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el Fierro is a home designed from the ground up
to be an upscale, energy-independent, passive solar
residence. The intention was to create a structure
capable of lasting centuries and one in which interior spaces
can be periodically updated as technology, lifestyles, and
trends change. The theory is that this would result in lower
lifetime costs with less overall environmental impact than a
similarly sized conventional home. Much of the project was
experimental with an eye on future scaled-down versions for
affordable housing projects.
64 home power 148 april & may 2012
High-DesertPerformanceDesigning & Constructing Del Fierro
Story & photos
by Edward Marue
D
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Developed by Paul Schwam, a local architect/builder, lava concrete (LC) is a made from a
porous, lightweight volcanic aggregate called scoria, which is mixed to form a self-supporting
semi-fluid. When cured, it becomes a lightweight stone. Scorias porosity holds pockets ofwater to help cure the cement. Once the water has evaporated, tiny air cavities remain to
provide sound and thermal insulation. LC is used for foundations, floors, walls, and roofs. It
provides the structure, insulation, thermal mass, and the finishall in a single and seamless,
raw-to-finish process.
Because LC starts as an easily workable semifluid, any shape or detail can be added to
architectural elementsplanes and curves; textures and details; and accessories, such as
fireplaces, furniture, moldings, drip edges, window/door trim, niches, and artwork. Builders
can integrate details such as ledgers, beam pockets, utility raceways, and block-outs
eliminating material and labor steps along the way.
LC helps optimize the sequence of construction. Electrical, plumbing, and other utilities install
first into empty forms. The structural wall is cast as the final step. The permanence of the
concrete and stone structure will remain fresh, even with periodic future upgrades to utilities,
internal layout, and furnishings.
What isLava Concrete?
The thermal mass and trapped air space of lava concrete lends itself to tempering the thermal swings of a desert climate.
The lava concrete mix is pretty dry, and
needs minimal hand work.
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After site preparation and installation of subgrade utilities and
footings, rebar and forming went directly on top of the concrete
foundations. Forms were constructed from treated 4-by-8 plywood
with cut-outs for window and door openings. Once utility raceways
were installed, pouring was done by three people: one operatedthe delivery chute; another tended to the formed details, lightly
compacting the LC to assure no voids; and the third monitored the
mixing machine and conveyors.
The mixing machine was key to the process and specifically
designed to combine the cinder sand (scoria) and Portland cement
with injected water to form a relatively dry mix. Conveyors delivered
the LC to the forms, where it was poured 2 to 8 feet high in lifts. A
24-foot-tall wall may require three to 12 pours.
The lava sand used was a deep red color, from Flagstaff, Arizona.
Blended with about 20% Portland cement, the poured fill cures
similarly to regular concrete. Sandblasting the surface restored much
of the natural volcanic color that was grayed by the cement. Once
sealed (water-based Okon was used for Del Fierro), the exteriorrequires no additional maintenance.
One of the advancements developed during our project was a post/
anchor bolt system, which eliminated the need for precast anchors, and
greatly simplified the hand-off to other trades. It allowed operations,
such as attaching ledgers, to be done simply by pre-drilling and
insertion of large bolts. During construction, tests were performed
on the strength of the attachment system, under the supervision of
structural engineers to comply with the local building department.
Windows and doors were installed in the blanked-out openings and
secured with polyurethane foam and an LC-based grout.
Due to the complexity and volumetric size of this house, the cost
per square foot for the basic shell was about $85. For modest-scale
projects, LC can be constructed more cost-competitively.
Right: Because lava concrete starts as
an easily workable semifluid, any shape
or detail can be added to architectural
elements.
Below right and lower right: Scaffolding
is set up to accommodate pumping lava
concrete in lifts.
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PV Tech SpecsOverview
System type:Batteryless, grid-tied solar-electric
Location:Tucson, Arizona
Solar resource:9 average daily peak sun-hours (for dual-axistracking system)
Record low temperature:6F
Average high temperature:100F
Average monthly production:1,104 AC kWh
Utility electricity offset annually:87%
Photovoltaic System Components
Modules:32 BP SX170B, 170 W STC, 35.4 Vmp, 4.8 Imp,
44.2 Voc, 5.0 Isc
Array:Four eight-module series strings, 5,440 W STC total,
283.2 Vmp, 19.2 Imp, 353.6 Voc, 20.0 Isc
Array combiner box:Square D HU361RB
Array installation:Array Technologies AZ-225 Wattsun dual-
axis trackers on pole mounts
Inverters:Two Xantrex GT 3.0, 3 kW rated output 600 VDC
maximum input, 195-550 VDC MPPT operating range, 240
VAC output
System Cost
Initial cost:$45,064
Less incentives:$16,320 TEP utility rebate
Final installed cost:$28,744
Major Load Information
Space heating:Electric forced-air heat pump
Cooling: Electric forced-air heat pump; evaporative cooling
tower (electric fan)
Water heating:Solar hot water system, with propane backup
Cooking:Electric
Clothes drying:Electric
Refrigeration:Electric
For keeping a home cool, it is critical to reduce its exposure
to the sun, keeping the surface temperature of the shell as
low as possible. This reduces thermal conduction through the
envelope and minimizes charging of thermal mass. This can
be accomplished by shading with overhangs, vegetation, the
use of reflective coatings, light colors, and radiant barriers.
Compromises for cooling over heating are made, since saving
energy on cooling was the deciding objective. Tucson has almosttwice as many cooling degree days (the number of days where
mechanical cooling is needed) as heating degree days (2,954 vs.
1,678). Compare this to Ann Arbor, Michigan, which has 7,864
heating degree days and 289 cooling degree days, and youll see
the drastic differences in passive design requirements.
An Inherently Cool DesignReadily available earthen-based building materials, such as
adobe and rammed earth, have been used in the Southwest
throughout history. Modern performance-driven designs
fully benefit from their inherently high thermal mass, fire
resistance, and durability. I originally intended to use rammedearth, which involves compressing a mixture of earth, chalk,
lime, and gravel (and sometimes a cement stabilizer). But at
3,450 square feet, costs for rammed earth were prohibitive
due to the volume and complexity of the house, with its high
ceilings, parapets, and intricate architecture.
A member of my design team introduced me to lava
concrete (LC), a lightweight cast-in-place concrete made from
volcanic cinder sand, very abundant in Arizona and other
parts of the world. The construction is similar to casting
rammed earth, but much less labor-intensive. Once cured,
the steel-reinforced structure is strong and considerably
more durable than rammed earth or other earthen building
systems. With an R-value of 3.27 per inch, our 18-inch-
thick walls would be nearly R-59. The LC also provides
thermal mass and soundproofing. The inside was finished
with a natural clay plaster from American Clay, which also
contributes to the homes thermal mass.
A cooling tower provides evaporative cooling and much-
needed humidity in the dry months of March through June.
A mister injects moisture at the top of the tower, wherein the
cooler, dense air falls by gravity, displacing warmer household
air out through open windows at each end of the house, and a
small electric fan augments the airflow. The cooling tower also
works at night when the outside ambient air temperature is
cooler than the displaced inside ambient room air.
Another passive cooling techniquedeep (8- to 12-foot)
overhangshelp shade south- and west-facing walls and
windows in the warmer months. Helping overall building
efficiency, the roof includes a 2-inch, R-14 sprayed-on layer
of urethane foam over a double layer of OSB over I-joists and
trusses filled with loose-fill fiberglass insulation, for a value
of R-45 to R-60. A white polymer reflective paint was sprayed
over the urethane. Pella InsulShield triple-pane, argon-filled,
low-e windows and doors retard heat transfer.The cooling tower uses evaporative phase-change and
reverse-chimney convection to draw cool air into the house.
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Mechanical SystemsAll appliances (range, oven, microwave, dishwasher,
refrigerators, washer, and dryer) are Energy Star-rated, and
CFL and LED lighting is used throughout.
Two 40-gallon batch solar water heaters are backed up
with two Bosch 2400E LP on-demand propane-fired units.
The SHW system can either feed the backup unit or send the
solar-heated water directly to the house. We have it route
through the backup units, which are temperature-modulated.When needed, mechanical cooling and heating is provided
by three high-efficiency (16 SEER-rated) fresh or recirculated
air-source heat pumps, 2-, 4- and 5-ton units, respectively.
A grid-tied 5.44 kW, dual-axis, tracked PV array provides
most of the homes electricity. (The array size was based on
the most PV modules I could get on a pair of Wattsun AZ-225
trackers.) PVWatts calculates that our system should generate
13,253 kWh annually. Since PV module prices have dropped,
I plan on adding a ground-mounted PV system to cover the
propertys entire electricity requirements, including the guest
house and shop.
For the first year of data gathered, ending in July 2010,our usage exceeded our original energy budget. During that
H2G H1
H1 H2G
100 KWH
100 KWH
100 KWH
AC Service Entrance:
To 120/240 VAC loadsProductionMeter (Array 1)
ProductionMeter (Array 2)
GroundGround
Ground
Lightning Arrestors:Two, LA302 MOVs
kWh Meter:Utility
Combiner Boxes/DC Disconnects:
Single-pole,single-throw switch
Inverter 2:Xantrex GT 3.0,
600 VDC max input,3 kW at 240 VAC output
Inverter 1:Xantrex GT 3.0,
600 VDC max input,
3 kW at 240 VAC output
AC Disconnect(Array 2)
AC Disconnect(Array 1)
To/From Utility
Two PV Arrays:Each with two series strings
of eight modules,BP SX170B, 170 W each,
wired for 5,440 W totalat 283 Vmp
PV Mounting:Two Array
TechnologiesAZ-225Wattsundual-axistrackers
20A
20A
Note:All numbers are rated,manufacturers specifications, or nominal
unless otherwise specified.
Del Fierro Batteryless Grid-Tied PV System
Two identical PV
systems, each
using a 3 kW
Xantrex grid-tied
inverter, combine
at the main AC
service entrance.
The meters here
are, left to right,
an auxiliary circuit
used during
construction and
testing, the utility
main meter for net
metering, and the
solar production
meter for the
utility.
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SHW Tech Specs
OverviewSystem type:Integrated storage tank system, 80 gal. total
capacity
Solar resource:6.5 average daily peak sun-hours
Production:UnknownPercentage of hot water produced annually:95%
(estimated)
EquipmentCollectors:Two Thermal Conversion Technology PT 40 CN,
40 gal.
Collector installation:Roof-mounted; south-facing; 34 tilt
Backup DWH:Two Bosch 2400E-LP, propane-fired,
tankless
period, thermostats were set for 80F in summer and 68F in
winter. The HVAC system measured 2,382 kWh over what
was anticipated.
To evaluate the buildings thermal performance,
thermocouples were embedded in eight strategic locations
one inch from the inner and outer surfaces of the exterior
walls. Data was recorded on a computer every 15 minutes
over a period of a year. This was helpful in understanding
where heat transfer was significant so adjustments could be
made. One zone with south and west exposure consumedpower disproportionately29% of the HVAC power used
for 11% of the building floor space. Planting shade trees on
Three-Way
Valve
Three-Way Valve(Solar bypass)
DrainValve
DrainValve
DrainValve
DrainValve
T&PReleaseValve
T&PReleaseValve
VacuumBreaker
Solar Collector / Storage:Thermal Conversion Technology,
PT 40 CN,4 x 8 ft., 40 gal.
Backup Heater:Bosch, 2400E-LP,
170,000 Btu,propane-fired,
on-demand(tankless)
ExpansionTank
TemperingValve
ColdSupply In
Hot toHouse
BOSCH
Note: Only one completesystem shown
Del FierroSHW System
Above left: At first glance, this collector looks like a typical flat-plate unit. However, this is a batch-type collectora progressive-
tube, integrated storage/collector. Two (only one shown) each hold 40 gallons of water, eliminating the need for a separate storagetank. (Note: These are only used in nonfreezing climates.) Above right: The system has only a few simple components beyond the
collectorsan expansion tank and backup tankless heater.
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those sides of the house largely corrected this problem. It
was also found that the default settings on the heat pumps
caused the variable-speed fan motors to operate all the time,
which was unnecessary for comfort. Filters were changed
more frequently and a fan was added to the cooling tower,
and used in the spring and early summer months, providing
needed humidity. Implementing these strategies reduced the
zone to 19%.
About a cord of wood was burned in the fireplace during
the coldest months, reducing the nighttime heating load.These changes had no effect on lifestyle and helped keep us
under the winter HVAC budget by about 4%.
PV production was improved by fixing a tracker problem
and replacing several defective PV modules. Three module
failures affected three strings of modules. With two identical
systems, its easy to spot a problem. Trees were removed
that had grown and shaded one of the arrays during the late-
afternoon hours. We also cleaned the surface of the modules
more frequently, resulting in improving PV production from
12,317 kWh annually to 13,307 kWh.
The SHW systems consist of two 40-gallon ICS batch units at
each end of the house, backed up by Bosch propane on-demandunits. The systems performance can be measured by the backup
fuel that hasnt been used. The 300-gallon underground propane
tank was filled to 80% of capacity at
move-in. It has never been refilled, and
today its just under 60% full. That means
about 60 gallons of LPG was used in 32
months (or 1.9 gallons per month). This
consumption also includes a frequently
used gas grill, which I suspect accounts
for most of the gas usage.
This put us within 2% of our objective
of meeting 90% of our energy needs
without any compromise in lifestyle. We
could squeeze out the last bit by managing
computer systems, entertainment systems,
and the phantom loads.
AccessA UCLA physics graduate, Edward A. Marue
designs off-grid power systems for remote
communications and border security sites. He
is also a principal in Solar Lava Development
Company, specializing in advanced green
design and construction.
Deep roof overhangs provide ample shade for outdoor spaces.
Additional thermal mass in the concrete floors and earthen clay wall finish helps
mitigate temperature swings in interior spaces.
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