How to Shop and Compare for Your Wind Turbine
Small wind turbines come in a range of sizes - and prices. The proper turbine for your situation will depend on three basic factors: 1) your energy consumption; 2) the wind turbine performance; and 3) your wind resource. You'll need to determine each of these three factors to help you assess what any particular wind turbine can do for you so that you can decide exactly what your money will buy.
Energy Consumption
The amount of electric energy you use is tracked by your electric utility and is available for you to review on your electric bill. If you have bills dating back months or years, these will be the easiest, quickest and most accurate means for you to assess your annual electricity consumption. Once you know how much energy you typically consume you have completed the first step toward choosing the size of wind turbine necessary to offset (partially or totally) your electricity usage.
In designing the Endurance Wind Turbine we took the average electricity usage of a household in North America and found that it was somewhere between 9,000 and 12,000 KWH per/yr. We then focused our design and engineering to create an affordable yet efficient wind turbine that in a Class 2 or Class 3 wind zone would generate this amount of energy for an American Household, with the objective of essentially eliminating the costs of utility power for a family.
Your Wind Resource
The next step in determining the right turbine for you is to know how much wind energy you have available to capture at the site you want to install the turbine. You can very accurately determine this with expensive meteorological equipment over several years, but this approach is prohibitively expensive and rarely warranted. All that is really needed is an annual wind speed average, which can often be determined from wind resource maps or nearby facilities that collect wind data such as airports. The final factor in defining the wind resource is knowing the distribution, or how often the wind blows at each particular speed. However, in most places the wind will follow a fairly well-known curve called a Rayleigh (RAY-lee) distribution. Knowing this and your average annual wind speed completes the wind resource picture.
Wind Turbine Size
The final part of the puzzle is determining the size of wind turbines you are considering. This sounds easier than it actually is. Unfortunately, in regards to a small wind turbine size, its usually described in terms of power. What we really are interested in, however, is energy - the thing you pay for on your electric bill. The two are related, but when it comes to wind turbines, more power does not always mean more energy. There are several reasons for this, and to know how to wade through the confusion you'll need to know a little bit about power, energy and wind.
Power and Energy
Two important terms in discussing electricity, whether generating or consuming it, are power and energy. Power is measured in watts (W) or kilowatts (kW = 1,000W) and tells you how much electricity something uses or makes any given instant. A 100W light bulb uses 100W whenever it is on. The longer it is on, however, the more you'll pay for electricity. That is because you pay for energy.
Energy is power multiplied by time, and is measured in kilowatt-hours (kWh). So that 100W light bulb you turned on will use 1 kWh every ten hours it is on. If you turn on ten 100W bulbs, you'll use 1 kWh every hour. You can see how many kWh you use by looking on your electric bill.
Power in the Wind
The power of the wind changes with the speed of the wind. Therefore, the power of any wind turbine can produce changes with the wind speed as well. How the wind turbine power changes with wind speed is described in the wind turbine power curve. This presents the picture of what a wind turbine can provide you from the wind passing through the rotor. The rotor is the fan blades and dome and the dimension of both.
The power a wind turbine generates basically depends on two factors: how strong the wind is blowing, and how large the wind turbine rotor is. Obviously the turbine has no control over the wind speed, so in general, the larger the rotor (or swept) area of the turbine the more power it can extract for any given wind speed. This is a scientifically proven fact. So do not be fooled by a vendors specifications sheet. Rotor Dimensions are directly correlated to amount of Power Generation Potential of a Windmill Turbine.
The power in the wind increases with the cube of the wind speed. That means there is eight times the wind power every time the wind speed doubles. Some wind turbine manufacturers take advantage of this fact by rating a turbine at a higher wind speed. The wind however, tends to blow much more often at slow speeds than high speeds. The wind turbine must find a balance between trying to capture a lot of very low winds that have little power and very high winds that rarely occur. This is why it is important to know the details of a turbine before you invest in it. We'll describe some of the important details to help you in this respect.
Power Curves Determine Performance
(But Buyers Beware)
As mentioned, the power curve of a wind turbine tells the story of how much power the wind turbine will produce at any given wind speed. But a power curve is only as good as the data that supports it. When choosing a wind turbine, ask to see real data that validates the power curve used in promotional material. It is a sad fact that many power curves published by wind turbine sales departments are inflated. If supporting data is not available, be wary of any claims made by anyone with a vested interested in your purchase of the turbine.
Energy is The Important Thing
Energy is what you pay for and get paid for when your wind turbine operates, not power, so don't be fooled by claims of high power. Keep your focus on energy, which is measured in kWh not kW. Given a valid power curve, and knowing your average annual wind speed (assuming a Rayleigh distribution as mentioned above), the annual estimated energy capture for the turbine can be determined. This is the number you want to compare to your annual energy consumption (from your electric bills) to figure out how much the turbine will offset in regards to your consumption.
There are other finer points you may be interested in knowing, and these are covered below for your edification.
Power Curve Nomenclature
Rated (or peak) power:
Many wind turbines are still designated by their peak power rating in watts or kilowatts. This number can, and historically has, represented a plethora of essentially meaningless things from name-plate generator rating to peak instantaneous power or even output at some very high wind speed. Despite its almost ubiquitous use, don't be so concerned about a wind turbine's rated size defined in kW - it often adds more to confusion than clarification.
Start-up Wind Speed:
This is the wind speed required to get the turbine rotor spinning. Because wind turbines are designed to be aerodynamically efficient when spinning, they do not always start spinning easily from rest when the wind starts blowing. Therefore, some small turbines need a strong wind to get them spinning at full speed and producing power. A wind turbine capable of motoring up to speed can capture the more frequent lower winds. The start-up wind speed can also become quite high for turbines that do not have a tail or rudder to point them into the wind. The Endurance Wind Turbine design uses a tail and is unique in that it motors up to speed lowering the start-up wind speed.
Cut-in Wind Speed:
This is the lowest wind speed at which a wind turbine can produce power. Although some manufacturers like to emphasize a very low cut-in wind speed they are typically publishing generator output and not power delivered to the grid. There is so little energy available in the wind below 9 mph (4 m/s) that most, if not all of it is lost in electrical system inefficiencies leaving you little if any usable energy.
Cut-out Wind Speed:
This is the wind speed at which the turbine stops producing power. Because there is so little time at wind speeds above 40 mph - ~1 hour/year at a typical wind site - it is best to protect the equipment instead of trying to capture the minuscule amount of energy. Although most small wind turbines are not capable of stopping the rotor in high winds they will require some means of protecting the rotor in these winds. Most often that method is furling. The furling wind speed is similar to the cut-out wind speed. The Endurance is unique in that it applies the brakes and parks the rotor in winds above 54 mph (24 m/s). Do not be fooled by claims of a high cut-out wind speed. Just think how often the wind blows at 50mph at your site ' if you had to think about it, it's not often enough.
Binned Power Curve Data:
The proper way to create a meaningful power curve is to average collected power and wind speed data for one minute, and then place each of those data points in wind speed bins. Like the name suggests, the bins collect power data points for a similar wind speed and then the data points in each bin are averaged to create the power curve. The bins usually cover a range of ½ m/s (1.1 mph). As an example, imagine the wind turbine runs for one minute and the average power and wind speed are 3kW and 10.2 m/s respectively. That point - 3 kW - goes into the 10 to 10.5 m/s bin. In each bin a minimum of 30 data points (0.5 hours) is needed to provide meaningful results. When the average power for each bin is plotted on the power curve against the average wind speed of the bin, we have a useful power curve. We use the average power as that represents what you would expect the turbine to produce at that wind speed on average. Some small turbine manufacturers use the misleading practice of plotting the maximum power in each bin, giving an inflated power curve that will only lead to disappointment to you, the customer, when the machine fails to live up to your expectations.
