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. <EOS>