Polytunnel structure

Abstract

A polytunnel structure 10 is provided with a retractable sheet 20 that can be over a window or hatch opening 18. The retractable sheet 20 has an inflatable chamber and can be double skin defining a vo

Description

Field of the Invention The present invention relates to polytunnels, particularly to double-skin polytunnels filled with gas or air for insulation purposes. More specifically, the invention relates to temporary openings, such as flaps or vents for such double-skin structures. Background A common form of polytunnel structure of the type used for cultivation of plants and crops comprises a series of pairs of support legs supporting a hoop or arched structure over which a cover material such as a polypropylene sheet is mounted to provide a generally semi-cylindrical or arched roof. The sheet may be provided in double skin form, providing two layers of material that define between them an interstitial volume providing a pocket to be filled with air or other gas for insulation purposes. Such double-skin polytunnels may require windows for access and servicing and / or venting. To reduce the effect on insulation qualities provided by the double-skin structure, such windows may be provided with access flaps, doors, or other suitable covers to allow the windows to be temporarily opened or closed. The present invention seeks to provide a system that helps to better maintain insulation properties. Summary of the Invention In accordance with a first aspect of the invention, there is provided polytunnel structure as defined in claim 1, provided with a retractable sheet, the retractable sheet comprising an inflatable chamber, wherein the retractable sheet is deployable into a deployed condition and retractable into a retracted condition, and wherein the retractable sheet is provided with a deflation port. The polytunnel structure may be a polytunnel of the type used in the cultivation of crops and soft fruit, typically comprising a support scaffold comprised of support hoops of arches supported on legs, the legs being separate structures or integrally formed as ends of the hoops or arches, to support a cover sheet in the form of sheet material, typically of polypropylene or polyethylene. The invention relates to a form of retractable sheet that is provided with an inflatable chamber. Such polytunnel cover sheets may be provided as a multilayer structure, commonly referenced as “double skin” structure, comprising an inter-sheet volume to be filled with gas such as ambient air. The polytunnel may be operatively connected to an inflation system, such as a fan-assisted inflation system, to transfer gas such as ambient air into the double skin structure, i.e. to inflate a chamber provided by the intersheet volume. The double-skin structure or chamber is not necessarily gastight and may comprise small openings or passages permitting leakage. In use, an inflation system serves to maintain the double skin structure inflated, whereas the inflation system can be operated to increase a rate of inflation for inflating the double skin layers, to maintain a rate of inflation, and likewise to decrease the rate of inflation and / or deactivate altogether. In this manner, a double skin or other multi-layer structure comprises an inflatable chamber. If the inflation rate is lower than the gas leakage rate, or turned off, it will be understood that this may result in gas escaping from the multilayer structure via the openings or other passages faster than it is replenished. In this manner, the double skin structure can be deflated if desired. As will be appreciated, retracting and deploying the retractable sheet may alter the volume of the inflatable chamber thereof. The present disclosure suggests providing the, or each, retractable sheet with a deflation port. The deflation port is understood to be a port structure that can be adjusted, e.g. manually opened and closed, to alter its deflation properties. As will be appreciated, the port may, in a simple form, be a structure of a more rigid material than the retractable sheet, so as to maintain open an aperture of predetermined size for deflation. The deflation port may comprise a flap or lid that is openable manually to facilitate deflation, and closeable manually so as to avoid interfering with inflation. The provision of a deflation port was found to allow for a faster retraction of the retractable sheet, without having to rely on unassisted deflation. In some embodiments, the polytunnel structure is provided with a cover sheet comprising and opening, the opening being coverable by deploying the retractable sheet. As will be appreciated, polytunnel cover sheets are installed more or less permanently, and may comprise apertures or openings for access, servicing and / or venting. Herein, a cover for such an apertures or openings is referred to as a retractable sheet, which may be understood as a cover panel or hatch, of sheet form, for covering and uncovering an opening in the cover of the polytunnel while the polytunnel remains installed. The opening may be provided by an aperture or recess in the cover sheet of the polytunnel, and / or may be provided by a gap between sections of polytunnel material. For instance, a polytunnel may comprise both a permanent polytunnel cover sheet and one or more retractable sheets, and both cover sheet and retractable sheet may be of double skin design. However, the invention is not necessarily limited to window covers. In some embodiments, the full length of a polytunnel installation may be covered by one or more retractable sheets. In some embodiments, the polytunnel structure is covered by one or more of the retractable sheets along its longitudinal extension. The retractable sheet may be of the same material as polytunnel cover sheet, e.g. of polypropylene or polyethylene form. The retractable sheet is of a material that is sufficiently flexible, rollable and / or pliable to allow it to be rolled up and / or folded, to retract it by rolling it up or folding it, respectively. In this manner, the cover panel may be deployed to cover an opening, and retracted to uncover the opening. In some embodiments, the retractable sheet is a double-skinned sheet. The inflatable chamber of the retractable sheet may be formed by a double skin structure defining an inflatable volume between two sheets. In some embodiments, the retractable sheet is retractable by folding one or multiple times. In some embodiments, the retractable sheet is retractable by rolling. In some embodiments, the retractable sheet comprises multiple chambers. In some embodiments, the sheet comprises at least one deflation port per chamber. The sheet panel may be comprised of one or more chambers. As will be appreciated, each one of the chambers may be provided with at least one deflation port. In some embodiments, the retractable sheet is retractable towards a mounting location on the polytunnel structure, the deflation port being located near the mounting location. In some embodiments, the deflation port is located at the mounting location. In some embodiments, the retractable sheet is openable upwards, the mounting location being at its in-use upper end. In some embodiments, the deflation port is located at or near the in-use upper end of the retractable sheet. In some embodiments, the deflation port is provided by a pressure relief valve. By providing a deflation port in the form of a pressure relief valve, it was found that a deflation-assisting function can be provided automatically, without requiring manual actuation of a port opening mechanism. An appreciation by the inventor was that the pressure relief valve may be set to actuate to open at or above a relief pressure threshold. The threshold is understood to be set above a normal operating pressure level, and at a level expected to occur during retraction (e.g., during rolling up) of a retractable cover. In some embodiments, the pressure relief valve is adjustable to allow its actuation threshold to be altered. In this manner, the actuation threshold, i.e. the relief pressure threshold, of the valve may be adjusted. For instance, the valve may be adjustable manually to alter the threshold at which the pressure relief valve expels air, to account for different inflation system characteristics, sheet size or panel size, and / or retraction speed levels. This allows the pressure relief valve to be adjusted for different climates, and / or if necessary to account for different area or volume sizes of the sheet, to account for the speed of a retraction mechanism and / or the power of an inflation system. In some embodiments, the polytunnel structure comprises a motorised actuator operatively connected to the retractable sheet for deployment and retraction. In some embodiments, the polytunnel structure comprises an inflation system for inflating the retractable sheet. For instance, an inflation system of the polytunnel structure may comprise a separate duct specifically to adjust the inflation level of the retractable sheet. In some embodiments, the inflation system is fan assisted. In some embodiments, the polytunnel structure comprises a controller in communication with an external control system, configured to receive commands for the operation of the motorised actuator, to operate the motorised actuator to deploy the sheet panel and to retract the sheet panel. In accordance with a second aspect of the invention, there is provided a polytunnel installation comprising one or more polytunnel structures according to any one of the embodiments of the first aspect. In accordance with a further aspect of the invention, there is provided a retrofit kit for equipping a polytunnel, the kit comprising an inflatable cover reconfigurable into an extended configuration and into a retracted configuration, wherein the extended configuration permits more inflation than the retracted configuration, and a motorised actuator operatively connectable to the inflatable cover for deployment and retraction, wherein the inflatable cover further comprises a deflation port positioned to remain accessible in the retracted configuration. In some embodiments, the deflation port is constituted by a pressure relief valve. In some embodiments, the pressure relief valve is adjustable to allow its actuation threshold to be altered. Any one or more of the embodiments disclosed in relation with the first aspect may be combined with the respective second or further aspects. For instance, a retrofit kit according to a further aspect may further include an inflation system and / or a controller for the operation of a motorised actuator and / or inflation system. Description of the Figures Exemplary embodiments of the invention will now be described with reference to the Figures, in which: Figures 1 and 2 are schematic illustrations of a polytunnel installation; Figures 3 and 4 are sections of valve elements; Figure 5 is a schematic illustration of a retractable double skin panel; and Figures 6A to 6D show steps of a retraction sequence of different configurations. Description Referring to Figures 1 and 2, a polytunnel installation 10 comprises a series of support legs 12 supporting hoops 14, the support legs 12 and hoops 14 providing a support structure for a cover 16. The cover 16 may be of a suitable sheet material, such as polypropylene or polyethylene. A polytunnel of this form may have a width of several meters, widths of 4, 6 or ten metres not being uncommon, and a length of several tens of metres, for instance in the region of around 100 metres. The cover 16 may be a double skin cover comprising an internal layer for gas, particularly ambient air, for improved insulation. Depending on the design, the double skin layer may have varying degrees of airtightness. The double skin may be connected to a fan assisted inflation system 28 to replenish air or other gas. In practice, an installation of this type is not necessarily airtight and may be ‘leaky’ to some extent, such that inflation, in regular intervals and / or continuously at low flow rates, is used to maintain a desired degree of inflation. The inflation system 28 will be adjusted to maintain the cover inflated at a working pressure. While the cover 16 is described as double skin structure, this is not necessarily a requirement of the invention. The polytunnel installation 10 comprises an openable panel 20, in the form of a retractable sheet, covering an opening 18 (see Figure 2) in the cover 16, the panel 20 providing a hatch or window cover, to allow a portion of the opening 18 to be uncovered if required for venting or access. Conveniently, the panel 20 is from the same material as the cover 16, however this is not necessarily a requirement of all embodiments. The panel 20 is a design that is deployable so as to cover the opening 18 to close it, and retractable so as to uncover the opening 18 to open it. A practical form of such a panel design is to use a rollup mechanism, using a roll bar 22, driven by a motor 24 connected to the roll bar 22 via an articulated translation arm 26, for instance using two or more Kardan joints allowing the roll bar 22 to be rotated while moving towards and away from a mounting point of the panel 20. Operation of the motor 24, to rotate the articulated translation arm 26, results in the roll bar 22 being wound up, the material of the panel 20 being rolled up in the manner of a blind mechanism. The motor 24 may be controlled to operate between two end points corresponding to an extended configuration of the panel 20 and to a retracted configuration of the panel 20, and therefore corresponding to an open configuration of the opening 18 and a closed configuration thereof. While the present example illustrates a rollup mechanism, it will be appreciated that other retraction mechanisms, such as folding or concertina-style retraction mechanisms, or others, may be appropriate alternatives. In an endeavour to increase the size of such panels 20, it was found desirable by the inventors that the thermal insulation properties of the panel material are increased, for instance to match or exceed those of the cover 16 so as to avoid compromising thermal insulation. The use of double skinned, inflated structures was found to be suitable. A difficulty appreciated by the inventors was that rapid retraction (e.g. by rolling up) may create pockets of gas of increased pressure where this is unable to escape, or unable to escape fast enough, through existing leakage passages. The polytunnel installation is provided with a valve 30, constituting an example of a deflation port. Herein, the deflation port is described in a form of a pressure relief valve, connected with an air chamber of the panel 20, providing a gas passage between the air chamber of the panel 20 and its exterior side. In other variants, the deflation port may be provided by a port structure comprising a lid that is manually openable to create a passage that is maintained open to assist deflation, and closeable to shut the passage to assist inflation. As will be appreciated, the lid will provide a sufficiently gastight seal to block the deflation port, however the lid may not be designed to block overall gas leakage from the cover. Referring to Figures 3 and 4, the pressure relief valve 30 presented in this embodiment comprises a cylindrical body 32 of hollow form closed at one side with a valve plate 34 mounted on a support rod 36 and displaceable against the inherent resilience of a biasing element 38, here in the form of a helical spring. The cylindrical body 32 can be understood as a body of more rigid material than the panel material, ensuring that a minimum passage diameter is maintained. The biasing element 38 is held in a prestressed condition between two mounting elements 40, 42, here in threaded engagement with the support rod 36. For instance, the mounting elements may be provided in the form of adjustable nuts and washers. The support rod 36 comprises a threaded region along which mounting elements 40, 42 may be moved. As will be appreciated, adjusting the position of one or both of the mounting elements 40, 42 along the support rod 36 allows a pre-stress condition of the biasing element 38 to be adjusted, and therefore allows the relief pressure threshold to be adjusted at which the pressure relief valve 30 actuates. The adjustment may be carried out manually, e.g., by adjusting one or both of the mounting elements 40, 42. With reference to Figure 5, the panel 20 may be arranged to be mounted to the polytunnel via a mounting end 23, which may be part of a frame or track structure. Herein, the mounting end 23 is located in the in-use upper end of the panel 20, although this is not necessarily a requirement of all embodiments. For a rollup mechanism as illustrated herein, a retraction direction R may be, in use, upwards, from a lower, deployed, end, to the upper, retracted, mounting end 23. The valve 30 is located close to, or at, the mounting end 23, i.e. at a free side of the panel 20 so as to remain unobstructed during retraction and / or deployment of the panel 20. While the system has been described using polypropylene or polyethylene sheet, other materials may be used, including non-plastic material. The sheet material may not necessarily be provided as a pliable or rollable material. The relief pressure threshold may not necessarily be high. It is believed that, during normal operation, a closed panel 20 may be at or only slightly above atmospheric pressure, or higher, e.g. up to five times atmospheric pressure, being inflated by an inflation system 28 regularly merely for replenishing purposes. As mentioned before, because the panel 20 may not necessarily be airtight, reduction or deactivation of the inflation system 28 in normal use would be expected to result in deflation of the cover over time. Upon retraction of the panel 20, for example by relatively rapid rolling-up, the relief pressure threshold of the valve 30 may be exceeded, even if only temporarily, resulting in automated activation of the pressure relief valve 30 to vent gas from within the panel 20. To provide an illustrative indication of magnitudes, the panel 20 may be of a size covering up to 100 metres length and 10 metres high, able to open an entire section or side of a polytunnel. In this case, the retraction from deployed condition (e.g. Figure 1) to retracted condition (e.g. Figure 2) may be completed, depending on motor speed and panel size, within one to ten minutes. As will be appreciated, a panel 20 of such size may hold several hundreds or thousands of litres of gas when deployed. The provision of the pressure relief valve 30 allows the panel 20 to be retracted more rapidly than would otherwise be the case. Figures 6A and 6B illustrate, schematically, moments in a rollup procedure of a prototype panel without deflation port. The panel 20 is of double-skin form comprising a volume Vi inflated with gas. The panel 20 is attached to a polytunnel structure via its mounting end 23, here the upper end. The opposite, free end of the panel 20 is connected to a roll bar 22. In Figure 6B, the roll bar 22 has been wound up partially, in a retraction direction R, for instance by means of a motor 24. Due to leakage, some of the gas in the panel 20 is expelled, resulting in a reduced volume V2. However, depending on the rollup speed, the remaining gas results in additional pressure on the sheets forming the panel 20. The additional pressure may resist rolling up and / or may even cause damage to components. Figures 6C and 6D illustrate, schematically, a similar rollup procedure as in Figures 6A and 6B, however provided with a deflation port. The panel 20 is of double-skin form comprising a volume V1 inflated with gas. The panel 20 is attached to a polytunnel structure via its mounting end 23. Near, or practically at, the mounting end 23, a valve 30 is provided. The valve 30 may be a deflation port that can be manually opened or may comprise a pressure relieve valve that actuates upon reaching a threshold pressure. The panel end opposite the mounting end 23 is attached to a roll bar 22. In Figure 6D, the roll bar 22 has been wound up in a retraction direction R. As the retraction of the panel 20 reduces the available volume of the panel 20, this leads to expulsion of excess gas Ve, the remaining gas or residual volume Vr placing less strain on the sheet walls of the panel 20. Furthermore, as the gas in the remaining volume Vr is not under as high a pressure, due to the operation of the valve 30, the remaining gas volume Vr presents less resistance to the retraction mechanism. As such, the rollup mechanism, or retraction mechanism, can operate faster than in the absence of a deflation port. If it is desired to retract the panel 20 only partially, partway along the retraction distance, the valve 30 provides that a residual volume Vr remains in the chamber while its pressure is below the threshold pressure, thereby maintaining the inflated characteristics and / or reducing replenishing time upon subsequent closing of the panel 20. The relief pressure threshold of the valve 30 is set above the working pressure of the double skin structure. As the cover materials have a certain degree of flexibility, the relief pressure threshold may be set to be somewhat higher than the maximum working pressure, e.g. at least 0.5 bar or 1.0 above the maximum working pressure. In test calculations, it was found that a single pressure relief valve will suffice for a relatively large retractable panel of several square metres size. However, the invention is not necessarily so limited, and a panel may be provided with multiple pressure relief valves. When it is desired to close the opening 18, the motor 24 is controlled to unwind the panel 20. The operation of the motor 24 may be coordinated with an inflation system 28, whereby the panel 20 is being inflated as it is being unfolded, or unwound. In initial trials, it was found that a control system to coordinate motor and inflation system may not be required in all configurations. Typical inflation systems may provide relatively low flow rates, and may suffice to increase, over time, the gas held in the chamber of the panel 20, which during its expansion remains under the relief pressure threshold of the valve 30. Depending on how inflation chambers of the polytunnel installation 10 are interconnected, it was found that an inflation system 28 may remain active even during retraction of the panel 20, the pressure relief system provided by the valve 30 allowing excess gas to vent practically automatically as required. While the illustrated panel 20 is shown as a vertically retractable structure, the mechanism including the deflation port may also be used on other sheet structures that extend horizontally or at an angel. As such, the deflation mechanism may be integrated with retractable ceiling or roof cover portions of a polytunnel. The invention enables the use of larger retractable inflatable covers. As such, the invention may be used to provide retractable inflatable covers along the full length of a polytunnel installation, either as a single retractable cover, or in the form of a series of retractable covers. If provided in the form of a series of retractable covers, a control mechanism may be configured to deploy (to close) or to retract (to open) individual ones of the covers independently of the others, and / or to deploy or retract multiple covers in a coordinated manner, e.g. simultaneously or sequentially. Whilst the principle of the invention has been illustrated using exemplary embodiments, 5 it will be understood that the invention is not so limited, and that the invention may be embodied by other variants defined within the scope of the appended claims.

Claims

1. A polytunnel structure provided with a retractable sheet, the retractable sheet comprising an inflatable chamber, wherein the retractable sheet is deployable into a deployed condition and retractable into a retracted condition, and wherein the retractable sheet is provided with a deflation port.

2. The polytunnel structure according to claim 1, wherein the polytunnel structure is provided with a cover sheet comprising and opening, the opening being coverable by deploying the retractable sheet.

3. The polytunnel structure according to claim 1 or 2, wherein the polytunnel structure is covered by one or more of the retractable sheets along its longitudinal extension.

4. The polytunnel structure according to any one of the preceding claims, wherein the retractable sheet is a double-skinned sheet.

5. The polytunnel structure according to any one of the preceding claims, wherein the retractable sheet is retractable by folding one or multiple times.

6. The polytunnel structure according to any one of the preceding claims, wherein the retractable sheet is retractable by rolling.

7. The polytunnel structure according to any one of the preceding claims, wherein the retractable sheet comprises multiple chambers.

8. The polytunnel structure according to claim 7, comprising at least one deflation port per chamber.

9. The polytunnel structure according to any one of the preceding claims, wherein the retractable sheet is retractable towards a mounting location on the polytunnel structure, the deflation port being located near the mounting location.

10. The polytunnel structure according to claim 9, wherein the deflation port is located at the mounting location.

11. The polytunnel structure according to claim 9 or 10, wherein the retractable sheet is openable upwards, the mounting location being at its in-use upper end.

12. The polytunnel structure according to claim 11, wherein the deflation port is located at or near the in-use upper end of the retractable sheet.

13. The polytunnel structure according to any one of the preceding claims, wherein the deflation port is provided by a pressure relief valve.

14. The polytunnel structure according to claim 13, wherein the pressure relief valve is adjustable to allow its actuation threshold to be altered.

15. The polytunnel structure according to any one of the preceding claims, comprising a motorised actuator operatively connected to the retractable sheet for deployment and retraction.

16. The polytunnel structure according to any one of the preceding claims, comprising an inflation system for inflating the sheet panel.

17. The polytunnel structure according to claim 16, wherein the inflation system is fan assisted.

18. The polytunnel structure according to any one of claim 15 to 17, comprising a controller in communication with an external control system, configured to receive commands for the operation of the motorised actuator, to operate the motorised actuator to deploy the sheet panel and to retract the sheet panel.

19. A polytunnel installation comprising one or more polytunnel structures according to any one of the preceding claims.

20. A retrofit kit for equipping a polytunnel, the kit comprising an inflatable cover reconfigurable into an extended configuration and into a retracted configuration,wherein the extended configuration permits more inflation than the retracted configuration, and a motorised actuator operatively connectable to the inflatable cover for deployment and retraction, wherein the inflatable cover further comprises a deflation port positioned to remain accessible in the retracted configuration.

519. The retro-fit kit according to claim 18, wherein the deflation port is constituted by a pressure relief valve.

20. The retro-fit kit according to claim 19, wherein the pressure relief valve is 10 adjustable to allow its actuation threshold to be altered.16

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