**Contents**


## **About the Special Issue Editor**

**Attila R. Imre** received his M.Sc. in Physics from Eotv ¨ os University, Budapest in 1990; four years ¨ later, he obtained a Ph.D. from the same institution. He has spent several years in various U.S. and German universities, like the University of Tennesee, Johannes Gutenberg Universitat Mainz, and ¨ Universitat zu K ¨ oln. InHungary, he held various positions in the Atomic Energy Research Institute. ¨ Later, this institute became part of the newly formed Centre for Energy Research, where A.R. Imre is a Scientific Advisor and the Head of the Scientific Council of the research centre. After obtaining a Doctor of Science title from the Hungarian Academy of Science in 2015, he was appointed a Full Professor of Energy Engineering with the Budapest University of Technology and Economics. His present research topics cover various fields of energy production and storage, including the use of low-temperature heat sources (like geothermal, solar, and industrial waste heat) by ORC and similar methods and the application of power-to-gas technologies in energy storage. He has authored around 100 papers in various international journals.

#### **Preface to "Working Fluid Selection for Organic Rankine Cycle and Other Related Cycles"**

To create a more environmental-friendly and sustainable society, previously improperly used "low-quality" energy sources can be used. In the last few decades, power generation from low-temperature heat sources (below 300 ◦C), like thermal, solar, geothermal, biomass, or waste heat, has become increasingly significant. The traditional Rankine cycle using water as a working fluid cannot be used with sufficient efficiency at low temperatures. Therefore, finding novel working fluids for organic Rankine cycles or similar but less frequently used thermodynamic cycles (like trilateral flash cycles) has become a priority.

Traditionally, the working fluid for a given ORC process is selected using a trial-and-error procedure through experience from chemically similar materials. This method, however, risks excluding novel and previously unused working fluids that could be more suitable for the given heat source than any of the traditional fluids. In this Special Issue, more sophisticated methods are presented using optimization models, thermodynamic analyses, equation-of-state parameters, and molecular properties. Expanders and working fluids should be selected together; therefore, some discussion related to special ORC-expanders is also included.

We aim to present a reliable source for researchers, innovators, and developers working on ORC-related fields to help them to find the proper working fluid/expander pairs for any given heat source.

> **Attila R. Imre** *Special Issue Editor*

## **Thermodynamic Selection of the Optimal Working Fluid for Organic Rankine Cycles**

#### **Attila R. Imre 1,2,\*, Réka Kustán 1 and Axel Groniewsky 1**


Received: 6 May 2019; Accepted: 24 May 2019; Published: 27 May 2019

**Abstract:** A novel method proposed to choose the optimal working fluid—solely from the point of view of expansion route—for a given heat source and heat sink (characterized by a maximum and minimum temperature). The basis of this method is the novel classification of working fluids using the sequences of their characteristic points on temperature-entropy space. The most suitable existing working fluid can be selected, where an ideal adiabatic (isentropic) expansion step 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. Problems related to the presence of droplets or superheated dry steam in the final expansion state can be avoided or minimized by using the working fluid chosen with this method. Results obtained with real materials are compared with those gained with model (van der Waals) fluids; based on the results obtained with model fluids, erroneous experimental data-sets can be pinpointed. Since most of the known working fluids have optimal expansion routes at low temperatures, presently the method is most suitable to choose working fluids for cryogenic cycles, applied for example for heat recovery during LNG-regasification. Some of the materials, however, can be applied in ranges located at relatively higher temperatures, therefore the method can also be applied in some limited manner for the utilization of other low temperature heat sources (like geothermal or waste heat) as well.

**Keywords:** adiabatic expansion; isentropic expansion; T-s diagram; working fluid classification; optimization
