Wind Resources

World wind energy capacity has been doubling every three years during the last decade and growth rates in the last two years have been even faster. It is doubtful whether any other energy technology is growing, or has grown, at such a rate. Total world wind capacity at the end of 2003 was around 27,500 MW and generation from wind now approximately equates to annual consumption of electricity in Chile or Singapore. Germany, with over 9,000 MW, has the highest capacity but Denmark, with over 4,000 MW, has the highest level per capita and the production accounts for about 20% of Danish electricity.

Wind energy is recognized worldwide as a proven technology to meet increasing electricity demands in a sustainable and clean way. Offshore wind energy has the added attraction that it has minimal environmental effects and, broadly speaking, the best resources are reasonably well located relative to the centers of electricity demand. Moreover, higher wind speeds at sea mean an increased energy production, as energy output is a function of the cube of wind speed. Average offshore wind energy increases range from 10-20%.

Offshore wind velocity in the U.S. varies with latitude, usually higher in latitude, the greater in wind velocity. The East and West coasts of the U.S. have optimum wind conditions; the Gulf of Mexico has minimal wind conditions.

Ocean Current Resources

The global marine current energy resource is mostly driven by the tides and to a lesser extent by thermal and density effects. The tides cause water to flow inwards twice each day (flood tide) and seawards twice each day (ebb tide) with a period of approximately 12 hours and 24 minutes (a semi-diurnal tide), or once both inwards and seawards in approximately 24 hours and 48 minutes (a diurnal tide). In most locations the tides are a combination of the semi-diurnal and diurnal effects, with the tide being named after the most dominant type.

The strength of the currents varies, depending on the proximity of the moon and sun relative to Earth. The magnitude of the tide-generating force is about 68% moon and 32% sun due to their respective masses and distance from Earth. Where the semi-diurnal tide is dominant, the largest marine currents occur at new moon and full moon (spring tides) and the lowest at the first and third quarters of the moon (neap tides). With diurnal tides, the current strength varies with the declination of the moon (position of the moon relative to the equator). The largest currents occur at the extreme declination of the moon and lowest currents at zero declination. Further differences occur due to changes between the distances of the moon and sun from Earth, their relative positions with reference to Earth and varying angles of declination. These occur with a periodicity of two weeks, one month, one year or longer, and are entirely predictable.

Generally the marine current resource follows a sinusoidal curve with the largest currents generated during the mid-tide. The ebb tide often has slightly larger currents than the flood tide. At the turn of the tide (slack tide), the marine currents stop and change direction by approximately 180°.

The strength of the marine currents generated by the tide varies, depending on the position of a site on the earth, the shape of the coastline and the bathymetry (shape of the sea bed). Along straight coastlines and in the middle of deep oceans, the tidal range and marine currents are typically low. Generally, but not always, the strength of the currents is directly related to the tidal height of the location. However, in land-locked seas such as the Mediterranean, where the tidal range is small, some sizeable marine currents exist.

There are some locations where the water flows continuously in one direction only, and the strength is largely independent of the moon's phase. These currents are dependent on large thermal movements and run generally from the equator to cooler areas. The most obvious example is the Gulf Stream, which moves approximately 60-80 million cubic meters of water per second. Another example is the Strait of Gibraltar where in the upper layer, a constant flow of water passes into the Mediterranean basin from the Atlantic (and a constant outflow in the lower layer).

Areas that typically experience high marine current flows are in narrow straits, between islands and around headlands. Entrances to lochs, bays and large harbors often also have high marine current flows. Generally the resource is largest where the water depth is relatively shallow and a good tidal range exists. In particular, large marine current flows exist where there is a significant phase difference between the tides that flow on either side of large islands.

There are many sites world-wide with velocities of 5 knots (2.5 m/s) and greater. Countries with an exceptionally high resource include the UK, Ireland, and Italy, the Philippines, Japan and parts of the United States. Few studies have been carried out to determine the total global marine current resource, although it is estimated to exceed 450 GW.