This paper presents a new modeling and algorithm to analyze and optimize a Central Receiver System (CRS) for solar thermal energy collection and storage. The analysis includes associated modeling of the heliostat field and the heat transfer at the receiver to couple the energy in the two subsystems. Transient optical performance of the heliostat field and predicted heat flux at the receiver is provided. The modeling analyzed a case which is a medium temperature solar concentrating system with pressurized water as heat transfer fluid and thermal storage medium. A tubular external solar receiver is considered and the modeling determines the geometry to maintain a best thermal efficiency. The transient optical performance and heat flux mapping are conducted using Monte Carlo Ray Tracing (MCRT) technique. The new receiver modeling and approach of optimization design was verified through analysis of two reference solar thermal power plants with molten salt heat transfer fluid (i.e. SolarTwo and GemaSolar Plants) with a satisfactory agreement to the reported design parameters. The computations and algorithm was implemented in a use-friendly interface via Visual Basic. The modeling and algorithm provides an efficient tool for solar thermal power plant design in industry.