Every two years the U.S. Department of Energy hosts a Solar Decathlon on the National Mall in Washington, D.C. in which twenty college and university teams compete to design, build, and operate the most attractive and energy-efficient solar-powered house. A great deal of information is available from the Solar Decathlon website, and from the websites for the teams which are linked from there, so there’s no need to repeat any of that here. But we would like to share some observations about which ideas need further development, and which may be ready to become standard architectural practice.
The competition rules were improved this year by eliminating the transportation contest, and tying the houses into the grid. Transportation is a large component of our energy use, but it is a vast subject unto itself, and deserves its own event. At the Solar Decathlon it was just a distraction. In previous years each home had to have an elaborate and expensive power storage system. That’s great for people living “off-the-grid” but for most of us it’s much simpler and cheaper to instal a net meter, use your solar power to run the meter backwards during the day, and get your nighttime power off the grid. Or looking at it in another way, we are nowhere near to generating all of our daytime power needs from the sun, so there’s no need to store it for use at night. Below is a table summarizing the differences in scoring between 2007 and 2009.
It is clear that every effort was made to judge the entries fairly. Nevertheless, in the Architecture and Market Viability contests the judges seemed to give preference to conventional and familiar approaches. We would like to have seen them give extra credit to the most innovative and original thinking. Most teams optimized their house designs for their local environment, and so they may have been at a disadvantage if their native climate differed significantly from that of Washington. For the competition, the performance of the homes could only be monitored for a two week span in October, so how they would fare on a year-round basis is yet to be determined.
There are a few design themes common to most of the houses. Each of them needed some sort of central control system to monitor and operate all the mechanical systems. Such controllers are not readily available off the shelf, so most teams cobbled something together with custom hardware and software. Many homes pursued the current trend of helping consumers reduce their energy use by increasing their “energy consciousness”. This usually takes the form of prominently displayed energy-use monitors to remind occupants of how much power they’re using. Tracking energy generation and consumption is a great way to detect maintenance issues, but a building should adjust to the needs of its occupants, not the other way around. We need to find ways to make energy efficiency easy, not to bombard people with new information and new constraints on their behavior. If we do want to influence people’s energy use habits, we should do it by making conservation the easier and more natural choice.
Because it is very difficult to make an exterior wall completely waterproof, several of the houses had rain screens – barriers that stand off from the exterior wall, preventing most water from reaching it. In between the rain screen and the water resistant wall there is an air gap that allows for evaporation of any moisture that makes it through the outer barrier. Some people have expressed the concern that debris, insects, and other pests might take up residence in the air gap, but it has not been determined if this is a problem in practice.
Several teams used a microinverter system from Enphase Energy. Conventional solar-electric installations use a single power inverter to convert the direct current (DC) from the panels to alternating current (AC) like what we get from power companies. The problem with this is that anything that impairs the performance of one panel (such as dirt or a shadow) reduces the amount of power generated by all the panels. By equipping each panel with its own inverter, maximum power output can be attained. Another advantage is that the performance of each panel can be monitored independently, so you can easily see if one panel is damaged or not performing as it should, without having to get up on the roof with test equipment. Unfortunately the only way to monitor and manage the solar power system is through an Enphase proprietary website. That makes you completely dependent on the company continuing to provide this service.
An important take-home message from the event is that it is cheeper and easier to reduce power consumption in the home than it is to increase power generation from sunlight. One workshop speaker went so far as to refer to solar-electric panels as “eco-bling”. You could see this in the two-fold strategy that was used by each team: generate as much power as possible, but also reduce power use as much as possible. If you can’t afford to do both, you’ll get more bang for your buck by increasing energy efficiency.
While each home was unique, there is a sort of canonical solar house design that has emerged. It includes a wedge shape with roof tilted toward the sun & clearstory windows at the top of the taller wall, passive ventilation, super insulation, thermal storage, thermal recovery ventilation, induction cooktops, front-loading washing machines, LED (or CFL) lights, solar-thermal panels, and solar-electric panels.
Besides energy efficiency and solar power, there were several other popular design objectives, such as affordability, use of recycled and recyclable construction materials, wheelchair accessibility, hurricane resistance, and water conservation. Recycled construction materials are good but, if amortized over the lifetime of a house, their significance is limited. A stylistic trend that has nothing to do with energy, is dishwashers and refrigerators that mount flush, and can be faced with the same material as the kitchen cabinets so that they blend in seamlessly. Very few of the homes used any form of solar tracking system. Since the homes were limited by competition rules to no more than 800 square feet, many of the teams used Murphy beds and movable cabinets or partitions to create reconfigurable multi-use rooms. Many also used retractable glass walls so that in good weather the outdoors could be used as additional living space. Many of the homes made use of flammable materials such as wood, polycarbonate, or PET plastic, so fire safety does not seem to have been a major concern. Several of the students commented to me that their houses cost more than a production house because they needed to be disassembled, transported, and reassembled. This meant that the individual modules needed more structural integrity than would be necessary in a site-built house. But this extra strength would be quite helpful in an earthquake or tornado.
Some of the houses had unique features that deserve special mention here:
Instead of trying to describe the “Sol Abode” house in a brochure, Team Alberta (University of Calgary, SAIT Polytechnic, Alberta College of Art + Design, & Mount Royal College) gave out a card with their URL printed on hand-made recycled paper with wildflower seeds imbedded. I will be planting the card in the spring, and I’ll let you know what happens.
The third place winner was the “Refract House” from Team California (Santa Clara University and California College of the Arts) The bathroom walls were made of large sheets of opaque glass.
The Ohio State University house featured bi-directional solar panels that generate power from light that falls on the top side of the panel, and also from light that reflects off the white roof onto the back of the panel.
The University of Arizona entry, called the “Solar Energy-Efficient Dwelling” (SEED) [Pod], was one of the most striking and original designs. Its south wall was made of large panels of clear plastic made from recycled soda bottles, filled with water to provide thermal mass. The kitchen had distinctive individual hexagonal cooktop burners. The chimney effect was used to cool the solar panels. (Solar cells are less efficient when they’re hot.) The angle of the roof can be adjusted for optimum performance at different latitudes.
The first place winner was the “SurPlus” house from Team Germany (Technische Universität Darmstadt). It used vacuum insulated panels, phase change materials, and while it had traditional mono-crystalline photo-voltaic panels on the roof, it used thin-film panels on all the exterior walls. Thin-film panels perform better at high temperatures and in cloudy weather. I heard that the judges were quite impressed one cloudy day to find that this house was generating more power than it was using. It looks like we’ll be seeing a lot more thin-film solar panels at future Solar Decathlons. The brochure for this house had a game you could play. And this was the only house that had a built-in automatic espresso machine. (Frankly I think that’s why they won. After all, students can get by for a few days without sunlight, but caffeine?)
Team Ontario/BC (University of Waterloo, Ryerson University, & Simon Fraser University) put their photo-voltaic panels on the south, east, and west sides of their “North House” instead of on the roof where snow would interfere with their use. It also had R-60 insulation, quadruple-glazed, floor-to-ceiling windows, salt hydrate phase-change material in the floor, a bed that retracts to the ceiling, and Corian on interior walls. There was a steel grate on the north side that admitted light from the northwest while blocking it from northeast..
“The Black and White House” from Team Spain (Universidad Politécnica de Madrid) was one of the most handsome and iconic of all the houses, and the only one with a solar panel that could be pointed directly at the sun at any time of day in any season, regardless of how the house is oriented. And it can do that in a variety of latitudes without any change to the design. It was also perhaps the most impressive in its symbolism and attention to detail – from the sculpture and reflecting pool to the custom-designed “orbital” furniture and the lavishly produced brochure, including a game for children. The north and south interior walls had Corian panels engraved with maps of Washington, DC and Madrid. It had a square footprint to reduce the surface-area-to-enclosed-volume ratio. The main room was big, but you had to pull out the TV cabinet before you could lower the Murphy bed. The kitchen and bathroom were reduced in size to leave more space for the great room. There were solar-thermal collectors on the facade and edge of the roof. The facade PV panels were manually operated.
The Universidad de Puerto Rico “Caribbean Affordable Solar House” (CASH) had a steel frame with walls made of Structural Insulated Panels (SIPs) and a radiant ceiling. This house had a large cabinet with Murphy bed that could be moved back-and-forth along a track by turning a crank. But other teams found simpler ways to deal with this problem. Some such as Arizona and Germany used and open plan. Others like Virginia had identifiable rooms, but did not feel cramped.
The University of Kentucky’s “S.Ky Blue House” had occupancy sensors flush-mounted in the ceiling, and a translucent back-splash on the kitchen counter. It also used the chimney-effect to cool the back side of the solar panels.
The brochure for Virginia Tech’s “LumenHaus” asked, “Does your home dress for the weather?” This design concept, called “Responsive Architecture”, retrieves the local weather forecast from the Internet, and uses it to proactively adjust motorized sliding sun shades to regulate the absorption of solar energy. In contrast, passive solar architecture usually uses roof overhangs or louvers to provide extra shade on the exterior walls when the sun is high. Since the sun tends to be higher in the summer months, this provides some advantage. But on cloudy winter days, you really want all the sun you can get. And in the heat of summer, even the early morning or late afternoon sun can be more than you need. So this responsive strategy has the potential to provide improved performance. For the floor, they used a special concrete formulation that allowed for some flexing without cracking during transport.
The houses were closed to the public at night, but there was nothing to stop any visitor to the National Mall from walking through the Solar Village. The evening of Friday, October 9 was downright balmy, so I took the opportunity to visit the solar village after closing to see how it was functioning as a village. Competing teams visiting with each other in their houses, or out on their decks, and everything was brightly lit. It was a charming sight.