GB2642323A

GB2642323A - Solar heating system

Info

Publication number: GB2642323A
Application number: GB2409573.9A
Authority: GB
Inventors: Squire John
Original assignee: Solar Steam Ltd
Current assignee: Solar Steam Ltd
Priority date: 2024-07-02
Filing date: 2024-07-02
Publication date: 2026-01-07
Also published as: GB202409573D0

Abstract

A solar heating system 1 comprises plural solar heating units 2 including a tubular receiver (5, fig 4) having an input and output side, a support assembly (7), and a lens array (3) focussing solar radiation onto the tubular receiver. The lens array is pivotally attached to the support assembly. The system also comprises a heating circuit 100 having a heat transfer fluid storage tank 104, heat exchanger 108, first (cold) piping network 102 connecting an output of the heat exchanger with an input of the storage tank and connecting an output of the storage tank to the input of each tubular receiver, and a second (hot) piping network 106 connecting the output of each tubular receiver to an input of the heat exchanger. At least one pump 110 circulates the heat transfer fluid through the storage tank, heat exchanger, first and second piping networks and tubular receivers. The lens arrays focus the solar radiation onto respective tubular receivers heating the transfer fluid in the tubular receivers, and the at least one pump pumps the heated transfer fluid to the heat exchanger and heat extracted to heat air. A blower 400 blows the heated air to a drying zone.

Description

[0001] SOLAR HEATING SYSTEM

[0002] TECHNICAL FIELD

[0003] The present invention relates to a solar heating system and a method for drying moist substances such as foodstuff ingredients

[0004] BACKGROUND TO THE INVENTION

[0005] The utilisation of thermal radiation from the sun, referred to herein as solar radiation, for heating is well known. For example, solar heating systems can be arranged to heat water in a tube for the purposes of producing steam and, subsequently used in various processes is generally known.

[0006] For example, W02010097637 (Boyle) discloses an apparatus with a Fresnel lens mounted on a frame for selectively directing solar radiation onto a thermosensitive member and a generator means for converting the resulting thermal expansion/contraction into electrical power. While expansion/contraction of the thermosensitive member is the main drive means for the generator, this document also describes an embodiment where sunlight is focussed through an array of Fresnel lenses onto a tube carrying water for the purposes of producing steam. The steam can be used for various purposes, for example the steam is capable of powering an electrical generator, and also for utilisation as a desalination process. It is also known to generate hot air for use in a drying step in a foodstuff manufacturing process. For example, when manufacturing tea it is known to dry leaves from tea plants (sometimes referred to as tea shrubs or tea trees) by generating hot air and exposing the leaves to the hot air. The hot air is typically heated by wood fired boilers. In one manufacturing facility, the preferred choice of fuel for the boilers is eucalyptus trees. The boilers used are very large, generating up to 9MW of power per hour and burning between 10 to 20 tonnes of trees per hour to generate steam and subsequently hot air. The eucalyptus trees take up a significant area of land, which could be given up to other purposes, such as growing more tea plants. Growing the eucalyptus trees takes up a significant amount of water from the local ecosystem, and cutting the trees down and transporting them to the factory generates a significant amount of CO2. Buring the eucalyptus trees in boilers generates further CO2 and adds other pollutants to atmosphere, which is undesirable The present invention seeks to improve upon the basic principles outlined in the prior art for the purposes of producing a solar heating system, which is particularly suited to heating air for use in drying processes.

[0007] SUMMARY OF THE INVENTION

[0008] The invention seeks to provide a solar heating system and a method for drying moist substances, such as at least one foodstuff ingredient, that mitigates at least one of the abovementioned problems, or at least provides an alternative system and method. In some embodiments the invention also seeks to provide a solar heating system that is arranged to generate hot air that can be used in a method for drying leaves, such as tea plant leaves, that mitigates at least one of the abovementioned problems.

[0009] According to one aspect of the invention there is provided a solar heating system according to claim 1. The invention is arranged to produce hot air and deliver the hot air to a drying zone in an efficient and cost-effective manner.

[0010] According to another aspect there is provided a solar heating system The solar heating system can include a plurality of solar heating units Each solar heating unit can include a tubular receiver having an input side and an output side. Each solar heating unit can include a support assembly, and Each solar heating unit can include a lens array having a plurality of lens assemblies arranged to focus, in use, solar radiation on to the tubular receiver. The lens array can be pivotally attached to the support assembly. The lens array can be arranged to pivot with respect to the support assembly about a first pivot axis; The solar heating system can include a heating circuit The heating circuit can include a storage tank arranged to store heat transfer fluid. The heating circuit can include a heat exchanger.

[0011] The heating circuit can include a first piping network connecting an output side of the heat exchanger with an input side of the storage tank and connecting an output side of the storage tank to the input side of each tubular receiver. The input side each tubular receiver can be directly connected to the first piping network. That is, the input side of each tubular receiver can be connected in parallel to the first piping network such that no tubular receivers are connected together in series.

[0012] The heating circuit can include a second piping network arranged to connect the output side of each tubular receiver to an input side of the heat exchanger. The output side each tubular receiver can be directly connected to the second piping network. That is, the output side of each tubular receiver can be connected in parallel to the second piping network such that no tubular receivers are connected together in series.

[0013] The heating circuit can include at least one pump arranged to selectively circulate heat transfer fluid through the storage tank, heat exchanger, first and second piping networks and tubular receivers In use, the lenses can be arranged to focus solar radiation on to their respective tubular receivers to heat the transfer fluid in the tubular receivers.

[0014] The at least one pump can be arranged to pump the heated transfer fluid to the input side of the heat exchanger.

[0015] The heat exchanger can be arranged to extract heat from the heat transfer fluid and to heat air using said heat.

[0016] The heating system can include a blower arranged to blow the heated air to a drying zone. In some embodiments the air is blown into ducting and the ducting directs the heated air to the drying zone The blower can include at least one fan, and preferably a plurality of fans. The heat exchanger can include a radiator through which, in use, the heat transfer fluid flows and the fan can be arranged to blow air over the radiator, thereby heating the air. The radiator can comprise at least one pipe, and preferably a plurality of pipes, containing the heat transfer fluid, and the fan can be arranged to, in use, blow air over the pipe(s), thereby heating the air. The heat exchanger can include a least one heat transfer formation arranged to promote heat transfer from the heat transfer fluid to the air, for example the heat exchanger can include at least one fin, and preferably a plurality of fins, and the fan is arranged to blow air over the at least one fin, thereby heating the air. The fins can be mounted on the radiator, for example the fins can be mounted on the at least one pipe.

[0017] The heat exchanger can include at least one radiator. At least one fan can be arranged to blow air over the at least one radiator thereby heating the air.

[0018] The system can include at least one temperature sensor arranged to monitor the temperature of the heat transfer fluid in the heating circuit. For example, the system can include a temperature sensor that is arranged to monitor the temperature of the heat transfer fluid in the storage tank. The system can include a temperature sensor that is arranged to monitor the temperature of the heat transfer fluid at the heat exchanger.

[0019] The system can include a controller, such as an electronic controller, arranged to control operation of the heating circuit.

[0020] The controller can be arranged to receive signals from the at least one temperature sensor. The controller can be arranged to control operation of the at least one pump in response to signals received from the at least one temperature sensor to control the flow rate of heat transfer fluid through the tubular receivers, thereby controlling an operating temperature of the heat exchanger. The control signals can be based on any one of the temperature sensors alone, or any combination of the temperature sensors.

[0021] The heat exchanger can be arranged to heat air to a temperature Tair. Tair can be greater than or equal to 50C, greater than or equal to 55, greater than or equal to 60C. Tair can be less than or equal to 130C, less than or equal to 125C, less than or equal to 120C.

[0022] The controller can be arranged to control the flow rate of heat transfer fluid through the tubular receivers such that the flow rate is greater than or equal to 2 litres per minute, greater than or equal to 3 litres per minute, greater than or equal to 4 litres per minute The controller can be arranged to control the flow rate of heat transfer fluid through the tubular receivers such that the flow rate is wherein the flow rate of heat transfer fluid through the tubular receivers is less than or equal to 15 litres per minute, less than or equal to 14 litres per minute, less than or equal to 13 litres per minute, less than or equal to 12 litres per minute.

[0023] The system can include at least one flow meter arranged to monitor the flow rate of heat transfer fluid within the heating circuit, for example the flow rate of heat transfer fluid through the tubular receivers.

[0024] The controller can be arranged to receive signals from the at least one flow meter. The controller can be arranged to control operation of the at least one pump in response to signals received.

[0025] The heating system can include a drive system arranged to adjust the orientation of the lens assembly by pivoting the lens array about the first pivot axis. The drive system can comprise a linear actuator. The drive system can comprise a slewing ring. The drive system can comprise an electric motor. In some embodiments the electric motor can be connected to a transmission system.

[0026] The first pivot axis can be a generally horizontal axis, for example when the solar heating unit is mounted on flat horizontal ground. The horizontal axis can be sometimes referred to as a roll axis. This enables the lens array to adjust its orientation to accommodate changes in elevation of the sun In some embodiments, the first axis arranged parallel with a central longitudinal axis of the tubular receiver. In some embodiments, the first pivot axis can be coaxial with a central longitudinal axis of the tubular receiver.

[0027] The heating system can include n solar heating units, wherein n can be greater than or equal to 5, greater than or equal to 10, greater than or equal to 20, greater than or equal to 30, greater than or equal to 40, or greater than or equal to 50 The heating system can include n solar heating units, wherein n can be less than or equal to 100, less than or equal to 90, less than or equal to 80, less than or equal to 70, or less than or equal to 60. In some embodiments the output of the solar heating system can be in excess of 1MW. In some embodiments the storage tank can have a 10,000 litre capacity.

[0028] The heat exchanger can comprise an assembly of a plurality of heat exchanger units. In some embodiments the heat exchanger units can be arranged in series. Each heat exchanger unit can be arranged to remove some heat from the heat transfer fluid to heat the air.

[0029] The heating system can include ducting to guide air from the heat exchanger to the drying zone.

[0030] The heating system can include a second heat exchanger, for example located between the heat exchanger and the storage tank.

[0031] The heating system can include a cooling circuit.

[0032] The cooling circuit can include a second storage tank arranged to store a second heat transfer fluid.

[0033] The cooling circuit can include a first conduit connecting an output side of the second storage tank to an input side of the second heat exchanger.

[0034] The cooling circuit can include a second conduit connecting an output side of the second heat exchanger to an input side of the second storage tank.

[0035] The cooling circuit can include a pump arranged do circulate the second heat transfer fluid through the first conduit, the second heat exchanger, the second conduit and the second storage tank.

[0036] In use, the second heat transfer fluid has a lower temperature than the heat transfer fluid, and therefore can extract heat from the heat transfer fluid in the heating circuit.

[0037] The cooling circuit can include at least one temperature sensor arranged to monitor the temperature of the second heat transfer fluid in the cooling circuit, and the controller can be arranged to receive signals from the at least one temperature sensor. In some embodiments the controller can be arranged to control operation of the cooling circuit pump in response to signals received from the cooling circuit temperature sensor.

[0038] The tubular receiver can include a tubular absorber having an input side and an output side. The tubular receiver can include an optically transparent casing housing at least part of the tubular absorber. The tubular receiver can include a space between the optically transparent casing and the tubular absorber, wherein air can be evacuated from said space to create a vacuum. A receiver arranged in this manner has been found to have an efficiency of around 98.4%. The optically transparent casing helps to prevent light escaping. The optically transparent casing can be coaxial with the tubular absorber. The casing can be made from glass. The casing can have an ant reflective coating to increase solar transmittance. In some embodiments the tubular receiver can be mounted on the support assembly below the lens array. In some embodiments the tubular receiver can be arranged generally horizontally, for example when the solar heating unit is mounted on flat horizontal ground.

[0039] The tubular absorber can be metallic, and can be preferably made from steel. The tubular absorber can be coated with an absorber coating to minimise heat loss, for example infrared heat loss. The coating can comprise a nano-coating. Nano-coatings can be particularly effective at reducing heat loss. The coating can be a dark colour, such as black.

[0040] The heating system can include an annular member mounted on the tubular absorber co-axially therewith, the annular member can be arranged to support a first end of the optically transparent casing The annular member can include a first annular part sealably attached to the tubular absorber, a second annular part sealably attached to the optically transparent casing, and an expandable member sealably attached to the first and second annular parts. The expandable flexible member can have a folded concertina arrangement. The expandable member accounts for

[0041] S

[0042] differences in thermal expansion between the tubular receiver and the optically transparent casing. In some embodiments the system can include an elongate reflector mounted parallel with the tubular receiver to redirect sunlight towards the tubular receiver.

[0043] The tubular receiver can include a second annular member mounted on the tubular receiver co-axially therewith. The second annular member can be arranged to support a second end of the optically transparent casing. The second annual member can include a first annular part sealably attached to the tubular absorber. The second annual member can include a second annular part sealably attached to the optically transparent casing. The tubular receiver can include an expandable flexible member folded in a concertina arrangement. The system can include an elongate reflector mounted parallel with the tubular receiver on a side of the tubular receiver that can be opposite to the side facing towards the lens array. The reflector redirects light towards the tubular receiver.

[0044] The lens array can include at least one lens assembly, and preferably at least one Fresnel lens assembly. The lens assembly can include a supporting substrate. The lens assembly can include a film applied to a surface of the supporting substrate, the film having at least one lens formed therein. The lens array can include a plurality of lens assemblies, and preferably a plurality of Fresnel lens assemblies. In some embodiments, the lens array can include m lens assemblies, wherein m is in the range 8 to 50.

[0045] The film can include a plastics material, and preferably a thermoplastics such as polymethyl methacrylate (PM1v4A).

[0046] The film can have a thickness in the range 30 to 250 microns. The lens can be formed in the film by a casting drum.

[0047] The film can include at least one Fresnel lens formed therein. In some embodiments, the film can include a plurality of Fresnel lenses formed therein The supporting substrate can comprise a plastics material, and preferably a thermoplastics such as polymethyl methacrylate (PMMA), thermoplastics polyurethane (TPU), polyethylene terephthalate (PET) or polycarbonates (PC) In some preferred embodiments the substrate is made from PMMA In some preferred embodiments the substrate is made from polycarbonate. The substrate typically has a thickness in the range 3mm to omm. In some embodiments the thickness is around 4mm.

[0048] The tubular receiver can be located at a focal length of the at least one lens assembly. In some embodiments the lens array can include at least one lens assembly having a first focal length. In some embodiments the lens array can include at least one lens assembly having a second focal length. In some embodiments the lens array can include at least one lens assembly having a third focal length. Having different focal lengths helps to ensure that sunlight can be focused on to the tubular receiver. In some embodiments each lens assembly has a focal length FL, wherein FL can be in the range 1m to 2m, preferably 1.2m to 1.9m, and more preferably still 1.4m to 1.8m.

[0049] The lens array can include a first part can comprise a plurality of lens assemblies, wherein the lens assemblies can be arranged in a flat array. The lens array can include a second part, which can be inclined downwards from the first part. For example, a first row of lens assemblies that can be inclined downwards from the flat array of lens assemblies. The lens array can include a third part, which can be inclined downwards from the first part. For example, a second row of lens assemblies that can be inclined downwards from the flat array of lens assemblies. The first row of lens assemblies can abut a first edge of the flat array of lens assemblies. The second row of lens assemblies can abut a second edge of the flat array of lens assemblies. The first edge can be located opposite to the second edge. The inclined lens assemblies help to direct light on to the tubular receiver when the lens array can be pivoted to some operational orientations.

[0050] The system of any one of the preceding claims, wherein the support assembly can include at least one support post, and preferably a plurality of support posts.

[0051] The support assembly can include a frame having a first part and a second part. The first part supports the lens array. The first part can be mounted on the second part and can be pivotally attached thereto. In some embodiments the articulation of the first part with respect to the second part provides the first pivot axis. Optionally, the first part of the frame can include a counter weight to balance the first part of the frame.

[0052] The second part can be arranged to support the tubular receiver. The first part of the frame can be arranged to pivot through an angle of approximately 90 degrees. In some embodiments the first part of the frame can be arranged to pivot through an angle of approximately +90 degrees. The first part of the frame can be arranged to pivot between a first position wherein the lens assembly is in a horizontal orientation and a second position wherein the lens assembly is in a vertical orientation, for example when the solar heating unit is mounted on flat horizontal ground. The first part of the frame can be arranged to move to a least one intermediate position that can be intermediate between the first and second positions.

[0053] At least some of the plurality of solar heater units can be grouped together to form a first solar heater assembly. The first solar heater assembly can include a first base and each solar heater unit in the first solar heater assembly can be mounted on to the first base. The first base can be arranged to pivot about a second pivot axis. The solar heating units can be arranged in an array on the first base.

[0054] The second pivot axis can be arranged perpendicular to the first pivot axis.

[0055] The second pivot axis can be a vertical axis, for example when the solar heating unit is mounted on flat horizontal ground. The vertical pivot axis enables the system to track the sun from east to west. The second pivot axis can be sometimes referred to as a yaw axis The first base can be mounted on roller elements, such as castors and/or wheels. The roller elements can be mounted on a track, for example a circular track.

[0056] The solar heater assembly can include a second drive system. The second drive system can be arranged to pivot the base, and hence each of the solar heater units in the first solar heater assembly, about the second pivot axis. The second drive system can include a slewing ring and a slewing ring post. The second drive system can be arranged to drive the roller elements along the track in clockwise and anticlockwise directions.

[0057] The first solar heater assembly can include any practicable number of solar heating units.

[0058] II

[0059] The first solar heater assembly can include n solar heating units, wherein n can be greater than or equal to 2, greater than or equal to 3, greater than or equal to 4, greater than or equal to 5, or greater than or equal to 6, greater than or equal to 7, or greater than or equal to 8.

[0060] The solar heater assembly can include n solar heating units, wherein n can be less than or equal to 15, less than or equal to 14, less than or equal to 13, less than or equal to 12, less than or equal to 11, less than or equal to 10.

[0061] The first base can include an aperture and conduits, such as pipes or flexible hoses, which connect the input side of the tubular receivers to the cold side pipe network and the output side of the tubular receivers to the hot side pipe network, pass through the aperture.

[0062] The vertical height of some of the solar heating units can be greater than the vertical heights of others solar heating units. For example, a first row of solar heating units can have a first vertical height. A second row of solar heating units can have a second vertical height, wherein the second vertical height can be larger than the first vertical height. A third row of solar heating units can have a third vertical height, wherein the third vertical height can be larger than the second vertical height. In some embodiments the support assembly for second row of solar heating units can have a larger vertical height than the support assembly in the first row of solar heating units. In some embodiments the support assembly for third row of solar heating units can have a larger vertical height than the support assembly in the second row of solar heating units. In some embodiments the second part of the support frame for second row of solar heating units can have a larger vertical height than the second part of the support frame in the first row of solar heating units. In some embodiments the second part of the support frame for the third row of solar heating units can have a larger vertical height than the second part of the support frame in the second row of solar heating units.

[0063] At least some of the plurality of solar heater units can be grouped together to form a second solar heater assembly. The second solar heater assembly can include a second base and each solar heater unit in the second solar heater assembly can be mounted on to the second base. The second base can be arranged to pivot about a third pivot axis, which is preferably a vertical pivot axis, for example when the solar heating unit is mounted on flat horizontal ground. The solar heating units can be arranged in an array on the second base. The second solar heating assembly can be arranged similarly to the first solar heating assembly.

[0064] At least some of the plurality of solar heater units can be grouped together to form a further solar heater assembly. The further solar heater assembly can include a further base and each solar heater unit in the further solar heater assembly can be mounted on to the further base. The further base can be arranged to pivot about a third pivot axis, which is preferably a vertical pivot axis, for example when the solar heating unit is mounted on flat horizontal ground. The solar heating units can be arranged in an array on the further base. The further solar heating assembly can be arranged similarly to the first solar heating assembly.

[0065] The heating system can include any practicable number of solar heating assemblies For example, the solar heating system can include n solar heating assemblies wherein n is greater than or equal to 2, greater than or equal to 3, greater than or equal to 4, greater than or equal to 5, greater than or equal to 6, greater than or equal to 7, greater than or equal to 8.

[0066] The heating system can include any practicable number of solar heating assemblies For example, the solar heating system can include n solar heating assemblies wherein n is less than or equal to 15, less than or equal to 14, less than or equal to 13, less than or equal to 12, less than or equal to 11, less than or equal to 10 The solar heating system can include a tracking system arranged to automatically adjust the orientation of the lens array about the first pivot axis, and optionally about the second pivot axis. The tracking system can be arranged to automatically operate the drive system to adjust the orientation of the lens array about the first pivot axis. In some embodiments the tracking system can be arranged to automatically operate the second drive system to adjust the orientation of the lens array about the second pivot axis.

[0067] The tracking system can be arranged to automatically adjust the orientation of the lens array to keep the lenses facing towards the sun. This helps to maximise the amount of light focused on the tubular receiver.

[0068] The tracking system can comprise an open-loop arrangement.

[0069] The tracking system can include an astronomical algorithm and a real-time clock to control the orientation of the lens array. The astronomical algorithm can be arranged to calculate the correct position of the sun in the sky at any given point in time. The real-time clock can be used to determine the calendar date and time of day. The tracking system can be arranged to use an output signal from the real-time clock to enable the astronomical algorithm to determine the correct position of the sun in the sky for the date and time indicated by the output signal from the real-time clock.

[0070] The tracking system can be arranged to automatically adjust at least one of the linear actuator, electrical motor and the slewing ring to adjust the orientation of the lens array. The tracking system can be arranged to automatically operate at least one of the linear actuator, electric motor and the slewing ring to keep the lens array facing towards the sun. The tracking system can be arranged to automatically adjust at least one of the linear actuator and the slewing ring to adjust the orientation of the lens array in response to the tracking system calculating at least one of the solar azimuth and zenith angles of the sun.

[0071] The solar heating system can include at least one limit switch arranged to limit movement of the linear actuator; and/or at least one limit switch arranged to limit movement of the slewing ring The tracking system can include a wind protection mode. The tracking system can include wind speed measurement device, and can be arranged to move the lens assembly in response to an output signal from wind speed measurement device. The tracking system can be arranged to move the lens assembly to a safe orientation in response to receiving a signal from the wind speed measurement means that indicates that the wind speed can be greater than or equal to a threshold value. Typically the safe orientation can be a horizontal orientation, for example when the solar heating unit is mounted on flat horizontal ground. When the lens assembly can be in the horizontal orientation, typically the first part of the frame can be in a vertical orientation.

[0072] The controller can be arranged to send control signals to the tracking system to adjust the position of the lens array in response to receipt of a signal from the at least one of the temperature sensor indicating that the temperature has reached a threshold value. The tracking system can be arranged to adjust the position of the lens array in response to receipt of the control signal from the controller. The tracking system can be arranged to move the lens array to a position that decreases the amount of solar energy directed on to the receiver in response to determining that the temperature has reached the threshold value, and preferably the tracking system can be arranged to move the lens array to a position wherein the solar array faces away from the sun.

[0073] According to another aspect there is provided a method according to claim 38 According to another aspect there is provided a method for drying at least one foodstuff ingredient. The method can include providing a solar heating system according to any configuration described herein. The method can include heating the heat transfer fluid in the tubular receivers by focusing sunlight on to the tubular receivers. The method can include pumping the heated heat transfer fluid to the input side of the heat exchanger. The method can include the heat exchanger extracting heat from the heat transfer fluid and heating air adjacent to the heat exchanger using said heat. The method can include blowing heated air to a drying zone having at least one foodstuff ingredient, thereby drying the at least one foodstuff ingredient.

[0074] According to another aspect there is provided a method for drying leaves from a tree plant, including providing a solar heating system according to any one of the preceding claims; heating the heat transfer fluid in the tubular receivers by focusing sunlight on to the tubular receivers; pumping the heated heat transfer fluid to the input side of the heat exchanger the heat exchanger extracting heat from the heat transfer fluid and heating air adjacent to the heat exchanger using said heat; blowing heated air to a drying zone having leaves from tea plants, thereby drying the leaves.

[0075] According to another aspect there is provided a method for drying at least one moist substance. The method can include providing a solar heating system according to any configuration described herein. The method can include heating the heat transfer fluid in the tubular receivers by focusing sunlight on to the tubular receivers. The method can include pumping the heated heat transfer fluid to the input side of the heat exchanger. The method can include the heat exchanger extracting heat from the heat transfer fluid and heating air adjacent to the heat exchanger using said heat. The method can include blowing heated air to a drying zone having at least one moist substance, thereby drying the at least one moist substance.

[0076] According to another aspect there is provided a solar heating system according to claim 4L The solar heating system according to this aspect can include any solar heating system feature described herein.

[0077] BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Embodiments of the invention will be described with reference to the following drawings by way of example only: Figure 1 is a diagrammatic representation of a solar heating system according to a first embodiment of the invention, the solar heating system having a multiple solar heating units and a fluid heating circuit heated by the solar heating units, the heating circuit including a heat exchanger arranged to heat air; Figure 2 is a simplified diagrammatic representation of the heating circuit shown in Figure 1, together with an optional cooling circuit arranged to cool the fluid in the heating circuit; Figure 3 is a diagrammatic representation of a control system, which is arranged to control circulation of the fluid within the heating circuit, and optionally the cooling circuit; Figure 4 is an isometric view of an individual solar heating unit used in the solar heating system of Figure 1, the solar heating unit having a lens array and a tubular receiver; Figure 5 is a plan view from above to the solar heating unit of Figure 4; Figure 6 is an end view of the solar heating unit of Figure 4, Figure 7 is a front side view of the solar heating unit of Figure 4; Figure 8 is cross-sectional view and an end view of a drive system used to adjust the orientation of the lens array; Figure 9 is an enlarged sectional view of part of the tubular receiver; Figure 10 is an enlarged diagrammatic view of a part of a lens included in the lens array of Figure 4; Figure 11 is a schematic view of part of the lens array of Figure 4; Figure 12 are side, top and end views of the heat exchanger used in heating circuit of the embodiment of Figure 1; Figure 13 is a diagrammatic view of a solar heating system according to a second embodiment of the invention, the solar heating system having a plurality of solar heating assemblies, each solar heating assembly including a plurality of solar heating units mounted on a track, and a fluid heating circuit heated by the solar heating units, the heating circuit including a heat exchanger arranged to heat air; Figure 14 is an isometric view of a solar heater assembly used in the embodiment of Figure 13; Figures 15 is an isometric view of part of the solar heater assembly of Figure 14; Figure 16 is a side view of the solar heater assembly of Figure 14; Figure 17 is a front view of the solar heater assembly of Figure 14; Figure 18 is a plan view of the solar heater assembly of Figure 14; Figure 19 is cross-sectional view of part of the circular track used in the solar heater assembly of Figure 14; Figure 20 is an isometric view of a stewing ring post; Figure 21 is plan view of a slewing ring, which is used to move the solar heater units along the circular track, thereby rotating the solar heater units about a vertical axis; Figure 22 is an isometric view of an individual solar heater unit used in the solar heater assembly of Figure 14, the solar heater unit having a lens array; Figure 23 is a side view of the solar heater of Figure 22; Figures 24 to 16 is a front view of the solar heater unit of Figure 22; Figure 25 is a view of a linear actuator that is arranged to pivot the lens array of the solar heater unit of Figure 14 about a horizontal pivot axis; and Figure 26 is a control system for tracking movement of the sun and automatically adjusting the orientation of the solar heater units about the vertical axis and each lens array about its respective the horizontal axis.

[0079] DETAILED DESCRIPTION OF THE INVENTION

[0080] Figure 1 shows diagrammatically a solar heating system 1 in accordance with a first embodiment of the invention.

[0081] The solar heating system 1 includes multiple solar heating units 2 and a heating circuit 100 Each solar heating unit 2 includes a lens array 3, a tubular receiver 5 arranged to receive solar energy from the lens array 3, and a support frame 7. The heating circuit 100 includes a heat transfer fluid, a storage tank 104 for storing the heat transfer fluid, the tubular receivers 5, a heat exchanger 108 system for generating hot air, a first (cold) side pipeline network 102 arranged to supply heat transfer fluid from the storage tank 104 to the tubular receivers 5, and a second (hot) side pipeline network 106 arranged to supply heated heat transfer fluid from the receivers 5 to the heat exchanger system 108.

[0082] The tubular receiver 5 of each solar heating unit 2 has an input side and an output side. The cold side pipeline network 102 connects the storage tank 104 to the input side of each tubular receiver 5. The hot side pipeline network 106 connects the output side of each tubular receiver 5 to the heat exchanger system 108. The cold side pipeline network 102 is arranged supply the heat transfer fluid from the storage tank 104 at relatively cold temperature to the input side of each tubular receiver 5. The heat transfer fluid in the tubular receiver 5 is heated by solar radiation directed on to the tubular receiver 5 by the lens array 3. The heated transfer fluid exits the output side of the tubular receiver at a relatively higher temperature than received at the input side. The hot side pipeline network 106 is arranged to supply the heated transfer fluid from the output side of each tubular receiver 5 to the heat exchanger system 108.

[0083] Typically, each tubular receiver 5 is connected to the cold side pipeline network 102 in parallel. That is, the cold side pipeline network 102 and tubular receivers 5 are arranged such that relatively cold heat transfer fluid is supplied to each tubular receiver 5 from the storage tank 104 without having passed through another tubular receiver 5. In this arrangement, each tubular receiver 5 receives the heat transfer fluid at approximately the same temperature, save for any temperature variations as the transfer fluid travels through different parts of the cold side pipeline network 102.

[0084] Typically, each tubular receiver 5 is connected to the hot side pipeline network 106 in parallel. That is, the hot side pipeline network 106 and the tubular receivers 5 are arranged such that relatively hot heat transfer fluid is supplied from each tubular receiver 5 to the hot side pipeline network 106 without having passed through another tubular receiver 5. In this arrangement, the heat transfer fluid supplied to the tubular receiver 5 of a particular solar heating unit 2 is heated by the lens array 3 of that solar heating unit 2 only. This helps to ensure that the heat transfer fluid remains within its safe operating temperature, while at the same time heating a large volume of the heat transfer fluid The solar heating system 1 includes multiple solar heating units 2. Any practicable number of solar heating units 2 can be included. For example, the solar heating system 1 can include n solar heating units, wherein n is in the range 10 to 100 The solar heating units 2 are typically arranged in an array. Figure 1 shows 60 solar heating units 2 in a 6 by 10 array to illustrate the arrangement. It can be seen from Figure 1 that each row R1, R2... R6 of solar heating units 2 has a respective cold side pipe 102-1, 102-2...102-6, and for each solar heating unit 2 in a given row, the input side of the tubular receiver 5 is connected to the respective cold side pipe 102-1, 102-2...102-6. For example, for each solar heating unit 2 in the first row R1, the input side of the tubular receiver 5 is connected to cold side pipe 102-1, for each solar heating unit 2 in the second row R2, the input side of the tubular receiver 5 is connected to cold side pipe 102-2, and so on. It can be seen from Figure 1 that each column Cl, C2...0 1 0 of solar heating units 2 has a respective hot side pipe 106-1, 106-2...106-6, and for each solar heating unit 2 in a given column, the output side of the tubular receiver 5 is connected to the respective hot side pipe 106-1, 106-2...106-10. For example, for each solar heating unit 2 in the first column Cl, the output side of the tubular receiver 5 is connected to hot side pipe 106-1, for each solar heating unit 2 in the second column C2, the output side of the tubular receiver 5 is connected to hot side pipe 106-2, and so on. It will be appreciated that in some embodiments the cold side pipes 102-1...102-6 can be arranged to supply relatively cold heat transfer fluid to columns of solar heating units 2, and the hot side pipes 106-1...106-10 can be arranged to receive heat transfer fluid from the output sides of rows of solar heating units 2.

[0085] If additional solar heating capacity is required, either further solar heating units 2 can be added to the array or, if desirable, a further array of solar heating units can be assembled, having its own separate heating circuit 100.

[0086] The heating circuit 100 can include at least one pump to circulate heat transfer fluid within the circuit 100. For example, the heating circuit 100 can include at least one pump 110 arranged to pump heat transfer fluid from the storage tank 104 to the tubular receivers 5. The heating circuit 100 can include at least one pump 110 arranged to pump heat transfer fluid from the tubular receivers 5 to heat exchanger system 108. In some embodiments, the same pump 110 can be arranged to pump heat transfer fluid from the storage tank 104 to the tubular receivers 5 and from the tubular receivers 5 to the heat exchanger system 108. In some embodiments, the pumps 110 can be arranged in series. In some embodiments the pumps can be arranged in parallel. In some embodiments the heating circuit 100 can include a pair of pumps arranged in parallel and at least one pump arranged in series with the pair of pumps arranged in parallel.

[0087] The heating circuit 100 can include at least one valve to control the flow of fluid within the circuit 100. For example, the heating circuit can include at least on non-return valve 112 to control the direction of flow. The heating circuit 100 can include at least gate valve 114, which can be used for example to isolate heating circuit 100 components, such as the pumps 110. The cold side piping network 102 can include a non-return valve 112 located between the heating transfer fluid storage tank 4 and the receivers 5. For example, the cold side piping network 102 can include the storage tank 4, the non-return valve 112, the pump 110 and the receivers 5 arranged in series, in that order. The hot side piping network 106 can include a non-return valve 112 located between the receivers 5 and the heat exchanger system 108. For example, the hot side piping network 106 can include the receivers 5, the non-return valve 112, a pump 110 and the heat exchanger system 108, in that order. In some embodiments, the hot side piping network 106 can include the receivers 5, a gate valve 114, a junction splitting the flow along first and second paths, each of the first and second paths having a gate valve 114, a pump 110, a gate valve 114, and a non-return valve 112, followed by a junction combining the first and second paths together, a gate valve 114 and the heat exchanger system 108, in that order. The gate valves 114 enables the pumps 110 to be isolated, for example for the purposes of cleaning, maintenance and replacement.

[0088] The inventors have determined that a hydrocarbon based heat transfer fluid, which is designed for use in a liquid phase, such as cumene, is particularly suited for use as the heat transfer fluid in the heating circuit 100. Cumene is commercially available and is sold by one supplier under the trade name Marlothenn0 XC. Cumene has excellent thermal stability, and while it has a boiling point of 152C, it has a bulk temperature of 300C. Cumene has an operating temperature of -90C to 300C.

[0089] The heat exchanger system 108 is arranged to remove heat from the heat transfer fluid and to use that heat to heat air for use in a downstream process, such as a drying process. The heat exchanger system 108 removes a significant amount of heat from the transfer fluid. For example, a typical input temperature of the heat transfer fluid to the heat exchanger system 108 can be around 185C, whereas a typical output temperature for the heat transfer fluid is around 50C. The heat exchanger system 108 includes at least one heat exchange unit 108-1 (see Figure 12), and preferably a plurality of heat exchanger units. The plurality of heat exchanger units can be arranged in series, with the heat exchanger fluid passing through each heat exchanger unit in turn, after which the heat transfer fluid is returned to the storage tank 104. Each heat exchanger unit removes some heat from the transfer fluid which is used to heat air. For example, in one embodiment the heat exchanger system 108 includes a first heat exchanger unit 108-1 and a second heat exchanger unit arranged in series with one another. In another embodiment the heat exchanger system 108 includes first, second and third heat exchange units arranged in series with one another. In another embodiment the heat exchanger system 108 includes first, second, third, fourth and fifth heat exchanger units arranged in series. For example, the first heat exchanger unit can be arranged to receive at input side the heat transfer fluid transfer at a temperature of around 185C. The transfer fluid exits the output side first heat exchanger unit at a temperature of around 160C, and enters the input side of the second heat exchanger unit. The transfer fluid exits the output side of the second heat exchanger unit at a temperature of around 140C, and enters the input side of the third heat exchanger unit. The transfer fluid exits the output side of the third heat exchanger unit at a temperature of around 120C, and enters the input side of the fourth heat exchanger unit. The transfer fluid exits the output side of the fourth heat exchanger unit at a temperature of around 90C, and enters the input side of the fifth heat exchanger unit. The transfer fluid exits the output side of the fifth heat exchanger unit at a temperature of around 50C, and returns to the cold side pipe network 102.

[0090] The heat exchanger system 108 is mounted adjacent one or more blowers 400, each of which includes at least one fan arranged to blow the heated air to the location of the downstream process. For example, the blower 400 can be arranged to blow the heated air along ducting to a drying room, which stores the moist substance to be dried. In some embodiments, the hot air is used to dry a foodstuff ingredient such as leaves from tea plants. The heat exchanger 108 can include a radiator through which, in use, the heat transfer fluid flows and the blower 400 can be arranged to blow air over the radiator, thereby heating the air. The radiator can comprise at least one pipe, and preferably a plurality of pipes, containing the heat transfer fluid, and the blower 400 can be arranged to, in use, blow air over the pipe(s), thereby heating the air. The heat exchanger 108 can include a least one heat transfer formation arranged to promote heat transfer from the heat transfer fluid to the air, for example the heat exchanger 108 can include at least one fin, and preferably a plurality of fins, and the blower 400 is arranged to blow air over the at least one fin, thereby heating the air. The fins can be mounted on the radiator, for example the fins can be mounted on the at least one pipe.

[0091] The heating circuit 100 can be monitored by at least one temperature sensor TS1,TS2. For example, the heating circuit 100 can include a temperature sensor TS1 that is arranged to monitor the temperature of the heat transfer fluid in the storage tank 104. The heating circuit 100 can include a temperature sensor TS2 that is arranged to monitor the temperature of the heat transfer fluid at the heat exchanger system 108.This helps to determine the stability of the heating circuit 100, in particular to determine if there is a temperature differential between the cold and hot sides of the circuit 102,106. The temperature sensors TS1,TS2 are connected to an electronic controller 200. The electronic controller 200 is arranged to adjust the flow rate of heat transfer fluid within the heating circuit 100 in response to signals received from the temperatures sensors TS1,TS2. For example, the electronic controller 200 can be arranged to control operation of at least one circuit pump 110 in response to signals received from the temperatures sensors TS1,TS2. This enables the electronic controller 200 to control the temperature at which the heat exchanger 108 heats the air, and can help prevent the system from overheating. For example, in some embodiments it is desirable for the heat exchanger 108 to heat the air to a temperature Tat, within a range of 60C to 120C. When it is required to complete dry a moist sub stance, such as tea plant leaves, the electronic controller 200 can be arranged to operate the heat exchanger 108 at temperature in the range 100C to 120C. Whereas when it is required to undertake a willowing process, the electronic control 200 can be arranged to operate the heat exchanger 108 at temperature in the range 60C to 70C. The electronic controller 200 controls the operating temperature of the heat exchanger 200 by controlling operation of the pump 110 in order to control the flow rate at which heat transfer fluid passes through the tubular receivers 5. The slower the flow rate of fluid passing through the tubular receivers 5, the more the heat transfer fluid will heat up and the greater the operating temperature of the heat exchanger 108. Likewise, the faster the flow rate of fluid passing through the tubular receivers 5, the less the heat transfer fluid will heat up and the lower the operating temperature of the heat exchanger 108. In some embodiments, to achieve a heat exchanger 108 operating temperature of around 60C the flow rate of heat transfer fluid through the tubular receivers should be around 12 litres per minute. In some embodiments, to achieve a heat exchanger 108 operating temperature of around 120C the flow rate of heat transfer fluid through the tubular receivers should be around 4 litres per minute.

[0092] In some embodiments, the heating circuit 100 can include at least one flow meter, which is arranged to measure the flow rate of heat transfer fluid within the heating circuit. In some embodiments, the flow meter is located adjacent to the tubular receivers 5, either upstream or downstream of the tubular receivers. In some embodiments the heating circuit can include a plurality of flow meters that are arranged to measure the flow rate of heat transfer fluid within the heating circuit. Signals from the flow meters can be provided to the electronic controller 200. The electronic controller 200 can be arranged to control operation of the pumps 110 in response to signals received from the flow meters.

[0093] Optionally, the heating circuit can include at least one controllable valve 116 and the electronic controller 200 can be arranged to control operation of the or each controllable valve 116 in response signals received from the temperature sensors TS1,TS2. For example, the valves can be used to help control the flow rate of fluid within the circuit. Additionally, or alternatively, controllable valves 116 can be used to isolate one or more solar heating units 2 from the circuit temporarily as a way of reducing the solar heating effect. For example, an entire row and/or column of solar heating units 2 can be isolated from the heating circuit 100 temporarily.

[0094] Optionally, each solar heating unit 2 can include a drive system 300 that is arranged to adjust the orientation of the of the lens array 3, thereby adjusting the amount of solar radiation that is incident on tubular receiver 5. The electronic controller 200 can be arranged to control operation of the drive system 300 in response to signals received by the temperature sensors TS1,TS2 Optionally, the solar heating system can include a cooling circuit 500. The cooling circuit can be provided in embodiments wherein it is necessary to further reduce the temperature of the heat transfer fluid before it is returned to the storage tank 104. The cooling circuit 500 includes a heat exchanger system 508, a storage tank 504 for storing a second heat transfer fluid, which is preferably cold water, cold side piping 502 connecting the storage tank 504 to an input side of the heat exchanger system 508 and warm side piping 506 connecting the output side of the heat exchanger system 508 with the storage tank 504. The cooling circuit 500 includes a pump 510 to circulate water from the storage tank 504 to the input side of the heat exchanger system 508 and from the output side of the heat exchanger system 508 back to the water tank 504. In some embodiments, the pump 510 is located in the warm side piping 506. In other embodiments, the pump 510 is located in the cold side piping 502. A temperature sensor TS3 monitors the temperature of the heat transfer fluid at the cool circuit heat exchanger system 508. Signals from the temperature sensor TS3 are provided to the electronic controller 200. In response to the signals from the temperature sensor TS3, the electronic controller 200 is arranged to control operation of the cooling circuit pump 510 in order to start the pump to commence a cooling operation, and during a cooling operation to adjust the flow rate of water within the cooling circuit to control the rate at which heat is removed from the heat transfer fluid.

[0095] Figures 4-11 show the main components of a solar heating unit 2 used in the solar heating system 1 of Figure 1. Each solar heating unit 2 included in the solar heating system 101 can be arranged similarly, for example as described below.

[0096] The lens array 3 includes a plurality of lens assemblies, such as Fresnel lens assemblies 9. Typically the lens array 3 includes 10 to 50 lens assemblies 9. In Figure 4, the arrangement has 36 lens assemblies 9. The number of lens assemblies 9 is selected according to the heating requirements. Each Fresnel lens 9 includes a substrate 11 and a film 13 (see Figure 12). The substrate 11 provides support for the film 13. The substrate typically comprises a panel, and preferably a planar panel. Typically the panel is rectangular in plan view. The panel is typically made from a plastics material, and preferably a thermoplastics material, such as polymethyl methacrylate (PM:MA), thermoplastics polyurethane (TPU), polyethylene terephthalate (PET) or polycarbonates (PC). In some preferred embodiments the substrate 11 comprises a PMMA panel. In some preferred embodiments the substrate 11 comprises a polycarbonate panel. The substrate 11 typically has a thickness in the range 3mm to 6mm. In some embodiments the thickness is around 4mm.

[0097] The film 13 includes an arrangement of lens elements 15 formed therein. Each lens element 15 typically comprises a micro-prism. The overall arrangement of the lens elements 15 produces the Fresnel lens. The film 13 typically has a thickness in the range 30-250 microns. The film 13 is typically made from polymethyl methacrylate (PMMA). The film 13 is applied to a major surface of the substrate 11. The film 13 is preferably bonded to the substrate 11, for example by a solvent such as methyl chloride.

[0098] The film 13 is produced by forming a lens pattern into a curved surface of a casting drum. The casting drum can be made from copper or high phosphorus nickel, the lens pattern is typically cut into the drum using diamond cutters that are computer controlled according to a program arranged to provide a lens with a specific focal length. The casting drum is mounted in a roll to roll UV casting machine. A web of film material is fed over the casting drum and the lens pattern on the casting drum forms the arrangement of lens elements 15 in the web of film material. The film 13 is then cut to size and each film portion is mounted on to a respective substrate 11. Each Fresnel lens is approximately 1.2m long 0.7m wide. Each lens has a specific focal length, which is typically in the range lm to 2m, preferably 1.2m to 1.8m, and is preferably around 1.65m.

[0099] Optionally, a transparent wear film or coating can be applied to the film 13 to protect the optical surfaces.

[0100] The Fresnel lens assemblies 9 are mounted in the frame 7. The lens array 3 includes a 6 x 4 array wherein each Fresnel lens assembly 9 is arranged in a common plane, and at a first end of the 6x 4 array there is a first row 9a of inclined lens assemblies 9 and a second end of the 6 x 4 array there is a second row 9b of inclined lens assemblies 9. The first row of lens assemblies 9a is inclined downwards from the plane of lenses by an angle a, wherein the angle a is typically around 5 to 40 degrees, and is preferably 10 to 30 degrees. The second row of lens assemblies 9b is inclined downwards from the plane of lenses by an angle a, wherein the angle a is typically around 5 to 40 degrees, and is preferably 10 to 30 degrees. Having the first and second rows of lens assemblies 9a,9b inclined at the correct angle for the application helps to maximise the incidence of solar energy on the tubular receiver 5. The frame 7 includes an arrangement of frame members 21,23 that engage and support edges of the Fresnel lens assemblies 9. The frame 7 includes a trestle 25, which supports the frame members 21,23 and the array of Fresnel lens assemblies 9.

[0101] The frame 7 of the lens assembly is pivotally attached to two vertical support posts 17 by an axle 19 mounted in bearings 20, and the lens assembly 3 is arranged to pivot relative to the support posts 17 about a generally horizontal axis Z-Z, for example when the solar heating unit is mounted on flat horizontal ground. This enables the orientation of the Fresnel lens assemblies 9 to be adjusted along an arcuate path about the generally horizontal axis Z-Z. Typically the Fresnel lens assemblies 9 are arranged to move through a maximum arc of around +50 degrees, for example while tracking the sun, thereby maximizing the amount of solar radiation that is incident upon the tubular receiver 5. A single pivot axis Z-Z is particularly suited for solar heating systems lthat are located close to the equator.

[0102] The solar heating unit 2 includes a drive system 300 that is arranged to pivot the lens array 3 about the horizontal axis Z-Z. The drive system 300 can include a drive mechanism 302, which includes a worm 304 and a worm gear 306 (see Figure 8). The worm 304 is driven by an electric motor (not shown). The worm gear 306 is splined to the axle 19 and the lens array is fixed to the axle 19. Accordingly, driving the electric motor causes the axle 19 to rotate, thereby changing the orientation of the lens assembly 3 about the horizontal axis Z-Z. In some embodiments, a stewing ring can be used to rotate the axle 19.

[0103] The tubular receiver 5 is attached to the support posts 17 towards lower ends of the support posts 17. The tubular receiver 5 is arranged substantially horizontally and is located at the focal points of each lens so that solar radiation is focused on to the tubular receiver 5.

[0104] The tubular receiver 5 is elongate. In use, the lens assemblies 9 focus solar energy onto the tubular receiver 5 to heat a transfer fluid stored therein. Figure 11 illustrates diagrammatically the lens assemblies 9 focusing light towards the tubular receiver 5.

[0105] The tubular receiver 5 comprises a thermally conductive absorber tube 49, which is typically made from metal, such as steel (see Figure 9) The absorber tube 49 has a transparent outer casing 51, which is typically made from glass. The outer casing 51 is arranged co-axially with the absorber tube 49. The outer casing 51 has a larger diameter that the absorber tube 49 such that the inner surface of the outer casing 51 is spaced apart from the outer surface of the absorber tube 49. The space between the outer casing 51 and the absorber tube 49 is evacuated to create a vacuum. The vacuum supresses gas heat conduction, and convection within the space. Each end of the tubular receiver 5 has end assembly 53. The end assembly 53 includes a ring assembly 55, which separates the outer casing 51 from the absorber tube 49 and a bellows assembly 57 which allows for differences in the rates of thermal expansion and contraction between the absorber tube 49 and the outer casing 51. The ring assembly 55 comprises a first part 55a sealed to the absorber tube 49. The metal ring 55 includes a second part 55b sealed to the outer casing 51. The bellows assembly 57 is sealed to both the first and second parts of the ring. The receiver 5 can include getter material in order to maintain the vacuum in the receiver. The getter material is arranged to absorb free gases in the space between the outer casing 51 and the absorber tube 49. In use, the absorber tube 49 carries a transfer fluid, which is heated up by the solar energy directed on to it by the Fresnel lens assemblies 9. The heat generated can be sufficient for the fluid to change state, for example the fluid can be inserted into the absorber tube 49 in the form of a liquid and can change state to a gas. For example, liquid water can be changed to steam.

[0106] A pipe portion is attached to input side of absorber tube 49, and fluidly couples the input side of the absorber tube 49 to the cold side pipe network 102. A pipe portion is attached to the output side of absorber tube 49, and fluidly couples the output side of the absorber tube 49 to the hot side pipe network 106 The outer diameter of the outer casing 51 is typically around 100mm to 150mm and is preferably around 125mm. The outer diameter of the tubular absorber is typically in the range 50mm to 100mm, and is preferably around 70mm. The absorber is typically between 3m and 5m long, and is preferably around 4.00m long. The tubular receiver is very efficient at converting solar energy to heat the fluid. The arrangement has an efficiency of 98.4%.

[0107] The absorber tube 49 can include a coating to promote heat absorption. For example, the absorber tube 49 can include a coating of nanoparticles to promote heat absorption The outer casing 51 can include an anti-reflective coating to reduce the amount of light reflected by the casing 51 The solar heating unit 2 can include a reflector 52, which is in the form of a semi-cylindrical element arranged coaxial with the tubular receiver 5 (see Figures 4 and 6). The reflector 52 provides a reflective surface to redirect light and heat energy back to tubular receiver 5.

[0108] The system I can include a solar tracking system 59. The tracking system 59 is arranged to automatically adjust the orientation of the Fresnel lens assemblies 9 to track the position of the sun, thereby increasing the amount of solar energy directed to the tubular receiver 5. The tracking system 59 includes memory and at least one processor. Preferably the tracking system 59 has an open-loop control arrangement. The tracking system 59 is preferably includes a real-time clock and is programmed with an algorithm that enables the processor to calculate the solar zenith angles of the sun. These angles are then used by the processor for positioning the Linear Fresnel Lenses to point toward the sun. The algorithm is mathematical based on astronomical data instead of utilizing real-time light-intensity readings. Data specific to the location is stored in the memory, so the mathematical calculations are performed accurately. The data includes values relating to the Timezone (TZ), Longitude and Latitude of the desired location. The processor also receives input from the real-time clock to determine the time of day, and optionally the date. The time and/or date information is used to calculate the solar azimuth and zenith angles of the sun.

[0109] The tracking system 59 controls operation of the drive system 300, in accordance with the calculated solar zenith angle of the sun to adjust the orientation of the lens array 7 about the horizontal axis Z-Z, thereby optimising solar energy incident on the tubular receiver 5.

[0110] For an arrangement having 74 solar heating units 2 arranged in an array, wherein the heat exchanger system 108 and fans 400 are located at the near side of a processing building, where the drying process takes place, and the solar heating system is located relatively close to the equator, the storage tank 104 has a capacity of 10,000 litres and it is estimated that the combined power generated by the solar heating units 2 would be approximately 1.2 MW1.4MW per hour of heat energy.

[0111] The invention also relates to a method for drying a foodstuff ingredient, such as leaves from a plant, in particular leaves from a tea plant. The foodstuff ingredient to be dried can be located in an enclosure, such as a building. The solar heating system 1 is used to generate hot air, which can be driven into the enclosure in order to dry the foodstuff ingredient. This is achieved by operating the solar heating units 2 to adjust the orientation of their lens arrays to focus sunlight on to their respective tubular receivers 5, thereby heating the heat transfer fluid located within the tubular receivers. The transfer fluid is pumped to the heat exchanger 108, which is arranged to extract heat from the heat transfer fluid and to use that heat to warm air adjacent to the heat exchanger 108. The blowers 400 are arranged to drive the heated air to the foodstuff ingredient located in the enclosure, for example along ducting. The heated air dries the foodstuff ingredient, which can then be collected for further processing steps or packaging.

[0112] Figures 13 to 26 show a solar heating system 601 in accordance with a second embodiment of the invention.

[0113] The solar heating system 601 includes a heating circuit 600 and a plurality of solar heating assemblies 700 each comprising a plurality of solar heating units 602 The solar heating system 601 can include any practicable number of solar heating assemblies 700 to meet the heating requirements of the system. Typically, the solar heating system includes in solar heating assemblies wherein m is in the range 2 to 20. Each solar heating assembly 700 can include any practicable number of solar heating units 602. Typically, each solar heating assembly includes n solar heating units 602, wherein n is in the range 2 to 15. As an example, Figure 13 shows an arrangement having 9 solar heating assemblies 700, with each solar heating assembly 700 having 9 solar heating units 602, giving a total of 81 solar heating units 602. Each solar heating unit 602 includes a lens array 3, a tubular receiver 5 arranged to receive solar energy from the lens array 3, and a support frame 7.

[0114] The heating circuit 600 includes a heat transfer fluid, a storage tank 604 for storing the heat transfer fluid, the tubular receivers 5, a heat exchanger 608 system for generating hot air, a cold side pipeline network 622 arranged to supply heat transfer fluid from the storage tank 604 to the tubular receivers 5, and a hot side pipeline network 606 arranged to supply heated heat transfer fluid from the receivers 5 to the heat exchanger system 608.

[0115] The heat transfer fluid is Cumene is commercially available and is sold by one supplier under the trade name Marlothermg XC.

[0116] The tubular receiver 5 of each solar heating unit 602 has an input side and an output side. The cold side pipeline network 622 connects the storage tank 604 to the input side of each tubular receiver 5. The hot side pipeline network 606 connects the output side of each tubular receiver 5 to the heat exchanger system 608. The cold side pipeline network 622 is arranged supply the heat transfer fluid from the storage tank 604 at relatively cold temperature to the input side of each tubular receiver 5. The heat transfer fluid in the tubular receiver 5 is heated by solar radiation directed on to the tubular receiver 5 by the lens array 3. The heated transfer fluid exits the output side of the tubular receiver at a relatively higher temperature than received at the input side. The hot side pipeline network 606 is arranged to supply the heated transfer fluid from the output side of each tubular receiver 5 to the heat exchanger system 608.

[0117] Typically, each tubular receiver 5 is connected to the cold side pipeline network 622 in parallel. That is, the cold side pipeline network 622 and tubular receivers 5 are arranged such that relatively cold heat transfer fluid is supplied to each tubular receiver 5 from the storage tank 604 without having passed through another tubular receiver 5. In this arrangement, each tubular receiver 5 receives the heat transfer fluid at approximately the same temperature, save for any temperature variations as the transfer fluid travels through different parts of the cold side pipeline network 622.

[0118] Typically, each tubular receiver 5 is connected to the hot side pipeline network 606 in parallel. That is, the hot side pipeline network 606 and the tubular receivers 5 are arranged such that relatively hot heat transfer fluid is supplied from each tubular receiver 5 to the hot side pipeline network 606 without having passed through another tubular receiver 5. In this arrangement, the heat transfer fluid supplied to the tubular receiver 5 of a particular solar heating unit 2 is heated by the lens array 3 of that solar heating unit 2 only. This helps to ensure that the heat transfer fluid remains within its safe operating temperature, while at the same time heating a large volume of the heat transfer fluid The electronic control system 200 shown in the first embodiment can be used in a similar manner for the second embodiment to control the flow rate of heat exchange fluid through the tubular receivers 5, and hence control the operating temperature of the heat exchanger 608, thereby controlling the temperature to which the air is heated.

[0119] Figures 14-21 show the arrangement of one solar heating assembly 700 and Figures 22 to 26 show the main components of a solar heating unit 602 used in the solar heating assembly 700.

[0120] Each solar heating unit 602 in a solar heating assembly 700 can be arranged as follows. The lens array 3 includes a plurality of lens assemblies, such as Fresnel lens assemblies 9. Typically the lens array 3 includes four to twenty lens assemblies 9. In Figure 22, the arrangement has 36 lens assemblies 9. The number of lenses is selected according to the heating requirements. Each Fresnel lens assembly 9 can be arranged similarly to the lens assembly 9 in the first embodiment.

[0121] The Fresnel lens assemblies 9 are mounted in the frame 7. The frame 7 includes a first and second parts 17,19. The first part 17 is pivotally attached to the second part 19 by bearings. The first part 17 can be pivotally attached to the second part in a manner that enables the first part 17 to pivot relative to the second part 19 about a generally horizontal axis Z-Z. The first part 17 supports the Fresnel lens assemblies 9. This enables the orientation of the Fresnel lens assemblies 9 to be adjusted along an arcuate path about the generally horizontal axis Z-Z. Typically the Fresnel lens assemblies 9 are arranged to move through an arc of around 90 degrees, for example while tracking the sun. Starting at a first orientation at 0 degrees of rotation, wherein the Fresnel lens assemblies 9 are arranged in a generally vertical plane, the Fresnel lens assemblies 9 are able to pivot along the arcuate path to a final orientation at 90 degrees of rotation wherein the Fresnel lenses are in a generally horizontal plane. The Fresnel lens assemblies 9 are arranged to move along the arcuate path in both directions.

[0122] Each solar heating unit 602 includes drive means for pivoting the first part of the frame 17 relative to the second part of the frame 19, and hence pivoting the lens assembly 3 about the horizontal axis Z-Z. The drive means can include any suitable driver for pivoting the lens assembly 3 about the horizontal axis, such as a linear actuator 41 (see Figure 25). The linear actuator 41 is arranged to pivot the first part 17 of the frame relative to the second part 19 of the frame about the generally horizontal axis. The linear actuator 41 is preferably mounted on the second part of the frame 19. Additionally, or alternatively, the drive means can include a slewing ring to pivot the lens assembly 3 about the horizontal axis Z-Z and/or an electric motor and transmission system. 3.)

[0123] Typically, the horizontal pivot axis Z-Z is coaxial with a longitudinal centre line of the tubular receiver 5. Thus the lens assembly 3 is arranged to pivot about the tubular receiver 5, to adjust the orientation of the lens assembly, however the lens continue to focus light on to the tubular receiver 5.

[0124] The Fresnel lens assemblies 9 are mounted in the frame 7. The lens array 3 includes a 6 x 4 array wherein each Fresnel lens 9 is arranged in a common plane, and at a first end of the 6 x 4 array there is a first row 9a of inclined lens assemblies 9 and a second end of the 6 x 4 array there is a second row 9b of inclined lens assemblies 9. The first row of lens assemblies 9a is inclined downwards from the plane of lenses by an angle a, wherein the angle a is typically around 5 to 40 degrees, and preferably 10 to 30 degrees. The second row of lens assemblies 9b is inclined downwards from the plane of lenses by an angle a, wherein the angle a is typically around 5 to 40 degrees, and preferably 10 to 30 degrees. Having the first and second rows of lens assemblies 9a,9b inclined at the correct angle for the application helps to maximise the incidence of solar energy on the tubular receiver 5. The frame 7 includes an arrangement of frame members 21,23 that engage and support edges of the Fresnel lens assemblies 9. The frame 7 includes a trestle 25, which supports the frame members 21,23 and the array of Fresnel lens 9. This provides the first part of the frame 17 with a space frame having a generally triangular prismatic arrangement. A counterweight is located at an apex of the trestle 25. The counterweight can comprise first and second parts 31a,31b. The counterweight provides a means of balancing the Fresnel lens 9 about its pivot axis Z-Z to reduce the torque on any pivoting connection, which may be mechanised to enable raising and lowering of the Fresnel lens assemblies 9 in an arc about the pivot axis Z-Z. The first part of the counterweight 31a extends between a first pair of frame members 27a-b and the second part of the counter weight 31b extends between a second pair of frame members 29a-b. Each part of the counterweight 31a,3 lb comprises an elongate member having an arrangement of weights mounted thereon. The counterweight 3 la,3 lb is arranged generally parallel with the tubular receiver 5. The counterweight is attached to the trestle 25 by first, second and third plates 33,35,38. The first plate 33 is an end plate, which is fixed to a first end of a lower part of the trestle 25 and the frame member 27a is attached to the first plate 33. The second plate 35 is located centrally in a lower portion of the trestle 25 and is fixedly attached thereto. The frame members 27b and 29a are each attached to the second plate 35. The third plate 38 is an end plate which is fixed to a second end of a lower part of the trestle 25 and the frame member 29b is attached to the third plate 38. The first, second and third plates 33,35,38 have a triangular shape in plan. The counterweight pivots with the trestle 25.

[0125] The tubular receiver 5 is mounted on the first part 17 of the frame. The tubular receiver 5 is elongate. The tubular receiver 5 is centrally located on the second part 19 of the frame. A central longitudinal axis of the tubular receiver 5 is co-axial with the generally horizontal axis Z-Z which the first part 17 of the frame pivots about relative to the second part 19 of the frame. The tubular receiver 5 is located at a focal line of the Fresnel lens assemblies 9. In use, the lens assemblies 9 focus solar energy onto the tubular receiver 5 to heat a fluid stored therein. Figure 11 from the first embodiment illustrates diagrammatically the lens assemblies 9 focusing light towards the tubular receiver 5 in the second embodiment.

[0126] The tubular receiver 5 comprises a thermally conductive absorber tube 49, which is typically made from metal, such as steel (see Figure 9 from the first embodiment). The absorber tube 49 has a transparent outer casing 51, which is typically made from glass. The outer casing 51 is arranged co-axially with the absorber tube 49. The outer casing 51 has a larger diameter that the absorber tube 49 such that the inner surface of the outer casing 51 is spaced apart from the outer surface of the absorber tube 49. The space between the outer casing 51 and the absorber tube 49 is evacuated to create a vacuum. The vacuum supresses gas heat conduction, and convection within the space. Each end of the tubular receiver 5 has end assembly 53. The end assembly 53 includes a ring assembly 55, which separates the outer casing 51 from the absorber tube 49 and a bellows assembly 57 which allows for differences in the rates of thermal expansion and contraction between the absorber tube 49 and the outer casing 51. The ring assembly 55 comprises a first part 55a sealed to the absorber tube 49. The metal ring 55 includes a second part 55b sealed to the outer casing 51. The bellows assembly 57 is sealed to both the first and second parts of the ring. The receiver 5 can include getter material in order to maintain the vacuum in the receiver. The getter material is arranged to absorb free gases in the space between the outer casing 51 and the absorber tube 49. In use, the absorber tube 49 carries a heat transfer fluid, which is heated up by the solar energy directed on to it by the Fresnel lens assemblies 9. The heat generated can be sufficient for the fluid to change state, for example the fluid can be inserted into the absorber tube 49 in the form of a liquid and can change state to a gas. For example, liquid water can be changed to steam.

[0127] Pipe or flexible hose portions are attached to each of the end plates and are arranged co-axially with the tubular receiver 5. Each pipe or flexible hose portion is connected to a respective end of the absorber tube 49 and is in fluid communication therewith. The pipe of flexible hose portions connect the tubular receiver 5 to a fluid circuit, such as a steam circuit.

[0128] The outer diameter of the outer casing 51 is typically around 100mm to 150mm and is preferably around 125mm. The outer diameter of the tubular absorber is typically in the range 50mm to 100mm, and is preferably around 70mm. The absorber is typically between 3m and 5m long, and is preferably around 4m long. The tubular receiver is very efficient at converting solar energy to heat the fluid. The arrangement has an efficiency of 98.4%.

[0129] The absorber tube 49 can include a coating to promote heat absorption. For example, the absorber tube 49 can include a coating of nanoparticles to promote heat absorption. The outer casing 51 can include an anti-reflective coating to reduce the amount of light reflected by the casing 51 A reflector 52 is shown in Figure 22 which is in the form of a semi-cylindrical element coaxial with the receiver, providing a reflective surface to direct light and heat energy back upon tubular receiver 5.

[0130] A solar heating assembly 700 includes a plurality of solar heating units 602. Typically, one solar heating assembly includes n solar heating units 602, wherein n is in the range 2 to 20, and preferably 5 to 15. In the arrangements shown in Figures 13 to 15 each solar heating assembly 700 includes 9 solar heating units 602, arranged in a 3 x 3 array. The solar heating units 602 in one of the solar heating assemblies 700 are mounted on to a common base 702, which can be in the form of a space frame. Typically, each solar heating unit 602 in given row of solar heating units 602 is mounted on the base 702 such that the tubular receivers 5 are arranged generally coaxially, but are spaced apart from one another in the axial direction to enable the input side of each tubular receiver 5 to be connected to the cold side pipeline network 622 and each output side of each tubular receiver 5 to be connected to the hot side pipeline network 606, for example by pipes or flexible hoses. Accordingly, for each solar heating unit 602 in the row of solar heating units 602 is arranged to pivot in generally the same direction. The pipes and/or flexible hoses that connect the tubular receivers to the cold side pipeline network 622 and the hot side pipeline network 606 can be arranged to pass through an aperture formed in the base 702.

[0131] Each solar heating unit 602 in the first row of solar heating units 602 is arranged such that the second part of the frame 19 is mounted directly on to the base 702. Each solar heating unit 602 in the second row of solar heating units 602 is arranged such that the second part of the frame 19 is mounted on to a first subframe 704, which in turn is mounted on to the base 702. The first subframe 704 increases the vertical height of each solar heating unit 602 in the second row of solar heating units 602. This helps to prevent the lens assemblies 3 in the first row of solar heating units 602 clashing with the lens arrays 3 in the second row of solar heating units 602, and helps to ensure that the lens arrays 3 in the first row of solar heating units 602 do not overshadow the lens arrays 3 in the second row of solar heating units 602. Likewise, each solar heating unit 602 in the third row of solar heating units 602 is arranged such that the second part of the frame 19 is mounted on to a second subframe 706, which has a greater vertical height than the first subframe 704, which in turn is mounted on to the base 702. The second subframe 706 increases the vertical height of each solar heating unit 602 in the third row of solar heating units 602 compared to the solar heating units in the second row of solar heating units 602. This helps to prevent the lens assemblies 3 in the third row of solar heating units 602 clashing with the lens assemblies 33 in the second row of solar heating units 602, and helps to ensure that the lens assemblies 3 in the second row of solar heating units 602 do not overshadow the lens assemblies 33 in the third row of solar heating units 602.

[0132] The base 702 is mounted on roller elements 708, such as casters or wheels. The roller elements 708 are mounted on a circular track 710. The roller elements 708 and track 710 enable the solar heater assembly 700 to rotate about a second pivot axis, which can be a generally vertical axis Y-Y, in clockwise and anti-clockwise directions. The axis Y-Y is vertical when the track 710 is mounted on flat horizontal ground. The generally vertical axis Y-Y is arranged centrally relative to the base 702. This enables the orientation of the Fresnel lens assemblies 9 to be adjusted along a second arcuate path about the vertical axis Y-Y. Typically the Fresnel lens assemblies 9 are arranged to move through an arc of up to 180 degrees. For example, when located in-situ, the second part 19 of the frame can have a starting orientation (0 degrees) wherein the Fresnel lens assemblies 9 face in a generally easterly direction and can be rotated slowly about the vertical axis Y-Y while tracking the sun to a final orientation (180 degrees) wherein the Fresnel lenses face in a generally westerly direction. This type of movement is particularly useful for installations located a significant distance away from the equator.

[0133] The solar heater assembly 700 includes a slewing ring post 712 and a slewing ring 39 (see Figures 16, 20 and 21). The slewing ring 39 is arranged to act on the slewing ring post 712 and drive the base 702 and the solar heating units 602 mounted thereon about the generally vertical axis Y-Y.

[0134] The system 1 includes a solar tracking system 59 (see Figure 26). The tracking system 59 is arranged to automatically adjust the orientation of the Fresnel lens assemblies 9 to track the position of the sun, thereby increasing the amount of solar energy directed to the tubular receiver 5. The tracking system 59 includes memory and at least one processor. Preferably the tracking system 59 has an open-loop control arrangement. The tracking system 59 is preferably includes a real-time clock and is programmed with an algorithm that enables the processor to calculate the solar azimuth and zenith angles of the sun. These angles are then used by the processor for positioning the Linear Fresnel Lenses to point toward the sun. The algorithm is mathematical based on astronomical data instead of utilizing real-time light-intensity readings. Data specific to the location is stored in the memory, so the mathematical calculations are performed accurately. The data includes values relating to the Timezone (TZ), Longitude and Latitude of the desired location. The processor also receives input from the real-time clock to determine the time of day, and optionally the date. The time and/or date information is used to calculate the solar azimuth and zenith angles of the sun The tracking system 59 controls operation of the linear actuator 41 and the slew drive 39, in accordance with the calculated solar azimuth and zenith angles of the sun. The tracking system 59 operates a slew drive motor 61 in order to rotate the Fresnel lenses about the vertical axis. The tracking system 59 operates a linear actuator motor 63 in order to rotate the Fresnel lens assemblies 9 about the horizontal axis. The tracking system 59 moves the Fresnel lens assemblies 9 to face the sun at the optimum angles.

[0135] Due to the two axes of movement of the Fresnel lens assemblies 9, the lens assemblies 9 may track the movement of the sun during daylight hours in both elevation and from East to West.

[0136] The tracking system 59 can include a limit switch that limits movement of the linear actuator 41. The tracking system 59 can include a limit switch that limits movement of the slewing ring 39 Optionally, the tracking system 59 can include a wind protection system that is arranged to move the lens array 3 into a safe orientation in the event of high winds. The tracking system 59 can include a wind speed measurement device, or at least be arranged to receive an input from a separate device. The tracking system is arranged to monitor the wind speed. When the tracking system 59 determines that the wind speed is greater than or equal to a threshold value, in response the tracking system moves the lens array 3 into a safe orientation, for example by actuating at least on the linear actuator 41 and the slewing ring 39. A safe orientation is when the lens array 3 is in a generally horizontal orientation.

[0137] It will be appreciated that the above examples can be modified while still falling within the scope of the invention. For example, a different number of Fresnel lens assemblies 9 can be included in the array. The lens assemblies 9 can have a different focal length. The lens panels can have a different size and/or shape.

[0138] The second embodiment can be used in a similar way to the first embodiment to in a method for drying a moist substance, for example a foodstuff ingredient, such as leaves from a plant, in particular leaves from a tea plant.

[0139] The description presents exemplary embodiments and, together with the drawings, serves to explain principles of the invention. However, the scope of the invention is not intended to be limited to the precise details of the embodiments or exact adherence with all method installation steps, since variations will be apparent to a skilled person and are deemed also to be covered by the claims. Terms for components used herein should be given a broad interpretation that also encompasses equivalent functions and features. In some cases, several alternative terms (synonyms) for structural features have been provided but such terms are not intended to be exhaustive.

[0140] Descriptive terms should also be given the broadest possible interpretation; e.g. the term "comprising" as used in this specification means "including" such that interpreting each statement in this specification that includes the term "comprising", features other than that or those prefaced by the term may also be present. Related terms such as "comprise" and "comprises" are to be interpreted in the same manner. Directional terms such as "vertical", "horizontal", -up", "down", "upper" and "lower" may be used for convenience of explanation usually with reference to the illustrations and are not intended to be ultimately limiting if an equivalent function can be achieved with an alternative dimension and/or direction.

[0141] The description herein refers to embodiments with particular combinations of configuration steps or features, however, it is envisaged that further combinations and cross-combinations of compatible steps or features between embodiments will be possible indeed, isolated features may function independently as an invention from other features and not necessarily require implementation as a complete combination. Any feature from an embodiment can be isolated from that embodiment and included in any other embodiment.

[0142] The term "at least one of' is to be interpreted in the sense of "and/or". For example, the term "at least one of a linear actuator and a slewing ring" is to be interpreted as meaning any one of the following: the linear actuator alone; the slewing ring alone; or the combination of the linear actuator and stewing ring. As another example, the term "at least one of the linear actuator, the stewing ring and the transmission" is to be interpreted as meaning any one of the following: the linear actuator alone; the slewing ring alone; the transmission alone; the combination of the linear actuator and the slewing ring; the combination of the linear actuator and the transmission; the combination of the stewing ring and the transmission; or the combination of the linear actuator, the slewing ring and transmission.

Claims

1. CLAIMS: 1. A solar heating system, including: a plurality of solar heating units, wherein each solar heating unit includes: a tubular receiver having an input side and an output side; a support assembly. and a lens array including arranged to focus, in use, solar radiation on to the tubular receiver, wherein the lens array is pivotally attached to the support assembly and is arranged to pivot with respect to the support assembly about a first pivot axis; a heating circuitincluding: a storage tank arranged to store heat transfer fluid, a heat exchanger; a first piping network connecting an output side of the heat exchanger with an input side of the storage tank and connecting an output side of the storage tank to the input side of each tubular receiver; a second piping network arranged to connect the output side of each tubular receiver to an input side of the heat exchanger; and at least one pump arranged to selectively circulate heat transfer fluid through the storage tank, heat exchanger, first and second piping networks and tubular receivers; wherein, in use, the lens arrays are arranged to focus solar radiation on to their respective tubular receivers to heat the transfer fluid in the tubular receivers, the at least one pump is arranged to pump the heated transfer fluid to the input side of the heat exchanger, the heat exchanger is arranged to extract heat from the heat transfer fluid and to heat air using said heat; and a blower arranged to blow the heated air to a drying zone.

2 The system of claim 1, wherein the heat exchanger includes at least one radiator, the blower includes at least one fan that is arranged to blow air over the at least one radiator thereby heating the air.The system of any one of the preceding claims, including at least one temperature sensor arranged to monitor the temperature of the heat transfer fluid in the heating circuit 4 The system of claim 3, wherein the at least one temperature sensor includes a temperature sensor arranged to monitor the temperature of the heat exchanger.The system according to any one of the preceding claims, including a controller arranged to control operation of the heating circuit.6 The system of claim 5, when dependent on claim 3 or 4, wherein the controller is arranged to receive signals from the at least one temperature sensor, and the controller is arranged to control operation of the at least one pump in response to signals received from the at least one temperature sensor to control the flow rate of heat transfer fluid through the tubular receivers, thereby controlling an operating temperature of the heat exchanger.The system of claim 6, wherein the heat exchanger is arranged to heat air to a temperature Taff, wherein Tan is in the range 60C to 120C.The system of claim 6 or 7, wherein the controller is arranged to control the flow rate of heat transfer fluid through the tubular receivers is greater than or equal to 2 litres per minute, greater than or equal to 3 litres per minute, greater than or equal to 4 litres per minute, and/or wherein the flow rate of heat transfer fluid through the tubular receivers is less than or equal to 15 litres per minute, less than or equal to 14 litres per minute, less than or equal to 13 litres per minute, less than or equal to 12 litres per minute.9 The system of any one of the preceding claims, including at least one flow meter arranged to monitor the flow rate of heat transfer fluid within the heating circuit, for example the flow rate of heat transfer fluid through the tubular receivers 10. The system of claim 9 when dependent on any one of claims 5 to 8, wherein controller is arranged to receive signals from the at least one flow meter, and optionally the controller is arranged to control operation of the at least one pump in response to signals received 11. The system of any one of the preceding claims, wherein the heat transfer fluid comprises cumene.12. The system of any one of the proceeding claims, a drive system arranged to adjust the orientation of the lens array by pivoting the lens array about the first pivot axis.13. The system of any one of the proceeding claims, wherein the first pivot axis is a generally horizontal axis.14. The system of any one of the proceeding claims, including n solar heating units, wherein n is greater than or equal to 5, greater than or equal to 10, greater than or equal to 20, greater than or equal to 30, greater than or equal to 40, or greater than or equal to 50; and / or n solar heating units, wherein n is less than or equal to 100, less than or equal to 90, less than or equal to 80, less than or equal to 70, or less than or equal to 60.15. The system of any one of the proceeding claims, wherein the heat exchanger comprises an assembly of a plurality of heat exchanger units.16. The system of any one of the proceeding claims, including ducting to guide air from the heat exchanger to the drying zone.17. The system of any one of the proceeding claims, wherein the heating circuit includes a second heat exchanger, for example located between the heat exchanger and the storage tank, a cooling circuit having a storage tank arranged to store a second heat transfer fluid, a first conduit connecting an output side of the second storage tank to an input side of the second heat exchanger, a second conduit connecting an output side of the second heat exchanger to an input side of the second storage tank, and a pump arranged do circulate the second heat transfer fluid through the first conduit, the second heat exchanger, the second conduit and the second storage tank, wherein, in use, the second heat transfer fluid has a lower temperature than the heat transfer fluid.18. The system of any one of the proceeding claims, wherein the tubular receiver includes: a tubular absorber having an input side and an output side, an optically transparent casing housing at least part of the tubular absorber, and a space between the optically transparent casing and the tubular absorber, wherein air is evacuated from said space to create a vacuum.19. The system of any one of the preceding claims, including an annular member mounted on the tubular absorber co-axially therewith, the annular member is arranged to support a first end of the optically transparent casing.20. The system of claim 19, wherein the annular member includes a first annular part sealably attached to the tubular absorber, a second annular part sealably attached to the optically transparent casing, and an expandable member sealably attached to the first and second annular parts.21. The system of any one of the proceeding claims, wherein the lens array includes at least one lens assembly, the lens assembly including a supporting substrate and a film applied to a surface of the supporting substrate, the film having at least one lens formed therein.22. The system of claim 21, wherein the film includes a plastics material, and preferably a thermoplastics such as polymethyl methacrylate (PMMA).23. The system of claim 21 or 22, wherein the film has a thickness in the range 30 to 250 microns.24. The system of any one of claims 21 to 23, wherein the lens is formed in the film by a casting drum.25. The system of any one of claims 21 to 24, wherein comprises a Fresnel lens assembly. The film includes at least one Fresnel lens.26. The system of any one of claims 21 to 25, wherein the supporting substrate comprises a plastics material, and preferably a thermoplastics such as polymethyl methacrylate (PMM A), thermoplastics polyurethane (TPU), polyethylene terephthal ate (PET) or polycarbonates (PC).27. The system of any one of the preceding claims, wherein the at least one lens assembly comprises a Fresnel lens assembly, the film having at least one Fresnel lens formed therein.28. The system of any one of the preceding claims, wherein the support assembly includes a pair of support posts.29. The system of any one of the preceding claims, wherein the support assembly includes a frame having a first part and a second part, the first part supports the lens array and the first part is mounted on the second part and is pivotally attached thereto.30. The system of any one of the preceding claims, wherein at least some of the plurality of solar heater units are grouped together to form a first solar heater assembly, the first solar heater assembly includes a first base and each solar heater unit in the first solar heater assembly is mounted on to the base, wherein the base is arranged to pivot about a second pivot axis.31. The base according to claim 30, wherein the second pivot axis is arranged perpendicular to the first pivot axis.32. The base according to claim 30 or 31, wherein the second pivot axis is a vertical axis.33. The base according to any one of claims 30 to 32, wherein the base is mounted on roller elements, such as castors and/or wheels, the roller elements are mounted on a circular track.34. The base according to any one of claims 30 to 33, including a second drive system arranged to pivot the base, and hence each of the solar heater units in the first solar heater assembly, about the second pivot axis.35. The base according to any one of claims 30 to 34, wherein the solar heater assembly includes n solar heating units, wherein n is greater than or equal to 2, greater than or equal to 3, greater than or equal to 4, greater than or equal to 5, or greater than or equal to 6, greater than or equal to 7, or greater than or equal to 8, and/or the solar heater assembly includes n solar heating units, wherein n is less than or equal to 15, less than or equal to 14, less than or equal to 13, less than or equal to 12, less than or equal to 11, less than or equal to 10.36. The base according to any one of claims 30 to 35, wherein the base includes an aperture and conduits, such as pipes or flexible hoses, which connect the input side of the tubular receivers to the cold side pipe network and the output side of the tubular receivers to the hot side pipe network, pass through the aperture.37. The system of any one of the preceding claims, wherein the vertical height of some of the solar heating units is greater than others.38. The system of any one of the preceding claims, including a tracking system arranged to automatically adjust the orientation of the lens array about the first pivot axis, and optionally about the second pivot axis.39. The system of any one of the preceding claims, wherein the tracking system includes a wind protection mode, wherein the tracking system includes wind speed measurement means, and is arranged to move the lens array to a horizontal orientation in response to an output signal from wind speed measurement means.The system according to claim 38 or 39 when dependent on claim 5, wherein the controller is arranged to send control signals to the tracking system to adjust the position of the lens array in response to receipt of a signal from the at least one of the temperature sensor indicating that the temperature has reached a threshold value, and the tracking system is arranged to adjust the position of the lens array in response to receipt of the control signal from the controller.41. The system according to claim 40, the tracking system is arranged to move the lens array to a position that decreases the amount of solar energy directed on to the receiver in response to determining that the temperature has reached the threshold value, and preferably the tracking system is arranged to move the lens array to a position wherein the solar array faces away from the sun.42. A method for drying at least one foodstuff ingredient, including providing a system according to any one of the preceding claims; heating the heat transfer fluid in the tubular receivers by focusing sunlight on to the tubular receivers, pumping the heated heat transfer fluid to the input side of the heat exchanger, the heat exchanger extracting heat from the heat transfer fluid and heating air adjacent to the heat exchanger using said heat; blowing heated air to a drying zone having at least one foodstuff ingredient, thereby drying the at least one foodstuff ingredient.43. A method according to claim 42, wherein the at least one foodstuff ingredient comprises leaves, and preferably leaves from a tea plant.44. According to another aspect there is provided a method for drying at least one moist substance, the method including providing a solar heating system according to any of the preceding claims; heating the heat transfer fluid in the tubular receivers by focusing sunlight on to the tubular receivers; pumping the heated heat transfer fluid to the input side of the heat exchanger; the heat exchanger extracting heat from the heat transfer fluid and heating air using said heat; blowing heated air to a drying zone having at least one moist substance, thereby drying the at least one moist substance.45. A solar heating system, including: a plurality of solar heating units, wherein each solar heating unit includes: a tubular receiver having an input side and an output side; a support assembly; and a lens array including a plurality of Fresnel lens assemblies arranged to focus, in use, solar radiation on to the tubular receiver, wherein the lens array is pivotally attached to the support assembly and is arranged to pivot with respect to the support assembly about a first pivot axis; a heating circuit including: a storage tank arranged to store heat transfer fluid; a heat exchanger; a first piping network connecting an output side of the heat exchanger with an input side of the storage tank and connecting an output side of the storage tank to the input side of each tubular receiver, wherein the input side each tubular receiver is directly connected to the first piping network; a second piping network arranged to connect the output side of each tubular receiver to an input side of the heat exchanger, wherein the output side each tubular receiver is directly connected to the second piping network; and at least one pump arranged to circulate heat transfer fluid through the heating circuit and tubular receivers; wherein, in use, the lens arrays are arranged to focus solar radiation on to their respective tubular receivers to heat the transfer fluid in the tubular receivers, the at least one pump is arranged to pump the heated transfer fluid to the input side of the heat exchanger, the heat exchanger is arranged to extract heat from the heat transfer fluid and to heat air using said heat; and a blower arranged to blow the heated air to a drying zone.