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02‐06‐20141

Energy storage system – design and modeling considerations

Dong Wang, Kaiyuan LuDept. of Energy Technology, Aalborg University

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• Purpose and role in the whole project

• Hydraulic energy storage – design and optimization System topology  Verification Case study ‐WaveStar Optimization

Content

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Purpose and role in the whole project

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It behaviors likes a filter (buffer) – smoothing the energy generated by irregular energy sources (e.g. waves).

• The output power to the grid is more stable, predicable, and controllable.

• Secure the energy supply. 

• Extra economic benefits like moving off‐peak power to on‐peak periods with higher electricity prices. (smart grid)

Advantages offered by an energy storage system 

Purpose

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• Understand the performances of energy storage systems

• Understand their interactions with the rest of the Wave Energy Converter (WEC). – require a proper model that   can be inserted in the complete WEC mode.

In this project, we need to

Role in the project

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Hydraulic energy storage – design and optimization

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• hydraulic accumulator• hydraulic pump/motor• Reservoir• connecting lines• controller

Basic configuration

System topology

Gas

Oil

Pump/Motor

Reservoir

Controller Mechanicalinput / output

Hydraulicinput / output

ShaftConnecting linesControl signal

Accumulator

Hydraulic energy storage system

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• A regeneration system is used for verification

Mathematical model is established in Simulink

System modeling

[A. Pourmovahed, etc. all, 1992], Ref [1]

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• A regeneration system is used for verification

Mathematical model is established in Simulink

System modeling

Hydraulic Energy Regeneration System

Scope Ppm_alpha

Scope Ppm_Ta

Scope Ppm_Qa

Scope FW_N

Scope Eff_rt

Scope Acc_p

Ppm_Qa Res_p

Reservoir

FW_N

Ppm_alpha

Acc_p

CL_pl

Res_p

Ppm_Qa

Ppm_Ta

Pump/Motor

Ppm_Ta

FW_N

Ef f _rt

Flywheel

Acc_v

FW_N

Ppm_alpha

Control

Ppm_Qa CL_pl

Connecting Lines

Ppm_Qa Leav e Acc

Acc_v

Acc_p

Accumulator

[A. Pourmovahed, etc. all, 1992], Ref [1]

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10 02‐06‐2014

Model of hydraulic accumulator

System modeling

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11 02‐06‐2014

Simulation and reference results comparison

Verification

0 20 40 60 80 100 1200

200

400

600

800

1000

1200

1400

1600

1800

2000

Time (s)

Flyw

heel

Spe

ed (r

/min

)

P/M Angle = 20 Deg, Cold Oil

0 20 40 60 80 100 1204

6

8

10

12

14

16

18

20

Time (s)G

as P

ress

ure

(MP

a)

P/M Angle = 20 Deg, Cold Oil

0 5 10 15 20-100

-80

-60

-40

-20

0

20

40

60

80

100

Cycle Number, i

Rou

nd-tr

ip E

ffici

ency

, %

P/M Angle = 20 Deg, Cold Oil

[A. Pourmovahed, etc. all, 1992], Ref [2]

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System topology• The excessive wave energy 

enters the energy overflow system, where energy storage system is installed

• Hydraulic pump/motor works at motor mode only

• Output power to generator is controlled by adjusting the swivel angle

Case study - WaveStar

[R. H. Hansen, etc. all, 2011], Ref [3]

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13 02‐06‐2014

System model

Case study - WaveStarHydraulic Energy Storage System

Wave_no

Wave Type1, 2 or 3

Scope Ppm_alpha

Scope Ppm_Ta

Scope Ppm_Qa

Scope Gen_T

Scope Gen_N

Scope Eff_Sys

Scope CC_Qa

Ppm_Qa

CC_Qa

Res_p

Reservoir

FW_N

Ppm_alpha

Acc_p

CL_pl

Res_p

Ppm_Qa

Ppm_Ta

Pump/Motor

Ppm_Ta

Gen_N

Gen_T

Gen_Pin

Electric Generator

Gen_Pin

Cy l_P

CC_P

Whl_Ef f

Sy s_Ef f

Efficiency Calculation

Acc_p

Wav _t

Res_p

Cy l_P

CC_Qa

CC_P

Cylinder and Check Valve

Acc_v

Gen_T

Ppm_alpha

Control

Ppm_Qa CL_pl

Connecting Lines

Ppm_Qa Leav e Acc

CC_Qa

Acc_v

Acc_p

Accumulator

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Work procedure

Case study - WaveStar

910 920 930 940 950 960-1

-0.5

0

x

Fraction of Maximum Unit Capacity

910 920 930 940 950 9600

20

40

60

Time (s)

Sha

ft Pow

er [k

W]

Shaft Power and Hydraulic Input Power

910 920 930 940 950 9600

100

200

300

Time (s)

Hyd

raulic P

ower [k

W]

910 920 930 940 950 9600

0.01

0.02

0.03

0.04

Gas

Volum

e [m

3 ]

Gas Volume, Flow Rate and Pressure

910 920 930 940 950 960-5

0

5

10

15x 10

-3

Time (s)

Flow

Rate [m

3 /s]

910 920 930 940 950 9600

10

20

30

40

Time (s)

Press

ure [M

Pa]

1234 5 6 7

1. Too much input, relief valve open to limit the system pressure to max; swivel angle (capacity) is limited in order to limit the shaft power output;

2. No input, system pressure drops as stored energy is used; no pump capacity limitation; 

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Key parameters regarding system efficiency optimization• System maximum allowable pressure –

‒ How much energy can be stored in accumulator‒ How fast hydraulic motor can convert energy

• Accumulator size –‒ How much energy can be stored

• Hydraulic motor displacement (power) –‒ How fast it can convert energy – from hydraulic to 

mechanical

Optimization

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System maximum allowable pressure

Optimization

0100

200300

400 4050

6070

0.7

0.75

0.8

0.85

0.9

0.95

Hydraulic MotorDisplacement [cm3/rev]

System EfficiencyWave 3, 40kW Generator, System Max. Pressure 35MPa

Accumulator MaximumGas Volume [liter] 0

100200

300400 40

5060

70

0.7

0.75

0.8

0.85

0.9

0.95

Hydraulic MotorDisplacement [cm3/rev]

System EfficiencyWave 3, 40kW Generator, System Max. Pressure 45MPa

Accumulator MaximumGas Volume [liter]

0100

200300

400 4050

6070

0.7

0.75

0.8

0.85

0.9

0.95

Hydraulic MotorDisplacement [cm3/rev]

System EfficiencyWave 2, 40kW Generator, System Max. Pressure 35MPa

Accumulator MaximumGas Volume [liter] 0

100200

300400 40

5060

70

0.7

0.75

0.8

0.85

0.9

0.95

Hydraulic MotorDisplacement [cm3/rev]

System EfficiencyWave 2, 40kW Generator, System Max. Pressure 45MPa

Accumulator MaximumGas Volume [liter]

At sea state 3:

• ↑ → ↑

• ↑ → ↑

• ↑ → ↓

At sea state 2:

• ↑ → ↓→ ↓→ ↓

• ↑ → ↓→ ↓→ ↓

Structural Design of Wave Energy Devices  –

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0100

200300

400 4050

6070

0.7

0.75

0.8

0.85

0.9

0.95

Hydraulic MotorDisplacement [cm3/rev]

System EfficiencyWave 3, 30kW Generator, System Max. Pressure 35MPa

Accumulator MaximumGas Volume [liter] 0

100200

300400 40

5060

70

0.7

0.75

0.8

0.85

0.9

0.95

Hydraulic MotorDisplacement [cm3/rev]

System EfficiencyWave 3, 40kW Generator, System Max. Pressure 35MPa

Accumulator MaximumGas Volume [liter]

0100

200300

400 4050

6070

0.7

0.75

0.8

0.85

0.9

0.95

Hydraulic MotorDisplacement [cm3/rev]

System EfficiencyWave 2, 30kW Generator, System Max. Pressure 35MPa

Accumulator MaximumGas Volume [liter] 0

100200

300400 40

5060

70

0.7

0.75

0.8

0.85

0.9

0.95

Hydraulic MotorDisplacement [cm3/rev]

System EfficiencyWave 2, 40kW Generator, System Max. Pressure 35MPa

Accumulator MaximumGas Volume [liter]

17 02‐06‐2014

Generator rated power

Optimization

At sea state 3:

• ↑ → ↑

• ↑ → ↑

• ↑ → ↑

At sea state 2:

• 30kW is big enough

• At very small ,  drops 

due to waste of energy

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Discussions• Balance sea state 1, 2 and 3 to obtain the highest 

possible average system efficiency• Relative low hydraulic motor capacity and high system 

pressure is preferred• It can ensure that the hydraulic motor works with a relative 

high average efficiency and give much smooth output torque

• Balance the system max. allowable pressure and size to obtain optimal cost• High  requires high quality component, thus high cost• High  can reduce the requirement of accumulator size and 

hydraulic motor displacement, which may reduce cost• High  can give high system efficiency, which save money

Optimization

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Summary• The model can be used to analyse the performance of 

hydraulic energy storage system, and give reasonable accuracy

• Key parameters regarding system efficiency optimization are found and discussed

• A system efficiency around 85 percent seems to be an attainable theoretical value. In a real system, system efficiency of 80 percent may be practically expected.

Summary

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Reference1. A. Pourmovahed, N. H. Beachley, F. J. Fronczak, “Modeling of a Hydraulic Energy Regeneration 

System – Part I: Analytical Treatment”, Journal of Dynamic Systems, Measurement, and Control, vol. 114, pp. 155‐159, Mar. 1992.

2. A. Pourmovahed, N. H. Beachley, F. J. Fronczak, “Modeling of a Hydraulic Energy Regeneration System – Part II: Experimental Program”, Journal of Dynamic Systems, Measurement, and Control, vol. 114, pp. 160‐165, Mar. 1992.

3. R. H. Hansen, T. O. Andersen, and H. C. Pedersen, Model Based Design of Efficient Power Take‐off Systems for Wave Energy Converters, in The 12th Scandinavian International Conference on Fluid Power, May 18‐20, Tampere, Finland, 2011

Funded by

The International Research Alliance

2102‐06‐2014