Renewable Energy Technology I

Wind Turbine Laboratory Report

Rajesh Ramesh

MSc Electrical Engineering for Sustainable and Renewable Energy

2015-2016

Department of Electrical and Electronic Engineering

University of Nottingham

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Abstract

When attempting to obtain the maximum efficiency from a wind turbine generator, it

becomes apposite to understand the correlation between wind velocity and generating

capacity. This know-how of the relationship between the bespoke parameters allows for

simplified analysis of how a wind turbine operates under varying physical conditions. This

experiment, conducted with a Rutland 913 Turbine Generator, helps in identifying key

similarities and differences in the theoretical and real-time power flow (both of the wind

(Wk) and of the electricity generated (We)) in comparison to the wind velocity. A detailed

analysis is performed with the help of data plots and the respective congruencies or

deviations are explained with suitable standard data and practical examples.

Keywords: analysis, electricity, power, Rutland 913, wind turbine, wind velocity

Introduction

Taking a review of several policies and programmes to promote wind energy based

generation around the world, it could be noted that wind turbine performance monitoring

is a vital aspect which governs the lifespan and efficiency [1] [2]. No one single parameter

alone could be considered when turbines are manufactured or operated and so a

combination of these parameters, related through equations as would be explained in

later sections, establish correlations that allow analysis and resulting design /

performance optimization.

While parameters such as wind (or) air velocity (c), mass of air (m), density of air (ρ),

wind temperature (T), voltage (V), current (I) are considered in this experiment, many

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more such as power co-efficient as a function of pitch angle, blade tip speed, climatic

factors etc. are also taken into account in other real-world, macro cases to ensure

maximization of output [3].

It is interesting to note that the theoretical maximum limit to energy that could be

extracted from wind energy has been established to be 59.3% by the Betz Law and has

been instrumental in the design of wind turbines since the 1920s [4]. In real time, wind

turbines achieve about 75% - 80% of the Bertz Co-efficient Value.

The kinetic energy (Ek) present in the wind, driven by the convection currents generated

due to uneven heating and cooling of the atmosphere by solar radiation, strike the blades

of the wind turbine with a velocity (c). The mass of air (m) flowing over the turbine is a

vital component and the overall kinetic energy is given by:

Ek = ½ mc 2 (1)

This kinetic energy is converted into mechanical energy as per the law of conservation of

energy and since this is a real time system, losses would be recorded which could be

primarily electrical and / or structural (mechanical) in nature.

A Wind Energy Primer by E.ON Energy states that “a 20% increase in blade length leads

to a 44% increase in the power produced” [5]. Considering this, the total available power

(W) is determined by the mass of air flowing over the turbine blades in 1 second (M)

given by:

M = ρAc (2)

where ρ is the density of air and A is the area that the turbine blades sweep out which is

given by:

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A = πr2 (3)

where ‘r’ is the radius of the circle swept by the blades of the turbine with its centre at

the coupling junction for all the three blades. Fig.2 illustrates the blade radius and swept

area measurements. The swept area is also referred to as the ‘disc’ by engineers and

industry experts.

Figure 1. Blade Radius and Swept Area of a Wind Turbine | Source: http://xtremewind.blogspot.co.uk/2015/06/wind-turbine-

blade-swept-area-versus.html

Hence, from (1), (2) and (3), the total power flow in the wind (Wk) is given by the

equation:

Wk = ½ ρAc 3 (4)

It is important to note here that the power generated by the turbine is proportional to

the a) area swept by the turbines and b) cube of the wind speed.

! Why do most turbines have only 3 blades and not lesser number or more

number of turbines? [Appendix I]

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The electrical power produced is given by the equation:

WE = VI (5)

Where V is the voltage and I is the current produced by the generator set located at the

hub behind the turbine setup atop the mast.

Hence, turbine efficiency (η) is given by:

η = WE / WK (6)

The above calculations are performed keeping in mind the main objectives of the

experiment which are as follows:

a. To understand and appreciate the experimental setup of the Rutland 913 Wind

Turbine along with the Wind Tunnel Setup.

b. To calculate parameters such as electrical power, mechanical power and overall

generating capacity of the wind turbine with an aim to establish a relationship

between it and wind velocity.

c. To calculate the wind turbine efficiency under different settings of wind velocity

It is expected that the graph plotting the data such as power produced versus the average

wind velocity should be uniformly exponential in nature when depicted as a 2nd or 3rd

order polynomial equation.

Experimental Setup and Procedure

a. Setup

Fig.2 below illustrates the experimental setup of the Wind Tunnel Simulator & Wind

Turbine.

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Figure 2. Experimental Setup

List of Parts / Components

Wind Tunnel Wind Tunnel VFD Motor

Control Panel Rutland 913 Wind

Turbine Analogue Ammeter

Analogue Voltmeter Ni Cd Battery Pack

(Load) Connecting Wires Digital Anemometer

Actual Experimental Setup and Descriptions

1. Wind Turbine

Image 1. Rutland 913 Wind Turbine in action in front of the Wind Tunnel’s Conversion Cone

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The Rutland 913 Wind Turbine is a UK Manufactured 12 V / 24 V 250 W low start speed

(5 knots) wind turbine primarily used to charge on board batteries in boats which are

over 10 m in hull length.

The ideal dimensions for this turbine are given below:

1. 6 efficient aerofoil blades

2. Turbine diameter = 910mm

3. Length = 608mm

4. Weight = 10.5Kg

5. Power output = Maximum 250W

6. Available as a 12V or 24V system

! Why do we use a 6 blade design here? [Appendix II]

2. Variable Frequency Drive

Image 2. ABB SAMI Ministar Variable Speed Drive

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Figure 3. Variable Frequency Speed Drive Control

The variable frequency drive used in this setup allows for smooth control of output power

which in turn controls the speed of the wind tunnel fan setup which acts as the wind

source [6][7]. As shown, the knob upon which the experimenter’s hands are placed

(Img.2), is used to control the frequency which in turn controls the speed with which the

rotor of the motor rotates (angular frequency control). The display indicates that the

speed control is done in terms of percentage. The following equation stipulates how the

percentage speed control is effected.

𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝑅𝑒𝑔𝑢𝑙𝑎𝑡𝑖𝑜𝑛 =𝑁𝑜 𝐿𝑜𝑎𝑑 𝑆𝑝𝑒𝑒𝑑 − 𝐹𝑢𝑙𝑙 𝐿𝑜𝑎𝑑 𝑆𝑝𝑒𝑒𝑑

𝐵𝑎𝑠𝑒 𝑆𝑝𝑒𝑒𝑑∗ 100 [7]

3. Wind Tunnel

Image 4. Sample Experimental Setup - Similar to University of

Nottingham – RE Lab's Wind Tunnel

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The Renewable Energy Laboratory’s Open Circuit Wind Tunnel consists of the following

parts (in order from back to front). It is open circuit since new air is let inwards as the

setup operates.

1. Drive Section – As shown in Img. 3, the drive section which is spaced inside from

the back section houses the fan connected to the rotor of the variable frequency

drive motor (ABB).

2. Diffuser – This region keeps the air moving smoothly along a fixed path. It is to

be noted here that the wind is not in a steady path.

3. Contraction Cone – This region comes right before the settling chamber which is

right at the front of the tunnel. Here is where the wind speed rises from a more

constricted setting to an area with more magnitude of space. Since the wind flow

is outward, this could also be termed as the Expansion Cone since the outflowing

wind is expansive in nature.

Image 3. A. Reverse View - Wind Tunnel - Showing Protection Mesh and Drive Fan Section |

B. Full Reverse View of the Variable Speed Drive Motor Fan Setup

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4. Settling Chamber – As seen in Img. 1, this region holds the honey-comb mesh

(made of Aluminum or its alloy), which brings down the air turbulence from the

diffuser and contraction cone alongside straightening out the air.

Figure 4. An Industrial Scale Wind Tunnel

4. Digital Anemometer

Image 3. Airflow - Digital Anemometer with Readings

Front

Back

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The digital anemometer is attached with a fan whose rotational velocity is recorded in a

handheld device as shown above by the experimenter in Img.3. Most of these devices

are also used as flow sensors to measure rate of flow of fluids.

5. Battery Setup

b. Procedure

The group consisting of 4 students as experimenters were allotted individual tasks to

perform in the experiment. The tasks namely were as follows:

Stage I: Wind Velocity Readings

1. VFD Motor Fan Speed Control

2. Digital Anemometer Data Reading

3. Recording of values in lab sheet for appropriate calculations

Stage II: Voltage and Current Readings

1. VFD Motor Fan Speed Control

2. Analogue Voltmeter and Ammeter Data Reading

3. Recording of values in lab sheet for appropriate calculations

Figure 5. Battery Topologies - a. Parallel and b. Series

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The Lab Instructor had completed a demonstration session following which the student

team took the lead in performing the experiment under the various conditions outlined.

Stage I

Step 1: The Digital Anemometer was calibrated.

Step 2: The Student Team was advised to remove the Wind Turbine from its mast as the

Stage I of the experiment did not require its operation and also posed a risk while

recording wind velocity data using the digital anemometer. The Lab Instructor aided the

Student Team with this task.

Step 3: A Member of the Student Team switched ON the Wind Tunnel Fan using the flip

switch of the VFD Speed Control Panel as shown in Img.2 and the fan speed controller

setting was initiated at 10%. It is to be noted here that the knob controlling the

percentage speed setting has been designed to be sensitive and so it is advised to avoid

rapid changes in speed settings which have the potential to damage the drive. As the

speed was increased, the noise level of the wind tunnel also proportionately increased.

Step 4: At each speed percentage setting, five values are to be recorded at five different

locations as illustrated below.

I

II

III

IV V

Figure 6. Locations for Anemometer Data Readings

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The wind velocity data for each speed percentage setting should be recorded for each of

the above locations (which could be understood as the (in order) 12’O, 3’O, 6’O, 9’O Clock

locations and the Centre of the Clock). The digital anemometer’s fan is to be held for at

least ten seconds facing the honey-comb mesh with a gap not more than 2 cm to record

accurate readings.

NOTE: Due to excessive wind tunnel operating noise, it is advised either to perform the

experiment as quickly as possible or wear ear plugs for the course of Stage I of the

experiment.

Step 5: The first set of readings for wind speed need to be recorded.

Step 6: Following this, the speed setting is to be ramped up by 5% to 15% from the

previous value and Step 4 and Step 5 are to be repeated. This is to be continued until a

speed setting value of 50% has been reached.

Step 7: Upon completion of data recording, the speed setting for the wind tunnel is to be

slowly throttled down using the knob as shown in Img.2. and brought down to 10%

before switching OFF the apparatus.

Step 8: Using the five different wind speed readings for each speed setting of the wind

tunnel fan, the average wind velocity is calculated and recorded.

Stage II

Step 1: The Wind Turbine is re-hoisted onto the mast and is locked in to face the exit

section of the wind tunnel (the settling chamber, i.e. the honey comb mesh).

Step 2: The connecting wires to the ammeter and voltmeter are connected to the wind

turbine as in Fig.2.

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Step 3: The setup is inspected by the Lab Instructor and the wind tunnel fan is switched

ON.

Step 4: As in the previous Stage I, the wind speed setting is ramped up gradually by 5%

from a value of 10% to 50%. As this is done, the voltage and current values are to be

recorded from the voltmeter and ammeter respectively in the lab sheet provided to aid

with future calculations of power. The Student Team is to face the measuring instruments

vertically with respect to the analogue value pointers in order to avoid misplaced readings

due to parallax error. For each controller setting allow a short period of time for the turbine

to pick up speed and for the load to respond to the generated power before taking the

readings. NOTE: When increasing the speed setting from 25% to 30%, an accurate 30%

setting might not be achieved. This is due to the error present in the system.

Step 5: After the values have been recorded, the speed control knob is adjusted to bring

the speed back to 10% and the apparatus is switched OFF.

Results

Table 1. displays the recorded and calculated values from the experiment.

Table 1. Wind Turbine Laboratory – Data Sheet

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Formulae and Sample Calculations (at 15% Speed Control):

𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑊𝑖𝑛𝑑 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦 =[ ∑ 𝑃𝑜𝑠𝑖𝑡𝑖𝑜𝑛(𝑛)5

𝑛=1 ]

5 𝑚/𝑠

𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑊𝑖𝑛𝑑 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦 (𝐴𝑊𝑉) = (2.65 + 2.45 + 2.4 + 2.9 + 3.4

5) 𝑚/𝑠

𝑨𝑾𝑽 = 𝟐. 𝟕𝟔 𝒎/𝒔

𝑃𝑜𝑤𝑒𝑟 𝐹𝑙𝑜𝑤 (𝑊𝑘) =1

2𝜌𝐴𝑐3 𝑊𝑎𝑡𝑡

𝑊𝑘 =1

2∗ 1.2534 ∗ 0.567 ∗ 2.763

𝑾𝒌 = 𝟕. 𝟒𝟕 𝑾

𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑎𝑙 𝑃𝑜𝑤𝑒𝑟 (𝑊𝐸) = 𝑉 ∗ 𝐼

𝑊𝐸 = 12.3 ∗ 0.1

𝑾𝑬 = 𝟏. 𝟐𝟑 𝑾

𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 (𝜂) =𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑎𝑙 𝑃𝑜𝑤𝑒𝑟 (𝑊𝐸)

𝑃𝑜𝑤𝑒𝑟 𝐹𝑙𝑜𝑤 (𝑊𝑘)∗ 100

𝜂 =1.23

7.47∗ 100

𝜼 = 𝟏𝟔. 𝟒𝟔 %

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Graphs:

1. Power Flow vs Wind Velocity

Figure 7. Power Flow vs Wind Velocity Graph | Legend: X-Axis - Wind Velocity: Y-Axis – Power Flow

2. Output Electrical Power vs Wind Velocity

Figure 8. Electrical Power (W) vs Wind Velocity (m/s) | Legend: X-axis - Wind Velocity ; Y-axis - Output Power

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3. Efficiency vs Average Wind Velocity

Figure 9. Efficiency vs Average Wind Velocity Graph | Legend: X-Axis - Wind Velocity: Y-Axis – Efficiency

4. Wind Velocity (m/s) vs Current (A)

Figure 10. Wind Velocity vs Current Graph

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Discussion

Figure 11. Manufacturer's Measured Output Power vs. Wind Speed for a Rutland 913 Wind Turbine | Source: Rutland 913

Turbine User’s Manual (Link: http://www.bluemoment.com/manuals/Rutland_913.pdf)

Figure 12. Wind Speed vs Current Graph | Source: Rutland 913 Turbine User’s Manual (Link:

http://www.bluemoment.com/manuals/Rutland_913.pdf)

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Explanation

The measured and calculated relationship between power flow (mechanical & electrical)

and wind velocity is shown in Fig. 7 and 8. The wind velocity here represents the average

wind speed, the power flow (Wk) represents the mechanical power and the output power

(WE) represents the electrical power generated by the turbine. The relationship between

efficiency of the wind turbine at varying speeds is established in Fig. 9. Fig. 10 shows the

curves outlining the relationship between the output current and wind velocity.

Fig. 11 and 12 represent the ideal relationships between wind velocity and a. power flow

and b. charge current to batteries respectively. The fact that these graphs have been

obtained under test / laboratory conditions has been considered when comparing the

real-time graphical outputs from the system parameters.

Analysis

1. Power Flow (WK) vs Wind Velocity (m/s)

A near perfect match between the power flow and wind velocity values is displayed as

seen from the overlap of the 3rd order polynomial trend line. This proportional increase in

power flow suggests and consolidates the direct correlation between itself and velocity of

the wind striking the blades of the turbine.

2. Output Power (WE) vs Wind Velocity (m/s)

Deviations from the 3rd order polynomial trend curve at the points – 24.7 W and 49.5 W

in Fig. 8 are noticed which is otherwise not seen in Fig. 11 (which represents the Rutland

913 Turbine’s manufacturer ratings). This coincides with the minor turbulence in wind

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speeds noted during experimentation (Table 1 – Average Wind Speed values show minor

variation between 25% - 35% wind tunnel speed settings). At the same point, an error

in wind speed control was noted between the 25% to 35% wind speed settings wherein

a perfect value was unable to be set at the 30% point.

3. Efficiency (%) vs Average Wind Velocity (m/s)

The theoretical relationship curve is maintained excepting the point where the efficiency

drops to 44.13%. A large gap in the power flow (in the turbine versus the electrical power

output from the turbine) due to the increased rotational velocity of the turbine (at an

average wind speed of ~ 5 m/s) leads to a stark reduction in the efficiency which

compares the power flow versus output power. The efficiency yet again reduces as the

turbine approaches higher speeds. The peak efficiency is maintained at 5.98 m/s

(65.12%).

Interpretation

Discrepancies in output power with respect to wind velocity could be directly attributed

to mechanical losses (wear and tear) of turbine parts which emerge as a result of

prolonged usage. A method to reduce this would be to increase the maintenance

frequency of greasing the machine parts to reduce friction and also to replace rotatory

parts of the turbine across a regular time period.

Sensitivity of wind speed control has both advantages (allows for smoother and more

accurate control of the speed of the VFD Motor Fan) and disadvantages (digital errors

add up over time periods leading to a sudden rise / fall in values (as seen in the 25% to

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35% case). This could be corrected by incorporating regular calibration of the control

device.

Despite the use of the honey comb mesh structure to reduce turbulence, the wind turbine

was in fact exposed to the outer atmosphere. Minor external wind currents were recorded

on the day of experimentation which leads the Student Team to believe that this has had

a detrimental effect on the performance of the turbine. Alongside this, increased speed

also leads to increased noise in the overall system. These losses too need to be considered

when trying to explain the reduction / variation in power output. The wind exhaust coming

out of the honey comb mesh although reduced in turbulence has been non-uniform (as

seen from the five different readings taken for wind speed). This non-uniformity leads to

uneven angles of attack (angle at which the wind strikes the turbines usually measured

between the directional vector of wind flow and the surface horizontal of the turbine),

thus leading to variations in rotation.

Between 25% and 30% of the wind speed control setting, a sudden jump in power flow

from 22.87 W to 55.97 W is recorded. This is attributed to the fact that at 5+ m/s, the

wind turbine’s cut-through speed is achieved and hence the rotational reverse inertia is

overcome, leading to a more, resistance-less, rotatory action. This could also be noted

from Fig. 12 where in a sharp rise in current value recorded is seen between 5 m/s and

6 m/s wind speed from the wind tunnel.

While higher wind speeds are preferred for increased power output, in real time, the

speeds are limited to keep the rotational speed of the wind turbines well under the sonic

limits (i.e. speed of sound). The efficiency of performance of the turbine reduces after

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5.98 m/s and onwards. This is attributed to the braking system that is activated in order

to reduce damage to the rotors [8].

Conclusion

Hence, as aimed, the operation and experimental setup of the Rutland 913 Speed Charger

Wind Turbine along with the Wind Tunnel Setup has been understood and studied. The

parameters required to analyze the performance of the wind turbine with respect to wind

velocity have been recorded and secondary parameters have been calculated using

appropriate formulae and methodology. Following this, the efficiency of the wind turbine

as against wind velocity has also been recorded. It could be concluded with a greater

threshold of wind speeds alongside a well-maintained or operated wind turbine lead to

better efficiency and performance. It could also be stated here that mechanical and

electrical stresses / deviations in the system cause variations in the output which could

be reduced with proper maintenance and control strategies. Studies such as these help

appreciate the importance of renewable energy and hence need to be conducted more

regularly to help students understand and appreciate the need for a fossil-fuel free

economy.

References

[1] Moriarty, P. (NREL). (2015). NWTC Aerodynamics Studies Improve Energy Capture and Lower Costs of Wind-Generated Electricity. Retrieved from http://www.nrel.gov/docs/fy15osti/64731.pdf

[2] Ministry of New & Renewable Energy, G. of I. (2015). 2015 National Off-Shore Wind Energy Policy, 58(51). Retrieved from http://mnre.gov.in/file-manager/UserFiles/National-Offshore-Wind-Energy-Policy.pdf

[3] Badran, O., Abdulhadi, E., Mamlook, R., & Science, F. (2003). Evaluation of parameters affecting wind turbine power generation. The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan.

[4] E.ON. (2013). E.ON Wind Turbine Technology and Operations Factbook, (September). Retrieved from http://www.eon.com/content/dam/eon-content-pool/eon/company-asset-finder/company-profiles/ecr/WindTurbineFactbook_SinglePages.pdf

[5] Gorban’, A. N., Gorlov, A. M., & Silantyev, V. M. (2001). Limits of the Turbine Efficiency for Free Fluid Flow. Journal of Energy Resources Technology, 123 (4), 311. http://doi.org/10.1115/1.1414137

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[6] ABB. (n.d.). Technical Guide No . 100 High Performance Drives -- speed and torque regulation, (100). Retrieved

from https://library.e.abb.com/public/d3c711ec2acddb18c125788f002cf5da/ABB_Technical_guide_No_4_REVC.pdf

[7] ABB. (2011). Technical guide No. 4 Guide to variable speed drives, (4), 1–44. Retrieved from https://library.e.abb.com/public/8492cb383e941d57852571c3006b0194/Speedtorq.pdf

[8] The British Wind Energy Association. (2005). British Wind Energy Association Briefing Sheet, 4. Retrieved from https://www.nottingham.ac.uk/renewableenergyproject/documents/windturbinetechnology.pdf

APPENDIX

I

A four blade design offers only a slightly better efficiency improvement in the case of

wind turbines. With increased blade numbers, the air disturbance for the clock-wise or

anti-clockwise consecutive blade increases and thus has a stark effect on the turbine

efficiency. Increased number of blades also increase the speed to near sound velocity

which is an efficiency reduction factor. The minimum required number of blades for overall

turbine’s balance comes to about two. Keeping the above facts in mind, the three blade

design offers the following advantages without a compromise on performance:

1. Reduced air disturbance to the other blades

2. Speed is kept below sonic levels

3. Proportionate mast height

4. Reasonable amount of energy harvested and converted

II

Low hub height, low wind speeds and for sake of experimentation.