NOVELTY - The back-up boiler system (25) has a convection section (80) in fluid communication with a combustion chamber (70). A first heat exchanger (92) is provided in the convection section to heat a molten salt mixture of the solar thermal power plant and a second heat exchanger (90) to preheat boiler feed water of the plant. The back-up boiler system is configured to allow selection between only providing heat to either the first or second heat exchanger, or both, depending on the availability of solar radiation or on the power generation demand.    USE - Back-up boiler system for solar thermal power plant (claimed).    ADVANTAGE - Enables provision of a boiler system that operates on a molten salts mixture and which allows efficient generation of electricity, preferably under any weather condition and at a low sun level, such as at night. The provision of the back-up boiler system extends the operation time of the solar thermal power plant, while enabling accurate control over the temperature of the superheated steam and the boiler feed water temperature. The boiler system ensures that the solar thermal power plant complies with start-up conditions, e.g. minimizing the start-up time and start-up trouble of the solar powered generation system, daily operating conditions, night or absence of sun operating conditions, thus providing full back-up conditions to be able to continue generation of electricity, and heating of the molten salts mixture when the sun is absent. The back-up boiler system ensures the continuous operation of low-pressure stream for operation of the desalination unit of the solar thermal power plant. Ensures that the solar thermal power plant will operate with a more stable temperature of the steam at the steam turbine, thus overall having a more stable operation and will have a short starting-up period.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) a solar thermal power plant; and    (2) a method for operating a solar thermal power plant.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic diagram of the convection system of the back-up boiler system.    Back-up boiler system (25)    Combustion chamber (70)    Convection section (80)    Second heat exchanger (90)    First heat exchanger (92)