photo attribution: CalderOliver at en.wikipedia

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. This series of posts is 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. Part 1 of the post can be found here

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 2: Zero Net Source Energy

what this is: A source zero net energy building  produces at least as much energy as it uses in a year when accounted for at the source.

what this means: “Source energy” refers to both the energy used by the building and the energy lost in the generation and delivery of the energy to the building. To illustrate the point, think of carrying a bucket of water with a small leak across a room; you then water a plant with it. The ZNE site definition would only be concerned with the water that was applied to the plant; the ZNE source definition would be concerned with both the water that was applied to the plant, and the water that leaked out onto the floor.

pros, cons & considerations:

• A ZNE source definition can be a benefit if one is striving to be extra conscientious about accounting for energy use and could have benefits depending benefits depending on the fuel mix of your building.

further reading:

P. Torcellini et al., Zero Energy Buildings: A Critical Look at the Definition, National Renewable Energy Lab, 2006.

This post is part of our definitions series on “eco-lingo” and technical terms