A while back I tried to figure out whether we can keep running things the way we're running them: do we need to transition off of fossil fuels, how fast, and would we be able to provide today's level of energy supply using alternatives?  I didn't include many of the details in that post, so I'd like to consider a few additional questions and look at this topic in greater depth.


  • How fast do we need to transition off of fossil fuels?
  • What industrial capacity is available today for different alternative energy technologies and what is likely to be available in the future?
  • What might we do if we can't replace fossil fuels with alternatives fast enough, and what might the consequences be?

I finally got around to re-doing these calculations (so I could present them in a talk last week), and wanted to go through the numbers.

How fast do we need to transition?

In his excellent talk on climate change and energy, Saul Griffith analyzes how fast climate change might require a transition off of fossil fuels.  While there is good reason to think that 350 ppm of CO2 should be our global target, we passed that point in the late 1980s and have shown no signs of turning back.  Ultimately the goal is to avoid exceeding 2C of warming, as it marks the rough threshold at which many climate feedbacks are likely to kick in, including permafrost melt and loss of the Amazon.  Griffith observes that 400 ppm should be a target since it gives us a reasonable shot of staying below 2C, but we're only 2-3 years away from 400 ppm (we'll be at about393 ppm by the end of the year) so it seems that is unlikely to happen.  Griffith does his calculations assuming a target of 450 ppm, which by various projections only gives us a 1/3 to 1/2 chance of staying below 2C of warming.  (As a point of reference, the "agreement" at the Copenhagen climate conference would have taken us to 770 ppm, which equates to roughly 5C of warming.)

Given a target of 450 ppm, Griffith calculates we need to transition in about 20 years.  I think about it as follows: we're increasing CO2 by about 2.5 ppm / year at the moment (a little less when the economy isn't growing quickly - this is a whole other subject I'd like to explore).  Any real-world deployment of alternatives would necessarily face some ramp-up and would be producing much more towards the end of the transition, and thus we'd probably keep fossil fuel generation going until near the end of the transition period.  As a result, there'd be little drop-off in emissions until near the end, and so we should aim to transition over a period of about 20 years to avoid overshooting 450 ppm.  As for transitioning faster, we can look to the Hirsch report, which argued that a 20 year energy crash program is about as fast as it can be done.  So 20 years it is.

Industrial capacity

I had done some quick calculations last time around, but I wanted to delve into more detail on industrial production numbers.  Specifically, what does the alternative energy industry say?  I figure that each industry is its own best advocate, so it's likely that their numbers will be on the optimistic end of the spectrum.

First, how much would we need to build to provide 15 TW globally using mostly alternatives in 20 years?  Griffith calculates that we'd need to build 2 TW each of Solar PV, Solar Thermal, Wind, and Geothermal, 3 TW of Nuclear, and 0.5 TW of Algae fuel (we'd also keep some existing fossil fuel and other production capacity).  While I could quibble with the particulars, it's a fairly balanced profile and is a reasonable starting point for a calculation.

Let's start with Solar PV, with this blurb that says we'll be at 28 GW / year of (nameplate) production by 2012.  Let's round that up to 30 GW / year and use a capacity factor of 15% (considering the 200 W / m2 that's available in most temperate zones).  That yields about 4.5 GW / year of production.  Let's allow for a steady 20% yearly increase in production over the next two decades.  Combined production over 20 years will thus produce about 745 GW of PV capacity.

I'm going to assume that Solar Thermal can easily meet its 2 TW production slice, since there's not much to it (mirrors, motors, pumps, generators).

How about Wind?  The Global Wind Energy Council expects 2500 GW of wind nameplate capacity by the 2030s.  Using a standard 30% capacity factor gives us 750 GW of Wind capacity in 20 years.

Geothermal is a bit complicated, as there really aren't that many easy places to tap geothermal energy.  The International Geothermal Association expects a 9 GW nameplate capacity increase over 5 years.  Assuming the same growth trend, this yields 94.5 GW of capacity.  Including a 65% capacity factor, this yields about 61 GW of new geothermal capacity in 20 years.

Biofuels are also complicated.  While there are numerous current-day biofuels, including ethanol from corn and sugarcane, Griffith rightly considers better options such as algae-based oil.  Biofuels digest estimates a production capacity of 1.6 billion gallons / year by 2014.  That's about half of one day's global production of crude oil, per year.  Given the 15% production growth rate, this yields 26 billion gallons / year of capacity in 20 years, which is about 1.7 million barrels of oil per day (about 2% of today's oil production).  Assuming the same energy density as gasoline, this yields about 105 GW of capacity in 20 years.

Finally, nuclear.  There are no good sources for expected production capacity, so to be optimistic (side note: I don't actually think we should build any more nuclear, but that's another issue) let's use the peak rate of construction ever achieved,30 GW / year of nameplate capacity construction (MacKay looks at a 60 year construction horizon, which is far too long).  This would yield about 600 GW of capacity in 20 years, assuming no loss thanks to nuclear's high capacity factor.

In total this yields about 4.2 TW of new capacity to add to an existing 2 TW of fossil fuels, 1 TW of nuclear, and 0.5 of hydro, yielding 7.7 TW - about half of the target.  That is, even assuming optimistic rates of production of alternative energy sources, we'd be about 50% short of our energy target in 20 years.

A few scenarios

Doing the analysis above reminded me that a wholesale transition to alternatives seems unlikely to deliver energy at current levels of consumption/production.  I'd like to briefly consider a few possible trajectories / scenarios here, which I'll explore in more depth another time.

  • Business-as-usual: we'll just keep on going with fossil fuels until we can no longer do so; that is we'll follow the oil depletion curve down and try to substitute with coal, tar sands, and other dirty fuels.  We may not build new coal plants in the United States, but we probably won't decommission them as fast as we should to deal with climate issues, and China, India, and other countries will continue using coal at breakneck rates.  This scenario might, in the short term, maximize global economic output (though it will likely still be decreasing on a long-term basis given the economic impact of oil depletion).  It will however cause us to overshoot 450 ppm of CO2, taking us to perhaps 500 or 550 ppm, which is probably past the point of no return in terms of warming - natural feedbacks are likely to take over.  (I think we probably are unlikely to go much further than that, since we'll start running out of cheap coal at that point.)
  • Unmanaged descent: we'll keep using fossil fuels, but the economic contraction due to oil depletion will hit hard enough that we'll end up using less energy overall.  In this way, we'll haphazardly decrease our energy use at the expense of global human hardship.  In this scenario, we'd probably avoid exceeding 450 ppm of CO2 simply due to a non-functioning economy, though we also won't be able to build alternatives at anything near the rate I describe above.
  • Managed descent: there are a lot of things that need to be done just right to manage our descent.  First, we'd need policy-based solutions, either in the form of a carbon tax (or the equivalent) or energy quotas.  Second, we'd need to stabilize swings in oil prices as I discussed before.  Third, we'd need to invest in alternatives that have the highest capacity yield per unit time.  From what I can tell, solar thermal is one of the best options, as it doesn't require particularly advanced technology and therefore could probably be ramped up quickly.  It is however only viable in desert regions.  My preference would be to target solar thermal, wind, and algae fuel as the three main alternative sources; solar PV can help at the household scale.

Until next time...

Originally posted to barath on Wed Aug 24, 2011 at 07:11 AM PDT.

Also republished by Community Spotlight.

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