For Marilynne: Carbon Savings

>> Monday, September 14, 2009

Marilynne asked: Solar. Our inverter for our solar panels says we have not contributed 600 pounds of carbon to the environment in the few weeks we've had it. Where does this carbon come from? Power plants or something else?

Presumably, your inverter has calculated the power you've used via the solar panels and there is a standard value assigned as the amount of carbon burned used per kilowatt hour. For instance, my power company (because I use Green Mountain Energy which is 100% renewable energy) says I've saved 855 pounds of carbon last month. Here's the source for their value: U.S.
Environmental Protection Agency, Energy Information Administration, EGRID 2004 database, Texas State Carbon Dioxide Emission Rate of 1.420 lbs of CO2 /kWh.

In theory, this represents how much carbon dioxide would be produced if that same energy were produced by fossil fuels. Chance are your own are is using a similar value in keeping with local norms. I'm not sure if all fossil fuels produce the same amount of carbon per kW, but it's a reasonable estimate.

So this represents the amount of carbon your utility company is not having to produce to provide the power you used. Congratulations!


  • Jeff King

    yes nice work Marilynne, and thx for the info steph.

  • Richard

    This is another question I couldn't help but chime in on. I wrote a lengthy comment on the post about the cost benefit of solar power. Then I pressed back instead of submit and lost it all. Didn't have the heart to re-write it. But this one I'll get right!

    Carbon auditing is a real challenge. It can be difficult to know where to draw your boundaries. Does the fuel that tanker trucks and railroad engines burned transporting the coal to the power plant count against the electric utility or the freight haulers? What about the machinery used to mine the fuels, or the cars that workers drove to remote mine sites? Like an onion, there are many layers to consider, and the more you peel, the more you cry.

    But I'll leave those considerations aside for the moment and just look at the fuel combustion process. Not all fossil fuels are created equal. Coal and natural gas are the two most prevalent fossil fuels used in generation of stationary electricity (the kind that comes from your electric outlets). Coal plants generate about half of US power, while natural gas plants supply about 20%. This varies from country to country and even state to state.

    Although there are variations within grades of coal and natural gas, coal typically has about twice the carbon intensity of natural gas per unit of energy.

    Why you might ask? You can answer that question if you remember your chemistry.

    Coal is predominantly pure carbon bound together in long chains. (There's also a bit of mercury, uranium, sulfur, and host of other trace elements mixed in at fractions of a percent levels but I won't go into the hazards those represent here.) When we burn coal, we break up those carbon to carbon bonds and release heat. The carbon then binds to oxygen, producing carbon dioxide (C02), the single largest byproduct of coal combustion.

    By contrast, natural gas is mostly methane - CH4. That means 4 parts hydrogen for every one part carbon (Again I'm ignoring the problematic trace elements that contribute to other forms of pollution. I'm also ignoring the fact that methane itself is a pretty nasty greenhouse gas if you let it leak into the atmosphere instead of burning it.) When we burn natural gas, we break the carbon to hydrogen bonds to release heat. The carbon and hydrogen then bind to oxygen to produce one part CO2 and two parts H20 or water vapor.

    This brings us to the end of our simplified chemistry lesson. To sum up: natural gas has lower carbon intensity than coal because there's just less carbon in it to begin with.

  • Stephanie B

    Thank you, Richard. Is suspected as much but didn't do more digging like I should have. But I have some of the best commenters and they often chime in with incredibly useful data.

    The Mother's come through many times as well.

    I'm glad you did. Never hesitate if you have something to contribute!

  • Writer Lady

    Thanks everyone. I just wondered how it was calculated. Many sunny days we put more into the grid than we use. That's payback for us.

    This first month of being solar, our utility bill was mostly add-on costs. We didn't go over the first tier of energy costs. (We are charged more per kWhr if we use more electricity (another tier or two).)

    Compared to last year's bill (when prices were a bit lower) we paid 18@ of our enery costs compared to last year. We also used 20% more energy.

    Thanks for explaining about the carbon.

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