WO1979001004A1

WO1979001004A1 - Thermal energy accumulator

Info

Publication number: WO1979001004A1
Application number: PCT/US1979/000276
Authority: WO
Inventors: R Marke
Original assignee: Solar Dynamics Ltd
Priority date: 1978-04-28
Filing date: 1979-04-30
Publication date: 1979-11-29
Also published as: EP0015979A1

Abstract

A solar energy system (10) including a collector (12) for collecting and concentrating solar radiation, a receiver (14) associated with the collector for converting the radiation concentrated by the collector into thermal energy, a thermal energy accumulator (16), and a thermal energy transfer system (18) for transferring thermal energy from the receiver to the thermal energy accumulator is disclosed. The thermal energy accumulator comprises a mixture of fusible salts (20) which begin nucleation at different temperatures whereby the latent heat of fusion of the contained salts can be used to spread the time over which usable heat can be extracted from the thermal energy accumulator, while maintaining the total mixture in a flowable state.

Description

Thermal Energy Accumulator

Technical Field

The present invention relates generally to solar energy systems, and in one of its aspects, to a high temperature collector system using a salt mixture storage. Another aspect of the invention relates to a method and apparatus for tracking the movement of a radiation source such as the movement of the sun across the sky.

Background Art

The recovery of useful energy directly from solar radiation has been the subject of much work in recent years. Collector systems including collector systems using parabolic-shaped collectors are known. Various kinds of thermal energy accumulators are known including those using mineral storage, oil and mineral storage and liquid storage. Salt eutectics, also known as salt eutectic mixtures, have also been used for thermal energy storage as shown in ϋ. S. Patent No. 3,709,209 issued to Schroder.

Mineral storage is limited to sensible heat only since there is no realistic operating range wherein the heat of fusion might be utilized. Further, since there is air in the mineral media itself, expanding gases must be dealt with. Further, the transfer of thermal energy within the mineral media itself is slow.

Oil and mineral storage of thermal energy is not suitable for high temperatures due to the flammability of most oils. Additionally, oil and mineral storage systems pose drastic environmental problems in the event of either a fire or a spill. Liquid thermal energy storage systems normally have low boiling points and excessive vapor pressure and often present flammability problems.

Salt eutectics overcome many of the problems presented by the other thermal storage systems, but usually have narrow thermal bandwidths and cannot be utilized at extremely high temperatures.

Compound parabolic structures are known for light collectors as described by Winston, "Light Collection within the Framework of Geometric Optics", J. Optical Soc. Am., Vol. 60, pp. 245-247 (1970). The structures described by Winston were, however, developed for compound parabolic troughs of such a nature that they can be used for solar radiation collection in a fixed position relative to the earth and not have to track the sun. The present invention, on the other hand, makes use of a compound paraboloidal geometry for a radiation absorbing receiver which has no exit aperture, substantially does not reflect and is not fixed relative to the earth.

A sun tracking system is shown in U. S. Patent No. 3,996,917 issued to Trihey which includes two pairs . of light sensitive elements disposed on opposite sides of an optical axis so that the two elements of each pair will be exposed to a different degree of solar radiation when the optical axis is misaligned with the direction of the sun, and a shading element to increase the sensitivity of the tracker to small movements of the sun. The shading element is fixed in size and position relative to the light-sensitive elements and does not take into account the differences between sunny days where the solar image is small but intense and cloudy days when the solar image is large but less intense. Disclosure of Invention

The present invention concerns a system for utilizing solar energy by collecting and concentrating solar radiation and converting the ' radiation concentrated into thermal energy and then storing that thermal energy at a high temperature. The solar energy system utilizes at least one collector for collecting and concentrating the solar radiation and a receiver associated with the at least one collector for converting the radiation concentrated by the collector into thermal energy at a high temperature. The thermal energy is then transferred from the receiver to a thermal energy accumulator which includes a mixture of fusible salts. The salts are ^' fused by the high temperature of the solar energy system so that during periods of reduced sunshine, the system can continue to supply thermal energy not only from the sensible heat of the salts but also from the salts giving up the heat of fusion as various salts in the mixture nucleate. The salts of- the mixture are chosen in such a way that they nucleate at different temperatures, thus spreading the time over which significant energy can be extracted from the thermal energy accumulator and maintaining the mixture in a fluid state over a wide range of temperature.

One important feature of t^'he invention is a collector for concentrating radiation from sub¬ stantially a single direction which is used as the collector for collecting and concentrating solar radiation. The collector includes a primary reflector including a concave substantially paraboloid-shaped reflecting surface for concentrating radiation and a secondary reflector for directing to the receiver the radiation concentrated by the primary reflector. The primary reflector forms an aperture substantially at the vertex of the substantially paraboloid-shaped reflecting surface, and the receiver is disposed outside of the primary reflector opposite the aperture whereby the radiation reflected from the secondary reflector is directed through the aperture to the receiver. The primary reflector also includes a removable reflective liner for lining the concentrating side of the reflector so that the reflecting surface can actually be removed and replaced rather than polished, thus simplifying maintenance of the collector. One embodiment of the solar energy system utilizes at least one heat pipe for transferring thermal energy from the receiver to the thermal energy accumulator. The at least one heat pipe includes a means for substantially stopping the transfer of thermal energy. According to another important feature of the invention, a radiation seeker is used as a sun-seeker, and as part of a control system, the sun-seeker can be used for guiding the collector. The sun-seeker includes three photosensitive elements disposed around a central axis and an adjustable aperture along the axis so that the elements are exposed to a different amount of solar radiation when the tracker is misaligned with the direction of maximum usable solar radiation and to the same amount of solar radiation when the radiation axis is aligned with the direction of maximum usable solar radiation. This arrangement also prevents the tracker from mistakenly tracking a bright cloud rather than the sun.

The solar energy system also utilizes a control system which adjusts the position of the primary reflector with respect to its support and can stop the transfer of thermal energy from the receiver to the thermal energy accumulator as well as perform other system functions. The control system is responsive to the expected position of the sun and the position of the sun as tracked by the sun-seeker, as well as certain predetermined potentially dangerous conditions.

The novel features which characterize the invention are defined by the appended claims. The foregoing and other objects, advantages and features of the invention will hereinafter appear, and for purposes of illustration of the invention, but not of limitation, an exemplary embodiment of the invention is shown in the appended drawings. _ _/""BUR

. OM WΪ Brief Description of Drawings

Figure 1 is a schematic diagram of the overall system;

Figure 2 is an elevational view of a collector according to this invention;

Figure 3 is a plan view of a collector according to this invention with the collector oriented to receive radiation from straight overhead;

Figure 4 is a sectional view of a collector, a receiver and part of a thermal energy transfer system utilized in the present invention;

Figure 5 is a detail view taken from FIGURE 4; Figure 6 is a perspective view of a receiver and part of a thermal transfer system according to this invention;

Figure 7 is a pictorial representation of a meridian view of a compound paraboloid of revolution;

Figure 8 is a detail section view taken from Figure 1; Figure 9 is an isometric view in partial section of a sun-seeker according to this invention; *-

Figure 10 diagramatically shows a sun-seeker according to the present invention;

Figure 11 is a schematic diagram, of the overall system showing much of the major feedback and control flow of the control system; and

Figure 12 is a schematic diagram of a commercial size power station utilizing the present invention.

_* Best Mode for Carrying Out the Invention Referring now to the drawings, a solar energy system constructed according to the present invention is indicated generally by reference numeral 10. The solar energy system 10 includes at least one collector

12 for collecting and concentrating solar radiation, a receiver 14 associated with said at least one collector

12 for converting the radiation concentrated by the collector into thermal energy, a thermal energy accumulator 16, and a thermal energy transfer system 18 for transferring thermal energy from the receiver to the thermal energy accumulator. The thermal energy accumulator 16 comprises a mixture of fusible salts 20 wherein the nucleation temperature of at least one salt of the mixture differs from the nucleation temperature of at least one other salt of the mixture.

One embodiment of solar energy system 10 further comprises a means 22 thermally coupled to the fusible^* salt mixture for producing electric potential from the thermal energy stored in fusible salt mixture 20. Means 22 for producing electric potential includes a working fluid 24, a heat exchanger 26 thermally coupled to the fusible salt mixture for transferring heat from fusible salt mixture 20 to working fluid 24, and a means 28 driven by working fluid 24 for producing electric potential.

Collector 12 of this invention is generally a collector for concentrating radiation from substantially a single direction and directing the concentrated radiation to a receiver such as receiver 14. In this embodiment, collector 12 is used for concerrtrating solar radiation which is substantially from a single direction at any given moment and generally comprises a pivotal means for receiving solar ^• radiation and directing the solar radiation to receiver 14. In this embodiment, collector 12 includes a pivotal joint 15 to allow the collector to pivot. Pivotal joint 15 also houses receiver 14.

Collector 12 comprises a primary reflector 30 including a concave substantially paraboloid-shaped reflecting surface 32 for concentrating radiation, a secondary reflector 34 for directing to receiver 14 the radiation concentrated by a primary reflector 30, and a means for fixing the position of secondary reflector 34 with respect to primary reflector 30 wherein the secondary reflector reflects substantially all of the radiation reflected from the primary reflector. Preferably, secondary reflector 34 is a convex reflector positioned to refle.ct the radiation reflected from primary reflector 30 before the radiation passes through focus. In one preferred embodiment, secondary reflector 34 is a convex parabolic reflector with a diameter roughly 10 percent as large as that of primary reflector 30. Means for fixing the position of secondary reflector 34 with respect to primary reflector 30 includes three support elements 36 disposed peripherally to primary reflector 30 and secondary reflector 34. Support elements 36 include a means for adjusting the position of secondary reflector 34 with respect to primary reflector 30, the means in this case consisting of extensors 38.

Primary reflector 30 forms an aperture 40 substantially at the vertex of paraboloid-shaped reflecting surface 32. Receiver 14 is disposed outside of primary reflector 30 opposite aperture 40 whereby the radiation reflected from secondary reflector 34 is directed through aperture 40 to receiver 14. The radiation reflected from secondary reflector 34 is preferably focused at a point interior to^' receiver 14.

Referring now to Figures 2 through 5, collector 12 is used in conjunction with a support element 42. Collector 12 further comprises a base support 44 for pivotally mounting primary reflector 30, and a plurality of linear actuators which in this case consist of three hydraulic jacks 46 for adjusting the position of primary reflector 30 with respect to base support 44 , the linear actuators being disposed from one another between primary reflector 30 and support element 42.

Primary reflector 30 comprises a removable liner 48, better shown in Figure 5, for lining the concentrating side of reflector 30, the liner having a reflective side for concentrating the radiation. When in place, the reflective side of liner 48 becomes reflecting surface 32 for concentrating radiation. Primary reflector 30 further comprises a means 50 for evacuating air from between liner 48 and the adjacent portion of the rest of primary reflector 30 whereby liner 48 is held in place by air pressure. A ring 52, preferably made of silicone or thermo resistant plastic, is also included in primary reflector 30 in this embodiment to aid in holding liner 48 in place and to assist in installing a new removable liner.

Primary reflector 30 comprises a reinforced plastic shell 54 which forms a concave substantially paraboloid-shaped surface 56 for receiving removable liner 48. Rubber ring 52 grips outer edge 58 of reinforced plastic shell 54 and grips removable liner 48 in slit 59. Thus, when replacing removable liner 48, a workman slides a new liner 48 into slit 59 and then establishes a vacuum underneath the liner by means 50. Means 50, in one embodiment, includes a plurality of porous areas through which air can be evacuated while still providing support to liner 48. By means of this invention, maintaining a high quality reflecting surface for a collector becomes a simple matter which can be routinely performed by relatively unskilled labor. Maintaining the reflecting surface -no longer requires elaborate cleaning or buffing operations. Removable liner 48 is made of metal coated plastic such as aluminized plastic,., and secondary reflector 34 comprises a highly polished surface such as the mirrors used in high powered lasers, although suitable substitute materials such as an electroplated metal stamping might be found. Referring now to Figures 6 and 7, a preferred embodiment of receiver 14 forms a cavity 60 with an aperture 62 for receiving the collected and concentrated solar radiation into cavity 60. Aperture 62 is oppositely disposed to vertex aperture 40 formed by primary reflector 30. Receiver 14 further comprises a plurality of radiation absorptive and thermally conductive fins 64 projecting from the wall of cavity 60. The wall of cavity 60 formed by the receiver is generally a compound paraboloid of revolution. The generating compound parabola of this embodiment is composed of four parabolic sections 66, 68, 70 and 72 which intersect at intersection points 74, 76, and 78. The generating compound parabola is revolved around an axis of revolution 81 to generate the compound paraboloid surface which describes the wall of cavity 60. The extensions of the parabolic sections beyond the intersection points are shown for illustration only. The curve formed by the inner edge of one of fins 64 is geometrically similar to the generating compound parabola. The solar radiation directed into receiver 14 by secondary reflector 34 is preferably focused at a point 80 which is interior to cavity 60.

Receiver 14 further comprises a means for increasing the radiation absorptive surface area of the cavity wall and plurality of fins 64. In this case the means includes surface roughness of the radiation absorptive surfaces. It can thus be seen, that due to its compound paraboloid of revolution shape along with the use of fins and surface roughness all aiding in its . radiation absorptive and thermally conductive characteristics, that receiver 14 is well suited for converting the radiation concentrated by collector 12 into thermal energy. Due to the nature and characteristics of the receiver, very little of the energy reradiated from the surface of the wall of cavity 60 and fins 64 will be lost from the receiver. The wall of cavity 60 and the plurality of fins 64 are preferably colored black to aid in radiation absorption. Receiver 14 is substantially enclosed in thermal insulation 65 for preventing loss of thermal energy. A preferred embodiment of thermal energy transfer system 18 comprises at least one heat pipe 82, heat pipe 82 including an evaporator section 84 thermally coupled to receiver 14 and a condensor section 86 thermally coupled to thermal energy accumulator 16 by means of terminator 87. Terminator 87, in a preferred embodiment, comprises a ceramic material such as berrylium oxide. At least one heat pipe 82 comprises a liquid metal working fluid for high temperature applications.

Thermal energy transport system 18 further comprises a thermally conducting coupling 88 thermally coupled to receiver 14 and the at least one heat pipe 82, wherein coupling 88 rotates freely about the at least one heat pipe 82 and coupling 88 rotates freely within base 90 of receiver 14. The axis of rotation 92 of coupling 88 is in a direction substantially perpendicular to the axis of rotation 94 of receiver 14. It can thus be seen that heat is transferred from receiver 14 to the at least one heat pipe 82, and receiver 14 pivots freely with respect to the at least one heat pipe 82 so that the collector heat pipe combination is free to track the sun. In one embodiment of thermal energy accumulator 16, mixture of fusible salts 20 comprises a mixture of sodium salts. The mixture of sodium salts consists mainly of sodium sulphate and at least one salt from the group of sodium sulfide, sodium chloride and sodium metasilicate. Preferably, the mixture of sodium -salts consists mainly of sodium sulphate, sodium sulfide, sodium chloride and sodium metasilicate. Sodium sulphate has a relatively low fusion temperature of 31°C. Once the sodium sulphate has fused, the entire mixture can be kept in a flowable condition since, if the salts of the mixture are carefully chosen, the nonfused salts will remain in "gelling pockets" throughout the mixture. Once the entire mixture is brought to its normal operating temperature, all of the salts of the mixture will be in a liquid state. During periods when the energy received by the solar energy system from the sun is inadequate to keep up with the demands on the system by the system's energy thermal energy accumulator 16 can continue to supply usable energy over a long period of time and considerable temperature range since mixture of fusible salts 20 can yield not only sensible heat, but can also yield the heat of fusion for each of the salts as each of the salts nucleates. Since at least one of the salts of the mixture remains in a liquid state throughout all of the useful operating temperatures of the mixture, transfer of thermal energy within the mixture of fusible salts 20 itself and transfer of thermal energy from mixture of fusible salts 20 to heat exchanger 26 is facilitated. The mixture of fusible salts 20 of this embodiment can be kept at an average temperature between 1000°C and the boiling temperature of the entire mixture. In another embodiment of the present invention, mixture of fusible salts 20 is a mixture of sodium salts which consist mainly of sodium chloride, sodium nitrate, sodium hydroxide, sodium sulfide, sodium sulphate and sodium metasilicate. Referring now to Figure 1 and Figure 8, thermal energy accumulator 16 further comprises a refractory material lining 96 for substantially enclosing mixture of fusible salts 20, a plurality of metal plates 98 for supporting refractory material . lining 96, and a plurality of structural members such as H-beam 100 external to metal plates 98 and engaging metal plates 98 for providing structural strength to thermal energy accumulator 16. A preferred embodiment of refractory material lining 96 comprises fused cast aluminum oxide although other suitable refractories are known in the art.

Thermal energy accumulator 16 also comprises a reinforced concrete wall 102 substantially enclosing the structure made up of refractory material lining 96, plurality of metal plates 98 and the plurality of structural members such as H-beam 100. The structural members such as H-beam 100 act as spacers between plurality of metal plates 98 and reinforced concrete wall 102. This allows the use of an insulation such as alumina-silica insulation between plurality of metal plates 98 and reinforced concrete wall 102. As used here the term "metal plates" includes but is not limited to steel plates. Reinforced concrete wall 102 is preferably a steel-reinforced concrete wall. The term "wall" includes the floor and ceiling structure also. Reinforced concrete wall 102 is not necessary for systems that are sufficiently small. Referring again to Figure 1, one means for producing electric potential 22 includes a Rankine cycle power system which includes means 28 driven by working fluid 24 for producing electric potential. Means 28 includes a turbine 108 and a generator 106 driven by turbine 108 through gearing means 110. In this configuration, heat exchanger 26 acts as the "boiler" or "heater" for the Rankine cycle power system. For further efficiency, means 22 for producing electric potential can also include a second cycle, the second cycle including heat exchanger 112 and working fluid 1.14 in a heat exchange relationship with working fluid 24. Heat exchanger 112 thus acts as the condenser for the first cycle as in a normal binary cycle system. Pump 25 is included in the first cycle to recompress wprking fluid 24 and pump 115 is added in the second cycle to recompress working fluid 114 after working fluid 114 has been passed through a condenser 113.

A preferred embodiment of means 22 for producing electric potential includes a relief valve 116 for releasing working fluid 24 in the event of excessive pressure in working fluid 24 caused by some malfunction in the overall system such as an overheating of mixture of fusible salts 20. Relief valve 116 should only be opened in extreme emergency since the release of working fluid 24 into the environment would normally be undesirable. Means 22 further includes normally open bypass valve 117 for bypassing turbine 108 during fURE

_QMPI normal operating conditions with some percentage of working fluid 24 in its high pressure gas stage. Turbine 108 can immediately respond to increased load by closing normally open bypass valve 117. A preferred embodiment of solar energy system 10 also includes a control system. Referring now to Figure 11, the control system includes a means 118 for driving the plurality of linear actuators such as hydraulic jacks 46 whereby the control system adjusts the position of primary reflector 30 with respect to base support 44.

The control system further comprises a means for tracking the expected direction of maximum usable solar radiation relative to the axis of substantially paraboloid-shaped reflecting surface 32 of the primary reflector which will also be referred to as the axis of the primary reflector. The tracking means comprises basically a calculator or table lookup for computing or storing information regarding the solar ephemeris. The calculations or lookup can be performed by cams or a computer. The tracking means is cooperatively coupled to means 118 for driving the_ plurality of linear actuators and is responsive to misalignments between the primary reflector and the expected direction of maximum usable solar radiation.

The control system further comprises a means for sensing the true orientation of primary reflector 30 relative to base support 44, and a means for detecting a predetermined potentially damaging condition affecting the solar energy system. One embodiment of the means for " sensing the true orientation of primary reflector 30 relative to base support 44 includes a plurality of linear displacement transducers 120 disposed between support element 42 and primary reflector 30. Each linear displacement transducer 120 measures the length of a cord .122 between a given point on support element 42 and a given point on primary reflector 30. One type of such linear displacement transducer includes a rotary encoder such as a shaft positioned encoder which is turned as cord 122 is pulled out by primary reflector 30. or drawn back by a spring element. The location of the given points and the lengths of cord 122 give the true orientation of primary reflector 30. The orientation sensing means and the detecting means are cooperatively coupled to means 118 for driving the plurality of linear actuators such as hydraulic jacks 46 whereby the means 118 for driving the plurality of linear actuators is responsive to the predetermined potentially damaging condition whereby the control system adjusts the position of primary reflector 30 with respect to base support to lessen the effects of the predetermined potentially damaging condition and adjusts the position of the primary reflector for normal operation when the predetermined potentially dangerous condition has passed.

One embodiment of the means for detecting a predetermined potentially damaging condition affecting solar energy system 10 comprises a means for detecting potentially damaging wind strain on the collector. The means for detecting potentially damaging wind strain on the collector includes strain gauges 124 and data link 126 to transmit data from strain gauges 124 to the decision making portion of the control system, whereby the control system adjusts the position of primary reflector 30 with respect to base support 44 to lessen the effects of the wind strain. One embodiment of the means for detecting a predetermined potentially dangerous condition affecting solar energy system 10 includes a means 128 for detecting potentially damaging high temperature in mixture of fusible salts 20 whereby the control system adjusts the position of primary reflector 30 with respect to base support 44 to misalign primary reflector 30 and the sun in order to decrease the energy flow from the collector to mixture of fusible

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_ salts 20. This misaligning of primary reflector 30 and the sun prevents the further overheating of mixture of fusible salts 20 and allows means for producing electric potential 22 or other means for using the stored thermal energy to reduce the temperature within mixture of fusible salts 20. Means 128 for detecting potentially damaging high temperature in mixture of fusible salts 20 can also include means for detecting potentially damaging pressure, thermal loss or salt leakage.

The control system further comprises a radiation seeker 130 used as a sun seeker. Sun seeker 130 is cooperatively coupled to means 118 for driving the plurality of linear actuators, in this case hydraulic jacks 46. Seeker 130 includes at least three photosensitive elements 132 and a means for shading photosensitive elements 132, the means for shading comprising a substantially encircling wall 134 and a variable shading means 136 which forms aperture 138. In this embodiment of radiation seeker 130, the at least three photosensitive elements 132 consist of three roughly poplanar. photosensitive elements. The locus of points equidistant from three photosensitive elements 132 defines a radiation axis so that three photosensitive elements 132 are substantially equally spaced from one another around the radiation axis, and aperture 138 is substantially concentric about the radiation axis, wherein the elements 132 are exposed to a different amount of radiation when the radiation axis is misaligned with the direction of maximum usable radiation and to the same amount of radiation when the radiation axis is aligned with the direction of maximum usable radiation. At least three photosensitive elements 132 are preferably somewhat angled in toward the radiation axis.

Radiation seeker 130 is cooperatively coupled to means 118 for driving the plurality of linear actuators through the decision making portion of the control system and data link 140. The decision making portion of the control system includes differential amplifier 142 and comparators 143a, 143b and 143c. ' The voltage output 144 of differential amplifier 142 is the average of the output voltages of elements 130a, 130b and 130c. Each of the comparators will have a non-zero output whenever the voltage output of its associated element 130 exceeds the average voltage as indicated by output 144. The outputs 145 of comparators 143 are used to drive hydraulic jacks 46, and in this case, are part of the means for driving the plurality of linear actuators. Thus, means 118 for driving the plurality of linear actuators is responsive to misalignments between primary reflector 30 and the direction of maximum usable solar radiation. As used here, the term "photosensitive" is used in a broad sense of meaning not just light sensitive, but rather sensitive to all radiation in the range of interest. Similarly, how well radiation seeker 130 aligns with the direction of maximum usable radiation is dependent upon how sensitive photosensitive elements 132 are to radiation in the usable range of frequencies.

Variable shading means 136 is responsive to the amount of available usable radiation whereby the size of aperture 138 is responsive to actual radiation conditions. When radiation seeker 130 is used to track the sun, the size of aperture 138 is responsive to the size of the solar image as affected by distance from the sun, clouds, etc. as well as other conditions, thus aiding in more accurately tracking the maximum usable solar radiation. Additionally, by varying the size of aperture 138 according to the amount of available usable radiation, radiation seeker 130 will track the actual solar image and not mistake a bright cloud for the image to be followed. The radiation of interest for solar energy system 10 is solar radiation, but the principals used in radiation seeker 130 work equally well for other forms of radiation. At least one heat pipe 82 includes a means 146 for substantially stopping the transfer of thermal energy from receiver 14 to thermal energy accumulator 16. The control system includes a means 148 for detecting a predetermined level of solar radiation cooperatively coupled to means 146 for substantially stopping the transfer of thermal energy whereby means 146 for substantially stopping the transfer of thermal energy is responsive to the predetermined level of solar radiation. Normally, the predetermined level of solar radiation is a level of solar radiation so low that it takes more energy to operate solar energy system 10 than is received by the system, so that there would be a net energy drain on the system to try to continue to operate. This situation would exist between a certain time in the evening and a certain time in the morning as well as on extremely overcast days. Means 146 for substantially stopping the transfer of thermal energy is also used to prevent reverse heat flow in heat pipe 82 which would drain heat from accumulator 16.

Means 128 fo.r detecting a predetermined temperature within thermal energy accumulator 16 is also cooperatively coupled ' to means 146 for substantially stopping the transfer of thermal energy whereby means 146 for substantially stopping the transfer of thermal energy is responsive to the predetermined temperature within thermal energy accumulator 16. Thus, if the predetermined temperature is a dangerously high -temperature, means 146 can be actuated to substantially stop the transfer of thermal energy from receiver 14 to thermal energy accumulator 16 so that the temperature in thermal energy accumulator 16 is not raised further.

One suitable means 146 for substantially stopping the transfer of thermal energy is a magnetically operated butterfly valve within at least one heat pipe 82, although other suitable means for thermally switching heat pipes are known. One embodiment of the control system also includes a data link 150 for transmitting information regarding the pressure, temperature and leakage of working fluid 24 within heat exchanger 26, data link 152 for reporting the speed and torque of turbine 108, and data link 154 for reporting the current and frequency of generator 106. The term "data link" is used throughout to refer to both digital and analog data. Data link 150 is also another means for detecting a predetermined potentially damaging condition.

The control system of this embodiment also includes a means 156 for controlling relief valve 116 responsive to data link 150 for reporting pressure, temperature and leakage of working fluid 24 within heat exchanger 26. Thus, dangerously high working fluid pressure can be relieved by opening relief valve 116. The control system further comprises means 158 for throttling the flow of working fluid 24.

Solar energy system 10 further comprises retaining wall 168 which in cooperation with thermal energy accumulator 16 forms moat 170 for retaining salt spills or at least impeding the flow of released salts and, thus, preventing ecological havoc. In general, small installations such as residential installations would not have as much need for retaining wall 168 as would large installations.

Although a preferred embodiment of the invention has been described in detail, it should be understood that various changes, substitutions, and alterations can be made therein without departing from the spirit and scope of the invention as defined by the appended claims. In general, collector 12 is a means for directing concentrated solar radiation to receiver 14. Thermal energy accumulator 16 in combination with ■ either means 22 for producing electric potential or any other means for utilizing the thermal energy from thermal energy accumulator 16 make up one means for utilizing thermal energy. In embodiments in which at least one collector 12 must substantially face a certain direction with respect to the sun, the embodiment of at least one collector 12 illustrated is only one pivotal means for receiving solar radiation and directing the solar radiation to receiver 14.

In an alternative embodiment of at least one collector 12, primary reflector 30 comprises a substantially paraboloid-shaped reflecting surface and secondary reflector 34 comprises a hyperbolic reflecting surface, changing the diameter of secondary reflector 34 relative to the diameter of primary reflector 30 to approximately 50 percent.

Many features and subcombinations of this invention are of utility and other applications are contemplated within the scope of the claims. For instance, radiation seeker 130 can also be used, for surveying as in laser alignment of large pipelines now being done by conventional optical methods or for the precise guidance of telescopes. At least one collector 12 can be used to provide commercial or residential heat directly.

Referring now to Figure 12, solar energy system 160 includes an array 162 of collectors 12, suitable for commercial power generation, to collect and concentrate solar radiation. A receiver 14 is associated with each of said collectors for converting the radiation concentrated by the collectors into thermal energy. Thermal energy transfer system 18 transfers thermal energy from receivers 14 to thermal energy accumulator 16. Solar energy system 160 further comprises a means 22 thermally coupled to mixture of fusible salts 20 within thermal energy accumulator 16 for producing electric potential from the thermal energy stored in the fusible salt mixture. Solar energy system 160 further comprises a cooling tower 164 for cooling working fluid 24 used by solar energy system 160, and a maintenance building 166. Thus, it will be appreciated that the present solar energy system solves the problems of many collector type systems of the past. First, there is no need to suspend a boiler or any liquid carrying element at the focal point of the primary reflector. Secondly, the receiver which converts radiation into thermal energy is situated at the base of the primary reflector, near the thermal energy transfer system, rather than at the focal point of the primary reflector. Further, the receiver itself does not need to transfer any liquid medium to the thermal energy transfer system, although the receiver could be filled with such medium to be heated in certain applications. Additionally, a preferred embodiment of the system is one in which the only liquid transfer is the liquid metal of the liquid metal heat pipe which is highly efficient and of high reliability. Still further, the thermal energy accumulator not only yields sensible heat, but also yields the heat of fusion of the salts in the mixture over a wide range of temperatures while still remaining in a liquid state. Besides these advantages, the primary collector of the system can be maintained in good condition by simply replacing the reflective liner. Other advantages of the solar energy system of this invention are obvious from the description and the appended claims.

Although a preferred embodiment of the invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made therein without departing from the spirit and scope of the invention as defined ^*by the appended claims.

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Claims

1. A thermal energy accumulator comprising a mixture of fusible salts, wherein the nucleation temperature of at least one salt of the mixture differs from the nucleation temperature of at least one other salt of the mixture.

2. A thermal energy accumulator according to Claim 1 wherein the mixture of fusible salts comprises a mixture of sodium salts.

3. A thermal energy accumulator according to Claim 2 further comprising, in combination: a refractory material lining for substantially enclosing the mixture of fusible salts; a plurality of metal plates for supporting the refractory material lining; and a plurality of structural members external to the metal plates and engaging the metal plates for providing structural strength to the thermal energy .accumulator.

4. A thermal energy accumulator according to Claim 3 wherein the refractory material lining comprises fused cast aluminum oxide.

5. A thermal energy accumulator according to Claim 3 further comprising, in combination: a reinforced concrete wall substantially enclosing the structure made up of the refractory material lining, the plurality of metal plates and the plurality of structural members whereby the structural members act as spacers between the plurality of metal plates and the reinforced concrete wall; and alumina-silica insulation between the plurality αf metal plates and the reinforced concrete wall.

6. A thermal energy accumulator according to Claim 2 wherein the mixture of sodium salts consists mainly of sodium sulfate and at least one salt from the group of sodium sulfide, sodium chloride and sodium metasilicate.

7. A thermal energy accumulator according to Claim 6 wherein the mixture of sodium salts consists mainly of sodium sulfate, sodium sulfide, sodium chloride and sodium metasilicate.

8. A thermal energy accumulator according to Claim 2 wherein the mixture of sodium salts consists mainly of sodium chloride, sodium nitrate, sodium hydroxide, sodium sulfide, sodium sulfate and sodium metasilicate.

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