Laddermill, a novel concept to exploit the energy in the airspace

doi:10.1016/S1369-8869(01)00002-7

Aircraft Design

Volume 4, Issues 2–3, June–September 2001, Pages 81-97

https://doi.org/10.1016/S1369-8869(01)00002-7 Get rights and content

Abstract

A very significant amount of wind energy is contained in the movements of air at high altitudes. A particular invention (Laddermill Patent Ned. 1004508. Nov. 1996 applications Europe and USA), named “laddermill”, is described that allows exploiting this energy using the winds up to possibly the tropopause. A “laddermill” is a self-supporting system that consists of an endless cable connected to a series of high-lifting wings or kites moving up in a linear fashion, combined with a series of low-lifting wings or kites going down. The cable drives an energy generator placed on the ground. Dutch measured wind statistics are presented that show the immense power source at high altitudes. Some general physical considerations are given for the laddermill. Three simulation programmes were developed independently by different institutions. The results give a good consistency of the laddermill shape and power production. Design solutions are indicated for the wing attitude control and stability and a concept for the ground station is presented providing wing and cable handling. Adaptation to weather is given by flexible retrieval and deployment capability. Comparisons are shown with existing wind turbines. An operational model and related cost model have been made that include an operation strategy that optimises the economical effectiveness of the wings, cable and ground station. This operational model has been applied to the actual wind measurements over a period of 10 years. The results show, in comparison to the existing horizontal axes wind turbines, (i) a potential for significantly larger amount of wind energy production and (ii) an indication that this can be done at significantly lower cost. The public acceptance has been assessed, resulting in a positive perception of elegance of the low speed and silent movements combined with the excitement from reaching impressive altitudes. Safety and potential aviation interference are also addressed.

Section snippets

The laddermill design concept

The “laddermill” consists of a self-supporting series of high-lifting wings or kites that move up in a linear fashion, combined with a series of low-lifting wings or kites that go down, called the translator, and a ground station that contains the (electric) energy generator being driven by the translator.

Forces and cable diameter (one cable version)

For n wings going up, the total lift and drag force is given by $L=n q C_{L} S,$ $D=n q C_{D} S,$ where $q = 0.5ρV^{2}$ is the dynamic pressure averaged over the altitude covered by the laddermill.

The aerodynamic force on the cable can be split into the perpendicular drag force and the along side friction force. Wind tunnel measurements have confirmed [5] that these forces can be seen resulting from the wind speed in the perpendicular, respectively, along side direction. The drag component perpendicular to the cable is

Power production estimate

A rough assessment can be made about the power production and economics of the laddermill. The available power is given by $P=V_{cable} (F_{up} −F_{down}),$ where the forces will depend on the dynamic pressure and wing attitudes and somewhat on the cable speed (see Fig. 6).

The KNMI data [6] shows (see also Section 5) that the dynamic pressure averaged over the altitude range 0–10 km corresponds to 11 m/s at sea level, i.e. $〈 q 〉=90 N/m^{2}$ . The 10 percentile value corresponds to 4 m/s, i.e. $〈 q 〉=10 N/m^{2}$ and the 90

The laddermill simulations

Three simulation programmes were independently developed:

•
a simple Pascal programme LADDER by O-Mill [9] (made by the author), giving the power as a function of cable speed and the overall shape of the cable(s) [10],
•
a programme JLADDER, developed by the Delft University of Technology, under contract of O-Mill, that shows the full dynamics of the cable(s) as a function of the cable speed [11],
•
a more detailed programme developed by ECN, also under contract of O-Mill, that involves a particular

Wind statistics

The KNMI, Dutch Meteorological Institute, has made available the wind measurements that are made by weather balloon sondes, which have been launched during the period 1951–1980 (noon and midnight) [7]. The launch site was De Bilt, in the center of the Netherlands. The winds at lower altitudes, i.e. below 500 m, are therefore influenced by the roughness of the terrain. The upper winds are very much the same as would be found at other less rough places such as along the coast. The average wind

Model

The Pascal programme LADDER [10] has been included in an operation simulator. The output gives the power production for the actual wind profiles, while adapting each time the laddermill.

The input wind altitude profiles for the calculation are taken from the actual measured wind profiles data base at sequential days.

Further inputs are

•
Wing parameters: surface, weight, aspect ratio, lift and drag coefficients,
•
Cable parameters: diameter, strength, safety factor, density, and maximum length.

The

Discussion

For the actual realisation of the laddermill, a substantial design effort has to take place. The laddermill is a completely novel concept that covers a large variety of multidisciplinary aspects. Some elements are “historical”, i.e. the cable connections and slow motion combined with large forces, whereas other elements need advanced technologies such as very light weight structures and advanced control systems.

Most critical areas are the stability of the translator, the safety and the ground

Conclusion

The laddermill concept is a totally new concept that allows exploiting the unequalled durable energy resource of the wind in the troposphere. Physical limitations are shown, indicating maximum power production and optimal cable speed. Some design considerations are given and a particular operation simulation is presented using actual Dutch wind measurements over a 20 yr long period of time. An example of a rather small laddermill is included, giving indications of potentially very low cost per kW

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¹: Also at: O-Mill BV Laddermill development comp. Boekenroodeweg 45, 2111HK Aerdenhout, Netherlands.

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