Prototype testing the flying electric generators in Australia in May 1986, showing the powered craft almost in autorotation at a wind speed of 8 m/s. Electricity generation was achieved briefly in another test. The craft, which had a total mass of 29 kg, had two rotating hubs, each radiating a lifting rotor blade and a shorter streamlined blade with a counter-balancing mass at its tip.
Bryan W. Roberts Former Foundation Professor of Mechanical Automation Engineering
University of Western Sydney Australia
tashypines@bigpond.com
Quad-Rotorcraft to Harness High Altitude Wind Energy
Bryan W. Roberts It is well known that powerful and persistent winds
ex-ist aloft with annual average power densities as high as 20 kW/m2. This figure far exceeds the power density of any other large-scale renewable resource found on Earth. A tethered rotorcraft is proposed to capture this energy. These rotorcraft would develop sufficient lift to keep the system aloft, while simultaneously generating an electri-cal output for transmission to a ground station [1]. Two jet streams exist in each Earth hemisphere, called the sub-tropical and polar-front jet. The former are relevant as they exist around latitude 30 degrees in bands approx-imately 1000 km wide over the Mediterranean, Northern India, China, Southern Japan, North America, Africa, Aus-tralia, South America and elsewhere. These streams have enormous energy and persistence compared to near sur-face winds.
Upper wind data statistics for Australia and the USA have been prepared. This data is presented as cumulative probability distributions. The paper will show that due to the persistence of these winds it is possible to achieve near base-load electrical outputs at capacity (or generat-ing) factors of between 70 and 80%.
The paper will detail early Australian work with a towed generating rotor, wind tunnel tests and with atmospheric, low altitude test vehicles. These tests confirm the fea-sibility of kite-like flight with the rotors simultaneously generating electricity. It is envisaged that a craft having twin or quadruple rotors can generate electricity at alti-tude when the rotors are inclined at a disk incidence to the on-coming wind. In general, the rotor disks operate
at an adjustable angle up to about 40 degrees. With suit-able electrical switching it is possible to fly the system, helicopter- style, using power supplied from the ground through the electro-mechanical tether. This allows easy ascent to altitude through any low-speed, near-ground wind and also to allow descents during short term lulls at altitude.
The paper will next detail, in a manageable manner, the basic mathematics underlying the technology. It will rely on classical helicopter theory to derive the rotor thrusts and the rotors’ limits to power generation, through the established technique of retreating blade incidence. The range of useful tip-speed ratios will be presented for the complete range of disk incidences. Power and thrust co-efficients will be derived. This mathematics can be used to propose a small quad-rotorcraft for demonstration of the technology at low altitude. It would then be possi-ble to escalate the size to obtain a multi-megawatt output from a single machine.
The final section of the paper will give a dynamic analysis of the system in order to describe a control strategy for the craft’s power output, pitch, roll and yaw, using purely blade collective pitch action. Wind axis derivatives will be derived and transformed into tether axes to develop a full set of control equations. Controller gains for stable oper-ation will be demonstrated in a manageable fashion. References:
[1] Roberts B. W., Shepard D. H., Caldeira K., Cannon M. E., Eccles D. G., Grenier A. J., Freidin J. F.: Harnessing High-Altitude Wind Power. IEEE Transaction on Energy Conversion, Vol. 22, No. 1 (2007)
