Energy Alternatives29 Jul 2011
The kinetic energy of the wind is thought to have carried Australian Aborigines from the mainland of southeast Asia to Australia 40 000 years ago. Travel by boat, using both muscle power and wind power, brought about the ?rst contacts between distant people and created trade routes for merchandise and knowledge. Early civilizations harnessed wind power for sailing, but it was not until about 2000 years ago that the ?rst windmills were constructed in China, Afghanistan, and Persia. The windmills were used mainly to pump water for irrigation of farmland. Over 1000 years ago, people of the country we now know as Holland used windmills to pump out large inland lakes that were prone to ?ooding. The land left behind became fertile farmland.
Wind power is currently used in many parts of the world to generate electricity. By 1996, the wind-turbine installation at Californias windy Altamont Pass had a total generating capacity of 3000 MW. This generating capacity is the same as the capacity of one of the Bruce nuclear plants in Ontario. A wind-farm test site, capable of generating 6 MW, has also been installed in Tarifa in southern Spain, a location known as a popular windsur?ng destination. The purpose of the test site was not only to determine the suitability of the site, but also to develop locally the required infrastructure of technologies, ?nancing, and materials. Should the tests be successful, the site will provide the community of Tarifa with the ability to implement an 8000 MW facility by 2005. The 6 MW site has already contributed power to the local electrical grid and reduced the use of local oil-?red plants. During the ?rst year of operation, the emissions of CO2 were reduced by about 12 000 t, SO2 by 5 t, and NOx gases by 4 t. Currently, Ontario operates a 0.6 MW facility in Tiverton.
Hydrogen Fuel Cells
The most promising, and currently the most actively researched, method of non-fossil fuel power generation is the fuel cell. A fuel cell requires hydrogen as fuel at the anode and oxygen, or air, at the cathode. A reaction splits the hydrogen atom into an electron and a proton in the presence of platinum, which acts as a catalyst. The electrons take a different path from the anode to the cathode than do the protons. The electrons may therefore be used as electric current. The only waste product from the reaction is water vapour.
The technology to create fuel cells was ?rst demonstrated at the University of Cambridge in 1839. It was not until the U.S. drive to put someone on the Moon that the technology was developed to a point of usefulness beyond a university laboratory. NASA ?rst considered fuel-cell technology simply to produce potable water for the astronauts during the long trip to the Moon. The ?nished fuel cell versions that ?ew into space were used to produce both water and electric power on board the space capsule.
Fuel cell research and large-scale production have been limited due to the need for large amounts of very expensive platinum. In the past, platinum sheets were required, but new technology allows engineers to coat cheaper materials with a very thin layer of platinum. This method produces a large surface area of platinum, required for sustained reactions, at signi?cantly reduced cost.
After World War II, nuclear power was retooled for peaceful uses. In 1955, a U.S. Navy submarine, Nautilus, travelled almost 100 000 km powered by the controlled nuclear ?ssion of a lump of uranium the size of a golf ball. Nuclear energy promised to be a clean, ef?cient energy source to meet rising global energy demands. Compared to coal, the amount of uranium that would need to be extracted from the ground was almost negligible. Further, electricity produced by nuclear ?ssion did not involve the release of greenhouse gases into the environment.
Nuclear ?ssion is the process of splitting extremely large atoms into two or more pieces, which releases an enormous amount of energy in the form of radiation or heat. The heat is used to boil water that eventually turns an electrical generator. Canadas CANDU reactor is a very popular choice worldwide because it has the capability of using unenriched uranium as fuel and has the fortunate record of never having had a catastrophic accident.
Nuclear power is currently used to generate approximately 16% of the global energy demand, which falls far short of the early visions of what nuclear power was to be. Nuclear power production faces two major problems: waste and safety. Some waste products of nuclear ?ssion are extremely radioactive and will remain that way for thousands of years. Proponents of nuclear power note that the most radioactive by-products disintegrate within 100 years, leaving the remaining waste products much less hazardous to living creatures. They also point out that all of the nuclear waste ever stockpiled in Canada would barely ?ll two ice hockey rinks to the height of the boards. The safety issue is not as easily dismissed. Memories of the 1979 Three Mile Island accident in Pennsylvania and the 1986 disaster at Chernobyl in the former U.S.S.R. do not soon fade. The catastrophic consequences of a fullblown core meltdown similar to Chernobyl have led Sweden and Germany to legislate the reduction of nuclear generating facilities in their countries. Canada, although never having experienced a serious nuclear accident, has also
scaled back reactor use because of both ?nancial and safety reasons. Unfortunately, green house gas-belching plants ?red by coal are now generating the electrical power that would have been generated by the nuclear plants.
Nuclear fusion could produce almost endless amounts of energy without greenhouse gas emissions or long-lived dangerous radioactive waste products. A 1000 MW fusion generator would have a yearly fuel consumption of only 150 kg of deuterium and 400 kg of lithium. However, scientists and engineers have yet to develop a method to sustain and control nuclear fusion reactions. Some researchers predict that within 40 to 50 years, fusion power will be available. Unfortunately, to date the only application of fusion is the hydrogen thermonuclear bomb. Will nuclear fusion, the energy that powers our Sun, become a mainstream power source?
The gravitational attraction of the moon as it orbits Earth causes bulges in the oceans nearest and farthest from the Moon. As our world rotates completely around on its axis once every 24 h, these two bulges become tides twice a day. Capturing high-tide waters, only to release them through turbines during low tide, is another method for generating electrical power from the ocean.
The ?rst modern tidal power generation plant, with a capacity of 0.04 MW, was constructed in China in 1956. Since then, eight more tidal power stations have been built in China, with a total generating capacity of 6.2 MW. Canada built North Americas ?rst tidal power facility in 1994. The 17.8 MW plant was built on the Annapolis River west of Halifax in Nova Scotia. The Bay of Fundy Tidal Power Review studied several potential sites throughout Nova Scotia and New Brunswick. One of the most promising sites could generate an estimated 3800 MW of power. However, there are no current plans to construct tidal facilities because of the high production costs and the availability of several untapped sites that are suitable for hydro-electricity generation.
Physicists have calculated that rising and falling tides dissipate energy at a rate of two to three million megawatts. Unfortunately, only a small fraction of that energy is recoverable, approximately 23 000 MW worldwide, or about 1% of the available power from hydro generation. This, and the fact that tidal facilities are viable in only a few locations around the world, means that tidal power will not become a global energy resource anytime soon.
Wave power devices serve coastal communities in two important ways. One type of device, created in Japan and called The Mighty Whale, is capable of extracting 60% of the waves energy and, in doing so, reducing the waves height by 80%. This provides both electric energy generation and shoreline protection. The sheltered region of shoreline may be suitable for aquaculture farming of speci?c ?sh species. A Norwegian plant was successfully generating 0.5 MW of power until 1988 when 10 m high storm swells destroyed the facility.
Electric energy generation using the gravitational potential energy of water is known as hydroelectric generation. In Ontario, seven separate facilities generate up to 2278 MW of power from the fast-moving waters approaching Niagara Falls. Ontarians are so familiar with the concept of generating power using water power that the term hydro is often used in place of electricity. The force of gravity does work on the water, pulling it down and providing it with a tremendous amount of kinetic energy. This kinetic energy is transformed into electric energy by very large turbines.
The production of electric energy in this way appears to be a perfect solution, with minimal cost to the environment. This perception is false. In northern Qubec, a massive hydro-electric generating dam project called the Canadian La Grande Complex was started in 1973. When completed, it will be the largest dam project in the world. The plan calls for the diversion of three rivers, reversing the ?ow of a fourth, and then channelling the water from all of those rivers into the La Grand-Rivire, which ?ows into James Bay. The Chinese government is currently constructing the Three Gorges Dam, which will be the largest single dam in the world when completed.
Such large reservoirs sometimes ?ood thousands of hectares of farmland. In other cases, they drastically alter the ecosystem with unknown consequences. For many years, engineers did not have the technology to economically use smaller reservoirs for generating electricity. With improvements in technology, smaller generation facilities are becoming much more popular. Table 6.6 lists the number and capacity of small hydro-electric facilities in selected countries throughout the world.
Two thirds of Earth is covered by water, undoubtedly the largest solar collector available. The waves that ripple the oceans surface only hint at the amount of energy collected and stored by the water. A better window for glimpsing the power that is stored as thermal energy in the oceans is to study the seasonal weather systems that form out in the middle of the Atlantic and Paci?c Oceans hurricanes and cyclones. The destructive power of hurricanes is demonstrated every year, as coastal communities are besieged by storms with winds of over 180 km/h. The ultimate source of the energy for these giant storms is the thermal energy of the ocean waters.
Tapping into the energy of the ocean in a reliable and predictable way has been approached in three basic ways wave power, tidal power, and ocean thermal power. Tides were used as early as the eleventh century along the coast of present-day England to operate ?our-grinding mills. Japan started researching the extraction of power from the ocean as a means to generate electricity as far back as 1945. Since then, engineers in several countries have studied numerous techniques for taking advantage of the power of the oceans. The invention of special turbines and hydraulic machines are opening the way toward practical uses of energy stored in the oceans.
In 1839, French physicist Edmund Becquerel (18201891) ?rst discovered that generating electric current directly from sunlight was possible. Another 100 years would pass before commercial applications of the technology would appear. Today, solar cells are used to power everything from calculators and watches to small cities. Sunlight, the fuel required by solar cells, is 100% free and allows for electric energy generation that is completely free of greenhouse gas emissions.
Photovoltaic cells are composed of semiconductors, such as silicon. The sunlight knocks an electron from the crystal structure. Impurities added to the semiconductor do not allow the electron to fall directly back into place. The liberated electron will therefore follow the path of least resistance, which in the case of semiconductors is an external circuit. The ?ow of electrons in the external circuit can be used directly or stored in batteries for later use.
Several different materials and techniques are used to create photovoltaic cells. Very expensive cells are able to convert sunlight into electrical energy with ef?ciencies approaching 50 percent. Other procedures use thin ?lms of amorphous silicon deposited on a variety of bases. These cells are much more economical to produce and have the advantage of being ?exible and more durable. The weakness of amorphous silicon cells is that they degrade with exposure to sunlight, losing up to 50 percent of their ef?ciency over time.
Photovoltaic cells have been used very successfully in space, to provide power for satellites and space stations. The variety of different types of solar collectors has also allowed successful implementations throughout the world, from pole to pole. Small-scale power generation, for road signs or individual homes is gaining popularity as storage battery systems improve. Large-scale solar power generation has been slower off the mark. This slow progress is due to the variability of sunlight. In addition, weather has a dramatic effect on a photovoltaic cells ability to generate electric current. During nighttime hours, energy must be supplied by solar energy stored during the day. This type of intermittent power supply is a drawback of most renewable sources.
Solar cells are widely used in developing countries, where systematic power-delivery infrastructure does not exist. Individual homes consume the power to cook and cool their food and heat their homes. Industrialized countries demand signi?cantly more power during the workday, when the sun is shining, than at night, so perhaps solar power generation will offer a supplement to help reduce the use of fossil fuels.
Geothermal energy is the energy recovered from Earths core. The thermal energy contained within Earths core results from energy trapped almost 5 billion years ago during the formation of the planet, and from the heating effects of naturally decaying radioactive elements. The rate of heat ?ow through Earths crust is 5000 times slower than the rate of energy arriving from the Sun. Therefore, Earths surface temperature is the result of energy from the Sun, and not the heat ?owing from its core.
Geothermal energy is not, in fact, a renewable energy source, because heat is extracted much more quickly than it is replaced. Although the total thermal energy is ?nite, the time it would take to deplete this resource would be measured
in millennia because of the enormous size of Earth. For this reason, geothermal energy is considered to be renewable.
The four basic geologic formations that allow for the extraction of geothermal energy are hydrothermal; geopressurized; hot, dry rock; and magma. The distribution of favourable sites around the globe tends to be localized near regions of geologic instability that often experience active volcanoes or tectonic movement.