US20020073987A1

US20020073987A1 - Solar energy collector assembly

Info

Publication number: US20020073987A1
Application number: US09/780,255
Authority: US
Inventors: Heiji Fukutake; Hitoshi Yano; Takeo Saitoh
Original assignee: Exedy Corp
Current assignee: Exedy Corp
Priority date: 2000-02-10
Filing date: 2001-02-09
Publication date: 2002-06-20

Abstract

In a solar energy collector assembly comprising a tubular heat collector member and a reflector extending along a length of the tubular heat collector member, the reflector consists of a modified compound parabolic concentrator having a bottom part defined by a linear segment in cross section. Thus, the shape of the reflector is significantly simplified without substantially impairing the efficiency in the collection of heat.

Description

TECHNICAL FIELD

The present invention relates to a solar energy collector assembly, and in particular to an assembly comprising at least one solar collector module which includes a tubular solar collector member defining a fluid passage for conducting fluid such as water for absorbing solar energy and a reflector for converging the incident solar energy radiation onto the solar collector member.

BACKGROUND OF THE INVENTION

Conventionally, solar energy collector assemblies consisting of a tubular solar collector member and a reflector for converging the solar radiation energy onto the solar collector member are known, typically for heating utility water. Thermal efficiency is an important factor in designing a solar energy collector assembly, and various efforts have been made to increase the efficiency.

The reflector called as the compound parabolic concentrator (CPC) is known to provide a high efficiency for such purposes. The CPC is formed by combining a pair of parabolic segments which are angled relative to each other by a certain angle, and an involute curve joining them. For the details of the CPC, the reference should be made to U.S. Pat. No. 4,002,499 issued Jan. 11, 1977 to R. Winston.

The reflector is typically formed by stamp forming sheet metal having a surface of a high reflectivity, but the CPC requires the reflector surface to have a complex contour including a cusp in the middle part thereof. Therefore, it is difficult to form the reflector surface as designed, and any deviation from the design has been known to lead to a reduction in the efficiency. Also, because the reflector is typically made of thin sheet metal to reduce cost and weight, a certain measure is required to maintain the surface contour as designed, and ribs are typically used for this purpose. The ribs are likewise required to have a complex contour.

Another important consideration in the design of a solar energy collector assembly is to achieve favorable heat transfer between the heat collector member and the fluid therein such as water for carrying the heat. The fluid has a certain viscosity, and has a tendency to heat up rapidly in the part of the internal passage adjacent to the heat collector member, but remain cold in the middle part of the passage remote from the surrounding wall or the surface of the heat collector member. Because of the relatively flat temperature gradient in the interior of the passage, the heat generated in the heat collector member would not be transferred to the internal fluid as favorably as one would expect. Such a problem can be mitigated by increasing the flow speed of the fluid and/or creating turbulences in the fluid by using suitable means. However, in either case, forcing the flow of the fluid requires a pump of a corresponding capacity and an additional power consumption. Also, the resulting increase in the internal pressure requires an increased mechanical strength of the heat collector member.

BRIEF SUMMARY OF THE INVENTION

In view of such problems of the prior art, a primary object of the present invention is to provide a solar energy collector assembly which has a high thermal efficiency.

A second object of the present invention is to provide a solar energy collector assembly which is easy and economical to manufacture.

According to the present invention, such objects can be accomplished by providing a solar energy collector assembly, comprising a tubular heat collector member defining an internal passage for fluid, and a reflector extending along a length of the tubular heat collector member, the reflector consisting of a modified compound parabolic concentrator having a bottom part defined by a linear segment in cross section. Thus, the shape of the reflector is significantly simplified without substantially impairing the efficiency in the collection of heat.

If the linear segment consists of a line joining a pair of compound parabolic concentrator segments in a bottom part of the reflector, the shape of the reflector is highly simplified. If the linear segment consists of a pair of lines joining a pair of compound parabolic concentrator segments in a bottom part of the reflector so as to linearly approximate a complete compound parabolic concentrator segment, the simplicity can be attained without substantially affecting the efficiency in collecting heat.

For a favorable heat transfer, a steep temperature gradient is necessary. If the tubular heat collector member comprises a double-walled tubular member defining an annular internal passage, the flow speed is increased for a given outer profile of the heat collector member, and the rapid replacement of the fluid ensures a steep temperature gradient, and hence a favorable heat transfer. Also, because the entire part of the fluid is located in close proximity to the outer wall of the heat collector member, and this also promotes a favorable heat conduction.

Similar results can be achieved by using slightly different arrangements. For instance, the tubular heat collector member may be provided with an irregular cross section which is deformed from a circular cross section so that the fabrication of the heat collector member may be simplified.

For structural and thermal advantages, a fin may be provided so as to at least partly extend along the length of the tubular heat collector member and to thereby support the tubular heat collector member relative to the reflector. The fin itself also provides a surface for absorbing the radiation energy reflected by the reflector. For optimum thermal efficiency, the fin may be located at least partly in an area which is to be nominally occupied by the tubular heat collector member in association with the CPC.

According to a preferred embodiment, the heat collector member comprises a pair of tubular members extending in a vertically spaced and mutually parallel relationship along the length of the reflector. These tubular members may be retained to the reflector by using a fin which extends along the length of the tubular heat collector member. This arrangement provides the advantage in mechanical stability and thermal efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

Now the present invention is described in the following with reference to the appended drawings, in which: [0014]
FIG. 1 is a perspective view of a solar energy collector assembly embodying the present invention; [0015]
FIG. 2 is a sectional view of a solar energy collector module; [0016]
FIG. 3 is a view similar to FIG. 2 showing a second embodiment of the present invention; [0017]
FIG. 4 is a graph comparing the efficiency of the first and second embodiments of the present invention with a comparable conventional arrangement using a complete CPC; [0018]
FIG. 5 is a view similar to FIG. 2 showing a third embodiment of the present invention; [0019]
FIG. 6 is a partly broken away fragmentary perspective view of a part of the third embodiment of the present invention; [0020]
FIG. 7 is a view similar to FIG. 2 showing a fourth embodiment of the present invention: [0021]
FIG. 8 is a sectional side view of the fourth embodiment of the present invention; [0022]
FIG. 9 is a view similar to FIG. 2 showing a fifth embodiment of the present invention; [0023]
FIG. 10 is a view similar to FIG. 2 showing a sixth embodiment of the present invention; [0024]
FIG. 11 is a view similar to FIG. 2 showing a seventh embodiment of the present invention; and [0025]
FIGS. 12[0026] a to 12 c are sectional views of variously modified heat collector members according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates an overall view of a solar energy collector assembly embodying the present invention. This solar energy collector assembly comprises a [0027] casing 1 consisting of a shallow box having an open top end, and a plurality of heat collection modules 2. Each heat collection module 2 comprises a tubular heat collector member 3 having a circular cross section and a reflector 4 for concentrating solar heat onto the heat collector member 3. The heat collector members 3 each extend linearly, and are arranged in parallel with one another. Each heat collector member 3 is connected to the adjacent heat collector members via a U-shaped tubular member 3 a at each end so that the heat collector members are connected in series as a whole, and fluid such as water which is introduced from an end of the first heat collector member passes through all of the heat collector members before it flows out of the last heat collector member. Typically, the water from a cold water source is heated as it is conducted through the series of heat collector members 3, and is finally released from the last heat collector member to a hot water tank or directly to user devices such as room heaters and bath tubs. The open top end of the casing 1 may be closed by a pane or a plurality of panes of glass 5, and the casing 1 itself may be appropriately insulated so as to minimize heat loss. If desired, thermal insulation can be improved by using a double glass pane or other glass panes providing a high level of thermal insulation.
Referring also to FIG. 2, the [0028] reflector 4 of each heat collection module 2 consists of a trough shaped sheet metal which is provided with a mirror surface on a concave surface thereof. As shown in the sectional view of FIG. 2, the reflector 4 consists of a modified compound parabolic surface which smoothly connects a substantially planar segment 4 b provided in a bottom part of the mirror surface, and a pair of compound parabolic surface segments 4 a provided on either side of the middle planar segment 4 b. The reflector 4 is provided with a plurality of ribs 7 made of plate members having an edge conforming to the prescribed profile of the reflector 4 and arranged perpendicularly along the axial line of the reflector 4 at a regular interval. The compound parabolic surface as used herein means a combination of a pair of symmetrically shaped and angled parabolic lines joined by involute curves. Such a mirror surface is known to be able to converge the solar heat received from the opening of the reflector onto a heat collector module placed near the bottom surface of the reflector in a highly efficient manner. Normally, a cusp is generated in the region where the two compound parabolic segments meet in the middle as indicated by imaginary lines t in FIG. 2.
According to simulations conducted by the inventors, a reflector consisting of a pure compound parabolic reflection surface failed to converge about 2.5% of the solar light received from the opening of the reflector onto the heat collector member. The reflector of the present embodiment failed to converge about 10% of the received light onto the heat collector member (FIG. 4). There is a difference of about 7.5%, but this is an insignificant difference in view of the inevitable inaccuracy in achieving a compound parabolic surface in the practical situation and the resulting loss in the collection of the solar heat. [0029]
FIG. 3 shows a second embodiment of the present invention, and the parts corresponding to the previous embodiment are denoted with like numerals. This embodiment is different from the previous embodiment only in the structure of the reflector, and the parts corresponding to those of the previous embodiment are denoted with like numerals. In this case, the [0030] reflector 14 includes a roof-shaped middle segment 14 b which is defined by a pair of symmetric oblique lines meeting at the middle point, and a pair of compound parabolic surface segments 14 a provided on either side of the roof-shaped middle segment 14 b. In other words, the middle segment 14 b is a linear approximation of the cusp which is required in the genuine compound parabolic surface.
According to the simulations conducted by the inventors, it was found that this approximate compound parabolic surface is capable of converging the light received from the opening onto the heat collector member substantially as efficiently as a pure compound parabolic surface (FIG. 4). [0031]
FIGS. 5 and 6 show a third embodiment of the present invention. The reflector of this embodiment is similar to that of the first embodiment illustrated in FIG. 2. The [0032] heat collector member 3 in this embodiment consists of a simple tube having a relatively small diameter which is held in position by a holder 6 in the form of a fin. The holder 6 comprises a pair of clamping members 6 a which are made of sheet metal and extend substantially over the entire axial length of the reflector 4. The free ends of the clamp members 6 a jointly define a receptacle into which the heat collector member 3 can be snap fitted and resiliently retained. The base end of the clamp members 6 a are each provided with a plurality of tabs 6 b at a regular interval along the length thereof which can be passed each into a slot provided in the bottom portion of the reflector 4. The tabs 6 b are curved and provided with a suitable resiliency so that the clamp members 6 a along with the heat collector member 3 can be firmly retained by the reflector 4. The holder 6 supports the heat collector member 3 against bending deformation, and eliminates the need for the heat collector member 3 to be able to withstand a bonding deformation by itself. The holder 6 also contributes to a stable and accurate positioning of the heat collector member 3 which in turn contributes to a high thermal efficiency.
Because the [0033] heat collector member 3 consists of a tubular member having a relatively small diameter, the temperature difference of the fluid flowing through the heat collector member 3 between the central part and peripheral part is relatively small, and an efficient heat transfer from the heat collector member 3 to the fluid in the internal passage can be ensured. The use of a tubular member having a smaller diameter contributes to the reduction in the material cost. Furthermore, the small heat collector member 3 is located in an upper end of an area which would be occupied by a normal heat collector member for a compound parabolic concentrator, and the holder in the form of a fin extends to the lower end of this area. Therefore, the solar heat which converges upon the holder as well as the heat collector member itself can be effectively used for heating the fluid flowing inside the heat collector member. Therefore, according to this embodiment, even though the heat collector member 3 has a relatively small diameter, a thermal efficiency comparable to that obtained by using a heat collector member of a normal size can be achieved.
FIGS. 7 and 8 show a fourth embodiment of the present invention, and the parts corresponding to the previous embodiments are denoted with like numerals. This embodiment is similar to the previous embodiment except for that the shape of the holder is different, and a pair of [0034] heat collector members 13 a and 13 b are used instead of one. The upper heat collector member 13 a is located in an upper end of an area which would be occupied by a normal heat collector member, and the lower heat collector member 13 b is located in a lower end of the area which would be occupied by a normal heat collector member. The holder 16 comprises a pair of clamp members 16 a which are adapted to retain the two heat collector members 13 a and 13 b in a vertically spaced and mutually parallel relationship. The lower parts of the clamp members 16 a are provided with tabs 16 b similar to those of the previous embodiment to attach the holder 16 along with the heat collector members 13 a and 13 b to the reflector 4.
According to this embodiment, one ends of the paired [0035] heat collector members 13 a and 13 b are joined by a U-shaped tubular member 13 c, and the heat collector modules can be connected in series along one side of a group of similar heat collector modules, as opposed to the previous embodiments in which the adjacent heat collector modules are connected along two lateral sides in an alternate fashion. This contributes to a compact design of the solar energy collector assembly, and a simplicity of the assembly work.
FIG. 9 shows a fifth embodiment of the present invention which is similar to the third embodiment. The parts corresponding to those of the previous embodiments are denoted with like numerals. In this embodiment, the [0036] holder 26 comprises a fin 26 a formed by bending a single plate member so as to wrap around the heat collector member 23, and tabs 26 b provided in a lower part thereof. This provides the advantage of requiring fewer component parts. The structures of the fin 26 a and tabs 26 b are similar to those of the previous embodiment. In particular, the holder 26 generally presents a triangular cross section having a side facing away from the reflector and a pair of symmetric sides which face the reflector at a certain angle.
FIG. 10 shows a sixth embodiment of the present invention, and the parts corresponding to those of the previous embodiments are denoted with like numerals. The heat collector member [0037] 33 in this embodiment consists of a double-walled tubular member including an inner tube 33 a and an outer tube 33 b coaxially surrounding the inner tube 33 a, and an annular passage 8 for the fluid (water) is defined between the inner and outer tubes 33 a and 33 b. The two ends of the inner tube are closed in a water-tight manner (not shown in the drawing). The interior of the inner tube 23 a may be either vacant or filled with heat insulating material. According to this embodiment, the heat transfer is improved because a larger part of the fluid is located near the wall of the outer tube 33 b. Also, the flow speed of the fluid is increased because the cross sectional area of the passage is smaller for a given outer profile of the heat collector member, and this contributes to a favorable heat transfer. The reflector 4 of this embodiment consists of a normal CPC, but may also consist of any other reflector including a simple parabolic reflector, a circular reflector, and a modified CPC such as those described above. The same is true with any of the embodiments illustrated from FIGS. 5 to 12.
FIG. 11 shows a seventh embodiment of the present invention, and the parts corresponding to those of the previous embodiments are denoted with like numerals. The [0038] heat collector member 43 in this embodiment consists of a tube having an irregular cross section. In this case, the tube originally consisted of a circular tube as indicated by an imaginary line, and two 120-degree segments of the outer wall thereof symmetrically facing the reflector 4 are inwardly pressed, and generally presents a fan shaped cross section. This reduces the cross sectional area of the heat collector member 43 while maintaining a substantially same surface area. Therefore, this embodiment provides similar advantages as those of the sixth embodiment. The outer profile of the heat collector member 43 is also not significantly changed so that there is no significant reduction in the amount of solar radiation which the heat collector member receives from the reflector.
FIG. 12[0039] a shows a modification from the embodiment illustrated in FIG. 11. The remaining segment of the outer wall of the heat collector member 42 facing away from the reflector is also pressed inward so as to have a generally Y-shaped cross section. This also provides similar advantages as those of the embodiment illustrated in FIG. 11. FIGS. 12b and 12 c show additional modifications from the embodiment illustrated in FIG. 11. The tube originally having a tubular cross section is slightly collapses so as to have an elliptic cross section which may be compressed either longitudinally (FIG. 12b) or laterally (FIG. 12c) as viewed from the bottom of the reflector. The elliptic shape as used herein may include any slightly collapsed shape including racetrack, triangular, lozenge, and other similar shapes.
Although the present invention has been described in terms of preferred embodiments thereof, it is obvious to a person skilled in the art that various alterations and modifications are possible without departing from the scope of the present invention which is set forth in the appended claims. [0040]

Claims

1. A solar energy collector assembly, comprising a tubular heat collector member defining an internal passage for fluid, and a reflector extending along a length of the tubular heat collector member, the reflector consisting of a modified compound parabolic concentrator having a bottom part defined by a linear segment in cross section.

2. A solar energy collector assembly according to claim 1, wherein the linear segment consists of a line joining a pair of compound parabolic concentrator segments in a bottom part of the reflector.

3. A solar energy collector assembly according to claim 1, wherein the linear segment consists of a pair of lines joining a pair of compound parabolic concentrator segments in a bottom part of the reflector so as to linearly approximate a complete compound parabolic concentrator segment.

4. A solar energy collector assembly according to claim 1, wherein the tubular heat collector member comprises a double-walled tubular member defining an annular internal passage.

5. A solar energy collector assembly according to claim 1, wherein the tubular heat collector member is provided with an irregular cross section which is deformed from a circular cross section.

6. A solar energy collector assembly according to claim 1, further comprising a fin which at least partly extends along the length of the tubular heat collector member to support the tubular heat collector member relative to the reflector.

7. A solar energy collector assembly according to claim 6, wherein the fin is located at least partly in an area which is to be nominally occupied by the tubular heat collector member.

8. A solar energy collector assembly according to claim 6, wherein the heat collector member comprises a pair of tubular members extending in a vertically spaced and mutually parallel relationship along the length of the reflector, the tubular members being retained to the reflector by using a fin which at least partly extends along the length of the tubular heat collector member.