Net-Zero Performance: Page 3 of 5

Year 1
Beginner

Inside this Article

The Riggins family is net-zero energy, ...including their car.
The Riggins family is net-zero energy, ...including their car.
A solar hot-air collector heats the garage space passively.
A solar hot-air collector, built from plans in Home Power (HP109), heats the garage space passively.
Low U-factor, high SHGC windows on the south side admit the sun’s energy.
Low U-factor, high SHGC windows on the south side admit the sun’s energy, which is absorbed by the concrete slab floor.
The wood heater has proven more useful for setting ambiance than as necessary for auxiliary heating.
The wood heater has proven more useful for setting ambiance than as necessary for auxiliary heating.
Closed-cell spray-on polyurethane foam was used on all inside sheathing faces for an airtight, high R-value insulation.
Closed-cell spray-on polyurethane foam was used on all inside sheathing faces for an airtight, high R-value insulation.
A water-to-air heat exchanger connected to the earth tube outlet
A water-to-air heat exchanger connected to the earth tube outlet isn’t needed, and was never connected to the solar thermal system.
From the earth tube, air passes through an ERV
From the earth tube, air passes through an ERV, which transfers energy (and humidity) from outgoing to incoming air.
The 100-foot-long earth tube was placed in a 10-foot-deep trench.
The 100-foot-long earth tube was placed in a 10-foot-deep trench. The earth tube tempers incoming air—cooling it in the summer, and warming it in the winter.
A minisplit heat exchanger was installed as a backup space heating system
A minisplit heat exchanger was installed as a backup space heating system, but has never proven necessary.
A minisplit heat exchanger was installed as a backup space heating system
A minisplit heat exchanger was installed as a backup space heating system, but has never proven necessary.
Passive space heating, solar water heating, solar electricity, solar clothes drying, and solar cooking (with a portable solar oven).
The home’s “solar side”: Passive space heating, solar water heating, solar electricity, solar clothes drying, and solar cooking (with a portable solar oven). Garage air heater not shown.
A 15-gallon drainback tank sits in the conditioned attic space.
A 15-gallon drainback tank sits in the conditioned attic space.
A 120-gallon, dual heat-exchanger storage tank with a backup electrical element.
A 120-gallon, dual heat-exchanger storage tank with a backup electrical element.
Enphase M190 microinverters installed and ready for PV modules.
Enphase M190 microinverters installed and ready for PV modules.
Properly sized window overhangs help prevent summer overheating.
Properly sized window overhangs help prevent summer overheating. A mix of good passive design and high-quality active systems make a net-zero success story.
The Riggins family is net-zero energy, ...including their car.
A solar hot-air collector heats the garage space passively.
Low U-factor, high SHGC windows on the south side admit the sun’s energy.
The wood heater has proven more useful for setting ambiance than as necessary for auxiliary heating.
Closed-cell spray-on polyurethane foam was used on all inside sheathing faces for an airtight, high R-value insulation.
A water-to-air heat exchanger connected to the earth tube outlet
From the earth tube, air passes through an ERV
The 100-foot-long earth tube was placed in a 10-foot-deep trench.
A minisplit heat exchanger was installed as a backup space heating system
A minisplit heat exchanger was installed as a backup space heating system
Passive space heating, solar water heating, solar electricity, solar clothes drying, and solar cooking (with a portable solar oven).
A 15-gallon drainback tank sits in the conditioned attic space.
A 120-gallon, dual heat-exchanger storage tank with a backup electrical element.
Enphase M190 microinverters installed and ready for PV modules.
Properly sized window overhangs help prevent summer overheating.

Space Cooling

Chimney Effect. The Colorado climate provides cool, dry nights through the summer, so we had no trouble meeting 100% of our cooling load through passive means, even with a week of record-high temperatures (in the mid-90s). The house includes a central open staircase that serves as a thermal chimney. At night, we open three small lower-floor windows on the north and south faces, and two windows at the highest point at the top of the stairwell. The convective chimney effect, plus north-south prevailing winds, pull cool air across the concrete floor and exhaust hot air through the upper windows. If we properly time closing the windows in the morning, the house does not get hotter than 76°F, since the thermal mass floor moderates the temperature. On the couple of occasions that we delayed closing the windows until mid-morning, temperatures reached 80°F inside. With the low humidity and the occasional use of an Energy Star-rated ceiling fan, however, even these temperatures feel very comfortable.

Energy Recovery Ventilator. The RecoupAerator ERV has an “econo-cool” mode that shuts off energy transfer between incoming and exhaust air. Combined with the earth tube inlet, the ERV supplied comfortable, cool air during days that were too hot to ventilate the home via the windows.

Overhangs. Roof overhangs were designed to block most of the high summer sun, but allow maximum heat gain in the winter. We were in the second week of August before the sun started to appear on the windowsills, and into September before the sun reached the concrete floor at midday.

Domestic Hot Water

The goal of producing 100% of our hot water demand year-round with SHW was met. The backup electric element has not been used. The 120-gallon Vaughn storage tank with three SunEarth EC-40 collectors kept the water at an average temperature of 165°F in the winter, peaking at 170°+ in January and February. The average summer temperature was between 140°F and 150°F due to collector high-temperature limits. The lowest temperature at the top of the tank was 128°F during a rare three-day period of cloud cover and fog.

The SHW system is an unpressurized, indirect drainback design with distilled water as the heat-transfer fluid. Three roof-mounted (39° pitch) SunEarth collectors form the heart of the system. A 15-gallon drainback tank sits in a conditioned attic, 10 feet below the top of the collectors. A Vaughn 120-gallon, dual-heat exchanger tank provides storage and backup electric water heating. A Caleffi iSolar Plus controls the system.

Hot water production is only one side of the efficiency equation—distribution and demand is the other. Every inch of hot water line in the house is insulated with 3/8-inch-thick foam pipe insulation, and we installed high-efficiency water appliances and plumbing fixtures (see “Water Conservation” sidebar).

Solar Electricity

The grid-tied PV system has 20 Sharp 224-watt modules, with each connected to an Enphase 190-watt microinverter. Total rated output, given the nominal limits (see below) of the microinverters, is 3.8 kW. We mounted the modules on our standing-seam metal roof using nonpenetrating SolarMount S5! clamps to secure the rails.

A key element in the design process was to estimate the system’s energy production. The graph on the following page shows the system’s month-by-month predicted production (generated by NREL’s PVWatts program) and actual production to date. I used a 0.82 total DC-to-AC derate factor instead of the default 0.77 derate, due to the better efficiency of the microinverters compared to a central inverter. With the exception of October 2011, the actual energy production exceeded the predicted values. October’s lower production values were due to a failed roof-mounted disconnect switch, which took out 10 modules’ production over 10 days.

Comments (0)

Advertisement

X
You may login with either your assigned username or your e-mail address.
The password field is case sensitive.
Loading