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Environmental technology > Chapter 7 > Flashcards

Chapter 7 Flashcards

1
Q

In terms of wind speed outline the difference between a HAWT and VAWT.

A

HAWT needs high wind speed and VAWT requires low wind speeds

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2
Q

In terms of wind direction outline the difference between a HAWT and VAWT.

A

HAWT requires a relatively consistent wind direction where VAWT can have wind from any direction.

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3
Q

In terms of RPM outline the difference between a HAWT and VAWT.

A

HAWT has high RPM where VAWT has low RPM

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4
Q

In terms of vibration outline the difference between a HAWT and VAWT.

A

HAWT has high, VAWT has low

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5
Q

In terms of Noise outline the difference between a HAWT and VAWT.

A

HAWT has high noise levels where as VAWT has low noise levels.

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6
Q

In terms of rotor axis outline the difference between a HAWT and VAWT.

A

HAWT is on a horizontal plane where as VAWT is on a vertical plane

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7
Q

What does HAWT and VAWT stand for

A

HAWT= horizontal axis wind turbine
VAWT= vertical axis wind turbine

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8
Q

What is the equation for kinetic energy and give the units of each property

A

Ke= 1/2 x m x v2

Ke= in joules
M= in kg
V= m/s

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9
Q

What is the equation for velocity

A

V= square root of 2ke/m

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10
Q

What is the equation for mass

A

M= 2ke/ v2

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11
Q

How do you convert grams into kg

A

Divide by 1000 ie (9000g / 1000 = 9kg)

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12
Q

How do you convert tones into kg

A

Multiply by 1000 ie (0.075 x 1000= 75kg

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13
Q

How do you turn kilojoules into joules.

A

Multiply by 1000

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14
Q

State the equation for kinetic energy.

A

Ke= 1/2 m v square
Ke= kinetic energy in joules
M= mass in kg
V= velocity in m/s

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15
Q

State the equation for mass.

A

2ke/ v squared = M

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16
Q

What is the upper limit Albert betz came up with?

A

59.3%

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17
Q

What is meant by the betz limit?

A

The betz limit is the maximum amount of the wind turbines kinetic energy that a hawt can convert to mechanical energy turning a rotor.

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18
Q

Why can most modern win turbines only convert 35-45%?

A

Sound and thermal losses within the gear box and speed shafts.

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19
Q

Define rotor collected energy.

A

The exact amount of energy extracted from the wind

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20
Q

2 reasons for energy shortfall between rotor collected energy and rated energy output.

A

1- mechanical- friction- Any moving parts rub against each other, causing some of the kinetic energy to change to thermal energy (heat) which increases the temperature of the material. These thermal losses can be minimised by the use of bearings and lubrication on the low speed shaft,

2-Electrical- resistance- Any current moving through a cable will encounter resistance. This causes the wire to heat up and so some electrical energy is converted to thermal energy.

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21
Q

Explain the relationship between power output and swept area.

A

The swept area is directly proportional to the power output

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22
Q

What is the calculation used to calculate swept area?

A

A= pi x r squared

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23
Q

Explain the relationship between wind speed and power production.

A

Pout= 1/2 P A COP v cubed
P out is directly proportional to v3 this means when the wind speed is doubled pout increases by s factor of 8.

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24
Q

What happens p out when wind speeds drop?

A

If wind speeds our divided by 3 then the power output drops by a factor of 27.

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25
Q

Why is there flat lines on a power graph?

A

The flat line at 0 on power axis is because the wind turbine hasn’t reached sufficient cut in speeds.
The flat line at the top is because the wind turbine has reached its maximum rated power.

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26
Q

State the relationship between P out and diameter of a wind turbine.

A

P out is directly proportional to diameter squared this means when the diameter is doubled the power output increases by a factor of 4

27
Q

Describe how the power output is affected by air density for a wind turbine.

A

Using equation 1 it can be said P out is directly proportional to air density as it is not a constant in this equation.

28
Q

State the relationship between P out and temperature when the temperature is high.

A

A higher temperature means the air molecules have more kinetic energy therefore move with greater speed and are more spread out which decreases the air density. Therefore as p out is directly proportional to air density it can be stated that Higher temperature = lower P out.

29
Q

State the relationship between P out and temperature when temperatures are cold.

A

A low temperature, such as in winter time, means the molecules of the gases in air have less kinetic energy and so move with smaller speeds between collisions. This makes the gas contract and so the molecules move closer together This low temperature gives a higher air density as there is a higher mass per unit volume. This effectively means that: Low temperature = Higher Pout

30
Q

State the relationship between P out and altitude.

A

At lower altitude air molecules are closer together and therefore have a higher air density which as seen before will increase P out.
At higher altitudes air molecules are more spread out as the air is thinner and this leads to a lower air density which decreases P out.

31
Q

Define what altitude is and state where it is measured from.

A

Altitude is how high a specific point is above the surface of the Earth. Most often sea level is used as the datum point on the Earth’s surface. The measurement is taken vertically upwards from the surface.

32
Q

State the 8 parameters that effect the maximum output of energy from a wind turbine.

A

The parameters of air density, swept area, wind speed and Coefficient of Performance all impact on the maximum energy that can be produced also nearby obstructions, hub height, temperature and altitude.

33
Q

Explain how wind speed affected maximum energy output of a wind turbine

A

Wind speed is the most critical parameter as power output is directly proportional to v3.
If wind speed our doubled p out will increase by a factor of 8

34
Q

Explain how air density affected maximum energy output of a wind turbine

A

Air density is directly proportional to power output; however, its value will only change by approximately 10% from winter to summer time. As such, its relatively constant value means that it is not a critical consideration when undertaking the CBA.

35
Q

Explain how COP affected maximum energy output of a wind turbine

A

It has been established that some highly efficient wind turbines have a COP of up to 0.45, with those of poor design only reaching a COP of 0.2. This will have a huge impact on the maximum energy that can be generated by the wind turbine and careful selection of the turbine is required.

36
Q

Explain how swept area will affect maximum energy output of a wind turbine

A

The power output is directly proportional to the swept area. While this would suggest that installing a larger turbine is thereby desirable, the larger size will impact purchase cost, price per unit of electrical energy generated, cut in speeds and cut out speeds.

37
Q

Explain how obstructions will affect maximum energy output of a wind turbine

A

The proximity of the site to nearby obstructions such as hedgerows, trees and buildings could have a devastating impact upon maximum energy production. The flow of air around these obstructions produces turbulent air flow which negatively impacts upon the maximum energy that can be produced.

38
Q

Explain how Hub height will affect maximum energy output of a wind turbine

A

Wind speeds are higher in the air than on the ground so higher hub height will enable to maximize wind speeds to make more use out of the wind turbine. Also will help with over coming obstructions.

39
Q

State the equation for repayment period in years for a CBA.

A

Repayment period in years = Project costs / revenue produced per year.

40
Q

State equation one and what the values means.

A

Pout = ½ ρ A v3 COP {Eqn 1}
Where: Pout = Useful power output of the turbine in W.
ρ = Density of air in kg/m3.
A = Swept area of the wind turbine in m2.
v = Velocity of the wind in m/s.
COP = Coefficient of Performance.

41
Q

Define what hub height means.

A

Hub height is the distance from the ground to the center of the hub.

42
Q

Define rotor diameter.

A

Rotor diameter is the diameter of the swept area.

43
Q

What are the four factors used to determine hub height

A

Wind resource assessment, terrain, wind turbine size and visual impact.

44
Q

Explain what a wind resource assessment does to determine the hub height

A

An anemometer (device that measures wind speeds) is placed at different heights on a vertical pole to measure wind speeds at different heights this is to asses weather it is feasible to get a larger tower and spend more money. Also a wind vane is placed to measure wind direction.

45
Q

Explain how terrain would affect the decision to determine the hub height

A

An obstruction like a house or a hill can cause turbulent to 2 times the height vertical and 20 times the height horizontally.

46
Q

Explain how wind turbine size determines the hub height

A

Larger the wind turbine larger the tower tends to be it is usually 1:4 ratio of rotor diameter to hub height

47
Q

Explain visual impact would determine the hub height

A

The taller the hub height the more it will stick out and be an eye sore to community’s which might make it hard to get planning permission.

48
Q

What equation is used to show that the mass of a turbine is approximately proportional to the cube of its blade length.

A

Mn = Mo (BLn / BLo)3 Where: Mn = Mass of the new turbine in kg or tonnes. Mo = Mass of the old existing turbine in kg or tonnes. BLn = Blade length of a new wind turbine in m. BLo = Blade length of an old wind turbine in m.

49
Q

What three factors influence wind turbine performance

A

Blade length, strength of materials and siting requirements.

50
Q

Explain how blade length influences the performance of a wind turbine

A

P out is directly proportional to radius squared thus meaning as the radius goes up Toby a factor of 2 power out put goes up by a factor of 2 also as larger blades have more mass this means cut in speeds are also larger.

51
Q

Explain how blade materials influences the performance of a wind turbine

A

Stronger materials will enable the turbine to withstand higher wind speeds but it will also raise the cut in speed as stronger materials will raise the weight. Lighter materials would also not be affective as they would break easily.

52
Q

Explain how siting requirements influences the performance of a wind turbine

A

Ideally a wind turbine would preform best with no obstructions in an exposed place with high wind speeds.

53
Q

Define wind survival speed.

A

‘The maximum wind speed that a turbine is designed to withstand before sustaining damage.’

54
Q

What are the two elements of power control a turbine has?

A

Pitching and yawing

55
Q

What is pitching

A

Pitching changes the orientation of the blades to face away from the wind direction to slow the turbine down

56
Q

What is yawing and how can maximum output be achieved using this.

A

When a wind turbine can control where the swept area is facing to increase or decrease power output.
As if the swept area is perpendicular to the wind maximum output is achieved

57
Q

What is option 1 of yawing and state 1 benefit of this.

A

A wind vane which is passive yawing is used on small scale wind turbines. An advantage of this is that no electricity or maintenance costs will take play with this then.

58
Q

What is option 2 of yawing and state an advantage of this.

A

Active yawing which uses a motor in the nacelle to turn the swept area into the wind a benefit of this is that even though it raises initial costs it increases the efficiency massively.

59
Q

Give 1 implications of not using yawing.

A

If the swept area isn’t perpendicular to the wind the force on the blades are not evenly spread and the upstream blade faces more stress and pressure this can lead to torsional forces on the hub or tower over a long period of time this can cause the turbine to break.

60
Q

Discuss issues affecting energy output of a wind turbine in terms of size

A

The blade length of the wind turbine will affect energy output. Also the orientation of the site as in the wind speeds available also altitudes which is affectively the air density.

61
Q

Discuss issues affecting costs.

A

Installation costs: which require a path and turning circle for the location of the turbine
Grid connection costs: the costs to connect to the national grid could make a project infeasible depending how close the nearest transformer is as of 2015 the cables are £65000 per km.
Servicing the turbine can cost thousands per year
End of life costs: the owner is responsible for decommissioning the turbine and disposing and transporting the wind turbine.
Surveys: these include one for wind speeds in area and also shadow flicker and how it would affect the sourrounding area.

62
Q

Discuss environmental and social issues about wind turbines

A

Visual issues: planners and local residence can reject planning permission as of an eyesore.
Noise pollution: this is controlled by having a restriction on how close a turbine can be to an existing house or a house that has planning permission past.
Communication links: the swept area may be placed in infrastructure communications links which could affect other business like water wastage information system.
Environmental impact assessment: needs surveys to see if habitats are destroyed
Local objections.

63
Q

In terms of the betz limit what is the actual percentage of energy can be converted to mechanical and electrical energy and why is this.

A

35% to 45% this is because of energy losses within the gear box of the wind turbine

Environmental technology (9 decks)

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