Study of mixtures based on hydrocarbons used in ORC (Organic Rankine Cycle) for engine waste heat recovery

Energy 74 (2014) 428e438

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Study of mixtures based on hydrocarbons used in ORC (Organic Rankine Cycle) for engine waste heat recovery Gequn Shu*, Yuanyuan Gao, Hua Tian*, Haiqiao Wei, Xingyu Liang State Key Laboratory of Engines, Tianjin University, People's Republic of China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 January 2014 Received in revised form 2 July 2014 Accepted 3 July 2014 Available online 14 August 2014

For high temperature ORC (Organic Rankine Cycle) used in engine waste heat recovery, it's very critical to select a high temperature working fluid. HCs (Hydrocarbons) usually have excellent cycle performance, but the flammability limits their practical application. Considering that some retardants can be used to suppress flammability, the paper presents an application of mixtures based on hydrocarbons blending with refrigerant retardants to engine waste heat ORC. Three pure hydrocarbons (cyclopentane, cyclohexane, benzene) and two retardants (R11, R123) are selected for combination. Thermal efficiency and exergy loss are selected as the main objective functions. Based on thermodynamic model, the effects of retardants mass fraction, evaporation temperature and IHE (internal heat exchanger) are investigated. Results show that zeotropic mixtures do have higher thermal efficiency and lower exergy loss than pure fluids, at a certain mixture ratio. There exists the OMR (optimal mixture ratio) for different mixtures, and it changes with the evaporation temperature. When adding IHE to system, cycle performance could be obviously improved, and for benzene/R11 (0.7/0.3), the efficiency growth is about 7.12%~9.72%. Using it, the maximum thermal efficiency of the system can achieve 16.7%, and minimum exergy loss is only 30.76 kW. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Engine waste heat Organic Rankine Cycle (ORC) Mixtures based on hydrocarbons Thermodynamic analysis

1. Introduction As global energy consumption kept increasing, energy saving, emission reduction and energy efficiency improvement are becoming important measures to keep the sustainable development of economy. ICEs (Internal combustion engines) are main oilconsuming equipment. When such an engine runs, about one-third of the heat energy is wasted via exhaust gas. The efficiency of an engine will be greatly enhanced if such heat energy is effectively reused, which will certainly bring great economic and environmental benefits. ORC (Organic Rankine Cycle), as a mature method of waste heat recovery, can increase the energy usage by transforming heat into electric energy. In nowadays, it has paid a high compliment in the internal combustion engine industry. Because the properties of working fluids to determine the performance of the ORC system to a great extent, it is an important subject to adopt what kind of

* Corresponding authors. State Key Laboratory of Engines, Tianjin University, No. 92, Weijin Road, Nankai Region, Tianjin 300072, People's Republic of China. Tel.: þ86 022 27409558. E-mail addresses: [email protected] (G. Shu), [email protected], [email protected]. cn (H. Tian). http://dx.doi.org/10.1016/j.energy.2014.07.007 0360-5442/© 2014 Elsevier Ltd. All rights reserved.

working fluids as a recycling transmitter in order to achieve maximum heat recovery efficiency. The temperature of engine exhaust is relative high, which is easy to make some organic fluids decompose. Besides, the temperature difference between exhaust and working fluids is large, so the exhaust waste heat can't be fully utilized. That means high temperature ORC should be used in engine exhaust WHR (waste heat recovery). In previous studies, HCs (hydrocarbons) such as alkanes and aromatics show good character in high temperature ORC [1]. Besides, some refrigerants, are also often used in ORC. Siddiqi [1] investigated hydrocarbons from n-pentane to n-dodecane used in ORC, in comparison to water, benzene and toluene. Vaja [2] compared benzene, R11 and R134a used in internal combustion engine bottoming with ORC. Carcasci [3] selected toluene, benzene, cyclohexane and cyclopentane used in ORC for heat recovery from gas turbines. Shu et al. [4] found alkanes-based working fluids may be more attractive for engine exhaust heat recovery. Chacartegui [5] considered toluene, cyclohexane, isopentane, isobutene, R113 and R245 for low temperature Organic Rankine Cycle, and the first two perform better. Wang et al. [6] indicated R11, R141b, R113 and R123 manifest higher thermodynamic performances than other refrigerants. Tian et al. [7] analyzed 20 low-boiling fluids and found R141b, R123 and R245fa present well. Shu et al. [8] used R123 in

G. Shu et al. / Energy 74 (2014) 428e438

Nomenclature

Abbreviation ICE internal combustion engine ORC Organic Rankine Cycle WHR waste heat recovery HC hydrocarbon ODP ozone depletion potential GWP global warming potential IHE internal heat exchanger OMR optimal mixture ratio RD relative deviation Symbols M P T W m h I s

molar mass (g/mol) pressure (MPa) temperature (K) work input or output (kW) mass flow (kg/s) specific enthalpy (kJ/kg) exergy loss (kW) specific entropy (kJ/kg K)

waste heat recovery based on thermoelectric generator and Organic Rankine Cycle. Yu et al. [9] simulated a combined system of diesel engine with bottoming ORC utilizing R123. However, HCs with zero ODP (ozone depletion potential) and low GWP (global warming potential) are usually flammable and explosive, which limit the practical application. And refrigerants with low critical temperature and high GWP, have poor cycle performance and environmental benefits. Thus, we proposed a combination of hydrocarbons and non-inflammable refrigerants. The latter is used to suppress the flammability of the former, extend its application range, and its GWP can also be reduced. The idea is supported by the work of Oellrich [10] who studied flammability suppression of propane with perfluoropropane for application in refrigeration. Besides, Yang et al. [11] studied the inert effect of nonflammable refrigerants on flammable refrigerants based on experimental studies, and found with the number of nonflammable components increase, the area of the nonflammable zone expands. Besides, zeotropic mixtures have great suitable superiority on ORC application. Perhaps the largest shortcoming of pure fluids is the constant temperature phase change, resulting in a bad thermal match between the working fluid and the heat source or heat sink, leading to large exergy loss [12]. Using mixtures can partially solve the problem. For zeotropic mixtures, there exist a certain degree of temperature glide during the phase change process, providing a better thermal match with the heat source and sink profiles, reducing the exergy loss and promising a better cycle performance. Presently, researches on zeotropic mixtures continue to rise. M. Chys [12] examined the effects of mixtures used in ORCs. The results show that for heat sources at 423.15 K and 523.15 K, cycle efficiency can increase 16% and 6% compared with pure fluids. Angelino and Paliano [13] compared n-pentane and mixture of nbutane/n-hexane (50%/50%) and found the mixture gives 6.8% more electricity than n-pentane. Bliem [14] investigated the use of R114/ R22 for geothermal power generation. The mixture shows 3%e8% higher efficiency compared to R114. Yang et al. [15] designed an ORCeWHR (waste heat recovery) system to recover a diesel engine

Q ref est

429

heat absorbed or released (kW) reference value estimated value

Greek letters h efficiency (%) ε effectiveness of IHE (%) Subscript cri critical point 0 reference state 1e6,2s,2a,5s,5a state point in cycle p pump f working fluid e evaporation exh exhaust gas in come in out come out c condenser w water i IHE net net output

exhaust energy, and discussed the effect of zeotropic mixtures on cycle performance under various operating conditions. Results show that when engine operates at low load and high speed, R415B has the maximum exergy efficiency of 39.88%. Florian [16] compared the second law efficiency between mixtures and pure fluids and found the former leads to an increase of 15% for heat source temperature below 393.15 K. Li et al. [17] compared the cycle efficiency of R141b/RC318 mixtures with three pure fluids. It was found that the use of mixtures allows a wider selection of working fluids. Wang et al. [18] used zeotropic mixtures and pure fluid R245fa for a solar Rankine cycle, based on the experimental prototype. The results show that the collector efficiency and thermal efficiency of zeotropic mixtures are comparatively higher than pure R245fa in the experimental condition. Chen et al. [19] proposed a supercritical Rankine cycle using zeotropic mixtures, compared with subcritical cycle used pure fluids. The results show that the former can improve the efficiency of 10%e30% over the latter. Wang et al. [20] used zeotropic mixtures to analyze low temperature solar cycles. Investigation shows that thermal efficiencies of zeotropic mixtures can have a significant increase when superheating is combined with IHE (internal heat exchanger). Dai et al. [21] compared pure CO2 and zeotropic mixtures composed of CO2 and traditional working fluids used in transcritical Rankine cycle to recover low grade heat energy. Results show that the high operation pressure can be significantly reduced and thermal efficiency is improved by using zeotropic mixtures in comparison with pure CO2. Still, the research on zeotropic mixtures is limited and mainly focused on low temperature application. The selected components are either hydrocarbons or refrigerants, restricted to blending with the same kind. So the paper proposed an application of mixtures based on hydrocarbons blending with refrigerant retardants used in engine waste heat ORC, which can extend the application range on the basis of improving cycle efficiency, and at the same time, reduce exergy loss and improve exergy efficiency. In this paper, the energy and exergy theoretical model of high temperature ORC is

430

G. Shu et al. / Energy 74 (2014) 428e438

Table 1 Properties of the pure working fluids. Fluids

Molar mass M (g/mol)

Tcri (K)

Pcri (MPa)

Combustibility

ODP

GWP

Cyclopentane Cyclohexane Benzene R123 R11

70.133 84.161 78.112 152.93 137.37

511.69 553.64 562.02 456.83 471.11

4.515 4.075 4.9063 3.6618 4.4076

Flammable Flammable Flammable Non-flammable Non-flammable

0 0 0 0.012 1

11 Small Small 77 1500

built by MATLAB software. Hydrocarbons blending with nonflammable refrigerants are selected as the working fluids. Thermal efficiency and exergy loss are mainly studied and compared under different mass fraction of retardants and evaporation temperature. Also, another emphasis of the paper is to research the effect of internal heat exchanger (IHE) on ORC system, and results are compared to reach conclusions.

2. Mixtures based on hydrocarbons In ORCs applications, the choice of a working fluid is crucial. Because the fluid must not only have a good thermophysical property, but also have an adequate chemical stability at the desired temperature. To insure a chemically and operationally

stable working fluid, each component of a mixture needs to meet the same requirements [12]. For high temperature ORC, set the minimum decomposition temperature to 600 K, R123 and R11 are chosen as the retardants. In accordance with them, considering that condensation pressure can't be too low, and the creative temperature glide should be moderate, flammable hydrocarbons, cyclopentane, cyclohexane and benzene are chosen as the major components. The properties of the considered fluids are shown in Table 1. Because the presented fluids are all dry working fluids, which feature a positive slope of the saturated liquid and vapor line in the temperatureeentropy diagram, there is no need to superheat the fluids. Mago [22] showed that superheating dry fluids results in a decrease in efficiency. The phase change process of zeotropic mixtures is different with that of pure working fluids, which has obvious temperature glide. This character of the mixture favors better performance in ORC. This is explained in Fig. 1. Take cyclohexane and the mixture cyclohexane/R123 (mass fraction 0.5/0.5) for example, see Fig. 1. It's obvious that the temperature profile of the mixture [Fig. 1(b)] follows the heat source profile better than that of the pure fluid [Fig. 1(a)]. This leads to a higher evaporator outlet temperature (no.4) because of the smaller temperature difference across the heat exchanger, and the mean evaporation temperature is greater than that of the pure fluid. In the condenser, the mixture has a lower condenser outlet temperature (no.1), leading to a lower mean condensation temperature.

800 Texh,in

Temperature (K)

700 mexh

600

3

500

4

Texh,out

400

2s 2

300

Tw1

1

-0.4

0.0

5

5s

6

Tw2

mw

0.4

0.8

1.2

Entropy (kJ kg-1 K-1)

(a)

(a)

800

800 Texh,in

Texh,in

700

600

mexh

500

4

3

Texh,out

400

2s 2 1

300

6

0.4

600

mexh

500 Texh,out

400

2s 2 1

300 0.8

1.2

-1

-1

4

3

Tw2

mw

Tw1

5 5s

Temperature (K)

Temperature (K)

700

5

6

5s

Tw2

mw

Tw1

1.6

Entropy (kJ kg K )

(b) Fig. 1. Temperatureeentropy diagrams of the ORC process for (a) the pure cyclohexane and for (b) the mixture cyclohexane/R123 (0.5/0.5) as working fluid.

0.4

0.8

1.2

Entropy (kJ kg

-1

-1

1.6

K )

(b) Fig. 2. System diagram and Tes diagram of the basic ORC.

G. Shu et al. / Energy 74 (2014) 428e438

431

The both reasons result in a higher efficiency and lower exergy loss for the mixture. 3. System description and modeling 3.1. System description The system structure of a basic ORC is represented in Fig. 2(a). The working fluid is pumped to a high pressure by the pump (1e2).Next, in the evaporator (3e4), the heat source heats the fluid to its evaporation point, converting it to saturated vapor. Operational high pressures and related fluid temperatures lie on the temperature of heat source, specified at in and outlet of the heat exchanger. Then, saturated vapor enters the turbine (4e5), where it expands to a superheated vapor, and delivers work to generate electricity. Finally, the superheat vapor leaves the turbine and enters into the condenser (5e1), where it is condensed into saturated liquid by the cooling water. Thus a cycle completes. The corresponding temperatureeentropy diagram is shown in Fig. 2(b). Fig. 3 is the system layout of an ORC with an IHE and corresponding temperatureeentropy diagram. The included IHE is a frequently used component to improve the performance of an ORC. The IHE transfers heat from the superheated vapor at the turbine outlet (5e5a) to the liquid at the pump outlet, which flows into the IHE (2e2a).

(a) 800 Texh,in

Temperature (K)

700

3.2. Conditions and assumptions Initial conditions and boundary conditions setting: The exhaust gas from the diesel engine serves as a high temperature heat source for this system. The composition of exhaust gas on mass basis has been calculated at: CO2 ¼ 10.78%, H2O ¼ 3.83%, N2 ¼ 74.16%, O2 ¼ 11.23%. Its initial temperature is 792.15 K, and mass flow rate is about 0.2752 kg/s. To avoid acid dew point for fuels containing sulfur, the outlet temperature of the exhaust is at least 393.15 K. In addition, in order to simplify the computing and its complicacy, some assumptions are given: (1) The whole system is under equilibrium state and stable. (2) Ignore the pressure loss and heat radiation in the heat exchangers. (3) The temperature glide of the mixtures distributes linearly along heat exchangers. (4) The flowing compositions of the mixtures don't change in every process of the cycle. (5) The isentropic efficiency of the turbine is set to 70% and for the pump is 80%. (6) The ambient temperature is set to 298.15 K. (7) The effectiveness of IHE is assumed to 50%. In order to analyze the performance of the working fluids, a given operating condition is necessary. Because of the changeable temperature of mixtures during phase change process, in order to contrast with pure fluids, condensation dew point temperature T6 is set as cycle condensation temperature, and evaporation bubble point temperature T3 is set as evaporation temperature. Generally speaking, the lower the condensation pressure is, the higher the efficiency is. The entire cycle should operate above atmospheric pressure to keep ambient air out, so the condensation pressure of major components is set to 0.1 MPa, and the corresponding condensation temperature, for cyclopentane is 321.99 K, for cyclohexane is 353.45 K and for benzene is 352.79 K. And for the mixtures formed by them, respective condensation temperatures are also determined.

mexh

600 500

4

3

Texh,out

400

2s 2 1

300

2a

6 mw

Tw1

0.4

0.8

1.2

-1

5 5a Tw2

5s

1.6

-1

Entropy (kJ kg K )

(b) Fig. 3. System diagram and Tes diagram of the ORC with IHE.

3.3. Thermodynamic model On the basis of the first and the second laws of thermodynamics, the following formulas show the energy and exergy analysis in detail. First, the exergy for each steady state of ORC is defined:

_ i  h0 Þ  T0 ðsi  s0 Þ E_ i ¼ m½ðh

(1)

where 0 subscript means the reference state, which is set as ambient pressure and temperature in this paper. Then, the analysis for each component is given below. Input work of pump:

Table 2 Comparison between present results and those in Ref. [18].

Expansion work output (kJ/kg) Net power output (kJ/kg) Heat input (kJ/kg) Rankine Cycle efficiency hR (%) Carnot efficiency hC (%) Thermodynamics perfection hR/hC (%) a

Ref. [18]

Present

RDa (%)

21.68 21.28 245.1 8.682 16.76 51.802

21.6 21.07 242.16 8.7 16.75 51.93

0.37 0.99 1.2 0.21 0.06 0.25

RD: Relative difference, see equation (21).

Thermal efficiency (%)

15.6 14.4 13.2 12.0 cyclopentane/R123 cyclohexane/R123 benzene/R123 cyclopentane/R11 cyclohexane/R11 benzene/R11

10.8 9.6 0.0

0.2

0.4

0.6

0.8

1.0

Mass fraction of retardants Fig. 4. Thermal efficiency vs. mass fraction.

. _ p ¼ m_ ðh  h Þ ¼ m_ ðh  h Þ h W 1 1 p f 2 f 2s

(2)

Pump exergy loss rate:

I_p ¼ m_ f T0 ðs2  s1 Þ

(3)

Heat absorbed in evaporator: Without IHE:

  Q_ e ¼ m_ exh hexh;in  hexh;out ¼ m_ f ðh4  h2 Þ

(4)

With IHE:

  Q_ e ¼ m_ exh hexh;in  hexh;out ¼ m_ f ðh4  h2a Þ

(5)

Exergy loss of evaporator: Without IHE:

    Ie ¼ m_ exh hexh;in  hexh;out  T0 sexh;in  sexh;out  m_ f ½ðh4  h2 Þ  T0 ðs4  s2 Þ

(6)

With IHE:

    Ie ¼ m_ exh hexh;in  hexh;out  T0 sexh;in  sexh;out  m_ f ½ðh4  h2a Þ  T0 ðs4  s2a Þ

(7)

Output work of turbine:

_ t ¼ m_ ðh  h5 Þ ¼ m_ ðh  h5s Þh W t f 4 f 4

(8)

Exergy loss of turbine:

I_t ¼ m_ f T0 ðs5  s4 Þ

Exergy loss of condenser: Without IHE:

Condensation temperature glide

5 4 3 2

Evaporation temperature glide

cyclopentane/R123 cyclopentane/R11 cyclopentane/R123 cyclopentane/R11

1 0 0.0

0.2

0.4

0.6

0.8

1.0

Mass fraction of retardants

25 Condensation temperature glide 20 15 Evaporation temperature glide

10 cyclohexane/R123 cyclohexane/R11 cyclohexane/R123 cyclohexane/R11

5 0 0.0

0.2

0.4

0.6

0.8

1.0

Mass fraction of retardants 25

Condensation temperature glide

20

15 Evaporation temperature glide

10

benzene/R123 benzene/R11 benzene/R123 benzene/R11

5

0 0.0

0.2

0.4

0.6

0.8

1.0

Mass fraction of retardants Fig. 5. Temperature glide of different mixtures vs. mass fraction.

(10)

With IHE:

Q_ c ¼ m_ f ðh5a  h1 Þ

6

(9)

Heat released in the condenser: Without IHE:

Q_ c ¼ m_ f ðh5  h1 Þ

Temperature glide of cyclohexane mixtures (K)

Temperature glide of cyclopentane mixtures (K)

G. Shu et al. / Energy 74 (2014) 428e438

Temperature glide of benzene mixtures (K)

432

I_c ¼ m_ f ½ðh5  h1 Þ  T0 ðs5  s1 Þ  m_ w ½ðhw2  hw1 Þ  T0 ðsw2  sw1 Þ

(12)

With IHE:

(11)

I_c ¼ m_ f ½ðh5a  h1 Þ  T0 ðs5a  s1 Þ  m_ w ½ðhw2  hw1 Þ  T0 ðsw2  sw1 Þ Heat recovery in the IHE:

(13)

G. Shu et al. / Energy 74 (2014) 428e438

Q_ i ¼ m_ f ðh2a  h2 Þ ¼ m_ f ðh5  h5a Þ

(14)

33.0

(15)

Exergy loss of IHE:

I_i ¼ m_ f T0 ðs2a  s2 þ s5a  s5 Þ

(16)

Thermal efficiency of ORC system:

   _ net Q_ e ¼ W _ p Q_ e _ tW h¼W

total

exh;in

 E_ exh;out þ E_ win  E_ wout



(18) Total exergy loss of ORC system: Without IHE:

I_ ¼

X

I_i ¼ I_p þ I_e þ I_t þ I_c

(19)

With IHE:

I_ ¼

X

I_i ¼ I_p þ I_e þ I_t þ I_c þ I_i

(20)

3.4. Model validation The RD (relative deviation) is determined as.

RD ¼

ðref  estÞ  100% ref

(21)

where ref symbol means the reference value, est symbol means the estimated value. According to the equations above, a program is written by MATLAB R2011a, and the fluids' state parameters are calculated by REFPROP (reference fluid thermodynamic and transport properties) 9.0 [24]. The model is verified with the same operating parameters in Ref. [18] to validate the results. R245fa/R152a (mass fraction 0.9/0.1) is selected as the working fluid. The present results have good coherence to those in Ref. [18]. Results are shown in Table 2. The discrepancies are very small, mainly due to the difference of software version. In Ref. [18], REFPROP 7.0 is called to calculate the fluid' parameters, while REFPROP 9.0 is used in present study. 4. Results and discussion 4.1. Performance comparison of different mixtures Thermal efficiency is the most important index used to estimate cycle performance. In this part, evaporation temperature is set to 450 K, and thermal efficiency of different mixtures is plotted in Fig. 4. From Fig. 4, thermal efficiencies of two mixtures containing cyclopentane are significantly higher than that of the remaining four mixtures, and with the mass fraction of retardants increases,

30.0 cyclopentane/R123 cyclohexane/R123 benzene/R123 cyclopentane/R11 cyclohexane/R11 benzene/R11

28.5 27.0

0.0

0.2

0.4

0.6

0.8

1.0

Mass fraction of retardant Fig. 6. Exergy efficiency vs. mass fraction.

the efficiency gradually decreases. This is because they have a lower condensation temperature at 0.1 MPa, leading to a higher efficiency. Fig. 5 illustrates temperature glides of all mixtures during evaporation and condensation processes. From Fig. 5, we can see that condensation temperature glides of HCs/R11 are all higher than that of HCs/R123, while evaporation temperature glides show no significant differences. As a result, thermal efficiencies of HCs/R11 mixtures are all higher than HCs/R123. Cyclopentane/R123 can be considered as near-azeotropic mixture, since its maximum temperature glide is about 5 K, which contributes little to the improvement of cycle performance. Besides, the property of mixture is between pure cyclopentane and R123, so the mixture has lower efficiency than pure cyclopentane, and R123 is the lowest. So for the mixture cyclopentane/R11. For other four mixtures, with the increase mass fraction of retardants, thermal efficiency firstly increases, then decreases. The maximum thermal efficiency appears in different locations based on different compounds, and for cyclohexane/R123, it exists at 0.4/0.6, for benzene/R123, 0.5/0.5, for cyclohexane/R11, 0.3/0.7 and for benzene/R11, 0.4/0.6. The reasons are explained as below: in Fig. 5, for the four mixtures, there is an obvious temperature glide during phase change. The characteristic implies that the average evaporation temperature of the mixture will increase, while the average condensation temperature will decrease. Therefore, thermal efficiencies of the mixtures are higher than pure fluids. In addition, with the R123 mass fraction increases, temperature glide firstly increases, then decreases, and condensation temperature glide is greater than evaporation temperature glide. As a result, expansion work will increase first and then 39.6 38.7

Exergy loss (kW)

hex

   _ t W _ p = E_ ¼ W

31.5

25.5

(17)

Exergy efficiency of ORC system: Exergy efficiency of ORC system is the ratio of the total exergy used to the total available exergy [23]. In this paper, exergy used refers to the net output work, and available exergy means the exergy variation of the external heat and cool source going into and out of the system.

 _ net E_ ¼W

Exergy efficiency (%)

Effectiveness of IHE:

ε ¼ T5  T5a =T5  T2

433

37.8

cyclopentane/R123 cyclohexane/R123 benzene/R123 cyclopentane/R11 cyclohexane/R11 benzene/R11

36.9 36.0 35.1 0.0

0.2

0.4

0.6

0.8

Mass fraction of retardants Fig. 7. Exergy loss vs. mass fraction.

1.0

434

G. Shu et al. / Energy 74 (2014) 428e438

Mass fraction of benzene

Mass fraction of cyclohexane 14.0

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.7

0.6

0.5

0.4

0.3

0.2

0.1

12.6

11.9

Thermal efficiency (%)

Thermal efficiency (%)

14.7 430K 440K 450K 460K 470K 480K 490K 500K

13.3

11.2

10.5

430K 440K 450K 460K 470K 480K 490K 500K

14.0 13.3 12.6 11.9 11.2

0.3

0.7

0.4

0.5

0.6

0.7

0.8

0.9

0.3

0.4

0.5

0.6

0.7

Mass fraction of R123

Mass fraction of R123

Mass fraction of cyclohexane

Mass fraction of benzene

0.6

0.5

0.4

0.3

0.2

0.1

0.7

0.6

0.5

0.4

0.3

0.8

0.9

0.2

0.1

15.4 14.7

430K 440K 450K 460K 470K 480K 490K 500K

13.3 12.6 11.9

Thermal efficiency (%)

Thermal efficiency (%)

14.0

11.2 10.5

0.3

0.4

0.5

0.6

0.7

0.8

0.9

430K 440K 450K 460K 470K 480K 490K 500K

14.0 13.3 12.6 11.9 11.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Mass fraction of R11

Mass fraction of R11 Fig. 8. Thermal efficiency vs. evaporation temperature.

reduce, while pump work will continuously increase. Thus, net output work will also increase first and then reduce, but the peak point will differ from that of expansion work. For fixed values of recovered heat, maximum efficiency corresponds with maximum net output work, so thermal efficiency has such changes. Furthermore, we show the relation between the thermal efficiency and the non-isothermal phase change glide. For benzene/ R11, the maximum thermal efficiency coincides with the maximum temperature glides. The corresponding mixture composition 0.4/ 0.6 delivers the best match with temperature profiles of the heat

Table 3 The optimal mixture ratio (OMR) corresponding to maximal thermal efficiency. Evaporation temperature (K)

OMR of cyclohexane/ R123

OMR of benzene/ R123

OMR of cyclohexane/ R11

OMR of benzene/ R11

430 440

0.4/0.6 0.4/0.6

0.3/0.7 0.3/0.7

0.4/0.6 0.4/0.6

450 460 470 480 490 500

0.4/0.6 0.4/0.6 0.4/0.6 0.4/0.6 0.5/0.5 0.5/0.5 0.6/0.4

0.4/0.6 0.4/0.6 0.5/0.5 0.5/0.5 0.5/0.5 0.6/0.4 0.7/0.3 0.7/0.3 0.7/0.3

0.3/0.7 0.3/0.7 0.3/0.7 0.3/0.7 0.4/0.6 0.4/0.6

0.4/0.6 0.4/0.6 0.4/0.6 0.4/0.6 0.5/0.5 0.5/0.5 0.6/0.4

source and sink. In general, maximum glides don't equal to an optimal match and maximum efficiency. Take benzene/R123 as an example, composition 0.4/0.6 corresponds to maximum temperature glide, while 0.5/0.5 corresponds to maximum efficiency. Exergy is the maximum amount of work that can be done by a system as it reaches thermodynamic equilibrium with its surroundings by an order of reversible process. Exergy efficiency of different mixtures is shown in Fig. 6. Exergy efficiencies of HCs/R11 mixtures are all higher than that of HCs/R123. With the mass fraction of retardants increases, cyclopentane/R11 is always the highest, and changes slowly. The second-placed cyclopentane/R123 gradually declines. The exergy efficiencies of remaining mixtures firstly increase, then apparently decrease. Cyclohexane/R123 is always the least. Exergy efficiency is not only relevant to the net output work, but also to the exergy variation of the heat and cool source. And outlet parameters of the cooling water are very key factors of the latter. With the mass fraction of retardants increases, temperature glides during phase change process change the parameters of the working fluids at the outlet of the turbine and condenser, and the heat exchange in the condenser increases. While the outlet temperature of the cooling water continues to drop, the mass flow rate of cooling water ðm_ w Þ for each mixture at first changes slowly, then clearly increases. As a result, the total exergy variation is increasing. On the other hand, the change of the net output work is just like that of thermal efficiency. The change of exergy efficiency is the result influenced by the above two aspects.

G. Shu et al. / Energy 74 (2014) 428e438

435

Mass fraction of benzene

Mass fraction of cyclohexane 0.7

0.6

0.5

0.4

0.3

Exergy loss (kW)

36

0.2

0.1

34

0.7

0.6

0.5

0.4

0.3

36

430K 440K 450K 460K 470K 480K 490K 500K

35

37

Exergy loss (kW)

37

0.2

0.1

430K 440K 450K 460K 470K 480K 490K 500K

35 34 33

33 32 32 0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.3

0.4

Mass fraction of cyclohexane 0.7

0.6

0.5

0.4

0.6

0.7

0.8

0.9

0.2

0.1

Mass fraction of benzene

0.3

0.2

0.7

0.6

0.5

0.4

0.3

36

36 430K 440K 450K 460K 470K 480K 490K 500K

35

34

Exergy loss (kW)

Exergy loss (kW)

0.5

Mass fraction of R123

Mass fraction of R123

430K 440K 450K 460K 470K 480K 490K 500K

35 34 33

33 32 32 0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.3

Mass fraction of R11

0.4

0.5

0.6

0.7

0.8

0.9

Mass fraction of R11 Fig. 9. Exergy loss vs. evaporation temperature.

Exergy loss represents the loss of quality of energy. Exergy losses of the mixtures are given in Fig. 7. Two mixtures with cyclopentane have relatively large exergy losses. And with the increase of retardants contents, exergy losses increase more and more obviously. Exergy loss of cyclopentane/R123 is always larger than that of cyclopentane/R11. For other four mixtures, exergy losses exhibit a similar magnitude and change trend, that is, firstly decreases, then increases. Benzene/R11 has the lowest loss all along. The results show that the non-isothermal phase change of zeotropic mixtures reduces exergy loss in comparison to pure fluids. This is caused by a better glide matching of the temperature profiles in the heat exchangers. In order to take advantage of non-isothermal phase

Table 4 The optimal mixture ratio (OMR) corresponding to minimum exergy loss. Evaporation temperature (K)

OMR of cyclohexane/ R123

OMR of benzene/ R123

OMR of cyclohexane/ R11

OMR of benzene/ R11

430 440 450 460 470 480 490 500

0.4/0.6 0.5/0.5 0.5/0.5 0.5/0.5 0.6/0.4 0.7/0.3 0.7/0.3 0.7/0.3

0.4/0.6 0.4/0.6 0.5/0.5 0.5/0.5 0.6/0.4 0.7/0.3 0.7/0.3 0.7/0.3

0.4/0.6 0.4/0.6 0.4/0.6 0.5/0.5 0.5/0.5 0.5/0.5 0.6/0.4 0.7/0.3

0.4/0.6 0.4/0.6 0.4/0.6 0.5/0.5 0.5/0.5 0.6/0.4 0.7/0.3 0.7/0.3

change, in the following analysis, cyclopentane/R123 and cyclopentane/R11 will not be considered. 4.2. Effects of the evaporation temperature In this part, we mainly discuss the impacts of evaporation temperature on cycle performance. Zabetakis [25] investigated flammability envelopes of several hydrocarbons when blended with inert components, and show that generally 0.3 volume fraction is often adequate to take most hydrocarbons outside the flammability envelope. Pardeep Garg [26] deduced that as the molecular weight of inert additive increases, a smaller mole fraction of inert additive is adequate to suppress flammability, only 0.18 mol fraction of R245fa can have the same suppression effect as 0.3 mol fraction of CO2. In the absence of experimental data, we assume that a 0.3 mass fraction of retardants R123 and R11 is set as the minimum limit. So in this section, we just focus on the mass fraction of retardants ranging from 0.3 to 0.7. Thermal efficiencies of the mixtures, varying with evaporation temperature, are given in Fig. 8. From the vertical, with the increasing evaporation temperature, efficiency of each pair of mixtures gradually increases, and their respective increasing rate gets smaller. That is because with the increasing evaporation temperature, the average endothermic temperature and turbine inlet temperature increase, leading to an increase in enthalpy drop across turbine, meaning a higher output work per unit mass flux, so

15

0.7/0.3(-IHE) 0.3/0.7(-IHE) 0.7/0.3(+IHE) 0.3/0.7(+IHE)

14 13 12 11 420

440

460

480

500

16 15

16 15

13 12 11 420

0.7/0.3(-IHE) 0.3/0.7(-IHE) 0.7/0.3(+IHE) 0.3/0.7(+IHE)

13 12 11 440

460

480

440

460

480

500

Evaporation temperature (K)

14

420

0.7/0.3(-IHE) 0.3/0.7(-IHE) 0.7/0.3(+IHE) 0.3/0.7(+IHE)

14

Evaporation temperature (K)

500

Evaporation temperature (K)

Thernal efficiency of benzene/R11 (%)

Thernal efficiency of cyclohexane/R11 (%)

16

Thernal efficiency of benzene/R123 (%)

G. Shu et al. / Energy 74 (2014) 428e438

Thermal efficiency of cyclohexane/R123 (%)

436

16 15

0.7/0.3(-IHE) 0.3/0.7(-IHE) 0.7/0.3(+IHE) 0.3/0.7(+IHE)

14 13 12

420

440

460

480

500

Evaporation temperature (K)

Fig. 10. Thermal efficiency vs. evaporation temperature.

the thermal efficiency also increases. However, for a fixed percentage mixture, evaporation temperature glide decreases with the increasing evaporation temperature, causing a drop in increasing rate. From the horizontal, efficiencies of four sets of mixtures change differently with the increasing mass fraction of retardants. In Fig. 8, the enlarged points represent the maximum efficiency at different evaporation temperature. Overall, the OMR (optimal mixture ratio), corresponding to maximum efficiency, slowly approaches the side of fewer retardants. As the equation (17) shows, thermal efficiency of the ORC system depends on the enthalpy of each state point, which primarily has relationship with fluids' characteristics and system's components properties. With the increasing evaporation temperature, the effect of temperature glides keeps diminishing, so thermal efficiency is mainly affected by the properties of the mixtures. The major components have a relatively high efficiency, so the less the mass fraction of retardants, the higher efficiency the corresponding mixtures have. Table 3 lists the OMR (corresponding to maximum efficiency) of different mixtures at different evaporation temperatures. Fig. 9 illustrates the exergy losses of mixtures varying with evaporation temperature. From the vertical, with the increasing evaporation temperature, exergy loss of each pair of mixtures gradually decreases, and their respective reduce rate gets smaller. Hence, a higher evaporation temperature will result in smaller total exergy loss. The total exergy loss largely depends on the reduction of exergy loss in the evaporator and increment of exergy loss in the turbine. The exergy loss in the evaporator drops strongly with increasing evaporation temperature, while the exergy loss in the turbine goes roughly in the opposite direction, but with a lower slope. So the total exergy loss is reduced. However, for a fixed

percentage mixture, a gradual fading effect of evaporation temperature glide results in a drop in reduce rate. From the horizontal, when evaporation temperature is lower, with the increasing mass fraction of retardants, exergy losses of four sets of mixtures firstly decrease slowly, then increase. But at a higher evaporation temperature, exergy loss increases relatively quickly. In Fig. 9, the enlarged points represent the minimum exergy loss at different evaporation temperature. The optimal mixture ratio corresponding to minimum exergy loss, gradually approach the side of fewer retardants. The OMRs (corresponding to minimum exergy loss) of different mixtures at different evaporation temperatures are listed in Table 4. By comparison with Table 3, maximal efficiencies of each mixture are not always connected to minimum exergy loss. This is because they depend on different influence factors. 4.3. Effect of the IHE In this section, the influence of IHE based on different evaporation temperature on the total thermal efficiency and exergy loss are plotted in Figs. 10 and 11. In order to highlight the influence of IHE, for four sets of mixtures, we only select two mixture ratios, 0.7/ 0.3 and 0.3/0.7. As show in Fig. 10, after adding IHE to the system, thermal efficiency of each mixture has a significant increase, and the increasing rate gets greater with evaporation temperature increasing. Reasons are as follows: After equipped with IHE, the evaporator inlet temperature of the working fluid is higher than that of the ORC without IHE. That also means a higher outlet temperature of the exhaust gas and less quantity of heat transfer in the exchanger, so the heat load of the evaporator declines. Besides,

437

37

Exergy loss of benzene/R123 (kW)

Exergy loss of cyclohexane/R123 (kW)

G. Shu et al. / Energy 74 (2014) 428e438

36 35 34 33 32 31 420

0.7/0.3(-IHE) 0.3/0.7(-IHE) 0.7/0.3(+IHE) 0.3/0.7(+IHE)

440

460

480

500

36 35 34 33 32 31

0.7/0.3(-IHE) 0.3/0.7(-IHE) 0.7/0.3(+IHE) 0.3/0.7(+IHE)

420

440

36 35 34 33 32

0.7/0.3(-IHE) 0.3/0.7(-IHE) 0.7/0.3(+IHE) 0.3/0.7(+IHE)

31 420

440

460

480

460

480

500

Evaporation temperature (K)

Exergy loss of benzene/R11 (kW)

Exergy loss of cyclohexane/R11 (kW)

Evaporation temperature (K)

500

36 35 34 33 32 31

0.7/0.3(-IHE) 0.3/0.7(-IHE) 0.7/0.3(+IHE) 0.3/0.7(+IHE)

420

Evaporation temperature (K)

440

460

480

500

Evaporation temperature (K)

Fig. 11. Exergy loss vs. evaporation temperature.

superheated vapor out of the turbine can be used to preheat the fluid out of the pump, this can recover part of the heat energy and reduce heat load of the condenser. It's the two reasons that lead to an improvement in thermal efficiency. The increasing evaporation temperature results in a better thermal match between the working fluid and the heat source, so the increasing rate gets greater gradually. In details, take cyclohexane/R123 as an example. For pro ¼ 0.7/ 0.3, at 430 K, without IHE the efficiency is 10.63%, in contrast to 11.88% with IHE, representing an efficiency increase of 11.77%. While at 500 K, the former efficiency is 13.73%, and the latter is 15.94%, an increase of 16.15%. So a higher evaporation temperature is good for performance improvement. For pro ¼ 0.3/0.7, the efficiency tends to reach the maximum near the critical temperature. The efficiency can reach 13.25% without IHE and 14.71% with IHE. It shows the advantage of the critical ORC, but near the critical point, the property of some working fluids may change, so the state is difficult to reach. It's not wise to pursuit an excessive evaporation temperature in a practical application. It's apparent from Fig. 11 that with IHE, the exergy loss of each mixture decreases notably, and the decreasing rate gradually gets greater with evaporation temperature increasing. It could be indicated that most of the exergy loss takes place in the evaporator. If IHE is used, it could make the mean transferring heat temperature difference between the heat source and the working fluid smaller in the evaporator, and lead to a higher exhaust gas outlet temperature, resulting in less irreversibility. Besides, more energy of superheated vapor out of the turbine is recovered by IHE, leading to a lower exergy loss in the condenser. So it's beneficial to add the IHE especially when the vapor out of the turbine is superheated.

For the four sets of mixtures, pro ¼ 0.7/0.3 falls more quickly than pro ¼ 0.3/0.7 with the increasing temperature. At low temperatures, pro ¼ 0.7/0.3 has a larger loss than pro ¼ 0.3/0.7, but beyond a certain point, the opposite happens. Adding IHE makes the phenomenon more remarkable. So pro ¼ 0.7/0.3 has the advantage over the pro ¼ 0.3/0.7 on the exergy loss performance. For cyclohexane/R123, at 430 K, without IHE, pro ¼ 0.7/0.3 has exergy loss of 36.62 kW, while pro ¼ 0.3/0.7 has exergy loss of 36.46 kW, and the difference is 0.16. With IHE, the former is 36.23 kW, and the latter is 36.04 kW, a difference of 0.19 (an increase of 18.75%). At 490 K, without IHE, the difference is 1.21, and with IHE, it's 1.53 (an increase of 26.45%). Table 5 shows the optimal results with and without IHE. The mixture ratio and the evaporation temperature are also optimal. Temperature, pressure and enthalpy of each state point are given. The table clearly quantifies the influence of IHE on thermal efficiency, heat input and exergy loss. It can be seen whether with IHE or not, benzene/R11 (0.7/0.3) shows the best performance, followed by benzene/ R123, cyclohexane/R11 and cyclohexane/R123. The maximum thermal efficiency is 16.7% and minimum exergy loss is 30.76 kW.

5. Conclusions In this paper, the use of mixtures in high temperature ORC system has been proposed. Energy and exergy analysis on the cycle is simulated. The effects of the retardants mass fraction, evaporation temperature and IHE on the cycle performance are analyzed. According to the investigation, following important conclusions are drawn:

438

G. Shu et al. / Energy 74 (2014) 428e438

Table 5 Comparison of proposed zeotropic mixtures without (-IHE) and with internal heat exchanger (þIHE). Components

Cyclohexane/ R123

Benzene/ R123

Cyclohexane/ R11

Benzene/ R11

Mixture ratio T3 (K) T4 (K) Evaporation pressure P3 (MPa) T1 (K) T6 (K) Condensation pressure P1 (MPa) T2 (K) T2a (K) T5 (K) T5a (K) h2  h1 (kJ/kg) h4  h5 (kJ/kg) Qe (IHE) (kW) Qe (þIHE) (kW) Thermal efficiency h (IHE) (%) Thermal efficiency h (þIHE) (%) I (IHE)(kW) I (þIHE)(kW)

0.7/0.3 500 509.27 2.7958

0.7/0.3 500 508.01 2.8584

0.7/0.3 500 508.92 2.7983

0.7/0.3 500 508.14 2.8940

335.66 353.45 0.1202

337.99 352.79 0.1180

334.15 353.45 0.1224

335.38 352.79 0.1204

336.96 375.0 428.29 382.62 3.90 73.02 124.96 121.39 13.73

339.30 367.21 412.64 375.97 3.62 77.44 124.96 123.73 15.03

335.46 372.801 425.53 380.49 3.89 74.00 124.96 122.05 13.97

336.72 363.97 408.83 372.77 3.65 78.38 124.96 124.69 15.23

15.94

16.58

16.1

16.7

32.12 31.04

31.65 30.79

32.05 30.98

31.60 30.76

(1) For the azeotropic mixture (maximum temperature glide <5 K), the efficiency is lower than that of major pure working fluids, and the exergy loss is larger. For the zeotropic mixtures, they have a higher efficiency and lower exergy loss than the relative major pure working fluids at a certain mixture ratio. The retardants generally have the poor performance. (2) With the mass fraction of retardants increases, efficiencies of zeotropic mixtures firstly increase and then decrease, and the exergy loss has the opposite change trend. There exist optimal ratios for different zeotropic mixtures. With the evaporation temperature increases, optimal mixture ratios corresponding to maximum efficiency gradually approach the side of fewer retardants. So does the minimum exergy loss. But maximum efficiencies of each mixture are not always equivalent to minimum exergy loss. (3) Adding IHE to the system, thermal efficiency of each mixture has a significant increase, and the increasing rate gets greater with evaporation temperature increasing, and the exergy loss has the opposite change trend. So the IHE has a positive effect on the cycle performance. Take benzene/R11 (0.7/0.3) as example, with IHE, the efficiency growth is about 7.12% ~9.72%. Using it, the maximum thermal efficiency is 16.7%, and minimum exergy loss is 30.76 kW. Acknowledgments This work was supported by a grant from the National Basic Research Program of China (973 Program) (No. 2011CB707201), and the National Natural Science Foundation of China (No. 51206117).

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