FIELD OF THE INVENTION

This invention relates to a heating system which utilises solar energy and in which heat energy is transferred in a downward direction from an upper energy collector to a lower energy transfer point. The invention is applicable to a closed system, i.e., one in which a heat transfer fluid is re-cycled, and it has been developed to obviate the need for motor driven pumps and the like.

The invention has been developed in the context of a cooker for use in situations where skilled technologists and tradesmen are not permanently available to provide maintenance or breakdown repairs, where electrical or other such energy forms are not available and/or where combustible fuel reserves have been depleted. However, whilst the invention is to be hereinafter described in such context, it will be understood that it does have broader application and that it need not be limited to the cooker application. PRIOR ART

The inventor has already undertaken significant work in the context of cookers that utilise the heating effects of solar energy. Thus, one system has been developed in which a solar collector is located slightly above ground level and in which the collector is used to generate steam which is piped into a storage vessel

1 SUBSTITUTE SHEET

which is located at the same level as or slightly higher than the collector. The storage vessel is topped by a cooking surface in the form of a flat plate or bowl and cooking is effected by steam rising into contact with the underside of the cooking surface.

This system has been found to work in a satisfactory way, in terms of providing the requisite energy necessary for cooking under various ambient conditions, but it does rely on the upward movement of steam from the collector to the cooking surface and the downward movement of condensate from the cooking surface to the collector. This, in turn, requires that the collector be located at or near ground level where it is exposed to the risk of damage or interference. AIM OF THE INVENTION

The present invention seeks to avoid this problem by providing a system in which a collector may be located in an out-of-reach situation and in which heat is transported downwardly from the collector to a zone at which the heat is to be utilised. SUMMARY OF THE INVENTION

Thus, the present invention provides a solar heating system which comprises an insulated vessel containing a heat exchange fluid, and a heat transfer region located in heat exchange relationship with and exposed to the heating influence of fluid within the vessel. A reservoir is located at a level above the vessel and contains a further quantity of the heat

exchange fluid, and a single valveless conduit connects an upper region of the reservoir with a lower region of the vessel. Additionally, a solar collector element is located in heat exchange relationship with the fluid in the reservoir and is arranged to effect heating of the fluid within the reservoir during times when the collector element is exposed to incident solar radiation. The total liquid-phase volume of the fluid is less than the total contained volume of the vessel, reservoir and conduit whereby a vapour phase of the fluid transfers downwardly from the reservoir to the vessel when the pressure within the reservoir exceeds that in the vessel, and a liquid phase of the fluid transfers upwardly from the vessel to the reservoir when the pressure within the vessel exceeds that in the reservoir.

In operation of the system, a liquid phase of the fluid within the reservoir is heated by the solar collector and is converted to vapour when the collector is exposed to solar radiation. The pressure within the reservoir thus rises above that existing in the lower vessel, and this results in the downward flow of vapour from the reservoir to the lower vessel. On entering the vessel, the vapour condenses, giving up sensible and latent heat to the fluid within the lower vessel and the heat acquired by the fluid within the vessel is then used to heat the heat transfer region. For as long as the solar collector continues to be exposed to incident

radiation, the liquid phase of the fluid within the upper reservoir will continue to boil away and transfer as vapour to the lower vessel, causing the level of fluid in the vessel to gradually rise. At the end of each day, when the incident solar radiation diminishes, the temperature of the depleted fluid within the upper reservoir will gradually drop to a level approaching ambient temperature and, thus, the temperature and pressure in the upper reservoir will fall below the temperature and pressure in the lower vessel. The resultant pressure differential then acts to force liquid out from the lower storage vessel and up the conduit to re-enter and fill the upper reservoir. This condition is then maintained until the collector element is again exposed to solar radiation, when the cycle will be recommenced.

A pressure differential of one atmosphere will raise a column of water approximately ten metres and it has been established that pressure differentials of this order can readily be achieved.

Thus, the present invention embodies a pumping system which utilises the day-night cycle to facilitate the downward transport of vapour to the lower storage vessel and the upward transfer of liquid to the upper reservoir. This pumping system functions without there being any need for any moving parts to assist in the pumping operation.

SUBSTITUTE SHEET

The heat exchange fluid would normally be water, for convenience of replenishment in the event of any losses from the system, but other fluids or fluid mixtures may be employed. The volume contained by the lower storage vessel will be dependent upon the amount of heat to be stored in and transferred out from the system, and the day-night cycle of the system. Also, the size requirement for the upper reservoir will be dependent, at least in part, on the rate at which heat is to be extracted from the lower vessel. When the total input heat is extracted from the lower vessel during the daylight period, such as may occur in an industrial heating process, the upper reservoir may be required to have a volume approaching that of the lower vessel. However, it is expected that in most applications of the system the upper reservoir will have a volume less than that of the lower vessel. In any case, the system should be structured in such a way that when heat input from the solar collector ceases, the rate of temperature drop at the upper reservoir will be greater than that at the lower vessel, so that the pressure in the upper reservoir will drop more rapidly than the pressure in the lower vessel.

The total volume within the system should be arranged such that total boil-off from the upper reservoir does not occur, in order that fluid will always be present in the collector element to limit or prevent overheating.

SUBSTITUTESHEET

Although the solar heating system does have various applications, it has particular application in the context of a solar cooker, in which case the heat transfer region which is exposed to the heating effect of the fluid within the lower storage vessel will be constituted by one or more hotplates on which food may be cooked. The or each hotplate may simply be exposed to rising heat from the fluid within the vessel, or the heating effect of the fluid may be channelled to the hotplate. In the latter case, the hotplate may be isolated from the main body of the fluid and vapour may be channelled to the hotplate by a conduit. The quantity of heat delivered to the hotplate may be _ regulated by locating a control valve in the conduit. The invention will be more fully understood from the following description of a preferred embodiment of the solar heating system. DETAILED DESCRIPTION OF THE DRAWINGS

The description is given with reference to the accompanying drawings in which:

Figure 1 shows a schematic representation of a solar cooker system under conditions that exist prior to the input of solar energy;

Figure 2 shows the same representation as Figure 1 but under a condition that exists following input of solar energy;

Figure 3 shows an end elevation of a collector arrangement in operative relationship to a reservoir as used in the system of Figures 1 and 2; and

Figure 4 shows a front elevation view of the collector arrangement which is illustrated in Figure 3.

As illustrated, -the solar cooker system comprises a lower insulated storage vessel 10 which contains a heat exchange fluid in the form of water which occupies a lower volume 11 of the vessel. An upper volume 12 of the vessel provides a space into which steam may rise.

A conduit 13 extends into the vapour zone 12 and is used to convey steam into a flat spiral wound element (not shown) that is located at the underside of a hotplate 14. A manually operable control valve 15 is located in the conduit 13 for regulating the flow of steam through the spiral wound element, and a return conduit 16 extends from the trailing end of the spiral element for returning condensate to the bottom of the vessel 10 after latent and sensible heat has been given up to the hotplate 14 by the steam.

An over-pressure safety valve 17 connects with the interior of the vessel 10 for releasing excessive pressure from the vessel and/or bleeding any air that may be in the system during an initial charging operation.

The storage vessel 10 would normally be located near ground level in a building (not shown) and it is connected by a conduit 18 to an upper fluid containing reservoir 19. The reservoir 19 would normally be located on the roof of the building or, in any case, in a position such that it may be connected to a solar

SUB ^S TITUT

collector unit 20 that is exposed to solar radiation. The reservoir 19 also is insulated, and the conduit 18 is lagged to minimise heat losses when transferring energy from the reservoir 19 to the vessel 10. However, the reservoir 19 does have a surface area to volume ratio that is greater than that for storage vessel 10, so that it will lose temperature at a rate greater than that of the storage vessel 10. Additionally, the fluid within the reservoir 19 is exposed to the ambient by way of the collector unit 20, this also providing for a more rapid loss of heat from the reservoir 19 than from the vessel 10. These characteristics are important in the context of the operating cycle of the system.

The storage vessel 10 typically has a contained volume in the order of 15 litres and the reservoir 19 has a contained volume in the order of 4 to 5 litres. The interconnecting conduit 18 has a bore diameter of approximately 4mm and it connects an upper level 21 of the reservoir 19 to the lower level 11 of the vessel 10. As illustrated in Figures 3 and 4, the solar collector unit 20 comprises two oppositely extending, generally horizontal, metal tubes 22 which are closed at their outer ends and which are connected by upper and lower feed pipes 23 and 24 to the interior of the reservoir 19. Each of the tubes 22 is slideably located within a surrounding collector element 25 which extends for substantially the full length of the tube. In Figure 4 the collector elements are shown to be

displaced outwardly along the tubes 22, so as to expose the tubes, but the collector elements would not normally be so positioned.

Each collector element 25 comprises an inner tube, 5 an outer glass tube, a solar selective surface coating on the outer surface of the inner tube, and an evacuated space between the two tubes. The solar selective surface coating functions to absorb solar radiation and to transfer the heat energy of the radiation through the 10 collector element to the tubes 22.

Heat energy that is collected by the collector elements 25 is transferred through the tubes 22 and into the liquid which, in operation of the system, flows into and out from the tubes 22 by way of the connecting pipes 15 23 and 24. Parabolic reflectors 26 are located behind ^• the collector elements 25 for concentrating solar radiation onto the collector elements.

The parabolic reflectors 26 are supported by the collector elements 25 which, in turn, are supported by - 20 the tubes 22 and outrigger bearings 27. The reflectors 26 are pivotable about the collector elements 25 so that they may be orientated in an optimum position for the collection of solar radiation at different times during the year. 25 In operation of the system, and assuming the starting conditions shown in Figure 1, when solar . radiation falls on the collector unit 20 the liquid within the collector unit will heat rapidly and a

S

convection flow will be established between the collector unit 20 and the interior of the reservoir 19. Then, as the water within the reservoir 19 heats up and eventually boils-off, the pressure within the reservoir will increase to exceed that prevailing in the vessel

10, and the steam generated within the reservoir 19 will be caused to flow down the conduit 18 to enter the vessel 10. On entering the vessel 10, the steam will give up sensible and latent heat to the liquid within the lower region 11 of the vessel 10, and this heat will be used gradually to build up the temperature bf the liquid within the storage vessel.

For as long as the solar collector unit 20 continues to be exposed to solar radiation, the liquid within the upper reservoir will continue to boil away and transfer as steam to the lower vessel, this causing the level of liquid within the lower vessel to gradually rise and causing the liquid temperature within the vessel to increase. The above described stage of the operating cycle will commence at the beginning of each day and extend through until sunset. Then, at the end of each day, when the incident solar radiation diminishes, the temperature of the depleted fluid within the reservoir 19 will drop to a level aproaching the then prevailing ambient temperature and, thus, the pressure in the upper reservoir 19 will fall below that in the lower vessel 10. The resultant pressure differential will then act

to force liquid out from the storage vessel 10 and up the conduit 18 to re-enter and fill the upper reservoir. This condition will be maintained until the collector unit 20 is again exposed to solar radiation when the cycle will recommence.

SUBSTITUTESHEET