GB2053455A

GB2053455A - Collectors for Solar Energy

Info

Publication number: GB2053455A
Application number: GB8016985A
Authority: GB
Original assignee: Millar J H
Current assignee: Millar J H
Priority date: 1979-07-04
Filing date: 1980-05-22
Publication date: 1981-02-04
Also published as: GB2053455B; FR2461212B3; DE3025826A1; FR2461212A1

Abstract

A collector for solar energy comprising an outer glass tube (2) having therein a compound parabolic reflector (4) extending substantially along the length of said tube and a straight conduit (6), which may take the form of a heat pipe or may contain a heat pipe, extending along the focus of said reflector, the ends of the outer glass tube being sealed and the space within the sealed outer glass tube being evacuated. <IMAGE>

Description

SPECIFICATION Collectors for Solar Energy This invention relates to collectors for solar energy and in particular to vacuum collector tubes with an integral compound parabolic reflector.

According to the present invention there is provided a collector for solar energy comprising an outer glass tube having therein a compound parabolic reflector extending substantially along the length of the tube and a straight conduit extending along the focus of said reflector, the ends of the outer glass tube being sealed and the space within the sealed outer glass tube being evacuated.

The outer glass tube for use in the invention may be any glass tube, e.g. borosilicate glass, of suitable diameter and length.

The reflector used in the invention is a compound parabolic reflector designed to reflect the incident sunlight to its focus. The reflector may conveniently be made of high polished aluminium having a high specular reflectance. The reflector is preferably rigid and self-supporting so it may simply be held in place within the outer glass tube by means of spring clips. The reflector is preferably of the compound parabolic type designed in accordance with the principles disclosed in United States Patent Specification Nos. 4002 499 and 4 003 638.

The conduit may-take the form of an open-ended tube adapted for insertion into a circuit in which the heat-absorbing fluid is circulated, an open-ended tube in which a heat pipe may be fitted or it may take the form of a heat pipe which is designed to rapidly convey the heat absorbed by the collector to a suitable heat reservoir.

In accordance with one embodiment of the invention the conduit comprises a simple glass tube extending beyond the ends of the outer glass tube, the outer glass tube being sealed to the surface of the inner glass tube with the provision of suitable means to absorb differential expansion between the two tubes. The inner glass tube is connected into a suitable circuit for passage of heat absorbing fluid in the same manner as known collectors which employ circulating fluid.

In a second embodiment of the invention the conduit is in the form of a heat pipe. Heat pipes are known and comprise a sealed tube which is partially evacuated and contains a small quantity of a liquid and a tubular wick extending throughout its length held firmly in contact with the inner wall of the tube. When heat is applied to one end of the heat pipe the liquid vapourises and travels along the length of the pipe condensing at the cool end and thereby heating that end. The condensed liquid is recirculated to the other end via the wick. The use of a heat pipe allow rapid transfer of heat from one end of the pipe to the other.

In accordance with a further embodiment of the invention the conduit comprises a glass tube having an open end welded flush with the outer glass tube and a heat pipe is inserted within the tube extending beyond the end thereof.

In either embodiment of the invention the heat pipe is arranged with one end within the outer glass tube and the other end projecting therefrom and adapted for insertion into a heat exchanger or heat reservoir. The portion of the heat pipe within the outer glass tube or the glass tube containing said portion is preferably coated with a selective coating of high absorbivity and low emissivity to improve efficiency. The outer glass tube is sealed at one end with a suitable end piece and sealed at the other end to the surface of the heat pipe or glass tube. The inner end of the heat pipe is supported on a suitable support and further supports may optionally be provided along the length of the heat pipe. The exposed end of the heat pipe for insertion into the heat exchanger is preferably provided with a heat radiating coating and may optionally include fins.The heat pipe may be constructed of metal, e.g. copper, or glass.

The invention will now be illustrated with reference to the accompanying drawings, in which: Figure 1 represents a transverse cross-section through a collector tube in accordance with the invention, Figure 2 represents a longitudinal section through a collector in accordance with the invention, and Figure 3 represents a longitudinal section through a second type of collector in accordance with the invention.

Referring to Figure 1 , the collector comprises an outer glass tube 2 within which is positioned a compound parabolic reflector of polished aluminium 4 and a conduit 6, which may be a tube or a heat pipe, positioned at the focus of the reflector.

The reflector 4 is an optimized non-imaging concentrator of the compound parabolic type designed in accordance with the principles of United States Patent Specification Nos. 4 002 499 and 4 003 638. This designflconcentrates radiant energy incident in the angular range O1SOSS2 onto the conduit 6. The shadow lines FD and GB define the angular range. Section CD is an involute of the circular cross-section of the conduit. Section DE is designed to have substantially maximum slope for reflecting radiation incident at angle O onto the conduit 6.

Similarly section BC is an involute of the circular cross-section of the conduit and section BA has substantially maximum slope for reflecting radiation incident at angle 62 onto the conduit 6.

The reflector is truncated at points A and E to fit inside the glass tube 2. Both symmetric and asymmetric configurations are contemplated so that O may differ from 62. The configuration shown in Figure 1 has 0 = 62 = 350 which is a practical value for stationary solar collector. It attains a concentration ratio C = 1.5 which is adequate for efficient operation at elevated temperature.

For mechanical and thermal reasons, it may be desirable to interpose a gap between receiver and reflector at point C. In the vicinity of the gap, one may trap radiation with a cavity or design the involute portion from a virtual receiver. For example, the virtual receiver may be comprised of the minimal convex surface encompassing the actual receiver and the closest point of approach of the reflecting wall to the receiver.

In the embodiment of the invention illustrated in Figure 2 the conduit 6 is in the form of a heat pipe. The heat pipe comprises a sealed tube which is partially evacuated and contains a small quantity of liquid, e.g. water, and a wick 40, e.g.

copper gauze, extending throughout its length held firmly in place against the inner surface of the tube. The inner end 42 of the tube is supported within the outer glass tube 2 by means of a support 44. The support 44 preferably comprises a heat insulating material to prevent heat losses by conduction. The end 45 of the outer glass tube is sealed, e.g. with the provision of an end cap.

The other end 46 of the heat pipe extends beyond the outer tube 2 into a heat exchanger 47.

The outer glass tube 2 is sealed to the exterior of the inner glass tube 6 by means of a weld 48. The heat pipe is straight throughout its length to ensure maximum efficiency and therefore weld 48 is not central on the end of the outer tube.

The heat exchanger 47 may conveniently comprise a tube, shown in cross-section in Figure 2, through which is conveyed a heat-absorbing liquid. A series of collectors may be arranged with the heat pipes extending into a tubular heat exchanger. Heat-absorbing fluid may be passed into the heat exchanger from both ends of the tube and may be withdrawn from an intermediate point from the tube to ensure maximum transfer of heat to the heat-absorbing fluid. The heat exchanger 47 is preferably lagged with insulating material 49. The point of entry of the heat pipe into the heat exchanger 47 is sealed with a heat and liquid resistant gasket 50.

The heat pipe 6 may comprise a glass tube, a metal tube, or may be formed of lengths of both glass and metal tubes. It is preferable that at least the portion of the heat pipe within the outer glass tube is made of glass so that the weld 48 may be a glass to glass weld which is considerably easier to effect than a glass to metal weld. The portion of the heat pipe within the outer glass tube is preferably provided with a selective coating in order to absorb the maximum amount of radiation. The portion of heat pipe within the heat exchanger 47 is preferably provided with a heat radiating coating. If the heat pipe is made of glass, a metal sleeve may be inserted over the end of the heat pipe within the heat exchanger to facilitate transfer of heat. The end of the heat pipe within the heat exchanger may be provided with fins, several of which are shown at 52, in order to ensure a rapid transfer of heat. Whilst an exposed portion of heat pipe is shown between the weld 48 and the gasket 50 in the interests of clarity, this exposed portion is kept to a minimum in practice and if necessary is insulated to prevent heat loss.

In the embodiment shown in Figure 3 the conduit 6 is in the form of a glass tube, one end of which 42 is closed and supported within the outer glass tube 2 by a support 44 and the other end is sealed to the outer glass tube 2 by means of a weld 48. A metal heat pipe 39 is accommodated within the glass tube 6 and extends beyond the end thereof into a heat exchanger of the type described with reference to Figure 2.

Claims

1-. A collector for solar energy comprising an outer glass tube having therein a compound parabolic reflector extending substantially along the length of said tube and a straight conduit extending along the focus of said reflector, the ends of the outer glass tube being sealed and the space within the sealed outer glass tube being evacuated.

2. A collector as claimed in Claim 1 in which the conduit comprises an open-ended tube the ends of which extend beyond the outer glass tube and are adapted to be connected into a circuit for circulating heat-absorbing fluid.

3. A collector as claimed in Claim 1 in which the conduit comprises a heat pipe having one end supported within the outer glass tube and a second end extending beyond the outer glass tube.

4. A collector as claimed in Claim 1 in which the conduit is an open-ended tube having a heat pipe accommodated therein and extending beyond one end thereof.

5. A collector as claimed in any preceding claim in which the outer surface of the portion of the conduit within the outer glass tube is provided with a selective coating to improve heat absorption.

6. A collector as claimed in any one of claims 3 to 5 in which the end of the heat pipe extending beyond the outer glass tube is provided with a heat radiating coating.

7. A collector as claimed in any one of claims 3 to 6 in which the end of the heat pipe extending beyond the outer glass tube is provided with fins.

8. A collector as claimed in any one of claims 3 to 7 in which at least the end of the heat pipe extending beyond the outer glass tube is constructed of metal.

9. A collector for solar energy substantially as herein described with reference to Figures 1 to 3.

10. Apparatus comprising a plurality of collectors as claimed in any one of claims 3 to 9 in side by side relationship, in which the ends of the heat pipes extending beyond the outer glass tubes are positioned within a heat exchanger element through which cooling fluid is conveyed.