(This is the second of a two-part series. See Part 1: Students prove solar-powered utopia does exist.)

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 student teams that competed in the Solar Decathlon last month on the Mall in Washington, D.C. The teams, about equally divided between engineering and architecture students, represented schools in the United States, Canada and Spain.

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. Converting sunlight into heat for space heating and hot water is also a relatively easy task. The technology for this is also readily available, not nearly as costly, but still not widely utilized. 

The challenge for the Solar Decathlon teams was meeting the criteria established by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy, which sponsored the event. The house had to include all the creature comforts that homeowners today expect, and the house, its overhangs and all its energy-generating equipment had to fit within an 800-square-foot area. To win, the teams also had to meet the exacting requirements of the decathlon contests. 

In five of the 10 events, the houses had to perform at a much higher level than the average homeowner would expect. For example, the comfort contest required that the interior temperature and humidity level be maintained within a very narrow range. The appliance contest required that refrigerators and freezers maintain specific temperatures, and the students had to perform specified household tasks using the most energy-efficient appliances their team could garner. The performance of each house and each appliance was measured by sensors that were placed in the houses and monitored by DOE staff in a nearby trailer. 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 to the exacting standards of judges who were experts in each category. (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 amount of livable area in most of the entries, and most would 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 and one that every homeowner should pursue is a tight and energy-efficient building envelope. This significantly reduces the amount of energy required to heat and cool the house, and all that’s required is a conscientious effort to plug up all those air leaks that bring in unwanted hot or cold air and beef up the insulation.

The teams did this and then some.

Instead of using larger quantities of conventional insulation materials, the teams chose ones with even higher R-values, (the R-value indicates a material’s resistance to heat flow, the higher the R value, the better), and many of their choices were experimental. For example, the University of Colorado team used a wall panel made of soy-based foam insulation sandwiched between two soundboard panels made from 100 percent post-consumer waste paper.

The Solar Decathlon houses were so well sealed that each one required a mechanical means to bring in fresh air when all the windows were closed. Rather than use a simple air intake and exhaust fan, the solution that most homeowners would employ, all the teams installed an air-to-air heat exchanger. This captures the heat or cool from indoor air before expelling it and then transfers the recovered energy to the incoming air. As much as 70 percent of the energy can be recovered, a significant savings when you are trying to minimize energy consumption and a sensible investment if you live in a place like Vermont with very cold winters and an 80-degree difference between indoor and outdoor temperatures.

Energy efficiencies were also realized in the way each team designed its house and oriented it, and homeowners can do this as well. The teams from temperate climate zones had large openings on the south side of the house to capture the sun’s warmth during cold winter and generous overhangs to keep the hot sun out in hot summer weather. Teams from regions with humid summers located door and window openings to maximize cross ventilation. To avoid using electric lights during the day, all the teams located windows to maximize daylight levels inside.

Another energy-saving strategy that any homeowner can follow is selecting energy-efficient appliances. Unbeknownst to most homeowners, appliances in a typical house can consume as much as 30 percent to 40 percent of all the energy used. The Solar Decathlon teams selected Energy Star appliances or ones that were even more efficient. For example, many houses had an induction cooktop, which uses about 30 percent less energy than a conventional electric cooktop with a glass top or coil burners.

Substituting readily available compact fluorescent light bulbs (usually called CFLs) for incandescent bulbs also saved energy. The CFLs use about one-third as much energy, and they are color corrected to give the same quality of light.

To generate electricity, the teams used panels of photovoltaic cells, commonly called PVs, which convert solar energy into electricity. The PVs were installed on the roof. They produce DC-current, which is run through an inverter to convert it into the AC-current needed to run household equipment. The great shortcoming of this otherwise ingenious system is that when the sun sets, the system shuts 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. In the course of a year, the average household sells more power than it purchases.

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 the 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 entire array. The Solar Decathlon houses had anywhere from 20 to 44 batteries, depending on the electricity needs of the house and the size of the batteries.

The New York Institute of Technology team used a more complex system for storing their electricity, but one with potentially wider application. They chose to store their surplus electricity in the form of hydrogen, which they generated by sending their surplus electricity through an electrolyzer. This synthesizes hydrogen from water and then pumps the hydrogen gas into storage tanks. At night the hydrogen is drawn down to power a fuel cell that generates electricity. The tantalizing potential advantage of this system, which is decades away from mass production, is that the hydrogen produced could also be used to supply a hydrogen-fuel-celled automobile.

The Solar Decathlon teams could also have used electricity for space heating and hot water. But, a more efficient strategy is to convert sunlight directly into heat and then transfer it through one or more mediums until it reaches the 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 the roof. As the liquid heats up, electrically operated sensors signal an electric pump to move the hot liquid to a hot 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 household use. Solar collector systems for domestic hot water are widely available and can be installed in any house in the country.

The final take-away lesson for homeowners is the impact of these 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 United States each year, and it significantly contributes to global warming. In contrast, these solar-powered houses do not use any fossil fuels, and they do not affect the atmosphere at all.

Questions, queries or a house-building story you’d like to share? Katherine Salant can be contacted at www.katherinesalant.com.

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