Design Report: Wind Turbine Group 11 Manager: Robert Hudd
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Contents Summary (Claire)………………………………………………………………………………………..3 Introduction (Claire)…………………………………………………………………………..…..…4 1. Design and Simulation………………………………………………………………….……….5 Rotor (Arif)…………………………………………………………………………….………….6 Structure (Emily)………………………………………………………………..…………….10 Braking (Eliot)………………………………………………………………………..…………15 Power Generation (Mark & Sammy)……………………………………..………...21 Control (Alex)………………………………………………………………………….…..….26 Systems Integration (Arif & Yimiao)…………………………………………..…….31
2. Manufacturing and Assembly (Umar & Claire)……………………………….36 3. Discussion Rotor (Umar)…………………………….…………………………………………………....39 Structure (Claire)………………………………………………………………………….…40 Braking (Eliot)…………………………………………………………………………….…..41 Power (Mark & Sammy)………………………………………………………………….42 Control (Alex)………………………………………………………………………………….43 Systems (Arif)………………………………………………………………………………….44
Conclusion (Claire)…………………………………………………………………………………45 Appendices…………………………………………………….……………………………………….46 References………………………………………………………………………………………………47
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Summary The purpose of this design project was to design and build a wind turbine prototype that could operate at various wind speeds and generate an electric power output. With the guidance of a fourth year manager each team member was allocated a job role, and thus the group worked together to produce a horizontal axis wind turbine with the ability to generate 10.4W of power at a wind speed of 15m/s.
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Introduction Currently the world relies heavily on fossil fuels to generate energy in the form of electricity and gas for our appliances. However, since these sources are non-renewable and levels are running low, alternative renewable sources are slowly being introduced to replace them. To continue with the theme of energy generation, for our second year design project was to design and build a model wind turbine out of a 0.5 m x 1.0 m x 0.7 mm thick aluminum sheet with a budget of ÂŁ250. The prototype would have the ability to operate at a variety of wind speeds, generating a power output of up to 24V at wind speeds of 15m/s. The turbine also needed to be able to monitor the wind speed, and thus automatically brake (stop spinning) when the wind speed exceeded 17m/s. This report provides the overall progress of the team during this project, including the design process, manufacturing, and the lessons that were learnt along the way.
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1. Design and Simulation Each team member was allocated a sub-system of the wind turbine in which they would be responsible for concept ideas and designing. The job roles were assigned as follows: Rotor
–
Umar Dar Arif Mohamad Jam Ali
Structure
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Emily Shephard Claire Miller
Braking
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Eliot Ball
Power Generation
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Mark Jones Sammy Mintah
Control
–
Alex Robinson
Systems Integration
–
Yimiao Ning
Each sub-system was required to conceive a concept idea and then implement and modify it into a final design that conformed to all of the technical specified details requested. These parts would then be manufactured and assembled together to produce the final wind turbine prototype.
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Rotor
For this project it was important that the wind turbine was able to conform to the following specification: 1. 2. 3. 4. 5.
The maximum overall size (of turbine): 0.4m x 0.4m x 0.4m. Material to be used: 0.5m x 1.0m x 0.7mm thick 1050 aluminium alloy (H14) sheet. Withstand a maximum wind speed of 18m/s. Air temperature: 10 ≤ TA ≤ 30 °C. Air pressure: 90 kPa ≤ pa ≤ 110 kPa.
The initial design selected during the concept selection phase featured three flat rectangular 15cm x 8 cm x 1.4cm blades (double the thickness of the aluminium sheet for structural rigidity). The effective surface area facing the oncoming wind was 120cm 2, which allowed for maximum wind capture. Each blade had a 1.8cm x 3cm ‘neck’ to secure the blade into a cavity on the hub. The hub was to be designed from a piece of 5cm round steel bar with length of 5cm. A 1.2cm hole, for the shaft, was to be extruded through the center and 3 cavities of dimensions 1.8cm x 3cm x 1.4cm were to be made at 35° to the horizontal to ensure that the deflected wind’s kinetic energy was used to rotate the blades and shaft. 1
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60
0°
A
A
°
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0 .6 0
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R0
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U N L E S S O T H E R W IS E S P E C IF IE D : D IM E N S IO N S A R E IN M IL L I M E T E R S S U R F A C E F IN IS H : TO LER A N C ES: L IN E A R : A N G U LA R :
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NA M E
D EBUR AN D BREA K SH A RP ED G ES
F IN I S H :
S IG N A T U R E
D A TE
D O N O T S C A L E D R A W IN G
R E V IS IO N
T IT L E :
DRA W N C H K 'D
1 .8 0
D
2 .5 0
A P P V 'D M FG Q .A
M A T E R IA L :
DW G N O .
3 1
2
W E IG H T :
R o t o r A s s e m b ly
S C A L E :1 :5
A 4
SHEET 1 O F 1
Figure - Blade design carried forward form the Concept Selection Report
Once the drawings for the design had been produced, the group’s technician was consulted. Unfortunately the workshop did not possess a drill bit of the size that was needed, and so the slits could not be created on the hub, so the blades could not attach to the hub in a manner to allow rotation.
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The hub was redesigned with a 10cm shoulder for the blade to be attached to, and the blades were given longer necks so they could be bent to their ‘wind-deflective’ angles. Six M5mm holes were to be drilled into the hub to securely fasten each blade to the hub. Figures 2 and 3 represent the drawings for the new hub design: 1
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3
4
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6 x 5 m m D ia m e t e r T h r e a d e d H o le s , 2 0 m m D e e p A
A
B
B
3 x 5 m m D ia m e t e r T h r e a d e d s c r e w s , 1 9 m m d e e p
50 30 C
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C
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U N L E S S O T H E R W IS E S P E C I F IE D : D I M E N S I O N S A R E IN M IL L I M E T E R S S U R F A C E F IN IS H : TO LE R A N C ES: L IN E A R : A N G U LA R : NAM E
DEBUR A ND BREA K SH A RP ED G ES
F IN IS H :
S IG N A T U R E
D A TE
D O N O T S C A L E D R A W IN G
R E V IS IO N
T IT L E :
DRAW N C H K 'D
D
A P P V 'D M FG Q .A
1
2
M A T E R IA L :
W E IG H T :
D W G N O .
S C A L E :1 :1
Hub
A4
SHEET 1 O F 1
Figure - Drawing for new hub design
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1
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70 A
150
A
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U N L E S S O T H E R W I S E S P E C IF IE D : D I M E N S I O N S A R E I N M IL L I M E T E R S S U R F A C E F I N IS H : TO LE R A N C E S: L IN E A R : A N G U LA R : NAM E
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1 .4 0
C
D EBUR AN D B R EA K SH A R P ED G ES
F IN IS H :
S IG N A T U R E
D A TE
D O N O T S C A L E D R A W IN G
R E V IS IO N
T IT L E :
DRAW N C H K 'D
D
A P P V 'D M FG Q .A
1
2
M A T E R IA L :
W E IG H T :
DW G NO .
S C A L E :1 :2
B la d e 3
A4
SH EET 1 O F 1
Figure - Drawing for new blade Furthermore, on selecting the design above, simulations via finite-element stress strain analyses and calculations were done in order to determine the structural strength and the estimated torque generated by wind onto the turbine blades. Simulations: The 2D plot below shows the computational fluid flow simulation of the turbine blade at 17m/s.
Figure – 2D cut plot of the CFD analysis via viscous, compressible Navier-Stokes equations
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The presence of a turbulent wake behind the turbine blade concludes that the calculation done above would not be very accurate as the flow was assumed to be inviscid and incompressible. But this may be ignored as the above estimation would give larger forces, and hence a good overestimation of the stresses acting on the blades. The overestimation in the RPM values would be reduced due to the efficiency used. Prior to these calculations, stress-strain simulations via finite element method were done as shown by the plot below. The figure below shows the stress-strain plots on the rotor section when subjected to a force of 1N on each turbine blade.
Figure 6 – stress analysis From figure 6 above, it can be seen that the maximum stress acting on the rotor section is approximately 2.68MPa. This is much lower than the 0.2% proof stress of H-14 Aluminum; which is 85MPa. This simulation hence shows that the rotor is safe enough for use in the wind tunnel.
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Figure 7 – strain analysis
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Structure
In the concept stage of this project, the final concept design was chosen using the analysis of all the concepts, the initial technical specification and the feedback from the visiting design professor.
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The technical specification, which was established at the beginning of the project, gave the structural team an opportunity to assess the different strengths of each concept design knowing what was needed from the structure. The feedback from the visiting design professor gave us the chance of hearing from a professional who works in industry his opinion on the designs. The opinion given for the chosen design was positive as we had been able to prove through simulations how the structure would perform in the final testing stages. Since the initial design was chosen in the concept phase of this project there have been some changes. These have mainly been made so that we could ensure the integration of all the different components went smoothly.
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Figure 1 – Concept design of structure The initial concept design consisted of a table-inspired structure with two shelves for the braking, power, and control systems to sit on. However, the results from simulations showed a large amount of wind pressure building up behind the rotor blades, and so we decided on an open structure to prevent this from happening. We also removed the shelves, bringing the drive shaft to the top of the structure supported by two ball bearings, and added cut away sections for the wheel of the braking system and the driver pulley for the motor. For the sole purpose of supporting the eye bolt, we designed a triangular rear structure that would take the majority of the 10kg load. Through further simulations and by communicating with the other sub-systems, the design was adapted to fit the requirements of the specification, while still maintaining the table top concept we started off with. The adaptations can be seen below in Figure 2:
Figure 2 – Table structure with added bearings and triangle rear structure To support the motor and the solenoid, we decided on an aluminium sling system that consisted of two side brackets and a U-shaped. This helped with the overall stability of the structure as the side brackets meant that the two sets of side columns were held together and the sling was slightly off the ground to allow room for the nuts and bolts to attach the structure to the mounting base (Figure 3).
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Figure 3 – Final design of structure After these modifications, we created a design on SolidWorks of the full assembly (Figure 4); this allowed us to establish any issues that may arise when all sections were integrated together.
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Figure 4 – Assembly model of wind turbine in SolidWorks The full assembly design allowed us to assess the criteria set out on the initial specification. We found that not enough room had been allocated for the rotor structure on the front and therefore the overall structure was 10mm too large in overall length. As no manufacturing had yet taken place on the rear structure we were able to decrease the angle of this section slightly thus making the length shorter. We did further simulations on this to ensure this would not affect the stability of the structure. The final design also included the motor being offset from the centre, which meant the top hole had to be enlarged slightly to take into account the pulley being set at an angle. The motor was moved to being off centre to allow enough room for the solenoid and the braking system.
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Braking
After an extensive concept report with ideas such as using a disc brake, bicycle brake and electromagnetic brake it was decided that the concept to be developed was the band brake. The reasons for choosing this brake were because it was cheap, lightweight and innovative.
[1]
The idea was to mount a wheel onto the shaft and connect a band around it attached to a lever. When a force is applied in the direction of ‘P’ shown in figure 1 the lever tightens the band around the wheel causing friction and ultimately stopping the wheel and subsequently the wind turbine.
Later on in the design it was decided to make the brake a failsafe system, so that if power to the system was Figure 1 – Band brake lost then the brake would be activated and the wind turbine would come to a halt. When the solenoid has voltage running through it the band is loose around the wheel and the shaft can spin freely. As soon as the wind speed exceeds 17 −1 ms the power to the solenoid shuts off and the spring pulls the lever in the direction of ‘P’ and the band is tightened around the wheel. Components • Wheel • Band • Brackets • Lever • Bicycle inner tube • Solenoid • Spring
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Wheel – The wheel is made from polyethylene and was designed with a groove down the centre so that the band has a track to run through.
Figure 2
It is constructed by gluing 2 identical wheels (shown above) together using cyanoacrylate and forming a track shown in the figure 3 below.
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Figure 3
Lever- The lever was made from the 0.7mm thick aluminium; it was designed to get as much of a moment to pull the band around the wheel. The design is shown below in figure 4.
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A
B
C
Figure 4
Brackets – These were designed to give a pivot for the lever at the point labelled ‘A’ on figure 4. The bracket is shown below in figure 5 and there will be 2 of them, both attached
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to the underside of the roof of the structure and then to the lever to give it a pivot.
Band- The band is made out of a leather Figure 5 belt; it creates a lot of friction when rubbing along the rubber inner tube which is glued to the wheel track. It is attached to the wheel by drilling holes in the wheel then using a nut and bolt it is screwed onto the lever in holes ‘B’ labeled on figure 4. Solenoid – It was ordered from RS-Components with a 12Vdc limit and a pulling force of 56N, this is a sufficient amount of force to extend the spring. It is attached to the lever in hole ‘C’ labeled on figure 4 by using a nut and bolt. It is also screwed onto the base of the structure using nuts and bolts. The overall braking design is shown in figure 6 in solid works. The band is attached around the wheel attached to the two holes in the wheel that are not attached to anything.
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Figure 6
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Power Generation
This is a simple flowchart diagram of my construction design. Power in the form of kinetic energy from the wind is inputted into the turbine where it is converted into mechanical energy. Then from the turbine the power moves into the gearbox. Next the same mechanical energy power transfers to braking system. Afterward, the power transfers from the gearbox to the generator. Inside the generator the mechanical power takes the form of electrical energy and then the electrical power is outputted. Selecting of PM Motor In the process of selecting the most suitable motor for the design, certain criteria had to be met. The Maximum output voltage would be 24V DC and only for a permanent magnet motor. Looking at the different motors, we compared the RPM, power efficiencies and their armature resistance. After a series of consecutive calculations we were able to find the ideal motor (generator) for our system. RS Stock No: 420-631
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Table of motor data taken from the datasheet of the RE-975 geared motor from MFA / Como Drills (available from Rapid) Further Calculations of the motor can be found in the appendix Tests on motor Equipment Used: Tachometer Voltmeter Ammeter 10 â„Ś resistor with heat sync Motor Drill Machine Oscilloscope Tests on the motor When the motor has arrived I did run some tests to see if the motor will produce enough voltage at the correct rpm. It was done by connecting the voltmeter in parallel to the load resistance which was 10 â„Ś and in series with the motor and it was then connected to the ammeter which current and voltage reading was recorded. The hand drill was attached to the end part of the shaft of motor to generate speed of 1025rpm and based on that speed the values were recorded as below. The Tachometer was used to calibrate speed in which need for the motor to spin. From the tests, the power output can be determined and compared with the specification value.
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Reading values from testing (Sampling test) Frequency(Hz) 3.33 4.16 5.0 5.83 6.67
RPM 200 250 300 350 400
Voltage(V) 1.97 2.00 2.75 3.5 3.6
Power(W) 0.39 0.4 0.8 1.23 1.296
Current(A) 0.197 0.2 0.290 0.351 0.36
Actual Calculations For motor Voltage2 Power= Resistance Load
I=
V2 = R
P V
Resistance Load 10â„Ś
Estimation calculations 800rpm produces 6.5Vand 0.4A Power = voltage x current =6.5x0.4 =2.6W
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Testing for the Torque
At length of motor from clamp is 5cm and weight is 98.5gram Torque = force x distance −2 =0.967N x 5x 10 m Torque =0.048Nm 0.048Nm is the estimated calculated torque which the motor shaft needed to spin to generate power of 2.6W The torque calculated from motor is an equivalent to the torque of the rotor which was given by rotor engineers is 0.05Nm and also estimated rpm giving by the rotor engineers is 800-1500rpm. The specification requirement of power is 2W and as compare to what we will generating is quite reasonable and it makes the motor suitable for the project. From the Rotor engineers theoretical calculation approximately 2W power would be produced at 800rpm. After testing, 1.296W was produces at 400rpm. This is different to the predicated value from the theoretical Rotor engineer’s calculation hence there is slight issue. After discussing with the Rotor, systems and my colleague (i.e electrical assistant) engineers there are a few potential solutions that have been pursued. One way to produce more power is to attach an additional external 4:1gearbox to the motor. These will double the power output from 1.296 to 2.6W and hence enough power will be produced. However, due to the time constraints of this project this option is not feasible as the delivery for a 4:1 can take sufficient amount of time. Alternately, the calculated torque for the motor seems reasonable as the motor will spin easily. 28
Motor choice has been altered from the previous choices. The first chosen motor met the power requirements from the specification however the motor was too heavy and strength to weight ratio is required to be high in the specification. Additionally, this motor was ÂŁ31.50 which was very expensive which meant that the other procurement would have been running on a very tight budget. A second motor was chosen which again met the specification and the price was fairly reasonable. However, there was a wait for 3weeks wait for the gears that would be attached on to the motor hence the motor was disregarded. Finally a choice was made working with the systems engineer and a geared motor was selected which was very reasonable in price and met the specification. Whilst selecting the motor, we had to take into account the torque produced by the main rotor. This value had to be greater than the stall torque of the motor; otherwise the shaft of the motor would not turn. As the motor did not sit directly in-line with the main rotor shaft, a solution had to be devised in order to connect the two systems. A timing belt system was used. The belt had teeth such that they would align with the grooves in the timing gears, which would prevent the belt from slipping; during the motion of the wind turbine. The timing gears were connected to their corresponding shafts by a grub screw; a flat surface would be machined into the shafts so that the screw would have something to grab onto.
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Control
The purpose of the control system in this design was monitoring of important variables and controlling the storm protection brake. The part of the specification that applied to control is as follows 1. Free-stream wind speed: 15 m/s ≤ u∞ ≤ 18 m/s. 2. Monitoring: • Rotor Speed (rpm) • Wind Speed (m/s) • Output Voltage (V) • Output Power (W) • Estimate of the overall electromechanical efficiency, based on the available wind power • Operational status ( standby, generating, blocked (due to storm conditions), no wind) u∞ 3. The system must have a storm protection blocking brake that is active at ≥ 17m/s. 4. Two PP3 9V batteries can be used to power the electronic components of the system. However, batteries are not allowed anywhere in the power output. Braking control As described in braking a 12VDC solenoid was selected. It was determined through testing that running 18V (2x 9V batteries in series) through the solenoid was safe and the following circuit was adapted for this system based on a design found on the internet. (1)
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Figure 24 – Circuit diagram for the solenoid control The circuit is designed so that when the digital pin on the Arduino circuit board is set to HIGH (+5 Volts) the TIP102 transistor closes the circuit and current flows through the solenoid. The 12TQ045 is a Shottkey diode with a reverse breakdown voltage that is enough to protect the circuit in the event the solenoid tries to induce a high voltage than the circuit is designed for. Voltage and Power Sensing The specification required that the control system be able to measure and display the voltage and power being produced by the wind turbine. It was decided that the best way to represent the power output would be as a function of the voltage as we were using a fixed 10Ω load. Therefore the following equation was used to compute the power output. 2
P=
V R
The main design limitation on the voltage sensor circuit is that the Arduino board’s analogue input has a maximum input voltage of 5VDC where the designed maximum output of the generator that was selected for power generation was 12V. This was solved by using a potential divider to limit give a proportional reading which did not exceed the board’s limits as shown in Figure 25.
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Figure 25 – Circuit diagram for voltage sensing A Zener diode with a reverse breakdown voltage of 4.7Volts was added across the second resistor to protect the Arduino board in case the generator voltage exceeded 12V.
Rotor Speed Rotor speed was measured by adapting an existing design (2) . The circuit is shown in Figure 26.
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Figure 26 – Circuit diagram for Tachometer The Tachometer works by using an infra-red (IR) LED to emit IR light which is received by the IR-sensitive phototransistor. This causes Q3 to close causing Q4 to stop conducting due to the voltage dropping which causes the signal going to the Arduino pin to fall. Two transistors are used instead of one to prevent bouncing in the Arduino pin.
Full Circuit Diagram The 3 shown circuits come together with the Arduino board as shown in Figure 27.
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Figure 27 – Circuit diagram of the full system with the Arduino board Efficiency The efficiency, η was calculated using the energy being provided by the wind and the energy being harnessed by the turbine as follows. η ( )=
Where
P ( turbine ) ∗100 P ( wind )
P ( tubine )=
V2 R
and
1 P ( wind )= ρA v 3 2
CITATION Phy14 \l 2057
(3)
Provided that air density, ρ, rotor area (as a disk), A, and load on the generator, R remained constant then the efficiency could be measured measuring the wind velocity, v and output voltage, V. Therefore: V2 ∗2 v3 η( )= ∗100 ρAR 34
Program and Display The specification specifies that six items have to be displayed in some fashion and that the board is required to activate the brake. It was decided after the last design report that displaying data on a program called “Processing” was a more effective way of displaying than a LCD. This is because Processing already has integration with Arduino serial communication; it did not require use of shift registers and was more customizable. The display that was made is shown in Figure 28 which is displaying example data.
Figure 28 – Image of the Processing Display
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Systems Integration
This section explains the integration of all sub-systems and the wind measurement system implemented. The relevant specification for this part of the design is as follows: 1. Free-stream wind speed: 15 m/s ≤ u∞ ≤ 18 m/s. 2. Monitoring: • Rotor Speed (rpm) • Wind Speed (m/s) • Output Voltage (V) • Output Power (W) • Estimate of the overall electromechanical efficiency, based on the available wind power • Operational status ( standby, generating, blocked (due to storm conditions), no wind) u∞ 5. The system must have a storm protection blocking brake that is active at ≥ 17m/s. 6. Two PP3 9V batteries can be used to power the electronic components of the system. However, batteries are not allowed anywhere in the power output. A hot-wire anemometer was chosen to determine the wind speed of the oncoming wind. The circuit diagram for the sensor and the dimensions are shown below:
Figure - Circuit diagram for the hot-wire anemometer 36
Figure - Dimensions of the sensor and a picture of the sensor The sensor above is heated up as an input of 18V is applied to the circuit via two 9V batteries, the heating would cause a change in its resistance and hence the output. When subjected to the wind flow inside the wind tunnel, the sensor would cool down and its resistance would change again. This change in resistance would give a change in the output, which is measured and then converted to display the measured wind speed. The sensor will be placed on the side of the structure as shown below. The advantage of placing the sensor as shown is so that can be placed high enough so it doesn’t get affected by the boundary layer formed from the ground (bottom mount of wind turbine). The disadvantage is that it will slightly be affected by the vortices generated by the turbine blades when it spins. In addition to that, another disadvantage of using this hot-wire anemometer is its ability to detect fast fluctuations in velocity. If the local fluid velocity changes very rapidly, the response of the sensor will lag behind the actual change due to its thermal inertia. A backup system has been planned out in case the hot-wire anemometer doesn’t work. Previously on the ‘Controls’ section of this report, it was pointed out that a tachometer would be used to measure the RPM of the turbine blades. In addition to that, from the calculations of the ‘Rotor’ section, the RPM of the turbine can be calculated as:
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ω=5.5 v ∞ /(170 mm) ≈
32 . 35 v ∞
(Since TSR =
v Tip /v ∞
=
rω v∞
, where
r=170mm.) v Hence, a conversion from RPM to ω then to ∞ may be applied to give;
v∞=
ω 32.353(
2π ) 60
m/s.
This can then be used to measure the oncoming wind speed in the wind turbine and u determine the critical wind speed ( ∞ ≥ 17m/s.) at which the braking system would be activated. The advantage of using the tachometer itself is due to fact that it is already part of the control system and can be easily implemented; its only disadvantage is its accuracy in measuring the wind speed. Since the turbine is not 100% efficient, the conversion as shown above would not be very accurate due to viscous fluid-skin interaction. The main circuit diagram of the hot wire anemometer is as follows:
Circuit diagram for the hot-wire anemometer
Components: Heraeus M 1020 6.8 O platinum micro heater Amplifier LM 741CN Resistance of 1.5k Ω (2) and 2kΩ Strip board Cost: ￡5.55 When wind comes, it will cool down the hot wire (micro heater). As temperature goes down, the resistance of hot wire will decrease. This will lead to a decrease in electric potential at point B. And a potential deference will appear between point B and point C( 38
R2 Rs − R2 +Rw R1 + R s ) ). After amplifier, this potential deference and battery will cause ∆U =K ( E B− E c ) =K ¿ a bigger current through hot wire, which will reheat the hot wire and lead the bridge to a new balance. The wind speed is measured by monitoring the potential difference. We set wind speed as V =15 m/s 15m/s ( w ). At room temperature 20 ℃ and standard atmospheric pressure, the dynamic viscosity coefficient of air is −6 2 v = 15.06* 10 m / s . So we can calculate Reynolds number as: V w d 15× 3.5× 10−3 Re = = =3486 .(d is the diameter of hot wire) v 15.06× 10−6 n According to King’s equation: N u =A+ B Re .
When 1<
Re <4000
, N u =0.43+0.48 √ R e
a h= (0.43+0.48 √ R e ) .(a is the physical parameters of the air) So d Rf t =85.3 ℃ We set rate of overheating as 1.5, α= Rw =1.5 .So working temperature is w
Taking radiation into consideration, power of radiation per unit area is −8 T=85.3 +273.15 =358.45K, ϵ ≈ 0.8 , σ =5.67× 10 . So
E r =ϵσ T 4 .
E r =724W / m2 and total
−3 −3 radiation power is E= E r πdl=724 ×π × 3.5× 10 × 4 × 9× 10 =0.2 W , which can be
ignored. For the heat balance of hot wire: a I 2 R w =h A ( t w −t f ) = ( 0.43+ 0.48 √ Re )∗( t w −t f ) =( A+ B √ V w ) ( t w −t f ) , A=0.43 d
√
πla , B=0.48 πal d v
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t w −t f
And
Rw −R f = R f CT
Average temperature is
2
I Rw , so R −R = A+ B √ V w . w f t f +t w 20+85.3 = =52.7 ℃ . At this temperature, the physical 2 2
−2 −6 2 parameters of the air is a=2.9 ×10 W/mK,v=18.97 ×10 m s . So A=0.43 πla =1.4 −3
×10
√
d , B=0.48 πal v =0.02
When there is a little change of wind speed ∆V ,which cause a little change of current through hot wire ∆ I. It should satisfy: 2
( I + ∆ I ) Rw = A+ B √ V w + ∆V Rw − R f
( I 2 + 2 I ∆ I ) Rw
Using Taylor expansion, we get:
Rw −R f
=A+ B √ V w +
B ∆V 2 √V w
BV w ∆I ∆V = ∙ Rearrange the equation: I 4( A √ V w + B V w ) V w
Applying A, B and
Vw
According to Ohm's law,
, we get
∆V ∆I =¿ 0.25 Vw . I
∆U R ∆ I ∆I ∆U R = ∙UR . .So = I UR I 2
2
2
2
The turbine should stop working when wind speed is over 18m/s. So we design the anemometer to monitor the wind speed change as 0.1m/s. ∆I ∆U R So = I ∙ U R =0.25 2
2
R2 ∆V ∆V 0.1 1500 ∙U R =0.25 ∙ ∙ E=0.25 × × ×18=0.026V . Vw V w R w + R2 15 1500+6.8 2
The accuracy of this design is therefore enough to measure wind speed.
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2. Manufacturing and Assembly During the design period it was clear that the parts for the wind turbine would be relatively simple to manufacture in the sense that it would mainly consist of cutting straight edges, drilling holes and bending the sheet at right angles, minimising departmental labour as much as possible. However, the number of parts to make â&#x20AC;&#x201C; along with the precision required of some of the measurements â&#x20AC;&#x201C; caused a great deal of stress in the manufacturing group. Each design group was allocated a day where their manufacturing staff would be allowed access to the workshop and construct the wind turbine; this way no more than eight groups would be in the workshop at a time. Despite this, it was still very apparent to the team that there was a lot that needed to be done in just four two-hour sessions, so we came up with tasks to be completed in preparation for the first practical session: 41
1. Manufacturing staff Our manager assigned Umar, Eliot and Claire to manufacture the wind turbine. Since rotor, structure and braking were the major areas in terms of manufacturing, this put us at an advantage since there would always be at least one person who knew exactly how a part needed to be. 2. Cutting list and planning parts onto the aluminium sheet This meant that we would be able to cut every part out on the guillotine as soon as possible and know how much material we had left over for reinforcements. 3. Compiling technical drawings All subsystems were asked to produce annotated technical drawings of parts they required manufacturing. 4. Ordering extra materials in advance We needed to order extra materials and components for the wind turbine, including bearings to support the drive shaft, and steel round bar for the hub for the rotor blades. By ordering them in advance they were able to arrive in time for manufacturing. 5. Planning of manufacturing methods We conferred with the technicians prior to the first practical session to make sure we would have the tools and machinery available to make the parts how we wanted them. Although carrying out these steps meant we knew what we needed to do in each practical session, time management was still a major issue. What with a capacity of 24 students in the workshop at a time, the hand tools and power drills were on high demand, which meant that sometimes we would be spending half an hour waiting to drill a few holes. A few mistakes were made due to miscalculations in tolerances and errors in measurements, however these were minor enough to be resolved using the large amount of aluminium sheet that was left over.
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Bill of materials ordered.
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Gantt chart showing the planned schedule for manufacturing
Table showing the total estimated technician time used for manufacturing
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3. Discussion Rotor
Fault table analysis diagram: Rotor The main issue with the design for the hub was that the workshop did not possess the tools required to make the 1.5mm diameter, 2mm long slits in the hub. Other options such as creating slits using a cutter running the full length of the hub were considered but they were later decided against because no viable method for joining the blades to the hub could be determined. As a result the hub was redesigned. The overall dimensions were kept the same, the only difference was that the blades would now be placed on a shoulder and they would be twisted to provide the necessary angle. This also resulted in a lighter hub component.
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Structure
The majority of issues experienced with the structural side of the wind turbine have been due to human error in measurements whilst manufacturing, or because of a change in another sub-system. However one problem which affected the overall wind turbine the most was perhaps the chosen ball bearings. At the time of ordering materials, we failed to research fully into different types of ball bearings, and as a consequence ordered two P 12TF ball bearings. After further research into them, we found out that this series of bearings are in fact designed for use in pulleys and airframes with PTFE seals and CRES seal caps [2]. Ideally, the ball bearings we should have ordered were either the cylindrical or tapered roller bearings. However, by keeping the P series y-bearings loosely around the shaft, the turbine was able to rotate quite freely while still being supported by the bearings.
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Braking
Fault table analysis diagram: Braking A few problems were found during the design and manufacturing process. First of all the lever had to be modified because it was interfering with the wheel, this was easily done by trimming the top. Secondly there was trouble finding a way to spring load the solenoid, the solenoid did not work when the spring was inside it so an expansion spring was found and attached which solved the problem. The surface of the wheel was not giving as much friction as was necessary, the solution was to glue the rubber from a bicycle inner tube around the track as it has a higher coefficient of friction. There may be a problem with the band causing friction when it is not in the braking position which would affect the overall torque. However from the calculations shown in the appendix and the fact that the structure and brake have been manufactured well, there will â&#x2C6;&#x2019;1 be a sufficient braking torque applied to stop the wind turbine at 17 m s .
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Power Generation
Fault table analysis diagram: Power Generation Various problems arose with the positioning and the mounting of the motor to the structure. Firstly, the motor was very large and occupied a lot of space under the table of the structure. This meant there was no space for the braking system. The motor had to be re-positioned to the side of the structure, in order to give space for the braking system. Also, the motor did not come with a mounting bracket and so a part had to be manufactured so that the motor could be attached the structure. The mounting bracket was made out of wood and the motor would fit between the two brackets, as shown in the pictures below:
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Control
Figure X - Tachometer Fault tree analysis
Figure Y â&#x20AC;&#x201C; Solenoid Control fault tree analysis.
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50
Systems Integration
Fault tree analysis for the wind sensor There were various problems face when designing this hot-wire anemometer. Initially, the first problem which arose was the heating state of the anemometer. This problem was solved by replacing the initial arrangement of resistors. Another problem faced was the connection method for the anemometer to the circuit. Since the anemometer was very small and prone to damage, wires were contacted onto the anemometer leads, then taped in order to secure them. The longer part of the wire had to then be soldered to the taped wires in order to secure them together and ensure that they are long enough to reach the terminal block behind the structure when inside the wind tunnel. Despite tremendous efforts, the hot-wire anemometer concept had to be abandoned in the end due to a number of difficulties. The backup concept then had to be implemented.
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Conclusion In the wind tunnel unfortunately there must have been a problem with a connection because the solenoid for the braking system, which had worked fine for the practice test, wasnâ&#x20AC;&#x2122;t working. This meant that we had to continue the test without being able to block the turbine after 17m/s. On a positive note, however, the wind turbine generated 10.4W of power at 15m/s, which had been predicted by the rotor engineers initially through their calculations. The design project has been a considerable learning curve for many of us. It has been an insight into working under pressure in a team, allowed us to experience applying for engineering-related job roles, and has allowed us to reflect on ourselves and find our strengths and weaknesses as individuals as well as team members.
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References [1] http://www.mdp.eng.cam.ac.uk/web/library/enginfo/textbooks_dvd_only/DAN/brakes/intr o/bandSketch.gif [2] http://www.rbcbearings.com/aero-pdfs/Rolling/025_054/ACBB_PSeries.pdf ‘Innovation in Wind Turbine Design’ Peter Jamieson, ISBN: 9780470699812
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Appendix A – Rotor The calculations below estimate the force acting on each blade, and subsequently the torque on the rotor section about the center of rotation of the rotor blades. The figure below shows the dimensions of each turbine blade; the darker parts show the area which will mostly contribute to the ‘deflection’ of the oncoming wind. The force acting on each blade shall be estimated using two-dimensional, inviscid and incompressible control volume methods as shown below; Power from oncoming wind: 1 E w = m v 2 , therefore: 2 Power =
P w=
d E dt w
And since the mass flow rate is: m= ́ ρAv The power from the wind becomes: 1 P w = ρA v 3 Where; ρ = Fluid density (Air, in this case) 2 A = Area of flow v = Flow velocity
The blades have a bend of 35⁰, hence the area that the oncoming wind ‘sees’ is: A= AT sin 35
Where;
AT
= Area hitting the wind (Dark grey on diagram) on each
blade. From the diagram shown below, the vertical components of the fluid velocities v2
are 0 and
v 2y =v 2 cos 35
v1
and
respectively. The power ‘captured’ by the blades and the
momentum flux in the vertical direction shall now be determined;
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Figure - Diagram of control volume (box) and fluid velocities
Power captured: 1 P c = ρ( Asin 35)[ v 13−( v 1 cos 35 )3 ] 2
Where;
v 1=v ∞
Hence, the force acting on each blade in the vertical direction is: F y =v ∞ 2 ρA(sin 35)2 Subsequently, the torque generated by each blade is: v∞ [¿ ¿ 2 ρA(sin 35)2 ]d Where; d = Distance from center of rotation to centroid of τ y =F y d =¿ the darker area (Marked in red on figure 1) From figure 1 above, the area of the darker region was determined and used to calculate these P Fy τ values ( c , , y ). The values are tabulated below, these values are calculated by assuming a turbine efficiency of 32%, as estimated on the ‘Systems Integration’ of this report and a tip-speed ratio of 5.5. rω v Tip /v ∞ r≈ NB: Tip speed ratio = TSR = = v∞ Where; 170mm ∴
v∞
32.35 v ∞
ω=5.5v ∞ /(170mm)≈
(m/s)
Pc
(W)
τy
(Nm)
Fy
(N)
ω (rad −1
s
15 17 18
5.99 8.70 10.32
0.094 0.121 0.138
1.015 1.303 1.460
157 170 188
)
RPM ( min−1 ) 1500 1700 1800
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Appendix B â&#x20AC;&#x201C; Structure Technical drawings of parts
Table
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Table legs/columns
Rear structure 1 and 2
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Rear structure 3
Rear structure 4
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Motor and solenoid â&#x20AC;&#x153;slingâ&#x20AC;?
SolidWorks load simulation study showing that the rear structure alone would be able to support the 10kg load
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Appendix C – Power Generation V t ¿ E a −I a Ra
Vt is the power at terminal voltage
T em =K I a
Tem is electromagnetic torque
E a =Kω
Ea (back e.m.f)
P em= P m−P loss=ω ( T m −T loss )
K motor constant Pem is electromagnetic power in motor Ia is armature current
At no load: Speed =700
( 2π60 )
rad/s
Armature current =0.9A Terminal voltage =12V Mechanical power=0 2 2 V I a=I a Ra + ω K fr 2
(0.9)(12)= ( 0.9 ) Ra 10.8=0.81
Ra + ¿
(
+
2
2π ×700 K fr 60
)
2
0.5373 ×10 K fr
(1)
At maximum efficiency 2 2 V I a=I a Ra + ω K fr + P m 2 (12)(5.5)= ( 5.5 ) Ra +
To get
Ra
and
K fr
(
2 2π ×5700 K fr 60
)
+¿ 41.3 ( 2 )
in equations 1 and 2 must solved simultaneously −5
Ra = 0.809 Ω and Kfr = 1.887 ×10
Nms
Motor Constant K The motor constant K can be obtained from Kirchhoff’s voltage law and the linear relation of Ea to ω K=
V t −I a R a ω 60
K=0.015 The frictional resistance Rfr can be calculated from K and Kfr using K R fr = K fr R fr
=814Ω
Back emf E a =Kω (0.015)
( 2π60 ×5700)
a=¿ E ¿ 8.95V
SOLVING FOR MECHANICAL AND ELECTRICAL POWER LOSS P loss ( elec ) = I a2 Ra P mech
=24.47W 2 = K fr ω
=6.72W The power loss= 24.47+6.72 =31.19W Pm P e + Plm + p¿ = 41.3=24.47+6.72+ P El
P EL
=10.11W
Efficiency =
10.11 × 100 41.3
24.5%
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