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Winds can be harnessed to provide power, though extreme winds may damage structures (Note 14.B), and some of the wind's energy goes into eroding the land, which is the other aspect we will consider in this section.

Wind Power

The power of wind at velocity V (m/s) through an area of a square metre equals ?.V3/2 watts (Note 14.F), where ? is the air's density (about

1.25 kg/m3 at sea-level). For instance, a breeze of 5 m/s blowing onto the rotating blades of 3 m radius of a wind turbine contains 78 W/m2 of power (i.e. 0.5x53x1.25), so the blades (sweeping an area of 28 m2) receive 2.2 kW. Such a wind turbine with an efficiency of 40 per cent could power fifteen 60W lightbulbs (i.e. 2,200x0.4/60).

The dependence on the cube of the wind speed means that a small change of velocity makes a large difference of power. If the wind were 10 m/s in the example above, instead of 5 m/s, the collected power would be eight times as much. So it is worth while taking trouble to find the windiest spot, and erecting a tall mast to benefit from the stronger winds away from the ground's friction (Section 14.1).

Unfortunately, the strongest winds occur at sea, where it difficult to mount a wind turbine. Daily average winds across the Atlantic and Indian oceans at 40°S exceed 15 m/s on 30 per cent of days. The long-term average is over 12 m/s in the gap at about 56°S between South America and Antarctica.

Figure 14.7 and Figure 14.15 show that winds are relatively modest inland, bearing in mind that 200 km/day, for instance, averages only 2.3 m/s. The world's windiest place onshore appears to be at Commonwealth Bay at 67°S, 141°E, where three different stations have measured annual mean winds of 11-18 m/s.

Critics of wind power complain that wind farms are noisy, kill birds, spoil TV reception locally and blight areas of the upland or coastal wilderness, where the strongest winds tend to be found. There is also the economic problem of the variability of winds, between seasons and between times of day, so that the output from any place is fluctuating. The effect of this can be reduced by linking widely separated wind farms, where there are different patterns of wind. In Britain, for instance, there are more than eighteen wind farms connected to the national grid, generating up to 30 megawatts each, and thereby reducing the dependence on fossil fuels, with their undesirable outputs of carbon dioxide and suphur dioxide (Chapter 15).

Blowing Particles

Winds disturb loose materials on the ground and blow them around. A speed of 7 m/s is usually sufficient to lift fresh powdery snow and reduce visibility. Piles of stored coal have to be sprayed with water to prevent clouds of dust when winds exceed 10 m/s.

Australia has occasional dust storms, when visibility is reduced to less than 1 km by a cloud of red soil. They occur especially in the arid centre (Figure 14.17), where Alice Springs averages over ten annually, mostly in summer. A dust cloud covering 3,000 km2 to a height of 3 km and containing 0.2 grams of particles per cubic metre, removes 2 million tonnes of topsoil and the cloud may be blown to New Zealand and even Fiji, some 4,000 km downwind.

400 Isohyet
Figure 14.17 The frequency of dust storms in parts of Australia. The darkest shading represents over 5 dust storms annually, the next lightest 2-5, the next 0.5-2, and the unshaded part has less than one dust storm in two years, on average. The dashed line is the 400 mm/a isohyet.

Dust consists of particles of 1-10 microns diameter, and dust storms usually need winds of at least 12 m/s or so, along with a strongly unstable atmosphere. In general, conditions for dust storms are the same as for heat waves (Section 3.2), drought (Section 10.7) or an extreme risk of bushfire, i.e. prolonged, hot and dry weather.

Sand storms involve surface particles about a hundred times larger in diameter, so winds need to be stronger, the threshold depending on the particles' dryness, shape and density. Winds of only 3 m/s can move the smaller grains of dry sand across the surface in a hopping motion called saltation, where each dislodged grain jumps some centimetres into the air and then falls to the ground and thereby loosens another one or two grains, as well as dust. Indeed, bombardment by saltating sand is probably the main process raising dust from the ground in dust storms. But most movement of sand dunes is caused by winds above 12 m/s.

Soil erosion is caused by wind drying the surface (Chapter 4) and then lifting particles into the air. The rate of 'aeolian soil erosion' (i.e. erosion by wind) is proportional to the wind's power, i.e. to the cube of the wind speed, so it is greatly reduced by decreasing the surface wind. The decrease caused by the friction of even a partial cover of vegetation reduces soil erosion considerably, and it is the rare high wind that does most of the damage.

Water in the surface soil reduces aeolian erosion by holding the grains together, as well as by promoting the growth of a vegetative cover. Even 2 per cent of moisture in bare soil typically raises the threshold wind speed for erosion by more than 2 m/s.

Windbreaks

Winds across a field are commonly reduced by a windbreak, consisting of a wall or hedge, or a shelter belt of a row of trees. These lessen

Melbourne Dust Storm 1983
Plate 14.1 An unusual dust storm over Melbourne on 8 February 1983.

evaporation and soil erosion downwind, and give shade. The wind protection that is provided by a windbreak of height H is indicated by Figure 14.18; it depends on the porosity of the windbreak and on the distance downwind. About 10H away, crop yields tend be 10 per cent more than elsewhere, but the yield nearer to the windbreak is reduced by the windbreak's own demand for sunshine, moisture and nutriment.

14.6 SEA WAVES

More of the energy in the winds at sea goes into creating waves than into driving ocean currents. As a result, sea waves contain enormous power.

Ripples form on water when winds are only 1 m/s or so (Table 14.1). But strong winds drive the waves accordingly, and their speed is inherently related to the eventual wave height and to the distance between crests—the wavelength. For instance, a wave driven to a speed of 5 m/s is finally 0.5 m high from crest to trough and has a wavelength of 16 m, whilst a 12 m wave in the vicinity of a tropical cyclone is 400 m long and travels at 25 m/s. (These speeds do not mean that water actually travels along, it is simply the wave pattern that moves, like the undulations along a fixed rope that is shaken.) Usually the wave speed is slightly less than the speed of the wind over the ocean surface.

Waves do not reach their full height, wavelength and speed in immediate response

Figure 14.18 The wind speeds around a windbreak, as percentages of the speed at the height H of the top of the windbreak, and 20 times H upwind, showing elevation and plan views, with either a solid wall or one through which almost half the air can flow.

to changes of wind speed. The degree of adjustment depends on the duration of the wind regime, or, in a confined body of water, the distance upwind to land—the fetch. Figure 14.19 illustrates that the fetch required for the sea to adjust to a wind of 6 m/s, for instance, is about 80 km, and the time needed about five hours. After that, waves are about 0.7 m high and have a wavelength of 12 m and a speed of 4 m/s, so the period of the waves is three seconds (i.e. 12/4). The fetch and time to adjust fully are shorter with strong winds.

Long waves are called swell, and originate in distant major storms. They travel faster than the storm, so their arrival can give useful warning. Swell reaching Australian coasts comes mostly from midlatitude storms, and waves higher than 7 m are occasionally experienced on the south coast.

What is called a 'moderate sea' is one with waves 1-2 m high, whereas a 'heavy swell' has waves over 4 m high. Often there is a combination of swell from distant storms and smaller waves due to local winds, and the occasional synchronisation of swell and local waves leads to unusually large waves from time to time.

The power in kilowatts in each metre along the crest of a wave is about H2.P, where H is the

Plate 14.2 The night-time collision of sea breezes from both east and west coasts of Cape York (in the northeast of Australia) causes parallel, low-level, long rolls of circulation, subsequently carried westwards in the Trade winds, over the Gulf of Carpentaria. The forward rising edge of each roll lifts moist air from sea-level, forming a long cloud, producing the 'Morning Glory'. This photograph was taken at 7.45 a.m. and shows the first two of about five rolls oriented approximately north-west to south-east. The Sun in the east casts a shadow ahead of the clouds as they advance east of Burketown.

wave height (m) and P the period (seconds). Thus, for example, waves which are 0.7 m high and have a period of three seconds (from a wind of about 5 m/s) contain 1.5 kW per metre length, which is similar to the figure for a wind turbine with 3 m propellors in the same wind (Section 14.5.). A doubling of the wind speed leads to an approximate quadrupling of the wave height (Figure 14.19), and wave power is proportional to the square of the wave height, so the power is very sensitive to the wind speed, e.g. doubling the wind speed increases power about sixteenfold, if the wind speed is maintained over a sufficient time and fetch. Short-lived winds with a short fetch, like sea breezes, yield little wave power.

Such sea-wave power is unleashed in the course of coast erosion. It would be good to

Figure 14.19 Diagram for the estimation of the eventual wave height, period (i.e. time between crests), length and speed, for a given wind, when the fetch and duration are sufficient.

harness the energy, sustainably available to generate electricity close to coastal cities. Unfortunately, the practical problems of tides, corrosion, the vagaries of wave conditions, reduced wave height inshore and the awesome force of the worst storms have so far prevented our using the energy in sea waves.

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Renewable Energy 101

Renewable Energy 101

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. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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