Patent Publication Number: US-2021164697-A1

Title: Multi-Stage Falling Particle Receivers

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of and claims priority to U.S. Non-Provisional patent application Ser. No. 16/385,993, filed Apr. 16, 2019, entitled “Multi-Stage Falling Particle Receivers,” and U.S. Provisional Patent Application Ser. No. 62/658,340, filed Apr. 16, 2018, entitled “Multi-Stage Falling Particle Receivers,” and is related to U.S. Provisional Patent Application Ser. No. 62/145,136, “Falling Particle Solar Receivers,” filed on Apr. 9, 2015, and to U.S. patent application Ser. No. 15/095,738, “Falling Particle Solar Receivers,” filed on Apr. 11, 2016, the disclosures of which are incorporated by reference herein in their entireties. 
    
    
     STATEMENT CONCERNING FEDERALLY SPONSORED RESEARCH 
     The United States Government has rights in this invention pursuant to Contract No. DE-AC04-94AL85000 between the United States Department of Energy and Sandia Corporation, and pursuant to Contract No. DE-NA0003525 between the United State Department of Energy and National Technology and Engineering Solutions of Sandia, LLC, for the operation of the Sandia National Laboratories. 
     The Australian Government has rights in this invention as partial funding for this work was provided to the Commonwealth Scientific and Industrial Research Organisation by the Australian Government. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure is generally directed to solar energy, and more particularly directed to falling particle concentrated solar receivers. 
     BACKGROUND OF THE INVENTION 
     Solar power systems offer much promise for clean energy, with few, or zero, carbon emissions. These systems collect incident sunlight and convert this sunlight into a usable form of power, such as heat or electricity. Solar energy offers a clean, inexhaustible, sustainable solution to energy demands and has the potential to supply a very significant fraction of U.S. and global electricity consumption. While the U.S. and global solar power potential is known to be immense, solar power systems have not been economically competitive without government support, to date. Challenges remain to devise solar technologies that can lower installation costs, increase power output, and lower the marginal cost per unit energy produced, for a lower levelized cost of energy. 
     Emerging solar power systems include particle solar receivers that heat particles for energy conversion, energy storage, thermochemical processes, electricity production, and process heating. However, previously disclosed processes and systems are either not efficient in capturing solar energy to heat particles or require complex structures or fluidization, which increase costs and parasitic electricity consumption. 
     The particle receivers utilize solid particles as the heat transfer medium to absorb the incident concentrated solar energy. One type of receiver uses a freely falling particle curtain directly exposed to concentrated solar energy through an aperture open between the particles and sunlight to heat the particles. However, there are some inherent drawbacks with the free-falling particles, such as the decreased volume fraction and opacity of the falling particles caused by gravitational acceleration that increases downward velocity and dispersion and reduces the residence time of the particles. 
     Low particle volume fraction increases the transmittance of the particle curtain. Additionally, solar energy not absorbed by the particles is lost either by being reflected back out the receiver aperture to the outside surroundings or by being absorbed by the walls and materials other than the particles. This problem is affected by a combination of multiple factors including particle falling height, flow rate, and particle size. Flow instability of the particle curtain is an additional issue with the free-falling particle receiver. As the particles fall, a stable and dense initial clustered formation is perturbed and transforms into a wider and unstable particle curtain, creating undesired impact on particle hydrodynamics and heat transfer. Uneven heating across the falling particle curtain is one of the main drawbacks of a free-falling particle receiver. 
     The need remains, therefore, for falling particle solar receivers and methods of falling particle heating to address these and other limitations and that efficiently capture solar energy to heat particles for energy conversion, storage, and thermal processes. 
     SUMMARY OF THE INVENTION 
     The disclosure is directed to multi-stage falling particle receivers and methods of falling particle heating. As the particles fall through the receiver, the particles are periodically collected in a trough or funnel, where they are slowly released again. 
     According to an embodiment of the disclosure, a solar receiver is disclosed that includes a housing comprising at least one opening for receiving concentrated solar irradiance and a front wall and a rear wall, one or more openings in the housing for receiving irradiance, and a flow retarding system comprising one or more flow retention devices disposed within the housing for receiving and releasing particles as the particles fall through the solar receiver. 
     According to an embodiment of the disclosure, a solar heating method is disclosed that includes providing particles to a solar receiver and collecting and releasing particles in one or more flow retarding devices as the particles fall through the solar receiver and are heated by solar irradiance. 
     An advantage of the disclosed system and method is that it reduces the particle velocity and amount of vertical dispersion that a free-falling particle curtain experiences due to gravitational acceleration. 
     Another advantage is that the multistage system increases particle flow stability and reduces the impact of wind and chance of dust formation. 
     Another advantage of the disclosure is that the collection troughs can be used to mix the particles to enhance heat transfer and uniformity of the particle temperatures as they fall through the receiver. 
     Another advantage over a large array of continuous discrete obstructions is that the present disclosure uses less material to slow down the particles, which is beneficial for cost, repair, and maintenance, and the collection troughs can be positioned so that they are not within the direct irradiance. 
     Another advantage is that our invention is designed to cause falling particles to impinge on other particles that have accumulated in the trough, funnel, or ledge below (rather than impinge on the walls or surfaces of the trough, funnel, or ledge themselves) when being collected and slowed down, which protects surfaces from erosion and damage. The accumulation of particles in the trough, funnels, or ledges can be controlled actively (motorized) or passively (gravimetric counterweights, springs, variable slot apertures), as described in the application. 
     Another advantage is that in some embodiments, collection troughs can also be designed to be hidden behind the falling parting curtain to avoid direct exposure to the concentrated solar energy. 
     Another advantage is that our invention enables variable particle mass flow rate while still maintaining the desired high opacity. In previous systems that use a continuous array of porous structures, if the particle flow rate is lower than the design point, the mesh or porous structures will be exposed to direct irradiance since the particle flow will be insufficient to cover all the obstructions. If the particle flow is greater than the design point, particles will begin to waterfall over the leading edge of the top row of obstructions, and there will be significant reduction in the opacity of the waterfalling particles due to free-fall acceleration and dispersion. Our invention can be designed to handle a wide range of flow rates and still perform the periodic catch-and-release behavior to reduce downward particle velocities and increase opacity of the particle curtain. 
     Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The figures depict embodiments of the present invention for purposes of illustration only and are not necessarily drawn to scale. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein. 
         FIG. 1A  is a perspective view of a multi-stage falling particle receiver according to an embodiment of the disclosure. 
         FIG. 1B  is a front view of the multi-stage falling particle receiver of  FIG. 1A . 
         FIG. 1C  is a side view of the multi-stage falling particle receiver of  FIG. 1A . 
         FIG. 2A  is a perspective view of a trough according to an embodiment of the disclosure. 
         FIG. 2B  is a top perspective view of the trough of  FIG. 2A . 
         FIG. 2C  is a bottom perspective view of the trough of  FIG. 2A . 
         FIG. 2D  is a side view of the trough of  FIG. 2A . 
         FIG. 3A  shows another embodiment of a trough particle flow retarding device according to an embodiment of the disclosure. 
         FIG. 3B  is a cross-sectional view of the trough particle flow retarding device of  FIG. 3A . 
         FIG. 4  is a simplified schematic of an embodiment of a flow retarding system disposed within a receiver according to embodiments of the disclosure. 
         FIG. 5  is a simplified side profile of a multi-stage falling particle receiver with V-shaped troughs. 
         FIG. 6  is a simplified side profile of a multi-stage falling particle receiver with wall-assisted troughs. 
         FIG. 7  is a simplified side profile of a multi-stage falling particle receiver with troughs and wall-attached deflectors. 
         FIG. 8  is a simplified side profile of a multi-stage falling particle receiver with wall mounted troughs. 
         FIG. 9  is a simplified side profile of a multi-stage falling particle receiver with inclined wall and wall-attached troughs. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present disclosure is directed to multi-stage falling particle receivers and methods of falling particle heating. As the particles fall through a falling particle solar receiver (receiver), the particles are periodically collected and released from rest (near-zero downward velocity) through the receiver by a flow retarding system that includes one or more catch-and-release or flow retarding devices that collect and release the particles as they continue to flow through the receiver. In various embodiments, the flow retarding system may include one or more flow retarding devices. In some embodiments the flow retarding system may include three or more flow retarding devices. In yet other embodiments, the flow retarding system may include between 10 and 90 flow retarding devices determined by factors including, but not limited to receiver cavity size, particle flow rate and irradiance. 
     The periodic collection and release prevent dispersion and acceleration as they fall through the receiver cavity and are heated by concentrated sunlight. For example, periodic collection and release in large-scale solar receiver systems (receiver size ˜10-20 m) can prevent increased vertical and horizontal dispersion as the particles accelerate under gravity. In addition, the periodic collection and release reduces vertical particle dispersion that may lead to increased light transmittance through the particles, and horizontal dispersion may lead to particle loss through the aperture. Both vertical and horizontal dispersion cause increased heat loss resulting in decreased receiver performance. 
     In various embodiment, the flow retarding device may be funnels or troughs, ledges, grooves, wedges, or other structures or surfaces and combinations thereof that collect, retard and release the particles falling through the receiver under gravity. The flow retention devices are housed in a solar receiver that includes a housing, one or more openings in the housing for receiving irradiance or sunlight. In an embodiment, the flow retention devices are one or more troughs disposed within the housing for receiving and discharging particles as the particles fall through the solar receiver. The one or more openings may be slots, holes or other passages of variable sizes that allow for the release of the particles. In another embodiment, the flow retarding devices may be one or more tilted or inclined ledges or surfaces that collect and release the falling particles as the particles overfill the device (creating a “waterfall” effect”). In yet another embodiment, the flow retarding device may be any two or more of the above disclosed devices. 
     The present disclosure is further directed to solar particle heating methods that include collecting, retarding and releasing, which can be collectively referred to as “retarding” particles as they fall through a solar receiver. The particles may be retarded one or more times as they fall through the receiver. The particles are retarded to reduce vertical particle dispersion that may lead to increased light transmittance through the particles, and to reduce horizontal dispersion may lead to particle loss through the aperture. 
     In various embodiments, the flow retention devices may have no or one or more openings. An opening is for this disclosure an opening other than the top opening for receiving falling particles. As such, in some embodiments, some flow retention devices include no openings so that collected particles “spill out” of the top opening. As such, in some other embodiments, the flow retention devices include side and/or a bottom opening for releasing particles. The openings are sized for the flow rate so that the particles are retained within the flow retention devices for some amount of time greater than zero before being released. In both embodiments, there is some accumulation of particles for some predetermined amount of time based on flow rate by the flow retention devices. For example, troughs having no openings accumulate particles until the particle level reaches a height in the trough that allows particles to spill out of the trough. In another example, particles accumulate on ledges to a predetermined amount before the particles spill off of the ledges. For troughs with openings, the particles accumulate in the troughs until the particles are released through the openings. One or more openings may be passively or actively controlled to adjust the size of the openings, such as for example passively adjusted by collected weight of particles or actively adjusted by actuators and/or other mechanical devices by an operator based on factors including, but not limited to flow rate, particle characteristics and irradiance. 
     In an embodiment, a series of particle collection troughs or other flow retention devices may be placed inside a receiver to collect the particles at intermittent intervals before the particles can accelerate and disperse too much. In another embodiment, one or more particle collection troughs may be used. The receiver can be comprised of a single aperture or multiple apertures to accommodate various sections of the particle flow as defined by the troughs. By aiming heliostat beams through multiple apertures, direct irradiance on the troughs (which can be placed in between the apertures) can be minimized to prevent overheating of the troughs. Some amount of incident light on the troughs may actually be good to heat the particles, and the troughs can be transparent or porous to allow light to heat the particles directly. In some embodiments, retarding troughs can be protected from direct irradiance by overflowing particles that flow over the edge facing incident concentrated solar energy, in other words, the waterfall of particles over the edge block/absorb the sunlight from the troughs. 
     The collection troughs can be designed to accommodate variable particle mass flow rates. The objective is to decelerate the particles before they are released again. If the trough contains just a single aperture at the bottom, the particle mass flow is prescribed by the aperture size. If the particle flow is less than the capacity of the aperture, then the particles will flow through the open aperture without significant deceleration. If the particle mass flow is greater than the capacity through the aperture, then the particles will accumulate and overfill the trough. Particles that impinge and flow over the mound above the trough will also not be decelerated to the fullest extent. In an embodiment, the troughs may include vertical slots that enable variable particle mass flow rates to be collected and released from a near-zero vertical velocity. In other embodiments, variable particle flow rates can be accommodated by designing the retarding troughs to allow one-side overflowing, creating a waterfall effect. 
     The troughs may mix the particles to enhance heat transfer and provide uniformity of the particle temperatures as they fall through the receiver. The collection troughs can be opaque, or they can be transparent or porous to allow direct heating of the particles by incident light. In an embodiment, the collection troughs can be positioned so that they are not within the direct irradiance. The aperture size of the collection troughs can be fixed, actively controlled, or passively controlled to enable variable mass flow rates of particles to be released from rest or near rest from each trough. In other embodiments including overflowing troughs, when overflowing is designed to occur on the solar irradiance side, the collection troughs can be efficiently protected from solar irradiance by the overflowing particles. The troughs can be designed to accommodate variable particle mass flow rates while maintaining a high particle curtain opacity. In contrast, with continuous discrete obstructions, only a small range of particle mass flow rates can be accommodated. 
     Computer modeling has shown that excessive vertical and horizontal dispersion is mitigated as a result of the flow retention devices, such as periodic trough collectors. As the particles fall, a high opacity of the particle curtain is desired to intercept the incident sunlight. The multistage system increases the particle opacity. 
     Other embodiments provide for various arrangements and staggering of the flow retention devices through the receiver. The angle of the troughs can be symmetric or skewed toward one size, and the location of the troughs can be located anywhere within the receiver or against the back wall to mitigate particle loss and the impacts of wind. 
     The present disclosure if further directed to methods for heating particles falling within a multi-stage falling particle receiver. As the particles fall through the receiver, the particles are periodically collected and released, preventing further dispersion and acceleration as they fall through the cavity of the receiver. This is important, especially in large-scale systems (receiver size ˜10-20 m), to prevent increasing vertical dispersion as the particles accelerate under gravity, which leads to increased light transmittance through the particles and horizontal dispersion, which may lead to particle loss through the aperture. 
       FIGS. 1A-1C  show different views of a multi-stage falling particle receiver (receiver)  100  according to an embodiment of the disclosure. As can be seen in  FIGS. 1A-1C , the receiver  100  includes a housing  102  and a flow retarding system  104  disposed therewithin. The housing  102  includes a front panel  103 A and a rear panel  103 B. The housing  102  further includes an inlet  106  disposed within a housing top structure  107  and an outlet  108  disposed in a collection bin  109  in fluid communication to an internal chamber or cavity  110 . The housing  102  further includes windows, apertures or openings  112  in the front panel  103 A that allow directed, concentrated solar irradiance  113  to enter the cavity  110  and heat particles (not shown) falling therethrough. The concentrated solar irradiance  113  is from a solar source (not shown), such as a plurality of mirrors. In this exemplary embodiment, the front panel  103  includes four openings  112  that allow concentrated solar irradiance  113  to enter the cavity  110 , and those openings  112  are separated by portions of the front panel  103  arranged to block the concentrated solar irradiance  113  from impinging on the flow retarding devices  104 .  FIG. 1B  shows a frontal view wherein the flow retarding system  104  is visible, however, it should be noted that as seen from the perspective of the angle of irradiance of sunlight into the receiver, the flow retarding system  104  would be not visible as it would be blocked by portions of the front panel  103 A In other embodiments, the receiver  100  may include one or more openings that may or may not be arranged within the front panel to block solar irradiance from impinging on the flow retarding devices. 
     The flow retarding system  104  retards particles falling through the receiver  100 . In this exemplary embodiment, the flow retarding system  104  includes three flow retarding devices  105 , respectively referred to as troughs  104 A,  104 B and  104 C. In another embodiment, the receiver  100  may include one or more flow retarding devices. In yet another embodiment, the receiver  100  may include two or more flow retarding devices. 
     In this embodiment, the flow retarding devices  104  are funnels or troughs  104 . The troughs  104  collect and retain the falling of particles for a predetermined amount of time, and then release and allow the particles to continue to fall. In such a manner, the particles fall is retarded. As the particles fall and horizontally (measured from the front or opening side of the receiver to the opposing back of the receiver) disperse, the particles are collected by the trough and released in a curtain, veil or other shape that has a predetermined horizontal length. In such a manner, the falling particle dispersement can be corrected to a predetermined width. In an embodiment, the predetermined width is the same as the initial width the particle curtain has as the particles enter the chamber  110 . In another embodiment, the predetermined width is less than or greater than the initial width the particle curtain has as it enters the chamber  110 . In such a manner, the particle curtain width can be controlled to accommodate the spatially non-uniform solar flux distribution entering the receiver. 
     The troughs  104  are disposed within the housing  102  and vertically arranged so that the most upper trough  104 A receives particles from the inlet  106 , and outputs or releases those particles to a next in sequence or second trough  104 B disposed underneath thereof. The next in sequence, trough  104 B thereafter releases those collected particles to a third trough  104 C, which releases those particles to the collection bin  109  or other particle collection device or system located proximate the bottom of the receiver  100 . 
     In such a manner, particles falling into the upper most or first trough  104 A receive irradiance, and the particles falling between the troughs and from the third trough also receive irradiance. In other embodiments, two or more openings  112  may be placed to allow irradiance to be received by particles at two or more locations in the falling particle arrangement. A single aperture could also be used with multiple heliostat aim points to heat the particles falling between collection troughs. 
       FIGS. 2A-D  show a trough  200  according to an embodiment of the disclosure. As can be seen in  FIGS. 2A-2D , the trough  200  include apertures, slots or openings  204 . In this exemplary embodiment, the trough  200  includes a top opening  202  for receiving falling particles, and a plurality of apertures, slots or openings  204  for releasing collected particles. In this exemplary embodiment, the trough  200  includes six openings  204 . In other embodiments, the trough  200  may include one or more openings  204 . In this exemplary embodiment, the openings are horizonal slots having a length L approximately equal to the length of the inlet  106 . 
     The size of the slot apertures can be varied to accommodate more mass release as the slots progress towards the top. The configuration and size of the slot apertures may be optimized to create particle release configurations that take advantage of volumetric heating. Furthermore, the collective opening area is sized so that the total volume release is greater than the mass flow into the receiver. In such a manner, a trough cannot overflow from not being able to release more than the received volume of flow. In addition, the spacing of the slots from the front (side closest to impinging light) to the back (side farthest from impinging light) can be varied so as to maintain the width of the particle flow curtain, increase or decrease the width. Furthermore, the distance between the troughs can be varied to optimize the heating of the particles in conjunction with the irradiance distribution from the heliostat aiming strategy. In particular, the troughs are separate components that do not form a continuous retention of particle falling flow but provide for a discontinuous catch-and-release system of retardation. 
     Referring back to  FIG. 1C , it can be seen how particles (indicated by the mass collected in the troughs and dashed lines) collect and are retained for a period of time greater than zero in the troughs  104  and are released from the openings to continue to fall within receiver  102 . 
       FIGS. 3A and 3B  show another trough  300  according to an embodiment of the disclosure.  FIG. 3B  is a cross-sectional view of  FIG. 3A  and is a mirror image of the opposing section. As can be seen in  FIGS. 3A and 3B , the trough  300  includes a front panel  310  and a rear panel  312 . The trough  300  further includes a top opening  314  for receiving falling particles, and a bottom opening  316  for discharging or releasing particles. The trough  300  also includes a central divider  318  that directs falling particles either towards the front panel  310  or the rear panel  312 . 
     The central divider  318  includes a front divider panel  320  and a rear divider panel  322 . As can be seen in  FIG. 3B , the front divider panel  320  includes slots or openings  324  that allow particles directed towards the front panel  310  to accumulate and be released into the bottom opening  316  for discharge. The rear divider panel  322  mirrors the front divider panel  320 . This particular embodiment causes the particles to be released through a single opening, creating a single particle curtain. If the particle flow rate exceeds the discharge capacity of the bottom slot of panel  320 , the particles will further accumulate and discharge through the slot above the bottom slot of panel  320 , and so on. The height and number of the slot openings in panel  320  can be variable and designed to handle any range of expected particle flow rates of the system. In one embodiment, there could be as few as one slot opening in front divider panel  320  and rear divider panel  322 . 
     According to another embodiment of the disclosure, the flow retarding system may include one or more flow retarding devices that are inclined planes.  FIG. 4  is a simplified schematic of an embodiment of a flow retarding system  400  disposed within a receiver  402 . The flow retarding system includes flow retention devices  404  that are planes or ledges that are. In this exemplary embodiment, the ledges are inclined downward from the rear to the front of the receiver  402 . In other embodiments, the ledges may be horizontal or inclined downward. In such a manner, falling particles  406  can cascade through the interior of the receiver while being irradiated by concentrated solar irradiance. In this exemplary embodiment, the inclined planes are inclined downward by fifteen degrees. In another embodiment, the inclined planes may be inclined between 0 and less than 90 degrees. The inclined planes minimize unnecessary particle collection. 
       FIG. 5  illustrates a simplified schematic of a flow retarding system  500  arranged within a solar receiver  501  having a rear wall  510  according to another embodiment of the disclosure. As can be seen in  FIG. 5 , the flow retarding system  500  includes troughs  502  having a V-shaped profile formed by opposing walls  504  with an aperture or opening  506  at the bottom of the troughs  502 . The troughs  502  are installed in series fully exposed to solar irradiance  508 . In this exemplary embodiment, the troughs are vertically offset. In other embodiments, the troughs may be vertically alighted. In order to maintain proper particle level to allow efficient retardation and stable release of particles, the trough opening  506  can be controlled actively by an actuator or passively by particle weight in the trough. For example, the troughs may be actively or passively controlled to allow the trough sides to swing as shown by the arrows to increase or decrease the bottom opening size in order to control flow rate and the amount of accumulation of particles  550  (indicated by the mass shown contained within the troughs. 
       FIG. 6  illustrates another simplified schematic of a flow retarding system  600  arranged within a solar receiver  601  having a rear wall  610  according to another embodiment of the disclosure. As can be seen in  FIG. 6 , the flow retarding system  600  includes ledges  602  inclined or angled downward towards the rear wall  610 . The reference system in this and other figures disclosed herein, referring to  FIG. 1 , is that the direction from the inlet  106  to the outlet  108  is downward. The inclined ledges  602  and rear wall  610  define an opening  606  that allow particles collected on the ledges  602  to flow towards the rear wall  610  and then downward. The inclined ledges  602  may be actively or passively controlled to adjust incline angle and/or distance from the rear wall  610  to control particle flow. The inclined ledges  602  include an inclined flat portion  602   a  and a barrier portion  602   b . The barrier portion  602   b  includes ridges, walls, or other flow impediments that slow or impede the flow of particles off of the ledges  602 . In other embodiments, the ledges  602  may only include an inclined flat portion and no barrier portion. In other embodiments, the inclined ledges  602  may only include a barrier portion and no flat portion. As shown in  FIG. 6 , the angle of the troughs can be actively or passively increased or decreased as shown by the arrows to control the amount of particles accumulated  650  on the ledges and to control the release or flow rate from the ledges. 
       FIG. 7  illustrates another simplified schematic of a flow retarding system  700  arranged within a solar receiver  701  having a rear wall  710  according to another embodiment of the disclosure. As can be seen in  FIG. 7 , the flow retarding system  700  includes flow diversion structures  709  and corresponding flow retarding devices  702 . The flow diversion structures  709  include an inclined surface  709   a  that divert falling particles downward away from the rear wall  710  and into the flow retarding devices  702 . The flow retarding devices  702  have a non-symmetrical V-shaped cross-section including a high side  702   a  and a lower side  702   b  that directs particles filling the flow retarding devices  702  towards the rear wall  710 . The flow retarding system  700  allows efficient retardation and stable release of particles regardless of particle flow rate without needing an active or passive flow control mechanism in the trough. Direct particle falling through the gap between wall and trough is prevented by deflectors attached to the wall just above each trough. It should be understood that particles fill the space above and contained by the flow retarding devices  702  in a manner that when the space if full, the particles impact contained particles, and may mix, and then particles overflow as shown by the flow represented by the dashed line. 
       FIG. 8  illustrates another simplified schematic of a flow retarding system  800  arranged within a solar receiver  801  having a rear wall  810  according to another embodiment of the disclosure. As can be seen in  FIG. 8 , the flow retarding system  800  includes flow retarding devices  802  configured to release particles in a cascade flow away from the rear wall  810 . The flow retarding devices  802  include a trough portion  804  and a mount portion  808  that connects the trough portion  804  to the rear wall  810 . The trough portion  804  increases in length as the flow retarding devices  802  progress downward, allowing for particles to cascade. In this exemplary embodiment, the first (numbering from top to bottom) flow retarding device has not mount portion. In other embodiments, the mount portions may start with zero or greater than zero length. In such a manner the flow retarding devices are designed to allow overflowing of particles in the direction of solar energy reception. In this exemplary embodiment, the mount portions are inclined downward in order to minimize unnecessary particle collection. Particles may impact and accumulate over the mount portion  808  and/or trough portion  804 . In other embodiments, the mount portions may be horizontal or inclined downward. The flow retarding system provides for efficient retardation and stable release of particles regardless of particle flow rate. An additional benefit of this design is that the troughs and mounts are protected from direct solar irradiance by overflowing particles. The protected trough prevents heat loss from the particles and material damage caused by direct exposure of trough to high flux solar irradiance. It should be understood that particles fill the space above and contained by the flow retarding devices  802  in a manner that when the space if full, the particles impact contained particles, and may mix, and then particles overflow as shown by the flow represented by the dashed line. 
       FIG. 9  illustrates another simplified schematic of a flow retarding system  900  arranged within a solar receiver  901  having a rear wall  910  according to another embodiment of the disclosure. As can be seen in  FIG. 9 , the flow retarding system  900  includes flow retarding devices  902  configured to release particles in a cascade flow away from the rear wall  910 . The flow retarding devices  902  include a retention portion  904  and a mount or base portion  908  that connects the retention portion  904  to the rear wall  910 . In this exemplary embodiment, the rear wall  910  is inclined downward towards the direction of incident light, allowing for particles to cascade. In such a manner the flow retarding devices in conjunction with the inclined rear wall are designed to allow overflowing of particles in the direction of solar energy reception that protect the devices from direct solar irradiance. Optimum angle of the rear wall inclination is determined by a combination of particle flow rate and the distance between troughs. It should be understood that particles fill the space above and contained by the flow retarding devices  902  in a manner that when the space if full, the particles impact contained particles, and may mix, and then particles overflow as shown by the flow represented by the dashed line. 
     The disclosed embodiments include multiple flow retarding devices, however, it should be understood that a flow retarding system may include one or more of one or more of the various embodiments of flow retarding devices. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.