離岸式風力發電系統之生命週期評估與淨能源分析

Abstract

再生能源中的風力發電為世界上發展最快的能源系統，其中離岸式風力發電位於面積廣闊的海域，擁有風期長、平均風速大、風力平穩且不受遮擋等優點，已成為近年來世界各國相當重視的能源開發方案之一。臺灣經濟部能源局也為此積極準備，於2012年公布「風力發電離岸系統示範獎勵辦法」，鼓勵業者進行離岸式風力發電示範開發，期望藉此達成能源多元化、改善環境品質、二氧化碳減量及國家永續發展之目標。 由於臺灣尚未有離岸式風力發電實廠的相關資料，因此本研究參考國內廠商已提交環境影響評估之「福海離岸風力發電計畫」，根據可能選用的電力輸送方式，模擬離岸式風力發電廠，並依照是否裝設海上變壓站，區分兩個案例進行探討，案例一為52部離岸式風力機，以2條32kV海底纜線將電力傳輸至岸上；案例二為52部離岸式風力機，經由海上變壓站後利用150kV海底纜線將電力傳輸至岸上。 研究方法選用生命週期評估軟體SimaPro 7.1，盤查分析使用文獻回顧及軟體內建Ecoinvent資料庫，經由Eco-indicator 99評估模式量化其潛在衝擊，並計算其累積能源需求、能源投資報酬以及能源回收期等淨能源指標。最後針對研究結果進行敏感度分析及蒙地卡羅模擬計算不確定性。 結果顯示，風力機的環境衝擊主要來自鋼鐵材料，以基座採用的鋼筋為主，塔架及機艙使用的鋼材次之，衝擊項目以化石燃料與可吸入無機物最高。比較生命週期各階段的環境衝擊，由大到小依序為組件製造、建造運輸、退役及運轉維修階段。由單項得點結果比較，含有海上變壓站之案例二單項得點(0.00139 Pt)約為案例一(0.00126 Pt)的1.1倍，特別是氣候變遷衝擊項目，案例二約大於案例一29%，其差異來自於海上變壓站大量使用的混凝土材料。 在淨能源計算部分，離岸式風電廠每產出1度電，案例一需要0.192 MJ能源投入，案例二需要0.216 MJ能源投入；案例一EROI為18.7，案例二EROI為16.7；案例一之能源回收期為12.8個月，案例二之能源回收期為14.4個月。若考慮回收階段帶來的效益，整體來說，可為案例一及案例二減少25%環境衝擊，減少約30%的能源耗用，縮短了約4個月的能源回收期。 雖然含有海上變壓站之案例二，不論是環境衝擊或者能源損耗皆大於案例一，但是考慮電力傳輸損失減低之效益，或許不失為可行之方案。最後，若要進一步降低風電廠帶來之環境衝擊，可以針對風力機材料進行改良，特別是鋼鐵材料的部分可尋找更環保的材料替代，或者進行鋼鐵、混凝土製程改善、電力結構調整以及提升建造階段之海上船隻燃油使用效率。

Wind energy is among the fastest growing sources of electricity generation worldwide. Because of optimal wind conditions at sea, long operating times, strong average wind speeds, and unobstructed wind fields, offshore wind power technology has become a focal program of energy development. Taiwan has strong winds, particularly offshore of the west coast. The Energy Bureau is committed to promoting offshore wind energy and propelling Taiwan into a sustainably green era. To evaluate the environmental burden and energy balance of using wind power technology, this study developed and assessed life cycle inventories for two conceptual offshore wind-power plants that depend on electricity transmission systems. Life cycle assessment software SimaPro 7.1 and Eco-indicator 99 methodology were applied to evaluate functional units of 1 kWh offshore wind power electricity. Furthermore, net energy indicators, such as cumulative energy demand, energy return on investment (EROI), and energy payback time, were calculated. The results indicate that the environmental impact of Vestas V80 wind turbines is mainly due to ferrous metals used in the foundations of wind turbines and towers. The most relevant impact categories are fossil fuels and respiratory inorganics. Regarding the single scores, the environmental impact is derived from production, subsequent installation, end-of-life, and O&M. Regarding Case 2, the impact of an offshore wind-power plant employing an offshore transformer and a 150 kV submarine cable was 1.1 times greater than the impact calculated for Case 1, which involved using an offshore wind-power plant equipped with two 32 kV submarine cables. In the climate change category, Case 2 indicated a 129% greater environmental impact than that of Case 1. Regarding net energy analysis, the energy required was 0.192 MJ/kWh for Case 1 and 0.216 MJ/kWh for Case 2, and the EROI was 18.7 and 16.7, respectively. Moreover, the energy payback time were 12.8 months for Case 1 and 14.4 months for Case 2. For the both cases, considering the recycling benefits can reduce 25% environment impact, 30% energy required and 4 months energy payback time. Case 2 demonstrated environmental and energy impacts that were more significant than those of Case 1. Considering the effectiveness of reducing power transmission loss, Case 2 may be feasible. Overall, to reduce the impact of offshore wind-power plants, the materials of wind turbines can be improved, particularly steel and concrete. In addition, during the installation phase, the electricity structure can be adjusted and the vessel fuel efficiency can be upgraded.