Calculating the Need for Storage

Calculating symbols

Current Calculations

Currently each country and grid calculates its need for storage in very complicated manner, by creating a model with various scenarios, projecting different generation mixes all based on huge assumptions as to what will be rolled out in future. Using the UK’s National Grid’s annual Future Energy Scenarios as an example, every year their estimate of the storage needed by 2050 increases over the previous year’s estimate.

In round numbers it is currently at 20-40GW storage (across all scenarios, including the do-nothing “steady progression” scenario) for a projected 80GW grid, with that requirement still rising. This begs two questions:

  • Is there a simpler way to calculate storage requirements;
  • If forecasts are increasing continually, what is the end point to which they are trending?

Urgency of the Need

This is vitally important because if the requirement is underestimated, insufficient political, financial and operational investment will be put into it. Large volumes of storage will require a massive roll-out of new plants, however large these plants are. Lead times for financing, building and commissioning large-scale long-duration storage are long, so work needs to be started soon if this roll-out will be sufficient to achieve 2050 emissions targets.

There are two factors to be calculated: power (GW = Gigawatts) and energy (GWh = Gigawatt hours). It is easiest and clearest to calculate them separately. They must be calculated for the longest low-generation period forecastable – i.e. the kalte Dunkelflaute, which is a regular weather pattern (every 2 or 3 years) in which a high pressure system stations itself over most of Western Europe for periods of up to two weeks mid-winter (i.e. during peak demand and minimal solar generation) and during which maximum renewable generation is reduced by 90% or more.

Calculating Storage Power

Stored power = {peak demand} + {10-15% supply margin} – {total zero-carbon dispatchable generation capacity}.
The supply margin exists to accommodate failures in any part of the network or the plants attached to it, and/or unusual spikes in demand.

Calculating Storage Energy

Stored energy = {total demand} – {total zero-carbon dispatchable generation}.
This should potentially be up-rated for (a) deterioration of stored energy such as battery self-discharge or cooling of stored heat, and (b) any possibility of a follow-on extreme weather period before the stores are sufficiently re-charged.

Calculating Plant Sizes

Actual plant sizes will vary. Some plants need to have sufficient duration to operate as baseload during the weather pattern; others will provide for variable demand, at various utilisation rates. This is not a rigid distinction: shorter-duration plants, for example, can be used in a relay through the period, and plants can operate sometimes for an hour or less, and at other times for many hours.

The best way to calculate actual plant-size requirements is to model both supply and demand during the most extreme weather event. This should be upgraded on consideration of:

  • Any degradation of stored energy, e.g. thermal cooling, or battery self-discharge;
  • Any probability of a follow-on extreme weather period occurring before storage is sufficiently re-charged.

The final stage is to input the actual projects that are proposed, and re-run the scenario / calculation to determine the sizes of plants that remain to be developed.

What if the Target is Not Net Zero?

There are two answers to this: technical and strategic.

Technically, add all the permitted emitting generation to the total dispatchable generation factor in the two equations.

Strategically, the electricity grid is much easier and cheaper to decarbonise than many other sectors such as aviation, shipping, heating, industry. Therefore to minimise the cost and disruption of the energy transition we should target Net Zero grids in order to permit some excess emissions from other sectors, to enable the entire economy to meet 2050 targets.

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