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A Novel Design of LNG Regasification Power Plant Integrated with Cryogenic Energy Storage System Jinwoo Park, Inkyu Lee, and Il Moon Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04157 • Publication Date (Web): 12 Jan 2017 Downloaded from http://pubs.acs.org on January 21, 2017

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Industrial & Engineering Chemistry Research

A Novel Design of LNG Regasification Power Plant Integrated with Cryogenic Energy Storage System Jinwoo Park, Inkyu Lee, and Il Moon* Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea

KEYWORDS: Liquefied natural gas, cryogenic energy storage, system integration, optimization

ABSTRACT: Natural gas is transported in its liquid state over long distances and thus must be gasified before use. This study focused on the alternative use of cold energy in an LNG regasification power plant integrated with a cryogenic energy storage (LPCES) system that supports variation over time. Energy demands change over time; these dynamics must be considered to improve overall energy efficiency. During off-peak times, the LNG cold energy is stored in the cryogenic energy storage (CES) system. In contrast, during on-peak times, the stored cryogenic energy is released as electricity to meet higher energy demands. To evaluate the efficiency of the proposed LPCES system, the total power used in the CES system was optimized and a thermodynamic analysis was conducted. In addition, a case study was performed to investigate the effect of the LPCES system with respect to an hourly reserve margin. The results indicated a 95.2% round-trip efficiency for the proposed LPCES system, which is higher than the efficiencies (up to 75%) offered by existing bulk power management systems using hydropower and compressed air.

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1. Introduction Natural gas is one of the cleanest energy sources, offering relatively low carbon dioxide emissions compared with other alternative fossil fuels.1 Natural gas, with a boiling point of approximately -160 °C, is generally transported in its liquid state over long distances, which decreases its volume by more than 600 times.2 Despite the challenges of long-distance cryogenic shipping, liquefied natural gas (LNG) trade is predicted to increase by 300% in the next 30 years.3 Therefore, many studies have focused on the natural gas liquefaction process (i.e., energy concentration process).4 In addition, transported LNG must be returned to its gaseous state before use or further transport through onshore pipelines. Thus, research has also focused on the regasification process at LNG receiving terminals using LNG cold energy as power. Dispenza et al.5 and Rocca6 investigated electricity production using LNG cryogenic energy but mainly focused on cold heat transfer. Szargut et al.7 considered cascading LNG cold power cycles using ethane as the working fluid. Comparatively, Choi et al.8 used methane, ethane, and propane as working fluids to produce electricity from LNG. Garcia et al.9 investigated a cascading LNG cold power plant to determine the feasibility of expanded natural gas use for industrial electricity demand. Subsequently, Garcia et al.10 considered the application of residual heat to an LNG cold power plant. Each of these studies focused primarily on electricity generation using LNG cold energy. Concurrently, significant research has focused on energy storage technologies. Energy storage plays an important role in current energy system structures, substituting a rise in energy production. The cryogenic energy storage (CES) system shows significant potential. CES system is a huge energy storage system that stores and releases energy by use of a cryogenic fluid as a storage medium. The CES system was first introduced by Smith11 using air or nitrogen. Subsequently, Yang et al.12 and Preston et al.13 investigated the wall temperatures and thermal insulation of LNG cryogenic storage tanks, respectively. Kishimoto et al.14 used a pilot plant to test CES system feasibility in 1998. Li et al.15, 16 performed


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thermodynamic and economic analyses on a CES system. Abdo et al.17 evaluated the performance of various CES systems. According to Zhang et al.18, a CES system has a relatively high energy density (100– 200 Wh/kg), a low capital cost per unit energy, a benign effect on the environment, and a relatively long storage period. Recent studies have considered time variances with CES systems—storing energy at off-peak times and releasing stored energy at on-peak times. Li et al.19 proposed an integrated nuclear power plant and CES system that stores electricity at off-peak times and releases electricity at on-peak times. Fazlollahi et al.20 proposed an integrated carbon capture process and CES system using natural gas as the working fluid that stores energy at off-peak times and captures carbon at on-peak times. The CES system’s cryogenic characteristics provide an advantage when used in conjunction with an LNG regasification power plant. In this study, an LNG regasification power plant integrated with a CES (LPCES) system was designed and mathematically optimized. To meet residential and industrial natural gas demands, LNG regasification power plants operate continuously. However, this continuous electricity generation is not efficient during off-peak times when energy demands and prices are relatively low and results in surplus electricity production. To address this issue, this study also considered variations over time in the use of LNG cold energy. During off-peak times, electricity is stored in the CES system and during on-peak times, this stored electricity is released to meet high energy demands and balance the overall electricity supply.

2. Proposed LPCES system design

2.1 System description The proposed LPCES system comprises a regasification power unit and a CES unit. Figure 1 provides a schematic of the proposed LPCES system including its energy storage and energy release operational modes. Three different time periods were defined based on energy demands: (1) conventional, (2) off-


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peak, and (3) on-peak. The off-peak time represents the lowest electricity demand, which typically occurs at night when people are inactive. Comparatively, the on-peak time represents the highest electricity demand, which typically occurs in the afternoon. Conventional time includes all other times outside of these on-peak and off-peak periods. During the conventional times, the proposed LPCES system uses LNG cold energy to liquefy the working fluid in the regasification power unit to produce electricity. During the off-peak times, the proposed LPCES system operates in its energy storage mode. During this time, the regasification power unit is inoperable; all LNG cold energy is used to liquefy the air in the CES unit for electricity storage. During the on-peak times, the proposed LPCES system operates in its energy release mode to discharge stored liquid air. During this time, the cold energy of liquid air is used to produce electricity using air turbines. Concurrently, LNG cold energy is used in the regasification unit to generate additional electricity. The Aspen HYSYS process simulation software was used to depict the three different time-based operational modes: (1) conventional, (2) energy storage, and (3) energy release. The mass flow of LNG was assumed as 105 kg/s (approximately 3 million ton/year), which is equal to the capacity of a commercial LNG regasification power plant.21 In the CES unit, the liquid air was stored at 5 °C below its boiling point. Cold energy loss (0.05%/day in storage tank) was ignored.22 Furthermore, power consumption on utility system was not considered in this study. Table 1 summarizes the additional operating conditions used for the proposed LPCES system, which is derived from Garcia et al.9


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Figure 1. Schematic diagram of LPCES system


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Table 1. Design basis of LPCES system9

LNG inlet temperature

-162 °C

LNG inlet pressure

1.3 bar

LNG mass flow

105 kg/s

LNG mass composition Nitrogen

0.0019

Methane

0.8182

Ethane

0.0933

Propane

0.0532

n-Butane

0.0167

i-butane

0.0167

Natural gas outlet pressure

70 bar

Natural gas outlet temperature

-10 °C

Isentropic efficiency of air compressors

0.90

Isentropic efficiency of air turbines

0.92

Isentropic efficiency of power generator turbines

0.92

Isentropic efficiency of natural gas expanders

0.90

Isentropic efficiency of pumps

0.90

Minimum temperature approach in heat exchangers

5 °C

Sea water temperature

15 °C


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2.1.1 Conventional mode During conventional times, the regasification power unit produces electricity. Figure 2 depicts this process. Regasification power unit is based on the model by Garcia et al.9 because they provided an improved model compared with the most recent studies. Two generators power the regasification power unit based on the Rankine cycle. The LNG is initially fed to a pump and pressurized to 300 bar. Next, the LNG passes through the power generators, transferring LNG cold energy to produce electricity. The first and second power generators use argon (Ar) and methane (CH4) as the working fluids, respectively. After passing through the two power generators, the LNG becomes a high-pressure natural gas. The residual pressure is discharged using electricity generation expanders. Through this overall process, the regasification power unit produced 14.136 MW of electricity.

Figure 2. Conventional mode of LPCES system


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2.1.2 Energy storage mode During off-peak times, the CES unit stores electricity. Figure 3 depicts this process. The air is initially pressurized using air compressors powered by surplus electricity. When the pressure reaches 25.44 bar, the air is liquefied by the LNG through a heat exchanger. The liquid air is stored at -154.5 °C (5 °C below its boiling point). Concurrently, this heat exchange process gasifies the LNG. This process has allowable temperature difference in heat exchanger.23 The resulting natural gas is discharged as same condition as described for the conventional operation mode.

Figure 3. Energy storage mode of LPCES system


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2.1.3 Energy release mode During the energy release mode, both the regasification power and CES units generate electricity. Figure 4 depicts this process. The regasification power unit operates as it did in the conventional mode. The CES unit releases stored liquid air to the pump where it is pressurized to 120 bar. Residual heat from the energy storage mode heats the air to 60 °C to maximize air turbine efficiency. The gasified air at 120 bar is depressurized to 1 bar using the air turbines to produce electricity. By generating additional electricity in the CES unit, the proposed LPCES system can supply a large amount of electricity during the on-peak demand times.

Figure 4. Energy release mode of LPCES system


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2.2 Optimization modeling To maximize energy storage efficiency in the CES system, the electricity consumed by the air compressors must be minimized and the electricity generated by the air turbines must be maximized. Pressure changes in each type of equipment affects power quantities. Thus, one objective of this study was to determine optimal compression and expansion ratios for the proposed LPCES system. Two commercial software packages were used to determine these optimal ratios: (1) gPROMS that solves the optimization problem and (2) Multiflash that calculates the thermodynamic properties of air using the Peng–Robinson equation of state model.

2.2.1 Compressor model The compression ratio of each compressor affects the amount of power stored. To minimize the total power consumed while operating in the energy storage mode, it can be optimized as follows: min: = ∑

(1)

where is the power consumed by the compressor and the subscript, , indicates the th compressor in the CES unit. The compressor was modeled based on a, model used in a previous study 2, namely in terms of isentropic compression. The power consumed by each compressor is calculated as follows: = ( − ) =

, !"#$ %! & !"#$,$'#" "

(2) +

(3)

where is the mass flow rate, is the specific enthalpy, and ) is the isentropic efficiency. The subscripts, out, in, and isentropic, indicate outlet stream, inlet stream, and isentropic conditions, respectively. The parameter, ,*+,-. , is defined by the thermodynamic properties of air as follows: /,*+,-. = /

(4)

/,*+,-. = 0(1,*+,-. , 2 )

(5)


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,*+,-. = 0(1,*+,-. , 2 ) 34567738 59:3 =

(6)

; ;!

(7)

where / is the specific entropy, 1 is the temperature, 2 is the pressure, and 0 represents a function.

2.2.2 Turbine model Similarly, the expansion ratio of each turbine affects the amount of power released. To maximize the amount of power generated, the expansion ratio can be optimized as follows: max: G = ∑ ?@

(8)

where ? is the power generated by the air turbine and the subscript, 1, indicates the th air turbine in the CES unit. The power generated by each air turbine is calculated as follows: ? = ( − )

(9)

= A,*+,-. − B )*+,-.,,C+ +

(10)

The parameter, ,*+,-. , is calculated using the same relationships defined previously for the compressor model where: /,*+,-. = /

(11)

/,*+,-. = 0(1,*+,-. , 2 )

(12)

,*+,-. = 0(1,*+,-. , 2 )

(13)

DE498738 59:3 =

; ;!

(14)

2.2.3 Constraints In both the compressor and turbine models, constraints must be defined to prevent intractable states. For example, regarding discharge temperatures, previous research has determined that compression ratios cannot exceed 3.24 In this study, the following constraints were used: 1 ≤ 34567738 59:3 ≤ 3

(15)

0 ≤ DE498738 59:3 ≤ 1

(16)


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In addition, the equipment’s inlet and outlet streams cannot be in a liquid state as follows: JK = 1

(17)

JK = 1

(18)

where JK is the vapor fraction.

3. Optimization results The proposed integrated regasification power plant and CES system was optimized using the models presented in Section 2. For the non-linear optimization, nonlinear sequential quadratic programming optimization solver (optimization tolerance = 0.001) was used. Total solution time took 16 seconds; Process, Intel Core i7-6700K CPU @ 4.00 GHz; RAM, 16 GB; 64 Bit OS. Table 2 presents the simulation results following optimization, which include the compression ratio, expansion ratio, stored power, and released power of the CES unit. These results indicate a total stored power of 79.655 MW and a total released power of 75.864 MW in the CES unit suggesting a significant ability to store electricity.

Table 2. Simulation results for optimized CES unit


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Compression ratio

Stored power (MW)

Air Compressor 1

2.244

19.73

Air Compressor 2

2.250

19.46

Air Compressor 3

2.247

19.41

Air Compressor 4

2.242

19.31

Pump (LNG)

1.745

Total

79.655 Expansion ratio

Released power (MW)

Air Turbine 1

0.315

18.86

Air Turbine 2

0.302

19.91

Air Turbine 3

0.297

20.40

Air Turbine 4

0.295

20.59

Pump (Air)

-3.896

Total

75.864

As one of the most feasible indicators used to assess energy storage technology, round-trip efficiency was used. According to Li et al.19, such a parameter represents the conversion efficiency during energy storage and energy recover cycle. Round-trip efficiency is determined as follows: )L@ =

MN O P %MQ!R !!PO MS"PT

(19)

where )L@ is the round-trip efficiency and L+ + *+ is the power released, U+ is the power generated, and V, W+ is the power stored when the system is in energy release, conventional, and energy storage modes. Table 3 shows the calculated powers and round-trip efficiency from the Aspen HYSYS software. The 95.2% round-trip efficiency for the proposed system is substantially higher than other bulk power management systems. Previous studies have estimated 71.26%19 and 75%25 round-trip efficiencies for an integrated nuclear power plant and CES system and a compressed air energy storage system, respectively.


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Table 3. Per mode powers and round-trip efficiency

Power (MW) V, W+

79.655

L+ + *+

90.000

U+

14.136

)L@

95.2%

4. Thermodynamic analysis Following optimization of the proposed LPCES system, a thermodynamic analysis was performed. Figure 5 provides a schematic of the CES unit that includes numbered streams. When operating in the energy storage mode, air is compressed in streams 1–9. Because the post-compressor cooling uses sea water, the cooling temperature in streams 1–9 is limited to 20 °C. The air is then liquefied by LNG cold energy in streams 9–10. Next, the liquid air is injected to the liquid air storage. When the air is released from the liquid air storage, it is pressurized to 120 bar in streams 10–11. Next, the air operates the air turbines in streams 11–19. Finally, depressurized air (to 1 bar) is discharged to atmosphere.


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Figure 5. Schematic diagram of CES unit with numbered streams


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Figure 6(a–b) presents temperature-entropy and pressure-entropy diagrams for air. The bubble and dew point lines were based on the Multiflash software results. These diagrams explain the behavior of air through the stream lines and reveal a significant entropy change in streams 9–10 where the LNG cold energy is used to liquefy the air.

(a)

(b)

Figure 6. Air behavior during energy storage and energy release modes: (a) temperature-entropy (T-S) diagram and (b) pressure-entropy (P-S) diagram


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Comparatively, Figure 7(a–b) presents temperature-enthalpy and pressure-enthalpy diagrams for air. Enthalpy and entropy behave similarly but enthalpy is less affected by pressure changes. Streams 1–9 and streams 12–19 indicate that enthalpy is primarily affected by changes in temperature.

(a)

(b)

Figure 7. Air behavior during energy storage and energy release modes: (a) temperature-enthalpy (T-H) diagram and (b) pressure-enthalpy (P-H) diagram


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5. Case study for LPCES system Table 4 indicates hourly electricity demand and generation in Ontario, Canada.26 A case study was conducted to compare the base case of Ontario with the LPCES system proposed in the present study. According to the Ontario energy board, the natural gas demand in Ontario corresponded to approximately 2.9 Bcfd.27 This is equivalent to 29.6 million tons of natural gas that 10 LPCES systems assumed as activated. System operation times were divided into conventional (8-13, 22-23), off-peak (14-21), and on-peak (24-7) times to analyze system effects.

Table 4. Hourly electricity demand and generation in Ontario, Canada26


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Hours

Demand (MW)

Generation (MW)

1

14,068

23,347

2

13,492

22,993

3

13,160

22,974

4

13,437

22,922

5

13,831

22,980

6

14,696

23,033

7

15,946

22,949

8

16,819

23,164

9

17,724

23,170

10

18,289

23,341

11

18,692

23,481

12

19,045

23,634

13

19,505

23,700

14

19,600

24,117

15

19,846

24,272

16

20,294

24,345

17

20,749

24,424

18

20,567

24,494

19

20,225

24,291

20

20,290

24,294

21

19,676

24,043

22

18,455

23,734

23

16,965

23,403

24

15,776

23,191

Figure 8 shows the stored and released electricity with respect to time variances. In this case study, the proposed LPCES system decreased electricity use in the off-peak time by 937.91MW/h. Specifically, it included a sum of stored energy and energy deficit from stationary power generators in the proposed LPCES system. Conversely, the proposed LPCES system increased electricity use in the peak time by 758.64 MW/h.


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Figure 8. Time dependencies of stored and released electricity in the case study


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To stabilize the electricity supply, reserve margin plays an important role. Figure 9 compares the hourly reserve margin of the base case with that in the case of the proposed LPCES system. Due to relatively low energy demands and continuously operating power plants, the reserve margin has a high value at off-peak times. The proposed LPCES system can store the surplus electricity at off-peak times and release the stored energy during on-peak times. Consequently, the minimum reserve margin increased from 17.7% to 21.4%, thereby accommodating increased energy demand. Thus, the application of the proposed LPCES system to the LNG regasification power plant improved electricity flexibility.

Figure 9. Hourly reserve margin from the surplus electricity supply


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6. Conclusion In this study, we proposed a novel LPCES system that alternatively uses cold energy in an LNG regasification power plant integrated with a CES system and supports variance over time. The proposed LPCES system operates in three different modes: (1) conventional, (2) off-peak, and (3) on-peak and utilizes the LNG regasification power plant as an energy bank, which minimizes wastage of surplus electricity during off-peak times. In this study, the compression ratio of the compressors and the expansion ratio of the air turbines in the CES system were optimized. These efforts resulted in a 95.2% round-trip efficiency for the proposed LPCES system—a significantly higher efficiency level than the levels achieved (up to 75%) by either hydropower or compressed air energy storage systems.18, 25 In conclusion, this study not only contributed to energy saving technology but also to the environment by efficiently storing surplus electricity and reducing the total amount of fuel consumed. Furthermore, the results of this study can potentially be used to create additional profits by exploiting electricity based on its flexibility. Further studies should investigate the economic aspects of this integrated system, as well as the optimization of the operating time variances. AUTHOR INFORMATION Corresponding Author *Il Moon, Tel.: +82 2 2123 2761, Fax: +82 2 312 6401, E-mail: [email protected] Notes

The authors declare no competing financial interests. ACKNOWLEDGMENT This research was supported by a grant from the LNG Plant R&D Center, funded by the Korean Ministry of Land, Infrastructure, and Transport (MOLIT), and by the BK 21 Program, funded by the Korean Ministry of Education (MOE).


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NOMENCLATURE Variables Power consumed by compressors in CES unit (MW) ? Power generated by air turbines in CES unit (MW) Mass flow rate (kg/h) Specific enthalpy (kJ/kg) ) Isentropic efficiency 0 Function / Specific entropy (kJ/kg °C) 1 Temperature (°C) 2 Pressure (bar) JK Vapor fraction )L@ Round-trip efficiency U+ Power generated when system is in conventional mode (MW) V, W+ Power stored when system is in energy storage mode (MW) L+ + *+ Power released when system is in energy release mode (MW) Subscripts th compressor in CES unit 1 th air turbine in CES unit 8 Inlet stream 3X: Outlet stream 768:534Y Isentropic condition Y34567735 Compressor :X5Z86 Air turbine


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Abbreviations LNG Liquefied natural gas CES Cryogenic energy storage LPCES LNG regasification power plant integrated with a cryogenic energy storage REFERENCES (1) Lim, W.; Choi, K.; Moon, I. Current Status and Perspectives of Liquefied Natural Gas (LNG) Plant Design. Ind. Eng. Chem. Res. 2013, 52 (9), 3065−3088.

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Table of Contents (TOC)


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A Novel Design of Liquefied Natural Gas (LNG ... - ACS Publications

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