THIS INVENTION relates to solar collectors and in partic a focussing solar collector-

Focussing collectors can be divided broadly into t systems.

(a) Troughs

(b) Dishes

(c) Heliostat/power tower combinations

The first two systems can be further subdivided as follow

Troughs

(a) Tracking on one axis.

(b) Tracking on two axes.

Dishes

(a) Parabolic

(b) Spherical

The main problem with troughs is the relatively low max temperatures attainable because a line focus rather tha point focus is produced. This applies to both types alth the second type is better in this regard than the first. temperatures cause heat transfer problems and give low th odyna ic efficiency and in endeavouring to overcome t problems by employing two axes tracking other complicat arise namely extracting heat at high temperatures from focus of a system which is swinging around as the sun tracked. The hot, exotic heat transfer medium tends to past the seals in pipes undergoing relative movement. ( exotic heat transfer fluids are necessary because pressure of steam at even the relatively low operat temperatures, typically 570 F, is 1,200 p.s.i., which enough to cause a blow out). This is apart from the act tracking problem - the rate of swing on the two axes can

be preset as it varies for each axis during the day, bein maximum for the elevation axis and a minimum for the azim axis' at dawn and the reverse at noon, then back to the d situation at sunset. Furthermore, the changing relations between the two axial swings changes over successive 24 h periods during the course of 12 months. The usual track method is to adjust the elevation and azimuth at frequ intervals during the day, this being accomplished automat ally via input from sensors. However, this results in jerky movement and a lot of the time an appreciable perce age of the reflected incident radiation misses the target.

Parabolic dishes produce a point focus resulting in h temperatures but they suffer the other disadvantages of axes troughs, namely a swinging focus, leaky exotic h transfer fluid lines (exacerbated by the higher temperatu and tracking problems (exacerbated by the generally big unit size and the greater precision required). In additi it is expensive to fabricate and support a large parabo dish - each panel in the dish is different to every ot not on its particular "latitude zone".

Spherical dishes are an improvement in many respects. instance they can be fabricated out of a large number o few types of panels and the tracking requirements simple. A spherical dish has an infinity of optical axis, opposed to a paraboloid which has one so that even completely immobile it is, in a sense, always perfec aligned with the sun. An immobile dish of course would o receive a fraction of the total possible energy int during the day so in practice the dish is tilted and rota around a vertical axis through the centre of curvature that it is facing in the general direction of the sun at times during the day. The trade off for these advantages the loss of the point focus - the "non paraxial" incid rays (ie; those parallel but at some distance from optical axis) are reflected to points on the axis progre

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ively further and further from the paraxial focus as t become more and more "non paraxial". In section reflected rays form a caustic curve. The result is that order to collect the reflected radiation a heat trans fluid pipe must at all times be positioned coincident w the optical axis of the moment. In other words the sys reverts to a low temperature line focus with a swing target. In this instance the target swing is independent the reflector and is relatively straight forward - rotates around a North-South axis through the centre curvature inclined at the latitude angle of the site at same speed as the earth rotates with respect to the which is essentially constant at 15° per hour. It a swings on a second axis through the centre of curvature right angles to the first axis to maintain the angle betw the first axis and the pipe essentially equal to the decli tion of the sun as it changes with the seasons. This known as an equatorial mounting and provides the simpl possible tracking.

The heliostat/power tower system is currently the m favoured in large solar power installations. It consists a large central tower with an immobile target sitting top. The tower is surrounded at ground level by a la number of plane reflecting surfaces which reflect sunli to the target. This system has many inherent advantages the tracking problem is multiplied by the number of ind vidual reflectors used - each reflector has to track elevation and azimuth and its attitude at any time different to all the other reflectors. At least with t axes troughs and parabolic dishes all units have the sa attitude at the same time - one unit can act as the mast tracking unit which all the others follow using the ser principal.

Obviously one system is required that incorporates the go points of the various existing systems without the problem

The desirable attributes are:

(a) Point focus (b) Immobile target (allowing direct generation of ste without the use of an intermediate exotic he transfer fluid and a steam generator).

(c) Ease of construction (d) Simple tracking

It has been found that this could be achieved by introduci a secondary reflecting surface into a spherical dish syst which gives the spherical dish a point focus at the cent of curvature.

In one form the invention relates to a solar collec comprising a hemisphere having a reflective inter surface; a caustical conical concentrator located at focus of the hemisphere and having a concentrated sec focus for radiation reflected onto its surface from s hemisphere, a heat exchanger located at the second foc said caustical conical concentrator being mounted to rot about a substantially North-South axis passing through s second focus and being driven by a drive such that w movement of the sun, incident solar radiation is constan directed at said second focus.

According to a preferred feature of the invention, second focus is located at the centre of curvature of hemisphere.

According to a preferred feature of the invention, hemisphere is inclined at an angle from the horizon substantially equal to the angle of latitude of the locat of the solar collector.

According to a preferred feature of the invention North-South axis is inclined at an angle to the horizon

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substantially equal to the latitude of the location of t solar collector.

According to a preferred feature of the invention s hemisphere is rotatable about said North-South axis.

According to a further preferred feature of the inventi the hemisphere is inclined at an angle from the horizont substantially equal to the angle of latitude of the locati of the solar collector and the lower portion of the hemi phere is fixed while the upper portion can rotate over t lower portion a second drive being associated with the up portion to move said upper portion such that said hemisphe is substantially constantly directed towards the sun duri its movement.

According to a further preferred feature of the inventio said caustic conical concentrator is rotatable about East-West axis and is capable of being driven by a thi drive to maintain the caustical conical concentrator at sa first focus as the sun varies its declination.

According to a further preferred feature of the inventi the caustical conical concentrator is associated with sensor located adjacent the light path between the caustic conical concentrator and the second focus to cause activ tion of the drives for said caustical conical concentrat on said light path intersecting said sensor.

According to a preferred feature of the invention t collector is spherical and the other hemisphere thereof substantially transparent. •

According to a preferred feature of the previous featur the caustical conical concentrator is associated with refractive and/or reflective concentrator mounted to t other side of the second focus in opposed relation to t

caustical conical concentrator and mounted to move opposed relation with the caustical conical concentrator having as its focus the second focus.

According to a preferred feature of the invention collector is spherical and is formed by a flexible membr inflated to maintain its shape.

According to a preferred feature of the invention, collector is spherical and is formed of two hemispheres rigid plastics material interconnected by a rigid circu frame supported by said diametric axial member.

The invention will be more fully understood in the light the following description of several specific embodimen The description is made with reference to the accompany drawings of which;

Figure 1 is a schematic sectional elevation of the fi embodiment;

Figure 2 shows a ray trace of solar radiation incident the collection of Figure 1; and

Figure 3 is a part sectional elevation showing the c struction of the shell of the second embodiment. Figure 4 is a schematic sectional elevation of the th embodiment;

Figure 5 is a ray trace applicable to the third embodime Figure 6 is a part sectional elevation of the third embo ment showing the boiler configuration;

Figure 7 is a part plan view of the third embodim showing the truss mounting thereof; and

Figure 8 is a schematic layout of an energy collect means utilising a bank of collectors according to third embodiment.

The embodiment of Figure 1 comprises a solar collec comprising a spherical shell 11 formed from an infla

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flexible membrane. The shell comprises one hemisphere having a reflective inner face, the other hemisphere lib substantially transparent. The shell is supported on a di etric shaft 12 having a substantially North-South orien tion and extending between the junctions of the hemisphe and is inclined at an angle to the horizontal substantia equal to the latitude of the location of the collector. E end of the shaft 12 is pivotally supported in suita bearings 13 and is driven by a drive motor to cause sphere to revolve about the central axis of the shaft. drive for said motor is controlled such that the reflect hemisphere is directed constantly towards the sun. At end of each day the sphere is caused to revolve t position at which the reflective hemisphere 11a will directed towards the sun when it rises the following d The drive for the motor may be controlled by a suita sensor to maintain the reflective hemisphere in position alternatively may be calibrated according to the s predicted movement.

The diametric shaft 12 supports a diametric frame 15 subst tially perpendicular to the shaft 12 and rotatable about centre of curvature of the sphere about a North-South a (principal axis) and an axis perpendicular thereto (sw axis) to ensure that the diametric frame remains in ali ment with the sun as it moves across the sky and varies angle of declination respectively. One end of the diamet frame 15 adjacent the reflective hemisphere 11a support reflective caustical conical concentrator 16 which located at the focus of the reflective hemisphere. curvature of the caustical conical cone is such that so radiation incident thereon from the reflective hemisphere reflected to a substantially point focus at the centre curvature of the hemisphere. The pivotal movement of diametric frame 15 on the diametric shaft 12 is control by a hydraulic means which is in turn controlled by suitable sensor to maintain the radiation reflected from

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caustical cone 16 incident upon the centre of curvature the hemisphere.

A spherical heat exchanger 20 is located at the point focus and heat exchange fluid is introduced to and extrac from the heat exchanger by means of fluid lines loca within the shaft 12. The heated fluid from the h exchanger passes to a means 17 for extracting the heat f the fluid which is then returned to the heat exchanger b return conduit 18.

The other end of the diametric frame remote from the cau ical conical concentrator supports a lens 23 which dire the solar radiation incident thereon onto the heat exchan 20. The other end also supports an annular array of conc tric parabolic reflectors 19 which shadows the causti cone from incident solar and reflects the incident radiat onto the heat exchanger 20.

The underside of the caustical conical concentrator formed at its outer periphery with an annular convex refle ing surface 21 which reflects radiation reflected from hemisphere to a small reflective cone 22 located centra below the caustical conical concentrator 16. The apex of caustical conical concentrator is cut away to permit rad tion reflected from the small reflective cone 22 to incident onto the heat exchanger 20.

The path of the solar radiation incident on the collector the embodiment may be more fully understood by reference Figure 2.

As shown at Figure 1, when in position the collector associated with a support structure 27 wich supports upper end of the shaft and accommodates the appropri equipment for extraction of the heat from the heat excha fluid and for controlling the movement of the collector.

addition, the concentrator is associated with a w deflector 28 which surrounds the lower portion of collector to protect it from distortion and damage due wind.

The construction of the spherical shell may be effected construction of a set of suitably shaped panels formed f a desirable flexible membrane and interconnected them by desired technique.

^• The second embodiment differs from the first embodiment o in relation to the nature and method of construction of

' spherical shell. The shell of the second embodim comprises a circular frame member 30 which is fixed to diametric shaft 31. The circular frame member 30 supports pair of hemispheres formed of a rigid plastics material. circular frame 30 is formed from a circular channel sect 32 where each flange 33 supports a pair of membranes 34 35 which extend across the circular frame 30 and are cla ingly mounted at their periphery to the respective flange of the frame by means of circular angle members 36. circular spacer 37 is located between each membrane. forming each hemisphere after each membrane is fixed to circular frame 30, a quantity of acrylic resin and catal is mixed and injected into the space between each pair membranes to fill the space therebetween. During the filli step, air displaced by the incoming resin is permitted escape. On completion of the filling step compressed air introduced into the space defined by. the circular frame and membranes to inflate each set of membranes to a hemi pherical shape. Pressure is maintained in the structu until the resin trapped between each pair of membranes h set hard. On completion of the forming step, one or both the hemispheres can be removed to permit installation of t internal mechanism of the collector. If desired, the reflec ive surface of the reflective hemisphere may be formed using a reflective material as the inner membrane 34 or

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providing a suitable reflective coating to the inner surfa after the forming process.

The concentrator of the third embodiment of Figures 4 to consists of a hemispherical reflector 111 with its o diametric face inclined to the horizontal at an angle eq to the latitude of the site. The reflector is set in ground slightly deeper than its lowest edge and the part the reflector above ground level (the shell 11A) is separ from the part in the ground (the bowl 111B) and can revo around a vertical axis through the centre of the hemispher

A spherical boiler 120 is located at the centre of hemisphere 111 with an insulated water inlet pipe lead into it and an adjacent insulated steam outlet pipe lead out of it. The pipes are inclined above the horizontal at angle equal to the latitude of the site and are alig North-South and pass out of the hemisphere slightly be ground level. The line of contact between the pipes co-incident with the North-South diametric axis (the prin pal axis) across the face of the hemisphere 111. At gro level the pipes are fixed by a concrete collar wh surrounds the hemisphere and are supported above the hem phere by suitable columns and struts 140.

The boiler 120 is surrounded by a diametric truss 115 wh can rotate around the principal axis and also swing on axis coincident with the East-West diametric axis of hemisphere (the swing axis). A reflecting caustic correct cone 116 is located at one end of the truss adjacent reflective face of the hemisphere 111 and a paraboloid/hyp boloid combination 119 (a counterweight collector) at other end. The curvature of the caustic correction cone is such that solar radiaton reflected from the hemisph will impinge on the cone 116 and is reflected to the cent boiler 120. The cone is shadowed from direct radiation f the sun by the annular paraboloid reflector 119A wh

reflects the radiation to a central hyperboloid reflec 119B which in turn reflects the radiation through the cen of the annular paraboloid 119A to the central boiler 120.

The boiler 120 is ^' surrounded. by a spherical heat shroud which rotates with the truss. Radiation from the cone the counterweight collector passes through fused sil covered slots 142 in the shroud to the boiler 120. The sp between the boiler 120 and shroud 141 is evacuated and interior of the opaque portion of the shroud 141 is silver

In operation the axis of ^' the truss 115 supporting the c 116 and counterwight collector system 119 is kept alig with the centre of the sun during the course of the d This is done by rotating the truss 115 about the princi axis at a speed equal to the rate of rotation of the ea with respect to the sun and maintaining the angle betw the principal axis and the cone counterweight axis ess tially equal to the declination of the sun by periodica swinging the truss on the swing axis. The movements accomplished automatically by motors and solenoids respo ing to signals from a sensor which detects any misalignme During the course of the day the shell 111A is periodica traversed on rollers around the horizontal rim of the b 111B so that it is essentially facing the sun at all tim In this manner most of the radiation impinging on collector is concentrated at the central ^• boiler 120 wh remains immobile at all times.

Water is pumped into the boiler 120 by a circulating p 143 attached to a central steam accumulator 150 (see Fig 8) and steam from the boiler discharges into the st accumulator 150. A first group of concentrators A are ser by the steam accumulator 150. Steam from the accumulator drawn off and passed at constant pressure through a sec group of concentrators B where it is super-heated bef utilisation in a turbine/alternator sub-system 144. Af

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partial expansion in the turbine some steam can be drawn and re-heated in a third group of concentrators C. The th groups of concentrators constitute a collector field wh acts as a supplementary source of heat to a fossil f fired steam turbine power station.

As shown at Figure 6, the boiler 120 is located at centre of the hemisphere and consists of an outer spheri shell 151 and a concentric inner spherical shell 152 wit gap between the two shells. A straight water inlet pipe passes through the outer shell 151 and discharges into inner shell 152. Steam discharges from the inner shell into the space between the inner and outer shells at a po opposite the water inlet, via a short internal pipe 154 is constrained by ribs between the inner and outer shell travel in a spiral path to a point adjacent to the wa inlet pipe 153 where it enters a steam outlet pipe 156 into the outer shell. The steam outlet pipe 156 lies contact with and parallel to the water inlet pipe 153. B pipes are insulated and the insulation is jacketed b jacket 157 which is set at its lower end in a concr collar 158 surrounding the hemisphere at an inclination the horizontal equal to the latitude of the site and has orientation along the North-South axis. The fully insula steam and water pipes pass through the collar 158 to a po outside the bank surrounding the bowl 111b where t connect into the field pipe system leading to the cent steam accumulator (see Figure 8). The centre line of jac 157 is coincident with a diameter of the plane defined the open face of the hemisphere 111 and the jacket 157 supported by legs 140 and struts running back to the rim the bowl on both sides (not shown) .

As stated previously, in order to ensure that solar rad tion reflected from the spherical surfaces of the bowl shell impinge on the central energy receptor it is necess to interpose a secondary reflecting surface, or caus

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correction cone 116. The resultant ray path is shown Figure 5. The correction cone 116 collects all refle radiation from an annular zone of the hemisphere bet central angle 45.3° and central angle 127° which is preferable size of the cone. Reflection from an appreci annular zone of the hemisphere 111 concentric with the 116 impinges on the back of the cone 116 and to utilise radiation an annular spherical reflecting surface redirects it to a small plane sided axial cone 122 and h to the central energy receptor as shown in Figure 5. S radiation incident directly onto the face of the cone would normally be dissipated but this is prevented by in posing a cassegrainian telescope type arrangement 119 a of the boiler 120 comprising a paraboloid reflector which reflects primary radiation (ie; radiation direct the sun) to hyperboloid reflector 119B and thence to boiler 120. A convex lens 123a and concave lens 123b uti radiation which would otherwise be blocked by hyperboloid.

As disclosed previously the caustic correction cone 116 the cassegrainian arrangement 119 (with associated lens are mounted on opposite ends of truss 115 which can be s on the swing axis corresponding to a diameter of the boi 120 and at right angles to the centre line of the jac 157. The truss 115 can also rotate about the principal a corresponding to the centre line of jacket 157. The ab movements are achieved by first providing a hoop 170 wh surrounds the energy receptor and is supported by diametr ally opposed sleeves 171 and 172. The lowermost sleeve can rotate on bearings around the jacket 157 and the ot sleeve 172 can rotate around an insulated pin 173 project from the receptor at the other end. A diametrically oppo pair of pins 174 project from the hoop 170 perpendicular the jacket 157 and the truss swings on bearings 175 on th pins. This is a type of equatorial mounting and provide simple means of keeping the axis of the cone/cassegrain

system aligned with the sun at all times.

A single drive unit (not shown) on the concrete collar at the edge of the bowl powers both rotation and sw movements and comprises a small electric motor equipped ^"" w an eddy current dynamometer type brake which drives the gear of an epicyclic gear train at practically no load an identical motor similarly equipped which drives the an lus gear of the train. Depending on the relative speeds the two motors (varied by the degree of braking up to f load) the carrier shaft of the train can be made to revo at an appropriate slow rate either forwards or backwar The carrier shaft drives through a double reduction w gear box and a set of bevel gears, a drive shaft 180 runn in bearings mounted on jacket 157. A pinion -182 on the of the drive shaft 180 drives a 180 internal gear wheel attached to sleeve 171, thus effecting rotation of the tr about the principal axis. Rotation about the principal a must be continuous during the day, at a mean speed of per hour, but rotation about the swing axis (which accom dates the changing declination of the sun during the ye need only occur intermittently - an adjustment once every minutes limits misalignment of the cone axis to a maximu 15 seconds of arc which produces an undetectable depart from circularity of the sun's image at the energy recept Rotation about the swing axis as accomplished by gear between a quadrant worm wheel 183 and a worm gear 184 bearing 175 and a half spur gear on jacket 157. The gear which is supported by brackets on hoop 170 consists o pinion 185 running on a ring gear 186 and a set of be gears 187 taking the drive off at right angles int solenoid activated reversing gear box 188 another set bevel gears 189 after the reversing box again taking drive through 90° to the sun gear of an epicyclic gear t 190 with a basic ratio of -1. The annulus gear of epicyclic gear train 190 normally free wheels on the car shaft but when adjustment is required it can be locked b

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solenoid activated dog clutch. This causes the planet gear carrier to rotate which drives the worm 184 mating worm wheel 183 thus swinging truss 115 about the swing a As truss 115 is fully balanced (the weight of the caus cone arm is less than the weight of the cassegrai support) very little power is required to operate movements.

As disclosed previously, the boiler 120 is shrouded minimise heat loss and the shroud 141 is a spherical me shell surrounding the boiler 120 and supporting the slee 171 and 172. The surface of the shroud coincides with circle of least confusion in the envelope of rays reflec from the caustic correction cone 116 and is silvered on inside surface, apart from two diametrically opposed elo ated slots 142 and 142 which admit radiation from caustic correction cone 116 and the cassegrainian t collector 119. The slots 142 are covered by fused sil windows coated with a ulti layer dielectric film to mi mise reflection losses. The outer rim of sleeve 171 equipped with a stuffing box type seal 195 against jac 157 to provide an air tight seal.

The operation of the system is as follows:-

At dawn on any day of the year the axis of truss supporting the correction cone and cassegrainian system aligned with a point on the horizon where the centre of sun is expected to rise. (The cone is west of the cent energy receptor) . At sunrise rotation of truss 115 initiated at a speed equal to the rotation of the earth w respect to the sun at the particular time of the year. the southern hemisphere rotation is clockwise looking Sou Because the axis of rotation is inclined above the ho zontal at an angle equal to the latitude of the site rotation maintains the cone axis aligned with the centre the sun as it travels across the sky provided the an

between the cone axis and the rotational axis is alte every 15 minutes or so during the day so that it maintained essentially equal to the sun's declination at time. Corrections to rotational speed and axis angle made automatically during the day in response to sign from a sensor on the caustic cone supporting arm. The sen consists of a number of thermocouple junctions arranged i ring around the reflected ray envelope at some point betw the cone and the heat shroud. When the cone axis correctly aligned the ray envelope is circular in cro section at any point and passes through the ring of ther couples without activating any of them. If a misalignm occurs the envelope becomes distorted and some of the ther couple junctions will be heated causing currents to flow those circuits affected. These currents are fed to a mic processor which senses which segment of the ring they orig ate from and what corrective action is required to rest circularity. It then sends the appropriate signals to circuits controlling current to the motors, eddy curr brakes and solenoids. As soon as circularity is resto signals from the sensors stop and so does the correct action, leaving rotation to continue at a constant sp until such time as further corrections are called up by sensor ring.

When a group of dishes are working together only one ne to be equipped with sensors, micro-processor and cont circuits - this acts as a "master dish" and the cont wiring is arranged in such a way that all the other colle ors in the group act precisely as the master dish acts.

While the cone axis is tracking the sun in a rather prec manner the reflecting shell is also being independen moved around its axis so that at any time of the day it facing in the general direction of the sun. The outcome that practically continuously during the day the axis of cone/cassegrainian system is pointing at the centre of

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sun and for most of the day the cone/cassegrainian syste backed, by the full effective hemispherical reflect surface which is a zone subtending 127° at the centre of hemisphere. Thus for most of the day all direct radiat from the sun on a circular area of Tf (R sin 63.5) un normal to the sun's rays and centred on the centre of hemisphere is concentrated, minus reflection and ot losses, on the central immobile energy receptor. (R = rad of the hemisphere). This radiation is absorbed by the ene receptor and converted to heat which is transferred thro an insulated steam line in the form of essentially satura steam at 600 p.s.i. to a central steam accumulator. A boi circulating pump supplies water from the accumulator thro an insulated main to the energy receptors in a field concentrators. Saturated steam is drawn off the accumula and passed at constant pressure through other concentrat where it is superheated to 850 F. Under these conditions steam is suitable for utilisation in the turbine circuit a thermal power station up to 30 megawatts capacity. A la field of collectors with extensive heat storage would required to supply this amount of power alone but a hund

20 metre diameter dishes in a 350 x 350 array wo supply 42,900 tonnes of steam per year at 600 p.s.i.

850 F to supplement the output of steam from the fossil f fired boilers of the power station. Where the boilers fired by fuel oil this is an attractive econom proposition.

Another application is in the provision of high gr process heat. Future applications could be in the dir conversion of heat to electricity via thermionic converte These devices have not been perfected at this time but t show good potential to achieve efficiencies of 407 _o at temp atures of 1000 to 1500 C which is not much less than best fossil fuel fired thermal stations. They would be b suited to operation in conjunction with concentrating so collectors.

The criterior for the configuration of the caustic c corrector and other optical features of the invention w now be discussed.

Caustic Correction Cone

The generating curve of the caustic correction cone 116 parametric form is given below:-

X = 1 ₊ sln 2 ⁽ B -fr ⁾ . cos 2-θ-

2 cos -θ- sin 2(2-θ-- B)

Y = R sin -Q- sin 2(B--Θ-) sin 2(2-β-B)

where

R - radius of the sphere

-G-= angle subtended at the centre of the sphere by arc of the sphere in the plane of the co-ordin system measured from the X axis to a point wher ray parallel to the X axis (the incident ray) int sects the sphere.

β = tan- slope of the generating curve at the po (X,Y) corresponding to the point where the reflec ray resulting from the above incident ray interse the generating curve.

_&) = a log-θ- - b (log-θ-) ² - (c log-θ-+ d) ( S (log-β-) ² + g log-θ-+ h) - k log (

- - - - log-0-+ m -t- n/f (log"θ-) + g log-0"+ h) -

a = 2.97376 k = 0.246541 b = 0.255737 >t = 3.65602 c = 0.205587 m = 1.60789 d = 0.0904155 n = 2.70408 f = 1.82801 p = 1.22642 g = 1.60789 h = 1.97494

Theta, -0" . is in radians and logs are to the base e

The limits of the generating curve are:

X ^• = 0.830384, Y = 0.3851 and X = 0.585203, Y = 0.029691 based on a hemisphere of unit radius.

Figure 9 depicts a half longitudinal section of two envelopes as reflected from the caustic correction cone. origin of the rays is the periphery of the sun's disk it has its maximum semi diameter of 16.3 minutes of arc. steep envelope is formed by rays reflected from the max circumference of the cone and the flat envelope is forme rays reflected from the minimum circumference. It can seen that the diameter of the sun's image as formed o plane through the origin at right angles to the cone axi 0.086 (for a hemisphere of unit radius) or 86 cm fo hemisphere of 20 diameter. Also, the circle of l confusion occurs at a point on the X axis 80 cm from centre of a 20 m diameter hemisphere. The circumference the circle of least confusion is co-incident with surface of a sphere 1.76 diameter (for a 20 m diam hemisphere) and this fixes the diameter of the heat shr The cone substrate is formed out of rigid self skin polyurethane foam 10 mm thick. The density of the foam 185 kg per cubic metre giving a total weight of the s strate of 98.4 kg. This weight can be moulded in one shot.

The silver reflecting surface is vacuum deposited on

face of the cone and protected by a clear acrylic f polymerized in place.

Annular Convex Spherical Mirror

The equation of the generating curve of the annular con spherical reflecting surface 121 backing the caustic corr tion cone is (X + 1.33238) ² + (Y - 0.926749) ² = 4.97094

The limits are X = 0.830384, Y = 0.3851 and

X = 0.793318, Y = 0.254182

This is formed as an integral part of the caustic correct cone in the one moulding operation.

Axial Cone

The equation of the generating curve for the axial cone which redirects reflected radiation from annular spheri mirror 121 to the central energy receptor 120 is:

Y = 1.7882X - 0.980432

The limits are X = 0.869114, Y = 0.0440965 and

X = 0.835488, Y = 0.004458

The substrate is glass 6 mm thick silvered on the ins surface. The silver film is protected by a coating aliphatic polyurethane paint.

Paraboloid

The generating curve of paraboloid 119A is:

y ² = -0.570392 (x + 0.2)

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The limits are x = -0.46, y = 0.3851 and x = -0.2086026, y = 0.07005

The paraboloid is formed out of the same material and in same manner as the cone.

Hyperboloid

The generating curve of the hyperboloid 119B is:

y ² = 0.327036 |(x + 0.1713) ² - 0.022112

The limits are x = -0.363956, y = 0.07005 and x = -0.321265, y = 0.0111511

It is constructed of the same material as the axial cone (ie; glass silvered on its inside surface).

Convex Lens

The generating curve of the spherical surface of the con lens 123A is (x + 0.481613) ² + y ² = 0.0526

The limits are x = -0.7, y = 0.07005 and x = -0.710959, y = 0

The lens is 6.96 cm thick at its edge (for a 20 m diame hemisphere) and the plane back is stepped inwards in series of concentric zones to cut down weight. The lens moulded from methyl methacrylate with a refractive index 1.49 giving a focal length for paraxial rays of 4.6 metres for a 20 m diameter hemisphere.

Concave Lens

The concave lens 123B is made of glass of refractive ind 1.46 and for a 20 m diameter hemisphere is 3.29 cm thick the edge and 2.0 cm thick at the middle. The front surfa

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is plane and the generating curve for the rear surface (x + 0.2731) ² + y ² = 0.002131

The limits are x = -0.317971, y = 0.0108 and x = -0.319265, y = 0

The focal length for a hemisphere of 20 diameter is 1.00 metres.

It should be appreciated that the scope of this inventi need not be limited to the particular scope of the embod ment described above.