Energy and the New Reality, Volume 2: C-Free Energy Supply Chapter 3: Wind Energy L. D. Danny Harvey [e-mail]

1
Energy and the New Reality, Volume 2C-Free
Energy Supply Chapter 3 Wind EnergyL. D.
Danny Harveyharvey_at_geog.utoronto.ca
Publisher Earthscan, UKHomepage
www.earthscan.co.uk/?tabid101808

• This material is intended for use in lectures,
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2
Figure 3.1a Annual additions to wind energy
capacity
3
Figure 3.1b Growth in total wind energy capacity
4
Figure 3.2a Breakdown of installed capacity at
the end of 2009
5
Figure 3.2b Capacity (MW) installed in 2009
6
Figure 3.3 Wind farm at Pincher creek, Alberta
Source Garry Sowerby
7
Table 3.1 Market shares of the worlds leading
wind turbine manufacturers. Source BTM Consult
Press Release, March 2007
8
Components of a Wind Turbine

• Foundation
• Tower
• Rotor
• Nacelle
• Gearbox (usually)
• High speed shaft
• Generator
• Control system, cooling unit, anemometer
• Yaw mechanism

9
Turbine characteristics

• Rotor diameter up to 120 m
• Hub height up to 120 m
• Peak electrical power output up to 5 MW now, up
  to 10 MW foreseen
• Cut-in wind speed (typically 3-4 m/s)
• Rated wind speed (typically 15 m/s)
• Cut-out wind speed (typically 25 m/s)

10
Figure 3.4 Progression of rotor sizes over time
11
Figure 3.5a Rotor diameter vs rated power
12
Figure 3.5b Hub height vs rated power
13
Figure 3.6 Minimum hub height vs rotor diameter
14
Figure 3.7a Power curves for wind turbines with
80-m, 87-m, and 90-m rotors and a 2.0-MW generator
15
Figure 3.7b Power curves for wind turbines with
different rotor-generator combinations
16
Wind turbine aerodynamics

• Lift, not a pushing force, is what makes the
  rotor rotate
• Thus, the aerodynamics of a wind turbine have
  much in common with the aerodynamics of an
  airplane wing

17
Figure 3.8 Airflow Past Wing
18
Figure 3.9 Forces acting on a turbine rotor blade
Source Danish Wind Turbine Manufacturers
Association
19
Efficiency of a wind turbine this is the ratio
of the electrical power produced (W) to the power
of the wind passing through the area swept by the
rotor blades. It is the product of three factors

• Aerodynamic efficiency (ratio of mechanical power
  of the rotor to wind power)
• Mechanical efficiency (ratio of mechanical power
  of the generator axis to the mechanical power of
  the rotor axis)
• Electrical efficiency (ratio of electrical power
  fed into the grid to the mechanical power of the
  generator axis)

20

• The maximum possible aerodynamic efficiency, as
  given by Betz Law, is 59.3, and occurs if the
  turbine slows the wind down to 2/3 of its
  original speed. The aerodynamic efficiency of a
  real turbine varies with wind speed, having a
  typical peak value of 44 and a typical value
  averaged over all wind speeds of 25
• A typical mechanical efficiency is 96-99
• A typical electrical efficiency is 96-97
• Multiply the efficiencies (expressed as a
  fraction) to get the overall efficiency

21
Figure 3.10 Variation of power output and
efficiency with wind speed for the Nordex N90-2.3
turbine
22
Turbine generators

• Synchronous
• Asynchronous (induction)
• Variable speed

23
Synchronous generators

• Common in fossil fuel powerplants, but rare in
  wind turbines
• Rotation speed is synchronized with the grid
  frequency

24
Recap Volume 1 Figure 3.2 Two Pole Synchronous
Generator
25
Recap Volume 1 Figure 3.1 Three Phase AC Current
26
Asynchronous (induction) generators

• If the rotor were to rotate at the same frequency
  as the electric field in the stator, no
  electricity would be produced
• When the rotor of the generator rotates faster
  than the stator, a strong current is induced in
  the rotor
• The harder one cranks on the rotor, the more
  power that is transferred as electromagnetic
  force to the stator, converted to electricity,
  and fed to the grid
• The difference in the rotation speed between no
  power and peak power is about 1, but this slip
  reduces stress on the rotor and smoothes out
  power variations

27
Variable Speed Generators

• Becoming more common
• Rotation rate of rotor varies with wind speed
  from 8 rpm to 16 rpm
• Results in less stress on the structure and more
  uniform variation in power output
• Requires more complex electronics and gearbox to
  always produce electricity at the fixed grid
  frequency

28
Characteristics of wind

• Variation of mean wind speed with height
• Variation of turbulence intensity with height
• Weibull probability distribution function for
  wind speed

29
Figure 3.11 Logarithmic velocity profile

• U plots as a straight line on semi-log paper,
  with slope u/?. zo is the height at which U
  extrapolates to zero

30
Figure 3.12 Effect of surface roughness on
velocity profiles

• Wind speed 100-200 m above the surface is fixed
  (governed by the large scale pressure patterns)
• Rougher surface the air feels the surface to
  a greater height, so wind speeds are slower at
  all heights within the first 100-200 m.

31
An alternative mathematical representation of the
variation of wind speed with height is using a
power relationship,Uh/Uref (H/href)nThe
logarithmic relationship is theoretically valid
in a neutral atmosphere only.The power
relationship has no theoretical basis but
provides a good fit to observed atmospheric wind
profiles
32
Figure 3.13 Turbulence intensity (wind speed
standard deviation divided by mean wind speed) vs
height
Source Soker et al (2000, Offshore Wind Energy
in the North Sea Technical Possibilities and
Ecological Considerations - A Study for
Greenpeace)
33
Power output from a wind turbine

• Kinetic energy of a moving mass ½ mv2
• Power density of wind ½ ?V3
• The efficiency of a wind turbine is defined as
  the ratio of power output to the power of the
  wind in the area swept by the rotating rotor.
  Thus,
• Power output of a wind turbine
• efficiency x swept area x power density of
  wind, or
• P1/2 ?(pR2) ? V3

34
Weibull Distribution Function

• Gives the probability of a wind speed occurring
  per unit of wind-speed interval
• Thus, the units are 1/(m/s)
• The value of the function times the width of the
  interval gives the probability of the wind speed
  occurring in that interval
• The function is
• f(u)k/c(u/c)k-1exp(-(u/c)k)
• where c is the scale parameter and k is the
  shape parameter

35
Figure 3.15 Weibull wind speed distribution with
c5 m/s and k1.6
36
Figure 3.14 Distribution of best-fit Weibull
scale factor (c) and shape factor (k) deduced
from observed wind velocity variations at various
sites
37
Figure 3.16 Weibull wind speed probability
distributions
38
Because wind power varies non-linearly with wind
speed

• The mean (average) wind power for a given mean
  wind speed depends on the shape of the
  probability distribution on either side of the
  mean wind speed
• The mean wind power (based on wind power
  computed at many different wind speeds and then
  weighted by the probabilities) is about twice the
  wind power computed once at the mean wind speed

39
Figure 3.17 Mean wind power vs mean wind speed.A
smaller k means a more spread out wind speed
distribution, so more winds at both very high and
very low wind speeds, but the high wind speeds
disproportionately contribute to wind power (due
to the cubic dependence), so the mean wind power
is greater at a given mean wind speed with
smaller k
40
Table 3.3. Comparison of wind power computed at
the average wind speed with the average wind
power computed over a distribution of wind speeds
giving the same average wind speed.
41
Mean Efficiency

• The power output at any given wind speed is given
  by the wind power x swept area x efficiency, so
  the efficiencies matter more when the wind power
  is larger than when it is smaller
• Thus, the appropriate mean efficiency involves
  the efficiency at each wind speed times the
  probability of that wind speed interval times the
  wind power at that wind speed, divided by the
  mean wind power

42
Figure 3.18a Mean efficiency vs wind speed,
computed from the turbine power curve and the
Weibull wind speed probability distribution using
3 different shape parameters
43
Figure 3.18b Mean turbine efficiency vs mean
wind speed for three turbines with similar
generator ratings
44
Capacity Factor

• This is the mean (average) power output of the
  turbine divided by the peak (or rated) power
  output
• The mean power output is computed as the power
  output in the centre of each wind speed interval,
  times the probability of that interval, summed
  over all intervals and divided by the total
  probability (which is 1.0)

45
Figure 3.19a Variation of capacity factor with
wind speed for 3 different Weibull shape
parameters
46
Figure 3.19b Variation of capacity factor with
wind speed for three different turbines
47
Table 3.4 Average wind turbine capacity factors
in 2001. Source BTM Consult (2002).
48
Figure 3.20 Mean wind speed over North America at
a height of 100 m.
Wind speed (m/s)
Source for this and other wind maps Prepared
from data file at power.larc.nasa.gov (go to
Sustainable Buildings, Global Datasets)
49
Figure 3.21 Mean wind speed over Europe at a
height of 100 m.
Wind speed (m/s)
50
Figure 3.22 Mean wind speed over China and
surrounding regions at a height of 100 m.
Wind Speed (m/s)
51
Supplemental Figure Mean wind speed over North
Africa and the Middle East at a height of 100 m.
Wind speed (m/s)
52
Supplemental figure Mean wind speed over
southern Africa at a height of 100 m.
Wind speed (m/s)
53
Supplemental figure Mean wind speed over
Australia, Indonesia and adjoining regions at a
height of 100 m.
Wind speed (m/s)
54
Supplemental figure Mean wind speed over South
America at a height of 100 m.
Wind speed (m/s)
55
Windfarms

• Clustering of many wind turbines in a regular
  array in a region of good winds
• Turbines are typically spaced 5-9 rotor diameters
  apart in the along-wind direction, and 3-5 rotor
  diameters apart in the cross-wind direction
• Clustering reduces costs (economies of scale for
  installation), and takes better advantage of the
  best wind sites

56
Impact of windfarms on weather and climate

• Large wind farms (involving hundreds of wind
  turbines at the closest permitted spacing (i.e.,
  separated by 7 rotor diameters) would have a
  noticeable effect on regional winds and hence on
  vertical fluxes of heat and moisture in the
  atmosphere, thereby changing the surface air
  temperature in the region of the wind farm and
  downstream from the wind farm
• Interference with the winds might reduce the
  overall power output from a wind farm by up to
  30 compared to the case where the winds are
  assumed to be unaffected by the wind farm

57
Scaling Relationships

• Intercepted wind power varies with rotor diameter
  squared, so with constant efficiency, power
  output would also vary with D2
• If turbines are spaced apart at a constant
  multiple of the rotor diameter (say, 7D x 7D),
  then the land area also increases with D2
• Thus, the wind farm capacity (that is, the peak
  power output at high wind speeds) per unit of
  land area will be independent of the rotor
  diameter
• However, larger turbines also have a greater hub
  height and so greater mean wind speed
• Wind power varies with V3, so 15 greater wind
  speed results in 52 greater wind power
• Thus, as turbines have gotten bigger (and
  higher), the energy production (kWh/yr) per unit
  of land area has increased
• Capacity factors would also be larger for larger
  turbines, all else being equal, due to the
  greater wind speeds at a greater turbine height.

58
Figure 3.23 Variation of rotor swept area with
rated power for turbines listed in Table 3.2
59
Figure 3.24a Peak wind farm power per km2 for
various turbines, assuming the turbines to be
arranged on a grid with a spacing of 7D x 7D,
where Drotor diameter
60
Figure 3.24b Annual electricity production per
km2 for various turbines, assuming the turbines
to be arranged on a grid with a spacing of 7D x
7D, where Drotor diameter
61
Offshore wind farms

• Wind turbines mounted on the seabed in water up
  to 50 m deep
• Can double or triple the cost of the wind turbine
  connections to the grid, but there can easily
  be twice the electricity production
• Net result electricity for about the same to
  50 higher price but with twice the capacity
  factor (i.e., 40-50 instead of 20-25)
• Turbines especially designed for offshore
  conditions have been built

62
Table 3.7 Additional costs of offshore wind
energy as a function of distance from the north
German coast.
Source Soker et al (2000, Offshore Wind Energy
in the North Sea Technical Possibilities and
Ecological Considerations - A Study for
Greenpeace)
63
Figure 3.25 Middelgrunden wind farm, next to
Copenhagen
Source Danny Harvey
64
Figure 3.26 European offshore wind atlas
Source Risø National Laboratory, Wind Atlases of
the World (www.windatlas.dk)
65
Figure 3.27 Types of foundations
Source Soker et al (2000, Offshore Wind Energy
in the North Sea Technical Possibilities and
Ecological Considerations - A Study for
Greenpeace)
66
Floating wind turbines

• Under development using technology transferred
  from the North Sea offshore oil industry (which
  is winding down)
• One concept (WindSea) involves a
  triangular-shaped floating platform with a 3.2MW
  turbine at each corner on an outward-inclined
  tower
• One rotor would be on an airfoil-shaper tower
  that would act like the tail of an airplane,
  serving to continuously and automatically orient
  all three rotors perpendicular to the wind

67
Figure 3.28 Existing and planned (as of late
2009) offshore wind farms (total capacity
44,600 MW)
Source Richard Harrod, 4COffshore
68
Fluctuations in Wind Electricity Production

• Because there might be times when the wind speed
  might be less than the cut-in wind speed (so that
  no electricity is produced), some amount of
  non-wind backup capacity is needed (how much will
  be discussed later)
• As well, because wind is variable, some power
  units that can go up and down to offset the
  variations in wind electricity production are
  needed
• The problem is, the units most able to fluctuate
  rapidly (such as simple-cycle natural gas
  turbines) tend to be less efficient than the
  units that would normally be used (such as coal
  steam turbines or combined-cycle natural gas
  systems)
• Thus, it is desirable to minimize the variations
  in the electricity production from wind

69
Minimizing rapid (seconds to minutes)
fluctuations in wind output

• Use of variable-speed turbines provides some
  smoothing of output on a time scale of seconds
• Link together several turbines in a wind farm
  provides some cancellation of fluctuations at a
  time scale of up to a minute or so
• Implement short-term storage of excess energy
• - flywheels, supercapacitors, plug-in hybrid
  vehicles with a two-way connection to the grid

70
Dealing with longer fluctuations (hours to days
and months)

• Link together wind farms over a broad region
• Use electrolyzers and fuels cells (making and
  using H2)
• Use flow batteries (regenerative fuel cells)
• Use underground compressed air energy storage
  (CAES)
• Use hydro-electric reservoirs
• Use heat pumps and thermal energy storage in
  district energy systems
• Use other flexible end-use electric loads

71
Table 3.10 Impact on the statistical properties
of wind energy of spreading wind farms over
increasingly larger areas in and around Europe
Source Czisch and Giebel (2000, Wind Power for
the 21st Century, Kassel)
72
Figure 3.29 Amalgamation of dispersed wind farms
Source Czisch and Giebel (2000, Wind Power for
the 21st Century, Kassel)
73
Table 3.11 Largest variation in wind power over
different time periods and averaged over
differently-sized regions.
Source EWEA (2005, Large-scale Integration of
Wind Energy in the European Power Supply,
www.ewea.org)
74
Making Use of Short-term Wind Forecasts

• The variability in electricity output that
  remains after making use of the various
  strategies outlined in the previous slides can be
  better handled if the variation can be predicted
  several hours in advance, as this permits
  scheduling of slowly responding backup fossil
  fuel power units
• Thus, improving local wind forecasts with
  high-resolution meteorological computer
  forecasting models is an intensive area of
  research at present

75
Electrolyzers and fuel cells

• Electrolyzers generate hydrogen by splitting
  water using electricity, and so could use excess
  wind electricity
• The hydrogen can be stored as a compressed gas
• When there is a shortage of wind electricity,
  additional electricity can be generated by
  running the electrolyzer backwards as a fuel cell
• Rapid variations in output degrade the
  performance and shorten the lifespan of
  electrolyzers and fuel cells, so a battery would
  likely be used to smooth out the electricity
  input to the electrolyzer and smooth the demand
  for extra electricity from the fuel cell

76
Recap Figures 3.8 and 3.9 from Volume 1
Source (right) www.utcfuelcells.com
77
Figure 3.30 Regenerative fuel cell (or flow
battery)
Source Modified from Lotspeich and Holde (2002,
Proceedings of the 2002 ACEEE Summer Study on
Energy Efficiency in Buildings 3, American
Council for an Energy Efficient Economy,
Washington)
78
Compressed air energy storage (CAES)

• With electricity generation using a gas turbine,
  about half of the turbine power is used to
  compress the air needed for combustion, and only
  about half is used to drive the generator that
  produces electricity
• If excess wind energy is used to compress air and
  store it underground, the compressed air could be
  directly used in a gas turbine to generate
  electricity when there is a shortage of wind
  power
• This would more than double the efficiency of
  using natural gas to produce electricity from
  about 37 (with a simple-cycle turbine) to 84

79
CAES was initially developed in the 1970s and
1980s as a means of absorbing excess nuclear
electricity (the output from a nuclear powerplant
is largely fixed, while electricity demand
varies, so if the plant meets a large fraction of
peak demand, there would be excess power at
times)Only two such plants were built (one in
Germany, one in Alabama)Now there is a revival
of CAES, with many plants under construction or
planned (especially in Texas) for storage of
excess wind energy
80
Geological formations suitable for CAES

• Salt domes, salt beds and porous sedimentary
  rocks are best
• These underlie 75 of the land area of the US,
  including many of the best wind regions
• Salt domes closely coincide with the best wind
  resource regions in Europe
• Caverns can also be excavated in hard rock, but
  these would be considerably more expensive

81
Supplemental Figure Location of geology suitable
for CAES and of good wind resources in the US,
and CAES sites.
Source Succar and Williams (2008, Compressed Air
Energy Storage Theory, Resources, and
Applications for Wind Power, Princeton
Environment Institute, Princeton University,
Princeton, New Jersey)
82
Figure 3.31 Wind CAES energy flow
Source Denholm (2006, Renewable Energy 31,
13551370, http//www.sciencedirect.com/science/jo
urnal/09601481)
83
Existing and planned CAES facilities require some
supplemental fuel (natural gas at present, but it
could be gasified biomass in the future)A new
system under development is called advanced
adiabatic CAES (AA-CAES)In this system, no (or
very little) supplemental fuel is required.
Instead, the heat that is produced when air is
compressed is stored in the form of hot ( 650ºC)
molten salts in an insulated tank, and used
instead of fuel along with the compressed air to
generate electricity when needed
84
Use of hydro-electric reservoirs

• Most hydro-electric reservoirs are not running at
  full capacity all the time, because there is not
  enough water
• Thus, when there is excess wind, the water flow
  and hence electricity production can be reduced,
  and when there is a shortage, greater water flow
  than would otherwise be the case (using the saved
  water) can be allowed
• This entails no energy loss, and in fact can
  slightly increase the annual hydro-electric
  energy production from the same annual water
  flow, because the average reservoir level will be
  greater (hydro-electric power production depends
  on flow rate x elevation drop)

85

• Many areas of the world that are lousy for CAES
  (such as the hard pre-Cambrian rocks of the
  Canadian Shield) have excellent hydro-electric
  resources or excellent potential hydro-electric
  resources

86
Pumped hydro

• When there is no reservoir to hold back the river
  flow, build a dam in some mountain valley and
  pump water up behind the dam using excess wind
  electricity, creating a reservoir
• Let water drain the reservoir through a
  conventional hydro-electric turbine at times of
  wind electricity deficit
• This is used in Europe (in the Alps, for example)

87
Use of heat pumps

• One way to provide heat with electricity is
  through electric resistance heating, but this is
  not efficient (only 1 unit of heat per unit of
  electricity used)
• A better method is to use electric heat pumps,
  which provide 3-4 units of heat per unit of
  electricity used
• If we have well-insulated buildings (such that
  they can drift for a few hours without the
  heating system on), then heat pumps could be used
  when there is excess wind and the heating turned
  off altogether when wind energy drops
• Similarly, heat pumps (or chillers) can be used
  for cooling purposes (in the summer) at times of
  excess wind and turned off at times of deficit
• In effect, excess wind energy is being stored as
  thermal energy (heat or coldness)

88

• There is no loss of energy in this way, so the
  storage efficiency is 100
• If, however, heat or coldness is stored in a
  large insulated tank outside the building (as in
  some district energy systems), there would be
  some loss of stored heat or coldness to the
  environment
• The storage efficiency in this case is lt 100,
  perhaps 95

89
Other dispatchable loads using dynamic demand

• The power industry talks about dispatchable
  power sources those that can be quickly brought
  on line and varied in output to meet fluctuating
  electricity demand
• The other side is to have dispatchable demand
  demand that can be reduced by the power utility
  to compensate for surges in demand elsewhere or
  for the loss of power units
• This is already done with things like electric
  resistance water heaters a signal can be sent
  from the utility to temporarily turn them off

90

• When electricity demand exceeds electricity
  supply, the voltage and frequency drop (and vice
  versa when supply exceeds demand)
• Equipment with compressors (such as refrigerators
  and air conditioners) can sense changes in
  frequency and can now be designed to
  automatically shut down when the frequency drops
  below some threshold
• This could compensate for sudden drops in wind
  power until the wind power resumes or backup
  systems come on line
• This is called dynamic demand

91
Present round-trip efficiency of various energy
storage option (energy taken out vs energy put in)

• Electrolyzer/hydrogen/fuel cell system 32-42
• Flywheels (store for 1 day) 45
• Pumped hydro 65-80
• Flow batteries 75
• CAES 70-75
• AA-CAES 70
• Flywheels (store for seconds) 85
• Batteries 85-90
• Capacitors 95
• Thermal energy storage 95-100
• Hydro-electric reservoirs 100

92
Improving system stability through the addition
of wind turbines

• Until recently, wind turbines had been thought of
  only as a liability as far as system stability is
  concerned (due to the fluctuating and partly
  unpredictable nature of wind)
• However, modern wind turbines can be used to
  improve overall system stability by
• - compensating for shifts in reactive power
  (related to the phase shift between voltage and
  current oscillations) caused by other supply
  sources or loads in the system
• - maintaining connection to the grid and
  continuing to produce power when faults elsewhere
  cause large transient variations in voltage (this
  is called fault ride through (FRT) capability,
  and is still under development)
• - varying their output in response to changes in
  grid frequency (this comes with a cost output
  has to be restrained slightly under normal
  conditions)

93
Transmission
94
Transmission basics

• Transmitted power Voltage (V) x Current (I)
• Resistance loss varies with I2, so,
• For a given energy flow, the resistance loss
  varies with 1/V2
• Thus, the key to minimizing resistance losses is
  to transmit electricity at high voltage
• There were be some offsetting losses in the
  transformers from low to high and back to low
  voltage
• Typical voltages for long distance transmission
• 500-800 kV, compared to 30 kV for local
  distribution

95
HVDC (high voltage DC)

• Less expensive transmission lines with smaller
  resistance losses, but more expensive
  transformers with greater losses
• Thus, HVDC costs less and entails less overall
  loss only for transmission beyond some minimum
  distance, namely,
• HVDC costs less beyond about 750 km distance, and
  entails less loss beyond about 250 km distance
  (the exact break-even distance for cost depends
  on the terrain and local market conditions)

96
Other pros and cons of HVDC compared to HVAC

• HVDC
• - has a much narrower right of way
• - generates negligible magnetic fields
  (concern over which has been one source of public
  opposition to new transmission lines)
• - has better reactive power control and full
  control of where the power flows (unlike AC mesh
  grids)
• - an offshore grid for offshore wind farms would
  permit the wind turbines to operate at a greater
  range of speeds, which in turn would permit more
  efficient operation (no need for synchronization
  of the power output with the land AC grid)
• However,
• - branching of DC lines is difficult, as is the
  construction of multiple terminals, although
  these problem should be solvable in a few years

97
Figure 3.32 Typical DC and AC Transmission Pylons
Source GAC (2006, Trans-Mediterranean
Interconnection for Concentrating Solar Power,
Final Report, GAC, www.dlr.de/tt/trans-csp)
98
Figure 3.33 Transmission corridors transmitting
10 GW of electric power
Source GAC (2006, Trans-Mediterranean
Interconnection for Concentrating Solar Power,
Final Report, GAC, www.dlr.de/tt/trans-csp)
99
Economics
100
Direct Cost of Wind EnergyThe cost per kWh is
the annual revenue requirement per kW of capacity
divided by the number of kWhs sold per year per
kW of capacity. That is,C (CRFOM)CCwt/(?sfa
8760CF) where CRFcost recovery factor
i/(1-(1i)(-N)) i
interest rate (expressed as a fraction per
year) N number of years over which the wind
project is financed OM annual
operation and maintenance cost as a fraction of
the initial capital cost CCwt, CCwt
initial capital cost given as /kW ( per kW of
turbine capacity)
101
8760 number of hours in a year ?s is an
efficiency that takes into account various losses
that are not accounted for in the turbine power
curve (such as dirt on the blades, imperfect
tracking of the wind direction by the yaw
mechanism, or wake effects in wind farms) fa
is the fraction of time that the turbine is
available CF capacity factor (the average
power output as a fraction of the peak output or
capacity)a 1kW turbine running full out all the
time would produce 1kW x 8760 hr/yr 8760 kWh/yr
of electricity
102
Units in the previous equation(yr)-1 (for OM
and CRF) x /kW, divided by kWh/kW/yr
gives/kWh
103
Figure 3.34 Total cost (including installation
and grid connection) of wind turbines in various
countries in 2006
Source Krohn et al (2009, The Economics of Wind
Energy, A Report by the European Wind Energy
Association, European Wind Energy Association,
Brussels, www.ewea.org )
104
Figure 3.35 Trend in turbine and non-turbine
costs and in the cost of wind-generated
electricity in Denmark
Source Krohn et al (2009, The Economics of Wind
Energy, A Report by the European Wind Energy
Association, European Wind Energy Association,
Brussels, www.ewea.org )
105
Figure 3.36 Illustrative costs of wind
electricity for various rates of return (ROI) in
the investment and for various capital costs,
assuming a CF of 0.35, 20-year financing and?s
fa 1.0
106
Figure 3.37a Costs of offshore wind farms as a
function of the size of the turbines used in the
wind farm
Source Redrafted from Snyder and Kaiser (2009,
Renewable Energy 34, 1567-1578,
http//www.sciencedirect.com/science/journal/09601
481)
107
Figure 3.37b Costs of offshore wind farms as a
function of the size of the wind farm
Source Redrafted from Snyder and Kaiser (2009,
Renewable Energy 34, 1567-1578,
http//www.sciencedirect.com/science/journal/09601
481)
108
In spite of the lack of any relationship between
the cost of offshore wind farms and the size of
the wind farm, the purchase price of turbines has
been reduced by up to 45 from the list price for
orders of 500-1600 turbines
109
Progress Ratio

• This is the fraction by which the cost of
  something is multiplied for every doubling in
  cumulative production.
• The cost of a wide range of manufactured products
  follows this relationship
• For wind turbines, the observed progress ratio up
  to 2005 was about 0.8, meaning a 20 reduction in
  cost for every doubling in cumulative global
  production

110
Figure 3.43 Reduction in cost with a progress
ratio of 0.81and growth in capacity decreasing
linearly from20/yr in 2008 to 0/yr in 2050
111
However, prices of onshore wind turbines in
Europe increased by 75 between 2005-2008, while
that of offshore turbines increased by almost
50. There are two reasons for this

• Demand greater than supply (perhaps growth has
  been too rapid global installed capacity had
  been growing by 26/yr from 2000-2007)
• Spikes in the costs of steel and copper
  (affecting fossil fuel and nuclear power plant
  costs too)
• Prices have subsequently fallen a little

112
Indirect costs of wind turbines

• Reduced electricity output by non-wind generators
  (which increases the unit cost of their
  electricity), partly offset by a reduction in the
  need for non-wind generators
• Wasted wind electricity generation potential

113
It is often thought that the addition of wind
energy does not allow any reduction in the amount
(capacity) of other power sources, because there
could be zero wind production near times of peak
demand. That is, the capacity credit of wind is
often assumed to be zero. However, this is not
correct.Instead, the amount of non-wind
capacity that is needed is calculated so as to
have the same loss-of-load probability as when
there is no wind capacity with, instead, the full
non-wind capacity
114
The result is that, for small wind penetration
(that is, small wind capacity compared to the
total capacity), the capacity credit is roughly
equal to the capacity factor. So, if 100 MW of
wind power capacity is added to a very large
system and the CF (average output as a fraction
of peak output) of the wind turbines is 20, then
the non-wind capacity can be reduced by 20 MW
while still having the same overall reliability.
115
As the wind penetration increases,the capacity
credit as a fraction of the capacity factor
becomes progressively smallerSo, at 1000 MW and
the same 20 capacity factor, the capacity credit
might be only 10 instead of 20, so the non-wind
capacity can be reduced by only 100 MW The
capacity credit for wind is non-zero only because
the backup fossil fuel powerplants are themselves
not 100 reliable, as seen in the next table
116
Table 3.16 Outage rates for various electricity
generators in the US.
117
Figure 3.38 Capacity credit for wind as a
function of the wind penetration and capacity
factor
118
Figure 3.39 Capacity credit for wind as a
function of wind penetration and the degree of
geographical dispersion of the wind turbines
119
To recap,

• The addition of wind means that the existing
  fossil fuel powerplant is used less, which
  increases the unit cost of that portion of the
  electricity from the fossil fuel plant
• However, less fossil fuel powerplant is needed
  (which is not helpful if the fossil capacity has
  already been built) due to the non-zero capacity
  credit from wind
• Other indirect costs of wind include
• - wasted wind electricity potential due to the
  need to maintain a minimum fossil fuel output
• - reduction in the efficiency of the fossil fuel
  powerplant when wind is added (either because it
  is operating at lower average load and thus less
  efficiency, or because of larger swings in output)

120
Figure 3.40 Wasted wind energy potential as a
function of wind energy penetration for Danish
conditions
Source Redlinger et al (2002, Wind Energy in the
21st Century Economics, Policy, Technology and
the Changing Electricity Industry, Palgrave,
Basingstoke)
121
Cost of transmission

• Direct related to the investment cost for
  transmission equipment and annual operation and
  maintenance costs
• Indirect related to the loss of electricity
  during transmission. This has to be made up by
  generating more electricity than if there were no
  transmission losses. The indirect cost is equal
  to the required extra electricity generation x
  the cost of the electricity, which includes the
  powerplant and the direct transmission costs

122

• If Tloss is the fractional electricity loss when
  the transmission line is transmitting at its full
  capacity, and CFtr is the transmission line
  capacity factor (average power transmitted over
  peak power transmission capacity), then the
  average loss is CFtrTloss
• The amount of electricity that needs to be
  generated in order to deliver X kWhs at the end
  of the transmission line is thus
• X / (1- CFtrTloss)
• Similarly, if C is the cost before transmission
  losses (but including the cost of the
  transmission equipment), then the cost after
  accounting for transmission losses becomes
• C / (1- CFtrTloss) C / (1 e), where e
  CFtrTloss

123

• Cost C/(1 e)
• C(1 e e2 e3 .)
• (this is a Taylor Series expansion)
• C eC (1 e e2 .)
• C eC/(1 e)
• (using (1 e e2 e3 ) 1/(1-e))
• Thus, the extra cost due to transmission losses
  is
• eC/(1 e)
• where, remember, C is the cost of electricity
  before accounting for transmission losses, and
  includes the transmission line capital cost.

124
Figure 3.41a Transmission energy loss using GAC
(2006) data
125
Figure 3.41b Absolute transmission investment cost
126
Figure 3.42 Cost of transmission assuming a line
capital cost of 560/kW and 8.6 loss at full
capacity
127
Strategies for Baseload Wind Energy

• Oversized wind farms compared to the transmission
  link can give capacity factors at the receiving
  end of the link of 0.6-0.7
• Compressed air energy storage
• Use of dispatchable loads (such as reverse
  osmosis for desalination or heat pumps in
  district heating and cooling systems)

128
Figure 3.44 Oversizing Concept
129
Figure 3.45a Wind farm capacity factor as a
function of mean wind speed for various degrees
of over-sizing
1
2
3
4
130
Figure 3.45b Wasted wind energy potential as a
function of mean wind speed for various degrees
of wind farm over-sizing
4
3
2
131
Figure 3.45c Transmission line capacity factor as
a function of mean wind speed for various degrees
of wind farm over-sizing
4
3
2
1
132
Figure 3.46 Cost of electricity from 2000-km
distant wind farms oversized by various factors
4
3
2
1
133
Figure 3.47 Contribution to the cost of
electricity
Capacity Factor 0.57 0.72
0.77 0.81
134
Figure 3.48 Comparison of electricity costs from
local and distant oversized wind farms vs wind
speed
135
Capital Cost Estimates

• Natural gas combined cycle 660/kW
• Wind natural gas hybrid 1640/kW
• Wind natural gas CAES 2270/kW
• (the last is cheaper than many recent nuclear
  power plants)

136
Figure 3.49 Cost with electricity from natural
gascombined cycle and from wind with natural gas
or CAES
Source Greenblatt et al (2007, Energy Policy 35,
14741492, http//www.sciencedirect.com/science/jo
urnal/03014215)
137
Desalination of seawater

• The most efficient method is reverse osmosis,
  using electricity (the energy requirement is 2-3
  kWh/m3)
• Desalination is a load that could come on when
  there is excess wind, thereby allowing a bigger
  wind plant in order to meet a larger fraction of
  the non-desalination loads, without wasting
  energy
• However, the desalination equipment capacity
  factor would be small, increasing the capital
  cost contribution to the cost of desalinated
  water
• But since the wind that is used would otherwise
  be wasted, it would have to be sold at a deep
  discount

138
Figure 3.50a Cost of desalinated water using
wind-generated electricity and reverse osmosis
139
Because most of the cost of desalinated water is
from the capital cost rather than the energy
cost, even making the electricity free would not
offset the impact of a low utilization of the
desalination equipment (low desalination capacity
factor)Instead, it is better to have dedicated,
even over-sized, windfarms to power the
desalination equipment
140
Figure 3.50b Cost of desalinated water
usingelectricity from oversized windfarms
141
Energy Payback Time

• This is the time required for the amount of
  primary energy saved by the wind turbine to
  offset the total primary required to produce the
  turbine
• Saved primary energy per year electrical energy
  produced per year divided by the efficiency of
  the powerplant that would otherwise be used to
  produce electricity

142

• Generally speaking, calculated payback times for
  wind turbines are 2-8 months
• The payback time would be significantly longer if
  the components need to be transported 1000 km or
  more by truck

143
Noise and impacts on birds and bats

• Bird mortality is miniscule compared to many
  other human causes of bird deaths (see table to
  follow)
• Noise level at a distance of 350 m is less than
  the typical background level in a home
• Impacts on bats need further study

144
Table 3.24 Main human-related causes of bird
deaths in the US.
Source GWEC (2006, Global Wind Energy Outlook
2006, www.gwec.net)
145
Wind energy potential and global scenario
146
Figure 3.51 Wind energy potential in various US
states. The total potential from the 10 states
shown here is more than twice the total US
electricity demand of 4200 billion kWh in 2004.
147
Table 3.26 Land areas, percent of area with Class
3 or better winds (6.49 m/s) at a height of 80
m, and annual electricity production using 1-MW
and 5-MW wind turbines compared with current
demand (in 1000s TWh/yr).
Based on Archer and Jacobson (2005, Journal of
Geophysical Research 108, D9, 4289)
148
Figure 3.52a Scenario whereby the global wind
powerplant grows to a capacity of 12500 GW
following a logistic growth curve with a growth
parameter of 0.2 (20/yr initial growth)
149
Figure 3.52b Scenario whereby the global wind
powerplant grows to a capacity of 12500 GW
following a logistic growth curve with a growth
parameter of 0.2 (20/yr initial growth)
150
12500 GW 12.5 TWAssume an average capacity
factor of 0.312.5 TW x 8760 hr/yr x 0.3 32350
TWh/yrCurrent world electricity demand is
18500 TWh/yr