A cold and power cogeneration system utilizing LNG cryogenic energy and low-temperature waste heat

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Energy Procedia 158 Energy Procedia 00(2019) (2017)2335–2340 000–000 www.elsevier.com/locate/procedia

10th th

International Conference on Applied Energy (ICAE2018), 22-25 August 2018, Hong Kong, 10 International Conference on Applied Energy China(ICAE2018), 22-25 August 2018, Hong Kong, China

A cold and power cogeneration system utilizing LNG cryogenic Thepower 15th International Symposium on District Heating and Cooling A cold and cogeneration system utilizing LNG cryogenic energy and low-temperature waste heat energy and low-temperature waste heat Assessing the feasibility of using the heat demand-outdoor a Yongyi Lia, Guoqiang Zhangaa, Yujia Liubb, Xiaowei Songaa, Yongping Yangaa* Yongyi Li , Guoqiang Liu , Xiaowei Song , Yongping Yang * temperature functionZhang for a, Yujia long-term district heat demand forecast Key Laboratory of Condition Monitoring and Control for Power Plant Equipment of Ministry of Education (North China Electric Power

a

Key Laboratory of Condition MonitoringUniversity), and ControlChangping for PowerDistrict, Plant Equipment of Ministry of Education (North China Electric Power Beijing 102206, China a,b,c a a b c c b University), Changping District, Beijing 102206, China Tongfang Environment Co.,Ltd, TsinghuaTongfang Hi-tech Plaza, Wangzhuanglu Road No.1, Beijing 100083, China b Tongfang Environment Co.,Ltd, TsinghuaTongfang Hi-tech Plaza, Wangzhuanglu Road No.1, Beijing 100083, China a IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France Abstract a

I. Andrić

*, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Corre

Abstract A novel cold and power cogeneration system is proposed in this paper. In the system, a “special” absorption refrigeration cycle is A novel cold andispower cogeneration system is proposed in this paper. In the system, a “special” absorption refrigeration cycle is designed which driven by liquefied natural gas (LNG) cryogenic energy with ammonia-water as working fluid. And this Abstract designed which two is driven liquefiedinnatural gas (LNG) energy with ammonia-water as working fluid. And this system contains powerby processes, which ammonia andcryogenic natural gas are heated respectively by low-temperature heat source system power processes, in which ammonia and natural are heatedsystem respectively by low-temperature source and thencontains expansetwo in turbines. The thermodynamic performance of the gas cogeneration is simulated and analyzed. heat The results District heating networks commonly addressed ingood the thermal literature as one of the most solutions decreasing the and then turbines.are The thermodynamic performance of theperformance. cogeneration system iseffective simulated and The results show thatexpanse the newin cogeneration system demonstrations The exergy efficiency of analyzed. the for system is as high greenhouse gas emissions the building sector. These systems which areefficiency returned through the heat show that the new cogeneration system demonstrations good thermalrequire performance. The efficiency of the system is as high as 36.1%. The coefficient offrom performance (COP) of the refrigeration cycle high is 1.6investments and exergy the generating of power cycle Due to coefficient the changed climate conditions building renovation policies, heat demand in efficiency the future could decrease, assales. 36.1%. The of performance (COP)toand ofevaluate the refrigeration cycle isparameters 1.6 and theon generating of power cycle reaches 38.8%. Sensitive analyses are performed the effects of key the performance of the cogeneration prolonging the investment return period. reaches 38.8%. Sensitive are performed evaluate the effects of key parameters onmain the performance of thecharacters cogeneration system. Some values of analyses the key parameters aretorecommended and the variations of the thermodynamic are The main scope ofcogeneration this is system toparameters assess the feasibility of using for the heat demand – outdoor temperature heat demand system. Some values ofpaper the key arearecommended andthe theefficient variations of the of main thermodynamic characters are outlined. This new provides new approach utilization LNG cold function energy. for forecast.This Thenew district of Alvalade, located in Lisbon (Portugal), used asutilization a case study. Thecold district is consisted of 665 outlined. cogeneration system provides a new approach for was the efficient of LNG energy. buildings ©that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district Copyright 2018 Elsevier Ltd. All rights reserved. © 2019 The Authors. Published by Elsevier Ltd. intermediate, deep). To estimate the error, renovation scenarios were developed (shallow, obtained heat demand on values were Copyright © 2018 Elsevier Ltd. Allresponsibility rights reserved. Selection and peer-review under of the scientific committee of the 10th International Conference Applied This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) th International compared with results from a dynamic heat demand model, previously developed and validated by the authors. Selection and peer-review under responsibility of the scientific committee of the 10 Conference on Applied Energy (ICAE2018). Peer-review under responsibility of the scientific committee of ICAE2018 – The of 10th International Conferencefor onsome Applied Energy. The results showed that when only weather change is considered, the margin error could be acceptable applications Energy (ICAE2018). (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation Keywords: LNG cryogenic energy; cold and power cogeneration; ammonia-water absorption; low-temperature waste heat scenarios,LNG the cryogenic error value increased uppower to 59.5% (depending on the weather and low-temperature renovation scenarios combination considered). Keywords: energy; cold and cogeneration; ammonia-water absorption; waste heat The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and 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 Cooling. * Corresponding author. Tel.: +86-10-61772011; fax: +86-10-61772011. * E-mail Corresponding Tel.: +86-10-61772011; fax: +86-10-61772011. address:author. [email protected] Keywords: Heat demand; Forecast; Climate change E-mail address: [email protected]

1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. 1876-6102 Copyright © 2018 Elsevier Ltd. All of rights reserved. committee of the 10th International Conference on Applied Energy (ICAE2018). Selection and peer-review under responsibility the scientific Selection and peer-review under responsibility of the scientific committee of the 10th International Conference on Applied Energy (ICAE2018). 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 © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of ICAE2018 – The 10th International Conference on Applied Energy. 10.1016/j.egypro.2019.01.280

Yongyi Li et al. / Energy Procedia 158 (2019) 2335–2340 Author name / Energy Procedia 00 (2018) 000–000

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1. Introduction Nearly 25% of international natural gas trade is transported by the way of LNG [1]. However, most of the cold energy within the LNG has not been utilized, which is an enormous waste of energy. Many scholars have proposed various ways of recycling LNG cooling energy in refrigeration, power generation, food engineering and other fields [2-12], and these applications are very effective. In our industrial society, the industrial waste gas from factories contains a lot of low-grade heat, which is also a waste of energy. Upon careful study we find that combined cold and power cogeneration is a valid method to recycle the pressure energy and thermal energy of the LNG and lowtemperature waste heat. The present work aims to design an efficient thermal system to achieve the above objectives and simulate and analyze its performances. 2. The novel cold and power cogeneration cycle description The proposed cold and power cogeneration cycle, taking ammonia-water as working fluid, utilizes LNG cryogenic energy and low-temperature waste heat to generate cold energy and electricity. The schematic diagram of this system is shown in Fig. 1. In this system, concentrated ammonia (‘1’) coming from the absorber enters into the generator, then the ammonia vapor (‘2’) distilled in the generator is sent into the condenser where the ammonia vapor is cooled by LNG and condensed. After pressurized, the ammonia vaporizes in the evaporator, and in this process it can be used as a cold source for cooling. The ammonia vapor, further heated by low-temperature waste heat (such as the exhaust gas of factory or fossil-fired power plant), expands in turbine and generate electric power. The turbine exhaust and the heated low concentration ammonia produced in generator are sent into the absorber in which the concentrated ammonia is generated. As shown in Fig. 1, LNG, which serves as the cold source for the ammonia condenser and the refrigeration process successively, is further heated by the waste heat and then expands in turbine to produce electricity. The LNG cold energy is used to drive the ammonia absorption refrigeration cycle and to provide cooling, the low temperature heat source in this system further heats natural gas and ammonia which expand in turbine and generate electricity. More low-grade cold energy is produced by a small amount of high-grade cold energy (LNG). And the ammonia gas outlet turbine is absorbed by ammonia water directly without the need to create a low temperature environment for condensation. LNG Tank

13

Cold Energy Recoverer

14

15

16

3 2

Natural Gas Heater

17

5

18 Ammonia Heater

4

1

Throttle Valve

19

6

Generator 9

10 11

Ammonia Turbine

Solution Heat Exchanger

Natural Gas Turbine

20

Evaporator

Condenser

21

Natural Gas Supply

Low-Temperature Heat

7

12 Absorber 8

Fig. 1 the scheme diagram of the proposed combined cold and power system



Yongyi Li et al. / Energy Procedia 158 (2019) 2335–2340 Author name / Energy Procedia 00 (2018) 000–000

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3. Thermodynamic models and analytical method Models of the components and evaluation methods have been carried out respectively. The thermodynamic models of the system are based on mass conservation and energy conservation [13]. We assume that all the heat exchangers and pipes have no heat and mass loss, and pressure drop in pipes is neglected. In the thermodynamics calculation, we assume that the LNG is pure methane considered as a steady flow state. The calculation models are built with software of REFPROP 9.0. The first law and the second law of thermodynamics were used to evaluate the performance of the system. In the model of the second law, exergy efficiency of the system, which is the ratio of the system output exergy to the exergy input into the system, is the main indicator to evaluate the efficiency of energy conversion. The distillation rate of generator is defined as the ratio of the mass flow of distilled ammonia vapor to mass of ammonia in ammonia-water strong solution. At the design point, the distillation rate and refrigeration temperature of generator are taken as 0.185 and 0 ℃, which decide the operating pressure of generator. Main parameters of the proposed system for the calculations are shown in Table 1. Table.1 Main parameters for the calculations Parameters

Value

Ambient temperature /℃

15

Ambient pressure /MPa

0.1013

Operation pressure of generator /MPa

0.033

Operation pressure of absorber /MPa

0.15

Supply pressure of natural gas /MPa Ammonia-water mass flow rate of basic solution /kg·s

2 -1

6.5

Ammonia mass fraction of basic solution

0.38

Heat source temperature /℃

130

Minimal heat transfer temperature difference of condenser /℃

15

Minimal heat transfer temperature difference of low-temperature heater/℃

10

Relative internal efficiency of turbine

0.85

Pressure loss of generator, evaporator, condenser and absorber

3%

Pressure loss of low-temperature heater and cold energy recoverer

2%

4. Result and discussion 4.1. Thermodynamic analysis of the system Based on the assumptions, parameters and thermodynamic analysis models, performance of the system was calculated. Under design condition, COP of the system is 1.6. The ammonia turbine produces a power output of 165.5 kW with an efficiency of 51.3%. And the natural gas turbine outputs 340.5 kW with an efficiency of 34.2%. The high-grade cold energy input to system is 2440.8 kW while large amount of low-grade cold energy with 3858.2 kW is produced. The exergy efficiency of the system is 36.1% under design condition. Fig. 2 shows the exergy loss efficiency of components. The maximum exergy loss of 745.1 kW exists in the condenser with proportion of 37.7%, which is caused by a large heat transfer temperature difference. The T-Q diagram of heat transfer of LNG and ammonia in condenser is presented in Fig. 3. The exergy loss in cold energy recoverer cannot be ignored because of the large heat transfer temperature difference. Low-grade thermal energy released by absorber cannot be used, which is the third largest loss of the system.

Yongyi Li et al. / Energy Procedia 158 (2019) 2335–2340 Author name / Energy Procedia 00 (2018) 000–000

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4.2. System sensitive analysis In the initial state, the LNG pressure at the condenser inlet and ammonia turbine inlet pressure is set to 4.5 MPa and 0.4 MPa respectively, and the refrigeration temperature of generator and evaporator is set to 0 ℃. Other thermal conditions keep the values at design condition and the minimal heat transfer temperature difference of condenser remains 15 ℃. 0 Solution heat exchanger 0.002 0.019

Cold energy recoverer Ammonia heater Natural gas turbine Ammonia turbine Evaporator

-60

0.018 0.027 0.013

Absorber

0.055

-80 -100

methane

-120

0.025

Condenser

-140

0.377

Generator

ammonia

-40

0.061

T (℃)

Natural gas heater

-20

-160

0.041

0

Throttle valve 0.002 0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

500

1000

1500

2000

Q (kW)

0.40

Exergy loss coefficiency

Fig. 3. T-Q diagram of condenser

Fig. 2. Exergy loss efficiency of components

0.035

0.24

0.030

0.22

0.025 0.020

0.20

0.15

0.18

0.21

Distillation ratio of generator

Fig. 4. Effects of distillation ratio on operating pressure and concentration

0.18 0.24

-56

4.0

-58

3.5

-60 -62

3.0

-64

2.5

-66 -68

0.14

0.16

0.18

0.20

Distillation ratio of generator

0.22

2.0 0.24

Fig. 5. Effects of distillation ratio on condensing temperature and LNG mass flow

1.61 1.60

41

1.59

40

1.58

39

1.57 1.56

38

COP

0.26

Exergy efficiency Electrical efficiency COP

42

Efficiency(%)

0.040

0.28

4.5

Ammonia condensing temperature Mass flow rate of LNG

-54

Mass flow rate of LNG(kg/s)

Operation pressure of generator Concentration of NH3-H2O

Concentration of NH3-H2O

Operation pressure of generator(MPa)

0.045

Ammonia condensing temperature(℃)

As the distillation ratio of generator increases from 0.134 to 0.23，the operating pressure of generator and the concentration of weak solution out of the generator decrease, which are shown in Fig. 4. As shown in Fig. 5, the ammonia condensing temperature and mass flow of LNG change in opposite directions. The performance of the proposed system was calculated as the distillation ratio increases from 0.134 to 0.23. As shown in Fig. 6, generating efficiency, exergy efficiency and COP rise as the distillation ratio increases. The increase of mass flow rate of the distilled ammonia vapor enables ammonia turbine to increase its power output and the generating efficiency. And the LNG mass flow rate increases to provide sufficient heat exchange quantity for condensation. This increases the refrigerating capacity and COP. The decreasing operating pressure brings down saturation temperature, which makes heat transfer temperature difference decline. It can be seen that increased distillation ratio improves the thermal and refrigeration performance. But limited by the heat transfer conditions of condenser, the system performance reaches its limit when distillation ratio is close to 0.23.

1.55

37

1.54

36

1.53 0.14

0.16

0.18

0.20

0.22

0.24

Distillation ratio of generator

Fig. 6. Effects of distillation ratio on the performance

The ammonia turbine inlet pressure, which affects the power output of the ammonia turbine and the evaporation temperature of evaporator, is controlled by the solution pump downstream of the condenser. We calculate the performance of the system when the ammonia turbine inlet pressure increases from 0.3 MPa to 0.42 MPa, and the results are shown in Fig. 7. The generating efficiency and the evaporation temperature rise as the inlet pressure increases. However, on the whole, exergy efficiency is falling. In order to keep quality of cold exergy and exergy efficiency in high level, the ammonia turbine inlet pressure should be taken around 0.4 MPa. We calculate the performance of the system when hot water temperature varies from 110 to 140 ℃ (the ammonia turbine inlet temperature varies from 100 to 130 ℃) and the minimal heat transfer temperature difference of ammonia heater is taken as 10 ℃. As shown in Fig. 8, the efficiency of ammonia turbine is improved. The



Yongyi Li et al. / Energy Procedia 158 (2019) 2335–2340 Author name / Energy Procedia 00 (2018) 000–000

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generating efficiency and exergy efficiency increase from 37.2% and 36.05% to 41.9% and 37.7% respectively. However, as the exergy loss of ammonia heater increases, the increment of exergy efficiency decreases gradually. The performance of the system is near to the height when the ammonia turbine inlet temperature is between 120130 ℃.

0

0.41

-2 -3 -4 -5 -6 -7

37

-8 36

0.30

0.32

0.34

0.36

0.38

0.40

Ammonia turbine inlet pressure (MPa)

0.42

0.050 0.045 0.040

3.6

0.035 3.4

0.030 0.025

3.2

0.020 3.0

0.015 0.010

2.8

0.30

0.32

0.34

0.36

0.38

0.40

0.38 0.37 0.36 105

110

115

120

125

130

Ammonia turbine inlet temperature (℃)

Operating pressure of generator (MPa)

Mass flow rate of LNG (kg/s)

3.8

0.39

100

0.055 Mass flow rate of LNG Operating pressure of generator

0.40

-9

Fig. 7. Effects of ammonia turbine inlet pressure on performance 4.0

Exergy efficiency Electrical efficiency

0.42

Fig. 8. Effects of ammonia turbine inlet temperature on performance 52

1.60

Exergy efficiency Electrical efficiency COP

50

1.58

48 46

1.56

COP

38

Exergy efficiency (%)

39

Efficiency(%)

0.42

-1

40

Efficiency(%)

1

Evaporation temperature (℃)

41

0.43

2

Exergy efficiency Electrical efficiency Evaporation temperature

44 1.54

42 40

1.52

38 36

0.30

0.32

0.34

0.36

0.38

0.40

0.42

1.50

Ammonia concentration of generator inlet solution

Ammonia concentration of generator inlet solution

Fig. 9. Effects of ammonia turbine inlet temperature on performance

Fig. 10. Effects of ammonia concentration on performance

-58 3.2

-59 -60 -61

3.0

-62 -63 4.2

4.4

4.6

4.8

5.0

5.2

5.4

5.6

LNG import pressure (MPa)

Fig.11. Effects of LNG pressure on saturation temperature and mass flow rate

Efficiency(%)

-57

4.0

1.61 1.60

42

Mass flow of LNG (kg/s)

Natural gas temperature (℃)

3.4

1.59 1.58

40

1.57 1.56

38

COP

Natural gas temperature Mass flow of LNG

-56

3.8

Exergy efficiency Electrical efficiency COP

44

-55

1.55 36 34 3.8

1.54 1.53 4.0

4.2

4.4

4.6

4.8

5.0

5.2

5.4

5.6

LNG import pressure (MPa)

Fig.12. Effects of LNG import pressure on performance

As we know, the ammonia concentration of the inlet generator affects the operating pressure of generator. As shown in Fig. 9, the mass flow rate of LNG drops as the increase of the concentration because of the increasing operating pressure and saturation temperature. As shown in Fig. 10, the decline of the cold energy inserted into the system increases the COP. The exergy efficiency is slowly declining while the concentration of generator inlet solution increases.

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Yongyi Li et al. / Energy Procedia 158 (2019) 2335–2340 Author name / Energy Procedia 00 (2018) 000–000

LNG pressure (‘14’) also affects the performance of the system. Increasing LNG pressure can increase the mass flow rate of LNG and reduce the natural gas temperature at the outlet of condenser as the Fig. 11 shown. Effects of LNG pressure on system performance are shown in Fig. 12. Generating efficiency and power output are improved by the increased LNG pressure. As the mass flow rate of LNG increases, COP sharply declines. The exergy efficiency increase slows down when the LNG pressure is over 4.5 MPa. Therefore, considering the thermal performance and refrigeration performance, it is better to set LNG pressure at 4.5-4.6 MPa. 5. Conclusion A combined power and cold system is proposed, which can efficiently recycle low-temperature waste heat and make full use of the cold energy of LNG. The thermodynamic system and analysis models are built and the performances of the system with several influence factors are analyzed. Under design condition, the COP of the system reaches 1.6 and generating efficiency reaches 38.8% with power output 506.0 kW. And the exergy efficiency of the system can reach 36.1%. However, there is a large heat transfer temperature difference in the condenser, which is a great potential for optimization. In addition, a higher distillation rate of generator helps to increase the performance of the system. The increase of the ammonia turbine inlet pressure can improve the generating efficiency of the system. Higher ammonia concentration inlet of the generator benefits the refrigeration performance, but it is a liability to generating efficiency and power output. In terms of parameter design, the performance almost reaches the best when the ammonia turbine inlet temperature is between 120-130 ℃, and LNG pressure (‘14’) should be taken as 4.5-4.6 MPa. Acknowledgements This study was supported by National Nature Science Fund of China (Grant No. 51436006); National Nature Science Fund of China (Grant No. 51306049); Fundamental Research Funds for the Central Universities (No.2017MS15); Fundamental Research Funds for the Central Universities (No. 2017XS044). References [1] Geng. JB. Research on International Natural Gas Market and China's LNG Supply Security. University of Science and Technology of China, 2014 [in Chinese]. [2] Miyazaki T, Kang Y T, Akisawa A, et al. A combined power cycle using refuse incineration and LNG cold energy. Energy, 2000, 25(7):639655. [3] Wang Q, Li Y, Wang J. Analysis of power cycle based on cold energy of liquefied natural gas and low-grade heat source. Applied Thermal Engineering, 2004, 24(4):539-548. [4] Rao W J, Zhao L J, Liu C, et al. A combined cycle utilizing LNG and low-temperature solar energy. Applied Thermal Engineering, 2013, 60(1-2):51-60. [5] Cheng WL, ITO Takehiro, Chen ZS. A Cryogenic Power Generation Cycle for Recovering Cold Energy of LNG. Journal of China University of Science and Technology, 1999, 29(6):671-676 [in Chinese]. [6] García R F, Carril J C, Gomez J R, et al. Power plant based on three series Rankine cycles combined with a direct expander using LNG cold as heat sink. Energy Conversion & Management, 2015, 101(8):285-294. [7] García R F, Carril J C, Gomez J R, et al. Combined cascaded Rankine and direct expander based power units using LNG (liquefied natural gas) cold as heat sink in LNG regasification. Energy, 2016, 105:16-24. [8] Shi X, Che D. A combined power cycle utilizing low-temperature waste heat and LNG cold energy. Energy Conversion & Management, 2009, 50(3):567-575. [9] Messineo A, Panno G. LNG cold energy use in agro-food industry: A case study in Sicily. Journal of Natural Gas Science & Engineering, 2011, 3(1):356-363. [10] Rocca V L. Cold recovery during regasification of LNG part one: Cold utilization far from the regasification facility. Energy, 2010, 35(5):2049-2058. [11] Rocca V L. Cold recovery during regasification of LNG part two: applications in an Agro Food Industry and a Hypermarket. Energy, 2011, 36(8):4897-4908. [12] Zhang G, Zheng J, Yang Y, et al. A novel LNG cryogenic energy utilization method for inlet air cooling to improve the performance of combined cycle. Applied Energy, 2016, 179(1):638-649. [13] Chen GM, Chen GB. Principle of Refrigeration and Low Temperature, Mechanical industry press, 2010 [in Chinese].