The invention relates to a heat accumulator (1) for storing thermal energy, which comprises a container (2) having a horizontally extending longitudinal axis, wherein the container (2) comprises a thermal storage material (5). According to the invention, the container has a first opening (81) for inflow and/or outflow of a fluid and a second opening (82) offset vertically opposite the first opening, wherein the container (2) comprises at least one fluid-impermeable plate (42, 43) which is inclined against an inflow and/or an outflow direction (20, 21) of the fluid.