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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'How does the wind blow behind wind turbines and in wind farms?' taking place on Tuesday, 11 March 2014 at 16:30-18:00. The meet-the-authors will take place in the poster area.

Christopher Steele University of East Anglia, United Kingdom
Co-authors:
Christopher Steele (1) F P Steve Dorling (1)
(1) University of East Anglia, Norwich, United Kingdom

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Abstract

SEA BREEZE IMPACTS ON WIND RESOURCE AND TURBINE PERFORMANCE IN COMPLEX OFFSHORE ENVIRONMENTS

Introduction

We have previously developed model-based sea breeze climatologies for multiple coastlines in the Southern North Sea. Verification of the simulations has been undertaken using mast observational data, both onshore and offshore, and we have also begun quantifying the impact of these circulations on wind energy output from both resource assessment and site-specific forecasting perspectives (Steele et al., In Press). This work has highlighted the important spatial and temporal scales of sea breezes (Steele et al., 2013) and demonstrated the impact which adjacent and opposite coastlines have on each other in terms of the potential establishment of sea breeze circulations.



Approach

The sea breeze is a circulation which forms as a result of the land surface becoming warmer than the sea, producing a local pressure gradient which overcomes the initial large scale background flow. It has the potential to either contribute to wind energy output on a day with low wind speeds, or reduce the potential output in an otherwise moderate large scale flow. Consequently it is important to assess these circulations in the context of wind energy.

The sea breeze identification method of Steele et al. (In Press) is based on both the magnitude of the land-sea temperature contrast and the strength of the large scale wind. Using a series of filters on different spatial scales, the conditions that are favourable for the development of sea breezes are determined. Furthermore, days which are cyclonic or which involve rapidly changing large scale flow patterns are also excluded from selection. Consequently, an archive of sea breeze days is created along with an additional archive of those days which do not meet the criteria. The method is also unique as it also classifies sea breeze type, in which important differences in the offshore wind fields are differentiated.

In this paper, we extend the 2002-2012 sea breeze identification method of Steele et al. (2013) to:

(a) construct an additional sea breeze climatology for the coast of Lincolnshire; a region of multiple offshore wind farm developments (for example, Lynn Dowsing, Links and Hornsea).

(b) quantify the spatial impacts of different sea breeze types on offshore wind energy through comparison of power output between sea breeze and non-sea breeze days. An additional comparison is made with observations from offshore wind turbines and masts, including the Egmond aan Zee tower off the Dutch coast. Since interactions between sea breezes on opposing coastlines, such as the Lincolnshire and Dutch coasts, are important, we also construct vertical cross-sections of horizontal wind speed spanning the width of the southern North Sea. Finally, we compare composite diurnal cycles of each sea breeze type with those of non-sea breeze days.

Main body of abstract

From the perspective of considering the potential establishment of sea breeze circulations, the Lincolnshire coast in eastern England is particularly complex, due in part to the presence to the south and south-east of multiple coastlines of different orientation in The Wash and the North Norfolk adjacent regions . The large estuary of the River Humber, at the northward limit of the county of Lincolnshire, adds further to this complexity since it is an area which is vulnerable to the development and impact of coastal wind speed maxima known as jets, associated with the sudden change of coastal orientation (Orr et al., 2005; Capon, 2003). Coastal jets can be significant with respect to their predictability, their impact upon wind energy resource and their generation of associated localised horizontal and vertical wind shear, with obvious related challenges for the wind energy industry.

Figure presents the impact potential of the nearby previously mentioned coasts. The figure, in this case, portrays the averaged 10m wind field for the 117 identified “pure” sea breeze events (2002-2012) for the adjacent coast of North Norfolk at 10:00, 12:00 and 15:00 UTC. Clearly evident in the figure is the westward movement through the day, close to the coast, of the 10m wind speed maximum towards the coast of Lincolnshire.

We present our latest findings in which we establish the climatology of sea breeze circulations, and associated jet features, offshore from the Lincolnshire coast. Furthermore, we relate our sea breeze event record in terms of its specific impact, between 2002-2012, to the performance of an operational offshore wind farm, both in terms of wind climate and of wind farm efficiency through associated wind shear and turbulence. The formation of sea breezes causes the wind direction to vary substantially during a twenty-four hour period, with resulting potential impacts on the pattern of any wake effects within and around a wind farm.

Impacts on predicted wind power output are shown to be largest for “corkscrew” type sea breezes, relative to the other “pure” and “backdoor” sea breeze types, due to the formation of coastal jets and the relatively large spatial footprint of the “corkscrew” sea breeze on the atmospheric wind field. For the “corkscrew” type sea breeze, events are not solely confined to low wind speed periods and so it is demonstrated that the impact of sea breezes, in terms of total power production during the Spring and Summer months, is not insignificant and needs to be quantified.

The average diurnal cycle associated with each sea breeze type also differs from that observed on non-sea breeze days, both in terms of the timing and magnitude of the peaks in power output. The frequency of each sea breeze type is shown to vary substantially between nearby coasts, for example the coast of North Norfolk experienced 264 sea breeze events in the 2002-2012 eleven year period, compared with 335 on the adjacent East Norfolk coast. These frequencies also show large inter-annual and month-to-month variability, therefore implying that the sea breeze impact on wind power output also varies strongly with time and needs to be accounted for.


Conclusion

Sea breezes can either contribute to, through the generation of coastal jets, or hinder because of the formation of offshore calm zones, the generation of offshore wind power, depending on the orientation of the large scale wind flow. Those forming on both the Continental coasts and on the coasts of the UK impact one another and so sea breezes can potentially affect the recent large scale wind farm developments across the southern North Sea.

Real sea breeze characteristics do not always fit their classical description in text books and are not always confined to light intermediate wind flow days. Therefore the effects of the different sea breeze types on offshore wind fields needs to be accounted for. The maximum impact of coastal jets, associated with corkscrew sea breezes, is close to the coastline, whereas the "calm zones" are most prevalent approximately 30km offshore.

The timing of the diurnal wind speed maximum is also different for each sea breeze type when compared to the averaged diurnal cycle for non-sea breeze days. For example, the "backdoor" sea breeze type is able to form earlier in the day than the "pure" sea breeze type since it forms under light wind speed conditions.

Furthermore, the complex Lincolnshire coast experiences coastal circulation effects from the nearby Humber Estuary, The Wash and North Norfolk which, in the context of the recent developments in offshore wind energy in the region, represent a significant challenge to wind power forecasting for farms and the individual turbines within them.

The considerable inter-annual variability of sea breeze events reinforces the importance of having an adequate length of wind speed record in wind farm resource assessment studies.


Learning objectives
1. An appreciation of how the distinctive shape of each coastline, relative to the neighbouring one and to the larger scale wind flow, is important in determining the local offshore wind climate

2. That different types of sea breeze impact offshore wind power generation in different ways

3. An understanding of the sea breeze impact on wind energy during the last decade

4. Meteorological model setup requirements for realistic simulation of sea breeze circulation features




References
Capon, R.A. (2003) Wind speed-up in the Dover Straits with the Met Office New Dynamics Model. Meteorological Applications 10, 229-237.

Orr, A., Hunt, J., Capon, R., Sommeria, J., Cresswell, D. and Owinoh, A. (2005) Coriolis effects on wind jets and cloudiness along coasts. Weather 60 (10), 291-299.

Steele, C.J., Dorling, S.R., von Glasow, R. and Bacon, J.D. (2013) Idealized WRF model sensitivity simulations of sea breeze types and their effects on offshore windfields. Atmospheric Chemistry and Physics, 13, 443-461.

Steele, C.J., Dorling, S.R., von Glasow, R. and Bacon, J. (In Press) New insights on sea breeze types and characteristics in the offshore environment: Important implications for stakeholders. Quarterly Journal of the Royal Meteorological Society