Friday, September 26, 2008

Carbon Footprint Size Varies

A recent University of Washington study found that when the same values were used with 10 different online calculators, the results varied greatly. In one category, the bottom line for a typical American homeowner varied by more than 32,800 pounds of carbon produced per year.

One reason for the wide range is that emissions from air travel, the couple's largest carbon producer, are often calculated very differently, said Clark Williams-Derry, research director at Sightline Institute, a nonprofit research center that studies carbon calculators.

Another part of the variation stems from the fact that different calculators include different behaviors in their calculations. For example, some ask for the amount of garbage a household produces each year, while others use the national average (1,606 pounds), and others don't include garbage at all.

Another reason for the differences is that most calculators use different internal numbers as conversion factors and standards to come up with their results, Steinemann said.

"There are so many different ways to calculate, using different variables, different standards and different assumptions," she said. "There's no one absolute right best number, so each calculator seems to use something different."

Web sites offering the calculators also often don't let the user see what those numbers are.

"The other problem is that individual calculators don't tell you the assumptions behind their calculations," Steinemann said. "Even if there isn't one standard calculator, they should at least be able to be transparent, so people know what's being included, what isn't being included, and what's not being calculated."
Given how complex it is to calculate a footprint and all the assumptions involved, I am not surprised that there is so much difference. I think the calculators do need to be more transparent and let you know exactly how they calculate their value and what assumptions were made.

Links to 10 carbon calculators in the article.

via Seattle PI


Rebelfish said...

After reading this, I went to the "impact" calculator Facebook app with the intent of removing it, but I found that CarbonMinder actually explicitly tells you where the numbers come from.

Fat Knowledge said...


Yeah, as long as they state the assumptions then I think it is useful.

Oh, and I finally got a chance to read that Solar Power without Storage document you told me about a while back. Interesting stuff, thanks for sharing.

The one thing I didn't understand though is if you could reduce the number of dispatchable natural gas plants if you had more solar power, or if you would still need the same amount because there would be a few times where the sun wasn't shining and then you would need to go all nat gas.

If that is the case, then you would really just be substituting solar power for the natural gas that didn't need to be burned, and I wonder how the numbers would look in that case?

Another way of looking at this would be that the capacity factor of the natural gas plants would go down, so that capital costs of building the plant would need to be recouped somehow, maybe as higher electricity generation prices.

If on the other hand you could decrease the number of nat gas power plants needed, then solar with storage would be more economically competitive.

Rebelfish said...

Hey, thanks for reading.

What I look at in the first case is that all of the baseload plants would continue to operate as they do now. All of the dispatchable plants would still be maintained and ready to go, but the amount of energy produced would be reduced (and so then would fuel costs and emissions). Generally, we couldn't reduce the number of plants because in the early evenings, the demand is still fairly high, but the sun is not.

Because the capacity factor of the gas plants goes down, the cost per kWh goes up. However, natural gas plants are actually quite cheap to build (per MW capacity) compared to coal/nuke. Since they are basically an enormous jet engine hooked up to a generator, you don' need to worry about building all steam ducts and cooling towers and 12 story coal furnace or nuclear reactors. So with a high fuel cost and (comparatively) low capital cost, the cost-vs-energy graph has a much lower "y-intercept" than the baseload plants.

However, at some point, the reduced operation will bring the cost/kWh up where it's too high and a few plants will be dismantled and replaced by batteries or compressed air storage. On the other hand, the utility graphs of electricity production cost versus percent of plants running looks something like this: running those last 20% of plants that are only used in demand emergencies costs almost as much as running the remaining 80%. So if those last-resort utility-scale diesel generators ran a lot less, I don't think anyone would be too upset.

And I agree that storage will eventually be needed for solar (and especially wind), but that time isn't yet. I'm working on a expanded version of the paper now (don' worry -- I won't ask you to read it) where I'm looking at the current curve of solar panel production and estimating the time until the 8% of all energy comes from the sun, and storage is needed.

Fat Knowledge said...


I guess what I was wondering is: if you assume you can't get rid of any nat gas plants, then could you look at the feasibility of solar as follows:
1) determine # of kWh of peak power being produced by solar cells (power created at low demand times is just thrown out) in a year
2) determine how much nat gas would have been needed to provide that peak power
3) take that amount of nat gas * price of nat gas to give you the amount of money saved by using those solar cells
4) taking a look at how long you think the solar cells will last, and the price of the solar cells, determine what the ROI of the investment will be
5) compare ROI with other investments to see if a good place to put money

Difference here being that the solar cells are just used as a way to offset fuel, not as a way to decrease number of power plants.

I am interested to know what you conclude on batteries. I also wonder if batteries get cheaper, will it be more cost effective to just use base load plants + batteries and not use peak load plants at all? Run your coal and nuke plants a little higher, save up the electricity that isn't being used in low demand times in batteries and then use it at peak times.

Rebelfish said...

In my report, I'm assuming that 95% of the power comes at peak times and 5% when the demand is too low for it to be useful. I'm finding that the 95% is equal to a little over 300 billion kWh each year (7.9% of the total electricity consumed).

If the installed cost of the panels is $5/W, repaying the whole cost over 25 years (which is what most panels are warrantied for) with 5% interest puts the cost at $.17 to $.26 per kWh. According to this paper (the graph on page 40), peaking plants cost between $.15 and $.30 a kWh to run, which is fairly similar. (On the other hand, a year ago gas cost $7.72/mmbtu, which is $7.32/GJ. When run thru a 30-45% efficient plant, that works out to 10 to 7 cents/kWh. Maybe the study includes labor and maintenance costs.)

But burning natural gas also produces 1.314 lb CO_2/kWh. If a CO_2 tax of $50/ton is brought about, that adds about 3 cents/kWh to natural gas's cost.

I suspect that batteries will not get cheap enough to replace peak plants, but mechanical energy storage will. There's actually a CAES plant in Alabama that uses electricity from nearby coal plants to compress air at night, and then use that "pressure energy" during the high-demand day. And there's lots of pumped storage hydro plants in the US (but most of the good locations to build those have already been taken).

I did find that the maximum electricity consumption always occurred when there was more than a little sun. If we assume existing power plants are perfectly sized to the demand now, the addition of these solar fields mean the grid could support about 5% greater demand than it does now. I didn't report this in my paper because it assumes that any increase in consumption would simply be a linear scale factor applied over the entire year, which isn't necessarily true.

Fat Knowledge said...


Thanks for the good info.

Interesting to know that only 5% of solar power comes when demand is low.

On the nat gas prices, I wonder if labor and maintenance costs would go down with capacity factor, or if they are more or less fixed so that really it is just the nat gas itself which is on the margin. It isn't clear to me whether to compare the solar with the 7-10¢/kWh of nat gas that isn't used or the 15-30¢ of cost to run a plant. But that assumption is key to the conclusion of the report.

Thanks for the info on batteries vs. mechanical energy storage. I wonder if the economics look good for further investment in mechanical storage right now. Start it off as a way to utilize more coal and nuke power right now, but then allow for transition to intermittent solar and wind in the future.

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