Engineering360: "The value of vertical axis wind turbines"

Wind generation system type Economics 101:The question of purchase & maintenance costs between the two rotations quickly rises. Simple equation is:

$WLv - $PMLv > $WLh - $PMLh

$WLv: lifetime profit per watt vertical

$WLh: lifetime profit per watt horizontal

$OMLv: lifetime cost purchase & operations vertical

$OMLh: lifetime cost purchase & operations horizontal

There may well be, and probably is, a duration factor between the lifetimes of the two systems. Purchase Costs include both manufacturing & installation costs.

If 3-diameter spacing is optimal, why does increasing the spacing improve efficiency?