8'4: NonFossil Fuel Power Production

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8.4 Non-Fossil Fuel Power Production

• Hydroelectric Power
• By Mic Chan

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Table of Contents

• Definition
• 8.415 - Hydroelectric schemes
• Pumped storage hydroelectricity
• Run-of-the-river plants
• Tidal power
• 8.416 - Energy transformation
• 8.417 - Solving problems involving hydroelectric
  schemes

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Definition

• Power produced through the use of the
  gravitational forces of falling or flowing water
• Most widely used form of renewable energy
• Produces no direct waste
• Considerably different output level of carbon
  dioxide than fossil fueled energy plants
• Produced 19 of the worlds electricity in 2005

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Hydroelectric Schemes

• 3 types to be discussed
• Pumped storage hydroelectricity
• Run-of-the-river plants
• Tidal power

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Pumped storage hydroelectricity

• Energy stored in the form of water, pumped from a
  lower elevation reservoir to a higher elevation
• Low-cost off-peak electric power is used to run
  the pumps
• When there is higher demand, water is released
  back into the lower reservoir through a turbine,
  generating electricity
• Reversible turbine/generator assemblies act as
  pump and turbine

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Pumped storage hydroelectricity (continued)

• Although the losses of pumping process makes the
  plant a net consumer of energy overall, the
  system increases revenues by selling more
  electricity during periods of peak demand, when
  electricity prices are highest
• Pumped storage is the largest-capacity form of
  grid energy storage now available
• Taking into account evaporation losses from the
  exposed water surface and conversion losses,
  approximately 70 to 85 of the electrical energy
  used to pump the water into the elevated
  reservoir can be regained
• The technique is currently the most
  cost-effective means of storing large amounts of
  electrical energy on an operating basis

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Diagram of the TVA pumped storage facility at
Raccoon Mountain Pumped-Storage Plant
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Run-of-the-river plants

• The natural flow and elevation drop of a river
  are used to generate electricity
• Power stations of this type are built on rivers
  with a consistent and steady flow, either natural
  or through the use of a large reservoir at the
  head of the river which then can provide a
  regulated steady flow for stations down-river

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Run-of-the-river concept

• Power stations on rivers with great seasonal
  fluctuations require a large reservoir in order
  to operate during the dry season, resulting in
  the necessity to impound and flood large tracts
  of land
• In contrast, run of river projects do not require
  a large impoundment of water
• Instead, some of the water is diverted from a
  river, and sent into a pipe called a penstock
• The penstock feeds the water downhill to the
  power station's turbines

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Run-of-the-river concept (continued)

• Because of the difference in altitude, potential
  energy from the water up river is transformed
  into kinetic energy while it flows downriver
  through the penstock, giving it the speed
  required to spin the turbines that in turn
  transform this kinetic energy into electrical
  energy
• The water leaves the generating station and is
  returned to the river without altering the
  existing flow or water levels
• Most run-of-river power plants will have a dam
  across the full width of the river to utilize all
  the river's water for electricity generation
• Such installations will have a reservoir behind
  the dam but since flooding is minimal, they can
  be considered "run-of-river"

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Chief Joseph Dam near Bridgeport, Washington,
USA, is a major run-of-river station without a
sizeable reservoir.
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Run-of-the-river advantages

• Flooding the upper part of the river is not
  required as it doesn't need a large reservoir
• As a result, people living at or near the river
  don't need to be relocated and natural habitats
  are preserved, reducing the environmental impact
  as compared to reservoirs

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Run-of-the-river disadvantages

• The output of the power plant is highly dependent
  on natural run-off
• Spring melts will create a lot of energy while
  dry seasons will create relatively little energy
• This disadvantage can be reduced if a site with
  consistent flow is chosen
• A run-of-the-river power plant has little or no
  capacity for energy storage and hence can't
  co-ordinate the output of electricity generation
  to match consumer demand

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Tidal Power

• Converts the energy of tides into electricity or
  other useful forms of power
• Although not yet widely used, tidal power has
  potential for future electricity generation
• Tides are more predictable than wind energy and
  solar power

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Tidal power (continued)

• Tidal power is the only form of energy which
  derives directly from the relative motions of the
  Earth-Moon system, and to a lesser extent from
  the Earth-Sun system
• Periodic changes of water levels, and associated
  tidal currents, are due to the gravitational
  attraction by the Sun and Moon
• The magnitude of the tide at a location is the
  result of
• The changing positions of the Moon and Sun
  relative to the Earth
• The effects of Earth rotation
• The local shape of the sea floor and coastlines

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Tidal power (continued)

• Earth's tides are caused by the tidal forces due
  to gravitational interaction with the Moon and
  Sun, and the Earth's rotation
• Tidal power is practically inexhaustible
• Classified as a renewable energy source
• A tidal energy generator uses this phenomenon to
  generate energy
• The stronger the tide, either in water level
  height or tidal current velocities, the greater
  the potential for tidal energy generation

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Tidal power (continued)

• Tidal power can be classified into two main
  types
• Tidal stream systems make use of the kinetic
  energy of moving water to power turbines, in a
  similar way to windmills that use moving air
• This method is gaining in popularity because of
  the lower cost and lower ecological impact
  compared to barrages

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Tidal power (continued)

• Barrages make use of the potential energy in the
  difference in height between high and low tides
• Barrages are essentially dams across the full
  width of a tidal estuary
• Disadvantages
• Suffer from very high civil infrastructure costs
• A worldwide shortage of viable sites
• Environmental issues
• Tidal lagoons, are similar to barrages, but can
  be constructed as self contained structures, not
  fully across an estuary
• Are claimed to incur much lower cost and impact
  overall
• Furthermore they can be configured to generate
  continuously which is not the case with barrages

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Tidal power - environmental effects

• 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
• 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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Tidal power - fish

• Fish may move through sluices safely, but when
  these are closed, fish will seek out turbines and
  attempt to swim through them
• 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.)
• 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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Energy transformation

• Run-of-the-river
• Water is diverted from a river, and sent into the
  penstock, which feeds the water downhill to the
  power station's turbines
• Because of the difference in altitude, potential
  energy from the water up river is transformed
  into kinetic energy while it flows downriver
  through the penstock
• This gives the water the speed required to spin
  the turbines that in turn transform this kinetic
  energy into electrical energy

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Solving problems using hydroelectric schemes

• The energy available from barrage is dependent on
  the volume of water. The potential energy
  contained in a volume of water is
• where
• h is the vertical tidal range
• A is the horizontal area of the barrage basin
• ? is the density of water 1025 kg per cubic
  meter (seawater varies between 1021 and 1030 kg
  per cubic meter)
• g is the acceleration due to the Earths gravity
  9.81 meters per second squared
• The factor half is due to the fact, that as the
  basin flows empty through the turbines, the
  hydraulic head over the dam reduces. The maximum
  head is only available at the moment of low
  water, assuming the high water level is still
  present in the basin.

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Example calculation of tidal power generation

• Assumptions
• Let us assume that the tidal range of tide at a
  particular place is 32 feet 10 m (approx)
• The surface of the tidal energy harnessing plant
  is 9 km2 (3 km ? 3 km) 3000 m ? 3000 m 9 ?
  106 m2
• Specific density of sea water 1025.18 kg/m3
• Mass of the water volume of water ? specific
  gravity
• (area ? tidal range) of water ? mass density
• (9 ? 106 m2 ? 10 m) ? 1025.18 kg/m3
• 92 ? 109 kg (approx)

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Example calculation of tidal power generation
(continued)

• Potential energy content of the water in the
  basin at high tide 1/2 area ? density ?
  gravitational acceleration ? tidal range squared
• 1/2 ? 9 ? 106 m2 ? 1025 kg/m3 ? 9.81 m/s2 ?(10
  m)2
• 4.5 ? 1012 J (approx)
• Now we have 2 high tides and 2 low tides every
  day. At low tide the potential energy is zero.
• Therefore the total energy potential per day
  Energy for a single high tide ? 2
• 4.5 ? 1012 J ? 2
• 9 ? 1012 J

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Example calculation of tidal power generation
(continued)

• Therefore, the mean power generation potential
  Energy generation potential / time in 1 day
• 9 ?1012 J / 86400 s
• 104 MW
• Assuming the power conversion efficiency to be
  30 The daily-average power generated 104 MW
  30 / 100
• 31 MW (approx)