ENERGY STORAGE DEVICE

Abstract

The invention provides an energy storage apparatus comprising: a sensible heat storage body having a heat exchanger channel and a heating element channel adapted to receive a removable heating element; and a heat exchanger having an inlet and an outlet, wherein at least a portion of the heat exchanger is disposed along the channel. Also provided are methods or reversibly storing and/or extracting energy, a heating element and an energy storage array comprising a plurality of energy storage apparatus as described herein.

Claims

1. A method of reversibly storing and/or extracting energy comprising the steps of: heating an inner region of a sensible heat storage body using a removable heating element thereby storing energy, wherein a portion of the heating element is in contact with the inner region of the sensible heat storage body; extracting energy by flowing a heat transfer medium through an inlet and an outlet of the sensible heat storage body having an inlet temperature below that of said sensible heat storage body such that energy is transferred from the sensible heat storage body to the heat transfer medium having an outlet temperature, thereby providing reversible energy storage and extraction.

2. A method according to claim 1, further comprising a step of regulating the outlet temperature of the heat transfer medium to provide a regulated outlet temperature.

3. A method according to claim 2, wherein the step of regulating the outlet temperature comprises mixing the heat transfer medium and a stream of cooling liquid having a temperature below an initial outlet temperature in an attemperation unit comprising a first-stage mixing chamber and optionally a second-stage mixing chamber to provide the regulated outlet temperature.

4. A method according to claim 3, wherein the mixing is performed at a ratio of the heat transfer medium and the cooling liquid between about 20:1 to 1:20.

5. A method according to claim 3, wherein the stream of cooling liquid is introduced to the attemperation unit through at least one flow valve.

6. A method according to claim 4, wherein the flow valve is a fixed flow valve or a variable flow valve.

7. A method according to claim 3, wherein the stream of cooling liquid is introduced to the attemperation unit by an atomiser, a nozzle or an injector.

8. A method according to claim 5, wherein the stream of cooling liquid is introduced to the attemperation unit by an atomiser, a nozzle or an injector downstream of the at least one flow valve.

9. A method according to claim 3, wherein the regulated outlet temperature is below a threshold temperature, wherein the threshold temperature is between 25% and 99% of the initial outlet temperature.

10. A method according to claim 9, further comprising the step of: reducing the regulated outlet temperature by mixing the heat transfer medium and the stream of cooling liquid in a second-stage mixing chamber to provide an application temperature.

11. A method according to claim 10, wherein the stream of cooling liquid is introduced to the second-stage mixing chamber through at least one flow valve. 12 A method according to claim 11, wherein the mixing is performed at a ratio of the heat transfer medium and the cooling liquid between about 20:1 to 1:20.

13. A method according to claim 11, wherein the flow valve is a fixed flow valve or a variable flow valve.

14. A method according to claim 11, wherein the stream of cooling liquid is introduced to the attemperation unit comprising the second-stage mixing chamber by an atomiser, a nozzle or an injector.

15. An energy storage apparatus comprising: a sensible heat storage body having a heat exchanger channel and a heating element channel adapted to receive a removable heating element, wherein the heating element channel is located internally of the sensible heat storage body; and a heat exchanger having an inlet and an outlet, wherein at least a portion of the heat exchanger is disposed along the channel; an attemperation unit comprising a first-stage mixing chamber and optionally a second mixing chamber, wherein the attemperation unit is in fluid communication with the outlet of the heat exchanger for regulating the outlet temperature of the heat transfer medium to provide a regulated outlet temperature.

16. An energy storage apparatus according to claim 15, wherein the attemperation unit is adapted to receive a stream of cooling liquid for regulating the outlet temperature of the heat transfer medium.

17. An energy storage apparatus according to claim 16, wherein the attemperation unit comprises at least one flow valve for introducing the stream of cooling liquid to the attemperation unit.

18. An energy storage apparatus according to claim 17, wherein the flow valve is a fixed flow valve or a variable flow valve.

19. An energy storage apparatus according to claim 16, wherein the attemperation unit comprises an atomiser, a nozzle or an injector for introducing the stream of cooling liquid to the attemperation unit.

20. An energy storage apparatus according to claim 16, wherein the first-stage mixing chamber is in fluid communication with the outlet of the heat exchanger to reduce the outlet temperature of the heat transfer medium to a regulated outlet temperature by mixing the heat transfer medium with a stream of cooling liquid having a temperature below an initial outlet temperature in the first-stage mixing chamber; and further comprising a second-stage mixing chamber in fluid communication with the first mixing chamber to reduce the regulated outlet temperature to provide an application temperature by mixing the heat transfer medium and the stream of cooling liquid in the second-stage mixing chamber.

21. An energy storage apparatus according to claim 20, wherein the first-stage mixing chamber is proximal to the outlet of the heat exchanger, and the second-stage mixing chamber is distal to the first-stage mixing chamber.

22. An energy storage array comprising: a plurality of energy storage apparatus according to claim 15.

23. An energy storage array according to claim 22, wherein the energy storage array comprises at least one second-stage mixing chamber in fluid communication with at least one of the first-stage mixing chambers for reducing the regulated outlet temperature of the heat transfer medium to provide an application temperature by mixing the heat transfer medium and a stream of cooling liquid in the second-stage mixing chamber.

24. An energy storage array according to claim 23, wherein the first-stage mixing chamber is proximal to the outlet of the heat exchanger, and the second-stage mixing chamber is distal to the first-stage mixing chamber.

25. An energy storage array according to claim 24, wherein each energy storage apparatus comprises at least one first-stage mixing chamber in fluid communication with the outlet of the heat exchanger of each energy storage apparatus; and wherein the energy storage array comprise one second-stage mixing chamber in fluid communication with each first-stage mixing chambers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0155] Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

[0156] FIG. 1 shows an embodiment of the energy storage apparatus of the present invention. a) side perspective view; b) cross-sectional perspective view taken along the line A-A of FIG. 1a; and c) cross-sectional perspective view taken along the line B-B of FIG. 1a without heat exchanger tubing and heating elements present.

[0157] FIG. 2 shows an embodiment of the energy storage apparatus of the present invention encased in an enclosure. a) side perspective view of front; and b) side perspective view of back.

[0158] FIG. 3a shows an embodiment of the heating element of the present invention. FIG. 3b shows an end view of the heating element of FIG. 3a in use when inserted into a heating element channel.

[0159] FIG. 4a shows an embodiment of the heating element of the present invention in use when inserted into an embodiment of the energy storage apparatus. FIG. 4b shows the opposite end of the heating element of FIG. 4a.

[0160] FIG. 5 shows an embodiment of the heating element mount pad of the present invention.

[0161] FIG. 6 shows an embodiment of a heat exchanger design in the form of a serpentine coil. a) side perspective view; and b) cross-sectional perspective view taken along the line A-A of FIG. 6a.

[0162] FIG. 7a shows an embodiment of a component of the enclosure having an aperture to receive the heat exchanger. FIG. 7b shows the assembly of a prior art embodiment of an over-pressure vent panel.

[0163] FIG. 8 shows an embodiment of the bellows sealing configuration for sealing engagement between the heat exchanger and the enclosure.

[0164] FIG. 9 shows attemperation of fluid outlet temperature.

[0165] FIG. 10a shows an embodiment of a manifold assembly for attemperation. FIG. 10b shows an embodiment of an energy storage unit comprising a 20 ft HC intermodal container.

[0166] FIGS. 11a-d shows schematics of different embodiments of using pressure regulators for an inert gas blanketing system. FIG. 11e shows an alternative pressure regulator using a water column.

[0167] FIG. 12 shows a process flow diagram for an embodiment (Test Rig 1) for an inert gas blanketing system.

[0168] FIG. 13 shows a process flow diagram to FIG. 12 for an embodiment (Test Rig 2) for an inert gas blanketing system. In this embodiment, a pilot operated back pressure regulator replaces the fabricated backpressure device of Test Rig 1.

[0169] FIG. 14 shows an alternative process flow diagram for an embodiment of an inert gas blanketing system of the present invention.

[0170] FIG. 15 shows an embodiment of an energy storage array comprising three energy storage apparatus, three first-stage mixing chambers and one second-stage mixing chamber, for regulating the outlet temperature.

[0171] FIG. 16 shows an embodiment of the first-stage attemperation.

[0172] FIG. 17 shows an embodiment of the second-stage attemperation.

[0173] FIG. 18 shows a comparison of outlet temperature with no attemperation, first-stage and second-stage attemperation.

DETAILED DESCRIPTION OF THE INVENTION

[0174] The skilled addressee will understand that the invention comprises the embodiments and features disclosed herein as well as all combinations and/or permutations of the disclosed embodiments and features.

EXAMPLES

Example 1Energy Storage Apparatus

[0175] Referring to FIG. 1a, there is shown a sensible heat storage body 102 for use as an energy apparatus 100. The sensible heat storage body 102 has a heating element channel 104 for receiving a removable heating element 106 (not shown). The sensible heat storage body 102 also has a heat exchanger channel 108 for receiving the heat exchanger 110. The sensible heat storage body 102 is assembled by component parts and can be milled, machined or the like to provide the heating element channel 104 and heat exchanger channel 108 having at least two open ends within the sensible heat storage body. The sensible heat storage body 102 is in the form of a graphite panel comprised of component slabs of graphite machined to snugly receive a heat exchanger 110 as well as a heating element 106.

[0176] Referring to FIGS. 1b-c, the energy storage apparatus 100 shows an embodiment comprising four layers of insulation 112. Each piece of insulation is layered in a staggered overlapping manner to minimise the amount of heat and hot gas leakage from the energy storage apparatus. As the temperature of each subsequent insulation layer 112 from the hot graphite body 102 reduces, lower temperature insulating material 112 can be used, optimising the performance/cost ratio. The energy storage apparatus is then enclosed by an enclosure 114 (not shown).

[0177] In use, the removable heating element 106 heats the inner region of the sensible heat storage body 102 and the heat exchanger 110 is encased within the heat exchanger channel 108 of the sensible heat storage body 102 such that a heat transfer medium can flow from the inlet to the outlet of the heat exchanger 110 through the body 102.

[0178] FIG. 2a-b shows the front and back view of the resulting encased energy storage apparatus 100 wherein the enclosure comprises structural reinforcement 116 such as in the form of a rib or case stiffener to increase the structural integrity of the enclosure to allow for greater internal pressures.

Example 2Heating Element

[0179] Referring to FIG. 3a, there is shown a heating element 106 having an elongated heating portion 106a at one end and a thermally insulated portion 106b at an opposite end. The thermally insulated portion has an electrical conductor (not shown) adapted to be in electrical communication with an electrical terminal 107. The thermally insulated portion 106b of the heating element 106 has two steps. The heating element is an electrical resistor wherein the heating portion 106a is in the form of tubular loops.

[0180] FIG. 3b shows an end view of the heating element 106 in use when inserted into the heating element channel 104. In use, the heating element 106 expands and contacts the surface of the inner region of the heating element channel 104. This allows for efficient conduction of the heat from the heater elements to the graphite body.

[0181] The heating element 106 comprises a resistance wire 106a, typically nichrome, surrounded by compacted magnesium oxide powder which is thermally conductive but electrically insulative. This is then encased in a heating element casing which is in the form of a sealed tubular metal sheath made from high temperature alloy material such as Inconel or Incoloy. Since graphite has low emissivity, high thermal conductivity and high specific heat which is the preferred material of the sensible heat storage body 102, the heating element 106 can be of high watt density, reducing the heating element surface area required and reducing the number of heating elements required and subsequent cost.

[0182] The heating element provides for easy removal and replacement of each heater from and into the energy storage apparatus.

[0183] FIG. 4a shows insertion of the heating element 106 into the energy storage apparatus 102. The heating element 106 has a heating element flange 106c which is secured to the heating element mount pad 105 welded to the enclosure 114 using a clamping plate 115 and bolted. The heating element 106 has a length of cold leg (thermally insulated portion 106b) where there is no resistance wire and only has a conducting wire/pin and this cold leg section is thermally insulated. A portion of the length of the cold leg is outside the enclosure 114 such that it is exposed to ambient temperatures to keep the electrical terminal 107 cool.

[0184] In alternative configurations, the heating element can be secured to the energy storage apparatus 100 by a tapered screw coupling for example.

[0185] A sealing gasket 118 can be provided between the clamping plate 115 and heating element mount pad 105 to provide a gas tight seal. Insulation 112 is provided similar to example 1 between the sensible heat storage body 102 and enclosure 114. This combination of features can ensure the electrical terminals 107 is adequately cooled by the surrounding air and thermally insulated to prolong the life of heating element 106 by reducing or preventing overheating of and hot gas migrating to the electrical terminals 107.

[0186] FIG. 4b shows the opposite end of the energy storage apparatus 100 where the heating element channel 104 of the sensible heat storage body further comprises a bore 120. The bore 120 is located opposite the opening of the heating element channel which receives the removable heating element 106. The bore 120 allows gas present in the heating element channel 104 to egress when heating the inner region of the sensible heat storage body during use by avoiding gas pressure build-up. This can avoid compromise of the gas tight seal due to overpressure. The bore 120 can also allow the heating element channel 104 to breathe out expanded gas (such as inert gas) when hot and breathe in gas when cooled. The heating element channel 104 also allows the longitudinal expansion of the heating element when heated.

[0187] FIG. 5 shows an embodiment of the heating element mount pad 105. The heating element mount pad is a single piece with multiple apertures for receiving each individual heating element 106. The aperture of the heating element mount pad 105, aperture of the enclosure for receiving heating element 106 and diameter of the heating element channel 104 is larger than the thermally insulated portion of the heating element 106b diameter so that it can accommodate any misalignment from construction/assembly tolerances. The gasket bore is also larger than the chamfered step of the thermally insulated portion of the heating element 106b and compressed to seal the assembly using a clamping plate and bolts and washers. In this configuration, by removing the bolts, washers and clamping plate 115 allows each heater to be removed and replaced individually.

Example 3Heat Exchanger

[0188] FIG. 6a-b shows an embodiment of a heat exchanger 110 design in the form of a serpentine coil. High tensile strength materials suitable for the heat exchanger 110 at elevated operating temperatures have reduced ductility and the bend radii needs to be larger than for typical highly ductile steel pipes for steam (HTF).

[0189] ASME B31.3 recommends a bend radius of 3D where D is the outside diameter of the pipe. For example, for a DN20 Sch 80 heat exchanger pipe, the bend radius would need to be 160 mm. For a 2 m high body of graphite as the sensible heat storage body, a vertical coil design (where alternating passes are in the same plane) provide about 12 horizontal runs per heat exchanger pipe encased in the graphite limiting the contact surface area of heat exchanger pipe in the graphite body.

[0190] In contrast, a rising serpentine heat exchanger coil design as shown in FIGS. 6a-b provides more flexibility in tailoring the contact area of heat exchanger pipe with the graphite body. For example, if the rise is about 50 mm (and assuming corresponding to an about 50 mm thick graphite slab component), a 160 mm bend radius can be achieved while having about 40 horizontal runs of pipe in a 2 m high graphite body. This is particularly significant as the extraction rate of thermal energy by the heat exchanger is 3 times that of a vertical coil design.

[0191] A further advantage to a serpentine coil heat exchanger design is that on heat extraction as the heat transfer fluid and/or working fluid flows through the heat exchanger conduit/pipes, heat in the graphite body transfers from left to right and from bottom to top, creating a more uniform temperature profile across the graphite body.

[0192] FIG. 7a shows a component of the enclosure 114 having an aperture to receive the heat exchanger 110 to be sealingly engaged using a bellows sealing configuration. The aperture of the enclosure 114 has a bellows sealing pad 122 for sealing engagement with the heat exchanger 110. A further over-pressure vent panel opening 124 is provided in the enclosure 114 in the event of over-pressure of inert gas within the enclosure 114 during operation. FIG. 7b shows the assembly of an embodiment of the over-pressure vent panel 126.

[0193] FIG. 8 shows the bellows sealing configuration for sealing engagement between the heat exchanger 110 and the enclosure 114. In this configuration, M1050 mm SS grub screws screw entirely into the threaded holes of the sealing surfaces of the bellows sealing pad 122 which is welded to the enclosure 114. A sealing gasket 118a is inserted through the M10 grub screws and insulation discs 112a are threaded onto each heat exchanger pipe 110. A bellows sub-assembly 127 is placed over the insulation discs 112a and the heat exchanger pipe 110 through a tube connector. The bellows sub assembly 127 comprises the sealing gasket 118a, compression fitting 128, SS bellows ferrules or compression olive 130 and bellows flange 132. The compression olive or ferrules 132 is fitted over heat exchanger pipe 110 and compressed with the compression fitting 128.

[0194] When the heat exchanger 110 is sealingly engaged with the enclosure 114 of the energy storage apparatus 100, heat can be retained within the graphite body 102 and not leak via hot heat exchanger pipes 110 contacting the enclosure 114 because of the provision of a gas tight seal and thermal insulation.

Example 4Attemperation

[0195] With sensible heat storage extraction, the discharge heat transfer fluid and/or working fluid temperature starts at the maximum storage temperature and reduces to the minimum operating temperature as the heat is extracted from the graphite body. In this configuration, a portion of the cooler inlet working fluid is mixed with the hotter outlet fluid maintaining a set temperature for a longer duration as shown in FIG. 9.

[0196] Attemperation can be provided by an embodiment as shown in FIG. 10a using a manifold assembly 133. A flow control valve 134 is disposed between the inlet manifold 136 and the outlet manifold 138. Temperature sensors (not shown) in the graphite body 102, inlet 136 and outlet manifolds 138 can determine the proportion of inlet manifold flow to be mixed with the outlet manifold flow of heat transfer fluid.

[0197] FIG. 10b shows a unit embodiment comprising a 20 ft HC intermodal container. Access to the housing 140 provide for insertion, removal and replacement of the tubular heating elements 106. Access to the housing can also provide for installation of the manifold assembly and service the flow control valve 134 which is disposed between two energy storage apparatus 100. The energy storage apparatus in the form of graphite panels 102 are secured within the container and the whole unit can be assembled and tested off site and transported to site.

[0198] The unit configuration of two, three or multiple graphite panels 102 can be stacked on top of each other to provide a high footprint storage density. A plurality of these graphite panels 102 are connected together with a manifold 133 and are housed in an intermodal container with standard openings to access the heating elements 106 and control valves 134. This design can provide for medium volume manufacture and ease of manufacturing and assembly off site and ease of transportation.

Example 5Inert Gas Blanketing

[0199] An energy storage apparatus of the present invention can be enclosed in an enclosure in an inert gas environment. Since the graphite panel is sealed gas-tight, increases in working/operating temperature will result in higher internal gas pressures. This pressure must be relieved to minimise the stresses caused by the panel enclosure expanding and contracting. Conversely, as the graphite panel cools the inert or surrounding gas will contract raising the possibility of vacuum being created, and the volume will need to be compensated.

[0200] The purpose of the inert gas blanketing system is to provide an inert gas supply such as argon to the graphite panel and maintain the pressure at a minimum value, and to further relieve the argon should the pressure rise above a pre-set minimum as shown in FIG. 11a.

[0201] A pressure regulator can maintain pressure within the graphite panel. The pressure required is low and needs only to prevent the ingress of air. Blanket regulators are typically set at a few inches water column. In this system 1-2 WC (2.5-5.0 mbar) is sufficient.

[0202] During the heating/storage phase, the internal pressure of the graphite panel will increase. The backpressure regulator will relieve the internal pressure to a predetermined value. The factors that influence the value of the backpressure setting include:

[0203] minimising the consumption of inert gas;

[0204] reducing the cycling stresses on the graphite panel enclosure; and

[0205] to provide sufficient margin between the normal operating pressure of the graphite panel and the burst pressure.

[0206] A value of 10 WC (25 mbar) can be used in one embodiment. This could be increased if the burst pressure of the graphite panel is significantly higher (3-4 psi/200-300 mbar).

[0207] While the inert gas blanket system is required to prevent material degradation in the graphite panel, its failure could result in system damage or catastrophic failure of the graphite panels.

[0208] At least two components of a pressure regulator can fail which are the diaphragm and the spring. The consequence of a spring failure is for the pressure regulator to close, i.e. reduce the pressure and prevent the flow of inert gas. While this mode is suboptimal, it does not pose a significant risk of damage to the energy storage apparatus. Damage and failure can be mitigated by having another pressure regulator in parallel that will take over the supply of regulated inert gas.

[0209] The consequence of a diaphragm failure is that the regulator will not open. This failure mode causes the downstream pressure to increase and can have an extreme impact to the operation of the present invention if not mitigated.

[0210] The first level of mitigation can be installing a relief valve on the inert gas manifold set at a pressure setting which will not cause damage to the graphite panels. The second level of mitigation can be to ensure that the wide-open CV (the flow coefficient of a device and is a relative measure of its efficiency at allowing fluid flow) of the backpressure regulator is greater than the wide-open CV of the pressure regulator. This will prevent the accumulation of pressure in the graphite panel. A suitable pressure regulator can be one which has a minimum orifice CV of 1. Table 2 below shows the flow through the regulator in the wide-open (failure) condition.

TABLE-US-00002 TABLE 2 Different pressure regulator conditions for wide-open failure Inlet pressure CV = 1 Psig bar Kg/cm.sup.2 kPa SCFH Nm.sup.3/h 25 1.7 1.76 172 1130 30.3 30 2.4 2.11 207 1280 34.3 40 2.8 2.81 276 1680 45.0 50 3.5 3.52 345 2050 54.9 60 4.1 4.22 414 2330 62.4 70 4.8 4.92 483 2670 71.6 80 5.5 4.92 483 2670 71.6 90 6.2 6.33 621 3410 91.4

[0211] If the backpressure regulator has a CV of greater than 1 (based on a calculation of maximum flow and a differential pressure referencing the safety pressure of the graphite panel), then there will be no accumulation of pressure in the panel. This system is shown in FIGS. 11b and 11c for example.

[0212] The failure modes for the backpressure regulator are the same as the pressure regulator; however, the consequences are different. A failure of the diaphragm will have the regulator wide open and causing it not to maintain pressure. This will result in a loss of inert gas, but not an accumulation of pressure. A failure of the spring, however, will result in the failed regulator to close and prevent it from relieving the pressure. The mitigation for this condition is to have a redundant back pressure regulator in parallel to the primary. The redundant regulator will take over control in the event of the failure of the primary as shown in FIG. 11d.

[0213] An alternative option of achieving backpressure to the storage panel is to connect the output line of the graphite panel to a water column as shown in FIG. 11e.

[0214] The principle of operation is based on the pressure effect of the weight of water. The pressure the base of the water column is equal to the height of water above it. If the height of the water were for example 10, it would require an argon pressure greater than 10 WC (25 mbar) for it to be relieved through the water column. In this configuration, the materials needed for this type of system are inexpensive as the system only encounters low pressure. For example, the water column type valve could be constructed from a thin-walled steel pipe or PVC piping.

[0215] The unit has a surge chamber above the water column. The purpose of this chamber is to prevent water from entering the graphite panel in the event of a rapid depressurising. The surge chamber has been nominally sized at twice the volume of liquid in the column.

[0216] Also, the liquid used does not need to be water. Other fluids, such as ethylene glycol could be used with the column height adjusted to account for the change in specific gravity. The disadvantage of using water is that biomaterial could accumulate, causing the unit to fail. Other liquids which do not support biomass development can address this issue. Also, these liquids can be are coloured which will aid inspection if a sight glass is installed in the column.

(a) Test Rig 1

[0217] A test rig is shown in FIG. 12. In this embodiment, pressure needed to maintain a blanket of inert gas in the graphite panel is low, in the order of 1 to 2 inches water column (wcWC) (2.5-5.0 mbar).

[0218] The graphite panel is a sealed container that is heated. Since it is a sealed container, the pressure of the gas inside the container will increase when heated. To prevent an over pressure inside the graphite panel, a back-pressure regulator is installed that will relieve the gas if the pressure exceeds a predetermined pressure level. For this setup, a fabricated pressure maintaining device will be used in place of a backpressure regulator to carry out that function. The relieving pressure of the regulator should be set sufficiently high so not to relieve the inert gas unnecessarily, but at a point that will not stress the graphite panel enclosure during the heating and cooling cycles. In this configuration, the backpressure is assumed to be 10 WC (25 mbar).

[0219] Argon is supplied in high-pressure bottles with an integrated pressure regulator to reduce the pressure (1). The argon bottles are connected to the argon header line by a flexible hose connection (2). The pilot regulator requires a maximum upstream pressure of 10 barg. If the bottle regulator cannot provide this pressure, a separate regulator (3) should be installed upstream of the pilot regulator to reduce the pressure to 10 barg (or lower). The accuracy of this regulator is not critical, as the pilot regulator will maintain an accurate downstream pressure regardless of the upstream pressure.

[0220] The pressure to the graphite panel is maintained by a low-pressure pilot operated regulator (4) set at 2WC (5.0 mbar). The regulators pilot is connected to a downstream point with SST tubing (5). The location of the connection is not critical but should be sufficiently downstream so it is not affected by the turbulent flow from the pilot regulator output, and is sufficiently close to the inert gas inlet to ensure that the pressure regulation reflects the pressure of inert gas in the graphite panel.

[0221] A pressure switch and solenoid (6) is installed downstream of the pilot regulator to cut off the supply of inert gas to the graphite panel in the event of an overpressure (most likely caused by a regulator failure). The pressure switch setting should be above the backpressure setting and below the safety margin (pressure) of the graphite panel. In this instance, it is assumed to be 15WC (37 mbar). The solenoid is energised to open, with a high-pressure signal de-energising the solenoid valve. Since the capacity (CV) of the backpressure device is much larger than the CV of the pilot pressure regulator, the likelihood of pressure increase in the graphite panel caused by a failure of the pilot regulator is low.

[0222] The connection of the inert gas from the inert gas header to the graphite panel, and from the graphite panel to the backpressure device is by a SST tube (7) (8) (9). The size of the table is sufficient to pass the require volume of inert gas.

(b) Test Rig 2

[0223] This system as shown in FIG. 13 replaces the fabricated backpressure device with a pilot operated back pressure regulator. If the regulator fails closed and the pressure rises. If it rises above the value of PS 2, valve V2 will open to vent the inert gas to the atmosphere.

(c) Principle of OperationProcess flow diagram (PFD)

[0224] The PFD shown in FIG. 14 is a simplified PFD and omits non-return and manual isolation valves as well as instrumentation except for the pressure switches associated with manifold relief or inert gas isolation. The graphite panels are assembled into units. The PFD assumes that one unit contains four panels (number not essential to describe operation) and the energy storage array is shown with four units.

[0225] Inert gas such as argon to the graphite panels is supplied by the gas bottles or the recovered and compressed gas from the graphite panels. If there is leakage from the graphite panels while inert gas is supplied by recompression, the inert gas from the gas bottles will be blended with the recompressed inert gas to replace the lost inert gas. The method of detecting inert gas loss and combining bottled inert is not shown or described, however, would be known by persons skilled in the art.

[0226] Each unit can be isolated from the inert gas system if taken out of service. If isolated, both the inlet and outlet valves must be closed. In the case of Unit 1, this is V5 and V9.

[0227] There are two pressure reducing regulators in parallel to supply inert gas at a controlled pressure to the graphite panels. R1 is the primary regulator and set at 2 WC and R2 is the secondary regulator set at 1 WC.

[0228] If R1 fails in the closed configuration, the outlet pressure will fall. When it drops to 1 WC, R2 will take over control.

[0229] If R1 or R2 fails in the open configuration, the pressure will rise. If the pressure increases above a predetermined value (PS 1), the solenoid valve V3 will close, and the flow of inert gas to the graphite panels will stop.

[0230] The pressure is maintained in the graphite panels by the backpressure regulators R3 (primary set at 10 WC) and R4 (secondary set at 12 WC). If R3 fails in the closed configuration, the pressure will rise. When it rises to 12 WC, R4 will take over control.

[0231] If both R3 and R4 fail in the closed configuration, the pressure will rise. When it rises to a predetermined value (PS 2) V4 will open and vent inert gas to the atmosphere.

[0232] During normal operation, the inert gas outlet from R3 (or R4) is accumulated in T1. The inert gas is then compressed, filtered and dried and buffered in tank T2.

[0233] (d) Graphite Oxidation and Fire

[0234] The use of an inert gas blanketing system can avoid graphite oxidation which occurs in the presence of oxygen at temperatures above 450 C. Further, use of an inert gas blanketing system can prevent graphite fires. The four conditions which are all required for a graphite fire are:

[0235] High temperatures >1100 C.;

[0236] large mass of graphite;

[0237] exposure to adequate supply of oxygen; and

[0238] unchecked source of heat.

[0239] The energy storage apparatus of the present invention cannot trigger or sustain a graphite fire and each of these conditions have been designed to eliminate or reduce the risks.

[0240] Each heating element has a thermocouple welded to a sheath of the tubular element. This temperature is used to control the power input of the heater. Further, the heating element is designed to fail when sheath temperature reaches 1000 C.

[0241] The unit of the present invention has a maximum gross weight of 30 tonnes and each graphite panel is limited to 12 tonnes of graphite.

[0242] Each unit has an inert gas management system which monitors the oxygen level in the graphite panel with inert gas injection. The graphite body is encased in a gas tight enclosure and allowed to breath expelling hot inert gas and breathing in cool inert gas.

[0243] The operating temperature range of the energy storage apparatus is preferably from 500 to 800 C. (although variances to operating temperature out of this range is also possible depending on application), and when maximum set temperature is reached the power to the heating elements are cut off. The heater controls are fail-safe in that failure of the control system causes power to be cut off from the heating elements.

[0244] Further, heat can be extracted out of the graphite panels by flowing a heat transfer fluid and/or working fluid through the heat exchanger.

[0245] Example 6Material Selection for sCO.sub.2 Heat Exchanger Piping

[0246] The Applicant has evaluated 20 potential heat exchanger materials suitable for supercritical CO2 based on the following operating criteria:

[0247] Temperature between 500 to 800 C.;

[0248] Pressure from 100 to 250 bar (and above)

[0249] sCO.sub.2 and air as heat transfer fluids; and

[0250] Heat exchanger piping embedded in solid graphite crucible.

[0251] In order to determine the suitability, each of the heat exchanger materials was evaluated and ranked with regards to their temperature/pressure performance, carburisation resistance, weldability, bendability, availability, cost, compatibility with sCO.sub.2 and compatibility with molten aluminium. The materials shortlisted and ranked based on the above criteria (in descending order) are alloys 625, 740H, 230, 617 and 800HT. However, depending on application of the energy storage apparatus, the other heat exchanger materials may also be suitable for use in the energy storage apparatus of the present invention.

[0252] The following alloy materials are preferred:

[0253] 625 is a preferred heat exchanger material due to its high ranking in most categories;

[0254] 740H is another preferred heat exchanger material due to its high allowable stress at operating temperature;

[0255] 230 remains in consideration as a substitute for 740H;

[0256] 617; and

[0257] 800HT remains in consideration for lower temperature and pressure applications, due to its low comparative cost and ready availability, this material is suitable if the temperature and pressure of the application are reduced and extent of carburisation can be quantified.

[0258] As would be appreciated by a person skilled in the art, the selection of a heat exchanger material can depend on the operating parameters of the energy storage apparatus. The preferred heat exchanger material can be application dependent due to factors such as operating conditions, project requirements and manufacturing environment. However, energy storage apparatus of the present invention is largely indifferent to heat exchanger material selection (i.e. only minor design changes are required for a different piping material).

[0259] To maximise energy conversion efficiency when the energy storage apparatus is used for supercritical fluids such as sCO2, the energy storage apparatus can be operated between 500 to 800 C. (and potentially above) and from 100 to 250 bar (and potentially above).

[0260] The heat exchanger piping is embedded in the solid graphite (assembled by component parts) and is used as the conduit for heat extraction, with sCO.sub.2 and air considered for the heat transfer fluids (HTFs) at these high temperature and pressure conditions.

[0261] The energy storage apparatus of the present invention can be designed to comply with the following standards, ASME BPVC (relevant sections), ASME B31.3 and EU Pressure equipment Directive PED 2014/68/EU.

[0262] As the heat exchanger piping is in contact with graphite at high temperatures, the material is preferably carburisation resistant.

Example 7Regulating Outlet Temperature/Multistage Attemperation

[0263] FIG. 15 shows an embodiment of an energy storage array comprising three energy storage apparatus (TES_1, TES_2, TES_3). Each energy storage apparatus is in fluid communication with a first-stage mixing chamber (first-stage mixing chamber_1, first-stage mixing chamber_2, first-stage mixing chamber_3), and a second-stage mixing chamber is in fluid communication with all the first-stage mixing chambers.

[0264] In operation, the three streams of heat transfer medium having an initial outlet temperature (1500, 1501, 1502) are mixed with a cooling liquid (not shown) in the first-stage mixing chambers to provide streams of heat transfer medium (1503, 1504, 1505) having a temperature below threshold temperature. These streams are then provided to a second-stage mixing chamber as a single stream (1506), where it is mixed with a cooling liquid to provide heat transfer medium stream (1507) at the application temperature. The skilled person would appreciate that the conditions of streams 1503, 1504 and 1505 are not necessarily the same and are dependent on the operating conditions including the temperature of the energy storage apparatus and the cooling liquid.

[0265] FIG. 16 shows an embodiment of the first-stage attemperation. An outlet stream (1601) from an energy storage apparatus is mixed with a cooling liquid stream (1603) in a first-stage mixing chamber to provide a stream of heat transfer medium (1602) with a temperature below a threshold temperature. The cooling liquid stream is introduced to the first-stage mixing chamber via three fixed flow valves. The valves may have different fixed flow rates to provide a total of 8 different levels of flow control.

[0266] FIG. 17 shows an embodiment of the second-stage attemperation. A stream (1701) from a first-stage mixing chamber is mixed with a cooling liquid stream (1703) in a second-stage mixing chamber to provide a stream of heat transfer medium (1702) with a regulated temperature. The cooling liquid stream is introduced to the second-stage mixing chamber via a variable flow valve to allow fine tuning of the regulated temperature.

[0267] Table 3 shows nominal operating condition ranges of different streams shown in FIGS. 15 to 17.

TABLE-US-00003 TABLE 3 Nominal operating conditions Temperature Pressure Flowrate Stream ( C.) (kPa) (kg/hr) 1500, 1501, 1502 200-750 1400-1600 0-1000 1503, 1504, 1505 200-500 1400-1600 0-1000 1506 200-500 1400-1600 0-3000 1507 120-250 1400-1600 0-3000 1603, 1703 20-159 2400-2600 0-250

[0268] FIG. 18 shows a comparison of outlet temperature with no attemperation, first-stage and second-stage attemperation.

[0269] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.