HIGHLIGHTS
• A mesoscale modeling study was conducted to evaluate the influence of intra-farm and inter-farm wakes on offshore wind power generation.
• Data on wind speed, temperature and atmospheric pressure was obtained in evaluating five types of wake scenarios.
• Wake speeds led to a total power reduction between 34% and 38% over the one-year analysis period. Intra-wakes lowered power output at twice the degree than inter-wakes. 

The growth of wind energy is leading to the expansion of wind farms offshore. This trend is placing more stress on the lubricants used to lubricate wind turbines as they are not only operating in very challenging operating conditions, but providing maintenance is more difficult in most cases compared to land-based counterparts. 

In a previous TLT article,¹ a study describing how offshore wind energy can affect the power grid servicing the New England region of the U.S. during an extreme cold snap and from the years 1949 to 2018 was discussed. The modeling study assessed changes in wholesale electricity price changes for the extreme cold snap based on zero contribution from offshore wind energy and hypothetical contributions of 800 and 4,000 megawatts. For the long-term study, the researchers determined that wholesale electricity pricing would have been reduced if 4,000 megawatts of offshore wind power was available. The authors found that the benefit would be lower during the summer than the winter due to lower wind speeds and greater electricity demand.

The geographical location offshore from the Northeast U.S. is well-suited to wind power development, according to Dr. Julie Lundquist, professor in the department of atmospheric and ocean sciences at the University of Colorado in Boulder, Colo. She says, “The geographical area from the coast of the state of New Jersey northeast to the state of Rhode Island is very amenable to wind power because of fast wind speeds and relatively low turbulence.” This region is known as the Mid-Atlantic Outer Continental Shelf (OCS). 

One factor that needs to be carefully evaluated to better understand the potential efficiency of an individual wind turbine in generating electricity is wakes. Lundquist explains the concept of a wake by using analogies involving water. She says, “As water in a stream moves over rocks, the flow becomes more turbulent as it moves downstream. A second way to picture a wake is to consider what happens as a boat moves through water and disrupts the water flow behind the boat.”

Atmospheric wakes generated by individual wind turbines can reduce wind speeds and power generation for those turbines downwind. In the OCS, this issue becomes magnified because of the number of wind farms clustered together, which brings up the concern about intra-farm (within one farm) and inter-farm (between wind farms) wakes. Lundquist says, “The wind direction in the OCS is on average southwesterly, which means that designing a southwest-to-northeast wind farm orientation will reduce the potential influence of inter-farm wakes on individual turbines.”

Lundquist and her colleagues conducted a mesoscale modeling study to evaluate the influence of intra-farm and inter-farm wakes on offshore wind power production over an annual basis.



Weather Research and Forecasting Model
The researchers used the Weather Research and Forecasting Model (WRF) in calculating the impact of wakes on power generation. Weather data between September 2019 and September 2020 was utilized in the study giving the researchers a full picture of how power generation will change over a yearly period. 

Simulations were carried out for five types of wake scenarios. The first is a control with no wind farms. A second analysis was based on one wind farm situated in the OCS that contains 12 megawatt turbines with a 138-meter hub height and a 215-meter rotor diameter. The objective was to have the wind farm generate a power density of 3.14 megawatts per kilometer. The third scenario involved wind farms situated in one lease area within the block off the coast of the states of Rhode Island and Massachusetts with the same power density as the one wind farm. To accomplish this goal, wind turbines were situated in the model one nautical mile or 8.6 rotor diameters apart, and an additional 0.5 nautical mile from the lease area boundaries.

A fourth scenario expanded the number of wind farms to all lease areas in the OCS and the final scenario expanded the number of wind farms to all cover those in the call areas, which have been identified as potential locations for wind farms within the OCS. 

Lundquist says, “In doing our analysis, we calculated data on wind speed, temperature and atmospheric pressure among other variables. To quantify the effect of wakes on power output, we subtracted the wind speed found in the no wind farm scenario from the wind speeds calculated in the various wind farm scenarios where wakes are included using the Fitch wind farm parmeterization approach.”

Atmospheric stratification is a factor that changes on a seasonal basis. Differences in the air temperature over land near the coast and the water temperature can impact instability in the atmosphere, which will affect wind speed. Lundquist says, “During the winter, stratification is more unstable due to the land air temperature being colder than the ocean water temperature. Stratification is more stable because warm air from the land blows over colder water. The result is that wakes are stronger and propagate further downwind during the summer.”

In calculating the impact of wake speeds, the researchers found a total power reduction between 34% and 38% was found over the one-year analysis period. Intra-wakes lowered power output at more than twice the degree than inter-wakes. 

The significance of this work is to give developers the opportunity to establish wind farms that minimize the negative influence of wakes, and to give grid operators knowledge to predict and therefore manage wakes. This should result in reducing the stress on individual wind turbines, helping to improve the operating conditions for the lubricants and also reducing the need for maintenance. 

Future work will focus on attaching instrumentation to offshore wind turbines to better understand how they react to wakes. Additional information can be found in a recent article² or by contacting Lundquist at Julie.Lundquist@colorado.edu. 

REFERENCES
1. Canter, N. (2022), “Offshore wind energy’s impact on energy cost,” TLT, 78 (8), pp. 12-13. Available here.
2. Rosencrans, D., Lundquist, J., Optis, M., Rybchuk, A., Bodini, N. and Rossol, M. (2024), “Seasonal variability of wake impacts on US mid-Atlantic offshore wind plant power production,” Wind Energy Science, 9 (3), pp. 555-583.