LTA Windpower

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Technology - The Airship

The airship is a nonrigid envelope filled with a lighter-than-air lifting gas, typically helium although the PowerShip is specifically designed to safely accomodate hydrogen instead. It takes its shape thanks to the pressure of the gas inside.

As a tethered flying body, a PowerShip needs to be stable with respect to three axes of motion: pitch, yaw, and roll. Ift may be feasible to get by with a passive tail structure. In the passive approach the blimp fins should be large enough to provide the required stability with respect to pitch and yaw, so the system doesn't wobble in the wind when in service, and stays oriented with the tail downwind. If necessary, the fins can be supplemented with active (i.e. controllable) surfaces. For instance, elevators may be used to adjust pitch. A rudder may be used to offset asymmetries in the wind drag on the wings and their generators, although this situation should seldom arise. Controlling roll using active surfaces is essential, e.g. in the form of "flapperons" on the wings. These devices actually do triple duty by controlling roll (in the role of ailerons), increasing lift (in the role of flaps) and decreasing lift particularly when the system is sitting on the ground (in the role of air brakes).

Powership on the ground

The envelope also houses one or two ballonets, as can be found in normal airships. In this design, the ballonet serves two functions. One is to compensate for changes in the temperature of the lifting gas. The other is to deliberately change the buoyancy of the airborne wind turbine overall. If two ballonets are used, one at the front and one at the rear, this gives an additional mechanism for adjusting pitch. The design is such that with the ballonet maximally inflated, the airborne wind turbine system is just slightly negatively buoyant, so it will land gently even if there is no wind. With the ballonet deflated, the system has enough positive buoyancy to lift off by itself and drag an appreciable length of tether along with it. The overall buoyancy of the airship is controlled by the ballonet with a target range of about 5% negative (ballonet inflated) to 15% positive (ballonet deflated).

The Lifting Gas

There are two viable lifting gases, helium and hydrogen. It is important to understand the importance of designing the PowerShip from the very beginning for compatibility with hydrogen as the lifting gas, instead of treating this option as a "me too!" afterthought. Helium is desirable because it is non-flammable, and memories of the Hindenburg disaster still linger. In that accident, static electricity that built up on the skin of the airship ignited the highly flammable skin when it made contact with ground structures. Part of the hydrogen fill burned too, but wasn't the immediate cause of the accident. The main problem with hydrogen is thus political, not scientific, although there are legitimate safety issues.

The problem with reliance on helium is even more serious, but different in character. This gas is in short supply, and we can forecast with confidence that in the long run the situation will only get worse, and helium costs will go up. Why? Helium is so light, that it can escape the Earth's gravitational field. For this reason, its concentration in the atmosphere is too low for recovery. The only commercially viable source of helium is its production as a by-product from natural gas wells where it was trapped by geological structures along with the methane. Helium recovery is currently happening in quantity mostly in the United States where production of methane and helium are declining. Meanwhile demand for helium is increasing, most of all as a coolant for medical diagnostic equipment using superconducting magnets and detectors. In 2006, consumption exceeded production for the first time, with the balance being met from US Government strategic reserves. When serious shortages start to occur, the medical and research applications will demand and receive priority for available helium over new applications like airborne power generation. In any case, power generation is a much more cost-sensitive industry than medicine. As a result of these considerations, compatibility with the much more plentiful (and much less expensive) hydrogen is a must. To boot, hydrogen is also more buoyant.

What then are realistic engineering and political requirements for an airship to use hydrogen instead of helium as a lifting gas? The airship should: (1) be unmanned, and not require a ground crew to approach it until after it has reached the ground; (2) be equipped with a properly grounded lightning arrestor at its highest points and be able to land quickly in the event of an electrical storm; and (3) have all components that run hot or which represent potential sources of ignition be installed far from the hydrogen-filled envelope. With these conditions satisfied, we believe it should be possible to persuade regulatory authorities that the airship can safely operate with hydrogen as the lifting gas. Uniquely among its competitors, the PowerShip does satisfy these requirements, but of course it can also use helium.