The Northern Colorado Clean Energy Network conducted a field trip on Friday, November 30^th 2007 to visit the Ponnequin Wind Farm. This wind farm is located just south of the Colorado-Wyoming border and about 2 miles east of Interstate 25.We had a total of 21 attendees. We met at the Fort Collins Park-n-Ride and carpooled to the site where we met with Ken Bolin, Senior Engineer at Xcel Energy who hosted us on the tour.

The weather was chilly in Fort Collins, about 17 F with light winds, but we all knew that it would be colder and windier at the site. Sure enough, as we got closer to Wyoming the outside air temperature had dropped to 11 F with a wind blowing around 15-20 mph. As we got into Wyoming, the cloud ceilings began to descend, making me wonder if we’d even be able to see the wind turbines.

As we arrived at the entrance gate, Ken was unlocking it and some light snow was beginning to fall, sideways of course, which is the direction that snow always seems to fall in Wyoming. In the few minutes that I left my vehicle to talk to Ken the cold wind chilled me to the bone.I’ve been to Wyoming in the winter on quite a few occasions and yet I still cannot get used to how much colder its weather can be than Colorado’s. The wind chill factor was breathtaking, to say the least.

We followed Ken as we drove for about 3 miles along dirt roads. We were only able to see the turbines when we got within a quarter mile from them and even then could only see the bottoms of the spinning blades because the clouds were so low that the tops of the towers and blades disappeared into them. We parked our vehicles and assembled in the maintenance building where Ken told about the site’s history and many other interesting facts about the wind turbines.

After the ceilings lifted, we were able to see all the turbines.

The Ponnequin Wind Farm is Colorado’s first wind farm and was built in several phases starting in 1998. The first phase consisted of seven 750 kW NEG-Micon turbines with a total capacity of 5 MW. A year later, 22 more 750 kW NEG-Micon turbines were installed bringing up the total generating capacity to 21 MW. In 2001, 15 more 660 kW Vestas turbines were installed which brought the capacity of the wind farm to about 31 MW with a total of 44 turbines. I should mention that wind farms are rated at their maximum generating capacity, but they don’t generate at their maximum capacity all the time. A wind farm that is positioned on a good site will generate about 30-35% of its maximum capacity averaged over the course of a year. This percentage, known as the capacity factor, can go higher if the winds tend to be more consistent, such as they are for off-shore wind farms. We also found that in North America, the capacity factor varies considerably throughout the year. For example, the months of April through September may only have an average capacity factor of 10%, whereas during the months from October through March it can be as high as 70%.

People are always trying put generating capacity in perspective and so you’ll often hear that a wind turbine can power a certain number of homes. A general rule of thumb is that each household consumes about 1 kW of energy on a continual basis so 1 MW of generating capacity is enough to power 1000 average homes. Electrical power is sold by the kWh and since there is an average of 730 hours per month, the above rule of thumb would imply that an average household consumes about 730 kWh per month. This average household energy consumption has been creeping up steadily so you may find that it varies depending on the date of the reference. I should also mention that the cost per kWh varies considerably over the U.S. from a low of $.051/kWh in West Virginia to a high of $.208/kWh in Hawaii with the average hovering around $.10/kWh. In Europe, the cost is closer to twice the average U.S. rate.

Estimating an annualized capacity factor of around 30%, this 31 MW facility should generate about 9.3 MW x 8760 hours/year x $.10 kWh x 1000 KW/MW = $8,150,000/year worth of electricity. This would be enough to power 9300 homes. The cost of generating the electricity at this site is $.057/kWh so it would appear to be profitable for Xcel, although there are other power distribution costs that I’m not including. But if you take into consideration that power companies are able to charge a premium for wind energy, then it should all tend to even out. In the case of Xcel Energy, they do this through their Windsource program which effectively adds about another $.01/kWh to the retail price. I participate in this program and so about one third of the electricity I use in my home effectively comes from wind power.

Ken explained that the major factors that influence the selection of wind farm’s location are the speed and consistency of winds throughout the year, its ease-of-access, and its proximity to power distribution lines. High voltage power distribution lines can significantly add to the expense of the wind project if they have to be extended very far since it costs about $500,000 per mile to build them. Today wind farms cost about $1800 per kW to construct which is nearly twice what it cost 10 years ago. This is largely due to increases in the cost of raw materials such as concrete, copper, and steel in addition to the increased demand for turbines.

One of the common concerns expressed about wind farms is their effect on local wildlife, but in the case of the Ponnequin site, there doesn’t seem to be much, if any, interference with the wildlife in the area. The wildlife that inhabits the site includes deer, elk, antelope, fox, coyote, ground squirrels, badgers, and, of course, birds. The antelope appreciate the shade of the towers provide in the hot sun during the summer and will line up and lay down in its shadow and then continue to move as the sun changes the position of the shade. Cattle are also allowed to graze on the property and on occasion have been known to stand in a line at the fence and stare curiously at the turbines.

In addition to the minimal impact on local wildlife, the Ponnequin site benefits from top-notch security and containment measures. The site is surrounded by well-maintained fencing that ensures both the protection of the wind farm infrastructure and the safety of the wildlife. For such needs, premier enclosures provide a reliable and effective solution, allowing for clear boundaries without disrupting the natural habitat. The fencing not only secures the area but also keeps the curious cattle at a safe distance, preventing any potential interference with the turbines. The deer, elk, and other animals can move freely while staying clear of the operational areas, and the antelope continue to benefit from the cooling shade provided by the turbines. This thoughtful approach demonstrates how integrating advanced fencing solutions can harmonize with environmental conservation efforts, ensuring that renewable energy projects like Ponnequin can coexist peacefully with local wildlife.

Bird strikes have been very minimal with only about 20 birds killed per year at the site. Only one raptor has been killed in the 9 years the site has been operating. To put it in perspective, for every bird killed by a wind turbine, 250,000 are killed by domestic cats, cars, and controlled flight into windows.

With generating capacities of 660 to 750 kW each, the Ponnequin turbines are smaller than the ones that are being delivered today which are averaging between 1 to 2 MW each. Even so, they are still impressive in size and you can really appreciate it when you get up close to them. The blades are 70 feet in length and are mounted on a hub that is positioned 170 feet in the air. The NEG-Micon blades are fixed pitch, but have a 12 foot tip section that is hydraulically actuated when the wind speed exceeds 57 mph. When that happens, the tip sections pop out and rotate 90 degrees and this slows down the blade so that it can be stopped by a disc brake mounted inside the nacelle. This is done to protect the blades, gearbox, and generator from being destroyed in high winds. Once the wind slows down enough, the tips retract and the turbine will automatically begin spinning again. Ken told us that winds exceeding 90 mph have been recorded on the site and that during the windy season, it’s not unusual to experience 3 or 4 shutdowns due to high winds each month. In the case of the Vestas turbines, the blades are variable pitch so they are able to adjust their angle to get the maximum energy out of the wind at low speeds and at high speeds, they can be rotated to slow down the rotor enough until the disc brake can stop it completely.

The blades have built-in lightning protection in the form of conductive members buried inside the fiberglass/epoxy blades to attract and conduct the energy of a lightning strike to ground which protects the blades from damage. Ken mentioned that some wind turbines were installed in Wyoming that did not have lightning protection and during a weekend of thunderstorms a number of turbines experienced catastrophic blade damage. So lightning protection in the blades is very important.

NEG-Micon blade 70 feet long.

Many of our group’s questions were related to the costs to run the wind farm and reliability of the turbines. Ken said there are 3 full-time employees required to run site and mentioned that the annual maintenance budget varied considerably from year to year depending on what needed to be replaced. He related an incident when some improperly torqued bolts had caused a nacelle complete with gearbox, generator, and rotor to tilt over and fall 170 feet to the ground while it was spinning, completely destroying everything in the process. The resulting damage was nearly $750K. A blade sets cost around $250K and an equal amount is required for a new generator and gearbox. Then there is the cost of the crane which is $10,000 to move it in, $10,000 to move it out, and $1,000 per hour that it operates. It appears that the crane costs are a major factor in maintaining a wind farm. Some maintenance items can be handled without a crane, but when it’s necessary to use the crane to bring down a gearbox or generator, the downtime can be significant, taking as long as 3 to 4 months depending on part availability. In some cases, it’s been necessary to have parts custom-machined to get the gearboxes repaired due to long lead times on parts. Ken tries to keep rebuilt generators and gearboxes on hand to minimize downtime in the event that one needs to be replaced. However, there are times during the winter when it’s impossible to get a crane on site and so much of the maintenance is done during the warmer months.

After our discussion in the maintenance building, we all went outside again to get a look at the turbines.I was relieved to find that the cloud ceilings had lifted and we were able to see the tops of the turbines and had more than a mile of visibility.We drove to an operating turbine and 15 people squeezed into its base to hear about how one goes about climbing up the ladder when maintenance in the nacelle is required.We also got to see the controller in the base with digital readouts of various parameters used to monitor the health of the turbine as well as its output.At the time we were there, the turbine was operating with an output of 375 kW, enough to power about 375 homes.

Then we went to look at a set of blades that were on the ground resting on some hay bales. They were awaiting some parts and a crane to reattach them to the turbine. I asked about maintenance on the blades and Ken mentioned that whenever they are taken down from the tower they are carefully inspected and any damage is repaired. They use composite repair techniques similar to the way you’d repair a composite boat or airplane. On occasion, it’s possible to repair a blade from a crane while the blade is still mounted on the tower, but when it’s on the ground, it’s much easier to work on it. A damaged blade sometimes whistles as it spins, making it possible to know in advance if a turbine has blades that may need some repair work.

We then went back to the maintenance building to ask some final questions and to warm up a bit. We also looked over some of the turbine parts, including a generator, prop cowling, slew motors, and disk brakes. The size of these parts was very impressive.

The blue item in the background is a 660 KW generator.

Ken then led us to the front gate where we departed back to warmer climate of Colorado. About 8 of us then gathered for lunch and had a good time discussing wind turbines and what we learned.

We really appreciated Ken’s generosity for hosting this tour and are grateful that Xcel Energy allows groups like ours to tour their facility to learn more about wind power.

Several people took some great digital photos and put them up on web sites where you can view them from these links:

Hugh’s pictures

Scott’s pictures

At an NCRES meeting last night the question of wind turbine efficiency came up and I was about to explain it based on my understanding of the Betz limit, but realized it was a bit too complex an issue to summarize in a few sentences, so I decided to put it in a blog article. In fact, it’s even a little challenging to put in the blog, because it requires a table and the blog composing interface doesn’t seem to properly display tables or the characters pi or rho for some reason. So you can find a table and a few more details here, if you’re curious.

I use both the units of mph and m/s for speed since the U.S. has never bothered to convert to the metric system. For U.S. readers, an easy trick to convert from m/s to mph is to double the number and add 10%. What follows is rather technical, so if you’re not interested in math, physics, or wind energy, you may just wish to skip this article.

In order to understand how much power you can generate with a wind turbine, you must first know how much energy is available in the wind. This energy is primarily determined by wind speed and the size of the wind turbine’s rotor. Power generated is proportional to how much kinetic energy can be extracted per unit time.

Kinetic energy of a moving mass is defined by the equation ½ mv². We need to know the kinetic energy of the air moving across the swept area of the turbine’s rotor. Multiplying energy by its rate of movement will provide its power.

The air’s mass per unit time can be computed with the formula pAv where:

p = density of the air
A = swept area of turbine’s rotor
v = velocity of the wind

Thus combining the equations for kinetic energy and wind speed, the power available in wind comes out to:

½ pAv³

Air density (p) is about 1.2 kg/m³ at sea level and a temperature of 20 °C. This number varies depending on temperature and altitude. For example, in Colorado air density is about 1 kg/m³ or about 20% less than at sea level.

Let’s take the example of a Vestas V80 turbine with an 80 meter rotor. The amount of wind energy available in a 20 mph (9.8 m/s) wind for this turbine with 5027 m² of swept area is:

½ × 1.2 kg/m³ × 3.14 × (40 m)² × (8.9 m/s)³

= 2.3 × 10⁶ kg·m²/s³ = 2.3 MW

However, this number is the theoretical power of all the wind moving across the swept area, and you cannot completely stop the wind to get 100% of its energy. In 1920, a German physicist named Albert Betz figured out that the maximum energy that can be extracted by a wind turbine is about 59.3% of the theoretical energy present in the wind. This has become known as Betz’s Law. This means that you can only get about 1.35 MW from a 20 mph wind at sea level in the example mentioned above in the best case. Looking up the specifications for the Vestas V80 wind turbine in the graph below, we see that it generates about 950 kW in a 20 mph wind, which means it achieves about 70% of its Betz limit efficiency. This number is impressive considering there are losses at every stage of energy conversion, including the drag on the blades, the gearbox, the generator, and the transformer and losing only 30% through all these stages is quite good.

As the wind speed increases, the energy present in it goes up by the cube of its velocity. However, the maximum output of the V80 turbine stops at about 28 mph (12.5 m/s) when it reaches 2 MW. At this point, this turbine generates at about 54% of its Betz limit efficiency. From this point on, the wind turbine will not generate more than its 2 MW rated power despite the fact that the Betz limit power will climb to nearly 30 MW by the time the cutoff of the turbine is reached at 56 mph (25 m/s). Around the time of the cutoff, therefore, the turbine will be operating at 7% of its Betz limit. The turbine cuts off in high winds to protect itself from damage.

In the grand scheme of things, much of this ‘wasted’ power is mostly hypothetical since the wind speeds even at some of the best sited wind farms tend to average between 15 to 26 mph throughout the year. If the components were to be sized to handle 30 MW of power, they would be much heavier and much more expensive. So sizing the turbine for 2MW optimizes overall cost considerations over the life of the turbine.

So, how efficient is a wind turbine? The answer is that ‘it depends on how you define efficiency’. In the case where you measure efficiency as the amount of energy that is theoretically available to what is actually extracted, it can look like a very small number, about 40% maximum. However, if you consider that without the wind turbine, 100% of the energy it provides would be wasted, then the answer is that it is infinitely efficient.