Chapter 1

Introduction

For any power plant to generate electricity, it needs fuel. For a wind power plant, that fuel is the wind.

Wind resource assessment is the process of estimating how much fuel will be available for a wind power plant over the course of its useful life. This process is the single most important step for determining how much energy the plant will produce, and ultimately how much money it will earn for its owners. For a wind project to be successful, accurate wind resource assessment is therefore essential.

Technologies for measuring wind speeds have been available for centuries. The cup anemometer—the most commonly used type for wind resource assessment—was developed in the mid nineteenth century, and its basic design (three or four cups attached to a vertical, rotating axis) has scarcely changed since.

Yet, an accurate estimate of the energy production of a large wind project depends on much more than being able to measure the wind speed at a particular time and place. The requirement is to characterize atmospheric conditions at the wind project site over a wide range of spatial and temporal scales—from meters to kilometers and from seconds to years. This entails a blend of techniques from the mundane to the sophisticated, honed through years of experience into a rigorous process.

The details of this process are the subject of this book. Before diving into them, however, we should back up a little and set wind resource assessment in context. Where does the wind come from? What are its key characteristics? And how is it converted to electricity in a wind power plant?

1.1 Where do Winds Come From?

The simple answer to this question is that air moves in response to pressure differences, or gradients, between different parts of the earth's surface. An air mass tends to move toward a zone of low pressure and away from a zone of high pressure. Left alone, the resulting wind would eventually equalize the pressure difference and die away.

The reason air pressure gradients never completely disappear is that they are continually being powered by the uneven solar heating of the earth's surface. When the surface heats up, the air above it expands and rises, and the pressure drops. When there is surface cooling, the opposite process occurs, and the pressure rises. Owing to differences in the amount of solar radiation received and retained at different points on the earth's surface, variations in surface temperature and pressure, large and small, are continually being created. Thus, there is always wind somewhere on the planet.

While uneven solar heating is ultimately the wind's driving force, the earth's rotation also plays a key role. The Coriolis effect1 causes air moving toward the poles to veer to the east, while air heading for the equator veers to the west. Its influence means that the wind never moves directly toward a zone of low pressure but rather, at heights above the influence of the earth's surface, it circles around it along the lines of constant pressure. This is the origin of the cyclonic winds in hurricanes.

By far, the most important temperature gradient driving global wind patterns is that between the equator and the poles. Combined with the Coriolis effect, it is responsible for the well-known equatorial trade winds and midlatitude westerlies (Fig. 1.1). At the equator, relatively warm, moist air has a tendency to rise through convection to a high altitude. This draws air in from middle latitudes toward the equator and thereby sets up a circulation known as a Hadley cell (after the nineteenth century meteorologist who first explained the phenomenon). Because of the Coriolis effect, the inflowing air turns toward the west, creating the easterly trade winds.2

Figure 1.1 The main global atmospheric circulations. Source: NASA/JPL-Caltech.

A similar circulation pattern known as a polar cell is set up between high latitudes and the poles. Lying between the polar and Hadley cells are the midlatitude (Ferrel) cells, which circulate in the opposite direction. Unlike the others, they are not driven by convection but rather by the action of sinking and rising air from the adjacent cells. Once again the Coriolis effect asserts itself as the air flowing poleward along the surface turns east, creating the westerlies. The westerlies are the reason wind resources tend to be so good in the temperate and high latitudes (around 35–65 °N) of North America, Europe, and Asia, as well as the southern extremes of Africa, South America, and Australia.

Superimposed on these global circulation patterns are many regional patterns. Large land masses heat up and cool down more rapidly than the oceans, and even within land masses, there are variations in surface heating, for instance, between a snow-covered mountain top and a green valley below or between a desert and a cultivated plain. The resulting temperature gradients set up what are called mesoscale atmospheric circulations—mesoscale because they are in between the global scale and the local scale, or microscale.

The most familiar mesoscale circulation is the sea breeze. During a typical summer day, the land becomes warmer than the ocean, the pressure drops as the air above it expands and rises, and relatively cool, dense air is pulled in from the ocean. At night, the process reverses, resulting in a land breeze. Normally, sea breezes are weak, but where the wind is concentrated by terrain, they can have a powerful effect. This is the primary mechanism behind the very strong winds found in coastal mountain passes in the US states of California, Oregon, and Washington, and in comparable passes in other countries.

While temperature and pressure differences create the wind, it can be strongly influenced by topography and land surface conditions as well, as the example of coastal mountain passes attests. Where the wind is driven over a rise in the terrain, and especially over a ridge that lies transverse to the flow, there can be a significant acceleration, as the air mass is “squeezed” through a more restricted vertical space. Thanks to this effect, many of the best wind sites in the world are on elevated hilltops, ridges, mesas, and other terrain features. However, where the air near the surface tends to be cooler and heavier than the air it is displacing, as in the sea breeze example, it has a tendency to find paths around the high ground rather than over it. In such situations, it is often the mountain passes rather than the mountain tops that have the best wind resource.

Surface vegetation and other elements of land cover, such as houses and other structures, also play an important role. This role is often represented in meteorology by a parameter called the surface roughness length, or simply the roughness. Because of the friction, or drag, exerted on the lower atmosphere, wind speeds near the ground tend to be lower in areas of higher roughness. This is one of the main reasons why the eastern United States has fewer good wind sites than, for example, the Great Plains. Conversely, the relatively low roughness of open water helps explain why wind resources generally improve with distance offshore.

1.2 Key Characteristics of the Wind

The annual average wind speed is often mentioned as a way to rate or rank wind project sites, and indeed, it can be a convenient metric. These days, most wind project development takes place at sites with a mean wind speed at the hub height of the turbine of 6.5 m/s or greater, although in regions with relatively high prices of competing power or other favorable market conditions, sites with a lower wind resource may be viable. However, the mean speed is only a rough measure of the wind resource. To provide the basis for an accurate estimate of energy production, the wind resource must also be characterized by the variations in speed and direction, as well as air density, in time and space.

1.2.1 The Temporal Dimension

The very short timescales of seconds and less is the domain of turbulence, the general term for rapid fluctuations in wind speed and direction caused by passing pressure disturbances, or eddies, which we typically experience as brief wind gusts and lulls. Turbulence is a critical mechanism by which the atmosphere gradually sheds the energy built up by solar radiation. Unfortunately, it has little positive role in power production because wind turbines cannot respond fast enough to the speed variations. In fact, high turbulence can cause a decrease in power output as the turbine finds itself with the wrong pitch setting or not pointing directly into the wind. In addition, turbulence contributes to wear in mechanical components such as pitch actuators and yaw motors. For this reason, manufacturers may not warrant their turbines at sites where the turbulence exceeds the design range. Knowledge of turbulence at a site is thus very important for resource assessment.

Fluctuations in wind speed and direction also occur over periods of minutes to hours. Unlike true turbulence, however, these variations are readily captured by wind turbines, resulting in changes in output. This is a time frame of great interest for electric power system operators, who must respond to the wind fluctuations with corresponding changes in the output of other plants on their systems to maintain steady power delivery to their customers. It is consequently a focus of short-term wind energy forecasting.

On a timescale of 12–24 h, we see variations associated with the daily pattern of solar heating and radiative cooling of the earth's surface....