opaque panels to try out various glazing schemes.

For the solar panel study, the designers placed the larger model under the heliodon alone. They wanted to locate the panels for optimum solar exposure, while using them to shade the building interior from direct sunlight.

Unlike Pacific Energy’s heliodon, the university’s Skydome moves both Earth and sun. The sun rises and sets along a vertical slot cut into half of the dome. Models, fixed to Earth, ride a turntable. The combination yields two degrees of freedom, Alexander said: azimuth (or compass) and altitude.

Designers enter the Skydome through this portal in the geodesic surface. Bottom of the sun slot is visible along far inside wall. A model sits at the dome's center.

Grahic Jump LocationDesigners enter the Skydome through this portal in the geodesic surface. Bottom of the sun slot is visible along far inside wall. A model sits at the dome's center.

Although the Skydome takes up a lot more room than the sun machine at the Energy Center does, it also adds realism. Designers kneel down on the turntable right alongside their models. Depending on the season, the Skydome runs a day in two to five minutes. The designer watches the run, makes a correction or two, then verifies any improvement. The session can be taped, of course, but there’s nothing like a live performance.

Immediate interaction with physical models is a big benefit of the Skydome. “We argue that one crucial aspect of scale modeling is dealing with tangible objects in real time,” Alexander said. “We can interact with the designer in the facility—get him playing with his model, shining a light on it from different angles—and he can react interactively.

“In my experience, we still haven’t got the generation of designers fully happy with virtual worlds—with working on computers,” he said. “They have their technicians do that. They’re happier building models.”

Modeling the southern facade of a low-energy factory in Wales, designers wanted to locate photovoltaic panels (blue) so they would both gather sun and shield windows. A smaller model served the daylighting study.

Grahic Jump LocationModeling the southern facade of a low-energy factory in Wales, designers wanted to locate photovoltaic panels (blue) so they would both gather sun and shield windows. A smaller model served the daylighting study.

The Skydome involves architects with daylighting and shading early, so that afterward engineers need not correct inadequacies with extra lights and conditioned air. “The building designer is presenting the engineers with fewer problems,” Alexander said.

About the time the architecture school began contemplating its Skydome, the technology behind dim-mable, compact fluorescent lighting was reaching a state of commercial viability. The dome needed lights capable of operating over a 50:1 dimming range. In that way, the dome would produce the extremes of sky type—from cloudy to blue.

Surprisingly, blue sky is the more complex of the two types to reproduce, Alexander said. A cloudy sky doesn’t change with azimuth, only with altitude. It is brightest directly overhead, then drops to about a third of its intensity down around eye level. So daylighting in England, where cloudiness prevails, is a different problem than it is in the American Southwest. To study daylighting in the tropics, the university recently dispatched a research team to Malaysia, he said. The team will study changeable skies using a combination of scale- and computer-modeling techniques.

New Day Dawning

As they do almost everywhere, computers are supplanting the role of heliodons in daylighting and energy studies. Lawrence Berkeley National Laboratory, near San Francisco, has idled its own sky simulator, for instance.

Stephen Selkowitz, who heads the lab’s building technologies department, said that most commercial CAD programs for architecture calculate sun penetration quite convincingly. In addition, the lab itself offers several programs for lighting simulation and for solar heat load calculations.

Intended for designers wanting to assess the illumination in their buildings, the lighting software called Radiance has even caught the attention of mechanical engineers interested in knowing how sunlight shining on an inside wall might later add to a room’s temperature, for example. NASA used it to determine if astronauts, working in space’s deep contrast between shadow and light, could see well enough to repair the Hubble telescope.

Radiance uses ray tracing techniques to compute the amount of light traveling through a point in a given direction. The program begins working at the point of view and traces the path of direct and indirect lighting back through the surfaces in a room to their sources. Photograph-quality images result, Selkowitz said. In that its output is highly visual, Radiance, among the lab’s software collection, is the program that most closely simulates a heliodon.

An architect and his clients study the performance of a solar shade covering a residence.

Grahic Jump LocationAn architect and his clients study the performance of a solar shade covering a residence.

The lab’s other two programs are not visually based. EnergyPlus simulates a building’s HVAC, lighting, window, and daylighting systems, letting a designer match the efficiencies of various components to a site’s specific weather. Released this past spring to replace the earlier building energy simulator, DOE-2, the program is able to simulate window blinds and electrochromatic glazing.

Much work at the lab’s building technologies department revolves around these last two areas.

In a study for the Federal Aviation Administration, LBNL researchers used Radiance software to simulate the light in an airport control tower. The study investigated new display systems and shading devices.

Grahic Jump LocationIn a study for the Federal Aviation Administration, LBNL researchers used Radiance software to simulate the light in an airport control tower. The study investigated new display systems and shading devices.

Electrochromatic windows replicate the sunblocking function of window shades, but do so under electronic control, Selkowitz explained. Fit a room with light sensors, tie them into a control system for motorized blinds or electronic glazing, and the possibility of controlling the sunlight coming into a room by season, by time of day, and by occupancy, becomes a very real one.

Selkowitz said that the building technologies department was designing full-size rooms for testing the performance of electrochromatic windows alongside standard windows. By exposing both kinds of windows to identical, realistic sun conditions, researchers expect to demonstrate how advanced glazing systems can reduce power use by letting the winter sun in, keeping the summer sun out, and borrowing light from it the whole year long.

Sun machines are likely to always have a part in architecture, Selkowitz said. As instrumentation improves, sky simulators will see their roles increasingly directed toward comparative analyses. He expects the simulators to see use in hybrid systems. A silvered mesh, for example, while difficult to describe optically, could be tested easily in a sky simulator. Then, designers could feed the material’s optical characteristics into a program like Radiance, and proceed to simulate its effect on a room’s light.

Radiance produced this evaluation of energy-efficient, emergency lighting systems on a Navy cruiser.

Grahic Jump LocationRadiance produced this evaluation of energy-efficient, emergency lighting systems on a Navy cruiser.

An LBNL study for the California Institute of Energy Efficiency produced this image of daylight control systems.

Grahic Jump LocationAn LBNL study for the California Institute of Energy Efficiency produced this image of daylight control systems.

Meanwhile, architect Todd Jersey of Berkeley, Calif., still spends half a day with the heliodon at Pacific Energy Center for just about every project he works on. He has every intention of continuing that practice, he said.

Specializing in green designs, Jersey said that the heliodon has taught his firm a great deal about orienting overhangs and windows to control shade and sun. In California’s Bay Area, springs are usually cool while autumns are generally hot. But in either season the sun follows the same path. When a home’s windows face southeast, the morning sun can penetrate the house to heat the chilly springtime air within. In the fall, however, the hot afternoon sun falls only upon the building’s exterior, while the inside stays shady.

The heliodon provides Jersey, and architects like him, an almost intuitive method of studying the sun as it makes its way across the seasonal skies. Into the sun machine are built the mathematics of solar position and seasonal variation, of night and day. Design changes are just a snip of cardboard and a dab of glue away.

Copyright © 2001 by ASME
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