AU738957B2 - Improved solar water heater - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT Name of Applicant: Actual Inventors: Address for Service: GOUGH INDUSTRIES PTY LTD Harry Suehrcke Andrew Robert Winston Gough Simon John Gough Rodney William Lowe CULLEN CO., Patent Trade Mark Attorneys, 239 George Street, Brisbane, QId. 4000, Australia.

Invention Title: IMPROVED SOLAR WATER HEATER The following statement is a full description of this invention, including the best method of performing it known to us: 2 TECHNICAL FIELD This invention relates to solar water heaters. In particular, the invention relates to solar water heaters of the integral collector-storage type.

BACKGROUND ART Solar heaters for the provision of hot water are common and have been commercially available for many years. Such heaters consist of a collector whereby solar energy can be used to heat the water and a vessel for storage of the heated water. The collector and storage vessel can be separate, or integrated into a single unit-an "integral collector-storage heater".

10 In their simplest form, integral collector-storage heaters comprise a water tank with a black energy absorbing upper surface which is exposed to solar radiation through a transparent cover, the tank being insulated at the bottom and sides. Using e such a device, cold water which is feed into the tank can be heated during the sunlight hours and drawn off on demand for the hot water supply. Despite the 15 apparent simplicity of the integral collector-storage design concept, water heaters with 0go• separate collector and storage vessels predominate in most parts of the world.

The performance of integral collector-storage heaters has been limited in the past by the comparatively large night time heat losses through the transparent cover, (ii) the thermal resistance of the water when it is heated from above by the absorbing surface, and (iii) the need to pressurise the hot supply water. The first problem, the control of night time heat losses, has been largely solved by the availability of transparent insulation materials Goetzberger M. Rommel, Solar Energy, pp. 211-219, Sept. 1987). The other two problems remain current and their solution constitutes part of this invention.

The second problem, the thermal resistance of the water when it is heated from above, stems from the fact that current commercial integral collector-storage heaters, which need to be inclined towards the sun and mounted on sloped roofs, invariably use non-transparent black tank absorber surfaces. This is unlike solar pond water heaters which are limited to near horizontal operation. The use of nontransparent absorber surfaces may in part be understood from the preference to use metal tanks which offer good structural support against internal fluid pressure forces and which enable the application of a selective surface to reduce the radiative heat 3 loss. Moreover, unlike plastic materials, metal materials do not introduce significant thermal resistance in the tank absorber wall.

The third problem, the need to pressurise the hot supply water, is usually addressed by the use of tubular water tanks or in some cases by the use of a heat exchanger Tsilingiris, Solar Energy, pp. 245-256, June 1997). However, the heat exchanger increases the cost of the unit and does not eliminate the static water pressures acting on inner walls of the water tank when the unit is inclined.

Apart from the costs of applying a selective surface to the absorber of solar collectors, the overall high cost of solar water heaters conceivably deters many home 10 owners from installing such systems. There is thus a need for a solar water heater S which is more competitively priced with respect to heaters which use other forms of energy. This is particularly the case when the environmental benefits of the use of solar energy are taken into account.

S. SUMMARY OF THE INVENTION 15 The object of the present invention is to provide a solar water heater which can be produced at lower cost than existing heaters and which is at least as efficient as existing heaters. Part of this invention is the design of a heater which takes advantage of low cost materials and low cost manufacturing processes.

In a first aspect, the invention provides a solar water heater comprising: 20 a thermally insulated liquid-filled chamber for storage of absorbed solar energy by said liquid, said chamber having a solar energy-absorbing inside bottom surface and a top which is transparent to solar radiation, wherein said top is adapted to withstand the fluid pressure of said chamber contents on tilting of said heater, and said chamber is sealed to contain said liquid; and a heat exchanger extending through at least a portion of said chamber within said liquid and comprising 0.3 to 10% of the total volume of said chamber, said heat exchanger having a water inlet and a water outlet external to said chamber.

The water heater defined above can be likened to a glazed solar pond. The performance advantage of solar pond heaters over heaters with a non-transparent (black) top has been noted in the literature see "Water pillow heater with transparent and black plastic film top", I. Tanishita, ISES Conference, Melbourne, Australia, 1970). However, the structural features of the heaters according to the invention allow them to be used at angles other than horizontal and to remain of very simple construction. The heaters can thus be used on a tilted surface such as the roof of a building. Furthermore, the ability to tilt the heaters increases the solar radiation received, particularly during winter months, as the sun sees a greater projected area of the collector. The advantage of the transparency of the water tank top surface will be discussed in detail below. However, at this point it can be noted that the transparent top of the chamber significantly reduces the top surface temperature (heat loss) and the tendency of the water to boil, thereby overcoming the earlier identified problem (ii).

i Water heaters according to the invention are distinguishable from heaters that 10 use transparent chambers through which a darkened fluid flows such as described, for example, in German patent No. 26 08 302, "Verfahren und Vorrichtung zurn Auffangen von Sonnenenergie". This type of design does not store the energy absorbed from the sun within the transparent chamber, but employs an external heat S. exchanger with continuous fluid circulation to transfer the energy to a secondary fluid.

o.0 15 This design does not have the inherent simplicity of the integral-collector storage concept where no special fluid or forced fluid circulation is required.

The chamber of heaters according to the invention includes a base that is advantageously prepared by a rotational or injection moulding process. In the first of these processes, a powdered polymer is placed into a metal female mould. The metal 20 mould is then externally air heated and slowly rotated through two axes. The thermoplastic material becomes molten when it contacts the hot metal surface and covers the inside of the mould with a layer of even wall thickness. After the heating cycle, the mould is cooled and removed. In this manner a hollow box-including metal inserts, such as threads-can be formed in a one step process at low cost. A preferred material for the formation of the base is polypropylene. However, any plastics material can be used. Insulation can also be incorporated into the base during the rotational moulding process (which further reduces production costs). A preferred insulating material is polypropylene foam. The rotational moulding process is described in Rotational Moulding of Plastics Crawford, ed., Wiley, New York, 1996), the entire content of which is incorporated herein by cross-reference.

The base of the heater can be of any shape, but is preferably rectangular. In this preferred form, the base is essentially a shallow open box. One of the advantages of the invention is that the base of the heater can simultaneously act as the outer case, the insulation and the water tank. This is unlike existing integral collectorstorage designs that use a separate water tank and insulating box. This feature reduces the manufacturing cost.

A chamber base having a unitary outer case, insulation and water tank as described in the previous paragraph is not essential to the invention. The chamber can comprise a solar radiation transparent tank that is contained within an insulated box having a solar energy-absorbing inside bottom surface. However, a chamber base having a unitary outer case, insulation and water tank has the advantage that the insulation can play a structural role.

S10 The inner sides of the chamber of heaters according to the invention are *lol.

S advantageously solar energy-absorbing like the chamber bottom. Solar energyabsorbence is enhanced by colouring the appropriate surfaces of the chamber black.

The solar energy-absorbing surfaces preferably have a matt finish rather than a gloss finish. However, the exterior surfaces of the chamber sides and bottom can be any 15 colour.

In addition to a base, the chamber of heaters according to the invention have a sheet of (transparent) material as an upper surface. The sheet of material is sealed at its edges to the chamber base so that the chamber is leak proof. The sheet of material forming the upper surface of the heater chamber can be glass or a plastics 20 material. A preferred material is toughened glass. As will be detailed below, the chamber top is also insulated with (transparent) insulation above the upper surface of the chamber.

The transparent insulation above the upper surface of the heater chamber preferably consists of a transparent cover and a transparent insulation panel. The transparent insulation, as well as aiding retention of solar energy absorbed by the liquid in the chamber, can also play a structural role in that it can reinforce the glazed upper surface of the heater chamber. Glazing of solar ponds by plastic films) to reduce their heat loss is known Clark and W.C. Dickinson, Solar Energy Technology Handbook, Part A, Chapt 12., Marcel Dekker, 1980) However, tilting of a solar collector places additional constraints on the heater design as allowance must be made for the static fluid pressure against the glazing. Existing solar ponds are not capable of withstanding this static fluid pressure and for this reason are sometimes referred to as "horizontal flat plate collectors" (Clark and Dickson, supra, p. 379).

6 The transparent insulation does not necessarily have to be in contact with the chamber upper surface. Indeed, an air gap of the order of 10 mm is advantageous as this gap reduces the heat transfer from the chamber upper surface to the transparent insulation, particularly when the chamber upper surface includes a low emittance coating.

However, because of the efficient transfer of heat throughout the chamber of heaters according to the invention, the upper surface of the chamber generally has a temperature that is low enough for the transparent insulation to be in contact with the upper surface. This contributes to the structural integrity of the top.

10 The outer transparent cover of the transparent insulation as defined in the preceding two paragraphs is preferably a low iron glass to maximise the solar transmission. However, the cover can be a sheet of any transparent material such as acrylic glass, or a plastics material such as a polycarbonate sheet. Indeed, the cover per se can act as the insulation, particularly when the sheet of transparent material is S 15 duplicated. Such sheets are typically about 10 to 15 mm above the chamber upper surface and any additional sheets are spaced from the first sheet by a similar distance. The sheets typically have a thickness of about 4 mm.

When glass is used as the transparent sheet of material forming the upper surface of the chamber, low emittance glass Pilkington K-glass with emissivity 20 0.16, K-glass information sheet, 1993) can be used. The inclusion of such glass has the same effect as a selective surface has for a non-transparent absorber which greatly reduces the thermal radiative heat loss from the collector. Furthermore, the use of standard low emittance glass is generally less costly than having to apply a low emittance coating on non-transparent surfaces of known solar heaters. However, the use of a low emittance coating somewhat increases the solar reflectivity of the upper chamber (glass) surface and may not be necessary when the transparent insulation above the chamber provides sufficient suppression of radiative heat loss.

The transparent insulation panel can consist of transparent sheets, a honeycomb, silica aerogel or a thin film of material such as a plastics material. A honeycomb has the advantage that it can act as a transparent insulator and simultaneously provide structural reinforcement. Nevertheless, heaters according to the invention perform satisfactorily even with modest transparent insulation such as two acrylic glass sheets above the upper glazed surface of the heater chamber as described above.

A honeycomb can be made up of a plurality of transparent polycarbonate straws that are bonded together to form a panel (Plascore Inc., Polycarbonate Honeycomb Manufacturer, Zeeland, Michigan, USA). The main working principle of the honeycomb is that the cell boundaries constrain the air movement inside the honeycomb sufficiently so that the convective heat transport is suppressed. However, this only works if both the cell diameter (in the order of 10 mm) and the temperature i difference across the honeycomb are small enough. The cell size may be used to limit 10 the risk of boiling. High transparency to solar radiation is achieved by placing the Shoneycomb so that the open cells face the sun and forward reflect the solar radiation to the upper chamber surface. The honycomb cells, depending on the wall material, can also reduce the thermal radiative heat loss (Hollands et al., Int. J. Heat Mass Transfer, Vol. 27, pp. 2119-2131, 1984).

15 The preferred fluid in the chamber of heaters according to the invention is water because of its high heat capacitance and its relative transparency to solar radiation. For a water layer thickness of 10 cm, approximately 50% of solar radiation is absorbed within the water while the remaining 50% is transmitted Siegel and J.R. Howell, Thermal Radiation Heat Transfer, 2nd ed., pp. 156-157, McGraw-Hill, 20 1981) and absorbed at the (preferably black) bottom of the tank. This water property means that the transparent chamber is largely heated from below which causes the water density distribution to become unstable and the water to become well mixed through natural convection. As a consequence, the water temperature inside the tank is essentially uniform and the top chamber surface temperature is minimised (which reduces the heat loss). Experimental evidence for this behaviour is presented below.

It will be appreciated by those of skill in the art that the fluid in the chamber of heaters according to the invention is not pressurised and merely acts as a storage medium for solar energy. However, water to be heated is passed through the heat exchanger and can be pressurised. Indeed, mains pressure can be applied to the water inside the heat exchanger without use of a pressure reducing valve.

To maximise heat transfer from the fluid in the chamber to water in the heat exchanger of heaters according to the invention, the external surfaces of the exchanger are advantageously finned. A preferred heat exchanger is a serpentine 8 copper tube with rolled on fins. Such tubes are described, for example, in a 1996 sales brochure available from Wieland-Werke AG, Ulm, Germany. When cold water enters the heat exchanger tube, heat flows from the hot water inside the storage chamber to the water inside the tube, and a natural convection circulation is set up inside the chamber to maintain this heat flow. In this manner heat can be extracted with relatively little heat exchange area and the hot water supply temperature can be kept relatively constant.

An advantage of using a heat exchanger is that scaling inside the heater chamber, particularly at the transparent upper surface, is minimised when water is 10 used as the fluid in the chamber, as this water is external to the heat exchanger and is not replaced during operation of the heater.

Heaters according to the invention can be made to any size compatible with the structural integrity of the chamber glazing alone or the chamber glazing in combination with the transparent insulation panel as will be appreciated by one of skill 15 in the art. For example, with a 6 mm thick sheet of toughened glass as the chamber upper surface, heaters with an area of at least 1 square metre can be prepared. Even without any structural support from the transparent insulation-such as insulation

I•IO

l comprising two acrylic glass sheets-such heaters can still be tilted without fluid pressure damage to the glazing. Heaters typically have a chamber volume of 100 to 20 200 litres. The heat exchanger of heaters according to the second aspect can comprise 0.3 to 10% of the chamber volume.

Heaters according to the invention preferably include a dump valve in combination with the heat exchanger for protection against boiling. When the temperature in the heat exchanger reaches a predetermined valve, the dump valve opens so that unheated water flows through the heat exchanger. This prevents further temperature increase and protects against boiling.

A heater according to the invention can be connected in series with at least one other heater. Hence, outlet water from the heat exchanger of the first heater can be further heated by the second in series heater and so on.

Heaters according to the invention can be used in series with an external booster heater as will be known to one of skill in the art. The booster heater will normally be in the outlet line of the solar water heater. A booster heater can also be 9 incorporated inside the water chamber (for example, inside an enlarged section of the heat exchanger tube).

It will be appreciated from the above description that heaters according to the invention can be fabricated from readily available materials. The materials used for fabrication are also recyclable.

Having broadly described the invention, heaters will now be exemplified with reference to the accompanying drawings briefly described hereafter. Reference will also be made to figures showing the performance of a heater according to the invention and prior art heaters.

:i 10 BRIEF DESCRIPTION OF THE DRAWINGS S• Figure 1 is an exploded perspective view of a heater according to the invention.

Figure 2 is a plan view of the heater shown in Figure 1 with a portion of the S transparent top broken away.

S 15 Figure 3 is a partial elevational view in cross-section of the heater shown in Figure 1 at plane A-A.

Figure 4 comprises graphs showing numerically calculated temperature o*oo profiles for a horizontal heater having a black (non-transparent) chamber top and a transparent top with a black bottom surface in the chamber.

20 Figure 5 is a graph showing the experimentally measured temperature rise for two solar water heaters tilted 200 towards the equator, one having a non-transparent chamber top and the other a transparent chamber top.

Figure 6 is a perspective cross-sectional view of an alternative form of heater according to the invention.

Figure 7 is an enlarged elevational view of the cross-section in Figure 6.

Figure 8 is a partially exploded perspective view of the heater of Figures 6 and 7.

Figure 9 is an enlarged cross-sectional view of the seal shown in Figure 7.

DETAILED DESCRIPTION OF THE INVENTION Where a particular feature of the heater is shown in more than one drawing, the same item number is used for that feature.

Figures 1 and 2 show solar heater 1 comprising chamber base 2, a top consisting of transparent panel assembly 3, and heat exchanger 4. It can be appreciated from Figure 1 that transparent panel assembly 3 is made up of a sheet of low iron glass 5, a transparent insulation panel 6, a clamp 7 made up of a plurality of angle sections, and a sheet of low emissivity glass 8. Glass sheet 8 constitutes the upper surface of the chamber base described earlier. Panel assembly 3, which, it will be appreciated, is shown exploded in the figure, is secured to chamber 2 with screws or the like through clamping frame 9 and clamp angle 7 into the walls of chamber 2.

Transparent insulation panel 6 is a polycarbonate honeycomb with 10 mm cell size and 30 mm thickness. The honeycomb can be appreciated from Figure 3 in which the cross-section is along an axial plane along a row of cells.

10 Heat exchanger 4 is a copper tube with an undulating inner surface, a nominal inner diameter of 12 mm, and a nominal 23 mm outer fin diameter nominally having 11 fins per 25.4 mm. It can be appreciated from Figure 2 that the heat exchanger is a tubular serpentine that extends through about 50% of chamber 2. The finned nature of the heat exchanger can be appreciated from the cross sectional view of Figure 3 in which heat exchanger 4, and fin 10 can be seen. The heat exchanger tubing has a length within chamber 2 of approximately 4 m and hence a storage capacity of approximately 0.5 litre.

The juxtaposition of components in an assembled heater can be appreciated from Figure 3. This figure shows portion of heater 1, in which portions of chamber 2 20 and transparent panel assembly 3 can be seen. Components of panel assembly 3 visible are glass sheets 5 and 8, transparent insulation panel 6, and angle clamp 7.

With further reference to Figure 3, chamber 2 comprises a core of polypropylene foam 11 and polypropylene outer layers 12 and 13. Layer 12, which is on the inside of the chamber, is coloured black while layer 13, the exterior layer, can be any colour. A rebate 14 is provided on the internal edges of the walls of the tray making up chamber 2, which receives the edges of glass sheet 8 and insulating panel 7. A sealing strip 15 of elastomeric material is provided between the portion of the rebate 16 returning from the opening in the chamber and glass sheet 8. Clamp angle.

7 has a section 17 which extends between the upward extent of the rebate and insulation panel 6 to contact glass sheet 8. An outwardly extending flange 18 on clamp angle 7 lies between glass sheet 5 and the lip of the chamber but stands sufficiently away therefrom so that pressure applied via clamping frame 9 applies pressure to glass sheet 8 and sealing strip 16 to effect a fluid-tight seal. Clamping 11 frame 9 is secured to chamber 2 by bolts, one of which is indicated at 19. A sealing strip 20 is also provided between glass sheet 5 and flange 18. The low iron glass sheet 5 and the (toughened) low emittance glass sheet 8 have thicknesses of 4 mm and 6 mm, respectively.

The heater as shown in Figures 1 and 2 has overall dimensions of 1400 mm length, 900 mm width and 210 mm height. Chamber 2 contains about 95 litres of water. The chamber can be filled and the water replaced via a port in the wall of the chamber.

Water flow through the heater according to Figures 1 to 3 can be appreciated 10 from Figure 2. Water to be heated enters heater 1 at 21 to flow through the heat S exchanger 4 to exit the heater at 22, this flow being generally indicated by the arrows **:**adjacent the inlet and outlet.

As indicated above, heaters according to the invention are generally tilted in use to increase the amount of solar energy received. In the case of a heater *15 according to Figures 1 to 3, when tilted the uppermost part of the heater would normally be side 23 (see Figure This positions the heat exchanger at a higher level within the heater and maintains a higher temperature during water draw off.

Heaters as shown in the figures typically include a dump valve for protection against boiling, and pressure relief valve. These valves are typically at the heat 20 exchanger outlet. A sub-chamber into which the heat exchanger water flows can be provided for this purpose. The sub-chamber can also include a booster element connected to an electricity supply or other energy source. However, when used as preheaters-a use for which heaters according to the invention are particularly suited-a booster element is not required.

In a variant of the heaters shown in Figures 1 to 3, the heat exchanger extends throughout the heater chamber. With a heater of the size exemplified above, a heat exchanger of nominal inner diameter of 12 mm has a length within the chamber of about 8 m and a storage capacity of 1.0 litres.

It will be appreciated that heaters according to the first aspect of the invention are essentially the same as the heater according to Figures 1 to 3 save that the heat exchanger is omitted. With such heaters, the water inlet and outlet are normally adjacent diagonally opposite corners of the chamber.

Performance of heaters according to the invention will now be detailed.

A key feature of the water heater according to the invention is the transparent upper surface of the water tank. Numerical simulations and experimental measurements were carried out to compare the performance of a heater according to the invention with a heater having a non-transparent, black top. The results of the numerical simulations here are limited to a comparison between horizontal heaters with black and transparent tank surface so that heat transfer to the water of the black heater can be assumed to be by conduction only. The experimental results, however, include the effect of surface tilt.

A numerical determination of the vertical water temperature distribution within a horizontal tank was carried out by exposing both a transparent and non-transparent tank to the same value of solar radiation for a period of 3 hours. A top heat loss 999".

coefficient of 3 W/m 2 K was assumed for both tanks. Initially the water in both tanks had uniform temperature equal to the environment temperature. The solar radiation was varied in the simulation so that the heat flux into the black tank remained constant at 500 W/m 2 The results of these numerical experiments are presented in Figures 4a and 4b in which the former figure represents the results obtained with a iheater having a non-transparent upper surface while the latter figure relates to a heater according to the invention. It is apparent from Figures 4a and 4b that at the end of the heating period, the top surface temperature (and therefore the heat loss) of 20 the tank with a black surface is much higher than that of the transparent tank which remains at essentially uniform temperature (neglecting solar radiation absorption in the transparent tank top). When the thermal resistance of the polypropylene tank wall is included (4 mm), the surface temperature of the black tank at the end of the time interval is almost 3 times that of the transparent tank! The performance advantage through the reduction of this heat loss is best illustrated by stating the basic equation governing the solar collection Duffie and W.A. Beckman, Solar Engineering of Thermal Processes, 2nd ed., p. 251, Wiley, 1991): Useful energy Absorbed solar radiation Heat loss When it is considered that a substantial part of the collector heat loss is the top loss and that this heat loss is approximately proportional to the temperature difference between the ambient and the tank surface temperature, the advantage of a greatly reduced surface temperature becomes apparent from the above equation. The tank surface to ambient temperature difference is shown in Figure 4 for both the black and 13 transparent tank. It is important to note that the high surface temperature of the black tank tends to persist during night time and that high performance transparent insulation would maintain the high top surface temperature for a longer period. When the collector base is tilted the performance advantage of a heater with transparent top chamber compared to a black one is somewhat reduced and some thermal stratification can occur when the absorption in the transparent tank top is not negligible. However, preliminary numerical results indicate that at 200 tilt the heat loss for the black unit still remains about 2 times that of the transparent unit.

The only mechanism for heat transfer through the (stably stratified) horizontal 10 water layer with a black top surface is by conduction. The transparent tank, on the other hand, permits the penetration of sunlight which is absorbed inside and below .the water body as discussed above. This means that the transparent tank is largely heated from below which causes the water layer to be unstable and become well mixed.

15 Apart from a lower heat loss, a lower tank surface temperature also means lower stresses on the materials involved. This is particularly important for heaters ii 4° according to the invention as this enables the transparent insulation panel to be brought in to contact with the glass cover thereby giving structural support to the glass cover. As a result heaters with relatively large surface area can be tilted and 20 withstand the static pressures generated due to the weight of the water. However, heater tanks according to the invention do not necessarily have to rely on the support by the transparent insulation as in many cases-particularly with heater tanks of small size-the use of a toughened glass sheet is sufficient.

Experimental measurements were carried out to further assess the effect of a transparent tank top on the water heating performance. Two identical rectangular water containers of clear plastic material (PET) with approximately 1 mm wall thickness and 100 mm height were prepared. The top surface of one container was painted black while for the other container the bottom surface was painted black. The two containers were filled with water of ambient temperature and positioned side by side in the open with an inclination of approximately 200 towards the equator. The uncovered containers were exposed to solar radiation in still air during a partly cloudy day for approximately 5 hours. The temperature rise within the containers was measured via a thermometer positioned at the centre of each container and half way between the top and bottom. At the end of the test period, each container was thoroughly shaken to ensure complete dispersion of any stratas within the water.

The results of the experiment are presented in Figure 5. The results show that only in the container with a non-transparent top was a change in temperature noted when the container was shaken at the end of the test period. This suggests that the container with the transparent top essentially was at uniform temperature while the container with the black top had significant thermal stratification with most of the hot water near the top surface. A further observation is that, after shaking, the overall temperature rise in the tank is representative of the useful energy added to the tank 0 10 which is approximately 40% greater for the transparent tank than for the black one. It so :e Is is important to realise that the effect of the heat loss due to high surface temperatures is cumulative and increases with time. Moreover, the effect of the transparency on the thermal performance is somewhat dependent on the tilt of the tank and the top heat loss coefficient.

S 15 A heater with an alternative seal arrangement to the heater shown in Figures 1 to 3 is depicted in Figures 6, to 8. The cross-section in Figures 6 and 7 is at about point A of Figure 8. Some components of the heater have been omitted from Figure S I 8.

The Figures 6 to 8 heater 23 comprises chamber base 24, a top panel assembly 25 and a heat exchanger 26. The top panel assembly 25 consists of a sheet of ordinary glass 27, a transparent insulation panel 28 and a sheet of polycarbonate material 29 which acts as a cover for the insulation and the heater per se. Other than sheet 29, heater components are of essentially the same nature as the heater described above with reference to Figures 1 to 3. The heater has dimensions of 210 mm x 970 mm x 1,620 mm and contains about 125 1 of water.

To effect a fluid-tight junction between glass sheet 27 and the fluid-filled interior of the heater, an elastomeric seal 31-to be described in greater detail below-is provided about the edges of glass sheet 27. The seal is located within a groove 32 formed in the surfaces of three of the sidewalls of the heater chamber. The fourth sidewall 33 is recessed via which recess glass sheet 27 with seal 31 thereabout can be slid into the groove (see Figure The seal on the side of the glass sheet proximal recessed wall 33 can contact the flat upper surface 34 of the sidewall. A retainer 35 having a rebated edge is secured into the recess with screws or the like, the rebate 36 in conjunction with flat surface 34 in effect forming a groove for the section 37 of the seal in this portion of the chamber. It can be seen from Figure 8 that a separate portion of seal material comprises section 37. This facilitates fitting the seal to glass sheet 27.

As can be appreciated from Figure 7, insulating panel 28 is located between glass sheet 27 and upper sheet 29, and within the space formed by the upper extremities of the grooved sidewalls and retainer 35. Sheet 29 has sloping outer edges by which it is affixed to corresponding surfaces on the heater chamber by screws.

10 With reference to Figure 9 which is a cross-sectional view of seal 31 of Figure :i 7, the seal has a channel 38 for receiving an edge of glass sheet 27 and an air cavity 39. This cavity can be evacuated for fitting the glass sheet with seal thereabout into S the groove in the heater chamber, the evacuation causing collapse of the seal in the Sarea of the cavity. When the vacuum is released after installation of the sheet and seal, the seal expands in the groove and elastically contacts opposite walls of the groove.

The portion 40 of the seal opposite the side of glass sheet 27 which contacts the liquid in the heater chamber does not have a collapsible air cavity. This helps to reduce the unloading of the seal on the liquid-contact side which occurs when fluid pressure is applied to the glass sheet. The sealing pressure of seal 31 is enhanced by the inclusion of ribs, one of which is indicated at 41.

In addition to the ease of assembling a heater using a seal such as seal 31, a further advantage is that in use a double seal is provided-that is, in the event of failure of the seal portion on one side of the glass sheet, the portion on the other side of the sheet can still effect a fluid-tight junction between the sheet and the chamber.

Moreover, because the seal force is evenly distributed along the seal, the design can cope with some irregularity in the sealing groove provided that the sealing sheet (the glazing) allows some elastic deflection.

It will be appreciated by one of skill in the art that many modifications and additions can be made to the heaters as exemplified above without departing from the broad ambit and scopy of the invention. For example at least one baffle plate can be included in the heater chamber.

Claims (25)

1. A solar water heater comprising: a thermally insulated liquid-filled chamber for storage of absorbed solar energy by said liquid, said chamber having a solar energy-absorbing inside bottom surface and a top which is transparent to solar radiation, wherein said top is adapted to withstand the fluid pressure of said chamber contents on tilting of said heater, and said chamber is sealed to contain said liquid; and a heat exchanger extending through at least a portion of said chamber within said liquid and comprising 0.3 to 10% of the total volume of said chamber, said heat exchanger having a water inlet and a water outlet external to said chamber.

2. The heater according to claim 1, wherein said heat exchanger comprises 0.5 to of the total volume of said chamber.

3. The heater according to claim 1 or claim 2, wherein said water chamber comprises a tray fabricated from a core of foam insulation between outer layers of a plastics material.

4. The heater according to any one of claims 1 to 3, wherein said solar energy- absorbing surface is black. The heater according to any one of claims 1 to 4, wherein a sheet of glass forms the upper surface of said chamber.

6. The heater according to claim 5, wherein said glass is low emissivity glass.

7. The heater according to claim 5 or claim 6, wherein said glass is toughened glass.

8. The heater according to any one of claims 1 to 4, wherein said top comprises at least one sheet of transparent material as insulation above a sheet of transparent material that forms the upper surface of said heater chamber.

9. The heater according to claim 8, comprising two said sheets of transparent material 25 as insulation. The heater according to any one of claims 1 to 4, wherein said top comprises a panel assembly having upper and lower sheets of transparent material and a transparent insulation panel sandwiched between said sheets.

11. The heater according to claim 10, wherein said upper sheet is low iron glass.

12. The heater according to claim 10 or claim 11, wherein said lower sheet is low emissivity glass.

13. The heater according to any one of claims 10 to 12, wherein said transparent insulation panel is selected from a honeycomb, silica aerogel, or a film of plastics material.

14. The heater according to any one of claims 10 to 13, wherein: 1said panel assembly is secured to said chamber by a clamping frame which contacts said upper glass sheet, said lower glass sheet and said transparent insulation panel being located in a rebate in the walls of said chamber with a sealing strip between said lower glass sheet and a surface of said rebate; and said lower glass sheet is brought into fluid-tight contact with said sealing strip by a plurality of angle clamps each having a section which extends between the upward extent of said rebate and the edge of said transparent insulation panel to said lower glass sheet, and an outwardly extending flange which lies between said upper glass sheet and the lip of said chamber, whereby pressure applied to said clamping frame applies pressure to said lower glass sheet via said plurality of angle clamps.

15. The heater according to claim 14, wherein said tray comprising said water chamber is rectangular.

16. The heater according to claim 10, wherein: said panel assembly lower sheet has a seal thereabout, which seal is received in a groove in the surface of three sidewalls of a rectangularly shaped chamber tray and is retained therein by a member fixed in sealing contact with said lower sheet at a fourth sidewall of said chamber; and said upper sheet is affixed to said chamber to retain said insulation panel between said upper and lower sheets.

17. The heater according to claim 16, wherein said upper sheet is polycarbonate 0:0*0 20 material.

18. The heater according to claim 16 or 17, wherein said lower sheet is low emissivity glass.

19. The heater according to any one of claims 16 to 18, wherein said transparent insulation panel is selected from a honeycomb, silica aerogel, or a film of a plastics material.

20. The heater according to any one of claims 16 to 19, wherein said seal is divided into approximate halves by a channel for receiving edges of said lower sheet and one of said halves includes an evacuable cavity.

21. The heater according to any one of the preceding claims, wherein the inside walls of said chamber are solar energy-absorbing.

22. The heater according to any one of the preceding claims, wherein said heat exchanger extends through about half of said chamber.

23. The heater according to any one of claims 1 to 21, wherein said heat exchanger extends throughout said chamber. The heater accordingly to any one of the preceding claims, wherein said heat 435 anger comprises a finned tubular serpentine. 18 The heater according to any one of the preceding claims, wherein said chamber storage fluid is water.

26. The heater according to any one of the preceding claims, wherein said chamber further includes a booster heater.

27. The heater according to any one of the preceding claims, wherein said heat exchanger includes a dump valve at the outlet thereof.

28. The heater according to any one of the preceding claims in series with a booster heater.

29. A solar water heater substantially as hereinbefore described with reference to Figures 1 to 3 or Figures 6 and 7. A solar water heater substantially as hereinbefore described with reference to Figures 6 and 7 including a seal as depicted in Figure 8. DATED this 2 5 th day of July 2001 15 GOUGH INDUSTRIES PTY LTD By its Patent Attorneys CULLEN CO. 0 9 2 0 •o o*o* o oo *i