Project

Thermal stability tests on organic working fluids

Goal: At the moment we are carrying out a series of measures to analyse the "heat resistance" of working fluids potentially usable in Rankine cycles.

The greater the thermal stability, the greater is the maximum temperature at which the fluid can be used. Higher operating temperatures extend the field of applications of the engines and their efficiency increases too.

The Organic Rankine Cycle power systems can offer viable solutions to generate electric/mechanical power from a wide variety of energy sources. Different applications need different working fluids.

Methods: isoteniscope, vapour pressure

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Project log

Costante Mario Invernizzi
added an update
We have recently reconsidered the thermal stability of some siloxanes by two different approaches: (1) by analysing the variation of the boiling point and (2) by the Differential Scanning Calorimeter,
This second modality could be very interesting for a fast screening of different fluids and classes of fluids. Some results are reported in
In the same paper we carried out also some thermodynamic evaluations on siloxane mixtures.
 
Costante Mario Invernizzi
added an update
The thermochemical decomposition of a fluid occurs through many complex reactions and, as in all chemical reactions, the activation energy plays an important role. So, in a given environment, compounds apparently chemically unstable (with a positive enthalpy of formation) can show an acceptable chemical and thermochemical stability.
Anyway, the enthalpy of formation can give an appropriate useful indication in a first rough classification of working fluids for the thermal stability point of view.
In the Figure, some values of the standard mean enthalpy of formation per bond as a function of the bond number in the molecule for different fluids are reported.
 
Costante Mario Invernizzi
added an update
We did some tests on the thermal stability of pure carbon dioxide according to a new procedure.
Instead to refer to the variation of the vapour pressure, we organised measures along lines at constant volume.
A detailed description of our new approach is in the following paper:
The enclosed Figure gives an example of the obtainable results.
 
Costante Mario Invernizzi
added an update
We investigated the thermal stability of 2,2,2-trifluoroethanol and of n-butanol during a search of compounds to mix with water.
The addition of a solute to water changes its thermodynamic properties (the molecular weight, the parameter of molecular complexity, ec.), attenuating its drawbacks.
 
Costante Mario Invernizzi
added a research item
This work explores the possibility to adopt in organic Rankine cycle (ORC) plants mixtures of water (acting as solvent) plus an organic compound (acting as solute) as the working fluid. Initially an evaluation of the thermodynamic properties of the mixtures is performed, in order to assess their properties, and to point out the molar fractions which entail a near-azeotropic behaviour. Four species from three different classes of chemical compounds are investigated: 2,2,2-trifluoroethanol and n-butanol for alcohols, where the first is fluorinated, acetonitrile for nitrile class and 2-methylpyrazine as a heterocyclic aromatic compound. Simultaneously, the thermal stability of the pure substances considered as the possible solute for the mixtures is experimentally investigated in order to estimate the temperature applicability range. The ORC plant performance, from a low-enthalpy geothermal heat source (hot water stream from 100 to 200 °C), adopting the selected mixtures as the working fluid is finally evaluated, and the analysis includes a preliminary discussion on the turbine design; results are compared with respect to the reference case of a hypothetical plant adopting water as the working fluid.
Costante Mario Invernizzi
added an update
We analysed the thermal stability of some other fluids: (1) n-butanol; (2) tetrahydrofuran; (3) 2,2,2-trifluoroethanol. See Figure 1.
n-butanol is a primary alcohol and tetrahydrofuran is a cyclic ether.
2,2,2-trifluoroethanol results acceptably stable up to 300 degree. It is completely miscible in water and (see Figure 2) different quantities of 2,2,2-trifluoroethanol change significantly and continuously the thermodynamic properties of the resulting mixture.
Mixtures of water and 2,2,2-trifluoroethanol are known as “fluorinols” and were considered in the past as working fluids for ORC. See, for example:
A. Verneau, Recovery from exhaust gas on a diesel engine. In VDI Berichte 539, ORC-HP-Technology. Working Fluid Problems. Proceedings of the International VDI-Seminar held in Zurich, 10-12 September 1984.
 
Costante Mario Invernizzi
added an update
In the Figure an alcohol and a fluoroalcohol are compared (toluene is reported as a reference).
As expected, the fluorine increase the thermal stability of the fluid.
The measures continue with other representative fluids, but a problem arises: how do the values of the parameter on the ordinate affect the cycle performances?
 
Costante Mario Invernizzi
added an update
The Figure compares results of thermal stability of toluene with the results for an alcohol.
The experimental procedure is described in:
At a temperature of 300 °C the decomposition rate of the considered alcohol is 10-20 times greater than the corresponding rate for toluene.
 
Costante Mario Invernizzi
added a research item
In this paper we investigate the thermal stability of three representative hydrocarbons used as working fluids in organic Rankine cycles (ORC): n-pentane, cyclo-pentane and toluene. The experimental used method is a “static” one, based on the recording of the pressure during the permanence of the fluid sample at constant temperature and on the measure of the differences in the vapour pressure in comparison with the reference values for the virgin pure fluid. The sample container and the circuit are in stainless steel.
Costante Mario Invernizzi
added an update
We published recently the following paper:
in the article a comparison is carried out among three representative hydrocarbons: three typical working fluids for Rankine cycles.
a copy of the paper can be downloaded from:
 
Costante Mario Invernizzi
added an update
The results we will obtain could be similar to those in Figure 1.
In Figure 1 a parameter k*, proportional to the constant of the velocity reaction, is reported as a function of the (reciprocal) test temperature. The series of measures in Figure 1 are relative to a linear hydrocarbon, to a linear perfluoro-carbon and to an aromatic hydrocarbon. It is common knowledge perfluoro-carbons and aromatic hydrocarbons, as a general rule, are more thermally stable than linear hydrocarbons and the results in Figure confirm this conclusions. The thermal stability of the aromatic hydrocarbon is all similar to that of the perfluoro-carbon.
So, the (quite simple) method and the procedure we utilise are able to discriminate between different fluids on the ground of their thermal stability.
 
Costante Mario Invernizzi
added an update
Our aim is to inspect systematically several families of organic (and inorganic) compounds to classify them from the thermal stability point of view.
We are beginning with some representative well known hydrocarbons: linear, cyclic and aromatic.
The method we are using is a static method, as described in:
Invernizzi, Costante M., Bonalumi D, Thermal stability of organic fluids for Organic Rankine Cycle systems, in Organic Rankine Cycle (ORC) Power Systems - Technologies and Applications, Ennio Macchi and Marco Astolfi eds. Elsevier - Woodhead Publishing. September 12th 2016
 
Costante Mario Invernizzi
added a project goal
At the moment we are carrying out a series of measures to analyse the "heat resistance" of working fluids potentially usable in Rankine cycles.
The greater the thermal stability, the greater is the maximum temperature at which the fluid can be used. Higher operating temperatures extend the field of applications of the engines and their efficiency increases too.
The Organic Rankine Cycle power systems can offer viable solutions to generate electric/mechanical power from a wide variety of energy sources. Different applications need different working fluids.