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In Search of the Zero-Energy Holy Grail
a Real-world yet hypothetical design problem in a tough climate challenges engineers to think about the measures necessary for reducing a building’s environmental footprint

By Joann Gonchar, AIA

When architectural record’s editors “commissioned” a team from Arup to design a zero-energy building to present at this year’s Innovation Conference, held in New York City in October, we did not specify a site, a client, a program, or a budget. But, even though we left the assignment open for the engineers to define, they set for themselves a very real-world, challenging design problem.

To examine strategies that might be used to reach zero, Arup developed a scheme for a generic mixed-use project in hot and humid Houston. “We decided not to pick a climate that was too easy,” said Fiona Cousins, Arup principal and mechanical engineer. It is much simpler to achieve a building that operates without fossil fuel in a dry climate, “where winters are warm, summers are cool, and nighttime temperatures are moderate,” she explained when presenting the hypothetical building at Innovation.

 

Images: Courtesy Arup  

 

Cousins and her colleagues could have chosen to design a zero-energy hut in a field but instead developed a scheme for a million-square-foot building that included six stories of parking, eight stories of offices, and 20 stories of residential space. In terms of configuration, they hewed closely to buildings currently under construction or already existing, reasoning that program and financial considerations usually drive building form more strongly than the desire to conserve energy or to provide views. For example, the building has a deep office floor plate in order to maximize rentable space.

The engineers first analyzed the building’s annual energy consumption. Lighting accounted for 35 percent, followed by office equipment at 23 percent, and cooling at 25 percent. Results would have been different if the floor plate was shallow and there was more opportunity for daylighting, said Cousins.

The team modeled the effect of cumulatively adding features such as underfloor air, a high-performance facade, daylighting controls, and occupancy sensors. After tallying the impact of these measures, energy use was significantly reduced but was still at 60 percent of an ASHRAE 90.1 base building. For a structure of this size in this climate, “there is no magic bullet,” said Cousins.

 

Images: Courtesy Arup
Zero energy has different definitions, depending on the scale of investigation. At the scale of a person using a computer, zero energy is almost impossible to achieve, unless he or she pedals to generate power. At the building scale, zero energy could be defined as a structure that uses no energy—a realistic goal for a barn or warehouse. At the global scale, where the only input to the system is solar, arguably every building is zero energy. However, this analysis ignores the world’s dependence on fossil fuel, the solar income of past generations, says Arup.

 

The results of this analysis prompted copresenter Gary Lawrence, an Arup principal and its urban strategies leader, to question the premise of the design exercise. “Is zero energy at the building scale really the right question?” he asked.

The team then turned its attention to the building’s energy supply and to carbon-reduction strategies, first looking at the impact of adding a cogeneration plant. The addition shifted fuel use from the utility to the site and technically increased the building’s power use. However, because power loss is associated with the delivery of utility-generated electricity, the cogeneration plant is a more efficient source. Cogeneration reduces the “source” energy, or the overall amount of fossil fuel used, and therefore also reduces the amount of carbon dioxide emitted. The cogeneration scheme brought source energy consumption down to 55 percent of a code-compliant building.

Arup next added photovoltaics and wind turbines. But even with these site-generated renewables, the structure still consumed 45 percent of the energy of a base building, making it clear that reaching zero at the site would be very difficult without moving beyond the building.

To reach zero carbon or zero energy, “we should move toward a regional renewable power infrastructure,” said Lawrence, warning that even renewable resources should be used wisely. We need to “bear in mind the embodied energy required to construct such a network.”

 

 

Arup’s first step was to analyze the energy consumption of the hypothetical building over the course of a year (below). The team then modeled the cumulative effect of various energy-conservation strategies, including high-performance glazing and an underfloor air system. The reduction in site energy use of all of the measures taken together was only 60 percent over a code-compliant building (middle). To try to reduce energy use further, the design team added cogeneration to the scheme. The cogeneration plant would actually increase the “site” energy used, because fuel use is shifted from the utility to the site. However, because power is generated more efficiently on-site, overall energy, or the building’s “source energy,” is reduced to 55 percent (bottom).
Images: Courtesy Arup

 

 

 

 

 

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