Solar, water and wind power represent the most promising, as well as the most powerful, sources of renewable energy. These energy sources flow around us constantly, as sunlight falls upon the Earth's surface; as rivers flow to the sea and the tides rise and fall and the surface of the ocean perpetually moves up and down in waves; and as the wind blows over the Earth’s surface.
These energy sources do not depend on extracting materials from the Earth, except those used in constructing the energy generation equipment. Solar power and the power contained in moving air and water result from the interplay of natural forces in the universe: solar radiation; the gravitational attraction of the moon which causes tidal variation; and the rotation of the Earth, as well as temperature and air pressure differentials which cause weather and wind.
These renewable energy sources that arise from natural forces are distributed worldwide. While there are regional and local differences in average winds, height of tidal rise and fall, and days of sunlight received annually, these energy sources have the potential to provide power for the Earth that is clean, inexhaustible, and environmentally sound.
Renewable Energy: Solar
Solar power usually refers to the conversion of sunlight into electricity, either through the photovoltaic effect, or by heating a fluid to produce steam to turn a generator. Solar photovoltaics, or solar cells, currently provide about .04% of the world’s energy usage. The price per watt of electricity generated has fallen significantly over time, as researchers have increased the efficiency of photovoltaic cells.
Solar cells generate power in large installations around the world, some of which cover many acres. Long used to power satellites and the Space Station, smaller-scale solar panels are providing power in a wide range of applications: roof-top installations on homes; cruising sailboats far at sea; highway signs; remote sensing stations; small, self-contained garden lights.
Solar thermal installations are large-scale arrays of mirrors or lenses that are programmed to move during the day so they track the sun through the sky. The light collected is beamed to a central receiver, often a tower, where fluid is circulated and heated by the focused beams of sunlight to produce steam. This steam is then used to turn power-generating turbines or to provide heat.
Renewable Energy: Hydro-electric and Tidal Power
The power of moving water has been used for millennia to operate grain mills, textile machines, sawmills and many other applications in which the torque of a turning water wheel can be usefully adapted. Of necessity, this use of water power required that the mill or machinery be located very near the moving water.
During the 20th century, dams were constructed around the world on small and large rivers so the impounded water could be used to turn large turbines to generate electricity. This produced low-cost energy that can be used far from the source. A more recent development is the design of microhydro generators – small but powerful generators that can be turned by small streams to power individual homes or businesses.
Modern research and development into harnessing the power of moving water is now looking at the enormous amount of water that is constantly moving in the ocean, in the form of tidal flows, tidal rise and fall, and wave motion.
Tidal flows, that usually occur twice a day in each direction, are predictable and powerful. There are many locations in the world where these flows are concentrated by passing between obstructions, for example at the entrance to bays and rivers, around headlands, and within narrow straits.
Some locations that appear promising include the Bay of Fundy in Nova Scotia, the East River in New York, the Strait of Gibralter, the Bosporus, Torres Strait in Australia, and many more. Prototype tidal stream power generating installations are being tested in the Strait of Messina, Italy; the Gold Coast, Queensland, Australia; off the coast at Devon, England; the Kvalsund, in Norway; and in the East River in New York City.
The potential power contained in tidal rise and fall may be tapped by constructing a barrage, a type of dam which has sluice gates that can open and close, across a strait or river that has a large tidal range. The gates, when shut, hold back the tidal flow and force it to fall through turbines, much as traditional hydroelectric power is produced.
Since the tides move in and out, the barrages are constructed so they can operate in both directions. Tidal barrages are currently in place in the Rance River, France; the Bay of Fundy, Nova Scotia; and Kislaya Guba, a fjord on the Kola Peninsula near Murmansk, Russia.
Wave power, the up and down motion of the surface of the ocean, is also a potential source for renewable energy. Pacific Gas and Electric recently announced plans to build America’s first commercial wave power plant off the coast of Northern California. It will consist of eight buoys anchored two miles offshore. Each buoy will generate electricity as it rises and falls with the waves. Other wave farms are planned or approved for locations off Portugal, northern Scotland, and the north coast of Cornwall, England.
Because tidal flows, the rise and fall of tides, and wave motion in the sea are so powerful – and subject to large variations due to storms and heavy weather – these approaches to power generation pose significant engineering and economic challenges. Yet the potential for generating power is enormous. Oceans cover three-fourths of the Earth’s surface and are in constant motion.
Renewable Energy: Wind
Wind has been used to power mills for many centuries, providing power to crush grain or to pump water. Holland’s picturesque windmills were built to pump the North Sea waters out lands enclosed by dikes in ambitious land reclamation projects.
More recently, wind turbines have been used to generate significant amounts of electricity: worldwide capacity of wind-powered generators was 74 gigawatts at the end of 2006. Wind power is produced in wind farms, large-scale arrays of towering windmills connected to electrical grids.
Although wind currently produces just over 1% of electricity globally, it accounts for 20% of electricity production in Denmark, 9% in Spain, and 7% in Germany. Worldwide, wind power generation quadrupled between 2000 and 2006 – currently the fastest-growing means of renewable energy production.
The U.S. has added more wind-generated energy to its power grid than any other country in recent years. Capacity grew by 3 gigawatts in 2007 alone, up 32% over the previous year. Tax incentives, accelerated depreciation of equipment, green credits and other financial measures are encouraging the rapid development of this renewable energy source.
The theoretical potential of wind power is much greater than current world energy consumption: over fifteen times the world’s total energy use and 40 times the current electricity use.
These are exciting times as renewable energy projects are being built worldwide. Research and development is producing cheaper and more efficient generating devices. Difficult policy and economic issues are being addressed, as renewable energy generation capacity is becoming more viable.
The books selected in the sections on Solar Power, Wind Power and Hydro-electric and Tidal Power provide an introduction to policy and economic issues concerning renewable energy generation projects, technological developments affecting solar, water and wind power; history of the growth of these alternative energies; and small-scale applications accessible to the average citizen who is interested in going “off the grid.”