Photovoltaic system Energy calculations Kilowatt-hour consumption (net) over 31 months: 2,163 Kilowatt-hour consumption (net) per month: 70 Average electricity bill per month: $8 PV shading compromise calculations 2007-2008 6,128 k Wh/12 months Mar.-Feb. 8,400 PV watts-predicted production 73% of theoretical, or 27% reduction 2008-2009 6,372 k Wh/12 months Apr.-Mar. 8,400 PV Watts-predicted production 76% of theoretical, or 24% reduction
and by using natural materials that won’t emit off-gassed chemicals.
To provide a constant stream of fresh air to the interior, we installed
an efficient energy-recovery ventilator (ERV). Motorized dampers, in all
ducts connected to the outdoors, close automatically when the ERV is
off. Direct-to-exterior exhaust fans vent the bathrooms and kitchen, also
with motorized dampers.
We were careful in selecting interior finishes, avoiding volatile organic
compounds (VOC). We used low- and zero-VOC paints and stains,
water-based floor finishes, clay plaster, linoleum tile with low-VOC adhesives, formaldehyde-free cabinets, concrete and sorghum-based counter-tops, oriented-strand board sub-floors and natural wool carpet. These
produced beautiful results.
Most of these finishes are manufactured using renewable resources,
and this, coupled with recycled materials acquired via the deconstruction
process, did wonders in reducing the project’s carbon footprint. An audit
revealed that 90 percent of our construction waste was diverted through
on-site reuse, salvaging and recycling. Most of the framing lumber was
Forest Stewardship Council-certified, although the cost premium led
the Grahams to forgo using it exclusively in favor of investing more in
renewable energy systems. Finally, dual-flush toilets, xeriscaping and drip
irrigation were employed to conserve water.
Projected and Actual Performance Differ
A web energy logger (WEL) was permanently installed to monitor the
overall energy performance and to collect data that might help in planning for improvements. The retrofit aimed to make the Graham residence
a net energy-producing home, but recent WEL analysis confirmed that
the home is not performing as well as projected. We believe the discrepancy can be attributed to a couple of problems.
First, the new high-performance windows use foam-filled fiberglass
frames and are well-sealed against air leakage. Because they offer overall
unit U-values of 0.15, the Grahams felt that window treatments would not
be necessary for comfort or energy performance. Actual experience is that
the 700-square-foot addition is not “coasting” as well as it could if it had
movable nighttime insulation on the glazing — which, significantly, is 30
percent of floor area. We later calculated that on a 5°F (- 15°C) night, the
heat loss prevented by cellular shades would be about equal to the heat
produced by burning wood in the EPA-rated stove for one to two hours per
night. In late 2009, the Grahams planned to add window treatments.
An additional factor is that the PV contractor, hired by the Grahams independently of the integrated design team, did not take into
account solar panel shading issues at certain times of the day, which
compromises the renewable energy production by about 25 percent.
With this shading problem solved, it is reasonable to believe that the
Graham’s home could achieve net energy production on an annual basis.
rebates Help reduce Project cost
Overall costs covered a high-end addition and whole-house remodel,
with heating energy per square foot cut in half. About 25 percent of
expenses were for the efficiency measures and on-site renewable energy
sources that brought the home to near-zero net energy. Much of this
efficiency budget was spent to provide the benefits the Grahams were
after anyway: Better indoor air quality, improved thermal comfort and
durable assemblies to manage moisture.
The budget included $25,000 for PV (after Xcel Energy’s $4.50/
watt Solar Rewards rebate but before the 30 percent federal tax credit),
$34,000 for the solar thermal installation (after the $2,000 federal
tax credit) and $44,880 for insulation and window/door upgrades. In
total, the project had $103,880 in direct costs and about $95,500 after
tax credits. ST