A Description of the Main Flow Features
Forested areas often have considerable potential wind power available, and because they are sparsely populated they have the advantage that there are fewer people to object to any wind turbines. Also, these onshore sites are convenient because of the low maintenance costs when compared to offshore wind farms. However, it is generally recognised that the wind flow over a forest will be highly turbulent, leading to more severe fatigue loads on the wind turbine blades than those encountered on other flatter, more homogeneous sites. This article attempts to provide a description of the main characteristic wind flow features in forested areas, together with an overview of the current methods being used to evaluate the quality of a given forest site.
By Antonio Segalini, the Linné FLOW Centre of the Royal Institute of Technology (KTH), Sweden
Wind resource evaluation is a critical element when assessing the potential performance of wind turbines at a given site because the energy available in a wind stream is proportional to the cube of its speed (i.e. doubling the wind speed increases the available energy by a factor of eight).
The recent exploitation of wind energy has triggered significant attention on the design of wind turbines and the environment where they will operate. Classical theories of windmills are still relevant, but they all assume an ideal inflow, namely a constant wind field with no fluctuations ahead of the turbine. Despite this simplifying restriction, their aerodynamics are still not simple.
The aerodynamics are further complicated by the fact that wind turbines operate in the surface layer of the atmosphere, the layer closest to the ground and approximately 200–500 metres thick, which is affected by friction, thermal stability and Coriolis forces due to the Earth's rotation. Even neglecting the last two contributions, friction is important enough to establish a boundary layer close to the ground where the wind speed changes from the higher values of the upper atmosphere to the almost negligible values found close to the ground. The presence of trees or buildings makes this transition layer much thicker, so it can be reasonably expected that smooth terrain will have more wind than forested areas at the same height above ground level. Nevertheless, the continuous growth of wind power production has driven attention towards more complex terrains, mostly because they are common but also because the human population density around such areas is usually low or even zero.
It is also well known that turbulence has an important effect on wind turbines’ performance for a variety of reasons, but especially because it enhances the fatigue loads and thereby reduces the lifespan of turbines operating inside the surface layer. It is becoming generally accepted that wind turbine failures are most strongly correlated with intense atmospheric turbulence events. This underlines the need for not only a statistical characterisation of the expected inflow, but also the necessity of providing information about extreme events. To give an idea of the turbulence encountered over a forest, Figure 1 shows the turbulence intensity profile (defined as the wind speed standard deviation, ?u, divided by the mean velocity, U) above a forest at different heights (normalised with the canopy height, hc). By assuming a tree height of the order of 20 metres, it is easily demonstrated that most of the wind turbines located near forested areas must withstand a significant level of turbulence intensity, which will inevitably lead to enhanced fatigue.
Fatigue is therefore the most important concern, but the energy yield is also significantly affected by the turbulent environment. This issue can be observed in some recent wind tunnel measurements using a wind turbine model with different hub heights over a forest model, shown in Figure 2. By decreasing the hub height both wind shear and turbulence intensity will increase, reducing the power coefficient (namely the rated power normalised with the 0.5rAbUhub3, where r is the air density, Ab is the rotor area and Uhub is the wind speed at hub height) of the turbine model and consequently the efficiency with which it converts wind energy into electricity.
The wind quality assessment of a site is usually made by placing some meteorological towers equipped with cup/sonic anemometers close to the area being investigated. The measurement campaign generally takes about a year, after which the expected wind can be computed. These towers are usually expensive and therefore they are placed at a few key points, while the remaining area is modelled by means of numerical models based on the averaged fluid dynamics equations (Reynolds Averaged Navier–Stokes, RANS, models).
At the moment there is no general consensus about how the forest canopy absorbs momentum and turbulence and how to account for that in numerical models. The simplest approach adopted in weather models is to assume the forest is a rough surface, with a given distribution of roughness height, but this approach only becomes realistic at heights above the canopy far greater than those at which turbines are able to operate. A more detailed method (and more computationally expensive) is to assume that trees have some drag proportional to the square of the local velocity, and that this force acts as a momentum sink in the equations that determine the mean flow evolution. However, it is not clear to what extent this model replicates the forest features, making the outcomes of the numerical methods still doubtful.
Actual atmospheric measurements provide data that can be used to validate such numerical models but are affected by numerous issues like the unsteadiness of the atmospheric conditions, the diurnal cycle, the different thermal stratification conditions and the often-neglected change of the foliage density during the various seasons of the year. A feasible alternative to such measurements is provided by wind tunnel experiments that provide almost perfect control of the operating conditions, but require the creation of a forest model that can replicate the effects of a real one. Nevertheless, a significant number of experiments have been carried out over the last few years and it is becoming evident that the structure of atmospheric turbulence over canopies has many more physical complications than are found over flat, homogeneous lands. Indeed, only a detailed characterisation of the velocity field can provide enough statistical evidence to improve the actual comprehension of this complex flow scenario.
The incoming wind velocity can generally be imagined as the superposition of the mean wind speed profile, together with some velocity fluctuations. A further scrutiny of the measured atmospheric data reveals that large-scale wind structures are usually embedded in the wind speed signal, where they are often referred to as 'gusts', a phenomenon that is particularly intense over a forest. Therefore, turbulence can be further decomposed into large-scale coherent motions and small-scale random motions; these, in turn, can be statistically characterised with plots similar to the one shown in Figure 1. The large-scale motions have been observed to occur almost every minute over a typical forest with a sudden velocity change of the order of 4m/s in less than 20s, and with a 'shape' similar to the one shown in Figure 3. Where manufacturers use evaluation methods that couple the predicted incoming flow characteristics with the dynamic loads on the blades to improve fatigue estimation these wind structures must be taken into account.
The flow over forest does indeed appear to be complex; it has been shown that the turbulence intensity level (up to as much as 20% at hub height) is high enough to influence the wind turbine efficiency, besides enhancing fatigue loads. In order to estimate such effects a full description of the forest area being considered must be provided using direct measurements. Whether or not different forests share a universal behaviour (even as a first approximation), regardless of the neighbourhood forest conformation, is still an open challenge that motivates most of the recent research in the topic. However, to place wind turbines above the forest canopy is still considered to be economically convenient because of the factors mentioned above.
Acknowledgement
This work is part of Vindforsk III, a research programme sponsored by the Swedish Energy Agency.
Biography of the Author
Dr Antonio Segalini earned his doctoral degree in fluid dynamics at the University of Bologna in 2010. He is currently working as a postdoctoral researcher at the Linné FLOW Centre of the Royal Institute of Technology (KTH) of Stockholm in the characterisation of the flow over forests for wind energy.{/access}
Forested areas often have considerable potential wind power available, and because they are sparsely populated they have the advantage that there are fewer people to object to any wind turbines. Also, these onshore sites are convenient because of the low maintenance costs when compared to offshore wind farms. However, it is generally recognised that the wind flow over a forest will be highly turbulent, leading to more severe fatigue loads on the wind turbine blades than those encountered on other flatter, more homogeneous sites. This article attempts to provide a description of the main characteristic wind flow features in forested areas, together with an overview of the current methods being used to evaluate the quality of a given forest site.By Antonio Segalini, the Linné FLOW Centre of the Royal Institute of Technology (KTH), Sweden
{access view=!registered}Only logged in users can view the full text of the article.{/access}{access view=registered}
Wind resource evaluation is a critical element when assessing the potential performance of wind turbines at a given site because the energy available in a wind stream is proportional to the cube of its speed (i.e. doubling the wind speed increases the available energy by a factor of eight).
The recent exploitation of wind energy has triggered significant attention on the design of wind turbines and the environment where they will operate. Classical theories of windmills are still relevant, but they all assume an ideal inflow, namely a constant wind field with no fluctuations ahead of the turbine. Despite this simplifying restriction, their aerodynamics are still not simple.
The aerodynamics are further complicated by the fact that wind turbines operate in the surface layer of the atmosphere, the layer closest to the ground and approximately 200–500 metres thick, which is affected by friction, thermal stability and Coriolis forces due to the Earth's rotation. Even neglecting the last two contributions, friction is important enough to establish a boundary layer close to the ground where the wind speed changes from the higher values of the upper atmosphere to the almost negligible values found close to the ground. The presence of trees or buildings makes this transition layer much thicker, so it can be reasonably expected that smooth terrain will have more wind than forested areas at the same height above ground level. Nevertheless, the continuous growth of wind power production has driven attention towards more complex terrains, mostly because they are common but also because the human population density around such areas is usually low or even zero.
It is also well known that turbulence has an important effect on wind turbines’ performance for a variety of reasons, but especially because it enhances the fatigue loads and thereby reduces the lifespan of turbines operating inside the surface layer. It is becoming generally accepted that wind turbine failures are most strongly correlated with intense atmospheric turbulence events. This underlines the need for not only a statistical characterisation of the expected inflow, but also the necessity of providing information about extreme events. To give an idea of the turbulence encountered over a forest, Figure 1 shows the turbulence intensity profile (defined as the wind speed standard deviation, ?u, divided by the mean velocity, U) above a forest at different heights (normalised with the canopy height, hc). By assuming a tree height of the order of 20 metres, it is easily demonstrated that most of the wind turbines located near forested areas must withstand a significant level of turbulence intensity, which will inevitably lead to enhanced fatigue.
Fatigue is therefore the most important concern, but the energy yield is also significantly affected by the turbulent environment. This issue can be observed in some recent wind tunnel measurements using a wind turbine model with different hub heights over a forest model, shown in Figure 2. By decreasing the hub height both wind shear and turbulence intensity will increase, reducing the power coefficient (namely the rated power normalised with the 0.5rAbUhub3, where r is the air density, Ab is the rotor area and Uhub is the wind speed at hub height) of the turbine model and consequently the efficiency with which it converts wind energy into electricity.
The wind quality assessment of a site is usually made by placing some meteorological towers equipped with cup/sonic anemometers close to the area being investigated. The measurement campaign generally takes about a year, after which the expected wind can be computed. These towers are usually expensive and therefore they are placed at a few key points, while the remaining area is modelled by means of numerical models based on the averaged fluid dynamics equations (Reynolds Averaged Navier–Stokes, RANS, models).
At the moment there is no general consensus about how the forest canopy absorbs momentum and turbulence and how to account for that in numerical models. The simplest approach adopted in weather models is to assume the forest is a rough surface, with a given distribution of roughness height, but this approach only becomes realistic at heights above the canopy far greater than those at which turbines are able to operate. A more detailed method (and more computationally expensive) is to assume that trees have some drag proportional to the square of the local velocity, and that this force acts as a momentum sink in the equations that determine the mean flow evolution. However, it is not clear to what extent this model replicates the forest features, making the outcomes of the numerical methods still doubtful.
Actual atmospheric measurements provide data that can be used to validate such numerical models but are affected by numerous issues like the unsteadiness of the atmospheric conditions, the diurnal cycle, the different thermal stratification conditions and the often-neglected change of the foliage density during the various seasons of the year. A feasible alternative to such measurements is provided by wind tunnel experiments that provide almost perfect control of the operating conditions, but require the creation of a forest model that can replicate the effects of a real one. Nevertheless, a significant number of experiments have been carried out over the last few years and it is becoming evident that the structure of atmospheric turbulence over canopies has many more physical complications than are found over flat, homogeneous lands. Indeed, only a detailed characterisation of the velocity field can provide enough statistical evidence to improve the actual comprehension of this complex flow scenario.
The incoming wind velocity can generally be imagined as the superposition of the mean wind speed profile, together with some velocity fluctuations. A further scrutiny of the measured atmospheric data reveals that large-scale wind structures are usually embedded in the wind speed signal, where they are often referred to as 'gusts', a phenomenon that is particularly intense over a forest. Therefore, turbulence can be further decomposed into large-scale coherent motions and small-scale random motions; these, in turn, can be statistically characterised with plots similar to the one shown in Figure 1. The large-scale motions have been observed to occur almost every minute over a typical forest with a sudden velocity change of the order of 4m/s in less than 20s, and with a 'shape' similar to the one shown in Figure 3. Where manufacturers use evaluation methods that couple the predicted incoming flow characteristics with the dynamic loads on the blades to improve fatigue estimation these wind structures must be taken into account.
The flow over forest does indeed appear to be complex; it has been shown that the turbulence intensity level (up to as much as 20% at hub height) is high enough to influence the wind turbine efficiency, besides enhancing fatigue loads. In order to estimate such effects a full description of the forest area being considered must be provided using direct measurements. Whether or not different forests share a universal behaviour (even as a first approximation), regardless of the neighbourhood forest conformation, is still an open challenge that motivates most of the recent research in the topic. However, to place wind turbines above the forest canopy is still considered to be economically convenient because of the factors mentioned above.
Acknowledgement
This work is part of Vindforsk III, a research programme sponsored by the Swedish Energy Agency.
Biography of the Author
Dr Antonio Segalini earned his doctoral degree in fluid dynamics at the University of Bologna in 2010. He is currently working as a postdoctoral researcher at the Linné FLOW Centre of the Royal Institute of Technology (KTH) of Stockholm in the characterisation of the flow over forests for wind energy.{/access}
