An integrated design team led by HOK and energy and daylighting consultant The Weidt Group undertook a 10-month virtual design charrette to create a market-rate, zero-emissions design for a class A commercial office building in St. Louis. The team used the U.S. Department of Energy’s definition of a zero emissions building: “A building that produces and exports at least as much emissions-free renewable energy as it imports and uses from emission-producing energy sources annually.”
Specific design goals for Net Zero Co2urt included:
- 150,00-200,000-sq.-ft. multi-tenant office building
- Affordable (10-year payback)
- Designed based on today’s technology, materials, systems and codes
- Flexible and adaptable
- Use ‘Life’s Principles’ to inform design
- Compare to base case LEED-NC Certified building of similar size
The team selected a potentially developable site in midtown St. Louis, Mo., for the project. They selected this site for its challenging four-season climate, because electricity costs in Missouri are among the lowest in the country — a factor that challenged the team’s ability to make the design affordable — and because St Louis’ electrical fuel profile is 81 percent coal.
The team believed that if they could create a market-rate carbon neutral design on this difficult site, they would be able replicate the process in any location.
The building site is part of an emerging biotech corridor. To the west is Barnes-Jewish Hospital at Washington University Medical Center, and to the east is Saint Louis University. Immediately to the north is the city’s historic Central West End, a thriving commercial and residential district.
The site is on a bus line and adjacent to the St. Louis MetroLink rapid transit system near a planned new stop in the next improvement cycle.
Before beginning design, the team evaluated energy use data for similar buildings and set an energy efficiency target of 80 percent compared to a benchmark office building in the St. Louis climate region. Establishing this aggressive energy efficiency target enabled them to produce the amount of on-site renewable energy required to reach a net zero carbon emissions footprint for the building.
Comparative analysis guided all decisions, major or minor, by explicating emissions data and creating a design driven by performance metrics. The mantra was, “model, measure and manage.”
Creating iterative virtual building models and measuring performance at every step of the design forced the team to manage its expectations until a working design emerged. Subsequent iterations refined initial decisions and resulted in a solution that achieved 73 percent energy use reduction through efficiency measures, with the balance satisfied by renewable energy sources.
An early climate and context study analyzed local solar intensity, wind potential, temperature and humidity ranges and cycles, and the effects of the St. Louis latitude and building type on daylighting potential and shading options. With 150-180 cloudy days per year classifying St. Louis as an “overcast sky” zone, the team designed to an 18.5 percent window-to-floor area and 35.2 percent window-to-wall ratio to achieve a minimum 1.5 percent daylight factor across a 60-foot-wide floor plate.
The program organized most optimally into two four-story, 300-foot-long office bars oriented east-west and joined by two links that enclose a 60-foot-wide landscaped courtyard. The north and south facades optimize vision and daylight glazing with insulated opaque areas to leverage natural light while maintaining a high performance envelope. The east and west facades are essentially solid, blocking glare at low sun angles and adding to the average R-value of the building skin.
Exterior walls of the office bars consist of R-40 rain screen construction with tile façades to the east and west. Vision and daylight panels are triple-glazed, double-low E with Argon fill set in wood frames to provide optimized and controlled daylight into the workspace, views to the exterior and maximal R-value.
On southern facades, evacuated solar thermal tube panels provide both a unique aesthetic and a heat source for the building. The roof is sloped at 10 degrees south and incorporates solar PV and solar thermal panels over an R-30 insulated roof. The east and west facades of the links are faced with a vegetated wall to provide aesthetic continuity to the natural aspects of the courtyard, moderate the outdoor climate for tenants and even soak up a few additional carbon dioxide molecules (not included in the total analysis).
The design solution features extensive use of natural light. Features including the massing, orientation, floor-to-floor height, window sizes, quality of glass and landscaping are optimized to ensure that the building can be illuminated without electricity during daylight hours and can mitigate the carbon emissions related to electrical energy consumption.
Because architectural solutions greatly reduced HVAC loads, the team was able to design an in-slab radiant heating and cooling system that is integrated with an underfloor air distribution (UFAD) system. As the radiant heating and cooling system provides temperature control for the space, air handling systems are primarily providing only ventilation and thus are greatly downsized.
The dedicated outdoor air system (DOAS) air handling units include total energy recovery wheels as well as dehumidification and ventilation for building occupants. A demand control ventilation (DCV) system monitors and adjusts the volume of outside air introduced into the building based on demand. This reduces heating, cooling and dehumidification loads attributed to outdoor air.
In addition to the DCV, building automation control strategies include photocell and occupancy sensor control of lighting and occupant control of plug loads. A raised floor provides complete flexibility for a multi-tenant layout. Manually operated windows allow for natural ventilation during the spring and fall shoulder seasons.
The integrated design of Net Zero Co2urt reduced carbon emissions by 76 percent through energy efficiency strategies, with only minor additional first costs compared to a conventional office building.
To provide the remaining clean energy required to reach zero carbon emissions, the team identified on-site renewable energy systems that include approximately 51,800 square feet of rooftop and wall-mounted photovoltaic panels and 15,000 square feet of solar thermal tubes on the southern building facades and roof.