Production of wind turbines and wind farms continues to increase as this energy source is considered to be a viable option now and in the future. One of the problems in generating energy in this manner is determining the best approach to maximize the wind present. As it turns out, geographical location plays a major role in where to best position wind farms. 

This issue also is related to the stresses placed on the gearboxes that perform the critical function in converting low-speed wind energy to high-speed energy. Reducing the stress placed on a lubricant may help to extend the operating life of a turbine. This becomes important due to the difficulty in doing maintenance on wind turbines in remote locations. 

In a previous TLT article, a computer program was used to simulate the performance of an off-shore wind farm near the coast of Sweden (1). Researchers found that the positions of each wind turbine in the wind farm significantly impacted the power generated. By positioning wind farms in a staggered fashion, the number of turbines was reduced by 75% while the power generated exceeded the predicted rate.

Dr. Ken Caldeira in the department of global ecology at the Carnegie Institution of Science in Stanford, Calif., indicates that wind farms generate electricity by deriving kinetic energy from the wind. He says, “Wind energy is produced in the atmosphere through contrasts in temperature. At the north and south poles, air is cooler and denser while at the equator, air is warmer and less dense. When the two air masses interact a temperature gradient exists as the dense cooler air moves under the less dense warm air. This action converts potential energy into the kinetic energy of the wind.”

Wind farms have been built on land, but there are concerns with maximizing the power output from them. Caldeira says, “Wind can easily be disrupted on land due to the presence of trees and buildings. Even with building wind farms in open spaces, the wind moves horizontally and can be disrupted due to the size of the wind turbines, the separation distance between turbines and how each wind turbine is spaced with respect to the others. Each turbine gives off a wake that can impact the wind and as a consequence, electricity generation.”

The question then needs to be asked: Where are the most consistent winds in the world? The focus is determining which region will enable wind kinetic energy to flow downward from the troposphere to the boundary layer in the atmosphere where wind turbines are situated. This kinetic energy is very important in sustaining wind turbines and enabling high levels of power to be generated. 

Open oceans appear to be an option that have not been examined. Caldeira and his associate, Anna Possner, have now used sophisticated modeling tools to assess the power output from a wind farm situated in the middle of the North Atlantic Ocean.

Kinetic energy extraction
Caldeira and Possner determined that positioning a wind farm in the North Atlantic Ocean will lead to a higher electricity output than wind farms of comparable size situated in the U.S. state of Kansas. He says, “We decided to evaluate open ocean wind farms in the North Atlantic Ocean because of the heat that pours out of the Atlantic Ocean due to the presence of the Gulf Stream that flows from off the coast of the U.S. state of Florida northeast across the Atlantic Ocean past the U.K. and off the coast of Norway. Kansas wind farms were used as reference points because a number of past studies were done in this region.”

The heat interacts with the cold present in the atmosphere to create the atmospheric conditions that lead to the generation of wind energy. An important parameter to evaluate is the kinetic energy extraction (KEE) rate, which measures the amount of wind energy available for use by the wind turbines. Caldeira says, “Past studies report that electricity generated by land-based wind farms may be limited to 1.5 watts per square meter.”

The researchers found through their modeling that wind farms in the North Atlantic Ocean could realize a KEE value of 6 watts per square meter. In contrast, KEE values that are much lower were seen with land-based wind farms. Caldeira says, “We found that the atmosphere over wind farms in the North Atlantic can sustain KEE values three times as high as those on land. The key was that open ocean wind farms can extract more wind energy coming down from the troposphere.”

The researchers also studied the impact of climate changes on KEE. Caldeira says, “Our studies indicate that climate changes have only a minor effect on wind power generation.” 

Caldeira also noted that wind power generation will be particularly strong due to seasonal variability in the North Atlantic Ocean between September and April. 

This study shows the potential for open ocean wind farms, but commercialization is still not viable at this point. Caldeira points out that the world’s first ocean-floating wind farm just started generating electricity off the coast of Scotland in October 2017 (see Figure 3). 


Figure 3. Two floating wind turbines were being prepared to be used in the world’s first ocean-floating wind farm, which started generating electricity off the coast of Scotland in October 2017. A recent study showed that open ocean wind farms can generate more electricity than those on land. (Figure courtesy of the Carnegie Institution of Science.)

Caldeira indicates that the researchers will be doing further work on two issues related to open ocean wind farms. He says, “We will be evaluating the spatial orientation of open ocean wind farms to answer such questions as how far apart should the wind farms be spaced and how they should vary in size and positioning. We also will be doing theoretical work to determine the mechanism for how heat flow out of the ocean impacts the kinetic energy available in the upper atmosphere that can be used by the wind farms.”

Additional information can be found in a recent article (2) or by contacting Caldeira at kcaldeira@carnegiescience.edu. 

REFERENCES
1. Canter, N. (2014), “Staggered wind turbines,” TLT, 70 (2), pp. 12-13.
2. Possner, A. and Caldeira, K. (2017), “Geophysical potential for wind energy over the open oceans,” Proceedings of the National Academy of Sciences, 114 (43), pp. 11338-11443.