Learning about electricity from wind

Lately I am reading reading some reports, finding them mostly quite informative. This was also the case with "Wind Vision: A new era for wind power in the United States", so I decided to summarize some key elements here in case you don't have the time to read the full report. Please, note that I am not an expert on wind electricity, so proceed reading with caution.

Previously, I saw data about installed capacities in Europe, and it struck me that some countries were having comparatively large capacities and still managed to produce less electricity from wind than countries with much smaller capacities. So I wondered why this is the case and secretly hoped that the report could shed more light on it.

Wind is only one of the available resources that can be used to generate power. It is more ecological than burning fossil fuels, for instance. It can greatly reduce greenhouse gas emissions, but not completely eliminate them (as it causes some air pollution with SO2, NO, PM). But generating electricity from wind, turns out, has its own challenges. Wind, similar to sunlight, isn't a resource whose availability can be predicted. Therefore, operating a wind turbine has a lot to do with knowing where and when this resource becomes available. Understanding wind flow is important, where Doppler and scanning LIDAR are used to measure wind speeds by scattering beams of laser light at a distance. For instance, tests have shown that the highest wind speeds in USA are in the middle of the country. The amount of power that can be extracted from the wind by the turbine increases with the cube of the wind speed. This means that even small changes in wind speeds can greatly affect the electricity produced. Electricity generation is said to be possible with wind speeds starting from 3-5 m/s, where the turbine can reach its peak mostly at 11-14 m/s. At 25 m/s, the control system stops the turbine to protect it from damage, where this can happen gradually or abruptly. Interestingly, wind direction is said to be correlated with wind speed, turbulence and vertical shear. Sometimes it is possible to optimize even for weather "windows".

Wind turbines come in various sizes where the average rotor diameter can be 50-100 m or more. The advantage of higher installations with larger motors is that they capture energy in a bigger area, enabling electricity production at lower wind speeds, so that more sites can use them. In 2013 around 75% of the newly installed turbines in USA had rotor diameters above 100 m, where the hub heights were 80-90m on average. But the larger the turbines, the more costly and complex their transportation becomes, which is why they are often manufactured domestically. The report had a striking image of a very long blade that was blocking two main streets at a turning point. Larger rotor diameters mean that larger height hubs need to be installed too. A 3MW nacelle assembly is said to weight 78 tonnes, while a 5MW one comes at — 130 tonnes. To this weight comes a gearbox of 104 tonnes and a generator of 173 tonnes. Since the trend is towards bigger and bigger weights, you can imagine that this requires specialized and very costly cranes that become progressively unavailable. For instance, back then (2013), USA had 85 cranes for 600 tonnes, allowing the installation of 3MW/140m motor hub height, but only 10 cranes capable of moving 1250-1600 tonnes for 5MW/150m or 3MW/160m installations. Due to the challenges in the logistics, installing a 140m high tower can become 2.5x times more expensive than installing a 120m one.

Organizations like ERCOT and MISO show that large amounts of wind energy can be reliably integrated into the power system. But strong fluctuations in the energy prices at which electricity is bought from the power plants can make their maintenance less economical and they may be forced to close. For a period of only 2-3 years in USA, 14 wind plants stopped operation for various reasons. Policies, availability of qualified workers in less densely populated areas can also influence new installations and the willingness to continue operations. The Great plains region, for instance, has high average wind speeds and vast areas of open land, but lack of transmission is inhibiting operations. Densely populated areas are said to be unsuitable for wind power plants.

Companies which already purchase electricity from wind or have invested in their own installations include Google, Microsoft, IKEA, Intel, Staples, Unilever, Wal-Mart and others. This shows that the trend towards sustainable energy production and usage will continue.

The capital cost for an older wind turbine in 2013 was around 1000$/kW, while for newer ones it was 1300$/kW. Replacements of faulty components can cost a lot. A generator replacement in 2013 was needed by 3.5% of the available turbines at an estimated cost of 310000$, while gearbox replacement was needed by 5% at an estimated cost of 380000$. Replacement of a blade costed 240000$ on average. Operational damage due to machine misalignment, lightning strikes, blades susceptible to corrosion and others is also possible. In 1980 the price of the electricity generated from wind in USA was 0.50$/kWh, while in 2013 it was 0.045$/kWh.

One measure of wind power plant productivity is the capacity factor:

capacity factor = amount of energy the wind plant produces over time /
amount of energy that would have been produced at full capacity

Between 2006 and 2013 the capacity factor of wind power plants in USA was ≈32.1%; between 2000 and 2005 it was 30.3%. Interestingly, operating a power plant has an electricity cost, irrespective of whether electricity is generated or not. This is one reason why a lower specific power rating for a turbine yields a higher capacity factor. Common values are less than 220W/m2.

In USA, the states Iowa and South Dakota generated more than 25% of their electricity from wind. But in absolute numbers (2013), Texas produced 12.3% of the total electricity from wind in the country, California 5.8% and Iowa 5.2%. The country has planned to increase the cumulative wind capacity from 61GW in 2013 to 113GW by 2020, 224GW by 2030 and 404GW by 2050. With this data, we can calculate that the highest slope is between 2020 and 2030: (224-113) / (2030-2020) = 11.0. European countries that already cover a large percentage of their needs from wind energy include Denmark (32.7%), Portugal (23.5%), Spain (20.9%), Ireland (16.3%) and Germany (8.9%).

Floating wind turbine installations, positioned slightly over water level are also possible and already used. Unfortunately, having such installations reach deeper and deeper into water creates some aesthetic problems and also exposes the turbines to extreme weather (hurricanes, ice etc.). Additionally, turbines can produce noise (which is why airfoils or other aerodynamic solutions exist) and vibrations. Wind power plants can also contribute to increasing the bird mortality rate, but the report claims that this effect is negligible compared to the number of birds killed by other towers, power lines and buildings.

At the end the project economics and the approval from all key stakeholders will determine whether a plant will be built or not. Every decision will be subjected to some kind of trade-off.

If you want more information, you can still read the report.