Nuclear power plants have the largest share in the Ukrainian generation balance (more than 50%), but their capacity may not be changed during the day. Thermal power plants are the same uncontrolled source. The amount of electricity they produce depends on the heat production schedule, as the electricity is a by-product for them.
Thus, there are only 2 sources of balancing capacity left - TPPs and HPPs.
The hydroelectric power plant is the most maneuverable, because it can be put into operation and shut down during the day as much as needed: it quickly increases and decreases generation capacity. The capacities of hydroelectric power plants could be used freely and cover all the balancing needs, but, unfortunately, they are scarce in Ukraine. Therefore, we can use HPPs to regulate only consumption peaks when another generation is missing.
The situation is difficult with thermal power plants as well. Commissioning a thermal unit is a complex and resource demanding process. They cannot be put into operation and shut down daily. This is done only in emergency situations, but it significantly affects equipment runout and is economically impractical. Common in Ukraine, the TPP with a capacity of 300 MW operates in the range from 170 MW to 250-300 MW (depending on its technical condition). It cannot work below 170 MW, this is its base.
Thus, the minimum level of daily consumption is covered by the base of atomic and thermal generation. The minimum composition of TPP units is determined by their maximum capacity during peak hours of the day, taking into account the capacity of HPPs. When we want to put into operation many nuclear power plants, the balancing problem is not always solved, because TPP units during peak hours cannot always fit with their minimum capacity in the energy balance during minimum consumption hours. Increasing the capacity of solar power plants further complicates the problem. With a significant increase in solar power capacity during the day, consumption for other power plants remains even lower than at night, so the inequality of the daily schedule increases, and the required balancing capacity also goes up. More thermal units have to be put into operation and more nuclear ones have to be shut down.
Hydroaccumulation stations also operate in the power system. Their principle of operation is based on the fact that in the charging mode the water is pumped into the upper reservoir, and in the generation mode it is discharged into the lower one. They are used for daily balancing: in the mode of generation surplus, they work as a consumer, and in the mode of generation deficit, as generators. This is a convenient system, but requires a long construction period and large capital investments.
There is an urgent need for energy transfer technologies over time during the day. Batteries handle this perfectly. However, with batteries based on lithium cells, this is still not economically justified. In the current situation (market without market conditions), the minimum and maximum energy price is fixed, so when there is a deficit, there are no volunteers willing to close this deficit - because the price is limited. Conversely, when there is a surplus, and it seems possible to sell cheaper, but conditions do not allow. There is a paradox. When the market really works, with a significant difference in price during periods of surplus and generation deficit - it will be profitable for the owners of the storage capacities to provide their services.
A flow battery is ideal for daily balancing. It is an electrochemical converter that has a certain capacity. The number of hours it works is determined by the amount of electrolyte that flows through it. An electrolyte of one chemical form enters it, and another flows out. When this conversion occurs - it generates current, and vice versa, when the same electrolyte flows in a different direction - current is consumed. The capacity of such batteries is limited only by the size of the electrolyte tanks. This technology can compete with maneuverable generation. The flow battery developed at KNESS RnD Center has already gone through laboratory tests. In the future, flow batteries can play the role of weekly balancing.