LTA Windpower

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Technology - The Ground Structure

The PowerShip does not require any winches or vehicles on the ground, although it can use them if desired. All the PowerShip does need is a solid tether point than can withstand the load, mostly wind drag, that it will experience, and a rotatable joint that prevents excess flexure of the tether. The tether loading is expected to be on the order of 1000 lbs per 1000 watts of nameplate capacity for small systems, and decrease (in terms of lbs/W) for larger systems, particularly for those equipped with variable-pitch propellers.

For high output systems, additional considerations may apply to enable the generated power to be conducted to ground at higher voltage (and lower ohmic losses) than typical 600V or 2kV cable insulation will allow.

Powership Anchor on the ground

The anchoring system for the tether post can take a variety of forms, depending on the type of terrain and the intended permanence (or lack thereof) of the given deployment. For example, for long term deployments over unstable soil, the tether post may be embedded in a mass of concrete on the ground. Other possibilities include having the tether post attached to a plate or space frame which is in turn fixed to the soil using multiple ground anchors or reusable soil screws. In the event that solid rock is present on site, it may be possible to anchor the tether post to that.

Compare the above with the complexity, cost, and weight of a winching system. A unified tether as is required by a winched airborne generator cannot tolerate a sharp bending radius - 10's of inches may be required. That, in turn, means having a large drum with a termination and slip rings for the electrical conductors. It also requires that the winch re-orient itself as the wind shifts, which requires a large steerable turntable underneath the winch or a "funnel" above it. The loading on the winch is in the 10's of thousands of pounds or more and it itself weighs tonnes. A powerful winch needs to have a suitably powerful engine driving it. This can be a large electric motor with a source of electricity to power it, or an internal combustion engine with its fuel. The need for either greatly complicates a winched airborne power generator's suitability for use in isolated or temporary applications. It also greatly inflates the overall system and deployment costs.

The PowerShip's ground-structure requirements are also quite different from those of terrestrial wind turbines. The fixed, immobile portion of the system is expensive for the terrestrical turbine (a tower) and relatively inexpensive for the PowerShip (a tethering post). Furthermore, the provision of large construction machines (backhoes, cranes, etc.) to build the tower is expensive in its own right, particularly in remote locations. In the case of the PowerShip, much more modest construction equipment is sufficient. The tethering post can be built by a small lightly equipped crew, and the airborne system can be brought in by truck or ship and inflated on site, or perhaps even be towed in pre-inflated on a windless day. Also, the PowerShip system can subsequently be re-located, except perhaps for components not easily removed from the ground such as a concrete base or ground anchors.

Unattended Operation

Sketch Powership on the ground and in the air

The key to unattended operation is automated yet reliable takeoff and landing. Many years of experience with conventional heavier-than-air aircraft help to show that safe takeoffs and landings are possible. During their 20+ year service life, airliners may take off and land twice a day, accomplishing over 10,000 lifetime takeoff and landing operations, and most airliners do so without ever crashing. The landings sometimes involve complications like tailwinds and crosswinds, but the fact that their airspeeds greatly exceed ambient windspeeds helps airliners to manage the effect of winds.

How does one apply this to tethered airborne wind turbines, which cannot take advantage of airspeeds in excess of the windspeed? The two speeds are necessarily the same, given that the tethered body's groundspeed is zero. Furthermore, the fact that windspeed tends to be lowest and least stable close to the ground would seem to make landing a challenge even with a skilled crew on hand. This is the reason for replacing the plentiful Bernoulli lift offered to airliners by high airspeeds with lift from another source (i.e. buoyant lift) for the tethered wind turbine. With sufficient buoyant lift to make the tethered body neutrally buoyant, a soft landing is possible at any windspeed down to zero, and with otherwise aircraft-like flight control systems, the reliability of the landing process in nonzero winds is assured. Tethered gliders and kites reliant solely on Bernoulli lift are at much greater risk of crash landing sometime during their planned lifetime, and possibly destroying much of the investment they represent.