GB2120065A - Heating plants - Google Patents

Abstract

Plants are directly heated by the action of microwaves upon cell-water in the plants. The plant-growing area is enclosed by a light-transparent membrane or mesh which reflects microwaves and microwave generators are positioned in the area. The membrane may also reflect infra-red rays, and additional transparent membranes may be used to reduce convection currents within the area.

Description

SPECIFICATION A method of heating for horticulture I, Michael Frederick Huber, a British subject, of Woodview, Standford Hill, Bordon, Hampshire, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a method of heating growing plants as required,forexample, by horticulture and in particularto the application of microwaves to obtain this heating, in a safe and efficient manner.

Existing methods of heating greenhouses generally rely on oil for fuel and heat the plants largely by producing warm air. Unfortunately, a greenhouse is intrinsically very poor at containing heat and so large amounts of energy are used and effectivelywasted in heating the plants. In an effortto reduce heating costs infra-red radiant heating has been tried since the rays should heat the plants ratherthan air, so that with the interior air staying cooler less heat should be lost from the greenhouse.Some energy savings have been claimedforthis method but it has not been generally adopted forthe following reasons: the rays are not reflected to any extent by glass orthe plastic glazing used, so heat is radiated out ofthe greenhouse; the rays only heat objects in their path and do not easily penetratethe leaf canopy and this gives uneven heating; the plants themselves re-radiate and also lose heat by convection to sufficient extent to reduce energy savings below that anticipated.

Now it is well known that microwaves because of their longer wavelength are more easily passed around obstacles by diffraction than infra-red rays and unlike infra-red rays are not absorbed at surfaces but penetrate many materials, being particularly absorbed bywaterorwatery materials. Such materials may be heated to a depth depending on wavelength and absorption, by the action of microwaves upon the water molecules they contain.

It is an object of the present invention to exploit these properties of microwaves to heat plants economically in a greenhouse and also in situations where previously artificial heating was considered uneconomic.

Microwaves when employed to heat growing plants and particularly by the fastest growing parts ofthem, due to the waterthey contain. Furthermore, the leaves of plants heated in this way are kept from being wet, which otherwise can happen particularly if a lower growing temperature is attempted without adequate ventilation. This is due to thefactthatthe leaves are warmerthan the air so water will not condense on them and also because any water on leaves will be gently heated by the microwaves until it evaporates.

Cold wet leaves are liable to fungal diseases so microwave heating enables a lower growing temperatu re without increasing the risk of such disease.

Compared to infra-red heating, microwave heating is also much more uniform. This is because microwaves at the industrial heating wavelengths of 12cm.

and 30 cm. are able to completely penetrate several layers of leaves causing heating by partial absorption at each layer. Such waves are also partly absorbed by the soil, causing some heating, and partly reflected.

Furthermore, by diffraction the microwaves can in effect pass around obstacles.

According to the invention, the sides and roof of a greenhouse are lined with a membrane or mesh substantiallytransparentto optical radiation but very good at reflecting microwaves. One or more microwave generators radiate into the space so enclosed, in such a way as to set up by multiple reflections a complex pattern of standing waves filling the space.

The lined greenhouse becomes in effect a very large microwave cavity, damped by a load largely created by the plants and to a smaller extent by the soil. Soil not to be heated can be covered with a reflector of microwaves. A substantially uniform and fairly low radiation density can therefore easily be produced and if required even greater uniformity oftime averaged field strength can be obtained by using rotating or oscillating mode stirrers.

The plants in the enclosed space are heated by absorption of microwaves and then lose heat primarily by re-radiation on long infra-red rays and by convection. Any initial differences in temperature between plants, due to non-uniformityofthe micro wavefield or differing absorption, are largely levelled out due to inter-radiation and convection between the closely spaced leaves.

Internal bulkairtemperature remains belowthatof the plants when they are heated according to the invention, so their temperature is controlled by sensing the actual planttemperature rather than the airtemperature, and then suitably varying the average power of the microwave generators. This may be done bytemperature sensors, such as for example, thermo- couples, thermistors or platinum resistance thermometers, in contact with or buried in plants, or material which imitates growing plants in its ability to absorb microwaves and lose heat by infra-red radiation and convection.Another method, which has the advan tage of sensing the bulktemperature of the heated plants rather than that of particular leaves, is to use an infra-red sensor, such as for example, a thermistor or platinum film bolometer or a semiconductor pyroelec tric detecto r, situ ated at the focus of an infra-redlens or a mirror not able to focus microwaves at the sensor.

Theirtemperature can then be sensed, either by their total infra-red emission with direct comparison or previous calibration against a source of known temperature and emmissivity, or by using in front of the sensor a rotating chopper disc carrying filters, so as to sense the ratio of shortto long wavelengths of their infra-red emission. A simpler but less accurate method isto control the microwave generators by temperature sensors or thermostats buried in the soil orsituated in the air around the plants. This relies on previously finding the relationship between soil and planttemperature or soil and airtemperature. It to be noted here that by the nature otthe heating means employed according to the invention, as well as the lowerthermal mass created by the heated plants as compared to a heated greenhouse, the control re sponsefrom these arrangements is both faster and more accuratethanthat of conventional heating systems.

Although the radiation density in a greenhouse heated according to the invention would be fairly low it could still be above allowed safety limits. Where such heating is of spaces that humans may enter, the microwave generators are automatically shut off before persons enterthe greenhouse by a keyswitch arrangement as explained later, or by a doppler movement detector or otherwise.

Furthermore, signs and warning lamps indicate when microwave radiation is present. Audible warning can also be given. The membrane or mesh lining the greenhouse covers all apertures when the door is closed, the ventilators having mesh behind them so that ventilation can be accomplished without leakage of microwaves. The field strength outsidethe greenhouse will therefore be very low indeed and well below safety limits providing that the mesh and membrane lining the greenhouse are substantially opaque to microwaves. An electrically conducting membrane is therefore required, such as for example, metallized or metal oxide coated, clear, plastic film and if mesh is used it must also be electrically conducting, for example made of metal ormetallized plastic and have aperture sizes of less than one tenth the shortest microwave wavelength being used.

For preference the greenhouse is lined bya membrane so asto create an air gap between itself and the glazing, afterthe manner of double glazing.

This membrane is substantially transpa rent to light but is a good reflector of microwaves and infra-red rays.Asuitable materialforthis membrane is, for example, metal oxide coated, clear, plastictilm. A conducting mesh is then only used where necessary to enable ventilation to take place. By using this type of membrane as a component in the invention as described above, efficiency is greater so that lower microwave powers can be used. This improvement in efficiency is due partly to increasing the thermal insulation of the greenhouse and partly to the fact that the heated plants lose a large proportion of their heat by infra-red radiation. Infra-red radiation out of the greenhouse is now much reduced and the infra-red radiation is instead reflected around likethe microwaves.Furthermore, extremely uniform heating by radiation is achieved by using the two kinds of radiation. Control, by sensing the infra-red radiation density, is also now made easiersincethis is substantially uniform.

Growing plants are often protected by cloches and frames. These are heated bysolarradiation but cloches in particular are not generally heated by artificial means since the most convenient heat sources are electrical heaters, and conventional electrical heating is uneconomic. This limits the use of cloches and frames since even reliable protection againstfrost cannot be given. Heated cloches and frames could assume some of the tasks now performed bygreenhouseswith a large saving in capital cost. Microwave heating can be applied to cloches and frames as described abovefora greenhouse and later, if lined byorglazed bya microwave reflecting mesh or membrane.

#fruit bushes are enclosed in mesh as described above and after the manner of a fruit cage, they may be protected againstfrost by one or more microwave generators radiating within the enclosed space and controlled as described above or simply switched on at a predetermined ambient temperature by an air or soil thermostat.

Trees in an orchard or crops growing in a field may be protected against frost by surrounding the area to be protected buy a perimeterfenceclad in mesh ableto reflect microwaves, and radiating microwaves within the perimeter in such a way as to give as far as possible a low uniform field strength within the space up to the height ofthe perimeterfence with preferably a lowerfield strength above this height. This can be accomplished by,forexample,choosing antennas that radiate fan shaped beams whose angular distribution of energy is greatest in the horizontal plane and by inclining the top part of the mesh fence inwards. The generators may be controlled as described above for the protection of fruit bushes.

Since plants are heated according to the invention by radiation, the reduction of convection heat loss from the plantswill decreasetheheating power required. This, of course, is not possible with conventional heating which relies on warm airto heatthe plants. Furthermore, as previously explained, such restriction of air movement is less likely, with microwave heating than with conventional heating, to increase the riskoffungal disease. In anyofthe applications of microwave heating described above, thin membranes transparent to light, infra-red radiation and microwaves, for example very thin, clear plasticsheet, can be arranged aboveoraround plants to reduce convection heat loss.These membranes may be perforated, if required, to allow some air movement or the passage of water. An air thermostat nearto the plants and inside th is membrane can now be used to control the microwave generators with reasonable accuracy.

It may be convenient to use the microwave generators installed for one of the applications described above to also supply microwave energy for another separate application. For example, quite low power would be sufficient to protect the contents of well designed horticultural cloches from the effect of frost.

It would probably be more economic to supply microwave energy generated elsewhere, for example in a heated greenhouse orframe, bywaveguide or coaxial cable to antennas in nearbycloches rather than to employ a separate generatorto supply them.

Embodiments ofthe invention will now be de scribed, by way of example,with reference to the accompanying diagrammatic drawings in which: Figure lisa sectional view of a greenhouse embodying the invention; Figure 2 is a block diagram of a control unit suitable for the embodiments described; Figure 3 is a block diagram showing power supply and safety arrangements suitable for a greenhouse embodying the invention; Figure 4 is a view ofthe interior of the end of a horticultural frame embodying the invention; Figure 5 is a planviewofa number of horticultural cloches togetherwith a frame embodying the inven tion; Figure 6 is an end view of a cloche embodying the invention.

Referring to Figure 1, a magnetron oscillator 0 radiates microwaves from a horn antenna A, upwards and inwards so as to create by multiple reflections from the reflecting liner L, as uniform a radiation density as possible. The liner is positioned so as to give a small air gap of not less than two centimetres between itself andthe glazing G and may conveniently be attached to the inside of the glazing bars B. The lining consists of metal oxide coated polyesterfilm except behind ventilators V where it consists of metal mesh N whose apertures for a 12 cm. band magnetron are less than 1 cm. For largergreenhouses it may be more convenient not to line the gables but to make an internal flat ceiling with the membrane liner below the ventilators.Ventilation may then be accomplished by opening traps in this ceiling when the ventilators are opened. Such traps will be covered with metal mesh when open if heating and ventilation are required.

The number and the powerofthe magnetron oscillators used will depend on the size of the greenhouse and the purpose to which it is put. In Figure 1 which represents a fairly small greenhouse of around 20 square metres a single magnetron with a power of 1 kW and a wavelength in the 12 cm. band is used. The soil is also heated to some extent by the microwaves so any soil our surface it is not required to heat is made a good reflector by covering itwith metal foil F. The plants to be heated P may be covered by a thin film C of thin, clear, polythene sheetwhich is transparentto light, infra-red rays and microwaves, forthe purpose of reducing convection heat loss.

Such a film may be perforated to allow some ventilation and the passag e of water. For low growing crops it is sufficient to laythe film over the crops but for taller crops such astomatoes the film may be drapedoverthe plantsupports S. The magnetron oscillator 0 is cooled by air and its waste heat utilized to heat the interior of the greenhouse. Water cooling is however preferable in terms of efficiency since there is then less air movement.

The magnetron oscillator is controlled as follows.

An opaque box has an aperture facing the plants covered with an infra-red filter. Athermistor at the focus of an infra-red lens behind thefilterforms an infra-red detector I. Referring to Figure 21 is electrically part of a bridge circuit BR. The output of this bridge, which is proportional to the infra-red radiation from the plants, is connected to a differential amplifier AM which uses an integrated operational amplifier. An output from this drives a meterTto indicate plant temperature. Another output goes to a level detector LE consisting of another integrated operational amplifier connected afterthe manner of a Schmitt trigger circuit. This is provided with a control to set the level at which the transistor relay driver DR connected to it closes the relay R.The hysteresis, that is the temperature difference between which relay R closes and opens is also set by the design of LE. The relay R closes the primary circuit of the magnetron high tension supply transformer H. Full circuit details of this control arrangement and the supply and safety arrangementfollowfng are not described here since any parbsufarly cirnurtis not a necessary part of the invention and these detaiis will be obvious to an electrical engineer.

With reference to Figure 1,thedoorD is lined inside with metal mesh liner N arranged to give an effective seal againstmicrowavesescaping from the green house when the door is closed. When closed, the door is locked by a keyswitch so completing an electrical circuit. With reference to Figure 3, the electrical supply is switched on by the main switch M situated inside the g reenhouse. This connects the supply to the magnetron cooling motor BL,the magnetron heatertransformer LT and the power supply U forthe control unit. A lamp PWalso is lit to indicate that power is available.One supplyfrom the control power supply U is routed via the keyswitch and a number of safety switches in prominent places around the greenhouse, all switches K being in series, to the timer unit TI. This contains a timer circuit and relay. When the main switch is closed and all switches K are closed this supply is routed for a shorttimeto theaudiblewarningdeviceAW.Attheend ofthis time the relay in time TI removes this control supply from the warning device and applies itto the control unit CT. If temperature is below that set on the control unit, this unit will now cause relay R to close applying powerto the primary of the high tension supply transformer H and rectifier,forthe magnetron oscillator 0. Indicator lamps Rare also lit to indicate that radiation is present. The timer TI is reset when power is removed.

With the above arrangement unauthorized persons cannot enterthe greenhouse since it is locked, however the door can always be opened from the inside if necessary. When workers need to enter the N greenhousethedoorin unlocked, so cutting off the radiation,the door is opened and the key taken in by a worker. The main switch shou Id then be put off. On leaving the greenhouse the worker responsible for the key checks that all persons have leftthe green- house before switching on the power and closing and locking the door. Should an error occur so that someone is still in the greenhouse, the first warning will bethe power available lamp PW lighting and the sound of the magnetron cooling motors BL.When the door is closed and locked a loud warning will be given by the device AW, giving ample time for one of the safety switches K situated around the greenhouse to be broken. Sincethe radiation density is in any case rather low and unlikely to be harmful, these precau tions should be more than adequate.

A second embodiment of the invention is con cernedwith the heating of horticultural frames.

Referring to Figure 4the end E of such a frame is shown from the inside. The ends and sides of the frame are made of wood and lined with thin sheet aluminium. The sheet aluminium is bent outwards along the top edges of the ends and sides so asto lie flat covering these edges and then at the outside of these edges bent upwards so asto form a lip LI a few centimetres in height. The frame lights then locate inside this lip. The frame light or lights are covered overthe whoie of thei r bottom surfacewith metal oxide coated polyesterfilm, the glazing bars and glass being above the film. The object this arrangement is to seal the frame against the escape of microwaves when the frame lights are located inside the aluminium lip running around the top of the frame.When so located the end glazing bars ofthe frame lights bearthrough the coated film on to microswitches K1. Furthermicroswitches may be locatedsimilarlyattheotherend of the frame and partway along the sides. All microswitches are wired in series to close a circuit only when the frame lights are properly located so as to seal against the leakage of microwaves from the interior. Power to the frame is supplied through a buried cableY rising into a splashproof boxX containing the magnetron and associated apparatus for power, control and cooling.

A magnetron oscillatorworking on the 12 cm. band and with a power output of around 100 W can be used here. Radiation is from the horn antenna Al. The temperature is controlled by the dummy leaf temperature detector DP. This consists of a platinum film resistance thermometer 11, varnished and arranged between two layers of thin, dark coloured, wet, fibrous material, for example cardboard, to form a flat package, with the two layers offibrous material glued in contact with one another, exceptwheretheirinside faces are in contact one with each side ofthe flat substrate carrying the platinum film.The object of this arrangement is to obtain good thermal contact between the platinum film resistance thermometer and the fibrous material which for temperature control purposes takes the place of a leaf of a plant.

After moistening the fibrous material it is kept moist by being covered in a thin film of plastic which is transparentto light, infra-red radiation and microwaves and attached to a short cane which can be pushed into the soil to suitably locate the dummy leaf.

When the switch M1 located on the outside of the frame is switched on, and providing the frame lights are properly located so as to close all microswitches Kl ,the magnetron is controlled by a similar arrange menu to that shown by Figure 2 but with the control signal originating from the platinum resistance thermometer 11. When the temperature ofthis is belowthat required,the magnetron is caused to radiate into the frame and the radiation warning lamp W is lit. Ventilation oftheframewill not normally be required atthe same time as heating so can be accomplished in the normal manner by raising the frame lights or sliding them partway off.

Athird embodimentofthe invention concerns the heating of horticultural clochesforthe main purpose of preventing frost damage to plants inside them.

This may be done most conveniently by feeding powerto them from a magnetron located inside a frame heated as described above. With reference to Figure Swhich shows such an arrangement in plan view,the magnetron 01 is more powerful than that required to heat the frame FR alone. This is now largely heated by the waste heatfrom the magnetron and radio frequency energy is taken from the magnetron by low loss coaxial cables Y1 to a number of helix antennas A2 situated inside and at intervals along the cloches CL. In a convenient embodiment of the invention, the cloches are formed of rectangular frames of wood completely covered on one side with metal oxide coated polyester film, fixed by stapling to the wood.Onthe outer edge of the frame and in the centre of each side a small brass plate is attached, flush to the wood, having a portion protruding which is pierced buy a hole. With reference to Figure 6, these plates PLenabletwosuchframesto befastened together, by using a clip or piece ofwire, with the glazing film G1 on the inside, to form atentcloche.

The plates PL on the opposite sides are fixed to the ground with wire spikes SP. A continuous run of such cloches may be made byfastening them end to end, using the plates PL situated halfway up the cloches.

The cloche is completed at each end, by a triangular frame shaped to fit, glazed as the rectangular frames, with the glazing on the inside. The triangular frames also have similarfixing plates, placed in such positions so asto lie against the unused plates PL half way up the end rectangularframes. By attaching contiguous plates together therefore, a cloche is formed which has no gaps to allowthe escape of microwavesorinfra-red radiation. Frames are however easily separated to give access to the enclosed plants and to allow watering. The antennas A2 for the cloches are held in position by ground- spikes GS, with the feed cables Y1 passing underneath the end of the cloche and along to each antenna.Asimple method of controlling the magnetron is by using a soil or air thermostat in one of the cloches.

This invention may be exploited in horticulturefor the purposes offrost protection, enabling or increasing growth and ripening of plants or parts thereof. If the microwave generators are designed so that much higher radiation densities can be produced in smaller volumes, then they may have otheruses such as soil sterilization, deinfestation and crop drying. The invention may be particularly appropriate to islands in the far North or South having no fossil fuel but fairly constant wind and long hours of Summer sunshine. Wind generation could easily provide the moderate amounts of electricity needed, according to the invention,to enable greenhouse crops to be grown.

The invention may also be used in silviculture to raise tree seed lings and inagriculturetoprotectfield crops or even to provided warmth for livestock.

Claims (9)

1. Within any kind of horticultural greenhouse or any building or structure used for horticultural or agricultural purposes, including horticultural frames and cloches, a membrane or a mesh, both ofwhich are able to transmit light and reflect microwaves, ora contiguous combination of mesh and membrane, forming a single layer contiguousto the perimeter of an area of ground or floor so as to form over the ground orfloor an enclosed volume, in conjunction with apparatus for the generation of microwaves and their radiation within the said enclosed volume.

2. An enclosedvolumewith apparatus as claimed in claim 1 in which the enclosure is partlyformed by sheet metal or other electrically conducting material which is contiguous to the ground and the mesh or membrane which completes the enclosure.

3. An enclosed volume with apparatus as claimed inclaim 1 or claim 2 in which the membrane is also able to reflect infra-red rays.

4. An enclosed volume with arpparatus as claimed in claims 1,2 or3 in which the membrane forms the glazing of the building or structure.

5. Aspace not inside a building such as a piece of cultivated land, enclosed around its perimeter after the mannerofafenceorenclosed above and around its perimeter afterthe manner of a cage, by a mesh able to transmit light and reflect microwaves, in conjunction with apparatusforthe generation of microwaves and their radiation within the volume enclosed or partly enclosed by the mesh and the land within.

6. Within any enclosed or partly enclosed volume as claimed in any preceding claim, one or a plurality of membranes transparent to light, infra-red rays and microwaves, arranged so as to reduce convection currents in the airwithin the said volume.

7. A method of control for the apparatus as claimed in any preceding claim in which the microwave generators are controlled by the output from one or a plurality of infra-red sensors positioned so as to intercept infra-red radiation from items heated by microwaves within the enclosed volume.

8. A method of control forthe apparatus as claimed in any preceding claim in which the microwave generators are controlled by the output from one or a plurality of temperature sensors placed in contactwith or buried in plants heated by microwaves within the enclosed volume, or in which said sensors are placed in contact with or buried in material within the enclosed volume which imitates growing plants in its ability to absorb microwave energy and lose heat by radiation and convection.

9. A method of control for the apparatus as claimed in any preceding claim in which the microwave generators are controlled by the output of one or a plurality oftemperature sensors or thermostats buried in the soil or situated in the air within the enclosed volume.