ing ▁the ▁challeng es ▁steel ▁faces ▁during ▁transport ation . ▁One ▁down side ▁of ▁concrete ▁to wers ▁is ▁the ▁higher ▁CO 2 ▁em issions ▁during ▁concrete ▁production ▁as ▁compared ▁to ▁steel . ▁However , ▁the ▁overall ▁environmental ▁benefit ▁should ▁be ▁higher ▁if ▁concrete ▁to wers ▁can ▁double ▁the ▁wind ▁tur bine ▁lifetime . ▁ ▁Wood ▁is ▁being ▁investig ated ▁as ▁a ▁material ▁for ▁wind ▁tur bine ▁to wers , ▁and ▁a ▁ 1 0 0 ▁met re ▁tall ▁tower ▁supporting ▁a ▁ 1 . 5 ▁M W ▁tur bine ▁has ▁been ▁ere cted ▁in ▁Germany . ▁The ▁wood ▁tower ▁shares ▁the ▁same ▁transport ation ▁benefits ▁of ▁the ▁segment ed ▁steel ▁shell ▁tower , ▁but ▁without ▁the ▁steel ▁resource ▁consumption . ▁ ▁Connection ▁to ▁the ▁electric ▁grid ▁ ▁All ▁grid - connected ▁wind ▁tur b ines , ▁from ▁the ▁first ▁one ▁in ▁ 1 9 3 9 ▁until ▁the ▁development ▁of ▁variable - speed ▁grid - connected ▁wind ▁tur b ines ▁in ▁the ▁ 1 9 7 0 s , ▁were ▁fixed - speed ▁wind ▁tur b ines . ▁As ▁recently ▁as ▁ 2 0 0 3 , ▁nearly ▁all ▁grid - connected ▁wind ▁tur b ines ▁operated ▁at ▁exactly ▁constant ▁speed ▁( s ynchronous ▁gener ators ) ▁or ▁within ▁a ▁few ▁percent ▁of ▁constant ▁speed ▁( indu ction ▁gener ators ). ▁As ▁of ▁ 2 0 1 1 , ▁many ▁oper ational ▁wind ▁tur b ines ▁used ▁fixed ▁speed ▁induction ▁gener ators ▁( F SI G ). ▁As ▁of ▁ 2 0 1