ECE 7800: Renewable Energy Systems Topic 16: Tidal Power Spring 2010 © Pritpal Singh, 2010

Acknowledgement Much of the material in this topic, including most of the figures, is taken from the book “Renewable Energy – Power for a Sustainable Future”, 2 nd edition, edited by Godfrey Boyle (Oxford University Press).

Introduction The ocean represents a renewable source of energy for coastal regions. Tidal power harnesses the energy of rising and falling tides to generate power. Small tidal mills were quite widely used on sections of rivers in the Middle Ages but only recently has the full power of using tides in estuaries been proposed.

Introduction (cont’d) H igh tide and low tide occur once a day. During high tide water comes into an estuary and during low tide water goes out. The basic concept of tidal power is to convert the water flowing in and out into electrical energy (see below): Source:

Introduction (cont’d) The first significant tidal project was a tidal power plant on the Rance Estuary in Brittany, France. This 240 MW plant came on line in 1966.

Introduction (cont’d) The first (and only) tidal power site in North America is the Annapolis Royal Generating Station, which opened in 1984 on an inlet of the Bay of Fundy. The North Atlantic waters are funneled up through a dam across the Annapolis River between the Canadian provinces of New Brunswick and Nova Scotia. Source:

Introduction (cont’d) Sluice gates are opened at high tide to allow water from the sea to flow into the river. The gates are then closed. When the water level in the ocean drops at low tide, the gates are opened and the water flows back into the sea turning a large turbine and generating a peak power of 20MW. Source:

Tides There are two components to the tides: 1)A centrifugal effect 2)A gravitational effect The centrifugal effect causes a larger bulge on the side away from the moon due to the rotation of the earth, whereas the gravitational effect causes a larger bulge on the moon side of the earth. This results in approximately equal bulges on both sides of the earth.

Tides (cont’d) The gravitational attraction of the moon on the oceans (and to a lesser extent the Sun) changes about every two weeks (half a lunar cycle). When the Sun and moon pull at 90º to each other, the tides are relatively low (neap tides). When they pull together, the tides are much higher (spring tides).

Tides (cont’d) In mid-ocean, the typical tidal range is ~ 0.5 m. However, in coastal regions, tidal changes due to local topographic variations can be much larger. As the tide approaches the shore and the water level gets shallower, the tidal flow is more concentrated and can be much higher, typically ~ 3 m. In a suitably shaped estuary, the water can be funneled, leading to further amplification of the tidal water levels to 10-15m at some sites (e.g. the Severn estuary) (see next slide).

Tides (cont’d)

Tidal Power Generation Tidal barrages (dams) across suitable estuaries are designed to extract energy from the rise and fall of the tides using turbines located in the barrages (as shown earlier). The amount of power that can be extracted depends on the amount of water flowing in and out of the estuary.

Tidal Power Generation (cont’d) Consider a rectangular basin behind a barrage of constant surface area A and a high-to-low range R, see figure below:

Tidal Power Generation (cont’d) Thus, the mass of water that flows through the turbines with the change of tides = ρAR, where ρ = density of water. Thus the max. potential energy change, assuming the entire mass falls a distance R/2 (from its center of gravity) is given by: (PE) max = ρAR x gR/2 = ρgAR 2 /2 Of course, the larger is R, the more energy can be converted.

Tidal Power Generation (cont’d) The basic technology for generating power is similar to that used in low- head hydroelectric plants. An artist’s impression of a typical power generating scheme is shown below:

Tidal Power Generation (cont’d) There are several ways of implementing the designs. The approach used at La Rance comprises a turbine generator mounted in a bulb-shaped enclosure. The water flows around the bulb and must be interrupted for maintenance.

Tidal Power Generation (cont’d) These problems are avoided in a rim generator (or Straflo) type turbine (the type used in the Annapolis Royal plant). Here the generator is mounted radially around the rim and only the runner (turbine blades) are in the flow. Although more efficient than the bulb-type, sealing issues b/w the runner blades and the radial generator is a problem.

Tidal Power Generation (cont’d) Another approach is to use a tubular turbine configuration. Here the runner is set at an angle so that the power can be fed to an externally mounted generator (see below).

Tidal Power Generation (cont’d) Although this type of design has been used in hydroelectric plants, the long runners needed for tidal power plants results in significant vibration problems. Thus the most popular type of turbine used have been bulb-type.

Environmental Factors The construction of a large barrier across an estuary will obviously have a large impact on the local ecosystem. It particularly impacts fish and birds. Some birds feed in mud flats exposed by the water in the estuary. However, the turbidity of the water would be improved because of less silting of the estuary waters, resulting in more penetration of sunlight enabling more biological productivity of the water. This would result in more food being available for the birds.

Tidal Power Economics The economics of tidal power systems include the high initial capital costs of constructing the barrage and the plant itself. The running O&M costs must also be included in an economic assessment of a tidal power system. A proposed 8,640MW system across the Severn estuary near Bristol, in the U.K. is shown on the next slide. A cost breakdown for building this system is shown on the following slide.

Tidal Power Economics (cont’d)

In 2002, the total cost had inflated to £10-15 billion! Because the power is only generated once or twice a day, the effective capacity factor of such a plant is only ~23% compared to approx. 77% for nuclear power plants and about 84% for combined cycle gas turbines.

Tidal Power Economics (cont’d) The unit cost of electricity will vary with discount rate as shown below:

Tidal Power Economics (cont’d) This “discount rate” basis for assessing a tidal power plant (or large scale hydroelectric plant) is criticized because once the initial capital costs have been paid off, the plant will continue to generate power for more than 100 years with the low-speed turbines requiring replacement every 30 years. However, the large capital investment for the Severn estuary project (or one on the Mersey estuary) have not been forthcoming to date.

Tidal Streams Another approach to harnessing the energy of the tides is to use tidal streams. These are fast flowing volumes of water caused by the motion of the tide. These usually occur in a shallow sea where a natural constriction forces the water to speed up. The conceptual approach is very similar to wind power.

Tidal Streams (cont’d) However, water is 800 times denser than air and has a much slower flow rate but the turbine experiences much larger forces and moments. This results in turbines with much smaller diameters. The turbines must either be able to generate power on both ebbs of the tide or be able to withstand the structural strain. This technology is still in its infancy.

Tidal Streams (cont’d) Underwater water fluid farms (like wind farms) offer many advantages over other means of harnessing tidal power. The turbines are submerged in the water and are therefore out of sight. They don’t pose a problem for navigation and shipping and require the use of much less material in construction. They are also less harmful to the environment.

Tidal Streams (cont’d) The optimal water velocity is m/s. Above this level the turbine experiences heavy structural loads and below this insufficient power is generated. Artist’s impressions of tidal turbine farms are shown on the next two slides.

Tidal Streams (cont’d) Source:

Tidal Streams (cont’d) Source: Also see: