Back-up boiler system for a solar thermal power plant based on molten salt technology, a solar thermal power plant and a method for operating a solar thermal power plant

A back-up boiler system (25) for a solar thermal power plant (201) for transferring solar energy into electricity, said back-up boiler system comprising:

• a combustion chamber (70), and
• a convection section (80) in fluid connection with said combustion chamber (70), wherein in the convection section (80) at least a first heat exchanger (92) is provided for heating a molten salts mixture of the solar thermal power plant and a second heat exchanger (90) is provided for pre-heating boiler feed water of the solar thermal power plant,
• wherein the back-up boiler system (25) is configured to allow selection between only providing heat to the first heat exchanger (92), only providing heat to the second heat exchanger (90) and providing heat to both heat exchangers (90, 92), dependent on availability of solar radiation and/or dependent on demand of power generation;
• wherein the first heat exchanger (92) is operatively coupled to a heat transfer fluid circuit (2) comprising the molten salts mixture and wherein the second heat exchanger (90) is operatively coupled to a boiler feed water circuit (3) comprising the boiler feed water;
• and
• wherein the back-up boiler system further comprises at least two valves (81, 83) configured to selectively direct flue gas coming from the combustion chamber along at least one of the first heat exchanger, the second heat exchanger, and both of the first heat exchanger and the second heat exchanger.