GB2494380A

GB2494380A - Freeze tolerant solar collector with frame and clamping system

Info

Publication number: GB2494380A
Application number: GB1114337.7A
Authority: GB
Inventors: Andrew John Fell; Sophie Charlotte Slade
Original assignee: Individual
Current assignee: Individual
Priority date: 2011-08-19
Filing date: 2011-08-19
Publication date: 2013-03-13
Also published as: GB201114337D0

Abstract

A solar water heating panel 1 comprises an absorber plate 2, a companion plate 3, a gasket 4, a frame 5 and a means for securing the frame to the perimeter of the companion plate. The companion plate is located below the absorber plate, with the gasket sandwiched between the absorber plate and the companion plate. The frame surrounds the absorber plate, the companion plate and the gasket, and may comprise a flange portion 12. The securing means is provided in order that the absorber plate and the gasket are clamped between the frame and the companion plate to form a watertight seal around their edges. Preferably, the securing means are fasteners 13, such as screws, bolts or rivets. In use, fluid can pass directly underneath the absorber plate and absorb heat from solar radiation hitting the absorber plate, and the absorber plate can bend to accommodate the expansion of fluid during freezing. Preferably, the absorber plate is able to bend within its elastic limit without any mechanical damage occurring.

Description

FREEZE-TOLERANT SOLAR COLLECTOR WITH INTEGRATED FRAME AND

CLAMPING SYSTEM

FIELD OF THE INVENTION

The invention relates to the field of solar energy collectors and more specifically to flat plate collectors for use in heating water.

BACKGROUND OF THE INVENTION

Solar energy has been used to heat domestic water for many years and solar thermal collectors are a well known and documented technology.

Solar thermal technologies currently on the market consist of two main types -the evacuated tube collector and the flat plate collector. Both types comprise of piping through which a heat receiving fluid is circulated. In an evacuated tube collector the piping is contained within a transparent, evacuated tube to reduce convective heat losses. In a flat plate collector, the piping is connected to a heat absorbing plate to increase heat absorption and this is usually housed within a frame, with a heat insulating cover and base. There are a number of drawbacks to these conventional systems, which are discussed below.

In conventional systems the fluid-carrying piping is often produced from a thermally conductive material such as copper or aluminium. When water freezes within these pipes, there s not enough elasticity in the material to accommodate expansion of the water, resulting in permanent damage. A number of solutions have been developed to deal with this problem, each with their own drawbacks. Antifreeze solution is often used in place of water. However, this requires the instaUation of an expensive dual coil cylinder, as well as regular replacement of the antifreeze solution throughout the lifetime of the system. Other designs use a drain back system', where the piping is drained when temperatures fall below a certain level. However, these systems often use extra components, increasing costs and complexity at manufacture and installation. The final option is to use flexible piping, which has enough elasticity to withstand expansion during freezing. However, the materials currently available for flexible piping have low thermal conductivity, decreasing the efficiency of the solar collector. Gonventional evacuated tube collectors are expensive to manufacture and are prone to vacuum loss, which significantly reduces performance.

In a conventional flat plate collector, heat is absorbed by a metal plate and is transferred via thermal conduction to the fluid carrying tubes that are welded to its underside. This is inefficient in a number of ways. Firstly, there is often poor thermal contact between the tubes and heat absorbing plate, reducing heat transfer. Secondly, the heat must follow a long heat transfer path to reach the fluid within the tubes. Since this longer path has a greater thermal resistance, the fluid does not receive as much of the heat energy. Heat that is not absorbed by the fluid is transferred back into the internal space, causing a rise in box temperature and subsequently a rise in the temperature differential between the box and the ambient air temperature. This process drives convective and radiative losses, reducing the useful gain of the collector.

Due to their design, conventional flat plate collectors can obtain stagnation temperatures of up to 30000. For this reason metals are most commonly used to produce the outer frame. However, since metals have a high thermal conductivity, heat is rapidly lost through the sides of the collector. To alleviate this, significant amounts of insulation and thermal breaks are often required. A further disadvantage associated with the metal frame is its considerable weight, making the collector cumbersome and difficult to install.

A number of inventors have attempted to overcome these problems and this is evident in the patent literature. A number of designs attempt to improve the efficiency of solar collectors by allowing direct contact between the absorber plate and the heat absorbing fluid below. US200810277096 and US20091025709 claim improved efficiency by shortening the flow path from solar radiation to the heat absorbing fluid below, However, in both these designs, the absorber plate is welded to a lower plate along its seanis, and the flow paths between the two plates are unable to accommodate expansion caused by freezing. Therefore, the systems would not be suitable for use with water unless used in conjunction with a drain back' system.

The system described in G82404241 A uses two parallel sheets, with one or both of them formed to produce parallel water channels. The two sheets are joined together around the edges and along the channel seams. The height to width aspect ratio of the channels is such that the sheets are able to bend to accommodate the expansion of water during freezing. The inventor, Hilder, recommends the use of stainless steel plates, joined by welding, but also suggests that non-metallic sheets joined by adhesives can be used.

Where stainless steel is used, heat losses will occur through the base plate due to its relatively high thermal conductivity. Where non-metallic plates are used, the adhesives used to bond the plates along the seams must be capable of joining two dissimilar materials and withstanding repeated pressure and thermal cycling. Adhesives such as these are, at present, costly and have a limited useful lifetime under the aforementioned conditions.

The system described in W02010077815 comprises two parallel aluminium sheets between which heat absorbing fluid is passed. The sheets are spaced by an elastic gasket and a watertight seal is formed around the edge by an extruded clip or a series of machine screws and nuts. A serpentine flow path is formed by elastic baffles between the plates, held in place by bolts and nuts. The absorber is in direct contact with the heat absorbing fluid and is able to bend to accommodate expansion of the fluid during freezing.

However, there are a number of drawbacks to this system. The inventor, Kunczynski, specifies the use of aluminium for both the absorber and base plate. As aluminium has high thermal conductivity, heat would be rapidly lost through the base plate, decreasing efficiency. The serpentine flow path uses more energy to pump the water through the system, reducing the system efficiency. The design relies on baffles to provide spacing and water channels between the two sheets, increasing the number of components and associated assembly time. The bolts and nuts used to secure the baffles in place penetrate both aluminium plates, increasing the likelihood of leakage.

BRIEF SUMMARY OF THE INVENTION

A solar water heating panel, comprising an absorber plate overlaying a companion plate, with a gasket positioned between the two. The system is surrounded by a frame and the frame is secured to the companion sheet around its perimeter. The absorber plate and gasket are clamped between the frame and companion plate to form a watertight seal.

The absorber sheet can be formed to produce fluid carrying channels and heat absorbing fluid is able to flow through these channels in direct contact wah the absorber plate. The absorber plate can bend within its elastic limit to accommodate expansion ot water caused by freezing.

ADVANTAGES OF THE INVENTION

Accordingly, several advantages of one or more aspects are as follows: 1. Direct contact between the absorber plate and heat absorbing fluid reduces the heat transfer path, meaning more heat is able to be quickly absorbed by the fluid and less is transferred back into the space above. This improves overall collector gain by reducing heat losses and increasing the rate of heat transfer into the fluid.

2. The absorber plate is able to bend within its elastic limit to accommodate the 9% volumetric expansion which occurs during the freezing of water. This allows potable watet to be used in the system in place of anti-freeze solution, eliminating the need for an expensive dual coil cylinder.

3. The absorber plate is sealed to its companion plate by a peripheral clamping system and is not physically welded to any component. The plate has some flexibility within the clamp, allowing it to accommodate expansion of the water without putting significant strain on any components. This eliminates the potential for premature failure ol joints from expansion of the water.

4. The unique clamping system removes the need for welding, which means the lower companion plate need not be produced from metal. Therefore it can be a thermally insulating material, reducing the heat lost through the base via conduction. This increases the efficiency of the collector.

5. The number of components in the system is minimal for two reasons; the frame is used for both reducing heat losses and to form the clamping system between the absorber plate and gasket; and the fluid carrying channels are created by shaping the absorber plate, eliminating the need for additional baffles or separators. This decreases manufacturing costs.

8. There are no fasteners penetrating the absorber plate, ruinimising risk of leakage.

This creates an inherently robust design.

7. Parallel channelling running up the panel allows the water to be driven by natural convection. This improves the system's efficiency as less energy is drawn by the pump.

8. A higher volume of water is pumped through the system than in conventional tube and fin designs. This continuously removes heat energy from the internal box. By varying the flow of water through the panel it can be ensured that the internal box temperature can be kept below a set limit. There are two advantages of this.

Firstly, there is a broader range of materials from which the collector frame can be produced. Secondly, the collector efficiency is greatly improved by driving heat into the water and reducing thermal losses.

Other advantages of one or more aspects will become apparent from consideration of the

drawings and ensuing description.

DRAWING FIGURES

FIG. 1 is a perspective view o the assembled solar collector.

FIG. 2 is an exploded perspective view of the solar collector.

FIG. 3 is a cross sectional view of a portion of the solar collector indicated by the section lines 3-3 iii Fig 1.

FIG. 4 is a close up cross-sectional view of the watertight clamping system between the absorber and companion plate.

FIG. S is a perspective view of the absorber plate to show the channel formation.

FIG. 6 is a cross-sectional view of a portion of the solar collector indicated by the section lines 6-6, showing how the cover is inserted into the top of the solar collector.

DRAWINGS -REFERENCE NUMERALS

1. Solar Collector 13. Fasteners 2. Absorber Plate 14. Ridge 3. Companion Plate 15. Protective Strip 4. Gasket 16. Fluid Carrying Channels 5. Frame 17. Lower Manifold 6. Clamping System 18. Upper Manifold 7. Inlet Poit 19. Peripheral Strip 8. Outlet Port 20. Lip 9. Cover 21. Frame corners 10. Base Plate 22. Sealing Strip 11. Brackets 23. Baffles 12. Flange

DETAILED DESCRIPTION

Fics1,2and3 One embodiment of the solar collector 1 as shown in Figs 1, 2 and 3, comprises an absorber plate 2 overlaying a companion plate 3, with a gasket 4 sandwiched between the two. A frame 5 houses the three ayers, preventing heat losses from the sides, while also forming an integral part of a clamping system 6, which forms a watertight compression seal between the edges of the absorber plate 2 and gasket 4. Heat absorbing fluid is introduced from a fluid supply to the watertight space beneath the absorber plate 2 via an inlet port 7. The heat absorbing fluid passes underneath the absorber plate 2 to an outlet port 8 positioned at the opposite end of the collector 1, during which time it is able to absorb heat from solar radiation hitting the absorber plate 2. A cover S can optionally be positioned above the absorber plate 2, secured in place by the frame 5 to provide insulation, A base plate 10 can optionafly be positioned beneath the companion plate 3, secured to the frame 5 to provide structural support and insulation.

The frame S is produced from a single length of extruded material, which is cut into four lengths, mitred and joined together by brackets 11. Any suitable joining technique, such as bolts, screws or rivets, will connect the bracket to the sides of the frame 5. It is currently envisaged that the frame 5 will be produced from UV stabilised polycarbonate, extruded at a uniform thickness of between 2mm and 5mm. This material is UV stable and has a glass transition temperature of 150°C, which is much higher than the normal operating temperature of the collector. It will therefore not deteriorate nor deform during operation. It also has low thermal conductivity, reducing heat losses from within the collector 1. Alternative materials can include aluminium, steel and other high temperature plastics at any suitable thickness.

Figs 3 and 4 The clamping system 6 is formed by a flange 12 protruding from the inside of the frame 5, which can best be seen in Figs 3 and 4. The flange 12 sits on top of the absorber plate 2, overhanging its perimeter. The overhang of the flange 12 is secured to the companion plate 3 perimeter by a series of fasteners 13. Rivets are a good choice of fastener as they are quick to install during manufacture, but screws or bolts can also be used. The fasteners 13 pull the flange 12 tight to the companion plate 3 and this clamps together the absorber plate 2 and gasket 4 around the edges, forming a watertight compression seal between the two. A small ridge 14 can be pressed into the perimeter of the absorber plate 2, which pushes into the gasket 4, improving the compression seal. Tile fasteners 13 should be spaced at a suitable distance to ensure that there is no leakage. A protective strip 15 can be positioned on the underside of the frame flange 12. This serves two putposes; to provide insulation to the frame from the heat of the absorber plate 2 and to increase flexibility within the clamping system 6 during freezing. This should be produced from a material with high temperature resistance and a high strain to failure, such as silicone rubber. The absorber plate 2 is able to bend within its elastic limit to accommodate expansion of the fluid during freezing and return to its original shape alter thawing.

It is currently envisaged that the absorber plate 2 will be produced from aluminium with a thickness of between 0.5mm and 2mm. Aluminium a has high thermal conductivity, is fairly low cost and is flexible enough to bend within its elastic limit in order to accommodate expansion of the heat absorbing fluid within the system. Alternative materials include steel, copper or a plastic with suitably high glass transition temperature.

The underside of the absorber plate 2 can optionally be covered with a food grade non-stick coating, for example PTFE. The non-stick' coating will help to reduce the build-up of limescale deposits, maintaining the efficiency of the collector over its lifespan. The upper surface of the absorber plate 2 can optionally be covered with a heat absorbing coating to increase the solar radiation absorbed.

It is currently envisaged that the gasket 4 will be a sheet of peroxide cured silicone rubber, covering companion plate 3. The material can be between 30 and 70 Shore A hardness and with a thickness between 0.5mm and 2mm. This material has a very high temperature resistance and is food grade during prolonged contact with hot water. In combination with the non-stick' coating on the absorber plate 2, food-grade channels 16 are created, through which the heat absorbing fluid can flow. This allows potable water to be used in the collector I. The peroxide cured silicone rubber also has sufficient elasticity to form a compression seal and has very low thermal conductivity, minimising heat losses through the companion plate 3. In cases where the use of potable water is not required, the gasket 4 can consist of a strip of material positioned around the edges of the companion plate 3, in which case the fluid can flow directly over the companion plateS.

It is currently envisaged that the companion plate 3 and base plate 10 will be produced from sheets of Acrylonitrile Butadiene Styrene (ABS) at a thickness of between 3mm and 10mm. This material is cheap to produce, has adequate mechanical strength and low thermal conductivity, reducing convective heat losses. Many alternative materials can be used to produce the companion plate 3 and base plate 10, including most plastics and metals. The base plate 10 can be attached to the lower edge of the frame 5 by any suitable method, such as solvent welding or the use of fasteners. Fig 5

The absorber plate 2 will be formed to produce fluid carrying channels 16 shown in Fig 5.

If produced from metal, metal pressing is a suitable process for forming the absorber plate 2. When the collector 1 is positioned at an angle to the vertical, for example when on a pitched roof, the fluid carrying channels 16 will be orientated as outlined in the following description. The fluid arrives through the inlet port 7 into the lower manifold 17 wrining horizontally along the lower end of the absorber plate 2. The fluid carrying channels 16 run vertically up the absorber plate 2 and guide the flow of fluid from the lower manifold I? to the upper manifold 18, assisted by thermo-siphoning. Guiding the fluid in this way encourages uniform heating and prevents regions from stagnating. The fluid runs horizontally along the upper manifold iSto the outlet port 8. The channels 16 pressed into the absorber plate 2 also increase the surface area. The geometry of the channels 16 will be such that the depth is large enough to mitigate any limescale build-up, but shallow enough to reduce shading on the top surface of the absorber plate 2. It is noted that the geometry of the channels 16 also affects the flow rate through the collector I for a given temperature rise.

There will be a strip 19 around the periphery of the absorber plate 2 that remains flat, clamped between the frame flange 12 and companion plate 3. The strip 19 will extend a short distance from the clamping system 6 into the collector I and is designed to alleviate bending stresses.

Figs 3 and 6 As seen in Fig 3 and 6, the cover 9 in this embodiment is held in place by the frame 5.

The cover 9 will be nserted nto the top of the frame 5 after the damping system 6 has been assembled, but before the frame S corners have been secured by brackets 11. The frame 5 has a lip 20 running around the top edge. Once the cover 9 has been inserted, the lip 20 holds it in place. To assemble, the lip 20 is pulled back simultaneously along all four sides to create an opening wide enough to insert the cover 9, then released. In order to prevent the yield stress of the frame 5 being exceeded, stress concentrations are reduced by the use of rounded corners 21 in the frame cross section. A sealing strip 22 will be included between the lip 20 and cover 9 to ensure the collector I is waterproof.

Alternative methods of securing the cover 9 to the frame 5 include using fasteners such as bolts or screws, or solvent welding.

It is currently envisaged that the cover 9 will be produced from a sheet of UV stable twin wall polycarbonate. This material has good optical properties such as high transparency and low emissivity. It is also lightweight, cheap and has a glass transftion temperature of 150°C, which is above the maximum internal temperature of the collector 1 during normal operation. The baffles 23 between the twin walls of the envisaged polycarbonate cover 9 reduce convective heat losses. The cover 9 can also be produced from alternative materials such as glass.

CONCLUSIONS, RAMIFICATIONS AND SCOPE

From the above, it can be seen that at least one embodiment of the solar collector 1 provides a design that is freeze tolerant, able to be produced from cheaper, lighter-weight materials and with greater efficiency than existing designs.

Although the embodiments have been described with relation to a solar collector for heating domestic water, it should be appredated that the solar collector 1 has a number of alternative uses, such as for heating water for commercial use, swimming pools, horticulture or for space heating.

The solar collector 1 can be produced to cover any area depending on the requirements for heated water. ii

Claims

<claim-text>CLAIMS1. A solar water heating panel, described in Figs ito 6, comprising: an absorber plate; a companion plate disposed below the absorber plate; a gasket sandwiched between the absorber plate and the companion plate; a frame surrounding the absorber plate, the companion plate and the gasket; and a means for securing the frame to the perimeter of the companion plate in order that the absorber plate and the gasket are clamped between them to form a watertight seal around their edges, whereby fluid can pass directly underneath the absorber plate and absorb heat from radiation hitting the absorber plate, and the absorber plate can bend to accommodate expansion of fluid during freezing.</claim-text> <claim-text>2. A solar water heating panel as described in Claim 1, wherein the frame is produced from a length of extruded material cut into four shorter lengths, mitred and joined together by brackets.</claim-text> <claim-text>3. A solar water heating panel as described in Claim 2, wherein the extruded material is UV stabilised polycarbonate.</claim-text> <claim-text>4. A solar water heating panel as described in Claim 1, wherein the frame is formed to include a flange protruding from its inner face, which is secured to the perimeter of the companion plate.</claim-text> <claim-text>5. A solar water heating panel as described in Claim 1, wherein the frame is secured to the companion plate by a series of fasteners.</claim-text> <claim-text>6. A solar water heating panel as described in Claim 5, wherein the fasteners are rivets.</claim-text> <claim-text>7. A solar water heating panel as described in Claim I, wherein the absorber plate is formed to include a ridge around its perimeter, which will press into the gasket, thereby improving the watertight seal.</claim-text> <claim-text>8. A solar water heating panel as described in Claim 1, further including a strip of material with high strain to failure sandwiched between the frame and the absorber plate, thereby giving the absorber plate some flexibility to bend within its constraints to accommodate expansion of fluid during freezing.</claim-text> <claim-text>9. A solar water heating panel as described in Claim 1, wherein the underside of the absorber plate is coated with a layer of food-grade, non-stick material, thereby allowing safe contact with potable water, 10. A solar water heating panel as described in Claim 1, wherein the companion plate is formed from a material with low thermal conductivity, thereby reducing heat losses from the fluid above.11. A solar water heating panel as described in Claim 1 wherein the gasket is formed from a material with suitably high strain to failure, thereby improving the watertight seal.12. A solar water heating panel as described in Claim 1, wherein the gasket is a layer of food-grade material covering the companion plate, thereby allowing sate contactwith potable water.13. A solar water heating panel as described in Claim 12, wherein the layer of food grade material is a sheet of peroxide cured silicone rubber.14. A solar water heating panel as described in Claim 1, wherein the gasket is a strip of material disposed around the perimeter of the companion plate.15. A solar water heating panel as described in Claim 1, wherein the absorber plate is formed to produce parallel fluid-carrying channels, connected at both ends by a manifold channel.16. A solar water heating panel as described in Claim 15, wherein the channels are formed in the absorber plate at a predetermined distance inside of where the absorber plate is clamped between the frame and the companion plate, thereby afleviating bending stresses to the absorber plate during expansion of the fluid below.17. A solar water heating panel as described in Claim 1, further including a cover disposed in spaced relation above the absorber plate to provide insulation.18. A solar water heating panel as described in Claim 17, wherein the cover is held in place by a lip around the frame, 19, A solar water heating panel as described in Claim 17, wherein the cover is inserted alter the frame has been secured to the companion plate, by simultaneously bending all four edges of the frame to allow the cover to be inserted, before releasing the frame to aflow the lip to fit over the cover, holding it in place.</claim-text>