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An LCA based indicator for evaluation of alternative energy routes
Applied Energy 88 (2011) 630–635
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
An LCA based indicator for evaluation of alternative energy routes M.A. Rubio Rodríguez a,⇑, J. De Ruyck b, P. Roque Díaz a, V.K. Verma b, S. Bram b,c a
Universidad Central ‘‘Marta Abreu” de Las Villas, Carretera a Camajuani km 5, 54830 Santa Clara, Cuba Department of Mechanical Engineering, Faculty of Applied Sciences, Vrije Universiteit Brussel (VUB), Pleinlaan 2, 1050 Brussel, Belgium c Department of Industrial Sciences and Technology, Erasmushogeschool Brussel, Nijverheidskaai 170, 1070 Brussel, Belgium b
a r t i c l e
i n f o
Article history: Received 3 February 2010 Received in revised form 7 August 2010 Accepted 13 August 2010 Available online 15 September 2010 Keywords: Energy system Energy route Energy sustainability LCA ELCA
a b s t r a c t Politicians and policymakers are searching for alternatives for current energy conversion systems in order to reduce environmental pollution whilst preserving scarce natural resources. When defining new alternatives it is important to find out how environmentally friendly and sustainable these alternatives are, and which of them will ensure a major improvement in environmental issue. To answer such questions the present work proposes a new type of indicator based on exergy life cycle data. Alternative energy routes towards different services and commodities are compared in terms of environmental impact and indirect natural resource costs. Two case studies are presented using data from the Ecoinvent database (v2.01 2007) to compare alternatives to petrol transport and fuel oil power generation. The alternative transport routes assessed were methanol and biogas fueled cars, and wind electricity in the case of power generation. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Mankind is realizing that dependence on fossil fuels can no longer be sustained. Consequently many studies have been performed to plan the way back to the usage of renewable energy sources [1–7]. Care must however be taken to go for alternatives which offer the highest degree of sustainability in order to use the limited resources in the most optimal way. One trend has been the search for a sustainability function that might be applied in parallel with a classical economic function. First there is the category of indicators and indexes, integrated or not, which represent a state of economic, social and environmental development in a defined region, often the national level [8–10], secondly there are those product-related assessment tools with the focus on the material and/or energy flows of a product or service from a life cycle perspective [10–12]. Within the frame of the second approach, present work has been developed. A wide collection of those product-related assessment tools are implemented on Ecoinvent database [13]. Some of these assessment tools are CML 2001, eco-indicator 99, ecological-footprint, and some others like cumulative exergy demand and cumulative
Abbreviations: CExC, cumulative exergy consumption; CExD, cumulative exergy demand; EI, environmental impact; ELCA, exergy life cycle assessment; LCA, life cycle assessment; LCI, life cycle impact; LCIA, life cycle impact assessment; SPA, system perturbation analysis. ⇑ Corresponding author. Tel.: +53 42281194. E-mail address: [email protected] (M.A. Rubio Rodríguez). 0306-2619/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2010.08.013
energy demand. All these indexes are also formed by subcategories which are calculated using linear weight functions. These assessments aim to quantify specific environmental troubles as well as the total impact. The main disadvantage of these assessments is the use of weight function which is obtained by subjective procedures like experts opinions. On the other hand exergy life cycle based indicators have been proposed not only as a measure for economic losses and dematerialization, but for waste accounting and ecotoxicity as well. The reason for this is that exergy embodied in resources, products, and waste materials, has the potential to cause change in both the industrial environment and the natural ecosystem [14]. The advantages of use exergy as measurement of environmental impact are widely recognized [14,15]. But the use of the exergy entering the techno-sphere as base of net exergy efficiency and the no distinction between renewable and non-renewable primary resources bear to a contradiction due to fact that ‘‘their main disadvantage (renewable energy systems) lies in their incapability to take advantage of a big part of the available energy” [16]. So an increment in the renewability share of primary resources could be canceled by a decrease of net exergy efficiency, despite the fact that probably it would be preferable to decrease the use of non-renewable resources in spite of an efficiency decrement. This investigation aims at demonstrating the convenience of a quite different exergy indicator, based on indirect exergetic cost and environmental exergetic cost, when it is necessary to assess the exergetic sustainability of energy transformation strategies to different final services or commodities. This new perspective
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would contribute to later elaborate a new optimization function for energy matrixes.
it to a number of polymers. These works represent the present concern on sustainability issue and the consensus around the successful use of exergy concept to face up the sustainability problem.
2. Methodology
2.2. Exergetic costs of sustainability
2.1. Background
It was possible to define three exergetic cost function for an energy transformation route, taking into account the life cycle exergy consumption and the exergy neutralization cost dependence on pollutant exergy. The first one was based on Dewulf’s idea [20] about a renewability parameter, the cumulative exergy consumption (CExC) of Szargut [18] and it was related to the service or good produced instead of the total exergy required. This cost can be expressed as:
The present sustainability measuring procedure for alternative energy transformation routes is based on four approaches supported on life cycle assessment (LCA) [17]: Szargut’s cumulative exergy consumption (CExC) [18], zero-exergy emission from Cornelissen [15,19], Dewulf’s renewability idea [20] and the indirect exergy consumption approach from Wall [4]. The first approach was the cumulative exergy consumption (CExC) from Szargut [18] and its implementation as a life cycle assessment (LCIA) on the Ecoinvent database [21]. In order to quantify the life cycle exergy demand of a product, cumulative exergy demand (CExD) is defined as the sum of exergy of all resources required to provide a service or product [22]. CExD is equivalent to the definition of CExC, both quantifying the total exergy requirement of a product. Szargut calculated CExC by adding up the total exergy requirement of a process over a defined period of time. The exergy requirement of one unit of process output was then obtained by dividing the total exergy requirement by the number of unit outputs during the concerned period. The emergence of large life cycle databases such as Ecoinvent enables and facilitates a product-specific approach, since such databases provide the resource demand for each unit process. The second approach is the zero-exergy emission from Cornelissen also used by Sciubba’s extended exergy accounting [15,19]. This work addressed the accounting of pollution impact based on exergy approach. The essence of this idea is that: ‘‘the potential environmental impact of an effluent is represented by the cumulative amount of exergetic resources that must be consumed to attain an ideal, zero-impact disposal of both the effluent itself and the equipment that handles it”. Respecting this issue researchers have been looking for relation between pollutants exergy and environmental damage, and a qualitative relation has been found, but more research is needed to determine a quantitative relation [23]. On the issue of sustainability measuring Dewulf’s works and Ocaña’s PhD thesis [11] will be used as Refs. [14,20]. From both authors exergy accounting idea has been retained to achieve consistency owing to the possibility of this thermodynamic function to express disequilibrium and potential interaction with surroundings. This includes the notion of renewability index and efficiency as important factors on sustainability issues. Finally he indirect exergy concept proposed by Wall will be applied. This exergy would be whole exergy which is not the one that is directly transformed to obtain the final energy service [4]. An important concept that will be followed along this paper, used in Bram’s work [24], is the concept of energy route or energy path. This route or path was defined as all transformations necessary to convert a primary natural resource into a final service or commodity. For instance it is the path from crude oil under ground as joules of exergy, to transported passengers as passenger-kilometer, or the route from energy in the wind as joules, to electricity served at consumer as Watt–hour. Many other references on the sustainability field and exergy analysis were considered such as Hoang’s work [25] which proposed the notion of sustainability efficiency (SE) as the ratio of the minimum total cumulative exergy content (TCExC) to an observed TCExC. Another latest attempt to overcome de sustainability problem applied to the polymer manufacturing can be found in Dewulf’s work [26]. Dewulf proposed a cumulative degree of perfection (CDP) as the cumulative exergy efficiency and applied
kF ¼ ðCExDF Þ=ðFmÞ ½J=fu
ð1Þ
where kF is the fossil exergetic cost in joule of fossil cumulative exergy, CExDF is the fossil part of CExD and Fm is a proper functional magnitude chosen as a final desired product or service, for instance passengers-kilometer, tons-kilometer, kW h, etc.; and fu is the corresponding functional unit. The same concept was used by Ocaña [11] to create a fossil exergy consumption index for a system, but it accounted only for the exergy of energy flows in the assessed system lacking of the life cycle approach of the CExD concept which considered the cumulative amount of exergy ‘‘embodied” in the product or service by the successive contribution of each step in the production chain [14]. The second exergetic cost was based on the CExD of renewable resources. This is similar to the previous but accounting for the renewable portion of the CExD, thus the expression is:
kR ¼ ðCExDR Þ=ðFmÞ ½J=fu
ð2Þ
where as it was said, CExDR is the renewable part of the life cycle exergy demand in resources. Consequently if both exergy costs are summed one obtains the cumulative exergy demand due to the consumption of all natural resources chargeable to a final service.
kC ¼ kF þ kR
½J=fu
ð3Þ
It would be valuable also to account for externalities due to the pollutants emission and associated environmental consequences. It is Cornelissen’s theory that to achieve a zero environmental impact, pollutants would have to be brought to both thermal- and chemical equilibrium with the surroundings, which can be technically achieved in several ways: but in any case, the exergetic cost of the zero-impact (i.e., its extended exergy) will be proportional to the physical exergy of the effluent. . . [23,27]. This approach has two main shortcomings. Firstly the proportional factor needed for an accurate evaluation of the exergy neutralization cost would be difficult to determine because of the various treatment process availability and the process that are not available for certain pollutants. Furthermore, the exergy cost of avoiding pollutants does not represent a precise measurement of the damage caused by those which would not be avoided. However, it is also important to notice that this idea would be a consistent incorporation of the effects of effluent treatment in the exergetic cost balance of an energy route. Moreover if the cumulative exergy by means of non-renewable primary natural resources have been accepted as environmental degradation measure, it would not be hard to assume the exergetic cost for zero environmental impact as a measure of pollution impact. Thus an expression to calculate an exergetic cost due to zero-exergy impact of an energy transformation chain is:
kP ¼ Rðmi yi Þ=ðFmÞ ½J=fu
ð4Þ
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where mi is the mass of each computable pollutant emitted during the life cycle of the energy route assessed and yi would be the exergy mitigation cost or zero-exergy cost for each pollutant. In spite of the lack of this term to be proportional to the real magnitude of environmental impact, it is obvious that kP = 0 implies a non-polluting scenario or technology. Now if the pollutant-impact exergetic cost (kp) is added to the previous defined exergetic cost of the energy route kR, a different concept is obtained.
Indirect exergetic cost represents resources that should be decided where to be invested. This is the cost in natural resources to run an energy route, natural resources that could be better to use on some route else. A comparison between this concept and the economic cost can be made and probably there would be a relation, but they are not equivalent consequently an economic assessment always has to be done in order to complement any kind of assessment.
kRT ¼ kF þ kR þ kP
2.3. Assessing alternative energy routes
½J=fu
ð5Þ
This total exergetic cost (kRT) is quite similar to the extended exergy accounting (EEA) from Sciubba [28] the main difference is that there is no accounting for labor. This was because to do so it would be necessary to make some considerations implying nonaccurate results. There is also the possibility to group just the zero-impact exergetic cost kP and the fossil exergetic cost kF, the new category could be called environmental exergetic cost and it would be part of the sustainability concept that is supported in this work, and the expression would be as follows:
A cardinal problem today is the assessment of substituting current fossil based energy routes by other more sustainable alternatives. It is also important to know how much each alternative route contributes to the sustainability and which of them are the most suitable to be implemented independently of the final service or product. The environmental exergetic cost (kE) was defined previously as a measure of the impact on sustainability of a certain energy route. So it was possible to define the variation in terms of this indicator when it is decided to asses an alternative route:
kE ¼ kF þ kP
DkE ¼ kE kE
½J=fu
ð6Þ
This term accounts for cumulative exergy from fossil fuels and that of zero-impact cost, both during the life cycle of one functional unit of final energy route. It is important to remark that the environmental exergetic cost kE embodies the influence of the life cycle efficiency because and increment of the efficiency without any other change, obviously decreases environmental exergetic cost kE. An idea supported by the authors, which is similar to that from Wall [4], is represented in Fig. 1. The additional primary resources are those which indirectly enter the considered system during the transformation process of a primary resource, which itself is directly related with the supply of an energy service either inside the techno-sphere or the society. The difference in the present work is that Wall’s idea was adapted to be implemented using LCI databases which already accounts for cumulative exergy demand of several processes. According to the concepts presented above, the category that will be called indirect exergetic cost (kT) was obtained by subtracting the exergy of the primary resource directly related with the energy route itself from the CExC accounted for the route, that is:
kT ¼ kC BPR =Fm ½J=fu
ð7Þ
where BPR is the exergy of the primary resource directly related with the energy route.
Environmental impact (EI)
Final Energy Service
Energy
route
Directly Primary resources
Additional primary resources for transformation Indirectly
Society
Techno-sphere
Nature Fig. 1. Energy route model.
0
½J=fu
ð8Þ
where kE is the environmental exergetic cost of the alternative route 0 and kE is of the current one or an appropriated base to be used, a positive value of DkE implies a reduction in the environmental impact of the alternative route. Consequently the alternative routes showing a positive DkE will be superior to the current one and that with the highest variance would be the most advantageous. It is important to keep in mind that alternatives are needed also for coproducts if they are generated along the energy route, leading to a complete balance when the base alternative is substituted by the assessed one [21]. Finally, it is important to express DkE in proportion to the realized exergetic cost reduction, which leads to the following sustainability index
SIC ¼ DkE =kT
ð9Þ
SIC represents joules of exergy potentially avoided in terms of ‘‘environmental impact” per joule of exergy invested in terms of the natural resources which can be chosen where to invest. This indicator embodies three important concepts that should be present on an indicator which aims at expressing sustainability improvements. First, the efficiency in using non-renewable resources used during the life cycle of the assessed services, products or commodities. Second, the efficiency using renewable and nonrenewable resources which indirectly contribute to the energy route. And finally the proposed indicator includes the influence of abatement exergy cost of the pollutants emitted to the environment. 3. Case study: replacing gasoline by methanol or biogas for transport The present approach is used to analyze whether so called sustainable alternative fuels would really be a viable way to enhance environmental sustainability. Biofuels are widely treated in the literature [29–32], and they are seen as a way to improve CO2 balance and avoid fossil fuel dependence. Cars driven by methanol and biogas from biowaste are considered as alternatives for gasoline. The life cycle database used was Ecoinvent [33], and all data were processed by means of software developed for this purpose. The base case chosen in this sample was transportation by personal petrol car, and the selected process in the Ecoinvent database
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was the 6590 named: transport, passenger car, petrol, EURO5. The life cycle impact assessment for this service is shown in Table 1. The primary exergy resulting from crude oil to produce the gasoline consumed to obtain one passenger-kilometer (pkm), was 1489 MJ-Eq. Exergy removal costs for CO2, SO2 and NOX were taken from Cornelissen [23]. Ninety percentage of SO2 abatement would cost 57 MJ/kg, 80% abatement of NOX 16 MJ/kg, and 90% abatement of CO2 3 MJ/kg on the base of removal by compression and storage in empty gas field. With these primary data and the specific zero-exergy cost of the pollutants accounted (carbon dioxide equivalent, sulfur dioxide and nitrogen oxides) the non-renewable exergy demand, renewable exergy demand, indirect exergy demand and the cumulative zero-exergy emission were calculated, everything per passengerkilometer and shown in Table 2. The same procedure was applied to the other two alternatives, methanol and methane fueled cars. The methanol was supposed to be obtained from biomass synthetic gas and the methane from biowaste anaerobic digestion. The results are presented in Tables 3 and 4. Chart 1 compares kF, kR, kC, BPR, kT and kP. Savings in terms of non-renewable resources (kF), correspond to 45% for methanol alternative and 38% for the methane alternative. The abatement exergy (kP) would be also reduced by 65% and 63% respectively. Primary exergy (BPR) is 3.6 times higher in the case of methanol alternative and 1.5 times higher in the case of methane, comparing with the petrol base case, but these are renewable sources. Indirect exergy (kT) which represents the cost in terms of natural resources
Table 3 ELCA summary for methanol alternative. Concept
Unit
Value
Non-renewable cumulative exergy demand Renewable cumulative exergy demand Cumulative exergy demand Total primary exergy Indirect cumulative exergy demand Zero-exergy pollution cost
MJ-Eq/pkm MJ-Eq/pkm MJ-Eq/pkm MJ-Eq/pkm MJ-Eq/pkm MJ-Eq/pkm
1.602 5.705 7.308 5.415 1.893 0.1718
Concept
Unit
Value
Non-renewable cumulative exergy demand Renewable cumulative exergy demand Cumulative exergy demand Total primary exergy Indirect cumulative exergy demand Zero-exergy pollution cost
MJ-Eq/pkm MJ-Eq/pkm MJ-Eq/pkm MJ-Eq/pkm MJ-Eq/pkm MJ-Eq/pkm
1.796 2.449 4.245 2.195 2.050 0.1838
Table 4 ELCA summary for methane from biowaste alternative.
Table 1 LCA summary for petrol passenger car base case. Process or assessment name
Unit
Value
Transport, passenger car, petrol, EURO5 CO2, fossil Nitrogen oxides Sulfur dioxide Non-renewable exergy resources, fossil
pkm kg kg kg MJEq MJEq MJEq MJEq MJEq MJEq MJEq MJEq MJEq MJEq
1 1.593E01 1.861E04 2.735E04 2.485E+00
Non-renewable exergy resources, nuclear Renewable exergy resources, kinetic (in wind), converted Renewable exergy resources, solar, converted Renewable exergy resources, potential (in barrage water), converted Non-renewable exergy resources, primary forest Renewable exergy resources, biomass Renewable material resources, water Non-renewable material resources, metals Non-renewable material resources, minerals
3.700E01 2.711E03 5.103E05 7.281E02 7.611E06
Chart 1. ELCA comparison among petrol, methanol and methane alternatives.
to drive the alternative routes is 21% higher in the case of methanol option and 31% in the case of methane. Eq. (9) yields the exergy representing non-renewable resources and pollution abatement per mega joule of indirect exergy for each alternative (SIC). The value for methanol alternative is 0.85 and for methane alternative is 0.69. So this means it is possible to obtain higher improvement on sustainability in the case of methanol with the same amount of resources.
8.101E03 7.937E02 3.006E02 6.159E03
Table 2 ELCA and zero-exergy emission for petrol passenger car base case. Concept
Unit
Value
Non-renewable cumulative exergy demand Renewable cumulative exergy demand Cumulative exergy demand Total primary exergy Indirect cumulative exergy demand Zero-exergy pollution cost
MJ-Eq/pkm MJ-Eq/pkm MJ-Eq/pkm MJ-Eq/pkm MJ-Eq/pkm MJ-Eq/pkm
2.892 0.1630 3.055 1.489 1.566 0.4964
4. Case study: replacing electricity from an oil plant by wind energy The two alternatives for the use of fossil oil discussed in the previous section can also be compared with another one not related with transport but with any other service or product, and maybe such alternative will lead to a higher reduction of environmental impact when using the same amount of indirect exergy. To illustrate this issue the case of substituting electricity produced in an oil plant by electricity produced with wind power technology is analyzed. The data shown below were also taken from Ecoinvent [34] and the process numbers are 1612 for electricity produced at oil plant, and 6849 for electricity produced at wind power plant. Results are presented in Table 5. It is not possible to compare the data from electricity production alternatives with those related to transport because in the
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Table 5 ELCA summary for electricity from oil and wind.
Acknowledgments
Concepts
Unit
Oil electricity
Wind electricity
Non-renewable cumulative exergy demand Renewable cumulative exergy demand Cumulative exergy demand Total primary exergy Indirect cumulative exergy demand Cumulative pollutants exergy
MJ-Eq/kW h
13.44
0.2266
MJ-Eq/kW h
0.0321
This work was supported by VLIR program through the project: Environmental education and development of clean technologies. And it also results from the collaboration between Vrije Universiteit Brussel and Universidad Central ‘Marta Abreu’ de Las Villas.
3.901
MJ-Eq/kW h
13.47
4.128
MJ-Eq/kW h MJ-Eq/kW h
11.22 2.250
3.870 0.2575
MJ-Eq/kW h
3.043
0.0345
cases of transport alternatives they are referred to one passengerkilometer and in the cases of electricity production alternatives it is referred to 1 kW h. But when the proposed sustainability index is calculated, then it is possible to compare all alternative over the same base, because all of them would be referred to 1 MJ of indirect exergy. So in the case of electricity from wind it is obtained that it is possible to avoid 63 MJ of exergy cost in terms of environmental impact per MJ of indirect exergy cost (SIC), and that is 74 times more than the environmental impact avoided by methanol substituting petrol. This means that the same resources put on substituting electricity from oil by electricity from wind would ensure a higher reduction in terms of environmental impact than those assessed alternative concerning transport. Hence, a possible strategy can be that alternatives which will contribute with the highest environmental improvement at the lowest resources cost should be implemented first. Something to be considered is that this analysis was carried out using a database which was elaborated on the base of actual statistics, so it corresponds to the current Swiss energy transformation matrix. Hence it is a fact that these results are consistent only for small perturbation of the actual Swiss energy matrix. In the case of an intended optimization of the energy transformation system with large perturbation on the system, it will be necessary to elaborate a flexible LCA model to account for influence of the alteration on the whole energy matrix. This complementary analysis would be suitable since major energy matrix perturbations could lead to dissimilar results. 5. Conclusions The indicator proposed in this paper successfully accomplished the purpose of comparing alternatives to different final services which imply different functional magnitudes. The indicator also successfully embodies three sustainability criteria: exergy efficiency of non-renewable resources, indirect resources cost (part of the economic cost) and the abatement exergy of pollutants. A comparison among alternatives to petrol personal car and fuel oil electricity generation was carried out. Results showed that substituting petrol driven cars by methanol ones from wood synthetic gas, generates and environmental benefit of 0.85 MJ of exergy in terms of non-renewable natural resources and pollutants abatement, per MJ of cumulative indirect exergy used to drive the alternative. In the case of biogas from biowaste it results in a benefit of 0.69 MJ per MJ of indirect exergy. However, when it was assessed the wind electricity alternative substituting fuel oil electricity generation, it was found that by this way it is possible to obtain an environmental benefit of 63 MJ of exergy in terms of non-renewable resources and pollutant abatement per MJ of cumulative indirect exergy cost.
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
An LCA based indicator for evaluation of alternative energy routes M.A. Rubio Rodríguez a,⇑, J. De Ruyck b, P. Roque Díaz a, V.K. Verma b, S. Bram b,c a
Universidad Central ‘‘Marta Abreu” de Las Villas, Carretera a Camajuani km 5, 54830 Santa Clara, Cuba Department of Mechanical Engineering, Faculty of Applied Sciences, Vrije Universiteit Brussel (VUB), Pleinlaan 2, 1050 Brussel, Belgium c Department of Industrial Sciences and Technology, Erasmushogeschool Brussel, Nijverheidskaai 170, 1070 Brussel, Belgium b
a r t i c l e
i n f o
Article history: Received 3 February 2010 Received in revised form 7 August 2010 Accepted 13 August 2010 Available online 15 September 2010 Keywords: Energy system Energy route Energy sustainability LCA ELCA
a b s t r a c t Politicians and policymakers are searching for alternatives for current energy conversion systems in order to reduce environmental pollution whilst preserving scarce natural resources. When defining new alternatives it is important to find out how environmentally friendly and sustainable these alternatives are, and which of them will ensure a major improvement in environmental issue. To answer such questions the present work proposes a new type of indicator based on exergy life cycle data. Alternative energy routes towards different services and commodities are compared in terms of environmental impact and indirect natural resource costs. Two case studies are presented using data from the Ecoinvent database (v2.01 2007) to compare alternatives to petrol transport and fuel oil power generation. The alternative transport routes assessed were methanol and biogas fueled cars, and wind electricity in the case of power generation. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Mankind is realizing that dependence on fossil fuels can no longer be sustained. Consequently many studies have been performed to plan the way back to the usage of renewable energy sources [1–7]. Care must however be taken to go for alternatives which offer the highest degree of sustainability in order to use the limited resources in the most optimal way. One trend has been the search for a sustainability function that might be applied in parallel with a classical economic function. First there is the category of indicators and indexes, integrated or not, which represent a state of economic, social and environmental development in a defined region, often the national level [8–10], secondly there are those product-related assessment tools with the focus on the material and/or energy flows of a product or service from a life cycle perspective [10–12]. Within the frame of the second approach, present work has been developed. A wide collection of those product-related assessment tools are implemented on Ecoinvent database [13]. Some of these assessment tools are CML 2001, eco-indicator 99, ecological-footprint, and some others like cumulative exergy demand and cumulative
Abbreviations: CExC, cumulative exergy consumption; CExD, cumulative exergy demand; EI, environmental impact; ELCA, exergy life cycle assessment; LCA, life cycle assessment; LCI, life cycle impact; LCIA, life cycle impact assessment; SPA, system perturbation analysis. ⇑ Corresponding author. Tel.: +53 42281194. E-mail address: [email protected] (M.A. Rubio Rodríguez). 0306-2619/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2010.08.013
energy demand. All these indexes are also formed by subcategories which are calculated using linear weight functions. These assessments aim to quantify specific environmental troubles as well as the total impact. The main disadvantage of these assessments is the use of weight function which is obtained by subjective procedures like experts opinions. On the other hand exergy life cycle based indicators have been proposed not only as a measure for economic losses and dematerialization, but for waste accounting and ecotoxicity as well. The reason for this is that exergy embodied in resources, products, and waste materials, has the potential to cause change in both the industrial environment and the natural ecosystem [14]. The advantages of use exergy as measurement of environmental impact are widely recognized [14,15]. But the use of the exergy entering the techno-sphere as base of net exergy efficiency and the no distinction between renewable and non-renewable primary resources bear to a contradiction due to fact that ‘‘their main disadvantage (renewable energy systems) lies in their incapability to take advantage of a big part of the available energy” [16]. So an increment in the renewability share of primary resources could be canceled by a decrease of net exergy efficiency, despite the fact that probably it would be preferable to decrease the use of non-renewable resources in spite of an efficiency decrement. This investigation aims at demonstrating the convenience of a quite different exergy indicator, based on indirect exergetic cost and environmental exergetic cost, when it is necessary to assess the exergetic sustainability of energy transformation strategies to different final services or commodities. This new perspective
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would contribute to later elaborate a new optimization function for energy matrixes.
it to a number of polymers. These works represent the present concern on sustainability issue and the consensus around the successful use of exergy concept to face up the sustainability problem.
2. Methodology
2.2. Exergetic costs of sustainability
2.1. Background
It was possible to define three exergetic cost function for an energy transformation route, taking into account the life cycle exergy consumption and the exergy neutralization cost dependence on pollutant exergy. The first one was based on Dewulf’s idea [20] about a renewability parameter, the cumulative exergy consumption (CExC) of Szargut [18] and it was related to the service or good produced instead of the total exergy required. This cost can be expressed as:
The present sustainability measuring procedure for alternative energy transformation routes is based on four approaches supported on life cycle assessment (LCA) [17]: Szargut’s cumulative exergy consumption (CExC) [18], zero-exergy emission from Cornelissen [15,19], Dewulf’s renewability idea [20] and the indirect exergy consumption approach from Wall [4]. The first approach was the cumulative exergy consumption (CExC) from Szargut [18] and its implementation as a life cycle assessment (LCIA) on the Ecoinvent database [21]. In order to quantify the life cycle exergy demand of a product, cumulative exergy demand (CExD) is defined as the sum of exergy of all resources required to provide a service or product [22]. CExD is equivalent to the definition of CExC, both quantifying the total exergy requirement of a product. Szargut calculated CExC by adding up the total exergy requirement of a process over a defined period of time. The exergy requirement of one unit of process output was then obtained by dividing the total exergy requirement by the number of unit outputs during the concerned period. The emergence of large life cycle databases such as Ecoinvent enables and facilitates a product-specific approach, since such databases provide the resource demand for each unit process. The second approach is the zero-exergy emission from Cornelissen also used by Sciubba’s extended exergy accounting [15,19]. This work addressed the accounting of pollution impact based on exergy approach. The essence of this idea is that: ‘‘the potential environmental impact of an effluent is represented by the cumulative amount of exergetic resources that must be consumed to attain an ideal, zero-impact disposal of both the effluent itself and the equipment that handles it”. Respecting this issue researchers have been looking for relation between pollutants exergy and environmental damage, and a qualitative relation has been found, but more research is needed to determine a quantitative relation [23]. On the issue of sustainability measuring Dewulf’s works and Ocaña’s PhD thesis [11] will be used as Refs. [14,20]. From both authors exergy accounting idea has been retained to achieve consistency owing to the possibility of this thermodynamic function to express disequilibrium and potential interaction with surroundings. This includes the notion of renewability index and efficiency as important factors on sustainability issues. Finally he indirect exergy concept proposed by Wall will be applied. This exergy would be whole exergy which is not the one that is directly transformed to obtain the final energy service [4]. An important concept that will be followed along this paper, used in Bram’s work [24], is the concept of energy route or energy path. This route or path was defined as all transformations necessary to convert a primary natural resource into a final service or commodity. For instance it is the path from crude oil under ground as joules of exergy, to transported passengers as passenger-kilometer, or the route from energy in the wind as joules, to electricity served at consumer as Watt–hour. Many other references on the sustainability field and exergy analysis were considered such as Hoang’s work [25] which proposed the notion of sustainability efficiency (SE) as the ratio of the minimum total cumulative exergy content (TCExC) to an observed TCExC. Another latest attempt to overcome de sustainability problem applied to the polymer manufacturing can be found in Dewulf’s work [26]. Dewulf proposed a cumulative degree of perfection (CDP) as the cumulative exergy efficiency and applied
kF ¼ ðCExDF Þ=ðFmÞ ½J=fu
ð1Þ
where kF is the fossil exergetic cost in joule of fossil cumulative exergy, CExDF is the fossil part of CExD and Fm is a proper functional magnitude chosen as a final desired product or service, for instance passengers-kilometer, tons-kilometer, kW h, etc.; and fu is the corresponding functional unit. The same concept was used by Ocaña [11] to create a fossil exergy consumption index for a system, but it accounted only for the exergy of energy flows in the assessed system lacking of the life cycle approach of the CExD concept which considered the cumulative amount of exergy ‘‘embodied” in the product or service by the successive contribution of each step in the production chain [14]. The second exergetic cost was based on the CExD of renewable resources. This is similar to the previous but accounting for the renewable portion of the CExD, thus the expression is:
kR ¼ ðCExDR Þ=ðFmÞ ½J=fu
ð2Þ
where as it was said, CExDR is the renewable part of the life cycle exergy demand in resources. Consequently if both exergy costs are summed one obtains the cumulative exergy demand due to the consumption of all natural resources chargeable to a final service.
kC ¼ kF þ kR
½J=fu
ð3Þ
It would be valuable also to account for externalities due to the pollutants emission and associated environmental consequences. It is Cornelissen’s theory that to achieve a zero environmental impact, pollutants would have to be brought to both thermal- and chemical equilibrium with the surroundings, which can be technically achieved in several ways: but in any case, the exergetic cost of the zero-impact (i.e., its extended exergy) will be proportional to the physical exergy of the effluent. . . [23,27]. This approach has two main shortcomings. Firstly the proportional factor needed for an accurate evaluation of the exergy neutralization cost would be difficult to determine because of the various treatment process availability and the process that are not available for certain pollutants. Furthermore, the exergy cost of avoiding pollutants does not represent a precise measurement of the damage caused by those which would not be avoided. However, it is also important to notice that this idea would be a consistent incorporation of the effects of effluent treatment in the exergetic cost balance of an energy route. Moreover if the cumulative exergy by means of non-renewable primary natural resources have been accepted as environmental degradation measure, it would not be hard to assume the exergetic cost for zero environmental impact as a measure of pollution impact. Thus an expression to calculate an exergetic cost due to zero-exergy impact of an energy transformation chain is:
kP ¼ Rðmi yi Þ=ðFmÞ ½J=fu
ð4Þ
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where mi is the mass of each computable pollutant emitted during the life cycle of the energy route assessed and yi would be the exergy mitigation cost or zero-exergy cost for each pollutant. In spite of the lack of this term to be proportional to the real magnitude of environmental impact, it is obvious that kP = 0 implies a non-polluting scenario or technology. Now if the pollutant-impact exergetic cost (kp) is added to the previous defined exergetic cost of the energy route kR, a different concept is obtained.
Indirect exergetic cost represents resources that should be decided where to be invested. This is the cost in natural resources to run an energy route, natural resources that could be better to use on some route else. A comparison between this concept and the economic cost can be made and probably there would be a relation, but they are not equivalent consequently an economic assessment always has to be done in order to complement any kind of assessment.
kRT ¼ kF þ kR þ kP
2.3. Assessing alternative energy routes
½J=fu
ð5Þ
This total exergetic cost (kRT) is quite similar to the extended exergy accounting (EEA) from Sciubba [28] the main difference is that there is no accounting for labor. This was because to do so it would be necessary to make some considerations implying nonaccurate results. There is also the possibility to group just the zero-impact exergetic cost kP and the fossil exergetic cost kF, the new category could be called environmental exergetic cost and it would be part of the sustainability concept that is supported in this work, and the expression would be as follows:
A cardinal problem today is the assessment of substituting current fossil based energy routes by other more sustainable alternatives. It is also important to know how much each alternative route contributes to the sustainability and which of them are the most suitable to be implemented independently of the final service or product. The environmental exergetic cost (kE) was defined previously as a measure of the impact on sustainability of a certain energy route. So it was possible to define the variation in terms of this indicator when it is decided to asses an alternative route:
kE ¼ kF þ kP
DkE ¼ kE kE
½J=fu
ð6Þ
This term accounts for cumulative exergy from fossil fuels and that of zero-impact cost, both during the life cycle of one functional unit of final energy route. It is important to remark that the environmental exergetic cost kE embodies the influence of the life cycle efficiency because and increment of the efficiency without any other change, obviously decreases environmental exergetic cost kE. An idea supported by the authors, which is similar to that from Wall [4], is represented in Fig. 1. The additional primary resources are those which indirectly enter the considered system during the transformation process of a primary resource, which itself is directly related with the supply of an energy service either inside the techno-sphere or the society. The difference in the present work is that Wall’s idea was adapted to be implemented using LCI databases which already accounts for cumulative exergy demand of several processes. According to the concepts presented above, the category that will be called indirect exergetic cost (kT) was obtained by subtracting the exergy of the primary resource directly related with the energy route itself from the CExC accounted for the route, that is:
kT ¼ kC BPR =Fm ½J=fu
ð7Þ
where BPR is the exergy of the primary resource directly related with the energy route.
Environmental impact (EI)
Final Energy Service
Energy
route
Directly Primary resources
Additional primary resources for transformation Indirectly
Society
Techno-sphere
Nature Fig. 1. Energy route model.
0
½J=fu
ð8Þ
where kE is the environmental exergetic cost of the alternative route 0 and kE is of the current one or an appropriated base to be used, a positive value of DkE implies a reduction in the environmental impact of the alternative route. Consequently the alternative routes showing a positive DkE will be superior to the current one and that with the highest variance would be the most advantageous. It is important to keep in mind that alternatives are needed also for coproducts if they are generated along the energy route, leading to a complete balance when the base alternative is substituted by the assessed one [21]. Finally, it is important to express DkE in proportion to the realized exergetic cost reduction, which leads to the following sustainability index
SIC ¼ DkE =kT
ð9Þ
SIC represents joules of exergy potentially avoided in terms of ‘‘environmental impact” per joule of exergy invested in terms of the natural resources which can be chosen where to invest. This indicator embodies three important concepts that should be present on an indicator which aims at expressing sustainability improvements. First, the efficiency in using non-renewable resources used during the life cycle of the assessed services, products or commodities. Second, the efficiency using renewable and nonrenewable resources which indirectly contribute to the energy route. And finally the proposed indicator includes the influence of abatement exergy cost of the pollutants emitted to the environment. 3. Case study: replacing gasoline by methanol or biogas for transport The present approach is used to analyze whether so called sustainable alternative fuels would really be a viable way to enhance environmental sustainability. Biofuels are widely treated in the literature [29–32], and they are seen as a way to improve CO2 balance and avoid fossil fuel dependence. Cars driven by methanol and biogas from biowaste are considered as alternatives for gasoline. The life cycle database used was Ecoinvent [33], and all data were processed by means of software developed for this purpose. The base case chosen in this sample was transportation by personal petrol car, and the selected process in the Ecoinvent database
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was the 6590 named: transport, passenger car, petrol, EURO5. The life cycle impact assessment for this service is shown in Table 1. The primary exergy resulting from crude oil to produce the gasoline consumed to obtain one passenger-kilometer (pkm), was 1489 MJ-Eq. Exergy removal costs for CO2, SO2 and NOX were taken from Cornelissen [23]. Ninety percentage of SO2 abatement would cost 57 MJ/kg, 80% abatement of NOX 16 MJ/kg, and 90% abatement of CO2 3 MJ/kg on the base of removal by compression and storage in empty gas field. With these primary data and the specific zero-exergy cost of the pollutants accounted (carbon dioxide equivalent, sulfur dioxide and nitrogen oxides) the non-renewable exergy demand, renewable exergy demand, indirect exergy demand and the cumulative zero-exergy emission were calculated, everything per passengerkilometer and shown in Table 2. The same procedure was applied to the other two alternatives, methanol and methane fueled cars. The methanol was supposed to be obtained from biomass synthetic gas and the methane from biowaste anaerobic digestion. The results are presented in Tables 3 and 4. Chart 1 compares kF, kR, kC, BPR, kT and kP. Savings in terms of non-renewable resources (kF), correspond to 45% for methanol alternative and 38% for the methane alternative. The abatement exergy (kP) would be also reduced by 65% and 63% respectively. Primary exergy (BPR) is 3.6 times higher in the case of methanol alternative and 1.5 times higher in the case of methane, comparing with the petrol base case, but these are renewable sources. Indirect exergy (kT) which represents the cost in terms of natural resources
Table 3 ELCA summary for methanol alternative. Concept
Unit
Value
Non-renewable cumulative exergy demand Renewable cumulative exergy demand Cumulative exergy demand Total primary exergy Indirect cumulative exergy demand Zero-exergy pollution cost
MJ-Eq/pkm MJ-Eq/pkm MJ-Eq/pkm MJ-Eq/pkm MJ-Eq/pkm MJ-Eq/pkm
1.602 5.705 7.308 5.415 1.893 0.1718
Concept
Unit
Value
Non-renewable cumulative exergy demand Renewable cumulative exergy demand Cumulative exergy demand Total primary exergy Indirect cumulative exergy demand Zero-exergy pollution cost
MJ-Eq/pkm MJ-Eq/pkm MJ-Eq/pkm MJ-Eq/pkm MJ-Eq/pkm MJ-Eq/pkm
1.796 2.449 4.245 2.195 2.050 0.1838
Table 4 ELCA summary for methane from biowaste alternative.
Table 1 LCA summary for petrol passenger car base case. Process or assessment name
Unit
Value
Transport, passenger car, petrol, EURO5 CO2, fossil Nitrogen oxides Sulfur dioxide Non-renewable exergy resources, fossil
pkm kg kg kg MJEq MJEq MJEq MJEq MJEq MJEq MJEq MJEq MJEq MJEq
1 1.593E01 1.861E04 2.735E04 2.485E+00
Non-renewable exergy resources, nuclear Renewable exergy resources, kinetic (in wind), converted Renewable exergy resources, solar, converted Renewable exergy resources, potential (in barrage water), converted Non-renewable exergy resources, primary forest Renewable exergy resources, biomass Renewable material resources, water Non-renewable material resources, metals Non-renewable material resources, minerals
3.700E01 2.711E03 5.103E05 7.281E02 7.611E06
Chart 1. ELCA comparison among petrol, methanol and methane alternatives.
to drive the alternative routes is 21% higher in the case of methanol option and 31% in the case of methane. Eq. (9) yields the exergy representing non-renewable resources and pollution abatement per mega joule of indirect exergy for each alternative (SIC). The value for methanol alternative is 0.85 and for methane alternative is 0.69. So this means it is possible to obtain higher improvement on sustainability in the case of methanol with the same amount of resources.
8.101E03 7.937E02 3.006E02 6.159E03
Table 2 ELCA and zero-exergy emission for petrol passenger car base case. Concept
Unit
Value
Non-renewable cumulative exergy demand Renewable cumulative exergy demand Cumulative exergy demand Total primary exergy Indirect cumulative exergy demand Zero-exergy pollution cost
MJ-Eq/pkm MJ-Eq/pkm MJ-Eq/pkm MJ-Eq/pkm MJ-Eq/pkm MJ-Eq/pkm
2.892 0.1630 3.055 1.489 1.566 0.4964
4. Case study: replacing electricity from an oil plant by wind energy The two alternatives for the use of fossil oil discussed in the previous section can also be compared with another one not related with transport but with any other service or product, and maybe such alternative will lead to a higher reduction of environmental impact when using the same amount of indirect exergy. To illustrate this issue the case of substituting electricity produced in an oil plant by electricity produced with wind power technology is analyzed. The data shown below were also taken from Ecoinvent [34] and the process numbers are 1612 for electricity produced at oil plant, and 6849 for electricity produced at wind power plant. Results are presented in Table 5. It is not possible to compare the data from electricity production alternatives with those related to transport because in the
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Table 5 ELCA summary for electricity from oil and wind.
Acknowledgments
Concepts
Unit
Oil electricity
Wind electricity
Non-renewable cumulative exergy demand Renewable cumulative exergy demand Cumulative exergy demand Total primary exergy Indirect cumulative exergy demand Cumulative pollutants exergy
MJ-Eq/kW h
13.44
0.2266
MJ-Eq/kW h
0.0321
This work was supported by VLIR program through the project: Environmental education and development of clean technologies. And it also results from the collaboration between Vrije Universiteit Brussel and Universidad Central ‘Marta Abreu’ de Las Villas.
3.901
MJ-Eq/kW h
13.47
4.128
MJ-Eq/kW h MJ-Eq/kW h
11.22 2.250
3.870 0.2575
MJ-Eq/kW h
3.043
0.0345
cases of transport alternatives they are referred to one passengerkilometer and in the cases of electricity production alternatives it is referred to 1 kW h. But when the proposed sustainability index is calculated, then it is possible to compare all alternative over the same base, because all of them would be referred to 1 MJ of indirect exergy. So in the case of electricity from wind it is obtained that it is possible to avoid 63 MJ of exergy cost in terms of environmental impact per MJ of indirect exergy cost (SIC), and that is 74 times more than the environmental impact avoided by methanol substituting petrol. This means that the same resources put on substituting electricity from oil by electricity from wind would ensure a higher reduction in terms of environmental impact than those assessed alternative concerning transport. Hence, a possible strategy can be that alternatives which will contribute with the highest environmental improvement at the lowest resources cost should be implemented first. Something to be considered is that this analysis was carried out using a database which was elaborated on the base of actual statistics, so it corresponds to the current Swiss energy transformation matrix. Hence it is a fact that these results are consistent only for small perturbation of the actual Swiss energy matrix. In the case of an intended optimization of the energy transformation system with large perturbation on the system, it will be necessary to elaborate a flexible LCA model to account for influence of the alteration on the whole energy matrix. This complementary analysis would be suitable since major energy matrix perturbations could lead to dissimilar results. 5. Conclusions The indicator proposed in this paper successfully accomplished the purpose of comparing alternatives to different final services which imply different functional magnitudes. The indicator also successfully embodies three sustainability criteria: exergy efficiency of non-renewable resources, indirect resources cost (part of the economic cost) and the abatement exergy of pollutants. A comparison among alternatives to petrol personal car and fuel oil electricity generation was carried out. Results showed that substituting petrol driven cars by methanol ones from wood synthetic gas, generates and environmental benefit of 0.85 MJ of exergy in terms of non-renewable natural resources and pollutants abatement, per MJ of cumulative indirect exergy used to drive the alternative. In the case of biogas from biowaste it results in a benefit of 0.69 MJ per MJ of indirect exergy. However, when it was assessed the wind electricity alternative substituting fuel oil electricity generation, it was found that by this way it is possible to obtain an environmental benefit of 63 MJ of exergy in terms of non-renewable resources and pollutant abatement per MJ of cumulative indirect exergy cost.
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