Solar receiver support structure

Process for manufacturing a support structure and a solar receiver includes the following steps:

TECHNICAL FIELD

The present invention relates to a method for easy manufacturing of inexpensive high quality solar troughs. It also relates to the arrangement of the above mentioned solar troughs in series in order to maximize the amount of energy produced.

STATE OF THE ART

During the next decades, large scale generation of sustainable energy will become one of the main challenges of our civilization. The challenge is underlined both by the ongoing depletion of fossil fuels (coal, oil and gas) as well as the global warming due to increased amount of CO2in the air, CO2being an unavoidable by-product of energy generation through fossil fuels. Among the many renewable energy alternatives, solar energy currently offers the most potential.

Today, three main classes of solar energy systems are used:Photovoltaic panels (PV panels),Thermo-solar collectors,Concentrated Solar Power systems (CSP).

PV panels are expensive and the efficiency of their energy conversion is still quite low, but on the other hand PV panels can directly provide electricity without intermediate conversion. Thermo-solar collectors heat up liquids (essentially water) at temperatures that rarely exceed 90° C. A large quantity of liquid is contained in the tubes. The thermal inertia of thermo-solar collectors is therefore quite high and makes them slow to heat up. For this reason, they are mostly used for domestic warm water supply. They can also be used for pre-heating water which is later brought to higher temperatures with other means. Thermo-solar collectors are therefore not used for bulk energy generation and/or for electricity generation.

On the other hand, Concentrated Solar Power (CSP) are very suitable for large scale applications such as solar power plants. These systems are arranged to focus sunlight onto a smaller area in order to produce electricity either through high-efficiency photovoltaic cells (Concentrated Photovoltaics), or through heat carried by a transfer fluid in a tube (essentially water) or by air in a Stirling engine.

CSP includes systems as diverse as solar troughs, solar dishes and solar towers.

Most large scale solar power plants use solar troughs and provide energy at a cost per kWh still significantly higher than conventional fossil energy sources. One main reason is the manufacturing cost of the solar troughs induced by the precise shapes to be obtained. Another reason is the cost of the sun tracking equipment to follow the sun. Such equipment generally comprises expensive optoelectronic systems and electromechanical actuators. Finally, a third reason is the need for large ground areas between troughs to compensate for the shadowing effect when the sun is at a low elevation. Conversely, when the sun elevation is high (sun close to the zenith), these areas are hit by solar rays lost to the CSP.

Most CSP systems use parabolic-shaped reflective troughs arranged to follow the elevation of the sun during daylight and to concentrate the incident rays onto the receiving tubes located along the trough's focal line. The tube contains a fluid (water for example) heated by the reflected sun rays. The resulting hot water (or steam, depending on the pressure and temperature) can be used for heating, cooling (with a chiller), desalination of salted water, or electricity generation through a steam turbine and an electrical generator.

The first requirement for a solar trough is to follow an accurate shape. Defects of curvature influence the position of the focal point. Therefore these defects reduce the efficiency of the solar receiver when the tubes are misaligned with the focal lines. In general, the troughs' shapes are parabolic. Non-parabolic troughs also exist such as circular troughs which are less sensitive to the sun's elevation although they concentrate the sun on a slightly more diffuse and vertical area. In the current state of the art, the shape and the structural stability of the trough is usually obtained with structural frameworks involving ribs and stiffeners. A large number of ribs and stiffeners are needed in order to obtain the exact desired shape and to resist to the wind force. In addition to this, the way the reflective sheet is attached to the ribs and stiffeners is not straightforward. It can be done, among other solutions, with screws, rivets or glue. This leads to significant local stress on the reflective sheet and, hence, to optical inaccuracies.

A second requirement for a solar trough is to achieve a low manufacturing cost for mass production in order to reach a competitive cost per kWh compared to conventional energy sources. This requirement is difficult to achieve with current systems as reflective sheets, ribs and stiffeners are expensive to produce and to mount.

Attempts have been made to reduce the costs of the troughs by using honeycomb panels or by molding the whole trough in one piece or in two halves. Still the resulting shape of the trough often lacks accuracy and the manufacturing process requires expensive and complex equipment.

The present invention aims to solve the above mentioned problems.

DESCRIPTION OF THE INVENTION

The present invention relates to a manufacturing process of a support structure for a solar receiver (solar trough) according to the following steps.a) Providing a flat sheet of material20with a definite shape. It can be rectangular shape as inFIG. 1, or any other shapes as inFIGS. 3 and 19.b) Performing on the flat sheet a first groove21along a definite curve running from one side of the sheet to the opposite side of the sheet, a second groove22along another definite curve running from one side of the sheet to the opposite side of the sheet. The grooves divide the sheet in three surfaces: a central surface24and first and second lateral surfaces23and25. Both grooves are arranged so that they do not intersect. Each groove is respectively etched or milled on one side of the sheet, without traversing it. According to the material and its rigidity, the deepness of the groove is determined by a skilled person so that the sheet does not risk being broken during the manufacturing process or later.c) Bending the first lateral surface23of the sheet with respect to the central surface24and the first groove21. The first groove21will define the first edge31of the support structure. The bending angle θ1can be 90° as inFIG. 2or it can be an angle different from 90°. Moreover, the angle θ1can actually vary along the groove21. It can also advantageously be adjustable with a mechanism as it will be explained hereafter.d) Bending the second lateral surface25of the sheet with respect to the central surface24and the second groove22. The second groove will define the second edge32of the support structure. The bending angle θ2can be 90° as inFIG. 2or it can be a different angle. Likewise, the angle θ2can vary along groove22. It can equally be adjustable with a mechanism. The central surface24as well as the first and the second lateral surfaces23and25form accurate three-dimensional geometrical surfaces that depend on the shapes of the first and the second grooves as well as the bending angles θ1and θ2. If both grooves are symmetrical along a central line26, the central surface24can be used has an accurate profile supporting the flexible reflective sheets that will make up the solar trough such as inFIG. 6showing an example of a solar trough using three such supports. It should be noted that the support structure with a parabolic shape are obtained from grooves made on a flat surface as inFIG. 1or inFIG. 3that are not parabolic curves.
Mode(S) For Carrying Out The Invention

The manufacturing process can further comprise the step of truncating the sheet corners41as shown inFIG. 3andFIG. 4. This yields a lighter yet rigid support structure taking less volume. Another step in the manufacturing process may include the stiffening of the support structure. As shown inFIG. 6, two transversal ribs71can be attached to the mounting supports42on which holes are drilled. This enables a proper alignment of the supports structures50before attaching the flexible reflective sheet81as inFIG. 7. A preferred attachment method of the flexible reflective sheet is through gluing of the flexible reflective sheet to the area24of the support structure. The wide area24enables a very stable and rigid gluing of the flexible reflective sheet which is superior to the current state of the art through ribs and stiffeners attached with screws or rivets. It must be noted that in order to strengthen the support structure, stiffening ribs72,91asFIGS. 6 and 8can be bolted to holes drilled in the lateral surfaces23and25. Contrary to most of the ribs used for solar receivers in the state of the art, these ribs are not required to have accurate shapes and are therefore cheaper to manufacture. Alternatively, holes43as inFIG. 5can be drilled to attach cables or strings to rigidify the support structure as surfaces23and25tend to spread. Other means, accessible to a person skilled in the art, could be applied to achieve similar results. Additionally, a mechanism with pulleys can advantageously be used to change the length of the strings change the shape24and to adjust the focal point of the resulting trough.

Aluminium is well suited for this manufacturing process. A sandwich material with two external sides of aluminum and a layer of epoxy in between is preferred.

The materials previously described as well as any other materials that a skilled person would find appropriate for that purpose, are low cost and easily etched or milled. Additionally, they are flexible and bend with no difficulty. They offer durability and high strength-to-weight ratio.

Another embodiment of the present invention is shown inFIG. 10where the initial sheet110presents also two grooves21and22. Their axe of symmetry is line26. The resulting solar receiver as shown inFIG. 11is two half-support structures combined and spaced to form a gutter. Advantageously, this gutter121allows the parabolic trough to be cleaned by a natural process: dust and rainwater would run down the parabolic profile and be disposed of in the gutter. The additional effect is to lower the costs of maintenance. Moreover, it has no performance penalty as the gutter is arranged under the cast shadow of the receiver tube. Another advantage, compared with the previous embodiment, is that, given the same manufacturing equipment handling the same rigid sheets, it is possible to build solar receivers about twice as big. In the subsequent embodiments of the present invention, one shall always consider that a solar receiver can be obtained from one single structure or from the combination of two or more structures comprising a gutter.

FIGS. 13 and 14show another embodiment of a solar receiver according to the invention. It comprises concave and convex edges obtained from a first and a second groove141,142. The first lateral surface143is bent upward and the second lateral surface144is bent downward. The advantage to use such a solar receiver150will be later apparent. It can be further noticed that the solar receiver150includes additional stiffening means145,146.

Another object of the present invention is shown inFIG. 15. It involves a set of solar receivers150supported by support structure50(two in this drawing, but it can be more. This configuration has many advantages. When the focal lines are aligned with the sun azimuth, and the set is tilted with respect to the sun's mean position, a highly effective optical coupling takes place from one solar receiver to the next even for small incident solar angles, yielding very high energy efficiency. The concave and convex edges are here particularly advantageous. The staircase arrangement of the solar receivers ensures that an incident beam is either reflected from one solar receiver to its focal line or further reflected onto the next solar receiver and eventually to another focal line. Moreover, the cast shadow is also accounted for by the staircase arrangement, resulting in an improved use factor of the area. Additionally, for a given energy production, the overall height of such set of solar receivers remains quite low in comparison to the state of the art solar receivers. Therefore the mechanical structure can be lighter and less expensive to manufacture.

Another aspect of the present invention is to obtain a solar receiver by carrying out the manufacturing process as previously described with a sheet of rigid material chosen to have a reflective surface. Alternatively, the sheet can have no reflective surface, but the process comprises an additional step consisting in a treatment applied to the sheet to produce a reflective surface. As a result, the parabolic profile defines a focal line along which a receiver tube can be installed. A solar receiver is therefore obtained with a precise parabolic profile or parabolic trough's shape.

An additional aspect of the invention relates to a further arrangement of such solar receivers150and180to yield solar power plants. Solar power plants using a plurality of sets of solar receivers are another object of the invention.FIG. 18shows a solar power plant according to the invention. Sets of solar receivers are arranged so that their vertical plans are parallel. They can be mounted on a Solar Island, which is an artificial circular island of potentially large dimensions. It consists of an outer torus wrapped in a membrane, on which the sets of solar receivers are mounted. The island is able to rotate in order to follow the sun azimuth movement. Thus, the solar receivers are precisely aligned to the sun. This avoids fitting the concentrating solar receivers with dedicated and expensive sun tracking systems. In order to optimize the use factor of the area, individual solar receivers are also used. They are arranged so that their focal lines are parallel to the sets vertical plans. A solar power plant is therefore obtained with a limited height, which meets the requirements of local building regulations, high efficiency and increased power production for a given area. The cost factors to manufacture and maintain the power plant are also reduced and the cost per kWh becomes competitive against fossil energy sources.