What does net zero energy really mean?
For a grid-tied net-zero-energy building, this is about offsetting the energy consumed with energy produced by renewable, emissions-free means. Why not offset by purchasing renewable energy or just buying carbon offsets? This could be called a Net-Zero Off-Site Energy Building. While there is no standard or third-party verification system for net zero energy, the four most common definitions do not allow offsets for off-site renewable energy. A 2006 paper (PDF) by the National Renewable Energy Laboratory explains these definitions in detail.
- Net Zero Site Energy: A site 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 at least as much energy as it uses in a year, when accounted for at the source. Source energy refers to the primary energy used to 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.
- Net Zero Energy Costs: In a cost ZEB, the amount of money the utility pays the building owner for the 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 at least as much emissions-free renewable energy as it uses from emissions-producing energy sources.
One design implication of a site ZEB is that this definition favors electric equipment that is more efficient at the site than its gas counterpart. Using the site ZEB definition, a 95% efficient gas boiler consumes 1053 kWh to produce 1000 kWh [3412 BTU] of heat, while an air-source heat pump with a coefficient of performance of 2.85 only consumes 351 kWh to produce the same amount of heat. However, using the source ZEB definition, the two are equivalent. 1000 kWh of heat requires 1158 kWh of source energy in both cases, assuming a source-energy factor of 3.3 for electric and 1.1 for natural gas.
The NREL paper ignores the embodied energy required to produce photovoltaic and solar thermal equipment. The source energy factor for PV including embodied energy is 0.7 according to the standards used by the Passive House Institute. Additional source energy factors per PHI are 2.7 for electric, 1.1 for gas, and 0.2 for wood. The PHI factor for electric is lower than the US average because it is based on the average for Europe, which uses less coal than the US. Determining the correct source energy factor can be difficult. For instance, in New York, should the factor be based on a state-wide average or on average for the entire Eastern US/Canada power grid?
HOK and The Weidt Group designed the Net Zero Court project to be Net Zero Energy Emissions and therefore carbon neutral. The office building still has an estimated energy bill of $0.01/SF so it is not Net Zero Energy Cost. In St. Louis, where the project is located, 81% of electricity comes from coal, so a carbon-neutral building would have a large impact. Their design methodology parallels that of Passive House.
- Start with optimizing the building orientation and thermal envelope to improve energy efficiency.
- Then use efficient mechanical systems.
- Finally, offset the remaining emissions with on-site renewable energy. In this case, with a large four-story structure, they had to rely on PV panels in the parking lot as well as on the roof in order to produce enough electricity to balance that used by the building.
See their PDF report here.
Off-grid buildings—also called energy autarkic buildings—use more total energy over 80 years because of the embodied energy for the PV and batteries which must be replaced every 30 years on average. [source]
A building constructed to Passive House standards can be a Net Zero Energy Emissions building by adding photovoltaic arrays or a wind turbine. The Passive House Planning Package (PHPP) software provides a tool to calculate what is required for carbon neutrality. All other things being equal, the Passive House building will cost less to operate.