Oceanic energy sources

Ocean currents

Ocean currents are largely solar driven, but appear to depend critically on the global climate. Unlike tidal schemes, projects that tap ocean currents can generate continuously.

The Gulf Stream is driven by tropical waters being heated and by salination increasing the seawater density as ice is formed at the Antarctic and off Greenland. There are American schemes afoot to tap the Gulf Stream with submerged water turbines, which sounds like a good idea at present. Because its not tidal, there are no gaps in the energy supply and hence no bulk electricity storage problem. However, if climate change shuts down the Gulf Stream, and there is evidence that this may occur soon, then all you are left with is a heap of junk growing seaweed at the bottom of the Atlantic. Not helpful.

There is a continual surface flow through the Strait of Gibraltar into the Mediterranean and a matching opposite flow of bottom water. The surface current was originally thought to be the result of replacing water lost by evaporation, but this was disproved when the lower, more saline, current was discovered. The outflow is driven by gravity as the dense, saline water pours down the western slopes into the Atlantic. The surface flow is required both to replace the outflow and to replace water lost by evaporation. Both flows are powerful enough to overcome tidal effects. In addition, the high ground on either side of the Strait funnels wind through it, raising the velocity by 2-3 Beaufort points in the throat of the strait. On the face of it, this would be another good place to install both submerged turbines and windmills to generate electricity. There are already wind farms on the Spanish coast and a floating bridge across the Strait has been proposed. It would carry both water and wind turbine generators.

Oceanic Thermal energy

Ocean Thermal Energy Conversion (OTEC)

The sun heats the upper layers of oceans and lakes as well as raising waves and driving currents and this temperature difference can also be exploited. The idea was first proposed by Jaques D'Arsonville in 1881 and later played a central role in The Shining ones, Arthur C Clarke's short story, written in 1962: this is probably the the inspiration for current OTEC projects.

Sea water from a depth of 1000m has a more or less constant temperature of 5°C while at the surface it is at 15°C or warmer depending on lattitude and, outside the tropics, the season. This temperature difference can be exploited, though as it is small, relatively large amounts of water must be passed through the system. Clarke suggested simply exploiting thermoelectric effects to generate electricity directly. However, all the experimental plants built to date have used the warm surface water to vapourise a working fluid with a low boiling point, such as water/ammonia mixture, which is used to drive a turbine. The turbine exhaust gas is then condensed to liquid form using cold bottom water and recycled within a closed system. The major advantage of OTEC over wind and tidal power generation is that the systems will provide continuous power at a more or less constant output level, though outside the tropics the output will show a seasonal variation as the temperature of surface water changes with the seasons. The main problems with OTEC are engineering ones:

In 1979 Lockheed Martin and Makai Ocean Engineering ran an experimental system in a barge off Hawaii that generated 50kW of electricity. In 1981 a Japanese group built a 120 kW experimental unit off Nauru. Work stopped at this point when a drop in oil price made OTEC un economic.

In 2008 rising oil prices again made OTEC economically attractive and several projects are being designed. Lockheed Martin and Makai Ocean Engineering are again involved in planning a 10-20MW plant to be installed off Waimanalo in Hawaii, using a 4m pipe to pick up cold water from 1000m down. Meanwhile OCEES International are about to build a plant for the US military base on Diego Garcia. This should produce 8MW of electricity and desalinate 4.73 million litres of seawater a day. Energy Harvesting Systems is planning a 100MW plant for Indonesia, though this system is said to be using OTEC generated electricity to make hydrogen, thus avoiding problems with the large submarine power cable that would otherwise be needed. However, Indonesia doesn't have a hydrogen economy....

Sea Water Air Conditioning (SWAC)

It is also possible to use cold bottom water as an energy-efficient way of cooling buildings in summer. Toronto has reduced its summer power consumption by 60 MW by pumping water at 4°C from a depth of 80m in Lake Ontario and using it to run air chilling systems. A similar system is planned for Honolulu. Makai appear to be one of the prime movers behind SWAC technology.