Wind Variability

One of the most critical features of wind generation is the variability of wind. Wind speeds vary with time of day, time of year, height above ground, and location on the earth’s surface. This makes wind generators into what might be called energy producers rather than power producers.

That is, it is easier to estimate the energy production for the next month or year than it is to estimate the power that will be produced at 4:00 PM next Tuesday. Wind power is not dispatchable in the same manner as a gas turbine. A gas turbine can be scheduled to come on at a given time and to be turned off at a later time, with full power production in between. A wind turbine produces only when the wind is available.

At a good site, the power output will be zero (or very small) for perhaps 10% of the time, rated for perhaps another 10% of the time, and at some intermediate value the remaining 80% of the time.

This variability means that some sort of storage is necessary for a utility to meet the demands of its customers, when wind turbines are supplying part of the energy. This is not a problem for penetrations of wind turbines less than a few percent of the utility peak demand. In small concentrations, wind turbines act like negative load. That is, an increase in wind speed is no different in its effect than a customer turning off load. The control systems on the other utility generation sense that generation is greater than load, and decrease the fuel supply to bring generation into equilibrium with load. In this case, storage is in the form of coal in the pile or natural gas in the well.

 

An excellent form of storage is water in a hydroelectric lake. Most hydroelectric plants are sized large enough to not be able to operate full-time at peak power. They therefore must cut back part of the time because of the lack of water.

A combination hydro and wind plant can conserve water when the wind is blowing, and use the water later, when the wind is not blowing. When high-temperature superconductors become a little less expensive, energy storage in a magnetic field will be an exciting possibility. Each wind turbine can have its own superconducting coil storage unit.

This immediately converts the wind generator from an energy producer to a peak power producer, fully dis-patchable. Dis-patchable peak power is always worth more than the fuel cost savings of an energy producer. Utilities with adequate base load generation (at low fuel costs) would become more interested in wind power if it were a dis-patchable peak power generator.

The variation of wind speed with time of day is called the diurnal cycle. Near the earth’s surface, winds are usually greater during the middle of the day and decrease at night. This is due to solar heating, which causes “bubbles” of warm air to rise. The rising air is replaced by cooler air from above. This thermal mixing causes wind speeds to have only a slight increase with height for the first hundred meters or so above the earth. At night, however, the mixing stops, the air near the earth slows to a stop, and the winds above some height (usually 30 to 100 m) actually increase over the daytime value. A turbine on a short tower will produce a greater proportion of its energy during daylight hours, while a turbine on a very tall tower will produce a greater proportion at night.

As tower height is increased, a given generator will produce substantially more energy.

However, most of the extra energy will be produced at night, when it is not worth very much. Standard heights have been increasing in recent years, from 50 to 65 m or even more. A taller tower gets the blades into less turbulent air, a definite advantage.

The disadvantages are extra cost and more danger from overturning in high winds. A very careful look should be given the economics before buying a tower that is significantly taller than whatever is sold as a standard height for a given turbine.

Wind speeds also vary strongly with time of year. In the southern Great Plains (Kansas, Oklahoma, and Texas), the winds are strongest in the spring (March and April) and weakest in the summer (July and August). Utilities here are summer peaking, and hence need the most power when winds are the lowest and the least power when winds are highest. The diurnal variation of wind power is thus a fairly good match to utility needs, while the yearly variation is not.

TABLE 1
Monthly Average Wind Speed in MPH and Projected Energy Production at 65 m, at a Good Site in Southern Kansas

	10 m	60 m	Energy		10 m	60 m	Energy
Month	Speed	Speed	(MWh)	Month	Speed	Speed	(MWh)
1/96	14.9	20.3	256	1/97	15.8	21.2	269
2/96	16.2	22.4	290	2/97	14.7	19.0	207
3/96	17.6	22.3	281	3/97	17.4	22.8	291
4/96	19.8	25.2	322	4/97	15.9	20.4	242
5/96	18.4	23.1	297	5/97	15.2	19.8	236
6/96	13.5	18.2	203	6/97	11.9	16.3	167
7/96	12.5	16.5	169	7/97	13.3	18.5	212
8/96	11.6	16.0	156	8/97	11.7	16.9	176
9/96	12.4	17.2	182	9/97	13.6	19.0	211
10/96	17.1	23.3	320	10/97	15.0	21.1	265
11/96	15.3	20.0	235	11/97	14.3	19.7	239
12/96	15.1	20.1	247	12/97	13.6	19.5	235

The variability of wind with month of year and height above ground is illustrated in Table 1. These are actual wind speed data for a good site in Kansas, and projected electrical generation of a Vestas turbine (V47-660) at that site. Anemometers were located at 10, 40, and 60 m above ground. Wind speeds at 40 and 60 m were used to estimate the wind speed at 65 m (the nominal tower height of the V47-660) and to calculate the expected energy production from this turbine at this height. Data have been normalized for a 30-day month. There can be a factor of two between a poor month and an excellent month (156 MWh in 8/96 to 322 MWh in 4/96). There will not be as much variation from one year to the next, perhaps 10 to 20%.

A wind power plant developer would like to have as long a data set as possible, with an absolute minimum of one year. If the one year of data happens to be for the best year in the decade, followed by several below average years, a developer could easily get into financial trouble. The risk gets smaller if the data set is at least two years long.

One would think that long-term airport data could be used to predict whether a given data set was collected in a high or low wind period for a given part of the country, but this is not always true. One study showed that the correlation between average annual wind speeds at Russell, Kansas, and Dodge City, Kansas, was 0.596 while the correlation between Russell and Wichita was 0.115.

The terrain around Russell is very similar to that around Wichita, and there is no obvious reason why wind speeds should be high at one site and low at the other for one year, and then swap roles the next year.

There is also concern about long-term variation in wind speeds. There appears to be an increase in global temperatures over the past decade or so, which would probably have an impact on wind speeds. It also appears that wind speeds have been somewhat lower as temperatures have risen, at least in Kansas. It appears that wind speeds can vary significantly over relatively short distances. A good data set at one location may under-predict or over-predict the winds at a site a few miles away by as much as 10 to 20%. Airport data collected on a 7-m tower in a flat river valley may underestimate the true surrounding hilltop winds by a factor of two.

If economics are critical, a wind power plant developer needs to acquire rights to a site and collect wind speed data for at least one or two years before committing to actually constructing turbines there.

Aerodynamics of Horizontal-Axis Wind Turbines

A wind turbine is a device for extracting kinetic energy from the wind. By removing some of its kinetic energy the wind must slow down but only that mass of air which passes through the rotor disc is affected. Assuming that the affected mass of air remains separate from the air which does not pass through the rotor disc and does not slow down a boundary surface can be drawn containing the affected air mass and this boundary can be extended upstream as well as downstream forming a long stream-tube of circular cross section. No air flows across the boundary and so the mass flow rate of the air flowing along the stream-tube will be the same for all stream-wise positions along the stream-tube. Because the air within the stream-tube slows down, but does not become compressed, the cross-sectional area of the stream-tube must expand to accommodate the slower moving air (Figure 1).

Although kinetic energy is extracted from the airflow, a sudden step change in velocity is neither possible nor desirable because of the enormous accelerations and forces this would require. Pressure energy can be extracted in a step-like manner, however, and all wind turbines, whatever their design, operate in this way.

Figure 1 The Energy Extracting Stream-tube of a Wind Turbine

The presence of the turbine causes the approaching air, upstream, gradually to slow down such that when the air arrives at the rotor disc its velocity is already lower than the free-stream wind speed.

The stream-tube expands as a result of the slowing down and, because no work has yet been done on, or by, the air its static pressure rises to absorb the decrease in kinetic energy. As the air passes through the rotor disc, by design, there is a drop in static pressure such that, on leaving, the air is below the atmospheric pressure level. The air then proceeds downstream with reduced speed and static pressure – this region of the flow is called the wake.

Eventually, far downstream, the static pressure in the wake must return to the atmospheric level for equilibrium to be achieved. The rise in static pressure is at the expense of the kinetic energy and so causes a further slowing down of the wind. Thus, between the far upstream and far wake conditions, no change in static pressure exists but there is a reduction in kinetic energy.

Siemens Wind Turbine SWT-2.3-82

The SWT-2.3-82 turbine is the classical workhorse for utility-scale wind turbine projects, designed on the basis of our unique experience from more than 25 years of leading-edge design and construction of wind turbines.

The turbine represents the best of the qualities for which we are known throughout the wind industry – an efficient and reliable machine combining a solid and conservative design approach with high-performance technical features, such as the unique CombiStall® power regulation system and IntegralBlade® technology.

The SWT-2.3-82 wind turbine is equally well suited for tough and demanding applications onshore and offshore. One of the projects featuring the turbine is the Nysted Offshore Wind Farm – at the time of installation the world’s largest offshore wind farm.

Nacelle arrangement

1. Spinner
2. Spinner bracket
3. Blade
4. Pitch bearing
5. Rotor hub
6. Main bearing
7. Main shaft

8. Gearbox
9. Brake disc
10. Coupling
11. Service crane
12. Generator
13. Meteorological sensors
14. Yaw gear

15. Yaw ring
16. Tower
17. Nacelle bedplate
18. Canopy
19. Oil filter
20. Generator fan
21. Oil cooler

Technical description

General design

The overall design of the SWT-2.3-82 is based on the so-called “Danish Concept” – a three-bladed upwind rotor with stall regulation and constant rotor speed, an asynchronous generator coupled directly to the grid, and fail-safe safety systems with automatic air brakes and hydraulic disc brakes. Even though many other manufacturers use this concept, special design features, such as heavier dimensions on major components have characterized Siemens turbines through the era of modern electricity producing wind turbines.

Rotor

The SWT-2.3-82 turbine has a three-bladed rotor with CombiStall® regulation for power output optimization and control.

Blades

The B40 blades are made of fiberglassreinforced epoxy in Siemens’ proprietary IntegralBlade® manufacturing process. In this process, the blades are cast in one piece, leaving no weak points at glue joints and providing optimum quality. The aerodynamic design represents state-ofthe- art wind turbine technology, and the structural design has special Siemens safety factors over and above all normal industry and customer requirements. The design has been thoroughly verified by static and dynamic testing of both prototype and serial production blades.

Rotor hub

The rotor hub is cast in nodular cast iron and is fitted to the main shaft with a flange connection. The hub is large enough to provide a comfortable working environment inside the structure for two service technicians during maintenance of bolt connections and pitch bearings.

Blade pitch system

The blade pitch arrangement is used to optimize and regulate power output through the operating range. The blades are feathered to minimize wind loads during standstill under extreme wind conditions.

Main shaft and bearing

The main shaft is forged in alloy steel and is hollow for the transfer of power and signals to the blade pitching system. The main shaft is supported by a self-aligning double spherical roller bearing, grease lubricated from an automatic lubrication system. The bearing seals are maintenancefree labyrinth seals.

Gearbox

The gearbox is a custom-built, three-stage planetary-helical design. The planetaryhelical, high-torque stage provides a compact high-performance construction. The intermediary and high-speed stages are normal helical stages arranged with an offset of the high-speed shaft and thus allowing passage of power and control signals to the pitch systems. The gearbox is equipped with large-capacity cooling and filtering systems that ensure optimum operating conditions.

Generator

The generator is a fully-enclosed asynchronous machine with squirrel-cage rotor, which does not require slip rings.
The generator rotor construction and stator windings are specially designed for high efficiency at partial loads. The generator is internally ventilated and cooled with an air-to-air heat exchanger.

Mechanical brake

The mechanical brake represents the secondary safety system of the turbine. It is fitted to the gearbox high-speed shaft and has two hydraulic calipers.

Yaw system

The yaw bearing is an externally geared ring with a friction bearing. Eight electric planetary gear motors drive the yawing. The yaw gear motors are fitted with brakes, assisting the passive friction of the bearing for stable maintenance of the yaw position.

Controller

A standard industrial computer is the basis of the turbine controller. The controller is self-diagnosing and includes a keyboard and display for easy status readout and adjustment of settings.

Tower

The SWT-2.3-82 turbine is mounted on a tapered tubular steel tower. The tower can be fitted with a personnel hoist as an option.

Operation

The wind turbine operates automatically, self-starting when the wind reaches an average speed of about 3 – 5 m/s. During operation below rated power, the pitch angle and rotor speed are continuously adjusted to maximize the aerodynamic efficiency. Rated power is reached at a wind speed of about 13 – 14 m/s, and at higher wind speeds the output is regulated at rated power. If the average wind speed exceeds the maximum operational limit of 25 m/s, the turbine is shut down by feathering of the blades. When the wind drops back below the restart speed, the safety systems reset automatically.

Remote control

The SWT-2.3-82 turbine is equipped with the unique WebWPS SCADA system. This system offers remote control and a variety of status views and useful reports from a standard Internet Web browser. The status views present electrical and mechanical data, operation and fault status, meteorological data and grid station data. Primary level users can be granted access to any of the server’s features, including full control over the turbines.

Turbine Condition Monitoring

In addition to the WebWPS SCADA system, the turbine is equipped with a Web-based Turbine Condition Monitoring (TCM) system. The TCM system carries out precise condition diagnostics on main turbine components continuously and in real time. It gives early warning of possible component failures by continuous comparison of current vibration spectra with previously established reference spectra.
The TCM system has various alarm levels, from informative through alerting level to turbine shutdown.

Grid compliance

The SWT-2.3-82 turbine complies with all currently valid grid code requirements on relevant markets. The power factor can be controlled over a wide range by the use of thyristor-controlled capacitors, and the turbine has fault ride through capability for all normal faults. Voltage and frequency control and other grid-related adjustments can be implemented by the integrated Park Pilot facility in the WebWPS SCADA system.

Technical Specifications

What Do Wind Systems Cost?

A small turbine can cost anywhere from $3,000 to $35,000 installed, depending on size, application, and service agreements with the manufacturer. (The American Wind Energy Association [AWEA] says a typical home wind system costs approximately $32,000 (10 kW); a comparable photovoltaic [PV] solar system would cost over $80,000.)

A general rule of thumb for estimating the cost of a residential turbine is $1,000 to $3,000 per kilowatt. Wind energy becomes more cost effective as the size of the turbine’s rotor increases. Although small turbines cost less in initial outlay, they are proportionally more expensive. The cost of an installed residential wind energy system that comes with an 80-foot tower, batteries, and inverter, typically ranges from $13,000 to $40,000 for a 3 to 10 kW wind turbine.

Although wind energy systems involve a significant initial investment, they can be competitive with conventional energy sources when you account for a lifetime of reduced or avoided utility costs. The length of the payback period – the time before the savings resulting from your system equal the cost of the system itself – depends on the system you choose, the wind resource on your site, electricity costs in your area, and how you use your wind system.

For example, if you live in California and have received the 50% buydown of your small wind system, have net metering, and an average annual wind speed of 15 miles per hour (mph) (6.7 meters per second [m/s]), your simple payback would be approximately 6 years.

Things to Consider When Purchasing a Wind Turbine

Once you determine you can install a wind energy system in compliance with local land use requirements, you can begin pricing systems and components. Comparatively shop for a wind system as you would any major purchase. Obtain and review the product literature from several manufacturers.

As mentioned earlier, lists of manufacturers are available from AWEA, but not all small turbine manufacturers are members of AWEA. Check the yellow pages for wind energy system dealers in your area. Once you have narrowed the field, research a few companies to be sure they are recognized wind energy businesses and that parts and service will be available when you need them. You may wish to contact the Better Business Bureau to check on the company’s integrity and ask for references of past customers with installations similar to the one you are considering.

Ask the system owners about performance, reliability, and maintenance and repair requirements, and whether the system is meeting their expectations. Also, find out how long the warranty lasts and what it includes.

Is There Enough Wind on My Site?

Is there enough wind on my site? What do I need to know?

Does the wind blow hard and consistently enough at my site to make a small wind turbine system economically worthwhile? That is a key question and not always easily answered.

The wind resource can vary significantly over an area of just a few miles because of local terrain influences on the wind flow. Yet, there are steps you can take that will go a long way towards answering the above question.

As a first step, wind resource maps can be used to estimate the wind resource in your region. The highest average wind speeds in the United States are generally found along seacoasts, on ridge lines, and on the Great Plains; however, many areas have wind resources strong enough to power a small wind turbine economically. The wind resource estimates on this map generally apply to terrain features that are well exposed to the wind, such as plains, hilltops, and ridge crests. Local terrain features may cause the wind resource at a specific site to differ considerably from these estimates.

More detailed wind resource information, including the Wind Energy Resource Atlas of United States, published by the U.S. Department of Energy (DOE), can be found at the National Wind Technology Center, and the DOE Windpowering America.

Another way to indirectly quantify the wind resource is to obtain average wind speed information from a nearby airport. However, caution should be used because local terrain influences and other factors may cause the wind speed recorded at an airport to be different from your particular location.

Airport wind data are generally measured at heights about 20–33 ft (6–10 m) above ground.

Average wind speed

Average wind speeds increase with height and may be 15%–25% greater at a typical wind turbine hub-height of 80 ft (24 m) than those measured at airport anemometer heights. The National Climatic Data Center collects data from airports in the United States and makes wind data summaries available for purchase.

Summaries of wind data from almost 1000 U.S. airports are also included in the Wind Energy Resource Atlas of the United States (see For More Information). Another useful indirect measurement of the wind resource is the observation of an area’s vegetation. Trees, especially conifers or evergreens, can be permanently deformed by strong winds. This deformity, known as “flagging,” has been used to estimate the average wind speed for an area.

Flagging, the effect of strong winds on area vegetation, can help determine area wind speeds.

Direct monitoring by a wind resource measurement system at a site provides the clearest picture of the available resource. Wind measurement systems are available for costs as low as $600 to $1200.

This expense may or may not be hard to justify depending on the exact nature of the proposed small wind turbine system. The measurement equipment must be set high enough to avoid turbulence created by trees, buildings, and other obstructions. The most useful readings are those taken at hub-height, the elevation at the top of the tower where the wind turbine is going to be installed.

If there is a small wind turbine system in your area, you may be able to obtain information on the annual output of the system and also wind speed data if available.

Can I Connect Wind Turbine to the Utility Grid?

Small wind energy systems can be connected to the electricity distribution system and are called gridconnected systems. A grid-connected wind turbine can reduce your consumption of utility-supplied electricity for lighting, appliances, and electric heat. If the turbine cannot deliver the amount of energy you need, the utility makes up the difference.

When the wind system produces more electricity than the household requires, the excess is sent or sold to the utility.

Grid-connected systems can be practical if the following conditions exist:

• You live in an area with average annual wind speed of at least 10 mph (4.5 m/s).
• Utility-supplied electricity is expensive in your area (about 10 to 15 cents per kilowatt-hour).
• The utility’s requirements for connecting your system to its grid are not prohibitively expensive.
• There are good incentives for the sale of excess electricity or for the purchase of wind turbines.

Federal regulations (specifically, the Public Utility Regulatory Policies Act of 1978, or PURPA) require utilities to connect with and purchase power from small wind energy systems. However, you should contact your utility before connecting to their distribution lines to address any power quality and safety concerns. Your utility can provide you with a list of requirements for connecting your system to the grid. The American Wind Energy Association is another good source for information on utility interconnection requirements.

The following information about utility grid connection requirements was taken from AWEA’s Web site.

Net Metering

The concept of net metering programs is to allow the electric meters of customers with generating facilities to turn backwards when their generators are producing more energy than the customers’ demand.

Net metering allows customers to use their generation to offset their consumption over the entire billing period, not just instantaneously. This offset would enable customers with generating facilities to receive retail prices for more of the electricity they generate. Net metering varies by state and by utility company, depending on whether net metering was legislated or directed by the Public Utility Commission.

Net metering programs all specify a way to handle the net excess generation (NEG) in terms of payment for electricity and/or length of time allowed for NEG credit. If the net metering requirements define NEG on a monthly basis, the consumer can only get credit for their excess that month. But if the net metering rules allow for annual NEG, the NEG credit can be carried for up to a year.

Most of North America gets more wind in the winter than in the summer. For people using wind energy to displace a large load in the summer like air-conditioning or irrigation water pumping, having an annual NEG credit allows them to produce NEG in the winter and be credited in the summer.

Safety Requirements

Whether or not your wind turbine is connected to the utility grid, the installation and operation of the wind turbine is probably subject to the electrical codes that your local government (city or county) or in some instances your state government has in place.

The government’s principal concern is with the safety of the facility, so these code requirements emphasize proper wiring and installation, and the use of components that have been certified for fire and electrical safety by approved testing laboratories, such as Underwriters Laboratories. Most local electrical codes requirements are based on the National Electrical Code (NEC), which is published by the National Fire Protection Association. As of 1999, the latest version of the NEC did not have any sections specific to the installation of wind energy facilities, consequently wind energy installations are governed by the generic provisions of the NEC.

If your wind turbine is connected to the local utility grid so that any of the power produced by your wind turbine is delivered to the grid, then your utility also has legitimate concerns about safety and power quality that need to be addressed.

The utility’s principal concern is that your wind turbine automatically stops delivering any electricity to its power lines during a power outage. Otherwise line workers and the public, thinking that the line is “dead,” might not take normal precautions and might be hurt or even killed by the power from your turbine. Another concern among utilities is that the power from your facility synchronize properly with the utility grid, and that it match the utility’s own power in terms of voltage, frequency, and power quality.

A few years ago, some state governments started developing new standardized interconnection requirements for small renewable energy generating facilities (including wind turbines).

In most cases the new requirements have been based on consensus- based standards and testing procedures developed by independent third-party authorities, such as the Institute of Electrical and Electronic Engineers and Underwriters Laboratories.

Interconnection Requirements

Most utilities and other electricity providers require you to enter into a formal agreement with them before you interconnect your wind turbine with the utility grid. In states that have retail competition for electricity service (e.g., your utility operates the local wires, but you have a choice of electricity provider) you may have to sign a separate agreement with each company.

Usually these agreements are written by the utility or the electricity provider. In the case of private (investor-owned) utilities, the terms and conditions in these agreements must be reviewed and approved by state regulatory authorities.

Insurance

Some utilities require small wind turbine owners to maintain liability insurance in amounts of $1 million or more. Utilities consider these requirements are necessary to protect them from liability for facilities they do not own and have no control over. Others consider the insurance requirements excessive and unduly burdensome, making wind energy uneconomic. In the 21 years since utilities have been required to allow small wind systems to interconnect with the grid there has never been a liability claim, let alone a monetary award, relating to electrical safety. In six states (California, Maryland, Nevada, Oklahoma, Oregon, and Washington), laws or regulatory authorities prohibit utilities from imposing any insurance requirements on small wind systems that qualify for “net metering.”

In at least three other states (Idaho, New York, Virginia) regulatory authorities have allowed utilities to impose insurance requirements, but have reduced the required coverage amounts to levels consistent with conventional residential or commercial insurance policies (e.g., $100,000 to $300,000).

If your insurance amounts seem excessive, you can ask for a reconsideration from regulatory authorities (in the case of private investor-owned utilities) or to the utility’s governing board (in the case of publicly-owned utilities).

Indemnification

An indemnity is an agreement between two parties where one agrees to secure the other against loss or damage arising from some act or some assumed responsibility. In the context of customer-owned generating facilities, utilities often want customers to indemnify them for any potential liability arising from the operation of the customer’s generating facility.

Although the basic principle is sound-utilities should not be held responsible for property damage or personal injury attributable to someone else-indemnity provisions should not favor the utility but should be fair to both parties.

Look for language that says, “each party shall indemnify the other…” rather than “the customer shall indemnify the utility . . .”

Customer Charges

Customer charges can take a variety of forms, including interconnection charges, metering charges, and standby charges, among others. You should not hesitate to question any charges that seem inappropriate to you.

Federal law (Public Utility Regulatory Policies Act of 1978, or PURPA, Section 210) prohibits utilities from assessing discriminatory charges to customers who have their own generation facilities.

How Do I Choose the Best Site for My Wind Turbine?

You can have varied wind resources within the same property. In addition to measuring or finding out about the annual wind speeds, you need to know about the prevailing directions of the wind at your site. If you live in complex terrain, take care in selecting the installation site. If you site your wind turbine on the top of or on the windy side of a hill, for example, you will have more access to prevailing winds than in a gully or on the leeward (sheltered) side of a hill on the same property.

In addition to geologic formations, you need to consider existing obstacles such as trees, houses, and sheds, and you need to plan for future obstructions such as new buildings or trees that have not reached their full height. Your turbine needs to be sited upwind of buildings and trees, and it needs to be 30 feet above anything within 300 feet. You also need enough room to raise and lower the tower for maintenance, and if your tower is guyed, you must allow room for the guy wires.

Whether the system is stand-alone or grid-connected, you will also need to take the length of the wire run between the turbine and the load (house, batteries, water pumps, etc.) into consideration. A substantial amount of electricity can be lost as a result of the wire resistance—the longer the wire run, the more electricity is lost.

Using more or larger wire will also increase your installation cost. Your wire run losses are greater when you have direct current (DC) instead of alternating current (AC).

So, if you have a long wire run, it is advisable to invert DC to AC.

World’s largest wind turbine blade developed by Alstom and LM Wind Power

Global leader in power generation equipment and services, Alstom, and LM Wind Power, leading wind turbine blade supplier, have entered a strategic partnership to develop the world’s longest wind turbine blade ever produced. The new blade is a unique development designed specifically for Alstom’s next generation 6-megawatt (MW) offshore wind turbine.
Development of the blade will require more than 20,000 hours of work by LM Wind Power’s specialist teams focusing especially on aerodynamics, structural design, and production processes of this giant. The use of specifically developed material compounds will enable LM Wind Power to maximize strength and durability while producing an exceptionally light blade. Furthermore, it will feature a structural design specifically tailored for Alstom’s turbine, as well as LM Wind Power’s proprietary aerodynamic profiles based on its latest GloBlade, which offers an additional 4-5 percent of annual energy production compared to standard designs. The geometry of the new blade has already been validated in LM Wind Power’s own wind tunnel – the world’s only wind tunnel customized for aerodynamic testing of wind turbine blades.
The prototype blades will be produced in LM Wind Power’s Danish factory in Lunderskov and will be ready for installation at Alstom prototype sites in Europe to start testing during the winter of 2011.

LM Wind Power is one of the pioneers in offshore wind. Its first blades installed offshore have been operating since 1991 at the world’s first offshore wind farm, Vindeby, Denmark. Since 2004, LM Wind Power has supplied its 61.5-meter long blades for offshore. One example is the Beatrice Wind Farm Demonstrator project 25 kilometers off the coast of Scotland.
These blades are currently the world’s longest in serial production and have been installed in offshore wind farms across Europe, where they achieve very high availability.
Alstom’s 6 MW turbine has been optimized for the UK’s Crown Estate Round 3 and other North Sea markets. Two prototypes will be installed in 2011 and 2012, a per-series in 2013 (the final roll out step before full commercialization), and series production in 2014. In January Alstom and EDF Energies Nouvelles (EDF-EN) announced that, using this turbine, they would bid jointly for projects under the recently launched 3 GW French offshore wind tender.
The turbine incorporates leading edge technologies to meet the challenges of the tough marine environment and bring down the cost of energy (COE). These include the turbine’s very large rotor diameter and 6 MW power output for higher energy output. High yield helps offset wind farm investment and operating costs, lowering COE. The turbine’s weight has also been optimized to reduce installation and infrastructure costs.
The turbine features Alstom’s unique ALSTOM PURE TORQUE™ technology to protect and improve the performance of the generator. The technology protects the turbine’s drive train by deflecting unwanted stresses from the wind safely to the tower. Only turning force, or torque, is transmitted to the generator thereby boosting the turbine’s reliability.
Furthermore the turbine’s innovative permanent magnet direct drive system enables a compact, lightweight design that reduces service costs and improves operating efficiency. The system’s low number of rotating parts increases reliability, to maximize turbine availability and further reduce maintenance costs.
Alfonso Faubel, Alstom Power’s Wind business Vice President, said: “LM Wind Power’s quality technology perfectly complements our innovative wind turbine development capability. We have a partner of undoubted pedigree, a proven track record on offshore blade development and the global manufacturing presence and development resources to support our offshore growth plans. The close collaboration of both companies in offshore wind will deliver best in class technology to help bring down the COE, and a highly competitive market offering that will help us become a leader in the global offshore wind energy market.”
Roland Sundén, CEO LM Wind Power Group, said: “I am confident that Alstom’s innovative turbines flying the world’s longest blades from LM Wind Power will give them a clear COE advantage, and we are very pleased to extend our excellent working relations with Alstom into this prestigious project. The key to success in offshore, given that you have a quality wind turbine, is strength of balance sheet, project management skills and power engineering experience. LM Wind Power recognizes Alstom’s clear advantages in this respect compared to many others in offshore.”

About LM Wind Power

LM Wind Power Group is the world’s largest component supplier to the wind industry comprising a blades, brakes and service business and operating from or close to the major wind energy markets.
LM Wind Power has produced more than 130,300 blades in the course of more than 30 years corresponding to approximately 43 GW installed wind power capacity.

What happens when the wind doesn’t blow?

One of the questions most often asked about wind power is ‘what happens when the wind doesn’t blow’. In the big picture wind is a vast untapped resource capable of supplying the world’s electricity needs many times over.

In practical terms, in an optimum, clean energy future, wind will be an important part of a mix of renewable energy technologies, playing a more dominant role in some regions than in others. However, it is worthwhile to step back for a minute and consider the enormity of the resource.

Researchers at Stanford University’s Global Climate and Energy Project recently did an evaluation of the global potential of wind power, using five years of data from the US National Climatic Data Center and the Forecasts Systems Laboratory 1). They estimated that the world’s wind resources can generate more than enough power to satisfy total global energy demand.

Using only 20% of this potential resource for power generation, the report concluded that wind energy could satisfy the world’s electricity demand in the year 2000 seven times over.

After collecting measurements from 7,500 surface and 500 balloon-launch monitoring stations to determine global wind speeds at 80 meters above ground level, they found that nearly 13% had an average wind speed above 6.9 meters per second (Class 3), sufficient for economical wind power generation.

Offshore Resources

North America was found to have the greatest wind power potential, although some of the strongest winds were observed in Northern Europe, while the southern tip of South America and the Australian island of Tasmania also recorded significant and sustained strong winds. To be clear, however, there are extraordinarily large untapped wind resources on all continents, and in most countries; and while this study included some island observation points, it did not include offshore resources, which are enormous.

Scale of large wind turbines

For example, looking at the resource potential in the shallow waters on the continental shelf off the densely populated east coast of the US , from Massachusetts to North Carolina, the average potential resource was found to be approximately four times the total energy demand in what is one of the most urbanized, densely populated and highest-electricity consuming regions of the world 2).

The WBGU calculations of the technical potential were based on average values of wind speeds from meteorological data collected over a 14 year period (1979–1992). They also assumed that advanced multi-megawatt wind energy converters would be used. Limitations to the potential came through excluding all urban areas and natural features such as forests, wetlands, nature reserves, glaciers and sand dunes. Agriculture, on the other hand, was not regarded as competition for wind energy in terms of land use.

Looking in more detail at the solar and wind resource in 13 developing countries, the SWERA (Solar and Wind Energy Resource Assessment) project, supported by the United Nations Environment Programme, has found the potential, for instance, for 7,000 MW of wind capacity in Guatemala and 26,000 MW in Sri Lanka. Neither country has yet started to seriously exploit this large resource.

After this initial pilot programme, SWERA has expanded since 2006 into a larger programme with the aim of providing high quality information on renewable energy resources for countries and regions around the world, along with the tools needed to apply this data in ways that facilitate renewable energy policies and investments. The private sector is also getting into the resource-mapping business, with Seattle based 3Tier launching its ‘mapping the world’ programme in 2008, with the goal of making accessible resource assessments available for the entire world by 2010.

In summary, wind power is a practically unlimited, clean and emissions free power source, of which only a tiny fraction is currently being exploited.

Offshore wind farms – transmission cables

The European wind power industry is increasingly turning to the offshore wind resource, and the United States will draw on the Europeans’ experience as we begin to plan offshore wind farms. Short of generating hydrogen, or otherwise using or storing the energy offshore, it must be conducted to the on-shore load centers by submarine cables.

Offshore transmission has proved to be challenging and costly in Europe, and will present additional challenges in the US because of the lack of domestic manufacturers of high-voltage, high-capacity submarine cable, and lack of equipment for and experience in installing this type of cable.

Submarine transmission cables are common in the US for other applications, but this experience has a limited applicability to wind farms.

The offshore gas and drilling industry uses lower power levels and low (under 10 kV) to medium voltage (10-100 kV), whereas the trend in offshore wind power is toward high voltage transmission. A number of medium and high voltage transmission cables have been installed in the US to power islands but submarine transmission from generation offers different problems than transmission to a load.

For instance, windfarms usually have high reactive current demands, since most wind turbines employ induction generators. This can cause resonance with the capacitance of the cables. Economies of scale are driving up the size of offshore windfarms. Larger farms will both allow and demand more sophisticated electrical transmission systems, as wind power makes a greater impact on the onshore electrical grid. As power electronics are being developed, we may expect to see them play a greater role in offshore windfarm transmission and distribution designs, including the introduction of high voltage direct current (HVDC) transmission.

The following is a brief introduction to cable types and components as it pertains to offshore wind installations.

Insulation

Three types of cable insulation are in common use for submarine transmission for long distances (at least several kilometers.) While insulation construction and thickness vary based on voltage, all three types discussed here are used for both medium and high voltages. Insulation is characterized by their insulation material, their construction, and whether the dielectric (i.e. insulation) is lapped or extruded.

Low-pressure oil-filled (LPOF), or fluid-filled (LPFF) cables, insulated with fluid-impregnated paper, have historically been the most commonly used cables in the US for submarine AC transmission. The insulation is impregnated with synthetic oil whose pressure is typically maintained by pumping stations on either end. The pressurized fluid prevents voids from forming in the insulation when the conductor expands and contracts as the loading changes. The auxiliary pressurizing equipment represents a significant portion of the system cost. LPFF cables run the risk of fluid leakage, which is an environmental hazard.

Fluid-filled cables can be made up to about 50 km (30 mi.) in length. They are rarely used for DC applications, which are generally longer than practical for pressurizing. While LPFF cables are widely installed worldwide, the cost of the auxiliary equipment, the environmental risks, and the development of lower-cost alternatives with lower losses, have all contributed to the reduced use of LPFF cables in recent years.

Similar in construction are the solid, mass-impregnated paper-insulated cables, which are traditionally used for HVDC transmission. The lapped paper insulation is impregnated with a high-viscosity fluid and these cables do not have the LPOF cable’s risk of leakage.

Extruded insulation is replacing lapped installation as the favored options in many applications. Cross-linked polyethylene (XLPE, also called PEX) is lower cost than LPOF of a similar rating and has lower capacitance, leading to lower losses for AC applications. XLPE can be manufactured in longer lengths than LPFF (Gilbertson 2000.)

Until recently XLPE was not an option for DC transmission, since it broke down quickly in the presence of a DC current, but recent improvements allow its use for DC as well. Figure 1 shows an example of an XLPE cable.

Figure 1: Anatomy of a single-core XLPE cable (from ABB’s Long Island Cross Sound cable)

Another extruded insulation used in submarine cables is ethylene propylene rubber (EPR), which has similar properties to XLPE at lower voltages, but at 69 kV and above, has higher capacitance (Gilbertson, 2000). High-voltage submarine XLPE cable is not manufactured in the North or South America. LPOF cables are manufactured here but are not available in the sizes and lengths that will be required for an economically sized offshore wind farm. Currently any offshore windfarm in the US (or anywhere else in the Western Hemisphere) will have to import cables from Europe or Japan.

With cables that may weigh more than 75 kg/m (50 lbm/ft), the transportation costs will be a significant portion of the cost of the cable.

Conductors

The conductor in medium and high-voltage cables is copper, or less commonly aluminum, which has a lower current carrying capacity (ampacity) and so requires a greater diameter. Ampacity increases proportionally with the cross sectional area, which can range up to about 2000 mm2 (3 in2, i.e. 50 mm (2 in) in diameter) before the cable becomes unwieldy and the bending radius is too great. Large cables may have a bending radius as large as 6 m (20 ft).

The design amperage is a function not only of the voltage and the power to be carried, but also the cable length, insulation type, laying formation, burial depth, soil type, and electrical losses. Gilbertson (2000) offers a thorough technical reference on these subjects. The issues of length and losses are discussed in more detail below.

Number of Conductors

When possible in AC-cable applications, all three phases are bundled into one “three-core” cable. A three-core cable reduces cable and laying costs. It also produces weaker electromagnetic fields outside the cable and has lower induced current losses than three single core cables laid separately. As the load requirements and conductor diameter rise, however, a three-core cable becomes unwieldy and no longer feasible.

One advantage of single-core cables is that it is easier and cheaper to run a spare, fourth wire. Another advantage is that longer lengths can be made without splices or joints. Figure 2 shows a three-core cable.

Screening

A semi conductive screening layer, of paper or extruded polymer, is placed around the conductor to smooth the electric field and avoid concentrations of electrical stress, and also to assure a complete bond of the insulation to the conductor.

Figure 2: Three-core cable (Nexans)

Figure 1 shows screening on a single-core cable, and Figure 3 shows a three-core cable with screening on both the individual conductors and the three-core bundle.

Sheathing

Outside the screening of all the conductors is a metallic sheathing, which plays several roles. It helps to ground the cable as a whole and carries fault current if the cable is damaged. It also creates a moisture barrier. In AC cables, current will be induced in this sheath, leading to circulating sheath losses; various sheath-grounding schemes have been developed to reduce circulating currents that arise in the sheath.

Unlike other cable types, EPR insulation does not require a metal sheath.

Figure 3: Three-core cable (Okonite)

Table 1: Capacities of high voltage cable (Häusler, ABB, 2002)

System	AC 3 single-core cables		DC bipolar operation, 2 cables
Cable insulation type	XLPE polymer	LPOF: Oil- filled paper	LPOF: Oil- filled paper	Mass imp. Paper	XLPE polymer
Maximum Voltage	400 kV	500 kV	600 kV	500 kV	150 kV
Maximum Power	1200 MVA*	1500 MVA*	2400 MW	2000 MW	500 MW
Max. length, km (mi.)	100 (62)	60 (37)	80 (50)	Unlimited	Unlimited

* Losses may be excessive at these powers

Armor

An overall jacket and then armoring complete the construction. Corrosion protection will be applied to the armor; this may include a biocide to inhibit destruction by marine creatures such as marine borers that are present in Southeast US waters, and have recently been reported in the Northeast (Fox Islands, 2001).

Fiber optic cables for communications and control can be bundled into the cables. Note the bundled fiber optic line in Figure 2. Table 1 summarizes the current availability and limitations of AC & DC cables.

Can I Go Off-Grid With Hybrid Systems?

Hybrid wind energy systems can provide reliable off-grid power for homes, farms or even entire communities (a co-housing project, for example) that are far from the nearest utility lines. According to many renewable energy experts, a “hybrid” system that combines wind and photovoltaic (PV) technologies offers several advantages over either single system.

In much of the United States, wind speeds are low in the summer when the sun shines brightest and longest. The wind is strong in the winter when there is less sunlight available. Because the peak operating times for wind and PV occur at different times of the day and year, hybrid systems are more likely to produce power when you need it.

For the times when neither the wind nor the PV modules are producing, most hybrid systems provide power through batteries and/or an engine-generator powered by conventional fuels such as diesel. If the batteries run low, the engine-generator can provide power and recharge the batteries. Adding an engine-generator makes the system more complex, but modern electronic controllers can operate these systems automatically.

An engine-generator can also reduce the size of the other components needed for the system.

Hybrid Power Systems - Combine multiple sources to deliver non-intermittent electric power

Keep in mind that the storage capacity must be large enough to supply electrical needs during non-charging periods. Battery banks are typically sized to supply the electric load for one to three days.

An off-grid hybrid system may be practical for you if:

• You live in an area with average annual wind speed of at least 9 mph (4.0 m/s).
• A grid connection is not available or can only be made through an expensive extension. The cost of running a power line to a remote site to connect with the utility grid can be prohibitive, ranging from $15,000 to more than $50,000 per mile, depending on terrain.
• You would like to gain energy independence from the utility.
• You would like to generate clean power.

Wind Turbine Control and Operation with Ovation SCADA

Wind is an infinitely renewable supply of power that can be harnessed as an outstanding energy source given the proper location and use of the latest turbine generation technology. Major power producers world-wide have adopted wind power, a clean source of energy that can last up to 20 years or more, as an economically viable alternative to fossil-fueled power generation.

Wind farms can fluctuate in size depending upon the availability and strength of their surrounding airstreams. Wind farms are beginning to increase in magnitude as utilities are starting to see the significant amount of electrical generating capacity that can be added to their overall production. Within a farm, individual wind turbine generators are interconnected with a medium voltage collection system and communications network.

This medium-voltage electricity is then stepped up with a transformer to a high voltage transmission system and the electric grid. As the number of wind turbine generators increases within a farm’s configuration, the need for managing those assets becomes progressively more important.

Emerson’s Ovation SCADA solution can manage your wind farm generation resources to help minimize turbine generator downtime and maximize their availability. Based on the Ovation SCADA Communication Server, our solution features state-of-the-art control and diagnostic functions to optimize your wind farm operations.

Wind Turbine Control and Operation

Supervisory Control

Supervisory control of each wind turbine generator includes start, stop, reset, and tag out functions. An interactive wind farm overview graphic provides convenient operator access to these functions. This graphic can be customized to match the configuration of any wind farm. Select any one of the wind turbine generator symbols within the display to access a graphic control window. This window contains built-in buttons that help the operator manually control the turbine through start, stop, and reset functions.

A tag out button is provided to place a turbine into maintenance mode. Permissive interlocks are built into this function for accomplishing this activity in a safe manner. A reset button clears the tag out condition once the maintenance has been completed.

The Ovation SCADA server control window also provides a means to send a speed set point to the wind turbine generator. This function can be used to activate a change in speed (faster or slower) to within a range specified by the turbine’s original equipment manufacturer. While the control window offers these functions for manual operator intervention, the Ovation SCADA system is designed to accomplish many of these functions automatically.

The wind turbine generator symbol on the overview graphic changes color depending on the state of the selected turbine.

Protective Supervisory Shutdown

Protective supervisory shutdown is an inherent function in the Ovation SCADA solution that protects the wind turbine generator. This function is automatically performed by the Ovation SCADA Server when certain predefined site conditions are reached. For example, the direction and speed of wind on a farm can randomly change at any time. A typical wind farm configuration consists of rows and columns of turbines that in some instances can be located close together. Strong winds that blow directly parallel to a row of turbines can potentially cause damaging vibrations to the turbine blades. Vibration conditions can be monitored through meteorological data obtained from a Met tower interface to the Ovation SCADA system.

When the vibrations exceed a safe operating limit, the Ovation SCADA system activates a shutdown of the turbine to avoid any equipment or area damage.

Once a turbine is shutdown by the protective supervisory shutdown function, it can only be manually reset by the operator provided all of the adverse conditions that caused the shutdown have been restored.

Programmed Supervisory Stop

A programmed supervisory stop is similar to the supervisory shutdown function except that it is based on other conditions that may be present at a wind farm site that are not related to safety issues. For instance, depending on the direction and angle of the sunlight, the rotating blades of a wind turbine can sometimes have an adverse stroboscopic effect on humans looking at the rotating turbines. Consequently, in some wind farm locations, due to local regulations, one turbine or a group of turbines may have to be stopped at certain times of the day to alleviate the stroboscopic condition. The Ovation SCADA system allows the user to program supervisory stops to accommodate issues such as the stroboscopic effect caused by local conditions.

Another key difference between the supervisory stop and shutdown functions is that the user can choose to let the SCADA system automatically restart from a programmed stop or elect to use a manual start.

Features

• Supervisory control of each wind turbine generator
• Protective supervisory shutdown
• Programmed supervisory stop
• Operating status detail for every wind turbine generator
• Power curves for:
  - Documenting data
  - Optimizing machine performance
  - Facilitating scheduling blade cleaning service
• Identifies production potential
• Supervisory control of switch gear
• Economic dispatch
• Wind turbine generator maintenance management
• Wind turbine diagnostics and alarms

Is Wind Power the Answer to Small-Scale Renewable Energy?

Wind Power has the potential to be used in a home environment only in the right circumstances, and has the potential to power everything your home needs, from electric radiators to hot water. In this instance, I’m going to focus on the electrical side of things, and how you can use mainly use wind turbines to power the electricity in your home, and even make money selling it back to the grid.

The Basics

Ok, so it makes sense to begin at the beginning; that is, explain the basics of wind power and the ideas behind it. So, wind power is the conversion of wind into a useful form of energy e.g. using wind turbines to create electricity. But this is not the only form of energy that it is capable of being converted into, other forms such as using wind mills for mechanical power, wind pumps for pumping water or drainage, or sails to propel ships with kinetic energy are also applicable. At the end of 2010, worldwide capacity of wind-powered generators was 196.6 GW, with over 90 countries worldwide using wind power commercially.

Some, however, are recently starting to doubt the ability of wind power to provide enough energy on a global scale (power output vs. Needs of nation) for it to be economically viable (see recent article in the Telegraph).

However, it is still unquestionably one of the most popular methods of renewable energy on a personal scale, owing to a number of factors that will be discussed later. In terms of electrical uses on a large-scale, wind farms are the key. These are a collection of many wind turbines (as pictured) to collect as much wind power from one economically-viable area as possible. Wind Farms are connected to the electric power transmission network, whereas smaller facilities are used to provide electricity to isolated locations. Utility companies increasingly buy back surplus electricity produced by small domestic turbines.

Wind park on sea

Wind energy, as an alternative to fossil fuels, has plenty of positive factors. It is plentiful, clean, renewable, widely distributed, and produces no greenhouse gas emissions during operation. However, the construction of wind farms/turbines has been frowned upon in some communities and areas, as it can have a large visual impact aesthetically.

Yet, with some of the most viable areas for wind farming out to sea (and thus more wind farms being built at sea) this could be a small complaint of the future. Any effects on the environment, nevertheless, are generally among the least problematic of any power source, making it a favourite amongst the more environmentally friendly.

Generating Electricity

In a wind farm, individual turbines are interconnected with a medium voltage, power collection system and communications network. At a substation, this medium-voltage electric current is increased in voltage with a transformer for connection to the high voltage electric power transmission system.

In small scale wind energy generation, the surplus power produced by domestic microgenerators can be fed into the network and sold to the utility company, producing a retail credit for the microgenerators’ owners to offset their energy costs. This works on a retail credit system, the same as that used in small scale solar power generation.

Grid management

Induction generators are often used for wind power. These systems include substantial capacitor banks for power factor correction. Different types of wind turbine generators behave differently during transmission grid disturbances. However, induction generators cannot support the system voltage during faults, unlike steam or hydro turbine-driven synchronous generators, making this a sight weakness of the system.

Capacity factor

Since wind speed is not constant, a wind farm’s annual energy production is never as much as the sum of the generator nameplate ratings 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. Typical capacity factors are 20–40%, with values at the upper end of the range in particularly favourable sites.

The inherent properties of wind limit the capacity factors, unlike fuelled generating plants. Capacity factors of other types of power plant are based mostly on fuel cost, with a small amount of downtime for maintenance. Nuclear plants, for instance, have low incremental fuel cost, and so are run at full output and achieve a 90% capacity factor.

Variability and intermittency

Electricity generated from wind power can be highly variable at several different timescales: from hour to hour, daily, and seasonally. Annual variation also exists, but is not as significant. Related to variability is the short-term (hourly or daily) predictability of wind plant output. Like other electricity sources, wind energy must be “scheduled”.

Wind power forecasting methods are used, but predictability of wind plant output remains low for short-term operation.

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Wind Turbines and Personal Usage

Wind turbine for home use

Now that the ins-and-outs of generating electricity using wind have been covered, it’s time to take a look at the practicalities of using it on a personal scale. To do this we have to look at what is called Microgeneration. This is described as the small-scale generation of heat and power by individuals, small businesses and communities to meet their own needs, as alternatives or supplements to traditional centralized grid-connected power. Although this may be motivated by practical considerations (such as unreliable grid power or long distance from the grid) it is primarily used to describe the environmentally-conscious and their approaches to generating power that aspire to zero or low-carbon footprints.

Additionally, studies have backed-up the environmental impacts of wind power on a small-scale in the UK.  A new study by the Carbon Trust into the potential of small-scale wind energy has found that small wind turbines could provide up to 1.5 terawatt hours (tW/h) per year of electricity (0.4% of total UK electricity consumption). This is saving 0.6 million tonnes of carbon dioxide (Mt CO2) emission. (This data is based on the assumption that 10% of households would install turbines at costs competitive with grid electricity, around 12p a kW/h). These figures show it not only makes financial sense to make use of wind power, but makes a massive difference environmentally.

Wind turbines have been used for household electricity generation in conjunction with battery storage over many decades in remote areas.

Small wind turbine - basic parts

In terms of the practicalities, turbines should be mounted on a suitable tower to raise them above any nearby obstacles. Another approach to positioning a small turbine is to use a ‘shelter model’ to predict how nearby obstacles will affect local wind conditions. Models of this type are general and can be applied to any site. They are often developed based on actual wind measurements (obviously preferable) and can estimate flow properties, such as mean wind speed and turbulence levels at a potential turbine location. They do this by taking into account the size, shape, and distance to any nearby obstacles.

A small wind turbine can also be installed on a roof. But installation issues then include the strength of the roof, vibration, and the turbulence caused by the roof ledge. Small-scale rooftop turbines suffer from turbulence and rarely generate significant amounts of power, especially in towns and cities.

Also, in locations near or around a group of high-rise buildings, wind shear generates areas of intense turbulence, especially at street-level. The risks associated with mechanical or catastrophic failure have thus plagued urban wind development in densely populated areas, rendering the costs of insuring urban wind systems prohibitive. Moreover, quantifying the amount of wind in urban areas has been difficult, as little is known about the actual wind resources of towns and cities.

Conclusion

So it seems that whilst wind turbines seem like a great idea for urban personal renewable energy, it may be best left to those in areas high above sea level, or in less densely populated urban areas, and of course the most environmentally conscious.

However, due to factors like; ease of installation, benefits to the environment and ability (in the right conditions) to easily (and effectively) produce electricity in many different environments, wind power will continue to be a popular source of personal renewable energy.

Integrating wind energy into the electricity network

One of the most frequent misunderstandings occurring in the public discussion about integrating wind energy into the electricity network is that it is treated in isolation. An electricity system is in practice much like a massive bath tub, with hundreds of taps (power stations) providing the input and millions of plug holes (consumers) draining the output. The taps and plugs are opening and closing all the time.

For the grid operators, the task is to make sure there is enough water in the bath to maintain system security. It is therefore the combined effects of all technologies, as well as the demand patterns, that matter. Power systems have always had to deal with these sudden output variations from large power plants, and the procedures put in place can be applied to deal with variations in wind power production as well.

The issue is therefore not one of variability in itself, but how to predict, manage this variability, and what tools can be used to improve efficiency.

Experience has shown that the established control methods and system reserves available for dealing with variable demand and supply are more than adequate for coping with the additional variability from wind energy up to penetration levels of around 20%, depending of the nature of the system in question. This 20% figure is merely indicative, and the reality will vary widely from system to system. The more flexible a power system in terms of responding to variations both on the demand and the supply side, the easier the integration of variable generation sources such as wind energy.

In practice, such flexible systems, which tend to have higher levels of hydro power and gas generation in their power mix, will find that significantly higher levels of wind power can be integrated without major system changes.

Within Europe, Denmark already gets 21% of its gross electricity demand from the wind, Spain almost 12%, Portugal 9%, Ireland 8% and Germany 7%. Some regions achieve much higher penetrations. In the western half of Denmark, for example, more than 100% of demand is sometimes met by wind power.

Grid operators in a number of European countries, including Spain and Portugal, have now introduced central control centers which can monitor and manage efficiently the entire national fleet of wind turbines.

The present levels of wind power connected to electricity systems already show that it is feasible to integrate the technology to a significant extent. Experience with almost 60 GW installed in Europe, for example, has shown where areas of high, medium and low penetration levels take place in different conditions, and which bottlenecks and challenges occur.

Another frequent misunderstanding concerning wind power relates to the amount of ‘back up’ generation capacity required, as the inherent variability of wind power needs to be balanced in a system.
Wind power does indeed have an impact on the other generation plants in a given power system, the magnitude of which will depend on the power system size, generation mix, load variations, demand size management and degree of gird interconnection. However, large power systems can take advantage of the natural diversity of variable sources, however. They have flexible mechanisms to follow the varying load and plant outages that cannot always be accurately predicted.

Studies and practice demonstrate that the need for additional reserve capacity with growing wind penetration very modest. Up to around 20% of wind power penetration, unpredicted imbalances can be countered with reserves existing in the system. Several national and regional studies indicate additional balancing costs in the order of 0 to 3 €/MWh for levels of wind power up to 20%. In Spain, with 12% of wind penetration, the cost of balancing power was assessed in 2007 at 1.4 €/MWh 4).

The additional balancing costs associated with large-scale wind integration tend to amount to less than 10% of wind power generation costs 5), depending on the power system flexibility, the accuracy of short-term forecasting and gate-closure times in the individual power market. The effect of this to the consumer power price is close to zero.

In order to reduce the extra costs of integrating high levels of wind, the flexibility of power systems is key. This can be achieved by a combination of flexible generation units, storage systems, flexibility on the demand side, interconnections with other power systems and more flexible rules in the power market.