Let’s set a milestone goal of replacing 50% of our coal and gas fired electricity generation with solar and wind and see what that might require. First we look at the actual generation from coal+gas over about three years to get a feel for the challenge. All of these charts are daily totals for the lower 48 states.
The most notable feature is the seasonal rise and fall of generation. The peak in summer is due to hot weather where spring and, to a lesser extent, fall have the lowest generation rates. Winter has a smaller peak that is much more variable than the summer peak. We can see that there is a definite pattern to our usage through the months and seasons, but also variations that correspond closely with ambient temperature.
Two notable winter peaks are labelled which show abnormally high demand with the first in 2019 and the second in 2021 which became famous as the Texas Freeze. The 2021 peak was really no worse than the 2019 peak but only Texas experienced disastrous outages due to generating plants freezing and stopping. This is the value of a broad electrical interconnection — areas under stress can be supported by surrounding areas leading to increased reliability.
Now let’s look at the actual solar and wind generation over about a year and a half. The chart has two vertical axis, one for solar (left side) and the other for wind (right side). This effectively multiplies the solar generation by 4 to make the total megawatthours generated equal between the two. The result are generation lines that overlap and are easier to visually compare. There is currently four times more wind capacity than solar capacity so this adjustment creates a more apples to apples comparison.
The first observation is that solar has much less variability than wind. Day to day and week to week solar is very boring and regular for output — something that engineers like. Wind has much bigger highs and lows which makes for a greater challenge. We also see that, on a seasonal basis, solar has a flat peak in the summer and a low in the winter. Wind peaks in the winter and has a low in the summer so the two are somewhat complimentary.
The problem is that we have a huge peak in demand during the summer. Solar rises fast in the spring but then flattens out for four months. Wind production decreases during summer so that hurts for meeting that peak demand. We will see in the next chart how that causes a mismatch but I think there is an explanation for that line. Most of the current solar is installed in northern and west coast states which may account for that flat output during the summer. I think (really just guessing here) that when more PV capacity is installed in the south and south west states then solar will have a more pronounced peak during the summer. Matching the supply and demand is the big goal.
We now look at one scenario for replacing 50% of coal+gas with solar+wind in the next chart. The blue line is the coal+gas generation that we are trying to replace with solar+wind shown by the orange line. The springtime portion of the curves show why I selected 50% as they get very close but do not overlap. This means that large amounts of storage or curtailment is not needed which results in the lowest cost situation.
I use solar at 7 times our current installed capacity and wind at 1.3 times our current wind capacity as an attempt to match the demand line as best possible but there are still big gaps in the summer and winter. Solar PV capacity is way behind wind capacity so solar is emphasized to get more summer generation. There is no way to get the orange solar+wind curve any closer to the blue coal+gas curve but I hope that more solar installed in the super sunny states will help.
We may be able to push that a bit higher if we accept some small curtailment during the spring months. The next chart uses 9X solar and 1.4X wind to get 60% replacement and we can go even further with the only downside as increasing curtailment during that mild spring weather.
If this analysis is correct then we can reduce emissions by at least 50% nationwide for electricity generation without needing large scale storage. More than 50% replacement is also possible with increasing curtailment during the spring. Curtailment is strictly a financial factor as increasing amounts of curtailment increases the cost of electricity but reducing emissions may be worth the slightly higher cost and we can install more storage in the future as batteries get less expensive. It is best to match supply with demand as closely as possible but we humans don’t always cooperate. Getting to 60% replacement will take a lot of work but it is possible if we can get past the political roadblocks. An upcoming article will look at going above 50% replacement.