How can we store excess electricity generated by wind and solar power?

There are many solutions for short-term storage (from a few hours to a few weeks): local storage in batteries, or centralised storage using surplus electricity, either to produce hydrogen that will be injected into the natural gas grid or to pump water from our alpine storage lakes (pumped storage). For long-term (seasonal) storage, as the capacity of our storage lakes is not sufficient, some of our dams would have to be upgraded to increase their storage volume. Another option would be to convert electricity to liquid synthetic fuels, such as methanol, which can be easily stored.

The amount of solar and wind electricity fed into the European grid today is high enough to ensure that electricity supply regularly exceeds demand. This “green electricity” is in addition to the production of many facilities (nuclear, coal-fired, run-of-river hydro) that operate continuously and cannot be easily shut down. No one wants to buy this surplus electricity when it is generated, so we will have to store it if we do not want to lose it.

If we want to store on a daily or weekly timescale, the simplest and most economical option is pumped storage, as we already have some of the necessary infrastructure in place (such as dams and turbines). This capacity is expected to increase sharply, with plans to build new pumping facilities for almost 10 GW, some of which are already under construction [→ Q73].

Another approach is to use electric batteries. This option is currently being investigated for the storage of electricity from low-capacity photovoltaic installations (e.g. for single-family homes). Another option would be compressed air storage. This technology, which is under development, consists of using electricity to compress air that is stored under high pressure, for example in ad hoc caverns. Later, this compressed air can be expanded, thus releasing part of the compression energy, which can be reconverted into electricity. To date, no study of the technical potential and economic viability of compressed air storage has been carried out for Switzerland. There has, however, been a pilot project in the Ticino Alps to test the technical and economic feasibility of this concept on a large scale.

We could also store electricity in the batteries of electric cars, a particularly interesting solution in terms of energy efficiency. But this option presupposes that a large number of electric cars are connected to a charging station at the moment when the excess electricity has to be taken off the grid. In Switzerland, however, surplus electricity from renewable sources will be produced mainly by solar panels during the day, while electric cars will be connected to charging stations mainly at night. This cross-fertilisation severely limits the potential of this solution… unless there is an incentive to recharge batteries during the day at workplaces, sports centres and other shopping centres.

And what about hydrogen, an excellent fuel? It is also a possibility. It would involve using excess electricity to electrolyse water. The water then splits into its two components, hydrogen and oxygen. This hydrogen could be injected, in limited quantities, into the natural gas network, which would reduce our gas imports. It can also be converted into synthetic natural gas, mostly methane, which would make it possible to circumvent these limitations. The gas network is large enough to provide ample short-term storage capacity. This is the so-called “power-to-gas” solution, which is of great interest to energy companies today (in French, literally: “conversion of electricity into combustible gas”).

Instead of injecting this hydrogen into the gas grid, we could also use it to produce an easily storable liquid fuel, such as methanol. Later, this methanol could be converted directly into electricity. This would allow large quantities of electricity to be stored indirectly, for long periods of time, thus offering an interesting potential for much-needed seasonal storage [→ Q75].

Our storage dams can also be used for seasonal storage. However, the current storage capacity of our dams is insufficient and will have to be increased if they are to make a greater contribution to seasonal storage. This will involve upgrading some of our dams to increase the volume of water they can hold [→ Q76].

In fact, the main challenge of electricity storage is more economic than technical. Only a significant increase in the price of electricity could guarantee the profitability of these different options.