Technical Fiefd

The present invention relates to the field of energy storage and in particular to devices fo storage of energy generated by renewable sources such as photovoltaies, wind and wave power but may be used with any source which generates power in excess of demand for periods of time.

Background of the Invention

Worldwide there is an increasing awareness of the need to reduce reliance on fossil fuels and increase the use of renewable energy sources. One major renewable energy source that is effectively unlimited in the foreseeable future i solar energy, however solar energy has the disadvantage that it is not available at night and during cloudy periods and so conversion systems need to include some form of energy storage if they are to become a viable replacement for fossil fuel as source of energy.

Other renewable energy sources such as wind, wave and tidal power also have variable output at best and in some cases are unpredictably variable. In order to ensure availability of capacity to meet demand, some means of storage is required to match supply with demand occurring at different times. One method that has been proposed for energy storage is to heat a body when energy production exceeds demand and to recover the heat and convert it to electricity when demand exceeds supply. Various materials have been proposed for use in heat storage bodies hut graphite is particularly useful, in this role. However graphite is combustible at certain conditions at very high temperature and presents special challenges when used as a heat storage medium.

Carbon in the form of graphite is used in a variety of applications to store heat or buffer heat generation in high temperature, plant. A continual risk in such applications is the possibilit of a graphite fire if the graphite at high temperature comes into contact with, oxygen (or air).

Throughout this specification, unless otherwise specified, thermal energy storage panels will be described in a vertical orientation with vertical side walls at least one of which is a radiant energy receiving wall. The panels and their components will be described as having a top and a bottom and tw ends relative to the vertical side walls, and will include a top and bottom walls, and end walls, however the panels may be used in •7

other orientations in which, for example a horizontal orientatio in which the side wall raa he at the top and a top wall may be at the side.

Summary

According to one aspect, the present invention consist in a thermal energy storage module comprising a plurality of -spaced thermal energy storage panel separated by one or more heater assemblies, eac thermal energy storage panel comprising a graphite core, a substantially gas tight housing encasing the graphite core, a heat exchanger comprising heat exchanger tubing including a heat exchanger inlet and a heat exchanger outlet, the heat exchanger tubing at least partially embedded in the graphite core, the heat exchanger inlet and the heat exchanger outlet extending through the housing and the housing sealed around the heat exchanger inlet and the heat exchanger outlet, whereby each graphite core is indirectly heated by one or more of the heater assemblies radiating heat onto the respective gas tight housing of the thermal energy storage panel containin the graphite core.

According to another aspect, the present invention consists in method of controlling a thermal energy storage module comprising a plurality of spaced thermal energy storage panels separated by one or more heater assemblies, each thermal energy storage panel comprising a graphite core, a substantiall gas tight housing encasing the graphite core, a heat exchanger comprising heat exchanger tubing including a heat exchanger inlet and a heat exchanger outlet, the heat exchanger tubing at least partially embedded in the graphite core, the heat exchanger inlet and the heat exchanger outlet extending through the housin and the housing sealed around the heat exchanger inlet and the heat exchanger outlet, whereby each graphite core is indirectly heated by one or more of the heater assemblies radiating heat onto the respective gas tight housing of the thermal energy storage panel containing the graphite core, the method comprising:

establishing an electrical connection between the heater assemblies and a supply of electrical power whereby the heater assemblies to heat the thermal, energy storage panels;

monitoring a temperature in the graphite core of each thermal energy storage panel; and

when any graphite core attains a temperature greater than a predetermined limit, disconnecting the heater assemblies adjacent to the given thermal energ storage panel from the supply of electrical power. The thermal energy storage panels may be mounted by each being suspended from a thermal energy storage module housing.

The thermal energy storage panels may each contain no more than 5000 k of graphite and each may contain between 2000 and 3800 kg or between 2000 and 3000 of graphite. The thermal energy storage module may comprise between 2 and 40 thermal energ storage panels and preferably between 4 and 16 thermal energy storage panels. Each thermal energy storage panel may have an oxygen or inert gas sensor for monitoring the level of an inert gas (such as argon) used to fill voids in the thermal energy storage panel and/or detectin oxygen within the thermal energy storage panel. Methods of testing the condition of the inert gas may include:

i) when temperature is stable, conducting a pressure hold test;

ii) using an oxygen sensor to detect presence of oxygen within the panel;

in) measuring flow of the inert gas into die panel to detect abnormal inflow ⁷ rates. The thermal energy storage module may include an inlet manifold connecting heat exchanger inlet of the plurality of thermal energy storage panels. An inlet manifold temperature sensor may measure inlet manifold temperature. The thermal energy storage module ma also include an outlet manifold connecting heat exchanger outlets of the plurality of thermal energ storage panels. An outlet manifold temperature sensor may measure outlet manifold temperature.

The heater assemblies may each comprise a plurality of heaters which may be arranged in a vertical array. The heaters may comprise bundle of heater elements, and may be arranged in vertical columns, with one vertical column of heaters between each adjacent pair of thermal energy storage panels. Each vertical column of heaters may comprise 2 to 10 heaters. The heaters ma be designed to fail (i.e. stop heating) if the temperature rises above an upper limit (above the other protection limits) to prevent thermal runaway in the system.

The supply of electrical power to each heater or each column of the heaters may be switched by a manual isolator to permit total electrical isolation of any or all of the heaters of a respective column of heaters. The supply of electrical power to the heaters may also be switched by a main manual isolator contactor to permit total electrical isolation of all of the heaters of the thermal energy storage module. One or more temperature sensors, such as thermocouples, may be provided on each of the heaters to measure heater temperature. The suppl of electrical power to each, heater or column of heaters may be

controlled by one or more electroiiic power control devices such as tliyristors, each thermal energy storage panel may have one or more temperature sensors to measure a temperature of the graphite and a programmable logic controller (PLC) may be provided, whereby signals from the temperature sensors may be connected to the PLC and electronic power control devices are controlled by the PLC and the PLC ma be programmed to monitor the temperature sensors and control the electronic power control devices which control the suppl of power to the heaters.

The method of controlling the heaters may comprise interrupting the electrical connection to the heaters adjacent a thermal energy storage panel if any one of the temperature sensors of the respective thermal energy storage panel measures a temperature greater than the predetermined limit or if an average of the temperatures measured by the temperature sensors of the respective thermal energy storage panel is greater than the predetermined limit. The method of controlling the heaters may also comprise interrupting the electrical connection to all of the heaters if any one of the temperature sensors of any thermal energy storage panel measures a temperature greater than the predetermined limit.

Sensors for measuring a condition of an inert gas such as argon in the thermal energy storage panels may also be connected to the PLC and the PLC may be programmed to monitor the sensors and to control the electronic power control devices to isolate the supply of power to a heater if the condition of the inert gas in a thermal energy storage panel adjacent to that heater deteriorates below a predetermined level such as by pressure dropping below a predetermined level or pressure or decreasing rapidly. Alternatively a flow meter may b used on an inert gas inlet line to monitor gas consumption and operate the electronic power control devices if gas supply suddenly increases indicating a possible breach of the ski of the thermal energy storage panel . Detection of the presence of oxygen within a thermal energy storage panel may also be used to operate the electronic power control devices.

Each heater may also be provided with one or more temperature sensors and the PLC may also be programmed to interrupt the electrical connection to one of the heaters if the temperature sensor of the respective heater measures a temperature greater titan di predetermined limit. The PLC may also interrupt the supply of electrical power a column of the heaters if the temperature sensor of one of the heaters in the respecti ve column measures a temperature greater than the predetermined limit. The PLC may be programmed to provide signal outputs and inputs for transmission to and from system level controllers such as a Distributed Control System (DCS) and displays providing control functions and indicating measured and calculated parameters including one or more of:

Module Average Graphite Temperature

Module Max Graphite Temperature (indicating which temperature sensor on which

Panel)

Module Min Graphite Temperature (indicating which temperature sensor on which Panel)

Module State of Charge percentage;

Module State of Thermal Charge kWh _t ;

Heater status; offline, active, faulty;

Module Charge Current and Power;

Inert Gas (e.g. argon) Pressure and or Flow rate;

Inlet manifold and outlet manifold temperature;

System generated commands to start or stop heating.

A local display may be provided t display the outputs from the PLC,

The PLC may measure inlet manifold temperature and transmit the inlet manifold temperature to a central controller. The PLC ma also measure outlet manifold

temperature and transmit the outlet manifold temperature to a central controller.

Brief description of the drawings

Embodiments of the invention will now be described b wa of example with reference to the accompanying drawings in which:

Figure 1 shows a storag module for converting electricity to thermal energy and storing the thermal energy in graphite for later use;

Figure 2 shows a therraal energy storage panel used in the thermal energ storage module of Figure 1 shown in perspective.

Figure 3 shows the thermal energy storage panel of Figur 2 in plan (Figure 3a), elevation (Figure 3b) and end elevation (Figure 3c);

Figure 4 shows a perspective view of a heat exchanger coil used in the panel of Figure 2 & 3; Figure 5 shows a partial perspective view of the heat exchanger coil of Figure 4 sitting on a base capping graphite plank and showing insertion of a graphite plank adjacent to the base capping plank :

Figure 6 shows a partial perspective view of the heat exchanger coi! of figures 4 & 5 with a number of a graphite planks inserted;

Figure 7 shows a perspective view of the heat exchanger coil of figures 4, 5 & 6 fully embedded in graphite planks, with a graphite plank partially inserted the underside;

Figure 8 shows the thermal energy storage panel of Figures 2 & 3 with a surface of the housing removed;

Figure 9 shows a cross-section of two of the planks seen in Figures 5, 6, 7 and 8 illustratin a half obround groove in which the heat exchanger tubing is contained;

Figure 10a & 1.0b, shows side and rear views of the heater assembly used in the thermal energy storage module of Figure 1 , with a rear mounting panel removed in the rear view (Figure 10b) for clarity;

Figure 11 shows a heater which forms part of the heater assembly of Figure 10; and

Figure 12 schematically illustrates the electrical circuit used to operate the thermal energy storage module of Fipire 1..

Detailed description

Referring to Figure 1 , an energy storage module 100 i illustrated. The thermal energy storage module 100 is housed in a housing 1.01. having die dimensions of standard intermodal shipping container making the unit relatively easy to tra sport using

conventional transportation equipment. The housing 101 would typically have an outer skin and insulation within, which are not shown in Figure 1 to permit a view of internal components. Within the housing a plurality of discrete thermal energ storage panels 102 are alternated with heater assemblies 106 (described in greater detail below). Each thennal energy storage panel 102 has a metal shell containing a graphite body and embedded tubes for heat recover also described in detail below.

The thermal energy storage panels 102 are suspended from mounting frames 1 5 to which they are bolted. The mounting frames 105 are in turn suspended from cross members 104 supported between upper rails 103 of the housing 1 1 of the thermal energy st rage module 1 0- The heater assemblies 106 between adjacent thermal energy storage panels 102 comprise a plurality of electric heaters 107 producing radiant energy that is absorbed through the shells of the thermal energy storage panels 102 to heat the internal graphite bodies,

Thus the heaters 107 of the thermal energy storage module 10 may be connected to a renewable energ supply or a network energy supply, such that when suppl exceeds demand for energy, the heaters 107 are connected to the supply to use the excess energy to generate heat which is radiated through air towards the thermal energy storage panel 102, The thermal energy storage panels 102 absorb the heat in the graphite encased in each thermal energy storage panel 102 t store excess energy for later use.

Each of the thermal energy storage panels 102 includes, embedded tubes, which carry a hea transfer fluid and e ble heat to be recovered from the thermal energy storage panels. Inlet tubes 113, 114 deliver heat transfer fluid to each thermal energy storage panel 102 from inlet manifold 1 15, and after being heated, the heat transfer fluid is passed from each thermal energy storage panel 102 via outlet tubes 7, 1.18 connected to outlet manifolds 119.

When tire demand for electrical energy exceeds the supply, a heat transfer fluid is passed through .the tubes embedded in the graphite to extract the stored heat for use to warm up the power generating system (e.g. HRSG, piping and steam turbine) and or to drive a power generating machine. Typically the heat transfer fluid will be water/steam, although other possibilities exist such as carbon dioxide, compressed air, heat transfer oil, refrigerant, etc.

A plurality of thermal energy storag modules 100 may be used in a system with different thermal energy storage modules being switched in to receive excess energy as the amount of excess energy increases. Similarly different thermal energy storage modules 100 may be brought on-line to permit recovery of stored energy as demand increases above the available supply of energy.

The use of a plurality of thermal energy storage panel in the thermal energ storage module, of the embodiments described herein and the method of their operation allows the parameters of the thermal energy storage module t be constrained such that the possibility a graphite fire is eliminated.

The graphi te in each thermal energy storage panel may be encased in a gas tight high temperature stainless steel skin filled with an inert gas, such as argon gas. The condition of the inert gas may be continuously monitored and the module unit shut down or its operating temperature reduced when the condition of the inert gas in a thermal energy storage panel is lost. For example die pressure of the inert gas may be monitored and the module shut down if the pressure in one thermal energ storage panel drops below a predetermined level, or if while temperature is stable the pressure does not remain within predefined limits. The thermal energy storage panels may also include an oxygen sensor to be monitor for presence of oxygen and the heating may be shut down if oxygen is detected in any significant amount.

Each thermal energy storage panel may have a plurality of temperature sensors (e.g. 3), such as thermocouples TKl to T8-3, (as seen in Figure 12) to measure graphite temperature at multiple locations within the panel. The graphite is heated to a maximum operating temperature (e.g. 650 ^e C), which is well below a temperature at which a graphite fire can be initiated or sustained fie >1400 ^° C).

The heating elements in the thermal energy storage module may be sized to reach the maximum operating temperature in 4 hours. Thermal energy input is preferably stopped when die average graphite temperature reaches me maximum operating

temperature (e.g. 650°C). In the event that thi safety mechanism fails, heating elements of the heater 107 are designed to fail if their ambient temperature (in the middle of the heater elements) reaches 1000 °C.

The thermal energy storage module may comprise 8 thermal energy storage panels each containing 2200kg of graphite. Each thermal energy storage panel is separated from adjacent energy storage panels and each energy storage panel is encased by a high temperature steel skin. This separates the graphite mas into small sub-units, which are each below the critical mass required for ini tiation or maintenance of a graphite fire.

The thermal energy storage module is designed to extract heat efficiently through the embedded heat exchanger tubes in the graphite of each thermal energy storage panels. The current embodiment of the thermal energy storage module has been rated to extract 3.6MW of thermal energy over 4 hours but can be designed to extract more or less oyer a shorter or longer period of time depending on the various parameters (e.g. heat transfer fluid, flow rate, etc.) chosen to suit the particular application, without departing form the fundamental design principles discussed herein. In the event of graphite temperature exceeding the maximum average operating temperature (i.e. of 650°C in this example) the heating elements may be shut off. In the improbable event that this safety mechanism fails, the heating elements may also be capable of being manually shut off and the heat may be extracted out of the graphite through the heat transfer fluid fed to the units.

The present thermal energy storage module uses purely sensible heat storage in an inert materi l. .

In a preferred embodiment the heaters would be rated at between 15kW to. 40kW however this depends on the design time for charging the system and may be varied for particular applications (the shorter charge time the larger the heater capacity required). The heaters are typically powered by 3 phase power with control by electronic control device such as thyristors but may have simple ON / OFF control. In the illustrated embodiment each heater assembly 106 comprises a column of 5 heaters 107 which are divided into 7 groups of 5 heaters.

Th-yristor control provides variable power to the heaters 107 of the heater assemblies 106, with each column of 5 heaters 107 connected to one thyristor (TY1-7 in Figure 12). The thyristors TYl-7 may for example allow power to trickle int the thermal energy storage panel 102 as they approach the maximum design temperature to ensure the temperature does not overshoot the set maximum limits. A central control system may provide commands to a local controller to operate the thyristors to turn heaters on or off or to heat at a reduced level.

Apart for temperature sensors in the thermal energy storage panels there may be one or more temperature sensors per heater 107, such as thermocouples HE 1-HE35, linked to its temperature controller to ensure the element does not overheat shortening its useful life.

Typically the heaters may be designed for 415 Volt 50 Hz or 460 V 60Hz 3 phase operation, but may vary from country to country and site to site based on specifications of the .available power supply. For example the supply may not be 3 phase, may operate at a different current and voltage and may be a different frequency or ma even be designed for Direct Current

At the plant storage system level thermal energy storage modules may be connected in 'trains' where a 'train' consists of thermal energy storage modules connected in series and/or in parallel depending on the output conditions required for that plant.

In Figure 2 an example of the outer housin of a thermal energy storage panel 102 is i llustrated in perspective view. The panel of Figure 2 is also illustrated in Figure 3 in plan (Figure 3a), elevation (Figure 3b). and end elevation (Figure 3c) views. The thermal energy storage panel housing comprises two large substantially flat parallel side walls 212, 213 bounded by a bottom wall 214, end walls 215, 21.6 and a top wall 217 to form a closed container. In use the panel 102 will typically be oriented vertically with the bottom wall

214 typically located at a lower end of the panel. With reference to Figure 2 and Figure 3a, b, & c, in one form the housing has dimensions of 2200mm (C) x 1.800mm (B) x 400mm (A) (see Figure 3), however these dimensions may vary to optimize usage of graphite cut from standard dimension blank and to optimize packing of complete thermal energ storage panels into containers of different sizes.

The bottom wall 214 of the housing may be integrally formed with the two side walls 212, 213 by bending a single piece of wall material into a "IF shape in which the base transitions into each of the side walls via a curved bend 271 of radius R which in the present example is in the range of 50 t 180 mm and nominally 80 mm. The wall material is preferably a sheet steel material capable of retaining structural integrity to support the enclosed graphite core, the heat exchanger and any heat exchange fluid contained therein at elevated temperatures of at least 1 00°C.

'The -walls of the housings in Figures 2 and 3 are preferably fabricated from stainless steel (316/304) or 253MA ausicnitic stainles steel (or any suitable high temperature thermally conductive material such as 800.H austenitic steel or alloys such as Inconel) .finished to mill finish class 2B. The surfaces 212, 213, 214, 215, 216 & 217 of the thermal energy storage panels 102 may have a natural finish to the stainless steel material (specific emissivity 0. 7) or a polished surface (specific .emissivity 0,2 - 0,3), or may be provided with another suitable surface coating or treatment (specific emissivity in the range of 0.3 - 0.8). The surfaces 212, 213, 214, 215, 216 & 217 may also be coated with a robust high temperature heat absorbing (e.g. black - specific absorptivity in the range of 0.8 - 1.0, preferabl 0.90 - L0) paint, surface treatment or other suitable coating.

Mounting flanges 121 are provided extending from the tops of the end walls 215,

216 and include respective mounting holes 223. The flanges 121 are used to suspend the panel 102 from the mounting frame 105 by bolting them to the mounting frame via the mounting holes 223. Each flange may comprise an extension of one of the end walls 215, 216 beyond the respective side wall 213 to which it is joined (i.e. the flange may be cut from the same piece of sheet material as the end walls 21.5, 216 from which they extend). By suspendin the thermal energ storage panel from the flanges 121 rather than supporting it from below, the resulting tension in the side walls due to gravit of the graphite core acting on the housing allows them to resist buckling to maintain good thermal communication with the graphite core. The curved shape of the housing where the side walls 215, 216 join the bottom wall 214 through a bend 271 also tends to keep the metal walls pressed against the graphite core.

Vents 251 are provided In the top wail 217 of the housing to allow venting during welding together of the housing walls. These holes ma be plugged (e.g. by welding after the panel walls are joined), or they may be used to accommodate sealed cable ports throug the wall to pass instrumentation cables such as thermocouple wires into the housing, as fill ports to provide an argon blanket to the graphite core, to accommodate a filling nozzle to fill the void space and/ or an internal reservoir with graphite powder or other thermall conductive media, or to accommodate a connection to an external reservoir to maintain the level of such materials when the graphite core and housing expand and contract during thermal cycling. In the illustrated embodiment, one of the vents 251 is used to accommodate sealed cable ports 161 through the wall to pass instrumentation cables such as thermocouple wires into the housing. The cable port 161 is also used as fill ports t provide the argon blanket to the graphite core. A second vent 251 is used to accommodate a filling nozzle 163 to fill the void space and/or an internal reservoir with the graphite powder or other thermally conducti ve media.

Further holes 252 & 253 are provided in the top wall 217 of the housing t allow passage of the heat exchanger outlets 117 & 118 respectively. Similarly holes 254, 255 are provided in the side wall 216 of the housing to allow passage of the heat exchanger inlets 114 & 113 respectively.

Referring to Figure 4, a heat exchanger 420 is shown in perspective. The heat exchanger 420 is embedded in a graphite core as seen in Figures 5, 6 & 7. The heat exchanger 420 comprises heat exchanger tubing 425, 426, 427, 438, 439, 440 and first and second heat exchanger inlet 113, 114 and first and second heat exchanger outlet 1 17, 118. The first and second heat exchanger inlet 113, 114 and first and second heat exchanger 117, 118 are interchangeable as inlet or outlet depending on the direction in which it i desired to flow the heat exchange fluid through the heat exchanger in a particular application. The heat exchanger inlets 113, 114 terminate straight tube portions 440 which form part of a first serpentine shaped tube portion 425 comprisin sequential "TJ" shaped sections 428. The first serpentine shaped tube portions 425, of which: there are two in parallel, are joined with welded joins 437 to a plurality of intermediate serpentine shaped tube portions 426, similarly joined together by welded joins 437, Final serpentine shaped tube portions 426 are joined to final serpentine shaped tube portions 427 by further welded joins 437. The final serpentine shaped tube portions 427 each terminate in outlet sections 438 & 439 which extend to the outlets 117 & 118 respectively.

The number of "U" shaped sections 428 provided in the serpentine portions 425, 426, 427 can vary depending on the application. For example for low flow rates with long discharge durations, the fewer the number of "U" shaped sections 428 may be required and conversely for high flow rates with short discharge durations more "IT shaped sections 428 may be required.

The heat exchanger tubes may be made, for example, from 253MA austenitic stainless steel (or any suitable high temperature thermally conductive material such as 800H austenitic steel or alloys such as Inconel), and may have a nominal outside diameter in the range of for example 26.67mm to 42.1 mm. In the present embodiment the nominal outside diameter is 33.4mm but the outside diameter may vary to be greater or smaller than this depending on the particular circumstance of the application. The heat exchanger tubing 426, 439, 440, and associated inlet tubes 113, 1 14 and first and second heat exchanger outlet tubes 117, 118 are preferably formed with at least some sections of the tube assembl taking a coiled or serpentine form suitable for compression (like a spring) during assembly (e.g. the serpentine portions 425, 426, 427 and the outlet sections 438, 439), such that when the housing 102 expands due to thermal expansion, the resulting stresses from the movement of the pipe configuration does not exceed the mechanical properties of the pipe material.

Referring to Figure 4, the heat exchanger 420 comprises two parallel serpentine shaped tube assemblie each having an independent inputs 113 & 114 and outputs 1 17 & 1 18, however applications may require differing numbers of coils such a L 2, 3, 4 coils etc. The heat exchanger 420 is almost fully embedded in a graphite core as seen in Figures 5, 6 & 7. The heat exchanger 420 comprises heat exchanger tubing 425, 426, 427, 438, 439, 440, 1 17, 1 18, 113 & 114. The lower tube ends 113 & ί 14 provide the two inlets and connect to the lower end of the main tube assembly comprisin tube portions 425, 426 &. 427. The heat exchanger inlets 113 & 114 may also act as drains. The upper tube ends 117 & 1 18 provide the two outlets and terminate tube sections 439 & 440 extending from the upper end of the main tube assembly comprising tube portions 427. The tube portions 425, 426, 427, are joined together by welds 437. The flow may be reversed in various applications such that the inlets may be 117 and 118 and the outlets may be 113 and 114. The heat exchanger tubes may be made, for example, from 253MA austenitic stainless steel {or any suitable high temperature thermally conductive material such as 800H austenitic steel or alloys such as Ineonel), and may have a nominal outside diameter of for example 33.4mm in this embodiment but the outside diameter may vary to be greater or smaller than thi depending on the particular circumstances of the application.

Referring to Figures 5, 6 and 7, the heat exchanger inlets 113 & 114 extend through the ends of grooves 51 1 in a bottom graphite capping plank 509. The "Li" shaped bends 428 in the tube portions 426 are accommodated in recesses 513 in the ends of the graphite planks 512, A holes 522 is also provided in the graphite planks 512 to permit the insertion of a locating tube (not shown) to maintain the location of the graphite planks after assembly. Referring to Figure 8, the heat exchanger outlet tubes 117 & 118 extend through openings 252, & 253 in the top wall 117 of the housing 102 and the heat exchanger inlet tubes 113 & 114 extend through openings 255, & 254 in the bottom of the end wall 216 of the housing 102. The tubing portions 425, 426, 427 are able to move to accommodate expansion of the heat exchanger tubing in use, without exceeding the material limits of the tubing.

The housing is sealed around the heat exchanger inlet tubes 1 13 & 1 14 and outlet tubes 117 & 1 1 8 where the exit the housing through the holes 252, 253, 255 & 254 such that, air cannot enter the housing after it. is sealed. The plurality of openings 251 in the top wall 217 of the housing (as seen in Figure 8) act as vents during welding together of the wall panels. These vents may be sealed by welding after the rest of the panel has been welded togethe or they may be used as sealed cable ports for sensors such as thermocouples used to monitor conditions inside the panel in operation, as fill and purge ports to provide argon blanket to graphite core or as filling nozzle to fill void space with graphite powder or other thermally conductive media.

After the heat exchanger is fabricated, pre-shaped planks of graphite 509, 512, are positioned to encompass most of the heat exchanger tubes. Referring to Figure 5, first a lower capping plank 509 is positioned beneath the lowest tubes 440 which extend to the inlets 113 <&i l4.

The lower capping plank 509 is grooved 511 on one (upper) surface with the grooves having a semicircular (or preferably obround) cross-section conforming to the shape and radius of the lowest tube sections 440 of the heat exchanger. The lower edges 506 of the lower capping plank 509, between the face opposite the grooved surface (i.e. the downward facing surface in. Figures 5, 6 & 7) have a radius corresponding with the transition 271 between the side walls 212, 213 and the base wall 214 of the housing (see Figure 8). The edges 506 may have a radius in the range of 50-150 mm and in the proposed embodiment will have a radius of 80mm.

Referring to Figures. 5,· 6, 7 & 9, the bulk of the graphite planks 512 are positioned between the rows of tubes in the tube portions 425, 426, 427. The graphite planks 512 each include two opposite surfaces in which the semicircular (or preferably semi-obround) grooves 511, 516 are formed, conforming to the shape and radius of the tubes of tube portions 425, 426, 427. When semi-obround grooves are used they are elongated in the vertical direction (i.e. two grooves abut to form an obround cross section with a vertical major axis) to accommodate expansion of the tube assembly in the vertical direction (as viewed in Figure 7). Referring to Figure 9, a partial cross section of two abutting planks 512 shows two pairs of aligned semi-obround grooves (511, 516) encompassing pair of tubes 426.

Referring to Figure 8, after the remaining graphite planks 512 are in position a void

802 will remain above planks to accommodate the tube sections 438 & 439. A volume of graphite powder 801 is deposited over the upper tube sections 438 & 439 in the void 802 t accommodate expansion and contraction of the housing as the temperature of the assembly changes. The graphite powder ma not completely fill the void 802 leaving a small space above the graphite powder 801.

Preferably the abutting surfaces of the graphite planks of Figures 5, 6 & 7 will have a surface finish which is N8 or better (ISO 1302). Such that when assembled between rows of straight tube portions adjacent pairs of the planks encompass and closely conform to the respective straight tube portion and first connecting tube portions at the internal working temperature of the panel, which is up to 800°C, the grooves are made approximately 1.6% bigger than the nominal outside diameter of the tubes with a tolerance of approximately +0.00/~1.00%. For example, when the heat exchanger tube are made from 253MA austenitic stainless steel (any suitable high temperature thermally conducti ve material such as 800H austenitic steel or alloys such as Inconel) and have a nominal outside diameter of 33,4mm, the grooves will preferably be 33.9mm (+0.00/-0,25mm) in diameter. Alternatively, when the heat exchanger tubes are made from the same or similar material and have a nominal outside diameter of 26.67mm, the grooves will preferably be 27.1mm (+0.00/-0.25mm) in diameter and when the heat exchanger tubes have a nominal outside diameter of 42.16mm, the grooves will preferably be 42,9 (+0.00/-0.2 ram.) in diameter. To achieve a high contact surface without excessive expense, the surface of the graphite within the grooves will preferably have a surface finish which is N7 or better (ISO 1302). By maximising the contact of the graphite- with the surface of the grooves by designing the grooves to be sized appropriately for the tube diameter at the working temperature and by providing appropriate surface finish, the operation of the heat, exchanger within the graphite is enhanced.

The graphite planks 509, 512, are assembled to encompass the heat exchanger 420, in the open housing, and the locating tube is inserted into the hole 522 extending through all of the planks to maintain alignment. The locating tube may engage a locating pin projecting from the base of the housing (not shown) to locate the graphite core 509, 512, withi the housing. The housing is then welded closed, including sealing the openings 255, 254, 252, 253 through which the inlet tubes 113 & 114 and outlet tubes 117 & 118 pass through the housing, to form the finished panel 102 (see Figures3 & 8). The vent holes 251 ma also be sealed either by welding or by inserting sealing plugs or a port fitting that allows sealed passage of transducer cables such as thermocouple wires into the interior of the panel. The vent holes 251 might also be fitted with port fittings to be used as fill ports to provide argon blanket to graphite core or as filling nozzles to fill void space 802 with graphite powder or other thermally conductive media.

Because the graphite planks extend to the ends of the housing and almost fully occupy the space within the housing, the load of the graphite is spread evenly across the botto wall 214 of the housing, allowing thinner material to be used. Also by maximizing the area of graphite in contact with the walls and consequentially minimizing void space, the heat transfer into the graphite by conduction may be maximized. Minimizing void space also minimizes the amount of trapped air that is available to react with the graphite when the panel is heated to it operating temperature. In th present embodiment the volume of void spaces within the housing not occupied by graphite or tubing is generally in the range of 4- 10% and typically 5-7% of the internal volume of the housing (at the working temperature). Correspondingly the side panel of the housing, which is the irradiated surface of the panel when in use, is generally backed by the graphite core over till but 1-5 % of its area and typically 2-3% (at the working temperature) in the preferred embmiiment. In the top wall of the panels, openings 251 allow expansion of the internal air during manufacture and may be welded closed or used as ports. One of the openings 25 i is shown with a filling nozzle 163 attached to permit filling of void spaces with graphite powder (refer to description of Figure 8 below),

Figure 8 shows a thermal energy storage panel 102 with one side wall removed showing the graphite planks 509, 512, forming the graphite core. Voids will exist between the graphite planks and the walls of the housing (e.g. between the planks 509. 512, visible in Figure 8 and the vertical walls 212, 213, 215, 216, including the wall 213 which has been removed). A larger void 802 forms a reservoir between the top of graphite core and the top of the housing. The reservoir 802 and the voids in this case are at least partly filled with graphite powder 801. The graphite powder 801 enhances heat transfer between walls of the housing and the graphite core. A filling nozzle 163 is in communication with the reservoir 802 to enable filling of the voids in the housing and topping up of the reservoir 802. The reservoir 802 stores additional graphite powder which prevent spaces opening up when expansion and contraction of the housing and core occur during thermal cycling. This arrangement may be employed in any of the previously described embodiments.

^' Referring to Figures 10a & 10b, side and rear views are illustrated of one of .the heater assemblies 1 6, used in the thermal energy storage module of Figure 1. The rear view is shown with a rear mounting panel 1002 removed. The heater assembly comprises a plurality of heaters 107 mounted through a front mounting panel 1001 and supported by support rails 101 1 which are supported by the rear mounting panel 1002. Figure 11 is a perspective view .of one of the heaters 107 seen in the heater assembly 106 of Figure 10a. The heaters 107 comprise Thermal Tubular-Cairod™ heating elements 1003, or equivalent, which are manufactured with a resistance coil of nickel-chrome wire centrally located within a metal sheath tube. Terminal pins are fusion welded to each end of the resistance coil. These terminal pins form the cold zone 1113 of the heating element 1003, seen in Figure 1 1. The resistance coil assembly i stretched within the tube and filled with a magnesium oxide powder, which electrically insulates the assembly from the outer sheath of the element. The magnesium oxide powder has excellent heat transfer properties and when combined with an evenly stretched resistance coil, a. uniform, heat is achieved along the length of the heating element 1.003. Once filled, the heating element 1003 is then roll compacted, which compresses the magnesium oxide powde to a rock hard construction. This protects the resistance assembly from atmospheric corrosion and mechanical damage. The heating element 1003 is then trimmed at each end t expos the terrainal pin. Silicone insulators are used to insulate the terminal pin from the outer sheath of the heating element 1003 and a terminal connection is fitted. The heating elements 1003 are then bent into "Tj" shapes and fitted into the assembly 107 in which the heater elements 1003 pass through a pluralit of intermediate spacers 1004. Cold legs 111 are provided at the terminal ends of the elements 1003 which pass through a mounting flange 1005

The heaters 107 are mounted in the thermal energy storage module 100 via the front mounting panel 1001 a top panel 10.10 and the rear mounting panel 1002. The front panel lOOl, the top panel 1010 and rear panel 1002 may be formed from a single folded sheet of high temperature plate steel. In the case of the front mounting panel 1001 , tubes 1008 extend from the mounting panel and terminate in flanges 1009 to which mounting flanges 1005 of the heaters are bolted. The cold legs 1113 of the heater elements pass through the tubes 1008 such that the tubes are not heated excessively. Connection to the individual elements 1003 of the heaters i made to the heater element terminals 1007 mounted on the heating element mounting flange 1005. The thermal, energy storage module 100 may include an internal cabling tray (not shown) and each of the heaters 107 (flanged heating element bundle (FHE's)) ma be wired up with a cable looped at the end of the module to which an Electrical Controller (ECB) 1200 is mounted, inside a removable hatch at the end of the module.

Referring to Figure 12, the example thermal energy storage module 100 illustrated in Figure 1 may comprise;

i. 8 thermal energy storage panels each containing 2200 kg of graphite;

ii. 7 vertical columns of FHEs 107;

iii. Each vertical column of heaters Comprises 5 FHE's 107:

iv. Each FHE 107 is rated at 30kW (3 phase 415 V, 50 Hz) providing 150 W per column or 1,05 MW per thermal energy storage module 100) heating both sides of Panels 1 2 (except the end panels which are heated on one side only);

v. All FHE 1.07 connections are made on one side of the Module .1.00 and are spaced 200mm inside the insulated face panel (not illustrated) of the module:

vi. Each thermal energy storage panel 1.02 has 3 thermocouples, (or a total of 24

thermocouples per thermal energy storage module 100: TP 1 -1. to TP8-3);

vii . Each FHE 1 7 has a single thermocouple located within the bundle of heater

elements 1003 (HE1 to HE35); viii. The thermal energy storage module 100 also includes at least one inlet manifold thermocouple Til , and at least one outlet manifold thermocouple TQI ;

ix. Each thermal energy storage panel 102 has a sensor for measuring the condition of the inert gas (e.g. argon) pressure within the thermal energ storage panel (e.g. 8 pressure transmitters per module: PT1-PT8) and optionally one oxygen sensor (ΟΤΊ-ΟΤ8);

The Electrical Controller (ECB) 1200 may be attached to an end or the side of the thermal energy storage module 100 and may comprise:

i. A weatherproof Electrical Control Bo or housing 1202 having for example

dimensions of 2.43m W x 2.89m H (max) x any suitable depth. If the height of the housing is less than the height of the thermal energy storage module 100, an awning may be installed above the housing for shade;

ii. The ECB will preferably be suitabl Ingress Protection (IP) rated;

in. A main manual isolator Contactor S3 may be provided for the therm l energy

storage module to permit total electrical isolation of all of the heaters 106 of the module 100.

iv, A manual isolator S2-8 may be provided for each column of FHE's 107;

v. A Programmable Logic Controller (PLC) 1201 programmed to:

a. monitor the thermocouples Tl-1 to T8-3;

b. The thermocouples HE3 to HE35; and

c. The pressure transmitters PT1 to PT8;

d. Control power to each column of FHEs (or each FPtE) by controlling

thyristors TYI-7 (or TY 1-35) which control power to the heaters;

e. Provid sequence control for the switching of the FHE to avoid switching multiple heaters simultaneously.

f. Provide signal outputs 1203 and inputs 1204 for transmission to and from system level controllers (DCS) and displays providing control function and indicating measured and calculated parameters such as:

• Module Average Graphite Temperature

• Module ax Graphite Temperature (which thermocouple on which Panel)

• Module Min Graphite Temperature (which thermocouple on which Panel) • Module State of Charge %

• Module State of Thermal Charge k Wh _t

» Each FHE status: offline, active, faulty

• Module Charg Current and Power

* Inert gas (e.g. argon) Pressure PI to P8

• Inlet manifold and outlet manifold temperature

• System generated commands to start or stop heating

vi. A local display to display the outputs from PLC;

It will be appreciated by person skilled in the art that numerous variations and or modifications may be made to the above-described embodiments, without departing from the broad general scope of the present disclosure. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.