A back-up boiler system 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) 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), preferably dependent on availability of solar radiation and/or dependent on demand of power generation. The invention also relates to a solar thermal power plant (201) for transferring solar energy into electricity and a method for operating a solar thermal power plant.

A method and a device for recovering heat from an engine and for converting the heat into mechanical energy using an expansion machine. A heat accumulator fluid is guided in a primary circuit by means of a primary pump and is firstly heated by the waste heat of the combustion engine by means of a heat exchanger, is transferred into a heat accumulator and recirculated to the first heat exchanger, and, secondly, the heat accumulator fluid is guided in a secondary circuit by said heat accumulator fluid being extracted in the vapor state from the heat accumulator and being supplied to the expansion machine, and being condensed by means of a condenser downstream thereof and being recirculated into the heat accumulator by means of a secondary pump. The primary circuit of the heat accumulator fluid is connected to the secondary circuit exclusively via the heat accumulator.