Review 正式发布 Versions 3 Vol 28 (4) : 597-607 2019
Review of the Working Fluid Thermal Stability for Organic Rankine Cycles;有机朗肯循环工质热稳定性研究进展综述
: 2019 - 07 - 05
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Abstract & Keywords
Abstract: The organic Rankine cycle (ORC) is an efficient power generation technology and has been widely used for renewable energy utilization and industrial waste heat recovery. Thermal stability is a significant property of ORC working fluids and is the primary limitation for working fluid selection and system design. This paper presents a review of the working fluid thermal stability for ORCs, including an analysis of the main theoretical method for thermal stability, a summary of the main experimental method for thermal stability, a summary of the decomposition experimental results for working fluids, and a discussion of the decomposition influence on ORC systems. Further research trends of thermal stability are also discussed in this paper.有机朗肯循环(ORC)技术被认为是一种高效的能源回收利用技术,目前在可再生能源和工业余热资源利用领域都得到广泛的应用。有机工质的热稳定性是非常重要的工质热物性,是ORC工质筛选和系统设计研究中都是重要限制因素。本文主要综述了目前针对有机朗肯循环工质热稳定性的研究进展,包括了对热稳定性的理论研究方法分析、对有机工质热稳定性主要实验方法的总结、对现有的有机工质热分解实验数据的总结以及讨论了有机工质热分解后对ORC的影响机制和程度。同时分析讨论了有机工质热稳定性研究进一步可能的发展方向。
Keywords: organic Rankine cycle (ORC), working fluid, thermal stability
1. Introduction
Renewable energy utilization and industrial waste heat recovery are significant methods for solving global energy and environmental problems. However, some heat sources of industrial waste heat and renewable energy face problems including low energy density, scattered distribution, and unstable energy outputs. The traditional steam power cycle cannot solve these problems and has a relatively low thermal efficiency below 350℃. For these heat sources and temperature extents, the organic Rankine cycle (ORC) is considered to be an efficient power generation technology, compared with other methods because of its advantages in thermal efficiency, variable load adaptability, system cost, and operational maintenance [1-6].
An ORC system with a heat recuperator is shown in Fig. 1. The organic working fluid absorbs heat from the heat source in the evaporator and becomes gaseous under the high pressure and temperature (Process 4-1). The gas flows into the expander and expands to work (Process 1-2). The gas is then cooled to a liquid in the condenser by the heat sink (Process 2-3). The pressure of the liquid is increased by the fluid pump and a new cycle starts (Process 3-4). Although the recuperator is not a basic component of ORC systems, it was widely used in most ORC systems because of the efficiency improvement.
The ORC has been a mature power generation technology after development over several decades and is widely used in solar power, geothermal power, biomass power and industrial waste heat recovery systems [7-11]. By the end of the year 2016, there have been 1754 commercial ORC units and the gross installed capacity is 2701 MW [12].
The ORCs can be divided into subcritical and supercritical ORC by the relationship between the fluid critical properties and the evaporation parameters as shown in Fig. 2. Supercritical ORCs can obtain higher evaporation parameters, which lead to higher theoretical thermal efficiency. Supercritical ORCs also match better with the heat source temperatures for some heat sources with significantly variable temperature ranges. Thus, supercritical ORCs with high heat source temperatures have attracted much interest recently [13-16]. The thermal stability of working fluids becomes more significant because of the rising heat source temperatures.

Fig. 1 Schematic diagram of the ORC system with a heat recuperator

Fig. 2 T-s diagrams of (a) subcritical and (b) supercritical ORCs
2. ORC Working Fluid and Thermal Stability
The organic working fluid is the main characteristic of ORCs. The selection of working fluids is an important research content for the studies of ORCs. The general requirements for ORC working fluids include the following [17-20].
(1) Desirable thermodynamic properties. The working fluids can lead to good cycle thermal efficiency and power output.
(2) Good thermal stability and material compatibility. The working fluids cannot decompose in ORC systems and do not react with system materials.
(3) Good environmental properties. The working fluids should have 0 ozone depleting potential (ODP) and small global warming potential (GWP), and meet other environmental requirements.
(4) No safety problems. No or low flammability and toxicity is better for working fluid selection.
(5) Low cost.
There are many studies about the working fluid selection because of its significance for ORCs, and various organics are considered to be suitable ORC working fluids. At the beginning of ORC research, chlorofluorocarbons (CFCs) and aromatic hydrocarbons were the frequently-used working fluids. Because of increasingly stringent environmental and safety requirements, CFCs have been eliminated for ORC working fluids. In recent studies, suitable working fluids include hydrofluorocarbons (HFCs), hydrocarbons (HCs), hydrofluoroolefins (HFOs), and siloxanes, which have different characteristics and apply to different cycle conditions [21-28].
(1) HFCs. HFCs have been widely used as ORC working fluids in some commercial units because they have been well studied in the fields of refrigeration and heat pumps. HFCs have good thermodynamic properties as ORC working fluids. However, some HFCs with high GWP values will be eliminated in the future because of stringent environmental requirements.
(2) HCs. HCs usually have lower GWP than HFCs, which are good environmental properties for ORC working fluids. HCs also have relatively high critical temperatures and can be suitable in a wide heat source temperature range. Thus, HCs, such as butanes and pentanes, have been widely used in some commercial ORC systems. The main disadvantage of HCs is the flammability problem.
(3) HFOs. HFOs are the potential substitute working fluids for HFCs with similar thermodynamic properties and considerably lower GWP. Even though the new HFOs have high costs at present, their development merits more attention.
(4) Siloxanes. Siloxanes are considered to be suitable choices for high temperature ORCs because of their high critical temperatures, low flammability and toxicity, and good thermal stability.
All organics will decompose at a sufficiently high temperature to different extents. If the working fluids decompose in the ORC systems, the operational parameters will change from the design values to affect the system efficiency and power output. The decomposition products may affect the system components, and may even cause safety problems. Thus, the thermal stability of working fluids is the key property for ORC fluid selection.
The importance of working fluid thermal stability has been recognized in initial ORC studies. For example, Curran et al. [29] and Badr et al. [30] both mentioned that the thermal stability of working fluids should be considered primarily because of the possible decomposition. Curran et al. [29] summarized the approximate decomposition temperatures of 20 fluids, including R11, R21, R113, and toluene. Badr et al. [30] also summarized the approximate decomposition temperatures of some CFCs and HCs. However, these thermal stability results were collected mainly in different research fields with different experimental systems, conditions, and procedures. The decomposition temperatures were called “approximate” because of the large differences among the results for the same fluid. For example, the decomposition temperatures of R12 were 120℃, 204℃, and 300℃ in different literatures by the statistics of Badr et al. The thermal stability studies in other research fields focused on different aims and temperature extents, therefore, the results could not meet the requirements of ORC research.
Even though the thermal stability of working fluids are important for ORCs, there are few specialized thermal stability experiments for ORCs in the initial studies because of the low cycle temperatures during this period. Because of the rising ORC heat source temperatures and more suitable working fluids, studies about the thermal stability of working fluids have become urgent and specialized results have been increasing in recent years.
3. Theoretical Method for Thermal Stability
The decomposition of the working fluid is a unimolecular reaction in chemical theory. There are many theoretical results in the study field regarding the chemistry of working fluids, especially for HCs. For example, Doty et al. [31] summarized three main decomposition paths for HCs, including dehydrogenation, catalytic decomposition and thermal decomposition. Thermal decomposition was confirmed to be the main decomposition paths in ORC temperature ranges. Schroeder et al. [32] summarized the reaction rate constants of some HCs at high temperatures and analyzed the effects of molecular structures on thermal stability. However, Schroeder et al. indicated that these conclusions were based on the experimental results at higher temperatures; therefore, they could not be applied to the ORC temperature extents.
The first order reaction model is the main theoretical method in chemical kinetics for ORC working fluid thermal stability [33]. The definition of the first order reaction is that the reaction rate is proportional to the reactant concentrations as shown in Eq. (1).
where CA is the reactant concentration; t is the time; and k is the reaction rate constant. The other integral form of Eq. (1) can be shown as Eq. (2).
where C0 is the original reactant concentration, and x is the decomposition ratio.
The reaction rate constant is related only to the temperature. The Arrhenius Equation is used to calculate reaction rate constants at different temperatures.
where A is the pre-exponential factor; Ea is the activation energy; R is the gas constant and T is the temperature. The decomposition ratio can be expressed as a function of time and temperature by combining Eqs. (2) and (3).
Eq. (4) is the expression of the first order reaction model. The decomposition ratio at any time and temperature can be calculated if the chemical kinetics factors are obtained by experiments.
Dai et al. [33] verified the first order reaction model method with n-pentane as the test fluid. A chemical kinetics experimental system including heating and heat preservation parts was built to obtain the chemical kinetics factors of n-pentane. The C-t curves of n-pentane at different temperatures in the ORC temperature ranges were measured, and the chemical kinetics factors A and Ea of n-pentane were fitted by the C-t data. The first order reaction model of n-pentane in the ORC temperature ranges is shown as Eq. (5) and was verified by comparison with results from other literatures [33]. The foremost advantage of the first order reaction model method is that it can calculate the effects of time on decomposition. The experimental results in short periods can be used to predict the conditions in the whole operational periods of ORC systems, which are highly important for the evaluation of ORC working fluid thermal stability.
The first order reaction model method is an apparent chemical kinetics model, which does not involve detailed reaction mechanisms. The studies about the reaction mechanism and paths in the molecular levels are helpful to confirm the decomposition products and to analyze the influence factors. The density function theory (DFT) and molecular dynamics (MD) methods are both common reaction simulation methods that have been used for reaction mechanism studies of ORC working fluid thermal stability. Zhang et al. [34] studied the possible reaction paths of R1234yf using the DFT simulation method. The results showed that the direct decomposition products of R1234yf were mainly CF3H, HF, and CF4, and further decomposition products included CF3C≡CH, CF2=C=CH2, and CH≡CF. Cao et al. [35] studied the oxidation reaction mechanism of R1234yf using the ReaxFF MD simulation method. The results showed that the main oxidation products of R1234yf were HF, COF2, and CO2. Huo et al. [36,37] simulated the thermal and oxidation decompositions of R1336mzz-Z using the ReaxFF MD simulation method. The results showed that the thermal decomposition products of R1336mzz-Z were mainly HF, CF≡CF, CF2=CF2, CF4, and CHF3 and the main oxidation products were HF, COF2, and CO2.
The simulation methods are complex and difficult, and the simulation results cannot represent the thermal stability of working fluids directly. Thus, another research idea is to choose a suitable parameter of working fluids to represent the relative thermal stability. Dai et al. [38] chose the minimum dissociation energy value of all the fluid molecule chemical bonds (named as Em ) to represent the relative thermal stability of the working fluid. The Em values of some working fluids were calculated using a chemical simulation software to verify this assumption. The results showed that the Em values had a roughly linear relationship with decomposition temperatures by comparing them with experimental results. However, this representation relationship was weak with similar molecule structures. Thus, accurate thermal stability results still should be obtained experimentally.
4. Experimental Method for Thermal Stability
4.1   Dynamic circuit method
The dynamic circuit method for fluid thermal stability in this paper refers to the experimental method to measure the decomposition in a real ORC system or using a forced dynamic circuit test system that simulates the real ORCs. The decomposition results from real ORC units with long operational periods are significant for fluid thermal stability studies. For example, Curran et al. [29] investigated some running ORC units and fluid decomposition and material corrosion were found in parts of these systems. A more detailed example was from Erhart et al. [39], who investigated eight running ORC plants using MDM as the working fluids. The heat source temperatures of these eight ORC plants were 290-315°C, and the gross operational periods were 6-12 years. The fluid purities of each ORC plant were measured, and decompositions were found in all plants with the maximum decomposition ratio up to 34%. The decomposition affected the ORCs obviously, and the thermal efficiency of one sample plant decreased from 11.5% to 5.04% for 6 years. The decomposition products were also measured by Erhart et al., and other kinds of siloxanes were found to be the main compositions of the MDM decomposition products.
The real ORC system can also be simulated using a forced dynamic circuit test system in the laboratory. A typical example was the experiments for cyclopentane thermal stability by Ginosar et al. [40]. A forced dynamic circuit test system including fluid pumps, control valves, and an electric heating tube was built. The cyclopentane circulated between high and low temperatures uninterruptedly in this test system. The experimental periods were 300 h, and the effective reaction periods were approximately 17-30 h at different temperatures. The decomposition ratios at 240°C, 300°C, and 350°C were measured. Except for the small hydrocarbon gaseous products, some residual products on the tube surface were also detected as heavy saturated hydrocarbons.
4.2   Sealed glass tube method
Because the experimental periods of the dynamic circuit method are lengthy, more experiments for fluid thermal stability are made by static test methods. The sealed glass tube method is a common experimental method for the thermal stability and material compatibility of refrigerants. The detailed introduction for the method is described in the ASHRAE Standard 1997-2002. The refrigerants are generally filled with the metals, air, moisture, or lubricant oils in the same glass tube to study the effects of different materials on fluid decomposition.
The sealed glass tube method was used mainly to study the thermal stability of HFOs in existing literatures. Konstantinos et al. [41] measured the thermal stability of R1234yf using the sealed glass tube method. No decomposition product was detected at 200°C after 14 days and the same results were obtained at 175°C after 14 days with POE lubricants. Konstantinos et al. [42] also compared the thermal stability of R245fa, R1233zd-E, and R1336mzz-Z using the sealed glass tube method. The carbon steel, copper, and aluminum samples were placed in the glass tubes with test fluids at 250°C for 7 days. The results showed that R245fa and R1336mzz-Z both had good thermal stability at 250°C, while the decomposition extent of R1233zd-E was obviously larger than that of the other two fluids at 250°C. Minor et al. [43] added R123 and HFE7100 as comparisons under the same experimental conditions as that of Konstantinos et al. [42]. The results showed that the R245fa and R1336mzz-Z had better thermal stability than R123 and HFE7100 at 250°C.
4.3   Pressure method
The pressure method for fluid thermal stability in this paper refers to the experimental method using fluid pressure changes as the decomposition indicator. The fluid pressure changes include the experimental pressure change during the experimental periods and the difference in the vapor pressure before and after the tests. This method has been widely used for kinds of working fluids and includes a precise measurement system and the quantification analysis method of the decomposition [44].
The typical test system of the pressure method included a reactor, a thermostatic bath and a muffle furnace [45]. The test fluid was filled into the reactor, and the vapor pressure at low temperatures was first measured in the thermostatic bath. The reactor was then placed into the muffle furnace at set temperatures and the pressure in the reactor was recorded during the test period. The reactor was cooled to room temperature after the test. The vapor pressure at low temperatures was measured again in the thermostatic bath and was compared with the original value before the test.
Pressure is the only parameter that is measured in the pressure method, so it can be used to estimate the decomposition temperatures for various kinds of working fluids. Calderazzi et al. [46] measured the decomposition temperatures of some working fluids using the pressure method. The decomposition temperature of R134a was 368-389°C, that of R141b was 90-122°C, that of R13I1 was 102-123°C, and that of R125 was 396-422°C. Angelino et al. [47] measured the decomposition temperatures of some HFCs using the pressure method. The decomposition temperatures of R227ea, R23, R236fa, R143a, and R245fa were measured to be 400-450°C, 425-450°C, 400-450°C, 350-400°C, and 300-330°C, respectively. Pasetti et al. [45] used the same method to measure the decomposition temperatures of cyclopentane, iso-pentane and n-butane, which were found to be 275-300°C, 290-310°C, and 310-330°C, respectively. Invernizzi et al. [48] chose TiCl4 as the test fluid using the pressure method and no obvious pressure change was detected until the test temperature rose to be 500°C.
4.4   Rapid experimental method
Except for the pressure changes, the decomposition of different fluids can produce different decomposition indicators. The decomposition temperatures can be measured using the most sensitive decomposition indicator, which is selected by theoretical analysis and calculation. The experimental method using a sensitive decomposition indicator is collectively called the rapid experimental method in this paper.
The key procedure of the rapid experimental method is the selection of the decomposition indicator. Dai et al. [49] analyzed the decomposition of HCs by the chain thermal decomposition reaction theory and the methane and hydrogen were confirmed to be sensitive decomposition indicators. The decomposition temperatures of some HCs were measured using a rapid experimental system. The results showed that the decomposition temperatures of n-butane and isobutane were 300-320°C, that of n-pentane was 280-300°C, and those of n-hexane, isopentane and cyclopentane were 260-280°C. The results also verified the sensitivity of methane and hydrogen as decomposition indicators. Dai et al. [50] also chose the fluoride ion concentration in product aqueous solutions to be the indicator of HFC decomposition by chemical theory analysis. The decomposition temperature of R245fa was measured to be 300-320°C, that of R152a was 160-180°C, that of R134a was 340-360°C, and that of R236fa was 380- 400°C. The decomposition temperature results by Dai et al. [50] were relatively lower than the results obtained by the pressure method, which meant the fluoride ion concentration was a more sensitive decomposition indicator than pressure.
5. Experimental Results of Thermal Stability
Some experimental results of ORC working fluids are summarized in Table 1. The results of some CFCs are not listed because of their eliminations for environmental requirements. Some results were obtained at higher temperatures than ORCs in other research yields [58-64], or were obtained indirectly by theoretical prediction based on other test results [65-68]. However, these results cannot apply to the ORC temperature extent directly, so they are not listed here.
The results show that the fluid thermal stability has obvious correlativity with the fluid molecular structure. For example, the thermal stability of n-alkanes is better than that of isomerized structures for linear HCs with the same carbon numbers, and longer carbon chains have worse thermal stability for linear HCs with similar structures. For HFCs with similar structures, fluids with more fluorine atoms have better thermal stability, and the fluorine atom number has a larger effect on the thermal stability than the carbon atom number. These laws are favorable for the thermal stability prediction of fluids with similar structures.
Table 1 Experimental results for ORC working fluids
Fluid category and namesResult categoryExperimental results
HFCsR125Decomposition temperature extent396-422°C [46]
R134aDecomposition temperature extent368-389°C [46]
Decomposition temperature extent340-360°C [50]
R143aDecomposition temperature extent350-400°C [47]
R152aDecomposition temperature extent160-180°C [50]
R227eaDecomposition temperature extent400-450°C [47]
R23Decomposition temperature extent425-450°C [47]
R236faDecomposition temperature extent400-450°C [47]
Decomposition temperature extent380-400°C [50]
R245faDecomposition temperature extent300-330°C [47]
Decomposition temperature extent300-320°C [50]
Stable temperature for 14 days250°C [42]
HFEsHFE7100Decomposition temperature250°C [43]
Decomposition temperature extent250-300°C [51]
HFE7200Decomposition temperature extent160-200°C; 200°C, 6.4 mmol/kg [52]
HFE7500Decomposition temperature extent200-230°C; 230°C, <12 mmol/kg [52]
HFOsR1336mzz-ZStable temperature for 14 days250°C [42]
Stable temperature for 7 days with metals, air and moisture250°C [53]
R1234yfStable temperature for 14 days200°C [41]
Stable temperature for 25 h250°C [53]
R1233zdDecomposition temperature250°C [42]
HCsn-butaneDecomposition temperature extent310-330°C [45]
Decomposition temperature extent300-320°C [49]
isobutaneDecomposition temperature extent300-320°C [49]
n-pentaneReaction rate constant315°C: 4.7×10-9 s-1 [55]
Decomposition temperature extent280-300°C [49]
isopentaneReaction rate constant315°C: 7.9×10-9 s-1 [55]
Decomposition temperature extent270-290°C [45]
Decomposition temperature extent260-280°C [49]
neopentaneReaction rate constant315°C: 1.5×10-8 s-1 [55]
cyclopentaneDecomposition temperature extent275-300°C [45]
Decomposition temperature extent260-280°C [49]
Decomposition ratio for 300 h350°C: 0.15% [40]
n-hexaneDecomposition temperature extent260-280°C [49]
n-heptaneDecomposition temperature260°C [49]
benzeneReaction rate constant315°C: 6.3×10-11 s-1 [55]
tolueneReaction rate constant315°C: 6.6×10-9 s-1 [55]
SiloxanesMMDecomposition temperature extent240-300°C [56]
MDMDecomposition temperature290°C [39]
Decomposition temperature250°C [57]
OthersR13I1Decomposition temperature extent102-123°C [46]
R141bDecomposition temperature extent90-122°C [46]
R7146Decomposition temperature extent204-296°C [46]
R123Decomposition temperature extent200-220°C [50]
Decomposition temperature250°C [42]
PerfluorohexaneDecomposition temperature extent350-400°C [58]
TitaniumStable temperature for 80 h500°C [48]
The materials and impurities in ORC systems can also affect the thermal stability of working fluids. For example, Ginosar et al. [40] studied the effects of air on the cyclopentane decomposition. The results showed that the decomposition ratio of cyclopentane with air rose to be 504 ppm, while the ratio was only 274 ppm without air under the same conditions. The air can accelerate the decomposition and change the reaction mechanism. Dai et al. [69] measured the effects of copper and aluminum on the n-pentane decomposition. The results showed that copper and aluminum both had obvious catalysis effects on the n-pentane decomposition and the catalysis effect of copper was stronger than that of aluminum. Erhart et al. [39] analyzed the compositions of the MDM decomposition products from real ORC plants. The residues of lubricants were detected in the decomposition products. The decomposition ratio of MDM was found to have a positive relationship with the mass fraction of lubricant residue, which indicated that the lubricant residue might accelerate the decomposition of MDM. In summary, the air, lubricants, and materials used in the ORC systems all can affect the thermal stability of working fluids.
In addition to the temperature, pressure is also considered to be a possible factor of fluid thermal stability. Andersen et al. [55] measured the effects of pressure on the decomposition of n-pentane and toluene. The results showed that the fluid decomposition ratio had no obvious change when the experimental pressure changed to a very large extent, which indicated that the pressure had little effect on the decomposition of n-pentane and toluene. The same conclusion was also verified by Dai et al. [33] with the measurement of n-pentane decomposition under different pressures.
6. Effect of Fluid Decomposition on ORC Systems
The possible effects of fluid decomposition on ORC systems include the following [71].
(1) The decomposition products change the properties of the original fluids. The operation parameters of ORC systems deviate from the design values, which decrease the thermal efficiency and work output of the systems.
(2) The gaseous products stay in the condenser as non-condensable gas and increase the condensation pressure, which decrease the thermal efficiency and work output of the systems.
(3) The non-condensable gas or solid deposit products cover the surface of the heat exchanger, which causes heat transfer deterioration in the heat exchanger.
(4) The solid products block the tubes in the ORC systems or destroy the moving components, such as the expander, which causes serious safety problems.
(5) The decomposition products are corrosive or poisonous, which can bring serious safety problems for the systems or the environment.
The influence mechanism and the extent are significant for the accurate evaluation of the decomposition experimental results. Thus, studies about the effects of fluid decomposition on ORC systems are important for ORC research.
Dai et al. [71] studied the effects of n-pentane decomposition on ORC systems. The decomposition product composition of n-pentane was first confirmed experimentally. The products of n-pentane included gaseous and solid products and the small molecule gaseous products were the main composition. An ORC system simulation model was then built to calculate the influence extent of the gaseous products. The results showed that the main influence mechanism of the n-pentane decomposition was that the gaseous products increased the condensation pressure as non-condensable gases. A small decomposition ratio could cause relatively large decreases of thermal stability and system outputs by this influence mechanism. Rajabloo et al. [72] built an off-design ORC simulation model to calculate the influence extents of isopentane and MDM decomposition. The effects of heat and cold source conditions and the fluid mass flow rate changes were also considered. The results showed that the decomposition had large effects on ORC system performance, especially the effects of products with low boiling points.
Dai et al. [50] also studied the effects of HFC decomposition with R152a as the test fluid. The experimental results showed that some water insoluble liquid products were found in the aqueous solutions of R152a decomposition products. The reactor was cleaned by ethanol after the tests, and then the ethanol became yellow and washed out a large amount of black residues. The inner wall of the reactor also became black after the tests, and the black residues were measured to be corrosion products of stainless steel. Thus, the products of HFCs have serious corrosion effects on the system materials, which cannot be tolerant for ORC units.
The main decomposition influence mechanisms and products of some working fluids are listed in Table 2.
For HCs, the main decomposition influence mechanism is the condensation pressure change that is caused by gaseous products with low boiling points. For siloxanes, the main decomposition influence mechanism is the cycle parameter deviations that are caused by similar siloxane products. The decompositions of HCs and siloxanes only affect the cycle operation parameters, while they do not affect the components and materials of ORC systems. Thus, the decomposition effects of HCs and siloxanes can be weakened by adjusting the cycle operation parameters. For example, Erhart et al. [39]
Table 2 Decomposition influence mechanisms and products of some working fluids
Fluid categoryMain influence productMain influence mechanismPossible improvement method
HCsGaseous products like methaneCondensation pressure change caused by non-condensable gasesRemove non-condensable gases
HFCsHFCorrosive products corrode the components and materialsIrreversible damages
SiloxanesOther siloxanesCycle parameter deviations caused by siloxane productsRemove the liquid products
designed a distilling treatment system to exclude the decomposition products in different ORC components. The results showed that the removal of the decomposition products could obviously improve the thermal efficiency and work losses. However, the decomposition products of HFCs and HFOs contain HF, which are corrosive gases. The decomposition products can corrode the components and materials of ORC systems, which are irreversible damages and cannot be restored by only adjusting the cycle operation parameters. Thus, the decomposition of HFCs and HFOs should be strictly avoided in ORCs.
7. Conclusions
This paper presents a review of the working fluid thermal stability for ORCs. The theoretical method for thermal stability mainly include the first order reaction model method and the reaction mechanism studies using reaction simulation methods, such as DFT and MD. The main experimental methods for thermal stability are classified as the dynamic circuit method, the sealed glass tube method, the pressure method, and the rapid experimental method. The theoretical and experimental studies investigating the effects of fluid decomposition on ORC systems are also summarized.
The existing decomposition experimental results are mainly about HCs and HFCs, while the results on HFOs and siloxanes are relatively scarce. Thus, more basic decomposition experimental results are needed. The effects of metal materials, air, and lubricants on the fluid thermal stability in real ORC systems also need further studies.
Except for the first order reaction model method, other theoretical methods do not build relationships between decomposition ratios and time, so the results cannot refer to the conditions under long operational periods. Thus, the decomposition reaction laws for long periods need further studies and the relationships between the experimental results and the conditions of the real systems should be built.
The decomposition influence mechanism and extent are significant for working fluid thermal stability studies. The accurate decomposition influence mechanism should be analyzed by confirming the detail product composition and by considering the actual operational conditions of the systems. The improvement method for working fluid thermal stability also merits further investigation.
This work was supported by the National Natural Science Foundation of China (51806117, 51236004), China Postdoctoral Science Foundation funded project (2018M630155), the Science Fund for Creative Research Group (No. 51621062).
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Article and author information
DAI Xiaoye
SHI Lin*
QIAN Weizhong
This work was supported by the National Natural Science Foundation of China (51806117, 51236004), China Postdoctoral Science Foundation funded project (2018M630155), the Science Fund for Creative Research Group (No. 51621062).
Publication records
Published: July 5, 2019 (Versions3
Journal of Thermal Science