How to store excess energy?

The problem with both solar and wind energy is that the production is necessarily intermittent. Day and night follow each other, and some days are cloudy. Some days are windy and others are calm. Consequently, sometimes energy is produced much over consumption and at other times renewable energy production remains at much lower level than what is needed. Because of this, it has often been said that wind and solar energy production need constant energy production, such as nuclear power. Also, the intermittence makes it difficult to become completely free from natural gas, oil and coal.

However, the situation would be much better if all the excess energy produced could be stored for later use. The immediate idea is to use batteries. Although batteries are a common solution for small scale storage, they are relatively expensive and a lot of natural resources are needed for making them. A less expensive, large scale storage solution would be to utilize gravity. For a long period of time this has been almost completely restricted to pumping water to reservoirs uphill when excess energy has been available, and then letting water flow downwards when energy production is needed. Instead of pumping water, any material can be moved upward with excess energy, and the potential energy thus stored released when the weight is moved downwards. Such gravity-utilizing energy storage systems are, in principle, cheap and easy to operate. Consequently, they should become an important feature of wind and solar power plants