Long Duration Energy Storage – The missing link in America’s decarbonization pathway

Representatives from Hydrostor, Energy Dome, and Sumitomo – all LDES Council members – presented on their technologies and the importance of long duration energy storage for decarbonization in a closing panel session discussion at the Energy Storage North America conference on January 18.? This is a summary, with some key points highlighted, from that discussion.

• Long duration energy storage resources provide reliability to the grid

Grids that are decarbonizing need solutions to maintain grid reliability, without building generators that burn fossil fuels.? When initially decarbonizing legacy fossil fuel burning resources, grids may have relatively small needs – i.e. generating capacity equal to the retiring generator during very specific critical times on very select days.? These gaps can be filled by storage resources with relatively short durations.? As more traditional capacity retires, longer durations of storage resources become necessary.?

Long duration energy storage resources are necessary for least cost decarbonization.

Fully decarbonized, grids require long duration energy resources.? For example, if a grid only generates power from solar generating resources, generation occurs for 8 to 12 hours per day during periods when the sun is shining.? This system would require storage resources with durations of at least 16 hours to ensure that demand is served during all times of the day.? Longer durations would be required to serve energy in periods when there is persistent cloud cover or extreme weather threatening other elements of the system.

• Long duration energy storage is viable and ready for deployment

The Hydrostor, Energy Dome, and Sumitomo solutions are all commercially viable and have demonstration projects deployed around the world.? Full scale projects are being developed and some have anticipated delivery dates later this year.? The technologies utilized by these three companies are similar – each are mechanical storage solutions using compressed gasses or liquids – and provide “mid-duration” storage solutions providing continuous output for up to 8-20 hours.?

Hydrostor’s technology lends itself to larger projects (100+ MW) utilizing sub terranean caverns, while Energy Dome and Sumitomo have above ground storage reservoirs for building projects with smaller outputs (20 MW-50 MW).

• These long duration storage solutions offer more than simply energy

Each of the technologies from these companies have machinery with rotating mass.? This can be an essential feature for grid stability, as rotating mass has been the cornerstone for maintaining a specific operating frequency for decades.? Deviations from operating frequency targets can cause costly disruptions to downstream users of energy from the grid, and thus operators must follow strict rules to maintain specific operating frequency.? Most of the traditional fossil resource retirements have rotating mass and contribute materially to grid stability. Replacement inverter-based resources, like batteries and solar, do not have rotating mass and could increase the potential exposure to frequency deviation.? Some work has been done on synthetic inertia, which would allow these inverter-based resources to help maintain target system operating frequencies, but these solutions are not yet ready for widespread deployment.

• Grid scale projects benefit from tax credits, but more needs to be done for buildout

Grid planning models must include long duration energy storage

Defining solution for decarbonizing grids begins with clearly defining the problem and understanding options for potential solutions.? For most grid planning, this means that long duration energy storage must be included in resource mix considered for decarbonization.? Thus, models for decarbonized grids must be constructed in a way that considers procurement of short duration storage solutions, long duration storage solutions with 8-20 hour of discharge capability – like the technologies discussed in this panel, and long duration storage solutions with very long 100+ hour durations.? Further, these models must take into consideration time challenges to get new resources deployed, challenges from retiring existing fossil resources, and the critical need for grid reliability while this transition happens.? Developing planning processes that are two short sighted will result in sub-optimal costs for procurement and missed decarbonization goals.

Existing resource adequacy constructs do not sufficiently differentiate between short and long duration storage

Resource adequacy models that do not call out specific values for buildout of storage capacity with specific durations should not expect sufficient buildout of storage.? It is very difficult for utilities and end-users of energy to contract for long duration energy storage when frameworks do not require these resources.? Prices for 8-hour duration storage – for lithium-ion and these long duration technologies – tend to be higher than prices for 4-hour duration storage resources.? If there are no other differentiators or requirements for these types of storage it is irrational to purchase resources with longer storage durations.? A significant amount of work needs to be completed to ensure that long duration energy storage is procured in correct amounts for grids trying to decarbonize.

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The landscape for storage, and particularly long duration energy storage, is changing rapidly.? New technologies are maturing and preparing for grid scale commercial deployment.? Development and deployment will lead to realized economies of scale and put downward pressure on costs for these technologies.? This will drive expanded growth and development.? Utilities, grid operators, and regulators serious about decarbonizing must stay agile while planning decarbonization pathways and must arm themselves with advanced modeling techniques and the most current information on existing technologies.

This article was contributed by Gabe Murtaugh .

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