Flow Energy Harvesting by Means of Oscillating Airfoils

Maryam ZahediOctober 2015

Motivation & Objective• Aquatic animals, birds, and insects utilize

oscillatory motions with fins or wings for propelling

• Through these oscillatory motions, it is possible to extract energy from the incoming vortices

• Bio-inspired energy harvesting devices based on the oscillatory motions may be used to extract energy from the unsteady flow fields generated by free surface waves

• Scientist are interesting in this method of energy extraction due to the necessity of substituting the fossil fuel energy conversion with renewable methods

Energy Harvesting by means of Oscillating Foils (EHOF)

Mathematical Model & Governing Equations

0 Heaving motion : ( ) sinh t h t

0 Pitching motion: α(t) α sin(ωt ) 30Power coefficient: /((1/ 2)opC P U cs

. .1Cycle - avraged power: [ ( ) ( ) ( ) ]t T

tP N t h t M t dt

T

30Efficiency: /(1/ 2) pP U h s

Where, is the chord length, is the span length and is the difference between highest and lowest point .

c sph

According to the Betz’s limit, the efficiency is Limited to 60% in wind turbines. Same thing was addressed by W.McKinney and J. DeLaurier*

Fig1. Schematic of wingmill analytical model *

*J. D. William McKinney, “The wingmill: an oscillating -wing windmill,” Energy, vol. 5, pp. 109–115, 1981.

Different Type of the EHOF

(a) System with combined forced heaving and pitching motions(b) System with forced pitching but induced heaving motions(c) Self sustained pitching and heaving motion

Fig.2. Schematics of different EHOF *

*Q. Z. Qing Xiao, “A review on flow energy harvesters based on flapping foils,” Fluids Struct., vol. 46, pp. 174–191, 2014

Flapping Foils As Energy Harvesters Vs. Propulsors

Propulsors Energy harvesters

Energy flux is from foil to fluid Energy flux is from fluid to foil

Increase the energy of the fluid

Removes energy from the fluid

Velocity of the downstream increase

Velocity of the fluid decrease

Performance is measured in terms of thrust

Performance is measured in terms of percentage of energy extracted from the incoming flow

Rotary Blades vs. Oscillating foils• The oscillating foils is believed to me more environmentally friendly • A rectangular fluid inlet cross section of flow swept by an oscillating

foil may enable the extraction of more energy in the same available location compared to rotary blades which have a circular fluid inlet cross section

• The swept area can be wide and shallow for a single blade of device. Thus, they can be utilized in shallow water enabling a wider range of tidal resources to be used.

• Practical utility-scale wind turbines can achieve about 75% to 80% of the Betz limit while the average efficiency of the oscillating foils is reported about 28%, which only approximately 46% of the Betz limit.

Optimal Design for

Wind/Water Energy

Extraction

Rotary Blades

Aerodynamic Optimization

1. Rotor Diameter2. Number of Blades3. Pitch Angle4. Tip Speed Ratio5. Application of Wake instabilities6. Shape of the Foil7. Reynolds Number

EconomicalFactors

1. Material2. Installation3. Mechanical operation4. Maintenance

Environmental

Issues

Oscillating Airfoils

Aerodynamic Optimization

1. Foil Chord Length2. Number of Foils3. Angle Of Attack4. Frequency5. Application of Wake Instabilities6. Foil Shape Effect7. Reynolds Number

Economical Factors

1. Material2. Installation3. Mechanical operation4. Maintenance

Environmental Issues

Chart 1. A comparison pattern which articulates the parameters that affect the efficiency to find the optimal design for water/wind energy harvesting devices

Identification of the Effective Parameters on Efficiency: Aerodynamic parameters

• Oscillating frequency • Heaving amplitude• Angle of attack• Phase lag• Location of pitching axis • Wake instabilities• Foil shape• 3D effect • Reynolds number

Applying Other Mechanisms such as: Multiple Foil Arrangement Corrugated Foils Non – Sinusoidal Motion Flexible Structures

Aerodynamic parameters Effect on the Efficiency of the Oscillating Foils

Oscillating frequency The optimal point of maximum efficiency is within the range between 0.10 and 0.15

Heaving amplitude At low heaving amplitude (h0), efficiency increases with h0 until it reaches one chord length, then, the efficiency reduces

Angle of attack (AOA) It is necessary to consider the effect of AOA in an integrated way

Phase lag If phase lag between pitch and heave motions is equal to 900 , the maximum energy can be harvested

Location of pitching axis

The optimum efficiency will occur if the pivot location is in front of the mid-chord position.

Wake instabilities The maximum energy harvesting will be reached if the flapping frequency and the frequency of the most unstable mode occur simultaneously

Foil shape Not sensitive

3D effect A monotonic trend between aspect ration decrement and efficiency reduction was observed

Reynolds number Energy harvesting efficiency increased from 32.7% to 36.4% when Reynolds Number rises from 500 to 10000

Identification of the Effective Parameters on EHOF Efficiency: Economical Factors

• Material• Installation• Operational Cost• Maintenance

http://www.simens.comhttp://www.pulsetidal.com/

The mechanical systems of EHOF is also very complicated which lead to high capital cost of fabrication and high operating costs in terms of maintenance

Complex mechanical system also contribute to the efficiency of the EHOF

overall mechanical aerodynamic

It is crucial to have a comprehensive comparison between rotary blades and oscillating foils energy harvesters in terms of aerodynamic optimization,

economical factors and environmental parameters

The impact of key parameters on the efficiency and cost can be tested experimentally using the “Dynamic Similarity“ method

Important items in dimensionless analysis of the EHOF :

• Reynolds Number• Strouhal Number • Flow coefficient• Power coefficient These parameters have to be developed according to

the configuration of device

( )fLU

Example for rotary blades

22

3 3

3 5 3 5

1Reynolds Number: Re , if 10

Flow Coefficient: , 100

Power Coefficient 100

p p pm m mm p

m p

p pm mm p

m m p p

pmp m

m m m p p p

V LV LL L

V DV DN N

N D N D

PP P PN D N D

Optimal Design for Wind/Water

Energy Extraction

Aerodynamic Optimization

1. Blades Diameter/Chord2. Number of Blades3. Pitch Angle4. Tip Speed Ratio5. Application of Wake instabilities6. Shape of the Foil7. Reynolds Number

EconomicalFactors

1. Material2. Installation3. Mechanical operation4. Maintenance

Environmental Issues

Rota

ry

Blad

esO

scillating Foils

Thank you for your time!