WO1999020951A1

WO1999020951A1 - Solar water heater

Info

Publication number: WO1999020951A1
Application number: PCT/AU1998/000861
Authority: WO
Inventors: Harry Suehrcke; Andrew Robert Winston Gough
Original assignee: Gough Industries Pty. Ltd.
Priority date: 1997-10-17
Filing date: 1998-10-16
Publication date: 1999-04-29
Also published as: ZA989477B; CN1276857A; BR9812724A; BR9812724C1; AUPO987697A0

Abstract

The invention relates to a solar water heater comprising a thermally insulated liquid-filled chamber (2) having a solar energy-absorbing inside bottom surface and a top (3) 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 a heat exchanger (4) extending through at least a portion of said chamber, said heat exchanger having a water inlet and a water outlet external to said chamber. The invention also provides a solar water heater similar to that described above save that the heater does not include a heat exchanger.

Description

SOLAR WATER HEATER 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".

In their simplest form, integral collector-storage heaters comprise a water tank with a black energy absorbing surface which is exposed to solar radiation at the top through a transparent cover, the tank being insulated at the bottom and sides. Using 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 apparent simplicity of integral collector- storage construction, water heaters with 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 (i) the comparatively large night time heat losses through the transparent cover, (ii) the thermal stratification 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 (A. 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 stratification 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 non-transparent 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 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 (P.T.

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 owners from installing such systems. There is thus a need for a solar water heater 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.

SUMMARY OF THE INVENTION 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 a thermally insulated water chamber having a solar energy-absorbing inside bottom surface and a top which is transparent to solar radiation, a water inlet, and a water outlet, wherein said top is adapted to withstand the fluid pressure of said chamber contents on tilting of said heater. In a second aspect, the invention provides a solar water heater comprising: a thermally insulated liquid-filled 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 a tubular heat exchanger extending through at least a portion of said chamber, said heat exchanger having a water inlet and a water outlet external to said chamber.

The water heaters 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 (e.g., 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 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).

The above water heaters are distinguishable from heaters that use transparent chambers through which a darkened fluid flows such as described, for example, in German patent No. 26 08 302, "Verfahren und Vorrichtung zum 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 exchanger with continuous fluid circulation to transfer the energy to a secondary fluid. 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 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 (R.J. 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 collector-storage 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.

The inner sides of the chamber of heaters according to the invention are advantageously solar energy-absorbing like the chamber bottom. Solar energy-absorbence 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 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 can be a plastics material, glass, or a thin film of a material such as a plastics material. A preferred material is toughened glass. As will be detailed below, the chamber top is also insulated with the 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 (e.g., by plastic films) to reduce their heat loss is known (A.F. 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).

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 transfer of heat from the chamber upper surface to the transparent insulation. 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. 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 duplicated. Such sheets are typically about 10 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 (e.g, Pilkington K-glass with emissivity = 0.16, K-glass information sheet, 1993) is typically 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.

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 difference across the honeycomb are small enough. The cell size may be used to limit the risk of boiling. High transparency to solar radiation is achieved by placing the honeycomb so that the open cells face the sun.

The preferred fluid in the chamber of heaters according to the second aspect of the invention is water because of its high heat capacitance and its relative transparency to solar radiation. It must be noted, however, that the characteristics of high heat capacitance and relative transparency to solar radiation also apply to the water in the chamber of heaters according to the first aspect. For a water layer thickness of 10 cm, approximately 50% of solar radiation is absorbed within the water while the remaining 50% is transmitted (R. Siegel and J.R. Howell, Thermal Radiation Heat Transfer, 2nd ed., pp. 156-157, McGraw-Hill, 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 heated water in the chamber of a heater according to the first aspect of the invention is drawn off for use and replaced by water from a cold water supply. To avoid damage to the heater, the pressure of the cold water supply must be restricted to a pressure consistent with the heater design. The cold water supply is usually restricted to about 10 kPa. This pressure restriction can be effected by a pressure reducing valve in the supply line.

The fluid in the chamber of heaters according to the second aspect of the invention is not pressurised and merely acts as a store 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 second aspect, the external surfaces of the exchanger are advantageously finned. A preferred heat exchanger is a serpentine 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 is minimised when water is 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 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 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 200 litres. The heat exchanger of heaters according to the second aspect can comprise 0.5 to 10% of the chamber volume. Heaters according to the second aspect of 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 second aspect of the invention can be connected in series with at least one other heater of the second aspect. 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.

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.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an exploded perspective view of a heater according to the second aspect of the invention. Figure 2 is a plan view of the heater shown in Figure 1 with a portion of the transparent top broken away.

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 profiles for a horizontal heater having (a) a black (non-transparent) chamber top and (b) a transparent top with a black bottom surface in the chamber.

Figure 5 is a graph showing the experimentally measured temperature rise for two solar water heaters tilted 20 towards the equator, one having a non-transparent chamber top and the other a transparent chamber top. 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 2 in which the cross- section is along an axial plane along a row of cells.

Heat exchanger 4 is a copper tube with an undulating inner surface, a nominal inner diameter of 16 mm, and a nominal 30 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 0.8 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 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 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 from Figure 2. Water to be heated enters heater 1 at 21 to flow through the heat 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 according to Figures 1 to 3, when tilted the uppermost part of the heater would normally be side 23 (see Figure 2). 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 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 16 mm has a length within the chamber of about 8 m and a storage capacity of 1.6 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 experiments 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 coefficient of 3 W/m² 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/m². The results of these numerical experiments are presented in Figures 4a and 4b in which the former figure represents the results obtained with a heater 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 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 (J.A. 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 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 20° 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 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.

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 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 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 20 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 a transparent top essentially is at uniform temperature while for the container with black top there is significant stratification with most of the hot water being at 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 which is approximately 40% greater for the transparent tank than for the black one. It 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.

Claims

1. A solar water heater comprising a thermally insulated water chamber having a solar energy-absorbing inside bottom surface and a top which is transparent to solar radiation, a water inlet, and a water outlet, wherein said top is adapted to withstand the fluid pressure of said chamber contents on tilting of said heater.

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

3. The heater according to claim 1 , wherein said solar energy-absorbing surface is black.

4. The heater according to claim 1 , wherein a sheet of glass forms the upper surface of said chamber

5. The heater according to claim 4, wherein said glass is low emissivity glass.

6. The heater according to claim 4, wherein said glass is toughened glass.

7. The heater according to claim 1 , wherein said top comprises at least one sheet of transparent material as insulation above a sheet of transparent material as the upper surface of said heater chamber.

8. The heater according to claim 7, comprising two said sheets of transparent material as insulation.

9. The heater according to claim 1 , wherein said top comprises a panel assembly having upper and lower sheets of transparent material and a transparent insulation panel sandwiched between said sheets.

10. The heater according to claim 9, wherein said upper sheet is low iron glass.

11. The heater according to claim 9, wherein said lower sheet is low emissivity glass.

12. The heater according to claim 9, wherein said transparent insulation panel is selected from a honeycomb, silica aerogel, or a film of a plastics material.

13. The heater according to claim 9, wherein: said 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.

14. The heater according to claim 1 , wherein the inside walls of said chamber are solar energy-absorbing.

15. A solar water heater comprising: a thermally insulated liquid-filled 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 a heat exchanger extending through at least a portion of said chamber, said heat exchanger having a water inlet and a water outlet external to said chamber.

16. The heater according to claim 15, wherein said water chamber comprises a tray fabricated from a core of foam insulation between outer layers of a plastics material.

17. The heater according to claim 15, wherein said solar energy-absorbing surface is black.

18. The heater according to claim 15, wherein a sheet of glass forms the upper surface of said chamber.

19. The heater according to claim 18, wherein said glass is low emissivity glass.

20. The heater according to claim 18, wherein said glass is toughened glass.

21. The heater according to claim 15, wherein said top comprises at least one sheet of transparent material as insulation above a sheet of transparent material as the upper surface of said heater chamber.

22. The heater according to claim 21 , comprising two said sheets of transparent material as insulation.

23. The heater according to claim 15, wherein said top comprises a panel assembly having upper and lower sheets of transparent material and a transparent insulation panel sandwiched between said sheets.

24. The heater according to claim 23, wherein said upper sheet is low iron glass.

25. The heater according to claim 23, wherein said lower sheet is low emissivity glass.

26. The heater according to claim 23, wherein said transparent insulation panel is selected from a honeycomb, silica aerogel, or a film of a plastics material.

27. The heater according to claim 23, wherein: said 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.

28. The heater according to claim 15, wherein the inside walls of said chamber are solar energy-absorbing.

29. The heater according to claim 15, wherein said heat exchanger extends through about half of said chamber.

30. The heater according to claim 15, wherein said heat exchanger extends throughout said chamber.

31. The heater according to claim 15, wherein said heat exchanger comprises a finned tubular serpentine.

32. The heater according to claim 15, wherein said chamber fluid is water.

33. The heater according to claim 15, wherein said chamber includes a booster heater adjacent the outlet of said heat exchanger.

34. The heater according to claim 15, wherein said heat exchanger includes a dump valve at the outlet thereof

35. The heater according to claim 15 in series with a booster heater.

36. The heater according to claim 15 in series with at least one other heater according to claim 11.