Life cycle assessment of hydrogen fuel cell and gasoline vehicles

Life cycle assessment of hydrogen fuel cell and gasoline vehicles

International Journal of Hydrogen Energy 31 (2006) 337 – 352 www.elsevier.com/locate/ijhydene Life cycle assessment of hydrogen fuel cell and gasolin...

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International Journal of Hydrogen Energy 31 (2006) 337 – 352 www.elsevier.com/locate/ijhydene

Life cycle assessment of hydrogen fuel cell and gasoline vehicles Mikhail Granovskii, Ibrahim Dincer∗ , Marc A. Rosen Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ont., Canada L1H 7K4 Received 25 February 2005; received in revised form 6 July 2005; accepted 7 October 2005 Available online 22 November 2005

Abstract A life cycle assessment of hydrogen and gasoline vehicles, including fuel production and utilization in vehicles powered by fuel cells and internal combustion engines, is conducted to evaluate and compare their efficiencies and environmental impacts. Fossil fuel and renewable technologies are investigated, and the assessment is divided into various stages. Energy efficiencies and greenhouse gas emissions are evaluated in each step for crude oil and natural gas pipeline transportation, crude oil distillation and natural gas reformation, wind and solar electricity generation, hydrogen production through water electrolysis, and gasoline and hydrogen distribution and utilization. The results indicate that, when taking into account fossil fuel energy consumption and greenhouse gas emissions, the efficiency of a fuel cell vehicle employing hydrogen from natural gas should be at least 25–30% higher than a gasoline one to be competitive. Wind electricity generation to produce hydrogen via electrolysis, and its application in a PEMFC vehicle, has the lowest greenhouse gas emissions and fossil fuel energy consumption. However, the economic attractiveness of renewable wind technology is shown to depend significantly on the ratio in costs between hydrogen and natural gas; when this ratio is 2:1, a financial investment to produce hydrogen via natural gas is about five times more profitable than to do so via wind energy. 䉷 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Life cycle assessment; Hydrogen; Renewable energy; Fuel cells

1. Introduction Growing concerns about the global warming effect and exhausting crude oil stocks have led to an interest in hydrogen as a fuel for transportation applications. The efficiency of applying new hydrogen technologies, e.g., proton exchange membrane fuel cell (PEMFC) vehicles [1], depends on the characteristics of the many steps and chains involved, which include production, ∗ Corresponding author. Tel.: +1 905 721 3209; fax: +1 905 721 3140. E-mail addresses: [email protected] (M. Granovskii), [email protected] (I. Dincer), [email protected] (M.A. Rosen).

distribution and, finally, conversion of the chemical energy of hydrogen into mechanical work in a vehicle. Adequate evaluation of environmental impact and energy consumption throughout the overall hydrogen production and utilization life cycle (from “cradle-tograve”), in comparison with that for gasoline, is critical for making proper strategic decisions about its competitiveness in the future. Life cycle assessment (LCA) is a methodology for this type of assessment, and represents a systematic set of procedures for compiling and examining the inputs and outputs of materials and energy and the associated environmental impacts directly attributable to the functioning of a product or service system throughout its life cycle. A life cycle is the interlinked stages of a product or service

0360-3199/$30.00 䉷 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2005.10.004

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Nomenclature AC CH4 CH2 CNG CO2 DC d EEQ EMB E EH2 EOP G GWP H2 H2 O ICE IEE IPM k l L LCA LFT LHV n NG O2 p patm PEMFC Q

alternating current methane cost of hydrogen energy cost of natural gas energy carbon dioxide direct current diameter of pipeline energy equivalent of construction materials and devices embodied energy energy energy of hydrogen operation energy to install, operate, maintain, etc. equipment capacity of crude oil distillation global warming potential hydrogen water internal combustion engine intensity of embodied energy investments to produce construction materials and devices number of the intermediate compressions distance between consecutive compressions pipeline length life cycle assessment lifetime of a technological unit lower heating value mole flow rate natural gas oxygen pressure pressure of the natural environment, 1atm proton exchange membrane fuel cell heat

system, from the extraction of natural resources to final disposal [2]. Numerous LCA studies of gasoline and hydrogen vehicles have been reported in the literature [3–9]. However, the reasons for and necessities of energy losses throughout the life cycle of the fuels, starting from production and finishing with utilization in a vehicle, have not been carefully considered. As a consequence, comprehensive assessments have not been available of why renewable technologies for hydrogen production are economically less attractive than traditional ones.

R Re T0 V W E p

universal gas constant, 8.314 J/mol K Reynolds number temperature of the natural environment, 298 K volumetric flow capacity of hydrogen production energy consumed pressure losses

Greek letters      w

ratio of cost of hydrogen to natural gas capital investments efficiency energy efficiency viscosity density linear velocity

Subscripts av cmp dir el ind in ir fr gt max min ng oil out pmp

average compressor direct electricity indirect input internal resources friction gas turbine maximal minimal natural gas crude oil output pump

Superscript dir

direct

It is especially important for advanced countries to define ways to reduce greenhouse gas emissions from vehicles because of the large amounts of energy used in such transportation. For instance, the transportation sector of Canada is responsible for approximately 25% of Canada’s total emissions, and three quarters of these emissions come from road transport, primarily via personal vehicle trips. An internal combustion motor vehicle produces an average of 0.35 kg of greenhouse gases for every kilometer traveled and there were over 17 million passenger vehicles on Canadian roads in 2002 [10].

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In this article, hydrogen life cycles, for a variety of methods for hydrogen production, are compared with the life cycle of gasoline. This comparison is done to obtain the critical efficiency of a hydrogen PEMFC stack which allows it to be energetically competitive with the internal combustion engine (ICE). The comparison is performed based on fossil fuel energy consumption and greenhouse gas emissions. Conventional technology for natural gas conversion to hydrogen is compared with the utilization of solar and wind energies to produce hydrogen through electrolysis, and advantages and disadvantages of the technologies are considered.

2. Analysis Hydrogen and gasoline represent the final products of several modern technologies. The conventional production methods are reforming of natural gas (hydrogen) and crude oil distillation (gasoline). The principal technological steps in crude oil and natural gas utilization in transportation are presented in Fig. 1. Use of environmentally benign renewable technologies to generate hydrogen is considered attractive. The principal technological steps in the utilization of solar and wind energy to produce hydrogen are presented in Fig. 2. Each technological step in Figs. 1 and 2 is accompanied by a consumption of fossil fuel energy E which can be expressed as E = Edir + Eind ,

(1)

where Edir is the energy content of the fuels directly transformed into the final products, as well as the fuels consumed to perform such transformation, and Eind is the fossil fuel energy embodied in construction materials and apparatuses, as well as in installation, operation, maintenance, decommissioning, etc. In this paper, fuel energy represents the heat released by the complete burning of all fuel components to CO2 and H2 O (steam), i.e., its lower heating value (LHV). Physical and chemical properties of fossil fuels and final fuels are given in Table 1 and are used throughout this article. The energy balance of each technological operation in the crude oil and natural gas utilization scheme (Fig. 1) can be presented as E = Ein − Eout ,

(2)

where Ein is the energy content of the fuels consumed (directly and indirectly) and Eout is the energy content of the fuels produced.

Fig. 1. Principal technological steps in utilizing fossil fuels in two vehicle types.

The energy efficiency  is then defined as  = Eout /Ein .

(3)

In order to compare fossil fuel and renewable technologies more correctly, it should be acknowledged that construction materials are also produced from mineral sources (ores, limestone, etc.) which, like fossil fuels, have value. To account for this, the energy equivalent of construction materials and devices (EEQ) is introduced and the following procedure proposed to calculate the indirect energy Eind . The intensities of embodied energies (cost of construction materials and devices per consumed fossil fuel energy to produce them) (IEE) are obtained using Economic Input–Output Life Cycle Assessment Software

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energy equivalents of construction materials and devices (EEQ) are calculated as follows: IPM IEE · EMB = , (5) PNG CNG where the values of embodied energies EMB are taken from [12] and CNG is the industrial cost of natural gas, equal in 1992 to 0.00316 US$/MJ [13] (this year is chosen in order to be consistent with the data obtained by using Life Cycle Assessment Software [11]). The indirect energy to perform a technological operation is evaluated with the following expression:  EEQ + EOP Eind = , (6) LFT  where EEQ is the summation of the energy equivalents of construction materials and devices related to a given technological operation, EOP is the operation energy, i.e., the fossil fuel energy required for installation, construction, operation, maintenance, decommissioning, etc. of the equipment, and LFT is the lifetime of the unit performing a technological operation. The fossil fuel and renewable technologies for hydrogen production are generally distinguished by (1) source of energy consumed, (2) efficiency of hydrogen production per unit of energy consumed, and (3) capital investments made per unit of hydrogen produced. To account for all of these differences a new indicator, the capital investments effectiveness , is introduced as a measure of economic efficiency. This indicator is proportional to the relationship between the gain and investments and is equal to EEQ =

Fig. 2. Principal technological steps in utilizing solar and wind energies in transportation.

Table 1 Physical and chemical properties of fuels Fuel

Density  (g/l)

Crude oila Natural gas (p=1 atm, T0 = 298 K) Conventional gasolinea Conventional diesela Hydrogen (p = 1 atm, T0 = 298 K)

Lower heating value (MJ/kg)

Carbon percentage (%)

845 0.654

42.8 50.1

85.0 75.0

737 856 0.0818

43.7 41.8 121.0

85.5 87.0 0

(7)

CH2 . (8) CNG Also, W is the capacity of hydrogen production expressed in units of hydrogen LHV per second, Eind is the indirect energy which is proportional to the capital investments in a technology, and  is the energy efficiency which is expressible as =

[11], where they are expressed in US$ 1992 per unit of energy: IPM , EMB

W ( − 1/) . Eind

The numerator of the fraction in Eq. (7) is proportional to the gain from the exploitation of a technology and the denominator to the investments made in it. Here,  is the ratio in costs of hydrogen (CH2 ) and natural gas (CNG):

a Source: [9].

IEE =

=

(4)

where IPM denotes investments to produce construction materials or devices and EMB denotes energy embodied in construction materials and devices. The fossil fuel

=

EH2 . Eng + E

(9)

In Eq. (9), EH2 is the energy content of hydrogen, Eng is the energy content of natural gas, which is transformed

M. Granovskii et al. / International Journal of Hydrogen Energy 31 (2006) 337 – 352 Table 2 Global warming potential (GWP) of several greenhouse gases Compound

GWP relative to CO2

CO2 CH4 N2 O

1 21 310

Source: [14].

directly to hydrogen, and E is the fossil fuel energy consumption (direct and indirect). Note that Eng = EH2 for natural gas reforming and Eng = 0 for “renewable” hydrogen. The initial solar and wind energies do not have any price and, therefore, are not included in the denominator of Eq. (9) for renewable technologies. As a result, the values of  for these technologies can exceed 1. To obtain capital investments effectivenesses  for different technologies, it is necessary to consider the steps involved in the transformation of the initial energy sources to the final fuels. The direct and indirect fossil fuel energy consumptions lead to different kinds of harmful pollution. In mass terms greenhouse gases (CO2 , CH4 , N2 O) constitute the greatest part of them. The global warming potentials of these substances, according to the International Panel on Climate Change, are given in Table 2 [14]. To perform a LCA of the fuels considered here, it is necessary to consider the steps involved in the transformation, starting from production and finishing with utilization in vehicles. 2.1. Natural gas and crude oil transportation In 2002, 95% of Canadian oil and gas were transported by pipelines, representing 180 billion cubic meters of gas and 103 million cubic meters (860 million barrels) of oil, 60% of which was exported to the use USA. The length of operated pipelines was over 100,000 km [15]. These two fossil fuel technologies (Fig. 1) are characterized by their pipeline transportation efficiencies. To evaluate and compare the energy consumption and environmental impact of natural gas and crude oil pipeline transportation, equal lengths of pipelines are considered. Pertinent average data are compiled [16–19] and calculated (see Table 3). Mechanical work or electricity required to perform pipeline transportation is assumed produced by a gas turbine unit with an average efficiency gt = 0.33 [16]. In a gas turbine unit, the heat of natural gas combustion is converted into mechanical

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work and emissions of various gases occur. The average values of emission of gases from natural gas combustion are given in Table 4. 2.1.1. Natural gas pipeline transportation The heat of natural gas combustion is converted in gas turbine units into mechanical work to compress gas to transport it along the pipeline. This mechanical work is consumed for the initial compression of natural gas from 1 atm to pmax and to overcome friction resistance. The consumption of the natural gas energy (Ecmp ) has been evaluated by the formula for isothermal compression: Ecmp =

nRT 0 (ln(pmax ) + k ln(pmax /pmin )), cmp gt

(10)

where n is the molar flow of natural gas, R = 8.314 J/mol K is the universal gas constant, and k is the number of intermediate compressions needed. Applying the ideal gas model to the molar flow of gas, n is calculated as n = pav V /RT 0 ,

(11)

where pav = (pmax + pmin )/2 is the average pressure in the pipeline between two intermediate compressions, V = (d 2 /4)w is the average volumetric flow of natural gas, d is the diameter of the gas pipeline, and w is the linear velocity. The number of the intermediate compressions is defined by applying of the Darcy–Weisbach formula for gas transportation: k = L/ l =

f Lw 2  , 2pfr d

(12)

where L is the length of pipeline, l is the distance between intermediate compressions, pfr = pmax − pmin is the pressure drop as a result of friction,  is the average density of gas, expressible as  = (patm , T0 )pav , f is the Fanning friction factor, which depends on quality of pipes and the Reynolds number Re which is equal to Re = wd/.

(13)

For smooth pipes and Re = 2.0 × 107 (calculated using data in Table 3), the friction factor f = 0.0074. The calculated number of intermediate compressions is k = 4.4. The loss of methane in pipelines is estimated by [20] as 1.4% of the stream. As a result, the fossil fuel energy consumed within the natural gas transportation system equals E = Edir + Eind ,

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Table 3 Typical characteristics for crude oil and natural gas pipeline transportation Characteristic

Natural gas

Crude oil

Velocity in pipeline w, m/s Diameter of pipeline d, m Length of pipeline L, m Viscosity of crude oil , mPa s Efficiency of isothermal compressors (NG) and pumps (CO) Maximal pressure in natural gas pipeline pmax , atm Minimal pressure in natural gas pipeline pmin , atm Reference − environment temperature T0 , K Energy of input flow, MJ/s Mass of pipeline, metric tonnes Embodied energy in pipeline, GJ Lifetime of pipeline, years Embodied energy in compressors and pumps, GJ Lifetime of pumps/compressors, years

7.0 0.8 1.0 × 106 0.011 0.65 70.0

2.0 0.4 1.0 × 106 60.0 0.65 –

50.0



298 6914 126,102 4,337,912 80 1,513,728 20

298 90,849 64,739 2,227,022 80 4,014,163 20

Sources: [16–18].

Table 4 Typical emissions from natural gas combustion Pollutant

Emission, g/g (NG)

CO2 CO CH4 N2 O NOx SO2 Volatile organic compounds (VOC) Particulates

2.69 1.88 × 10−3 5.15 × 10−5 4.93 × 10−5 2.24 × 10−3 1.34 × 10−5 12.3 × 10−5 17.0 × 10−5

Source: [19].

E ≈ Edir , dir Ein ≈ Ein .

where Edir = 0.014V Qng + Ecmp

(14)

and Qng is the lower heating value of natural gas. In compliance with the energy conservation law, if the output energy is equal to Eout = V Qng

Epmp = (16)

Table 5 presents the resulting indirect energy consumption and indirect greenhouse gas emissions for the natural gas transportation stage. It has been assumed

(17)

2.1.2. Crude oil pipeline transportation If the electricity or mechanical work is produced by a gas turbine unit and used by a pump to provide the crude oil transportation, the fuel energy consumption can be evaluated by the following formula of the pump capacity:

(15)

dir is the input direct energy Ein dir Ein = Eout + Edir .

that the operation energy to install, maintain and operate the equipment is equal to the embodied energy which is consumed to produce it. Table 6 presents the resulting direct energy consumption. The direct greenhouse gas emissions are associated with the loss of methane (natural gas) along the transportation process and its combustion in the gas turbine unit to generate mechanical work to perform the gas transportation. It can be seen in Tables 5 and 6 that the values for indirect energy and greenhouse gas emissions are negligible compared to the same values for direct energy:

V p , pmp gt

(18)

where Epmp is the fuel energy consumed by the pump, V = (d 2 /4)w is the volumetric flow of oil, pmp is the average efficiency of the oil pumps, and p is the pressure transferred from the pump to the transported oil.

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Table 5 Indirect energy and greenhouse gas emissions for natural gas and crude oil pipeline transportation Indirect energy of material and equipment in natural gas pipeline

Embodied energy per second of lifetime (MJ/s)

Embodied energy intensity, 1992$/MJ

Energy equivalent per second of lifetime (MJ/s)

CO2 -equiv. emission (g/s)

Natural gas pipeline Natural gas compressors Indirect energy of installation, operation, etc. in natural gas pipeline Natural gas pipeline and compressors Total indirect energy and greenhouse gas emissions per MJ of natural gas transported 1 MJ/s of Eout (natural gas) Indirect energy of material and equipment in crude oil pipeline

1.72 2.4 Operation energy per second of lifetime (MJ/s)

0.0201a 0.0980b

10.9 74.4

123.5 176.7 CO2 -equiv. emission (g/s)

Crude oil pipeline Crude oil pumps Indirect energy of installation, operation, etc. in crude oil pipeline Crude oil pipeline and pumps Total indirect energy and greenhouse gas emissions per MJ of crude oil transported 1 MJ/s of Eout (crude oil)

4.12

300.2

Total indirect energy per second of lifetime (MJ/s) 0.013 Embodied energy per second of lifetime (MJ/s) 0.88 6.37 Operation energy per second of lifetime (MJ/s) 7.25 Total indirect energy per second of lifetime (MJ/s) 0.0023

Total CO2 -equiv. emission (g/s) 0.086 CO2 -equiv. emission (g/s)

Embodied energy intensity, 1992$/MJ 0.0201 0.0980

Energy equivalent per second of lifetime (MJ/s) 5.60 197.5

63.4 469.0 CO2 -equiv. emission (g/s) 532.4 Total CO2 -equiv. emission (g/s) 0.0118

a From Ref. [11] for sector: blast furnaces and steel mills. b From Ref. [11] for sector: pumps and compressors.

Table 6 Direct energy and direct greenhouse gas emissions for crude oil and natural gas pipeline transportation dir Direct energy and greenhouse Ein gas emissions per (kJ/s) MJ of natural gas and crude oil transported

1 MJ/s of Eout (natural gas) 1 MJ/s of Eout (crude oil)

Edir CO2 -equiv.  (%) (kJ/s)

1095.3 95.3 1016.7 16.7

emission (g/s)

10.19 0.90

91.3 98.4

E = Edir + Eind ,

In the general case of pipeline oil transport, p is mainly caused by the pressure drop pfr due to friction resistance. Thus, p ≈ pfr , where pfr can be evaluated by the Darcy–Weisbach formula: p ≈ pfr =

f Lw 2  , 2d

where L is the length of pipeline, d is the diameter of the crude oil pipeline, w is the linear velocity,  is the density of crude oil, and f is Fanning friction factor, which depends on the quality of pipes and the Reynolds number Re. For smooth pipes, Re = 11.3 × 103 and the respective friction factor f = 0.0306. The energy consumed in the crude oil transportation system is

(19)

where Edir = Epmp .

(20)

By the energy conservation law, if the output energy is equal to Eout = V Qoil ,

(21)

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2

2

2

4

2 2

2

2

2

2

2 4

2

Fig. 3. Simplified process for hydrogen production through natural gas reforming.

where Qoil is the lower heating value and  the density of crude oil, the input direct energy is dir Ein = V Qoil + Edir .

(22)

Table 5 presents the resulting indirect energy consumption and indirect greenhouse gas emissions associated with the crude oil transportation stage. It has been assumed that the operation energy to install, maintain and operate the equipment is equal to the embodied energy to make it. Table 6 presents the resulting direct energy. The direct greenhouse gas emissions are only associated with the methane combustion in a gas turbine unit to generate energy Epmp to transport the oil. As in the case of natural gas transport, the values of indirect energy and indirect greenhouse gas emissions are negligible compared to the values for direct energy:

4 2

4

2

Fig. 4. Energy balance for hydrogen production through natural gas reforming [21].

E ≈ Edir ≈ Epmp , dir Ein ≈ Ein .

(23)

2.2. Natural gas reformation and crude oil distillation 2.2.1. Reformation of natural gas Natural gas reforming to produce hydrogen is often the first stage in large-scale manufacturing of ammonia, methanol and other synthetic fuels. A simplified diagram of hydrogen production by natural gas reforming is presented in Fig. 3. The sum of the reactions to produce hydrogen through natural gas reforming is the following: t≈950 ◦ C

CH4 + 2H2 O −→ 4H2 + CO2 − 165 kJ.

(24)

As seen in Eq. (24), a high-temperature heat supply is necessary for reforming. The required heat is provided by the heat released from natural gas combustion, according to CH4 + O2 → CO2 + 2H2 O + 802 kJ.

(25)

Table 7 Hydrogen plant material requirements (base case), assuming a 20year lifetime and 1.5 million Nm3 /day hydrogen production capacity (hydrogen LHV production 187.562 MJ/s) Material

Amount required (metric tonnes)

Embodied energy (GJ/t)

Concrete Steel Aluminium Iron

10,242 3272 27 40

1.4 34.4 201.4 23.5

Source: [22].

Neglecting the energy consumption for CH4 feed desulphurization, an energy balance for this hydrogen production process was presented (Fig. 4) by Rosen [21]. The calculations here of direct energy consumption and greenhouse gas emissions are based on this diagram. The overall results for the reforming and transportation stages are presented in Table 10. The indirect energy evaluation is based on the data given in [22]. In Table 7, the material requirements of

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345

Table 8 Energy equivalents and greenhouse gas emissions for a natural gas reforming plant

(Edir ) and the indirect greenhouse gas emissions comprise only 0.5% from the direct ones.

Indirect energy of materials in natural gas reforming plant

Embodied energy consumption per second of lifetime (MJ/s)

Embodied energy intensity, 1992$/MJ

Energy CO2 -equiv. equivalent per emission second of (g/s) lifetime (MJ/s)

Concrete Steel Aluminium Iron Total

0.0227 0.178 0.00862 0.00149

0.0483a 0.0201b 0.0205c 0.0201b

0.347 1.135 0.0559 0.00948 1.548

2.2.2. Crude oil distillation Crude oil is directed from the oil field to a refinery plant where, by means of mechanical, physical and chemical processes, it is converted to a finished fuel—gasoline and numerous other valuable products. A comprehensive LCA of an entire refinery is not available, likely because of the wide variety of processes involved. The energy efficiency and environmental impact to produce 1 MJ of gasoline have been estimated (Table 10) according to the energy consumption and carbon-dioxide emission data of all petroleum refineries in the US in 1996 [23] (Table 11). The internal resources and internal resource energy (Eir ) in Table 11 represent the fuel products which are obtained from crude oil and then burned to drive refinery processes. The values of the direct energy consumption and the direct greenhouse gas emissions for crude oil transportation and distillation processes are evaluated by taking into account the efficiency of crude oil and natural gas transportation and applying the energy conservation law:

8.44 12.82 0.569 0.107 21.94

a From Ref. [11] for sector: concrete products, except block and brick. b From Ref. [11] for sector: blast furnaces and steel mills. c From Ref. [11] for sector: primary aluminium.

Table 9a Energy for installation, operation, etc., and respective greenhouse gas emissions for the natural gas reforming plant Energy for installation, operation, etc. (MJ/s)

CO2 -equiv. emission (g/s)

0.211

21.9

Table 9b Indirect energy and indirect greenhouse gas emissions for the natural gas transportation and reforming stages Indirect energy for natural gas Indirect energy Total CO2 -equiv. reforming and transportation (MJ/s) emission (g/s) processes (energy of natural gas flow: 217.9 MJ/s based on LHV) Natural gas reforming Natural gas transportation Total 1 MJ/s of Eout (hydrogen)

1.759 2.833 4.59 0.024

43.8 18.7 62.5 0.333

the natural gas reformation plant are presented. The values of the energy embodied in the materials and greenhouse gas emissions which accompany this kind of energy consumption have been taken from [12]. In Tables 8 and 9a, 9b, the indirect energy calculations are presented. The same assumption, that the operation energy and emission to install, maintain and operate the equipment is equal to its embodied energy, has been applied. Comparing the values of direct and indirect energies and their emissions, it can be observed that indirect energy (Eind ) is more than the 10 times less than direct one

Eoil = GQoil , dir Ein = Eoil + Eng + Eel /gt , Eout = Eoil − Eir , dir Edir = Ein − Eout .

(26)

Here, G, Qoil and Eoil denote mass flow rate, lower heating value and energy content of crude oil, respectively; Eng is the energy content of natural gas, Eel is dir is the the electricity required to refine the crude oil, Ein input flow of fossil fuel energy, and Eout is the output flow of fossil fuel. Accounting for the fact that refinery production is dominated by production of gasoline and different fuels (more than 80%), we assume that the energy consumption is proportionally distributed between all refinery products depending on their energy contents (lower heating values). In this case the energy losses are attributed to refinery products and, in a first approximation, to gasoline. The resulting values are presented in Table 10 (second row). The greenhouse gas emissions are calculated as the sum of values for the crude oil refinery and the crude oil and required natural gas transport processes. No data are available to calculate the indirect energy consumption for the crude oil refinery process. However, as seen in [24], the capital cost of crude oil

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Table 10 Total direct energy and direct greenhouse gas emissions for natural gas and crude oil transportation and reforming (distillation) processes Direct transportation and reforming energy per MJ of hydrogen and gasoline

dir Ein reforming (MJ/s)

dir Ein total (MJ/s)

Edir (MJ/s)

CO2 -equiv. emission (g/s)

 (%)

1 MJ/s of Eout (hydrogen) 1 MJ/s of Eout (gasoline)

1.162 1.151

1.273 1173

0.273 0.173

75.7 12.1

78.5 85.3

distillation is lower than that for natural gas reforming. Therefore, it can be concluded that, as in the case of the natural gas reforming, the indirect energy and indirect greenhouse gas emissions are negligible compared to direct ones.

Table 11 Basic parameters of oil refinery process in US, where capacity G is 58.3 × 1010 kg crude oil per year over 355 stream days

2.3. Wind and solar energy utilization to produce hydrogen

Eir —internal energy resources (refinery gas, residual fuel oil, liquid petroleum gases, etc.) Eng —energy of natural gas (including energy of purchased steam and a little coal) Eel —electricity

Hydrogen can also be produced using renewable wind and solar energies to generate electricity and then electrolysis of water to produce hydrogen. In these cases there are no direct fossil fuel energy consumption and direct greenhouse gas emissions, and only the indirect energy is responsible for the environmental impact. 2.3.1. Renewable hydrogen production via wind energy The energy of wind is converted to mechanical work by wind turbines and then transformed by an alternator to AC electricity which is transmitted to the power grid (Fig. 2). The efficiency of wind turbines depends on the location. Applications of wind energy normally make sense only in the areas with a high wind activity. The data for a 6 MW wind power generation plant was taken from [25] for use here. Tables 12 and 13 present the material requirements, indirect energy consumption, indirect greenhouse gas emissions for this plant. Based on data in [26] for electrolysis to produce hydrogen with a 72% efficiency (on an LHV basis), accounting for 7% electricity loss during transmission, the efficiency of hydrogen production is 66.9%. The indirect energy and greenhouse gas emissions are 6.61% and 5.64%, respectively, of those for a wind power generation plant [26]. Thus, a 6 MW wind power plant combined with electrolysis of water could produce 4.01 MJ/s of hydrogen. Table 14a, b presents the values of indirect energy and indirect greenhouse gas emissions of hydrogen production through the wind power plant and water electrolysis. The efficiency of fossil fuel energy consumption

Energy source

Quantity, 1012 kJ

CO2 -equivalent emissions (metric tonnes)

2197.2

170349.0

887.8

51049.5

117.4

22497.9

 reaches 3.46, meaning that the consumed fossil fuel energy (embodied in materials, equipment, etc.) is 3.46 less than the energy of hydrogen produced. This value should not be confused with the energy efficiency of wind power generation plants, which is about 12–25% and usually calculated as the ratio of electricity produced to the sum of all sources of input energy (mainly kinetic energy of wind). 2.3.2. Renewable hydrogen production via solar energy The indirect energy consumption and indirect greenhouse gas emissions of a photovoltaic system have been evaluated. The photovoltaic elements convert solar energy into direct current (DC) electricity, which is transformed by inverters to alternating current (AC) electricity and transmitted to the power grid. At fuelling stations, AC electricity is used to electrolyze water to produce hydrogen (Fig. 2). Data for a 1.231kW (average) building-integrated photovoltaic system in Silverthorne, Colorado [18], which utilizes thin-film amorphous silicon technology, are considered here. Tables 15 and 16a, b present the material requirements, indirect energy consumption and indirect greenhouse gas emissions for hydrogen production through photovoltaic power generation and

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Table 12 Material requirements and corresponding greenhouse gas emissions for a 6 MW wind power generation plant Material

Amount required (metric tonnes)

Embodied energy (GJ/t)

CO2 -equiv. emission (kg/ton of material)

Concrete Copper Fibreglass Steel-carbon/low alloy Steel-stainless

7647.3 5.275 496.6 1888.0 226.2

1.4 131 13 34.4 53

520 7450 804 2471 3280

Source: [26].

Table 13 Indirect energy and greenhouse gas emissions of wind power generation plant, for capacity of 6 MW and lifetime of 25 years Indirect energy of materials and equipment

Embodied energy consumption per second of lifetime (MJ/s)

Embodied energy intensity, 1992$/MJ

Energy equivalent per second of lifetime (MJ/s)

CO2 -equiv. emission per second of lifetime (g/s)

Concrete Copper Fibreglass Steel-carbon/low alloy Steel-stainless Total

0.0136 0.000876 0.00819 0.0824 0.0152

0.0483a 0.0412b 0.045c 0.0201d 0.0201d

0.208 0.0114 0.117 0.524 0.0967 0.956

5.044 0.0498 0.506 5.922 0.940 12.462

Total indirect energy and greenhouse gas emissions of wind power generation plant

Energy for installation, operation, etc. (MJ/s)

CO2 -equiv. emission (g/s)

Total indirect energy (MJ/s)

Total CO2 -equiv. emission (g/s)

6 MW wind power generation plant

0.131

13.60

1.087

26.06

a From b From c From d From

Ref. Ref. Ref. Ref.

[11] [11] [11] [11]

for for for for

sector: sector: sector: sector:

concrete products, except block and brick. primary smelting and refining of copper. glass and glass products, except containers. blast furnaces and steel mills.

water electrolysis. A procedure similar to that used for the wind power plant is applied to evaluate the indirect energy and greenhouse gas emissions, which are associated with electrolysis. Taking into account the efficiency of electrolysis and transmission losses, the 1.231-kW photovoltaic system combined with water electrolysis can produce 823.5 J/s of hydrogen energy. 2.4. Hydrogen compression The density of hydrogen at standard conditions is low. To assist in storage and utilization as a fuel, the density is increased via compression. The direct fossil fuel (natural gas) energy consumption E to compress isothermally 1 mol of hydrogen can be expressed,

assuming ideal gas behaviour, as Edir =

RT 0 ln(pmax /pmin ). cmp gt

(27)

Here, T0 = 298 K is the standard environmental temperature and R = 8.314 kJ/mol K is the universal gas constant. Also, cmp denotes isothermal compression efficiency and gt gas-turbine power plant efficiency. The direct energy consumed and direct greenhouse gas emissions to compress hydrogen are evaluated (see Table 17), assuming an isothermal compression efficiency cmp of 0.65 and a typical gas-turbine power plant efficiency gt of 0.33. A typical maximal pressure pmax = 200 atm [27] has been considered. Minimal pressures before compression of pmin = 1 and 20 atm

348

M. Granovskii et al. / International Journal of Hydrogen Energy 31 (2006) 337 – 352

Table 14 Indirect energy and indirect greenhouse gas emissions for (a) hydrogen production via a wind power plant and water electrolysis and for (b) per 1 MJ of hydrogen produced via wind power generation plant and water electrolysis (a) Indirect energy and greenhouse gas emissions to produce hydrogen utilizing wind energy 6 MW wind power generation plant Electrolysis Total (b) Indirect energy and greenhouse gas emissions per 1 MJ of hydrogen produced 1 MJ/s of Eout (hydrogen)

Eind (MJ/s)

CO2 -equiv. emission (g/s)

1.087 0.072 1.159

26.06 1.47 27.5

Eind

CO2 -equiv. emission (g/s)

 (efficiency of fossil

(MJ/s)

0.289

6.85

3.46

fuel energy consumption)

Table 15 Indirect energy and greenhouse gas emissions for 1231-kW thin film photovoltaic system, with 157.2 m2 of surface area and lifetime of 30 years Indirect energy of the solar cell unit

Embodied energy (MJ/m2 )

Embodied energy intensity, 1992$/MJ

Energy equivalent (MJ/m2 )

Embodied energy in manufacturing (MJ/m2 )

Embodied energy in solar cell block (GJ/unit)

Encapsulation Substrate Deposition materials Busbar Back reflector Grid Conductive oxide Total

0.2119 × 103 0.0256 × 103 0.0188 × 103 0.0051 × 103 0.0007 × 103 – –

0.0498a 0.0201b 0.0498a 0.0498a 0.0498a – –

3.338 × 103 0.163 × 103 0.296 × 103 0.0803 × 103 0.011 × 103 – –

0.1372 × 103 0.0564 × 103 0.0925 × 103 0 0.074 × 103 0.0342 × 103 0.0969 × 103

546.27 34.46 61.09 12.63 13.37 5.38 15.23 688.44

Indirect energy and greenhouse gas emissions of system

Embodied energy (MJ/unit)

Embodied energy intensity, 1992$/MJ

Energy equivalent (GJ/unit)

CO2 -equiv. emission (kg/unit)

CO2 -equiv. emission (g/s)

Inverters Wiring Solar cell block Total

40 × 103 2.9 × 103

0.0880c 0.0509d

111.4 46.7 688.4 846.5

290 180 7320 7790

0.000306 0.000190 0.00774 0.00823

Total indirect energy and green house gas emission for 1.231 kW thin film PV system

Energy for installation, operation etc. (J/s)

CO2 -equiv. emission per second of lifetime (g/s)

Energy equivalent per second of lifetime (J/s)

Total indirect energy per second of lifetime (J/s)

Total CO2 -equiv. emission per second of lifetime (g/s)

1231 J/s electricity generation

78.957

0.00498

894.81

973.77

0.0132

a From b From c From d From

Ref. Ref. Ref. Ref.

[11]. [11]. [11]. [11].

Sector: Sector: Sector: Sector:

primary nonferrous metals. blast furnaces and steel mills. wiring devices. miscellaneous fabricated wire products.

M. Granovskii et al. / International Journal of Hydrogen Energy 31 (2006) 337 – 352

349

Table 16 Indirect energy and indirect greenhouse gas emissions for (a) hydrogen production via solar energy utilization and water electrolysis and (b) per 1 MJ of hydrogen produced via solar energy utilization and water electrolysis (a) Indirect energy and greenhouse gas emissions to produce hydrogen utilizing solar energy

1.231-kW photovoltaic system Electrolysis Total (b) Indirect energy and greenhouse gas emissions per 1 MJ of hydrogen produced

1 MJ/s of Eout (hydrogen)

Eind

CO2 -equiv. emission

(J/s)

(g/s)

973.7 64.4 1038.1

0.0132 0.0007 0.0139

Eind

CO2 -equiv. emission

(J/s)

(g/s)

1261 × 103

16.88

Table 17 Direct energy consumed and direct greenhouse gas emissions to compress hydrogen Direct energy and greenhouse gas emissions to compress hydrogen Hydrogen compression from 1 to 200 atm (wind and solar energy) 1 MJ/s of Eout (hydrogen compressed) Hydrogen compression from 20 to 200 atm (natural gas reforming) 1 MJ/s of Eout (hydrogen compressed)

Edir (MJ/s)

CO2 -equiv. emission (g/s)

 (%)

0.253

13.7

79.8

0.110

5.94

90.1

are taken for hydrogen production through electrolysis and natural gas reformation [22], respectively. 2.5. Hydrogen and gasoline distribution Hydrogen distribution is replaced by electricity distribution in cases using wind and solar energy (seeFig. 2) and such distribution has been accounted for

H2 Container H2 Container H2 Container

 (efficiency of fossil fuel energy consumption)

0.79

Table 18 Direct energy consumed and greenhouse gas emissions to distribute hydrogen and gasoline to refueling stations Direct energy and greenhouse gas emissions to distribute hydrogen and gasoline

Edir

CO2 -equiv.  (%) (MJ/s) emission (g/s)

1 MJ/s of Eout 0.041 3.13 (hydrogen compressed, p = 200 atm) 0.0025 0.19 1 MJ/s of Eout (gasoline)

99.75

in hydrogen production. The distribution of compressed hydrogen after its production via natural gas reforming (Fig. 5) is similar to that for liquid gasoline, but compressed hydrogen is characterized by a lower volumetric energy capacity and higher material requirements for a hydrogen tank. According to the 1997 Vehicle Inventory and Use Survey, the average “heavy–heavy” truck in the US ran 6.1 miles per gallon of diesel fuel [28]. The direct fuel (diesel) energy consumption, assuming a distance of 300 km is travelled before refuelling for a truck with a 50 m3 tank, has been evaluated (see Table 18). Trucks

Cars

Trucks

Cars

Trucks

Cars

H2 Container H2 Container

96.1

Fig. 5. Illustration of hydrogen distribution from a plant for hydrogen production through natural gas reforming.

350

M. Granovskii et al. / International Journal of Hydrogen Energy 31 (2006) 337 – 352

The direct energy consumption and greenhouse gas emissions are associated with the combustion of the diesel fuel.

The results are summarized illustratively in Figs. 6–10. Fig. 6 presents the fossil fuel energy consumption to produce 1 MJ of gasoline and 1 MJ of hydrogen applying different technologies. The production of gasoline from crude oil is seen to be more efficient than the production of hydrogen from natural gas. Crude oil and natural gas are the main energy commodities used in these processes. The use of renewable technologies

70

50 40 30

10 0 Gasoline

Hydrogen-nat.gas Hydrogen-wind

Hydrogen-solar

Fig. 8. Greenhouse gas emissions accompanying the production and utilization of 1 MJ (based on LHV) of gasoline and hydrogen in ICE and PEMFC vehicles, respectively.

500 initial fossil fuels fuels distribution hydrogen compression fuels production

1200

gasoline hydrogen from natural gas hydrogen from wind energy hydrogen from solar energy

400 CO2-equivalent, g/MJ

1400 Energy Consumption, kJ

60

20

1600

1000 800 600 400

300

200

100

200 0

fuels utilization fuels distribution hydrogen compression fuels production

80 CO2-eqivalent, g

3. Results and discussion

90

Gasoline Hydrogen-nat.gas Hydrogen-wind Hydrogen-solar

Fig. 6. Fossil fuel energy consumption to produce 1 MJ (based on LHV) of gasoline and hydrogen for utilization in ICE and PEMFC vehicles.

0 0.2

0.3

0.4 Efficiency

0.5

0.6

Fig. 9. The life cycle greenhouse gas emissions corresponding to 1 MJ of mechanical work generated in a vehicle, as a function of ICE (gasoline) or PEMFC (hydrogen) engine efficiency.

Mechanical Work, MJ

1.2 gasoline hydrogen from natural gas hydrogen from wind energy hydrogen from solar energy

1 0.8 0.6 0.4 0.2 0 0.2

0.25

0.3

0.35 0.4 0.45 Efficiency

0.5

0.55

0.6

Fig. 7. Mechanical work produced, as a function of ICE (gasoline) and PEMFC (hydrogen) vehicle efficiency, corresponding to the fossil fuel energy used to produce 1 MJ (based on LHV) of gasoline and hydrogen.

for hydrogen production is characterized by the absence of the direct fossil fuel use. Hydrogen production via wind power and electrolysis has the lowest indirect fossil fuel energy use. The photovoltaic system has the highest fossil fuel energy embodied in materials and equipment. The chemical energies of gasoline and hydrogen are converted to work with different efficiencies in an internal combustion engine (ICE) vehicle and a PEMFC vehicle. The efficiency ranges from 0.2 to 0.3 for an ICE engine [16] and from 0.4 to 0.6 for a PEMFC engine [1]. Fig. 7 shows the mechanical work produced per unit fossil fuel energy consumption as a function of engine efficiency. This figure indicates that the efficiency of a PEMFC vehicle operating on hydrogen from

M. Granovskii et al. / International Journal of Hydrogen Energy 31 (2006) 337 – 352

materials requirements of wind per unit of electricity generated. A fair assessment when comparing different renewable technologies requires consideration of both energy efficiency (ability to convert renewable energy into mechanical work or electricity) and the efficiency of construction materials and equipment exploitation.

35 hydrogen from natural gas hydrogen from wind energy hydrogen from solar energy

30 25

351

γ

20 15 10

4. Conclusion

5 0 1

1.5

α

2

Fig. 10. Capital investment efficiency  for several hydrogen production processes as a function of the cost ratio  for hydrogen and natural gas.

natural gas must be at least 25–30% greater than that for an ICE gasoline engine to be competitive. Treating wind energy as free, hydrogen via wind is characterized by a high (more than 1) ratio of mechanical work production per unit fossil fuel energy consumption. The diagram in Fig. 8 illustrates the greenhouse gas emissions for the life cycles of fuels, from production to utilization in vehicles. Since the use of 1 MJ of gasoline is accompanied by the release of 71.7 g of CO2 , gasoline and hydrogen from natural gas are seen to be characterized by almost the same greenhouse gas emissions. Hydrogen production via wind and solar energy reduces greenhouse gas emissions considerably. Accounting for the higher efficiency of a PEMFC engine (Fig. 9), greenhouse gas emissions can be reduced from 40% where hydrogen is obtained from natural gas to 85% where hydrogen is obtained from renewable wind energy. Technologies for hydrogen production via wind and solar energy are not more widespread due to economic challenges. To better understand these challenges, we consider the capital investments efficiency  introduced in Eq. (7). Fig. 10 presents , as a function of the cost ratio  for hydrogen and natural gas, for energy efficiencies of  = 0.785 for hydrogen via natural gas (Table 10), of  = 3.46 for hydrogen via wind energy (Table 14) and of  = 0.79 for hydrogen via solar energy (Table 16). Since the cost of 1 MJ of hydrogen (on the basis of LHV) is presently about two times more than that of natural gas [29], it follows from Fig. 10 at  = 2 that the capital investment needed to produce hydrogen via natural gas is about five less than that needed to produce hydrogen via wind energy. This situation could be altered by reducing the construction

Life cycle assessment has been used to compare hydrogen-fuelled PEMFC vehicles to traditional internal combustion engine vehicles operating on gasoline. Energy efficiencies and greenhouse gas emissions have been evaluated during all process steps, including crude oil and natural gas pipeline transportation, crude oil distillation and natural gas reformation, wind and solar electricity generation, hydrogen production through water electrolysis, and gasoline and hydrogen distribution and utilization. The use of wind power to produce hydrogen via electrolysis, and its application in a PEMFC vehicle, is characterized by the lowest greenhouse gas emissions and fossil fuel energy consumption. However, the economic attractiveness of wind technology depends significantly on the ratio in costs for hydrogen and natural gas. At a cost ratio of 2, capital investments are about five times lower to produce hydrogen via natural gas than to produce hydrogen via wind energy. We plan to analyse in the future the air pollutants for the technologies considered here to evaluate the relationship between their environmental impact and economic attractiveness.

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