GB2341675A - Solar collector and cooker - Google Patents

Abstract

A solar collector comprises a spherical reflector dish focussing on to an element to be heated or a photovoltaic generator. The spherical reflector dish is mounted on a base and has a sun locating means 3. The solar collector may be formed from flat triangular elements which may be laser cut and scored on their non-reflective side so that they can be folded. The sun locating means 3 may comprise a square tube which intersects the dish creating when it is aligned with the sun an equal sized image on the ground. This may used to aligned the dish towards the sun. The element may be attached to the dish using studs 14 and a cord and hook arrangement 13.

Description

2341675 SOLAR COLLECTOR AND COOKER This invention relates to a solar

concentrating collector and cooker comprising a reflector dish and an integral base unit.

Solar cookers have been proposed since the nineteenth century, relying either on the greenhouse effect, in the form of insulated boxes, whose interior is painted black, closed at the top by a sheet of glass, in which food to be prepared cooks slowly at temperatures around 100" C, or on concentrators which, either by parabolic, spherical or other geometries focus solar radiation on the vessels to be heated or foods to be cooked at much higher temperatures.

According to the present invention, there is provided a solar concentrator / cooker which consists of the concentrator dish, assembled from any number of coherent elements, of the central focus location system, and of a base which both supports and gives shape to the dish, but allows it to be correctly aligned with the sun, pivoting about its base and set in the optimal position by adjustable wedges so that the linear or point focus receives the greatest insolation, this being insured by an in built solar tracking device. The dish, the focus location and the base form an integral whole, which is designed to be made up from scored and cut flat sheet material, being easy and compact to pack, and cheaper to manufacture than three dimensionally formed components.

In a preferred embodiment, there is provided a spherical or parabolic geometry reflector, with its compatible base, concentrating direct solar radiation onto its focus, which is cylindrical and half the radius of the sphere in height in spherical geometry, or theoretically point-like in parabolic geometry.

Spherical geometry has the advantage of a cylindrical focus, so that any cylindrical element, for instance, a pot, a pressure cooker, a needlesterilising pressure vessel or a bottle will be intensely heated at the focus, the focussing occurring as the base is moved and aligned to the sun. The reflector can be made up of a number of flat mirrors, these being disposed in varying methods, such as placed along the radii of the circle, then of truncated triangular shapes, or made up of triangles divided into smaller triangles, both different types of sphere being formed from flat elements being drawn together.

A preferred way of joining these elements is by demountable plastic ties that draw the surfaces together between a front mounted and a rear mounted pierced tab to prevent overlapping, which would weaken the dish and distort its optics. When disassembled, the mirror elements can again be flat packed for ease of transport, The food to be cooked, for instance in a cylindrical metal oven, or the element to be heated are placed at the focus on support pads. As the concentrator / cooker is gradually inclined and turned to track the sun, restraining dowels tied down to the base prevent the elements from slipping, locating them firmly at the dish focus.

2 A cord and hook assembly hooked to handles on these elements and attached to the upper part of the dish prevent these from overturning, the hook being simply released from the handle or grip to allow the oven or other vessel to be lifted out of its focal location. In this way, the element to be heated is always located at the focus, whatever the concentrator / cooker inclination and orientation.

in another embodiment of this invention, a cylindrical or polygonal array of high temperature photovoltaic (PV) cells, of a height of half the radius of the sphere geometry, are located at the focus so that cooling air can circulate behind and around them, and from a small area, they can produce electricity more efficiently than could a flat PV panel.

In a preferred embodiment, a pentagonal base is chosen because it provides a pivot line at the front, and a high point at the back well suited to the tracking device position, and to the high attachment for the cord and hook assembly. However, other base geometries can be proposed, for instance, square, triangular or hexagonal, depending on the upper reflector geometry.

Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings of which Figure 1 shows a sectional plan view of the reflector dish and its base structure.

Figure 2 shows an elevation or side view of the reflector dish attached to its structured base.

Figure 3 shows a section through the assembly with the reflector dish and its focus, the position of the tracking device and the base structure details.

Figure 4 shows the inclined assembly in section with the reflector dish, its base with its front location pad, the aiming device lined up with the sun, and the inclination wedges.

Figure 5 shows a section through the inclined collector / cooker, illustrating the reflector dish, the base with its continuous triangular ground beam, the aiming device, the focus base with its support and restraining studs, a typical heating oven painted black externally, with its cord and hook restraining device.

Figure 6 shows the aiming device, a square hollow tube which, placed on the diagonal, and aligned with the sun, defines, by the shadow it casts and the light it allows to pass, either a perfect alignment, or allows the collector / cooker to be "advanced" by, say, half an hour, and for its sun point to vanish 1 hour thereafter.

Figure 7 shows one of two wedges designed to accept the assembly base triangular beam.

Figure 8 shows a method of bringing together two flat sheets and of tying them firmly so that they cannot overlap.

3 Figure 9 shows a perspective view of the focus base, and oven located, supported and restrained at its focus, as well as the dish, base and orientation wedges.

Figure 10 shows the perforated centre piece as well as the method of fixing the locating and support studs. It also shows in section a variant in which the support and locating studs are elongated around a focus base whose centre has been raised so that incoming reflected radiation is diverted upwards onto the base of the cooking vessel, thus permitting frying, as in a "wok".

Figure 11 shows an array of photovoltaic cells located at the reflector focus. These are lightweight, and are held firmly at the base, which is perforated in its centre so that a cooling air current can flow through the hollow element.

Figure 12 shows a method of packing down the laser scored and cut component elements making up the collector / cooker. The dish is preferably made of triangular elements which can be folded down around a central triangle as shown. The base edge elements can be folded in two, whilst the ribs and wedges as well as the tabs, ties, cord and hook can all be made to fit, say, in a 50 cm by 50 cm square flat box. In an illustrated variant, the bottom of the box can also contain four edge panels, say also 50 cm by 50 cm, so that, when tied together, they can form a complementary "hot box" to keep, with added locally available insulation, foods or liquids warm, avoiding packaging waste and providing a useful extension to the cooker, Referring to the drawings, the solar collector 1 cooker is made up from a reflective dish, 1, generally circular or polygonal,, as shown in Figure 1, which can be made up of a number of flat triangular or polygonal elements, 25, as shown in Figure 12, forming when assembled a generally spherical, or paraboloid surface. The triangular or polygonal elements are made from flat sandwich material and are scored at the back of the reflective surface so that they can fold in one direction to form a curved section, this being achieved by drawing together edges either side of the slits, 36, these being drawn together as shown in Figure 8 such that the edges of the reflective surface, 17, are sandwiched between two hinged and perforated tabs, 16, such that a demountable tie, 15, can pull them together without the possibility of them overlapping. For instance, 5 triangles can so be tied such that each forms one fifth of a spherical dish, made up of a number of flat focussing reflective surfaces.

The material used needs to be reflective on the concave side, so that incident solar radiation is reflected by the triangular or other polygonal shaped basic elements.

A cheap solar cooker can be so made from a composite material such as a reflective film on one side,a waterproof card or foamed plastic core at the centre, and a balancing plastic film on the other side. The same packaging type material can be used for the dish as well as for the base and the packing / "hot box".

4 A more expensive but more durable reflective dish and cooker can be made from reflective metal sheet, the cut elements being hinged and hooked or clipped together with appropriate fasteners, the dish resting on a metal collapsible frame, the tracking device having the same properties.

Spherical geometry is such that all radiation failing on the dish orientated perpendicularly to it is reflected onto a linear focus whose height from the surface is half the radius of the sphere. An element, 10, located to intercept reflected radiation at that focus will be heated to a high temperature, typically over 2000C. This element is held always in the same relationship to the spherical dish, by the studs,14, and cord and hook arrangement, 13, shown in Figure 5. Other fixing methods can be used, such as, for instance a metal cylindrical cage attached to the base and allowing the element to be heated to be slotted into such a cage, insuring that the element is always at the fixed focus. In parabolic geometry, the point focus is above the surface of the dish, and a metal sub frame such as a tripod needs to be built to support the element to be heated, but again keeping it in the same relationship to the dish.

The solar collector 1 cooker, as particularly shown in Figures 2, 3, and 4 comprises an integral base 2, made up from edge panels, 35, which have perforated slits, 34, which permit the base of the edge panel to be folded back and fixed to the tab, 34, such that a triangular base beam, 5, is formed, giving great stiffness to the whole base assembly.

Figure 1 shows a plan of the base, with, in this example, five edge panels, 35, linked together and five radial ribs,6, horizontal along their base, but spherically curved along their upper surface, 30, to insure the correct shape of the attached dish,l. The ribs join at the centre and there support the pentagonal perforated focus support, 7.

A pentagonal base as shown in Figure 1 is well suited to this collector 1 cooker in that it offers a long flat pivot line along axis AA in Figure 1. This pivot line is reinforced along its base by an L shaped unit, 18, shown in Figure 5. At the centre of this pivot line AA is a perforated front fixing tab,4, which can be pegged down, so that the assembly cannot slide forward, but can only pivot around the peg, 11, to follow the sun. This allows the wedges,9, shown in Figure 7, also possibly pegged down, to be brought right up to the cooker to give an inclination of up to 6W to the horizontal, more than is ever likely to be necessary. Instead of wedges, an adjustable prop, 42, can be used at the rear, as shown in Figure 5.

At all inclinations, the element to be heated, 10, shown in Figure 5, remains in the same optimum position in relation to the reflector dish,l. When six frequency geodesic geometry is used, the five triangles constituting the dish are made up of 175 mirrors whose reflected energy is concentrated onto the focus. A variety of other uses can be made of this concentrated heat and radiation, such as PV cells, thermoelectric generators, Stirling engine heat sinks, various heaters and dryers.

A hollow cylindrical vessel, 10, say, 30 cm high by 16 cm diameter and containing over 5 litres, painted with high temperature matt black paint will reach internal temperatures of 2500C, given a dish diameter of 140 cm, and so act as an oven, cooking food in the same time as an electric oven set at that temperature. Water can be boiled in such a vessel, ( needing to be fitted with a lid to prevent over spilling when inclined,) thus allowing water to be sterilised. If a condenser is placed above the boiling vessel, distilled water can be produced. Temperatures reached are high enough to cook in pressure cookers, and for pressure vessels for needle and other medical sterilisers. It can so be useful where bottled gas is not available. Also, it can replace wood or charcoal fires, and thus have a positive effect on deforestation and its consequences.

The dish is preferably constituted of flat mirrors so as to be optically safe, whereas curved concave mirrors can be dangerous in that light at their point focus can damage the retina. To prevent glare however, it is to be recommended that sunglasses be worn whilst cooking, and that children be kept at least 5 metres away from the concentrator 1 cooker. Every energy source can be dangerous. Warning must be given not to touch the heated element, and not to place ones' head at the focus.

In another embodiment show in Figure 6, a tracking device,3, is fitted to the rear part of the base, to intersect the dish, 1. It is formed of a square tube, set on the diagonal. It creates, when its "window" is aligned with the sun, an equal size square on the diagonal image on the ground, easy to perceive in the shadow cast by the dish, shown in dash and dotted lines. In a preferred embodiment of this device, the angle between opposite top and bottom corners can be 7,50, casting a very small square of light at this extreme point. The 7.50 displacement to the centre represents a time of half an hour, whilst the movement of the sun to the opposite extremity of the device provides a further half hour lapse of time. It is thus possible, using the tracking device, to align the collector 1 cooker exactly with the sun, but also, to preset it, using the light square sizes as markers, by up to half an hour to the east, giving it a one hour time lapse until the light square disappears to the opposite side equal size square. This time lapse can be less, when the light square goes from a median size to its opposite equal size, various cooking times without moving the cooker being thus possible, depending on the length of cooking requirements, and the intervals at which the cooker is realigned. Because a spherical focus is less sharp than that of a paraboloid, a spherical dish collector 1 cooker may need to be aligned, say, every half hour, whilst its paraboloid counterpart will need adjusting every five minutes. The smaller altitude adjustments are effected using the tracking device and the base wedges. Thus, the in built tracking and timing device is an essential and integral part of the collector 1 cooker.

Figure 7 shows the wedges, 9, which are formed by two triangular edge pieces, with a central recessed triangular spacer, 19, of such width that it can support the base triangular ground beam, 5. The wedges'base plate, which is not recessed, 6 extends beyond the wedge base, and is provided with holes front and back so that it can be pegged down if necessary. In high winds, the unit, pegged at the front, can be tied down by cords from the fixings of the base rear corners to pegs or weights.

Figure 9 shows an overall perspective view of the collector / cooker in operation. In this example, a container, which could be a clean paint can, preferably twice as high as it is wide, and painted matt black externally to absorb maximum radiation is shown acting as an oven. It is located at the focus of a five "petalled" reflector dish, 1, by four studs tied down to the pentagonal centre piece so that, independently of inclination, its base cannot slide. As the inclination increases, the tension on the 'cord and hook', 13 increases and prevents the can from failing over. To remove or replace the can in position, the hook is released or repositioned.

Figure 9 also shows the base panels, 35, with their triangle base forming tabs, 34, as well as the inclination location wedges, 9, and the front base pegged pivot 4. It shows the reinforcing base angle, 18, running along the pivotal front edge. The aperture of the aiming device 3, is at the top of the base below the inclined dish, and in turn lower than the attachment point to the highest dish tie of the 'cord and hook' assembly, 13. Depending on the heating vessel and its handles or attachment points, other fixing methods can be devised, so long as they insure that the vessel, or vessels if superimposed, are properly maintained at the focus.

Figure 10 illustrates the centre piece of a pentagonal dish which is perforated by twinned holes such that a variety of studs, 14, can be fixed with demountable ties on any axial location. High studs serve to locate the heating vessel, whilst low studs serve as support pads to keep it off the mirror surface. The ties are demountable, and different vessels can thus be used readily. In a variant, the pentagonal centre piece can have added above it a version with 5 triangular surfaces slightly inclined upwards to an apex 24, such that, with high support studs, a frying pan or wok can be used, since radiation is thus reflected upwards to heat the vessel bases, to allow foods to be fried.

Figure 11 shows a pentagonal array of photovoltaic cells, 37, arranged so as to form a hollow tube so that apertures in the base, 39, and at the top, 40, allow air to circulate freely at the back of the panels, the array being firmly tied down to the base by five spurs,38. In a variation, the PV cells may need to be pivoted around pivot points, 41, so as to better capture glancing radiation. PV cells are expensive, and the use of high temperature cells at the focus of a concentrator can improve their cost efficiency. Small scale electricity generation is important to the developing world, for example, for recharging batteries, and can be an important function of the concentrator.

It is important that such a collector / cooker can be packed down compactly so as to be sent to distant locations in readily handleable form, and that it ran be carried with ease if demounted.

7 The method proposed shown in Figure 12, is to use laser scored flat sandwich panels, scored only on the non reflective side so that, in some cases, the reflective film acts as a hinge. In this way, a six frequency geodesic component,25, with the cut slits, 36, that can be drawn together to give it its required shape, can lie flat, with three triangular elements 26, folded back on each other giving a single four thickness flat triangle, 27, which is then of moderate size, the longest edge being say, 50 cm long. The base panel, 35, can also be folded down the middle, 28, so that it will also fit in a compact box, say 50 cm by 50 cm on plan, as can also fit the short radial ribs, 30, the wedges, 29,as well as the jointing tabs, the ties, the cord and hook, and the locating and support studs. Typically, If the material used is 3 mm thick, the depth of the box will be 12 cm deep. The packed dimension of one cooker / hot box is 50 cm by 50 cm by 12cm, 31, which also contains the four side panels of approximately 50 cm by 50 cm to make up a cubic' hot box' in which a locally found insulator, for instance, straw with a clay binder, or cork, or cellular plastics can be used to keep the solar cooked foods warm until they are consumed.

A preferred reflector dish has a diameter of 1,4 metres allowing this compact packing, but intercepting 1 kW of solar energy which is directed to the focus.

The product is seen as particularly advantageous to many developing world countries, where sun is plentiful, and there is a dearth of fuel, and where wood burning causes environmental havoc, leading sometimes to deforestation, and at its worst, desertification. Such use of solar energy is also beneficial to polluted atmospheres, and has no adverse effects on the ozone layer.

8

Claims (13)

1. A solar collector / cooker comprising a spherical or parabolic reflector dish focussing onto an element to be heated or a photovoltaic generator located at the central focus, the reflector being firmly attached to a base structure provided with a sun tracking device such that it supports the dish and allows the assembly to be orientated accurately to achieve optimum insolation.
2. 2 A solar collector I cooker comprising a dish and a coherent base, as in claim 1, made from flat packed elements which, when tied together, form the reflective concave dish and its rigid support base.
3. 3, A solar collector / cooker, as in claim 2, whereby flat sandwich material is laser cut and scored on its non reflective side to allow subcomponents to hinge and be tied together to provide the required geometry.
4. 4, A solar collector / cooker, as in claim 3, where the dish and base are made up from flat material elements tied together with demountable ties, where free surface edges being pulled together are sandwiched between perforated hinged tabs to prevent them overlapping, as shown in Figure 8, the whole assembly being demountable.
5. 5. A solar collector / cooker as described in claims 1, 2, and 3, whereby the bottom of the base panel is folded back twice to provide a rigid triangular base beam, giving stiffness to the whole assembly and allowing the front base reinforced edge to take the loads imposed on it as it pivots to face the sun. Radial beams stiffen the base and are cut to the correct curvature to insure a firm and accurate match between the base and its reflector dish.
6. 6. A solar collector / cooker as described in claims 1, 2, and 3 which is fitted with an integral aiming device as shown in Fig 6 allowing alignment and presetting of the assembly with the sun.
7. 7. A solar collector / cooker as previously claimed equipped with a central locating plate as shown in Fig 8, perforated to take base locating and support studs for vessels to be heated and photovoltaic panels.
8. 8. A solar collector / cooker as described in claims 1 to 7 which, when inclined to follow the sun in both inclination and azimuth, insures that the vessel or receiver placed at its central focus always receives maximum reflected insolation and does not move in relation to the reflector dish, being withheld from toppling by a' cord and hook 'assembly, as shown in Fig 9. or other arrangement..

   9. A solar collector / concentrator whose dish and base assembly are held in the required position by a base front pivot point shown in Fig 9, and triangular wedges shown in Fig 7, or by an adjustable prop at the rear.
9. 9
10. 10. A solar collector 1 cooker as in claims 1 to 3 made up of flat packed elements that can fit into a compact box, which can also contain four side sheets of material so that a complementary insulated hot box ' can be constituted from the packaging.
11. 11. A method of heating an article or generating electricity, by using concentrated solar radiation from a dish and base assembly as according to claims 1 to 10, so that the article to be irradiated follows the same movement as the assembly.
12. 12. A method of generating electricity or heating an article to a high temperature, sufficient for oven cooking, pressure cooking, boiling water, and pressure vessel sterilisation, as substantially herein described.
13. 13. A method of generating electricity or heating an article by means of solar radiation substantially as herein described.