Wind Power II – Trends, Systems, and Costs Dr. Carl Elks ENGR 1559 Lecture 5 Previous Lecture from Professor Bean • Professor Bean – – – – Wind and Hydro fundamentals Wind power physics Land area calculations Types of turbines • This lecture builds on his lecture – – – – Emerging trends Wind conversion Energy Systems Perceived Impact to Environment. Costs Wind Energy Conversion Systems: The Wind, Sound and the Fury • Electricity is generated from other energy forms to electricity through the energy conversion process. • The most common conversion process to generate electricity is to convert mechanical energy using a generator. • This process is called “Electromagnetic induction”. You learned about this from Dr. Beans lecture… • The mechanical energy comes from turning the turbine (prime mover). Prime MoverGenerator Turbine 8/19/2012 3 History of US Wind Energy • • In 1888, the first use of large windmill to generate electricity is in Cleveland, Ohio. Rotor diameter was 17meters. The windmill produced 12 kW. 1930s and 1940s, small wind system used in rural areas. – Grandpa’s Knob in Vermont,1941, 1250kW, – 175-ft diameter, failed in 1945. • As utility grid expanded and become more reliable, electricity price declined. – Wind energy popularity fluctuates with the price of fossil fuels. – After World War II, oil prices declined so as the wind energy popularity. • Oil crisis in the 1970s stimulated worldwide interest in wind turbine generators. Source: http://www.telosnet.com/wind/early.html Grandpa Knob Capacity and Trends in Wind Energy - 2013 Cumulative Capacity By Region Installed Capacity Drop – Due to change tax credits, an policy uncertainty?? Near Term Historical Costs Costs – Three ways to look at it • Levelized Cost of Electricity (LCOE) — the utility way (the average cost over the lifespan of the project, initial investments plus operation and maintenance costs, not including externalities). • Wholesale price — hard to get complete numbers on this; many sources will not divulge them. • “All In” — taking into account externalities such as health and environmental costs (yes, these are real costs that we pay but which are not included in the price of electricity). Unsubsidized Levelized Cost of Energy Comparison For 2011-2013 – wind is averaging about 6 cents/KWh for 20 years. For 30 year – 4 cents. Cheaper than coal, on par with natural gas. “All-in-Costs or Full-Cost Accounting” • • • • If you take the full health costs and environmental costs of various energy sources into account, wind comes out looking even better. A recent study out of Harvard found that if one adds in the hidden costs of coal then its actual price in the U.S. is more like 9-27 cents higher per kilowatt hour. So, real cost is between 16 and 34 cents per kWh. Wind Power is about 6 cents per KWh. Harvard Report is the often cited.. Methodology is somewhat vague, but it captures the essence of “full-cost accounting”.. http://www.chgeharvard.org/about/people/paul-r-epstein • • The full-cost accounting is slowly gaining traction for most energy source/production sectors. Remember are sustainability metric – Full cost captures the impacts of the 3 pillars, Wind Power Systems • Terminology – Wind-driven generator – Wind generator – Wind turbine – Wind-turbine generator (WTG) – Wind energy conversion system (WECS) • Types – Horizontal axis wind turbines – Vertical axis wind turbines Number of Blades • Multi-blade windmill need high starting torque and low wind speed for continuous water pumping function. • As rpm increases, turbulence caused by one blade affects efficiency of the blade that follows • Fewer blades allow the turbine to spin faster => smaller generator. • Two and three blades are the most common in modern wind turbine. 1 3 Power in the Wind Kinetic energy ? 14 Power Density Mass flow rate: ρ = Air density (kg/m³) = 1.225 kg/m³ at 15°C and 1 atm is power in the wind (watts) Power density (specific power) = power per square meter (Watts/m²) 15 Power vs Swept Area • Power increases as proportional to swept area of the rotor. • This implies that power is proportional to square of the diameter; the bigger, the better. • This explains economies of scale of wind turbines. 16 Observations from Power Equation • Power in the wind depends on, – Air density, – Area that wind flow through (i.e. swept area of the turbine rotor), and – Wind speed. Cubed !!! – This is ideal, the real world is a little more involved… 20 WIND SPEED STATISTICS AND AVERAGE POWER IN THE WIND • • • In the real world, we modify the basic power to account for the probabilities of a certain wind type blowing for a duration of the year. Leads to probability distributions or histograms of collected data. We never calculate for a single wind speed…The wind never blows at fixed speed. Average Wind Speed from Histogram Wind Speed Distribution Function Recall Power in the Wind Example: Calculate Average Power Example: Calculate Average Power Realistic Overall Efficiency of Turbine Assume that rotor blades has 45% efficiency and gearbox and generator has 2/3 efficiency Average power in the wind (W/m²) 100% 9 Power extracted from the wind (W/m²), depending turbine blade (rotor) efficiency Electrical power output (W/m²), depending on the gearbox and generator efficiency 45% Overall efficiency = 30%!! 24 Power Density with Multiple Towers • Wind turbines have fairly high power densities when the rate measures the flux of wind’s kinetic energy moving through the working surface (the area swept by blades) – up to 1000 W/m2 (in really windy places!!)… • But, they need to be spaced!! – Reduce wake vortices and interface between turbine towers – Get clean air (no turbulence) • How does this impact our Footprint Recommended Spacing Rectangular with a few long rows facing wind, each row with many turbines. Offsetting/staggerring 1 Power Capacity/Per Turbine = Pdensity= 2 𝜌𝐴𝑣 3 𝐶𝑝 = ½𝜌 power density in the swept area of the turbine π 4 𝐷2 𝑣 3 𝐶𝑝 = 26 Wind Farm – How much Land • Wind turbines have fairly high power densities when the rate measures the flux of wind’s kinetic energy moving through the working surface (the area swept by blades) – up to 1000 W/m2 (in really windy places!!)… • The average footprint area occupied by a turbine is small (.2 - 3 Hectares) • However, Optimal Spacing to reduce turbulence effects eats up land. • Modern wind farms use the 5D by 10D array rule to space the turbines in a staggered array. This is for HAWT not VAWT !! • Under this rule, an area of 13-20 hectares/MW, equal to a capacity density of 5-8 MW/km2 or 5 to 8 W/m2 (Denholm 2006) is achievable. These estimates represent minimum spacing to optimize energy extraction. Main Components of a Wind Energy Conversion Systems (WECS) Power extracted by the rotor at low rotational speed, typically at 10-15 rpm. Gearbox is used to increase rotational speed to a higher rpm to generate 50/60Hz electricity. Generators and power converters are the main components to make sure that WECS produce high quality voltage supplies. Transformers are used to step-up the voltage to connect the generator to the grid. When wind speed varies, how can optimally extract wind energy while maintaining a constant voltage frequency? WCES Energy Conversion and Control Rotor speed Voltage frequency Mechanical power Kinetic energy to to Electrical power Mechanical power The Challenge is to design a machine that can accommodate variable motor speeds at a fixed generator speed Energy Conversion and Control • Turbine speed control • Goal: To achieve highest rotor efficiency (extract the highest amount of wind energy ) i.e. operate at optimal Tip Speed Ratio TSR. • To protect the turbine from strong wind. • How? – Adjust angle of attack at the turbine blades. – Stall or pitch control. • Generator speed control • Goal: To maintain constant voltage frequency i.e. operate at 50/60 • Hz. • How? – Multiple gearboxes design for different rotor speed to generator speed ratios. – Different generator designs and power convertors are used to adjust the voltage frequency to be the same as the grid frequency. Types of Turbines Designs • Wind turbine generators can be divided two big categories: Geared systems and direct drive systems. Geared Systems fall into into 5 types.  Direct Drive (Permanent Magnet Generator PMG) • Geared (Electrically Excited EE) – Type 1: Fixed speed (1-2% variation) – Type 2: Limited variable speed (10% variation) – Type 3: Variable speed with partial power electronic conversion (30% variation)  Type 4: Variable speed with full power electronic conversion. (full variation)  Type 5: Variable speed with mechanical torque converter to control synchronous speed. (full variation) • Different generator systems can be arranged in the order of high cost as EESG DD > PMSG DD > DFIG 3G (Type 4 and 5) Which Turbine Designs are Dominating the Market • • • The Dual-Fed Induction Generator (DFIG), an industry standard since the late 1990s, currently rules the roost in volume terms but its need for a high-speed gearbox, extra maintenance and difficulty in complying with grid codes means turbine manufacturers have been looking for new directions.. A Very Promising Newcomer - Permanent Magnet Generator (PMGs) and in other synchronous designs where the electrical energy is generated at a variable frequency related to the rotational speed of the rotor, the output must be converted to match the frequency of the grid. Here the Power electronics must deal with the full power output, demanding full power converters which are considerably more expensive than partial converters of DFIG. The geared generator system has the advantages in terms of the cost, size, and legacy parts. The direct-drive generator system, especially direct-drive PM generator system, is more superior in terms of the energy yield, reliability, and maintenance. Direct vs Geared Large PM Large PM – No field windings No gear box Lighter weight Less moving parts – by 20-40% Higher power outputs Harder to manufacture - air gap between stator and rotor is a few mm Complex power electronics Partial Power Conversion Electronics Long legacy of use – the standard for now Cheaper cost Heavier than PMG Not as efficient at converting wind to power as compared to PMG Horizontal Wind Axis Turbines (HAWT) Upwind HAWT Downwind HAWT Principal Subsystems of HAWTs • Rotor – Rotor blades, rotor hub that capture kinetic energy from wind. • Power train – Mechanical and electrical components to convert mechanical power received from rotor hub to electrical power. • Nacelle structure – Steel structure enclosing the power train. • Tower – Raise rotor and power train to a specified elevation. • Ground equipment station – Interface HAWTs with electric utility. HAWTs: Upwind VS Downwind Upwind • Complex yaw control system. • Keep blade facing wind. • Operate more smoothly. • Deliver more power • Most Modern Turbines are • Upwind type… Downwind • Let the wind control leftright motion (the yaw). • Orient itself correctly to wind direction. • Wind shadowing effect by the tower, cause the blade to flex. • Increase noise and reduce power output. Vertical Axis Wind Turbines (VAWT) Vertical Axis Turbines (Good for Low to Medium Wind Speed) Total height – at least 90 m Rotor height– 50 m Rotor diameter – 100 m Area occupied- 28,300 sq. m. Minimal working speed – 1.5 m/s Nominal working speed – 9.5 m/s Power on wind speed of 2 m/s – 11 kW 1Mw Principal Subsystems of VAWTs • Rotor – Typically contains 2-3 blades, symmetrical in cross-section. – Rotor height is usually 15-30% larger than diameter. • Power train – Mechanical and electrical components to convert mechanical power received from rotor hub to electrical power. • Support structure – Upper and lower rotor bearings, 3-4 structural cables (guy wire) at an elevation angle of 30-40 degree with tensioning devices, and a support stand. • Ground equipment station – Interface VAWTs with electric utility, similar to HAWTs HWAT vs VAWT HWAT • The turbines need to be align with the wind direction. • Capture wind energy at higher power. • Power-train equipment located above ground..Costly maintenance. VAWT • Capture wind energy from any direction because of the turbine is symmetry about its vertical axis. don’t require yaw control • Lower wind energy capture • Can’t capture wind energy at high altitude. • Power-train equipment are located at or near the ground… Easier maintenance. VAWTs for Off-Shore? www.sandia.gov “The economics of offshore windpower are different from landbased turbines, due to installation and operational challenges. VAWTs offer three big advantages that could reduce the cost of wind energy: a lower turbine center of gravity; reduced machine complexity; and better scalability to very large sizes. A lower center of gravity means improved stability afloat and lower gravitational fatigue loads. Additionally, the drivetrain on a VAWT is at or near the surface, potentially making maintenance easier and less time-consuming. Fewer parts, lower fatigue loads and simpler maintenance all lead to reduced maintenance costs .”