(12) INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT) (19) World Intellectual Property Organization International Bureau (10) International Publication Number (43) International Publication Date WO 2016/138389 Al 1 September 2016 (01.09.2016) P O P C T

(51) International Patent Classification: in, L.; 325 S. San Dimas Canyon Dr., Apartment #95, San H02N 11/00 (2006.01) Dimas, CA 91773 (US). DOTY, Benjamin, J.; 187 S. Marengo Ave., #2, Pasadena, CA 9 1101 (US). MOLINA, (21) International Application Number: Gabriel; 1200 E. California Blvd., Pasadena, CA 9 1125 PCT/US20 16/0 19786 (US). CORMARKOVIC, Velibor; 128 N. Oak Knoll (22) International Filing Date: Ave, Unit 108, Pasadena, CA 9 1101 (US). KEYAWA, 26 February 2016 (26.02.2016) Nicholas, R.; 11026 Baird Avenue, Porter Ranch, CA 91326 (US). KULCZYCKI, Eric, A.; 4607 Castle Rd., La (25) Filing Language: English Canada Flintridge, CA 9101 1 (US). PAIK, Jong-Ah; 210 (26) Publication Language: English S. Oak Knoll Ave #9, Pasadena, CA 9 1101 (US). FIR- DOSY, Samad; 2753 Harmony Place, La Crescenta, CA (30) Priority Data: 91214 (US). 62/121,084 26 February 2015 (26.02.2015) US (74) Agent: PECK, John, W.; KPPB LLP, 2400 E. Katella, (71) Applicant: CALIFORNIA INSTITUTE OF TECHNO¬ Suite 1050, Anaheim, CA 92806 (US). LOGY [US/US]; 1200 E. California Boulevard, M/C 6-32, Pasadena, CA 1125 (US). (81) Designated States (unless otherwise indicated, for every kind of national protection available): AE, AG, AL, AM, (72) Inventors: EWELL, Richard, C ; 1053 East Mendocino AO, AT, AU, AZ, BA, BB, BG, BH, BN, BR, BW, BY, Street, Altadena, CA 91001 (US). FLEURIAL, Jean-Pi¬ BZ, CA, CH, CL, CN, CO, CR, CU, CZ, DE, DK, DM, erre; 1947 N. Roosevelt, Altadena, CA 91001 (US). PA- DO, DZ, EC, EE, EG, ES, FI, GB, GD, GE, GH, GM, GT, TEL, Jagdishbhai, U.; 420 N. Milton Dr., San Gabriel, HN, HR, HU, ID, IL, IN, IR, IS, JP, KE, KG, KN, KP, KR, CA 91775 (US). NESMITH, Bill, J.; 6809 Estepa Dr, KZ, LA, LC, LK, LR, LS, LU, LY, MA, MD, ME, MG, Tujunga, CA 91042 (US). LI, Billy, Chun-Yip; 1341 MK, MN, MW, MX, MY, MZ, NA, NG, NI, NO, NZ, OM, Wakeforest Ave, Walnut, CA 91789 (US). SMITH, Kev¬ PA, PE, PG, PH, PL, PT, QA, RO, RS, RU, RW, SA, SC,

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(54) Title: RADIOISOTOPE THERMOELECTRIC GENERATOR (57) Abstract: Thermoelectric generators (TEGs) and systems of such TEGs, and more particularly thermoelectric generators incorporating radiological heat sources (RTGs) capable of being utilized in inaccessible and confined spaces, and methods of producing such RTGs, are provided. The RTGs are configured to operate in high temperature, high pressure, and high vibration environments (e.g., within a drill string down-hole in a drilling environment) where the ambient operating temperatures of the RTGs may exceed -150° C, and the hydrostatic pressure of the environment within which the TEG is located may exceed 30,000 psi. w o 2016/138389 A i III II I I 11 I I lllll 111 I II III lllll lllll II lllll lllll 111 llll 11llll

SD, SE, SG, SK, SL, SM, ST, SV, SY, TH, TJ, TM, TN, DK, EE, ES, FI, FR, GB, GR, HR, HU, IE, IS, IT, LT, TR, TT, TZ, UA, UG, US, UZ, VC, VN, ZA, ZM, ZW. LU, LV, MC, MK, MT, NL, NO, PL, PT, RO, RS, SE, SI, SK, SM, TR), OAPI (BF, BJ, CF, CG, CI, CM, GA, (84) Designated States (unless otherwise indicated, for every GN, GQ, GW, KM, ML, MR, NE, SN, TD, TG). kind of regional protection available): ARIPO (BW, GH, GM, KE, LR, LS, MW, MZ, NA, RW, SD, SL, ST, SZ, Published: TZ, UG, ZM, ZW), Eurasian (AM, AZ, BY, KG, KZ, RU, — with international search report (Art. 21(3)) TJ, TM), European (AL, AT, BE, BG, CH, CY, CZ, DE, RADIOISOTOPE THERMOELECTRIC GENERATOR

STATEMENT REGARDING FEDERAL FUNDING [0001] The invention described herein was made in the performance of work under a NASA contract NNN12AA01C, and is subject to the provisions of Public Law 96-517 (35 USC 202) in which the Contractor has elected to retain title.

CROSS-REFERENCE TO RELATED APPLICATIONS [0002] This application claims priority to U.S. Provisional App. No. 62/121,084, filed, February 26, 2015, the disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION [0003] The present invention generally relates to radioisotope thermoelectric generators, and more particularly to miniaturized versions of such systems for providing in-situ power for space constrained applications.

BACKGROUND [0004] Thermoelectric generators, or (TEGs) are solid state devices that convert heat (i.e., temperature differences) into electrical energy through a thermoelectric phenomenon called the Seebeck effect. In its most basic form a thermoelectric generator includes a circuit consisting of two dissimilar thermoelectric materials (i.e., an n-type (negatively charged) semiconductor and a p-type (positively charged) semiconductor) joined at their ends. A direct electric current flows in the circuit when there is a temperature gradient across the two materials. Generally, the current magnitude has a proportional relationship with the temperature difference (i.e., the more the temperature difference, the higher the current.) A radioisotope thermoelectric generator (RTG) is a TEG where the heat source is derived from the heat released by the decay of a suitable radiological material. [0005] Regardless of the heat source, there are many challenges in designing reliable TEG and RTG systems. For example, because RTGs and TEGs operate optimally in very high temperature gradients, the modules are subject to large thermal induced stresses and strains. RTGs and TEGs are also subject to mechanical fatigue caused by thermal cycling. As a result, achieving high efficiency in the system requires extensive engineering design in order to balance between the heat flow through the modules and maximizing the temperature gradient across them. The typical solution is to design a sophisticated heat exchange system. However, such systems add bulk and complexity to these systems.

SUMMARY OF THE INVENTION [0006] Systems, methods and apparatus in accordance with embodiments of the invention implement miniaturized radioisotope thermoelectric systems. In embodiments, the mini-RTG systems may be implemented within a drill casing making them suitable for use in drilling applications as down-hole power sources. [0007] Some embodiments of the disclosure are directed to radioisotope thermoelectric generators including: • an enclosure having an inner wall defining an enclosure volume; • a heat source casing having an outer wall and defining an internal casing volume; • at least one mechanical linkage interconnected between the heat source casing and the enclosure, the at least one mechanical linkage securely suspending the heat source casing within the enclosure volume, isolated from the inner walls of the enclosure; • at least one radiological material disposed within the internal casing volume, the at least one radiological material emitting heat from the radioactive decay thereof such that the outer wall of the heat source casing is at a heated temperature; • at least one thermopile comprised of a plurality of elongated thermocouples each of the plurality of thermocouples being a pair of thermoelectric elements wherein one of the thermoelectric elements of each pair is formed of a p-type thermoelectric material and wherein one of the thermoelectric elements of each pair is formed of an n-type thermoelectric material, and wherein the adjacent ends of the thermoelectric elements of the elongated thermocouples are conductively interconnected into a circuit; • at least one electrically insulating element disposed on each of the opposing ends of the conductively interconnected thermocouples of each of the thermopiles, and wherein at least a first electrically insulating element is disposed on a first side of the thermopile and is mechanically engaged with the outer wall of the heat source casing such that the first side of the thermopile is thermally interconnected therewith, and wherein at least a second electrically insulating element is disposed on a second side of the thermopile and is mechanically engaged with the inner wall of the enclosure such that the second side of the thermopile is thermally interconnected therewith; • at least one conductive element electrically interconnecting the circuit of the at least one thermopile to an external circuit; and • wherein the temperature of the inner wall of the enclosure is at a temperature lower than the heated temperature of the outer wall of the heat source casing such that a direct current is established in the circuit of the at least one thermopile via the Seebeck effect. [0008] In other embodiments at least the first electrically insulating element is fixedly secured to the heat source casing, and wherein at least the second electrically insulating element is mechanically compliant and slidingly engaged with the inner wall of the enclosure. [0009] In still other embodiments the second electrically insulating element is one of either a spring or a carbon fiber pad. In many such embodiments the first electrically insulating element is fixedly attached to the heat source casing via brazing. [0010] In yet other embodiments the radioisotope thermoelectric generator further includes a thermal insulating material disposed between the outer wall of the heat source casing and the inner wall of the enclosure and filling the unoccupied portion of the enclosure volume. In some such embodiments the insulating material is aerogel that may include the use of a noble gas. [0011] In still yet other embodiments the radioisotope thermoelectric generator further includes a plurality of thermopiles conductively interconnected in series. [0012] In still yet other embodiments the radiological material is selected from the group consisting of plutonium-238, curium-244, strontium-90, polonium-210, promethium-147, caesium- 137, cerium- 144, ruthenium- 106, cobalt-60, curium-242, americium-241, thulium isotopes. [0013] In still yet other embodiments the thermoelectric materials are skutterudite. [0014] In still yet other embodiments the outer wall of the enclosure includes a radiation shielding material. [0015] In still yet other embodiments the thermoelectric elements of the thermopile are affixed via brazing. [0016] In still yet other embodiments the enclosure is disposed in the annular space about the circumference of a drilling pipe. In many such embodiments the radial dimension of the enclosure is no greater than 15 mm. In other such embodiments the circumferential dimension of the enclosure is no greater than 40 mm. [0017] In still yet other embodiments the mechanical linkage comprises a pair of cooperative pins and brackets disposed at opposing end of the heat source casing, wherein one of either the pin or the bracket is mechanically interconnected with the heat source casing, and wherein the other of the pin or the bracket is mechanically interconnected with the enclosure wall. In many such embodiments the pin is formed into the heat source casing and the bracket is inserted into a cooperative slot in the enclosure wall, and wherein the pin cooperatively engages with a hole in the bracket. In other such embodiments the bracket comprises a ribbed body. [0018] In still yet other embodiments the mechanical linkage is a plurality of tensionable cables interconnected between the heat source casing and the enclosure wall. [0019] Other embodiments of the disclosure are directed to radioisotope thermoelectric generation systems including a plurality of the radioisotope thermoelectric generators wherein the radioisotope thermoelectric generators are electrically interconnected in series. [0020] In other embodiments each of the enclosures are disposed in the annular space about the circumference of a drilling pipe, and wherein the radial dimension of each enclosure is no greater than 15 mm, and wherein the circumferential dimension of each enclosure is no greater than 40 mm. [0021] In still other embodiments the radiological source is Sr-90, or a titanate, oxide, fluoride, or mixture thereof, wherein the thermoelectric material is a skutterudite. [0022] Still other embodiments of the disclosure are directed to methods of assembling a radioisotope thermoelectric generator including: • providing an enclosure having an inner wall defining an enclosure volume; • assembling a heat source assembly including: o a heat source casing having an outer wall and defining an internal casing volume, o at least one thermopile comprised of a plurality of elongated thermocouples each of the plurality of thermocouples being a pair of thermoelectric elements wherein one of the thermoelectric elements of each pair is formed of a p-type thermoelectric material and wherein one of the thermoelectric elements of each pair is formed of an n-type thermoelectric material, and wherein the adjacent ends of the thermoelectric elements of the elongated thermocouples are conductively interconnected into a circuit, and o at least one electrically insulating element disposed on each of the opposing ends of the conductively interconnected thermocouples of each of the thermopiles, and wherein at least a first electrically insulating element is disposed on a first side of the thermopile and is mechanically engaged with the outer wall of the heat source casing such that the first side of the thermopile is thermally interconnected therewith, and wherein at least a second electrically insulating element is disposed on a second side of the thermopile and is mechanically engaged with the inner wall of the enclosure such that the second side of the thermopile is thermally interconnected therewith; • inserting the heat source assembly into the enclosure such that the second electrically insulating element mechanically engages and slides along the inner wall of the enclosure such that the second electrically insulating element is mechanically engaged with the inner wall of the enclosure such that the second side of the thermopile is thermally interconnected therewith; • depositing at least one radiological material within the internal casing volume, the at least one radiological material emitting heat from the radioactive decay thereof such that the outer wall of the heat source casing is at a heated temperature, such that the temperature of the inner wall of the enclosure is at a temperature lower than the heated temperature of the outer wall of the heat source casing such that a direct current is established in the circuit of the at least one thermopile via the Seebeck effect.; • sealing the heat source casing; • attaching at least first and second mechanical linkages between opposing ends of the heat source casing and the enclosure, the mechanical linkages securely suspending the heat source casing within the enclosure volume, isolated from the inner walls of the enclosure; • depositing a thermal insulation material between the outer wall of the heat source casing and the inner wall of the enclosure and filling the unoccupied portion of the enclosure volume; • interconnecting at least one conductive element electrically interconnecting the circuit of the at least one thermopile to an external circuit; and • sealing the enclosure. [0023] Additional embodiments and features are set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the invention. A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings, which forms a part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS [0024] The description will be more fully understood with reference to the following figures, which are presented as exemplary embodiments of the invention and should not be construed as a complete recitation of the scope of the invention, wherein: [0025] FIG. 1 provides a schematic illustration of a thermoelectric generator in accordance with embodiments. [0026] FIGs. 2A to 2D provide schematic cross-sections of a RTG showing a top view of the internal cavity of a heat source in accordance with embodiments. [0027] FIG. 3A provides a schematic of a RTG incorporating a bracket holding assembly in accordance with embodiments. [0028] FIG. 3B provides a schematic of a RTG incorporating a tension wire holding assembly in accordance with embodiments. [0029] FIG. 4 provides a schematic illustration of a thermopile in accordance with embodiments. [0030] FIG. 5A provides a schematic of a spring slidable interface element in accordance with embodiments. [0031] FIG. 5B provides an image of a carbon-fiber slidable interface element in accordance with embodiments. [0032] FIG. 6 provides a schematic of an assembled thermopile for a RTG in accordance with embodiments. [0033] FIG. 7 provides a schematic of a disassembled thermopile for a RTG in accordance with embodiments. [0034] FIG. 8 provides a schematic illustration of a RTG system in accordance with embodiments. [0035] FIGs. 9A to 9C provide schematics of an assembly process for a RTG in accordance with embodiments. [0036] FIG. 10 provides a schematic illustration of a drilling rig incorporating a RTG system in accordance with embodiments. [0037] FIG. 11 provides a schematic cross-section of a RTG incorporated within an annular housing disposed about a drill pipe in accordance with embodiments. [0038] FIG. 12A provides a schematic cross-section of a drill pipe incorporating an annular housing in accordance with embodiments. [0039] FIG. 12B provides a schematic cross-section of a drill pipe incorporating an annular housing in accordance with embodiments.

[0040] FIGs. 13A to 13C provide schematics of a RTG configured for use in a down-hole application in accordance with embodiments. [0041] FIGs. 14A to 14C provide detailed schematics of assembled brackets for holding a RTG heat source in accordance with embodiments. [0042] FIG. 15 provides a schematic of a RTG incorporating a wire holding assembly in accordance with embodiments. [0043] FIG. 16 provides a data plot of the performance of a thermopile as a function of temperature differential in accordance with embodiments.

[0044] FIGs. 17A to 171 provide schematics of an assembly process for a RTG in accordance with embodiments.

DETAILED DESCRIPTION [0045] Turning now to the drawings, embodiments of thermoelectric generators (TEGs) and systems of such TEGs, and more particularly thermoelectric generators incorporating radiological heat sources (RTGs) capable of being utilized in inaccessible and confined spaces, and methods of producing such RTGs, are provided. In many embodiments the RTGs are configured to operate in high temperature, high pressure, and high vibration environments (e.g., within a drill string down-hole in a drilling environment). In some embodiments the ambient operating temperatures of the RTGs may exceed -150° C . In other embodiments the hydrostatic pressure of the environment within which the TEG is located may exceed 30,000 psi. [0046] In many embodiments the RTGs comprise a radiological heat source disposed within an enclosure having an outer wall exposed to an external environmental temperature lower than that produced by the heat source. In many such embodiments the radiological heat source comprises a container configured to be mechanically suspended within the open volume of the enclosure. In various embodiments the radiological heat source is secured via a mechanical interlink such as tensioned wires or a pin and bracket system. In some embodiments the heat source is secured such that longitudinal expansion of the heat source is permitted, but out of axis movement is constrained. In embodiments the radiological heat source contains a radiological material such as, for example, Sr-90 or Pu-238 [0047] In many embodiments the RTG further comprises one or more thermopiles having a first end engaged with the container of the radiological heat source and a second end engaged with the wall of the enclosure of the RTG. In various embodiments the thermopiles comprise at least one pair of thermoelectrical elements, each pair being made of dissimilar thermoelectric materials (i.e., a n-type (negative) semiconductor and a p-type (positive) semiconductor) the elements being electrically interconnected at each end to form a circuit. In various such embodiments the thermoelectric materials are formed from skutterudite. In some embodiments the RTGs are formed of at least two such thermopiles. In other embodiments the thermopiles are formed of at least two pairs of thermoelectric elements. In embodiments the thermopile may be permanently or securely interconnected to the radiological heat source via brazing, welding, and or secure fixtures such as, for example, bolts, screws, etc. In other embodiments the thermopile may be interconnected to the enclosure wall via a heat conducting element slidingly engaged to the enclosure wall, such as for example, a spring element or an element comprising a plurality of deformable carbon fibers. [0048] In many embodiments the thermopile and heat source are surrounded by an insulating material, such as, for example an aerogel, including a supercritical aerogel. In other embodiments the enclosure may further be filled with an inert gas, such as, for example, a noble gas (e.g., helium, argon, xenon, etc.). [0049] In many embodiments a plurality of RTGs may be interconnected in a system. In many embodiments each of the RTGs of such a system are disposed in a separate enclosure and interconnected via conductive elements. In some such embodiments the separate RTGs are connected in series or in parallel. In various embodiments the RTG systems are configured to operate in a drill string within a down-hole environment. In many such embodiments the enclosures are arranged circumferentially within the wall of the drill pipe. In some such embodiments the radial dimension of each of the enclosures is constrained to be no greater than -15 mm. In other such embodiments the circumferential dimension of each of the enclosures is no greater than 40 mm. [0050] Embodiments are also directed to methods of manufacturing and assembling RTGs and RTG systems in accordance with embodiments. In various such embodiments methods are provided to allow for the pre-positioning and securing of an unfueled RTG within the enclosure for shipping. In other embodiments the RTG is configured to allow for insertion of fuel within the pre-positioned heat source container followed by the sealing of the container, the insulation of the heat source within the enclosure and the sealing of the enclosure to the external environment. RTG Technology Overview and Design Considerations [0051] Radioisotope Thermoelectric Generators (RTGs) have the potential to be an ideal power source for potential applications in remote areas on Earth or in space that require little to no maintenance and stable electrical power over the course of decades. As thermoelectric technology improves, RTG designs are pushing the boundaries in how small they can become in order to meet applications that would require a few watts of electrical power in tight and compact areas. However, in order to enable such compact RTG designs, a number of challenges must be addressed. In particular, because RTGs operate optimally in very high temperature gradients, there is a need to have excellent thermal insulation between the heat source (i.e., hot side of the RTG) and the low temperature region (i.e., cold side) of the RTG. The typical solution has been to provide a sophisticated heat exchange system that allows for a very high temperature differential between the sides of the thermopile of the RTG (e.g., that keeps the cold side of the RTG thermopile cold and hot side of the RTG thermopile hot). However, such systems add bulk and complexity to these RTG systems, which are incompatible with the size requirements for such robust miniaturized RTGs. In addition, because many of the applications for such miniaturized RTG systems are designed to be used in applications where substantial mechanical stresses may be encountered, the RTG systems also need to provide compliance in order to minimize load stresses in the thermoelectric materials due to environment vibrations and shocks (e.g., from potential dropping/handling). Conventional systems, with their complex and bulky heat exchangers and heat sources are ill-suited for such applications. [0052] Accordingly, in many embodiments RTGs and RTG systems are provided that allow for deployment in highly space-constrained, inaccessible locations under extreme environmental conditions (e.g., pressure and temperatures) for the generation of electrical power over long lifetimes.

RTG Systems

[0053] Turning now to the construction of a RTG, as shown in FIG. 1, in many embodiments a RTG (10) generally comprises a sealable ( 11) enclosure (12) defining an enclosable RTG volume (14) disposed therein. Within the enclosable RTG volume (14) is slidably disposed a radiological heat source (16) itself comprising a sealable (17) heat source container (18) defining an enclosable heat source volume (20) within which may be confined a radiological material (22). The heat source container (18) in accordance with embodiments is mechanically suspended via at least one mechanical linkage (24) within the enclosable RTG volume such that it remains physically isolated from the interior walls of the sealable enclosure (12) except via the mechanical linkage. At least one thermopile (26), but in many embodiments (as shown in FIG. 1) a plurality of thermopiles are conductively interconnected between the heat source and the interior wall of the enclosure (12) such that a temperature differential is created between the portion of the thermopile proximal (28) the heat source (16) and the portion of the thermopile (30) proximal the interior wall of the enclosure. [0054] Although specific cross-sections and relative dimensions for the enclosure and the heat container are shown in FIG. 1, it should be understood that any relative configuration can be provided such that an insulating volume encompasses the heat source, and such that minimal contact is made between the heat source and the interior wall of the enclosure. For example, the heat container (18) could be any geometric shape suitable to conform to the contours of the enclosable RTG volume (14). Similarly, although one configuration of heat source volume (20) is shown in FIG. 1, it will be understood that the internal contour of the heat source volume (20) may take any configuration suitable to the shape of the radiological material (22) to be disposed therein. Exemplary, but non-exclusive, embodiments of some heat source volume cross-sections are provided in FIGs. 2A to 2D. As shown, the configuration can be chosen to maximize the volume of radiological material within the heat source container and can be reconfigured such that similarly the heat source container is able to conform to the internal cavity of the RTG enclosure thus maximizing the insulating space between the heat source and the interior wall of the enclosure. [0055] Likewise, although the heat source container is shown in embodiments as being suspended between two longitudinally opposed pins (24) anchored to the walls of the enclosure, it should be understood that any suitable mechanical mounting elements may be used capable of suspending the heat source container within the enclosable RTG volume and preventing contact between the heat source and the interior wall of the enclosure. Exemplary embodiments of such mechanical mounting elements may include, for example, tensioned wires (FIG. 3B), pin and brackets (FIG. 3A), mechanical clamps, etc. [0056] As shown in FIG. 3A, in many embodiments the mechanical linkages (24) may incorporate a pair of pins (32) disposed at opposing ends of the heat source (16). Although single, cylindrical pins are shown in the exemplary embodiment, it will be understood that any structural member extending from opposing ends of the heat source may be utilized in such embodiments. In additional, although in FIG. 1 the mechanical linkages are shown directly engaging the walls of the RTG enclosure, it should be understood that in other embodiments a separate enclosure interface linkage (34), such as, for example, a bracket or clamp, may be used as an interface between the mechanical linkage and the enclosure body. Such an enclosure interface linkage may be configured to engage a cooperative portion of the wall of the enclosure (36) and the mechanical linkage of the heat source. Although any suitable enclosure linkage configuration capable of securely engaging the mechanical linkage of the heat source and the enclosure wall, and to create a stable interconnection therebetween may be used, in many embodiments the materials and geometry of the enclosure linkage may be formed to minimize the heat transfer between the heat source and the enclosure. For example, as shown in FIG. 3A the enclosure linkage (34) may be formed as a ribbed structure rather than a solid body to reduce heat transfer. In addition, although the engagement of the mechanical and enclosure linkages are configured to prevent lateral movement of the heat source relative to the enclosure wall, thereby to prevent undue stresses from being applied to the thermopile elements, in various embodiments the enclosure linkage is configured to allow longitudinal movement (see arrow) of the mechanical linkage and thus heat source relative to the enclosure wall for thermal expansion and contraction that may occur during operation. Although not shown, the mechanical and enclosure linkages may further incorporate alignment pins to ensure that the body of the heat source is aligned as desired relative to the enclosure walls. The mechanical linkage and the enclosure linkage may be formed of any suitable materials that provide high strength and high corrosion resistance, as will be described in greater detail below with respect to the heat source container. [0057] As shown in FIG. 3B, in other embodiments the heat source (16) is suspended inside the enclosure of the RTG utilizing tension wires or cables (37), such as, for example, stainless steel cables. In such an embodiment the tension cables are attached to opposing ends of the heat source (either directly or through a separate bracket that is itself attached to the heat source container 38). The tension cables are then attached to cooperative cable holds in either the wall of the enclosure or a supporting bracket and tensioned until tight, and set in place with a locking screw. Once the tension cables on both ends of the heat source have been made taught, the assembly fixture is stabilized against unwanted out-of-plane movement. Although eight tension cables (four at either end) are shown in the exemplary embodiment provided in FIG. 3B, it should be understood that the number and arrangement of such tension cables make take any configuration suitable to provide sufficient movement stability to the heat source. [0058] Turning to the heat source itself, the radiological material of the heat source may be any radioactive source suitable for use in the RTG. Considerations for suitable radiological materials include: 1) a half-life long enough so that it will release heat energy at a relatively constant rate for an amount of time suitable for the intended use (where the amount of energy released per time (power) of a given quantity is inversely proportional to half-life); 2) for size- constrained applications (e.g., space, down-hole tools) the radiological material must produce a sufficient amount of power per mass and volume (density) for the intended use (where the decay energy can be calculated if the energy of radioactive radiation or the mass loss before and after radioactive decay is known); and 3) the radiation must be of a type easily absorbed and transformed into thermal radiation (e.g., alpha radiation, or beta radiation where sufficient shielding can be provided to prevent release of excess amounts of the gamma/X-ray radiation produced from such sources through bremsstrahlung secondary radiation production, and not producing significant amounts of gamma, neutron radiation or penetrating radiation in general through other decay modes or decay chain products). Exemplary radiological materials include, for example, the following nuclides: plutonium-238, curium-244, strontium-90, polonium-210, promethium-147, caesium- 137, cerium- 144, ruthenium- 106, cobalt-60, curium-242, americium- 241, thulium isotopes, and derivatives and mixtures thereof, such as titanates, oxides and flourides (e.g., SrTi0 3, Sr2Ti0 4, SrF2, SrO, etc.). [0059] The heat source container is preferable formed of a high strength material having excellent heat transfer properties such that as much of the heat from the decay of the radiological material is conductively passed through the walls of the heat source container to the thermopile. Exemplary materials, include, for example, high corrosion resistance alloys, such as, alloy materials containing, for example, chrome, stainless steel, cobalt, nickel, iron, titanium and/or molybdenum. In several embodiments a high corrosion resistant Ni-based alloy such as, for example, Hastelloy X® made by Haynes International. In various embodiments where radiological materials are used in the heat source that are subject to beta decay, or where the RTG is to be in an environment sensitive to radiation release (e.g., being handled by unshielded personnel), the exterior walls of the enclosure may include suitable radiation shielding materials. This shielding may be permanent or removable. In such embodiments, materials and shielding thicknesses are preferably selected to prevent the release of ionizing particles from the radiological material to the environment. Examples of suitable materials, include, high atomic number materials, such as, for example, Pb, Ta, W, etc. [0060] As shown in FIG. 4, the thermopile (40), in many embodiments, generally comprises at least two pairs (42) of thermoelectric elements (44) or thermoelectric couples, each of the thermoelectric elements of each of the couples being formed of a dissimilar thermoelectric material (i.e., an n-type (negative semiconductor and a p-type (positive) semiconductor), each of the ends of the pairs being electrically interconnected via a conductive element (46) such that a circuit is formed between the various thermoelectric elements of the thermopile. In such a configuration, a direct electric current will flow in the circuit when there is a temperature gradient along each of the thermoelectric elements of the module. Generally, the current magnitude has a proportional relationship with the temperature difference (i.e., the more the temperature difference, the higher the current.) [0061] In such embodiments, thermoelectric materials generate power directly from heat by converting temperature differences into electric voltage. These materials must have both high electrical conductivity (σ) and low thermal conductivity (κ) to be good thermoelectric materials. Having low thermal conductivity ensures that when one side is made hot, the other side stays cold, which helps to generate a large voltage while in a temperature gradient. The measure of the magnitude of the voltage in response to a temperature difference across that material is given by the Seebeck coefficient (S). This voltage will result in current flow when the RTG is electrically connected to an external load. The efficiency of a given material to produce a thermoelectric power is governed by its "figure of merit,' which is given by: where (zT) is the figure of merit. The mechanical properties of the materials must be considered and the coefficient of thermal expansion of the n and p-type material must be matched reasonably well. In segmented thermoelectric generators, the material's compatibility must also be considered. A material's compatibility factor is defined in accordance with: s = ((l-zT)l/2-l)/(ST) (EQ. 2) When the compatibility factor from one segment to the next differs by more than a factor of about two, the device will not operate efficiently. The material parameters determining s (as well as zT) are temperature dependent, so the compatibility factor may change from the hot side to the cold side of the device, even in one segment. This behavior is referred to as self-compatibility and may become important in devices design for low temperature operation. Examples of suitable thermoelectric materials include, for example, BiTe, PbTe, inorganic clathrates, magnesium group IV compounds, skutterudites, silicides, oxides, half-Heusler alloys, electrically conducting organic materials, SiGe, Na-coabltate, etc. Materials must be selected for use within suitable temperature environments and within given mechanical requirements as will be understood within the art. For example, thermoelectric materials can be divided into three groups based on the temperature range of operation: low temperature materials (operate up to around 450K), such as, alloys based on Bismuth (Bi) in combinations with Antimony (Sb), Tellurium (Te) or Selenium (Se); intermediate temperature materials (operate up to 850K), such as, materials based on alloys of Lead (Pb); and high temperature materials (operate up to 1300K), such as, materials fabricated from silicon germanium (SiGe) alloys. [0062] During use the thermopiles (26) are disposed between the heat source and the interior wall of the RTG enclosure, exposing the thermopiles to the heat differential between the heat source and the enclosure wall such that a differential voltage is generated in the dissimilar thermoelectric materials of the pairs (42) through the Seebeck effect. The end (28) of the thermopile proximal to the heat source is referred to herein as the "hot side" and the end (30) of the thermopile proximal to the wall of the enclosure is referred to herein as the "cold side." Because the temperature differential experienced by each end of the thermopile is important to the efficient operation of thermopiles, in embodiments good heat conductive contact between the thermopiles and the heat source on one end and the interior of the enclosure on the other is maintained. Accordingly, in many embodiments heat transfer elements (FIG. 4, 48 & 50) are disposed adjacent to the conductive element or elements (46) on the respective ends of the thermopiles (26). In embodiments these heat transfer elements are electrically isolating (e.g., to prevent shorting) such that one of these heat transfer elements (48 & 50) may be fixedly attached to its corresponding surface (i.e., heat source or interior wall of the enclosure), while the other may be slidably disposed relative to its corresponding surface such that the heat source can be inserted within the enclosure. In various embodiments the electrically isolating heat transfer element (50) affixed to the conductive element (46) on the hot side (28) of the thermopile is fixedly attached to the heat source via a suitable fixation technique, such as, for example, brazing, welding, etc., or through a suitable fixation device, such as, for example, a bolt, clamp, screw, rivet, etc. [0063] In various other embodiments the electrically isolating heat transfer element (48) affixed to the conductive element (46) on the cold side (30) of the thermopile is formed of a resilient material or element that may be slidingly engaged with the interior wall of the enclosure (as shown schematically in FIG. 1). In such embodiments any suitable thermally conductive element that is resilient and slidingly engagable with the interior wall of the enclosure and able to create a good thermal contact interface between the cold side of the thermopile and the interior wall of the enclosure may be used. Specifically, due to the assembly procedure of the Mini-RTG, the thermal contact interface needs to be able to maintain good thermal contact with the wall while sliding along the wall's surface. In addition, the interface must provide some spring load compliance to absorb vibrations and shock that the RTG may experience during handling and operation. However, the spring force must not be too high in order to minimize shear loads on the thermopile. In addition, the thermal contact interface needs to bridge the gap between the cold side of the thermopile and the interior wall of the enclosure such that good thermal contact is maintained. In many embodiments, the thermal requirements for the thermal contact interface are as follows: • The nominal hot side temperature of the thermopile is expected to be at least about -350C. • The temperature difference between the cold side of the thermopile and the interior wall of the enclosure is to not exceed ~20C. • The nominal enclosure wall temperature is expected to be about 100 to 200C. • The heat flux through the thermopile is on the order of magnitude of around 2W/cm2. [0064] Exemplary embodiments of cold side thermal contact interfaces include, for example, springs (e.g., coil, leaf or flat springs), and a carbon fiber pads. In some embodiments where a spring, such as a metal spring is used, a flat spring (FIG. 5A) may be used. In such an embodiment, in order for the legs (52) of the flat spring (54) to maintain full contact with the surface under operational load, the legs maybe given a slight bent. Although any suitable metal with a high elastic limit may be utilized, in some embodiments the springs may be fabricated out of stainless steel, such as 302 stainless steel. In embodiments where a carbon fiber pad is utilized, the carbon fiber pad may comprise a plurality of nano carbon fibers attached to a thin carbon sheet that allows for thermal contact on sliding interfaces (FIG. 5B). The carbon fiber pad has the capability of being tailored to obtain desired thermal contact characteristics. Properties that can be adjusted include the diameter of the carbon fibers, shape of the carbon fiber contact tip, carbon fiber compact density, carbon sheet thickness, and carbon fiber length. These carbon fiber pads can typically achieve a thermal contact resistance of 5 K*cm /W. Regardless of the material used in forming the resilient element in such embodiments these elements may be electrically isolated from the circuit utilizing a suitable electrically insulating material, such as, for example, alumina. [0065] Although two couples (42) of pairs of thermoelectric elements (44) arranged in a row are shown in FIG. 4, it will be understood that any number and arrangement of thermoelectric couples may be used such that a continuous circuit is formed through the various elements and couples of each of the thermopiles. For example, the thermoelectric couples could be formed in a grid array, a circular array, a serpentine array, etc. In addition, each thermopile may be formed of a plurality of such interconnected grids of couples, where the various grids are interconnected together to form a single circuit, or isolated from each other to form separate circuits. An alternative embodiment of such a thermopile (60) comprising a 4x5 grid of thermoelectric elements (62) is depicted in FIG. 6 . As shown, the arrangement and number of the couples (64), while increasing the overall voltage and power output of the RTG, does not impact the overall operation of the thermopile. [0066] Indeed, as shown FIG. 7 the construction of any thermopile (70) regardless of the number thermoelectric elements (72) in the thermopile array may be described in relation to a series of layers. As shown in thermopiles in accordance with embodiments of the invention, an array of thermoelectric elements (72) formed of dissimilar thermoelectric materials is first arranged. The adjacent ends of pairs of these thermoelectric elements (72) formed of such dissimilar thermoelectric materials are conductively interconnected via a series of conductive elements, e.g., electrodes, (74) to form a circuit of couples (76). In many embodiments a mating material (78) is disposed between the thermoelectric elements and the conductive elements to affix them together. In such embodiments, any suitable mating material may be used such as an conductive epoxy or in high temperature applications a brazing material, such as, for example, a braze foil such as pure metals (e.g., noble metals such as Ag, Al, Au, Pd, Zn, etc.), and alloys including, for example, Ag-Cu, Ag-Zn, Ag-Cu-Zn, Cu-P, Ag, Cu-P, Au-Ag, Au-Cu, Au-Ni, Au- Pd, Pd, Ni, Co, Al-Si and active metals, such as Ti or V. On each side of the thermopile, on the face of the conductive element (74) distal the thermoelectric elements (72) a heat transfer element (80) is disposed, this heat transfer element being configured to engage either the heat source or the interior wall of the enclosure and to electrically isolate the conductive elements of the thermopile from the heat source or interior wall. In many such embodiments at least one of the heat transfer elements being configured to fixedly attach and the other to slidingly engage. As with the conductive elements, these heat transfer elements (80) may be affixed to the conductive elements via a mating material (82), such as, for example, an epoxy or brazing foil, as appropriate for the operational temperature of the RTG. In various embodiments one of the sets of conductive elements may further comprise a connection point (84) that extends outward from the side of the thermopile to allow for the electrical interconnection of the thermopiles to an outgoing power conduit as desired. (See also, FIGs. 1, 4 and 6) [0067] Because many applications of the RTGs in accordance with the disclosure require operation at high temperature, quite often direct brazing between thermoelectric elements and the conductive elements and heat transfer elements will be required. In such cases, axial alignment of the thermoelectric elements is difficult, however, because even a minor gap between the end of the thermoelectric element and the conductive element and/or heat transfer element can lead to a significant loss of efficiency. Accordingly, in various embodiments, a compliant conductive material (such as for example a metal foam, such as a Ni metal foam, as disclosed in U.S. Patent App. No. 13/913,233, the disclosure of which is incorporated herein by reference) may be disposed between the heat transfer element and the conductive element such that perfect axial alignment is not required, thereby increasing the likelihood of good heat transfer between the thermoelectric element and the heat transfer element and low resistance between the thermoelectric elements (72) and the conductive elements (74). [0068] In many embodiments, to allow more heat to travel through the thermopile, an insulating material may be inserted into the RTG volume (14) of the enclosure (12) to surround the elements disposed therein, including the thermopile, heat source, mechanical linkages, etc. It should be understood that any insulating material suitable for operation in the conditions for which the RTG is designed may be used. In many embodiments the insulating material may be an aerogel, such as for example, a supercritical aerogel. To prevent oxidation of elements therein, the volume of the enclosure remaining after filling with the insulation material may be pressurized with a noble gas, such as, for example, He, Ne, Xe, Ar, etc. In such embodiments it will be understood that the cap portions ( 11 & 17) of the enclosure (12) and container (18) may include hermetic seals to prevent leakage of the materials disposed therein. Any suitable hermetic seal may be used, such as for example, a metal gasket seal, including, for example, a copper metal gasket seal. [0069] Although the above discussion has focused on the construction of individual RTGs, it will be understood that embodiments of the invention are also directed to systems of RTGs. As shown in FIG. 8, in some embodiments of such systems a plurality of RTGs (90) may be interconnected via suitable conductive elements (92) via electric feedthroughs disposed in the bodies of the RTGs, such that a circuit (e.g., series or parallel) of such RTGs may be combined into an overall power generation system. Although three RTGs connected via multiple conductive elements are depicted in FIG. 8, it should be understood that any number or arrangement of RTGs and conductive elements may be used to form a wide-variety of RTG systems. [0070] Embodiments are also directed to methods for constructing and assembling RTGs that are to be deployed in confined enclosures in larger devices. In such applications, the RTG, according to embodiments, must be designed to be inserted in an assembled stated into the disclosure, and then secured therewithin. As shown in FIGs. 9A to 9C, in one embodiment, once the heat source (100) is assembled with the thermopile (102) cold side slidable heat transfer interface (104), the heat source assembly is integrated inside the designed enclosure (106) of the RTG. As shown in FIG. 9B, in this exemplary embodiment the heat source is suspended inside the enclosure utilizing tension cables (although other methods and constructs may be used, as discussed above). To accomplish this, in accordance with various embodiments, an installation assembly fixture (108) shown in FIGs 9A and 9B is used to insert and secure the heat source within the enclosure. The fixture, that in this embodiment spans the length of the cavity, may be used to guide the heat source into the enclosure. The heat source with the mechanical linkage ( 110) (e.g., tension cables) and thermopiles is gently slid into place using the fixture. The already attached mechanical linkages ( 110) are then attached to the securing elements associated with the enclosure. Once the mechanical linkages on both ends of the heat source have been made secure, the assembly fixture (108) is removed. Once the heat source has been installed (with all the appropriate wiring) into the enclosure, an enclosure cap ( 112) is set in place on the open end(s) of the enclosure as shown in FIG. 9C. This allows the enclosure to be filled with an insulation material (e.g., aerogel) and pressurized with a suitable gas (e.g., Xe).

EXEMPLARY EMB ODEVIENTS [0071] Exemplary embodiments of systems and apparatus in accordance with the disclosure herein were implemented. Although the following discussion will focus on embodiments of RTG systems for use in down-hole environments for drilling applications, it will be understood that the designs and configurations of the individual RTG elements and overall RTG systems, described above, are equally applicable for any application where a RTG would need to be confined within a very small space, such as, for example, in space applications, such as, surface probes, small landers, instruments, or CubeSats. [0072] A drilling rig is used to create boreholes or wells (also called a wellbore) in the earth's sub-surface, for example in order to extract natural resources such as gas or oil. During such drilling it is useful to obtain data from drilling rig sensors of conditions within the borehole for a range of purposes such as: monitoring and managing the smooth operation of drilling; providing information about the geologic formations penetrated by a borehole; generating operations statistics and performance benchmarks, and providing historical operations-performance data for future well operations. Often these measurements require that electrical power be delivered to sensors or other tools down-hole. In the past such down-hole electrical power has been chiefly provided by battery units (lithium) or continuous conductive wires that connect to the surface. For various reasons these conventional solutions present problems: batteries must be replaced regularly, which requires withdrawal of the battery unit from the borehole and disruption of the drilling process; and wired systems require complex interconnections down the entire length of the borehole. [0073] The goal is to replace these temporary or complex wired systems with in-situ power for a down-hole device to monitor and control tools down-hole for decades of continuous operation. Previous terrestrial RTGs utilized Sr-90 to be cost effective, but they were typically large and weighed about a 1000 kg and operated on the surface. The large mass needed was because these terrestrial RTGs typically produced several hundred Watts of power. This allowed for the radioisotope to be concentrated increasing the power density and allowing conventional individual high temperature thermocouples to be used. The large available volume also allowed for a large amount of radiation shielding to be used to simplify transport and operations. Space RTGs have used Pu-238 because of the much higher power density and longer life. This considerably simplifies the design since Pu-238 generates its heat from alpha decay and this minimizes the amount of shielding required. In addition, because space RTGs have also been designed to produce a couple of hundred Watts of electric power they also have been able to use individual high temperature couples and did not require the use of thermopiles. However, space RTGs are extremely costly, because of the use of Pu-238 and also require a large amount of volume. As such current RTGs do not provide a suitable solution to the issue of long-term, in- situ down-hole power. [0074] Accordingly, many embodiments are directed to RTGs for use as in-situ, long-term power supplies for down-hole applications. In many such embodiments the RTG is a miniature radioisotope thermoelectric generator (Mini-RTG) that will fit in the annular space of a completed well and that will provide continuous power for at least the 20 years of operation of a power requiring down-hole system, such as, for example, a smart pipe. FIG. 10 provides a general schematic of the placement of such a RTG within the drill string of a drilling operation. The design and operational requirements of the down-hole environment put a number of requirements and restrictions on the configuration of such RTGs. Specifically, in many embodiments the down-hole RTG is configured to fit within the annular space of a roughly 210 mm outside diameter pipe that is about 30,000 feet down-hole. The ambient down-hole environment is about 150 C with about 30,000 psi of hydrostatic pressure. In addition, the volume available for the RTG is within the pipe wall significantly restricting the radial dimension of the RTD. Moreover, the high pressure environment means that this space must be divided into multiple cavities to ensure sufficient structural support to resist the high hydrostatic pressure. [0075] Accordingly, embodiments of an RTG and RTG system are provided capable of operating in the severe down-hole environment, within a very small volume. As a result of the small volume and the need to have a relatively high voltage compared to the power output, the thermoelectric converters must have many small legs connected in series. Embodiments have provided RTGs designed around thermopiles rather than being individual thermocouples as are a typically used for RTGs. As a result of the high temperature down-hole environment embodiments cannot use normally preferred thermoelectric materials (e.g., BiTe). Accordingly, embodiments have been provided that use a new higher temperature thermoelectric materials, e.g., skutterudite. In addition, because of the high temperatures, epoxy cannot be used to bond the couples into a thermopile. Accordingly, in many embodiments the thermopiles are formed by individually brazing the thermoelectric elements. Furthermore because the RTGs are inaccessible once in position and must survive for 20 years in a high radiation and high shock environment, the elements of the RTG need to be shock isolated. Accordingly, in embodiments the heat source is mechanically isolated from the enclosure, and the thermopiles are engaged to the enclosure wall via a mechanically compliant linkage. The small enclosure volume afforded also means that there is not room for typical thermal insulation or heat exchange mechanisms. Accordingly, in many embodiments a thermal insulation (e.g., an aerogel) may be used to improve thermal efficiency. Finally, because of the high radiation given off by the radiological material (e.g., Sr-90) and the small volume available, a unique configuration of internal and auxiliary radiation shields is employed for transport, deployment, and operation. Specific details of exemplary RTGs and RTG systems are provided below. [0076] As shown in FIG. 11 in various embodiments an RTG system (150) would comprise one or more RTGs (152) incorporated within one or more annular enclosures (154) disposed about the circumference of a well pipe (156) thus allowing for power to be positioned in-situ with down-hole tools. In many such embodiments, as shown in cross-section in FIG. 12A, the RTG is configured to fit within a plurality of enclosures (154) that are arranged around the annular space. In many embodiments, to fit within the operational profile of standard drill pipe, the enclosures are constrained to no more than 15 mm in the radial direction, and to provide sufficient structural strength the RTG enclosures are constrained to no more than 40 mm in the circumferential direction. Although greater flexibility is available in the axial direction, a nominal dimension for the enclosures may be on the order of 300 mm. Although FIGs. 11 and 12A provide specific enclosure configurations, it will be understood that many different cross- sectional configurations and arrangements of enclosures may be contemplated within the embodiments. For example, FIG. 12B provides an enclosure (154) having a curved or arced cross-section that may be contemplated within embodiments of the invention. It will be understood that as long as the enclosures fit within the allowable annular space about the circumference of the drill pipe, and as long the walls of the enclosures provide sufficient structural support to resist the hydrostatic pressures experienced by the RTG system in down- hole conditions, any suitable enclosure cross-section may be used including, circular, ovoid, square, rectangular, etc.

[0077] As shown in FIGs. 13A to 13D, each of the enclosures (154) contains a radioisotope heat source (158) (e.g., Sr-90) within a heat source container or casing (160) that is mechanically suspended within the middle of the cavity utilizing a plurality of opposing mechanical linkages (162) that engage cooperative enclosure linkages (164). A plurality (e.g., 2 to 4) thermopiles (166) are fixedly bonded to each heat source casing (160) and thermally connected to the enclosure wall (168) via a slidable heat transfer element (170) (e.g., carbon fiber element) disposed on the cold-side of the thermopile (i.e., proximal to the enclosure wall (168). [0078] In many embodiments, each of the thermopiles (166) consists of a plurality of thermoelectric elements (172) formed of n-type and p-type thermoelectric material. Given the 150C environmental temperature and the 300 to 350C temperature of the heat source, in many embodiments a skutterudite thermoelectric material is suitable for use with the down-hole RTGs of the disclosure. Although any suitable number and arrangement of thermoelectric couples (e.g., thermocouples) may be used, in many embodiments each thermopile consists of 10 thermocouples (e.g., 10 n-type elements and 10 p-type elements) electrically interconnected in series. Although any suitable dimension and configuration of thermoelectric element may be used, in many embodiments each of the thermoelectric elements has a square cross-section of about 1mm x 1mm with a length of about 10mm. The thermopiles are, in turn, bonded to hot-side and cold-side electrical isolation/ heat transfer elements (174 & 176), formed for example of a suitable heat conductive electric insulator, such as, alumina. Thermoelectric elements are electrically interconnected via a plurality of conductive elements on the side of the electrical isolation/ heat transfer elements that the thermoelectric elements are bonded. The hot side electrical isolation element (174) is then fixedly bonded to the heat source casing (160) and the cold side alumina electrical isolation plate is slidably connected to the enclosure wall. [0079] The RTGs in accordance with embodiments must withstand a severe shock and vibration environment during transportation and deployment down-hole. Since, the thermopiles are relatively fragile; they must be isolated from the wall shock environment. Accordingly, as shown in FIGs. 14A to 14C and 15 in many embodiments the thermopiles (166) are bonded to a heat source casing (160) that is suspended within the enclosure (154) via a plurality of fixed mechanical linkages (162). In many embodiments, as previously discussed, the linkage may comprise a plurality of pins (162) disposed on or in association with the heat source casing (160) configured to cooperatively engage a plurality of brackets or clamps (164) that themselves cooperatively engage channels (169) in the walls of the enclosure (160) such that a robust linkage is provided between the heat casing and the enclosure wall to prevent interaction therebetween (see FIG. 14C for a detail view and 14B for a cross-section of the bracket showing the interconnection of the pin and bracket). In many embodiments the various linkages are configured to minimize heat transfer between the heat source and the enclosure, such as via a ribbed construction, for example. In other embodiments, as shown in FIG. 15, and as also previously disclosed, the heat source casing (160) may be suspended within the enclosure (160) via a series of tension cables (178) attached directly or indirectly (e.g., through a bracket, clamp, etc.) (179) between the casing and enclosure. [0080] Regardless of the manner in which the heat source is suspended within the enclosure, the thermopiles are themselves engaged with the walls of the enclosure (on the cold-side) through an interface (180) that provides a compliant mechanical bond therebetween. In embodiments, this compliant interface to the enclosure wall is obtained through the use of, for example, a spring, graphite fiber felt, or a combination thereof. Utilizing such a mechanically resilient element results in a compliant mechanical interface while maintaining a high thermal conductance. In addition, such a compliant element allows for the heat source and thermopile assembly to slide longitudinally relative to the enclosure providing for assembly of the RTG. [0081] Regardless of the specific design of the individual RTGs, the thermopiles of the RTGs within the system are electrically connected in series through one or more channels connecting the enclosures of the system. To provide improved thermal insulation of the heat source, the enclosure volume may be filled with a thermal insulation (e.g., aerogel) and/or a pressurizing gas (e.g., 1 atmosphere of Ar). [0082] Utilizing such an RTG system, in accordance with embodiments, and with a nominal down-hole environmental temperature of about 150C, and a nominal heat production from the heat source of from about 300 to 350C, this results in about a 150C to 200C temperature differential across the thermoelectric modules, such that the RTG will generate about 2 Watts of electrical power at 5 Volts at the end of twenty years (assuming a suitable radiological material, such as, for example, Sr-90 having a 29 year half-life is employed within the heat source), the RTG will generate about twice as much power at the beginning of life. Such performance data in accordance with embodiments is provided in the data plot of FIG. 16. [0083] Embodiments of such down-hole RTGs are also directed to methods of their construction and deployment. As discussed, the constraints placed on the size and configuration of the enclosures within the drill pipe place severe constraints on access to the enclosure. As shown in FIGs. 17A to 171, in some embodiments, the method of assembling each of the RTGs in the RTG system require a series of steps. As shown in FIG. 17A, in a first step an assembled heat source assembly (200), including thermopiles (202) and lower enclosure linkage (204) is inserted into the enclosure (206). Heat source casing cap (208) and upper enclosure linkage (210) are held in aligned position above the upper enclosure opening (212), as shown in FIG. 17B. A thermal insulator (214) (e.g., aerogel or the like) is then injected into the enclosure about the heat source assemble (200), as shown in FIG. 17C. The radiological heat source material (216) is then inserted into the heat source casing (218), as shown in FIGs. 17D and 17E. The heat source casing cap (208) is then mounted atop the heat source casing (218) and the casing sealed, as shown in FIG. 17F. The upper enclosure linkage (210) is then engaged with the upper mechanical linkage (220), and the remainder of the enclosure filled with a thermal insulation material (221), as shown in FIGs. 17G and 17H. Finally the guide rods (222) on which the elements of the RTG have been mounted and along which the elements of the RTG have been moving during the installation are removed, and the enclosure is sealed, as shown in FIG. 171. [0084] Although alternative methods of assembling the RTGs and RTG system may be considered and implemented without departing from the spirit of the disclosure, for example, some steps may be done in a different order or the directions of insertion may be reversed, it should be understood that all such methods must provide for a method of inserting all elements of the RTG into an enclosure with limited accessibility, and where precision of placement is important to the system's efficient operation.

DOCTRINE OF EQUIVALENTS [0085] As can be inferred from the above discussion, the above-mentioned concepts can be implemented in a variety of arrangements in accordance with embodiments of the invention. Accordingly, although the present invention has been described in certain specific aspects, many additional modifications and variations would be apparent to those skilled in the art. It is therefore to be understood that the present invention may be practiced otherwise than specifically described. Thus, embodiments of the present invention should be considered in all respects as illustrative and not restrictive. WHAT CLAIMED IS:

1. A radioisotope thermoelectric generator comprising: an enclosure having an inner wall defining an enclosure volume; a heat source casing having an outer wall and defining an internal casing volume; at least one mechanical linkage interconnected between the heat source casing and the enclosure, the at least one mechanical linkage securely suspending the heat source casing within the enclosure volume, isolated from the inner walls of the enclosure; at least one radiological material disposed within the internal casing volume, the at least one radiological material emitting heat from the radioactive decay thereof such that the outer wall of the heat source casing is at a heated temperature; at least one thermopile comprised of a plurality of elongated thermocouples each of the plurality of thermocouples being a pair of thermoelectric elements wherein one of the thermoelectric elements of each pair is formed of a p-type thermoelectric material and wherein one of the thermoelectric elements of each pair is formed of an n-type thermoelectric material, and wherein the adjacent ends of the thermoelectric elements of the elongated thermocouples are conductively interconnected into a circuit; at least one electrically insulating element disposed on each of the opposing ends of the conductively interconnected thermocouples of each of the thermopiles, and wherein at least a first electrically insulating element is disposed on a first side of the thermopile and is mechanically engaged with the outer wall of the heat source casing such that the first side of the thermopile is thermally interconnected therewith, and wherein at least a second electrically insulating element is disposed on a second side of the thermopile and is mechanically engaged with the inner wall of the enclosure such that the second side of the thermopile is thermally interconnected therewith; at least one conductive element electrically interconnecting the circuit of the at least one thermopile to an external circuit; and wherein the temperature of the inner wall of the enclosure is at a temperature lower than the heated temperature of the outer wall of the heat source casing such that a direct current is established in the circuit of the at least one thermopile via the Seebeck effect. 2 . The radioisotope thermoelectric generator of claim 1, wherein at least the first electrically insulating element is fixedly secured to the heat source casing, and wherein at least the second electrically insulating element is mechanically compliant and slidingly engaged with the inner wall of the enclosure.

3 . The radioisotope thermoelectric generator of claim 2, wherein the second electrically insulating element is one of either a spring or a carbon fiber pad.

4 . The radioisotope thermoelectric generator of claim 2, wherein the first electrically insulating element is fixedly attached to the heat source casing via brazing.

5. The radioisotope thermoelectric generator of claim 1, further comprising a thermal insulating material disposed between the outer wall of the heat source casing and the inner wall of the enclosure and filling the unoccupied portion of the enclosure volume.

6 . The radioisotope thermoelectric generator of claim 5, wherein the insulating material is aerogel that may include the use of a noble gas.

7 . The radioisotope thermoelectric generator of claim 1, comprising a plurality of thermopiles conductively interconnected in series.

8. The radioisotope thermoelectric generator of claim 1, wherein the radiological material is selected from the group consisting of plutonium-238, curium-244, strontium-90, polonium-210, promethium-147, caesium- 137, cerium- 144, ruthenium- 106, cobalt-60, curium-242, americium- 241, thulium isotopes.

9 . The radioisotope thermoelectric generator of claim 1, wherein the thermoelectric materials are skutterudite. 10. The radioisotope thermoelectric generator of claim 1, wherein the outer wall of the enclosure includes a radiation shielding material.

11. The radioisotope thermoelectric generator of claim 1, wherein the thermoelectric elements of the thermopile are affixed via brazing.

12. The radioisotope thermoelectric generator of claim 1, wherein the enclosure is disposed in the annular space about the circumference of a drilling pipe.

13. The radioisotope thermoelectric generator of claim 12, wherein the radial dimension of the enclosure is no greater than 15 mm.

14. The radioisotope thermoelectric generator of claim 12, wherein the circumferential dimension of the enclosure is no greater than 40 mm.

15. The radioisotope thermoelectric generator of claim 1, wherein the mechanical linkage comprises a pair of cooperative pins and brackets disposed at opposing end of the heat source casing, wherein one of either the pin or the bracket is mechanically interconnected with the heat source casing, and wherein the other of the pin or the bracket is mechanically interconnected with the enclosure wall.

16. The radioisotope thermoelectric generator of claim 15, wherein the pin is formed into the heat source casing and the bracket is inserted into a cooperative slot in the enclosure wall, and wherein the pin cooperatively engages with a hole in the bracket.

17. The radioisotope thermoelectric generator of claim 16, wherein the bracket comprises a ribbed body. 18. The radioisotope thermoelectric generator of claim 1, wherein the mechanical linkage is a plurality of tensionable cables interconnected between the heat source casing and the enclosure wall.

19. A radioisotope thermoelectric generation system comprising a plurality of the radioisotope thermoelectric generators of claim 1, wherein the radioisotope thermoelectric generators are electrically interconnected in series.

20. The radioisotope thermoelectric generation system of claim 19, wherein each of the enclosures are disposed in the annular space about the circumference of a drilling pipe, and wherein the radial dimension of each enclosure is no greater than 15 mm, and wherein the circumferential dimension of each enclosure is no greater than 40 mm.

21. The radioisotope thermoelectric generation system of claim 20, wherein the radiological source is Sr-90, or a titanate, oxide, fluoride, or mixture thereof, wherein the thermoelectric material is a skutterudite.

22. A method of assembling a radioisotope thermoelectric generator comprising: providing an enclosure having an inner wall defining an enclosure volume; assembling a heat source assembly comprising: a heat source casing having an outer wall and defining an internal casing volume, at least one thermopile comprised of a plurality of elongated thermocouples each of the plurality of thermocouples being a pair of thermoelectric elements wherein one of the thermoelectric elements of each pair is formed of a p-type thermoelectric material and wherein one of the thermoelectric elements of each pair is formed of an n-type thermoelectric material, and wherein the adjacent ends of the thermoelectric elements of the elongated thermocouples are conductively interconnected into a circuit, and at least one electrically insulating element disposed on each of the opposing ends of the conductively interconnected thermocouples of each of the thermopiles, and wherein at least a first electrically insulating element is disposed on a first side of the thermopile and is mechanically engaged with the outer wall of the heat source casing such that the first side of the thermopile is thermally interconnected therewith, and wherein at least a second electrically insulating element is disposed on a second side of the thermopile and is mechanically engaged with the inner wall of the enclosure such that the second side of the thermopile is thermally interconnected therewith; inserting the heat source assembly into the enclosure such that the second electrically insulating element mechanically engages and slides along the inner wall of the enclosure such that the second electrically insulating element is mechanically engaged with the inner wall of the enclosure such that the second side of the thermopile is thermally interconnected therewith; depositing at least one radiological material within the internal casing volume, the at least one radiological material emitting heat from the radioactive decay thereof such that the outer wall of the heat source casing is at a heated temperature, such that the temperature of the inner wall of the enclosure is at a temperature lower than the heated temperature of the outer wall of the heat source casing such that a direct current is established in the circuit of the at least one thermopile via the Seebeck effect.; sealing the heat source casing; attaching at least first and second mechanical linkages between opposing ends of the heat source casing and the enclosure, the mechanical linkages securely suspending the heat source casing within the enclosure volume, isolated from the inner walls of the enclosure; depositing a thermal insulation material between the outer wall of the heat source casing and the inner wall of the enclosure and filling the unoccupied portion of the enclosure volume; interconnecting at least one conductive element electrically interconnecting the circuit of the at least one thermopile to an external circuit; and sealing the enclosure.

A. CLASSIFICATION OF SUBJECT MATTER H02N ll/00(2006.01)i

According to International Patent Classification (IPC) or to both national classification and IPC B. FIELDS SEARCHED Minimum documentation searched (classification system followed by classification symbols) H02N 11/00; H01L 35/30; H01L 35/00; H01L 35/32; H01L 35/34; H01L 35/12; H01L 37/00

Documentation searched other than minimum documentation to the extent that such documents are included in the fields searched Korean utility models and applications for utility models Japanese utility models and applications for utility models

Electronic data base consulted during the international search (name of data base and, where practicable, search terms used) eKOMPASS(KIPO internal) & keywords: thermoelectric, generator, radiological, enclosure, heat source, thermopile, Seebeck

DOCUMENTS CONSIDERED TO BE RELEVANT

Category' Citation of document, with indication, where appropriate, of the relevant passages Relevant to claim No.

US 5246505 A (ALFRED L . MOWERY, JR. ) 2 1 September 1993 1-22 See c o lumn 4 , l ines 34-43 , and c l aim 1 .

US 2009-0020148 Al (AKRAM BOUKAI e t a l . ) 22 January 2009 1-22 See paragraph 62, c l aim 15 , and f i gure 1A.

J P 2014-529285 A ( INCUBE LABS , LLC) 3 0 Oct ober 2014 1-22 See paragraphs 8-12 , c l aims 1-3 , and f i gures 1-2A.

EP 1976034 A2 (STICKING IMEC NEDERLAND) 0 1 Oct ober 2008 1-22 See paragraphs 42-44, c l aim 1 , and f i gures 3-5 .

US 2010-0229910 Al (JONG- AH PAIK e t a l . ) 16 September 2010 1-22 See paragraphs 27-30, and f igure 1 .

I IFurther documents are listed in the continuation of Box C . See patent family annex.

* Special categories of cited documents: "T" later document published after the international filing date or priority "A" document defining the general state of the art which is not considered date and not in conflict with the application but cited to understand to be of particular relevance the principle or theory underlying the invention "E" earlier application or patent but published on or after the international "X" document of particular relevance; the claimed invention cannot be filing date considered novel or cannot be considered to involve an inventive "L" document which may throw doubts on priority claim(s) or which is step when the document is taken alone cited to establish the publication date of another citation or other "Y" document of particular relevance; the claimed invention cannot be special reason (as specified) considered to involve an inventive step when the document is "O" document referring to an oral disclosure, use, exhibition or other combined with one or more other such documents,such combination means being obvious to a person skilled in the art "P" document published prior to the international filing date but later "&" document member of the same patent family than the priority date claimed

Date of the actual completion of the international search Date of mailing of the international search report 30 May 2016 (30.05.2016) 31 May 2016 (31.05.2016)

Name and mailing address of the ISA/KR Authorized officer - " / International Application Division Korean Intellectual Property Office PARK, H y e Lyun f ¾ ¾ | 189 Cheongsa-ro, Seo-gu, Daejeon, 35208, Republic of Korea

Facsimile No. +82-42-481-8578 Telephone No. +82-42-481-3463

Form PCT/ISA/210 (second sheet) (January 2015) Information on patent family members PCT/US2016/019786

Patent document Publication Patent family Publication cited in search report date member(s) date

US 5246505 A 21/09/1993 None

US 2009-0020148 Al 22/01/2009 US 9209375 B2 08/12/2015 WO 2009-014985 A2 29/01/2009 WO 2009-014985 A3 02/04/2009

JP 2014-529285 A 30/10/2014 EP 2745334 Al 25/06/2014 EP 2745334 A4 17/12/2014 US 2013-0205780 Al 15/08/2013 WO 2013-025843 Al 21/02/2013

EP 1976034 A2 01/10/2008 EP 1976034 A3 09/11/2011 US 2008-0271772 Al 06/11/2008 US 7875791 B2 25/01/2011

US 2010-0229910 Al 16/09/2010 US 8791353 B2 29/07/2014

Form PCT/ISA/2 10 (patent family annex) (January 20 15)