A Simple Method of Finding New Dry and Isentropic Working Fluids for Organic Rankine Cycle

Gábor Györke,Attila Imre,Axel Groniewsky

doi:10.3390/en12030480

Gábor Györke, Attila Imre + Show 1 more

Open Access

https://doi.org/10.3390/en12030480

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Abstract

One of the most crucial challenges of sustainable development is the use of low-temperature heat sources (60–200 °C), such as thermal solar, geothermal, biomass, or waste heat, for electricity production. Since conventional water-based thermodynamic cycles are not suitable in this temperature range or at least operate with very low efficiency, other working fluids need to be applied. Organic Rankine Cycle (ORC) uses organic working fluids, which results in higher thermal efficiency for low-temperature heat sources. Traditionally, new working fluids are found using a trial-and-error procedure through experience among chemically similar materials. This approach, however, carries a high risk of excluding the ideal working fluid. Therefore, a new method and a simple rule of thumb—based on a correlation related to molar isochoric specific heat capacity of saturated vapor states—were developed. With the application of this thumb rule, novel isentropic and dry working fluids can be found applicable for given low-temperature heat sources. Additionally, the importance of molar quantities—usually ignored by energy engineers—was demonstrated.

Highlights

In the last few decades, power generation started to turn to alternative energy sources, such as wind energy and photovoltaics, as well as unconventional sources of heat, such as geothermal energy, solar heat, biomass-firing plants, or waste heat from other industrial processes
Organic Rankine Cycle (ORC) provides an alternative solution, because it uses non-conventional working fluids, mainly organic ones. These unconventional working fluids are suitable for power generation from various low-temperature heat sources [1,2], including geothermal heat [3,4]
The working fluid is called dry if an isentropic expansion starting from a saturated vapor state will end in the single-phase vapor region, which is called

Summary

Introduction

In the last few decades, power generation started to turn to alternative energy sources, such as wind energy and photovoltaics, as well as unconventional sources of heat, such as geothermal energy, solar heat, biomass-firing plants, or waste heat from other industrial processes. Organic Rankine Cycle (ORC) provides an alternative solution, because it uses non-conventional working fluids, mainly organic ones These unconventional working fluids are suitable for power generation from various low-temperature heat sources [1,2], including geothermal heat [3,4]. Using one or two of these points and correlating them with the isochoric molar specific heat capacities of the equilibrium vapor phase, we are able to give a “rule of thumb” to select working fluids, where droplet formation during expansion and, the potentially harmful droplet erosion of turbine blades—common for traditional and some organic Rankine processes using wet working fluids [19,20]—can be avoided In this way, several new potential working fluids can be found and used for further evaluation, using general criteria for ORC working fluid selection including thermodynamics [18,21], safety and environmental impact [22,23], availability, compatibility, and cost-effectiveness [24], or most often, a combination of these criteria

Traditional Classification

Characteristic Points

Application and Correlations

Conclusions