Part 2 – Zero Energy vs Net Zero Case Study
In our last Mindshare post, we discussed the introduction of a new designation in the energy world, a “zero energy” project. This in contrast to the “net zero” goal that has been more widely pursued within the industry. In this post, we focus on what it would mean for a single building to achieve zero energy status versus net zero status.
While multiple definitions of the term are used in the industry, “net zero” usually means a building produces as much renewable energy on-site as it consumes on-site on an annual basis. For example if a project uses 200,000 kBTU of electricity and 100,000 kBTU of natural gas, it would have to produce 300,000 kBTU of on-site renewable energy, typically in the form of PV electric or solar thermal. The typical “net zero” designation does not require any on-site storage of energy, nor does it preclude being connected to standard utilities. In contrast, “zero energy,” as defined by the Department of Energy, means that a building produces as much renewable energy on-site as it consumes in source energy (accounting for the different amounts energy needed to obtain each type of fuel and deliver to the project site) on an annual basis.
The DOE has provided the following table to convert site-energy use to equivalent source energy required, based on national averages. For example, every unit of electricity that is consumed on-site requires 3.15 units of energy at the source to account for extracting fuel, converting it to electricity (~30-40% efficient typically), and transmitting it across the grid . Our regional efficiency in the Pacific Northwest is better than the national average because we are so reliant on hydropower, but for the purpose of this standard, we are to use the averages to maintain a level playing field.
Going back to our example project mentioned above, using 200,000 kBTU of electricity from the grid and 100,000 kBTU of natural gas, the source energy required is:
200,000 kBTU (electricity) *3.15 + 100,000 kBTU (natural gas) * 1.09 = 739,000 kBTU
To be zero energy, the project has to produce (and export) at least that quantity of energy, but it also gets to use the same conversion factors for any exported energy. Let’s examine how much PV-produced electricity the building would have to export to qualify as zero-energy.
739,000 kBTU/3.15 = 234,603 kBTU of PV electricity
So for this particular example, the site balance would be 300,000 kBTU consumed, but “only” 234,603 kBTU of renewable energy (PV electricity) produced.
Because of the conversion factor for electricity (3.15), 234,603 kBTU of exported PV electricity produced makes up for the full 739,000 kBTU in source energy from offsite sources.
While it might seem paradoxical at first that less energy is produced onsite than consumed from offsite sources, looking at the conversion factors and making the zero energy calculation allows us to see the real impact of source energy production. Produced as usable electricity that feeds directly back to the grid, renewable energy generated on site demands no additional energy for extraction or conversion, and transmission losses are greatly reduced because energy is produced locally to where it is consumed. Conversely, the energy used by the building from the grid has to go through each of those processes before reaching the site, losing efficiency at each step.