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System flexibility from Medium-Duration Energy Storage

  • Task 42
  • Currently running
  • Energy storage in energy systems


Energy storage will be required over a vast range of timescales to support future net-zero electricity grids. The discharge times range from as little as 1 second to as long as several years – a factor of around 226 seconds. Different types of technology are important for different for different parts of the spectrum. It is easier to describe distinct parts of the spectrum using times rather than frequencies – since the time values are so large relative to the SI unit, 1 second.

The terminology for the different parts of the spectrum is not yet standardised and this presents the first challenge for Task 42. There is reasonable agreement that “short duration energy storage” (SDES) has discharge times up to a few hours and down to a few tens of seconds. SDES systems are very suited to Lithium-Ion and other non-flow batteries. For this part of the spectrum, we need high turnaround efficiency and cycle lifetimes upwards of a few thousand cycles. The key metric is cost-per-unit-power ($/kW).

Below timescales of a few tens of seconds, we encounter “very short duration storage”. (VSDES). Here the priority shifts towards technologies that can endure vast numbers (sometimes billions) of cycles and that can have exceptionally high turnaround efficiency. Flywheels and super-capacitors dominate this area and the key metric is actually cost-per-unit-power-slew-rate ($/(kW/s)).

For discharge times that are much longer than a few hours (and this may actually mean longer than 1000 hours), we clearly need “Long Duration Energy Storage” (LDES) and the key metric of success is very simple – cost-per-unit-of-electrical-energy-that-can-be-released ($/(kWh)). Values in the order of $1/kWh are essential here so that electricity systems that will rely primarily on generation from renewables can withstand large variability in the renewable resources spanning years. Virtually all of the technologies in this space are based on combustible fuels with hydrogen being the main contender. Very large scale thermal energy storage can potentially reach into the bottom end of this. The turnaround efficiency is relatively unimportant for this set of technologies because the storage will be charged-up mainly at times when electricity has low value anyway.

Sandwiched between SDES and LDES is Medium Duration Energy Storage (MDES). At present, the range of discharge times being associated with MDES is 4hrs – 200hrs but these boundaries (with SDES and LDES respectively) are not yet crisply defined and both may need to move. MDES technologies are mostly based around (thermo-)mechanical technologies but flow-batteries also play into the bottom end of this space. MDES technologies may be more expensive per unit power than batteries and their associated power-electronic converters. They may also be more expensive than LDES systems per unit of electrical energy that may be released. Notwithstanding both of these, there is a very important part of the spectrum in which MDES systems are the natural choice.

The main goal of IEA ES Task 42 on Large Scale, Medium Duration Energy Storage is to establish the rightful place of MDES technologies within the array of technologies that are available to support transition to Net Zero electricity grids. Although any electricity grid could technically operate without realising any MDES technologies at all, this is invariably far from being cost optimal and it is vital that policy-makers and grid-operators understand this crucial point. The cost differentials are typically in the order of 5%-10% of total costs between systems that do or do-not include MDES. It is also interesting to note that in the cost optimal solutions, significantly more energy will pass out from MDES technologies than from VSDES, SDES or LDES over any given long period of time – and significantly more investment will be drawn in to realise MDES systems.

At present, information about Task 42 is being communicated through a shared folder on Google Drive accessible through www.TinyURL.com/LS-MDES-task.  This shared folder contains individual folders for each of the following sub-tasks:

ST1:     Definitions and Taxonomy

ST2:     MDES Technologies

ST3:     Regional Dimensions

ST4:     Roles served by MDES systems

ST5:     Market and policy frameworks

You can also find links through www.TinyURL.com/LS-MDES-task to past and forthcoming events.

Task manager(s)

  • Seamus Garvey
  • University of Nottingham, United Kingdom


For requests and information about Task 42, please use the contactform below.