A thermal energy storage plant (1, 1', 1") for storing thermal energy from an energy source, the storage plant (1, 1', 1") comprising: a liquid reservoir for being coupled to an energy source and comprising a reservoir volume with a top side, said top side coinciding with a liquid level of a thermal energy storage liquid in the liquid reservoir, and a surface cover (2, 2', 2") comprising an insulating material, such as extruded polystyrene, for retaining heat energy stored in said liquid, said surface cover (2, 2', 2") at least partly covering said top side, said surface cover (2, 2', 2") being divided into at least a first and a second surface cover segment (2a, 2a', 2a", 2b, 2b', 2b") positioned adjacently to each other and each comprising a circumferential periphery, characterized in that the first surface cover segment (2a, 2a', 2a") further comprises a first tilting device (21a, 21a', 21a"), a weight of the first tilting device (21a, 21a', 21a") tilting a draining surface of the first surface cover segment (2a, 2a', 2a") from its circumferential periphery downwards towards a first draining location of the first surface cover segment (2a, 2a', 2a"), whereby water from precipitate falling on the first surface cover segment (2a, 2a', 2a") will flow by gravity towards the first draining location where it can be drained off from the first surface cover segment (2a, 2a', 2a"), that the second surface cover segment (2b, 2b', 2b") similarly further comprises a second tilting device (21b, 21b', 21b"), a weight of the second tilting device (21b, 21b', 21b") tilting a draining surface of the second surface cover segment (2b, 2b', 2b") from its circumferential periphery downwards towards a second draining location of the second surface cover segment (2b, 2b', 2b"), whereby water from precipitate falling on the second surface cover segment (2b, 2b', 2b") will flow by gravity towards the second draining location where it can be drained off from the second surface cover segment (2b, 2b', 2b"), and wherein the weight of the tilting device (21a, 21a', 21a", 21b, 21b', 21b") increases from a respective circumferential periphery towards the respective draining location (27a, 27b) of the first and/or second surface cover segment (2a, 2a', 2a", 2b, 2b', 2b"). a liquid reservoir for being coupled to an energy source and comprising a reservoir volume with a top side, said top side coinciding with a liquid level of a thermal energy storage liquid in the liquid reservoir, and a surface cover (2, 2', 2") comprising an insulating material, such as extruded polystyrene, for retaining heat energy stored in said liquid, said surface cover (2, 2', 2") at least partly covering said top side, said surface cover (2, 2', 2") being divided into at least a first and a second surface cover segment (2a, 2a', 2a", 2b, 2b', 2b") positioned adjacently to each other and each comprising a circumferential periphery, characterized in that the first surface cover segment (2a, 2a', 2a") further comprises a first tilting device (21a, 21a', 21a"), a weight of the first tilting device (21a, 21a', 21a") tilting a draining surface of the first surface cover segment (2a, 2a', 2a") from its circumferential periphery downwards towards a first draining location of the first surface cover segment (2a, 2a', 2a"), whereby water from precipitate falling on the first surface cover segment (2a, 2a', 2a") will flow by gravity towards the first draining location where it can be drained off from the first surface cover segment (2a, 2a', 2a"), that the second surface cover segment (2b, 2b', 2b") similarly further comprises a second tilting device (21b, 21b', 21b"), a weight of the second tilting device (21b, 21b', 21b") tilting a draining surface of the second surface cover segment (2b, 2b', 2b") from its circumferential periphery downwards towards a second draining location of the second surface cover segment (2b, 2b', 2b"), whereby water from precipitate falling on the second surface cover segment (2b, 2b', 2b") will flow by gravity towards the second draining location where it can be drained off from the second surface cover segment (2b, 2b', 2b"), and wherein the weight of the tilting device (21a, 21a', 21a", 21b, 21b', 21b") increases from a respective circumferential periphery towards the respective draining location (27a, 27b) of the first and/or second surface cover segment (2a, 2a', 2a", 2b, 2b', 2b").