Homes that generate more energy than they consume are becoming more common and could become mainstream once the economy improves, according to a green building expert who spoke at the recent Ecobuild conference in Washington, D.C.
And Bion Howard of California-based Building Environmental Science and Technology foresees high-performance houses going beyond net-zero energy usage to net-positive, supplying power to the utility company from their on-site solar, wind, or hydroelectric systems.
“The idea is to be selling in some seasons, buying in others,” he said, with a goal of selling enough back to the grid to pay for the home’s utility bill averaged over the year.
In Howard’s vision, a net-positive-energy (NPE) home is ultra-insulated and precisely sited to reduce the need for mechanical heating and cooling so that a renewable energy system can generate more than enough power for the home.
“In some climates you may need no traditional HVAC system, maybe just a dehumidifier with an ERV and baseboard heat,” he said. “That will save a ton of money to put back on the table to do other things with.”
Building an NPE house requires superior team communication, Howard said, particularly in the early planning stages. In addition, design costs are higher than a traditional high-performance home and the process can be very time consuming, with an average of 32 computer runs required just during the assessment phase.
“This does significantly have a cost impact on what you’re going to build,” he said, estimating up to an 18% increase in construction cost—mainly in homes with solar panels and geothermal systems--although he’s familiar with some prototypes that have shown no increased cost, at least on paper.
Some of Howard's strategies for building an NPE home can be applied to any energy-efficient dwelling:
--Tune siding and roofing materials color selection to the local climate. Howard noted that this simple tactic can provide up to a 15% reduction in heating and cooling costs, especially in hot areas where light-colored cool roofs help offset the need for mechanical air conditioning.
--Decrease electric demand by specing right-sized appliances, mechanical systems, and plug loads.
--Make sure the home’s passive solar penalty (the amount of sunlight beyond what is needed for adequate daylighting) does not exceed 10% in order to minimize excess air conditioning demand.
--Use Energy Star standards as a starting point when specing appliances and lighting. Howard recommends the ACEEE Consumer Guide to Home Energy Savings Online. “This is the next step in energy efficiency, beyond green programs such as those from the NAHB, Energy Star, and LEED,” said Howard.
--Explore energy modeling software during the assessment process to discover how small changes in a home’s design can save or cost thousands of dollars. Howard recommends programs such as Energy-10 v.1.8, which includes a WeatherMaker feature; EnergyGauge USA, which computes heating and cooling system part-load performance and detailed hourly modeling; and, for novice or intermediate users, REM/Design v. 12.2, which provides for automatic and editable sizing of HVAC equipment and detailed solar systems modeling.
--Because there are diminishing returns with any energy efficiency upgrade, Howard recommends using life-cycle cost analysis to evaluate each upgrade’s worth over time.
For now, net-positive-energy homes are rare, although one, the Equinox House in Urbana, Ill., is nearly complete. Designed by architect and LEED-AP Jean Ascoli, the ultra-insulated four-bedroom dwelling will generate enough solar energy to power itself and an electric car.
Still, the mortgage crisis and recession have put plans on hold for most homeowners interested in net-positive living, Howard said, especially since energy efficient mortgages that could help finance these custom homes have dried up.
“I believe that in better times these houses would be fundable because their cash flow is always positive,” he said. “But for right now we haven’t figured out how to bring this to the mainstream.”
For a detailed analysis of energy-producing prototypes in Baltimore, Atlanta, and Grand Junction, Colo., click here.
Jennifer Goodman is Senior Editor, Online for EcoHome.