The Department of Energy's Wind Vision report released in March details a scenario where 10% of the United States' end-use demand can be met with wind power by 2020, 20% by 2030, and 35% by 2050. Achieving these milestones would be challenging, says DOE, but doable provided the right policies are maintained (i.e. public subsides), project permitting is streamlined, community acceptance increased, and technology advancements that improve turbine output become mainstream.
DOE's Vision was followed-up last month with Enabling Wind Power Nationwide, a sister document aimed at explaining how we can unlock the nation's vast wind riches by increasing hub heights up to 140 meters (460 feet). That's hub height, which means towers only, absent the blades, standing taller than almost every turbine operating in the U.S. today. If we can get to the prevailing winds 500-feet above the earth's surface, we should achieve marked increases in average capacity factors, or so DOE claims.
But not so fast!
The Germany Experience: Tall Towers, Low Production
In Enabling, DOE continually points to Germany's success at being able to install larger turbines. And it's true, there are bigger turbines, with average hub heights in 2014 of 116 meters. But what DOE omits from its document is that Germany has not experienced any notable increase in capacity factors with the taller turbines. Conservatively, Germany's capacity factors are dismal, wallowing at under 20%.
* Capacity factors are computed using installed MW from the prior year to avoid projects with partial year production.
Lower capacity factors mean more turbines, but apparently Germany's focus is on national performance with little regard for the sheer number of turbines needed. Turbine forests are beginning to unnerve even green-minded Germans who are protesting the towers and related transmission.
As in Germany, taller hub heights in the US have not always translated into higher production numbers. The Hardscrabble wind project (100 m hub height) in New York state has produced at or under 28% capacity factor since it went online in 2011. This is in line with other projects in the state with 80 m hub heights. Iowa's Pocahontas Prairie wind farm, also with 100 m hub heights, produced at 35.4% in 2014. While better than the NY project, it's well below other Iowa projects with 80 m hub heights.
Transporting Bigger Turbines
DOE's Enabling report does a reasonable job explaining the physical constraints of transporting larger turbine parts. Blades longer than 53 m (174 feet) are at the limit for road corners and curves. Overhead obstructions limit tower sections to no more than 4.6 meters in diameter (15 feet) on roads and 4 meters (13 feet) for rail. Alternate road routes may be found but generally have weight limits that restrict access.
Moving to segmented blades could help the transport issues, as would using concrete towers that can be manufactured on site but these carry their own set of issues. DOE suggests that it serve as the research arm -- and funder -- for new wind technology. After all, such projects "are usually low in terms of technology readiness level making them generally beyond the scope of existing industry," writes the DOE. This is another wind power cost that's rarely debated.
Other Barriers Stalling Wind
There are a multitude of barriers to achieving wider geographic development of wind in the US aside from those already cited.
Mandates. When the cash grant program was in effect, we saw a massive build-out of wind resulting in a number of states meeting or coming close to satisfying their renewable energy mandates. Michigan's RPS expires this year as does New York's. Kansas and Texas met their mandates and other states, including Ohio and North Carolina, are reevaluating their laws. West Virginia, meanwhile, has repealed it's RPS altogether.
Economics. Fewer utilities are signing contracts for wind and we are seeing a shift to corporate contracts for wind including IKEA and Microsoft. The economics of wind are worse today with low natural gas prices. Utilities are not as willing to sign above-market contracts which is putting pressure on developers to drop costs and keep profits up. This means building mainly in states where the permit rules are not onerous and where the wind regime assures more production tax credits. Texas, Oklahoma and Iowa are obvious fits. It's no surprise that several approved projects in Minnesota (here and here) had their permits revoked for failing to find a buyer.
The output of DOE's models are easy to promote but reality paints a very different picture. DOE's Vision assumes 7 GW of wind built per year between 2014 and 2020, followed by 12 gigawatts per year between 2020 and 2030, and 17 GW every year after until 2050. The Agency points to the progress since 2009 as proof that a more aggressive wind roll-out is possible. But in many ways, the success of U.S. wind in those years is the very reason wind development will not grow, but continue to slow.
 At the end of 2014, wind represented just 4.4% of US demand (66 gigawatts installed) with 50% sited in five states: Texas (14.2 GW), California (5.9 GW), Iowa (5.7 GW), Oklahoma (3.8 GW), and Illinois (3.5 GW), By 2050, DOE's models suggest that Texas, Iowa and Illinois could EACH have 60 GW or more of wind installed.
 Some concerns with DOE's Vision are addressed here.
 98% of the turbines sited in the U.S. have hub heights at 80 meters or less. To our knowledge, none stand taller than 100 meters.