Wind turbines are obviously one of the main options for clean energy. The amount of energy that a wind turbine can produce is critical to economics and can decide whether a turbine is a worthwhile investment. There are quite a few factors that determine this energy, and these need to be considered carefully when evaluating the potential of a wind turbine.
The most basic specification for a wind turbine is a power rating.
A residential wind turbine might be rated at 5kW, and much bigger wind farm turbines might be rated at several MWs each. However, the turbine will not produce this rated power all the time. The power output is fairly obviously dependent on how much wind is blowing. Thus the rated power of a wind turbine is the power that the turbine will produce at a particular wind speed.
The curve below shows an example ‘power curve’ for a wind turbine rated at 1000W. You’ll note that the power doesn’t start increasing at zero wind speed: each turbine has a ‘cut-in’ wind speed at which it starts to produce power. The power increases with wind speed up to it’s rated power, which is at a defined wind speed (wind turbine specifications will state the rated power and the wind speed for the rated power). The power then stays fairly constant with increasing wind speed until the turbine is shut down for safety reasons. Typically shut-down speeds are about 25 m/s.
To put these speeds in perspective, maximum power is at about 11 m/s which is 24.6 mph or 21 knots. Pretty windy. A shut-down wind speed of 25 m/s is 56 mph or 48.6 knots. This shows that wind turbines have a wide operating window for stronger winds. At the lower end, a minimum wind speed of about 5 m/s is often considered necessary for a wind turbine to be viable. This is 11 mph or 9.7 knots.
Commercial wind turbines have different power curves depending on whether they’re designed to operate at lower or higher wind speeds.
However all this relates to power, not energy. Your electricity bill is based on how much energy you use: if you look at the bill you will be charged per kWh (short for kilowatt-hour) you use. Energy is power multiplied by time. The units of power are watts, and units of energy watt-hours. For example, if a turbine runs for 1 hour at 1000W, it will generate 1000 watt-hours of energy. A higher rated power will give you more energy, but you also need the wind to blow at a good speed for lots of time.
So what determines rated power? The biggest factor is the size of the turbine. Wind turbines work by converting the wind that passes through the spinning turbines into energy. They’re about 40-50% efficient at doing this. The spinning turbines define a circle, with wind passing through the area of the circle being converted to energy. Remembering some basic high school maths, the area of a circle is pi x r2. In this case r is the length of the turbine blades.
This equation is important because it shows just how much the power and energy a turbine produces is dependent on the length of the blade. If you double the length of the blade, you will get four times the amount of power and energy.
Homes and commercial sites have space and regulatory considerations that limit the lengths of the blades. Large wind farms have much less restrictions and there is a continual push to make larger turbine blades and thus more powerful turbines. The world record length is currently set at a whopping 107m on the General Electric 12 MW Haliade-X turbine. These things have all sorts of transport issues and are certainly not cheap!
How much Wind?
Wind speed is obviously critical: the longer the wind blows at higher speeds the more energy the wind turbine produces. So how do we figure out how much wind is at a particular site?
There are plenty of wind maps around showing average wind speeds at different locations, for example see the excellent NREL site at https://www.nrel.gov/gis/wind.html. There are a few things that need to be considered when using such maps. First is the height above ground. This has a massive effect on average wind speed, with wind increasing substantially as height increases. Three average wind speed maps of the United States are shown below, for heights of 10, 40 and 80m above the ground. These maps show just how important height is. Take Nebraska, for example. At 10m above the ground, the average wind speed is about 4-4.9 m/s. This is close to the edge of viability for a wind turbine. At 40m high, the average wind speed increases to about 6-6.9 m/s. And at 80m, its up to 7-7.9 m/s.
The figure below shows a power curve for a commercial wind turbine with a rated power of 4000 W. At a wind speed of 4.5 m/s, the turbine only outputs about 230W. At 6.5 m/s this increases to about 900W. At 7.5 m/s, the power output is about 1500W. A massive difference in power output and therefore energy as the height above ground increases.
Commercial wind farms with very large blades (80m plus) have their hubs more than 80m high and thus can reach the higher wind regions. For smaller home and business systems, heights can be limited by local regulation, and of course economics. However for wind turbines, to maximise power and therefore energy, its important to go as high as regulations and/or economics allow.
Another factor that can be important when looking at average speeds is how constant the wind is. If the wind is reasonably constant then average wind speed is a good indicator. If, however, the site has mostly very light wind, but the average speed is boosted by frequent violent storms with wind speeds above the shutdown speed, then power and energy output will be much lower.
How much Energy?
We’ve seen that energy output from a wind turbine is dependent on the power rating of the turbine but also on how strong the wind is and how long it blows. So how can we figure out how much energy to expect out of a turbine? We need this to evaluate the economic performance.
Engineers use a term called ‘Capacity Factor’ to calculate the amount of energy from a wind turbine. The capacity factor, expressed as a percentage, is the actual energy output from a turbine over a year, divided by the energy output that would be obtained by the turbine operating at its rated power over a year.
For example, let’s look at a 5kW turbine. If the turbine operated at 5kW for a whole year, the energy output would be 5kW x 24 hours per day x 365 days per year equals 43,800 kWh. As we’ve seen the turbine doesn’t actually do this. Suppose the turbine actually produced 20,000 kWh over the year. The capacity factor could be 20,000/43,800 = 45.7%.
On land, capacity factors range between about 25-50%. In the US, the average capacity factor for wind turbines is about 33%.
To run the economics of a wind turbine it is necessary to have an estimate of the capacity factor so we can estimate the amount of output energy. The average wind speed combined with the power curve is one way of doing this. For example, using the power curve above, an average wind speed of 6 m/s gives a power output of 200W, which is 20% of the rated 1000W. Thus the capacity factor is 20%. In this situation, the turbine would produce about 20% x 1000W x 24 h per day x 365 days per year = 1,752 kWh. This estimation gets better with more constant wind.
Local Site Factors
Average wind maps are all well and good, but local factors can play an important role and can guide where to best site the turbine. Structures such as trees and buildings disturb the airflow, making it more turbulent. Wind turbines are less efficient in turbulent airflow and will therefore give less energy.
As a rule of thumb, a structure will create turbulent airflow to about twice its height. Thus a wind turbine should be at least this high at the lowest part of the blades. The turbulence will also continue downwind for a distance of about twenty times the length of the structure.
Local structures can also be used to advantage. For example, a sufficiently smooth hill will not create turbulent airflow but can compress airflow at the top of the hill, increasing average wind speed.
Wind vs Solar
Let’s compare wind and solar systems, both with rated power of 4 kW, based at Wichita, Kansas. Using solar modelling software, the 4 kW solar panel system outputs about 5,679 kWh per year, or 15.6 kWh per day on average. For the 4 kW wind turbine, we’ll assume the turbine is 40m high. Average wind speed is about 6.5 m/s, giving an average power output of 900W (from power curve). Average energy per day is 900W x 24h = 21,600 Wh or 21.6 kWh.
Obviously the relative energy outputs are very dependent on location. If we take Atlanta, Georgia as another example, average wind speed is only 5.5 m/s, giving a power output of about 500W. This gives only about 12 kWh per day. The solar system gives about 13.7 kWh.
When is wind worth it?
A small wind turbine can cost between $3,000 and $5,000 per kW rated power fully installed (American Wind Energy Association). Nost homeowners using wind as a primary source of electricity will install between 5 to 15 kW, at a total coast between $15,000 and $75,000. With this sort of capital outlay it’s important to run some economics.
We can use a levelized cost of energy calculator (https://www.nrel.gov/analysis/tech-lcoe.html) to get an estimate of the cost of energy from the wind turbine over it’s life. Wind turbines are expected to last at least 20 years so we used 20 years as the lifetime. An average cost of $4,000 per kW is used. The table below lists the approximate cost of energy vs average wind speeds.
Ave. Wind Speed m/s Ave. Wind Speed mph Approx. Capacity Factor % Cost of Energy c/kWh
|Ave. Wind Speed m/s||Ave. Wind Speed mph||Approx. Capacity Factor %||Cost of Energy c/kWh|
Obviously buyers need to run these numbers with specific power curves and prices for products they are looking at, however this example gives a good idea of the economics.
The average wind speed is clearly critical. For this turbine, an average wind speed of 6 m/s is where the energy starts to be about the same as grid electricity. Moving from 6 to 7 m/s drastically drops the cost of energy to the point where the investment almost becomes a no-brainer.
Coming back to our wind map (40m high), and using the 6-7 m/s bracket as where the turbine becomes viable, we can see that large parts of the US aren’t really well suited to residential and commercial wind turbines that can’t go 80m into the sky. Luckily a lot of these areas do have good solar resources.
However there are plenty of areas where wind can make sense. In a lot of these areas solar is also good, so hybrid systems using both wind and solar may well be worth consideration.
There are quite a few factors that determine how much energy a wind turbine will generate. The big ones are rated power and average wind speed.
A thorough economic analysis should be run for specific wind turbines in specific locations. How much energy the turbine generates is critical to these economics.
While large parts of the US are not really suited to wind turbines, there are plenty of sites where wind turbines make sense.
A commonly used lowest average wind speed is 5 m/s or about 11 mph. However an economic analysis should be carried out for specific products and prices. Obviously these economics get better as the average wind increases, and become very good at about 7 m/s or 15.7 mph.
Yes. Turbines will shut down at a certain wind speed, normally around 25 m/s or 56 mph. You should also check the maximum wind speed specified for your turbine to make sure it doesn’t get blown apart!
This really depends on your location and requires a proper analysis. One potential advantage of wind over solar is that it can deliver power when the sun isn’t shining. It might be worth considering a combination of wind and solar.
As high as possible! Wind gets stronger with height. However in residential and commercial sites, the allowable height will be regulated. Economics and available area also influence the height that can be practically achieved.
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