I am often asked how our airborne wind turbines stay in the air. Our understanding of physics hinges on the observation that energy is never created or destroyed. If we generate power to put it on the grid, we have to take it from somewhere, which in the case of an AWT or a wind turbine, is the kinetic energy carried by the wind. When you are driving down the freeway and hit the brake pedal, the brakes are extracting kinetic energy from the car that heat the brake rotors, or in a hybrid car, charge the batteries. For a given volume of air interacting with the wing, it slows down the volume of air and extracts energy.
The wing harvests kinetic energy by creating lift. One rough approximation, close enough to reality for our purposes, is that the wing pushes on a tube of air equal in diameter to the span of the wing. Because the lift is not exactly opposite the wind, the air is actually pushed slightly downward or back from the wing. This force leaves some momentum in the air, moving opposite the direction of motion of the wing.
This is not the whole story. The wing is gaining energy, which would make it accelerate. However, the onboard turbines are dragging the volume of air they interact with along with them, generating power and keeping the wing at a constant velocity. The volume of air that interacts with just the wing is left moving slightly opposite the motion of the wing, and the volume of air which interacts with both the wing and the turbines is left moving slightly with the wing. In this idealized example, if the turbines somehow interacted with the exact same volume of air as the wing, the wake could leave no energy behind. This points to one of the nice synergies of our system: Because the rotors are interacting with much of the same air as the wing, the overall efficiency of the system is higher than it would be if the two components were separated.
We can tie this idea of energy and slowing a volume of air to an image we are more familiar with. A drop of water hitting a pool has momentum, and behaves in much the same way as the air the AWT interacts with: the ambient fluid must push out of the way of the disturbed fluid. The two begin to roll up and mix. This transfer of momentum is called vortex shedding or wake rollup. It is what we see in classic images of things as large as a 747 or the M600, or as small as a drop of dye in a cup of water.
