From Wikipedia, the free encyclopedia.
Wind
farm on South Point, Big Island of Hawaii
Wind power is the kinetic
energy of wind,
or the extraction of this energy by wind
turbines. In 2004, wind power became the least expensive form of
new power generation, dipping below the cost per kilowatt-hour of
coal-fired plants[1].
Wind power is growing quickly, at about 37%[2],
up from 25% growth in 2002. In the United States, as of 2003, wind
power was the fastest growing form of electricity generation on a
percentage basis[3].
In the late-1990s, the cost of wind power was about five times what it
is in 2005, and that downward trend is expected to continue as larger
multi-megawatt turbines are mass-produced.[4]
 | 1 Wind
energy
 | 1.1 Wind
variability and turbine power |
| 1.2 Wind
power density classes |
|
| 2 Turbine
siting
| 2.1 Onshore |
| 2.2 Offshore |
| 2.3 Airborne |
|
| 3 Utilization
| 3.1 Large
scale |
| 3.2 Small
scale |
|
| 4 Controversy
| 4.1 Arguments
of opponents
 | 4.1.1 Economics |
| 4.1.2 Yield |
| 4.1.3 Ecological
footprint |
| 4.1.4 Scalability |
| 4.1.5 Aesthetics |
|
| 4.2 Arguments
of supporters
| 4.2.1 Yield |
| 4.2.2 Coping
with intermittent wind power |
| 4.2.3 Ecological
footprint |
| 4.2.4 Economic
feasibility |
| 4.2.5 Aesthetics |
|
|
| 5 See
also |
| 6 Sources
| 6.1 Technical |
| 6.2 Political |
| 6.3 Wind
Power projects |
|
| 7 External
links |
|
Wind energy
- Main article: Wind
An estimated 1 to 3 percent of the energy from the Sun
that hits the earth is converted into wind energy. This is about 50 to
100 times more energy than what is converted into biomass by all the
plants on earth through photosynthesis.
Most of this wind energy can be found at high altitudes where
continuous wind speeds of over 160 km/h (100 mph) are common.
Eventually, the wind energy is converted through friction into diffuse
heat all through the earth's surface and atmosphere.
While the exact kinetics of wind are extremely complicated and
relatively little understood, the basics of its origins are relatively
simple. The earth is not heated evenly by the sun. Not only do the poles
receive less energy from the sun than the equator
does, but dry land heats up (and cools down) more quickly than the
seas do. The differential heating powers a global atmospheric
convection system reaching from the earth's surface to the stratosphere
which acts as a virtual ceiling.
The change of seasons, change of day and night, the Coriolis
effect, the irregular albedo
(reflectivity) of land and water, humidity, and the friction of wind
over different terrain are some of the many factors which complicate
the flow of wind over the surface.
Wind variability and turbine power
The power in the wind can be extracted by having it act on moving
wings that exert torque on the rotor. The amount of power transferred
depends on the wind speed (cubed), the swept area (linearly), and the
density of the air (linearly).
The mass
flow of air that travels through the swept area of a wind turbine
varies with the wind speed and air density. As an example, on a cool
15 °C (59 °F) day at sea level, air density is about 1.22 kilograms
per cubic metre (it gets less dense with higher humidity). An 8 m/s
breeze blowing through a 100 meter diameter rotor would move about
76,000 kilograms of air per second through the swept area.
The kinetic
energy of a given mass varies with the square of its velocity.
Because the mass flow increases linearly with the wind speed, the wind
energy available to a wind turbine increases as the cube of the wind
speed. The power of the example breeze above through the example rotor
would be about 2.5 megawatts.
As the wind turbine extracts energy from the air flow, the air is
slowed down, which causes it to spread out and diverts it around the
wind turbine to some extent. A German physicist, Albert Betz,
determined in 1919 that a wind turbine can extract at most 59% of the
energy that would otherwise flow through the turbine's cross section.
The Betz limit applies regardless of the design of the turbine. More
recent work [5]
by Gorlov shows a theoretical limit of about 30% for propeller-type
turbines. Actual efficiencies range from 10% to 20% for propeller-type
turbines, and are as high as 35% for three-dimensional vertical-axis
turbines like Darrieus
or Gorlov turbines.
Distribution of wind speed (red) and energy (blue) for all of
2002 at the Lee Ranch facility in Colorado. The histogram shows
measured data, while the curve is the Raleigh model distribution
for the same average wind speed. Energy is the Betz limit
through a 100 meter diameter circle facing directly into the
wind. Total energy for the year through that circle was 15.4 gigawatt-hours.
Windiness varies, and an average value for a given location is not
in itself a clear indication of the amount of energy a wind turbine
could yield there. The distribution model most frequently used is the
Raleigh model, an example of which is plotted to the right against an
actual measured dataset.
Because so much power is generated by higher windspeed, much of the
average power available to a windmill comes in short bursts. The 2002
Lee Ranch sample is telling: half of the energy available arrived in
just 15% of the operating time.
Since wind speed is not constant, a wind generator's annual energy
production is never as much as its nameplate rating multiplied by the
total hours in a year. The ratio of actual productivity in a year to
this theoretical maximum is called the capacity factor. A well-sited
wind generator will have a capacity factor of as much as 35%. When
comparing the size of wind turbine plants to fueled power
plants, it is important to note that 1000 kW of wind-turbine
potential power would be expected to produce as much energy in a year
as approximately 350 kW of fuel-fired generation. Though the
short-term (hours or days) output of a wind-plant is not completely
predictable, the annual energy of output tends to vary only a few
percent between years.
Wind power density classes
Wind maps in the United States and Europe classify areas into seven
arbitrarily defined classes of wind power density, analogous to the
five classes of hurricane force.
Each class is a range of power densities, so that an area rated as
class 4, for example, would have an average power density from 200 to
250 W/m2 at 10 m above ground. Generally, economic
development of wind power for electricity generation takes place in
areas rated Class 3 or higher.
Table 1-1 Classes of wind power density at 10 m and 50 m(a).
[6]
Wind Power
Class |
10 m (33 ft) |
50 m (164 ft) |
| Wind power density (W/m2) |
Speed(b) m/s (mph) |
Wind power density (W/m2) |
Speed(b) m/s (mph) |
| 1 |
0 |
0 |
0 |
0 |
| 100 |
4.4 (9.8) |
200 |
5.6 (12.5) |
| 2 |
| 150 |
5.1 (11.5) |
300 |
6.4 (14.3) |
| 3 |
| 200 |
5.6 (12.5) |
400 |
7.0 (15.7) |
| 4 |
| 250 |
6.0 (13.4) |
500 |
7.5 (16.8) |
| 5 |
| 300 |
6.4 (14.3) |
600 |
8.0 (17.9) |
| 6 |
| 400 |
7.0 (15.7) |
800 |
8.8 (19.7) |
| 7 |
| 1000 |
9.4 (21.1) |
2000 |
11.9 (26.6) |
- (a) Vertical extrapolation of wind speed based on the 1/7
power law.
- (b) Mean wind speed is based on Rayleigh speed distribution
of equivalent mean wind power density. Wind speed is for standard
sea-level conditions. To maintain the same power density, speed
increases 3%/1000 m (5%/5000 ft) elevation.
Turbine siting
Map of available wind power over the United States. Color
codes indicate wind power density class.
As a general rule, wind generators are practical where the average
wind speed is greater than 20 km/h (5.5 m/s or 12.5 mph). Obviously, meteorology
plays an important part in determining possible locations for wind
parks, though it has great accuracy limitations. Meteorological wind
data is not usually sufficient for accurate siting of a large wind
power project. An 'ideal' location would have a near constant flow of
non-turbulent wind throughout the year, and wouldn't suffer too many
sudden powerful bursts of wind.
The wind blows faster at higher altitudes because of the reduced
influence of drag of the surface (sea or land) and the reduced
viscosity of the air. The variation in velocity with altitude, called wind
shear is most dramatic near the surface. Typically, the variation
follows the 1/7th power law, which predicts that wind speed
rises proportionally to the seventh root of altitude. Doubling the
altitude of a turbine, then, increases the expected wind speeds by 10%
and the expected power by 34%.
Wind farms or wind parks often have many turbines installed. Since
each turbine extracts some of the energy of the wind, it is important
to provide adequate spacing between turbines to avoid excess energy
loss. Where land area is sufficient, turbines are spaced three to five
rotor diameters apart perpendicular to the prevailing wind, and five
to ten rotor diameters apart in the direction of the prevailing wind,
to minimize efficiency loss. The "wind park effect" loss can
be as low as 2% of the combined nameplate rating of the
turbines.
Utility-scale wind turbine generators have low temperature
operating limits which restrict the application in areas that
routinely experience temperatures less than −20 °C. Wind
turbines must be protected from ice accumulation, which can make anemometer
readings inaccurate and which can cause high structure loads and
damage. Some turbine manufacturers offer low-temperature packages at a
cost of a few percent of the turbine cost, which include internal
heaters, different lubricants, and different alloys for structural
elements, to make it possible to operate the turbines at lower
temperatures. If the low-temperature interval is combined with a
low-wind condition, the wind turbine will require station service
power, equivalent to a few percent of its output rating, to maintain
internal temperatures during the cold snap. For example, the St. Leon,
Manitoba
project has a total rating of 99 MW and is estimated to need up to 3
MW (around 3% of capacity) of station service power a few days a year
for temperatures down to −30 °C. This factor affects the
economics of wind turbine operation in cold climates.[1]
Onshore
Onshore turbine installations tend to be along mountain ridges or
passes, or at the top of cliff faces. The change in ground elevation
causes the wind velocities to be generally higher in these areas,
although there may be a lot of variation over relatively short
distances (a difference of 30 m can sometimes mean a doubling in
output). Local winds are often monitored for a year or more with anemometers
and detailed wind maps constructed before wind generators are
installed.
For smaller installations where such data collection is too
expensive or time consuming, the normal way of prospecting
for wind-power sites is to directly look for trees or vegetation that
is permanently "cast" or deformed by the prevailing winds.
Another way is to use a wind-speed survey map, or historical data from
a nearby meteorological station, although these methods are less
reliable.
Sea shores also tend to be windy areas and good sites for turbine
installation, because a primary source of wind is convection from the
differential heating and cooling of land and sea over the course of
day and night. Winds at sea level carry somewhat more energy than
winds of the same speed in mountainous areas because the air at sea
level is more dense.
Unfortunately, windy areas tend to be picturesque, and so there is
a great deal of opposition to the installation of wind turbines on
what would otherwise appear to be ideal sites.
Offshore
Wind often flows briskly and smoothly over water since there
are no obstructions. The large and slow turning turbines of this
offshore wind farm near Copenhagen take advantage of the
moderate yet constant breezes at this location.
Offshore wind turbines are considered to be less unsightly (they
can be invisible from shore), and because the winds are usually more
potent offshore, such turbines don´t need to reach quite as high into
the air. However, offshore conditions are harsh, abrasive, and
corrosive, and it is often impossible or near-impossible to repair a
broken down turbine in open waters.
In stormy areas with extended shallow continental shelves and sand
banks (such as Denmark),
turbines are reasonably easy to install, and give good service -
Denmark's offshore wind generation provides about 20% of total
electricity demand in the country, while generating more than 20,000
jobs [7].
At the site shown, the wind is not especially strong but is very
consistent. The largest offshore wind turbines in the world are 3.6MW
rated machines that are installed in a small group of seven turbines
off the east coast of Ireland about 60km south of Dublin. The turbines
are located on a sandbank approximately 10km from the coast that has
the potential for the installation of 500MW of generation capacity. As
of 2006, the largest offshore wind farm is Horns Rev which is
located 15km west of Jutland,
Denmark
[8].
Airborne
- Main article: Airborne
wind turbine
It has been suggested that wind turbines might be flown in high
speed winds at high altitude. No such systems currently exist in the
marketplace. An Ontario company, Magenn Power, Inc., is attempting to
commercialize tethered aerial turbines suspended with helium.
Utilization
Large scale
| Total windpower capacity (late 2004)[9] |
| Rank |
Nation |
Windpower capacity (MW) |
| 01 |
Germany |
16,628 (18,428 in 2005) |
| 02 |
Spain |
8,263 (10,028 in 2005) |
| 03 |
USA |
6,752 (9,149 in 2005) |
| 04 |
Denmark |
3,118 |
| 05 |
India |
2,983 |
| 06 |
Italy |
1,265 |
| 07 |
Netherlands |
1,078 |
| 08 |
Japan |
940 |
| 09 |
United Kingdom |
897 |
| 10 |
China |
764 |
| 11 |
Austria |
607 |
| 12 |
Portugal |
523 |
| 13 |
Greenland |
466 |
| 14 |
Canada |
444 |
| 15 |
Sweden |
442 |
| 16 |
France |
390 |
| 17 |
Australia |
380 |
| 18 |
Ireland |
353 |
| 19 |
New Zealand |
170 |
| 20 |
Norway |
160 |
| |
Other |
951 |
| |
World total |
47,574 |
There are now many thousands of wind turbines operating in various
parts of the world, with a total capacity of over 47,317 MW of which Europe
accounts for 72% (2005). It was the most rapidly-growing means of
alternative electricity generation at the turn of the century and
provides a valuable complement to large-scale base-load power
stations. World wind generation capacity quadrupled between 1997 and
2002. 90% of wind power installations are in the US and Europe.
Germany,
Spain, Denmark,
and the United
States have made considerable investments in wind generated
electricity. Denmark is especially a leader in the manufacturing and
use of wind turbines, with a commitment made in the 1970s
to eventually produce half of the country's power by wind. In December
2003, General
Electric installed the world's largest offshore wind turbines in
Ireland, and plans are being made for more such installations on the
west coast, including the possible the use of floating turbines.
Germany already produces 40% of the entire world's wind power, and
the hope is that by 2010, wind will meet 12.5% of German electricity
needs. Germany has 16,000 wind turbines, mostly concentrated in the
north of the country, near the border with Denmark - including the
biggest in the world, owned by the Repower company. In 2005, the
goverment of Spain approved a new national goal for installed wind
power capacity of 20,000 MW by 2012. While the United States
government lost interest when the price of oil dropped after the 1970s
oil crisis, the Danes and Germans continued their efforts and now are
a leading exporter of large turbines (each generating 0.66 to 5.0
megawatt).
Wind accounts for 0.4% of the total electricity production on a
global scale (2002). Germany is the leading producer of wind power
with 35% of the total world capacity in 2005 (10% of German
electricity). Spain
and the United
States are next in terms of installed capacity. According to the
American Wind Energy Association, wind generated enough electricity to
power 0.4% (1.6 million households) of total electricity in US, up
from less than 0.1% in 1999. Germany's Schleswig-Holstein
province generates 25% of its power with wind turbines. Denmark
generates over 20% of its electricity with wind turbines, the highest
percentage of any country and is fourth in the world in total power
generation. In 2005, both Germany and Spain have produced more
electricity from wind power than from hydropower
plants.
After Germany, Spain, the United States, and Denmark, India
ranks 5th in the world with a total wind power capacity of 3500 MW.
Almost half of this capacity (1600 MW) was added in the last two
years, and of new electricity capacity additions in the country, wind
power accounted for over 20% of the total in that period. Currently
wind power generates 3% of all electricity produced in India. Unlike
the others in the top 5, India's estimated wind power potential is
pretty low at just 45 gigawatts, while world wide potential is
estimated at 72 terawatts, with the US and Northern Europe among the
regions with the maximum potential.
On August
15, 2005,
China
announced it would build a 1000-megawatt wind farm in Hebei for
completion in 2020.
China reportedly has set a generating target of 20 million kilowatts
by 2020 from renewable energy sources - it says indigenous wind power
could generate up to 253 million kilowatts.[10]
Another growing market is Brazil, with a wind potential of 143 GW.[11]
The federal government has created an incentive program, called Proinfa[12],
to build production capacity of 3300 MW of renewable energy for 2008,
of which 1422 MW through wind energy. The program seeks to produce 10%
of Brazillian electricity through renewable sources. Brazil produced
320 TWh in
2004.
Small scale
This rooftop-mounted urban wind turbine charges a 12 volt battery
and runs various 12 volt appliances within the building on which
it is installed.
Wind turbines have been used for household electricity generation
in conjunction with battery storage over many decades in remote areas.
Household generator units of more than 1 kW are now functioning in
several countries.
To compensate for the varying power output, grid-connected wind
turbines utilise some sort of grid
energy storage. Off-grid systems either adapt to intermittent
power or use photovoltaic
or diesel
systems to supplement the wind turbine.
Wind turbines range from small four hundred watt generators for
residential use to several megawatt machines for wind farms and
offshore. The small ones have direct drive generators, direct
current output, aeroelastic
blades, lifetime bearings and use a vane to point into the wind;
while the larger ones generally have geared power trains, alternating
current output, flaps and are actively pointed into the wind. As
technology progresses, large generators are becoming as simple as
small generators. Direct drive generators and aeroelastic blades for
large wind turbines are being researched and direct current generators
are sometimes used.
In urban locations, where it is difficult to obtain large amounts
of wind energy, smaller systems may still be used to run low power
equipment. Distributed
power from rooftop mounted wind turbines can also alleviate power
distribution problems, as well as provide resilience to power
failures. Equipment such as wireless internet gateways may be powered
by a wind turbine that charges a small battery, replacing the need for
a connection to the power grid and/or maintaining service despite
possible power grid failures.
Small-scale wind power in rural Indiana.
The Lakota turbine by Aeromax is approximately 7 feet (2 m) in
diameter and produces 900 watts of three
phase power. It uses a three phase rectifier and charge controller
so that it is free to spin at whatever speed is optimal for a given
wind condition. Lightweight materials (the entire turbine weighs only
16kg (35 pounds)) allow it to respond quickly to the gusts of wind
typical of urban settings. It attaches to a size 9 structural pipe
(similar to a TV antenna mast). The Lakota is very quiet. Even when
standing up on the roof right next to the mast it is inaudible.
Climbing up the mast, it is still inaudible from just a few feet under
the turbine. A dynamic
braking system regulates the speed by dumping excess energy, so
that the turbine continues to produce electricity even in high winds.
The dynamic braking resistor may be installed inside the building, so
that the 'heat loss' will heat the inside of the building (i.e. during
high winds when more heat is lost by the building, more heat is also
produced by the braking resistor). The proximal location makes low
voltage (12 volt, or the like) energy distribution practical, e.g. in
a typical installation the braking resistor can be located just inside
to where the mast is attached to the building. Such small-scale
renewable energy sources also impart a beneficial psychological effect
on building owners, so that they begin to take on a keen awareness of
electricity consumption, possibly reducing their consumption down to
the average level that the turbine can produce.
Controversy
The debate around wind energy is heated and often emotional.
Arguments of both parties are listed below.
Arguments of opponents
Some of the over 4000 wind turbines at Altamont
Pass, in California. Developed during a period of tax
incentives in the 1980s, this wind farm has more turbines than
any other in the United States. These kilowatt turbines cost
several times more per kw/h and spin much more quickly than
modern megawatt turbines, endangering birds and making noise.
These units are likely Enertech E44-40kWs.
Economics
| In order to compete with traditional sources of energy, wind
power often receives financial incentives. In the United States,
wind power receives a tax credit of 1.9 cents per kilowatt-hour
produced, with a year inflationary adjustment. However, in 2004
when the U.S. production tax credit had lapsed for nine months,
wind power was still a rapidly growing form of electrical
generation. Another tax benefit is accelerated depreciation. Many
American states also provide incentives, such as exemption from
property tax, mandated purchases, and additional markets for
"green credits." Countries such as Canada
and Germany
also provide tax credits and other incentives for wind turbine
construction. Although other energy sources are also subsidized,
the amount per kilowatt-hour may be much higher for wind. |
| Maintenance of wind turbines can be difficult and expensive.
Repairs require a much more complicated and expensive operation
than ground based generation. |
| Many potential sites for wind farms are far from demand centers,
requiring substantially more money to construct new transmission
lines and substations. |
Yield
| The goals of renewable energy development are reduction of
reliance on fossil and nuclear fuels, reduction of greenhouse gas
and other emissions, and establishment of more sustainable sources
of energy. Some critics question wind energy's ability to
significantly move society towards these goals. They point out
that 25-30% annual load factor is typical for wind facilities. The
intermittent and nondispatchable nature of wind turbine power
requires that "spinning reserves" are kept burning for
supply security. More frequent ramping of such plants means lower
efficiency and possibly greater emissions. |
| Electric power production is only part (about one to two fifths[13])
of a country's energy use, and wind power does nothing to mitigate
the larger part of the effects of energy use. For example, despite
aggressive installation of wind facilities in the U.K., that
country's CO2 emissions continued to rise in 2002 and
2003 (Department of Trade and Industry). Greenhouse gas emissions
in Denmark rose 6.2% in 2003 (National Environmental Research
Institute). |
| Groups such as the UN's
Intergovernmental
Panel on Climate Change state that the desired mitigation
goals can be achieved at lower cost and to a greater degree by
continued improvements in general efficiency — in building,
manufacturing, and transport — than by wind power. A study by
the Irish Grid into expanding wind power similarly concluded that,
"The cost of CO2 abatement arising from using
large levels of wind energy penetration appears high relative to
other alternatives." |
Ecological footprint
| The construction of a large facility is also far from
ecologically neutral if the location has no previous development.
It requires roads, foundations, clearing of trees, and
construction of power lines. The clearing of trees is necessary
since obstructions within a distance ten times the height of the
turbine reduce yield dramatically. A distance of twenty times is
preferred. |
| A wind farm that produces the energy equivalent of a
conventional power plant would have to cover an area of
approximately 300 square miles. [14] |
| Offshore sites eliminate some of these objections, but raise
others such as dangers to navigation
and the possible adverse effect of low-frequency vibration and
shadow flicker on aquatic mammals. |
| Another important complaint is that windmills kill too many
birds, especially birds
of prey. Siting generally takes into account bird flight
patterns, but most paths of bird
migration, particularly for birds that fly by night, are
unknown. Although a Danish survey in 2005 (Biology Letters
2005:336) showed that less than 1% of migrating birds passing a
wind farm in Rønde, Denmark, got close to collision, the site was
studied only during low-wind non-twilight conditions. A survey at
Altamont Pass, California conducted by a California Energy
Commission in 2004 showed that turbines killed 4,700 birds
annually (1,300 of which are birds of prey). Radar studies of
proposed sites in the eastern U.S. have shown that migrating
songbirds fly well within the reach of large modern turbines. |
| The numbers of bats killed by existing facilities has troubled
even industry personnel [15].
A six-week study [16]
in 2004 estimated that over 2200 bats were killed by 63 turbines
at two sites in the Eastern US. This study suggests some site
locations may be particularly hazardous to local bat populations,
and that more research is urgently needed. |
Scalability
| The large number of turbines required for a viable wind plant,
and the huge number of plants required to meet the ambitious goals
of the wind industry and governments, ensures that more people and
wildlife habitat will be affected by them. |
Aesthetics
| There is resistance to the establishment of land based wind
farms owing to perceptions that they are noisy and contribute to
"visual pollution." Moving the turbines offshore
mitigates the problem, but offshore wind farms are more expensive
to maintain and there is an increase in transmission loss due to
longer distances of power lines. |
| The large installations of a modern wind facility are typically
100 m high to the tip of the rotor blade, and, besides the
continuous motion of the 35-m-long rotor blades through the air,
each time the blade passes the tower a deep subsonic thump is
produced, which is a form of noise
pollution. |
| Some residents near windmills complain of "shadow
flicker," which is the alternating pattern of sun and shade
caused by a rotating windmill casting a shadow over residences.
Efforts are made when siting turbines to avoid this problem. |
Arguments of supporters
Supporters of wind energy state that:
Yield
| Wind's long-term technical potential is believed 5 times current
global energy consumption or 40 times current electricity demand.
This requires 12.7% of all land area, or that land area with Class
3 or greater potential at a height of 80 meters. It assumes that
the land is covered with 6 large wind turbines per square
kilometer. Offshore resources experience mean wind speeds ~90%
greater than that of land, so offshore resources could contribute
substantially more energy.[17][18].
This number could also increase with higher altitude ground based
or airborne wind turbines [19]. |
Coping with intermittent wind power
| As the fraction of energy produced by wind
("penetration") increases, different technical and
economic factors affect the electric power grid and storage
facilities. Large networks, connected to multiple wind plants at
widely separated geographic locations, may accept a higher
penetration of wind than small networks or those without storage
systems or economical methods of compensating for the variability
of wind. In systems with significant amounts of existing pumped
storage (e.g. UK, eastern US) this proportion may be higher;
isolated, relatively small systems with only a few wind plants may
only be stable and economic with a lower fraction of wind energy
(e.g. Ireland). |
| On most large power systems a moderate proportion of wind
generation (typically <10%) can be connected without the
need for storage of any kind. This is explained below. For larger
proportions (>20%). storage may be economically attractive or
even technically necessary. |
| Long-term storage of electrical energy involves substantial
capital costs, space for storage facilities, and some portion of
the stored power will be lost during conversion and transmission.
The percentage retrievable from stored power is called the
"efficiency of storage." The cost incurred to
"shape" intermittent wind power for reliable delivery is
about a 20% premium for most wind applications on large grids, but
approaches 50% of the cost of generation when wind comprises more
than 70% of the local grid's input power. Large scale energy
storage includes pumped hydro and other mechanical forms of
storage, as well as stationary hydrogen electrolysis and fuel
cells and other forms of chemical storage. |
| Electricity demand is variable but generally very predictable on
larger grids—errors in demand forecasting are typically no more
than 2%. Because conventional powerplants can drop off the grid
within a few seconds,for example due to equipment failures, in
most systems the output of some coal or gas powerplants is
intentionally part-loaded to follow demand and to replace rapidly
lost generation. The ability to follow demand (by maintaining
constant frequency) is termed "response." The ability to
quickly replace lost generation, typically within timescales of 30
seconds to 30 minutes, is termed "reserve." Nuclear
power plants in contrast are not very flexible and are not
intentionally part-loaded. A power plant that operates in a steady
fashion, usually for many days continuously, is termed a
"base load" plant. As a consequence of these
arrangements most grids' production systems are already equipped
with an array of substantial, quickly adjustable back-up
techniques. |
| What happens in practice therefore is that as the power output
from wind varies, part-loaded conventional plants, which must be
there anyway due to changing demand, adjust their output to
compensate; they do this in response to small changes in the
frequency (nominally 50 or 60 Hz) of the grid. In this sense wind
acts like "negative" load or demand. |
| The maximum proportion of wind power allowable in a power system
will thus depend on many factors, including the size of the
system, the attainable geographical diversity of wind, the
conventional plant mix (coal, gas, nuclear) and seasonal load
factors (heating in winter, air-conditioning in summer) and their
statistical correlation with wind output. For most large systems
the allowable penetration fraction (wind nameplate rating divided
by system peak demand) is thus at least 15% without the need for
any energy storage whatsoever. Note that the interconnected
electrical system may be much larger than the particular country
or state (e.g. Denmark, California) being considered. |
| The allowable penetration may of course be further increased by
increasing the amount of part-loaded generation available, or by
using energy storage facilities, although if purpose-built for
wind energy these may significantly increase the overall cost of
wind power. |
| Existing European hydroelectric
power plants can store enough energy to supply one month's worth
of European electricity consumption. Improvement of the
international grid would allow using this in the relatively short
term at low cost, supplementing wind power. Excess wind power
could even be used to pump water up into collection basins for
later use. |
| Energy
Demand Management or Demand-Side Management refers to the use
of communication and switching devices which can release
deferrable loads quickly to correct supply/demand imbalances.
Incentives can be created for the use of these systems, such as
favorable rates or capital cost assistance, encouraging consumers
with large loads to take advantage of renewable energy by
adjusting their loads to coincide with resource availability. For
example, pumping water to pressurize municipal water systems is an
electricity intensive application that can be performed when
electricity is available.[20] |
|
