Dr. Ron DiPippo presented a report at the last Select Board meeting on the findings of the Alternative Energy Committee regarding wind power possibilities in Dartmouth's future. Bob Barboza of the Chronicle wrote about the meeting and report in this article.
I have some background in power generation, and as an engineer, I'd like to go over a few points about the presentation. As Dr. DiPippo presented, the average wind speed as measured or calculated was about 6.93 meters per second at 100 meters (330 ft) above the ground. That information in itself does not tell the whole story. Wind turbine power is a function of wind speed. Most commercial sized wind turbines have a cut-in speed of about 3.5 to 4 m/s, they generate nothing below that wind speed. Commercial sized wind turbines have a cut-out speed of about 12-14 m/s. They generate nothing above that wind speed. Let's look at why this is so.
At the low end, you must overcome inertia and friction to start and keep the blades turning. At the high end, the wind power is so great that it will accelerate the turbine blades to dangerous speeds because the generator is maxed out on power and cannot produce more electricity, added power beyond that goes into blade acceleration. The turbine blades spin faster and faster until it flies apart. This cut-in/cut-out band is caused by a trade-off between the two ends. Increasing the turbine's generator size, and therefore the cut-out wind speed, increases the inertia needed to start the blades turning and raises the cut-in wind speed. A smaller turbine has less inertia and a lower cut-in wind speed but reaches maximum output sooner, and therefore the cut-out wind speed is lower. 
The usable wind speed band is very narrow due to the relationship of available power from the wind and wind speed. The power available from wind varies with the cube of wind speed. Stated another way, wind power increases 8 times (2X2X2) for every doubling of wind speed.
The available energy from a 2 m/s (4.5mph) breeze is about 5 watts per square meter(W/m2). Double that wind speed to 4 m/s (9mph) and you have about 40 W/m2 available. Double wind speed again to 8m/s (18mph) and there are about 320 W/m2 available. And at wind speed of 16 m/s (36mph), the available power is a whopping 2560 W/m2. Doubling the speed again to 72mph, just below hurricane strength, and you have an eye-popping 20,460 W/m2. No wonder hurricanes are so destructive! The graph at the left is the power curve for a 1.5 MW GE wind turbine. The y axis units are kilowatts (kW.) More after the jump ...
As you can see the power of a wind turbine does not vary linearly with wind speed, so the average wind speed is not as important as the distribution of wind speed. Most of the wind turbines generation takes place at times of high wind speed and none below the cut-in speed. Using the average wind speed will cause the predicted output to be low. Happily, there are statistical methods to make the correction. The one most commonly used is to assume a Rayleigh distribution for the wind speed from a given mean or average. The chart at the right shows the predicted annual power output of a 1.5MW GE wind turbine versus average wind speed. The distribution of wind speed is assumed to be a standard Rayleigh distribution. The units of the y axis are kilowatt/hours(kW/h). From the graph, Dartmouth could expect annual generation of about 4,000,000kW/h at the 6.93 m/s average wind speed measured. Given that annual generation and assuming the cost of electricity is $.10 kW/h, the town could save $400,000 annually on electricity costs. We would save on transmission tariffs as well which further add to the savings. Dr. DiPippo has the wind speed data at 10 minute intervals for a year, so an engineering firm could more precisely calculate the generation. The engineers would also factor in the initial cost to procure and erect the turbine, maintenance and repair costs, and the cost of the borrowing to determine if a wind turbine installation makes economic sense for the town. That study is what Mr. Gagne has commissioned. Here are some resources to learn more about wind turbines, GE Wind Turbines, Vesta wind turbines, Siemens Wind Turbines, Gamesa Wind Turbines, Wikipedia article about wind turbines
Saturday, November 22, 2008
Wind turbines in Dartmouth's future?
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A regional energy meeting (wind turbines)took place in Marion ,Sept 2008 .Nils Bolgen from MTC made an announcement that most towns on the SouthCoast do not have the wind power inland to support commercial wind turbines bolgen@masstech.org . The representative from Dartmouth produced a short description of the wind results they have as of date. During the presentation it became clear that the months of July and August were missing from the equation. The study was done from Sept to June . The study was concluded leaving out the two slower wind speed months . Are these the same facts presented to the town of Dartmouth last week? This question was asked at the Marion meeting in Sept . Should the wind study be a complete year of wind data ?
Bill, Does the $400k number of savings that you posted as potential energy savings take into account the original capital expenses to get the turbines up and running?
If not, what is the cost of erecting a turbine? I think I remember Dr. D. saying that it was around $3.5Mil.
Documentation of all the months average wind speed should be taken into consideration for at least 12 to 14 months to make up for any lost months. Sodar is another method of checking for wind speeds that average at least 14.7 mph at heights over 260 feet. Both the Sodar or Meteorological tower data must be over 12 to 14 months and not exclude the slower months .
Lets say you had a 1.65 mw turbine . The blades spin all the time and stop at around 40 mph . While the blades are spinning from 0 mph to 14.7 mph no energy is being produced . At 15 mph you start to produce power and as the wind speed increases to say 35 mph your power increases. The problem is if you have a 1.65 mw turbine with an average yearly wind speed lees than 14.7 you hava problems.
http://www.mtpc.org/Project%20Deliverables/Comm_Wind/Dartmouth/Dartmouth_Preliminary_Site_Analysis.pdf
Wind Power in Dartmouth:Siting considerations for a Met Tower and for a Wind Turbine
Bill you will see on page 7 of this report the wind speed predictions by the MTC are between 13.4 to 14.5 mph for all four locations in Dartmouth . these are all the lowest level even if they make that . The report speaks for itself.
Frank1, I think that you are jumping the gun by concluding that the wind resource is not there based on a lecture. We have empirical data and the analysis of that data will be the proof. That is the study which has been commissioned. As the chart included in my post shows, the wind available would generate some 4 million kW-hrs. The question now is whether the actual wind distribution as measured will provide that sort of generation and whether the cost associated with installing and operating the turbine justify the investment.
To answer wally's question, the $400K figure is the gross revenue, not the net after expenses.
The 14.7mph figure which you cite is equal 6.7m/s. The average wind speed that the Alternative Energy Committee reported was 6.93m/s or 15mph. Also your contention that no power is generated below 14.7 mph is incorrect. The cut-in wind speed for the GE turbines whose power curves appear above is 3.5 to 4m/s(7.7 to 8.8mph). Above those wind speed that turbine produce power albeit at low rates but the power production increases with the cube of wind speed.
To answer wally's second question, I don't know what the cost of the turbine and its installation are.
Bill, Recently, last Monday, the town approved $5000.00 for the engineering of one or two commercial wind turbines. There have been questions recently about what exactly the executive administrator does in Dartmouth. In a study prepared by the University of Dartmouth a request was made to review turbine sites with only the executive administrator.
The question here is why would the review be done with only the executive administrator? The sites were chosen by the Dartmouth Alternative Energy Committee why were they not included in the review? Here is the first item with a quote from the first paragraph in the study:
http://www.mtpc.org/Project%20Deliverables/Comm_Wind/Dartmouth/Dartmouth_Preliminary_Site_Analysis.pdf
"1. Introduction
At the request of the Massachusetts Technology Collaborative’s Renewable Energy Trust, Sally Wright of the UMass Renewable Energy Research Lab (RERL) visited several potential wind turbine and/or wind monitoring sites in Dartmouth in October of 2004, along with the town’s Executive Administrator, Mike Gagne. This report focuses on the siting considerations for wind-monitoring towers (met towers) as well as some of the logistics for met tower installation. Additionally, it takes into consideration a few logistical considerations in a broad “fatal flaw” analysis for potential wind turbine installations. This report is not intended as and should not be considered a wind turbine siting study."
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