**5. Conclusions**

A novel method has been proposed to choose the best working fluid—from the point of view of expansion route—for a given heat source and heat sink (characterized by a maximum and minimum

temperature). The method is based on the novel classification of working fluids using the sequences of their characteristic points on temperature-entropy space [11]. Using this simple method, a possible expansion route can be defined between an upper and lower temperature values; these temperatures are characterizing the given heat source, heat sink and the set of heat exchangers. In this way, one can select a working fluid (from a database, see [14]), where an ideal adiabatic (isentropic) expansion process between a given upper and lower temperature is possible in a way, that the initial and final states are both saturated vapour states and the ideal (isentropic) expansion line runs in the superheated dry vapour region all along the expansion. In this way, it is possible to avoid problems related to the presence of droplets or superheated dry steam in the final expansion state, just before the condensation. Therefore, one can use the most simple ORC layout, using only a pump, two heat exchangers (evaporator, condenser) and an expander, avoiding the use of superheater (or droplet separator) and recuperator; since the fluid is always in dry condition during expansion, droplet erosion can also be avoided.

Study of potential expansion routes in van der Waals model fluids and comparison of the results with the one obtained with real working fluids can help us to single out erroneous data of working fluids presented in various databases.

Presently we have a database of nearly 30 pure fluids with *T-s* data taken from the NIST Chemistry WebBook [15] and from RefProp 9.1 [16]. Most of these fluids were termed formerly as "dry", while in the novel classification they are in various isentropic sub-classes, namely in ANCMZ, ACNMZ, ANZCM and ANCZM. Due to the fact, that the characteristic point responsible for the existence of these expansion routes (point N) is usually located at low temperature for most of the materials of our database, presently this method with the existing, highly accurate database can mostly be used to choose the working fluids thermodynamically most suitable for cryogenic cycles (applied for example for heat recovery during LNG-regasification, see for example [27,28], although some of the materials can be used in higher temperatures (covering part of the temperature range for geothermal and waste heat utilization).

Extension of the method (including internal efficiencies below 1 for the expander as well as acceptable wetness for the final vapour state) and expansion of the database with more material are in progress. After a proper extension of the database, this method will be available to choose ideal working fluids for other temperatures (like the ones characteristic for various geothermal or waste heat applications).

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1073/12/10/2028/s1, "Cycle efficiencies of ORCs using working fluids presented in Figures 5 and 6".

**Author Contributions:** Conceptualization, A.R.I.; analysis: R.K. and A.G; writing: A.R.I., A.G. and R.K.

**Funding:** This work was performed in the frame of FIEK\_16-1-2016-0007 project, implemented with the support provided from the National Research, Development and Innovation Fund of Hungary, financed under the FIEK\_16 funding scheme. Some part of the research reported in this paper was supported by the Higher Education Excellence Program of the Ministry of Human Capacities in the frame of Nanotechnology research area of Budapest University of Technology and Economics (BME FIKP-NANO) Partial financial supports of the Hungarian National Innovation Office gran<sup>t</sup> (NKFIH, gran<sup>t</sup> No. K116375) is also acknowledged

**Conflicts of Interest:** The Authors declare no conflict of interest.
