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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.
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."
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.
Wind power Airflows 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
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.
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.
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.
Water power Energy 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.
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.
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.
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.
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.
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.
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.
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.
. 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.
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.
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.
Term project: Research on Banqiao Dam failure. Describe why it failed.
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.
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.
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.
Formula: Power (in kW/m) = k H2 T ~ 0.5 H2 T, • where k = constant, H = wave height (crest to trough) in 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.
When an object bobs up and down on a ripple in a pond, it experiences an elliptical trajectory
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.
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.
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 osmosisdesalination.
Challenges Waves overtopping a test wave generator of the Wave Dragon in Nissum-Bredning, Denmark. Image courtesy of Earth Vision. See Wave Dragon
The challenges of wave power are: • efficiently converting wave motion into electricity --generally 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.
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.
Wave farms • Portugal claims the world's first commercial wave farm, 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.
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.
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.
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.
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.
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.
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.
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.
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%.
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.
View of a land based OTEC facility at Keahole Point on the Kona coast of Hawaii
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.
Left: Pipes used for OTEC.Right: Floating OTEC plant constructed in India in 2000
Closed-cycle Diagram of a closed cycle OTEC plant
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.
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.
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.
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.
