Defining the Net Zero Concept
One of the most interesting aspects of the net-zero conversation is the definition of net zero itself. The initial definition from EO 13514 was not quite explicit enough. The National Renewable Energy Laboratory (NREL) published Net-Zero Energy Buildings: A Classification System Based on Renewable Energy Supply Options.
The publication provides several sub-definitions of net zero. My favorite set of definitions comes from another NREL publication, Getting to Net Zero. There are four categories of net-zero-energy buildings (NZEB):
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 the amount it uses in a year, when accounted for at the source. Source energy refers to the primary energy that is used to extract, process, generate, and deliver the energy to the site. To calculate a building's total source energy, managers need to multiply imported and exported energy 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 customer for the renewable energy the building exports to the grid is at least equal to the amount the customer pays the utility for the energy services and energy used over the year.
Net-zero emissions. An NZEB produces — or purchases — enough emissions-free renewable energy to offset the emissions from all of the energy the building uses 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.