Abstract:
This report will be based on a 7 MW wind turbine cooling circuit produced by Siemens and contains the theory behind a cooling circuit, a simulation of a serial and a parallel configuration using EES and an experiment with a cross flow heat exchanger provided by Aalborg University. These elements will serve to provide a better understanding of the functions of a heat exchanger. The purpose of the report is to investigate if it is possible to make a more cost effective solution for the pipe system and the heating surface area, based on variations of the area and mass flow. At the end of the report, the goal is to conclude on what kind of circuit configuration has the most favorable performance and what approach is best suited to achieve a more cost effective heating surface area.
Foreword
This project is written by group C2-21 in the period from the 10th of October to the 19th of December. The project was chosen due to the common interest to the subject of the request from Siemens.
Thanks to Siemens for the project and the visit in Brande. It has been an inspiring project. At the same time we want to thank our supervisors, Kim and Shobhana for their guidance through the project, their inputs, corrections and help.
Thanks to AAU university for allowing us to use the facilities and Flemming for all the help he has been putting in the experiment with the heat exchanger.
Reading guide
The report is written in chapters with belonging sections. The division of chapters and sections can be found in the table of contents. At first the nomenclature is presented which will work as a encyclopaedia and describe all the abbreviations. When reading the report the literature will be referenced in accordance to the Vancouver method. All equations, figures and tables will have their own reference number used throughout the report. Although the report is written in English some Danish mathematical ground rules will be applied, meaning that in our tables and calculation comma will be used instead of period.
4 Introduction
The world is moving towards relying on renewable energy instead of fossil fuels. One area, which has great potential for partly covering the world’s energy need is wind energy. Wind turbines are one of man’s first inventions to exploit the forces of nature. Today’s society primarily use them to produce electricity, and it has in the last couple of decades become one of the most exported goods in Denmark. There are many both commercial and environmental gains in wind energy that Denmark and other countries can benefit from. After the setup of the first electricity-producing wind turbines in Denmark in 1891, wind turbines became an important part of the Danish electricity production.
During the first oil crisis, Denmark learned how difficult it is if a country’s energy supply is dependent on only one energy source from a certain area, on which you do not have any influence. One of the consequences of the oil crisis is the increased awareness of the importance of developing alternative forms of energy. In Denmark, a political agreement proposes the reaching of independence from importing fossil fuels is of uttermost importance. The most competitive and developed form of sustainable energy in Denmark is wind energy and is an important part of effectuating the conversion of our energy supply.
It is favourable to harvest energy from the kinetic energy of the wind, however, this brings with it its own challenges that need solutions regarding the optimizations of the cost of wind turbines. Reducing the overall cost of the cooling circuit allows the manufacturer to produce cheaper wind turbines.
On a daily basis, energy is produced to keep our laptops, televisions and etc. supplied with electricity. A lot of this energy comes from wind turbine energy production. The reason for this is that there is great potential for wind energy in Denmark, and because this is a sustainable energy source and therefore it is favourable for the environment.
An area where improvements can be made to the wind turbine is on the cooling circuit, which results in lowered costs. Since it is possible to improve the cooling circuit on different parameters. The main focus is optimizing the cost of the cooling circuit to reduce the overall cost of Siemens 7 MW wind turbine.
In order to analyse this problem it is necessary to make a model of the cooling circuit in EES to show how heat is transferred within the cooling circuit. By doing this it is possible to variate different parameters within the EES model, to see how it will affect the cost of the cooling circuit. This is done to determine if a changed parameter is an improvement.
1 Statement of problem
Building and maintaining wind turbines is an expensive establishment. Can the cost of the cooling circuit be reduced by studying thermodynamics through theory, experiments and using EES simulations?
2 Object clause
Due to a high amount of heat transfer in the Siemens 7 MW wind turbine, it is relevant to look at some optimizations for the cooling circuit. We want to highlight these optimizations by simulating a cooling circuit in EES and vary different parameters in order to produce a cooling circuit at lower costs.
There will be several stakeholders interested in our project, Siemens, the government of Denmark and environmental organizations. Siemens, who suggested this project, have an interest in a more effective and cheaper cooling circuit for their own turbines. The government of Denmark have an interest in increasing the production of sustainable energy and lower the costs of these. The different environmental organizations are interested in making sure that the project stays within moral and ethical guidelines. The solution will be constructed from laboratory experiments and mathematical calculations with the data on the wind turbine provided by Siemens. This data will be compared to data from our own experiment and data from our simulations in EES and end up with the most cost efficient solution.
3 Problem definition
In the predesign of our project, it has been deduced that our main problem as the design and optimizations of the cooling circuit, more accurately the pipes and heat transfer areas between the different components within the circuit. This is chosen in order to make a more cost efficient circuit with either cheaper materials or a decreasing pipe dimensions. This will be achieved through a basic understanding on some mathematical and physic-related aspects of heat transfer and furthermore mathematical simulations where varying different parameters will determine if it will be cost efficient to make the chosen optimizations. Focusing on the cost efficient aspects our project will be based on quantitative methods such as EES simulations and a lab experiment. The materials of both the pipe and heat surface areas have been determined as stainless steel, even though this would be an interesting consideration, the time does not allow further investigation. Furthermore would it have been interesting to look if choosing another cooling fluid could have cheapened the overall cost. At the same time we have decided to look aside any environmental aspects. Even though optimizing the effectiveness of the components within the cooling circuit or altering the fluid too gain a more profitable result but due to the deadline requirements this will not be further investigated.
5 Theory
As our problem revolves around a desire to make the cooling circuit in the Siemens 7 MW wind turbine more cost effective, it is a necessity to have knowledge about some basic theory within the fields of thermodynamics and energy-tech to make the right assumptions when implementing modelling and experiments on a heat exchanger.
1 A wind turbine’s construction
Having a basic understanding of the wind turbine and the elements within such as; the generator, transformer, condenser and moreover the cooling circuit in general, is necessary when wanting to locate or calculate on the real problem. In this section a wind turbine’s construction will be described, moreover some features in the Siemens 7 MW mill will be concluded.
Today wind turbines is used to produce electricity from the winds kinetic energy. A construction of a wind turbine is shown in fig. 3.1. It is divided into four parts – the foundation, the tower, the rotor (with often 3 wings) and the nacelle .
Figure 1: A wind turbine construction [5,redrawn from].
The first part is the foundation, where the cables to the grid are located. The cables leads through the tower, where it will be linked to the generator in the nacelle. Outside the nacelle the rotor is placed. The rotor combines, the wings to the nacelle, which is combined with the generator. It transforms the kinetic energy to rotation energy, where the generator will transform it into electricity. In this project, the focus will be on the 7 MW offshore wind turbine from Siemens, which is direct drive, it means the rotor is converted directly to the generator . A direct drive wind turbine is shown in fig. 3.2.
Figure 2: A nacelle in a direct drive wind turbine .
Without a gearbox, the wind turbine weighs less, has fewer components that rotate and smaller dimensions which result in easier maintenance of the wind turbine and lower costs for transportation and installation of the wind turbine . It is a permanent magnet generator that is installed in the wind turbine. The generator consists of a rotor and a stator. The rotor consists of permanent magnets. The rotor which consists of copper winding rotates inside the stator. The process affects the copper winding that makes the electrons move. In that way the mechanical energy is converted into electricity, which is AC. After the generator the converter converts the AC to DC and back to AC again. Then the transformer transforms AC to the right voltage for the grid. Above the nacelle, the wind vane is placed. It is provided with a sensor, which notes the direction of the wind. This information will be send to the jaw motor and it will move the nacelle, so it points towards the wind. The movement of the wings will start when the wind blows with 4 m/s . The aerodynamic shape of the wings determine how well the wings can convert the kinetic energy from the wind into an increase in the rotation speed in the rotor. The wind turbine is built to have a maximum performance when the wind blows with 12-14 m/s, this is called the wind turbines rated maximum net power. If the wind speed is above 12-14 m/s, the wind turbine will have to let the excess wind pass the wings to avoid overload and therefore the extra wind will not be exploited. The wind turbine can let the wind pass in one of the two following methods; pitch regulation, which is when the wings can rotate around its own axis, so the wind will be broken, and the wings cannot rotate faster. When the wind speed decreases, the wings will rotate back again. The other method is stall regulation but in this wind turbine, the 7 MW offshore wind turbine, is produced with the pitch regulation, so the stall regulation will not be described any further .
Knowing the construction and function of the different elements is a necessity when modelling in ESS and doing experiments. When this is established, it is beneficial to take a closer look into the physics that take place in the cooling circuit.
2 Laws of thermodynamics
Thermodynamics is the study of relations involving mechanical work, heat and other aspects of energy and energy transfer. The laws of thermodynamics are the fundamentals of all thermodynamic calculations. There are four laws of thermodynamics and in this project only the first and second law will be interesting to take a closer look at.
The first law of thermodynamics states that energy can neither be created nor destroyed. It is basically an expression of the conservation of energy principle that states; While energy can change from one form to another, the total amount of energy will remain the same[5] cha. 1.
Examples of this law in practice are objects falling from the edge of a table to the floor, where potential energy is converted to kinetic energy or the energy contained in an amount of fuel that is converted to thermal energy upon a chemical reaction. To calculate the difference between states of energy this equation is used:
(1)
The second law of thermodynamics states that there is a distinction between energy quality and energy quantity, so a process only occurs in the direction of a decrease in the quality of energy. An example of the second law is a cup of hot chocolate placed on a table. The cup of chocolate will at some point cool down to room temperature, but a cold cup of chocolate in the same room will never get hot by itself. So the high-temperature energy present in the hot chocolate, will be degraded to a lower quality form at a lower temperature when transformed to the surrounding air[5] cha. 1.
Knowing that energy can not be used, but only transfer from one state to another, and that there is a distinct difference between quality and quantity. It is now relevant to talk about other aspects of energy.
3 Enthalpy
When calculating the performance of the different elements in a cooling circuit it is relevant to look at the enthalpy of the working fluid.
Enthalpy is a state variable constructed from other state variables. It is defined as the internal energy added with the product of pressure and volume.
(2)
Enthalpy does not depend on what path the circuit underwent to get to the state. So every time you return to point A in the circuit it will have the same value. This indicates that enthalpy is a state variable.
Now to clarify this even more see the fig. 3.3 where it is visualised that the network done by the circuit is the shaded area on the figure.
Figure 3: The shaded area shows the net work done by the circuit.
The internal energy at point A has not changed when the cycle is complete. That means that the change in internal energy is zero. This leads to the following:
(3)
and now knowing that internal energy is defined as:
(4)
and since has been defined in eq. 3.3 as being equal to zero, then the following equation appears
(5)
By isolating Q the following expression appears and it states that the work that the circuit receives is equal to the heat:
(6)
Since enthalpy is defined as in eq (3.2) and internal energy, pressure and volume are state variables, enthalpy is a state variable.
Enthalpy can be calculated, but in most cases it is not desirable to measure enthalpy at a specific state but rather the change in enthalpy from one state to another. Therefore, expression (3.2) can be written as followed:
(7)
Knowing that internal work can be defined as pressure times the change in volume:
(8)
And this gives the following expression for enthalpy
(9)
Sources: [4] and [5] page 158-170]
4 Energy balance
If the rate of energy going into a system is different from the rate of energy leaving the system, the energy content in the system will either increase or decrease. This results in an accumulation of energy in the cooling system which would eventually cause the generator to surpass its thermal limits.
The equation for the energy balance will change to reflect the imbalance:
(10)
A system that has an imbalance of energy is referred to as being non-stationary.
The first law of thermodynamics; conservation of energy in a thermodynamic system, states, that net energy crossing a system boundary equals the energy change inside the system.
Energy balance, under steady conditions, is where the total rate of all forms of energy entering is equal to the total rate of all forms of energy leaving the system. A system that is in balance is referred to as being stationary. The equation for such a system is the following:
(11)
Where the sum of energy going out, designated as, , is subtracted from the energy going in designated as as shown in equation 3.11. The result need to be zero for there to be a balance of energy so that a system is stationary.
The information gathered in this section on energy balance and the two previously is what we base our data processing, modelling and simulation around.
5 Genneral heat transfer
Heat transfer is the transfer of heat from one media to another. In the 7 MW wind turbine s ooling circuit, heat transfer takes place between the different components and the working fluid of the cooling circuit.
5.1 Conduction
In a heat exchanger, there will be a working fluid in motion in which interaction between particles will cause heat transfer by the phenomena conduction.
Conduction is transfer of energy that takes place between the more energetic particles of a substance to the adjacent and less active ones, the transfer takes place as the two particles interact. Conduction takes place in gases, liquids and solids. In solids, conduction occurs as a combination of vibrations in molecules in a grid and the energy transported by free electrons. In gases and liquids, conduction is a result of collisions and diffusion of the molecules in motion. It is possible to determine the rate of conduction through a medium. The rate of conduction depends on;
• Geometry (of the media).
• Thickness.
• Material
• The temperature difference from one side of the medium to the other.
Fig. shows a wall with an area A and a thickness Thickness , on a wall similar to the one on the figure, the temperature difference across the wall can be found:
(12)
The rate of heat conduction through a wall is proportional to across the wall and the heat transfer area A, but it is inversely proportional to the thickness of the wall.
Figure 4: Conduction through a large plane wall with thickness dx and area A.
That leads to an equation that describes the rate of heat transfer;
(13)
The thermal conductivity of a material, k, is a measure of the ability of the material to conduct heat. A high value of thermal conductivity shows that the material is a good heat conductor, and a low value indicates that the material is a poor conductor. In a limiting case where moves towards 0, meaning the wall is very thin. The equation above can be reduced to.
(14)
This equation is known as Fourier s law of conduction. is the temperature gradient, the slope of the temperature curve on a diagram (fig. 3.4), of the rate of change in T of x, at location x. This indicates that the rate of conduction in a certain direction is proportional to the temperature gradient in that direction. Heat will be conducted in the direction of decreasing temperature and the temperature gradient becomes negative when x is increasing and the temperature is decreasing [5] page 636-637 chapter 16.
5.2 Convection
Heat transfer between a solid surface and the adjacent fluid in motion, is called convection. It involves the combined effects of conduction and fluid motion. The faster the fluid flows, the better heat transfer convection. If there was not any mass with fluid motion, the heat transfer taking place between the surface and the adjacent fluid would have been pure conduction. There are two types of convection, forced convection and natural convection. Forced convection is when the fluid is forced to flow over the surface by an external source such as a fan. Natural convection is a non influenced motion, caused by buoyancy forces provoked by the density differences as a result of the varying temperature of a fluid.
The best way to describe convection is by an example; it is wanted to cool a hot block, that can be done in two ways; forced or natural convection. With forced convection, a fan will blow cold air over the blocks top surface. First heat transfers to the air adjacent to the block, by conduction. The energy stored in the air is now carried away by convection, meaning that the combined effects of conduction in the air and the macroscopic motion of the air, is what removes the heated air allowing new and cold air to replace it.
Figure 5: Forced convection.
Figure 6: Free convection.
If you now remove the fan from the heated block, then the process at hand will be natural convection. Since, any motion in the air would be due to the rise of the warmer, therefore lighter, air close to the surface and the fall of the cooler, thus heavier, air will take its place. Heat transfer processes involving a phase change are also considered to be convection, due to the fluid in motion induced in the process such as the rise of vapour bubbles under boiling.
Convection is a complex phenomena but is observed to be proportional to the difference in temperature, and can be described by Newton s law of cooling
(15)
h is the convection heat transfer coefficient, h depends on the following;
• Type of fluid.
• Flow proporties (such as velocity).
• Type of flow.
• Geometry.
[5] page 636-637 chapter 16.
5.3 Heat transfer coefficient
The pipes of a cooling circuit consist of materials which have an inherent thermal resistance and a considerable influence on the processes of a heat exchanger. The resistance in the pipe is a key concept when calculating heat transfer.
The flow through the thermal resistance associated with this heat transfer process involves two convection and one conduction resistances fig 3.8 . Furthermore;
(16)
(17)
Figure 7: Sketch of a double-pipe heat exchanger.
Figure 8: Heat transfer though a wall section of a pipe.
When dealing with a double piped heat exchanger, as shown on picture 3.7, the thermal resistance of the tube wall is
(18)
After defining the thermal resistance on the wall, the total thermal resistance can be defined as:
(19)
When calculating different parameters in heat exchangers, it is convenient to combine all the thermal resistances in the path of the heat flow from the hot fluid to the cold fluid into one resistance R. The rate of heat transfer is expressed between the two fluids as:
(20)
U s unit is identical to the unit of the ordinary convection coefficient h. The convection heat transfer coefficient – h – depends on:
• Type of fluid.
• The flows.
• Temperature dependent properties.
6 Fouling
During operations with fluids and gasses a layer of dirt can build op on the surfaces of the heat exchanger. This layer of dirt is called fouling[6].
The fouling factor represents the theoretical resistance to the heat flow, due to accumulation of fouling on the surfaces of the heat exchanger.
It is called the fouling factor . Fouling is typically classified into four common types[8].
1. Chemical fouling: When chemical changes within a fluid causes a fouling layer to build up on the surface of the heat exchanger.
2. Biological fouling: When organisms grow within a fluid it causes a fouling layer to build up on the surfaces of the heat exchanger.
3. Deposition fouling: When the particles contained within the fluid is deposited on the surface of the heat exchanger, since the fluid velocity drops beneath a critical level.
4. Corrosion fouling: When a layer of corrosive products accumulate on the surfaces of the tube, forming an extra layer of high thermal resistance material.
Obviously the Fouling factor is 0 for a new heat exchanger, but accumulation will happen, and the fouling factor will increase with time. Accumulation cannot be prevented, but with the right materials, heat velocities and temperature can be maintained throughout the heat exchange s run time[5]. However, that ties into a economically discussion
The fouling factor depends on: The operating temperature, the velocity of the fluids and the length of service. Fouling growths with increasing temperature and decreasing velocity[5].
Figure 9: Illustrates where fouling is deposited within a heat exchanger.
The fouling factor can be determined as
(21)
Thermal conductance of clean heat exchanger can also be expressed as[8]:
(22)
The overall heat transfer coefficient relation is valid when discussing clean surfaces. To account for the effects of fouling for both the inner and the outer surface, it is necessary to modify the equation.
For an unfinned shell-and-tube heat exchanger it will be expressed as:
(23)
[5][6][8]