Saturday, November 9, 2013

New England wind capacity factors

New England's wind industry is in its youth--still at a very rapid rate of expansion. New Englanders who don't live near one of the wind plants tend not to know much about the dimensions of the industry and sometimes lack vocabulary to describe it. "Capacity factor" is one of the industry's key phrases.

The capacity factor for a power source is its actual output expressed as a fraction or percentage of its rated capacity. Although that is straightforward, in New England, at least, wind-energy promoters seem to avoid it. Instead they tend to confuse people by citing how many homes their projects could serve--whatever "homes" might mean. Factories, stores, offices, street lights and homes of different sizes don't seem to interest them. An accurate way to characterize a power source is through the average amount of power it can produce: the rated capacity, multiplied by the capacity factor.

It is usual to find the capacity of a wind turbine rated according to its peak output--during the strongest winds it can tolerate--rather than by its average output as installed, even though average output is usually what matters most. A knowledgeable reader encountering a capacity rating in "MW" will interpret that as meaning "peak MW." Capacity factors for a single turbine, a group of turbines or a whole wind plant are figured against their rarely achieved peak outputs.

Since in most places wind speeds are usually much lower than peak winds, capacity factors for wind turbines and wind plants are usually much less than 100 percent. When reporting capacity factors achieved by wind, with its great variability, measured outputs are often averaged over full operating years, avoiding false impressions resulting from seasonal changes. For example, in New England during a midsummer week a wind turbine may produce only a third the electricity that it produces during a midwinter week.

Accurately estimating a wind plant's capacity factor is a critical element for financial success. When a wind plant falls short of its expected capacity factor, its revenue typically falls short in proportion. To the same degree, it fails to displace conventional generation, with its pollution and greenhouse-gas emissions. So far, New England has only modest operating experience with wind plants of commercial sizes. Estimating the capacity factors for the region's wind plants, even averaged over several years, remains somewhat uncertain.

As of late 2013, there were ten New England wind plants of commercial size--rated 10 peak MW or more--with at least one full calendar year of operation. At that time, only three of the plants, with 78 turbines among them rated at 123 peak MW, had been operating for three or more full calendar years. Following are the numbers of commercial-size wind plants in each state, with their numbers of full calendar years of operation. [1] [2]

                 ME  (2) 1-year  (2) 2-year  (1) 3-year  (1) 5-year
                 NH  (1) 1-year  (1) 4-year
                 VT  (1) 1-year
                 MA  (1) 1-year

For the full calendar years 2010 through 2012 during which they operated, New England's commercial-size wind plants achieved a combined capacity factor of 26 percent. Those plants have a combined rated capacity of 515 peak MW, so their average combined output would have been about 134 MW. That is far less than the region's 20 large natural gas-fired plants, which generated, on average, more than 6,000 MW. Following are the plant names, states and opening years, rated capacities and capacity factors of individual wind plants for those years. [2]

           New England wind-power plants                                                                                    Capacity factors
Plant name State Capacity 2012 2011 2010
Mars Hill ME 2007 42 peak MW 36% 35% 33%
Stetson I ME 2009 57 peak MW 21% 31% 31%
Kibby Mountain ME 2010 132 peak MW 23% 24%  
Stetson II ME 2010 26 peak MW 19% 27%  
Rollins Mountain ME 2011 60 peak MW 24%    
Spruce Mountain ME 2011 20 peak MW 32%    
Lempster Mountain NH 2008 24 peak MW 30% 31% 36%
Granite Reliable NH 2011 99 peak MW 16%    
Sheffield VT 2011 40 peak MW 23%    
Berkshire Wind Plant MA 2011 15 peak MW 36%    

When data for calendar 2013 become available, six more commercial-size wind plants will appear in a similar list: Record Hill and Bull Hill in Maine, Groton in New Hampshire, Lowell Montain and Georgia Mountain in Vermont, and Hoosac in Massachusetts (recently renamed New England Wind). These newer plants add a combined rated capacity of 234 peak MW--representing capacity growth of 45 percent in a year.

Operators of Stetson and Rollins Mountain in Maine have noted that those plants were idle or curtailed for substantial periods of 2012, while Central Maine Power installed upgrades to a transmission line, causing an estimated 30 percent loss in total output for the year. [3] [4] The new Lowell Mountain plant in Vermont also experienced severe curtailments. Its operators needed to install a $10.5 million synchronous condenser--a phase-shifting motor-generator--to avoid causing instability on the ISO New England grid. [5]

With a larger and longer base of operations, some countries of western Europe have encountered discrepancies between capacity factors projected for commercial-size wind plants and those experienced. A survey of estimates from project developers, compared with estimates produced by academic researchers and with actual experiences, found a typical developer estimate of 35 percent capacity factor, average academic researcher estimate of 37 percent and average experience of 21 percent. [6] A government survey of the five largest wind development regions in California found an average capacity factor for about 11,600 active wind turbines, during calendar years 2000 and 2001, of about 22 percent. [7]

Experiences in Europe and California were both influenced by many small turbines--under 0.1 peak MW--installed while government promotions during the 1970s and 1980s offered large subsidies. Some either were poorly located or were too closely spaced. [8] More professionally designed and managed wind plants, using the larger and more robust wind turbines produced since the mid-1990s, usually achieve somewhat higher capacity factors. [9]

Nevertheless, for New England we should expect overly optimistic projections from project promoters, and we should be prepared to discount estimates from academic researchers. Not all discrepancies are faults of wind turbines or plant design and operation. In addition to problems during power transmission projects, New England wind outputs have been curtailed to avoid overloading segments of the power grid, called "congestion." [10]

Many practical elements that reduce capacity factors are also often omitted from estimates. Wind turbines are complex machines requiring periodic maintenance. They are exposed to lightning strikes, drastic temerature changes and storms. They suffer electrical, mechanical and control failures. [11] In the snowy country from northeastern Vermont through west central Maine, some roads may remain unplowed in winter. Heavy snow drifts can make back-country roads impassible until spring melt, so that power losses may extend over weeks. [12]

If their rotating components or automatic orientation (yaw control) systems fail, wind turbines can become candidates for disasters. [13] In some accidents, lubricating oil has ignited; turbine compartments have been completely incinerated. [12] In others, heavy blades snapped by strong winds have collided with turbine towers that folded and collapsed. There have been deaths, injuries and wildfires. [14] When turbines of a wind plant are damaged, obviously the plant's capacity factor suffers too.

[1] Existing and planned generators, Form EIA-860, U.S. Energy Information Administration, 2013, at

[2] Power plant operating data, Form EIA-923, U.S. Energy Information Administration, 1970-2012, at

[3] Tux Turkel, Inadequate transmission lines affect some Maine wind power, Portland (ME) Press Herald, August 4, 2013, at

[4] Tux Turkel, Wind industry says tax breaks are key, Portland (ME) Press Herald, December 31, 2012, at

[5] Willem Post, Was Vermont's Lowell Mountain wind turbine facility a good idea?, Energy Collective, July 15, 2011, at

[6] Nicolas Bocard (Department of Economics, University of Girona, Spain), Capacity factor of wind power: realized values vs. estimates, Energy Policy 37:2679–2688 (in English), 2009, available at

[7] Dora Yen-Nakafuji, California wind resources, California Energy Commission, 2005, at

[8] Paul Gipe, Wind Energy Comes of Age, Wiley, 1995, excerpted at

[9] Berthold Hahn, Michael Durstewitz and Kurt Rohrig, Reliability of wind turbines, University of Kassel (Germany), 2005, available at

[10] John Dillon, Grid constraints mean less power output from wind projects, Vermont Public Radio, January 30, 2013, at

[11] Kevin Alewine (Shermco Industries), Wind turbine generator failure modes, National Renewable Energy Laboratory, U.S. Department of Energy, September 19, 2011, at

[12] Whit Richardson, $4-million turbine fire at Kibby Mountain, Bangor (ME) Daily News, April 23, 2013, at

[13] Andred Walden, Wind energy's ghosts, American Thinker, February 10, 2010, at

[14] Summary of wind turbine accidents, Caithness (UK) Windfarm Information Forum, 2013, at

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