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High Performing Buildings Magazine | Case Study – Bullitt Center
Source: High Performing Buildings Magazine
3-15-2016 | News
In 2009, the Bullitt Foundation set out to change the world with a building. Led by president Denis Hayes, organizer of the first Earth Day, the Foundation’s plans called for the Bullitt Center to be the first speculative development to achieve Living Building Challenge certification, meaning it would have to meet the toughest environmental sustainability requirements while also attracting tenants to make it financially sustainable. More than two years into operation, those lofty goals are becoming reality. It is achieving net positive energy use, challenging regulatory hurdles in pursuit of using harvested rainwater as its water source, and raising the sustainability bar as it seeks to live up to its moniker as “The Greenest Office Building in the World.”
Net Zero Energy
In design, predicted energy use for the Bullitt Center was driven down to an annual energy use intensity (EUI) of 15–16 kBtu/ft2. This matched estimates of what could be reasonably generated on site, following detailed parametric studies of potential solar photovoltaic configurations. The resulting 242 kW canopy PV array is a distinguishing design feature, extending beyond the building ’s foot-print and encompassing 14,303 ft2.
In the past year, the six-story building has been operating at an EUI near 11 kBtu/ft2—an astonishing feat given that its performance is 79% below the stringent 2009 Seattle Energy Code baseline (Figure 1). The successful net positive energy operation is achieved through attention to every driver of energy use in the building, from the architecture, to HVAC systems and delivery of thermal comfort, to lighting and plug loads, and ultimately to the occupants and their decision-making around energy consumption (Figure 2).
From a design perspective, the building massing is driven by the goal of using daylighting as the primary means of illumination, with glazed façades oriented for solar control. The thermally efficient skin achieves a heat loss rate 30% better than Seattle’s code; the largest enhancement comes from the triple-glazed aluminum curtainwall systems. Automated exterior blinds enhance solar control for much of the glazing.
The building’s mechanical system includes 26 closed-loop geothermal bores that are 400 ft deep and connect to water-to-water heat pumps. Three heat pumps serve the changeover radiant floor system for space heating and cooling, one heat pump serves the 5,200 cfm heat recovery unit (HRU), and one heat pump provides domestic hot water. The HRU includes a 70% effective sensible heat recovery wheel.
Automated windows are operated via direct digital control to provide the first stage of space cooling.
Modeling during design was per-formed with two different building analysis software programs for energy and comfort modeling. Both models used a Seattle TMY2 weather file and identical geometries.
The building was assumed to be fully occupied with 150 ft2/person peak occupancy in the open office spaces. Occupant density significantly affected the energy loads since each person was assumed to work at a computer. The model assumed a mix of equipment use: desktop computer stations with two monitors each (65%), laptops plus a monitor (20%) and thin-clients plus a monitor (15%).
Other typical office equipment was also included in the model. Schedules were applied to simulate the building operating at about 80% of peak occupancy from 8 a.m. to 5 p.m. weekdays.
The Bullitt Center’s energy use is measured and monitored at the individual circuit level. Each circuit is designated as a building load or a tenant load and categorized by type of use, such as HVAC or lighting.
These data are displayed in an inter-active public dashboard at the Bullitt Center and on the Web. Unfortunately, the submetering system wasn’t fully commissioned and has only recently been providing useful end use data, so the data presented in this article are from the utility net metering system.
Without end use breakdowns, it is not easy to explain where the better-than-expected energy use is coming from, but the design and ownership team believe it is primarily tenant-related (Figure 3). Less dense office occupancies and more efficient computing equipment have been identified as likely sources of the discrepancy.
Typical office floor, Levels 3 through 6. Timber structure, daylight, concrete floors, white-painted gypsum wallboard, and views to the outside form the basis of the architecture. Tenants may develop space within the limits of the Living Building Challenge materials list and their specific energy budgets.
Net Zero Water
The building ’s once-through water system is designed to operate independently of any utility, using collected rainwater as its water source and treating wastewater on site. Rainwater is collected from the roof and stored in a 56,000 gallon cistern.
The rainwater is designed to be filtered, disinfected with UV and chlorine, and stored as potable water in a 500-gallon day tank. While the filtration and disinfection is sufficient to bring the water to potable quality, current regulations have thus far prevented the project from providing rainwater to potable water fixtures, and municipal water is provided instead.
The building owner is working to establish an independent water district for eventual approval to the rain-water-to-potable system. Rainwater is provided to the building’s foam flush toilets.
Graywater is collected from the building’s showers, sinks, floor drains, and lavatories, but not reused.
Instead, it is stored in a 500-gallon tank, then pumped through a constructed wetland on the building’s north terrace. The water recirculates through the wetland and finally flows into swales at grade to replenish the local groundwater aquifer.
All toilets in the building feed into the composting waste system. The foam flush toilets use approximately 0.04 gallons of rainwater per flush. The toilets prewash the bowl with foam when they are approached; the foam lubricates the bowl and feeds continuously during use to allow solids to pass through the fixture.
Ten composting bins in the basement collect nutrients from the toilets. The composters are continuously exhausted, manually turned daily, and have excess leachate pumped to a storage tank. The leachate is hauled away approximately every 12 to 18 months for treatment and reuse as fertilizer.
Daylight is intended to be the primary source of illumination in all tenant spaces. Building massing, window specifications, floor plate organization, and space planning are designed so no workstations are more than 20 ft from a window.
To manage dynamic sunlight, a weather-responsive automated exterior horizontal blind system is included. This system enables sun-light diffusion, glare control, and dynamic solar shading based on current solar exposure. And, importantly, it reverts to maximum unobstructed aperture area under Seattle’s overcast skies or during clear sky conditions on façades that are not receiving direct sunlight.
The Bullitt Center design reflects a modeled 67% reduction in electric lighting power consumption over a code building. This includes a connected lighting power density target of 0.4 W/ft² with additional reductions in lighting power via photocell-controlled continuous dimming, vacancy sensing, time clock-based sweep controls, and manual wall switches.
The persistent delivery of sufficient, visually comfortable daylight illuminance and luminance for significant periods of the occupied times is critical to meeting net zero annual energy. Accordingly, proper operation of the automated exterior blinds must be maintained for comfort and energy performance.
Blind slat tilt-control and deployment schedules are pre-programmed based on latitude, longitude, and façade orientations. A solar radiation sensor on the roof deploys or retracts the shading system based on sky conditions.
Technically, tenants may choose to install any lighting and lighting controls system that meets the Seattle Energy Code (SEC), so long as they do not exceed a maximum annual total power allowance codified in the tenant “green” lease.
Current lighting energy use is less than anticipated, primarily due to tenants manually turning off ambi-ent overhead lighting more than expected. This is likely due to the conservation-oriented nature of many of the building tenant organizations and because lighting is a particularly “visible” source of energy use building occupants can directly impact.
Natural Ventilation and Passive Cooling
The building is provided with actuated windows for indoor comfort. During occupied hours, the windows operate during suitable conditions to maintain 70°F.
A night flush mode maintains 60°F zone air temperature. Occupant override allows user control of the windows and reverts to automatic control after a set time limit.
The building’s natural ventilation performance was modeled to under-stand cooling comfort and window size requirements (Figure 4). Tenants are encouraged to maintain a mini-mum amount of openings in partition walls to maintain the designed cross-ventilation rate.
Window system installation in progress. Air tightness at the interface between the curtainwall and fluid-applied air/weather barrier is achieved with ethylene propylene diene terpolymer (EPDM) flap integrated into aluminum framing. The curtainwall incorporates 4 ft by 10 ft parallel arm vents with motorized actuators and operable windows, which extend 7 in.
The Living Building Challenge presents challenging requirements for materials procurement. The Red List imperative enumerates 14 chemicals to avoid in building components, with the intent of removing chemicals harmful to humans and the environment from the entire building life cycle.
The Appropriate Sourcing imperative promotes development of ecological and regional building solutions by limiting the distance materials may travel to the building site. The result is a built work that derives its aesthetic from its structure and a minimal materials palette, in this case showcasing regionally sourced wood and concrete. The design and construction team worked with sub-contractors and material suppliers in screening materials.
The User Experience
Thermal comfort is anecdotally reported to be very good, even during summer 2015, which had a record-setting number of days with elevated temperatures. The thermally massive floors (3 in. of concrete over 6 in. of solid wood) provide an excellent heat sink in the summer and warmth in the winter, but in swing seasons may feel too cool when the radiant heating isn’t energized. Heating setpoints were adjusted upward to solve this comfort issue.
Visually, the operable blinds provide excellent glare control. The design team expected some tenants might install interior blinds for more localized control of daylight. However, as of this writing, blinds have only been installed on the sixth floor where exterior blinds were not included due to the shading effect of the 20 ft overhang of the rooftop PV array.
Users say the Bullitt Center restrooms have no odors, even though they contain composting toilets. They are exhausted at the point of use through the toilet bowl. The restrooms are under continuous negative pressure to ensure composter vapors never reach the occupants.
Adherence to the materials Red List helps to achieve a high level of indoor air quality. However, other than testing for respirable suspended particulates and volatile organic compounds, no data is available to support this contention.
Project Successes/ Resulting Changes
With its own water supply and treatment system and its own energy plant, the construction of the Bullitt Center came at a cost premium. Because of the integrated design, however, it is not possible to calculate exactly what portion of its $350/ft2 cost might be attributed to its environmental features or its high performance. The Bullitt Center website (www.bullitcenter.org) has a more in-depth discussion of project costs.
More importantly, the cost of the project met the owner’s requirements for economic feasibility. With a fully leased building, the project is making positive returns at typical class A rents in Seattle.
From a structural perspective, an example of change is the project spurred the City of Seattle to create the Living Building Pilot Program, which grants certain departures, such as additional floor area ratio (the square footage of a building divided by the lot’s square footage) and building height, for projects targeting Living Building certification. The building height departure was the only departure used in this project, and the extra 10 ft of height was divided among each of the floors to increase structure height for better daylight penetration.
When approval is finally in place for operating as a water district, the project will be the first commercial building in the region to run on rain-water. It has also proven that using composting toilets in a multistory urban building is viable.
The project is the first in the region in which the public utility has contracted with a speculative building developer supplying high levels of energy efficiency as a grid resource. Using the term “nega-watt-hours,” the Bullitt Center receives monthly payments for the difference between its consumption and what a code building would have used, while passing on to tenants the full energy cost (which has been zero).
In the future, this type of arrangement would allow other building owners and developers a more direct and predictable return on additional investment in efficiency.
The Bullitt Center has exceeded its original goals. It has operated on a net positive energy basis, was the seventh and largest Living Building when it was certified in Spring 2015, and is the first urban commercial project to be so recognized. And, since opening in 2013, the building has hosted more than 6,000 visitors interested in learning how they can achieve similar results in their own communities.