Electricity supply part 2 - how much storage for 100% renewables?

 Let's "spill" that excess renewable generation into storage

This is part 2 of a short series of posts thinking about energy storage needs for a renewable electricity grid. We'll develop the ideas first, then explore what happens when we change various things, making the results more realistic at the end.

In Part 1, we saw that, for the first week of April 2022 in NSW Australia, once we expand renewable generation much more than about 5 times current generation, there are times we would be generating more electricity than is required by the state. In order to not cause a grid failure, we have to switch off some renewable generation at those times.

There was another time, just before dawn with no solar yet available, when there was very little wind  power being generated across the entire state.

It seems clear that we need to store that energy when it's available in excess, in order to use at times when the sun isn't shining and the wind isn't blowing. 

I'm going to redraw the graphs we had previously, to show what's going on more clearly as we start thinking about storage. Figure 1 shows just two data series, not stacked. The orange is the demand level, and the green is the total renewable generation. This is the same data as shown in Figure 1 of part 1, except we are just showing the renewables total, not the breakdown by type.

Figure 1 - total electricity demand and total renewable (total of solar and wind) generation in NSW Australia for the first week of April, 2022. The difference between the two curves is currently met by fossil fuel generation, but would need to be withdrawn from energy storage in a fully renewable scenario.

Suppose for simplicity that we just keep the current proportions of solar and wind the same, but multiply the renewable output by 4. We can see in Figure 2 below that supply exceeds demand in the middle of the day, and falls below at night. Whenever the green generation curve is higher than the orange demand one we are charging our energy storage system (instead of spilling the electricity), and when the green curve is below the orange curve, we need to discharge from storage to keep the  lights on.

Figure 2 - First week of April, imagining that we've scaled up renewable generation by a factor of four. When the green generation curve is above the orange demand curve we send excess energy to storage, when it is below the renewable generation is not enough to satisfy demand and needs to be withdrawn from storage.

How much storage is needed?

First, lets work out the power flows needed to and from storage, by subtracting the demand curve from the supply curve. If this is positive, it's a measure of the excess power from renewable generation. If it's negative, power is being withdrawn from storage to support the grid. I've shown this in Figure 3, with the area shaded to make it obvious when it's above or below zero.

Figure 3 - storage power required to balance renewables at 4 times the current generation rate in NSW, for the first week of April, 2022

Remember that energy is power multiplied by time is energy. So although the height on the graph represents power (in MW), the shaded area represents energy in MWh. It looks to me like the total energy withdrawn from the storage (area of the blue shading below the zero mark) might be a bit larger than the energy stored (area of blue shading above zero), but we can't be sure just eyeballing the graph.

What we need to do is add all those areas together - to integrate that storage curve to show how much energy we've accumulated since the beginning of the week. Setting the beginning of the week at zero arbitrarily, we can see the energy storage deficit in MWh below in Figure 4. You can see that the energy stored keeps dropping because renewable generation is not keeping up with demand. 

Figure 4 - Cumulative energy storage in NSW for the first week of April 2022, with four-fold increase of renewable energy supposed. Four-fold is clearly not enough to keep up with demand over this week, as the storage level continues to deplete through the week. Compare to Figure 3 - during the day when renewable generation exceeds  energy demand, the storage level increases, during the night it decreases.

This is easily fixed by building more renewables, If we assume a five-fold increase in renewable generation instead of four-fold then the energy stored accumulates over time. If we go with a 4.7 fold increase in renewable generation then we get the result shown in Figure 5, where we end the week with about the same level of stored energy as we started with ("zero"). This means that we have just enough generation such that the average generation over the week matches the average demand over the week, with sufficient storage allowing us to soak up the variability in both.

Figure 5 - Cumulative energy storage in NSW for the first week of April 2022, with 4.7-fold increase of renewable energy supposed. The storage ends the week at the same storage level at the beginning, indicating that, averaged over the week, 100% of demand was met by renewables. The difference between the highest and lowest stored energy states tells us the absolute minimum size for the energy storage - 96,000 MWh, equivalent to about 14 hours of power stored, at state average power consumption.

Where are we at?

So we're getting a picture of what's needed for a fully renewable grid:

• About 4.7 times the renewable generation infrastructure that we currently have installed, so that we generate enough power over the week (Figure 5)
• At least 96,000 MWh of energy storage - able to power the whole state, by itself, for 14 hours (difference between  highest and lowest values on Figure 5)
• Energy storage able to handle discharge rates of 9,000 MW (highest point on Figure 3)

But there are a few problems with what we've done here:

• This analysis is only for the first week of April 2022. I don't expect that week to be special or unusual in any way, but special and unusual times do occur regularly and the grid needs to be able to deal with them. It also needs to deal with seasonal variations that don't show up in a single week.
• We've just scaled the renewable generation all together, we  might be able to do much better if we build proportionally more wind and less solar, for example
• We've assumed that the storage should be able to absorb all of the renewable energy we throw at it, and that the renewable generation should be enough to meet 100% of demand. This might be an expensive solution. It might be cheaper, for example, to over-build the generation by 20% (which means we waste 20% of what's generated) so that we can make do with a smaller amount of storage.

Next time, we'll develop the analysis to answer these questions and get some more nuanced results.