Flexible backup capacity is better option; Need for power storage overstated

Published by Renewables International. View original article

A recent publication by Fraunhofer ISE shows how little wind and solar power would need to be stored at various levels of grid penetration. We are years away from such situations. Before we need power storage, we will need flexible backup capacity. In fact, that’s what we need already.

For my international audience, I should probably point out that Fraunhofer is not a single institute, but a “research society.” The other study I have been talking about this week was done by Fraunhofer IWES of Kassel. Today, I focus on a paper published in November by researchers at Fraunhofer ISE of Freiburg.

The researchers took a look at how much of Germany’s current installed capacity is must-run, meaning that the power plants technically cannot be ramped down any further. While prices on the power exchange plummet as soon as conventional power generation dips below 25 GW, the researchers estimate that the technical lower limit is 20 GW. Now take a look at this chart:

 - Click to enlarge

Going up the Y axis, we see installed wind capacity. The x-axis shows solar capacity. Germany currently has around 33 GW of each, so you can quickly estimate that there is absolutely no need for power storage today, though a bit of wind power is being curtailed – less than one percent. The area between 100 percent and 99 percent is slightly different for wind and solar because all of solar power is consumed around midday, when power demand is generally higher.

There are two percentages in the chart. The colored areas show the share of combined solar + wind in total power supply; the dotted lines, the amount of solar plus wind that can be immediately consumed without being curtailed or having to be stored. What do we see?

First, we can reach around 18 percent solar + wind power (the current level is closer to 13 percent) without losing much power at all. In fact, we would even be able to directly consume at 99 percent (one percent curtailed or stored) at a penetration level above 25 percent. With more than 33 percent solar + wind, we would only be losing/storing five percent of this electricity.

Tweak the amount of must-run capacity, and you get a much different outcome.

 - Click to enlarge

Here, we see that more flexible backup capacity means that the 99 percent dotted line shifts from around 25 percent in the chart above to nearly 40 percent here. We could therefore triple the share of solar and wind power above the current level without any significant need for storage or curtailment. Likewise, the 95 percent dotted line is moved from 33 percent to nearly 50 percent solar + wind.

In conclusion, the need for power storage is years away, and it also depends on the unflexibility of conventional power generation. Put differently, power storage is not needed for solar and wind, but for inflexible coal and nuclear in combination with solar and wind. These findings are not particularly new or surprising; I remember reading similar things in Photon magazine years ago. If Germany started building gas turbines instead of new coal plants, the need for power storage could be pushed into the distant future. The problem is that baseload coal and nuclear power is incompatible with fluctuating wind + solar. (Craig Morris)

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