Michael H. Fox
Why We Need Nuclear Power
The Environmental Case
Read Chapter 4 for more information
Wind energy ultimately comes from the sun because it depends on the
temperature and pressure differences created by the sun shining on the
oceans and land. The wind turns the blades that drive a gearbox to run a
generator that produces electricity. The gearbox and generator are in a bus-
sized nacelle perched atop the tower. The energy of the wind varies by the
cubed power of wind speed, so the electrical power output increases
dramatically as the wind speed increases to the turbine’s rated wind speed,
typically 11 to 14 meters per second (25 to 31 mph), and then remains flat
until the cut-out speed. For example, if the wind speed doubled from 10 to 20
mph, the electrical power output would increase by eight times! Of course,
the output also drops off rapidly as the wind falls below the rated wind
speed. This makes wind power fluctuate rapidly as wind speeds constantly
vary. Wind can also blow too hard. When wind speeds exceed about 60 mph
(the cut-out speed), the turbines shut down to avoid permanent damage.
The rated output of most wind turbines is 1.5–2.5 MW, which is specified as
the rated wind speed.
Location: A big problem for wind power, as it is for solar power, is that the
best resources are not where most of the people live. This is obvious when
you compare the NREL wind map with the NASA map of lights at night. To
get wind power to large population centers requires building large
transmission lines. Winds are best in the low density central plains of the
country and areas of Wyoming and Montana. Ridges of mountains are also
good for wind power, especially in the Northeast. Both coasts of the US
have excellent offshore wind resources but the West Coast is not amenable
to wind development because of the rapid dropoff of the continental shelf.
Wind resources are classified in a range from 1 to 7 by NREL, based on the
range of wind speeds. Areas that have a wind power classification of 4 or
higher are areas where it might make sense to use wind energy to produce
Capacity Factor: As is the case with solar power, wind farms are specified
by their peak output but that is not what they produce. Intermittency is a big
problem for wind—the wind doesn’t always blow, so there has to be backup
sources of power that can adjust rapidly to make up the shortfall. The
capacity factor for wind is the proportion of the total installed power that is
actually available to be used in the electrical grid when it is needed.
Because of variations in wind on hourly, daily, and monthly time frames, the
average capacity factor for the US wind power in 2010 was 27%. In other
words, of the 60 GW installed in the US, only slightly over one-quarter of that
is available on average, or about 16 GW. The actual capacity factor depends
on the wind power classification, so wind farms in Class 3 areas might have
a capacity factor of only 10%, while wind farms in Class 7 areas might be
near 40% (see wind map in Figure 4.5). According to the EIA (Energy
Information Administration), the capacity factor for wind turbines built in 2019
is expected to average 35%.
Wind varies throughout the day, but in many areas it is stronger at night than
during the day when the peak electrical demand occurs. Thus it is not well
matched with the demand curve. As an example, grid operators in Texas
count on just 8.7 MW for each 100 MW of installed wind capacity to be
available during the peak demand on hot days, a capacity factor of just 8.7%.
This might be partly balanced with solar power that is highest during the day,
but most states do not have both good wind and good solar resources.
A good example of the variation in wind energy throughout the day in
Minnesota in the spring of 2010 is shown below.
Figure 4.7 Spring 2010 wind power in Minnesota. Source: US DOE
Wind also varies during the seasons, being stronger in the winter than in
the summer when there is a greater need for electricity to power air
conditioners. The Platte River Power Authority in Fort Collins, Colorado,
owns an 8.3 MW wind farm in Medicine Bow, Wyoming -- one of the
windiest places in the country. If the wind turbines were running at full
capacity for 24 hours a day, the wind farm should generate 6.0 million
kWh per month. I have plotted the actual output over a 5-year period
below. It is immediately obvious that the wind blows much more strongly
in the winter months than it does during the summer months, generating
about three times as much energy during the winter as during the
summer. This is fairly typical of wind resources, and it is a problem
because much of the additional energy demands are for air conditioning
during the summer.
The graph also illustrates how variable wind energy is, even when
averaged over a monthly basis. Given that the maximum generation
capacity is 6.0 million kWh per month, the actual output (capacity factor)
was half of that for only 6 months out of 5 years! In the summer the
capacity factor is frequently about 15%, while in the winter it averages
closer to 40%. The overall monthly output averaged over 5 years is 1.8
million kWh or a capacity factor of 30%, slightly better than the US average.
Figure 4.6. Monthly power generation from a wind farm in Medicine Bow,
Wyoming. Data from Platte River Power Authority.
The NREL modeled the effects of up to 30% wind and 5% solar in the
West on electrical grid operations. Two months were modeled. During the
windy month of April, with highly variable wind, it becomes very difficult for
a grid operator to meet the net load requirements. In July—when winds
are low, wind contributes only 10–15% of the power, and solar matches
the load much better—it is much easier for the grid operator to match the
load requirements. Overall, a combined contribution of 23% solar and
wind was feasible but increasing that to 35% caused severe problems in
matching the load because of the large fluctuations in wind power.
Furthermore, the main impact of the wind and solar power is to reduce
the most efficient and least carbon intensive fossil fuel plants—natural
gas turbine and combined cycle plants—rather than reducing coal-fired
power plants, which is the biggest problem for CO2 and other emissions.
And you still have to have most of the natural gas power plant capacity
available to meet the summer demand.