If I asked you to name a solar power Nirvana, Seattle probably wouldn’t be your first thought. Nevermind its rainy reputation; Seattle’s Bullitt Center is a net-zero energy building whose solar panels generate as much electricity as the building uses. But more than that, it’s an exemplary model of eco-friendly design and green living.
Located in downtown Seattle, the Bullitt Center is easily accessible by foot, bicycle, and mass transit. Its solar array generates as much energy as the building uses and its energy-efficient design ensures that it uses as little energy as possible. Building materials come from environmentally-friendly sources and are free from hazardous materials. Fresh air and natural daylight are abundant, and the building’s design encourages walking. And to top it off, water conservation and rainwater collection minimize the waste of one of the planet’s most precious resources: fresh water.
Solar Power
The Bullitt Center’s roof consists of 575 photovoltaic panels, a 1300 m2 (14000 ft2) array capable of producing 230 megawatt-hours of electricity per year. Rather than aiming the array due south with a tilt angle equal to its latitude, as conventional wisdom dictates, the designers realized that Seattle’s cloudy character is more prevalent in the winter, so they chose to take advantage of its sunnier disposition in the summer months. The array is pointed west and tilted just slightly so that the summer midday sunlight is perpendicular to the array, maximizing summer energy production.
This means that the building will generate a lot of power during the summer months - much more power than it needs - and not generate much during the winter. To achieve its net-zero energy status, the Bullitt Center sells excess electricity to the grid during the summer and buys power from the grid in the winter, effectively using the grid as a “virtual energy storage” system. You can see its annual production and consumption on the site’s dashboard:
Ground-Sourced Heating and Cooling
Just as it uses the grid to “store” electricity, the Bullitt Center takes advantage of the world’s largest thermal mass - the Earth itself - for its heating and cooling. Ground-sourced (“geothermal”) heating and cooling allow it to draw heat from underground during the winter and sink excess heat during the summer.
Cross-linked polyethylene (PEX) tubes in the floors carry a mixture of water and glycol throughout the building and 120 meters (400 feet) underground. Five heat pumps act as the heart of the system - four for space heating and cooling, and the fifth for water heating. The closed-loop system provides radiant heat in the winter and works in reverse to cool the building in the summer. PEX, although a petroleum product, has been deemed environmentally safe. The heat pumps are electric, but since the system extracts heat from the ground, it uses only a fraction of the energy of a natural gas heating system.
In the cooler months, the HVAC system includes heat exchangers made of honeycombed rotating drums to extract heat from the warm air before it’s vented outside. This system allows the building to recover 65% of the heat from that air - heat that would otherwise have gone to waste. Likewise, in the summer months, the same system extracts heat from outdoor air and pumps the heat back outside rather than bringing it into the building.
It Has Its Ups and Downs
To encourage a healthy lifestyle and reduce energy consumption, the Bullitt Center’s glass-enclosed stairways are easily accessible and inviting. Near the stairs are ADA-compliant elevators that include regenerative braking - just like you find in hybrid-electric vehicles - making the elevators 60% more efficient than standard elevators.
Built to Last, with Green Materials
The Bullitt Center is designed for a 250 year lifespan. All of its material was obtained within a 600 mile radius of the building itself. In order to minimize the use of concrete, whose production has a large carbon footprint, heavy timber provides much of the building’s vertical structure. The timber is Forest Stewardship Council (FSC) certified, which means that it came from a responsibly managed forest. A steel frame reinforces the entire structure. Concrete is only used for the foundation and base of the building.
Water
Seattle actually receives less annual rainfall than cities like New York, Boston, or Atlanta. To conserve this limited resource, the Bullitt Center has a 212,000 liter (56,000 gallon) cistern at the bottom of its rainwater collection system. The rainwater is currently used for all non-potable applications such as toilets and irrigation systems. Although it has the capacity to provide enough potable water for the entire building using a system of filters and ultraviolet light, the permitting of such a system hasn’t yet been approved. Until that time, drinking water comes from Seattle’s municipal supply. Composting toilets use natural soaps and only 30 ml (2 tablespoons) of rainwater per flush. Solid waste is aerobically composted and shipped to a facility that turns it into fertilizer.
Integrated Wetlands
The third floor terrace of the Bullitt Center serves as its greywater filtration system. The filtering is provided by a man-made wetland consisting of horsetail plants, a Seattle area native species. Nearly 1500 liters (400 gallons) of water is recirculated through the wetlands each day, allowing the plants to absorb organic material and purify the water. After several cycles, the water is clean enough to put back into the aquifer.
If I Only Had a Brain
The Bullitt Center’s Building Management System (BMS) is a sophisticated computer network that includes sensors and actuators to monitor and control its heating, cooling, and ventilation systems. It also manipulates external louvers to control how much sunlight enters the building, and it can open and close windows to circulate fresh air when the weather permits. In addition, the BMS monitors the greywater system and controls the composting system.
Cost
A net-zero energy building costs more money up front with a payback from energy savings as the return on investment. Authorities estimate that the Bullitt Center’s cost was about $50 per square foot more than a “regular” building, adding around $2.5M to the $30M building. Considering its energy efficiency and limited water usage compared to other buildings, coupled with the solar energy production from the PV array, I’m estimating a 25 year payback period. Not bad for a building designed to last 250 years!
The Future Is Now
Efficient use of energy and water is the key to a healthy and productive future. Seattle’s Bullitt Center will provide valuable lessons on the design and maintenance of buildings that make the most of limited resources. In fact, the building includes a teaching facility and research laboratory, assuring that the next generation of architects and civil engineers will benefit from this experiment in green building techniques.
Images courtesy of The Bullitt Foundation