The world’s growing power usage, coupled with the rapid depletion of fossil fuels and increased atmospheric CO2concentrations increase the demand for renewable energy solutions. Marine hydrokinetic (MHK) energy has potential as an effective and efficient source of renewable power, but a full evaluation of ecological and economic considerations should be undertaken before installing MHK technology. Researchers at Sandia National Laboratories recognized the need for knowledge of near and far field hydrodynamics in exploring the optimal placement of MHK technologies (James et al. 2011). They simulated an experimental water flume, which was calibrated against the results of a previous flume experiment at the University of Southampton. They studied the impacts of MHK energy generation on factors such as water flow, sediment dynamics, and water quality using SNL-EFDC. Their model successfully imitated the original flume studied at Southampton, suggesting is can be used to perform MHK-array site optimization studies. —Meredith Reisfield
James, S.C., Barco, J., Johnson, E., Roberts, J.D., Lefantzi, S., 2011. Verifying marine-hydro-kinetic energy generation simulations using SNL-EFDC. Oceans, 1-9.
Sources of marine hydrokinetic power, such as water turbines and wave energy converters, can be implemented at a reasonable cost, with higher predictability and decreased intrusion on the environment than many traditional power systems. MHK technology has not been implemented on a wide scale, so there is a high availability and low exploitation of available sites. Many of these potential sites are located near population centers, which can help to keep transmission costs at a minimum. James and his colleagues stressed the importance of economic and environmental cost-benefit analyses before installing MHK technologies, and designed their model in the hopes of enabling these types of examinations. Further research will be needed to understand and quantify the environmental impacts of MHK devices. MHK devices remove energy from marine systems, so they can impact volumetric flows, tidal ranges and sediment dynamics. Changes in water circulation patterns can change, altering flushing rates and the concentration of nutrients and dissolved gasses in a marine ecosystem, possibly altering algae growth. Changes in sediment dynamics can alter patterns of deposition and erosion, as well as the size and composition of eroded particles. Additionally, acoustic energy and electromagnetic waves generated by MHK devices could disturb wildlife. Taken together, these factors may interfere with biological activities such as migration, life cycles, communication, and resource availability. Conversely, an MHK installation could benefit an ecosystem by creating a reef-like presence that could provide shelter and food resources, or by causing an area to be designated a marine sanctuary, thereby reducing other intrusive effects. The authors also discuss the importance of evaluating economies of scale associated with the diminishing levels of return of installing additional turbine at an MHK installation.
James and his colleagues applied SNL-EFDC, an upgraded version of the US Environmental Protection Agency’s environmental flow and transport code, Environmental Fluid Dynamics Code (EFDC) enhanced with the US Army Corp of Engineers’ water quality code, and a sediment dynamics code developed by researchers at the University of California, Santa Barbara. EFDC is an open-source 3D flow and transport code that directly combines sediment transport and water quality calculations. The various components of the model had also been used successfully at numerous sites. The module considered energy removal by MHK devices with changes in turbulent kinetic energy and the turbulent kinetic energy dissipation rate, in addition to being able to map levels of dissolved CO2 and O2, algae, and components of carbon, nitrogen, phosphorous and silica. The original University of Southampton study the authors reference measured the flow field in the wake of a tidal current turbine in the circulating water channel at Boulogne-sur-Mer in France. An SNL-EFDC was built to simulate this experiment. An MHK device and support structure was built to represent the turbine used in this experiment. James and his colleagues ensured that the difference in total energy, or kinetic and potential energy, upstream and downstream from the device was equal to the energy converted by the MHK plus frictional losses to the MHK support structure, so that the flume sidewalls could be specified as frictionless. They compared their model output to the wake data from the original flume in France, and found they had successfully simulated the original experiment. This model could perform simulations with the ultimate goal of optimizing design and minimizing environmental damage. Their model does have some limitations. SNL-EFDC is designed for macro-scale systems and cannot simulate a fine level of detail. However, the model could be very appropriate for systems with large amounts of turbulence.