Wave and Tidal

The ocean can produce two types of energy: thermal energy from the sun’s heat, and mechanical energy from the tides and waves. To date deployment is limited as oceans are a difficult environment to harness energy. The ocean carries enough energy to sustain every country on Earth.

Oceans cover more than 70% of Earth’s surface, making them the world’s largest solar collectors. The sun’s heat warms the surface water a lot more than the deep ocean water, and this temperature difference creates thermal energy. Just a small portion of the heat trapped in the ocean could power the world. Ocean thermal energy is used for many applications, including electricity generation.

Ocean mechanical energy is driven by tides which are moving due to the gravitational pull of the moon, and waves which are driven primarily by the winds. As a result, tides and waves are intermittent sources of energy, while ocean thermal energy is fairly constant. Also, unlike thermal energy, the electricity conversion of both tidal and wave energy usually involves mechanical devices.

Australian thermal energy resource

Best areas available for ocean thermal energy is in the northern parts of Australia.

Ocean Thermal Technologies

  • Use the temperature gradients and a working fluid with a low boiling point (i.e. ammonia)
  • Use the surface water to generate vapour and the cooler water below to condense it again (essentially a reverse heat pump)

Wave Energy

Wave power is the transport of energy by ocean surface waves, and the capture of that energy is to do useful work – for example, electricity generation, water desalination, or the pumping of water (into reservoirs). Machinery able to exploit wave power is generally known as a wave energy converter (WEC).

 4.1 wave energy

Source: CSIRO

Power in waves (kW per m of wave front is written as

P = 0.49 TeHs2

Where Te is the mean wave energy period, measured in seconds per interval and Hs is the significant wave height in metres (upper third of the instantaneous wave field). An approximation based on assumed fetch.

Australia’s Wave Resource

The CSIRO identified Western Australia, Victoria, South Australia and Tasmania as having the best wave resources. These also happen to be near our largest energy markets. However in these areas there are differing demographics and existing energy resources which may affect take-up of wave energy.

Tasmania’s west coast alone is nearly 300km of constant waves which produce more than 12 times the state’s current consumption of energy each year. But its population size and the existence of hydroelectric sources mean that it’s not likely to need wave energy. That is unless it’s as an industry that can be linked via Bass Link, the high voltage DC line to the mainland.

Victoria, though it lacks the powerful wave resource of Tasmania’s west coast, is predicted by CSIRO modelling to have the greatest amount of wave energy. Victoria has high energy demands and needs to replace its brown-coal fuelled electricity supply with low-emission energy sources by 2050.

Wave Technologies

4.2 Oscilliating Water Column1) Oscilliating Water Column

  • Partly submerged structure fixed tot he sea floor or cliff face
  • Open below the surface and as teh waves move up and down, pushes the air through the turbined
  • Examples today:
    • Oceanlinx Mk1 – 2.5 MW (see video below)
    • Wells Turbine: a self rectifying turbine that works with air flow from either incoming wave forces air out of OWC or retracting waves sucking air back into OWC

2) Oscilliating Bodies

  • Either floating or submerged buoyant body that goes up and down with wave motion
  • Can have vrtical motion or pitching
  • Vertical motion can be used to drive a hydraulic fluid or a direct linear generator
  • Examples:
    • Carnegie CETO (WA) – movement of the buoys in ocean drive hydraulic cylinder with work transferred to motor/generator
    • Wave Rider (SA) – Floating structure with surface buoys that drive an axel turning a generator
    • Linear Generator (magnets drives induced current)
    • Hinged technologies
      • Pelamis (Portugal) – Accumulates pressure to drive power take-off unit4.2 Oscilliating Bodies
      • Mace
      • Searev
      • Oyster (UK)

3) Overtopping

  • Waves are channelled up a ramp into a reservoir which then drains through a low head hydraulic generator
  • Examples:
    • Wave Dragon

4.4 Overtopping

Tidal Energy

Tidal power is a form of hydropower that converts the energy of tides into useful forms of power – mainly electricity. Although not yet widely used, tidal power has potential for future electricity generation. Tides are more predictable than wind energy and solar power.

 4.5 Tidal Energy in Australia

Source: National Tidal Facility at http://www.bom.gov.au/oceanography/

Similar to wind, the kinetic energy is related to the density and velocity.

K = 0.5πr2Dv3

Where: v = 2 m/s, D = 1023 kgm3, r (blade length) = 16 m/w

4.6 Tidal TurbinesAustralia’s Tidal Resource

Tidal resource is modest compared to voerseas where tide can reach up to 5-8m. Tides reach 4m in the Kimberly (WA), 3m in Broad Sound (QLD) and 1.2m in Northern Tasmania.

Tidal Technologies

            1. 1. Tidal Barrage (hydro dam) is typically used to convert tidal energy into electricity by forcing the water through turbines, activating a generator
            2. 2. Tidal turbines

Advantages and Disadvantages

Advantages Disadvantage
Wave and Tidal
  • Minimum maintainance and monitoring required
  • Very reliable
  • Wave is predictable and not volatile like wind and solar. Hence, can be used as baseload power with the right energy mix
  • Small visual profile
  • Environmental impact can be lowered for those devices with no moving parts and no contaminants (i.e. Oceanlinx OWC)
  • Coastlines have many uses that compete with ability to harvest wave power, i.e. fishing, recreation, native title and land rights, shipping, etc.
  • Environmental risk such as change in local current and sediment patterns, risk to marine animals, leakage of hydraulic fluids and visual impacts
  • Multitude of legislation which governs Marine energy projects. Complexity in project approval and management
  • Challenges in deciding where to put wave energy converters (i.e. close to shore or away from shore)
  • Immature technology that is very expensive
  •  Best sites expensive and few locations
  • Electricity grid expensive
  • Low energy density

Current Legislative challenge

14 State and Federal Acts relating to oceans and marine environment. Currently no framework for approving and reviewing ocean technologies. This makes it difficult to navigate the current political landscape and complete against off-shore wind which is a more mature technology.

Trends in Australia – Wave after wave of cheap, clean power Australian Financial Review, Page: 2                        By Derek Parker Tuesday, 18 September 2012

The portfolio of renewable energy options is slowly expanding, with wave energy moving towards commercial viability following a series of successful test projects based on new technology in the field. “Although this sector started its real development a fair time after wind and solar, its rate of maturity is much greater due to advances in IT and computational capabilities,” says Ali Baghaei, chief executive officer of Oceanlinx, the company that has developed the new technology. The technology is known as the oscillating water column system. It might be compared to an artificial blowhole in which sea water is forced into a funnel by its own energy, creating a rush of air which turns a bi-directional turbine. As the water sweeps back, the air pressure differential turns the turbine again. “One of the key elements of this is that we get energy generated by the wave going both in and out,” Baghaei says. “That, plus the fact that the turbine is turned by air pressure rather than water, provides much greater efficiency and durability than other wave energy systems.”

Oceanlinx is now working on a facility known as the 1MW Commercial Wave Energy Demonstrator, which will be located 4 kilometres off the South Australian coast, not far from Port MacDonnell. The facility, which will connect to the national electricity grid, builds on the knowledge gained from a series of pilot projects at Port Kembla, NSW. The facility is designed to deliver 25 years of continuous operation and to survive one-in- 100-years storms. Significantly, it can be fabricated with off-the-shelf parts. “We learned a great deal from the Port Kembla projects, both about the technology and about issues such as scalability and grid connection,” Baghaei says. “We anticipate final installation by this time next year and grid connection by the end of 2013. We will reach full capacity by March 2014.” Operating at full capacity, the project will be able to generate more than 2.5 gigawatt hours of electricity a year with zero emissions.

Oceanlinx recently received a grant of just under $4 million from the Emerging Renewables Program, which will be about half of the capital expenditure for the Port MacDonnell project. “We see that as a substantial vote of faith in the technology,” Baghaei says. “Once the Port MacDonnell project is demonstrated successfully, a small array is predicted to produce power for 1 5q~ per kilowatt hour, much better than any other wave energy options and not far from the onshore wind cost of around 1 O~ per KWh. “Once we’re over the initial capital outlay, the costs should go down even further and put us in the range of fossil fuel costs, especially if we can start building arrays of 50MW or larger rather than standalone facilities.” Baghaei believes wave energy could provide up to 10 per cent of Australia’s total electricity needs before 2050, and much earlier if the grid infrastructure can be provided.

Sources: National Renewable Energy Laboratory and the Department of Energy at World.Com


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