An Airborne Wind Turbine Engineering Essay

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An Airborne Wind Turbine is a design concept for a wind turbine that is supported in the air without a tower. Airborne wind turbines may operate in low or high altitudes; they are part of a wider class of airborne wind energy systems (AWE) addressed by high altitude wind power. When the generator is on the ground then the tethered aircraft need not carry the generator mass or have a conductive tether. When the generator is aloft, then a conductive tether would be used to transmit energy to the ground or used aloft or beamed to receivers using microwave or laser. Airborned turbine systems would have the advantage of tapping an almost constant wind, without requirements for slip rings or yaw mechanism, and without the expense of tower construction. Kites and 'helicopters' come down when there is insufficient wind; kytoons and blimps resolve the matter. Also, bad weather such as lightning or thunderstorms, could temporarily suspend use of the machines, probably requiring them to be brought back down to the ground and covered. Some schemes require a long power cable and, if the turbine is high enough, an aircraft exclusion zone. When the generator is ground-based, the tether need not be conductive. As of 2008, no commercial airborne wind turbines are in regular operation.

Introduction

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At present fossil fuels, petroleum, coal, natural gas which are regarded as conventional resources will get dried up in coming 50 years as per estimates. So the world should work together united to overcome this problem before the world activities come to a stand still. For coal extraction the technology is in sufficient and risky. So man has no choice except developing technologies for the utilization of abundant inexhaustible resources like solar, biomass, tidal, wind .Electric energy is the most versatile energy

Here we are going to discuss about wind energy. The basic principle of wind energy is to extract kinetic energy of wind and by means of wind turbines we are creating electricity.

Convectional wind turbines have many limitations some are listed below:

These wind mills can't operate if the speed of wind is greater than 69 mph because generators get over heated.

Low capacity factor.

Noise pollution is created due to continuous rotation of blades.

Many birds and bats are killed.

In order to overcome these limitations researchers looking to replace existing convectional systems. Taking all the factors in mind, a system, which will be really effective to solve the problem, has been designed. That "air borne wind turbines" which uses high altitude wind power. "If we were able to tap 1% of the wind energy at high altitude, that would be enough to supply all the world's energy needs," says David Shepard, president of renewable energy startup Sky WindPower, in Coronado, California.

The two main design frameworks they came up with are still with us today. 1. The first is essentially a power plant in the sky, generating electricity aloft and sending it down to Earth via a conductive tether.

2. The second is more like a kite, transmitting mechanical energy to the ground, where generators turn it into electricity. In this paper the first type is discussed in detail.

Prototype:

These technologies de-couple the energy extraction from the electricity generation through the use of kite/airfoils creating tension in a tether causing the rotation of a ground-based generator as the tether reels out from a drum. Such proposals fly the kite in a crosswind pattern which is shown to increase the apparent wind experienced by the kite by a ratio defined by the liftdrag ratio of the kite. Typical lift-drag ratios for the surfkites used in simulations and prototyping are 5.8 meaning that a kite will experience an apparent wind of 58m/s when flown in a true wind condition of 10m/s.

Architecture:

The helium filled ARS is a buoyant turbine made of vectran - a bulletproof material that is stronger than steel of the same thickness - and is connected to the ground by an insulated conductive tether. The unit can rise to a height of 300 to 1,000 feet to take advantage of more constant and higher wind speeds at higher altitudes that conventional wind turbines are unable to reach.

Principle:

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An aerostat-type wind power system relies at least in part on buoyancy to support the wind-collecting elements. Aerostats vary in their designs and resulting lift-over-drag aerodynamic characteristic; the kiting effect of higher lift-over-drag shapes for the aerostat can effectively keep an airborne turbine aloft.

Working:

Air Rotor System (ARS) is filled with helium gas, which is inert and non-flammable. The lifting gas creates a lift force that is in excess of the total weight of the system. The helium provides at least twice the positive lift versus the overall weight of the ARS unit. Additional lift is also created when the rotor is spinning in a wind. The aerodynamic effect that produces additional lift is called the Magnus Effect.

Magnus effect:

This is the same effect, discovered in the mid 1800's, that creates lift when a spherical or cylindrical object is spun while moving in a fluid. A dimpled golf ball, hit properly, has a back spin that causes it to lift in flight. A baseball curve ball pitch uses the Magnus effect. Basically, a back spin causes a low pressure region to form above the object and high pressure to form below, resulting in lift. A large object like the Magenn Air Rotor creates substantial lift, so much so that the device should actually work in a wind, without using a lifting gas.

ARS is a lighter-than-air tethered wind turbine that rotates about a horizontal axis in response to wind, generating electrical energy. This electrical energy is transferred down the 1000-foot tether for immediate use, or to a set of batteries for later use, or to the power grid. Helium sustains ARS and allows it to ascend to a higher altitude than traditional wind turbines. ARS captures the energy available in the 600 to 1000-foot low level and nocturnal jet streams that exist almost everywhere. ARS rotation also generates the "Magnus effect" which provides additional lift, keeps the ARS stabilized.

Depending on size, either DC or AC generators will be used, with rectification as necessary.

The ARS looks something like one of those blimps you see at sporting events. Except this one is not free flying. It's tethered to the ground on both ends, while the center part - equipped with large fins - spins in the wind.

The mechanical energy of the spinning is captured and converted to electricity in generators located at both ends. The electricity zips down to the ground by way of the tether, where it is delivered to a transformer. From there it can be sent for immediate consumption to a battery or to the power grid.

For maintenance, a worker can press a button on a winch located at the base station, and reel the balloon down to Earth.

Advantages :

Air Rotor System is less expensive per unit of actual electrical energy output than competing wind power systems. From an embodied-energy perspective the proposed kite systems compare favourably with terrestrial wind-turbines. As the kite/tether replaces the rotor of a terrestrial windturbine and no tower structure is required, it is estimated that the embodied-energy would be reduced by 74%

Power Air Rotor System will deliver time-averaged output much closer to its rated capacity than the capacity factor typical with conventional designs. Efficiency will be 25 to 60 percent. This is hugely important, since doubling capacity factor cuts the cost of each delivered watt by half.

Wind farms can be placed closer to demand centers, reducing transmission line costs and transmission line loses.

Air Rotors are operable between 2 meter/sec and in excess of 28 meters/sec.

Air Rotors can be raised to higher altitudes, thus capitalizing on higher winds aloft. Altitudes from 400-ft to 1,000-ft above ground level are possible, without having to build an expensive tower, or use a crane to perform maintenance.

Air Rotors are mobile and can be easily moved to different locations to correspond to changing wind patterns. Mobility is also useful in emergency deployment and disaster relief situations.

Wind energy costs are continuously decreasing due to technology development and scale enlargement

Capture of High Altitude Wind Energy Can Supply the World's Energy Needs and Achieve Kyoto Goals Through Economics

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Clearing wooded areas is unnecessary.

Farmers lease their land to companies and receive royalties.

Farm land can still be used for farming and cattle grazing.

Relative bird safety. Air borne wind turbine will not cause any bird and bat

Disadvantages:

The major drawback of the aforementioned systems is the fact that the power generating equipment is located at the energy source, i.e. at high altitudes. This means that the energy extracted from the wind must first be sufficient to keep the power-generating equipment airborne before any excess is available for transmission to the ground via the tether. The safety concerns raised by heavy airborne systems coupled with the risks/costs associated with prototype development have hampered the development of these proposed systems.

A higher altitude means that the turbine will have to contend with lower flying airplanes, more damaging ultraviolet light and particles in the atmosphere, and potentially winds that are too strong.

The risk posed by lightning must be addressed in any operational system. Although the proposed tether material,

Dyneema, is not itself a conductor, a wet tether will provide a route to ground for lightning. The option of landing all kite-based systems prior to lightning conditions may have a serious impact on the overall capacity factor and may be an important element in site selection.

Conclusion:

Capture of High Altitude Wind Energy Can Supply the World's Energy Needs and Achieve Kyoto Goals Through Economics

An array of 600 Air Rotar System (Flying Electric Generators) rated at 20MW each, a total of twelve thousand megawatts in capacity, operating over a ground space of a ten by twenty mile rectangle, would produce approximately three times as many megawatt hours per year as the 28,572,902 MWh produced by the Palos Verde Arizona nuclear facility in the year 2003, the most electricity produced by a single generating plant in the U.S. that year.

Additional arrays in the most remote places in the temperate zones, including off shore, may be used to generate hydrogen.

Because vehicles will probably best be powered electrically using new battery technology, the most important use of this hydrogen probably will be to supply energy for use in tropical regions where the high altitude winds are insufficient to supply this energy to civilization directly below. This use of hydrogen in the tropics shipped from the temperate zones is in the world's interests, as the global warming from green house gases there are as undesirable as anywhere else, and the non-global warming hydrogen should be able to be economically competitve in that use even though shipped from elsewhere, as fossil fuel is now.

Thus, the number of ARS/FEG arrays required to meet future world needs will increase significantly.

Clean Electricity for the Future World