Abstract:
A heat conveyance system particularly suitable for solar applications is described, based on the mechanical conveyance of heat-storage solid bodies containing a bulk that is capable of undergoing phase change. 
     The invention covers the conveyance system itself, and means of inserting and extracting heat into and out of it.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional patent applications 61/349,859 filed on May 30, 2010, and 61/351,941 filed on Jun. 7, 2010, the entirety of which is incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The present application is of the field of thermal power transfer and storage systems. 
       BACKGROUND 
       [0003]    Transferring heat from a heat producer to a heat consumer (e.g. heater and boiler, respectively) is a common task in many industrial applications, especially in the field of solar thermal power generation. For example, in solar power generation (e.g. heliostat fields or dish receivers) heat collection is optimal typically at scales smaller than the optimal scale of the turbines it drives. There is also the need to store heat for use when the sun is not shining—either night use or during times of cloud coverage. Heat conveyance is also required in other energy fields, such as metal production, cement production, and nuclear power plants. 
         [0004]    Heat conveyance and storage is typically done using a variety of thermal transfer fluids, including steam, oil, and molten nitrate salts. 
         [0005]    Turbines become more efficient at higher working temperatures. Steam turbines with steel blades typically work at 560-580 C, using superheated steam (this is the creep limit for steel). Ceramic bladed gas turbines can work at much higher temperatures and achieve higher efficiencies. When only low temperature heat is available, less efficient condensing steam turbines are used. 
         [0006]    Every heat conveyance system has a hot end (where it consumes the heat) and a cold end (where it relinquishes it). Since heat exchangers require a temperature difference to operate, and become large and expensive when the temperature difference is small, the heat conveyance system has to have its cold end significantly hotter than the steam it is generating, and its hot end colder than the heat generation temperature. Every additional medium transfer that requires additional heat exchanger (e.g. steam-to-salt) adds to these temperature differences, and thus increases cost and reduces the efficiency of the turbine. 
         [0007]    The most common method of heat conveyance in solar fields today is by piping oil in tubes, which has a temperature limit lower than 400 C. A substitute for oil is molten salt (commonly a mixture of Sodium nitrate and Potassium nitrate) which melts at 220 C and allows for working temperatures of up to about 500 C. More advanced fluoride-based heat conveyance fluids promise higher working temperatures. When pumping molten salt through pipes, care must be taken that the temperature never drops below the melting temperature of the salt, or it will freeze in the pipes, and the system is not able to take advantage of the latent heat of the phase change. 
         [0008]    As the working temperature increases, however, fluid handling (pumping, valving, sealing) becomes progressively more difficult. Additionally, with all molten fluid systems, the risk of fluid freeze-out in case of a malfunction and drop in temperature is an ever-present problem. 
         [0009]    When superheated steam is used as the working fluid, there is no freeze-out problem, but the combination of high temperature and high pressure also makes the pumping and sealing difficult and expensive. Additionally, the heat capacity of steam is low relative to the salts. 
         [0010]    With a fluid phase-change system, the system cannot take advantage of the solid-liquid phase transition, since the solid medium cannot flow. Instead (as in the case of steam) the system takes advantage of the liquid-gas phase change, which typically carries less latent heat. 
       SUMMARY 
       [0011]    The following summary of the invention is included in order to provide a basic understanding of some aspects and features of the invention. This summary is not an extensive overview of the invention and as such it is not intended to particularly identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented below. 
         [0012]    The invention described herein is a heat conveyance and storage system based on discrete spherical pods filled with a phase-change medium, each capable of transitioning between the solid and liquid phases, and thus storing large amounts of latent heat, but being transported as individual solid objects. 
         [0013]    In various embodiments of this invention, the pods are placed inside tubular conduits. Once inside the conduits, the pods act as their own heat exchangers into fluids (gas or liquid) that flows in the conduit, since they present a large surface area and induce turbulent flow in flowing medium. The tube itself only has to sustain the temperature of the flowing medium, not of the molten salt. 
         [0014]    In various embodiments of this invention, The pods are transported either by rolling them on rails inside the tubular conduits, or by moving entire sections of tubular conduits with pods in them. The system allows transfer of heat at high temperatures (exceeding 1000 C in some configurations) and over large distances, and so works well, for example, for collecting heat from solar dishes and into a central steam generator. 
         [0015]    In various embodiments of this invention, Each pod is comprised of an inner heat-storing medium which undergoes a solid-to-liquid phase change somewhat below the temperature at the hot end of the system, and an outer structural shell which is capable of containing the heat storage medium and supporting the rolling of the pod at the high operating temperature. By wholly encapsulating the phase-change medium by a solid shell, the heat-storage medium is equally transportable in both its liquid and solid states, and so the system is able to take advantage of latent heat storage from the solid-liquid phase transition. Additionally, the hot fluid is not at risk of being contaminated by exposure to a long conduit system. 
         [0016]    In addition to describing the pods and conduits, this application also describes a steam or gas heat-exchangers for turbines and Stirling engines that operate with the pods. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0017]    The accompanying drawings, which are incorporated in and constitute a part of this specification, exemplify the embodiments of the present invention and, together with the description, serve to explain and illustrate principles of the invention. The drawings are intended to illustrate major features of the exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not drawn to scale. 
           [0018]      FIG. 1 : Power transfer pod according to an embodiment of the invention 
           [0019]      FIG. 2 : Transfer conduit according to an embodiment of the invention 
           [0020]      FIG. 3 : Heat Exchanger according to an embodiment of the invention 
           [0021]      FIG. 4 : Conduit with stationary pods according to an embodiment of the invention 
           [0022]      FIG. 5 : Liquid-based heat exchanger according to an embodiment of the invention 
           [0023]      FIG. 6 : None-spherical pods according to an embodiment of the invention 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    The invention described herein is heat conveyance and storage system based on the enclosure of a liquid-solid phase-change medium inside a solid heat-resistance shell, and handling the conveying the entire structure, labeled a “pod”, so that the phase change medium never comes in contact with anything but the shell. In this manner, the conveyance of the heat is decoupled from the heat-storing medium. 
         [0025]    Each pod is thus comprised of an inner heat-storing medium which undergoes a solid-to-liquid phase change between the temperatures of the cold and the hot ends of the conveyance system, and an outer structural shell which is capable of containing the heat storage medium and supporting it structurally at the high operating temperature. 
         [0026]    By wholly encapsulating the phase-change medium by a solid shell, the heat-storage medium is equally transportable in both its liquid and solid states, and so the system is able to take advantage of latent heat storage from the solid-liquid phase transition. Additionally, the hot fluid is not at risk of being contaminated by exposure to a long conduit system. 
         [0027]    The conveyance system, meanwhile, has only to handle solid objects and can do so by contacting them only along small areas, thus being able to keep the contact points cold, and minimizing heat loss. The conveyance system is not required to seal, valve, or pump hot liquid. This allows the system to transport heat at very high temperatures. 
         [0028]    In one embodiment described here, the phase change medium is simple ionic salts such as NaCl (sea salt) which are formed by one element from the first column of the period table (Alkali metals such as Na, K, Li) and one element from the second-to-last column (Halogens such as F, Cl, Br, I). The melting temperature of these materials is in the 700-800 C range, the latent heat is high, and they are generally abundant, inexpensive, and non-toxic. In other embodiments, the heat-transfer medium can be a metal such as Copper, with a melting point of 1000 C. 
         [0029]    In an embodiment of this invention, the outer shell is made out of SiC (Silicon Carbide), which has good thermal conductivity and can operate at temperatures in excess of 1500 C. Other materials can be used for the shell including other carbides, ceramics such as Alumina, or high temperature metals ranging from Steel to Tungsten. The outer surface of the shell is optionally pitted, to improve heat transfer to and from it. 
         [0030]    In an embodiment of this invention, to improve thermal conductivity of the heat storage medium, especially when it is in solid form, a heat conductive structure is embedded inside of it. This structure is made out of copper, or other high-temperature conductive materials. 
         [0031]    In an embodiment of this invention, if necessary, a thin inert isolation layer is added around the heat-storage bulk to prevent any chemical interaction between it and any residual components of the shell. Such a layer can be made from a material such as Quartz. 
         [0032]    In an embodiment of this invention, the pod is heated directly by radiation, and so an absorbent layer is added around the structural shell, made from a material such as graphite, and a thin transparent protection layer placed around it. 
         [0033]      FIG. 1  shows the structure of the pod in cross section. The inner heat storage bulk [ 10 ] is enclosed within the outer shell [ 11 ], and a conductivity aid [ 13 ] is embedded in the heat storage bulk. A void [ 12 ] is left in the solid heat-storage bulk to accommodate thermal expansion and phase-change expansion. The inert isolation layer is shown as [ 16 ]. 
         [0034]    In solar applications, if the shell material is reflective (e.g. Alumina) it is coated with a thin absorption layer [ 14 ] made out of graphite, and finally a thin and transparent outer roll-bearing layer [ 15 ] is added, made out of Alumina, Quartz, or from high purity SiC. If the shell material is absorbent enough (e.g. black SiC) then no such layer is necessary. 
         [0035]      FIG. 2  shows a cross section of an embodiment of the invention comprising a transfer conduit for spherical pods that allows the pods [ 25 ] to roll inside of it. The tubular conduit [ 20 ] provides isolation from the environment and is purged with Nitrogen to suppress oxidation at high temperatures. Reduced pressure can also be employed to reduce heat transfer to the walls of the conduit, but it is more cost effective to insulate the conduit using an external layer [ 21 ]. The conduit has two creases in it [ 22 ] with Alumina or Carbide lining to resist the temperature of the pod. The creases are supported by external rails [ 23 ] which also serve as heat sinks to prevent the conduit wall from reaching high temperatures at the point of contact. A gap [ 24 ] at the bottom of the conduit prevents any particulate contamination from hindering the rolling motion of the pods. Motion of the pods [ 25 ] in the conduit is induced either by gravity, or by pneumatic pressure. 
         [0036]      FIG. 3  shows the cross section of the steam heat exchanger that uses rolling pods. A long conduit [ 30 ] holds the pods [ 31 ] while steam [ 32 ] is counter-flowed [ 33 ] over them, so that the cold steam meets the cold pods [ 34 ], and the hot steam meets the hot pods [ 35 ]. Two load-locks [ 36 ][ 37 ] manage inserting and extracting the pods from the conduit, which operates under high pressure. The conduit is slanted so the pods move against the flow of the steam by gravity. The conduit is constructed from a thin metallic wall, fiber-reinforced along its circumference to resist the pressure. 
         [0037]    In other embodiments, the conduit itself is filled with stationary pods that are not able to move inside of it, but the conduit itself can be carried from the location of the heat producer (such as the bottom of a central heliostat tower) to the location of the heat consumer (such as the boiler that powered a turbine). The purpose of the conduit in this case is simply to contain the pods and allow fluid to flow across them. In these embodiments, the pods are much smaller than the diameter of the conduit. 
         [0038]      FIG. 4  shows such an embodiment that uses stationary pods [ 41 ], and a movable section of conduit [ 40 ]. Instead of load-locks there are simple gate valves [ 42 ] that allow the conduit to be coupled to either a heat producer or a heat consumer, and the entire section of conduit containing the hot pods is moved on wheels [ 43 ] from one to the other. Once connected, steam or another transfer fluid is then flowed through the gate valves [ 42 ] and into the conduit section to either heat the pods or be heated by them. On the consumer side of the system, once the pods cool down to below the phase-change temperature, the entire section of conduit is taken back to the heat producer, and vice versa. Since the pods do not move within the conduit they can be of any shape, such as for example elongated tubes parallel the axis of the conduit. 
         [0039]      FIG. 5  shows a different embodiment, the pods traverse a trench filled with heat-transfer fluid [ 50 ] with a lower melting temperature than the cold side of the steam generator (possibly a Nitrate salt) so it remains liquid throughout the process. When the pods exit [ 51 ] the trench (aided by a mechanical lift, not shown), an air-blade cleans off the exceed fluid that might be present on their outer surface. Steam pipes [ 52 ] are immersed in the same trench, parallel and in proximity to the path of the pods, with steam flowing in the opposite direction [ 53 ] to the thermal gradient in the trench. In this embodiment, the pods do not have to enter the high-pressure conduit, and so the need for load-locks is eliminated and the steam system remains isolated from the pods. The heat-transfer medium, being liquid, can transfer heat from the pods faster than direct steam, and then distribute it efficiently to the steam tubes which can be made small and numerous to increase the heat transfer area. The steam tubes can also be made to coil around the path of the pods to increase the dwell time of the steam. 
         [0040]    Finally, since the pods can operate at very high temperatures, the turbine can operate using a working gas other than steam, such as ambient air or a gas such as Helium. In these case, the steam generator will become a gas heater, and the turbine will be a Brayton cycle gas turbine rather than a Rankine cycle steam turbine. 
         [0041]    It is also possible to store hot pods inside an insulated holding chamber or conduit for later (overnight) use. The walls of a holding chamber will rise in temperature close to the temperature of the pods, and so are made out of a ceramic or other high-temperature material. 
         [0042]    In other embodiments, pods can be of shapes shown in  FIG. 6 , including cylindrical, barrel shaped, or even non-round. 
         [0043]    In these embodiments, the dimension of the pod is between 0.1 and 0.5 m. However, the system can be used at much different scales, both smaller and larger. 
         [0044]    Other embodiments of the system can be used with other power sources such as nuclear reactors where the bulk of the pod can be heated up by absorbing energetic particles, or using a heat exchanger similar to the one used on the electricity-generation side of the system.