Wind Power and Wind Variability Offshore UK: Is Wind Dependable Enough?

 

     The waters offshore of the UK are reputed to have the best wind power resources in Europe and some of the best in the world. UK wind farms produced a record amount of power in 2022. Most of the deployed offshore wind power is in the Irish Sea west of England, the North Sea West of Scotland, and on down to the English Channel west of England. However, most wind under construction and proposed for the future is offshore the east coast from the North Sea to the English Channel. UK wind speeds are typically higher in the winter months and lower in the summer months. Variations of 10-15% are typical for Scottish wind power. In the past the UK’s power network has been most sensitive to temperature. Now it is transitioning to becoming most sensitive to wind speeds. Some studies have shown that wind variability can be mitigated somewhat by geographic dispersal of wind farm locations. That can be considered a kind of overbuilding to account for variability. The UK also has significant onshore wind, but offshore wind capacity is overtaking onshore capacity due to higher wind speeds offshore and resultant higher capacity factors (rates of utilization). In 2019 UK offshore wind achieved an average capacity factor of 39.6% compared to 26.2% onshore. New data from 2017-2021 by the UK Dept. of Business, Energy, and Industrial Strategy (BEIS) has offshore capacity factor at 40.22% and onshore capacity factor at 26.3%. Overall avg. CF is at 31.84%. BEIS also notes that the load factor (similar to capacity factor) for new offshore wind builds from 2023-2025 is expected to be 58.4%. That is a large increase above the average. It is offshore wind that will dominate future deployments in the UK. It is variability of weather systems that leads to variability of wind speeds. Near-shore wind farms are more vulnerable to multi-day periods of low wind speeds. Thus, it is likely that future wind deployments will be further offshore.

     The current total operational capacity of UK wind power is 14.326GW of onshore capacity and 13.660GW of offshore capacity. The UK has a goal of deploying 50GW of total offshore wind capacity by 2030 which means it is expected to triple in less than 7 years. Somewhere between 1 and 2 GW is expected in 2023 so that deployment goal is expected to be daunting and not easy to meet. Total power produced currently is 78,059,303 MWh per year, enough to provide electric power to nearly 21 million homes. In 2022 UK wind power accounted for 26.8% of total electricity generation, the highest ever. Natural gas accounted for the highest percentage of electricity at 38.5%. Solar accounted for 4.4%. The UK goal for 2050 is to have 70% of power needs met by wind and solar from the current 31.2%. Going forward grid integration issues are expected to increase significantly. That will require both an overbuild of offshore wind and a large buildout of energy storage capacity. As of May 2021, the UK had 1.3GW of battery storage capacity and 2.5GW of pumped storage capacity. Low wind events in 2021 required an increase of natural gas use at a time of high natural gas prices and constrained supply. In terms of primary energy consumption in the UK which includes residential and commercial heating and industrial use, natural gas represents 43% and renewables a mere 4%. Winter residential heating in the UK uses the most natural gas. During the mid-December 2022 cold snap gas use skyrocketed to heat homes and on at least one day over 60% of grid power was provided by natural gas. An unexpected low wind event contributed. Electricity and natural gas prices skyrocketed as well. Since then, the weather has been milder, gas and electricity prices have dropped, the UK says their gas storage levels should end the year in good shape, and they should be able to refill storage to 90% of full by November. Even though lots of anti-wind pundits cite such events as a reason to disparage wind energy, those events are not common.  

     One issue with both wind and solar that certainly increases as their presence on a power grid increases, is system inertia. NREL explains system inertia as follows:

 

“Inertia in power systems refers to the energy stored in large rotating generators and some industrial motors, which gives them the tendency to remain rotating. This stored energy can be particularly valuable when a large power plant fails, as it can temporarily make up for the power lost from the failed generator. This temporary response—which is typically available for a few seconds—allows the mechanical systems that control most power plants time to detect and respond to the failure.”

 

     Both wind and solar are inverter-based resources that do not provide what is known as “spinning reserve.” According to Science Direct:

 

“Spinning reserve is defined as unloaded generation that is rotating in synchronism with a utility-grid, i.e., the spinning generator is rotating at a speed that will produce power at precisely the same frequency as the frequency of the grid power. Thus, spinning reserve can be brought online within minutes to serve additional load demand or to compensate for the unanticipated loss of an operating generator.”

 

     It is spinning reserve that best provides short-term frequency balancing. Since system inertia must be maintained to provide frequency balancing over sub hourly to hourly time intervals, it often leads to curtailment of wind and solar resources when they are making too much of the power on a grid. Again, geographical dispersion of wind resources can offer some help. The UK is exploring other frequency response measures, but it is still likely that natural gas will provide the bulk of frequency response.

     A big concern for the UK and other North Sea countries are high pressure systems and easterly winds that can result in persistent calm conditions with low wind speeds for multiple days. These most often happen in the summer and fall but can also occur in winter when power demand is high. When this happens there is a need for alternate energy resources like natural gas that need to be ramped up and ready to go. Wind speeds can sometimes be greater during these times in nearby regions not under the influence of the calming weather system so interconnectivity of resources and importing of power could provide some relief.

     Another issue with wind power is turbine degradation which is significant. Basic wear and tear and the erosion of the aerodynamic surfaces of the turbine blades decrease reliability. Studies have found that turbine power output degrades at a rate of 1.57% per year even accounting for technology improvements and changes in wind conditions. In addition to wind speed variation other factors like low electricity demand and downtime for maintenance affect wind capacity factors. During periods of high winds and low electricity demand curtailment of wind is prevalent.

     Seasonally in the UK the highest wind speeds occur during the highest electricity demand periods in winter so that is a positive correlation that makes UK wind more valuable as a grid resource.

Fig. 1 Seasonal variability of UK offshore wind vs. Seasonal electricity demand. Source: Spatial and temporal variability characteristics of offshore wind energy in the United Kingdom. Panit Potisomporn, Christopher R. Vogel. First published October 4, 2021. Wind Energy, Vol 25. Issue 3, March 2022. https://doi.org/10.1002/we.2685

 

     UK offshore wind daily variability is less pronounced than seasonal variability. Wind speeds are highest at night, peaking at 9PM to 10PM and lowest in the morning around 7AM to 8AM. Daily wind variability is highest in the spring and summer months and more consistent in the autumn and winter months. This greater consistency also correlates to higher power demand times. In the autumn and winter peak wind output is from 8PM to 11PM and minimum output is at midday.

Fig. 2 Seasonal variability of UK offshore wind vs. daily electricity demand. Source: Spatial and temporal variability characteristics of offshore wind energy in the United Kingdom. Panit Potisomporn, Christopher R. Vogel. First published October 4, 2021. Wind Energy, Vol.25, Issue 3, March 2022. https://doi.org/10.1002/we.2685

 

     Onshore UK wind speeds are a reversal of offshore wind speeds due to the different factors such as topography influencing onshore winds. Onshore wind does not match electricity demand as offshore wind does. However, onshore wind resources can help to complement offshore winds when they are low by providing wind onshore when it is not available offshore.

     Low wind generation events include both prolonged low wind speeds and prolonged high wind speeds. Wind speeds below a certain threshold are known as the cut-in speed of 4ms^-1 and wind speeds above a certain threshold are known as the cut-out limit of 24ms^-1. Beyond these limits turbines are shut down. In the UK low wind speed events account for 96% of such shutoffs and this amounted to about 7% of the time in an 18-year period between 2000 and 2017. These low wind speed shutdowns occur more in the spring and summer and typically last up to 72 hours with the bulk of them lasting less than 24 hours. The low wind speed threshold below the cut-in speed corresponds to a capacity factor of 5.67%. If low wind events are defined by a capacity factor of below 20%, which is suggested as a cutoff for when alternate resources are required for back-up, then the data for 2000-2017 show that such events occurred 16.57% of the time. Events below the 20% capacity factor cutoff may last for much longer periods and it has been noted that some of these events in the 2000-2017 data set lasted for more than a week. The authors of the main paper cited here note that “the challenge of persistently low, but nonzero, offshore wind energy production may therefore be a significant factor when considering requirement for alternative generation and energy storage systems.” Thus, when we say that low wind output leads to increased need for backup resources such as natural gas, this is implied. This is more likely to happen in spring and summer, including late summer. However, it can occasionally happen in winter as it did in December 2022 during a cold snap. If it happens in late summer during a heat wave where electricity demand is high, then wind resources will need to be backed up. Energy storage may provide relief for short-term low wind events below 20% capacity factor but there are not nearly enough storage resources to account for longer events of this magnitude.

     Even though there was a slump in wind output during the cold snap in December 2022, wind output on December 30 hit record levels and on January 10 total wind output broke that record. Between 6pm and 6:30pm on that day wind generated 21.6GW of power, providing 50.4% of UK power output. However, at times wind output was too high for grid demand and had to be curtailed. In these situations, generators receive constraint payments to curtail generation. The UK's National Grid payed out 82M pounds in constraint payments to wind generators in December. Even so, from January through November constraint payments to wind generators amounted to just 9% of total constraint payments for that period, compared to 36% to natural gas, and 47% to interconnectors. Thus, wind curtailment is less of an issue than may be perceived. I suspect a percentage of natural gas constraint payments were due to forcing natural gas generators to provide backup power and peak demand power only so that preferred lower emissions power can be prioritized. If that is the case, then much of the natural gas constraint payments are due to their necessity of use to back renewables and as I have long argued this use should be accounted as part of the grid integration costs of variable intermittent generation, or solar and wind.

     Recommendations for wind fleet optimization include geographic diversification as mentioned previously, maximization of deployment in regions with the lowest wind variability, and maximization of deployment in regions with the best match to seasonal and daily electricity demand. If wind speed variability is similar in nearby regions, then geographically diversifying resources will be less effective.

     So back to the question – Is wind dependable enough in the UK? I think it is but there will still be a need for more energy storage, natural gas power plants to be ramped up and ready for anticipated weather events, redundancy of generation, and overbuild of renewables. All the costs associated with these needs should be accounted into future projections as well.

 

References:

 

Seasonal variability of UK offshore wind vs. Seasonal electricity demand. Source: Spatial and temporal variability characteristics of offshore wind energy in the United Kingdom. Panit Potisomporn, Christopher R. Vogel. First published October 4, 2021. Wind Energy, Vol25, Issue 3, March 2022. https://doi.org/10.1002/we.2685

 

Spinning Reserve. Science Direct. Accessed 2023. Spinning Reserve - an overview | ScienceDirect Topics

 

Inertia and the Power Grid: A Guide Without the Spin. National Renewable Energy Laboratory. May 28, 2020. Inertia and the Power Grid: A Guide Without the Spin | News | NREL

 

Wind Energy Statistics. Renewable UK. Wind Energy Statistics - RenewableUK

 

Wind energy in the UK: June 2021. UK Office for National Statistics. Accessed 2023. Wind energy in the UK - Office for National Statistics (ons.gov.uk)

 

European gas storage plans for 2023 still stand despite cold snap. Wood MacKenzie. December 14, 2022. European gas storage plans for 2023 still stand | | Wood Mackenzie

 

UK Peak Demands for Natural Gas. Paul Homewood, December 12, 2023. Watts Up with That. UK Peak Demands for Natural Gas - Watts Up With That?

 

The UK Produced a Record mount of Wind Power in 2022, Easing Gas Crisis. William Mathis. Bloomberg. December 22, 2022. The UK Produced a Record Amount of Wind Power in 2022, Easing Gas Crisis - BNN Bloomberg

 

Wind Power in the United Kingdom. Wikipedia. Accessed 2023. Wind power in the United Kingdom - Wikipedia

National Grid pays wind farm operators £82m to turn off fans amid calls to ramp up battery plans. Nicholas Earl, January 17, 2023. City AM. National Grid pays wind farm operators £82m to turn off fans amid calls to ramp up battery plans (msn.com)