IE44135B1

IE44135B1 - Solar energy collector

Info

Publication number: IE44135B1
Application number: IE2640/76A
Authority: IE
Inventors: Farefaletti Casali Flaviano; E Aeanovitch; P Gritzmann; Peter Friedrich
Original assignee: Euratom
Priority date: 1975-12-04
Filing date: 1976-12-02
Publication date: 1981-08-26
Also published as: BE848974A; DK543076A; FR2334070B1; NL7613490A; LU76317A1; IT1052506B; IE44135L; GB1509476A; FR2334070A1

Abstract

1509476 Solar heaters EUROPEAN ATOMIC ENERGY COMMUNITY (EURATOM) 3 Dec 1976 [4 Dec 1975] 50595/76 Heading F4U A solar collector for heating a fluid comprises an evacuated transparent cylindrical container 8 through which passes a U-shaped tube 1 for circulating the fluid to be heated, the straight portions of the tube being parallel to the container axis. Associated with each straight portion of the tube there is a mirror 7 having a cross-section formed of two partial ellipses. In an alternative embodiment, Fig. 3 (not shown), the container is provided with a straight tube which passes through the ends of the container. The tube 1 may be a stainless steel tube blackened by varnish or provided with a coating of metal oxide. The container is formed of glass. The straight tube, Fig. 3 (not shown), may alternatively be formed of glass and the fluid may be a black liquid.

Description

This invention is concerned with the construction of a solar energy collector which is air-tight and therefore very efficient and which can also make maximum use of the incident solar light without requiring special features.

The simplest solar-energy collectors, which are most frequently used for producing hot water or for heating fluids in general, usually comprise flat panels made up of a number of layers, i.e. a transparent plate, a black opaque plate absorbing radiation and in contact with moving water or in more general terms, the fluid to be heated, and a layer of insulation underneath.

The transparent plate, which is usually of glass but may also be of plastics material, bounds an insulating space a few centimetres thick above the absorbing plate, thus reducing heat losses through return of heat to the exterior. Usually the space is a simple stationary air cavity, but it may also be a vacuum chamber, provided it has a suitable sealing system.

A vacuum chamber disposed around the heat absorbent body will eliminate heat losses through conduction Or convection towards the exterior, thus greatly increasing the efficiency of the system.

In practice however, it is impossible to construct a flat panel owing to the need for a vacuum chamber which can remain evacuated for an indefinite time without ι requiring continuous or intermittent operation of a vacuum pump. The cost of the panel would be prohibitive. 2. owing to the need for static vacuum seals, usually substantially rectangular, between the transparent plate and the heat absorbent plate, and the need for an internal structure capable of supporting the transparent plate under atmospheric pressure. For this reason flat panels are not generally provided with a vacuum chamber.

Another disadvantage of flat panels is that they absorb considerable energy when the solar radiation is at right angles to their surface or when it strikes the surface at a small angle of incidence but the absorption decreases greatly when the sun moves substantially away from the vertical direction. This is partly because of the reduction in the surface affected by the sun, but mainly because, when light falls on the transparent plate in a substantially oblique direction, a large part of the light radiation is lost by reflection. Of course, this last-mentioned disadvantage could be obviated by constructing panels which can move during the entire day, from dawn to sunset, and thus continuously follow the approximate motion of the sun, but this would again increase the cost in a prohibitive manner, and make it necessary to use mechanisms and automatic devices which might fail if exposed to the weather for long periods.

According to the invention there is provided an air-tight solar energy collector for direct heating of fluids, comprising an evacuated cylindrical container made of a material which is transparent to light radiation, a tube for circulating the fluid to be heated, the tube being positioned within the container and parallel to the container axis, a shaped mirror surface within the container having a 3. 413 3 composite elliptical cross-section the shaped mirror surface being disposed around the tube, and retaining and centering elements for the shaped mirror surface, the inlet and the outlet ends of tlie fluid tube being secured in a sealed-tight manner to the cylindrical container.

According to a preferred aspect of the invention the shaped mirror surface is made up of two similar elliptical shapes having parallel major axes and staggered by an amount equal to two-thirds of the minor semi1° axis.

The solar energy collector according to the invention as exemplified hereafter obviates most of the aforementioned disadvantages of prior art solar energy collectors. More particularly, the strong base structure, which is also the outer casing of the entire system, comprises a completely sealed-tight cylindrical glass tube having a circular cross-section. The only apertures between the outside and inside of the tube are two brass-metal, i.e. perfectly sealed-tight, rings forming the inlet and outlet of the fluid for being heated. The fluid (usually water) flows in an opaque blackened stainless-steel tube which, at the inlet and outlet of the glass casing, is directly welded to the corresponding glass-metal ring. As can clearly be seen, the described system does not require any seal, but only a perfectly sealed-tight weld.

The first advantage of the system as specifically described hereafter is that it can retain its vacuum for a practically unlimited time, without the use of very expensive suction devices.

Another advantageous feature is that, since the 4. Ί4135 casing is transparent and cylindrical and the inner collector is disposed along its central region, if the casing is suitably disposed with the axial plane of symmetry of the collector facing the south so that the incident light rays along the axial plane of symmetry are perpendicular to the casing axis during the motion of the sun for most of the day, the incident radiation will always strike a large region of the transparent cylindrical area in a substantially vertical direction, i.e. with a small angle of incidence and therefore with small losses by reflection, since the transparent surface comprises a cylindrical surface region disposed around the generatrix facing the sun.

However, the main advantage of the system is due to the fact that the inner collector is constructed so that, without moving, it can absorb and hold a large part of the incident solar radiation when the radiation itself moves along an arc of approximately 90° (45° on each side) around the axial plane of symmetry, corresponding to a few hours in the day around midday.

Of course the time during which solar radiation is absorbed can be increased if the system can take up two or three different positions, e.g. if the system can take up only three different positions about 30° from one another, one in the maming, one at midday and one in the afternoon,solar energy can be efficiently absorbed during practically the entire day from dawn to sunset, without the sun's path having to be followed gradually and continuously, as is desirable in the case of a flat panel.

. The solar energy collector described herein can be used to cover vertical surfaces such as south-facing walls of a building; in this case a number of collectors may be disposed parallel to the horizontal axis and adjacent one another along the vertical wall of the building the advantage of this arrangement is that vertical walls as well as roofs can be used to collect solar energy, and it is easier to protect the collectors from the weather, e.g. from hail.

The solar collector according to the invention will now be described with reference to the accompanying drawings:Figure 1 is a perspective view of an element forming a solar energy collector in accordance v/ith the invention, as seen through the transparent cylinder, which is an air-tight glass casing about 100 mm in diameter.

Figure 2 is a cross-section through the structure shown in Figure 1 at right angles to the cylinder axis.

Figure 3 is a perspective view and cross-section of an alternative embodiment of the invention.

Figure 1 shows a externally blackened, opaque stainless steel tube 1, inside which the fluid to be heated flows. The tube is bent in the form of a U inside the glass casing 8 so as to extend through the collector in both the outgoing and the return directions of the fluid, so as to reduce the effect of thermal forces on the glass. For the same reason, the curve connecting the outgoing and return portions is given a lyre shape so as to absorb differences in 6. thermal expansion between the two tube portions. The tube has an outer diameter of 12 mm and an inner diameter of 10 mm; the total length of the tube, including both outgoing and return portions, is about 3 ra.

The inlet end 2 of the tube for fluid to be heated is disposed outside the glass casing and is adapted to be supplied in series or in parallel with other solar collectors. The outlet end 3 of the tube is also disposed outside the glass casing and is beside the inlet end (see arrows).

Reference numeral 4 indicates a sealing ring for the inlet end 2, the metal part of the ring being welded around a stainless-steel tube and the glass part being secured to the glass of the casing, thus bounding the evacuated space inside the casing, around the collector tube.

The sealing ring 5 for the outlet end 3 is similar to ring 4 and disposed beside it; collector tube 1 is centred inside the glass casing by a centreing plate 6.

The collector mirror 7 is made of aluminium mirror sheet-metal, and is simply secured and held in a predetermined position inside the casing by a retaining plate 9 disposed underneath (see Figure 2).

The mirror has the shape shown in Figure 2. The mirror is double,corresponding to the two portions of the tube disposed in parallel inside the collector (i.e. the outgoing and return portions of the tube for the fluid to be heated); in this way better use is made of the space available inside the glass casing. Consequently, one mirror forms a reflecting surface around the outgoing portion of the tube and the other 7. forms a reflecting surface around the return portion of the tube..

In such situations it would be natural to think of parabolic mirrors, the tube for heating being disposed at the focus. A parabolic mirror will operate very well provided the parabolic mirror can continuously follow the sun's course, so that the plane of symmetry extending through the parabola axis is always parallel to the incident radiation. However, even if there are small deviations, a large part of the incident radiation may escape from the tube and return to the exterior after a few reflections. Use can be made of mirrors having very complex curves but these are difficult to construct.

The possibility of using a mirror obtained in a relatively simple manner but capable of satisfying requirements has therefore been considered.

After examining various possible shapes it has been discovered that, in the case of a stationary collector, a most suitable mirror has a shape obtained by placing two elliptical shapes together side by side.

In this case there is no focus but a more extended region inside which all the incident rays travel after one or more reflections. Accordingly, the tube containing liquid to be heated is placed in this region.

An advantage, of this system however, is that when the incident rays do not arrive in the direction of the plane of symmetry, they still pass through the restricted region after one or more reflections. Of course, this applies only within certain limits of the deflection angle, i.e. in the present case between 8. approximately 45° on each side of the plane of symmetry. However, this range is more than sufficient, not only for a collector which does not have to follow the sun's course exactly, but even for a stationary collector, if installed so that its axial plane of symmetry faces the south.

In this manner, even using a stationary collector, it is possible to collect most of the incident radiation during a fairly long time, i.e. of the order of 6 hours around midday.

A prototype has been constructed having the following dimensions: Outer diameter of glass tube: 100 mm.

Inner diameter: 93 mm. Length: 1.5 m. The stainlesssteel tube, which contains the liquid to be heated and is opaque with a blackened outer surface, has an outer diameter of 12 mm and an inner diameter of 10 mm. The . two tube portions (outgoing and return) are disposed inside the glass tube at a distance of 43 mm between axes, and the axes of the two tube portions are 26 mm below the axis of the glass tube. Two aluminium mirrors (i.e. the mirror for the outgoing portion of the tube and the mirror for the return portion of the tube) are parallel and side-by-side and are obtained by bending mirror aluminium sheet-metal having a thickness of about 1 mm.

Each mirror has a shape obtained by placing together two elliptical shapes forming part of an ellipse having a 48 mm major semi-axis and a 15 mm minor semi-axis.

The distance between the centres of the two adjacent elliptical shapes is 10 mm, so that the resulting shape has a cusp in its bottom centre region 9. under the heating tube, i.e. at the point of intersection between the two elliptical shapes. The height of the mirrors is equal to the major semi-axis of the ellipse, and the mirrors are disposed so that their upper edge is about 12 mm above the axis of the glass tube.

The two mirrors are secured together by spot welding or rivets to a vertical central plate 9 acting as a support inside the glass tube. The plate can be recessed and bent back in some regions to form lateral projections 10 for improved centreing of the mirrors. The heating tube can be centred at its two ends inside the glass tube by means of two transverse clamping plates 6.

The solar energy collector can be used in seriesparallel batteries with other similar collectors disposed side-by-side and adjacent one another so as to cover a given area. The surface is placed at a suitable angle between the horizontal and the vertical so as to obtain maximum efficiency, allowing for the geographical lattitude of the installation and the season.

It can be shown by a simple geometrical check based on the elementary laws of reflection that, at an excursion of approximately 90° of the incident radiation around the axial plane of symmetry of the system, the radiation practically always strikes the heating tube.

The aforementioned system can be improved if the heating tube, instead of being a simple stainless-steel tube having a matt outer surface and blackened by varnish, has a selectively absorbing surface, i.e. an outer surface which is treated so as to obtain maximum absorption of solar radiation and minimum emission in the infra-red. This can be . done by covering the outer surface with a thin film of metal oxides.

In addition, the outer absorbent surface of the tube can be corrugated by ribs or scoring parallel to the tube axis in order further to increase the resultant absoption and reduce losses by reflection from the absorbing surface.

In a variant of the afore-described system, the fluid to be heated can travel through a glass tube having an outer surface which is made absorbent and selective by suitable treatment.

This variant, in which the entire structural part of the collector (i.e. the container and the tube for j the cooling fluid) is made of glass, eliminates the two > glass-metal junctions at the inlet and outlet of the ' cooling tube, thus obtaining a considerable saving. ι Two other variants of the system can also be obtained by using a glass cooling tube.

In the first variant, the fluid to be heated does not travel through a glass tube having a blackened absorbent outer surface, but through a transparent glass tube, in which case the fluid to be heated is black and itself absorbs visible radiation. The visible radiation falling on the collector is reflected from the aluminium mirror, strikes the transparent inner tube and directly heats the black fluid travelling inside.

This variant, in which the tube is made of transparent glass, can be used to obtain another variant of the system. The reason for the U-shaped tube inside the collector, in which the fluid flows in an outgoing and a return direction, is mainly to avoid excessive thermal forces between the inner (or hot) tube 11. and the actual container (which is cold), but undoubtedly increases the technical complication. In addition, in the case of a metal or blackened-glass cooling tube, the U-shapa is needed mainly in case the flow of fluid to be heated stopped even briefly, as a result of damage to the circuit. In the event of a stoppage of flow of fluid to be heated the temperature of the blackened tube exposed to the sun would quickly increase to a value likely to break the collector, if the inner tube extended through the collector from one end to the other and was rigidly secured at the ends.

If, on the other hand, the tube is made of transparent glass, the proposed system can be varied in that the tube may be disposed in a straight line with its axis parallel to the collector axis and extending only once from one end of the collector to the other.

Even if the fluid to be heated Stops flowing for any reason, the transparent tube will not capture incident radiation and will thus avoid excessive temperature variations between the different parts of the collector. The collector constructed according to the last-mentioned variant corresponds to the embodiment shown in Figure 2, but is fairly simple and has a cross-section as shown in Figure 3.

In any case the solar energy collector according to the invention is not limited to the specific embodiments described in the preceding description, but further variants and features can be applied without departing from the scope of the invention as claimed hereafter.

Claims

1. CLAIMS:1. An air-tight solar energy collector for direct heating of fluids, comprising an evacuated cylindrical container made of a material which is transparent to light radiation, a tube for circulating the fluid to be heated, the tube being positioned within the container and parallel to the container axis, a shaped mirror surface within the container having a composite elliptical cross-section, the shaped mirror surface being disposed around the tube, and retaining and centreing elements for the shaped mirror surface, the inlet and the outlet ends of the fluid tube being secured in a sealed-tight manner to the cylindrical container.

2. An air-tight solar energy collector as claimed in claim 1 wherein the tube is adapted to carry the fluid first in one direction and then in the opposite direction parallel to the container axis, a shaped mirror surface having a composite elliptical cross-section being provided for each direction along the path of a fluid.

3. An air-tight solar energy collector as claimed in claim 1 or claim 2 wherein the shaped mirror surface comprises two similar elliptical shapes having parallel major axes and staggered by an amount equal to two-thirds of the minor semi-axis.

4. An air-tight solar energy collector as claimed in claim 3 wherein the fluid tube is disposed along the cusp formed by the intersection of the two elliptical shapes.

5. An air-tight solar energy collector as claimed in any one of the preceding claims wherein the fluid tube is made of metal having a smooth outer surface which has been processed to obtain a selectively absorbing surface as hereinbefore defined.

6. An air-tight solar energy collector as claimed in any one of claims 1 to 4 wherein the fluid tube is made of glass having a smooth outer surface which has been processed to obtain a selectively absorbing surface as hereinbefore defined.

7. An air-tight solar energy collector as claimed in any one 13 44135 of claims 1 to 4 wherein the fluid tube is made of metal having a corrugated outer surface which has been treated to obtain a selectively absorbing surface as hereinbefore defined.

8. An air-tight solar energy collector as claimed in any 5 one of claims 1 to 4 wherein the fluid tube is made of glass having a corrugated outer surface which has been treated to obtain a selectively absorbing surface as hereinbefore defined.

9. An air-tight solar energy collector as claimed in any one of claims 1 to 4 wherein the fluid tube is made of transparent 10. Glass and the tube contains a fluid to be heated which itself is suitably processed, by blackening, to form a means absorbing Incident light radiation.

10. An air-tight solar energy collector as claimed in claim 1 substantially as hereinbefore described with reference to and 15 as illustrated in Figures 1 and 2 of the accompanying drawings.

11. An air-tight solar energy collector as claimed in claim 1 substantially as hereinbefore described with reference to and as illustrated in Figure 3 of the accompanying drawings.