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Long-Term Energy Storage

Long duration ES for a renewable power grid


Energy storage (ES) is key to addressing i) the intermittency problem of variable renewable energy (VRE) sources plugged in the grid, allowing CO2-free dispatchable power, ii) the electrification of the transport sector, and iii) the decarbonization of the global economy.


As a result, several R&D efforts are focusing on making (various sorts of) ES more efficient and cost effective, addressing a market size of USD 435.32 billion by 2030 (includes grid mgmt, transportation, and end user) [1].


Some technologies provide short-term (or short duration) ES (storage of up to 4 hours), while others can last longer.  Today, batteries get all the hype, but they are not the only way to provide continuous power, nor the most adequate to support 100% (or almost 100%) renewable energy in the area you live or even an entire city.

ES classification

ES techs can be classified with respect to their energy to power ratio (E/P), i.e., the relationship btw the ES capacity (MWh) and the power rating (MW) of the storage [2].  ES techs with E/P ratios up to around 4 hours are referred to as short-term.  In this case, the system is able to discharge energy for up to 4 hours at its rated power (maximum continuous electrical power output).


The U.S. Department of Energy (DOE) defines long-duration as 10+ hours of storage capacity [3].  Thus, systems with 4 to 8 or 10 hours are classified as mid-term ES, and above that as long-term ES.  Wood Mackenzie, which excludes Li-ion battery techs from long-term ES, states that a duration of 8 to 100 hours holds great promise as a (low-cost) solution to enable a grid with more renewable sources [4].


Long-term ES techs


Long-term ES (a.k.a. long duration energy storage - LDES - or seasonal ES) systems shift the storage time between charging & discharging on scales of multiple hours, days, weeks, or seasons, balancing supply & demand over long periods of time.  The combination of VRE with long-term ES can create opportunities for transition to a carbon-free energy future.


Pumped hydro, stacked blocks, underground compressed air, and flow batteries are options for long-term ES.  The latter, which can be found in 25 kW to 100+ MW capacities, may give manufactures an edge in utility grid applications.  But they all lack attributes such as modularity and high degree of siting flexibility.


Thermal ES (TES)


Thermal energy storage (TES) refers to the process of heating or cooling a medium to use the energy when needed.  Three kinds of TES systems are available: i) sensible heat storage, based on storing thermal energy by heating or cooling a liquid or solid storage medium, ii) latent heat storage, that uses phase change materials (PCM), e.g., from solid to liquid state, and iii) thermo-chemical storage, which uses chemical reactions to store & release thermal energy [5].


TES systems can approach 100% efficiencies and can be used by all customers using electricity for heating or air conditioning.  Since heating and air conditioning represent a major component of peak demand loads, TES can have a major impact.


Chemical ES (CES)


Classified as seasonal ES solutions, chemical ES (CES) approaches such as i) power-to-gas (P2G), with gaseous energy carriers as hydrogen and methane, and ii) power-to-liquid (P2L), with energy carriers as liquid methanol and (refrigerated) liquid ammonia, have the strengths of high energy density and modularity, and can be somewhat easily converted into electrical energy.


In CES systems, the energy is stored in the chemical bonds between the materials’ atoms and molecules, and the stored chemical energy is released during chemical reactions [6].  The downside of CES is its low round-trip efficiency (E2E < 50%).


CES techs are likely to have a significant role in the decarbonization of the future energy system.  For instance, hydrogen can be key to a strong climate protection either as an energy carrier, molecule, feedstock, or in its derivative chemicals due to its ability to link the electricity sector to transport, industry, and commercial/residential sectors [7].


Energy surplus


It’s worth note that renewable energy not absorbed by the grid, due to peak moments of nighttime wind production and/or periods close to noon in solar plants, can be used to produce green hydrogen.  This process, built around the P2G concept, appears as an opportunity to compensate for the imbalance between energy production & consumption.










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