Introduction to Ocean Energy Systems
What are Ocean Energy Systems (OES)?
Ocean Energy Systems transform the energy available in the marine environment into energy which may be used by mankind. The most obvious source is from waves. Other sources are marine and tidal currents, the temperature gradient between deep water and surface water, and the pressure gradient between seawater and fresh water, known as the salinity gradient.
There are many ways to capture and transform this energy. The first concept was patented in 1799 (Girard & Son, France). Systems can be deployed either at the shoreline or near shore (water depths < 40m) or offshore (water depths > 40m).
What are the main concepts?
There are numerous concepts under development but the following generalisations are applicable:
- Wave Energy: The energy stored in a wave is proportional to the wave height. The energy is transformed mechanically into electrical energy by means of:
- Oscillating Water Column:
At its simplest this is a column with one side in the water and the other open to the air. When the water level is displaced, the air column is also displaced. If an air turbine is placed in the open side of the column, the airflow will generate power. Such a column can be located either offshore or onshore.
- Over-topping device:
In this concept the wave rolls over a slope to fill an artificial basin. The captured water is then piped through a water turbine to generate electricity.
- Point Absorbers:
In this concept a "point", such as a floating buoy, responds to the sea surface movement. This movement is then converted into mechanical or hydraulic power.
- Marine and tidal current:
This device is very similar conceptually to a wind turbine in which the water flow drives rotating blades connected to a generator.
- Temperature Gradient:
The temperature difference between surface water and deep water can be used to capture energy in a conventional Rankine cycle.
- Salinity Gradient:
Energy can be captured by exploiting the pressure difference at the boundary between freshwater and saltwater.
What are the main OES sub-systems?
Sub-systems vary according to the location of devices.
Power Take-off system: Transforms the ocean energy source into mechanical or hydraulic energy.
Power Conversion system: Converts the mechanical or the hydraulic energy into electricity.
Housing structure: This is the enclosure in which all the sub-systems are housed (power offtake, electronics, control systems and data acquisition systems).
Supporting structure: This installation supports the housing structure. It can be either floating or fixed to the sea floor.
Mooring: This keeps the floating structure in position whatever the sea conditions.
Transmission: Energy produced may be transported ashore by electric cables or another energy carrier.
What are the operating principles of ocean energy systems?
- Wave Energy: Wave movement is transformed into a mechanical or a hydraulic force. A connecting "link" transmits the force to a power conversion system which generates electricity. The amount of power generated depends on the wave height and the nature of the conversion system.
- Marine and tidal currents: Ocean currents are like underwater winds and can generate electricity by powering a conventional rotating element. The amount of power generated depends on the current speed and the rotor diameter.
- Ocean Thermal Energy Conversion: This uses the temperature difference between cold, deep seawater and warm, shallow seawater to source a thermodynamic cycle which can be used to generate electricity.
- Salinity Gradient: The difference in salinity between seawater and fresh water creates a pressure difference. If a semipermeable membrane is placed at the boundary between seawater and freshwater, the freshwater will slowly permeate through it by osmosis. Energy may be extracted by exploiting the pressure difference. The amount of energy extracted is proportional to this pressure difference.
What are the current uses of OES technology?
- It is an energy source for the 'post-fossil fuel era'.
- Prototypes and models have been built and tested for short periods since the 1970s. Long-term tests have been conducted on only two devices and just one has been in operation for one year. Both have been funded under EC Framework Programme projects. However, neither has achieved commercial viability because of the technical difficulties in adapting to marine conditions.
- The technology has improved as a result of national and international R&D. At current technologies power generation costs are estimated at around 0.25€/kWh for a one-off prototype. But once this technology is proved costs could fall to below 0.04€/kWh, opening up investment opportunities and renewed interest worldwide.
- The installed capacity of Ocean Energy Systems currently under development totals 900kWe. These only produce electricity.