It's windy out there...but is it windy enough?

Much is being made of wind energy as a key renewable resource.  The question is whether or not there is enough of it to provide for our needs.  Wind is not a constant - even though it might seem like that sometimes!  In fact it varies hugely in most places from day to day and week to week.  It also varies a great deal from place to place.

David Mackay's estimates for onshore wind energy are an average of 2W/square metre across the UK.  To turn that into enough useful energy for the UK will require a lot of wind turbines.  With those numbers, even if we covered 10% of the UK land area with wind turbines (not very likely) we would generate only 20 kWh per day per person - a long way short of the 125 kWh per day average that we use today.  That might be enough to provide about half of our typical domestic needs.

As with waves, once wind has been used to do work by turning a turbine, it is slowed down and is not then able to be used again.

Wind farms are springing up in a number of places and lots of companies are busy building them, with plenty more to come.  How many windmills will we need?  Where can we put them?

There are plenty of impressive sounding 10 or even 100 mega-watt numbers being thrown around for individual wind farm output - but UK energy consumption is in the 10's of giga-watt range.  The other problem is that these tend to be peak numbers - you can only achieve that when the wind is blowing within a fairly narrow range of optimum speeds.

Wind energy captured by a turbine is proportional to the area of the blades multiplied by the cube of the wind speed.  (Cyclists know this relationship well - the wind resistance goes up cubically as you pedal harder.)

This cubic relationship is one of the things that makes wind farm design a bit tricky.  Too much wind and generators and transmission capacity can be overloaded and the turbine blades themselves could be damaged - so over a certain limit the turbines have to be shut down.  Too little and the turbines won't even turn.  A lot of work has gone into making wind turbines operate across wide ranges of wind speeds - feathering the blades for example - but it is still a challenge.  In many ways achieving a predictable output is more important from a practical perspective than achieving the highest possible output.

The industry and the press tend to talk about two sources of wind energy - offshore and onshore, which funnily enough are either in the sea or on land.    Mackay's book estimates an average 3 watts per square metre for offshore wind.  The main benefit is that the wind is steadier and more consistent and not affected by mountains and trees.  The engineering challenges of offshore wind farms are not insignificant, there's a cool marketing video on the RWE website showing the building of the North Hoyle offshore wind farm.  This farm will generate a peak of 60 MegaWatts of electricity.  Compare that with a typical coal fired power station generating on the order of 1 to 2 GigaWatts - 20 times as much in a much smaller area.

In summary, wind energy is useful - but on it's own it isn't going to be enough to get us off our fossil fuel habit.