BACKGROUND OF THE INVENTION

[0001] This invention relates generally to a solar water heater.

[0002] Many designs for solar water heaters have been proposed. Generally, an incentive for a solar water heater lies in the low operating cost thereof and in the capability of providing heated water without making use of an energy source other than, of course, the sun.

[0003] In most instances a compromise must be struck between the effectiveness of a heating system and its capability to store hot water, and the cost of the system. For example, a system which is based on the use of an insulated water storage vessel which is separate from a collector is usually more efficient than an integrated collector storage heater. The higher efficiency, however, comes with an increased cost. For this reason attention has been directed to techniques for improving the efficiency of the relatively lower cost integrated collector storage heaters.

[0004] An analysis of integrated collector storage solar water heaters can be found in a publication of the same name, by M. Smyth, P C Eames and B Norton. The paper examines aspects of the integrated approach and performance characteristics. In essence, to be viable, an integrated collector solar water heater must maximise solar radiation collection while minimising thermal loss, in a cost-effective manner.

[0005] An object of the present invention is to provide a solar heating arrangement which attempts to address the aforementioned requirements. SUMMARY OF INVENTION

[0006] The invention provides a solar heating arrangement which includes at least one vessel, a fluid inside the vessel, phase change material in a heat-transfer relationship with the fluid, and at least a first housing, formed at least partly from a light transmissive material, which encloses at least part of the vessel and the phase change material.

[0007] A second light transmissive housing may enclose the first housing and, similarly, a third light transmissive housing may enclose the second housing, and so on.

[0008] As the housings are light transmissive, each housing_exhibits_the_pr-operty-of- allowing solar radiation to pass through the housing, with minimal attenuation and, thereafter, due to the "greenhouse effect", heat energy is trapped in each housing.

Each housing thus acts as an insulator to assist in reducing conduction, convection, and the re-radiation, of heat energy which has passed to the heat absorbing storage vessel.

[0009] Each housing may be of any suitable material and may for example be made from glass. It is preferred however for cost and structural reasons to make each housing from a suitable clear plastics material, for example polycarbonate or the like.

[0010] Each housing may be formed in any suitable way. In one form of the invention the housings are integrally formed, for example by means of an extrusion process. In another form of the invention each housing is separately formed, for example from a suitable pipe, and the housings (pipes) are then positioned one inside the other to conform to the aforementioned requirements. In a third approach a sheet or film of a suitable material with a thickness, say, of from 50 micron to 5mm, is wound around a core, for example a storage vessel, and successive windings of the sheet material constitute the respective housings.

[0011] Spacers may be provided between successive housings to ensure that they are correctly positioned relative to each other.

[0012] The housings are preferably positioned so that, in use, in a vertical direction, the spacing between opposing surfaces of adjacent housings is at a maximum to contain rising heat energy.

[0013] Each housing may comprise a generally cylindrical section _with_cl,o.s.ures-at- opposed ends. In cross-section the cylindrical section may be circular or oval.

[0014] The closures may be insulated.

[0015] The vessel may function as a storage vessel. The vessel is, itself, preferably heat-absorbing and may be of any suitable kind. The vessel may be cylindrical and may be made from a suitable heat-absorbing material such as aluminium or the like. The material may be darkened to increase its heat-absorbing capability.

[0016] The vessel should preferably be constructed to withstand normal pressures in a water supply system i.e. it should be capable of acting either at a low pressure or as a so-called "high pressure" vessel.

[0017] In one form of the invention the vessel is intended to act as a collector and as a storage vessel. For the aforementioned purpose the dimensions of the vessel should be such that a relatively large surface area is presented to incident radiation, in relation to the volume of the vessel. The optimization of this aspect can however result in a relatively high cost and a compromise should be struck between heat-absorbing capability (area presented to incident radiation) and internal storage volume. The heat absorbing capability can however be enhanced by internally or externally coating surfaces of, at least, the outermost housing. Sunlight which does not strike the vessel and which impinges on a reflective coating can thereby be directed onto a heat- absorbing surface of the vessel.

[0018] Preferably the phase change material changes phase at a temperature which is greater than 30°C. For example the phase change may occur in a temperature range of from 45°C to 85°C. A suitable value is of the order of 60°C. This is associated with a relatively high water temperatur-e-suited-in-qeneral-for-domestic-use-

[0019] The phase change material may change from a solid to a liquid phase. A suitable material is a paraffin-type wax. This material is of a relatively low cost, and has a high specific heat storage capability. This type of material is exemplary only and is non-limiting.

[0020] The phase change material may be specifically selected to be efficient at collecting solar radiation. Alternatively or additionally the material may be pigmented, coloured or altered in any appropriate way to optimize or increase its capability to absorb heat from incident solar radiation. For example, carbon black or any other suitable dye or pigment may be added to a paraffin-type wax to make the phase change material as black as is practical.

[0021] The phase change material is thus preferably contained in a light transmissive housing which abuts the vessel. [0022] In one form of the invention the vessel includes a plurality of formations for enhancing heat transfer between the phase change material and the fluid.

[0023] In another form of the invention the arrangement includes at least one conduit formed from a heat transferring material which is exposed to the phase change material and through which the fluid can pass. The conduit may, itself, be sized and adapted so that it comprises the vessel or at least a part thereof.

[0024] In another form of the invention the arrangement includes a jacket which surrounds at least part of the vessel and the phase change material is located in a volume formed between opposing surfaces of the jacket and the vessel. [0025] The fluid in the vessel may be water.

[0026] Alternatively the fluid, e.g. a suitable oil or other liquid medium is used to transfer heat to another system.

[0027] In each variation an important aspect lies in the fact that phase change material is used to store latent heat. Phase change material is at least partly enclosed in one or more light transmissive housings. Reflective surfaces are used to direct heat which might bypass the phase change material onto the phase change material. Water is heated by being directly exposed to the phase change material and through an intermediate medium.

BRIEF DESCRIPTION OF THE DRAWINGS [0028] The invention is further described by way of examples with reference to the accompanying drawings in which: Figure 1 is a schematic side view in section of an integrated collector storage solar water heater according to the invention;

Figure 2 is a view in cross-section on an enlarged scale of the solar water heater taken on a line 2-2 in Figure 1 ;

Figure 3 illustrates an alternative construction of the solar water heater;

Figures 4, 5 and 6 depict variations of the invention; and

Figures 7 and 8 are a side view in cross-section and a plan view, respectively, of another form of the invention.

DESCRIPTION OF PREFERRED E BODIME TS-

[0029] Figures 1 and 2 of the accompanying drawings are longitudinal and transverse cross-sectional views, respectively, of a solar water heating arrangement 10, according to the invention, which includes a water storage vessel 12, a jacket 14 which surrounds the vessel and a number of housings 16A, 16B ... 16N which successively enclose one another.

[0030] The vessel 12 is made from a suitable heat-absorbent material such as aluminium, copper or stainless steel which may be blackened, using any suitable process, so that it has an effective heat-absorbing capability. The vessel 12 is engineered to be resistant to forces exerted by water pressure at levels which are encountered in normal domestic water supply systems.

[0031] The vessel includes an elongate cylindrical body 20 and opposed ends 22 and 24 respectively which seal the body. These ends may be domed to increase their capability to contain water pressure. The vessel encloses a water storage volume 26. A water inlet 28 and a water outlet 30 are made to the vessel. The inlet 28 typically comes from a water supply source e.g. a main supply and the outlet 30 typically goes to one or more user points, or to a geyser, (not shown). In the latter instance the heater arrangement 10 is used as a preheating system for an electrically-based system. This type of application is however known in the art and is mentioned merely by way of example.

[0032] The dimensions and relative sizes of the vessel are optimized to ensure that an effective compromise is struck between the size of the volume 26 and the area of an external surface of the body which is presented to incident solar radiation.

[0033] The jacket 14 is made from a light transmissive material e.g. polycarbonate or glass. A suitable phase change material 36 is positioned between the jacket 14 and the vessel 2. By way of example only the phase change material may be a paraffin-based wax which exhibits a phase change characteristic at a temperature which lies between 55°C and 65°C. The wax is pigmented by the addition of a suitable colorant such as carbon black so that it is directly heat-absorbent. Opposed surfaces of the vessel and the jacket define a volume which determines the quantity of wax which can be placed around the vessel. This quantity is based on various criteria, some of which may be established on an empirical basis. Generally the volume of the wax will lie between 60% to 80% of the size of the volume 26.

[0034] The housings 16A, 16B ... 16N can be made in any appropriate manner. In one example of the invention the housings are integrally formed by winding an elongate strip or film of a flexible plastic material such as polycarbonate e.g. of the kind sold under the trade mark LEXAN, of a suitable thickness, around a core. A first winding is closed on itself at a location 40A to form the housing 16A which is of a relatively small diameter. The winding is continued around a longer circumferential path to form the housing 16B and, near the location 40A, the winding is close to or adjacent the previous winding. The process is continued in this way to form a number of housings, which are positioned one inside the other, around the jacket and the vessel. There is a practical limit to the number of housings which are formed in this way but typically at least three housings would be formed.

[0035] The vessel 12 may include formations 12A which extend into an interior of the vessel and formations 12B which extend into an interior of the jacket. These formations help-to-transferheat ^' between ^' the^hase change matefial ^" arid ^_ the contentsl>ffhe vessel?

[0036] Figure 2 is a cross-sectional view showing the centrally positioned vessel, the jacket, the phase change material and a number of the housings. In general terms each housing has a configuration in cross-section which is substantially circular. This results, despite the fact that the housings are formed by spiral windings of suitable strip material, by bringing the successive windings fairly close to each other at the location 40A.

[0037] The circles/spirals are centred on a vertical line 44. Thus, for each adjacent pair of windings, a respective gap 46, which is between opposed surfaces of the adjacent windings, has a greatest magnitude on this vertical line.

[0038] A dotted line 48 is at an angle of about 35° to the horizontal. The line passes through the diametrical centre of the outermost housing 16N. An inner surface 50 of the outermost winding, to the right of, and below, the line 48 (in Figure 2) has a reflective mirror coating 52 applied to it. [0039] The angle of 35° is determined by the latitude at which the solar water heating arrangement is to be used. In this instance the angle is at 35° so that the heater can be used at a latitude of 35°. Clearly this angle is exemplary only and is non-limiting.

[0040] It is also possible to include in the solar heating arrangement angled mirrored surfaces 54 and 56 the dimensions and locations of which are carefully determined. These surfaces extend the length of the housing and are used to enhance the collector's capability of trapping light. Incident radiation from the left side passing below the phase change material is reflected upwardly by the reflective coating 52 onto the lower mirrored surface, which then reflects this radiation onto the phase change material. Incident radiation which directly impinges on the upper mirrored surface is reflected to the phase change material.

[0041] Referring to Figure 1 closures or caps 58A, 58B ... 58N are engaged with opposed ends of each winding. In this way each housing is made effectively airtight. Each cap is made from any suitable material, for example a suitable plastics material which may be covered with insulation e.g. of polyurethane. Thus, in succession, the vessel 12 is enclosed by the jacket, the jacket is enclosed by the first housing 16A, the second housing 16B encloses the first housing, and so on. As each housing is made from a light transmissive material, solar radiation can pass in succession through the housings and the jacket 14, with minimal attenuation. Heat which is absorbed by the pigmented wax raises the temperature of the wax to above ambient temperature. Minimal heat is however radiated from the wax due to the greenhouse effect displayed by the jacket and successive housings which act as insulators and which inhibit the re- radiation of heat energy. [0042] Heat energy trapped between adjacent pairs of housings naturally tends to rise. The configuration shown in Figure 2 thus automatically provides the longest path, in respect of each gap between adjacent housings, for the heat energy to escape. In other words the thickest insulation is maintained directly above the water vessel.

[0043] Substantial heat energy is required to convert the solid wax to liquid wax. This heat energy is stored in the form of latent heat during the solid to liquid phase change. During and after this process the temperature of water in the vessel 12 is constantly being raised, a process which is enhanced by the heat-absorbing capability of the material from which the vessel is made.

[0044] The vessel 12 thus acts as a heat absorber for the water in the volume 26 and is a storage container for the heated water.

[0045] Water is drawn from the outlet 30 by a user, according to requirement. When this occurs colder water entering via the inlet lowers the temperature of the water in the volume 26. The liquid wax, around the vessel, acts to store heat energy and if the water temperature is lower than the wax temperature heat energy flows from the wax to the water. During the day the wax is of course constantly exposed to solar radiation and absorbs heat energy depending on the weather etc.

[0046] Typically, in most households, a relatively large quantity of hot water is used in the evening. A smaller volume is required during the night and during the following morning. If water usage is substantial the temperature of the water in the volume 26 is lowered to a noticeable extent. When this occurs a cooling effect is exerted on the liquid wax which then tends to solidify. As this phase change takes place the wax which solidifies gives off its latent heat and, in the process, the water in the vessel is heated. Heat radiation to atmosphere by the hot wax is kept at a minimum due to the insulating effect of the various housings. When the wax changes phase to a solid it becomes an effective insulation material which enhances the insulation properties of the system to assist in keeping the water in the volume 26 at an elevated value.

[0047] Depending on the quantity of wax, in relation to the size of the volume 26, the heat energy which is released can be sufficient to keep the water temperature in the volume at a high value. In most situations, adequate hot water will be available for morning usage even if relatively high hot water usage took place the preceding evening.

[0048] The density of the phase change material decreases by 10% to 5% from solid to liquid, accordingly increasing the containment volume by 10% to 15%. An expansion chamber or ulage bag can be incorporated into the design to accommodate the volumetric change. Alternatively the vessel and first housing can be designed to be airtight and withstand the vacuum created when the phase change material transforms from a liquid to a solid. For example the first housing could be ribbed and constructed from a thicker material. The advantage of incorporating a vacuum into the design is that it provides excellent insulation. As the wax solidifies and settles to the bottom of the first housing, the resulting vacuum at the top of the first housing will prevent rising heat from escaping.

[0049] Figure 3 illustrates an alternative form of constructing a plurality of housings 70A, 70B, 70C etc. which surround a centrally positioned vessel 12. The housings are formed from cylinders which are extruded, or otherwise separately formed, from a clear plastics material. Adjacent housings are interconnected by means of spacers 72. The housings are offset relative to each other, in a manner similar to that which has been described in connection with Figure 2, to ensure that air gaps 76A, 76B, 76C etc. of a maximum size are aligned in a vertical direction. Prefabricated insulated end caps, not shown, are engaged with the extruded cylinders to make each housing substantially airtight.

[0050] A primary application of the solar heating arrangement of the invention is for the direct heating of water and the storage of the heated water. It is known in the art that it is possible to supplement the heating which is due to solar radiation by the inclusion of an electrical heating element in the vessel. The use of an element would normally require the concomitant use of a pressure relief valve. It would be important however to ensure that the element only functions to heat the water when, for whatever reason, the heating effect derived from the solar radiation is insufficient for a particular requirement.

[0051] The water which is heated can be used in a normal way, for example in a domestic dwelling. Another application however is to make use of the heated water for the space heating of a home or office. To achieve this objective heated water drawn from the vessel would be circulated by means of a small pump or by using some other principle, through heat exchangers (radiators) designed to transfer heat from the water into a surrounding space.

[0052] In a variation 10A of the aforementioned arrangements, shown in Figure 4, the heating arrangement 10 is used, not to heat water directly but, instead, a suitable fluid 80. The fluid is then used to transfer heat in a controlled way to a secondary device. For example suitable oil could be used in the vessel for heating by solar radiation. The inlet and outlets 28 and 30 could then be connected to a coil 82, which is positioned in heat-exchanging contact with water 84 inside a geyser 86. By causing the heated oil to circulate through the coil 82, heat from the oil is transferred to the water 84 and the water in the geyser 86 is heated.

[0053] In a variation of the Figure 4 technique, shown in Figure 5, a heating arrangement 10B has a conduit 90 which is made from a suitable heat-conductive material e.g. aluminium, copper or an equivalent material. A coiled part 90A of the conduit which has a relatively large surface area is embedded in phase change material 36. The remaining construction of the arrangement 10B is substantially the same as what has been described. The conduit extends through an insulated water storage vessel or geyser 86 and, as is the case with the Figure 4 embodiment, a coil 82 inside the geyser is used to heat water 84 inside the geyser.

[0054] Figure 6 illustrates another possible form 10C of the invention. A coil 94 made from a heat-conductive material with a large surface area is used in place of the vessel 12 shown in Figure 1. The coil is embedded in phase change material 36 contained in a jacket 14. The remaining components of the heating arrangement are similar to those shown in Figure 1. Thus the coil 94 in Figure 6 constitutes the water storage vessel and is connected directly to an inlet 28 and an outlet 30 as is the case with the Figure 1 embodiment. Alternatively a plurality of conduits made from heat-conductive material with a large surface area can be used in place of the vessel 12.

[0055] Figure 7 is a side view in cross-section of ajieating arrangement 100 according to another form of the invention while Figure 8 illustrates, in plan, part of the arrangement 100.

[0056] The arrangement 100 includes a spherical vessel 102 made of copper or a similar material, which is surrounded by a first housing 104. Wax (phase change material) 106 is located in a volume 108 formed between opposing surfaces of the vessel and the first housing. The wax, when changing from the solid phase to the liquid phase increases in volume by up to 15%. In liquid form the wax effectively fills the volume 108.

[0057] In the solid phase the wax volume decreases, and a void 110 is formed, at an upper end of the volume. The void is at a low pressure and may be regarded as vacuum.

[0058] The first housing 104 may be surrounded by successive housings, as has been described hereinbefore, which are notionally indicated by a dotted line 1 12.

[0059] The arrangement 100 includes a water inlet 1 14 to the vessel 102 and a water outlet 1 16 from the vessel.

[0060] The volume 108, occupied by the phase change material, is connected to the atmosphere via a non-return valve 1 18. This will allow air to escape to the atmosphere, when in liquid phase, which will assist in creating a vacuum in the confined volume 108, when in the solid phase. The vacuum produced in the liquid to solid phase change additionally acts as an effective insulator in containing the heat in the storage vessel. A non-return valve need not necessarily be included in the design, but it can also be a safety mechanism in that, if for whatever reason, pressure builds up inside the volume 108, pressure release can occur.

[0061] The vessel may include formations such as fins or the like, which are not shown, which project into the water and into the wax to enhance heat transfer characteristics. [0062] A reflective mirror 120, of any suitable shape e.g. hemispherical or parabolic, is aimed at the sun and is used to concentrate sunlight onto the phase change material and the vessel. The hemispherical reflective surface could form an inner surface of an outermost housing. That housing can be made in any appropriate way e.g. with an outer skin of metal or plastic and with a polyurethane core insulation. A transparent closure e.g of polycarbonate or multiwalled sheet can be used to enhance the insulation properties of the arrangement.

[0063] The heating arrangement of the invention comprises an integrated storage and absorber combination. The plurality of housings which surround the storage vessel help to minimise the radiation of absorbed heat. The phase change material, suitably pigmented, adjacent the vessel 12 acts as a heat storage medium which helps to increase the quantity of water which is available to a user at an elevated temperature for an extended period the phase change material in the solid phase, additionally acts as an effective insulator in containing the heat in the storage vessel. [0064] When the phase change material changes to a state in which latent heat is stored i.e. from a liquid to a solid or, as the case may be, from a gas to a liquid, the formation of a vacuum (reduced pressure) in a zone in contact with the phase change material is significant for this zone provides excellent thermal insulation.

[0065] The heating arrangement of the invention may be operated in a heat transfer relationship with a heat pump.