Ch. 5: Wind Energy
What values of wind power density make a location a poor site for a wind farm? What values make for a good site? What values make for a great site?
Poor: WPD < 100 W/m^2 Good: WPD ~ 400 W/m^2 Great: WPD > 700 W/m^2
How does air temperature impact the power potential of a wind turbine? Warm versus cold?
Power potential is proportional to air density, therefore: - Cold air has a higher wind power potential than warm air
What is the range of wind turbine speed? What is the capacity of wind turbines? What is the average rotor diameter?
Rotation Speed: - Under 40 RPM - Less than 20 RPM for large turbines Capacity: - Commercial - 100 kW - 3.2 MW of electrical power - Largest ones - 8MW capacity Diameter: - 320 ft / 100 m - Boeing Engineering (3.2MW capacity turbine)
What design feature are included in the design of HAWT blades to maximize performance?
Tapering and twist (T and T) Image: think of the Shanghai Tower
In addition to the fluid mechanics, what else influences the power performance curve?
The inefficiencies in: - electrical generators - gear boxes - structural limitations
What happens when the wind speeds are greater than the cut-out speed? Why?
The turbine blades are stopped by a braking mechanism to avoid damage and for safety reasons.
How must wind turbines be spaced in a wind park? Why? What is the optimum spacing?
Too Close: - Flow from one turbine effects flow to next turbine and thus turbine efficiency is decreased Too Far: - Poor use of the site, greater number of turbines could be installed in same area Optimum Spacing: - 3-5 blade diameters between turbines in a row - 5-9 blade diameters between rows of turbines
How are wind turbines typically categorized? Types? What are alternative classifications? Types?
Typical: They are categorized by the orientation of their axis of rotation: 1) Horizontal axis wind turbines (HAWT) 2) Vertical axis wind turbines (VAWT) Alternative: by the mechanism that provides torque to the rotating shaft. 1) Lift = aerodynamic type 2) Drag = impact type
What are typical values for the gearbox/generator efficiency in a wind turbine?
Typically greater than 80%
Compare and contrast the up-wind versus the down-wind turbine designs. Draw a diagram of each.
Up-Wind: - Typical design - More complex - More effective Down-Wind: - Simpler - Less effective
How does velocity effect the power potential of a wind turbine? How does blade diameter effect the power potential of a wind turbine?
Velocity: power potential = cube of wind velocity Blade diameter: power potential = square of blade diameter
What is a wind turbine?
a device used for electrical power generation. How to Remember: - Image: a wind turbine connected to electrical lines that run to a power station and into a home.
What is the purpose of a nozzle?
accelerates the flow at the expense of pressure - Velocity goes up - Pressure goes down
If you double the wind speed, how is the power potential of the wind turbine impacted? If you double the blade diameter, how is the power potential of the wind turbine impacted?
- Doubling wind velocity increases the power potential by a factor of 8 - Doubling blade diameter increase power potential by a factor of 4
How does the cost of wind turbines break down?
- Largest cost is due to gearbox, brake, and electrical system
What conditions must be considered when planning to build a wind farm?
1. Ground conditions - Vegetation, topography, ground roughness 2. Access - Convenience, load capacity of soil, earthquake characteristics 3. Traveling birds - Avoid known migration paths
Why can you not get 100% efficiency with a wind turbine? What would the velocity of the wind behind a turbine with an efficiency of 100% be?
1. Imperfections, frictional losses, etc. 2. Wind speed would be zero behind a 100% efficient wind turbine. This is not possible. There would be no mass flow at the exit of the turbine if this was true.
What are some of the challenges with wind power technology?
1. Increasing the size of the wind turbine 2. Designing the turbines specifically for a given site 3. Developing new structural dynamic concepts 4. Developing custom generators and power electronics 5. Improved manufacturing technology 6. Better control strategies -grid
What developments have led wind power to become a viable source of electricity generation?
1. Introduction of 3-bladed turbines 2. Replacement of DC generators with AC generators
What are the advantages of a wind farm/park?
1. Reduced site development costs 2. Simplified transmission 3. Centralized access for operation and maintenance
In real life, it is not possible to achieve the efficiency known as the Betz limit. What are the 3 primary effects that explain why this cannot be reached?
1. Rotation of the wake behind the rotor (swirl) 2. Finite number of rotor blades and their associated tip losses (tip vortices are generated in the wake of rotor blades for the same reason they are generated on a finite airplane wings sing both produce lift) 3. Non-zero aerodynamic drag on the rotor blades (frictional as well as induced drag)
What design qualifications are needed for a wind tower?
1. Strong enough to support turbine weight and withstand forces during high wind speeds and the thrust on the wind turbine. 2. Tall in order to minimize the turbulence induced and provide flexibility in siting.
Give some examples of wind turbine applications.
1. Wind farms 2. Building installations: Bahrain World Trade Center
Consider a location where the wind with a density of 1.2 kg/m^3 is blowing at a velocity of 4 m/s. What is the maximum power a wind turbine with a rotor diameter of 1.03 m can generate? If the wind velocity is doubled, what is the available power?
32 W 256 W
What is a typical value for the rotor tip speed of a turbine blade?
452 miles per hour (20 rpm)(π*200m/rev)(60 min/hr)(1km/1000m) = 753 km/hr = 452 mph
What is the minimum wind speed needed at a location in order to justify a wind turbine investment?
A steady wind velocity of 6 m/s or higher
What is largest wind farm? Where is it located? How much electricity does it produce?
Altamont Pass in California - 15,000 wind turbines - 3 billion kWh of electricity per year - Enough power to meet the electricity needs of San Francisco Imagery: - Sounds like ultimate. - It's the ultimate wind farm! - Image: wind turbines playing and "passing" ultimate frisbees.
What are some examples of electrical storage systems?
Battery Technologies: - Lead acid batteries: medium performance characteristics - Lithium ion batteries - Experimental technologies (saltwater batteries) Hydrogen: generated by electrolysis when power supply exceeds demand and stored - Pressurized gas - Cryogenic fluid - In a metal hydride - In a chemical bound- such as ammonia (NH_3) Used as Fuel: - Adaptive lean burn natural gas engines - Hybrid fuel (biodiesel/H_2) for stationary diesel engines - Gas turbines - Fuel Cells Potential Energy Storage: - Pump water to high altitude reservoirs to feed hydropower plants - Compressed gas storage
What effects do blade diameter and generator size have on a wind turbine? Draw diagrams of each.
Blade Diameter: - Increasing blade diameter moves power curve to the left - This allows for the maximum power(rated power) to be achieved at lower wind speeds - Effectively, a horizontal shift in Power vs. Wind Speed Curve Generator Size: - Larger generator = a larger rated power - Thus a larger generator increases the maximum turbine power output if the turbine mostly operates at high wind speeds - Effectively, a vertical shift in Power vs. Wind Speed Curve
5-2: Wind Turbine Types and Power Performance Curve
COMPLETE
5-5: Wind Turbine Efficiency
COMPLETE
5-6: Considerations in Wind Power Applications
COMPLETE
Compare and contrast the concentrator versus the diffuser turbine designs. Draw a diagram of each.
Concentrator = nozzle - increases velocity, decreases pressure Diffuser - Increases pressure, decreases velocity
What is typically used as a backup to wind power for remote sites using autonomous power systems (not connected to the grid)? Why?
Diesel engines - Can be put in operation relatively quickly - Units and small and inexpensive, especially when compared to other fossil fuel systems such as gas and steam turbines
How does the time of day impact the power generation of a wind turbine? Is there a way to reduce fluctuations associated with the time of day?
Early morning hours: 80% of average power Afternoon: 120% of average power These fluctuations can be reduced by using taller towers.
How does the mechanical energy equation change when applying it to a wind turbine? Why? Explain.
Flow energy and potential energy go to zero. Kinetic energy is all that remains. - Therefore, a wind turbine converts kinetic energy of fluid into power. Why?: - Pressures at the inlet and exit of a wind turbine are both equal to atmospheric pressure. (i.e. no change in flow energy) - The elevation does not change across a wind turbine (i.e. no change in potential energy)
What is crucial for wind turbines in relation to electrical power generation?
Grid stability. - 10-20% of the power can be lost due to power grid instability
What does HAWT stand for? What does VAWT stand for?
HAWT = horizontal axis wind turbine VAWT = vertical axis wind turbine
Where can wind turbines be located? What are the limitations?
Location: - Must be located where the wind blows Limitations: - Best locations are often far from traditional power grids which requires construction of new power lines - Power output from a turbine is not constant. Only produce power when the wind is blowing
What are the different types of HAWT's? Sketch a diagram of each. Most common?
Most common: 3 bladed For other HAWT designs, see page 141 in textbook
What are some examples of VAWT designs? Draw diagrams of them. What is noteworthy about VAWT designs?
Note: VAWT designs are ineffective when compared to HAWT
What is the purpose of a diffuser?
increase pressure by slowing down the flow down - Pressure goes up - Velocity goes down
What is wind power density? What is it used for? Equation?
the available wind power per unit area - Used for the comparison of wind turbines and locations
What is a load factor? What is the load factor for a wind turbine?
the average load divided by the peak load - 25-30% for wind turbines How to Remember: - A 5'9" man stand over a peak
What is the disk area of a wind turbine?
the circular area that the turbines blades make as they rotate
What is the power potential of a wind turbine proportional to?
the density of air
What is the overall wind turbine efficiency? Equation? What are typical values for the overall wind turbine efficiency?
the electrical power output divided by the available wind power Typical Values: 30-40%
What is wind turbine efficiency? Equation? Define each term in the equation.
the fraction of available power potential converted into actual shaft power
What is the most commonly used / most successful type of turbine?
the lift type - horizontal axis wind turbine (HAWT)
What is the Betz limit for wind turbine efficiency? What is its value?
the maximum possible efficiency of any wind turbine 59.26%
What is the available wind power/power potential for a wind turbine? What is the equation?
the maximum power a wind turbine can generate for the given wind velocity V
What is the cut-out speed?
the maximum wind speed at which the wind turbine is designed to produce power.
What is the cut-in speed?
the minimum wind speed at which useful power can be generated Image: - Think of power performance curve
What is the efficiency of a wind turbine usually referred to as?
the power coefficient C_p
What is the rated speed?
the wind speed that delivers the rated power - usually the maximum power
Why do we use tall wind towers?
wind speed increases with tower height - Higher wind velocities = higher power outputs
Compare and contrast a windmill and a wind turbine.
windmill = mechanical power generation wind turbine = electrical power generation Technically, both devices are turbines since they extract energy from a fluid.
What is a wind farm or a wind park?
clusters of wind turbines
What is a windmill? Examples?
device used for mechanical power generation Examples: - Grinding grain - Pumping water How to Remember: - mill and mechanical both start with the letter M - Image: little house on the prairie
What is the function of a turbine? What does this tell us in relation to windmills and wind turbines?
extract energy from a fluid. - Technically, both windmills and wind turbines are turbines since they extract energy from a fluid. How to Remember: - Velocity behind turbine is less than in from due to less energy - Image: wind turbine in CFD with large velocity vectors in front and small ones behind it
Define mechanical energy. What is the equation? Define each component in the equation.
form of energy that can converted to mechanical work completely and directly by an ideal mechanical device. mechanical energy = flow energy + kinetic energy + potential energy
What other factors affect the choice of a wind turbine site besides wind power density?
- Atmospheric turbulence intensity - Terrain - Obstacles (buildings, trees) - Environmental impact
Where is wind energy generation located in the world?
- China produces the most - US is second - Germany, India, and Spain are next 3 in line US, Germany, Denmark, and Spain account for 75% of current wind energy generation capacity world wide 10% of Denmark's national electricity is supplied by wind turbines