Tidal energy is an emerging field of the energy sector, with rapid advances currently being made. Although very few tidal energy systems are in place today, and the cost of developing and building them is very high, tidal energy has the potential to be a much more reliable source of energy than other renewables such as wind and solar, as tidal patterns are more predictable than the weather.
At the moment, only two tidal power plants exist which generate electricity at a rate >50MW: the Rance Tidal Power Station in France (240MW), and the Sihwa Lake Tidal Power station in South Korea (254MW). Both of these plants are tidal barrage systems, and have very high costs associated with construction. Adjusting for inflation, the Rance station cost $915 million, meaning that it cost roughly $3.8 in construction for every watt it generated, whereas large solar farms tend to cost $1 in construction for every watt generated.
The more recently built Sihwa Lake Tidal Power Station cost $641 million adjusted for inflation, meaning that it cost $2.5 in construction for every watt. While this is still not as cost-effective as solar power, it is a significant improvement over La Rance. In addition, tidal barrage systems such as La Rance last much longer than wind or solar farms; a wind turbine has an average lifespan of 20-25 years, while solar panels have degraded significantly by this point. In contrast, La Rance is still operational 54 years after construction.
Potentially the most important advantage of tidal power is that electricity generation from a tidal power plant can be predicted months in advance, and far more precisely than the weather that affects the power production of wind and solar. Tides are the result of the interaction between the gravitational pull of the Sun and the Moon, and as the movement of these two objects relative to the Earth is known, tides can be predicted very accurately. The shape and depth of the seafloor, as well as landmasses nearby also need to be taken into account in order to get a precise indication of the tides (this is why the Bay of Fundy has much larger tides than the west coast of Finland). Wind also helps determine tides and current speeds, and is the least predictable part of the equation to find the height of the tides. Nevertheless, a reasonably accurate prediction can be made in advance. Moreover, tidal power increases during the winter, when electricity demand is highest, and solar power generation is lowest, helping to plug the shortfall in electricity demand.
Unfortunately, tidal barrage systems have significant detrimental impacts on the local wildlife. During the construction phase of La Rance, the river ecosystem collapsed, and sand-eels, plaice, sea-bass and cuttlefish disappeared. In addition, heavy sedimentation of the river bed took place, and changes in salinity damaged the local flora. It took about 10 years for the ecosystem to reach a new equilibrium, and sand-eels and plaice are now permanently gone from the Rance estuary. The disappearance of sand-eels and plaice could have an impact on seabirds and minke whales, which feed on sand-eels, and ospreys, which prey on flatfish such as plaice.
Undersea turbines are a potential solution to the environmental and economic implications of building tidal barrages, but come with their own challenges. The British-Singaporean company SIMEC Atlantis has created several prototype turbines, and trialled them in several places in the British Isles. In 2008, they installed two turbines in Strangford Lough in Northern Ireland, where the narrow opening between the lake and the sea acts as a bottleneck, leading to much higher tides than usual. The turbines were much heavier than wind turbines, at 1000 tonnes each, with a span of 43m. The cost of the entire project, including R&D, manufacturing, installation and monitoring of aquatic wildlife cost $12 million, while the turbines only generated electricity at a rate of 1.2MW. However, the project was deemed to be successful, as it demonstrated that underwater turbines were possible and could generate large quantities of electricity. In addition, the energy produced was severely reduced due to a need to constantly test the turbines and interfere with them in general, so power generation could be higher. If these turbines were produced on a large scale, the cost could be reduced to the point of being competitive. It is important to note that technologies such as solar panels have only recently become economically competitive within the last 10 years, and that with further research into tidal turbines, they could also become very competitive.
Another advantage of tidal turbines over wind turbines is that due to the much higher density of water, tidal turbines can generate electricity at a much lower current speed, again making them more reliable than wind turbines. Moreover, tidal turbines are completely submerged and are offshore, generally away from people, meaning that complaints about noise or disruption of the countryside will be much less frequent, and meaning that tidal power can be rolled out on a larger scale than wind without opposition.
Unfortunately, during the project, algal growth on the turbines was disrupting efficiency. While normally they would have been allowed to coat the turbine blades with a herbicide, this would have impacted the local wildlife, and therefore they needed a different solution. Eventually, they decided to use a low-friction coating, which would mean that when the algal growth became too heavy, it would fall off the blade itself, due to gravity.
SIMEC Atlantis is also planning to create a massive tidal power plant in the Scottish Highlands, using the same turbine technology, but with more efficient turbines, and more turbines. In the initial phase, 8 turbines, each with the capacity to generate 1.5MW of power, have been installed. The agreement signed with the Scottish Government gives SIMEC the right to create a power plant which can generate up to 400MW of electricity, which would make it the largest tidal power plant in the world.
As with SeaGen, MeyGen has faced significant environmental concerns, the greatest of which concerns damage to marine mammals. Due to the noise, seals will completely avoid going within 40m of the turbines, and may avoid going within 200m of the turbines. Because of the large number of seal beaches nearby, the placement of the turbines needs to be carefully thought out, so that seals are not driven from the beaches and still have a place to breed. Whales are even more likely to avoid the turbines, and will not go within 500m of a turbine. While at MeyGen the placement of the turbines does not limit their feeding grounds, this will need to be taken into account in the future. The greatest environmental impact is likely to happen not during the use of the turbines, but beforehand, during construction, when noise is much louder, and may disrupt seal and whale activity greatly.
Overall, tidal turbines are a useful and promising new technology, which can help fill the gaps in electricity generation in an energy mix dominated by wind and solar. In addition, there is huge potential for electricity generation offshore in some countries: the UK could theoretically, if its total tidal resources were exploited, generate between 25-30GW (mostly in the Severn estuary), which would make up 12% of the UK's electricity demand. However, for most countries, especially landlocked countries, total tidal reserves are just too small to be a major part of their energy mix, especially with rapidly increasing electricity demands. Therefore, tidal power will only play a major role in the future supply of electricity in some countries, such as the UK and Russia, where tidal power potential is high.
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