Storing large amounts of electricity for very long times is fundamental to increased uptake of intermittent renewable energy such as wind and solar, without resorting to backup such as natural gas. Energy storage has been widely used by power utilities for many years to help regulate the electricity supply. Its use to store electricity produced by renewables is small, but this is changing fast, with energy storage expected to grow at a CAGR of around 6%.
There are a number of technologies used to store electricity on a grid scale, these are briefly summarised below.
Solid State Batteries
Sodium Sulphur (NaS) batteries comprise a molten sulphur positive electrode and a molten sodium negative electrode, separated by a ceramic that acts as the electrolyte. The battery must be kept at >300oC to function but it has efficiency of around 90%.
Lithium ion batteries come in a variety of design, but all are characterised by transfer of lithium ions for charge and discharge. Metal oxide/carbon batteries give the best energy density but are also the least safe as they can catch fire.
Electrochemical capacitors are poorly understood, which has restricted their use in the past. They are low cost, very efficient and have a very long cycle life. They can be designed for rapid cycling or long duration cycling, the latter most suited to pairing with renewables. Say, charging during the day and discharging at night.
Solid state batteries store energy as the electrode whereas flow batteries store it as the electrolyte. Possibly the best known flow battery is the vanadium redox battery (VRB). Vanadium can exist in 5 oxidation states – V2+/V3+ is the negative and V$+/V5+ is the positive, the two being separated by a membrane.
The VRB has a relatively high cell voltage and good energy density but can suffer chemical stress, particularly at higher temperatures. It is suitable for power systems up to around 100MW.
Other flow batteries include zinc-bromine, iron-chromium and redox flow.
There are many technologies used in thermal energy storage, ranging in scale from a single home to an entire town, and in storage time from hours to months. Only a couple of the more common technologies will be discussed here.
A common household scale form of storage is electric thermal. The storage heater, consisting of ceramic or feolite (sintered iron oxide) bricks, uses cheap night-time electricity to heat the bricks which then discharge the contained heat during the daytime.
Seasonal thermal energy storage allows storage of heat or cold for many months., releasing heat in winter and cold in summer. This can be ao a scale of a single building to a suburb scale. There are a variety of methods used, but typically water is heated or cooled in in solar collectors and then pumped into the storage medium. This can be an aquifer, bedrock itself or a cavern.
Typically, water is pumped uphill to a surface storage dam, using low cost off-peak power, and then released downhill through a turbine to generate electricity upon demand. The equipment is usually a constant speed combined pump-turbine, acting as a pump in one direction and a turbine in the other.
Materials other than water can also be used. For example, using mechanical systems to move gravel uphill and then mechanically generating electricity when the gravel is released downhill.
Compressed air energy storage is comparable to pumped hydro in scale and application. In essence air is compressed to around 1,000psi and the stored underground in a cavern or similar. It is not common and requires removal of heat upon compression and addition of heat when releasing the gas through a turbine.
Flywheel energy storage systems comprise a rotating mass that is accelerated by electricity input through a motor-generator and decelerated through the discharge of electricity. Higher rotation speeds can store more energy, but while dense materials such as steel can store more energy, they cannot rotate as fast as low density flywheels. Thus, while a steel flywheel might rotate at up to 10,000RPM, a carbon fibre flywheel may rotate at over 100,000RPM. Flywheels have fast response times, and can be fully charged or discharged in a matter of seconds. They are long lasting and reliable.
Most of the above methods consume energy to store it. In a 100% renewables future that would require considerably more capacity than is needed by the grid. Therefore, battery technology would seem the superior storage route for large storage systems. And of the two, flow batteries look superior.