To illustrate how this impacts the operation of the electricity grid, consider five different levels of available electricity production from RES, as shown in Figure. Note that there is not only a capacity of power towards the storage (charging the storage) but also a capacity of power from the storage to the grid (discharging the storage). It is part of the role of the system operator to decide which of the two should be chosen at any moment in time. Some thoughts are given below, based on the supply capacity in relation to the demand.

Supply level 1. The total supply capacity, directly from renewable sources plus by discharging the storage, is not enough to cover the power demand. The result is that not all the power demand can be fulfilled. All the available storage discharging capacity will be used to limit the amount of demand that is not fulfilled.

Supply level 2. The amount of supply capacity directly from renewables is not sufficient to cover the power demand, but by using part of the discharging capacity of the storage the power demand can be supplied. The remaining storage capacity can either be saved for later use or be used to cover some of the energy demand. This will be an optimization issue, where the state of charge of the storage, the expected future demand and the expected future production from renewables will have to be considered.

Supply level 3. The amount of supply capacity directly from renewables is sufficient to cover the total power demand. The remainder can be used to supply part of the energy demand and/or to charge the storage. When there is sufficient energy in the storage, the stored energy can even be used to supply the total energy demand. The optimisation of the charging/discharging of the storage versus supplying the energy demand is one of the tasks of the system operator.

Supply level 4. The amount of supply capacity directly from renewables exceeds the sum of power demand and energy demand. In that case the total power demand will be supplied and the remainder will be used to charge the storage.

Supply level 5. The amount of supply capacity directly from renewables exceeds to sum of power demand, energy demand, and charging capacity of the storage. In that case all demand should be fulfilled and the remaining amount of renewable energy will be curtailed.

Source: Antonio Moreno-Munoz. “Large Scale Grid Integration of Renewable Energy Sources”. The Institution of Engineering and Technology. 2017.

Dr. Jorge Mírez
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