A high temperature thermal energy storage includes a foundation comprising thermal insulation, at least one self-supported cassette arranged on the foundation. At least one cassette is a self-supporting frame or assembled structure with respect to transport and installation, containing a number of concrete thermal energy storage elements, some or all of the elements include heat exchangers embedded in the concrete of the elements. A pipe system includes an inlet and an outlet for thermal input to and output from the storage, respectively. The pipe system is fluidly coupled to the heat exchangers for circulating fluid through the heat exchangers for thermal energy input to or output from the elements and thermal insulation around and on top of the at least one self-supported cassette containing concrete thermal energy storage elements.