Tidal influence on offshore and coastal wind resource predictions at North Sea Barbara Jimenez 1,2, Bernhard Lange 3, and Detlev Heinemann 1. 1 ForWind - Center for Wind Energy Research, University of Oldenburg, 26111 Oldenburg, Germany, tel: +49-441- 36116732, fax: +49-441-36116739 e-mail: barbara.jimenez@forwind.de 2 CRES - Center for Renewable Energy Sources, Greece 3 ISET - Institut für Solare Energieversorgungstechnik, Kassel, Germany Abstract: Only the higher wind resource compared to land sites can make offshore wind farms economically feasible. Accurate knowledge of the offshore wind resource is, therefore, of crucial importance for estimating the wind power potential and planning of wind farms. The most commonly used method for wind resource assessment is the European Wind Atlas method with the WAsP program. Despite its strong simplifications for offshore sites (constant sea surface roughness, a simple wind profile and a stability independent internal boundary layer model), it showed only small deviations in practical applications in comparison with wind measurements in the Baltic Sea. The main influence of the tides close to the coast is that of a roughness change in the intertidal zone, i.e. areas which are covered by water at high tide, but fall dry at low tide. The Wadden Sea is a large intertidal zone at the German North Sea coast, which influences the wind regime at he coast, the islands and the offshore areas near the coast. Additionally, as the tide rises and falls, the offshore wind turbine effectively moves through the wind shear profile. Also, the tidal current leads to a moving surface, which influences the wind. The aim of this study is to identify these tidal influences in mean wind speed and to assess how this knowledge may influence on the offshore and coastal wind resource predictions at prospective sites. Wind speed of several stations (offshore, onshore and islands stations) are modelled with WAsP and compared with the measurements. Comparative study of offshore wind resource predictions are presented for low and high tide at offshore, onshore and islands sites. 1. Introduction As wind project developers move offshore, new issues arise in estimating the long-term wind resource at prospective sites. One such issue is the influence of the roughness change of the intertidal zone, i.e. areas which are covered by water at high tide, but fall dry at low tide. The Wadden Sea is a large intertidal zone at the German North Sea coast, which influences the wind regime at the coast, the islands and the offshore areas near the coast. The Wind Atlas Analysis and Application Program (WAsP) is a computer program for predicting wind climates and power productions from wind turbines and wind farms [1]. The predictions are based on wind data measured at stations in the same region. The program includes a complex terrain flow model, a roughness change model and a model for sheltering obstacles. In offshore areas, away from the influence of the coast, it gives good predictions in comparison with observed mean wind speeds and the wind speed profile [2]. A comparison with offshore masts in the Baltic Sea [3] showed a generally good performance, but also differences from the measurements for certain wind directions. The aim of this study is to present the evidence of the tidal influence on offshore and coastal wind resource predictions, and to quantify this effect at North Sea using WAsP method and measured data from offshore, onshore and island sites. WAsP is used to assess the wind resource over the German North Sea region where data from onshore (Wilhelmshaven, and Nordholz), coastline (Buessum and Strucklahnunghoern), offshore (EMS lightship) and island (Norderney, Spiekeroog and Hallig Hooge) measurements are available. Predictions are compared with each other and with the measured data. The structure of the paper is as follows: In the next section the measurements are described. WAsP method is briefly outlined in the following section. Section 4 contains the analysis of the results. Finally, conclusions are drawn in the last section. 2. Measurements Observations from one offshore, three island and four land sites are used in this study. The locations of the measurement sites are shown in figure 1. The stations are equipped with cup anemometers and wind vanes at different heights. Together with the coordinates of the stations, the measurement heights are shown in table 1. Photographs of the measurement stations are shown in figure 2.
Figure 1: Locations of the measurement sites in and around the North Sea. Table 1: Location and heights of the measurements (the anemometer heights used in this study are marked with *). Site Location (Geografical coordinates) Height (m) Hallig Hooge 54 35 N, 8 31 E 12* Spiekeroog 53 46.047 N, 7 40.339 E 10* Norderney 53 42 44.6 N, 7 09 06.6 E 12* WHV 53 36 59 N, 08 03 E 32, 62*, 92, 126, 130 EMS 54 10 N, 6 21 E 10* Nordholz 53 45.9 N, 08 39.2 E 10* Büsum 54 07 09.9 N, 08 51 30 E 10* Strucklahnungshoer n 54 29 47.5 N, 08 48 25.0 E 10* The Wilhelmshaven (WHV) and Nordholz (ND) land mast are located in Northern Germany, about 5 km from the coast. WHV is a high meteorological measurement mast run by the company Projekt GmbH. It is situated in a wind turbine test site near Wilhelmshaven with presently nineteen wind turbine prototypes of various European manufacturers. The Strucklahnungshoern (ST) and Buessum (BU) measurement sites are just located in the east coastline of the North Sea. EMS is a lightship measurement located in the south-eastern part of the North Sea. The Norderney (NR), Spiekeroog (SP) and Hallig Hooge (HH) measurement sites are situated on islands. NR and SP are located in the southern part of the North Sea, about 8 km from the coast. HH is located in the east of the North Sea about 5 km from the coast. All island sites, the lightship and ND, ST, BU on land sites are meteorological measurement sites run by the German Weather Service (DWD). For more information on the DWD sites, see www.dwd.de. Data from January 1995 to December 2004 have been collected at all the measurement sites. Hourly mean data are used. All data have been quality controlled by visual inspection of the time series. Time periods where one measurement was erroneous or missing were taken out of consideration at all sites. In this way, time series with identical observation periods (time and date) have been obtained at all the measurement stations. The data completeness of the simultaneous time series for each month and year is shown in table 2. The wind roses and wind speed histograms of the eight measurement stations used are shown in figure 3. Wilhelmshaven data have been split up in two different periods; the reasons will be explained in the next section.
Figure 2: Photographs of measurement masts: Hallig Hooge (HH), Norderney (NR), Spiekeroog (SP), lightship Ems (EMS), Nordholz (ND), Wilhelmshaven (WHV), Buessum (BU) and Strucklahnungshoern (ST).
Figure 3: Wind roses and wind speed histograms (with fitted Weibull distributions) for: Hallig Hooge, Norderney, Spiekeroog, Wilhelmshaven, Nordholz, Buessum, Strucklahnungshoern and EMS.
Figure 3: Continuation. Table 2: Data completeness used in the different measurement sites. Data completeness per month (%) 1 2 3 4 5 6 7 8 9 10 11 12 1997 72.4 1.9 93.9 1998 95.7 97.6 93.9 95 97.3 97.2 94.1 94.5 97.3 72.3 14.1 29.4 1999 0.3 0.3 64.4 92.7 95.4 94.8 85.1 44.6 19 2000 23.2 75.3 64.5 2001 67.3 96.4 61.4 6.4 2002 43 89.6 94.5 83.5 95.4 81.9 34.1 71.5 70 96.1 95.7 77 2003 83 95.7 96.5 50 96.4 97.6 95.7 44.4 35.2 74 46.7 2004 33.9 48.2 97 95.5 95.7 72.3 84 93.3 93.8 94.7 3. Application of the WAsP model WAsP estimations were calculated using the eight different measurement stations described in section 2 as input. Each station was visited to obtain, within a radius of approximately 10 km, an accurate description of the obstacles and vegetation around the meteorological mast. Orographic effects have been neglected, since the area is very flat. Different roughness map of the coastal area around the German Bight has been established; one map corresponds to the coastline for the high tide, and other represents the intertidal zone for the low tide (see figure 4). The intertidal zone in the eastern part of the North Sea, which involves Hallig Hooge (HH), Buessum (BU) and Strucklahnungshoern (ST), can be extended up to 10 km. Whereas, in the southern part of the North Sea, where
Norderney (NR) and Spiekeroog (SP) are included, this area reach up to 7 km in the South and some hundred meters in the North of the islands. For each of the measurement stations, a detailed roughness description has been made on the basis of maps and a site visit. The following roughness classes are distinguished: water areas with roughness 0.00002 m, intertidal zone with roughness 0.004 m, lakes with roughness 0.003, islands in the South coast with roughness 0.042 m, islands in the East coast with roughness 0.017 m, farm land with wind breaks with roughness 0.086 m, towns on the islands and small woods with roughness 0.209 m, woods and cities with roughness 0.4 m. Corrections due to obstacles were necessary in Norderney and Spiekeroog sites (table 3). Wind speed measurements at the Wilhelmshaven mast (WHV) is disturbed for some wind direction sectors by wakes of the wind turbines of the nearby wind farm. For a detailed description of the wind farm see http://www.dewi.de/. The measured wind speeds are therefore corrected for the shading effect of the turbines when they are bin-averaged for 30 wind direction sectors. For the calculations, the WHV data were split up in two different periods, 1997-2002 and 2003-2004, since new wind turbines were built in the wind farm close to the mast during the last period. Corrections factors for the site of the measurement mast have been established using the PARK model of WAsP (table 4). The default parameters of WAsP were used for the calculations. The average wind climatologies derived with WAsP from the eight stations were applied to calculate the wind resource over the German Bight and the wind speed profiles at the measurement sites. The model has been applied to a digitized map with a grid mesh of about 190 198 Km for the region under examination. Figure 4: The roughness map of the coastal area around the German Bight. A roughness map for high tide; B for low tide. Table 3: Corrections due to obstacles in Norderney (NR) and Spiekeroog (SP). Sector 1 2 3 4 5 6 7 8 9 10 11 12 Angle 0 30 60 90 120 150 180 210 240 270 300 330 NR -0.53-0.53-0.92-8.46-3.17 SP -0.63-30.75-35.41-12.69-10.19-21.25-2.43 Table 4: Corrections due to the shading effect of the wind farm in Wilhelmshaven (WHV). Sector 1 2 3 4 5 6 7 8 9 10 11 12 Angle 0 30 60 90 120 150 180 210 240 270 300 330 WHV (1997-2002) -7.29-11.51-12.34 WHV (2003-2004) -27.57-14.25-0.09-7.38-14.29-12.97-0.01-10.57
4. Results The assessment of the influence of the tides on the wind resource estimations by WAsP will be performed by means of the comparison between: Measured wind speed and WAsP predictions at high tide. Mean wind speed predictions at low and high tide. 4.1. WAsP intercomparison high tide An intercomparison study has been performed with the WAsP model using each of the eight measurements to predict the mean wind speed at the other seven sites. Errors in the mean wind speeds were calculated between the WAsP predictions and measured data at the height of the measurements: u predicted umeasured error(%) = *100 u predicted Results of the comparison are shown in Table 5. As it can be observed, it is clear that Wilhelmshaven (WHV) and Nordholz (ND), on land sites located to a distance of 5 km from the coast, differ rather much from all other sites. They are overpredicted by all other sites and themselves severely underpredicts the other sites up to 18% in some cases. On the contrary, they show relatively small differences, up to 4%, when estimating each other. The coastal stations, Buessum (BU) and Strucklahnungshoern (ST), which are just situated on the coastline, show relatively small differences when estimating each other, as well as, the island and offshore sites. They are underpredicted by all other sites up to 4% and themselves overpredict the other sites up to 5%. On the other hand, the three islands, Norderney (NR), Spiekeroog (SP) and Hallig Hooge (HH), and the lightship EMS also exhibit small differences when estimating each other. Therefore, the WAsP predictions at high tide based on the offshore, coastline and island measurements agree rather well with the site of the island, offshore and coastline stations. Table 5: Percentage errors in the mean wind speed between WAsP predictions and measurements at high tide. Name Norderney (NR) Hallig Hooge (HH) Spiekeroog (SP) Wilhelmshaven (WHV) Büsum (BU) Nordholz (ND) Strucklahnungshörn (ST) Lightship EMS Predicted site Meas. Height Meas. wind speed WAsP-Reference station NR HH SP WHV BU ND ST EMS 12 m 5.68 1.58% 4.05% 6.51% -6.69% 8.27% -4.23% 9.33% 10.04% 12 m 7.43-2.69% -0.67% 1.75% -10.6% 3.50% -8.75% 5.11% 3.23% 10 m 6.89-4.79% -1.45% 0.15% -10.4% 0.58% -8.13% 4.35% 4.06% 62 m 6.44 10.87% 13.04% 15.84% 0.93% 18.17% 4.04% 18.79% 18.79% 10 m 7.21-5.55% -2.91% -0.14% -13.0% -0.28% -11.1% 4.72% 2.22% 10 m 4.56 6.80% 9.65% 12.28% -2.41% 14.04% 0.22% 17.32% 16.23% 10 m 6.88-6.69% -4.07% -2.62% -13.5% -0.73% -19.0% 0.73% 1.74% 10 m 8.22-7.79% -5.72% -3.16% -16.1% -2.19% -14.1% 0.24% -0.36% 4.2. Comparison between low and high tide The difference of the mean wind speed predicted by WAsP has been calculated. Errors in the mean wind speeds were calculated between the WAsP predictions at low and high tide: error( m / s) = u * predicted u predicted Where, ū * is the wind speed predictions at low tide and ū at high tide. Results of the comparison are shown in Table 6. As it can be observed, the biggest differences appear in the predictions based on island and coastline stations up to approximately 0.8 m/s, as well as, when they are predicted by the other sites, especially in the eastern part of the
North Sea, where the extension of the intertidal zone is wider. On the contrary, the onshore and offshore stations exhibit low differences when predicting each other. Wind resource maps have been calculated for the area of the German Bight with WAsP for the high and low tide with a grid resolution of 9 km (figure 5). The WAsP maps were calculated on basis of data from the Spiekeroog (SP), Strucklahnungshoern (ST), Nordholz (ND) and EMS. Large differences of the predictions between low and high tide can be again seen for island (SP) and coastline (ST) stations as input. Higher wind speed predictions are found for low tide. Whereas the predictions performed by onshore (ND) and offshore (EMS) stations are practically similar. Table 6: Errors in the mean wind speed (m/s) between WAsP predictions at low and high tide. Name Predicted site Meas. Height WAsP-Reference station NR HH SP WHV BU ND ST EMS Norderney 12 m 0.02 0.5 0.22-0.09 0.36 0.06 0.52-0.13 Hallig Hooge 12 m -0.52-0.06-0.31-0.64-0.21-0.55-0.07-0.78 Spiekeroog 10 m -0.21 0.28 0-0.36 0.15-0.23 0.3-0.44 Wilhelmshaven 62 m 0.17 0.61 0.37-0.02 0.4 0.1 0.61-0.06 Büsum 10 m -0.38 0.04-0.2-0.52-0.06-0.46-0.01-0.64 Nordholz 10 m 0.07 0.42 0.25-0.05 0.32 0.01 0.42-0.1 Strucklahnungshörn 10 m -0.54-0.11-0.34-0.67-0.25-0.43-0.06-0.79 Lightship Ems 10 m 0.26 0.79 0.51 0.07 0.64 0.18 0.79 0 5. Conclusions To identify the tidal influence on offshore and coastal wind resources predictions using WAsP method is the main objective of this study. Wind speed of several stations (offshore, onshore and islands stations) are modelled with WAsP and compared with the measurements. WAsP estimations were calculated on basis of eight different stations: three islands, one offshore and four on land. As far as the WAsP intercomparison study is concerned, the differences between the predictions performed by the WAsP model at different sites depend on the measurement station used as reference. In the case of high tide, six of the stations, three island (Norderney, Spiekeroog and Hallig Hooge), one offshore (EMS) and two coastal sites (Strucklahnungshoern and Buessum), predict each other with a rather small error up to ±5%, despite their large geographical distance. On the contrary, two stations show rather high deviation; Wilhelmshaven (WHV) and Nordholz (ND) stations situated both onshore. They are overpredicted by all other sites and themselves severely underpredicts the other sites up to 18% in some cases. However, they show relatively small differences, up to 4%, when estimating each other. Comparing the WAsP predictions between high and low tide, the largest differences appear when island (SP, HH and NR) and coastline (ST and BU) stations are used as input or predicted by other sites, whereas predictions based on onshore (WHV and ND) and offshore (EMS) stations are practically similar. It has also been observed that coastline and island sites located in the eastern part of the North Sea exhibit the largest differences between the high and low tide predictions when predicting each other and by the others sites. This area has an important influence of the tidal, since the intertidal zone is broader, reaching up to 10 km. In conclusion, the WAsP predictions based on coastline and island stations seem to be suitable for predicting the offshore wind resource from land-based meteorological measurements. Stations on island, South and East of the German Bight, coastline and offshore agree well with each other, whereas on land stations differ pretty much. Evidence of the influence of the tidal has been shown, mainly in the island and coastline sites and, particularly in the eastern part of the North where the largest intertidal zone can be found.
Acknowledgement This work has been supported by the EC "Wind Energy Assessment Studies and Wind Engineering" (WINDENG) Training Network (contract n. HPRN-CT-2002-00215). The observations used have been provided by the German Weather Service (DWD), Projekt GmbH and Federal Office for naval and hidrography (BSH). German Wind Energy Institute (DEWI), Naval Air Station at Nordholz and the office for the rural region of Hussum (Amt für ländliche Räume Husum) are gratefully thanked for very helpful information provided. References 1. Mortensen N.G., Landberg L., Troen I., and Petersen E.L. (1993). Wind Atlas Analysis and Application Program (WAsP). Vol 1: Getting Started. Risoe National Laboratory User Guide: Risoe-I-666(EN)(v.1). 2. Petersen, E.L. (1992). Wind resources of Europe (Offshore and coastal resources). EWEA Special Topics Conference. 3. Lange B., and Højstrup J. (2001). Evaluation of the wind-resource estimation program WAsP for offshore applications. Journal of Wind Engineering & Industrial Aerodynamics, 89 3-4 271-291.
Figure 5: Mean wind speed predicted at 100 m height for high and low tide. Measurements at Spiekeroog, Strcuklahnungshoern, Nordholz and EMS are used as reference in WAsP.