The Economics

The potential superiority of electricity generated at high altitude is demonstrated in the adjacent table, using the well-known method of levelised cost of energy (LCOE). The computations shown in the table have been applied to three 100 MW electrical power stations powered by different generating methods: coal, ground-based wind and altitude wind. The installation costs for each have been taken from published figures. In these calculations, money is borrowed at rates appropriate to the type of venture involved. Capital is returned over twenty years, while coal is supplied at a subsidised price of $31/tonne. All facilities are depreciated linearly at a rate of 5%/annum.

The costs relate to commercial, electrical generating stations each with nominal outputs of 100 MW. For comparison purposes it was assumed that in both wind systems the 100MW is generated using say 20 individual units, each with a 5.0 MW  output at their ground receiving stations. In the case of Altitude Wind, the published charts of the Annual Probability Distribution of Wind Velocity, for typically good Australian sites, show that rated wind speeds of 19.8 m/s will give capacity factors equal to 70% at an altitude of around 4.6km. Importantly, the cost per kg of craft was found following conventional procedures. This procedure is the same as that adopted by aerospace organisations (particularly in US Advanced Project Offices) for estimating the cost competitiveness of aircraft designs. The procedure is essentially costing components on previously known costs of similar components based on a cost per unit of weight method. In the current LCOE estimates, the empty weights of conventional, production-line helicopters were used as the similar components. Thus it has been conservatively assumed that a cost of $850 per kg is appropriate. In addition, this basic cost has been increased to allow for the electromechanical tethers and the ground facilities. In total the installation costs are $1,590,000 per MW.   Furthermore, the unit costings are in general agreement with the method given in the US National Renewable Energy Laboratory (NREL) guidelines at a manufacturing rate of 250 units per annum.

The adjacent figure does not include LCOE calculations for solar and other more expensive renewable systems. It can be seen that without any carbon tax, altitude wind is exceedingly cost effective. This is primarily due to the large capacity or availability factor of 70% (see row 4 of the table). This capacity factor, compared to 28% for ground based wind, means that the altitude wind system produces two and a half times more saleable energy per annum. In other words, the airborne system is more expensive to install and operate than ground-based wind turbines, but it gives an outstanding increase in energy output per annum. This produces the vastly superior $-cost per MWh quoted in the table. Fundamentally, this is due to the greater persistence of the winds at altitude.

Table of Levelised Cost of Energy

Table of Levelised Cost of Energy

Early Developments

An atmospheric test vehicle was designed and constructed, known as Gyromilll Mk2, and employed twin, single bladed, counterweighted rotors of solidity 2.2%. The rotors were 3.65m in diameter and the all-up weight was 29 kg. Again no cyclic pitch capability was employed.

Thus in hover and wind it was found necessary to use three parallel tethers to maintain craft attitude. These three tethers were used in all tests, but the craft was difficult to control particularly in low winds. Two tethers were attached near the rotor thrust lines while the third tether was attached well forward on the forward pointing boom.

Prototype flying

Recent Technological Advances

In 2004 the US patent office granted a patent entitled “Windmill Kite”. This, in essence, detailed a quad-rotorcraft with the rotors assembled at the corners of a square, as shown in the accompanying sketch. The single tether in the sketch is attached to the craft at four places directly under each rotor’s thrust-line in order to reduce stresses in the craft’s structure. More recently this quad-rotor assembly has been extended with an Australian patent, wherein the rotors are arranged more or less in-line with two rotors somewhat ahead of two rear rotors. This assembly is also shown in an accompanying sketch.

Each of these craft are restrained by a single tether and both crafts’ attitude in pitch, roll and yaw can be controlled by differential collective pitch on the various rotors. For example, the craft’s pitch, or nose-up angle, can be increased  by increasing the thrust on the two front rotors while reducing it by the same amount on the two rear rotors. These thrust changes are made through differential blade pitch action on the rotors. This technique is not dis-similar to the means used for controlling the power output of a conventional ground-based wind turbine. It is also important to note that using this blade pitch action on the rotors, the total thrust and the total power output of the craft are unchanged. So the craft’s altitude and power output are unchanged during the abovementioned changes, assuming all other factors are unchanged.

Computer control of the craft’s attitude and power output, while managing the tether’s tension, can be found in another granted Australian patent entitled “Control System for a Windmill Kite”. This patent uses the above control strategy.

Optimal Location to Harness Winds

The abovementioned power and persistence levels exist in a band about 1000 kilometres wide, running along an axis from Perth to Brisbane, where the average wind power density is around 20 kW/m2. This wind energy also exists around latitude 30 degrees in both Earth hemispheres.

The winds occur in bands approximately 1,000 km wide; other bands, for example, run over the Mediterranean, Northern India, China, Southern Japan, North and South America and Africa. Furthermore, compared to the winds available to ground-based turbines, these streams offer annual energy outputs about two orders of magnitude greater than can be achieved by ground-based windmills of the same rotor area.

Extensive studies of wind probability statistics for Australia, using Bureau of Meteorology radiosonde data, gave annual average power densities of up to 19 kW/m2 in the 1,000 km band, along an axis extending from Perth to Brisbane. The adjacent diagram shows the isopleths of power density over Australia at a pressure altitude of 250 mb. The power distribution is spatially well organised because of the lack of high mountains, which tend to upset the orderly flow of air over a continent.

Map of Australian Isopleths

Additional Market Opportunities

A whole range of further applications can be envisaged for our unmanned platforms which ride on the persistent winds that give near-infinite endurance.

Military and civilian surveillance, telecommunications, electricity production for remote sites and disaster areas are possible.

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