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Quick Start Wind Energy Primer

Before we get started, I just wanted to mention that a Table of Contents (TOC) for the FAQ does exist.  A link to it is in the footer of each FAQ page.  If you want to see the site structure, or get confused, try using the FAQ Table of Contents.

Basic types of wind energy systems.  Wind can be harvested as a source of energy in many parts of the world.  There are electricity generating wind turbines and water pumping wind turbines.  The wind turbines we talk about most on this list are electricity generating turbines.  From here on out, when I say wind turbine, this is what I am referring to.  There are two basic ways of using wind energy.  The first is what is called a utility or grid intertie.  This is where the turbine (through various electronics) can output its energy as standard household current.  The second method is where the wind generator generates DC power to charge a battery.  Each type of system has its pros and cons.  Obviously for a grid connection, you must have the utility grid.  In many parts of the world, this is not an option.  Battery charging systems require more maintenance for the batteries, the batteries are a danger that needs to be accommodated, and the batteries can become a significant portion of the cost of a renewable energy system.

Wind energy...how much energy is in the wind?  The energy available in the wind is proportional to the wind speed (velocity) cubed.  That's right...V*V*V.  That means every time the wind speed doubles, there's 8 times more energy available in the wind.  The majority of the world is going to produce the majority of  their energy in wind speeds from about 11-20 mph (5-9 m/s).  Even though there is a lot of energy in higher winds, those wind speeds are not very common, and harvesting that energy is generally not economical.  Remember that you want to put a piece of machinery in the wind, and you want it to remain in one piece.  As the winds get stronger, there's lots more energy in the wind.  All turbines need to shed some of that energy just in order to survive.  Small turbines tend to "govern", or get out of the wind, letting lots of the energy pass.  Commercial machines have hydraulic brakes to stop them in strong winds.  Most turbines are designed to survive 120 mph (54 m/s) winds.  That is no small task for any machine, since there is more than 500 times the energy in 120 mph (54 m/s) winds than in 15 mph (6.7 m/s) winds.  Think about it for a moment.  That is a lot of energy.

I said that the energy available in the wind is proportional to velocity cubed.  However, since the wind speed is constantly changing, we need to look at the power available in the wind.  Energy is power times time.  Stated another way, power is the instantaneous energy available in the wind.  

       P= 0.5 * rho * A*(V^3)

       where rho = air density

                    A = swept area of the rotor

                    V = velocity

A wind turbine, of course is NOT 100% efficient.  The MAXIMUM energy that a wind turbine can get out of the wind is 59.6% of what is available.  That is called the Betz limit, and was proven many, many years ago.  Simply put, if you extract all of the energy out of the air with a wind turbine, the air is no longer moving on the downwind side of the blades.  If the air isn't moving, you cannot do any useful work (generating energy).  In this case, you might just as well put up a piece of plywood where the blades are.  The air will go around the wood, just like it would the turbine if you extracted 100% of the energy available in the wind.  On the other hand, if you don't slow the air down at all, you can't accomplish any useful work, either.  It's like not having a turbine in the air.  The compromise between slowing the air down and keeping it moving is the Betz limit.  

The equation for the power out of a wind turbine is the same equation as above, but adds a lot of efficiencies to the equation.  The design of a turbine will include efficiencies of the airfoil, the alternator, brushes and other items.  It is most simple and easiest to remember that the two most important factors in collecting energy from the wind is that the energy you can get is proportional to the swept area and to the velocity cubed.

Why are wind turbines put on towers?  A tower's main purpose is to get a turbine into smoother flowing air.  Generally speaking, the higher into the air you put a turbine, the faster the air is blowing.  A small increase in wind speed will give a large increase in power (because power is proportional to velocity cubed).  At one of our sites, I am measuring wind speeds at roughly  35 feet (11m), and 75 feet (22.9m).  There is a 3 mph (1.3 m/s) wind speed difference between these two turbines.  The energy available at the top of the taller tower is significantly higher than the energy available on the shorter tower.

Aren't there several different kinds of towers?  Yes.  There are two general types of towers: freestanding towers and guyed towers.  Freestanding towers generally have three or four legs or a large tubular body.  Guyed towers use sets of guy wires to hold a very thin tubular structure (relative to a freestanding tubular structure) or lattice structure in the air.  Guyed towers are cheaper than freestanding towers, but require more space.  There are actually two different general classes of guyed towers, also.  Those are tilt-up towers which can be tilted up and down with a winch, pickup truck or tractor.  These towers have 4 sets of guy wires.  There are also lattice towers (like cell and radio towers) which have 3 guy wires which must be raised in place or with a crane.  The 3 guy wire towers are generally cheaper, but generally require a crane for installation.

How do I know if I have enough wind to install a wind turbine?  There are wind maps that the DOE (Department of Enegy) has put out in the United States.  They are available through the DOE.  They are general maps, but local terrain (hills, trees, buildings, etc) will alter the winds on your site.  The manufacturers generally have links to these resources from their home pages (www.bergey.com has this data available).  There are local weather stations across the globe, where you can get their historical data.  Even though it is not at your site, these general values can be pretty useful, and in many cases are sufficient for most people.  Why?  It depends on the level of investment you wish to make into your wind energy system.  In many cases, it makes more sense to spend your money on the turbine than on instrumenting your site to determine your actual wind speed.  To monitor your site for one year could EASILY cost 10% of a complete (US home sized) wind energy system, depening on who is going to set up an anemometer to measure the wind speed, where it is going to be placed, etc.  If you want to put in a small wind energy system, monitoring your site can easily cost HALF of what your system may cost.  Rather than spending the money on the instrumentation, the general rule of thumb says...take a good guess at your wind speed, and put the money to instrument your site into the next biggest turbine if you can.  That sounds rather crude, but it works.  Many "cheap" units for measuring wind speeds are just not accurate enough to matter a lot.  "Local" wind data can give you a ballpark figure of what your site winds might be like, even if that "local" data is for someplace some distance away from you.

How would I go about comparing wind turbines?  The rated output from a wind turbine is pretty much a meaningless figure all on its own.  Most turbines are rated for wind speeds in excess of 25 mph (11 m/s).  It doesn't make a lot of sense to try to compare a wind turbine rated at 25 mph to another turbine rated at 28 mph, because there is a lot more energy (40% more) in the 28 mph winds.  A turbine that has a higher rated output in higher winds isn't necessarily any better than a turbine that has lower rated output in lower winds.  This is especially true since most sites will produce the bulk of their energy in 11-20 mph (5-9 m/s) winds...so why do you want to compare machines that are rated at much higher wind speeds?  The most useful piece of information you need to determine how much energy you will get from a wind turbine is the swept area.  The area that a wind turbine's blades sweep out as it turns is the collector in your wind energy system.  A turbine that collects energy from 50 square feet of area (4.6 square meters) will likely produce twice as much energy as a turbine that collects energy from 25 square feet of area (2.3 square meters).  Swept area is a more consistent way of comparing turbines.  Power curves in small turbines generally are not guaranteed by any manufacturer nor are they tested to a small turbine testing standard.

Does the number of blades affect the output?  For all practical purposes, no.  Theoretically, yes.  More energy can theoretically be extracted from machines with fewer blades.  More blades yields less theoretical efficiency. A single blade would be able to extract that most energy out of the wind.  However, a single bladed machine would be useless because it is unbalanced, and wouldn't last very long in the wind.  Two blades is the smallest number of blades that will work on a turbine.  Two-bladed machines are dynamically active, however.  They are susceptible to "chatter" as the turbine turns to follow the wind.  Three bladed machines are the minimum number of blades for a dynamically stable and balanced machine.  

How do I know how big of a  wind turbine I need?  Generally speaking, you would be purchasing a wind turbine because you want to generate electricity.  The best place to start is your last energy bill if you are connected to the grid right now.  Your energy bill states how many kilowatt-hours (kWh) of electricity you used.  You would want to add up the number of kWh's you used over the last year, then try to find a turbine that would generate that number of kWh's annually.  Manufacturers generally have an estimate of energy produced per month for varying wind speeds.

I will warn you...wind turbines are not cheap.  They are an investment in the future energy supply of the world.  There is a payback lifecycle on wind turbines, however, it is generally a very wise thing to try to economize on your utilization of energy than to create your own.  If you can cut your energy bill in half (not a difficult thing to do for many Americans), you can purchase a much smaller turbine, and your system will be much more affordable.  Unfortunately, I'm not going to go into how to do that right now, but energy efficient appliances and energy efficient light bulbs such as compact fluorescents are a great place to start.  An average American household is probably going to need a 10 kW turbine as a rough starting point.  You are NOT going to power a standard refrigerator with the smallest wind turbine on the market.  You are not going to power an average American household on a $2000 investment...it would probably take around $20,000+.  Let's keep this in perspective.  You say sticker shock?  How much does an automobile cost?  SUV's and pickups are pretty popular these days and a new one can start at $30-$50,000 and up!  You may wonder how people can afford wind turbines, but personally, I wonder how people can afford to buy new vehicles!  It is all a matter of priorities.  I have a higher priority for producing my own energy than for the vehicle I drive.  Don't get me wrong.  I need a dependable, reliable vehicle, especially for all of this windmill work...but I have one.  It just wasn't brand new.

If you don't have an energy bill right now, you need to figure out how much energy you are going to need to generate from the turbine in order to determine what size turbine to purchase.  If you aren't going to be on the utility system, you will need to consider purchasing a deep cycle lead acid battery pack (most commonly used battery pack) and perhaps an inverter to power household appliances in addition to the wind turbine and tower.

Now what?  On the other pages of this FAQ, you will find lots of resources and discussions we have had in the past.  Here's a rough outline of the FAQ part of this site.  I am always willing to improve this site to make it easier for you to find the information you need.  If I am failing miserably, please let me know how I can make it better.  Breaking up the FAQ from a single page to multiple pages (in March 2001) should make it easier for me to make it easier for you!  My contact information is in the footer.

FAQ Home

       Quick Start (this is where we are now)

       Wind Systems Information (this will give you more information about wind systems)

               Basic Information (normal issues with installing wind turbines)

               More Turbine Information (other discussions about wind turbines and installations)

               Manufacturers and Testimonials (these are the manufacturers we know of that are

                                        building turbines these days, and customer testimonials about these products)

               Net Billing Q and A (if you connect to the utility, you might want to learn more about

                                       net billing)

               Build Your Own Turbine (if you are interested in building your own turbine, there

                                       are a bunch of great discussions here)

       General Information (you'll find lots of information that isn't necessarily specific to

                                        wind energy systems)

               How do I learn more? (here are common suggestions for books to read)

               Common Terminology (a small dictionary)

               Useful Calculations (some useful calculations that have nothing directly to do

                                       with wind energy)

               Other Resources (links to vendors and items discussed on this list in the past)

               Example Introductions (some examples of who we are and how we introduced

                                       ourselves to the list - please let us know who you are in your first post!)

Copyright 2001, Michael A. Klemen                                                 contact info: webmaster                FAQ Table of Contents