Carbon Intensity of the Dollar

The debt of clean energy

by Matthew Formby

If a project meant to lower carbon emissions promises to immediately help the environment--it's probably being overly optimistic. Unless its energy, manufacturing, transportation of employees and resources... essentially too many things to account for, are all from carbon-free sources. So, with our currently carbon intense diet, even building new wind, nuclear, or solar will start with a carbon debt to pay off with its clean energy. 

To keep track of how such a project pays off this debt, two kinds of carbon intensity can be used. First is that of a places GDP (CO2/$), because any economic activity spurs activity in emissions sources. The second is that of an areas energy (CO2/KWh), which is a baseline of how much more carbon would be produced without the clean energy source. Turned around, the carbon intensity of energy production is like the rate at which a clean energy project pays off its carbon debt.

The U.S. has a carbon intensity in its GDP of 370.4 tons of CO2 per million dollars spent, or just under 3/4 of a pound per dollar(1). Other sources have this a little bit lower on average, at as low as 2/3 of a pound of CO2 per dollar(2). With this, you can even see how something like going to the movies may cause a few pounds of CO2 to be emitted--from running the lights, cooking popcorn, and so on.

So a 1,000 MWe nuclear power plant, which may cost around $5 billion to construct would begin with a carbon debt of 1.85 million tons. After including the average costs from decommissioning the nuclear power plant ($500 million(3)), including site restoration and fuel handling, the total carbon debt goes up to 2.04 million tons. A utility scale solar plant at similar capacity would cost about $3.42 billion ($3.42 per watt(4) * 1 Gigawatt). The solar plant's carbon debt would be less, at about 1.27 million tons of CO2. These are very rough estimates based off of averages only, but the general comparison will not change drastically.

Though solar starts with a much smaller debt, its capacity, and as a result it's repayment is misleading. The key is how quickly and reliably these debts are repaid. The current grid heavily favors dispatchable energy, which can be called to respond to changes in demand and offer power at any time it may be needed. Solar, wind, and hydroelectric instead rely on resource availability, or provide the same, constant amount. The other problem is that the example 1 GW solar plant only gets that much energy at peak times of day for the season and faces drops in efficiency as soon as a cloud comes by.

Because of these favorable conditions for dispatchable energy like nuclear, and the unfavorable conditions for solar, their capacity factors are generally around 90% and 25% respectively(5). Again these numbers are broad, and can vary significantly depending on the area and technologies used. Using U.S. average carbon intensity of energy generated(6), it would take the nuclear power plant 5.5 months to pay off its carbon debt, and the solar plant 12.3 months. These are rough numbers, and could be shifted by omissions like operating costs, which for nuclear, wind, and solar (with the exception of solar thermal) are very cheep. Also, the carbon debt isn't to say that both projects were truly causing more carbon emissions before that point, as they both should have been filling an energy need that would have otherwise been filled by even more carbon intense energy.

A sidebar:

Often, natural gas is supported even by environmentally conscious people because it's both cheap (often less expensive than coal) and produces far fewer emissions. It can even help compensate for intermittency issues of clean renewables. To see just how long it would take for a new natural gas plant to really create a positive impact on the environment, consider these figures for an advanced combined cycle natural gas power plant:

Capital cost: $917 Million(7)
Carbon Debt: 339,644 Tons of CO2
Capacity: 1 GW
Capacity Factor: 87%
Carbon Intensity: 1.22 pounds per KWh(8)
Monthly Emissions Prevented: -24.4 thousand tons (a net increase in carbon emissions) or 39.1 thousand tons(9).

Emissions prevented is a difficult number that makes certain assumptions behind the scenes, such as the efficiency of heat energy conversions into electrical energy. The first number uses a baseline of 571 tons of CO2 per KWh from the EPA source used throughout this article. The other number makes the same calculation using 672 tons of CO2 per KWh from the IPCC. Both sources are generally reputable, and the discrepancy could have to do with the methods used or year of publication. 

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