WO2011110415A1

WO2011110415A1 - Solar collection system

Info

Publication number: WO2011110415A1
Application number: PCT/EP2011/052481
Authority: WO
Inventors: Gabriel Cohen; Rami Ezer
Original assignee: Siemens Aktiengesellschaft; Siemens Concentrated Solar Power Ltd.
Priority date: 2010-03-09
Filing date: 2011-02-21
Publication date: 2011-09-15
Also published as: CN102869928A; EP2521885A1

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 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. The concentrator comprises a light rectifying arrangement configured to specularly reflect the incident solar radiation and increase the elevation angle thereof.

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, which may extend horizontally (i.e., parallelly to level ground) , designed to reflect at least a portion of the incident solar radiation toward a focus line thereof; and

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

wherein the concentrator comprises a light rectifying arrangement configured to specularly reflect the incident solar radiation and increase the elevation angle thereof.

The light rectifying arrangement may comprise a plurality of teeth formed on an inner surface of the concentrator which faces the HCE, the teeth comprising front, solar radiation-facing front surfaces and rear surfaces.

The front surfaces may be configured for increasing the elevation angle of impinging solar radiation reflection therefrom. The rear surfaces may be designed so as to not interfere with solar radiation reflected by the front surfaces.

The rear surfaces may be disposed substantially perpendicularly to the direction in which the concentrator extends .

The concentrator may have a parabolic cross-section in a direction perpendicular to the direction in which it extends, the teeth extending along the parabola.

The concentrator may be configured to track the sun along a single tracking axis, which is perpendicular to the direction in which the concentrator extends, the teeth extending within a plane containing the tracking axis and perpendicular to the HCE (i.e., perpendicularly to the direction in which the concentrator extends.

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

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 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. 6A is schematic perspective view of a concentrator of the solar collector illustrated in Fig. 2;

Fig. 6B is a partial, close-up cross-sectional of the concentrator taken along line VI-VI in Fig. 6A;

Fig. 6C is a partial, close-up cross-sectional of the concentrator taken along line VI-VI in Fig. 6A according to a modification; and

Fig. 7 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.

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, i.e., being parabolic in a cross- section which is perpendicular to the direction in which the concentrator extends, 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.

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.

As illustrated in Figs. 6A and 6B, some or all of the concentrators 22 may comprise such means. According to one example, the inner surfaces 22A thereof may be formed with teeth 32, constituting a light rectifying arrangement. As best seen in Fig. 6B, each tooth 32 comprises a front surface 34 and a rear surface. The teeth 32 are formed such that they extend perpendicularly to the length of the concentrator 22 (along the length of the parabola), i.e., perpendicularly to the direction in which the HCE 20 extends. Thus, the teeth 32 extend within a plane which is perpendicular to the tracking axis T and perpendicular to the direction in which the HCE 20 and the concentrator 22 extend.

The front surface 34 is disposed so as to face the impinging solar radiation and configured to specularly reflect solar radiation impinging thereupon. The rear surface 36 is disposed facing away from the impinging solar radiation, and may be formed perpendicularly to the length of the concentrator 22 (i.e., to the direction in which it extends), or at a slight angle. In any event, the teeth 32 are designed so as to not interfere with solar radiation reflected by the front surfaces 34, i.e., such that incident solar radiation which reflects off of the front surface of one tooth 32 does not impinge upon the rear surface 36 of an adjacent tooth.

As illustrated in Fig. 6B, incident solar radiation

(indicated at A) impinging on the inclined portions 32 of the concentrator 22 at an elevation angle ψι, reflects therefrom at an increased elevation angle <p_∑. (For reference and illustration, a ray A' illustrates the path that the incident solar radiation A would follow after reflecting off of a "flat" concentrator, i.e., one lacking any light rectifying arrangement.) As the declination angle Θ is in general a complementary angle to the elevation angle, it will be appreciated that such an increase in the elevation angle will decrease the incidence angle losses.

The geometry of the inclined portions 32, including the angles at which the front and rear surfaces 34, 36 thereof are formed, their size, etc., is determined by the designer, taking into account, inter alia, the elevation angles of the sun over the course of the year.

According to a modification, as illustrated in Fig. 6C, the inner surface 22a of the concentrator 22 may be flat, with the light rectifying arrangement therebelow. According to this modification, a transparent cover 35, for example made of glass, PMMA, or any other suitable material, is provided over the teeth 32. The top side 35a thereof is flat, while the bottom side 35b thereof is formed so as to contact the teeth 32. According to this modification, the light rectifying arrangement operates to carry out its function, while the inner surface 22a is flat, facilitating, for example, simple cleaning thereof.

As illustrated in Fig. 7, a light rectifying arrangement as described above with reference to Figs. 6A and 6B, which reflects 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 electricity produced by the solar thermal power plant 10) may "over-reflect" solar radiation during the summer months, i.e., reflect it such that it leaves the light rectifying arrangement (and thus impinges on the concentrator 22) with a declination angle θ_∑ which is larger than that of solar radiation which reflects off of a "flat" concentrator (the declination angle thereof being indicated by θι; it will be appreciated that the present discussion is in terms of declination angle for clarity only, and that a larger declination angle is equivalent to a smaller elevation angle) , albeit angled away from the direction from which the solar radiation impinges.

The phenomenon described above with reference to Fig. 7 may be addressed by the designer considering and designing to decrease the overall incidence angle losses over the course of the year.

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 38, to heat a working fluid which drives one or more turbines 40 driving a generator 42 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; and

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

wherein said concentrator comprises a light rectifying arrangement configured to specularly reflect said incident solar radiation and increase the elevation angle thereof.

2. The solar collection system according to Claim 1, wherein said concentrator extends horizontally.

3. The solar collection system according to any one of Claims 1 and 2, wherein said light rectifying arrangement comprises a plurality of teeth formed on an inner surface of the concentrator which faces said HCE, said teeth comprising front, solar radiation-facing front surfaces and rear surfaces .

4. The solar collection system according to Claim 3, wherein said front surfaces are configured for increasing the elevation angle of impinging solar radiation reflection therefrom.

5. The solar collection system according to any one of Claims 3 and 4, wherein said rear surfaces are designed so as to not interfere with solar radiation reflected by said front surfaces .

6. The solar collection system according to Claim 5, wherein said rear surfaces are disposed substantially perpendicularly to the direction in which the concentrator extends .

7. The solar collection system according to any one of Claims 3 through 6, wherein said concentrator has a parabolic cross-section in a direction perpendicular to the direction in which it extends, said teeth extending along said parabola .

8. The solar collection system according to any one of Claims 3 through 7, wherein said concentrator is configured to track the sun along a single tracking axis, said teeth extending within a plane containing said tracking axis and perpendicular to said HCE .

9. The solar collection system according to Claim 8, wherein said concentrator is configured to track the sun by pivoting about a pivot axis perpendicular to said tracking axis .

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

11. 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.

12. 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.

13. 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.

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

15. 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.