A heat accumulator for storing thermal energy may include a container with a horizontally-extending longitudinal axis, and a heat storage material in the form of a plurality of stone-like elements contained in the container. The container may include a first opening formed at a first portion of the container, and a second opening formed at a second portion of the container, the second opening being vertically offset with respect to the first opening. An average diameter of the stone-like elements arranged in the first portion of the container may be larger than an average diameter of the stone-like elements arranged in the second portion of the container.