A method of operating a solar thermal power system comprising a solar receiver, a multistage steam turbine, and a steam generator arrangement, wherein the steam generator arrangement comprises an economizer section, an evaporator section and a superheater section, the method including the steps of: circulating a thermal energy storage fluid, included in a thermal energy storage arrangement, through the solar receiver to store thermal energy; operating the multistage steam turbine by variable pressure steam, the variable pressure steam being generated from water supplied from a feedwater supply via the thermal energy storage fluid; utilizing the heat of the hot thermal energy storage fluid at an entrance of the evaporator section to generate and supply the variable pressure steam to the superheater section and further to the multistage steam turbine; the method characterised in the steps of: recirculating, around the economizer section, the heated water to an inlet of an economizer section; increasing heat load on the economizer section in turn increasing pressure range of the variable pressure steam in the steam generator arrangement, while maintaining the temperature of the thermal energy storage fluid at an inlet of the evaporator section and at an outlet of the economizer section, the feedwater temperature to the economizer section at desired values, by adjusting recirculation flow through the recirculating line to control water inlet temperature to the economiser section; and maintaining the temperature of the thermal energy storage fluid leaving the economizer section above a freezing temperature of the thermal energy storage fluid. circulating a thermal energy storage fluid, included in a thermal energy storage arrangement, through the solar receiver to store thermal energy; operating the multistage steam turbine by variable pressure steam, the variable pressure steam being generated from water supplied from a feedwater supply via the thermal energy storage fluid; utilizing the heat of the hot thermal energy storage fluid at an entrance of the evaporator section to generate and supply the variable pressure steam to the superheater section and further to the multistage steam turbine; the method characterised in the steps of: recirculating, around the economizer section, the heated water to an inlet of an economizer section; increasing heat load on the economizer section in turn increasing pressure range of the variable pressure steam in the steam generator arrangement, while maintaining the temperature of the thermal energy storage fluid at an inlet of the evaporator section and at an outlet of the economizer section, the feedwater temperature to the economizer section at desired values, by adjusting recirculation flow through the recirculating line to control water inlet temperature to the economiser section; and maintaining the temperature of the thermal energy storage fluid leaving the economizer section above a freezing temperature of the thermal energy storage fluid. recirculating, around the economizer section, the heated water to an inlet of an economizer section; increasing heat load on the economizer section in turn increasing pressure range of the variable pressure steam in the steam generator arrangement, while maintaining the temperature of the thermal energy storage fluid at an inlet of the evaporator section and at an outlet of the economizer section, the feedwater temperature to the economizer section at desired values, by adjusting recirculation flow through the recirculating line to control water inlet temperature to the economiser section; and maintaining the temperature of the thermal energy storage fluid leaving the economizer section above a freezing temperature of the thermal energy storage fluid.