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Renewable energy is energy derived from resources that are regenerative or for all practical purposes cannot be depleted. For this reason, renewable energy sources are fundamentally different from fossil fuels, and do not produce as many greenhouse gases and other pollutants as fossil fuel combustion.

Mankind's traditional uses of wind, water, and solar energy are widespread in developed and developing countries; but the mass production of electricity using renewable energy sources has become more commonplace recently, reflecting the major threats of climate change due to pollution, exhaustion of fossil fuel, and the environmental, social and political risks of fossil fuels and nuclear power.

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• Renewable energy use

Renewable energy flows involve natural phenomena such as sunlight, wind, tides and geothermal heat. Each of these sources has unique characteristics which influence how and where they are used.

The majority of renewable energy technologies are directly or indirectly powered by the Sun. The Earth-Atmosphere system is in equilibrium such that heat radiation into space is equal to incoming solar radiation, the resulting level of energy within the Earth-Atmosphere system can roughly be described as the Earth's "climate."

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The hydrosphere (water) absorbs a major fraction of the incoming radiation. Most radiation is absorbed at low latitudes around the equator, but this energy is dissipated around the globe in the form of winds and ocean currents. Wave motion may play a role in the process of transferring mechanical energy between the atmosphere and the ocean through wind stress. Solar energy is also responsible for the distribution of precipitation which is tapped by hydroelectric projects, and for the growth of plants used to create biofuels.

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Wind powerAirflows can be used to run wind turbines and some are capable of producing 5 MW of power. Turbines with rated output of 1.5-3 MW have become the most common for commercial use. The power output of a turbine is a function of the cube of the wind speed, so as wind speed increases, power output increases dramatically. Areas where winds are stronger and more constant, such as offshore and high altitude sites, are preferred locations for wind farms.

Offshore wind turbines near Copenhagen

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Wind power is the fastest growing of the renewable energy technologies. Over the past decade, global installed maximum capacity increased from 2,500 MW in 1992 to just over 40,000 MW at the end of 2003, at an annual growth rate of near 30%. Due to the intermittency of wind resources, most deployed turbines in the EU produce electricity an average of 25% of their rated maximum power, (a load factor of 25%), but under favourable wind regimes some reach 35% or higher. The load factor is generally higher in winter. It would mean that a typical 5 MW turbine in the EU would have an average output of 1.7 MW.

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Globally, the long-term technical potential of wind energy is believed to be five times current production global energy comsumption or 40 times current electricity demand. This could require large amounts of land to be utilized for wind turbines, particularly in areas of higher wind resources. Offshore resources experience mean wind speeds of ~90% greater than that of land, so offshore resources could contribute substantially more energy. This number could also increase with higher altitude ground-based or airborne wind turbines.

Wind power is renewable and produces no greenhouse gases during operation, such as carbon dioxide and methane.

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Wind strengths near the Earth's surface vary and thus cannot guarantee continuous power unless combined with other energy sources or storage systems. Some estimates suggest that 1,000 MW of conventional wind generation capacity can be relied on for just 333 MW of continuous power. While this might change as technology evolves, advocates have suggested incorporating wind power with other power sources, or the use of energy storage techniques, with this in mind. It is best used in the context of a system that has significant reserve capacity such as hydro, or reserve load, such as a desalination plant, to mitigate the economic effects of resource variability.

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• Water powerEnergy in water (in the form of motive energy or temperature differences) can be harnessed and used. Since water is about a thousand times denser than air, even a slow flowing stream of water, or moderate sea swell, can yield considerable amounts of energy.

There are many forms of water energy:

Hydroelectric energy is a term usually reserved for large-scale hydroelectric dams.

Micro hydro systems are hydroelectric system installations that typically produce up to 100 kW of power. They are often used in water rich areas as a Remote Area Power Supply (RAPS). There are many of these installations around the world, including several delivering around 50 kW in the Soloman Island.

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• Wave power uses the energy in waves. The waves will usually make large pontoons go up and down in the water, leaving an area with reduced wave height in the "shadow". Wave power has now reached commercialization.

• Tidal power captures energy from the tides in a vertical direction. Tides come in, raise water levels in a basin, and tides roll out. Around low tide, the water in the basin is discharged through a turbine.

• Tidal stream power captures energy from the flow of tides, usually using underwater plant resembling a small wind turbine. Tidal stream power demonstration projects exist, but large scale development requires additional capital.

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• Ocean thermal energy conversion (OTEC) uses the temperature difference between the warmer surface of the ocean and the colder lower recesses. To this end, it employs a cyclic heat engine. OTEC has not been field-tested on a large scale.

• Deep lake water cooling, although not technically an energy generation method, can save a lot of energy in summer. It uses submerged pipes as a heat sink for climate control systems. Lake-bottom water is a year-round local constant of about 4 °C.

• Blue energy is the reverse of desalination. This form of energy is in research.

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• Hydroelectricity is electricity produced by hydropower. Most hydroelectric power comes from the potential energy of dammed water driving a water turbine and generator, although less common variations use water's kinetic energy or dammed sources, such as tidal power. Hydroelectricity is a renewable energy source.

• The energy extracted from water depends not only on the volume but on the difference in height between the source and the water's outflow. This height difference is called the head. The amount of potential energy in water is proportional to the head. To obtain very high head, water for a hydraulic turbine may be run through a large pipe called a penstock.

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Advantages• The major advantage of hydro systems is elimination of

the cost of fuel. Hydroelectric plants are immune to price increases for fossil fuels such as oil, natural gas or coal, and do not require imported fuel. Hydroelectric plants tend to have longer lives than fuel-fired generation, with some plants now in service having been built 50 to 100 years ago. Operating labor cost is usually low since plants are automated and have few personnel on site during normal operation.

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Pumped storage plants currently provide the only commercially important means for energy storage on a scale useful for a utility. Low-value generation in off-peak times occurs because fossil-fuel and nuclear plants cannot be entirely shut down on a daily basis. This energy is used to store water that can be released during high load daily peaks. Operation of pumped-storage plants improves the daily load factor of the generation system.

Reservoirs created by hydroelectric schemes often provide facilities for water sports, and become tourist attractions in themselves. Multi-use dams installed for irrigation, flood control, or recreation, may have a hydroelectric plant added with relatively low construction cost, providing a useful revenue stream to offset the cost of dam operation.

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Disadvantages

Hydroelectric projects can be disruptive to surrounding aquatic ecosystems. For instance, studies have shown that dams along the Atlantic and Pacific coasts of North America have reduced salmon populations by preventing access to spawning grounds upstream, even though most dams in salmon habitat have fish ladders installed. Salmon smolt are also harmed on their migration to sea when they must pass through turbines. This has led to some areas barging smolt downstream during parts of the year. Turbine and power-plant designs that are easier on aquatic life are an active area of research. Large-scale hydroelectric dams, such as the Aswan Dam and the Three Gorges Dam, have created environmental problems both upstream and downstream.

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Generation of hydroelectric power impacts on the downstream river environment. Water exiting a turbine usually contains very little suspended sediment, which can lead to scouring of river beds and loss of riverbanks. Since turbines are often opened intermittently, rapid or even daily fluctuations in river flow are observed. For example, in the Grand Canyon, the daily cyclic flow variation caused by Glen Canyon Dam was found to be contributing to erosion of sand bars. Dissolved oxygen content of the water may change from pre-construction conditions. Water exiting from turbines is typically much colder than the pre-dam water, which can change aquatic faunal populations, including endangered species.

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. The reservoirs of hydroelectric power plants in tropical regions may produce substantial amounts of methane( 甲烷 沼氣 ) and carbon dioxide. This is due to plant material in flooded areas decaying in an anaerobic ( 厭氧性 ) environment, and forming methane, a very potent greenhouse gas. According to the World Commission on Dams report, where the reservoir is large compared to the generating capacity (less than 100 watts per square metre of surface area) and no clearing of the forests in the area was undertaken prior to impoundment of the reservoir, greenhouse gas emissions from the reservoir may be higher than those of a conventional oil-fired thermal generation plant.

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Another disadvantage of hydroelectric dams is the need to relocate the people living where the reservoirs are planned. In many cases, no amount of compensation can replace ancestral and cultural attachments to places that have spiritual value to the displaced population. Additionally, historically and culturally important sites can be flooded and lost. Such problems have arisen at the Three Gorges Dam project in China, the Clyde Dam in New Zealand and the Ilısu Dam in Southeastern Turkey.

• Recreational users of the reservoir or downstream areas are exposed to hazards due to changing water levels, and must be wary of power plant intakes and spillway operation.

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• Some hydroelectric projects also utilize canals, typically to divert a river at a shallower gradient to increase the head of the scheme. In some cases, the entire river may be diverted leaving a dry riverbed. The creation of a dam in a geologically inappropriate location may cause disasters like the one of the Vajont Dam in Italy, where almost 2000 people died, in 1963. Failures of large dams, while rare, are potentially serious — the Banqiao Dam failure in China resulted in the deaths of 171,000 people and left millions homeless, more than some estimates of the death toll from the Chernobyl disaster. Though the dams can be built stronger, at greater cost, they are still prone to sabotage and terrorism. Smaller dams and micro hydro facilities are less vulnerable to these threats.

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Term project:

Research on Banqiao Dam failure. Describe why it failed.

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• Wave power refers to the energy of ocean surface waves and the capture of that energy to do useful work - including electricity generation, desalination, and the pumping of water (into reservoirs). Wave power is a form of renewable energy. Though often co-mingled, wave power is distinct from the diurnal flux of tidal power and the steady gyre of ocean currents. Wave power generation is not a widely employed technology, with the world's first commercial wave farm, the Aguçadora Wave Park in Portugal, being established in 2006.

• The north and south temperate zones have the best sites for capturing wave power. The prevailing westerlies ( 西風帶 ) in these zones blow strongest in winter.

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• In general, large waves are more powerful. Specifically, wave power is determined by wave height, wave speed, wavelength, and water density.

• Wave size is determined by wind speed and fetch (the distance over which the wind excites the waves) and by the depth and topography of the seafloor (which can focus or disperse the energy of the waves). A given wind speed has a matching practical limit over which time or distance will not produce larger waves. This limit is called a "fully developed sea."

• Wave motion is highest at the surface and diminishes exponentially with depth; however, wave energy is also present as pressure waves in deeper water.

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• The potential energy of a set of waves is proportional to wave height squared times wave period (the time between wave crests). Longer period waves have relatively longer wavelengths and move faster. The potential energy is equal to the kinetic energy (that can be expended). Wave power is expressed in kilowatts per meter (at a location such as a shoreline).

• The formula below shows how wave power can be calculated. Excluding waves created by major storms, the largest waves are about 15 meters high and have a period of about 15 seconds. According to the formula, such waves carry about 1700 kilowatts of potential power across each meter of wavefront. A good wave power location will have an average flux much less than this: perhaps about 50 kW/m.

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• Formula: Power (in kW/m) = k H2 T ~ 0.5 H2 T, • where k = constant, H = wave height (crest to trough) i

n meters, and T = wave period (crest to crest) in seconds.

Pelamis machine pointing into the waves: it attenuates the waves, gathering more energy than its narrow profile suggests.

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When an object bobs up and down on a ripple in a pond, it experiences an elliptical trajectory

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Modern Technology• Wave power devices are generally categorized by the

method used to capture the energy of the waves. they can also be categorized by location and power take-off system. Method types are point absorber or buoy; surfacing following or attenuator; terminator, lining perpendicular to wave propagation; oscillating water column; and overtopping. Locations are shoreline, nearshore and offshore. Types of power take-off include: hydraulic ram, elastomeric hose pump, pump-to-shore, hydroelectric turbine, air turbine, and linear electrical generator.

• Some of these designs incorporate a parabolic reflectors as a means of increasing the wave energy at the point of capture.

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These are descriptions of some wave power systems:• An example of a surface following device is the Pelamis

Wave Energy Converter. The sections of the device articulate with the movement of the waves, each resisting motion between it and the next section, driving a hydraulic ram which drives a hydraulic motor.

• With the Wave Dragon wave energy converter large "arms" focus waves up a ramp into an offshore reservoir. The water returns to the ocean by the force of garvity via hydroelectric generators.

• The AquaBuOY wave energy device: Energy transfer takes place by converting the vertical component of wave kinetic energy into pressurized seawater by means of two-stroke hose pumps.

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• Pressurized seawater is directed into a conversion system consisting of a turbine driving an electrical generator. The power is transmitted to shore by means of a secure, undersea transmission line.

• A device called CETO, currently being tested off Fremantle, Western Australia, has a seafloor pressure transducer coupled to a high-pressure hydraulic pump, which pumps water to shore for driving hydraulic generators or running reverse osmosis desalination.

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Challenges

Waves overtopping a test wave generator of the Wave Dragon in Nissum-Bredning, Denmark. Image courtesy of Earth Vision. See Wave Dragon

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The challenges of wave power are:• efficiently converting wave motion into electricity --gen

erally speaking, wave power is available in low-speed, high forces and motion is not in a single direction. Most readily-available electric generators like to operate at higher speeds, with lower input forces, and they prefer to rotate in a single direction.

• constructing devices that can survive storm damage and saltwater corrosion. Likely sources of failure include seized bearings, broken welds, and snapped mooring lines. Knowing this, designers may create prototypes that are so overbuilt that materials costs prohibit affordable production.

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• low total cost of electricity... wave power will only be competitive when total cost of generation (p/kWhr) is reduced. The winning team will be the one that develops the lowest-cost system (which includes the primary converter, power takeoff system, mooring system, installation & maintenance procedures)

• While the industry has suffered many failures, it has benefited in recent years from increases in support from governments, universities, and angel investors. Several promising prototypes are now in operation.

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Wave farms• Portugal claims the world's first commercial wave far

m, the Aguçadora Wave Park near Póvoa de Varzim, established in 2006. The farm will initially use three Pelmis P-750 machines generating 2.25 MW. Initial costs are put at 8,5 million euro. Subject to successful operation, a further 70 million euro is likely to be invested before 2009 on a further 28 machines to generate 72.5 MW.

• Funding for a wave farm in Scotland was announced on February 20, 2007 by the Scottish Executive, at a cost of over 4 million pounds, as part of a £13 million funding packages for marine power in Scotland. The farm will be the world's largest with a capacity of 3MW generated by four Pelamis machines.

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Potential• Wave power could yield much more energy than

tidal power. Tidal dissipation (friction, measured by the slowing of the lunar orbit) is 2.5 terawatts. The energy potential of waves is certainly greater, and wave power can be exploited in many more locations. Countries with large coastlines and strong prevailing winds (notably, Ireland and the UK) could produce five percent or more of their electricity from wave power. Excess capacity (a problem common with intermittent energy sources) could be used to produce hydrogen or smelt aluminum.

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Local environmental impact• The placement of a barrage into an estuary has a

considerable effect on the water inside the basin and on the fish. A tidal current turbine will have a much lower impact.

Turbidity• Turbidity (the amount of matter in suspension in the

water) decreases as a result of smaller volume of water being exchanged between the basin and the sea. This lets light from the Sun to penetrate the water further, improving conditions for the phytoplankton. The changes propagate up the food chain, causing a general change in the ecosystem.

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Salinity• As a result of less water exchange with the sea, the

average salinity inside the basin decreases, also affecting the ecosystem. "Tidal Lagoons" do not suffer from this problem.

Sediment movements• Estuaries often have high volume of sediments

moving through them, from the rivers to the sea. The introduction of a barrage into an estuary may result in sediment accumulation within the barrage, affecting the ecosystem and also the operation of the barrage.

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With turbine generation, taking its power from the flow of the tidal stream, there will likely be a swirl of water down stream of the turbine. If this horizontal vortex touches the bottom, it will cause erosion. While the amount of sediment added to the tidal stream will likely be insignificant, this could, over time, erode the foundation of the turbine. Turbines held down with pilings would be largely immune to this problem but turbines held by heavy weights sitting on the bottom could eventually tip over.

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Pollutants• Again, as a result of reduced volume, the pollutants

accumulating in the basin may be less efficiently dispersed, so their concentrations may increase. For biodegradable pollutants, such as sewage, an increase in concentration is likely to lead to increased bacteria growth in the basin, having impacts on the health of the human community and the ecosystem.

Fish• Fish may move through sluices safely, but when these

are closed, fish will seek out turbines and attempt to swim through them. Also, some fish will be unable to escape the water speed near a turbine and will be sucked through.

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• Even with the most fish-friendly turbine design, fish mortality per pass is approximately 15% (from pressure drop, contact with blades, cavitation, etc.). This can be acceptable for a spawning run, but is devastating for local fish who pass in and out of the basin on a daily basis. Alternative passage technologies (fish ladders, fish lifts, etc.) have so far failed to solve this problem for tidal barrages, either offering extremely expensive solutions, or ones which are used by a small fraction of fish only. Research in sonic guidance of fish is ongoing.

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Global environmental impact• A tidal power scheme is a long-term source of

electricity. A proposal for the Severn Barrage, if built, has been projected to save 18 million tons of coal per year of operation. This decreases the output of greenhouse gases into the atmosphere. More importantly, as the fossil fuel resource is likely to be eliminated by the end of the twenty-first century, tidal power is one of the alternative source of energy that will need to be developed to satisfy the human demand for energy.

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Ocean thermal energy conversion• OTEC utilizes the temperature difference that exists

between deep and shallow waters — within 20° of the equator in the tropics — to run a heat engine. Because the oceans are continually heated by the sun and cover nearly 70% of the Earth's surface, this temperature difference contains a vast amount of solar energy which could potentially be tapped for human use. The total energy available is one or two orders of magnitude higher than other ocean energy options such as wave power, but the small size of the temperature difference makes energy extraction difficult and expensive. Hence, existing OTEC systems have an overall efficiency of only 1 to 3%.

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The concept of a heat engine is very common in engineering, and nearly all energy utilized by humans uses it in some form. A heat engine involves a device placed between a high temperature reservoir (such as a container) and a low temperature reservoir. As heat flows from one to the other, the engine extracts some of the heat in the form of work. This same general principle is used in steam turbines and internal combustion engines, while refrigerators reverse the natural flow of heat by "spending" energy. Rather than using heat energy from the burning of fuel, OTEC power draws on temperature differences caused by the sun's warming of the ocean surface.

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View of a land based OTEC facility at Keahole Point on the Kona

coast of Hawaii

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How OTEC works• Some energy experts believe that if it could become

cost-competitive with conventional power technologies, OTEC could produce gigawatts of electrical power. Bringing costs into line is still a huge challenge, however. All OTEC plants require an expensive, large diameter intake pipe, which is submerged a mile or more into the ocean's depths, to bring very cold water to the surface.

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Left: Pipes used for OTEC.Right: Floating OTEC plant constructed in India in 2000

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Closed-cycle

Diagram of a closed cycle OTEC plant

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Closed-cycle systems use fluid with a low boiling point, such as ammonia, to rotate a turbine to generate electricity. Warm surface seawater is pumped through a heat exchanger where the low-boiling-point fluid is vaporized. The expanding vapor turns the turbo-generator. Then, cold, deep seawater—pumped through a second heat exchanger—condenses the vapor back into a liquid, which is then recycled through the system. In 1979, the Natural Energy Laboratory and several private-sector partners developed the mini OTEC experiment, which achieved the first successful at-sea production net electrical power from closed-cycle OTEC. The mini OTEC vessel was moored 1.5 miles (2.4 km) off the Hawaiian coast and produced enough net electricity to illuminate the ship's light bulbs, and run its computers and televisions.

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Open-cycle• Open-cycle OTEC uses the tropical oceans' warm

surface water to make electricity. When warm seawater is placed in a low-pressure container, it boils. The expanding steam drives a low-pressure turbine attached to an electrical generator. The steam, which has left its salt behind in the low-pressure container, is almost pure fresh water. It is condensed back into a liquid by exposure to cold temperatures from deep-ocean water.

• In 1984, the Solar Energy Research Institute (now the National Renewable Energy Laboratory) developed a vertical-spout evaporator to convert warm seawater into low-pressure steam for open-cycle plants.

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Hybrid• A hybrid cycle combines the features of both the

closed-cycle and open-cycle systems. In a hybrid OTEC system, warm seawater enters a vacuum chamber where it is flash-evaporated into steam, which is similar to the open-cycle evaporation process. The steam vaporizes the working fluid of a closed-cycle loop on the other side of an ammonia vaporizer. The vaporized fluid then drives a turbine that produces electricity. The steam condenses within the heat exchanger and provides desalinated water.

• The electricity produced by the system can be delivered to a utility grid or used to manufacture methanol, hydrogen, refined metals, ammonia, and similar products.

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Solar energy

A photovoltaic (PV) module that is

composed of multiple PV cells. Two or

more interconnected PV modules create

an array.

A laundromat in California with

flat-plate solar water heating

collectors on its roof.

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•On a global scale solar radiation reaches the Earth's

upper atmosphere at a rate of 174 PW. While traveling through the atmosphere, 6% of the incoming solar radiation (insolation) is reflected and 16% is absorbed. Average atmospheric conditions (clouds, dust, pollutants) further reduce insolation by 20% through reflection and 3% through absorption. The absorption of solar energy in the atmosphere is not a loss of available energy. This energy produces our climate through its capture within derivative effects such as the hydrologic cycle, wind and ocean currents.

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• Atmospheric conditions not only reduce the quantity of insolation reaching the Earth's surface but also affect the quality of insolation by diffusing incoming light and altering its spectrum. After passing through the Earth's atmosphere approximately half the insolation is in the visible electromagnetic spectrum with the other half mostly in the infrared and ultraviolet spectrum.

• Insolation is a measure of solar radiation incident on a surface. It is the amount of solar energy received over a given area in a given time. It is commonly expressed in kilowatt-hours per square meter per day (kW•h/m²/day) or watts per square meter (W/m²).

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Solar power as it is dispersed on the planet and radiated back to space.

Values are in PW =1015 W

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Annual average insolation at the top of Earth's atmosphere (top) and at the

surface (bottom). The black dots represent the land area required to replace

the total primary energy supply with electricity from solar cells.

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The map on the right (top) shows how solar radiation at the top of the earth's atmosphere varies with latitude. The bottom map shows annual average ground level insolation. For example, in North America the average insolation at ground level over an entire year (including nights and periods of cloudy weather) lies between 125 and 375 W/m² (3 to 9 kWh/m²/day). At present, photovoltaic panels typically convert about 15% of incident sunlight into electricity; therefore, a solar panel in the contiguous United States on average delivers 19 to 56 W/m² or 0.45 - 1.35 kWh/m²/day.

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In this context, "solar energy" refers to energy that is

collected from sunlight. Solar energy can be applied in

many ways, including to:• Generate electricity using photovoltaic solar cells. • Generate electricity using concentrated solar power. • Generate electricity by heating trapped air which rotates

turbines in a Solar updraft tower. • Heat buildings, directly, through passive solar design. • Heat foodstuffs, through solar ovens. • Heat water or air for domestic hot water and space

heating needs using solar-thermal panels. • Heat and cool air through use of solar chimneys.

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• A solar cell or photovoltaic cell is a device that converts light energy into electrical energy. Sometimes the term solar cell is reserved for devices intended specifically to capture energy from sunlight, while the term photovoltaic cell is used when the light source is unspecified.

• Fundamentally, the device needs to fulfill only two functions: photogeneration of charge carriers (electrons and holes) in a light-absorbing material, and separation of the charge carriers to a conductive contact that will transmit the electricity. This conversion is called the photovoltaic effect, and the field of research related to solar cells is known as photovoltaics.

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• Solar cells have many applications. They have long been used in situations where electrical power from the grid is unavailable, such as in remote area power systems, Earth-orbiting satellites and space probes, consumer systems, e.g. handheld calculators or wrist watches, remote radiotelephones and water pumping applications. More recently, they are starting to be used in assemblies of solar modules (photovoltaic arrays) connected to the electricity grid through an inverter, often in combination with a net metering arrangement.

• Solar cells are regarded as one of the key technologies towards a sustainable energy supply.

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A solar cell, made from a monocrystalline silicon wafer

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Theory of photovoltaic cell• Photons in sunlight hit the solar panel and are absorbe

d by semiconducting materials, such as silicon. • Electrons (negatively charged) are knocked loose from

their atoms, allowing them to flow through the material to produce electricity. The complementary positive charges that are also created (like bubbles) are called holes and flow in the direction opposite of the electrons in a silicon solar panel.

• An array of solar panels converts solar energy into a usable amount of direct current (DC) electricity.

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Optionally:• The DC current enters an inverter. • The inverter turns DC electricity into 120 or 240-volt

AC (alternating current) electricity needed for home appliances.

• The AC power enters the utility panel in the house. • The electricity is then distributed to appliances or ligh

ts in the house. • The electricity that is not used will be recycled and re

used in other facilities.

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Architecture• Solar architecture is designed to use the sun as much a

s possible for temperature control, lighting and ventilation while minimizing negative effects such as overheating and glare. The basic elements of solar architecture are building orientation, proportion, thermal mass and window placement

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The Zion National Park Visitor's Center incorporates several aspects of

solar design.

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• A solar building's axis should run lengthwise east to west and the structure should be twice as long as wide.

• In the northern hemisphere, north facing windows should be minimized and south facing windows should be equal to 5-7% of the building's floor space. In the southern hemisphere, the south facing windows should be minimized and north facing windows should also be equal to 5-7% of the building's floor space.

• The thermal mass in the building should be sized to smooth out temperature swings.

• Spaces can be designed to naturally circulate air. Cooling elements such as a solar chimney can be incorporated to help with ventillation.

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Water Heating • On a technical level, solar water heating is highly efficie

nt (up to 87%) and is particularly appropriate for low temperature (75-150F) applications such as domestic hot water, heating swimming pools and space heating.

• The basic components of a solar water heating systems are solar thermal collectors, a storage tank and a circulation loop. The three basic classifications of solar water heaters are:

A. Batch systems which consist of a tank that is directly heated by sunlight. These are the oldest and simplest solar water heater designs, however; the exposed tank can be vulnerable to cooldown.

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B. Active systems which use pumps to circulate water or a heat transfer fluid.

C. Passive systems which circulate water or a heat transfer fluid by natural circulation. These are also called thermosiphon systems

Solar water heaters, on a

rooftop In Jerusalem, Israel

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Solar power plants• Solar power plants use a variety of methods to collect s

unlight and convert this energy into electricity, distill water or provide heat for industrial processes.

• Concentrating Solar Thermal (CST) systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. CST technologies require direct insolation to perform properly. This requirement makes them inappropriate for significantly overcast locations.

• The three basic CST technologies are the solar trough, solar power tower and parabolic dish. Each technology is capable of producing high temperatures and correspondingly high thermodynamic efficiencies but they vary in the way they track the sun and focus light.

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Solar Two power tower

surrounded by a field of

heliostats.

Parabolic dishes use Stirling

engines for power conversion.

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Geothermal power

Geothermal power is energy generated by heat stored beneath the Earth's surface. Geothermal power supplies 0.416% of the world's energy. Geothermal comes from the Greek words geo, meaning earth, and therme, meaning heat. Prince Piero Ginori Conti tested the first geothermal power plant on 4 July 1904, at the Larderello dry steam field in Italy. The largest group of geothermal power plants in the world is located in The Geysers, a geothermal field in California.

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Krafla Geothermal Station in northeast Iceland

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Electricity Generation• Three different types of power plants - dry steam, fl

ash, and binary - are used to generate electricity from geothermal energy, depending on temperature, depth, and quality of the water and steam in the area. In all cases the condensed steam and remaining geothermal fluid is injected back into the ground to pick up more heat. In some locations, the natural supply of water producing steam from the hot underground magma deposits has been exhausted and processed waste water is injected to replenish the supply. Most geothermal fields have more fluid recharge than heat, so re-injection can cool the resource, unless it is carefully managed.

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Dry steam

A dry steam power plant uses hot steam, typically above 235°C, to directly power its turbines. This is the oldest type of geothermal power plant and is still in use today. Dry steam plants emit small amounts of excess steam and gases.

Flash steam

Flash steam power plants use hot water above 182°C from geothermal reservoirs. The high pressure underground keeps the water in liquid form, even though it is well above the boiling point for water at sea level. As the water is pumped from the reservoir to the power plant, the drop in pressure causes the water to convert, or "flash", into steam to power the turbine. Any water not flashed into steam is injected back into the reservoir for reuse.

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Binary-cycle• The water used in binary-cycle power plants is cooler t

han that of flash steam plants. The hot fluid from geothermal reservoirs is passed through a heat exchanger which transfers heat to a separate pipe containing fluids with a much lower boiling point. These fluids, usually Iso-butane or Iso-pentane, are vaporized to power the turbine. The advantage to binary-cycle power plants is their lower cost and increased efficiency. These plants also do not emit any excess gas and are able to utilize lower temperature reservoirs, which are much more common. Most geothermal power plants planned for construction are binary-cycle.

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Advantages• From an environmental standpoint, the energy harnes

sed is clean and safe for the surrounding environment. It is also sustainable because the hot water used in the geothermal process can be re-injected into the ground to produce more steam. In addition, geothermal power plants are unaffected by changing weather conditions. Geothermal power works continually, day and night, providing baseload power. From an economic view, geothermal energy is extremely price competitive in some areas and reduces reliance on fossil fuels and their inherent price unpredictability.

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Disadvantages• There are several environmental concerns behind geot

hermal energy. Construction of the power plants can adversely affect land stability in the surrounding region. This is mainly a concern with Enhanced Geothermal Systems, as they involve drilling very deep and injecting water into hot dry rock where no water was before. Dry steam and flash steam power plants also emit low levels of carbon dioxide, nitric oxide, and sulfur, although at roughly 5% of the levels emitted by fossil fuel power plants. Geothermal plants can be built with emissions-controlling systems that can inject these gases back into the earth, thereby reducing carbon emissions to less than 0.1% of those from fossil fuel power plants.

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• History of wind power

• Wind machines were used for grinding grain in Persia as early as 200 B.C. This type of machine was introduced into the Roman Empire by 250 A.D. By the 14th century Dutch windmills were in use to drain areas of the Rhine River delta. The largest machines were on 24 m (79 ft) towers with four-bladed 23 m (75 ft) diameter rotors. In Denmark wind power was an important part of a decentralized electrification in the first quarter of the 20th century.

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• Wind turbines can also be classified by the location in which they are to be used. Onshore, offshore, or even aerial wind turbines have unique design characteristics, which are explained in more detail in the section on turbine design and construction.

• Wind turbines may also be used in conjunction with a solar collector to extract the energy due to air heated by the Sun and rising through a large vertical solar updraft tower.

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• The mass flow of air that travels through the swept area of a wind turbine varies with the wind speed and air density. As an example, on a cool 15°C day at sea level, air density is 1.225 kilograms per cubic metre. An 8 m/s breeze blowing through a 100 meter diameter rotor would move almost 77,000 kilograms of air per second through the swept area

• As the wind turbine extracts energy from the air flow, the air is slowed down, which causes it to spread out and diverts it around the wind turbine to some extent. Albert Betz, a German physicist, determined in 1919 (see Betz' law) that a wind turbine can extract at most 59% of the energy that would otherwise flow through the turbine's cross section. The Betz limit applies regardless of the design of the turbine

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• Mesas, hilltops, ridgelines and passes can have higher and more powerful winds near the ground than up high because of the speed up effect of winds moving up a slope or funneling into a pass combining with the winds moving directly into the site. In these places, VAWTs placed close to the ground can produce more power than HAWTs placed higher up.

• Smaller VAWTs can be much easier to transport and install.

• Does not need a free standing tower so is much less expensive and stronger in high winds that are close to the ground.

• Usually have a lower Tip-Speed ratio so less likely to break in high winds.

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• Most VAWTS need to be installed on a relatively flat piece of land and some sites could be too steep for them but are still usable by HAWTs.

• Most VAWT's have low starting torque.• A VAWT that uses guyed wires to hold it in place puts st

ress on the bottom bearing as all the weight of the rotor is on the bearing. Guyed wires attached to the top bearing increase downward thrust in wind gusts. Solving this problem requires a superstructure to hold a top bearing in place to eliminate the downward thrusts of gust events in guyed wired models.

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• Tall tower allows access to stronger wind in sites with wind shear. In some wind shear sites, every ten meters up, the wind speed can increase by 20% and the power output by 34%.

• Tall tower allows placement on uneven land or in offshore locations.

• Can be sited in forests above the treeline. .• Most are self-starting.• Can be cheaper because of higher production volu

me, larger sizes and, in general higher capacity factors and efficiencies.

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• Supply of HAWTs is less than demand and between 2004 and 2006, turbine prices increased up to 60%. At the end of 2006, all major manufacturers were booked up with orders through 2008.

• The FAA has raised concerns about tall HAWTs effects on radar in proximity to air force bases.

• Their height can create local opposition based on impacts to viewsheds.

• Offshore towers can be a navigation problem and must be installed in shallow seas. HAWTs can't be floated on barges.

• Downwind variants suffer from fatigue and structural failure caused by turbulence.

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風力產生

• 風是如何產生的呢？風是由於地球自轉以及太陽熱輻射不均所引起的空氣循環流動。太陽輻射能將地表的空氣加溫，空氣受熱膨脹後變輕上升，熱空氣上升冷空氣橫向進入，再次因為加溫而上升，如此循環作用後造成空氣的流動而產生了風

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風力應用發電 ()

• 風能轉換過程是利用風力機將風的動能轉換為機械力，再將機械力變換為電力貯藏利用。這方面技術牽扯到電力儲存的技術問題，一般適用於小型發電系統。小型風力發電系統將風能轉換之電力以蓄電池貯藏，當負載需求時由蓄電池提供電力。

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小型風力機

• 具景觀性• 高轉速噪音大• 適用偏遠無市電的獨立系統• 葉片長度越長所能擷取的風量越大。例如 400kw :1.2m( 葉片長度 )

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大型風力機發電介紹

• 商業化主流為水平軸 , 以及三葉式翼型風力發電機

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大型風力機結構

Vestas V52-850 千 瓦 風 力 發 電 機 機 艙 內 的 組 成 部 份

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大型風力機與小型風力機比較

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獨立發電系統

• 獨立型是由風力機產生電能經控制器將電力儲存於蓄電池中，當負載需求時由控制器將電力送至負載，而一般家庭使用可與市電做搭配，取得穩定電源並能減少對市電的需求 獨立發電系統一般常見的有噴水池抽水馬達、電力充電站等小電力需求端或電力品質要求不高的地方，他能提供全部或部份的電力需求。

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混合發電系統

• 混合發電系統是由風力發電機與不同的發電設備結合可同步或非同步供給電力一般可使用於偏遠而電力無法到達的地區，目前皆採用蓄電池貯能，家用電器的用電都由蓄電池提供，或與柴油發電機、太陽光電等混合供電，用電時的原則是，蓄電池放電後能及時由風力發電機給以補充。 而市面常見的風力與太陽光電搭配的路燈系統，偏遠而電力無法到達的地區如彭佳嶼、東吉島氣象觀測站，都是用這樣的系統提供生活用電，氣象觀測等需求。

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併聯發電系統

• 大型的風力發電廠由於並沒有適當的儲能設備，所以多數大型風力發電廠的用戶會採取與市電併聯方式（如圖所示），一方面可以防止原本僅仰賴風力發電之用戶無電可用的情形發生，一方面又可以將風力發電廠所產生多餘的電力送至電力系統供其他用戶使用。

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大型風力發電應用

• 大型風力發電裝置係由葉片將風力變換為旋轉機械能，並使用發電機將機械能轉換成電能，再透過電力轉換裝置轉換為穩定的電壓、電流，對負載或市電供給電力。風力發電廠與電力公司系統併聯時，必須考慮各種可能發生的問題，並且要有適當的防範措施與保護設備，以確保風力發電廠、電力公司及用戶之設備安全、供電品質及供電可靠性。 目前台電及英華威公司的風力發電系統皆是此架構。

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大型風力發電機趨勢

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風力發電機原理

• 風力發電機如何擷取風能轉換為機械能呢，主要利用流過風力機葉片表面的空氣發生直接影響風力機性能的二力－升力與阻力。升力在流入空氣流的垂直方向作用，阻力在空氣流的平行方向作用。風力機之葉片，須選發生最大升力與最小阻力的葉片，風能理論是 1919 年德國物理學家貝茲發表的，他由理論計算出理想的高速風機（無摩擦）其風能利用係數可達到貝茲極限（ Cp=0.593 ），在經電力轉換與機械損耗等輸出 效率約 20～ 40％ 。

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關於塔架高度與葉片長度及發電量

• 風車卒年的規模越來越大 , 而且風車半徑越大 , 風車半徑截面積所受的風力也越大 , 這樣能促使風車能轉換更大的功率以及年發電量 ,塔高也是跟著半徑增加或減少

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風力葉片發展趨勢• 風力機係藉著空氣的流動帶動葉片，因此，站在第一線的葉片之成敗即是風力

機的成敗，其涉及氣動力學、複合材料及結構安全 選旋翼之理由係僅用 5%掃面積材料就得近 50% 風能。 可調螺距是目前葉片之主流，翼內結構須抵抗頻繁轉動的扭力。 葉尖造型及葉尾厚薄適當處理與速度限制可以有效控制噪音的產生。 巨型化的葉片為減重，環氧樹脂成為主流，縱樑改採碳纖維補強。 目前三明治構造以夾 Balsa木為多，塑膠泡棉有崛起之勢。 翼形表面採光滑塗料。 根部大都以 T 型生根件之螺栓。

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葉片控制

• 為了不同需求目前大型風機大部分具有可調整之葉片，風力機可隨著風速大小、風向變化而自動調整風機起動、關機、煞車、或迎向風向，並具備遠距監控及緊急異狀保護之功能。

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何時會運轉或者是停機

• 大型風力發電機的運轉因機械結構及電力轉換等因素，大型風力機的輸出並不像小型風力機會與風速成一定比率，目前大型風力機的運作都由電腦來控制。 在風力機頂端會裝置風速風向量測裝置，由量測裝置回饋訊號給電腦，經電腦判斷進行運轉控制。

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風車風速運轉條件

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風力發電考量 ( 一 )• 風力地理條件（風期長、平均風速大、風力平穩）

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風力發電考量 (二 )• 風力機配置時需考量尾流效應，這是由於風經過風力機葉片後會改變後方的風向而在機身後方形成的紊流 當風力機受到前方風力機的尾流時，因為流過風力機的氣流並不平穩，會干擾後方風機迎風面的風場，並容易使後方風力機因紊流而產生震動，進而造成損壞 因此若有多部風力機須考量每部風力機之間的擾流 .(左下圖是機後的尾流效應 )

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風力發電考量 ( 三 )• 另外風力機設置位置最好在空曠無障礙的地方 若有障礙物將發生短距離內風向、風速明顯變化而產生亂流的現象。此類亂流大小與風速和阻礙物大小及粗糙度有關。風速愈大，阻礙物愈大、愈粗糙，則亂流愈強。 (擾流 )

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風力發電考量 (四 )• 我國環保署於民國 85年規定之音量標準與各國訂定方式略有不同，但其音量限制與各國大致相似，最大音量因時間及地點不同介於 30~80db 之間。一般噪音是以音強或音壓來表示大小，通常以分貝（ db ） 風力發電機所發出的噪音來自於齒輪箱及發動機發出的機械噪音和旋轉葉片所發出的空氣動力噪音

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風力發電考量 (五 )

• 研究學者在丹麥西部一個裝有 2MW其葉片轉子直徑 60公尺的風機場址做了一個雷達的研究，發現無論在白天或晚上鳥類會在它們離風機約 100~200公尺左右，改變它們的飛行方向，而從一個風機上方的安全高度飛過。

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風力發電考量 (六 )

風力機本身所造成之電磁波，則非常小 ,除了電磁波之外 ,也會通訊干擾

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風力發電考量 (七 )

• 風力發電場在環境景觀與視覺上的影響，因環境背景不同而有見仁見智之論點，風力機聳立於田野間雖非自然景觀，然而其優美造型，對一般適合開發風力發電場場址，常因強風致使草木不易成長之地形，反而有視覺點綴及景觀改造功效

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水平軸式 (Horizontal axis)

• 此型轉動軸與風平行 , 此型為逆風 (upwind) 型 ,就事業片對這著風向 , 大部分水平軸式風立輪葉會隨風向變化而調整位子

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垂直軸式 (Vertical axis)

• 此行之優點為設計簡單 , 因為不必隨風向改變而轉動調整風向 ,但是此系統無法抽取大量風能並需要大量材料是缺點 , 此型為打蛋形 (Darrieus)

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岸上式 (Onshore)

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離岸式 (Offshore)

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離岸式圖片

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離岸式風力發電機• 一般離岸式風力發電廠採取之風力機基礎包含 .重力式 , 以及單椿式基礎與

三腳式基礎 . 這三種必須綜合考量水深 , 地質條件及施工便利性等來選擇 .

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有齒輪箱與無齒輪箱式風力機比較• 有齒輪箱式風力機成本比較低 , 而且技術成熟發電容量提升較容易 ,預期近期內會成

為主流 . 而無齒輪是風力機輸出電量更穩定 ,效率更高 , 而且啟動風速需求較低 , 所需運轉維修較少 .但是龐大永久磁鐵發電機造價偏高 ,重量也重 ,更大型化是否可以耐離岸式環境有待技術發展 ,若能突破一些瓶頸並降低成本 ,預期長程會成為主流

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離岸與岸上成本比較

• 往海岸發展原因為 :缺乏陸上的廠址 ,離岸具有比較高且穩定的速度 ,海上蘊藏巨大的風力資源 , 表面粗糙度低可以採用較低的架塔 ,亂流比較小可以使風力機有比較長的壽命

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風農場 (Wind farm)

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Total installed windpower capacity

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風力市場的成長

• 從 2005年 (47686MW) 到 2006年 (59004MW) 成長了 11318MW,2007年 (73904MW) 比 2006又多了 14900MW, 成長會持續上升

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風力發電成本• 風力發電成本逐年下降 ,隨著風電受到重視，工業技術的提昇，提高了風力發電的效率，及設備大型化影響，連帶的也使得風力發電的成本得以逐年下降

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效益與特色• (1) 減少傳統化石能源消耗 : 例如風力發電一度可減少 0.25公升的燃油• (2)潔淨能源不排放污染物質 :每發一度電約較傳統燃煤發電減少 1公斤二氧化碳 ,3.5公克的氮氧化

合物 , 以及 6.1克的氧化硫等排放污染• (3) 風能是取之不盡用之不歇 :依目前石油剩50年 ,天然氣 70年 ,煤炭200年 , 而風能為永續 資源• (4) 風能是自然資源 : 可減少對石油天然氣煤炭等依賴• (5)各種發電成本如圖下• (6) 可扮演分散式發電系統角色 : 風力發電機可以分散設置 ,近負載端可減少輸電距離 , 提供輔助電力• (7) 風力發電具有觀光效益

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風能與轉換分析• 假設在風能利用的風速範圍內 , 氣流為不可壓縮 , 具有風

速 V知該氣流的動能為 K:• K=(1/2)mV2 :其中 m 為氣體質量 (kg),V 為氣流速度 (m/

s)• 若考慮氣流的速度為 V,垂直流過的截面積 A(m2) 的假想

面 ,在時間 t(s) 內 , 氣流流過的距離 L(m), 則流過該節面積的氣流體積 C(m3) 為 :

• C=AL=AVt , 而氣體質量 m 為 m=ρC=ρAVt , 式中 :ρ為空氣密度 ,kg/m3

• 因此 ,在時間 t 內流過該截面的封鎖具有的動能為 K=(1/2)mV2=(1/2)ρAVtV2=(1/2)ρAV3t 於是在單位時間流過的該截面的風能 ,即為風功率 PW(單位是 W)

• PW=(1/2)ρAV3

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• 當一氣流柱通過風力機葉片旋轉掃過之圓型面積時 ,該氣流柱之每單位時間之動能為

• ((MV2)/2)/t=MV2/2t --(1)• 式中 M 為氣流質量 ,V 為該氣流移動速度亦即風速 ,t

為時間﹒而該氣流柱質量為• M=ρ(πD2)L/4 –(2)﹒上式中 L 為該氣流柱長度 ,D 為

風力機葉片直徑 , ρ 則是空氣密度• 由 (2) 帶入 (1) 可得氣流柱之每單位時間之動能為 (π

D2Lρ)V2/2t,又因為風速 V=L/t• 再帶入 ((πD2)Lρ(L/t)2)/2t=(πD2ρV3)/8=((πD2)/4)(1/2)

ρV3

• 總動能之理論最大轉換率為 59.3%亦即為 0.593 乘((πD2)/4)(1/2)ρV3

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Environmental and social considerations• While most renewable energy sources do not produce

pollution directly, the materials, industrial processes, and construction equipment used to create them may generate waste and pollution. Some renewable energy systems actually create environmental problems. For instance, older wind turbines can be hazardous to flying birds.

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Land area required• Another environmental issue, particularly with biomass

and biofuels, is the large amount of land required to harvest energy, which otherwise could be used for other purposes or left as undeveloped land. However, it should be pointed out that these fuels may reduce the need for harvesting non-renewable energy sources, such as vast strip-mined areas and slag mountains for coal, safety zones around nuclear plants, and hundreds of square miles being strip-mined for oil sands.

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Hydroelectric Dams• There are several major disadvantages of

hydroelectric systems. These include: dislocation of people living where the reservoirs are planned, release of significant amounts of carbon dioxide at construction and flooding of the reservoir, disruption of aquatic ecosystems and birdlife, adverse impacts on the river environment, potential risks of sabotage and terrorism, and in rare cases catastrophic failure of the dam wall.

• Hydroelectric power is now more difficult to site in developed nations because most major sites within these nations are either already being exploited or may be unavailable for other reasons such as environmental considerations.

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Concluding comment• The U.S. electric power industry now relies on large,

central power stations, including coal, natural gas, nuclear, and hydropower plants that together generate more than 95 percent of the nation’s electricity. Over the next few decades uses of renewable energy could help to diversify the nation’s bulk power supply. Already, appropriate renewable resources (which excludes large hydropower) produce 12 percent of northern California’s electricity.

• Although most of today’s electricity comes from large, central-station power plants, new technologies offer a range of options for generating electricity nearer to where it is needed, saving on the cost of transmitting and distributing power and improving the overall efficiency and reliability of the system.

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• Improving energy efficiency represents the most immediate and often the most cost-effectiveway to reduce oil dependence, improve energy security, and reduce the health and environmental impact of the energy system. By reducing the total energy requirements of the economy, improved energy efficiency will make increased reliance on renewable energy sources more practical and affordable.