Low Density Tidal Energy Arrays Minimize Impact

by Cassandra Burgess

The configuration of a tidal energy array partially determines the level of environmental impact. In determining the optimal configuration for a particular area, it is important to consider not only power output, but also environmental impacts. Fallon, et al. discuss the impacts of a tidal energy array located in Ireland in their 2014 paper. They use a two dimensional model and average speeds over the depth of the channel. This simplifies the modeling process, but it also overestimates some of the impacts. They then analyze grid spacing for turbines, with turbines spaced 0.5, 2, and 5 times their diameters apart. The model indicated that the 5 diameter spacing had the least environmental impact. It decreased velocities by 19.9% less than the 0.5 diameter spacing outside the grid, and increased flood velocities by 27.3% less. The 5 diameter spacing also changed the tidal range of heights by only 1% while the 0.5 diameter changed them by 6.4%. Because the 5 diameter spacing has significantly less impact on the hydrodynamic environment around the turbines, the authors conclude that it is desirable to use low density arrays when possible. Continue reading

Marine Energy Device Mooring and Foundation Designs

by Cassandra Burgess

The deployment of marine renewable energy (MRE) devices requires new approaches to offshore mooring and foundations. Mooring and foundations are essential components of MRE device structures, enabling a device to remain in one spot while responding to the movement of tides or waves. Foundations and moorings often account for up to 10% of the total cost of building and maintaining an MRE device; designing a foundation of mooring that can be built at minimal cost requires analyzing the site characteristics, the direction and magnitude of forces on the device, and responses of different technologies to these forces, and the ease of installing and decommissioning the device. These steps present a novel challenge for MRE devices, because such devices have different requirements than traditional offshore technologies. One of the main challenges in applying traditional technologies to this problem is that MRE devices often need to be able to move. This is because the response of the device itself to tides and waves, which is necessary for energy production, is generally linked to the response of the foundation or mooring system. Thus these systems are required to endure loads both horizontally and vertically and to move with the device in some cases. This differs from more common offshore technologies, such as drilling platforms, where movement is not desired. Because of this novel challenge, Karimirad, Koushan, Weller, Hardwick, and Johanning suggest in their 2014 paper that new guidelines need to be developed to aid in the design of MRE device structures. Continue reading

Optimizing Tidal Energy Converters

by Cassandra Burgess

In order to make tidal energy converters economic enough to compete in the energy market, it is essential to build them as efficiently as possible, but also important to design them to avoid environmental impacts on the habitats in which they are installed. These impacts can be more difficult to predict when planning an array of tidal energy converters than a single turbine. Roberts, Nelson, Jones, and James worked to solve these two problems by creating a modeling framework that optimizes the placement of tidal energy converters in Cobscook Bay, Maine. The model uses restrictions on water height and velocity based on the region so it can be applied to other regional sites as well. It also allows researchers to input environmental restrictions on the decrease in velocity due to the turbines, and on changes in the bed shear stress at the site. These constraints represent points at which the turbines might change fish behavior by causing fish to congregate in the turbine wakes, and at which erosion of the ocean floor becomes serious. Using these restraints the researchers found that the non-environmentally constrained system had an output 19% higher than the originally planned placement, and the environmentally constrained system had an output of 16% higher.

For the purposes of this modeling process the environmental constraints were set arbitrarily. In future models, research would be necessary prior to the planning of the tidal energy converters to determine what levels of change the ecosystems could reasonably withstand. Once this is determined, the model can optimize the placement of tidal energy converters while minimizing the environmental impact. This model differs from previous models because it is on a much finer scale. While previous models have been able to accurately predict the impacts of tidal energy converters on a broad scale, this model looks at the fluid dynamics near the turbines themselves. This improvement allows for analysis of the environmental impacts near the turbines, as well as for better information on the turbulence and velocity changes created, both of which affect the power output of nearby turbines. Because this model was able to optimize both energy output and environmental impact, two areas most concerning when constructing a tidal energy array, the researchers recommend that it be used in the planning for future array sites.

 

Roberts, Jesse, Nelson, Kurt, Jones, Craig, James, Scott, 2014. A Framework for Optimizing the Placement of Current Energy Converters. 2nd Marine Energy Technology Symposium, April 15-18, 2014. [GSSS: Optimization Tidal Roberts]

https://vtechworks.lib.vt.edu/bitstream/handle/10919/49219/99-Roberts.pdf?sequence=1

 

Accuracy of Models for Wind and Tidal Turbines

by Cassandra Burgess

Computation Fluid Dynamic models are used to investigate the influence of rotating wind and tidal energy generator turbines on the surrounding environments. Johnson et al. (2014), compared current analytical and numerical models and experimental findings to a new computational fluid dynamic (CFD) model, and found that the CFD model agreed well with a simple conservation of momentum model, but did not closely match the experimental and numerical findings on reactions to the spinning turbines. This result was especially pronounced far from the turbine. The numerical and experimental findings predict much more turbulence downstream from a turbine, and larger changes in velocity. Continue reading

Fish Behavior near Tidal Energy Turbines

by Cassandra Burgess

Any man-made structure in a marine environment has the potential to impact the organisms living there. Previous research has shown that fish actively avoid trawlers and boats, and that abandoned oil platforms often become a place for fish to congregate. It has yet to be determined how fish will react to the presence of tidal energy generators. This may be an important design consideration. Vietnam and Zydlewski (2014) conducted research on fish behavior at a turbine in Cobscook Bay, Maine. This bay is known for high biodiversity, and provided the chance to study a range of fish species. The study found that over 50 percent of the fish they monitored in the area interacted with the turbine in some way, and that 34.8 percent were observed to enter or exit the turbine during the 22 hours study. They also found large fish (greater than 10 cm in length) were more likely to avoid the turbine at night than small fish. At night small fish had only a 0.002 probability of avoiding the turbine, while large fish had a probability of 0.109 of doing so. Continue reading