Comparison of Life Cycle Carbon Dioxide Emissions and Embodied

Jul 16, 2009 - emissions), and that tidal power generation was associated with 42.3 ... Renewable generation systems still consume energy and produce ...
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Environ. Sci. Technol. 2009, 43, 6406–6413

Comparison of Life Cycle Carbon Dioxide Emissions and Embodied Energy in Four Renewable Electricity Generation Technologies in New Zealand B R I D G E T M . R U L E , * ,† Z E B J . W O R T H , ‡ AND CAROL A. BOYLE† Department of Civil and Environmental Engineering, The University of Auckland, Private Bag 92019, Auckland Mail Centre, Auckland 1142, New Zealand, Opus International Consultants Ltd., PO Box 5848 Wellesley Street, Auckland 1141, New Zealand

Received January 14, 2009. Revised manuscript received June 16, 2009. Accepted June 24, 2009.

In order to make the best choice between renewable energy technologies, it is important to be able to compare these technologies on the basis of their sustainability, which may include a variety of social, environmental, and economic indicators. This study examined the comparative sustainability of four renewable electricity technologies in terms of their life cycle CO2 emissions and embodied energy, from construction to decommissioning and including maintenance (periodic component replacement plus machinery use), using life cycle analysis. The models developed were based on case studies of power plants in New Zealand, comprising geothermal, large-scale hydroelectric, tidal (a proposed scheme), and wind-farm electricity generation. The comparative results showed that tidal power generation was associated with 1.8 g of CO2/kWh, wind with 3.0 g of CO2/kWh, hydroelectric with 4.6 g of CO2/kWh, and geothermal with 5.6 g of CO2/kWh (not including fugitive emissions), and that tidal power generation was associated with 42.3 kJ/kWh, wind with 70.2 kJ/kWh, hydroelectric with 55.0 kJ/kWh, and geothermal with 94.6 kJ/kWh. Other environmental indicators, as well as social and economic indicators, should be applied to gain a complete picture of the technologies studied.

Introduction Due to increasing concerns over greenhouse gas emissions and declining fossil fuel stocks, interest in renewable energy technologies is growing rapidly (1). Many countries are now looking to invest heavily in renewable energy sources in order to meet the electricity requirements of their populations and reduce their contribution to global greenhouse gas emissions. While this trend has been progressing in Europe for some years now, for example with the German Renewable Energy Act, a prominent recent example has been United States’ President Obama’s announcement to reduce greenhouse gas emissions to 1990 levels by 2020, through a cap-and-trade system (2). While the legislation remains under consideration * Corresponding author phone: +64 27 327 3327; fax: +64 9 373 7462; e-mail: [email protected]. † University of Auckland. ‡ Opus International Consultants Ltd. 6406

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by Congress, this is a clear message that implementation of renewable technologies is being taken seriously. While electricity generation technologies that rely on renewable resources are perceived to be “clean”, a more appropriate definition would be to say that they are “cleaner” than fossil fuel-based technologies (3), by a significant margin. Renewable generation systems still consume energy and produce emissions during the construction, maintenance, and deconstruction/disposal (i.e., the whole life cycle) of the required infrastructure. Because the number of sites for new large-scale hydrodams and wind farms in New Zealand is limited and because hydroelectric schemes are vulnerable to drought, it is likely that new electricity developments will look to alternative technologies. A tidal power scheme comprising 200 marine turbines in the Kaipara Harbour near Auckland has been proposed, to supply up to 200 MW to the national grid (4). A comparison of the sustainability of tidal power to that of other renewable technologies currently in operation in New Zealand would provide an indication of suitable technologies and identify issues which need to be resolved. The objective of this study was to compare the differences in the life cycle sustainability of four renewable energy technologies in a New Zealand-specific context, using the indicators of associated CO2 emissions and total primary energy (embodied energy) consumed. This includes the construction, maintenance, and decommissioning phases, and the transport associated with each phase. It is recognized that other factors influence the sustainability of a product, but an evaluation of both life cycle CO2 and energy will provide an initial comparison. CO2 emissions and embodied energy can reveal the importance of “hidden” processes and materials pertaining to an individual product or component. This in turn gives an indication of the environmental impact of each technology in terms of their contribution to environmental effects such as climate change and resource depletion. However, because sustainability is an extremely broad field, although these indicators provide a salient overview of each system, relying on just two indicators limits the possible measure of sustainability. Many other environmental sustainability indicators (such as ecotoxicity) were not quantified, and social and economic indicators were also ignored. Embodied Energy. Although embodied energy (sometimes referred to as “emergy”) is not yet widely used as an indicator of sustainability, it provides a good indication of the level of resource consumption required to create or extract a product and to transport it to its final destination. Embodied energy includes all primary energy used by a product or process, including fuel and electricity. These energy inputs come from various sources, such as the machinery used in the extraction of raw materials, transport of the raw materials to the processing plant, manufacturing processes, and transport associated with the final end use. In this study, the initial emergy (intrinsic energy stored in raw materials) of the raw materials has been excluded, mainly due to the difficulty associated with quantifying this energy. This, however, maintains consistency with the main source of materials and process data used for this study (5). Carbon Dioxide Emissions. It is generally accepted that anthropogenic CO2 emissions are causing the global warming effect. Given New Zealand’s obligation under the Kyoto Protocol to reduce total greenhouse gas emissions to pre1990 levels by 2012 (6), the Ministry for Economic Development (MED) has advocated a transitional phasing out of all fossil fuel-based electricity generation (7). Thus, it is likely 10.1021/es900125e CCC: $40.75

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FIGURE 1. New Zealand electricity mix for the year ending in June 2007 (9). that the vast majority of investment in future electricity generation infrastructure in New Zealand will be targeted toward renewable technologies. The MED states that CO2-equivalent emissions from electricity generation (including fugitive emissions from geothermal fields, which were not quantified in this study) amounted to 7.5 million tons in 2005. This was 23.2% of New Zealand’s total CO2-equivalent emissions of 32 million tons (excluding emissions from international transport). This is a 17.3% increase from 2004 (8). Figure 1 shows the currently used types of electricity generation in New Zealand and their contribution to total electricity generation. Renewables form the majority of electricity production, with 64% of the country’s total generation; of this, hydroelectric power is by far the most significant source. However, there is still a heavy reliance on gas- and coal-powered generation, which together account for more than a third of electricity generation.

Life Cycle Analysis Methodology ISO Standard 14044 is the standardized method for life cycle analysis (LCA). It consists of four steps: goal and scope definition, inventory analysis, impact assessment, and interpretation. This methodology was followed as closely as possible for this study. A very simplified method of LCA was chosen for this project, whereby basic models were developed, the inventories were entered into LCA software, and the results were compared on a normalized basis (per kilowatt-hour). As suggested by SETAC (10), a level one approach was taken for this study, in which the third step of impact assessment was not applied due to the comparative structure of the study. SimaPro 7 software was chosen for this study to quantify the embodied energy and CO2 emissions for each type of generation technology. This program has the advantage of being a widely used, process-based software (11). A library was created in this program using data from ref 5, based on New Zealand-specific values, and for which travel from overseas had already been incorporated into materials where applicable. (The only exception to this was for fiberglass, for which ref 12 was used.) Where additional travel was applicable, it was incorporated into the pertinent model, as described in the section relevant to each case study. In ref 5, coefficients for both CO2 emissions and embodied energy were calculated using a process-based hybrid energy analysis. Notably, solar energy, human labor, and the calorific value of physical feedstocks were not included. The inputs for this software were materials quantities in tons, travel types, and distances in kilometers or ton-kilometers, and energy (such as diesel used by a crane) in megajoules. System Boundaries. As this study was of a comparative nature, the LCA system boundaries were drawn around the unique aspects of each technology, and apparatuses common to all types of power generation were ignored, including components such as paints and lubricants, as well as

transmission and distribution infrastructure such as switchyards. The cutoff point for inclusion of cables and transmission was at the point of linkage to the national grid, assumed to be at the step-up to 11 kV. CO2 emissions and embodied energy related to the manufacture of materials were included but assembly or fabrication of whole components was not. For example, energy and CO2 emissions related to the fabrication of steel for a wind tower were counted but welding the panels together was not. Rough sensitivity analyses suggested that the effects of including this type of contribution to both embodied energy and CO2 emissions changed the final values by less than one percent for each technology, meaning that the error for each model would be higher than the effect of this omission. Additionally, energy consumed and CO2 produced in processes were quantified but not the life cycles of the machinery carrying out these processes, as the machinery has a separate life cycle. For example, transportation of a drill rig and the diesel used to drill a borehole were included but not the components of the drill rig itself, as it was assumed that manufacturing emissions and energy would be proportional to the emissions and embodied energy of the materials and, in this context, the omission of manufacturing requirements would have a negligible effect on the relative performance of the compared technologies. Fugitive emissions from geothermal fields were noted, though not added to the result for geothermal power generation, but all other “CO2 emissions” pertaining to this study arose from construction, maintenance, and decommissioning of power stations, since renewable technologies (apart from geothermal) do not emit CO2 during normal operation. Geographical and Temporal Effects. Materials used in construction (such as the choice between imported and locally sourced materials), distance and type of transportation, and the electricity mix (whether dominated by renewable or fossil fuel sources) vary depending on geographical location. Legislation, standards, facilities, and technical capability (or lack thereof) can differ locally, regionally, and nationally, such as the opportunity for recycling or reuse (which influences decommissioning decisions). Therefore, a specified geographic setting is important. In this study, current New Zealand-wide averages of CO2 emissions and embodied energy data, both sourced from ref 5, were used for the materials and process data, and the relevant estimated transport distances were applied for individual case studies. This normalized the case studies to the present day and ensured that all information used was relevant to New Zealand. Temporal bias was further removed using a nominal 100year lifespan, which was considered a reasonable design lifespan for a hydrodam, the longest of any single component included in the study. The shorter lifespans of all other VOL. 43, NO. 16, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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individual components were factored up to this, allowing for inclusion of periodical replacement of components as well as the additional CO2 emissions and embodied energy related to this maintenance. Actual expected lifespans, rather than economic lifespans, were used. For example, the tidal power system included array cables estimated by the manufacturer to have an expected lifespan of 40 years, while the cables would sit on a concrete bed which would not have to be replaced during the lifespan of the system. As 100/40 ) 2.5, for the array cable component, the 100-year life cycle analysis includes the initial concrete and cable, as well as two and a half additional cables which would be required for the tidal system if it were to last 100 years. This allowed fair comparison between power stations of varying predicted lifespans. Functional Unit/Load Factor. As the power station models were based on real case studies, there was a natural level of disparity between the stations, each having a different age, installed capacity, and load factor (the percentage of time a power station is actually generating power). In order to eliminate the effects of the varying individual characteristics of the power stations so that the results could be realistically compared, they were normalized to a functional unit of grams of CO2 per kilowatt-hour and kilojoules consumed per kilowatt-hour. For this, the total life cycle CO2 emissions and embodied energy of each technology (as calculated using SimaPro) were divided by the total electricity generated over each power station’s nominal 100-year lifespan. Kilowatt-hours are adopted as the functional unit for the majority of LCAs involving electricity generation (13) and therefore application of this functional unit is in line with conventional research. Life Cycle Phases. For the construction phase, common, easily accessible materials such as diesel, wood, and concrete were assumed to be sourced and transported from the nearest major center. It was assumed that all steel components would be transported from the nearest port, because although steel is a common material, components for power stations are highly specific and so would require individual fabrication. It was assumed that no material waste was produced during the manufacture of components or that any waste produced was recycled for subsequent manufacturing. The exception to this was fiberglass components, where a waste rate of 10% was assumed, as the cured fiberglass cannot be recycled or reused. The decommissioning breakdown was assumed to be the same for each power station, except where specific circumstances applied. A disposal scenario of 50% recycling and 50% landfilling was assumed for materials that could be recycled, such as aluminum and steel (except for reinforcing steel), and a 100% landfill scenario was assumed for all other materials (including plastics, concrete, and fiberglass). Concrete reinforcing steel was assumed to be disposed of 100% by landfill at decommissioning, because of the difficulties associated with separating and recycling this material. The actual percentage of materials recycled may be higher than those assumed, in some cases as high as 90% (14). However, given the lack of available data for recycling of specific components in New Zealand, it was decided to adopt a conservative approach to avoid under-representing the end-of-life phase. Due to the nature of tidal power, hydropower, and geothermal extraction, some equipment (or parts thereof) is left in situ at decommissioning; additional materials, such as the concrete used to fill geothermal boreholes, was also considered in these cases. Reuse of individual components was included where this was likely, as well as the transport (presumably by rigid truck) involved in the scenario. A flat distance of 50 km was generally assumed for recycling and landfilling transport (except where specific cases differed from this), but for reuse scenarios, it was assumed that these 6408

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components would be transported to Auckland, the most likely place for disassembly and/or reuse.

Case Study Models Geothermal Electricity Generation. The geothermal power station was modeled on Wairakei Power Station, a 162 MW plant commissioned in 1958. Wairakei’s two power houses, of 67.2 MW and 90 MW capacities, produce around 1550 GWh of electricity per year. Over the assumed 100-year life cycle, it was estimated that Wairakei would produce a total of 143 × 109 kWh. Wairakei has a particularly high load factor of 93%, which means that it generates at close to full capacity for the majority of the time. As the power station was altered in the early stages of design, the eleven turbines have varying capacity, so for simplification, the median-sized 11.2 MW turbines were modeled. Because of the corrosion and build-up of silica from geothermal fluid, recycling was not considered for disposal of field equipment, although it was considered for powerhouse machinery as this equipment does not experience the same level of corrosion; as such, reuse of half of the turbines was included in this model. Wairakei’s binary cycle plant greatly improves its efficiency. Production of isopentane for the binary cycle plant was not included due to the difficulty associated with quantification, although this is most likely small compared to the production of the machinery involved in the binary cycle. Borehole drilling included production wells, reinjection wells, and monitoring wells, as well as exploration. The total value for drilling was sourced from boreholes drilled in the past, and the average borehole depth of 660 m was used for future drilling (some of which will be deeper, some shallower). On the basis of the literature data, it was assumed that a production well would last 17 years before it would be “replaced” by another well, following which a new borehole would be drilled (derived from ref 15). Although up to 1110 TJ/year of waste heat from Wairakei is piped to neighboring businesses (16) including a prawn farm and a tourist operator, this heat would be prohibitively expensive to generate without the presence of the geothermal plant. As such, this “avoided” electricity generation does not really count as avoided for life cycle analysis purposes and so was ignored for this study. Fugitive CO2 emissions from geothermal electricity generation were not added to the final result in this study. Hydroelectric Generation. The life cycle inventory for hydroelectric power generation was based on Clyde Dam and power station. Clyde Dam is located on the Clutha River in the Central Otago region of the South Island. Construction began on the Clyde Dam and power station in 1980 with all four generating units being finally commissioned in 1992 (17). The power station uses four 120 MVA AC generators, each driven by a 116 MW vertical Francis turbine (18). The total installed capacity of the four turbines at Clyde is 432 MW. Electricity generated at the Clyde power station is stepped up via an integrated switchyard, and transmitted by 220 kV lines to the national grid. A conservative load factor of 50% percent was assumed for this study. Although each turbine is rated at 116 MW, the maximum generating capacity is limited to 108 MW. This is due to energy being lost through heat, noise, and vibration during transmission from the turbines to the generators (19). This gives the turbines at the Clyde power station an efficiency of 92%. Using the stated assumptions, Clyde power station was calculated to have a total output of 186 × 109 kWh over the assumed 100-year life of the power station. For this model, a total of 50 km of roads was constructed in parallel with the dam and power station construction to replace roads that would be inundated by the storage reservoir. However, it was assumed that, had the dam not been constructed, these or similar roads would eventually

TABLE 1. Total and Normalized Indicators and Energy Generation over the 100-year Assumed Lifespan for Each Technology indicator

geothermal

hydroelectric

tidal

total energy generation (kWh) total CO2 emissions (kg of CO2) normalized CO2 emissions (g of CO2/kWh) total embodied energy (MJ) normalized embodied energy (kJ/kWh)

143 × 10 801 × 106 5.6 13.5 × 109 94.6

186 × 10 861 × 106 4.6 10.2 × 109 55.0

67.5 × 10 120 × 106 1.8 2.85 × 109 42.3

have been built in the area. It was considered appropriate to include 50% of the materials used for the roads in the inventory (i.e., equivalent to 25 km of road) to account for the additional requirements for roads attributable to the dam (i.e., increased width due to higher heavy vehicle volumes, etc.). For the Clyde dam case study, the decommissioning of the dam itself was not considered in the LCA, as it is common practice in New Zealand to leave hydrodams in place, with the reservoir becoming a recreational lake. Because the lake does not get drained, any methane produced from flooded vegetation never actually gets released, as it remains trapped in the river sediments. This may or may not apply to other hydrodam case studies. Tidal Electricity Generation. The tidal power generation system was modeled on the scheme proposed by Crest Energy, in which 200 marine turbines would be sited in the Kaipara Harbour north of Auckland, to produce a peak of approximately 200 MW of electricity for supply to the national grid. The load factor for the tidal scheme was taken as 37% after taking tidal flux into consideration, equating to 77 MW on average across the tide or 0.385 MW per turbine unit (4). This figure also includes maintenance considerations, being that each turbine will be removed for maintenance for 1 week every 4 years. The turbines will operate for 16 h per day, during tidal flux. The total predicted output over an assumed 100-year lifespan was 67.5 × 109 kWh. Note that as this is a proposed scheme, the actual values are subject to change and, as such, the results should be treated with care. Tidal electricity generation is similar in its configuration to wind electricity generation, however the same flow generates several hundred times more power. The turbines would be located in the middle of the harbor channel, which means that undersea cables would be required to transfer the electricity generated to the nearest substation, and because they generate in DC, a DC-AC converter would be required. Because the scheme’s turbine, generator, rotor, and stator are all in one unit, there is just one moving part, and therefore the system will have an overall low maintenance requirement. The inert gravity bases can be left on the seabed to simplify the decommissioning process, and there would be opportunity for cable reuse and steel recycling. As this scheme has not yet been built, all components were assumed to be manufactured in Auckland (except the cables, which are intended to be manufactured in New Plymouth), as this was the most probable quantifiable scenario. Wind Electricity Generation. The case study for wind electricity generation was Te Apiti wind farm, located in the lower North Island of New Zealand at the southern end of the Ruahine Range. The wind farm became fully operational in October 2004 with a total installed capacity of 90.75 MW. Electricity is generated by 55 wind turbines, each with a capacity of 1.65 MW, and fed to a nearby Transpower substation at Woodville for distribution to the national grid. Each turbine was assumed to generate electricity for 90% of the year. This availability factor takes into account that the turbines can only generate power under appropriate wind conditions, typically 4-24 m/s. However, within this range of wind speeds, the turbines can only deliver their full

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30.9 × 109 93.8 × 106 3.0 2.17 × 109 70.2

generation capacity at a speed of approximately 15 m/s. To account for this, an annual load factor of 45% was applied; this being generally accepted as the ratio of actual generation to that the turbine would produce at full capacity during a given year under New Zealand wind conditions (20). An estimated annual maintenance requirement of 350 h per year was then subtracted from the total potential generating hours per year (21). During this time the turbine does not generate electricity. The total output from the wind farm over the 100-year study life was calculated to be 30.9 × 109 kWh.

Results and Discussion Results. Table 1 shows the estimated total CO2 emissions, embodied energy, and energy generation for each of the four renewable energy technologies studied, over the whole 100year life cycle assumed for this study. Using the life cycle energy generation for each system, the total life cycle CO2 emissions and embodied energy were normalized to obtain the CO2 emissions and embodied energy per kilowatt-hour of output. Wind electricity generation, with the lowest total life cycle CO2 emissions and total embodied energy, also had the lowest total life cycle energy generation of the four case studies. The life cycle CO2 emissions and embodied energy for tidal electricity generation were only slightly higher, compared to the very high values of all three calculated for hydroelectric and geothermal electricity. While hydroelectric and geothermal electricity generation both showed significantly higher life cycle CO2 emissions and embodied energy than wind and tidal electricity generation, the total energy generation for hydroelectricity was higher than that calculated for geothermal electricity. Geothermal and hydroelectric electricity generation benefit from their higher load factors, which allow a much greater output in the 100-year plant lifespan compared to tidal and wind electricity generation. So, although there is a much higher materials investment in geothermal and hydroelectric power, the load factor brings the values for both CO2 emissions and embodied energy into close proximity with the values for tidal and wind electricity generation. Overall, tidal electricity generation had the lowest of both CO2 emissions and embodied energy per kilowatt-hour compared to the other technologies studied. Conversely, geothermal electricity generation had the highest values of CO2 emissions and embodied energy per unit output. The difference between wind electricity generation and hydroelectric generation was difficult to determine, since while wind electricity generation had lower CO2 emissions per kWh, hydroelectric generation performed better in terms of embodied energy per kilowatt hour. Figure 2shows the breakdown of percent contribution to total CO2 emissions and embodied energy by component for each technology, with each component further subdivided into percent contribution of each life cycle phase. Where the decommissioning phase accounts for negative contribution, such as in the case of recycling, this is shown as a negative bar on the left-hand side of the chart. Discussion. As this was a comparative study and components common to all power stations (such as transmissions VOL. 43, NO. 16, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Continued. 6410

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FIGURE 2. Breakdown of percentage contribution to life cycle CO2 emissions (left) and embodied energy (right) by life cycle phase for each major component. Cutoff is set at 1% of combined CO2 emissions and embodied energy contributions (1% of 200%). Negative contributions can arise from recycling or reuse of components at the end-of-life phase. systems) were not considered, the results may be used for was quantitative rather than comparative) (24). On the other comparison but not as definitive values. Additionally, hand, another study calculated a CO2 emissions factor (including fugitive emissions) of 37.8 g of CO2/kWh for hot manufacture of materials for each plant was quantified but dry-rock geothermal electricity (23). Taken altogether, these fabrication of components from those materials was not. figures suggest that the greenhouse gas emissions for This means that the results calculated are probably slightly geothermal electricity generation originate primarily from lower than the reality. fugitive CO2 emissions. Although geothermal wells allow CO2 A previous study undertaken by the International Energy to become concentrated at the surface of a well, these Agency (22) suggests CO2 emissions factors of anywhere between 1.9-35 g of CO2/kWh for wind farms and 1.3-11.3 emissions are generated by natural processes rather than by g of CO2/kWh for concrete reservoir type hydroelectric the presence of a power plant and, hence, may be considered schemes. Another study calculated the CO2 emissions coefnatural rather than anthropogenic (26), which means they ficient at 10.0 g of CO2/kWh for large-scale hydroelectricity do not pertain to the UNFCCC (8), and as such, they have and 10.2 g of CO2/kWh for on-shore windfarms (23). A Japannot been added to this study’s result for geothermal power based study found that the CO2 emissions coefficient was generation. The fugitive emissions refer to operational CO2, 11 g of CO2/kWh for hydroelectricity and 29 g of CO2/kWh rather than life cycle CO2; the latter comprises the CO2 emissions resulting from the lifespan of plant equipment for wind-generated electricity, but noted that in future, the and its maintenance. However, these additional emissions factor for wind would drop to around 20 g of CO2/kWh due to the larger size of turbines, which leads to higher energy should be taken into account when considering the susoutput per unit of material input (24). While these results tainability in terms of CO2 emissions from geothermal power generation. The CO2 emissions from geothermal fields are vary quite widely, it was felt the results obtained from this highly site-specific, and so CO2 release will vary widely study generally conformed, considering the comparative between geothermal power plants. In New Zealand, geonature of the study. As nonbarrage tidal power generation thermal fugitive CO2 emissions are estimated annually by is in its infancy worldwide and several different types of tidal the MED as ranging from 30 to 570 g of CO2/kWh, the average electricity generation exist, no data were available for a being 80 g of CO2/kWh; Wairakei Power Station is at the configuration similar to that used in this study, and so, no lower end with an estimated 40 g of CO2/kWh (27). comparison can be made. Likewise, in hydroelectric power systems, methane However, in addition to its 5.6 g of CO2/kWh, geothermal electricity generation also releases around 30 g of CO2/kWh released from the decomposing of flooded organic matter during normal operation (25). None of the other technologies increases the technology’s associated greenhouse gas studied release CO2 during operation (apart from mainterelease. In some cases, trees are cut down before the dam nance, which is included in life cycle CO2). A Japanese study is filled, reducing the associated methane production; while calculated a CO2 emissions factor (not including fugitive in other cases, vegetation is left as is, and depending on emissions) of 15.0 g of CO2/kWh for geothermal plants (this the location of the dam, there may be more or less may be higher than the value found in this study as the former vegetation per unit area, as well as more or less flooded VOL. 43, NO. 16, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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area. The equivalent CO2 production associated with hydrodam flooding has been calculated as 34 g of CO2/ kWh and 65-72 g of CO2/kWh in Canada and Finland, respectively (28). If the geothermal fugitive emissions were added to the CO2 emissions result for geothermal power production, fairness would dictate that the figures for fugitive methane production should be added to the result for the hydroelectricity case study. However, as the fugitive methane emissions from Clyde Dam are not known, the results would be skewed by the inclusion of only the geothermal fugitive emissions. Hence, this effect should be noted, even though it has not been added to the results. No specific data were available to verify the validity of the embodied energy calculations. However, interpolating energy payback ratios given by ref 13 suggests values of 45 kJ/kWh and 18 kJ/kWh for wind electricity generation and hydroelectric generation, respectively. These values are both lower than those obtained from the current study. As the current study was specific to New Zealand conditions and the emissions values compared well with those from other studies, the embodied energy values obtained were considered to be valid. Life cycle analysis on electricity generation technologies has not been carried out in New Zealand, so it is not possible to compare the results of this study with others for either renewable or nonrenewable energy technologies. However, data for the raw materials of gas and coal (to the power station gate) have been calculated; from this, it was found that the New Zealand CO2 emissions coefficient for coal is 325.7 g of CO2/kWh and for natural gas is 193.6 CO2/kWh (including fugitive emissions) (5). The embodied energy coefficient for coal mining is 3714 kJ/kWh and for natural gas is 7608 kJ/kWh (3679 kJ/kWh for extraction and a further 3929 kJ/kWh for treatment and distribution) (5). For embodied energy, the electricity generation infrastructure component is not included in these values but would be on a similar order of magnitude to the results from this study, which are significantly smaller than the intrinsic embodied energy and CO2 emissions values for the feedstock fossil fuels themselves. These values are corroborated by overseas studies (29, 30), with values for CO2 emissions and embodied energy being approximately 2 orders of magnitude higher than the values obtained for renewable technologies in this study. The nature of fossil fuels themselves translates almost all of the CO2 emissions and embodied energy to the operational phase. This demonstrates that in terms of the two indicators investigated, the extremely low CO2 output and embodied energy of all four renewable technologies studied (including geothermal power generation even with the addition of fugitive emissions) make fossil fuel-based technologies far inferior. This study does not address supply issues. Tidal power can only generate electricity for 16 h per day, during tidal flux, whereas geothermal electricity generation is constantly available and does not depend on weather conditions like hydroelectricity and wind electricity generation. In conclusion, the objective of comparing New Zealand’s four foremost renewable energy technologies through their life cycle CO2 emissions and embodied energy was achieved, and it may be said that on the basis of these two indicators, tidal power is the more sustainable technology, and geothermal power is the least, especially when considering fugitive emissions. However, this does not present a holistic picture of any of the four technologies discussed, since other environmental indicators, as well as economic and social indicators, have not been studied. These other indicators may be significant, especially in considering effects such as the impact of tidal power generation on livelihoods from fishing, the often high land use of arable areas by hydrodams, 6412

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the noise nuisance of wind farms, and the fugitive emissions of geothermal power generation. Limitations. The main limitation of this study was a lack of information, and the main source of error corresponding to this limitation was the need to extrapolate data and make assumptions to simplify the models and fill in any gaps. The nature of LCA means that its accuracy depends heavily on its assumptions; hence, the assumptions developed for this project were aimed at being as reasonable and realistic as possible, in an attempt to minimize the error inherent in modeling complex systems like those studied here. Because this study focused on CO2 emissions and embodied energy, many other environmental impacts of electricity generation were not quantified. Social and economic factors were also excluded. This limits the applicability of the results; they should not be used in isolation but examined as part of a complete impact assessment. Further investigation into weighting and other social, economic, and environmental indicators (such as reliability, land use, noise, and aesthetics) is needed to determine the overall sustainability of each technology. Methods such as sensitivity analysis have been developed in attempts to reduce the uncertainty associated with LCA, of which the effectiveness, like LCA itself, seem to depend on the assumptions and boundaries set. Informal sensitivity analyses were performed on the models used in this study to determine whether there was a significant variation in the levels of CO2 and embodied energy associated with each technology. It was found that since, for each technology, the vast majority of CO2 emissions and embodied energy derived from a few major components, other components which were not accounted for here, such as the related life cycle of a crane used to drill boreholes (as distinct from the diesel consumed in drilling, which was accounted for), constituted negligible additions to the results found (on the order of 1%). Hence, statistical sensitivity analyses were not performed on the results obtained in this study. However, in definitive LCA (which includes every aspect of power generation, rather than solely the unique aspects of each technology, as did this study), this type of error analysis may provide more insight into the distribution and potential minimization of CO2 emissions and embodied energy in each system.

Acknowledgments The authors wish to thank Crest Energy Ltd and the Foundation for Research Science and Technology New Zealand for their contributions to this study. The lead author also wishes to thank the anonymous referees and Dr. C. P. Hue.

Supporting Information Available The materials flow diagrams generated by SimaPro for this study are included as Supporting Information. This material is available free of charge via the Internet at http://pubs. acs.org.

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