Efficiency of Wind Turbines

Some of the anti wind materials try to describe wind turbines as “inefficient”, or claim that wind turbines only produce power 1/3 of the time. Let’s examine the merits of these claims, by understanding what efficiency means with respect to energy, and also what capacity factor is.

A definition I found that is a good one for energy is : The ratio of the effective or useful output to the total input in any system.
b. The ratio of the energy delivered by a machine to the energy supplied for its operation.

Based on this definition, wind turbines are among the most efficient converters of energy (in this case wind energy) to electricity ever invented. Betz law says that the most energy that you can capture from wind is about 59% – if you build a wall, you catch no energy – you must let some pass in order to capture the energy. Today’s modern wind turbines convert about 50% of the energy in the wind, so that is 50/59 of the theoretical maximum, or about 85% of the capturable energy. This would be close to what we get from waterpower. Steam generation (eg. fossil or nuclear) converts only about 35% to electricity (except in the case of certain natural gas technologies, such as combined cycle, or co-generation, which can approach 60%). The rest of the energy (chemical in the case of fossil, or nuclear) is lost in the form of heat that is sent up the stack, or out into a lake. Commercially available Solar cells convert 9-17% of the sun’s light energy to electricity.

What some people incorrectly confuse with efficiency is capacity factor. These are two very different things. Capacity factor is the percentage of the theoretical maximum that is captured. For example, a 1 Megawatt wind turbine that produces 1 Megawatt for an hour would have 100% capacity factor. If it produces 300 Kilowatts (.3 MW), then its capacity factor for that hour is 30%. Usually capacity factor must be measured over a year to have any relevance, as winds vary over the course of a year, and the percentage of theoretical maximum also varies. Wind turbines produce more power with more wind. A 1 MW turbine might produce 50 kW at 5 m/sec wind, 400 kW at 8 m/sec wind and 1 MW at 12 m/sec winds. Each model of wind turbine is different – they have different blade lengths, different cut in/cut out wind speeds, different generator sizes. One model of turbine with long blades and a small generator might have a 35% capacity factor in a particular location, whereas a different model, with shorter blades and a big generator might be only 26% in that same location. Comparing capacity factors of wind turbines is not relevant unless you are comparing the same model of turbine. And of course I haven’t even brought up economic efficiency! If the 26% capacity factor model is half the cost per installed MW, then this model will produce more kWh per dollar invested.

You might want to read this for a real life example.

Some people confuse capacity factor with the percentage of time that a device produces. This is definitely not true. The Ferndale wind farm produces power about 80% of the time. But its capacity factor is in the low 30’s.

Efficiency matters a great deal with fossil and nuclear. After all, if the efficiency of a coal or nuclear plant is low, then it must burn more fuel, with more cost, more emissions, and more waste. But with wind, the efficiency, or capacity factor of the turbine does not make a difference to cost – the fuel (wind) is free. And there is no waste, or emissions.

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