Seawater-Based Hydraulics for
Offshore Wind Turbines
N.F.B. Diepeveen (TU-Delft)
(We@Sea project 2004-012)
WE@Sea Progress Report
Seawater-Based Hydraulics for Offshore Wind
Turbines
August 2009
N.F.B. Diepeveen
Preface
Purpose of This Document
The purpose of this document is to report on the research which has done by the author as
part of the We@Sea program. It covers the period from August 15th 2008 to August 15th
2009. This period coincides with the first year of PhD research performed by the author.
During the first three months, the plan for the Delft Offshore Turbines (DOT) project was
constructed. It was written to lay the foundation for several PhD topics. The research topic
of the author concentrates on the hydraulic energy transmission using seawater. This has so
far resulted in two conference papers, one focused on hydraulic transmission in wind turbines,
the other specifically on the pump requirements.
Outline of This Document
This document is contains the following four parts:
- Executive Summary
An extensive summary is given of the set up of the DOT project and the related research
activities in the first year. Preliminary conclusions and an outline of the future research
efforts are also provided.
1. DOT Project Plan
Author: ir. N.F.B. Diepeveen,
Supervisor: dr.ir. J. van der Tempel
2. Closed-Loop Fluid Pumping as a Means to Transfer Wind Energy
Conference paper for the European Wind Energy Conference 2009, submitted on March
16th 2009.
Author: ir. N.F.B. Diepeveen.
3. Pump Design Requirements for Seawater-Based Hydraulic Power Transmission for Offshore Wind Turbines
Conference paper for the European Offshore Wind (EOW) Conference 2009, submitted
on September 14th 2009.
Author: ir. N.F.B. Diepeveen.
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Seawater-Based Hydraulics for Offshore Wind Turbines
WE@Sea Progress Report
Executive Summary
Introduction
Delft University of Technology is taking a radical step away from incremental development
of offshore wind turbines. It has started a research project on using a 2 bladed, fixed pitch
turbine (5 − 10M W ) to directly drive a water pump in the nacelle. By channeling the pressurized water of all turbines to one transformer platform, electricity generation is centralized.
The design goal is to reduce the number of components in the offshore turbines drastically to
come to the ultimate offshore turbine.
Current offshore wind turbines are marinized land turbines with only a few add-on features
to keep out the salty air. Improvements of turbine technology are only incremental and do
not take full benefit of the offshore environment. The Delft University of Technology has
a history in offshore wind research of over 25 years and has formulated a radical concept
change of offshore wind energy conversion that helps develop a completely new system and
spark revolutionary developments on sub-system and component level.
Typically, offshore wind farms have a generator platform that gathers all electricity of the
different turbines, steps up the voltage and feeds the power through shore connection cables
to the onshore grid. The DOT takes boundary conditions from this existing configuration:
horizontal axis turbine with blades and a platform where the combined electrical power is fed
to the onshore grid. Everything in between can be changed. The DOTs focuses on radical
technology changes. To facilitate this, a short list of design pointers has been defined to test
all developments against and to keep as life line throughout the project execution. Offshore,
one thing is abundant: water. The current turbine technology sees the nacelle weight increase
steadily giving increasing challenges in support structure design and installation. Furthermore, power electronics help harness wind power slightly more efficiently, but also add weight
and components (that can fail) to the turbine system.
Offshore wind energy has high potential. Currently the price for placing turbines offshore is
too high. Projects are not yet economically feasible without government subsidies.
The overall goal of the Delft Offshore Turbines project is therefore to design a wind turbine
infrastructure specifically for offshore purposes and thereby rendering offshore wind energy
more economically attractive. This translates to a design goal of the overall project which
is to reduce the number of components in the offshore turbines drastically to come to the
ultimate offshore turbine, which is characterized by:
- Very low maintenance
- High availability; as a direct consequence of high reliability.
- High efficiency
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iv
- Easy installation
- Low production costs
The Delft Offshore Turbines (DOTs) project aims to circumvent the need of the generator by
using the rotor shaft torque to power a pump in the nacelle. This, along with the proposal
to have a two-bladed rotor, will lead to a significant reduction in weight of the rotor-nacelle
assembly.
One of the fundamental parts of the DOT is the hydraulic transmission. The PhD research
project in this report focuses on the design of a seawater-based high pressure hydraulic energy
transmission system, from the rotor shaft to the generator platform.
This PhD project was set up as follows. First a plan for the overall DOT project was drafted
(part I) to lay the foundation on the basis of which up to 8 PhD students can select research
topics. The next step was to select one of these topic for my own research project.
Before any form of design could start a study was performed to look at the possibilities of
using fluid power for wind energy transmission. This was the subject of the European Wind
Energy Conference (EWEC) 2009 paper (part II). The purpose was to gain insight in how
fluid power circuits operate. This meant mapping which type of systems exist, which is the
most efficient and why and what the key performance indicators are. Gain insight in the
applications and the potential of fluid power circuits, i.e. what has already been done with
fluid power in similar applications, in general and hydraulic wind turbines in particular.
The next step was to perform a research based selection of the pump type and a first look
at seawater as hydraulic fluid. This was the subject of the European Offshore Wind (EOW)
Conference 2009 paper (part III). The goal was to select the best suitable pumping principles
and investigate their commercial availability. Next to that insight was gained into which
challenges arise from using seawater as hydraulic fluid.
Fluid Power
Applications
High pressure fluid power has been applied for many years in many industries. The number of
applications continues to grow. One of the earliest large scale projects were the Victorian age
tap-water hydraulics. Pumping stations outside the center of cities like London, New York
and Melbourne delivered water pressurized up to 60 bar through underground mains to power
facilities like elevators, cranes and even theater curtains. Nowadays, fluid power is used in
shredders, feeders, roll mills, cranes, bulldozers, jack-up systems, etcetera. These applications
use electricity to efficiently acquire power in the form of high torque through high pressure
fluid transmission. The DOT energy transmission concept is the exact opposite. High torque
is converted into a high pressure flow.
Classification of pumps
Pumps can be divided in two general categories: kinetic (or hydrodynamic) and positive displacement pumps. In hydrodynamic pumps such as centrifugal pumps, the flow is continuous
Seawater-Based Hydraulics for Offshore Wind Turbines
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v
from inlet to outlet and results from kinetic impulse given to the fluid stream. The output
is characterized by low pressure and high volume. Inefficiency and easy stalling as a result
of back-pressure make these pumps unsuitable for control. In positive displacement pumps,
fluid flows through an inlet into a chamber. As the pump shaft rotates, the (positive or
definite) volume of fluid is sealed from the inlet and transported to the outlet where it is
subsequently discharged. The essential difference between these two main categories is that
kinetic pumps are for fluid transport systems and PD drive systems are for fluid power systems. The power-to-weight ratio of pd pumps is much higher than that of the generators used
in wind turbines. This is without taking into account all the extra components required for
the use of an electricity generator.
Hydraulic Fluids
Seawater is preferable in to hydraulic oil in terms of dynamic performance. This is due to the
higher bulk modulus of seawater. Having an open-loop system means that the temperature
of the water will remain well within its liquid range. However, low viscosity of seawater also
means poor lubricity and high potential of wear due to erosion and corrosion.
Hydraulic Turbines
Hydraulic turbines are not new. Using seawater as hydraulic fluid is. Currently ChapDrive AS
in Norway, Artemis in Scotland and Voith Turbo (WinDrive) in Germany are all developing
hydraulic gears for wind turbines. The most similar to the DOTs project is the ChapDrive,
having its generator placed at the foot of the turbine tower. These systems all use hydraulic
oil as medium.
DOT Concepts
The Delft Offshore Turbines (DOTs) convert wind energy into a high pressure flow of water.
A pump is connected to the rotor directly, generating a high pressure flow. The pressurized
water is collected at a transformer platform, where generators are located comparable to a
hydro plant. The platform can be fitted with limited water storage/accumulation capacity to
smoothen energy variations. From this platform, an electricity cable connects to the onshore
grid. So far, two concepts have been derived:
1. closed-loop + open-loop
A high pressure fluid power circuit forms a closed loop between the pump connected to
the rotor shaft in the nacelle and a motor just below sea-level. The motor is connected
to a second pump which pumps seawater to the central generator platform. Having a
closed-loop systems requires subsystems for cooling & pressurizing. This adds significantly to the total number of components.
2. open-loop
The pump connected to the rotor shaft generates a high pressure flow. At the base, a
WE@Sea Progress Report
vi
small portion of this pressure is used to power a booster pump which ensures the flow
of sea water to the pump in the nacelle. The rest of the pressure is used to generate
electricity at the central platform.
Selection of Pumping Principle
Candidate pump types for the Delft Offshore Turbines are the vane pump and the radial
piston pump. Vane pumps can cope with low viscosity fluids like water but are limited in
terms of pressure (< 100bar). The radial piston pump can generate high pressure (> 500bar)
and can be designed to operate efficiently (> 95%) at rotation rates matching those of the
wind turbines. However, in particular the clearance between the piston and its casing is a
concern when using seawater. Corrosion and erosion of a pump will lead to a rapid decline
in efficiency. A solution must be found to prevent these phenomena from occurring.
Conclusion
The main challenge for the DOT hydraulic energy transmission is to have a robust, yet efficient
system.
Hydraulic drive systems have been applied in many industries for many years. High performance systems are characterized by high efficiencies and low maintenance needs. The power
production of a Delft Offshore Turbine can increase beyond rated due to the characteristics
of hydraulic drive systems. The cut-out wind speed is determined by the control characteristics of the rotor (blades). The current maximum size of suitable pumps is under 2 MW and
requires the use of hydraulic oils as power fluid. Larger systems are technically feasible. They
have not yet been produced due to lack in demand.
Despite the additional mass of the hoses and power fluid, having only a pump in the nacelle
significantly reduces the total mass of the wind turbine. For concept evaluation purposes,
further research needs to be done on the reduction of the nacelle mass and subsequently the
support structure mass.
A 5M W turbine will require a volume flow of close to 10, 000 liters per minute.
Since the idea behind the DOTs project is to design a turbine specifically for offshore purposes,
oil will not be the preferred power fluid in the long run. Instead we aim to use the offshore
environment to our advantage and use seawater.
The research presented in part III shows that multi-MW high pressure seawater pumps do
not yet exist. The main reason for this is that there has never been a real need for them. To
make the DOTs a reality, such a pump will need to be designed. The main challenge is how
to design for the use of seawater as hydraulic fluid.
For this the radial piston pump appears to be the best suited. A pump with rated capacity
of 5MW is not yet commercially available, let alone one capable of pumping seawater for long
periods of time without maintenance. In terms of power production, the Hägglunds CBP
210, with a little over 2.3M W rated power at 350bar pressure is the state of the art. More
powerful systems do already exist in the form of prototypes. However, all require hydraulic
Seawater-Based Hydraulics for Offshore Wind Turbines
WE@Sea Progress Report
vii
fluids with high viscosity. The conclusion therefore is that a specific system for high pressure
5M W seawater pumping will have to be designed. Important design considerations are:
- The Hägglunds CBP 210 (2.3M W ) operates with efficiency of 9096%. In these systems,
the larger the piston, the higher the efficiency. Therefore ever more efficiency can be
expected of a 5M W pump.
- The low viscosity of seawater will lead to an increase leakage flow, reducing the efficiently
slightly.
- The high bulk modulus of seawater should significantly benefit the overall efficiency.
- The pump has to operate efficiently also at very low rpm. Trends in radial piston pumps
and commercial wind turbines indicate that designing for matching rpms is feasible. One
of the first steps in the design process should be to find a solution for the poor lubricity
and the corrosive and erosive characteristics of seawater. The logical first step is to
look for a structural material that is sufficiently damage resistant for the fluid carrying
parts of the pump. Finding this material is potentially crucial to the development of
the project.
The next phase of the project will be focused on finding a suitable structural materials for
the high pressure seawater pump.
WE@Sea Progress Report
viii
Seawater-Based Hydraulics for Offshore Wind Turbines
WE@Sea Progress Report
Part I
DOTs Project Plan
roman
Contents
1 Introduction
1.1 The Future of Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2
1.3
Current Wind Turbine Technology . . . . . . . . . . . . . . . . . . . . . . . . . . .
The Proposed Idea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
1
2
4
2 Design Rules
5
3 Design Options
7
4 How DOTs Work
9
4.1
4.2
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Power Transmission: Wind to Water to Electric . . . . . . . . . . . . . . . . . . . .
9
10
4.2.1
4.2.2
A: The Rotor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
B: The Closed-Loop System . . . . . . . . . . . . . . . . . . . . . . . . . . .
10
11
4.2.3
4.2.4
C: The Open-Loop System . . . . . . . . . . . . . . . . . . . . . . . . . . .
D: The Generator Platform . . . . . . . . . . . . . . . . . . . . . . . . . . .
14
14
Simulation Results for Varying Wind Speeds . . . . . . . . . . . . . . . . . . . . .
15
5 Plan of Execution
5.1 DUWIND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
19
4.3
5.2
5.3
DOTs Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Time Planning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
20
5.4
Potential PhD Research Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21
References
23
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Contents
DOT Project Plan
ii
1 Introduction
1.1
The Future of Energy
Europe is becoming increasingly dependent on imported energy. The current level of 50% is expected to rise to 70% by 2030 by the European Wind Energy Association (EWEA) [2]. Meanwhile,
energy prices are soaring and a future energy crisis is looming.
Nearly all imported energy is derived from the burning of imported fossil fuels, releasing vast
amounts of carbon-dioxide (CO2 ) into the atmosphere. The global consensus is that CO2 emissions lead to global warming.
Of the energy consumed in Europe, only about 9% comes from renewable sources [1]. To become more independent and reduce CO2 emissions it is necessary for Europe and to produce more
and cleaner energy. An increasingly popular, clean and renewable energy source is wind, and in
particular offshore wind.
Targets
In 2007 the European Union and the EWEA set the following targets. Of the total energy consumed in Europe in 2020, 20% should come from renewable sources (EU).
The goal is to let 5% of this renewable energy come from offshore wind farms. According to
the EWEA offshore wind has the potential to deliver up to 25% by 2020. This translates to an
estimated total target of 40GW from offshore wind farms. Assuming one wind turbine produces
an average of 4M W , a total of 10,000 structures is therefore required. So far, around 530 offshore
wind turbines have been placed, mainly around Denmark and Great Britain.
In order to meet these requirements, the completion of these 10,000 structures needs to be realized within 10 years. Compare this to the oil and gas industry where worldwide 7,000 offshore
structures were built in 70 years [3].
Challenges
To achieve these targets many challenges have to be overcome. The essential technical design challenge is to improve performance, particularly in terms of reliability. Other design considerations
include,
- Wind energy economics. To make wind energy (more) profitable, the high costs of fabrication
and installation must be reduced.
- Impact on the local environment. The long term effects of offshore wind turbines on local
flora and fauna are not yet known. A popular theory is that they can function as an artificial
1
Introduction
Current Wind Turbine Technology
Figure 1.1: Operational and under construction offshore wind farms around Europe.
reefs, supporting marine life. However, for birds and their flight paths, offshore wind farms
(OWFs) may be a nuisance.
- Competition for space with other marine users. A popular location to place OWFs is on sand
banks. Unfortunately, these relatively shallow sites are often also popular with fishermen.
Once a site is officially assigned to a OWF developer, it becomes a restricted area and can
thus not be accessed anymore for fishing.1 Other common objections to OWFs are the fear
of obstruction of seaways and possible interference with surface radar.
- Installation logistics. Currently there is a shortage of vessels which can be used for OWF
installation. Hence, there is a need to optimize the use of cranes and other offshore construction vessels.
- Compatibility with the European grid infrastructure.
1.2
Current Wind Turbine Technology
The conventional model for wind turbines both onshore and offshore is to install a generator in
the nacelle. Supplying each turbine with its own generator has several disadvantages.
1 Organisations such as Greenpeace actually promote OWFs to create safe-havens for marine life and combat
overfishing
DOT Project Plan
2
Introduction
Current Wind Turbine Technology
1. It demands large amounts of copper, making wind farms expensive.
2. The nacelle becomes heavy and thus requires a strong support structure.
3. Continuous efficient conversion from kinetic to electric energy requires huge amounts of
switch gear. This severely complicates the installation and maintenance.
4. Many components are leading to many failures.
To make wind energy truly beneficial requires more than incremental improvements.
Figure 1.2: Layout of the nacelle of the Vestas V90 wind turbine [8]
DOT Project Plan
3
Introduction
1.3
The Proposed Idea
The Proposed Idea
The idea proposed in this project plan is the following. Picture a wind turbine in an offshore wind
farm. We take practically everything out of the nacelle. All that remains is the idea of rotating
kinetic energy. This kinetic energy is at some stage transformed into electric energy. The whole
process in between is for us to design.
Figure 1.3: Offshore wind farm at Egmond aan Zee, 10 km from the Dutch coast.
DOT Project Plan
4
2 Design Rules
The main idea behind the DOTs is to get more energy from wind in a less labor-intensive manner.
To realize this, the focus of the design process will not only be on turbine energy production,
but also on important economical and practical issues such as the optimization of production,
assembly and maintenance.
The DOTs project aims to bring about a radical change in (offshore) wind turbine technology.
It will not be performed for standard incremental improvements. However, research will not be
limited to one specific grand design. Many of the developed concepts will also be applicable to
current wind turbine technologies. For instance, one likely target for research is the concept of
boundary layer suction along the rotor blades to reduce drag. This research will be part of the
DOTs project, but the results can just as well be applied to other (common) wind turbines.
Already in this early phase a number of initial design rules have been determined.
1. The wind turbine will have two blades. The current standard for onshore wind turbines
is to have three blades. The main reason for this is that human spectators experience three
blades as the more tranquil view. Offshore, where spectators do not have to be taken into
account, the advantages of having two blades can be exploited. Such advantages are:
- lower production costs
- easier to assemble
- higher rotational speed
2. The blades will have a fixed pitch. This simplifies the design of the rotor significantly. Theoretically, pitch regulation enables higher turbine efficiency at varying wind speeds. However,
advanced aerodynamic profiling of the blades will be applied to minimize the difference.
3. The transport of energy from the nacelle to the base will be done using a closed-loop
system of pumps and pipelines.
4. A single pipe will transport seawater from each turbine base to the generator platform.
This makes the wind farm an open-loop system.
5. The conversion from kinetic to electric energy will happen on a central platform. Hence
the cable to the shore can be plugged in directly to the power grid.
6. As few components as possible will be used to simplify the assembly.
Figure 2.1 demonstrates how these rules can be translated to an initial design concept.
5
Design Rules
Figure 2.1: A functional flow diagram of a DOT proposal
DOT Project Plan
6
3 Design Options
The design rules of chapter 2 form a platform from which to start the actual design process.
During this process, many important choices will be made, such as:
Turbine design
- The actual size of the rotor.
- Placing the rotor up- or downwind.
- How to optimize flow around the blades with a minimum of moving parts.
Support structure
- Using a truss or a monopile as foundation.
- If and how to apply the slip joint connections.
- How to maximize the ease of installation.
Pumping system/energy transmission
- What kind of pumps to use.
- If and how to install a superconductor pipeline.
- The design of the transformer station.
General design choices
- If and how to include an option to store energy.
- Whether to design the system as self-installing.
- Which materials to use.
- How to deal with marine growth.
- How to minimize and ease maintenance requirements.
7
Design Options
DOT Project Plan
8
4 How DOTs Work
4.1
Introduction
In this chapter the theory behind the DOTs is presented. The basis for the theory is the initial
concept design in figure 2.1.
To review this design piece by piece, the entire system is split-up into the following subsystems:
A The rotor, which turns as a result of wind blowing.
B The closed-loop system, which transfers power from the rotor shaft in the nacelle, to the
base of the structure. Its main components are:
• pump A, which is directly driven by the rotor shaft. Its function is to induce a flow in
pipe 1.
• pipe 1, where fluid flows from the nacelle to the base.
• motor B extracts mechanical power from the pressure flow in pipe 1 at the base of the
structure.
Figure 4.1: Identification of the subsystems of the DOT concept from figure 2.1
9
How DOTs Work
Power Transmission: Wind to Water to Electric
C The open-loop system, where seawater is pumped from the base of a structure to the generator platform through pipe 2. Pump C is directly powered by motor B.
D The generator platform, where the seawater flow from all pipes 2 is collected and used to
generate electricity.
An overview of these subsystems is given in figure 4.1.
Section 4.2 presents the theoretical power transmission throughout the entire system. The initial
simulation results are presented in section 4.3. The (potential) initial conditions of this simulation
are defined in table 4.2.
4.2
Power Transmission: Wind to Water to Electric
4.2.1
A: The Rotor
Figure 4.2: Subsystem A: the rotor
Conventional turbines have three blades with variable pitch. The rotor of a Delft Offshore Turbine
(DOT) will have two blades with fixed pitch.
The power Protor extracted from the wind is a function of the wind speed v, the rotor radius r,
the air density ρair and the induction factor cp .
Protor = cp ·
1
· ρair · π · r2 · v 3
2
(4.1)
The induction factor cp is in fact a pressure coefficient. Its theoretical maximum value is 0.59.
This is known as the Betz Limit [6]. The latest designs of turbine blades have inductions factors
larger than 0.50. As part of the DOT project, research will be done on the application of boundary
layer suction, to further increase the cp .
The rotation of the rotor gives the rotor shaft a torque T at a rotation rate ω.
Protor = T · ω
DOT Project Plan
(4.2)
10
How DOTs Work
4.2.2
Power Transmission: Wind to Water to Electric
B: The Closed-Loop System
(a) layout sketch
(b) functional diagram
Figure 4.3: Subsystem B: the closed-loop system
Pump A
For conventional turbines, electricity is already generated in the nacelle. A system of gears and
a generator converts the power in the rotor shaft to electric power. The electricity is conducted
through a cable, along the base of the turbine support structure, to a generator platform.
The closed-loop system of the DOT (see figure 4.3) is in fact a hydraulic gear which transmits the
power from the top of the wind turbine to the base. Pump A is directly linked to the rotor shaft.
Its purpose is to generate, as efficiently as possible, high pressure in pipe 1.
The specific pumping technique has yet to be determined. A likely candidate for pump A is a
radial piston-design unit. Currently, the most power P any hydraulic unit is able to transfer is
around 1.75M W (the Hägglunds MB 3200 [12]). However, these units can be applied in parallel
to cope with higher torque/power.1 Despite their relative small size, such hydraulic units are
designed to operate under pressures of up to 350bar with high efficiencies (ηpump > 95%).
PpumpA = ηpumpA · Protor
(4.3)
Essentially, what pump A does is initiate a volume flow Q at high pressure p in the closed-loop
pipe. At the end of the entire system is the generator. The harder it is to turn the rotor of a
generator (the more torque is required), the more power it produces. The torque required to power
the generator determines the pressure in the open-loop system. The pressure and the volume flow
of the fluid in the open-loop system determine the torque that motor B is required to produce. The
1 The rotation rate of hydromotors is in the same order as the rotation rate of the rotor shaft. Hence, no gearbox
is required. The MB 3200 operates at a maximum of 16rpm. The popular Vestas V90 turbine (3.0M W ) operates
in the range of 8.6 − 18.4rpm.
DOT Project Plan
11
How DOTs Work
Power Transmission: Wind to Water to Electric
torque motor B is required to produce determines the pressures in the closed-loop system. Hence,
the torque required to power the generator can be translated into flow resistance throughout the
entire system. This flow resistance determines the pressure.
Within pump A, the height difference ∆z (head) is assumed to be zero. So, the pressure change
over pump A in the nacelle is,
∆p = phigh − plow
(4.4)
Pipe 1
Pump A generates a flow in pipe 1 at the nacelle. Assumptions with respect to flow through a
pipe:
• the fluid inside is incompressible
• the inner surface of the pipe is smooth (roughness factor e = 0)
From equation 4.5, it is evident that for constant power P , the volume flow Q decreases as pressure
change ∆p increases.
PpumpA = ∆p · Q
(4.5)
The volume flow Q in a pipe is a function of the pipe diameter D and the flow velocity v of the
fluid.
A=π·
D
2
Q=A·v
2
(4.6)
(4.7)
An overview of how several flow characteristics change for different ranges of power is given in
table 4.1. Here, Dmin is the minimal pipe diameter for laminar flow (< 6m/s).
Power P
1.7 M W
3.0 M W
5.0 M W
10.0 M W
Pressure p
350 bar
350 bar
350 bar
350 bar
Volume flow Q
0.049 m3 /s
0.086 m3 /s
0.143 m3 /s
0.286 m3 /s
2910 l/min
5140 l/min
8570 l/min
17100 l/min
Dmin
0.102 m
0.135 m
0.174 m
0.246 m
4.00
5.31
6.85
9.69
in
in
in
in
Table 4.1: An overview of flow characteristics for different turbine power capacities.
Losses Due To Friction
To gain high efficiency, friction must be minimized wherever possible. The pressure loss due to
friction in the pipeline is related to the value of the Reynolds number Re of the transported fluid.
Re =
v·D
ν
(4.8)
Here ν is the kinematic viscosity.
DOT Project Plan
12
How DOTs Work
Power Transmission: Wind to Water to Electric
The friction factor f for flow through a pipe is derived from references [5] and [7]. A distinction is made between disturbed (turbulent) and undisturbed (laminar) flow (see figure 4.4).
• For laminar flow (Re < 2300 in pipes):
f=
64
Re
(4.9)
• For turbulent flow (Re > 4000 in pipes):
1
√ = −2 · log
f
2.51
e/D
√
+
3.7
Re · f
(4.10)
Turbulent flow is undesirable because within a pipe the flow is subject to much more friction than
in the case of laminar flow.
(a) Laminar flow
(b) Turbulent flow
Figure 4.4: The two types of flow through a pipe
The loss of pressure due to friction is modeled as,
ploss = f ·
L 1
· · ρf luid · v 2
D 2
(4.11)
Notice that the length of the pipe L is directly proportional to the pressure loss.
Motor B
At the base of a DOT, motor B uses the pressure difference in pipe 1 to generate mechanical
power. The low pressure part of the closed-loop pipeline experiences the same head ∆z as the
high pressure part. So, the pressure change over motor B at the base is the same as at pump A
(equation 4.4) minus the losses in pipe 1 (equation 4.11).
∆p = ∆p − ploss
(4.12)
This pressure change is converted to mechanical power by motor B.
PmotorB = ηmotorB · ∆p · Q
(4.13)
The extracted power PmotorB is used to drive pump C.
DOT Project Plan
13
How DOTs Work
4.2.3
Power Transmission: Wind to Water to Electric
C: The Open-Loop System
Figure 4.5: Subsystem C: the open-loop system
Pump C
At the base of the structure, power from the closed-loop system in pipe 1 is used by pump C
to pump seawater into a second pipe. The open-loop system hence uses seawater as a means to
transfer energy from the base of a DOT to the generator platform.
The functioning of pump C is similar to pump A. It is mechanically powered by motor B and
it generates pressure in the open-loop pipe 2.
PpumpC = ηpumpC · PmotorB
(4.14)
Pipe 2
Pipe 2 connects the base of a DOT to the central generator platform (see figure 2.1). As with
pipe 1, the power PpumpC from pump C can be expressed in terms of the pressure change ∆p and
the volume flow Q.
PpumpC = ∆p · Q
(4.15)
The radius r and hence the area of the cross section A is likely to be larger than that of pipe
1. However, the main difference between the two pipes is that the length of pipe 2 will be much
greater. Using the exact same method as with pipe 1, the losses due to friction can be found.
4.2.4
D: The Generator Platform
Figure 4.6: Subsystem D: the generator platform
In DOT farms, all (N ) pipes 2 come together at the generator platform. Here the pressurized
seawater is distributed over several generator turbines. As with motor B, the power extracted by
DOT Project Plan
14
How DOTs Work
Simulation Results for Varying Wind Speeds
a generator is a function of the pressure change ∆p over it, and the volume flow Q through it.
Assuming no further losses before the generator turbines, the total power generated in the wind
farm is expressed as:
Pgen = ηgen · N · ∆p · Q
4.3
(4.16)
Simulation Results for Varying Wind Speeds
To better understand the (theoretic) performance of a DOT, a numeric input is given to the theory
of the previous section. The wind speed is varied from 0 to 20m/s. All other initial conditions
are derived from existing techniques. They are summed up in table 4.2.
Parameter
general
νsw
νf l
ρsw
ρf l
rotor
cp
r
pumps
ppumpA
ppumpC
ηpumpA
ηmotorB
ηpumpC
pipes
D1
D2
L1
L2
pmin
generator
ηgen
Value
Units
Description
1.17E − 6
4.20E − 4
1030
1000
[m2 /s]
[m2 /s]
[kg/m3 ]
[kg/m3 ]
0.50
63.0
[−]
[m]
induction factor
radius
35.0E6
10.0E6
0.95
0.90
0.80
[P a]
[P a]
[−]
[−]
[−]
pressure generated by pump A
pressure generated by pump C
efficiency pump A
efficiency motor B
efficiency pump C
0.30
0.50
100
500
2.0E6
[m]
[m]
[m]
[m]
[P a]
diameter pipe 1
diameter pipe 2
length pipe 1
length pipe 2
minimum pressure in pipe 1
0.90
[−]
efficiency generator
kin. viscosity seawater at 15◦ C
kin. viscosity of fluid in pipe 1
density of seawater
density of fluid in pipe 1
Table 4.2: Initial parameter values
The two main output parameters of interest are:
- the power production of a DOT
- the volume flow of seawater into the generator platform from pipe(s) 2
The amount of water flowing into the generator station is dependent on the pressure in the openloop pipe 2. As section 4.2 explains, for the purpose of efficiency, it is beneficial to use high
pressure and subsequent low velocity flow. For flow calculations (figure 4.7), the critical value of
the Reynolds number Re is assumed as 2800.
DOT Project Plan
15
How DOTs Work
Simulation Results for Varying Wind Speeds
−1
10
Pipe No.1
Pipe No.2
Friction factor f [−]
Laminar
Turbulent
−2
10
3
4
10
5
10
6
10
10
Re = v⋅D/ν [−]
Figure 4.7: The friction factor versus the Reynolds number for flow regimes in both pipes.
Figure 4.8 shows the power curves of a DOT (unrestricted) and the REpower 5M [13].2 Both
turbines have the same rotor diameter.
7
P
rotor
P
pump
6
B
Power [MW]
P
gen
5
REpower 5MW
4
3
2
1
0
0
2
4
6
8
10
12
Windspeed [m/s]
14
16
18
20
Figure 4.8: Power curves of the DOT and the REpower 5M
In figure 4.9 the volume discharge is plotted against the wind speed. For the (assumed) rated
wind speed of 13m/s the volume flow of seawater to the generator is approximately 650liters/s.
In every energy generating system, some energy is lost on its way to the consumer. A percentage
of the DOT’s kinetic energy at the rotor is lost on its way to the generator platform. Most of
these losses will be due to friction in the pipelines. The key to increasing efficiency in theory is to
improve the pumping systems and minimize losses in the pipelines.
The key to increasing efficiency in practice is to reduce maintenance and increase uptime.
2 The REpower 5M is the largest turbines currently in operation. So far these turbines have been installed at
offshore locations near Scotland (Beatrice) and Belgium (Thornton Bank).
DOT Project Plan
16
How DOTs Work
Simulation Results for Varying Wind Speeds
2.5
Pipe No.1
Pipe No.2
3
Discharge Q [m /s]
2
1.5
1
0.5
0
0
2
4
6
8
10
12
Windspeed [m/s]
14
16
18
20
Figure 4.9: The volume flow in the open-loop and closed-loop pipelines of a DOT.
DOT Project Plan
17
How DOTs Work
DOT Project Plan
Simulation Results for Varying Wind Speeds
18
5 Plan of Execution
5.1
DUWIND
The Delft University of Technology has joined together the research groups involved in wind energy research into the Delft University Wind Energy Research Institute, DUWIND. This institute
envelops sections of 5 of the 8 faculties of the DUT as shown in figure 5.1. DUWIND totals 60
Figure 5.1: Overview of participants in DUWIND
FTE of whom 30 FTE are PhD students. The topics covered range from offshore engineering,
aerodynamics, aeroelastics and material engineering to control systems, generators, grid and policy. DUWIND is one of the largest institutes on wind energy in the world with a particularly large
focus on offshore wind energy.
DUWIND drafted its first research plan in 2002 with a scope of 15 PhD students on all new
topics. This resulted in the current double of that in 2008. At this moment, the new research plan
is being finished with a goal for the next 5 years to reach 60 PhD students. Within this plan, the
DOTs also has its place. It has been identified as a showcase of combining scientific knowledge
and bring it to the market.
5.2
DOTs Organization
With the DOT project being an integrated part of DUWIND, the DOT team members will reside
under the different sections within DUWIND. Each PhD student will have a specialist professor to
act as coach and promotor. The PhD students will participate in their specific section in research
and education. On top of that, the group of 8 will also have a shared work place to increase
the team spirit and focus the group attention to reaching their common goal. The group will
furthermore be supervised by 1 supervisor, who is responsible for the day-to-day co-ordination of
the DOT project progress. Figure 5.2 shows the team structure organization chart.
19
Plan of Execution
Time Planning
Figure 5.2: DOT Team structure/organogram
5.3
Time Planning
The project started with the first PhD student, Niels Diepeveen, commencing his work on 1 August
2008. This detailed plan is the first work from his hand in shaping the DOT process. Over the next
months, financing needs to be secured for the project and new PhD students are being recruited.
The plan is to start full force in January 2009. The typical PhD study time has been set to 3
years, which is somewhat shorter than the ”normal” Delft duration. The PhD students will start
consecutively over a 1.5 year period, stretching the DOT project from 1-8-2008 to 1-8-2012 as
shown in figure 5.3.
Figure 5.3: Gantt chart
The goal of the DOT project is to develop all components for the new offshore wind turbine
technology. The combined outcome of the 8 PhD projects is a blueprint for the construction of
a first demonstration offshore wind farm. The project therefore does not end with the last PhD.
Early 2012, efforts will start to acquire funding for a demonstration farm in the order of magnitude
of 50 - 100 M e, to be constructed in 2013/2014. Following this planning, the DOTs will be ready
for commercial application in the second half of the next decade, exactly when the exponential
increase in offshore wind turbine installations is anticipated. As the goal of the DOT is not only to
create an entirely new wind turbine system, but also be unrestricted in component improvements
DOT Project Plan
20
Plan of Execution
Potential PhD Research Projects
that can be re-applied in current turbine technology, the result of the project will give a boost to
the wind energy industry as a whole.
5.4
Potential PhD Research Projects
The DOT project will be realized by 8 PhD students, each having their own research topic. These
topics will be detailed further as all PhD students start their work during the next 6 months. For
the moment, the following areas of research have been identified:
Aerodynamics (2 PhDs)
The design of the rotor is critical for the performance of a wind turbine. Possible topics include
boundary layer suction (to reduce drag of the blades) and the effects of stall behavior.
Hydraulics (2 PhDs)
The central theme in the DOT project is the use of hydraulics to conduct energy to a central
electricity generator. The main items in this system are the pumps and the pipes.
Mechanics (2 PhDs)
Of vital importance to the success of the DOT project are the required installation methods and
the overall energy balance. Both will require extensive research.
Support Structure Design (1 PhD)
So far, the design of offshore support structures for wind turbines is largely based on norms set
by the oil & gas industry. Possible topics include structural materials and soil mechanics.
Electronic Engineering (1 PhD)
Eventually, the mechanical power generated by the wind at the rotor of a DOT has to be converted
to electricity. This requires the design of the generator platform and all its components.
DOT Project Plan
21
Plan of Execution
DOT Project Plan
Potential PhD Research Projects
22
References
[1] European Wind Energy Association, Pure Power: Wind Energy Scenarios up to 2030,
March 2008.
[2] European Wind Energy Association, Delivering Offshore Windpower in Europe: Policy
Recommendations For Large-Scale Deployment Of Offshore Wind Power In Europe By 2020,
December 2007.
[3] Veldman, H., Lagers, G., 50 Years Offshore, Foundation for Offshore Studies, Delft, 1997
[4] Anderson JR., J.D., Fundamentals of Aerodynamics, Second Edition, 1991
[5] Battjes, J.A., Fluid Mechanics, Lecture Notes, Delft, March 2000
[6] Betz, A., Das maximum der theoretisch möglichen Auswendung des Windes durch Windmotoren, Zeitschrift für gesamte Turbinewesen, vol. 26, 1920
[7] Colebrook, C.F., Turbulent Flow in Pipes, Journal of the Inst. Civil Eng. (11), 1938
[8] www.vestas.com
[9] www.randstad380kv.nl
[10] www.ewea.org
[11] www.efunda.com
[12] www.hagglunds.com
[13] www.repower.de
23
Delft Offshore Turbines
Re-designing wind turbine technology for more
power, better performance and better economy
Wind energy is booming. The focus is
The DOTs project aims to bring about a
moving to offshore production. Current
radical change in (offshore) wind turbine
technology
technology. It will not be performed for
is
far
from
optimal.
standard incremental improvements.
The main idea behind the DOTs is to get
more energy from wind in a less labor-
Research will not be limited to one specific
intensive manner. The focus of the design
grand design. Many of the developed
process will be on wind farm energy
concepts will also be applicable to current
production, but also on important
wind turbine technologies.
economical and practical issues such as
the optimization of production, assembly
For more information, visit us at:
and maintenance.
www.offshore.tudelft.nl/offshorewind
Design rules
Two rotor blades - lower production cost easier assembly, higher rotational speed
The blades have a fixed pitch angle and use boundary layer suction (Actiflow)
Transport of energy from nacelle to base through closed-loop hydraulic system.
A single pipe transports seawater from each turbine base to the generator
Conversion to electricity happens on a central platform
As few components as possible
rotor
Functional diagram
Pumping fluid at high pressure in
combination with low volume displacement
allows for highly efficient energy transfer.
S o - c a l l e d f l u i d p o w e r i s a p r ove n
technology in many industrial sectors.
low pressure
high pressure
pump
generator
platform
Civil Eng
Mechanic Eng
power to shore
motor
Electrical Eng
Tech Management
Aerospace Eng
seawater
pump
high pressure
The DOT team members will all be PhD students,
residing under different sections within DUWIND.
Part II
Conference Paper EWEC
2009
roman
CLOSED-LOOP FLUID PUMPING AS A MEANS TO TRANSFER WIND ENERGY
N.F.B. Diepeveen
DUWIND, Faculty of Civil Engineering and Geosciences, Delft University of Technology
Stevinweg 1, 2628 CN Delft, The Netherlands
Tel.: +31 15 27 88030, E-mail: n.f.b.diepeveen@tudelft.nl
SUMMARY
The current standard for wind turbines is to have a generator placed in the nacelle. The Delft
Offshore Turbines project aims to circumvent the need of the generator by using the rotor shaft
torque to power a pump in the nacelle. This pump adds pressure to the liquid in a closed-loop
pipe circuit, creating a flow. At the base of the offshore wind turbine, the pressure (energy) is
taken out of the flow by a motor. Most of the energy losses between the nacelle and the base of
the OWT will occur due to friction in the closed loop pipeline and mechanical & volumetric
losses in the hydraulic drive systems.
The success of this new turbine concept depends in part on the efficiency of the energy transfer.
Considering the environment they are placed in, the drive systems need to be efficient, robust
and requiring low maintenance. This paper presents the modelling results of a 5MW DOT
concept which applies closed-loop fluid pumping as a means to transfer wind energy. The key
elements in this system are the hydraulic drive systems, the hoses between them and the power
fluid. Since suitable drive systems for this sort of multi-MW application does not yet exist,
modelling is done using extrapolated properties of systems that do.
This paper identifies significant design challenges and required system properties of a 5MW
offshore hydraulic wind turbine by modelling the concept of a closed-loop fluid power circuit. The
resulting general characteristics are compared to those of traditional offshore wind turbines.
Keywords: Fluid power, hydraulic drive systems, closed-loop
1
INTRODUCTION
A new concept for the design of offshore wind
turbines/farms incorporates the idea of using
fluids to transfer energy.
The current standard for wind turbines is to
have a generator placed in the nacelle. The
need for additional support systems results in
a large nacelle mass. This and frequent
component failures are not beneficial to the
economics of offshore wind farms.
-
losses. The high availability should
however lead to a large overall
increase in relative power production.
Easy installation
Low production costs
One design concept in which this can be
applied is to split the wind farm components
in two types of systems.
1. The closed-loop hydraulic wind
turbine. The pump in the nacelle adds
pressure to the flow in the circuit,
creating a power flow. At the base of
the offshore wind turbine, the
pressure is taken out of the flow by a
motor and converted to mechanical
energy.
2. The open-loop hydro-power system.
The motor at the base drives a
second pump which pumps freestream seawater to a central power
hub where this open-loop power flow
is converted to electricity.
The Delft Offshore Turbines (DOTs) project
aims to circumvent the need of the generator
by using the rotor shaft torque to power a
pump in the nacelle. This, along with the
proposal to have a two-bladed rotor, will lead
to a significant reduction in weight of the
rotor-nacelle assembly.
The overall goal of the Delft Offshore
Turbines project is to design a wind turbine
infrastructure
specifically
for
offshore
purposes and thereby rendering offshore
wind energy more economically attractive.
This means:
- Very low maintenance
- High availability; as a direct
consequence of high reliability.
- Reasonable efficiency;
hydraulic
drive systems often experience small
Fluid power circuits have been applied
successfully on different scales in many
industries.
The idea of applying this method to wind
turbines is not new. Literature on the topic
1
can be found from as early as 1981 [2]. So far
a successful launch has not yet occurred
mainly because the required components are
were not readily available. They still aren’t for
multi-MW systems.
The aim of this paper is to identify significant
design challenges and required system
properties of a 5MW offshore hydraulic wind
turbine by modelling the concept of a closed1
loop fluid power circuit. The resulting general
characteristics are compared to those of
traditional offshore wind turbines.
Rotor
increased chord and blade thickness, same
relative thickness. A thicker blade is a
stronger blade, hence less structural material
is required, making the rotor lighter. This
reduction in weight means the rotor has a
lower moment of inertia. This leads to
relatively higher angular acceleration and
consequently a higher rotational velocity.
Number
of
blades
Rotor diameter
Rated
wind
speed
Rated power
Rated
rotor
speed
Pump
Repower 5M
3
DOT
2
126m
13.0 m/s
126m
13.0 m/s
5 MW
12.1 RPM
5 MW
18.15 RPM
Table 1: Turbine characteristics
Fluid Power Circuit Characteristics
The main components of the circuit are [1]:
- a pump;
- a high pressure hose
- a motor to extract the power from the
flow
- a low pressure hose
- a combined cooling/boosting system
which
o keeps the temperature of the
fluid in the system below a
predefined maximum
o keeps the pressure at the
entrance of the pump at the
required level.
For multi-MW (>2MW) turbines, suitable
pumps/motors are not yet commercially
available. There however appears to be no
technical reason why they have not yet been
produced.
Motor
Pump
Seawater
To central
power hub
Figure 1: DOT functional diagram
2
SYSTEM DEFINITION & MODELING
Turbine Characteristics
Initially, the modelling of the DOT is done with
reference to an existing wind turbine, the
Repower 5M. The DOT is given the same
rotor diameter as the Repower, 63m. The
main difference is that the DOT has 2 blades
instead of three. They also share the same
rated wind speed.
The Delft Offshore Turbine will have two
blades to improve ease of installation and
reduce rotor mass. The size and shape of the
two blades is such that together they produce
the same amount of lift (& torque) as the
three-bladed rotor of the Repower.
The necessary blade area is distributed over
two blades instead of three. This results in an
Drive systems: pumps & motors
There are many types of hydraulic drives. For
these types of systems, the positive
displacement pump is most suitable. Criteria
for selecting a pump:
1. General purpose - the general
purpose of the pump is to transfer
energy through high pressured flow
as efficiently as possible whilst
bringing a significant reduction in
weight to the nacelle.
2. Amount of the fluid - this depends on
the size of the rotor (wind turbine) ,
the nominal operating pressure and
the length & diameter of the hoses.
3. Fluid properties – in this early stage
hydraulic oil is used.
1
Note that this paper presents the modeling of one possible
concept and by no means the final design.
2
4. Required head - again, this depends
on the rotor size.
5. Specify type of flow – preferably
laminar, to maximize efficiency.
6. Power supply - wind (the rotor)
7. Cost - not yet considered.
8. Efficiency - this is one of the most
important properties. Slightly lower
efficiencies are acceptable if required
maintenance is significantly less.
9. Cost compared to efficiency – not
relevant at this stage
10. Lifespan - minimum of 20 years
11. Noise level - for offshore application
noise is not considered as a form of
hindrance, only as a means of energy
loss.
12. Operating pressure - as high as
realistically possible. This is one of
the most important properties.
13. RPM - this will be a dynamic signal,
whose exact nature is to be
determined at a later stage.
demonstration, the motor has a swept volume
5 times smaller than the pump. The result of
this is that the motor will turn 5 times faster
than the pump.
Constant swept volume
Previous research has been done on the
application of variable stroke/swept volumes
[6].
This way the pressure in the high pressure
hose can be kept as high as possible to
minimize flow velocity and thus maximize
efficiency (higher pressure difference = lower
volume flow).
(1)
P = ∆p ⋅ Q
For example, at the start-up, when the rotor
begins to turn, the swept volume of the pump
and/or the motor is very small. Hereby a very
low velocity, high pressure flow is initiated,
which is beneficial in terms of efficiency.
For the system described in this paper
however, the drive systems will have constant
swept volumes albeit of different magnitudes.
The main reasons for this are that:
- The drive systems will need less
moving parts
- The reduction in efficiency will only
occur at lower wind speeds where it
will be minimal.
At start-up, the pressure throughout the
system will be at the charge-level. Once the
rotor begins to spin, the pressure in the
system builds up very quickly. Figure 5 shows
that the losses at start-up are minimal.
Displacement
Rated
Speed
Max. speed
Max pressure
Max torque
Rated power
Max power
For the modelling of the drive systems, the
characteristics of the Hägglunds’ Compact
CB series have been used [4]. These
systems are also known as radial pistondesign units. They can operate efficiently
(>95%) at high pressure.
Vi
nrated
nmax
pmax
Tmax
Pra
Pm
ted
ax
l/rev
rpm
rpm
bar
kNm
Pump
472
18.1
27.2
350
2480
M
W
5.0
M
W
7.1
Motor
118
72.6
108.9
350
620
5.0
7.1
Pipe/Hose-flow modelling
The standard formula for pressure loss in a
pipe or hose is given by equation 2 [5].
ploss = f ⋅
L 1
⋅ ⋅ ρ ⋅ v2
D 2
(2)
To minimize losses the velocity needs to be
as low as possible. Using the distance
between the hub and the base and the rated
capacity, the optimal pipe/hose diameter can
be determined. This optimal diameter is
chosen for a flow velocity at rated power
where the flow is in the laminar to turbulent
region, where the friction factor is minimal
(see Figure 6).
As base case a hub height of 90 m and a
hose diameter of 0.3 m are taken.
The power fluid of choice in this phase is a
type of hydraulic oil, which is assumed to be
incompressible. The hose itself is assumed to
be hydraulically smooth [3].
Table 2: Properties of modelled drive systems
with a gear ratio of 5
Since no system of this series is (yet)
available in the 5MW range, all relevant
system properties have been extrapolated to
meet requirements. The properties of these
non-existent drive systems are listed in Table
2: Properties of modelled drive systems.
The stroke volume of the pump is determined
by dividing the rated power by the maximum
pressure and the rated rotational velocity.
The motor at the other end of the circuit is the
same type of system, but used in reverse and
has a different stroke volume. The effect of
this is that the whole system functions as a
gear for the rotor shaft. For the purpose of
3
3. Once the maximum RPM is reached
the flow of the wind around the
blades is manipulated to also keep
the rotation rate of the rotor shaft and
therefore the power output at a
constant maximum. At the point
where the wind speed is too high for
the power output to remain at it’s
maximum, the blades will stall and
the system shuts off.
8
p
hydraulic turbine
traditional turbine
7
max
,n
max
power [MW]
6
pmax ,nnom
5
4
3
2
1
0
M
Cooling & charge pressure
Closed-loop
fluid
power
circuits
produce heat due
to internal friction
and thus require
cooling.
This
problem is initially
solved by including
a
drainage
reservoir
which
(together with a
small storage tank)
is connected to an
extra pump. This
pump adds charge
pressure
and
cooled fluid to the
low pressure hose
of the circuit. The
charge
pressure
before the pump
prevents cavitation
and increases the
pump’s efficiency.
0
5
10
15
wind speed [m/s]
20
25
30
Figure 3: The power curve of a traditional and
a hydraulic turbine
This three-step modulation leads to a different
look of the wind turbine power curve, as
demonstrated in Figure 3. The upper limit of
the power curve is not determined by
electrical components. Instead the maximum
allowable pumping conditions (pmax,nmax)
define the shape of the curve.
This
necessary
Figure 2: CL circuit
extra system does
however mean that
with cooling/boosting
the
amount
of
required components increases significantly.
It also reduces the overall efficiency of the
system, but it is a necessary requirement to
compensate for small leaks and over-heating.
Whether the subsequent heat dissipation is
sufficient requires further investigation.
400
pmax ,nnom
350
force [kN]
300
250
200
150
100
3
SIMULATION RESULTS
Using the system definition, a simulation
model was constructed in MATLAB to
analyze general characteristics.
Load modelling
1. For a constant angle of attack, the lift
and drag forces of blades are directly
proportional to the wind velocity
squared. Hence the torque produced
by the rotor is also directly
proportional to the wind velocity
squared. Since the power in the rotor
is directly proportional to the third
power wind speed, the rotational
speed of the rotor shaft is directly
proportional to the wind speed.
2. After vrated, the torque produced by
the blades may not increase, only the
RPM. This is done by changing the
flow around the blades.
50
0
0
5
10
15
wind speed [m/s]
20
25
30
Figure 4: The rotor force
The same maximum force (fFigure 4) occurs
at the rated wind speed. This is where the
pump performs nominally (pmax,nnom). The
flow around the blades is now regulated to
avoid exceeding the maximum pressure until
the maximum rotation rate of the pump is
reached. From this point (pmax,nmax) the power
output is kept constant until the rotor blades
stall. Figure 5 demonstrates:
- The build-up of pressure difference
between the input of the pump and
the input of the motor. The pressure
build-up ∆p is directly proportional to
the rotor torque build-up.
4
The increase in volume flow Q
increases as long as the rotor RPM
increases.
The efficiency of the system is high
for all rotor speeds except instantly
after start-up.
-
Q [m/s 3]
∆ p [bar]
-
number of moving components without
adding significant benefits and is therefore
not desired.
Despite the additional mass of the hoses and
power fluid, having only a pump in the nacelle
significantly reduces the total mass of the
wind turbine. For concept evaluation
purposes, further research needs to be done
on the reduction of the nacelle mass and
subsequently the support structure mass.
The necessity of a subsystem for cooling &
pressurizing adds significantly to the total
number of components.
A 5 MW turbine will require a volume flow of
close to 10,000 litres per minute.
The closed-loop fluid power circuit as
described here requires the use of hydraulic
oil. For future concept designs the use of
seawater as power fluid will be addressed.
Since the idea behind the DOTs project is to
design a turbine specifically for offshore
purposes, oil will not be the preferred power
fluid in the long run. Instead we aim to use
the offshore environment to our advantage
and use seawater. Research will therefore be
done on the design requirements for
seawater-based (wind turbine driven) fluid
power circuits.
400
200
0
0
5
10
15
20
25
30
35
40
45
0
5
10
15
20
25
30
35
40
45
0
5
10
15
30
35
40
45
0.4
0.2
0
η [-]
1
0.5
0
20
25
nrotor [RPM]
Figure 5: The rotor RPM vs. pressure change,
volume flow & efficiency
2
10
1
f [-]
10
0
10
-1
10
REFERENCES
[1] Cundiff JS. Fluid Power Circuits and
Controls, Fundamentals and
Applications. Virginia Polytechnic
Institute & State University Blacksburg.
[2] Unknown. Hydraulic Wind Energy
Conversion. Jacobs Energy Research,
Audubon. July, 1981.
[3] Albers PS, et al. Vademecum
Hydrauliek. Koopman & Kraaijenbrink
Publishing. September 2008.
[4] Hägglunds. Compact CB. Product
Manual.
[5] Batjes JA. Fluid Mechanics. Lecture
Notes, Delft University of Technology.
[6] Rademakers, LWMM. Possibilities of
Variable Transmissions in Wind
Turbines. MSc Thesis. Laboratory of
Power Transmission, Eindhoven
University of Technology
-2
10
-1
10
0
10
1
2
10
10
3
10
4
10
Re [-]
Figure 6: The Reynolds number vs. the
friction factor for flow in a straight DOT hose
4
PRELIMINARY CONCLUSIONS
From researching and the modelling of this
initial concept a number of conclusions can
be drawn.
Hydraulic drive systems have been applied in
many industries for many years. High
performance systems are characterized by
high efficiencies and low maintenance needs.
The power production of a Delft Offshore
Turbine can increase beyond rated due to the
characteristics of hydraulic drive systems.
The cut-out wind speed is determined by the
control characteristics of the rotor (blades).
The current maximum size of suitable pumps
is under 2 MW and requires the use of
hydraulic oils as power fluid. Larger systems
are technically feasible. They have not yet
been produced due to lack in demand.
The efficiency of the system can be slightly
improved by using more sophisticated drive
systems. However, this would increase the
5
Part III
Conference Paper EOW
2009 [DRAFT]
roman
Design Considerations for a Wind-Powered
Seawater Pump
N.F.B. Diepeveen
DUWIND, Faculty of Civil Engineering and Geosciences,
Delft University of Technology
Stevinweg 1, 2628 CN Delft, The Netherlands
Tel.: +31 15 27 88030, E-mail: n.f.b.diepeveen@tudelft.nl
Summary
Offshore, one thing is abundant: water. The current turbine technology sees the nacelle weight increase steadily giving increasing challenges in
support structure design and installation. Furthermore, power electronics
help harness wind power slightly more efficiently, but also add weight and
components (that can fail) to the turbine system.
The Delft Offshore Turbines (DOTs) convert wind energy into a high
pressure flow of water. A pump is connected to the rotor directly, generating a high pressure flow. The pressurized water is collected at a
transformer platform, where generators are located comparable to a hydro
plant. The platform can be fitted with limited water storage/accumulation
capacity to smoothen energy variations. From this platform, an electricity
cable connects to the onshore grid.
High pressure fluid power is used in shredders, feeders, roll mills,
cranes, bulldozers, jack-up systems, etcetera. These applications efficiently acquire power in the form of high torque through high pressure
(op to 500 bar) fluid transmission. The DOT energy transmission concept is the exact opposite. High torque is converted into a high pressure
flow.
Currently ChapDrive AS in Norway, Artemis in Scotland and Voith
Turbo (WinDrive) in Germany are all developing hydraulic gears for wind
turbines. The most similar to the DOTs project is the ChapDrive, having
its generator placed at the foot of the turbine tower. These systems all
use hydraulic oil as medium.
Seawater is preferable in to hydraulic oil in terms of dynamic performance. This is due to the higher bulk modulus of seawater. However, low
viscosity of seawater also means poor lubricity and high potential of wear
due to erosion and corrosion.
The research presented in this paper shows that multi-MW high pressure seawater pumps do not yet exist. The main reason for this is that
there has never been a real need for them. To make the DOTs a reality,
such a pump will need to be designed. The main challenge is how to
design for the use of seawater as hydraulic fluid.
1
Contents
1 Introduction
2
2 Delft Offshore Turbines
2.1 General Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Energy Transmission System Requirements . . . . . . . . . . . .
2.3 Motivation for Using Seawater as Hydraulic Fluid . . . . . . . . .
4
4
5
6
3 Fluid Power Circuits
3.1 Introduction to Fluid Power . . . . .
3.2 Basic circuit components . . . . . . .
3.3 Hydraulic Fluids . . . . . . . . . . .
3.4 Classification of Pumps . . . . . . .
3.5 Applications of Fluid Power Systems
6
6
6
7
8
8
4 Hydraulic Wind Turbines
4.1 Early Ideas . . . . . . . . .
4.2 Current Developments . . .
4.3 Advantages & disadvantages
wind turbines . . . . . . . .
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9
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5 Seawater as Hydraulic Fluid
10
5.1 Basic Properties of Seawater . . . . . . . . . . . . . . . . . . . . . 10
5.2 Systems for seawater pumping . . . . . . . . . . . . . . . . . . . . 12
6 Selection of Pumping Principle
13
6.1 Design Criteria for Seawater-based Positive Displacement Pumps 13
6.2 Characteristics of PD Pumps . . . . . . . . . . . . . . . . . . . . 13
6.3 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7 Conclusion
15
References
16
1
Introduction
aanleiding
The EWEA has set the target for an installed capacity of 40 GW for offshore
wind turbines in 2020 []. Some of the main obstacles in achieving this target
are the costs of the offshore support structures, installation and maintenance.
According to critics [], the root of the problem is that the turbines being installed
offshore were not initially designed for this environment.
The goal of the Delft Offshore Turbines (DOT) project is to design and
build a wind turbine (5+ MW) specifically for the offshore situation, thereby
stepping away from incremental improvements. This goal translates directly into
the driving project requirements: easy installation, robustness/low maintenance
and high efficiency energy conversion. Eventually the implementation of these
requirements will lead to lower IRR for offshore wind farms. To achieve these
demands, the working strategy is to minimize the total number of systems,
2
minimize the use of expensive materials and where possible use the elements to
our advantage.
One of the major differences in the DOT design with respect to current
mainstream wind turbine technology is the energy conversion method. Instead
of applying electricity generators and all related components, the DOT will use
a pump to convert wind power to high pressure flow of seawater. In other words,
the idea for the energy conversion is to transmit power through a seawater-based
fluid power system to a central generator platform in the wind farm.
probleem
Fluid power systems are used in many industries throughout the world. What
makes the application different for the DOT is the fact that it is a multi-MW
wind-driven and that the power transmission medium is seawater.
The basic components of any fluid power circuit are: a pump (mechanical
to power flow, a motor/generator (power flow to mechanic/electric), conductors
(pipes or hoses) and a hydraulic fluid as power transmission medium. The first
and most critical component is the pump driven by the turbine rotor.
In recent years, the number of industrial applications of fluid power has risen
dramatically.1 Hydraulic transmissions have often been considered for wind
turbines, mainly because of the low maintenance requirements. The part-load
efficiencies of available hydraulic drive systems however have so far been too poor
to make them commercially attractive. The main initial system requirements
are high power (5 + M W ), robustness/low maintenance and high efficiency.
Belang
Offshore wind energy has high potential. Currently the price for placing turbines offshore is too high. Projects are not yet economically feasible without
government subsidies.
Doelstelling
The overall goal of the project is to make offshore wind economically viable. This
translates to a design goal of the overall project which is to reduce the number of
components in the offshore turbines drastically to come to the ultimate offshore
turbine. One way in which this can be achieved is by having only one component
(other than the rotor shaft and its support) in the nacelle: a pump. Hydraulic
turbines are not new. Using seawater as hydraulic fluid is. The goal of the
research for this paper is therefore to find a pump type suitable to be used for
a DOT. The main requirements for this pump are: Efficient performance at low
wind speeds as well as high Very low maintenance Considering that the principle
hydraulic fluid is seawater, an essentially robust yet high performance pump is
required. The goal of this research for this paper is therefore to find a pump
type suitable to be used for a DOT.
Hoofdvraag
Werkwijze
From the analysis of the main DOT requirements, the main pump functions
& requirements are derived. Through the investigation of fluid power applications in general and in wind turbines specifically, the pumping principle which
allows for the highest efficiency is selected. One of the most challenging design
requirements is that the hydraulic fluid is seawater.
The approach for this research was to:
• Gain insight in how fluid power circuits operate. This means mapping
1 Currently
it is even being considered for power transmission in cars [9].
3
which type of systems exist, which is the most efficient and why and what
the fundamental performance indicators are.
• Gain insight in the applications and the potential of fluid power circuits,
i.e. what has already been done with fluid power in similar applications,
in general and hydraulic wind turbines in particular.
• Select the prime candidates for the best pumping principle and investigate
their commercial availability.
• Gain a general insight into which challenges arise from using seawater as
hydraulic fluid.
Randvoorwaarden
Structuurbeschrijving
After elaborating on the Delft Offshore Turbines Project, the basic characteristics, application and components of fluid power systems are discussed. The next
step is to look at current developments of hydraulic wind turbines and analyze
the pros and cons of hydraulic power transmission. The most unique feature
of the DOTs energy transfer system is that it uses seawater as hydraulic fluid.
With this in mind a selection of the preferred pumping principle is performed.
Figure 1: The research map
2
2.1
Delft Offshore Turbines
General Purpose
Delft University of Technology is taking a radical step away from incremental
development of offshore wind turbines. It has started a research project on using
a 2 bladed, fixed pitch turbine (5-10MW) to directly drive a water pump in the
nacelle. By channelling the pressurized water of all turbines to one transformer
platform, electricity generation is centralised. The design goal is to reduce the
number of components in the offshore turbines drastically to come to the ultimate offshore turbine. Current offshore wind turbines are marinized land turbines with only a few add-on features to keep out the salty air. Improvements of
turbine technology are only incremental and do not take full benefit of the offshore environment. The Delft University of Technology has a history in offshore
wind research of over 25 years and has formulated a radical concept change of
offshore wind energy conversion that helps develop a completely new system and
spark revolutionary developments on sub-system and component level. Typically,
offshore wind farms have a generator platform that gathers all electricity of the
4
different turbines, steps up the voltage and feeds the power through shore connection cables to the onshore grid. The DOT takes boundary conditions from
this existing configuration: horizontal axis turbine with blades and a platform
where the combined electrical power is fed to the onshore grid. Everything in
between can be changed. The DOTs focuses on radical technology changes. To
facilitate this, a short list of design pointers has been defined to test all developments against and to keep as life line throughout the project execution. Offshore,
one thing is abundant: water. The current turbine technology sees the nacelle
weight increase steadily giving increasing challenges in support structure design
and installation. Furthermore, power electronics help harness wind power more
efficiently, but also add weight and components (that can fail) to the turbine system. The DOTs convert wind energy into flowing water. A pump is connected
to the rotor directly, generating a high pressure flow. The pressurized water is
collected at a transformer platform, where generators are located comparable to
a hydro plant. The platform can be fitted with limited water storage capacity to
smooth energy variations. From this platform, an electricity cable connects to
the onshore grid.
2.2
Energy Transmission System Requirements
- Low cost - the ultimate purpose of the DOTs is to make offshore wind a
competitive energy source.
- Low maintenance is a fundamental requirement for lowering costs of operation
- Long lifespan, is arbitrary. Sometimes it becomes more viable to upgrade/replace a system before it is written off. If the payback time of
a DOT can be driven back to under 6 years, placing a new & improved
system every 10 years could be beneficial.
- High efficiency. This is both scientifically and economically also an arbitrary issue. How does one define efficiency? In this case the most straightforward answer is the rate at which wind energy is converted to electric
energy, which is then transported to the shore. Looking at offshore wind
turbines, a system can function very efficiently. However, once it breaks
down, its overall efficiency reduces. If there is not a weather window which
allows for repairs for some time, this overall efficiency drops significantly.
Efficiency can also be measured economically. If a lot of maintenance is
required, the time and energy (cost) that requires directly cuts into the
overall efficiency. So, by stating the requirement for high efficiency, this
refers to the entire system, including all related costs such as for installation, operation, maintenance and decommissioning. High efficiency thus
translates to every component of the entire system. The hydraulic energy
transmission system starts with the hydraulic pump in the nacelle. For
high efficiency during normal operation, a pumping system is required
that is also efficient in case of partial loading.
• ”use the elements/environment” + minimize No. of systems + minimize
expensive stuff like copper
• apply basic fluid power system using seawater
5
The mayor advantage of using seawater as a medium is that in this energy
transmission system can be applied to any kind of offshore project.
The idea behind DOTs is to make wind energy offshore more economically
viable. This can be achieved by:
• reducing the number of components
• (thereby) reducing the weight of the nacelle and subsequently the support
structure
• improving robustness
Here, we make a case for choosing reliability as the number one priority. Efficiency is secondary. To make offshore wind a competitive energy source for the
future.
DOT Functional Concepts
Motivation for the choice to pump seawater
Mission pump seawater
- OL ’ sw pump at rotor
- CL+OL ’ sw pump at base
In CL, if medium = oil, pumps already exist.
[1] [2]
2.3
Motivation for Using Seawater as Hydraulic Fluid
Pumping seawater - Disadvantages - Advantages
possible effects: - slight local sea temperature rise. effects?
3
Fluid Power Circuits
3.1
Introduction to Fluid Power
A fluid is any substance that flows or deforms under an applied shear stress [].
Power can be defined as the manifestation of control. For mechanical applications power is expressed in terms of energy over time.The basic theory for fluid
power is found in Pascal’s law: Pressure applied to a confined fluid in transmitted undiminished in all directions, acts with equal force on equal areas and at
right angles to them.
Fluid power is defined as the change in pressure of a volume of fluid times
the flow rate of that volume over time (P = δp · Q) Fluid or hydraulic power
circuits are found in a wide range of industrial machinery. The most common
application is for motion control.
3.2
Basic circuit components
A fluid power circuit (FPC) is defined as a system in which pressure and/or
flow speed are the primary forms of output control. Common components are:
• Pump - To pump is to use pressure to displace a fluid.
• Hydraulic fluid - the energy transmission medium
6
• Motor - a (positive displacement) pump creates an energy flow, a motor
extracts energy from the flow.
• Pipes/hoses - to contain and direct the flow
• Extras - valves, filters, accumulators and the sorts all are fundamentally
important to the functioning of a FPC.
The characteristics of a fluid power system are predominantly determined by
the characteristics of the pump the motor and the fluid medium.
3.3
Hydraulic Fluids
In fluid power circuits, the hydraulic fluid is used as a medium to transfer
mechanical power. Liquids are virtually incompressible, yet flow with little
frictional resistance. This makes them an ideal medium for the transmission of
power. Convention hydraulic fluids can be split up in two categories:
• petroleum base fluids (hydrocarbons): highly flammable, restricted operational range
• synthetic fluids: chemically compounded or water base fluids, resistant to
burning
Merritt [3]:
Water is a poor hydraulic fluid because of its restrictive liquid range,
low viscosity and lubricity and rusting capability
However, the use of freshwater for high pressure hydraulics has recently gained
new interest. As discussed in [4], water is environmentally friendly (thus readily
disposable), non-toxic, non-flammable, inexpensive, and readily obtained. With
health, safety and environment becoming ever more important in modern industry, it is likely that water-based hydraulics will gradually replace oil hydraulics.
An additional advantage is the high bulk modulus of water compared to oil,
resulting in better performance. The main drawbacks of water are the need
to use corrosion resistant materials and the low viscosity which leads to bad
lubrication.
The application of water as hydraulic fluid is mainly reserved for closed loop
systems. The way in which water is used is either as
• ”dead” water
• water based - glycol or another agent which improves lubrication.
•
no open-loops, water needs to be clean, low viscosity required finer filters
Hydrauvision
So, what about seawater? probably the worst choice?
The relation pressure, density (volume) and temperature is described by the
equation of state [3].
increasing pressure leads to a higher boiling point
According to Merrit:
7
the bulk modulus is the most important fluid property in determining the dynamic performance of hydraulic systems.
This is because β is a measure for the stiffness of the fluid. It is the inverse of
the compressibility.
Viscosity is an important property. Positive displacement pumps all employ
close-fitting surfaces. If viscosity is too low, leakage flows increase If it is too
high, power loss due to fluid friction occurs.
3.4
Classification of Pumps
Figure 2: Schematic classification of pumps
Pumps can be divided in two general categories: kinetic (or hydrodynamic)
and positive displacement pumps. In hydrodynamic pumps such as centrifugal
pumps, the flow is continuous from inlet to outlet and results from kinetic
impulse given to the fluid stream. The output is characterized by low pressure
and high volume. Inefficiency and easy stalling as a result of back-pressure make
these pumps unsuitable for control. In positive displacement pumps, fluid flows
through an inlet into a chamber. As the pump shaft rotates, the (positive or
definite) volume of fluid is sealed from the inlet and transported to the outlet
where it is subsequently discharged. The essential difference between these two
main categories is that kinetic pumps are for fluid transport systems and PD
drive systems are for fluid power systems.
By far the most widely used type of pump is the centrifugal pump (kinetic).
Centrifugal pumps are used for all kinds of flows including sludge and slurry.
Positive displacement pumps only account for about 10% [7]. For the DOT,
a pump is required that can cope with 5 + M W of power. To minimize the
flow speed and the necessary size of the pipe or hose diameter, high pressure
is required. This and the superior performance in terms of efficiency make the
positive displacement pump the prime candidate.
3.5
Applications of Fluid Power Systems
Evidence of the use of water power dates back to 250 BC. The most common
application up to well into the 20th century was in the form watermills, which
were used to grind grains. The use of high pressure in hydraulics was introduced
on a large scale in the second half of the 19th century. In major cities throughout
the world, hydraulic mains (first cast-iron, later steel) were installed beneath
the streets. Pressure was maintained by five hydraulic power stations, originally
driven by coal-fired steam engines. Short-term energy storage was provided by
8
hydraulic accumulators, which were large vertical pistons loaded with heavy
weights and tanks in high towers. Applications included cranes, elevators and
even theater curtains [13]. At its peak in 1939, the pumping stations in London
were supplying an average flow of around 14,000 liters of water per min at nearly
60 bar pressure. This translates to an average power production of around
1.35MW. Wartime bomb damage, the departure of manufacturing firms from
the city center and the rise of power electronics gradually led to the shut down
of the last pumping station in 1977.
Find out main complications of Victorian age tap water hydraulics: which
parts needed to be serviced most?
Modern industrial applications Heavy Lifting Machines Cranes Bulldozers Hagglunds drives CB/M Offshore: Jack-up hydraulic systems. Fluid power for
leverage.
Special application Taipei’s 101 tower [].
Mining
Pressure regimes
Power Regimes
Rpm regimes (GRAPH)
Power to weight ratio pump - see Hagglunds product manual generator ABB
4
4.1
Hydraulic Wind Turbines
Early Ideas
Nasa paper [8]
Sir Henrey Lawson-Tancred in Yorkshire. variable hydraulic drives
Luc Rademakers
1.3MW Bendix/Schachle turbine in the USA. variable hydraulic drives
4.2
Current Developments
Currently ChapDrive AS in Norway, Artemis in Scotland and Voith Turbo (WinDrive) in Germany are all developing hydraulic gears for wind turbines. The
most similar to the DOTs project is the ChapDrive, having its generator placed
at the foot of the turbine tower. These systems all use hydraulic oil as medium.
Seawater is preferable in to hydraulic oil in terms of dynamic performance.
This is due to the higher bulk modulus of seawater. Having an open-loop system means that the temperature of the water is likely to remain well within its
liquid range. However, low viscosity of seawater also means poor lubricity and
high potential of wear due to erosion and corrosion.
Artemis - Digital Displacement Wind Turbine Transmissions
replacing mechanical gearbox by a hydraulic transmission
Artemis Intelligent Power Ltd. [9] This system is being developed with the
aim to replace the traditional gear- box in the conventional wind turbine layout.
One of the main advantages of a hydraulic drive over a gearbox lies in the ability
to handle large shocks. This directly relates to the ruggedness and reli- ability
disadvantage of hydraulic drives is low efficiency at part-loading A prototype is
9
currently under development and scheduled to be ready in ... Pump properties
nominal pressure/Q/rpm
Voith - WinDrive
The variable speed of the wind turbine is transformed to constant rotational
speed by the hydraulic motor. The generator can therefor be connected directly
to the AC grid. No power converters are required, since only the strength of
the electric current increases as the motor builds up more torque at higher
windspeeds. Pump properties nominal pressure/Q/rpm [10]
ChapDrive
Compare performances! Also to other types of pump applications What happens
in the event of - failures - partial load - cut-out wind speed
[11]
4.3
Advantages & disadvantages of hydraulic power transmission for wind turbines
Advantages
• Heat generated by internal losses is a basic limitation of any machine.
• Hydraulic fluid acts also as a lubricant. This translates to long component life. The choice of hydraulic fluid and structural materials of the
component obviously play an important part here
Disadvantages
• It is not possible to keep the fluid free from contamination. Filtering is
required. The level of sophistication of the filter depends on the robustness
of the system components.
•
5
Seawater as Hydraulic Fluid
5.1
Basic Properties of Seawater
About 97% of the water on Earth is sea water. Almost every natural substance
known to man is found in the world’s oceans and seas, mostly in very small
concentrations [5]. The most notable characteristic component of seawater with
respect to freshwater is salt. Although the vast majority of seawater has a
salinity of between 3.1% and 3.8%, this number can vary significantly, for instance in response to addition of freshwater from rain and runoff, and removal
of freshwater through evaporation.
Despite small compositional irregularities, seawater behaves as a Newtonian
fluid, which is beneficial in terms of performance as a power fluid. For the use
as hydraulic fluid, it is important to note that seawater contains
• suspended solids, practically any form of debris
10
T vs. ρ at atmospheric pressure
Density ρ [kg/m3]
1035
salinity 40
1030
salinity 35
1025
salinity 30
1020
salinity 25
1015
1010
salinity 20
0
5
10
15
Temperature T [°C]
20
25
p vs. ρ at T = 0°C, salinity 35
1080
Density ρ [kg/m3]
1070
1060
1050
1040
1030
1020
0
200
400
600
Pressure p [bar]
800
1000
Figure 3: Nonlinear density changes of seawater with pressure [12]
• organic substances, one effect being marine growth
• dissolved gases, ...
The density of surface seawater ranges from about 1020 to 1029kg/m3 , depending on the temperature and salinity (figure 3). The pH of Seawater falls
in the range 7.5 to 8.4. Compare this to freshwater which has a pH of approximately 7, depending on temperature. Ocean acidification (like other changes in
oceanic composition) is a serious concern, but the time scale for it to be influential is to grand for this research. Also left out of this research are the additional
benefits of pressurizing seawater. They will be investigated in due time2 .
Seawater density depends on temperature, salinity and pressure. Colder
water is denser. Saltier water is denser. High pressure increases density.
The nonlinearity of the equation of state is apparent in contours of constant
density in the plane of temperature and salinity (at constant pressure) - they are
curved. They are concave towards higher salinity and lower temperature.
Cold water is more compressible than warm water. That is, it is easier to
deform a cold parcel than a warm parcel. Therefore cold water becomes denser
than warm water when they are both submerged to the same pressure. Therefore
various reference pressures are necessary. We use a pressure which is relatively
close to the depth we are interested in studying. The compressibility effect is
apparent when we look at contours of density at say 4000 dbar compared with
those at 0 dbar.
The freezing point of seawater is lower than that of freshwater, at around
2 Magnesium, bromine and sodium chloride (table salt) are all extracted from the sea on a
global scale. In theory desalted seawater can provide a limitless supply of drinking water. So
far this has been restricted due to the high processing costs.[14]
11
Element
Oxygen
Hydrogen
Chlorine
Sodium
Magnesium
Sulfur
Calcium
Potassium
Bromine
Carbon
Percent [%]
85.84
10.82
1.94
1.08
0.1292
0.091
0.04
0.04
0.0067
0.0028
Table 1: Seawater composition (by mass) (salinity = 35)
?2?C? As sea water freezes, it forms pockets of salt. The salt (brine) leaches
out of the bottom of the ice and the brine drips into the water below the ice.
5.2
Systems for seawater pumping
Check: triplex mudmotors Schaalvergroting van 2 tot 4 is max toegestaan Later
kijken naar low maintenance
Dredging/centrifugal pumps ? − > lifetime
Useful Properties of Centrifugal Pumps
General description
Dredging is an underwater excavation technique. Its main working principle
is the gathering up of bottom sediments and disposing of them at a different
location.
In essence, dredging is not a form of fluid power application but a mass
transfer method. Fluid, usually in the form of sweat or salt water, is used as a
lubricant to avoid cavitation and aeration.
Centrifugal pumps. This is probably the most applied type of pump in the
world. The fact that the
The size of sediment particles in a flow is typically defined in microns (micrometers).
What makes these pumps suitable? (Essential properties)
Resistance to erosion - no
Resistance to corrosion - to a certain extend
Dredging/centrifugal pumps − > lifetime Useful Properties of Centrifugal
Pumps
In contrast to centrifugal pumps, pd pumps are able to build up high pressure. Centrifugal pumps stall when the pressure inside a system becomes too
high. Since there are no tight fit with sealing in a centrifugal pump stall will
occur at relatively low pressures.
A pump suitable for a Delft Offshore Turbine does not yet exist. Therefor
either one has to be designed or an existing design is adapted to cope with
seawater as hydraulic fluid. The logical next step is to determine the type of
positive displacement pumping which is optimal for the DOT application.
12
6
Selection of Pumping Principle
6.1
Design Criteria for Seawater-based Positive Displacement Pumps
The main requirements for this pump are:
• Positive displacement. The only pumps which are suitable for fluid power
are positive displacement pumps.
• High pressure
• Operate for relatively large range of rpm
• Robust
• High efficiency
The selection of the positive displacement pump mechanism depends on characteristics like
• the potential power that it can deliver.
• the pressure regime
• the flow characteristics
• the applicability of seawater as medium
• the response to partial loading
6.2
Characteristics of PD Pumps
Reciprocating piston plunger pump
Triplex pumps for near-continuous flow PLAATJE Axial piston pumps are used
for smaller scale wind powered osmosis plants []
Reciprocating pumps require a crankshaft. This is unfavorable in terms of
load eccentricities due to asymmetrical loading. CHECK
Reciprocating diaphragm pump
PLAATJE
Rotary vane pump
low viscosity, non-lubricating liquids, restricted pressure
PLAATJE
Rotary helix pump
PLAATJE
Rotary piston pump
PLAATJE
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Lobe / gear pumps
Although these are more basic (less expensive) mechanisms, they only function
at a limited rpm range. At low rpm for instance, the leakage is to great to
generate a pressure flow.
PLAATJE
Radial Piston Pumps
(Triplex ’ Crankshaft)
Compare efficiency curves. Compare partial loading efficiencies.
Model efficiencies of different systems Find efficiency relations with torque
and rotational speed.
The main area of concern in terms of lubrication and wear due to corrosion
and erosion is the clearance between the piston and the cylinder. Once this
clearance begins to increase, the resulting volumetric losses will cause the overall
efficiency to drop.
next step: find a material suitable for the fabrication of hydraulic components resistant to seawater
7
Rated power [MW]
Hagglund CBP pumps
Wind Turbines
6
E−126
5
Bard 5.0
Repower 5M
4
SWT−3.6
3
V90
2
E−70
V80
1
E−53
E−33
0
0
50
100
150
200
Rated rotation rate [RPM]
250
300
350
Figure 4: Power vs. rotor rpm of commercial wind turbines and Hägglunds
CBP pumps
Figure 5: A Hägglunds pump/motor of the radial piston rotating case type
6.3
Analysis
The pump types best suited for the Delft Offshore Turbine are vane pump
and the radial piston pump. Vane pumps can cope with low viscosity fluids
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Figure 6: Performance efficiency of Hägglunds CA 210 (4 ports) pump/motor
[15]
but is limited in terms of pressure. The radial piston pump can generate high
pressure and can be designed to operate efficiently at rotation rates matching
those of the wind turbines. However, the clearance between the piston and its
casing is a concern when using seawater. The higher pressure regimes of piston
pumps result in a much larger power-to-weight ratio. This is the main reason for
selection the radial piston pump as the preferred choice for the system connected
to the rotor shaft of the Delft Offshore Turbines.
7
Conclusion
The main challenge for the DOT hydraulic energy transmission is to have a
robust, yet efficient system. For this the radial piston pump is the best suited.
A pump with rated capacity of 5MW is not yet commercially available, let alone
one capable of pumping seawater for long periods of time without maintenance.
In terms of power production, the Hgglunds CBP 210, with a little over 2.3M W
rated power at 350bar pressure is the state of the art. More powerful systems
do already exist in the form of prototypes. However, all require hydraulic fluids
with high viscosity. The conclusion therefore is that a specific system for high
pressure 5M W seawater pumping will have to be designed. Important design
considerations are:
- The Hgglunds CBP 210 (2.3 MW) operates with efficiency of 9096%. In
these systems, the larger the piston, the higher the efficiency. Therefore
ever more efficiency can be expected of a 5M W pump.
- The power-to-weight ratio of these types of pumps is much higher than
that of the generators in wind turbines. This is without taking into account
all the extra components required for the use of an electricity generator.
- The low viscosity of seawater will lead to an increase leakage flow, reducing
the efficiently slightly.
- The high bulk modulus of seawater should significantly benefit the overall
efficiency.
15
- The pump has to operate efficiently also at very low rpm. Trends in radial
piston pumps and commercial wind turbines indicate that designing for
matching rotation rates is feasible.
- One of the first steps in the design process should be to find a solution for
the poor lubricity and the corrosive and erosive characteristics of seawater.
The logical first step is to look for a structural material that is sufficiently
damage resistant for the fluid carrying parts of the pump. Finding this
material is crucial to the development of the project.
[Turbine No. of components reduction and thereby reduce the turbine mass
and subsequently the support structure mass.]
A this moment a pump fitting these requirement is not commercially available. The main challenge is to have a robust, yet efficient system.
References
[1] Diepeveen, NFB, Closed-Loop Fluid Pumping as a Means to Transfer Wind
Energy, DUWIND, Delft University of Technology, Proceedings EWEC 2009
[2] Diepeveen, NFB, Van der Tempel, J, Delft Offshore Turbines, Project
Plan, DUWIND, Delft University of Technology, www.offshore.tudelft.
nl/offshorewind
[3] Merrit, EH, Hydraulic Control Systems, 1967, John Wiley & Sons Inc.
[4] Lim, GH, Chua, PSK, He, YB, Modern water hydraulicsthe new energytransmission technology in fluid power, Nanyang Technological University, School
of Mechanical and Production Engineering, 1 February 2003
[5] Turekian, K, Oceans, 1976, Prentice-Hall
[6] Anderson jr., JD, Fundamentals of Aerodynamics, Second edition, 1991
[7] Cundiff, JS, Fluid power Circuits and Controls, Fundamentals and Applications 2002
[8] Unknown, Hydraulic wind energy conversion system, NASA STI/Recon
Technical Report, July 1981
[9] Rampen, W, Gearless Transmissions for Large Turbines - The History and
Future of Hydraulic Drives Artemis IP Ltd, Scotland, www.artemisip.com
[10] Müller, H, Pöller, M, Basteck A, Tilscher, M, Pfister, J, Grid Compatibility
of Variable Speed Wind Turbines with Directly Coupled Synchronous Generator and Hydro-Dynamically Controlled Gearbox, Proceedings Sixth International Workshop on Large-Scale Integration of Wind Power and Transmission
Networks for Offshore Wind Farms, 26-28 October 2006, Delft, NL
[11] ChapDrive AS, www.chapdrive.com date of access: August 24th 2009
[12] Physical properties of sea water, http://www.kayelaby.npl.co.uk/
general_physics/2_7/2_7_9.html date of access: August 24th 2009
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[13] www.subbrit.org.uk/sb-sites/sites/h/hydraulic_power_in_
london/, date of access: August 24th 2009
[14] Encyclopedia Britannica (online), www.britannica.com, search term “seawater”
[15] Hägglunds Product Manual for Compact CA Motors 2004 www.hagglunds.
com
[16] Hägglunds Installation and Maintenance Manual for Compact CBP Motors
2004 www.hagglunds.com
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