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Title:
HIGH RATIO LATENT ENERGY DEVICE AND METHODS THEREOF
Document Type and Number:
WIPO Patent Application WO/2019/034980
Kind Code:
A1
Abstract:
Exemplary embodiments of the present disclosure are directed towards high ratio latent heat thermal energy storage and recovery devices comprising a reservoir for holding a phase change material, a lid, a plurality of heating elements, one or more heat conducting members and an inert gas system. The reservoir walls and the lids of the device are three-layered with an inner low thermally conductive carbon fibre composite layer, a middle insulation layer and an outer layer made of a high strength material such as fibreglass. Use of CFC renders the device into a high ratio latent energy device. The reservoirs comprise hollow projections for insertion of the heat conducting members. The projection has an opening at its distal end for letting the heat conducting members come in contact with suitable devices such as heat engines that can utilise the stored thermal energy for other purposes.

Inventors:
CONDREN, James Jeffrey (4289, Kingstown Roa, Kingstown New South Wales 2358, 2358, AU)
Application Number:
IB2018/056080
Publication Date:
February 21, 2019
Filing Date:
August 13, 2018
Export Citation:
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Assignee:
GLOBAL ENERGY STORAGE TECHNOLOGY FZC (P.O.Box No, HFZA, Sharjah 51517, 51517, AE)
International Classes:
F28D20/02; B32B9/00; C09K5/06; F28F21/02
Domestic Patent References:
WO2018104896A12018-06-14
Foreign References:
US20150256119A12015-09-10
US20130153169A12013-06-20
DE102012000225A12013-07-11
US20150053367A12015-02-26
Download PDF:
Claims:
CLAIMS:

I claim:

1. A latent heat thermal energy storage and recovery device comprising: a) a reservoir defining a cavity and configured for holding a phase change material within the cavity, the reservoir thereof comprising an inner layer of low thermally conductive carbon fibre composite material, a middle insulation layer and an outer high strength layer, the reservoir thereof having at least one of a hollow projection with at least one of a distal opening; b) a lid substantially covering the reservoir and comprising an inner layer of low thermally conductive carbon fibre composite material, a middle insulation layer and an outer high strength layer; c) at least one of a heating element being configured for converting electrical energy from an external energy source into thermal energy and for efficiently providing thermal energy to the phase change material; d) at least one of a heat conducting member being positioned substantially within the hollow projection, the heat conducting member thereof being highly thermally conductive and configured for being in thermal contact with the phase change material at a proximal end and a thermal energy conversion system at a distal end through the distal opening, the heat conducting member thereof being further configured for transferring thermal energy from the phase change material to the thermal energy conversion system whereby the thermal energy conversion system converts thermal energy into electric energy; and e) an inert gas system configured to provide an inert atmosphere within the device for withstanding the high ambient temperatures reached during phase transition of the phase change material, the device thereof being configured for storing and retrieving thermal energy by using the phase change material.

2. The device as claimed in claim 1, wherein the reservoir has four sloping walls meeting at a reservoir base.

3. The device as claimed in claim 1, wherein the base of the lid is planar with a plurality of the heating elements being positioned underneath the base in a horizontal orientation.

4. The device as claimed in claim 1 , wherein the lid has a recess at the base with a plurality of the heating elements being positioned substantially within the recess in a horizontal orientation.

5. The device as claimed in claim 1, wherein the heating element is a sheathed heating element comprising an inner conductor unit, a middle insulation layer and an outer sheath.

6. The device as claimed in claim 5, wherein the insulation layer comprises of boron nitride.

7. The device as claimed in claim 5, wherein the outer sheath comprises of graphite.

8. The device as claimed in claim 6, wherein the outer sheath comprises of boron nitride.

9. The device as claimed in claim 1, wherein the heat conducting member is secured within the hollow projection by interference fit.

10. The device as claimed in claim 1, wherein the heat conducting member has a cylindrical body, the proximal end thereof having a sloped face, the distal end thereof having a substantially planar face with a central recess, a tubular member protruding from a mid-point in the base of the recess, the tubular member thereof being configured for thermal contact with the thermal energy conversion system.

11. The device as claimed in claim 1 , wherein the heat conducting member is made of highly thermally conductive carbon fibre composite material.

12. The device as claimed in claim 1, further comprising at least one of a mixing means positioned within the cavity and configured for mixing the phase change material.

13. The device as claimed in claim 12, wherein the mixing means is at least one of a helix mixer and a paddle wheel mixer.

14. The device as claimed in claim 1, further comprising at least one of baffle plates positioned within the cavity and configured to limit sloshing of the molten phase change material.

15. The device as claimed in claim 1, further comprising at least one of a valve, the valve thereof being configured for draining the molten phase change material from the reservoir.

16. The device as claimed in claim 1, wherein the phase change material is a silicon metalloid.

17. The device as claimed in claim 1, wherein the inner layer is a monolithic continuous structure.

18. The device as claimed in claim 1, wherein the inner layer comprises of a plurality of conjoined panels.

19. A method for storing and retrieving thermal energy comprising: a) providing a latent heat thermal energy storage and recovery device comprising of: i) a reservoir defining a cavity and configured for holding a phase change material within the cavity, the reservoir thereof comprising an inner layer of low thermally conductive carbon fibre composite material, a middle insulation layer and an outer high strength layer, the reservoir thereof having at least one of a hollow projection with at least one of a distal opening; ii) a lid substantially covering the reservoir and comprising an inner layer of low thermally conductive carbon fibre composite material, a middle insulation layer and an outer high strength layer; iii) at least one of a heating element being configured for converting electrical energy from an external energy source into thermal energy and for efficiently providing thermal energy to the phase change material; iv) at least one of a heat conducting member being positioned substantially within the hollow projection, the heat conducting member thereof being highly thermally conductive and configured for being in thermal contact with the phase change material at a proximal end and a thermal energy conversion system at a distal end through the distal opening, the heat conducting member thereof being further configured for transferring thermal energy from the phase change material to the thermal energy conversion system whereby the thermal energy conversion system converts thermal energy into electric energy; and v) an inert gas system configured to provide an inert atmosphere within the device for withstanding the high ambient temperatures reached during phase transition of the phase change material, the device thereof being configured for storing and retrieving thermal energy by using the phase change material; b) converting electrical energy from the external energy source into thermal energy by the heating element; c) storing of thermal energy by the phase change material during solid to liquid phase transition as latent heat; and d) transferring latent heat thermal energy released from the phase change material during liquid to solid phase transition through the heat conducting member to the thermal energy conversion system, whereby the thermal energy conversion system converts thermal energy into electrical energy.

20. The method as claimed in claim 19, wherein the reservoir has four sloping walls meeting at a reservoir base.

21. The method as claimed in claim 19, wherein the base of the lid is planar with a plurality of the heating elements being positioned underneath the base in a horizontal orientation.

22. The method as claimed in claim 19, wherein the lid has a recess at the base with a plurality of the heating elements being positioned substantially within the recess in a horizontal orientation.

23. The method as claimed in claim 19, wherein the heating element is a sheathed heating element comprising an inner conductor unit, a middle insulation layer and an outer sheath.

24. The method as claimed in claim 23, wherein the insulation layer comprises of boron nitride.

25. The method as claimed in claim 23, wherein the outer sheath comprises of graphite.

26. The method as claimed in claim 24, wherein the outer sheath comprises of boron nitride.

27. The method as claimed in claim 19, wherein the heat conducting member is secured within the hollow projection by interference fit.

28. The method as claimed in claim 19, wherein the heat conducting member has a cylindrical body, the proximal end thereof having a sloped face, the distal end thereof having a substantially planar face with a central recess, a tubular member protruding from a mid-point in the base of the recess, the tubular member thereof being configured for thermal contact with the thermal energy conversion system.

29. The method as claimed in claim 19, wherein the heat conducting member is made of highly thermally conductive carbon fibre composite material.

30. The method as claimed in claim 19, further comprising at least one of a mixing means positioned within the cavity and configured for mixing the phase change material.

31. The method as claimed in claim 19, wherein the phase change material is a silicon metalloid.

32. The method as claimed in claim 19, wherein the inner layer is a monolithic continuous structure.

33. The method as claimed in claim 19, wherein the inner layer comprises of a plurality of conjoined panels.

Description:
HIGH RATIO LATENT ENERGY DEVICE AND METHODS THEREOF

TECHNICAL FIELD

[0001] The present disclosure generally relates to energy storage and recovery systems. More particularly, the present disclosure relates to a high ratio latent heat thermal energy storage and recovery devices and methods thereof.

BACKGROUND

[0002] There are a number of devices that store energy in molten salt, brine solution, high pressure liquid or heated graphite. All of the above are used almost exclusively with Rankin cycle since Rankin Cycle maintains its efficiency over a wider variation of temperatures than Carnot Cycle.

[0003] The Rankin Cycle has a major limitation due to the phase change of water and the loss of energy over that phase change due to condensing of steam into water. Carnot cycle does not suffer from this disadvantage, and will continue to perform at very high efficiencies with a constant temperature as provided by latent heat of phase change.

[0004] The latent heat thermal energy devices known in the art use graphite as a medium to contain and store thermal energy but have the disadvantage of being limited to the stress forces placed upon the graphite. The size of graphite blocks and thermal conductivity of large graphite blocks are also a limiting factor to the amount of energy that can stored by such devices.

[0005] The present invention discloses high ratio latent heat thermal energy storage and recovery devices that are made of high strength carbon fibre composite (CFC) materials instead of graphite that overcome the above mentioned disadvantages.

BRIEF SUMMARY

[0006] The following presents a simplified summary of the disclosure in order to provide a basic understanding to the reader. This summary is not an extensive overview of the disclosure and it does not identify key/critical elements of the invention or delineate the scope of the invention. Its sole purpose is to present some concepts disclosed herein in a simplified form as a prelude to the more detailed description that is presented later.

[0007] Exemplary embodiments of the present disclosure are directed towards latent heat thermal energy storage and recovery devices that are configured for storing and recovering thermal energy by the phase change material. The device comprises of a reservoir defining a cavity that is configured for storing a phase change material, a lid over the reservoir and a plurality of heating elements configured for converting electrical energy from an external energy source into thermal energy and for efficiently providing thermal energy to the phase change material. The reservoir comprises of one or more of hollow projections with distal openings. Highly thermally conductive heat conducting members are positioned within the hollow projections. The heat conducting members are configured for being in thermal contact with the phase change material at a proximal end and a thermal energy conversion system at a distal end through the distal opening. The heat conducting members transfer thermal energy from the phase change material to the thermal energy conversion system whereby the thermal energy conversion system converts thermal energy into electric energy. The device has an inert gas system that provides an inert atmosphere within the device for withstanding the high ambient temperatures reached during phase transition of the phase change material. The reservoir walls and the lids of the device are three-layered with an inner low thermally conductive carbon fibre composite layer, a middle insulation layer and an outer layer made of a high strength material such as fibreglass. Use of CFC renders the device into a high ratio latent energy device.

[0008] Exemplary embodiments of the present disclosure are directed towards methods for storing and recovering thermal energy. The method begins by providing a latent heat thermal energy storage and recovery device as given in the previous para. Then electrical energy from an external energy source is converted into thermal energy by the heating elements. The phase change material (PCM) stored in the reservoir has a high heat of fusion and it absorbs large amount of heat during solid to liquid phase transition thus storing the thermal energy. During transition from liquid to solid, the PCM releases the stored thermal energy which is transferred via the heat conducting member to a thermal energy conversion system that converts thermal energy into mechanical/electrical energy. [0009] It is an object of the present invention to disclose a latent heat thermal energy device that has a high ratio of phase change material to the reservoir thus rendering it compact, energy dense, efficient and cost effective.

[0010] It is another object of the present invention to disclose a latent heat thermal energy device with high structural integrity to overcome the stress forces played upon by the repeated phase transitions of the phase change material.

[0011] It is another object of the present invention to disclose a light weight latent heat thermal energy device.

[0012] It is another object of the present invention to disclose a latent heat thermal energy device that is suitable for mass production.

[0013] It is another object of the present invention to disclose a latent heat thermal energy device with improved insulation to prevent loss of thermal energy.

BRIEF DESCRIPTION OF DRAWINGS

[0014] Other objects and advantages of the present invention will become apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments, in conjunction with the accompanying drawings, wherein like reference numerals have been used to designate like elements, and wherein:

[0015] Figures 1A and IB are simplistic representations of longitudinal sections of latent heat thermal energy (LHTE) storage and recovery devices, in accordance with different non-limiting exemplary embodiments of the present disclosure.

[0016] Figures 2A, 2B and 2C are schematic representations of different isometric views of latent heat thermal energy (LHTE) storage and recovery devices, in accordance with a non-limiting exemplary embodiment of the present disclosure.

[0017] Fig. 3 is a schematic representation of a cross section of latent heat thermal energy (LHTE) storage and recovery device depicted in Fig. 2A. [0018] Figures 4A, 4B and 4C are schematic representations of top view, side view and isometric view respectively of the reservoir of the device depicted in Fig. 2A.

[0019] Figures 5A, 5B and 5C are schematic representations of top isometric view, side view and bottom isometric view of a top hat lid of a latent heat thermal energy (LHTE) storage and recovery device, in accordance with a non-limiting exemplary embodiment of the present disclosure.

[0020] Figures 6A and 6B are schematic representations of top isometric view and bottom isometric view of an atrium lid of a latent heat thermal energy (LHTE) storage and recovery device, in accordance with a non-limiting exemplary embodiment of the present disclosure.

[0021] Figures 7 A and 7B are schematic representations of reservoirs with a helix mixer and a paddle wheel mixer respectively, in accordance with different non-limiting exemplary embodiments of the present disclosure.

[0022] Fig. 8 is a schematic representation of baffle plates, in accordance with a non-limiting exemplary embodiment of the present disclosure.

[0023] Fig. 9A is a simplistic representation of a blank for heat conducting member and Figures 9B and 9C are simplistic representations of heat conducting members in accordance with different non-limiting exemplary embodiments of the present disclosure.

[0024] Figures 10A to 10D are schematic representations of different isometric views of a heat conducting member, in accordance with a non-limiting exemplary embodiment of the present disclosure.

[0025] Figures 11A and 11B are partially cut away isometric views of heating elements, in accordance with different non-limiting exemplary embodiments of the present disclosure.

DETAILED DESCRIPTION

[0026] It is to be understood that the present disclosure is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The present disclosure is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.

[0027] The use of "including", "comprising" or "having" and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms "a" and "an" herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. Further, the use of terms "first", "second", and "third", and the like, herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another.

[0028] According to different non limiting exemplary embodiments of the present disclosure, high ratio latent heat thermal energy (LHTE) storage and recovery devices and methods thereof are disclosed.

[0029] Exemplary embodiments of the present disclosure are directed towards latent heat thermal energy storage and recovery devices that are configured for storing and recovering thermal energy by the phase change material. The device comprises of a reservoir defining a cavity that is configured for storing a phase change material, a lid over the reservoir and a plurality of heating elements configured for converting electrical energy from an external energy source into thermal energy and for efficiently providing thermal energy to the phase change material. The reservoir comprises of one or more of hollow projections with distal openings. Highly thermally conductive heat conducting members are positioned within the hollow projections. The heat conducting members are configured for being in thermal contact with the phase change material at a proximal end and a thermal energy conversion system at a distal end through the distal opening. The heat conducting members transfer thermal energy from the phase change material to the thermal energy conversion system whereby the thermal energy conversion system converts thermal energy into electric energy. The device has an inert gas system that provides an inert atmosphere within the device for withstanding the high ambient temperatures reached during phase transition of the phase change material. The reservoir walls and the lids of the device are three-layered with an inner low thermally conductive carbon fibre composite layer, a middle insulation layer and an outer layer made of a high strength material such as fibreglass. Use of CFC renders the device into a high ratio latent energy device.

[0030] Referring to figures 1A and IB, they depict LHTE storage and recovery devices 100 and 200, wherein the device comprises of a reservoir 102, 202 configured for holding a phase change material (PCM), a lid 104, 204 covering the reservoir 102, 202 and a plurality of heating elements 106, 206 attached to the lid 104, 204 configured for converting electrical energy from an external energy source into thermal energy and for heating up the phase change material. The figures further depict a hollow projection 108, 208 in the reservoir wall for insertion of a heat conducting member. The heat conducting members being highly thermally conductive are configured for being in thermal contact with the phase change material at a proximal end and a thermal energy conversion system at a distal end through a distal opening 110, 210. The heat conducting member is further configured for transferring thermal energy from the phase change material to the thermal energy conversion system such as any suitable heat engine(s) for conversion of thermal energy into mechanical or electrical energy. In different embodiments, the thermal conductivity of the heat conducting member ranges from about 100 to 300 W/m 2 K depending on the material used for fabricating the heat conducting member.

[0031] The reservoir walls and the lids of the devices as depicted in the above embodiments and in the embodiment of Fig. 3, 300 comprise of three distinct layers. The inner layer 112a, 112b, 212a, 212b, 312a, 312b is made of low thermally conductive carbon fibre composite (CFC) materials. The term "low thermal conductivity" used herein refers to thermal conductivity ranging from about 0 to 30 W/m 2 K. Use of CFC in the inner layer of this device enables it to withstand very high temperatures of up to 2200 degrees C while maintaining a very high structural integrity. CFC further enables rapid construction of devices with complex conformations suitable for high latent energy storage. The middle layer 114a, 114b, 214a, 214b, 314a, 314b is made of a graphite based insulation material though a ceramic based insulation material or any other suitable insulation material known in the art can be used without limiting the scope of the present disclosure. The outer layer 116a, 116b, 216a, 216b, 316a, 316b is made of carbon fibre though Kevlar, fibreglass, aluminium, other metals or any other material with high structural strength with an yield strength greater than 50 MPa and preferably cost-effective that is known in the art can be used without limiting the scope of the present disclosure.

[0032] The term carbon fibre composite (CFC) disclosed herein refers to carbon fibre -reinforced carbon. It is a composite material consisting of carbon fibre reinforced in a matrix of graphite. It also includes carbon fibre reinforced polymer, carbon fibre reinforced plastic and carbon fibre reinforced thermoplastic. Any other CFC material known in the art with high strength-to-weight ratio, rigidity and high temperature tolerance can be used without limiting the scope of the present disclosure.

[0033] In a particular embodiment, CFC is manufactured by processing a matrix of carbon fibres in the presence of a binder such as a plastic binder and then baked or pyrolyzed to remove the volatiles. This is followed by re-infiltration with a resin or pitch. This step is repeated until the desired concentration of carbon residue is reached between the fibres of carbon fibre. Once the desired concentration of carbon is reached, the matrix is baked at higher temperatures to process the carbon into graphite.

[0034] Referring to figures 2A to 4C, the reservoir 302 of the LHTE storage and recovery device 300 has four sloping walls that join a reservoir base 322 thus defining a generally cuboidal shaped cavity 324 therein. The four sloping walls include two longitudinal walls 318a, 318b and two transverse walls 320a, 320b. The length and breadth of the cavity are longer at the top of the reservoir as compared with the bottom of the reservoir. The reservoir and cavity can be of any suitable shape or size/dimensions that ensure a high volume ratio of phase change material to the reservoir and that can withstand the pressure changes associated with the repetitive phase change of the PCM without limiting the scope of the present disclosure. The device, in general, can be customised to any suitable shape and size/dimensions depending on energy requirements.

[0035] In this particular embodiment, there are eight hollow projections 308 for insertion of eight heat conducting members, three on each of the longitudinal walls 318a, 318b and one on each of the transverse walls 320a, 320b. Each projection 308 has a distal opening 310 for transferring the thermal energy from the heat conducting member to a suitable device such as heat engines. The number and dimensions of the heat conducting members and the number and dimensions of the corresponding projections in the device would vary depending on the device design and energy storage and recovery requirements without limiting the scope of the present disclosure.

[0036] The inner layer 112a of the reservoir 102 depicted in Fig. 1A is a monolithic continuous structure without any joints where multiple CFC layers are moulded to form the depicted structure. On the other hand, the inner layer 212a of the reservoir 202 depicted in Fig. IB has a panel construction with joints between the walls and the bottom face where pre-moulded panels of CFC are bonded to form the depicted structure.

[0037] Referring to Figures 1A, 5A, 5B and 5C, the top hat lid 104, 404 depicted therein has a plurality of heating elements 106, 406 attached to a planar base 405 of the lid by means of hooks 109, 409. Referring to Figures IB, 6 A and 6B, the atrium lid 204, 504 depicted therein has a plurality of heating elements 206, 506 positioned within a recess 507 in the lid by means of hooks 209, 509. The atrium lid offers the advantage of easy replacement of heating elements as compared with the top hat lid. In these embodiments, the heating elements are mounted in a horizontal orientation to the base of the lid though they can be positioned in any suitable direction or orientation or in any suitable position within the device that would allow efficient resistive heating of the phase change material stored in the reservoir.

[0038] Referring to figures 1A, IB, 5B, 5C and 6B, the heating elements are affixed with the lid by means of hooks 109, 209, 409, 509 though any other affixing means known in the art of furnace material that could withstand the high ambient temperatures within the device could be used without limiting the scope of the present disclosure.

[0039] Referring to figures 7 A and 7B, they depict reservoirs 702, 802 having mixing means that are configured for mixing the PCM. The mixing extends the period of liquid to solid phase transition of the PCM and thus reduces the mechanical stress on the walls during solidification of the PCM. In some embodiments, the devices with mixers have parallel longitudinal walls and parallel transverse walls due to the reduction in the mechanical stress. The embodiment depicted in figure 7A has a helix mixer 726 and that of figure 7B has a paddle wheel mixer 826 though any other suitable mixer known in the art can be used without limiting the scope of the present disclosure. The mixing means can be made of CFC or any other suitable material known in the art that can withstand the high temperatures of the phase change material without limiting the scope of the present disclosure. The number and position of the mixing wheels would vary depending on the size and shape of the reservoir, the kind of the mixing means used and other relevant parameters.

[0040] Referring to figure 8, they depict baffle plates 828 that are configured to be inserted into the reservoir cavity to limit sloshing of the molten phase change material and to stabilize them. In this particular embodiment, baffle plates are made of CFC though any other suitable material known in the art that can withstand the high temperatures of the phase change material can be used without limiting the scope of the present disclosure.

[0041] Referring to figure 9A, it depicts a blank 830 for a heat conducting member. In this embodiment, the heat conducting member is made of highly thermally conductive CFC though any other high thermal conductivity material such as graphite, boron nitride or other suitable material known in the art that can withstand high temperatures ambient in the device can be used without limiting the scope of the present disclosure. The diameter of the heat conducting member is directly proportional to the energy to be recovered and the thermal conductivity of the heat conducting member.

[0042] It is to be noted that CFC and Graphite are used for low thermal conductivity purposes as in reservoir walls as well as for high thermal conductivity purposes as in heat conducting members. This is because CFC and graphite are anisotropic in nature i.e. they have directionally dependent properties. Their physical properties like thermal conductivity depend on the orientation of the carbon fibres in CFC and graphene layers in graphite. Thus the reservoir walls, heat conducting members and other components comprising CFC or graphite material are suitably configured to fulfil low thermally conductive or high thermally conductive purposes.

[0043] Referring to figures 9B and 9C, they depict different configurations of heat conducting members 832, 932. They are made from a blank as depicted in figure 9A. In the embodiments depicted in figures 9B and 9C, the heat conducting members 832, 932 have a substantially cylindrical body 834, 934, the proximal end 836, 936 having a sloped face and a distal end 838, 938 having a substantially planar face. The body 834 depicted in figure 9B has a gradually tapering diameter from the proximal end 836 towards the distal end 838. The body 934 depicted in figure 9C does not taper initially but after a certain length starts tapering gradually towards the distal end 938.

[0044] Referring to figures 10A to 10D, they depict a heat conducting member 1032 having a body 1034 with a gradually tapering diameter from a sloped proximal end 1036 towards a distal end 1038. The distal end 1038 has a substantially planar face 1040 with a central recess 1042. The central recess 1042 has a tubular member 1044 protruding from a mid-point of its base, the tubular member 1044 thereof having a length that is smaller than the depth of the recess 1042. In this particular embodiment, the tubular member 1044 is configured for connecting with a heat engine that recovers and converts thermal energy into mechanical energy.

[0045] The dimensions of the heat conducting members correspond substantially with that of the spaces within the hollow projections. In the above embodiments, the heat conducting members are secured within the hollow projections in the reservoir by means of interference fit. The heat conducting members are initially shrunk by dipping into low temperature liquids such as liquid nitrogen and then pushed into the projections where upon warming up they expand and interfere thus having a leak-proof secure fit with the reservoir.

[0046] Referring to figures 11 A and 1 IB, the cylindrical heating elements 1106, 1206 are sheathed elements and comprise of an inner conductor unit 1146, 1246, a middle insulation layer 1148, 1248 and an outer sheath 1150, 1250. The longitudinal inner conductor unit 1146, 1246 is axially positioned and configured for the conversion of electrical energy into thermal energy by resistive heating and can be a coil conductor or a solid conductor. The conductor unit in figure 11A is a solid conductor 1146 made of graphite and in figure 11B the conductor unit is a coil conductor 1246 made of tungsten. Conductor units can be made of CFC, molybdenum disilicide (MoSi2), silicon carbide (SiC), any other suitable metals, any other suitable ceramics, any suitable polymers, any other suitable composites or any other suitable material known in the art that is capable of efficient resistive heating without limiting the scope of the present disclosure. [0047] The generally tubular middle insulation layer envelops the conductor unit and is configured to provide electrical insulation between the conductor unit and the outer sheath and for maintaining the conductor unit in an axial position relative to the outer sheath. In these embodiments, it is made from compacted boron nitride powder that is filled into the outer sheath to envelop the conductor unit. Any other suitable electrical insulating material known in the art that can withstand high temperatures can be used in the middle layer without limiting the scope of the present disclosure.

[0048] The tubular outer sheath envelops and contains the insulation layer and the conductor unit. In these particular embodiments, it is made of graphite, though CFC, boron nitride or any other suitable highly thermally conductive material known in the art that can withstand the high temperatures ambient in the device, that can provide structural integrity to the heating element to prevent it from sagging and that can efficiently transfer the thermal energy that is generated by resistive heating to the phase change material can be used without limiting the scope of the present disclosure. When the heating elements having graphite sheaths and CFC sheaths are compared, graphite sheaths have higher thermal conductivity but are thicker to achieve the desired structural integrity. CFC sheaths are thinner in cross section since they are stronger than graphite but have lower thermal conductivity comparatively. The choice between the two is made based on the device design and the energy requirements.

[0049] In some embodiments, both the insulation layer and the outer sheath are made of boron nitride. In such embodiments, the outer sheath is made by extruding, pressing or machining a solid tube of boron nitride and then compacted boron nitride powder is filled into the outer sheath to serve as the insulation layer with the conductor being placed in an axial position. Alternatively, the pre-fabricated conductor is fed along with compacted boron nitride material into a co-extrusion machine to form the insulator layer enveloping the conductor unit.

[0050] The sheathed heating elements disclosed herein can be mounted horizontally or vertically in any desired orientation or desired position within the device without sagging or bending because of the structural integrity provided by the outer sheath and the insulation layer. Otherwise the high ambient temperatures within the device would cause the conductor units to sag or bend which might lead to short-circuiting. In such cases, to avoid short-circuiting, free spaces should be provided around the conductor units to avoid them from coming into contact with any nearby surface when they sag or bend thus rendering the energy storage and recovery devices larger in size and costlier. The sheathed heating elements disclosed herein allow the elements to be in close proximity or in contact with the reservoir lids or walls. In a particular embodiment, the heating elements directly pass through the CFC inner layer of the reservoir lid. These advantages make the energy storage and recovery device disclosed herein more compact, energy dense, efficient and hence cost-effective. Other than the sheathed heating elements disclosed herein, other heating elements that are known in the art can also be used in the device without limiting the scope of the present disclosure.

[0051] In some embodiments, the LHTE storage and recovery devices are provided with valves for draining the molten phase change material from the reservoir. The valves can be positioned in any convenient location within the reservoir that would allow efficient draining of the molten material. Any suitable valves such as ball type valve, plug type valve or other valves known in the art that can regulate, direct or control the flow of the molten phase change material can be used without limiting the scope of the present disclosure. In some embodiments, the valves are made of CFC or graphite though any other suitable material known in the art that can withstand high temperatures can be used without limiting the scope of the present disclosure.

[0052] In some embodiments, the LHTE storage and recovery devices have further accessories such as vibration reduction means to reduce transfer of vibrations of externally connected devices such as heat engines to the LHTE storage and recovery devices. Any vibration reduction means known in the art such as a flexible ring like silicon seal, O-rings or bellow or any other means that can withstand high temperatures of the device can be used without limiting the scope of the present disclosure.

[0053] In some embodiments, the LHTE storage and recovery devices are configured to have an inert atmosphere within the apparatus to withstand the high ambient temperatures within the device by means of an inert gas system. The system comprises of an inert gas source supplying inert gas through pipes to the device and optionally further comprises of an expansion chamber for recycling the inert gas. The inert atmosphere is provided by any one or a mixture of the following gases comprising nitrogen, argon, helium or carbon dioxide. Any other gas or gaseous mixture known in the art that can provide a non-reactive inert atmosphere and that can reduce free oxygen levels within the apparatus can be used without limiting the scope of the present disclosure.

[0054] The devices are preferably provided with air-tight sealing means such as spacer rods, silicon rings, fasteners and/or any other suitable sealing means to seal the lid with the reservoir in an air-tight manner.

[0055] The thermal energy stored in the phase change material is transferred by heat conducting members to the heat receiving sections of heat engines. Among heat engines, Stirling engine is preferred, though any other closed or open cycle heat engines can be used without limiting the scope of the present disclosure.

[0056] Exemplary embodiments of the present disclosure are directed towards methods for storing and recovering thermal energy. The method begins by providing a latent heat thermal energy storage and recovery device as disclosed herein. Then electrical energy from an external energy source is converted into thermal energy by the heating elements. The phase change material (PCM) stored in the reservoir has a high heat of fusion and it absorbs large amount of heat during solid to liquid phase transition thus storing the thermal energy. During transition from liquid to solid, the PCM releases the stored thermal energy which is transferred via the heat conducting member to a thermal energy conversion system that converts thermal energy into mechanical/electrical energy.

[0057] The external energy source can be solar energy, wind energy or other forms of energy without limiting the scope of the present disclosure. The electric current that is supplied to the heating elements could be a DC current or an AC current. For example, the electric current could be a DC current from a photovoltaic array or an AC current from a wind turbine. The AC current or the DC current shall be suitably rectified to meet the requirements and specifications of the type of heating element that is used. The latent energy storage medium is a phase change material and preferably a silicon metalloid or silicon based eutectic composition.

[0058] The latent heat thermal energy (LHTE) storage and recovery devices were tested with four different volumes of phase change material and the following results show their storage capacity and output. a) Test cell 1= Thermal energy storage 412 kWh, electrical at 50% efficiency with cycling at 650°C. 152 kWh with 6 kWe output over 24 hours (use cell tower)

b) Test Cell 2 =Thermal energy storage 1947 kWh, electrical at 50% efficiency with cycling at 650°C. 734 kWh with 150 kWe output approx. 600 klms range (use Bus/Trucks) c) Test Cell 3 = Thermal energy storage 4209 kWh, electrical at 50% efficiency with cycling at 650°C. 1630 kWh with 42 kWe output over 24 hours (ferry hybrid use)

d) Test Cell 4 = Thermal energy storage 12416 kWh, electrical at 50% efficiency with cycling at 650°C. 4829 kWh with 240 kWe output over 24 hours (Utility scale energy storage)

[0059] The device disclosed herein uses the latent heat energy of the phase change material (latent heat storage medium) with over 80% increase in energy density or an extra 496.5 Watts of energy storage per kilogram when compared with sensible heat energy produced by the same mass of sensible heat storage medium.

[0060] The latent energy device disclosed herein has a high volume ratio of phase change material to the reservoir with a reduced mass of containment vessel/reservoir when compared with the thermal energy storage devices known in the prior art. This is because of the use of light weight yet high strength and high temperature tolerant CFC in the inner shell of the reservoir. The above advantage along with improved insulation of the device renders it cost effective and suitable for mass production as compared with conventional thermal energy storage devices such as those made of sintered graphite.

[0061] Although the present disclosure has been described in terms of certain preferred embodiments and illustrations thereof, other embodiments and modifications to preferred embodiments may be possible that are within the principles and spirit of the invention. The above descriptions and figures are therefore to be regarded as illustrative and not restrictive.

[0062] Thus the scope of the present disclosure is defined by the appended claims and includes both combinations and sub combinations of the various features described herein above as well as variations and modifications thereof, which would occur to persons skilled in the art upon reading the foregoing description.