rotating mechanisms and electric generators placed on the ground. The system – composed of the electric drives, drums, on-board sensors and all the hardware needed to control a single kite – is denoted as the kite steering unit (KSU). This is at the core of kitenergy technology (see Figure 1A).
The KSU can be used to produce electricity in different ways. In this paper, the so-called KE-yoyo configuration will be considered. In such a configuration, energy is obtained by continuously performing a two-phase cycle, depicted in Figure 1B.
In the traction phase, the wing is driven to fly fast in crosswind direction, with figure-of-eight trajectories. This generates high traction forces that unroll the lines. During this phase, the electric drives act as generators, driven by the rotation of the drums.
When the maximum line length is reached, the passive phase begins and the drives act as motors, spending a minimal amount (about 10 %) of the previously generated energy to roll back the lines and to drive the wing into a position that is suitable to start another traction phase.
The KE-yoyo small-scale prototype built at Politecnico di Torino (see Figure 2) is now being tested with 6–20 m2 area wings to validate the experimental modelling calculations. Videos of the experimental tests performed are available online.12–13
The key idea of kitenergy is to harvest HAW energy with the minimal effort in terms of generator structure, cost and land occupation. In wind towers, the outermost 30 % of the blade surface contributes about
The idea of harnessing high-altitude wind (HAW) power using a tethered aircraft was proposed at least as far back as the 1970s.
80 % of the power generated. Thus, the tower and the inner part of the blades do not directly contribute to energy generation. Despite this, the structure of a wind tower determines most of its cost and imposes a limit on the elevation that can be reached, as previously discussed.
To understand the concept of kitenergy, one can imagine removing all of the bulky structure of a wind tower and just keeping the outer part of the blade. This part of the blade becomes a much lighter wing, flying fast in crosswind conditions (see Figure 3A), connected to the ground by the cables alone. Thus, the rotor and the tower of the present wind technology are replaced in kitenergy technology by the wing and its cables, creating a wind generator that is significantly lighter and cheaper to produce.
For example, in a 2 MW wind turbine, the weight of the rotor and the tower is typically about 300 tons. A KE-yoyo generator of the same rated power can be obtained using a 500 m2 wing and 1,000 m long cables, with a total weight of only 5–6 tons. It is therefore expected that the construction costs of a KE-yoyo generator will be dramatically lower than those of a wind tower of the same rated power.
MODERN ENERGY REVIEW – VOLUME 3 ISSUE 2
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