Are Wave and Tidal Energy Plants New Green Technologies

Article pubs.acs.org/est

Are Wave and Tidal Energy Plants New Green Technologies? Mélanie Douziech,*,†,‡ Stefanie Hellweg,† and Francesca Verones§ †

ETH Zurich, Institute of Environmental Engineering, CH-8093 Zurich, Switzerland Radboud University Nijmegen, Department of Environmental Science, Institute for Water and Wetland Research, 6500 GL Nijmegen, The Netherlands § Industrial Ecology Programme and Department of Energy and Process Engineering, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway ‡

S Supporting Information *

ABSTRACT: Wave and tidal energy plants are upcoming, potentially green technologies. This study aims at quantifying their various potential environmental impacts. Three tidal stream devices, one tidal range plant and one wave energy harnessing device are analyzed over their entire life cycles, using the ReCiPe 2008 methodology at midpoint level. The impacts of the tidal range plant were on average 1.6 times higher than the ones of hydro-power plants (without considering natural land transformation). A similar ratio was found when comparing the results of the three tidal stream devices to offshore wind power plants (without considering water depletion). The wave energy harnessing device had on average 3.5 times higher impacts than offshore wind power. On the contrary, the considered plants have on average 8 (wave energy) to 20 (tidal stream), or even 115 times (tidal range) lower impact than electricity generated from coal power. Further, testing the sensitivity of the results highlighted the advantage of long lifetimes and small material requirements. Overall, this study supports the potential of wave and tidal energy plants as alternative green technologies. However, potential unknown effects, such as the impact of turbulence or noise on marine ecosystems, should be further explored in future research.

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INTRODUCTION Concerns about climate change drive the search for energy generating technologies with little environmental impacts. New technologies using the power from tides and waves seem to be a promising alternative to conventional fossil-based technologies in this regard. The technically available amount of tidal energy is estimated to 1TW worldwide.1 There is a projected 2TW that can be captured with wave energy.2 Assuming an average, overall efficiency for the installed plants of 40%3 these amounts translate to installed capacities of around 3500 and 7000 TWh annual electricity production, respectively. As a comparison, the worldwide electricity consumption for the year 2013 was 21 538 TWh.4 Wave and tidal energy could therefore contribute to satisfying the world energy demand. In addition to the high potential energy harvest, wave and tidal energy plants benefit from a reliable power generation scheme. Compared to other renewable energy sources, wave and tidal energy generation can be well predicted.2,3 Despite these advantages, only few devices have been tested offshore and none to the extent of large farms, like offshore wind power for example. Still, in the past years, research has evolved and many different potential plant designs were developed, making the broad use of wave and tidal energy more possible. However, the following two questions remain. At what environmental cost does this additional energy provision come? And can wave and tidal power plants be called “green © XXXX American Chemical Society

technologies”? To address the latter question, the following definition will be used.5 A green technology should not alter the climate, should conserve resources, and have no harmful effect on human health or ecosystems. Further, one expects green technologies to be less harmful to the environment than more traditional means of energy generation. In our study, both questions will be answered by assessing the environmental impacts of three tidal stream devices, one tidal range plant, and one wave energy harnessing device and by comparing them to conventional energy conversion systems. Life cycle assessment (LCA) is a commonly used method to quantify the environmental impacts of a product over its entire life cycle. Some LCA studies of wave and tidal power plants exist. However, up to now only the carbon and energy intensity of selected designs of wave and tidal energy plants have been quantified in these studies.6−8 Our study is the first to compare three different designs of tidal energy harnessing devices and to take impacts beyond climate change into account. Apart from ref 7, no other study so far compares the performance of wave and tidal energy plants. Another novelty of our study is the use of Received: January 11, 2016 Revised: May 23, 2016 Accepted: June 13, 2016

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DOI: 10.1021/acs.est.6b00156 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology Table 1. Key Data for the Five Analyzed Wave and Tidal Energy Plants plant

power output (kw)

lifetime (years)

frequency of service (years)

efficiencya (%)

annual expected power generation (GWh)

mass (tonnes)

Annapolis tidal plant SeaGen HS1000

20 000

75

25

29

50.8

1.74 × 106

1200 1000

25 25

5b 5

44 40

4.6 3.5

549 1.15 × 103

HydraTidal Oyster800

1500 800

25 20

5b 5b

38 40

5.0 2.8

712 1.93 × 103

location Annapolis (CA) Ireland Scotland (UK) Norway Scotland (UK)

references 24,25 26,27 28,29 30,31 20

a

The efficiency describes the ratio between the installed capacity and the electricity delivered to the grid by the plant. bA semiannual inspection is also planned. The service encompasses the maintenance operations. They differ from one plant to the other and are explained in more details in the following chapters.

three tidal stream devices: the Andritz Hydro Hammerfest HS1000 deployed in Scotland, the Marine Current Ltd. SeaGen plant in Ireland, and the Straum Group HydraTidal plant in Norway. Illustrations of the plants can be found in ref 15−19. Whereas the designs of tidal energy plants show some similarities, wave energy harnessing devices have various ways of functioning. The study analyzed the Oyster800 plant operated by Aquamarine Ltd., in Scotland. It works according to the oscillating wave surge converter principle.20 A hinged flap is attached to the seabed and moves forward and backward with the waves thus activating a piston pumping freshwater ashore. This water then goes through a turbine which activates a generator.19 The choice of the plants was driven primarily by the readiness of the manufacturers to share data about their plants, which was the case for the Annapolis tidal range, the HS1000, and the HydraTidal plant. The reason that motivated the investigation of the SeaGen and the Oyster800 plant was the availability of literature about their carbon footprints6,7 to which the result for the climate change impact category derived here will be compared. Goal and Scope. The functional unit used is the production of 1 kWh electricity fed into the grid by a wave or tidal energy harnessing plant. Table 1 shows the characteristics of the five plants assessed in this report. For each plant, the LCA includes the construction, including the installation, operation, maintenance and decommissioning phases. For each phase, as many processes as possible were included either through direct modeling or through the use of already existing processes from the ecoinvent database.10 Examples of processes taken from ecoinvent are the metal or steel product manufacturing processes used to represent the material and energy need to produce some elements. The recycling of the materials was modeled according to the ISO/TS 14067 closed-loop procedure.21 The input flows were modeled to represent only virgin materials in order to allow taking credit for the avoided primary production at the end-of-life as new material is produced out of the scrap. The end-of-life (EoL) scenario of steel was modeled with 90% of secondary steel production, in the form of unalloyed steel, and 10% of steel disposal to inert material landfill.6,22 In accordance with the ISO/ TS 14 067 standard, credits were granted for the avoided primary production of unalloyed steel. An overview of all implemented EoL procedures can be found in the Supporting Information (SI), S1. Another important assumption is the choice of the electricity used as input in some processes. This electricity was always taken from the country where the plant operates. Therefore, the electricity mixes from Great Britain, Norway, and the U.S. (as replacement for the Canadian mix not available in the ecoinvent database v2.2) were used. Ecoinvent v.3

scenarios to assess the influence of design decisions on the environmental impacts of the plant. First, we will compare the plants to each other to highlight their trade-offs in terms of environmental impacts. Then, we will compare the results to established renewable energy technologies like wind power plants and photovoltaic (PV) panels and also to more traditional ways of electricity generation, like coal and natural gas-based power plants. This will help answer the question whether wave and tidal energy harnessing devices can be considered green technologies in comparison to existing and operating technologies.

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MATERIALS AND METHODS This study used the ARDA software developed at NTNU9 and the ecoinvent database v.2.210 for background database processes to conduct attributional, process-based life cycle assessments of five different designs of wave and tidal energy harnessing plants. Where available, inventory data about the foreground system was gathered from plant developers and literature. For the life cycle impact assessment (LCIA), the ReCiPe 2008 methodology at a midpoint level with a Hierarchist perspective11 was used. This methodology was chosen because of the wide range of potential impacts it covers and its existing implementation in the ARDA software. ARDA is an internal matrix-based software running life-cycle calculations for a system, whose foreground and background inventory data have to be provided in an Excel template. Foreground inventory data relate all the modeled system’s components to each other, while background data list the ecoinvent process requirements for all foreground processes. Both matrices are combined to describe all the inputs required from the technosphere. Direct stressor emissions of the foreground are also provided in the Excel template. The calculation steps then implemented by the software are extensively described in ref 12. The ARDA software also implements the methodology of structural path analysis (SPA) to identify the contributions of single processes to the overall result of an impact category. More details on this procedure are given in ref 13. Description of the Powerplants. Tidal energy can be harnessed either with tidal range plants or with tidal stream devices.14 While the former devices are very similar to hydropower dams, the latter function like wind power plants under water, using the tidal current to rotate a turbine and activate a generator. Tidal range plants require larger construction works and are only suitable in places where the tidal amplitudes are significant. We investigate the Annapolis tidal range plant operated by NovaScotia Power in Canada and B


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profile and trade-offs for wave and tidal power, which can be fully achieved without addressing end points. In addition, we made use of the option for structural path analysis implemented in the ARDA software to identify the processes contributing most to the final values.13 We used this on the one hand to explain the impact category results, and on the other hand to divide the total impact category value into six life cycle stages. Processes were assigned to either maintenance, operation, end-of-life benefits and burdens, and transport in this order. The paths not assigned to any of the aforementioned life cycle stages, were classified as belonging to the construction phase. The sequence according to which the paths were allocated to the different life cycle stages was chosen primarily because maintenance, operation, and endof-life were or consisted of user-defined processes, thus could easily be classified. In addition, end-of-life and transport ecoinvent processes were categorized accordingly in the database used by ARDA. This again, made the allocation straightforward. As a result, the EoL of the tailings at the mine and of the materials used in the fabrication of plant elements was accounted for in the EoL stage, and not in the construction stage. Around 10% of all identified processes could not be attributed to a single life cycle stage path because of their very small contribution to the overall results. They were assigned to the category “not classified.” Sensitivity Analysis. To test the sensitivity of the results, six alternative scenarios were applied to all five plants. The LIFE scenario assumed a 20% shorter plant lifetime, and the PROD scenario a 20% lower power output. In the RECY scenario, the influence of a 10% lower recycling rate for steel, copper, cast iron, and aluminum was quantified, meaning a 10% higher amount of metal was landfilled. In the TRAVEL scenario the travel distances for the installation, maintenance, EoL, and material transport increased by 10%. In the ELE scenario, the electricity need that was modeled was directly replaced for all plants by the electricity mix from the Union for the Co-ordination of Transmission of Electricity (UCTE). The UCTE electricity mix was chosen because it was the most representative of an averaged European consumption in the version 2.2 of the ecoinvent database. Finally, the MANU scenario increased the material input to the processes of steel and metal product manufacturing by 10%. An overview of the scenarios is given in SI, S2. In addition, three scenarios were developed for the Oyster800 device only, and three others only for the HydraTidal device. For the Oyster800 plant, 1/2MASS denotes a scenario in which the material masses taken from ref 20 were halved. This scenario was motivated by contradicting information found in ref 20 and on the Fugro Seacore Web site49 about the masses of the piles supporting the device. Further, the 50% scenario assumed an efficiency of 50% for the Oyster800 plant as in ref 7. Finally, the 3Plants scenario assessed the influence of adding two other devices to the installed freshwater pipes which corresponds to the final plans of Aquamarine.20 The scenarios developed for the HydraTidal plant aimed at quantifying the influence of the choice of wooden blades. Scenario 1/2Blades assumed that only half of the blades were replaced upon maintenance. Scenario COMPBLA replaced the wooden blades by blades made out of the same composite material used for the SeaGen plant. The reduced maintenance operations were also taken into account. Finally, in scenario COMPBLAGB the blades were replaced by composite ones and the Norwegian electricity mix was replaced by the one from Great Britain to make the results more comparable to the ones of the SeaGen plant.

accounts for a Canadian electricity mix. The latter has, in contrast to the American mix, a much higher share of hydropower (58% compared to 7%).23 This difference leads to 2.5 times higher LCIA results, on average, for the production of 1 kWh of US electricity mix compared to Canadian electricity, and this over all impact categories of the ReCiPe methodology. This should be recalled when analyzing the impact assessment results of the Annapolis tidal range plant. In addition, a sensitivity analysis is conducted replacing the electricity mix with a standard one for all plants, in order to cancel out some of the potential differences. Life Cycle Inventory. Here, some of the important assumptions made upon the life cycle inventory for each plant will be highlighted. More details are provided in SI, S3. The three tidal stream devices have, as previously mentioned, a similar design. Only few modeling and material differences exist between the SeaGen and the HS1000 plant. Both have rotorblades made out of composite material,6,32 a rotorhub of cast iron,32,33 bearings,34,35 a gearbox,6,28 a generator,36 and other similar electronic equipment.37,38 Furthermore, their supporting structures are both made out of steel.6,39 An important difference is their anchoring on the seabed. Whereas, SeaGen has a quadropod structure grouted onto the seabed, HS1000 uses only ballast weights. Even though its design is similar to the other two tidal stream devices, the HydraTidal plant shows some specificities: The rotorblades are made out of wood,31 and neither gearbox nor bearings are needed as two pump units are used per powertrain to bring oil to the hydraulic motor.38,40 The supporting and anchoring structures are very similar to the ones used for the SeaGen plant.30,41 For all three plants, offshore cables were installed to transport the electricity ashore.6,42 The Oyster800 harnesses the mechanical energy by a hinged flap made out of carbon steel, and glass-reinforced plastic.20 Its movement activates four hydraulic modules36,43,44 that pump water ashore through a pipe installed on the seabed.20,45,46 The installation ashore consists of a turbine47,48 and a generator coupled to a transformer.36,37 The entire structure is anchored on the seabed.20,49,50 The Annapolis tidal range plant requires the modeling of a Straflo turbine ensemble including a generator, seals, filtering mechanisms, and wicket gates with activating servomotors.51 The causeway52 linking both sides of the Annapolis river was modeled assuming time requirements and using ref10 for the materials needed. The transformation from sea and ocean to traffic area was implemented as well. Common to all plants is the need for sacrificial anodes,53 lubrication,10 and paint.10,54 Most of the manufacturing steps were modeled using the processes steel or metal product manufacturing from ecoinvent.10 Still, some manufacturing operations were modeled separately. Sand blasting,55 grouting,56 drilling,57 or the prepreg process,58 were modeled from literature, and in single cases, the ecoinvent processes of molding, rolling or welding of steel plates were used. The transport steps were mostly modeled using information about the location of the subcontractors and indications on the type of transport from the different companies. When the latter was missing, the assumptions of ecoinvent were used (e.g., using 30% transport by train and 70% by lorry as in the cable manufacturing process of ecoinvent to account for the transport of the various cables needed by the plants). Life Cycle Impact Assessment. The results for all impact categories of the ReCiPe 2008 methodology (midpoint) are compared for the five different plants. We remained on the Midpoint level, as the goal of the study was to discuss the impact C


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Environmental Science & Technology Carbon Payback Time. To compute the carbon payback time for each plant, the results of the climate change impact category were used. eq 1 defines this carbon payback time.6 CO2 payback =

Oyster800 differs because of its high mass and the diversified material use. Finally, the material breakdown of the Annapolis tidal range plant is dominated by the need for gravel and rock fill for the building of the causeway. The water used for the lubrication of the seals over the entire plant’s lifetime is also a major element. After these materials, steel is an important component for the plant. Life Cycle Impact Assessment. The impact categories climate change, human toxicity, marine ecotoxicity, metal depletion, and particulate matter formation (Figure 2) were used to correspond to the definition of green technologies derived in the Introduction. The results of all impact categories can be found in the SI, S5. The results of the five plants are compared to established energy generating forms namely offshore wind energy,59 hydro power from Great Britain (GB),10 solar photovoltaic from GB,10 natural gas from GB,10 and electricity from hard coal from UCTE.10 (SI, S6) The construction and EoL burden phases dominate the values of the displayed impact categories clearly (Figure 2). The EoL burdens contribute mostly to the toxicity impact categories because of the disposal of the sulfidic tailings from the use of copper, especially in the metal product manufacturing process of ecoinvent. The disposal of nickel smelter slag was also found to be important. The influence of the construction phase can be explained by the need for steel and the bound extraction of pig iron or other raw materials like the use of ferronickel for chromium steel. What can be noticed as well from Figure 2, are the small impact scores for the Annapolis tidal range plant, and the high value of the Oyster800 plant across all impact categories. The HS1000 plant also shows high impacts compared to the HydraTidal and SeaGen plants, which have similar performances. The influence of the EoL benefits on the final results was high. Accounting for benefits for the avoided primary production of metals, for example, for steel, reduces the score of the HS1000, Oyster800, HydraTidal and SeaGen plant noticeably. The Annapolis tidal range plant, which performs best in all impact categories (Figure 2), has on average twice as high impact category results as hydropower (Figure 3). For the displayed impact categories in Figure 3, the other plants have on average 1.4 times (SeaGen plant) to 1.8 times (HS1000 plant), higher results than an offshore wind power plant. However, the tidal stream devices have on average 13 (HS1000) to 21 times (HydraTidal) lower impacts than electricity from hard coal, except for metal depletion. For the latter, the tidal stream devices have a roughly 10 times higher impact than electricity from hard coal. Similarly, the Oyster800 plant has on average 8 times lower impact and the Annapolis plant even 115 times than electricity from hard coal. The comparison to natural gas is more varied. Whereas electricity from natural gas has a 34, 11, and 2 times lower impact on metal depletion, human and marine ecotoxicity than all analyzed plants, the plants have on average 4 and 38 times lower impacts on particulate matter formation and climate change. For more detailed results see SI, S5. Sensitivity Analysis. Due to specific design elements, the results of the scenarios varied across the plants (SI, S8). In the following, the results of the scenarios are presented in Table 2 for the HS1000 plant and the five chosen impact categories. Only the results of the HS1000 plant are presented here because of the good inventory data available and its high sensitivity to the scenarios. Decreasing the lifetime (LIFE scenario) or the production capacity (PROD scenario) by 20% increases the score for all

life cycle embodied CO2[kg CO2eq] CO2 avoided[kg CO2 eq/year]

(1)

The value of the carbon payback time varies. It is highly dependent on the type of energy the renewable technology replaces, which is represented by the “CO2 avoided” in eq 1. In this study, the carbon payback times were computed for the electricity mixes of Great Britain, Norway, the United States, and the UCTE. These results were then compared to the carbon payback times of wind power, using data from ref 59, and to photovoltaic (PV), using the ecoinvent process of “electricity, production mix photovoltaic”. Because ecoinvent provides the electricity production of a PV cell in kWh/kWp, a value of 2 kW peak (kWp)60 was used to derive the total kWh production of the PV cells.10

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RESULTS

Material Breakdown. The material breakdowns of the five plants show that steel is the main material used for four out of the five plants (Figure 1). The exact mass breakdowns of the five plants can be found in SI, S4. Not all plants use the same shares of the different steels. The HydraTidal plant uses a larger share of chromium steel than the other technologies. Otherwise, the material breakdowns of the HS1000, HydraTidal, and SeaGen are similar (Figure 1). The

Figure 1. Material breakdowns for the five analyzed plants: (A) HS1000 plant, total mass 1.15 × 103 tons, (B) HydraTidal plant, total mass 712 tons, (C) Oyster800 plant, total mass 1.93 × 103 tons, (D) SeaGen plant, total mass 549 tons, (E) Annapolis plant, total mass 1.74 × 106 tons. The enlargement displayed for the Annapolis tidal range plant shows which other materials are used for the plant. D


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Figure 2. Results of the chosen impact categories for the five analyzed plants per kWh electricity. The results are divided into six different life cycle stages. (A) Climate change, (B) Human toxicity, (C) Marine ecotoxicity, (D) Metal depletion, and (E) Particulate matter formation. EoL stands for End of Life.

Figure 3. Comparison of electricity generated from oil (UCTE), natural gas (GB), solar power (GB), hydropower (GB), and wind power59 to the minimum and maximum values of the analyzed wave and tidal power plants for the chosen impact categories: (A) Climate change, (B) Human toxicity, (C) Marine ecotoxicity, (D) Metal depletion, and (E) Particulate matter formation.

around 4% of the impact category results, except for particulate matter formation or metal depletion, with 6 and 8% increase, respectively. The MANU scenario led to an average increase of 2% over all impact categories and for all plants. Only the

impact categories and all plants by 25%. The small differences between these two scenarios come from different maintenance requirements. The TRAVEL scenario was the one with least influence on the results. The RECY scenario led to increases E


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Table 2. Results of the Sensitivity Scenarios for the HS1000 Plant Comparing the Percent Change from the Reference Level for the Five Chosen Impact Categories impact category

unit

climate change % change human toxicity % change marine ecotoxicity % change metal depletion % change particulate matter formation % change

reference

ELE

LIFE

PROD

RECY

TRAVEL

MANU

g CO2-eq

37.02

g 1,4-DB-eq

30.15

g 1,4-DB-eq

1.12

35.62 −3.77 34.38 14.03 1.21 8.65 35.61 0.05 0.08 2.26

46.20 24.79 37.67 24.96 1.40 24.97 44.49 25.00 0.10 24.87

46.38 25.28 37.76 25.26 1.40 25.15 44.50 25.01 0.10 25.17

38.57 4.19 31.49 4.45 1.14 2.38 37.74 6.04 0.09 8.40

37.24 0.61 30.18 0.10 1.12 0.07 35.60 0.02 0.08 0.84

37.50 1.29 31.06 3.03 1.14 2.04 35.97 1.04 0.08 1.24

g Fe-eq

35.59

g PM10-eq

0.08

Table 3. Comparison of the Climate Change Impact Category Results for the Five Plants Analyzed to Values Found in Literaturea SeaGen

HS1000

Hydra Tidal

Oyster800

Annapolis

SeaGen6

65.5

3.9

15

Oyster3507

750 kW Pelamis8

7 MW Wave Dragon8

g CO2-eq/kWh

g CO2-eq/kWh

19.6

28.2

Climate change g CO2-eq/kWh 25.5

37.0

20.1

g CO2/kWh 25

The results for climate change impact category of the five plants analyzed are shown in the first five columns. The other values show carbon inventory results for other plants found in literature. a

PV power plants. The Annapolis (6 months), SeaGen (13.1 months), and HydraTidal (10.1 months) plants have lower carbon payback times than wind power (14.4 months) (SI, S9). Absolute payback times depend on the reference electriciy mix. The U.S. electricity mix, with a high share of electricity from coal, leads to a carbon payback time of 20.9 months for the Oyster800 technology. At the same time, this value increases to 30.5 months when using the average European electricity (UCTE). In Norway, with an energy mix of almost 100% hydropower, the carbon payback time is increased to 488.4 months, so nearly 40 years. Still, wave and tidal energy technologies can become promising alternative green technologies and be of great use to address the global CO2 emission reduction target. Comparison to Literature Values. A comparison of the climate change impact category results to literature values (Table 3) shows that our estimates are higher than other directly comparable ones from the literature (see Discussion section for potential reasons for these differences).

Oyster800 device experiences a mean increase over all impact categories of around 4%. The results of the ELE scenario reflected the influence of the choice of the electricity mix. The LCIA scores of the HydraTidal plant using mostly the almost fossil fuel-free electricity mix of Norway increased on average by 14%. For their part, the increase of the scores of the Oyster800, SeaGen, and HS1000 plants ranged between 2% and 8%. On the contrary, the influence of the ELE scenario on the Annapolis tidal range plant using the US electricity mix was either close to zero or led to a decrease of the LCIA values. The ELE scenario provides a first basis to quantify the influence of the limitation in the use of the electricity mixes induced by the use of the ecoinvent v.2.2 database. SI, S7 compares the LCIA results of the generation of 1 kWh US, Canadian, UCTE, and ENTSO-E electricity mixes. Except for the agricultural and industrial land use impact categories, where the LCIA results of the Canadian mix are four times higher than the U.S., the Canadian electricity mix leads to on average 2 times smaller LCIA results. Thus, according to the results in the ELE scenario, using the Canadian electricity mix instead of the U.S., could reduce the LCIA results of the Annapolis tidal range plant to up to 6% depending on the impact category. The differences in the LCIA results of the UCTE and the ENTSO-E electricity mixes varies across impact categories (SI, S7). Apart from a 10times and 5times higher value for the agricultural land occupation and the terrestrial ecotoxicity impact categories respectively, the difference in LCIA score varies between ±20%. Upon the ELE scenario such differences have led to reduction in LCIA scores of around 1.5%. The results of the specific scenarios for the Oyster800 and the HydraTidal plant can be found in the SI, S8. For the Oyster800, halving the material mass leads to the largest influence with a 25% decrease for all impact category values. Increasing the size of the array to three plants leads to a mean decrease of 16% of the LCIA results. The COMPBLAGB scenario applied for the HydraTidal plant induces substantial changes in impact and leads to results on average 1.2× higher than the values of the HS1000 plant. Carbon Payback Time. The carbon payback times for the five analyzed plants, using the British electricity mix to quantify the avoided CO2 emissions, are two to nine times lower than for

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DISCUSSION Material Breakdown. The material breakdowns presented in Figure 1 were compared to literature for the SeaGen,6 the Oyster800,7 and the Annapolis tidal range plant.25 The comparison for the SeaGen plant showed similar results. The Annapolis tidal range plant with high amounts of cement, gravel, and water shows similar values to the material consumption indicated in ref 25 for hydropower dams. The inventory of the Oyster800 was different from that shown in ref 7. Explanations could be that a previous design of the Oyster plant was analyzed in ref 7 or that some parts of the device were not considered. For the HydraTidal and HS1000 plant direct supervision from Anders Tørud, CEO of the Straum Group, and Craig Love, Engineering Manager at Andritz Hydro Hammerfest, respectively, ensures that the material breakdowns are good representations of the reality. Life Cycle Impact Assessment. The influence of steel on the impact category results of the tidal stream and wave energy plants appeared clearly especially the contributions of raw material extraction like pig iron, ferromanganese, or ferrochroF


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potential improvement lies in the modeling of the degradation of the antifouling coatings used on the structures. No comparison with state-of-the-art inventory values was possible for the wave and tidal stream devices. The main reason is the unavailability of such values for these novel technologies. No standard design is available for wave and tidal stream plants, thus the results presented are highly plant-dependent. Yet, for the HS1000, the HydraTidal, the Annapolis tidal range, and the SeaGen plant the material breakdowns were checked for consistency with the manufacturers and/or available literature. In addition, the design similarity of the Annapolis tidal range plant with hydropower plant, which was reflected in the life cycle impact assessment results, do support the rationale of the assumptions made throughout this work. On the contrary, the inventory data available for the Oyster800 plant could not be verified satisfyingly. Therefore, to test the assumptions made for the inventory of this plant, plant-specific scenarios were run. Overall, the results presented here help to understand the environmental performance of such plants and sensitive parameters, and potentially guide their technical development. Are Wave and Tidal Energy Plants Green Technologies? The LCIA results and the computed carbon payback times give a potential answer to the question whether the analyzed plants are green technologies. Considering the influence on the climate using the climate change impact category, all plants perform similar to wind and photovoltaic solar power plants. The Annapolis tidal range plant performs even better than the other analyzed plants with comparable values to hydropower. The computed carbon payback times also supports the comparisons to wind, solar, and hydropower. For the impact category of particulate matter formation, impacting human health, the results of the wave and tidal energy plants were again comparable to wind and photovoltaic power plants. For the human toxicity and marine ecotoxicity impact categories, the results are less clear but still highlight a good performance of the analyzed wave and tidal energy plants. Only the metal depletion impact category casts a shadow over the good performance of the five devices. Ref 61 also highlights that renewable energy technolgies like wind and photovoltaic cells require more metal than coal- and gas- based electricity. They state, however, that this additional requirement results in small emissions compared to the direct emissions of fossil fuel-based power plants. Still, additional concerns exist induced by the aim of developing wave farms, the lack of adequate impact categories, and the missing knowledge about further potential environmental impacts. If wave farms are developed, impacts, especially on marine ecosystems, may increase. Currently, only one extensive environmental monitoring study has been conducted for the SeaGen plant,62 which provides monitoring data on potential environmental impacts.63,64 Examples of included impacts in this study are the disturbance of local marine mammals and bird communities upon installation and operation of the plant or the potential damage to the auditory system of marine mammals due to the noise produced by the plant during operation. Changes in sedimentation patterns, the influence on the velocity field, or on the current direction were not observed upon this monitoring. Two aspects not addressed in the environmental monitoring program include electromagnetic effects of the subsea cables and toxic effects due to antifouling and anticorrosion measures. Unfortunately, most of the above cited environmental effects are not considered in todays available impact categories. It is therefore likely that environmental costs of the use of wave and tidal energy are higher than currently assumed. However, this

mium. The EoL benefits were shown to reduce the results noticeably, especially for the particulate matter formation with a reduction ranging between 23 and 90% for the SeaGen and HS1000 plant, respectively. It should be kept in mind that only benefits for the avoided production of unalloyed steel were accounted for. Therefore, the extraction of ferrochromium used in the low-alloyed and chromium steel is not compensated and influences all impact categories. For the Annapolis tidal range plant, concrete was the material with most impact on the LCIA results. Scenario Analysis. The different scenarios developed helped derive the following recommendations. A long lifetime and high power output of the plants should be the main aim. The EoL scenario also has an influence, therefore high recycling rates at the EoL should be aimed for. When displaying the results, the method used for EoL accounting should be reported carefully. The electricity mix used had a considerable influence on the environmental performance of the plant. This highlights the need for comparisons at standard sites with similar conditions (same electricity mix, standardized tide, and wave conditions) when making a comparative analysis of wave and tidal technologies. Moreover, the scenarios together with the more plant specific ones helped identify possible explanations for the relatively large impact of the Oyster800 device. The small electricity output, the short lifetime, the high mass, the low recycling rate and the use of long pipelines for a single plant are all contributing reasons. Finally, the scenarios developed for the HydraTidal plant support the choice of wooden blades compared to composite materials even when considering a higher maintenance need. Comparison to Literature Values and Limitations of the Study. The result of the climate change impact category found in literature, ranging from 15 to 25 g CO2/kWh and from 19.6 to 28.2 g CO2-eq/kWh are in the lower range of the results computed in this report for the SeaGen, HydraTidal, and HS1000 plants (Table 3). The Oyster800 plant and its high score, 65.5 g CO2-eq/kWh, and the low value of the Annapolis tidal range plant, 3.9 g CO2-eq/kWh, diverge from these literature values. Explanations for the variation in the values are diverse. First, the difference in the units should be highlighted. While the climate change impact category considers all greenhouse gas emissions, refs 6 and 7 only consider CO2. The difference induced is expected to be quite small, however. Differences in the carbon intensity of the materials used may also contribute to the different results. Further, differences in the assumed power output or lifetime also influence the results. Finally, differences in the electricity mixes used, could also be an explanation for the differences between our values and literature values. The low value of the Annapolis tidal range plant can be explained with its similarities to hydropower dams. Swiss run-ofriver power plants cause greenhouse gas emissions of 3.7 gCO2eq/kWh and storage power plants emissions of 10.8 gCO2-eq/ kWh,25 a range which includes the result of the Annapolis tidal range plant. These differences highlight some of the limitations of the study. While care was given to model as much of the processes entirely, this was not always possible due to data constraints. The sensitivity analysis showed that in some cases using the default processes of ecoinvent, as for the “steel product manufacturing”, was acceptable due to their small influence on the end results. On the other hand, the importance of the electricity mix was shown. This highlights the need for a comparison at standard sites with standard processes. Moreover, uncertainty induced by material data constraints should be analyzed further in future work. A G



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limitation also applies to other renewable energy technologies, like offshore wind power or hydropower. This highlights the need for more research to clarify these potential impacts and allow more meaningful comparisons of the environmental impacts of renewable energy technologies. Still, taking into account the results of the wave and tidal energy plants for the currently available ReCiPe 2008 impact categories their carbon payback times and the improvements they may undergo as a consequence of learning and upscaling, it is fair to consider these new forms of energy production as a green technology in comparison to fossil fuel-based technologies. Hence our study highlights the potential to further invest and conduct research on wave and tidal energy plants, in order to move toward a more sustainable society. To reach the Paris agreement signed recently on 22 April 2016 in New York, where 195 countries pledged to keep the global temperature increase below 2 °C, such alternative energy sources are strongly needed.65 In addition, the poor air quality increasingly reported in big cities and its impact on human health66 are another reason to encourage the development of alternative energy sources.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b00156. Summary of the end-of-life procedures applied; overview of the scenarios for the sensitivity analysis; description of the life cycle inventory; results for all plants and all impact categories divided into the six life cycle stages; ReCiPe results for five traditional energy generation forms; results of the different scenarios for all plants and impact categories as well as the special scenarios for the Oyster800 and HydraTidal plant; carbon footprint results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +31 (0)24−356 20 60; e-mail: M.Douziech@science. ru.nl. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Anders Tørud (HydraTidal), Craig Love (HS1000), and Robert Duran (Annapolis tidal range plant) for providing precious information that made this work possible. We would also like to thank the valuable inputs of Keith Morris, design engineer at ETA; Helmut Albers, from the technical service support of the ZF company; Chris Head, from Schaeffler; Anders Budtz, senior project manager from the Bladt Industries; Simon Thompson, consulting engineer at Wartsila UK Ltd; Robert Lloyd, continuous improvement manager of Burntisland Fabrications Ltd; Marcus Royle, business development manager at Gurit Composite Components; and all the other experts who preferred not to be cited directly. A final thank goes to Carine Lausselet for her help during the revision process. H


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Are Wave and Tidal Energy Plants New Green Technologies

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