Polytunnel ubiquity

Polytunnels facilitate growing sensitive, tropical fruiting plants in northern climes - where much of the planet's landmass and population reside.

Polytunnel problems

These usually serve dual roles: supporting the structure against gravity and supporting the polythene.

The result is often significant stress points - where the force in the polythene is concentrated many times over.

Polytunnel stress points

These stress points are where the covering ultimately goes through.

Cable tunnel

Using a net

The basic idea is to divorce the roles of structural support for the whole structure and supporting the polythene - and use differing structures for each task.

Essentially, this is likely to means using some sort of net to provide an external curved surface - and then stretching the polythene over that.

This fabric then takes the strain from the structural elements - rather having it transmited straight into the polythene.

The fabric should be substantially less elastic than polythene - so it results in a surface that is close to being cylindrical.

Because such a fabric would have more regular structural elements, there would be no large areas where the polythene is unsupported - and so the strains where it is supported would be reduced.

Such a net could also play the role of insulator - thermally insulating the polythene from the structural elements.

Longitudinal elements

That's essentially because the polythene curves perpendicular to this direction - and shouldn't curve much in any other direction.

Cable tunnel

Net drawbacks

• Material costs

  The material of the net costs money - and nets take time to install, and create an additional light barrier.

  However the idea of using a net is to provide better support for the polythene cover - and maintenance costs associated with the cover are responsible for many of the costs of operating a tunnel.

  More structural support for the polythene from a net could result in a reduced need for primary structural elements - since they no longer have to play the role of directly supporting the polythene. In theory, these could be drastically reduced - so the tunnel comes to consist primarily of polythene-covered net.

  Since the polythene is no longer subjected to such great stresses - it should be possible to either use cheaper coverings - or get a longer lifespan.

• Installation costs

  The net takes some time to install. Hopefully it will pay for itself in terms of decreased maintenance costs associated with the cover it acts to protect.

• Blocking light

  Use of a net is likely to result in some on the incident light being blocked.

  However the net does not need to be very dense to perform its primary function.

  The effect is quantified later on this page.

• Drips

  Lastly, a net interferes with the passage of condensation down the inside of the polythene, and increases the quantity of water that will drip into the main body of the structure.

Is it practical to use only the net to insulate the polythene cover?

The idea here would be to make the net play the role of thermal insulator - as well as the role of physical support.

The maths involved is fairly simple:

In the following, R1 is the radius of the tunnel, R2 is the radius of the cable.

To present a textual summary:

• Thinner strands block less light (despite the fact that more of them are needed).

• Using a polytunnel with a larger radius decreases the proportion of incident light blocked.

Material

Such wire is often built for applications such as washing lines - and is available inexpensively.

Woven tunnels