SOLAR TODAY®
november/december 2011
voL. 25, no. 9
A yearlong study of eight high-performance
homes confirms that performance ultimately
comes down to household size and behavior.
By Marc rosenbauM, P.e
5
4
6
3
7
8
RanDi BaiRD
Lowest production is seen at houses 2, 3 and 8, and highest at houses 5 and 6. How much of this variation is equipment performance and how much is light shading or off-south orientation is unclear. Lowest
production is in the one house that has installed a satellite dish on the open portion of the roof adjacent
to the PV array. It’s not clear how much, if at all, this dish shades the array.
with a solar heat gain coefficient of 0.62; all others are 0.48). blower-door results range from
117 to 184 cubic feet per minute of air leakage
at 50 pascals of pressure (cFm50) for the seven
South mountain homes and 236 cFm50 for the
Habitat home. These results were achieved with
no mechanical openings taped off and with the
heat recovery ventilator running.
Heating and cooling are provided by a dai-kin single-zone mini-split heat pump (rXS24
dvJU) with a wall cassette in the main living
area. For supplemental heating, we used ceiling-mounted enerjoy electric radiant ceiling panels.
The houses are designed such that the single-point source of heat — the heat pump — should
be able to provide all the required heating, as
long as the doors to the bedrooms are left open
to allow heating by natural convection. The radiant panels allow heating in the event of a doors-closed operation, or to provide supplemental
heating in extreme cold conditions.
ventilation is provided by a constantly operating Fantech 704 heat recovery ventilator. This unit
draws about 30-35 watts, exhausts 25 cFm from
each bathroom and supplies 15 cFm to each bedroom. (In the case of the two-bedroom units, 15
cFm is also supplied to the living area.)
domestic hot water is supplied by a 50-gallon
marathon electric water heater. This polybuty-
lene-lined tank is insulated with 2. 5 inches of
closed-cell foam.
The homes face close to due south and have
essentially full solar access. variation in Pv production from house to house is small. The massachusetts clean energy center (masscec) Pv
production calculator for these arrays estimates
an annual output of 6,247 k Wh. The average
output of the eight systems was 6,873 k Wh, 9
percent higher than the estimate. We don’t know
how much of this is due to better-than-average
solar insolation and how much is due to the premium SunPower product. Production exceeded
the estimate for 10 of the 12 months and fell
short for two months.
not surprisingly, Pv production reached a
minimum in november and december and a
maximum in July, although march was an excellent month. every household achieved at least
one month as a net exporter. All households
were net importers in the four coldest, cloudiest
months. As a neighborhood, the total annual net
import was 13,680 k Wh.
Occupant Choices
Drive Energy Usage
The production of the Pv array sets the total
annual energy budget that a household can
use if zero-net energy is the goal. As shown in
Marc Rosenbaum P.E. (mrosenbaum@south
mountain.com) is the director of engineering at South
Mountain Co., an employee-owned company offering integrated development, architecture, building,
interiors and renewable energy services on Martha’s
Vineyard and nearby communities. Rosenbaum is a
longtime student of making great buildings. He uses
an integrated systems design approach to help create
buildings and communities that connect us to the
natural world, and support both personal and planetary health. He is a Passive House Consultant and
trainer and his work has been recognized by ASHRAE,
AIA, EEBA and NESEA.
This article was adapted from a full report on
Eliakim’s Way by South Mountain Co. titled, “
Zero-Net Possible? Yes!” To view the full report, visit south
mountain.com.
