In the last period Kitemill's operation have shifted to endurance flights, from short flights to enable functionality. Flights with full operational cycles in various wind conditions gives some interesting operational statistics. Operational performance is not yet is the focus, but we can track some good trends.
The flight testing in 2022 changed to endurance targets after a period with shorter flights mainly to enable functionality. The prolonged test flights now includes the main operational phases, production, transitions and retraction phase. As net produced power has been traced in a variety of wind conditions the shape of a power curve can be traced. Though, the power curve is not yet the main focus when we here addresses the key take-aways so far. The three plots in this article are based on the same flight.
Net power
The KM1 system produces power at decent levels prior to tuning for power optimization. The average power of >1 hour of operation with continuous cycles has been measured at 5,5 kW. A targeted average of 20 kW (KM1 system rating) is some tuning ahead, but it already peaks regular beyond 20 kW.
One of the most significant observations is that the cycle efficiency is 80 %, meaning the consumed energy in the retraction phase is 20% of the produced energy in production phase.
Wind at trajectory height
Kitemill is fortunate to have the best lidar in the world located on the site as a part of the EU funded project LIKE.
Measurements from trajectory height shows that the wind speed was around 16 m/s during the flight shown above.
This day the wind at 300 m was less than in lower altitudes. Ideally the production could be maintained at 150 m average altitude.
Picking the optimal altitude is an option for the kite systems and which will increase the yield from the "average wind data" normally used to predict wind sites and production potential.
Automatic interaction with measured wind data will be implemented in the future. After this flight the operators have got easier access to real time lidar data.
Power curve
Tracing a power curve requires operation of the whole wind range over time with a rather static configuration. For wind turbines this normally takes one year of operation. Currently the configuration changes rapidly as the system is in continues development.
It is anticipated that flight, used as example above, performed at 65 % of its potential of control tuning. Well tuned it is capable to deliver 10 - 11 kW in average in these wind conditions.
Further improvements would also be possible addressing the aerodynamically performance. An obvious fix is the alignment of the two center propellers in the picture above. The KM1 Spark kite was designed in 2015 and flight testing reviled it could benefit of some lift features as wing slats to achieve average power of 20 kW. These observations are input to the 100 kW rated KM2 system currently under development.
Flight trajectory
The targeted power curve is based on the mean trajectory height . The current flights are performed at an average 220 m altitude. For a similar sized wind turbine this is 10 x the height. In these heights the wind is stronger and more consistent.
The flight trajectory is one of the items which will be optimized on later to improve the power curve.
Altitude, interaction with weather data will improve power. If the operator monitored the wind gradient, the operation would have been kept at the lowest altitude. The gradually increased altitude is one of the reason for the gradual power drop in each production cycle you can see on the first graph on this page.
What we now focus on is load control, test team performance, development cycles/processes, equipment availability and other operational key performance indicators.
Load control is the basic
The past years the main challenge has been structural integrity and load control.
Most of the recent flights have been at reduced load. By gradually increasing the load the kite is operating at its design load.
Load control allows the forces to be handled efficiently and is the main contributor to the power curve performance.
The power produces reaches the rated power as the kite flies towards the ground, then it sometimes changes to consumption in the upward part of the circle. The tension reaches maximum in the bottom of the circle and has a low spot on the top. The loads can be manipulated by the winch controls, kite static settings, kite active controls, flight path strategies, etc.
Great progress has been made lately. This is valuable experience and expected to be a core area of development until megawatt scale, as for conventional wind.
More details of current state of development and how the system development being planned will be presented by Espen Oland, who is Program Manager for KM1 and KM2 development, under the Airborne Wind Energy Conference in Milano June 2022.
