Students Build the Solar Homes of the Future

For the Solar Decathlon, 20 teams of college students strive to build the most technologically savvy sun-powered house. For two weeks in October their homes go head to head in a contest to consume the least energy.

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On a hot and bright California day, the red steel frame of a half-built house, its footprint a jagged stamp on the landscape, glints in the sunshine beaming down on Santa Clara University. Crawling around the crescent-shaped structure, workers holler measurements, fasten sheathing and snap chalk lines. They're wearing hard hats and tool belts, and the noise of steel swatting plywood sounds unmistakably like a job site. But this is hardly a construction crew out of central casting. Swinging a sledgehammer in this gravel lot somewhere between the quad and the cafeteria, a student volunteer with arms as skinny as the tool's handle tries in vain to pound a steel spike into the ground. He swings, misses and repeats, while the crew snickers. Each stroke gets more and more desperate, until, finally, a cartwheel swing sends the kid headlong. His oversized hard hat flies off and the crowd cracks up, prompting structural and construction manager Dan Ruffoni, a 22-year-old senior at Santa Clara, to intervene. Ruffoni sinks the spike in a few clean blows and progress resumes, but the sledgehammer slapstick has cost the rookie contractors 7 precious minutes. They really don't have 7 minutes to spare.

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The students have been working 100-hour weeks all semester, running to the job site after class and doffing hard hats to take midterms. So far, they've managed to finance the project and file 138 pages of engineering drawings on deadline--not to mention pass their exams. But the big deadline, though still months away, is approaching fast. That's when the half-clad structure now rising from the campus will have to function as a superefficient, sun-powered home of the future at the Solar Decathlon, in Washington, D.C. Sponsored by the U.S. Department of Energy this fall, the competition will pit 20 teams of college students against each other in an international showdown of innovative engineering. Team California's young roster tops out at age 23, whereas some schools attract Ph.D. candidates, or students Ruffoni describes as "old enough to look like they're already in the homebuilding business." As team project manager Allison Kopf puts it: "We don't have a construction background. We're undergrads--we have no background, period." But the students don't see that as a serious impediment; if anything, it has increased their drive to win.

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Like many of his teammates, 21-year-old Preet Anand says he "works on the project full-time and does school on the side." While workers whistle to "Layla" from a dusty boombox, Anand, the team's water systems leader, gestures to a rectangle of blue sky visible through the ceiling beams in the asymmetrical structure. "We're putting solar photovoltaics and flat-plate solar thermal collectors on the roof," he says, "and clerestory windows will provide simple daylighting. Our architecture is not only performance-driven, we want the house to look good." He describes his team's strategy as a "young, daring Silicon Valley approach"--and it's not without its challenges. Window shutters with thin-film solar collectors along the louvers had to be axed because of safety concerns. Certain products have proven to be so obscure that it was unclear whether they could be shipped to the U.S. For other prototypes, students have had to prove that new materials are up to code.

All this before crossing the last major hurdle: simply showing up at the competition. The members of Team California (which also includes students from California College of the Arts) must dismantle the house once they finally complete it and transport the pieces in three wide loads on a meticulously planned, 2848-mile journey across the country, only to reassemble the structure in just six days upon arrival at the National Mall. Then, they must move into their house for the duration of the competition. In the end, they're not just building a solar-powered home--they're proving that people can live in solar-powered homes, comfortably and without sacrifice. Teams have to shower in their houses, watch TV, do laundry and even host other teams for two dinner parties and a movie night.

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During the competition, the DOE will open the solar village's 20 homes for public tours from Oct. 9 to 13 and Oct. 15 to 18. On Oct. 14, expert judges in 10 categories--including architecture, engineering and market viability--will comb through each home, scrutinizing details down to the placement of the lighting. This year, there's an added twist: The houses will be plugged into the Potomac Electric Power Company's grid, where real-time meters will monitor both the amount of energy consumed and the power each home feeds back into the system. Nationally, meters like these could help utilities respond more quickly to power outages or prevent rolling brownouts. At the Solar Decathlon, they will keep score--50 additional points go to houses that generate more energy than they consume.

How It Works

Remarkable ingenuity defines every Solar Decathlon house. But these three standouts employ fresh technology and radical architecture to solve specific challenges.


Hot fluid from the rooftop solar thermal system transfers heat to water in PEX tubing in the floor panels, warming rooms. A second array of tubing in the ceiling can provide radiant cooling--an unusual twist. Cold water routed from a heat pump flows overhead, absorbing heat from the rooms and lowering the indoor temperature.

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Flat-plate solar thermal collectors, on the roof alongside photovoltaic panels, refract sunlight through heat-transmitting glass and a radiation-trapping absorptive coating. The process heats a freeze-proof fluid, which transfers heat to water in the domestic hot-water tank and radiant tubing in the floor. Radiant cooling, to be driven by solar thermal in the original plan, went electric when the manufacturer of a solar-powered absorption chiller couldn't deliver.


Graywater draining from the home's washing machine, sink and shower enters a planter that passively filters it through sand, loam and gravel. This filtered water irrigates an edible garden. A collection system channels rainwater to a pond and tank beneath the house, reducing runoff and aiding irrigation.

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Solar Decathlon Standouts >>>


Team California's heating and cooling system may help the house hover near that perfect zero. "Doing all the heating and cooling with electricity would take a lot of power from our solar panels," 23-year-old thermal design leader Tim Sennott says. So the students drew up an elaborate system in which solar thermal collectors heat and cool the house through radiant tubing in the floor and ceiling. An energy-recovery ventilator maintains a constant indoor temperature and humidity, further reducing HVAC demands. "What we designed is a whole ton more complicated than something we would ever put in a residential home," Sennott says. "We're trying to do stuff with technology that's not mainstream yet." The tactic can be risky. In July, the supplier of the cooling system's solar-powered absorption chiller pulled out, leaving students scrambling for an alternative.

The Solar Decathlon has a precedent of such edgy approaches, according to its director, Richard King. Students in years past have adopted solar technologies that were "hardly even heard of at the time," King says. The competition gives the public a chance to see these new technologies work together as a system--some, like California's radiant cooling, for the first time.

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Santa Clara's 2007 Solar Decathlon house, its boxy frame crowned with solar panels, stands a short walk from the current job site. In its sunny living room, sitting in a chair made from reclaimed wine-barrel staves, Anand pulls a tape measure from his back pocket and points to handsome bamboo joists along the ceiling. The 2007 team wanted to develop bamboo-based structural materials, he says. That idea caught the eye of the company Teragren, which sent unfinished bamboo for the students to test in a Tinius Olsen machine--a massive contraption that stresses I-beams to evaluate tensile strength and shear-load capacities. That year, underdog Santa Clara took third place. The students' joists, now certified under multiple international building codes, will soon be on the market.

"It's one thing to manufacture a product, but it's another to have it installed in a building that meets code," says Tom Goodham, Teragren's vice president of operations. "The beauty of putting products into an actual building is that it helps people understand it's not just conceptual. This is a material that's ready to be used, that can be integrated into design today." Working with Santa Clara on this year's house, Teragren revised the joist to create open-web floor trusses. The elegant, trestle-shaped bamboo beams have openings to run conduit, simplifying installation.

But really, all the cutting-edge technology eclipses the core challenge--building a house isn't easy. Try doing it on top of a full college course load. Despite the occasional decision Ruffoni describes as "so embarrassingly wrong that only students would do it," his teammate Anand credits their team's inexperience as a major asset. Unfamiliar with the sobering constraints of actual construction, he says, "The only thing we had to apply was creativity."

3 Solar Decathlon Standout Competitors

Team California's systems harness both the sun and rain--to keep the house's consumption at net-zero, nothing can go to waste.


Team Boston, comprising Boston Architectural College and Tufts University, wanted to keep its house's market price below $300,000. So students opted for a modular $200 Enphase micro-inverter--which converts DC into AC power--rather than a $3000 converter that typically handles the whole photovoltaic array. Ross Trethewey, the team's solar thermal engineer, also specified flat-panel thermal collectors at a third of the price of comparable evacuated tubes. "This isn't as flashy, but it's going to be affordable," Trethewey says, "and that's sustainable."

Sensors on the concrete floor inside Virginia Tech's house and a weather station on the roof detect temperature changes inside and out; a series of sliding panels, inspired by functional shutters, move across the front of the structure to adjust the interior climate accordingly. "The 'responsive architecture' strategy allows the user to easily manipulate shade and insulation screens to block harsh weather conditions, as well as open the house up when the weather is good," team leader Alden Haley says.

The sun doesn't hit the top of a house at quite the same angle in Duluth, Minn., as it does in, say, Tucson, Ariz. So the University of Arizona's house has a pivoting hinge that can adjust the one-piece southern wall and roof assembly to catch rays at the optimal angle. An airflow channel and a layer of insulation beneath the panels lowers their temperature. "Rooftop surface temperatures around here can get up to 150 F or 160 F," project manager Matt Gindle-sparger says. "PV panels are rated to tolerate even higher temperatures. But they're a lot happier when they're cooler."

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