Solar/Rankine steam cycle hybrid concentrating solar power (CSP) systems and methods for designing or retrofitting existent natural circulation boilers using saturated or superheated steam produced by direct steam generation (DSG) or Heat Transfer Fluid (HTF) steam generators and CSP solar field technology systems are described. Additionally, methods and processes of retrofitting the existent Heat Recovery Steam Generators (HRSG) or biomass, gas, oil or coal fired boilers to operate integrated to a molten salt/water-steam heat exchangers are disclosed. The hybrid CSP systems are highly efficient due to the increase of steam generated by a heating section comprising either the DSG receiver or the molten salt-water-steam sequential heat exchangers, heaters, boiler/saturated steam generators, super-heaters and re-heaters. The additional saturated, superheated and reheated steam produced is directed to a Rankine cycle according to its pressure, temperature and steam quality significantly reducing the fuel consumption within a cogeneration or Combine Cycle Power Plant.