1. A method for controlling a phase change refrigeration system having an evaporator, employing a refrigerant and a non-volatile liquid mixed with the refrigerant in at least a portion of the refrigeration system, comprising: storing a map relating evaporator efficiency according to a predetermined metric, evaporator heat load, refrigerant amount in the evaporator, and a concentration or amount of the non-volatile liquid mixed with the refrigerant in the evaporator; and selectively altering an operating point of the evaporator, by partitioning refrigerant between an accumulator and the evaporator and thereby altering a concentration of non-volatile liquid mixed with the refrigerant in the evaporator, during operation of the evaporator, by retrieving the stored map with at least one automated processor and processing data corresponding to at least a measured evaporator heat load during operation and in conjunction with the stored map, to selectively transition the operating point of the refrigeration system toward an optimum operating point.