Building footprint boundary
When boundary conditions are defined by the building footprint, only renewable systems installed on rooftops and building facade can be considered in calculating energy neutrality.
Renewable technologies installed on building footprint are the most assured supply of renewable energy to a NZEB through its lifetime as the footprint is secure from encroachment or any other change in surroundings.
Theoretically, rooftops and facades can be used to harness energy from sun, wind, hydroelectric and biofuels. Due to technological limitations, hydroelectric technologies have not been attempted on either so far. Wind energy is so site specific because of its dependence on wind direction and velocity, that it can hardly be a viable option for most buildings in urban areas. Rooftops are used quite commonly for solar PV and solar hot water, and depending on location, for wind. Similar technological constraints limit the use of facades only for solar PV. Biofuels like biodiesel and others, when procured from outside cannot be considered as on-site energy sources. Bio waste generated on site and converted to energy on building rooftop or façade will qualify under these boundary conditions.
Site energy, energy cost and energy emissions can use the footprint as a boundary. Buildings with high energy demand may be unable to offset their energy use by on-site or on-building to qualify as a NZEB and hence need broader boundaries extending beyond the footprint.
In this option, renewable energy sources on the site and areas adjacent to the site but not on the building itself are considered while measuring energy neutrality. Examples are parking lot based PV systems, solar thermal systems, small hydro units, tower mounted wind turbines and on-site biogas plants that uses waste generated on the site.
Defining the site is of utmost importance when using this boundary condition, especially when renewable sources near to the site are being used to evaluate renewable energy supply. Adjoining properties can be considered as a single site even. Non-adjoining sites owned by same organization or owner can be considered as a single site as long as the owner has unrestricted, non-public routes of access between such properties.
Energy supply from onsite renewable sources is assured unless construction on adjacent sites obstructs access to sun and wind.
When on-site or on building renewable energy sources are not adequate, project owners can find alternatives outside the site boundary.
RE supply needed to become NZEB can be supplemented by buying renewable energy produced elsewhere. Essentially, renewable energy credits or certificates (RECs) can be purchased from an off-site renewable energy generation plant.
Alternatively, renewable fuels can be imported from outside the boundary and used to generate renewable energy on-site. Possible renewable fuels that can be procured from outside are biodiesel, methane, biodegradable waste from other places. It should be noted that energy used and emissions in producing and transporting renewable fuels to the site are high, and may not be easily quantifiable.
Energy produced in either case will be measured when evaluating total supply of renewable energy to the project. Offsite boundaries may be necessary for buildings with high energy use or large projects like campuses with multiple NZEBs. Such projects do not have adequate on-site renewable resources to offset their energy demand.
While RECs are tradable, for the purpose of claiming NZEB status, they can only be considered over project lifetime if the project owner does not sell them. In case these are sold, project can claim to be NZEB if the old RECs are replaced by an equivalent number of new RECs.
Off-site boundaries will be inherently synchronous with energy emissions NZEB and source energy NZEB.