Distance miles

FIGURE 3-15: Example sea breeze winds, late afternoon, with no winds aloft.

During periods of light winds aloft in spring and summer, surface winds blow from ocean to land during the day (sea breeze) and in the reverse direction at night (land breeze). These breezes, as anyone near a large body of water knows, can be substantial winds. In winter, the land breeze may occur in the daytime as well as at night. Figure 3-2 roughly illustrates sea and land breezes. During a 24-hour period, the cycles of wind and temperature are much smaller over water than over land.

The development of a sea breeze is roughly as follows. Assuming there is jf®' wind aloft, the sky is cliar, the daytime has arrived, the sun^lll start heating the water and land. This heat is absorbed into several feet of water, but only into a fraction of an inch of earth, so the latter warms up much faster. The land heats the air at ground level, but this air gradually rises many hundreds of feet. The warmer air is lighter than the air over the 3ea so, as in the case of an open refrigerator door, the cold air rushes from the sea onto the land while the partially heated air far above the land moves out to sea to replace the incoming cold air. The circulation pattern has been completed; a sea breeze has been created. Starting as a local disturbance, this circulation pattern will extend many miles landward and seaward during each day.

A feeling for possible wind speeds in a sea breeze can be obtained from Figure 3-15. This shows the wind speed at 5 P.M. with no winds aloft in an area extending about 1 mile high by 20 miles out to the sea and 50 miles inland. In this example, winds from shoreline to about 11 miles inland are about 10 mph. Wind speeds of 15 mph caused by these sea breezes are common, even when winds aloft are still. Thus, a wind turbine at a seaside location can gain a great amount of power from this sealand breeze phenomenon and not rely solely on high winds aloft to be transmitted to the ground. Large lakes also create similar but smaller breezes.

The complex nature of winds in valleys is briefly described here. When a strong wind aloft is blowing in a direction more or less parallel to a valley, there is a funneling effect. Winds are often stronger in the valley than over level country at these tunes, particularly where the valley narrows or its sides steepen. When the wind blows perpendicular to the valley, very complex flow patterns develop and often large areas in the valley will experience a great commotion in the air called turbulence.

Next, we consider what happens if the wind aloft is light. At night the air on the sloping sides of the valley will cool near the ground and, being heavier, will flow to the valley floor. When the slopes of the valley are warmed during the day, the wind will reverse direction. Complex combinations of these flows will occur as shown in Figure 3-16. The above effects cause most of the wind energy available in a valley to be aligned with the direction of the valley. Therefore, when siting a wind turbine, care should be taken to obtain the best location for capturing these winds.

If a valley narrows at its lower end, the cold air may drain out of the upper, broader end of the valley. A study of night wind profiles in a number of Vermont valleys indicated that maximum winds on most nights were found at heights of 100 to 1000 feet above the ground, often about two-thirds the height of the surrounding hills. The intensity of the wind gradually increases with increasing distance from the head of the valley.

In spite of the frequent valley winds, if prevailing wind directions are roughly at right angles zo tne valley, chances are that there is more wind energy available on the plateaus above the valley. The valley wind patterns sketched in Figure 3-16 may not contain a significant amount of recoverable wind energy.

It is easy to see that wind records from a meteorological station at another part of a valley from your location (or at a different distance from a coastline) may give little indication of your own winds.

The flow over a long isolated mountain ridge that faces the wind (and tends to block it) is another interesting case. Near the ground (at wind turbine height) the wind speed decreases as the toe of the ridge is approached, then speeds up to greater than

FIGURE 3-16: Daily wind cycle in a valley facing a plain. No wind aloft.

FIGU RE 3-17: How the mountains of Oahu affect the wind.

the average in the region of the ridge line. If the sides of the ridge are very steep the increase in wind speed at the top will not be as great as for a ridge of moderate steepness. Also, the wind on the back side may be very turbulent and unsuitable for wind turbines. Near the toe of the hill on the upwind side the w?. power may be reduced to 50 percent, and near the top it can be double the average wind speed depending on the slope of the sides. Some specific cases will be described in Reference 10.

What happens to the wind near the ends of a ridge that faces the prevailing winds? The Hawaiian island of Oahu provides an excellent example of this. Recently, a large computer was used to predict the flow over and around this island. For much of the year, such a strong wind blows across the island that the sea breeze influence is not particularly significant. This wind is confronted by a range of mountains 30 miles long (Fig. 3-17) that can be described as a ridge about 2500 feet high, with occasional peaks several hundred feet higher. The ridge line is oriented onl about 13° counter-clockwise from a right angle to the prevailing wind direction. At each end, the ridge slopes down to sea level within a space of 5 to 10 miles.

The wind path lines at 500 feet above the surface of Oahu and the surrounding ocean are indicated in Figure 3-17. This ridge is very long - about 60 times as long as it is high - but even so, the wind path lines indicate that about one-third of the air approaching the island at the 500 foot height is deflected around the ends of the ridge. There are some good wind turbine sites along the top of the ridge line where the wind speed is about twice that of the approaching wind. The north and south ends of the ridge are particularly attractive sites, however, as the airflow that is deflected around the ends of the ridge speeds up to approximately twice the approaching wind speed for large regions at each end of the ridge. Thus, if your location is near the end of a ridge that tends to block prevailing winds, you may have considerably more wind power than your neighbors out on the flat land.

The wind near the top of an isolated hill is quite different than the wind over a long ridge. There is a strong tendency for the lower layers of the wind speed profile to split and go around the hill, particularly during the night and early morning when the ground has cooled off. For a wind turbine near the top, the hill acts quite a bit like a giant tower.

There is another characteristic of the wind that is helpful for increasing the wind power available on high hills. Reexamining Figure 3-12, the average daily wind curves above Oak Ridge, Tennessee, notice how the wind above 200 meters is greater at night than during the day while the reverse is true below 100 meters. This is a very general condition around much of the country. A wind turbine on a hilltop may produce power all night while below in the flat country the air will be nearly calm.

Structures and trees are likely to be in the vicinity of a wind turbine. How close can a wind turbine be placed to these, and how much penalty is paid if these obstacles cannot be avoided? Both loss of wind speed and the wind turbulence downwind of these obstructions are important. Wind turbine generators spin at high speeds and tend to have long, thin blades. This makes them much more susceptible to damage from wind turbulence than water-pumping windmills. Older water-pumping windmills are often found very close to trees and apparently are able to withstand the resulting turbulence. A rule of thumb generally used for wind turbine generator placement is: if the location is not well above all surrounding obstacles, the wind turbine should be placed at least 10 obstacle heights or widths away from the obstacles. This is a reasonable rule. The results of some available wind measurements will be presented in Reference 10 to give a better feeling for how the wind is disrupted when it passes over and around obstacles.

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

Get My Free Ebook

Post a comment