PUTNEY, VT. — Students at The Putney School, along with faculty, architects and engineers, were involved in the design process of the 16,800-sq.ft. Putney School Field House, which is not only a net-zero building powered by solar energy, but also U.S. Green Building Council LEED-Platinum certified — the school just received certification in May — making it the only independent school building in the U.S. that is net-zero with LEED Platinum status.
According to The Putney School website, the Field House has used 48,374 kWh of electricity while the sun-tracking photovoltaic cells that power it have produced 51,371 kWh, therefore, nearly 3,000 more kilowatt hours of energy was produced than the building used in one year’s time.
"We want to show the world that net-zero energy technology for public buildings exists right now," said The Putney School Director Emily Jones. "It's time to move net-zero energy buildings from the theoretical realm into reality."
“The uniqueness of this project is the knowledge that was gained by everyone involved in the process,” said Danielle Petter, research director at Maclay Architects, the firm that designed The Putney School Field House. “This project led all of us to the conclusion that net-zero building is really possible at an institutional level. There is now a group of people in Vermont and the Northeast that know that this can be done, that net-zero is not just a goal, but something that can be reached for large scale buildings and the techniques and solutions explored and implemented on this project are practical for other buildings in our cold climate.”
The photovoltaic system is made up of 160 SunPower 230 watt modules mounted on16 passive trackers made by Zomeworks, six SMA Sunny Boy inverters, and a data acquisition system.
“SunPower cell efficiency is about 19%, since you want to get the most power on a tracker you can, using this high-efficiency module made sense,” explained Leigh Seddon, vice president of engineering at Alteris Renewables Inc.
“What’s great about the trackers is they have no motor drive but are moved by two Freon counter-balance tubes on either side,” said Seddon. “If they aren’t pointing directly towards the sun, the Freon counter balances it and makes it face towards the sun. The trackers move east to west, following the sun. When the sun goes down they are all facing west. In the morning, it takes about 20 minutes for them to wake up. The sun comes on them, the Freon moves around, and then they turn towards the east. There is no noise, no power loss to a motor, they just gracefully track the sun using solar energy.”
According to Seddon, PV modules on trackers produce about 25% more energy than a fixed roof mounted system, however, trackers are not right for every project. It depends on each individual project and its geographic location.
“This project had a vaulted roof, and the building didn’t face true south, so the ground mounted trackers made sense,” said Seddon. “The trackers do add cost, however, so you have to weigh all these factors to figure out what is best for a specific project.”
Seddon and the architect did an analysis comparing a fixed array versus a tracking array.
“The goal was to produce about 50,000 kW a year,” said Seddon. “The fixed array needed to be considerably larger to produce this amount of energy, so we were able to reduce the array footprint and balance of system cost by using trackers.
“Green Mountain Power serves The Putney School, and the utility has a special solar program where they pay an extra six cents per kilowatt hour for all solar electricity generated,” said Seddon. “This is what helped make it cost effective for the school. The system produces about 50,000 kW a year, and with net metering, any extra solar energy that the building doesn’t need flows right back out the meter, spinning it backwards.
“This is basically an all electric building, and this is the easiest way to get to a net-zero building: use heat pumps, high-efficiency lighting and make everything electrical. The school is so pleased with the building since it basically powers itself that they are discussing taking the entire campus net-zero now.”
Other sustainable features
The Field House also utilizes an air-to-air heat pump system and a natural ventilation system.
An air-to-air heat pump system was selected for this project because it was a net-zero project with PV generation of electricity, so an electric-based heating system was needed, according to Dan Lewis of Kohler & Lewis, the project’s mechanical contractor.
“The cost of PVs are high enough that it made sense to spend money reducing all the loads and increasing the efficiency of all HVAC components, hence heat pumps,” said Lewis. “We used two Daikin VRV systems, heat pump and heat recovery heat pumps. And the main heat recovery system for the ventilation air is an enthalpy wheel system made by Semco.”
Automatic natural ventilation where windows are on automatic night time flushing is also utilized in the building, along with carbon dioxide sensors.
“Automatic and manual natural ventilation for passive cooling are incorporated through windows with electric operators that are controlled through the direct digital control (DDC) system,” explained Lewis.
“Automatic high and low windows open when it is warm out and when the outdoors is cooler than indoors," added Lewis. "Manual control can be gained for all mechanical operators with a crank-timer, which allows manual control for the selected time period, with reversion to automatic control after.”
Other technologies include low-water fixtures and composting toilets; a white reflective roof to reduce “heat island” effect; super-insulated envelope with R20 under slab insulation, R20 foundation wall insulation, R45 walls, R60 roofs, R5 fiberglass windows, and sky lighting for approximately 40% of floor area; and detailing to avoid air infiltration and thermal bridging.
Student attendance in the building planning process was voluntary. For more than five months, students participated in four charrettes, the first one a brainstorming session to define the mission of the Field House construction and what elements would be included in an ideal world. Architects also sat in on Environmental Ecology classes to teach students about current best practices in environmental building design.
“After each charrette, the architects would go home, process student and faculty input, then offer us options at the beginning of the next charrette, which we would then choose from,” said Don Cuerdon, director of communications at The Putney School. “This was largely how the final site was chosen, how the program space was arranged in the floor plan, what the building looked like from the outside, and how the social space was laid out.”
Student trustees, who had full voting rights on the school’s board of trustees, were also on the building committee.
“Three students, including myself, were on the building committee,” said Shotaro Nakamura, an alumni of The Putney School who is now attending Dartmouth. “We were essentially responsible for most of the steps in the designing process. So that was another way to make sure student voices were present at every stage of the process.”
When asked what was most memorable about being involved in this project, Nakamura told CONTRACTOR that making a commitment to building sustainable was the right thing to do.
“As for the sustainability part, it is obviously the highlight of the building, and I believe that the trustees made a right call by committing to sustainability,” said Nakamura. “Environmental awareness had been increasing rapidly in the few years before the beginning of the designing process, and there was enough interest among the school community in making a higher level of commitment for the cause.”
According to Architect Bill Maclay, president and founder of Maclay Architects, such buildings not only protect the environment by minimizing energy use and reducing the need for outside (fossil fuel-based) energy sources, they pay for themselves through improved efficiency and lower operating costs and help avoid the risks associated with fuel price volatility.
"There are only a handful of net-zero energy buildings in the nation," said Maclay. “Our hope is that this will be one of the first of many such buildings. This will show the world that our buildings can and should be built to meet much stronger energy standards to not only protect the environment, but also to improve an organization's bottom line."
At the moment, Maclay Architects is in the process of helping The Putney School to develop a strategy for looking at the possibility of a net-zero campus.
“The focus of this project is to embed the development of a sustainable campus in deferred maintenance projects, so that as buildings receive renovation or expansion they are brought slowly up to the net-zero building standards that we have developed through the design of the Field House,” said Petter.
Home at Green Acres community surpasses zero-energy goal
Near-zero Lab Home paves way to eradicating energy bills