by Cassandra Burgess
As tidal energy develops throughout the world several different designs for Tidal Energy Converters (TECs) have been developed. The main classifications are reciprocating and rotating devices, of which rotating are the most common. Within this category the TECs can be either floating and anchored to the bottom or fixed to the bottom by a rigid structure. Each of these designs has different impacts on the environment it is placed in. Sanchez et al. (2014) tested these impacts in Northwest Spain in Ria de Ortigueria by using a three-dimensional model to examine the impacts of two plants, one floating and the other bottom-fixed. The researchers found that both plants had little effect on water more than 4 kilometers away, but large impacts near the plants. The plants also exhibited very different patterns in changes in flow near the plants. Thus the authors argue that because TECs clearly have an impact on the flow of water around them, further investigation will be necessary to find the impacts of this change in flow on ecosystems.
The research focused on two plants in this region. The estuary in the region experiences the strong current and water depth that are ideal for tidal energy generation, and follows a tidal cycle characteristic of much of Europe, making it a good indicator for possible environmental impacts of TECs in other areas of the world. Both plants were the same size and had the same mechanical features. One was floating and placed in the upper half of the water column while the other was bottom fixed and placed in the lower half of the water column. The Delft3D-FLOW model was then applied. The model spanned an area about 10km away from the plant, and was verified by testing it on actual data from 2010. The model was run without the TECs present, with a floating plant present, and with a bottom-fixed plant present. In each run the boundary conditions were forced to maintain a typical sea surface level and flow rate of the river.
The model predictions indicate that while both plants created small (1/100 meter/second) reductions in flow rate on areas far away, the floating plant created a large decrease in the flow rate at the surface that wasn’t present in the floating plant simulation. Both designs caused an increase in velocity south of the plant in all depths of water. The model also predicted that the floating plant would produce approximately 1000 MWh more than the bottom-fixed plant, because of differences in flow rates at different water depths.
The results indicate the tidal energy plants can have large impacts on flow rate in areas near them, and these impacts vary significantly with the design of the plant. The impacts can also reach unexpected places, indicating that modeling should be done to determine the effects of plants before they are built. Depending on the ecosystem the plant will be built in the least impactful model and positioning can then be selected. In order to fully understand the environmental impacts of tidal energy plants, further study must be conducted on how variations in flow, like those the study describes, affect organisms living in the region.
Sanchez, M., Carballo, R., Ramos, V., Iglesias, G., 2014. Floating vs. bottom-fixed turbines for tidal stream energy: A comparative impact assessment. Energy 72, 691-701. [GSSS Sanchez carballo turbine]