The following article is taken from a newsletter published by Estes McClure and Assoc Inc., Nov 2012.
In the ongoing quest to decrease the impact buildings have on the environment, we have begun to hear more about the concept of net-zero-energy buildings. The term itself sounds impressive. After all, when looking at low-energy use, how can you beat zero?
But exactly how do you define a "net-zero-energy building"? A clean measurable definition is needed because the way the "zero-energy" goal is defined affects the decisions Architects, Engineers and Owners make to achieve and measure success. Let's look at some of the definitions in the market place today.
Net-Zero-Site Energy: A site Net-Zero-Energy Building (ZEB) produces at least as much energy as it uses in a year when accounted for at the site.
Net-Zero-Source Energy: A source ZEB produces as much energy as it uses in a year when accounted for at the source. Source energy refers to primary energy used to generate and deliver energy to the site (from a power plant, etc).
Net-Zero-Energy Cost: In a cost ZEB, the amount of money the utility pays the building owner for the energy the building supplies to the electrical grid is at least equal to the amount the owner pays the utility for energy services and energy used in a year. Carefully examine utility rate structure, as most utilities do not allow any offset in costs for fixed charges and demand charges.
Net-Zero-Energy Emissions: A net-zero-energy emissions building produces at least as much emissions-free renewable energy as it uses from emissions producing resources (i.e. an electric utility)
Net-Zero-Offsite Energy: A building may be considered a ZEB even if 100% of the energy it purchases comes from renewable energy sources, even if the energy is generated off-site.
The logical conclusion is to lean toward the Net-Zero-Site Energy definition because the only way to measure "ZEB" is to look at the energy crossing the site. Definitions that include emission, cost, source, etc. are based on this measured information and then calculations are needed to get to the metric or definition selected. In fact, through an agreement of understanding ASHRAE (American Society of Heating, Refrigeration and Air Conditioning Engineers), AIA (American Institute of Architects), the US Green Building Council, and IES (Illuminating Engineering Society) have chosen site-energy measurements.
ASHRAE's Vision 2020 defines ZEB as one that produces as much energy as it uses when measured at the site.On an annual basis, it produces or consumes as much energy from renewable sources as it uses while maintaining an acceptable level of service and functionality. Per ASHRAE a ZEB can exchange energy with the power grid as long as the net energy balance is zero on an annual basis. ASHRAE recently confirmed with Estes McClure and Assoc that ZEB's, on an annual basis, use no more energy than is provided by on-site renewable energy sources. Remember ZEB's includes only the energy flow of the building, not the overall sustainability of the building. A net-zero-energy building will have a higher first cost.
The best strategy for a ZEB using ASHRAE's on-site definition is to make the building as energy efficient s one sensibly can, then apply the on-site renewable energy. For more information you can contact Mike Clendenin at www.estesmcclure.com
Michael D. King AIA
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