Energy Storage - Cryogenics

In the world of cryogenic energy storage, there are three types that come to mind: flywheels (with superconducting bearings), SMES, and LAES.

Liquid Air Energy Storage (LAES) is a long-duration, large scale, locatable, scalable and clean technology. LAES systems comprise a charging system, an energy store and a discharging system. The charging system is an industrial air liquefaction plant where electrical energy (off-peak or excess electricity) is used to draw ambient air from the environment and it is then cooled to -196 degrees Celsius until it becomes liquid air (“cryogen”). The liquid air is stored in an insulated tank at low pressure, which functions as the energy store. When power is required, liquid air is drawn from the tank, pumped to high pressure and evaporated. This produces gaseous air that can be used to drive a piston engine or turbine to do useful work that can be used to generate electricity.

Superconducting Magnetic Energy Storage (SMES) provides another example where cryogenics enables high efficiency energy storage. In this case energy from the AC electric grid is converted to DC current, and stored in a coil of superconducting wire. One can associate the stored energy with the electric current and equivalently with the corresponding magnetic field that is produced. SMES projects in Asia, Europe, and the US have been on-going for many decades and provide energy storage for both short and long term applications. Although early projects utilized low temperature superconductors and required the use of liquid helium, today’s devices are designed and fabricated with high temperature superconductors and utilize various hybrid cryocooling systems.

Flywheel energy storage with enhanced efficiency from superconducting bearings is being aggressively developed in many places around the world. Some of the largest storage systems produced in Japan provide 100 kW, 100 kWh capacity with rotation rates approaching 6000 rpm. The superconducting bearings typically utilize YBCO materials cooled to around 50 K. The modular nature of the flywheel storage systems allow storage capacities from 7 kWh to as large as 2 MWh. The stored energy, in the form of kinetic (rotational) energy is roughly equal to the mass times the velocity squared. Because the energy increases as the square of the velocity, systems achieve substantial amounts of energy storage by utilizing lightweight, high strength materials such as carbon fiber reinforced plastic (CFRP), but operate at high rotational speeds, even in the 10,000 rpm range.