GB2196104A - Solar heater - Google Patents

Abstract

A solar heater has a tubular copper pipe deformed along one longitudinal line to form two concentric semi-tubular walls (12, 14) spaced-apart by a distance of 4mm. The radially inner wall (12) is concave in cross-section and this provides a greater surface area for transferring energy from the solar radiation to the fluid within the pipe. Since the walls (12, 14) are closely spaced, the pipe contains only a small amount of water which therefore may be heated to a higher temperature than in existing heaters. The pipe is embedded in an elongate block (16) of insulating foam, and the foam and the pipe are both enclosed in a plastics film (34) to increase the absorption of energy from the solar radiation. <IMAGE>

Description

SPECIFICATION Solar heater The present invention relates to a solar heater, and in particular, but not exclusively, to a solar heater for domestic use.

Conventional solar heaters operate either by trickling water down a flat surface which is exposed to sunlight, or by passing water through pipes surrounded by elongate concave mirrors which focus the radiation onto the pipes. The solar heaters are usually situated on the roof of a building, inclined skywards. In either case, the radiation absorbers (the flat surface or the pipes) are covered with a transparent sheet, e.g. glass, to reduce heat loss through conduction and convection ard to reflect back some of the radiation which is reemitted from the radiation absorbers.

However, conventional solar heaters can only heat water up to a temperature of about 65"C, which means that additional forms of heating the water must be employed to bring the water to a temperature which is adequate for a hot water supply. Conventional solar heaters also suffer from the disadvantage that they require a pump to pump the water through the solar heater, which means that an additional external source of energy is necessary. Moreover, conventional solar heaters are necessarily bulky, due to the relatively large surface which is required.

It is an object of the present invention to provide a solar heater which overcomes or reduces the above-mentioned disadvantages.

In accordance with the present invention, a solar heater comprises an elongate conduit for carrying fluid to be heated, the side of the conduit intended to receive incident radiation being at least partially concave in transverse cross-section.

A concave portion of the conduit provides a greater surface area for transferring energy from the solar radiation, to the fluid within the pipe, and results in more efficient absorbtion of radiation.

Preferably, the transverse cross-section of the concave portion is constant for most of its length.

In one emodiment, the conduit comprises two substantially concentric, part tubular, spaced-apart walls, the concave outer surface of one of the walls being the collecting surface for incident radiation.

This emodiment is in the form of alongate trough as the collecting surface, with a containment wall slightly spaced-apart from the collecting wall. This may be achieved by deforming an elongate, cylinderical tube, for example a metal or plastics tube at least partially along one side. The outwardly concave surface is preferably blackened to increase the absorption of the collecting surface.

In a preferred embodiment, the pipe is em bedded in an insulating material, for example an insulating plastics foam, and the pipe and insulating material may be enveloped in a plastics film, which encloses the concave portion of the pipe. The film is used to create an enclosed air space, which heats up quickly due to the so-called "greenhouse effect".

By way of example only, a specific embodiment of the present invention will now be described, with reference to the accompanying drawings, in which: Figure 1 is a side elevation of an embodiment of solar heater in accordance with the present invention, in position on the roof of a house; and Figure 2 is a cross-section of the water heater of Fig. 1., in the direction of arrows Il-Il in Fig. 1.

The solar heater comprises a tubular copper pipe 10 of 10.8 cm diameter, which has been deformed for most of its length along one longitudinal line to form two approximately concentric, semi-tubular walls 12, 14, which are spaced apart by a distance of approximately 4 millimetres. The radially inner wall 12, which is concave when viewed externally, is provided with a matt black coating, for better absorption of radiation.

The pipe 10 is embedded in an elongate block 16 of closed cell polyurethane foam, which is shaped to receive the pipe 10, and the radially outer, convex, wall 14 is secured thereto. Both ends of the pipe 10 are left undeformed and are thus tubular, and are closed off by upper and lower circular end walls 18, 20. A hook 22 is attached to the upper end of the insulated pipe and is adapted to be hooked over the ridge tile 24 of a roof 26. The closed cell polyurethane is quite flexible, and assumes the contours of the tiles on the roof, which ensures that the solar water heater is firmly seated on the roof. A pipe 28 leads from the base of the upper end wall 18, and passes through the insulation 16 to the bottom of the insulation where it emerges and is directed underneath the roof 26.A second pipe 30 leads to the base of the lower enclosing portion 20, and supplies cool water to the solar water heater from a hot water tank, as will be explained. Also provided in the upper portion 18 is a manually adjustable air vent 32.

The pipe 10 and the insulation 16 are both enclosed in a plastics film 34, which is applied by enveloping the water heater in a tubular plastics film and shrink-wrapping the plastics film onto the water heater. This defines an enclosed air gap 36 between the blackened outer surface of the radially innermost wall 12 and the upper surface of the plastics film 34.

The air space 36 is substantially hermetically sealed, since the plastics film also extends over the undeformed tubular end portions of the pipe 10.

In use, the solar water heater is positioned on a roof of a house, as illustrated in Fig. 2.

The solar water heater is filled with fluid, in this case water, which runs through a closed system. The pipes 28 and 30 lead to and from a hot water tank respectively, but the hot water obtained from the solar water heater is not used directly, but is used to heat cooler water already in the hot water tank by passing the pipes 28, 30 through the hot water tank. It will be noted that an air gap 38 is left at the upper end of the pipe 10, in order to allow for expansion and contraction.

The air gap 38 is adjustable by means of the previously mentioned air vent 32.

When radiation is incident on the solar water heater, most of the radiation passes through the upper surface of the plastics film 34, and is incident on the blackened surface of the radially innermost wall 12. The wall is thus heated, and transfers its heat to- the water which is in the gap defined by the radially inner and outer walls 14, 12. Some radiation is re-emitted from the blackened surface, and this is partially reflected by the plastics film 34, and, because it has been re-emitted from the wall 12, is of a wavelength which is reflected more efficiently by the plastics film 34. Thus, a large proportion of the radiation which is re-radiated from the blackened surface is reflected from the film 34 back to the blackened surface. Furthermore, air 36 heats up and transfers its heat to the pipe 10 and the water within the pipe, similar to the action of a greenhouse.The insulation 16 is provided to reduce the heat losses from the heater.

-The concave nature of the matt black sur face allows sunlight to be incident at a small angle of incidence for a much longer period of time. Obviously, the longer the incident radiation is incident at a small angle of incidence, the more efficient will be the absorbtion of energy from the radiatìon.Thus, as the sun moves across the sky, the solar water heaterwill still be able to absorb energy efficiently.

Since only a relatively small volume of water is being heated (the cross-sectional area of the pipe is approximately 5 cm2), as compared with a cylindrical pipe of the same diameter (the cross-sectional area would be approximately 365 cm2) and since the surface area of the radiation collecting portion is approximately the same as a cylindrical pipe of the same diameter, the water heats up much more quickly and attains a much higher temperature than in known solar heaters. A temperature of up to 95"C may be expected if the weather conditions are favourable.

As the water heats up, it rises to the top of the pipe 10 by convection. It is then forced through the pipe 28, at the top of the heater and descends due to gravity through the pipe 28, which passes down through the insulator, to the base of the solar water heater. The pipe 28 is then fed to the hot water tank which heats up water in the water tank, in a conventional manner and as previously described.

The pipe 30 is the return pipe from the hot water tank, and returns relatively cool water to the base of the solar water heater, the water then being heated and repeating the above cycle.

A number of solar water heaters facing in different directions may be used, to allow water to be heated as the sun moves during the day The invention is not restricted to the details of the foregoing embodiment. For example, since the fluid passing through the solar water heater is only used to heat water in a hot water tank, the fluid -in the water heater need not be water but may be a different fluid, for example a gas or a different liquid. In particular, the fluid may be one which has a lower specific heat capacity than water, so that it is heated to a higher temperature than water would be, and/or may be of a liquid which has a higher boiling point than that of water, so that the liquid may be heated to a temperature in excess of the boiling point of water.

Claims (16)

1. A solar heater comprising a elongate conduit for carrying fluid to be heated, the side of the conduit intended to receive instant radiation being at least partially concave in transverse cross-section.

2. A solar heater as claimed in claim 1, wherein the transverse cross-section of the concave portion is constant for most of its length.

3. A solar heater as claimed in claim 1 or claim 2, wherein the conduit comprises two substantially concentric, spaced-apart walls, the concave outer surface of one of the walls being intended to receive incident radiation.

4. A solar heater as claimed in claim 3, wherein the conduit comprises an elongate, cylindrical tube which has been deformed at least partially along one side to produce the two substantially concentric walls.

5. A solar heater as claimed in any of claims 1 to 4, wherein the concave portion of the solar heater is treated to increase the absorption of solar radiation.

6. A solar heater as claimed in claim 5, wherein the concave portion is blackened.

7. A solar heater as claimed in any of claims 1 to 6, further comprising a plastics film which encloses the side of the conduit intended to receive incident radiation.

8. A solar heater as claimed in any of claims 1 to 7, wherein the conduit is embedded in an insulating material.

9. A solar heater as claimed in claim 8, wherein the insulating material comprises a plastics foam.

10. A solar heater as claimed in claim 9, wherein the plastics foam is a closed cell polyurethane foam.

11. A solar heater as claimed in any of the preceding claims, wherein the cross-sectional of the conduit is less than or equal to 5cm2.

12. A solar heater as claimed in claim 3 or any of claims 4 to 11 when appendent to claim 3, wherein the walls are spaced-apart by a distance less than or equal to 0.4cm.

13. A solar heater as claimed in any of the preceding claims, wherein the conduit comprises plastics.

14. A solar heater as claimed in any of the preceding claims, wherein the conduit comprises metal.

15. A solar heater as claimed in claim 14, wherein the metal comprises copper.

16. A solar heater substantially as herein described, with reference to and as illustrated in the accompanying drawings.