Not Spaghetti

Can you cite a source for your 20% number? The report I started looking at here was this Oak Ridge study from December 2008: Combined Heat and Power: Effective Energy Solutions for a Sustainable Future. This itself contains some misleading comparisons as I'll hopefully get into in my next post on the topic. But the actual numbers it boils down to in terms of reducing growth in CO2 emissions seem to be real - and they are far less than a 20% reduction in total CO2, and the time frame was "by 2030". Al Gore has (inconsistently) suggested we have the ability to entirely remove our use of fossil fuels in generating electricity by 2020. I've run the numbers on that, and it really is (both financially and technically) possible. That would be far greater good, twice as fast. That should be one of our focuses - improved efficiency is the other.

The McKinsey report also has CHP on its list of alternatives to reduce CO2 emissions (their iconic graph is in the Oak Ridge report, with CHP components highlighted) - but it is neither the most cost effective nor a particularly large component of the solution.

I would also note I consider the typical discussion of combined cycle gas turbines substituting for coal plants as not really having anything to do with CHP - the improvement there is primarily in the gas turbine, since it takes more complete advantage of the high temperature output of combustion than a regular steam turbine can; then you can tack on the steam turbine to catch more useful energy from the lower temperature heat as well - that's really an efficiency improvement in a generator, not combining heat and power. But whatever, it's included in some of these numbers, so that's what we've got to work with...

Here are the actual numbers from the Oak Ridge report:
* First of all, the McKinsey graph (p. 13 of the report): potential CHP-related reductions, 0.15 Gt/yr CO2. That is out of total US emissions of almost 6 Gt/yr, so that suggests cost-effective CHP potential can reduce our emissions by about 2.5%.
* The same page (p. 13 of report) shows numbers from a DOE Annual Energy Outlook report, that suggests if CHP increases from 9% to 20% of electric generation capacity by 2030, it would reduce total US emissions in 2030 from 6.85 Gt to 6.25 Gt - that's an 8.8% reduction (but still an increase over present emissions!)
* On p. 21 it notes that CHP (mostly gas turbines) already produces 12.6% of US electricity generation (MWh - as opposed to capacity, measured in MW) - but I haven't pinned down what they're assuming for 2030 generation as opposed to capacity. Looking at the numbers in the two tables on p. 12, the CO2 reduction claimed for 2030 seems to be 3.4 times the amount claimed for 2006, but then total electric use in 2030 should be higher than in 2006 as well. If 2030 electric use is 50% more than in 2006, it looks to me like they're saying CHP should be producing something close to 30% of the generation (MWh) using only 20% of capacity. Not sure that makes sense, but those are the numbers they have.

And note most of the 8.8% in the AEO numbers comes from replacement of coal by natural gas. That is not just a "re-use" add-on to an existing plant, that is whole-sale replacement of coal-fired power with an alternative technology. If we're doing that, why not go all the way to wind or solar?

In any case, switching to natural gas from coal can certainly lower CO2 emissions, but that has nothing to do with CHP. The CHP component of improved efficiency is something that gives us a few percent - perhaps somewhere between the 2.5% of McKinsey's estimate and the 8.8% from Oak Ridge. Like I said, "some help", but not a major solution.