Technical

Wind turbine power curves need to be generated from data collected from the wind, and the wind turbine, while the machine in question is under test in a real world environment. Power curves obtained from wind tunnel tests, or elaborate rigs mounted on the back of trucks while one drives down a beach, cannot simulate the real world environment. The power curve is the fundamental performance indicator for the wind turbine and must represent the power actually generated, as it would in the field while connected to your renewable energy system.

Gusto wind turbines are tested using a specialised data acquisition system that monitors the wind speed and direction using transducers that are mounted at the same height as the wind turbine but about 30 metres away. At the same time, the wind turbine voltage, current and RPM are recorded, all at a rate of 7 times every second. This information is collected and used to plot a "data point" that represents the concurrent average value of the windspeed and power for a 10 second period. This is the average useful power the machine actually generated in that time for that average windspeed. Now, take thousands of these data points, plot them on a graph, draw a line through the middle of them all, and you get the green line in the power curve (Gusto 2.0 kW [mean]). The yellow line in the power curve represents a line through all those data points that are the maximum power for a given windspeed. The yellow line provides some idea of the scatter for all the data points due to the fact that the wind is constantly changing magnitude and direction, as one would expect in a real world environment.

Energy is simply the product of power and time, so if you multiply the power a wind turbine is producing by the length of time it produces that power, you get the energy it produces in that time. For example, if your wind turbine produces 2 kW for 10 hours then it generates 20 kWhs (kilowatt-hours) of energy, or 20 units of energy. The amount of energy a wind turbine produces in a year therefore depends on how big your wind turbine is, and how much wind there is at your intended site.

For most people who get their energy from the local power company, the units of energy you use are recorded by the meter man when he comes to read your meter, usually on the outside of the house. Your electricity retailer charges you for this energy (usually around 12 cents per unit in NZ), which covers the cost of generating the energy at the country's power plants, and also the cost of getting it to you. Most network-connected households would use between 5,000 and 15,000 units of energy a year. This however can be significantly reduced if energy efficient appliances are used, and the inhabitants are energy conscious.

If however your energy comes from a wind turbine then you need to know how much energy it produces. Using standard wind engineering procedures based on countless studies of the wind and originally derived from real wind data, one can assume the wind strength varies about a particular average value in a characteristic way, as shown above. This graph is simply a mathematical model of what the wind does, given certain assumptions, and shows how annual energy output can be determined for a particular wind turbine. In this case we are assuming that we know the average windspeed at our intended site is 8 m/s (metres per second), or 16 knots. You can see from this curve how long the wind should blow in a 12 month period at various windspeeds, given the average windspeed at the site is 16 knots. The wind should blow at the average windspeed for about 750 hours, and should blow at 24 knots for about 350 hours. In a similar way one can build up a picture of what the wind would be expected to do at all windspeeds, given that the average value was known to be 16 knots.

If this information is combined with the power curve for a wind turbine, then you can determine the total energy produced in a year, based on the wind turbine performance at any given windspeed. Using our example above and the blue line on the power curve for maximum power, if the average annual windspeed was known to be 16 knots (8 m/s) then the Gusto 2.0 kW wind turbine would produce 825 kWhs worth of energy while it was running for 750 hours at an output of about 1.1 kW (look at the blue line of the power curve and you'll see that the maximum power at 8 m/s is about 1.1 kW). Do the same thing for all the other times that the machine is expected to run at other power outputs (because the windspeed is constantly changing), and you'll get the total of about 8000 kWhs at 8 m/s in the Annual Energy Output curve. Notice that you get about 700 kWhs maximum when the wind is at 24 knots (12 m/s) because the power curve shows a value of about 2.0 kW at this windspeed, so the total energy produced soon mounts up.