WO2015104702A1

WO2015104702A1 - Solar power system

Info

Publication number: WO2015104702A1
Application number: PCT/IL2014/051042
Authority: WO
Inventors: Eldad Dagan
Original assignee: Individual
Current assignee: Individual
Priority date: 2014-01-13
Filing date: 2014-12-01
Publication date: 2015-07-16
Anticipated expiration: 2016-07-13
Also published as: IL230446A

Abstract

The invention is a solar power system based on the integration of parabolic trough technology and its piping design with solar dish technology. The invention exploits the structural chassis of a parabolic trough based on the design of a torque tube together with its rotational drive unit. An array composed of multiple parabolic dishes is mounted along the torque tube with appropriate individual receivers located next to the focus of each dish. The basic design of the system enables it to be used either to heat thermal fluid flowing through a system of pipes, to concentrate radiation onto Concentrated Photo Voltaic (CPV) cells, or for other dish applications.

Description

SOLAR POWER SYSTEM Field of the Invention

The invention is from the field of alternative energy. Specifically the invention is from the field of concentrating solar power for conversion of solar energy into other forms of usable energy such as electricity. Background of the Invention

There are presently three basic viable and reliable commercial technologies for Concentrating Solar Power (CSP) applications. The first of these, based on parabolic trough reflectors has a proven operational experience of almost 30 years mainly in south western USA and Spain. In this technology, the solar field is composed of many parallel rows of solar troughs, each of which is oriented and uses one axis tracking to concentrate incoming solar radiation onto a collector pipe that is coaxial with the trough. Main cold headers feed cold thermal fluid into parallel rows of collector pipes through which the thermal fluid flows through the solar field while the fluid is heated up to 400°C by the solar energy concentrated on the collectors. Hot headers accumulate the high temperature fluid, which is fed into a power block in which the heat is used to yield superheated steam for operating a turbine connected via shaft to an electrical generator. The cold fluid is then re-circulated into the solar field. The flow distribution via piping into the large scale solar field and out of it into the power block is based on a simple and neat design concept.

In this technology the parabolic troughs provide one axis tracking of the sun's motion to concentrate the reflected solar radiation into a line. This results in a modest concentration ratio and necessitates the introduction of collector pipes in the form of sealed elongated units known as Heat Collection Elements (HCE's). The HCE's share the single axis rotational movement with the parabolic trough mirrors. The HCE is comprised of a selectively coated inner metal pipe surrounded by a glass envelope with a vacuum in between to reduce heat losses out of the piping. This technology involves glass to metal connections at both ends of the HCE and means to maintain the vacuum for long term operation. Special flexible bellows are located at the ends of each HCE to compensate for the different thermal expansion rate of the metal pipe and the glass envelope. HCE's are highly expensive to manufacture, to maintain, and to replace since they may suffer failures of various types. Other drawbacks of the parabolic trough technology are: the glass to metal connection occupies about 4% of the total length of each unit and this introduces inactive area in the collector; and the reflected rays must pass through the glass envelop which diminishes the amount of energy reaching the thermal fluid by 4% to 8%. Additionally the vacuum existing between the glass envelope and the inner metal pipe can be spoiled due to various leaks and hydrogen permeation through the metal pipe. Hydrogen permeation rate is much higher in the application of Direct Steam Generation (DSG) since chemical reactions of the water with the inner wall of the metal pipe produces hydrogen which permeates into the vacuum zone.

The second commercial technology of CSP is that of central receiver (central tower). The central receiver is placed on the top of a tower surrounded by a distributed heliostat field. A heliostat contains a local control unit which is responsible for two axes tracking of the heliostat mirrors to project the solar reflected rays upon the central receiver. One typical type of receiver is a molten salt receiver which can be used for energy storage. A molten salt receiver also feeds its excess energy into a huge tank of hot salt from which energy is extracted during the night or when the solar insolation is low. Another technology involves a receiver containing sections for boiling and superheating steam to be fed into a steam turbine.

Medium size commercial plants of the central receiver type are present in Spain, e.g. The GEMASOLAR plant exploits a 30 MWE molten salt receiver with 24 hour storage capability. Plants of more than 100 MWE are found in south western USA, e.g. three plants of the Bright Source company that employ steam receivers are located in California next to Las Vegas. The Solar reserve company is completing the construction of a central tower plant, based on molten salt receiver, in Nevada (projected to be completed during 2014).

While enjoying the advantage of one single central receiver (in contrast to distributed receivers all over the solar field in the other two CSP technologies) plants of the central receiver type suffer from certain problems. The construction of very high towers and mounting and maintaining the receivers at the top is a severe challenge. The mirrors of the heliostats are not facing the sun since they should reflect the beam onto the fixed target located on the top of the tower. Thus the so called cos(theta) effect reduces the optical efficiency in comparison with mirrors facing the sun as found in the case of parabolic dishes as depicted below. Some of the reflected energy is lost in the long path between the heliostat and the receiver. There is also an effect called spillage, which refers to some of the reflected rays missing the frame of the receiver. Only further operational experience will provide information about the actual performance of large scale central receiver plants.

The third technology, which is based on parabolic dishes, uses two axes solar tracking. The use of two axes tracking and the fact that there is no need for a glass barrier in front of the absorbing surface provides a much higher energy concentration ratio than parabolic troughs. Parabolic dish units with mirror surface areas of up to 500 m² have been built to operate dish Sterling engines and to concentrate radiation onto Concentrated Photo Voltaic (CPV) cells for direct conversion of solar energy into electricity. While the concentration ratio with this technology is high the commercial exploitation of parabolic dish technology is based upon the installation of arrays of many distributed isolated units.

Disadvantages of large area parabolic dish technology relative to parabolic troughs are mainly related to the size of the dishes, which require a massive framework for supporting the dishes and to keep them stable during high wind conditions. A heavy duty mechanism is required to carry out the tracking function by moving the dishes. Also, since solar thermal power installations are commonly located at sites that are subject to frequent dust and sand storms, for large dishes it is more difficult and expensive to clean the reflecting surfaces of the dishes and to repair or replace the dishes if damaged than the troughs.

As can be understood from the discussion herein above, some of the strengths of the parabolic trough and dish technologies could provide a possible solution to some of the drawbacks of the other.

One idea to integrate parallel rows with dishes appears in US 2013/0098354. The system described in this patent application is comprised of parallel rows of reflectors and receivers. Each row of the system is comprised of an array of individual reflectors, each of which focuses the incoming radiation onto a receiver located at discrete points on a pipe through which a thermal energy heat transfer fluid flows. Very general and not well specified proposals for rotation of the reflectors about two mutually perpendicular axes are provided. It is a purpose of the present invention to provide a clear embodiment of solar power system based on the integration of parabolic troughs technology and its piping design with solar dish technology.

Further purposes and advantages of this invention will appear description proceeds.

Summary of the Invention

In a first aspect the invention is a unit of a solar power system. The unit of the invention is based on the integration of parabolic trough technology and its piping design with solar dish technology. The unit comprises:

a. a torque tube;

b. at least one dish comprised of two reflecting mirror segments, each of the dish segments supported with a gap between them symmetrically on opposite sides of the torque tube by a framework that is pivotally attached to the torque tube in a manner that allows the framework and the attached dish section to be tilted around an axis perpendicular to and passing through the center of the torque tube; c. a rigid pipe, that is held parallel to and attached to the torque tube segment by struts attached at their upper ends to the rigid pipe and at their lower ends to the torque tube and by rigid rods that are attached at their top ends to the rigid pipe and at their lower ends to a corner of framework that supports the dish segment; wherein the top and bottom ends of the struts and the rigid rods are connected to the rigid pipe, the torque tube, and the frameworks respectively with connections that allow the rigid pipe to be moved back and forth relative to the torque tube while remaining parallel to it; and d. a receiver supported by the long beam at the focal point of each of the dish segments. In embodiments of the unit of the invention the geometry of the reflecting surface of the reflecting mirror segments is that of a non-symmetric portion of a parabola, which is formed by rotating a two dimensional parabola around its symmetry axis. The optical axis of the parabola is located off center of the reflecting surface in the north south orientation and is located closer to the northern edge of the reflecting surface (in the northern hemisphere).

In a second aspect the invention is a solar power system comprised of at least two of the units of the first aspect joined together in series at the ends of their torque tubes and rigid pipes.

The system of the second aspect of the invention comprises:

a. at least two pylons each of which comprises at its top end a connection that both supports the torque tube and enables the torque tube to be rotated about its longitudinal axis; and

b. one drive pylon on which mechanisms for rotation and tilting of the dishes towards the sun are mounted. In embodiments of the system of the second aspect of the invention:

a. the mechanism for rotation of the dishes comprises two hydraulic pistons connected at their lower ends to the drive pylon and at their upper ends to the torque tube through a linkage such that a push-pull activation of the two hydraulic pistons causes the torque tube and everything that is attached to it to rotate about its longitudinal axis in both clockwise and counterclockwise directions; and

b. the mechanism for tilting the dishes is a third hydraulic piston that is pivotally attached to the torque tube above the rotation components on drive pylon 38, wherein the other end of the third hydraulic piston is pivotally connected to a reinforced strut that supports the rigid pipe above the torque tube such that expansion and contraction motion of the third hydraulic piston causes the reinforced strut to pivot about its connection to the torque tube.

In embodiments of the system of the second aspect of the invention all three pistons can be an integral part of a single hydraulic oil system comprising an oil tank and an oil pressure pump.

Embodiments of the system of the second aspect of the invention comprise a dual axes sun sensor or two one axis sun sensors configured to allow closed loop control of the tracking.

Embodiments of the system of the second aspect of the invention are aligned in parallel to a north-south axis. These embodiments comprise a gap separating two adjacent dishes, wherein the dishes that are attached to the torque tube segment in each unit of the system are located, in the northern hemisphere, as far north as possible relative to the pylon located to the south of them in the southern hemisphere as far south as possible relative to the pylon located to the north of them. In embodiments of the system of the second aspect of the invention the receivers are located on the optical axis of the parabola, which is off center of the reflecting surface to yield a permanent deflection of reflected rays towards the north in the northern hemisphere and to the south in the southern hemisphere.

Embodiments of the system of the second aspect of the invention are configured for use in a solar system based on Concentrating Photo Voltaic (CPV) cells. These embodiments comprise:

a. photo voltaic receivers attached to the rigid rods, wherein the receivers comprise a hollow shell and an aperture facing the reflecting dishes that is lined with high quality photo voltaic cells; b. a coolant supply pipe that supplies coolant liquid to the interior of the hollow shell and also serves as the rigid pipe;

c. a coolant return pipe that returns coolant liquid heated by the excess of energy not used by the cells back to a refrigeration unit and circulation pump at a central location;

d. flexible pipes configured to convey the cold coolant flowing in the coolant supply pipe into the hollow shells of the receivers and to remove heated coolant from reflector shell of the receivers to the coolant return pipe.

Embodiments of the system of the second aspect of the invention are configured for use in a solar system based on CPV cells comprise one of: a. one CPV receiver for each dish, the receiver configured to receive energy reflected from both dish segments; and

b. two CPV receivers for each dish, wherein each CPV receiver is configured to receive energy reflected from one of the dish segments.

Embodiments of the system of the second aspect of the invention are configured for use in a thermal solar system based on heating of a fluid flowing through a heat pipe that serves as the rigid pipe. These embodiments of the system comprise a receiver comprising a hollow shell through which at least a part of the fluid flowing through the heat pipe flows and a concave surface with high quality absorptive coating, the receiver fixedly attached to the heat pipe for each dish. These embodiments of the system are characterized by high pressure and high temperature of the fluid.

In embodiments of the system of the second aspect of the invention that are configured for use in a thermal solar system each receiver comprises one of a single concave surface facing both mirror segments of a dish or two concave surfaces each facing one of the two mirror segments of the dish. The flow of fluid through the receiver can be in either the south-north direction or in the north-south direction.

Embodiments of the system of the second aspect of the invention that are configured for use in a thermal solar system comprise a piping assembly comprised of four 90° bends and three ball joints at each end of the system.

Embodiments of the system of the second aspect of the invention that are configured for use in a thermal solar system comprise insulation surrounding the heat pipe through which the fluid flows.

Embodiments of the system of the second aspect of the invention that are configured for use in a thermal solar system comprise a compensation mechanism to compensate for shifting of the light rays reflected from the dish segments towards the receivers as the fluid flowing through the heat pipe heats up causing thermal expansion of the heat pipe.

In embodiments of the system of the second aspect of the invention that are configured for use in a thermal solar system the compensation mechanism comprises sliding hinges that connect the tops of the rigid rods to the heat pipe. The sliding hinges are configured to introduce a different amount of movement for each of the sliding hinges along the length of the heat pipe. In embodiments of the compensation mechanism the movement of the hinges is realized in one of the following ways:

a. an active correction caused by local motors and gears or belts for each hinge; and

b. a passive compensation mechanism which transforms the thermal expansion movement into the required change of position for each hinge. In a third aspect the invention is solar field plant comprising many parallel systems of the second aspect of the invention connected together in series.

All the above and other characteristics and advantages of the invention will be further understood through the following illustrative and non-limitative description of embodiments thereof, with reference to the appended drawings. Brief Description of the Drawings

— Fig. 1 schematically shows one unit of a system according to an embodiment of the invention;

— Fig. 2 schematically shows the support structure for the segments of the dishes of the unit shown in Fig. 1;

— Fig. 3 schematically shows a short system comprised of two of the units shown in Fig. 1 connected to each other in series;

— Fig. 4A and Fig. 4B schematically show views from two different angles of an embodiment of the mechanisms for causing rotation and tilting of the dishes;

— Fig. 5A and Fig. 5B schematically show views from two different angles of the connections of the piping system at each end of the short system shown in Fig. 3;

— Fig. 6 schematically shows an illustrative, but not limiting, embodiment of a long system comprised of eight of the units shown in Fig. 1 connected to each other in series;

— Fig. 7 symbolically shows a section of a unit of the system of the invention configured for use with CPV cells;

— Fig. 8A is an enlarged view of area B in Fig. 7;

— Fig. 8B is a cross-section of the receiver shown in Fig. 8A;

— Fig. 9A and Fig. 9C symbolically show in perspective and bottom views respectively an embodiment of a quasi-static receiver; — Fig. 9B is a cross-sectional view of the receiver of Fig. 9A;

— Fig. 10A and Fig. 10B schematically illustrate the permanent off center receiver angle, which characterizes the angular deviation of the location of the receiver relative to the center of the dish; and

— Fig. 11 A, Fig. 11B, and Fig. 11C schematically show the correction mechanism for compensation for thermal expansion of the piping.

Detailed Description of Embodiments of the Invention

The solar power system of this invention is based on the integration of parabolic trough technology and its piping design with solar dish technology. The invention is based upon the idea of exploiting the structural chassis of a parabolic trough based on the design of a torque tube together with its rotational drive unit. An array composed of multiple parabolic dishes is mounted along the torque tube with appropriate individual receivers located next to the focus of each dish. The basic design of the system enables it to be used either to heat thermal fluid flowing through a system of pipes, to concentrate radiation onto Concentrated Photo Voltaic (CPV) cells, or for other dish applications. The principal features of the solar system of the invention are shown in Fig. 1 to Fig. 4. The system is a modular one built up of individual units 10 shown in Fig. 1. These units 10 are connected together in series to form a short system 100 comprised of two units (Fig. 3) or a long system of eight units 200 (Fig. 6). Longer systems, called herein rows, comprised of more than eight units can be constructed on the same principles. Each unit 10 comprises two dishes 14, each of which is comprised of two segments 14a and 14b, mounted on a section of torque tube 12. The individual aperture area of each dish 14 is moderate but the total accumulated aperture area of a row is large. Thus the system is based on many distributed moderate size dishes 14 mounted upon one common tracking structure to present an alternative to a huge size single dish. In the embodiment of the invention shown in Fig. 1, each unit 10 comprises two dishes 14, each of which is composed of two rectangular shaped reflecting mirror segments 14a and 14b separated by a gap along the torque tube. The geometry of the surfaces of the mirror segments 14a and 14b is that of a non symmetric portion of a parabola. The mirror segments 14a and 14b are symmetrical relative to the torque tube, but asymmetrical along the north-south orientation. The optical axis of the parabola for each mirror segment is closer to the northern edge of its reflecting surface to enable an off center receiver. Each segment has its own focal point in order to optimize the concentration of solar energy reflected onto the receivers 28.

The gap between the mirror segments 14a and 14b allows tilting the dishes, while also avoiding blocking by torque tube 12 of reflected rays on their way towards the receiver. Since each tilted dish may cast some shadow upon its neighbor, a second gap separates two consecutive dishes along the length of the torque tube. The size of this second gap is chosen to yield reasonable shadowing effects throughout the year. The shadowing effect depends on the details relevant for each solar site, e.g. its latitude and the solar radiation characteristics throughout the year. It is noted that in other embodiments the number of dishes located on each segment of torque tube may be only one or more than two, depending upon the length of the segment, and that other parameters and dimensions such as the apertures of the dishes may be different.

Each dish segment 14a and 14b is supported by a framework 30 that is pivotally attached to the torque tube 12 in a manner that allows framework 30 and the attached dish section to be tilted around an axis perpendicular to and passing through the center of the torque tube. A long rigid pipe 24, that is parallel to and attached to the torque tube segment by struts 26, supports receivers 28 located at the focal points of the dishes. The edge of each dish section 14a and 14b that is parallel to and furthest from the torque tube 12 is also supported by two rigid rods 32 that are attached at their lower ends to a corner of framework 30 that supports the dish segment and at their upper ends to long rigid pipe 24 adjacent to the two sides of receiver 28.

In a real design the framework 30 is stiffened and the frameworks supporting both mirror segments are connected to each other. Thus both struts 26 together and rigid rods 32 share the task of supporting the long beam 24. The top and bottom ends of struts 26 and rigid rods 32 are connected to long rigid pipe 24 and to torque tube 12 and frameworks 30 respectively with a connection, e.g. a hinge or pivot, that allows long rigid pipe 24 to be moved back and forth relative to torque tube 12 while remaining parallel to it. Thus, if long rigid pipe 24 is made to move back and forth relative to torque tube 12, the mirror segments will be tilted. The motion of the long rigid pipe relative to the torque tube is caused by piston 46c, which is connected to a reinforced strut 26a as will be described herein below.

Fig. 2 is an enlarged view showing the support structure for the segments 14a and 14b of the dishes 14. The geometry of the surfaces of the reflecting dish segments 14a and 14b and that of the receivers 28 is designed to yield the optimal flux distribution upon the surface of the receivers.

Long rigid pipe 24 is one of the principal components of a mechanism that allows tilting of the dishes 14 as will be explained herein below. Long rigid pipe 24 can have different structures depending on the technology employed by the system to utilize the collected solar energy. If the system is used to heat thermal fluid flowing through a system of pipes, then long rigid pipe 24 conveys the thermal fluid. In this case long rigid pipe 24 is typically surrounded by a layer of thermally insulating material, which is symbolically shown as insulation 34 in the figures (most of long rigid pipe 24 is shown uncovered by insulation in order not to obscure other details of the construction). An embodiment of long rigid pipe 24 used in a system in which the receivers 28 are CPV cells will be described herein below. Fig. 3 shows a complete autonomous system 100 of the invention made by joining in series the ends of the torque tubes 12 and the long rigid pipes 24 of two units 10. The system is supported at each of its ends by pylons 36. At the top of each pylon 36 is a connection (not shown in the figures), e.g. a ball bearing, that both supports and enables the torque tube 12 to be rotated about its longitudinal axis thereby allowing rotational one axis tracking of the attached dishes. In the center of system 100 there is a drive pylon 38 on which all the mechanisms for rotation and tilting of the dishes towards the sun are mounted. In order to minimize shadowing by the pylons the dishes attached to the torque tube segment in each unit of the system are located asymmetrically on the torque tube unit 12. The dishes are located as far north (in the northern hemisphere— south in the southern hemisphere) as possible relative to the pylon located to the south of them.

Fig. 4A and Fig. 4B schematically show views from two different angles of an embodiment of the mechanisms for causing rotation and tilting of the dishes on the top of drive pylon unit 38 of the system 100 of the solar power system of the invention. In the embodiment shown in Fig. 4A and 4B, the drive mechanism responsible for the rotation of the torque tube 12 comprises two hydraulic pistons 46a and 46b connected on their lower ends to the drive pylon 36 and at their upper ends to the torque tube 12 through a linkage such that a push-pull activation of hydraulic pistons 46a and 46b will allow torque tube 12, and everything that is attached to it, to be rotated about a longitudinal axis in both clockwise and counterclockwise directions. This mechanism is the same as that used for the one axis tracking of parabolic trough systems. Pivotally attached to the torque tube 12 above the rotation components on drive pylon 38 is a third hydraulic piston 46c. The other end of hydraulic piston 46c is pivotally connected to a reinforced strut 26a (see Fig. 1) such that expansion and contraction motion of the hydraulic piston 46c causes strut 26a to pivot about its connection to the torque tube 12. Rotational motion of strut 26a causes longitudinal motion of long rigid pipe 24, which tilts the dish segments 14a and 14b as described herein above. Also shown symbolically in Fig. 4A and Fig. 4B are special reinforced structural parts 66 of the support by which piston 46c is attached to torque tube 12 to prevent twisting when piston 46c is activated. It is to be noted that parts like 66 and piston 46c are not needed for parabolic trough systems, which experience only axial movement.

All three pistons are an integral part of a single hydraulic oil system comprising an oil tank and an oil pressure pump. The embodiment of hydraulic drive mechanism described with reference to Fig. 4A and Fig. 4B is one example of drive systems that could be used with the invention. Other embodiments of drive systems that are based on motors, gears, cables, etc. may be employed to obtain the same results.

Fig. 5A and Fig. 5B schematically show views from two different angles of an enlarged view of the area enclosed in circles labeled "A" at both ends of the system 100 in Fig. 3. System 100 is designed to heat a circulating thermal fluid, e.g. water and steam, and employs a piping system similar to those used in convention trough systems. A ball joint 40a is used to connect the top of the header 42 to the long rigid pipe 24, which in this system is a heat pipe. A second ball joint 40b and a third ball joint 40c connect the bottom of the header to the supply pipe (not shown) running along the ground. The introduction of the ball joints 40a and 40b at the ends of system 100 enables the longitudinal movement of the heat pipe 24 and the movement of the attached header 42 relative to the torque tube. Ball joint 40c enables the rotation of heat pipe 24 and attached header 42 around a longitudinal axis at the center of the heat pipe while the system is tracking the motion of the sun. In other embodiments of the system flex hoses can be used instead of ball joints. In Fig. 5 A and Fig. 5B one can also observe that four 90° piping bends are present in the design together with the ball joints.

Fig. 6 schematically shows an illustrative, but not limiting, embodiment of a long system 200 of a solar energy system according to the present invention. In this embodiment four 12 m long torque tube units 10 are connected together on each side of the drive pylon 38 to form a torque tube 12 having a total length of 96 m. Two double segment dishes 14, with a total aperture of 50 m²> are mounted on each torque tube segment. Therefore, each row comprises eight long torque tube units 10 carrying 16 dishes with a total aperture area of 400 m². Four simple pylons 36 are present on each side of the drive pylon 38 to support the torque tubes 12.

Typically the rows of the system are oriented in a north-south direction (in both hemispheres). The drive mechanisms on drive pylon 38 and torque tube 12 arrangement provides the ability to rotate the dishes 14 to track the sun in its daily east-west motion across the sky. A second degree of freedom is introduced into the system of the invention by means of a third drive mechanism to cause linear motion of the long rigid pipe 24 to enable tilting the dishes 14 relative to the torque tube 12 in order to provide north-south tracking of the sun as its elevation angle relative to the torque tube changes along the day throughout the year.

As in the cases of parabolic trough, isolated dish and solar tower heliostat technologies a local controller is found on the drive unit. Each controller communicates with the central solar field control room, but may also show some independent capacities. It is recommended to employ a dual axes sun sensor, or two one axis sun sensors mounted on the top of the mechanism for rotating the torque tube 12 and on the strut 26a attached to hydraulic piston 46c that causes the longitudinal motion of long beam 24. The exploitation of sun sensors provides closed loop control of the tracking although non-closed loop means for tracking can also be employed.

A typical solar field plant according to the present embodiment of the invention comprises many parallel rows of dishes. Each row length can take the values up to 500-1000 m, i.e. 5 to 10 consecutive long systems of the type described in relation to Fig. 6, to yield a much cheaper investment in the solar field piping system. In contrast to this, in a solar plant based on deployment of many distributed isolated dishes the piping must feed the flow into each dish separately, i.e. climbing and descending piping is needed for each dish. Thus the system of the invention shares the same design characteristics of economic layout of the piping as that of the prior art parabolic troughs solar plants and also shares the two axis tracking system of the prior art dish technology.

For commercial applications in which many rows of solar units are involved, a staggered layout of the rows of the solar fields is recommended. In this layout the edge of each long row is somewhat shifted toward the north or toward the south, relative to its neighbors. This feature of the solar field layout design allows better optical collection efficiency of reflected sun rays during early morning and late afternoon when there is a shadowing effect of one row upon its neighbors, because the sun's rays will be able to pass through the gaps between the mirrors of one row to hit upon the mirrors of the adjacent row.

The basic design of the system as described herein above is suitable for a variety of applications. It is especially appropriate when flows of fluids into and out of the receivers 28 are involved, e.g. water for Direct Steam Generation (DSG) or cooling fluid for Concentrating Photo Voltaic (CPV) systems. The rigid long pipes 24 to be used are designed to have the appropriate stiffness to transfer the longitudinal forces needed to exert the proper moments, which cause the movement that changes the elevation, i.e. tilt, angle. Unlike the pipes in the technology of parabolic trough, the piping in the present invention plays an active role in tracking the sun.

Two categories of receivers can be used with the system of the invention. The first category is receivers that share the two axes angular rotations and tilting of the dishes and the second category are quasi-static receivers, which are static relative to the piping or the longitudinal beam.

Receivers sharing the two axes angular rotations of the dish can be used in a solar system based on Concentrating Photo Voltaic cells (CPV). Fig. 7 symbolically shows a section of a unit of the system of the invention configured for use with CPV cells. The concentrated light beam that is formed by the reflections of solar rays from each dish mirror segment 14a (and from mirror segment 14b, which is not shown) impinges upon a receiver 28a. Receiver 28a comprises a hollow shell and an aperture facing the reflecting dishes that is lined with high quality photo voltaic cells, e.g. multi junction cells. The excess of energy not used by the cells is absorbed as heat transferred into a coolant liquid, which flows into the interior of receiver 28a from a coolant supply pipe 44 to keep the PV cells at as at low a temperature as possible for better efficiency. The heated coolant liquid flows back to a refrigeration unit and circulation pump at a central location in the field via coolant return pipe 48. In this configuration coolant supply pipe 44 also serves as the long rigid pipe that is a part of the dish tilting mechanism. The photo voltaic receiver 28a is attached to the rigid rods 32 to share the same alignment as that of the dish. In this configuration the orientation of the incoming rays into the receiver is fixed (relative to the aperture of the receiver) during the tracking. Flexible pipes 50 (see figures 8A and 8B) convey the cold coolant flowing in the rigid supply pipe 44 into the multiple receivers present in the system. The heated coolant is then removed from the receivers through other flexible pipes 52 (see figures 8A and 8B) into a return pipe 48 located above the cold pipe. Such a design assures savings in piping investment.

Fig. 8 A is an enlarged view of area B in Fig. 7. Fig. 8B is a cross-section of receiver 28a, showing hollow interior 54. In embodiments of the invention, there is one CPV receiver for both dish segments. An alternative design may introduce instead two CPV receivers, wherein each dish segment reflects light onto its own CPV receiver.

Quasi-static receivers are suitable for solar thermal applications in which the fluid in the piping is characterized by high pressures and high temperatures, e.g. a solar thermal system of the invention configured for Direct Steam Generation (DSG). For every dish 14 composed of two mirror segments 14a and 14b there is installed a receiver comprised of a hollow shell with a pair of cavity like surfaces 28b facing each of the two dish segments, located symmetrically on both sides of the pipe 24 (see figures 9 A and 9C). The surfaces of cavities 28b are coated with a high quality absorptive layer designed for maximum absorption of reflected rays with wavelengths in the solar spectrum. The best design for the absorptive layer calls for the application of a selective coating that provides maximum absorption of light with wavelengths in the solar spectrum, together with low emissivity of infrared radiation having wavelengths characteristic of the operational temperatures of the absorbing surface. The focus of the reflected rays, from each mirror segment, takes place at some point away of the absorptive receiver surface 28b in order to assure the proper energy distribution upon the absorptive receiver surface and to prevent too high a flux, which may result in high values of temperature and temperature gradients across the receiver, which may cause distortions and cracks in the neighborhood of the receiver. In an embodiment of the invention each absorptive surface 28b has the geometry of the interior surface of a segment of a spherical shell. The focal point of the concentrated beam is located at the center of the sphere. The value of the radius of the sphere as well the location of center of the sphere relative to the rigid pipe are chosen to assure the optimal non-disturbed entrance of the reflected rays and the desired energy distribution upon the surface of the receiver during tracking throughout the day and the year. The absorptive surfaces are located a little bit below and to the side of the center of the rigid pipe 24 to avoid any blocking of the path of reflected rays toward the receiver.

Fig. 9A and Fig. 9C symbolically show in perspective and bottom views respectively an embodiment of a quasi-static receiver. Fig. 9B is a cross- sectional view of Fig. 9A. The receiver 28 has a hollow shell design to allow stable flow of relatively cool fluid, or fluid at boiling temperature through apertures 56 from the main rigid pipe 24 into the interior of the shell on both sides of pipe 24 and of relatively hot fluid and steam back into the pipe 24 to assure the proper temperature and temperature variations through all parts of the receiver and also to match the designed allowed metallurgical tolerances. The temperatures and temperature gradients along the receiver walls depend upon the irradiation distribution on its inner surface together with the flow regime characteristics next to the outer wall surface. The receiver 28 can be designed for flow in the south to north direction and for flow in the north to south direction. As mentioned above, part of the flow in pipe 24 is deflected into the receiver via the apertures 56. For two phase flow some liquid is always present in the low section of the receiver (for all tracking positions). Turbulence, waves and stirring should be considered within the hollow interior of the receiver to assure permanent wetting of the inner walls whose outer part is exposed to solar irradiation flux. The design of the receiver structure should maintain the rigidity demands of the long rigid pipe 24 to accommodate the forces involved in the longitudinal movements that are responsible for adjusting the tilt of the dishes. Unlike the receivers that share the two axes angular rotations and tilting of the dishes, for the quasi-static receivers the orientation with which the incoming rays impinge upon the absorbing surface changes while the system moves to track the sun. The hinge attached to the pipe next to the receiver connected to the supports 32 (as can be seen in figures 1-3) in each dish pass through the centerline of the rigid piping. Since the focal point of the dishes does not lie on the hinge axis, the locus of the focal point of the incoming beam changes, while tracking, relative to the receiver (because the focal point is located at some distance away of the rigid pipe centerline). The design of the receiver should cope with these optical features resulting from changes of the tilt angle. Some further adjustment of the reflections into the absorbing surfaces may be achieved by an additional global slight movement of the rigid piping 24 for each unit 100.

The use of quasi-static receivers provides the system of the invention with three major important advantages over prior art solar thermal parabolic trough technology:

1. The present system exploits dishes and two axis tracking to concentrate the solar radiation with high concentration ratio, i.e. essentially point concentration, onto the receivers. The high concentration rate enables the receiver to be exposed to the ambient atmosphere, while keeping the rate of heat losses relatively modest. This is as compared to parabolic troughs having only one axis tracking that concentrate the reflected rays onto a line. This results in smaller concentration ratio and necessitates the introduction of sealed elongated unit, the so called HCE (Heat Collection Element). The disadvantages of the HCE's have been discussed herein above. In the system of the invention one can avoid the introduction of a glass cover in front of the absorbing surface allowing more of the incoming energy to be absorbed by the receiver and transferred to the thermal fluid. The exposed surface of the receiver also enables simple access from ground level for maintenance and renovation of the coating if needed as opposed to replacement of the whole receiver.

To summarize this paragraph it is clear that heat losses out of the exposed receivers are relatively small due to the high concentration ratio typical to solar dishes. Other heat losses are minimized by installing thermal insulation upon the piping.

2. The optical design of the cavity like receivers used in the system of the invention leads to good impact angles of the reflected rays upon the absorbing surfaces with only modest deviations from normal impact of a reflected ray. In contrast, for HCE's of parabolic troughs one finds oblique impact angles of rays upon the cylindrical shaped receiver occur with the situation turning worse in winter when the sun's tilt angle becomes larger.

3. The small dimensions of the receivers in the present invention make it feasible to design local wind and dust shields adjacent to each receiver.

These shields minimize the thermal losses due to wind induced air flows and also protect the coating from intensive erosion.

The design of the shield may include a rigid isolated enclosure attached to the supports of the piping, next to the receiver, with a narrow elongated aperture whose center coincides with the focal point of the reflected rays. Two such rigid enclosures with their apertures exist on both sides of the receiver. For quasi static receivers the apertures keep the same spatial orientation with respect to the dish segments to allow a free path for the reflected rays into the receiver. In addition to the rigid enclosure a flexible sheet with thermal insulation can be connected both to the rigid enclosure and to the piping insulation to provide full shielding of the receiver, while assuring free movement of the shield during changes of tilt angles.

It is also possible to install shutters on the apertures of the enclosure to be closed during non-operating hours, and especially during storms.

Center of mass considerations are included in the design of the system of the invention. Specifically the center of mass of the system is located along the center of the torque tube 12, which also coincides with the rotational axis of tracking. Such considerations lead to a design in which the center of mass of each dish together with the relevant piping and its supports coincides with the rotation axis, passing through the center of the torque tube. These center of mass design considerations avoid unnecessary extra rotational moments which exert opposing forces on the tracking mechanisms.

Considerations related to the optical design together with the need to locate the center of mass of each dish in the center of the torque tube result in the observation that, when the system of dishes is facing the sky, most of the reflecting surfaces of the dish segments are located below the torque tube. In this configuration, light reflected from the dish surface towards the receiver will be blocked by the torque tube during tracking. The desire to minimize blocking by the torque tube together with the advantage of getting better penetration of the reflected rays into the receiver led the inventor to a design in which the dishes are comprised of two separated symmetrical mirror segments located on both sides of the torque tube. With this design the receiver for the reflected light comprises two surfaces for absorbing the reflected flux located on opposite sides at the bottom of long rigid pipe 24 and separated laterally to allow for each mirror segment to concentrate the reflected rays upon two different focal points.

The degree of freedom related to the tilt necessitates a longitudinal movement of the rigid piping 24 or 44. This movement is transformed into changes of the tilt angles of the mirror segments. Unlike the rotational movement, the variation of the tilt angles is much more limited. The tilt angle of the sun, relative to the zenith, at solar noon along the year, changes by 46 degrees. The tilt angle at solar noon in the northern hemisphere takes its minimal value on 21^st of June and its maximal value on 21^st of December. These two extreme angles coincide with those of the tilt in the solar power system of the invention at solar noon. Due to the northern location of the sun during sunrise and sunset in the summer, some extra tilt angles towards the north direction are needed during early morning and late afternoon in summertime. This limited angular range simplifies the design of the drive unit that causes the longitudinal movement, which is thus also limited in the distance that it has to move the pipe 24. The maximum tilt angle of the system during the year, which takes place at solar noon on the 21^st of December, is equal to the sum of the local latitude angle and 23 degrees (e.g. in latitude of 37 degrees the maximum tilt angle is 37+23=60 degrees).

The value of the maximum tilt of the system can be lessened if the design of the system includes an off center receiver to yield a permanent deflection of reflected rays towards the north.

Fig. 10A and Fig. 10B schematically illustrate the permanent deflection of reflected rays towards the north resulting from the off center location of the receiver. In most existing dish systems the receiver is located on the optical axis of the parabolic dish, which also passes through the center of the reflecting surface. In the system of the invention the receiver is also located on the optical axis of a parabolic dish, which is shifted north of the center of the reflecting surface to form a permanent angular deviation of angle d.

In Fig. 10A the reflecting segment is shown in a vertical position, i.e. pointing towards the sky with no tilt. The optical axis 57 of the dish is parallel to the zenith and is located north of the center of the reflecting surface . If the sun 58 were in the zenith the ray coming from the sun's center that impinges on the center of the reflecting surface will be reflected towards the receiver. Such a reflected ray will form an angle of d = 12 degrees with respect to the incoming ray. The receiver is located north of the center of the reflecting surface.. The angle d is defined as the off center angle. The value d=12 degrees is specific for the embodiment of figures 10A and 10B. All parallel rays originating from the sun 58, which are also parallel to the optical axis 57, impinging on the surface of the dish will, according to the law of reflection for a parabolic surface, be reflected toward the receiver located on the optical axis 57. The geometry of the reflecting surface of a dish segment is that of a non symmetric portion of a parabolic dish. In Fig. 10B the sun's inclination at noon forms an angle of 60 degrees with respect to the zenith (this is the situation on the 21^st of December in the northern hemisphere for a system located at latitude of 37°). However, since the receiver is located off center at a permanent angular deflection angle, i.e.12 degrees towards the north, the tilt of the dish towards the south forms only an angle of 48° relative to the zenith. Thus the introduction of an off axis receiver, associated with permanent angular deviation of reflected rays towards the north, decreases the maximal angle through which the dishes of the system have to be tilted. It is noted that the previous discussion is for the case of a north-south orientation of a system located at a specific location in the northern hemisphere but it should be clear that a similar calculation can be carried out to determine the permanent angular deflection of reflected rays towards the south in the southern hemisphere. The design of the off center receiver has some mechanical as well as optical benefits. Lessening the maximal tilt angle decreases the longitudinal forces needed for tilting. Since the longitudinal force transformed into the moment needed for tilting increases with the tilt angle due to shortening of the lever arm, the maximal longitudinal force at the maximal tilt angle is reduced in comparison with the case of greater tilt angle.

Another advantage of the off center receiver is related to the necessity of stiffening the structure supporting the two mirror segments 14a and 14b in order to assure the simultaneous tracking accuracy of both. A possible design option for stiffening the structure is to add additional secondary torque tubes for each dish, located below the mirrors, whose orientation is normal to the longitudinal torque tube of the system. Lessening the maximal tilt angle helps the designer to avoid the secondary torque tube interfering with the longitudinal torque tube.

The reduced value of the maximal tilt angle also assures that all reflected rays will enter safely into the receiver as can be seen in Fig. 10B. Even the reflected rays coming up from the northern edge of the dish can travel freely into the receiver well below the piping 24.

The introduction of a permanent angular deflection of reflected rays for a symmetrical dish reflector with one radius of curvature, i.e. the curvature obtained by rotating a parabola around the optical axis, results in an astigmatic effect (astigmatism). Such astigmatic effect causes a somewhat blurred spot instead of a bright well concentrated spot typical for a dish facing directly at the sun. Astigmatism is avoided by shifting the optical axis of the parabola towards the north edge of the reflecting surface. Thus the geometry of the reflecting surface is that of a nonsymmetrical portion of a parabola. The dish segments 14a and 14b of the present invention are manufactured to have the proper geometry to allow them to provide a high quality concentrated beam onto the off center receiver as described above. The details of the geometry of the reflecting surface of the dish segments should be determined together with the design of the receiver.

The design for solar thermal applications should also provide a mechanism related to the extra longitudinal movement caused by the thermal expansion of the piping to realize optical corrections needed for the shifting of the reflected rays, which results in non-uniform optical deviations of the focal points relative to the apertures of the array of receivers along the system. When the pipe in a thermal application warms up, the thermal expansion causes an extra longitudinal movement of each receiver together with the adjacent hinge connecting the rigid rod 32 to the pipe 24. The amount of this movement increases as we move away from the central drive pylon. The movement due to thermal expansion takes the south direction for the pipe segment located south of the drive pylon and takes the north direction for the pipe segment located north of the drive pylon. Fortunately this movement also deflects the reflections of light rays out of the mirrors into the same direction of the receiver movement (assuming that, due to warming up, the receiver changes its angle relative to the cold pipe case by the amount alpha). But according to the law of reflection the reflecting dish should change its angle only by half the value of alpha to assure that the reflected rays will enter properly into the receiver.

For solar thermal applications that the inventor proposes that location of the hinges connecting rods 32 to pipe 24 be sliding, i.e. the location of each hinge is movable and thus not fixed to pipe 24, to compensate for the thermal expansion. The longitudinal shift of the hinge will assures that the reflected rays from the dish mirrors will enter into all receiver apertures while the heated piping is expanding. Such a mechanism introduces different movement for each of the sliding hinges since the effect of thermal expansion is gradual along the system. As explained above, as the receiver is located further away from the drive pylon, the changes of the location of the hinges due to thermal expansion become greater and thus necessitate different adjustment for each receiver. The movement of the hinges can be realized in two ways: An active correction caused by local motors, gears etc. for each hinge next to the receiver, or by introducing a passive compensation mechanism which transforms the thermal expansion movement itself to yield the required change of position for each hinge.

An embodiment of passive thermal expansion compensation mechanism for moving the hinges is shown in Fig. 11A, Fig. 11B, and Fig. 11C. These figures show an embodiment of the mechanism for the southern part of the system, i.e. the part of the system located south of the drive pylon. In the figures some of the features described herein below are omitted for clarity and because of the difficulty of showing them in two dimensional drawings. Fig. 11A schematically shows an embodiment of the thermal expansion compensation mechanism for the first dish segment 14a on the south side of the drive pylon 38. The components of the mechanism (inside rectangle 70 and described in detail with respect to Fig. 11B) are attached to a structural base plate 67 which is rigidly attached to the heat pipe 24. An identical mechanism is present on the other side of heat pipe 24 for dish segment 14b for each of the other dish segments on both the south and north sides of the drive pylon. Note that the edges of the structural beam 64 are free at the south as well as at the north ends. This means that the amount by which the receiver shifts due to thermal expansion of pipe 24 increases linearly from the drive pylon 38 in both directions. A long structural beam 64 is added into the design to be located above the thermal isolation of the pipe 24. The new beam 64, which is made of a light structural material, is rigidly connected by part 69 to the thermal pipe 24 next to the hinge attaching the lever arm 26a responsible for the longitudinal tracking movement. This connection establishes a mechanical "fixed point" for all pipe temperatures and shares the same longitudinal movement related to the tilt with the pipe at this point. In an embodiment, beam 64 is made of aluminum with a hollow rectangular cross section. The beam 64 remains at the constant ambient temperature when thermal pipe 24 heats up and expands.

As shown in Fig. 11B, the thermal compensation mechanism comprises a downward extending part 68 of base plate 67, two plates 62a and 62b and four hinges, i.e. pivots, 60a, 60b, 60c, and 60d. The hinge 60d, connected to part 68 of base plate 67, shares the same longitudinal movement of the pipe 24 and thus moves to the south when thermal expansion is realized (for all tilt angles). The hinge 60b is located on the new structural beam 64 and its position is unchanged when the pipe temperature gets warmer (for every tilt angle) since beam 64 experiences no thermal expansion. Plate 62a connects hinge 60b with hinge 60d, while another hinge 60c is located in the middle of plate 62a. Plate 62a supports beam 64 in its location next to the pipe 24.

Due to thermal expansion, the hinge 60d and the pipe 24 move towards the south longitudinally by a distance D relative to hinge 60b located on the structural ambient temperature beam 64. The hinge 60c, at the center of plate 62a, moves only by the amount of -D. A second plate 62b is constrained to move horizontally and transfers the same longitudinal movement -D to the hinge 60a through which the pipe 24 is connected by rod 32 to the dish segment. Only one support 32 of the dish segment is shown in Fig. 11B and the receiver is also not shown to avoid unnecessary details for explaining the mechanism. Plate 62b is needed to keep most of the mechanical parts away from the hinge 60a, which is very close to the receiver in order to avoid exposure to intensive radiation. In practice, the upper ends of the rods 32, which are connected at their lower ends to the framework 30 (see Fig. 11 A), are attached on either side of the receiver to a structural plate (not shown) that is attached to base plate 67 by hinge 60a in such a way that it is spaced away from base plate 67 to allow it to pivot, pulling one rod 32 up while pushing the other rod 32 down, without hitting the receiver.

Such a mechanism allows for gradual adjustments of the locations of the hinges 60a along the piping. Assuming that the heat pipe 24 is made from carbon steel 1020 with thermal expansion of 0.0000126, the farthest dish from the central drive pylon is located 50m away from the central drive pylon, and a Direct Steam Generation (DSG) application with fluid/saturated steam temperature of 350° C then the temperature change with respect to initial ambient temperature of 20° C will be 330° C. Thus the correction needed for the southernmost dish and its receiver as well as the northernmost dish and receiver at the other end of the row, which is the greatest correction needed for the system can be estimated to be:

D=50 x 0.0000126 x 330 = 0.2m = 20cm while the hing °e 60a movement is - 2D = 10 cm.

Fig llC schematically shows the southernmost receiver with its thermal expansion compensation mechanism in 5 typical consecutive positions: detail A refers to the initial cold position when thermal pipe 24 has the same temperature as the beam 64; detail E corresponds to maximal temperature of pipe 24, while beam 64 maintains the ambient temperature; details B to D show intermediate stages as pipe 24 heats up to its maximal temperature. Non-sliding supports of the struts may cause distortions and bending of pipe 24. If this occurs, the compensation mechanism described herein can be applied also on the struts 26 which are also supporting the pipe 24 to the torque tube 12 to allow for a gradual change of the locations of the hinges that connect the struts 26 to the heat pipe 24.

Although embodiments of the invention have been described by way of illustration, it will be understood that the invention may be carried out with many variations, modifications, and adaptations, without exceeding the scope of the claims.

Claims

1. A unit of a solar power system based on the integration of parabolic trough technology and its piping design with solar dish technology, said unit comprising:

a. a torque tube;

b. at least one dish comprised of two reflecting mirror segments, each of the dish segments supported with a gap between them symmetrically on opposite sides of the torque tube by a framework that is pivotally attached to the torque tube in a manner that allows the framework and the attached dish section to be tilted around an axis perpendicular to and passing through the center of the torque tube; c. a rigid pipe, that is held parallel to and attached to the torque tube segment by struts attached at their upper ends to the rigid pipe and at their lower ends to the torque tube and by rigid rods that are attached at their top ends to the rigid pipe and at their lower ends to a corner of framework that supports the dish segment; wherein the top and bottom ends of the struts and the rigid rods are connected to the rigid pipe, the torque tube, and the frameworks respectively with connections that allow the rigid pipe to be moved back and forth relative to the torque tube while remaining parallel to it; and d. a receiver supported by the rigid pipe at the focal point of each of the dish segments;

wherein, the rigid pipe is designed to carry out two functions:

i. to carry a flow of either thermal fluid in a thermal solar system or cooling fluid in a concentrating photo voltaic system; and ii. to play an active role in tracking the sun by changing the elevation angle of the mirror segments when the rigid pipe moves back and forth relative to the torque tube while remaining parallel to it.

2. The unit of claim 1, wherein the geometry of the mirrors surfaces is that of a non-symmetric portion of a parabola, which is formed by rotating a two dimensional parabola around its symmetry axis.

3. A solar power system comprised of at least two of the units of claim 1 joined together in series at the ends of their torque tubes and rigid pipes.

4. The system of claim 3, comprising:

b. one drive pylon on which mechanisms for rotation and tilting of the dishes towards the sun are mounted.

5. The system of claim 4, wherein:

b. the mechanism for tilting the dishes is a third hydraulic piston that is pivotally attached to the torque tube above the rotation components on drive pylon, wherein the other end of the third hydraulic piston is pivotally connected to a reinforced strut that supports the rigid pipe above the torque tube such that expansion and contraction motion of the third hydraulic piston causes the reinforced strut to pivot about its connection to the torque tube.

6. The system of claim 5, wherein all three pistons are an integral part of a single hydraulic oil system comprising an oil tank and an oil pressure pump.

7. The system of claim 4, comprising a dual axes sun sensor or two one axis sun sensors configured to allow closed loop control of the tracking.

8. The system of claim 4 aligned in parallel to a north-south axis, said system comprising a gap separating two adjacent dishes, wherein the dishes that are attached to the torque tube segment in each unit of the system are located, in the northern hemisphere, as far north as possible relative to the pylon located to the south of them in the southern hemisphere as far south as possible relative to the pylon located to the north of them.

9. The system of claim 4, wherein the receivers are located off center of the reflecting surface to yield a permanent deflection of reflected rays towards the north in the northern hemisphere and to the south in the southern hemisphere.

10. The system of claim 3 configured for use in a solar system based on Concentrating Photo Voltaic cells (CPV), said system comprising:

a. photo voltaic receivers attached to the rigid rods, wherein the receivers comprise a hollow shell and an aperture facing the reflecting dishes that is lined with high quality photo voltaic cells;

b. a coolant supply pipe that supplies coolant liquid to the interior of the hollow shell and also serves as the rigid pipe;

c. a coolant return pipe that returns coolant liquid heated by the excess of energy not used by the cells back to a refrigeration unit and circulation pump at a central location; d. flexible pipes configured to convey the cold coolant flowing in the coolant supply pipe into the hollow shells of the receivers and to remove heated coolant from reflector shell of the receivers to the coolant return pipe.

11. The system of claim 10 comprising one of:

a. one CPV receiver for each dish, the receiver configured to receive energy reflected from both dish segments; and

12. The system of claim 3 configured for use in a thermal solar system based on heating of a fluid flowing through a rigid pipe that serves as the long rigid pipe, said system comprising a receiver comprising a hollow shell through which at least a part of the fluid flowing through the rigid pipe flows and a concave surface with high quality absorptive coating, the receiver fixedly attached to the rigid pipe for each dish and characterized by high pressure and high temperature of the fluid.

13. The system of claim 12, wherein each receiver comprises one of: a single concave surface facing both mirror segments of a dish or two concave surfaces each surface facing one of the two mirror segments of the dish.

14. The system of claim 12, wherein the flow of fluid through the receiver can be in either the south-north direction or in the north-south direction.

15. The system of claim 12 comprising a piping assembly comprised of four 90° bends and three ball joints at each end of the system, thereby allowing rotational tracking as well as longitudinal motion of the pipes associated with tilting of the dishes and thermal expansion of the rigid pipe.

16. The system of claim 12 comprising insulation surrounding the rigid pipe through which the fluid flows.

17. The system of claim 12 comprising a compensation mechanism to provide a different amount of compensation for each receiver for the shifting of the light rays reflected from the dish segments towards the receivers as the fluid flowing through the rigid pipe heats up causing thermal expansion of the rigid pipe.

18. The system of claim 17, wherein the compensation mechanism comprises sliding hinges that connect the tops of the rigid rods to the rigid pipe.

19. The system of claim 18, wherein the sliding hinges are configured to introduce a different amount of movement for each of the sliding hinges along the length of the rigid pipe.

20. The system of claim 18, wherein the movement of the hinges is realized in one of the following ways:

b. a passive compensation mechanism which transforms the thermal expansion movement into the required local amount of change of position for each hinge.

21. The system of claim 1 comprising a local wind and dust shield adjacent to each receiver.

22. The system of claim 21 wherein the shield comprises a rigid isolated enclosure attached to the rigid rods next to the receiver, the enclosure comprising an elongated aperture whose center coincides with the focal point of the rays reflected from the mirror segments.

23. A solar field plant comprising many parallel systems of claim 3 connected together in series.