Green Power Generation: Computer Aided Design & Life Cycle ...

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Wind turbine design has been presented as a source of green power ... gas impact than simply estimating plant emissions for various green power generation.
Green Power Generation: Computer Aided Design & Life Cycle Analysis (LCA) of Wind Turbine Nand K Jha1 and Patrick Drennan2 Professor of Mechanical Engineering, Manhattan College, United States 2 Manhattan College, New York, United States 2 Corresponding Author: [email protected]

1

Abstract Wind turbine design has been presented as a source of green power generation through solid modeling. The components of wind turbine have been designed and created through CAD software. Then each component has been tested for its strength through finite element analysis (FEA). Life cycle analysis and life cost analysis have been performed on the entire wind turbine including embodied energy analysis, turbine manufacturing and dismantling have been presented. Embodied energy of wind farm operation to maintenance has been presented. From total cost, the proportionate cost for different categories like administration, insurance etc. are shown in life cycle cost analysis.

1. Introduction: Scientific opinion on climate change has reached a new level of concern. The Intergovernmental Panel on Climate Change (IPCC) has concluded that Earth’s climate will change, though it may not be agreed by when and by how much. Global mean surface temperature has increased in the range of 0.3 – 0.6° C since the 19th century. In addition, global sea levels have risen by 10-25 cm during the same period; much of which may be related to the temperature increase. According to IPCC, these changes are “unlikely to entirely natural in origin”. The balance of evidence suggests an identifiable human influence on global climate. Global warming is mostly credited to greenhouse gases, which allows solar radiation to penetrate the atmosphere, and absorbs the infrared radiation reflected back by the Earth’s surface. These infrared radiations trapped in atmosphere causes the air temperature to rise. During the industrial revolution and after that greenhouse gases from human sources (anthropogenic) have been accelerating, in proportion to the growing use of fossil fuels. Carbon dioxide (CO2) is the most polluting gas and subsequent global warming, followed by methane (CH4) and nitrous oxide (N2O). The concentration of CO2 in the atmosphere has gone up by 28% during the last 1000 years. It is estimated that global temperature will rise by 1-3.5° C, and sea level rise by 15-95 cm by the year 2100. The United States is the world’s largest greenhouse gas contributor, accounting for 25% of global emissions. The vast majority of greenhouse gas emissions in US in the result of energy consumption of which electricity comprises a significant proportion. It is estimated that 40% of U.S. CO2 emissions were due to combustion of fossil fuels by electric utilities. Therefore, it is obvious that generation of electricity should be diverted to renewable sources as far as possible or green power generation.

1.1 Greenhouse Gas Emission Rates: The energy requirements for each phase of the life-cycle can be used to estimate the greenhouse emissions. This methodology provides a better estimate of greenhouse gas impact than simply estimating plant emissions for various green power generation technologies. Carbon dioxide is a byproduct of fossil fuel combustion. Because the vast majority of U.S. energy is produced by using fossil fuels, each energy input within the life-cycle has corresponding CO2 emissions. Carbon dioxide is the most significant greenhouse gas based on total global emissions. Methane (CH4) and Nitrous oxide (N2O) are actually stronger warming agents, but have far lower global emission rates. When averaged over 100 years, CH4 has a 21 times stronger global warming potential than CO2, meaning 1 ton of CH4 emissions can be equated to 21 tons of CO2-equivalent emissions. N2O has a 310 times stronger global warming potential than CO2.

2. Wind Power Generation: There is significant variation in wind turbine size depending on purpose. Wind energy is green energy because during the generation of power, the environment is not polluted. Wind energy is renewable energy simply because the ‘fuel’ used to generate electricity, i.e. the wind, is practically unlimited. Smaller turbines are generally used to power a single household and have a capacity under 100 kilowatts – most commonly around 2 kilowatts. Commercially sized turbines have a capacity of up to 5 million watts. In order to convert wind energy into electricity, an average wind speed of 14 mph is required. Wind turbine blade length and height are the main differences between commercial and residential turbines. Residential turbines generally stand at around 10 meters tall, while commercial turbines are anywhere from 30 to 100 meters tall. Wind turbines or wind generators are used to convert the natural kinetic energy produced by the wind into useful mechanical energy. The central purpose of the wind turbine is to turn the generator located within the main housing just behind the three large blades; this main housing is called the nacelle. The generator will then take the mechanical energy and create electrical power. The power is transmitted within the nacelle through a series of shafts connected by a gear box. The lower speed shaft is connected to the rotor which holds the three large blades as seen in Figure 1. As mentioned above, a wind turbine consists of 4 main parts. They are the rotor, nacelle, tower, and the foundation. The complete design and finite element analysis of all main mechanical components is presented. The rotor blades intersect the wind and capture the energy it contains, which causes them to rotate in a vertical plane about the shaft axis. The slow rotation of the shaft is normally increased by use of a gearbox, by which the rotational motion is delivered to a generator. The electrical output from the generator is then transferred through cables and down the turbine tower to a substation where the power is eventually fed into the electricity grid. The mechanical components housed at the top of the turbine tower- the rotor, gearbox, and generator – are all mounted in the nacelle that can pivot, or yaw, about the vertical axis, so that the rotor shaft is always aligned with the wind direction. Components of a Wind Turbine System are shown in the Figure 1. The specialized shape of the blades is what allows the movement of the entire system. Certain points along the cross-section of the blades are shaped like an airfoil, and like with the cross-section of an airplane wing, this shape creates a force perpendicular to the length of the blade or tangential to the swept area of the blades. Figure 3 shows the shape and forces involved with the blades.

Figure 1: Wind Turbine System (U.S. Dept. of Energy) Blade Cross-section: Flift

Wind Turbine ::(front): Frontal Area

V win

Frontal Area

Figure 2: A Typical Wind Turbine

Figure 3: Blade Design

Blade Cross-

Flift

Most wind turbines can produce a velocity tangential to the swept area at the blade tip (Vt) of between 4060m/s, giving the low speed shaft an angular velocity of only 4-6 radians per second (40-60 rpm) for a 21m diameter swept area. The generator however requires much faster speeds to produce electric power, therefore the low speed shaft is equipped with a large gear with many teeth and then that gear is in contact with a much smaller pinion gear that is then attached to the high speed shaft which is in direct connection with the generator. The effect of the gear box is that the high speed shaft will move at a higher velocity directly proportional to the number of teeth of the larger gear over the number of teeth of the smaller gear. The gear box in Figure 1 has a ratio of only 1:5 which is low for typical turbines which have ratios between 1:25 and 1:50. Low speed generators do exist; however, they are very expensive. The gear teeth carry loads proportional to their respective torque over the gear radius. The teeth must be made strong enough to endure repeated stresses. The base of the turbine, called the tower, is a long and hollow cylindrical piece which acts as a column to support the blades and nacelle. The tower must be designed to avoid buckling under the weight on top of it and to avoid bending possibly produced by the wind forces hitting the front of the wind turbine. The wind also creates stresses in the blades due to bending moment. The dynamic stresses are all due to the turning of the blades and both shafts. The shafts must be sized appropriately to avoid deformation due to torque. Making wind turbines a more significant source of energy in the future depends on the arrival of better and lower cost materials which can be produced in high volumes and still be reliable. There is trend towards lighter weight materials but sophisticated materials are often too expensive to justify using. The turbine tower can represent up to 65% of the weight of the entire turbine. Low cost materials such as reinforced concrete are now being used for the tower.

3. Mechanical Components Design: Most component design parameters are adapted from (Budynas et al., 2008) and (Tester et al., 2005). Siemens NX is used for both CAD and FEA to assess the strength of the turbine components. The full wind turbine has not been assessed in FEA, only components are evaluated separately. No code standard has been used to perform the FEA assessment. The components evaluated in FEA include: tower, blade, and nacelle. The first component to be analyzed is the tower which, as mentioned before, makes up most of the entire weight. The tower has to support the weight of the nacelle and blades which for most wind turbines is approximately 73 tons (716 kN). Assuming a 50m tower made of steel which is fixed at the base, the Euler column formula (Eqn. 1) shows that the second moment of area would have to equal: 2  4  4 4 4 4 4 F L2 716000N 50m  C 2 EI (2) F   I  cr   0.0038m 4 (1) I  d o  d i   2.2  2 m  0.365m cr

L2

C 2 E



1 2 π 190  109 Pa 4



64

64

A simple hollow cylinder was created in NX-9 by first drawing two circles with the same center point in the same plane. The larger and smaller circles were made 2200mm and 2000mm respectively and extruded together 50m. Figure 4 shows the tower model. FEA was performed in NX-9 to test the tower for buckling. The results showed a buckling load factor of 225 as shown in Figure 5 at right. Tower may bend due to strong winds creating a pressure over the entire frontal area of the turbine. The frontal area would be approximately equal to the 3 blades, nacelle, and tower added together. The combined force on the turbine is shown below using a wind speed of 15m/s at sea level:

Figure 4: Tower Model Figure 5: FEA of (3) Tower 2 2 2 (4) Aturbine  3 Ablade  Atower  Anacelle  3 7.125m  30m2.2m   2.3m  107m

Ablade  rd  7.125m1m  7.125m2





2

kg  m   2 Fm   airVmax Aturbine  1.23 3 15  104m 2   28.78kN m  s  

(5)

The material chosen is SAE 1006 hot rolled steel with yield strength of 170 MPa. The bending stress is calculated below shows very high factor of safety.





Side View:

c



28.88 *10 N 30m 1.1m Mc FLc    23.03MPa (6) I  d o4  d i4 0.365m 4 64





4

F

L Possible Cross Sections:

h Three blades have been found to be the most efficient when considering aerodynamic efficiency and cost. Aerodynamic efficiency increases six di do b percent from one to two blades and three percent from two to three blades and then less after that. Selecting a blade length is directly related to the Figure 6: Beam Dimensions desired power output of the wind turbine. A certain amount of power is available in the wind depending on its speed and density. The wind turbine can capture that power but the actual power output is reduced by the turbine efficiency, generally about 35%. Therefore the power required from the wind for a 10hp output is equivalent to 7.45 kW. The wind power is shown Eqn. 7. Pwind = Real Power / Turbine Efficiency 

7.45kW  21.286kW .35

(7)

The radius of swept area which is approximately blade length is calculated below.

Pwind 

1 2 3  air AsweptVwind , where Aswept  r  2

,

and r = 7.142m

A blade was created by drawing five different cross-sections, four of which are airfoils and one circle, initially in the same plane (MIT, 2011). Each cross-section was then moved in the z-direction in increments that would add up to 7.25m. Figure 7 shows the blade cross-sections from two views: Front View:

Cross-Sections: 1 z 1

5

2

3

2 2

4

3

5

4 x

y

2.0m

2.0m

Figure 7: Blade Design Detail

2.0m

1.25m

y

Figure 8: FE Blade Analysis

The blade was approximated by a square cross-section. The force and normal stress associated with the wind and blade can be seen below:

Figure 9: Rotor Solid Model

Fblade  3.506kN , bending stress was calculated as   1.6MPa An FE analysis was also modeled in NX and showed no major structural concerns as shown in Figure 8. Figure 9 shows the completed solid model of the rotor. The round shaped creates an ideal aerodynamic shape to minimize drag. The nacelle was created in a similar fashion as the blade in that two circular cross-sections were created and then lofted together. This shape, like the rotor, is aerodynamically effective. Wind turbine blades generally move with a tangential velocity at the ends (Vt) of 50m/s. Higher values are used to make gears and pinion. Using this value an angular velocity (ω) can be obtained for the low speed shaft and then a torque based on the power expected from the wind. The force on a tooth on the first gear is shown in Figure 10.

Wt  33.16N and stress on the gear tooth is calculated as   77.7Pa

An FE analysis of the gear tooth is safe as shown below:

Figure 10: FE Analysis Gear Tooth Solid Modeling of components of Wind Turbine:

Figure 11: Solid Model Gears

Figure 12: Tower

Figure 13: Rotor

Figure 14: Blade

Figure 15: Low Speed Shaft

Figure 16: Main Gear

Figure 17: Pinion Gear

Figure 18: High Speed Shaft

Figure 19: Nacelle

Table 1: Percentage of Materials for Large Turbines (Kutz, 2007) *Glass Reinforced Plastic, **Carbon Filament Reinforced Plastic Component /Materials

Permanent Magnetic

Concrete

Steel

Al

(95)-100 5 (65)-80 98-(100) (20)-65 85-(74)

(5)

Cu

GRP*

Wood Epoxy

CFRP**

95 1-(2)

(95)

(95)

Rotor Hub Blades Nacelle Gearbox Generator Frame, Machinery and Shell Tower

(17) (50)

2

98

3-4 2 9(50) (2)

14 2 (30)-35 4-(12)

3-(5)

4. Life Cycle Analysis of Wind Turbine: 4.1 Embodied Energy Analysis of Wind Power Generation: The life cycle of a wind farm consists of turbine production; turbine transportation to the site; site construction (which includes wind farm fixed assets); wind farm operation and maintenance; and dismantling, scrapping, and land reclamation. The various phases of the life cycle with energy inputs and electrical energy output (Jha, 2016), which is the only useful product, are illustrated in Figure 20 below. A sample of data is presented below in Table 2 for lifecycle analysis. Table 2 shows the operational characteristics and the estimated lifecycle electrical energy output of the Wind Power Generation system presented earlier. The useful life has been assumed to be 20 years and with an availability factor of 90%, a full power lifetime of 18 years is calculated. Normally, the turbine only operates when wind speeds are in the range 4-25 m/s. Within this range it is assumed the turbine can only generate electricity at full generation capacity at a nominal speed of 15 m/s. The full power life of the wind farm is calculated as 20x0.90=18 yrs. The net power output of the wind farm is calculated by multiplying the capacity of wind farm by annual load factor. Table 2: Useful Wind Farm Energy Data Description Wind Power Capacity Lifetime Availability Factor Annual Load Factor (Turbine efficiency) Full Power Lifetime Net Power Output Lifetime Net Power Output

Amount 10 hp (7.45 k W) 20 years 90% 35% 18 33556.5 W 604017 W

The lifecycle electrical energy output of 604017 W was calculated by multiplying the full power life time by net power outut. Figure 20: Lifecycle Energy Flow of a Wind Farm

4.2 Turbine Manufacturing and Dismantling: The embodied energy of turbine manufacturing and dismantling is calculated as shown below. Tower, shaft, and gears are made of steel. The volume of the steel used in the turbine manufacture is shown below: Steel: Nacelle Volume=  / 4 * 4.6 2 *15 =249.3m3 π2.22  2 2 m 2 Vtower   30m  19m 3 Frame Volume: The length of complete frame and 4 shell over generator, gearbox, and cone is assumed to  π2.92 π0.4252  2 be 3 times the length of slow speed shaft. The slow m  0.8m  5.4m3 Vgears     speed shaft is of 6000 mm and hence the frame length 4 4   is about 6000mm. There is likely to be of oval shape 2  π0.3  2  π0.82  2 3 and it is assumed that largest diameter of 1000mm m  7.2m  Vshafts  5.2m   4 m  4m 4 (twice the size of the main gear). The volume of     material of the frame and shell is presented at right. Steel: Frame volume = π / 4x12x6 =18.846 m3, Total steel in turbine = (19+5.4+4) m3 x 7860kg/m3=334x106g=223 ton Glass Fiber: Volume of Blade =7.125x3=21.375m3, Blade mass =21.375m3x2440kg/m3=52 ton Aluminum: From Table 1, Aluminum is used in Hub-2%, Nacelle-4%, Gearbox-2%, Frames & Shells50%, and Tower-2%. The mass of Al estimated: Al=.02x33m3+.04x249.3+.02x5.4+.5x18.846+.02x33 =718.9m3, mass of Al=116 tons Copper: Volume =.14x249.3+0.02x9.4+0.35x100 (volume of generator assumed) +0.17x18.846=73.3m3, Total mass of copper used=73.3x8940 kg/m3=65 ton Permanent Magnetic material materials are used in Generator=50% and Nacelle=17%. The volume of magnetic materials used in Wind Turbine; Magnetic material volume=.17x249.3+.5x100=42.931m3, mass of permanent magnet=7.7g/cm3x42.931=322ton (Density of the Permanent Magnet is taken from Hunghon Dunben Magnetic Co. Ltd, China website) Table 3: Embodied Energy of Wind Turbine Construction Material Material

Mass (Ton)

Energy Intensity (GJ/ton)+

Embodied Energy (GJ)

Aluminum Copper Steel Glass Fiber Perm. Mag.

116 65 223 52 332

200 -240 68-74 29-35 23.8-26.3 28-31 Total Embodied Energy

2.32x104-2.784x104 4.42x103-4.81x103 6.467x103-7.805x103 1.238x103-1.368x103 9.296x103-1.029x104 4.462x104-5.211x104

Percentage of Total (%) 52-53.4 9.2-9.9* 14.5-15.0 2.8-2.6* 19.7-20.8*

*It is noticed that increase in the embodied energy does not increase the percentage increase. + Energy intensity data is taken from (Ashby, 2009). In addition to the embodied energy calculated above in Table 3, the embodied energy in production of turbine along with its transportation to the wind farm site should also be calculated. This data was not readily available. This analysis is for a 7.45kW turbine. Larger turbines will have proportionately larger embodied energy.

4.3 Site Preparation and Construction: The embodied energy of site preparation and construction includes site construction building materials, non-material related site preparation, construction processes, fixed assets, and equipment (excluding wind turbine). The site construction building materials include the wind turbine foundations, cable trenches and cables, paths and roads, and site office. The results are presented below in Table 4.

Table 4: Embodied Energy of Site Preparation and Construction Description (Construction Materials) Turbine Foundation (Steel, Concrete)* Cable Trenches and Cable** Paths and Road*** Site Office Total Embodied Energy

Materials (Ton) Concrete – 720 Steel – 30 Corrosion Resistant Steel – 0.136 Soil – 50 Stone – 100 Concrete – 10 Steel – 5

Embodied Energy (GJ/Ton) 1.1 – 1.2 29 – 35 (Low C) 32 – 38 (Low Alloy)

Embodied Energy (GJ) 792 – 864 870 – 1050 4.352 – 5.168

Percentage (%)

2.2 – 2.3 4.9 – 6.4 1.1 – 1.2 29 - 35

110 – 115 490 – 640 11 – 12 145 – 175 2422 – 2861

4.0 – 4.5 20.2 – 22.4 0.4194 – 0.4542 6.0 – 6.1

30.2 – 32.7 35.9 – 36.7 0.01797 – 0.01806

* The amount of concrete in foundation of a turbine is 300 m3. It was assumed it has a density of 2400kg/m3. Hence the mass of concrete for each foundation is calculated as 720 tons in addition to about 30 tons of steel. **The tower height is 50 m and the length of cable is assumed to be 55 m. The material of the cable is corrosion resistant steel with cast steel as sheave material. The standard wire diameter could selected 3/8 in. and weight per foot is 1.7d2 lbf. From this we can calculate the weight of 55 m cable = 50/.305x1.7(3/8)2x12=470 lbs or 136 kg. *** The path and road construction materials are assumed to be stone and soil. The mass of soil and stone will change with the length of roads and path. It is assumed here that a 5 Km road was built and the total mass of soil is equal to 50 tons and the total mass of stone is equal to 100 tons. **** The material used in site office construction is again concrete and steel. The mass of materials will depend upon the rooms at the site; however, it could just be the main transformer room. The mass of steel and concrete materials used are assumed to be Steel = 5 tons and Concrete = 10 tons.

4.4 Wind Farm Operation and Maintenance: The embodied energy of wind farm operation and maintenance are calculated as follows: Steel: Total steel = 223+30+0.136+5 = 258.136 tons, Steel cost = 258.136 tons*1000 $/ton = $258136 Copper: Total copper = 65 tons, Copper cost = 65*7000=$ 455000 Aluminum: Total aluminum = 116 tons, Cost of Aluminum = $ 348000 Glass Fiber: Total glass fiber used = 52 tons, Cost of glass fiber =$ 338000 Permanent Magnet: Total permanent magnet used = 332 tons, 10kW permanent magnet for wind farm can be bought at about $14,000. (Anhui Hummer Dynamo Co. Ltd, China). Concrete: Total concrete used = 750 tons, Total concrete cost = $ 48750 Stone: Total stone used = 100 tons, Stone Cost =100 tons * 1000 $/ton = $ 100000 Total Material Cost = $ 1.67*106 The total cost is calculated as follows: Total cost = Material Cost + Overhead cost; Overhead cost = 25% of Material cost Total cost = $1.67*106 + 0.25(1.67*106) = $ 2.0875*106 The average proportion of cost associated with Table 5: Wind Farm Activities as Proportion of Cost the operation and maintenance of wind farm is Category Proportion (%) as follows: From the total cost, proportionate Administration 21 costs for different categories are calculated. Insurance 13 Categories are; Administration, electricity Land Rent 18 generation and distribution; Insurance; Miscellaneous 17 Miscellaneous, electricity generation and Power from Grid 5 distribution; Service and spare parts, machinery Service and Spare parts 26 and equipment maintenance; Power from Grid, electricity generation and distribution.

Life Cycle O& M costs = Total cost* Energy Intensity (GJ/$) * Plant life (20yeras) The energy intensity coefficients from (Ashby, 2009) were used to calculate the embodied energy.

4.5 Wind Farm Decommissioning and Land Reclamation: The energy embodied in wind farm decommissioning and land reclamation was assumed to be zero. The energy associated with wind turbine dismantling was already calculated in turbine production and dismantling. It is also stated in (Kutz, 2007) and (Jha, 2016) that the recycling and scrapping of the wind turbine has a negative embodied energy. This represents the energy gain because of the lower energy embodied in recycling the materials when compared to the extraction of the materials in their raw state from earth. It was assumed that the energy embodied in the other stages of plant decommissioning cancels out with the energy gain in recycling of turbine materials. The land reclamation embodied energy is assumed to be nil.

5. Conclusion: The proportion of energy embodied in the various phases of the Wind Farm’s life cycle is shown below in Fig. 21. The wind farm’s life cycle is divided into 4 main phases. They are turbine production, turbine transportation, site construction and fixed assets, and operation and maintenance. The wind farm decommissioning and reclamation embodied energy is assumed equal to zero. The highest portion of the wind farm’s life cycle energy is embodied in turbine production and dismantling, accounting for nearly 43% of the wind farm’s total embodied energy. Turbine transportation and site construction and fixed assets contributed about 30% and 20%, respectively, of the total embodied energy. Wind farm operation and maintenance contribute only 7% to the total embodied energy. The direct energy of the wind farm corresponds to operation and maintenance, while indirect energy includes all the other phases of the life cycle. The life cycle cost will change according to the optimized structural design of the wind turbine. It will however, be dependent on design philosophy, whether for minimum weight or maximum strength. More research is needed to compare the different optimized design objectives. 7% 20%

43%

30%

Total Energy: 10x104 – 12x104 GJ Production Transportaion Construction and Fixed Assets Operation and Maintenance

Figure 21: Proportion of Embodied Energy

References Ashby, Michael F. 2009. Materials and the Environment: Eco-informed Material Choice. Elsevier Inc. Budynas, Richard G., J. Keith Nisbett, and Joseph Edward Shigley. 2008. Shigley's Mechanical Engineering Design. 8th edn. McGraw-Hill. Jha, Nand K. 2016. Green Design and Manufacturing for Sustainability. CRC Press. Kutz, Myer. 2007. Environmentally Conscious Mechanical Design. Wiley & Sons. MIT. 2011. Composite Wind Blade Engineering and Manufacturing. http://web.mit.edu/windenergy/windweek/Presentations/Nolet_Blades.pdf Siemens. NX 9.0. Siemens Product Lifecycle Management Software Inc. Tester, Jefferson, et al. 2005. Sustainable Energy; Choosing Among Options. The MIT Press, Cambridge, Mass, USA. U.S. Department of Energy. 2006. Wind and Hydro Images. http://www1.eere.energy.gov/windandhydro/images/illust_large_turbine.gif