Abstract
Abstract. This paper quantifies the backward effect on the ocean currents caused by a tidal stream farm located in the open shallow sea. Recent studies in channels with 1-D models have indicated that the power potential is not given purely by the flux of kinetic energy, as has been commonly assumed. In this study, a 3-D ocean circulation model is used to estimate (i) practically extractable energy resource at different levels of rated generation capacity of the farm, (ii) changes in the strength of currents due to energy extraction, and (iii) alterations in the pattern of residual currents and the pathways of passive tracers. As well as tidal streams, the model also takes into account the wind-driven and density-driven ocean currents. Numerical modelling has been carried out for a hypothetical tidal farm located in the Celtic Sea north of Cornwall, an area known for its high level of tidal energy. Modelling results clearly indicate that the extracted power does not grow linearly with the increase in the rated capacity of the farm. For the case study covered in this paper, a 100-fold increase in the rated generation capacity of the farm results in only 7-fold increase in extracted power. In the case of a high power farm, kinetic energy of currents is altered significantly as far as 10–20 km away from the farm. At high levels of extracted energy the currents tend to avoid flowing through the farm, an effect which is not captured with 1-D models. Residual currents are altered as far as a hundred kilometres away. The magnitude of changes in the dispersion of tracers is highly sensitive to the location. Some of the passive drifters analysed in this study experience significant variations in the end-to-start distance due to energy extraction ranging from 13% to 238% while others are practically unaffected. This study shows that both energy extraction estimates and effects on region wide circulation depend on a complex combination of factors, and the specific figures given in the paper should be generally considered as first estimates.
Highlights
Ocean tides are driven by the Earth’s rotation in combination with gravitational forces from the Sun and the Moon
Tidal energy is almost inexhaustible despite the gradual reduction of kinetic energy of the Earth’s rotation due to the dissipation of tidal currents via friction and turbulence
This paper uses a 3-D ocean circulation model (i) to assess alterations to the circulation pattern in the shelf sea by a hypothetical tidal stream energy farm, (ii) to estimate how these alterations impact on the availability of extractable ocean energy, and (iii) to explore how the process of energy extraction changes both instantaneous and residual water flow in the wider region up to a hundred kilometres away from the farm
Summary
Ocean tides are driven by the Earth’s rotation in combination with gravitational forces from the Sun and the Moon. This paper uses a 3-D ocean circulation model (i) to assess alterations to the circulation pattern in the shelf sea by a hypothetical tidal stream energy farm, (ii) to estimate how these alterations impact on the availability of extractable ocean energy, and (iii) to explore how the process of energy extraction changes both instantaneous and residual (tidally averaged) water flow in the wider region up to a hundred kilometres away from the farm. It has to be noted that despite being named “tidal energy converters” the tidal stream devices located on the shelf extract energy from the total incoming current which is composed of wind driven and density driven currents as well as the tidal stream This notion is important in the assessment of the alteration to residual currents and the pathways of dispersion of pollutants on the shelf
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