Technology

 

A big energy reservoir: the altitude wind

Wind velocity is a function of altitude. The power associated to Sun radiance on the Earth is some 174,000 TW. Wind is heat transformed in mechanical energy by the biggest solar collector we know: the atmosphere of the Earth. The total power associated to the wind has been calculated between 1,700 and 3,500 TW. To better understand the value of these figures, one has to consider that the demand of primary energy of the mankind is around 14 TW*Year.

Wind energy is not uniformly distributed. Due to the Earth's rotation and to temperature variations from the Poles to the Equator, wind energy is mainly concentrated in the high troposphere stratums, into two bands of 4,000/5,000 km width, that spin around the Earth blowing toward East: one in the South Hemisphere, at Fire Earth latitude and the other one in the North Hemisphere, upon North America and Europe.

The wind highest velocity, around 30 m/s, happens at an altitude of 12,000 m (200 millibar), at latitudes close to 30° North and 45° degrees South. The best altitude from the energy point of view is a bit lower, at about 10,000 meters.

Frome these altitudes the velocity and the energy of the wind decreases away toward the ground, giving back heat generated by the friction amongst the air masses themselves and with the orography. At 80 m from the ground, where the last generation of aeolian towers work, the average wind velocity is about 4,59 m/s, not enough for industrial exploitation; at 10 m from the ground it is about 3,31 m/s.

Advantages and drawbacks of high altitude wind can be so summarized:

  • Faster wind - Advantage: The wind power grows with the cube of its velocity.
  • Lower air density - Dissadvantage: The wind power decreases linearly with the air density, that, at 1,000 meters of altitude is still the 90% of that at ground level

Notwithstanding that the considerations about this energy bed have just started, to exploit high altitude wind should be already profitable around 800 meters, where the its average velocity is about 7,2 m/s.

At this altitude, as the previous Diagram shows, is about four times that available to the traditional aeolian generators. As a highest importance consequence, the localization problem of the wind power plant should be reduced: in average, at every point of the Earth surface, 800 meters upon itself, there is enough wind power, against the traditional aeolian farms, that needs a more severe selection of the favourable sites.

It is called geostrophic wind the altitude wind resulting from what is called the geostrophic balance between the Coriolis force and the pressure gradient force acting on a parcel of air, causing the wind to blow parallel to isobars of pressure in the earth's atmosphere. Thus, the isobars must be straight in pure geostrophic flow.

Despite this balance is rarely found exactly in nature, due to other forces acting on the wind, such as friction from the ground, or the centrifugal force from curved fluid flow, much of the atmosphere outside the tropics is close to geostrophic flow much of the time and it is a valuable first approximation. When the isobars aren’t straight, the wind blowing parallel to them is referred to as gradient wind and blows at sub-geosotrophical speed. Moreover, air fluxes sent away from the air thermal expansion at ground level, which cannot be caught by conventional sails, can be harvested by the KITVES Project solution.

The KITVES Project identifies wind in altitude as an unexploited energy repository. At the altitude at which the KITVES aims to operate (500 meters and above), average winds are more constant and intense than those reachable by conventional sails. At 500/1000 m above the sea level wind velocity is two times higher than at 100 m. Harvestable wind power grows with the cube of its velocity; therefore, the wind power corresponding at 800/1000 m is 8 times greater than wind power at 80/100 m.

The two-stage manoeuvre which allows the KSU to cyclically produce energy will let the wing array lift up to 400-800 m while sweeping the wind front, so that, at the same time, both the wind velocity and the wind front area grow up during the cycle. To harvest the air fluxes sent away from the air thermal expansion at ground level, which cannot be caught by conventional sails. The yearly availability strictly depends on the wind availability.

Being able to catch the wind in altitude in any navigation condition, considering the same atmospheric conditions, the yearly availability, in respect to traditional sail navigation, could rise from 3000 h/y up to 6000 h/y and above.

 
 

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KitVes is a collaborative project of the Seventh Framework Programme of the European Commission.


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