Case study: Zero-net energy transit center
The John W. Olver Transit Center zero-net energy (ZNE) project is a high-performance building designed with reduced energy loads, passive design strategies, maximized efficiency of mechanical and electrical systems, and on-site power generation.
- Understand net zero energy buildings (NZEBs), and how they are defined.
- Learn the codes, standards, and guidelines that offer engineers a path toward zero-net energy (ZNE) design.
- Via a case study, see an example on how to design a ZNE facility.
Located in the heart of Greenfield, Mass., the 3-year-old John W. Olver Transit Center (OTC)—an intermodal depot for all of the area’s fixed-route bus lines and private intercity, taxi, and paratransit (community transport) services—is an example of a successful zero-net energy (ZNE) project. It is the first ZNE building of its type in the United States.
At a project cost of a little less than $11 million (including the site), the two-story, 24,000-sq-ft building houses the offices of the Franklin Regional Transit Authority (the public transportation provider for this part of Massachusetts and client for the OTC) and the Franklin Regional Council of Governments. The building comprises offices, community space, a waiting area, a café, storage, and restrooms.
Project goals were developed by both the client and design team in the early stages of planning, and were identified to achieve a ZNE building; use energy-efficient, user-friendly systems; be sustainable in operation; and optimize capital and running costs.
With objectives and goals outlined at the outset, the iterative process of aiming for high-performance was fully incorporated into the design. This strategy involved reducing loads, employing passive design strategies, maximizing efficiency of mechanical and electrical systems, and generating power on-site from renewable sources. When designing a ZNE building, one of the first design decisions to make is its orientation, which can impact heating, lighting, and cooling costs. The building orientation was fixed by the requirements for the bus driveway and parking.
Because the orientation was fixed, the Arup team worked closely with Charles Rose Architects (CRA) to optimize the location of spaces within the building to create buffer spaces where possible. Elements such as storage, plant rooms, and bathrooms were located on the west side of the building, which insulated the offices on the east from the low angle solar gain. In addition, the façade insulation levels were increased beyond what is required by the Massachusetts Code with an R-42 roof and R-33 to R-36 walls. Where reasonably possible, wider temperature and humidity comfort bands were agreed upon with the client for the design of the HVAC and lighting systems.
As with any high-performance building, developing goals early—specifically those concerning ZNE—is critical to success:
- Begin the energy-modeling process at schematic design and continue to refine it through construction documents. Additionally, checking performance as equipment is selected by the contractor is recommended.
- Agree input assumptions to the model with the users such as occupancy schedules and client-purchased equipment, i.e., computers, servers, and process loads such as kitchen equipment. Reinforce this through the design and issue it for feedback at each phase.
- Ensure that the building is fully commissioned, and have a strategy for ongoing measurement and verification so that performance enhancements persist. Also educate both facilities staff and end-users so that they can contribute to the building’s success.
- Celebrate achieving ZNE. Providing feedback on strategies used and performance achieved will help future designers achieve similar goals.
The daylighting strategy for the OTC ground floor was fairly straightforward. Because the space primarily consists of a waiting area with transient users, the light level targets were lower than in the offices. The glazed eastern and southern façades provide most of the light for this waiting area. The larger plan of the building’s second floor creates an overhang above this glazing, reducing direct sunlight penetration and glare, aiding lighting levels and visual comfort, and reducing thermal loads on the façade glazing.
The second floor comprises office space on the north, east, and south sides, and program requirements dictated that private offices occupy the areas near the façades.
To assess the quantity of daylight throughout the second floor, an initial “daylight factor” study was completed. This is a measure of the amount of daylight at a point inside as compared with an unobstructed point outside; office daylight factor targets are typically 2% to 5%.
The south and east glazing for the second-floor offices is partially covered by perforated copper screen. This both reduces glare in the office space and the direct solar load. To reduce electric lighting in the open office space located in the center of the floor plan, daylight is provided through a combination of clerestory glazing. Skylights were added so that the entire space is illuminated.
To further understand the daylight performance and provide accurate input to the energy modeling, an annual illuminance simulation was performed for each hour of the day. Through the use of these various analysis tools, the potentially competing elements of shading and available daylight could be assessed.
This analysis determined the “daylight autonomy” for each workstation in the office space—the percentage of operating hours during a typical year when illuminance levels from daylight can be expected to exceed the light level design criteria, allowing electric lighting to be turned off. Daylight autonomy is expressed as a percentage.
The project team defined 75% daylight autonomy as an appropriate target; for 75% of all operating hours there would be enough daylight for electric lighting not to be required. It was calculated that 89% of the workstations on the second floor met this criterion, and 100% achieved a daylight autonomy of >50%.
The west-facing lobby glazing is screened by an extension of the brick cladding, which has selected bricks removed to form an external egg-crate-type shading device. Only the north-facing offices are fully glazed, providing indirect daylight and long views back to historic Greenfield.