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.
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.
Types of VAWT
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Darrieus wind turbine
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"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
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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
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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.
VAWT Advantages
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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.
VAWT disadvantages
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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.
Efficiencies based on blade type
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.
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
Offshore wind farms
Offshore wind farms
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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.
Cape Wind Project
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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.
Interesting co-generation idea with
cars and wind turbines
• Turbines suspended
over highways.
• Each turbine can light a
medium size apartment
TVA wind farm near Oak Ridge
Ocean Thermal Energy
• Energy is available from the ocean by
– Tapping ocean currents
– Using the ocean as a heat engine
– Tidal energy
– Wave energy
Energy from ocean currents
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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
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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.
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)
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.
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.
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.
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.
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
OTEC Plant on Keahole Point, Hawaii
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).