Readers may have noticed that I have been largely absent from these pages for a few weeks. That is because I’ve been doing a consulting job for KiteGen assisting with a presentation to be made to the CleanTech investor summit in Rotterdam in November (see disclaimer at end). In the course of doing this work certain things came to light that explains the power generated by kites, flight reliability and the move away from fabric to composite materials. I kick off with an amazing movie of a kite powered trimaran / hydrofoil (below the fold).
The video shows the kite flying cross wind. The kite is not a parachute simply dragging the boat down wind, but it is a wing where the aerofoil shape and the speed provides the power. The aerodynamic lift is creating large forces pulling on the boat. But since the boat has a keel the only direction it can move is forwards. This is exactly the same principle as sailing. The main difference is the kite catches a lot more wind at altitude and the hydrofoil reduces the drag of water on the keel. If you look at the picture up top you will see that the kite is flying in a direction roughly perpendicular to the wind as shown by the waves. The kite is flying cross wind in the power spot.
The sail boat videos are a good way to visualise the power created by aerodynamic lift (there are more videos at the end of this post). In sailing, the kinetic energy of the wind is converted to the kinetic energy of the boat. In kite power generation, the aerodynamic lift pulls rope off a drum connected to a generator making electricity.
Aerofoils are shaped so that the curvature of one surface gives it a larger area than the opposing surface. The pressure of the air flowing over the curved surface is reduced creating a net upwards force that is called lift. The question is, does this give us something for nothing?
“C” shaped sports kites and the KiteGen PowerWing have this aerofoil shape. The faster they fly, the more lift they generate.
And so to re-visit one of the main equations from my earlier post High Altitude Wind Power Reviewed:
force on the ropes = 1/2 * wind speed^2 * aerodynamic efficiency^2 * area of the wing * air_density
From which it should be clear that wind speed, aerodynamic efficiency and wing area are all key variables.
In the last post I did on high altitude wind, there was a lot of discussion in the comments about flight security and test flight data. Let’s say this was a matter of some irritation for Massimo Ippolito, the main innovator at KiteGen. In the course of many discussions with Massimo it has become clear that the fabric sports kites are mere toys compared with the composite PowerWing that has been designed and built for the industrial scale 3 MW machine. One variable that was missed in my earlier article was the tensile strength of the kites. Fabric sports kites are typically 10 kN while the composite PowerWing is 100 kN. The lower tensile strength of the fabric wing sports kites probably sets an upper limit of around 100 kW for power generation that is too low for a commercial generator. Bespoke fabric kites made from heavier duty material that is braced by bridles may exceed 100 kW, but the bridle system increases drag.
A relaxed Massimo Ippolito, Turin, September 2016.
“During tests using fabric sports kites we burst hundreds of kites that were simply not designed for the purpose of power generation.”
The Tables below compare the key variables for fabric and composite wings. The examples are worked with a wind speed of 15 m/s, which is not really that high, and shows that the PowerWing produces about 100 times the force of a sports kite. It is the higher aerodynamic efficiency of the PowerWing that gives it this enormous advantage. The 100kN tensile strength of the PowerWing provides a safety factor of 2.4 over the maximum forces expected.
The 400 kN total nominal force is reached at a wind speed of about 3.5 m/s. Once that force is exceeded the ropes must unspool on the drums converting the excess force to electricity. (Note this uses only the aerodynamic parameters of the wing / kite. The ropes add both mass and drag and this reduces the efficiency so that the nominal force is not reached until 5.4 m/s.)
The PowerWing reaches maximum 3 MW output at wind speed of 17 m/s. At higher wind speed it must then fly out of the power spot to contain the total force within machine design parameters.
The PowerWing is designed in this way so that it can produce maximum power output under lower wind conditions. Under low wind conditions, the kite will be flown towards the power spot where it generates maximum lift. In higher wind conditions it is flown away from the power spot, the power spot being defined as the direction of maximum lift when the wing is flying perpendicular to the wind direction.
I think it is worthwhile showing the following figure again. The upper panel (red) shows electric power production and the lower panel (blue) rope reel out velocity. The middle panel (green) shows the force curve. The cut-in wind speed is 2 m/s. That is the speed required for the kite to fly. Between two and 5.42 m/s the force on the ropes builds to the nominal force of 200kN on each rope. No power is produced at this time. >5.42 m/s the ropes begin to unwind (lower panel) and electricity is produced reaching maximum power output at 17 m/s. Beyond 17 m/s the kite must be manoeuvred to a position where it sees 17 m/s wind speed either by going lower or by flying out of the power spot.
I want to finish with three more movies that demonstrate the awesome power of aerodynamic lift using both kites and semi-rigid sails.
This amazing video shows a kite foiler playing in the surf 2 mins 39 secs.
This video 9 min 25 secs shows Vestas Sailrocket team going after the world speed sailing record which they break at 65 knots. The fun starts at about 5:30. This is achieved using a rigid sail on an out rigger. Language advisory!
This video 4 min 21 secs shows a land based version of the Sailrocket called Greenbird that goes at 126 mph!
will be strictly moderated. Comments should be strictly limited to the technical concept of aerodynamic lift and how this can be converted to useful work that may be used by Mankind. Does it provide something for nothing?
In the following months I will be working for KiteGen as an unsalaried consultant, assisting in a third round equity raising exercise. Remuneration will be in the form of commission upon a successful conclusion. I have worked for over ten years as an unsalaried blogger, hoping that a white knight may ride across the horizon one day to restore financial security. And so I am very grateful for this opportunity. If you wish to protest about this then please do so via email and not in the comments.
I hope to find time to write at least one post per week and to maintain the objectivity of the blog. There may be a couple more but not too many more articles on kites.
KiteGen are at an interesting and vital stage of their development. They have secured a new test site in The Alps with consent to fly up to 5000 m and space for 9 machines. The current plan is to deploy the first fully functional 3 MW stem next spring with field trials of the PowerWing over the summer months. They are also moving to new premises adjacent to open ground that may permit limited testing of PowerWing launch and retrieval procedures.