While solar and wind options often dominate the alternative energy conversation, a rarely discussed, though increasingly popular, third option exists in gathering energy from the tides.
In fact, not only does tidal power have the potential to provide a extremely large amount of energy, it offers a few things even the sun and the wind can’t provide.
Among its many attributes, tidal power or marine and hydrokinetic power (MKP) is cyclical. Twice a day, rain or shine, this option produces a predictable and consistent amount of energy. Just how much energy can tidal power offer? A lot. In fact, the U.S. Department of Energy (DOE) estimates up to 1,400 TWh of potential tidal energy per year.
How it works
Researchers have designed a few different types of tidal power generation methods including tidal stream, barrages, and lagoon, but they all work on the same general principle of gathering energy via hydrokinetic energy conversion through turbines and generators. For example, tidal stream power works by taking advantage of the tidal streams created by the flow of the tides. This works in essentially the same way as any wind turbine on land, but the tides cause the underwater turbine to rotate — quite slowly due to the high density of water — which drives the generators and creates electricity. The turbines are generally 180 to 240 feet below the surface of the water, which lowers the risk of having them damaged by waves or boats, but also allows them to take advantage of the best sources of tidal streams.
The devices currently available on the market convert hydrokinetic energy into electricity generating AC power in amounts anywhere from 200 Kw to 2 MW with the more advanced turbines averaging between 1.5 and 2 MW, says Mark Baker, sales manager in renewables focusing on global marine renewables and UK solar project opportunities for General Electric. This electricity is then exported to the grid via variable speed drive converter technology. For reference, current wind turbine technology can offer up to 8 MW output per turbine.
It may sound like a whimsical bedtime story, but taking advantage of the tides to produce energy is an idea that goes back centuries. While hydropower currently provides about seven percent of the U.S.’s electricity, according to the DOE, it’s not exactly a groundbreaking idea.
As for using tidal energy specifically, evidence of “tide mills” dates back to the Middle ages with the earliest known tide mill cropping up in London during Roman times on the— now subterranean — River Fleet. Given its availability, harnessing water — or in the case of tidal stations, harnessing the moon and tides — has long been considered a solution to a lack of energy options.
Even the modern idea of tidal energy isn’t all that modern. The La Rance Tidal power plant in France was built between 1960 and 1966. This plant has 24 turbines and a peak rating of 240 MW. The La Rance is a barrage-style power plant, which means the water is captured by a dam as the tide flows in and released as it flows out. The turbines capture energy as tide ebbs and flows. Until recently, it was the largest tidal power plant, but was dethroned in 2011 by the Sihwa Lake Tidal power plant in South Korea, which has an output capacity of 254 MW. The other three largest plants include: Swansea Bay Tidal Lagoon with a maximum output of 240 MW (currently awaiting final planning consent); May Gen Tidal Energy project in Scotland with a maximum output of 86 MW, and the Annapolis Roral generating station in the Bay of Fundi with a maximum of 20 MW.
Tidal power vs. solar and wind
Solar and wind can provide a cleaner alternative to traditional energy sources, but both are dependent on unpredictable weather conditions. Locations for solar plants and wind farms are selected carefully in order to optimize energy collection, but it’s not a foolproof system. Additionally, the randomness of the energy collection means a suitable storage system is essential, which is an additional complication. Solar and wind are able to collect large amounts of energy under specific conditions, but since the available storage technology hasn’t evolved quickly enough, they require backup systems.
The challenges
Tidal power generation faces similar challenges to it’s alternative energy competitors, including location, price, and maintenance.
Even if the idea of a tidal plant is welcome, planning requires a consortium of investors and a solid supply chain. Developers and consortiums must carefully select their locations from limited geographical options to ensure it offers high tidal race flow rates or significant tidal head differentials, like the ones found in lagoons.
Unfortunately, tidal power plants can be expensive to install — the Swansea Bay Tidal Lagoon is a $1.5 billion investment — and the underwater location makes operations and maintenance a challenge.
Additionally, the systems can be quite expensive to build and maintain. Though the price of the devices varies depending on the size and design, prices can range up to $30 million for a pre-commercial test device.
One of the biggest challenges with tidal stream turbines is maintenance given that the devices are located underwater. While the underwater location means the system won’t detract from the natural aesthetics of the region, it means upkeep can be complicated and expensive.
Some turbine devices still depend on support vessels, which can have a price tag of up to $150,000 per day, others can be towed to shore for a cheaper support option, according to Baker. Smaller support vessels, specifically for the deployment and maintenance retrieval of tidal stream devices, are being designed with dynamic positioning systems, which helps lower costs.
The solution to maintenance problems is designing systems that are built to last in a very harsh environment. The longer devices can go without requiring service and maintenance — excluding unexpected breakdowns — the cheaper they are to operate. For example, Tidal Lagoon power stations are designed for more than 100 years of operations and tidal stream generation devices have to be designed with the capability of operating for five years without maintenance, says Baker. The turbines, made of composite material similar to those used in the aircraft industry, must be anti-corrosive and high-quality to withstand harsh marine environments.
Unfortunately, current installation and maintenance costs are still too high, prohibitively so in some cases, which means any new projects require a lot of collaboration between governments, corporations, and local citizens. Like any large infrastructure project, it can take years and a lot of money to secure approvals and additional time to build the facility.
The environmental effect
The environmental effects of solar and wind plants have been well-documented. For example, birds struggle to see the wind turbines and solar panels can magnify the sun’s rays in a way that can be detrimental to local animals.
Tidal power plants have a few different concerns. The turbines can slow the rate of the tides they’re designed to capture and even a slight change could be detrimental to local sea and plant life. In particular danger are migratory routes and breeding/nursery grounds for certain types of fish. Mudflats and salt-marshes, which serve as habitats for birds, can also be negatively affected by these facilities, according to research from the Proudman Oceanographic Laboratory and the University of Liverpool based on a study of the Irish Sea. Additionally, the turbines may offer the same issues as their on-land counterparts, creating a hazard that animals can run into.
The devices could also cause modifications to the tidal and residual flows, and more alarmingly, a reduced flow could result in a buildup of contaminants and chemicals.
Because each location for this type of power plant must be carefully selected for the right tides, each area comes with a unique ecosystem and the impact on this area must be carefully considered. The environmental effect of underwater turbines are still being studied by the DOE, but projects in the UK and Europe have proceeded with careful considertion as to how they will affect the local fish and wildlife.
Future tidal power solutions
The future of tidal power solutions is a little murky. Regulations still need to be established and the process of planning a plant is complicated and time-consuming. All plans require effective consortiums, the right investors, and an open dialogue with local environmentalists and citizens.
Though these systems reduce the amount of CO2 in the air and have the potential to provide clean energy, it is a major undertaking the harness the power of the tides.
