Memoirs of an hourly emissions modeler: a six-part series

Chapter 1: Not All That Glitters Is Zero Carbon
By Karina Hershberg

My intent for this series is to give folks noodling on the mysteries of hourly grid emissions a place for discussion. After many years of pondering the intersection of grids and buildings, I know enough to know that I don’t know enough. I’d love to start collectively brainstorming as a community- building and utility industry together! We’re ready to tackle grid-interactive, zero carbon buildings and we’ll go farther faster if we do this together.

Chapter 1: Not All That Glitters Is Zero Carbon:

At the November Greenbuild conference in San Francisco, I spoke with Z Smith, Josh Radoff (in virtual sprit, although not in person) and Clark Brockman at the session Moving from "Net Zero Energy" to "Grid Adaptive." Clark put forward to the audience that:

“It’s possible, even likely, that most Net Zero Energy buildings in most locations are not Zero Carbon Emissions.”

For the green building professionals who have worked tirelessly to advance Net Zero Energy (NZE) buildings, this statement can be a hard truth to face. Our work has been based on the hope that energy was a good enough proxy for emissions—if we solved energy, we would solve building emissions.

As we now know, building emissions are more than just energy. They are materials, refrigerants, watergy, nutrients, transportation, and, yes, energy. But making the math of energy work on paper doesn’t necessarily solve the physics of the real world. And in the case of operating emissions, timescale matters. A lot.

The annualized hook we’ve been hanging our energy-to-emissions hat on, while beautifully simple, has also been elegantly wrong.

It’s time for us to embrace the messy, evolving world of time varying grid emissions.

One of the key issues with the translation from NZE to Zero Carbon (ZC) is that it assumes the electric grids are static, simple entities. Although this makes the math so much nicer, in reality, they are incredibly complex, multi-faceted systems that are not easily summarized as a single constant.

Grid emission rates are changing all the time.

There are daily differences and seasonal differences, all of which impact the emissions associated with a building’s energy. And if we don’t understand this relationship then we are likely undercounting the operating emissions of NZE buildings.

To hit this point home a bit harder, let’s dive deeper with a theoretical case study.

These are the outputs for a single day from a model for a net positive energy project in the United States Southeast region.

Since it is net positive (i.e. it generates more energy with onsite solar than it uses within a year) by our annualized math, this project has negative emissions, right?

Alas, that’s not where the math lands with an hourly analysis.

The reason is the photovoltaic panels (PV) offset the building load during the middle of the day when the grid is already seeing slightly lower emissions. When the building starts pulling from the grid in the evening, it’s using grid energy during a time with higher emissions.

Therefore, even though we have significantly decreased our operating emissions with onsite PV, we have not hit zero emissions.

The math inconveniently proves, then, that solving for annualized energy usage does not solve for carbon emissions.

In other words, the promise of net zero energy is left unfulfilled. We have work left to do.

Next up: Part 2: The Hourly Emissions Multiverse

A quick note: Electrification is the path to building energy decarbonization. Natural gas emissions are as equally complex as grid emissions, but a whole lot more horrifying– everything from fracking to zombie wells to flaring to super emitter events. The intent of this series is not to put that conclusion into question but simply to ensure we’re setting ourselves up to to make the electrification transition responsibly.


Also published on LinkedIn by Karina Hershberg