NOVELTY - The system has a containment vessel defining a cavity comprising an inner surface an outer surface. Two phase change materials with respective liquid densities are placed respectively in the cavity, where one phase change material is adjacent to and in contact with another phase change material, and the liquid density of the former material liquid is less than the liquid density of the latter material. A vapor space is defined by the inner surface and the former phase change material. A heat receiver of a power conversion unit is placed in the vapor space.    USE - Thermal energy transfer system for use in electric power generating system (claimed) i.e. solar power generating system.    ADVANTAGE - The system allows large amounts of energy to be transferred using a combination of liquid, solid and vapor phases and to store energy using phase change materials. The system can be placed within the heat pipes, thus reducing the cost and complexity of heat pipes to add heat energy to a thermal storage medium, and avoiding the utilization of heat pipes to transfer heat from the storage medium to the energy converter. The thermal energy transfer can be done simply, effectively and efficiently. The system is environment friendly and possesses good thermal storage properties.    DETAILED DESCRIPTION - The system utilizes one phase change material that is selected from a group consisting of sodium fluoride (NaF)/sodium chloride (NaCl), magnesium chloride (MgCl2), sodium fluoride (NaF)/calcium fluoride (CaF2)/magnesium fluoride (MgF2), potassium fluoride (KF)/sodium fluoride (NaF)/magnesium fluoride (MgF2), KF/NaF, KF/beryllium fluoride (BeF2), BeF2, NaF/MgF2, lithium fluoride (LiF)/MgF2, lithium carbonate (Li2CO3), lithium hydride (LiH), LiF/ NaF/MgF2, LiF, NaC1/nickel chloride (NiCl2), lithium hydroxide (LiOH)/LiF, LiOH, LiF/lithium chloride (LiCl), NaCl, potassium chloride (KCl)/NaCl, sodium tetraborate (Na2B4O7), NaF/MgF2/KF, NaF, BeF2, LiCl/ KCl, and another phase change material that is selected from a group consisting of a material selected from a group consisting of a eutectic material, a pure composition material, sodium, potassium, sodium-potassium alloy, lithium, rubidium (Rb), cesium, sulfur, silver and magnesium. INDEPENDENT CLAIMS are also included for the following:    (1) a method for transferring thermal energy between a heat source and a power conversion unit having a heat receiver    (2) a thermal energy storage system comprising a containment vessel.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram illustrating operating principles of a system for coupling heat from a heat source to a power conversion unit using two phase change materials that include a vapor or gas phase.    Thermal energy coupling system (10)