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Life cycle assessment and net energy analysis of offshore wind power systems
Accepted Manuscript Life cycle assessment and net energy analysis of offshore wind power systems
Yu-Fong Huang, Xing-Jia Gan, Pei-Te Chiueh PII:
S0960-1481(16)30915-6
DOI:
10.1016/j.renene.2016.10.050
Reference:
RENE 8238
To appear in:
Renewable Energy
Received Date:
17 September 2015
Revised Date:
16 October 2016
Accepted Date:
22 October 2016
Please cite this article as: Yu-Fong Huang, Xing-Jia Gan, Pei-Te Chiueh, Life cycle assessment and net energy analysis of offshore wind power systems, Renewable Energy (2016), doi: 10.1016/j. renene.2016.10.050
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Highlights:
Environmental impact and energy benefit of offshore wind power was evaluated.
Installing an offshore substation would lead to higher environmental impact.
The greatest environmental impact was attributable to the use of ferrous metal.
Energy return on investment can be as high as 27.
Environmental impact and energy input can be largely reduced by waste recycling.
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Life cycle assessment and net energy analysis of offshore wind power systems
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Yu-Fong Huang, Xing-Jia Gan, Pei-Te Chiueh*
Graduate Institute of Environmental Engineering, National Taiwan University, 71,
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Corresponding author. Tel.: +886 2 3366 2798; fax: +886 2 2392 8830 Email address: [email protected] (P.-T. Chiueh). 1
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Abstract
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This study attempted to evaluate the environmental impact and energy benefit of offshore wind power systems using life cycle assessment (LCA) and net energy
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analysis. The environmental impact of offshore wind power systems is based primarily
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on ferrous metal, which is used to install the foundations, towers, and nacelles. The
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impact categories with the greatest relevance were fossil fuels and respiratory
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inorganics. This study assumed that the life cycle of an offshore wind power system has
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four stages (production, installation, operation and maintenance, and end-of-life). Two
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scenarios were examined in this study. The major difference between the scenarios was
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that Scenario 2 included an offshore substation. The overall environmental impact in
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Scenario 2 was higher than that in Scenario 1 by approximately 10%. The net energy
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analysis in this study included the evaluations of cumulative energy demand (CED),
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energy return on investment (EROI), and energy payback time (EPT). For Scenarios 1
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and 2, CED was 0.192 and 0.216 MJ/kWh, EROI was 18.7 and 16.7, and EPT was 12.8
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and 14.4 months, respectively. Moreover, when the recycling of waste materials was
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considered, each scenario produced a 25% lower environmental impact, 30% lower
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energy requirement, and 4 months lower EPT.
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Keywords: Offshore wind power; Life cycle assessment; Net energy analysis;
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Sensitivity analysis
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1. Introduction
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Research and development of renewable energy is urgent because of concerns such as heavy dependence on nonrenewable fossil fuels and disasters caused by global
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climate change. During the past four decades, the consumption of fossil fuels and
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emission of carbon dioxide have nearly doubled [1]. One promising form of renewable
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energy is wind power, which is generated by wind turbines that convert the kinetic
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energy of wind to electrical energy by rotating blades [2]. The worldwide electricity
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generation of wind power has increased rapidly, and wind power has become the fastest
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developing renewable energy technology [3]. From 1996 to 2011, the average annual
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capacity of wind power increased by approximately 28% [4]. At the end of 2013, there
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were commercial wind power installations in more than 90 countries with total installed
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capacity of 318 GW, providing approximately 3% of global electricity supply [5]. The
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rapid development of the wind power market and technology has major implications for
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research, education, and professionals working in the electric utility and wind power
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industries [6]. Wind power is exploited not only onshore but also offshore, where wind
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speeds are higher and wind is typically available more regularly and for longer periods
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[7]. Therefore, offshore wind power has attracted increasing interest mainly because of
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the limited land for onshore wind farms. In 2013, total cumulative installations in the
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offshore sector reach to 7,046 MW, approximately 2.2% of the global installed wind
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turbine capacity [8]. Accordingly, a substantial growth of offshore wind power
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installations is expected.
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Wind power is driven by renewable energy flux, but in a life cycle perspective
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there are still non-renewable resource demands and harmful emissions associated with it
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[9], mainly due to the production and installation of wind turbines and related facilities.
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The possible environmental and resource impact caused by the utilization of wind
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power can be quantified by the method of life cycle assessment (LCA) [9]. Life cycle
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assessment (LCA) is a tool for identifying and evaluating the environmental impact of a
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product or service during its entire life cycle [10]. LCA is one of the most effective
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methods of evaluating environmental burdens by identifying energy and materials used
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as well as waste and emissions released in the environment [11]. LCA may be
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conducted under various circumstances, but it is mainly based on a careful and
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comprehensive account of all energy and material flow associated with a system or
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process [12]. The environmental impact and benefits of wind power systems have been
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evaluated and compared with those of other forms of renewable energy by using the
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LCA method [10,13–15]. Compared with other renewable energy technologies, such as
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photovoltaics, hydropower, and geothermal power, wind power systems are superior
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regarding the sustainability indicators of carbon emissions, water consumption, and
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social impact [16].
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Both nonrenewable and renewable energy systems consume energy throughout
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their life cycles. When considering the net benefit of an energy system, one major
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concern is whether output energy is higher than input energy. The reliability of
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renewable energy is often questioned because of its net energy benefit. One technique
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for evaluating net benefit is to compare output energy delivered and input energy
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required. The purpose of net energy analysis is to assess energy use for such activities
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as the extraction of raw materials, construction, operation and maintenance (O&M),
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disposal, and recycling. Various indicators are used for net energy analysis to measure
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energy use and performance. One indicator is referred to as energy return on investment
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(EROI), which is the ratio of energy delivered to energy cost [17]. The EROI of wind
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power can be much higher than that of other renewable energy technologies, such as
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solar thermal power, photovoltaics, geothermal power, and hydropower [18].
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To promote the development and application of offshore wind power systems,
understanding the benefits and threats of these systems to the environment and energy
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production is crucial. Previous LCA studies have evaluated the GHG emissions, energy
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performance, and environmental impacts associated with various aspects of the offshore
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wind power designs [19–21]. However, there is no single literature that
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comprehensively studies both LCA and net energy analysis of offshore wind power
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systems. Besides, while a previous study pointed out that the substructure of offshore
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wind power plants is an important factor for the CO2 emissions [20], the effect of
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installing an offshore substation on the environment and energy has not been studied so
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far. This study aimed to identify and evaluate the environmental impact and energy
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consumed at each stage of an offshore wind power system developed in Taiwan west
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coast. Two scenarios, with and without a substation in the offshore wind system were
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considered. LCA and net energy analysis were conducted to assess environmental
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impact and energy benefit throughout the life cycles of wind power systems, including
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component manufacturing, construction, transportation, operation, maintenance, and
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recycling after decommissioning.
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2. Methods
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The life cycle inventories of two offshore wind power systems were developed and assessed based on a functional unit of 1 kWh of electricity to evaluate environmental
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burden and energy balance in this study. The LCA software SimaPro 7.1 and Eco-
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indicator 99 method were applied. This study used a conventional process-type LCA
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model using the inventory data from the built-in Ecoinvent database of SimaPro 7.1 and
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the inventory data from literature reports as described later in Section 2.2. In fact, a
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systematic truncation error occurs in the process-type LCA. The truncation error is
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caused by the setting of system boundaries and consequently the omission of processes
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outside these boundaries [22–24]. The magnitude of the error varies with the type of
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product or activity considered, but can be in the order of 50% [23,24]. One way to avoid
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the truncation error is to incorporate an input–output analysis into the assessment
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framework, resulting in a hybrid LCA method [23]. Besides, the truncation error of a
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process-type LCA is often significantly higher than that in the input–output part of a
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hybrid assessment [23]. The use of input–output-based hybrid techniques can prevent
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uncertainties and errors in the process-type and input-output analyses of wind power
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[13,25]. Further discussion on the truncation error and the input–output analysis is
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beyond the scope of the study, but these aspects are important to assist the decision-
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making of the government by providing information and suggestion with better
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accuracy and precision. There is a literature report on Taiwan’s CO2 and CH4 emissions
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inventories in an integrated hybrid input-output LCA [26], which is a helpful reference
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for setting up hybrid input-output LCA models. Besides, it should be noted that this
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study is an attributional LCA study which normally cannot take comprehensive system
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changes into consideration [27]. Further extensive investigation and analysis would be
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necessary to carry out a consequential analysis. Moreover, net energy indicators,
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including cumulative energy demand (CED), energy return on investment (EROI), and
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energy payback time (EPT), were also calculated.
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2.1.1 System boundary
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The system boundary of this study was the overall life cycles of offshore wind
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power systems, including the phases of resource extraction, component manufacturing,
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construction, transportation, operation, maintenance, and disposal. This study simulated
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two offshore wind power system scenarios: one had an offshore electrical substation,
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and the other did not. The necessity of an offshore substation depends on the total
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installed capacity of a wind farm and its distance from the coast. Generally, when the
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total installed capacity is less than 30 MW, constructing an offshore substation is
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unnecessary. If the total installed capacity is more than 30 MW but less than 120 MW,
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an offshore substation should be constructed when the distance between the wind farm
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and the coast is greater than 10 km. An offshore substation is generally necessary when
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the total installed capacity is higher than 120 MW, regardless of the distance between
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the wind farm and the coast.
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In this study, offshore wind farms were assumed to include the installation of 52
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wind turbines (Vestas V80-2.0 MW), resulting in a total power capacity of 104 MW.
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Two scenarios were simulated. Scenarios 1 and 2 included wind turbines, internal
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cables, and electrical submarine cables. In Scenario 1, the wind turbines were connected
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with one another by 32 kV submarine cables, and the electricity was then transmitted to
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the coast by two 32 kV submarine cables. In Scenario 2, the electricity generated from
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the wind farm was first delivered to an offshore substation (AC/DC), and it was then
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transmitted to the coast by a 150 kV submarine cable. PEX-insulated cables were
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assumed to be used (PEX: cross-linked polyethylene). The schematic diagrams of the
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two offshore wind power systems are illustrated in Fig. 1.
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Wind turbines
Offshore substation
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32 kV submarine cables
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Wind turbines
Fig. 1
System boundary
Onshore substation
Scenario 2
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Scenario 1
System boundary
Onshore substation
150 kV submarine cable
Offshore wind power systems.
2.1.2 Functional unit
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For this study, 1 kWh generated by offshore wind power was selected as a 8
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functional unit. Input resources, energy consumption, and relative emissions throughout
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the life cycles of offshore wind power systems were calculated based on this functional
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unit.
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For conducting the LCA of offshore wind power systems, the following four assumptions were held:
The site of the offshore wind power system is located in the Taiwan Strait, near the coast of Fangyuan Township, Changhua County. The distance
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between the system and the coast ranges from 8 to 15 km. The depth of the
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sea area ranges from 20 to 45 m. The area of the system is approximately 20
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km2.
Taiwan in 2011 [28].
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Electricity supply and consumption is that of the power generation structure in
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The lifetime of wind turbines and internal submarine cables is 20 years, and
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the lifetime of submarine transmission cables and an offshore substation is 40
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years.
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Only domestic transportation is considered. The distance of land
transportation is assumed to be 150 km, which is approximately the average
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distance across three counties in Taiwan. Maritime transportation is between the offshore wind farm and Taichung Harbor, and the average distance is
assumed to be 50 km.
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2.2 Inventory analysis
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The life cycle inventory analyses of the offshore wind power systems were mainly based on the reports of Dong Energy and Vestas Wind Systems [29,30]. The inventory
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data for the stages of installation, transportation, O&M, decommissioning, and
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recycling were based on the literature [20,31]. In addition, the number of vessels and
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workdays required were adjusted at the construction stage. Further description of the
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inventory analysis at the production, installation, O&M, and end-of-life (EOL) stages is
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provided in the following text.
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191 2.2.1 Production stage
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The offshore wind farm primarily consisted of wind turbines and a power
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transmission system. A wind turbine was mainly composed of rotor blades, nacelles,
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and a tower and foundation. In Scenario 1, the power transmission system included 32
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kV submarine cables only. In Scenario 2, the power transmission system included
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32/150 kV submarine cables and an offshore substation. The major materials necessary
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for the installation of the offshore wind farm included cast iron, steel, glass fiber,
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plastics, resin, concrete, lead, copper, and aluminum. The components and materials
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used for the offshore wind farm are listed in Table 1. The quantities of the materials
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used are provided in the Appendix (Table A.1), based on the reports of Dong Energy
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and Vestas Wind Systems for the establishment of an offshore wind farm at Horns Reef,
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Denmark in 2002 [29,30]. It is important to understand the source of the data. LCAs
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vary significantly when background economies are changed [32]. Lenzen and
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Wachsmann have shown that the energy requirements in different countries can differ
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by a factor of more than 8 [32], so the choice of country for inventory data can make a
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big difference.
Table 1 The components and materials used of the offshore wind farm. Components
Materials
Rotor blade
Glass fiber, epoxy resin, PVC
Nacelle
Cast iron, steel, glass fiber, HDPE, epoxy resin
Tower
Steel, alkyd resin
Foundation
Steel, aluminum
32/150 kV submarine cable
Lead, copper, steel, HDPE
Offshore substation
Steel, concrete, cast iron, aluminum
2.2.2 Installation stage
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During the installation of offshore wind power facilities, the required
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transportation was assumed to include domestic land and maritime transportation.
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Foreign transportation for the importation of wind turbine components was neglected.
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The distance for domestic land transportation by truck was assumed to be 150 km, and
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the distance for domestic maritime transportation from Taichung Harbor to the offshore
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wind farm was assumed to be 50 km. The installation of the wind power facilities could
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be divided into three steps: (1) the construction of submarine infrastructures; (2) the
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installation of the towers, nacelles, and rotor blades; (3) the connection of the power
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grid. The number of vessels and workdays required at the installation stage are listed in
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the Appendix (Table A.2).
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2.2.3 Operation and maintenance stage
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To maintain high efficiency, the maintenance of offshore wind turbines typically consists of scheduled maintenance one to two times and unscheduled (corrective)
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maintenance one to four times per year per wind turbine [20]. Maintenance was
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assumed to occur five times per year in this study, with a vessel being used for
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transportation during maintenance once per year and a helicopter being used for
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transportation during maintenance four times per year. The distance of transportation
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was assumed to be 50 km. The unscheduled adding or replacing of lubricants was
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assumed to be necessary to maintain the function of wind turbines. During the life cycle
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of an offshore wind turbine (20 years), one rotor blade and one out of three nacelles on
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average was assumed to require replacement in this study. The numbers of vessels and
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workdays required in the O&M stage are listed in the Appendix (Table A.3).
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Furthermore, the lifetime of wind farms could increase to 25 or even 30 years,
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attributable to the progress of wind power technology. Although the increased lifetime
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would need more maintenance works, it is difficult to present the effect of lifetime
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because the function unit of this study was energy generated by the offshore wind
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power system.
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2.2.4 End-of-life stage
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The last stage of an offshore wind power system is the EOL stage. At this stage,
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the primary goal is to address waste recycling and disposal to minimize its impact on
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the environment. To choose an appropriate decommissioning method (e.g., full or
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partial demolition), the environmental characteristics of a wind farm must be assessed.
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For the purpose of restoring the site to its original state, based on existing technology, at
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the decommissioning stage, all of the offshore wind power facilities were assumed to be
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demolished in this study, including submarine cables and an offshore substation. The
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number of vessels and workdays required at the EOL stage are listed in the Appendix
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(Table A.4).
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The recyclable materials from the waste offshore wind power system were
assumed to include steel, cast iron, copper, aluminum, lead, and other metals, whereas
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glass fibers and plastics were assumed to be buried or incinerated. In most cases, the
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loss rate of the steel and metal recycling is less than 10%. In this study, the recycling
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rate was assumed to be 90%, and the loss rate was assumed to be 10%. The methods of
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recycling and disposing of the aforementioned materials are listed in Table 2.
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Table 2 The methods of recycling and disposing of the waste materials. Material
Recycling / Disposal
Steel / cast iron, stainless steel / high-strength steel, copper, aluminum, lead
90% recycling
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Rubber, plastics (except PVC)
100% incineration
Glass fiber, PVC
100% buried
2.3 Impact assessment
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The impact assessment model of this study was Eco-indicator 99, which is a damage-oriented (endpoint) method for life cycle impact assessment [33]. Eco-indicator
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99 comprises five steps: (1) characterization, (2) damage assessment, (3) normalization,
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(4) weighting, and (5) single score calculation. After the impact assessment, the
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magnitude of environmental impact can be expressed as a single score point (Pt). The
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inventory results can be classified and characterized as 11 impact categories (i.e.,
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carcinogens, respiratory organics, respiratory inorganics, climate change, radiation,
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ozone layer, ecotoxicity, acidification and eutrophication, land use, minerals, and fossil
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fuels), and these can then be further classified as three main damage categories (i.e.,
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human health, ecosystem quality, and resource damage) [34].
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In this study, the built-in Cumulative Energy Demand (CED) model of SimaPro 7.1 LCA software was used for net energy analysis to understand the energy balance of
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offshore wind power. The CED model comprises three steps: (1) characterization, (2)
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weighting, and (3) single score calculation. The inventory results were classified as five
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categories and converted to the same unit. Each impact category was assigned a
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weighting factor of 1 to obtain a total CED. The CED can be presented as a single score
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point (Pt) [34]. In addition to the CED model, the net energy indicators of EROI and
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EPT of offshore wind power systems were studied.
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3. Results and discussion
3.1 Life cycle assessment
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In this study, Scenarios 1 and 2 adopted Vestas V80 (2.0 MW) wind turbines, which are composed of three blades as well nacelles, a tower, and a mono-pile
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foundation. Approximately 430 tonnes of materials were assumed to be used per wind
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turbine. The materials used to construct a wind turbine are shown in Fig. 2. Wind
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turbines are composed mainly of ferrous metal materials (89.81%), of which the
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proportions of reinforcing steel, steel, and cast iron are 47.58%, 38.27%, and 3.96%,
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respectively. Nonferrous metal materials account for only 0.41% of wind turbines and
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primarily include aluminum, copper, and lead. Additionally, 3.20% of wind turbines are
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composed of synthetic materials, including glass fibers (used for the blades) and epoxy
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resins. Moreover, 6.58% of wind turbines are composed of other materials, which are
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primarily the water used in the manufacturing process and lubricants used in the cabins.
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In addition, the manufacturing process also consumes energy, such as electricity and
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natural gas.
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Fig. 2
Material shares of a wind turbine.
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The characterization results of offshore wind turbines verified that, in all
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categories, the greatest environmental impact was attributable to the use of ferrous
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metal materials. Most of the ferrous metal was steel used for foundation installation,
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followed by steel used for tower and nacelle installation. The synthetic materials
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contributed to a higher impact proportion on the land use category, which was
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attributable to the use of alkyd resin that was used for the painting of the foundation and
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tower. The alkyd resin was made from polyacid, polyalcohol, and soybean oil. The
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planting of soybeans requires land, which explains why the impact on the land use
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category was high.
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The results of damage assessment were similar to those of characterization. The
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highest impact in each category was attributable to the use of ferrous metal, particularly
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in the human health and ecosystem quality categories. This can be explained by the
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inventory data. The use of ferrous metal requires extensive pickling and leads to high
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dust emissions. Workers may thus inhale acidic gases and dusts, resulting in damage to
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respiratory organs in the human body. In the resource damage category, the relatively
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high impact was attributable to the use of fuels and synthetic materials. The process of
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steel rolling consumes a large amount of natural gas and thus results in resource
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damage.
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The single score results of offshore wind turbines are illustrated in Fig. 3. The total
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single score (i.e., the total environmental impact) proportion of the resources and fuels,
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from highest to lowest, were reinforcing steel (45.77%), steel (22.89%), synthetic
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materials (12.50%), energy use (11.47%), cast iron (3.91%), nonferrous metal (2.90%),
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and other parts (0.56%). The use of ferrous metal materials (including steel, reinforcing
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steel, and cast iron) thus produced a total single score proportion as high as
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approximately 73%. The main impact categories with higher single scores were fossil
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fuels and respiratory inorganics, which were primarily attributable to the use of steel
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and synthetic materials as well as electricity.
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Fig. 3
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3.1.1 Scenario 1
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Single scores of offshore wind turbines.
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The single scores of Scenario 1 for the offshore wind power system are shown in Fig. 4. The environmental impact of the stages throughout the life cycle, from highest to
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lowest, were production (35.5%), installation (34.2%), EOL (16.6%), and O&M
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(13.7%). The impact categories with higher single scores were fossil fuels and
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respiratory inorganics, which were mainly attributable to the use of steel material,
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electricity, and oil for transportation vessels. Moreover, the environmental impact for
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the ecotoxicity and minerals categories was mainly attributable to the copper used in the
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electrical transmission cables.
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Fig. 4
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Single scores of Scenario 1.
The production stage of the offshore wind power system included the production
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of wind turbines and transmission systems, the former of which caused a major impact.
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Furthermore, the main impact sources, from highest to lowest, were the production of
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the foundation, tower, nacelles, and blades. At the installation stage, fuel consumption
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associated with land transportation, maritime transportation, and construction operations
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was considered. The impact of vessels and machines was primarily attributable to their
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fuel consumption, and thus the impact of fossil fuels, minerals, and respiratory
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inorganics was much higher than that of other categories. The vessels used for scour
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protection of the foundation were a substantial source of impact at the installation stage.
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Furthermore, the impact of the tugboats used for wind turbine and foundation
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transportation and construction, the cranes used for foundation installation, and the
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vessels used for submarine cable transportation was also crucial. Overall, foundation
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installation had the highest proportion of total impact (70.9%), followed by wind
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turbine installation (24.7%). The proportions of total impact from cable installation
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(2.98%) and land transportation (1.4%) were much lower.
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3.1.2 Scenario 2
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The distinction between Scenarios 1 and 2 was the transmission system. In the transmission system of Scenario 2, the offshore substation had the highest impact,
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followed by 150 kV and 32 kV submarine cables. The most substantial impact occurred
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in the categories of, from highest to lowest, fossil fuels, respiratory inorganics, minerals,
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and climate change. The main source of impact was the large amount of concrete used
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for offshore substation construction, which affected fossil fuels, respiratory inorganics,
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and climate change. Moreover, the copper content in the submarine cables had impact
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on minerals, respiratory inorganics, climate change, and ecotoxicity.
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The vessels used for scour protection of the foundation caused the highest impact, followed by the tugboats used for wind turbine and foundation transportation and
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construction, the cranes used for foundation construction, the jack-up vessels used for
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foundation and substation installation, and the vessels used for submarine cable
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transportation. Overall, foundation installation had the highest proportion of total
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impact (70.4%), followed by wind turbine installation (24.5%). The proportions of total
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impact from transmission system installation (3.62%) and land transportation (1.48%)
389
were not as substantial.
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390 391
The single scores of Scenario 2 for the offshore wind power system are shown in Fig. 5. The environmental impact of production, installation, O&M, and EOL was
393
40.8%, 31.3%, 12.7%, and 15.3%, respectively. The main impact was observed in the
394
categories of fossil fuels and respiratory inorganics, which was primarily attributable to
395
the use of steel material, electricity, and fuel for transportation. The impact in the
396
categories of ecotoxicity and minerals was mainly attributable to the copper used in the
397
submarine cables and steel used in the wind turbines. Moreover, the impact on climate
398
change was attributable to the concrete used for the construction of the offshore
399
substation. In general, the single score of Scenario 2 (0.00139 Pt) was higher than that
400
of Scenario 1 (0.00126 Pt) by approximately 10%. However, after considering the
401
effectiveness of the reduction in electricity transmission loss, Scenario 2 may still be
402
feasible.
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404 405
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Fig. 5
Single scores of Scenario 2.
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3.1.3 Recycling benefit
408 409
According to the aforementioned results of LCA on the two offshore wind power systems, the ferrous metals used for wind turbines and copper, iron, and lead used for
411
submarine cables were the primary sources of environmental impact. In this study, the
412
ferrous metals (including steel, reinforcing steel, and cast iron) and nonferrous metals
413
(including copper, aluminum, and lead) were assumed to be recyclable. The recycling
414
rate was assumed to be 90%. Although the transmission system of Scenario 2 requires
415
more materials than that of Scenario 1, most of the materials can be recycled at the EOL
416
stage of the offshore wind power system. This indicates that, if the benefit of material
417
recycling is considered, environmental impact can be lower regardless of whether a
418
substation is installed. The overall environmental impact of Scenarios 1 and 2 can thus
419
be reduced by approximately 25% because of the recycling of waste materials.
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3.2 Net energy analysis
424 425
3.2.1 Cumulative energy demand
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CED is a calculation of the total energy input for an energy system throughout its life. Therefore, a CED analysis of an offshore wind power system must consider the
427
overall energy consumed at the production, installation, O&M, and EOL stages. The
428
CED shares of the four stages are shown in Fig. 6; 46% and 51% of the total energy
429
demand was required at the production stage for Scenarios 1 and 2, respectively. Nearly
430
half of the overall energy demand was thus attributable to the production of wind
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431
turbines. The energy demand at the installation stage was 27% and 24% for Scenarios 1
432
and 2, respectively. The energy input at the O&M and EOL stages was nearly the same
433
and constituted approximately 13% of the overall energy input.
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435
Fig. 6
437
Scenario 1 included 52 offshore wind turbines and two 32 kV submarine power
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CED shares.
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transmission cables. The 1 kWh electricity output from Scenario 1 in the life cycle
440
required 0.192 MJ energy input. When considering the recycling of waste materials, the
441
energy input was reduced by approximately 30%, thus totaling 0.135 MJ. Scenario 2
442
included 52 offshore wind turbines, one substation, and one 150 kV submarine power
443
transmission cable. The 1 kWh electricity output from Scenario 2 in the life cycle
444
required 0.216 MJ energy input, which was higher than that in Scenario 1 by 12.5%.
445
When the recycling of waste materials was considered, the energy input was reduced to
446
0.155 MJ, which was approximately 28% of the overall energy demand.
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3.2.2 Energy return on investment
449 EROI represents the ratio of energy output to energy input and is a crucial
451
indicator of potential energy benefit to society [35]. The EROI of an offshore wind
452
power system is the energy output from wind turbines divided by the required energy
453
input. EROI is thus dimensionless, and a higher EROI value indicates that more net
454
energy can be obtained. The EROI of Scenario 1 was 18.7. It increased to 26.7 when the
455
recycling of waste materials was considered. The EROI of Scenario 2 was 16.7, and it
456
increased to 23.2 with waste recycling. The EROI of Scenario 2 was lower primarily
457
because of the installation of an offshore substation. Kubiszewski et al. showed an
458
average EROI in all of their studies (operational and conceptual) of 25.2 and an average
459
EROI in only the operational studies of 19.8 [18]. The EROI determined in this study
460
should thus be reasonable and reliable. In addition, Hall et al. found that the minimum
461
EROI is 3 for industrial societies [36], suggesting that no more than 33% of social and
462
economic resources are required for energy production without undermining the
463
sustainability of the societies. Offshore wind power development is thus warranted
464
because of its high EROI, particularly when waste materials are adequately recycled.
467 468
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3.2.3 Energy payback time
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EPT represents the time required when the energy output of an offshore wind
469
power system equals the energy input at its production, installation, O&M, and EOL
470
stages. A lower EPT value indicates that energy input and output can be balanced in a
471
shorter time. Therefore, EPT should be as low as possible for economic efficiency. In
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this study, 52 2.0 MW offshore wind turbines were assumed to work for 3,000
473
equivalent full-load hours per year, which theoretically could produce electricity of
474
6,240 GWh in 20 years. Based on this energy output, the EPT of Scenarios 1 and 2 was
475
12.8 and 14.4 months, respectively. When the recycling of waste materials was
476
considered, the EPT of Scenarios 1 and 2 was shortened to 9.0 and 10.3 months,
477
respectively. Because of the installation of an offshore substation, more energy and
478
resources were consumed in Scenario 2, and thus its EPT was approximately 1.5 months
479
longer than that of Scenario 1. Furthermore, for Scenarios 1 and 2, the EPT was
480
approximately 4 months shorter when the waste materials were properly recycled.
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Martinez et al. reported that the EPT of a 2 MW onshore wind turbine running for
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2,000 equivalent full-load hours per year is 4.8 months [37]. According to Wagner et
484
al., the EPT of a 5 MW offshore wind turbine running for 3,000 equivalent full-load
485
hours per year is 8.8 months [19]. Furthermore, Garrett and Ronde found that the EPT
486
of Vestas 2 MW wind turbines ranges from 8 to 11 months depending on wind turbine
487
model and wind speed [38]. The EPT in this study was slightly longer than those in the
488
aforementioned studies. This could be attributable to the difference in the location and
489
meteorology of wind farms, type and model of wind turbines, operation time and
490
frequency, and other factors relating to system boundaries and conditions. However, the
491
EPT of offshore wind power systems should be lower when considering the recycling of
492
waste materials.
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3.3 Sensitivity analysis
495
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Sensitivity analysis is helpful in determining the impact of selected variables as they are changed [15]. In this study, four factors substantially influenced the impact of
498
offshore wind power systems: power capacity, distance from shore, steel consumption,
499
and electrical consumption. The results of sensitivity analysis, including the variations
500
in single score and CED, are listed in Table 3. In this study, power capacity most
501
influenced the environmental impact of offshore wind power systems. A 10% increase
502
in power capacity led to an approximately 10% decrease in environmental impact, and a
503
10% decrease in power capacity led to an approximately 10% increase in environmental
504
impact. Steel consumption was also substantial because of its direct influence on energy
505
and material consumption. The factors of distance from shore and electrical
506
consumption were not as substantial as the aforementioned factors. When the two
507
factors were increased or reduced by 20%, the variations in both single score and CED
508
were approximately 1%. For offshore wind power systems, the most crucial factors may
509
thus include wind condition, power generation performance, and operation time and
510
frequency.
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Table 3 Sensitivity analysis of the offshore wind power system. Single score
CED
variation
variation
variation
Power capacity
+10%
–10%
–9%
–10%
+10%
+11%
+20%
+0.8%
+0.6%
+100%
+3.4%
+1.5%
Steel consumption
+20%
+5%
+6%
–20%
–5%
–6%
+20%
+0.75%
+1.5%
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Distance from shore
Electrical consumption
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–20%
–0.75%
–1.5%
513 514
4. Conclusions
516
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515 To evaluate the environmental burden and energy balance of offshore wind power, this study developed and assessed life cycle inventories in two scenarios, which were
518
distinguishable because only one scenario included an offshore substation. The SimaPro
519
7.1 software and Eco-indicator 99 method were applied to perform LCA based on a
520
functional unit of 1 kWh of electricity. Furthermore, this study also involved the
521
calculation of net energy indicators, such as CED, EROI, and EPT. The results show
522
that the factors, including steel material for wind turbines, electricity consumption at the
523
production stage, fuel consumption and air emissions during maritime transportation
524
and construction, and concrete materials for an offshore substation, are the most
525
substantial sources of environmental impact. The overall environmental impact of
526
Scenario 2 was higher than that of Scenario 1. However, after considering the
527
effectiveness of the reduction in electricity transmission loss, Scenario 2 may still be
528
feasible. The environmental impact of offshore wind power systems can be further
529
reduced by lowering the amount of materials used for wind turbines, particularly steel
530
and concrete. Offshore wind power development is warranted because of its high EROI,
531
particularly when waste materials are adequately recycled.
533
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Acknowledgments
534 535 536
The authors gratefully acknowledge the financial support from the Ministry of Science and Technology, Taiwan (contract no. 101-ET-E-002-013-ET). 26
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[32] Lenzen M, Wachsmann U. Wind turbines in Brazil and Germany: an example of
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geographical variability in life-cycle assessment. Appl Energy 2004;77:119–30. [33] Goedkoop M, Spriensma R. The eco-indicator 99—A damage oriented method for
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[36] Hall CAS, Balogh S, Murphy DJR. What is the minimum EROI that a sustainable society must have? Energies 2009;2:25–47.
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Figure captions
622 Fig. 1
Offshore wind power systems.
Fig. 2
Material shares of a wind turbine.
Fig. 3
Single scores of offshore wind turbines.
Fig. 4
Single scores of Scenario 1.
Fig. 5
Single scores of Scenario 2.
Fig. 6
CED shares.
624 625 626 627 628 629
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Appendices
637 Table A.1 The quantities of materials used for the wind power system. Materials
Wind turbine
Transmission
Total
(kg/farm)
(kg/farm)
0
10550800
279362
8766271
0
878701
0
396349
46118
358769
400879
400965
317829
320283
0
82961
0.0092
0
0.4784
202900
1357800
11908600
163210
363876
8850785
16898
68000
946701
7622
0
396349
6013
134228
446879
1.661
451405
451491
47.2
413770
416224
1595
66300
149261
0.0092
1262.2
1262.7
(kg/turbine) Scenario 1 202900
Steel
163210
Cast iron
16898
Glass fiber
7622
Plastic
6013
Lead
1.661
Copper
47.2
Aluminum
1595
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Reinforcing steel
Zinc Scenario 2
ED
Reinforcing steel Steel
Plastic Copper
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Lead
PT
Cast iron Glass fiber
Aluminum
Zinc * Data based on Ref. [29,30].
AC
639 640
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643
Table A.2 The numbers of vessels and workdays and the usage of fuels in the installation stage. No. of vessels
Activity Foundation vessel, transport of rock for scour protection vessel, dumping of rock for scour protection jack-up vessel, transport and installation of foundations tugboats, transport of foundations and jack-up vessels
1 1 1 2
Wind turbine
Fuel type*
Fuel rate (L/hr)
Work days
6250
HFO
360
182
3700
HFO
210
208
1500
HFO
87
52
10000
MGO
320
104
1
2750
HFO
160
52
10000
MGO
320
104
13000 7200
MGO MGO
450 360
9 41
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crane vessel, installation of wind turbines tugboats, transport and installation of wind turbines
Engine power (kW)
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2
ED
Electrical connections (Scenario 1) cable lay vessel with plough vessel, tie-in of cables
1 1
PT
Electrical connections (Scenario 2, including a substation)
644 645
AC
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cable lay vessel with plough 1 vessel, tie-in of cables 1 jack-up vessel, transport and installation of foundations 1 (substation) tugboats, transport of jack-up 2 vessel, etc. crane vessel, installation of 1 substation topside * HFO: heavy fuel oil; MGO: marine gas oil. ** Data based on Ref. [20,31].
34
130000 7200
MGO MGO
450 360
7.5 33.5
1500
HFO
87
11
10000
MGO
320
6
2750
HFO
160
11
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648
Table A.3 The numbers of vessels and workdays and the usage of fuels in the O&M stage. No. of vessels
Activity Wind Turbine support vessel, maintenance of wind turbines crane vessel, replacement of large parts
1 1
Electrical connections (Scenario 1) vessel, inspection of cables
1
Engine power (kW)
Fuel type*
Fuel rate (L/hr)
Work days
2000
MGO
99
650
2750
HFO
160
48
2000
MGO
99
211
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Electrical connections (Scenario 2, including a substation) 2000
MGO
99
173
2000
MGO
99
75
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ED PT CE AC
649 650 651
vessel, inspection of cables 1 vessel for maintenance of 1 substation * HFO: heavy fuel oil; MGO: marine gas oil. ** Data based on Ref. [20,31].
35
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Table A.4 The numbers of vessels and workdays and the usage of fuels in the EOL stage. No. of vessels
Activity/vessel Foundation jack-up vessel, dismantling and transportation of foundations tugboats, transport of foundations and jack-up vessels
1 2
Wind Turbine crane vessel, dismantling of wind turbine tugboats, dismantling and transport of wind turbines
1 2
Fuel type*
Fuel rate (L/hr)
Work time (day)
1500
HFO
87
52
10000
MGO
320
104
2000
HFO
160
52
10000
MGO
320
104
MGO
450
9
MA
Electrical connections (Scenario 1) vessel, removal of cables
Engine power (kW)
NU SC RI PT
653
1
13000
Electrical connections (Scenario 2, including a substation)
CE AC
655 656 657
PT
ED
vessel, removal of cables 1 jack-up vessel, dismantling and 1 transportation of foundations tugboats, transport of 2 foundations and jack-up vessels * HFO: heavy fuel oil; MGO: marine gas oil. ** Data based on Ref. [20,31].
36
13000
MGO
450
9
1500
HFO
87
11.
10000
MGO
320
6
Yu-Fong Huang, Xing-Jia Gan, Pei-Te Chiueh PII:
S0960-1481(16)30915-6
DOI:
10.1016/j.renene.2016.10.050
Reference:
RENE 8238
To appear in:
Renewable Energy
Received Date:
17 September 2015
Revised Date:
16 October 2016
Accepted Date:
22 October 2016
Please cite this article as: Yu-Fong Huang, Xing-Jia Gan, Pei-Te Chiueh, Life cycle assessment and net energy analysis of offshore wind power systems, Renewable Energy (2016), doi: 10.1016/j. renene.2016.10.050
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Highlights:
Environmental impact and energy benefit of offshore wind power was evaluated.
Installing an offshore substation would lead to higher environmental impact.
The greatest environmental impact was attributable to the use of ferrous metal.
Energy return on investment can be as high as 27.
Environmental impact and energy input can be largely reduced by waste recycling.
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Life cycle assessment and net energy analysis of offshore wind power systems
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Yu-Fong Huang, Xing-Jia Gan, Pei-Te Chiueh*
Graduate Institute of Environmental Engineering, National Taiwan University, 71,
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Chou-Shan Rd., Taipei 106, Taiwan, ROC
*
Corresponding author. Tel.: +886 2 3366 2798; fax: +886 2 2392 8830 Email address: [email protected] (P.-T. Chiueh). 1
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1
Abstract
2 3
This study attempted to evaluate the environmental impact and energy benefit of offshore wind power systems using life cycle assessment (LCA) and net energy
5
analysis. The environmental impact of offshore wind power systems is based primarily
6
on ferrous metal, which is used to install the foundations, towers, and nacelles. The
7
impact categories with the greatest relevance were fossil fuels and respiratory
8
inorganics. This study assumed that the life cycle of an offshore wind power system has
9
four stages (production, installation, operation and maintenance, and end-of-life). Two
10
scenarios were examined in this study. The major difference between the scenarios was
11
that Scenario 2 included an offshore substation. The overall environmental impact in
12
Scenario 2 was higher than that in Scenario 1 by approximately 10%. The net energy
13
analysis in this study included the evaluations of cumulative energy demand (CED),
14
energy return on investment (EROI), and energy payback time (EPT). For Scenarios 1
15
and 2, CED was 0.192 and 0.216 MJ/kWh, EROI was 18.7 and 16.7, and EPT was 12.8
16
and 14.4 months, respectively. Moreover, when the recycling of waste materials was
17
considered, each scenario produced a 25% lower environmental impact, 30% lower
18
energy requirement, and 4 months lower EPT.
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Keywords: Offshore wind power; Life cycle assessment; Net energy analysis;
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Sensitivity analysis
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1. Introduction
25 26
Research and development of renewable energy is urgent because of concerns such as heavy dependence on nonrenewable fossil fuels and disasters caused by global
28
climate change. During the past four decades, the consumption of fossil fuels and
29
emission of carbon dioxide have nearly doubled [1]. One promising form of renewable
30
energy is wind power, which is generated by wind turbines that convert the kinetic
31
energy of wind to electrical energy by rotating blades [2]. The worldwide electricity
32
generation of wind power has increased rapidly, and wind power has become the fastest
33
developing renewable energy technology [3]. From 1996 to 2011, the average annual
34
capacity of wind power increased by approximately 28% [4]. At the end of 2013, there
35
were commercial wind power installations in more than 90 countries with total installed
36
capacity of 318 GW, providing approximately 3% of global electricity supply [5]. The
37
rapid development of the wind power market and technology has major implications for
38
research, education, and professionals working in the electric utility and wind power
39
industries [6]. Wind power is exploited not only onshore but also offshore, where wind
40
speeds are higher and wind is typically available more regularly and for longer periods
41
[7]. Therefore, offshore wind power has attracted increasing interest mainly because of
42
the limited land for onshore wind farms. In 2013, total cumulative installations in the
43
offshore sector reach to 7,046 MW, approximately 2.2% of the global installed wind
44
turbine capacity [8]. Accordingly, a substantial growth of offshore wind power
45
installations is expected.
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46 47
Wind power is driven by renewable energy flux, but in a life cycle perspective
3
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there are still non-renewable resource demands and harmful emissions associated with it
49
[9], mainly due to the production and installation of wind turbines and related facilities.
50
The possible environmental and resource impact caused by the utilization of wind
51
power can be quantified by the method of life cycle assessment (LCA) [9]. Life cycle
52
assessment (LCA) is a tool for identifying and evaluating the environmental impact of a
53
product or service during its entire life cycle [10]. LCA is one of the most effective
54
methods of evaluating environmental burdens by identifying energy and materials used
55
as well as waste and emissions released in the environment [11]. LCA may be
56
conducted under various circumstances, but it is mainly based on a careful and
57
comprehensive account of all energy and material flow associated with a system or
58
process [12]. The environmental impact and benefits of wind power systems have been
59
evaluated and compared with those of other forms of renewable energy by using the
60
LCA method [10,13–15]. Compared with other renewable energy technologies, such as
61
photovoltaics, hydropower, and geothermal power, wind power systems are superior
62
regarding the sustainability indicators of carbon emissions, water consumption, and
63
social impact [16].
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Both nonrenewable and renewable energy systems consume energy throughout
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their life cycles. When considering the net benefit of an energy system, one major
67
concern is whether output energy is higher than input energy. The reliability of
68
renewable energy is often questioned because of its net energy benefit. One technique
69
for evaluating net benefit is to compare output energy delivered and input energy
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required. The purpose of net energy analysis is to assess energy use for such activities
71
as the extraction of raw materials, construction, operation and maintenance (O&M),
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disposal, and recycling. Various indicators are used for net energy analysis to measure
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energy use and performance. One indicator is referred to as energy return on investment
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(EROI), which is the ratio of energy delivered to energy cost [17]. The EROI of wind
75
power can be much higher than that of other renewable energy technologies, such as
76
solar thermal power, photovoltaics, geothermal power, and hydropower [18].
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77 78
To promote the development and application of offshore wind power systems,
understanding the benefits and threats of these systems to the environment and energy
80
production is crucial. Previous LCA studies have evaluated the GHG emissions, energy
81
performance, and environmental impacts associated with various aspects of the offshore
82
wind power designs [19–21]. However, there is no single literature that
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comprehensively studies both LCA and net energy analysis of offshore wind power
84
systems. Besides, while a previous study pointed out that the substructure of offshore
85
wind power plants is an important factor for the CO2 emissions [20], the effect of
86
installing an offshore substation on the environment and energy has not been studied so
87
far. This study aimed to identify and evaluate the environmental impact and energy
88
consumed at each stage of an offshore wind power system developed in Taiwan west
89
coast. Two scenarios, with and without a substation in the offshore wind system were
90
considered. LCA and net energy analysis were conducted to assess environmental
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impact and energy benefit throughout the life cycles of wind power systems, including
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component manufacturing, construction, transportation, operation, maintenance, and
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recycling after decommissioning.
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2. Methods
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96 97
The life cycle inventories of two offshore wind power systems were developed and assessed based on a functional unit of 1 kWh of electricity to evaluate environmental
99
burden and energy balance in this study. The LCA software SimaPro 7.1 and Eco-
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indicator 99 method were applied. This study used a conventional process-type LCA
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model using the inventory data from the built-in Ecoinvent database of SimaPro 7.1 and
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the inventory data from literature reports as described later in Section 2.2. In fact, a
103
systematic truncation error occurs in the process-type LCA. The truncation error is
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caused by the setting of system boundaries and consequently the omission of processes
105
outside these boundaries [22–24]. The magnitude of the error varies with the type of
106
product or activity considered, but can be in the order of 50% [23,24]. One way to avoid
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the truncation error is to incorporate an input–output analysis into the assessment
108
framework, resulting in a hybrid LCA method [23]. Besides, the truncation error of a
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process-type LCA is often significantly higher than that in the input–output part of a
110
hybrid assessment [23]. The use of input–output-based hybrid techniques can prevent
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uncertainties and errors in the process-type and input-output analyses of wind power
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[13,25]. Further discussion on the truncation error and the input–output analysis is
113
beyond the scope of the study, but these aspects are important to assist the decision-
114
making of the government by providing information and suggestion with better
115
accuracy and precision. There is a literature report on Taiwan’s CO2 and CH4 emissions
116
inventories in an integrated hybrid input-output LCA [26], which is a helpful reference
117
for setting up hybrid input-output LCA models. Besides, it should be noted that this
118
study is an attributional LCA study which normally cannot take comprehensive system
119
changes into consideration [27]. Further extensive investigation and analysis would be
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necessary to carry out a consequential analysis. Moreover, net energy indicators,
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including cumulative energy demand (CED), energy return on investment (EROI), and
122
energy payback time (EPT), were also calculated.
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125 126
2.1.1 System boundary
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The system boundary of this study was the overall life cycles of offshore wind
129
power systems, including the phases of resource extraction, component manufacturing,
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construction, transportation, operation, maintenance, and disposal. This study simulated
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two offshore wind power system scenarios: one had an offshore electrical substation,
132
and the other did not. The necessity of an offshore substation depends on the total
133
installed capacity of a wind farm and its distance from the coast. Generally, when the
134
total installed capacity is less than 30 MW, constructing an offshore substation is
135
unnecessary. If the total installed capacity is more than 30 MW but less than 120 MW,
136
an offshore substation should be constructed when the distance between the wind farm
137
and the coast is greater than 10 km. An offshore substation is generally necessary when
138
the total installed capacity is higher than 120 MW, regardless of the distance between
139
the wind farm and the coast.
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In this study, offshore wind farms were assumed to include the installation of 52
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wind turbines (Vestas V80-2.0 MW), resulting in a total power capacity of 104 MW.
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Two scenarios were simulated. Scenarios 1 and 2 included wind turbines, internal
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cables, and electrical submarine cables. In Scenario 1, the wind turbines were connected
145
with one another by 32 kV submarine cables, and the electricity was then transmitted to
146
the coast by two 32 kV submarine cables. In Scenario 2, the electricity generated from
147
the wind farm was first delivered to an offshore substation (AC/DC), and it was then
148
transmitted to the coast by a 150 kV submarine cable. PEX-insulated cables were
149
assumed to be used (PEX: cross-linked polyethylene). The schematic diagrams of the
150
two offshore wind power systems are illustrated in Fig. 1.
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Wind turbines
Offshore substation
153 154 155
32 kV submarine cables
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Wind turbines
Fig. 1
System boundary
Onshore substation
Scenario 2
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Scenario 1
System boundary
Onshore substation
150 kV submarine cable
Offshore wind power systems.
2.1.2 Functional unit
156 157
For this study, 1 kWh generated by offshore wind power was selected as a 8
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functional unit. Input resources, energy consumption, and relative emissions throughout
159
the life cycles of offshore wind power systems were calculated based on this functional
160
unit.
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163 164 165 166
For conducting the LCA of offshore wind power systems, the following four assumptions were held:
The site of the offshore wind power system is located in the Taiwan Strait, near the coast of Fangyuan Township, Changhua County. The distance
168
between the system and the coast ranges from 8 to 15 km. The depth of the
169
sea area ranges from 20 to 45 m. The area of the system is approximately 20
170
km2.
Taiwan in 2011 [28].
172 173
Electricity supply and consumption is that of the power generation structure in
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The lifetime of wind turbines and internal submarine cables is 20 years, and
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the lifetime of submarine transmission cables and an offshore substation is 40
175
years.
177 178 179 180
Only domestic transportation is considered. The distance of land
transportation is assumed to be 150 km, which is approximately the average
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distance across three counties in Taiwan. Maritime transportation is between the offshore wind farm and Taichung Harbor, and the average distance is
assumed to be 50 km.
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2.2 Inventory analysis
183 184
The life cycle inventory analyses of the offshore wind power systems were mainly based on the reports of Dong Energy and Vestas Wind Systems [29,30]. The inventory
186
data for the stages of installation, transportation, O&M, decommissioning, and
187
recycling were based on the literature [20,31]. In addition, the number of vessels and
188
workdays required were adjusted at the construction stage. Further description of the
189
inventory analysis at the production, installation, O&M, and end-of-life (EOL) stages is
190
provided in the following text.
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191 2.2.1 Production stage
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The offshore wind farm primarily consisted of wind turbines and a power
195
transmission system. A wind turbine was mainly composed of rotor blades, nacelles,
196
and a tower and foundation. In Scenario 1, the power transmission system included 32
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kV submarine cables only. In Scenario 2, the power transmission system included
198
32/150 kV submarine cables and an offshore substation. The major materials necessary
199
for the installation of the offshore wind farm included cast iron, steel, glass fiber,
200
plastics, resin, concrete, lead, copper, and aluminum. The components and materials
201
used for the offshore wind farm are listed in Table 1. The quantities of the materials
202
used are provided in the Appendix (Table A.1), based on the reports of Dong Energy
203
and Vestas Wind Systems for the establishment of an offshore wind farm at Horns Reef,
204
Denmark in 2002 [29,30]. It is important to understand the source of the data. LCAs
205
vary significantly when background economies are changed [32]. Lenzen and
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Wachsmann have shown that the energy requirements in different countries can differ
207
by a factor of more than 8 [32], so the choice of country for inventory data can make a
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big difference.
Table 1 The components and materials used of the offshore wind farm. Components
Materials
Rotor blade
Glass fiber, epoxy resin, PVC
Nacelle
Cast iron, steel, glass fiber, HDPE, epoxy resin
Tower
Steel, alkyd resin
Foundation
Steel, aluminum
32/150 kV submarine cable
Lead, copper, steel, HDPE
Offshore substation
Steel, concrete, cast iron, aluminum
2.2.2 Installation stage
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During the installation of offshore wind power facilities, the required
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transportation was assumed to include domestic land and maritime transportation.
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Foreign transportation for the importation of wind turbine components was neglected.
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The distance for domestic land transportation by truck was assumed to be 150 km, and
218
the distance for domestic maritime transportation from Taichung Harbor to the offshore
219
wind farm was assumed to be 50 km. The installation of the wind power facilities could
220
be divided into three steps: (1) the construction of submarine infrastructures; (2) the
221
installation of the towers, nacelles, and rotor blades; (3) the connection of the power
222
grid. The number of vessels and workdays required at the installation stage are listed in
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the Appendix (Table A.2).
224 225
2.2.3 Operation and maintenance stage
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To maintain high efficiency, the maintenance of offshore wind turbines typically consists of scheduled maintenance one to two times and unscheduled (corrective)
229
maintenance one to four times per year per wind turbine [20]. Maintenance was
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assumed to occur five times per year in this study, with a vessel being used for
231
transportation during maintenance once per year and a helicopter being used for
232
transportation during maintenance four times per year. The distance of transportation
233
was assumed to be 50 km. The unscheduled adding or replacing of lubricants was
234
assumed to be necessary to maintain the function of wind turbines. During the life cycle
235
of an offshore wind turbine (20 years), one rotor blade and one out of three nacelles on
236
average was assumed to require replacement in this study. The numbers of vessels and
237
workdays required in the O&M stage are listed in the Appendix (Table A.3).
238
Furthermore, the lifetime of wind farms could increase to 25 or even 30 years,
239
attributable to the progress of wind power technology. Although the increased lifetime
240
would need more maintenance works, it is difficult to present the effect of lifetime
241
because the function unit of this study was energy generated by the offshore wind
242
power system.
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2.2.4 End-of-life stage
245 246
The last stage of an offshore wind power system is the EOL stage. At this stage,
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the primary goal is to address waste recycling and disposal to minimize its impact on
248
the environment. To choose an appropriate decommissioning method (e.g., full or
249
partial demolition), the environmental characteristics of a wind farm must be assessed.
250
For the purpose of restoring the site to its original state, based on existing technology, at
251
the decommissioning stage, all of the offshore wind power facilities were assumed to be
252
demolished in this study, including submarine cables and an offshore substation. The
253
number of vessels and workdays required at the EOL stage are listed in the Appendix
254
(Table A.4).
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255 256
The recyclable materials from the waste offshore wind power system were
assumed to include steel, cast iron, copper, aluminum, lead, and other metals, whereas
258
glass fibers and plastics were assumed to be buried or incinerated. In most cases, the
259
loss rate of the steel and metal recycling is less than 10%. In this study, the recycling
260
rate was assumed to be 90%, and the loss rate was assumed to be 10%. The methods of
261
recycling and disposing of the aforementioned materials are listed in Table 2.
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Table 2 The methods of recycling and disposing of the waste materials. Material
Recycling / Disposal
Steel / cast iron, stainless steel / high-strength steel, copper, aluminum, lead
90% recycling
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Rubber, plastics (except PVC)
100% incineration
Glass fiber, PVC
100% buried
2.3 Impact assessment
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The impact assessment model of this study was Eco-indicator 99, which is a damage-oriented (endpoint) method for life cycle impact assessment [33]. Eco-indicator
269
99 comprises five steps: (1) characterization, (2) damage assessment, (3) normalization,
270
(4) weighting, and (5) single score calculation. After the impact assessment, the
271
magnitude of environmental impact can be expressed as a single score point (Pt). The
272
inventory results can be classified and characterized as 11 impact categories (i.e.,
273
carcinogens, respiratory organics, respiratory inorganics, climate change, radiation,
274
ozone layer, ecotoxicity, acidification and eutrophication, land use, minerals, and fossil
275
fuels), and these can then be further classified as three main damage categories (i.e.,
276
human health, ecosystem quality, and resource damage) [34].
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In this study, the built-in Cumulative Energy Demand (CED) model of SimaPro 7.1 LCA software was used for net energy analysis to understand the energy balance of
280
offshore wind power. The CED model comprises three steps: (1) characterization, (2)
281
weighting, and (3) single score calculation. The inventory results were classified as five
282
categories and converted to the same unit. Each impact category was assigned a
283
weighting factor of 1 to obtain a total CED. The CED can be presented as a single score
284
point (Pt) [34]. In addition to the CED model, the net energy indicators of EROI and
285
EPT of offshore wind power systems were studied.
287 288 289
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3. Results and discussion
3.1 Life cycle assessment
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In this study, Scenarios 1 and 2 adopted Vestas V80 (2.0 MW) wind turbines, which are composed of three blades as well nacelles, a tower, and a mono-pile
293
foundation. Approximately 430 tonnes of materials were assumed to be used per wind
294
turbine. The materials used to construct a wind turbine are shown in Fig. 2. Wind
295
turbines are composed mainly of ferrous metal materials (89.81%), of which the
296
proportions of reinforcing steel, steel, and cast iron are 47.58%, 38.27%, and 3.96%,
297
respectively. Nonferrous metal materials account for only 0.41% of wind turbines and
298
primarily include aluminum, copper, and lead. Additionally, 3.20% of wind turbines are
299
composed of synthetic materials, including glass fibers (used for the blades) and epoxy
300
resins. Moreover, 6.58% of wind turbines are composed of other materials, which are
301
primarily the water used in the manufacturing process and lubricants used in the cabins.
302
In addition, the manufacturing process also consumes energy, such as electricity and
303
natural gas.
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305 306
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Fig. 2
Material shares of a wind turbine.
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The characterization results of offshore wind turbines verified that, in all
309
categories, the greatest environmental impact was attributable to the use of ferrous
310
metal materials. Most of the ferrous metal was steel used for foundation installation,
311
followed by steel used for tower and nacelle installation. The synthetic materials
312
contributed to a higher impact proportion on the land use category, which was
313
attributable to the use of alkyd resin that was used for the painting of the foundation and
314
tower. The alkyd resin was made from polyacid, polyalcohol, and soybean oil. The
315
planting of soybeans requires land, which explains why the impact on the land use
316
category was high.
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The results of damage assessment were similar to those of characterization. The
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highest impact in each category was attributable to the use of ferrous metal, particularly
320
in the human health and ecosystem quality categories. This can be explained by the
321
inventory data. The use of ferrous metal requires extensive pickling and leads to high
322
dust emissions. Workers may thus inhale acidic gases and dusts, resulting in damage to
323
respiratory organs in the human body. In the resource damage category, the relatively
324
high impact was attributable to the use of fuels and synthetic materials. The process of
325
steel rolling consumes a large amount of natural gas and thus results in resource
326
damage.
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The single score results of offshore wind turbines are illustrated in Fig. 3. The total
329
single score (i.e., the total environmental impact) proportion of the resources and fuels,
330
from highest to lowest, were reinforcing steel (45.77%), steel (22.89%), synthetic
331
materials (12.50%), energy use (11.47%), cast iron (3.91%), nonferrous metal (2.90%),
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and other parts (0.56%). The use of ferrous metal materials (including steel, reinforcing
333
steel, and cast iron) thus produced a total single score proportion as high as
334
approximately 73%. The main impact categories with higher single scores were fossil
335
fuels and respiratory inorganics, which were primarily attributable to the use of steel
336
and synthetic materials as well as electricity.
338 339
Fig. 3
342 343
3.1.1 Scenario 1
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Single scores of offshore wind turbines.
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The single scores of Scenario 1 for the offshore wind power system are shown in Fig. 4. The environmental impact of the stages throughout the life cycle, from highest to
345
lowest, were production (35.5%), installation (34.2%), EOL (16.6%), and O&M
346
(13.7%). The impact categories with higher single scores were fossil fuels and
347
respiratory inorganics, which were mainly attributable to the use of steel material,
348
electricity, and oil for transportation vessels. Moreover, the environmental impact for
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the ecotoxicity and minerals categories was mainly attributable to the copper used in the
350
electrical transmission cables.
352 353
Fig. 4
354
Single scores of Scenario 1.
The production stage of the offshore wind power system included the production
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of wind turbines and transmission systems, the former of which caused a major impact.
357
Furthermore, the main impact sources, from highest to lowest, were the production of
358
the foundation, tower, nacelles, and blades. At the installation stage, fuel consumption
359
associated with land transportation, maritime transportation, and construction operations
360
was considered. The impact of vessels and machines was primarily attributable to their
361
fuel consumption, and thus the impact of fossil fuels, minerals, and respiratory
362
inorganics was much higher than that of other categories. The vessels used for scour
363
protection of the foundation were a substantial source of impact at the installation stage.
364
Furthermore, the impact of the tugboats used for wind turbine and foundation
365
transportation and construction, the cranes used for foundation installation, and the
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vessels used for submarine cable transportation was also crucial. Overall, foundation
367
installation had the highest proportion of total impact (70.9%), followed by wind
368
turbine installation (24.7%). The proportions of total impact from cable installation
369
(2.98%) and land transportation (1.4%) were much lower.
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370 371
3.1.2 Scenario 2
372 373
The distinction between Scenarios 1 and 2 was the transmission system. In the transmission system of Scenario 2, the offshore substation had the highest impact,
375
followed by 150 kV and 32 kV submarine cables. The most substantial impact occurred
376
in the categories of, from highest to lowest, fossil fuels, respiratory inorganics, minerals,
377
and climate change. The main source of impact was the large amount of concrete used
378
for offshore substation construction, which affected fossil fuels, respiratory inorganics,
379
and climate change. Moreover, the copper content in the submarine cables had impact
380
on minerals, respiratory inorganics, climate change, and ecotoxicity.
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The vessels used for scour protection of the foundation caused the highest impact, followed by the tugboats used for wind turbine and foundation transportation and
384
construction, the cranes used for foundation construction, the jack-up vessels used for
385
foundation and substation installation, and the vessels used for submarine cable
386
transportation. Overall, foundation installation had the highest proportion of total
387
impact (70.4%), followed by wind turbine installation (24.5%). The proportions of total
388
impact from transmission system installation (3.62%) and land transportation (1.48%)
389
were not as substantial.
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390 391
The single scores of Scenario 2 for the offshore wind power system are shown in Fig. 5. The environmental impact of production, installation, O&M, and EOL was
393
40.8%, 31.3%, 12.7%, and 15.3%, respectively. The main impact was observed in the
394
categories of fossil fuels and respiratory inorganics, which was primarily attributable to
395
the use of steel material, electricity, and fuel for transportation. The impact in the
396
categories of ecotoxicity and minerals was mainly attributable to the copper used in the
397
submarine cables and steel used in the wind turbines. Moreover, the impact on climate
398
change was attributable to the concrete used for the construction of the offshore
399
substation. In general, the single score of Scenario 2 (0.00139 Pt) was higher than that
400
of Scenario 1 (0.00126 Pt) by approximately 10%. However, after considering the
401
effectiveness of the reduction in electricity transmission loss, Scenario 2 may still be
402
feasible.
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404 405
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Fig. 5
Single scores of Scenario 2.
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3.1.3 Recycling benefit
408 409
According to the aforementioned results of LCA on the two offshore wind power systems, the ferrous metals used for wind turbines and copper, iron, and lead used for
411
submarine cables were the primary sources of environmental impact. In this study, the
412
ferrous metals (including steel, reinforcing steel, and cast iron) and nonferrous metals
413
(including copper, aluminum, and lead) were assumed to be recyclable. The recycling
414
rate was assumed to be 90%. Although the transmission system of Scenario 2 requires
415
more materials than that of Scenario 1, most of the materials can be recycled at the EOL
416
stage of the offshore wind power system. This indicates that, if the benefit of material
417
recycling is considered, environmental impact can be lower regardless of whether a
418
substation is installed. The overall environmental impact of Scenarios 1 and 2 can thus
419
be reduced by approximately 25% because of the recycling of waste materials.
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3.2 Net energy analysis
424 425
3.2.1 Cumulative energy demand
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420
CED is a calculation of the total energy input for an energy system throughout its life. Therefore, a CED analysis of an offshore wind power system must consider the
427
overall energy consumed at the production, installation, O&M, and EOL stages. The
428
CED shares of the four stages are shown in Fig. 6; 46% and 51% of the total energy
429
demand was required at the production stage for Scenarios 1 and 2, respectively. Nearly
430
half of the overall energy demand was thus attributable to the production of wind
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turbines. The energy demand at the installation stage was 27% and 24% for Scenarios 1
432
and 2, respectively. The energy input at the O&M and EOL stages was nearly the same
433
and constituted approximately 13% of the overall energy input.
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435
Fig. 6
437
Scenario 1 included 52 offshore wind turbines and two 32 kV submarine power
PT
438
CED shares.
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436
transmission cables. The 1 kWh electricity output from Scenario 1 in the life cycle
440
required 0.192 MJ energy input. When considering the recycling of waste materials, the
441
energy input was reduced by approximately 30%, thus totaling 0.135 MJ. Scenario 2
442
included 52 offshore wind turbines, one substation, and one 150 kV submarine power
443
transmission cable. The 1 kWh electricity output from Scenario 2 in the life cycle
444
required 0.216 MJ energy input, which was higher than that in Scenario 1 by 12.5%.
445
When the recycling of waste materials was considered, the energy input was reduced to
446
0.155 MJ, which was approximately 28% of the overall energy demand.
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3.2.2 Energy return on investment
449 EROI represents the ratio of energy output to energy input and is a crucial
451
indicator of potential energy benefit to society [35]. The EROI of an offshore wind
452
power system is the energy output from wind turbines divided by the required energy
453
input. EROI is thus dimensionless, and a higher EROI value indicates that more net
454
energy can be obtained. The EROI of Scenario 1 was 18.7. It increased to 26.7 when the
455
recycling of waste materials was considered. The EROI of Scenario 2 was 16.7, and it
456
increased to 23.2 with waste recycling. The EROI of Scenario 2 was lower primarily
457
because of the installation of an offshore substation. Kubiszewski et al. showed an
458
average EROI in all of their studies (operational and conceptual) of 25.2 and an average
459
EROI in only the operational studies of 19.8 [18]. The EROI determined in this study
460
should thus be reasonable and reliable. In addition, Hall et al. found that the minimum
461
EROI is 3 for industrial societies [36], suggesting that no more than 33% of social and
462
economic resources are required for energy production without undermining the
463
sustainability of the societies. Offshore wind power development is thus warranted
464
because of its high EROI, particularly when waste materials are adequately recycled.
467 468
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3.2.3 Energy payback time
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EPT represents the time required when the energy output of an offshore wind
469
power system equals the energy input at its production, installation, O&M, and EOL
470
stages. A lower EPT value indicates that energy input and output can be balanced in a
471
shorter time. Therefore, EPT should be as low as possible for economic efficiency. In
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this study, 52 2.0 MW offshore wind turbines were assumed to work for 3,000
473
equivalent full-load hours per year, which theoretically could produce electricity of
474
6,240 GWh in 20 years. Based on this energy output, the EPT of Scenarios 1 and 2 was
475
12.8 and 14.4 months, respectively. When the recycling of waste materials was
476
considered, the EPT of Scenarios 1 and 2 was shortened to 9.0 and 10.3 months,
477
respectively. Because of the installation of an offshore substation, more energy and
478
resources were consumed in Scenario 2, and thus its EPT was approximately 1.5 months
479
longer than that of Scenario 1. Furthermore, for Scenarios 1 and 2, the EPT was
480
approximately 4 months shorter when the waste materials were properly recycled.
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Martinez et al. reported that the EPT of a 2 MW onshore wind turbine running for
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482
2,000 equivalent full-load hours per year is 4.8 months [37]. According to Wagner et
484
al., the EPT of a 5 MW offshore wind turbine running for 3,000 equivalent full-load
485
hours per year is 8.8 months [19]. Furthermore, Garrett and Ronde found that the EPT
486
of Vestas 2 MW wind turbines ranges from 8 to 11 months depending on wind turbine
487
model and wind speed [38]. The EPT in this study was slightly longer than those in the
488
aforementioned studies. This could be attributable to the difference in the location and
489
meteorology of wind farms, type and model of wind turbines, operation time and
490
frequency, and other factors relating to system boundaries and conditions. However, the
491
EPT of offshore wind power systems should be lower when considering the recycling of
492
waste materials.
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3.3 Sensitivity analysis
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Sensitivity analysis is helpful in determining the impact of selected variables as they are changed [15]. In this study, four factors substantially influenced the impact of
498
offshore wind power systems: power capacity, distance from shore, steel consumption,
499
and electrical consumption. The results of sensitivity analysis, including the variations
500
in single score and CED, are listed in Table 3. In this study, power capacity most
501
influenced the environmental impact of offshore wind power systems. A 10% increase
502
in power capacity led to an approximately 10% decrease in environmental impact, and a
503
10% decrease in power capacity led to an approximately 10% increase in environmental
504
impact. Steel consumption was also substantial because of its direct influence on energy
505
and material consumption. The factors of distance from shore and electrical
506
consumption were not as substantial as the aforementioned factors. When the two
507
factors were increased or reduced by 20%, the variations in both single score and CED
508
were approximately 1%. For offshore wind power systems, the most crucial factors may
509
thus include wind condition, power generation performance, and operation time and
510
frequency.
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Table 3 Sensitivity analysis of the offshore wind power system. Single score
CED
variation
variation
variation
Power capacity
+10%
–10%
–9%
–10%
+10%
+11%
+20%
+0.8%
+0.6%
+100%
+3.4%
+1.5%
Steel consumption
+20%
+5%
+6%
–20%
–5%
–6%
+20%
+0.75%
+1.5%
CE
Parameter
AC
512
PT
511
Distance from shore
Electrical consumption
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–20%
–0.75%
–1.5%
513 514
4. Conclusions
516
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515 To evaluate the environmental burden and energy balance of offshore wind power, this study developed and assessed life cycle inventories in two scenarios, which were
518
distinguishable because only one scenario included an offshore substation. The SimaPro
519
7.1 software and Eco-indicator 99 method were applied to perform LCA based on a
520
functional unit of 1 kWh of electricity. Furthermore, this study also involved the
521
calculation of net energy indicators, such as CED, EROI, and EPT. The results show
522
that the factors, including steel material for wind turbines, electricity consumption at the
523
production stage, fuel consumption and air emissions during maritime transportation
524
and construction, and concrete materials for an offshore substation, are the most
525
substantial sources of environmental impact. The overall environmental impact of
526
Scenario 2 was higher than that of Scenario 1. However, after considering the
527
effectiveness of the reduction in electricity transmission loss, Scenario 2 may still be
528
feasible. The environmental impact of offshore wind power systems can be further
529
reduced by lowering the amount of materials used for wind turbines, particularly steel
530
and concrete. Offshore wind power development is warranted because of its high EROI,
531
particularly when waste materials are adequately recycled.
533
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Acknowledgments
534 535 536
The authors gratefully acknowledge the financial support from the Ministry of Science and Technology, Taiwan (contract no. 101-ET-E-002-013-ET). 26
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Figure captions
622 Fig. 1
Offshore wind power systems.
Fig. 2
Material shares of a wind turbine.
Fig. 3
Single scores of offshore wind turbines.
Fig. 4
Single scores of Scenario 1.
Fig. 5
Single scores of Scenario 2.
Fig. 6
CED shares.
624 625 626 627 628 629
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Appendices
637 Table A.1 The quantities of materials used for the wind power system. Materials
Wind turbine
Transmission
Total
(kg/farm)
(kg/farm)
0
10550800
279362
8766271
0
878701
0
396349
46118
358769
400879
400965
317829
320283
0
82961
0.0092
0
0.4784
202900
1357800
11908600
163210
363876
8850785
16898
68000
946701
7622
0
396349
6013
134228
446879
1.661
451405
451491
47.2
413770
416224
1595
66300
149261
0.0092
1262.2
1262.7
(kg/turbine) Scenario 1 202900
Steel
163210
Cast iron
16898
Glass fiber
7622
Plastic
6013
Lead
1.661
Copper
47.2
Aluminum
1595
MA
Reinforcing steel
Zinc Scenario 2
ED
Reinforcing steel Steel
Plastic Copper
CE
Lead
PT
Cast iron Glass fiber
Aluminum
Zinc * Data based on Ref. [29,30].
AC
639 640
NU SC RI PT
638
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643
Table A.2 The numbers of vessels and workdays and the usage of fuels in the installation stage. No. of vessels
Activity Foundation vessel, transport of rock for scour protection vessel, dumping of rock for scour protection jack-up vessel, transport and installation of foundations tugboats, transport of foundations and jack-up vessels
1 1 1 2
Wind turbine
Fuel type*
Fuel rate (L/hr)
Work days
6250
HFO
360
182
3700
HFO
210
208
1500
HFO
87
52
10000
MGO
320
104
1
2750
HFO
160
52
10000
MGO
320
104
13000 7200
MGO MGO
450 360
9 41
MA
crane vessel, installation of wind turbines tugboats, transport and installation of wind turbines
Engine power (kW)
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2
ED
Electrical connections (Scenario 1) cable lay vessel with plough vessel, tie-in of cables
1 1
PT
Electrical connections (Scenario 2, including a substation)
644 645
AC
CE
cable lay vessel with plough 1 vessel, tie-in of cables 1 jack-up vessel, transport and installation of foundations 1 (substation) tugboats, transport of jack-up 2 vessel, etc. crane vessel, installation of 1 substation topside * HFO: heavy fuel oil; MGO: marine gas oil. ** Data based on Ref. [20,31].
34
130000 7200
MGO MGO
450 360
7.5 33.5
1500
HFO
87
11
10000
MGO
320
6
2750
HFO
160
11
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648
Table A.3 The numbers of vessels and workdays and the usage of fuels in the O&M stage. No. of vessels
Activity Wind Turbine support vessel, maintenance of wind turbines crane vessel, replacement of large parts
1 1
Electrical connections (Scenario 1) vessel, inspection of cables
1
Engine power (kW)
Fuel type*
Fuel rate (L/hr)
Work days
2000
MGO
99
650
2750
HFO
160
48
2000
MGO
99
211
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Electrical connections (Scenario 2, including a substation) 2000
MGO
99
173
2000
MGO
99
75
MA
ED PT CE AC
649 650 651
vessel, inspection of cables 1 vessel for maintenance of 1 substation * HFO: heavy fuel oil; MGO: marine gas oil. ** Data based on Ref. [20,31].
35
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654
Table A.4 The numbers of vessels and workdays and the usage of fuels in the EOL stage. No. of vessels
Activity/vessel Foundation jack-up vessel, dismantling and transportation of foundations tugboats, transport of foundations and jack-up vessels
1 2
Wind Turbine crane vessel, dismantling of wind turbine tugboats, dismantling and transport of wind turbines
1 2
Fuel type*
Fuel rate (L/hr)
Work time (day)
1500
HFO
87
52
10000
MGO
320
104
2000
HFO
160
52
10000
MGO
320
104
MGO
450
9
MA
Electrical connections (Scenario 1) vessel, removal of cables
Engine power (kW)
NU SC RI PT
653
1
13000
Electrical connections (Scenario 2, including a substation)
CE AC
655 656 657
PT
ED
vessel, removal of cables 1 jack-up vessel, dismantling and 1 transportation of foundations tugboats, transport of 2 foundations and jack-up vessels * HFO: heavy fuel oil; MGO: marine gas oil. ** Data based on Ref. [20,31].
36
13000
MGO
450
9
1500
HFO
87
11.
10000
MGO
320
6