A method is provided for converting heat from a heat source to mechanical or electrical energy. The method comprises heating a working fluid using heat supplied from the heat source; and expanding the heated working fluid to lower pressure of the working fluid and generating mechanical or electrical energy as the pressure of the working fluid is lowered. The method is characterized by using a working fluid comprising 1,3,3,4,4,4-hexafluoro-1-butene (HFO-1336ze). Also provided is a power cycle apparatus. The apparatus is characterized by containing a working fluid comprising HFO-1336ze. Also provided is a method for replacing HFC-245fa in a power cycle apparatus. The method comprises removing at least a portion of HFC-245fa and adding HFO-1336ze.