A process for recovering heat from a heat source and generating mechanical energy, comprising the steps of: (a) compressing a first working fluid in liquid phase; (b) passing said first working fluid from Step (a) through a heat exchanger or a fluid heater, wherein said heat exchanger or said fluid heater is in communication with said heat source that supplies said heat; (c) removing at least a portion of the heated said first working fluid from said heat exchanger or fluid heater; (d) passing said at least a portion of the heated said first working fluid to an expander, wherein at least portion of said heat is converted into mechanical energy, and wherein the pressure on said first at least a portion of the heated said first working fluid is reduced to below the critical pressure of said first working fluid, thereby rendering said at least a portion of the heated said first working fluid to a first working fluid vapor or a first working fluid mixture of vapor and liquid; (e) passing said first working fluid vapor or said first working fluid mixture of vapor and liquid from said expander to a condenser, wherein said at least a portion of said working fluid vapor or said working fluid mixture of vapor and liquid is fully condensed to a second working fluid in liquid phase; (f) compressing and mixing said second working fluid in the liquid phase with said first working fluid in liquid phase in Step (a); (g) repeating Steps (a) through (f); characterized in that the first working fluid consists of HFO-1336mzz-Z and that in step (a) the first working fluid in liquid phase is compressed above said first working fluid's critical pressure; and in step (b) said first working fluid is heated to a temperature in the range of from 200°C to 250°C. (a) compressing a first working fluid in liquid phase; (b) passing said first working fluid from Step (a) through a heat exchanger or a fluid heater, wherein said heat exchanger or said fluid heater is in communication with said heat source that supplies said heat; (c) removing at least a portion of the heated said first working fluid from said heat exchanger or fluid heater; (d) passing said at least a portion of the heated said first working fluid to an expander, wherein at least portion of said heat is converted into mechanical energy, and wherein the pressure on said first at least a portion of the heated said first working fluid is reduced to below the critical pressure of said first working fluid, thereby rendering said at least a portion of the heated said first working fluid to a first working fluid vapor or a first working fluid mixture of vapor and liquid; wherein at least portion of said heat is converted into mechanical energy, and wherein the pressure on said first at least a portion of the heated said first working fluid is reduced to below the critical pressure of said first working fluid, thereby rendering said at least a portion of the heated said first working fluid to a first working fluid vapor or a first working fluid mixture of vapor and liquid; (e) passing said first working fluid vapor or said first working fluid mixture of vapor and liquid from said expander to a condenser, wherein said at least a portion of said working fluid vapor or said working fluid mixture of vapor and liquid is fully condensed to a second working fluid in liquid phase; (f) compressing and mixing said second working fluid in the liquid phase with said first working fluid in liquid phase in Step (a); (g) repeating Steps (a) through (f); characterized in that the first working fluid consists of HFO-1336mzz-Z and that in step (a) the first working fluid in liquid phase is compressed above said first working fluid's critical pressure; and in step (b) said first working fluid is heated to a temperature in the range of from 200°C to 250°C.