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Renewable Energy > TEST 2 > Flashcards

TEST 2 Flashcards

1
Q

what and where was the fist wind harvesting system founded

A

ancient civilization in the near east / persia.

vertical axis windmill: sails connected to a vertical shaft connected to a grinding stone for milling

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2
Q

what is the current onshore wind turbine LCOE, what is it expected to reach by 2050 and why are the values actually higher

A

5 USC/kWh
2.5 USC/kWh in 2050
they are actually higher due to country ris, local regulations and wind regimes etc.

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3
Q

what are some other terminologies for wind turbines (5) and what can they be classified into?

A

Wind-driven generator
wind generator
wind turbine
wind turbine generator (WTG)
wind energy conversion system (WECS)

most turbines are horizontal axis wind turbines (HAWT)
some are Vertical axis (VAWT)

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4
Q

what are the (1) advantages and (2) disadvantages of downwind HAWT?

A

(tower in front of the rotor)
advantages:
letting wind control the yaw so it naturally orients itself correctly with respect to the wind direction

disadvantages:
every time a blade swings behind the tower, ti encounters a brief period of reduced wind, which causes the bladet o flex (shadowing effect)

flexing may lead to blade failure due to fatigue, increases blade noise and reduces power output

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5
Q

what is the advantage and disadvantage of upwind HAWT

A

(tower behind the rotor)

+ operate more smoothly than downwind machines and deliver more power

  • require complex yaw control to keep the blades facing into the wind
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6
Q

Describe the number of blades of a HAWT (4)?

A

Multi-bladed:
- used for water pumping on farms
- rotor has a large area facing into the wind, which is required for high starting torque and low speed operation for continuous water pumping
- as the speed of the turbine increases, the turbulence caused by one blade affects the efficiency of the blade that follows
- turbines with fewer blades are quieter than multi bladed turbines and have smoother operation

Fewer-bladed:
- turbines operate at much higher rotational speed (generators can be physically smaller in size and cheaper)

Two-bladed:
- initially developed by the Americans are cheaper to fabricate and easier to hoist up to the nacelle

One-bladed:
- turbines with counterweight have also been tried but never deemed worth pursuing

MOST MODERN WIND HAWTS USED ARE THREE ROTOR BLADED

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7
Q

describe the VAWT (8)

A

the shape of the blades is similar to holding a rope on both ends and spinning around a vertical axis

heavy machinery contained in the nacelle and can be located on the ground where it can be serviced easily

tower does not need to be structurally strong as that of HAWT and it can be lighter if guy wires are used

they do not need yaw control to. face the wind

blades of a Darrieus rotor can be relatively lightweight and inexpensive and do not have constant flexing

blades are relatively close to the ground where wind speeds are lower

cannot be mounted on a tower where speeds are higher

not as commercially and economically competitive as HAWTs

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8
Q

what are the (15) principal components of a wind turbine

A

pitch
low speed shaft
gear bod
generator
controller
anemometer
wind vane
nacelle
higher speed shaft
yaw drive
yaw motor
tower
brake
rotor
blades

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9
Q

how does a rotor blade extract energy from the wind

A

the ar on top of the airfoil moves faster causing its pressure to be lower than under the airfoil

this creates a lifting force that holds an airplane up or a wind turbine blade to rotate

an airfoil (blade of turbine) takes advantage of bernoulli’s principle to obtain lift from the ar moving towards it

air moving over the top of the airfoil has a greater distance to travel before it can rejoin the air that took the shorter cut under the foil

rotating blades sees air moving toward it not only from the wind itself but also from the relative motion of the blade as it rotates

the combination of the wind and blade motion is like adding two vectors with the resultant moving across the airfoil at the correct angle to obtain life that moves the rotor along

since the blade is moving much faster at. the tip than ner the hub, the blade must be twisted along tis length to keep the angles right

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10
Q

what is the angle of attack and how does increasing it and decreasing affect the airfoil

A

the between the airfoil and the wind is called the angel of attack

increasing the angle of attack up to a point improves the lift at the expense of increased drag

increasing the angle of attack too much can result in a phenomenon known as stall (airflow on to no longer sticks and the surface and the resulting turbulence destroys lift)

in a wind turbine, stall can be a good thing

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11
Q

what are the methods to achieve power shedding

A

active pitch-control system
passive & active stall-control design
passive & active yaw-control system

small machines require a combination of these methods

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12
Q

describe active pitch control

A

electronic systems monitor the generator output and if the power exceeds specifications, the pitch of the turbine blades is adjusted to shed some of the wind

active pitch-control uses hydraulic system that slowly rotates the blades about their longitudinal axes, turning them a few degrees at a time to reduce or increase their efficiency as conditions dictate

the strategy of active pitch control is to reduce the blade’s angle of attack when winds are high

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13
Q

describe passive and active stall

A

passive:

the blades are designed to reduce efficiency when wind speeds are excessive.

the aerodynamic design of the blades, especially the twist as a function of distance from the hub,gradually reduces lift as the blades rotate faster (simple and reliable because there are no moving parts)

sacrifices power at lower wind speeds

active:

increases the angle of attack of the blades to induce stall when winds exceed the rated wind speed for the generator

are justified for large machines (more than 1MW)

blades are equipped with spring loaded rotating tips, that at very high winds turn the blades 90 out of the wind to stop the machine within a few revolutions

once rotor has stopped, a mechanical brake, which is especially important for safety during maintenance, locks the rotor shaft in place

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14
Q

describe passive and active yaw control of wind turbines

A

passive:

used to limit the power captured by the turbine during high wind gusts

one method involves mountingthe turbine slightly to the side of the tower so that high winds push the entire machine around the tower

another approach uses vanes that are mounted in parallel to the plane of the blades to rotate the machine away from very strong winds

active:

used to move the nacelle and turbine blades according to the wind direction to capture the maximum available wind

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15
Q

why is passive yaw conrol no longer used

A

low speed of response due to the large amount of inertia of the nacelle and turbine rotor along the yaw axis

reduction of lifespan of the turbine caused by mechanincal stresses and vibrations on different parts of the turbine

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16
Q

when is the maximum efficiency point of a WECS reached

A

wind energy conversion system is reached when the rotor speed matches the wind speed

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17
Q

what is the difference between variable and fixed speed turbines

A

fixed speed turbines offer the simplest, least cost approach to a wind system, but suffer from the inefficiencies associated with not being able to operate the turbine at its maximum power point in addition to the mechanical stress which need a sturdier design

rotor speeds should be variable to maintain maximum efficiency, but that capability comes at a cost

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18
Q

categorize the types of wind turbines

A

variable speed turbines
variable speed turbines
indirect drive (with gearbox)
wound rotor synchronous generator (WRSG)
permanent magnet synchronous generator (PMSG)
squirrel cage induction generator (SCIG)
doubly fed induction generator (DFIG)
wound rotor inductino generator (WRIG) + Variable R

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19
Q

what is betz law

A

the maximum efficiency that can possibly be captured by a wind turbine (kinetic to mechanical)

20
Q

why can’t we extract all the enrgy from the wind?

A

if we did, the wind would come to a complete stop in front of the turbine meaning no wind would be extracted at all. we can only extract one third of the wind ideally

21
Q

how does rotor angular speed affect the efficiency

A

if the rotor spins too slow, it is allowing too much wind to pass by unaffected

if the rotor spins too fast, efficiency is reduced as the turbulence caused by one blade increasingly affects the blade that follows

22
Q

what is the cut in wind speed

A

the minimum speed needed to generate net power

low speed winds may not be enough to overcome friction in the drive train, or offset the power required by generator field windings

no power is generated at windspeeds below Vc

not much energy in tehm so not much is lost

23
Q

what is rated windpeed

A

when winds reach rated

windspeed, the generator delivers as much power as it is designed for

above Vr, wind power must be shed to avoid damaging the generator

24
Q

what is cut out or furling windspeed

A

when wind is so string that there is real danger to the wind turbine, the machine must be shut down

furling means folding up the sail

when wind speed reaches the limit of the turbine, the blades are completely pitched out of the wind to induce stall, and no power will be captured by the blades

once the rotor has stopped, a mechanical brake locks the rotor shaft in place

25
Q

how do rotor diameter and generator size affect idealized power curve? what is the better alternative

A

increasing rotor diameter, while keeping the same generator, shifts the power curve upward so that rater power is reached at lower wind speeds

keeping the same rotor, but increasing generator size allows the power curve to continue upward to the new rated power

for lower wind speeds, there isn’t much change, but with higher wind speeds, increasing the generator rated power is a good strategy (low wind speed -> large rotor/high wind speed ->generator rating)

26
Q

why is rated power rarely used

A

rounding curve in teh vicinity of rated power makes it difficult to determine an appropriate value of the rated wind speed

27
Q

what is the IEC

A

the international electrotechnical commision is a non governmental organization that establishes safety standards for a wide range of electric technologies, including wind energy systems

it covers:
- mechanical loads
- acoustics
- power quality
- construction safety

28
Q

how can we measure wind speed

A

with wind odometers called cup anemometers which spin at a rate proportional to the wind speed

advanced anemometers:
sonic measurements
doppler effect SODAR (SOnic Detection And Ranging)

29
Q

describe the sonic anemometers

A

sends three ultrasonic sound waves accross their measurement space and then measure the time differential at which the sound arrave at opposing sensors

teh can collect rapidly changing, real time wind data in two or three dimensions which make them suited for turbulence measurements

sonic anemometers can be mounted on meteorological towers as well as right on the nacelle of the wind turbine itself

30
Q

describe the SODAR

A

transmit pulses of sound into the air

the length of time required for a pulse to bounce off the atmospheric particles and make its way back to a receiver provides information on the altitude of the reflecting particles

if the particles are moving, a Doppler effect frequency shift in the received signals will detect that motion

vector analysis of the signals received by three sonic cones situated above the transmitter makes it possible to calculate horizontal and vertical wind speeds as well as direction

31
Q

what do k=1 and k=2 and k=3 mean in the context of weibull statistics for wind probability

A

k=1 exponential decay (most winds blow at low speed which is not good)

k=2 winds blow fairly consistent, but there are periods where the wind is well above most frequency wind speeds (MOST REALISTIC)

k=3 pdf looks like a normal bell-shaped function in which the winds
always blow at a fairly constant speed

32
Q

how does changing the average windspeed affect the rayleigh statistic chart

A

increasing average speeds flattens the curve and shift to the right

33
Q

what are the factors that determine how much energy can be captured by a wind turbine

A
  • characteristics of the machine (rotor, gearbox etc)
  • terrain
  • wind regime
34
Q

describe the three perspectives of deciding whether a wind turbine project is worth pursuing

A

policy perspective: specific turbine characteristics are less important than determining estimates of the potential contributions of the technology

potential investors: back of the envelope calculations are sufficient to see whether a proposed project merits further investigation

for engineers: comparing the likely performance of one turbine over another requires a much more carefully done, site specific analysis

35
Q

why is the capacity factor not a good indicator

A

Significant fraction of the wind’s energy is not being captured since the blades
are purposely shedding much of the wind to protect the generator

Larger generators which cost more capture those higher-speed winds →
capacity factor goes down but energy delivered increases

36
Q

what are the advatages of clustering wind turbines together at a windy site

A

reduced site development costs

simplified connections to transmission lines

centralized access for operation and maintenance

37
Q

what are teh direct and indirect land uses of area requirements of wind farms

A

Direct
- access roads
- turbine pads that will remain in place over the life of the project
- areas that are temporarily affected during construction but which can revert to other uses after project completion

indirect
areas mostly those required for turbine spacing and other boundary constraints

38
Q

what are the points that require evaluation when laying out a wind farm

A

prevailing wind directions
irregular terrain considerations
access roads
transmission and grid connection facilities
current and projected landu use for areas between towers

39
Q

what are the advantages of offshore wind farms? what is the disadvantage?(3/3)

A

closer access to large, coastal metropolitan load centers, which can avoid transmission costs and constraints

offshore winds tend to be stronger, steadier and less turbulent and they frequently blow in the afternoon, when most power is needed

if located far enough from the shore, visual and audible noise impacts can be less of an issue

disadvantage:
marine environment is harsh,

access for maintenance purposes can be difficult,

and costs are considerably higher than land-based installaltions

40
Q

what are three phase submarie cables

A

cables that provide transmission from offshore transformers to substations on shore. they have much higher shunt capacitance that typical overhead lines on shore. they also inject reactive power (VARs)

41
Q

what does HVDC transmission require and what are its advantages

A

HVDC requires offshore AC to DC converter along with an onshore DC to AC converter to connect the substation

advantage:
it allows variable speed turbines to seamlessly connect to the fixed frequency grid

it reduces transmission losses to almost negligible amounts even for long distances

dc cables carry more current

power electronics in the converters allow greater control of active and reactive power, much like synchronous generators powered by steam turbines

42
Q

what are the economic factors of wind power

A

capital cost of the complete system (construction, grid connections, engineering etc.)

variable costs (insurance, tax, land leasing etc)

location: whether is is a land based or offshore system (offshore costs about double)

capacity factor: which depends on the wind resource, hub height, and ratio of generator rated power to swept area

incentives, which may be in such forms as production or investment tax credits, utility rebates, renewable energy credits and accelerated depreciation benefits

financing, which includes th mix of debt and equity, as well as the structure of the investment recovery system

43
Q

what are 4 factors that might make local production of wind power undesirable

A

noise

spoiling the landscape

flickering effect

real estate value

44
Q

what is yawning

A

the rotation of the nacelle to face the wind

45
Q

what is the difference between pitch and stall regulation

A

pitch control rotates the blades to control the forces on each blade

stall control uses blades that are especially designed to limit the forces at high wind speeds

46
Q

list 5 advantages of variable speed turbines over fixex speed

A

higher power production

pitch control

reduced mechanical stress

improve power quality

reduced acoustic noise

Renewable Energy (3 decks)

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