The Energy Storage Conundrum

The purpose of storage is to fill and empty to balance supply with demand. But what if the supply falls short when the storage is empty?

If, like me, your parents sent you to Sunday School so they could have some peace on a Sunday afternoon, you would have heard the story of Joseph, the guy with the multi-coloured coat. Joseph became a hero when he interpreted the Pharoah’s dream and predicted seven years of bountiful harvests followed by seven years of famine.

Pharoah ordered the building of huge grain stores, filled those stores during the bountiful years and emptied them to feed the people during the years of famine.

That’s precisely what the proponents of renewable power want to do with energy. The plan is to store energy when the sun is shining, and the wind is blowing and use that energy to generate power when those other sources are absent.

But what would the Pharoah have done if the seven years of famine had been followed by an eighth year of famine when the grain stores were empty? What would have happened if the famine, instead of starting after seven years of plenty, had started after only three years before the grain stores were filled? How could he have designed his storage without knowing in advance the grain production over the next 14 years?

Unfortunately, designers of energy systems don’t have dreams that predict the future, or boys with coloured coats to interpret those dreams

The chart below has been derived from a study of 37 years of wind and solar capacity factors for the UK. It simulates a wind/solar and hydrogen storage system that has been optimized for the lowest power cost. It shows the amount of energy that would have been in storage over the 37 years if such a system were in place.

You can find a very similar chart in a paper on energy storage published by The Royal Society. The data is from the UK, which has good wind resources but poor solar resources and where peak power demand happens in winter. A chart from California or Australia would look very different, but this one is typical for many of the world’s more populated areas.

I have modelled the storage so that it never goes below zero, and the start and end points have the same quantity of energy in storage. It appears to work fine for the 37 years of data used in the model.

But there is one point in January 1988, when the storage is empty. What would have happened if 1988 had been a year of low wind? Fossil fuel back-up would have been needed.

It takes 12 years between 1988 and 2000 to fill the storage, but there is one period from January 2010 to February 2011 when over 50% of the storage is drawn down in 13 months. What happens if that year is followed by another similar period when the storage is less than half full? Don’t shut down those gas plants, they may be needed.

What if I take the same wind and solar generating capacity and the same storage but start at a different year? This is the chart starting in 1995 and going through to 2017.

As you can see, the storage never exceeds 70% full, and there are several periods where the storage is empty and backup power from fossil fuels would have been needed. At the end of the 22 years, the storage is only 3% full, there is no safety margin if winds are lighter than normal in the following year.

So, a system that appears to work with 37 years of data, doesn’t work with 22 years of data.

The concept of long-term storage is to have the storage full after a long period of high renewables generation and empty after a long period of low generation. But there is no guarantee that the storage can be refilled before the next long period of low generation.

Adding more storage doesn’t help because there is no guaranteed way to fill that storage, except by using fossil fuels, either to make hydrogen or generate electricity, which defeats the purpose of having the storage in the first place.

The only way to solve the problem, without using fossil fuels, is to size the generating capacity to be capable of providing enough power to cover the worst year of low renewables generation, with an adequate safety factor. That would reduce the storage needs but it adds significantly to the amount of wind and solar that must be installed, it results in vastly more curtailment, and it increases the total cost of the system. And it doesn’t guarantee that there will never be a year when renewables generation is even lower than the year used to design the system.

There is no low-cost solution to the energy storage conundrum. Most countries that are pursuing a net zero grid using primarily wind and solar will eventually be forced to conclude that such a grid is prohibitively expensive. They will either retain their fossil fuel as a backup or turn to nuclear power.