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

Juan A White,Santiago Velasco

doi:10.3390/en12173266

Abstract

Recently, we proposed an approximate expression for the liquid–vapor saturation curves of pure fluids in a temperature–entropy diagram that requires the use of parameters related to the molar heat capacity along the vapor branch of the saturation curve. In the present work, we establish a connection between these parameters and the ideal-gas isobaric molar heat capacity. The resulting new approximation yields good results for most working fluids in Organic Rankine Cycles, improving the previous approximation for very dry fluids. The ideal-gas isobaric molar heat capacity can be obtained from most Thermophysical Properties databases for a very large number of substances for which the present approximation scheme can be applied.

Highlights

There is an increasing interest in the use of Organic Rankine Cycles (ORCs) as a suitable way of generating power from low-temperature heat sources such as geothermal, solar thermal, biomass, waste heat, and bottoming cycles
One of the most relevant aspects in ORC working fluid selection is the analysis of the shape of the liquid–vapor saturation curve in a temperature–molar entropy (T-s) diagram because it has a direct influence both in the thermal efficiency and in the particular design of the cycle
Tcon, so that Tcon < Tev < Tc where Tc is the critical temperature. In this simple ORC, the isentropic expansion that takes place in the turbine and starts from a saturated vapor state at Tev can lead to three different situations depending on the shape of the saturation curve: (1) If the mean slope of the vapor branch of the T-s saturation curve between Tcon and Tev is negative, the working fluid has a wet fluid behavior and the isentropic expansion process in the turbine gives rise to condensation, i.e., it ends in the two-phase region of the T-s diagram

Summary

Introduction

There is an increasing interest in the use of Organic Rankine Cycles (ORCs) as a suitable way of generating power from low-temperature heat sources such as geothermal, solar thermal, biomass, waste heat, and bottoming cycles. Tcon , so that Tcon < Tev < Tc where Tc is the critical temperature In this simple ORC, the isentropic expansion that takes place in the turbine and starts from a saturated vapor state at Tev can lead to three different situations depending on the shape of the saturation curve: (1) If the mean slope of the vapor branch of the T-s saturation curve between Tcon and Tev is negative, the working fluid has a wet fluid behavior and the isentropic expansion process in the turbine gives rise to condensation, i.e., it ends in the two-phase region of the T-s diagram. This situation should be avoided (via superheating) since the mixture of vapor with liquid droplets could lead to damage of the turbine blades [7]

Objectives

Results

Conclusion