Anyone who has visited a beach can understand the importance of tides. At low tide, beach-goers can walk along the exposed shoreline or construct sandcastles; high tide brings water further up the beach, washing away the castles and forcing people to higher ground. When considering the use of the ocean for energy purposes, the ocean can produce two types of energy: thermal energy from the sun's heat, and mechanical energy from the tides and waves.
With renewables such as solar, hydro, geothermal, biomass, wind and tidal power, however, account for only about 13 per cent of global energy supplies with waves and tides barely measurable, according to the International Energy Agency, after oil, coal, gas and nuclear generation. The pioneering use of energy from the sea is a viable idea for those few areas which can utilize it.
Energy analysts have been taking a closer look at this form of energy supply and they believe that tidal power can only make a tiny contribution to the world's energy supply, because of the few suitable sites, the high construction costs and the risk of equipment destruction by saltwater corrosion. However, there are a few areas with the right conditions to produce tidal power. As of 2003, France and Canada own the largest tidal energy facilities.
Ocean mechanical energy is quite different from ocean thermal energy. Even though the sun affects all ocean activity, tides are driven primarily by the gravitational pull of the moon, and waves 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. A barrage (dam) is typically used to convert tidal energy into electricity by forcing the water through turbines, activating a generator.
For wave energy conversion, there are three basic systems:
- channel systems that funnel the waves into reservoirs;
- float systems that drive hydraulic pumps; and
- oscillating water column systems that use the waves to compress air within a container.
The mechanical power created from these systems either directly activates a generator or transfers to a working fluid, water, or air, which then drives a turbine/generator.
The most likely type of energy the ocean can produce is likely to be thermal energy from the sun's heat. 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. There are three types of electricity conversion systems: closed-cycle, open-cycle, and hybrid. Closed-cycle systems use the ocean's warm surface water to vaporize a working fluid, which has a low-boiling point, such as ammonia. The vapor expands and turns a turbine. The turbine then activates a generator to produce electricity. Open-cycle systems actually boil the seawater by operating at low pressures. This produces steam that passes through a turbine/generator. And hybrid systems combine both closed-cycle and open-cycle systems.
Marine power from tides and waves is an infant industry aiming to take advantage of the growing demand for clean renewable energy that reduces dependence on fossil fuels. As more research is done and new methods instituted for thermal and mechanical wave energy, it remains likely marine power generation could play a role, at the very least, in regional power production.