CO2 Accounting in Energy Stewards

Energy Stewards® reports annual carbon dioxide (CO2) associated with your building’s use of energy.   Where do the numbers come from and how good are they?

Energy Stewards uses the ENERGY STAR calculations for CO2 emissions.  In this post, we’ll touch on the major ideas.   For technical details, check here.

You see CO2 information in two places in Energy Stewards:   (1)  On the home page, there’s an estimate of tons CO2 over the last 12 months and the % change in CO2 from baseline.

(2) On the “Details” page, you see the plot of a moving annual total of estimated tons CO2, updated each month.

Actually, the ENERGY STAR adjusts the CO2 emissions numbers to account for a little bit of methane (CH4) and nitrous oxides (N2O), which are potent greenhouse gases–for example,  a kilogram of methane has the same greenhouse impact as 20 kilograms of CO2.

ENERGY STAR reports CO2e, “CO2 equivalent” mass, as a result of the adjustments.  For simplicity, Energy Stewards just uses the label CO2.

ENERGY STAR calculates CO2 emissions in different ways depending on the type of energy.  For electricity, ENERGY STAR estimates the emissions associated with electricity generation using your building’s zip code!

Map of electric grid regions The U.S. is divided into non-overlapping sections, corresponding to parts of the national electric “grid”, as shown in the map to the left.  ENERGY STAR matches zip code to map region.

Different regions have different mixes of electrical generation sources (from coal plants, nuclear power stations, hydroelectric generators, etc.)  Because greenhouse gas emissions depend on the sources of electrical generation, this means that different regions have different average amounts of CO2 emissions per kilowatt-hour.

ENERGY STAR of course uses average numbers—at any given moment, the power plants “on line” determine the amount of gases emitted but the averages are a good place to start.  Also, the numbers are a few years old (as of February 2012, the arithmetic is based on data about power plant sources from 2007) so new power plants don’t yet show up in the numbers.

For other sources of energy, like natural gas, ENERGY STAR uses a table of values that describes the components of greenhouse gases that you get when you burn the fuel; to the left, we show an excerpt  from the p. 3 of the technical documentation with the numbers for natural gas, fuel oil (No. 2) and wood.  You can see the relatively small adjustments made for methane (CH4) and nitrous oxides (N2O).

The table numbers do not account for the actual condition of your furnace but are based on average performance characteristics of commercial combustion equipment.

More importantly, the CO2 calculations only cover combustion and do not look at the life cycle environmental costs of the fuel.

In the last five years, extraction of natural gas by hydraulic fracturing (aka “fracking”) has increased dramatically in the U.S.  This increase has driven down the dollar price of natural gas and made it a more attractive fuel for both heating and electricity generation.

For natural gas, the actual extraction conditions makes a big difference in greenhouse gas impact.  Fracking releases some methane.  No one knows how much.   But it is not zero.  As we said earlier, methane is a potent GHG, with more than 20 times the climate changing impact of CO2.

Thus, the ENERGY STAR calculations that focus only on the combustion of natural gas and not the entire production history underestimate the total greenhouse gas emissions associated with the use of the fuel.  At this point, there are only educated guesses for the extraction impacts.

Similarly, the emissions associated with the extraction and transport of fuel used to make electricity are also excluded from the electricity CO2 calculations.

Does this make the GHG emissions numbers reported by ENERGY STAR and used by Energy Stewards useless?

No, for two reasons.

(1) Treat the numbers reported as “at least this much CO2”.  Despite their apparent precision (e.g. “277.3 tons of CO2”) the numbers are underestimates , perhaps by as much as 25% or more.

(2) The ENERGY STAR method is consistent with other protocols used around the world (see page 1 of the technical documentation), so you can use the numbers for relative comparisons (e.g. % change from baseline).

The 19th century scientist Lord Kelvin said the more you know what’s wrong with a number, the more useful it becomes.    That’s our attitude in working with CO2 numbers.

Energy Stewards helps you to think about and account for your building’s impact on the climate by tracking CO2 as well as dollars over time.


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