Water-hydraulic power transmission for offshore wind farms

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• Water-Hydraulic Power Transmission

for Offshore Wind Farms

Small-scale experiments with water-hydraulic

power transmission

N.RB. Diepeveen,

A. Jarquin Laguna en

A.S. Kempenaar

Offshore Engineering,

Technische Universiteit Delft

The Netherlands

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Platform Hydrauliek

2012 Dutch Fluid Power Conference

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N.F.B. Diepeveen, A . Jarquin Laguna, A.S. Kempenaar Water Hydraulic Power Transmission for Offshore Wind Fanns

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a a n d r i j v e n e n a u t o m a t i s e i e n

2012 Dutch Fluid Power Conference

a a n d r i j v e n e n a u t o m a t i s e i e n a a n d r i j v e n e n a u t o m a t i s e i e n

Abstract

The current state o f the art o f offshore wind turbine power transmission technology is expensive, heavy and maintenance intensive. The D e l f t Offshore Turbine project considers a radically new concept f o r power transmission in an offshore w i n d farm: using seawater as power transmission medium. For this project, small scale experiments have been conducted to show-case the functionality o f this concept and prove a novel wind turbine control strategy using water hydraulics.

1. Introduction

As a source o f renewable energy, offshore wind has enormous potential. However, the tech-nology currently used to exploit it is not cost effective without subsidies. The engineering challenge is to find technical solutions which are compatible w i t h free market economics: i.e. making offshore w i n d energy an economically competitive source o f energy.

A large (3 - 6 M W ) commercial offshore wind turbine is an enormously complex machine. Key points o f concern in modern wind turbine technology are the sheer number o f components, the use o f vast amounts o f copper and the amount o f software and switch gear required to nin all systems.

As the competition f o r sites to place offshore wind turbines is increasing, there is a gradual move to more remote and deeper locations. Mass and complexity w i l l therefore have an even bigger impact on the wind farm economics.

Keywords: Offshore wind Fluid power Hydraulic Iransiitissioii Water hydraulics Cenlralized generation Figure I. Offshore windfarm

Egmoiid aan Zee

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To minimize tiie cost o f energy, tlie power transmission system o f an offshore wind farm should be simple, robust, reliable and energy efficient.

The D e l f t Offshore Turbine (DOT) project aims to bring about a radical change i n the way energy f r o m wind is converted to electricity. The D O T project was launched in 2008 by D e l f t University Wind Energy Research Institute ( D U W I N D ) . The idea is to replace the conventional drive train by a hydraulic transmission system that pumps seawater f r o m individual turbines to one generator platform. For this project, knowledge f r o m several fields o f engineering is combined, including wind power, fluid power, hydro power and offshore o i l & gas.

I n this paper, the latest D O T project results and conclusions regarding water hydraulics are presented, as well as an outlook on planned fiiture developments.

2. The DOT transmission concept

2.1 Overview

The eventual goal o f the D O T project is to make offshore wind a competitive source o f energy by reducing complexity, mass, required maintenance and capital cost. With this i n mind a new transmission concept for wind farms was developed, see Figure 2.

The power conversion and transmission concept con-sists o f conventional hor-izontal axis wind turbine rotors that convert w i n d energy into rotary motion. Each rotor is directly coupled to a radial piston pump that converts this motion into a high pressure flow o f o i l . The high pressure line o f the closed o i l circuit transmits the power to the base o f the turbine. Here the o i l circuit is connected to an open seawater hydraulic circuit which transmits the power to a a generator station. A t this station, the high pressure flow o f multiple turbines comes together and is converted into electricity. Closed oil circuit

W i n d

Figure 2.

Simplified hydraulic diagram of the DOT transmission concept for an individual turbine

1 Radial piston oil pump 2 Oil motor 3 Seawater pump 4 Nozzle 5 Pelton turbine 6 Generator 6

Open seawater circuit

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2.2. Open-circuit seawater hydraulics;

cen tralized electricity production

Generator

platform

The high pressurized o i l at the base o f each turbine drives an o i l motor coupled to a seawater pump. The pressurized seawater from multiple turbines is then transmitted to the generator platform through an open seawater circuit as shown in Figure 3. I n this way the power transmitted from all the individual turbines is used to generate electricity in a centralized man-ner w i t h the help o f one or more hydro turbines. By centralizing electricity generation, all the individual generators and power electronics f r o m single turbines are replaced w i t h a few high rated generators similar to the ones used in conventional hydro power plants.

2.3. The offshore hydro power station

A t the generator station, the high pressure flows o f seawater are converted into high velocity seawater jets. These jets propel one or more Pelton turbines, converting fluid power into electricity.

O f the existing hydraulic turbines, the Pelton turbine is the most suitable for high pressures/high velocity jets. Pelton turbines

have been built up to a capacity o f 4 2 3 M W f o r a single machine, see Figure 4. High rotation speed, combined w i t h high torque, enables a compact design. Hence, a D O T farm requires only one or a f e w generators to produce electricity, using technology that is similar to what is used in hydro power plants.

Figure 3. Diagram of the DOT cemralized electricity

generation coincept

Figure 4. Pelton turbine used in the Bieudron hydroelectric power station in the Swiss

Alps: 423MWper unit with 1,883m head and 25

nrVs flow. (Image courtesy of the Renewables Grid Inilialive mvw. renewables-grid.eu)

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3. Experiments with water hydraulics

3.1. VaUdation of passive control metltod

These experiments sei-ved to prove a novel control strategy, whereby the torque (and thus the speed) o f a variable speed wind turbine rotor is controlled passively, i.e. without the manipulation o f the in place instruments.

The efficiency o f the energy extraction fi'om the wind by the rotor is maximized by continuously modifying the rotational speed o f the rotor depending on the wind speed conditions. I n modem wind turbines this is achieved using active control through the power electronics linked to the generator. I n the D O T transmission the rotor speed is passively controlled by matching the static characteristics o f the rotor with the load characteristics given by the combination o f the hydraulic drives and the nozzle. I n other words, passive control is imposed by accurately sizing the main components in the transmission. The passive control o f the rotor is thus a major simplification o f this system.

Figure 5.

Operational envelopes of the system

Both the energy at the input o f the wind turbine rotor and the energy at the nozzle exit are kinetic energy transported by a fluid. The torque-speed relations f o r the rotor operating at optimal rotor speed and the torque-speed relations o f the transmission are both a function o f rotor speed squared. Given the aerodynamic performance o f the rotor, these two relations are matched by selecting the proper nozzle diameter as shown in Figure 5.

The passive control strategy was validated through wind tunnel experiments at the Open Jet Facility (OJF) o f the T U Delft. The set-up contained a 1.8 m diameter wooden rotor directly coupled to a 12.5 cm^ axial piston water pump. The water flow exited the high pressure line through a nozzle. The build-up o f pressure i n

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Component Properties

Rotor Diameter: 1.8 m, airfoil: N A C A 4 2 1 2

Pump Danfoss PAH12.5, displacement: 12.5 cc/rev, nominal pressure: 160 bar, speed range 500-1800 rpm Line Length: 5m, diameter: 12.5mm (0.5in)

Nozzle Outlet diameter: 1.20/1.30/1.35/1.40/1.50/2.00 m m

Table 1. Passive conlrol

experiment -Component properties

the circuit and hence the braking torque o f the pump is determined by the diameter o f the nozzle mouth. Thehydraulic diagram o f the set-up is shown i n Figure 6.

H a h pressure work firic

PiCSiiiTL- scrrior m,i^.tijl U'wer IrvC pump

L U

'

Low p r c M ü r e f e e d W-v Submerjibis pump Figure 6. Hydraulic diagram of the experimental set-up

The o i l circuit o f the D O T transmission concept was thus not included in this set-up. This however had no influence on achieving the main goal o f these experiments aimed to validate the proposed control strategy, since the o i l circuit acts as a gear w i t h constant ratio.

A submersible pump was used to elevate the water to the positive displacement pump and thus prevent caviation and the pump f r o m running diy. The pressure in the feeding line amounted to approximately two bar at the level o f the reservoir l i d .

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3.2. MicroDOT 10kW demonstrator

To demonstrate the functionality o f the D O T transmission concept, a small scale (lOkW) version was assembled i n the water laboratory at the faculty o f C i v i l Engineering and Geosciences o f the T U D e l f t . The set-up includes a closed-loop hydraulic circuit as well as a freshwater open loop circuit and a small Pelton turbine. The wind turbine rotor was simulated by a speed-controlled electric motor driving the o i l pump. A hydraulic diagram and picture o f the constructed demonstration set-up are shown i n Figure 7 and Figure 8.

Generator platform

N.F.B. Diepeveen, A . Jarquin Laguna, A.S. ICempenaar Water Hydraulic Power Transmission for Offshore Wind Farms

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2012 Dutch Fluid Power Conference

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1 = Oil pump 2 = Pressure relief valve 3 = Manual start valve 4 = Solenoid brealï valve 5 = Oil motor 6 = Pressure relief valve

7 = Oil reservoir 8 = Boost pump 9 = Non retum valve 10 = Water pump 11 = Pressure relief valve 12 = Nozzle

13 = Unfiltered v/ater reservoir 14 = Water fillers 15 = Filtered water reservoir 16 = Pellon turbine 17 = Generator

V

- 0 p .

Low pressure water circuit High pressure v/ater circuit Low pressure oil circuit IHigh pressure oil circuit Boundary control volume

-0- 0

3>r

T j P; I 16 12l Figure 8. Hydraulic diagram of tlie lOldVmicroDOT Component Properties

Rotor Simulated by electric motor, 7.5 m diameter, 10 k W rated pow-er, speed ranae 40 - 150 rpm

Oil pump Bosch Rexroth AF2M180, displacement: 180 cmVrev, nominal pressure: 400bar. speed range: 0 - 3600rpm

Higti pressure o i l line Length: 10m, diameter : 0.75 inch

O i l motor Bosch Rexroth AF2M16, displacement: 16 cmVrev, nominal pressure: 400 bar, speed range: 0 - 8000 rpm Water pump Hydroton P60, displacement: 70.3 cmVrev,

nominal pressure: 160 bar, speed range: 0 - 2000 rpm High pressure water line Length: 10m, diameter: l.Oinch

Nozzle Diameter: 4.24 / 5.14 / 5.96mm

Pelton Wheel Pitch circle diameter: 0.4m, number o f buckets: 32 Generator Stamford synchronous generator PI044, rating: 8 k W

Table II. MicroDOT test setup

-Component properties

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Figure 9.

Steady-state results of tlie passive conlrol experiments

Figure 10. MicroDOT lOkW performance

2012 Dutch Fluid Power Conference

4. Results

4.1. Passive torque control for variable speed wind turbines

From the wind tunnel experiments steady-state values were obtained for different wind speed conditions and nozzle diameters. The results show good agreement w i t h the theoiy, and the control strategy was validated for a broad range o f operational conditions with a quick and stable response.

Small deviations f r o m the ideal quadratic relationships are observed due to the influence o f the mechanical and volumetric efficiencies o f the pump. Nevertheless the passive control strategy was validated for different nozzle diameters. Each one results in a different operational envelope o f the turbine, w i t h the nozzle diameter o f

1.35 mm coiTesponding to the optimal aerodynamic

D | ^ ^ ^ = 1 . 3 5 n i m ^nozzle " ^'^^ i

Va

D | ^ ^ ^ = 1 . 3 5 n i m ^nozzle " ^'^^

T l x

i

.}:\. D | ^ ^ ^ = 1 . 3 5 n i m ^nozzle " ^'^^ 5 0 0 rotor s p e e d t ) [ r p m ]

efficiency o f the rotor.

4.2. Transmission efficiency

During the M i c r o D O T experiments the performance o f the main components was measured for different operational conditions o f the rotor (in this case simulated through an electric motor"), as clearlv shown in Fisure 10.

The total efficiency o f the transmission, f r o m the o i l pump up to the nozzle exit, is between 43 and 48 % for rotor speeds above what corresponds to a wind speed o f approximately 4.7 m/s. Below this threshold, the efficiency is significantly lower, due to the reduced water pump efficiency. The nozzle efficiency is a result o f the geometric properties o f the tested nozzles which showed low

efficiency oil pump efficiency oil motor efficiency water pump efficiency nozzle total efficiency

60 80 100 rotor speed o [rpm]

140

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800

100 120 140 rotor speed o [rpm]

discharge coefficients. Thus, the efficiency o f the system could be improved by simply using a better geometrically shaped nozzle.

The rotor operating envelop, between 9 0 % and 100% o f the maximum rotor efficiency, is indicated by the grey area shown in Figure 11. The characteristic torque versus rotation speed cuives o f the transmission were measured for three different nozzle diameters. The curve con-esponding to the 5.96 m m

nozzle matches the rotor envelope f o r almost the complete range o f operation.

A n important finding f r o m the experiments with the M i c r o D O T set-up was the high start-up torque required to drive the water pump and therefore a high start-up torque for the wind turbine rotor was also necessaiy.

A t low rotational speeds, a minimum value o f torque (and therefore oil pressure) is required to run the water pump. Figure 12 shows how the torque is built-up to a level such that the oil pressure is enough to run the water pump. Once the water pump is rotating, the oil pressure drops causing the torque to decrease below the required minimum and the water pump stops rotating. The pressure starts to build up again and the process is repeated.

When the speed o f the oil pump is slightly increased, the torque at the water pump remains above the minimum

torque required and 250 normal operation o f the

system is realized as 2 0 0 observed by the constant | torque at the o i l pump T-shown in the last seconds | o f the graph f r o m Figure =

12. This high start-up ^ torque o f the water pump 1 w i l l limit the operation o f " ^ the mrbine at low wind

speeds. .50 1] m e a s u r e d

/

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V 13 l i m e [ s ] Figure 11. MicroDOT torque-rotational speed curves

matched lo the rotor envelope

Figure 12. Low efficiency of the water pump when

operated at low rotational speeds due to the high slart-iip torque required

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2012 Dutch Fluid Power Conference

5. Conclusions

The D O T power transmission concept for offshore wind fanns envisions centralized electricity production through the application o f seawater hydraulics. A t each m u l t i - M W w i n d turbine a pump generates a flow o f seawater to a hydro power station where pressure in the line is transformed to a high speed jet. The energy in the j e t is converted to electricity by a Pelton turbine.

To demonstrate the functionality o f this concept, a number o f small scale experiments have been carried out. Wind tunnel tests with a small rotor coupled to a water-hydraulic circuit proved that correct sizing o f hydraulic components and in particular the nozzle facilitates passive torque control on the wind turbine rotors.

Experiments with the l O k W M i c r o D O T demonstrator show an overall maximum power transmission efficiency o f around 48 %. This low efficiency also means the system response is strongly damped. It is estimated that the total efficiency can be improved up to 6 0 % with a custom made nozzle.

A n important lesson learned from the M i c r o D O T and wind hmnel experiments was that the start-up torque o f the axial-piston water pumps w i l l limit the operation o f the system at low wind speeds.

From the research and experiments conducted so far, no show stoppers have been identified. However, in the search for components it became clear that the market for (sea) water hydraulics is still a niche. In relation to similar products for o i l hydraulics, these components are veiy expensive.

Figure 13.

E.\periiueutal set-tip in tlw open jet wind tunnel

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6. Outlook

The idea o f using seawater as power transmission medium i n large offshore wind farms has great potential. To realize this requires convincing potential investors. The (often) only way to do this is through practical demonstration.

6.1. Next step of practical development

Improving nozzle efficiency by applying a custom-design nozzle.

Implementation o f the experimental transmission into a small wind turbine. Up-scaling the transmission and evaluating the benefits o f centralized electricity production through experiments & prototyping.

6.2. Gaps in knowledge

A n offshore wind turbine is subjected to continuous dynamic and stochastic loads from wind and waves. In order to design a transmission system capable to withstand the harsh environment and highly dynamic loading, yet operate efficiently with a minimum o f required maintenance, a great number o f question require answering. What w i l l happen when multiple DOTs are connected in series? H o w should the water hydraulic system o f the entire wind farm be configured to maximize energy efficiency? H o w w i l l the harsh environment and dynamic loading influence the lifetime o f the components?

Acknowledgment

For their support to the D O T project we thank, our sponsors Ampelmann, Bosch-Rexroth and Hydroton; Sander and Tonny o f the T U D e l f t Waterlab; Nico, Nando, Ruud, Joost and Nienke for their help with the wind tunnel experiments and Peter Albers f o r his expert advice.

Water-hydraulic power transmission for offshore wind farms

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