Powering our society with the ocean tides
By: Andreas Verbruggen
Tags: Oceans, Water, Sustainability, Energy, Environment

On an astronomical scale, the Moon is our neighbor. It is so close that its gravity affects the Earth’s dynamics considerably. One of its most apparent influences is the ocean tides. Tides are largely a product of the Moon’s gravity combined with the Earth’s rotation. They are the rise and fall of water levels roughly twice or once a day or something in between (yes, tides don’t always occur twice a day). The differences in water level can be surprisingly big. For instance, in the Bay of Fundy located in Nova Scotia, Canada the difference can be more than 15 metres. That’s a four story high building completely disappearing in a little bit more than six hours. Of course in general, tides are not that extreme. However, locally tide ranges can differ significantly due to the local bathymetry (which is just a fancy word for what the ocean floor looks like). In some places, the tidal range is big enough and the corresponding water flows powerful enough to be used for power generation. The difference in height can be seen as potential energy while the water flows contain kinetic energy.

The Bay of Fundy during low tide and high tide.

Both these forms of energy, potential and kinetic, can be converted into electrical energy. On one hand, the potential energy can be used by building a power plant which basically acts as a hydroelectric dam, with the distinction that the water difference is provided by the tides instead of a down-flowing river. On the other hand, in places where the generated water flows are powerful enough, turbines could be directly placed into these streams with the advantage of not needing a sizable structure that holds back the water.

The greatest advantage of this form of renewable energy certainly is its reliability compared to other forms such as solar and wind power. The relative positions of the Earth, the Moon, and the Sun (which also has a significant influence on the tides) are accurately predictable for thousands of years to come. Next to that, tidal power plants have a long lifespan which can be more than a hundred years. The first large scale tidal power plant already opened in 1966 in Brittany, a region in France, and is still in operation today.

La Rance tidal power plant in Brittany, France.

Over the decades, countries such as South Korea, the UK, and Canada also installed limited amounts of tidal power plants. Today, there is a capacity of approximately 1 GW installed worldwide, less than a hundred of a percent of worldwide energy consumption. However, estimations for the potential energy available in the world’s tides range from 0.1 TW to 1 TW. In relative terms, tidal power could make out 0.3% to 3% of the world’s energy supply.

Unfortunately, this way of power generation also has its drawbacks. First of all, it can’t just be applied in any body of water with tides. The height difference between high tide and low tide has to be big enough to be able to generate a significant amount of power. Secondly, since it’s a technology that’s not been widely researched yet, costs and risk are still high despite already being used for half a century. This does not make it easy to find investments for projects that envision the use of tidal energy. Lastly and most importantly, it could just replace one form of environmental damage with another one. Although no air pollution is emitted during power generation, it can harm its direct environment. Changing natural occurring water flows and differences in heights could lead to the disturbance of marine life, alterations in water salinity could render perfect habitats uninhabitable for some species, and the EMF emissions could disturb navigational abilities of sharks, rays, and other fish. Nonetheless, if more research is done into these technologies and its impact on the environment, tidal energy could be nice addition to the world’s energy supply.

Today, the already installed but limited capacity mostly exists out of tidal power plants, called tidal barrages, in locations where the potential energy is readily available. However, different solutions to harvest the tidal power could be used in the future, even in places where the initial conditions don’t look so promising. In places were a tidal barrage would not be the optimal solution for environmental or economical reasons, tidal stream generators can be used if the current is strong enough. They have the advantages of not disturbing the environment too much and having lower investment costs since no great additional structures are required. More ambitious projects are also a possibility such as a dynamic tidal power plant. Such a power plant would exist out of a barrier extending perpendicular from a coast line with a minimum of 30 kilometres into the ocean or sea. The tides dynamics would then cause a water level difference to occur on the different sides of the barrier which can then be used to generate electric power. Such plants are estimated to generate up to 15 GW per plant which means that one of these plants could replace all the currently installed tidal power plants 15 times. Though this sounds promising, its environmental, economic, and political implications should not be underestimated. It would require quite some research, investments, and talking for such a project to ever become reality.

An illustration of what a dynamic tidal power plant would look like with the effect on water levels.

There’s still a long way ahead of us before the full potential of tidal energy is unlocked. Though it will not solve our energy problem all by itself, it could be a fitting piece in the bigger sustainable energy puzzle. If we ever want to become a real sustainable society, it is obvious that we should envision to stop burning dinosaurs and start investing more in alternative resources in every possible way. Even though today it still might sound as being far removed from your personal life, you can contribute by educating yourself, by investing in the right opportunities, and by talking to other people about your ideas. Hopefully there will come a day that we can look back and laugh about how silly it was that we were ever burning fossil fuels.