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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 WindTurbines

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 aspart 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 wasconstructed. It was written to lay the foundation for several PhD topics. The research topicof the author concentrates on the hydraulic energy transmission using seawater. This has sofar 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 SummaryAn extensive summary is given of the set up of the DOT project and the related researchactivities in the first year. Preliminary conclusions and an outline of the future researchefforts are also provided.

1. DOT Project PlanAuthor: ir. N.F.B. Diepeveen,Supervisor: dr.ir. J. van der Tempel

2. Closed-Loop Fluid Pumping as a Means to Transfer Wind EnergyConference paper for the European Wind Energy Conference 2009, submitted on March16th 2009.Author: ir. N.F.B. Diepeveen.

3. Pump Design Requirements for Seawater-Based Hydraulic Power Transmis-sion for Offshore Wind TurbinesConference paper for the European Offshore Wind (EOW) Conference 2009, submittedon 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 developmentof offshore wind turbines. It has started a research project on using a 2 bladed, fixed pitchturbine (5− 10MW ) to directly drive a water pump in the nacelle. By channeling the pres-surized 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 tocome to the ultimate offshore turbine.Current offshore wind turbines are marinized land turbines with only a few add-on featuresto keep out the salty air. Improvements of turbine technology are only incremental and donot take full benefit of the offshore environment. The Delft University of Technology hasa history in offshore wind research of over 25 years and has formulated a radical conceptchange of offshore wind energy conversion that helps develop a completely new system andspark revolutionary developments on sub-system and component level.Typically, offshore wind farms have a generator platform that gathers all electricity of thedifferent turbines, steps up the voltage and feeds the power through shore connection cablesto 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 fedto the onshore grid. Everything in between can be changed. The DOTs focuses on radicaltechnology changes. To facilitate this, a short list of design pointers has been defined to testall 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 increasesteadily giving increasing challenges in support structure design and installation. Further-more, power electronics help harness wind power slightly more efficiently, but also add weightand components (that can fail) to the turbine system.

Offshore wind energy has high potential. Currently the price for placing turbines offshore istoo 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 turbineinfrastructure specifically for offshore purposes and thereby rendering offshore wind energymore economically attractive. This translates to a design goal of the overall project whichis to reduce the number of components in the offshore turbines drastically to come to theultimate offshore turbine, which is characterized by:

- Very low maintenance

- High availability; as a direct consequence of high reliability.

- High efficiency

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- Easy installation

- Low production costs

The Delft Offshore Turbines (DOTs) project aims to circumvent the need of the generator byusing the rotor shaft torque to power a pump in the nacelle. This, along with the proposalto have a two-bladed rotor, will lead to a significant reduction in weight of the rotor-nacelleassembly.One of the fundamental parts of the DOT is the hydraulic transmission. The PhD researchproject in this report focuses on the design of a seawater-based high pressure hydraulic energytransmission 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 researchtopics. 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 ofusing fluid power for wind energy transmission. This was the subject of the European WindEnergy Conference (EWEC) 2009 paper (part II). The purpose was to gain insight in howfluid power circuits operate. This meant mapping which type of systems exist, which is themost efficient and why and what the key performance indicators are. Gain insight in theapplications and the potential of fluid power circuits, i.e. what has already been done withfluid 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 lookat 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 principlesand investigate their commercial availability. Next to that insight was gained into whichchallenges 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 ofapplications continues to grow. One of the earliest large scale projects were the Victorian agetap-water hydraulics. Pumping stations outside the center of cities like London, New Yorkand Melbourne delivered water pressurized up to 60 bar through underground mains to powerfacilities like elevators, cranes and even theater curtains. Nowadays, fluid power is used inshredders, feeders, roll mills, cranes, bulldozers, jack-up systems, etcetera. These applicationsuse electricity to efficiently acquire power in the form of high torque through high pressurefluid transmission. The DOT energy transmission concept is the exact opposite. High torqueis converted into a high pressure flow.

Classification of pumps

Pumps can be divided in two general categories: kinetic (or hydrodynamic) and positive dis-placement pumps. In hydrodynamic pumps such as centrifugal pumps, the flow is continuous


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from inlet to outlet and results from kinetic impulse given to the fluid stream. The outputis characterized by low pressure and high volume. Inefficiency and easy stalling as a resultof 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 ordefinite) volume of fluid is sealed from the inlet and transported to the outlet where it issubsequently discharged. The essential difference between these two main categories is thatkinetic pumps are for fluid transport systems and PD drive systems are for fluid power sys-tems. The power-to-weight ratio of pd pumps is much higher than that of the generators usedin wind turbines. This is without taking into account all the extra components required forthe use of an electricity generator.

Hydraulic Fluids

Seawater is preferable in to hydraulic oil in terms of dynamic performance. This is due to thehigher bulk modulus of seawater. Having an open-loop system means that the temperatureof the water will remain well within its liquid range. However, low viscosity of seawater alsomeans 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 ASin Norway, Artemis in Scotland and Voith Turbo (WinDrive) in Germany are all developinghydraulic 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 hydraulicoil 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 pressurizedwater is collected at a transformer platform, where generators are located comparable to ahydro plant. The platform can be fitted with limited water storage/accumulation capacity tosmoothen energy variations. From this platform, an electricity cable connects to the onshoregrid. So far, two concepts have been derived:

1. closed-loop + open-loopA high pressure fluid power circuit forms a closed loop between the pump connected tothe rotor shaft in the nacelle and a motor just below sea-level. The motor is connectedto a second pump which pumps seawater to the central generator platform. Having aclosed-loop systems requires subsystems for cooling & pressurizing. This adds signifi-cantly to the total number of components.

2. open-loopThe pump connected to the rotor shaft generates a high pressure flow. At the base, a


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small portion of this pressure is used to power a booster pump which ensures the flowof sea water to the pump in the nacelle. The rest of the pressure is used to generateelectricity at the central platform.

Selection of Pumping Principle

Candidate pump types for the Delft Offshore Turbines are the vane pump and the radialpiston pump. Vane pumps can cope with low viscosity fluids like water but are limited interms 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 thewind turbines. However, in particular the clearance between the piston and its casing is aconcern when using seawater. Corrosion and erosion of a pump will lead to a rapid declinein 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 efficientsystem.

Hydraulic drive systems have been applied in many industries for many years. High perfor-mance systems are characterized by high efficiencies and low maintenance needs. The powerproduction of a Delft Offshore Turbine can increase beyond rated due to the characteristicsof hydraulic drive systems. The cut-out wind speed is determined by the control characteris-tics of the rotor (blades). The current maximum size of suitable pumps is under 2 MW andrequires the use of hydraulic oils as power fluid. Larger systems are technically feasible. Theyhave 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 nacellesignificantly 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 thesupport structure mass.

A 5MW 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 offshoreenvironment to our advantage and use seawater.

The research presented in part III shows that multi-MW high pressure seawater pumps donot yet exist. The main reason for this is that there has never been a real need for them. Tomake the DOTs a reality, such a pump will need to be designed. The main challenge is howto 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 capacityof 5MW is not yet commercially available, let alone one capable of pumping seawater for longperiods of time without maintenance. In terms of power production, the Hagglunds CBP210, with a little over 2.3MW rated power at 350bar pressure is the state of the art. Morepowerful systems do already exist in the form of prototypes. However, all require hydraulic


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fluids with high viscosity. The conclusion therefore is that a specific system for high pressure5MW seawater pumping will have to be designed. Important design considerations are:

- The Hagglunds CBP 210 (2.3MW ) operates with efficiency of 9096%. In these systems,the larger the piston, the higher the efficiency. Therefore ever more efficiency can beexpected of a 5MW pump.

- The low viscosity of seawater will lead to an increase leakage flow, reducing the efficientlyslightly.

- 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 pumpsand commercial wind turbines indicate that designing for matching rpms is feasible. Oneof the first steps in the design process should be to find a solution for the poor lubricityand the corrosive and erosive characteristics of seawater. The logical first step is tolook for a structural material that is sufficiently damage resistant for the fluid carryingparts of the pump. Finding this material is potentially crucial to the development ofthe project.

The next phase of the project will be focused on finding a suitable structural materials forthe high pressure seawater pump.


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Part I

DOTs Project Plan

roman

Contents

1 Introduction 1

1.1 The Future of Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Current Wind Turbine Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3 The Proposed Idea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Design Rules 5

3 Design Options 7

4 How DOTs Work 9

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94.2 Power Transmission: Wind to Water to Electric . . . . . . . . . . . . . . . . . . . . 10

4.2.1 A: The Rotor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104.2.2 B: The Closed-Loop System . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.2.3 C: The Open-Loop System . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.2.4 D: The Generator Platform . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.3 Simulation Results for Varying Wind Speeds . . . . . . . . . . . . . . . . . . . . . 15

5 Plan of Execution 19

5.1 DUWIND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195.2 DOTs Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195.3 Time Planning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205.4 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 ex-pected 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 vastamounts of carbon-dioxide (CO2) into the atmosphere. The global consensus is that CO2 emis-sions lead to global warming.

Of the energy consumed in Europe, only about 9% comes from renewable sources [1]. To be-come more independent and reduce CO2 emissions it is necessary for Europe and to produce moreand cleaner energy. An increasingly popular, clean and renewable energy source is wind, and inparticular offshore wind.

Targets

In 2007 the European Union and the EWEA set the following targets. Of the total energy con-sumed 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 tothe EWEA offshore wind has the potential to deliver up to 25% by 2020. This translates to anestimated total target of 40GW from offshore wind farms. Assuming one wind turbine producesan average of 4MW , a total of 10,000 structures is therefore required. So far, around 530 offshorewind 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 re-alized within 10 years. Compare this to the oil and gas industry where worldwide 7,000 offshorestructures were built in 70 years [3].

Challenges

To achieve these targets many challenges have to be overcome. The essential technical design chal-lenge is to improve performance, particularly in terms of reliability. Other design considerationsinclude,

- Wind energy economics. To make wind energy (more) profitable, the high costs of fabricationand installation must be reduced.

- Impact on the local environment. The long term effects of offshore wind turbines on localflora and fauna are not yet known. A popular theory is that they can function as an artificial

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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 sandbanks. 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 canthus not be accessed anymore for fishing.1 Other common objections to OWFs are the fearof obstruction of seaways and possible interference with surface radar.

- Installation logistics. Currently there is a shortage of vessels which can be used for OWFinstallation. Hence, there is a need to optimize the use of cranes and other offshore con-struction 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 inthe nacelle. Supplying each turbine with its own generator has several disadvantages.

1Organisations such as Greenpeace actually promote OWFs to create safe-havens for marine life and combatoverfishing

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 ofswitch 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 The Proposed Idea

1.3 The Proposed Idea

The idea proposed in this project plan is the following. Picture a wind turbine in an offshore windfarm. We take practically everything out of the nacelle. All that remains is the idea of rotatingkinetic 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 belimited to one specific grand design. Many of the developed concepts will also be applicable tocurrent wind turbine technologies. For instance, one likely target for research is the concept ofboundary layer suction along the rotor blades to reduce drag. This research will be part of theDOTs 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 turbinesis to have three blades. The main reason for this is that human spectators experience threeblades as the more tranquil view. Offshore, where spectators do not have to be taken intoaccount, 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. The-oretically, 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-loopsystem 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. Hencethe 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.

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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.

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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 initialconcept 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 thebase of the structure. Its main components are:

• pump A, which is directly driven by the rotor shaft. Its function is to induce a flow inpipe 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 thestructure.

Figure 4.1: Identification of the subsystems of the DOT concept from figure 2.1

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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 gener-ator 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 togenerate 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 initialsimulation results are presented in section 4.3. The (potential) initial conditions of this simulationare 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 ·12· ρair · π · r2 · v3 (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 factorslarger than 0.50. As part of the DOT project, research will be done on the application of boundarylayer 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 (4.2)

DOT Project Plan 10


4.2.2 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 anda generator converts the power in the rotor shaft to electric power. The electricity is conductedthrough 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 thepower 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 aradial piston-design unit. Currently, the most power P any hydraulic unit is able to transfer isaround 1.75MW (the Hagglunds MB 3200 [12]). However, these units can be applied in parallelto cope with higher torque/power.1 Despite their relative small size, such hydraulic units aredesigned 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-looppipe. At the end of the entire system is the generator. The harder it is to turn the rotor of agenerator (the more torque is required), the more power it produces. The torque required to powerthe generator determines the pressure in the open-loop system. The pressure and the volume flowof the fluid in the open-loop system determine the torque that motor B is required to produce. The

1The rotation rate of hydromotors is in the same order as the rotation rate of the rotor shaft. Hence, no gearboxis required. The MB 3200 operates at a maximum of 16rpm. The popular Vestas V90 turbine (3.0MW ) operatesin the range of 8.6− 18.4rpm.

DOT Project Plan 11


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 theentire system. This flow resistance determines the pressure.Within pump A, the height difference ∆z (head) is assumed to be zero. So, the pressure changeover 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 apipe:

• 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 pressurechange ∆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 thefluid.

A = π ·(

D

2

)2

(4.6)

Q = A · v (4.7)

An overview of how several flow characteristics change for different ranges of power is given intable 4.1. Here, Dmin is the minimal pipe diameter for laminar flow (< 6m/s).

Power P Pressure p Volume flow Q Dmin

1.7 MW 350 bar 0.049 m3/s 2910 l/min 0.102 m 4.00 in3.0 MW 350 bar 0.086 m3/s 5140 l/min 0.135 m 5.31 in5.0 MW 350 bar 0.143 m3/s 8570 l/min 0.174 m 6.85 in10.0 MW 350 bar 0.286 m3/s 17100 l/min 0.246 m 9.69 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 tofriction 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


The friction factor f for flow through a pipe is derived from references [5] and [7]. A distinc-tion is made between disturbed (turbulent) and undisturbed (laminar) flow (see figure 4.4).

• For laminar flow (Re < 2300 in pipes):

f =64Re

(4.9)

• For turbulent flow (Re > 4000 in pipes):

1√f

= −2 · log(

e/D

3.7+

2.51Re · √f

)(4.10)

Turbulent flow is undesirable because within a pipe the flow is subject to much more friction thanin 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

D· 12· ρfluid · v2 (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 mechanicalpower. The low pressure part of the closed-loop pipeline experiences the same head ∆z as thehigh 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 PmotorBis used to drive pump C.

DOT Project Plan 13


4.2.3 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 Cto pump seawater into a second pipe. The open-loop system hence uses seawater as a means totransfer 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 andit 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 withpipe 1, the power PpumpC

from pump C can be expressed in terms of the pressure change ∆p andthe 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 pipe1. However, the main difference between the two pipes is that the length of pipe 2 will be muchgreater. 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 pressurizedseawater 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 windfarm is expressed as:

Pgen = ηgen ·N ·∆p ·Q (4.16)

4.3 Simulation Results for Varying Wind Speeds

To better understand the (theoretic) performance of a DOT, a numeric input is given to the theoryof the previous section. The wind speed is varied from 0 to 20m/s. All other initial conditionsare derived from existing techniques. They are summed up in table 4.2.

Parameter Value Units Descriptiongeneral

νsw 1.17E − 6 [m2/s] kin. viscosity seawater at 15◦Cνfl 4.20E − 4 [m2/s] kin. viscosity of fluid in pipe 1ρsw 1030 [kg/m3] density of seawaterρfl 1000 [kg/m3] density of fluid in pipe 1

rotorcp 0.50 [−] induction factorr 63.0 [m] radius

pumpsppumpA

35.0E6 [Pa] pressure generated by pump AppumpC

10.0E6 [Pa] pressure generated by pump CηpumpA

0.95 [−] efficiency pump AηmotorB

0.90 [−] efficiency motor BηpumpC

0.80 [−] efficiency pump Cpipes

D1 0.30 [m] diameter pipe 1D2 0.50 [m] diameter pipe 2L1 100 [m] length pipe 1L2 500 [m] length pipe 2

pmin 2.0E6 [Pa] minimum pressure in pipe 1generator

ηgen 0.90 [−] efficiency generator

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 open-loop pipe 2. As section 4.2 explains, for the purpose of efficiency, it is beneficial to use highpressure and subsequent low velocity flow. For flow calculations (figure 4.7), the critical value ofthe Reynolds number Re is assumed as 2800.

DOT Project Plan 15


103

104

105

106

10−2

10−1

Re = v⋅D/ν [−]

Fric

tion

fact

or f

[−]

Laminar

Turbulent

Pipe No.1Pipe No.2

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 Bothturbines have the same rotor diameter.

0 2 4 6 8 10 12 14 16 18 200

1

2

3

4

5

6

7

Windspeed [m/s]

Pow

er [M

W]

P

rotorP

pumpB

Pgen

REpower 5MW

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) ratedwind 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 percentageof the DOT’s kinetic energy at the rotor is lost on its way to the generator platform. Most ofthese losses will be due to friction in the pipelines. The key to increasing efficiency in theory is toimprove the pumping systems and minimize losses in the pipelines.The key to increasing efficiency in practice is to reduce maintenance and increase uptime.

2The REpower 5M is the largest turbines currently in operation. So far these turbines have been installed atoffshore locations near Scotland (Beatrice) and Belgium (Thornton Bank).

DOT Project Plan 16


0 2 4 6 8 10 12 14 16 18 200

0.5

1

1.5

2

2.5

Windspeed [m/s]

Dis

char

ge Q

[m3 /s

]

Pipe No.1Pipe No.2

Figure 4.9: The volume flow in the open-loop and closed-loop pipelines of a DOT.

DOT Project Plan 17


DOT Project Plan 18

5 Plan of Execution

5.1 DUWIND

The Delft University of Technology has joined together the research groups involved in wind en-ergy research into the Delft University Wind Energy Research Institute, DUWIND. This instituteenvelops 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 pol-icy. DUWIND is one of the largest institutes on wind energy in the world with a particularly largefocus on offshore wind energy.

DUWIND drafted its first research plan in 2002 with a scope of 15 PhD students on all newtopics. This resulted in the current double of that in 2008. At this moment, the new research planis being finished with a goal for the next 5 years to reach 60 PhD students. Within this plan, theDOTs also has its place. It has been identified as a showcase of combining scientific knowledgeand bring it to the market.

5.2 DOTs Organization

With the DOT project being an integrated part of DUWIND, the DOT team members will resideunder the different sections within DUWIND. Each PhD student will have a specialist professor toact as coach and promotor. The PhD students will participate in their specific section in researchand education. On top of that, the group of 8 will also have a shared work place to increasethe team spirit and focus the group attention to reaching their common goal. The group willfurthermore be supervised by 1 supervisor, who is responsible for the day-to-day co-ordination ofthe 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 August2008. This detailed plan is the first work from his hand in shaping the DOT process. Over the nextmonths, 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 3years, which is somewhat shorter than the ”normal” Delft duration. The PhD students will startconsecutively over a 1.5 year period, stretching the DOT project from 1-8-2008 to 1-8-2012 asshown 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 turbinetechnology. The combined outcome of the 8 PhD projects is a blueprint for the construction ofa 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 magnitudeof 50 - 100 Me, to be constructed in 2013/2014. Following this planning, the DOTs will be readyfor commercial application in the second half of the next decade, exactly when the exponentialincrease in offshore wind turbine installations is anticipated. As the goal of the DOT is not only tocreate 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 tothe 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. Thesetopics will be detailed further as all PhD students start their work during the next 6 months. Forthe 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 includeboundary 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 centralelectricity 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 andthe 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 setby 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 convertedto electricity. This requires the design of the generator platform and all its components.

DOT Project Plan 21

Plan of Execution Potential PhD Research Projects

DOT Project Plan 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: PolicyRecommendations 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 moglichen Auswendung des Windes durch Windmo-toren, Zeitschrift fur 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

Wind energy is booming. The focus is

moving to offshore production. Current

technology is far f rom opt imal .

The main idea behind the DOTs is to get

more energy from wind in a less labor-

intensive manner. The focus of the design

process will be on wind farm energy

production, but also on important

economical and practical issues such as

the optimization of production, assembly

and maintenance.

Delft Offshore TurbinesRe-designing wind turbine technology for more

power, better performance and better economy

The DOTs project aims to bring about a

radical change in (offshore) wind turbine

technology. It will not be performed for

standard incremental improvements.

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 more information, visit us at:

www.offshore.tudelft.nl/offshorewind

The DOT team members will all be PhD students,

residing under different sections within DUWIND.

Civil Eng

Mechanic Eng

Electrical Eng

Tech Management

Aerospace Eng

power to shore

generatorplatform

high pressurepump

motor

hig

h p

ressure

low

pre

ssure

pump

rotor Functional diagram

Pumping fluid at high pressure in

combination with low volume displacement

allows for highly efficient energy transfer.

So-called f luid power is a proven

technology in many industrial sectors.

Design rulesTwo 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

seawater

Part II

Conference Paper EWEC2009

roman

1

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: [email protected]

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. 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

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 free-stream seawater to a central power hub where this open-loop power flow is converted to electricity.

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

2

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 closed-loop fluid power circuit.1 The resulting general characteristics are compared to those of traditional offshore wind turbines.

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

1 Note that this paper presents the modeling of one possible concept and by no means the final design.

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. Repower 5M DOT Number of blades

3 2

Rotor diameter 126m 126m Rated wind speed

13.0 m/s 13.0 m/s

Rated power 5 MW 5 MW Rated rotor speed

12.1 RPM 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.

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.

Pump

Motor

Pump

Rotor

SeawaterTo centralpower hub

3

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.

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 piston-design units. They can operate efficiently (>95%) at high pressure.

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

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). P p Q= ∆ ⋅ (1)

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.

Pipe/Hose-flow modelling The standard formula for pressure loss in a pipe or hose is given by equation 2 [5].

21

2loss

Lp f v

Dρ= ⋅ ⋅ ⋅ ⋅ (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].

Dis

plac

emen

t

Rat

ed

Spe

ed

Max

. spe

ed

Max

pre

ssur

e

Max

torq

ue

Rat

ed p

ower

Max

pow

er

Vi nrated nmax pmax Tmax Pra

ted

Pm

ax

l/rev rpm rpm bar kNm MW

MW

Pump 472 18.1 27.2 350 2480 5.0 7.1

Motor 118 72.6 108.9 350 620 5.0 7.1

4

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.

This necessary extra system does however mean that 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. 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.

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.

0 5 10 15 20 25 300

1

2

3

4

5

6

7

8

wind speed [m/s]

pow

er [

MW

] pmax,nnom

pmax

,nmax

hydraulic turbine

traditional turbine

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.

0 5 10 15 20 25 300

50

100

150

200

250

300

350

400 pmax,nnom

wind speed [m/s]

forc

e [k

N]

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.

M

Figure 2: CL circuit with cooling/boosting

5

- 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.

0 5 10 15 20 25 30 35 40 450

200

400

∆ p

[bar

]

0 5 10 15 20 25 30 35 40 450

0.2

0.4

Q [

m/s

3 ]

0 5 10 15 20 25 30 35 40 450

0.5

1

η [-

]

nrotor [RPM]

Figure 5: The rotor RPM vs. pressure change, volume flow & efficiency

10-1

100

101

102

103

104

10-2

10-1

100

101

102

f [-

]

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

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. 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

Part III

Conference Paper EOW2009 [DRAFT]

roman

Design Considerations for a Wind-Powered

Seawater Pump

N.F.B. DiepeveenDUWIND, Faculty of Civil Engineering and Geosciences,

Delft University of TechnologyStevinweg 1, 2628 CN Delft, The Netherlands

Tel.: +31 15 27 88030, E-mail: [email protected]

Summary

Offshore, one thing is abundant: water. The current turbine technol-ogy sees the nacelle weight increase steadily giving increasing challenges insupport structure design and installation. Furthermore, power electronicshelp harness wind power slightly more efficiently, but also add weight andcomponents (that can fail) to the turbine system.

The Delft Offshore Turbines (DOTs) convert wind energy into a highpressure flow of water. A pump is connected to the rotor directly, gen-erating a high pressure flow. The pressurized water is collected at atransformer platform, where generators are located comparable to a hydroplant. The platform can be fitted with limited water storage/accumulationcapacity to smoothen energy variations. From this platform, an electricitycable connects to the onshore grid.

High pressure fluid power is used in shredders, feeders, roll mills,cranes, bulldozers, jack-up systems, etcetera. These applications effi-ciently acquire power in the form of high torque through high pressure(op to 500 bar) fluid transmission. The DOT energy transmission con-cept is the exact opposite. High torque is converted into a high pressureflow.

Currently ChapDrive AS in Norway, Artemis in Scotland and VoithTurbo (WinDrive) in Germany are all developing hydraulic gears for windturbines. The most similar to the DOTs project is the ChapDrive, havingits generator placed at the foot of the turbine tower. These systems alluse hydraulic oil as medium.

Seawater is preferable in to hydraulic oil in terms of dynamic perfor-mance. This is due to the higher bulk modulus of seawater. However, lowviscosity of seawater also means poor lubricity and high potential of weardue to erosion and corrosion.

The research presented in this paper shows that multi-MW high pres-sure seawater pumps do not yet exist. The main reason for this is thatthere 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 todesign for the use of seawater as hydraulic fluid.

1

Contents

1 Introduction 2

2 Delft Offshore Turbines 42.1 General Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.2 Energy Transmission System Requirements . . . . . . . . . . . . 52.3 Motivation for Using Seawater as Hydraulic Fluid . . . . . . . . . 6

3 Fluid Power Circuits 63.1 Introduction to Fluid Power . . . . . . . . . . . . . . . . . . . . . 63.2 Basic circuit components . . . . . . . . . . . . . . . . . . . . . . . 63.3 Hydraulic Fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.4 Classification of Pumps . . . . . . . . . . . . . . . . . . . . . . . 83.5 Applications of Fluid Power Systems . . . . . . . . . . . . . . . . 8

4 Hydraulic Wind Turbines 94.1 Early Ideas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94.2 Current Developments . . . . . . . . . . . . . . . . . . . . . . . . 94.3 Advantages & disadvantages of hydraulic power transmission for

wind turbines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

5 Seawater as Hydraulic Fluid 105.1 Basic Properties of Seawater . . . . . . . . . . . . . . . . . . . . . 105.2 Systems for seawater pumping . . . . . . . . . . . . . . . . . . . . 12

6 Selection of Pumping Principle 136.1 Design Criteria for Seawater-based Positive Displacement Pumps 136.2 Characteristics of PD Pumps . . . . . . . . . . . . . . . . . . . . 136.3 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

7 Conclusion 15

References 16

1 Introduction

aanleidingThe EWEA has set the target for an installed capacity of 40 GW for offshorewind turbines in 2020 []. Some of the main obstacles in achieving this targetare the costs of the offshore support structures, installation and maintenance.According to critics [], the root of the problem is that the turbines being installedoffshore were not initially designed for this environment.

The goal of the Delft Offshore Turbines (DOT) project is to design andbuild a wind turbine (5+ MW) specifically for the offshore situation, therebystepping away from incremental improvements. This goal translates directly intothe driving project requirements: easy installation, robustness/low maintenanceand high efficiency energy conversion. Eventually the implementation of theserequirements will lead to lower IRR for offshore wind farms. To achieve thesedemands, the working strategy is to minimize the total number of systems,

2

minimize the use of expensive materials and where possible use the elements toour advantage.

One of the major differences in the DOT design with respect to currentmainstream wind turbine technology is the energy conversion method. Insteadof applying electricity generators and all related components, the DOT will usea 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-basedfluid power system to a central generator platform in the wind farm.

probleemFluid power systems are used in many industries throughout the world. Whatmakes the application different for the DOT is the fact that it is a multi-MWwind-driven and that the power transmission medium is seawater.

The basic components of any fluid power circuit are: a pump (mechanicalto power flow, a motor/generator (power flow to mechanic/electric), conductors(pipes or hoses) and a hydraulic fluid as power transmission medium. The firstand most critical component is the pump driven by the turbine rotor.

In recent years, the number of industrial applications of fluid power has risendramatically.1 Hydraulic transmissions have often been considered for windturbines, mainly because of the low maintenance requirements. The part-loadefficiencies of available hydraulic drive systems however have so far been too poorto make them commercially attractive. The main initial system requirementsare high power (5 + MW ), robustness/low maintenance and high efficiency.

BelangOffshore wind energy has high potential. Currently the price for placing tur-bines offshore is too high. Projects are not yet economically feasible withoutgovernment subsidies.

DoelstellingThe overall goal of the project is to make offshore wind economically viable. Thistranslates to a design goal of the overall project which is to reduce the number ofcomponents in the offshore turbines drastically to come to the ultimate offshoreturbine. 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. Hydraulicturbines are not new. Using seawater as hydraulic fluid is. The goal of theresearch for this paper is therefore to find a pump type suitable to be used fora DOT. The main requirements for this pump are: Efficient performance at lowwind speeds as well as high Very low maintenance Considering that the principlehydraulic fluid is seawater, an essentially robust yet high performance pump isrequired. The goal of this research for this paper is therefore to find a pumptype suitable to be used for a DOT.

HoofdvraagWerkwijzeFrom the analysis of the main DOT requirements, the main pump functions& requirements are derived. Through the investigation of fluid power applica-tions in general and in wind turbines specifically, the pumping principle whichallows for the highest efficiency is selected. One of the most challenging designrequirements is that the hydraulic fluid is seawater.

The approach for this research was to:

• Gain insight in how fluid power circuits operate. This means mapping1Currently 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 whatthe 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 investigatetheir commercial availability.

• Gain a general insight into which challenges arise from using seawater ashydraulic fluid.

Randvoorwaarden

StructuurbeschrijvingAfter elaborating on the Delft Offshore Turbines Project, the basic characteris-tics, application and components of fluid power systems are discussed. The nextstep is to look at current developments of hydraulic wind turbines and analyzethe pros and cons of hydraulic power transmission. The most unique featureof 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 Delft Offshore Turbines

2.1 General Purpose

Delft University of Technology is taking a radical step away from incrementaldevelopment of offshore wind turbines. It has started a research project on usinga 2 bladed, fixed pitch turbine (5-10MW) to directly drive a water pump in thenacelle. By channelling the pressurized water of all turbines to one transformerplatform, electricity generation is centralised. The design goal is to reduce thenumber of components in the offshore turbines drastically to come to the ulti-mate offshore turbine. Current offshore wind turbines are marinized land tur-bines with only a few add-on features to keep out the salty air. Improvements ofturbine technology are only incremental and do not take full benefit of the off-shore environment. The Delft University of Technology has a history in offshorewind research of over 25 years and has formulated a radical concept change ofoffshore wind energy conversion that helps develop a completely new system andspark 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 con-nection cables to the onshore grid. The DOT takes boundary conditions fromthis existing configuration: horizontal axis turbine with blades and a platformwhere the combined electrical power is fed to the onshore grid. Everything inbetween can be changed. The DOTs focuses on radical technology changes. Tofacilitate this, a short list of design pointers has been defined to test all develop-ments against and to keep as life line throughout the project execution. Offshore,one thing is abundant: water. The current turbine technology sees the nacelleweight increase steadily giving increasing challenges in support structure designand installation. Furthermore, power electronics help harness wind power moreefficiently, but also add weight and components (that can fail) to the turbine sys-tem. The DOTs convert wind energy into flowing water. A pump is connectedto the rotor directly, generating a high pressure flow. The pressurized water iscollected at a transformer platform, where generators are located comparable toa hydro plant. The platform can be fitted with limited water storage capacity tosmooth energy variations. From this platform, an electricity cable connects tothe onshore grid.

2.2 Energy Transmission System Requirements

- Low cost - the ultimate purpose of the DOTs is to make offshore wind acompetitive energy source.

- Low maintenance is a fundamental requirement for lowering costs of op-eration

- Long lifespan, is arbitrary. Sometimes it becomes more viable to up-grade/replace a system before it is written off. If the payback time ofa DOT can be driven back to under 6 years, placing a new & improvedsystem every 10 years could be beneficial.

- High efficiency. This is both scientifically and economically also an arbi-trary issue. How does one define efficiency? In this case the most straight-forward answer is the rate at which wind energy is converted to electricenergy, which is then transported to the shore. Looking at offshore windturbines, a system can function very efficiently. However, once it breaksdown, its overall efficiency reduces. If there is not a weather window whichallows for repairs for some time, this overall efficiency drops significantly.Efficiency can also be measured economically. If a lot of maintenance isrequired, the time and energy (cost) that requires directly cuts into theoverall efficiency. So, by stating the requirement for high efficiency, thisrefers to the entire system, including all related costs such as for installa-tion, operation, maintenance and decommissioning. High efficiency thustranslates to every component of the entire system. The hydraulic energytransmission system starts with the hydraulic pump in the nacelle. Forhigh efficiency during normal operation, a pumping system is requiredthat is also efficient in case of partial loading.

• ”use the elements/environment” + minimize No. of systems + minimizeexpensive stuff like copper

• apply basic fluid power system using seawater

5

The mayor advantage of using seawater as a medium is that in this energytransmission system can be applied to any kind of offshore project.

The idea behind DOTs is to make wind energy offshore more economicallyviable. This can be achieved by:

• reducing the number of components

• (thereby) reducing the weight of the nacelle and subsequently the supportstructure

• improving robustness

Here, we make a case for choosing reliability as the number one priority. Effi-ciency is secondary. To make offshore wind a competitive energy source for thefuture.

DOT Functional ConceptsMotivation for the choice to pump seawaterMission pump seawater- OL ’ sw pump at rotor- CL+OL ’ sw pump at baseIn CL, if medium = oil, pumps already exist.[1] [2]

2.3 Motivation for Using Seawater as Hydraulic Fluid

Pumping seawater - Disadvantages - Advantagespossible 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 applica-tions power is expressed in terms of energy over time.The basic theory for fluidpower is found in Pascal’s law: Pressure applied to a confined fluid in transmit-ted undiminished in all directions, acts with equal force on equal areas and atright angles to them.

Fluid power is defined as the change in pressure of a volume of fluid timesthe flow rate of that volume over time (P = δp · Q) Fluid or hydraulic powercircuits are found in a wide range of industrial machinery. The most commonapplication is for motion control.

3.2 Basic circuit components

A fluid power circuit (FPC) is defined as a system in which pressure and/orflow 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 motorextracts energy from the flow.

• Pipes/hoses - to contain and direct the flow

• Extras - valves, filters, accumulators and the sorts all are fundamentallyimportant to the functioning of a FPC.

The characteristics of a fluid power system are predominantly determined bythe 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 transfermechanical power. Liquids are virtually incompressible, yet flow with littlefrictional resistance. This makes them an ideal medium for the transmission ofpower. Convention hydraulic fluids can be split up in two categories:

• petroleum base fluids (hydrocarbons): highly flammable, restricted oper-ational range

• synthetic fluids: chemically compounded or water base fluids, resistant toburning

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 gainednew interest. As discussed in [4], water is environmentally friendly (thus readilydisposable), non-toxic, non-flammable, inexpensive, and readily obtained. Withhealth, safety and environment becoming ever more important in modern indus-try, 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 needto use corrosion resistant materials and the low viscosity which leads to badlubrication.

The application of water as hydraulic fluid is mainly reserved for closed loopsystems. 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 filtersHydrauvisionSo, 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 pointAccording to Merrit:

7

the bulk modulus is the most important fluid property in determin-ing the dynamic performance of hydraulic systems.

This is because β is a measure for the stiffness of the fluid. It is the inverse ofthe compressibility.

Viscosity is an important property. Positive displacement pumps all employclose-fitting surfaces. If viscosity is too low, leakage flows increase If it is toohigh, 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 centrifugalpumps, the flow is continuous from inlet to outlet and results from kineticimpulse given to the fluid stream. The output is characterized by low pressureand high volume. Inefficiency and easy stalling as a result of back-pressure makethese pumps unsuitable for control. In positive displacement pumps, fluid flowsthrough an inlet into a chamber. As the pump shaft rotates, the (positive ordefinite) volume of fluid is sealed from the inlet and transported to the outletwhere it is subsequently discharged. The essential difference between these twomain categories is that kinetic pumps are for fluid transport systems and PDdrive 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 + MW of power. To minimize theflow speed and the necessary size of the pipe or hose diameter, high pressureis required. This and the superior performance in terms of efficiency make thepositive 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 commonapplication up to well into the 20th century was in the form watermills, whichwere used to grind grains. The use of high pressure in hydraulics was introducedon a large scale in the second half of the 19th century. In major cities throughoutthe world, hydraulic mains (first cast-iron, later steel) were installed beneaththe streets. Pressure was maintained by five hydraulic power stations, originallydriven by coal-fired steam engines. Short-term energy storage was provided by

8

hydraulic accumulators, which were large vertical pistons loaded with heavyweights and tanks in high towers. Applications included cranes, elevators andeven theater curtains [13]. At its peak in 1939, the pumping stations in Londonwere supplying an average flow of around 14,000 liters of water per min at nearly60 bar pressure. This translates to an average power production of around1.35MW. Wartime bomb damage, the departure of manufacturing firms fromthe city center and the rise of power electronics gradually led to the shut downof the last pumping station in 1977.

Find out main complications of Victorian age tap water hydraulics: whichparts needed to be serviced most?

Modern industrial applications Heavy Lifting Machines Cranes Bulldozers -Hagglunds drives CB/M Offshore: Jack-up hydraulic systems. Fluid power forleverage.

Special application Taipei’s 101 tower [].MiningPressure regimesPower RegimesRpm regimes (GRAPH)Power to weight ratio pump - see Hagglunds product manual generator -

ABB

4 Hydraulic Wind Turbines

4.1 Early Ideas

Nasa paper [8]Sir Henrey Lawson-Tancred in Yorkshire. variable hydraulic drivesLuc Rademakers1.3MW Bendix/Schachle turbine in the USA. variable hydraulic drives

4.2 Current Developments

Currently ChapDrive AS in Norway, Artemis in Scotland and Voith Turbo (Win-Drive) in Germany are all developing hydraulic gears for wind turbines. Themost similar to the DOTs project is the ChapDrive, having its generator placedat 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 sys-tem means that the temperature of the water is likely to remain well within itsliquid range. However, low viscosity of seawater also means poor lubricity andhigh potential of wear due to erosion and corrosion.

Artemis - Digital Displacement Wind Turbine Transmissions

replacing mechanical gearbox by a hydraulic transmissionArtemis Intelligent Power Ltd. [9] This system is being developed with the

aim to replace the traditional gearbox in the conventional wind turbine layout.One of the main advantages of a hydraulic drive over a gearbox lies in the abilityto handle large shocks. This directly relates to the ruggedness and reli- abilitydisadvantage of hydraulic drives is low efficiency at part-loading A prototype is

9

currently under development and scheduled to be ready in ... Pump propertiesnominal pressure/Q/rpm

Voith - WinDrive

The variable speed of the wind turbine is transformed to constant rotationalspeed by the hydraulic motor. The generator can therefor be connected directlyto the AC grid. No power converters are required, since only the strength ofthe electric current increases as the motor builds up more torque at higherwindspeeds. Pump properties nominal pressure/Q/rpm [10]

ChapDrive

Compare performances! Also to other types of pump applications What happensin the event of - failures - partial load - cut-out wind speed

[11]

4.3 Advantages & disadvantages of hydraulic power trans-mission 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 compo-nent life. The choice of hydraulic fluid and structural materials of thecomponent obviously play an important part here

Disadvantages

• It is not possible to keep the fluid free from contamination. Filtering isrequired. The level of sophistication of the filter depends on the robustnessof 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 substanceknown to man is found in the world’s oceans and seas, mostly in very smallconcentrations [5]. The most notable characteristic component of seawater withrespect to freshwater is salt. Although the vast majority of seawater has asalinity of between 3.1% and 3.8%, this number can vary significantly, for in-stance in response to addition of freshwater from rain and runoff, and removalof freshwater through evaporation.

Despite small compositional irregularities, seawater behaves as a Newtonianfluid, which is beneficial in terms of performance as a power fluid. For the useas hydraulic fluid, it is important to note that seawater contains

• suspended solids, practically any form of debris

10

0 5 10 15 20 251010

1015

1020

1025

1030

1035

salinity 20

salinity 25

salinity 30

salinity 35

salinity 40

T vs. ρ at atmospheric pressure

Temperature T [°C]

Den

sity

ρ [

kg/m

3 ]

0 200 400 600 800 10001020

1030

1040

1050

1060

1070

1080p vs. ρ at T = 0°C, salinity 35

Pressure p [bar]

Den

sity

ρ [

kg/m

3 ]

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, de-pending on the temperature and salinity (figure 3). The pH of Seawater fallsin the range 7.5 to 8.4. Compare this to freshwater which has a pH of approxi-mately 7, depending on temperature. Ocean acidification (like other changes inoceanic composition) is a serious concern, but the time scale for it to be influen-tial is to grand for this research. Also left out of this research are the additionalbenefits of pressurizing seawater. They will be investigated in due time2.

Seawater density depends on temperature, salinity and pressure. Colderwater is denser. Saltier water is denser. High pressure increases density.

The nonlinearity of the equation of state is apparent in contours of constantdensity in the plane of temperature and salinity (at constant pressure) - they arecurved. They are concave towards higher salinity and lower temperature.

Cold water is more compressible than warm water. That is, it is easier todeform a cold parcel than a warm parcel. Therefore cold water becomes denserthan warm water when they are both submerged to the same pressure. Thereforevarious reference pressures are necessary. We use a pressure which is relativelyclose to the depth we are interested in studying. The compressibility effect isapparent when we look at contours of density at say 4000 dbar compared withthose at 0 dbar.

The freezing point of seawater is lower than that of freshwater, at around2Magnesium, 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. Sofar this has been restricted due to the high processing costs.[14]

11

Element Percent [%]Oxygen 85.84Hydrogen 10.82Chlorine 1.94Sodium 1.08Magnesium 0.1292Sulfur 0.091Calcium 0.04Potassium 0.04Bromine 0.0067Carbon 0.0028

Table 1: Seawater composition (by mass) (salinity = 35)

?2?C? As sea water freezes, it forms pockets of salt. The salt (brine) leachesout 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 Laterkijken naar low maintenance

Dredging/centrifugal pumps ? − > lifetimeUseful Properties of Centrifugal PumpsGeneral descriptionDredging is an underwater excavation technique. Its main working principle

is the gathering up of bottom sediments and disposing of them at a differentlocation.

In essence, dredging is not a form of fluid power application but a masstransfer method. Fluid, usually in the form of sweat or salt water, is used as alubricant to avoid cavitation and aeration.

Centrifugal pumps. This is probably the most applied type of pump in theworld. The fact that the

The size of sediment particles in a flow is typically defined in microns (mi-crometers).

What makes these pumps suitable? (Essential properties)Resistance to erosion - noResistance to corrosion - to a certain extendDredging/centrifugal pumps − > lifetime Useful Properties of Centrifugal

PumpsIn contrast to centrifugal pumps, pd pumps are able to build up high pres-

sure. Centrifugal pumps stall when the pressure inside a system becomes toohigh. Since there are no tight fit with sealing in a centrifugal pump stall willoccur at relatively low pressures.

A pump suitable for a Delft Offshore Turbine does not yet exist. Thereforeither one has to be designed or an existing design is adapted to cope withseawater as hydraulic fluid. The logical next step is to determine the type ofpositive displacement pumping which is optimal for the DOT application.

12

6 Selection of Pumping Principle

6.1 Design Criteria for Seawater-based Positive Displace-ment Pumps

The main requirements for this pump are:

• Positive displacement. The only pumps which are suitable for fluid powerare 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 char-acteristics 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 usedfor smaller scale wind powered osmosis plants []

Reciprocating pumps require a crankshaft. This is unfavorable in terms ofload eccentricities due to asymmetrical loading. CHECK

Reciprocating diaphragm pump

PLAATJE

Rotary vane pump

low viscosity, non-lubricating liquids, restricted pressurePLAATJE

Rotary helix pump

PLAATJE

Rotary piston pump

PLAATJE

13

Lobe / gear pumps

Although these are more basic (less expensive) mechanisms, they only functionat a limited rpm range. At low rpm for instance, the leakage is to great togenerate 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 thisclearance begins to increase, the resulting volumetric losses will cause the overallefficiency to drop.

next step: find a material suitable for the fabrication of hydraulic compo-nents resistant to seawater

0 50 100 150 200 250 300 3500

1

2

3

4

5

6

7

E−126

Bard 5.0

V80

V90

SWT−3.6

Repower 5M

E−33

E−53

E−70

Rated rotation rate [RPM]

Rat

ed p

ow

er [

MW

]

Hagglund CBP pumpsWind Turbines

Figure 4: Power vs. rotor rpm of commercial wind turbines and HagglundsCBP pumps

Figure 5: A Hagglunds pump/motor of the radial piston rotating case type

6.3 Analysis

The pump types best suited for the Delft Offshore Turbine are vane pumpand the radial piston pump. Vane pumps can cope with low viscosity fluids

14

Figure 6: Performance efficiency of Hagglunds CA 210 (4 ports) pump/motor[15]

but is limited in terms of pressure. The radial piston pump can generate highpressure and can be designed to operate efficiently at rotation rates matchingthose of the wind turbines. However, the clearance between the piston and itscasing is a concern when using seawater. The higher pressure regimes of pistonpumps result in a much larger power-to-weight ratio. This is the main reason forselection the radial piston pump as the preferred choice for the system connectedto the rotor shaft of the Delft Offshore Turbines.

7 Conclusion

The main challenge for the DOT hydraulic energy transmission is to have arobust, 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 aloneone capable of pumping seawater for long periods of time without maintenance.In terms of power production, the Hgglunds CBP 210, with a little over 2.3MWrated power at 350bar pressure is the state of the art. More powerful systemsdo already exist in the form of prototypes. However, all require hydraulic fluidswith high viscosity. The conclusion therefore is that a specific system for highpressure 5MW seawater pumping will have to be designed. Important designconsiderations are:

- The Hgglunds CBP 210 (2.3 MW) operates with efficiency of 9096%. Inthese systems, the larger the piston, the higher the efficiency. Thereforeever more efficiency can be expected of a 5MW pump.

- The power-to-weight ratio of these types of pumps is much higher thanthat of the generators in wind turbines. This is without taking into accountall the extra components required for the use of an electricity generator.

- The low viscosity of seawater will lead to an increase leakage flow, reducingthe efficiently slightly.

- The high bulk modulus of seawater should significantly benefit the overallefficiency.

15

- The pump has to operate efficiently also at very low rpm. Trends in radialpiston pumps and commercial wind turbines indicate that designing formatching rotation rates is feasible.

- One of the first steps in the design process should be to find a solution forthe poor lubricity and the corrosive and erosive characteristics of seawater.The logical first step is to look for a structural material that is sufficientlydamage resistant for the fluid carrying parts of the pump. Finding thismaterial is crucial to the development of the project.

[Turbine No. of components reduction and thereby reduce the turbine massand subsequently the support structure mass.]

A this moment a pump fitting these requirement is not commercially avail-able. The main challenge is to have a robust, yet efficient system.

References

[1] Diepeveen, NFB, Closed-Loop Fluid Pumping as a Means to Transfer WindEnergy, DUWIND, Delft University of Technology, Proceedings EWEC 2009

[2] Diepeveen, NFB, Van der Tempel, J, Delft Offshore Turbines, ProjectPlan, 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 energytrans-mission technology in fluid power, Nanyang Technological University, Schoolof 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 Applica-tions 2002

[8] Unknown, Hydraulic wind energy conversion system, NASA STI/ReconTechnical Report, July 1981

[9] Rampen, W, Gearless Transmissions for Large Turbines - The History andFuture of Hydraulic Drives Artemis IP Ltd, Scotland, www.artemisip.com

[10] Muller, H, Poller, M, Basteck A, Tilscher, M, Pfister, J, Grid Compatibilityof Variable Speed Wind Turbines with Directly Coupled Synchronous Gener-ator and Hydro-Dynamically Controlled Gearbox, Proceedings Sixth Interna-tional Workshop on Large-Scale Integration of Wind Power and TransmissionNetworks 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 “sea-water”

[15] Hagglunds Product Manual for Compact CA Motors 2004 www.hagglunds.com

[16] Hagglunds Installation and Maintenance Manual for Compact CBP Motors2004 www.hagglunds.com

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