Thoughts of a Lapsed Physicist

(Note: this is a continuation of the previous blog on ocean energy)

Part 2: Ocean Current Energy, Tidal Energy, OTEC
A second ocean energy technology receiving attention is marine current power. As with wave power useful energy is derived from the kinetic energy found in the oceans, but in this case it is derived from ocean currents flowing beneath the ocean’s surface. An example is the Gulf Stream that flows around Florida in the U.S. Other areas with high marine current flows that can be usefully tapped by underwater ‘turbines’ are between islands, around headlands, and entrances to bays and large harbors.

While few studies have been carried out to date on this resource’s global potential, one 2000 study (Blue Energy) put the number at 450 GW. A 2006 study from the U.S. Department of the Interior estimates that capturing just 1 part in 1000 of the available kinetic energy of the Gulf Stream would supply 1/3 of Florida’s electrical demand. Ocean current energy also offers two significant advantages over other renewable energy technologies – it can be utilized with little environmental impact and is a reasonably predictable energy resource, lending itself to base-load applications.

A related form of ocean energy is tidal energy which taps the kinetic energy in a fast-flowing body of water created by tides. Generally, strong tidal flows exist where the water depth is relatively shallow and there is a broad vertical range beneath high and low tides. These tides are created by gravitational interactions among the earth, moon and sun, and are also impacted by the earth’s rotation and regional ocean temperature differences. One form of tidal energy (barrage) captures water at high tide and releases it at low tide, a form of hydroelectricity.

While still in its infancy, and geographically limited, there are currently two operating commercial tidal power plants (in La Rance, France and Nova Scotia, Canada), with a third pilot plant being tested in Russia. While the U.S. has few attractive sites (there is one operational site in the East River of New York City), locations in Russis, Canada, France, and the UK offer much more potential.

Finally, I will say a few words about OTEC, a technology that was first demonstrated in the 1880’s and continues to be of interest today. The world’s only currently operating OTEC plant is in Japan (100 KW).

Wikipedia concisely describes the technology:
“Ocean thermal energy conversion (OTEC) uses the temperature difference between cooler deep and warmer shallow or surface ocean waters to run a heat engine and produce useful work, usually in the form of electricity. OTEC is a base load technology that allows for production of electricity on a constant basis. However, the temperature differential is small and this impacts the economic feasibility of ocean thermal energy for electricity generation.”

Geographically limited to the topics where surface water temperatures are highest (85-90F), maximum theoretical Carnot efficiencies are still in the low range of 6-7 percent, using cooling water from depths where the temperature is close to freezing. Actual efficiencies achieved to date are in the range 2-3 percent. Nevertheless, it is important to point out that a small percentage of a very large number (the thermal energy stored in the oceans) is still a large number. Where OTEC struggles is that the low conversion efficiencies require pumping large amounts of seawater and large heat exchanger surfaces to make the technology feasible. Large amounts of pumping impose requirements for large amounts of parasitic power on OTEC systems, and large heat exchangers built for reliability in ocean environments are expensive. To date these factors have kept the technology from commercial application.

The technology comes in two types, both of which require bringing near-freezing seawater to the ocean surface. One is the closed-cycle system that uses exchanged thermal energy to vaporize a fluid with a low boiling point (e.g., ammonia) to power a turbine to generate electricity (see below).

The second type is the open-cycle that directly vaporizes seawater by introducing it into a low-pressure container which causes it to expand and boil. The resulting ‘steam’ drives a low-pressure turbine-generator. This latter form of OTEC offers a significant benefit in addition to electricity – the steam is free of salt and other seawater contaminants and can be condensed as fresh water. In many applications, particularly for isolated island locations, the desalinized fresh water may be more valuable than the electricity.

It should also be noted that a hybrid version of OTEC has also been designed, incorporating features of both types. The OTEC-extracted deep, cold water can also be used for non-power applications such as air conditioning and aquaculture (chilled-soil agriculture, nutrient-rich biology).

Despite these challenges, tentative plans have been announced for plants off the coasts of China and the U.S. Navy base on the Indian Ocean island of Diego Garcia. A 10-MW closed cycle pilot plant has just become operational in Hawaii. Other OTEC power plants are also being considered.