GB2404241A

GB2404241A - Heat exchanger especially for solar water heating

Info

Publication number: GB2404241A
Application number: GB0317322A
Authority: GB
Inventors: Robin Matthew Hilder
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-07-24
Filing date: 2003-07-24
Publication date: 2005-01-26
Also published as: GB0317322D0

Abstract

A heat exchanger panel for solar water heating systems suitable for use with potable water that can be exposed to temperatures below 0{C without damage due to the expansion of the water as it freezes to form ice. The heat exchanger consists of a pair of parallel sheets (3a,3b) made of thermally conductive material suitably formed and joined to give a water flow gap. The gap may be in the form of channels (6c). The sheets deform their shape to accommodate the additional volume of the ice without being subjected to stresses beyond the yield strength of the material used. The sheets are preferably of stainless steel, but may also be of plastics, rubber, glass or ceramics. The sheets are relatively thin in the range 0.5mm to 10mm. The manifolds (1,4 Fig 1) may be of neoprene rubber.

Description

1 240424 1

Title of the Invention

Heat Exchanger Especially for Solar Water Heating

Field of the Invention

A heat exchanger panel for solar water heating systems suitable for use with potable water that can be exposed to temperatures below 0 C without damage due to the expansion of the water as it freezes to form ice. The heat exchanger consists of a pair of parallel surfaces made of thermally conductive material suitably formed and joined to give a water flow gap with an aspect ratio greater than unity such that the sheets may deform their shape to accommodate the additional volume of the ice without being subjected to stresses beyond the yield strength of the material used.

Background of the Invention

The use of solar radiation to heat domestic and industrial water is well known and documented and it is now used widely in parts of the world where temperatures below the freezing point of potable water are common in winter. Since water expands by approximately 10% on freezing to ice their design must take into account the potential for significant damage to the exposed parts of the system.

Although it is relatively simple to mount the storage tank in the building and thus protect it from frost the collector panels are almost invariably mounted outside, usually on the roof.

Conventional collector panels consist of tubing thermally connected to a conductive collector surface, usually a thin metal plate or fin, contained either in an insulated glazed box or indeed a transparent evacuated tube through which the heat exchange liquid, which is usually water, flows absorbing heat from the collector surface.

The problem with this design in respect of water freezing to ice is that tubes or pipes being circular in cross-section cannot accommodate the additional volume unless the walls of the tubing can stretch.

Unfortunately highly conductive metals, which have the best thermal properties, have only marginal elasticity and so tend to tear rather than stretch under the forces generated in this change in volume.

That leaves effectively just two strategies to deal with this problem, one of which is to drain the water from the panel at times when the temperature falls too low or otherwise treat the water with chemicals that depress the freezing point.

The main disadvantage of the former is the increased cost and complexity and also the inherent danger that failure of the draining system will lead to the destruction of the heat exchanger.

Use of anti-freeze additives on the other hand precludes the direct heating of potable water forcing the use of a secondary heat exchanger in the potable hot water storage tank. In addition their are numerous environmental and regulatory issues that make the use of water soluble materials to achieve this problematic.

A number of systems, for example GO 2280741, to address these shortcomings by the use of flexible tubing in place of the rigid metal pipes have been proposed; however, these are not as thermally efficient and the durability of these materials, particularly in the case where a failure of the heat transfer liquid pumping system can lead to intense temporary overheating, also makes them a less than ideal choice.

It should also be noted that the conventional tube and sheeVfin construction is particularly dependent on the thermal properties of the materials used. This is because length of the heat transfer path is relatively long and the geometry provides for a relatively small liquid/collector interface area.

The use of materials with relatively low thermal conductivity, particularly at this critical interface, can therefore lead to a significant thermal gradient being required to drive the heat from where it is absorbed through the structure and into the heat transfer fluid.

Since the losses through thermal radiation increase with the fourth power of the absolute temperature it is always advantageous to minimise this thermal gradient.

Object of the Invention A basic object of the present invention is the provision of an improved heat exchanger panel for use in solar water heating systems.

Summary of the Invention

A heat exchanger panel is provided comprising: i) A pair of sheets, or a single sheet suitably folded, providing a pair of largely parallel surfaces one or both of which are equipped with at least two largely parallel separating features extending along the desired direction of flow through the panel.

ii) A means of joining these surfaces via these separating features to leave one or more channels extending along the length of the surfaces closed around the perimeter about the direction of extension, but open at both ends. The cross-section of the channels around the axis of extension has an aspect ratio greater than unity with the separation between the plates forming the minor dimension.

iii) A pair of manifolds sealing about the open ends of the channels providing a flow path to one or more openings in each manifold. The manifolds are further characterized by their ability to accommodate the expansion of water contained within them without suffering mechanical damage.

Advantages of the Invention Since the aspect ratio of the channels is greater than unity, changes in shape resulting from bending of the channel walls can result in an increase in the volume of the channel without the need to increase the length of the channel perimeter about the axis of channel extension.

This means that unlike a tube or pipe with a circular cross-section, i.e. an aspect ratio of one, such increases in volume require the material to only be stable under bending and not under stretching as well.

Using this advantageous geometry it is therefore possible to select a wider range of materials of construction in particular metallic and polymeric ones that offer superior thermal, environmental, ageing and other properties, but lack the elasticity to allow a pronounced stretching deformation.

Another significant advantage of the invention is that the ratio of collector area to heat exchange surface in contact with the heat transfer fluid is greatly improved for any given volume of heat transfer fluid present in the heat exchange panel. There is also a much shorter heat transfer path from the collector surface where the heat enters the panel and the heat transfer fluid interface where it leaves it.

Both factors significantly reduce the temperature gradient required to move thermal energy from the collector surface to the heat transfer liquid resulting in lower surface temperatures for a given solar flux and water output temperature.

This lower surface temperature results in lower losses due to radiation which follow the fourth power of the absolute surface temperature.

Preferred or Optional Features of the Invention In its preferred form the heat exchanger panel is made from stainless steel sheet which is both cheaper and mechanically stronger than copper; but like copper it is suitable for direct contact with potable water.

In a pipe and Sheehan type construction stainless steel would be a poor choice since its thermal conductivity is only a 15th that of copper and so would lead to a large thermal gradient. However the reduction in the length of the heat transfer path and the increase in the heat transfer surface more than compensate for this.

For this reason it is also possible to use other materials including plastic, rubber, glass and ceramics which would normally be regarded as insulators in this application.

It is also not essential to use identical materials for both the top and bottom sheet providing at least one of the sheets is flexible enough to accommodate the expansion through a bending deformation and the other is capable of supporting the strains imposed by this deformation.

Where stainless steel sheet is used this would have a thickness in the range 0.1 mm to 3mm and preferably 0.2mm to 1 mm. The suitable and preferred thickness for other materials would be determined by their physical and commercial properties.

In order to provide an efficient transfer of heat and to limit the volume of water present in the panel at any one time, which will improve its response to variable sunshine, the separation of the sheets should be in the range 0.1mm to 20mm and preferably in the range 0.5mm to 10mm.

Since the use of relatively thin sheet material is envisaged the structure of the panel should include a regular series of separating features extending in the direction of flow that will limit the approach of the sheets and provide the joining surfaces for attaching one sheet to another.

In the preferred form these are made by forming one of the sheets, preferably the bottom sheet, to leave the top sheet largely flat optimising its heat absorption properties, although they could equally be provided as separate elements suitably attached to both sheets. An additional benefit of forming the lower sheet is to greatly improve its stiffness.

These separating features should be in the range 1mm to 1000mm and preferably between 10mm and 250mm.

The manifolds which connect the water inlet to the heat exchanger channels in the panel should preferably generate a back pressure in use that is significantly lower than the back pressure caused by the channels themselves. In this way they serve not only to make the necessary flow connection, but also help equalise the flow in each channel ensuring even heating of the water.

In doing so the desirable cross-section would be much closer to unity than the channels themselves and so in the preferred form they are made of material that is able to stretch to accommodate the expansion on freezing.

Suitable materials include silicone and neoprene rubber, but whatever material is selected it should, like the materials of construction of the heat exchanger panel itself, be appropriate for contact with potable water.

Brief Description of the Drawings

Figure 1 illustrates a preferred form of the heat exchanger. Inlet manifold 1 with inlet opening 2 is attached to heat exchanger panel 3 to which outlet manifold 4 with outlet opening 5 is likewise attached.

Figure 2 is the same panel, but with the manifolds 1 and 4 detached and placed adjacent to the heat exchanger panel 3 revealing the heat transfer fluid channels 6a, 6b, 6c and 6d.

Figure 3 shows one end of the heat exchanger panel 3 comprising a top sheet 3a and bottom sheet 3b joined to form four heat transfer fluid channels 6a, 6b, 6c and 6d.

Figure 4 shows one end of the heat exchanger panel 3 with a section of the top sheet 3a removed to allow a better view of the bottom sheet 3b which has been formed to provide a series of parallel ridges 13, 14 and 15.

Figure 5 is an end view of the heat exchanger panel 3 with water channel 6c formed by the joining of the bottom sheet 3b to the top sheet 3a via ridges 14 and 15 at points 7 and 8 respectively. The sections of top sheet 3a and bottom sheets 3b between the joints 7 and 8 are also identified as portions 11 and 12 respectively.

Figure 6 shows the same panel after the water within it has frozen and expanded deforming the channels to accommodate the additional volume.

Detailed Description of the Drawings

Referring to figure 1 water enters the heat exchanger panel through inlet opening 2 in the inlet manifold 1 and flows from there into the plurality of channels 6a, 6b, 6c and ad in the heat absorber section 3. After passing through this section it enters the discharge manifold 4 and discharges via the discharge opening 5.

Referring to figures 2, 3 and 4 the water flow channels 6a, fib, 6c and ad are created by the joining of two suitably folded sheets 3a and 3b of stainless steel or another suitable material which are joined by joining means such as welding or adhesives.

The strength of the assembly is such that under the strain imposed by water freezing in the flow channels the two sheets do not separate.

Referring to figure 5 taking channel 6c as an example its cross-section is defined by parts of the top sheet 3a and bottom sheet 3b, where the top sheet 3a is flat so as to present the best possible absorber surface and bottom sheet 3b has been formed to equip it with a series of ridges 13, 14 and 15 extending along its length.

In the case of channel 6c the top and bottom surfaces are defined by the sheet sections 11 and 12 between ridges 14 and 15 respectively, whilst the sides of the channel are provided by the ridges 14 and 15 themselves. To complete the structure of the channel the sheets are joined by the joining means at the top of the ridges 13 and 14atpoints7and8.

Attachment of the manifolds 2 and 5 to the heat absorber 3 is also by joining means such as adhesive or other such so that the manifolds likewise do not detach from the heat absorber under the strains imposed by water freezing in the heat absorber and the manifolds themselves.

Referring to figure 6 again looking at channel 6c as an example should the water in the heat absorber panel freeze then the cross-section is so that the expansion of the water in turning to ice is accommodated by a bending deformation of the sections of the top and bottom sheet 11 and 12. - 11

Claims

Claims 1. A heat exchange panel containing one or more water flow channels

such that the aspect ratio of their height to their width is sufficiently large that the change in volume required to accommodate the freezing of water within the channels can be provided by a bending rather than stretching deformation of the channel walls.
2. A heat exchange panel as described in Claim 1 equipped with a pair of manifolds sealing about the open ends of the channels providing a flow path to one or more openings in each manifold.
3. A heat exchange panel as described in Claim 2 where the manifolds are further characterized by their ability to accommodate the expansion of the water contained within them on freezing to ice without suffering mechanical damage.
4. A heat exchange panel as described in Claim 2 comprising a pair of sheets, or a single sheet suitably folded, providing a pair of largely parallel surfaces one or both of which are equipped with at least two largely parallel separating features extending along the desired direction of flow through the panel and a means of joining these surfaces via these separating features to form the water flow channels such that the strength of the joints is greater than the forces imposed by bending of the sheets to accommodate the expansion of water on forming ice.
5. A heat exchange panel as described in any of the preceding claims which is made of materials approved for direct contact with potable water.
6. A heat exchange panel as described in any of the preceding claims wherein the heat absorbing face is made of highly conductive material.
7. A heat exchange panel as described in any of the preceding claims wherein the heat absorbing face is made of a material with a thermal conductivity of less than W m' K-'.
8. A heat exchange panel as described in any of the preceding claims wherein the bottom face is made of a material with a thermal conductivity of less than 5 W m' K-.
9. A heat exchange panel as described in any of the preceding claims wherein the heat absorbing surface is treated or coated so that its surface has an absorption coefficient of more than 0.8 for electromagnetic radiation with a wavelength less than 0.3 to 2 m.
10.A heat exchange panel as described in any of the preceding claims wherein the heat absorbing surface is treated or coated so that its surface has an emissivity less than 0.6 for electromagnetic radiation with a wavelength greater than 5 m.
11.A heat exchange panel as described in any of the preceding claims comprising a pair of stainless steel sheets the top one of which is largely flat and the other is formed to provide at least two largely parallel separating features extending along the desired direction of flow through the panel, with the sheets being suitably joined by means of TIG welding, MIG welding, laser welding, spot welding, seam welding, adhesive or any combination thereof.
12.A heat exchange panel as described in any of the preceding claims equipped with a pair of manifolds made of silicone rubber.