Intermittent output of wind energy
As you might imagine, wind turbines are entirely dependent on the availability of their natural resource: wind. The requirements are actually quite strict. Like Goldilocks, we need the wind speed to be just right: not too slow, and not too fast.
If the wind speed is too slow, it is not economical to run the turbine. If the wind speed is too high, the wind turbines must be shut down due to safety concerns.
Wind turbines tend to start up after 3-5 metres per second (6.7 - 11.2 mph) and then turn off when the speed goes above about 25 metres per second (56mph). 1
Reproduced from the Danish Wind Industry Association. One of the disadvantages of wind energy: you can see how the power output of a typical 600kW wind turbine varies with wind speed, but it doesn't operate outside pre programmed limits.
Not only do wind turbines not operate all of the time, but they can also never extract 100% of the energy stored within the wind which passes through the turbine.
Betz' law (a physical law derived from conservation of mass passing through the turbine) states that the wind turbine can only convert up to 16/27 (about 59%) of the energy available in the wind to mechanical energy.
These limits govern both the amount of energy produced, and also the consistency of this energy.
Most importantly though, a wind turbine cannot generate energy constantly. Not only that, but the output of a wind turbine is not constant.
Can energy not be stored? Not on such a large scale. We use an amount of energy, and the supply in the grid is kept approximately constant.
When demand increases, power plants are switched on or increase output to keep the supply stable. If demand drops, power plants are eased off.
This is impossible with wind power, though. We cannot control the wind and just turn them on. It also means that if there were no other energy supplies, we would not be able to depend on wind turbines to provide that load, no matter how many wind turbines we put up.
This probably the most important of the disadvantages of wind energy, and its solution will require some large scale applications. One solution is a fleet of electric cars in a country. While enough of them are plugged in to charge, they can take or give energy to or from the grid to stabilise it.
Low energy density of wind energy
Wind is diffuse, it is spread out of a large area. To obtain a reasonable amount of energy from a wind farm, the turbines need to cover a large ground area. This problem is not just one of the disadvantages of wind energy, but tends of be a disadvantage of most of the alternative energy sources.
Not only that, but turbulence caused by each turbine creates a minimum required distance between turbines, increasing the wind farm size.
Photo courtesy of Taylor Dundee
Wildlife
There have long been arguments that wind turbines affect migratory birds, but more recently it's been discovered that they can make bats' lungs explode!
That aside, it's true that we have to consider the impact that many huge towers and blades could have on local wildlife. These problems can be mitigated, but of course this needs extra planning and research, which of course increases cost.
Aesthetics
This one is down to personal taste, but it should be included here because wind farms have often been banned for exactly this reason, and is often the most obvious of the disadvantages of wind energy. This, unfortunately, is not a scientific or engineering problem.
As pretty as we may be able to make wind turbines, it does not alter the fact that they must be large, and a certain shape. That means that the people who object to them now will always object to them for the same reasons.
Unfortunately, there may be nothing we can do about this, short of building all wind turbines off-shore.
National Security!
There was a discovery that wind energy can even affect national security!
It seems wind farms cause holes in RADAR coverage as the blades on the turbines confuse the system. Apparently they look like planes. It is so bad that they cause a large RADAR shadow behind them! This can surely be solved, though.
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