Liquefied natural gas (LNG) will contribute more in the future than in the past to the overall energy supply in the world. The paper discusses the application of advanced exergy-based analyses to a recently developed LNG-based cogeneration system. These analyses include advanced exergetic, advanced exergoeconomic, and advanced exergoenvironmental analyses in which thermodynamic inefficiencies (exergy destruction), costs, and environmental impacts have been split into avoidable and unavoidable parts. With the aid of these analyses, the potentials for improving the thermodynamic efficiency and for reducing the overall cost and the overall environmental impact are revealed. The objectives of this paper are to demonstrate (a) the potential for generating electricity while regasifying LNG and (b) some of the capabilities associated with advanced exergy-based methods. The most important subsystems and components are identified, and suggestions for improving them are made.

Methods

Simulation

The base case is shown in Figure ¹ . The overall system consists of three subsystems with the following initial data and assumptions:

· LNG subsystem (process 1–2 − 3–4) - LNG from the storage system is (a) compressed by an LNG pump (P), (b) vaporized in heat exchanger II (HE II) using the waste heat from the nitrogen power system, and (c) expanded in expander III (EX III).

· N2 subsystem (process 11–12 − 13–14) - The N2 subsystem is a closed-cycle gas-turbine power system. After being cooled in HE II, the nitrogen is compressed in compressor III (CM III), heated in heat exchanger I (HE I) using the waste heat from an open gas-turbine power system, and expanded in expander II (EX II).

· Open gas-turbine power subsystem (process 21 through 28) - Air after compression in compressor I (CM I) is cooled in the cooler (CL) transferring thermal energy to the environment and is compressed in compressor II (CM II). After the combustion process in the combustion chamber (CC), the combustion gases are expanded in expander I (EX I) and rejected to the atmosphere after being cooled in HE I. The open gas-turbine power subsystem is based on an LMS 100 gas turbine (General Electric Company, Fairfield, CT, USA) [ ⁴ ].

**Schematic of the cogeneration system for vaporizing LNG**

For the simulation and the exergetic analyses, the softwares GateCycle (General Electric Company, Fairfield, CT, USA) [ ⁵ ], Gatex (Institut für Energietechnik, Technische Universität Berlin, Germany) [ ⁶ ], and EES (F-Chart Software, LLC, Madison, WI, USA) [ ⁷ ] were used. Table ¹ presents some results obtained from the simulation [ ³ ]. The following assumptions were used: η _CMI = 90%, η _CMII = 90%, p24/p21 = 42, T ₂₆ = 1,290°C, η _{EX I} = 94%, ΔTHEImin=20K ∆p _HEI = 3%, η _{CM III} = 85%, p12/p11 = 15, η _{EX II} = 88%, ΔTHEImin=15K ∆p _HEII = 3%, η _P = 66.5%, and η _{EX III} = 85%.

Table 1

Thermodynamic data for the material streams in the base case

Material stream	State	m⋅(kg/s)	T(°C)	p(bar)	e^T(kJ/kg)	e^M(kJ/kg)	e^PH(kJ/kg)	e^CH(kJ/kg)	e (kJ/kg)
LNG	1	65.03	−160	10	669.7	339.3	1,009	-	1,009
LNG	2	65.03	−144	272	250.5	778.5	1,029	-	1,029
NG	3	65.03	86	270	25.3	777.7	803	-	803
NG	4	65.03	2	80	1.0	630.0	631	-	631
N₂	11	217	−129	2.85	58.6	88.4	147	-	147
N₂	12	217	70	42.75	5.0	319.0	324	-	324
N₂	13	217	415	40.61	162.2	314.8	477	-	477
N₂	14	217	101	2.99	11.4	92.6	104	-	104
Air	21	209	15	1.013	-	-	0	1	1
Air	22	209	242	6.66	-	-	217	1	218
Air	23	209	117	6.53	-	-	170	1	171
Air	24	209	416	43.47	-	-	468	1	469
CH₄	25	5.1	15	45	-	-	566	51,534	52,100
Combustion gases	26	214.1	1,290	41.95	-	-	1,281	9	1,290
Combustion gases	27	214.1	435	1.08	-	-	190	9	199
Combustion gases	28	214.1	90	1.025	-	-	18	9	27

Exergy-based analyses

Exergoeconomics is a unique combination of exergy analysis and cost analysis conducted at the component level to provide the designer or operator of an energy conversion system with information crucial to the design or operation of a cost-effective system. Decisions are made, however, at the plant component level [ ⁸ , ⁹ ]. A complete exergoeconomic analysis consists of (a) an exergetic analysis, (b) an economic analysis, and (c) an exergoeconomic evaluation.

An exergoenvironmental analysis is considered as one of the most promising tools to evaluate energy conversion processes from an environmental point of view [ ¹⁰ ]. Exergoenvironmental analysis is a proper combination of exergy analysis and life cycle assessment (LCA). The exergoenvironmental analysis consists of three steps: The first step is an exergy analysis. In the second step, an LCA of (a) each relevant system component and (b) all relevant input streams to the overall system is carried out. In the last step, the environmental impact obtained from the LCA is assigned to the exergy streams in the system.

Exergy analysis

Table ¹ shows the value of exergy for each material stream. The chemical exergies for material streams of the N ₂ and the LNG subsystems do not need to be considered in the exergetic analysis because only the physical exergy of the working fluid is used in the corresponding subsystems. We considered the chemical exergies only in the open gas-turbine subsystem, where combustion takes place. The physical exergies of LNG, NG, and N ₂ are split into their thermal and mechanical parts according to the approach presented in [ ¹¹ ].

The exergetic analysis has been conducted at the component level using the ‘exergy of the fuel’ and the ‘exergy of the product’ [ ⁸ , ⁹ ]. The definitions of E⋅F,k and E⋅P,k for each system component are given in [ ³ ]. Table ² shows some data obtained from the conventional exergetic analysis.

Table 2

Results obtained from the conventional exergetic analysis [ ³ ]

Component	E⋅F,k (MW)	E⋅P,k (MW)	E⋅D,k (MW)	ε_k (%)
CM I	48.418	45.434	2.984	93.84
CL	Dissipative component		9.993	-
CM II	65.224	62.458	2.766	95.76
CC	267.603	178.131	89.466	66.57
EX I	233.661	227.569	6.092	97.39
HE I	36.926	33.091	3.834	89.83
CM III	57.021	51.222	5.799	89.83
EX II	80.956	72.829	8.127	89.96
HE II	19.722	14.427	5.295	73.15
P	34.295	28.582	5.713	83.34
EX III	11.249	9.479	1.770	84.26
Overall system (E⋅L,tot=5.768MW)	311.415	163.801	141.846	52.60

Exergoeconomic analysis

The exergoeconomic model for an energy conversion system consists of cost balances written for the k th component and auxiliary equations based on the P and the F rules [ ⁸ , ⁹ ]. The cost balances can be written as

C⋅P,k=C⋅F,k+Z⋅k

cP,kE⋅P,k=cF,kE⋅F,k+Z⋅k,

where

Z⋅k=Z⋅kCI+Z⋅kOM.

To simplify the discussion, we assume that the contribution of Z⋅kOM remains constant when the design changes, and therefore, the changes in the value of Z⋅k are associated only with changes in the capital investment cost Z⋅kCI .

The real cost sources in an energy conversion system are the (a) capital investment (and operating maintenance expenses) for each component, (b) cost of exergy destruction within each component, and (c) cost of exergy loss from the overall system. The last two terms can be revealed only through an exergoeconomic analysis:

· The cost rate associated with exergy destruction within the k th component is

C⋅D,k=cF,k⋅E⋅D,k.

· The cost rate associated with exergy loss from the overall system is

C⋅L,tot=C⋅28.

The exergoeconomic model for the base case has been discussed in detail in [ ¹² ]. Table ³ shows selected data obtained from the conventional exergoeconomic analysis. Here, the cooler is considered together with the cooling tower ( Z⋅CT=3.25$/h ), and the pump is considered together with the electrical motor ( Z⋅EM=1.33$/h ).

Table 3

Exergoeconomic variables for the LNG-based cogeneration systems

Component	Z⋅k ($/h)	C⋅D,k ($/h)	Z⋅k+C⋅D,k ($/h)	c_F,k ($/GJ)
CM I	64.67	554	619	51.63
CL	11.10	Dissipative component
CM II	97.01	514	611	51.63
CC	92.39	9,493	9,585	29.47
EX I	207.90	1,097	1,305	50.04
HE I	16.04	691	707	50.04
CM III	16.23	3,007	3,023	144.00
EX II	20.49	4,133	4,154	14.13
HE II	13.76	920	934	48.26
P	7.01	783	788	38.06
EX III	2.65	432	435	67.76

For the economic analysis, the methodology presented in [ ⁸ ] is applied using the following assumptions and sources:

· The purchased equipment cost of turbomachinery is based on data from [ ⁸ , ¹³ ].

· The purchased equipment cost of heat exchangers is based on data from [ ⁸ ].

· The cost of LNG is equal to $12/GJ [ ¹⁴ ].

· The average cost of money is i _eff = 10%.

· The plant economic life is n = 15 years with 7,300 h/year.

· The average general inflation rate is r _n = 2.5%

Exergoenvironmental analysis

Exergy analysis provides a powerful tool for assessing the quality of a resource as well as the location, magnitude, and causes of thermodynamic inefficiencies. In addition, LCA supplies the environmental impacts associated with a component or an overall system during its entire useful life. In the exergoenvironmental analysis, the environmental impacts obtained by LCA are apportioned to the exergy streams pointing out the main system components with the highest environmental impact and possible improvements associated with these components. Finally, exergoenvironmental variables are calculated, and an exergoenvironmental evaluation is carried out.

Life cycle assessment is a technique for assessing the environmental aspects associated with a product over its life cycle. The LCA process consists of goal definition and scoping (defining the system under consideration), inventory analysis (identifying and quantifying the consumption and release of materials), and interpretation (evaluation of the results) [ ¹⁵ ].

In general, any of recently introduced indicators can be used for LCA. For this exergoenvironmental analysis, an impact analysis method called Eco-indicator 99 [ ¹⁶ ] has been selected because it considers many environmental aspects and uses average European data.

In order to identify the raw material inlet flows, it is first necessary to perform a sizing of the plant components and to collect information about the weights, main materials, production processes, and scrap outputs of all relevant pieces of equipment needed to build the plant. This information is usually not very widely published (compared with the corresponding cost information). In this way, only rough calculations of the employed main materials and corresponding weights can be conducted.

The data collected in [ ¹⁷ , ¹⁸ ] were generalized in the form of equations (Tables ⁴ and ⁵ ) and used for estimating the component-related environmental impact that occurs during the construction phase. If the materials of a component correspond to the data given in Table ⁴ , then the values of b― (relative environmental impact) and m― (relative mass) are equal to 1. If the selected material is different, then b―=bgiven materialbnew material and m―=ρgiven materialρnew material (where ρ is the density of the material, kg/m ³ ).

Table 4

Eco-indicator 99 values and material composition of components

Equipment	Construction material	Eco-indicator 99 (mPoints/kg)	Material composition (%w/w)
CM I, CM II	Steel	86	33.33
	Steel low alloy	110	44.45
	Cast iron	240	22.22
CL	Steel	86	100
CT	Concrete	3.8	91.00
CT	PVC	280	9.00
CC	Steel	86	33.34
	Steel high alloy	910	66.66
EX I, EX II, EX III	Steel	86	25.00
EX I, EX II, EX III	Steel high alloy	910	75.00
HE I, HE II	Steel	86	25.00
HE I, HE II	Steel low alloy	110	75.00
CM III	Steel	86	33.33
	Steel low alloy	110	44.45
	Cast iron	240	22.22
P	Steel	86	35.00
P	Cast iron	240	65.00

Table 5

Environmental impact functions of components for the construction phase

Component	Environmental impact function,Y_k (Pts of Eco-indicator 99)	Variables
CM 1, CM 2, CM 3		W⋅CM(kW)
CL		Heat exchange area, A (m²)
CT	YCT=3098⋅m⋅water⋅0.09⋅b―PVCfill⋅m―PVCfill+0.91⋅b―CON⋅m―CON	m⋅water (m³/h)
CC		m⋅comb.gas (kg/s)
EX I, EX II, EX III	YEX=0.2457⋅W⋅EX+15,5380.25⋅b―S⋅m―S+0.75⋅b―SHA⋅m―SHA	W⋅EX (kW)
HE I, HE II	YHE=11.28⋅A+119.07×0.75⋅b―SLAtubes⋅m―SLAtubes+0.25⋅b―Scasing⋅m―Scasing	Tubes of SLA Casing of steel Heat exchange area, A (m²)
P	YP=0.0016⋅W⋅P2−0.5559⋅W⋅P+12.66⋅b―CI⋅m―CI (for p_discharge > 5 bar)	W⋅P (kW)

For the LCA of the system being analyzed, we assumed in analogy with the economic analysis a life time of 15 years and 7,300 working hours per year at full capacity.

The exergoenvironmental model for an energy conversion system consists of environmental impact balances written for the k th component and auxiliary equations based on the P and F rules [ ¹⁰ ]. The environmental impact balances can be written as

B⋅P,k=B⋅F,k+Y⋅k+B⋅kPF

bP,kE⋅P,k=bF,kE⋅F,k+Y⋅k+B⋅kPF,

where Y⋅k is the environmental impacts that occur during the three life-cycle phases: Construction Y⋅kCO , operation and maintenance, Y⋅kOM , and disposal Y⋅kDI constitute the component-related environmental impact associated with the k th component Y⋅k :

Y⋅k=Y⋅kCO+Y⋅kOM+Y⋅kDI.

To simplify the discussion, we assume in this paper that the value of Y⋅k is mainly associated with Y⋅kCO .

To account for pollutant formation within the k th component, a new variable was recently introduced B⋅kPF [ ¹⁹ , ²⁰ ]. This term B⋅kPF is zero if no pollutants are formed within a process, i.e., for processes without a chemical reaction (compression, expansion, heat transfer, etc.). For components, where chemical reactions occur (combustion, for example), the value of B⋅kPF is

B⋅kPF=∑ibiPFm⋅i,out−m⋅i,in,

where only pollutant streams which will finally be emitted to the environment are taken into account: CO, CO ₂ , CH ₄ , N ₂ O, NO _x , and SO _x [ ¹⁰ ].

The environmental impact of exergy destruction B⋅D,k identifies the environmental impact due to the exergy destruction within the k th component [ ¹⁰ ]:

B⋅D,k=bF,kE⋅D,k.

To identify the most important components from the viewpoint of formation of environmental impacts, the sum of environmental impacts Y⋅k+B⋅kPF+B⋅D,k is used.

The detailed exergoenvironmental model for the LNG-based cogeneration system (Figure ¹ ) will be presented in a future publication. In this paper, some data obtained from the conventional exergoenvironmental analysis are given in Table ⁶ . Here, the cooler is considered together with the cooling tower ( Y⋅CT=0.065Pts/h ), and the pump is considered together with the electrical motor, Y⋅EM=0.363Pts/h .

Table 6

Exergoenvironmental variables for the LNG-based cogeneration systems

Component	Y⋅k (Pts/h)	B⋅D,k (Pts/h)	B⋅kPF (Pts/h)	Y⋅k+B⋅D,k+B⋅kPF (Pts/h)	b_F,k(Pts/GJ)
CM I	1.254	90.718	-	91.972	8.443
CL	0.090	Dissipative component
CM II	1.054	84.085	-	85.139	8.443
CC	1.200	1,128.000	1,345.320	2,474.520	3.501
EX I	4.175	180.241	-	184.416	8.218
HE I	19.732	114.450	-	134.182	8.218
CM III	1.307	185.565	-	186.871	8.880
EX II	1.953	283.449	-	285.402	9.678
HE II	6.828	31.053	-	37.881	1.643
P	1.454	1.913	-	3.367	0.114
EX III	1.043	2.650	-	3.693	0.416

Advanced exergy-based analyses

The real potential for improving the system from a thermodynamic, economic, and environmental impact point of view can be estimated when the following are split into avoidable/unavoidable parts [ ¹⁷ , ²¹ – ²³ ]:

· exergy destruction within each (important) system component,

· investment cost and environmental impact associated with such component, and

· cost of exergy destruction and environmental impact associated with the exergy destruction for each (important) system component.

The unavoidable exergy destruction cannot be further reduced due to technological limitations such as availability and cost of materials and manufacturing methods. The difference between total and unavoidable exergy destruction for a component is the avoidable exergy destruction. Only this value and not the total exergy destruction should be considered during the improvement procedure.

E⋅D,kAV=E⋅D,k−E⋅D,kUN

The unavoidable investment cost ( Z⋅kUN ) and unavoidable component-related environmental impact ( Y⋅kUN ) for a component can be calculated by assuming the minimum values of Z⋅kE⋅P.kUN and Y⋅kE⋅P.kUN , respectively. These values will always be exceeded as long as such a component is used in a real system. The avoidable investment cost and component-related environmental impact are the differences between the total value and unavoidable part of this variable, i.e.,

Z⋅kAV=Z⋅k−Z⋅kUN

Y⋅kAV=Y⋅k−Y⋅kUN.

The value of the unavoidable exergy destruction within the k th component is calculated using the ratio E⋅D,kE⋅P,kUN that refers to the case where only unavoidable exergy destruction occurs:

E⋅D,kUN=E⋅P,krealE⋅D,kE⋅P,kUN.

The values of unavoidable capital investment cost and component-related environmental impact can be calculated using similar equations:

Z⋅kUN=E⋅P,krealZ⋅kE⋅P,kUN

Y⋅kUN=E⋅P,krealY⋅kE⋅P,kUN.

The approaches for estimating the values of E⋅DE⋅PkUN Z⋅E⋅PkUN , and Y⋅E⋅PkUN are given in [ ³ , ¹² ].

Selected data obtained from the advanced exergy-based analyses for the LNG regasification system are given in Table ⁷ . In this paper, we assumed that components of the open gas-turbine subsystem cannot be improved because this subsystem represents an already commercially available unit.

Table 7

Selected data obtained from the advanced exergy-based analyses

Component	E⋅DE⋅PkUN	E⋅D,kUN (MW)	E⋅D,kAV (MW)	Z⋅E⋅PkUN ($/MJ)	Z⋅kUN ($/h)	Z⋅kAV ($/h)	C⋅D,kUN ($/h)	C⋅D,kAV ($/h)	Z⋅kAV+C⋅D,kAV ($/h)	Y⋅E⋅PkUN(Pts/MJ)	Y⋅kUN (Pts/h)	Y⋅kAV (Pts/h)	B⋅D,kUN (Pts/h)	B⋅D,kAV (Pts/h)	B⋅D,kAV+Y⋅kAV (Pts/h)
HE I	0.0451	1.492 (39%)	2.342 (61%)	0.231	7.66 (48%)	8.38 (52%)	269	422	430 (61%)	0.3290	10.879 (55%)	8.835 (45%)	44.140	70.309	79.144 (58%)
CM III	0.0593	3.037 (52%)	2.762 (48%)	0.098	5.03 (31%)	11.20 (69%)	1,574	1,433	1,444 (48%)	0.0063	0.323 (25%)	0.984 (75%)	97.087	88.478	89.462 (48%)
EX II	0.0511	3.722 (46%)	4.405 (54%)	0.114	8.29 (40%)	12.20 (60%)	1,901	2,232	2,244 (54%)	0.0059	0.433 (23%)	1.520 (77%)	129.677	153.772	155.292 (54%)
HE II	0.0973	1.404 (27%)	3.891 (73%)	0.469	6.77 (49%)	6.99 (51%)	244	676	623 (73%)	0.0740	1.071 (16%)	5.757 (84%)	8.304	22.748	28.505 (75%)
P	0.0874	2.498 (44%)	3.125 (56%)	0.082	2.33 (33%)	3.35 (67%)	342	441	444 (56%)	0.0155	0.444 (30%)	1.010 (70%)	0.842	1.071	2.081 (62%)
EX III	0.0913	0.865 (49%)	0.905 (51%)	0.126	1.20 (45%)	1.45 (55%)	211	221	222 (51%)	0.0240	0.228 (22%)	0.815 (78%)	1.298	1.351	2.166 (59%)

Acknowledgments

The authors would like to thank Professor Hartmut Griepentrog (Greif-Foundation, Gelsenkirchen, Germany) for developing the base case and for many helpful discussions regarding gas-turbine power systems and LNG vaporization.

Competing interests

Authors' contributions

TM co-supervised the work, conducted the exergy analysis, and drafted the manuscript. GT co-supervised the work, conducted the economic analysis, and corrected the manuscript. AB conducted the life cycle assessment. CG conducted the calculations according to the instructions provided by the supervisors. All authors read and approved the final manuscript.

Advanced exergy-based analyses applied to a system including LNG regasification and electricity generation

Abstract

Background

Methods

Simulation

Figure 1

Table 1

Exergy-based analyses

Exergy analysis

Table 2

Exergoeconomic analysis

Table 3

Exergoenvironmental analysis

Table 4

Table 5

Table 6

Advanced exergy-based analyses

Table 7

Results and discussion

Conventional exergy based analyses

Advanced exergy-based analyses

Conclusions

Nomenclature

Greek symbols

Superscripts

Subscripts

Authors' information

Acknowledgments

Competing interests

Authors' contributions

References