Rounding out the HVAC system is a heat-recovery ventilation (HRV) system, which is required to meet LEED Platinum indoor air-quality standards. The system simultaneously exhausts stale, humid indoor air to the outside and introduces fresh, outside air to interior spaces. Heat between the two air streams is exchanged, reducing heating energy losses due to ventilation. A solar wall along the southwest corner of the home was considered for heating ventilation air but ultimately ruled out. The findings showed that the solar wall would have reduced the cost of the PV array (by approximately $1,150), but the construction costs for the wall would have been significantly greater than the PV cost savings.
“We didn’t just want to tack solar modules on the house. We wanted to integrate them into the design,” Rude says. The team selected Sanyo’s HIT bifacial modules. The double-sided modules—which can harvest solar energy from both faces—increase energy production within the fixed rooftop space. The design staggers HIT modules and clear glass panels in the grid of a steel canopy. Enough light passes through to grow plants underneath, while providing adequate protection for shade-seekers. Plus, the light reflecting onto the back of the modules adds about 5% to the array’s annual output.
Rude designed the canopy for the 7.02-kilowatt system, which was sized to meet the home’s projected energy load of about 9,000 kWh annually. Florian, a Cincinnati, Ohio, greenhouse company that also builds solar structures, fabricated the custom aluminum frame that holds the 36 PV modules and clear glass panels in place. The finished canopy faces south and east and is tilted at 6° and 4°. Ideally, Rude says, the canopy’s southern tilt would have been tilted at 10° or greater, but neighborhood covenants restricted the canopy’s height. “At a 40° tilt [the location’s latitude], the system would have been rather obnoxious, and not well integrated into the design,” Rude says.
The frame for the canopy was assembled on site by a local welder and hoisted into place by a crane. An installation crew from Boulder-based Namaste Solar mounted the modules and ran the wiring down to the ground-floor mechanical room. A small battery backup system—four 105 Ah batteries—has enough energy to power critical loads (air circulation, refrigerator, microwave, laptop, and some LED lights) for a limited time during a utility outage.
The PV was not commissioned until February 2013. Battery-based grid-tied PV systems can be complicated to meter, since power is flowing to the battery bank (the critical load center) and to the other household loads, with any surplus going to the grid. However, what has been recorded by reading the utility bidirectional meter indicates that from February to late September 2013, the PV system has exported to the grid 2,453 kWh in excess of what the home has used (3,550 kWh). Brigham records the incoming and outgoing energy every day, straight from the utility meter. The exercise has become part of her daily routine, like drawing the shades on the southeast windows. While a full year’s worth of data hasn’t yet been collected, Brigham is optimistic that the PV system will offset her household consumption. So far, over the late spring and summer months, she has only paid the monthly service charge of $21.50 to the City of Longmont. “I’d be happy with net-zero, but I’m aiming for net-plus,” Brigham says.
Home Power Associate Editor Kelly Davidson lives in Longmont, Colorado, where she and her husband are upgrading their 1970s trilevel home with new insulation, doors, and windows, in preparation for a PV system in the coming years. This season’s project—two new windows—is being made possible by a low-interest loan through the local Energy Smart program.