GB2147408A

GB2147408A - Solar water heater

Info

Publication number: GB2147408A
Application number: GB08326550A
Authority: GB
Inventors: Dimos Maglaras
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-10-04
Filing date: 1983-10-04
Publication date: 1985-05-09
Also published as: GB8326550D0; FR2553872A1; DE3340239A1

Abstract

A solar water heater comprises a cylindrical collector (6) placed in the vicinity of the focal axis of a simple or double parabolic mirror (5). The collector is provided with a selective absorbent surface (8). The collector and parabolic mirror are placed within a housing (3) with insulation (4) and a double-glased panel (2). The water heater may be provided with a resistor for supplementary heating, a small resistor for protection from freezing and an optional magnesium anode for protection from corrosion. The water heater is supported by a frame inclined by 20-60 DEG , and can be rotated by means of a servomechanism to ensure the maximum thermal absorbance. <IMAGE>

Description

SPECIFICATION Solar water heater with collector, its tubular selectively coated boiler, placed in the vicinity of the focal space of a semicylindrical parabolic mirror The present invention relates to a solar water heater which uses as collector its boiler, placed in the vicinity of the focal axis of a semicylindrical parabolic mirror, where the boiler is coated with a selective surface foil, this foil exhibiting a high coefficient of absorbance and a low coefficient of thermal radiation (reemission E = 5-15%, where the above boiler is preferably covered either with a double-glazed panel with dry air in the gap, or with a coaxial cylindrical ring with vacuum in the gap.The invented solar water heater achieves high performances at higher water temperatures ranging within 40"-70"C, thereby solving the problems associated with the usage of hot water either for the installations supplying hot water in buildings, hotels, etc., or for various industrial applications.

The prior rt of solar water heaters has ot up today invented a similar object. Thus up today flat plate collectors are mainly used in the various applications of solar water heaters.

The flat plate collectors might be suitable for heating a swimming pool or for providing hot water to a single house or a flat, but they cannot exceed the temperature of 25-30"C maintaining a satisfactory efficiency and they are thereby led to antieconomic solutions.

Thus in many installations of large groups of buildings a great number of flat plate collectors were used with a substantially zero outcome.

The main problem of the flat plate collectors is that they work efficiently only during noon, when the sun rays fall normally. However, the manufacturers of flat plate collectors often present efficiency diagrams for these collectors, which are actually valid only during noon.

Another problem of the flat plate collectors is that associated with the percentage of wetted surface at a certain film of water of a small thickness, immediately underneath the selective surface, which is in many cases not fullfilled, either because the manufacturer does not have a high percentage of wetted surface, or because he does not have the film of water of the desired thickness, or because he does not have a selectively coated surface, or because the selective surface indirectly transmits the heat at the near by pipes, etc., Another problem of the flat plate collectors is that they have an approximately 20% loss of their performance due to the double closed circuit of anti-freezing medium etc., the lack of a double-glazed panel for protection from the conduction losses and an air tight construction to provide long life to the special selective surface.

These and other problems and disadvan tages of the prior art of flat plate collectors are overcome with the application of the present invention, the main objects of which are pre sented below: An object of the present invention is to provide a solar water heater with collector the boiler itshelf, placed in the focal space of a semicylindrical parabolic mirror, preferably in an upright position, so that during the course of the sun from East to West, the active surface of the mirrors can provide the maxi mum of the incident radiation, whereas in the same time the diffused radiation is concen trated towards the axis of the focal space.

Another object of the present invention is the coating of the said boiler-collector with a special selective surface, e.g. Black Chrome or Black Nickel, this providing a high rate of absorbance of the solar radiation (a = 85-95%) and a low rate of reemission (e = 5-15%), thereby limiting the radiation losses.

Another object of the present invention is the air-tight mounting of a transparent panel or alternatively of a transparent cylindrical tubee coaxial with the said boiler-collector, so that a gap of dry air or even a certain vacuum might be formed between the said boiler collector and the transparent panel or the transparent coaxial cylindrical tube.

Another object of the present invention is the employment of a double transparent panel, e.g. glass panel for covering the said boiler-collector, or alternatively the usage of a coaxial cylindrical ring, where within the said double panel or the said coaxial cylindrical ring, dry air or even some vacuum is formed.

Another object of the present invention is the manufacture of the invented collector with one single circuit (that of the mains) thereby obtaining an improvement in efficiency by 17%.

Another object of the present invention is to provide a solar water heater easily assembla ble and with an aesthetic appearance, even when many collectors are connected as is the case with central installations, where the solar i water heater of the invention occupies a lim ited space, since it corresponds to half the space occupied by the best flat plate collector for the same result.

A final object of the present invention is the I satisfaction of the above objects and advan tages with a solar water heater, which while offering the same result as other collectors has a comparatively small and competative cost and a long life.

These and other advantages, characteristics and aims of the present invention will be made clear in the analytical description below.

The present invention will be understood better by those skilled in the art by reference I to the accompanying drawings, which show in an illustrative and not limiting mode the object of the invention.

Figure I presents a perspective view of the invented solar water heater made according to an embodiment of the invention, mounted on a supporting base.

Figure 2 presents a lateral cross sectional view of the above solar water heater.

Figure 3 presents a longitudinal cross sectional view of the above solar water heater.

Figure 4 presents a lateral cross sectional view of the invented solar water heater, where according to an alternative embodiment of the invention the boiler-collector is placed in the focal space of a double parabolic mirror.

Figure 5 presents a diagram showing the energy incident daily and the rate of absorbance of the above energy (a) by a flat plate collector and (b) by the collector of the present invention, in KCAL/M2 of surface.

Figure 6 presents a diagram of the efficiency n of a collector according to the embodiment of the invention of Fig. I and of flat plate collectors for comparison reasons.

Figure 7 presents a longitudinal cross sectional view of a solar water collector, which according to an alternative embodiment of the present invention is made with a transparent coaxial cylindrical ring around the said boilercollector.

Figure 8 presents a lateral cross sectional view of the solar water heater of Fig. 7.

Reffering now to the drawings and in particular in the drawings 1, 2 and 3 we observe that according to a preferable embodiment of the invention the solar water heater comprises a cylindrical boiler (6) which is in the same time the storage of hot water at its interior (7).

The boiler-collector (6) is preferably made with a special stainless material with anti-corrosion properties, but it can also be made with any other suitable material (e.g. glass), or with the usual metal used in the construction of boilers, or with steel laminates which have been pretreated to obtain anti-corrosive properties. The material of the boiler-collector can also be anodised (magnesium electrode 10) for protection from electrolysis phenomena.

Indicatively and according to a preferable embodiment of the invention, the collectorboiler (6) has an internal diameter 300 mm and a height of 1600 mm. The circular bases of the cylindrical collector-boiler, which do not form part of the collecting surface can be insulated with plates of polyurethane or any other suitable insulating material.

The cylindrical collector-boiler (6) is circumferentially coated with a foil of selective surface (8) or it is quenched into a suitable bath for the formation of the selective surface (BLACK CHROME or BLACK NICKEL). The process of coating with a foil of selective surface is possibly preferable for economic reasons. A conventional selectively coated foil made from black chrome or black nickel or a combination of these materials, which is available commercially and is used in the construction of the invented collector has a coefficient of absorbance of the thermal radiation in the range of 95-98% and a coefficient of reemission of the thermal radiation in the range of 5-15%.Thus the coating of the collectorboiler (6) with the above selective surface foil (8) on the one hand increase its heat absorbent capacity and on the other hand reduces the radiation heat losses because of the low coefficient of reemission of the thermal radiation. If the circular bases of the cylindrical collector-boiler (6) are not insulated with another special insulating material, they can also be coated with the above selective surface foil (8).

As shown in Fig. 2, the collector-boiler (6) is placed in the focal space of a semicylindrical, preferably upright parabolic mirror (5), or according to another preferable embodiment of the invention it is placed in the focal space of a double parabolic mirror (I) as shown in Fig. 4. The parabolic mirror (5, II) is made from suitable glossy materials, which can thus reflect the solar radiation with an efficiency of up to 85% towards the back side of the collector-boiler (6+which is coated with the selective surface foil (8). During the installation of the cylindrical collector-boiler (6), care is taken so that one or two line portions of its surface coicide (or approach) the focal axis or the focal axes of the single (5) or the double parabolic mirror (II) respectively.In this way the entire surface of the cylindrical boiler (6) constitutes a collecting surface, because its front side is directly supplied with the solar radiation. (direct and diffused), whereas its back side is supplied with the solar radiation reflected by the parabolic mirror (5, Il). The back side of the parabolic mirror (5, II) can be covered with insulating material (4), such as polyurethane, mineral cotton, etc., to limiti the heat losses.

The whole system is enclosed into a metallic, plastic or fibre-glass housing (3), the front side of which extends into a special profile (1) within which a suitable glass panel or other transport material panel, or the specially designed plastic sheets conventionally used in solar collectors and according to a preferable embodiment of the invention a double-glased panel (24 with an intermediate gap of dry air or some vacuum for an optimum insulation can be fitted. The fitting of the glass panel (2) is made water-tight and preferably air-tight as well, so that the sun radiation is unhindered in its passage through the glass panel (2) and its incidence onto the collector-boiler (6) or the parabolic mirror (5, II). It is for this same reason that the glass panels can also be made with special glass.

According to a preferable embodiment of the invention, instead of the above doubleglased panel, the collector-boiler (6) can also be covered with a coaxial transparent tube (16) with a single or double wall, and with suitable adjustments form around the collecting surface one or two layers of dry air or one layer of dry air and a concentric ring with vacuum if the strength of the material allows for this vacuum, so that the system may be satisfactorily protected from conduction losses and therefore eliminate the requirement for an additional insulation. In this way the layer of insulating material (4) is eliminated. The only additional insulation is at the circular bases of the cylinder of the boiler and its coaxial housing, where plates of insulating material (15) are placed.

In the case of the above preferable embodiment of the invention which is illustrated in an indicative manner in Figs. 7 and 8, the efficiency will be higher because the entire in cident radiation will pass to the active surface of the parabolic mirror. In this case, the glossy surface of the mirror must be protected by a suitable film, so that it may not decay with time, be protected from corrosion and be capable of retaining the dust and the water droplets. Such materials are already commercially available and are usually used for the vacuum pipe collectors.The case of the present invention is however different compared to the well known systems of vacuum pipe collectors, since the cylindrical collector used is in the same time the boiler, whereas the whole system operates with one single circuit and it can work at the pressure of the mains, in opposition to the known systems which work with an indirect closed circuit and can stand pressures only up to 2 atm.

As shown in the longitudinal cross section of Fig. 3, the collector-boiler (6) is provided with an electric resistor (9) and a magnesium anode (10) steeped into its interior. The electric resistor (9) can provide the additional heating of the water necessary during winter days.

The introduction of the mains water into the solar water heater of the invention and the discharge of the water from the same can be implemented using the established classical technology, via the pipes (12) and (13) respectively.

As long as the temperature does not fall below 5"C, the system operates without any danger of freezing. Nevertheless if its front side is covered with a double-glased panel (2) with an intermediate gap of dry air, an additional protection is provided even at lower temperatures. The above additional protection is also provided if instead of the double-glased panel the coaxial cylindrical ring is used.

Alternatively and in order to ensure the maximum possible protection, a small electric resistor (in the range of 500 W) may be provided, placed within the collector-boiler (6) and operating in connection to a differential thermostat, thereby keeping the temperature of the water well above + 2"C and ensure that the water contained in the store (7) does not freeze. It must however be noted that in those cases where the protection with a layer of dry air and an air tight construction around the collector-boiler is implemented, this is sufficient and the additional resistor is not necessary.

As shown in Fig. 1, the solar water heater is mounted onto a metal frame (14), the inclination of which can be adjusted. To ensure the optimum collection of solar radiation, the water heater is placed facing the South, with an inclination of 20-60" to the ground, where the optimum inclination is determined by the terrestrial latitude of the area where the solar water heater is to be installed. According to another preferable embodiment of the invention the water heater is rotated by means of a servomechanism from East to West following the course of the sun and thereby ensuring the maximum possible collection of the solar radiation.

The above solar water heater can be supplied at different dimensions to suit different requirements of hot water. A number of such solar water heaters can be connected in a parallel or serial arrangement, to cover the requirements of large installations. For economic reasons two or more units of boilercollector (6) and parabolic mirror (5, II) can be arranged within the same housing, thereby providing a solar water heater of higher performance.

The invented solar water heater can moreover be connected to an available electric water heater, so that through a triode manual or electric sluice gate valve, hot water will be provided in the days with sufficient sunshine from the solar water heater and in the days with insufficient sunshine from the electric water heater, which can however be supplied with the warm water of the solar water heater instead of the cold water of the mains, thereby making use even of the reduced insufficient sunshine.

The solar water heater of the invention and particularly that which is shown in the attached Figs. 1 and 2, will be compared to the solar water heaters of the prior art by reference to the diagrams 5 and 6.

First refering to diagram of Fig. 5, we observe three curves A, B and C. The first curve (A) presents the energy supplied by the solar radiation throughout the day from 07.00 up to 17.00 hrs, in Kcal/m2. The curve (B) shows the absorbance of the above incident solar energy from the collector of the present invention and the curve (C) shows the absorbance of the incident solar energy from a flat plate collector. The total average efficiency of the flat plate collectors is taken from the above diagram to be 22.5%, and the total average efficiency of the collector of the present invention is taken to be 55%. Thus the superiority of the collector of the present invention is evident.

Refering now to the diagram of Fig. 6, we observe here the variation of the efficiency of a collector according to the present invention (line 1), for a flat plate collector selectively coated (line 2) and for a common flat plate collector (line 3), with the variation of the temperature of operation of each of the above collectors. The diagram corresponds to an incident radiation of 5.67 KWH/m2 and an ambient temperature of 1 0'C. The diagram has been divided into three zones of temperature, zone (A) (10-30"C), zone B (30-60"C), and zone C (60-120"C). Zone A is primarily used for heating a swimming pool, zone B for heating and for providing hot water, and zone C for industrial applications and for air conditioning.It is obvious that the flat plate collector (and to a much larger extent the flat plate collector without a selective coating) has a lower efficiency n as regards zones A and B than the collector of the present invention, whereas in zone C it is practically unsuitable.

It must hereby be noted that the present invention has been described by reference to illustrative and not limiting examples which are shown in the drawings. Thus any change or amendment, such as in the dimensions, form, size, design, materials and accessories used in the manufacture and the assembly processes, which does not comprise a new inventive step and does not contribute to the further development of the state of the art, does not limit the rights claimed for the present invention and is considered part of the scope and the aims of the present invention.

The rights for which the applicant of the present invention claims patent protection are summarised in the following claims.

Claims

1. Solar water heater which uses as collector its boiler, where the said collector-boiler is placed in the vicinity of the focal space of a parabolic mirror and is covered with a foil of selective surface, which exhibits a high coefficient of absorbance and a low coefficient of reemission of the solar thermal radiation, where the said collector-boiler is covered with a transparent covering means for the limitation of the conduction thermal losses.

2. Solar water heater according to the above claim 1, characterised by that the said parabolic mirror is semicylindrical upright parabolic mirror.

3. Solar water heater according to the above claim 1, characterised by that the said collector-boiler is made from stainless material or from glass or from steel laminates, which have been treated against corrosion.

4. Solar water heater according to the above Claim 1, characterised by that the said foil of selective surface is formed by using a commercially available selective surface foil.

5. Solar water heater according to the above claim 1, characterised by that the said foil of selective surface is formed by steeping the said boiler into a suitable bath.

6. Solar water heater according to the above claim 1, characterised by that the said foil of selective surface covers completely the said collector-boiler including its circular bases.

7. Solar water heater according to the above claim 1, characterised by that only the cylindrical circumferential surface of the said collector-boiler is covered with the said foil of selective surface, whereas the circular bases of the said collector-boiler are covered with plates of polyurethane or other suitable insulating material.

8. Solar water heater according to the above claims 1 and 2, characterised by that the said parabolic mirror is a simple parabolic mirror and the said collector-boiler is placed in the focal space of the said simple parabolic mirror in a way such as one longitudinal portion of its cylindrical surface coincides or approaches the focal axis of the said simple parabolic mirror.

9. Solar water heater according to the above claims 1 and 2, characterised by that the said parabolic mirror is a double parabolic mirror and the said collector-boiler is placed in the focal space of the said double parabolic mirror, in a way such that two longitudinal portions of its cylindrical surface coincide or approach the corresponding focal axes of the said double parabolic mirror.

10. Solar water heater according to the above claim 1, characterised by that the said collector-boiler and the said parabolic mirror are placed into a single housing, where the said housing has the semicylindircal shape of the said parabolic mirror and a transparent and solar radiation permeable covering means is mounted at the open side of the said housing.

11. Solar water heater according to the above claim 10, characterised by that an insulating material is provided between the said housing and the back surface of the said parabolic mirror.

1 2. Solar water heater according to the above claim 10, characterised by that the said transparent covering means is a glass panel made from common or special glass, which is air-tightly mounted onto the said open side of the said housing, so that a dry air intermediate gap or even some vacuum is formed between the said collector-boiler and the said glass panel.

1 3. Solar water heater according to the above claims 10 and 12, characterised by that the said transparent means of covering is a double-glased panel made from commom or special glass, where a dry air intermediate air gap or even some vacuum is formed between the two sheets of the said double-glased panel.

14. Solar water heater according to the above claims 10-1 3, characterised by that the said transparent covering means is made from other non-glass, transparent and permeable to the solar radiation material.

1 5. Solar water heater according to the above claim 1, characterised by that the said parabolic mirror is open to the atmosphere and the said collector-boiler is placed in the vincinity of the focal space of the said parabolic mirror, where the said covering means of the said collector-boiler is a transparent coaxial cylindrical tube permeable by the solar radiation.

1 6. Solar water heater according to the above claim 15, characterised by that the circular bases of the said collector-boiler and of the said coaxial cylindrical tube covering it, are covered with plates of insulating material.

1 7. Solar water heater according to the above claim 15, characterised by that the said transparent coaxial cylindrical tube is made from glass-common or special- and is airtightly mounted around the said collectorboiler, so that a dry air intermediate gap or even some vacuum is formed between the said collector-boiler and the said coaxial cylindrical tube.

1 8. Solar water heater according to the above claims 1 5 and 17, characterised by that the said transparent coaxial cylindrical tube is a cylindrical ring, made from common or special glass, where an intermediate dry air gap or even some vacuum is formed between the two walls of the said cylindrical ring.

19. Solar water heater according to the above claims 1 5-1 8, characterised by that the said transparent cylindrical coaxial tube is made from other, non-glass, transparent and permeable to the solar radiation material.

20. Solar water heater according to the above claims 1-19, characterised by that it operates with one single open circuit, that of the mains and at the pressure of the said mains circuit.

21. Solar water heater according to the above claims 1-20, charterised by that it is mounted onto a metallic frame and is placed at an approximate inclination of 20-60" to the ground, and by means of a servomechanism moves from East to West following the course of the sun.

22. Solar water heater according to the above claim 10, characterised by that two or more systems of collector-boiler and parabolic mirror units are arranged within the same metallic housing, thereby providing a solar water heater of higher capacity.

23. Solar water heater according to the above claims 1-22, characterised by that it is provided with an additional small electric resistor, steeped into the said collector-boiler for heating the water during winter days without sunshine.

24. Solar water heater according to the above claims 1-23, characterised by that it is provided with an additional small electric resistor connected to a differential thermostat and steeped into the said collector-boiler, by means of which it is ensured that the water temperature does not fll below + 2"C.

25. Solar water heater according to the above claims 1-24, characterised by that it is connected to an electric water heater and works alternately, where the alternation is controlled by means of a sluice gate valve.

26. Solar water heaters, substantially as hereinbefore described with reference to the accompanying drawings.

27. The features herein described, or their equivalents, in any patentably novel selection.