Zero Net Energy (ZNE) is a term that is increasingly heard throughout the architecture and building sector, but it is also a term that can mean different things to the different people who use it. In this series of posts I will give an overview of the four common definitions of ZNE, and a brief expansion on their respective implications in relation to policy structures and physical infrastructure.
The four common definitions of ZNE are: 1. Zero Net Site Energy, 2. Zero Net Source Energy, 3. Zero Net Energy Cost, and 4. Zero Net Energy Emissions. All four of these calculations are as measured over a calendar year, or on an annual basis. The difference is in the metric (the “thing” being measured) and the boundary (what is included in the calculation). All four definitions can be applied to –and calculated at– a “community” or multiple-building scale as well. In all cases the “net” part refers to how energy is accounted for at the grid level; low energy buildings that are not grid tied would therefore not be under a zero net energy designation.
Part 1. Zero Net Site Energy
what this is: A Zero Net Energy Building is one that uses no more energy than it can produce on-site within one calendar year (this is the most commonly used definition of the term “zero net energy” at present).
what this means: A “site” can be defined as either the building footprint itself or the building and the property it sits on. In this definition, the building/ building site would incorporate a form of on-site renewable energy such as solar (most common), wind, small hydro or biogas. As mentioned, a ZNE building is still tied to the larger energy grid. For example, a ZNE building that generates energy through the use of solar panels would create a surplus of power while the sun was shining (and the excess power would be fed back into the grid), but would have to draw power from the grid in the evening or during cloudy days. The goal here is for the overall power drawn within one calendar year to be less than or equal to the power generated.
pros, cons & considerations: The chief benefit of this definition is that it promotes deep efficiency at the single building scale. This is because in order to viably (and cost effectively) achieve this definition of ZNE, it is much more desirable to build the lowest energy-use building possible and then add a source or renewable generation. In addition, ZNE sets a concrete goal to achieve and thus can be a more useful target than trying to meet or best shifting baselines as building performance codes change.
However, buildings are built in many types and have many necessary functions- not all of which are compatible with the site definition of ZNE. For example, hospitals, restaurants, industrial activities, etc., may all have a very hard time achieving a ZNE facility because of unusually high energy demands. In addition, high-rise buildings and urban infill sites have their own challenges due to physical constraints (poor solar access, low rooftop-to-building ratio, proximity and density issues, etc.)
The implication here is that if a ZNE site-definition goal is in place on a policy level, at a certain point it becomes necessary to start looking at the issue from a multiple-building, or “community”scale, that would allow energy balancing between buildings to achieve an overall ZNE outcome (this is a simplification, but that is the main point).
P. Torcellini et al., Zero Energy Buildings: A Critical Look at the Definition, National Renewable Energy Lab, 2006.