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Wind Energy Honors CourseSpring 2011Iowa State University James D. McCalley Harpole Professor of Electrical & Computer Engineering Wind Power and Power Balance in the Grid
Outline Basic problems, potential solutions Wind power equation Variability System Control Comments on potential solutions
Basic problems with wind & power balance Wind is a variable resource when maximizing energy production Definition: NETLOAD.MW=LOAD.MW-WIND.MW Fact: Wind increases NETLOAD.MW variability in grid Fact: Grid requires GEN.MW=NETLOAD.MW always Fact: “Expensive” gens move (ramp) quickly, “cheap” gens don’t, some gens do not ramp at all. Problem: Increasing wind increases need for more and “faster” resources to meet variability, increasing cost of wind. Wind is an uncertain resource Fact: Market makes day-ahead decisions for “unit commitment” (UC) based on NETLOAD.MW forecast. Fact: Large forecast error requires available units compensate. Problem: Too many (under-forecast) or too few (over-forecast) units may be available, increasing the cost of wind.
Groups of 2-3, 10 minutes • Identify your preferred approach to the variability problem • Consider the below solutions, one, or combination, or other • Identify reasons (e.g., economics, effectiveness, sustainability) and have one person report to class at end of 10 minutes Solutions to variability & uncertainty We have always dealt with variability and uncertainty in the load, so no changes are needed. Increase MW control capability during periods of expected high variability via control of the wind power. Increase MW control capability during periods of expected high variability via more conventional generation. Increase MW control capability during periods of expected high variability using demand control. Increase MW control capability during periods of expected high variability using storage.
Power production Wind power equation Mass flow rate is the mass of substance which passes through a given surface per unit time. Swept area At of turbine blades: • The disks have larger cross sectional area from left to right because • v1 > vt > v2 and • the mass flow rate must be the same everywhere within the streamtube: • ρ=air density (kg/m3) • Therefore, A 1 < At < A 2 v1 vt v2 v x
Power production Wind power equation 2. Air mass flowing: 1. Wind velocity: 3. Mass flow rate at swept area: 4b. Force on turbine blades: 4a. Kinetic energy change: 5b. Power extracted: 5a. Power extracted: 6b. Substitute (3) into (5b): 6a. Substitute (3) into (5a): 7. Equate 8. Substitute (7) into (6b): 9. Factor out v13:
Power production Wind power equation 10. Define wind stream speed ratio, a: This ratio is fixed for a given turbine & control condition. 11. Substitute a into power expression of (9): 12. Differentiate and find a which maximizes function: 13. Find the maximum power by substituting a=1/3 into (11):
Power production Wind power equation 14. Define Cp, the power (or performance) coefficient, which gives the ratio of the power extracted by the converter, P, to the power of the air stream, Pin. power extracted by the converter power of the air stream 15. The maximum value of Cp occurs when its numerator is maximum, i.e., when a=1/3: The Betz Limit!
Power production Cp vs. λ and θ u: tangential velocity of blade tip Tip-speed ratio: ω: rotational velocity of blade R: rotor radius v1: wind speed Pitch: θ GE SLE 1.5 MW
Power production Wind Power Equation • So power extracted depends on • Design factors: • Swept area, At • Environmental factors: • Air density, ρ (~1.225kg/m3 at sea level) • Wind speed v3 • 2. Control factors: • Tip speed ratio through the rotor speed ω • Pitch θ
Power production Cp vs. λ and θ u: tangential velocity of blade tip Tip-speed ratio: ω: rotational velocity of blade R: rotor radius • Important concept #1: • The control strategy of all US turbines today is to operate turbine at point of maximum energy extraction, as indicated by the locus of points on the black solid line in the figure. • Important concepts #2: • This strategy maximizes the energy produced by a given wind turbine. • Any other strategy “spills” wind !!! • Important concepts #3: • Cut-in speed>0 because blades need minimum torque to rotate. • Generator should not exceed rated power • Cut-out speed protects turbine in high winds v1: wind speed GE SLE 1.5 MW
Power production Usable speed range STOPPED HERE Cut-in speed (6.7 mph) Cut-out speed (55 mph)
Wind Power Temporal & Spatial Variability JULY2006 JANUARY2006 Blue~VERY LOW POWER; Red~VERY HIGH POWER • Notice the temporal variability: • lots of cycling between blue and red; • January has a lot more high-wind power (red) than July; • Notice the spatial variability • “waves” of wind power move through the entire Eastern Interconnection; • red occurs more in the Midwest than in the East
= + Load Following Regulation MW-Hz Time Frames Source: Steve Enyeart, “Large Wind Integration Challenges for Operations / System Reliability,” presentation by Bonneville Power Administration, Feb 12, 2008, available at http://cialab.ee.washington.edu/nwess/2008/presentations/stephen.ppt.
How Does Power System Handle Variability Turbine-Gen N Turbine-Gen … Turbine-Gen 2 Turbine-Gen 1 ACE=∆Ptie +B∆f Primary control provides regulation Secondary control provides load following ∆f ∆Ptie
Characterizing Netload Variability ∆T HISTOGRAM Measure each ∆T variation for 1 yr (∆T=1min, 5min, 1 hr) Identify “variability bins” in MW Count # of intervals in each variability bin Plot # against variability bin Compute standard deviation σ. Regulation Load following Ref: Growing Wind; Final Report of the NYISO 2010 Wind Generation Study, Sep 2010. www.nyiso.com/public/webdocs/newsroom/press_releases/2010/GROWING_WIND_-_Final_Report_of_the_NYISO_2010_Wind_Generation_Study.pdf
Do nothing: fossil-plants provide reg & LF (and die ). • Increase control of the wind generation • Provide wind with primary control • Reg down (4%/sec), but spills wind following the control • Reg up, but spills wind continuously • Limit wind generation ramp rates • Limit of increasing ramp is easy to do • Limit of decreasing ramp is harder, but good forecasting can warn of impending decrease and plant can begin decreasing in advance • Increase non-wind MW ramping capability during periods of expected high variability using one or more of the below: • Conventional generation • Load control • Storage • Expand control areas Solutions to variability & uncertainty
Why Does Variability Matter? • NERC penalties for poor-performance • Consequences of increased frequency variblty: • Some loads may lose performance (induction motors) • Relays can operate to trip loads (UFLS), and gen (V/Hz) • Lifetime reduction of turbine blades • Frequency dip may increase for given loss of generation • Areas without wind may regulate for windy areas • Consequences of increased ACE variability (more frequent MW corrections): • Increased inadvertent flows • Increase control action of generators • Regulation moves “down the stack,” cycling!
Increasing wind penetration causes cycling Hydro peaking: http://hydropowerstation.com/?tag=hydropower-peaking-operations
First, primary frequency control for over-frequency conditions, which requires generation reduction, can be effectively handled by pitching the blades and thus reducing the power output of the machine. Although this action “spills” wind, it is effective in providing the necessary frequency control. Second, primary frequency control for under-frequency conditions requires some “headroom” so that the wind turbine can increase its power output. This means that it must be operating below its maximum power production capability on a continuous basis. This also implies a “spilling” of wind. Question: Should we “spill” wind in order to provide frequency control, in contrast to using all wind energy and relying on some other means to provide the frequency control? Answer: Need to compare system economics between increased production costs from spilled wind, and increased investment, maint, & production costs from using storage & conventional gen. How to decide?