Humans have harnessed the power of moving water for thousands of years, but today hydropower energy is gaining a renewed interest as a source of clean, renewable energy. Hydropower dams are the most well-known type of hydropower energy, but several other methods of harnessing the power of moving water are under development. The Earth’s oceans are being looked to for new types of hydropower energy.
The idea of using water as a source of energy dates back to at least ancient Greece, where the force of flowing rivers was captured by water wheels and used to grind wheat into flour. Similar devices were developed in other parts of the world, and early European and American mills and factories were powered by water wheels. Until the late 19th century, water supplied only mechanical energy; the motion of water wheels was used to turn saws and other machinery.
In the late 1800s, rapid advances were made in hydroelectric power, a form of hydropower energy in which the motion of water is converted into electricity. The U.S. alone had over two hundred hydroelectric plants by the turn of the century, most of them located at medium-to-large dams. Today, most of the best locations for large hydroelectric plants have been exploited in the developed world, so new projects are mostly taking place in developing nations like China. Smaller scale hydroelectric plants are being looked at in both the developed and developing world.
The oceans of the world also contain many potential sources of power that the hydropower energy could tap into. Tidal power aims to exploit the changing levels of the oceans in coastal areas. Although a large tidal power plant began operating in France in the late 1960s, high construction costs and a limited number of suitable locations have prevented tidal power from gaining widespread use.
Another potential option is wave energy, which can be harnessed in several different ways. One demonstration project in Norway uses the motion of waves to push and pull air through a pipe, which in turn spins a turbine to generate electricity. Portugal, Australia, and the UK have all experimented with wave farms — groups of devices that generate electricity from wave power. Some other devices attempt to capture marine currents far beneath the surface of the water.
Ocean thermal energy conversion (OTEC) differs from other types of hydropower energy in that it does not capture energy directly from the motion of water. Instead, OTEC takes advantage of the temperature differences found in different depths of the ocean. Warm water on the surface of the ocean is pressurized and turned into steam, or used to heat another fluid which is turned into steam. The steam can then be used to power turbines and generate power, and cool water is then pumped from greater depths, turning the steam back into liquid and restarting the cycle. Experimental OTEC systems have been built in places like Hawaii, but pumping cold water from great depths reduces the efficiency of the system, making it difficult for OTEC systems to be cost effective.