Rather surprisingly, wind power is now the cheapest form of electricity in the US:

Lazard LCOE Dec 2016. Click to enlarge.

This comes from the tree-hugging socialists at  Lazard Asset Management.  Unsubsidized wind comes in at $32 to $62 per MWh, depending on the site.   Natural gas is substantially more at $48 to $76, and coal and nuclear are right out at $60 to $143 and $97 to $136 respectively.  Wind is now generating 5% of all power in the US, and 45% in Denmark.  Its cost has fallen by 1/3 in the last five years.  Solar gets all the publicity, perhaps because those black panels look magical compared to the simple mechanics of a wind turbine, but wind is actually ahead, especially in the Midwest and Northeast.

So how did wind mills change from being a stereotype of communes to major industrial machinery?  It looks like a combination of standard engineering improvements and quite non-standard citizen involvement in Denmark.  The story gets told in the comprehensive new book “Wind Energy For the Rest of Us” by Paul Gipe:

Click for author site

It discusses the historical development of turbines since the 19th century, the reasons for their key features, how to evaluate a site, how to choose a machine, and how to finance it.  It ends by noting that it would take about 750,000 standard 2 MW turbines to power all of the US.  They’re about as hard to build as a heavy truck, and much easier than aircraft, and so could not only save money and cut pollution but revitalize manufacturing.

A key reason why they’re getting cheaper is geometrical scaling – a wind turbine that’s twice as big sweeps out four times the area and so gets four times the power.  If it costs less than four times as much to build, you’re ahead.  Every tweak that makes the blades longer or the towers taller is a win.  They’re now up to 195 m tall, with 80 m blades and a peak power of 8 MW, although those are only used off-shore.

Overall it looks like their development followed the typical pattern for a new machine: try lots of different approaches initially, then narrow down to an optimum one and drive for high volume and low cost.   The standard configuration today is an open rotor with three fiberglass blades on a horizontal axle facing upwind.  They have overspeed protection by twisting the blades and having brakes on the axle.   They feed a variable-speed under-sized generator, and it’s all on a tall cylindrical steel tower.   Every variation on these features has been tried:

• Put a duct around the rotor to increase wind speed (too heavy) or have an open rotor.
• The blade number: one (too heavy), two (too much vibration), four (too heavy), or three.
• The blade material: wood (rots), steel (too heavy), aluminum (too weak) or fiberglass (a hollow shell around a structural spar.  Does have to be made as one piece, making it hard to transport).
• The axle orientation: vertical (can’t furl in high winds) or horizontal (turn sideways to high winds to lessen the force on the rotor).
• The blade direction: facing downwind (makes a whomp noise every time a blade goes through the wind shadow of the tower) or upwind (have to worry about the blades hitting the tower, which is highly bad).
• For overspeed protection: little parachutes on the tips (really?), slats on the blades (too prone to jam), twist the whole blade to make the airfoil stall and put a brake on the axle as a last resort.
• Use a fixed-speed generator to match the 60 Hz of the grid (needs variable gearing or loses energy by turning rotor too slowly) or a variable-speed generator that’s converted to AC (possible with modern high-power switching transistors).
• Size the generator to handle high winds (adds cost and weight) or size for only medium winds (improves the average power output since it runs full out more of the time, thus needing less backup.   The average power is now up to 40-50% of the rated limit versus 30% with the old over-sized generators).
• The tower style: a lattice mast like a radio tower (rusts, gets covered in bird shit, makes wind noise, unsafe to climb), use guy wires (noise, needs land), or have a closed cylinder (which can be climbed in any weather).
• The tower material: wood (can’t get tall enough), concrete (ugly and slow to assemble), steel.

So it took a long time to figure it all out!  But now that the major parameters are set, you can really go to town on optimization.  If you’re building $10B of wind turbines a year (as happened in 2016 in the US), an 0.1% improvement is worth $10M, which sure pays for your salary.  For example, the Betz Limit says the maximum amount of power that can be taken out of an air stream is (1 – (2/3)^3) = ~60% of the kinetic energy in the stream.   Modern turbines get within 75% of that, so a tweak of the airfoil shape (perhaps depending on the wind speed distribution at a site) could get another percent or two.

And now that people have a lot of experience with the designs, they can estimate how long they’ll really last.   Early machines failed after only a couple of years, but modern ones last for twenty.  That makes a big difference in the financing, since you no longer need a risk premium for early failure.

What’s more interesting, though, is that it got started in a quite unconventional way – at a folk craft / technical school in Denmark, the Tvind School.   They built the first modern wind turbine, the Tvindkraft, with secondhand parts and student labor:

900 kW turbine and the Tvind School in Jutland. Click for link

It’s still working after almost 40 years!  It pioneered the use of cantilevered fiberglass blades and a particular kind of flange for the blades that uses glass fibers wrapped around the bolt holes.  It’s big, 900 kW, and generated so much power that the grid couldn’t handle it, and so they dumped it into heating the school.

It was built during the oil crises of the 1970s.  The Danes had to buy oil from the Mideast and coal from Germany, and liked neither option.   Nor did they like buying nuclear-generated electricity from Sweden, who put a reactor immediately across the strait from them.  It had to be wind, but as a small country they didn’t have the resources to do a big research program.

The US had revived wind research at the same time, but gave it to NASA, GE, and Boeing.  They applied aerospace ideas to it, thinking that it was just another airfoil.  But it’s not – weight doesn’t matter much.   What really matters is reliability, and the GE and Boeing machines failed after less than a year.

The Danes came at it from a cost and reliability standpoint instead of performance.  They concentrated on making the blades strong and durable – each of the Tvindkraft blades weighs about five tons.   Even more importantly, they published their designs. A lot of small firms sprang up making similar-looking mills.   Some even specialized in making just pieces like the blades.  Danish windmill owners also joined up in cooperatives and forced the builders to have standard features likes brakes on the generator.  Some people didn’t like the look of the towers, but once they got a share of the revenue they minded much less.

Eventually a farm-machinery company, Vestas, got involved and took it over.   They’re now about the largest wind company in the world, with 2016 sales of about 10 billion euros.  They’re closely followed by another Danish company, Bonus Energy, now a part of Siemens.  GE is still a large player, and is covering the Midwest in towers.  The Chinese bought Danish and German tech and now dominate the field, since they’re desperate to get away from coal.  At the end of 2015 there were more than 300,000 turbines operating around the world, with a capacity of more than 430 GW, and that grew by 63 GW in just that year.

So this huge industry started at an obscure school in Denmark, grew to a cluster of small companies, got taken up by some larger ones, was incentivized by feed-in tariffs in Europe and investment tax credits in the US, and over 40 years worked out all the kinks in the technology.  It benefited from natural geometric scaling, from airfoil simulations, from power electronics, and most of all from operational experience.  Another factor of two in cost reduction is in sight, and off-shore tech is coming on strong.   It’s the newest class of large machinery in the world (rockets and nuclear reactors come from the 50s, and jets and container ships from the 60s), and it’s taking over.