AN EVACUATED TRANSPARENT ENCLOSURE

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U. S. Provisional Patent Application No. 62/799,374, filed on January 31 , 2019, which is incorporated by reference herein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

[0002] Not Applicable.

BACKGROUND OF THE DISCLOSURE

[0003] Solar power systems are known in which a solar field is provided having a multiplicity (hundreds or thousands) of trough-type solar collector assemblies (known as a“SCAs”) arranged in a solar trough field. Each SCA typically has an elongate, preferably tubular, solar receiver that extends longitudinally within the SCA. Typically, the SCA also includes an elongate reflector, which may be of different shapes. Such reflectors are often elongate parabolic reflectors and the solar receiver is a pipe or tube that is located at or near (proximate) a focal point (or focal line) of the parabolic reflector so that the solar radiation striking the reflector is concentrated to impinge on the solar receiver tube. A heat transfer medium, such as water, mineral oil, synthetic oil, molten salt, or supercritical C0 ₂ flows through the tubular solar receiver and is heated to an elevated temperature by solar radiation focused on the receiver by the reflector. Conventionally, an SCA includes a metal support structure for supporting the solar receiver and/or the reflector. Still further, the SCA may include a solar tracking system including a sensor that tracks the movement of the sun across the sky over the course of the day or a season and that moves the reflector so as to follow the sun so as to maximize the amount of solar radiation that impinges or strikes the solar receiver pipe.

[0004] A so-called conventional Parabolic Trough Power Plant (PTPP) is illustrated in Fig. 1 in which a plurality of SCAs that comprise elongate parabolic reflectors that focus radiation from the sun onto an absorber pipe typically located at or near a focal point or line of the reflector. In a typical PTPP system, the heat transfer medium is heated to a temperature of about 400°C (752°F) as it flows through its respective SCAs. This high temperature heat transfer medium or working fluid may be used in a steam boiler to heat water and to generate steam, which, in turn, is used to power a steam turbine and generator set in the conventional manner to produce electrical power. The heat transfer medium is then recirculated through the SCA to be reheated and reused. The steam discharged from the turbine is condensed by a condenser in the conventional manner and is re used as feedwater for the steam boiler. In addition, storage tanks may be provided in which heat from the hot heat transfer medium may be stored for use when there is a high electrical demand or when solar radiation is not available.

[0005] However, certain shortcomings have been noted with SCAs used in such PTPP systems. First, convection thermal losses from the high temperature of the absorber pipe and its being exposed to the atmosphere diminish the thermal efficiency of the SCAs. Also, because the SCAs and their reflectors are located out of doors, the reflectors will collect dust, dirt, and water condensation (dew) that diminish their reflectivity, and markedly reduce the amount of solar radiation focused on the absorber pipe. Still further, because the SCAs of such systems are located outdoors, these systems typically require substantial mechanical supporting structures to support the SCAs and to resist additional forces of wind, snow loads and the like. To overcome these problems, one solution has been proposed, as shown in U. S. Patent 8,915,244, where the entire solar field, which may comprise many hundreds or even thousands of SCAs, is enclosed in a so-called “glasshouse” (i.e., a greenhouse). However, the surface area covered by such a solar field may be quite large. For example, in a PTPP having a net output of about 64 megawatts of electrical power may have a solar field area of about 357,000 square meters (about 88 acres). Of course, the cost of enclosing such a large solar field within such a glasshouse is considerable. Moreover, the structure of such a glasshouse reduces the amount of solar radiation reaching the SCAs therewithin due to the reflectivity of the glasshouse and the structure necessary to support the glasshouse partially shades the SCAs therewithin. Still further, the glass in such glasshouses is also subject to having dust and dirt accumulate thereon, which adversely affects the efficiency of the solar system. It has also been proposed to enclose the absorber pipe within a transparent tube and to evacuate the tube so that a vacuum or a partial vacuum surrounds the absorber pipe. While this approach may lessen convection thermal losses from the absorber pipe, it will not prevent the reflectors from becoming dirty and thus less efficient.

[0006] Still further, with such SCAs where the reflector is exposed to the atmosphere, it has been noted that at night condensation (dew) may accumulate on the reflector. In applications where the solar field is enclosed within a glasshouse, condensation may be present on the glass of the glasshouse. At sunrise, it will take some amount of time until this condensation evaporates during which time the efficiency of the SCA is markedly reduced. Also, if rain, ice, or snow accumulates on the reflectors or on the glass of the glasshouse, the overall operability of the entire system is compromised even after the weather event has passed, especially if there are accumulations of snow or ice on the reflectors or on the glass.

[0007] There has been a long-standing need for solar power systems, including but not limited to PTPP systems, for an economical solution so as to lessen convection thermal loses from such absorber pipes and to prevent lessening of the reflectivity of the reflectors due to dirt and dust accumulation on the reflectors. Also, it is important that the reflectors be able to be moved to track the position of the sun so as to maximize the amount of thermal energy transmitted to the absorber pipes of the SCAs. SUMMARY OF THE DISCLOSURE

[0008] In accordance with the instant disclosure, a solar collector assembly is provided having a solar reflector for concentrating solar energy, a solar receiver pipe for absorbing solar energy concentrated by the solar reflector. The solar receiver pipe is adapted to have a working fluid flow therethrough for being heated by the solar energy absorbed by the solar receiver pipe and to be transported from the solar collector assembly to perform a predetermined task or function, such as to generate steam or to serve as a heat reservoir. The reflector and the solar receiver pipe are enclosed within an elongate transparent tube. The tube has an end cap sealably secured to each end of the tube, and the solar receiving pipe extends through an opening in each of the end caps and is sealed with respect to the end caps in such manner as to permit the pipe to expand or contract with respect to the expansion or contraction of the transparent tube due to differences in thermal expansion between the transparent tube and the pipe. The transparent tube is capable of being substantially evacuated of air from therewithin so as to form a vacuum or a partial vacuum within the transparent tube thereby to minimize thermal losses from the pipe due to convection and to prevent condensation within the tube.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Fig. 1 is a schematic of a prior art Parabolic Trough Power Plant

(PTPP) having a solar trough field of a multiplicity solar collector assemblies (SCAs) each of which has a reflector, such as a parabolic reflector, that concentrates solar radiation on a solar receiver pipe or line located along the focal line of a respective reflector for absorbing the concentrated heated of solar radiation and transferring that heat to a heat transfer medium, such as a suitable oil or the like, flowing through the solar receiver line and using that heat in a boiler to generate steam to drive a steam turbine to generate electricity; [0010] Fig. 2 is a perspective view of a solar collector assembly (SCA) of in accord with the present disclosure in which a reflector, such as a parabolic reflector, a solar receiver pipe, and other components are located inside of a transparent or semitransparent tube in which a vacuum or a partial vacuum is drawn and maintained so as to reduce thermal convection heat losses from the solar receiver pipe and to protect the reflector from the environment to prevent dirt, dust, condensation and the like from accumulating thereon, which may adversely affect the efficiency of the solar collector assembly, which lessens the mechanical forces acting on the collector due to weather and the like, and which allows for a lower cost system, both in terms of capital investment and operational costs;

[0011] Fig. 3 is a transverse cross-sectional diagrammatic view of a solar collector assembly of the present disclosure illustrating a trough- shaped generally parabolic reflector mounted within a transparent or semi transparent tube with a solar receiver pipe mounted within the tube at or near the center of the tube and lying along or near a focal line of the reflector and with the reflector being mounted for rotational movement within the tube so as to be moved by a suitable drive system in response to the movement of the sun across the sky so as to maximize the concentration of solar radiation on the solar receiver pipe, with the reflector being shown in solid lines in an intermediate position so as to maximize the collection of solar radiation during the mid-day time and with the shape of the reflector being illustrated in dotted lines in first position so as to maximize the collection of solar radiation in the early morning, with the reflector being movable over the course of the day from the first position through its intermediate position to a second position, which is substantially a mirror image of the first position on the opposite side of the intermediate position so as to maximize the collection of solar radiation in the late afternoon; [0012] Fig. 4 is a transverse cross-sectional diagrammatical view of another embodiment of a solar collector assembly of the present disclosure having a generally parabolic reflector and other components mounted within a transparent (or semi-transparent tube) cylindrical tube with the reflector with an end cap sealably secured to each end of the tube, and with a solar receiver pipe positioned to extend along a focal line of the reflector and to extend out beyond the end caps, and with the tube and the reflector therein being rotatable about the longitudinal axis of the pipe so as to maximize the amount of solar collection concentrated on the pipe over the course of a day or over the seasons of the year; and

[0013] Fig. 5 is a transverse cross-sectional view of still another embodiment of a solar collector assembly of the present disclosure in which a generally parabolic reflector has a transparent (or semi transparent) cover sealably affixed to the upper edges of the reflector to form a non-cylindric tube with an end cap sealably secured to each end thereof such that the interior of the interior of the space defined by the reflector and the cover may be evacuated to form a vacuum or partial vacuum therewithin, and with a solar receiver pipe mounted within the non-cylindrical tube to extend along or near a focal line of the reflector and to extend out beyond the end caps, wherein the non-cylindrical tube may be rotated about the longitudinal axis of the pipe so as to maximize the amount of solar energy concentrated on the pipe over the course of a day or over the seasons of the year.

[0014] Corresponding reference characters indicated corresponding parts throughout the several views of the drawings.

DETAILED DESCRIBED OF PREFERRED EMBODIMENTS

[0015] Referring now to Fig. 1 , a prior art, so-called parabolic trough power plant (PTPP) is indicated in its entirety at 1 . This power plant includes a field 3 of solar collector assemblies (SCAs). Where each SCA is generally indicated at 5. Each SCA includes a generally parabolic solar reflector, as indicated at 7, which focuses the radiation from the sun on an elongate solar receiver pipe 9 having a conduit there through for fluid flow, which pipe absorbs the heat of the solar radiation and heats a working or heat transfer fluid, such as water, oil, molten salt, carbon dioxide (C02), supercritical carbon dioxide (sC02), or the like, flowing through the pipe. Such solar receiver pipes may be made of a suitable metal that has sufficient strength to withstand internal pressures, as may be occasioned upon heating of a working fluid within the receiver pipe above its boiling point. Preferably, such solar receiver pipes have a high emissivity surface so as to maximize the heat absorbed from the solar radiation impinging thereon.

[0016] Each SCA 5 has a reflector 7 preferably shaped to concentrate solar radiation or energy along an elongate line of concentration. More specifically, each reflector may be of a generally parabolic shape having an elongate focal line F - F that constitutes the above mentioned line of concentration. Preferably, the receiver pipe 9 is positioned to be within (coaxial with) the focal line of the reflector to concentrate the focus of solar radiation on the pipe to heat the working fluid within the pipe to a high temperature. As shown in Fig. 2, the reflector 7 and the pipe 9 are located within a transparent (or semi-transparent) tube 29 (as will be described in detail hereinafter). As shown in Fig. 2, the tube 29 has a longitudinal centerline A - A and the focal line F - F of reflector 7 shown therein is coaxial with the longitudinal centerline A - A of the cylindrical tube.

[0017] Emissivity is generally understood to mean the relative power of a surface to emit heat by radiation. Emissivity is the ratio of the radiant energy emitted by a surface to that emitted by a black body at the same temperature and is a measure of the efficiency in which a surface emits or absorbs thermal energy and more particularly radiation energy including electromagnetic energy in the infrared, visible, and ultraviolet wave length ranges. A black body is a material that is a perfect emitter or absorber of heat energy and has an emissivity value of 1 and a reflective value of 0. A material with an emissivity value of 0 would be considered a perfect thermal reflector. While reflectors 7 may be made of a variety of materials that are highly reflective, such as silver coated glass, more typically, reflectors in such a solar field are made of highly polished aluminum and have an emissivity factor as low as practicable, preferably of about 0.07 or lower, and more preferably about 0.05, thereby to maximize the amount of solar energy reflected by reflectors 7. It is further preferred that the emissivity of the surface of pipes 9 have an emissivity factor as high as practicable, preferably about 0.93 or higher thereby to maximize the absorption of the solar radiation concentrated thereon by its respective reflector 7. Those skilled in the art will recognize that solar radiation impinging on a body may do either one or three things or a combination of these three things. More specifically, solar radiation may be reflected, or absorbed, or transmitted through the body. These properties are referred to reflectivity, absorptivity or transmissivity, respectively.

[0018] While the solar reflectors 7 may be of any desired shape, a preferred shape is that the reflector is a parabolic reflector having a focal line F - F along which solar energy reflected by the reflector is concentrated. It is also preferred that the solar receiver pipe 9 be generally coaxial with the focal line F - F of its parabolic reflector so as to concentrate the solar energy reflected by the reflector on the surface of the pipe. It is preferred that the solar receiver pipe 9 be of a metal having sufficient strength to support itself within the focal line of the reflector 7 without substantial deflection between its supports including support at its ends and that the pipe be able to withstand the elevated temperatures to which it is heated within the tube 29 and to withstand internal pressures generated due to heating of the working fluid by the concentrated solar energy absorbed by the pipe.

[0019] The solar reflectors 7 may be moved by a solar tracking system, as will be hereinafter described, to follow the course of the sun as it moves across the sky to maximize the amount of solar radiation focused on the receiver pipe over the course of the day or over the course of the seasons. As further shown in Fig. 1 , the heat transfer fluid or working fluid flowing through solar receiver pipes 9 is heated by the solar collector assemblies SCAs. The hot working fluid is collected in an outlet manifold 1 1 and is piped either to a steam power plant, as generally indicated at 13, or to a heat storage system, as generally indicated at 15. The steam power plant 13 and the heat storage system 15 shown in Fig. 1 are simplified for purposes of illustration, but those of ordinary skill in the art will recognize that these systems may include many other components such as valves, pumps, feedwater heaters, steam drums, economizers, deareators and the like not needed for the description of preferred embodiments of this disclosure.

[0020] In general, in the steam power plant 13, the hot working fluid passes through a steam boiler 17 to heat water into steam that is then directed to the inlet of a conventional steam turbine 19, which in turn drives a generator 19a that produces electricity. The steam expelled from the turbine is condensed in a condenser 21 and the condensate is returned to the boiler as feedwater. The working fluid discharged from boiler 17 is returned to the SCAs 5 via a return manifold system 22 to be reheated by the SCAs.

[0021] During the day when the heat output of the solar field 3 exceeds the demand for electrical power, all or a portion of the heat generated or collected by the SCAs 5 may be stored in the heat storage system 15. All or a portion of the hot working fluid from manifold 1 1 , as controlled by a valve V1 , may enter the heat storage system where it passes through the shell side of a shell and tube heat exchanger 23 that heats molten salt (or any other suitable heat storage medium or fluid) flowing through the tubes of the heat exchanger. The hot molten salt (or any other suitable heat transfer medium) is then stored in an insulated storage tank 25. After the working fluid flows through heat exchanger 23, it is directed by a valve V2 to inlet manifold 22 so that it may be reheated in the SCAs. However, when additional heat is required to generate electricity, such as at night, during cloudy weather, or during periods of high electrical demand that cannot be met solely by the solar field 3, hot molten salt is pumped from storage tank 25 and through the tubes 23a of heat exchanger 23 to heat the working fluid. Hot working fluid from heat exchanger 23 is then directed to boiler 17 so as to generate steam which is then used to power turbine 19. After the hot molten salt flows through the tubes 23a of heat exchanger 23 to heat the working fluid, the now cooler molten salt is stored in an insulated storage tank 27. It will be appreciated that a supplemental heating system using a suitable fuel may supplement the steam generated by the hot working fluid heated by the storage system 15.

[0022] As shown in Fig. 1 , a plurality of SCAs 5 may be arranged in end-to-end relation to form a row R of SCAs with a single solar receiver pipe 9 extending from one SCA to the next so that the working fluid flowing through the pipe is serially heated by the SCAs in that row to a high temperature as it flows through each of the SCAs in the row. Of course, those skilled in the art will recognize that the temperature rise of the working fluid as it flows through each SCA depends on a number of factors, such as the length of each SCA, the size of the reflector in each SCA, the number of SCAs in each row R, the size of pipe 9, and the flow rate of the working fluid, and other factors. The field 3 of SCAs may include a plurality of rows Ri - R _n of SCAs. The number of SCAs in each row need not be the same. Thus, there may be“n” solar receiver pipes 9 in the solar field whose inlets are in communication with outlet manifold 1 1 and with inlet manifold 22 so that the working fluid flows through the pipes 9 to be heated by the SCAs. Each sequential solar receiver pipe may be of a different diameter or cross section so as to accommodate changing properties of the working fluid as it flows through the receiver pipes 9 and is heated or cooled.

[0023] It will be appreciated that solar power plants are typically located in regions that receive a lot of sunshine and where there are not many cloudy days. In addition, many of these solar power plants are located in lower latitude countries so as to minimize the differences in the hours of daylight from summer to winter thus insuring that there is adequate solar energy year round to meet the energy demands anticipated for the system. On account of this, many of these solar power plants are located in desert regions where blowing dust and dirt may accumulate on the reflectors. If the entire solar field is enclosed within one or more glass greenhouses (as described in U. S. Patent 8,915,244) that are intended to keep the dirt and dust off the reflectors, nevertheless the glass of such greenhouses require periodic cleaning of the greenhouse glass. Because there may be a multiplicity (e.g., thousands) of SCAs in a solar power field 3 and because the greenhouse may cover a substantial area (typically about 6,000 to about 8,000 m ² /MW), a major expense is incurred in keeping the SCAs or such glass greenhouses free of dust and dirt. In addition, snow, ice and condensation (e.g., dew) may accumulate on the glass of such greenhouses such that the transmission of solar energy to the reflectors therein is adversely affected. Also, in systems in which the solar field is not covered by such a greenhouse, snow, ice, condensation (dew), and dirt will accumulate on the reflectors of such solar fields that adversely affect the reflectivity of the reflectors for considerable lengths of time. Thus, such reflectors require at least periodic cleaning of the reflectors. The cleaning of reflectors not protected by a glass greenhouse, due to their large numbers in a typical solar field, is a very large task. It may not be practicable to provide an automated washing system for cleaning the reflectors such that cleaning of the reflectors must be carried out by personnel. If the reflectors are to be cleaned during daylight hours, the presence of personnel in the solar field among the tightly packed SCAs presents issues about personnel being injured by the intense solar radiation, exposed to high temperatures within the solar field, or burned by the hot solar receiver pipes. If the reflectors are cleaned at night, the danger to cleaning personnel may be reduced, but the closely packed SCAs still makes access to the reflectors difficult and it may be necessary to provide an illumination system that will add to the cost of the solar field.

[0024] As pointed out in the above Background section of this disclosure, convection heat losses from the solar receiver pipes 9 of the SCAs 5 are incurred due to the relatively high temperature of the solar receiver pipe having concentrated solar energy impinging on it and due to the solar receiver pipe being exposed to the atmosphere. This may be the primary thermal loss of the system. It is known to enclose the solar receiver pipe 9 within a transparent tube and to evacuate the tube to form a vacuum or a partial vacuum within the tube so as to minimize such convention losses. However, because of the differences in thermal expansion between a glass transparent tube and the metal solar receiver pipe, it was heretofore necessary to equip both ends of the tube with a flexible metal bellows so as to allow for differences in thermal expansion between the tube and the pipe. If there are multiple SCAs in each row R of SCAs, there must be two bellows for each tube. Such metal bellows are relatively expensive to purchase and vacuum leaks may still occur.

[0025] Referring now to Fig. 2, as previously stated, an SCA 5 of the present disclosure includes an elongate reflector as generally indicated at 7, preferably but not necessarily a parabolic reflector, that focuses its solar energy on an elongate respective solar receiver pipe 9 located to extend along (to be generally coaxial with) the focal line F - F (also referred to as a line of concentration) of the parabolic reflector. In accord with the present disclosure, both the reflector 7 and the solar receiver pipe 9 are housed within a transparent (or semi-transparent) tube, as generally indicated at 29. The term“elongate” will be understood to mean that the length of the reflector 7, pipe 9 or tube 29 is long in relation to its diameter or cross section. Tube 29 is preferably made of a relatively large diameter (e.g., 400 mm., 15.7 inch, or larger) transparent tube, such as tempered solar glass, having a low reflectivity and a high solar transmittance coefficient so as to maximize the amount of solar radiation transmitted through the tube to reflector 7 and focused on pipe 9. If larger diameter glass pipes are commercially available, the diameter of tubes 29 may be considerably greater than 400 mm. Of course, smaller diameter tubes may be used in accord with the present disclosure. Tube 29 preferably is a cylinder having a circular cross section. As previously noted, tube 29 has a longitudinal centerline A - A, which extends axially of the tube, and, as shown in Fig. 2, is coaxial with the focal line F - F of the reflector and with the centerline of pipe 9. As shown in Fig. 2, at each end of reflector 7 is a reflector end plate 8 that has an opening 10 for receiving pipe 9 thereby to keep the reflector properly positioned relative to the pipe so as to insure that the pipe remains in its desired position generally coaxial with the focal line F - F. As will be subsequently described in regard to Figs. 3 - 5, within the broader aspects of this disclosure, the longitudinal centerline A - A of the tube need not be coaxial with the focal line F - F. While pipe 9 and tube 29 are shown to have a generally circular cross- section, it will be understood that they may have any desired cross section or shape.

[0026] Tube 29 may be of any convenient length such as may be commercially available from glass tube manufacturers. Ideally, the length of a solar receiver tube 9 and of a reflector 7 is such that the working fluid flowing through the pipe will be heated to a desired maximum outlet temperature using as few tubes 29 and reflectors 7 as possible. Preferably, tube 29 is of such the length of receiver pipe 9 and the length of single reflector 7 housed within a single tube is such that the working fluid flowing through the receiver pipe is heated to a desired maximum temperature at the point where the working fluid within the pipe exits the SCA. However, if it is not possible to have tube 29 of a sufficient length such that only a single tube is required to heat the working fluid to its desired maximum temperature, a series of shorter SCAs and tubes 29 may be arranged in end-to-end relation may be utilized. Depending on the size and length of the reflectors and the flow rate of the working fluid flowing through the solar receiving pipe 9, the required length of a single reflector (or of a series of shorter reflectors) is determined by the desired exit temperature and pressure of the working fluid as the working fluid enters outlet manifold 1 1 . For example, if the solar field exit temperature of the working fluid is 390°C (734°F), this could be accomplished by one elongate SCA where one or a series of reflectors is (are) housed within a single transparent tube 29, or it could be accomplished by a series of transparent tubes 29 in which a single solar receiver pipe extends between the tubes. Of course, the section of pipe 9 between the tubes may be insulated to reduce heat loss. It will be understood that multiple reflectors arranged one after the other may be housed in a tube 29.

[0027] As shown in Fig. 2, an end cap, as generally indicated at 31 , is sealably affixed to each end of each of each transparent tube 29. Each end cap has an opening 33 therethrough that slidably, sealably receives pipe 9 within a suitable seal 33a, such as a packing gland or the like, thus sealing the pipe with respect to the end cap in such manner as to permit axial movement of the pipe relative to the end cap as may be occasioned by differences in thermal expansion between pipe 9 and the transparent tube 29. End caps may be of any suitable material, such as reinforced plastic or aluminum, which is sufficiently strong to withstand the pressure differential of the atmosphere and a vacuum drawn inside of the tube without undue deflection of the end cap that in turn may cause vacuum leaks between the end caps and the tube or between the pipe and the end cap. Of course, a suitable vacuum seal is provided between the end cap and the end of the tube. A relatively long tube 29 (e.g., 40 feet or 12.91 m.) may be preferred because it reduces the number of end caps 31 and their attendant sources of potential leakage that may occur and it reduces heat losses from pipe 9 in the sections between the end caps of adjacent tubes 29.

[0028] A support structure for reflector 7, as generally indicated at 35, is installed within tube 29 to support the reflector within the tube. The support structure 35 may be a series of wheels or the like that roll in the inside of tube 29 and support the reflector such that the focal line F - F is substantially coaxial with the longitudinal centerline of pipe 9 as the reflector rotates within the tube as it tracks the position of the sun across the sky. The reflector 7 is free to be rotated within the tube by a reflector drive 37 under the control of a sun tracking system so as to follow the sun across the sky over the course of the day or over the course of the seasons of the year to maximize the amount of solar radiation focused on the pipe 9. These sun tracking systems are well known in the prior art, such as disclosed in U.S. Patent 8,915,244 and in U. S. Patent Application Publication No. US 2016/014507. The reflector drive 37 shown in Fig. 2 may comprise a gear rack 39 installed on one end of reflector 7 (or on its support) and a servo drive motor 41 under the control of the solar tracking system. However, other types of reflector drive systems may be used in accord with this disclosure. If the servo motor 41 for drive 37 is located within tube 29, the electrical leads for the drive motor may sealably pass through one of the end caps 31 . Alternatively, the servo motor may be located outside of the tube, and a suitable drive shaft may sealably pass through an end cap and may engage the above-discussed gear rack where the drive shaft is rotatably sealed with respect to the end cap. It will be understood that the sun tracking system and its drive may be operated to track the position of the sun daily as the sun rises and sets and to adjust the position of the reflectors 7 so as to maximize the amount of solar radiation transmitted to the pipes 9, or the sun tracking system may track the position of the sun relative to the location of the solar field as the seasons change and adjust the position of the reflectors accordingly so as to maximize the amount of solar radiation collected. For example, a sun tracking system may have a sun sensor that determines the position of the sun for any given day and for any time of the day and adjusts the position of the reflector accordingly to maximize the amount of solar radiation concentrated on the solar receiver pipe 9. Or, a sun tracking system may merely adjust the position of the reflector periodically (e.g., every few days) so that the reflectors more generally track the position of the sun as the seasons change.

[0029] Further, it will be understood that the support 35 for reflector must have sufficient strength to support the reflector without significant deflection or bending of the reflector to minimize distortion of the reflector. Likewise, the sun tracking system must have sufficient power to move the reflector in its support. It will be understood that the weight of the reflector 7 may be substantial if its cross section and length are substantial. For example, if the transparent tube 29 is relatively long (e.g., 40 feet or 12.9 m.), the reflector may weigh a substantial amount such that it must be supported at spaced locations along the length of tube 29. Also, if the length of tube 29 is long, it may be necessary to not only support pipe 9 as it passes through end caps 31 at the ends of the tube, but also at intermediate locations along the tube to prevent sagging of the pipe and to maintain the position of the pipe relative to the focal line of the reflector. Because the reflector and its support 35 are located within the tube 29, the tube must have sufficient strength to support this weight. Of course, the tube may be supported at predetermined locations along its length.

[0030] In accord with this disclosure, the interior of tube 29 is evacuated so as to form a vacuum or a partial vacuum therewithin. One or both of the end caps 31 has a closable vacuum port 40 which may be connected to a suitable vacuum source or pump (not shown) to initially draw a vacuum or partial vacuum within tube 29 and/or to replenish the vacuum if, over time, some leakage may degrade the vacuum within the tube. Of course, if a vacuum is present within tube, convection heat losses from the pipe 9 are substantially reduced since convection heat losses are dependent upon a gas (air) surrounding the pipe 9 within tube 29 and circulating within the tube. It will be appreciated that port 40 may be provided with a valve fitting that would allow a hose (not shown) from a vacuum source to be sealably connected so as to evacuate the interior of tube 29 and then to be closed so as to maintain the vacuum within the tube.

[0031] Because the reflectors 7 are located within transparent tubes 29, it will be appreciated that with the system of the present disclosure, there are no large surface areas of glass that must be cleaned periodically such as may be the case with a glass greenhouse enclosing the solar field. However, because the tubes 29 are located out of doors and are exposed to the atmosphere, dirt, dust, ice, snow and moisture condensation (dew) may accumulate on the outer surface of cylindrical tubes 29. Thus, unlike the prior art glass greenhouses, typically only the top surfaces of the tubes may need to be occasionally cleaned, which will involve considerably less labor than cleaning the entire glass surface area of such greenhouses, or in cleaning reflectors that are out of doors. It is contemplated that in accord with the present disclosure, the cleaning of the tubes can be readily accomplished by an automatic washing system (not shown herein) without requiring personnel to enter the solar field. Usually, only the tops surfaces of tubes 29 must be cleaned. In such circumstances, an automatic cleaning system (not shown) may be used that is substantially less complicated and less expensive than such cleaning systems used to clean the glass of the above-described greenhouses. The cleaning of the tubes 29 with such an automatic cleaning system can be done at night and no illumination system is required and no personnel need enter the solar field.

[0032] Referring now to Fig. 3, an embodiment of the SCA 5 is shown in which the reflector 7 and the solar receiver pipe 9 are located within transparent tube 29 with the pipe 9 extending through the central openings 33 of the end caps 31 such that the pipe is generally coaxial with respect to the longitudinal center axis A - A of tube 29. In the embodiment of Fig. 3, the center axis A - A of the tube 29 and of pipe 9 and the focal line F - F of reflector 7 are substantially coaxial. As shown, the reflector 7 is movable by the sun tracking system (as previously described herein) from a first position tilted to the left in Fig. 3 (as shown by dotted lines) so as to orient the reflector to best reflect and concentrate the morning sun on pipe 9 extending along the focal line F - F. With the sun in its position at noon, the reflector is oriented in an intermediate position so as to best reflect the noon sun toward pipe 9. This intermediate position of the reflector is shown in solid lines in Fig. 3. The reflector is further movable to a second position tilted to the right in Fig. 3 (as also shown by dotted lines) so that the reflector best reflects the late afternoon sun toward pipe 9. It will be understood that the movement of the reflector may move in a number of increments by servo motor 41 under the control of the sun tracking system. It will be appreciated that it may not be necessary to actually track the position of the sun, but rather when the orientation of the reflectors is known, when the geographic location of the solar field 3 is known, and when the date is known, the position of the reflectors may be moved accordingly and the actual position of the sun is not needed because the position of the sun above the horizon is known and its track across the sky is also known. In such a system, the position of the reflector may be changed to approximate the position of the sun.

[0033] Another embodiment of the system of the present disclosure is shown in Fig. 4 in which reflector 7 and pipe 9 are located within transparent tube 29. However, in this embodiment, pipe 9 passes through openings 33 in end caps 31 that are radially offset from the center of the end caps and from the center of tube 29. In this embodiment, the reflectors are not rotated with respect to the tube, but rather the tube with the reflector fixedly mounted therewithin is rotated about the center axis A - A of pipe 9 to track the position of the sun. In addition, the shape of reflector 7 may be such that a line of concentration of solar energy will impinge upon pipe 9. In this embodiment, the drive 37 rotates the tube 29.

[0034] In Fig. 5, still another embodiment of the system of the present disclosure is shown in which a reflector 7’ forms part of a two-piece tube 43. The upper portion of this two-piece tube is formed by a transparent (or semi-transparent) cover 45 that is joined to and sealed with respect to the upper edges of reflector 7’. A modified end cap 31’ having the shape of the cross section of the two-piece tube 43 is sealably secured to each end of the tube. The tube 43 is evacuated to form a vacuum (or a partial vacuum) therewithin. The pipe 9 sealably passes through an opening 33 in the end cap 31 which may be radially offset from the center of the end plate (as shown in Fig. 5) or it may be generally coaxial with the center of the end plate. Reflector 7’ may be shaped to have a line of concentration generally coincident with pipe 9 so as to concentrate solar energy on the pipe. The entire two-piece tube 43 along with the reflector 7’ may be rotated about the longitudinal axis of pipe 9 and openings 33 to orient the reflector with respect to the position of the sun so as to maximize the amount of solar radiation that impinges upon pipe 9.

[0035] As various changes could be made in the above constructions without departing from the broad scope of the disclosure, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.