GeoExchange in a Zero Net Energy Home
(Below) These are photo galleries of (Quincy, California’s) GeoExchange in a Zero net energy home, completed in 2014. It is largely conventional but is carbonless and marries regular electrical loads and a geothermal heat pump system to a solar photovoltaic array, inter-tied to Pacific Gas and Electric with a Net Energy Metering connection. In its first, full year of operation, this 3,265 square foot house exported 1,503 Kilowatt-hours more energy than it consumed. Click on the first slide in each photo gallery below to advance with your keyboard’s arrows for enlargements w/captions. To access a PDF document summary of this project’s first year, click 1st year Summary, Quincy ZNE House, 2015. For a access to a 22-page detailed technical paper on this project’s initial three years of performance, click Quincy ZNE 3-year Performance 6-6-18.
CONSTRUCTION FEATURES
This house combined a slightly unconventional, wide wall system with generous insulation in walls-ceiling-floor of R-32, 49, and 30 with low-E coated, argon-filled insulated vinyl windows of U=.28. The cantilevered roof edges overhang walls for 30″ at gables and eaves, and all attic space was fed fresh air passing through continuous soffit vents that conduct it through upper frieze blocks of 16″ height due to raised heel trusses. All roof peak spines were completed with continuous ridge venting. The south facing “over-framed” roof surface supports 523 sf of solar photovoltaic panels at an orientation of South 5° West.
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GEO HEAT PUMP SYSTEM
Because of a robust thermal envelope, this home uses a much smaller geo heat pump than usual. It provides three tons of capacity for 3,265 square feet of conditioned floor space. Equipped with a two-speed compressor, this unit runs on 67% of capacity most of the time, consuming less than 1,800 watts. Because all the floor space is a single story and because the small heat pump produces only 900 cfm on low compressor and 1350 cfm on full compressor, the system was divided into three zones to improve airflow that 95% of the time is 900 cfm. The ceiling return air ducting is fed by four 20″ x 20″ intakes spread out within the building for better zone circulation and for noise reduction. The horizontal Sinky® ground loop at 7-foot depth is exceptionally large because of extremely poor soil type (sand, rubble and boulders) and is composed of four trenches, each with 800 feet of 3/4″ HDPE SDR-11 pipe, heat fused at all points up to the flow center inside the garage. Nine gallons of ground loop water/antifreeze per minute serves the heat pump and spends approximately 11 minutes underground per round trip. The ground loop averages above nameplate performance temperatures throughout the winter, boosting the unit’s efficiency above its COP of 4.9. The system integrates an HRV (heat recovery ventilator), steam humidifier, electronic air filter, and a 50-gallon hot water pre-heat tank connected to the heat pump’s de-superheater.
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SOLAR ELECTRIC PV SYSTEM (Net Energy Metering)
A solar PV system was inter-tied to the local electric utility via a net energy metering program (PG&E E-6 rate). It’s composed of 31 panels of 275 watts each (8,525 watts DC) and is rated for 7.4KW AC. The DC output from four series-wired strings of panels feed energy to a combiner box at 225-to-240 volts and thence to two 4,000 watt DC/AC inverters before traveling to the main panel’s bus bars, from which all household electric loads are fed. If generation exceeds use, the unused power is pushed back through the smart meter, turning it backwards, passing through the utility’s transformer and out on to 14,000 volt secondary transmission lines where neighbors can now receive it. Performance and retail credits are covered in a photo gallery beyond this one just below.
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PERFORMANCE DURING QZNE’s FIRST TRUE-UP YEAR (On Pacific Gas & Electric)
The Quincy Zero Net Energy house is all-electric and without any interior combustion with wood, propane, or oil. With the utility’s grid as its battery, it used significant portions of electricity during cold months for heating (that produced lots of pre-heated hot water), and small amounts of electricity in summer for cooling, pushing back lots of excess generation during long days. Under NEM rules, customers do not pay monthly bills, only receiving them as notifications of use; they complete their full year of use/generation and then pay for any net use that exceeds their actual production. When exporting power to the grid, NEM installations are credited at the retail electricity rate in California. The more export of total Kilowatt-hours, the higher the retail credit if this accumulation exceeds baseline or upper tier rates. This home exported over 80% of its power during the summer of 2015 during periods defined by the utility as either peak or partial peak times. Exports in July got large enough to be in Tier 3 at a 1pm to 7pm credit of 43.2¢/Kwh. Each one of such Kilowatt-hours buys nearly three KWH during the high (but off-peak) winter use period. Performance of the QZNE House is not just about electricity but also about its thermal energy exchange through its ground loop, which is featured in a written Quincy GHP Horizontal Ground Loop Testing report you may want to download as a PDF document.
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