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When the Wind Blows
The Wind does NOT Blow "Only 1/3 of the Time"

May 8, 2009
by Craig Severance

There are some things that "Everybody Knows" because we have all heard them from (parts of) the popular media:

  • "Everybody Knows" Barack Obama is a Muslim Socialist. [He's a Christian, locked in an FDR-like struggle to save capitalism.]
  • "Everybody Knows" America has all the oil we need, and environmentalists have kept us from drilling for it.  [America's production of crude oil in the lower 48 peaked in 1970-71  because of geological limits predicted in advance by renowned oil company geologist M. King Hubbert.]
  • "Everybody Knows" the wind only blows one third of the time.

Right.

The phrase "the wind only blows one third of the time" has been repeated so many times in blogs and interviews by nuclear hawks and Big Oil apologists, it has become "common knowledge". 

I have even seen this phrase creep into articles written by proponents of clean energy.  A cacophony of voices is now lamenting the intermittency of wind energy, leading many in the public to believe wind cannot offer a serious contribution to U.S. energy needs.      

20% of U.S. Electricity Needs Can Be Met by Wind by 2030.  This was the conclusion not of some Hollywood celebrity, but of the Bush Administration's Department of Energy, in a landmark study published in July 2008. 

DOE projects that U.S. wind energy can grow from installed capacity of 11.5 Gigawatts (1 GW = 1 million Kilowatts) supplying 0.8% of U.S. electricity in early 2007, to a total of 305 GW of capacity by 2030.  These wind farms would supply 20% of projected U.S. electricity needs in that year.   

The remarkable thing about the DOE study was that it envisions a major expansion of wind energy without any advancement in "energy storage" technology to "make up for" the intermittency of wind by attempting to store it for later use.

How can we do this when everyone knows "the wind does not blow all the time"?  Won't the lights go out on a calm day? 

How it Actually Works.  The DOE study calmly and methodically describes how wind energy works, and how our electric grid actually works.  The study used sophisticated models of all the nation's regional electric grids, to determine how the nation's electric utilities can integrate this much wind generation without major challenges to grid reliability or operations.

What "30% Capacity Factor" for Wind Actually Means.  The misconception that "wind only blows one third of the time" comes from the fact that wind farms now typically operate with an average "Capacity Factor" of around 30%.   A 30% Capacity Factor means that over the course of a year, a wind generator will produce total kWh's equivalent to 30% of the number of kWh's that would be produced if the windmill ran at its top rated output (Rated Capacity) for every single hour of the year. 

The Rated Capacity of a windmill is similar to how fast your car could go if you ran it at top cruising speed.  We all have speedometers that lead us to believe our cars are "made to go" somewhere over 100 mph.  However, we don't actually drive that fast.  Sometimes we drive 45 mph, sometimes 75 mph -- and we get where we need to go. 

If every time you got in your car, you floored it and cruised only at top speed, your car would have a Capacity Factor of 100%.  However, with the way you actually drive, it wouldn't be uncommon if your average driving speed was somewhere around 30% of the "Capacity" that could have been delivered at top cruising speed.  This does NOT mean you were only able to use your car 30% of the time.

The same is true for a wind generator.  A 30% Capacity Factor does not mean the generator is dead stalled 70% of the time, and operating at full Rated Capacity 30% of the time.  Rather, in the very windy locations where wind farms are built, the wind blows almost all the time. The wind turbines therefore operate almost all of the time, but over a range of speeds and power outputs. 

Power Increases by CUBE of Wind Speed Increase.  This physical law which is at the heart of wind turbine operations helps put a "30% Capacity Factor" into perspective very dramatically.  The 30% is actually a very impressive figure. 

Most wind turbines will begin producing power ("cut in") at about 12 miles per hour, and reach their maximum Rated Power Output at about 25-29 mph. 

Table 2-8: Typical Power Output Versus Wind Speed Curve 

Source: U.S. DOE, "20% Wind Energy by 2030", July 2008

If a wind turbine has its Rated Capacity at 25 mph, it would need wind speeds of at least 25 mph all the time for a 100% Capacity Factor. 

However,  a more typical average wind speed in many windy locations might be 15 mph. (I'm about to do some quick math, so please hold on.)  A 25 mph wind is 1.666 times the speed of a 15 mph wind.  However, since the power in the wind increases by the CUBE of the increase in wind speed, a 25 mph wind (needed to reach Rated Capacity) has (1.666 x 1.666 x 1.666)= 4.63 times the power of a 15 mph wind!  (You can see this on "Power" curve on the graph.)

This means that if the wind turbine operates 100% of the time (very useful power), but at 15 mph, it would have a Capacity Factor of only about 22%.   

Achieving a 30% Capacity Factor is thus a very impressive number.  Since Rated Capacity is so many multiples higher than the power produced at average wind speeds, the math tells you these wind farms are actually cranking out power most of the time -- NOT "only one third of the time".  

Of course, you don't really need math to find out the wind blows just about all the time in places like North Dakota or Oklahoma -- just ask anybody who lives there.  They would  laugh you off the ranch if you told them the wind is "dead still" 2/3 of the time!

How the Grid Can Use Wind Power Without Storage.  The DOE study makes the point that a regional power grid is always adjusting to variations in power supply vs. power demand.  The variability of the power demand from customers requires the system operators to constantly adjust power production from a complement of many different generators. DOE noted "electric system operators already deal with these factors on similar time scales with current power system loads."   System operators are constantly bringing on line generators to supply load, and taking them off line when not needed.

Figure 4-1.Hourly Load Shapes With & Without Wind Generation

Source: U.S. DOE - "20% Wind Energy by 2030", July 2008

Some generators are designed to run all the time, and are therefore quite inflexible to changes. These "baseload" power plants -- nuclear and coal -- are therefore not useful in responding to customer loads, or allowing renewable energy to take over a portion of energy generation.  (For this and other reasons, Federal Energy Regulatory Commission Chairman Jon Wellinghoff recently stated about new nuclear and coal plants: "We may not need any, ever.")

Every grid must have a good complement of load-following and rapid-response generators to allow it respond to normal hour-by-hour, and minute-by-minute, fluctuations. The primary impact of wind may be, as DOE noted, "A system with wind generation needs more active load-following generation capability than one without wind, or more load-management capability to offset the combined variability of load net of wind."  (Note that load-management capability is a primary benefit of the "Smart Grid", see here.)

This ability of any modern electric grid to flexibly respond to changes is how wind energy can be integrated up to at least a 20% level without the need for storage, DOE concluded.  Once the 20-25% level is exceeded, however, energy storage may be needed to expand renewable resources to significantly higher levels.  (In other words, we probably don't need to worry about storage until we get that far.)

Modern wind turbines are equipped with electronics that can actually help stabilize a grid, by participating in this grid regulation, adjusting power output of the wind generator to adjust to needs of the grid for more or less power.  

DOE concluded "Modern wind plants can be added to a power grid without degrading system perfromance.  In fact, they can contribute to improvements in system performance."
  
Specific Backup Not Required.  DOE specifically debunked the notion that every wind generator must have its own "back-up" generator ready to come on line when the wind is not blowing. 

Modern regional power generation grids are structured so there is no need to provide a full back-up for each generator in the system, since no event would occur where all the generators in a region would go down simultaneously.  

This general rule also applies to wind generators.  DOE noted "this capacity would not be necessary because the pooling of resources across an electric system eliminates the need to provide costly backup capacity for individual resources."  Instead, regional grids are designed with a Reserve Capacity intended to serve the back-up reserve needs of the entire regional system.

The Study also noted that wind farms are designed to provide energy, not capacity, and other types of generators (typically natural gas turbines) are used to guarantee system reliability.  Since the system is not relying upon the wind farm to supply capacity needs, back-up for the wind capacity is not required.  The wind farm's value is providing free (no fuel cost) energy, thus saving the need to burn expensive fuel. (Using the automobile analogy once again, the electric motor and battery system on my Prius is not needed to get my car down the road -- but it sure saves on gas.)

More Wind Power is More Stable.  As wind farms spread over a larger geographic area, the variability of the wind resource evens out and is more stable.  While the wind may not be blowing strongly at the moment in one part of a state or region, it may be blowing quite hard in another location.  DOE ran the variability of wind analysis with ever wider grids included, and found the wider the geographic region, the more stable the wind resource:

Figure 4-8 Annual Hourly Capacity Factor

 Source: U.S. DOE - "20% Wind Energy by 2030", July 2008

A Long Way to Go.  The U.S. can move from 11.5 GW of wind in early 2007 to over 300 GW of wind by 2030 with current technologies, using this very low cost carbon-free energy source to supply 20% of all our electricity needs.  

The U.S. is considered the "Saudia Arabia" of wind energy resources. DOE has studied wind's potential, and found a massive expansion of wind energy is feasible and economical.

This 20% level of "penetration" of wind energy into use on the nation's electric grid will allow the U.S. to begin cutting our greenhouse gas emissions from electricity use, while providing very economical power.

Everybody Knows - that's a good thing. 


This article was originally posted on May 8, 2009.   

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