Low Capacity Factor CO2 Intensity

Can 'zero' carbon sources emit more CO2 than gas?

Figure 1. CO2 emissions per kWh as a function of capacity factor (fraction).

In a recent post, I pointed out that wind, solar, and nuclear have higher CO2 emissions than gas if the capacity factor is low enough. I then backed up that claim with some wrong numbers. Figure 1 shows the corrected numbers, assuming the GKG Base Case CO2 intensities. The breakeven between gas and nuclear is between 0.1% and 0.2%.

These numbers are realizable for backup against unplanned outages and extreme peaking. In the Upper Keys, the Coop's diesel backup normally ran for about 30 hours per year (0.3%), unless a storm came along and took down the line to the mainland. In a fully implemented REPOWER grid, the best thing that could happen for the Coops is that their local backup is never used. That way they avoid paying the higher cost of locally generated power. Think of this back up the way you think of life insurance (which should be named early death insurance).

Figure 1 also shows the numbers for wind and solar. The breakeven capacity factor between solar or offshore wind and CCGT is about 2%. Of course, this is a bogus comparison. Talking about solar and wind for peaking and backup is oxymoronic. But it does make the point that, if you push hard enough for an all renewables grid, you will end up with an fairly CO2 intensive grid. In our German study, we ended up with CO2 intensities in the neighborhood of 150 grams/kWh. An all renewables grid is not even close to a zero CO2 grid.

Figure 2 shows an extreme example where we tried to run Germany on just renewables and batteries. Batteries are so expensive and so CO2 intensive, that the model opted to install massive amounts of wind and solar, and accept all the curtailment that that implied. This pushed the wind/solar capacity factors down to a few percent. The result was an eye watering grid cost of 56 cents/kWh, and an unimpressive CO2 intensity of 161 g/kWh. I admit this is a concocted example. Nobody in his right mind would try to run Germany on wind, sun, and batteries. Would they?

Figure 2. Optimal German Grid, only Renewables and Batteries, SCC=$800/ton. Capacity factors on top of bars is ratio of marketed power to nameplate power. Solid blue is either not available or curtailed.