THERMAL ENERGY STORE

A thermal energy store comprising a vessel (2) having at least one fluid flow path (3) extending therethrough, the fluid flow path (3) arranged to carry a heat transfer fluid for transferring energy to and/or from the thermal energy store (1), wherein the fluid flow path (3) is constructed and arranged in the vessel such that the fluid flow path defines at least one cartridge aperture (5) to receive a thermal energy storage cartridge (20) containing a heat storage material.