COP26 and the Growth of Wind Power Series – Part 2: Windmills old and new
In the second part of our series looking forward to the UN climate summit, Brixton Windmill volunteer Nick Weedon looks at the creative ways in which wind energy has been harnessed over the centuries
Vertical axis mill
September marks the start of four months when a strong steady wind blows across Iran’s Sistan region, close to Afghanistan. This is flour grinding time in the town of Nashtifan. On the top of the hill stands the world’s oldest and largest collection of windmills. The climate is very dry, which helps to preserve the structures built of clay, straw and timber. What we see on the surface is around 200 years old. This may only be as ‘young’ as Brixton Windmill, but these mills have been in a continuous cycle of maintenance that stretches back around 1,200 years to when they were first built.
These are vertical axis (‘panemone’) mills. There are thirty of them, set behind high walls with openings that funnel the wind onto wooden sails. The local geography means that the autumn wind blows consistently from one direction, allowing the enclosures to be constructed without worrying that the openings may not be catching the wind.
This is the earliest form of windmill in recorded history, but harnessing of wind power goes back further, with sailing vessels, thought to have originated around 6,000 years ago. The grinding of grain to produce flour is believed to have originated around 10,000 years ago – originally a manual process using saddle querns. This is laborious work, usually undertaken by women, and still in use in some countries today.
Circular rotary querns first appeared in ancient Persia and Rome. The grinding process is faster and more efficient. They were often driven by hand, but Nashtifan is the first known place where wind power met milling, so it’s remarkable that the complex survives so intact. Groups of around six people from different families or tribes used to operate each mill. It was listed as an Iranian national heritage site in 2002, but only a single custodian remains, making its future uncertain. A Friends of Nashtifan Windmills is very much needed.
The technology of windmills first spread through the Muslim world and was used for flour grinding and pumping water for irrigation. The forces of Genghis Khan are said to have compelled Persian millwrights to take the technology eastwards, where it became established in India and China, well before appearing in Europe.
We don’t know how and when the vertical axis mills of Asia translated to the horizontal ones of Europe that started appearing in the 12th century, with their trademark sets of four sails set outside the structure rather than paddles within. Trade and communication between the regions is well known, but whether the change took place before reaching our shores is not recorded.
The most common type of European windmill took the form of the sails set onto a large box enclosure supported on a central post, allowing the entire structure to be rotated so that the sails could face into the wind from any direction. The advantage of a rotating structure over a fixed one was enhanced by the gearing of the mechanism within. The sails turned a large cogwheel, which drove a smaller one connected to the stones, resulting in the stones turning at a greater speed than the sails. Faster stones made for more efficient grinding.
Mills such as Chillenden were constructed on open trestles set above the damp ground on brick pads. Many were also built to incorporate a brick enclosure on the ground, called a roundhouse, which helped to protect the post from the weather and provide more space for handling the grain and flour. The post mill was very common throughout Britain and Europe for several centuries, but the timber structures required a lot of maintenance, and were vulnerable to being blown over.
Another type of structure, called a ‘tower mill’, provided a much stronger option, constructed of brick or stone. The earliest recorded one was in Dover, built of stone in 1295. The sails were supported on a timber structure at the top, called the cap, which could rotate to face into the wind.
These mills remained a rarity until the 18th century, when brick versions, similar to Brixton’s, became commonplace. Some tower mills were constructed with vertical walls, a notable example being Chesterton mill in Warwickshire, which has open arches around the base. Many, such as Brixton Windmill, were constructed with tapering walls shaped like a cone. This is a stronger structural form than a cylinder, and more easily enables the sails to be fixed at a slight off-vertical angle. This helps distribute some of the weight over the footprint of the building, rather than concentrating it all outside of the structure. Having the sails off-vertical also makes it easier and safer for people to climb up, which was necessary for attaching the canvas for catching the wind.
The third type of mill, the ‘smock mill’ was developed to combine the advantage of a sturdy fixed structure
with that of a much less expensive timber frame.
The shape of the frame, splaying outwards on the corners, helps to brace the structure against the wind and also suited the setting of the sails at an angle. The earliest recorded smock mill dates from 1650, built at Lacey Green in Buckinghamshire. It survives today, and is open for visits, as is Brixton’s closest Smock neighbour at Upminster.
It is estimated that at the peak, these three types of windmill numbered more than 200,000 throughout Europe.
And some others
Not all conformed to these types. Brixton Windmill’s closest surviving neighbour, on Wimbledon Common (built at the same time) has elements of all three: A brick building at the base supports a smock mill-like structure, on top of which sits a cap that is structured like a post mill. It no longer works, but a visit to the museum in the base of this triple-stack is recommended.
One thing that these illustrations have in common is that all the mills have four sails. Some mills were built with three and five sails. Three would save money, and five would be more powerful.
The choice of four sails carries the advantage that if one becomes defective, taking the opposite one out of use will still enable the remaining two to turn with an even action, whereas with an uneven number of sails none are directly opposite each other, so would lead to an uneven action and more of a risk of stalling.
In the late 18th Century, Captain Stephen Hooper of Margate attempted to update the original Persian vertical axis type in a way to suit our more changeable weather conditions. It was akin to a cylindrical version of a smock mill, but with the walls clad in vertical shutters that could be adjusted like venetian blinds. This allowed any side of the wall to be opened to admit the wind onto a vertical axis sail set inside. Mills at Margate and Sheerness were followed by his largest example near St Mary’s Church, Battersea in 1788. It stood 120ft high and dominated the riverfront until it was taken down in 1825. It was said to require frequent repairs, and the owner invested in newer technology not long after it opened. It converted to steam power as early as the 1790s, over 110 years before Brixton did the same thing.
The next article in this series will look at how artificial power took over from the wind – and the unique legacy at Brixton that enables us to demonstrate the changing technology and to keep milling.
COP26 and the Growth of Wind Power Series – Part 1 – Uniting to Tackle Climate Change
UN Climate Change Conference of the Parties (COP26)
Main Picture caption: Brickfields Pencil Drawing Postcard