Marine Renewable Energy (MRE)
The movement of water in the world’s oceans creates a vast store of kinetic energy. Marine Renewable Energy (MRE), also known as Marine Hydrokinetics (MHK), can be harnessed to generate electricity to power homes, transport and industries. MRE encompasses wave power — power from the movement of surface waves, tidal power — power from the kinetic energy of large bodies of moving water, ocean current power — power from the kinetic energy of ocean current, and ocean thermal energy conversion (OTEC) — power from the heat differential of different thermal layers within a body of water.
Ocean surface waves are generated by wind passing over the ocean surface. The friction between the wind and ocean surface causes energy to be transferred from the faster moving air to the surface layer of the ocean. Wave development depends on the length of ocean, or “fetch,” over which the wind blows in a constant direction. Longer fetches with higher wind velocities will produce larger waves. Waves can travel thousands of miles with little energy loss and can combine with waves from storms and other wind-driven events to create very energetic seas. The energy of ocean waves is concentrated at the surface and decays rapidly with depth.
Current energy can be captured from tidal channels, ocean currents, or rivers. Ocean current energy technologies capture the energy from the relatively constant flow of ocean currents, which are driven by several factors, including wind, bathymetry, and the rotation of the Earth, as well as water temperature, density, and salinity. Tidal energy technologies capture the energy from flow induced by the rise and fall of tides, which is driven by gravitational influence of the moon and sun on the earth’s oceans. Land or subsea constrictions, such as straits and inlets, can create high velocity currents at specific sites, making them suitable for electricity generation. Riverine energy technologies extract the kinetic energy from flowing water in rivers to generate electricity. Although not technically a marine resource, as part of the natural hydrological cycle, precipitation from drainage basins, groundwater springs, and snow melt feed rivers that flow towards lakes, seas, and oceans.
Ocean thermal energy conversion (OTEC) technology converts solar energy stored in the layers of the tropical and subtropical oceans. Thermal heat engines use the temperature difference between the sun-warmed surface water and cold water in the deep ocean.
Salinity gradient technologies generate electricity from the chemical pressure differential created by differences in ionic concentration between freshwater and saltwater. Seawater has a higher osmotic pressure than freshwater due to its high concentration of salt. Two main technology types, Reverse Electro Dialysis (RED) and Pressure Retarded Osmosis (PRO), make use of semi-permeable membranes which generate an osmotic potential that can be used to generate electricity using turbines in deltas or fjords.