The invention relates to a method for operating a steam power plant, particularly a combined cycle power plant, which includes a gas turbine an a steam/water cycle with a heat recovery steam generator, through which the exhaust gases of the gas turbine flow, a water-cooled condenser, a feedwater pump and a steam turbine. A cooling water pump is provided for pumping cooling water through said water-cooled condenser. Evacuating means are connected to the water-cooled condenser for evacuating at least said water-cooled condenser. The method relates to a shut down and start-up of the power plant after the shutdown. The method includes: a) shut down of the steam turbine and gas turbine and/or a combustor of said gas turbine ; b) restoration of a good vacuum within the condenser by using said evacuating means; c) stopping said cooling water pump of said water-cooled condenser and said evacuating means, and filling up at least said condenser with steam up to slightly above atmospheric pressure; d) controlling the pressure with a flow of said cooling water; e) after a certain shut-down period starting the cooling water pump just before start-up of the plant; and f) starting the plant.

A thermal energy recovery device capable of suppressing a rapid increase of thermal stress generated in an evaporator when the operation is started and a start-up method thereof are provided. The thermal energy recovery device includes an evaporator, a preheater, an energy recovery unit, a circulating flow path, a pump, a heating medium flow path for supplying a heating medium to the evaporator and the preheater, a flow adjustment unit provided in a portion on the upstream side than the evaporator within the heating medium flow path, and a control unit. The control unit controls the flow adjustment unit so that the inflow amount of the heating medium in a gas-phase to the evaporator gradually increases, in a state that the pump is stopped, until the temperature of the evaporator becomes a specified value.