Zero Energy Homes, Made Affordable

[Dominick Penny]

How's this for a challenge? Create a zero net source energy (as
opposed to site energy) home as defined by the Department of Energy's
Building America program. Design it to operate in the extremes of
Denver's unpredictable climate, using off-the-shelf, readily available
technologies. Keep the mechanical systems as simple and uncomplicated
as possible. Incorporate energy-efficiency strategies that don't
require the home owners to be experts in sustainable building
operations or conduct any maintenance beyond that required of a
'normal' home.
Not overly difficult, you might say? Add that the design must be
replicable for future Habitat for Humanity homes, utilize low-cost
construction materials, and allow volunteer- friendly construction
techniques. Still with me? The clincher: The target market is the
affordable housing sector. Impossible, you might say? Challenging yes,
but not impossible for an integrated design team of NREL engineers and
Habitat for Humanity staff and volunteers. The finished product is a
1,200 square foot, three-bedroom reduced income home that actually
produces more source energy than it consumes!
Design Considerations
The combination of energy engineers, a construction manager a real
estate development manager and Habitat volunteers on the design helped
strike a balance between engineering ideals based on energy modeling,
cost realities inherent in affordable housing, and considerations
involved in a volunteer construction crew.
Habitat's volunteer labor advantage steered the design team to
approaches that favored low materials costs and high labor costs.
While this approach minimizes the substantial labor cost, when
combined with the affordable housing requirement it restricts the
range of sustainable strategies available. For example, strategies
such as structured insulated panels (SIPs) and insulated concrete
forms were not considered because of their high cost. Similarly, the
requirement for volunteer-friendly construction techniques and ease of
replication eliminated the option of using Straw Bale. The design
simplicity requirement eliminated the possibility of a combined solar
space heating and water heating system. Finally, the zero energy
requirement, given Denver's cold climate and the current high cost of
PV systems, required some trade offs that some 'purists' might
consider controversial.
Design Approach - Envelope
Given the considerations described above, the design team decided to
focus first on reducing the home's energy load as much as possible,
and then size the PV system to meet the remaining electricity needs.
The first place to look? Yes, you guessed it - a passive solar
orientation with a 'super insulated' envelope. Starting with a
standard Habitat three-bedroom, 26 x 46 square foot design with a
crawlspace, the team increased the South-facing glazing area and
reduced the North, East and West facing glazing area. Next, a double-
stud wall with fiberglass batt construction was selected to take
advantage of it's relatively low cost, volunteer-friendly technique
and Habitat's low construction labor cost. Blown fiberglass installed
in the attic achieved an R-60 rating and insulated floors achieved an
R-30 rating. While the double stud wall design, with exterior
structural studs spaced at 16 inches O.C. might not achieve LEED Homes
Advanced Framing Techniques points, the interior studs spaced at 24
inches O.C. certainly meet the requirement. The R-3 fiberglass batts
in the exterior wall cavities and the R-13 filling the space between
the exterior and interior walls as well as the interior wall cavities
definitely help optimize energy performance. An outer vapor-permeable
house wrap and fiber cement siding, with and an inner poly vapor
barrier plus drywall adds to a very 'tight' whole-wall-R value. Blower
tests yielded a natural infiltration rate result of 0.15 ACH, a very
'tight' indication.
Heating and Ventilation
With the house's heating energy needs drastically reduced through this
super-insulated shell, the design team then focused on the heating and
ventilation system. Note that I didn't mention heating, cooling and
ventilation system. Yet another design challenge! Habitat for Humanity
Metro Denver has a policy of not equipping its homes with air
conditioning. This meant that the final design had to maximize heat
reduction (for example by maximizing solar gain) without increasing
the cooling energy load.
To supply a proper amount of fresh air to the house while minimizing
potential for energy loss, the team opted for an energy recovery
ventilation (ERV) system with efficient electronically commutated
motors. The system exhausts air from the kitchen and bathroom, and
supplies fresh air to the living room and bedrooms. Heat loss from
ventilation is reduced because the ERV system heats the incoming air
with warmth from the exhaust air.
The design team soon discovered that a very low heating load is a
double edged sword. On the one hand very little energy is required to
heat the house. On the other hand, most commonly available heating
systems are oversized for such low heating needs, and overly
complicated or expensive systems cannot be cost-justified. After
carefully considering a variety of high-efficiency heating systems,
and much internal debate, the team decided to follow a hybrid approach
of electricity from the PV system, and natural gas.
Controversial Approach
Some of us who are 'purists' may turn up our noses at the thought of a
zero energy home using natural gas. However, the economics involved
convinced the design team that a hybrid approach was the best solution
(see side bar)
The PV system selected by the design team uses the local utility grid
for storage, thus eliminating the substantial cost of the storage
battery. When the system is producing more energy than is being used,
it delivers energy to the grid. When the system produces less energy
than it produces, it draws electricity from the grid.
When the system draws electricity from the grid, it is likely drawing
fossil-fuel generated electricity. Although a larger sized PV system
may minimize the volume of electricity drawn from the grid, the cost
of larger systems is prohibitive. The design team opted to include
natural gas in order to reduce the size of the PV system by 1.1 kW,
making it much for affordable for a Habitat Home. The team designed
the system to offset the natural gas used, thus achieving, and even
surpassing, the goal of net zero source energy.
The hybrid approach allowed the team to size the PV system that is
affordable, offsets the use of natural gas as well as any grid
generated electricity, and thus allows the home to achieve (and even
surpass) the goal of net zero source energy. The hybrid space heating
system combines a pointsource direct-vent natural gas furnace in the
dining room and living area, with small baseboard electric-resistance
heaters in the bedrooms.
Water Heating
The design team selected a solar water heating system - rather than a
combined space/water hearing system - for simplicity, backed up by a
natural gas tankless water heater. The team calculated that the 96
square foot collector area and 200 gallon water storage would result
in an annual solar-savings fraction of 88%. They opted for the
tankless natural gas back up heater after finding that the tankless
system uses zero heating energy whenever the solar water tank is at or
above 115 degree water delivery temperature.
The Crowning Element
Having reduced all possible energy loads as much as possible, the
design team zeroed in on the lighting, appliances and miscellaneous
electric loads (MELs). They installed compact fluorescent light bulbs
throughout the house, and ENERGY STAR label appliances. This left the
miscellaneous electric loads, from TV, hair dryer, toasters,
computers, and anything else that could be plugged in by the
occupants. Using Built America benchmark assumptions on MELs, the team
settled on a 4kW PV system. Because the Built America assumptions on
based upon a national average of a 'typical' American household, the
actual occupant use and local climate may either block the home from
achieving zero energy usage, or propel it to the ranks of 'net energy
producer'.
The Verdict
Initial test results were encouraging. From the February to July of
2006, the PV system produced 1,600 kWh more electricity than the house
consumed. Factoring in the natural gas used for space heating and
water heating backup, the house produced 75% more source energy than
it consumed. Although a longer testing period is required, it's a safe
bet to say that the house will be an annual net energy producer rather
than just achieve net zero energy user. However, this could change if
the occupants begin using more than the average calculated into the
Built America benchmark.
And the home owners? While it is true that the house is a net energy
producer, they unfortunately are not free from utility bills. There is
the monthly charge for the natural gas, as well as fixed charges for
the electric grid and natural gas connection fees. From October of
2005 to May of the 2006, the owners shelled out an average of $18.25
per month in energy bills. Because the fixed monthly charges averaged
80% of those bills, in actuality the family used on average $14.60
worth of energy.
For those of us who suffered through $200+ monthly energy bills during
that same period, those results are very compelling.
KEY RESIDENTIAL SUSTAINABLE FEATURES
Energy and Atmosphere
Passive Solar Design? The house was designed with increased glazing
area on the long South facing side, and reduced glazing area on the
North, East and West facing sides.
Renewable Energy? 4kW Photovoltaic system using utility power grid
storage to eliminate need for and associated high cost of, storage
battery
Insulation? Raised heel trusses in attic allow 2 ft of blown
fiberglass insulation, achieving R-60 rating for thermal envelope top?
Floors insulated to R-30? R-3 fiberglass batts in outer 2 x 4
structural stud wall cavities, and a second, interior 2x4 stud wall
with R-13 fiberglass batts placed horizontally between stud walls and
vertically in interior wall cavities? Outer vapor-permeable house wrap
and fiber cement siding? Inner poly vapor barrier and drywall
Space Heating? Hybrid natural gas/electric heating system, combining a
pointsource direct-vent natural gas furnace in the living room and
dining area, and small baseboard electric-resistance heaters in the
bedrooms. This combination provides the added bonus of zone heating,
as each appliance has its own independent thermostat.
Water Heating? Solar water heating system with 96 square feet
collector area and 200 gallon water storage tank as primary water
heating system, with natural gas tankless water heater as a back up
system
Windows? Double - glazed, low -e glass installed in South facing
windows, with U-factor of 0.3 and SHGC of 0.58. For the East, West and
North facing windows, Double - glazed, low - e glass was also used,
with a U-factor of 0.22 and SHGC value of 0.27. The U-factors of all
windows exceed (ENERGY STAR requirements by 20%). Appliances? ENERGY
STAR? appliances were installed.
Lighting? Compact fluorescent light bulbs deployed throughout the
house
Indoor Environmental Quality
Local Exhaust? Energy Recovery Ventilation (ERV) system with efficient
electronically commutated motors, exhausts air from the kitchen and
bathroom, and supplies fresh air to the living room and bedrooms.
Outdoor Air Ventilation? The ERV system heats incoming fresh air with
warmth of the exhaust air, thus significantly reducing heat loss from
ventilation. Materials and Resources
? Advanced Framing Techniques: Walls consist of inner 2x4 stud wall,
24 inches O.C.

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