Niels' Take On Wind Turbines

The heavy investment in wind turbines at the expense of better systems is a mistake. There are a number of reasons why this is the case, some of them simple enough to understand, e.g. that the energy density of moving air is low, hence the amount of energy you can extract by slowing it down per square meter of wind farm is quite low, no more than about 2W/m2. The reasons wind turbines are so large have to do with efficiency and with the fact that wind speed increases with hight above the surrounding topology. Roughly, wind speed increases by some 10% when you double the hight of the turbine, and the power of the wind increases by around 30% (power is proportional to wind speed cubed). Calculating wind shear is complex because it is influenced by so many factors but these figures are representative.

In practice, wind turbines do not deliver output proportional to the wind speed cubed but only operate efficiently within a specific range of wind speeds. Outside this range they will deliver less power than you would expect. A modern turbine will start to spin at a wind speed of around 3-4m/s to reach rated output at around 12m/s(!) and needs to be stopped and switched off at around 25m/s (gale). Now, this is very important. Rated output is reached at a wind speed of around 12m/s - this is the output figure brandished about when the capacities of wind parks are promoted.

If we look at the output curve for the state of the art Vestas V112-3.0MW turbine this tells us that at a wind speed of 5m/s the output actually delivered by the turbine is less than 10% of the rated output. Now, 5m/s is a fairly good estimate of the average wind speed in this country; 20-25% higher in the winter period when the electricity demand is higher - so that is good. The fact that the wind speed only reaches the 12m/s+ necessary to run these turbines at rated output in a small fraction of the time is not so good, though.

In Germany, where there are wind turbines in mountains and coastal areas, the average load factor, i.e. the ratio of actual output to rated output, is 19%, in The Netherlands 22% and in Denmark also 22% (mainly off-shore). Some proponents of wind power claim it to be higher in the UK but it is difficult to understand why this would be the case. The figures we have seen so far do not seem to support this claim either, so a figure in the 20-25% range is probably realistic, assuming state of the art turbines. It is true that areas exist (in Wales and the Scottish Highlands mainly) where the mean annual wind speed is higher but for wind farms to make any real contribution to our energy supply such a huge proportion of the country needs to be covered with turbines that it becomes completely unrealistic to base calculations on the relatively small areas that are suitable for wind power. More about that below.

Dr David JC MacKay of Cambrindge University, in his book Sustainable Energy - without the hot air published some calculations concerning the Whitelee wind farm near Glasgow. This farm comprises 140 turbines with a combined rated capacity of 322MW in an area of 55km corresponding to 6W/m2 peak. Assuming a (generous) load factor of 25% I estimate the actual average output to be 1.5W/m2.

However, a major issue is that wind power consumption and generation currently need to coincide for wind energy to be useful the way things are being done in this country (large vested interests hoovering tax payers' pockets as best they can, supported by government, and an almost total lack of holistic planning). This means that when there is no wind the whole output of the wind farms needs to be covered through other means (this is why the current strategy will eventually lead to - not prevent - brownouts or blackouts as more wind farms are introduced). It is difficult to calculate reliable estimates of figures for this factor. The result is almost entirely dependent on your assumptions. Of course the law of diminishing returns quickly set in as wind turbines generate a larger proportion of the energy in the grid.

For a tiny country like Denmark, which is a technology leader and to a large extent wind powered, this presents no problem because the neighbouring countries, Sweden and Norway, produce a surplus of hydro-energy, which Denmark simply buys when short of wind. Once international sources of surplus energy are taken up fully as larger countries build wind turbines, it becomes necessary to build other type of energy generators, e.g. nuclear, to cover periods of no wind - greatly reducing the attractiveness of wind turbines.

Grid management is quite efficient throughout Europe and the European grids well integrated so for the sake of the calculation let us assume that 70% of the potential power output can actually be consumed (this is not a factor related to wind but related to consumption patterns, maintenance downtime, etc, and assumes a more mature wind power system than that existing today). Multiplying this into the 1.5W/m2 above results in an average usable output from wind power of just 1.1W/m2.

Now, let's calculate how many turbines the country needs for them to make a real difference.

The current UK population is just about 70 million people distributed across 244000km2, i.e. the current UK population density is 287 persons/km2 corresponding to an area of 3486m2 per person. So, just to bring our figures down to a level where they are easy to relate to (and I have stolen this idea from Dr MacKay), let us now calculate how much energy we can get from wind power for each person, if we fill the country completely with turbines, from John O'Groat to the Scilley Islands, from East Anglia to North Ireland - all one big wind farm. Then we derive 1.1W/m2 x 3486m2 x 24 hours/day = 92030W/person and day or 92kWh for each person per day.

Let me try to put this into perspective. The current average energy consumption of every UK inhabitant; man, woman and child is around 125KWh/day. If we plaster wind turbines efficiently across every square inch of the country we can thus cover less than 75% of the current energy consumption. Now, turning the whole country into a large wind farm is of course not realistic. If we only use the most suitable sites, which, incidentally, are also often the most scenic, we could perhaps erect wind farm on 10% of the land, corresponding to around 61.000 large wind turbines, roughly 1.5 to 2 times as many as installed globally by the end of 2008. Draw your own conclusions.

Wind turbines, however, can be made more useful in combination with modern energy storage, allowing energy generated by wind turbines to be stored when their output is not required elsewhere. In Austria they use the unconsumed wind energy to pump water up to higher-laying reservoirs. This water is subsequently used to drive water turbines in periods of larger demand than the wind turbines can cover. Smart if you have large mountain lakes.

In this country this is not a good solution but hydrogen generation is, particularly in case of off-shore wind farms. These are surrounded by water which can be hydrolysed to produce hydrogen and oxygen. It is easy to pipe the hydrogen ashore and either compress it for later use, e.g. in fuel cells in houses, power plants or vehicles, or simply stick it into the national gas grid to produce a fuel more akin to the city gas of old.

The money invested in wind farms should instead largely be invested in energy-saving measures, primarily in insulating buildings and making these air tight - done correctly this can save about 80% of the heating costs. The basic idea should be to use less energy, not to generate more by different, more sustainable, means!