Energetic study and comparative analysis of two novel ORC cogeneration systems using gas ejectors

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Technologies and Materials for Renewable Energy, Environment and Sustainability, TMREES18, Technologies and Materials for Renewable Energy, and Sustainability, TMREES18, 19–21 September 2018,Environment Athens, Greece 19–21 September 2018, Athens, Greece

a a

Energetic study and comparative analysis of two novel ORC Energetic and comparative analysis of two Thestudy 15th International Symposium on District andnovel Cooling ORC cogeneration systems using gasHeating ejectors cogeneration systems using gas ejectors Afif aa, Adel Elamari bb, Nahla Bouaziz a,b * a,b Assessing Larbi the feasibility of using the heat demand-outdoor Larbi Afif , Adel Elamari , Nahla Bouaziz * Université de Tunis El Manar, Ecole Nationale d'Ingénieurs de Tunis, Unité de Recherche Energétique et Environnement, 1002, Tunis, Tunisie temperature for Préparatoire a long-term district demand forecast Université defunction Tunis Manar, Institut aux Etudes d’Ingénieurs d’Elheat Manar, 2092, Tunis, Tunisie Université de Tunis El Manar, EcoleElNationale d'Ingénieurs de Tunis, Unité de Recherche Energétique et Environnement, 1002, Tunis, Tunisie b b

Université de Tunis El Manar, Institut Préparatoire aux Etudes d’Ingénieurs d’El Manar, 2092, Tunis, Tunisie

I. Andrića,b,c*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc Abstract a IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Abstract Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c The present work proposesSystèmes two new ORC systems set up under- IMT the subcritical ensuring the production Département Énergétiques et Environnement Atlantique, 4and rue transcritical Alfred Kastler,regime, 44300 Nantes, France of electricity and refrigeration Theset fundamental idea of these and configurations based on the association of an The present work proposes twosimultaneously. new ORC systems up under the subcritical transcriticalisregime, ensuring the production organic Rankine with a simultaneously. refrigeration oneThe through the integration of a gas ejector. Consequently, newassociation thermodynamic of electricity and cycle refrigeration fundamental idea of these configurations is based on the of an organic Rankine cycleused withto aoptimize refrigeration one through the On integration a gas Consequently, new for thermodynamic parameters have been the system efficiency. the other of hand, newejector. working fluids are chosen this research Abstract because of their depletion (ODP) and low warming (GWP). two parameters ensure parameters have low beenozone used to optimizepotentials the system efficiency. Onglobal the other hand, potentials new working fluidsThese are chosen for this research that the chosen are less harmfulpotentials to the environment and therefore they help to preserve the These ecosystem. Furthermore, the because of their fluids low ozone depletion (ODP) and low global warming potentials (GWP). two parameters ensure District heating networks arebeen commonly addressed in theand literature as they one of the most effective solutions the that the chosen fluids arehave less harmful to the environment therefore help preserve the ecosystem. Furthermore, the proposed configurations analyzed from the viewpoint of energy in order totoobtain a comparative studyfor ofdecreasing the proposed greenhouse gas emissions from theanalyzed building sector. systems requireinhigh are returned through the heat cycles as well as the different working fluids.from proposed configurations have been theThese viewpoint of energy orderinvestments to obtain a which comparative study of the proposed sales. asDue climate conditions and building renovation policies, heat demand in the future could decrease, cycles welltoasthe the changed different working fluids. the investment return ©prolonging 2018 The Authors. Published byperiod. Elsevier Ltd. ©The 2019 The Authors. Published Ltd. scope of this paperunder isby to Elsevier assess feasibilitylicense of using(https://creativecommons.org/licenses/by-nc-nd/4.0/) the heat demand – outdoor temperature function for heat demand © 2018 The Authors. Published by Elsevier Ltd. This ismain an open access article the CCthe BY-NC-ND This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) forecast. The district of Alvalade, located inofLisbon (Portugal), was used as a case study. The district is consistedEnergy, of 665 This is an and openpeer-review access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection under responsibility the scientific committee of Technologies and Materials for Renewable Selection and peer-review under responsibility of the scientific committee of Technologies and Materials for Renewable Energy, buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district Selection and peer-review under responsibility of the scientific committee of Technologies and Materials for Renewable Energy, Environment TMREES18. Environment and and Sustainability, Sustainability, TMREES18. renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were Environment and Sustainability, TMREES18. comparedOrganic with results from a dynamic heat energy; demandCogeneration; model, previously developed and validated by the authors. Keywords: Rankine Cycle; Ejector; Solar The results showed that Cycle; when Ejector; only weather changeCogeneration; is considered, the margin of error could be acceptable for some applications Keywords: Organic Rankine Solar energy; (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). 1.scenarios, Introduction value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the 1.The Introduction decrease in the due number of heating hours duringand the the heating (depending on the combination of world, weather the and Nowadays, to the depletion of of the22-139h fossil fuels riseseason of energy consumption in the entire renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the Nowadays, of due to theresources depletionhas of the fossilthe fuels andobjective the rise of of several energy researches. consumptionScholars in the entire world, the sustainability energy become main have introduced coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and sustainability of energy resources has become the main objective of several researches. Scholars have introduced improve the accuracy of heat demand estimations. © 2017 The Authors. Published by Elsevier Ltd. * Corresponding author: Nahla Bouaziz Tel.: +21697452626; Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and E-mail address:author: [email protected] * Corresponding Nahla Bouaziz Tel.: +21697452626; Cooling. E-mail address: [email protected] 1876-6102 © 2018 The Authors. Published by Elsevier Ltd. Keywords: Heat demand; Forecast; Climate change This is an open access under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) 1876-6102 © 2018 Thearticle Authors. Published by Elsevier Ltd. Selection under responsibility of the scientific of Technologies and Materials for Renewable Energy, Environment This is an and openpeer-review access article under the CC BY-NC-ND licensecommittee (https://creativecommons.org/licenses/by-nc-nd/4.0/) and Sustainability, TMREES18. Selection and peer-review under responsibility of the scientific committee of Technologies and Materials for Renewable Energy, Environment and Sustainability, TMREES18. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. 1876-6102 © 2019 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of Technologies and Materials for Renewable Energy, Environment and Sustainability, TMREES18. 10.1016/j.egypro.2018.11.288

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cleaner energy cycles working at low heat source temperature [1-3]. These cycles exist in the following: Kalina cycle [4], Goswami cycle [5], TLC (trilateral cycle) [6] and ORCs (Organic Rankine cycles) [7]. Among these cycles, and within the framework of conversion heat into electricity power, Organic Rankine Cycle presents an interesting technology that offers relatively high energy efficiency for low thermal power inputs. In fact, Bombarda et al.[8] concluded that despite a similar obtained net powers for Kalina cycle and ORC, a very high maximum pressure is needed for the Kalina cycle in order to reach high thermodynamic performance (100 bars compared to about 10 bars for the ORC). Besides, for a power plant providing limited output power, ORC cycle would be advantageous for several reasons, including simplicity, reliability, low maintenance, and remote monitoring [9]. On the other hand, ORC systems has been widely adopted in many applications such as geothermal [10], biomass [11], waste heat recovery [12] and solar energy [13]. In addition, the ORC technology has an interesting advantage of being suitable for cogeneration plants working at low-medium temperature heat source and presenting high energy efficiency [14]. Solar ORC systems can be competitive from an economic point of view. In fact, concentrating solar power technology is highly suitable for solar ORCs since they require a lower investment cost and work at lower temperature. Therefore, the most appropriate concentrating solar power technology is parabolic troughs. This equipment is working at a low temperature (300°C-400°C) [15]. In this paper work, the proposed ORC configurations are cogeneration systems using solar heat energy to provide electricity power and refrigeration. Furthermore, the energy analysis is made by considering the following assumptions:     

The heat losses from the pipes and system components into the environment are negligible. The pressure drop in the heat exchangers is assumed negligible. The flow across the expansion valves is isenthalpic. All cycle processes are steady state condition. The ejector processes are assumed to be adiabatic.

The aim of this work is to investigate and compare the performance of the ORC configurations proposed under the optimization condition for each working fluid selected. Nomenclature IHE m m1 m2 PD PM PS QCD QE QG WP WT

Internal Heat Exchanger Total mass flow rate (Kg/s) Mass flow of the ORC cycle (Kg/s) Mass flow rate of the refrigeration cycle (Kg/s) Outlet pressure of the ejector (bar) Pressure of the motive flow of the ejector (bar) Pressure of the secondary flow of the ejector (bar) Heat power produced by the condenser (Kw) Refrigeration power consumed by the evaporator (Kw) Heat power consumed by the vapour generator (Kw) Electricity power consumed by the pump (Kw) Electricity power produced by the turbine (Kw)

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2. Description of the proposed ORC configurations:

Fig.1. (a) Configuration of the cogeneration system without IHE; (b) Configuration of the cogeneration system with IHE

As shown in Figure 1, two ORC cycles are investigated in this paper. Figure 1 (a) illustrates the first configuration is a basic ORC system combined with a refrigeration cycle. The association of these two cycles made a cogeneration system which can ensure the production of electricity power and refrigeration simultaneously. Besides, a gas ejector has been integrated in the proposed configuration to improve the system performance. The ejector presents the advantage of compressing the gas without consuming energy. In fact, the gas compression is activated thermally and therefore an ejector is also called a thermo-compressor. Figure 2 shows the geometric configuration of an ejector.

Fig.2. Geometric configuration of the ejector

As for the second configuration in figure 2 (b), an internal heat exchanger was added to recuperate heat from the extracted vapor from the turbine which is compressed by the ejector. Consequently, the heat consumption in the vapor generator is minimized by preheating the inlet fluid and therefore the efficiency of the cogeneration Rankine power is improved. Both ORC systems are using concentrated solar collectors as a heat source and the ambient air as a cooling source. Furthermore, these configurations are established under subcritical and transcritical regimes.

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3. Working fluids: The selection of working fluid has a tremendous effect on the performance of an ORC system. In fact, this choice is based on the heat source temperature and also the cooling source temperature [16, 17]. Besides, other criteria like critical conditions, flammability, toxicity and other environmental properties must be considered in the ORC process. On the other hand, fluids are classified in wet, dry and isentropic fluids due to the shape of the saturated line in the T-s diagram [18, 19]. Dry and isentropic fluids are considered as the best candidates for ORC cycles because after the expansion in the turbine, the extracted fluid is either saturated or superheated which improves the thermal efficiency of the system. In this work, R124, R236fa, R1234yf and R1234ze are chosen as working fluids. They are less harmful to the environment and help preserve our ecosystem due to their low ODP & GWP as shown in the Table 1 [20, 21]. Table1. Characteristics of working fluids Refrigerant

Critical Temperature (°C)

Critical Pressure (Bar)

Molar Mass (g/mol)

Safety Group

GWP

ODP

R124

124,5

36,6

136,5

A1

609

0,022

R236fa

122,92

32,192

152,04

A1

9810

0

R1234yf

94,7

33,82

114

A2L

4

0

R1234ze

109,36

36,62

114

A2L

6

0

4. Simulation and Modeling: The simulation of the proposed ORC systems is realized by ASPEN HYSYS which contain a database of thermodynamic properties of the chosen working fluids. The thermodynamic model utilized is based on Peng Robinson equation: P

RT a (T )  2 V m  b V m  2 bV m  b 2

(1)

Where a(T )  a *

RT R 2Tc2  (T ) And b  b * c Pc Pc

a *  0,45724 And b*  0,0778

 (T )  [1  m(1  Tr )]2 m  M 0 M 1  M 2 2

The Peng-Robinson equation is highly recommended for equilibrium calculations liquid-vapor of hydrocarbons under pressure. The equations used in the energy analysis of the two cogeneration systems proposed are presented in table 2.

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Table2. Energy and efficiency equations of the system Component\Efficiency

Equation

Vapor Generator

Q g  m 1 ( h4  h3 )

Evaporator

Q E  m 2 ( h8  h 7 )

Condenser

Q CD  m ( h1  h 9 )

Turbine

W T  m 1 ( h5  h 4 )

Pump

W P  m 1 ( h3  h 2 )

Ejector

m  h9  m 1  h5  m 2  h8

Valve

h7  h6

Thermal efficiency of the ORC cycle

 ORC 

WT  W P QG

Refrigeration Coefficient of performance

COPR 

QE QG

Thermal efficiency of the cogeneration system

CG 

WT  WP  QE QG

5. Results & Discussions: In addition to the temperatures of the heat and cooling sources, the performance of the proposed ORC configurations is affected by the thermodynamic parameters of the ejector. These new variables are presented in table 3 [22]. Table3. Thermodynamic parameters of the ejector Symbol

Parameter

Expression

μ

Entrainment ratio

m2/m1

τ

Compression ratio

PD/PS

Γ

Pressure ratio

PM/PS

ε

Motive ratio

PM/PD

The entrainment ratio is defined as the ratio between the primary and the secondary mass flow rates. The performance curve of the proposed configurations is illustrated in figure 3.

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R124 R236fa R1234yf R1234ze

80 70

R124 R236fa R1234yf R1234ze

90 80 70

CG(%)

60

CG(%)

1225

50 40

60 50 40

30

30

20

(a)

20 0,2

0,3

0,4

0,5

0,6

0,7

0,8

Entrainment ratio 

0,9

1,0

(b)

0,2

0,3

0,4

0,5

0,6

0,7

Entrainment ratio 

0,8

0,9

1,0

Fig.3. (a) Influence of the entrainment ratio on the thermal efficiency of cogeneration system without IHE; (b) Influence of the entrainment ratio on the thermal efficiency of cogeneration system with IHE

Figure 3 indicates that the thermal efficiency of the two proposed cogeneration systems is improving as a result of an increase in the entrainment ratio. This variation of the efficiency of the system is justified by a decrease of the heat consumed by the vapor generator and also an increase of the refrigeration coefficient of performance. In fact, the thermal efficiency of the ORC cycle is independent of the entrainment ratio. On the other hand, the fluid R1234yf presents the highest efficiency compared to other chosen fluids in both cogeneration systems. Actually, for an equal division of the total mass flow rate, thermal efficiency reaches about 95% in the cogeneration ORC system with an IHE, as shown in figure 3 (b).

(a)

R1234ze R124 R236fa 45

80

Ste 90 100 am 110 ge 120 ne r at 130 or Te 140 T mp G (° 150 era C) tur e

40

25

n Co

ra 35 pe m Te 30 n io (°C) at ns T CD de

re tu

(b)

CG (%)

R1234yf

47,00 45,61 44,10 42,59 41,09 39,58 38,07 36,57 35,06 33,55 32,04 30,54 29,03 27,52 26,01 24,51 23,00

Thermal efficiency of the cogeneration system

CG (%)

35,00 34,13 33,19 32,25 31,30 30,36 29,42 28,48 27,54 26,59 25,65 24,71 23,77 22,83 21,88 20,94 20,00

Thermal efficiency of the cogeneration system

In the rest of this work, to maintain electricity power nearly equal to refrigeration power, the entrainment ratio is kept constant at 0,25. The figure 4 shows the influence of the vapor generator temperature and condensation temperature on the performances of the two proposed cogeneration systems.

R1234yf R1234ze R124 R236fa 45

80

Ste 90 100 am 110 ge 120 ne rat 130 or Te 140 T mp G (° 150 era C) tur e

40 35

ra pe

re tu

m Te n ) tio (°C a s 25 T CD en d n Co 30

Fig.4. (a) Influence of the steam generator temperature and condensation temperature on the thermal efficiency of the cogeneration system without IHE; (b) Influence of the steam generator temperature and condensation temperature on the thermal efficiency of the cogeneration system with IHE

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According to figure 4, as the vapor generator temperature increases, thermal efficiency is optimal around a particular temperature for each working fluid. This temperature is actually the critical temperature of the refrigerant. In fact, this variation is explained by the transition from the subcritical regime to the transcritical regime [23, 24] where the thermal power consumed by the vapor generator is minimal at this point. On the other hand, with increasing the condensation temperature, the thermal efficiency of the ORC system without an IHE is improving, as shown in figure 4 (a), because the heat consumed by the vapor generator is reducing. However, in the case of the ORC cogeneration system with an IHE, the thermal efficiency decreases, as shown in figure 4 (b). In fact, despite a constant heat consumed by the vapor generator, the increase of the condensation temperature causes a reduction of the refrigeration power and consequently a reduction of the refrigeration coefficient of performance. Furthermore, the refrigerant R1234yf still presents the highest performance among other chosen fluids. Thermal efficiency of the ORC with an IHE reaches about 47% where the thermal efficiency of the ORC without an IHE reaches about 34%. Unlike the temperatures mentioned above, the evaporator temperature affects only the refrigeration coefficient of performance and consequently the thermal efficiency of the cogeneration system. On the other hand, the evaporator temperature affects directly the pressure ratio of the ejector. Figure 5 displays the variation of the refrigeration performance as a function of the evaporator temperature for each working fluid. R236fa

R1234yf

0,30

R1234ze

0,25

0,25

0,20

0,20

0,15

0,10

0,05

0,05

-20

-15

-10

-5

Evaporator Temperature Tev(°C)

0

R124

R236fa

R1234yf

R1234ze

0,15

0,10

0,00

(a)

R124

COPR

COPR

0,30

0,00

(b)

-20

-15

-10

-5

0

Evaporator Temperature Tev (°C)

Fig.5. (a) Influence of the evaporator temperature on the refrigeration coefficient of performance of the cogeneration system without IHE; (b) Influence of the evaporator temperature on the refrigeration coefficient of performance of the cogeneration system with IHE

As observed in figure 5, the refrigeration performance is increasing as the evaporator temperature increases. In fact, the thermal power consumed by the vapor generator is stable while the refrigeration power is increasing to reach its maximum at a temperature equal to 0°C. In addition, the ORC system with IHE presents the best performance and its refrigeration coefficient of performance reaches about 0, 27 using R1234yf as a working fluid. Unlike the refrigeration cycle, the thermal efficiency of the ORC cycle is independent of the variation of the evaporator temperature. However, it’s affected by the motive ratio. The variation of the thermal efficiency of the ORC cycle as a function of the motive ratio is plotted in figure 6.

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17 16 15 14 13 12

R124 R236fa R1234yf R1234ze

22 21 20

ORC efficiency (%)

ORC efficiency (%)

18

19 18 17 16 15 14 13

11

12

10

11 0,24

(a)

23

R124 R236fa R1234yf R1234ze

19

1227

0,28

0,32

0,36

0,40

Motive Ratio

0,44

0,48

0,52

0,15

(b)

0,18

0,21

0,24

Motive Ratio

0,27

0,30

0,33

Fig.6. (a) Influence of the motive ratio on the ORC cycle efficiency of cogeneration system without IHE; (b) Influence of the motive ratio on the ORC cycle efficiency of cogeneration system with IHE

-20

CG

(b)

) (°C

at

-10

3,0

3,5 ine 4,0 outle t pre ssu re (b

pe r

2,5

ur e

0 -5

Turb

-15

5,0

R236fa

to rT em

-10

3,0

3,5 Turb ine o 4,0 utle 4,5 t pre ssur e (b a r)

or at or Te m

2,5

R1234ze R124

-15 4,5

a r)

5,0

ap or a

-5

R1234yf

-20

Ev

pe ra tu re

0

( °C )

R236fa

50,00 48,67 47,22 45,78 44,33 42,89 41,44 40,00 38,56 37,11 35,67 34,22 32,78 31,33 29,89 28,44 27,00

Efficiency of the cogeneration system

R1234ze R124

Ev ap

(a)

R1234yf CG

46,00 44,72 43,34 41,96 40,58 39,20 37,82 36,43 35,05 33,67 32,29 30,91 29,53 28,14 26,76 25,38 24,00

Efficiency of the cogeneration system

According to figure 6, the ORC cycle efficiency reaches its maximum at the lowest value of the motive ratio. In fact, the lowest value of motive ratio corresponds to the lowest outlet pressure of the turbine and therefore the optimal output power from the ORC cycle. As a result, the ORC cycle is optimized and the efficiency reaches about 20% and 23% respectively in the first cogeneration system without an IHE and the second one with IHE. Since the evaporator temperature and the motive ratio affect in general the thermal efficiency of the system, figure 7 illustrates the evolution of the system performance as a function of the evaporator temperature and the turbine outlet pressure.

Fig.7. (a) Influence of the turbine outlet pressure and evaporator temperature on the thermal efficiency of the cogeneration system with IHE; (b) Influence of the turbine outlet pressure and evaporator temperature on the thermal efficiency of the cogeneration system without IHE

Figure 7 indicates that the thermal efficiency of the two cogeneration systems is improving as the evaporator temperature increases and the outlet pressure in the turbine decreases. For both proposed systems, the refrigerant R1234yf is always presenting the highest performance. This is because R1234yf has the minimal heat consumption due to its low critical temperature. Furthermore, results showed that the efficiency of the cogeneration system with an IHE is higher than the efficiency of the cogeneration system without the IHE. This proves the important role of an internal heat exchanger in recuperating heat and therefore reducing the heat consumed by the vapor generator and optimizing the system performance. In fact, thermal efficiency has reached 45% for the ORC system without the IHE and almost 49% for the ORC system with the IHE using R1234yf as a working fluid.

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6. Conclusion: The present work has shown an energetic performance analysis of two cogeneration systems based on ORC and refrigeration cycle. A comparative study of the proposed systems has demonstrated the important role of an internal heat exchanger in improving the thermal efficiency of the system by minimizing the heat consumption in the vapor generator. In addition, another comparative study was made for each configuration about the performance of the chosen working fluids. These fluids were chosen based on their low environmental impact and considering some thermodynamic criteria such as critical temperature and critical pressure of the fluid. The obtained results have shown that the fluid R1234yf is the best candidate considering its high thermal efficiency. In fact, the refrigerant R1234yf presents the minimal heat consumption compared to others because of its low critical temperature. However, R1234ze presents the optimal output electricity and refrigeration power which is about 25 Kw and 24 Kw respectively. In general, results have proved that besides their low environmental impact, HFO refrigerants present a higher efficiency compared to HFC refrigerants. On the other hand, the integration of the gas ejector has revealed several parameters such as the entrainment ratio and motive ratio. These parameters have been useful in the performance optimization of both cogeneration systems. Besides, the gas ejector has another advantage: it thermally compresses the secondary flow without consuming energy, which has made it possible to improve the efficiency of the refrigeration cycle in both cogeneration systems. 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