02 September 2011

Energy Efficiency and Carbon Emissions: Class Assignment

Here is the assignment I gave my students in my graduate seminar on energy efficiency, motivated by this well done article:
Here is an optional exercise for you to contemplate over the holiday weekend.  (It might make for a good final exam question, hint, hint).

1. The world consumes about 500 quads of energy today
2. Projections are that the world will consume 700 quads by 2030
3. Energy efficiency (EE) can be thought of as the inverse of energy intensity (EI) -- that is GDP/TE rather than TE/GDP -- so an increase in energy efficiency is equivalent to a decrease in energy intensity.
4. In the Kaya Identity CO2/GDP equals the product of energy intensity and carbon intensity -- CO2/GDP = TE/GDP * CO2/TE
5. Using your new-found appreciation of Excel and exponential growth equations, please calculate the rate of efficiency gains necessary to offset the projected increase in energy consumption by 2030 (#2 above)
6. Do the same thing for a 30% reduction in energy consumption from 2011
7. If the world consumed 355 quads in 1990, how do your projected rates of efficiency gain compare with reductions in EI from 1990-2011?
8. Convert your numbers in #5 and #6 into (a) carbon dioxide emissions and (b) nuclear power plants equivalent
9. What does this math say about the potential for efficiency to contribute to emissions reduction goals? If not efficiency, then what? (Hint look at the Kaya Identity)
10. BTW, the numbers in #1 and #2 leave 1.5 billion people in the dark with no energy access.  So redo the exercise assuming (a) these people gain energy access at 20% of today's per capita US level (i.e., add 100 quads to #2), (b) these people gain energy access at 50% of the 2011 US level (i.e., add 250 quads to #2).

Understanding these numbers is, I think, a prerequisite to having a complete understanding of the role of efficiency gains in the debate over emissions.

To be clear on my views -- from TCF and the magazine article that I shared with you will know that I think that (a) energy efficiency is a very good thing and should be pursued, and (b) it is very limited in its potential to contribute to goals for emissions reductions.  The math exercise above will help you to understand why I have come to this conclusion.


  1. 11. Imagine all the possible ways to improve energy efficiency. Line them up in order of cost effectiveness, from cheapest and most effective to most expensive and least effective. On which end would you expect to find the methods already used? On which end, the ones we have left to try?

  2. As John Sterman has noted in his ‘Bathtub Analogy’ of the global carbon cycle:


    equal rates of “filling” and “draining” of a tub can keep the water level in the bathtub constant – (can maintain a concentration of atmospheric CO2 in ‘dynamic-equilibrium’ – and therefore maintain near constant, average, global surface temperature).

    Filling involves transfers of CO2 to the atmosphere. Draining involves sequestration from the atmosphere.

    Following more than a century of large-scale industrialization, rates of filling have been higher than rates of draining, and the planet has been warming.

    By increasing ‘flow out’ above ‘flow in’, the global temperature can be lowered.

    Thus, for effective mitigation, VERY large-scale bio-sequestration, like




    can provide significant control of atmospheric CO2 concentrations and global temperatures.

    Of course, every increase in energy efficiency reduces the total required investment in bio-sequestration.

    Roger's exercise is valuable for the perspective it can generate on the scale necessary to significantly affect costs.

  3. FYI:

    In Holland natural gas energy consumption has halved (!) over the last 30 years and electricity consumption decreased lightly since 2008.

    I'm sure this was not anticipated in the "models" or "scenario's".

  4. Will we get to see the Instructor's solution so that we can check ours?