Why is it necessary to store energy and for how long?

As with all consumer goods, energy supply and consumption are generally not simultaneous. As a result, temporary storage capacity is needed, for a period of a few hours to a few weeks. We also build up energy reserves for a few months to compensate for possible supply disruptions. In addition, we have surplus energy resources in the summer, which we must store as much as possible for use the following winter, thus reducing our energy dependence on foreign countries.

The availability of energy on demand, without unexpected interruptions, is a prerequisite for the proper functioning of industrialised countries. Any disruption in supply entails very high costs, not only economically, but also socially (welfare, health, safety).

Liquids can be easily stored, which greatly facilitates the logistics of supplying fuels and combustibles. We store petrol and diesel at different points in the supply chain, including petrol stations and car tanks. In addition, Switzerland, like all countries, maintains strategic stocks of fuels and combustibles to ensure its supply for a few months in case of import disruptions [→ Q22].

The storage of electricity presents a particular supply challenge, as this energy cannot be stored as such. Therefore, the electricity produced must always equal the electricity consumed throughout the network. As demand fluctuates constantly, output must be adjusted up or down across the entire network. With the new intermittent renewable energies (solar and wind), production also varies, depending on weather conditions. Adequate reserve capacity is therefore a prerequisite for balancing supply and demand. In practice, this means a number of standby power plants that can be instantly switched on when needed. This is known as “adjustment capacity”. In a liberalised market, these facilities, which make their generating capacity available to the grid, are remunerated for not producing. The plants that do not produce are remunerated through an auction system defining the “adjustment market”. In Switzerland, 6% of our production capacity is permanently on standby, available for “adjustment capacity”.

Should this reserve capacity prove insufficient (a very rare critical situation), neighbouring countries will come to our rescue with their own adjustment capacity. As the grid is interconnected, it is in nobody’s interest for part of the system to fail, as the repercussions could become critical for the entire European grid. If a large imbalance occurs between production and consumption (e.g. if a power plant breaks down or a high-voltage line is cut by a falling tree), the result is a sudden voltage change, which can lead to a power cut in one part of the grid with the risk of propagation to the whole system.

The opposite situation can also occur, i.e. when electricity production exceeds consumption. This is particularly the case at night, when demand falls sharply but some power stations (nuclear power stations in particular) cannot interrupt their production for technical and financial reasons. In such cases, the power must be removed to avoid an imbalance in the network. Since electricity cannot be stored, the only solution is to convert it into another form of energy that can be stored. This is what Switzerland is doing with its pumped storage facilities [→ Q66], while pilot units exist to convert this electricity into synthetic fuel [→ Q74]. If there is a lack of storage capacity, then this surplus electricity will have to be consumed in one way or another. That is why Belgian motorways are lit up all night long, even though this waste has decreased somewhat in recent years.

Finally, we have very significant energy resources in summer, which globally exceed our needs at this time of the year (solar, hydraulic, waste heat). It is possible to store them for use the following winter. This seasonal storage, which we already practise to a lesser extent with our alpine dams but which could be increased [→ Q76], represents a very attractive strategy for increasing our relatively low level of energy independence [→ Q20].