Tidal turbines work like underwater windmills: the twice-daily ebb and flow of the tide in coastal waters turns submerged rotors, and generators convert this rotation into electricity.
Tidal barrages work in much the same way as hydroelectric dams, but are built across coastal bays or estuaries instead of damming rivers. When the tide creates a significant enough difference between water levels on either side of the dam, water is allowed to flow through the dam, spinning turbines that drive generators to produce electricity. Wave power devices harness the up-and-down movement of ocean waves.
Marine turbines are installed on the seabed near the shore, where tidal seawater will flow over the turbine rotor blades. To generate sufficient electricity, the blades need to turn at a steady quick pace, similar to a wind turbine. Marine turbines are installed where marine currents flow rapidly because they are focussed by headlands, straits, inlets and narrow channels in the seabed. The rotor blades are shaped so that they turn with both the ebbing and flowing of each tide.
The spinning blades are attached to a generator, which uses an electromagnetic field to convert rotational mechanical energy into electrical current. The operation of the marine turbine is monitored remotely.
Marine turbines can provide a steady supply of electrical energy four times in every 24-hour period (the ebb and flow of two daily tides). But they cannot increase their output to meet sudden peaks in demand.
Electrical energy is brought to shore via an undersea cable and converted by a transformer to the correct voltage for transmission. The national grid uses high voltages to transmit electricity efficiently through the power lines to the homes and businesses that need it. Here other transformers reduce the voltage to a usable level.
Wave and tidal technology is at a very early stage and has not yet reached commercial deployment. However, the Department of Energy & Climate Change (DECC) has identified that these technologies have significant potential, with possibly 200-300MW deployed by 2020, and at the higher end of the range of estimates, up to 27GW by 2050.
Tidal current power has the potential to make a major contribution to future renewable energy sources. One of the benefits of generating electricity from tidal currents is their predictability. Because operators know when tides will be flowing, they know when the turbines will be producing electricity. Their disadvantage is the high cost compared to other forms of generating technology.
Initial investigations suggest the technology has a low environmental impact. Given that most of the structure will be below sea level, visual impact should be minimised as well.
Marine power is a renewable energy source. Building and maintaining the equipment produces carbon emissions, but generating electricity does not. All types of marine electrical generation are to some extent weather-dependent and thus intermittent. If the technology develops and costs fall sufficiently, it could make a significant contribution to the UK electricity supply.
However, many of the technologies are currently experimental or yet to be deployed on any large scale. Marine power is thus expected to remain among the more costly energy sources for some years to come. Significant investment in research and development over many years will be required to bring down these costs and help realise the potential of marine power as a large-scale contributor of electricity.