Thermodynamic Selection of the Optimal Working Fluid for Organic Rankine Cycles

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.

Evaluation of the Economic and Environmental Performance of Low-Temperature Heat to Power Conversion using a Reverse Electrodialysis – Multi-Effect Distillation System

“…Gross The high-performing membranes bring more than a three-fold power production increase, and the use of KAc instead of NaCl accompanied by the reheating option greatly increases the thermal efficiency, reaching a value of 5.5%. This value is similar or even slightly higher compared to the ORC technology for the same heat source temperature of 100 • C [29][30][31]. The thermal efficiency of the RED-MED system under cases 5 and 6 can be increased further, reaching values of about 10%, as presented in Ref.…”

Approximating the Temperature–Entropy Saturation Curve of ORC Working Fluids From the Ideal Gas Isobaric Heat Capacity

“…Equating Equations (10) and (21) and taking as reference the reduced temperature T Mr = 0.81, we obtain…”

“…Finally, fluids like the refrigerants RE143a, R11, or R116 present a wide range of temperatures for which the saturated vapor curve is almost vertical. These fluids are usually termed as isentropic, although isentropic behavior can also be obtained with a particular class of dry fluids [8][9][10]. To summarize, the slope of the vapor branch of the T-s saturation curve gives rise to a basic classification of working fluids into three categories: wet, dry, and isentropic.…”

Zeotropic Mixture Selection for an Organic Rankine Cycle Using a Single Screw Expander

“…Organic substance is used as the working fluid of the organic Rankine cycle which is usually used for the exploitation and utilization of medium-low grade heat source whose temperature is lower than 250 • C. Unlike lower molecular weight fluids like water, turbine design considerations in less than 100 kW output capacities results in lower efficiencies compared to heavier molecular weight organic fluids. The thermodynamic performance, working condition, impact on the environment, and economic feasibility of an ORC system are greatly determined by the characteristics of working fluid [1,2]. CFCs that were invented in 1930s and have a high ozone depletion potential (ODP) and a highest Global Warming Potential (GWP), HCFCs that were invented in 1950s and have a lower ODP and a high GWP, and HFCs that were invented in 1990s and have no ODP but a high GWP, were dominant organic working fluids before the ratification of the Montreal Protocol in 1987 and the ratification of the Kyoto Protocol in 1997.…”