Energy-Efficient Design

Estimated reading time: 9 minutes

Figure 2 in the slideshow above shows simulation results for the closed arch in four locations representing different climates: Abu Dhabi (arid), Boston (cool), Singapore (tropical), and Sydney (temperate). Each diagram plots annual operational energy against embodied energy of the structure, both measured in gigajoules per square meter. Individual dots on the diagrams represent specific designs generated by the computer.

The series of dark dots on each diagram forms the “Pareto front” — the best collection of compromising designs where the designer can’t make one performance objective better without making the other one worse. The dark dot at the farthest left in each diagram minimizes structural embodied energy regardless of operational energy, while the dark dot at the farthest right minimizes operational energy regardless of embodied energy. Points in between represent designs that are compromises between those objectives for a given emphasis on one objective over the other (say, minimizing operational energy more than embodied energy).

Of particular interest are the shapes of the Pareto fronts. The front for Boston is the classic shape — sometimes called a banana curve. The results are on a continuum such that moving either way will enable the user to do a bit better on one objective while doing a bit worse on the other.

In contrast, the curve for Abu Dhabi contains a long, flat section and then an abrupt 90-degree turn at a point referred to as the knee. In that case, moving left along the Pareto front will enable the user to significantly reduce embodied energy without much sacrifice in operational energy — as far as the knee, when operational energy suddenly jumps up. The point at the knee is therefore likely to be a good choice, as it provides a good balance between the two variables. “A single-objective optimization for operational energy would produce the dot farthest to the right,” says Mueller. “But by considering both objectives, we find that with just a small increase in operational energy, we can decrease embodied energy by about a factor of two.”

Visualizing the options

Figure 3 in the slideshow above presents a “visual catalog” of the arch configurations that correspond to five selected points on the Pareto fronts in the previous image. The designs range from the most structurally efficient at the top to the most operationally efficient at the bottom. Bars beside each design indicate its structural embodied energy and operational energy, both measured in gigajoules per square meter.

The structurally efficient designs don’t differ dramatically from city to city, but the options with efficient operation do. In Abu Dhabi, Boston, and Singapore, efficient operation is achieved by decreasing the arch truss depth and height to reduce the interior conditioned volume and the envelope surface area — a change that also reduces structural efficiency. In contrast, the Sydney arch achieves higher operating efficiency by becoming taller to maximize its surface area. In the mild Sydney climate, exchanging more heat with the outside can stabilize temperatures inside.

The transition from embodied to operational energy efficiency is more gradual with the x-brace, as shown below. In Abu Dhabi and Singapore, all the solutions are fairly shallow, with small envelope surface areas and shading edges that curve down toward the windows they protect. In Boston, the main arch members become less curved, with flatter shading elements that allow more sunlight to enter and offset heating loads. In Sydney, those elements also become flat but at a higher angle, which generates taller walls and windows — again supporting greater surface area and more extensive heat exchange with the outdoors. Interestingly, in several cases the x-brace is noticeably asymmetrical so as to more effectively block out or let in the sun.

Considering other factors

The researchers think there’s more to be done with their methodology. Already they have performed a series of analyses to show how different assumptions about building lifetimes and operational efficiency can change the shape of the Pareto front. Factors such as monetary cost and constructability could also be considered and traded off. But they hope that their work to date will encourage architects and structural engineers to incorporate the MIT team’s methodology early in the design process, when it can push solutions in interesting and unexpected ways and lead to new building designs that are high-performance, innovative, and architecturally expressive.

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