Most people think of solar collectors as the typical silicon-based photovoltaic cells so commonly seen on residential and commercial buildings. But the largest solar collector is the ocean itself. Each day, the sun provides the equivalent of 250 billion barrels of oil in the form of thermal energy to Earth's oceans [source: Energy Efficiency and Renewable Energy, "Ocean Power" Lesson Plan]. Converting all of that potential energy into electricity requires a process scientists call ocean thermal energy conversion, or OTEC for short.
There are three types of OTEC systems. Each takes advantage of the temperature differential that occurs between warm surface water and colder, deeper ocean water, but they do it in slightly different ways.
Closed cycle systems contain a fluid that has a low boiling point. Ammonia, for example, has a boiling point of -28.01 degrees Fahrenheit (-33.34 degrees Celsius). When it's exposed to warm seawater in the system's heat exchanger, it immediately boils away into ammonia gas. The expanding vapor passes over a turbine, causing it to turn. Then it travels to a second heat exchanger into which cold seawater has been pumped. When the hot ammonia vapor encounters the cold water, it condenses back into a liquid and is ready for another cycle.
Get Your Juice -- with a Side of Water
One advantage of open or hybrid OTEC systems is that, on the way to making electricity, they also produce fresh water -- another resource in short supply in certain areas of the world. A single 2-megawatt OTEC plant could, in theory, produce 14,118.3 cubic feet (4,300 cubic meters) of desalinated water each day [source: Energy Efficiency and Renewable Energy].
Open cycle systems operate on a different principle. They start with warm surface water, which is placed into a vacuum chamber. As the vacuum pump removes air to create a low-pressure environment, the warm seawater boils. The resulting steam is almost pure water, and like the steam produced in a coal-fired power plant, it can be used to drive a turbine. Cold seawater, pumped from the ocean depths, cools the steam and changes it back into water.
Hybrid systems, as the name suggests, combine the best of their open and closed cousins. First, warm seawater changes into steam in a low-pressure container. The steam then vaporizes a low-boiling-point fluid in a closed-cycle loop that drives a turbine.
Currently, OTEC systems produce relatively small amounts of electricity. But the potential is enormous. Some experts believe that OTEC could produce billions of watts of power in the coming decades. As we'll see in the next section, these are the kinds of numbers that have driven energy companies to invest heavily in all types of ocean-power technologies.