The document is intended to provide you with some basic information about wind energy systems from approximately 500 W up to 50 kW.

SMALL-SCALE WIND ENERGY

Walter Hulshorst | Econ International

EarthToys Renewable Energy Article
The document is intended to provide you with some basic information about wind energy systems from approximately 500 W up to 50 kW.

Walter Hulshorst, Econ International


Introduction

Can I use wind energy to power my home or business? This is a question that is increasingly being asked around the world as more and more people look for affordable and reliable sourc­es of electricity. Advancements in wind turbine technology are being made every day. In the shadows of multi-megawatt wind turbines, there is another growing sector within this industry: systems for farms, companies and small business.

How to use this manual

The document is intended to provide you with some basic information about wind energy systems from approximately 500 W up to 50 kW. Along with being environmentally sound, wind energy system can lower your electricity bill, help you avoid the high costs of having utility power lines extended to remote locations and prevent power failures. Whatever your reason, this manual will help you decide if wind energy is a viable option for you. To do so, therefore, this guide will do the following:
  • offer some basic theory on how wind energy works
  • introduce some of the main components of a wind energy system
  • provide you with some tips on how to determine the optimal design and placement of a wind energy system
  • outline how you can determine if wind energy makes sense for you

Basic theory of wind energy

Wind is a very complex process, which nevertheless can be described in very simple terms. The sun heats the Earth’s surface at varying rates, depending on whether an area is overcast, is in direct sunlight or covered with water. The air above the warmer areas heats up, becomes less dense and rises. The rising air creates a low pressure area, which causes the cooler air from adjacent higher-pressure areas to move to the lower-pressure areas. This movement of the air is what we call wind. As shown in figure 1, power production from a wind turbine is a function of wind speed. The relationship between wind speed and power is defined by a power curve, which is unique to each turbine model and, in some cases, unique to site-specific settings. In general, most wind turbines begin to produce power at wind speeds of about 4 m/s, achieve rated power at approximately 13 m/s, and halt power production at 25 m/s. Variability in the wind resource results in the turbine operating at continually changing power levels.


Figure 1: From wind speed to electrical power (P-v curve)


Cut-in speed is the minimum wind speed at which the blades will rotate and generate usable power, typically between 3 and 4 m/s. The rated speed is the minimum wind speed at which the wind turbine will generate its designated rated power. A 10 kW wind turbine, for example, may not generate 10 kW until wind speeds reach the rated power wind speed. At wind speeds between cut-in and rated speeds, the power output is cubicle to the wind speed. At very high wind speeds, typically 25 m/s, most wind turbines cease power generation and shut down. The wind speed at which shut down occurs is called the cut-out speed. Having a cut-out speed is a safety feature which protects the wind turbine from damage. Shut down may occur in one of several ways. In some machines an automatic brake is activated by a wind speed sensor. Some machines twist or "pitch" the blades to spill the wind. Others use "spoilers": drag flaps mounted on the blades or the hub which are automatically activated by high rotor RPMs, or are mechanically activated by a spring loaded device which turns the machine sideways to the wind stream. Normal wind turbine operation usually resumes when the wind drops back to safe levels.

Wind energy technology

The wind turbine rotor is one of the most visible parts of a wind energy system. Most turbines manufactured today are horizontal axis upwind machines with two or three blades. The main propeller-like rotor has an axis that is parallel to the ground, and therefore horizontal to the wind (figure 2). For small wind energy systems vertical wind turbine can also be used. Vertical wind turbines have an axis perpendicular to the flow of the wind (figure 3). Small wind energy systems generally consists of a rotor, a generator mounted on a frame, a gearbox, a nacelle, a tale vane or yaw system and some control and protection systems.


Figure 2 Horizontal wind turbine1

Figure 3 Vertical wind turbine2

Within this manual we will mostly concentrate on horizontal wind turbine, however it must be mentioned that vertical systems are also used for small scale wind systems as shown in figure 3.

Figure 4 provides an overview of the parts of a horizontal wind turbine.


Figure 4: Main components of a wind turbine Rotor The rotor consists of blades with specially shaped, aerodynamic surfaces.

These rotors are usually made of composites of fibreglass, reinforced plastic or wood. The amount of power a turbine will produce is determined primarily by the diameter of its rotor. The diameter of the rotor defines its “swept area”, or the quantity of wind intercepted by the turbine. Blades are turned, or “pitched”, out of the wind to control the rotor speed and keep the rotor from rotating in winds that are either too high or too low to produce electricity. Generator The generator converts the rotating motion of a wind turbine's blades into electricity. Inside this component, coils of wire rotate in a magnetic field to produce electricity. Different generator designs produce either alternating current (AC) or direct current (DC), and are available in a large range of output power ratings. The generator's rating or size depends on the length of the wind turbine's blades: the longer the blade, the more energy is generated. Gearbox Many turbines (particularly those above 10 kW) use a gearbox to match the rotor speed to that of the generator. Nacelle A nacelle is an enclosure that protects the gearbox, generator and other components from the elements. The nacelle can also be removed for maintenance purposes. Tail vane (Yaw system) A yaw system aligns the wind turbine with the wind. Most small units use a simple tail vane that directs the rotor into the wind. Special release mechanisms can use the yaw system to turn the turbine out of dangerously high winds. Tower The tower holds the turbine in the path of the wind and is therefore an integral part of a wind energy system. Towers should be able to withstand lightning strikes, extreme winds, hail and icing. Because wind becomes less turbulent and increases in speed in relation to its elevation above the ground, and because power output increases substantially with wind speed, increasing tower height from 10 to 50 metres can double the available wind energy. There are two basic types of towers: self-supporting (free standing) and guyed. Most home wind power tower systems use a guyed tower. Guyed towers, which are the least expensive, can be constructed of lattice sections, pipe or tubing and supporting guy wires. Guyed towers are also easier to install than self supporting towers. However, guyed towers require sufficient space to accommodate them. Although tilt-down towers are more expensive, they offer an easy way to perform maintenance on smaller light-weight turbines (smaller than 5 kW). Control and protection systems Control systems range from switches, fuses and battery charge regulators to computerized systems for control of yaw systems. The sophistication of the control and protection system varies depending on the application of the wind turbine and the energy system it supports.

How to plan a wind turbine


The size of the wind turbine you will need depends on how you intend to use it. Turbines for residential use and industrial use range in size from 20 W to 50 kW. Smaller turbines are used in a variety of applications, such as for charging batteries for recreational vehicles and sailboats. For larger applications, a good idea is to set up an energy budget to help define the size of turbine you will need. Before you start considering a wind turbine, you should also be sure to ensure that you are obtaining optimum energy-efficiency in your home, farm or business. This in turn will lower the costs of your wind turbine. Another point to consider before investing in a wind energy system is the presence of any potential obstacles. Some jurisdictions, for example, restrict the height of the structures permitted in residential areas. To find out more information about zoning restrictions in your area, contact your local authorities. They can tell you if you will need to obtain a building permit and can also provide you with a list of requirements. In addition to zoning issues, your neighbours might object to a wind machine that blocks their view, or might be concerned about noise.

Wind assessment

To operate a wind turbine, you will need a fair amount of wind at your location. However, like the weather in general, wind patterns can be unpredictable, varying from place to place, and from moment to moment. Wind velocity can be affected by trees, buildings, hills and valleys around us. A wind turbine should not be placed in a location in which it will be subject to very turbulent air flow. For this reason, you must keep the turbine as clear of obstructions as possible. Even mild turbulence can decrease the performance of a wind turbine, since a turbine cannot react to rapid changes in wind directions, and heavy turbulence can reduce a turbine’s operational life. Wind is a diffuse energy source that can neither be contained or stored, nor used elsewhere at a later time. The wind challenges us to harness it, but it first requires a wind assessment study at a particular location. The best way to determine the wind conditions in your area is by means of an extended series of wind measurements, at least several months in duration, but preferably a year or longer, particularly if you hope to use the data as a basis of comparison with measurements collected at a nearby wind monitoring station. The instruments necessary to perform these measurements are available for purchase or hire, and are often provided as part of a site evaluation study to be carried out by consultants or wind turbine dealers. Remember that the power of the wind is a function of the cube of the wind speed. A 10 per cent error in a wind-speed estimate can mean a 33 per cent deviation in a wind power calculation. In general an annual average wind speed greater than 4 m/s is required to be able to consider a wind energy system. However, speeds above 4 m/s are desirable. Wind turbines should be installed in unobstructed, open areas with clear exposure to prevailing winds.

Figure 5 provides a sample wind map of Western Europe3. This diagram illustrates that Scandinavia, the UK, Ireland and the Atlantic coastline of Europe have the most favourable wind conditions for the development of wind energy4.

Figure 5: Sample wind atlas Europe (areas marked in grey indicate periods for which no data is yet available) For a wind turbine with a rated power of 3 kW, and a P-v curve according to figure 1 (cut in speed at 3 m/s and rated power speed at 13 m/s), the annual energy can be calculated by multiplying the output of the wind turbine for each wind speed with the total number of hours per yearly the wind speed is sustained, as shown in the following table. For an average wind speed of 6 m/s, for example, the annual energy is 4.508 kWh.

Wind speed (m/s)

Hours/year

Output (kW)

kWh/yr

0 – 1

242

0

0

1 – 2

686

0

0

2 – 3

1.023

0

0

3 – 4

1.213

0,09

109

4 – 5

1.244

0,18

224

5 – 6

1.154

0,33

381

6 – 7

978

0,54

528

7 – 8

765

0,82

629

8 – 9

556

1,19

644

9 – 10

377

1,66

626

10 – 11

239

2,22

531

11 – 12

142

2,76

392

12 – 13

79

3

238

13 – 14

42

3

125

14 – 15

21

3

62

Total

8.760


4.508


The following table lists the expected annual energy for a range of average wind speeds:

Wind speed

4 m/s

6 m/s

8 m/s

10 m/s

Annual energy

1.405 kWh

4.508 kWh

9.397 kWh

15.174 kWh

If you require a wind energy system that, for example, provides at least the average annual electricity consumption of a single household in Europe (3.500 kWh), the turbine size is inadequate in a region with an average wind speed of 4 m/s. The size of the turbine for a region with an average wind speed of 4 m/s should actually be about 8 kW to produce enough electricity to power a single household. The investment required for a wind energy system with similar electricity production at a location with a higher average wind speed will be lower in comparison with a location with a lower average wind speed.

How much energy do you require?

To determine the amount of energy you require, you must first know the total amount of energy you require (over the course of a year) to power all the appliances and equipment in your home. The size and generating capacity of a wind turbine for a particular installation depends on the amount of power required, as well as on the wind conditions at the site. Consider also the height of the tower: while a higher tower is more expensive, it also offers your turbine access to greater wind energy. A lower tower requires a larger turbine to generate the same amount of energy as a higher tower with a smaller, less expensive turbine. The type of tower you will need depends on your site: Is there sufficient room for the tower guy-wire anchors? Does the tower height allow the turbine to operate above nearby obstructions? Because most buildings are connected to a utility grid, many wind turbine owners have opted to interconnect their systems. In effect, wind turbine owners use the utility merely as a backup system. Excess electricity from the turbine is automatically fed back into to the utility and backup power is automatically supplied. While this does not constitute true storage, it does provide power on demand, at any time and in any amount. The process to obtain approval for interconnection from the utility company can, however, be a lengthy and complicated one, and requires careful planning. It is a good idea to investigate the possibilities for interconnection early in the process of researching a wind system. If you hope to produce surplus electricity, check first that your grid connection is adequate to feed the electricity back into the grid. The variability of your energy consumption and the amount of money you are willing to spend on a wind system should also guide your selection. For example, a user whose consumption is erratic or concentrated during short periods of the day should size a wind turbine differently than a user with a fairly constant energy demand. In the former case, wind turbine size should be a function of off-peak or average energy demand.

Wind turbine installation

The most ideal site for a wind turbine is mounted on a free standing mast in an exposed location. Many conventional designs of wind turbines are not recommended to be mounted on buildings. However, if the only site available is on the roof of a building, then installing a small wind system may nevertheless be feasible if mounted high enough to minimise turbulence, or if the wind regime at that particular location is favourable. In this case, it should be noted that performance will still be reduced as compared to an equivalent mast-mounted machine; the building itself will act as an obstruction and can cause turbulent air flow. For a building-mounted turbine site in an open area, wind speed can, theoretically, actually increase as it passes over the top of a building. However, this is only likely if the building itself is in a very exposed location. A building at the edge of a settlement may also be subject to acceptable wind speeds and reasonably smooth airflow on those occasions when the wind blows from the direction of exposed land; however, when the wind blows from the direction of the settlement, the wind regime will be poor. A building located at the middle of a settlement or built up area is unlikely to benefit from a good wind regime.

Operation and maintenance

Prior to and during operation, a number of issues should be addressed5,6 Safety: There are no specific safety considerations to bear in mind in relation to the operation of wind turbines. Fencing or other restrictions are unnecessary for safety considerations. People and animals can safely approach the base of the turbines. There is a very remote risk of injury to people or animals or damage to buildings as a result of flying fragments of ice (on the blades) or from a damaged blade. The turbine can only start after de-icing prior to beginning operation.

Birds: Birds can collide with the rotor blades of a turbine, or get caught in the turbulence behind the rotor. Research has shown that risks of collisions are relatively small. The estimated number of “collision casualties” at an installed power of 1.000 MW is approximately 21.000 annually. While this may at first appear to be a rather high number when considered annual, this figure actually dwarfs in comparison to the number of birds that fall victim to automobile traffic each year (2 million annually) or the number of birds lost each year to fatalities involving power lines (1 million annually). Most wind turbine casualties involving birds occur at night, at twilight or in bad weather. Birds know their forage and resting grounds very well, and know to avoid wind turbines. Nevertheless, when installing turbines, it is advisable to pay close consideration to the breeding and foraging areas of birds.

Electrical interference: Wind turbines, like all electrical equipment, produce electromagnetic radiation, which can interfere with broadcast communications. This interference can be overcome through the installation of deflectors or repeaters.

Shadow flickering: Wind turbines, like other tall structures, can also cast long shadows when the sun is low in the sky. The effect, which is known as shadow flicker, occurs when the blades of the wind turbine cast a shadow on a window of a nearby house, and the rotation of the blades chops the sunlight and causes flickering while the blades are in motion. This effect lasts for short periods and happens only in certain specific combined circumstances such as cases in which:

  • the sun is shining and is at a low angle (at dawn or at dusk) and
  • the turbine is directly between the sun and the affected property and
  • sufficient wind energy is available to ensure that the turbine blades are moving.

As a general rule of thumb, shadow flicker on neighbouring offices and dwellings within 500 metres should not exceed 30 hours per year and a maximum of 30 minutes per day7. At distances greater than 10 rotor diameters from a turbine, the potential for flicker is very low.

Noise: Two distinct noise sources are associated with the operation of wind turbines: aerodynamic noise, caused by blades passing through the air, and mechanical noise, created by the operation of mechanical elements in the nacelle (the generator, gearbox and so on). Aerodynamic noise is a function of many interacting factors, including blade design, rotational speed, wind speed and in-flow turbulence. Aerodynamic noise is generally broadband in nature and can display a certain “character”, often referred to as “swish”. Mechanical noise from a wind turbine is tonal in nature.

Maintenance: Most of the wind energy systems that are available require owner-intervention during operation. Many manufacturers offer maintenance service for the wind turbines they install. The manufacturer should at least have detailed information on maintenance procedures, and should be able to tell you when maintenance must be carried out. Most turbines can operate for long periods of time without troubleshooting or repair. Minor maintenance is usually carried out either on a quarterly basis or twice yearly. More comprehensive maintenance is required annually. Maintenance can range from a simple oil check, which just about anyone can do, up to intricate gear backlash or blade pitch settings inspections, which can require a high degree of expertise. When considering a wind energy system, be sure that you have the technical skill that would be necessary to maintain the installation.

Costs and benefits

Along with investment costs, an economic evaluation of wind energy systems should also include a number of other aspects that must also be taken into account:
  • Reduction of annual electricity costs as a result of electricity production by the wind energy system: you should also take into account future expectations of the electricity price;
  • Possible stimulus programs from the government, for example, subsidies or tax incentives, to encourage the use of wind energy systems;
  • Costs of CO2 pollution due to the production of electricity, which is zero for wind energy systems.

Investment costs

If you completed the assessment in chapter 2, you should have a fairly good idea of the basic configuration for your system. You can now calculate the price of the wind energy system.

As of 2008, the average price for small wind energy systems (up to 10 kW) are approximately 5 euros per W. For larger systems, the price is lower per W, while some studies estimate approximately 1 euro per W8. Suppliers can also indicate what spare parts are important for the system; therefore, it is probably a good idea to purchase them right away. In addition, depending on the size and complexity, there can also be a number of other initial costs, such as:

  • Costs for obtaining wind data or wind assessment
  • Transportation of the system
  • Construction and installation: larger systems can require special equipment such as a crane to set up
Some countries and grid operators also provide subsidies for the purchase of wind energy systems.

Operational costs

The most significant annual costs are the parts and labour required for system maintenance. However, depending on your specific application, they can also include land leasing, property taxes and insurance premiums. The annual operation and maintenance costs for a wind turbine can be estimated as a percentage of the initial capital cost of the installed equipment. Values typically range between 3 and 10 percent of the initial capital costs per year.6,9

Evaluations of a wind energy system

To get a quick indication of the generation costs for a wind energy system, simply divide the costs of a wind energy system by the amount of kWh produced during the lifetime of the system. With the wind energy system described in section 2.1, you can easily calculate the generation costs per kWh.

At an average wind speed of 6 m/s, a 3 kW wind turbine as described in section 2.1 will provide an annual energy supply equal to 4.508 kWh. During its life time of 20 years, this type of turbine will produce: 4.508 * 20 = 90.160 kWh. The investment costs for the system will be approximately 3000 W * euro 5 = euro 15.000. If we include an additional 10 per cent for other initial costs, the price comes to euro 16.500. The costs for 1 kWh will be: euro 16.500 / 90.160 = € 0,183 (not accounting for the time value of money). At an average wind speed of 8 m/s, the costs for 1 kWh comes to € 0,088. Obviously, higher wind speeds will reduce the price per kWh when using wind energy systems. The generation costs for wind energy systems are competitive with residential electricity prices. Electricity prices vary greatly across the 27 EU countries. According to Eurostat, the average price of electricity for an average household within the EU (as of January 2007) is approximately €0,1528 kWh10. At these prices, wind energy systems can be economically competitive in Europe. The costs of wind energy systems can also be expected to decrease, while the costs of electricity are only likely increase. Some countries and grid operators offer higher prices for kWh generated by wind energy systems, and feed this energy back into the grid. For this reason, it can make sense to sell electricity produced by wind energy to the grid.

Along with economic evaluation, wind energy also provides additional benefits, such as:

  • Increased efficiency of the electrical network: because power is generated close to the point of use, losses in the electricity grid decrease.
  • Lower utility costs: after your initial investment in wind energy, your monthly electricity bill will go down; wind, after all, is free.
  • Climate protection: wind energy systems emit zero carbon dioxide during their operation.
  • Security of supply: if you use a back up system (batteries), your wind energy system can operate while no electricity is delivered from the electricity grid.

Installation at your home, farm or business

Should you invest in a wind turbine? A well-informed decision will require some thought and personal research. Having read all of the chapters up to now, you now have enough information about wind energy to decide on your next step. The following is a review of the major points involved in a wind energy decision, condensed into a number of steps to be taken.

Evaluating energy-efficiency measures

Before considering a wind turbine, you should be sure to first perform an inspection to insure that you are obtaining optimum energy-efficiency in your home, farm or business. This advantage here is double, because you might find that the wind turbine you require is not as large or expensive as you might have thought.

Evaluating legal, social and environmental issues

A survey of these issues is crucial to your decision-making, because certain issues can alter or even put an end to your plans for a wind turbine. In general, rural areas are the least affected by these issues. To learn more about the applicable zoning ordinances and building permit requirements, contact the local zoning board, town clerk or building inspector in your area. You should also be sure to discuss liability coverage and insurance needs with a licensed insurance agent. To avoid unforeseen public objections to the sight of a wind turbine in the neighbourhood, discuss your plans with your neighbours. Have a title search carried out to determine whether prior agreements or easements exist which would prevent you from installing a wind turbine on your property.

Evaluating wind resources

The best way to determine the wind conditions in your area is by taking wind measurements for a period of at least several months, and preferably a year or longer, particularly if you are able to compare the data with that collected at a nearby wind monitoring station. The instruments you will need to conduct the measurements are available for purchase or for hire, and are often even provided as part of a site evaluation study performed by a consultant or wind turbine dealer. Remember that the power of the wind is a function of the cube of the speed. A 10 per cent error in a wind speed estimate can mean a 33 per cent deviation in a wind power calculation.

Determining wind system application

The next step is to determine the appropriate machine size, or generating capacity. As a rule of thumb, a wind turbine should be sized to supply anywhere between 25 and 75 per cent of your electrical needs. The amount you are willing to spend on a system will also affect size selection. If you are considering storing the electricity, for example, with batteries, then the daily variation in winds is less significant a factor. Most people choose electricity as the wind turbine's product. Interconnection of the wind system with the utility grid eliminates the need for separate storage and provides the convenience of an almost unlimited back-up power supply from an existing energy source.

Shopping for a wind system

Once you have decided how you would like wind energy to work for you, you should begin to explore the wide range of wind turbine products and accessories that are available on the market. If you have not already done so, now is the time to contact one or more dealers to discuss your particular interests and to get some preliminary cost estimates. Choosing a good wind dealer is perhaps just as important as selecting a good wind system, because most dealers service what they sell. Dealers should also provide you with references. Do not hesitate to demand a very high level of service. You should be as particular about the service and maintenance of your wind machine as you would be when purchasing an automobile. Become familiar with manufacturers’ product literature, and read everything you can about specific wind turbines in popular magazines, newspaper articles and elsewhere. Talk with local wind turbine owners, who can be your most reliable information source. When you talk to existing wind turbine owners, ask them how they chose their wind systems and whether the system they picked met their expectations. Find out what problems they have had with their machines' performance, utility cooperation and so on. Also be sure to ask how responsive the dealer has been to service calls since the initial installation. As you narrow your choices down to a few machines, compare wind turbine warranties and note the differences between those provided by the manufacturer and those offered by the dealer. Inquire whether the dealer's warranty is transferable or assumable by the manufacturer should the dealer go out of business. Compare the maintenance requirements of machines. Higher maintenance means higher annual costs. It is also wise to compare the terms and prices of service contracts offered by different dealers. An overview of smallest wind turbines (including technical specifications and an indication of the prices) is available at: www.allsmallwindturbines.com.

Determine the requirements for utility interconnection

If you plan to interconnect your wind system with the utility grid, you must first contact the local utility office. Some utilities do not like customers to install utility-connected power generation systems. Your local utility can also inform you about possible incentives for wind energy systems. Also, ask about the possibility of feeding electricity back onto the grid. Your local utility can also inform you about the availability of potential subsidies on investment and/or feed-in tariffs. The utility should provide you with a written description of the costs, as well as the terms and conditions involved with interconnection, such as double-metering. Find out what the requirements are for safety and power-conditioning devices, additional monthly service and demand charges, buy-back rates and electrical inspection of the installation. Be prepared to submit a detailed schematic diagram (including electrical plans) of the planned wind system.

7. Evaluating the economics of a wind system

Now is the time to evaluate the financial consequences of your pending decision. The initial costs and annual expenses of wind turbine ownership must be weighed against the benefits of long-term electricity cost savings.

4 For more information on specific wind speeds for various countries, go to: www.windatlas.dk.
5 Wind Energy Development Guidelines, Ireland Department of the Environment, Heritage and Local Government, Ireland:www.environ.ie
6 van der Wekken, T., “Wind power” (KEMA Consulting, Autumn 2006), at: www.leonardo-energy.org.
7 The shadow flicker recommendations provided here are based on research by Predac, a European Union-sponsored organisation promoting best practice in energy use and supply, which draws on experience from Belgium, Denmark, France, the Netherlands and Germany.
8 Wind Energy: The facts -- An analysis of Wind Energy in the EU-25, EWEA
9 Stand alone Wind Energy Systems: A Buyer’s Guide (Natural Resources Canada, 2003).

10 , “Electricity Prices for EU Households and Industrial Consumers on 1 January 2007”, at:www.epp.eurostat.ec.europe.eu.

 


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