An American Passive Home | Home Power Magazine
Earth Room
In addition to capturing solar gain, Pura Vida takes advantage of
another source of free heat (and cooling): from the relatively constant
temperature of the ground. In Europe, earth tubes are often buried
around the foundation of homes to provide tempered fresh air, but in the
United States, there has been concern that earth tubes can grow mold or
mildew. With Pura Vida, and subsequent buildings I have designed, an
“earth room”—a modified approach to earth tubes—has been successful.
In Pura Vida, the earth room lies below the front porch. A short,
12-inch-diameter tube brings fresh outdoor air into one end of the earth
room. The air flows the length of the 48-foot-long room where it is
preheated (in the fall and winter) or precooled (spring and summer)
through direct contact with the concrete walls prior to entering a heat
recovery ventilator (HRV). The earth room is like a large thermal
battery, storing heat in the summer for use in the fall and winter and
storing “coolness” in the winter for use in the spring and summer. The
earth room eliminates the need to use a conventional mechanical heating
system for about two months of the year (October and November) and
eliminates the need to run a cooling system from mid-May to mid-June.
Throughout the rest of year, the earth room significantly reduces the
heating and cooling loads.
After passing through the earth room, the air enters the Nu-Air
Ventilation Enerboss, a complete heating, filtration, and HRV system. An
efficient fan constantly pulls air from the bathrooms and kitchen,
which is exhausted, while the same amount of fresh air is evenly
distributed throughout the home by an airflow-balanced high-velocity
duct system. The system has operated flawlessly for the past five years.
The Enerboss system requires an external source of hot water for
heating the air to be distributed. In Pura Vida, we use a 4.5 kW
Marathon water heater to provide hot water to the heat exchanger coils
within the Enerboss. Air conditioning is provided by a 3-ton, 16 SEER
Lennox Elite. The evaporator for the AC unit is mounted on top of the
Enerboss.
Visitors to Pura Vida often comment how fresh the air is and how
quiet it is within the home. The comfort provided by constantly filtered
fresh air moving throughout the home makes it difficult to go back to
living with a conventional HVAC system.
Domestic Hot Water
A small Nyle Systems air-to-water heat pump mounted on a wall in the
earth room provides domestic water heating. A timer is programmed to
turn it on in the evenings when the time-of-use electricity price is
$0.02 to $0.03 per kWh. The hot water is stored in a well-insulated
105-gallon Marathon water heater for use during the day. The average
monthly cost for domestic hot water is about $5. (The timer is bypassed
when we have guests or need to use more hot water.)
We also have a GFX Technologies wastewater-to-water heat exchanger.
Although the concept of recovering the energy in hot water going down
the drain is an interesting one, the high cost of copper makes this
technology too expensive to be cost-effective for the amount of
preheated water the unit provides.
Demonstrated Performance
Six Lascar temperature and humidity sensors were placed throughout
the home, outside, and in the earth room. Data collected from these
sensors every 30 minutes for the past five years—along with energy use
data collected from several TED (The Energy Detective) units and our
utility bills—have validated the home’s performance.
The all-electric home’s advantage is that we can measure and directly
compare the energy use for every appliance and system. Having collected
data on the efficiency of the air-to-water heat pump in the earth room
for providing domestic hot water, I plan to modify the heating system to
include a second heat pump, instead of the existing water heater, for
space heating. This should reduce the electricity demand for space
heating by at least 60%.
When the home was built, we also decided to sign up for time-of-use
utility metering. This utility billing method provides us with cheaper
energy during off-peak times, and more expensive energy during peak
times. The risk we took with this decision was that if we needed to use
large amounts of energy during peak times—like running air conditioning
midday during the summer—the cost could be significantly higher. The
table (upper right) shows our average annual energy use for the various
portions of the property along with an approximate annual cost for each.
Because of the relatively high upfront cost of renewable electricity
systems, we felt it was important to first design and build a home that
is as efficient as possible—and then get a good understanding of the
home’s energy demands. A year after the home was completed, we applied
for and received a state grant to measure the effectiveness of
residential small wind systems.
We installed a 2.4 kW Southwest Windpower Skystream 3.7 turbine on a
60-foot tower (at the time, we did not understand that height is too
short for nearly
all applications). The utility account was
converted to net metering so that we could sell our excess energy back
to Commonwealth Edison. The energy production from the Skystream was
monitored through a Zigbee data system and logged on a computer.
We
believed we had a wind resource that was sufficient to justify a
wind-electric system. Our chosen turbine, the Skystream, is predicted to
produce about 2,600 kWh per year given an 11 mph resource, and about
900 kWh per year in 8 mph winds. After several years of operation, the
system has produced roughly 500 kWh per year, or the equivalent of $50
of energy, which is expensive electricity when you consider the $21,000
installation cost.
It is clear now that the information we started with was not
sufficient to justify using wind energy at our site. Factors
contributing to our turbine’s lower-than-anticipated production include a
tree line that sits about 800 feet to the west, resulting in turbulent
wind from the prevailing wind direction. A taller tower would
undoubtedly help with this problem, because the farther you get away
from the earth and its obstructions, the more wind there is. As a
well-known small-wind expert has said, we unknowingly chose expensive
energy over an expensive tower.
Over the past several years, the cost of photovoltaic systems has
come down dramatically. During that time, we collected information on
the energy demand for Pura Vida. We found that during the fall and
spring, when there was no energy use for heating or cooling, the average
demand was between 600 and 700 kWh per month.
In 2011, we installed a 4.3 kW grid-tied PV system that should
average about 400 kWh per month. Twenty-four batteries provide backup
for power outages and provide enough energy to live without the grid
periodically—assuming that the backup heating would be provided by the
wood heater instead of electricity. Performance data is being collected
by the OutBack MATE3 energy monitor and we should have an accurate
performance picture by the end of 2012.
Future of APH
We believe that building super-energy-efficient homes will have a
significant impact on U.S. energy use. If the building cost of an APH is
within 15% of a conventional building method, a conservative energy
cost savings estimate is more than $1,000 each year per home.
One outcome of the recent economic downturn has been that the
American consciousness has been awakened to the importance of
common-sense solutions to important challenges. For families paying to
heat and cool their homes—and as a nation that needs to use less fossil
fuel—emphasizing energy efficiency in homebuilding is a simple yet
promising way to address these challenges.
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Victor Zaderej
has been passionate about energy use in homes, businesses, and
transportation for 30 years. He holds two engineering degrees from MIT,
and an MBA. Through his company, Solar Homes, he helps design
energy-efficient buildings. He is currently the manager of Advanced
Solid State Lighting at Molex.
