The PV system consists of three Evergreen ES-170 SL modules totalling 510 W. The modules are wired in series, for a maximum output of 76 V. An OutBack MX60 charge controller steps this voltage down to 12 V for the MagnaSine 2,000 W inverter/charger and the battery bank, which consists of two 6 V Trojan L16E-AC batteries wired in series. Key was making the system components as accessible as possible for future workshops where the system will be disassembled and reassembled. The battery box, inverter, and DC panel are all located in a small, weather-tight enclosure on the outside of the building.
A TriMetric meter keeps track of the battery bank’s state of charge, and the office staff were all given instructions for how to work in an off-grid environment. There is no backup generator, and if there are multiple days of clouds—and with little PV charging—the office staff implements load management to keep the battery bank healthy. It has proven relatively easy to keep the system fully charged, since the office is closed at night and on the weekends, and loads are switched off then. One of the staff members has been instructed in how to safely water the flooded batteries on a regular schedule, and she checks them monthly and adds distilled water when necessary, taking the appropriate safety precautions.
The Piedmont area of North Carolina has a relatively mild climate overall, but winters get cold, with about 3,000 heating degree days on average. TAF hoped to provide for as much of their heating needs as possible using the sun. GDD had previously added two casement windows to the south side of the small building to capture some solar gain in winter, but there was room on the south-facing wall for additional solar gain, and TAF wanted to demonstrate both passive and active solar heating.
Passive solar heating is accomplished by installing windows with appropriate solar heat gain values in a south-facing wall, with thermal mass (usually stone or concrete) to absorb and store the resulting solar gain. A roof overhang (or other shading) of the appropriate depth allows the winter sun in, but helps block unwanted sun in the summer. A well-designed passive solar home is attractive and functional, with significantly reduced need for mechanical space heating. Plus, it has the additional benefit of eliminating or reducing the need for artificial lighting during the day. The drawback is that, during the night, when temperatures plunge, heat can be lost through the glazing.
Active solar air heaters are generally inexpensive, can be easily added to an existing building’s south-facing wall, and only operate when the sun is out (and hence don’t lose heat at night or during cloudy weather). The drawbacks are that they can be less aesthetically appealing than a passive solar designed space, they do not have heat storage, and they don’t contribute to daylighting.
The first decision TAF had to make was whether to purchase a premanufactured solar air heater or to build one from scratch. DIY solar heaters can be made inexpensively, and can incorporate recycled or reused materials. Pre-manufactured solar air heaters are likely to be more durable, quicker to install, and may qualify for federal and state incentives that can make them as inexpensive as homemade ones. For many incentive programs, the collector must be rated by the Solar Rating and Certification Corporation (SRCC).
The staff decided on a DIY heater so workshop participants could get hands-on experience with building one—and understand in detail how they function. Because access to the wiring inside the building was limited by the spray-foam insulation, we decided to power the heater’s DC blower with a small PV module. A DC fan, powered by the module, pushes air from the interior of the room into the bottom of the heater. As the air flows through the heater, it warms and returns into the room from the top of the heater. A wall-mounted heater is ideal because having the inlet and outlet heights far apart promotes air circulation, which is more difficult to do with a roof-mounted unit.
Baffles in the heater slow the air and create turbulence for better heat exchange between the collector’s hot interior surfaces and the air. A backdraft damper at the top of the heater keeps air from flowing into the heater when there is not heat to be gained. The design assumption is that if there is enough sun on the PV module to run the fan, then there is enough heat in the box to make it worthwhile to move air. The PV module must be placed so that it has solar access at the same time as the air collector, so it is mounted on the same wall. The more sunlight that is on the PV module, the more power goes to the blower, so more air is moved through the heater. A switch can turn off the blower if heat is not needed.
There are a myriad designs for a DIY solar air heater, and we chose a simple, 3- by 6-foot rectangular box. We sheathed one side of the 1-by-4 frame with 1/2-inch plywood. Inside the box, we put in some 1 by 2s for baffles and fiberglass batt insulation. A matte-black, prepainted sheet of aluminum serves as the absorber plate, which sits on top of the baffles. A clear acrylic sheet serves as glazing.
A year and a half later, the little office has performed admirably. Powering all of the electronics with the PV system has not been a problem, and the solar air heater helps keep the building warm on sunny winter days, typically raising the interior temperature by 10° to 15°F. On colder, cloudier days, a small auxiliary propane heater is needed. During our hot, humid summers, the building is excessively warm, so some kind of air conditioning, if not necessarily required, would make the space more comfortable (although this would probably overtax the existing PV system). Keeping the batteries full has not been a problem, but requires a bit of mindfulness on the part of the TAF crew, like using a power strip to shut off phantom loads at the end of the day; using laptops instead of desktop computers; and using efficient fans and compact fluorescent bulbs. While there are still kinks to work out, the workers at TAF are happy to occupy a small part of a renewable future.
Stephen Hren is a builder, teacher, and author focusing on sustainable design. He is the author of the upcoming book Tales from the Sustainable Underground and coauthor of The Solar Buyer’s Guide for the Home and Office and The Carbon-Free Home.
The Abundance Foundation • www.theabundancefoundation.org