Generating renewable power is vital to the world’s decarbonisation efforts. But so too will be developing the energy storage systems that are required at times when the intermittency of solar and wind power means that energy isn’t being produced.

Energy storage technologies already exist, of course. They span a wide range of time scales and requirements for flexible operation. This indicates that some energy storage technologies are more suitable for certain services than others.

But those with longer durations of days, weeks, and even months — long duration energy storage (LDES) – could enable cost-effective, deep decarbonisation of electric power systems, while ensuring high system reliability.

Lithium-ion batteries have been stealing the spotlight in electric vehicles and stationary energy storage sectors in the past few years. However, Wood Mackenzie understands that they are economically uncompetitive when it comes to long-duration energy storage applications, defined by periods longer than eight hours.

In addition, lithium-ion batteries have safety and sustainability issues. Extra measures are required to predict and prevent thermal runaways of lithium-ion batteries. Finally, high recycling costs and stubbornly high battery prices combine to weigh on the sustainability side.

There is a range of LDES technologies available, in particular redox flow batteries, metal-air batteries, thermal energy storage and mechanical storage technologies. Each is at different levels of maturity and market readiness.

Yet while they are promising, efforts should be made to reduce the costs of such technologies, while allowing them to be deployed on a larger scale in order to accelerate commercialisation.

Redox flow battery (RFB) technology

Redox flow batteries feature easy scalability, a long service life, and high operational safety, making them suitable for stationary storage. The key components of redox flow batteries are the circulating electrolytes and a species-selective membrane. A typical limitation of RFBs is lower energy density, compared to lithium-ion batteries.

All-vanadium redox flow batteries (VRFB) and zinc-bromine (Zn-Br) are the most researched and developed RFB chemistries.

The advantages of flow batteries include flexibility, longer duration, increased safety, lower levelised cost of storage, longer asset life and less concern when it comes to ambient temperatures.

Yet on the other hand, both of these types have so far only been developed on a small scale. In addition, both come with some technical and commercial challenges:

VRFB:

VRFBs have been the most deployed RFB technology so far. However, they only account for a tiny percentage of energy storage systems under development.

Faster market adoption of VRFBs has been hampered mainly by cost, including relatively high upfront capital costs — a situation that persisted in 2022. Costly chemicals, due to high and fluctuating vanadium prices and the use of expensive membranes, are also significant challenges.

Rapid scalability is needed in order to reduce costs and realise the potential of this technology. And while some flow battery developers are developing vertically integrated business models – by entering parts of the raw material supply chain, for example – this trend needs to be accelerated.

On the technical side, there is a need to develop novel membranes with higher ionic selectivity and conductivity. In addition, battery design can be improved to boost efficiency and energy density.

Zinc-bromine:

The zinc-bromine redox battery offers a high cell voltage for aqueous systems, exhibiting a relatively high energy density among flow batteries.

However, there are at least two technical challenges. One is to mitigate both hydrogen evolution and zinc metal dendrite formation, which reduce performance and can cause safety issues. Also, current zinc-bromine systems use expensive chemicals to reduce toxic bromine vapor emissions.

Commercially, more efforts are needed for zinc-bromine redox battery development as VRFBs have been the centre of RFB commercialisation work over the past decade.

Metal-air batteries

Metal air batteries are used to store electricity over multiple days. They have a theoretical energy density that is much higher than that of commercial lithium-ion batteries.

The materials involved (like zinc and iron) are safe, cost-competitive, and abundant on the planet, with the use of aqueous electrolytes. As a result, materials costs and associated system-level energy costs are low.

Yet such batteries have not fulfilled their full potential due to challenges associated with the metal anode, air cathode, and electrolyte. The round-trip efficiency of metal-air batteries is also lower than that of lithium-ion and flow batteries.

Thermal energy storage and mechanical storage

Compared with most other forms of storage technologies, thermal energy storage, like hot rocks energy storage, shows the advantage of using cheap and abundant materials. The main challenge for thermal energy storage technologies is converting heat back into electricity in an efficient and cost-effective way.

Pumped storage hydropower and compressed air energy storage are the two most discussed mechanical storage technologies. The energy density of mechanical storage technologies is much lower than that of electrochemical and chemical storage.

In addition, most mechanical storage technologies have to be established at sites with suitable topography. For example, a pumped storage hydropower station needs two water reservoirs at different elevations and a slope between the two reservoirs.

In many parts of the world, LDES technologies are also facing various financing, market, and policy barriers that limit deployment.