What?
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Optimisation of ABB’s Wind
Turbine Generator Performance
by Installing A
Heat Pipe
Heat Exchanger
Presented by: Haytham Abdulwahab
The United Arab Emirates University
Overview
- Increase in the temperature
difference safety margin, which
allows increasing the loading.
- extending the life of the insulation,
and thus the life of the generator.
Cold air
outlet
Cold ambient air
dragged by fan
fan
Internal circulating
hot air closed cycle
cycle
Internal air is
beeing cooled by
cold ambient air
after it absorbed
heat from
generator
Internal air
entres closed
cycle again
- Increase in the temperature
difference safety margin, which
allows increasing the loading.
- extending the life of the insulation,
and thus the life of the generator.
What?
What?
What’s a heat pipe?
What reasons behind choosing a heat pipe to solve the problem
What consideration should be taken in designing a heat pipe?
What could limit the operation of a heat pipe?
What procedures are taken to design a heat pipe?
What other things should we be awared of?
What do you need to ask more?
What?
Why a heat pipe was considered?
1. A heat pipe will transport the heat to a location where it
can be effectively dissipated by natural or forced
convection.
2. The heat pipe provides a thermal path through the
enclosure wall, while the internal air cycle is kept close.
3. There will be no need for extra cooling fan that would
consume extra power; since the original cooling fan used
to drag cooling air for the primary cooling unit is the one
to be used in the cooling of the heat sink by forced
convection.
4. The product maintenance requirements are eliminated
or reduced. And no noise source does exist.
Heat Pipe Fundamentals:
Thermal Design:
- Evaporator
- Adiabatic
- Condenser( Heat Sink)
Manufacturer:
- Container
- Working Fluid
- Wick Structure (operation
against gravity)
Materials Selection Creteria:
Container Material:
1. The container should isolate the working fluid from the
outside environment.
2. The container should also enable heat transfer to take
place from and into the working fluid.
3. The container material should be compatible with both
the working fluid and external environment.
4. A material with good fabrication properties including
weldibility, machineability and ductility, is preferable.
Materials Selection Creteria:
Working Fluid Material:
1. Compatibality of the working fluid with the container
material.
2. The thermal stability of the working fluid.
3. High latent heat a high latent heat of vaporization and
high thermal conductivity.
4. Low values of vapor and liquid viscosities to minimize
the resistance to fluid flow.
5. Acceptable freezing point in comparison to the
operating temperature range.
ethanol has a
oiling point of
o
6c
Temperature
Range (oC)
Working Fluid
Vessel
Material
-200 to -80
Liquid Nitrogen
Stainless
Steel
-70 to 60
Nicker,
Aluminum,
Liquid Ammonia
Stainless
Steel
-45 to 120
Methanol
Copper,
Nicker,
Stainless
Steel
5 to 230
Water
Copper,
Nickel
190 to 550
Mercury,
Magnesium
Stainless
Steel
400 to 800
Potassium
500 to 900
Sodium
900 to 1,500
Lithium
Nickel,
Stainless
Steel
Nickel,
Stainless
Steel
Niobium,
+15
Zirconium
Tantalum
Materials Selection Creteria:
- Methanol was fluid of choice.
Methanol would provide a temperature potential capable
of driving the required amount of heat because of its low
value of boiling point Tsat.
Since methanol freezes at a very low temperature, -97C,
it is useful in gravity-aided, pool boiling applications
where water heat pipes would be subject to freezing.
- Copper for evaporator tubes, and aluminum for
condenser fins.
materials with good fabrication properties and good
thermal properties in addition to compatability with
working fluid of choice.
Limits to Heat Pipe Operation
Description
Cause
Potential Solution
Viscosity
Viscous forces hinder vapor
flow in the heat pipe
Heat pipe operating below
recommended operating
temperature
Increase heat pipe
operating temperature
or find alternative
working fluid
Sonic
Vapor flow reaches sonic
velocity when exiting heat
pipe evaporator resulting in a
constant heat pipe transport
power and large temperature
gradients
Power/temperature
combination, too much
power at low operating
temperature
This is typically only a
problem at start-up.
large temperature
gradient will be
reduced as the heat
pipe warms up
Entrainment/Flooding
High velocity vapor flow
prevents condensate from
returning to evaporator
Heat pipe operating above
designed power input or at
too low operating
temperature
Increase vapor space
diameter or operating
temperature
Capillary
Sum of gravitational, liquid
and vapor flow pressure
drops exceed the capillary
pumping head of the heat
pipe wick structure
Heat pipe input power
exceeds the design heat
transport capacity of the
heat pipe
Modify heat pipe wick
structure design or
reduce power input
Boiling
Film boiling in heat pipe
evaporator would initiate
High radial heat flux causes
Use a wick with a
film boiling resulting in heat
higher heat flux
pipe dry-out and large
capacity or spread out
thermal resistances
the heat load
Tair in 
Astator ho (Tstator - (Tair in  Tair out ) / 2)  Qevaporator  Qrad
 air u .Cpair
Tair out 
1
Astator ho
2  air u .Cpair
NuD = 1.13C1C2 Remmax Pr 1/3
ST/D
1.25
SL /D
0.6
0.9
1
1.125
1.25
1.5
2
3
NL
C1
0.518
0.451
0.404
0.31
1
0.68
1.5
m
0.556
0.568
0.572
0.592
2
0.75
3
0.83
C1
0.497
0.505
0.46
0.416
0.356
4
0.89
2
m
0.558
0.554
0.562
0.568
0.58
5
0.92
C1
0.446
0.478
0.519
0.452
0.482
0.44
6
0.95
7
0.97
3
m
0.571
0.565
0.556
0.568
0.556
0.562
8
0.98
Δp=NLχ(χρairV2max/2)f
Evaporator Design
C1
0.213
0.401
0.518
0.522
0.488
0.449
0.428
9
0.99
M
0.636
0.581
0.56
0.562
0.568
0.57
0.574
Evaporator Tubes Design Summery:
All the dimensions and geometry details are
shown in the figure beside
18 circular pipes of 25mm. The cross section of
the pipe array is cantered a distance of
237.5mm from the axis of the generator.
Each pipe is made of copper and has a wall
thickness of 2mm.
The average convection heat transfer
coefficient based on the velocity of air at the
centre of the array is equal to 47.5 W/m2.oC
The total heat transfer to the evaporator tubes is
equal to 1692W
The total weigh of evaporator tubes filled with
methanol is 42kg. The empty tubes weigh 35kg.
The time required for the heat pipe to start
working is 8 minutes.
Nu 
η
hNC D
k air



 0.6 






0.387 Ra 1 / 6

8
/
27
9 / 16
  0.559



 
1  

Pr
air 
 


2
actual heat transfer rate from the fin
heat transfer rate from the fin if the entire
fin was at the temperature of the wall.
 g l (  l   v )h fg  0,68Cpl (Tsat  Twall ) k l 3 
ho  0.729

u l (Tsat  Twall ) D


Ri 
1/ 4
Average
velocity
14.8m/s
1 / hi
1
R h
N f Af  f  G i
Nf
High velocity area
Average
velocity
12.1m/s
Low velocity area
 N w Aw w  Aunfin
Condenser Design
Average
velocity
14.8m/s
High velocity area
Average
velocity
12.1m/s
Front
Side
Fin Dimensions
N = 49
Heat Sink Design Summery:
All the dimensions and geometry details are shown in the figure above.
49 L-shaped rectangular aluminium fins will be attached on the inner surface of the air
duct using a glue material that has a thermal conductivity of 0.95 W/m.oC. The methanol
vapor will be contained between this surface and a 510mm diameter concentric
cylindrical surface forming the heat pipe heat sink.
Front
Side
Fin Dimensions
N = 49
Heat Sink Design Summery:
The natural convection heat transfer coefficient on the outer surface of the heat sink was
found to be 4.77 W/m2.oC.
The emmisivity ε of paint material on the outer surface was taken to be 0.85
This gives us a total of heat transfer to the outside equal to 309W.
Front
Side
Fin Dimensions
N = 49
Heat Sink Design Summery:
Based on an average air velocity inside the duct of 13.443 m/s, the forced convection
heat transfer coefficient on the inner surface of the heat sink was found to be
33.3kW/m2.oC
Front
Side
Fin Dimensions
N = 49
Heat Sink Design Summery:
And thus the heat emitted by the fins is equal to 687W. While the heat emitted by the
wings is equal to 103W. The rest unfinned area emits 551W. A total amount of heat
transfer to the air driven by the duct fan equal to 1341W
The total amount of heat emitted by the heat sink is equal to 1650W. The temperature of
the inner surface of the heat sink is equal to 63.6 oC.
Alternative Design
[Acetone Heat Pipe]
Just 12 evaporator tubes!
But we will need more than 40 extra fins.
II
Recommendations!
It is highly recommended to check a safety data sheet
or a hazard sheet that provides information about
safety about dealing with methanol.
Recommendations!
Since the wind turbine will be used in a marine
environment, a surface coating is required to
protect the heat sink assembly, where dissimilar
materials are being attached to each other
(aluminum fins on steel wall), from galvanic
corrosion.
Recommendations!
When monitoring heat pipe performance, the key
parameter is the temperature difference between the
surfaces of the evaporator and the condenser.
Recommendations!
Don’t use the same heat pipe design for two different
working fluids.
Thanks for Listening!
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What questions do you have?
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