I just read an article about an electric vehicle having zero CO2 emissions and thought it’d be an opportune moment to emphasize the value of thermodynamics in critically assessing such claims. Let’s walk through how this is done, starting first with a recap of the foundational mass & energy conservation laws.
The conservation laws for mass and energy define what is and is not possible
Man’s failed attempts at alchemy and perpetual motion revealed the underlying mass and energy conservation laws that prevented each from happening. Alchemy was the effort to magically transform mass from one form to another that never succeeded but instead led to the wonderful research performed by Antoine Lavoisier in his Parisian laboratory in the late 1700s. In a series of experiments involving chemical reactions of gaseous species such as oxygen, nitrogen, hydrogen, and water, Lavoisier monitored the weights of the reactants and products very meticulously and so quantified highly accurate reaction stoichiometries. It was in the perfection of his mass balance methodology that he concluded, “nothing is created either in the operations of the laboratory, or in those of nature, and one can affirm as an axiom that, in every operation, there is an equal quantity of matter before and after the operation.” In addition to discovering the conservation of mass, his results also helped found modern chemistry and, later, helped validate the atomic theory, since not only is mass conserved but so too the numbers of atoms of each element involved (ignoring nuclear reactions).
As for perpetual motion, for hundreds of years, scientists sought in vain to create mechanically operated and gravity-driven devices and machines that could work forever. By around the mid-1700s, the frustration with repeated failures evolved into the realization that perpetual motion is impossible. A rudimentary form of the conservation of mechanical energy then emerged in which the sum of kinetic energies and potential energies (gravitational) remains constant for a system of moving and interacting bodies. It would take another 100 years for heat to be added to this summation based on the work of a small group of scientists. In 1850 Rudolf Clausius quantified these concepts with his famed equation, dU (change in energy) = Q (heat) – W (work). The equation became known as the 1st Law of Thermodynamics and solidified energy as the central property in the new field of thermodynamics.
The Mass & Energy Balance (M&EB)
Lavoisier’s and Clausius’s separate works eventually merged into a seemingly simple equation for any system with a defined boundary, Accumulation = IN – OUT (Figure 1). Since both mass and energy are conserved, this equation applies to each property individually and thus became the core of what we now call the Mass & Energy Balance (M&EB). Scientists use the M&EB as a necessary (but not sufficient) reality check on their work. If the equation fails, then scientists conclude that something must be wrong with their data and not the equation, thus prompting them to review their assumptions, their calculations, their equipment, and so on. The power of the M&EB revealed itself in 1925 when experimental results on beta-decay didn’t make sense from an energy-balance perspective, thus casting doubt on the conservation of energy itself. It was Wolfgang Pauli, who, “desperate” for a solution to save the energy law, hypothesized a way out by proposing in 1930 the existence of a new, difficult-to-detect particle to account for the missing energy. This particle, the anti-neutrino, was discovered in 1956 and not only provided yet more supporting evidence for the conservation of energy but also demonstrated the active use of this law as a reality check on experimental research.
While the M&EB can be applied to any system with a defined boundary, it’s usually applied to a continuous process. As most continuous processes operate at steady-state for which there’s no accumulation, then whatever you put IN to the process must eventually come OUT.
The M&EB is often used in the direction of either IN to OUT or OUT to IN. Here’s what I mean by this. In the former, you know what you put IN and so make sure that everything you measure OUT is accounted for. For example, a single reactant flows into a reactor and many products flow out, some of which might be hard to detect. You focus your attention on all of your measuring devices for flow and composition to make sure that all of the IN atoms are accounted for in the OUT. The same logic applies for energy. If you know the IN energy, then you know what the OUT must be, and if you’re not arriving at that answer, then something’s wrong. This was the logic used in the beta-decay discussion above. The power of this tool explains why it became one of the engineer’s guiding principles: in equals out at steady state.
The other approach for using the M&EB is when you know the OUT that you want. If you know you have to produce so much of a given OUT material, then you can work backwards to determine the IN required to do this. With regards to the automobile, for example, you know the OUT you want to achieve. It’s the energy required to move a person from one place to another against the resistant forces of wind and road friction. You know what OUT looks like and so can then work backwards to determine the IN to get there.
What is the appropriate boundary for the Mass & Energy Balance when evaluating CO2 reduction options?
While use of the M&EB seems pretty straightforward, there’s one complication involved. Notice the previously used words, “defined boundary.” Where exactly do you define the boundary? The answer is that it depends what your objective is, which brings us back to the electric vehicle.
Consider the typical gasoline car and our desire to reduce transportation CO2 emissions. Where would you draw the M&EB boundary to analyze this situation? Well, you could draw it around the car itself (Figure 2) and then consider how much CO2 flows out of the tailpipe. Assuming steady-state operation, the carbon contained in the gasoline IN stream determines the carbon contained in the CO2 OUT stream. Simple enough.
Now consider an electric car (Figure 3). What are the corresponding IN and OUT streams? Here you would have a continuous flow of electricity IN and zero CO2 emissions OUT. Whereas for the gasoline car, you have a certain amount of CO2 emitted, for the electric car, you have zero CO2 emitted. Is this a valid comparison on which to draw a conclusion about which option is more desirable? Again, the goal is to reduce CO2 emissions caused by driving. What is the real boundary to consider?
The answer is that the boundary should encompass not just the operation of the vehicle itself but also everything required to create both the vehicle and its energy source (Figure 4). Only in this way can you quantify which option results in the lowest total CO2. For example, are the energy source options used to power the automobile sitting in nature, ready to use? Clearly not. There’s no lake of gasoline in nature, nor is there a “lake” of electricity. When you plug a cord into the electric outlet, electricity isn’t just sitting there waiting to flow. Each of these resources must be manufactured or generated and so the system boundary should be drawn all the way back to the resources that are sitting there in nature waiting to be used. The crude oil, coal, or raw nuclear fuels buried underground. The water fall, the wind, the sunlight. Such a system boundary would then justifiably account for the construction and operation of the infrastructure required to extract the natural resources and refine them into usable forms.
And that’s not all. Don’t forget that there’s no “lake” of cars sitting around either. You need to account for the building and continuous re-charging of the cars, again starting with natural resources. You might think that a car’s a car and so there’d be no need to account for this when comparing options. But the material inputs used to manufacture a gasoline-powered car are quite different from those used to manufacture a battery-powered electric car. For each automobile option there will be many and often different natural-resource IN stream components. Also don’t forget that the options must be compared using the same rule book, meaning, for example, that the same regulatory criteria as regards safety and environmental impact must be applied to all activities comprising each option.
Life Cycle Analysis (LCA)
This example is very real and calls for an accurate M&EB analysis to determine which option is best for reducing total CO2 emissions associated with driving. This type of all-encompassing analysis is called a Life Cycle Analysis (LCA) as it covers every single cradle-to-grave step in the process, including the final step of handling the used, no-longer-working automobiles themselves, with either recycling or disposal. Each step of each option can be characterized by its own CO2 emission OUT stream, the sum of which becomes the metric by which to make an informed decision.
The discovery of mass, energy, and the conservation of each helped lay the foundation on which thermodynamics was built, including the Mass & Energy Balance and the Life Cycle Analysis. These methodologies offer the means by which informed decisions can be made by ensuring that all is accounted for and nothing ignored. I do not offer an opinion here about which car option is best as regards CO2 emissions but I do suggest that the final decision can only be made after a thorough LCA is completed.
Illustrations by Carly Sanker