Surplus of solar thermal energy is circulated in pipes in several shorter boreholes and the ground warms up. Depending on the surface, the energy can be stored for a long time and from one season to another. Examples of projects are: solar thermal ground storage
This methode combines the advantages of a heat pump that exchange heat with the ground (water to water HP). Solar thermal energy is circulated in the pipes and the ground is heated during the summer season. Later, when there is no sunshine, you use a heat pump to bring the solar energy back. Depending on the surface, the energy can be stored for long periods and from one season to another. The same method can be used with a borehole, but it assumes there is no water flow in the ground.
This method is still in a research and development stage. The solar thermal energy is bound in a salt / crystal structure and is released only by the addition of a small amount of energy. Thus the solar thermal energy can be stored virtually unlimited.
This method uses advanced molecular boiling temperature to absorb or emit heat. In this manner solar thermal energy is absorbed when we have too much of it as during the summer. Later, when we need the energy, the energy is released. The IEA estimates that new solar thermal energy systems storage methods will be on the market sometime between 2015 and 2020. IEA has also forecasted all new homes in the western world will be 100 % self-sufficient with solar energy already in 2030.