Wind power Flashcards
1
Q
Which country has the highest share of wind power in their energy mix?
A
China
2
Q
How has the development of wind turbines looked?
A
The installed capacity per turbine has increased
3
Q
Explain the concept of LCOE
A
LCOE is a measurement to compare different energy production technologies. It is the cost of producing 1 kWh of electricity.
Renewable power production sources have low LCOE and onshore wind is the lowest of them
4
Q
Which two types of wind turbines exists?
A
- Vertical-axis wind turbine (VAWT)
- Horizontal-axis wind turbine (HAWT)
5
Q
Discuss pros and cons of VAWT
A
- Smaller turbine
- Low capacity, but produces even at low wind speeds
- Resource heavy - hard to build to withstand forces
- Difficult to scale up
- If you don’t need as much power & want to produce more of the time, this could be a good choice
6
Q
Discuss pros and cons of HAWT
A
- Easy to scale up
- Commercial turbine
- Cannot produce at too low wind speeds
7
Q
What is rated wind and rated power?
A
The rated wind is the windsepeed at which turbine produces maximum power. Around 11-13 m/s
Rated power is the maximal power at the rated wind
8
Q
Explain the difference between operational hours, full load hours and availability
A
Operational hours: 8000. Hours turbine can produce something
Full load hours: 3200-3600. Hours that the wind turbine produces at maximum power
Availability: The actual number of hours in operation during operational hours
9
Q
How are the full load hours calculated?
A
Full load hours = Annual production*h/Max power
10
Q
How is the capacity factor calculated?
A
Annual production/(Max power*Hours of the year)
11
Q
What is the capacity factor of wind power?
A
Around 20-40% for onshore wind
12
Q
What is the purpose of the gear box?
A
The gearbox increases the main shaft’s rpm to the rotational speed of the generator. Usually a 3-stage gearbox
13
Q
What are the benefits of using a gearbox?
A
To match the speed between the rotor and the generator, the generator needs to be much bigger. Without gearbox, rotor speed & generator speed needs to be the same
14
Q
A
15
Q
What different foundations exists?
A
- Onshore:
Gravity foundation (concrete)
Rock foundation
- Offshore:
Gravity foundation
Jacket
Tripod
Monopile - one pile down (elonging the pile to the seabed)
Floating
16
Q
Why has the size of wind turbines increased over time?
A
- The wind speed increases with height, meaning that the higher you are, the more power you can get. If you also have a larger area you get more power. More efficient when larger and you can also gain from economic scaling, where increasing size leads to lower cost per kWh.
17
Q
What is the tip speed ratio?
A
- Ratio between blade blade tip velocity and wind speed
lambda = omega*r/v_wind
The tip-speed-ratio is given by how fast the blades rorate in relation to the speed of the wind.
Too slow rotation –> too little wind energy absorbtion
Too fast rotation –> forms a wall (no wind will pass)
Optimum TSR depends on number of blades. The fewer the blades, the faster the turbine must rotate to achieve maximum energy
18
Q
Discuss pros and cons of variable rotor speed
A
*Benefits:
- less noise (due to less mechanical noise)
- less stress
- smaller power variations
- possible to connect to weak network
- Disadvantages:
- more expensive electrical system
19
Q
Which are the most common generators used?
A
- Doubly fed induction generator (DFIG)
- Electrically excited synchronous generator
- Permanent magnet synchronous generator
20
Q
What is the purpose of power control?
A
Wind turbines are dimensioned for certain loads. If these are exceeded, the turbine will break down.
The loads will increase at wind speeds above rated wind speed. To compensate, the turbine will limit the power production by control activities
21
Q
Which power control activities exists?
A
- Pitch control: turn the blades
- Stall controll: rigid blades, turbulence limit the power
- Active stall: the pitch angle can be adjusted
22
Q
How are winds formed?
A
- Winds occur due to pressure differences in the air, which arise from temperature differences
- Pressure wants to equalize: air moves from high pressure areas to low pressure areas = wind
- Temperature differences occurs due to that different parts of the earth gets different amount of solar radiation
- The rotation of the earth gives us diurnal variations in temperature
- The axial tilt of the earth give rise to different seasons when the earth moves around the sun
23
Q
Where is the wind speed at its highest?
A
At the sea and the coasts
24
Q
How does the ground affect the wind?
A
- The ground will slow down the wind
- The closer the ground, the more friction
- The higher above the ground, the higher wind speed. The speed of the increase in wind speeds depends on the ground
- Different grounds have different friction
25
Explain the concept of turbulence
* Undisturbed wind will move straight in the horizontal plane
* If obstacles appear: vortices or waves will be created in the air
* These are short variations of the wind speed = turbulence
* Temperature variations can also create turbulence
* The disturbed wind after it has passed through the rotor is called a wake
26
How do you calculate the energy contained in wind?
P = 1/2*m_dot*v^2 = 1/2*rho*A*v^3
Thus, the power increases with the wind speed to the power of three.
To calculate the energy in the wind it is not enough to just know the average wind speed, you also need the frequency of different wind speeds.
27
Explain the concept of generating power in a wind turbine
* To transform the kinetic energy of the wind to electricity, it needs to slow down.
* All energy cannot be transformed, since then the wind speed at the rotor would be 0 and the wind wouldn't want to go through the rotor
P_before = P_el+P_after
0.5*dot_m*v1^2=P_el+0.5*dot_m*v2^2
P_el = 0.5*dot_m(v1^2-v2^2)
28
What is Betz law?
Betz law gives the maximal energy: at most 16/27=59.3% of the energy can be captured.
How efficient a turbine is given by the power coefficient Cp. The maximum value of Cp is 59.3
P = 1/2*rho*A*Cp*v^3
29
What is the drag force?
The drag force is the force that is aligned with the wind.
30
What is the lift force?
The lift force is the force that is perpendicular to the drag. This force is what makes the blades rotate.
31
How is thrust and torque related to drag and lift?
Thrust and torque are resulting forces of lift and drag. A stronger lift results in a higher torque and thrust. The thrust can be seen as the force which results due to the third law of Newton along the whole rotor blade. The torque is the cross product (F_L x r) of the lift force F_L and the radius of the rotor blade, seen from the hub.
The torque is the x-components of the lift and drag and thrust is the y-components.
32
What is the angle of attack and how does it affect the wind turbine?
The angle of attack is the angle between the incoming wind and the chord line. When we increase the angle of attack we increase the lift until we get to a certain angle where we have stalling. Stalling means that the flow is separated too early at the top of the airfoil, sharply reducing the lift coefficient and thus also the lift force.
33
What are vortex-induced vibrations? (VIV)
VIVs occurs when alternating vortex shedding behind a bluff or semi-bluff body induces oscillatory forces, exciting structural vibrations
When the wind hits the cylinder of the tower, vortices will be created inducing oscillatory forces, which (if matched with natural frequency) will generate resonance
34
35
Which different generations of wind power exists?
* Gen 1: Functional energy conversion systems
* Gen 2: Low-cost and reliable turbines
* Gen 3: Controllable wind plants supporting the grid (we are now here)
* Gen 4: Achieving a carbon-neutral energy system
36
Explain variability of wind
Wind varies from timescales of ms to years. The ms timescales will impact the airfoil whereas longer variations will impact wind turbines and wind farms
37
Why is wind plant flow management (WPFM) necessary?
* Increasing energy extraction
* Reducing turbine's structural loading
* Lifetime extension and reducing maintenance
* Improving grid integration
* Mitigation of environmental (societal impacts)
* How to achieve?
- Advanced plant design
- Innovative control strategies?
- Data-driven optimization
38
What are the 3 most important reasons for WPFM?
1. Increased energy production
2. Potential reduction of turbine structural loads and lifetime extension
3. Operations and maintenance (O&M) optimization
39
What factors affect the efficiency of wind turbines?
Atmospheric flows, local topograph and turbine wakes
40
What is the axial induction factor and how can it be affected?
The axial induction factor measures the reduction of wind speed by the rotor.
By changing the blade pitch angle or the generator torque, we can affect lambda which in turn affects the axial induction factor.
In other words: By changing pitch angle and rotational speed we can increase/decrease power production
41
What are wind farm flow control strategies?
* Strategies to control wind farms
* Physical control of the wake characteristics
* Optimize the performance of wind farms by adjusting the turbine set-points (e.g., yaw, induction, pitch, tilt)
42
What are the two main categories of WFFC?
* Static: adapts to slow changes in metrological variations, e.g., wind direction and speed, over the day
* Dynamic: reacts to faster flow physics, e.g., wind gusts and direct impact on the wake mixing
43
Explain the boundary-layer turbulence
* Atmospheric boundary layer (ABL):
- lower part of the atmosphere
- varying height and turbulence
* Wind farm- ABL integration:
- turbines as flow actuators
- large wind farms affect the ABL
We want the wake wind to be mixed with the surrounding air
44
Which different wake breakup mechanisms exists?
* Tip- and/or root-vortex instabilities: triggered by blade rotation modulation, helicoidal tip-vortex system dominating at low-turbulence. Tip-vortex works like a shield, making mixing of air easier (increases velocity by mixing)
* Wake meandering: triggered by large-scale motions in background turbulence, leads to increased dynamic loading on downstream turbines.
* Vortex rings: triggered by dynamic thrust modulation, draw high-speed air into the wake core
45
Which two wake-control mechanisms exists?
* Static wake control: slow changes of turbine control degrees of freedom to affect wake's time-averaged properties
* Dynamic wake control: Manipulation of wakes in wind farms for speeding up wake breakup to increase energy extraction
46
Which WFFC strategies exists?
* Static:
- Induction (derating): to reduce wake losses
- Yaw: to deflect wakes away from downstream turbines
- Yaw & induction: to optimize both power and load
* Dynamic:
- Induction: to increase wake mixing
- Yaw (and induction): to trigger the wake meandering, less effective in higher turbulence
- Individual pitch: to mitigate loading, improving mixing and thus power
47
What is axial-induction-based control?
* Adjusting the power and thrust setpoints (SPs) of turbine by changing the blade pitch or tip-speed ratio or both
* Under-induction: reduces the power and thrust SPs of upstream turbines, aiming to increase the kinetic energy in the wake and to enhance the power of downstream turbines
* Over-induction: increases the thrust and reduces the power SPs of upstream turbines, aiming to induce faster wake recovery and reduce wake losses of downstream turbines (more effective wake mitigation, but also higher structural loading)
48
Explain what wake steering using yaw offsets is?
* Most advanced control approach, commercially proven but encountering several research challenges
* Complexity of the wake response to yaw SP due to atmosheric conditions (stratification, wind veer)
* Inducing a lateral flow component, due to the curled and asymmetric wake shape, changing inflow direction for downstream turbines
* Beneficial for increased power but may increase loading on downstream turbines
49
Explain the principle of dynamic individual pitch control
* Principle is built on changing the angle of attack
* Cyclic individual pitch control to control turbine structural loads
* Actuation of helical wake modes to improve mixing, and thus power extraction, at downstream turbines
* Compared to collective dynamic induction control, smaller actuation amplitudes, lower structural loading and better wake breakup
50
Explain the open challenges & opportunities in WFFC
* Physics: Understanding and exploting quasi-steady and dynamic control effects on wake shape, turbulence and mesoscale phenomena
* Algorithms: Developing closed-loop control models, and ML methods to reduce uncertainty and model errors
* Validation: Testing and implementing new control ideas using simulations, wind tunnel and field campaigns, as well as ensuring safety and commercial value
51
Explain what a induction generator with soft-starter (thyristor is)
* Robust generator
* Low maintenance
* Simple system
* High mechanical forces
* Not so common on large machines > 1.5 MW
52
How is the flux related to the frequency in the windings of a motor?
The flux is proportional to the frequency
53
Which two types of induction generator rotors exists?
* Rotor windings with sliprings and brushes
* Short circuit rotor
54
What is the inrush current?
When you connect induction generator to the grid it will be an inrush current generating a flux. This inrush current is high
55
What is a soft starter?
* Device that is active during starting/connection of induction generator
* Works by letting small voltage pass first, inducing a small current & after a while it lets everything pass to get a normal operation
* It reduces the inrush current
56
What is the purpose of capacitors?
They reduce the reactive power from the grid
57
How does the strenght of the grid affect voltage stability?
A strong grid will lead to small voltage deviations --> higher stability
58
What happens to the grid voltage when you connect an induction generator?
In the beginning, the asynchronous motor will consume reactive power, causing the voltage to drop. After a while it will start producing active power which will increase the voltage again
59
What is the purpose of adding rotor resistances to and induction generator?
Increased resistance & increased speed gives more time to pitch blades.
The resistances generates heat (losses) and are thus not viable for larger turbines
60
What is the purpose of a thyristor?
To generate AC voltage from DC-voltage
61
What is the purpose of an IGBT?
* Can switch on and off the current
* AC/DC or DC/AC
* Control of active and reactive power
62
Explain the working principle of a DFIG (double fed induction generator)
The generator is both directly connected to the grid but also to a rectifier (AC to DC) and an inverter (DC to AC), which will then go into a transformer after which it will enter the grid
* Limited speed variation
* Limited power of the converter
* Good efficiency
* Good control of P and Q
* Slip rings = maintenance
63
What are the pros and cons of a full power converter
* Full control of P & Q
* All power through the converter
* Higher losses
* Generator AG, SG, PM
The generator is directly connected to a rectifier, which in turn is connected to an inverter that is connected to the grid
64
Discuss pros and cons of having no gearbox
* Lower the costs for the drive line
* Lower the losses in the energy conversion from mechanical rotation to electric power
* Increase the availability for the wind turbine, ther by higher energy production
* More heavy
* More costly
* Becoming more common offshore due to expensive maintenance
65
How does the development of wind farm connections look?
* Today: point to point connections
* Tomorrow: Meshed grids
- Depending on needs of power & production of power, it is possible to control direction of flow
66
Discuss pros and cons of using AC for transmission
* Well known
- "Limited" transmission distance
67
Discuss pros and cons of using DC for transmission
* Lower transmission losses
* Easy to connect to the grid
* More control is needed
* About the same cost as for AC
68
Which are the important components in a DC-grid?
* DC cable
* DC/AC converter
* DC/DC transformers
* DC breaker
The last two needs further development
69
How does a variable speed wind turbing with AC-output look?
* The generator is connected to a rectifier, which is connected to an inverter and then to a transformer to the grid
70
How does a variable speed wind turbine with DC-output look?
The generator is connected to the rectifier, then inverter. After the incerter, there is a transformer after which another rectifier is used to get DC
By having higher frequency after the inverter, the transformer will be smaller
71
Explain the working principles of a DC-based wind farm
* The wind turbines are connected to an internal DC bus
* After the bus, DC/DC converters are used to increase the voltage levels
* This enables lower weight for the transformer and longer cable distances possible
72
Explain the design of fullbridge converters
* Choice of switching frequency
* A trade-off between low weight and low losses
73
Describe the properties of autonomous power systems
* Isolated from main grid
* Low inertia in the system
* Fast and relative large power fluctuations
* Large voltage fluctuations
* Large frequency variations +/- 2 Hz (old system)
* Power electronic converters can increase power quality
74
Describe the environmental impacts of wind power
* Environmentally friendly
* Different kind of environmental impact:
* Impact on the ecosystem
- Chemical/physical environmental impact (e.g. acidification, eutrophication, greenhouse effect, environmental toxins) (In wind power and society)
- Impact on flaura and fauna (in impact on nature)
* Health and comfort (impact on humans)
- Disturbance for nearby residents (noise, shadows, ice)
- Recreation values
* Cultural environment
- Impact on the landscape (impact on landscape and tourism)
- Impact on cultural monuments
* Ecosystem
- Physical environmental impact: foundation, site, road, power lines
- The space around the plant can be used for agriculture, forestry, hunting, etc.
- No emissions that affect the environment
* Can be demolished without trace
* 3-6 months until it has produced energy required for production
* Can be local, regional or global
- Global: e.g. carbon dioxide emissions
- Local: e.g. mining
* Physical environmental impact
* No emissions from wind power
75
Describe wind power's impact on land use
* Does wind power take up a lot of space?
- Depends on what you compare with
- Possibility for coexisting farms etc.
- Depends on where they are placed
- Need plenty of them
* How does it compare to other sources?
- Compared to solar and hydro, wind doesn't take up a lot of space
- Fossil coal requires more place
- Fossil power plants: fuel = mines, transport
* Differentiate between net and gross area.
- Net: the foundation and site
- Gross: the area around the wind turbines in the park
- Compared with e.g. bioenergy, wind doesn't take up a lot of space
76
How does wind power impact birds and bats?
* Impact on birds
- Birds are killed by colliding with the rotor blades or the tower
- Traffic, buildings, power lines, even cats, kill more birds than wind turbines
* Impact on fowls
- Fowls have poorer flight skills than other birds and may risk flying into the tower if visibility is poor or if they are startled
- Buffer zones of 1 km around playgrounds with more than 5 rooster
- The best way to benefit hen birds is to benefit their habitats: playgrounds, places where the chicks are raised and other places where they stay
- Forestry has a greater impact on fowls than wind power
* Impact on birds of prey
- The risk of accidents increases if there is a large population
- More collisions at larger turbines (height and swept area)
- One solution: Buffer zones around nests and flight paths
- Different sizes for different birds: 500 m - 3 km
- The biggest knowledge gap is still the impact on populations
* Impact on birds
- Look at all things that impacts the birds, not just the wind power
- Set population goals for the different species, based on biology
- Focus on population and not on specific individuals
* Impact on bats
- Only some species of bats fly high enough to risk collide with the turbine blades
- The bats are drawn to the turbines by insects that gather there, and the insects are drawn to the turbine due to heat from the machines
- Bats can't fly in high wind speed, so when the wind speed is above 5-6 m/s there will be no bats out and about. Small overlap with wind turbine operation
- Solution:
- Turn of the turbines when the risk for collisions are the highest:
- At night in Aug-Sept
- Wind speed <5 m/s
- Temperature above 15 deg C
--> "bat mode"
77
What is the impact from offshore wind?
* Impact on fish:
- The fish is mostly disturbed during the building of the turbines
- They are affected by piling and that the water get muddy
- The disturbance with piling can be minimized by:
Choosing the right time
Scare away
- Artificial reefs
- Safe playgrounds
* Impact on porpoise
- Porpoises are mainly disturbed by the noise from the wind farm and mostly from piling
- Here, too, it is important to scare the animals away before piling is started
- However, bycatch in fisheries is considered to be the main threat to the porpoises
- Environmental toxins are also a problem for the porpoises
- Noise from boat traffic also disturbs the habitat
* Impact on reindeer
- Poorer grazing conditions
- Forestry, predators and other land use also affect the reindeer's choice of grazing
- A problem for reindeer husbandry is that wind power is always established in highland areas. These are especially important in poor snow conditions, which have become more common with climate change
- Avoids facilities during the calving and autumn periods, but continued to use the upland areas during the summer
78
How does wind power impact humans?
79
