xx. Energy Storage

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About

Notwithstanding the advantages, Renewables have a peculiar problem called intermittency problem which means it is not available round the clock.
Thus, energy storage becomes an important component of renewable shift.
The most important energy storage strategy is batteries.

Grid-level Battery Storage: Flow Batteries

The batteries we have discussed in the battery section including Li-ion batteries are not suitable to store energy for longer time.
The energy storage at the grid level requires us to store energy atleast for 8 hours (night).
The alternative suitable for grid-level storage is flow batteries.

Flow batteries

The basic difference between conventional batteries and flow batteries is that while in convectional batteries we derived chemical energy of the electrodes, in flow batteries the energy is derived out of the flow of electrolytes.
A flow battery uses two electrolyte solutions separated by a membrane to store electrical energy.
A flow battery consists of two tanks – one for the positively charged electrolyte solution (called the catholyte) and one for the negatively charged electrolyte solution (called the anolyte).
During charging the catholyte and anolyte flow on opposite sides of the membrane which is like going uphill.
During discharge, the process is reversed. The catholyte and anolyte flow back into their respective tanks, and the electrical energy that was stored in the battery is released to power a device or a system.

Advantage

Flow batteries are unique in their scalability which makes them suitable for grid-level storage.

Super Capacitors

Principle

A capacitor is a device that stores electric charges thereby electric energy
It is a set of two metal surfaces separated by an electric insulator.
Put positive electric charge on one plate, negative on the other, and the combination can store energy for a long time, much longer than batteries can.
Add more electric charge on the plate and you store more energy, but you also raise the voltage.
The trick for storing energy in capacitors is making the insulator very thin, so that you can have lots of energy per unit volume while keeping the voltage low.
These high energy-density capacitors are called supercapacitors or ultracapacitors.

Advantages

Since capacitors don’t depend on chemical reactions, they can release their energy extremely quickly thus suitable for EVs
Not subject to degradation with use and time.
Because they can be charged so quickly, supercapacitors can be used to improve the efficiency of regenerative braking; they absorb the energy in charges and release it when needed.

Disadvantage

Supercapacitors can store about 1/3rd the energy of a same-weight lithium-ion batteries.
Costs 3 times more than Li-ion batteries.