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.
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
(1)
or
(2)
where
(3)
To simplify the discussion, we assume that the contribution of remains constant when the design changes, and therefore, the changes in the value of are associated only with changes in the capital investment cost .
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
(4)
· The cost rate associated with exergy loss from the overall system is
(5)
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 (
), and the pump is considered together with the electrical motor (
).
Table 3
Exergoeconomic variables for the LNG-based cogeneration systems
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 ieff = 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
(relative environmental impact) and
(relative mass) are equal to 1. If the selected material is different, then
and
(where
ρ is the density of the material, kg/m
3).
Table 4
Eco-indicator 99 values and material composition of components
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 |
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 |
Steel high alloy | 910 | 75.00 |
HE I, HE II | Steel | 86 | 25.00 |
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 |
Cast iron | 240 | 65.00 |
Table 5
Environmental impact functions of components for the construction phase
CM 1, CM 2, CM 3 | | (kW) |
CL | | Heat exchange area, A (m2) |
CT |
| (m3/h) |
CC | | (kg/s) |
EX I, EX II, EX III |
| (kW) |
HE I, HE II |
| Tubes of SLA Casing of steel Heat exchange area, A (m2) |
P | (for pdischarge > 5 bar) | (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
(6)
or
(7)
where
is the environmental impacts that occur during the three life-cycle phases: Construction
, operation and maintenance,
, and disposal
constitute the component-related environmental impact associated with the
k th component
:
(8)
To simplify the discussion, we assume in this paper that the value of is mainly associated with .
To account for
pollutant formation within the
k th component, a new variable was recently introduced
[
19,
20]. This term
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
is
(9)
where only pollutant streams which will finally be emitted to the environment are taken into account: CO, CO
2, CH
4, N
2O, NO
x, and SO
x[
10].
The environmental impact of exergy destruction
identifies the environmental impact due to the exergy destruction within the
k th component [
10]:
(10)
To identify the most important components from the viewpoint of formation of environmental impacts, the sum of environmental impacts 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 (
), and the pump is considered together with the electrical motor,
.
Table 6
Exergoenvironmental variables for the LNG-based cogeneration systems
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 |