RETScreen - Wind Energy Project Analysis - Speaker's notes
SLIDE 1: Wind Energy Project Analysis
This is the Wind Energy Project Analysis Training Module of the RETScreen Clean Energy Project Analysis Course. In this presentation, we examine the use of wind energy, as generated by wind turbines, such as the one shown in this photo.
SLIDE 2: Objectives
This module has three objectives. These are first, to review the basics of wind energy systems; second, to illustrate key considerations in wind energy project analysis; and third, to introduce the RETScreen Wind Energy Project Model.
SLIDE 3: What do wind energy systems provide?
Quite simply, they provide electricity. This electricity can be used on central-grids or isolated-grids, or the turbine can serve as a remote power supply or source of energy for water pumping.
Many turbines around the world feed the electricity they generate onto the central-grid, by which we mean the interconnected network of generating stations and transmission facilities that provide electricity to consumers dispersed over a large area. For example, in this photo, wind turbines located in Palm Springs, California, are connected to the North American power grid. The electricity generated by such "central grid-tied" wind turbines may be used by consumers located far from the turbines themselves.
An isolated-grid is a smaller network of generation and distribution facilities, not interconnected with the central-grid, that supplies electricity to a limited area, such as a single remote community or the communities on an island. Wind turbines can be a very attractive power source for these isolated-grids, since the cost of electricity generation on such grids is typically quite high.
Sometimes, electricity is needed at a location far from any grid. A few examples of such loads are mountain top telecommunication repeater stations, navigation lights on small islands or reefs, and remote homes and cabins. Wind turbines can be cost-effective power sources for such loads.
Water often needs to be pumped from wells or sloughs, as a source of drinking water for people or livestock, or for irrigation. If the electricity grid is not nearby, wind power can be used to pump this water. In the past, North American agricultural areas were littered with hundreds of thousands of multi-vaned wind pumpers. While most of these mechanical pumpers have disappeared or fallen into disuse, in some locations the wind is today being used to generate electricity for remote water pumping.
Thus, the primary benefit delivered by wind turbines is electricity. But there are secondary benefits as well. Depending on the wind resource, wind turbines can be distributed around a grid, shoring up weak areas of the grid that are far from central generating stations. Many large-scale turbines are now being designed to improve grid power quality and stability. Distributed around a grid, wind turbines tend to feed electricity to loads relatively close to the point of generation, as opposed to massive conventional power plants, where economies of scale often dictate that one plant service an enormous area. As a consequence, the loss of electrical power when it is carried by transmission lines over a long distance can be reduced. Finally, wind, the fuel for wind turbines, is free, and thus protected from the price volatility that is always a concern with fossil fuels.
SLIDE 4: Wind Turbine Description
Wind turbines contain a number of components. Let's follow the flow of energy through these components, starting with the kinetic energy of the wind and ending with electrical energy. Then we will examine some of the components that do not participate in the flow of energy directly, but are nonetheless necessary.
When considered together, the individual blades of a wind turbine constitute the rotor. Wind blowing over a blade generates a force on the blade, called lift. This is the same force that permits airplanes to fly, and it is no coincidence that wind turbine blades are reminiscent of airplane wings. The blades of a turbine are angled in such a way that part of this lift force lies in the plane of the rotor, causing it to rotate. This rotational momentum is transmitted, by way of a shaft, to a generator, an electromagnetic device that converts the kinetic energy available from the shaft into electrical energy. In larger wind turbines, the rotor turns at 15 to 30 revolutions per minute, which is too slow for many types of generators that are to be connected to a grid operating at 50 to 60 Hz. In these turbines, a gearbox is often used to increase the rotational speed of the shaft between the rotor and the generator.
In many wind turbines, such as the one shown in this figure, the gearbox and generator are located in a nacelle, mounted atop the tower. The tower may be a tubular steel structure or, less commonly among large turbines, a lattice structure somewhat similar to an electricity transmission line tower. Smaller turbines may use guyed towers. When guys are not used, however, the tower will sit upon a massive foundation. Internally, the turbine may contain a number of controls to keep it operating properly through changing wind conditions.
The turbine rotor may rotate about a horizontal axis, as seen here, or about a vertical axis. The most popular of the latter configuration is also known as a Darrieus rotor, after its inventor, or less formally, as an eggbeater type, for its resemblance to a whisk. Although they permit the generator, gearbox, and other components to be situated near ground level, where they can be easily maintained, vertical axis turbines are relatively rare.
Instead, the vast majority of modern, large-scale turbines follow a design championed by Danish manufacturers, and therefore referred to as the Danish design. In this design, the rotor has three blades and turns about a horizontal axis. The rotor is kept upwind of the turbine by motors, controlled by input from an anemometer and a wind vane located on the nacelle.
It is important to recognize that the wind energy available to the turbine is directly proportional to the area swept by the rotor. Regardless of the size of the generator, the rotor diameter is the ultimate determinant of the maximum power that can be produced by the turbine at a given wind speed. Over the last twenty years, rotor sizes on utility scale turbines have increased dramatically. Today's largest turbines have rotor diameters of 80 m or more - that is, greater than the length or wingspan of a Boeing 747 jumbo jet. Similarly, tower heights have increased, with the hub height, or distance between the ground and the nacelle, being between 40 and 70 m for most utility scale turbines.
SLIDE 5: Utilisation of Wind Energy
Wind turbines are used in a variety of applications, and this is reflected in the size of the turbines available on the market.
The turbines used for off-grid power supplies tend to be small, ranging from as small as 50 W up to 10 kW. Since their output will vary with the strength of the wind, turbines are often used to charge batteries, which store the electricity generated during windy periods for use during calm periods. This photo shows a 10 kW turbine that supplies power to the off-grid Costa de Cocos resort in Mexico. Small turbines are also used to power water pumps.
For isolated-grid applications, the turbines are typically larger, ranging from about 10 to 200 kW. Often isolated-grids rely on diesel generators as their primary source of energy. The expense of diesel fuel, especially when inflated by the cost of transporting the fuel to a remote or inaccessible community, makes wind attractive: when the wind blows, less fuel is consumed. On those isolated-grids where the capacity of the wind turbines is relatively small compared to the typical loads, the diesel generators are responsible for maintaining grid stability in the face of changing demand for electricity. Such systems are referred to as low penetration wind/diesel hybrid systems. In contrast, high penetration systems have sufficient wind capacity that turbine output often exceeds electricity demand. In these cases, the diesel generator is shut down entirely, and non-critical dump loads are used to maintain grid stability.
The largest turbines are found on the central-grid. Some older turbines as small as 200 kW are still found, but most onshore turbines being installed today are rated at about 1 MW. Offshore machines as large as 2 MW are now being put into service. While turbines may be installed, owned, and operated individually, they are often grouped in windfarms of tens or even hundreds of turbines. This shares fixed costs among a large number of turbines.
SLIDE 6: Elements of Wind Energy Projects
There are a number of steps in wind energy project development. A very important first step, especially for large projects, is the wind resource assessment. This is the estimation of the frequency distribution of wind speed and direction for a potential site; that is, the assessment reveals how hard the wind blows and from which directions. The wind resource assessment for a major wind park may include the use of sophisticated computer models and the installation of one or more meteorological towers. The computer models apply input from numerical weather prediction models, such as those used for weather forecasting, to a topographical terrain model, to yield an estimate of the wind resource over a region. The meteorological towers are instrumented with anemometers and wind vanes that measure the wind speed at several heights above the ground. Met towers of 40 to 60 m are necessary to have a good understanding of the wind resource near the turbine hub height. They are generally left in place for 3 months to 2 years. The upper photo on this slide shows a guyed, 40 m met tower being tilted up into place. Unfortunately, computer models and met towers are usually prohibitively expensive for small off-grid applications.
Once a site for a utility scale project has been selected, an environmental assessment must be conducted and regulatory approval obtained. Financing must be secured and, for private developers selling electricity to a utility, a power purchase agreement must be negotiated. Prior to construction, the optimal wind farm layout must be determined and electrical and civil design finalized. While erection and commissioning of the turbines is relatively fast, access roads, transmission lines, and electrical substations may need to be built. The lower photo on this slide shows an electrical substation connecting the grid to the wind farm at Altamount Pass in California. With horizontal axis turbines turning in the distance, directly behind the substation a vertical axis turbine is just visible.
The development of a major wind energy project, from resource assessment through commissioning, takes 2 to 4 years on average.
SLIDE 7: Wind Resource
A good wind resource is critical to the success of a commercial wind energy project. The energy available from the wind is not linearly related to the wind speed, but rather increases in proportion to the cube of the wind speed. This means that doubling the wind speed results in 8 times more energy in the wind. The wind developer seeks strong steady winds, with few periods of calm and few storm periods when wind speeds are too high to be used by the turbine.
Wind speed typically increases with height above the ground, so when discussing wind speeds, the height of the wind speed measurement must be specified. At minimum, the annual average wind speed for a wind energy project should exceed 4 m/s, or 14 km/h, at a height of 10 m above the ground; commercial wind farms are usually sited at locations with average wind speeds significantly higher than this.
An accurate estimate of the wind speed at a site is very difficult to arrive at without a long term record of measurements. When measurements are not available, people tend to overestimate the windiness of a site, remembering those dramatic moments when the wind blew with force and forgetting the many hours when the site was calm.
Certain topographical features tend to accelerate the wind, and wind turbines are often located along these features. These include the crests of long, gradual slopes (but not cliffs), passes between mountains or hills, and valleys that channel winds. In addition, areas that present few obstructions to winds, such as the sea surface adjacent to coastal regions and flat, grassy plains, may have a better wind resource.
The wind speed often shows a marked diurnal and seasonal variation. Winds tend to be stronger during the day than the night, and stronger during the winter than the summer. Thus, the times of stronger winds tend to coincide with periods of elevated electricity demand, especially in cold climates.
The figure on this slide shows the power curve for a hypothetical 1 MW turbine. Along the x-axis, values of the instantaneous wind speed, in m/s, are indicated. Along the y-axis, the power output of the turbine, in kW, is shown. At wind speeds below about 4 m/s, no electricity is generated: the wind speed above which the turbine produces power is known as the cut-in speed. The output of the turbine rises rapidly at wind speeds between 7 m/s and 15 m/s, where it reaches a maximum. This wind speed is called the rated wind speed. At wind speeds above 25 m/s, the wind turbine shuts itself down, and this threshold is the cut-out wind speed.
SLIDE 8: Wind Energy System Costs
Wind turbines that are installed in a wind farm tend to have an installed cost of around $1,500 per kW. In addition to this up front expenditure, operation and maintenance costs average around 1 ¢/kWh of electricity generated. For comparison, the selling price of electricity from the turbines may be in the 4 to 10 ¢/kWh range.
The figure on this slide breaks down the costs of building a wind farm into costs for feasibility study, development, engineering, turbine purchase, and balance of plant. The length of the bar indicates the fraction of the total installed cost of the wind farm that goes into each of these 5 categories. Turbine purchase is by far the largest expense, accounting for about 70% of the installed cost of the wind farm. Balance of plant, which includes electricity transmission lines and substations, is the second most important item, at about 20% of the installed cost.
When single turbines are installed outside of a wind farm, or when turbines are installed on an isolated-grid, costs tend to be higher. The portions of the total installed cost that are accounted for by the feasibility study, development, and engineering also tend to increase.
In addition to the initial costs and the regular operation and maintenance costs, there will be the costs associated with major equipment repair. It can be expected that over the lifetime of the turbine at least one major component, costing 20 to 25% of the initial costs, will need replacement. Rotor blades and gearboxes are the components most likely to need attention.
SLIDE 9: Wind Energy Project Considerations
A number of factors must be considered when developing a wind energy project; here we discuss some of the most important.
First, as mentioned earlier, a good wind resource is essential to a commercial project: the average cost of generating a unit of electricity falls dramatically with increasing average wind speed. Because the wind resource plays such an important role in the profitability of wind energy, a thorough wind resource assessment is a worthwhile investment for a major wind project.
The profitability of a wind project can be significantly improved when the sale of electricity at the market rate is not the only source of revenue. Additional revenue sources may include renewable energy production credits offered by a utility or government, a greenpower rate premium, or the sale of emissions reductions credits (or ERC's). It is hoped that ERC's, which put a value on the greenhouse gas emissions that will be avoided by the turbine, will in the future become a significant source of income for wind energy projects.
A good wind resource and additional sources of revenue do not guarantee successful project development, however. Even the most promising project will face insurmountable hurdles if its planners have not ensured that the site is environmentally acceptable, that the local population supports wind energy, and that an affordable connection can be made to transmission or distribution lines having spare capacity to transport the windgenerated electricity to nearby load centers.
Financial parameters also play a major role in the profitability of a wind project. Since wind energy development involves a large initial expenditure that must pay for itself over the life of the project, low discount and interest rates make these projects much more attractive. Procuring financing and convincing investors that the project presents a reasonable risk can represent a significant effort. Currency exchange rates can also impact the profitability of a project: since most turbines are manufactured in the European Union, and Denmark in particular, developments outside of the EU must pay close attention to the exchange rate between the Euro and the local currency.
The photo on this page shows one of the 133 turbines installed in Eastern Quebec as part of the Le Nordais project, Canada's largest wind park.
SLIDE 10: Central-Grid Wind Energy Systems - Examples: Europe and USA
The photos on this slide show 2 wind developments that are tied to the central-grid. On the right, the 9.6 MW coastal windfarm at Lolland, Denmark, is shown. On the left, an early wind farm in Palm Springs, California is seen.
Although their output varies with the wind speed, wind turbines integrate into the electric grid relatively easily. Contrary to popular expectation, no storage systems are needed to buffer the varying output of the turbines. Rather, the grid accepts the wind generated electricity when it is available.
Electric grids are designed and constructed to accommodate electric demand that varies over time as consumers unpredictably turn loads on and off. Wind turbines can be considered negative loads that also vary with time. The same systems that compensate for changes in the amount of electricity that is being demanded from the grid by consumers can also compensate for changes in the electricity being supplied to the grid by wind turbines.
Furthermore, when wind turbines are dispersed over a large geographic area, their net output to the grid tends to change less rapidly, since winds in one area may be dying down as winds in another are picking up.
Obviously, there is a limit to how much wind capacity can be installed on a grid: if wind turbines were the only form of generation on the grid, then there would be no electricity available during calm periods. But the experience of Denmark and parts of Northern Germany indicates that this limit is, in fact, quite high: 17% of Denmark's electrical energy comes from wind power, and this has not required the construction of any additional spinning reserve, that is, capacity specifically meant to compensate for the output of the wind turbines. While the limit may be different in other parts of the world, there are few locations if any where the intermittent output of wind turbines is presently a real obstacle to their profitable use.
Studies have indicated that wind generation integrates especially well into grids which have significant hydroelectric resources, such as the grids of the Canadian provinces of Quebec, Manitoba, and British Columbia. Hydro reservoirs act as a form of storage that can be drawn down during calm periods and preserved during windy periods.
Wind power is protected from several types of risk associated with conventional generation, and thus is a valuable addition to a portfolio of generating sources. Obviously, since the wind is free and locally available, wind power is not subject to rapid and unpredictable spikes in fuel prices. But just as importantly, wind is a modular, scalable technology that can be put into place quickly and in any increment desired, from a few hundred kilowatts to hundreds of megawatts. Many conventional power plants, and nuclear power plants in particular, can only be built as large developments requiring many years to plan, construct, and put into operation. The 5 to 10 year planning horizon this imposes raises the risks that capacity will be built in anticipation of demand growth that does materialize, for example, due to an economic slowdown, or that electricity changes in the prices of fuel and electricity will make the new plant unprofitable. On the other hand, the 2 to 4 year planning horizon required for wind power is much more manageable and less subject to forecasting error.
Wind farms containing scores of machines having a combined capacity of hundreds of megawatts are clearly very large, and are spread over a large land area. It is important to recognise, however, that the turbines and associated access roads and electrical lines occupy only a very small part of this land. In fact, about 99% of the land of a wind park is available for other uses, such as agriculture. Thus, the land requirement for wind power is quite low.
While large wind farms are a common approach to wind development, it is possible for individuals, businesses and cooperatives to own and operate single turbines or a small number of dispersed turbines. For example, 80% of Denmark's turbines are owned by individuals or local cooperatives. In Toronto, Ontario, Canada, a citizen's renewable energy cooperative has recently installed a 700 kW turbine on the city's Lake Ontario waterfront.
SLIDE 11: Isolated-Grid Wind Energy Systems - Examples: India and Canada
Many communities, such as those found on islands, in sparsely populated regions of Canada and Alaska, and in many developing countries with limited infrastructure, are too far from the central-grid for a grid connection to be financially viable. Rather, these communities have a local, isolated-grid, typically energized by diesel-powered generators. The cost of electricity generation is high: diesel fuel is expensive, transporting it to the community inflates its cost, and the generators themselves are often quite inefficient.
Wind turbines can reduce the consumption of diesel fuel and the cost of providing electricity to these communities. The fluctuations in the output of the turbines and the electricity demand are compensated for either by the diesel genset itself or by dump loads, that is, loads, such as water heaters, that can be turned on and off over a short time-scale without impacting the consumer.
Turbines that operate on isolated-grids must be especially robust and reliable, and should have minimal maintenance requirements. Often the communities will have limited materials and expertise to dedicate to keeping the turbine in good working order. Sending parts and specialised labour to the community will be expensive. On the other hand, the operator of the isolated-grid must have a realistic plan in place for conducting the required regular inspections and maintenance. Otherwise, even the most robust and reliable turbine will eventually fail.
The photos on this slide show 2 very different isolated-grid applications of the same model of turbine, a 50 kW machine. On the left, a crew is installing the turbine as part of a small wind park for the isolated-grid of Sagar Island in West Bengal, India. On the right, the turbine is providing power to the very remote community of Rankin Inlet, in Nunavut Territory, in the far north of Canada.
SLIDE 12: Off-Grid Wind Energy Systems - Examples: USA, Brazil, and Chile
Numerous small electrical loads are found at locations far from the electricity grid. These include remote villages, resorts, homes and cottages, telecommunications and monitoring equipment, navigation aids, and water pumps. When these electrical loads are found in windy areas, a small wind turbine can be a much more cost-effective source of electricity than either grid extension or a prime power diesel generator.
The wind does not blow all the time, so some form of energy storage is required for loads that will not tolerate an intermittent source of power, that is, the majority of loads. Deepcycle lead acid or nickel cadmium batteries are typically used to buffer short term lulls in the wind. This adds some complexity to the system and, especially in the case of leadacid batteries, increases maintenance requirements and periodic replacement costs.
One load that can tolerate intermittent power, and thus does not need batteries, is a water pump that feeds a storage reservoir: the reservoir in effect fufills the same role as the battery. In the past, multi-vaned wind mills powered mechanical pumps. While this technology is still used today, it is also possible to use a wind turbine to generate electricity for an electric pump.
Wind turbines can be used in conjuction with other power sources in a so-called hybrid power system. These systems will typically include a battery for short-term storage, a fossil fuel-powered generator for long periods where there is little wind or unusually high loads, and possibly even a photovoltaic array. The advantages of hybrid systems are several. First, they combine the strengths of each of the power sources. Wind power and photovoltaics are expensive to purchase but have low operating costs; on the other hand, fossil fuel-powered generators are relatively inexpensive to buy but costly to operate. In a hybrid power system, the fossil fuel-powered generator reduces the size of the turbine required, and the turbine reduces the fuel costs of the generator. Second, the solar resource utilised by a photovoltaic array and the wind resource utilised by the turbine are often negatively correlated on a seasonal basis, such that when one is not available, the other is. Third, having power from multiple generators reduces the risk of complete loss of power should one power source fail.
Hybrid power systems are associated with a higher level of complexity, necessitating more careful design, more time for installation and commissioning, and more involved operation and maintenance. As a result, they tend to be used for larger or more crucial off-grid loads.
Three small off-grid wind turbine applications are shown on this slide. The one on the left is a 400 W turbine providing power for telecommunications equipment in Arizona, USA. In the centre, a hybrid power system provides power to an off-grid village on the Island of Marajo, along the north coast of Brazil. This hybrid system includes 50 kW of wind and photovoltaic capacity as well as a battery bank. The photo on the right shows two 1 kW turbines that charge batteries for a remote school at Villa Tehuelche, in Punta Arenas, Chile.
SLIDE 13: RETScreen Wind Energy Project Model
The RETScreen Wind Energy Project Model is a simple but very useful tool for a preliminary investigation of technical and financial feasibility of wind energy projects. For an installation anywhere in the world it can provide an analysis of the energy production, life-cycle costs, and green house gas emissions reductions. The installation can be connected to either the central-grid or an isolated-grid, a single turbine or a wind farm. At present, stand-alone turbines requiring battery storage are not covered by RETScreen.
To conduct this analysis, the user provides information about the wind speed distribution at the site. If the user has very little information about the wind resource, a typical wind speed distribution, known as the Rayleigh distribution, can be assumed, and the user needs to provide only an estimate of the annual average wind speed. If the user knows the shape of the wind speed distribution, this can be specified in terms of an additional shape parameter for the so-called Weibull distribution. If the user knows that the distribution is not modelled by the Weibull, he or she is free to enter a turbine energy curve corresponding to an arbitrary distribution. In this way, the user can have highly accurate estimates of annual energy production based on as little data as the average wind speed - while not being limited to typical distributions.
SLIDE 14: RETScreen Wind Energy Calculation
As mentioned in the previous slide, RETScreen permits the user to specify the wind speed distribution that characterises the site. When the user selects a Rayleigh or Weibull wind speed distribution, this fixes the shape of the distribution for a given average wind speed. This is the starting point for RETScreen's calculations in the energy model section of the software.
First, RETScreen combines the wind speed distribution with the power curve of the userselected turbine to generate an energy curve giving the turbine's annual production as a function of average wind speed. RETScreen includes a database of power curves for many different turbines, which relate the power output of a turbine to the wind speed.
Second, RETScreen determines the annual unadjusted energy production by locating the point on the turbine energy curve corresponding to the average wind speed specified by the user. This gives the unadjusted energy production for a single turbine; RETScreen multiplies this by the number of turbines in the installation.
Third, RETScreen adjusts this energy production estimate to account for the average density of the air at the site. This is found from the annual average air temperature and barometric pressure, both contained in RETScreen's weather database. The result is the gross energy production for the site.
Fourth, RETScreen revises the estimate of the gross energy production to account for losses due to the turbines being located in the wakes of other turbines, airfoil soiling, airfoil icing losses, and expected downtime. The result is an estimate of the energy collected by the turbines.
Fifth, RETScreen compares the energy collected by the turbines to the energy that can be absorbed by an isolated grid. If the turbines produce more than can be absorbed, the energy delivered will be less than the energy collected. In central-grid systems, the energy delivered is the same as the energy collected.
Finally, RETScreen calculates the values of various auxiliary quantities, such as the specific yield, the wind plant capacity factor, and the excess renewable energy available.
For more information, see the RETScreen Engineering & Cases Textbook, available online and free-of-charge.
SLIDE 15: Example Validation of the RETScreen Wind Energy Project Model
The RETScreen software has been validated in a number of ways. For example, RETScreen has been compared with the National Renewable Energy Laboratory's HOMER simulation tool, which uses hourly wind speed data. In particular, for an installation consisting of 10 turbines of 50 kW each, located on the isolated-grid of Kotzebue, Alaska, RETScreen and HOMER agreed to within 1.1% in their estimates of the annual energy production.
RETScreen's prediction of the annual energy and production for this installation was compared with monitored data from the installation. Over the years of 1998 through 2000, RETScreen underestimated the actual energy production by only 8 to 10%. This is sufficiently accurate for pre-feasibility purposes.
SLIDE 16: Conclusions
Wind turbines provide electricity on and off the electric grid at locations around the world. Wind turbines are a reliable, proven technology.
Since the energy in the wind increases in proportion to the cube of the wind speed, high average wind speeds are key to profitable wind project development.
In addition, government or utility production credits and green power rates significantly improve the profitability of on-grid wind projects.
The RETScreen software calculates energy production using the annual average windspeed but achieves an accuracy comparable to simulations based on hourly data. Thus, RETScreen can significantly reduce the cost of conducting preliminary feasibility studies of wind energy projects.
SLIDE 17: Questions?
This is the end of the Wind Energy Project Analysis Training Module in the RETScreen International Clean Energy Project Analysis Course.
This is the Wind Energy Project Analysis Training Module of the RETScreen Clean Energy Project Analysis Course. In this presentation, we examine the use of wind energy, as generated by wind turbines, such as the one shown in this photo.
SLIDE 2: Objectives
This module has three objectives. These are first, to review the basics of wind energy systems; second, to illustrate key considerations in wind energy project analysis; and third, to introduce the RETScreen Wind Energy Project Model.
SLIDE 3: What do wind energy systems provide?
Quite simply, they provide electricity. This electricity can be used on central-grids or isolated-grids, or the turbine can serve as a remote power supply or source of energy for water pumping.
Many turbines around the world feed the electricity they generate onto the central-grid, by which we mean the interconnected network of generating stations and transmission facilities that provide electricity to consumers dispersed over a large area. For example, in this photo, wind turbines located in Palm Springs, California, are connected to the North American power grid. The electricity generated by such "central grid-tied" wind turbines may be used by consumers located far from the turbines themselves.
An isolated-grid is a smaller network of generation and distribution facilities, not interconnected with the central-grid, that supplies electricity to a limited area, such as a single remote community or the communities on an island. Wind turbines can be a very attractive power source for these isolated-grids, since the cost of electricity generation on such grids is typically quite high.
Sometimes, electricity is needed at a location far from any grid. A few examples of such loads are mountain top telecommunication repeater stations, navigation lights on small islands or reefs, and remote homes and cabins. Wind turbines can be cost-effective power sources for such loads.
Water often needs to be pumped from wells or sloughs, as a source of drinking water for people or livestock, or for irrigation. If the electricity grid is not nearby, wind power can be used to pump this water. In the past, North American agricultural areas were littered with hundreds of thousands of multi-vaned wind pumpers. While most of these mechanical pumpers have disappeared or fallen into disuse, in some locations the wind is today being used to generate electricity for remote water pumping.
Thus, the primary benefit delivered by wind turbines is electricity. But there are secondary benefits as well. Depending on the wind resource, wind turbines can be distributed around a grid, shoring up weak areas of the grid that are far from central generating stations. Many large-scale turbines are now being designed to improve grid power quality and stability. Distributed around a grid, wind turbines tend to feed electricity to loads relatively close to the point of generation, as opposed to massive conventional power plants, where economies of scale often dictate that one plant service an enormous area. As a consequence, the loss of electrical power when it is carried by transmission lines over a long distance can be reduced. Finally, wind, the fuel for wind turbines, is free, and thus protected from the price volatility that is always a concern with fossil fuels.
SLIDE 4: Wind Turbine Description
Wind turbines contain a number of components. Let's follow the flow of energy through these components, starting with the kinetic energy of the wind and ending with electrical energy. Then we will examine some of the components that do not participate in the flow of energy directly, but are nonetheless necessary.
When considered together, the individual blades of a wind turbine constitute the rotor. Wind blowing over a blade generates a force on the blade, called lift. This is the same force that permits airplanes to fly, and it is no coincidence that wind turbine blades are reminiscent of airplane wings. The blades of a turbine are angled in such a way that part of this lift force lies in the plane of the rotor, causing it to rotate. This rotational momentum is transmitted, by way of a shaft, to a generator, an electromagnetic device that converts the kinetic energy available from the shaft into electrical energy. In larger wind turbines, the rotor turns at 15 to 30 revolutions per minute, which is too slow for many types of generators that are to be connected to a grid operating at 50 to 60 Hz. In these turbines, a gearbox is often used to increase the rotational speed of the shaft between the rotor and the generator.
In many wind turbines, such as the one shown in this figure, the gearbox and generator are located in a nacelle, mounted atop the tower. The tower may be a tubular steel structure or, less commonly among large turbines, a lattice structure somewhat similar to an electricity transmission line tower. Smaller turbines may use guyed towers. When guys are not used, however, the tower will sit upon a massive foundation. Internally, the turbine may contain a number of controls to keep it operating properly through changing wind conditions.
The turbine rotor may rotate about a horizontal axis, as seen here, or about a vertical axis. The most popular of the latter configuration is also known as a Darrieus rotor, after its inventor, or less formally, as an eggbeater type, for its resemblance to a whisk. Although they permit the generator, gearbox, and other components to be situated near ground level, where they can be easily maintained, vertical axis turbines are relatively rare.
Instead, the vast majority of modern, large-scale turbines follow a design championed by Danish manufacturers, and therefore referred to as the Danish design. In this design, the rotor has three blades and turns about a horizontal axis. The rotor is kept upwind of the turbine by motors, controlled by input from an anemometer and a wind vane located on the nacelle.
It is important to recognize that the wind energy available to the turbine is directly proportional to the area swept by the rotor. Regardless of the size of the generator, the rotor diameter is the ultimate determinant of the maximum power that can be produced by the turbine at a given wind speed. Over the last twenty years, rotor sizes on utility scale turbines have increased dramatically. Today's largest turbines have rotor diameters of 80 m or more - that is, greater than the length or wingspan of a Boeing 747 jumbo jet. Similarly, tower heights have increased, with the hub height, or distance between the ground and the nacelle, being between 40 and 70 m for most utility scale turbines.
SLIDE 5: Utilisation of Wind Energy
Wind turbines are used in a variety of applications, and this is reflected in the size of the turbines available on the market.
The turbines used for off-grid power supplies tend to be small, ranging from as small as 50 W up to 10 kW. Since their output will vary with the strength of the wind, turbines are often used to charge batteries, which store the electricity generated during windy periods for use during calm periods. This photo shows a 10 kW turbine that supplies power to the off-grid Costa de Cocos resort in Mexico. Small turbines are also used to power water pumps.
For isolated-grid applications, the turbines are typically larger, ranging from about 10 to 200 kW. Often isolated-grids rely on diesel generators as their primary source of energy. The expense of diesel fuel, especially when inflated by the cost of transporting the fuel to a remote or inaccessible community, makes wind attractive: when the wind blows, less fuel is consumed. On those isolated-grids where the capacity of the wind turbines is relatively small compared to the typical loads, the diesel generators are responsible for maintaining grid stability in the face of changing demand for electricity. Such systems are referred to as low penetration wind/diesel hybrid systems. In contrast, high penetration systems have sufficient wind capacity that turbine output often exceeds electricity demand. In these cases, the diesel generator is shut down entirely, and non-critical dump loads are used to maintain grid stability.
The largest turbines are found on the central-grid. Some older turbines as small as 200 kW are still found, but most onshore turbines being installed today are rated at about 1 MW. Offshore machines as large as 2 MW are now being put into service. While turbines may be installed, owned, and operated individually, they are often grouped in windfarms of tens or even hundreds of turbines. This shares fixed costs among a large number of turbines.
SLIDE 6: Elements of Wind Energy Projects
There are a number of steps in wind energy project development. A very important first step, especially for large projects, is the wind resource assessment. This is the estimation of the frequency distribution of wind speed and direction for a potential site; that is, the assessment reveals how hard the wind blows and from which directions. The wind resource assessment for a major wind park may include the use of sophisticated computer models and the installation of one or more meteorological towers. The computer models apply input from numerical weather prediction models, such as those used for weather forecasting, to a topographical terrain model, to yield an estimate of the wind resource over a region. The meteorological towers are instrumented with anemometers and wind vanes that measure the wind speed at several heights above the ground. Met towers of 40 to 60 m are necessary to have a good understanding of the wind resource near the turbine hub height. They are generally left in place for 3 months to 2 years. The upper photo on this slide shows a guyed, 40 m met tower being tilted up into place. Unfortunately, computer models and met towers are usually prohibitively expensive for small off-grid applications.
Once a site for a utility scale project has been selected, an environmental assessment must be conducted and regulatory approval obtained. Financing must be secured and, for private developers selling electricity to a utility, a power purchase agreement must be negotiated. Prior to construction, the optimal wind farm layout must be determined and electrical and civil design finalized. While erection and commissioning of the turbines is relatively fast, access roads, transmission lines, and electrical substations may need to be built. The lower photo on this slide shows an electrical substation connecting the grid to the wind farm at Altamount Pass in California. With horizontal axis turbines turning in the distance, directly behind the substation a vertical axis turbine is just visible.
The development of a major wind energy project, from resource assessment through commissioning, takes 2 to 4 years on average.
SLIDE 7: Wind Resource
A good wind resource is critical to the success of a commercial wind energy project. The energy available from the wind is not linearly related to the wind speed, but rather increases in proportion to the cube of the wind speed. This means that doubling the wind speed results in 8 times more energy in the wind. The wind developer seeks strong steady winds, with few periods of calm and few storm periods when wind speeds are too high to be used by the turbine.
Wind speed typically increases with height above the ground, so when discussing wind speeds, the height of the wind speed measurement must be specified. At minimum, the annual average wind speed for a wind energy project should exceed 4 m/s, or 14 km/h, at a height of 10 m above the ground; commercial wind farms are usually sited at locations with average wind speeds significantly higher than this.
An accurate estimate of the wind speed at a site is very difficult to arrive at without a long term record of measurements. When measurements are not available, people tend to overestimate the windiness of a site, remembering those dramatic moments when the wind blew with force and forgetting the many hours when the site was calm.
Certain topographical features tend to accelerate the wind, and wind turbines are often located along these features. These include the crests of long, gradual slopes (but not cliffs), passes between mountains or hills, and valleys that channel winds. In addition, areas that present few obstructions to winds, such as the sea surface adjacent to coastal regions and flat, grassy plains, may have a better wind resource.
The wind speed often shows a marked diurnal and seasonal variation. Winds tend to be stronger during the day than the night, and stronger during the winter than the summer. Thus, the times of stronger winds tend to coincide with periods of elevated electricity demand, especially in cold climates.
The figure on this slide shows the power curve for a hypothetical 1 MW turbine. Along the x-axis, values of the instantaneous wind speed, in m/s, are indicated. Along the y-axis, the power output of the turbine, in kW, is shown. At wind speeds below about 4 m/s, no electricity is generated: the wind speed above which the turbine produces power is known as the cut-in speed. The output of the turbine rises rapidly at wind speeds between 7 m/s and 15 m/s, where it reaches a maximum. This wind speed is called the rated wind speed. At wind speeds above 25 m/s, the wind turbine shuts itself down, and this threshold is the cut-out wind speed.
SLIDE 8: Wind Energy System Costs
Wind turbines that are installed in a wind farm tend to have an installed cost of around $1,500 per kW. In addition to this up front expenditure, operation and maintenance costs average around 1 ¢/kWh of electricity generated. For comparison, the selling price of electricity from the turbines may be in the 4 to 10 ¢/kWh range.
The figure on this slide breaks down the costs of building a wind farm into costs for feasibility study, development, engineering, turbine purchase, and balance of plant. The length of the bar indicates the fraction of the total installed cost of the wind farm that goes into each of these 5 categories. Turbine purchase is by far the largest expense, accounting for about 70% of the installed cost of the wind farm. Balance of plant, which includes electricity transmission lines and substations, is the second most important item, at about 20% of the installed cost.
When single turbines are installed outside of a wind farm, or when turbines are installed on an isolated-grid, costs tend to be higher. The portions of the total installed cost that are accounted for by the feasibility study, development, and engineering also tend to increase.
In addition to the initial costs and the regular operation and maintenance costs, there will be the costs associated with major equipment repair. It can be expected that over the lifetime of the turbine at least one major component, costing 20 to 25% of the initial costs, will need replacement. Rotor blades and gearboxes are the components most likely to need attention.
SLIDE 9: Wind Energy Project Considerations
A number of factors must be considered when developing a wind energy project; here we discuss some of the most important.
First, as mentioned earlier, a good wind resource is essential to a commercial project: the average cost of generating a unit of electricity falls dramatically with increasing average wind speed. Because the wind resource plays such an important role in the profitability of wind energy, a thorough wind resource assessment is a worthwhile investment for a major wind project.
The profitability of a wind project can be significantly improved when the sale of electricity at the market rate is not the only source of revenue. Additional revenue sources may include renewable energy production credits offered by a utility or government, a greenpower rate premium, or the sale of emissions reductions credits (or ERC's). It is hoped that ERC's, which put a value on the greenhouse gas emissions that will be avoided by the turbine, will in the future become a significant source of income for wind energy projects.
A good wind resource and additional sources of revenue do not guarantee successful project development, however. Even the most promising project will face insurmountable hurdles if its planners have not ensured that the site is environmentally acceptable, that the local population supports wind energy, and that an affordable connection can be made to transmission or distribution lines having spare capacity to transport the windgenerated electricity to nearby load centers.
Financial parameters also play a major role in the profitability of a wind project. Since wind energy development involves a large initial expenditure that must pay for itself over the life of the project, low discount and interest rates make these projects much more attractive. Procuring financing and convincing investors that the project presents a reasonable risk can represent a significant effort. Currency exchange rates can also impact the profitability of a project: since most turbines are manufactured in the European Union, and Denmark in particular, developments outside of the EU must pay close attention to the exchange rate between the Euro and the local currency.
The photo on this page shows one of the 133 turbines installed in Eastern Quebec as part of the Le Nordais project, Canada's largest wind park.
SLIDE 10: Central-Grid Wind Energy Systems - Examples: Europe and USA
The photos on this slide show 2 wind developments that are tied to the central-grid. On the right, the 9.6 MW coastal windfarm at Lolland, Denmark, is shown. On the left, an early wind farm in Palm Springs, California is seen.
Although their output varies with the wind speed, wind turbines integrate into the electric grid relatively easily. Contrary to popular expectation, no storage systems are needed to buffer the varying output of the turbines. Rather, the grid accepts the wind generated electricity when it is available.
Electric grids are designed and constructed to accommodate electric demand that varies over time as consumers unpredictably turn loads on and off. Wind turbines can be considered negative loads that also vary with time. The same systems that compensate for changes in the amount of electricity that is being demanded from the grid by consumers can also compensate for changes in the electricity being supplied to the grid by wind turbines.
Furthermore, when wind turbines are dispersed over a large geographic area, their net output to the grid tends to change less rapidly, since winds in one area may be dying down as winds in another are picking up.
Obviously, there is a limit to how much wind capacity can be installed on a grid: if wind turbines were the only form of generation on the grid, then there would be no electricity available during calm periods. But the experience of Denmark and parts of Northern Germany indicates that this limit is, in fact, quite high: 17% of Denmark's electrical energy comes from wind power, and this has not required the construction of any additional spinning reserve, that is, capacity specifically meant to compensate for the output of the wind turbines. While the limit may be different in other parts of the world, there are few locations if any where the intermittent output of wind turbines is presently a real obstacle to their profitable use.
Studies have indicated that wind generation integrates especially well into grids which have significant hydroelectric resources, such as the grids of the Canadian provinces of Quebec, Manitoba, and British Columbia. Hydro reservoirs act as a form of storage that can be drawn down during calm periods and preserved during windy periods.
Wind power is protected from several types of risk associated with conventional generation, and thus is a valuable addition to a portfolio of generating sources. Obviously, since the wind is free and locally available, wind power is not subject to rapid and unpredictable spikes in fuel prices. But just as importantly, wind is a modular, scalable technology that can be put into place quickly and in any increment desired, from a few hundred kilowatts to hundreds of megawatts. Many conventional power plants, and nuclear power plants in particular, can only be built as large developments requiring many years to plan, construct, and put into operation. The 5 to 10 year planning horizon this imposes raises the risks that capacity will be built in anticipation of demand growth that does materialize, for example, due to an economic slowdown, or that electricity changes in the prices of fuel and electricity will make the new plant unprofitable. On the other hand, the 2 to 4 year planning horizon required for wind power is much more manageable and less subject to forecasting error.
Wind farms containing scores of machines having a combined capacity of hundreds of megawatts are clearly very large, and are spread over a large land area. It is important to recognise, however, that the turbines and associated access roads and electrical lines occupy only a very small part of this land. In fact, about 99% of the land of a wind park is available for other uses, such as agriculture. Thus, the land requirement for wind power is quite low.
While large wind farms are a common approach to wind development, it is possible for individuals, businesses and cooperatives to own and operate single turbines or a small number of dispersed turbines. For example, 80% of Denmark's turbines are owned by individuals or local cooperatives. In Toronto, Ontario, Canada, a citizen's renewable energy cooperative has recently installed a 700 kW turbine on the city's Lake Ontario waterfront.
SLIDE 11: Isolated-Grid Wind Energy Systems - Examples: India and Canada
Many communities, such as those found on islands, in sparsely populated regions of Canada and Alaska, and in many developing countries with limited infrastructure, are too far from the central-grid for a grid connection to be financially viable. Rather, these communities have a local, isolated-grid, typically energized by diesel-powered generators. The cost of electricity generation is high: diesel fuel is expensive, transporting it to the community inflates its cost, and the generators themselves are often quite inefficient.
Wind turbines can reduce the consumption of diesel fuel and the cost of providing electricity to these communities. The fluctuations in the output of the turbines and the electricity demand are compensated for either by the diesel genset itself or by dump loads, that is, loads, such as water heaters, that can be turned on and off over a short time-scale without impacting the consumer.
Turbines that operate on isolated-grids must be especially robust and reliable, and should have minimal maintenance requirements. Often the communities will have limited materials and expertise to dedicate to keeping the turbine in good working order. Sending parts and specialised labour to the community will be expensive. On the other hand, the operator of the isolated-grid must have a realistic plan in place for conducting the required regular inspections and maintenance. Otherwise, even the most robust and reliable turbine will eventually fail.
The photos on this slide show 2 very different isolated-grid applications of the same model of turbine, a 50 kW machine. On the left, a crew is installing the turbine as part of a small wind park for the isolated-grid of Sagar Island in West Bengal, India. On the right, the turbine is providing power to the very remote community of Rankin Inlet, in Nunavut Territory, in the far north of Canada.
SLIDE 12: Off-Grid Wind Energy Systems - Examples: USA, Brazil, and Chile
Numerous small electrical loads are found at locations far from the electricity grid. These include remote villages, resorts, homes and cottages, telecommunications and monitoring equipment, navigation aids, and water pumps. When these electrical loads are found in windy areas, a small wind turbine can be a much more cost-effective source of electricity than either grid extension or a prime power diesel generator.
The wind does not blow all the time, so some form of energy storage is required for loads that will not tolerate an intermittent source of power, that is, the majority of loads. Deepcycle lead acid or nickel cadmium batteries are typically used to buffer short term lulls in the wind. This adds some complexity to the system and, especially in the case of leadacid batteries, increases maintenance requirements and periodic replacement costs.
One load that can tolerate intermittent power, and thus does not need batteries, is a water pump that feeds a storage reservoir: the reservoir in effect fufills the same role as the battery. In the past, multi-vaned wind mills powered mechanical pumps. While this technology is still used today, it is also possible to use a wind turbine to generate electricity for an electric pump.
Wind turbines can be used in conjuction with other power sources in a so-called hybrid power system. These systems will typically include a battery for short-term storage, a fossil fuel-powered generator for long periods where there is little wind or unusually high loads, and possibly even a photovoltaic array. The advantages of hybrid systems are several. First, they combine the strengths of each of the power sources. Wind power and photovoltaics are expensive to purchase but have low operating costs; on the other hand, fossil fuel-powered generators are relatively inexpensive to buy but costly to operate. In a hybrid power system, the fossil fuel-powered generator reduces the size of the turbine required, and the turbine reduces the fuel costs of the generator. Second, the solar resource utilised by a photovoltaic array and the wind resource utilised by the turbine are often negatively correlated on a seasonal basis, such that when one is not available, the other is. Third, having power from multiple generators reduces the risk of complete loss of power should one power source fail.
Hybrid power systems are associated with a higher level of complexity, necessitating more careful design, more time for installation and commissioning, and more involved operation and maintenance. As a result, they tend to be used for larger or more crucial off-grid loads.
Three small off-grid wind turbine applications are shown on this slide. The one on the left is a 400 W turbine providing power for telecommunications equipment in Arizona, USA. In the centre, a hybrid power system provides power to an off-grid village on the Island of Marajo, along the north coast of Brazil. This hybrid system includes 50 kW of wind and photovoltaic capacity as well as a battery bank. The photo on the right shows two 1 kW turbines that charge batteries for a remote school at Villa Tehuelche, in Punta Arenas, Chile.
SLIDE 13: RETScreen Wind Energy Project Model
The RETScreen Wind Energy Project Model is a simple but very useful tool for a preliminary investigation of technical and financial feasibility of wind energy projects. For an installation anywhere in the world it can provide an analysis of the energy production, life-cycle costs, and green house gas emissions reductions. The installation can be connected to either the central-grid or an isolated-grid, a single turbine or a wind farm. At present, stand-alone turbines requiring battery storage are not covered by RETScreen.
To conduct this analysis, the user provides information about the wind speed distribution at the site. If the user has very little information about the wind resource, a typical wind speed distribution, known as the Rayleigh distribution, can be assumed, and the user needs to provide only an estimate of the annual average wind speed. If the user knows the shape of the wind speed distribution, this can be specified in terms of an additional shape parameter for the so-called Weibull distribution. If the user knows that the distribution is not modelled by the Weibull, he or she is free to enter a turbine energy curve corresponding to an arbitrary distribution. In this way, the user can have highly accurate estimates of annual energy production based on as little data as the average wind speed - while not being limited to typical distributions.
SLIDE 14: RETScreen Wind Energy Calculation
As mentioned in the previous slide, RETScreen permits the user to specify the wind speed distribution that characterises the site. When the user selects a Rayleigh or Weibull wind speed distribution, this fixes the shape of the distribution for a given average wind speed. This is the starting point for RETScreen's calculations in the energy model section of the software.
First, RETScreen combines the wind speed distribution with the power curve of the userselected turbine to generate an energy curve giving the turbine's annual production as a function of average wind speed. RETScreen includes a database of power curves for many different turbines, which relate the power output of a turbine to the wind speed.
Second, RETScreen determines the annual unadjusted energy production by locating the point on the turbine energy curve corresponding to the average wind speed specified by the user. This gives the unadjusted energy production for a single turbine; RETScreen multiplies this by the number of turbines in the installation.
Third, RETScreen adjusts this energy production estimate to account for the average density of the air at the site. This is found from the annual average air temperature and barometric pressure, both contained in RETScreen's weather database. The result is the gross energy production for the site.
Fourth, RETScreen revises the estimate of the gross energy production to account for losses due to the turbines being located in the wakes of other turbines, airfoil soiling, airfoil icing losses, and expected downtime. The result is an estimate of the energy collected by the turbines.
Fifth, RETScreen compares the energy collected by the turbines to the energy that can be absorbed by an isolated grid. If the turbines produce more than can be absorbed, the energy delivered will be less than the energy collected. In central-grid systems, the energy delivered is the same as the energy collected.
Finally, RETScreen calculates the values of various auxiliary quantities, such as the specific yield, the wind plant capacity factor, and the excess renewable energy available.
For more information, see the RETScreen Engineering & Cases Textbook, available online and free-of-charge.
SLIDE 15: Example Validation of the RETScreen Wind Energy Project Model
The RETScreen software has been validated in a number of ways. For example, RETScreen has been compared with the National Renewable Energy Laboratory's HOMER simulation tool, which uses hourly wind speed data. In particular, for an installation consisting of 10 turbines of 50 kW each, located on the isolated-grid of Kotzebue, Alaska, RETScreen and HOMER agreed to within 1.1% in their estimates of the annual energy production.
RETScreen's prediction of the annual energy and production for this installation was compared with monitored data from the installation. Over the years of 1998 through 2000, RETScreen underestimated the actual energy production by only 8 to 10%. This is sufficiently accurate for pre-feasibility purposes.
SLIDE 16: Conclusions
Wind turbines provide electricity on and off the electric grid at locations around the world. Wind turbines are a reliable, proven technology.
Since the energy in the wind increases in proportion to the cube of the wind speed, high average wind speeds are key to profitable wind project development.
In addition, government or utility production credits and green power rates significantly improve the profitability of on-grid wind projects.
The RETScreen software calculates energy production using the annual average windspeed but achieves an accuracy comparable to simulations based on hourly data. Thus, RETScreen can significantly reduce the cost of conducting preliminary feasibility studies of wind energy projects.
SLIDE 17: Questions?
This is the end of the Wind Energy Project Analysis Training Module in the RETScreen International Clean Energy Project Analysis Course.
