GB2060926A - A device for utilising solar energy - Google Patents

Abstract

A solar energy utilisation device has a curved mirror 36 which focusses the sun's rays onto a heat exchanger 50. A temperature sensor is located adjacent to the exchanger and is used to control a drive mechanism 70 for effecting movement of the mirror about a substantially horizontal axis 44, to follow the sun's rising and setting apparent movement. The same or a different drive mechanism may be used to swing the mirror around a generally vertical axis. One motion may be obtained from the other using a cam and cam follower. Methods of making the mirror are described. In Figs. 1 and 2 (not shown) a reflective sheet is shaped by applying pressure or suction and is held in that shape by curing a plastics support in contact therewith. In Figs. 8 and 9 (not shown) two reflective sheets are used, and after curing, the assembly is separated into two mirrors. <IMAGE>

Description

SPECIFICATION A device for utilising solar energy This invention relates to a device for utilising solar energy.

It has been proposed to utilise solar energy by concentrating the sun's rays with a concave mirror. For good efficiency, it is necessary to arrange that the axis of the mirror should point at the sun, and hence the mirror must be moved relative to the ground as the day proceeds. It would be desirable if there was a relatively simple and largely automatic means of effecting such movement.

Another point which has hindered the practical employment of mirrors in solar energy utilisation devices is the cost of design and manufacture.

Conventionally, mirrors are made by shaping and polishing a metal surface, or by depositing silver on a suitably shaped surface.

However, curved (e.g. parabolic) mirrors are expensive to produce, and curved mirrors of relative large diameter, e.g. over one metre in diameter, are particularly expensive. The high cost of mirrors has been a deterrent to their employment in obtaining useful energy from the sun's rays. With the present high cost of oil, and the likely energy shortage in the future, it would be beneficial if a solar energy utilisation device could be produced comparatively cheaply.

According to the invention, there is provided a device for utilising solar energy which includes a curved mirror, a transducer located in the region of the focal point of the mirror, a temperature sensor located at a lower edge of the transducer, and a drive mechanism for rotating the mirror about a substantially horizontal axis, the drive mechanism being connected to the sensor so that it can be controlled in response to the temperature of the sensor.

Also according to the invention, there is provided a device for utilising solar energy which includes a concavely curved mirror, a transducer located in the region of the focal point of the mirror, a pair of temperature sensors located adjacent to the transducer, and two drive mechanisms for the mirror, one for rotating it about a vertical axis and arranged to be controlled by one of the sensors and the other for rotating it about a horizontal axis and arranged to be controlled by the other of the sensors.

It will be appreciated that at temperate latitutes, it is desirable that the mirror should be rotated about horizontal and vertical axes in order to point substantially at the sun during the day. However in an Equatorial zone it may well be adequate if the mirror is rotated solely about a horizontal axis and in this event only one sensor and one drive mechanism is required.

In a preferred embodiment of the invention, the mirror is formed by a highly reflective film on a plastics sheet, and the latter is supported by a substrate formed by a cured synthetic resin layer.

The mirror may have a drain hole therethrough.

In one embodiment of the invention the transducer may be a heat exchanger.

The heat exchanger may be supported by a spider formed by a plurality of rods extending between the periphery of the mirror and the heat exchanger.

In another embodiment of the invention, the transducer may include one or more, or preferably an array of semiconductor devices which generate electrical energy in response to changes of their temperature. It will be understood that transducers other than a heat exchanger or a semiconductor device may be employed.

Each drive mechanism may include a piston-cylinder assembly.

Each sensor, or at least one of the sensors, may be constituted by a chamber containing water and closed except for a pipe leading to an associated piston-cylinder assembly.

The mirror may be supported on a base by a ball joint.

There is particularly disclosed herein a method of making a mirror which includes the steps of: (a) placing a plastics sheet carrying thereon a film of highly reflective material over a circular frame which bounds a plenum chamber, and securing it around the frame with the film facing inwardly towards the chamber, (b) feeding air or gas into the plenum chamber until the sheet is distended outwardly to the desired extent, (c) placing a curable or hardenable layer of synthetic plastics material onto that surface of the sheet which is opposite to the film, (d) curing or hardening the layer, and (e) removing the mirror from the frame.

There is also particularly disclosed herein a method of making a mirror including: (a) placing a plastics sheet carrying thereon a film of highly reflective material over a circular frame which bounds a plenum chamber, and securing it around the frame in a gas tight manner with the film facing outwardly away from the chamber, (b) placing a curable or hardenable layer of synthetic plastics material onto that surface of the sheet which is opposite to the film, (c) sucking air or gas out of the plenum chamber until the sheet is distended inwardly to a desired extent, (d) curing or hardening the layer, and (e) removing the mirror from the frame.

The curable or hardenable layer may include glass fibres or chopped glass mat as a reinforcing material.

An alternative form of procedure enables one to make a pair of mirrors. This procedure includes placing a pair of plastics sheets one over the other, each of the sheets bearing a film of highly reflective material thereon and the films facing inwardly, welding together the sheets by two circular concentric bar welds, making a hole in one of the sheets within the inner bar weld and attaching a nozzle thereto, placing a circular stiffening member in the space defined between the sheets and the two bar welds, supplying air or gas to the distend the sheets away from one another, placing respective curable or hardenable layers of synthetic plastics material on the surface opposite to the films, curing or hardening the layers, and cutting the assembly around the rim to separate the two mirrors so formed.

The mirror may be provided with a drain hole so that any rain that collects therein can drain away.

The invention will be better understood from the following non-limiting description of an illustrative embodiment thereof, given with reference to the accompanying drawings, in which: Figure 1 is a central vertical cross-section through one example of frame usable in the method of the invention, showing a plastics sheet placed thereover; Figure 2 is a view similar to Fig. 1 showing a later stage in the procedure; Figure 3 is a side elevation view of a mirror in accordance with the invention built into a solar energy collecting device; Figure 4 is a front view of the device shown in Fig. 3; Figure 5 is a front view similar to Fig. 4 but of an alternative design of solar energy collecting device; Figure 6 is a view similar to Fig. 3 and showing a pantograph mechanism for swinging the mirror in a vertical plane;; Figure 7 is a schematic circuit diagram of a simple control circuit for effecting a desired movement of the mirror in response to movement of the sun; Figure 8 is a plan view of a pair of superposed plastics sheets used in a further embodiment of a method of manufacturing a mirror according to the invention; Figure 9 is a vertical cross-section, on the plane IX-IX of Fig. 8; Figure 10 is a vertical cross-section on an enlarged scale through part of a mirror in accordance with one example of the invention.

Figure 11 illustrates in perspective view another form of mirror which can be used in a solar energy device according to the invention. For a full description thereof the reader is referred to my Patent Application No.

7924078 dated 11th July 1979 (Reference N82/4), Fig. 1 illustrates a frame 10 in the form of a circular dish having upstanding side walls, the interior of the dish constituting a plenum chamber 12 and the dish having a hole 14 therein connected to a pipe 1 6 which is in turn connected to a pump 18.

The frame is circular about a vertical axis located in the plane of the paper. In operation of one example of a method according to the invention, a plastics sheet carrying thereon a film of highly reflective material is placed over this frame, and is secured thereto in a gas tight manner by an encircling strap 24. The strap 24 may be of flexible metal and may be tightened around the frame 10 by a nut and screw fitting, not shown. In other words, what is commonly known as a "Jubilee" clip of appropriate size may be employed. In fitting the plastics sheet over the frame it is desirable to smooth it out so that it is flat and free from wrinkles before finally tightening the "Jubilee" clip.

Next, air is supplied from the pump 1 8 into the plenum chamber 12, so distending the plastics sheet 22 into the shape approximately shown in Fig. 2. A curable or hardenable synthetic plastics material 26 is then applied to the outer surface of the plastics sheet 22, and optionally one may include glass fibres or glass fibre mat at this stage to provide reinforcement. The purpose of applying the plastics material 26 is to form, after it has hardened, a backing of adequate mechanical strength and rigidity of shape to carry the plastics sheet 22. In a preferred example of the invention, liquid synthetic resin may be applied using a roller in a plurality of layers, said resin including a hardener whose action is initiated by mild application of heat.When several layers have been applied and the glass fibre mat laid therein if desired, the resin is cured by an appropriate application of heat.

Thereafter, the sheet 22 is cut around the periphery at the zone indicated by reference numeral 28 in Fig. 2 and the resulting mirror is removed. Fig. 10 is an enlarged crosssection of part of such a mirror, consisting of a reflective film 32, a synthetic plastics sheet 34 and a supporting substrate 36 of hardened synthetic resin including glass fibres or glass fibre mat. If desired, metal stiffeners may be embeded in the resin 36, or extra layers may be laid on two crossing diameters, so building up ribs which provide extra strength and rigid- ity to the structure. No further detailed description need be given here, since the techniques of building up rigid curved structures with synthetic resin impregnated with glass fibre are well known in the boat building industry and these techniques may be applied.

In the above description, reference has been made to feeding air or gas under pressure into the chamber 1 2. According to an alternative embodiment of the invention, not illustrated, a suction pump may be used in replacement of the pump 1 8 and air may be sucked out of the chamber 1 2. In this case the sheet is placed with its reflective film on the surface facing away from the chamber and has a curable resin applied to its non-reflective surface in an uncured condition. The sheet carrying the curable resin is then applied to the frame 10 with the reflective film outwardly and air is sucked out of the chamber 1 2 using the suction pump. When the sheet has been deformed by the external air pressure so that it is concave to the desired extent, heat is applied so curing the resin.The resulting concave mirror may then be removed from the frame.

In yet a further alternative embodiment of the invention, instead of using suction one may arrange things in the manner described in the preceding paragraph and place water on the upper surface of the plastics sheet prior to curing the resin. This stretches it and it takes up a slightly concave form. The resin is then cured by application of heat, the strap 24 is released and the resulting mirror is removed from the frame 1 0.

According to a preferred feature of the invention, a drain hole is bored in the formed mirror to allow excape of any rain which may collect therein. Such rain water may be collected for drinking or irrigation purposes.

It will generally be desirable to choose a curvature for the mirror such that the focal point is out of reach of a person. This is for obvious safety reasons.

In general, the distension method using air under pressure is preferred.

If it is desired to make a convex mirror, the plastics sheet may be placed with its reflecting film facing inwardly and with the curable resin applied to the concave outer surface. The suction is then applied and held while the resin is cured. The mirror is then removed from the frame.

One use of a mirror in accordance with the invention is as a mirror in a solar energy device. An illustrative solar energy device is schematically depicted in Fig. 3. It comprises a base 40 bearing a cup 42 which co-operates with a ball 44 to provide a universallypivotable ball joint. A support pillar 46 carries a support 48 which is secured in any convenient way to the glass fibre resin 36 forming part of the mirror. The plastics sheet and film are shown at 34 and 32 in Fig. 3.

A spider is constituted by four support rods 52 extending radially inwards from four equally spaced points on the periphery of the mirror. The spider supports a heat exchanger 50. The heat exchanger 50 is disposed substantially at the focal point of the mirror surface 32.

Fig. 4 is a front view of the solar energy device shown in Fig. 3, with the mirror located so that its axis is substantially horizontal. As seen in Fig. 4, the heat exchanger 50 is of generally square shape seen in front elevation and carries two heat sensors 56 and 58 one along its lower side and one along its right hand side as seen from the front. Each of the heat sensors 56 and 58 contains a working fluid whose pressure rises as a result of increasing temperature of the sensor 56 or 58, and a pipe 60 connects the sensor 56 with a device capable of producing mechanical movement in one direction in response to a rise of pressure and in the other direction in response to a fall in pressure. Likewise, a pipe 62 connects the sensor 58 to a similar movement producing device. An entry conduit and an exit conduit 66, 68 carry working fluid to and from the heat exchanger 50.

The purpose of the sensors 56 and 58 is to automatically move the mirror to follow the apparent movement of the sun during the day. In other words, the system is designed to adjust the position of the mirror in both a horizontal and a vertical plane so that its axis at all times points substantially directly at the sun.

The position of the sensors 56 and 58 illustrated is appropriate for a temperature zone in the Northern Hemisphere.

In the case of a solar energy utilisation device intended for use in an Equatorial zone, the sensor 58 and the drive mechanism associated therewith can be omitted.

In the case of a device intended for use in the Southern Hemisphere, and not in an Equatorial zone, the sensors will be appropriately adjusted in position. That is to say, considering Fig. 5, the sensor 56 would be kept in its illustrated position, but the sensor 58 would be placed on the left-hand side of the heat exchanger 50 instead of on the right hand side as illustrated.

The mirror may be moved by an arrangement diagrammatically illustrated in Fig. 6, in which a pantograph linkage 70 extends from a pivotal connection 72 on the rim of the mirror backing 36 downwardly to a pistoncylinder device 74. One end of the pantograph linkage is is secured to the cylinder at a pivot 76 and the other arm at the lower end of the pantograph linkage is secured by a pivotal connection 78 to the free end of the piston rod 80 of the piston cylinder device 74. The latter has its working space connected to the sensor 56.

A similar pantograph linkage, not shown, may be connected between a point on the mirror periphery at 90 to the point of connection 72, and another piston-cylinder device may be provided in order to allow movement of the mirror around a vertical axis. Fig. 7 illustrates the system. Working fluid is fed to the heat exchanger 50 by the conduit 66 and withdrawn therefrom via the conduit 68. A further heat exchanger 84 permits, for example, water entering on line 86 to be heated to steam leaving on line 88. The steam 88 is used as a source of energy for example in a steam turbine, not shown. It will be understood that the energy in the heated working fluid may be extracted and used otherwise than by raising steam.The sensor 56 is connected by the pipe 60 to the piston cylinder device 74, the latter providing the actuating force for operating the pantograph linkage 70 (Fig. 6). In a similar way, the sensor 58 is connected by a pipe 62 to a second pistoncylinder device 90 whose piston rod 92 and whose cylinder 94 are respectively connected to a second pantograph linkage connected to the mirror as described above. The piston cylinder device 74 effects angular motion of the mirror about a horizontal axis passing through the centre of the ball 44 and the piston-cylinder device 90 effects angular motion of the mirror about a vertical axis passing through the centre of the ball 44.

With respect to the adjustment about a horizontal axis, it can be seen that as the sun rises, and assuming the mirror does not move, the sun's rays become focused at a point lower with respect to the centre of the heat exchanger 50 than earlier in the day. Therefore, the sensor 56 is heated and the pressure of the sensing fluid therein increases. Consequently this increased pressure is transmitted by the line 70 to the piston-cylinder device 74, and its piston rod is consequently forced inwardly, so pulling the pivot points 76 and 78 closer together and therefore increasing the height of the pantograph linkage 70, thus driving the mirror to follow the rising sun. As a consequence, the mirror swings upwardly about the horizontal axis through the centre of the ball 44 and the sensor 50 is positioned in substantial registry with the focal point of the sun's rays.The sensor 56 continues to operate in this way as the sun rises, continuously pivoting the mirror upwardly.

Adjustment of position of the mirror about a vertical axis through the centre of the ball 44 is carried out in a similar way, using a pistoncylinder device 90 linked to the lateral sensor 58, the device 90 being connected to a similar pantograph linkage attached to a point at half height of the mirror on the mirror periphery.

There has been described a mechanicism for swinging the mirror about a vertical and a horizontal axis, using two piston-cylinder devices. In an alternative embodiment of the invention, one piston-cylinder device could be actuated in the manner described above and could be used to provide the two desired motions. For example, by including a cam surface on or connected to the piston rod, its linear motion can be used directly to effect pivoting about one axis and it can also be used to drive a cam follower which effects, through a suitable linkage, motion of the mirror about the other axis. The skilled mechanical engineer will be aware of other possibilities for achieving the two desired motions from the linear motion of the piston rod.

An alternative configuration of heat exchanger 50 and sensors 56 and 58 is shown in Fig. 5, the principal distinction being that the heat exchanger 50 is of circular shape seen in front elevation and the sensors 56 and 58 are each arcuate, covering 90 of the periphery. It will be appreciated that it is not essential that the heat exchanger 50 be located exactly at the focal point. Good results can be achieved with the heat exchanger 50 being located either further from or nearer to the mirror than the focal point, and indeed in hot countries this will be preferable in order to prevent the temperature of the heat exchanger 50 being raised to a level at which it would be essential to make the heat exchanger 50 of expensive high temperature materials.It will be understood that the principle of control will still be applicable even though the heat exchanger 50 is not located in the focal plane.

In an advantageous embodiment of the invention, the initial setting up of the equipment is arranged so that the sun's rays at dawn or shortly thereafter are focused in a small circular area of which half overlaps the sensor 56 and half overlaps the sensor 58. Then, as the sun continues its apparent movement, the control of the mirror rotation immediately starts to take effect. In other words, the waiting period while the cone of focused rays to move to the edge of the heat exchanger 50 is avoided. This waiting period is of course a "dead-time" so far as control of the mirror rotation is concerned.

The heat exchanger 50 may be any conventional design of heat exchanger. It may, for example, be about 30 centimetres in diameter. As a preferred feature, its surface which faces towards the sun may be ribbed or finned in order to provide an increased surface area. The working fluid passed therethrough may be water. The working fluid pipes 66, 68 and the sensor conduits 60, 62 may be carried along the rods 52, along the back of the mirror support 36, and then via lengths of flexible hose, around the universal joint and to the base 40. It will be realised that it is important not to significantly restrain the pivoting movement of the mirror.

The synthetic plastics sheet bearing a highly reflective film, used as the mirror surface, may be a material of this kind developed by National Aeronautics and Space Agency of United States of America. This material is now commercially available.

Each piston-cylinder device 74, 90 may be provided with a pressure relief valve as a safety measure in case movement of the mirtor is stopped by an obstruction and hence pressure builds up in one of the piston cylinder devices. The pressure relief valves are not shown in the drawings. The working surface of the heat exchanger 50 will preferably be painted or otherwise coated to produce a matt black surface.

An alternative method of manufacture of a mirror according to the invention is illustrated in Figs. 8 and 9. In this method of making a mirror, two sheets of plastics material 100 and 102 each having a highly reflective film on one surface thereof are placed face to face with the reflecting films inwards. Then a pair of circular bar welds 104, 106 are made, so fusing together the two sheets at the indicated locations, and a wire cable 108 is placed in the space 110 between these welds. Then a hole 11 2 is punched or formed in the layer 100 and a nozzle 114 is welded to said layer.

A source of air or gas under pressure such as a pump is connected to the nozzle 114 and the space 11 6 within the inner bar weld is inflated. This causes the two plastics sheets to distend outwardly. The nozzle is then closed off and layers of synthetic resin incorporating glass fibre mat are built up on the outer surfaces 11 8 of the two sheets. The resin is cured to provide a firm support for the mirror and a circular cut is made at the location indicated by 1 22. In this way, two mirrors in accordance with the invention can be formed in one operation.

The production of a mirror in the manner particularly disclosed and illustrated herein gives a number of advantages. Firstly, mirrors of quite large size can be readily produced on site, so avoiding awkward transport problems.

The method of production is simple and uses established technology; it is sufficiently easy to be carried out by a competent "do it yourself" operator. Compared to highly polished mirrors prepared by conventional methods, the procedure is extremely cheap, for example, a curved mirror about two metres in diameter could be produced for a total cost of only a few hundred pounds, compared to a cost of several thousand pounds if it was produced by a conventional method. When applied in a solar energy device, a mirror according to the invention as particularly described herein will produce an immediate return of investment and gives an opportunity for a reasonably competent householder to himself build and use a solar energy device.

This is expected to be of increasing value as the cost of coal and petroleum-based fuels continues to rise. The curvature of mirrors produced by the distention method disclosed is of satisfactory accuracy.

In the description above, the mirror 32 (Fig.

3) has been described as supported by a base 40 and a universal ball joint 42, 44. In an alternative embodiment of the invention, not illustrated, the mirror may advantageously be supported by a universal gimbal arrangement.

Such an arrangement may consist for example, of an inner circular ring located in a substantially horizontal plane upon which the mirror, its support backing, and the heat exchanger carried by the spider rods are carried.

This ring is mounted concentrically within a second ring by a pivot mounting so that the two rings can pivot relative to each other about a common diametral axis. The second ring is mounted within a third ring for pivotal movement relative thereto about a second diametral axis, the latter being perpendicular to the first-mentioned diametral axis, and the third ring is supported on the ground or elsewhere by suitable legs or supports. This construction has the advantage that the ancillary equipment, Fig. 7, can be mounted beneath the third ring, leading to a compact arrangement. Also a firm and wind-resistant support for the mirror can be provided, and the closed circular rings are strong structurally.

The foregoing particular description is directed to the positional control of a concave, usually parabolic mirror. The invention may also be advantageously applied to the control of a part-cylindrical mirror. These mirrors are known and may be made by cutting a hollow tube in half along a diametral plane, and silvering or polishing the interior curved surface. Such a mirror when used in a device for utilising solar energy is mounted with the tube axis horizontal and has a linearly-extending and horizontally-extending heat exchanger located at or near its focal zone. By placing one sensor below the heat exchanger, the attitude of the mirror relative to a horizontal plane can be controlled according to a procedure similar to that described above.A second sensor may be included if it is desired to rotate the mirror about a vertical axis in addition, as will be required in a temperate zone. The general construction and manner of operation will be the same as described above in connection with Figs. 4 and 5.

Reference is made above to the use of a working fluid in the sensors 56, 58 and in the piston-cylinder devices 74, 90. This working fluid may be water and a chosen amount of glycerine or of glycol, for example, may be incorporated therein in order to alter the boiling point and hence allow some control in the pressure which is generated in the pistoncylinder device as a result of the associated sensor rising temperature by a given amount.

It will be realised that in very hot countries where the temperature change is great, a lesser amount of pressure generated per degree rise in temperature of the sensor will be adequate to properly control the movement of the mirror than would be the case in a cooler region of the Earth. The admixture of substances with raise or lower the N.T.P. boiling point of the working fluid offers a convenient means of compensating for these variations, and avoids the necessity of offering units of different mechanical design according to the zone in which they are intended to be used.

Claims (23)

1. A device for utilising solar energy which includes a curved mirror, a heat exchanger located in the region of the focal point of the mirror, a temperature sensor located at a lower edge of the heat exchanger, and a drive mechanism for rotating the mirror about a substantially horizontal axis, the drive mechanism being connected to the sensor so that it can be controlled in response to the temperature of the sensor.

2. A device for utilising solar energy which includes a concavely curved mirror, a heat exchanger located in the region of the focal point of the mirror, a pair of temperature sensors located adjacent to the heat exchanger, and two drive mechanisms for the mirror, one for rotating it about a vertical axis and arranged to be controlled by one of the sensors and the other for rotating it about a horizontal axis and arranged to be controlled by the other of the sensors.

3. A device according to claim 1 or 2 in which the mirror is formed by a highly reflective film on a plastics sheet, and the latter is supported by a substrate formed by a cured synthetic resin layer.

4. A device according to claim 1, 2 or 3 in which the mirror has a drain hole therethrough.

3. A device according to claim 1, 2, 3 or 4 in which the mirror is a generally circular concavely-curved mirror.

6. A device according to claim 5 in which the heat exchanger is supported by a spider formed by a plurality of rods extending between the periphery of the mirror and the heat exchanger.

7. A device according to claim 2 or any claim dependent thereon in which the drive mechanisms each include a piston-cylinder assembly.

8. A device according to claim 7 in which at least one of the sensors is constituted by a chamber containing a working fluid and closed except for a pipe leading to an associated piston-cylinder assembly.

9. A device according to any preceding claim in which the mirror is of over one metre in diameter.

10. A device according to any preceding claim in which the mirror is supported on a base by a spherical ball joint.

11. A device according to any of claims 1-9 in which the mirror is supported on a gimbal arrangement.

1 2. A device according to claim 10 when dependent upon claim 8 when dependent upon claim 6 in which conduits conducting working fluid to and from the heat exchanger and the pipes leading to the sensors are carried by the spider rods and are connected to flexible hoses at the region of the ball joint.

1 3. A device according to any preceding claim in which-at least one drive mechanism is connected to the mirror by a pantograph linkage.

14. A device according to claim 8 in which each sensor is partly-filled with water.

1 5. A device according to claim 1 4 in which the water is admixed with a substance for adjusting its boiling point.

16. A device according to claim 6 in which each sensor is filled with a liquid having a high temperature coefficient of expansion.

1 7. A solar energy utilisation device according to claim 2 or any claim dependent thereon in combination with a plurality of mirrors each mounted for rotation about a horizontal and a vertical axis, said plurality of mirrors being connected together to be controlled in unison in response to the temperature sensors.

1 8. A device according to any of claims 1, 2, 3, 4 and 6 in which the mirror is partcylindrical in shape.

1 9. A device for utilising solar energy substantially as herein particularly described with reference to and as illustrated in the accompanying drawings.

20. A modification of the solar energy utilisation device according to any preceding claim in which the heat exchanger is replaced by one or more transducers.

21. A modification according to claim 20 in which the or each transducer is a semiconductor which produces an electrical output when its temperature rises or falls.

22. A device for utilising solar energy which includes a curved mirror, a transducer located in the region of the focal point of the mirror, a temperature sensor located adjacent to the transducer, and a drive mechanism for rotating the mirror about a substantially horizontal axis, the drive mechanism being connected to the sensor so that it can be controlled in response to the temperature of the sensor, the drive mechanism also being connected to a mechanical linkage arranged to rotate the mirror about a substantially vertical axis.

23. A device according to claim 22 in which the mechanical linkage includes a cam and cam follower.