Tidal Stream Energy

Tidal streams are water movements caused by forces of gravity between the earth, the moon and the sun, and to a much lesser extent other planets. It is a purely astronomical phenomenon that is observable because the Earth rotates and has oceans.

Tidal current energy converters capture the kinetic energy of the tidal streams by driving a generator that converts the energy into electricity.

The Resource Potential

Tidal stream energy is as of yet a largely untapped resource with no commercial size application, although some are very close. According to a DTI report, the world's accessible tidal stream resources are around 90GW, representing 3% of the total tidal stream energy. That's less than the globally installed wind capacity in 2010. The best sites can be found in Korea, UK and North America. In fact, in the UK alone there is potential for 5 - 16GW that could account for up to 15% of the UK's electricity.

 

 

How much power is in tidal streams?

Power density - comparison between rivers and tidal streamsSimilar to wind power, the power in tidal streams,

depends on the density of water (ρ = 1,023kg/m³), a cross section of the flow and the cubed velocity of the flow.Good tidal stream sites have maximum flow speeds of 4 - 5 m/s, five times the flow speed in non-tidal rivers like the Amazon river, or a 100 times more power per square meter!

Tides are greatly influenced by size and location of land masses as well as the shape and depth of the ocean floors. 90% of tidal stream energy is found in deep waters (> 40m), but not on ground level. Luckily, some "good" sites are near shores rather than far away.

Tidal Stream Characteristics

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Where tidal streams occur, they are highly predictable, driven by patterns caused by the movements of the earth and moon.

Sun - Earth: Solar Tides

Within 24h, gravitational forces between the sun and Earth cause two high tides and two low tides. Maximum currents, and therefore maximum power, are reached at mid tide, every 6 hours (1). As the peak speeds when flowing in are higher than when flowing out, we can observe a 12-hour pattern (2). Plus, the two daily tidal bulges differ, accounting for a 24h pattern (3).

The seasonal tilt of the Earth (4) is responsible for maximum peaks around the equinoxes and minimal during solistices.

Finally, because of the elliptic shape of the Earth's orbit around the Sun, peaks observed during June solistice (when Earth is furthest away from the Sun) are less than during December solistice (5).

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Moon - Earth: Lunar Tides

Similar to solar tides, though following the lunar cycle of 24h 50' 24" (6,7,8). Due to the differences in mass and distance of Sun and Moon, the power of lunar tides outweighs the solar tides by around 7:3 with the dominant pattern following the 6h 12' 24" cycle.

The Moon- Earth- Sun- Constellation

Neap tides occur every 14 days and 18 hours when the Moon's orbit is at 90° to that of the sun, thus cancelling out some of the resulting tidal stream power (9). Maximum peak flows can be observed during spring tides when all three are aligned. During every other spring tide (i.e. every 29 days) Moon and Sun are on the same side of the Earth with combined gravitational pull causing another increase in the maximum peak flow (10).

So What?

Tidal stream power patterns are dominated by the 25h lunar day cycle, which means tidal power is not correlated to electricity demand that follows a 24h cycle. However, variations are highly predictable, and monthly averages are almost constant.

The amplitude, however, and therefore the available energy, of a stream varies hugely from site to site.

Technology Design Decisions
Whilst the underlying principle for converting a stream into a rotary movement driving a generator has long been applied in hydro or wind turbines, technical designs in tidal may still vary greatly due to some of the features of tidal streams. As with other marine technologies, the biggest challenge may not be the mechanical efficiency of the device, but the ease with which it can be installed or maintained.
Rotor Axis Direction
  • Horizontal Axis: The most advanced tidal devices are based on the same prinicple as most wind turbines. Most prominent example is Marine Current Turbines
  • Vertical Axis: Due to their geometry, these devices can cover a wider area in shallow waters. Their lower cut-in speed (in comparison to horizontal axis) is less of a problem in tidal because there is little energy lost.
  • Oscillating Device: The aerofoile-shaped blades of these devices are parallel to the water surface, and perpendicular to the stream. They osciallate up and down - that movement then drives the generator. Most prominent example is Pulse Tidal.
Anchoring & installation
  • Ground Mounted: Some devices are mounted directly on the seabed. With depths of hundred or more meters, this means that long pylons are not required. Disadvantage: Maximum speeds are rarely encountered close to the seabed.
  • Pylon: This is the "wind turbine under water" principle, where one or more devices are attached to one pylon. The device may be completely or paritally submerged
  • Floating: Floating devices are attached by steel cable or similar to the seabed. Obvious advantage: Less material required.
  • Suspended: River devices may be suspended from regular bridges over the river.
Maintenance
Mechanisms for maintenance are of huge importance, as to limit scheduled downtime and reduce operation costs.

 

  • Float:Mechanisms which allow the device to be floated
  • Raise above sea level: With help of some mechanism.
  • Crane: from a vessle.
Control Mechanism

Axial flow rotors can be stopped or limited to the rated power by pitching the blades to neutral position, dissipating the energy without increasing the thrust. Other mechanisms include hydraulic brakes.

 

 

 

 

Generator
  • Synchronous generator: Does not require a gearbox, but is more expensive. Works particularly well with vertical axis machines where the rotor frequency is low.
  • Asynchronous generators - mostly with gearbox. Disadvantage: mechanical parts under water

 

Yaw Mechanism

All tidal devices need to operate bi-directionally.

 

Horizontal axis machines need to have a mechanism to reverse the rotor pitch to work efficiently in both flow directions. Vertical axis machines do not require a special meachnism.

 

 

Technology Pipeline

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The technology stack is largely divided into two camps: Horizontal and Vertical.

With no commercial installation as of yet, there are still many technologies developed with no clear winner yet.

The most advanced technology is Seagen, developed by Marine Current Turbines.

Most early-stage technology developers have some association with academic institutions. Funding tends to come from either public sources or strategic investors. Due to the long time horizon, this is a sector void of venture capital. Strategic investors include utility companies such as e-on, but also wind energy companies like Siemens.

A comprehensive database of current projects and their status has been compiled by the U.S. Department of Energy.

Global Spread

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Organisations that develop tidal stream technologies tend to be in countries with a significant resource, especially U.S., Canada and the UK.

The exception is South Korea with huge tidal resources, but no home-grown companies. Instead, UK- based Lunar Energy has entered a joint venture with a Korean power company.

Tidal stream energy has been recognized in the UK as a vital source in future. As a result, tidal stream energy generators receive 5 ROCs (renewable energy certificates) per MWh in comparison to 2 ROCs foroff-shore wind energy.

 

Challenges and Opportunities
Challenges
  • Most sites are in remote areas that cannot consume all the energy. Strengthening of the grid is required.
  • Environmental aspect: How much energy is acceptable to be taken out of a particular site? Unlike in the wind energy industry, there is no established process for environmental impact analysis.
  • New technology developers will need to proof that their devices are significantly more efficient, robust or cheaper to operate.
  • The market may be limited to a small number of countries.
  • Requirements for navigational clearance may hinder installations.
Opportunities
  • Largely untapped source with fairly low technology risk and synergies with off-shore wind industry.
  • Unlike solar, tidal stream energy lends itself to clusters of large-scale projects, a business model favoured by utility companies.
  • As tidal energy is uncorrelated to wind energy, tidal capacity leads to an increase in the total capacity credit of a renewable energy portfolio, and therefore a reduction in required fossil back-up capacity. This could be as much as 3.3% cost saving per kWh (according to Carbon Trust).
  • High capacity factors of > 40% help make the technology economically competitive.
Capacity credit is the reduction of installed power capacity at thermal power stations so that the probability of loss of load at peaks is not increased and the security of supply remains ensured. For low penetration of renewable energy within a portfolio, the capacity credit = renewable capacity x capacity factor.
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