Very interesting from the Polish point of view are the Swedish experiences in the field of large scale, long-term thermal energy storage and active systems using solar energy to heat entire housing estates. These systems always work with a heat pump. One example is the development of four terraced and semi-detached houses in Kullavik. Annual demand for thermal energy for heating buildings and domestic hot water. totals 310 MWh, total area of collectors - 490 m2, and the ground-type thermal energy storage uses in total 8100 m3 gruntu. In the central part there is a zone of high temperatures, wherein 200 m3 of soil is heated to a temperature + 60°C. The system works with the extract heat pump 62 kW of electricity. It is a low temperature system, which covers the annual demand 85%.
Solar heating and storage system in Kullavik (Sweden)
Another example, where buildings are not used in any way for solar installations, there is an estate in Lyckebo, where from 1983 r. a kind of central solar heating plant is installed for 550 single-family houses. Installed there 4320 m2 of low-temperature solar collectors, which are connected to a long-term heat storage. This role is played by an artificial rock grotto 30 m below the surface of the earth, o 30 meters deep, 75 m in diameter and capacity equal to 100 000 m3. At the end of summer, the water accumulated in the cave reaches a temperature of +90°C in the upper part. The heat pump system and the control and measurement system are above, in a small cave. Hence, the traditional way is to distribute heat, which it covers 95% demand.
Solar system and heat storage in Lyckebo (Sweden)
Based on such a large number of implementations in the temperate-cold climate zone, it can be concluded, that the efficiency of active systems is still low, because it does not exceed 20%.
Active systems use the most solar energy at the beginning and end of the heating season (April-May and October-November). Systems are more efficient in winter, because it decreases during the summer due to the high storage temperature. The angle of inclination of the collectors allows receiving the maximum amount of energy in the period of the greatest demand.
In existing projects in Central and Northern Europe, solar power covers from 16 do 72% total demand for thermal energy needed for heating and hot water preparation.
Two examples of heat balance for single-family houses with active systems for heating and preparation of hot water. (Aramon - France - Latitude. 49°30’ i Zoetermer — Netherlands — 52°8’) shown in the figure 
The profitability of using active systems should be considered from two points of view:
— pierwszy, it's global (social) — saving non-renewable fuels, not polluting the environment,
— drugi to indywidualny, involving higher investment costs incurred once by the user, but compensated by lower operating costs.
In German conditions, the costs incurred for the installation of hot water. they return within 5 — 6 lat, and for heating - during 9 — 10 lat. The difference in the operating costs of traditional systems and active solar systems gives a measurable profit in the following years.