Pushing the envelope
A promising example of ORNL's work on integrating building technologies is progress in creating affordable zero-energy houses
Utility bill shows August 2004 electricity cost for a near-zero-energy house was only $13.98, thanks partly to rooftop solar panels.
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If ORNL's Jeff Christian could have his way, next-generation houses in East Tennessee would generate as much electricity as they consume. Such zero-energy homes, which eliminate electric bills by producing the power they need using solar panels, would be modeled after the four near-zero-energy Habitat for Humanity houses built in 2002-2004 near Lenoir City, Tennessee.
The master builders of the village are volunteers and researchers with the Department of Energy's Buildings Technology Center (BTC), which Christian leads. DOE and the Tennessee Valley Authority (TVA) jointly fund the program. The houses are the culmination of ORNL research for DOE on building envelopes, including roofs and walls, as well as appliances, such as refrigerators and systems for heating and cooling air inside buildings.
"The average daily energy bill for these houses is less than a dollar, possibly as low as 65 cents, for the most efficient house over time," Christian says. "The fifth and last house will take advantage of the latest energy-efficient technologies and lessons learned from the construction, monitoring, and analysis of the four near-zero-energy houses."
For each test house, the insulated metal roof assembly has three jobs: it provides enhanced moisture control using a drained layer, insulation for the residents, and electricity for the grid. Each roof has solar panels, with the electricity produced from the panels' photovoltaic cells sold to TVA's Green Power Generation Partnership Program. The electric bill for each house is exceptionally low, based on the power used for the house minus the credits TVA gives for the power generated by the solar panels for distribution on TVA power lines.
A data system that charts energy usage, temperatures, and flow of water and electricity for each house indicates that these houses use 50 to 70% less energy than typical new American homes.
The secret behind each house's exceptional energy efficiency lies in a well-insulated airtight envelope. The walls and roof incorporate structural insulated panels (SIPs) that keep indoor heat in during winter and outdoor heat out during summer. The SIPS in the fourth house, which contain pentane-blown polyisocyanurate insulation, minimize damage by moisture and, combined with integrated window wall panels, reduce losses of inside air already heated or cooled to comfortable living temperatures.
During Tennessee's cold season the house's thermal mass foundation walls store heat absorbed from the sun during the day and release the heat to the house interior at night. The air ducts are positioned inside the conditioned space, as recommended by ORNL research, providing 30% to 40% space heating and cooling energy savings.
Each house has efficient heating, ventilation, and air conditioning equipment, as well as highly efficient appliances. To keep residents comfortable, each super-efficient house has a heat pump, including a geothermal heat pump in the third house.
The fourth house has an integrated drop-in heat pump water heater, mechanical ventilation (to circulate fresh air in the airtight structure), and appliances rated at some of the highest efficiencies possible.
Other energy-saving technologies in the houses are compact fluorescent light bulbs, high-efficiency windows, and extended roof overhangs on the south side to shade the top-floor windows during the summer. The houses also contain sensors and controls to ensure that the mechanical ventilation system is providing airflow at rates that meet national standards.
Because of the more-efficient appliances and better photovoltaic system on the roof, the second- and third-generation test houses use about 20% less energy and produce about 11% more solar electricity than the first.
"We are building one more house that will be outfitted with energy-saving technologies," Christian says. "Based on analysis of the data we get from all the houses, we plan to determine the most cost-efficient methods of saving energy in houses and assembling small, well-crafted houses. The ultimate goal is the creation of zero-energy homes that are affordable to most Americans."
Integrating Building Technologies
Jeff Christian in one of the Habitat for Humanity near-zero-energy homes in Lenoir City, Tennessee.—Courtesy Mother Earth News/Allan B. Hunt
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Patrick Hughes, ORNL's Building Technologies Integration Manager, says DOE's Building Technologies (BT) Program, the foundation sponsor for ORNL's buildings research, is calling for integration of buildings technologies. "As Jeff Christian's Habitat for Humanity project illustrates, ORNL is well positioned to integrate building technologies to foster their advancement, transfer, and deployment. ORNL will continue to develop new and exciting technologies to integrate into buildings in the future."
Four groups in ORNL's Engineering Science and Technology Division—Building Equipment, Building Envelope, Residential Buildings, and Commercial Buildings—are developing breakthrough building technologies. For example, to help make thermal mass more available for mass market construction, ORNL researchers seek to increase the effective thermal mass in the insulation cavity through use of phase change material (PCM)-enhanced fiber insulation. To make SIPs less susceptible to termites, rotting, and warping, Laboratory researchers are working with industry to develop SIPs with steel instead of processed-wood facings. The new SIPS would also exhibit improved wind and fire resistance, dimensional stability, and energy savings. ORNL researchers are working on the development of an integrated residential heat pump that provides hot water, space heating, cooling, ventilation, and dehumidification. Combining experiments with computational modeling, ORNL, in partnership with industry, is improving air curtain design to reduce energy use by refrigerated display cases in supermarkets and other food retail outlets.
Other technologies under development include an advanced, low-cost geothermal heat pump, a lower-cost version of the heat pump water heater invented by ORNL researchers and industry, and low-cost sensors for adaptive controlled ventilation that could be tied into an energy management control system that turns off lights, air handlers, and other unneeded energy-consuming equipment in buildings.
By combining their efforts with those of a variety of partners, ORNL scientists anticipate a growing integration of energy-saving ideas with construction technologies.