Center for Ocean Renewable Energy, University of New Hampshire

Center for Ocean Renewable Energy,
University of New Hampshire
Alex Johnston
ajc32@unh.edu
Adviser: Martin Wosnik
martin.wosnik@unh.edu


Tidal currents are a predictable, renewable source
of energy
Cross-flow axis hydrokinetic turbines
◦ Favorable turbine geometry for deployment in shallow flows
(compared to horizontal, in-stream axis turbines)
◦ Can receive flow from any direction
 Do not have to yaw into flow or pitch blades when tidal current
changes directions
 Reduces complexity for underwater deployment
◦ Complex and interesting fluid dynamics
2

Straight-bladed (e.g. Darrieus)
◦ Severe lift/torque pulsations

Helical blades (e.g. Gorlov)
◦ Reduce lift/torque pulsations
◦ Circumferential overlap of blades
◦ With full overlap, lift/torque is
circumferentially averaged (there is
always some portion of a blade
experiencing maximum lift)
◦ LUCID GHT uses NACA 0020
symmetrical hydrofoil
Picture 1 from: http://www.energy-daily.com/reports/PacWind_To_Offer_Three_New_Proprietary_Applications_For_Wind_Energy_999.html
Picture 2: Lucid Energy, Gorlov Helical Turbine 1m x 1.25m
3
Geometry and Definitions
Free Stream
Velocity, U
Where,
V = radial velocity
W = relative wind
U = free stream velocity
L = lift force
D = drag force
N = normal force
T = tangential force
θ = angle of rotation
α = angle of attack
ω = 2πf
http://en.wikipedia.org/wiki/File:Forces_and_velocities.png
4
Tip Speed Ratio
R

U
Relative “wind”
(velocity experienced by blade element)
W  U 1  2 cos   2
Angle of Attack

Vector relations
derived from
geometry and written
in terms of the tip
speed ratio, λ
(for a blade element)
 sin  
  tan 

   cos  
1
5
Relative Velocity (constant Vinf=2m/s)
Angle of Attack
8
40
Desired values
TSR=1.57
TSR=1.83
TSR=2.09
TSR=2.36
TSR=2.62
TSR=2.88
30
20
alpha (rad)
λ
7
6
Wrel (m/s)
λ
10
TSR=1.57
TSR=1.83
TSR=2.09
TSR=2.35
TSR=2.62
TSR=2.88
theta=180
0
5
4
-10
3
-20
2
-30
-40
0
50
100
150
200
theta (deg)
250
 sin  



cos



  tan 1 
300
350
400
1
0
R
TSR:  
U
50
100
150
200
theta (deg)
250
300
350
400
W  U 1  2 cos   2
6




Foil data from Sandia National Laboratories
CL and CD values were generated using wind
tunnel data and extended using airfoil code
XFOIL
CL and CD functions of α for a full 360°
Data for several values of Reynolds number
and different airfoil shapes
7



NACA 0021 (symmetric) foil shape;
Reynolds number range: 40,000-5 million
Using
◦ The geometrically derived equations for α and W,
◦ CL/CD data,
◦ And equations for lift and drag forces

Calculate torque and power as functions of
rotation angle for a blade element
8





Breaks up blades into small individual
elements
Determine forces on each element
Forces are integrated over the full length of
the blade
Performed for all 3 (or any number of) blades
Determine torque and power for the entire
turbine as a function of rotation angle
9

Necessary because
◦ Free stream velocity and RPM are not coupled in model
◦ no losses accounted for

Scale model turbine specifications:
◦
◦
◦
◦

Diameter = 1m
Length = 1.1m
Blade chord = 140mm
Blade slope angle = 67°
Data was collected during push tests of turbine
(Lucid)
10
Acquire curve fits from experimental data
◦ RPM vs. Free Stream Velocity
◦ Power predicted by code (no losses) vs. Measured
Power
Predicted and Measured Power
14.0
12.0
Power (kW)

10.0
Measured
8.0
Power
6.0
Predicted
4.0
Power
2.0
1.5
2.5
3.5
4.5
5.5
Free Stream Velocity (m/s)
11


General Sullivan Bridge in the Great Bay Estuary, NH:
good test site, strong tidal currents (up to ~ 2.5
m/s, typically >2 m/s during each tidal cycle)
Data acquired by Carl Kammerer (UNH) using a
bottom deployed ADCP (Acoustic Doppler Current
Profiler)
Tidal Energy Test Site (under bridge)
Newington
N
Dover
ocean
12
Power Output and Free Stream Velocity over Time
Power Output and Free Stream Velocity over Time
1.5
1
1
0.5
0.5
0
2
4
6
8
Time (hours)
10
12
14
0
Power (kW)
1.5
2.5
2
2
Free Stream Velocity (m/s)
Power (kW)
2
0
2.5
2.5
2
1.5
1.5
1
1
0.5
0
Free Stream Velocity (m/s)
2.5
0.5
0
100
200
300
400
500
Time (hours)
600
700
0
800
Prediction for 30 days (full lunar cycle) for a 1.1m2 Lucid Energy GHT
(turbine diameter D=1m, height L=1.1m):
 Energy = 653.1 kWh
 Average Power: ~ 0.9 kW
 Note: average monthly electric energy consumption per household
in New Hampshire is 616 kWh (DOE EIA, 2008)
13



Cross-flow axis turbine blades have large
angle of attack variations as blades rotate
Flow separates (hydrofoil stalls) for certain
AoA ranges during rotation
Large increase in drag on hydrofoil results in:
◦ large lift/torque variations as blades rotate
◦ decrease performance

If stall avoided, hydrodynamic performance is
maximized
14



~ 0° AoA: no lift, small drag
(for symmetric airfoil sections)
~ 5°-15° AoA: lift>drag
~ 15°+ AoA: lift<drag
NOTE: AoA ranges are a function
of Reynolds number and airfoil
thickness
Picture from: http://www.aviation-history.com/theory/angle_of_attack.htm
15
Lift/Drag Ratio vs. AoA

50
CL/CD
40
30

20
10

0
0
5
10
15
Angle of Attack
20
25
For low AoA lift increases
with AoA, while drag does
not vary much
Once flow is separated,
large increase in drag while
large decrease in lift
Separation point depends
on foil shape and Reynolds
number
NACA0021, Re=360,000
16
NACA0021 Torque (1 blade)
NACA0021 Torque (3 blades)
600
1200
TSR=1.5708
TSR=1.8326
TSR=2.0944
TSR=2.3562
TSR=2.618
TSR=2.8798
500
1000
800
300
T (Nm)
T (Nm)
400
200
600
400
100
200
0
0
-100
0
50
100


150
200
theta (deg)
250
300
350
TSR=1.5708
TSR=1.8326
TSR=2.0944
TSR=2.3562
TSR=2.618
TSR=2.8798
400
-200
0
50
100
150
200
theta (deg)
250
300
350
400
Regions of stall = poor performance/large variations in
torque
Define “Critical Tip Speed Ratio” (λcrit)
◦ λ above which turbine torque no longer becomes negative
during rotation (except at AoA = 0, 180, where there is no lift)
17
NACA0021 Torque (1 blade)
NACA0021 Torque (3 blades)
700
1400
TSR=2.3562
TSR=2.618
TSR=2.8798
TSR=3.1416
TSR=3.4034
TSR=3.6652
600
500
TSR=2.3562
TSR=2.618
TSR=2.8798
TSR=3.1416
TSR=3.4034
TSR=3.6652
1200
1000
T (Nm)
T (Nm)
400
300
800
600
200
400
100
200
0
-100
0
50
100


150
200
theta (deg)
250
300
350
400
0
0
50
100
150
200
theta (deg)
250
300
350
400
Stall avoided when TSR high enough
Define “Optimum Tip Speed Ratio” (λopt)
◦ Where λ is sufficiently high to keep AoA below stall
angle throughout turbine rotation
18
Angle of Attack (AoA) throughout
rotation Angle
with
varying TSR
of Attack (constant Vinf=2m/s)
25
Desired values
TSR=2.35
TSR=2.62
TSR=2.88
TSR=3.14
TSR=3.40
TSR=3.67
theta=180
20
15
10
alpha (rad)

5
0
-5
-10
-15
-20
-25
50
100
150
200
250
theta (deg)
300
 sin  



cos



  tan 1 
350
400

Define range of desired
values based on stall angle
for a specific Reynolds
number
The TSR at which the
derived function for angle of
attack stays within the
desired range is the optimal
TSR (λopt)
Values of λcrit and λopt are
turbine-specific. For a D=1m,
67° sweep angle GHT the blade
element model predicts:
crit  2.24
opt  3.60
19
TSR v Vinf for No load and Peak Power TSR; Crit. and Opt. TSR
4
3.8
opt  3.60
3.6
Tip Speed Ratio
3.4
3.2
3
No Load TSR
Peak Power TSR
Critical TSR
Optimal TSR
2.8
2.6
2.4
UNH model
crit  2.24
2.2
2
0.6
Explanation for decrease in
TSR: due to increase in drag
on support struts?
0.7
0.8

0.9
1
1.1
1.2
Free Stream Velocity
1.3
1.4
1.5
Data from full scale LUCID turbine site testing in
Amesbury, MA in the Merrimack River (MTC 2005)
◦ Diameter=1m, sweep angle = 67°, Length = 2.5m
20



The model under development uses derived
equations and CL and CD data to predict torque
and power curves for cross-flow turbines
Model is calibrated using experimental
performance data from scale testing for a
specific turbine
With temporally/spatially varying flow data from
a deployment site, power/energy performance
can be accurately predicted
21


Stall of the hydrofoil sections during rotation can lead to
poor performance of the turbine
Two Tip Speed Ratios of interest were defined:
◦ the “critical tip speed ratio”, λcrit , for desirable performance
(torque remains positive during stall)
◦ the “optimum tip speed ratio” , λopt, for which performance is
maximized (dynamic stall avoided)

Working hypotheses:
◦ It appears that a turbine under no-load conditions (“freewheeling”) will try to rotate (“settle”) near λopt
 Hydrodynamic performance optimized
 Is a “range of AoA-weighted” lift/drag ratio maximized near λopt? (if
turbine moves off this operating point, will weighted lift/drag decrease?)
◦ It appears that a turbine will have its maximum power output (peak
performance) near λcrit
 If λ < λcrit, power drops due to regions of negative torque
22




Correlate data gathered from the UNH tow
tank for different turbines
Predict performances at the GSB test site
Compare predictions with prototype
deployments
Further investigate/validate hypotheses that
turbines will rotate at model-predicted critical
and optimal tip speed ratios when under noload and at peak power, respectively
23
[1]
Sheldahl, R. E. and Klimas, P. C., Aerodynamic Characteristics of Seven Airfoil Sections
Through 180 Degrees Angle of Attack for Use in Aerodynamic Analysis of Vertical
Axis Wind Turbines, SAND80-2114, March 1981, Sandia National Laboratories,
Albuquerque, New Mexico.
[2]
Gorlov, A.M., Professor of Engineering, Northeastern University.
[3]
Fox, R.W., Pritchard, P.J., and McDonald, A.T., Introduction to Fluid Mechanics, Seventh
Edition. John Wiley & Sons, Inc., 2009.
[4]
Verdant Power LLC, GCK Technology, Inc. Amesbury Tidal Energy Project: Integration
of the Gorlov Helical Turbine into Optimized Hardware/Software System Platform, MTC
Final Report. April 2005.
[5]
Lucid Energy Technologies, LLP, Push Test Data Acquisition for GHT, Vancouver, BC,
2007.
[6]
Carl Kammerer, NOAA/NOS/CO-OPS and UNH/Joint Hydrographic Center, Tidal
TurbinesWith a Twist
Currents in the Piscataqua River, NH: Preliminary findings from the 2007 National
Current Observation Program Survey. 2008
24

New England Marine Renewable Energy
Center (NE-MREC):
◦ Summer scholarship for research development

U.S. Department of Transportation (DOT):
◦ Graduate Fellowship through the New England
University Transportation Center (NE UTC)

Lucid Energy Technologies, LLP
◦ Turbines and Data

UNH Department of Mechanical Engineering
25