A new method used to evaluate organic working fluids

A new method used to evaluate organic working fluids

Energy 67 (2014) 363e369 Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy A new method used to eva...

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Energy 67 (2014) 363e369

Contents lists available at ScienceDirect

Energy journal homepage: www.elsevier.com/locate/energy

A new method used to evaluate organic working fluids Xinxin Zhang a, b, c, *, Maogang He b, Jingfu Wang a, c a Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, PR China b Key Laboratory of Thermo-Fluid Science and Engineering, Ministry of Education, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, PR China c Key Laboratory of Heat Transfer and Energy Conversion, Beijing Municipality, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 August 2013 Received in revised form 7 January 2014 Accepted 9 January 2014 Available online 4 February 2014

In this paper, we propose a method named “Weight Classification-Hasse Dominance” to evaluate organic working fluids. This new method combines the advantages of both the method of weight determination and the Hasse Diagram Technique (HDT). It can be used to evaluate the thermodynamic performance, environmental protection indicator, and safety requirement of organic working fluid simultaneously. This evaluation method can offer good reference for working fluid selection. Using this method, the organic working fluids which have been phased out and will be phased out by the Montreal Protocol including CFCs (chlorofluorocarbons), HCFCs (hydrochlorofluorocarbons), and HFCs (hydrofluorocarbons) were evaluated. Moreover, HCs (hydrocarbons) can be considered as a completely different kind of organic working fluid from CFCs, HCFCs, and HFCs according to the comparison based on this new evaluation method. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: Weight classification Hasse Diagram Technique Organic working fluid Evaluation method

1. Introduction Working fluid plays a very important role in thermodynamic cycle. The efficiency, the operation condition, the impact on the environment, and the economic feasibility of thermodynamic cycle are greatly affected by working fluid selection and the nature of the working fluid. As far as the organic working fluid is concerned, there are mainly two applications. One is used as refrigerant in refrigeration cycle which is the primary application of the organic working fluid. The other application is low-grade heat recovery through an organic Rankine cycle (ORC). In 1926, the first CFCs (chlorofluorocarbons), R12 was developed by Thomas Midgley [1]. Commercial production of R12 began in 1931, followed by R-11 in 1932 [2,3]. Compared with the early refrigerants such as sulfur dioxide, CFCs (chlorofluorocarbons) is nonflammable, non-toxic and efficient. CFCs (chlorofluorocarbons) and later, starting in the 1950s, HCFCs (hydrochlorofluorocarbons) dominated the refrigerants from 1931 to 1990s. In 1966, Ray and

* Corresponding author. Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, PR China. Tel.: þ86 10 6739 1612; fax: þ86 10 6739 2774. E-mail addresses: [email protected], [email protected] (X. Zhang). 0360-5442/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.energy.2014.01.030

Moss employed fluorochemicals as working fluids in a small Rankine cycle power unit [4]. This led to the birth of the concept of organic Rankine cycle (ORC) which has been drawing more attention in the field of low-grade heat utilization and recovery. ORC has become the second main application of the organic working fluid. However, by the mid 1970s, there had been concerns about the thinning of the ozone layer and whether CFCs may be in part responsible. These concerns led to the ratification of the Montreal Protocol in 1987 that required the phase out of CFCs and HCFCs [5]. New solutions were developed with HFCs (hydrofluorocarbons) taking on a major role as refrigerants. In the 1990s, global warming became a new threat to the human being. Although many factors can cause global warming, refrigerants were again involved in the discussion due to the significant energy consumption of air conditioning and refrigeration and greenhouse gas effect caused by refrigerants themselves. Global warming threat led to the ratification of the Kyoto Protocol in 1997 that set binding target for 37 industrialized countries and the European community for reducing greenhouse gas emissions of six primary greenhouse gases [6]. Many researchers conducted numerous works on the selection of organic fluids [7e17]. These works are generally based on particular application domain in which fluid-related and processrelated properties are studied by testing various available fluids in analysis and simulation models. For example, organic fluid is used in ocean thermal energy conversion (OTEC) system [7],

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in refrigeration machines [8], in vehicle exhaust waste heat recovery [9e12], and in recovery of low-temperature flue gas produced by a liquefied petroleum gas (LPG) stove [13]. Fewer works also consider the performance of several working fluids either through parametric studies or optimization of process operating parameters [14e17]. However, in most works, attention focused on thermal efficiency and exergy evaluations when selecting working fluids. The selection results are different due to the different application objectives. Therefore, an accurate methodology to evaluate organic working fluids comprehensively and systematically is of great significance. There are many decision making approaches which have been successfully applied in power and energy area [18e22]. However, only little work introduces the decision making approaches used in comprehensive and systematical working fluid evaluation and selection [23]. In this paper, we briefly describe the selection criteria of organic working fluid including thermodynamic performance, environmental protection indicator, and safety requirement. A mathematical method named Hasse Diagram Technique (HDT) which is used to assess order relationships of chemicals is introduced. On this basis, we propose a method named “Weight ClassificationHasse Dominance” to evaluate organic working fluids. This new method combines the advantages of both the method of weight determination and the Hasse Diagram Technique (HDT). It can be used to evaluate the thermodynamic performance, environmental protection indicator, and safety requirement of organic working fluid simultaneously. Using this method, the organic working fluids which have been phased out and will be phased out by the Montreal Protocol including CFCs (chlorofluorocarbons), HCFCs (hydrochlorofluorocarbons), and HFCs (hydrofluorocarbons) were evaluated. Moreover, HCs (hydrocarbons) were also compared with the above three kinds of organic working fluids using this new evaluation method. In real world, the proposed method can be used to evaluate working fluids comprehensively and systematically in terms of thermodynamic performance, environmental protection indicator, and safety requirement simultaneously. Thus, the drawback of subjective overestimation in some evaluation method can be avoided.

solidification of fluid can be avoided. (4) In temperatureeentropy (Tes) diagram, the slope of the saturation vapor curve of the working fluid should be positive or nearly infinitely large. After the isentropic expansion process to produce work, the working fluid with a positive slope of the saturation vapor curve is at a superheated state. The working fluid with a nearly infinitely large slope is at a saturated state. This character avoids the damage to the turbine’s blade caused by the wet vapor. (5) The working fluid should have big specific heat capacity, big density, low viscosity, and good heat transfer capacity. The expander size and heat transfer area of the heat exchanger can be reduced because of these characteristics. 2.2. Environmental protection indicator It can be said that since the ratification of the Montreal Protocol in 1987 and the Kyoto Protocol in 1997, more and more attention has been paid to the development of environmentally friendly working fluid. Indicators for quantitative comparison of the various working fluids are ODP (Ozone Depletion Potential) and GWP (Global Warming Potential), which are closely related to their atmospheric lifetime (ALT). The Ozone Depletion Potential (ODP) of a chemical compound is the relative amount of degradation to the ozone layer it can cause, with trichlorofluoromethane (R-11 or CFC-11) being fixed at an ODP of 1.0. The first proposal of ODP came from Wuebbles in 1981 [24]. It was defined as a measure of destructive effects of a substance compared to a reference substance. Precisely, ODP of a given substance is defined as the ratio of global loss of ozone due to given substance over the global loss of ozone due to CFC-11 of the same mass. Its definition can be given by the following equation.

ODPi h

global loss of ozone due to given substance i global loss of ozone due to CFC  11

A simple and purely physical GWP index, based on the timeintegrated global mean radiative forcing (RF) of a pulse emission of 1 kg of some compound (i) relative to that of 1 kg of the reference gas CO2 was developed [25] and adopted for use in the Kyoto Protocol [6]. The GWP of component i is defined by

ZTH

2.1. Thermodynamic performance Cycle efficiency is considered the most important in thermodynamic performance of an organic working fluid. Besides, the following factors should be considered when selecting a working fluid used in the ORC. (1) The critical point of the working fluid should be above the highest operating temperature in the thermodynamic cycle. Therefore, the problems caused by adopting transcritical cycle can be avoided. (2) The saturation pressure of the working fluid at the highest operating temperature of the cycle should be in an acceptable range. The condensation pressure of the fluid should not be very low, but approximately equal to the atmospheric pressure. Very high pressure or high vacuum has a tendency to impact the reliability of the cycle or increase the unnecessary cost of equipment. (3) The freezing point of the fluid must be below the lowest operating temperature in the cycle. Therefore, blockage and damage of equipment caused by

ZTH RFi ðtÞdt

2. Selection criteria of organic working fluid Generally speaking, there are three important considerations that need to be taken into account when selecting organic working fluid. These three considerations are thermodynamic performance, environmental protection indicator, and safety requirement.

(1)

GWPi h 0 ZTH

ai $½Ci ðtÞdt ¼

0

RFr ðtÞdt 0

(2)

ZTH ar $½Cr ðtÞdt 0

where, TH is the time horizon, RFi is the global mean RF of component i, ai is the RF per unit mass increase in atmospheric abundance of component i (radiative efficiency), [Ci(t)] is the timedependent abundance of i, and the corresponding quantities for the reference gas (r) in the denominator. Those organic working fluids with zero ODP and low GWP should be selected to use in refrigeration cycle and ORC. 2.3. Safety requirement The consideration of safety requirement of organic working fluid mainly includes toxicity, flammability, and chemical stability. Toxicity and flammability are the two key parameters used by American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) to indicate the safety level of an organic working fluid [26]. ASHRAE Standard 34 has adopted a matrix as shown in Table 1 that indicates the relative levels of these two parameters.

X. Zhang et al. / Energy 67 (2014) 363e369 Table 1 ASHRAE standard 34-organic working fluid safety classification.

Higher flammability Lower flammability No flame propagation

Lower toxicity

Higher toxicity

A3 A2 A1

B3 B2 B1

Organic working fluid may decompose when it reaches a certain temperature and pressure. Decomposition product might cause corrosion of equipment, even lead to an explosion and combustion. Therefore, chemical stability is a factor for selecting working fluid. The suitable working fluid can be selected according to the temperature range of waste heat. Generally speaking, the easier a fluid can decompose, the more toxicity it has. Non-toxic, non-flammable, and non-explosive working fluid should be selected to use in refrigeration cycle and ORC.

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Fig. 1(c) can be obtained. Although weighting analysis uses all descriptors simultaneously, the determination of weights is subjective. Therefore, it is necessary for us to establish a scientific approach to analyze all descriptors objectively and simultaneously. As for power and energy engineering area, some popular decision making approaches have been applied successfully such as analytical hierarchical process (AHP) and fuzzy AHP [18], the attack graph and multiple criteria decision making (MCDM) [19], the nonlinear optimization neural network approach [20], pairwise comparison of analytic hierarchy process (AHP) [21], and optimal remote terminal unit (RTU) placement [22]. However, only little work introduces the decision making approaches used in comprehensive and systematical working fluid evaluation and selection. A ranking method named Hasse Diagram Technique which previously used to assess the environmental relevance of organic and inorganic chemicals can avoid the drawback mentioned above [27e29].

3. A new method for evaluating organic working fluid 3.2. Hasse Diagram Technique (HDT) The last section described the selection criteria of organic working fluid. Unfortunately, we still cannot find an ideal organic working fluid which has good thermodynamic performance, zero ODP and low GWP and can meet non-toxic, non-flammable, and non-explosive requirement. Therefore, there is a compromise when selecting an organic working fluid. It is necessary for us to establish a scientific index to evaluate organic working fluids. A key to establish this index is to compare and rank fluids with different indicators simultaneously and independently. 3.1. Some ranking method [23] Objects a, b, . are gathered in a set G. Different descriptors q1, q2, ., qi are used to rank these objects. For instance, Fig. 1 depicts a data matrix used to describe a set of objects G ¼ {a, b, c, d, e, f, g}. Considering only one property will obtain a linear ranking. Fig. 1(a) depicts the linear ranking result when q1 is regarded and Fig. 1(b) for q2. Object a and b have the same descriptor q2 [q2(a) ¼ q2(b)], object e and g also have the same one[q2(e) ¼ q2(g)]. Therefore, each of theses pairs is equivalent (a w b and e w g) in a linear ranking result depicted by Fig. 1(b). In practical application, the objects need to be ranked always have several descriptors which have to be considered and compared simultaneously. Weighting analysis is a commonly used method in practical application. If a weight gi is given to each descriptor qi, then a new superdescriptor can be obP tained by calculating the weighted sum according to G ¼ gi  qi. If we give the same weight to q1 and q2, then a linear order showed in

Two groups of descriptors q1(x), q2(x), ., qi(x) and q1(y), q2(y), ., qi(y) are used to characterize two objects x and y, respectively. In an HDT ranking procedure, these two objects are compared in the following way [28,29]. If all the descriptors used to characterize x are higher than those of y (qi(x) > qi(y), for all i) or no less than one descriptor is higher for x while all others are equal (qj(x) > qj(y), for some j; qi(x) ¼ qi(y), for all others), then x is ranked higher than y (x > y). In this case, x and y are said to be comparable. Object x and y are equivalent if all their corresponding descriptors are equal [28]. When x > y and y > z, then x > z. x and y are said incomparable if one descriptor is higher for x while all others are opposite (qj(x) > qj(y), qi(x) < qi(y)) [28]. Such order relationships can be presented in a Hasse Diagram (HD) as Fig. 1(d). The lines connecting the objects in HD are its important features. Objects with the highest ranks only have lines in the downward direction, for example object a and d in Fig. 1(d). Similarly, objects with the lowest ranks only have lines in the upward direction (b and g in Fig. 1(d)). Two objects are incomparable if there are no lines between them. If no direct line is drawn between two objects but there is a sequence of lines connecting them in the same direction, for example, f and b, then these two objects are comparable (f > b) because there is a path f > c, c > b. In an HD, we can recognize the incomparable pairs easily because there are no lines between them, or they are connected by lines not following the same direction, for example, b and g in Fig. 1(d). In an HD, any comparison between two objects requires enough knowledge about the importance of the descriptors. Therefore, HDT can avoid the drawback of subjective overestimation in weighting analysis. 3.3. A new method for evaluating organic working fluid

Fig. 1. Data matrix of seven objects need to be ranked and ranking results according to (a) q1, (b) q2, (c) a weighted combination of q1 and q2, and (d) Hasse Diagram Technique.

From the introduction of HDT and its comparison with weighting analysis described in the last subsection, it can be seen that HDT can offer great help for the decision making when evaluating and selecting organic working fluids. In this paper, we try to combine the advantages of both the method of weight determination and the Hasse Diagram Technique (HDT) to propose a new method for evaluating organic working fluids. We named it “Weight Classification-Hasse Dominance” method. As mentioned earlier in this paper, there is no ideal organic working fluid that can meet all selection criteria discussed in the last section. Compromise must be made when selecting the fluids. Among all the selection criteria, thermodynamic performance,

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environmental protection indicator, and safety requirement are three equally important properties. We call them senior properties of working fluid. There is a series of properties belonging to each senior property or they can affect the senior property. We call it corresponding sub-property. For example, thermodynamic performance is a senior property of working fluid. Corresponding subproperty of working fluid includes critical temperature, saturation pressure, triple point temperature, viscosity, heat transfer coefficient, and so on. Moreover, environmental protection indicator is a senior property, while Ozone Depletion Potential (ODP), Global Warming Potential (GWP), and Atmospheric Lifetime (ALT) are three corresponding sub-properties. Similarly, safety requirement is a senior property, while toxicity, flammability, and chemical stability are three corresponding sub-properties. Three senior properties of working fluid are equally important and they are independent of each other. Therefore, any subjective choice involving weighting determination is not advisable, while the Hasse Diagram Technique is suitable for comparing and ranking different organic working fluids on the basis of these three senior properties. However, among all the sub-properties, there must be a compromise and a trade-off according to specific issues. Therefore, weighting analysis can be used to compare the sub-property of working fluid. For example, as for heat source with different temperature, selection of working fluid on the basis of thermodynamic performance will be different due to the difference of critical temperature, saturation pressure, latent heat, density, specific heat and so on. In such a selection procedure, a weight coefficient which represents “importance” can be set according to a specific problem for decision making. After the order of sub-property is obtained using weighting analysis, the Hasse Diagram Technique can be used for the further ranking analysis on the senior property of working fluid in order to obtain the final evaluation and decision. Fig. 2 shows the flow process of the evaluation method named “Weight Classification-Hasse Dominance”.

4.1. Weighting analysis on thermodynamic performance In order to make the calculation and analysis simple and clear, only thermal efficiency of cycle is considered when conducting the weighting analysis on thermodynamic performance of working fluid. That is to say the weight coefficient of thermal efficiency is set to be 1. Here we assume there is a special heat source that can make the highest temperature in cycle is the critical temperature (Tcr) of the working fluid used in the cycle and the lowest temperature in cycle is the standard boiling temperature (Tb) of the working fluid. Therefore, according to Carnot cycle efficiency, the thermal efficiency of cycle is approximately calculated as: h ¼ 1  Tcr/Tb. Table 2 lists the parameters used in weighting analysis on thermodynamic performance. According to the thermal efficiency of cycle, the ranking result of all the working fluid candidates is 7 > 1 > 13 > 2 > 3 > 12 > 10 > 11 > 17 > 16 > 4 > 8 > 15 > 9 > 5 > 6 > 14 > 20 > 21 > 19 > 18. Here we use the number of working fluid and symbol “>” to represent a superior relationship. For example, “X > Y” means X is better than Y. 4.2. Weighting analysis on environmental protection indicator ODP is the key indicator used in weighting analysis on environmental protection indicator. The smaller ODP of a working fluid, the higher rank it has. When the working fluids have the same ODP, the one with a small GWP has the higher rank. Table 3 lists the parameters used in weighting analysis on environmental protection indicator. ODP of all kinds of HFC working fluid is 0, therefore, the rank of HFC is higher than that of CFC and HCFC. According to the environmental protection indicator, the ranking result of all the working fluid candidates is 17 > 13 > 21 > 20 > 15 > 18 > 14 > 16 > 19 > 12 > 8 > 9 > 7 > 11 > 10 > 6 > 4 > 1 > 5 > 2 > 3.

4. Calculation example and discussion

4.3. Weighting analysis on safety requirement

Six kinds of CFC working fluids which have been phased out, five kinds of HCFC and ten kinds of HFC working fluids which will be phased out were chosen to be the candidates to demonstrate this new evaluation method. The final winner can be considered as the optimal working fluid besides HC working fluids.

The organic matter used as a working fluid generally has a good chemical stability. Therefore, only toxicity and flammability of

Fig. 2. Flow process of the evaluation method named “Weight Classification-Hasse Dominance”.

Table 2 Some parameters used in weighting analysis on thermodynamic performance. Number

Working fluid

Standard boiling point temperature, Tb/K

Critical temperature, Tcr/K

Thermal efficiency/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

CFC-11 CFC-12 CFC-13 CFC-113 CFC-114 CFC-115 HCFC-22 HCFC-123 HCFC-124 HCFC-141b HCFC-142b HFC-23 HFC-32 HFC-125 HFC-134a HFC-143a HFC-152a HFC-227ea HFC-236ea HFC-245fa HFC-365mfc

296.86 243.4 191.67 320.74 276.74 234.21 232.34 300.97 261.19 305.2 264.03 191.13 221.5 225.06 247.08 225.91 249.13 256.73 271.71 288.29 313.4

197.96 385.12 302.0 487.21 418.83 353.1 369.3 456.83 395.43 477.5 410.26 299.29 351.26 339.17 374.21 345.86 386.41 375.96 398.07 427.16 460.0

36.99 36.80 36.53 34.17 33.93 33.67 37.09 34.12 33.95 36.08 35.64 36.14 36.94 33.64 33.97 34.68 35.53 31.66 31.74 32.51 31.87

X. Zhang et al. / Energy 67 (2014) 363e369 Table 3 Some parameters used in weighting analysis on environmental protection indicator. Number

Working fluid

ODP

GWP

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

CFC-11 CFC-12 CFC-13 CFC-113 CFC-114 CFC-115 HCFC-22 HCFC-123 HCFC-124 HCFC-141b HCFC-142b HFC-23 HFC-32 HFC-125 HFC-134a HFC-143a HFC-152a HFC-227ea HFC-236ea HFC-245fa HFC-365mfc

1 1 1 0.8 1 0.6 0.055 0.02 0.022 0.11 0.065 0 0 0 0 0 0 0 0 0 0

4750 10,900 14,400 6130 10,000 7370 1810 77 609 725 2310 14,800 675 3500 1430 4470 124 3220 9810 1030 794

working fluid are considered when conducting weighting analysis on safety requirement. Data coming from ASHRAE Standard 34 are used for weighting analysis and Table 4 lists these data. For safety’s sake, HFC-141b and HFC-365mfc have the lowest rank due to the lack of safety data in ASHRAE Standard 34. The ranking result of all the working fluid candidates based on safety requirement is 1 ¼ 2 ¼ 3 ¼ 4 ¼ 5 ¼ 6 ¼ 7 ¼ 9 ¼ 12 ¼ 14 ¼ 15 ¼ 18 ¼ 19 > 11 ¼ 13 ¼ 16 ¼ 17 > 8 ¼ 20 > 10 > 21. Here symbol “¼” is used to represent the equal relationship. 4.4. Final ranking analysis based on Hasse Diagram Technique Working fluid winners of thermodynamic performance, environmental protection indicator, and safety requirement through weighting analysis are gathered in a group for the further analysis using the Hasse Diagram Technique. Considering that many working fluids belong to A1 safety classification, the group for the Hasse Diagram Technique analysis contains many working fluids. Their

Table 4 Some parameters used in weighting analysis on safety requirement. Number

Working fluid

Safety classification

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

CFC-11 CFC-12 CFC-13 CFC-113 CFC-114 CFC-115 HCFC-22 HCFC-123 HCFC-124 HCFC-141b HCFC-142b HFC-23 HFC-32 HFC-125 HFC-134a HFC-143a HFC-152a HFC-227ea HFC-236ea HFC-245fa HFC-365mfc

A1 A1 A1 A1 A1 A1 A1 B1 A1 N/A A2 A1 A2 A1 A1 A2 A2 A1 A1 B1 N/A

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Table 5 Some parameters used for the final Hasse Diagram Technique analysis. Number

Working fluid

Thermal efficiency/%

ODP

GWP

Safety classification

1 2 3 4 5 6 7 9 12 14 15 17 18 19

CFC-11 CFC-12 CFC-13 CFC-113 CFC-114 CFC-115 HCFC-22 HCFC-124 HFC-23 HFC-125 HFC-134a HFC-152a HFC-227ea HFC-236ea

36.99 36.80 36.53 34.17 33.93 33.67 37.09 33.95 36.14 33.64 33.97 35.53 31.66 31.74

1 1 1 0.8 1 0.6 0.055 0.022 0 0 0 0 0 0

4750 10,900 14,400 6130 10,000 7370 1810 609 14,800 3500 1430 124 3220 9810

A1 A1 A1 A1 A1 A1 A1 A1 A1 A1 A1 A2 A1 A1

properties used for the final Hasse Diagram Technique analysis are listed in Table 5. From Table 5, it can be seen that there are altogether 14 working fluids involved in the Hasse Diagram Technique analysis. Among these 14 working fluids, pairwise comparison has to be made and there are altogether C(14,2) ¼ 91 pairwise relationships. The Hasse Diagram Technique is used for the pairwise comparison. Here we use symbol “>” to represent a superior relationship, while “<” for an inferior relationship. The pairwise relationships obtained by conducting the Hasse Diagram Technique analysis are listed as follows. (The number represents the organic working fluid.) (1) 1 > 2, 1 > 3, 1 > 5, 1 < 7, and it is incomparable with other working fluid which has a bigger number; (2) 2 > 3, 2 < 7, and it is incomparable with other working fluid which has a bigger number; (3) 3 < 7 and it is incomparable with other working fluid which has a bigger number; (4) 4 > 5, 4 < 7, and it is incomparable with other working fluid which has a bigger number; (5) 5 > 7, 5 < 9, 5 < 15, and it is incomparable with other working fluid which has a bigger number; (6) 6 > 7, 6 < 9, 6 < 15, and it is incomparable with other working fluid which has a bigger number; (7) Except for the above pairwise relationships, 7 is incomparable with other working fluid which has a bigger number; (8) Except for the above pairwise relationships, 9 is incomparable with other working fluid which has a bigger number; (9) 12 is incomparable with all the working fluids; (10) 14 < 15, 14 > 19, and it is incomparable with other working fluid which has a bigger number; (11) 15 > 18, 15 > 19, and it is incomparable with other working fluid which has a bigger number; (12) 17 is incomparable with all the working fluids; (13) Except for the above pairwise relationships, 18 is incomparable with other working fluid which has a bigger number; (14) Except for the above pairwise relationships, 19 is incomparable with other working fluid. According to the above pairwise relationships, a Hasse Diagram is drawn as Fig. 3. From Fig. 3 it can be seen that among all the 91 pairwise relationships built by 14 working fluids, HCFC-22 (Number 7) has 7 superior relationships to the other working fluids. HFC134a (Number 15) has 6 superior relationships to the other working fluids. Therefore, among CFC, HCFC, and HFC working fluids, HCFC22 is the optimal.

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were evaluated. The result shows HCFC-22 and HFC-134a are two optimal working fluids among these working fluids. Moreover, HCs (hydrocarbons) can be considered as a completely different kind of organic working fluid from CFCs, HCFCs, and HFCs working fluids according to comparison based on this new evaluation method. Acknowledgment

Fig. 3. Hasse Diagram used for ranking the CFC, HCFC, and HFC working fluids.

This work was supported by the National Key Basic Research Program of China (973 Program, No. 2013CB228306). The authors gratefully acknowledge them for financial support of this work.

4.5. Some discussion References Nowadays, HC working fluids with R290 and R600a as the representatives have been widely used in refrigerators, freezers, and other small cryogenic refrigeration equipments. Now we add these two HC working fluids to the evaluation example. The calculation result shows the thermal efficiency of cycle using R290 is 37.56%. ODP of R290 is 0 and its GWP is 3. It belongs to A3 safety classification with a higher flammability and a lower toxicity. As for R600a, the thermal efficiency is 35.88%, ODP is 0, and GWP is 3.3. A higher flammability and a lower toxicity make it belong to A3 safety classification. Let us consider the first senior property, the thermodynamic performance (thermal efficiency) of all the working fluids including CFC, HCFC, HFC, and two HC working fluids. It can be seen that R290 has the highest thermal efficiency. When comparing the second senior property, the environmental protection indicator, two HC working fluids are the best. However, if the last senior property, the safety requirement is taken into account, then two HC working fluids are the worst working fluids due to their higher flammability. Therefore, according to the comparison principle of the Hasse Diagram Technique, these two HC working fluids are incomparable with CFC, HCFC, and HFC working fluids. This is because of the contradiction between the safety requirement and the environmental protection indicator. Unfortunately, some of the obvious solutions to the environmental concerns raise flammability and/or toxicity concerns. The HC working fluid is the example. In other words, HC working fluid can be deemed as a totally different kind of working fluid from CFC, HCFC, and HFC working fluids. 5. Conclusion Organic substance is widely used as the working fluid in refrigeration engineering and low-grade heat utilization through an ORC. The efficiency, the operation condition, the impact on the environment, and the economic feasibility of thermodynamic cycle are greatly affected by working fluid selection and the nature of the working fluid. Thermodynamic performance, environmental protection indicator, and safety requirement have to be considered simultaneously when selecting organic working fluids. Unfortunately, we still cannot find an ideal organic working fluid which has good thermodynamic performance, zero ODP and low GWP and can meet non-toxic, non-flammable, and non-explosive requirement. Therefore, it is necessary for us to rank and evaluate organic working fluids. Combining the advantages of both the method of weight determination and the Hasse Diagram Technique (HDT), a method named “Weight Classification-Hasse Dominance” was proposed to evaluate the thermodynamic performance, environmental protection indicator, and safety requirement of organic working fluid simultaneously. Using this method, CFCs (chlorofluorocarbons), HCFCs (hydrochlorofluorocarbons), and HFCs (hydrofluorocarbons)

[1] McQuay Co. Ltd. Application guide (AG 31-007) e refrigerants. McQuay International; 2002. [2] Downing RC. History of the organic fluorine industry. Kirk-Othmer encyclopedia of chemical technology. 2nd ed. New York: John Wiley and Sons; 1966. pp. 481e91. [3] Downing RC. Development of chlorofluorocarbon refrigerant. ASHRAE Trans 1984;90(2B):481e91. [4] Ray SK, Moss G. Fluorochemicals as working fluids for small Rankine cycle power units. Adv Energy Convers 1966;6(2):89e102. [5] Ozone Secretariat, United Nations Environment Programme. The Montreal protocol on substances that deplete the ozone layer; 2000. [6] United Nations. Kyoto protocol to the United Nations framework convention on climate chang; 1998. [7] Hung TC, Wang SK, Kuo CH, Pei BS, Tsai KF. A study of organic working fluids on system efficiency of an ORC using low-grade energy sources. Energy 2010;35(3):1403e11. [8] Morosuk T, Tsatsaronis G. Advanced exergetic evaluation of refrigeration machines using different working fluids. Energy 2009;(12):2248e58. [9] Domingues António, Santos Helder, Costa Mário. Analysis of vehicle exhaust waste heat recovery potential using a Rankine cycle. Energy 2013;49(1):71e 85. [10] Sipeng Zhu, Kangyao Deng, Shuan Qu. Energy and exergy analyses of a bottoming Rankine cycle for engine exhaust heat recovery. Energy 58(1):448e57. [11] Wang EH, Zhang HG, Fan BY, Ouyang MG, Zhao Y, Mu QH. Study of working fluid selection of organic Rankine cycle (ORC) for engine waste heat recovery. Energy 2011;36(5):3406e18. [12] Panesar Angad S, Morgan Robert E, Miché Nicolas DD, Heikal Morgan R. Working fluid selection for a subcritical bottoming cycle applied to a high exhaust gas recirculation engine. Energy 2013;60(1):388e400. [13] Zhou Naijun, Wang Xiaoyuan, Chen Zhuo, Wang Zhiqi. Experimental study on organic Rankine cycle for waste heat recovery from low-temperature flue gas. Energy 2013;55(15):216e25. [14] Victor Rachel Anne, Kim Jin-Kuk, Smith Robin. Composition optimisation of working fluids for organic Rankine cycles and Kalina cycles. Energy 2013;55(15):114e26. [15] Pierobon Leonardo, Nguyen Tuong-Van, Larsen Ulrik, Haglind Fredrik, Elmegaard Brian. Multi-objective optimization of organic Rankine cycles for waste heat recovery: application in an offshore platform. Energy 2013;58(1): 538e49. [16] Roy JP, Misra Ashok. Parametric optimization and performance analysis of a regenerative organic Rankine cycle using R-123 for waste heat recovery. Energy 2012;39(1):227e35. [17] Anh Lai Ngoc, Wendland Martin, Fischer Johann. Working fluids for hightemperature organic Rankine cycles. Energy 2011;36(1):199e211. [18] Dehghanian P, Fotuhi-Firuzabad M, Bagheri-Shouraki S, Razi Kazemi AA. Critical component identification in reliability centered asset management of distribution power systems via fuzzy AHP. IEEE Syst J 2012;6(4):593e602. [19] Liu Nian, Zhang Jianhua, Zhang Hao, Liu Wenxia. Security assessment for communication networks of power control systems using attack graph and MCDM. IEEE Trans Power Deliv 2010;25(3):1492e500. [20] Zhu JZ, Chang CS, Yan W, Xu GY. Reactive power optimization using an analytic hierarchical process and a nonlinear optimization neural network approach. Gener Transm Distrib IEE Proc 1998;145(1):89e97. [21] Tanaka Hideaki, Tsukao Shigeyuki, Yamashita Daiki, Niimura Takahide, Yokoyama Ryuichi. Multiple criteria assessment of substation conditions by pair-wise comparison of analytic hierarchy process. IEEE Trans Power Deliv 2010;25(4):3017e23. [22] Razi Kazemi AA, Dehghanian P. A practical approach on optimal RTU placement in power distribution systems incorporating fuzzy sets theory. Int J Electr Power Energy Syst 2012;37(1):31e42. [23] Restrepo G, Weckert M, Brüggemann R, Gerstmann S, Frank H. Ranking of refrigerants. Environ Sci Technol 2008;42:2925e30. [24] Wuebbles DJ. Relative efficiency of a number of halocarbons for destroying stratospheric ozone; 1981. [25] Intergovernmental Panel on Climate Change (IPCC) first assessment report (FAR); 1990.

X. Zhang et al. / Energy 67 (2014) 363e369 [26] American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Designation and safety classification of refrigerants; 2007. [27] Lerche D, Brüggemann R, Sørensen P, et al. A comparison of partial order technique with three methods of multi-criteria analysis for ranking of chemical substances. J Chem Inf Comput Sci 2002;42(5):1086e98.

369

[28] Brüggemann R, Bartel H-G. A theoretical concept to rank environmentally significant chemicals. J Chem Inf Comput Sci 1998;39(2):211e7. [29] Brüggemann R, Münzer B. A graph-theoretical tool for priority setting of chemicals. Chemosphere 1993;27(9):1729e36.