Multi-criteria evaluation of hydrogen and natural gas fuelled power plant technologies

Abstract

This paper evaluates nine types of electrical energy generation options with regard to seven criteria. The options use natural gas or hydrogen as a fuel. The Analytic Hierarchy Process was used to perform the evaluation, which allows decision-making when single or multiple criteria are considered.The options that were evaluated are the hydrogen combustion turbine, the hydrogen internal combustion engine, the hydrogen fuelled phosphoric acid fuel cell, the hydrogen fuelled solid oxide fuel cell, the natural gas fuelled phosphoric acid fuel cell, the natural gas fuelled solid oxide fuel cell, the natural gas turbine, the natural gas combined cycle and the natural gas internal combustion engine.The criteria used for the evaluation are CO2 emissions, NOX emissions, efficiency, capital cost, operation and maintenance costs, service life and produced electricity cost.A total of 19 scenarios were studied. In 15 of these scenarios, the hydrogen turbine ranked first and proved to be the most preferred electricity production technology. However since the hydrogen combustion turbine is still under research, the most preferred power generation technology which is available nowadays proved to be the natural gas combined cycle which ranked first in five scenarios and second in eight. The last in ranking electricity production technology proved to be the natural gas fuelled phosphoric acid fuel cell, which ranked in the last position in 13 scenarios.

Full text

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Multi-criteria evaluation of hydrogen and natural gas fuelled power plant technologies
Petros A. Pilavachi*, Stilianos D. Stephanidis, Vasilios A. Pappas, Naim H. Afgan1

Department of Engineering and Management of Energy Resources
University of Western Macedonia, 50100 Kozani, Greece

1Department of Mechanical Engineering, Instituto Superior Technico, Lisbon, Portugal.


Abstract
This paper evaluates nine types of electrical energy generation options with regard to
seven criteria. The options use natural gas or hydrogen as a fuel. The Analytic Hierarchy
Process was used to perform the evaluation, which allows decision-making when single or
multiple criteria are considered.
The options that were evaluated are the hydrogen combustion turbine, the hydrogen
internal combustion engine, the hydrogen fuelled phosphoric acid fuel cell, the hydrogen
fuelled solid oxide fuel cell, the natural gas fuelled phosphoric acid fuel cell, the natural gas
fuelled solid oxide fuel cell, the natural gas turbine, the natural gas combined cycle and the
natural gas internal combustion engine.
The criteria used for the evaluation are CO2 emissions, NOX emissions, efficiency, capital
cost, operation and maintenance costs, service life and produced electricity cost.
A total of 19 scenarios were studied. In 15 of these scenarios, the hydrogen turbine ranked
first and proved to be the most preferred electricity production technology. However since the
hydrogen combustion turbine is still under research, the most preferred power generation
technology which is available nowadays proved to be the natural gas combined cycle which
ranked first in five scenarios and second in eight. The last in ranking electricity production
technology proved to be the natural gas fuelled phosphoric acid fuel cell, which ranked in the
last position in 13 scenarios.

Keywords: Power generation; Hydrogen; Natural gas; Analytic Hierarchy Process; Single-
criterion analysis; Multi-criteria analysis



1. Introduction
Clean, low-cost power generation; these are the trends of the energy market today, in a
highly competitive environment with rising environmental concerns. Concepts like energy
policy and green house gas emissions reduction, that used to exist in scientific discussions
only, are now already a part of the national and international political scene. The warnings of
the scientific community are now been taken into consideration and have a permanent place in
conferences relevant to energy and the environment.
The increasing world power consumption, over 80% of which is generated by means of
fossil fuel combustion processes, has raised the CO2 concentration in the atmosphere more
than 30% above the level of the pre-industrial era [1]. Expected demand for electricity would
require during the coming two decades the installation of as much power generation capacity
as was installed in the entire 20th century [2]. Considering this, the world community has

*Corresponding author. Tel: +30 24610 56640, e-mail address: ppilavachi@uowm.gr (P.A.
Pilavachi)

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already taken measures to reduce CO2 and other green house gas emissions. Such reductions
are possible by developing more efficient technologies, using renewable energy sources and
utilizing new, cleaner fuels.
Natural gas is a widely used fossil fuel that is cleaner than coal and petrol. There is
abundance of natural gas and its utilization is constantly increasing in the last 50 years [3].
Power plants that utilize natural gas have significantly lower emissions than other fossil fuel
plants.
Hydrogen on the other hand, is viewed as the fuel of the future and has been recently
gaining a lot of attention. A great number of studies have been carried out concerning issues
to be addressed in order to facilitate the introduction of hydrogen in the energy balance [4].
Alternative fuels including hydrogen-enriched fuels were studied for use in power generation
[5]. The effect of hydrogen injection as additional fuel in gas turbine combustors was
evaluated [6]. Power plants that utilize hydrogen could potentially have absolutely zero
emissions. However, hydrogen is more of an energy carrier than a fuel (because its production
requires energy) and hydrogen technologies cannot yet be considered mature and are
relatively more expensive.
This paper uses the Analytic Hierarchy Process (AHP) methodology to evaluate different
power plant technologies that use natural gas or hydrogen as a fuel using economic,
environmental and technological criteria. The AHP is a common tool for single- and multi-
objective decision-making problems and has the ability to simplify complex problems. In the
past, the AHP has been used before in several studies to evaluate power generation plant
technologies, such as the evaluation of power plants with regard to their non-radioactive
emissions [7] and with regard to the impact on the living standard [8]. It has also been used to
perform a comparison between conventional and renewable power technologies [9] and to
make a sustainability comparison of fuel cell systems SOFC, PAFC, MCFC with respect to
environmental, societal and economic impacts [10]. Methods other than AHP have also been
used in the past to evaluate power plants. A multi-criteria evaluation of plants that produce
hydrogen and use it as a fuel was presented [11] and a sustainability assessment of the
phosphoric acid and solid oxide fuel cells was carried out and then compared with new and
renewable energy systems [12].

2. Description of Power Technologies
Nine different energy generation options were selected for evaluation. It should be noted
that there are differences in the levels of maturity of the technology of these options.
Therefore the data on emerging technologies may be preliminary and less reliable than the
data of more mature technologies. However, it is interesting to see how upcoming and
developing technologies perform compared to mature and established ones. It should also be
noted that for the hydrogen options under consideration, it is assumed that hydrogen is
supplied through a distribution network and is not produced on site. Below follows a brief
description of these options.
2.1. Hydrogen combustion turbines

In the recent years, there is an effort to build hydrogen fuelled turbine power plants and
companies are funded to study such technologies. Toshiba is currently developing a hydrogen
combustion turbine under the Japanese World Energy Network research program (WE-NET).
Toshiba’s technology uses combustion chambers to burn hydrogen with pure oxygen in order
to produce steam (H2 + ½ O2 = H2O). The steam is then used in steam turbines to produce
work.

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2.2. Hydrogen internal combustion engines

Hydrogen internal combustion engines (H2 ICE) operate under the same principles as all
reciprocating internal combustion engines. Due to hydrogen’s properties, H2 ICEs are
generally more efficient.
2.3. Hydrogen fuelled phosphoric acid fuel cells

Phosphoric acid fuel cells (PAFC) are chemical energy conversion devices. They directly
convert the chemical energy of a fuel into electricity. Hydrogen fuelled PAFCs are fed with
pure hydrogen and thus have no need for a fuel reformer. This lowers their capital as well as
operation and maintenance (O&M) costs, makes them more efficient and extends their service
life.
2.4. Hydrogen fuelled solid oxide fuel cells

As above, solid oxide fuel cells (SOFC) directly convert the chemical energy of a fuel into
electricity. SOFCs fed with pure hydrogen have lower capital and operation and maintenance
costs and are more efficient.
2.5. Natural gas fuelled phosphoric acid fuel cells

Natural gas fuelled PAFCs operate the same way like the hydrogen fuelled ones. Natural
gas fuelled PAFCs are mainly comprised of the energy conversion unit (fuel cell) and a fuel
reformer. Natural gas is fed into the fuel reformer and is converted into a hydrogen rich gas,
which is then fed in the energy conversion unit.
2.6. Natural gas fuelled solid oxide fuel cells

SOFCs can operate on a variety of fuels, including natural gas, without the need of an
external fuel reformer (unlike PAFCs). The fact that they operate at high temperatures (600-
1000oC), allows SOFCs to reform fuels into hydrogen rich gases internally, eliminating the
need for a complex reformer. Only a simple reformer is required to remove impurities from
the fuel.
2.7. Natural gas turbine

Natural gas turbines (NG turbines) burn natural gas with compressed air to produce high
temperature and pressure exhaust gasses, which rotate a turbine. The turbine produces work
which is used to rotate the air compressor and to power an electrical generator.
2.8. Natural gas combined cycle

Natural gas combined cycle power plants combine gas and steam turbine technologies.
They utilize the waste heat of a natural gas turbine to produce steam, which rotates a steam
turbine in order to produce additional energy. Other arrangements are possible as well.
2.9. Natural gas internal combustion engine

Natural gas internal combustion engines (NG ICE) burn natural gas in a combustion
chamber (cylinder) to produce thermal energy. The thermal energy is then converted into
work through an array of appropriate components.

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3. Overview of the Analytic Hierarchy Process
The Analytic Hierarchy Process (AHP) is a structured tool that helps the user deal with
complex decisions. It is based on mathematics and human psychology and was developed by
Thomas L. Saaty in the 1970s [13].
In the first step of the process, a hierarchy is built by analyzing the problem into a goal,
criteria and decision alternatives. In the next step, each decision alternative is evaluated with
regard to each criterion. After that, each criterion is given a numerical weight representing the
importance of the criterion. In the last step of the process, numerical values are calculated for
each decision alternatives. These values represent the ability of each alternative to achieve the
decision goal.

4. Description of criteria
The criteria used for the evaluation of the selected energy production technologies are
efficiency, CO2 emissions, NOX emissions, capital cost, operation and maintenance costs
(O&M costs), electricity cost and service life. All criteria data were collected from the
bibliography unless otherwise mentioned.
All economic data were found in US dollars and were all converted to February 2008 US
dollars and then in February 2008 Euros.

4.1. Efficiency

The efficiency criterion is the quality measure of the system. It represents the percentage
of the fuel’s lower heating value (LHV) that is converted to useful electrical energy. A
graphical representation of the efficiency values for all nine options is shown in Fig. 1.
As shown in Fig. 1, the option with the highest efficiency is the hydrogen combustion
turbine followed by the hydrogen fuelled PAFC and SOFC. The options with the lowest
efficiency are the natural gas internal combustion engine and the natural gas turbine.
4.2. CO2 emissions

The CO2 emissions criterion represents the amount of carbon dioxide that is released from
the power plant in the atmosphere as a byproduct of the energy conversion process. It is
measured in g/kWh. A graphical representation of the CO2 emissions for all nine options is
given in Fig. 2.
CO2 emissions for hydrogen turbine, hydrogen fuelled PAFC and hydrogen fuelled SOFC
are 0 g/kWh. This is because hydrogen is a carbon free fuel and thus no carbon oxides are
formed. Hydrogen ICEs emit traces of CO2 due to the combustion of the oil that leaks in the
engine’s cylinders. However these emissions are very low and are assumed to be 0 g/kWh in
this study. As shown in Fig. 2, the option with the highest CO2 emissions is the natural gas
internal combustion engine, followed by the natural gas fuelled phosphoric acid fuel cell.
Since the hydrogen combustion turbine and the hydrogen internal combustion engine do not
use a carbon-based fuel, they have zero CO2 emissions.

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4.3. NOX emissions

The NOX emissions criterion represents the amount of nitric oxides (NO) and nitrogen
dioxides (NO2) that is released from the power plant in the atmosphere as a byproduct of the
energy conversion process. It is measured in g/kWh. A graphical representation of the NOX
emissions for all nine options is given in Fig. 3.
As shown in Fig. 3, the option with the highest NOX emissions is the natural gas turbine.
The hydrogen combustion turbine combusts hydrogen with pure oxygen, therefore no NOx is
produced by the combustion process.
4.4. Capital cost

The capital cost criterion represents the total cost of the power plant and includes the cost
of all equipment and all installation costs. It is measured in euros per installed kilo-watt
(€/kW). A graphical representation of the capital cost for all nine options is given in Fig. 4.
As shown in Fig. 4, fuel cell technologies are extremely expensive compared to the other
technologies. The natural gas combined cycle and the natural gas turbine have the lowest
capital cost.
4.5. O&M costs

The O&M costs criterion represents the operation and maintenance costs and includes
replacement parts costs and labor costs for the operation and maintenance of the power plant.
It does not include fuel cost. It is measured in €/kWh. A graphical presentation of the O&M
costs for all nine options is given in Fig. 5.
No data for the O&M costs criterion of the hydrogen turbine could be found. Therefore the
O&M costs for the hydrogen turbine are assumed to be 0.0057 €/kWh, approximately 24%
higher than the natural gas turbine, in the same way as it’s capital cost. The natural gas
combined cycle and the natural gas turbine have the lowest O&M costs, whereas the
phosphoric acid fuel cell has the highest.
4.6. Electricity cost

The electricity cost criterion represents the cost of the produced electric energy of the
power plant and is calculated based on the fuel cost (assuming a cost of 0.04231 €/kWh for
natural gas and a cost of 0.122 €/kWh for hydrogen), the O&M costs, the power plant cost
and the power plant’s service life. It is measured in €/kWh. A graphical presentation of the
electricity cost for all nine options is given in Fig. 6.
The electricity cost for all nine options was calculated taking capital cost, O&M costs, fuel
costs and service life into consideration. As shown in Fig. 6, the natural gas combined cycle
has the lowest electricity cost, whereas the hydrogen internal combustion engine has the
highest.
4.7. Service life

The service life criterion refers to the number of years the power plant can operate before
the equipment needs to be replaced. It is measured in years. A graphical representation of the
service life for all nine options is given in Fig. 7.
As shown in Fig. 7, turbine power plants have the longest service life whereas fuel cells
have the shortest.
Table 1 shows all numerical data for all nine options.

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5. Hierarchy tree
In order to evaluate each energy generation option, a hierarchy tree has been built for the
application of the AHP. The hierarchy tree is shown in Fig. 8.
On the top level of the hierarchy tree is the goal, which is the choice of the best energy
generation option. On the next level are the criteria used for the evaluation, efficiency, CO2
emissions, NOX emissions, capital cost, O&M costs, electricity cost and service life. The
decision alternatives, H2 Turbine, H2 ICE, NG PAFC, NG SOFC, NG Turbine, NG ICE and
NG CC, appear at the lowest level.

7. Analysis of the results
For each case, the criteria weights are given in Table 2, while the results of the evaluation
are presented in Table 3.
7.1. Base case

In the base case, the weight factors were distributed subjectively. However, an attempt was
made for the weight factors to reflect the current trends of the energy market. Therefore, the
economic criteria (capital cost, O&M costs, electricity cost) were considered the most
significant, followed by the environmental criteria (CO2 emissions and NOx emissions).
7.2. Equally distributed weights

In this case (case 1) the weights were distributed evenly among the nine criteria. Each
criterion received 14.3% weight.
7.3. Single-criterion analysis

Seven single-criterion cases were studied (cases 2-8). In these cases, a single criterion
receives full emphasis while the other six criteria are ignored.
In cases 2-8, full emphasis is given, respectively, to the efficiency, the CO2 emissions, the
NOX emissions, the capital cost, the O&M costs, the electricity cost and the service life
criteria.
7.4. Multi-criteria analysis

Ten multi-criteria cases were studied. In cases 9-15, 60% emphasis is given, respectively,
to the efficiency, the CO2 emissions, the NOX emissions, the capital cost, the O&M costs, the
electricity cost and the service life criteria while the remaining 40% is equally distributed
among the rest of the criteria.
In case 16, 30% emphasis is given to the capital cost and the CO2 emissions criteria and
the remaining 40% is equally distributed among the rest of the criteria.
In case 17, 30% emphasis is given to the capital cost and the electricity cost criteria and
the remaining 40% is equally distributed among the rest of the criteria.
In case 18, 30% emphasis is given to the electricity cost and the CO2 emissions criteria and
the remaining 40% is equally distributed among the rest of the criteria.

8. Conclusions
In this paper, nine energy generation options were evaluated with regard to seven criteria.
The energy generation options were the hydrogen combustion turbine, the hydrogen internal
combustion engine, the hydrogen fuelled phosphoric acid fuel cell, the hydrogen fuelled solid
oxide fuel cell, the natural gas fuelled phosphoric acid fuel cell, the natural gas fuelled solid

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oxide fuel cell, the natural gas turbine, the natural gas combined cycle and the natural gas
internal combustion engine. The criteria used for the evaluation were efficiency, CO2
emissions, NOx emissions, capital cost, O&M costs, electricity cost and service life. The
Analytic Hierarchy Process was used to perform the evaluation. A total of 19 scenarios were
studied.
The most dominant electricity generation technology proved to be the hydrogen
combustion turbine, which ranked in the first place in 15 out of 19 scenarios. This was to be
expected since the hydrogen combustion turbine promises to deliver ultra clean and low-cost
power generation, despite the high price of hydrogen, which is about three times more
expensive than natural gas. Considering the fact that hydrogen prices are expected to drop as
its production methods are evolving [1], the cost of the hydrogen turbine’s generated
electricity is expected to be very competitive in the future. However the hydrogen turbine is
not a currently available technology, as it is still under research, and its actual performance
characteristics when it becomes commercially available could be very different from the ones
used in this paper.
The second most preferable power generation option proved to be the natural gas
combined cycle, which ranked first in 5 out of 19 scenarios and second in 8. Had the
hydrogen combustion turbine not been taken into consideration in this paper, the natural gas
combined cycle would have ranked first in 12 scenarios. In most of these scenarios, focus is
given primarily on the economic and secondarily on the environmental criteria. This shows
that the natural gas combined cycle is a very competitive power generation technology.
The phosphoric acid fuel cell ranked in the last place in 13 out of 19 scenarios. This was to
be expected since this cell has a very high capital cost and O&M costs combined with a short
service life and produces rather high emissions compared to other technologies.

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Table 1. Criterion values for all nine types of electricity generation technologies
Option
Efficiency
(%)
CO2
Emissions
(g/kWh)
NOX
Emissions
(g/kWh)
Capital
Cost
(€/kW)
O&M Cost
(€/kWh)
Electricity
Cost
(€/kWh)
Service
Life
(years)
H2 Turbine 70 [14] 0 0 680 [23] 0.0057 0.184 20 [27]
H2 ICE 45 [9, 15] 0 0.5 [22] 794 [24] 0.014 [24] 0.2929 12.5 [27]
H2 PAFC 60 [16] 0 0 [16] 2000 [16] 0.025 [16] 0.268 6.25 [16]
H2 SOFC 60 [16] 0 0 [16] 1600 [16] 0.012 [16] 0.24 8 [16]
NG PAFC 40 [11] 510 [19] 0.0135 [19] 2645 [25] 0.03 [28] 0.202 5 [17]
NG SOFC 55 [17] 410 [19] 0.023 [19] 2140 [26] 0.018 [26] 0.1284 8 [17]
NG Turbine 35 [11] 500 [20] 1.5 [20] 550 [27] 0.0046 [27] 0.129 20 [27]
NG CC 50 [20] 400 [20] 1.3 [20] 531 [27] 0.0046 [27] 0.09252 20 [27]
NG ICE 35 [18] 590 [18] 0.21 [21] 794 [27] 0.014 [27] 0.1429 12.5 [27]

Table 2. Criteria weights for each case studied.
Criterion
Capital
Cost
Case Efficiency
CO2
Emissions
NOX
emissions
O&M
Costs
Electricity
Cost
Service
Life
Base Case 10% 12.5% 10% 25% 10% 25% 7.5%
Case 1 14.3% 14.3% 14.3% 14.3% 14.3% 14.3% 14.3%
Case 2 100% 0% 0% 0% 0% 0% 0%
Case 3 0% 100% 0% 0% 0% 0% 0%
Case 4 0% 0% 100% 0% 0% 0% 0%
Case 5 0% 0% 0% 100% 0% 0% 0%
Case 6 0% 0% 0% 0% 100% 0% 0%
Case 7 0% 0% 0% 0% 0% 100% 0%
Case 8 0% 0% 0% 0% 0% 0% 100%
Case 9 60% 6.7% 6.7% 6.7% 6.7% 6.7% 6.7%
Case 10 6.7% 60% 6.7% 6.7% 6.7% 6.7% 6.7%
Case 11 6.7% 6.7% 60% 6.7% 6.7% 6.7% 6.7%
Case 12 6.7% 6.7% 6.7% 60% 6.7% 6.7% 6.7%
Case 13 6.7% 6.7% 6.7% 6.7% 60% 6.7% 6.7%
Case 14 6.7% 6.7% 6.7% 6.7% 6.7% 60% 6.7%
Case 15 6.7% 6.7% 6.7% 6.7% 6.7% 6.7% 60%
Case 16 8% 30% 8% 30% 8% 8% 8%

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Case 17 8% 8% 8% 30% 8% 30% 8%
Case 18 8% 30% 8% 8% 8% 30% 8%

Table 3. The results of the evaluation for each case studied.
Technology under evaluation

Case
H2
Turbine
H2 ICE
H2
PAFC
H2
SOFC
NG
PAFC
NG
SOFC
NG
Turbine
NG CC NG ICE
Base Case 16.7% 10.4% 8.2% 11% 4.7% 10% 12.5% 15% 11.4%
Case 1 18% 11% 9.4% 12.1% 4.8% 9.9% 11.1% 13.7% 10%
Case 2 25.9% 7.4% 18.5% 18.5% 3.7% 14.8% 0% 11.1% 0%
Case 3 20.3% 20.3% 20.3% 20.3% 2.8% 6.2% 3.1% 6.6% 0%
Case 4 15.1% 10% 15.1% 15.1% 14.9% 14.8% 0% 2% 13%
Case 5 16.3% 15.3% 5.3% 8.7% 0% 4.2% 17.4% 17.5% 15.3%
Case 6 17.1% 11.3% 3.5% 12.7% 0% 8.4% 17.9% 17.9% 11.3%
Case 7 11.4% 0% 2.4% 5.4% 9.4% 17.3% 17.3% 21.2% 15.8%
Case 8 23.6% 11% 0.4% 3.4% 0% 3.4% 23.6% 23.6% 11%
Case 9 21.7% 9.3% 13.6% 15.1% 4.3% 12.2% 5.9% 12.5% 5.4%
Case 10 19.2% 15.9% 15.1% 16.4% 3.7% 8% 6.9% 9.9% 4.8%
Case 11 16.2% 10.4% 12.8% 13.9% 10.9% 12.9% 4.5% 6.7% 11.8%
Case 12 17% 13.4% 7.1% 10.2% 2.1% 6.7% 14.6% 15.8% 13%
Case 13 17.5% 11.1% 6.1% 12.4% 2.1% 9.1% 14.9% 16% 10.7%
Case 14 14.6% 5.3% 5.8% 8.6% 7.2% 13.7% 14.3% 17.5% 13%
Case 15 20.7% 11% 5% 7.8% 2.5% 6.7% 17.2% 18.5% 10.5%
Case 16 18.1% 14% 10.7% 13% 3.2% 7.8% 10.9% 13.1% 9.2%
Case 17 16.2% 9.8% 7% 9.9% 4.6% 10% 13.9% 16.1% 12.5%
Case 18 17.1% 10.7% 10.3% 12.5% 5.3% 10.7% 10.6% 13.7% 9%

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0%
10%
20%
30%
40%
50%
60%
70%
80%
H2??  TurbineH2??  IC EH2??  P AF CH2??  S OF CNG??  P AF CNG??  S OF C NG??  turbineNG??  C C NG??  IC E

Fig. 1. Efficiency for nine types of electricity generation technologies

0
100
200
300
400
500
600
700
H2??  TurbineH2??  IC EH2??  P AF C H2??  S OF CNG??  P AF CNG??  S OF CNG??  Turbine NG??  C C NG??  IC E
Fig. 2. CO2 emissions for nine types of electricity generation technologies

0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
H2??  TurbineH2??  IC EH2??  P AF CH2??  S OF CNG??  P AF CNG??  S OF CNG??  TurbineNG??  C CNG??  IC E

Fig. 3. NOX emissions for nine types of electricity generation technologies
0
500
1000
1500
2000
2500
3000
H2??  Turbine H2??  IC EH2??  P AF CH2??  S OF C NG??  P AF C NG??  S OF CNG??  Turbine NG??  C C NG??  IC E

Fig. 4. Capital cost for nine types of electricity generation technologies
Efficiency (%)
CO2 emissions (g/kWh)
NOx emissions (g/kWh)
Capital cost (€/kW)

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0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
H2??  TurbineH2??  IC EH2??  PAF CH2??  S OF C NG??  PAF CNG??  S OF CNG??  Turbine NG??  C C NG??  IC E

Fig. 5. O&M costs for nine types of electricity generation technologies
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
H2??  TurbineH2??  IC EH2??  P AF CH2??  S OF CNG??  P AF CNG??  S OF CNG??  TurbineNG??  C C NG??  IC E

Fig. 6. Electricity cost for nine types of electricity generation technologies
0
5
10
15
20
25
H2??  TurbineH2??  IC EH2??  P AF C H2??  S OF C NG??  P AF CNG??  S OF C NG??  Turbine NG??  C C NG??  IC E

Fig. 7. Service life for nine types of electricity generation technologies


Fig. 8. The hierarchy tree of the problem
O&M costs (€/kWh)
Electricity cost (€/kWh)
Service life (years)