Innovative and Sustainable Natural Ventilation System Design for Protected Polyethene-Covered Structures (Polytunnels)

Williams, Michael Philip (2019) Innovative and Sustainable Natural Ventilation System Design for Protected Polyethene-Covered Structures (Polytunnels). Masters by Research thesis (MSc), Manchester Metropolitan University.

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Abstract

Protected cultivation has expanded globally in recent decades because cheap polyethene (PE)-clad structures (polytunnels) are cheaper to produce than traditional glass greenhouses and are therefore more widely available. The main challenge is to develop a lighter and more cost-effective polytunnel ventilation system. Current systems are heavy-duty traditional window-type designs and ventilation is optimal only if there is a constant flow of wind. Such designs are also costly and need to be supported by the metal framework of the polytunnel or by an additional robust structure. In the present study, a novel retrofit ventilation design is proposed that is lightweight, cheaper, and supported by the PE sheet alone. Many such vents could be installed according to requirements. The proposed vent operates by a butterfly valve mechanism using a circular rod frame and a flap pivoting around a central shaft, the ends of which are connected to the frame. It can be retrofitted to the existing PE sheet of the polytunnel by cutting a hole in it. A clip system was designed to attach the stretched PE sheet to the frame at the aperture edge. SolidWorks software was used to produce CAD models of the basic concept and more detailed designs of the individual components. CES Edupack software was used to select suitable material for the frame. A working model was used for experiments on stretching of the PE sheet under different loading conditions and temperatures to determine load tolerance. It was found that the PE could withstand forces up to 40 N at temperatures up to about 50oC. Finite element analysis (FEA) demonstrated that the ventilation frame could sustain the tension exerted by the stretched PE and revealed the stress distribution within the frame; an optimisation study enabled the correct rod dimensions to be selected. A prototype of the vent system was built using information obtained from the material selection, FEA and experimental studies. The practical applicability of the design was also assessed through Design for Manufacture (DFMA), cost and sustainability analyses. This new vent system design would provide a robust, retrofitable, affordable, mass-customisable ventilation solution for polytunnel users. All these qualities would help towards achieving sustainable crop production in polytunnels through natural ventilation.