The path forward for storage systems is being written day by day, thanks to research and testing already underway. Alongside the latest generation of flow batteries, there are promising solutions for mechanical energy storage, like compressed air energy storage (CAES) and liquid air energy storage (LAES).
The Challenge of Compressed Air
These two technologies are pretty similar: CAES uses compressed air to store energy, and LAES uses air that is compressed and cooled until liquefaction. In both cases, to ensure adequate overall efficiency, it is essential to accumulate the heat and cold developed during the compression and expansion of the air.
CAES systems have already been proven on an industrial scale, but growth has so far been limited by their need for natural reservoirs, like underground caverns, which are not as widespread and easy to use as we would like. LAES hopes to overcome this constraint and compete with other technologies in the challenge to conserve and reuse renewable energy better.
Flywheels, Gravity and Thermal Energy
Other storage systems of renewable energy in mechanical form are flywheels and gravitational storage systems. Flywheels – where energy is stored in rotational kinetic form – are particularly suitable in situations where a high discharge speed is required and have costs that make it difficult for them to compete with other alternatives. The same is true for gravity storage systems, where large masses are raised, accumulating potential energy, and then make it continuously and effectively available for long discharge durations, through the use of sophisticated control systems.
Another important family of non-battery-based energy storage systems are TES (thermal energy storage) systems. For these systems to function in a way similar to a battery, electrical energy is converted into heat during the charging phase and accumulated, to then be converted back into sustainable electricity during the discharge phase, through a thermodynamic cycle.
TES systems have many variants, mainly in the charging phase and storage system. In the simplest forms, charging takes place through resistors, while the principle of the heat pump is used in the most innovative systems to increase overall efficiency. As for the storage medium, this can range from molten salt, to solid systems based on rocks and cement, to fluidized sand beds and phase transition systems.
Beyond Lithium: What are the Advantages?
In general, these types of storage in development have several potential advantages, guaranteeing them a facilitating role in the future of green energy:
- Use of technologies that are partly established already;
- Absence of significant degradation in storage capacity over the life of the system;
- Greater sustainability: less use of raw materials that are critical from supply, environmental compatibility and safety perspectives.
The systems analyzed have costs that decrease significantly as system size and energy/power relationship increase, and are particularly promising in scenarios where there is greater need for large-scale and long-term storage. This prospect fits well with the need for increased production and use of renewable energy through 2030, as required by international agreements and the energy policies of several countries.