System for storing thermal energy

Thermal energy storage system (100) for storing thermal energy from a thermal energy source, the storage system (100)
a liquid reservoir (110) for coupling to a thermal energy source, comprising a reservoir volume with an open top (120), the open top (120) coinciding with a liquid level (130) of a thermal energy storage liquid in the liquid reservoir (110), and
a surface cover (200) comprising an insulating material (210), such as mineral wool, for retaining thermal energy stored in the liquid, the surface cover (200) at least partially covering the open top (120),
wherein the surface cover (200) is divided into surface cover segments (205), wherein a first surface cover segment comprises a drainage system (220) which is separated from a drainage system of a second surface cover segment by means of a substantially fluid-tight and / or liquid-tight barrier (230), so that Liquid on a surface of and / or in each of the first and second cover segments can be individually drained.