Thermodynamic analysis and optimization of a novel zeotropic organic Rankine Cycle

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9th International Conference on Applied Energy, ICAE2017, 21-24 August 2017, Cardiff, UK

Thermodynamic analysis and optimization of a novel zeotropic The 15th International Symposium on District Heating and Cooling organic Rankine Cycle Assessing the feasibility of using the heata demand-outdoor a a a Renlong Huanga, Xianglong Luoa*, Zhi Yanga, Ying Chena temperature function for a Soft long-term district heat demand forecast Guangdong Condensed Guangdong Provincial Provincial Key Key Laboratory Laboratory on on Functional Functional Soft Condensed Matter, Matter, School School of of Material Material and and Energy, Energy, Guangdong Guangdong University University of of

a a

Technology, Technology, No. No. 100 100 Waihuan Waihuan Xi Xi Road, Road, Guangzhou Guangzhou Higher Higher Education Education Mega Mega Center, Center, Panyu Panyu District, District, Guangzhou Guangzhou 510006, 510006, China China

I. Andrića,b,c*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc

a Abstract IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal Abstract b

Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France

c Département Systèmes Énergétiques Environnement - IMT Atlantique, 4 rue Alfred Nantes, France renewable The fossil reserves and of prices have in strong interest in The decrease decrease of of fossil energy energy reserves and the the etincrease increase of energy energy prices have resulted resulted in aaKastler, strong 44300 interest in utilizing utilizing renewable heat sources sources or or waste waste heat heat for for power power generation. generation. Organic Organic Rankine Rankine cycle cycle (ORC) (ORC) is is aa promising promising heat-to-power heat-to-power conversion conversion heat technology. Although Although significant significant efforts efforts have have been been devoted devoted to to improve improve the the thermo-economic thermo-economic performance performance of of the the pure pure fluid fluid ORC, ORC, technology. the the improvement improvement is is limited limited due due to to the the isothermal isothermal nature nature of of evaporation evaporation and and condensation. condensation. ORC ORC using using zeotropic zeotropic mixture mixture is is Abstract ORC using pure fluid in thermodynamic performance due to the lower irreversibility in heat transfer process of the superior superior to to ORC using pure fluid in thermodynamic performance due to the lower irreversibility in heat transfer process of the zeotropic mixture. mixture. In In the the present present study, study, aa novel novel zeotropic zeotropic ORC ORC with with liquid-separation liquid-separation condensation condensation and and multi-pressure multi-pressure zeotropic District heating networksThermodynamic are commonly addressed in the literature as one of for The decreasing the evaporation is proposed. proposed. analysis and and optimization model of the the most noveleffective ORC is is solutions developed. objective evaporation is Thermodynamic analysis optimization model of the novel ORC developed. The objective greenhouse gas emissions from the building sector. These systems require high investments which are returned through the heat function of the optimization is the maximization of net power output. A case study is elaborated to test the proposed methodology. function of the optimization is the maximization of net power output. A case study is elaborated to test the proposed methodology. sales. Dueshow to the climate conditions and building renovationimproved policies, compared heat demand the future could decrease, The results thatchanged the thermodynamic thermodynamic performance can be be significantly significantly to the theinbasic basic zeotropic ORC. The results show that the performance can improved compared to zeotropic ORC. prolonging the investment return period. © 2017 2017 The The Authors. Authors. Published Published by by Elsevier Elsevier Ltd. Ltd. © ©The 2017 Thescope Authors. Published Ltd. main this paper isby to Elsevier assess the feasibility of using demand –Conference outdoor temperature function Peer-review underofresponsibility responsibility of the scientific scientific committee of the thethe 9thheat International on Applied Applied Energy.for heat demand Peer-review under of the committee of 9th International Conference on on Energy. Peer-review under responsibility of thelocated scientific of the 9th International Applied Energy. forecast. The district of Alvalade, in committee Lisbon (Portugal), was used as aConference case study. The district is consisted of 665 buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district Keywords:Zoetrope Keywords:Zoetrope mixture, mixture, Liquid-vapour Liquid-vapour separation, separation, MultiMulti- pressure pressure evaporation; evaporation; renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were compared with results from a dynamic heat demand model, previously developed and validated by the authors. The results showed that when only weather change is considered, the margin of error could be acceptable for some applications 1. 1. Introduction Introduction (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). Strong interest has been in renewable heat sources or heat for power due Strong been exerted exerted in utilizing utilizing renewable heat sources or waste waste for decade, power generation generation due to totothe the The valueinterest of slopehas coefficient increased on average within the range of 3.8% up to heat 8% per that corresponds the decrease of fossil energy reserves and the increase of energy prices. Organic Rankine cycle (ORC) is a promising decrease fossil energy reserveshours and of the22-139h increase of energy prices. Organic Rankine cycle (ORC) isofa weather promising decrease of in the number of heating during the heating season (depending on the combination and power generation technology. Although significant efforts have increased been devoted devoted to improve improve the thermo-economic thermo-economic power generation significant efforts have been to the renovation scenariostechnology. considered). Although On the other hand, function intercept for 7.8-12.7% per decade (depending on the performance of the the The ORCvalues usingsuggested pure fluid fluid [1, 2, 2, 3], thetoimprovement improvement is limited limited due to to the the isothermal behavior of the the coupled scenarios). could be3], used modify the function parameters for isothermal the scenariosbehavior considered, and performance of ORC using pure [1, the is due of improve the accuracy of heat demand estimations.

© 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Corresponding author. author. Tel.: Tel.: +86-020-39322570. +86-020-39322570. ** Corresponding Cooling. E-mail E-mail address: address: [email protected]. [email protected]. (X. (X. Luo). Luo).

Keywords: Heat demand; Forecast; Climate change 1876-6102 © © 2017 2017 The The Authors. Authors. Published Published by by Elsevier Elsevier Ltd. Ltd. 1876-6102 Peer-review Peer-review under under responsibility responsibility of of the the scientific scientific committee committee of of the the 9th 9th International International Conference Conference on on Applied Applied Energy. Energy. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 9th International Conference on Applied Energy. 10.1016/j.egypro.2017.12.518

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evaporation and condensation. Fortunately, the ORC performance can be improved by using a zeotropic mixture, which exhibits non-isothermal characteristics during the evaporation and condensation processes [4]. Ghahosseini et al. [5] analyzed low-temperature ORC using pure and zeotropic working fluids. Abadi et al. [6] presented an experimental study of a 1kW ORC using R245fa/R134a. The performance comparison between the ORC using pure fluid and ORC using mixture was also performed. These previous researches showed that considerable net power output or the second law efficiency can be enhanced by using zeotropic mixture in the ORC. The cycle configuration improvement is also vital in improving the ORC performance in addition to the utilization of zeotropic mixtures. Shokati et al. [7] presented a comparison between a dual-pressure ORC and a basic ORC. They found that the net power output of the former is 15.22% higher than that of the latter. Song et al. [8] proposed a dual-loop ORC to recover waste heat from diesel engines. Their results indicated that the power output of the dual-loop ORC was increased by 11.6%. The thermodynamic performance improvement of dual-loop ORC over basic ORC is mainly attributed to the improved heat match of evaporation process. Compared to the research on the evaporation process, the research on the condensation process of the ORC is limited. Collings et al. [9] proposed a dynamic zeotropic fluid organic Rankine cycle. Their case study results showed that the annual average thermal efficiency can be improved by up to 23% over a conventional ORC. Liquid– vapor separation during condensation is a novel technology in enhancing heat transfer and/or reducing pressure drop of the condenser. Luo et al. [1, 10] proposed an ORC using liquid-separation condenser and conducted a performance comparison among liquid separation condenser (LSC), parallel flow condenser, and serpentine condenser in an ORC using pure fluid. Li et al. [11] introduced the liquid-separation condensation into ORC using R600/R601a mixtures. Their results showed that the liquid-separation condensation can increase the average condensation heat transfer coefficient by 23.8%. In this work, a novel zeotropic ORC with liquid-separation condensation and dual-pressure evaporation (LMZORC) is proposed. Thermodynamic analysis and optimization model of the novel ORC is developed. A case study is elaborated to validate the novel ORC and test the proposed methodology. 2. Problem description Fig.1 shows a representation of the basic zeotropic ORC (BZORC), Fig.2 shows a representation of multipressure evaporation zeotropic ORC (MZORC) and Fig.3 shows a representation of the zeotropic ORC with liquidseparation condensation and multi-pressure evaporation (LMZORC). As shown in Fig.3, the working fluid that exits the expander is partially condensed in the first part of the condenser. It then flows through a liquid separator equipped in the condenser. The liquid is separated in the liquid separator. Then the high-quality vapor flows into the next part of the condenser, and it is continually cooled down by heat sink. The first condensate stream is pumped into the high pressure evaporator while the second condensate stream is pumped into the low pressure evaporator. The high and low pressure vapors then expand in the expander. The exhaust vapor exit the expander enters the liquid-separation condenser. And then the cycle continues.

Fig. 1. Representation of the BZORC system: (a) schematic, (b) T-s diagram

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Fig. 2.Representation of the MZORC system: (a) schematic, (b) T-s diagram

Fig. 3.Representation of the LMZORC system: (a) schematic, (b) T-s diagram

As shown in Fig.3 (b), there are two advantages in addition to the improvement in heat transfer coefficient when applying liquid-separation condenser in the zeotropic ORC. The first one is the temperature glide can be controlled by adjusting the liquid-separation position to improve the heat match between working fluid and heat sink. The second one is the mixture composition can be controlled by adjusting the liquid-separation position to provide the potential of better heat match between heat source and working fluid. Thus, the liquid-vapor separation during condensation is a key factor in determining the system performance of the LMZORC. 3. Thermodynamic model of LMZORC Following assumptions are given before the modelling in order to optimize the BZORC, MZORC, LMZORC.  The friction pressure drop is neglected in pipelines and heat exchangers.  All cycles are assumed under steady state.  Energy loss during the mixing process of the zeotropic mixture in the expander is neglected.  No superheating in the evaporator and no sub-cooling in the condenser are considered. The thermodynamic model is developed based on the model developed in [12]. The optimization variables are evaporating pressures, inlet mixture composition of condenser, mass quality, and condensing pressure. The objective function is the maximization of the net power output for a waste heat driven ORC. The mass and energy balance can be written as Eqs (1) and (2), where m, Q, W and h stands for mass flow, heat load from heat source, the output power and enthalpy, respectively. ∑ 𝑚𝑚𝑖𝑖𝑖𝑖 = ∑ 𝑚𝑚𝑜𝑜𝑜𝑜𝑜𝑜 (1) (2) 𝑄𝑄 − 𝑊𝑊 = ∑ 𝑚𝑚𝑜𝑜𝑜𝑜𝑜𝑜 ℎ𝑜𝑜𝑜𝑜𝑜𝑜 − ∑ 𝑚𝑚𝑖𝑖𝑖𝑖 ℎ𝑖𝑖𝑖𝑖 The exergy balance can be expressed by Eqs (3) and (4), where Wnet is net power output, IRtotal denotes the total irreversible loss. Subscript o represents the thermodynamic property at the dead state. The objective function is the maximization of the net power output for a waste heat driven ORC. The model is formulated in GAMS 23.6 on a PC with a 3.0 GHz Intel(R) Core (TM) 2 processor.

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(3) (4)

𝑊𝑊𝑛𝑛𝑛𝑛𝑛𝑛 + 𝐼𝐼𝐼𝐼𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 = 𝐸𝐸𝐸𝐸𝑖𝑖𝑖𝑖 − 𝐸𝐸𝐸𝐸𝑜𝑜𝑜𝑜𝑜𝑜 𝐸𝐸𝐸𝐸 = 𝑚𝑚((ℎ − ℎ𝑜𝑜 ) − 𝑇𝑇𝑜𝑜 (𝑠𝑠 − 𝑠𝑠𝑜𝑜 ))

4. Result and discussion

In this paper, R245fa/R365mfc is selected as working fluid due to its zero ozone depletion, low toxicity, low cost, and liquid sate at room temperature. In this section, the optimization of LMZORC driven by low temperature waste heat is conducted. The reference design parameters are shown in Table 1. The inlet temperature of the waste heat is 150 oC at 0.5 MPa and the inlet temperature of cooling water is 20 oC at 0.4Mpa. Table 1. Main assumptions and given conditions [13, 14] Parameter item

Value

The inlet temperature of the waste heat water(℃)

150

The mass flow rate of the waste heat water(kg/s）

70

The inlet temperature of the cooling water(℃)

20

The outlet temperature of the cooling water(℃)

30

The heat pinch point temperature difference with high boiling mixture(℃)

10

The heat pinch point temperature difference with low boiling mixture(℃)

10

The turbine isentropic efficiency (%)

80

The Pump isentropic efficiency (%)

65

Ambient temperature (℃)

25

Ambient pressure(MPa)

0.1

4.1. Optimization and comparison of BZORC, MZORC, and LMZORC Table 2. Performance comparison of BZORC and LMZORC Items

BZORC

MZORC

LMZORC

Outlet temperature of heat source(℃)

76.49

61.51

61.80

Mass flow rate of cool water (kg/s)

478.73

572.40

570.73

R245fa/R365mfc(inlet mole composition)

0.536/0.464

0.471/0.529

0.413/0.587

R245fa/ R365mfc (liquid)

0.536/0.464

0.471/0.529

0.331/0.669

R245fa/ R365mfc (vapour)

0.536/0.464

0.471/0.529

0.539/0.461

Mass flow rate of working fluid

91.08

109.41

108.94

The mass ratio of the vapour and liquid

/

0.306

0.343

Evaporation pressure(MPa)

0.78

0.93/0.28

0.88/0.31

Condensation pressure (MPa)

0.15

0.14

0.13

Thermal efficiency (%)

8.19

7.59

7.58

Second thermodynamic efficiency (%)

28.09

32.03

32.04

Total irreversible loss (kW)

3185.61

3473.66

3436.26

Net power output (kW)

1787.70

1964.53

1965.32

The thermodynamic optimization for BZORC, MZORC and LMZORC are performed using GAMS under the given conditions and the results are listed in Table 2. The optimization variables are evaporating pressure, original mixture composition, mass quality, and condensing pressure. Table 2 gives the comparison of three optimal results of BZORC、MZORC and LMZORC. Among BZORC, MZORC and LMZORC, LMZORC generates the highest net power, whereas BZORC is the lowest. As shown in Table 2, LMZORC performs better than the BZORC in

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terms of net power output. And the net power output is increased by 9.94%. The MZORC and LMZORC can utilize more energy from waste heat to generate power. Meanwhile, LMZORC can generate a little bit more net power output than MZORC. 4.2. Irreversible loss comparison of BZORC, MZORC and LMZORC Fig.4 gives the Irreversible loss comparison of BZORC MZORC and LMZORC. Among the total irreversible loss of BZORC, MZORC and LMZORC, MZORC is the highest, whereas BZORC is lowest. It is evident that the evaporator accounts for the largest proportion of the irreversible loss, which reaches almost 50% of the total ORC irreversible loss. Moreover, the irreversible loss in the evaporator, expander, condenser and pump for MZORC, LMZORC is slightly reduced. For evaporator and condenser, changing mixture composition during condensation, the working fluid may match better with heat and sink temperature profile. For the expander, the irreversible loss is proportional to the working fluid mass flow rate. Moreover, the mass flow rate of LMZORC is slightly less than that of MZORC. The pump has similar variation trend with that of expander.

Fig. 4. Irreversible loss of BZORC MZORC and LMZORC: (a) irreversible loss, (b) irreversible loss ratio

4.3. The influence of liquid-separation during condensation In this section, the influence of liquid-separation on the cycle performance is conducted. For LMZORC, the design scheme is optimized at different mixture composition range from 0.01 to 0.99 with changing composition. For MZORC, the mass flow rates of fluid enter the high pressure and low pressure evaporator is the same as those of LMZORC. Noting that, the mixture composition in MZORC is not changed. Fig.5 gives the net power output comparison between MZORC and LMZORC. As shown in Fig.5, the net output work increases firstly and then decreases with mass quality for both MZORC and LMZORC. Obviously, the mass quality at liquid-vapor separation has significant influence on the net output work.

Fig. 5. The influence of the mass ratio of two streams at liquid-vapor separation

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5. Conclusions This paper presented a novel zeotropic ORC system with liquid-separation condensation and multi-pressure evaporation. The thermodynamic optimization of BZORC, LMZORC and MZORC were conducted and compared. The effects of vapor quality on net power output and irreversible loss were analyzed. Following conclusions were drawn.  The comparison between LMZORC and BZORC showed that the LMZORC is superior to BZORC in terms of net power output. The net power output of LMZORC is 9.94% higher than that of BZORC. Meanwhile, MZORC can bring a relative 9.89% improvement in net power output compared with BZORC.  Among these cycle configurations, the evaporator accounts for the largest proportion of the total irreversible loss. Meanwhile, LMZORC features less irreversible loss than MZORC.  The influence analysis of liquid-separation condensation on the cycle performance showed that the mass quality has significant influence on the net output work and the liquid-separation condensation is effective in control the mixture composition Acknowledgements The authors gratefully acknowledge the financial support from the Guangdong Special Funding for Applied Technology Research & Development (2016B020243010) and from the Science and Technology Major Project of Guangdong (No.2013A011402006). References [1] Luo X, Yi Z, Zhang B, Mo S, Wang C, Song M, Chen Y, Mathematical modelling and optimization of the liquid separation condenser used in organic Rankine cycle. Applied Energy 2017; 185:1309-23. [2] Chen H., Goswami D.Y., Stefanakos E.K., A review of thermodynamic cycles and working fluids for the conversion of low-grade heat. Renewable and Sustainable Energy Reviews 2010; 14(9); 3059-67. [3] Etemoglu A.B., Thermodynamic investigation of low-temperature industrial waste-heat recovery in combined heat and power generation systems. International Communications in Heat and Mass Transfer 2013; 42; 82-88. [4] Wang J.L., Zhao L., Wang X.D., A comparative study of pure and zeotropic mixtures in low-temperature solar Rankine cycle. Applied Energy 2010; 87(11); 3366-73. [5] Aghahosseini S., Dincer I., Comparative performance analysis of low-temperature Organic Rankine Cycle (ORC) using pure and zeotropic working fluids. Applied Thermal Engineering 2013; 54(1); 35-42. [6] Abadi, G. B., Yun, E., Kim, K. C., Experimental study of a 1kw organic rankine cycle with a zeotropic mixture of r245fa/r134a. Energy 2015; 93; 2363-73. [7] Shokati N, Ranjbar F, Yari M, Exergoeconomic analysis and optimization of basic, dual-pressure and dual-fluid ORCs and Kalina geothermal power plants: a comparative study. Renewable Energy 2015; 83: 527-542. [8] Song J, Gu C, Performance analysis of a dual-loop organic Rankine cycle (ORC) system with wet steam expansion for engine waste heat recovery. Applied Energy 2015; 156: 280-289. [9] Collings P., Yu Z., Wang E., A dynamic organic Rankine cycle using a zeotropic mixture as the working fluid with composition tuning to match changing ambient conditions. Applied Energy 2016; 171; 581-91. [10] Luo X, Yi Z, Chen Z, Chen Y, Mo S, Performance comparison of the liquid–vapor separation, parallel flow, and serpentine condensers in the organic Rankine cycle. Applied Thermal Engineering 2016; 94: 435-448. [11] Jian Li, Qiang Liu, Yuanyuan Duan, Zhen Yang, Performance analysis of organic Rankine cycles using R600/R601a mixtures with liquidseparated condensation, Applied Energy 2017; 190:376–389. [12] Liu Q., Shen A., Duan Y., Parametric optimization and performance analyses of geothermal organic Rankine cycles using R600a/R601a mixtures as working fluids. Applied Energy 2015; 148; 410-20. [13] Lecompte S., Ameel B., Ziviani D., Van den Broek M., De Paepe M., Exergy analysis of zeotropic mixtures as working fluids in Organic Rankine Cycles. Energy Conversion and Management 2014; 85; 727-39. [14] Chys M., Van den Broek M., Vanslambrouck B.,De Paepe M., Potential of zeotropic mixtures as working fluids in organic Rankine cycles, Energy, 44(2012) 623-632.

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Biography Xianglong Luo is a professor in the School of Material and Energy, Guangdong University of Technology. His research interests include energy system optimization and integration, waste heat utilization, and its application in energy conservation and emission reduction for energy-intensive industries.