WO2011089187A2

WO2011089187A2 - Solar collection system

Info

Publication number: WO2011089187A2
Application number: PCT/EP2011/050766
Authority: WO
Inventors: Gabriel Cohen
Original assignee: Siemens Concentrated Solar Power Ltd.; Siemens Aktiengesellschaft
Priority date: 2010-01-21
Filing date: 2011-01-20
Publication date: 2011-07-28
Also published as: WO2011089187A3

Abstract

A solar collection system configured for use with a solar thermal power plant is provided. The solar collection system is designed to facilitate capture of thermal energy of incident solar radiation by a heat transfer fluid (HTF) flowing therethrough and comprises a longitudinally extending concentrator (22) designed to reflect at least a portion of the incident solar radiation toward a focus line thereof, a heat collecting element (20) (HCE) coincident with the focus line (22a) and comprising a tube carrying the HTF, and a transparent enclosure surrounding the tube, and a light rectifier (32) located at an entrance of the concentrator (22), the light rectifier (32) being configured to increase the elevation angle of the impinging solar radiation passing therethrough.

Description

SOLAR COLLECTION SYSTEM FIELD OF THE INVENTION

This invention relates to solar thermal power plants, and in particular, to solar collection system designed for use with such plants to collect solar energy therefor.

BACKGROUND OF THE INVENTION

Amid concerns over global warming, and forecasts of both the depletion of non-renewable energy sources and rising power demand, suppliers of energy are increasingly seeking alternative primary sources of energy. One such source of energy is solar energy, and one way of utilizing solar energy is with a solar thermal power plant.

One type of solar power plant utilizes a "radiation concentrator collector" which concentrates the solar

radiation by focusing it onto a smaller area, e.g., using mirrored surfaces or lenses. In this system, a reflector, which is typically parabolic, receives and reflects (focuses) incoming solar radiation onto a radiation absorber, which is formed as a tube. The tube radiation absorber is

concentrically surrounded by a treated glass enclosure tube to limit the loss of heat. The collector system further includes means to track the sun.

To minimize the loss of heat through convection and conduction and to improve the solar radiation collection efficiency, the space between the tube radiation absorber and the glass enclosure tube is evacuated to very low pressure.

The tube radiation absorber is made of metal with a coating having a high solar radiation absorption coefficient to maximize the energy transfer imparted by the solar

radiation reflecting off the reflector. A heat transfer fluid constituting a heat transport medium, which is typically a liquid such as oil, flows within the tube radiation absorber.

The thermal energy transported by the thermal fluid is then is used to power a steam-electric power plant to drive one or more turbines thereof, in order to generate

electricity in a conventional way, e.g., by coupling the axle of each of the turbines to an electric generator.

SUMMARY OF THE INVENTION

According to one aspect of the presently disclosed subject matter, there is provided a solar collection system configured for use with a solar thermal power plant, the solar collection system being designed to facilitate capture of thermal energy of incident solar radiation by a heat transfer fluid (HTF) flowing therethrough and comprising:

• a longitudinally extending concentrator designed to reflect at least a portion of the incident solar radiation toward a focus line thereof;

• a heat collecting element (HCE) coincident with the focus line and comprising a tube carrying the HTF, and a transparent enclosure surrounding the tube; and

• a light rectifier located at an entrance of the

concentrator, the light rectifier being configured to increase the elevation angle of the impinging solar radiation passing therethrough.

It will be appreciated that the increase of the

elevation angle is with respect to the direction from which the solar radiation impinges. Thus, in a case wherein the elevation angle of the sun is increased to as to exceed 90°, it is still considered, for the purposes of the present disclosure, to be an increase in elevation angle, even though the magnitude of the elevation of the solar radiation having passed through the light rectifier is less than that of the solar radiation impinging on the light rectifier.

The concentrator may be configured to track the sun along a single tracking axis, the light rectifier being configured to accomplish the increase by refracting, i.e., bending, the solar radiation within a plane perpendicular to the tracking axis.

The light rectifier may comprise a plurality of prisms, which may be made of Polymethyl methacrylate (PMMA) or glass, each being configured to refract incident solar radiation, thereby increasing its elevation angle. Surfaces of the light rectifiers, especially those configured for allowing light to pass therethrough into and out of the light rectifier, may comprise an anti-reflective coating.

The prisms may each have a cross-section shaped as a right triangle and be arranged with an entrance aperture thereof generally facing the incident solar radiation, and an exit aperture generally facing the concentrator. It will be appreciated that the term "generally facing" as used herein describes a disposition wherein an element is turned toward a particular direction or object, but is not necessarily limited to facing it directly. A first cathetus of the right triangle of each prism may constitute the entrance aperture thereof, and a second cathetus of the right triangle of each prism may be disposed so as to be closer to the direction from which the solar radiation comes. The first cathetus may be the longer of the two catheti of the right triangle.

Alternatively, the triangle may be a right isosceles

triangle .

Each of the prisms may be associated with a critical range, wherein incident light impinging on the entrance aperture within the critical range is totally internally reflected within the prism by the exit aperture, the prisms being oriented such that direct solar radiation impinges outside the critical range.

The prisms may extend parallelly to the tracking axis. The concentrator may be configured to track the sun by pivoting about a pivot axis perpendicular to the tracking axis .

The prisms are formed as a single body. The body may comprise a plate with the prisms being formed on one surface thereof, i.e., the prisms may project from one surface of the plate.

The transparent enclosure of the HCE may be made of glass .

The tube and transparent enclosure of the HCE may be arranged as concentric cylinders.

The space between the tube and enclosure of the HCE may be evacuated.

The HTF may be selected from the group consisting of thermal oil, steam/water, molten salts, carbon dioxide, and helium.

The concentrator may comprise a reflecting surface having a parabolic cross-section.

According to another aspect of the presently disclosed subject matter, there is provided a solar thermal power plant comprising a thermal-electric power plant and a solar collection system as described above, in communication therewith to provide heat thereto for driving its operation.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting examples only, with

reference to the accompanying drawings, in which:

Fig. 1 is a schematic illustration of a solar thermal power plant according to the present disclosure; Fig. 2 is schematic perspective view of a solar collector of the solar thermal power plant illustrated in Fig. 1 ;

Fig. 3 is a schematic perspective view of a heat collecting element of the solar collector illustrated in Fig. 2; Fig. 4 is a partial cross-sectional view of the enclosure of the heat collecting element, taken along line VI-VI in

Fig. 3;

Fig. 5 is a graph illustrating incident angle losses at

various incident angles;

Fig. 6 illustrates solar radiation passing through a light rectifier of the solar collector illustrated in Fig. 2 ; Fig. 7 illustrates an example of a light rectifier of the

solar collector illustrated in Fig. 2 ;

Fig. 8 illustrates an example of incident solar radiation

having a high elevation angle passing through a light rectifier of the solar collector illustrated in Fig. 2 ; and

Fig. 9 is a schematic side view of a modification of the

solar collector illustrated in Fig. 2.

DETAILED DESCRIPTION OF EMBODIMENTS

As illustrated schematically in Fig. 1, there is

provided a solar thermal power plant, which is generally indicated at 10. The plant 10 comprises a solar collection system 12 and a steam-electric power plant 14. The plant further comprises a heating circuit 16.

The solar collection system 12 is configured to utilize impinging solar radiation to heat a heat transfer fluid

(HTF) . The heated HTF is carried, via the heating circuit 16, to the steam-electric power plant 14, wherein the heated HTF is utilized create steam to drive a turbine thereof, thereby produce electricity. Such systems are known in the art, and are provided, inter alia, by Siemens Concentrated Solar

Power, Ltd. The solar collection system 12 comprises one or more solar collectors 18 (only one of which is schematically illustrated in Fig. 1) each comprising an HCE 20 (which constitutes a portion of the heating circuit 16) and one or more longitudinally extending (i.e., elongate) concentrators 22. The HCEs 20 carry the HTF, which may be a thermal fluid such as oil (phenyls) which are commercially available, such as under the trade name Therminol® VP-1. Alternatively, the HTF may be one of steam/water, molten salts, carbon dioxide, and helium. The thermal fluid, according to any of the embodiments, is heated within the HCE 20 upon its exposure to solar radiation.

As illustrated in Fig. 2, the concentrators 22 each have a parabolic cross-section, defining an entrance area 24

between ends 26 thereof. The inner, (HCE-facing) surface 22a of each concentrator is highly reflective, and may be formed or provided with mirrors for this purpose. It is codisposed with the HCE 20 such that the HCE is located at and extends along its focal line. Solar radiation entering the

concentrator 22 perpendicularly thereto is reflected toward the HCE 20.

In order to maintain the proper orientation of impinging solar radiation to the concentrator 22, so that it will be properly reflected toward the HCE 20, tracking means (not illustrated) are provided, configured to pivot the

concentrator about an axis which is parallel to the HCE

(i.e., parallel to the focal line of the concentrator). Thus, the concentrators are considered to track the sun along a tracking axis T which is perpendicular to the HCE/focal line of the concentrator. The tracking is configured with the objective of maintaining each rays of impinging solar

radiation within a plane (one of which is schematically illustrated at P) until impinging upon the concentrator 22, and which are perpendicular to the tracking axis T , i.e., they contain the focal line of the concentrator and the sun.

As illustrated in Fig. 3, the HCE 20 comprises a

transparent enclosure 28, which may be made of, e.g., glass, surrounding a tube 30 carrying the HTF. It is these tubes in which the HTF is heated as it flows through the HCE 20. The enclosure 28 and tube 30 may be formed as concentrically arranged cylinders. The space between the enclosure 28 and the tube 30 is evacuated, thus thermally insulating the tube from the surrounding atmosphere. The tube may be provided with one or more coatings designed to increase the absorption or solar radiation and limit the amount of heat radiated thereby .

As illustrated in Fig. 4, solar radiation which impinges on the enclosure 28 passes therethrough, refracting at each surface thereof (at a 90° angle of incidence, the angle of refraction is 0°), and passing therethrough as represented by line A. At each surface, some of the radiation is reflected away and thus lost, as represented by lines A' . This

reflected radiation is not recovered, and results in less energy of the incident solar radiation being used to generate useful energy (i.e., by heating the HTF to a lower

temperature, which leads to a lower amount of electricity being produced by the solar thermal power plant 10) . The amount of radiation which is reflected is associated with the angle of incidence of the radiation with the enclosure.

Specifically, the greater the deviation from a 90° angle of incidence, the greater these losses will be.

Similarly, some light is lost when the solar radiation reflects off of the mirrored inner surface 22a of the

concentrator 22. The amount of losses associated with the mirrored inner surface 22a increases as well with a greater angle of deviation from a 90° angle of incidence from the cross-section thereof. Both of these losses (referred to as incidence angle losses) can together be quantified as the incidence angle modifier (IAM) , which depends on physical properties of the concentrator 22 and the enclosure 28, including, but not limited to, dimensions, materials, etc. In addition, it decreases with the elevation angle of the solar radiation impinging upon the various elements of the solar collector 18 (although it does not necessarily decrease linearly with the elevation angle) . The amount of solar radiation flux

impinging upon the tube 30 of the HCE 20 (and consequently the amount of electricity ultimately produced by the solar thermal power plant 10) taking into account these losses (compared to the amount which would impinge if these losses would not be present) is related to the IAM (i.e., a lower IAM is associated with a lower amount of solar radiation flux impinging upon the tube of the HCE) .

As seen in Fig. 5, at very high incident angles (and thus, as the incident and elevation angles are complementary, low elevation angles) of the sun, which are typical in winter months, the incidence angle losses, and thus the decrease in the amount of energy produced by the solar thermal power plant 10, is significant.

In order to mitigate the incidence angle losses, the solar collectors 18 may be provided with means designed to increase the elevation angle (i.e., decrease the declination angle) of impinging solar radiation.

Reverting to Fig. 2, some or all of the solar collectors 18 may comprise a light rectifier 32 located above and parallel to the entrance area 24 of the concentrators 22 thereof. As illustrated in Fig. 6, incident solar radiation (indicated at A) impinging on the light rectifier 32 at an elevation angle ψι, passes therethrough and leaves it at an increased elevation angle <p_∑. It will be appreciated that such an increase in the elevation angle will decrease the incidence angle losses. The increase in the elevation angle of the solar radiation may occur within a single plane, i.e., the light rectifier 32 may be designed such that each ray of solar radiation is maintained within a single same plane perpendicular to the tracking axis T (for example, plane P illustrated in and described with reference to Fig. 2 above) before impinging thereon and after passing therethrough.

As illustrated in Fig. 7, the light rectifier 32 may comprise a plurality of prisms 34, extending parallelly to the tracking axis T, optionally joined together by and projecting from one surface of a plate 36. It will be

appreciated that while the components of the light rectifier 32 are separated by dotted lines in Fig. 7, typically these components are formed as a single, monolithic element, with no optical boundaries therebetween. Thus, the prisms 34 may not be standalone elements, but constitute an integral part of a larger element. For illustrative purposes, a ray of solar radiation is represented in Fig. 7 at A.

The prisms 34 each comprise an entrance aperture 38 for receiving incident solar radiation, and an exit aperture 40, via which the solar radiation leaves the prism and continues on toward the concentrator 22. The prisms may be made of any suitable material, for example glass or PMMA.

The prisms 34 may be right prisms (isosceles or

scalene) , and be oriented such that one of the catheti thereof, which, in the case of an isosceles right triangle, may be the longer or shorter cathetus, constitutes the entrance aperture 38, and the hypotenuse thereof constitutes the exit aperture 40. In such a case, and as illustrated in Fig. 7, the exit aperture 40 faces the direction opposite that from which impinging solar radiation arrives (i.e., it generally faces away from the sun) .

Alternatively, the prisms may be non-right prisms, for example isosceles, equilateral, or scalene and be oriented such that any suitable side thereof constitutes the entrance aperture 38 and any other suitable side thereof constitutes the exit aperture 40.

In any event, the geometries of the prisms 34, including their shape, internal angles, and orientation thereof, is determine by the designer, taking into account, inter alia, the elevation angles of the sun over the course of the year, as well as optical properties of the prisms and the other components of the solar collectors 18 which impact the path of solar radiation toward the tube 30 of the HCE 20.

As illustrated in Fig. 8, a light rectifier 32 which refracts impinging solar radiation impinging during the winter months so as to bring its orientation closer to the vertical and thus decrease the incidence angle losses at this time (thereby leading to an increase in the amount of solar radiation flux impinging upon the tube 30 of the HCE 20) may "over-refract" solar radiation during the summer months, i.e., refract it such that it leaves the light rectifier (and thus impinges on the concentrator 22) with an elevation angle ψ₂ which is smaller than that of the un-refracted solar radiation, albeit angled away from the direction from which the solar radiation impinges.

The phenomenon described above with reference to Fig. 8 may be addressed by the designer considering and designing to decrease the overall incidence angle losses over the course of the year. Alternatively, the solar thermal power plant 10 may be provided with several light rectifiers 32 of different designs, each one being configured to refract impinging solar radiation at a different angle. For example, a "summer" light rectifier may be designed to refract incoming solar radiation to a lower degree than a "winter" light rectifier would. (It will be appreciated that a solar thermal power plant may comprise an suitable number of different types of light rectifiers which may each be installed/removed at any point during the year.) Alternatively, one type of light rectifier may be provided which is removed from the solar collectors 18 during the summer months when it would serve to lower the amount of solar radiation flux impinging upon the tube 30 of the HCE 20.

Alternatively, as illustrated in Fig. 9, several light rectifiers 32 may be provided along the length of a

concentrator 22. Pivoting means (not illustrated) may be provided to tilt the light rectifiers 32, as indicated by broken lines at 32', about an axis, which is indicated at 42, and which is parallel to the length of the prisms 34. This arrangement permits altering the relative angle between the various surfaces of the light rectifier 32 and impinging solar radiation, thereby permitting regulation of the effect that the light rectifier has on the solar radiation. A controller (not illustrated) may be provided to direct the pivoting means to bring the light rectifiers 32 to an

appropriate orientation.

Reverting to Fig. 1, the HTF which is heated within the HCEs 20 as described above is flows to the steam electric power plant 14. It is used therein, for example within one or more heat exchangers 44, to heat a working fluid which drives one or more turbines 46 driving a generator 48 to create electricity, as is well known in the art.

Those skilled in the art to which this invention

pertains will readily appreciate that numerous changes, variations and modifications can be made without departing from the scope of the invention mutatis mutandis .

Claims

Patent claims

1. A solar collection system configured for use with a solar thermal power plant, said solar collection system being designed to facilitate capture of thermal energy of incident solar radiation by a heat transfer fluid (HTF) flowing therethrough and comprising:

• a longitudinally extending concentrator designed to

reflect at least a portion of said incident solar radiation toward a focus line thereof;

• a heat collecting element (HCE) coincident with said focus line and comprising a tube carrying said HTF, and a transparent enclosure surrounding said tube; and

• a light rectifier located at an entrance of said

concentrator, said light rectifier being configured to increase the elevation angle of said impinging solar radiation passing therethrough.

2. The solar collection system according to Claim 1, wherein said concentrator is configured to track the sun along a single tracking axis, said light rectifier being configured to accomplish the increase by refracting said solar radiation within a plane perpendicular to said tracking axis.

3. The solar collection system according to any one of Claims 1 and 2, wherein said light rectifier comprises a plurality of prisms, each being configured to refract incident solar radiation, thereby increasing its elevation angle.

4. The solar collection system according to Claim 3, wherein surfaces of said light rectifier are provided with an anti- reflective coating.

5. The solar collection system according to any one of Claims 3 and 4, wherein said prisms each have a cross-section shaped as a right triangle and are arranged with an entrance

aperture thereof generally facing the incident solar

radiation, and an exit aperture generally facing the

concentrator .

6. The solar collection system according to Claim 5, wherein a first cathetus of the right triangle of each prism

constitutes said entrance aperture thereof.

7. The solar collection system according to Claim 6, wherein a second cathetus of the right triangle of each prism is disposed so as to be closer to the direction from which said solar radiation comes.

8. The solar collection system according to any one of Claims 5 through 7, wherein said first cathetus is the longer of the two catheti of the right triangle.

9. The solar collection system according to any one of Claims 3 through 8, each of said prisms being associated with a critical range, wherein incident light impinging on the entrance aperture within said critical range is totally internally reflected within the prism by the exit aperture, said prisms being oriented such that direct solar radiation impinges outside the critical range.

10. The solar collection system according to any one of Claims 3 through 9 when dependent on Claim 2, wherein said prisms extend parallelly to said tracking axis.

11. The solar collection system according to Claim 10, wherein said concentrator is configured to track the sun by pivoting about a pivot axis perpendicular to said tracking axis .

12. The solar collection system according to any one of Claims 3 through 11, wherein said prisms are formed as a single body.

13. The solar collection system according to Claim 11, wherein said body comprises a plate with said prisms being formed on one surface thereof.

14. The solar collection system according to any one of Claims 3 through 13, wherein said prisms are made of a material selected from PMMA (Polymethyl methacrylate) and glass.

15. The solar collection system according to any one of the preceding claims, wherein the transparent enclosure of the HCE is made of glass.

16. The solar collection system according to any one of the preceding claims, wherein the tube and transparent enclosure of the HCE are arranged as concentric cylinders.

17. The solar collection system according to any one of the preceding claims, wherein the space between the tube and enclosure of the HCE is evacuated.

18. The solar collection system according to any one of the preceding claims, wherein said HTF is selected from the group consisting of thermal oil, steam/water, molten salts, carbon dioxide, and helium.

19. The solar collection system according to any one of the preceding claim, wherein said concentrator comprises a reflecting surface having a parabolic cross-section.

20. A solar thermal power plant comprising a thermal-electric power plant and a solar collection system according to any one of the preceding claims in communication therewith to provide heat thereto for driving its operation.