Until recently, zero net-energy buildings were found only in areas having moderate to high utility rates, like California, or were owned by people concerned about “green” energy, even if it was more expensive. Now, net-zero has come to coal country, where electricity is as cheap as it gets.
Richardsville Elementary School, in Bowling Green, Kentucky, is the first net-zero energy public school in the United States. This 72,000-square-foot public school achieved net-zero energy status and stayed within the budgetary constraints of the Kentucky Department of Education. An energy goal of using no more than 18 kBtu per square foot per year was set at the beginning of the design phase. High- performance features, such as geothermal HVAC, occupancy controls, insulated concrete form walls, and a dedicated ventilation system with demand control, helped drive down energy usage.
A combination of clerestory windows and tubular daylighting devices admit natural light into teaching spaces. Supplemental artificial lighting is controlled by a dimming system. Computer labs and classroom computers were eliminated in favor of mobile laptop carts, which use less power and are easy to shut down when not being used.
One of the energy-reduction strategies was to eliminate the kitchen’s Type 1 exhaust hoods, since they are big energy consumers. Instead, steamers and convection ovens, which produce no grease-laden vapors, were selected, thus allowing only Type 2 hoods, which require much less exhaust and make-up air.
Once modeling showed that energy consumption was driven as low as economically practical, a 348 kW photovoltaic system was designed to completely offset all electricity usage. A 208 kW thin-film PV system was installed on the rooftop, and a 140 kW parking lot canopy supplements the rooftop system. The system will maintain the building’s net-zero energy status over its entire lifetime.
Richardsville Elementary takes advantage of the TVA Generation Partners program, which credits each kWh at $0.24. Between September 2011 and August 2012, the PV system generated $82,000 worth of credit. The PV system has a dedicated generation meter, and all solar electricity generated is reimbursed at $0.24 per kWh—about three times the retail electricity rate. Because this generation is presented as a credit toward the normally purchased electricity of the building, each utility bill has the normal energy charges plus a generation credit.
After two years of full occupancy, the building is operating at 17.3 kBtu per square foot per year—below the projected estimate and less than 25% of the energy used in a typical Kentucky school designed to current energy code, which specifies an energy usage of 72 kBtu per square foot per year or less. If this building had been designed to current energy code, the PV system required to offset all the school’s consumption would have cost more than $11 million. Due to its focus on implementing efficiency first, a smaller PV system was needed. The system was put out to bid in 2008 and cost $2.9 million.
To address this expense, the financial model recognized the value of cost-shifting elements of a “typical” school that were not needed in the Richardsville design. For example, eliminating computer labs saved on wiring infrastructure and removed 1,000 square feet of space. Also, the building envelope was designed to reduce exterior surface area compared to the building’s volume, resulting in a roughly rectangular shape. This helped reduce energy consumption and saved approximately $1.2 million on construction. The final building cost of Richardsville Elementary—with its PV system—was $206.50 per square foot—less than the national median ($216 per square foot) for a school building that would generate none of its own energy.