Turning the Tide in Green Energy Production

In recent years, the need for energy production that does not rely on fossil fuels such as coal, oil and gas has become more urgent. Major headway has been made in the area of wind power; costs to produce electricity from wind have dropped considerably and it has become competitive with other sources of power. Electricity produced from solar power is in the early stages of a similar drive toward reduced production costs, larger installation base and higher power output cells. One innovative energy source that may not always be considered is tidal power.

What is Tidal Power?

Tidal turbines are, in principal, fairly analogous to wind turbines; the ebb and flow of the tides cause the turbine blades to spin and electricity is harnessed from this action, as depicted in Figure 1, below. The first wave of tidal power, called “barrages”, created artificial hydroelectric reservoirs that are refilled twice daily by rising tides. They accomplished this by installing the barrage across estuaries or bays, as shown in Figures 2 and 3, below.

Basic Diagram of a Tidal Barrage

La Rance Barrage in Normandy

Water-level view of La Rance Barrage

La Rance barrage in Normandy, pictured above, has been able to produce up to 240 megawatts of power since 1966. To put that into perspective the Cottonwood Creek Wind Farm southwest of Sweetwater, Texas, currently has 181 operating wind turbines and produces an equivalent amount of power. Additionally, Nova Scotia Power has been generating up to 20 megawatts of power since 1984 at a tidal barrage in the Bay of Fundy; home to the world’s highest tides.

Tidal Power has Great Potential

Tidal power, like wind power, is a renewable energy source; it is a clean source of power and does not generate any on-site emissions. While wind energy has been gaining momentum worldwide, tidal power has two major advantages over wind that may eventually make it more attractive as a means of generating electricity: predictability and location.

Wind power was, until recently not always able to harness energy predictably. There were, of course, isolated locations where the wind blew ‘all the time’ such as some mountaintop installs and the westward slopes of the Hawaiian Islands, but these are not the majority. Only recently, with the push toward 45-50 meter blades were wind structures able to move into the 200-300 meter altitude winds and make power generation more predicable. In contrast, the tides come in and go out everyday; this makes tidal power generation a predictable source of power from day one. Such reliability is key to creating an electric grid that is not just green, but also capable of consistently meeting the demands of a modern world.

According to the United Nations, over half the world’s population lives within 200km of the coast and of the 10 largest cities in the world, 8 are coastal cities. This proximity makes power transmission less complicated. Compare that to wind energy where, for obvious reasons, human settlements are not typically near high-wind regions. When they are nearby, many people have taken a ‘not in my backyard’ attitude towards large wind farms, calling them unsightly and noisy. Additionally, service and repair of wind structures can be costly due to their height and remote location.

Tidal Power Not Immune to Environmental Concerns
Historically speaking, tidal barrages have been criticized for causing environmental issues of their own. Barrages can impede fish migration; fish such as salmon that spawn in fresh water and then migrate to salt water would, necessarily pass through the turbines of an intervening barrage at least twice (once coming in from the sea, once when returning). The mortality rate for fish passing through the low-head turbine is about 6%. Fish ladders are sometimes provided as an alternative means of bypassing the dam, but the mortality rate of fish ladders is slightly higher than that of passing through the turbines and most fish avoid them.

Additionally, barrages change the size and location of the intertidal zone. The intertidal zone is the area that is alternatively wet and dry during the tidal cycles. The wet/dry habitat is unique and only certain types of plants and creatures thrive there. A barrage re-times the tidal cycle and changes the water levels, thereby “moving” the wet/dry intertidal zone, obliging the plant and animal life to adapt or “move” to the new location.

Beyond the environmental issues, barrages block navigation. A barrage is essentially a dam across an inlet or estuary and blocks passage to and from the ocean. Locks must be installed to allow for traffic to come into and leave ports surrounded by barrages .

Innovative New Solutions

New technology under development by Marine Current Turbines Ltd. of Bristol, UK, consists of twin axial flow rotors of 15m to 20m in diameter, each driving a generator via a gearbox much like a wind turbine. The twin power units of each system are mounted on wing-like extensions either side of a tubular steel monopile, which is set into the seabed. Because water is 800 times denser than air, marine current turbines are of a smaller profile watt-for-watt, when compared to wind turbines. Figure 4 shows a 300 kW marine current turbine in a maintenance position above the waterline.

Marine Current Turbine

An advantage of the monopile design is that it effectively addresses the main concerns regarding barrages. There is no permanent blockade of estuaries or navigation channels and fish migration is not impeded. Additionally, these generating stations do not artificially raise or lower the tide in a local region; instead they sit quietly under water, well below ship keels, rotating with the ebb and flow of the tides.

New installations of this type have already begun. In April 2007, five 35kW turbines were installed in the East River in New York City; future plans include a ‘turbine farm’ of approximately 100 such turbines. With the close proximity to coastal cities and no obvious environmental drawbacks, the next generation of tidal turbines appears ready to meet the world’s demand for clean, dependable electricity.