P e r g a m o n

Energy Convers. Mgmt Voi. 37, Nos 6-8, pp. 831-836, 1996 Copyright © 1996 Elsevier Science Ltd

0 1 9 6 - 8 9 0 4 ( 9 5 ) 0 0 2 6 4 - 2 Printed in Great Britain. All rights reserved 0196-8904/96 $15.00 + 0.00

USING LIFE-CYCLE ASSESSMENTS FOR THE ENVIRONMENTAL

EVALUATION OF GREENHOUSE GAS MITIGATION OPTIONS

EVERT NIEUWLAAR t, ERIK ALSEMA and BAREND VAN ENGELENBURG

Department of Science, Technology and Society, Utrecht University, Padualaan 14, 3584 CH Utrecht, The Netherlands

Abstract - The complex structure of energy supply systems and the range of environmen- tal issues involved, make decisions regarding the use of new or improved energy r e s o u r c e s

and energy technologies far from being straightforward. A life-cycle approach is required to reveal the full potential for an option to realize increased energy performance and reduced emissions of greenhouse gases. In addition, the life-cycle assessment reveals possible bottlenecks regarding other environmental issues.

1. INTRODUCTION

The use of energy resources is a major source of environmental impacts. Many atmospheric emissions are

directly related to the use of fossil fuels. Combustion products like carbon dioxide (CO2), sulphur dioxide (SO2) and nitrogen oxides (NOx) contribute to the environmental themes of climate change and acidifica- tion. Under imperfect circumstances, products from incomplete combustion are produced like carbon monoxide (CO) and volatile organic compounds (VOC). Atmospheric releases of VOC also take p l a c e

before combustion through leaks (intended or not-intended). Nitrogen oxides, carbon monoxide and volatile organic compounds lead to the formation of photochemical oxidants in the troposphere. Control- ling these emissions has become a major task in energy policy making. The primary task of energy supply systems is nowadays not only to convert primary energy carriers (coal, crude oil etc.) into more manage- able secondary energy carders (electricity, gasoline etc.). The secondary energy carders are also required to have properties that lead to low or no pollution during end-use. In addition, the development and use of efficient and clean technologies for energy conversion, energy storage and energy transport is required to bring back emissions to acceptable levels. The energy system is complex. Many options exist for improving the energy system from the point of view of economics, reliability, efficiency and the environment. Within each of these perspectives, the right analysis tool is required in order to present carefully assessed information in a form that decision makers can use. This paper only deals with the environmental perspective. The complex nature of the energy system calls for assessments in which full energy chains are considered. Therefore, the methods of environmental life-cycle assessment are applied to energy technologies in order to reveal environmental opportunities and bottlenecks associated with greenhouse gas mitigation options. After discussing some of the methodological issues involved in energy life-cycle assessment a two-step procedure is presented for interpreting the results of energy LCA' s.: (I) assessment of new or improved energy technologies regarding

* To whom all correspondence should be addressed

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832 NIEUWLAAR et al.: USING LIFE-CYCLE ASSESSMENTS

their potential environmental impacts, and (II) the environmental assessment of integrating these technologies into the energy system. In the first step the method of life-cycle assessment on single energy technologies reveals: • the life-cycle energy use, • the cumulative emission of greenhouse gas; • potential bottlenecks regarding other environmental impacts; These issues will be illustrated with results from case studies on solar photovoltaics, gas engine cogeneration of heat and power, and coal gasification. In the second step the prospects for realizing increased energy performance and reduced emissions of greenhouse gases are assessed. In this step one has to account for the way in which the performance of new or improved energy technologies relates to the performance of competing energy technologies.

2. ENERGY LIFE-CYCLE ASSESSMENT

The economic price of electricity is an expression of accumulated cost that had to be made to extract energy resources from nature, transport them and convert them into electricity which is delivered through the grid. Similarly, the physical analysis of full energy chains helps us to express physical scarcity of energy in terms of cumulative energy resource use and cumulative environmental impacts. In the seventies the determination of cumulative energy requirements developed under the name of energy analysis ~'2. More recently, this analysis framework has been extended to incorporate environmental impacts under the name of environmental life-cycle assessment (LCA). In the same way as in energy analysis LCA follows the craddle-to-grave approach, examining all processes from natural resources (energy, minerals, biotic resources etc.) until waste handling. The determination of cumulative CO2 emissions is in fact a small extension of energy analysis since it only requires multiplication of fuel use with emission factors based on carbon content. Unfortunately, many energy analysis results from the past cannot be used now for two reasons: (I) they are outdated, and (II) the underlying fuel distribution data were not specified. The LCA-framework incorporates the determination of global warming equivalents along with factors relating to other environmental impacts. Four steps are distinguished in the LCA-methodology3: • Goal definition and scoping

In this step the type of application, the scope of the research and the functional unit are defined. In the energy field various types of application can be distinguished: energy technology analysis in order to reveal weaknesses and opportunities regarding environmental impacts, energy technology comparison, and energy system analysis. Scoping refers a.o. to the geographic and time frame for which the results may be considered valid. The functional unit is a description of the product delivered or the service rendered, along with the quantity in which it will be represented (e.g., 1 kWh electricity).

• Inventory analysis The goal of the inventory analysis is to map out the environmental interventions (a general term for emissions and all other inputs and outputs from and to the environment) per part of the life-cycle. For this, a process-tree of the life-cycle is drawn and environmental inputs and outputs per process are determined in physical units

• Impact assessment In this stage the results of the inventory analysis are translated into scores on a number of environmen- tal issues or themes (e.g., climate change, acidification). Table 1 gives a list of generally recognized environmental problems. The scores indicate the potential environmental harm of the interventions determined in the previous step. Examples are the global warming equivalents based on Global Warming Potentials (GWP's) and acidification equivalents based on acidification potentials. One of the results of the impact assessment step is the environmental profile (i.e., the list of impact scores)

• Improvement analysis The aim of the improvement analysis step is to identify potential bottlenecks of the life-cycle and possibly define improvements to overcome these difficulties.

NIEUWLAAR et al.: USING LIFE-CYCLE ASSESSMENTS

Table 1. Generally recognized environmental problems 4.

833

Depletion Abiotic resources Biotic resources

Pollution Ozone depletion Global warming Photochemical oxidant formation Acidification Human toxicity Ecotoxicity Nutrification Radiation Dispersion of heat Noise Smell Occupational health

Disturbances Desiccation Physical ecosystem degradation Landscape degradation Direct human victims

3. LIFE-CYCLE ASSESSMENT OF ENERGY TECHNOLOGIES

Methodology A streamlined procedure for the life-cycle assessment of energy technologies has been developed 5. The procedure incorporates a.o. the use of generic data for common inputs to the life-cycle and a limited number of environmental issues that are considered relevant for studied system. Streamlined life-cycle assessment can be seen as part of an iterative procedure: weaknesses in previous iterations may induce a new iteration. In the basic procedure at least the following indicators are determined: • energy use • waste production • exhaustion of raw materials • global warming • acidification Energy use and waste production are not considered as environmental impact categories in environmental science 3. The use of energy carriers indirectly leads to environmental impacts such as resource depletion, global warming, acidification. In much the same way, waste production is not an environmental impact category but the waste management technologies (incineration, recycling, landfill etc.) are to be considered as processes leading to environmental interventions (e.g., methane emissions from a landfill) that have to be assessed separately in the inventory analysis. In our streamlined procedure for energy LCA we choose to present energy use (expressed as fossil energy requirement) as a performance indicator as well as an impact category for energy resource depletion. Waste production was chosen as an indicator for the environmental problems corresponding with waste treatment beyond the chosen scope of analysis. It is given in weight units and subcategories like domestic waste, hazardous chemical waste, or radioactive waste can be made if necessary. Exhaustion of raw materials is a measure of non-energy resource depletion. The weighing factor often used in adding up quantities of raw materials is the inverse of the global reserve of the material 3. The exhaustion is thus expressed as a dimensionless number. Global warming and acidification are included because of the strong link between these environmental themes and the use of energy. For global warming, global warming potentials (100 years) are used for adding greenhouse gases together. For acidification the acidification potentials are used, which are based on the number of protons that a molecule of an acidifying gas can release. Other environmental themes can of course be added to the research when considered relevant (for instance, hydrocarbon emissions from gas engines contributing to photochemical oxidant formation have been included in one of the case studies).

The case studies Case studies were performed with the streamlined LCA method on three technologies for electricity production: gas engine cogeneration of heat and power, photovoltaic solar cell systems, and electricity production using coal gasification integrated with a combined cycle power plant. In all cases the technol- ogy is situated in the Netherlands. For the gas engine study 6 operating data for the gas engines at the Utrecht University campus were used. These engines produce electricity and heat for space heating. Since the engines were relatively large and

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had a high annual operating time because of underground heat storage, a more representative case has been derived: a 500 kW~ engine (electrical efficiency 0.36) operating 4500 hours per year. In addition to the 500 kW electric output, the system produces 670 kW thermal energy (120 °C) used for space heating. Processes included in the analysis are: raw material extraction and processing, gas engine and generator production, lube-oil production, natural gas extraction and delivery, gas engine operation and processing of waste materials, lube oil and cleaning water. In the solar cell system 7'8, electricity is produced by a roof-integrated system in the Netherlands, constituted of 30 m 2 modules connected to the electricity grid via a single inverter. The modules are based on amorphous silicon (a-Si) cells. The efficiency of the modules was assumed to be 10%, based on expected future technology. This is much higher than the current commercially available a-Si modules (5-6%). The modules are integrated into the roof with aluminium profiles. Processes included in the analysis are: raw material extraction, processing & transport, module production, support structure production, the power conditioner, cabling, conversion and decommissioning. In the Integrated Coal Gasification Combined Cycle (IGCC) study 9, one of the most modem techniques for coal power production was analyzed. Data were based on the demonstration plant in Buggenum (The Netherlands), scaled to a commercial installation of 600 MWe. Processes included in the analysis are a.o. coal mining and treatment (mostly in Australia and the USA; 50 % surface mining, 50 % underground mining), coal transport by train, bulk carrier and river vessel, building of the IGCC plant, operation and decommissioning of the IGCC plant.

Case study results A summary of the results from the case studies is given in table 2. For energy, global warming and acidification this table also shows data for average electricity production in the Netherlands. The results are given here per kWh of useful energy output. For the coal gasification power plant and for the solar cell system the output is electricity. In the gas engine CHP case the output is electricity plus a quantity of heat. Adding these two outputs together on an energy basis would be the same as neglecting the difference in thermodynamic quality and economic value of these distinct energy carders. The approach chosen here is therefore to add the electric and heat output on an exergy basis.

Table 2. Results of case studies. All results are related to an output of 1 kWh useful energy (n.a. = not available).

ICGCC gas engine solar cell

exhaustion ( ' 1 0 3 ) 0.0103 0.173 6.91

Energy (MJ) 9.25 7.96 0.74

Global Warming (kg CO2 equivalent) 0.80 0.48 0.047

acidification (g SO 2 equivalent) 1.14 0.87 0.23

Solid waste (g) 126 0.1 1.9

Photochemical oxidant creation n.a. 0.15 n.a. (g ethene equivalent)

electricity production

Netherlands average, 1987 l°

n.a

9.78

0.65

2.58

n.a.

n.a.

It must be noted that the results in table 2 can only be used as a starting point for comparison. Although all three technologies deliver electricity, they cannot easily be interchanged. The coal gasification power plant has a large scale and is most likely operated in base-load electricity production. The operation of the gas engine CHP plant strongly depends on local heat demand whereas the solar cell system as a power source has only a limited potential in replacing fuel based electric capacity. On the other hand, the results reveal strongnesses and weaknesses of the technologies from an environmental point of view. None of the technologies scores best on all themes considered. The coal gasification power plant scores best on raw material exhaustion but worst on all other themes. The difference is most profound for solid waste which is the result of the large amount of solids produced in the coal mining and purification process. The relative contributions of distinguished processes in the life-cycle analysis of the IGCC plant is given in Fig. 1. Next to the solid waste issue already mentioned, the acidification theme shows that only about 50% of the acidifying emissions take place during the operation of the IGCC power plant. Most of the other acidifying

NIEUWLAAR et al.: USING LIFE-CYCLEASSESSMENTS 835

emissions take place during coal transport overseas. Since the oceans are less vulnarable to acidification it cannot be concluded that on acidification, IGCC scores worse than gas engine CHP, at least in these cases.

1 r~t~

g

o

=- t~

Energy Exhaustion Global Warming Acidification Solid waste Environmental issue

ii~ Decommissioning ~ Inl. transp. & storage Electricity production [ ] Foreign transport

[ ] Building plant [ ] Coal mining

Fig. 1. Relative contribution of distinguished processes to the environmental profile of a Shell 600 MW~ IGCC power plant.

The insights one can gain from comparing energy technologies improves when further development of the technologies are taken into consideration. The development and commercialization of gas engines has progressed further than those for solar cells. The prospects for further improvement of solar cells have been further investigated. From table 2 it can be noticed that the solar cell system scores best on all items except raw materials exhaustion and solid waste. Furthermore it was noticed that the solar cell module as well as the supporting structure have a large influence on the results. Apart from options, like increased solar cell efficiency and longer life-times, that are beneficial on all environmental items, options that address the choice and efficiency of material use must therefore be taken into consideration. A second case was designed in which, a.o., the solar cell efficiency was raised to 15%, the lifetime of the system increased from 20 to 30 years and the modules integrated into the roof with plastic "tiles" instead of aluminium profiles. Because the roof structure in this case is similar to a conventional tiled roof, no additional zinc plating is necessary which strongly reduces the score'on exhaustion of raw materials. Furthermore, the connection to the electricity grid is more or less centralized: 25 systems share an inverter which is connected to the grid. The results of this case are shown in table 3 along with the results of the first case. From these results in can be concluded that a large potentential for further improvement exists for solar cell systems.

Table 3. Improvement of the solar cell system. Impact scores per kWh of useful energy output.

exhaustion (* 103)

Energy (M J)

Global Warming (kg CO2-equivalent)

Acidification (g SO2-equivalent)

Solid waste (g)

Base case Improved case

6.91 0.13

0.74 0.27

0.047 0.014

0.23 0.08

1.9 0.19

4. SYSTEM INTEGRATION

In various studies, the method of avoided interventions is used to determine environmental pros and cons of integrating an energy technology into the energy system~ In its most simple form, avoided emissions are calculated by determining the environmental interventions of the technology that is replaced by the technology under study. In other words, the environmental merits of a technology are determined by subtracting the environmental interventions of a reference technology from the environmental interventions

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of the technology itself. An example of this simplified procedure is the calculation of avoided CO~ emissions through solar cells by subtracting the cumulative CO2 emission per kWh from the CO2 emission caused by a natural gas fired power station. A problem with this procedure is that it is not allways obvious that natural gas fired power production is the relevant competing technology H. In a more accurate procedure, avoided interventions are determined by using reliability and simulation models of the electricity supply. These models are used to determine capacity credits, fuel savings and avoided emissions when increasing quantities of solar PV are integrated into the complete electricity supply system. In this way it is possible to determine the type and the amount of central electricity production capacity that can be avoided without endangering the reliability of the electricity production system (capacity credit). In addition, the fuel savings and avoided emissions are determined for the resulting mix of power plants. In the solar cell case, for example, calculations with the Dutch generation mix that is used in the year 2015 according to the Global Shift scenario showed that the capacity credit ranges from 25 to 8% of PV peak power when the PV capacity in the Netherlands increases from 100 to 5000 MWp. The fuel savings are about 8 TJ per MWp per year. Avoided emissions per MWp per year are 0.5 ktonne CO2, 0.3 tonne NOx and 0.12 tonne SO~J 2 Since one MWp can produce about 106 kWh, fuel savings per kWh are 8 MJ. Avoided emissions per kwh are 0.5 kg CO2, 0.3 g NOx and 0.12 g SO 2. The last two numbers can be combined into SO2 equivalents resulting in 0.33 g avoided SO2 equivalents.

CONCLUSIONS

Environmental life-cycle assessment is a useful tool for revealing potential environmental impacts of energy supply options. The full potential of options in the energy supply system to enhance energy performance and reduced emissions of greenhouse gases requires a careful assessment of integrating the new or improved options into existing energy supply systems.

REFERENCES

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3. R. Heijungs et al, Environmental life-cycle assessment of products - Guide. Center for Environmental Studies, Leiden University, The Netherlands, Report No. 9267, 1992.

4. Jeroen B. Guin6e, Development of a Methodology for the Environmental Life-Cycle Assessment of Products. Ph.D. thesis Leiden University, The Netherlands, 1995.

5. Margreet van Brummelen, Barend van Engelenburg and Evert Nieuwlaar, Methodology for the Life- Cycle Assessment of Energy Technologies, Second Edition. Utrecht University, Department of Science, Technology and Society, Report No. 94022, 1994.

6. Margreet van Brummelen and Evert Nieuwlaar, Life Cycle Assessment of Gas Engines for Cogeneration of Heat and Power. Utrecht University, Department of Science, Technology and Society, Report No. 94022, November 1994.

7. B.C.W. van Engelenburg and E.A. Alsema, Environmental Aspects and Risks of Amorpous Silicon Solar Cells. Utrecht University, Department of Science, Technology and Society, Report No. 93008, January 1994.

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9. Rob Smit and Evert Nieuwlaar, Life Cycle Assessment of Integrated Coal Gasification Combined Cycle. Utrecht University, Department of Science, Technology and Society, Report No, 94021, November 1994.

10. Evert Nieuwlaar, Assessment of energy use and atmospheric emissions by energy supply systems. 12th Annual Meeting of the International Association for Impact Assessment, August 19-12, Washington DC, 1992. ~.~.

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12. E.A, Alsema and M. van Brummelen, Minder CO2 door PV. Utrecht University, Department of Science, Technology and Society, Report No. 92038, November 1992.