What is the Difference Between Net-Zero and Zero Carbon Performance?
2020.02.10
There are various measures to identify highly energy efficient buildings. It is often cited that buildings are net-zero energy, or zero carbon – but what does this mean? There is a lot of variance in the industry with respect to understanding the important differences, and what they imply for buildings.
Net-zero energy performance means that 100% of the energy used by a building is provided by on-site renewable sources. Renewable energy is converted from solar energy, wind energy, or geothermal energy. This calculation is a net calculation, annualized over an operational year. For example, buildings located in high latitudes (i.e. cold climate) will generate much more solar energy in the summer but may need to draw from a local electrical grid in the winter. However, the draw-versus-supply balance summed over the year is averaged out to zero. The Living Building Challenge certifies net-zero energy performance.
This is a very rigorous and difficult design aspiration. The necessary on-site renewable production criterium forces the building to be ultra-low energy consumption. It also means the building orientation, blocking & stacking and site plan must accommodate the large amount of renewable energy generation (e.g. area of photovoltaic array). The certification process is based on measured results – not simulated design. Combustion of fuels is not allowed.
The Joyce Centre for Partnership & Innovation at Mohawk College (Architects: McCallumSather & B+H Architects, Design Principal-in-Charge: Kevin Stelzer) is a Net-Zero Energy Building. The building is fully electrically powered, running on a VRF ground-source heat pump. The facility has a 545 kW PV array. The building has an anticipated energy use intensity (EUI) of 71.5 kWh/m2/yr.
Zero Carbon performance measures the emissions associated with your building operations. For example, if a building is fully electrically powered, and the electricity grid is fully renewably powered – e.g. via hydro-electricity – then the building produces no carbon emissions associated with its operations; and therefore is carbon neutral (or zero carbon). Many electricity grids use some form of combustion to manage peak load, so the individual net-zero building needs to offset these emissions with on-site renewable power production. The CaGBC Zero Carbon Framework certifies net-zero carbon perfomance.
This certification focuses upon the impact of the building emissions. If a building uses combustion, then the associated emissions can be off-set through on-site renewables. The certification system uses Energy star to calculate the emissions associated with various fuels. This net-zero certification might be considered for urban buildings where large footprints are not feasible. For example, in Ontario, the on-site installation of 5% of total building energy consumption (for a fully electrically powered building) will net-out the emissions and achieve carbon neutrality. The system has other measures that ensures the certified buildings are ultra-efficient, including energy metrics for superb enclosure performance.
The Humber College BLDG Nx deep energy retrofit (Architects: B+H Architects, Principal-in-Charge: Kevin Stelzer, Project Manager: Kevin Pu) is a Zero Carbon Building. The building is fully electrically powered, running on a VRF air-source heat pump. The facility has a 25 kW PV array to offset all associated emissions produced by the electricity grid. The building is a Passivhaus design; and has an anticipated EUI of 64 kWh/m2/yr.