WO2017035043A1 - Solar receiver using heat pipes and granular thermal storage medium - Google Patents

Abstract

A receiver for a concentrating solar power plant uses heat pipes to transfer thermal energy to a heat transfer medium in a chamber. An evaporation zone of each of the heat pipes is outside the chamber, and the heat pipes extend through a front wall of the chamber so that their condensation zones are inside the chamber. The heat transfer medium substantially surrounds the heat pipes. In some installations, the total surface area of the heat pipes within the heat transfer medium may be larger than the projected area of the ends of the plurality of heat pipes receiving solar radiation onto the plane of the aperture of the receiver. In some installations, the heat transfer medium is a granular heat transfer medium, and the heat transfer medium and the heat pipes form a moving bed heat exchanger.

Description

SOLAR RECEIVER USING HEAT PIPES AND GRANULAR THERMAL STORAGE MEDIUM

Related Applications

The present application claims a priority benefit to U.S. Provisional Patent Application No.

62/209111 filed on August 24, 2016, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0001] Solar electric power generation systems offer a source of clean, renewable energy.

Various kinds of plants have been proposed and built for generating electricity from solar energy. However, the peak incidence of solar radiation does not necessarily occur when power is most needed. For example, incoming solar radiation may be at its peak near mid-day, but demand for electric power may peak in the late afternoon, and strong demand may continue into the evening and early nighttime hours when no solar radiation is being received.

[0002] Energy storage may be used to at least partially decouple the generation of electric power from the rate of incoming solar radiation, and greater efficiencies in storing thermal energy are desired. Previous thermal storage media, for example steam or molten salts, have disadvantages. For example, piping for handling these media must prevent leakage, and must withstand high thermal stresses over long periods of time, despite the corrosive nature of the conventional storage media and heat transfer fluids.

BRIEF SUMMARY OF THE INVENTION

[0003] In one aspect a solar energy receiver comprises an enclosed chamber having a front wall and a plurality of heat pipes. Each of the plurality of heat pipes has a first end and a second end, and the plurality of heat pipes are arranged generally parallel to each other in a bundle. An evaporation zone of each of the plurality of heat pipes is at the first end of the heat pipe disposed outside of the chamber and is configured for receiving concentrated solar radiation. Each of the plurality of heat pipes extends from its first end through the front wall of the chamber such that at least some of a condensation zone of each of the plurality of heat pipes is in the interior of the chamber. Within the chamber, the plurality of heat pipes are spaced from each other. The solar energy receiver further comprises a heat transfer medium within the chamber substantially filling the spaces between the plurality of heat pipes, such that the heat pipes and the heat transfer medium form a heat exchanger that heats the heat transfer medium.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004] FIG. 1 illustrates a simplified schematic view of a conventional concentrating solar thermal power plant. [0005] FIG. 2 illustrates a simplified cutaway schematic of a heat pipe.

[0006] FIG. 3 shows a perspective view of a solar receiver in accordance with a first embodiment of the invention.

[0007] FIG. 4 illustrates the solar receiver of FIG. 3 with a heat transfer medium in place.

[0008] FIG. 5 shows a heat pipe in accordance with the embodiment of FIG. 3. [0009] FIG. 6 shows a close-up view of some of the heat pipes of FIG. 3 at their penetration of a front wall of a chamber, in accordance with embodiments of the invention.

[0010] FIG. 7 shows some of the heat pipes of FIG. 6, viewed in the direction of the longitudinal axis of the heat pipes.

[0011] FIG. 8 shows a cross section view of some of the heat pipes of FIG. 3 taken within the chamber, and viewed in the direction of the longitudinal axis of the heat pipes.

[0012] FIG. 9 illustrates a cross section of heat pipes having an oblong cross section, in accordance with embodiments of the invention.

[0013] FIG. 10 illustrates a second heat pipe embodiment.

[0014] FIG. 11 shows a close-up view of some heat pipes as in FIG. 10 at their penetration of a front wall of a chamber, in accordance with embodiments of the invention.

[0015] FIG. 12 shows some of the heat pipes of FIG. 11, viewed in the direction of the longitudinal axis of the heat pipes.

[0016] FIG. 13 illustrates a third heat pipe embodiment. [0017] FIG. 14 shows a close-up view of some heat pipes as in FIG. 13 at their penetration of a front wall of a chamber, in accordance with embodiments of the invention.

[0018] FIG. 15 shows some of the heat pipes of FIG. 14, viewed in the direction of the longitudinal axis of the heat pipes.

[0019] FIG. 16 illustrates a fourth heat pipe embodiment.

[0020] FIG. 17 shows a close-up view of some heat pipes as in FIG. 16 at their penetration of a front wall of a chamber, in accordance with embodiments of the invention.

[0021] FIG. 18 shows some of the heat pipes of FIG. 17, viewed in the direction of the longitudinal axis of the heat pipes.

[0022] FIG. 19 illustrates a fifth heat pipe embodiment.

[0023] FIG. 20 shows a transverse cross section of the annular heat pipe of FIG. 19.

[0024] FIG. 21 shows an axial cross section of the annular heat pipe of FIG. 19.

[0025] FIG. 22 shows a close-up view of some heat pipes as in FIG. 19 at their penetration of a front wall of a chamber, in accordance with embodiments of the invention.

[0026] FIG. 23 illustrates an effect of the annular shape of the heat pipe of FIG. 19, in accordance with embodiments of the invention.

[0027] FIG. 24 illustrates a sixth heat pipe embodiment.

[0028] FIG. 25 shows a close-up view of some heat pipes as in FIG. 24 at their penetration of a front wall of a chamber, in accordance with embodiments of the invention.

[0029] FIG. 26 illustrates a seventh heat pipe embodiment.

[0030] FIG. 27 shows a close-up view of some heat pipes as in FIG. 26 at their penetration of a front wall of a chamber, in accordance with embodiments of the invention.

[0031] FIG. 28 shows a perspective view of a solar receiver in accordance with an eighth embodiment of the invention.

[0032] FIG. 29 shows an orthogonal view of the solar receiver of FIG. 28. DETAILED DESCRIPTION OF THE INVENTION

[0033] FIG. 1 illustrates a simplified schematic view of a conventional concentrating solar thermal power plant 100. Example power plant 100 uses a central receiver 113 in a "power tower" 101 to heat a heat transfer fluid such as a molten salt circulating in a first piping loop 102. Incoming solar radiation 103 is directed to receiver 113 by a field of heliostats 104. FIG. 1 is highly simplified, and in practice there may be hundreds or thousands of heliostats, and the heat transfer fluid can reach temperatures of hundreds of degrees Celsius. The heated fluid is passed through a steam generator 105, to generate steam in a second piping loop 106. The steam may be used to generate electricity, for example by turning a turbine 107, which in turn powers a generator 108, which supplies power to the grid 112. After passing through the turbine 107, the steam may be condensed 109 and reheated in steam generator 105.

[0034] During the day, the heat transfer fluid in its heated state may be accumulated in hot storage tank 110, for use at a later time. The system thus stores energy thermally by virtue of the specific heat of the heat transfer fluid and its elevated temperature. When no solar radiation is being received, the hot heat transfer fluid from the hot storage tank 110 can be passed through steam generator 105 to generate steam, and then accumulated in cold storage tank 111, to be heated again when sunlight is available. The thermal energy storage capacity of the system depends on many factors, including primarily the amount of heat transfer fluid that can be held in the hot storage tank 110, and the temperature differential between the cold storage tank 111 and the hot storage tank 110.

[0035] For the purposes of this disclosure, the terms "hot" and "cold" are to be understood in a relative sense. That is, the heat transfer fluid in the hot storage tank 110 is at a higher temperature than the heat transfer fluid in the cold storage tank 111, but even the "cold" heat transfer fluid may be considered very hot to human senses. [0036] In other plant designs, thermal energy may be stored using a stationary phase change material, and other working fluids may exchange heat with the phase change material to add energy to it (causing some of the phase change material to melt) or remove energy from it (causing some of the phase change material to freeze). Thus, the energy is stored by virtue of the heat of fusion of the phase change material. In other designs, energy may be stored in high pressure steam. [0037] Each of these prior systems involves handling of liquids that may be corrosive and may be at high temperature and pressure. In some designs, molten salts must be kept at elevated temperatures to avoid freezing within the plant piping.

[0038] The use of solid granules as a heat transfer and storage medium has been proposed. Solid granules may be chemically stable in air, avoiding the need for air tight plumbing systems and connections. In addition, because the granules are already solid, there is no freezing risk.

[0039] One proposed design provides a falling "curtain" of granules at the receiver aperture of a solar tower. The granules are heated by the incoming solar radiation as they fall past the aperture, thus capturing thermal energy from the incoming solar radiation by virtue of the specific heat of the granule material. The heated granules may be used immediately or may be stored for later use. Their thermal energy may be recovered, for example, through a heat exchanger to heat water to make steam, which can then be used conventionally to generate electricity.

[0040] However, existing solid granule systems also have difficulties. For example, the falling curtain design may be susceptible to granule loss due to wind or other causes, and the residence time of the granules at the receiver aperture may be short, so that multiple passes by the aperture may be necessary to achieve workable granule temperatures. Constructing a recirculation system capable of handling the heated granules may be difficult and expensive. In addition, the granules should be made of a material that is stable in air and can withstand the extreme temperatures involved. Even materials with good thermal conductivity may have a poor effective conductivity in granular form as the conductivity of the fluid that fills the inter granule voids may dominate the effective conductivity of the bulk granular material. Further, some materials such as ceramics that are otherwise suitable for use as the granules tend to have low thermal conductivity, making it difficult to transfer heat to and from them. Furthermore, some otherwise-suitable materials may be light in color, making it difficult to heat them by direct radiation.

[0041] One approach to addressing the difficulties of poorly-conducting solid granule media might be to increase the receiver size, thus increasing the heat transfer area for heating the granules. However, a larger receiver is subject to larger heat losses and other difficulties. Thus, it may be difficult to achieve sufficient heating of the granules using a receiver aperture that is sufficiently small to achieve the high concentration necessary to reach the required temperatures and avoid excessive thermal radiation losses. [0042] Embodiments of the invention use heat pipes to receive concentrated solar radiation and transfer heat to a granular or other heat transfer medium. The surface area available for heat transfer to the heat transfer medium can be much larger than the receiver area.

[0043] FIG. 2 illustrates a simplified cutaway schematic of a heat pipe 200. A heat pipe is a passive device for transferring heat much more efficiently than can be accomplished by thermal conduction. In the example of FIG. 2, a quantity of a working fluid 201 is enclosed in a hollow envelope 202, which also encloses a wick 203. The working fluid is selected to have a boiling point below the temperature of the heat source 204, but above the temperature of the item to be heated 205. When heat is applied from the heat source 204, the working fluid at the hot end 206 of the heat pipe vaporizes, taking advantage of its latent heat of vaporization, and flows in a gaseous state toward the cold end 207 of the heat pipe. The portion of the heat pipe where evaporation of the working fluid occurs may be called the evaporation zone. When the working fluid encounters the relatively cold walls of the enclosure at the cold end 207, the working fluid condenses, giving up its latent heat of vaporization to the walls of the heat pipe enclosure 202, from which heat is conducted to the item to be heated 205. The portion of the heat pipe where condensation of the working fluid occurs may be called the condensation zone. The condensed working fluid then flows due to surface tension through the wick 203 back toward the hot end 206 of the heat pipe 200, to be reused. This natural convection cycle enables very high heat transfer rates. Preferably, the wick 203 covers the interior surface of the envelope 202, ensuring that the entire surface is covered in liquid working fluid so that the entire evaporation zone is able to absorb the applied heat.

[0044] The heat pipe may operate at any temperature above the working fluid melting temperature up to the temperature at which the vapor pressure exceeds the maximum allowable stress in the envelope. The increase in boiling temperature with increase in vapor pressure provides the heat pipe a method of self-regulating. Also in some embodiments the heat pipes may be created with a negative internal pressure, to lower the boiling point of the liquid working fluid.

[0045] Some heat pipes may be oriented with their hot ends lower than their cold ends so that the condensed working fluid flows back toward the hot due to gravity, in addition to or instead of using a wick. In other installations, the evaporation zone of a heat pipe may be above the condensation zone, with the wick providing a mechanism for the working fluid to be transported against gravity back to the evaporation zone.

[0046] FIG. 3 shows a perspective view of a solar receiver 300 in accordance with a first embodiment of the invention. Receiver 300 may be placed, for example, at the top of a solar "power tower" such as tower 101 shown in FIG. 1. In example solar receiver 300, a chamber 301 is defined by walls including front wall 302, back wall 303, and side walls 304 and 305. A number of heat pipes 306 are placed parallel to each other in a bundle. Each heat pipe 306 has a first end and a second end, and the evaporation zone is at the first end, which is outside of chamber 301. The first ends of the heat pipes 306 are configured to receive solar radiation 307. Each heat pipe 306 extends through front wall 302 and into chamber 301, so that at least some of the condensation zone of the heat pipe 306 is within chamber 301. Within chamber 301, the heat pipes 306 are spaced apart from each other, so that there are spaces or gaps between the heat pipes 306, as is explained in more detail below. While a number of heat pipes 306 are shown, the actual number will depend on the generating capacity of the plant in which the receiver is used. Hundreds or even thousands of heat pipes may be present, and the exposed receiving area may be as large as 50 or more square meters. Baffles may also be present within chamber 301 for additional support of heat pipes 306, and may also act as fins increasing the heat transfer surface area. The walls and any baffles of chamber 301 are preferably made of a material capable of enduring the high temperatures involved, for example silicon carbide or alumina. [0047] FIG. 4 illustrates solar receiver 300 with a heat transfer medium 401 in place in chamber 301. Heat transfer medium 401 may be, for example a granular heat transfer medium comprising granules of a suitable material, for example alumina, silicon carbide, another ceramic material, or another suitable non-ceramic material. Mixtures of granules made of different materials may also be used. Any suitable size of granules may be used, but in some

embodiments, the granules may be between 100 microns and 1 millimeter in average diameter, for example 300 microns. Because the granules are not heated directly by the concentrated solar radiation, the color of the granules may be of little or no importance. While the principles of the invention are described herein using a granular heat transfer medium as an example, it will be recognized that other heat transfer media may be used in other embodiments, for example liquid or gaseous media. [0048] In operation, the exposed ends of heat pipes 306 receive direct concentrated solar radiation 307. The heat pipes transfer the absorbed energy efficiently to the inside of chamber 301, where heat is transferred to heat transfer medium 401. Although many other arrangements are possible, heat transfer medium 401 preferably surrounds heat pipes 306 within chamber 301, and substantially fills gaps and spaces between heat pipes 306. Thus, the area of the heat pipes within heat transfer medium 401 is larger than the area of the surface of the ends of heat pipes 306 receiving solar radiation, and is also larger than the projected area of the ends of heat pipes 306 receiving solar radiation, projected onto the plane of the aperture of the receiver. The aperture of a receiver is understood to be the physical aperture of a cavity receiver or the virtual aperture of an external receiver, which would be the plane through which the sun light would reach the receiver in the case it were enclosed in a cavity. An example of the projected area 404 of the ends of heat pipes receiving solar radiation, projected onto the plane of the aperture of a cavity receiver, is illustrated in FIG. 4.

[0049] Thus, the size of the solar receiver can be kept small, to contain losses, but heat is transferred effectively to heat transfer medium 401 via heat pipes 306. Because chamber 301 is closed, little or no loss of heat transfer medium 401 may occur, even when heat transfer medium 401 is granular, as the interior of chamber 301 is protected from wind.

[0050] Heat transfer medium 401 is preferably fed into the top 402 of chamber 301 by any suitable feeder, works its way past heat pipes 306, gathering heat, and exits the bottom 403 in a heated state. The flow of heat transfer medium 401 through the bundle of heat pipes 306 may be gravity fed, or gravity assisted. Heat transfer medium 401 may then be removed by any suitable removal mechanism. The heated heat transfer medium may be stored for later use, for example in a storage silo, or may be passed immediately to another process step where its stored thermal energy is extracted and exploited. For example, the hot heat transfer medium 401 exiting chamber 301 may be used to directly heat air for the operation of a Brayton cycle power plant, or may be used to heat steam for the operation of a Rankine cycle power plant. Once its thermal energy is extracted, heat transfer medium 401 can be carried back to and fed into the top of chamber 301, or can be stored in a cold storage silo and then fed into the top of chamber 301 at a later time, so that it can be heated again in an ongoing cycle. [0051] As is shown in FIGS. 3 and 4, heat pipes 306 are angled downward from front wall 302, to point approximately toward the heliostat field, from which the solar energy receiver is positioned to receive solar radiation 307. This orientation may also facilitate the operation of the heat pipes, as the condensed working fluid can flow back to the exposed absorption zones of the heat pipes assisted by gravity. In other embodiments, other heat pipe orientations may be used, for example horizontal or vertical. [0052] Heat pipes 306 may be of any suitable configuration. The example heat pipe embodiment of FIGS. 3 and 4 is shown in more detail in FIG. 5. Example heat pipe 306 has a tapered first end 501, at which the evaporation zone is disposed. Tapered first end 501 joins to an enlarged hexagonal section 502, which is then joined to the condensation portion 503 of heat pipe 306. Heat pipe 306 may be of any suitable dimensions determined by the thermal capacity of the receiver and other factors. In some embodiments, the heat pipes may be between 1 and 5 meters in length, for example about 2.5 meters. The diameter of condensation portion 503 may also be of any suitable size. In some embodiments, the diameter of condensation portion 503 may be between 20 and 100 millimeters, for example about 50 millimeters, with expanded hexagonal section 502 having faces spaced about 65 millimeters apart. Other cross sectional shapes, for example rectangular, square, triangular, pentagonal, etc. may be used. In a preferred alternative to the first embodiment the cross sectional shape is rectangular.

[0053] The envelope of heat pipe 306 may be made of any suitable material, depending on the temperature range at which they are expected to operate. For example, for temperatures up to about 1000 °C, molten/gaseous sodium may be used as the working fluid, and the envelope of heat pipe may be made of Inconel, stainless steel, a nickel alloy, or another suitable material. For higher temperatures, for example up to about 1300 °C molten/gaseous lithium may be used as the working fluid, in which case the envelope of heat pipe 306 may be made of a refractory material such as a zirconium alloy. The wick of heat pipe 306 is also preferably made of a porous material that can withstand the high temperatures involved, for example a sintered metal. [0054] The tapered first end 501 may facilitate efficient collection of the concentrated solar energy, as any reflections from the tapered surface tend to be directed deeper into the bundle of heat pipes 306, so that there are additional opportunities for absorption of most reflected energy. Thus, the receiver is a volumetric receiver, in that the receiving surfaces are not a simple plane presented to the incoming concentrated solar radiation, but include a depth component so that absorption occurs throughout the volume of the aggregated tapered sections. [0055] FIG. 6 shows a close-up view of some of heat pipes 306 at their penetration of front wall 302.

[0056] FIG. 7 shows some of heat pipes 306 "head on" in the direction of view 601 indicated in FIG. 6. As can be seen in FIG. 7, hexagonal sections 502 nest together to substantially block incoming solar radiation 307 from reaching front wall 302. That is, the interstices 701 left by the hexagonal sections are of minimal size. For example, the heat pipes and their hexagonal sections may obscure up to 95%, 98%, 99% or more of the projected area of front wall 302 within the heat pipe bundle, as viewed in the direction of the longitudinal axis of the heat pipes.

[0057] Within chamber 301, heat pipes 306 are spaced from each other. FIG. 8 shows a cross section view of some of heat pipes 306 taken within chamber 301, also viewed in the direction of the longitudinal axis of the heat pipes. Heat pipes 306 are substantially embedded in heat transfer medium 401, providing substantial contact between heat transfer medium 401 and the outer envelopes of heat pipes 306.

[0058] When heat transfer medium 401 is a granular heat transfer medium, heat transfer medium 401 and heat pipes 306 may form a moving bed heat exchanger, in which the "bed" of granules gradually flows through the bundle of heat pipes 306, being heated in the process by absorbing heat from heat pipes 306. Preferably, the moving bed is supported from below so that heat transfer medium 401 moves past heat pipes 306 without free falling any appreciable distance through air. [0059] The spacing between heat pipes 306 may be chosen to be large enough to avoid bridging of granules between the heat pipes, which may block the progress of heat transfer medium 401 through the heat pipe bundle. In addition, it may be desirable to keep the spacing small, to facilitate good heat transfer between heat pipes 306 and heat transfer medium 401 without relying unduly on heat transfer through the relatively non-thermally-conductive granules. In some embodiments, the spacing between heat pipes 306 may be three to 20 times the average granule diameter or more, for example about five times.

[0060] In some embodiments, heat pipes 306 may be oblong in cross section in their condensation regions. FIG. 9 illustrates a cross section of heat pipes having an oblong cross section within chamber 301. As compared with cylindrical heat pipes, heat pipes that are oblong in cross section may improve heat transfer to heat transfer medium 401, as the regions of low flow at the tops and bottoms of the heat pipes are reduced in size. [0061] In other embodiments, other heat pipe configurations may be used. FIG. 10 illustrates a second heat pipe embodiment 1001. Heat pipe 1001 includes a tapered first end 1002, an expanded region 1003, and a cylindrical condensation region 1004, such that the receiver is volumetric. Heat pipe 1001 is similar to example heat pipe 306, except that expanded region 1003 is cylindrical rather than hexagonal. In other embodiments, condensation region 1004 may have a different cross section, for example an oblong cross section.

[0062] FIG. 11 shows a close-up view of some of heat pipes 1001 at their penetration of front wall 302.

[0063] FIG. 12 shows some of heat pipes 1001 "head on" in the direction of view 1101 indicated in FIG. 11. As can be seen in FIG. 12, enlarged sections 1003 nest together to substantially block incoming solar radiation 307 from reaching front wall 302. That is, the interstices 1201 left by the enlarged sections 1003 are small, albeit somewhat larger than those left by heat pipes 306 having hexagonal enlarged sections.

[0064] FIG. 13 illustrates a third heat pipe embodiment 1301. Heat pipe 1301 is simply straight and is cylindrical, although other cross sectional shapes are also possible. FIG. 14 shows a close-up view of some of heat pipes 1301 at their penetration of front wall 302, and FIG.

15 shows some of heat pipes 1301 "head on" in the direction of view 1401 indicated in FIG. 14.

As can be seen, the interstices 1501 between heat pipes 1301 may be somewhat larger than for heat pipes having expanded sections, and thus a collector using heat pipes 1301 may not block as much direct concentrated solar radiation from reaching front wall 302 as would a collector using heat pipes with expanded sections. However, heat pipe 1301 may have an advantage in ease of manufacturing. FIG. 15 shows some of heat pipes 1301 "head on" in the direction of view indicated in FIG. 14.

[0065] FIG. 16 illustrates a fourth heat pipe embodiment 1601. Heat pipe 1601 includes a shaped end 1602, which in this example is shaped into a cross shape. Such a shape may be formed by swaging a cylindrical tube, or by other methods. The cross shape of end 1602 may provide additional absorbing area, and additional opportunities for absorption of reflected energy, especially for incoming concentrated solar radiation that is not aligned with the longitudinal axis of the heat pipes. The receiver is thus volumetric. FIG. 17 shows a close-up view of some of heat pipes 1601 at their penetration of front wall 302. FIG. 18 shows some of heat pipes 1601 "head on" in the direction of view 1701 indicated in FIG. 17. [0066] In other embodiments, annular heat pipes may be used. FIG. 19 illustrates a fifth heat pipe embodiment 1901. Heat pipe 1901 is annular, and FIGS. 20 and 21 show transverse and axial cross section views of annular heat pipe 1901. Annular heat pipe 1901 has a double- walled outer envelope 2001, defining annular space 2002 between the double walls. Wick 2003 and the working fluid reside in annular space 2002. Central bore 2004 is open to the air. The annular heat pipe central bore 2004 may be open to the air on both ends, may be open to the air only at the front end of the pipe, which is the side where the solar radiation impinges, or may be open to the air only at the back end of the pipe. When central bore 2004 is only open at the front end or at the back end, the annulus may continue in a cap at the back end or the front end of the heat pipe, with the same walls, comprising the wick and in fluid communication with the working fluid, allowing heat transfer directly to the working fluid of the heat pipe at the back end of the heat pipe.

[0067] FIG. 22 shows a close-up view of some of heat pipes 1901 at their penetration of front wall 302. FIG. 23 illustrates the effect of the annular shape of heat pipe 1901. Incoming ray 2301 entering heat pipe 1901 obliquely will strike the inner wall. Some of the thermal energy will be absorbed by envelope 2001, to evaporate some of the working fluid in annular space 2002. However, some of the incoming radiation may reflect from envelope 2001. Because ray 2301 is inside of heat pipe 1901, the reflected ray 2301a will also encounter envelope 2001, to be at least partially absorbed. The receiver is thus volumetric, and multiple reflections are possible. Thus an annular heat pipe may direct some of the incoming concentrated solar radiation relatively deeply into the annular heat pipe, so that a larger portion of the heat pipe absorbs radiation without increasing the projected area of the ends of the heat pipes receiving solar radiation, projected onto the plane of the aperture of the receiver. This may help maintain a more nearly isothermal temperature of the heat pipe and eliminate hot spots that might cause stress concentrations. Optionally, some of the interior wall of the heat pipe may be coated with a reflective surface, to promote travel of radiation along the heat pipe.

[0068] In addition, as is shown in FIG. 23, some radiation may traverse the entire length of heat pipe 1901 and reach the back end 2302 of the heat pipe, either by reflecting along the interior surface of heat pipe 1901 or, for rays arriving from certain heliostats aligned exactly with heat pipe 1901, by traversing the entire length of heat pipe 1901 without encountering the wall of heat pipe 1901. Ray 2301b in FIG. 23 is a ray that reaches back end 2302 after reflection. In some embodiments, a backing reflector 2303 may be positioned at back end 2302 to redirect radiation reaching reflector 2303 back into heat pipe 1901, so that the radiation has additional opportunity to be absorbed. Reflector 2303 may be a specular reflector or a diffuse reflector, and may include angles designed to reflect rays arriving from those heliostats aligned exactly with the heat pipe towards the envelope walls so the rays can reflect from the reflector to the annular heat pipe internal walls. Reflectors such as reflector 2303 may be used with any of the annular heat pipe embodiments described herein.

[0069] As will be apparent from FIG. 22, because annular heat pipe 1901 has a constant cross section, some radiation may arrive in the spaces between the heat pipes. Some of this radiation will be absorbed by the outer surfaces of the heat pipe envelopes, and some may reflect to the outer surfaces of other nearby heat pipes for additional absorption. However, radiation arriving coaxially with the heat pipes may strike front wall 302 and at least partially reflect out of the receiver.

[0070] To reduce this possibility, secondary reconcentrators may be formed on the exposed ends of some or all of the heat pipes, to direct radiation into the heat pipes that might otherwise reach front wall 302.

[0071] FIG. 24 illustrates a sixth heat pipe embodiment 2401. Annular heat pipe 2401 has a flared end 2402, allowing the receiving ends of the bundled heat pipes to nest more closely together, and to better block incoming solar radiation from reaching front wall 302 of chamber 301, as compared with simple cylindrical annular heat pipe 1901. FIG. 25 shows a close-up view of some of heat pipes 2401 at their penetration of front wall 302.

[0072] FIG. 26 illustrates a seventh heat pipe embodiment 2601. Annular heat pipe 2601 has a flared end 2602 that flares to a hexagonal shape, allowing the receiving ends of the bundled heat pipes to nest even more closely together, and to better block incoming solar radiation from reaching front wall 302 of chamber 301, as compared with flared annular heat pipe 2401. FIG. 27 shows a close-up view of some of heat pipes 2601 at their penetration of front wall 302. As is apparent, very little receiver area is not covered by the heat pipes.

[0073] In an alternative embodiment, rather than flaring the ends of the annular heat pipes, separate reconcentrators may be affixed to cylindrical heat pipes. The secondary reconcentrators are preferably made of a high-temperature material such as alumina or the like, and may optionally have reflective surfaces to promote reflection of radiation into the interior of the heat pipe. [0074] FIG. 28 shows a perspective view of a solar receiver 2800 in accordance with an eighth embodiment of the invention. Solar receiver 2800 is shown as using heat pipes 306 like those shown in FIGS. 3 and 5, but it will be recognized that other heat pipe embodiments may be used as well. In solar receiver 2800, heat pipes 306 are oriented toward the heliostat field as in receiver 300, and receive incoming solar radiation 2802. However, in receiver 2800, front wall 2801 is also angled to face the heliostat field, so that the axes of heat pipes 306 are perpendicular to front wall 2801. The exposed ends of heat pipes 306 thus define a plane that is also angled to face the heliostat field, from which the solar energy receiver is positioned to receive solar energy.

[0075] FIG. 29 shows an orthogonal view of solar receiver 2800, illustrating the tilt of front wall 2801 toward the heliostat field which is located in direction 2901, from which the solar energy receiver is positioned to receive solar energy.

[0076] While the annular heat pipes of FIGS. 19-26 are shown as having circular cross sections in their condensation regions, other cross section shapes are possible, for example an oblong cross section shape. [0077] It is further to be understood that any workable combination of the features and capabilities disclosed above in the various embodiments is also considered to be disclosed.

[0078] The invention has now been described in detail for the purposes of clarity and understanding. However, those skilled in the art will appreciate that certain changes and modifications may be practiced within the scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A solar energy receiver, comprising:

an enclosed chamber having a front wall;

a plurality of heat pipes each having a first end and a second end, the plurality of heat pipes arranged in a bundle, wherein an evaporation zone of each of the plurality of heat pipes is at the first end of the heat pipe disposed outside of the chamber and configured for receiving concentrated solar radiation, and wherein each of the plurality of heat pipes extends from its first end through the front wall of the chamber such that at least some of a condensation zone of each of the plurality of heat pipes is in the interior of the chamber, and wherein within the chamber, the plurality of heat pipes are spaced from each other; and

a heat transfer medium within the chamber substantially filling the spaces between the plurality of heat pipes, such that the plurality of heat pipes and the heat transfer medium form a heat exchanger that heats the heat transfer medium.

2. The solar energy receiver of claim 1 , wherein the plurality of heat pipes are arranged generally parallel to each other.

3. The solar energy receiver of claim 1, wherein the total surface area of the plurality of heat pipes within the heat transfer medium is larger than the projected area of the ends of the plurality of heat pipes receiving solar radiation onto the plane of the aperture of the receiver.

4. The solar energy receiver of claim 1 , wherein the heat transfer medium is a granular heat transfer medium comprising solid granules.

5. The solar energy receiver of claim 4, further comprising a feeder that feeds the granular heat transfer medium system into an upper portion of the chamber; and

a removal system that removes the heated granular heat transfer medium from a bottom portion of the chamber.

6. The solar energy receiver of claim 5, further comprising a hot storage silo for storing the heated granular heat transfer medium removed from the chamber.

7. The solar energy receiver of claim 1 , wherein the front wall is vertical, and wherein each of the plurality of heat pipes is disposed perpendicularly to the front wall of the receiver.

8. The solar energy receiver of claim 1, wherein the front wall is vertical, and wherein each of the plurality of heat pipes is disposed forming an angle with the front wall of the receiver and is angled downward with its longitudinal axis directed toward a heliostat field from which the solar energy receiver is positioned to receive solar energy.

9. The solar energy receiver of claim 1, wherein the front wall is angled to face a heliostat field from which the solar energy receiver is positioned to receive solar energy, and wherein each of the plurality of heat pipes is disposed perpendicularly to the front wall of the receiver and is angled downward with its longitudinal axis directed toward the heliostat field.

10. The solar energy receiver of claim 1, wherein the enclosed chamber comprises baffles for additional support of the heat pipes, and for acting as fins increasing the heat transfer surface area.

11. The solar energy receiver of claim 1 , wherein each of the plurality of heat pipes has a substantially constant cross section along its length.

12. The solar energy receiver of claim 1 , wherein each of the plurality of heat pipes comprises:

a tapered first portion at the first end such that the girth of the heat pipe increases with increasing distance from the first end, the tapered portion disposed outside the chamber;

a second portion adjacent the tapered first portion, the second portion having a substantially constant cross section; and

a third portion adjacent the second portion, the third portion having a substantially constant cross section smaller than the cross section of the second portion.

13. The solar energy receiver of claim 12, wherein the second portions of the plurality of heat pipes are disposed in close proximity to substantially block incoming solar radiation from reaching the front wall of the chamber.

14. The solar energy receiver of claim 12, wherein the second portions of the plurality of heat pipes are hexagonal or rectangular in cross section.

15. The solar energy receiver of claim 12, wherein the second portions of the plurality of heat pipes are circular or oblong in cross section.

16. The solar energy receiver of claim 1 , wherein the first end of each of the plurality of heat pipes is formed into a cross shape.

17. The solar energy receiver of claim 1, wherein at least some of the plurality of heat pipes are annular.

18. The solar energy receiver of claim 17, wherein each of at least some of the annular heat pipes comprises a secondary concentrator at its first end, configured to guide incoming solar radiation into the interior of the annular heat pipe.

19. The solar energy receiver of claim 18, wherein the secondary concentrators nest together to substantially block incoming solar radiation from reaching the front wall of the chamber.

20. The solar energy receiver of claim 17, wherein each of at least some of the annular heat pipes comprises a backing reflector positioned to redirect solar radiation reaching a back end of the annular heat pipe back into the annular heat pipe.