Definitions of Net Zero Energy
What is a net-zero energy building, precisely, and why are we hearing more about these buildings now than ever before?
I asked my customer, Michale Koman, director of sustainability for the University of South Carolina, to field this one. Here’s what he had to say:
A Net Zero Energy Building (NZEB) is a building (residential or commercial) that has significantly reduced energy needs. The high level of efficiency allows the energy that is needed to be supplied by renewable energy sources, such as solar. For us, it is simply the building will require as little energy as possible, and will produce whatever energy it does need through renewables. How that is done varies among projects, and each has benefits/drawbacks but most try to fit into several of DOE/NREL's classifications. The steps are the same for all. Address demand first, then supply.
Here are some standard definitions, via DOE and NREL:
Zero Energy Buildings: A Classification System based on Renewable Energy Supply Options classifies NZEBs based on the renewable energy sources used. At the top of the classification system is the NZEB: A - a building that offsets all of its energy use from renewable energy resources available within the footprint. At the lowest end is the NZEB:D - a building that achieves an NZEB definition through a combination of on-site renewables and off-site purchases of renewable energy credits.
Net Zero Energy Building Definitions
* Net Zero Site Energy: A site NZEB produces at least as much renewable energy as it uses in a year, when accounted for at the site.
* Net Zero Source Energy: A source NZEB produces (or purchases) at least as much renewable energy as it uses in a year, when accounted for at the source. Source energy refers to the primary energy used to extract, process, generate, and deliver the energy to the site. To calculate a building's total source energy, imported and exported energy is multiplied by the appropriate site-to-source conversion multipliers based on the utility's source energy type.
* Net Zero Energy Costs: In a cost NZEB, the amount of money the utility pays the building owner for the renewable energy the building exports to the grid is at least equal to the amount the owner pays the utility for the energy services and energy used over the year.
* Net Zero Energy Emissions: A net zero emissions building produces (or purchases) enough emissions-free renewable energy to offset emissions from all energy used in the building annually. Carbon, nitrogen oxides, and sulfur oxides are common emissions that NZEBs offset. To calculate a building's total emissions, imported and exported energy is multiplied by the appropriate emission multipliers based on the utility's emissions and on-site generation emissions (if there are any).
The reason we hear more now is because of the need (economic, energy independence/security and climate change) as well as the push from major players such as ASHRAE, DOE and AIA. All have some form of program that hope to see net zero buildings common place within a decade or so.