Is the way you think about emissions from purchased electricity wrong?
Electrons flowing down a wire. How many times have you heard this description in discussions on how electric power grids functions? Our greenhouse gas (GHG) accounting framework for indirect emissions from purchased electricity (i.e., Scope 2) is built around this mental model — the idea that electrons in the electric grid are analogous to water or natural gas in a pipe where we just replace molecules with electrons. Indeed, it is hard to find a reference in our field on the topic of indirect emissions that does not lean on or allude to this description of the physics of electricity.
Well, in the spirit of earlier semantic alerts I’ve posted on misused terminology in GHG accounting (see previous entries on “real” and “counterfactual”), in this blog post I’m going to come after another esoteric GHG-related linguistic bubble with a pointy stick.
First, let’s start with the basics, electricity generators that supply energy to a grid do not produce electrons that flow down wires where they eventually reach your house, building, or factory. Physical electrons exist in these wires. And these electrons (e.g., held in atoms of copper or aluminum) do move through the wire, but at a speed of roughly 0.03 cm/sec (or about 1 meter per hour, a snail’s pace …though only if you’re dealing with a particularly slow snail). In other words, there are moving electrons at play here, but that’s not the main story. It’s the electric field that propagates at the speed of light through the wire, and that does the work (in the thermodynamics sense of the word). The result: when you throw a switch it takes 1/1000 of a second for the electrons 300 km away to start moving at 0.03 cm/sec.
But, components of an electric field are not individually identifiable like electrons are (and according to Heisenberg, it’s not clear how easy it is to identify individual electrons, but I probably just got myself in heap of trouble with the physicists out there; quantum theory and engineers rarely mix). There is just more or less total field in any given place. Everything physically connected to the wire is contributing to the field. The consequence of this fact is that it is impossible to supply electricity from one generator to you and from another generator to your neighbor. All generators and grid operators are able to do is toss more or less energy from each generation source onto the grid and let everyone collectively tap into it. (It even gets a little more complicated because where you toss it in matters as well as other factors.)
This little physics of electromagnetism lesson is relevant to GHG management because the oversimplified and mistaken electrons-in-wires-as-water-in-a-pipe model has led us to an inappropriate framing of a key question. The question concerns how we should allocate indirect emissions from electricity consumption to end users. By approaching the problem with a mental model of electrons flowing down a wire, we have established what I would argue has grown into a full-fledged red herring.
Although the first law of thermodynamics does hold (i.e., total energy is conserved), this is very different from applying a conservation of mass framing to this question. A conservation of mass model would view the system as a bunch of generators contributing to a pool of electricity which then mixes and flows down wires and is sucked out —as if with a straw— by individual consumption devices. But this analogy that every “kilogram” of electricity that enters the grid also leaves the grid is the wrong way to think about it. “Electrical mass” is not conserved in this way. Instead energy enters and leaves the system through many different routes in the form of both work and heat (e.g., line losses) and is affected by many different variables (temperature, material properties, interference by other electromagnetic sources, distance between generators and consumers, etc.). So in the end, what goes into the lines from generators is not exactly the same as what is drawn out to light our houses and office buildings.
At this point you might find yourself nodding, while at the same time thinking I’m missing the point. After all, science is often simplified for policymaking. And while today’s model may not meet the muster of an academic physicist, by many measures it is “close enough” of a system: line losses are small (~7-10%); it’s convenient to ignore other, often immaterial, factors; and not least of all, it’s conceptually useful to bypass the minutiae of electromagnetics to the more infographic-friendly model pervasive in GHG accounting today. The problem? Basing GHG accounting on an incomplete understanding of science leads to poor reasoning, which in turn leads to ill-fitting policies. Case and point is the very supposition that green power markets and programs have been built upon: that every MWh of electricity is fungible with any other regardless of when or where it was added to the grid. The elaboration of this concept being, that this pool of “electrons” can just be added up over a year and their “ownership” allocated to individual consumers, ignoring the realities of space and time and the other variables I listed above. I get to claim the wind electrons and you get stuck with the coal electrons.
The point here is admittedly technical and esoteric (if you read the early posts in my series on misconceptions in GHG accounting, then the erudite nature of my conclusion should come as no surprise to you). But, as I have said before, definitions matter and so do concepts. Call me a purist, but I firmly believe it’s critically important to ground our reasoning when we create GHG accounting frameworks on a sound technical understanding of the underlying mechanics leading to emissions. As GHG professionals, it is important that we establish a rigorous tradition of technical credibility.
Working on implementing climate programs, we face enough adversity as it is. The last thing we need is scientists laughing at us for getting the basic physics wrong. If we want to compromise by assuming something slightly at odds with a purist interpretation of the science, then we should do so explicitly and with transparency regarding the implications of our simplifying assumptions.
So, the next time you hear a GHG accountant describe the “flow of electrons” to their facility, politely intervene and correct them. You may risk sounding a bit pretentious, but you will have taken a baby step to elevate the scientific reputation of our profession in the process.
Note: If you are interested a quick reference on the topic, see the following: http://c2.com/cgi/wiki?SpeedOfElectrons