GB2069686A

GB2069686A - Solar water heater

Info

Publication number: GB2069686A
Application number: GB8104009A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-02-14
Filing date: 1981-02-10
Publication date: 1981-08-26

Abstract

A solar water heater comprises a box 1 blackened on the inside and having a window 2. Inside the box a plano-convex lens 6 is formed by passing water to be heated along a trough made up of a sheet 3 of flexible transparent plastics material. Sun's rays 7 enter the box via a pivotable mirror and window 2 and are converged to a focus by means of the lens 6. A blackened pipe 8 is connected to receive water leaving the trough, said pipe being arranged coincident with the focus to receive heat resulting from the focussing of the sun's rays. A mirror may be used to reflect the convergent beam so that the focus lies immediately beneath the lens, and not far below as would otherwise be the case. Water passing through the heater is heated in three ways: firstly in the reservoir due to passage of the sun's rays therethrough, secondly in the heat exchanger pipe 8 due to concentration of the sun's rays at the focus of the lens, and thirdly in the reservoir and heat exchanger due to straightforward conduction from the air in the box whose temperature is maintained at a high level due to the "greenhouse" effect. <IMAGE>

Description

SPECIFICATION Solar water heater This invention relates to a solar water heater, that is a heater in which water is heated wholly or partially by energy from the sun or equivalent source.

According to the invention there isprovided a solarwater heater comprising a box having a window through which the sun's rays may be arranged to pass and, mounted within said box, a reservoir for water to be heated, said reservoir being so shaped as to act as a convergent lens for sun's rays passing through said window and a heat exchanger through which the water to be heated may pass, said heat exchanger being placed at or near the focus of such lens to thereby receive the concentrated energy from the sun's rays.

During the time that the water is in the reservoir, it is heated by absorption as the sun's rays pass therethrough. The reservoir acts as a preheater for the heat exchanger, and thus allows the efficiency of the conventional solar heater to be improved upon, thus saving energy.

The reservoir may be open or closed, but in an embodiment of the invention the reservoir takes the form of a shallow bowl or trough of transparent material into which the water is fed. Sun's rays incident on the top surface of the water pass through the water and through the bowl or trough, and are caused to converge due to the rounded bottom of the bowl or trough. The bowl or trough may be made of rigid plastics material, or flexible plastics material.

Heaters may be connected in series, with the output of one feeding the input of the next, in order to increase the water temperature.

In a preferred embodiment of the invention, a mirror is used to direct sunlight into the box. This mirror is preferably pivotted to enable the angle at which the sun's rays enter the lens to be maintained, either manually or automatically, reasonably constant.

In order that the invention may be better understood, three embodiments thereof will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 is a cross sectional elevation of one embodiment of a solar water heater according to the invention; Figure2 is a cross sectional elevation of a further embodiment of a solar water heater according to the invention; Figure 3 is a cross sectional elevation of a still further emodiment of a solar water heater according to the invention; and Figure 4 is a diagrammatic view of a hot water system to show one example of the use of the solar water heater of this invention.

Referring to Figure 1, the heater comprises a sealed box 1 of g.r.p. or similar, blackened on the inside, and having a transparent glass or plastics window 2 through which the rays 7 from the sun or other suitable source may pass. A bowl-shaped diaphragm 3 divides the interior of the box into two sections. The upper section, which is watertight has an inlet pipe 4 for water and, at a lower level, an outlet pipe 5. Water enters the upper section of the box through pipe 4 and falls into the diaphragm, thus forming a reservoir 6 of water in the shape of a plano-convex lens. When the water level in the reservoir reaches that of the pipe 5, the water overflows through the pipe, whence it is fed into the lower section of the box 1. Situated in the lower section of the box is a heat exchanger 8 through which the water in pipe 5 flows, eventually exiting through an outlet 9.The heat exchanger 8 may, for example, take the form of a flat spiral of blackened copper pipe through which the water flows.

The shape of the reservoir 6 is such as to form a convergent lens for sun's rays 7 entering through the window 2. As the rays pass through the water making up the reservoir, heat energy is transferred to the water by absorption, so that the water overflowing into pipe 5 is effectively preheated. The heat exchanger 8 is positioned at the approximate focus of the lens formed by reservoir 6 so that the maximum heat energy is transferred to the water in the heat exchanger.

Output 9 may be simply terminated by a water tap, or it may feed a reservoir tank in a larger heating system. To obtain higher water temperatures, two or more solar heaters may be connected in series such that water leaving output 9 of one heater enters inlet 4 of the next, and soon.

The diaphragm 3 may be fabricated cheaply of flexible plastics material which will take up a curvature, as shown, under the weight of water. However, a better defined focus would be obtained by use of rigid plastics material.

A plan view of the Figure 1 device is not shown in the accompanying drawing. In practice, the box 1 will be square in plan, with a circular diaphragm 3.

The box could also be rectangular, with the diaphragm, in the form of a trough, extending along the length of the box. In this latter case, an elongate linear focus is formed, and the heat exchanger 8 would be arranged to run along this focus to obtain maximum benefit from the available head energy.

Figure 2 shows a further embodiment in which the box is rectangular in plan, and has a diaphragm in the form of a trough. Figure 2 is a view of the trough in section and consequently the input pipe is not shown. The entrance to the outlet pipe is shown under reference 9. Apart from the shape of the reservoir, the embodiment of Figure 2 includes a number of features not present in the embodiment of Figure 1: Because the lens is in the form of a trough, the focus is linear, and the heat exchanger 8 therefore may take the form of a length of straight copper pipe blackened on the outside. The pipe is positioned to coincide with the focal plane of lens 6 which has been found in practice to lie a considerable distance below the lens itself, leading to the requirement for an excessively deep box 1.In order to avoid this problem, the sun's rays, after passing through lens 6 are reflected by a mirror 10 which is angled to cause the focus F to lie much higher up in the box than would otherwise be the case. The depth of the box is thereby reduced by half.

The second feature of the Figure 2 embodiment is a plane mirror 11 which is positioned above the box in order to direct the sun's rays into the lens 6. It has been assumed until now that the sun's rays will impinge vertically on the lens 6 but this is, of course, rarely the case. The angle at which the sun's rays fall depends on the latitude of the country in which the device is used, and on the time of year. The mirror 11 is pivoted about an axis P1 which is parallel to that of the lens and so can be adjusted, either automatically or manually, to take account of different angles of sunlight.

In adjusting the angle of mirror 11, it is desirable to maintain a reasonably constant angle of entry of the sun's rays into the box otherwise the position of the focus F will change. In order to minimize spherical aberration in the lens 6, the light should enter the lens at right angles. Although this could easily be achieved by suitable inclination of mirror 11, it is found that, during the summer months when the sun is highest above the horizon, the effective area of sunlight reflected by the mirror becomes small, and the size of the mirror too large to be practicable. In order to keep the mirror 11 within limits, the reflected light is made to enter the lens at an angle reasonably close to a right angle.The angle chosen depends to some extent on latitude but for Europe an angle of 28 is considered convenient, subject only to adjustments in the size of the mirror to cover equal areas.

A further problem which has arisen is due to the fact that the light entering a plano-convex lens with the plane side facing the light does not all converge to a sharp focus. This is due to spherical aberration of the lens and it has been found that the effect can be minimized by utilising only a portion of the lens for light transmission. This is illustrated in Figure 2, where it will be seen that only the left-hand 55% of the plane surface of the lens 6 is being used. It is possible to theoretically calculate the area of the lens which will form a sharp focus, but these calculations are complex and will not be repeated here. Suffice to say that the useful area is dependent upon the angle of incidence of the light beam on the lens.

It has been found during tests that the greenhouse effect within the box 1 causes the interior of the box to rise to a considerable temperature. This heat energy passes directly to the water passing through the box by simple conduction and the heat energy transferred by this mechanism has been found to be significant. Thus the embodiment of Figure 2 improves over that of Figure 1 in that it provides a greater window area through which the sun's rays may pass. In the Figure 2 embodiment, the window 2 takes up the whole of the top surface of the box and, in addition, the wall 12 of the box is made transparent so as to take full advantage of the inclination of the sun.In order to capitalise on the greenhouse effect, the two windows are preferably double glazed and, for cheapness, this may comprise a double layer of transparent sheet plastics material, with a small air space between the sheets.

The remaining walls are constructed in such a way as to absorb as much as possible of the sunlight which impinges on them since any heat energy retained thereby is available to heat the interior of the box and hence the water passing therethrough.

As mentioned above, the inside surfaces of the walls may be blackened, and a preferred method of construction is to form a sandwich of a material having reasonably good thermal absorption properties - e.g. wood - with a reflective material, arranged so that any heat which passes through the absorptive material will be reflected back into the box by the mirror. In one particular embodiment, a sandwich of hardboard-aluminised polyesterhardboard has been found satisfactory, and cheap.

The mirror side of the aluminised polyester is arranged to face the interior of the box to provide the required heat reflective properties.

Figure 3 shows a third embodiment of the invention, similar two that of Figure 1, but in which the window 2, is below the lens 6, and the sun's rays 7 are directed in an upwards direction through this window by means of a pivotable mirror 21. A further mirror 22 mounted at the top of the box 1 reflects the already converging light back through the lens where further converging takes place, resulting in a focus which is very close to the underside of the lens 6.

On the other hand, the solar energy entering the box is limited to that reflected by the pivoted mirror 21 underneath. When light enters a plano-convex lens with the convex side facing the light, the problem of spherical aberration is greatly reduced, so that virtually all light entering the lens can be concentrated to form an extremely hot focus. The Applications for this type of heater are thus different from those of Figures 1 and 2, and consist primarily in heating small amounts of water to boiling point. In one embodiment (not shown), the heat exchanger 8 in the focal plane is replaced by a container designed to hold, for instance, tea. Water from the lens reservoir is allowed to seep into the container, where it boils entirely by means of solar energy, and extracts the soluble properties of the tea to form a beverage.

Figure 4 shows a typical application for the above described solar water heater in a domestic plumbing system. The heater, shown diagrammatically under reference 13, is of the 'trough' type described with reference to Figure 2, However, it will be clear that the heaters of Figure 1 or 3 could equally well be incorporated in this system.

Water enters from the main into a conventional cold-water storage tank 14 having a ball-valve 15.

The output of the tank 14 is taken via a pipe 16 to the inlet 4 of the solar water heater. The water level in the lens 6 of the solar water heater is controlled by a ball-valve 17. The hot water outlet 9 of the solar water heater is taken either directly to a tap or taps 18 andzor to an indirect hot water cylinder 19 where a heat exchanger 20 of conventional type may be used to heat the water to its final required temperature. In this case, the solar water heater 13 acts as a pre-heaterfor the "cold" water entering the cylinder 19 and thus reduces the energy needed to heat the water, by conventional means, to the required temperature.

It is clear that the solar water heater described above will not be limited to use in domestic systems, but will find wide application in agriculture and industry for heating or preheating water for a variety of uses.

Claims

1. A solar water heater comprising a box having a window through which the sun's rays may be arranged to pass and, mounted within said box, a reservoir for water to be heated, said reservoir being so shaped as to act as a convergent lens for sun's rays passing through said window and a heat exchanger through which the water to be heated may pass, said heat exchanger being placed at or near the focus of such lens to thereby receive the concentrated energy from the sun's rays.

2. A solar water heater as claimed in claim 1 wherein the pipework for water to be heated is arranged such that the water enters the reservoir first, and is thence passed to the heat exchanger.

3. A solar water heater as claimed in either one of claims 1 or 2 wherein the reservoir takes the form of a shallow bowl of transparent material into which the water to be heated is fed, said bowl thus forming when filled, a circular plano-convex lens which acts to converge sun s rays passing therethrough to a point focus.

4. A solar water heater as claimed in claim 3 wherein the heat exchanger takes the form of a container containing water to be heated, said container being fed continuously or otherwise from the water in said reservoir.

5. A solar water heater as claimed in either one of claims 1 or 2 wherein the reservoir takes the form of a shallow elongate trough of transparent material into which the water to be heated is fed, said trough thus forming, when filled, an elongate lens, planoconvex in section, which acts to converge sun's rays passing therethrough to a linear focus.

6. A solar water heater as claimed in claim 5 wherein the heat exchanger comprises a pipe through which water to be heated passes, said pipe being positioned coincident with the focal plane of the lens.

7. A solar water heater as claimed in either one of claims 5 or 6 wherein water to be heated enters at one end of said trough and exits from the other.

8. A solar water heater as claimed in any one of claims 3 to 7 wherein the transparent material takes the form of a flexible plastics material which is mounted in the box in such a way that, when filled with water, the plastics material takes up a shape, due to the weight of the water, which renders the reservoir a convergent lens.

9. A solar water heater as claimed in any one of the preceding claims further comprising a mirror positioned so as to direct sunlight into said window.

10. A solar water heater as claimed in claim 9 wherein said mirror is pivotable so as to enable sun's rays to be directed through the lens at a reasonably constant angle irrespective of the angle of incidence of the sun's rays.

11. A solar water heater as claimed in any one of the preceding claims wherein within said box is further mounted an additional mirror to reflect the converging beam of light from the lens through an angle so that the focal plane of the lens is closer to the lens than it would otherwise have been.

12. A solar water heater as claimed in any one of claims 1 to 10 wherein said additional mirror is angled so as to cause light to pass back through the lens, causing the light to be refracted twice.

13. A solar water heater as claimed in either one of claims 11 or 12 wherein the light enters the box from above and said additional mirror is positioned beneath the lens.

14. A solar water heater as claimed in either one of claims 11 or 12 wherein the light enters the box from below and said additional mirror is positioned above the lens.

15. A solar water heater as claimed in any one of the preceding claims wherein the walls of the box are designed for maximum heat absorption.

16. A solar water heater as claimed in any one of the preceding claims wherein the water level in said reservoir is maintained by means of a float valve.

17. A solar water heater substantially as hereinbefore described with reference to the accompanying drawings.