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Hydropower is energy extracted from moving water. However it is essentially another way of trapping the sun's energy since it is the sun which drives the flow of water on the planet – the so called hydrologic cycle.The main water reservoirs are of course the oceans and the sun’s energy warms the ocean water and the degree of warming depends on location. The warmer the water, the greater the degree of evaporation and the higher the concentration of water vapor in the atmosphere. This process has an enormous effect on the earth’s climate and, for example, is the main driving force behind hurricanes. As the water vapor laden air rises, it cools, the water condenses and clouds form. The clouds are blown onshore and over land. If conditions are right, perhaps as the air mass rises over a mountain range, the clouds cool sufficiently for the condensed water to reach the point where rain, or even snow is formed. This precipitation eventually makes its way back downhill to the ocean. On its way, some of it replenishes natural underground reservoirs, irrigates agricultural lands, and some of it is used to produce power. Among renewables, water is the second greatest source of electricity production and hydroelectric plants operate wherever suitable waterways are available. In the developed world at least, many of the best of these sites have already been developed.
Generating electricity using water has several advantages. The major advantage is that water is a source of cheap power and requires no imported fuel. In addition, because there is no fuel combustion, there is little air pollution in comparison with fossil fuel plants and limited thermal pollution compared with nuclear plants. However the building of dams, floods land, reduces the river’s flow downstream, which affects the habitats of the local plant, fish, and animal life, and is one reason this form of renewable power has so many opponents. Thus, like other energy sources, the use of water for generation has limitations, including environmental impacts.
Conventional hydropower uses the controlled release of water from a reservoir to power water turbines and produce electricity. Most use a dam to create the reservoir although sometimes this is not necessary. Lake Manpouri, on the South island of New Zealand, is almost 200 feet above sea level and it is used to generate power in Meridian Energy's power plant; water is fed to the turbines through 200ft vertical shafts from the lake and then to Doubtful Sound via two 10km long tunnels (tailraces).
One refinement of the reservoir scheme is pumped storage: in these systems the water flows downhill as normal when needed to power the turbines and produce electricity but, when not being used to produce electricity, the water is pumped back uphill to replenish the reservoir.
Run-of-river systems do not use dams and simply use the force of water in the stream to power a turbine. Such systems are analogous to those used more than 2000 years ago by the Greeks to power grinding wheels or in the Industrial Revolution to power a multiplicity of machines.
Of course most of the earth's water is in the oceans and there are many ways being investigated to harness its energy. These approaches are introduced below but none yet contribute significantly to the total electricity generated via water.
The simplest systems, at least in concept, are analogous to wind turbines, and use water turbines placed in high flow tidal streams to generate power; an example of this is the Evood system illustrated to the left. A second system uses dams to form reservoirs, which are filled by the rising tide and then power turbines in the normal way; the newest and most novel approach is called Dynamic Tidal Power (DTP) by its inventors, H2iD, who are based in Emmeloord, The Netherlands. A DTP dam is a long dam of 30 to 60 km which is built perpendicular to the coast, running straight out into the ocean, without enclosing an area. The horizontal acceleration of the tides is blocked by the dam. In many coastal areas the main tidal movement runs parallel to the coast: the entire mass of the ocean water accelerates in one direction, and later in the day back the other way. A DTP dam is long enough to exert an influence on the horizontal tidal movement, which generates a water level differential (head) over both sides of the dam. The head can be converted into power using a long series of conventional low-head turbines installed in the dam.
Ocean Wave Power
Wave power generation is not currently a widely employed commercial technology although there have been attempts at using it since at least 1890. In 2008, an attempt was made at creating a commercial wave farm in Portugal, at the Aguçadoura Wave Park. The wave farm consisted of three 750kW Pelamis devices which are shown to the right.The sections of the device articulate with the movement of the waves, each resisting motion between it and the next section, creating pressurized oil to drive a hydraulic ram which drives a hydraulic motor.The machine is long and narrow (snake-like) and points into the waves; it attenuates the waves, gathering more energy than its narrow profile suggests. Its articulating sections drive internal hydraulic generators (through the use of pumps and accumulators). In November 2008, just two months after the official opening, the Pelamis machines were brought back to harbor due to problems with some of the bearings.
In the United States, the Pacific Northwest Generating Cooperative is funding the building of a commercial wave-power park at Reedsport, Oregon. The project will utilize the PowerBuoy technology from Ocean Power Technologies which consists of modular, ocean-going buoys. The rising and falling of the waves moves hydraulic fluid within the buoy; this motion is used to spin a generator, and the electricity is transmitted to shore over a submerged transmission line. A 150kW buoy has a diameter of 36feet (11 m) and is 145feet (44 m) tall, with approximately 30 feet of the unit rising above the ocean surface. Using a three-point mooring system, they are designed to be installed one to five miles (8km) offshore in water 100 to 200feet (60 m) deep.
The Oyster wave energy converter is a hydro-electric wave energy device currently being developed by Aquamarine Power. The wave energy device captures the energy found in nearshore waves and converts it into clean usable electricity. The systems consists of a hinged mechanical flap connected to the seabed at around 10m depth. Each passing wave moves the flap which drives hydraulic pistons to deliver high pressure water via a pipeline to an onshore turbine which generates electricity. In November 2009, the first full-scale demonstrator Oyster began producing power when it was launched at the European Marine Energy Centre (EMEC) on Orkney.
An Australian firm, Oceanlinx, is developing a deep-water technology to generate electricity from, ostensibly, easy-to-predict long-wavelength ocean swell oscillations. Oceanlinx recently began installation of a demonstration-scale, grid-connected unit near Sydney, a 2.5 MWe system, that is expected to go online in early 2010, when its power will be connected to the Australian grid. The companies much smaller first-generation prototype unit, in operation since 2006, is now being disassembled. For a comprehensive list of projects as of January 2010 see Figure 14.2 in WEC 2010.
The global wave power resource in deep water (i.e. 100 m or more) is estimated to be ~ 8 000–80 000 TWh compared to global electricity production of 19 855 TWh in 2007 (IEA, 2009).
The Earth's oceans are continually heated by the sun and cover over 70% of the planet's surface. Ocean Thermal Power (OTEC) uses the temperature difference that exists between deep and shallow waters to run a heat engine, providing access to an energy source one or two orders of magnitude greater than other ocean energy options such as wave power. However the temperature difference is small and this makes energy extraction difficult and expensive, due to low thermal efficiency. Earlier OTEC systems had an overall efficiency of only 1 to 3% (the theoretical maximum efficiency lies between 6 and 7%). As with any heat engine, the greatest efficiency and power is produced with the largest temperature difference. This temperature difference generally increases with decreasing latitude, i.e. near the equator, in the tropics. Historically, the main technical challenge of OTEC was to generate significant amounts of power efficiently from this very small temperature ratio but changes in efficiency of heat exchange in modern designs allow performance approaching the theoretical maximum efficiency.
Current designs under review will operate closer to the theoretical maximum efficiency. In May 1993, an open-cycle OTEC plant at Keahole Point, Hawaii, produced 50 kW of electricity during a net power-producing experiment. This broke the record of 40,000 watts set by a Japanese system in 1982. Today, scientists are developing new, cost-effective, state-of-the-art turbines for open-cycle OTEC systems.
Marine Current Power
Marine Current Power is a form of energy obtained from harnessing of the kinetic energy of ocean currents, such as the Gulf Stream. Although not widely used at present, marine current power has an important potential for future electricity generation since they are more predictable than wind and solar power.>
A 2006 report from the United States Department of the Interior estimates that capturing just 1/1,000th of the available energy from the Gulf Stream, which has 21,000 times more energy than Niagara Falls in a flow of water that is 50 times the total flow of all the world’s freshwater rivers, would supply Florida with 35% of its electrical needs.
Osmotic Power, also called salinity gradient power, is the energy available from the difference in the salt concentration between seawater and river water. Two practical methods for this are reverse electrodialysis (RED) and pressure retarded osmosis (PRO). Both processes rely on osmosis with ion specific membranes. The key waste product is brackish water, which is the result of natural forces that are being harnessed: the flow of fresh water into seas that are made up of salt water. The technologies have been confirmed in laboratory conditions. They are being developed into commercial use in the Netherlands (RED) and Norway (PRO). The cost of the membrane has been an obstacle. A new, cheap membrane, based on an electrically modified polyethylene plastic, made it fit for potential commercial use.
The world's first osmotic plant with capacity of 4kW was opened by Statkraft in 2009 in Tofte, Norway. This plant uses a polyimide membrane, and is able to produce 1W/m² of membrane. This amount of power is obtained at 10 l of water flowing through the membrane per second, and at a pressure of 10 bar. Both the increasing of the pressure as well as the flow rate of the water would make it possible to increase the power output. Hypothetically, the output of the SGP-plant
Hydropower is currently being utilized in over 160 countries. At end-2008, global installed hydropower capacity stood at about 874 GW (WEC 2010). As far as possible the data refer to net installed capacity excluding pumped-storage schemes. According to data made available to the IHA, this capacity is derived from some 11 000 stations, with around 27 000 generating units. A comparison with installed capacity is not the same as that of generation, as many countries rely on hydropower less for base-load supply and more for load-following operations; consequently, for example, Canada tends to generate more from hydropower than the U.S. (in 2008, Canada produced 377 TWh, whereas the U.S. produced 255 TWh). Asia, led by China, has overtaken Europe, while North America and South America take third and fourth place respectively. Africa remains the region with the poorest ratio of deployment to potential.
Orders for hydropower equipment clearly demonstrate that hydro development continues to show strong growth well into the future. While there is a dip in 2009-2010, it is reasonable to assume that this reflects recent financialuncertainty. From the period after 2010, growth is substantial, with worldwide hydro capacity expected to grow significantly over the period between 2011 and 2020. Again, an analysis of the regional distribution of this growth confirms earlier comments about financing. As Fig. 7.5 depicts, the growth trends per region remain uneven in the 2011-2020 period, with China, Asia and South America continuing to show strong growth. Africa’s share of new capacity remains small at 5%, compared, for example, with Europe at 13-14%.
The variable nature of the growing portfolio of renewables, as well as the costs associated with shutting down thermal energy options means that there is often excess power in a grid at times of low demand. This has led to an increasingly important role for pumped storage hydro, where, to store energy for use in periods of high demand, water is pumped from a lower to a higher reservoir. Currently, there are more than 127 GW of pumped storage throughout the world. It is anticipated that the market for pumped storage will increase by 60% over the next four years.
Total employment in hydropower was estimated at 39,000 in 2006 in a study by the Worldwatch Institute for UNEP. However this estimate may be for direct jobs only and may exclude those working in large hydropower, which is often regarded as not "green" because of its environmental impact. Currently, hydropower represents about 7 percent of U.S. electricity generation, with about 100 GW of capacity. However, Navigant Consulting research suggests hydropower's technical potential in the US is around 400,000 megawatts of capacity and with increased hydro capacity comes increased hydro industry employment. Currently, the U.S. supports 200,000 to 300,000 hydropower-related jobs, but achievement of a25 percent renewable energy standard by 2025 could lead to 700,000 jobs that could be directly or indirectly related to hydropower.
Given that this describes only the US potential and that most new hydropower expansion will be elsewhere, the prospects for hydropower employment appear excellent.
References and Useful Links
Green Jobs: Towards Decent Work in a Sustainable, Low-Carbon World, UNEP/ILO/IOE/ITUC, September 2008
IEA, 2009Energy Statistics of Non-OECD Countries, International Energy Agency.
Job Creation Opportunities in Hydropower, Lisa Frantzis, Navigant Consulting,National Hydropower Association Annual Conference, April 26, 2010
REN21. 2010Renewables 2010 Global Status Report (Paris: REN21 Secretariat).
WEC 2010 Survey of Energy Resources, World Energy Council, www.worldenergy.org/publications/3040.asp
British Hydropower Association
Canadian Hydropower Association
How Stuff Works
INEEL Hydropower program
International Hydropower Association
National Hydropower Association
US DOE Hydropower