The Answer My Friend Is Not Blowing In The Wind

Aug 03, 2016

A few years back, a friend of mine working for one of the larger cellular telephone companies asked if I would assist him in a project to import wind class data into a GIS mapping application. He was tasked with performing wind analysis studies to determine what cell sites could potentially run off of a wind power as part of his company’s green initiative. The project intrigued me so I agreed to assist. While I was working on importing the data I took an interest in what the data had to say.

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The U.S. Department of Energy classifies land suitability for wind power generation on a range of one (1) to seven (7). One being land that has little to no potential and 7 having the greatest. This classification is referred to as the NREL (National Renewable Energy Laboratory) Classification.

Maximum conversion of wind power to mechanical energy would occur if the blades of the wind turbine captured all of the wind energy, reducing the wind velocity behind the blades to zero. In practical application that’s impossible. Betz law tells us that the maximum achievable extraction of energy from wind using a wind turbine is 59%. When you factor in inefficiencies and losses in rotor blade friction and aerodynamic drag, gearbox losses, and generator and converter losses, todays wind turbines achieve around 32% to 35% conversion efficiency.

Curious about how extraction efficiency, wind classifications, and the power output curves of commercial wind turbine generators relate, I obtained the technical specifications from several manufacturers. They were all pretty much the same as I expected. This is the power curve of a Siemens 3.6 MW turbine, one of the more efficient designs in use today:

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The power curve chart tells us that “cut-in” wind speed for the turbine is between 3 and 5 m/s (meters per second) wind speed or 7 to 11 mph. That’s the amount of wind required to start the blades turning. Nominal (plated capacity) power is achieved at between 12 and 15 m/s (27 - 34 mph). Plated capacity is the maximum power output of the generator as shown on its affixed serial / model number plate.

Let's plant one of these wind turbines in each of the NREL land classes and see how it performs relative to plated capacity. We’ll start with class 3 because there are a small number of commercial wind turbine generator farms built on Class 3 land:

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From the above comparisons we can see that this wind turbine needs to be sited on class 7 tracts to achieve at least 49% or better plated capacity per its power curve. Unfortunately, wind turbine generators rarely meet the rated top end performance figures and tend to barely exceed the lower end. Most wind turbine farms operate at less than 20% plated capacity.

The performance gap isn’t due to overstated power curves but rather there are a lot of assumptions built into ratings like air temperature, barometric pressure, air density, altitude, air flow perturbation, and other physics that play a significant factor in the actual energy capacity of the wind. Taking into account only the average wind speed disregards a number of equally important variables in the wind energy budget.

How much viable land do we have in the U.S.A. to build wind turbine generation farms on? Let’s have a look:

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With this map, courtesy of NREL data, I’m showing only Class 4 and above. Class 3 and lower, with few exceptions, is generally not seen to be commercially attractive even with generous government subsidies (around $23 per megawatt - half the wholesale price of electricity).

For convenience, I placed the percentage of plated capacity of the Siemens wind turbine in the ledger. Note also that I’m showing only the lower 48 states. Alaska and Hawaii tell pretty much the same story. Their electric grids do not interconnect with the lower 48 states. Thus, being energy islands, they add nothing to the energy budget of the lower 48 where the majority of our population lives and manufacturing occurs.

Several things become readily apparent in this map. Most of the Class 6 and 7 wind is off-shore and there’s not a lot of it relative to the size of the country. There’s little land in the Eastern states that is viable for commercial wind energy production. There is some capacity off-shore along the mid to Northern Atlantic coastline. However, according to ArcGIS, narrow continental shelves, areas restricted for reasons of military use, shipping, hazardous waste disposal, environmental protection, and depths greater than 100 ft., make relatively little of the off-shore tracts developable. Almost no off-shore tracts can be developed along the Pacific coastline. As such, relatively little of the Class 6 and 7 off-shore areas depicted on the above map are suitable for developing wind energy.

The U.S. Department of Energy optimistically predicts that wind power can supply up to 20% of the nation’s power by 2030. Meeting that number will require massive government subsidy, planting wind turbines on almost every developable tract of land and sea, solving the problems of energy storage, integration of intermittent power sources, and reengineering the the existing grid, all at tremendous expense and soaring electric prices.

Lets work some simple math. Coal provides 33% of the 4 trillion kilowatt hours of electric power generated in the U.S. Nuclear provides 20%. Natural Gas provides 33%, hydroelectric provides ~6%, and petroleum provides less than 1%. Approximately 5% is provided by wind and less than 1% by solar (reference). Activists want to shut down coal, nuclear power and natural gas energy production, eliminating 86% of our nation’s generation capacity. And they think wind will save the day? It doesn’t add up. There simply isn’t enough developable wind energy on the map to reach even a modest fraction of this capacity.

This article is an updated reprint of my original work of the same title published at Suyts Space, an on-line science and politics venue. Not much has changed with the numbers presented herein since its original writing.

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