HAWT Disadvantages

1
HAWT Disadvantages

• The tall towers and blades up to 90 meters long
  are difficult to transport. Transportation can be
  20 of equipment costs.
• Tall HAWTs are difficult to install, needing very
  tall and expensive cranes and skilled operators.
• Massive tower construction is required to support
  the heavy blades, gearbox, and generator.
• Reflections from tall HAWTs may affect side lobes
  of radar installations creating signal clutter,
  although filtering can suppress it.
• Their height makes them obtrusively visible
  across large areas, disrupting the appearance of
  the landscape and sometimes creating local
  opposition.
• Downwind variants suffer from fatigue and
  structural failure caused by turbulence when a
  blade passes through the tower's wind shadow (for
  this reason, the majority of HAWTs use an upwind
  design, with the rotor facing the wind in front
  of the tower).
• HAWTs require an additional yaw control mechanism
  to turn the blades toward the wind.

2
VAWT

• Vertical Axis Wind Turbines
• have the main rotor shaft arranged vertically.
• turbine does not need to be pointed into the wind
  to be effective. This is an advantage on sites
  where the wind direction is highly variable.
• VAWTs can utilize winds from varying directions.

3
Types of VAWT

• Darrieus wind turbine
• "Eggbeater" turbines. They have good efficiency,
  but poor reliability. Also, they generally
  require some external power source, or an
  additional Savonius rotor, to start turning.
• Giromill
• A subtype of Darrieus turbine with straight, as
  opposed to curved, blades. The cycloturbine
  variety has variable pitch and is self-starting.
• more efficient operation in turbulent winds and
  a lower blade speed ratio which lowers blade
  bending stresses. Straight, V, or curved blades
  may be used.
• Savonius wind turbine
• These are drag-type devices with two (or more)
  scoops that are used in anemometers, Flettner
  vents (commonly seen on bus and van roofs), and
  in some high-reliability low-efficiency power
  turbines. They are always self-starting if there
  are at least three scoops. They sometimes have
  long helical scoops to give a smooth torque.

4
VAWT Advantages

• A massive tower structure is less frequently
  used, as VAWTs are more frequently mounted with
  the lower bearing mounted near the ground.
• A VAWT can be located nearer the ground, making
  it easier to maintain the moving parts.
• VAWTs have lower wind startup speeds than HAWTs.
  Typically, they start creating electricity at 6
  m.p.h. (10 km/h).
• VAWTs may be built at locations where taller
  structures are prohibited.
• VAWTs situated close to the ground can take
  advantage of locations where mesas, hilltops,
  ridgelines, and passes funnel the wind and
  increase wind velocity.
• VAWTs may have a lower noise signature.

5
VAWT disadvantages

• Most VAWTs produce energy at only 50 of the
  efficiency of HAWTs
• A VAWT that uses guy-wires to hold it in place
  puts stress on the bottom bearing as all the
  weight of the rotor is on the bearing. Guy 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 guy wired models.
• VAWTs' parts are located under the weight of the
  structure above it, which can make changing out
  parts nearly impossible without dismantling the
  structure if not designed properly.
• Because VAWTs are not commonly deployed due
  mainly to the serious disadvantages mentioned
  above, they appear novel to those not familiar
  with the wind industry. This has often made them
  the subject of wild claims and investment scams
  over the last 50 years.

6
Efficiencies based on blade type
7
Overall Criticisms of Wind Turbines

• wind power is an intermittent power source. The
  production from a wind turbine may increase or
  decrease dramatically over a short period of time
  with little or no warning. In the absence of
  large scale energy storage, the balance of the
  grid must be able to quickly compensate for this
  change. A proposed solution is a super grid of
  wind farms.
• Economics high quality wind resources are often
  located in areas inhospitable to people,
  logistics and transmission capacity can introduce
  significant obstacles to new installations.
• The impact of wind turbines on wildlife has often
  been cited as a disadvantage of wind
  installations. Wind turbines can pose a danger to
  birds and bats, though the magnitude and gravity
  of this danger may be much less than threats such
  as house cats or plate glass.

8
Wind Farms

• A group of turbines in the same location
• 3 types
• Onshore- within 30km of the shore line
• Near shore -within 3km of the shoreline or 10 km
  offshore
• Off shore -more than 10Km from land
• Noise is a big issue for onshore and near shore,
  as is aesthetics

9
Offshore wind farms
10
Offshore wind farms

• less obtrusive than turbines on land
• apparent size and noise is mitigated by distance.
  
• the average wind speed is usually considerably
  higher over open water.
• Offshore installation is more expensive than
  onshore
• Offshore towers are generally taller than onshore
  towers once the submerged height is included.
• Offshore foundations may be more expensive to
  build.
• Power transmission from offshore turbines is
  through undersea cable
• Offshore saltwater environments also raise
  maintenance costs by corroding the towers, but
  fresh-water locations such as the Great Lakes do
  not.
• Turbine components (rotor blades, tower sections)
  can be transported by barge, making large parts
  easier to transport offshore than on land, where
  turn clearances and underpass clearances of
  available roads limit the size of turbine
  components that can be moved by truck. Similarly,
  large construction cranes are difficult to move
  to remote wind farms on land, but crane vessels
  easily move over water.
• Offshore wind farms tend to be quite large,
  often involving over 100 turbines.

11
Cape Wind Project

• Approved offshore wind farm off of Cape Cod, MA.
• 130 wind turbines would produce a maximum of 454
  MW enough
• power for 420,000 homes.
  
• Would provide 75 of the electrical needs to Cape
  Cod. and the
  Islands
• Concerns included ruining the views from people's
  private property.
• Views from public property such as beaches (even
  though it would be about twenty or so miles
  offshore, people complained it would ruin their
  views of the horizon).
• decrease property values.
• ruining popular areas for yachting.
• the proposed wind farm would be located near
  shipping lanes.
• Local fishermen, cite the fact that for many of
  them, up to 60 of their annual income comes from
  catch caught on Horseshoe Shoals, which they
  claim would disappear and would have to be
  replaced by steaming to fishing grounds farther
  out to sea if the project is completed.
• Some who oppose the project are concerned about
  the corporate privatization of public
  property.

12
Interesting co-generation idea with cars and wind
turbines

• Turbines suspended over highways.
• Each turbine can light a medium size apartment

13
TVA wind farm near Oak Ridge
14
Ocean Thermal Energy

• Energy is available from the ocean by
• Tapping ocean currents
• Using the ocean as a heat engine
• Tidal energy
• Wave energy

15
Energy from ocean currents

• Ocean currents flow at a steady velocity
• Place turbines in these currents (like the gulf
  stream) that operate just like wind turbines
• Water is more than 800 times denser than air, so
  for the same surface area, water moving 12 miles
  per hour exerts about the same amount of force as
  a constant 110 mph wind.
• Expensive proposition
• Upkeep could be expensive and complicated
• Environmental concerns
• species protection (including fish and marine
  mammals) from injury from turning turbine blades.
  
• Consideration of shipping routes and present
  recreational uses of location
• Other considerations include risks from slowing
  the current flow by extracting energy.

16
The ocean as a heat engine

• There can be a 20 difference between ocean
  surface temps and the temp at 1000m
• The surface acts as the heat source, the deeper
  cold water acts as a heat sink.
• Temperature differences are very steady
• Florida, Puerto Rico, Hawaii and other pacific
  islands are well suited to take advantage of this
  idea.
• Called OTEC (Ocean Thermal Energy Conversion)

17
Types of Ocean heat engines

• Closed cycle system
• Heat from warm seawater causes a fluid like
  ammonia to be evaporated in an evaporator
• Expanding vapor rotates a turbine connected to an
  electric generator.
• Cold seawater is brought up and cools the ammonia
  vapor in a condenser. This liquid returns to the
  evaporator and the process repeats.

18
Types of OTECs

• Open Cycle Systems
• Working fluid is the seawater.
• Warm seawater is brought into a partial vacuum.
• In the vacuum, the warm seawater boils and the
  steam drives a turbine
• The steam enters a condenser, where it is cooled
  by cold seawater brought up form below and it
  condenses back into liquid and is discharged into
  the ocean.

19
Boiling water in a vacuum

• The boiling point of any liquid depends upon
  temperature and pressure.
• Boiling occurs when the molecules in the liquid
  have enough energy to break free from surrounding
  molecules
• If you reduce the pressure, you reduce the amount
  of energy needed for the molecules to break free.
  
• Creating a vacuum reduces the air pressure on the
  molecules and lowers the boiling point.

20
OTECs

• Carnot Efficiency is low, only about 7
• Net efficiency even lower, only about 2.5
• Low efficiencies require large water volumes to
  produce appreciable amount of electricity
• For 100 mW output, you would need 25 X 106
  liters/sec of warm and cold water.
• For a 40 mW plant, a 10 meter wide intake pipe is
  needed. This is the size of a traffic tunnel.

21
History of OTECs

• Jacques d Arsonval in 1881 first proposed the
  idea
• Completed by his student, Georges Claude in 1930.
  (Claude also invented the neon lightbulb)
• Claude built and tested the first OTEC system
• Not much further interest until the energy crisis
  of the 1970s.
• In the 1970s, US DOE financed large floating OTEC
  power plant to provide power to islands
• One was built in Hawaii.
• Little further support

22
OTEC Plant on Keahole Point, Hawaii
23
Other uses for OTEC plants

• Generate Hydrogen for use as a clean fuel source
• Generate fertilizer from biological nutrients
  that are drawn up from the ocean floor in the
  cold water intake.
• Source of ocean water to be used as drinking
  water via desalination (taking out the salt).