Abstract: Solar thermal energy is a type of non-conventional renewable energy (NCRE) that takes advantage of the sun's heat to heat a fluid called heat carrier, then in a heat exchanger it produces steam at high pressure and temperature that generates electricity in a conventional thermal process, this process being of crucial importance since the production of electrical energy on a large scale depends on its efficiency. The technology chosen here is that of parabolic cylinder thermosolar (PCT), that reflect solar radiation and concentrate it in a tube located in its focal line. Inside it is a thermal fluid heated up to almost 400 ºC. The objective of the present investigation was the thermodynamic modelling of the power cycle of PCT plant that works under a modified Rankin cycle. The research is quantitative in nature since it allowed us to assess how the different thermodynamic processes occur, and it is also non-experimental, descriptive, explanatory and propositional, establishing causal relationships based on secondary sources. The result was a real temperatureentropy diagram (T-S) in which the thermodynamic modelling of the power cycle is represented.