ASYMMETRIC SOLAR RECEIVER

Abstract

The invention relates to an asymmetric solar receiver suitable for its installation in heliostat tower solar power plants, wherein said receiver is adapted to cover at least an angular region of around a tower, and wherein the effective surface density of the receiver for receiving radiation varies depending on its orientation over the covered angular region thereof. The effective surface of said receiver is preferably made of one or more panels and, more preferably, the receiver comprises at least two panels, wherein at least one of the panels comprises a smaller height than the other panels. The invention also relates to a solar tower comprising said receiver, a solar field comprising said tower, a solar power plant comprising said solar field, and a process for installing a solar field.

Claims

1. An Asymmetric solar receiver suitable for its installation in heliostat tower solar power plants, said receiver being adapted to cover at least an angular region of 180° around a tower, wherein the receiver it comprises an effective surface comprising a plurality of panel structures, wherein one of the panel structures has a height lower than an adjacent panel structure, so that the effective surface density of the receiver for receiving radiation varies depending on its orientation over the covered angular region thereof.

2. The asymmetric solar receiver according to claim 1, wherein at least one of the panel structures of the receiver comprises one panel.

3. The asymmetric solar receiver according to claim 1, wherein at least one of the panel structures of the receiver comprises more than one panel, so that the height of the panel structure is the result of the number of panels comprised in the panel structure.

4. The asymmetric solar receiver according to claim 3, wherein at least one panel is rectangular.

5. The asymmetric solar receiver according to claim 1, wherein at least one of the panel structure is shortened in their upper and/or lower ends compared to the other panel structures.

6. The asymmetric solar receiver according to claim 1, wherein the angular region covered angular region covered by said receiver is at least 270°.

7. The asymmetric solar receiver according to claim 1, wherein the angular region covered angular region covered by said receiver is 360°.

8. A solar tower comprising at least an asymmetric receiver according to claim 1.

9. A solar field comprising the solar tower according to claim 8, and a plurality of heliostats pointing to the receiver of said tower.

10. The solar field according to claim 9, wherein the heliostats are arranged so that their solar spots over the receiver are distributed along a plurality of pointing lines.

11. The solar field according to claim 10, comprising at least one, two, three, four or five pointing lines.

12. The solar field according to claim 9, wherein the heliostats are arranged in a first group of heliostats at the north of the tower, and a second group of heliostats at the south of the tower.

13. The solar power plant comprising a solar field according to claim 9.

14. A process for installing a solar field according to claim 9, comprising installing a solar tower and arranging a plurality of heliostats around said tower, and wherein said process further comprises at least calculating the distribution of said heliostats over the solar field which maximizes their energy flux performance over the effective surface of the receiver.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] To complete the description and in order to provide for a better understanding of the invention, a set of drawings is provided. Said drawings form an integral part of the description and illustrate an embodiment of the invention, which should not be interpreted as restricting the scope of the invention, but just as an example of how the invention can be carried out. The drawings comprise the following figures:

[0030] FIG. 1 shows a schematic view of a central tower power plant.

[0031] FIG. 2 shows a flat projection of the surface of the solar receiver showing two sun spots in a pointing line and its possible vertical and horizontal pointing corrections. Sun spot (A) shows the standard pointing corrections while the sun spot (B) shows the pointing corrections according to the invention.

[0032] FIG. 3 and FIG. 4 show a flat projection of the surface of a solar receiver with three and five pointing lines, respectively.

[0033] FIGS. 5a and 5b show two schematic views of a known tower solar receiver with cylindrical configuration.

[0034] FIG. 6 shows the reflected solar energy distribution of a standard solar field on the cylindrical solar receiver (3) of FIGS. 5a-5b (flat projection surface). The angular distribution is represented from south to south.

[0035] FIGS. 7a and 7b show two schematic views of a known tower solar receiver with conical configuration.

[0036] FIGS. 8a and 8b show two schematic views of a tower solar receiver with asymmetric configuration according to the present invention.

[0037] FIG. 9 shows the energy flux distribution obtained with the asymmetric receiver of the invention in a flat projection. The angular distribution is represented from south to south.

[0038] FIGS. 10a-10d show four schematic views of two tower solar receivers with asymmetric configuration according to the present invention.

[0039] FIGS. 11a-11b show a comparison between a symmetrical solar field in the northern hemisphere, where the heliostats are homogeneously distributed around the receiver (FIG. 11a) and a field according to the present invention, where the number of heliostats in the north region is greater than the number of heliostats in the south region around the asymmetric receiver (FIG. 11b). In the figures, (N, S, E, W) denotes north, south, east and west, respectively.

[0040] Elements of the example embodiments are consistently denoted by the same reference numerals throughout the drawings and detailed description where appropriate: [0041] (10) Sun [0042] (11) Heliostat [0043] (12) Tower [0044] (13) Solar receiver [0045] (14) Sun spot [0046] (15) Pointing line [0047] (16) Vertical pointing correction [0048] (17) Horizontal pointing correction [0049] (18) Panel structure

DETAILED DESCRIPTION OF THE INVENTION

[0050] As described in the preceding sections, central tower solar concentration technologies comprise essentially a solar field made up of structures with multiple individual mirror elements, or heliostats (11), normally configuring a large spherical cap or paraboloid, at the focus of which a fixed receiving element is located. A schematic view of a tower solar concentration system is depicted in FIG. 1. By means of its specific configuration, the solar radiation that comes from the sun (10) strikes the reflecting surfaces of the heliostats (11) (which are normally composed by one or more facets), tracking the movement of the sun (10) and concentrating and focusing the sunrays on a tower (12) comprising a solar receiver (13) located at its top end. The receiver (13) thereby absorbs the radiation from the heliostats (11) and transfers it, in the form of thermal energy, to a carrier fluid, referred to as heat transfer fluid, to either be used directly in the corresponding thermal or thermodynamic process, or else to be stored as thermal energy, to be used at a later time.

[0051] As a general rule, the heliostats (11) forming the solar field have dual-axis tracking capacity, where the corresponding axes are designated as azimuth axis or axis of rotation, and zenith axis or axis of inclination with respect to the horizontal. The freedom of movement in both axes thereby assures that the radiation reflected strikes the effective surface of the receiver, regardless of the position of the sun. In turn, the aiming of the reflected radiation depends on the relative position of the reflective surface of the heliostats (11) and on the incident radiation, for the purpose of further concentrating radiation in the receiver (13). The reflected solar energy of a single heliostat (11) is usually known as sun spot (14). In this context, a minor angular error in the inclination of the reflective surfaces translates into a corresponding deviation of the reflected beam, which depends on the distance to the receiver (13) and on said angular error. Onwards the joint effect of all the angular errors of the heliostat (11) respect to the nominal values will be referred as heliostat optical quality. In addition, insufficient structural rigidity or excessive clearance entails deviation from the focal point where solar radiation is focused with respect to the region where it is intended to be concentrated at the receiver (13).

[0052] Additionally and in order to obtain further energy increase, the tower solar plants are normally configured with a specific pointing strategy, which means that each heliostat (11) is configured to aim its sun spot (14) to a predefined position on the solar receiver (13). The standard in current commercial tower solar plants is to define one or several pointing lines (15) where the sun spots (14) are to be positioned (see FIG. 2 for instance, where two sun spots (14) are shown over a single pointing line (15)). In the ideal scenario, the sun spots (14) can be displaced within the vertical (A) and/or within the horizontal (B) direction to homogenize at much as possible the energy onto the receiver (13), by means of vertical (16) or horizontal (17) pointing corrections, respectively. Note that the increase of the optical quality in current heliostats allows performing said corrections to a certain extent, and this technical advance implies that the surface of the receiver (13) can be taken into consideration in detail for further optical performance. Thus, current receivers (13) can be configured with several pointing lines (15), as shown in FIG. 3 (three pointing lines (15)) and in FIG. 4 (five pointing lines (15)).

[0053] In order to absorb as much energy as possible, multiple heliostats (11) are placed around the tower (12), wherein the solar receiver (13) is preferably a 360° receiver or where, alternatively, it is constrained to a specific angular subregion (for example, substantially equal or greater than 270°, or substantially equal or greater than 180°). It should be noted that the present invention is not preferably related to cavity-type receivers, but to open region receivers (13).

[0054] Under this configuration, the main layout of the heliostat field is dictated by the type of central receiver (13) and its opening angle. Cylindrical and, in general, open angular receivers (13) demand surrounding heliostat fields. In this context, to take advantage of heliostats (11) oppositely located to the sun, which have better incidence angles and thereby less energy losses due to cosine effect, these heliostat fields are biased to the north (in northern hemisphere). Two schematic views of a known tower solar receiver (13) with cylindrical configuration is shown in FIGS. 5a-5b. The receiver is preferably made of consecutive sections or panels (which are rectangular in this specific example).

[0055] With this configuration, the reflected solar energy distribution on the solar receiver (13) is represented in FIG. 6, which shows maximum energy flux distribution of about 800 W/m.sup.2 over its central horizontal region. Known alternatives to cylindrical configurations for 360° receivers (13) are, for example, conical configurations such as the one shown in FIGS. 7a-7b. Even though the use of these receivers (13) can improve the tower solar plant performance over the cylindrical receivers (13) (providing improved focus for the standard plant configuration of the heliostats), it still behaves similarly for the north and south heliostats of the solar field.

[0056] In order to provide tower solar plants with novel configurations which are able to optimize the global energy flux performance of the heliostat plant, the present invention proposes a novel receiver (13) configuration based on the asymmetry of its north and south effective surfaces, so that further focusing of the sun sports (14) can be achieved, thus leading to higher energy flux distributions compared to known receivers (13). This kind of receivers (13) will be designated as “asymmetric receivers”. In this context, an asymmetric receiver (13) is to be understood as an angular receiver (13) comprising an asymmetric angular distribution. This, in turn, implies that its effective surface density will vary depending on the orientation angle. The term “surface density” will be understood as the surface of the receiver (13) comprised per angular region.

[0057] With this asymmetry, regions with smaller surface densities will allow sunspots (14) to undergone smaller energy losses, obtaining increased focusing for heliostats (11) aiming to those regions. This advantage also impacts on the overall energy performance of the whole solar field so that, in order to obtain maximum performance for a given number of heliostats (11), some of them will have to be relocated in the solar field regions aiming to the effective surfaces with greater surface density. This way, the receiver (13) will be able to achieve higher energy flux distributions.

[0058] A first embodiment of asymmetric solar receiver (13) according to the invention is depicted in FIG. 8a-8b. As shown in the figure, the panel structures (18) forming the receiver (13) comprise smaller area in one of the regions, where the panel structures are shortened in their upper and lower ends. Said asymmetric receiver (13) is installed in the tower (12) with its smaller area region oriented towards the south, while its bigger area region is oriented north. Thus, the number of heliostats (11) in the solar field can be rearranged for optimal efficiency compared to the case of a cylindrical receiver (13), so that a number of them will be located in the north field instead of in the south field.

[0059] Despite in these figures this receiver (13) is formed of vertical panel structures (18) wherein each panel structure is formed by only one panel, in different embodiments, each panel structure could also be divided into several unitary panels, so that the height of each panel structure is the result of the number of unitary panels provided in each panel structure.

[0060] Under this embodiment, the energy flux distribution on the solar receiver (13) according to the invention is represented in FIG. 9, which shows maximum reflected energy flux of about 1200 W/m.sup.2 over its central horizontal region, for the same number of heliostats in the solar field of the example of FIGS. 5-6. As can be seen in FIG. 9, in order to obtain the same energy in the receiver (3), some of the panel structures (18) or modules thereof can be shortened, and so reducing its cost without losing energy, which is a further advantage of the invention. Moreover, as the energy is more concentrated than in the known receivers (13), there is less surface at high temperature, which in turn leads to lower thermal losses of the receiver (3), thereby increasing its efficiency.

[0061] Two further embodiments of the asymmetric solar receiver (13) according to the invention are shown in FIGS. 10a-10b and FIGS. 10c-10d, respectively. As depicted in the figures, the panels forming the receiver (13) comprise a smaller area in one of the regions, where the panels are shortened in their upper (FIGS. 10a-10b) or in their lower (FIGS. 10c-10d) ends.

[0062] FIGS. 11a-11b show a comparison between a symmetrical solar field in the northern hemisphere, where the heliostats are homogeneously distributed around the receiver (FIG. 11a) and a field according to the present invention, where the number of heliostats in the north region is greater than the number of heliostats in the south region around the asymmetric receiver (FIG. 11b). Thanks to the improved surface distribution of the asymmetric solar receiver, the north-south configuration of the heliostat field can be further optimized, typically for heliostats of optical qualities equal or better than 1 mrad. In these cases and for solar fields in the northern hemisphere, the panels with smaller height in the receiver will be located at the south regions of the tower, so that the use of highly focused heliostats (due to their improved optical qualities) can lead to higher energy concentration in a smaller surface of the receiver. Thus, the overall energy flux in the south region can be highly improved. At the same time, a higher number of heliostats can be located at the north region of the field compared to standard configurations of symmetrical receivers, thereby further optimizing the energy yield of the solar field.

[0063] The above example embodiments are described in sufficient detail to enable those of ordinary skill in the art to embody and implement the systems and processes herein described. However, it is important to understand that other embodiments of the invention can be provided in many alternate forms and should not be construed as limited to the examples set forth herein.

[0064] Accordingly, while embodiment can be modified in various ways and take on various alternative forms, there is no intent to limit to the particular forms disclosed. On the contrary, all modifications, equivalents, and alternatives falling within the scope of the appended claims should be included.