medium · GMAT Verbal

Passage: Tidal energy, harnessed from the relative motion of the Earth-Moon-Sun system, is often touted as a more predictable and reliable alternative to solar and wind power. There are two primary methods for capturing this energy: tidal barrages, which are dam-like structures built across estuaries, and tidal stream turbines, which function like underwater wind turbines. While barrages can generate large amounts of electricity, they have significant environmental impacts, including altering the salinity of estuaries and disrupting the migration patterns of fish. Tidal stream turbines are generally considered to have a lower environmental footprint, but they face significant engineering challenges due to the harsh, corrosive nature of the marine environment. Furthermore, the total amount of energy available from tides is physically limited by the rate of 'tidal dissipation'—the process by which tidal energy is converted into heat through friction on the ocean floor. While tidal power can be a valuable component of a regional energy portfolio, its potential to replace traditional base-load power on a global scale is constrained by both geographical requirements and the finite energy available in the tidal system. The passage mentions 'tidal dissipation' primarily in order to:

1. Contrast the salinity levels of estuaries with those of the open ocean.
2. Describe the primary method used to prevent the corrosion of underwater turbines.
3. Argue that tidal energy is a more sustainable alternative than traditional base-load power.
4. Identify a physical limit on the total amount of energy that can be extracted from the tides.
5. Explain why tidal stream turbines are more efficient than tidal barrages.

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