While tidal power is far more predictable than solar or wind, its periodic nature can still be an issue (unless the design allows for reversible flow, power is only generated during half of the tidal cycle, and, even if it can generate power in both directions, the relative pauses between spring and neap tides will not generate significant power), and placement of systems for optimal power still depend on the surrounding geography, since confined watercourses experience greater fluctuations in height and current.
According to Bernoulli’s principle, the velocity of a fluid flow and the cross-sectional area of the aperture through which it flows are inversely proportional; this holds in the case of water, even though it is not nearly as compressible as air. Further, in the case of barrage tidal power systems, the aperture size has a strong influence on the overall cost of the project. Thus, confined inlets and the like are an ideal location to consider.
As for environmental impacts, some of the non-monetary costs associated with barrage systems include destruction of habitat, interruption of organisms’ travel routes, potential electromagnetic interference (in the case of species that can sense electric fields), and potential acoustic pollution. In addition, hydroelectric systems are well-known for killing fish, and the waste heat that ends up in the water reduces its capacity to store dissolved oxygen, harming not only fish, but all organisms in the affected area. Despite all this, such systems are attractive to many because of their reliance on existing technology and knowledge.
In contrast, turbine-based systems do not have the same geographic constraints, do not disrupt the environment to nearly the same degree, have less of an impact on local organisms, and are quieter; however, the requisite technology is not nearly as mature, large-scale power generation is less efficient, and the costs (both short- and long-term) are more uncertain. Still, the advantages weigh in their favor, which is why a number of such systems are being tested around the world- the BioPower Systems and SeaGen approaches being two of the most visible examples. Indeed, the BioPower Systems designs seek to mimic nature and be as environmentally inobtrusive as possible. Hopefully, full-scale testing will validate both designs.
Tidal power is certainly not a panacaea and definitely has its limitations, but it certainly qualifies as a viable component of our alternative-energy toolkit.