Battery energy storage systems (BESS) have long been held as a vital part of the shift to renewable energy. Renewables like wind and solar PV are intermittent generation sources and are inherently unable to provide consistent power like generators with a fuel supply. Numerous chemistries exist, with benefits that vary from energy storage density (Lithium Ion to duration of storage (Iron).
The National Electrical Code 2020 edition doesn't actually have a section dedicated to BESS. Instead, there is a more general section related to all types of energy storage systems (ESS) in Article 706. Batteries are covered in NEC Article 480. Some examples of ESS that aren't battery-based (courtesy of the Environmental Protection Agency):
Gravity: Electrical energy is stored as gravitational potential energy by pushing materials (like weights or water) up to a specific elevation. Pumped hydroelectric is an example.
Flywheel: Electrical energy is stored as kinetic energy on a flywheel. The flywheels is tapped into later to extract the kinetic energy.
Thermal: Electrical energy is used to pump heat into a reservoir. A thermoelectric generator later uses this heat flux to produce energy.
Compressed Air: Electrical energy is used to compress a gas at artificially high pressures. The gas is then released later to extract energy.
All of these technologies have their own unique technical challenges. However, from the perspective of the power systems engineer, design around these components is relatively simple. All energy storage systems will come with ratings for input and output currents of their associated devices (e.g. inverters, converters, etc.). In any case, the standard NEC recommendation of sizing to 125% of the continuous load and 100% of the noncontinuous load applies. Likewise, standard overcurrent protection requirements apply.
One unique requirement for energy storage systems relates to a piece of equipment referred to as a diversion charge controller. This particular piece of equipment charges an energy storage system by diverting excess power to a load. This type of system is required to have a load rated for 150% of the charger rating and conductors with ampacity of 150% of the rated current.
Diversion Charge Controller Block Diagram. Red Arrows are the Flow of Power.
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