FIELD OF THE INVENTION

This invention relates to a solar water heater of the general type comprising a thermally insulated water storage tank with which integrates with a solar heating panel that typically, although not necessarily, comprises a series of generally parallel solar heat absorbing evacuated tube collectors.

A typical heat absorbing evacuated tube collector may be of the type that includes two concentric tubes, typically of glass, with a vacuum in between the two tubes. Heat absorption takes place at the inner concentric tube with the aid of a heat absorbent coating or layer over the inner tube.

The invention is particularly directed, although not exclusively, at direct thermo-siphon solar water heaters that are cost effective to manufacture, whilst being satisfactorily efficient in use; and that are generally, although not necessarily, of the low-pressure type operating on the basis that gravity causes water to leave the water storage tank on its way to a consumer outlet.

BACKGROUND TO THE INVENTION Numerous different solar water heaters have been proposed and are currently manufactured. For the most part, each of these has its own advantages and disadvantages; its own cost of manufacture; and its own thermal efficiency. In one common arrangement of a direct thermo-siphon solar water heater, a series of evacuated tube collectors extends directly into a circular cross- sectioned water storage tank that is typically made of metal. The axes of the evacuated tube collectors generally intersect the axis of the water storage tank in order to ensure that an effective seal is maintained between the outer surface of an evacuated tube collector and an aperture serving as the point of entry into the water storage tank. The problem with this arrangement is that there is a significant volume of water in the bottom of the tank that generally does not become heated satisfactorily. Also, the extent of the volume of water that is below the level of communication with the interior of the evacuated tube collectors varies according to the angle of inclination of the evacuated tube collectors to the horizontal in the installed position. The optimal angle of inclination in turn varies with the geographical latitude of location and the consequent median seasonal angle of inclination of the sun overhead at midday.

This problem may be avoided by making the series of evacuated tube collectors separate from the water storage tank so that it can be located below the bottom of the tank with connecting pipes communicating between them to provide for the flow of water. This, however, increases the cost and complexity of the installation and is not appropriate to inexpensive self- contained installations.

Many other types of evacuated tube collector solar water heaters exist in which a float controlled valve is contained within the water storage tank. This arrangement leaves vacant space above the water level that varies according to the angle of inclination at which the solar water heater is installed.

Another aspect of low-cost solar water heaters is the cost of manufacturing the water storage tank which, typically being made of metal, is rather costly. The outer casing that contains a layer of heat insulating material between it and the storage tank itself is also typically made of metal, mostly galvanised steel. There is thus a need for a cost-effective solar water heater that provides at least some advantages over existing low-pressure solar water heaters.

SUMMARY OF THE INVENTION

In accordance with one aspect of this invention there is provided a solar water heater of the type comprising a thermally insulated water storage tank and a series of evacuated tube collectors communicating directly with the interior of the water storage tank by way of apertures receiving an end region of each of the evacuated tube collectors, wherein the cross-sectional shape of the water storage tank is generally polygonal so as to provide a generally flat wall through which apertures are formed for receiving ends of the evacuated tube collectors at generally right angles to that wall so that the evacuated tube collectors communicate operatively with a lowermost region of the interior of the water storage tank and wherein a water inlet is provided with a water level maintenance arrangement such that a water level in the storage tank is maintained, in use.

Further features of the first aspect of the invention provide for the water level maintenance arrangement to be a float controlled inlet valve; in one alternative, for an upper wall of the water storage tank to have an outwardly directed bulge for at least partially accommodating the water level maintenance arrangement which may optionally be located centrally of the horizontal length of the water storage tank and in which instance a water inlet connection is preferably accessible from the exterior of the water storage tank in a central region thereof; in another alternative for the water level maintenance arrangement to be located in a separate inlet tank located above the water storage tank and preferably supported by the water storage tank; and for the cross-sectional shape of the water storage tank to be generally rectangular with a diagonal fifth side occupying of the order of 40% to 60%, and preferably about 50% of the height of the water storage tank taken in a direction at right angles to the axes of the evacuated tube collectors.

In accordance with a second aspect of the invention there is provided a thermally insulated water storage tank suitable for use in a solar water heater wherein the water storage tank has an inner tank that is made of a moulded plastics material that is optionally fibre reinforced, and an outer housing forming an outer skin that can also optionally be moulded from plastics material with a space between the outer surface of the inner tank and the inner surface of the outer housing and wherein the space is filled with heat insulating material.

Further features of the second aspect of the invention provide for the heat insulating material to be a foamed plastic insulating material of any appropriate type; for the inner tank to be blow moulded from a suitable plastics parison; for the plastics parison to be a coextruded parison with an inner layer selected for its water retention properties and an outer layer selected for its weather resisting properties, in particular UV resisting properties; for spigots defining access passages to be formed integral with the inner tank during the manufacturing process by means of two spaced blow pins or locating pins at one end of the inner tank and at least one, and preferably two, blow pins or locating pins at the other end of the inner tank with the spigots being positioned such that one may serve as a hot water outlet; one may serve as an optional cool water return; and one may serve as an overflow outlet; for external screw-threads to be moulded integral with selected spigots; and for the spigots to include enlarged zones that form spacers for defining a space between the inner tank and outer housing during assembly thereof as will be more fully apparent from what follows concerning the method of manufacture of the thermally insulated water storage tank.

A still further feature of the second aspect of the invention provides for the operatively upper wall of the water storage tank to have an outwardly directed bulge for at least partially accommodating a water level maintenance arrangement or, alternatively, for the upper wall of the outer housing to have moulded therein a recessed region for supporting a lower wall of a separate water inlet tank located above it.

In accordance with a third aspect of the invention there is provided a method of manufacturing a thermally insulated water storage tank as defined above wherein an inner tank is blow moulded from a suitable plastics parison of thermally softened material with two spaced blow pins or locating pins at one end of the inner tank and at least one blow pin of locating pin at the opposite end whereby spigots are formed at each end of the inner tank with selected spigots forming access passages that open into the interior of the inner tank, the spigots being suitable for connecting the appropriate access passages to piping, in use; separately forming an outer housing in two parts that can be assembled to enclose the inner tank with a space between its inner surface and the outer surface of the inner tank wherein holes are formed as may be necessary in the ends of the outer housing to receive the spigots formed by the blow pins or locating pins; securing the two parts of the outer housing together; and filling the space between the inner tank and outer housing with a suitable insulating material that is introduced by way of an inlet passage provided in the outer housing.

Further features of the third aspect of the invention provide for there to be four moulded spigots, two at each end of the inner tank, corresponding to the positions of each of the blow pins or locating pins wherein the moulded spigots together assist in locating the inner tank within the outer housing during assembly; for each of the spigots to include an enlarged zone adjacent the inner tank wherein the outer end of the enlarged zone serves as a stop to locate the inner tank within the outer housing and thereby provide a required space for insulating material between the end walls of the inner tank and outer housing; and for the plastics parison to be a coextruded parison with an inner layer selected for its water retention properties and an outer layer selected for its weather resisting properties, in particular UV resisting properties.

In order that the above and additional features of the invention may become more apparent, one embodiment of all of the aspects of the invention defined above will now be described with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:-

Figure 1 is a perspective illustration of one embodiment of the invention;

Figure 2 is a plan view thereof;

Figure 3 is a sectional side view thereof taken a long line III - III in

Figure 2;

Figure 4 is a sectional side view taken a long line IV - IV in Figure 2;

Figure 5 is a detailed section showing the entry of one evacuated tube collector into the storage tank;

Figure 6 is an exploded schematic illustration showing the blow moulding of the inner tank;

Figure 7 is a plan view of the completed inner tank; Figure 8 is a plan view of the outer housing illustrating the one end in exploded relationship and illustrating the assembly of the inner tank and outer housing; Figure 9 is a three-dimensional illustration of a second embodiment of the invention in which a separate inlet tank is employed as the water level maintenance arrangement;

Figure 10 is a cross-sectional view taken through the water storage tank and separate inlet tank mounted thereon; and,

Figure 1 1 is a three-dimensional illustration of the inner tank of the second embodiment of the invention that is illustrated in

Figures 9 and 10.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

In the embodiment of the invention illustrated in particular in Figures 1 to 5 of the drawings, a solar water heater, generally indicated by the numeral (1 ), comprises a thermally insulated water storage tank (2) and a series of parallel evacuated tube collectors (3) communicating directly with the interior of the water storage tank by way of apertures each of which receives an end region of one of the evacuated tube collectors.

The cross-sectional shape of the water storage tank is generally rectangular so as to provide a generally flat wall (4) through which the apertures are formed for receiving ends of the evacuated tube collectors at generally right angles to that wall. The evacuated tube collectors communicate operatively with a lowermost region of the interior of the water storage tank, as will be quite apparent from a reference to Figures 3 and 4 of the drawings. A diagonal fifth side (5) occupies about 50% of the height of the water storage tank as taken in a direction at right angles to the axes of the evacuated tube collectors in order to provide a neat appearance to the water storage tank.

In this embodiment of the invention the upper wall of the water storage tank has an outwardly directed bulge (7) located centrally along its horizontal length for at least partially accommodating a water level maintenance arrangement in the form of a float controlled valve (8). A water inlet connection (9) for supplying water to the float controlled valve is accessible from the exterior of the water storage tank in a central region thereof by way of an aperture (1 1 ) located near the top of the water tank remote from the evacuated tube collectors. A closable maintenance aperture (1 2) is provided through the wall of the bulge so that access is provided for installation, maintenance or replacement of the float controlled valve as may be necessary. An optional entry point (1 3) indicated in dotted lines in Figure 1 may be provided for an auxiliary electric heating element (not shown) should such a facility be required for use in prolonged bad weather conditions.

The water storage tank has an inner blow moulded plastics tank (14) and an outer housing (1 5) forming an outer skin that, in this embodiment of the invention, is also blow moulded from plastics material, and is proportioned such that a space that becomes filled with foamed plastics insulation material (1 6) is formed between the outer surface of the inner tank and the inner surface of the outer housing. Blow moulding in each case takes place from a suitable thermally softened plastics parison in a manner that is generally known in the art. However, as regards the inner tank, the parison is formed from a length of a co-extruded plastics material whereof the inner layer is of a plastic more appropriate to water retention, such as a polypropylene material, whilst the outer layer is of a plastic that is selected more for its weather resisting properties, especially its UV resistance, and is typically a suitable polyethylene material, the reasons for this arrangement will become apparent from the following. It is to be noted that the outer layer of the coextrusion is preferably formed from a starting material that embodies at least some granular or finely divided polypropylene material to ensure that an adequate bond is achieved between the two layers. This granular or finely divided polypropylene material may indeed be recycled reject mixed material of polypropylene and polyethylene together.

As regards the outer housing, that is produced from a simple extrusion of a plastics material selected for its weather resisting properties, especially its UV resistance, and is typically a suitable polyethylene material of generally well-known type or at a plastics material subsequently developed with the appropriate properties. The inner tank has two spaced spigots (17a, 17b, 17c, 17d) that may define access passages or blind passages (only those that are required to provide communication with the interior of the inner tank need to be open to form access passages) formed integral with the inner tank at each end during the manufacturing process. The spigots are formed by means of two spaced blow pins (or locating pins) at each end of the inner tank. The integral spigots are positioned such that one (17a) may serve as a hot water outlet from an upper region of the inner tank; one (17b) may serve as an optional cool water return; and one (17c) may serve as an overflow outlet. External screw-threads are moulded integral with required spigots in order to facilitate attachment of the spigots to co-operant piping in an installed condition. Furthermore, as shown clearly in Figure 7, the spigots have enlarged zones (19) immediately adjacent the inner tank to define a fixed space between the end walls of the inner tank and those of the outer housing. In this way integral spacers are formed for defining a space between the inner tank and outer housing during assembly thereof for receiving foamed plastic thermal insulation material. The spigots and cooperating holes in the outer housing also serve to locate the inner tank in the transverse direction so that a suitable space is provided all around the inner tank for receiving the foamed thermal insulation material. The inner tank also has a bulge (20) corresponding to the bulge of the composite water storage tank. The outer housing is also blow moulded in this embodiment of the invention, the housing being moulded as a single item whereof one end (21 ) is severed from the balance of the housing after moulding to remove a small portion (22) of the length of the housing and leave a flange (23) attached to the severed end. The arrangement is such that the flange will fit neatly over an adjacent reduced size of free end (24) of the outer housing and can be permanently attached to it by a suitable means such as ultrasonic or thermal welding or, alternatively, by means of an adhesive. Manufacture of a water tank therefore takes place by blow moulding the inner tank between a pair of closed blow moulding dies (25) (see Figure 6); blow moulding the outer housing in a generally conventional manner; machining holes in the ends of the outer housing to receive the spigots of the inner tank with the holes being dimensioned to abut the enlarged zones of the spigots; severing one end of the outer housing from the rest of the outer housing by removing said small portion (22) of the length of the outer housing; inserting the inner tank into the outer housing; installing the severed end of the outer housing in its final position; and thereafter introducing foamed plastics thermal insulating material to fill the space between the inner tank and outer housing.

It is to be noted that at least parts of the integral spigots that form the inlet and outlet to the interior of the inner tank by way of the outer housing and layer of insulation in between, also become exposed to the weather of the outdoors, especially UV radiation from the sun, and it is for this reason that the inner tank has an outer layer of weather resistant plastics material to ensure longevity of the spigots in keeping with the rest of the water tank assembly. The water tank is then completed by machining any required holes through the composite wall, especially the central maintenance aperture whereby the float controlled valve can be installed and serviced, as may be required, and the apertures for receiving the evacuated tube collectors.

It is to be noted that sealing of the outer surface of the end region of each of the evacuated tube collectors to the inner tank is achieved, in this instance and as illustrated in Figure 5, using a suitable elastomeric grommet (28) inserted into the aperture in the inner tank. The outer hole is simply covered by means of a suitable ring-shaped cover (29). It is envisaged that this arrangement of sealing the evacuated tube collectors to the inner tank will be superior to existing arrangements in which more complicated sleeves are used which can suffer from loss of effect over time in consequence of expansion and contraction.

It will be understood that installation of a water heater as described above is simple and may take place at a wide range of different inclinations (as indicated by the angle "o " in Figure 4), preferably facing generally north in the Southern hemisphere or South in the Northern hemisphere, without any adverse effect on the functioning of the solar water heater. The geometric arrangement of the water tank is such that substantially the entire volume of the water tank is available for use whatever the inclination of the evacuated tube collectors.

Also, because the evacuated tube collectors communicate with what in effect is the lowermost region of the water storage tank, there is no significant volume of cold water in the bottom of the tank that does not become effectively heated by entering the evacuated tube collectors during solar heating of the evacuated tube collectors.

It may happen that the arrangement described above with the water inlet arrangement positioned in the bulge in the water tank assembly does not comply with local regulations, or indeed local practical considerations. In such an instance the embodiment of the invention illustrated in Figures 9 to 1 1 may be used.

In this second embodiment of the invention the bulges are omitted and the inner tank (31 ) and outer housing (32) may have continuous outer surfaces between the ends of the water tank. Instead of the bulges to accommodate a float controlled valve (33), and external water inlet tank (34) is provided that is located on top of the outer housing towards the end there of from which a spigot (35a) forming the hot water outlet and a spigot (35b) forming a cold water inlet extend. The external water inlet tank has an outlet (36) towards its lower end and that is listed in the installed position to be cold water inlet of the inner tank of the hot water tank assembly. Conveniently the water inlet tank is located in a shallow recess (37) in the top of the outer housing (32) of the water tank assembly.

Numerous other variations may be made to the embodiment of the invention described above without departing from the scope hereof.