Richard Mollier (1863-1935) spent most of his working life at the Technische Hochschule in Dresden studying the properties of thermodynamic media and their effective representation in the form of charts and diagrams. His major contribution was in popularizing the use of enthalpy. In 1904, Mollier devised the first enthalpy-entropy chart still most closely associated with his name. However, he published a number of other enthalpy-based charts and in recognition of his work, the US Bureau of Standards recommended in 1923 that all such charts should be known as Mollier diagrams.
Mollier's H-S diagram (Enthalpy v Entropy) was a logical extension of the T-S diagram (Temperature v Entropy) first proposed by Gibbs, retaining the advantages of T-S diagrams but introducing several new advantages. A typical H-S Mollier diagram for a thermodynamic fluid such as steam is shown in Figure 1.
The advantages of such a diagram are that vertical lines represent reversible processes and horizontal lines represent lines of constant energy. Power generation and refrigeration cycles are most conveniently represented on H-S diagrams because work can be calculated directly from vertical distances as opposed to areas on T-S and P-V diagrams. Further the inefficiencies due to irreversibility in real processes are shown clearly on an H-S diagram as shown in Figure 2.