The physical wind potential of an area is affected by several factors including: yearly average wind speed, topology, land cover, and proximity to shorelines.  The net result can be summarized in a wind potential map and used to determine the capacity factor--the percentage of a turbine's power rating that can utilized over the course of a year--that one can expect a turbine to achieve.  The actual capacity factor of a turbine cannot be determined until it is placed into operation but one can accurately estimate that capacity factor by measuring wind speeds at the proposed turbine site for an entire year.

 

The Wind Power Map

The National Renewable Energy Laboratory, working with the Michigan Wind Working Group, has published the report Michigan Wind Resources (2004), which includes the most recent maps of wind potential in Michigan . Areas with good wind power potential includes the western Upper Peninsula.

 

The annual wind power estimates for this report were produced at the 50 meter height level and broken down into 7 classifications. These wind power classifications range from “poor potential” (class 1) to “superb potential” (class 7). The class ranges are color coded for easier referencing. The power available in the wind is proportional to the cube of the wind speed—and so strong sustained winds can significantly raise an area's wind power classification..

From the map, it is clear that the best wind potential in the western U.P. is along Lake Superior to the west side of the Keweenaw Peninsula starting near the Little Girl’s Point and continuing clear around Copper Harbor.   Offshore, the wind  potential is general rated as Class 5 (Excellent) and Class 6 (Outstanding). Directly on the western shoreline, the wind potential rating drops to Class 4 (Good) and Class 3 (Fair).   Further inland, the wind potential rating falls even lower to Class 2 (marginal) and Class 1 (Poor) due to the north to south running ridgeline and forested land cover. 

 

Capacity Factor

Wind turbines do not operate 24 hours a day, 365 days a year. A wind turbine at a typical wind farm operates 65-80 percent of the time, but usually at less than full capacity because the wind speed is not at optimum levels. Given that the power available in the wind is proportional to the cube of the wind speed, the power generated at low wind speeds is much less than that at higher speeds.  Small increases or decreases in wind speed can make a significant difference in the amount of power generated.

The capacity factor of a wind turbine, sometimes referred to as the load factor, is the ratio of the actual energy generated to the maximum possible.  More precisely, the capacity factor is the energy produced during a given period divided by the energy that would have been produced had the wind farm been running continually and at the maximum output:

 

   Capacity Factor = energy produced during measured period [watt-hrs]
                            rated capacity [kwatts] x no. of hours in period [hrs]

 

Thus, in terms of the energy produced over the course of a year, a 1 megawatt turbine that has an installed capacity factor of 30 percent is best thought of as a 300 kilowatt generator.  In general, the capacity factor can always be made higher by putting a smaller generator on the wind turbine, so that it ran at full output for a greater proportion of the time. However, power generation would be limited to the lower rating at times when the wind is capable of producing more electricity. 

The profile of the wind at a site, and hence the corresponding capacity factor, is one of the largest factors in determining how profitable a commercial wind farm will be. 

 

Need for Measurement

The wind map for the western U.P., while fairly accurate over large regions, is just a computer model based on relatively small amounts of data and cannot predict the wind profile of particular site with pinpoint accuracy.  Therefore, when wind turbines are proposed for specific sites, wind speed measurements taken over a year's time are highly desirable.

The more one can know about a site’s wind profile, the better. First, given that the power available in the wind is proportional to the cube of the wind speed, small differences in strong sustained winds can make a significant difference in the resulting capacity factor.  In addition, wind turbines that are optimized for the wind profile of a particular site can result in a higher capacity factor. Manufacturers optimize the design of wind turbines for particular wind conditions by adjusting things such as the rotor swept area.

Wind Power in the Western Upper Peninsula