16-05-2013, 04:56 PM
High Altitude Wind Energy Generation Using Controlled Power Kites
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Abstract
This paper presents simulation and experimental results
regarding a new class of wind energy generators, denoted as
KiteGen, which employ power kites to capture high altitude wind
power. A realistic kite model, which includes the kite aerodynamic
characteristics and the effects of line weight and drag forces, is
used to describe the system dynamics. Nonlinear model predictive
control techniques, together with an efficient implementation
based on set membership function approximation theory, are employed
to maximize the energy obtained by KiteGen, while satisfying
input and state constraints. Two different kinds of KiteGen
are investigated through numerical simulations, the yo-yo configuration
and the carousel configuration, respectively. For each configuration,
a generator with the same kite and nominal wind characteristics
is considered. A novel control strategy for the carousel
configuration, with respect to previous works, is also introduced.
The simulation results show that the power generation potentials
of the yo-yo and carousel configurations are very similar. Thus,
the choice between these two configurations for further development
of a medium-to-large scale generator will be made on the
basis of technical implementation problems and of other indexes
like construction costs and generated power density with respect to
land occupation. Experimental data, collected using the small-scale
KiteGen prototype built at Politecnico di Torino, are compared
to simulation results.
INTRODUCTION
GLOBAL wind power has the potential to meet the world’s
energy demand and, differently from fossil sources, it is
largely available almost everywhere (see, e.g., [1]). However,
the actual wind power technology, based on wind towers, has
several limitations that need to be overcome to make such energy
source competitive against fossil sources (for an overview
of the present wind technology, see, e.g., [2]). In particular, wind
towers require heavy foundations and huge blades, with massive
investments leading to higher energy production costs with respect
to thermal plants. Moreover, the average power density per
km obtained by the present wind farms is 200–300 times lower
than that of big thermal plants of the same rated power, leading
to significant land occupation and impact on the environment.
Finally, wind towers can operate at a maximum height of about
150 m, due to structural limits, and can therefore be used with
profit only in locations with “good” wind speed at 50–150 m of
height from the ground.
KITE GENERATORS
Yo-Yo Configuration
As a first step, in the KiteGen project a small scale prototype of
the yo-yo kite generator has been realized [see Fig. 1(a)]. In this
configuration, the K.S.U. is fixed with respect to the ground. Energy
is obtained by continuously performing a two-phase cycle
[depicted in Fig. 1(b)]: in the traction phase the kite exploits wind
power to unroll the lines and 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 minimum amount of the previously
generated energy, to recover the kite and to drive it in a position
which is suitable to start another traction phase, i.e., when the
kite is flying with wind advantage in a symmetric zone with respect
to the nominal wind direction.
KITE GENERATOR CONTROL
The control problem and related objectives are now described.
As highlighted in Section II, the main objective is to
generate energy by a suitable control action on the kite. In order
to accomplish this aim, a two-phase cycle has been designed
for each generator configuration. The two phases are referred
to as the traction phase and the passive phase for the yo-yo
and as the traction phase and the unroll phase for the carousel
configuration. A nonlinear Model Predictive Control (MPC,
see, e.g., [13]) strategy is designed for each phase, according to
its own cost function, state and input constraints and terminal
conditions.
CONCLUSION AND PERSPECTIVES
This paper presented simulation and experimental results
regarding a new class of wind energy generators, denoted as
KiteGen, which employ tethered airfoils to capture wind power.
More realistic models, with respect to previous works, have
been used for the kite and its lines and a novel control strategy
for the carousel configuration has been introduced too. The
presented simulation results show that the power generation
potentials of the yo-yo and carousel configurations are very
similar. Thus, the choice between these two configurations for
further development will be made on the basis of technical implementation
problems and of other indexes, like construction
costs and generated power density per kilometers squared. The
presented experimental data, collected using the small-scale
KiteGen prototype built at Politecnico di Torino, have been
compared to simulation results. The good matching between
simulation and real measured data increases the confidence
with the obtained simulation results.