Exploring the Wind

When I first became interested in sustainable energy, my first thoughts were “haw much wind and solar energy is available.  A large chunk of my life has been spent poking around datasets, mainly those related to hydrocarbon exploration and production, so for me a logical starting point was to seek out sources of wind and cloud data.  This post is collection of personal observations, anyone wishing to acquire a wider knowledge is encouraged to seek out more authoritative material.  For the sake of brevity, nuances and detail have been omitted.

If I was starting again, the first task would be to read a couple of textbooks on global climate and air circulation in particular.  Whilst browsing books at a car boot sale I came across “Climate, Soils and Vegetation” by D.C. Money, this and Wikipedia provided a useful, if belated, context for the maths and stats.

The actual starting point was a sample of Metar and similar weather reports and then attempting to reduce them to Weibull distributions.  One of the locations was an airfield which is approximately 10 km to the west of where I live.  The average wind speed there is around 5 m/s, the surrounding terrain is flat and having stood, with a simple anemometer, on the roof of a WW2 pillbox which overlooks the site, it appears that the air flow is smooth with little turbulence below 10 m/s. In contrast, where I live, the average speed is less than 3 m/s and very turbulent.  Weather data is collected to support human activity, frequently transport, where the objective is to provide pilots, sailors, road users with an indication of what forces of nature they will be exposed to.  This data may not be relevant to other nearby locations with different terrain and land cover.  It is a gross oversimplification, but the average wind speed for a selection of onshore locations  in Western Europe and North America was in the range 4 – 7 m/s with Weibull shape factors in the range 1.3 to 1.8.  The higher the shape factor, the greater the energy yield.  There is an inference that the lower shape factors are associated with complex terrains.  At some point I need to revisit the data and produce a better summary.

Next I looked at offshore data.  Onshore wind is fluid flow over a rough surface, the effects of which extend for a significant height.  In contrast, the fluid friction over water is much reduced.  In the areas between the tropical and polar circulations, the average wind speed might be in the range 6 – 10 m/s with wind speed increasing with poleward distance.  Often the Weibull shape factor is close to 2.0 allowing the use of the Rayleigh distribution (a special case of the Weibull distribution with the shape factor set to 2.0).  The greater energy yield offshore is offset by the higher cost of the installations necessary to capture it. Onshore wind turbines can be erected with general purpose equipment such cranes, diggers and lorries, offshore it is necessary to use specialist vessels for pile driving, lifting and cable laying which are both large and expensive. The nature of offshore structures is determined by the conditions in which they need to survive.  I had a brief career as a merchant seaman and have first hand experience of the violence of an ocean storm.

In addition to capturing temperature and dew point data, weather balloons also record wind speed and direction.  Unlike airports and buoys which usually report once an hour typically providing 8760 observations per year, an upper air sounding station might generate 200 – 300 observations per year for a given altitude.  At about 800m the flow of air is not influenced by the nature of the earth’s surface and this known as the planetary boundary layer.  For North America and Europe, the average wind speed at 800m was often in excess of 10 m/s and the distribution similar to that for offshore locations.  I am intrigued by the concept of airborne wind turbines, machines which generate electricity can be summarised as being large chunks of metal, Doing this safely and economically is fascinating technical challenge.

Close to earth are personal weather stations.  For large scale installations turbines can be mounted on masts which gets them away from the turbulent zone close to the surface, but for small installations, once you get above a few metres from the ground, the cost of the mast becomes significant, not to mention the attitude of local residents.  Many PWS are located in urban or residential areas, they often suggest low average wind speeds, lots of turbulence and long periods of calm.

Whilst I’m sadly fond of software and databases, the hours spent with them were suggesting complexity.  I found two ways of exploring this.  The first was simply to cycle around the local coastal area with a wind speed meter.  Close to the coast the wind when blowing off the sea was smooth and similar to that reported by the local airfield.  In urban parts, it was weak and turbulent (however, destructive gusts can and do occur in such places).  On the surrounding ridges and hills, the wind speed had some relationship to that on the coast but was noticeably more turbulent.  Taking a kite on these expeditions made them more fun.  If you fly a kite people will smile at you and look happy whilst messing with an anemometer (or small model Savonius wind turbine)  is a solitary activity.

A variation on this theme is to observe the angle of trees.  In an area where the wind speeds are low, trees grow vertically, as the wind speed increases they take on a slant away from the direction of the prevailing wind, the countryside records the weather.

The second route was to look at the location of windmills.  During the 19th many windmills were constructed, mainly to grind corn but also for pumping, sawing timber and towards the end of the period a few were used for generating electricity.  The siting of a mill was critical to its economic success and therefore it is reasonable to assume that millwrights had a good knowledge of the interaction of wind and terrain.  I’m still working on this, but it is appears that ridge and plains were the favoured locations.   I’m still looking at this, but it appears that many early airfields were also build on shallow hills and ridge with the runways oriented along the direction of the prevailing wind, thus giving aircraft with low power/weight ratios compared to today’s standards some assistance in getting aloft.

Wind is not constant and the energy available for conversion is proportional to the cube of its velocity, for example, wind blowing at 7.5 m/s represents approx 3.4 time more energy than at 5.0 m/s and at 10 m/s this increases to 8.0.  Wind energy arrives in “pulses” sometime separated by days or even weeks and subject to seasonal variation.

 

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About SolarBucket

I trained as a mechanical engineer in the 1970's and then spent most of the following 25 years doing sums and software for Oil and Gas Exploration. Current interests are the study of wind and solar resources.
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