Underground thermal energy storage

The invention provides an underground thermal energy storage having a shape selected from substantially cylindrical and an n-gonal prism, having an axial direction that in use is vertical, and comprising an inner volume for holding a liquid, said energy 5 storage device comprising a peripheral outer wall, a peripheral inner wall around said inner volume, and a filling layer between said inner wall and said outer wall, said inner wall comprising a series of modular wall parts provided with a heat exchanger for exchanging thermal energy with said liquid, said modular wall parts arranged in rings and said modular wall parts each having opposite radial surfaces that are in use 10 vertical, an inner tangential surface contacting said inner volume, an outer tangential surface directed towards said outer wall, and opposite axial surfaces that are in use horizontal, said modular wall parts comprising an elastic sealing between a joint of adjacent radial surfaces for limiting liquid flow between the inner volume and the filling layer and taking up thermal expansion of the modular wall parts, and said filling 15 layer comprising an insulating layer extending over at least part of a height of the underground energy storage, having an R value designed for providing said outer wall at a temperature of below 30°C when said inner volume is at a temperature of at least 90°C, and a structural layer for maintaining said insulating layer and said prefab inner wall parts in position.