This research project was undertaken to accurately estimate the power potential of the tides and tidal currents in the Minas Basin and Minas Channel regions of the Bay of Fundy. Its goal was to make a power estimate that is both attainable and sustainable; that is, an assessment based on available turbine technology that includes an estimate of the impact on the tidal range and currents. The results of our theoretical analysis and hydrodynamic modelling are producing critical information for many stakeholders. For developers, it will provide realistic power estimates that will determine if commercial scale turbine farms are financially viable. For resource users, it will provide estimates of the changes to the physical environment such power extraction will produce so that they can determine how their livelihood will be affected. For researchers, it will provide the baseline changes to tidal range and currents that will be necessary to examine changes in sediment, impacts on marine life, and other concerns.
The research, described in detail in this report, focused on four topics. First, we examined the improvement of the numerical simulations of the tides and tidal flows in the upper Bay of Fundy. The numerical models, FVCOM and RiCOM, have been run at high resolution in 2D and 3D. The models have been validated against data gathered from ADCPs. This has produced an accurate data set of tidal currents through Minas Passage that will be made publicly available through an ftp site in the coming month. The high resolution simulations has revealed that the flow through Minas Passage is turbulent, with large eddies forming around major bathymetric features. In particular, the slack after the flood tide is dominated by these eddies. The flood and ebb tides are very different, with the flood tide dominated by a strong jet coming off the tip of Cape Split.
Although the models have improved significantly, there is still significant issues that need to be addressed. Observations have shown that the bottom roughness varies across Minas Passage. Observations and simulations show that the throughout the water column is determined by the bottom drag, and therefore accurate modelling of the bottom roughness on the flow is critical. This requires further research. As well, the observations show large velocity fluctuations due to unsteady flow and turbulence. Further work needs to be done to include these in numerical simulations. As well, other effects such as waves and extreme weather events need to be included to get a complete analysis of the variations and extremes of the tidal currents in Minas Passage.
Secondly, we adapted previous theoretical analysis to the specific dynamics of Minas Pas- sage to predict the potential power of turbine fences. This analysis illustrates the importance of the blockage ratio, the portion of the channel cross-section that turbines occupy. If the blockage ratio is high, turbines can be designed to extract significantly more power from the flow. In such cases, the power the turbine fence generates can be over 500% the kinetic energy flux, rather than the 59% maximum of a single turbine. For more realistic smaller blockage ratios, similar large amounts of power can be extracted using many turbine fences. The theory highlights the difference between the power that is available to the turbine for power generation and the total power extracted from the flow. The extracted power exceeds the generation power because energy is lost in the turbine wake (and due to the drag of the turbine supporting structure). It is the extracted power that determines how much power can be taken from a given tidal flow and how extracting this power will change the tidal flow. It is therefore important to increase turbine efficiency, the ratio of the generated power to the extracted power. However, there is a trade-off between maximizing the generated power and increasing efficiency. For Minas Passage, where we expect blockage ratios are most likely to be less than 20%, the theory suggests that it is still possible for the generation power to be exceed 2500 MW for less than a 5% reduction in the flow, and approximately 800 MW for the first 1% reduction in the flow. These results confirm that previous results can be extended to realistic turbine fences.
Our third research topic was the analysis of power extraction using numerical simulations. Using a typical power curve for a 1.2 MW turbine, we used the water speed calculated by our numerical simulations to generate a map of the potential power generation over the Minas Passage. The resulting map shows that there are tens of thousands of locations where the turbine will generate a mean power exceeding 750 kW, a very high capacity factor. This region cover the northern portion of the passage with a significant portion in water depths that exceed 50 m. We also examined the power potential of a 3D complete turbine fence, illustrating that numerical simulations roughly agree with the theory analysis. We did show that increasing the numerical resolution could reduce the maximum power extracted, from 8 GW to 6 GW. This emphasizes that the numbers produced by simulations are not absolute, but will change as the models are adapted and improved. But, significantly, moving to 3D dimensional simulations has not changed qualitatively the 2D results. When we examined partial turbine fences, the simulations suggested that significantly more turbine power than the theory suggested was possible. In the simulations, the turbine drag could be increased to much higher values than the theory suggested. Why this is the case is not fully understood, we believe it is connected to the vertical profile of the flow. It is the focus of ongoing research.
Finally, we combined numerical and theoretical to construct a Turbine Array Model. This model was a relatively simple tool to quickly analyse what the power potential of turbine arrays in Minas Passage. Importantly, included a model of turbine wakes and therefore could assess realistic arrangement of turbines. This model further confirmed previous results. For a small number of turbines, the turbines can theoretically generate in excess of 1MW each. As the number of turbines increases, the turbines must be placed in locations of slower flow and will produce less power. This is especially true if the turbines are restricted to shallow water, for example if the turbines are restricted to water less than 50 m less than 500 turbines can be reasonably located. For all the tested arrays, the model suggests that about 800 MW of power is available for each 1% reduction in the flow through Minas Passage. Even given the reduction in efficiencies of real turbines, this suggests that hundreds of turbines producing hundreds of MW will results in a minimal, likely difficult to observe, reduction in flow through Minas Passage.
In conclusion, our research has supported previous results that in the range of 2000MWof power extracted from Minas Passage by an array of in-stream tidal turbines with a relatively small reduction in the flow through the passage, less than 5%. For smaller arrays, the turbines can be placed in locations with the fastest water and produce power more efficiently with even less impact. Of course, these are initial estimates. More work remains to be done to understand the turbulent nature of tidal flow, the complex interaction of turbines and the flow, and possibility of designing turbines to make arrays generate electricity with minimum power extraction.