A fully renewable grid, purely reliant on wind and solar generation, could require hundreds of terrawatt hours (TWh) of storage capacity to correct seasonal and multi-year imbalances between the supply and demand of electricity, according to a new study.
The analysis, carried out by Birmingham University with Dr Keith MacLean OBE of Providence Policy, shows that electrification of heat could double or even treble the scale of daily and cumulative imbalances across the system.
The “optimum” mix of wind and solar for minimising electricity imbalances differs according to both timescales and the extent to which heat is electrified.
A mix of wind and solar, with the latter making up 15 to 20 per cent, could show some positive correlation to patterns of electricity consumption, even though they are both inherently variable.
Solar generation is partially correlated to electricity demand, which means that an 80:20 wind:solar mix can halve the system imbalances that would be seen without any contribution from the latter.
However, once electrification of heat is added to the equation, these correlations go into reverse.
MacLean told Utility Week: “If we electrify heat those imbalances will be transferred into the electricity system.”
Solar output is seasonally anti-correlated to heat and reduces the benefits wind delivers in terms of cumulative system imbalances.
“A 20 per cent solar component almost doubles the cumulative system imbalance compared to a mix containing none, so completely eliminating solar enables the availability of electricity to better match heat demand,” says the report.
A grid composed only of wind and solar generation would require “grid-scale” balancing of up to 3 TWh on a daily basis, and tens or hundreds of TWh respectively over seasonal and multi-year timescales.
But just replacing the current daily balancing capability, provided by natural gas plants, would require 3-4 TWh of batteries costing over £1 trillion, based on grid-scale projects being installed in Australia and the UK.
Storage options like pumped storage and batteries work well at smaller scales and over shorter durations but fall “several orders of magnitude short” in terms of both capacity and duration as seasonal, grid-scale solutions.
The report concludes that it is “hard to see” how grid imbalances can be managed without extensive use of low or zero-carbon chemical fuels.
“Although these may be more expensive than natural gas, even at ten times the storage and transportation cost, such options would still remain orders of magnitude less expensive than batteries and thermomechanical alternatives.”
The report asks whether it would be more cost effective and efficient on a system basis to use renewables to directly produce hydrogen, which could then be stored and transported, like natural gas today, for flexible generation of electric power and for heat.
MacLean told Utility Week that hydrogen has to be looked at “very seriously” as a vector to transport and store energy.
He added that the system benefits that hydrogen could deliver may outweigh losses of efficiency and greater cost involved in its manufacture.