AU711669B2 - Integral tank solar water heater - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Applicant: Siang Teik TEOH Invention Title: INTEGRAL TANK SOLAR WATER HEATER The following statement is a full description of this invention, including the best method of performing it known to me/us: 0 00 ".0.0 0.0.

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Goo INTEGRAL TANK SOLAR WATER HEATER This invention relates to an improved solar water heating system.

TECHNICAL FIELD A solar water heating system to which the present invention particularly relates comprises a solar heat collector connected to hot water storage tank. The collector panel has parallel heat pipes or other channels embedded in a selective absorber black metal sheet inclined towards the sun and running vertically and opening into horizontal manifold or header pipes at the top and at the bottom of the solar collector panel. The sun shines on the black metal sheet and on the pipes or other channels embedded in the black metal sheet which heat up, the iheat is transferred through conduction into the water in the pipes which heats up, expands slightly and so its density is less than that of the rest of the (cooler) water in the other parts of the system. This heated water then begins to rise to the top of the inclined panel and enters the horizontal manifold, then the water rises from the upper manifold through a bend of one or two 90 degree elbow joints before entering the hot water tank. The new incoming hot water pushes its way to 15 the top of the hot water tank, displacing the cooler water existing in the hot water tank, which cooler water sinks to the bottom of the hot water tank. This cooler, denser, water continues to sink down a pipe at the side of the hot water tank, down the side of the solar collector panel, makes another 90 degree turn in an elbow joint, before entering the horizontal manifold at tlhe bottom of the solar collector panel, where it is heated again by the sun's rays and the process 20 begins anew. Thus the temperature in the hot water tank increases throughout the day.

BACKGROUND ART Existing thermosyphon based solar water heating systems of this kind normally consist of a solar collector panel with a separate insulated hot water tank placed immediately above the collector panel. An example of prior art of this type of solar water heater is disclosed in US Patent 4,084,578, Ishibashi, 18 April 1978, a drawing of his solar water heating system is shown in Figure 0 PRIOR ART. As shown in Ishibashi an attempt is made to increase collection efficiency by improving the selective surface of the absorber through special paints and coatings, special non reflecting glass, utilising different corrugation profile angles of collector sheet, and using the hot water tank as a reflector in winter.

Existing prior art does not.recognise the importance of a direct upward connection for the hot water to flow naturally and without unnecessary impedance into the lower level of the hot water storage tank, and doesnot recognise the importance of a direct downward separate return pipe(s) centrally placed to minimise horizontal flows.

For instance it is known to place the tank in close proximity immediately above the collector panel as is shown in US Patent 4,766,885, (Murumatsu). In such systems the hot water is forced to flow horizontally across the breadth of the panel, or even worse, across two panels if it is a two panel system, and the hot water is forced through one or two elbow joints with all their attendant increase in resistance to flow due to form drag (eddying and turbulence) and friction drag etc. on the thermosyphon flow, before entering the hot water storage tank.

It is well known that slowing down the natural thermosyphon flow reduces the efficiency of the heat collection, as the water in the horizontal manifold slows down as it is unable to move in its natural direction, that is upwards. It consequently gets hotter and hotter.

This relatively stagnant flow in the upper manifold and upper part of the heat pipes and in the elbow joints becomes (unnecessarily) very hot thus losing more heat through the glass, which 25 heat should have been allowed to flow smoothly into the insulated tank. The overall heat transfer coefficient of efficiency is therefore low.

US Patent 4,353,352 (Zinn) discloses an improved thermosyphon flow, in that there is a near direct connection from the heat pipes into the tank, but that system is flawed since the hot water is forced to travel in a roundabout way from the top of the panel, curving behind the tank, before entering the tank itself. In addition the hot water is also forced to flow downwards, against thermosyphon flow, upon entering the tank, as the hot water inlet pipes are at the very top of the tank, pointing downwards.

The importance of placing the hot water inlet pipes at the lower level of the hot water tank, pointing upwards as forms part of the present invention is not recognised in Zinn where the inlet pipes are at the very top pointing downwards. If the inlet pipes are at the top, the build-up of hot water in this area causes a congestion once some hot water has accumulated at the top of the tank. This is because any new incoming hot water has to displace this layer of hot water downwards. This layer of hot water would naturally be difficult to push downwards and so this "plug" of hot water around the top of the tank effectively slows down the thermosyphon flow even more. The inefficiency rises as the amount of hot water increases.

No less important is the direct downward flow from the bottom of the hot water storage tank to the bottom of the collector panel. Known-prior art systems, including Ishibashi and Zinn, have the return-pipe at the side of the collector panel, which slows down the thermosyphon flow, as horizontal runs, (which are against the natural upwards or downwards movement of thermosyphon flow) are then necessary across the width of the panel(s). In prior art, not only does the cooler water 20 (which wants to sink), have to travel horizontally across the length of the hot water tank before finding the outlet leading down to the panel, but once in the bottom of the panel, the cooler water has to travel horizontally back across the width of the panel before it can reach the furthest heat pipe..

Thus in prior art only a few of the heat pipes near the inlet and outlet are working at peak efficiency, but the major parts of the panel away from the inlets/outlets suffer from stagnant or retarded flow.

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In Murumatsu's system US Patent 4,724,826 of 16 February 1988 the heating fluid is allowed to flow upwards and return downwards together via a multiplicity of channels in a plate-like heat pipe absorber, but these return channels are the very same as the heating channels, hence the upwards and downwards flows clash with each other in the little channels, the cross channels with many junctions give rise to more turbulence, and the whole flow is inefficient. Having the return-flow exposed to the sun is also not efficient as there is little or no temperature differential between the upwards and downwards flows, with the result that the whole heat plate panel attains an even temperature across the glazed portion, with little thermosyphon flow happening.

Other systems which employ the "fully wetted" plate system such as Murumalsu's where the upwards and downwards flows occur unseparated in the same flat channel suffer from the inefficient mixing of flow, for the thermosyphon flow to work efficiently (laminar flow) the thickness of the flat channel heat pipe must be increased so that there is enough depth in the water channel for the two opposing laminar flows to work independently of each other, separated by a middle layer of water which is the turbulent boundary layer. This increase in thickness of water depth in the flat plate channel unnecessarily increases the total volume of water or heating fluid in the system so much so that the temperatures attained will not be very high and is thus only suitable for low temperature gains. Furthermore the volume of water or heating fluid exposed to night-time radiation loss through the glass is increased leading to greater heat loss at night. The hot water tank is also directly thermally connected to the whole 20 width of the flat plate channel which leads to increased heat loss at night through conduction into the flat plate channel and its subsequent radiation loss through the glass.

OBJECTS OF THE INVENTION It is therefore an object of this invention to provide an improved solar water heating system which will at least minimise the inherent inefficiencies in the thermosyphon flow in the prior 25 known systems, whilst still minimising heat losses at night.

SUMMARY OF THE INVENTION According to the present invention there is provided a solar water heating system comprising a core consisting of a plurality of substantially parallel heat pipes or other channels adapted in use to be inclined to the horizontal, characterised in that the upper end of each of the heat pipes extends directly into and opens upwards into a hot water storage tank at a level approximately one quarter the height (depth) of water in the tank measured from the bottom of the hot water tank, and wherein the bottom end of each heat pipe or other channel is connected to a manifold which extends transversely to the axis of the heat pipes and further characterised in that at least one cold water return pipe or other channel (separate from the heat pipes) located centrally under the heat pipes extends directly from the lowest portion of the said hot water storage tank intermediate of either end of the hot water storage tank and is joined to the said manifold intermediate of either 20 end of the manifold.

The solar water heating system can be used in direct and indirect heating systems. Indirect systems can be adapted in freezing environments wherein a closed system of heat 25 and return-pipes containing antifreeze solution or other fluid heats water flowing through an integrated heat o exchange tank. The water extracted from the heating system is therefore indirectly heated. Indirect and direct heating systems are well known in the art.

0 9 BRIEF DESCRIPTION OF THE DRAWINGS Two preferred forms of the invention will now be described with the aid of the accompanying drawings wherein, Figures 1A to 10A are of Model A designed for a flat roof surface \\melb-files\homeS\Moniqxie\Keep\speci\24885-95.doc 02/09/99 5a mounting, and Figures 1B to 10B are of Model B designed for a sloping tiled roof surface mounting. The Model A is shown with the cold water inlet introduced through the side end of the bottom manifold while Model B is shown with the cold water inlet introduced through a ball float valve fitted with a perforated baffle, both forms of cold water supply are interchangeable on the two models, the ball float valve is only required if there is no cold water supply balancing tank available above the solar heater; Figure 1A is a view of the complete solar heating system for a flat roof according to the present invention.

Figure 2A is a view similar to Figure 1A with the casing being partly cut away to show the collector panel and the interior of the hot water storage tank and the collector panel mounted on a stand.

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\\melbfies\homeS\Monique\eep\speci\24885-95.doc 02/08/99 6 Figure 3A is cutaway view with the casing removed of the core of the solar water heating system for a flat roof comprising the collector panel integrated to the hot water tank, with a single return pipe.

Figure 4A is similar to Figure 3 but with double return pipes.

Figure 5A is a view from below of the core of the solar heating system for a flat roof comprising collector panel integrated to hot water tank showing the double return pipes below the panel.

Figure 6A is a sectional side elevation of the solar water heating system for a flat roof with side casing removed.

Figure 7A is a transverse section through the complete unit.

Figure 8A is an edge detail of the transverse section through the collector panel.

Figure 9A is a cutaway plan of the solar heating system for a flat roof with the upper casing removed.

Figure 10A is a section through a flat roof showing the solar water heater fixed to the roof by means of a stand.

Figurd 1B is a view of the complete solar heating system for a sloped tile roof according to the present invention.

0Figure 2B is a view similar to Figure IB with the casing being partly cut away to show the 0...i1 collector panel and the interior of the hot water storage tank and the collector panel mounted on a stand.

20 Figure 3B is cutaway view with the casing removed of the core of the solar water heating system oo* for a sloped tile roof comprising the collector panel integrated to the hot water tank, with a single return pipe.

Figure 4B is similar to Figure 3B but with double return pipes.

Figure 5B is a view from below of the core of the solar heating system for a sloped tile roof 25 comprising collector panel integrated to hot water tank showing the double return pipes below the panel.

7 Figure 6B is a sectional side elevation of the solar water heating system for a sloped tile roof with side casing removed.

Figure 7B is a transverse section through the complete unit.

Figure 8B is an edge detail of the transverse section through the collector panel.

Figure 9B is a cutaway plan of the solar heating system for a sloped tile roof with the upper casing removed.

Figure 10B is a section through a sloped tile roof showing the solar water heater fixed to the roof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, the solar heating system as 20 shown for instance in Figure 1A 1B is designed to be installed as a single unit which will be inclined toward the sun to ensure every part is operating at a peak S: efficiency. The solar heating system comprises a core indicated generally at the core having a multiplicity of heat pipes or other channels which may be formed for instance from galvanised iron, stainless steel, copper or the like of 15 mm or other suitable diameter.

The heat pipes are preferably equidistantly spaced 30 apart as indicated in the drawings. In a highly preferred form the heat pipes or other channels are spaced at approximately 90 mm centres although this spacing can be varied to suit particular circumstances. The lower end of each heat pipe or channel is connected to a manifold (3) which for instance may be a 25 mm diameter galvanised iron or stainless steel or copper pipe.

\\melb-files\homes\Monique\Keep\speci\2488.95.doc 02/08/99 7a Each heat pipe is made of a heat absorbing and heat conducting material such as copper pipes coated with a matt black paint, and a multiplicity of heat pipes may alone form the collector panel core or each heat pipe is in the preferred embodiment embedded in good thermal contact with and in a heat absorbing/conducting medium which in a highly preferred form comprises a metal sheet preferably corrugated so formed so that its upper face, which in use will face the sun, is formed into a highly solar heat absorbent surface while the underside of the metal sheet is faced with a reflective surface.

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One preferred method of obtaining this is to paint the upper face of the metal sheet with a suitable matte black paint or selective absorber and to face the underside of the metal sheet with reflective silver foil. The metal sheet is preferably formed as is known in the art so that it will capture the maximum amount of solar radiation as the sun passes through the sky.

The core is suitably housed within a casing (10) which has a transparent cover (11) which may typically be of glass or the like. When the core is positioned within the casing (10) so that the cover (11) faces the sun, solar radiation which passes through the cover (I will be absorbed by the absorbent surface of the heat pipes and by the metal sheet which in turn will heat the water that will be contained within the heat pipes A suitable stand such as that indicated at may be provided to support the complete unit at the desired angle to the horizontal if the installation site is on a flat roof. If the installation site is on a sloping roof then the solar water heater is laid directly to the roof and fixed with metal brackets as is shown in Figure As can be seen particularly from Figure 2A 2B, each heat pipe extends into and opens into 15 the insulated hot water tank (15) at a level about one quarter (from the bottom) of the depth of the water in the hot water tank Consequently water heated within the heat pipes will rise without interruption into the tank and will avoid the hot water congestion "plug" at the top level of the tank by entering below the level of the hot water "plug".

Hot water entering into the hot water storage tank (15) is therefore not retarded in its circulation 20 because there are no horizontal flows or bends or superfluous piping lengths in the upward part of the thermosyphon circulation.

This will ensure that heat captured by the water in the heat pipes or other channels is directly transferred to the water in the hot water storage tank (15) with minimal heat loss.

Even though the heat pipes or other channels open into the hot water tank relatively close to the opening of the return pipes, that is at the lower quarter level of the water in the hot water tank, this slight vertical separation between the heat pipes and return pipes and also the continuous upright angle of the heat pipes when installed, will ensure that the incoming hot water will be prevented from being sucked back into the return flow pipes As can be seen from the drawings the entry to the return flow pipes (21) is adjacent to the bottom of the hot water storage tank (15) which will ensure that only the cooler water which is stratified at the bottom of the tank is sent back towards the collector panel to be reheated.

Although there is some minor conduction heat loss at night due to the hot water storage tank (15) being in close contact with the heat pipes this heat loss is much reduced due to the stratification effect of the water in the hot water tank (15) acting as insulation layers. Only the water at the lower part of the hot water tank (15) near the heat pipes would cool down slightly, but this is more than made up by the improved collection efficiency during the day. As the metal sheet itself which forms the major surface area exposed to the glass is not in thermal contact with the hot water tank, unlike a "fully wetted" panel or almost "fully wetted" panel such as Murumatsu's US Patent 4,724,826, the heat loss is still relatively negligible.

The placement of the panel below the hot water tank also prevents reverse circulation at night when thepanelis radiating.heat out into the night sky, as only the water inside the heat pipes is cooled and since this cool water cannot descend anymore, there is thus no reverse circulation.

•20 Thus at night, the hot water in the insulated tank above remains hot and does not flow into the panel as the cold water in, the lower part of the panel cannot be displaced, effcctivcly shutting down any circulation when the sun is not shining.

ft *o ae stecld wtritelwr ato h ae cn o edslcd efcieys utn o nayc rc lto h nt esu sntsii g For improved thermosyphon flow in the present invention, in order to minimise horizontal runs, and to avoid too many bends and drag inducing elbow joints, two return-pipes (21) (see Figure 5B) are preferably utilised which bring the cooler water that has settled at the bottom of the tank to the bottom of the panel, and are located directly beneath the heat pipes, instead of being laterally displaced at the side of the panel as was previously known. If only one return pipe (21) is utilised, it is located in the centre underneath the collector panel.

The return-pipes can be clearly seen in Figure No. 5A 5B, which shows the underside of the collector panel/tank combination. Such an arrangement will take maximum advantage of the difference in density between the heated water in the upward flowing heat pipes and the cooler water in the return downward flowing pipes (21) i.e. to have as vertical as possible an effective angle of thermosyphon flow. If the return pipes (21) are laterally displaced as was previously known, there is inevitably a large component of horizontal flow in the thermosyphon circulation which retards the circulation.

This vertical drop to the return-pipe (21) is preferred instead of a lateral displacement to the 15 return-pipe, the difference in flow rates between the two placements becomes even more marked as the day wears on. As the water becomes hotter and hotter towards the afternoon the temperature differential between the hot and cold pipes becomes less and less. If the return pipes (21) are laterally displaced, i.e. at the side of the panel, instead of directly below the heat pipes the flow would slow down considerably in the afternoon due to the horizontal flow and 20 reduced temperature differential.

Thermosyphon flow is not only inherently weak, but its natural movement is directly up or directly down, relying solely on the difference in densities between hot and not so hot water, so any impediment to its free flow upwards or downwards greatly reduces the overall efficiency of the solar heating system.

Another advantage of having the return-pipe(s) (21) under the collector panel is that the cooler return-pipe is shielded from the sun. The retum-pipe(s) (21) however, are well insulated against heat gain from the heat pipes by a layer of reflective silver foil on the metal sheet and by fibreglass insulation Suitable insulation such as fibreglass can also be employed to maintain as great a differential in temperature between the heat pipes and the return pipes (21) as possible. The placement of the return pipes (21) under the heat pipes will also allow the width of the panel to be reduced.

The return-pipes (21) are one or two or more in number, spaced evenly beneath the collector panel core to minimise any horizontal runs. In a system utilising two return pipes as described here and in part of the drawings, the horizontal runs are limited to one quarter the width of the collector panel core and occur only on the bottom header manifold as opposed to running the total width of the panel and on both top and bottom headers as in prior known systems.

The effective angle or ratio of vertical travel and horizontal travel (which has a direct correlation 15 on the speed of flow) of the thermosyphon circulation of the invention with two centrally and evenly spaced return pipes (21) directly under the collector panel, is thus considerably greater than that of prior art systems with one return pipe located at the side of the panel. This translates into considerably greater efficiency of thermosyphon flow over the prior art.

The efficiency would rise if more return pipes (21) were placed under the panel but that could run into diminishing returns as the weight of the collector becomes too unwieldy for transport, and manufacturing costs would increase. Even with only one centrally placed return pipe (21) under the collector panel core the horizontal travel component of the circulation system is halved, so that the circulation efficiency is greater than the prior art systems with a single returnpipe placed at the side.

l~i_ Cold water inlet from the supply tank (not shown) or the mains supply may be introduced through a ball float valve shown here on Model B fitted with a perforated baffle to prevent mixing of the cold water with the hot water.

If a balancing tank (not shown) is available above the position of the solar water heater the ball float valve may not be required, the cold water may then be introduced through the side end of the bottom manifold through a suitable supply pipe as shown in Figure 2A on Model A.

The access hatch would not be required in this case, to be replaced by a pressure relief vent or other vent (not shown). Hot water is drawn out (22) at the top of the tank in as shown in Figure 2A or through a floating outlet in as shown in Figure 2B which ensures that the outlet is always at the topmost (hottest) level of the water in the tank.

The collector panel and hot water tank (15) are manufactured as one single unit, simplifying transport and installation. There is no need for extra connections on site between the collector panel and the tank thus installation can be done by workers with simple plumbing skills. Due to the improved thermosyphon circulation, the collector panel can be reduced in size 15 and weight, reducing the cost of the unit and making installation feasible in small roof areas S* directly above the point of use. Because of the particular layout of the double return pipes (21) under the collector panel these return pipes (21) also act as structural stiffening reinforcement to the whole unit, improving the strength of the collector panel and hot water tank welded connection. The return-pipes directly underneath the heat pipes increase the structural 20 depth of the panel/tank unit. The resultant unit is very stiff and the welded joints are not prone to leakage.

On prior systems, this stiffening is not possible if the return-pipe is at the side of the collector panel, i.e. on the same plane as the heat pipes. The system disclosed in US Patent 4,353,352 (Zinn) of 23 October 1980 with its direct connection to the tank avoided the problem by having a flexible 25 collector panel comprised of flexible rubber tube strips as the heat pipes.

However, this system is not efficient due to the low coefficient of heat transfer of rubber tubing and interlocking rubber webbing used as heat collection fins.

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13 Also the poor durability of rubber tubing subject to high temperatures and extreme UV radiation would not make this system commercially viable.

By reason of the present invention a considerable increase in the efficiency of thermosyphon flow can be obtained. The system will allow hot water to flow upwards smoothly and directly without any inefficient twists or horizontal flow, and allows the return downward flow to be similarly direct and with minimal horizontal flow component.

Modification and improvements to the preferred forms of the invention as described herein will be apparent to those skilled in the art who come to understand the principles and precepts of the invention. All such modifications and improvements are intended to be part of the invention, the scope of which is defined in the appended claims.

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Claims (8)

1. A solar water heating system comprising a core consisting of a plurality of substantially parallel heat pipes or other channels adapted in use to be inclined to the horizontal, characterised in that the upper end of each of the heat pipes extends directly into and opens upwards into a hot water tank storage tank (15) at a level approximately one quarter the height (depth) of water in the tank measured from the bottom of the hot water tank and wherein the bottom end of each heat pipe or other channel is connected to a manifold (3) which extends transversely to the axis of the heat pipes and further characterised in that at least one cold water return pipe (21) or other channel (separate from the heat pipes) located centrally under the heat pipes extends directly from the lowest portion of the said hot watcr storage tank (15) intermediate of either end of the hot water storage tank (15) and is joined to the said manifold intermediate of either end of the manifold

2. The solar heating system as claimed in claim 1 wherein the said heat pipes or other channels are thermally joined to a heat conducting metal sheet which may be corrugated 15 vertically and wherein the said heat pipes or other channel and the upper face of the heat conducting metal sheet is provided with a heat absorbing surface and wherin the other lower face of the said metal sheet is provided with a heat reflective surface.

3. The solar heating system as claimed in claim 1 or claim 2 wherein the said return pipe (21) or other channel (separate from the heat pipes) located centrally under the heat pipes extends directly from the bottom of said hot water storage tank (15) intermediate of either end of the hot water tank (15) to said manifold in a direction substantially parallel to the axis of said a *heat pipes at a position approximately equidistant from the longitudinal edges of the said core 15

4. The solar heating system as claimed in claim 1 or claim 2 including two return pipes (21) or other channels separate from the heat pipes located underneath the heat pipes each having substantially parallel axes and being spaced apart a distance (centre to centre) substantially one half of the width of the said core (1) and being positioned substantially equidistantly from the longitudinal edges of the said core

5. The solar heating system as claimed in any one of the preceding claims wherein the said core is housed within a single casing (10) having a transparent cover (11) over the heat pipes and heat conducting metal sheet and heat insulation under the heat pipes and heat conducting metal sheet and also heat insulation (9) around the hot water tank

6. The solar heating system as claimed in any one of i the preceding claims wherein there are a plurality of 20 return pipes (21) or other channels (separate from the heat pipes) from the bottom of the hot water tank (15) directly to the bottom portion of the collector panel running under the collector panel core in a direction substantially parallel to the heat pipes and where the return pipes (21) are equidistantly spaced from each other centre -to centre and where the edgemost return pipes are located at a "distance from the longitudinal edges of the said core equal to half the equidistant centre to centre spacing of the return pipes (21). *t

7. The solar water heating system with rigid connections between the panel and tank as claimed in any one of the preceding claims wherein the placement of the return pipe(s) (21) underneath the collector panel core and the tank (15) structurally strengthens the fixed or welded connections between heat pipes tank (15) and return pipe(s) (21) enabling the solar water heating system \\melb-files\homes\Monique\Keep\peci\24885-9 .doc 02/08/99 15a to be transported and installed as one rigid integral complete unit.

8. A solar water heating system substantially as hereinbefore described with reference to and as illustrated by the accompanying drawings. Dated this 2nd day of August 1999 SIANG TEIK TEOH By their Patent Attorneys GRIFFITH HACK Fellows Institute of Patent and Trade Mark Attorneys of Australia \\melb-files\hone$\Monique\Keep\speci\24885-95.doc 02/08/99