Second of two articles

Is it possible to live well in a house powered entirely by energy from the sun?

The answer is yes, as demonstrated by the 18 houses designed and built by the teams of engineering and architecture students that competed in the U.S. Department of Energy's Solar Decathlon last month on the National Mall.

Is it hard to build such a house?

Not as hard as you might suspect. Though generating electricity from sunlight might seem to be the Solar Decathlon's most daunting challenge, the technology is readily available, but not widely utilized because of its cost. Using sunlight to heat rooms or hot water is also relatively easy. The technology is also readily available, not nearly as costly, but still not widely utilized.

The challenge for the Solar Decathlon teams was to include all the creature comforts that homeowners expect, meet the exacting requirements of each of the decathlon contests and keep the house within 800 square feet.

In five of the 10 contests, the houses had to perform at a much higher level than the average homeowner would require. For example, the comfort contest required that the interior temperature and humidity be maintained within a very narrow range. The appliance contest required that refrigerators and freezers maintain specific temperatures, and the students had to perform specific household tasks using the most energy-efficient appliances they could get.

For a sixth event, each team drove an electric car whose battery was charged by the team's solar electric system; the winning team clocked the highest mileage. For the other four events, the teams were subjected similarly exacting standards. (Disclaimer: I was a judge for the dwelling event.)

The Solar Decathlon did not present real-world conditions or expectations. Not many couples would choose to live in a 550-square-foot house, the livable area in most of the entries, and most would not be so rigid about their room and appliance temperatures. Nevertheless, there was much that anyone building a new house could emulate.

The first priority for all the teams was a tight and energy-efficient building envelope. That significantly reduces the amount of energy required to heat and cool the house, and all that's required is a conscientious effort to plug all those air leaks that bring in unwanted hot or cold air, and beef up the insulation. The teams did that and then some.

Instead of using larger quantities of conventional insulation materials, the teams chose ones with even higher R-values, and many of their choices were experimental. For example, the University of Colorado team, the overall decathlon winner, used a wall panel made of soy-based foam insulation sandwiched between two soundboard panels made from waste paper.

The Solar Decathlon houses were sealed so well that each required a mechanical means to bring in fresh air when all the windows were closed. Rather than use simple intake and exhaust fans, the solution that most homeowners would employ, all the teams installed air-to-air heat exchangers. They capture the heat or coolness from indoor air before expelling it and then transfer the recovered energy to the incoming air. As much as 70 percent of the energy can be recovered.

Energy efficiencies were also realized in the way each team designed its house and oriented it. The teams from temperate zones had large openings on the south side of their houses to capture the sun's warmth in winter and generous overhangs to keep the hot sun out in summer. Teams from regions with humid summers located door and window openings to maximize cross-ventilation. The teams also selected Energy Star appliances or ones that were even more efficient.

To generate electricity, the teams used panels of photovoltaic cells, commonly called PVs, which convert solar energy into electricity. The PVs were installed on roofs. The great shortcoming of such systems is that when the sun sets, they shut down.

When PVs are installed in a house that can be tied to a utility grid, the house operates under its own power during the day, typically selling any surplus to the utility. At night the homeowners buy power. Since the Solar Decathlon houses had to be self-sufficient, each team had to generate enough electricity to have a surplus that could be stored in batteries and drawn down at night.

All but one team stored its electricity in sealed lead acid batteries, which are similar to car batteries but with two significant differences. First, a car battery is designed to deliver a lot of power just long enough to start an engine, while these batteries are "deeply charged" and designed to deliver far less power for hours at a time. Second, a car needs only one battery, but a house requires an array. The Solar Decathlon houses had 20 to 44 batteries, depending on the electricity needs of the house and the size of the batteries.

The Solar Decathlon teams could also have used electricity for space heating and hot water. A more efficient strategy is to convert sunlight directly into heat and then transfer it through one or more mediums until it reaches a hot-water tank or a heating system, while using electricity sparingly as an "assist." For example, some of the teams produced hot water by placing solar collectors -- flat, glass-covered boxes with liquid-filled tubes -- on roofs. As the liquid heats up, sensors signal an electric pump to move the hot liquid to a water heater in the house where the heat is transferred to the water in the tank. During the day the liquid is recirculated many times between the roof and the tank, which keeps the stored water hot and ready for use. Solar collection systems for domestic hot water are widely available and can be installed in any house.

The lesson for homeowners is the impact of solar-powered houses on the environment. Almost all houses in the United States are powered directly or indirectly by fossil fuels. Collectively, their energy consumption accounts for about 21 percent of all energy consumed in the country each year, and it significantly contributes to global warming. In contrast, solar-powered houses do not use any fossil fuels, and they do not affect the atmosphere at all.

Katherine Salant can be contacted at

(c) 2005, Katherine Salant

Distributed by Inman News Features

A solar-powered, energy-efficient house won first place in the Solar Decathlon for a University of Colorado team. Eighteen teams competed.