Distributed generation: technologies available for energy storage
In a microgrid, storage of energy by means of a storage system is of fundamental importance. In fact, during potential load peaks, the microgrid may not be able to respond to load changes without an immediate availability of energy.
Storage systems become even essential when massive amounts of renewable generation are used. In fact, one of the major obstacles to the deployment of renewable energy sources is their discontinuity, especially with regard to solar, wind and, to some extent, mini and micro hydro.
In order to make full use of these primary sources, plants need storage systems with a charge-discharge cycle and with the highest net energy yield possible. In this sense, energy storage is valuable when the recovered net energy less the energy used for the operation of auxiliary equipment (pumps, heat exchangers, controls, etc.) is greater than 70% of energy input.
In classic power network, the simplest energy storage system is related to the inertia of the generators: when there is an increase of load in the network, the initial deficit of powers is satisfied by the inertia of the system, at the expense of a small reduction in the frequency of the network. In the case of a microgrid, in order to maintain the voltage and frequency within the limits, a rapid response is instead necessary. For this reason, the mostly used systems are batteries, flywheels and supercapacitors.
The batteries are the most traditional equipment for energy storage: in this case, since energy is in the form of a continuous current, an interface with inverter is needed. Otherwise, the flywheel accumulates energy at high speed (up to 100,000 rpm) and is coupled with the rotating motor / generator.
The rotation takes place at a high speed, since the amount of energy stored is actually proportional to the square of the speed. Finally, supercapacitors (high-capacity electrolytic capacitors) store energy in the form of electrostatic charge. These components have a considerable speed of discharge: therefore, they can be used for rapid load variations in the microgrid.
The following table shows an example of comparison between various characteristics of the main energy storage technologies.
Finally, some typical features of batteries and flywheels are shown in the table below.