Hello @PierreB,
I also think that the power-to-surface-area-used ratio is a useful metric, especially once AWE systems have reached higher TRLs and are operated in relevant environments. Only then we will have more certainty about the surface-area-used parameter.
The power harvesting factor \zeta=P/(P_wS) that is now often used in theoretical studies [1], was introduced as a result of dimensional analysis. It uses the wing surface area S because this is the only area measure that is constant for an AWE system. Other than wind turbines, where the blades of the rotor are always sweeping a constant area A=\pi l_b^2, an AWE system will not necessarily harvest from a constant swept area. This is also the reason why it is not straightforward to formulate the Betz law for AWE systems, which has been the subject of a number of recent studies.
While we can now determine the power output and annual energy production (AEP) of AWE systems with reasonably good accuracy, both by measurements as well as by computational simulations, the surface area used by AWE systems is a less tangible parameter. It will be influenced by the specific type of AWE system (e.g. rigid wing vs flexible membrane wing), operational safety characteristics and regulations. I think that you will agree that there is little data available on this. Also the cost of this surface area needs to be taken into account (offshore vs onshore). Your estimate of drawing a circle with the maximum tether length around the ground station is only a first guess, which will probably be contested by industry. In recent publications [2,3] about flexible wing systems we have introduced a spacial layout of kite parks that is requiring decidedly less space. See, for example, Fig. 4 in [2].
But I do agree that AWE architectures maximizing the surface area by concept used should be investigated more thoroughly.
References
- R. Schmehl, M. Noom, R. van der Vlugt: “Traction Power Generation with Tethered Wings”. In: U. Ahrens, M. Diehl, R. Schmehl (eds.) “Airborne Wind Energy”. Springer, Berlin Heidelberg, 2013. doi:10.1007/978-3-642-39965-7_2. Preprint accessible as pdf.
- V. Salma, F. Friedl, R. Schmehl: “Improving Reliability and Safety of Airborne Wind Energy Systems”. Wind Energy, in production, 2019. doi:10.1002/we.2433. Preprint accessible as pdf
- P. Faggiani, R. Schmehl: “Design and Economics of a Pumping Kite Wind Park”. In: R. Schmehl (ed.) “Airborne Wind Energy - Advances in Technology Development and Research”, Springer Nature, Singapore, pp. 391-411, 2018. doi:10.1007/978-981-10-1947-0_16. Preprint accessible as pdf