Tidal power is a source of renewable energy that is harnessed from the changing of tides, profiting from either the rise and fall of the sea level, or from the high speed currents created by the tide. The two most common ways of obtaining this energy are through technologies called tidal range and tidal stream, although a third way, called tidal lagoon, is in the process of construction. Coastal areas are able to utilize these technologies to harness energy from changing tides to supply them with electrical power. However, while tidal energy is efficient, these methods are costly, and have a negative impact to underwater ecosystems.Tides are created by the gravitational pull of the Moon (mainly) and the Sun, as well as Earth’s orbit. The ocean’s water bulges on one side of Earth due to the moon’s gravitational pull, while it also bulges on the opposing side due to Earth’s orbit. These two bulges are where the high tide is occurring, while the adjacent areas are where the low tide occurs. Since the moon orbits the Earth and the Earth orbits the sun, the areas where high tide occurs are changing, and each high tide lasts for about 6 hours and 12.5 minutes (Moon takes 24 hours and 50 minutes to orbit Earth, meaning the time interval of high tide is always changing). Seeing as tides are constant and can be predicted, tidal power is a dependable source of renewable energy. The most used way of harnessing this energy thus far is through the method of tidal range. It is quite similar to that of hydroelectric power, whereby it normally uses a type of barrage which has retractable gates (sluice gates) that open to allow the tide to flood into the barrage. The barrage is a very long structure (that can double as a road in some cases) projecting out from the coastal edge. As the water from the tide, which has mechanical energy, is entering the barrage, it passes through turbines (where the kinetic energy transforms into mechanical, while part is transformed into thermal due to friction, and a minimal amount into sound) which are attached to generators (which convert the mechanical energy into electrical energy, again some turns thermal due to friction), capable of creating electrical energy. When the maximum level of water is reached in the barrage, the gates close, sealing the water within. A head differential (the difference in pressure due to the difference in height of water level) is gradually created between the trapped water and the ocean level until it is great enough to transfer the gravitational potential energy of the barraged water into mechanical energy. At this stage, the sluice gates are reopened allowing the water to flow outwards (the potential energy of the water transforms into kinetic as it falls, and the process of entrance is reproduced). This energy is transferred into electrical energy thanks to the generators. This method is useful because it utilises the turbines in two directions, allowing energy to be obtained as the tides flow in and ebb out. Tidal stream technology (the newer version of harnessing tidal energy) differs from the tidal range method in that it utilises the currents created by the tides rather than creating a potential energy. Underwater structures resembling wind turbines are placed in a shallow area (20-30 metres deep) below water, where the kinetic energy from the tidal currents passes through the turbines (as the tide ebbs and flows) where it is turned to mechanical. Then it passes through a generator by the turbines where it is turned to electrical energy and put through power lines. Some of the kinetic turns to thermal and sound energy while it goes through the turbine, and again while it is transferred to electrical. Tidal energy power is around 80% efficient (similar to hydroelectric power), because it only transforms energy twice. However, tidal power plants are very costly (France’s La Rance plant $817 million; Korea’s Shiwa Lake at $217 million). The brightside to this is that once barrages are constructed, they need very little maintenance and last for ~100 years. Tidal stream technology on the other hand is equally efficient, with as large a cost. However it remains in early stages and expects the cost to decrease dramatically with technological advancement.Environmentally, tidal power plants are beneficial in the fact that they produce no greenhouse gases, however they are detrimental to aquatic ecosystems. The construction stage automatically kills off a large amount of surrounding marine life. Barrages reduce the tidal range which in turn reduces the intertidal zone (area of difference between where low and high tides hit) of the coast line. This affects breeding and feeding zones for various animals, which could lead to extinction or disruption of ecosystems and the food chain. Tidal lagoons are a newer alternative which place a barrage-like structure off the coast, which is not fully enclosed allowing a more natural flow of marine life. Tidal stream turbines are beneficial for they do not spin fast enough to kill fish (water density too high), however they are expected eventually to change current speeds and migration paths, as well as water pressures. All of these could be detrimental to ecosystems.