Long Duration Energy Storage

Grid options: beyond lithium-ion battery energy storage systems

Long duration energy storage (LDES) technologies are potentially more cost effective and safer alternatives to lithium-ion batteries for power grids that are increasingly reliant on intermittent renewable generation. The US Department of Energy (DOE) defines LDES technologies as systems capable of delivering energy for at least 10 hours at the system’s rated power. This includes intra-day, multi-day (more than 100 hours), multi-week and seasonal storage systems. A distinctive characteristic of LDES technologies is the potential to scale up energy storage capacity at significantly lower marginal costs compared to lithium-ion battery energy storage systems (BESS). Most LDES technologies are in the pilot/demonstration phase with some, such as redox flow batteries, already scaling up manufacturing capacity.

Applications and use cases

Applications refer to the services rendered by an energy storage system while use case refers to how the storage system is cycled (used). LDES technologies can be connected to the grid (grid-connected) or be independent of the grid (off-grid). LDES serves grid and off-grid applications such as renewables integration, transmission and distribution deferral and microgrid resiliency. Renewables integration entails energy shifting (storing excess renewable energy production for later use) and capacity firming (reducing fluctuations of renewable resources).

Use case covers factors such as the number of cycles per day, the depth of discharge (partial or full) and environmental conditions to be maintained within the storage system during both operation and rest (typically temperature and humidity).

Technology maturity

Technology readiness level (TRL) and commercial readiness index (CRI) are measures that capture a technology’s level of maturity. The two scales are coupled with CRI being the senior of the two. TRL has 9 levels with TRL 1 reflecting basic laboratory research and TRL 9 indicating system demonstration. On the other hand, CRI has 6 levels - CRI 1 shows that technical aspects of a technology have been proven while CRI 6 rating indicates a technology whose risks, standards and performance are well understood and whose uptake is primarily driven by market forces such as price and demand. Novel technologies typically depend on subsidies and various protection mechanisms for survival during their infancy.

LDES technologies fall under 4 main categories:

1. Electrochemical

2. Mechanical

3. Thermal

4. Chemical

Electrochemical LDES

Electrochemical LDES technologies leverage reduction-oxidation (redox) reactions similar to those utilized in lithium-ion batteries. These tend to be battery technologies that leverage metals other than lithium, for instance, vanadium, nickel, zinc, iron etc. Electrochemical non-lithium ion batteries are divided into four main categories: redox flow batteries (RFBs), metal-air batteries, high temperature batteries and zinc static batteries. Of the four categories, RFBs have achieved the highest commercial success so far (approximately CRI 5).

Mechanical LDES

Mechanical LDES technologies are the leading category in terms of TRL and CRI. In particular, pumped storage hydropower (PSH) has a TRL of 9 and CRI of 6. This category boasts the largest installed capacity despite having much fewer deployments compared to BESS. In addition to PSH, this category contains technologies such as compressed air energy storage (CAES), liquid air energy storage (LAES), compressed CO₂ energy storage and gravity-based energy storage (involves raising and lowering large masses such as bricks). The biggest hindrance to scaling most mechanical energy storage technologies is geographical constraints. PSH can only be built where there are suitable water reservoirs and terrain while CAES is specific to areas with salt caverns.

Thermal LDES

Thermal LDES technologies capture and store heat in three main media: sensible, latent and chemical. Sensible storage media are substances whose temperature changes with heat addition and removal. This includes molten salts, bricks, water, rock material and concrete. Latent storage media store heat by undergoing phase change instead of temperature change. Examples of such materials are water, paraffin, aluminum alloys and eutectic hydrate salts. Thermochemical storage media undergo reversible chemical or sorption reactions that absorb and release heat. Thermochemical storage substances include potassium oxide, lead oxide, zeolites and silica gel.

Chemical LDES

Chemical LDES technologies tend to utilize hydrogen or hydrogen derivatives such as metal hydrides as storage media. A green hydrogen energy storage system consists of electrolyzers (for hydrogen generation), hydrogen storage tanks and fuel cells (for electricity generation). Electrolyzers consume electricity during supply surplus while fuel cells release electricity back to the grid during periods of high demand.

Conclusion

Herein, we have looked at four modes of consuming, storing and releasing electrical energy back to the grid:

The next few articles will explore each of the four categories of LDES in greater detail.