Long-term thermal energy storage takes place by increasing the temperature of large masses of water or soil. There are several types of such trays:

A. Water reservoirs - acting as a collector and storage of thermal energy:
- shallow tanks (to ok. 15 Cm) with a black bottom and glazed from above are placed on the roofs of houses;
- "solar ponds" - unconventional (brine) water tanks, natural or artificial, O 2 into 5 m deep, containing sodium chloride or magnesium chloride,
for countries situated in the belt between 40 a 50° north latitude is recommended ponds with an area equal to the area of the heated house and depth 2 into 3 m; for a single-family house the pond should have ok. 200 m² area; for a prototype house with an area of 186 m² and a pond of 184 m², built near London, maximum temperature values in the state storage zone were obtained in mid-August and amounted to 89°C, namely in mid-March = 38°C; a heat pump was used to transfer heat from the solar pond to the house,
- stabilized liquid tank - the water in it becomes so concentrated (gelatinous), that her movements are impossible;
- a water reservoir divided by horizontal ones, transparent partitions, which prevent convective movements of water;
- tank with different layers of membranes, horizontal and vertical planes of the sheets, tubes made of Teflon.

Scheme of using soil aquifers as a heat reservoir.

B. Ground aquifers as a cheap and attractive reservoir.
C. Water reservoirs placed in the ground, rocks (arrowheads), workings and mining corridors; example from Sweden - Lykebo
D. The use of land as a thermal energy store. In 1m² of dry soil, approx. 8 kWh of thermal energy, and in moist soil up to 25 kWh of heat. Research by Swedish scholars has shown, that the most effective are ground bunkers with large volumes, and thus serving entire estates of single-family houses.