GB2225102A - Solar collector - Google Patents

Abstract

A Solar Collector assembly comprises opposing manifolds, and an array of collector tubes coupled to the manifolds. Each collector tube comprises a central copper tube (10) disposed in an evacuated housing (12) with an elongate copper plate (14) welded to the tube (10). A bellows arrangement is provided to compensate for any temperature differential. A mobile and watertight seal (16) is provided between joint pieces (10A, 10B) and manifolds to allow the plates to be rotatably set in an optimum position following installation, or be used in conjunction with a drive unit for solar tracking. A spring clip (24) is provided in order to prevent rotation. <IMAGE>

Description

SOLAR COLLECTOR The invention relates to a solar collector for harnessing the energy of the sun and converting this energy into heat energy in a fluid.

The use of solar panels for heating water is well known. The means for harnessing the solar energy may comprise panels of radiation absorbent material trapping the incoming radiation energy and transferring the heat generated to water flowing in U tubes attached to or integral with the absorbent panels.

Another class of solar collectors operates on the heat pipe principle. The energy of the sun is trapped by an absorbent surface and used to heat up and vaporise a liquid in one part of a heat pipe, the vapour condensing in another part of the heat pipe and the released latent heat of vaporisation being transferred to water to be heated via a heat exchanger.

However, both methods have disadvantages.

In particular the heat pipe construction has the disadvantage that if it is heated the liquid vaporises to an extent such that parts of the energy collecting portion of the heat pipe are saturated by vapour, thereby lowering the efficiency of the heat pipe and restricting its operational temperature range.

Heat pipes in solar collectors are only able to function efficiency if placed at approximately 600 to the horizontal. At lower angles of inclination a danger of vapour locks within the pipe exists.

Solar collectors in which the radiation energy of the sun is converted directly into heat energy in the fluid to be heated rather than indirectly by means of a heat pipe often take the form of radiation absorbing panels containing several fluid passages, Alternatively the fluid passages comprise elongated U-tubes housed in evacuated glass containers, each tube being provided with an integral heat absorbing surface. Such known collectors require mounting at an inclined angle to the horizontal to present the largest possible area in the direction of the sun and to prevent air locks in the fluid passages, and as a result tend to be unsightly when installed on a building unless mounted on an inclined roof facing generally in the direction of the sun.

Such collectors require some skill in installation to mount them at the correct angle with fluid-tight sealing. Skilled maintenance is also generally required periodically to check the sealing of joints.

It is an object of the invention to provide a solar collector which can be simply and conveniently installed in a variety of locations.

According to this invention, a solar collector has an array of elongate collector units coupled to first and second manifolds for containing fluid to be heated, wherein each unit comprises a pipe coupled at one of its ends to the first manifold and at the other of its ends to the second manifold, the pipe bearing a radiation absorbent surface, and having at least part of its length housed in a respective evacuated tubular radiation-transparent enclosure, and wherein each pipe is so coupled to the manifolds that it is rotatably adjustable about a longitudinal axis to vary the orientation of the radiation absorbent surface.

The invention also includes a solar collector unit comprising a pipe bearing a radiation absorbent surface, and an evacuated tubular radiation-transparent enclosure housing at least part of the pipe, wherein the pipe has inlet and outlet joint means at respective oppositely directed ends thereof arranged to allow rotation of the unit about a longitudinal axis thereof when coupled to fixed fluid supply conduits.

Since the collector units are rotatable, the array can be mounted in a variety of locations and orientations without significantly reducing the effective area of the radiation absorbing surfaces.

The invention will now be described by way of example with reference to the accompanying drawings, in which Figure 1 is a partly sectioned side view of a solar collector unit for use in a collector in accordance with the invention; Figure 2 is a longitudinally sectioned view of a joint for coupling the collector unit of Figure 1 to a fluid supply conduit; Figure 3 is a diagrammatic perspective view of a solar collector in accordance with the invention; Figures 4A and 4B are diagrams showing alternative installations of a solar collector in accordance with the invention; and, Figure 5 is a diagram of a solar-powered system for heating water in a tank.

Referring to Figures 1 and 2 of the drawings, a collector unit to form part of a solar collector which is constructed in accordance with the invention and which has a plurality of such units, has a central copper pipe 10 including joint pieces lOA, lOB at respective oppositely directed ends. Between the joint pieces 10A, 10B the pipe is housed in an evacuated tubular glass enclosure 12 having metallic, preferably stainless steel, end caps 12A, 12B bonded to the joint pieces 10A, 10B. Each end of the glass tube of the enclosure 12 is bonded inside an outer lip of the respective cap 12A, 12B using an epoxy resin adhesive selected for its low out-gassing properties. An evacuation nipple 12C is let into one of the end caps 12A to 12B for extraction of air.

The pipe 10 carries a fluid, generally water, which is raised in temperature as it passes through the pipe 10, due to absorption of solar radiation. Such absorption is maximised by welding to the pipe 10 an elongate copper plate 14 having a wavelength selective coating on its radiation absorbing surface.

Expansion and contraction of the pipe 10 relative to the enclosure 12 is accommodated by a flexible bellows portion 10C of the pipe within the enclosure 12.

When it forms part of an assembled solar collector, the collection unit of Figures 1 and 2 extends between two fluid supply conduits in the form of manifolds as will be described below. To obtain a fluid tight seal at its connection with the respective manifold, each joint piece 10A, 10B has a flexible O-ring 16A or 16B housed in an annular groove adjacent the end of the joint piece, the O-ring 16A or 16B bearing against the inner cylindrical surface of a socket in the manifold. One of the sockets 18A is shown in Figure 1, the other, 18B, in Figure 2, the latter being shown housed in a manifold portion 30. A brass washer 22 with a preselected internal aperture diameter allows the rate of flow of water in the unit to be preset, primarily to balance that flow rate with the flow rates in other collector units in the solar collector as a whole. A stainless steel spring clip 24 is fitted during assembly around joint piece 10A to retain the unit in the required position between the manifold for reasons that will become apparent below.

The arrangement of four such collector units 26 in a solar collector including first and second manifolds 28 and 30 is shown in Figure 3.

Ease of assembly and the ability to adjust the orientation of the radiation absorbing plate 14 is facilitated by the use of the O-rings 16A, 16B in combination with the cylindrical inner surfaces of the sockets 18A, 18B (Figures 1 and 2), the spacing of opposing pairs of sockets 18A, 18B when the manifolds 28, 30 are fitted together on a frame 32 (Figure 3) and the length of the inner recesses of the sockets 18A being such that the joint pieces ICA, 10B may be slid lengthwise back and forth in their respective sockets 18A, 18B.Thus, to assemble the collector units 26 between the manifolds 28, 30, the joint end pieces 10A, which are of reduced diameter proximally of the O-rings 16A, are first slid fully into the recesses 18A and then, with the opposite joint pieces 10B in registery with the other sockets 18B, are slid in the opposite direction so that the 0-rings 16B enter the recesses of the sockets 18B to an extent defined by shoulders 16C (Figures 1 and 2) abutting the ends of the sockets 18B. The length of the recesses of the sockets 18A is such that the O-rings 16A are still well inside those sockets.Next, the clips 24 are snapped around the joint pieces 10A between the ends of the sockets 18A and the enclosure end caps 12A to retain the joint pieces 10A, 10B in the required longitudinal positions.

In this way the collector units 26 can be mounted between the manifolds 28, 30 (Figure 3) at a required orientation without the need for soldered joints, and the units are very simply and rapidly replaced if necessary. The circular cross-section of the mating surfaces of the joints between each collector unit 26 and the respective sockets 18A, 18B on the manifolds 28, 30 and the coaxial positioning of those surfaces permits the required rotational adjustment of the unit about a longitudinal axis, preferably the central axis of the enclosure 12.

It will now be appreciated, that with collector units which can be easily rotated, the complete solar collector may be mounted in a number of orientations with the radiation absorbing plates 14 angled to present the maximum surface area in the direction of the sun. In particular the collector is not restricted to use in an inclined position; such a position tends to be useful only when the collector is placed against an inclined surface such as that of a pitched roof facing in the required direction.

The collector described herein may also be mounted horizontally on, for instance, a flat roof as shown in Figure 4, or vertically against a wall as shown in Figure 5. In both cases the absorption plates 14 are angled to receive maximum radiation.

If required, the spring clip 24 (shown in Figure 1) may be dimensioned such that rotation of the respective collector unit is prevented. However, improved performance can be achieved if the collector units 26 are all rotated during use to "follow" the sun. To achieve this, drive means in the form of a drive belt, strip or rod may be mounted transversely of the collector units 26 as shown in Figure 3, engaging each of the units to rotate them in synchronism when moved transversely. In the preferred embodiment shown, the drive means is a strip 32 which presses against rubber bands 34 (Figure 1) encircling the collector units 26.An electrical stepper motor 36 is coupled to the strip 32 by means of a crank, and electrical timing means (not shown) controls the position of the motor 36 to rotate the units 26 - in a pre-programmed cycle - according to the orientation of the collector and its geographical location.

Referring to Figure 6, a typical fluid circuit including the solar collector described above includes a fluid supply conduit 38, a return conduit 40, a pump 42 connected in the supply conduit 38, and a hot water tank 44 containing a heat exchanger coil 46 coupled to the conduits 38 and 40. The pump 42 is coupled in an electrical circuit (not shown) containing temperature sensors in the collector and the tank 44 so as to pump fluid through the collector when the difference between the two sensor temperatures exceeds a predetermined value, typically four degrees centigrade.

An alternative fluid circuit, as shown in Figure 7, dispenses with a pump and relies instead on thermosyphonic operation.

Since the collector units 26 of the solar collector are rotatable the collector may be conveniently and compactly mounted on a vertical wall of a building below the level of the hot water tank 44, yet close to the tank, preferably with the collector units 26 horizontal as shown in Figure 5. In this arrangement, the working fluid is drawn from the top end of one of the manifolds 28 and fed upwards via a supply conduit 38 to the heat exchanger coil 46 of the tank 44, and returns to the base of the other manifold 30 via return conduit 40.

Circulation occurs naturally when the fluid in the collector is heated by incident radiation due to the difference in height between the inlet and outlet of the collector and the placement of the tank 44 above the collector. Since the circuit contains no inverted bends, air locks can be avoided even if the system is allowed to run dry. In this embodiment, a header tank 48 is included.

Further constructional details of the preferred collector follow.

The outside diameter of the pipe 10 of each collector unit is 15mm, and the pipe 10 is bonded to a copper plate having a thickness of 0.2mm which forms the radiation absorbent plate 14.

The selective coating applied to the plate 14 is available from Inco Limited of Birmingham, West Midlands under the trade mark "Maxlam". This finish has a roughness of 0.075 microns. The enclosure 12 has a glass tube 1.814m in length and 65mm in diameter, and is evacuated to at least 10-3 bar (preferably less than 10-6 bar). The manifolds or headers 28, 30 are fabricated from stainless steel and have a square cross-section of 40mm x 40mm.

A typical collector has 20 collector units, the capacity of the complete collector being 90 litres, and that of the collector units alone being 55 litres.

The collector capacity allows a large amount of working fluid to be heated in comparison to the radiation absorbing area. This is of benefit for both pumped and thermosyphonic systems, particularly when radiation is first incident on the collector or when operating at low temperatures, as for example when coupled to a swimming pool.

Claims (7)

Claims

1. A Solar Collector comprising an array of elongated radiation absorbers, such absorbers housed within a sealed evacuated enclosure, two man ifolds situated one at either end of the enclosures and a means of attaching the said components together in a way to provide both a watertight and mobile seal together with a device to allow for expan sion and contraction.

2. A Collector as clamped in claim 1, wherein a solar absorbent plate is fastened to a working fluid conductor and housed within a glass enclosure.

3. A Collector as claimed in claims 1 and 2 wherein a bellow is fastened between the enclosure end cap and the fluid conductor for the provision of expansion and contraction.

4. A Collector as claimed in claims 1, 2 and 3 wherein the absorber plate, fluid conductor and bellow is sealed in a glass enclosure which is evacuated to 10-6 to provide insulation and protection to the absorbent surface.

5. A Collector as claimed in claims 1, 2, 3 and 4 wherein the watertight seal shall remain mobile between manifold and collector enclosure by means of male and female bushes and '0' ring.

6. A Collector as claimed in claims 1, 2, 3, 4 and 5 wherein a push-rod and drive unit be provided to allow individual absorber plates to track sun while collector module remains static.

7. A Collector as claimed in claims 1, 2, 3, 4, 5 and 6 wherein the absorber plates can be adjusted for maximum effect with the collector fixed in either horizontal or vertical stance.

:. A Collector as claimed in all aforementioned claims from 1 - 7 atid d5 illustrated in attached drawing from fig 1 to fig 6.