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According to the American Wind Energy Association (AWEA), the fastest-growing profession in the US today is wind-turbine technician. That becomes a less surprising statement when the state of the US wind power market is examined in detail. Simply put, there is a great deal of work out there.

In broad terms, the country contains nearly 500 wind farms of at least 10MW in capacity that are five or more years old. (Most US wind farms are much bigger and many are a good deal older.) Taken together, these older projects account for over 40GW of operating capacity and more than 30,000 wind turbines, most of which it can be assumed are out of warranty.

Those 30,000-odd turbines are the products of more than 20 manufacturers, several of which are no longer in business or have been swallowed-up by the industry's big players. Capacity, performance and reliability vary hugely, as does their design age and the time they have been generating electricity. They also operate in diverse regions with widely different weather conditions. Repairing or replacing a gearbox on a 500kW turbine in sunny California is hard to compare with doing the same on a 2MW unit in Wyoming in winter.

Market domination

The US wind market took a significant shift in 2001 when the first half-dozen 1.5MW GE turbines were installed at the Somerset wind farm in Pennsylvania. Over the next few years this model — and improved variants with longer rotor diameters — dominated new installations across the country.

By the end of 2010, GE hardware was providing around 40% of America's total installed wind capacity with more than 10,000 turbines operating, a quarter of those in one state alone, Texas.

The ubiquity of GE's 1.5MW turbine has gone a long way to simplifying the work of operations and maintenance (O&M) providers. They have a familiar and well-proven product, with few if any problems in sourcing replacement parts. Independent suppliers have added their expertise to aid performance too.

Last November for example, Moventas launched its XL (extra life) technology package for replacing GE 1.5MW gearboxes.

This features a case-carburised planetary stage with larger integrated planet wheel bearings, upgraded bearings and raw material specifications for the intermediate and high-speed gears and a high-spec vibration and oil-particle monitoring system. The package comes with a five-year warranty that covers turbine downtime and crane costs.

Up-tower repair solutions are also being developed to cut the cost of cranes when changing out large items such as generators. And a recent entrant to the wind industry, Sentient Science, has won contracts with major owners of older turbines to use its digital gearbox simulations to monitor and predict upcoming maintenance issues.

Repowering

Even the oldest of GE's 1.5MW turbines are modern by comparison with the machinery that powered the first wave of utility-scale wind power in the US.

California was the pioneer in this regard, and the state remains dotted with projects that are small in terms of overall capacity, but contain very large numbers of little, old turbines. Repowering projects reveal the rapid progress in wind technology.

The Shiloh IV wind farm in California's Solano County provides a good example. It started life in 1989 with 235 Kenetech 100kW turbines, but these were dismantled after more than 20 years of operation, and replaced by 50 Senvion 2.05MW MM92 machines in 2012.

The repowered project has over four times the capacity of the old one, but from less than a quarter of the number of turbines.

Replacing old turbines looks like the obvious solution to California's ageing turbine fleet, but it is rarely either easier or cheaper than developing a completely new project.

The cost of decommissioning the old machinery can sometimes be partly offset by selling it on the second-hand market - although the demand for 25-year-old 100kW turbines from a manufacturer that went out of business a long time ago is unlikely to be high - but developers still have to go through a complex permitting process for potentially larger and fewer turbines, and the infrastructure around the original project is also likely to need upgrading.

For this reason, operators are usually keen to keep old machinery in operation for as long as possible. The era of the lattice towered-mounted 100kW turbine is clearly coming to a close, but there is plenty of profitable life remaining in units such as the Vestas V47-660, of which there are around 1,500 examples still running in the US.

Arrivals from Europe

The average capacity of wind turbines in the US shifted upwards again from around 2005 as new models, mainly from Europe's leading OEMs, became available.

Siemens' SWT-2.3MW model, first with a 93-metre rotor diameter, then extended to 101 metres, found plenty of customers willing to look beyond GE's shop window. “uåX˜äŠÊ˜·³Ç Intelligence data points to around 1,500 units that have now been operating in the US for at least five years.

The same source indicates around 2,300 Vestas turbines in the 1.5-2MW range that have passed their fifth birthday, plus another 300 or so 3MW units. Gamesa's G87 2MW turbine sold in good numbers in this period, too. More than 1,200 were up and running by the end of 2010.

The only non-European manufacturer, apart from GE of course, to make much of an impact on the US market in this period was Mitsubishi. Close to 400 of the company's 2.4MW turbine went online in 2009-10.

The US has been surpassed by China in terms of total wind capacity, but it remains the biggest producer of wind-generated electricity.

Getting the most from the new generation of multi-megawatt turbines that came online five to ten years ago will be the key to ensuring that it holds on to that crown for a few more years at least.