US20150354856A1

US20150354856A1 - Trough collector with concentrator arrangement

Info

Publication number: US20150354856A1
Application number: US14/397,723
Authority: US
Inventors: Andrea Pedretti-Rodi; Gianluca Ambrosetti; Sergio Granzella
Original assignee: Airlight Energy IP SA
Current assignee: Airlight Energy IP SA
Priority date: 2012-05-01
Filing date: 2013-04-30
Publication date: 2015-12-10
Also published as: WO2013163771A1; CH706465A1; EP2844928A1; CN104471326A

Abstract

The invention relates to a trough collector with a primary concentrator, which concentrates solar radiation into a focal line region, with an arrangement for secondary concentration of the solar rays reflected by the primary concentrator, which arrangement concentrates the solar rays further into focal point regions. The arrangement for secondary concentration has a plurality of rows of secondary concentrators which have identical orientation in a row, but have differing orientation from row to row. Further, means are provided in order to keep one of the rows in the operating position and the other rows in a rest position. Thus, a range of the skew angle can be assigned to each of the rows and in the event of the change thereof, another row is brought into operating position.

Description

The present invention relates to a solar collector with a concentrator arrangement according to the preamble of claim 1.
Trough collectors are used inter alia in solar power plants, wherein arrangements for the secondary concentration for such trough collectors have been increasingly suggested.
Until now it has not been possible to generate solar electricity in an approximately cost-covering manner by using this technology, owing to the disadvantages of photovoltaics which have not been overcome. By contrast, for some time, solar power plants have already been producing power on an industrial scale at prices which, compared to photovoltaic methods, are close to the commercial prices now usual for power produced in the conventional manner.
In solar thermal power plants, the radiation of the sun is reflected using the concentrator through collectors and focused in a targeted manner on a location in which high temperatures (or high light density) arise as a result. The concentrated heat can be conducted away and used to operate thermal engines such as turbines which in turn drive the generators which generate electricity.
Three basic forms of solar thermal power plant are currently in use: dish/Stirling systems, solar tower plant systems and parabolic trough systems.
The dish/Sterling systems as small units in the range of up to 50 kW per module have generally not caught on.
Solar tower plant systems have a central absorber which is mounted in an elevated manner (on the “tower”) for the sunlight which is reflected to it by means of hundreds to thousands of individual mirrors, whereby the radiation energy of the sun is concentrated in the absorber by means of the many mirrors or concentrators and thus temperatures of up to 1300° C. should be reached, which is favourable for the efficiency of the downstream thermal engines (generally a steam or fluid turbine power plant for electricity generation). The “Solar two” system in California has an output of several MW. The PS20 system in California has an output of 20 MW.
Solar tower power plants have hitherto likewise not become relatively widespread (in spite of the high temperatures which can advantageously be reached).
Parabolic trough plants are however widespread and have large numbers of collectors which have long concentrators with small transverse dimensions and thus do not have a focal point but a focal line. These linear concentrators currently have a length of 20 m to 150 m. An absorber pipe runs in the focal line for the concentrated heat (up to almost 500° C.), which transports the heat to the power plant. Thermal oil, molten salts or superheated steam for example are possibilities for the transport medium.
The 9 SEGS parabolic trough plants in Southern California together produce an output of approx. 350 MW. The power plant “Nevada Solar One”, connected to the mains in 2007, has trough collectors with 182,400 curved mirrors, which are arranged on an area of 140 hectares, and produces 65 MW. Andasol 3 in Spain has been under construction since September 2009 and should enter into operation in 2011, so that the plants Andasol 1 to 3 will have a maximum output of 50 MW.
For the plant as a whole (Andasol 1 to 3), a peak efficiency of approx. 20% and also an annual average efficiency of about 15% is expected.
As indicated above, an essential parameter for the efficiency of a solar tower power plant is the temperature of the transport medium heated by the collectors, by means of which transport medium the heat obtained is transported away from the collector and is used for conversion into power for example: at relatively high temperature, a higher efficiency can be achieved during the conversion. The temperature that can be realised in the transport medium depends in turn on the concentration of the solar radiation reflected by the concentrator. A concentration of 50 means that in the focal range of the concentrator, an energy density per m²which corresponds to 50-times the energy irradiated from the sun to a m²of the earth's surface is achieved.
The theoretically maximum possible concentration depends on the Earth-Sun geometry, that is to say on the opening angle of the solar disc observed from the Earth. It follows from this opening angle of 0.27° that the theoretically maximum possible concentration factor lies at 213 for trough collectors.
Even with mirrors which are very complex in terms of production and therefore (too) expensive for industrial deployment and are well approximated to a parabola in cross section and therefore produce a focal line range with smallest diameter, it is not possible today to even reach this maximum concentration of 213 only approximately. A reliably achievable concentration of approx. 50 to 60 is however realistic and already allows the above-mentioned temperatures of approximately 500° C. in the absorber pipe of a parabolic trough power plant.
To increase the achievable temperature, secondary concentrators have now been suggested, which once again concentrate the solar radiation reflected by the primary (trough) concentrator in the longitudinal direction of the primary concentrator, so that the solar radiation is ultimately concentrated into a number of focal points, thus the concentration of the sunlight and the thus created temperature are higher and more than 600° C. should be achievable.
The same is true if the radiation should be concentrated onto photovoltaic cells, which however, as mentioned above, has hitherto not been realised on an industrial scale.
The construction of secondary concentrators is demanding however, as the so-called skew angle, i.e. the angle at which the sunlight falls onto the primary concentrator of a trough collector, changes seasonally and over the day, wherein the focal point regions created by the secondary concentrators can then shift for example, which is problematic in particular in the case of the use of an absorber pipe with thermal openings.
Secondary concentrators constructed as compound parabolic concentrators (CPCs) appear particularly suitable for a concentration in the longitudinal direction, but have the disadvantage that the achievable concentration is dependent on the angle of acceptance (θ_in) of the secondary concentrator (radiation which enters the secondary concentrator outside of the angle of acceptance does not reach the focal point region created thereby): the greater θ_inis, the smaller is the further concentration achievable by means of the CPC secondary concentrator.
It was suggested in US 2010/037953 to arrange the secondary concentrators, which are constructed as Fresnel lenses or parabolic reflectors, pivotably with respect to the primary concentrator, so that these can constantly be made to track the current skew angle.
The solution shown for the construction of the pivotable secondary concentrators has the disadvantage however that these can only be pivoted over a small part of the necessary pivoting range, without striking one another and thus blocking a further pivoting movement. It is conceivable to equip the arrangement shown with secondary concentrators which are spaced apart in the vertical position, which although it would allow the necessary pivotability, has the consequence that not all of the solar radiation reflected by the primary concentrator can be secondarily concentrated in the vertical position required during operation, which reduces the efficiency of the solar collector.
Accordingly, it is the object of the present invention to provide a trough collector with an improved arrangement for the secondary concentration of reflected solar rays.
This object is achieved by means of a trough collector with the features of claim 1.
The fact that, for incident radiation, variously orientated components can be positioned alternately in the path of the radiation reflected by the primary concentrator enables the use of secondary concentrators orientated in accordance with the current skew angle, without the same having to be of pivotable construction. Secondary concentrators of different orientation not currently required can be parked in a rest position outside of the path of the reflected radiation.
Accordingly, the constructive outlay is dispensed with for an arrangement of pivotable secondary concentrators both for the complete capture of all reflected rays and for the orientation which is always to be observed with high precision with the passage of time, but is constantly changing. This is of particular importance in a high-temperature environment, as is naturally unavoidable in the case of secondary concentrators which should generate temperatures above 600°. Likewise, or to a greater extent if photovoltaic cells are used, which by their very nature must be arranged directly at the exit of the secondary concentrators and therefore must be cooled, which creates additional design problems.
The invention is explained in more detail on the basis of the figures.

In the figures:

FIG. 1 a schematically shows a trough collector of known design with an arrangement of secondary concentrators,

FIG. 1 b schematically shows the daily path of the sun and the skew angle occurring,

FIG. 1 c schematically shows the skew angle in the collector,

FIG. 1 d shows a graph with the change of the skew angle over a year in a North/South orientation of a trough collector with the assumed location of Dubai,

FIG. 2 shows a preferred embodiment of a secondary concentrator,

FIG. 3 schematically shows a first preferred embodiment of the trough collector according to the present invention,

FIG. 4 shows a view from above onto the embodiment of FIG. 3,

FIG. 5 shows a view onto a section of the adjacently arranged rows of secondary concentrators according to the view of FIG. 4,

FIG. 6 shows a further embodiment according to the present invention, and

FIGS. 7 a to 7 c show a view from the side of a concentrating element modified according to the invention for various acceptance ranges.

FIG. 1 a shows a trough collector 1 according to the prior art with a primary concentrator 2 which rests in a frame which is not illustrated in any more detail so as not to overload the figure, is of pivotable construction and thus can be made to track the daily course of the sun.
The double arrow 3 shows the longitudinal direction, the double arrow 4 shows the transverse direction of the trough collector 1 and the double arrow 5 shows the pivoting directions of the collector 1.
Further illustrated is an arrangement 8 of secondary concentrators 9 here constructed as Fresnel lenses, and also a solar ray 10 which falls onto the primary concentrator 2, is reflected by means of the same as a ray 11 towards a focal line region of the primary concentrator 2 and after the passage through a secondary concentrator 9 is refracted in the longitudinal direction 3, so that it is finally directed as ray 12 onto a focal line region 13.
In other words, the incident solar rays are initially concentrated in the transverse direction 4, then in the longitudinal direction 3, wherein the thus arising focal point regions 13 are located on an absorber pipe 14 which absorbs the heat and dissipates the same via a heat-transporting medium.
The primary concentrator 2, here illustrated as a rigid mirror, can also be constructed as a flexible film clamped in a pressure cell, as is illustrated for example in WO 2010/037243. Instead of the absorber pipe 14 shown here, photovoltaic cells may also be provided for producing power at the location of the focal point regions 13.
As mentioned at the beginning, it is suggested in US 2010/037953 to construct the secondary concentrators 9 in a pivotable manner with a pivot axis running in the transverse direction 4, in order to constantly make the same track the skew angle (see FIG. 1 b).
FIG. 1 b shows the daily path of the sun in relation to a collector 1. Illustrated is the collector 1 orientated in the North/South direction with the horizon symbolised by the dashed line 20, as may be visible from the collector 1. Further illustrated is the path 21 of the sun on a summer's day which begins in the East at point 22 and ends in the West at point 23. Likewise, the path 25 of the sun on a winter's day beginning in the East at point 26 and ending in the West at point 27 can be seen.
By pivoting in accordance with the double arrow 5, the collector 1 is continuously orientated towards the sun over the day, i.e. in the morning, it is tilted to the left with reference to the FIG. 1 b, orientated horizontally at midday and tilted to the right in the evening.
In spite of this orientation, it is the case that in the summer the sun (here seen with reference to the drawing plane) rises in front of the collector 1 (that is to say North thereof), is behind the same (that is to say to the South) at midday and goes down in front of the same (that is to say in turn to the North) in the evening. In the winter, the sun is constantly behind the collector 1, that is to say to the South thereof.
This is illustrated by means of the normal 28 of the collector 1 which lies perpendicularly on the surface line 29 of the concentrator 2 which is marked dashed and runs in the longitudinal direction 3: for example it encloses a first angle S with the solar ray 30 (sun in winter) and it encloses a second angle S with the solar ray 31 (sun in summer). The angle S is known to the person skilled in the art as the skew angle and designates the incidence of the solar rats in the longitudinal direction 3 obliquely onto the concentrator of the collector, when the same is orientated towards the sun.
If a solar ray 31 falls from obliquely in front onto the collector 1, the skew angle is negative, if a solar ray 30 falls from obliquely behind onto the collector 1, the skew angle is positive. If a solar ray coincides with the normal 29 at midday, the skew angle is 0. This is shown in summary in FIGS. 1 c and 1 d: If the collector 1 is orientated towards the sun, the solar rays fall below the skew angle S onto the concentrator 2 in such a manner that they lie with the normal 29 in a plane E, wherein the reflected rays, which are not illustrated so as not to overload the figure, are concentrated into the focal line region of the concentrator 2.
FIG. 1 d shows a graph by way of example with the region of the skew angle based on the location of Dubai depending on the season: the season t is plotted on the horizontal axis, the value of the skew angle in degrees is plotted on the vertical axis.
The graph of FIG. 1 d is based on a North/South orientation of the collector 1, wherein the pivoting range of −70 to +70° is sufficient (0° corresponds to the horizontal orientation at midday).
From the relationships illustrated in FIG. 1 b, it is possible to read off that the skew angle S in summer, for example on 25 June, in the morning (point 22 in FIG. 1 b) at approx. −17°, at midday at approx. +4° and in the evening again at −17°. Over the day, the skew angle S changes by approx. 21°.
In winter, for example on 5 January, the skew angle S changes between approx. +32° and approx. +48°, changes therefore by approx. 26°.
FIG. 2 shows a secondary concentrator 40, as can be used in a collector 1 (FIG. 1 a) in the place of the secondary concentrators 9 constructed as Fresnel lenses shown schematically there. The secondary concentrator 40 has a front wall 41 and a rear wall 42, which are constructed as compound parabolic concentrators (CPCs). CPCs are fundamentally known to the person skilled in the art. The CPC is used for the secondary concentration of the solar rays 11, 11′ reflected by the primary concentrator 2 (FIG. 1 a), i.e. it concentrates the same in the longitudinal direction 3.
Further illustrated are a right side wall 43 and a left side wall 44, which are constructed as trumpet concentrators and allow solar rays 11 concentrated by the primary concentrator in the transverse direction 4 to additionally concentrate once more in the transverse direction 4. A trumpet concentrator is fundamentally known to a person skilled in the art.
As a result, highly concentrated rays 45 emerge from the upper opening 46 of the secondary concentrator 40 and according to the invention in a focal point region 47 hit an absorber pipe 14 (FIG. 1 a) or photovoltaic cells, which are omitted so as not to overload the figure.
The lower opening 48 of the secondary concentrator 40 has an acceptance range which is determined by the properties of the CPCs and also the trumpet concentrator, with the consequence that only rays 11 incident below the acceptance angle are concentrated into the focal point region 47, which is not the case for the ray 11′ lying outside the acceptance angle.
FIG. 3 shows a preferred embodiment of the present invention. Illustrated is a collector 50 with a flexible concentrator membrane 52 arranged in a pressure cell 51, which primarily concentrates the incident solar rays 53, 53′ and therefore reflects the same as rays 54, 54′ onto an arrangement for secondary concentration 55. The pressure cell 51 is clamped in a frame 59 of the collector 50.
In a framework 56 below the absorber pipe 57, a carriage 58 is arranged, which is arranged such that it can be displaced back and forth in the transverse direction 4 below the absorber pipe 57 and carries secondary concentrators 40 (FIG. 2), here in a plurality of mutually adjacent rows 60, 61 and 62, so that the secondary concentrators 40 are grouped in the rows 60 to 62. In each row or group 60 to 62, the associated secondary concentrators 40 then lie one behind the other and are thus arranged along the length of the primary concentrator.
So as not to overload the figure, a conventional drive of the carriage 58, which can readily be conceived by the person skilled in the art in accordance with the requirements on site, is omitted.
By means of suitable movement of the carriage 58, in each case one of the rows 60 to 62 is located below the absorber pipe 57, i.e. in the operating position in the path of the reflected radiation and the two other rows are located in the rest position i.e. outside of the path of the reflected radiation.
The secondary concentrators of each row (or group) 60 to 62 are orientated differently compared to those of another group, that is to say have a differently orientated acceptance range and are therefore suitable to secondarily concentrate radiation which corresponds to an associated predetermined range of the skew angle S, i.e. in a predetermined skew range:
Starting from the range of the skew angle at the actual location and according to the graph of FIG. 1 d, the person skilled in the art can therefore determine the complete skew range (which in the example of FIG. 1 d ranges from −18° to)+49°, divide the same into a suitable number of predetermined skew ranges (in the example according to the arrangement of FIG. 3, three thereof) and assign a row (or group) 60 to 62 of secondary concentrators 40 to each of these thus predetermined skew ranges, which secondary concentrators are orientated towards their skew range and which are then brought into operating position in the corresponding season by means of the displacement of the carriage 48.
FIG. 4 shows a view from above onto the collector 50 according to FIG. 3, wherein the absorber pipe 57 is omitted so as not to overload the figure. The position thereof is illustrated by means of the line 70.
According to the embodiment illustrated, three rows or groups 60 to 62 of secondary concentrators 40 are arranged in the carriage 58. As mentioned above, the secondary concentrators 40 are orientated in a respective row towards an associated skew range.
A section of the rows 60 to 62 of secondary concentrators, which is illustrated in more detail in FIG. 5, is designated by means of the dashed line 71.
As a result, the trough collector according to the invention has an arrangement 65 for secondary concentration of the solar rays 54, 54′ reflected by the primary concentrator 42, which arrangement concentrates the solar rays further into focal point regions 46 (FIG. 2), wherein the arrangement 65 for secondary concentration of the reflected radiation has a number of variously orientated concentrating components here constructed as secondary concentrators 40 and further has means in order to bring the concentrating components alternately into an operating position in the path of the reflected radiation or into a rest position outside of the path of the reflected radiation. These means are constructed in the embodiment according to FIG. 3 as framework 56, carriage 58 and drive for the carriage 58.
FIG. 5 shows the view onto the section according to the dashed line 71 made up of the rows 60 to 62 of secondary concentrators 40. So as not to overload the figure, the carriage 58 and all further elements of the collector 50 are omitted, only the position of the absorber pipe 57 is shown by the line 71. In the figure, the different orientation of the secondary concentrators of each of the rows 60 to 62 can be seen clearly.
As mentioned above, the present invention is not limited to the embodiment of a secondary concentrator illustrated in FIG. 2, any element, by means of which the radiation reflected by the primary concentrator is concentrated longitudinally into a focal point region, conforms with the invention. Further, the means for the displacement of the secondarily concentrating elements may be constructed differently, so for example in the place of a carriage 58 travelling over the framework 56, it is conceivable to arrange the secondarily concentrating elements on rotating rings placed around the absorber pipe. Likewise, photovoltaic cells may be arranged in the focal point regions formed by the secondarily concentrating elements.
The parabolic-trough shape of the primary concentrator means that the reflected rays do not fall parallel into the secondarily concentrating element as seen in the longitudinal direction, but rather at an angle of a few degrees, the value of which changes with the size of the skew angle. Accordingly, in a preferred embodiment, the acceptance ranges of the secondarily concentrating elements of the various rows or groups are constructed in an overlapping manner, so that when changing from one row to another row, the currently prevailing solar radiation can be concentrated completely into the focal point regions by both rows.
FIG. 6 shows a further embodiment of a collector 80 according to the present invention, in which the arrangement 65 for secondary concentration is modified. A single row 83 of secondarily concentrating elements, here of secondary concentrators 40 is fixedly arranged in the framework 56 by means of holding arms 84 between the absorber pipe 57 and the carriage 58. Arranged in the carriage 58 are rows 85 and 86 of attachment elements 87 and 89, which, depending on the position of the carriage 85 lie in an operating position (i.e. in the path of the radiation 44, 44′) and together with the secondary concentrators 40 form a modified element for secondary concentrators. The effect of the attachment element is such that the acceptance range of the secondary concentrators 40 changes so that, in turn, three different rows of secondarily concentrating elements exist, which in each case are assigned to a skew range and have the corresponding acceptance range.
Secondary concentrators 40 are shown schematically in the FIGS. 7 a to 7 c, wherein FIG. 7 a shows a secondary concentrator 40 without attachment element, the acceptance range of which corresponds to the dashed line 86. Illustrated in FIG. 7 b is an attachment element 87 which constitutes an asymmetric continuation of the front wall opposite the rear wall 42 of the secondary concentrator 40 and thus changes the direction of the acceptance range thereof in accordance with the dashed line 88. Likewise in FIG. 7 c, where the direction of the acceptance range of the secondary concentrator 40 illustrated there is changed even more strongly by means of the larger attachment element 89, as is illustrated by the dashed line 90. For example and in accordance with the graph according to FIG. 1 c, the row of secondary concentrators 40 according to FIG. 7 a can be laid out for a skew range of −18° to +10°, the row of secondary concentrators with attachment elements 87 according to FIG. 7 b can be laid out for a skew range of 5° to +33°, and the row of secondary concentrators with attachment elements 89 according to FIG. 7 c can be laid out for a skew range of +30° to +48°.
The person skilled in the art can determine the skew ranges depending on the actual conditions prevailing on site. Likewise, the person skilled in the art can determine the number of rows of secondarily concentrating elements; although the number of three rows shown in the present exemplary embodiments is seen as advantageous, only two or more than three, for example four to six rows, are conceivable.

Claims

1. A trough collector comprising:

a primary concentrator, which concentrates solar radiation into a focal line region, with an arrangement for secondary concentration of the solar rays reflected by the primary concentrator 2, which arrangement concentrates the solar rays further into focal point regions;

wherein the arrangement for secondary concentration of the reflected radiation has a number of components variously orientated for incident radiation and means in order to bring the variously orientated components alternately into an operating position in the path of the reflected radiation or into a rest position placed outside of the path of the reflected radiation.

2. The trough collector according to claim 1, wherein the variously orientated components in groups have the same acceptance range fixedly orientated towards a predetermined skew range, a plurality of groups are provided for in each case different skew ranges and means are constructed to position in each case one of the groups operationally in the path of the reflected radiation.

3. The trough collector according to claim 1, wherein the acceptance range of at least one group in the transverse direction is constructed differently to that of another group.

4. The trough collector according to claim 1, wherein the concentrating components are constructed as secondary concentrators with identical orientation within the group, but differing orientation from group to group, wherein the orientation of a group corresponds to a predetermined skew range and the secondary concentrators of a group are arranged one behind the other along the length of the primary concentrator.

5. The trough collector according to claim 1, wherein the concentrating components have a single group of fixedly orientated secondary concentrators and various groups of attachment elements for the secondary concentrators, which in connection with the secondary concentrators in each case effect the orientation of the acceptance range thereof for various skew ranges, and wherein the means are constructed to position in each case one group of attachment elements operationally in the path of the radiation upstream of the secondary concentrators.

6. The trough collector according to claim 4, wherein the secondary concentrators are constructed as compound parabolic concentrators.

7. The trough collector according to claim 1, wherein the arrangement for secondary concentration is constructed for the concentration of the reflected radiation in the transverse direction also and preferably has a trumpet concentrator for each focal point region.

8. The trough collector according to claim 1, wherein the arrangement for secondary concentration is constructed to concentrate sunlight, which is incident at a skew angle of −20° to +50° onto the primary concentrator, into focal point regions.

9. The trough collector according to claim 2, wherein the acceptance ranges for the various predetermined skew ranges overlap.

10. The trough collector according to claim 1, wherein an absorber pipe is provided with thermal openings and the focal point regions lie at the location of the thermal openings.

11. The trough collector according to claim 1, wherein photovoltaic cells are arranged at the location of the focal point regions.

12. The trough collector according to claim 1, wherein the primary concentrator is divided into a plurality of longitudinal regions and an arrangement for the secondary concentration of the sunlight reflected by the longitudinal region is assigned to each longitudinal region.