THERMOELECTRIC CONVERTER WITH PROJECTING CELL STACK
A thermoelectric converter is formed by a plenum divided into high and low pressure chambers by a partition and includes a stack of series-coupled alkali-metal thermoelectric cells that projects orthogonally from the partition into one of the chambers.
This application is a continuation of U.S. patent application Ser. No. 13/651,049 filed Oct. 12, 2012 and entitled “Thermoelectric Converter with Projecting Cell Stack,” which claims priority to U.S. Provisional Patent Application No. 61/627,949, filed Oct. 21, 2011 and entitled “Concentration-mode Thermoelectric Converter (C-TEC).” Each of the foregoing patent applications is hereby incorporated by reference.
TECHNICAL FIELDThe disclosure herein relates to thermal-to-electric power generation.
BACKGROUNDTo limit power-draining ohmic losses, wire-wrapping or other auxiliary current collection structures typically overlay the relatively low-conductance porous electrodes. Unfortunately, such structures tend to degrade prematurely in the thermally challenging environment of the converter. For example, wrapped wires tend to lose physical and electrical contact over time (e.g., due to non-uniform thermal expansion/contraction of the wires and structures they encircle), increasing I2R loss and thus degrading the power density of the converter.
In thermal-to-electric converter 120, shown in cross-section in
The annular design of converter 120 brings additional complications. For one, the cell stack and its interconnection to opposite ends of the plenum housing are subjected to significant stress/strain during thermal expansion/contraction (i.e., as the plenum housing and cell stack components tend to exhibit expand/contract non-uniformly), mechanical wear forces that tend to degrade device power density and lead to premature failure, particularly in applications that involve frequent temperature cycling. The coaxial heating arrangement also adds complexity (requiring heat to be injected into a blind hole) and tends to be thermally inefficient as heat radiates directly from the interior heat source toward the cold containment wall of the plenum. Perhaps more significantly, the large temperature gradient between the heat source and plenum wall (and relatively short distance between the cell stack and cold plenum wall) and makes it difficult to prevent sodium condensation on the cell stack surface, a highly problematic phenomenon as the electrically conductive sodium condensate can short the different-potential cells to one another, severely disrupting operation of the converter.
Yet other issues plague the implementation of
The disclosure herein is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:
A thermal-to-electric converter having a plenum divided into high and low pressure chambers by a partition and a stack of alkali-metal thermal-to-electric cells that projects orthogonally from the partition into one of the chambers is disclosed herein in various embodiments. In one embodiment, the partition is formed by a substantially planar member extending between side walls of the plenum to divide the plenum into high vapor pressure and low vapor pressure chambers, and having one or more openings covered by respective cell stacks that are capped on their unsecured ends. By securing the cell stack at only one end (i.e., at the partition opening), the cell stack is free to expand and contract without suffering the mechanical stress/strain forces that plague cell stacks secured at both ends. Further, by projecting the cell stack into the high vapor pressure chamber so that heat is directed inwardly from the plenum housing toward the cell stack, a highly efficient thermal arrangement is obtained (i.e., as heat energy radiating through the cell stack remains predominantly in the high vapor pressure chamber instead of flowing, for example, to a cold plenum wall) and maintains the cell stack at a temperature that avoids working fluid condensation. Also, in a number of embodiments, the cells in a given stack are electrically interconnected and sealed to one another by a continuous joint, simplifying construction of the cell stack and limiting inter-cell dead space. In other embodiments, shunt currents are suppressed by an isolation barrier that projects outwardly (and/or inwardly) from the wall of the cell stack to increase the electrical path length between cell electrodes and cell interconnects of different potential. In yet other embodiments, solid electrolyte components of individual cells are physically isolated from cell-to-cell interconnects via extensions of the projecting isolation barriers into the cell wall and/or by one or more ceramic joints to provide triple-point isolation. These and other embodiments and their features and benefits are discussed below.
A heat source 159 applied to the exterior surface of the plenum wall adjacent the high pressure chamber (although heat sources may alternatively or additionally projected into or be embedded within the high pressure chamber, and/or a remote boiler may be used) heats the high pressure alkali metal vapor therein (e.g., sodium vapor, potassium vapor or any other practicable alkali metal vapor) to a temperature that creates a pressure (or concentration) gradient between the two chambers, the driving force for device operation. More specifically, the openings in partition 153 effectively extend the low pressure chamber into the interiors of the cell stacks 160, establishing a pressure or concentration gradient across the solid electrolyte wall of each cell 161i that promotes alkali metal ionization and propels ionized alkali metal ions through the solid electrolyte to the interior surface of each cell. Electrons freed by the ionization reactions are blocked by the solid electrolyte and collected by a porous electrode (i.e., anode) on the exterior cell surface, thus developing an electrical potential between the exterior and interior surfaces of each cell.
Still referring to
The solid electrolyte plates that form walls of the cell stacks generally exhibit low thermal expansion rates in comparison with oxidation-resistant materials (e.g., stainless steel) used to form the plenum housing and partition 153. Accordingly, to account for potentially non-uniform thermal expansions of plenum 151, partition 153 and projecting cell stacks 160, a malleable stack-mount member 166 is provided to secure the initial cell (1611) of a given stack to the perimeter of the corresponding partition opening. In one embodiment, shown in detail views 170a and 170b for example, stack-mount 166 is implemented by a Niobium dome (Molybdenum, Tantalum, Titanium, various alloys, etc. may also be used) brazed or otherwise secured and electrically coupled between the perimeter of the partition opening and the underside of the initial cell. This malleable dome-shaped stack-mount 166 may be bonded directly to the low-thermal-expansion-rate solid electrolyte plate 172 without undue stress at joint 169, while the radius of stack-mount 166 will plastically deform to relieve the stress induced at the high-thermal-expansion-rate partition material in joint 168 (which may be formed within a notch, groove, channel or other recess disposed about an opening in partition 153) without transferring this stress to solid electrolyte plate 172. Accordingly, through this arrangement, joints 168 and 169 and malleable stack-mount 166 secure initial cell 1611 to partition 153 (sealing the partition opening and thus maintaining separation between the high and low pressure chambers) and electrically couple the partition to an anode electrode 171 of the initial cell. Comparing the two detail views 170a and 170b, which show the converter in idle (unheated) and operating (heated) states, malleable stack-mount member 166 plastically deforms about the dome radius (i.e., exhibiting a progressively larger interior radius at the stack-mount/partition joint 168 and thus flattening the dome) as the partition becomes hot (enlarging the partition opening as the partition and plenum wall expand), and contracts as the partition cools, thus protecting the joint 169 and the solid electrolyte plate of the base cell in the projecting cell stack from stress induced in the malleable stack-mount 166 over the temperature range seen by the projecting cell-stacks 160 and partition member 153.
As discussed above, the single-sided mounting of the projecting cell stacks arrangement of
With regard to the particular embodiment shown in
Brazed joints 212 and 214 are also provided at the mounting and free ends of the stack (i.e., at the initial and final cells in the stack), respectively, with joint 212 securing and electrically coupling the initial (or base) cell in the stack to an electrically conductive, malleable mount member 215 (and thus to the plenum partition) and joint 214 securing and electrically coupling the final cell in the stack to an electrically conductive lid 218 that caps the free end of the stack. Lid 218 may be implemented by an electrically conductive metal, or by composite materials (e.g., a glass and/or ceramic structure having a conductive underplate electrically coupled to joint 214). In the embodiment shown, a lead wire 221, conductive rod or the like is coupled to lid 218 (or a conductive component thereof) to form the positive terminal of the cell stack. In converters with multiple cell stacks, the lead wire 221 for each cell stack is routed through the corresponding opening in the plenum partition, and may either be joined with the lead wires for other cell stacks (if there is more than one stack) before egres sing from the plenum housing, or may exit directly from the plenum at a respective egress point (i.e., isolated from the plenum wall which is forms the negative terminal of the cell).
Still referring to
The projecting isolators shown in
In the foregoing description and in the accompanying drawings, specific terminology and drawing symbols have been set forth to provide a thorough understanding of the present invention. In some instances, the terminology and symbols may imply specific details that are not required to practice the invention. For example, any of the specific numbers of cells, cell stacks, cell dimensions, material types, component shapes, manner of interconnection or construction and the like may be different from those described above in alternative embodiments. The term “coupled” is used herein to express a direct connection as well as a connection through one or more intervening circuits or structures. The terms “exemplary” and “embodiment” are used to express an example, not a preference or requirement.
While the invention has been described with reference to specific embodiments thereof, it will be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope. For example, features or aspects of any of the embodiments may be applied, at least where practicable, in combination with any other of the embodiments or in place of counterpart features or aspects thereof. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.
Claims
1. A thermoelectric converter comprising:
- a housing;
- a partition that divides an interior region of the housing into first and second chambers; and
- a thermoelectric cell stack disposed over an opening in the partition and projecting orthogonally from the partition into the first chamber without contacting the housing such that the cell stack is free to expand and contract in the projecting direction, the thermoelectric cell stack having a plurality of alkali-metal thermoelectric conversion cells stacked on one another and electrically coupled in series to generate a collective voltage higher than a voltage generated by any one of the thermoelectric conversion cells.
2. The thermoelectric converter of claim 1 further comprising a lid coupled to an end of the thermoelectric cell stack furthest from the partition, the lid separating an interior region of the thermoelectric cell stack from the first chamber.
3. The thermoelectric converter of claim 2 wherein the housing is electrically coupled to an initial one of the thermoelectric conversion cells in the thermoelectric cell stack to form a first electric terminal of the thermoelectric converter, and wherein the lid is electrically coupled to a final one of the thermoelectric conversion cells in the thermoelectric cell stack, the thermoelectric converter further comprising a conductor extending from the lid to a point outside the housing to form a second electric terminal of the thermoelectric converter.
4. The thermoelectric converter of claim 2 wherein the plurality of thermoelectric conversion cells in the thermoelectric cell stack are mechanically joined to seal the interior region of the thermoelectric cell stack from the first chamber.
5. The thermoelectric converter of claim 1 wherein the housing comprises a vapor input to receive a flow of alkali metal vapor into the first chamber such that the first chamber is at a higher pressure than the second chamber, the pressure differential between the first and second chambers exerting a compression force on joints between the plurality of thermoelectric conversion cells.
6. The thermoelectric converter of claim 1 further comprising a mounting member that secures and electrically couples an initial one of the thermoelectric conversion cells in the thermoelectric cell stack to the partition.
7. The thermoelectric converter of claim 6 wherein the mounting member is malleable to permit the thermoelectric cell stack to thermally expand and contract at a different rate than the partition without disruptive stress.
8. The thermoelectric converter of claim 7 wherein the mounting member, partition and housing are electrically conductive such that the housing and the initial one of the thermoelectric conversion cells are at nominally the same electric potential.
9. The thermoelectric converter of claim 1 further comprising a plurality of additional thermoelectric cell stacks disposed respectively over additional openings in the partition, each additional thermoelectric cell stack projecting orthogonally from the partition into the first chamber without contacting the housing.
10. The thermoelectric converter of claim 1 wherein adjacent first and second thermoelectric conversion cells of the plurality of thermoelectric conversion cells are mounted to one another and each include a solid electrolyte member disposed between an anode and a cathode, and wherein the thermoelectric cell stack comprises:
- an electrical interconnect structure extending from the cathode of the first thermoelectric conversion cell to the anode of the second thermoelectric conversion cell; and
- a first electrically insulating barrier disposed between the anode of the first thermoelectric conversion cell and the electrical interconnect structure, the first electrically insulating barrier projecting away from an outer surface of the first thermoelectric conversion cell by a distance substantially greater than a thickness of the anode of the first thermoelectric conversion cell.
11. The thermoelectric converter of claim 10 wherein the thermoelectric cell stack further comprises a second electrically insulating barrier disposed between the cathode of the second thermoelectric conversion cell and the electrical interconnect structure and projecting away from the second thermoelectric conversion cell in a direction opposite the projection of the first electrically insulating barrier.
12. A method of operation within a thermoelectric converter having a housing divided into first and second chambers by a partition, the method comprising:
- receiving a flow of alkali-metal vapor into the first chamber to produce a pressure differential between the first and second chambers; and
- generating an electric potential by conducting alkali metal ions from the first chamber to the second chamber through solid electrolyte members of respective thermoelectric conversion cells coupled electrically in series and stacked over an opening in the partition to form a thermoelectric cell stack that projects orthogonally from the partition into the first chamber without contacting the housing, the electric potential being proportional to the number of thermoelectric conversion cells within the thermoelectric cell stack.
13. The method of claim 12 wherein generating the electric potential by conducting alkali metal ions from the first chamber to the second chamber comprises generating the electric potential by conducting the alkali metal ions from the first chamber to an interior region of the thermoelectric cell stack.
14. The method of claim 12 wherein generating the electric potential by conducting alkali metal ions from the first chamber to the second chamber comprises ionizing alkali-metal vapor to separate the alkali metal ions from corresponding electrons, the method further comprising conducting the electrons to a first terminal of the thermoelectric converter via a conduction path exterior to the thermoelectric cell stack.
15. The method of claim 14 further comprising receiving electrons via a second terminal of the thermoelectric converter, the second terminal being electrically coupled to and at the same electric potential as one of the thermoelectric conversion cells at an end of the thermoelectric cell stack furthest from the partition.
16. The method of claim 15 further comprising forming alkali metal vapor within the second chamber by recombining the electrons received via the second terminal with the alkali metal ions conducted to the second chamber through the solid electrolyte members of respective thermoelectric conversion cells.
17. The method of claim 16 further comprising condensing the alkali-metal vapor formed within the second chamber into liquid metal and pumping the liquid metal out of the second chamber to be vaporized and returned to the first chamber.
18. The method of claim 17 wherein condensing the alkali-metal vapor formed within the second chamber into liquid metal comprises collecting the liquid metal in a pool at an end of the housing opposite from an end of the housing toward which the thermoelectric cell stack projects.
19. The method of claim 12 further comprising applying a heat source to an exterior surface of the housing opposite the first chamber to maintain the first chamber at a temperature that prevents the alkali-metal vapor received within the first chamber from condensing on the thermoelectric cell stack.
20. A thermoelectric converter comprising:
- a housing;
- means for dividing the housing into first and second chambers;
- means for receiving a flow of alkali-metal vapor into the first chamber to produce a pressure differential between the first and second chambers; and
- means for generating an electric potential by conducting alkali metal ions from the first chamber to the second chamber via respective thermoelectric conversion cells coupled electrically in series and stacked over an opening in means for dividing the housing to form a thermoelectric cell stack that projects from the means for dividing the housing into the first chamber without contacting the housing, the electric potential being proportional to the number of thermoelectric conversion cells within the thermoelectric cell stack.
Type: Application
Filed: Oct 8, 2014
Publication Date: Jan 22, 2015
Inventors: David M. Rossi (San Francisco, CA), Michael P. Staskus (San Jose, CA), Derek W. Nam (Los Altos Hills, CA)
Application Number: 14/510,077
International Classification: H01L 35/32 (20060101);