Figure 1 Lake Ontario Offshore Study Area near East Toronto

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1 Wind Energy Resource Assessment and Forecasting for Sites on the Great Lakes Peter Taylor1,2, Jim Salmon2, Jack Simpson3, Wensong Weng1, Matthew Corkum1 and Hong Liu1 1 CRESS, York niversity, 2 Zephyr North Canada, 3 Toronto Hydro Energy Services The wind energy resource offshore is not well understood in the Great Lakes area. This is unfortunate as potential offshore wind farm sites are attracting greater interest. Therefore, in an effort to improve our knowledge of the offshore resource, data are now being collected in specific locations. nless there happens to be an offshore platform already in place, such as Cleveland s CRIB (Dykes et al, 2008) the cost of installing a platform for a tower or lidar profiler can be significant. Toronto Hydro are currently (September/October 2009) in the process of installing a lidar anemometer platform in Lake Ontario about 1.2 km offshore from east Toronto and data from that site will be a critical factor in decisions related to a prospective wind farm in that area. Figure 1 outlines the study area for the offshore wind research and shows the location of the lidar platform. Figure 2 shows an artists sketch of the platform to be located in 12 m water depth, and a photograph of the platform prior to installation. Power for the lidar will be sourced from small wind turbines and solar panels. Cell phone communication will be used for data transfer. Figure 1 Lake Ontario Offshore Study Area near East Toronto

2 Figure 2. Artists sketch of the lidar platform and the platform prior to shipment to the site. Preferred Great Lakes wind farm sites are in relatively shallow water reasonably close to shore. Airflow over these sites will often be within a transition zone as air flows from a rough land surface to a smoother lake surface. There will also often be changes in the thermal stratification of the air column within this Internal Boundary Layer (IBL) and there may be topographic effects at the shoreline. Most previous studies of internal boundary layers (see Garratt, 1992 or Kaimal and Finnigan, 1994) have concentrated on relatively short fetches and are based on a single roughness change with a constant flux layer as the undisturbed upstream flow. The same is true of the roughness change treatment in the European Wind Atlas (Troen and Petersen, 1989) and the WAsP model (see which is widely used in the wind energy community. As a simple application of those theories we can use a Guidelines (Walmsley et al, 1989) approach and the GLW software. Figure 3 gives an example of GLW output for flow from a rough land surface to a water surface with a 2 km fetch over water. GLW assumes neutral stratification and logarithmic velocity profile segments. The focus here is on winds at 80m and one can see that there is little change in the 80m wind speed after a 2 km fetch over the lake. Further out, or for longer over-water fetches there will be acceleration but the GLW model assumptions are less satisfactory for long fetches. Turbulence intensity also remains close to the upwind value (0.166) in this scenario. For longer fetches where the finite depth of the boundary layer and the limitations of the constant flux layer formulation come into play. Taylor (1969) made some early calculations with a simple mixing length formulation within a neutrally stratified Planetary Boundary Layer formulation while Jensen (1978) addressed the problem analytically.

3 Figure 3 GLW calculation of flow from a suburban surface (z0 = 0.2m) over a lake (z0 = m) at a fetch of 2 km. For a more advanced numerical approach we can formulate and solve the Reynolds averaged Navier-Stokes equations. We make boundary-layer approximations for pressure and assume that along-wind diffusion is minimal. In this model we are just considering changes in surface roughness and thermal properties over a flat, horizontal surface. Effects of gentle terrain can be added separately. If there is a sharp drop in elevation at the shoreline (as is the case for portions of the Toronto area), we assume that the effects are limited to a downwind region of order ten times the height of the cliff or bluff and only apply the model results downstream of that. The set of equations for velocity components (,V,W), potential temperature (Θ) and Turbulent Kinetic Energy, or TKS (E) plus closure hypotheses are given below. Shear stresses and vertical heat flux are <-uw>, <-vw> and <θw>. Further details are given in Weng et al (2009). Solving these equations for the same case that we considered with GLW, but for a range of fetches out to 0 km and a range of heights we obtain the results shown in Figure 4. If we also allow for thermal differences, characterized by the surface heat flux Hf, we obtain results such as those shown in Figure 5 for flow from a cool land surface over a warm lake. We can also generate results for temperature, velocity shear and turbulence intensity.

4 The governing equations solved in the model are: < uw > = f (V V g ) < vw > = f ( g ) Θ Θ < wϑ > W = + = 0 < uw > = K m K m = m (α E )1/ 2 < vw > = K m < wϑ > = K h Θ K h = m (α E )1/ 2 / Pr E E (α E ) 3 / 2 E = < uw > < vw > + β g < wϑ > + (Km ) d Normalized wind speed, 2 / g 1 z0 = m Height (m) Figure Distance from discontinuity in z0, x (m) Prediction of wind speed acceleration as air flows offshore, neutral stratification. 0000

5 z0 = m & Hf = 0 0 W m-2 Normalized wind speed, 2 / g Height (m) Distance from discontinuity in z0, x (m) 0000 Figure 5 Flow offshore from cool land to a warm lake. Both Figures 4 and 5 show that a relatively long fetch (say > km) is needed before there is significant acceleration of the 80m wind, and that wind speed increases continue out to distances of order 0 km. By that distance the full planetary boundary layer flow has adjusted to the underlying water surface and is in equilibrium with it. The implications for our site in Lake Ontario are that the greatest increases in hub-height winds, relative to an on-shore location, will be for wind directions from about degrees. There will however, also be benefits for winds coming from the NW (315 degrees) since lower level winds will see the effect of the smoother water surface and wind shear will be reduced. Over Lakes Erie and Ontario winds are frequently from the degree quadrant and this suggests that the wind resource should be better in the northern (Canadian) half of these lakes. We illustrate this with energy density plots for locations near lakes Erie and Ontario in Figure 6. Percentages of power from the quadrant are 64% at Port Colborne and 43% for Toronto Headland. Reliable wind forecasting will become a critical issue for Electricity System Operators as wind energy provides an increasing proportion of the supply. Our studies for an onshore site near Lake Erie (Liu et al, 2009) show that the model of flow over a roughness change described above can be used in conjunction with wind forecasts from a Numerical Weather Prediction model (the Canadian Global Environmental Model in this case) to improve site-specific, hub-height wind forecasts. The same approach is being implemented for our offshore site in Lake Ontario and will be evaluated against data obtained by the lidar profiler.

6 Figure 6. Energy Density roses at approximately 20m height based on data from Environment Canada sites at Port Colborne near Lake Erie (left) and Toronto Headland near Lake Ontario (right). Total values are for 4 years at Port Colborne and 6 years at Toronto Headland. Corresponding average power densities, at 20m, are 327 and 315 Wm-2. Acknowledgements Wind energy research by Professor Taylor s group at York niversity has been funded by grants from the Canadian Foundation for Climate and Atmospheric Sciences (CFCAS) and by Zephyr North and the MITACS program supporting mathematical research in collaboration with industry. Toronto Hydro acknowledge financial support and the loan of equipment from NRCAN for this Lake Ontario study. References Dykes, Katherine, Fletcher Miller, Bob Weinberg, AAron Godwin, Emily Sautter,,2008, A Wind Resource Assessment for Near-Shore Lake Erie, Report to Green Energy Ohio, see Garratt, J.R., 1992, The Atmospheric Boundary Layer, Cambridge niversity Press, 316pp. Jensen, N.O., Change of surface roughness and the planetary boundary layer. Quarterly Journal of the Royal Meteorological Society 4, Kaimal, J.C. and Finnigan, J.J., 1994, Atmospheric Boundary Layer Flows, Oxford niversity Press, 289pp. Liu, H., Wensong Weng, Peter A. Taylor and Jim Salmon, 2009, Coupling of Roughness Change Models with GEM to Improve Hub-Height Wind Speed Forecasts, CANWEA 2009 poster

7 Taylor, P.A., 1969: The planetary boundary layer above a change in surface roughness. J. Atmos. Sci., 26, Troen, I. and E.L. Petersen, European Wind Atlas. Risø National Laboratory, Roskilde, Denmark. ISBN pp. Walmsley, J.L., Taylor, P.A. and Salmon, J.R., 1989: Simple Guidelines for Estimating Wind Speed Variations due to Small-Scale Topographic Features - An pdate, Climatological Bulletin, 23(1), 314. Weng, W., Taylor. P.A. and Salmon, J.R., 2009, A 2-D numerical model of boundary-layer flow over single and multiple changes in surface roughness, J.Wind Engineering and Industrial Aerodynamics (in press)