Thermal energy storage and method for constructing such a thermal energy storage

A thermal energy storage with at least one artificial thermal energy storage volume and an additional volume of rock body surrounding the storage volume, the thermal energy storage comprising:

• at least one primary borehole extending from ground level to a first predetermined depth in a rock body;
• at least one set of secondary boreholes located in a cluster adjacent the at least one primary borehole at ground level;
• at least an upper and a lower fracture plane extending in a radial and/or oblique plane from the at least one primary borehole towards adjacent secondary boreholes, where at least one fracture plane permits a hydraulic flow between at least one of the secondary boreholes and the primary borehole;
• wherein each artificial thermal energy storage volume is defined by one set of secondary boreholes and its upper and lower fracture planes;
• wherein the set of secondary boreholes diverge away from the at least one primary borehole at each fractured plane level with increasing depth, without intersecting the at least one primary borehole;
• and
• wherein the surface area of consecutive fracture planes and the storage volume of the thermal energy storage are arranged to gradually increase with increasing depth, which fracture planes are connected by the secondary boreholes.