A centralized approach for electricity generation within a wind farm is explored through the use of fluid power technology. This concept considers a new way of generation, collection and transmission of wind energy inside a wind farm, in which electrical conversion does not occur during any intermediate conversion step before the energy has reached the offshore central platform. A numerical model was developed to capture the relevant physics from the dynamic interaction between different turbines coupled to a common hydraulic network and controller. This paper presents a few examples of the time domain simulation results for a hypothetical hydraulic wind farm subject to turbulent wind conditions. The performance and operational parameters of individual turbines are compared with those of a reference wind farm based on conventional wind turbine generator technology using the same wind farm layout and environmental conditions. For the presented case studies, results indicate that the individual wind turbines are able to operate within operational limits. Despite the stochastic turbulent wind conditions and wake effects, the hydraulic wind farm is able to produce electricity with reasonable performance in both below and above rated conditions. With the current pressure control concept, a continuous operation of the hydraulic wind farm is shown including the full stop of one or more turbines.
A typical offshore wind farm consists of an array of individual wind turbines several kilometres from shore. Each of these turbines captures the kinetic energy from the wind and converts it into electrical power in a similar way as is done with onshore technology. However, one main characteristic of a wind farm as a collection of individual turbines, is that electricity is still generated in a distributed manner. This means that the whole process of electricity generation occurs separately and the electricity is then collected, conditioned and transmitted to shore. When looking at a wind farm as a power plant, it seems reasonable to consider the use of only a few generators of larger capacity rather than around 100 generators of lower capacity. The potential benefits, challenges, and limitations of a centralized electricity generation scheme for an offshore wind farm are not known yet.
Conceptual comparison between a conventional and the proposed offshore wind farm.
This work explores a particular concept in which a centralized electricity
generation within a wind farm is proposed by means of a hydraulic network
using fluid power technology
The main motivation for introduction of a centralized offshore wind farm is
to reduce the complexity and capital cost for the individual rotor–nacelle
assemblies. It is also expected that by having the whole electrical
generation equipment in one offshore central platform instead of having it in
a constraint space hundreds of metres above sea level, would have a positive
impact regarding operation and maintenance costs. A conceptual comparison
between a conventional and the proposed offshore wind farm is shown in
Fig.
Hydraulic systems have already shown their effectiveness when used for
demanding applications where performance, durability, and reliability are
critical aspects. In particular, the efficient and easy generation of linear
movements, together with their good dynamic performance give hydraulic drives
a clear advantage over mechanical or electrical solutions. Furthermore,
hydraulic drives have the potential to facilitate the integration with energy
storage devices such as hydraulic accumulators which are important to smooth
the energy output from wind energy applications
For the proposed concept, using high pressure makes it possible to reduce the
top mass of the individual rotor–nacelle assemblies. For this reason, a high
potential exists to reduce the amount of structural steel needed in the
support structures as well; for a 5 MW turbine in 30 m water depth, 1.9 t
of structural steel of the monopile can be saved for every tonne of top mass
reduction
With the purpose to avoid fluid circulation, an open-loop circuit is
considered with seawater as hydraulic fluid. The choice of seawater as
hydraulic fluid is preferred because of its availability and environmental
friendly nature when compared to oil hydraulics. In this regard, it is
important to consider that seawater contains a high concentration of
minerals, which give it a high degree of hardness. It also contains dissolved
gases such as oxygen and chlorine which cause corrosion. Despite its
corrosive nature, the use of seawater hydraulics has already been used in
some industrial applications, where in terms of safety, water hydraulics
might be preferred due to potential fire hazards or risk of leakage as is the
case of the mining industry. An example in the offshore industry includes the
seawater hydraulic system for deep-sea pile driving incorporating high-pressure water pumps
The modelling and analysis of a single turbine with hydraulic technology has
been previously presented for variable-speed control strategies. Simulations
of an individual turbine with an oil-based hydrostatic transmission have been
presented in
The overall wind farm model incorporates the dynamic interaction between the individual turbines, the hydraulic network, the Pelton turbine, and the controller. The model is described as a set of coupled algebraic and non-linear ordinary differential equations which are solved by numeric integration using MATLAB–Simulink. The hydraulic wind power plant model is composed of the following subsystems.
The aerodynamic characteristics of a horizontal-axis wind turbine rotor are a
function of its rotational speed
This reduced-order model does not include any aero-elastic or unsteady aerodynamic effects. Although these aspects are important for the loading of both rotor and support structure, their effects on the aerodynamic torque are considered less relevant from the performance and control point of view of the overall wind farm. The relatively large mass moment of inertia of the rotor in the angular degree of freedom will absorb large peak fluctuations in the rotor speed derived from the unsteady aerodynamic effects on the rotor torque.
The hydraulic drive train consists of a large positive displacement water
pump directly coupled to the low-speed rotor shaft. Hence, the rotor-pump
angular acceleration is described through the balance of the aerodynamic
torque
The pump is mainly characterized through a variable volumetric displacement
Here
The variable
The yaw degree of freedom of the individual turbines is not considered.
Hence, the yaw controller of the turbines is not included. A schematic
showing the different subsystems of a single turbine is shown in
Fig.
Subsystem block diagram of a single turbine connected to the hydraulic network.
The pitch actuator is based on a pitch-servo model described by a
proportional regulator with constant
The motion of the top mass of the tower in the fore–aft direction
Schematic for parallel hydraulic lines connected to a common line.
Schematic of the spear valve and nozzle.
One of the key aspects for having a centralized electricity generation is the
use of hydraulic networks to collect and transport the pressurized water from
the individual wind turbines to the generator platform. Similarly to the
electrical inter-array cable system for a conventional offshore wind farm,
the design of the hydraulic layout should consider several practical and
economical aspects, such as reducing the number and length of pipelines,
operational losses, and installation methods. For wind farms with a large
number of turbines, it is expected that branched hydraulic networks using
parallel and common pipelines will result in the most convenient
configuration. The hydraulic network consists of a number of interconnected
pipelines represented by linear transmission line models. The approach to
construct this network for time domain simulations from individual pipelines
was previously presented in
The dynamic response of the compressible laminar flow of a Newtonian fluid through a rigid pipeline network is given by the following state-space model; the model includes inertia and compressibility effects which are necessary to describe the fluid transients or so-called “water-hammer” effects. The model uses the volumetric flow rates from the individual rotor driven pumps and at the nozzle as an input, and the pressures at the water pumps and nozzle as an output:
At the end of the hydraulic network, a nozzle and spear valve is used to
adapt the pressurized water flow into the Pelton turbine. The nozzle
characteristics are included as a first-order differential equation by taking
the momentum equation of a fluid particle into account along the nozzle
length
Cross-sectional area of the nozzle as function of the spear valve
linear position for different spear cone angles where
Theoretical Pelton efficiency for different values of friction
factor
Figure
Similarly to the pump actuator, the dynamics of the spear valve linear
actuator are approximated by a first-order differential equation in which a
constant
The hydraulic power at the nozzle
The hydraulic efficiency of the Pelton runner
The theoretical Pelton efficiency is shown in Fig.
For the proposed configuration the efficiency of the Pelton turbine is only
determined by the water jet velocity, which is simply the volumetric flow
rate divided by the cross-sectional area and multiplied by a vena contracta
coefficient
The dynamic wind flow models and wake effects for a given layout are based on
an open source toolbox developed for “Distributed Control of Large-Scale
Offshore Wind Farms” as part of the European FP7 project with the acronym
Aeolus
Three wake effects are considered: deficit, expansion, and centre, where wake
deficit is a measure of the decrease in downwind wind speed, wake expansion
describes the size of the downwind area affected by the wake, and wake centre
defines the lateral position (meandering) of the wake area. Expressions for
wake deficit, centre, and expansion were developed in
The so-called variable-speed operation is of particular interest for this concept because by removing the individual generators and power electronics from the turbines, the hydraulic drives need to replace the control actions to obtain the variable-speed functionality.
As shown in Eq. (
Layout of the proposed wind farm with five turbines of 5 MW each.
Snapshot of the wind field and wake effects.
Pressure control schematic based on the spear valve position of the nozzle.
Schematic overview of the structure of the controller. The control blocks from left to right: proportional–integral (PI), low-pass filter (LPF), notch filter 1 (NF1), and notch filter 2 (NF2).
A first-order low-pass filter on the pressure measurement is employed to
prevent actuation from the fluid transient fluctuations in the hydraulic
network with the following transfer function form:
In order to achieve a constant pressure in the hydraulic network, linear
actuation of the spear valve is used to constrict or release the flow rate
through the nozzle area. The pressure control is based on a proportional–integral (PI) feedback
controller and a cascade controller compensation to modify the linear
position of the spear valve. A schematic of the proposed controller is shown
in Fig.
The PI controller is augmented with a second-order low-pass filter and a
series of notch filters. A schematic showing the structure of the augmented
controller is shown in Fig.
The low-pass filter and the notch filters are described in the frequency
domain according to Eqs. (
Controller parameters of the spear valve augmented controller.
Simplified schematic with the main components involving the energy
conversion for a reference offshore wind turbine and the proposed hydraulic
concept.
Above rated wind speed, the rated rotor speed is maintained by pitching
collectively the rotor blades. A conventional PI pitch controller is proposed
using the rotor speed error instead of the generator speed error. Due to the
sensitivity of the aerodynamic response of the rotor to the pitch angle, the
value of the controller gains are modified as a function of the pitch angle
through a gain-scheduled approach. The gain scheduled PI controller is shown
in the next equations, where
The values of the different gains are obtained in a similar way as described
in
The model described in the previous sections is used to assess the performance and operating conditions of a small hydraulic wind farm under specific wind conditions. Five turbines of 5 MW each are interconnected, through a hydraulic network, to a 25 MW Pelton turbine located at an offshore platform within 1 km distance from the individual turbines.
Main design parameters for the offshore wind turbine with fluid power transmission.
Time domain results for a wind farm comprising of five turbines subject
to a wind field with a mean speed of 9 m s
Time domain results for a wind farm comprising of five turbines subject
to a wind field with a mean speed of 15 m s
Two different wind speeds corresponding to below and above rated conditions
are simulated. First, a wind field with a mean wind speed of 9 m s
Performance overview of time domain results for below rated conditions.
Power performance for the reference wind farm, below rated conditions.
Power performance for the hydraulic wind farm, below rated condition.
The results from the simulations are compared with those of a reference wind
farm comprising of 5 MW NREL turbines
The results of the time domain simulations are presented in terms of the main
operational parameters such as mechanical power, rotor speed, and pitch angle
for the five turbines. For below rated conditions
Fig.
Performance overview of time domain results for above rated conditions.
For above rated conditions, the simulation results are shown in
Fig.
Power performance for the reference wind farm, above rated conditions.
Power performance for the hydraulic wind farm, above rated conditions.
The performance of both wind farms for the considered conditions is
summarized in the bar charts of Figs.
Time domain results for a hydraulic wind farm subject to a wind
field with a mean speed of 15 m s
Operating parameters of a hydraulic wind farm subject to a wind
field with a mean speed of 15 m s
The first observation based on the general results for both wind farms is the reduced power performance of turbines WT2 and WT5. The performance of these two turbines is directly affected by the generated wake from turbines WT1 and WT4. In contrast, turbines WT1, WT3 and WT4 are not affected by any other wake interaction.
After including the performances of the main subsystems involved in the conversion and transmission of wind energy in a wind farm, the results show that the overall efficiency of a hydraulic wind farm is lower for a hydraulic concept compared to conventional technology. For the presented operating conditions the hydraulic wind farm overall efficiency was between 0.772 and 0.810 compared to 0.835 excluding aerodynamic performance. The most important losses in the hydraulic concept are attributed to the variable displacement pumps and friction losses in the hydraulic network. Despite having a slower response due to high water inertia, the hydraulic concept also showed higher standard deviations in the generated electrical power due to pressure transients in the hydraulic network.
In the proposed hydraulic wind farm, all turbines are coupled to the same hydraulic network. This means that the pressure response in the hydraulic network is influenced by the individual flow rates of each turbine water pump. At the same time, the transmitted torque to each rotor is influenced by the local pressure at the water pumps. When abrupt changes in flow or pressure are induced as a result of either accidental or normal operation, pressure transients in the form of travelling waves are introduced in the hydraulic network which have to be taken into account. Furthermore, with the “secondary control” strategy proposed for the hydraulic system, the main large system effect of having several turbines connected to the hydraulic network is mostly determined by the ability of the spear valve and its controller to keep a constant pressure in the system. From this perspective, if one or more turbines are brought to a full stop, the spear valve should be able to maintain a relatively constant pressure in order for the remaining turbines to keep operating within design limits.
The following simulation presents the results of the scenario in which two
turbines are brought to a full stop at different moments of time. Starting
from the same environmental wind conditions from the previous example, above rated conditions with mean speed of 15 m s
The operational parameters of each turbine including the full stop of WT1 and
WT4 are shown in Fig.
In order to compensate for the overall decrease in flow rate through the
hydraulic network, each of the water pumps from the remaining operating
turbines are required to increase their volumetric displacement as observed
in the normalized control signal
The numerical model of a hydraulic wind power plant aimed to generate electricity in a centralized manner has been presented. The model demonstrates that on the basis of physical principles, it is possible to centralize electricity generation by dedicating the individual turbines inside a wind farm to pressurize water into a hydraulic network and then use the pressurized flow in a Pelton turbine. A variable-speed operation of the turbine is proposed in combination with a pressure controller in the nozzle spear valve to avoid the excitation of flow and pressure dynamics in the hydraulic network. Furthermore, the constant-pressure system makes it possible to include a fixed-speed Pelton turbine which simplifies the integration with the electrical grid.
Despite the stochastic turbulent wind conditions and wake effects, the results of the presented case studies indicate that the individual wind turbines are able to operate within operational limits for both below- and above rated wind conditions. Compared to a reference wind farm based on conventional wind turbine generator technology, the hydraulic collection and transmission has a lower efficiency due to the losses induced by the variable displacement water pumps and friction losses in the hydraulic network. The continuous operation of the hydraulic wind farm has been shown by bringing two different turbines to a full stop in above rated wind conditions. Further work includes the evaluation of alternative control strategies to assist the performance evaluation of the proposed centralized electricity generation approach. Other prospects of the hydraulic concept include the development and integration of an energy storage system using hydraulic accumulators. It is expected that these hydraulic devices will minimize the electrical power fluctuations for turbulent wind conditions.
The MATLAB SimWindFarm toolbox used in this work to
generate and simulate the wind field and wake effects is part of the European
research project funded by the European Commission under the IST framework
programme 7, and is publicly available at
The author declares that they have no conflict of interest.
This article is part of the special issue “The Science of Making Torque from Wind (TORQUE) 2016”. It is a result of the The Science of Making Torque from Wind (TORQUE 2016), Munich, Germany, 5–7 October 2016.
The author would like to thank the staff members from the groups of Dynamics of Solids and Structures, and Offshore Engineering at TU Delft. Edited by: Carlo L. Bottasso Reviewed by: Michael Muskulus and one anonymous referee