WO2010004545A1

WO2010004545A1 - Apparatus and system for collecting concentrated solar radiation

Info

Publication number: WO2010004545A1
Application number: PCT/IL2009/000585
Authority: WO
Inventors: Refael Ridnik; Sharon Roded
Original assignee: Advanced Solar Power Israel Ltd.
Priority date: 2008-06-16
Filing date: 2009-06-14
Publication date: 2010-01-14
Also published as: IL192215A0

Abstract

A receiver (5) is provided that is configured to allow heating of a fluid flowing therethrough by collecting concentrated solar radiation. The receiver comprises a substantially tubular flow conduit (10) and an insulation layer (15) attached to that flow conduit, and wherein the insulation layer extends substantially along the receiver's longitudinal axis and surrounds part of flow conduit circumference.

Description

APPARATUS AND SYSTEM FOR COLLECTING CONCENTRATED SOLAR

RADIATION

Field of the Invention The present invention relates in general to the field of solar energy, and in particular to concentrated solar radiation systems for absorbing thermal energy.

Background of the Invention As the world social and economic infrastructures tend to become closely related to the planet' s energy balance, the need for developing and applying clean renewable energy sources, on a large scale, has become more apparent. Adding to that that the world may be facing the non-availability of fossil fuel supplies which prices are already the highest they have ever been, as well as a deteriorating ecological atmosphere, one can realize why the role of renewable energy sources is becoming increasingly critical. During the last decades, interest in solar energy solutions has increased because of the future role of solar energy among the energy alternatives became apparent. People become aware of the fact that many natural resources are finite, and may face exhaustion in the not too far future. In order to use solar energy on a large scale it has to be collected from large areas. For efficient high temperature applications the heat losses from the collecting systems have to be minimized, hence the use of solar concentrators. In general, for high temperature applications (such as producing process heating, converting solar heat into electricity and the like) , solar radiation is concentrated by using arrays of the any of the following concentrating devices such as parabolic troughs, parabolic dishes, or heliostats combined with a central receiver.

The parabolic trough type of collector consists of a long, reflecting parabolic trough having a tube mounted at the trough focal line, through which heat transfer fluid flows. Typically, one-axis tracking is used in order to simplify the system. Quite a few attempts have been made in order to improve the heat collecting efficiency in order to reach a more economically feasible solution. Such attempts were made by coating the absorbing tubes with a selective coating which has high absorbtivity of visible solar radiation but low emissivity of IR radiation, by enclosing the absorbing tubes in evacuated glass sleeves, etc. However, in order to reach a stage where the use of solar radiation as a source for energy is a widespread solution, it is yet desired to improve the existing solutions and increase the efficiency of collecting the solar radiation, as the costs associated with collecting the solar radiation is still the major factor whether solar energy can be used as a source to replace the conventional energy sources.

Summary of the Invention It is an object of the present invention to provide a device for use as an efficient collector of concentrated solar radiation.

It is another object of the present invention to provide a system comprising sun tracking reflectors and receivers for collecting solar radiation.

Other objects of the invention will become apparent as the description of the invention proceeds.

According to a first embodiment of the present invention there is provided receiver configured to allow heating of a fluid flowing therethrough by collecting (e.g. absorbing) concentrated solar radiation, wherein the receiver comprises a substantially tubular flow conduit through which the fluid flows and an insulation layer attached to the flow conduit, wherein the insulation layer extends substantially along the flow conduit's longitudinal axis and surrounds part of the flow conduit's circumference.

As will be appreciated by those skilled in the art, the solar radiation is concentrated by a corresponding solar concentrating reflector, e.g. a parabolic trough reflector, and is predominantly reflected onto the exposed part of the flow conduit (i.e. the non insulated part) which is mounted so as to face the corresponding reflector while the insulated part is directed away from the parabolic trough reflector, so that it remains unexposed to the concentrated solar radiation.

Preferably, a protection layer surrounds the outwardly facing side of the insulation layer, where this protection layer is made of, for example, aluminum or galvanized steel sheet to protect the insulation layer from absorbing or soaking water e.g. under rainy conditions or when water is used to wash the reflectors' mirrors . The insulation is preferably fixed to the flow conduit and is adapted to allow on one hand the axial thermal expansion of the flow conduit, and on the other hand to prevent the radial and circumferential movement of the insulation layer around the flow conduit. In accordance with another preferred embodiment of the invention, the insulation layer comprises a member of the group consisting of: ceramic fibers, glass fibers, rock wool, and the like. Preferably, the insulation layer is covered with a metal envelope which can be made of a sheet of e.g. galvanized steel, stainless steel or aluminum.

More preferably, the edges (e.g. flat edges) of the metal envelope covering the insulation layer are reflecting surfaces. The reflecting surfaces can be for example made of aluminum or silvered glass mirror. Preferably, further flat secondary reflectors are connected to the edges of the insulation assembly to collect any spillage radiation. According to still another embodiment, the insulation layer further comprises one or more reflecting surfaces that extend substantially along the receiver' s longitudinal axis and outwardly in a radial direction and are adapted to collect spillage of radiation that would otherwise not have reached the outer circumference of the receiver's flow conduit.

By yet another preferred embodiment of the invention at least the non-insulated part of the receiver' s flow conduit is coated with a selective coating having high absorptivity coefficient of visible solar radiation and low emissivity coefficient of IR radiation. When such a selective coating is only applied onto the exposed (non- insulated) part of the receiver tube, the costs involved in coating the flow conduit are reduced. In the alternative, and particularly while operating under relatively low concentration of solar radiation (i.e. operation under relatively low temperatures) , the coating that can be applied, may be a black paint.

According to still another embodiment of the invention, the receiver further comprises a glass covering the non-insulated part of the receiver' s flow conduit, and preferably, the space formed between the external surface of the non-insulated part of the flow conduit, the inner side of the covering glass and the two insulation layer side walls, is evacuated (e.g. operating at a pressure of less than 10^"5 torr) .

Preferably, the flow conduit is made of stainless steel coated with selective coating or steel coated with nickel layer to protect the steel base material, and a selective surface is applied on the protecting coating layer, i.e. the insulation layer.

According to another aspect of this invention, there is provided a solar system comprising a plurality of solar tracking reflectors operative to concentrate incident solar radiation onto respective receivers each configured to allow heating of a fluid flowing therethrough by collecting (e.g. absorbing) the concentrated solar radiation, wherein each of the receiver comprises a substantially tubular flow conduit and an insulation layer attached thereto and extending substantially along the receiver' s longitudinal axis and surrounding part of flow conduit circumference.

In accordance with a preferred embodiment, the plurality of solar tracking reflectors are adapted to be arranged in a configuration of a plurality of parabolic troughs and wherein each of said receivers is mounted so that concentrated solar radiation is reflected from the plurality of parabolic trough reflectors onto non- insulated parts of their corresponding receivers' flow conduits .

By another embodiment of the invention, the receivers are adapted to be mounted off the optical axis of their corresponding parabolic troughs. According to yet another preferred embodiment of the invention, the receivers are adapted to be mounted so that the imaginary focal line of each of the parabolic troughs is located tangentially to the corresponding receiver's flow conduit circumference. The heat transfer fluid may be a fluid with enough heat capacity to absorb the solar heat at the operating temperature. It may be thermal oil or molten salt that absorbs the solar energy by heating up. In the alternative, it may be a phase changing fluid such as water/steam system where the solar energy is absorbed through the evaporation of the liquid as well as through the heating up on the liquid and the generated steam, and the like. As will be appreciated by those skilled in the art, the solar radiation reflectors may be made of mirrors, polished aluminum or any other suitable material that is known in the art per se.

Brief Description of the Drawings

For a more complete understanding of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawings wherein: FIG. 1 - presents a schematic example of a cross section of a receiver constructed according to an embodiment of the present invention;

FIGs. 2 - FIG. 2A illustrates an example of a receiver support, whereas FIG. 2B illustrates an example of the insulation support;

FIG. 3 - presents a schematic view of an assembly which includes a receiver and a support for mounting the receiver;

FIG. 4 - presents a schematic illustration of a receiver and its corresponding reflector for concentrating solar radiation onto the receiver; and

FIG. 5 - presents a schematic example of system comprising a plurality of receivers and their corresponding parabolic trough reflectors. FIG. 6 - presents a schematic example of a receiver assembly associated with a secondary reflector.

Detailed Description of the Invention A better understanding of the present invention is obtained when the following non-limiting detailed description is considered in conjunction with the following figures.

The following are examples demonstrating certain ways of carrying out embodiments of the present invention .

FIG. 1 illustrates a cross section of a receiver 5 comprising a substantially tubular flow conduit 10 which is adapted to allow a flow of a fluid therethrough. The flow conduit is typically made of a metal that can withstand temperatures that would develop thereat due to the absorption of concentrated solar radiation, and should not be extensively affected by the daily thermal cycles which it undergoes (i.e. being at elevated temperature when exposed to concentrated solar radiation and at ambient temperature when no solar radiation is available, e.g. at night time) . Such a flow conduit could be made of stainless steel, Inconel, steel coated with protecting layer such as nickel and the like. Receiver 5 further comprises an insulation layer 15 attached to the flow conduit by using an insulation anchoring device 20 and is preferably held at its position by using an insulation fixing device 25 such as a metal band, etc. Insulation layer 15 extends substantially along the flow conduit's longitudinal axis and surrounds part of flow conduit circumference (clockwise from A' to A") . The remaining part of flow conduit 10 which is not insulated, is the part of the conduit at which the concentrated solar radiation will be reflected from solar reflector 30. In order to determine the angle (i.e. the section) out of the flow conduit circumference that will remain exposed and not insulated, the geometry of the overall system will have to be taken into account, e.g. the type of the solar reflector (trough in the present example) , the opening size of the solar reflector, its curvature and its focal distance, the length of the reflector's optical axis, the actual location of the receiver, i.e. the location of the flow conduit center relatively to the reflector focal line, etc. The receiver can be mounted also so that the focal axis of the reflecting trough does not coincide with the flow conduit central longitudinal axis, but in a way that the focal axis 50 of the reflecting trough 30 is located substantially adjacent to the flow conduit circumference (the flow conduit wall) . This configuration may provide a better homogeneity of the concentrated solar radiation incident on the exposed portion of flow conduit 10 (the part extending from A" to A') . Thus, when solar rays, such as rays 40', 40" and 40"' that are illustrated in this Fig., hit the reflecting surface of trough 30, they are reflected therefrom and the respective reflected rays 45' , 45" and 45"' reach the non-insulated part of flow conduit 10.

The determination of which part of the flow conduit circumference should be insulated and which part - should not, is taken while considering the above mentioned variables, as well as while applying some economical considerations which stem from the fact that the larger the exposed area is left, the more concentrated solar radiation may hit the flow conduit and can be absorbed by the flowing fluid. However, at the same time, the larger is the area from which heat is lost primarily by radiation and convention heat transfer mechanisms. FIGs. 2 exemplify supports for the receiver of the present invention. In the example shown in FIG. 2A, support 210 is used for supporting receiver 215. Receiver 215 illustrated in this FIG. comprises flow conduit (the tube) 220 and support 225 for the insulation layer (the latter is not shown in this FIG.) . For better understanding the geometry used, optical axis 230 is the optical axis of the receiver' s respective reflecting trough concentrator, whereas its focal point (or rather focal axis), is shown in 240. FIG. 2B illustrates an example of the insulation layer and its support. A typical insulation layer 250 as shown in this FIG. is e.g. about 10 cm thick, surrounded by a shell 260. The shell is fixed to the apparatus support by attachment means 270 whereas the receiver (not shown in this FIG.) is attached to the support by using attachments 280. The insulation layer which cross section is illustrated as an example in this FIG. surrounds about 200° out of the 360° representing the flow conduit circumference, whereas opening angle θ (of the exposed, non-insulated part of the receiver, the remaining 160° of the circumference.

FIG. 3 presents a schematic view of an assembly 310 which includes a receiver that comprises flow conduit 320 (and its supporting structure as illustrated in FIG. 2B) , the insulation layer 330 (with its supporting shell) and the receiver support 340 which ensures the placing of the exposed part of the receiver tube relative to the trough focal line. In the configuration illustrated herein the receiver support further includes also longitudinal expansion hinging. An example for the receiver tube length is about 12 meters per each parabolic trough module. By the example presented in the FIG., the tube is supported in two points, positioned 4 meters apart and connected by e.g. welding of one tube to the adjacent tube of the neighboring module. There may be an additional support structure in the space between each two adjacent trough modules. The single tube of each module can be made out of several pieces, typically three pieces, by welding them serially to create the 12 meters length of the module discussed in this example.

FIG. 4 presents a schematic illustration of a set up which includes the receiver with its corresponding concentrating reflector. The structure is comprised of mirrors' frame 410 on which the reflecting mirrors 420 for concentrating the solar rays are mounted. As illustrated in this FIG., mirrors 420 are arranged in a substantially parabolic trough shape so that the solar rays reaching the mirrors are reflected to the non- insulated (exposed) part of receiver 440. The receiver itself is mounted on support 430 which is attached to the mirrors' frame 410. The structure is provided with driving means (not shown in this FIG. ) to allow the mirrors to track the sun as a function of the time of the day, while the receiver itself remains fixed in its position.

FIG. 5 - presents a schematic example of system 500 comprising a plurality of receivers 510 and their corresponding parabolic trough reflectors 520, mounted on supporting frames 530.

FIG. 6 - presents a schematic example of a receiver that includes a secondary reflector 610. The focus of the parabola is at the center of the receiver tube. The arrangement illustrated in FIG. 6 increases the solar radiation interception as can be indicated by the trajectory of the rays 620 and 630 shown, as without the secondary reflector, these rays would have missed the receiver tube. Ray 620 is first reflected from the edge of the primary mirror, then it is reflected from the secondary reflector and therefrom, it is reflected again from the primary reflector before it hits the tube.

It is to be understood that the above description only includes some embodiments of the invention and serves for its illustration. Numerous other ways of carrying out the methods provided by the present invention may be construed by a person skilled in the art without departing from the scope of the invention, and are thus encompassed by the present invention. For example, it should be clear to any person skilled in the art that the functionalities required to carry out the present invention may be achieved by locating the trough focal axis differently relatively to the location of the fluid conduit, etc.

Claims

1. A receiver configured to allow heating of a fluid flowing therethrough by ^■ collecting concentrated solar radiation, wherein said receiver comprises a substantially tubular flow conduit and an insulation layer attached to said flow conduit, and wherein said insulation layer extends substantially along the receiver' s longitudinal axis and surrounds part of flow conduit circumference.

2. A receiver of claim 1, wherein at least a non- insulated part of said flow conduit is coated with a coating being a member of the group consisting of black paint and selective coating having high absorptivity coefficient of visible solar radiation and low emissivity coefficient of IR radiation.

3. A receiver according to claim 1, further comprising a glass covering at least said non-insulated part of said flow conduit.

4. A receiver according to claim 3, wherein the space formed between said non-insulated part of the flow conduit, said covering glass and the insulation layer side walls, is evacuated.

5. A receiver according to claim 1, wherein said insulation layer further comprising one or more reflecting surfaces extending substantially along the receiver' s longitudinal axis and outwardly in a radial direction and adapted to collect spillage radiation that would otherwise not have reached the outer circumference of said flow conduit.

6. A solar concentrating system comprising a plurality of solar tracking reflectors operative to concentrate incident solar radiation onto respective receivers, each configured to allow heating of a fluid flowing therethrough by collecting concentrated solar radiation, wherein each of said receivers comprises a substantially tubular flow conduit and an insulation layer attached thereto and wherein said receiver' s insulation layer extends substantially along the receiver's longitudinal axis and surrounds part of said flow conduit circumference .

7. A solar concentrating system according to claim 6, wherein said plurality of solar tracking reflectors are adapted to be arranged in a configuration of a plurality of parabolic troughs and wherein each of said receivers is mounted so that concentrated solar radiation is reflected from its respective parabolic trough reflector onto the non-insulated part of its flow conduit.

8. A solar concentrating system according to claim 6, wherein said receivers are adapted to be mounted off the optical axis of their corresponding solar tracking reflectors.

9. A solar concentrating system according to claim 7, wherein said receivers are adapted to be mounted so that the imaginary focal line of each of the parabolic troughs is located tangentially to the corresponding receiver' s flow conduit circumference.