The small French island of Mont Saint-Michel, a UNESCO World Heritage site, is famous for its fast and forcefully rising tides. The famous writer Victor Hugo once claimed them to be as fast as galloping horses. Tidal energy is all about capturing this energy of the sea. While large tidal projects have been proposed for centuries, actual progress has been slow.
The back and forth of the sea is caused by the attraction of celestial bodies, notably the sun and the moon. The understanding of these gravitational forces enables us to make quite accurate tide predictions today. Nevertheless, water levels are still influenced by factors such as the coastline’s shape, weather conditions or local currents. It is this very feature that distinguishes tidal energy from other renewable energy sources, such as wind, wave and solar energy. They are not only hard to predict, but also incredibly variable.
Although we can find tides on coastlines around the world, not all locations are suitable for tidal energy generation. Sites need to have substantial volumes of tidal water flow, typically found either in a narrow channel or a bay inlet, to qualify. Depth is another site selection criteria, since insufficient water depth can perturb the water flow and thus the productivity of the tidal project. An adequate sea bottom is essential to the design of tidal energy projects: For a tidal dam, for example, flat profiles are needed. Ideally, a site will be easy to access for maintenance and situated close to a location where repair can be performed as required. Like with any other offshore energy, the closer the site is located to a power distribution network, the better. Some people claim that it is this list of requirements that limits the amount of adequate sites and restrains the growth of tidal energy.
The technology for energy extraction from tides can be divided into two main categories. While one is based on the potential energy induced by the differences in water level, the other is based on the kinetic energy of water in motion. Dams for example are based on the concept of potential energy, as it is characterized by different water levels at each side. By allowing water to flow from the higher water level to the lower water level and directing it through a turbine, electric energy is generated. There are currently two tidal dams in operation: La Rance in France and Annapolis Royal in Canada. The UK has pondered the adoption of a similar project for several decades, the Severn Barrage, capable of generating about 5% of the UK’s electricity consumption, but has so far not moved forward.
But why has a giant potential energy project not been tapped yet? As so often with a mega project of this kind, environmental impact is a huge obstacle. Potential sites for tidal dams are generally marked by ecological systems based on a dynamic equilibrium between fresh and salt water. As the construction of a dam changes the tidal reach and causes the water level behind the dam to rise, it disturbs this fragile equilibrium. Consequently the habitat’s flora and fauna can become affected, as well as the migration of sea life. Furthermore, obstructing the water flow will affect sedimentation, since little silt particles cannot be kept in suspense when water slows down and can alter the coastline.
Nevertheless, some people would argue that a rise in a bay’s sea level would occur independently of a dam being built or not. Additionally, the environmental benefit and carbon offset of this type of damn is large enough to affect the thinking of ecological systems’ advocates.
The devices that extract the tide’s kinetic energy do not require as large civil structures as a dam. With tidal stream technology, the energy of flowing sea water is caught directly by turbines. Although this emerging technology has not yet settled on a single concept, some of the proposals are very similar to the technology used to extract wind energy. As a matter of fact, these are sometimes referred to as submerged wind turbines. Today, various types of devices are being proposed, including horizontal turbines, vertical turbines, cross flow units and even reciprocating devices. Ideally, these devices can generate energy electricity both in ebb and flow streams. Some of the main challenges for this technology have to do with the design of a device’s foundation, which is crucial to its viability. Even more important is the efficiency of a unit’s installation and maintenance. Access for repair works is an issue, especially when the turbines need to be brought to surface or shore.
As with other renewables, the competitiveness of tidal energy depends on the industry’s ability to drive down costs. This is likely to be achieved through technological progress and the accumulation of practical experience. However, it is important to realize that tidal energy has one advantage in comparison to its green alternatives, namely its relatively predictable character. This can give it a distinctive role in a country’s energy mix, especially when it can provide large amounts of energy like the 5% of UK energy consumption of a potential Severn type barrage. The question is, how much is society prepared to pay for its energy, not only in money but also in terms of sustainability. It will be interesting to follow the development of tidal energy.
Image: Tidal Energy via Shutterstock