The big problem with renewable energies is that these energies (solar and wind) vary with the time of day and weather conditions, and users want to have electricity at all times, regardless of weather conditions.
Electric companies must produce the same electrical energy that customers are going to consume. If it is produced in excess, the distribution system falls, and if it is produced less, too. And in both cases we are left without power and in the dark; and putting the system back into operation takes some time.
Electrical energy cannot be stored, and what is produced at a given moment must be consumed. That is why the energy distribution companies are continuously checking the demand and supplying the electricity network with the energy that is needed at that moment. Not more, but not less either.
And that variable "contribution" of electrical energy is made by consuming fossil or nuclear fuels.
And this is the biggest "problem" of renewable energies. How to have non-polluting energy when it gets dark or there is no wind. And the only alternative is to convert the renewable energy generated in excess into other sources that are capable of subsequently generating non-polluting electrical energy.
The first that was used was the transfer of water masses to higher areas, to later use that water to generate electricity passing through turbines.
The surplus energy can also be used to charge large amounts of batteries so that later (when needed) they can contribute that energy to the network; or to compress large volumes of air and then use that compressed air to drive turbines; or etc...
Not long ago I have seen a company that intends to use gravity to store and then generate non-polluting electrical energy and they do it in a very simple way. By means of a crane that raises a weight (35 tons) to a certain height and then by gravity when dropping that weight, it moves a motor and generates electricity.
The company is called Energy Vault and is doing demonstration tests in a Swiss canton. The system could not be simpler and all the necessary basic technology is more than proven. Now only the management of the system remains to be proved to provide electricity to the network when it is needed.
A video can be seen at: https://vimeo.com/647372871
The idea is simple and cheap, and also has the advantage that it can be placed where it is needed. And like all new ideas, it remains to be seen if it gets enough funding to go from testing to reality.
We are currently experiencing a revolution in the quest to generate electricity in a non-polluting way, and almost all the contributions are positive.
As a Society, we have the BIG PROBLEM of obtaining different means of storing the non-polluting energy generated, for later and at the time it is needed to obtain it again in a non-polluting way.
Gravity has been with us since creation and it would be great if it could help us with it to obtain clean energy.
I wish the best to the people of Energy Vault.
EN 
ES_ES 