Renewables and Grid Level Storage (updated Dec 2022)

Greg Brougham
4 min readSep 30, 2022
Photo by Matthew Henry on Unsplash

I’ve mentioned earlier that this decade is regarded as one of storage and discussed local or distributed storage but that leaves the question of storage at grid level storage and in particular the issue of curtailment.

US residential home consumes on average the order of 29kWh/day so a small 150MW coal station can supports the order of 5,000 homes and we are seeing large solar and wind deployment of several hundred MW and some GW deployments. Still, the issue is a coal fire station can run for a whole day while renewables tend to be peaky in nature. I know I’m comparing an average with effectively the peak generation capacity but we’ll come back to that.

This peakiness leads to what is called curtailment when the electricity operator asks the renewable generators to go offline, that is to disconnect from the grid. There are two issues here and the first is that obviously this is electricity that is wasted and the other is that operators are often required to reimburse the generators as they contracted at a fixed price for the supply. This cost can be hundreds of millions on an annualised basis at the moment. Form Energy analysis is that curtailment in California has increased from an average of 2.5 to 9.5 hours since 2014 and in 2022 and curtailment totals 1,860 GWh of wind and solar to date in 2022.

So the answer is obviously some form of storage that allows these peaks to be leveled out so that renewables can provide more of the base load and to avoid curtailment. This is where grid-level storage comes in and there are several options here but I’m discounting hydrogen production due to its low overall efficiency :

  • You can of course look to add some form of battery storage to the renewables
  • You can use another medium such as gas and there are several options here
  • Or you can look to combine generation with another ‘renewable’ to provide something like pumped hydro

The other thing that grid-level storage offers when combined with renewables is the ability to cover peaks. For example if you have 100MW of generation capacity and 50MW of storage you can for a period of time supply 150MW of capacity. Not only does storage allow you to extend coverage but it can help you meet peaks that exceed our capacity. This is basically what they have been doing with the ‘distributed’ grid in California over the summer.

Grid-level batteries have been in the press and one that got the most press coverage is the one deployed in Southern Australia as the result of a Twitter exchange between Atlassian chief Mike Cannon-Brookes and Elon Musk. This was a lithium-ion battery and these types of chemistry are more suited to supporting the operation (stability) of grids than storage. At scale there are other chemistries such as redox flow batteries that are more appropriate. Those based on Vanadium offer better economics of scale and also very high cycle expectancy, leading to an expected life of 25+ years. It is worth noting that the Chinese are due to bring a 200MW Vanadium flow battery online in October 2022 which will be the world’s largest redox flow battery to date. A recent article by Wood McKensize incorrectly characterised this as having 4 hours duration but this is missing leading as they have just used the initial 100MW/400MWh capacity to deduce this number whereas the technology support longer duration storage. If the energy draw was 1/4 of the storage capacity it would support 16 hours and so on.

With regards to other medium Energy Dome’s novel CO2 storage system looks very interesting as it does not need the cryogenic temperatures needed for air for storage and the architecture is modulator in nature offering 50MW, 100MW and 200MW of capacity. This show promises and offers round trip efficiencies of 75+%.

And we are seeing increasing use of pumped hydro energy storage (PHES), in particular in Australia. The main constraint here is the geology and this makes sense where there is an existing hydro facility or the natural geology offers a height differential that can be leveraged. One interesting development here is RheEnergise development of what they refer to as high-density hydro using a mineral-rich fluid that is 2.5 times denser than water. This means that the head or height difference only needs to be the order of 100m so is not so constrained from a geology perspective and can in principle be localised (my italics).

The bottom line is the deployment of grid-scale renewables without some form of storage makes no sense so why is there not a policy to ensure this in the UK and why are the electricity operators not looking at subsiding their installation instead of making curtailment payments?