WO2016038388A1

WO2016038388A1 - Solar water heater

Info

Publication number: WO2016038388A1
Application number: PCT/GB2015/052639
Authority: WO
Inventors: William Alan LUDGATE; Lydia LUDGATE
Original assignee: Ludgate William Alan; Ludgate Lydia
Priority date: 2014-09-11
Filing date: 2015-09-11
Publication date: 2016-03-17
Also published as: GB201416105D0

Abstract

A solar liquid heater comprising: a transparent outer shell; an opaque solar absorber positioned inside said transparent outer shell; a liquid inlet that feeds liquid into a space between an outer surface of said absorber and an inner surface of said shell at an upper end of said absorber; and a liquid outlet that collects liquid from said space at a lower end of said absorber; wherein the outer surface of said absorber has a bulbous shape which bulges outwards between said upper and lower ends. The bulbous shape means that in cross section its outer surface extends around more than 180 degrees. Thus its surface can simultaneously face a large portion of the sky, collecting solar radiation efficiently across a large range of orientations. The outer surface of the absorber may be substantially globe shaped. The outer shell and the absorber may have a shape substantially corresponding to at least a part of a sphere, said part being greater than a hemisphere. Sunlight passes through the outer shell. Liquid is fed into and extracted from the space between the outer shell and the absorber, continually passing between the two. Solar energy will be absorbed by the liquid. Energy will also pass through the liquid and be absorbed by the absorber. Any reflected component may be reflected back and forth between the absorber and the shell, eventually either escaping through the shell or being absorbed by the absorber or the liquid. Thus the absorber heats up and this energy is transferred to the liquid by conduction, raising the temperature of the liquid up to the temperature of the absorber.

Description

Solar water heater

The invention relates to solar water heaters, in particular solar water heaters suitable for use in vehicles such as boats and other recreational vehicles (RVs) such as motor homes, camper vans and caravans.

Solar water heaters are often used for providing hot water in a dwelling. These usually involve some form of solar collector that concentrates solar energy and transfers it via conduction to water passing through the unit. Such devices are typically situated in a location which has a good (minimally obstructed) view of the sky and are ideally directed towards the path that the sun will take during the course of a day (e.g. they will typically be south facing in the northern hemisphere). The roof of a dwelling is often selected as the best place for such solar collectors as the roof area has a good view of the sky and is often not in use for other purposes.

A problem exists for vehicles that are also used as living space (whether temporary or permanent). The problem is that vehicles move around and change orientation and so the angle of a solar collector cannot readily be optimized to face the sun all the time. The solar collector can be arranged substantially flat and with its main collecting face facing upwards so that it has the same collecting efficiency regardless of orientation of the vehicle. It may be fitted with an additional adjustable mechanism permitting a more optimal direction to be set once the vehicle has come to a stop. This can provide some improvement, compensating for the latitude of the vehicle. However, the optimal angle changes throughout the day too. Compensating for that requires continuous adjustment and thus the equipment becomes expensive. Moreover, on a boat the angle of the solar collector to the sun changes on a much more continual basis due to changes in heading (e.g. when tacking) as well as due to the pitch and/or roll of the boat. It is not practical to compensate for these movements by tilting the solar collector as such a system would be expensive as well as being heavy and bulky. Size and weight are important considerations, particularly on smaller vessels.

According to a first aspect, the invention provides a solar liquid heater comprising: a transparent outer shell; an opaque solar absorber positioned inside said transparent outer shell; a liquid inlet that feeds liquid into a space between an outer surface of said absorber and an inner surface of said shell at an upper end of said absorber; and a liquid outlet that collects liquid from said space at a lower end of said absorber; wherein the outer surface of said absorber has a bulbous shape which bulges outwards between said upper and lower ends.

The bulbous shape of the absorber means that in cross section its outer surface extends around more than 180 degrees (and in three dimensions its surface faces more than two pi steradians of solid angle). Thus its surface can simultaneously face a large portion of the sky and thus can collect solar radiation efficiently across a large range of orientations.

The outer surface of the absorber may be substantially globe shaped.

The outer shell and the absorber may each have an outer surface with a shape substantially corresponding to at least a part of a sphere, said part being greater than a hemisphere.

In use, sunlight passes through the transparent outer shell. Liquid is fed into the space between the outer shell and the absorber and is extracted from the same space, thus continually passing between the two. Some of the energy from the sunlight (i.e. solar energy) will be absorbed directly by the liquid. However, to the extent that the liquid is also transparent (or translucent), a significant portion of the sunlight's energy will pass through the liquid and will be absorbed or reflected by the absorber. Any reflected component may be reflected back and forth between the absorber and the outer shell, eventually either escaping through the shell or being absorbed by the absorber or the liquid. The energy that is absorbed by the absorber heats up the absorber and this energy can thus be transferred to the liquid by conduction. In this way, the temperature of the liquid passing between the shell and the absorber can be raised up to the temperature of the absorber. The more sunlight reaches the absorber, the higher its temperature will be. The liquid can be cycled round, making several passes through the heater so as to gradually raise its temperature.

A solar liquid heater at least partially in the shape of a sphere has symmetry in the horizontal plane and can absorb sunlight from numerous directions. This is an important consideration for boats and recreational vehicles where the relative orientation of the heater with respect to the sun is not determined at the point of installation and does not remain constant, e.g. during travel. The globe-like shape means that the sun's light continues to be collected even as the direction of the vehicle (and thus any heater mounted to it) changes. Having a greater surface area than a hemisphere means that the heater can also collect sunlight efficiently at certain angles of tilt to the horizontal. The heater may take the shape of a spherical cap or truncated globe, i.e. the intersection of a sphere and a plane. Larger globes (i.e. with less of the sphere cut off) can accommodate larger ranges of tilt efficiently. Tilt to the horizontal is particularly important on boats which roll and pitch in anything but calm water. While sailing, boats may also have a more permanent roll to one side, known as heel, e.g. caused by wind pushing on the sails. The globe shaped solar collector with a surface area greater than a hemisphere is able to accommodate such tilt without significantly compromising the solar collection efficiency. For maximum solar collection, the heater ideally presents a full hemisphere shape towards the sun so that the maximum surface area of the absorber is able to receive and absorb direct sunlight. The increased range of tilt angles at which the heater can operate at maximum efficiency is also beneficial in a static deployment as the heater can operate at maximum efficiency when the sun is at a lower angle in the sky (and therefore can operate at maximum efficiency for a longer period of time throughout the day).

In some preferred embodiments the surface shape of the shell and/or the absorber corresponds substantially to a spherical cap with a height of at least 60% of the spherical diameter. Such a shape can be tilted up to 1 1.5 degrees from the horizontal and still present a full hemisphere directly upwards. More preferably, the surface shape of the shell and/or the absorber corresponds substantially to a spherical cap with a height of at least 75% of the spherical diameter, more preferably 80%, still more preferably 90%. At 75% the heater can be tilted up to 30 degrees from the horizontal and still present a full hemisphere of absorber vertically upwards. At 80%, the heater can be tilted up to 37 degrees and at 90% it can be tilted up to 53 degrees. More generally, the greater the spherical area (solid angle) presented to the sky, the more efficient the heater is at collecting solar energy and the more relative orientations with respect to the sun result in maximum solar collection. The shell and absorber can be non-concentric if desired, but for better symmetry it is preferred that the shell and the absorber are concentric (i.e. they are both partial spherical surfaces of spheres of different radii, but with a common centre). In some embodiments the shell and/or absorber may take the form of a stretched sphere or a stretched partial sphere. In other words it may take the form of a sphere or partial sphere (spherical cap) that has been separated at its maximum diameter in a plane perpendicular to its axis of symmetry, the two parts separated and an extension cylinder of corresponding diameter inserted between them. The cylinder may take various lengths. The shell and absorber may take substantially the same stretched sphere shape. However in some embodiments the absorber may be smaller, e.g. it may be a sphere or spherical cap. In such cases the absorber is preferably located towards the top of the shell so that the lower region of the shell acts as an enlarged sump area to be filled with liquid in use. The enlarged sump (or liquid reservoir) area allows for greater tilting of the solar heater, while keeping the outlet pipe under the liquid level. This is particularly

advantageous in embodiments where the outlet pipe is pumped and it is desirable not to run the pump dry. The absorber may be any colour. However, for maximum solar absorption it is preferred that the absorber is black. Black materials absorb the greatest range of wavelengths in the visible spectrum.

The liquid inlet may pass through the absorber, i.e. through the inside of the absorber. With this arrangement the inlet does not obstruct sunlight from reaching the outside of the solar heater (i.e. it does not cast shadows upon the outer surface of the solar heater).

The solar liquid heater may further comprise a liquid reservoir located inside the spherical shell and below the absorber. The reservoir collects and holds the heated liquid as it flows down between the absorber and the shell. As the solar heater is tilted, the reservoir flows within the bottom of the shell and thus moves relative to the outlet. The reservoir is preferably large enough that when the heater is tilted through the tilt ranges discussed above, the liquid level remains above the outlet. The outlet is thus always able to vacate fluid from the absorber. It will be appreciated that the terms "upper", "lower", "below", etc. are used here with reference to the installed position. For example, the "lower" part is the place in which liquid collects under gravity. Preferably the liquid inlet is located at an upper portion of said heater and the liquid outlet is located at a lower portion of said heater. The inlet may be situated to deliver liquid to the top of the absorber and the outlet may be situated to collect water from the bottom of the absorber.

Preferably at least one groove is formed on the outside surface of the absorber, the groove having both a horizontal component and a vertical component when the heater is in an upright orientation. The groove(s) serve to encourage liquid to flow not just directly down the absorber under gravity, but also around the absorber in a horizontal direction, thus increasing the path length that the liquid takes from the inlet to the outlet. The increased path length provides a greater contact time between the liquid and the absorber and thereby allows for greater transfer of thermal energy through conduction. The groove(s) may be made by removing material from the surface of the absorber (e.g. by turning or etching) or by building up the surface of the absorber (e.g. by deposition processes) or they may simply be molded into the absorber surface during manufacture.

The groove(s) may take a helical path around said absorber, i.e. may progress around the absorber in a spiral or helix from top to bottom. Several grooves may be provided in parallel. A groove does not need to form a single continuous path from top to bottom, but may be formed from several partial paths that together encourage liquid to flow in a generally helical path around the absorber.

The groove(s) may be angled at between 10 degrees and 45 degrees to the horizontal. The optimum angle will depend on a number of factors such as the size of the absorber, the liquid that the absorber is intended to heat and the material of the absorber. Ideally the angle is selected so as to allow maximum heat transfer between the absorber and the liquid without unnecessarily slowing down the liquid flow from inlet to outlet so as to keep the throughput high. In some preferred embodiments the groove is angled at between 15 degrees and 30 degrees to the horizontal (when the solar heater is in an upright position. The groove may be angled so as to maximize the heat exchange of liquid with the surface of the absorber in use.

The grooves serve to encourage liquid flow around the surface of the absorber, but do not prevent it from taking an alternative path, e.g. directly downwards under gravity. This is important when the solar heater is tilted at an angle greater than the angle of the grooves as liquid. In such situations, if the alternative / direct path were not available, liquid could be unable to flow between the inlet and the outlet. The outer shell may be a double walled shell. The space between the walls of the shell may be a vacuum or may be filled with a noble gas or other insulating gas. The double walled shell serves as an insulating layer, producing a greenhouse to reflect radiated heat back to the absorber, thus accumulating heat within the solar heater.

The outer shell may be made from plastics or glass. The choice of material will depend on the intended use. For example, glass is heavier and more breakable so plastics are preferred for mobile or temporary units such as on boats or RVs. For stationary units glass may be a better material due to its scratch resistance and the fact that it does not discolour over time. Suitable plastics include Polycarbonate and PMMA (poly(methyl methacrylate)) and variants thereof.

The solar liquid heater may further comprise an insulating material inside said absorber. The insulating material minimizes heat conduction from the absorber into the interior of the apparatus, thus encouraging heat transfer from the absorber to the liquid in order to increase the temperature of the liquid as much as possible. The absorber may be thin walled, i.e. in the form of a shell. The absorber is typically a denser and heavier material than any insulation inside and so this keeps the weight of the heater down. The insulation may be formed as a layer inside the absorber (i.e. also a shell of material) or it may fully fill the interior of the absorber.

In the latter case, the insulation may be a foam that can be sprayed into the interior after the absorber has been formed, then allowed to set.

In some embodiments the absorber is a shell and holes are formed through the absorber so as to channel liquid from the outside of the absorber to the inside thereof and at least one liquid channel is provided on the inside surface of the absorber that has both a horizontal component and a vertical component when the heater is in an upright orientation. The provision of liquid channels inside the absorber allows for heat conduction from the absorber to the liquid from an increased surface area, i.e. from its outer surface and its inner surface. This allows more rapid and efficient heat transfer from the absorber to the liquid. While a portion of the liquid may be fed from the inlet directly to the inside of the absorber, it is simpler from a manufacturing perspective to provide all the liquid to the outside surface of the absorber and then to create holes that transfer a portion of the liquid to the inside surface of the absorber. While the liquid may be allowed simply to run down the inside surface of the absorber under gravity, the liquid channel with both horizontal and vertical components encourages a longer contact path between the liquid and the absorber, providing a greater opportunity for heat exchange.

Preferably the liquid channel takes a helical path around the absorber. The liquid channel may be angled at between 10 degrees and 45 degrees to the horizontal, more preferably between 15 degrees and 30 degrees to the horizontal. The liquid channel may be angled so as to maximize the heat exchange of liquid with the surface of the absorber in use. The liquid channel may have one or more downward passages therethrough so that when the heater is tilted at an angle greater than the angle of the channel, liquid that is caught in the channel can still flow downwards towards the outlet.

The globe shaped solar liquid heater is able to collect a large quantity of incident solar radiation from a large range of orientations. However, for increased collection, it preferably further comprises one or more reflectors around the base of the transparent outer shell to reflect sunlight through the shell and onto the absorber. The reflectors may be in a ring arrangement around the base of the heater for reflecting sunlight from above up onto the lower part of the absorber. The solar liquid heater may further comprise a motor arranged to rotate the absorber. Rotating the absorber (around an axis which is vertical in normal, non- tilted use), allows a greater proportion of the absorber to receive direct sunlight. During normal use, while some of the absorber faces the sun, the opposite side is in shadow. Rotating the absorber brings the side that is in shadow round into the sunlight allowing it to be directly heated by absorbing radiation. Once heated above the temperature of the liquid, this portion of the absorber will continue to conduct heat to the liquid when it has been rotated back to the shaded side. It will be appreciated that the outer shell may also be rotated with the absorber as this may be practically simpler to implement (however it is not strictly necessary - the absorber alone could be rotated).

Preferably the solar liquid heater is connected to a liquid reservoir and a pump is arranged to circulate liquid from the reservoir to the inlet, through the heater and from the outlet to the reservoir. As the liquid is continuously circulated, it continuously absorbs heat from the absorber, gradually increasing in temperature. The reservoir (tank) will normally be insulated so as to minimize heat loss from the system. Preferably the pipes and/or the pump are also insulated to minimize heat loss from the system. The invention also extends to an array of solar liquid heaters as described above, including any of the optional features, all connected to a common liquid reservoir and with one or more pumps arranged to circulate liquid from the reservoir through the heaters and back to the reservoir. An array of heaters (meaning two or more, but optionally three, four or more (possibly many more) individual solar heaters) can be used to keep size and weight down when a larger heat input (i.e. larger solar collection area) is required.

The invention extends to use of a solar liquid heater as described above (including any of the option features) for heating water on a boat or recreational vehicle.

The invention extends to use of a solar liquid heater as described above (including any of the option features) for heating swimming pool water. The swimming pool is essentially a large reservoir and the water can be pumped into the solar liquid heater for heating before being fed back into the pool. The water is thus continually cycled through the heater to raise its temperature.

It will be appreciated that the size of the solar liquid heater will depend on the application. For transportation such as boats and recreational land vehicles (RVs) size and weight are important considerations. Therefore in some preferred embodiments the diameter of the outer shell is no more than 50 cm. While in some embodiments the diameter of the outer shell may be as small as 20 cm, in other embodiments it is preferred that the diameter of the outer shell be at least 30 cm so as to provide sufficient solar collection area. For larger applications such as for heating a swimming pool, the diameter of the solar globe heater may be significantly larger as space and weight are not so restricted. Such a heater is likely to be ground mounted. For such applications the diameter of the outer shell may be at least 50 cm and may be up to 1 metre, or more. Where the heater employs a motor to rotate the absorber, this may be particularly aesthetically pleasing, essentially acting like a fountain or other water feature, thus making the heater less of an intrusion upon users of the pool. The heater therefore does not have to be mounted out of sight and can be located right next to the pool for a high efficiency (lower heat loss in transfer piping). It will be appreciated that while the solar liquid heater described above may be used for many applications for heating any liquid, one of the predominant uses is expected to be for water heating. Heated water is an important commodity on boats and RVs which have limited power sources and need to carry limited quantities of fuel with them. Therefore access to solar power to effectively heat water is highly desirable. Therefore in many preferred embodiments of the invention the liquid is water.

According to another aspect, the invention provides a method of making a solar liquid heater comprising: forming an opaque solar absorber; forming a transparent outer shell around said absorber; wherein said absorber has an outer surface with a substantially bulbous shape, bulging out between an upper end and a lower end; and wherein forming said transparent outer shell comprises forming said shell in two parts and joining them together around said absorber. The two parts may be shaped in any suitable manner such as be moulding, casting, vacuum forming, injection moulding, etching, turning etc. The two parts may be attached together in any liquid tight bonding manner. For example, they may be welded or bonded together with an adhesive. The same principles apply to the absorber. Therefore preferably forming the absorber comprises forming the absorber in two parts and joining them together. Again, the absorber parts may be formed and bonded in any suitable fashion. The absorber may be formed and bonded first with the shell then formed around the absorber. Alternatively the two shell parts and the two absorber parts may be bonded in a single step, i.e. with one absorber part located inside one shell part and the other absorber part located inside the other shell part, these structures then being brought together and bonded in a single bonding step. The absorber may comprise a liquid inlet that passes through the interior of the absorber and feeds liquid to the exterior surface of the absorber. This liquid inlet passage may be formed as part of the moulding process, e.g. formed as two parts which are then bonded together to form the passage. Alternatively a separate tube may later be inserted into the absorber through suitable formed holes so as to channel liquid through the inside of the absorber.

As discussed above, the inside of the absorber may be filled with an insulating material. Therefore preferably the method further comprises injecting an insulating material inside the absorber. The insulating material may be a foam which sets inside the absorber.

It will be appreciated that all of the other preferred features described above in relation to the apparatus apply equally to the method. Certain preferred embodiments of the invention will now be described, by way of example only and with reference to the accompanying drawings in which:

Fig. 1 shows a first embodiment;

Fig. 2 shows a second embodiment;

Fig. 3 shows a third embodiment;

Fig. 4 shows a heater system; and

Fig. 5 shows a motor driven heater.

The figures illustrate embodiments of a solar water heater that can be used on boats, RV vehicles, cabins, caravans, etc and that is designed to heat water from sunlight regardless of the position and angle of the installation site, even heeled up to 45 degrees from the horizontal. The unit is inexpensive to produce and is robust enough to withstand a harsh or rugged environment. On boats, RV vehicles, etc, power is normally supplied by batteries that are contained in the vehicle. With the exception of particularly large vehicles, these batteries tend to have limited capacity. Heating water (and keeping it hot) requires large amounts of energy that cannot normally be supplied by such batteries, at least not for extended periods of time. This limited power supply can only be renewed by running the engine, or by charging through solar panels, wind generators, etc. The inefficiency of such devices generally results in a very limited power source being available.

Another major problem is that such vehicles are always changing their aspect to the sun. This is particularly the case with sailing boats which do not stay in a horizontal state (they roll and pitch as well as changing heading).

Ideally a heating unit needs to be robust and relatively small so as to be able to be sited easily in any position and drawing very little (if any) energy from the vehicle's batteries to heat the water.

In other applications, the water heater described here may be applied in static (non- vehicle) installations. The heater may be particularly useful in remote (especially third world) situations where energy is not readily available to heat water.

Fig. 1 shows a cross-section through a first embodiment of a solar water heater. A mounting plate 10 is provided for mounting the solar heater to a fixed structure such as the deck of a boat, the roof of a cabin or the roof of a recreational vehicle such as a camper van or caravan. Equally the solar heater may be fixed to the roof of a house or other building. A base 20 is attached to the mounting plate 10. This may be a permanent attachment, e.g. the base 20 may be welded to the mounting plate 10 or integrally formed therewith. The base 20 provides an upper concave surface on which a spherical globe-shaped transparent outer sphere 30 is mounted.

Preferably the outer sphere 30 is permanently or semi-permanently mounted to the base 20, e.g. by adhesive. Inside the transparent outer sphere 30 is an opaque spherical absorber 40. The absorber 40 is coloured black so as to absorb the maximum amount of solar energy that is incident upon it. The colouring may be achieved via dyes, pigments or paints or the absorber 40 may be formed from a naturally black material. The inside of the absorber 40 is filled with a thermally insulating foam 45 that restricts heat transfer away from the outer surface of absorber 40 that is in thermal contact with the water. It will be appreciated that a lining of foam 45 may be sufficient to ensure thermal insulation without completely filling the inside of absorber 40, although it may facilitate manufacture simply to fill the whole interior of the absorber 40, thus also surrounding and insulating the inlet pipe 50 as it passes through the absorber 40.

A water inlet 50 feeds water (or other liquid) into the unit via a pipe that passes through the centre of the absorber 40. The fact that the pipe 50 travels through the inside of the absorber rather than outside the absorber 40 means that it does not obscure or cast shadow on any part of the absorber's outer surface. The inlet pipe 50 emerges at the top of the absorber 40, thus feeding water directly onto the outer surface of the absorber 40, into the space between the absorber 40 and the outer shell 30. The water is then free to flow downwards over the surface of the absorber 40 under gravity to a collection reservoir 60 formed at the bottom of the unit (still between the outer shell 30 and the absorber 40). The reservoir is enlarged by the absorber 40 taking the form of a spherical cap, i.e. a truncated sphere (the intersection of a sphere and a plane, leaving more than a hemisphere). The reservoir (or sump) 60 is drained by an outlet pipe 70 that removes water from the unit. The inlet pipe 50 and outlet pipe 70 pass through one or more openings in the base 20 and mounting plate 10. The water inlet 50 and outlet 70 may be connected to a larger insulated storage tank (not shown) where it is stored until needed. The water may be pumped round continuously or intermittently so as to add heat to the water in the tank. Pumping may be terminated overnight when no heat is available from the sun.

Spiral grooves 80 are shown in Fig. 1 on the outer surface of the absorber 40 on a section which has not been cut away in the cross-section. These (or similar) grooves 80 are provided over the entire surface of the absorber 40. They may be continuous grooves 80 forming a continual spiral around the globe surface or they may be intermittent, i.e. a large number of shorter grooves 80 that together form a spiral pattern over and around the whole surface of the globe. The purpose of these grooves 80 is to encourage the flow of water to move around the surface of the globe, not just directly downwards under gravity. It should be noted that the direct path downwards under gravity is not blocked so that if the grooves 80 overflow, water can still travel from the inlet pipe 50 at the top of the absorber 40 down to the collection reservoir 60 at the bottom of the absorber 40. The grooves 80 serve to increase the path length of water flowing from the inlet pipe 50 to the reservoir 60, thus increasing the time that it is exposed both to the incoming solar radiation and the time that it is in contact with the absorber 40. As the absorber 40 is designed to absorb the solar radiation and heat up, contact between the absorber 40 and the water is important for transferring heat energy via thermal conduction.

Fig. 1 also shows reflectors 90 that are generally circular and surround the base 20 underneath the transparent outer shell 30. The reflectors 90 serve to collect radiation from a larger area of the sky and reflect that additional radiation back up onto the bottom of the absorber 40 which would otherwise not so often see directly incident radiation. This increases the amount of energy that can be collected by absorber 40, thus increasing its temperature and thereby increasing the

temperature of the water passing over it.

Fig. 2 shows another embodiment of the solar water heater, similar in many respects to the embodiment of Fig. 1. Like elements are indicated with like reference numerals. It will be noted that Fig. 2 shows no reflectors 90 underneath the shell 30, but these could optionally be added if desired. It will also be noted that the grooves 80 on the outer surface of the absorber 40 are not shown in Fig. 2 although they are a feature of this embodiment and are merely not visible in the drawing.

One significant difference of this embodiment that is illustrated in Fig. 2 is that the outer shell 30 is a double walled shell 30 comprising an outer shell 31 and an inner shell 32. The double walled construction increases the amount of solar energy that is retained within the globe by increasing back reflection of radiation emitted form absorber 40, creating a greenhouse effect. This increases the total energy absorption of the water and of absorber 40 and thereby increases the temperature of both. The space between the inner shell 32 and outer shell 31 may be a vacuum or it may simply be air. Alternatively it may be a noble gas as used in many double glazed windows and doors.

Another feature illustrated in Fig. 2 and which is not shown in Fig. 1 is a plurality of holes 85 that allow liquid flow from the outer surface of absorber 40 through to the inner surface of absorber 40. All of the liquid form inlet pipe 50 is delivered to the outside surface of absorber 40. By allowing some of the liquid to pass back to the inside surface of absorber 40, the total contact area between liquid and absorber is effectively nearly doubled. This allows much more efficient heat transfer from the absorber 40 to the water as it travels from the top of the absorber 40 to the bottom. Water on the inside of the absorber 40 sticks to the inside surface through surface tension. However in the same way as on the outer surface of absorber 40, spiral grooves are provided on the inside surface of absorber 40 so as to encourage liquid to flow in a spiral or helical path around the inside of the absorber 40, thus increasing its path length from top to bottom and thereby increasing its thermal contact time with the absorber 40. At the bottom of the absorber 40, exit holes 86 are formed which allow water to pass back out from the inside of the absorber 40 to the outside of the absorber 40 and into the collection reservoir 60. A guide lip 87 also helps to guide flow through these holes 86, preventing or limiting liquid overflow in case the flow rate is temporarily greater than the holes 86 permit.

By way of example, the globe solar water heater shown in Fig. 2 is approximately 40 cm in diameter. This size provides a good compromise between a large surface area for solar collection and absorber/water contact and a small enough unit to be portable, light weight and not take up too much space when mounted on a vehicle (where space is often limited).

It can also be seen in Fig. 2 that the outer shell 30 is formed from two hemispherical halves, an upper half 33 and a lower half 34. The lower half 34 is mounted to the base 20 via fixings 21. The upper half 33 is mounted to the lower half 34 via a clip 35 that holds together an outwardly extending flange on each of the upper half 33 and lower half 34. Forming the outer shell 30 from two halves 33, 34 makes manufacture and assembly much easier as the two halves 33, 34 can be moulded in straightforward moulds and can then be brought together around the absorber 40. Although not shown, the absorber 40 may also be formed from two individually moulded halves which are fixed together. The absorber halves may be

permanently fixed to one another e.g. by welding or with adhesive.

Fig. 3a shows illustrates another embodiment of the solar absorber with an elongated shape. This modification may readily be applied to either of the embodiments of Figs. 1 and 2. The outer shell 30 is shown with the same upper hemispherical part 33 and lower hemispherical part 34 as are shown in Fig. 2. However instead of mounting these two parts directly to one another, an intermediate cylinder 36 is provided that elongates the shape of the outer shell 30 so that it takes the form of a stretched sphere. The hemispheres 33, 34 are attached to the cylinder part 36 via flanges in a similar manner to that shown in Fig. 2.

The elongated shape maintains the benefits of the hemispherical (globe-like) top and bottom (i.e. that the unit is efficient in terms of its sky-facing surface area and is resilient to tilts and direction changes), but also allows for an enlarged sump or collection reservoir 60. This extended reservoir 60 allows the unit to hold much more water and be much more resilient to tilts such that the level of water in the sump less likely to fall below the outlet pipe 70 even when the unit experiences a high tilt angle.

Fig. 3b shows the elongated shell 30 with the absorber 40 inside and located towards the upper end, thus creating the enlarged sump 60 underneath the absorber 40.

The extension cylinder 36 may take various lengths, but it is generally designed to have a height of around 30 to 100% of the radius of the hemispherical parts 33, 34.

The outer shell 30 is preferably made from a clear plastic material for reduced weight, e.g. Perspex™.

Fig. 4 shows a solar globe water heater unit 100 (which may be as shown in any of Figs. 1 , 2 or 3 above) connected to a tank 120 via a pump 110. A temperature sensor 130 is submerged within the tank 120 for monitoring the temperature of the water. When the temperature sensor 130 detects a drop in temperature below a predetermined acceptable value (the cut-in temperature), the pump 110 may be activated to pump water through the solar globe water heater 100 via its inlet pipe 50. Water from the outlet pipe 70 of globe 100 is brought back to tank 120. The pump 110 may continue in operation until the temperature sensor 130 detects that the water in tank 120 has reached a cut-off temperature. Note that the cut-off temperature may be higher than the cut-in temperature so as to avoid continual switching of the pump 1 10. It will be appreciated that more sophisticated logic may be programmed into the system e.g. to take account of time and location (e.g. via GPS or manually set) so as to avoid pumping water through the solar heater 100 when the solar energy will be insufficient to heat the water. This will be the case after sunset and before sunrise, but may also be the case at times when the sun is low in the sky just before sunset and just after sunrise. Another alternative for control of the pump 1 10 is to use a light detector 140 to detect the amount of available light. If there is insufficient light, the pump 110 will not be activated.

The pump 1 10 may be a 12 V or 240 V pump driven from a mains supply or battery (e.g. a vehicle battery). It may also be solar powered, thereby only operating when there is sufficient sunlight. It will be appreciated that it does not matter what angle the globe 100 is at with respect to the horizontal at any given time as the water within the globe 100 will always run down into the collecting reservoir 60 before returning to the hot tank 120. Solar radiation will fall onto the unit 100 onto at least 50% of its surface area regardless of the position or angle of the unit to the sun. The reflector 90 around the base of the globe can reflect about a further 20% of available light that would otherwise not be collected into the collector and therefore increases the collected energy that can be used for heating the water. Water can be continuously circulated from the hot tank 120 to the solar collector 100 by the pump 1 10 while sunlight is available and until the water temperature has reached the required heat then the pump will stop pumping.

Fig. 5 shows a solar heater 100 in which the absorber 40 is rotatably driven by a motor 200. The other features of solar heater 100 may be as shown in any or all of Figs. 1 , 2, 3 or 4 and described above. As the motor 200 rotates the absorber 40, different parts of the surface of absorber 40 will come to face the sun (or the optimum direction for solar energy absorption). This brings the full surface area and the full thermal mass of the absorber 40 into play without relying on thermal conduction within the absorber 40. The temperature of the absorber 40 will be raised more uniformly and the efficiency of heat transfer from the absorber 40 to the liquid is increased. The absorber 40 need not rotate rapidly and thus the motor 200 may be a low power motor that uses very little energy. The motor 200 may be controlled via the same or similar logic that drives the pump 1 10 so that it is not driven unnecessarily when the heater 100 is not in use. The unit 100 may be manufactured in many different sizes. The appropriate size will depend on the application, e.g. whether it is a static unit or a vehicle mounted unit, and it will also depend on the size of hot tank (and therefore the amount of water to be kept at the required temperature). The unit 100 can be mounted on any flat surface or permanent structure. In the case of a boat, the cabin roof or rails are strong enough to mount the unit.

Similarly, the roof of a recreational vehicle (RV) is a suitable mounting point for the unit 100. Such mobile units will typically be kept to a smaller size due to the limited space available for mounting, the weight considerations for being sufficiently portable and the reduced size of water tanks that are practical in vehicles. For example, typical solar globe heaters for vehicle applications are less than 50 cm in diameter.

Larger units 100 for heating dwellings or swimming pools can be mounted in static situations, e.g. on flat roofs or on the ground as long as the unit is not in a shaded position. Such units are for heating larger quantities of water and do not suffer from the weight limitations of vehicles. Therefore such units 100 may be 1 metre in diameter or greater. This contrasts with normal solar heating units that tend to be sited on sloping roofs of dwellings that are south facing (in the northern

hemisphere) so as to maximize the solar collection.

As mentioned above, larger units (e.g. greater than 1 metre in diameter) are particularly advantageous for water heating in remote or third world areas as the units 100 are inexpensive to make, are lightweight for transport out to the location, are easy to assemble and inexpensive to run, e.g. from a 12 V battery. A further advantage of the solar globe heater 100 (applicable in all applications) is that with a plastic outer shell 30 that allows UV to pass, a degree of UV sterilization will take place as the water passes over the outer surface of the absorber 40.

Claims

1. A solar liquid heater comprising:

a transparent outer shell;

an opaque solar absorber positioned inside said transparent outer shell; a base for mounting said solar liquid heater;

a liquid inlet that feeds liquid into a space between an outer surface of said absorber and an inner surface of said shell at an upper end of said absorber; and a liquid outlet that collects liquid from said space at a lower end of said absorber;

wherein the outer surface of said absorber has a bulbous shape which bulges outwards between said upper and lower ends.

2. A solar liquid heater as claimed in claim 1 , wherein the outer surface of said absorber is substantially globe shaped.

3. A solar liquid heater as claimed in claim 1 or 2, wherein said outer shell and said absorber each have an outer surface with a shape substantially corresponding to at least a part of a sphere, said part being greater than a hemisphere.

4. A solar liquid heater as claimed in claim 3, wherein the surface shape of said shell and/or said absorber corresponds substantially to a spherical cap with a height of at least 60% of the spherical diameter.

5. A solar liquid heater as claimed in claim 4, wherein the surface shape of said shell and/or said absorber corresponds substantially to a spherical cap with a height of at least 80% of the spherical diameter.

6. A solar liquid heater as claimed in claim 5, wherein the surface shape of said shell and/or said absorber corresponds substantially to a spherical cap with a height of at least 90% of the spherical diameter.

7. A solar liquid heater as claimed in any preceding claim, wherein said shell and said absorber are concentric.

8. A solar liquid heater as claimed in any preceding claim, wherein the absorber is black.

9. A solar liquid heater as claimed in any preceding claim, wherein said liquid inlet passes through said absorber.

10. A solar liquid heater as claimed in any preceding claim, further comprising a liquid reservoir located inside said spherical shell and below said absorber.

1 1. A solar liquid heater as claimed in any preceding claim, wherein said liquid inlet is located at an upper portion of said heater and wherein said liquid outlet is located at a lower portion of said heater.

12. A solar liquid heater as claimed in any preceding claim, wherein at least one groove is formed on the outside surface of said absorber, said groove having both a horizontal component and a vertical component when the heater is in an upright orientation.

13. A solar liquid heater as claimed in claim 12, wherein said groove takes a helical path around said absorber.

14. A solar liquid heater as claimed in claim 12 or 13, wherein said groove is angled at between 10 degrees and 45 degrees to the horizontal.

15. A solar liquid heater as claimed in claim 14, wherein said groove is angled at between 15 degrees and 30 degrees to the horizontal.

16. A solar liquid heater as claimed in any of claims 12 to 15, wherein said groove is angled so as to maximize the heat exchange of liquid with the surface of the absorber in use.

17. A solar liquid heater as claimed in any preceding claim, wherein said outer shell is a double walled shell.

18. A solar liquid heater as claimed in any preceding claim, wherein said outer shell is made from plastics or glass.

19. A solar liquid heater as claimed in any preceding claim, further comprising an insulating material inside said absorber.

20. A solar liquid heater as claimed in any preceding claim, wherein said absorber is a shell and wherein holes are formed through said absorber so as to channel liquid from the outside of said absorber to the inside thereof and wherein at least one liquid channel is provided on the inside surface of said absorber that has both a horizontal component and a vertical component when the heater is in an upright orientation.

21. A solar liquid heater as claimed in claim 20, wherein said liquid channel takes a helical path around said absorber.

22. A solar liquid heater as claimed in claim 20 or 21 , wherein said liquid channel is angled at between 10 degrees and 45 degrees to the horizontal.

23. A solar liquid heater as claimed in claim 22, wherein said liquid channel is angled at between 15 degrees and 30 degrees to the horizontal.

24. A solar liquid heater as claimed in any of claims 20 to 23, wherein said liquid channel is angled so as to maximize the heat exchange of liquid with the surface of the absorber in use.

25. A solar liquid heater as claimed in any of claims 20 to 24, wherein said liquid channel has one or more downward passages therethrough.

26. A solar liquid heater as claimed in any preceding claim, further comprising one or more reflectors around the base of said shell to reflect sunlight through said shell.

27. A solar liquid heater as claimed in any preceding claim, further comprising a motor arranged to rotate the absorber.

28. A solar liquid heater as claimed in any preceding claim connected to a liquid reservoir and with a pump arranged to circulate liquid from said reservoir to said inlet, through said heater and from said outlet to said reservoir.

29. A solar liquid heater as claimed in any preceding claim, wherein the outer shell has a diameter of at least 30 cm and up to 100 cm.

30. An array of solar liquid heaters each as claimed in any of claims 1 to 29, all connected to a common liquid reservoir and with one or more pumps arranged to circulate liquid from said reservoir through said heaters and back to said reservoir.

31. Use of a solar liquid heater as claimed in any of claims 1 to 29 for heating water on a boat or recreational vehicle.

32. Use of a solar liquid heater as claimed in any of claims 1 to 29 for heating swimming pool water.

33. A method of making a solar liquid heater comprising:

forming an opaque solar absorber;

forming a transparent outer shell around said absorber;

wherein said absorber has an outer surface with a substantially bulbous shape, bulging out between an upper end and a lower end; and

wherein forming said transparent outer shell comprises forming said shell in two parts and joining them together around said absorber.

34. A method as claimed in claim 33, wherein forming said absorber comprises forming said absorber in two parts and joining them together.

35. A method as claimed in claim 34, wherein the absorber comprises a liquid inlet that passes through the interior of the absorber and feeds liquid to the exterior surface of said absorber.

36. A method as claimed in any of claims 33, 34 or 35, wherein said absorber is a shell and further comprising injecting an insulating material inside said absorber.