The problem of thermal energy storage is as important as the problem of solar radiation absorption. Globally, the amount of solar energy falling on the surface of a single-family house during the year is sufficient to meet the needs related to heating and domestic hot water preparation..
The main disadvantage limiting the possibilities of using energy is the unevenness of solar radiation depending on the weather, time of day and year.

Heat demand for heating and domestic hot water preparation. for a single-family house and solar radiation in the annual cycle

Heat demand for heating on a clear January day, and the intensity of solar radiation on that day.

To compensate for the uneven supply of solar radiation energy, conventional energy sources are used in combination with a solar heating system and heat accumulators. Heat storage tanks for a period from one day to several days are called short-term. The long-term compensate for the differences in the supply and demand for thermal energy in the annual cycle. In the existing warehouses, the problem of accumulating energy by:
1) increasing the temperature of the agent used with a specific heat capacity (perceptible warmth),
2) phase transformations of the medium or chemical reactions taking place inside the material (latent heat),
3) substances, in which reversible physical reactions take place (heat pumps).

Comparison of types of thermal energy storage

Comparison of the three types of heat storage, in which an equal amount of thermal energy has been stored - 4,18 x 10 kJ, gives the opportunity to visualize the benefits of using chemical reservoirs, or using phase transitions.
It follows from the table above, that materials that accumulate energy in the form of latent heat or chemical compounds that emit heat when decomposing or combining are best suited for storing heat.