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Wind speed only tells part of the story for site selection

While modern technology is offering developers capacity factors for low wind sites they could only have dreamed of a few years ago, they need to consider other factors, such as transport and meteorological conditions, when planning projects.

Logistics challenge… Modern low-wind turbines tend to be massive, with very long blades and other components to be transported (pic:Nordex)
Logistics challenge… Modern low-wind turbines tend to be massive, with very long blades and other components to be transported (pic:Nordex)

When the Rockland wind project became operational in the US state of Idaho in December 2011, its developer, Ridgeline Energy, was taking advantage of one of a new generation of wind turbines designed for low and moderate wind resource sites: the Vestas V100-1.8MW. It is partly thanks to this machine, whose rotor span is about the length of a football field, that the project could be financed and built.

As wind energy use grows worldwide, developers are increasingly exploiting sites with a low or moderate wind resource. This trend reflects a multitude of market influences as well as changes in wind-turbine technology. But whatever its causes, developers need to be aware of the challenges they will face.

What can be defined as low wind varies greatly depending on the market context. There are two defining factors that will make a low-wind site viable: the price of competing generation resources and policy support for wind power. Where competing power is cheap, the minimum resource required for viable wind projects is higher. Conversely, policy supports such as feed-in tariffs and renewable portfolio standards can sometimes overcome obstacles to developing even the most resource-challenged sites.

These days, if I had to distinguish between low, moderate and good sites within the low-to-medium-wind category, I would pick the following wind speed ranges at a nominal height of 80 metres above ground: low = 5.5-6.5 m/s; moderate = 6.5-7.5 m/s; good = more than 7.5 m/s.

The "good" category, which corresponds loosely to IEC Class II and I, dominated development through the first decade of the 21st century. Many sites being developed today are "moderate" Class III sites, and some fall in the "low" range. Note that the speed ranges ignore air density, the speed-frequency distribution, and other factors that influence production, but they are a handy guideline for most situations.

The map shows how major areas of these resource categories are distributed around the world. Large stretches of China, south-east Asia, Brazil, and other populous regions with fast growing economies are where much of the low and medium wind-power future lies.

Challenges for developers

Technical and financial hurdles to overcome tend to be more acute for low-wind projects. The main challenge is the higher cost of energy compared with windier sites. All things being equal, the same capital investment must be spread over a smaller amount of energy generated, making it harder for low-wind projects to compete against other generation sources.

The advent of new turbine models designed specifically for low winds has helped to lower this barrier. To improve performance at low-wind sites, manufacturers have raised towers to reach better winds aloft, lengthened blades to capture more energy and kept the generators small to reduce weight and cost. There have also been efficiency gains through improvements in blades and other components, although their impact has been much smaller.

Impressive performance

All of these strategies have raised the production per turbine and per unit of swept rotor area, as well as, in many cases, the average capacity factor (average annual output divided by the maximum possible output).

For example, the Vestas V90 has a 90-metre rotor diameter, hub heights ranging from 80 to 105 metres, and generator ratings of 1.8MW, 2MW, and 3MW. The chart below illustrates how variants of this turbine perform compared with the older, much smaller 660kW V47.

Vestas low wind chart

The most productive new model, the V90-3.0, generates almost five times as much energy as the V47-660, yet its gross capacity factor is not much greater than the V47's. The capacity factor is a flexible quantity that depends on how the manufacturer has chosen to configure the turbine for the market segment it is targeting. At the same hub height, simply reducing the generator size for the V90-1.8 raises the capacity factor to 38%. Increasing the hub height to 105 metres boosts the capacity factor to 42.4%.

Trade-offs

A developer might have dozens of turbine vendors and models to choose from. It is tempting to pick the option with the highest capacity factor, but it is important to recognise that performance gains come at a cost.

Larger blades tend to be heavier and more costly than smaller blades. The added mass increases the weight and cost of components such as the tower, hub, and yaw and pitch systems. The US National Renewable Energy Laboratory (NREL) calculates that extending the rotor diameter from 90 metres to 100 metres increases the overall capital cost of a multi-megawatt turbine by some $300,000. Similarly, taller towers add cost and require stronger foundations, with NREL calculating a hub raised from 80 metres to 100 metres costing about $100,000. Whether the increased cost is justified depends in part on the wind shear: sites with greater shear generally justify taller towers.

The practical hurdles of developing projects at low-wind sites should not be overlooked when gazing starry-eyed at the impressive capacity factors offered by some of the new turbines. Low-wind machines are enormous, as tall and wide as a jumbo jetliner, and require appropriate roads and equipment for installation. The tower sections and blades must be shipped on special trucks, which require a minimum clearance and turning radius. Access roads must often be reinforced, regraded and recut - the larger the turbine, the more such civil engineering is required.

On site, a wide and flat clearing is needed for rotor assembly, usually requiring more cutting, filling and tree-clearing. And tall cranes are needed to mount the tower sections and rotor. In some remote or developing regions, the nearest cranes of sufficient size must be brought from hundreds of kilometres away, and they may encounter road limitations.

Storm watch

The meteorological conditions in which low-wind turbines often operate must also be considered. While the average speed might be low, these turbines may well experience extreme gusts and storms that could damage them. Low-wind site turbines typically have a lower survival gust threshold - such as a three-second maximum gust of 52.5m/s rather than 59.5m/s, which can be a challenge at semi-tropical sites prone to hurricanes.

Light winds tend to be more variable and turbulent in proportion to the average speed, and with the turbines often operating in the lower part of their power curves, this can mean sudden changes in output are larger and more frequent. Yaw systems may have to work overtime to keep the turbine pointing upwind in light, shifting breezes, causing more wear and higher-than-usual losses.

It is more difficult to estimate power production accurately in such circumstances. In the steepest part of the power curve, where low-wind turbines frequently operate, even small errors in expected speed can mean sizable errors in power output. Also, standard power curves are calibrated to a fairly narrow range of conditions. AWS Truepower is involved in studies to extend power curves to unusual or extreme conditions of both turbulence and shear.

None of these concerns should stop development in low-wind sites. But the savvy developer will be aware of them in advance, and will take appropriate measures to mitigate risk and ensure maximum rewards.

Michael C Brower is chief technical officer at renewable energy consultancy AWS Truepower, which carried out consultancy work on the Rockland wind project in Idaho, US

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