Amidst growing interest in electrical energy storage, it’s often unclear what the it means. Here’s a quick guide to the three main categories.

Bulk storage

Normally located within the transmission network these technologies provide storage and discharge of electricity on the large scale to provide a variety of functions supporting the electricity grid.

Compressed air energy storage (CAES). Air compressed using electricity is stored above ground or in underground vessels. The stored air can be expanded through turbine alternators to generate electricity as required. Some variations pre-heat the air, and combine this with thermal energy storage.

Pumped hydro storage. The most mature of the storage technologies; it comprises two reservoirs. Water is pumped from the lower to the upper to provide gravitational potential energy. Energy is generated by releasing the water and allowing it to flow through the turbine and back to the lower reservoir. Pumped storage systems can have fast response times in the order of seconds, but cannot switch rapidly between charge and discharge due to mechanical inertia of the turbine. The system is charged when demand for electricity is low i.e. at night. Some pumped hydro schemes use the sea as the lower reservoir.

Distributed storage

Distributed storage technologies tend to be smaller than bulk storage systems and are generally used to connect to medium or low voltage electrical distribution networks.

A vanadium redox flow battery – photo Sumitomo Electric

Vanadium redox flow battery. Vanadium ions are present in An aqueous acidic solution, in which vanadium ions are present, is pumped through a stack of cells within which an electrochemical reaction takes place.

The electrolytes are pumped to the cell stack as required and stored in external tanks. One of the advantages of this type of flow battery is that self-discharge is minimal and there is no cross contamination of the electrolytes. Vanadium flow batteries have started to emerge in grid-scale applications.

Lithium ion batteries. One of the fastest growing storage technologies, and the one that is generating most excitement in terms of its use in transport and static applications below 50MW.

The advent of large scale manufacturing, with a significant scaling up expected in the next 5 years, is leading to rapid cost reductions per MW and MWh of capacity.

The Lithium Ion battery pack that powers a Tesla Model S. Source: Tesla

Lithium ion batteries work on the transfer of lithium ions from one electrode to another. There is a wide choice of lithium oxide materials used in the cathode and a smaller number of materials in use as the anode.

Liquid air storage. Air is liquified using an electrically driven air liquefier. To generate power the liquid air is exposed to the ambient air which leads to rapid expansion as the fluid turns to gas. This is used to drive a turbine. One the main advantages of this approach is that it uses processes widely deployed in industry.

Sodium sulphur (NaS) batteries. Molten sodium and sulphur electrodes are combined with a ceramic separator acting as a conductive electrolyte. This type of storage has a high life cycle, long discharge times and quick response capability.

Fast storage

Fast storage systems provide high power for very short discharge durations in the order of milliseconds to seconds. This makes them suitable for applications such as real-time voltage stabilisation.

Super capacitors. These have higher energy storage capacity than conventional capacitors and are capable of discharging over longer time periods. They are able to respond very quickly through charge and discharge cycles and can be used to provide a high output within a very short response time, making them suitable for frequency regulation. Lifetime usage is up to a million charge-discharge cycles.

Flywheels. Electrical energy is converted to kinetic energy by increasing the rotational speed of a disk or rotor on an axis.

Stored energy is proportional to the square of the rotational speed and the flywheel’s mass. Flywheels can be used on their own or combined with batteries for use in uninterrupted power applications (UPS) due to their instantaneous response time. Their ability respond rapidly coupled with up and down regulation makes them suitable for grid frequency regulation functions.