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Only an engineer can operate a zero-energy house

REALITY: ORNL's high-tech houses use low-tech operation

Jeff Christian has a mission and a vision. Over the past five years the Oak Ridge National Laboratory engineer has directed the design, construction and energy monitoring of five high-performance houses for low-income families. Aware that many people are not as "high-tech" as his colleagues at the Laboratory, Christian has also assessed the occupants' reactions to a suite of energy-efficient technologies developed by ORNL especially for the homes. Located in Lenoir City, Tennessee, and built for approximately $110,000 by Habitat for Humanity, the houses are providing the families year round with dramatically lower electric bills.

near-zero-energy house concept

"When we started this project in 2002 in cooperation with the Tennessee Valley Authority, the cost of electricity was 7 cents a kilowatt hour," Christian says. "One of the first four houses, which were quite small, had electric bills amounting to about 50 cents a day. The local electric rate in 2008 is 9 cents a kilowatt hour. Our largest near-zero-energy house, which has 2600 sq. ft., has an electric bill of $1.16 to $1.60 per day or about $35 to $48 a month." For these working families, the annual cost savings is more than $2,000 a year.

The savings are made possible by a combination of affordable technologies. Rooftop solar panels generate electricity that can be transferred back to TVA's electric grid, occasionally making the electric meter actually spin backwards. Other special energy features include a solar water heater, a foundation geothermal heat pump installed in the excavated space as the house is being built, highly efficient appliances with Energy Star ratings, compact fluorescent lights, windows facing south toward the sun and a variety of insulation technologies inside and outside walls to keep warm air in during winter and hot air out in summer.

The goal is an affordable zero-energy home—a house that in the course of a year generates as much electricity as it consumes. And, Christian emphasizes, he wants a house with technologies that the average American can manage easily.

Christian's job involves more than planning and overseeing house construction, tweaking installed energy-saving equipment in Habitat houses and writing scientific papers on energy efficiency. He is frequently on the lecture circuit preaching the gospel about ways to use less energy in commercial and residential buildings.

"I talk to hundreds of people who are interested in the zero-energy house. Sometimes, the reaction I get is that my talk was fascinating, but the idea of building a zero-energy house sounds really complicated and a bit intimidating.

"We have had Habitat for Humanity families in homes with advanced energy technologies since 2002, and they are doing fine. They do not have any special knowledge about mechanics and electrical features beyond the average homeowner. Equally important, they do not have to pay special attention to the package of energy technologies in these homes."

Christian concedes that on occasion after a family has moved into a near-zero-energy house, he has received calls on Saturday evening about the air or water inside being too hot or too cool. He has been willing to drive to the house and do some tweaking, dialing, and switching to improve the house's performance for the safety and comfort of the family.

Despite the simplicity of operation, the houses are different from conventional houses in several ways. House 5 has a utility wall that takes advantage of appliances that release heat—such as a refrigerator and freezer—by locating them next to those that use heat to raise the temperature of air or water, such as a dryer and dishwasher. House 5 has a well-insulated basement with concrete blocks that provide thermal mass to enhance occupant comfort because the heat-storing blocks are insulated on the outside by a fiberglass drainage board and exterior finish system. Above-grade walls are 6-in.-thick structural insulated panels, which are slightly thicker than the typical 2 by 4 in. wall system of a conventional house.

Residents in one of ORNLs near-zero-energy houses
Residents in one of ORNL's near-zero-energy houses.

Few houses have both solar panels on the roof and a foundation geothermal heat pump below ground with a compressor inside the basement. Most geothermal heat pumps for houses draw their heat from the ground after vertical wells have been dug as deep as 300 ft. Christian has found it much less expensive to install three horizontal loops made of high-density polyethylene pipes placed 5 ft. deep. The backhoe used to remove soil and rocks to provide space for the foundation can contribute to cost savings by also excavating additional space to accommodate the geothermal heat pump loops and the 200 ft.-long trenches to the sewer tap and water tap at the street.

One simple strategy is to take advantage of existing ground temperatures to supplement heating and cooling. In Tennessee, the winter temperature 5 ft. deep in undisturbed soil is as low as 45F. In summer the soil at the same depth can be as high as 82F. Throughout House 5's heating season from November through March, heat is drawn from the below-ground loops, causing their temperature to drop as the house warms inside. By summer, the heat in the house has migrated to the colder loops, reducing the need for air conditioning in the living space. Electric bills for cooling are thus lower.

With permission of the residents, Christian tried an experiment on House 5 during the hottest day of 2007. He wished to find out which interventions would minimize this house's effect on the Tennessee Valley Authority's critical peak period—the time between 5 and 7 p.m. when average customers use the most electricity, primarily for cooking, lighting, laundry and television. The resident stipulated a house temperature no higher than 73F by noon.

"Because of House 5's excellent thermal envelope with masonry inside, we were able to pre-cool the house to 71F by noon and hold the temperature at 71F until 5 p.m. when TVA's critical peak period begins." Christian explains. "We had programmed the thermostat to shut off the air conditioning until the cooling temperature reached 76. The indoor air temperature drifted to 74F as the temperature outside soared to 102F. We continued to bring in outside fresh air, but the house temperature that day never exceeded 74.5F."

The experiment showed that, during the hottest part of summer, occupants of House 5 can be comfortable during TVA's critical peak period without using electricity for air conditioning. Overall, the house used 0.75 kilowatt less power on the hottest day of last summer while the rooftop solar panels generated 0.75 kW. Doing without the heat pump for cooling saved 2kW, and discharging the storage batteries in the house sent 3kW to TVA's electric grid.

Christian, TVA, the Department of Energy and two building developers share a common vision. If one house can reduce TVA's peak load by 7 kW during an unusually hot day, many larger houses capable of near-zero-energy performance might cut TVA's peak power load enough to satisfy one of the agency's five-year strategic goals: to reduce demand for peak power by 1400 megawatts. A reduction of this scale would allow TVA to avoid the purchase of expensive power from other utilities or the construction of a nuclear power plant—both costly options for TVA customers.

Thanks to an increase in funding, Christian is optimistic that five new prototype houses will be built near ORNL by the end of the year. These houses would have solar panels on the rooftop, a solar water heater, a geothermal heat pump 5 ft deep, and structural insulation panels as insulation for the walls. One of the houses would have an internal utility wall called a ZEH Cor wall—steel frames containing pipes, wires and pumps to extract heat from the ground or reject excess heat in the house to the ground-source heat pump. A "feedback meter" would show the occupants the amount of electricity being used or generated. The three other houses would have different levels of energy efficiency.

This shared vision, if realized, would demonstrate further that residents without engineering degrees could live comfortably in high-tech homes. —Carolyn Krause


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