Modeling Software Helps Design Net-Zero Houses
Tom Lombardo posted on September 22, 2013 | 10531 views

Happy Autumnal Equinox, everyone! (And for our readers in the southern hemisphere, happy Vernal Equinox!)

Build Equinox is a company that specializes in energy-efficient design and construction, energy analysis, and indoor air quality. Using its experience with residential and commercial construction, Build Equinox developed a web-based computer modeling tool that estimates the energy consumption - and production, if you’re generating energy - of a building. ZEROs (Zero Energy Residence Optimization software) helps architects design buildings that minimize their energy consumption. Using ZEROs, Build Equinox has designed and constructed the Equinox House, a net-zero home in Urbana Illinois.

The Equinox House

Much like the Harvest Home, the Equinox House (pictured above) was designed to maximize passive heating, cooling, and lighting, and to minimize energy and water waste. South-facing clerestory windows allow indirect sunlight to provide daylighting throughout the year, and their small overall size helps reduce the loss in overall R-value for the house. A white metal roof reflects even more light into the clerestory windows. The walls are 12-inch thick structural insulated panels (SIPs). Concrete floors add to the thermal mass, helping to regulate the house’s temperatures throughout the year. An air-source heat pump provides winter warmth, using about the same amount of energy as two hair dryers. I notice a lot of south-facing walls without windows. I might have added a passive heating system to the south side of the house, although that might detract from the home’s appearance. Aesthetics may be the reason they didn’t include that.

Water is heated not with an exterior solar water heater, but with a heat pump water heater (HPWH). Just like a heat pump uses ambient heat in the outside air to heat the house, a HPWH takes ambient heat in the room and uses it to heat water. This is great in the summer, when the room air is warm anyway, but in the winter you’re “robbing Peter to pay Paul” since you’re moving heat twice. I’m not sure why they didn’t consider a solar collector water heater - I’m guessing that was also an aesthetic decision.

Speaking of water, rainwater is filtered and collected in a 1700 gallon cistern buried below the frost point. Due to state health department regulations, collected rainwater can only be used for toilets, gardening, and other uses that don’t require potable water, so the house is also connected to the local city water supplier to provide water to the sinks and showers.

Something interesting about this house is Build Equinox’s innovative CERV system that monitors and controls indoor air quality. In a well insulated and tightly constructed house, there should be no air leakage. But for health reasons every home needs a certain number of air changes over a given period of time, so efficient houses include some form of mechanical ventilation. CERV (Conditioning Energy Recovery Ventilator) monitors a variety of indoor air factors and controls the ventilation. CERV includes heat exchangers to minimize the effects of bringing in fresh cold air in the winter and warm air in the summer.

The Equinox House sports an 8.2 kW ground-mounted photovoltaic array that provides about 10,000 kWh of energy every year. The house itself uses around 8000 kWh each year. The owners plan to use excess energy to charge an electric vehicle.

The Equinox House was featured in a series of articles in the ASHRAE Journal.

ZEROs allows designers to choose various design features of the house, including the home’s location, its building envelope, windows, heating and cooling systems, electrical appliances, and even the homeowners’ desired comfort levels. For now ZEROs only considers electric heat pumps as heating sources, due to their lower environmental impact compared to furnaces powered by natural gas or other fossil fuels. (To see why, check out “Can We Live on Sustainable Energy Alone?”)

Building Envelope:

Comfort Profile:

After entering the information, ZEROs runs a simulation and generates a detailed report showing an overall summary of the house’s energy performance, as well as a detailed analysis of its thermal and electric loads. The overview is shown. Notice the tabs at the top showing the detailed reports available.

ZEROs Report:

If you plan to add photovoltaic panels, a solar water heater, and/or light tubes to the house, ZEROs will take those into account as well. Its objective is to help you design a net-zero energy home - one that produces at least as much energy as it uses.

The images above are screen captures from the free demo version of ZEROs. While the demo version is limited in function, if you’re designing your own house it would probably prove itself useful. If you’re a professional, you might consider paying for a subscription.

Images: Build Equinox

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