Design and Analysis of Wind Turbine Tower

Design of Wind Turbine Tower

The tower of a wind turbine is not only a constructional member which is there to endure the static and dynamic load of the wind turbine system. 

Keeping in view the high weight of the nacelle and the heavy operational load of the turbine, turbine tower designing is given special importance in the designing stage of the whole system [N. Bazeos, 2002]. 

The tower for a small-scale wind turbine is given a simple cylindrical shape whereas, for medium and large-scale turbines, the shape of the tower becomes complex i.e. conical [Gwon, 2011]. 

Complexion in the making of the tower for medium and large-scale turbines arises due to large height and high weight. Due to its large weight, the tower is made in multiple sections which are later assembled in the factory or on-site [Guo, 2011]. The thickness of the wall and the diameter of the tower varies along its length. 

The diameter of the tower and wall thickness decreases from the bottom to the top section of the tower [Yang, 2014]. The process of designing any wind turbine consists of estimating different parameters of the tower such as material, length, diameter, and wall thickness. 

The height of the tower is usually fixed with the locality and power requirement whereas wall thickness and diameter can vary depending upon the material selected.

According to Satish's (2017) work the analytical and theoretical deflection of a wind turbine tower with no joints was about 232.8 mm which is less than 1% of the height of the tower. 

However, the Finite element analysis report generated after conducting the static structural analysis of the tower model in Ansys Workbench showed that the deflection in the tower was 253.69 mm under wind load. The deviation in the theoretical and Ansys workbench deflection of the tower is 15.5%. 

In the second case when the tower was modeled with a finite number of joints, then the theoretical and analytical deflection was observed to be 232.8 mm. 

Whereas the deflection of towers reported in Ansys workbench analysis is to be 274.09 mm. Resulting in a variation of 17.7% in tower deflection from the calculated data.  

 

It is observed that in both cases the deflection in wind turbine tower under wind flow load has a variation of 17.7% in the case of the tower model with the finite number of joints and 15.5% in the case of the tower model without any joint from their theoretically calculated value to finite element analysis value. 

This kind of variation is observed due to the assumptions made while doing the theoretical calculations. The assumptions made were that the whole tower is a single entity in the case of the tower without joints and a finite number of joints in the case of the tower with joints. 

This resulted in the assumption that the tower is stiff in both cases yielding the same theoretical deflection of 232.8 mm in both cases. 

Whereas the FEA showed that the tower is not stiff as assumed and the result showed the real variation in the tower under the wind load.  

FEM of Wind Turbine Tower

According to Satish's (2017) work which was carried out on the wind turbine tower of two structural variances, It was observed that the theoretical deflection in the tower was identical in both the tower models that are 232.8 mm under the given wind load and conditions. 

However, when the tower models were subjected to finite element analysis in Ansys workbench, the result showed that the variation in the tower model without any joint was a minimum of 253.69 mm and a maximum deflection of 3.75 mm. 

In the case of a tower model with a finite number of joints, the analysis reported that the minimum variation in the tower was 260.22 mm and the maximum deflection was 260.25 mm after that the wind turbine tower doesn't deflect in both cases. 

 

Deflection of the wind Turbine tower with joints in the direction of the wind

The variation in deflection of the tower from FEA data to the theoretical data for the tower without any joint is 15.5 % due to the assumptions made during the calculation. 

These assumptions made the tower to be stiff theoretically and the deflection calculated was less than that of FEA. 

Similarly, in the case of a tower model with a finite number of joints, the deflection of the tower from FEA data to the theoretical data for the tower without any joint is 17.7 % due to the assumptions made during the calculation. 

These assumptions made the tower to be stiff theoretically and the deflection calculated was less than that of FEA. 

 

Deflection of wind Turbine tower without joints

The tower under finite element analysis is modeled as a single entity without any joint. 

The analysis is carried out in Ansys workbench for a different number of elements. It was observed that the number of elements that is mesh increased on the tower the deflection value increased. 

The increase in deflection was observed to be to a certain value after which the graph of deflection in the tower became constant and didn't increase. 

The result revealed that for 1000 mesh the deflection in the tower was observed to be 253.69 mm and for 2000 mesh it was 253.7 mm and for 3000 mesh it was reported as 253.75 mm and after which the deflection didn’t change, stating that the maximum deflection of the modeled monopole tower is 253.75 mm under the wind load.  

 The tower under finite element analysis is modeled with a finite number of joints. The analysis is carried out in Ansys workbench for a different number of elements. 

It was observed that as the number of elements that is mesh increased on the tower the deflection value increased. The increase in deflection was observed to be till a certain value after which the graph of deflection in the tower became constant and didn't increase. 

The result revealed that for 1000 mesh the deflection in the tower was observed to be 260.22mm and for 2000 mesh it was 260.25mm and after which the deflection didn’t change, stating that the maximum deflection of the modeled tower is 260.22 mm under the wind load. 

These results suggest that when we construct a wind turbine tower, the theoretical deflection will be less due to the constraints in the assumptions. Hence we need to do a proper FEA analysis before physical construction and determining the materials to be used. 

The slight variation in the tower determination of the maximum deflection a tower will show under the given wind load can affect the efficiency and life of the tower.  From the above analysis, we find that the tower model with a finite number of joints will undergo minimum deflection even if we increase the mesh amount. 

It was reported that the towehasve joints, will show less deflection because the joints will absorb and discontinue the deflection from migrating and accumulating from one part to another, because of the joints the deflection from the previous part will end at the joint and the deflection has to again start from next part resulting in less deflection. 

Also, the mesh amount should be maximum because as the mesh densitincreasesed the point of analysis increases resulting in a better analysis report. 

Hence it is reported that to obtain better efficiency and life of the wind turbine tower, we need to conduct an in-depth finite element analysis, where the number of mesh is kept to be maximum and the proper details such as wind load conditions, materials to be used in the construction of tower should be fed properly and based on the result from the analysis a proper material to be used, to minimize the cost of construction and future maintained and produce maximum efficiency with the prolonged life cycle.

Design Parameters of Wind Turbine Tower

The tower of wind turbine is one of the very important component of wind turbine system. The tower of wind turbine is the only support available to the whole turbine system. 

Wind turbine tower supports the both static and dynamic load of the turbine. It also provides housing for different other mechanical and electrical components of the turbine [F. Knox, 2010]. 

The tower does not only bear the weight of the entire turbine but it also provide resistance against acute weather conditions .i.e. rain, storm, snow etc. 

Life of a wind turbine directly or indirectly depends upon its tower, therefore, the tower’s material properties such as strength, density and stiffness become important parameters in the design of a sustainable wind turbine system [J. Manwell, 2009]. 

The tower of wind turbine is also the most expensive part of the whole wind turbine structure due to its larger weight. For the sake of decreasing the expenses, the dimensions of tower are optimized in each case. 

In the process of optimization parameters like stiffness, strength etc. are taken into account to ensure the safety of whole system. Cost is usually lessened without compromising on strength and stiffness of the tower.

Diameter of Wind Turbine Tower

Tower of wind turbine supports the nacelle of the wind turbine that contains its blades, generator, gear assembly and other important components [F. Knox, 2010]. 

The turbine’s diameter should be selected in such a way that it has enough space to install nacelle of the turbine properly along with yawning mechanism so that nacelle can be rotated properly to make the turbine face the wind to produce maximum power. 

At the bottom, a turbine have transformer and other necessary electronic equipment as shown in the figure. There is a door at the bottom section of the tower to enter the tower for maintenance and other purposes. 

In the middle section there is a staircase which is built to allow workers to reach different sections of the system for maintenance purposes [J Manwell, 2009]. The tower height and weight of nacelle are also taken into account while deciding the tower’s diameter. 

If the size and weight of nacelle is greater, the tower’s diameter is also kept larger. Similarly if the tower has a large height, the diameter of the tower should also be kept larger to prevent its buckling. 

The requirements which have been discussed above are usually a concern when we are dealing with the larger turbines because for the smaller turbines the electrical and mechanical components are simple and can be installed on the outside. 

An NREL type horizontal axis wind turbine (HAWT) with the power rating of 5 MW has a diameter of 6 meters at the bottom and 3.87 meters at the top section [Kackman 2005]. 

Similarly a large horizontal axis wind turbine (HAWT) with the power rating of 10 MW has a diameter of 8.3 meters at the bottom and 5.5 meters at the top section [Bac 2013].

Wall Thickness of Wind Turbine Tower

Wall thickness of wind turbine is an important factor which affects the strength of the wind turbine tower. Increasing the wall thickness also increases the strength but it also increases its weight [Yang 2014]. 

We have a limit to which wall thickness of the tower can be increased or decreased to obtain the necessary strength [Huskey 2005]. A medium size turbine having power rating of 5MW must have 20mm of wall thickness to endure the stresses produced due to the nacelle’s weight and during operation. 

In the similar manner, the wall thickness for two turbines of 10 MW and 8 MW power rating must have a minimum of 38 mm and 36 mm respectively [Cian Desmond, 2016]. Stiffness rings, now a days, are being used to acquire the increased strength without increasing the thickness and weight of wind turbine tower’s wall [Yang 2014]. 

Stiffness rings are very efficient against compressive loads but less effective when it comes to bending stresses. Predicting the exact integer of stiffness rings for a specific wind turbine is very difficult therefore we have to compromise on the strength of tower and if we don’t, the cost of system becomes very high [Yang 2014]. 

Material used in the making of wind turbine tower greatly impacts the wall thickness of the tower. Any material whose stiffness and strength are high will require small thickness of wall as compared to the material which have low stiffness and strength to withstand the amount of operational and static load. 

Thus, wall thickness is a tradeoff between stiffness, density and strength [Gwon, 2011].

Height of Wind Turbine Tower

Dimensions of wind turbine, amount of power to be produced, available wind speed in the locality where wind turbine is being installed and area in the vicinity are important factors that have a role in deciding the height of wind turbine tower [F. Knox, 2010]. 

Wind turbines that are intended for small scale power production generally have small towers but in the urban areas where neighboring have obstacles i.e. buildings and houses etc. 

It is necessary to build a tower of large height to avoid shear that comes into play due to fluid and solid interaction. For large scale wind turbines there is a limitation to tower height due to large weight of the system. 

Height and type of wind turbine tower changes from turbine to turbine [Manwell 2009]. The tower height of a small scale wind turbine can vary from 15 meters to 25 meters but this limit is not fixed. 

Based on the wind speed available, tower’s height can be increased or decreased. It is also effected by the chord length of blades so there is a limit to tower height in case of medium and large size wind turbines. 

Tower height for small and large scale turbines is taken equal to the diameter of turbine or 1.5 times the diameter of wind turbine (Diameter of rotor). 

For medium size wind turbines of power rating of around 5MW the approximate tower height is around 80 meters which increase to above 100 meters for a turbine of power rating greater than 8 MW. 

For turbines that have power rating greater than 8MW, the tower height is increased to 120 meters [Cian Desmond 2016]. 

In addition to the factors discussed above the process of manufacturing and transporting the material to the site are also very important in deciding the dimensions of the tower. 

Obviously, it is difficult to build large towers and construction costs for building them is also very high.

Types of Wind Turbines Towers for Wind Turbine Power Plants

Electrical energy in the current period is a very simple and most needed facility. Because of a variety of reasons, including the growth of international population, the growing trend of mobile devices and more individuals embracing modernity, the burden on the power generation sector has risen to a-time high.

Figure 1 Energy consumption [U.S. Energy information, 2018]

Although there are two energy generating methods, one using fossil fuel consumable raw materials, and the other using natural resources such as running water and air currents. 

The use of second category of services is very useful for long-term use because their services are usable for lifelong consumption and therefore do not cost whatsoever. 

 

Figure 2 resource wise energy product for year 2017

Wind turbines are the world's primary producer of green electricity, next to only hydro-power plants. 

In a potential region where wind turbines can generate a significant amount of electricity with little effects on environment, they have become an outstanding source of renewable energy. 

The wind turbine is a mechanism that transforms wind-blowing kinetic energy into electrical energy. 

Figure 3 Types of wind turbine based on energy produce in KW

Wind Turbine working and structure

The wind turbine is a basic mechanical mechanism that transforms the existing energy existing in the flowing air flow into its blades' rotational kinetic energy. 

The blades are connected to a shaft that goes into the generator via a gear box. Kinetic energy is transformed by a generator to useful output, that can then be provided for delivery to the grid. 

The wind turbine has four operating parts, first blade, the next shaft, the third gearbox, the final generator. Also there is a fifth wind turbine part that does not function in it, but because of it the other systems work, it is the wind turbine tower.

There are two major types of wind turbines depending on the types of wind turbine blades and their rotating axis; the horizontal axis wind turbine and the vertical axis wind turbine. 

There are two key distinctions between all styles depending on their tower, one dealing with the tower and the other dealing with the height of the tower. The weight of the blades is only for the vertical axis wind turbine tower and their height is normally very limited relative to HAWT. 

The tower carries the weight of all parts of blades, gearbox, shaft and generator for horizontal axis wind turbine and their size is much larger than VAWT typically in metres (Babcock 1994). 

Owing to the operation and elevation, the horizontal axis wind turbine tower design is far more significant.

Figure 4 wind turbine power plant parts

Types of Wind Turbine Tower 

Guy wired tower

For horizontal axis wind turbine, there are four major types of towers, one is a guy wired tower, the next is a lattice tower, the third is a tubular tower and the last is a hybrid tower. Often recognized as the free standing towers, Guy wired towers derive their name from the supporting wire that they use for stability. 

The quantity of wires is connected to its top end, while the lower end with the specially made foundation is fixed properly. Just small-scale wind turbines should use the free-standing tower or the Guy wired towers because they can not bear much weight (Gipe, n, d). 

As wind turbine blade wheels drag force to generate the bending moment in the tower, the wire connected to the tower offers resistance to the bending moment by supplying the equivalent and opposite tension. 

This style of tower is quite simple to produce and deliver, due to its geometry and limited cross section, but it took a lot of time to build as each component needed to be installed physically. 

The Man wired towers or the free standing tower possess small upfront investment but heavy maintenance expenses.

Figure 5 Free standing tower or the Guy wired towers

Lattice Tower

Lattice Tower is generally a series of welded or bolted linked trusses. For small to medium sized wind turbines, lattice towers likewise be utilized. 

As trusses are made of plain steel bars, they are welded or bolted securely, and there's no structural limitation in terms of tower length or tower cross-section specifications. 

They can be shipped pre-assembled or could be configured as needed at the construction site. Such versatility makes the technique of tower quite cost-effective in terms of development and shipping, however at the expense of prolonged assembly time and higher assembly resources needed on site. So their greatest gain is that they can be shipped very quickly and have fewer initial costs. 

They need a lot of assembly time and high cost of labor, their greatest drawback (Malcolm 2004). 

Figure 6 Lattice Tower

Tubular wind

The tubular wind turbine tower takes its title from its circular cross section and the form of the tube along the span of the tower. For any form and size of wind turbine, tubular towers seem to be the most commonly used towers. 

The tubular tower with a standard cross-section long the size of the tower is used for small-scale wind turbines where the static and dynamic load is not significant as they are easy in development and cost-effective terms. 

The cylindrical tower with non-uniform cross section long the length of the tower is used for medium to large wind turbines where the static and dynamic load is very significant or may be assumed to be conical shape tower. 

They can withstand heavy stresses due to larger diameter at the base, and minimal diameter at the top end decreases tower mass. 

Since they have a complicated shape, they are very difficult to manufacture and have high original development, shipping and construction costs. So their greatest advantage is quick configuration and operation, and it is hard to ship drawbacks. 

Figure 7 Tubular Tower

Hybrid towers

Owing to the aesthetic problems of lattice tower and tubular tower construction drawbacks, a new form of tower is made with all types of characteristics. 

Hybrid towers are a synthesis of tubular and lattice towers intended to minimise the costs and disadvantages of construction, shipping and implementation in particular situations. 

They are planned to eliminate the size limitations of the mixture of tubular and lattice towers (Manwell, 2010). These could be the best alternative suitable for uneven terrain of development or installation in low depth water bodies. 

They could be used off wind turbine grid farms where it is difficult for tubular towers to run independently. The hydride tower can also contain the guy wires for reinforcement in certain situations where dynamic load is heavy.

Figure 8 Hybrid Tower

Organic Rankine Cycle and Vehicle Energy Recovery System

Increase in human population and rapid industrialization has increased the burden on already limited natural resources of energy production like fossil fuel. Industrial process wastes a lot of energy which can be extracted and used to meet human need. Organic Rankine Cycle ORC is one of the processes which can be operated even on low waste energy. Organic Rankine Cycle ORC are also being installed in vehicles to extract waste heat from radiator and on diesel engine exhaust. Fluids for Organic Rankine Cycle ORC either refrigerant or any organic fluid which is selected on basic of ISO 817:2014/AMD 1:2017, ISO 17584:2005 and ISO 11650:1999 standards.

Introduction

Today’s world has a never ending demand of energy due to continuous increase in the world population, rapid industrialization and highly dependence of humans of electrical and thermal devices to meet day to day needs. This never ending demand of the energy has forced the maximum utilization of the limited natural resource for the energy production purpose. The quantity of the fossil fuel is limited but the conventional energy generating systems are not effectively converting and utilizing the energy produce by burning the fossil fuel due to design and process limitation. According to the world energy consumption report about the use of energy by the geographical regions and the use of natural resources to meet the need of energy, the requirement of energy will only increase in future but the availability of the natural resources will decrease. So there is a need of setup which can extract the maximum energy from the existing energy generation or processing systems to meet the world energy need.

 

Figure 1 global energy consumption [(energy matter, 2017).

 

Figure 2 energy consumption by resources (energy.gov, 2017).

One of the methods to extract the maximum energy out of the system is to utilize the energy being wasted into the environment. The wasted energy can be utilized to do useful work which either part of the same process like ventilation cooling system that utilize the low temperature air to cool down the fresh air or the completely separate system like waste energy recovery organic Rankine cycle power plant that utilize the waste heat rejected by a system to heat an organic fluid which then is used to power turbine for energy production. 

Organic Rankine Cycle for Waste Energy Recovery

Organic Rankine cycle power plant are just like a standard Rankine cycle power plant with the only difference is that an organic fluid is used as a working fluid instead of water. Benefit of utilizing the organic fluid instead of water is that organic fluid require very less heat to get vaporize as compared to water which enable this power plant to operate at the low waste heat of different process for energy recovery process. Recently a lot of work is being done to develop small scale organic Rankine cycle power plant which can extract waste energy from low heat waste process. Yinji (2018) worked on the development of the organic Rankine cycle power plant which can run of the waste heat of the vehicle exhaust and engine waste heat of radiator. According to work 30% of the heat developed inside engine is wasted by the radiator which can be utilized by the specially designed organic Rankine cycle to produce and store electrical energy. The electrical energy produce can be utilized to power the vehicle after engine is turn off.

 

Figure 3 ORC integrated in Honda vehicle

Other than the vehicle the internal combustion engine which use diesel as a fuel are used for the electricity production purpose in many industries. These engines waste about one third of the heat produce in engine cylinder through the exhaust gases. Cipollone (2015) worked to develop an organic Rankine cycle power plant which can operate on the exhaust gases of the internal combustion engine to produce electricity using its own turbine and generator system. Regular work conditions (regular load) and off work conditions (maximum load) were tested in this setup and result show the overall efficiency of 4% which includes 4.8% for the regular working conditions and 4.4 percent for off work conditions. As organic Rankine cycle combines two different engineering fields which include refrigerants and power plant setup so two different ISO standards are used to develop, evaluate and improve these power plants. A special purpose turbine will be used for ORC so ISO 10437:2003 revised in 2017 will be used for development, evaluation and improvement of ORC along with the ISO 10494:2018 for noise pollution. As per energy related product directive (ERP) of European commission the minimum efficiency of the machined used for energy recovery system should 75 % by the year 2015 and should reach 80 to 85 % by the year 2017.

Organic Fluids of ORC

Organic fluid used in organic Rankine cycle is one of the most important factors in defining the efficiency of the plant. Kamyar (2015) worked on nine different fluids of organic Rankine cycle which includes refrigerants (R 134a, R123, R227ea, R245fa, R600) and organic fluid (Iso butane, Iso pentane, n pentane and toluene) in order to find the best working fluid. Result show that for the given condition refrigerant R134a and organic fluid n-butane provide best result in terms of overall efficiency of 21.3 % and 21.9 % respectively. Working fluids for organic Rankine cycle are selected based on criteria which includes requirements like environment sustainability, ozone depletion potential, global warming potential, safety (non-toxic, non-corrosive and non-flammable) and properties like critical vapor pressure, critical temperature and thermal stability (Kamyar, 2015). Hung (2019) worked on the development of a small scale organic Rankine cycle for waste heat recovery and investigated the effect of pure refrigerant and mixture of refrigerant on the performance of the organic Rankine cycle power plant. Two pure liquids R 245 fa and R 123 and two mixture 67 % R 245 fa with 33 % R 123 and 33% R 245 fa with 67 % R 123 were test and result show that the performance of pure fluid or mixture depends on the working parameters of the organic Rankine cycle. Working parameters includes working pressure, temperature, pressure ratio and the mass flow rate of the fluid. For refrigerants ISO 817:2014/AMD 1:2017 designate and classify the safety of refrigerant, ISO 17584:2005 defines the properties and ISO 11650:1999 define the refrigerants recycling and recovery. All these refrigerants are used to select the proper refrigerant for ORC. Other than this general ISO rules which includes, ISO 9001:2015 for standard quality, ISO 14001:2015 for environmental protection and ISO 45001:2018 for safety and health standard are used for the advancement and future development of energy recovery organic Rankine cycle.