Abstract: A working fluid heating system that utilizes solar energy and fuel-fired heaters to heat the working fluid is provided. The system may have a fuel heating plant that has a first fuel-fired heater to heat a first portion of working fluid, a solar heating plant that has both a solar thermal-energy heater and a second fuel-fired heater to heat a second portion of working fluid. The first and second portions may join in a pipeline to supply heated working fluid to a facility such as an electrical generation facility, desalination facility, petrochemical facility, enhanced oil recovery facility, or air conditioning facility.