Wind Turbines

SUR Energy prefers to sell wind turbines based on actual data collected over time rather than the sales literature presented by the manufacturers. We explain why here. This presentation is a bit simplistic, and is written for the benefit of the non-statistician.

How much energy will a Turbine generate?

Rated Wind Speed

Learn About Different Types of Turbines

How much do Turbines cost?

Wind System Components

How much energy will a Turbine generate?

All reputable wind turbine manufacturers (and some less than reputable ones) publish what is called an instantaneous power curve or power chart. The chart shows what energy was coming out of the turbine in different wind speeds. The power out is never exact at any given wind speed, so they plot these samples, and look at the curve formed by the plots. They average the samples into a single line and call it the power curve for that turbine. The curve shows, on average, how much electricity will come out of the turbine at a given wind speed. Wind speed is all-important because the power in the wind increases at a rate that grows exponentially. That means that there is TWICE as much energy in a 10 MPH wind than in an 8 MPH wind. Twice the energy for winds moving 2 MPH faster. If you look over several power curves for several turbines youll find that the turbine outputs twice the electricity for this small increase in speed. How will that affect your electric bill? Here is the tricky part.

The turbine companies not only present us with instantaneous power curves, they also do calculations to move from this instantaneous chart to how much energy is produced over a month or a years time for each speed. NOTE: these monthly charts are NOT actual data; instead they are the result of calculations made on the instantaneous charts. Lets look at a couple of examples.

SWWP Skystream, 13 MPH and 16.2 MPH winds

As you can see from the graphs in a 13 MPH wind the Skystream outputs about 400 watts. In a 16.2 MPH wind the Skystream outputs about 800 watts. If we take the 400 watt average (for a 13 MPH wind) times 24 hours per day, 30.4 days per month, our multiplication comes out to 29184 Watt-hours of electricity, divide by 1000 gives 291 kilowatt-hours of electricity per month. kWhs is the unit on your electric bill. (Average per household is around 850/month). If we read the chart on the left for 13 MPH average wind speed, the number given for monthly production is more like 440 kilowatt-hours of electricity, much higher than our 291 number. Where does this exaggeration of production come from?

The answer is that this and all small wind manufacturers use statistics of wind energy to get from one chart to the other. The statistics have to do with the distribution of wind for a given average. In other words, if there is a 12 MPH average, and we imagine it as 16 MPH half the time and eight miles per hour the other half of the time, you will see that the production is higher than a constant 12 MPH wind. This massaging of the statistical is considered valid and is done in one way or another by all small wind manufacturers. Lets look at some other examples.

WTIC, Wind Turbine Industries Corporation, makes a 20 kilowatt machine. Here are some charts from one of their brochures.

They also have the following table in the sales literature for annual energy production.

Estimated Annual Output

Wind Speed (MPH)	10	11	12	13	14	15	16	17	18
Energy Output (KWH)	19727	25704	32297	39289	46468	53646	60665	67398	73743

The upper chart shows that 2400 Watts is expected in a 13 MPH wind. If we take that times 24 hours per day, 30.4 days per month, our multiplication comes out to 1751040 Watt-hours of electricity, divide by 1000 gives 1751 kilowatt-hours of electricity per month. If we read the table for 13 MPH average wind speed, the number given is 39,289 kilowatt-hours of electricity per year divided by 12 months gives 3,274 kWhs per month. How does this compare to our previous example? The Skystream chart came out to 1.5 times the multiplied amount (440kWhs stated divided by 291 kWhs calculated). Here our average comes out to about 1.9 times higher than our calculation (3,274 kWhs stated divided by 1751 kWhs calculated here). The WTIC literature also includes a power increase by height that starts at 30. Their number may be higher for assuming average wind speed comes from airport data collected at 30. They may be assuming the turbine will be higher, and therefore in faster winds. So if 13 MPH comes from 30, and you buy a 100 tower, your turbine will be in winds 19% higher, or 13 MPH times 1.19= about 15.5 MPH winds. Their web site lists appropriate disclaimers around these charts.

Lets wrap this portion up with one more example. Abundant Renewable Energy has been building a nice 2.5kilowatt machine called the ARE 110. Their power curve and monthly output is shown on their web site. At about 13 MPH winds the curve shows output of around 500 watts. 500 watts times 24 hours times 30.4 days in a month would equal about 364,800 watts, or 365 kilowatt hours in a month's time. If we look at their monthly production chart it shows about 500kWhs for the same time period. The ratio they are giving us is 1.36 (500kWhs stated divided by 365kWhs calculated). This to us seems the most realistic of the three statements listed by the manufacturers.

Why don't they all simply measure the output in a typical wind regime for a month that comes out to around 13 MPH at hub height and report the output over a month's time? If any one of them strayed from what the others do they would look much worse than the others, even if their turbine out-performed their competitors - a bad business move. We hate to say this but we have found that some machines perform well below the claims of the manufacturers in their monthly charts, even though the instantaneous charts are likely accurate.

Why would SUR want their clients to know about the products in this much detail? We are not just looking for the highest sales in the immediate future, but also to have a lot of repeat business, excellent referrals, and a nights sleep that doesn't include lying awake for all the wrong reasons. Having learned from the past, and having watched other installers add their own exaggerations to the manufacturers exaggerated claims, we feel there is a great need for someone to stand up and try to determine more accurate cost benefits for our clients. Now, off of the soapbox and on to more wind turbine facts.

Rated Wind Speed

Many turbines are referred to by wattage. For example, there are several one kilowatt machines out there, about 9 to 11 diameter, and several 10kW models with diameters over 20. It turns out that turbines with the same rating can be very different from one another. They all should list something called rated wind speed. This is the speed at which they achieve the output of their rated wattage. For example, the Bergey 10 kilowatt has a rated wind speed of 29 MPH. The ARE 442 has a rated wind speed of 25 miles per hour to get to 10 kilowatts of output. Therefore, it is safe to assume the Bergey would output something less than the ARE in a 25 MPH wind. Keep in mind that no machines with the same wattage rating are actually equal.

Learn About Different Types of Turbines

As soon as the government starts throwing money at renewables we see all sorts of ideas surfacing, especially in the area of wind power. Some of them might be genuine innovation, but more often than not they smell a little fishy. If it sounds too good to be true (a 5 diameter unit can power the whole house!) it very likely is too good to be true. Here is how we determine a decent and realistic machine.

The first principal to look for is solidity. Don't pay a dime for anything that has high solidity. Solidity means the wind cannot pass through the swept area. Intuitively people think that more surface area covered by the rotor is good because the wind has more to hit. What happens instead is all machines with high solidity cause turbulence, and the wind ends up going around them rather than passing through and dropping off energy. Good turbine design is walking a tightrope between drawing as much power out of the wind as you can without slowing the wind down. While the old water pumping windmills of the past have a nostalgic feel to them, and the solid vertical axis machines might seem new and inventive, in fact both have been shown to not do as well as their counterparts with low solidity.

While we don't have anything against vertical machines, the benefits are usually outweighed by the drawbacks. The thing that really offends us is verticals with high solidity. We talked to one buyer that spent $100,000 on a 10kW machine and never saw it produce more than 1kW. They were also told that the 16 by 3 turbine would generate 30,000 kWhs per year. We calculated that, in order to do so, a great turbine (30% efficient) of this size would need 36 MPH winds 24/7, 365 days a year to generate 30,000 kWhs. This is not at all realistic. The winds would have to be faster than 36 MPH because turbines of this type would never be 30% efficient. This always ends up being a black eye for the entire small wind industry.

How much do Turbines cost?

Many turbine manufacturers list prices on their website. Installation typically more than doubles the cost, especially to put one on a nice tall tower. It is best not to skimp on tower height, if budget allows. A turbine at 120 will generate as much as two turbines at 30. As a rule we do not push turbine installation in suburban or urban areas. While there is some potential to get a small amount of energy this way it is not a focus of SUR. There are fewer areas with good winds (remember, a 10 MPH wind has TWICE the energy of an 8 MPH wind) and it is harder to get the turbine up into laminar airflow, at least 30-40 feet above surrounding obstructions. If you decide to pursue this with other installers we suggest you ask for real world data rather than just reading the output charts provided by the manufacturers. See above.

Wind System Components

The primary components of a wind power system are the turbine itself, the tower, some sort of voltage clamp, a dump load, an inverter, and batteries may or may not be a part of the mix. To understand the system types go to our Electric Systems Page. The parts specific to wind power are covered below.

Most of the turbines we sell are three-blade up wind machines meant to be mounted in a field or yard high above any obstructions to the wind. Most turbines are permanent magnet alternators as the custom windings available result in more power and lower cost of energy. The generators output wild AC, which is alternating current that alternates depending on the rotational speed of the turbine, which goes up and down depending on wind speed.

Current and voltage coming out of the turbine will be too inconsistent to be useful, so we have to control it one way or another. First, the AC current is rectified to DC. Once this is done the power can go into a battery bank DC bus, or to the inverter. In a battery system, the batteries act as a voltage clamp to regulate the voltage out of the turbine. For a grid-tied system without batteries, we have to clamp the voltage within the DC voltage window expected by the inverter.

Wind towers come in many shapes, sizes, heights and costs. Monopoles tend to be the nicest looking and most expensive for a given height because they use the most steel. Truss towers without guy wires are typically the second most expensive. Smaller truss towers use guy wires to add strength and lower cost, but the tower then requires a larger footprint, whose diameter can be as large as the tower height. All of the towers discussed so far are typically installed using a crane, unless the turbine is small enough and the tower low enough to tilt up into position using a hinge arrangement at the base.

Most small turbine manufacturers supply a tilt up tower (typical for less than 10kW machines) that uses guy wires attached at four locations and allow one to tilt the turbine and tower up into place. This is nice if the turbine ever needs service because it can be tilted back down to the ground in order to service it. Also, with guy wires you can get the same strength out of a tower that uses much less steel because the thrust loads from the wind are spread out over a larger area, which uses leverage to hold the turbine up in place. Tilt up towers on locally purchased pipes tend to be the lowest cost/highest value option. In our opinion they are often less visually obtrusive than the heavier more expensive options of monopoles and lattice towers. From a short distance away the guy wires are lost visually.

To read about how the turbines connect into the home, visit our Electric Systems Page.