According to present invention, a method is provided for operating a multi-heat source power plant using a low-medium temperature heat source fluid, wherein the multi-heat source power plant includes a turbine or expander run by an organic motive fluid, comprising preheating the organic motive fluid using the low-medium temperature heat source fluid and thereafter providing further heat from an additional heat source to vaporize the motive fluid which is supplied to the turbine or expander. Furthermore, in an embodiment of the present invention, the present invention provides an apparatus comprising a heat exchanger suitable to pre-heat an organic motive fluid with a low-medium temperature geothermal fluid, and solar energy collecting means suitable to directly or indirectly provide heat to the pre-heated organic motive fluid for heating and vaporizing the motive fluid. In addition, in a further embodiment of the present invention, the present invention provides an apparatus comprising a heat exchanger suitable to pre-heat an organic motive fluid with a low-medium temperature unused industrial heat streams, and solar energy collecting means suitable to directly or indirectly provide heat to the pre-heated organic motive fluid for heating and vaporizing as well as superheating the motive fluid.