Tuesday, 29 September 2015

Le Puits Aerien - an elegant failure

Whenever I travel from Les Arcs-sur-Argens to Trans-en-Provence, I am struck by the appearance of a giant stone beehive on the hill opposite my descent into the town.

Achille Knapen's puits aerien at Trans-en-Provence
I always thought it was some kind of ancient well, but instead of it being centuries old, it was actually constructed in 1930-31.

It was a sort-of 'folly' created by a Belgian engineer, called Achille Knapen.

A 'folly' is generally thought of as something useless but beautiful, yet that was not how Monsieur Knapen saw his work - even if it turned out that way.

I learned it actually was a well, but it was one that collected the moisture in the air by condensation, rather than water itself, hence the name 'puits aerien' - literally 'air wells'.

Monsieur Knapen had attended a conference in Algeria in 1928, on drought and had put forward his idea to develop these puits aeriens in Africa for countries suffering drought.

Simply, they work as cold night air enters a metallic tube that runs down the centre of the well, while during the day, warm air enters the building through the hundreds of orifices on the sides of the well.
Inside the puits, showing the central tube.

When the warm air hits the cold central tube, it condenses, forming droplets of water which are collected in a reservoir underneath.

The African project did not go ahead and M. Knapen brought his ideas to France. He decided on a site at Trans-en-Provence as the most suitable place for his first experiment with le puits aerien.

The town was elevated enough and it was not only exposed to cool night winds from the mountains to the north, but also to the warm, moist winds from the sea.

When the dome-shaped structure was finished in 1931 it measured 12 metres across and 12m high, like a giant upturned stone bell.

However after 18 months of use, his efforts had not borne fruit. On the best nights, M. Knapen only collected about a bucket of water - certainly insufficient for the residents of Trans-en-Provence.
The archway through to the inner workings.

So why did his experiment go so wrong?

Somehow the engineer had forgotten to take into account the night temperatures in the Var. He needed it to be between 4'C and 11'C at night for the condenser to work, as it is in desert regions. The area is just too warm!

The other tragedy for M. Knapen, was that Trans-en-Provence is already well served by the Nartuby River which flows all year round through the centre of the town supplying water to residents and businesses alike.

Le puits areien remains - its elegant silhouette graces the hillside opposite the entrance to the town. It is fascinating to visit and walk around and see the amount of work that went into it.

But despite never having served its original purpose, the redoubtable 'air well' has carved out a new one - that of a tourist attraction and I am sure M Knapen would be pleased to know that is now listed as part of the town's heritage.


  1. Yes, I'd seen it there for several years before investigating and learning its story.