Europe replaces old wind farms; More power from fewer, bigger turbines

The construction crane hauling wind turbines at the 20‑year-old Nørrekær Enge wind farm in northern Jutland, Denmark, is performing a highly energetic sleight of hand. By next summer it will have transformed a 77-turbine facility into one with just 13. Amazingly, this reduction will help double the farm's energy production. The trick is in the scale of the replacement turbines. At 2.3 megawatts, they should each generate as much peak power from Nørrekær Enge's winds as 8 to 15 of the vintage turbines installed on the site in 1988 and 1990.

The wind farm, owned by Stockholm-based utility giant Vattenfall, is the largest project to date under a Danish incentive program to promote the "repowering" of wind sites. It's an early example of what will soon become one of the largest sources of additional wind energy in Denmark, Germany, and California, whose governments pioneered wind energy in the 1970s and 1980s. Bundesverband WindEnergie (BWE), Germany's wind-energy industry association, set a goal of adding at least 15 000 MW of new wind-power capacity through repowering by 2020. That's 50 percent more than BWE expects to be added over the same period at new wind sites on land or at offshore wind farms.

The big challenge, wind developers acknowledge, is modifying site permits that restrict the spacing and height of turbines. Neighbors are often put off by the visual impact of the turbines themselves, which have morphed from blade spans of just a dozen meters or so in the early 1980s to as large as 126 meters today. And the new rotors ride higher than ever, perched on towers exceeding 100 meters, whereas hub heights generally maxed out at 60 to 70 meters in the 1990s.

Higher hubs are key to repowering's profitability, because they place the turbines in stronger, more consistent winds, thereby increasing the number of hours per year that a given turbine will run. As a result, a repowered wind farm with double the power capacity of the original wind farm can deliver as much as four times as much energy. Held to the height restrictions commonly included in the wind-farm permits of previous decades, however, a repowered site is likely to generate just 50 percent more energy, according to Claudia Grotz, a BWE senior policy advisor. Grotz says that's a nonstarter for wind developers. "If people can't harvest the full potential on a site, then they won't repower it," says Grotz. "They'll let their old turbines run and make more money."

Grotz looks forward to a conference of regional planning authorities that the German government is organizing for early 2009, which she hopes will accelerate the process of approving wind-farm repowering.

Wind developers face an additional hurdle to repower in Denmark, where legislation going into effect next year will require wind-park operators to compensate residents if wind turbines reduce their property values. That's not an issue at Nørrekær Enge, according to Vattenfall spokesperson Arne Rahbek, because the site's repowering plan predates the new requirement.

So what happens to the old turbines littering the ground at Nørrekær Enge? Rahbek says they will be dismantled for shipping and sold, to resume making electricity in emerging markets like Eastern Europe and Cuba.