WO2017035042A1

WO2017035042A1 - Granule curtain solar receiver

Info

Publication number: WO2017035042A1
Application number: PCT/US2016/047941
Authority: WO
Inventors: Cory HODGHINS
Original assignee: Abengoa Solar Llc.
Priority date: 2015-08-24
Filing date: 2016-08-22
Publication date: 2017-03-02

Abstract

A receiver for a concentrating solar power plant includes a number of falling curtains of a granular heat transfer medium. The curtains are not "face on" to the heliostat field, and thus the receiver has a volumetric effect. Flow interrupters may be present to slow the downward progress of the falling granules, allowing more time for absorption of concentrated solar radiation.

Description

GRANULE CURTAIN SOLAR RECEIVER

Related Applications

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

62/209120 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 for use in a concentrating solar thermal power plant includes a plurality of falling curtains of a granular heat transfer medium. The plurality of falling curtains of the granular heat transfer medium are positioned to receive concentrated solar radiation from a heliostat field. Each of the plurality of falling curtains is positioned so that its largest face does not face the heliostat field. BRIEF DESCRIPTION OF THE DRAWINGS

[0004] FIG. 1 illustrates a simplified schematic view of conventional concentrating solar thermal power plant.

[0005] FIG. 2 illustrates a solar energy receiver in accordance with embodiments of the invention.

[0006] FIG. 3 illustrates a top view of two particular falling curtains in the receiver of FIG. 2, and an obliquely incoming solar energy ray.

[0007] FIG. 4 shows a volumetric receiver in accordance with another embodiment of the invention, including a backing curtain of falling granular heat transfer medium.

[0008] FIG. 5 shows a top view of two particular falling curtains in the receiver of FIG. 4, and an incoming solar radiation ray.

[0009] FIG. 6 shows a partial top view of a solar energy receiver according to another embodiment.

[0010] FIG. 7 illustrates a solar energy receiver according to other embodiments.

[0011] FIG. 8 shows a partial top view of the receiver of FIG. 7 and an incoming ray.

[0012] FIG. 9 illustrates a volumetric solar energy receiver in accordance with other embodiments of the invention.

[0013] FIG. 10 shows an orthogonal view of the solar energy receiver of FIG. 9, from the front of the receiver.

[0014] FIG. 11 shows a volumetric solar energy receiver in accordance with other

embodiments of the invention.

[0015] FIG. 12 illustrates a simplified cutaway schematic of a heat pipe.

[0016] FIG. 13 shows an orthogonal view of the solar energy receiver of FIG. 11, from the front of the receiver.

[0017] FIG. 14 illustrates an orthogonal front view of another volumetric solar energy receiver, in accordance with other embodiments of the invention. [0018] FIG. 15 illustrates a solar energy receiver in accordance with other embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0019] FIG. 1 illustrates a simplified schematic view of a conventional concentrating solar thermal power plant 100. Example power plant 100 uses a central receiver or "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. [0020] 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.

[0021] 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.

[0022] 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. Each of these prior systems involves handling of liquids that may be corrosive and may be at high temperature and pressure.

[0023] The use of solid particles or 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. [0024] One prior 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.

[0025] However, existing solid granule systems also have difficulties. For example, the residence time of the granules at the receiver aperture may be short, so that multiple passes through 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 chemically 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 granular voids ends up dominating 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. [0026] One approach to addressing the difficulties of using solid granule media having poor effective conductivity 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.

[0027] Previous designs having a single curtain of granules oriented perpendicular to the incoming flux may have a theoretical performance limit near that of a flat plate made of the granule material. Embodiments of the invention provide receivers that utilize a volumetric effect to increase the effective absorptivity of falling granule curtains. For example, multiple curtains of falling granular heat transfer material may be created by feeding the granules through an array of slot nozzles. In this manner the incoming flux will see multiple deep, narrow cavities with a small view factor (similar in principle to a blackbody furnace or blackbody receiver). With sufficient depth, these pseudo cavities may increase the effective absorptivity of the receiver (from the perspective of the aperture) and as such the absorptivity of the granules themselves becomes less important. Embodiments of the invention may allow other granule materials to be considered that would otherwise have to be ruled out due to unacceptable low absorptivity for a typical falling curtain design.

[0028] FIG. 2 illustrates a solar energy receiver 200 in accordance with a first embodiment of the invention. Receiver 200 may be placed, for example, at the top of a solar "power tower" such as tower 101 shown in FIG. 1 , and concentrated solar radiation 201 may reach receiver 200 from a field of heliostats. Concentrated solar radiation 201 enters the front of receiver 200.

[0029] Example receiver 200 includes a series of slot nozzles 202 into which a granular heat transfer medium 203 is fed, by any appropriate feeder. Granular heat transfer medium 203 may be made, for example, of alumina, silicon carbide, another ceramic material, or another suitable non-ceramic material. Granular heat transfer medium is preferably a dark color, to promote absorption of solar radiation, but this is not a requirement. 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 3 millimeters in average diameter, for example 300 microns. [0030] Granular heat transfer medium 203 falls through slot nozzles 202 to form a plurality of falling curtains 204 of granular heat transfer medium 203. In the example of FIG. 2, the curtains form parallel planes that are perpendicular (90 degrees) to a vertical plane 205 (containing the zenith) that faces the heliostat field, wherein the heliostat field is the region of the solar field that the receiver views. The rays of solar radiation 201 incoming from the heliostat field intersect plane 205 as they converge toward receiver 200. In other embodiments, the planes formed by falling curtains 204 may form an angle other than 90 degrees with plane 205. For example, the planes formed by the falling curtains 204 may form an angle of 80 to 100 degrees with plane 205, may form an angle of 70 to 110 degrees with plane 205, or may form another angle with plane 205. The planes formed by the falling curtains may all be parallel to each other (as in FIG. 2), or may not be all parallel with each other.

[0031] While some of incoming concentrated solar radiation 201 will arrive exactly parallel to the planes of the falling curtains shown in FIG. 2, the majority will arrive obliquely to the falling curtains. FIG. 3 illustrates a top view of two particular falling curtains 204a and 204b, and an obliquely incoming solar energy ray 301. Ray 301 strikes curtain 204a, and some of its energy is absorbed into the granules of curtain 204a. However, some of the energy is reflected or scattered from the granules, and strikes the adjacent curtain 204b, where again some of the incident energy is absorbed into the granules of curtain 204b, and some is reflected or scattered back toward curtain 204a. This process may continue well into the receiver. 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 falling curtains. The performance of the receiver is therefore less dependent on the optical properties of the heat transfer medium.

[0032] Heat transfer medium 203 is heated as it falls through receiver 200, and 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 203 exiting receiver 200 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 203 can be carried back to and fed into the top of receiver 200, or can be stored in a cold storage silo until such time as solar insolation is available and then fed into the top of receiver 200, so that it can be heated again in an ongoing cycle.

[0033] While only a few slot nozzles 202 and falling curtains 204 are shown in FIG. 2 for ease of illustration, many more may be present, and the receiving area of a receiver aperture embodying the invention may be as large as 50 or more square meters.

[0034] While much of the incoming solar radiation 201 is absorbed in example receiver 200, a portion of incoming solar radiation 201 may reach the back of receiver 200 without having struck any of falling curtains 204 or after having been scattered from one or more of falling curtains 204. FIG. 4 shows a volumetric receiver 400 in accordance with a second embodiment of the invention, including a backing curtain 401 of falling granular heat transfer medium 203 situated at the back of the cavity of the receiver, opposite the solar radiation incidence aperture at the front of the receiver. The function of backing curtain 401 is explained below.

[0035] FIG. 5 shows a top view of two particular falling curtains 204a and 204b of receiver 400, and an incoming solar radiation ray 501 that happens to be parallel to the planes of the falling curtains. Ray 501 thus reaches backing curtain 401 without having struck either of curtains 204a or 204b. Some of the radiation of ray 501 is absorbed into backing curtain 401, heating the granules in backing curtain 401. In addition, some of the radiation from ray 501 is reflected or scattered from backing curtain 401 toward curtain 204a or 204b. The reflected radiation is at least partially absorbed by curtains 204a and 204b, and may be partially reflected or scattered one or more times between curtains 204a and 204b. Thus, much of the radiation in ray 501 that may have been lost in the absence of backing curtain 401 can be captured for heating heat transfer medium 203. Backing curtain 401 may also absorb or redirect to other curtains solar radiation reaching it after being scattered from other curtains, thus preventing the loss of at least some such radiation. The granules of backing curtain 401 may be collected along with the granules of the other curtains for subsequent use.

[0036] FIG. 6 shows a partial top view of a solar energy receiver 600 according to a third embodiment of the invention. Receiver 600 also addresses incoming solar radiation that reaches the back of the receiver, but in a different way than receiver 400. In FIG. 6, an incoming solar radiation ray 601 is parallel to the planes of the falling curtains, and reaches the back of receiver 600 without striking curtain 204a or 204b. At the back of receiver 600 is a backing reflector 602. Example backing reflector 602 includes planar reflective surfaces 603 angled with respect to the curtains 204, so that incoming radiation striking backing reflector 602 is directed toward the curtains. Surfaces 603 may be specularly reflective or diffusely reflective to any suitable degree, and do not need to be planar. Other kinds of backing reflectors may be used as well. For example, a simple flat diffuse reflector at the back of the receiver will scatter incoming light so that most of it strikes curtains 204.

[0037] FIG. 7 illustrates a solar energy receiver 700 according to a fourth embodiment of the invention. In example receiver 700, a "zig zag" or sawtooth shaped slot nozzle 701 is fed with granular heat transfer medium 203. Heat transfer medium 203 falls through slot nozzle 701 to form a plurality of falling curtains 702 of heat transfer medium 203. In this embodiment, curtains 702 are not perpendicular with vertical plane 205, and are also not all parallel with each other. [0038] FIG. 8 shows a partial top view of receiver 700, and an incoming ray 801, which is oriented like rays 501 and 601 discussed above. That is, ray 801 would be parallel to a falling curtain oriented perpendicular to plane 205. However, each of curtains 702 is non-perpendicular to plane 205. That is, each of curtains 702 is angled with respect to plane 205, and in the example of FIG. 8, forms an angle Θ of about 80 (or 100) degrees with plane 205. In other embodiments, each of curtains 702 may be angled with respect to plane 205 by 88 to 92 degrees , by 87 to 93 degrees, by 85 to 95 degrees, by 80 to 100 degrees, by 70 to 110 degrees, or by another amount. As can be seen in FIG. 8, incoming ray 801 may travel within curtains 702 before striking one of them. Some of the energy of ray 801 is absorbed and some is scattered to other curtains. Thus, receiver 700 is also volumetric. [0039] While curtains 702 are shown as joining at their edges, this is not a requirement. A backing curtain or backing reflector as described above may be used with angled curtains such as curtains 702, and may be especially helpful if gaps exist between curtains 702. [0040] In some embodiments, flow interrupters may be placed to slow the downward flow of granules in the falling curtains, so that the residence time of the heat transfer medium within the receiver is increased, allowing additional time for absorption of incoming solar radiation.

[0041] FIG. 9 illustrates a volumetric solar energy receiver 900 in accordance with a fifth embodiment of the invention. Receiver 900 may be placed, for example, at the top of a solar "power tower" such as tower 101 shown in FIG. 1 , and concentrated solar radiation 901 may reach receiver 900 from a field of heliostats.

[0042] Example receiver 900 includes a series of slot nozzles 902 into which a granular heat transfer medium 903 is fed, by any appropriate feeder. Granular heat transfer medium 903 may be made, for example, of alumina, silicon carbide, another ceramic material, or another suitable non-ceramic material. Granular heat transfer medium 903 is preferably a dark color, to promote absorption of solar radiation. 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 3 millimeters in average diameter, for example 300 microns. [0043] Granular heat transfer medium 903 falls through slot nozzles 902 to form a plurality of falling curtains 904 of granular heat transfer medium 903. A plurality of flow interrupters 905 are placed below slot nozzles 902. Flow interrupters 905 may be placed directly below falling curtains 904. When one of falling curtains 904 encounters a flow interrupter, the downward flow of the falling curtain is temporarily stopped or significantly slowed. [0044] FIG. 10 shows an orthogonal view of solar energy receiver 900, from the front of the receiver. In this example, several tiers of flow interrupters 905 are present, each tier impeding the downward flow of heat transfer medium 903 falling from the flow interrupter tier above, or from the corresponding slot nozzle 902. The flow interrupters 905 may be arranged in multiple tiers of flow interrupters spaced vertically from each other. The flow of heat transfer medium 903 through receiver 900 is thus significantly slowed, allowing heat transfer medium 903 to reside within receiver 900 for a longer time than in a comparable receiver without flow interrupters 905, and thus allowing additional time for absorption of incoming solar radiation. In the example of FIG. 9, flow interrupters 905 also divide the flows striking them, although this is not a requirement. [0045] A solar energy receiver having flow interrupters 905, such as solar energy receiver 900, may also use a backing curtain or backing reflector as described above. A solar energy receiver having flow interrupters 905, such as solar energy receiver 900, may also have nozzles disposed in any of the configurations of the embodiments described above, generating configurations with parallel or non-parallel curtains and curtains angled or perpendicular to plane 205.

[0046] According to other embodiments, a solar energy receiver may use heat pipes as flow interrupters.

[0047] FIG. 11 illustrates a volumetric solar energy receiver 1100 in accordance with a sixth embodiment of the invention. Receiver 1100 may be placed, for example, at the top of a solar "power tower" such as tower 101 shown in FIG. 1, and concentrated solar radiation 1101 may reach receiver 1100 from a field of heliostats.

[0048] Example receiver 1100 includes a series of slot nozzles 1102 into which a granular heat transfer medium 1103 is fed, by any appropriate feeder. Granular heat transfer medium 1103 may be made, for example, of alumina, silicon carbide, another ceramic material, or another suitable non-ceramic material. Granular heat transfer medium 1103 is preferably a dark color, to promote absorption of solar radiation, but this is not a requirement. 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 3 millimeters in average diameter, for example 300 microns. [0049] Granular heat transfer medium 1103 falls through slot nozzles 1102 to form a plurality of falling curtains 1104 of granular heat transfer medium 1103. A plurality of heat pipes 1105 are placed below slot nozzles 1102, and serve as flow interrupters.

[0050] FIG. 12 illustrates a simplified cutaway schematic of a heat pipe 1200. 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. 12, a quantity of a working fluid 1201 is enclosed in a hollow envelope 1202, which also encloses a wick 1203. The working fluid is selected to have a boiling point below the temperature of the heat source 1204, but above the temperature of the item to be heated 1205. When heat is applied from the heat source 1204, the working fluid at the hot end 1206 of the heat pipe vaporizes, taking advantage of its latent heat of vaporization, and flows in a gaseous state toward the cold end 1207 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 1207, the working fluid condenses, giving up its latent heat of vaporization to the walls of the heat pipe enclosure 1202, from which heat is conducted to the item to be heated 1205. 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 1203 back toward the hot end 1206 of the heat pipe 1200, to be reused. This natural convection cycle enables very high heat transfer rates. Preferably, the wick 1203 covers the interior surface of the envelope 1202, ensuring that the entire surface is covered in liquid working fluid so that the entire evaporation zone is able to absorb the applied heat.

[0051] 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.

[0052] 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.

[0053] FIG. 13 shows an orthogonal view of solar energy receiver 1100, from the front of the receiver. In this example, several tiers of heat pipes 1105 are present, each tier impeding the downward flow of heat transfer medium 1103 falling from the heat pipe tier above, or from the corresponding slot nozzle 1102. The flow of heat transfer medium 1103 through receiver 1100 is thus significantly slowed, allowing heat transfer medium 1103 to reside within receiver 1100 for a longer time than in a comparable receiver without heat pipes 1105, and thus allowing additional time for absorption of incoming solar radiation. [0054] In this example embodiment, heat pipes 1105 are positioned so that the interstices between heat pipes 1105 align with the planes of falling curtains 1104. The flow of heat transfer medium 1103 is gauged so that a pool 1301 of heat transfer medium 1103 accumulates in the gap between each pair of heat pipes 1105 between which a falling curtain 1104 falls. Thus, a portion of heat transfer medium dwells against the heat pipes for a time on its way through receiver 1100.

[0055] Referring again to FIG. 1 1, heat pipes 1105 also receive some of incoming solar radiation 1101 on their receiving ends 1106, which also include the evaporation zones of heat pipes 1105. As is explained above, the working fluid within heat pipes 1105 evaporates and circulates throughout heat pipes 1105, including to the condensation zones 1107 of heat pipes 1105, where the working fluid condenses and gives up its latent heat of vaporization to the envelopes of heat pipes 1105, and thus to the pools 1301 of heat transfer medium 1103 resting on heat pipes 1105. In this way, heat is transferred efficiently to the back of receiver 1100, and may result in more uniform heating of heat transfer medium 1103. Thus heat pipes 1105 perform a dual function as flow interrupters and heat transfer devices. [0056] Heat pipes 1105 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 zone 1107 may also be of any suitable size. In some embodiments, the diameter of condensation zone 1107 may be between 20 and 100 millimeters, for example about 50 millimeters. Other cross sectional shapes and sizes may be used.

[0057] The envelopes of heat pipes 1105 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 heat pipe working fluid, in which case the envelopes of heat pipes 1105 may be made of a refractory material such as a zirconium alloy. The wicks of heat pipes 1105 are also preferably made of a porous material that can withstand the high temperatures involved, for example a sintered metal. [0058] A solar energy receiver having heat pipes 1105, such as solar energy receiver 1100, may also use a backing curtain or backing reflector as described above. A solar energy receiver having heat pipes 1105, such as solar energy receiver 1100, may also have the nozzle disposed in any of the configurations of the embodiments described above, generating configurations with parallel or non-parallel curtains and curtains angled or perpendicular to plane 205.

[0059] FIG. 14 illustrates an orthogonal front view of another volumetric solar energy receiver 1400, in accordance with a seventh embodiment of the invention. Receiver 1400 is similar in many ways to receiver 1100 shown in FIGS. 12 and 13, but in receiver 1400, heat pipes 1105 are aligned with respective planes of falling curtains 1104. Pools 1401 of heat transfer medium 1103 may still form between heat pipes 1105.

[0060] FIG. 15 illustrates a solar energy receiver 1500 in accordance with an eighth embodiment of the invention. Similar to receivers 1100 and 1400 discussed above, receiver 1500 includes falling curtains 1501 of a granular heat transfer medium, and also include heat pipes 1502 that perform the dual function of interrupting the downward flow of curtains 1501 and transporting heat from the front of receiver 1500 to the rear of receiver 1500.

[0061] In addition, heat pipes 1502 extend rearward beyond falling curtains 1501, into a preheater 1503 outside the receiver. Preheater 1503 uses heat from heat pipes 1502 to preheat the granular heat transfer medium, before it is dispersed into falling curtains 1501 to be heated by direct incoming concentrated solar radiation. Preheater 1503 may, for example, entrain the granular heat transfer medium in air heated by heat pipes 1502, to carry the heat transfer medium to the top of receiver 1500. In other embodiments, preheater may carry the granular heat transfer medium using a conveyor or lift system through an air volume heated by heat pipes 1502. Many other preheating arrangements are possible.

[0062] A backing curtain or backing reflector as described above may be used with receiver 1500 as well. A solar energy receiver having heat pipes 1502 and a preheater 1503, such as solar energy receiver 1500, may also have nozzles disposed in any of the configurations of the embodiments described above, generating configurations with parallel or non-parallel curtains and curtains angled or perpendicular to plane 205. [0063] It is to be understood that any workable combination of the features and capabilities disclosed above in the various embodiments is also considered to be disclosed.

[0064] 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 for use in a concentrating solar thermal power plant, the solar energy receiver comprising:

a plurality of falling curtains of a granular heat transfer medium, the plurality of falling curtains of the granular heat transfer medium positioned to receive concentrated solar radiation from a heliostat field, wherein each of the plurality of falling curtains is positioned so that its largest face does not face the heliostat field.

2. The solar energy receiver of claim 1 , wherein the plurality of falling curtains form parallel planes that are perpendicular to the vertical plane that faces the heliostat field.

3. The solar energy receiver of claim 1, wherein the plurality of falling curtains form parallel planes that are not perpendicular to the vertical plane that faces the heliostat field.

4. The solar energy receiver of claim 1, further comprising a backing falling curtain of the granular heat transfer medium positioned behind the plurality of falling curtains of the granular heat transfer medium, such that at least some direct concentrated solar radiation that does not strike any of the plurality of falling curtains of the granular heat transfer medium directly strikes the backing falling curtain.

5. The solar energy receiver of claim 1, further comprising a backing reflector positioned behind the plurality of falling curtains of the granular heat transfer medium, such that at least some concentrated solar radiation that reaches the back of the receiver without having been absorbed into the granular heat transfer medium is reflected toward the plurality of falling curtains of the granular heat transfer medium.

6. The solar energy receiver of claim 5, wherein the backing reflector comprises a plurality of planar reflective surfaces angled with respect to the plurality of falling curtains of the granular heat transfer medium.

7. The solar energy receiver of claim 1, wherein each of the plurality of falling curtains of the granular heat transfer medium is angled with respect to the vertical plane that faces the heliostat field.

8. The solar energy receiver of claim 7, wherein each of the plurality of falling curtains of the granular heat transfer medium is angled by between 70 and 110 degrees with respect to the vertical plane that faces the heliostat field.

9. The solar energy receiver of claim 8, wherein each of the plurality of falling curtains of the granular heat transfer medium is angled by between 80 and 100 degrees with respect to the vertical plane that faces the heliostat field.

10. The solar energy receiver of claim 7, wherein the plurality of falling curtains of the granular heat transfer medium join at their edges.

11. The solar energy receiver of claim 7, wherein the plurality of falling curtains of the granular heat transfer medium do not join at their edges.

12. The solar energy receiver of claim 1, further comprising a plurality of flow interrupters positioned such that each of the plurality of falling curtains of the granular heat transfer medium strikes one of the flow interrupters while within the receiver, slowing the downward flow of the granular heat transfer medium.

13. The solar energy receiver of claim 12, wherein each of the plurality of flow interrupters divides the flow of the granular heat transfer medium striking the flow interrupter.

14. The solar energy receiver of claim 12, wherein the plurality of flow interrupters are arranged in multiple tiers of flow interrupters spaced vertically from each other.

15. The solar energy receiver of claim 12, wherein the plurality of flow interrupters comprises a plurality of heat pipes.

16. The solar energy receiver of claim 15, wherein each of the plurality of heat pipes is aligned with the respective plane of one of the plurality of falling curtains of the granular heat transfer medium.

17. The solar energy receiver of claim 15, wherein the plurality of heat pipes are positioned such that interstices between the heat pipes align with the respective planes of at least some of the falling curtains of the granular heat transfer medium.

18. The solar energy receiver of claim 15, wherein the plurality of heat pipes extend outside the solar energy receiver, and are used to preheat the granular heat transfer medium.

19. The solar energy receiver of claim 1, further comprising at least one slot nozzle through which the granular heat transfer medium falls to form the plurality of falling curtains of the granular heat transfer medium.