Capturing CO2 from ambient air - David Keith

5.2 Lessons for assessing of future energy technologies
Assessing the future of energy technologies has long been an important task. But with climate change,
greenhouse gas regulation, high oil prices, and strife in the Middle East, there is arguably more pressure
than ever to radically change the energy system. The climate change issue in particular means that policymakers
are making decisions with century-scale consequences. Understanding the potential of particular
nascent energy technologies can help direct R&D funding to effective projects and can inform various
climate and energy forecasting models. Here we briefly consider the steps taken in this thesis – an assessment
of a future energy technology – as they may apply to such assessments in general. We do not claim
that this is an optimal strategy, but this is what we used.
We assume that we are trying to answer questions like “Could this technology be feasible at large
scale?” and “Could it be made less expensive than alternatives?” in order to answer more immediate
questions like “Should further research on this technology be funded?” and “How should the cost of this
technology be represented in this forecasting model?” The general approach is one of bounding. We try to
set an upper limit on the performance of the technology by considering the fundamentals of the process.
If, because of some fundamental physical or economic constraint, the upper limit is not satisfactory, then
the answer to the first two questions is “no”, and we are done. If not, then we try to set a lower limit on
performance (where low performance is bad) based on current knowledge and technology. If the lower
limit is high enough, then we can answer “yes” and, though we may not be done, we have provided useful
information to the decision maker. If neither answer is possible, then the door is open for further analysis
and technology development.
Steps we applied:
1. Calculate thermodynamic limits. Assuming an ideal process is developed, what is the potential
of this technology? There are many examples of technologies that, when mature, approach their
thermodynamic limits of efficiency. The calciner in lime production is one. Were one assessing
the feasibility of natural gas turbines for electricity today, one would calculate the Carnot efficiency
and multiply by the fuel cost. One might conclude immediately that the technology is not worth
pursuing. For air capture, we presented this calculation in Section 1.3
2. Assess natural and practical limits to large-scale deployment. Are rare materials necessary?
Is there excessive land use? Are other scarce resources involved? Is siting limited or difficult?
We answered some of these questions for air capture in Section 1.3. Also a survey of industrial
sodium carbonate and calcite production was performed. Had we found that global deployment of
air capture would use all of the world’s sodium, we would have considered the example system
impractical. These are the kinds of questions that would keep biodiesel from restaurant waste oils
from being a key part of the national energy agenda.
3. Look for industrial analogies – processes that are similar or work with similar principles. Do
these analogous systems work and why or why not? No technology appears in isolation. There
are typically major components that have been previously engineered or have evolved from similar
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