Heat transfer system with phase change composition

A method of transferring heat, comprising

• flowing a phase change composition through a flow path in a heat exchanger between an inlet and an outlet of the heat exchanger, and a fluid circulation loop external to the heat exchanger connecting the outlet to the inlet;
• wherein the phase change composition comprises a first PCM that is a plant or animal or paraffinic oil having a first melting point and a second PCM that is a plant or animal or paraffinic oil having a second melting point lower than the first melting point;
• wherein a surface of the flow path through the heat exchanger in contact with the phase change composition has a temperature at or above the second melting point in a heat absorption mode, or has a temperature at or below the first melting point in a heat rejection mode;
• wherein the heat absorption mode includes a first heat absorption phase change mode as successively larger portions of the second PCM melt, then a heat absorption non-phase changing mode as the temperature of both of the liquid second PCM and the still solid first PCM increases, and then a second heat absorption phase change mode as the melting point of the first PCM is reached and the first PCM begins to melt;
• wherein the heat rejection mode includes a first heat rejection phase change mode as successively larger portions of the first PCM solidify, then a heat rejection non-phase changing mode as the temperature of the solidified first PCM and the still liquid second PCM drops, and then a second heat rejection phase change mode as the solidification temperature of the second PCM is reached and the second PCM begins to solidify.